City and County of San Francisco 2030 Sewer System Master Plan

Transcription

City and County of San Francisco
2030 Sewer System Master Plan
TASK 600
TECHNICAL MEMORANDUM NO. 607
LONG-TERM BIOSOLIDS MANAGEMENT PLAN
FINAL DRAFT
August 2009
2700 YGNACIO VALLEY ROAD • SUITE 300 • WALNUT CREEK, CALIFORNIA 94598 • (925) 932-1710 • FAX (925) 930-0208
pw://Carollo/Documents/Client/CA/SFPUC/7240A00/Final Draft PM-TM/600 System Configurations/Task600TM607_LongTermBiosolidsMgtPlan.doc (FinalDraft)
CITY AND COUNTY OF SAN FRANCISCO
2030 SEWER SYSTEM MASTER PLAN
TASK 600
TECHNICAL MEMORANDUM
NO. 607
TABLE OF CONTENTS
Page No.
1.0
BACKGROUND..................................................................................................... 607-1
2.0
WASTEWATER SYSTEM OVERVIEW ................................................................ 607-1
2.1 Southeast Water Pollution Control Plant ...................................................... 607-2
2.2 Oceanside Water Pollution Control Plant ..................................................... 607-2
2.3 North Point Water Pollution Control Plant .................................................... 607-4
2.4 Solids Processing Facilities.......................................................................... 607-4
2.5 Recent Biosolids Quantities ......................................................................... 607-5
2.6 Current Biosolids Management Practices .................................................... 607-5
2.7 Recent Solids Characteristics ...................................................................... 607-5
2.8 Current Solids Management Costs ............................................................ 607-11
2.9 Biosolids Management Program Goals...................................................... 607-12
2.10 Previous Biosolids Management Planning for San Francisco.................... 607-12
2.11 References................................................................................................. 607-13
3.0
SOLIDS PROJECTIONS..................................................................................... 607-13
3.1 Future Raw Wastewater Solids Quantities and Characteristics ................. 607-13
3.2 Future Digested Solids Quantities.............................................................. 607-14
3.3 Product Quantity Projections for Alternative Treatment Options................ 607-15
3.4 Future Biogas Production Rates ................................................................ 607-16
3.5 Fats, Oils, and Grease Quantities .............................................................. 607-16
3.6 Organic Waste Material.............................................................................. 607-17
3.7 References................................................................................................. 607-18
4.0
REGULATORY AND PUBLIC FRAMEWORK .................................................... 607-18
4.1 Regulatory Considerations ......................................................................... 607-18
4.2 Policy Considerations................................................................................. 607-22
4.3 Recent Contracts ....................................................................................... 607-23
4.4 Annual 40 CFR 503 Report ........................................................................ 607-24
4.5 Public Input and Involvement ..................................................................... 607-24
4.6 Industry Trends .......................................................................................... 607-24
5.0
BIOSOLIDS MARKETS AND DISPOSITION ...................................................... 607-25
5.1 Biosolids Products...................................................................................... 607-25
5.2 Agricultural Land Application Market – Class B Biosolids .......................... 607-26
5.3 Agricultural Land Application Market – Improved Biosolids Products ........ 607-29
DRAFT - September 14, 2009
i
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
Landfill Markets .......................................................................................... 607-30
Horticulture and Silviculture - Product Distribution and Marketing ............. 607-32
Land and Mine Reclamation Market .......................................................... 607-34
Construction Products Market .................................................................... 607-34
Fuel and Energy Markets ........................................................................... 607-35
Dedicated Disposal .................................................................................... 607-36
Overall Market and Product Assessment ................................................... 607-36
References................................................................................................. 607-40
6.0
STATUS ON ORGANIC WASTE PROCESSING ............................................... 607-40
6.1 Organic Waste Situation at San Francisco ................................................ 607-40
6.2 Digestion of Organic Waste ....................................................................... 607-41
6.3 Economic and Non-economic Assessment for Organic Waste
Processing/Digestion ............................................................................................ 607-46
6.4 Summary .................................................................................................... 607-46
6.5 References................................................................................................. 607-46
7.0
PROCESSING TECHNOLOGIES ....................................................................... 607-47
7.1 Thickening Technologies ........................................................................... 607-47
7.2 Digestion Stabilization Technologies.......................................................... 607-50
7.3 Non-Digestion Stabilization Technologies.................................................. 607-57
7.4 Dewatering and Drying Technologies ........................................................ 607-63
7.5 Other Solids Processing Technologies ...................................................... 607-71
7.6 Biogas Processing and Use Technologies................................................. 607-72
7.7 Screening Criteria ...................................................................................... 607-76
7.8 Screening to Identify Viable Technologies ................................................. 607-78
7.9 Recommendations on Viable Technologies ............................................... 607-78
7.10 Solids Processing Approach ...................................................................... 607-78
7.11 References................................................................................................. 607-91
8.0
SOLIDS PROCESSING SITES........................................................................... 607-92
8.1 Dispersed Versus Centralized Solids Processing ...................................... 607-92
8.2 Land Area Needs for Centralized Processing ............................................ 607-93
8.3 Bayside Solids Site Alternatives................................................................. 607-94
8.4 Oceanside Solids Siting ............................................................................. 607-95
8.5 Evaluation of Bayside Site Options ............................................................ 607-98
8.6 References............................................................................................... 607-100
9.0
EVALUATION OF BIOSOLIDS MANAGEMENT ALTERNATIVES................... 607-100
9.1 Alternative B-1 - Retain/Upgrade Existing Class B Program.................... 607-102
9.2 Alternative B-2 - Upgrade To Class A Program and Expand Uses .......... 607-105
9.3 Alternative B-3 - Create Marketable Products for ~Half of the
Production ................................................................................................ 607-107
9.4 Alternative B-4 - Create Marketable Products for Entire Production ........ 607-110
9.5 Alternative B-5 - Utilize Thermal Processing............................................ 607-112
9.6 Economic Comparisons ........................................................................... 607-113
9.7 Evaluation of the Alternatives................................................................... 607-117
9.8 Recommendations ................................................................................... 607-121
9.9 References............................................................................................... 607-122
ii
10.0 IMPLEMENTATION PROGRAM....................................................................... 607-122
10.1 Program Description ................................................................................ 607-122
10.2 Site Selection for BBC.............................................................................. 607-124
10.3 Products and Markets .............................................................................. 607-124
10.4 Technology Follow-Up ............................................................................. 607-126
LIST OF TABLES
Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Table 8
Table 9
Table 10
Table 11
Table 12
Table 13
Table 14
Table 15
Table 16
Table 17
Table 18
Table 19
Table 20
Table 21
Table 22
Table 23
Table 24
Table 25
Table 26
Table 27
Table 28
Table 29
Table 30
Table 31
Table 32
Table 33
Table 34
Table 35
Table 36
Table 37
Table 38
Recent San Francisco Biosolids Production ................................................ 607-5
San Francisco Biosolids Management Summary, 2005............................... 607-6
San Francisco Biosolids Beneficial Use, 2005 ............................................. 607-6
Average 2005 SEWPCP Biosolids Metal Concentrations ............................ 607-7
Class B Pathogen Density Compliance at SEWPCP, 2005 ......................... 607-7
Vector Attraction Reduction Compliance at SEWPCP, 2005 ....................... 607-8
SEWPCP Nutrient Monitoring Results, 2005 ............................................... 607-8
Average 2005 OSWPCP Biosolids Metal Concentrations............................ 607-9
Class B Pathogen Density Compliance at OSWPCP, 2005....................... 607-10
Vector Attraction Reduction Compliance at OSWPCP, 2005..................... 607-10
OSWPCP Nutrient Monitoring Results, 2005 ............................................. 607-11
Estimated Current Annual Solids Management Costs ............................... 607-11
Current Hauling and Tipping Fees ............................................................. 607-11
Bayside Raw Wastewater Solids Projections, Year 2030 .......................... 607-13
Oceanside Raw Wastewater Solids Projections, Year 2030...................... 607-14
Bayside Digested Solids Projections, Year 2030 ....................................... 607-15
Oceanside Digested Solids Projections, Year 2030................................... 607-15
Comparison of Current and Future Biosolids Production Rates................. 607-15
Average Annual Product Quantities, Year 2030......................................... 607-16
Biogas Production Estimates, Year 2030................................................... 607-16
San Francisco FOG Estimates................................................................... 607-17
Estimated Organic Waste Quantities in San Francisco.............................. 607-17
Local Regulation of Biosolids Land Application in Northern California....... 607-22
Current San Francisco Biosolids Management Practices .......................... 607-23
Available Landfill Options ........................................................................... 607-31
Potential Demand for Biosolids Product Within San Francisco.................. 607-34
Market Assessment.................................................................................... 607-37
Product and Market Compatibility .............................................................. 607-39
Technology Screening ............................................................................... 607-79
Technology Recommendations Summary ................................................. 607-87
Sites Considered for BBC .......................................................................... 607-95
BBC Siting Criteria ..................................................................................... 607-98
BBC Site Screening Evaluation.................................................................. 607-99
Suitable Bayside Sites - Advantages and Disadvantages........................ 607-101
Outline of Five Categorical Biosolids Alternatives.................................... 607-102
Costs for Biosolids Management Alternatives.......................................... 607-115
Evaluation Criteria for the Biosolids Management Alternatives................ 607-118
Ratings of Biosolids Management Alternatives ........................................ 607-120
iii
LIST OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
The San Francisco Wastewater System ...................................................... 607-2
SEWPCP Process Flow Diagram ................................................................ 607-3
OSWPCP Process Flow Diagram ................................................................ 607-3
Temperature Phased Anaerobic Digestion ................................................ 607-51
Acid/Gas Phased Digestion Options .......................................................... 607-53
Class A Thermophilic Digestion Options .................................................... 607-55
Belt Filter Press .......................................................................................... 607-64
Centrifuge................................................................................................... 607-65
Screw Press ............................................................................................... 607-65
Direct Thermal Drum Dryer Producing Graded Pellet Product................... 607-69
Indirect Thermal Dryer Producing Ungraded Product ................................ 607-70
Sites Considered for Bayside Biosolids Center (1)..................................... 607-96
Sites Considered for Bayside Biosolids Center (2)..................................... 607-97
Price Range for Biosolids Cake Disposition in California ......................... 607-104
Biosolids Management Alternatives ......................................................... 607-117
BBC/OBC Simplified Solids Process Schematic ...................................... 607-123
OSWPCP Simplified Solids Process Schematic ...................................... 607-124
Potential Sites for Bayside Biosolids Center ............................................ 607-125
iv
Technical Memorandum No. 607
Please note this memo was created in February of 2007 and was not updated. It was
determined by the SFPUC and the consultants that it was important to capture the
information at the time of development so the reviewers could see the progression of
information and decisions made at the time of the memo development. Please also
note that the word 'alternative' was used instead of 'configurations' for the memos
reflecting the existing wording at the time it was written. In the Summary Report, the
term was updated to 'configuration' so as not to confuse the CEQA review process.
The configurations mentioned herein may have changed or been eliminated and are
not considered full CEQA alternatives.
1.0
BACKGROUND
Wastewater treatment plants serve to remove the waste from wastewater, so that clean
plant effluent can be safely discharged to waterways or be reused for beneficial purposes.
The wastewater treatment process concentrates the waste materials present in sewage,
creating wastewater solids or sewage sludge as a residual material. The wastewater solids
are processed to create “biosolids.” Biosolids are wastewater solids that comply with
standards developed by the United States Environmental Protection Agency (USEPA) for
beneficial use.
This technical memorandum addresses wastewater solids processing needs to create
biosolids products, evaluates alternative biosolids management methods and approaches
for San Francisco, and includes recommendations for improvements and future needs
relating to San Francisco’s wastewater solids processing and biosolids management.
2.0
WASTEWATER SYSTEM OVERVIEW
The San Francisco sewer system is primarily a combined sewer system whereby storm
water and wastewater are conveyed in the same pipes. Many cities, including New York,
Philadelphia, Boston, Seattle, and Sacramento, have combined sewers. Other cities have
one system of pipes for sewage and another system of pipes for storm water. San
Francisco developed a relatively unique system of transport/storage boxes and related
infrastructure for the City’s combined wastewater system in the 1970s and 1980s.
Transport/storage boxes collect the combined wastewater and stormwater and transport it
to pumping stations, which deliver it to three treatment plants shown in Figure 1. Combined
sewer discharges (CSDs) occur when the flows exceed the treatment and storage capacity
of the system. The three treatment plants are described below.
607-1
Figure 1
2.1
The San Francisco Wastewater System
Southeast Water Pollution Control Plant
The Southeast Water Pollution Control Plant (SEWPCP) has a dry weather average flow
capacity of 85 million gallons per day (mgd). During wet weather conditions plant can
provide up to 150 mgd of secondary treatment, and an additional 100 mgd of primary
treatment. Flows up to 110 mgd are discharged through an outfall to San Francisco Bay.
Flows greater than 110 mgd are discharged to Islais Creek. Only secondary effluent is
discharged to Islais Creek; all primary effluent discharges are routed to the San Francisco
Bay outfall. The liquid treatment processes include screening, grit removal, primary
sedimentation, pure oxygen activated sludge, secondary clarification, and sodium
hypochlorite disinfection, and sodium bisulfite dechlorination, as shown in Figure 2.
2.2
Oceanside Water Pollution Control Plant
The Oceanside Water Pollution Control Plant (OSWPCP) has a dry weather average flow
capacity of 21 mgd and peak wet weather flow capacity of 65 mgd. Treated water is
discharged through an ocean outfall. The liquid treatment processes include screening, grit
removal, primary sedimentation, pure oxygen activated sludge, secondary clarification, and
sodium hypochlorite disinfection, as shown in Figure 3.
607-2
Figure 2
SEWPCP Process Flow Diagram
Figure 3
OSWPCP Process Flow Diagram
607-3
2.3
North Point Water Pollution Control Plant
The North Point Water Pollution Control Plant (NPWPCP) provides wet weather treatment
only. The peak wet weather capacity is 150 mgd. The effluent is discharged to the San
Francisco Bay through four outfalls. The NPWPCP provides primary level treatment only for
wet weather flows.
2.4
Solids Processing Facilities
The existing processes used to turn wastewater solids into biosolids are described below.
2.4.1
The existing SEWPCP solids processing facilities are shown in Figure 2. Primary sludge is
thickened in the primary clarifiers. Waste activated sludge (WAS) is thickened using gravity
belt thickeners. The two thickened sludges are mixed in a solids blending tank prior to being
fed to the anaerobic digestion system.
The anaerobic digestion system consists of ten tanks. Seven tanks are active digesters,
and two tanks are storage digesters. One digestion tank is currently out of service due to a
collapsed roof. The sludge within the tanks is heated to mesophilic temperatures (95 to 100
degrees Fahrenheit) and continuously mixed. The typical sludge residence time within the
anaerobic digestion system exceeds 15 days. Anaerobic bacteria in the tanks degrade
volatile solids in the sludge. The wastewater solids are stabilized in the anaerobic digestion
process and pathogen densities are significantly reduced. Water is removed from the
biosolids that exit the digesters using centrifuges. The dewatered biosolids have a
gelatinous consistency and are called biosolids “cake.” The cake is loaded into covered
trucks for transport to the end-use or disposal sites.
The anaerobic bacteria in the digestion system create biogas as a byproduct of the
digestion process. The biogas is a mixture of methane and carbon dioxide. The biogas is
collected and used as fuel for boilers, and as fuel for a cogeneration system that uses an
internal combustion engine and generator for electrical power production with waste heat
from the engine providing heat for the digestion process. In addition, the SFPUC is
implementing a 600 kW molten carbonate fuel cell project at the SEWPCP to convert
biogas into electricity.
2.4.2
The OSWPCP solids processing facilities are shown in Figure 3. Primary and waste
activated sludges are mixed and co-thickened using gravity belt thickeners prior to being
fed to the anaerobic digestion system. The anaerobic digestion system consists of four eggshaped digesters. Biogas from the anaerobic digestion process fuels boilers and a
cogeneration system that supplies about 30 percent of the electricity needs of the
607-4
wastewater treatment plant. Digested biosolids are dewatered using belt filter presses prior
to being loaded into covered trucks for transport to the end-use or disposal sites.
2.4.3
North Point Water Pollution Control Plant
Solids from the NPWPCP are returned to the combined sewer system during and following
wet weather events for transport to the SEWPCP.
2.5
Recent Biosolids Quantities
Table 1 presents the quantity of biosolids produced by San Francisco during the period
2003 through 2005.
Table 1
2003
Recent San Francisco Biosolids Production
SEWPCP
OSWPCP
Wet
Dry
Wet
Dry
Year
Tons
Tons
Tons
Tons
66,558
17,189
22,261
3,613
Total City
Wet
Dry
Tons
Tons
88,819
20,802
2004
60,138
14,535
21,401
3,293
81,539
17,828
2005
58,269
13,738
24,845
3,814
83,114
17,552
Annual average
61,655
15,154
22,836
3,573
84,491
18,727
169
42
63
10
231
52
Daily Average
2.6
Current Biosolids Management Practices
San Francisco has beneficially reused all of the biosolids produced in recent years. The
biosolids are transported to Alameda, Contra Costa, Solano, and Sonoma Counties for
agricultural land application, use as landfill alternative daily cover (ADC), or other landfill
beneficial use. Agricultural land application occurs during the months of April through
October each year. Landfill use occurs throughout the year. Table 2 provides a summary of
biosolids management practices in 2005. Table 3 summarizes the 2005 data by beneficial
use.
2.7
Recent Solids Characteristics
The solids characteristics most pertinent to biosolids recycling include solids concentration,
metals concentrations, pathogen densities, and vector attraction reduction. Recent solids
characteristics from the San Francisco Water Pollution Control Plants are described below.
607-5
Table 2
San Francisco Biosolids Management Summary, 2005
County
Site
Beneficial Use
Tons
(wet weight)
Percent
of Total
Alameda
Vasco Road Landfill
ADC
175
0.2
Contra
Costa
Western Contra Costa County
Landfill
ADC
9,668
11.6
ADC + other
43,260
52.1
ADC
4,243
5.1
Synagro
Land
Application
19,204
23.1
Synagro
Land
Application
6,564
7.9
83,114
100
Hay Road Landfill
Solano
Sonoma
Potrero Hills Landfill
Totals
Table 3
San Francisco Biosolids Beneficial Use, 2005
Beneficial Use
Tons (wet weight basis)
Percent of Total
Landfill ADC and other uses
57,346
69
Land Application
25,768
31
Totals
83,114
100
2.7.1
The total solids concentrations for biosolids from the SEWPCP averaged 23.6 percent
during 2005.
Average metal concentrations for 2005 are shown in Table 4, along with the 40 CFR 503.13
“Table 3” pollutant concentrations. As shown in Table 4, the biosolids produced at the
SEWPCP have low metals concentrations, well below “Table 3” limits, which are the most
stringent established by the USEPA.
The SEWPCP achieved compliance with 40 CFR 503 Class B pathogen density
requirements during 2005 by maintaining anaerobic digestion detention times greater than
15 days and digestion temperatures above 95°F. Table 5 summarizes the 2005 results.
607-6
Table 4
Average 2005 SEWPCP Biosolids Metal Concentrations
SEWPCP 2005
Pollutant
Average
Concentration Limit(2)
(mg/kg(1))
(mg/kg(1))
Constituent
Compliance?
Arsenic
<6.74
41
Yes
Cadmium
<5.58
39
Yes
Copper
517
1500
Yes
Lead
115
300
Yes
Mercury
1.63
17
Yes
(3)
Molybdenum
11.4
Yes
Nickel
28.6
420
Yes
Selenium
<6.24
100
Yes
Zinc
882
2800
Yes
Source: SFPUC, February 20, 2006.
(1) Milligrams per kilogram, dry weight basis.
(2) From 40 CFR 503.13, Table 3.
(3) Limit is under reconsideration by USEPA. Biosolids may not exceed 75 mg/kg
molybdenum until a new pollutant concentration limit is established.
Table 5
Class B Pathogen Density Compliance at SEWPCP, 2005
Average Digester
Class B
Digestion Detention
Temperature
Compliance?(1)
Month
Time (days)
(°F)
Jan
20.9
95
Yes
Feb
19.3
97
Yes
Mar
17.6
96
Yes
Apr
18.0
98
Yes
May
20.5
99
Yes
Jun
21.4
99
Yes
Jul
19.7
98
Yes
Aug
23.3
97
Yes
Sep
24.4
98
Yes
Oct
24.8
98
Yes
Nov
23.1
97
Yes
Dec
21.7
96
Yes
(1) 15 days detention time at or above 95°F required per 40 CFR 503.32(b)(3).
607-7
The SEWPCP achieved compliance with 40 CFR 503 vector attraction reduction
requirements during 2005 by reducing volatile solids concentrations by 38 percent or
greater in the anaerobic digestion process. Table 6 summarizes the 2005 results.
Table 6
Vector Attraction Reduction Compliance at SEWPCP, 2005
Average Volatile Solids
Month
Reduction(1)
Compliance? (2)
Jan
40.1
Yes
Feb
56.3
Yes
Mar
45.9
Yes
Apr
51.5
Yes
May
52.3
Yes
Jun
43.4
Yes
Jul
45.2
Yes
Aug
48.5
Yes
Sep
47.6
Yes
(3)
Oct
56. 8
Yes
Nov
51.2
Yes
Dec
51.5
Yes
(1) Van Kleeck Method used to calculate volatile solids reduction, except where noted.
(2) 38 percent or greater reduction required per 40 CFR 503.33(b)(1).
(3) Approximate mass balance method used to calculate volatile solids reduction.
Nutrient monitoring is required for the land application of biosolids, so that the biosolids
loading rate to the soil can meet the fertilizer needs of the crop that is grown. Table 7
summarizes the 2005 nutrient monitoring results for the SEWPCP.
Table 7
SEWPCP Nutrient Monitoring Results, 2005
Average Concentration
Parameter
(%(1))
Total Kjeldahl Nitrogen(2)
5.65
Ammonia
1.54
Total Phosphorus
1.56
(1) Dry weight basis.
(2) Measurement of organic nitrogen plus ammonia nitrogen.
607-8
2.7.2
The total solids concentrations for biosolids from the OSWPCP averaged 15.4 percent
during 2005.
Average metal concentrations for 2005 are shown in Table 8, along with the 40 CFR 503.13
“Table 3” pollutant concentrations. As shown in Table 8, the biosolids produced at the
OSWPCP have low metals concentrations, well below the “Table 3” limits, which are the
most stringent established by the USEPA.
Table 8
Average 2005 OSWPCP Biosolids Metal Concentrations
SEWPCP 2005
Pollutant Concentration
(1)
Average (mg/kg )
Limit(2) (mg/kg(1))
Constituent
Compliance?
Arsenic
<4.36
41
Yes
Cadmium
5.9
39
Yes
Copper
779
1500
Yes
Lead
151
300
Yes
Mercury
1.57
17
Yes
Molybdenum
13
-(3)
Yes
Nickel
43
420
Yes
Selenium
9.32
100
Yes
Zinc
1325
2800
Yes
(1) Milligrams per kilogram, dry weight basis.
(2) From 40 CFR 503.13, Table 3.
(3) Limit is under reconsideration by USEPA. Biosolids may not exceed 75 mg/kg
molybdenum until a new pollutant concentration limit is established
The OSWPCP achieved compliance with 40 CFR 503 Class B pathogen density
requirements during 2005 by maintaining anaerobic digestion detention times greater than
15 days and digestion temperatures above 95°F. Table 9 summarizes the 2005 results.
The OSWPCP achieved compliance with 40 CFR 503 vector attraction reduction
requirements during 2005 by reducing volatile solids concentrations by 38 percent or
greater in the anaerobic digestion process. Table 10 summarizes the 2005 results.
Table 11 summarizes the 2005 nutrient monitoring results for the OSWPCP.
607-9
Table 9
Class B Pathogen Density Compliance at OSWPCP, 2005
Average Digester
Temperature
Class B
Digestion Detention
Compliance? (1)
Time (days)
(°F)
Month
Jan
28
98
Yes
Feb
24
98
Yes
Mar
23
98
Yes
Apr
27
98
Yes
May
28
98
Yes
Jun
30
98
Yes
Jul
32
99
Yes
Aug
22
98
Yes
Sep
37
99
Yes
Oct
32
98
Yes
Nov
25
98
Yes
Dec
28
97
Yes
(1)
15 days detention time at or above 95°F required per 40 CFR 503.32(b)(3).
Table 10
Vector Attraction Reduction Compliance at OSWPCP, 2005
Average Volatile Solids
Month
Reduction(1)
Compliance? (2)
Jan
58.5
Yes
Feb
62.3
Yes
Mar
66.9
Yes
Apr
66.2
Yes
May
70.2
Yes
Jun
66.1
Yes
Jul
68.8
Yes
Aug
62.0
Yes
Sep
65.2
Yes
Oct
70.8
Yes
Nov
65.1
Yes
Dec
57.3
Yes
(1) Van Kleeck Method used to calculate volatile solids reduction.
(2) 38 percent or greater reduction required per 40 CFR 503.33(b)(1).
607-10
Table 11
OSWPCP Nutrient Monitoring Results, 2005
Average Concentration
(%(1))
Parameter
7.17
Total Kjeldahl Nitrogen(2)
Ammonia
2.19
Total Phosphorus
1.50
(1) Dry weight basis.
(2) Measurement of organic nitrogen plus ammonia nitrogen.
2.8
Current Solids Management Costs
Estimated current annual solids management costs (excluding amortized capital) for the
SEWPCP and OSWPCP are summarized in Table 12. The costs reflect the thickening,
digestion, and dewatering processes at the wastewater treatment plant sites, as well as
offsite hauling and final disposition of the biosolids by contractors. The current offsite
hauling and tipping fees are summarized in Table 13 (SFPUC, 2005).
Table 12
Estimated Current Annual Solids Management Costs
Description
Labor
Energy
Chemicals
Materials and Services
Contract Hauling and End Use
Total Annual Cost
Table 13
County
Alameda
Contra
Costa
Solano
Sonoma
SEWPCP
$3,814,000
$1,174,000
$983,000
$452,000
$2,050,000
$8,473,000
Current Hauling and Tipping Fees
Hauling ($/ wet ton)
Reuse Site
SEWPCP OSWPCP
Vasco Road Landfill
$16.01
$17.34
Western Contra Costa
$14.34
$15.93
County Landfill
Hay Road Landfill
$24.50
$25.72
$22.46
$24.97
Synagro Land
$23.78
$25.57
Application
Synagro Land
$22.70
$18.74
Application
OSWPCP
$1,653,000
$402,000
$265,000
$128,000
$888,000
$3,336,000
Tipping
($/ton)
$15.95
$19.00
Total ($/wet ton)
SEWPCP OSWPCP
$31.96
$33.29
$33.34
$34.93
$10.75
$15.95
$12.95
$35.25
$38.41
$36.73
$36.47
$40.92
$38.52
$22.50
$45.20
$41.24
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2.9
Biosolids Management Program Goals
The SFPUC has established the following goals for its biosolids management program:
•
Create a sustainable, flexible and diversified program of biosolids management.
•
Maximize beneficial and cost-effective reuse of the City’s biosolids and any system
byproducts (i.e., digester gas or biogas).
•
Maximize energy efficiency and recovery in the processing of biosolids.
•
Provide a high degree of protection for health and safety.
•
Evaluate Class A treatment processes and techniques to determine feasibility and
cost-effectiveness for City facilities.
•
Evaluate the complete range of environmental impacts of biosolids management and
choose options that minimize negative impacts.
•
Manage biosolids operations (treatment processing, transportation and reuse) in a
manner consistent with being a good neighbor to the local community.
•
Complete & certify Biosolids Environmental Management System.
•
Investigate feasible markets for biosolids reuse within the City and County of San
Francisco.
•
Consider opportunities for co-management with other waste streams (grease, food
waste, etc.).
•
Develop a ten-year maintenance plan for the existing solids handling facility to ensure
reliability and compliance.
2.10 Previous Biosolids Management Planning for San Francisco
Several biosolids management and sludge processing studies have been conducted by
San Francisco over the past 25 years. The current biosolids management system of the
City has evolved from a variety of activities and studies over many years. The Long-Term
Biosolids Management Plan, prepared by Carollo Engineers in 1997 (Carollo Engineers,
1997) is the most recent long-term, overall study for the City. The 1997 Long-Term
Biosolids Management Plan recommended a diversified biosolids management strategy
that included Class B biosolids recycling as ADC at landfills and agricultural land
application.
607-12
2.11 References
Carollo Engineers. City and County of San Francisco, Long-Term Biosolids Management
Plan. December 1997.
SFPUC. City and County of San Francisco – Cost and Solids Information for 2005.
SFPUC. City and County of San Francisco, Annual Report for Sludge Generators.
February 20, 2006.
3.0
SOLIDS PROJECTIONS
Future wastewater solids quantities and characteristics are discussed in this section, so that
alternatives can be sized and evaluated for appropriate future needs. Also estimated here
are quantities of trucked grease and related wastes which are likely to be co-processed
along with the wastewater solids. A third type of material is the separately-collectable
organic waste material from City residential and commercial sources. This organic waste
material is discussed here because there is also potential for this material to be processed
with wastewater solids.
3.1
Future Raw Wastewater Solids Quantities and Characteristics
Future raw wastewater solids quantities were developed by the SFPUC based on
population projections, water supply projections, pollutant load projections, and historical
treatment plant performance data (SFPUC, May 25, 2006). The raw solids projections are
shown in Tables 14 and 15 for the Bayside and Oceanside, respectively. The projections
assume continued use of primary sedimentation tanks and high-purity oxygen activated
sludge system biological treatment at the City’s wastewater plants. Alternate treatment
methods for wastewater processing could occur in the future, which would alter these
projections. To provide a safety factor for this eventuality, a 10 percent contingency is
added into the year 2030 projections, as indicated in the tables in this section.
Table 14
Bayside Raw Wastewater Solids Projections, Year 2030
Fixed Solids (3)
Peaking Total Solids(1)
Volatile Solids(2)
(klbs/d)
(klbs/d)
Description
Factor
(klbs/d)
Average annual
1.0
265
207
58
Peak month
1.31
347
271
76
Peak 2-week
1.35
358
279
79
Peak day
2.18
578
451
127
Notes:
(1)
10 percent contingency factor was added as a safety factor
(2)
Volatile solids assumed to be 78 percent of total solids, based on historical data.
(3)
Total solids less volatile solids.
607-13
Table 15
Oceanside Raw Wastewater Solids Projections, Year 2030
Fixed Solids (3)
Peaking
Total Solids(1)
Volatile Solids(2)
(klbs/d)
(klbs/d)
Description
Factor
(klbs/d)
Average annual
1.0
55
43
12
Peak month
1.31
72
56
16
Peak 2-week
1.44
79
62
17
Peak day
2.59
142
111
31
Notes:
(1) 10 percent contingency factor was added as a safety factor
(2) Volatile solids assumed to be 78 percent of total solids, based on historical data.
(3) Total solids less volatile solids.
The characteristics of the future raw sludge/solids are assumed at this time to be similar to
current characteristics. It is possible that, in the future, alternate wastewater treatment
methods such as using a membrane bioreactor (MBR) might be used which would change
the characteristics of the biological solids to some extent. Also, effluent filtration could be
added which would increase quantities slightly. If chemical precipitation was added, this
would create more inorganic sludge quantities but would not cause much change in the
total organic wastewater solids.
The potential changes in wastewater processing and the resulting impact on sludge
characteristics, are not so severe as to cause a problem in the evaluation of
sludge/biosolids alternatives or in the development or recommendations of this Biosolids
Management Plan. It is possible that eventual design and operating criteria for solids
processing facilities could be impacted by such changes in sludge characteristics.
3.2
Future Digested Solids Quantities
Assuming that anaerobic digestion will be used to stabilize the raw wastewater solids in the
future, quantities are developed here. (A section evaluating sludge stabilization options is
included in Section 5 of this Report.) Stabilization reduces the odor of the solids and makes
them less attractive to vectors such as flies and rodents. The digestion process greatly
reduces the volatile content of the solids, thereby reducing the total mass of solids. Future
digested solids quantities are shown in Tables 16 and 17 for the Bayside and Oceanside,
respectively. The digestion and solids processing system provides equalization of sludge
production to some extent, and, therefore, peak day production is not presented for
digested quantities.
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Table 16
Bayside Digested Solids Projections, Year 2030
Volatile Solids(1)
Fixed Solids
Total Solids(2)
(klbs/d)
(klbs/d)
(klbs/d)
Description
Average annual
93
58
151
Peak month
122
76
198
Peak 2-week
126
79
204
Notes:
(1) Assumes 55 percent volatile solids reduction in an anaerobic digestion process.
(2) Volatile solids + fixed solids.
Table 17
Oceanside Digested Solids Projections, Year 2030
Fixed Solids
Total Solids(2)
Volatile Solids(1)
(klbs/d)
(klbs/d)
Description
(klbs/d)
Average annual
19
12
31
Peak month
25
16
41
Peak 2-week
28
17
45
Notes:
(1) Assumes 55 percent volatile solids reduction in an anaerobic digestion process.
(2) Volatile solids + fixed solids.
Table 18 presents comparisons of current and future biosolids production rates. Significant
increases in biosolids quantities are expected during the planning period.
Comparison of Current and Future Biosolids Production Rates
Current Average
Year 2030 Projection(1)
Increase
Source
(dry tons/d)
(dry tons/d)
(%)
Bayside
42
72
71
Oceanside
10
15
50
Total City
52
87
67
Note:
(1) Assumes 95 percent capture rate in dewatering process.
Table 18
3.3
Product Quantity Projections for Alternative Treatment Options
Digested solids receive further treatment to reduce water content and change the product
characteristics so that it will be suitable for the end use or disposition. Table 19 presents the
average annual product quantities from the Bayside and Oceanside for various product
moisture contents. The table also shows the total number of truckloads per day required to
transport the entire mass of processed solids from the City.
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Table 19
Average Annual Product Quantities, Year 2030
Bayside
Oceanside
Moisture
(wet tons/
(wet tons/
day)
day)
Product
Content
Dewatered cake
85%
479
99
Dewatered cake
75%
288
60
Thermally dried product
10%
80
17
Combustion ash
0%
29
6
Note:
(1) Assumes 20 wet tons per truckload.
3.4
Total City
(truckloads/
day)(1)
29
18
5
2
Future Biogas Production Rates
Biogas production estimates from the anaerobic digestion process are summarized in
Table 20. This projection assumes 55 percent volatile solids reduction. Somewhat greater
gas production is likely to occur from advanced digestion processes and Class A anaerobic
digestion processes.
Table 20
Biogas Production Estimates, Year 2030
Description
Bayside(KSCF/d)(1)
Oceanside (KSCF/d) (1)
Average annual
1820
378
Peak month
2384
495
Peak 2-week
2457
544
Peak day
3967
978
Note:
(1)
Assumes 16 cubic feet per pound of volatile solids destroyed.
3.5
Fats, Oils, and Grease Quantities
Fats, oils, and grease (FOG) consist of both “yellow grease” and “brown grease.” Yellow
grease is FOG waste generated by restaurants and collected for recycling by rendering
companies, which use it to make tallow and other products. Therefore, yellow grease
quantities are not expected to be available for use within the sludge processing system at
the City.
Brown grease is FOG discharged to sewers or collected from grease traps, and it becomes
contaminated with sewage. The City is expecting to institute additional rules in the near
future to minimize the amount of brown grease that is discharged to the sewers. In the
future, the brown grease is expected to be collected separately in trucks to prevent its
potential clogging and other negative impacts in the sewers. Trucks that collect brown
grease could be logically directed to bring this material to the anaerobic digestion system
for wastewater solids. This has proven cost-effective at other wastewater agencies in
607-16
California. The quantity of brown grease is not added onto the raw solids quantities
presented in Tables 1 and 2, since this material has largely been accounted for already in
those projections. Table 21 presents estimates of the FOG material quantities produced in
San Francisco, based on typical FOG production rates (National Renewable Energy
Laboratory).
Table 21
San Francisco FOG Estimates
Yellow Grease(1)
Location
(lbs/d)
Total City
19,000
Bayside
Brown Grease(2)
(lbs/d)
27,000
n/a
22,000
Oceanside
n/a
5,000
Notes:
(1) Assumes 9 lbs per person per year (National Renewable Energy Laboratory,
1998), and population of 750,000 persons.
(2) Assumes 13 lbs per person per year (National Renewable Energy Laboratory,
1998). The brown grease, in the future, may be trucked to the City’s wastewater
sludge digesters for processing.
3.6
Organic Waste Material
Organic waste materials represent a major potential digestion feedstock to greatly increase
biogas production and produce renewable energy. Norcal collects organic wastes within the
City and County of San Francisco as part of the solid waste collection system. Norcal
estimates that about 400 wet tons per day of organic waste material could be collected in
the future. Table 22 summarizes the organic waste materials potentially available.
Comparison with Table 1 shows that the quantity of organic waste volatile solids is similar
to the year 2030 city-wide volatile solids projections for wastewater solids; therefore, these
organic wastes represent a major potential source of biogas and energy production.
Table 22
Estimated Organic Waste Quantities in San Francisco
Description
Separated Source Organics
2
Quantity1
% Solids
% VS/TS
200 wet tons/day
~30
85
3
Mixed Organics
200 wet tons/day
~50
80
Notes:
(1) 6 day/week basis
(2) SSO = mostly commercial food waste
(3) MO = food and yard waste, plus miscellaneous other wastes including paper
These organic waste materials would need pre-processing before they could be fed to
anaerobic digestion systems, and these costs need to be determined to evaluate the overall
607-17
economics of digesting this material. Discussions between the SFPUC staff and Norcal are
ongoing to further discuss and evaluate options of processing and handling these organic
waste materials in San Francisco, and to discuss potential for co-locating such digestion
and biogas utilization facilities along with wastewater solids processing facilities. Chapter 5
of this technical memorandum provides further details on conceptual planning on potential
organic waste processing and digestion at San Francisco.
3.7
References
National Renewable Energy Laboratory. Urban Waste Grease Resource Assessment.
November 1998.
SFPUC. Wastewater Flow and Load Projections SFPUC Wastewater Master Plan.
May 25, 2006.
Norcal, 2007. Estimates of organic waste production within the City and County of
San Francisco.
4.0
REGULATORY AND PUBLIC FRAMEWORK
This section addresses regulatory, policy, and public perception issues associated with
biosolids management.
4.1
Regulatory Considerations
There are a number of regulatory considerations associated with biosolids management.
Detailed descriptions of the regulatory requirements are provided in a separate Project
Memorandum (REFERENCE). Brief discussion is presented below to provide a regulatory
context to the biosolids planning efforts described in this report. Biosolids use and disposal
is regulated at the Federal, State, and local levels, as described below.
4.1.1
United States Environmental Protection Agency
The USEPA regulates biosolids use under Section 503 of Chapter 40 of the Code of
Federal Regulations (40 CFR 503). The 40 CFR 503 regulations address land application,
surface disposal, and incineration of biosolids. The 40 CFR 503 regulations are selfimplementing and include monitoring, certification, and reporting requirements. Although a
permit application must be submitted, USEPA Region 9 does not typically issue permits.
Agencies are required to send an annual report to USEPA Region 9 summarizing and
certifying their compliance with the rule.
The 40 CFR 503 regulations establish metal concentration limitations, pathogen density
reduction requirements, vector attraction reduction requirements, and site management
practices for land application of biosolids. Land application refers to the beneficial use of
biosolids for their nutrient and organic matter content. Biosolids land application rates
607-18
cannot exceed the fertilizer (nitrogen) needs of the vegetation that will be grown. The metal
concentration limitations are based on a risk assessment prepared by USEPA. The
pathogen density and vector attraction reduction requirements are based on past
successful experience. Biosolids are classified as either “Class B” or “Class A” with respect
to pathogen density. Class B biosolids have significantly reduced pathogen densities (as
compared to raw sludge), but require application site management to ensure protection of
public health and the environment. Class A biosolids have further reduced pathogen
densities and do not require application site management to ensure protection of public
health and the environment. Biosolids that meet the pollutant concentration, Class A
pathogen, and vector attraction reduction requirements in 40 CFR 503 are typically called
“Exceptional Quality Biosolids”, and can be sold or given away in bulk or bags without
additional regulation by USEPA.
The 40 CFR 503 regulations also establish requirements for surface disposal of biosolids.
Surface disposal includes monofills, surface impoundments, lagoons used for final disposal
as opposed to treatment, waste piles, dedicated disposal sites, and dedicated beneficial
use sites. In general, surface disposal of biosolids refers to application at high rates – in
excess of crop nutrient requirements, if a crop is grown – as a management practice. The
regulation establishes metal concentration limitations, pathogen density reduction
requirements, vector attraction reduction requirements, and site management practices.
Incineration refers to combustion of sewage sludge or biosolids at high temperatures in an
enclosed device. The 40 CFR 503 regulations establish metals concentration limits, total
hydrocarbon emission limits, and management practices. The use or disposal of
nonhazardous incinerator ash is not covered by 40 CFR 503; other Federal regulations (40
CFR 257 and 40 CFR 258) cover these practices.
4.1.2
United States Department of Agriculture
The United States Department of Agriculture (USDA) has established national organic food
standards (7 CFR 205) that govern the production and marketing of fresh and processed
food that is labeled “organic.” Although biosolids are essentially all organic matter, biosolids
or products containing biosolids may not be used in organic food production.
4.1.3
State Water Resources Control Board
State regulation of biosolids land application is more stringent than Federal regulation. The
California State Water Resources Control Board (SWRCB) has adopted General Waste
Discharge Requirements (WDRs) for the Discharge of Biosolids to Land for use as a Soil
Amendment in Agricultural, Silvicultural, Horticultural, and Land Reclamation Activities
(Biosolids General Order). The Biosolids General Order can be used by Regional Water
Quality Control Boards (RWQCBs) for streamlined permitting of biosolids land application
sites. The RWQCBs may also elect to create site-specific WDRs for biosolids land
application sites. The Biosolids General Order applies to Class B land application sites and
607-19
sites where Class A Exceptional Quality biosolids will be applied at rates greater than 10
dry tons per acre per year to a field that is larger than 20 acres in size. The Biosolids
General Order goes beyond the requirements of 40 CFR 503 by requiring additional
biosolids testing, soil testing, and groundwater sampling.
4.1.4
Regional Water Quality Control Boards
The RWQCBs protect water quality within their jurisdictions by issuing WDRs to regulate
the discharge of waste to land, including agricultural land application of biosolids and
biosolids co-disposal in landfills.
The RWQCBs regulate biosolids use or disposal by issuing WDRs for sites where biosolids
are to be applied. The adoption of the Biosolids General Order has led to increased
consistency between WDRs, however, the RWQCBs can adopt site-specific WDRs if
conditions warrant.
The SWRCB and the RWQCBs generally recognize that highly treated Class A, Exceptional
Quality biosolids products such as heat dried pellets or properly prepared composts are
commercial products and their use is not regulated.
Landfills are classified by the nature of their lining systems and the types of waste they can
accept. Class III landfills are designed to accept typical municipal solid waste, whereas
Class I landfills are designed for hazardous waste.
Non-hazardous biosolids can be co-disposed at a Class III landfill if:
•
The biosolids are at least 20% solids for primary sludge only, or at least 15% solids if
a mixture of primary and secondary sludges; and,
•
The landfill maintains a minimum 5:1 solids to liquids ratio in the co-disposed waste;
and,
•
The landfill has a leachate collection and removal system.
Most Class III landfills do not have leachate collection and removal systems, and therefore
cannot accept dewatered cake biosolids for co-disposal. The RWQCBs can allow codisposal of biosolids greater than 50 percent solids in Class III landfills without leachate
collection and removal systems.
Class III landfills may also have site-specific waste acceptance criteria established in their
WDRs. Biosolids that exceed the acceptance criteria cannot be co-disposed at that
particular landfill.
4.1.5
California Integrated Waste Management Board
The California Integrated Waste Management Board (CIWMB) is responsible for reducing
California’s use of landfills for waste disposal by increasing waste diversion for recycling.
607-20
The CIWMB regulates co-disposal of biosolids in landfills, use of biosolids for Alternative
Daily Cover (ADC), and biosolids composting facilities.
Some landfills are permitted to use biosolids as Alternative Daily Cover (ADC). At these
landfills biosolids are mixed with other materials to serve as a daily cover for the solid waste
placed in the landfill, reducing the need to use soil for that purpose. ADC is considered to
be a beneficial use, even though the materials are ultimately entombed in a landfill. ADC
use is regulated by the California Integrated Waste Management Board, and is limited to
25 percent of the total landfill cover requirements.
Bioreactor landfills are designed and operated in ways to rapidly degrade organic waste.
The increase in waste degradation and stabilization is accomplished through the addition of
liquid and air to enhance microbial processes and increase the production of landfill gas.
The bioreactor landfill concept is very different from conventional sanitary landfilling
practices and regulations that emphasize minimizing liquid addition and creating “dry tomb”
conditions within landfills. The addition of the moisture, organic matter, and nutrients in
biosolids to bioreactor landfills can potentially increase landfill gas production, which in turn
can be used to produce electricity.
There is currently only one bioreactor landfill project in California, located in Yolo County,
but the California Integrated Waste Management Board reports significant interest in
utilizing bioreactor landfill technologies at other locations in California. The USEPA has
proposed to issue a Federal rule that will allow states to issue site-specific research,
development, and demonstration permits to landfills that will allow the addition of liquids to
landfills. If the proposed regulations are adopted there may be more bioreactor landfill
projects in California, which in turn could increase the number of landfills that accept
biosolids.
4.1.6
Air Quality Management Districts
Air Quality Management Districts (AQMDs) are regional agencies tasked with reducing air
pollution within their jurisdictions. AQMD regulations can affect biosolids management
programs by requiring permits and emission control systems at cogeneration, heat drying,
composting, and thermal conversion facilities.
4.1.7
California Department of Food and Agriculture
The California Department of Food and Agriculture (CDFA) regulates nutrient guarantees of
fertilizer materials and agricultural minerals. CDFA licensing is required for all producers of
fertilizing materials and agricultural minerals. San Francisco biosolids can either be
classified as a fertilizer material or an agricultural mineral, depending on the moisture
content. Heat dried biosolids will qualify as a fertilizing material under the CDFA
regulations. Dewatered cake biosolids are considered to be an agricultural mineral.
607-21
4.1.8
Delta Protection Commission
The Delta Protection Commission is a state agency tasked with regional planning for the
Sacramento/San Joaquin River Delta area, a large, fertile agricultural area located relatively
close to the Bay Area. The Delta Protection Commission has adopted regulations that
prohibit biosolids land application within the Primary Zone of the Delta, as defined in
Section 12220 of the California Water Code. The Primary Zone includes portions of Solano,
Yolo, Sacramento, San Joaquin, and Contra Costa counties. The five counties have
incorporated the requirements of the Delta Protection Commission within their land use
plans and zoning codes.
4.1.9
Local Regulation
Northern California counties have enacted local regulation of biosolids land application in
various forms, as summarized in Table 23. Some counties require a Conditional Use Permit
(CUP) be obtained for a site, which triggers an environmental review in accordance with the
California Environmental Quality Act (CEQA) and allows the county to apply site-specific
conditions to the proposed operation. Other counties have enacted biosolids ordinances to
address local concerns. The ordinances range from complete banning of biosolids land
application to allowing Class A or Class B biosolids to be applied. Each county’s
requirements are unique and must be studied carefully; some ordinances even ban the use
of high quality products like compost or fertilizer pellets derived from biosolids. In general,
the trend in California has been towards increasingly restrictive local regulation or bans of
biosolids land application.
Table 23
County
Alameda
Local Regulation of Biosolids Land Application in Northern California
Biosolids Ordinance
CUP Required
Ban
Class A Only
Class B Allowed
♦
Merced
Sacramento
♦
♦
San Joaquin
♦
Solano
♦
Sonoma
♦
Stanislaus
Yolo
4.2
♦
♦
Policy Considerations
There are policies established by non-governmental organizations that should be
considered for biosolids management planning purposes.
607-22
4.2.1
Food Processor Policies
A number of food processors will not accept crops that are grown using biosolids. Some
company policies go so far as not accepting crops grown on land where biosolids have ever
been applied. The food processing companies are concerned that their products could be
viewed as tainted by human waste, even though extensive research has shown that the use
of biosolids in accordance with the established regulations is safe for human health and the
environment.
4.2.2
California Farm Bureau
The California Farm Bureau Federation urges a cautious approach to biosolids use, due to
the presence of heavy metals and pathogens and perceived questions over the safety of
the material. While heat dried biosolids address the pathogen issue by sterilizing the
product, heavy metals will still be present. Local, including County, Farm Bureaus are free
to establish their own policies towards biosolids use, and established policies range from
support for biosolids recycling to calls for bans on the practice.
4.3
Recent Contracts
All San Francisco biosolids are currently hauled by Sunset Scavenger, a subsidiary of
Norcal Waste Systems. Table 24 summarizes the current biosolids management practices.
As shown in the table, biosolids are used as ADC during the wet season, when land
application is not permitted due to wet field conditions. During the dry season land
application sites are used as much as possible, and any excess material is sent to landfills
for use as ADC.
Table 24
Current San Francisco Biosolids Management Practices
Disposition
Perio88d
Weekdays
Weekends
Wet Season
♦ Hay Road Landfill ADC + other uses (Solano County)
(November – April)
Dry Season
♦ Synagro Land Application (Solano
♦ Synagro Land
(May – October)
County)(1)
Application
♦ Landfill ADC as needed (Alameda,
(Sonoma County)
Contra Costa, and Solano Counties)
Note:
(1)
Strategy is to maximize use of this outlet
During the summer of 2006 San Francisco also began transporting small (pilot) quantities of
biosolids to Merced County land application and composting sites. The land application
sites are managed by Synagro and Solid Solutions. The composting facility is operated by
Synagro.
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4.4
Annual 40 CFR 503 Report
San Francisco submits an annual report to USEPA each February that documents the
biosolids management practices for the previous year and demonstrates compliance with
the 40 CFR 503 regulations, including pollutant concentrations, pathogen requirements,
and vector attraction reduction requirements. The report is certified by the SFPUC
management prior to submittal to USEPA.
4.5
Public Input and Involvement
Significant outreach efforts have been made by SFPUC staff in recent years to address
issues and concerns expressed in the counties where land application of San Francisco
biosolids is taking place, particularly Solano County. The outreach efforts have included
contact with members of the Board of Supervisors, discussions with Solano County
Environmental Management staff, attendance at public meetings and hearings, providing
information to interested members of the public, etc.
4.6
Industry Trends
General industry trends towards biosolids management in California’s more urban locations
include the following:
•
Biosolids quantities are increasing due to population growth and increasingly tighter
clean water regulations.
•
Most wastewater agencies remain committed to recycling biosolids rather than
disposing of them.
•
There is a general recognition that agricultural land application of Class B dewatered
cake is not a long-term biosolids management solution. Local ordinances increasingly
limit the practice or ban it outright. Some county bans include Class A biosolids
products.
•
The shrinking inventory of permitted land application sites and increasing county
restrictions in California have forced wastewater agencies to haul biosolids greater
distances, raising transportation costs. Increased competition for available sites has
increased application costs.
•
Large wastewater agencies are increasingly turning to high-solids centrifuges for
dewatering to reduce the moisture content of their biosolids and reduce hauling costs.
•
Some wastewater agencies are converting to advanced anaerobic digestion
processes, such as thermophilic or temperature-phased digestion, to achieve Class A
pathogen status, increase volatile solids destruction, and increase biogas production.
607-24
•
Large wastewater agencies in California are often relying on private sector
involvement in their biosolids management programs downstream of the dewatering
function, including hauling, land application, composting, heat drying, product
marketing and distribution, and thermal conversion processes.
•
Wastewater agencies are increasingly considering production of biosolids products
with improved aesthetic qualities, such as compost or heat dried pellets, for their
recycling programs.
•
Wastewater agencies are pursuing other, sometime unique, outlets for biosolids
besides agriculture, including biosolids as renewable fuel in cement kilns, or deep
well injection in petroleum oil fields to enhance natural gas production.
•
Wastewater agencies are identifying the need for, and pursuing, regional solutions to
biosolids management.
5.0
BIOSOLIDS MARKETS AND DISPOSITION
This section considers potential outlets for biosolids products produced by San Francisco;
whether for beneficial use or disposal. The discussion begins with consideration of the
characteristics of the products that could potentially be produced, followed by discussion of
the potential markets for the products.
5.1
Biosolids Products
Biosolids products can take a number of different forms, as described below.
5.1.1
Dewatered Cake
Dewatered cake represents the most basic and most common form of biosolids products.
Dewatered cake is produced using mechanical dewatering technologies, such as belt filter
presses or centrifuges. Dewatered cake products typically consist of 85 to 70 percent
moisture (15 to 30 percent solids) and have a gelatinous, bread dough consistency. The
color, odor, and pathogen density characteristics of dewatered cake products are a function
of the processes used to treat the biosolids prior to dewatering. Dewatered cake products
can be produced that have pathogen densities that achieve Class A standards. The typical
reaction by the general public to the overall appearance of dewatered cake varies widely
from curiosity and fascination to suspicion and revulsion.
5.1.2
Soil Amendments
Dewatered cake biosolids can be mixed with various other materials and processed to
create soil amendments (such as compost) or topsoil replacement products. The list of
potential feedstock materials that can be used include green waste, wood chips, sawdust,
sand, lime, cement kiln dust, wood ash, and others. Soil amendment products are generally
607-25
treated to Class A pathogen density standards. The soil amendment class of products
usually has a pleasant, earthy odor and pleasing overall appearance to the general public.
5.1.3
Dried Products and Fertilizers
Dewatered cake biosolids can be dried to form fertilizer products. Drying methods include
solar drying and thermal drying. This class of products can take a wide variety of forms.
Solar dried biosolids typically contain less than 40 percent moisture and can have a dusty,
soil-like appearance. Solar dried products may meet Class A pathogen density standards.
Solar drying is usually land-intensive and therefore may not be a practical option for an
urbanized city such as San Francisco.
Thermally dried biosolids products generally contain less than 10 percent moisture. The
product appearance is a function of the drying technology used, and can range from
uniform spherical pellets with little dust to angular, non-uniform, dusty products. The
thermally-dried biosolids products generally have a slightly stronger, more pungent odor
than the soil amendment products, but fewer odors than dewatered cake. The overall
appearance of thermally-dried products is generally acceptable to the general public.
Uniform, spherical products with low dust content are generally preferred over angular, nonuniform, dusty products.
5.1.4
Other Products
Several other types of biosolids products can result from specific biosolids treatment
processes, including:
•
Ash: The end product of biosolids combustion for energy recovery or disposal is ash.
•
Lightweight Aggregate: Vitrification processes (e.g., Minergy) create a lightweight
glass aggregate product.
•
Fuel: Pyrolysis processes create char or oil fuel products.
5.2
Agricultural Land Application Market – Class B Biosolids
Agricultural land application refers to the use of biosolids in bulk as a soil amendment or
fertilizer to grow agricultural crops. Biosolids are applied at or below the agronomic rates to
ensure that the nutrients in the biosolids are used up by the crop, rather than accumulating
in the soil and leaching to groundwater. The biosolids add organic matter to the soil, which
is a valuable addition to many California soils that are typically very low in organic matter.
Class B biosolids that are to be recycled through agricultural land application generally take
the form of dewatered cake. San Francisco biosolids are currently Class B dewatered cake.
Land application of Class B dewatered cake has been attractive to wastewater agencies
because it has been one of the lowest cost ways to manage biosolids. It has also become
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increasingly controversial in California and has been banned or restricted by a number of
counties, as described in the previous section.
In light of the food processor policies towards biosolids, a farmer’s use of biosolids could
potentially affect the value of prime farmland where vegetables could be grown and
marketed to food processors. Many farmers perceive that the potential benefits from using
biosolids do not outweigh the risks associated with crop and land values. Therefore, much
of the agricultural land application occurs on marginal ground where the growth of highvalue crops is not possible due to soil quality characteristics. Low value crops, such as hay
used for animal feed, are typically grown.
Land application of Class B biosolids is mostly accomplished by firms that specialize in
biosolids management. The firms are under contract with the municipal wastewater
agencies. The firms solicit interested farmers and obtain the required permits. The firm
spreads the biosolids and completes the monitoring required by the permits. The farmer
receives free fertilizer, but is generally not paid a tipping fee. Proactive public outreach is
generally required in communities where land application is to occur because dewatered
cake biosolids do not look or smell like materials commonly used in agriculture. Neighbors
of land application sites may react with fear and concerns about the practice if not given
proper information on the safety and benefits.
Agricultural land application is a seasonal market in Northern California. Land application
activities are generally not possible (and may be prohibited by local regulations) during the
wet season, November through April. Farm fields are usually too wet during this time of the
year to allow access to the heavy equipment needed to spread biosolids. Dry season
application (May through October) must be scheduled around the growth cycle of the crops;
biosolids cannot be applied while a crop is being grown. Farm land that is not irrigated
(dryland farming) is ideal for biosolids land application because biosolids can be applied
throughout much of the dry season; the farmer plants his crop just prior to the wet season
and harvests the crop in late spring or early summer.
Agricultural land application does not appear to be a sustainable biosolids management
practice for wastewater agencies that serve large urban areas, such as the San Francisco
Bay area and the greater Los Angeles area. Rural communities in California are becoming
increasingly resistant to accepting waste products that are transported from distant urban
centers, particularly with dewatered cake products. Agricultural land application of
dewatered cake may provide a short-term outlet for San Francisco biosolids, but should not
be considered a permanent biosolids management solution. As counties located close to
the Bay Area place greater restrictions on agricultural land application of Class B biosolids
the SFPUC will be forced to haul dewatered cake longer distances. Counties that ban or
restrict Class B biosolids reuse may also limit Class A biosolids products. It is prudent for
San Francisco to focus efforts on creating higher-quality biosolids products that are morereadily accepted by the communities that receive and use them, rather than focusing on
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maximizing the short-term economic advantages provided by Class B biosolids recycling in
agriculture.
5.2.1
Dedicated Land Application Sites
Land application on land owned by wastewater agencies appears to be more sustainable
than land application on distant private property. Many small wastewater agencies in
California apply their biosolids to property they own that is adjacent to or near the
wastewater treatment plant of origin. Often these dedicated land application sites are
located within the incorporated limits of the city that operates the site.
Dedicated land application sites are generally accepted by the local agricultural community,
provided that they remain a good neighbor with respect to odors, dust, and other nuisance
conditions. The agricultural community’s concern over the fate of heavy metals in biosolids
and soil contamination is addressed by permanent public agency ownership of the land.
Purchase of farmland outside the wastewater agency’s county presents greater risk than
development within the city or county of origin. Vallejo Sanitation and Flood Control Agency
owns and operates a farm on Tubbs Island, located in adjacent Sonoma County. The
award-winning project has a long, successful operating history. However, the City of Los
Angeles’ purchase of an established site in Kern County has not appeared to reduce Kern
County resident’s resistance to land application of biosolids originating from urban Southern
California. The “Green Acres” farm is located within unincorporated Kern County, and is
subject to the provisions of a land application ban that was approved by Kern County voters
through the local initiative process in June 2006. Therefore, development of a dedicated
land application site by San Francisco in another county presents considerable risk and is
not considered feasible.
5.2.2
Exportation Out-of-State
Exporting biosolids out of the state of origin is or has been practiced by a number of large
wastewater agencies, including the City of New York, District of Colombia Water and
Sewage Authority, City of Los Angeles, and Orange County Sanitation District. Transport
can be by truck or rail, depending on the haul distance. Biosolids from Southern California
have been successfully exported by truck to several Arizona counties for beneficial use.
However, Orange County Sanitation District encountered significant local opposition in
2003 when it began exporting biosolids by truck to Nye County, Nevada. The Sacramento
Regional County Sanitation District received a proposal from a large, fully-permitted ranch
located north of Reno (in Nevada) to accept biosolids transported by rail as a long-term (15
year) solution. The proposal was not accepted, however it demonstrates that out-of-state
exportation may be a viable alternative for Northern California wastewater agencies. Pursuit
of out-of-state markets for San Francisco Class B biosolids is not recommended at this
time, but should be considered in the future if solutions located within California become
infeasible or prohibitively expensive.
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5.3
Agricultural Land Application Market – Improved Biosolids Products
Biosolids can be processed to create products with improved characteristics when
compared with the existing Class B dewatered cake. The improved products can range
from Class A dewatered cake to heat dried pellets, compost, or other soil amendments. The
aesthetic qualities of this broad category of “improved products” vary widely, as will the
marketability of the products for agricultural land application.
5.3.1
Class A Dewatered Cake
Upgrading treatment to produce Class A dewatered cake reduces the pathogen density in
the biosolids, but does not improve the aesthetic qualities of the product. From a State and
Federal regulatory perspective Class A dewatered cake is a product that does not require
regulation to protect human health and the environment. However, some counties in
California have chosen to regulate (or ban) the use of Class A biosolids in agriculture within
their jurisdictions. Similarly, food processing company policies against biosolids apply
equally to all products irrespective of pathogen density or product aesthetic qualities.
Therefore, the market for Class A dewatered cake is somewhat similar to the market for
Class B dewatered cake, although with less regulation.
Neighbors of land application sites cannot distinguish between Class A and Class B
dewatered cake products, because they look and smell the same. Therefore, production of
a Class A dewatered cake product does not relieve responsibility for providing proactive
public outreach to communities where application will occur.
5.3.2
Dried Biosolids - Pellets and Granules
The only Northern California heat drying facility in operation is located at the Sacramento
Regional Wastewater Treatment Plant. Fertilizer pellets produced at the facility are used in
bulk for agricultural purposes to grow animal feed crops within Sacramento County. The
pellets are similar in size and shape to conventional granular fertilizer materials, and
conventional spreading equipment is used. The use of the product is not regulated at the
Federal, state, or local (Sacramento County) levels. The contractor that produces and
distributes the product maintains a low profile. Nearby local biosolids bans (e.g., San
Joaquin County, Delta Protection Commission) and food processing company policies do
not discriminate between types of biosolids products and therefore apply to the heat dried
pellet product. Nevertheless, use of the product in bulk agriculture appears to be
successful.
The market potential for heat dried pellets in agriculture appears to be greater than for
Class A dewatered cake due to the improved aesthetic qualities of the product. The product
appearance and use resembles fertilizer rather than manure. The pellets can be produced
to be similar in size and shape to conventional fertilizer materials. The product contains
minimal moisture, so truck traffic is minimized. Conventional spreading equipment is used
to apply the product. Neighbors of sites where the product is used are less likely to react
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negatively. The target market for the product would be similar to dewatered cake biosolids;
marginal soils used to grow animal feed crops. Product revenue is expected to be minimal
due to the low cost of competing conventional fertilizing materials, but the use of the
product will likely prove to be more-acceptable to the receiving communities than
dewatered cake.
5.3.3
Compost and Other Soil Amendments
State mandates to divert waste from landfills has resulted in large quantities of green waste
compost flooding the soil amendment markets. Soil amendments are generally only used in
agriculture to correct soil problems. The market for compost and other soil amendment
products derived from biosolids in agriculture is expected to be limited due to the availability
of competing products.
A subclass of biosolids soil amendment products has high residual pH values due to the
use of alkaline materials (e.g., lime) in the treatment process. In some parts of the country
high pH biosolids products are popular with growers due to their need to raise the pH of
acidic soils and the low cost of biosolids products compared with other liming agents.
However, there is little market in Northern California for high pH biosolids products due to
generally calcareous soils and availability of low cost liming products (e.g., sugar beet lime)
that are more-readily accepted by the agricultural community than biosolids.
5.4
Landfill Markets
Biosolids products may be either disposed or put to beneficial use in landfills, as described
below. A dewatered cake product is generally the most economical form of biosolids to
dispose or use at landfills.
5.4.1
Disposal
Some landfills allow disposal of biosolids. Each landfill has its own requirements for
biosolids disposal with respect to total solids content and specific chemical constituent
concentrations. SFPUC staff conducted a survey to determine landfill availability for
biosolids disposal within 200 miles of San Francisco (Jones and Schepis, 2006). The
results of the survey are summarized in Table 25.
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Table 25
Available Landfill Options
Landfill Name
Altamont Landfill
Vasco Road Sanitary Landfill
Keller Canyon Landfill
Salinas Valley Solid Waste
Authority
Forward Landfill
Newby Island Sanitary
Landfill
Hay Road Landfill
County
Dispo
sal
ADC
Alameda
♦
♦
Alameda
♦
♦
Contra
Costa
♦
Monterey
♦
San
Joaquin
Santa
Clara
Solano
Yolo
♦
Not
specified
1,000
tons/week
1,000
tons/week
♦
♦
♦
♦
♦
Unused
Biosolids
Capacity
1,000
tons/week
♦
Solano
Yolo County Landfill
Permitted
Biosolids
Capacity
Not
specified
500
tons/day
250
tons/day
40,000
tons/yr
Not
specified
Source: Jones and Schepis, 2006.
5.4.2
Alternative Daily Cover
Some landfills are permitted to use biosolids as Alternative Daily Cover (ADC), as shown in
Table 25. At these landfills biosolids are mixed with other materials to serve as a daily cover
for the solid waste placed in the landfill, reducing the need to use soil for that purpose. ADC
is considered to be a beneficial use, even though the materials are ultimately entombed
within the landfill. ADC use is regulated by the California Integrated Waste Management
Board, and is limited to 25 percent of the total landfill cover requirements. Therefore, there
is limited ADC capacity available for use by Bay Area wastewater agencies.
5.4.3
Bioreactor Landfills
Bioreactor landfills are operated in ways to rapidly degrade organic waste. The increase in
waste degradation and stabilization is accomplished through the addition of liquid to
enhance microbial processes and increase the production of landfill gas. Air is also
sometimes added to bioreactor landfills to enhance aerobic decomposition of organic
wastes. The bioreactor landfill concept is very different from conventional sanitary landfilling
practices and regulations that emphasize minimizing liquid addition and creating “dry tomb”
conditions within landfills. The addition of the moisture, organic matter, and nutrients in
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biosolids to bioreactor landfills can potentially increase landfill gas production, which in turn
can be used to produce electricity.
There is currently only one bioreactor landfill project in California, located in Yolo County,
but the California Integrated Waste Management Board reports significant interest in
utilizing bioreactor landfill technologies at other locations in California. The USEPA has
proposed to issue a Federal rule that will allow states to issue site-specific research,
development, and demonstration permits to landfills that will allow the addition of liquids to
landfills. If the proposed regulations are adopted there may be more bioreactor landfill
projects in California, which in turn could increase the number of landfills that accept
biosolids.
5.5
Horticulture and Silviculture - Product Distribution and Marketing
High quality Class A biosolids fertilizer or soil amendment products can be distributed and
marketed to horticulture and silviculture (tree farming) users. This broad category of users
comprises most other users besides commercial agriculture.
5.5.1
Dried Pellet Products
The Bay Area Clean Water Agencies (BACWA, 2006) and Sacramento Regional County
Sanitation District (SRCSD, 1996) have both conducted extensive studies to investigate
potential markets for heat dried pellet biosolids products in Northern California. Both studies
identified substantial market potential for high quality heat dried biosolids pellets. The
potential markets include:
•
Fertilizer blending operations;
•
Parks and golf courses; and,
•
Bagged retail sales.
Both studies identified potential market opportunities that far exceed the annual volume of
biosolids produced by San Francisco. Furthermore, heat drying removes most of the
moisture from the biosolids, reducing the total mass of product and therefore substantially
increasing the radius of economical truck transport. The SRCSD is currently the only
producer of heat dried biosolids pellets in Northern California. The entire SRCSD product is
used in bulk use in agriculture within Sacramento County; therefore substantial market
potential remains for other wastewater agencies.
5.5.2
Soil Amendment Products
The City of Santa Rosa currently operates the only biosolids composting operation in the
Bay Area, processing approximately 5 dry tons of biosolids daily in an agitated bed system
(City of Santa Rosa, 2003). The product is marketed in bulk to local users. Another notable
compost producer historically was the East Bay Municipal Utility District, which operated a
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successful composting program for many years until their aerated static pile composting
facility was shut down in the 1990s, primarily due to nuisance odor conditions.
Biosolids soil amendment products must compete with similar products produced from
other feedstock, such as green waste compost. Some existing products are labeled as “not
produced from biosolids.” The potential markets for biosolids soil amendment products
include:
•
Soil blending operations;
•
Landscape contractors;
•
Parks and golf courses; and,
•
Bagged retail sales.
Solid waste agencies have been required to divert green waste from landfills to achieve
mandated diversion goals. The result has been a major increase in the volume of compost
produced in California, which in some cases has flooded markets. Further market study is
advised prior to substantial investment in a soil amendment production system to manage
San Francisco biosolids.
5.5.3
Markets within San Francisco
High quality biosolids products could potentially be used on parks, golf courses,
playgrounds, schools and other landscaped areas within San Francisco. The SFPUC
inventoried publicly-owned irrigated acreage as part of its recycled water master planning
efforts. The inventory results are summarized in Table 26, along with the theoretical and
realistic demands for biosolids, based on average annual nitrogen fertilizer demands of 100
lbs. N per acre per year. As shown in Table 26, the theoretical demand is between 2,000
and 2,100 dry tons per year; however, for planning purposes no more than about 50
percent of the theoretical demand for parks, golf courses, and playgrounds can be
realistically relied upon. Because the theoretical school use is small and potentially
controversial it is excluded from the estimated demand. Therefore, a reasonable
expectation is for a demand of approximately 1,000 dry tons per year. Some additional
private use could also occur, increasing the total demand to perhaps 1,100 to 1,200 dry
tons per year – which represents less than five percent of the total mass of biosolids
produced by the City.
The information in Table 26 assumes that a well-digested, Class A, high-quality, heat dried
biosolids fertilizer product is produced, bagged, and actively marketed. The demand would
likely be highly seasonal, with most use occurring during the spring and autumn months.
There would likely be less demand for a soil amendment product such as compost, due to
the difficulty in effectively using compost products on established turf areas.
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Table 26
Potential Demand for Biosolids Product Within San Francisco
Annual Biosolids
Demand
(dry tons/year)
Irrigated
Description
Area(1)
Theoretical(2) Realistic(3)
Parks, golf courses, and playgrounds
2038 acres
2038
1000
Schools
44 acres
44
0
Total
2083 acres
2083
1000
Notes:
(1) Source: San Francisco Department of Public Works (March 2006).
(2) Assumes 5% N guaranteed analysis. Corresponding product application rate is 1
dry ton per acre per year.
(3) Approximately 50 percent of theoretical demand, excluding school use.
5.6
Land and Mine Reclamation Market
Biosolids have been used successfully to reclaim land damaged by mining operations,
particularly in the mid-Atlantic area (Pennsylvania) and in British Columbia (Canada). The
biosolids add vital nutrients and organic matter to the damaged soils, enhancing restoration
efforts. Class B dewatered cake biosolids are added in a one-time application prior to
seeding with a mixture of grasses and legumes. Research has found that a high application
rate of biosolids is required to provide sufficient organic matter and nutrients to ensure a
sustainable vegetative cover. The high application rate can result in a nitrate spike in
underlying groundwater, but this is seen as less of a water quality problem in the states
where the land reclamation activities are pursued than the surface water quality problems
caused by lands disturbed by the mining activities.
There are currently no land or mine reclamation projects using biosolids in California,
although significant land areas exist that are disturbed by mining operations. California has
an anti-degradation policy towards groundwater that could prove to be an obstacle to the
high rates of biosolids application found to be successful in the mid-Atlantic area.
Significant effort would be required to obtain regulatory approvals in California, due to the
lack of project precedent.
5.7
Construction Products Market
Potential markets for products created from vitrification processes are specific to the
characteristics of the materials. Lightweight glass aggregate products from vitrification can
be used in the manufacture of ceramic floor tile, abrasives, concrete additives, asphalt
paving mixtures, or composite roofing shingles. Generally these types of processes have
been implemented by private companies with long-term contracts to receive dewatered
cake biosolids from wastewater agencies. The implementing companies are responsible for
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marketing the products they produce. Additional market study is highly recommended prior
to substantial investment by San Francisco in a publicly owned and operated facility of this
nature.
Beneficial uses for non-hazardous ash from biosolids incineration include use as an
additive in the production of blocks used for erosion prevention, bricks, and novelty
products. Neither of the two agencies using incinerators located in Northern California uses
their ash in these ways. The two agencies are the Central Costa County Sanitary District
(CCCSD) and the City of Palo Alto. The CCCSD disposes its incinerator ash in a landfill.
The Palo Alto incinerator ash contains sufficient phosphorus to make it attractive as a soil
amendment additive. Palo Alto therefore recycles its incinerator ash by transporting it to a
soil blender, who mixes it with compost to form a soil amendment product.
5.8
Fuel and Energy Markets
Energy can be derived from biosolids or from biogas generated from biosolids processing,
as described below.
5.8.1
Markets for Biosolids
Potential markets for products created from pyrolysis processes are specific to the
characteristics of the materials. Char or oil fuel products from pyrolysis can potentially be
used for energy production or in cement kilns. Generally these types of processes have
been implemented by private companies with long-term contracts to receive dewatered
cake biosolids from wastewater agencies. The implementing companies are responsible for
marketing the products they produce. Additional market study is recommended prior to
substantial investment by San Francisco in a publicly owned and operated facility of this
nature.
Dried biosolids pellets have an energy content of approximately 9,500 BTU per pound and
can potentially be used as fuel. Potential future markets for dried biosolids pellets include
waste-to-energy facilities and cement kilns. The cement industry has recently become
interested in biosolids as a renewable fuel source. User requirements are specific to each
cement kiln. The combustion ash is integrated into the cement product.
Increasing interest in renewable energy sources could lead to increased production of
biomass and bioenergy crops, such as hybrid poplar trees, corn for ethanol production, or
seed crops to create the vegetable oils used in the production of biodiesel. The use of
biosolids to grow these crops would be subject to local agricultural regulations and use
restrictions.
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5.8.2
Markets for Biogas
Anaerobic digestion processes create a biogas product, which is a mixture of methane and
carbon dioxide. Biogas can be used as fuel in internal combustion or gas turbines that are
connected to generators that produce electricity. Fuel cells are another technology that can
be used to create electricity from biogas.
Biogas is considered to be a renewable energy source, and increasing awareness of global
warming issues and rising fossil fuel prices is leading to increased interest in creating new
sources of biogas and enhancing biogas production at existing anaerobic digestion
facilities. Programs are being developed to enhance biogas production in wastewater
treatment plant anaerobic digestion systems by direct addition of fats, oils, and grease
(FOG) and liquefied food wastes. EBMUD has a successful program that incorporates
trucked-in FOG and food processing wastes. South Bayside System Authority in Redwood
City has been adding trucked-in FOG directly to anaerobic digesters since the 1990s.
5.9
Dedicated Disposal
Surface disposal of biosolids is practiced by several wastewater agencies in Northern
California, including Sacramento Regional County Sanitation District, Dublin-San Ramon
Services District, and Las Gallinas Valley Sanitation District. All of the surface disposal sites
are located on treatment plant property. Biosolids are mixed into the soil at these dedicated
land disposal sites at high rates. Vegetation is not grown, but soil microbes decompose the
biosolids and use or transform much of the nutrients. The dedicated land disposal sites are
lined or otherwise highly controlled, depending on the subsurface geological conditions.
Surface disposal is not considered to be feasible for San Francisco due to the landintensive nature of the process. Development of a surface disposal site in another county is
not viewed to be politically feasible.
5.10 Overall Market and Product Assessment
Table 27 presents a simplified assessment of the current markets for biosolids products in
Northern California, as well as opinions of the future market potential. The table reflects the
increasing wastewater industry awareness of limits to agricultural land application of
biosolids and a needed shift to other markets.
Table 28 presents the compatibility of various biosolids products with the future markets. A
product that is compatible with multiple markets presents lower risk to the wastewater
agency than a product that is compatible with only a few. The table shows that heat dried
pellets and compost (including compost-like soil amendments) have the greatest
compatibility with multiple markets.
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Table 27
Market Assessment
Market
Current Market Assessment
Agricultural Land Application
Increasingly problematic,
trending towards increased
local restrictions and/or bans.
Food processor policies render
prime farmland unavailable.
Best opportunities are on
marginal soils growing animal
feed crops.
Landfill – ADC
Good, but limited ADC capacity
available.
Currently no available projects
in California.
Landfill – Bioreactors
Landfill – Co-disposal
Horticulture and Silviculture – Distribution
and Marketing
Land and Mine Reclamation
Construction Products
Limited availability. Good backup option.
High quality product and active
marketing required
Currently no projects in
California.
California.
Opinion of Future Market Potential
Dewatered Cake Products
Trends towards local
restrictions and/or bans likely
to continue.
Class A cake market
potential somewhat better
than Class B cake due to
reduced regulatory burden.
Improved Products
More-likely to be accepted by
receiving communities
(compared to dewatered cake
products) due to improved
product aesthetics. Local
restrictions and/or bans may
apply to improved products.
Dried products can be
economically hauled further
than cake products.
Increasing demands for limited capacity likely to continue as
agricultural land application becomes less feasible.
Interest in renewable energy could increase number of
bioreactor landfills. Proposed regulatory changes could
increase number of bioreactor landfills available.
Limited availability. Good back-up option.
High quality product and active marketing required.
Lack of precedent in California. Regulatory hurdles to
implementation.
Lack of precedent in California. Markets are product-specific.
Active marketing required.
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Table 27
Market Assessment
Market
Current Market Assessment
Fuel and Energy - Biosolids
California.
Fuel and Energy - Biogas
Biogas commonly used to cogenerate electric power and
provide heat.
Currently no projects available
to San Francisco
Dedicated Disposal
Opinion of Future Market Potential
Projects currently being developed in California. Regulatory
mandates for power companies to increase renewable energy
portfolios combined with rising prices for fossil fuels could
significantly increase interest in biosolids as a fuel source.
Cement industry interest in biosolids as renewable fuel source
is increasing.
Increasingly valuable biogas uses for co-generation systems,
fuel cells, and direct energy to dry biosolids.
Implementation by San Francisco considered infeasible.
Color Key:
Highest potential
Medium potential, some concerns
Lowest potential, significant barriers
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Table 28
Product and Market Compatibility
Products
Agricultural
Land
Application
Class B dewatered cake
Landfill
♦
ADC
♦
Bioreactors
♦
CoDisposal
♦
Class A dewatered cake
♦
♦
♦
♦
Compost
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
Alkaline soil amendment
Heat dried pellets
Construction products
♦
♦
Markets
Horticulture
and Silviculture
– Distribution
and Marketing
Construction
Energy
♦
♦
♦
♦
♦
♦
♦
♦
♦
Char and/or oil
Ash
Land and
Mine
Reclamation
♦
♦
♦
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5.11 References
Bay Area Clean Water Agencies. “Bay Area Regional Biosolids Management Program
Initial Market Assessment.” April 2006.
Jones, Bonnie M., Gerald Schepis. “Review of Current Landfill Options for Reuse and
Disposal of Bay Area Biosolids.” 2006.
Sacramento Regional County Sanitation District. “Biosolids Heat Drying/Chemical
Treatment Project.” May 1996.
San Francisco Department of Public Works. “Recycled Water Master Plan for the City and
County of San Francisco.” March 2006.
Santa Rosa, City of. “Laguna Subregional Water Reclamation Facility, Biosolids Program
Phase 2.” July 2003.
6.0
STATUS ON ORGANIC WASTE PROCESSING
This section is prepared based on conceptual information available on potential organic
waste processing and digestion at San Francisco. There are several factors coalescing to
push this concept forward. There are also factors that suggest a combined anaerobic
digestion and biogas utilization facility for organic wastes and wastewater solids could be a
cost-effective and sensible approach at San Francisco. This section is a status report on
the conceptual planning to date.
6.1
Organic Waste Situation at San Francisco
Norcal handles, by charter, solid waste collection and solid waste management for the City
and County of San Francisco (City). The Department of Public Works and a Charterestablished Rate Board oversee rates for solid waste services, and the City’s Department of
the Environment undertakes planning services (with Norcal) for solid waste recycling.
Norcal already provides a high degree of solid waste recycling activities for the City;
however, there are additional City-established diversion goals Norcal is facing:
•
75 percent landfill diversion by 2010
•
100 percent landfill diversion by 2020
The major category of waste material that needs to be recycled to reach these extreme
diversion goals is the category of organic wastes (typically wet or partially wet materials).
This includes a large quantity of food waste material in San Francisco from restaurants, but
also from grocery stores and other commercial establishments, along with yard wastes and
other mixed organic materials.
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Currently, about 300 wet tons/day of these organic wastes are transported from the San
Francisco Recycling and Disposal Facility (SFRD) to various composting facilities. The
primary food waste composting occurs at a Norcal facility in the Central Valley. This
facility’s ability to increase capacity is restricted due to air emission limitations (VOC
emission limits), yet there is a need to handle perhaps 600 wet tons/day or more.
Furthermore, this is a large quantity of waste to transport out of San Francisco. Therefore,
alternative processing of organic wastes to reduce volume and extract energy appears
advantageous. If the wastes were anaerobically digested at San Francisco to produce
biogas/energy, the residual digested (and dewatered) material could be transported to the
Valley for composting. In this manner, Norcal indicates the composting operation would be
more manageable and cost-effective and the final composting product would be
significantly improved. Also, air emissions would be significantly reduced, and a major
renewable energy supply created at the digestion/biogas facility.
6.2
Digestion of Organic Waste
The approach conceptualized here for organic waste processing has evolved from Europe
over the last 10 to 15 years. Process technologies have been developed and patented and
are licensed for use in various countries around the world. The technologies are still not
extensively used, even world-wide. North America has only a tiny number of examples of
such organic waste processing/digestion for biogas production. This situation makes it
challenging to develop decisions quickly at San Francisco. The most pertinent North
American location using this concept full-scale is at Toronto, Canada, where two facilities
have been operating for a few years (See Van Opstal references for Toronto-Dufferin
facility). At some facilities world-wide such as at Toronto-Dufferin, only certain organic
fractions of municipal solid waste (MSW) are digested. At other facilities, co-digestion is
used, whereby organic fractions of MSW are digested along with materials such as cow
manure, wastewater sludges, or other organic feedstock.
Fortunately, work on organic waste digestion has been underway the past several years by
Norcal, in preparing food waste material for digestion at the EBMUD wastewater plant
digesters in Oakland (see Yoloye et al article, 2006). A pilot program has been underway
since 2004, and Norcal has provided up to one truckload per day of pre-sorted, screened
food waste to EBMUD with many of the contaminants removed. At EBMUD, plant staff
conduct additional screening and mashing/pulping, and dilute the material to about 6 or 8
percent solids for feeding to the anaerobic digesters. At EBMUD, the relatively limited
quantity of food waste in this pilot program is mixed with the sludges and other organic
wastes fed to all digesters at the plant. EBMUD indicates this program is highly successful
in producing additional biogas which has helped them greatly boost their power production
in recent years. At EBMUD, the biogas is used in cogeneration engines (internal
combustion engines) to produce electrical power and hot water for digester heating.
EBMUD intends to expand this program of processing/digesting food waste materials and
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bring other organic waste materials to the digestion facility to further increase biogas
production and electrical power production over the next 5 years.
6.2.1
Organic Waste Quantities and Characteristics at San Francisco
The organic wastes under consideration here are the wet or partially wet organic waste
materials collected. San Francisco currently has basically a 3-can/bin system for solid
waste collection – all collected on a weekly basis:
•
Organics bin (yard waste, food waste, etc.)
•
Recycle bin (plastics, glass, metals, etc.)
•
Trash bin
In this arrangement, it is the Organics bin that is collecting the potential waste of interest. In
the future, however, San Francisco may change to a 2-bin arrangement which would likely
involve the following: (1) a wet bin (containing all food/yard waste and other organic
wastes); and (2) a dry bin containing mostly dry recyclables. Obviously, collection details,
public education, and other factors affect quantities and characteristics of the wastes, and
the estimates provided here are the best available at this time.
The quantities/characteristics estimated by Norcal for organic waste collection in the nearterm future (within about 5 years) are broken into two source categories:
•
Source Separated Organics (SSO), which is largely commercial food waste. 200 wet
tons/day (6 day/week basis), with 30 % solids content, and 85 % VS/TS.
•
Mixed Organics (MO), which is food/yard waste, plus miscellaneous other organic
wastes including paper). 200 wet tons/day (6 day/week basis), with 50 % solids
content, and 80 % VS/TS.
In total, this is 125,000 tons of organic wastes per year. The two source categories above
are estimated to have somewhat different moisture content as indicated, and, therefore,
initial processing of the as-received wastes could be different for these two organics
streams. Norcal is currently evaluating options to initially process these materials at its
SFRD facility in the southeast portion of the City. The SFRD is currently a large transfer and
recycling facility for San Francisco solid wastes.
Initial processing of these organic streams needs to include pre-sorting and screening to
remove large material perhaps over 4” in size. This would include removal of obvious large
items, large yard wastes (limbs/sticks/etc), larger wood wastes, as well as some plastics,
rags, metals, and miscellaneous debris. When coupled with other reject materials below,
Norcal believes about 10 percent of the as-received material will not proceed to digestion.
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6.2.2
Pre-Processing and Digestion
After the initial presorting/screening described above, the organic wastes can be further
processed within one of several optional technologies and fed to anaerobic digesters. The
functions of these pre-processing steps are to further remove contaminants from the waste
stream (especially plastics, grit, glass, and metal), shred and/or pulp the material to much
smaller particle sizes for effective digestion, and dilute the relatively thick material received
down to a slurry that digesters can handle – typically 6 to 10 percent solids for feeding. The
Toronto Dufferin example (see references) indicates 6 to 7 percent solids feed has been
their common feed thickness. These final pre-processing steps (including pulping/slurrying)
probably need to be located adjacent to the digesters because the organic waste slurry is
likely to be difficult to pump or transport long distances.
The layout and sizing of a pre-processing facility located at the digester complex has not
been completed. However, it would need to encompass functions including the following:
truck delivery/storage of initially-processed organic wastes, equipment for the preprocessing described above, loadout for reject material, handling and storage for recycle
process liquids, tankage for storing processed and slurried material ready for digestion, as
well as all support facilities necessary including odor control.
As indicated, the quantity of contaminants and debris (i.e., rejects) from the initial
processing and pre-processing steps is estimated by Norcal to be 10 percent based on wet
weight of as-received material. This would eliminate 40 wet tons/day from the total of 400
wet tons/day of as-received wastes.
Therefore, digester feed material would be as follows on a 6 day/week basis:
•
SSO – 200 wet tons/day = 60 dry tons/day of solids. After rejects = 54 dry tons/day
digester feed.
•
MO – 200 wet tons/day = 100 dry tons/day of solids. After rejects = 90 dry tons/day
digester feed.
These two streams total 144 dry tons/day. Assuming 7 to 8 percent solids feed to digestion,
this would be a flowrate of about 400,000 to 500,000 gallons/day. For a 15- to 18-day
Hydraulic Residence Time for digestion, this would require between 6 and 9 million gallons
of digestion volume. If feedrates were equalized throughout the week (for a 7 day/week
basis versus 6 day/week basis), these quantities would be reduced by about 15 percent.
Volatile solids loading rates to digestion would be within acceptable ranges. At least one of
the prominent technologies from Germany promotes recuperative thickening on the
digestion process, so that the Solids Residence Times are boosted to perhaps 25 to 30
days, while the Hydraulic Residence Time remains in the 15 to 18 day range. The reason
for this is to achieve greater volatile solids reduction and gas production, and perhaps to
keep higher solids content or solids density within the digester for improved performance.
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Bayside Biosolids Center (BBC) planning indicates that 15 million gallons of digester
volume (6 tanks at 2.5 mil gal each) is needed for wastewater solids digestion. Therefore,
addition of organic waste digestion would require 3 or 4 similar-sized digester tanks to be
added to the digester complex.
Digesting the organic waste materials within separate digesters would keep the organic
waste materials segregated from wastewater-derived biosolids, In this manner, two
separate digestion facilities would be operated at the same site, and the digested products
would need to be separately handled, dewatered, and loaded out to avoid crosscontamination. The digesters built and used for organic waste digestion might be somewhat
different in design than the sludge digesters, and contain different mixing systems. The
organic waste digestion system that is envisioned would use mesophilic digestion, not the
higher-temperature thermopohilic operation being planned for the wastewater solids.
All digesters need to be fed 7 days per week and 24 hours per day, and, therefore, storage
of processed organic waste feedstock would be required for night-time feeding and to allow
Sunday/weekend feeding of digesters. Feedrates may be somewhat reduced below
average on the weekend, however, consistent feeding of digesters produces consistent
biogas production, which is critical for maximum biogas utilization.
6.2.3
Biogas Production and Utilization
If loading to the organic waste digesters is fully equalized on a 7-day/week basis, then
average volatile solids (VS) loading would be about 201,000 lbs/day (91,000 kg VS/day). All
additional information below assumes that loading is equalized for 7 day/week feeding.
The destruction of volatile solids within digestion is subject to many variables and there are
significant different percentages reported in the literature for similar types of feedstocks.
Some of this work is from full-scale, operating facilities, and some is from pilot or lab-scale
research and development. Due to the conceptual nature of this work, an estimated range
of VS reduction (VSR) is used as follows for the two wastestreams:
•
For SSO, VSR is estimated to be 60 percent (low end) and 80 percent (high end) to
encompass the likely range. The quantity of VS reduced or destroyed would then
range from 47,000 to 62,000 lbs/day for the SSO material.
•
For MO, VSR is estimated to be 55 percent (low end) and 70 percent (high end). The
slightly reduced VSR range for MO is due to expected paper and other fractions in
this waste. The quantity of VS reduced or destroyed would then range from 68,000 to
86,000 lbs/day for the MO material.
Total VS reduced or destroyed in the organic waste digesters would be the sum of these, or
115,000 to 148,000 lbs/day of Volatile Solids. It should be noted that Norcal is undertaking
additional testing work currently to better determine digestability of the waste steams,
volatile solids reduction, and gas production rates.
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As for biogas production rates for these organic wastes, a reasonable range for estimating
purposes would be between 12 and 16 cubic feet of biogas per pound of VS destroyed.
When these assumptions are used for calculations, the biogas production estimates are a
range as follows:
•
1.4 million cubic feet per day of biogas (low end average condition)
•
2.4 million cubic feet per day of biogas (high end average condition)
Gas production can vary considerably, of course, based on loading rates, characteristics of
feedstocks, and operating conditions. However, based on the loads provided, the above
biogas estimates are reasonable.
The biogas is estimated to contain about 64 percent methane and have an energy value of
about 600 BTU per cubic foot. Biogas processing would likely need to include the following,
assuming that biogas is used in engine cogeneration equipment:
•
Hydrogen sulfide removal (perhaps iron chloride addition to digestion, and perhaps
iron sponge treatment of the biogas)
•
Moisture removal through condensation
•
Possible siloxane removal (via activated carbon)
•
Compression for feeding engines
•
Biogas storage of perhaps 1 hour or less of biogas production volume
The two different biogas streams (from the two different digestion processes), will have
somewhat different characteristics. Co-treatment of the biogas from wastewater solids
digestion and organic waste digestion is likely to be cost-effective, but this needs to be
evaluated. Once treated, the biogas from both digestion processes is likely to be used
within a common cogeneration system. However, again, the details of this need to be
evaluated.
Based on recent cogeneration engine performance, a value of 9000 BTU per KW-hr is used
here for estimating power production. Using the total gas production quantities from above,
and an energy value of 600 BTU per cubic foot, the biogas from organic waste digestion
would provide between 4 and 6½ megawatts (MW) of continuous electrical power
production. Waste heat from the engines would likely be used to produce hot water which is
then used in digester heating and perhaps for other purposes in the facility. This power
production estimate does not include the biogas energy from wastewater solids digestion –
that information is contained within other SSMP documentation, which shows that the
sludge digestion biogas can also produce a few megawatts of electrical power.
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6.2.4
Solids Dewatering Operation
Digested organic wastes would need to be mechanically dewatered, and the dewatered
material would be trucked to composting facilities. Dewatering equipment options are being
evaluated, however, centrifuges can probably perform this dewatering operation effectively
and would probably achieve 25 to 30 percent solids content material, assuming appropriate
polymer conditioning dose. Quantities of dewatered material would vary depending on the
volatile solids reduction actually achieved within digestion. Based on the VSR estimates
provided in this memorandum, the range of dewatered material would be between 160 and
260 wet tons per day. This would require about 7 to 11 truckloads per day.
Part of the centrate from the dewatering operation would be recycled for use in slurrying the
organic waste material for digestion. The remaining portion of the centrate would be
discharged to the sewer system.
6.3
Economic and Non-economic Assessment for Organic Waste
Processing/Digestion
Cost estimates have not yet been determined for the construction or operation of facilities
needed to process/digest these organic waste streams or for the biogas utilization system.
Electrical power production economics will also require assessment. Overall economic
assessment will need to consider multiple issues associated with this conceptual approach.
And non-economic factors will also be crucial to further evaluation.
6.4
Summary
This section is a status report as of March 30, 2007 for conceptual work to date on organic
waste processing, digestion and biogas utilization at San Francisco. It is intended as
information to be used in the development of the SFPUC Sewer System Master Plan.
Projections of solids reduction and biogas production within the digestion process are
presented here as a range because limited testing results are available to date on the
specific organic wastestreams. Additional data should be available soon to help narrow the
range in these projections, and allow more detailed evaluation of processing issues.
6.5
References
Van Opstal, 2006. Evaluating AD System Performance for MSW Organics. Article from
November 2006 Biocycle magazine. (Toronto – Dufferin facility)
Van Opstal, 2006. Managing AD System Logistics for MSW Organics. Article from
December 2006 Biocycle magazine. (Toronto – Dufferin facility).
Norcal, 2007. Discussions with Norcal representatives over the period late 2006 through
March 2007.
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Yoloye, O., Kiang, D., Gray, D., and Hendry T., 2006. “Don’t Waste Your Food!” Article in
the Water Environment Federation Biosolids Technical Bulletin, Vol. 11, No. 2, March/April
2006
7.0
PROCESSING TECHNOLOGIES
This section describes and evaluates a wide range of technologies that are available in the
field of wastewater sludge processing. Some or many of these may be feasible for use in
San Francisco. The processing technologies are discussed within the following categories:
•
Thickening Technologies
•
Digestion Stabilization Technologies
•
Non-Digestion Stabilization Technologies
•
Dewatering and Drying Technologies
•
Other Solids Processing Technologies
•
Biogas Processing and Use Technologies
The technologies discussed and evaluated here include those that are commonly used in
the industry (either in North America or in Europe). This evaluation also includes
technologies that are considered innovative and are undergoing further
improvement/development, as long as they have promising features and there are
examples of full-scale experience. In general, processes that are at the research or
embryonic stage of their development are not included here, since it is too early to
determine if these processes will ever move to full-scale use.
Screening criteria are established later in this section. These criteria are applied to the
processes described to develop technology recommendations.
7.1
Thickening Technologies
A common first step in processing wastewater solids is to conduct thickening operations to
bring thin solids slurries up to several percent solids. Thickened sludges are still slurries
and are almost always pumped to move or convey them.
7.1.1
Thickening Primary Sludge in the Primary Clarifier
Thickening of primary sludge within the primary clarifier is a very common technique at
wastewater treatment plants, and is currently the method used at the SEWPCP. There are,
however, some drawbacks to this option. Thickening performance varies and often the
thickened solids are less than four percent solids. Limited thickening makes the following
processes larger and more costly, so that thickening has become a much more important
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process than it was historically. There are processes described below that can be used to
thicken primary sludge to a higher degree. An advantage of thickening within the primary
clarifier is that no polymer is required.
7.1.2
Gravity Belt Thickener
A gravity belt thickener (GBT) consists of a fabric belt that rotates slowly around a series of
rollers driven by electric variable speed motors. The top surface is flat, and slopes gradually
upward from the feed end to the discharge end of the machine. A series of “plows”
suspended from a frame above the belt gently mix the sludge as it passes to separate
water from the flocculated solids.
Water is separated from the solids by gravity drainage, and also by the capillary suction
forces exerted by the belt pores. Thickened sludge typically discharges to a hopper from
where positive displacement pumps convey it downstream to the next process. Polymer
must be added to the sludge for the proper operation of a GBT. It can be fed to the sludge
in a small mixing chamber directly upstream of the sludge feed point, into the sludge feed
box, or at a point in the feed piping upstream of the unit. Having several injection points
allows optimization of polymer performance.
Through adjustment of the operating variables such as belt speed, polymer dosing, and
sludge feed rate, thickened sludge concentrations of 4 to 7 percent are commonly
achievable. Thickening of waste activated sludge (WAS) is very common with GBTs, and
this is the method currently used at the SEWPCP to thicken the WAS. However, cothickening of primary and WAS on GBTs has been conducted successfully, and is used
currently by the City at the OSWPCP.
7.1.3
Dissolved Air Flotation
In dissolved air flotation thickening (DAFT) the solids are floated to the surface of the tank
by using air bubbles to alter their specific gravity. In the DAFT process, a source of air for
flotation is provided by pressurizing a stream of liquid (process water or DAFT effluent) and
saturating the liquid with air. When the pressurization system is discharged into the tank,
the pressure is reduced and turbulence is created. Air in excess of that required for
saturation at atmospheric pressure leaves solution as very small bubbles (the effect is
similar to that of removing the cap from a bottle of seltzer water). A key requirement is a
depressurizing zone at a location where the bubbles that form upon release of pressure
contact the solids entering the DAFT. The bubbles adhere to the suspended particles or
become enmeshed in the solids matrix. Since the average density of the solids-air
aggregate is less than that of water, the agglomerate floats to the surface and is skimmed
off the top.
Polymer is commonly used to enhance the performance of the DAFT. DAFTs typically
achieve thickened sludge solids concentrations ranging from 4 to 6 percent, depending
upon the characteristics of the sludge, the air to solids (A/S) ratio, polymer dosage, and
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various design and operating refinements. WAS is commonly thickened in DAFT units, but
DAFTs are also effective in co-thickening of primary and WAS together.
7.1.4
Centrifuge Thickening
Centrifuges are typically used for thickening applications where high capacity is required
within a small footprint, or where fully-contained thickening is critical for odor control. The
machines use more electrical power than other thickening options and usually require more
maintenance. Thickened solids of five percent or more can be achieved, and polymer
conditioning is required. Centrifuge thickening is used for biological sludges, and has also
been used by a few agencies to thicken primary solids, and even for co-thickening service.
The very large City of Los Angeles’ Hyperion plant uses centrifuges for WAS thickening.
7.1.5
Gravity Thickening
Separate gravity thickeners have been most often used to thicken primary sludge. They are
used less often today than historically, mostly because of improved thickening performance
of other processes. Gravity thickening for primary sludge might be able to achieve five
percent solids at San Francisco, but the thickening is likely to be variable in performance. It
is also an odorous process, as the sludge is retained within the thickener for several hours.
Thickening of biological sludge within gravity thickeners is not advisable because thickening
performance would deteriorate and increased odors are likely. The gravity thickening
process offers little advantage at San Francisco and is not considered further.
7.1.6
Rotary Drum Thickening
Rotary drum thickening, sometimes called rotary screen thickening, has been used
primarily at smaller wastewater plants. It functions similar to gravity belt thickening, in that
free water from a flocculated sludge is drained through porous media. Polymer use is
required, and in general, better performance is achieved on sludges with more fiber
content. An advantage is its small footprint requirement. Concerns for the process include
odor control, sensitivity to polymer type, operator attention needs, and use primarily at
smaller scale plants than San Francisco.
7.1.7
Membrane Thickening
Membrane thickening has been tested for waste activated sludge and implemented at a few
wastewater plants in the USA. This approach involves the use of membranes to separate
liquid from a WAS slurry. Thickening to over four percent solids has been reported. For the
type of sludge currently produced in San Francisco, this would not be an acceptable option.
However, if membrane bioreactors were utilized in the future for wastewater treatment, then
membrane thickening could be considered.
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7.2
Digestion Stabilization Technologies
Both anaerobic and aerobic digestion technologies have a long history of use in sludge
processing in North America, although anaerobic digestion is much more common at larger
wastewater treatment plants of the size at San Francisco. There are several distinctive
digestion processes which are discussed in this section
7.2.1
Anaerobic Digestion – Mesophilic
Mesophilic anaerobic digestion is the most common stabilization process used in North
America (and in Europe), and is the process that has been used at the SEWPCP and the
OSWPCP. Anaerobic digesters are large covered tanks equipped with mixing, heating, and
biogas collection systems. Anaerobic bacteria in the digesters convert organic matter into
methane, carbon dioxide, and water; pathogen densities are reduced; and a stabilized
sludge is produced. Modern high-rate digesters are typically single-stage reactors.
Mesophilic anaerobic digesters are typically operated at temperatures between 35 and
38°C. USEPA estimates that over 50 percent of the wastewater treatment plants in the
United States use this process. Mesophilic digestion systems produce a Class B biosolids
product if the solids retention time is greater than 15 days.
Two-stage mesophilic anaerobic digestion, where digesters are operated in series,
improves process performance. The second-stage anaerobic digester generally has less
solids retention time (SRT) than the first stage. The advantages of this process
configuration are slightly improved volatile solids reduction, a product with reduced
pathogen content, and less product odor potential.
7.2.2
Pasteurization/Mesophilic Anaerobic Digestion
Pasteurization consists of heating sludge to 70°C or greater for 30 minutes or longer to
inactivate bacteria, enteric viruses, and other pathogens – this definition by the USEPA
describes a process that will produce Class A biosolids. Pasteurization is almost always
accomplished in batches to prevent short-circuiting of pathogens through the process.
Pasteurization temperatures are achieved using heat exchangers or with direct steam
injection. Mesophilic digestion follows the pasteurization step to stabilize the solids.
Pasteurization as described here is quite limited in North America but is gaining more
interest. It has been implemented more in Europe.
7.2.3
Anaerobic Digestion – Thermophilic
Thermophilic anaerobic digestion is similar to mesophilic anaerobic digestion except that
the reactors are operated at temperatures ranging from 50 to 57 degrees C. In general,
different categories of microorganisms are working at these different temperature ranges
(thermophilic versus mesophilic). At present, there are at least a dozen full-scale
wastewater plants using thermophilic anaerobic digestion in the US and Canada. California
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installations include the City of Los Angeles’ Hyperion and Terminal Island treatment plants
and one of the Inland Empire Utilities Agency plants.
The major differences between thermophilic and mesophilic digestion are the requirements
for sludge heating, feed control, and digester gas management. Mixing and temperature
control also become more important for thermophilic digestion. The major advantages of
thermophilic digestion are faster reaction rates, additional volatile solids reduction and gas
production, and the potential to meet Class A pathogen density requirements depending on
process configuration. Thermophilic digestion can be undertaken in single-stage systems or
in multiple-stage systems. Multiple stage systems have advantages in pathogen control.
7.2.4
Temperature Phased Anaerobic Digestion
The temperature phased digestion process includes two anaerobic digestion systems - one
operating at thermophilic temperature and one operating at mesophilic temperature. Since
each phase has largely different sets of bacteria and since thermophilic reactions proceed
at a faster rate, greater overall volatile solids reduction can be achieved. Within each
phase, all digestion processes (hydrolysis, acidification, and methane formation) occur and
should be in balance. Therefore, the pH is not depressed and acid/alkalinity ratios are
monitored to show proper health of each phase of the process. The phases can be in either
order, but sufficient solids retention time must be provided in each phase for full methane
production.
The more common process order for temperature phased digestion is for the thermophilic
system to be used before the mesophilic system, as shown in Figure 4. This is primarily
because of the desire to reduce volatile acids concentrations as low as possible, and
thereby have a digested product with minimum odor level. This process arrangement may
also add to the energy efficiency by allowing for heat recovery between the two phases. A
minor variation of this arrangement would be to operate the first phase in the thermophilic
range, and then leave the second phase unheated, allowing it to cool down naturally and
operate between mesophilic and thermophilic temperatures. However, performance may
suffer at some operating temperatures.
Figure 4
Temperature Phased Anaerobic Digestion
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The process of thermophilic followed by mesophilic digestion evolved in Germany in the
1980s and some process developers there have promoted minimum solids retention times
in the thermophilic phase of only 2 to 3 days. However, essentially all German facilities
using the process (about 10 are reported to exist) have thermophilic phase solids retention
times greater than this (Dichtl, 1997). In the United States, the process has been promoted
by Iowa State University (ISU), which holds a patent on it. ISU calls the process
Temperature Phased Anaerobic Digestion or TPAD. About a dozen TPAD systems are now
operating in the USA; however, several of these have just recently come on-line and data at
others are quite limited. Many of the USA plants are located in the Upper Midwest and
Great Plains states. Omaha Nebraska’s Papillion Creek WWTP (about 55 mgd) has one of
the largest temperature phased digestion systems in the United States.
Significant increases in volatile solids reduction are being achieved by moving from a
single-stage mesophilic system to a temperature phased process. Plants seem to be
achieving between 15 and 20 percent additional volatile solids reduction by making this
change. For instance, a plant which had been recording 50 percent volatile solids reduction
with single-stage mesophilic digestion, may achieve about 58 to 60 percent volatile solids
reduction by changing to this temperature phased process, assuming total SRT remains
similar.
An advantage of the process is that it operates well at a wide variety of retention times for
each phase. To implement the process in the USA, existing digesters have often been
converted from mesophilic to thermophilic service, thereby providing fairly long retention
times for thermophilic digestion (greater than 12 to 15 days, typically). For plants wishing to
maximize performance, thermophilic retention times in the range of 5 to 10 days may be
more appropriate. Mesophilic retention times may be reduced to as low as 8 to 10 days.
The above-described temperature phased digestion system produces a material that would
not meet Class A, Alternative 1 requirements of 40 CFR 503 because the thermophilic
digester is not operated in a batch mode. With continuous or near-continuous feeding and
withdrawal from a digester, short-circuiting of pathogens through the reactor prevents Class
A designation of the process. Therefore, additional tanks are required to meet the Class A
Alternative 1 standards. Thermophilic batch tanks can be expected to meet the Class A
requirement, as described in Section 5.2.6 below.
7.2.5
Acid/Gas Phased Digestion
The anaerobic digestion process proceeds through definable phases. These phases
include: (1) hydrolysis – the solubilization of particulate material; (2) acidification –
production of volatile acids; and (3) methanogenesis – the production of methane gas. Each
phase has groups of micro-organisms that are primarily responsible for these activities. To
some extent, these phases can be separated so that the bacteria are grown within
desirable or even optimum conditions. The most workable process separation developed to
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date is to utilize an acid-phase and a methane or biogas-production phase, as shown in
Figure 5. The primary characteristics of this phasing approach are the following:
•
The first phase (acid generation) has a short SRT and high volatile loading to
maximize acid production within an acidic environment – generally pH 6 or less. The
result is that little methane gas and relatively little total gas is produced. High
concentrations of volatile acids occur in this phase.
•
The second phase (methane gas generation) has a considerably longer SRT
because methane-generating bacteria require longer growth times. The pH is typically
at or slightly above neutral and the vast majority of methane, and total gas, is
produced in this phase.
Figure 5
Acid/Gas Phased Digestion Options
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The primary advantage of this approach is that greater volatile solids reduction is possible
(compared to single stage mesophilic digestion) and the approach helps to limit foam
production within the process. The process was originally developed at mesophilic
temperatures for both phases, but either phase can be at thermophilic temperatures.
This process evolved in the 1970s under the name Acimet, and patents were developed for
the process. The treatment plant that has the longest history of using this process is the
Woodridge-Greene Valley WWTP in DuPage County, Illinois. For over 10 years this plant
has operated with acid/gas phased digestion. More recently, a few other facilities in the
USA are moving toward using the process or are pilot testing the process. For instance, the
Inland Empire Utilities Agency in California has tested and implemented the process at its
Regional Plant 1. A specially built and specially operated acid phase reactor may be
required to achieve the short SRTs and the maximum benefit from the process.
The gas from the first phase reactor in DuPage County contains about 3,000 to 10,000 ppm
of hydrogen sulfide. This gas is burned directly in a flare. Other options may be required at
plants located in air basins where SOx limits are more severe. Also, any leakage of the gas
from the acid-phase reactor is likely to be a significant odor problem, due to its extremely
high odor level (much more odorous than typical digester gas from mesophilic digestion).
Three-phase digestion is a variant of the acid-gas phased digestion process. This process
is also shown in Figure 6. The three-phase process is used at the Inland Empire plant as
follows: (1) acid phase at mesophilic temperature: (2) methane phase at thermophilic
temperature: and (3) methane phase at a variable temperature regime, which is sometimes
thermophilic and sometimes between thermophilic and mesophilic.
7.2.6
Class A Thermophilic Digestion Options
Several options have been developed and implemented to provide Class A biosolids from
thermophilic digestion configurations. Figure 7 presents three different configurations that
have been developed. The first option shown on the figure meets the USEPA Class A
Alternative 1 (time/temperature equation) in a direct manner with required batch step at the
necessary time/temp. This is used at the City of Los Angeles Terminal Island Plant. The
second option was developed through research and testing at Columbus, Georgia and has
a Conditional PFRP-Equivalency status from the USEPA. The third option is used at
Vancouver, Canada, and has 4-stages of thermophilic digestion to minimize short-circuiting
of sludge through the process train. Each of these options has shown its ability to meet
Class A biosolids requirements. There are pros and cons to each one of these options. The
first option is obviously easiest to meet EPA’s regulatory requirements, but requires the use
of frequent valve changes for the batch-operated tankage. From an operational standpoint,
it is better to avoid valve changes, and have a process (such as options 2 and 3) that is
operating continuously and has the needed flexibility to handle all loading variations.
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Figure 6
Class A Thermophilic Digestion Options
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7.2.7
Thermal Hydrolysis/Anaerobic Digestion
Thermal hydrolysis is a sludge pre-digestion process aimed at achieving more efficient
anaerobic digestion and producing Class A digested biosolids and a well-dewatered
product. The Cambi® Process is the thermal hydrolysis process that has been developed
and marketed world-wide. It was developed by a Norwegian company and has been
implemented full-scale at several plants in Northern European countries (UK, Ireland,
Norway, Denmark, etc.).
Cambi® thermal hydrolysis consists of first dewatering the sludge (raw primary and WAS)
to about 15 percent solids content. The dewatered sludge is fed into a batch hydrolysis
vessel, where it is subjected to high heat (320°F) and pressure (100 psi). The pressure is
released in a flash tank which helps destroys pathogens and breaks down the cell
structures in the sludge. Following hydrolysis, the sludge is anaerobically digested usually
at mesophilic temperature. Digestion feedstock for this process is typically about 9 percent
solids, thus reducing the digestion tankage requirements. This is one of the key advantages
of the process. Another key advantage is a very well-dewatered product (typically 30 to 35
percent solids), and a product that is well-stabilized.
The Cambi® process was pilot tested at San Francisco’s SEWPCP in 2001. The pilot test
was successful and achieved increased volatile solids reduction over mesophilic, Class B
digestion.
7.2.8
Aerobic Digestion
Aerobic digestion consists of aerating thickened or unthickened sludge in a tank for an
extended period of time. Volatile solids are oxidized in the process, stabilizing the sludge
and reducing the total mass of solids that must be managed by recycling or disposal.
Pathogen densities are also reduced. The process does not produce a methane-rich
biogas.
Many small (less than 5 mgd) wastewater treatment plants in the United States use aerobic
digestion to stabilize solids, due to its relatively low capital costs, simplicity, and
compatibility with the certain liquid treatment processes. The aerobic digestion process
requires substantial energy input in the form of aeration blowers, and therefore is not
typically used at larger wastewater plants.
7.2.9
Auto-thermal Thermophilic Aerobic Digestion
The Auto-thermal Thermophilic Aerobic Digestion (ATAD) process uses the energy
released from the oxidation process to raise the temperature of the sludge above 50 to
55 degrees C. The process is effective at volatile solids reduction and greatly reduces
pathogen densities. Sludge is first thickened prior to the ATAD process to about 5 to
6 percent solids. The ATAD process typically consists of at least two covered reactors in
series. Air is mixed with the sludge to maintain vigorous mixing and to insure maximum
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aerobic conditions and oxidation to achieve thermophilic temperatures. The detention time
is typically 8 to 10 days. Process temperature is controlled by adjusting the amount of
air/oxygen added. The ATAD process has been used primarily at small plants – it has rarely
been used at plants over about 5 to 10 mgd in size. There are significant odor issues to
overcome in the design and operation since maintaining truly aerobic conditions within the
reactors under all conditions is challenging. For these reasons, it is not considered
appropriate for San Francisco.
7.2.10
Dual Digestion
Dual digestion is a combination of aerobic and anaerobic digestion processes. The first
stage consists of high-purity oxygen aerobic digestion with about a one-day detention time,
which heats the sludge to thermophilic temperatures. Process temperature is controlled by
adjusting the amount of oxygen added to the reactor. The first-stage thermophilic aerobic
digestion provides a high-degree of pathogen reduction and conditions the sludge to
provide a more-stable and reliable second-stage anaerobic process.
The second-stage of the dual digestion process consists of an anaerobic digestion system
with typically 15 days of SRT. Sludge can be cooled to mesophilic temperatures prior to
introduction to the second-stage anaerobic digestion system. The dual digestion process
can be designed to produce a Class A biosolids product. The only West Coast plant to
employ this process is at Tacoma, Washington. Tacoma staff developed a multi-stage
anaerobic digestion portion of the process which includes thermophilic, then mesophilic,
anaerobic digestion.
7.2.11
Anaerobic/Aerobic Digestion
Recent research by Virginia Polytechnic Institute and State University (Virginia Tech) has
investigated the concept of anaerobic digestion followed by aerobic digestion. Virginia Tech
has found that some sludge solids are degraded only or primarily by anaerobic digestion,
and some solids are digested primarily during aerobic digestion. Providing an aerobic
digestion process following conventional mesophilic anaerobic digestion has been found to
achieve up to 65 percent volatile solids reduction. The studies also suggest that providing
aerobic digestion after anaerobic digestion can remove significant amounts of ammonia and
total nitrogen from the biosolids, thus greatly limiting the recycle stream impact from
dewatering. The process is in the development stage.
7.3
Non-Digestion Stabilization Technologies
There are a host of non-digestion processing technologies which are used to stabilize
sludge. These range from alkaline processes to composting, and also include many thermal
processes. Some of these processes are quite specific in terms of producing a certain
product (such as a fuel product or construction aggregate). Other processes are more
general in terms of the final product and its end use or disposition.
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7.3.1
Alkaline Stabilization (PSRP, or Class B process)
Alkaline stabilization consists of adding sufficient quantities of quicklime (CaO) or other
alkaline materials to sludge to raise the pH of the mixture above 12 for two hours or more.
The high pH significantly reduces or eliminates biological activity and destroys pathogens.
Biological activity in the mixture can resume if the pH of the mixture is allowed to decline
over time, so alkaline-stabilized biosolids cannot be stored for long periods of time. Raising
the pH of sludge releases ammonia; therefore, air collection and odor control equipment is
frequently required for an alkaline stabilization process. Alkaline stabilization is not a
common practice in California due to product market limitations.
7.3.2
Alkaline Treatment (Class A process)
Several proprietary processes are available that use a combination of heat and high pH to
create Class A soil amendment products. Quicklime, cement kiln dust, or other alkaline
materials are mixed with dewatered biosolids in heated or insulated reactors. The high heat
of the chemical reaction (or supplemental heat addition) destroys pathogens. Raising the
pH of biosolids releases ammonia and sometimes other odorous compounds; therefore, air
collection and odor control equipment is frequently required for an alkaline stabilization
process. The appearance of the finished product varies with each proprietary process;
some products are significantly more aesthetically agreeable than others. As indicated
above, alkaline stabilization is not common in California.
7.3.3
Composting – Unconfined
Composting is the controlled aerobic decomposition of organic matter to produce a humuslike material. Thermophilic temperatures are achieved through auto-heating during the
composting process, destroying pathogens. Bulking agents are mixed with dewatered cake
to increase the porosity of the mixture and add carbon. Typical bulking agents include wood
chips or sawdust. In some co-composting operations the bulking agent is municipal green
waste. Unconfined composting is accomplished outside of an enclosed building or vessel.
The lowest-cost composting technique is normally the use of mixed windrows, however, this
technique has high odor emissions. Open windrow composting should therefore not be
considered unless a very remote site can be located and odor transport potential carefully
evaluated. In San Francisco, this is not considered possible.
7.3.4
Composting – Confined
Confined composting is composting within an enclosed building or vessel. The advantage
of confined composting is that odors can be controlled. There are many different
arrangements for confined composting – as simple as aerated static pile composting within
a building, to systems using automated, mechanical mixing and transport during the
process.
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One proven process is the agitated bed system manufactured by a number of companies.
With this technology the composting takes place within concrete bays that measure
approximately 10 feet wide by 6 feet deep by 200 feet long. Automated machinery
periodically mixes and moves the composting mixture. Feed materials are introduced at one
end of the bay and the finished compost is removed at the other end. The system is
enclosed within a building so that foul air can be collected and treated in a biofilter. The City
of Santa Rosa has had a successful agitated bed biosolids composting program for many
years.
Finding adequate space/footprint within San Francisco is likely to be one of the key
challenges for a local composting facility, and insuring that odors are adequately controlled.
Also, bulking agent would need to be brought in for the operation. The market for a compost
product is probably one of the first things to be evaluated before proceeding further with
evaluations.
7.3.5
Vermiculture
Vermiculture is the process of converting biosolids into soil conditioner material using
earthworms. The earthworms consume the biosolids and produce castings (earthworm
feces). Earthworm castings have a mild odor and are similar in appearance to high-quality
topsoil.
Biosolids are mixed with a bulking agent, such as green waste or wood chips, to increase
porosity and create an ideal aerobic environment suitable for the earthworms. The mixture
is typically spread in low windrows on top of a mixture of castings and earthworms. The
earthworms migrate vertically into the biosolids and begin making castings and
reproducing. After a period of time (depending on the volume of biosolids and the
earthworm density) the earthworms are separated from the castings and the process
begins again. There are significant space requirements and related odor control issues to
be resolved. Work to date has been on smaller scale facilities. For these reasons,
vermiculture is not recommended for further evaluation in San Francisco.
7.3.6
Slurry-Carb® Process
EnerTech Environmental, Inc. has patented their Slurry-Carb® process to create renewable
fuel from biosolids. Dewatered biosolids are subjected to high heat and pressure to break
down the cellular structure of the material, releasing carbon dioxide gas. After
depressurization and partial cooling the resulting slurry is dewatered using centrifuges to
about 50 percent total solids content, and then dried to over 90 percent total solids content
to create E-Fuel®. The E-Fuel is considered to be a renewable fuel in California, and can
be used in cement kilns and coal-fired processes. EnerTech Environmental, Inc. claims the
process uses less energy to create renewable fuel than the heat drying processes
described above.
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The first full-scale Slurry-Carb facility is being planned for a site at Rialto, California
adjacent to the Rialto Wastewater Treatment Plant. Several Southern California wastewater
agencies have signed contracts to provide dewatered biosolids to the privately-owned,
operated, and financed facility at negotiated tip fees, if the facility is actually implemented in
a timely fashion. The cost to these agencies is expected to be in the range of $70 to $80
per wet ton. The Rialto facility is expected process 675 wet tons of biosolids cake daily, and
occupies approximately 2 acres. The facility is currently being designed and site permits
negotiated. EnerTech indicates the facility could be operating by 2008. All of the E-Fuel
produced at the Rialto facility will be used at a nearby cement kiln, and the fuel residuals
would become part of the cement product.
7.3.7
Pyrolysis
Pyrolysis processes consist of subjecting dried biosolids to very high temperatures in the
absence of oxygen. Unlike the Slurry-Carb process described above, the biosolids are first
dried to remove most of the moisture. The dried biosolids are then heated to 1200°F to
1800°F in a reactor, creating a char product that can be used as a fuel.
7.3.8
Gasification
Gasification is also called “starved air combustion.” Gasification consists of exposing
biosolids to high heat and pressure without adequate oxygen to complete combustion. The
biogas that is produced has heating value. Other products from the process include char
and/or oil, depending on the temperature and pressure used. There are currently no
biosolids gasification facilities in the United States.
7.3.9
Sludge-to-Oil Technology
The Enersludge® process, a proprietary system by Environmental Solutions International,
Ltd., of Australia is a pyrolysis process designed to create an oil product. Dried biosolids
are subjected to a temperature of 850°F for 30 minuets, creating oil, biogas, and char. The
char and biogas are burned to provide energy for the process. The low-grade oil product is
marketed. There is currently one facility operational in Australia.
7.3.10
Thermal Depolymerization and Thermal Conversion Process
Changing World Technologies, Inc. offers a patented technology called “thermal
depolymerization” or “thermal conversion” that converts complex organic materials into a
light crude oil. Potential feedstock materials for the process include food processing wastes,
sewage sludge, mixed plastics, and old tires. The feedstock is ground into small pieces and
mixed with water, if dry. The feedstock is fed into a reactor and subjected to high
temperature and pressure (250°C and 600 psi, respectively) for about 15 minutes, after
which the pressure is rapidly reduced to boil off most of the water. The end result is a
mixture of crude hydrocarbons and solid minerals that are separated out. The hydrocarbons
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are refined further in a second reactor that is heated to 500°C. The oil product is suitable for
use in electrical generation equipment.
The first full-scale facility is operating in Carthage, Missouri, processing turkey offal from an
adjacent slaughterhouse. The facility has experienced odor complaints since its startup in
February 2005. There are currently no full-scale facilities that process sewage sludge.
7.3.11
Thermal Processing with Energy Recovery
The most direct method of exploiting the energy value of biosolids is thermal processing
with energy recovery. Thermal processing with energy recovery consists of the complete
combustion of biosolids in fluidized bed or multiple hearth incinerators. Exhaust from the
combustion reaction passes through heat exchangers to recover energy. Usually the
energy that is recovered is directed back to the combustion process to reduce or eliminate
supplemental fuel requirements. Supplemental fuel requirements are very low if raw sludge
is dewatered to about 30 to 32 percent solids. For digested sludge, higher solids content
would be required to avoid supplemental fuel needs. Air pollution control devices, such as
wet scrubbers, dry and wet electrostatic precipitators, fabric filters, and afterburners are
used to reduce emissions to acceptable levels.
The USEPA estimates that approximately 20 percent of the biosolids generated in the
United States are combusted for disposal. In California biosolids incinerators are operated
by the City of Palo Alto and the Central Contra Costa Sanitary District.
7.3.12
Thermal Conditioning and Heat Treatment
Organic matter can be oxidized in the presence of water by subjecting it to high
temperatures, pressure, and sufficient oxygen. The end product of “wet air oxidation” is a
sterile ash that can be readily dewatered. The Zimpro® process was installed at a number
of wastewater treatment plants in the 1970s and 1980s. Thickened sludge and air were
pumped into a reactor, and subjected to temperatures and pressures of up to 500°F and
1,800 psi for 40 to 60 minutes. The oxidized slurry was then cooled in a heat exchanger,
and the gases reduced to atmospheric pressure through a pressure control valve. The
processed sludge could then be dewatered using conventional methods, such as
centrifuges or belt filter presses (USEPA , September 1979). Unfortunately, the Zimpro
process, which operated at subcritical temperatures and pressures proved to be very
odorous, and this technology is being phased out.
Two manufacturers are developing wet air oxidation systems that operate at super critical
temperatures and pressures. The HydroProcessing and Chematur Engineering AB systems
operate at temperatures and pressures up to 1,100°F and 3,700 psi to achieve greater
oxidation than the older Zimpro process with less odors. Neither process has an operational
full-scale facility.
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7.3.13
Wet Air Oxidation in Deep Well
A variant of the wet air oxidation concept uses a deep well to achieve high pressure and
temperature, rather than pumps. GeneSyst International Inc. offers a technology that
consists of two concentric tubes that extends to a depth of 6,000 to 8,000 feet in the
ground. Thickened sludge is pumped down the outer tube, and the treated product is
withdrawn from the top of the inner tube. The sludge is subjected to increasing temperature
and pressure as it travels down the outer tube. Oxygen is injected to the bottom of the well,
where the temperature and pressure is the highest, causing the oxidation reaction to occur.
The treated slurry cools as it travels up the inner tube. The chief advantage of the system is
that it does not require the use of high pressure sludge pumping systems. Unfortunately,
maintenance of deep well structures is difficult and specialized. In addition, the well cannot
be installed across an earthquake fault. There are no full-scale installations in operation in
North America.
7.3.14
Irradiation
Sludge can be disinfected to Class A standards by subjecting it to beta or gamma radiation.
The 40 CFR 503 regulations require a 1.0 megarad dose at room temperatures. Beta rays
can be generated by a particle accelerator. Gamma rays are emitted by radioactive
isotopes like Cobalt 60 or Cesium 137. Additional treatment is required after irradiation to
achieve vector attraction reduction requirements (USEPA, July 2003). There are no fullscale applications of this technology in North America.
7.3.15
High-Temperature Melting and Vitrification
The Minergy® process is designed to create a glass aggregate product from biosolids. The
Minergy process is owned by the Minergy Corporation, a subsidiary of Wisconsin Energy
Corporation. Biosolids are first dried to 90 percent solids and then combusted in an oxygenrich atmosphere at temperatures of 2,600 °F to 2900 °F. At those high temperatures the
ash from the biosolids melts and is subsequently cooled to create an inert glass aggregate
product that is black in color. Air emissions control equipment includes a condenser, carbon
bed, and particulate, NOx and SOx control equipment. A full-scale (1,200 wet tons per day)
Minergy facility has been in operation at Neenah, Wisconsin since 1998. In addition, a
smaller (187 wet tons per day) facility located at Zion, Illinois started operation in 2006.
7.3.16
Bio-Brick Production
Bio-bricks are created by mixing dewatered biosolids with the conventional clay and shale
ingredients used to make bricks. The resulting mixture is formed into brick shapes, dried,
and then fired in the conventional brick-making manner. The kiln temperatures reach 1,100
°C during the firing process, causing the combustion of the organic matter in the biosolids.
Therefore, the biosolids provide some energy value to the kiln. The interstitial voids in the
bio-bricks add to the freeze-thaw durability of the bricks and improve mortar adhesion.
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Pilot testing is required to determine the acceptable quantity of biosolids that can be
incorporated into a brick mixture while maintaining acceptable compressive strength, water
absorption, and freeze-thaw resistance criteria. The chemical characteristics of the
biosolids can also affect the color of the bio-bricks. Upgraded emission control equipment
may be required for the bio-brick kiln.
7.3.17
Lagooning
Plants with sufficient area have lagooned their digested biosolids for months or even years
to achieve further stabilization and destruction of volatile solids. Pathogen reduction to
Class A standards has also been accomplished. The San Jose/Santa Clara WPCP
continues to use large-scale lagooning and air drying on a large site with several hundred
acres for this type of operation. The Dublin San Ramon Services District also uses
lagooning for biosolids processing. These are unique sites with sufficient land and buffer
area. Since the land and buffer area required are not available in San Francisco, lagooning
is not considered a viable option.
7.4
Dewatering and Drying Technologies
Dewatering technologies remove water from biosolids to create materials having about 15
to 30 percent solids content. Drying can involve a number of options, and the solids content
of dried biosolids can range from 40 or 50 percent for partially dried, and up to 95+ percent
solids for fully-dried material.
7.4.1
Belt Filter Press
Belt filter presses employ moving porous belts to continuously dewater sludge. Belt filter
presses are currently used at the OCWPCP to dewater biosolids, and are capable of
achieving approximately 15 percent total solids content. Figure 7 illustrates the belt filter
press process. The process consists of three distinct phases. In the first phase, polymer is
mixed with the sludge for conditioning purposes. The second phase consists of gravity
drainage through a single belt to a non-fluid consistency. The sludge is pressed between
two belts and rollers in the third stage to produce the dewatered cake. Belt widths of 1.0
and 2.0 meters are generally used, although machines using belts up to 3.0 meters wide
are manufactured. High pressure water sprays continuously clean the belts. Required
ancillary equipment includes polymer feed systems, wash water systems, sludge feed
pumps, odor containment and control systems, and dewatered cake conveyance systems.
Belt filter press dewatering has worked satisfactorily in San Francisco, however, if prices for
transport and product use continue to rise, it may be more economical to consider
technologies that provide a higher cake solids content.
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Figure 7
7.4.2
Belt Filter Press
Centrifuge
Centrifuges are another commonly used dewatering technology. Centrifuges are currently
used at the SEWPCP to dewater biosolids. The newer high-solids centrifuge machines
typically achieve dewatered cake of approximately 23 to 28 percent total solids content. The
process is illustrated in Figure 8. Centrifugal force of 500 to 3,000 times the force of gravity
is applied to the biosolids within the centrifuge, separating liquid from the solids. The
centrifuge has a solid bowl that spins at a high rate. Liquid sludge, conditioned with
polymer, is introduced within the rotating bowl. The sludge spins with the bowl, separating
into liquid and solid fractions. A screw conveyor mechanism spins within the rotating bowl at
a slightly faster or slower speed than the bowl to facilitate moving the solids fraction
towards one end of the bowl, where it is discharged. The centrate (removed liquid) is
discharged through another port. The process operates continuously. Required ancillary
equipment includes sludge feed pumps, polymer feed systems, and sludge cake
conveyance systems.
Centrifuges are sized based on hydraulic and solids throughput. Machines are available to
dewater sludge flow rates ranging from 25 gpm to 700 gpm. Newer high-solids machines
can produce a very well-dewatered material.
7.4.3
Screw Press
The screw press represents a relatively new technology for dewatering municipal
wastewater solids, although the technology has been used successfully in industrial, pulp
and paper production, chemical, and food processing applications.
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Figure 8
Centrifuge
Figure 9 is a diagram of a screw press. Thickened sludge, conditioned with polymer, is
introduced to the machine in the head box at the inlet end. The mixture is conveyed from
the inlet end to the outlet end of the press by the rotating screw. As the material is
conveyed along the length of the press it is squeezed between the tapered screw shell and
the screen drums. The dewatered solids exit the press at the discharge end and fall down
the discharge box. The adjustable pressure cone provides back pressure within the
machine, particularly when the machine is initially filled. For municipal wastewater solids
applications the pressure cone is typically not needed after the machine is filled; the
dewatered sludge provides sufficient back pressure. The liquid that was forced out through
the screens is returned to the liquid treatment process.
Figure 9
Screw Press
Unlike the centrifuge, the screw press operates at a very slow rotational speed. The screw
rotation is usually one-half of a revolution per minute or less for municipal wastewater
solids. Water is slowly forced from the sludge by squeezing action – similar to a belt filter
press – but for much longer periods of time. The solids retention time in a screw press can
be on the order of two hours. The long solids retention time make screw presses practical
for small wastewater treatment plants only, due to the large size of the machine relative to
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amount of biosolids throughput. Therefore, screw presses are likely not a practical solution
for San Francisco.
7.4.4
Rotary Press
The rotary press technology operates by feeding flocculated sludge between two parallel, 4foot diameter rotating, chrome-plated, stainless steel fine screens which rotate very slowly
on a single shaft (typically between 1 to 3 rpm). Each disk set is called a channel. Filtrate
passes through the screens as the sludge advances around the channel. The frictional
force at the sludge/screen interface coupled with increased pressure caused by an outlet
restriction produces the dewatered sludge cake.
The rotary press has proven to be an effective dewatering device for blends of primary and
secondary sludge as well as digested sludge. Developed in Canada, there are several
plants running for over ten years with proven performance. The rotary press can typically
achieve higher solids content than belt filter presses with only slightly higher polymer and
power requirements. The low rotational speed has resulted in low maintenance
requirements. The rotary press has enclosed dewatering channels minimizing odor control
requirements.
As with the screw press, the rotary press has a relatively long solids retention time,
resulting in a large machine relative to the amount of biosolids throughput. Therefore, rotary
presses are not a practical solution for San Francisco.
7.4.5
Plate and Frame Pressure Filter
A plate and frame pressure filter consists of a series of metal plates sandwiched together in
a heavy steel frame. Pressure filters operate by using a positive pressure differential as the
driving force to separate water from the sludge slurry. Conditioned sludge is pumped into
the pressure filter (between the plates) until the pressure reaches 100 lbs/in2, at which time
pumping is discontinued and the press plates are separated and the dewatered sludge is
normally scraped off and falls into a truck trailer below.
The use of pressure filters has fallen out of favor for dewatering municipal wastewater
sludges for a variety of reasons. The traditional process requires addition of large amounts
of lime and ferric chloride that increases the volume of sludge that must be disposed. Many
installations modified their operation years ago to use polymer to condition sludge instead
of lime and ferric chloride, creating a lower solids content cake. Filter presses are a batch
process requiring more operator attention than the favored continuous-flow processes
described above. A typical problem has been the labor required to scrape the dewatered
material off the plates.
A fairly recent variation of the plate and frame pressure filter is the vacuum/heat filter press.
Following the pressure dewatering described above, steam or hot water is circulated inside
the plates to heat the biosolids. At the same time a vacuum is pulled on the biosolids,
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causing the water in the biosolids to evaporate. When the solids are dry (upwards of 80
percent solids is reported), the vacuum and heat are removed and the plates are separated
to allow the dried product to be removed from the press. The process should be capable of
producing a Class A biosolids product, but long-term maintenance issues remain and the
process has not been proven at large scale.
7.4.6
Air/Solar Drying – Open Systems
Biosolids can be dewatered and dried using open system drying beds. Drying bed area
requirements are a function of the mass of water that must be removed and the climatic
characteristics of the site. Covers to limit rainfall on the bed can be used in areas of higher
precipitation. Regulatory agencies typically require that newly constructed drying beds be
lined to prevent groundwater contamination by nutrients or salts in the biosolids. Asphalt
concrete pavement and other materials have been used successfully for this purpose.
Paved beds work well as they allow mechanical equipment to work on the beds.
Biosolids must be stabilized prior to air drying to limit odor emissions. And in more urban
areas, uncovered/uncontained drying beds are usually limited in size. Sometimes,
dewatering precedes drying beds to limit the area required.
Open system drying beds are not a practical solution for San Francisco due to the landintensive nature of the process and odor emissions.
7.4.7
Air/Solar Drying – Within Structure
Recent innovations in air/solar dewatering/drying operations involve handling biosolids
within a greenhouse or hot-house structure equipped with forced-air ventilation and
automatic mechanical mixing. Humidity and air temperature are monitored within the
greenhouse and ventilation fans are energized as needed to maintain suitable drying
conditions. The mechanical mixing systems vary in type and complexity. Treatment of the
discharged airstream is required for sites with close neighbors. One small system has been
successfully implemented at the Town of Discovery Bay in Northern California.
Air/solar drying within a structure is probably not a practical solution for San Francisco due
to the area requirements and odor issues. However, advancements in the technology are
making evaporation more efficient and cost-effective, and there could be a future option,
depending on space requirements, for a portion of the City’s biosolids product to be dried or
partially dried with a technology in this category.
7.4.8
Heat Drying – Graded Pellet Product
Heat drying technologies use thermal energy to evaporate almost all moisture from
biosolids to create a Class A product. There are wide varieties of dryer technologies
available; for master planning purposes the technologies can be divided into processes that
create graded pellet products similar to commercial fertilizer products, and those that create
ungraded products.
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Most of the heat drying technologies that create graded pellet products are “direct” dryers.
In direct dryers, moisture removal is achieved predominantly by convective heat transfer. A
hot air/gas mixture is generated by a fuel-burning furnace, which exhausts the hot gases
directly into the drying vessel. The hot gases come into direct contact with the dewatered
sludge, causing the water to evaporate. Direct drum dryers are capable of making a highquality biosolids product consisting of uniform, hard, spherical pellets similar in appearance
(with the exception of color and odor) to commercial inorganic fertilizer products. Most of
the largest thermal drying operations in the United States use direct drum dryers to create
their biosolids products.
A process schematic of a direct thermal drum dryer system is shown in Figure 10.
Dewatered biosolids are first mixed with already-dried biosolids pellets upstream of the
drying drum to control the moisture content of the mixture within the dryer. This first step in
the drying process accomplishes two important functions. First, it provides a means of
reincorporating “fines” and undersized particles that are separated from the product in the
screening step following the dryer. Second, the physical form of the biosolids is altered so it
does not stick to the internal parts of the drying drum. This preliminary mixing step is critical
to producing a pellet product from the dryer. The triple-pass drying drum rotates as hot air
and the sludge particles pass through. The biosolids particles exiting the drum are screened
to separate product of the desired particle size for cooling and temporary storage while
awaiting distribution to market outlets. Oversized particles are crushed and returned to the
head-end of the process, along with undersized particles and fines. The dryer off-gases are
treated with a condenser prior to recycling back to the furnace or discharging to the
atmosphere following treatment in a regenerative thermal oxidizer. Recycling a large portion
of the process air serves to decrease the volume of air requiring treatment prior to
discharge and to increase the thermal efficiency of the process.
Heat dried biosolids products must be stored properly or they can catch on fire. If a pile of
heat dried biosolids absorbs moisture it can autoheat and combust, therefore, proper
design of product storage facilities is vital. Product storage silos are generally equipped with
temperature sensors and inert gas blanketing to reduce fire potential.
7.4.9
Heat Drying – Ungraded Product
Heat drying technologies are also available that produce ungraded products containing
wider variation in particle sizes and shapes. In general, ungraded heat dried biosolids
products contain higher percentages of fines, creating dustier products. Ungraded product
particles tend to be more angular in appearance and therefore less-similar to commercial
fertilizer than graded products. Ungraded products will be more-difficult to market as
fertilizer than graded products because of these differences. Most of the ungraded products
are produced using indirect drying technology.
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Figure 10
Direct Thermal Drum Dryer Producing Graded Pellet Product
Indirect dryers achieve moisture removal predominantly by conductive heat transfer, and
the biosolids are kept separate from the primary heated drying medium (typically oil or
steam). The drying medium is heated in a boiler or heat exchanger by the hot combustion
gases from a fuel-burning furnace. An indirect dryer consists of a stationary vessel with an
internal agitator and stirring assembly. The dewatered biosolids cake enters the stationary
vessel of the indirect dryer and is continuously agitated and stirred during the drying cycle.
The heat is then transferred from the drying medium to the sludge by circulating the
medium through the stirring mechanisms, augers, shafts, disks, dryer casing, or other
equipment that comes into contact with the sludge.
A process diagram of a typical indirect thermal drying system is shown in Figure 11.
Dewatered biosolids are introduced to the drying chamber, which is heated with hot oil or
steam. Moisture evaporates from the biosolids as they move through the machine. Dried
biosolids exit the dryer, and are cooled prior to temporary storage in a silo while awaiting
distribution to market outlets. Vapor from the dryer passes through a condenser prior to
treatment in a biofilter or other odor control process and discharge to the atmosphere. The
volume of air that must be treated is significantly smaller than the direct drying systems
because the furnace air does not come into contact with the drying biosolids.
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Figure 11
Indirect Thermal Dryer Producing Ungraded Product
Unlike the direct dryers, the indirect drying systems generally do not include product
screening and recycle. The product storage silo must include temperature sensors and
provisions for inert gas blanketing for fire prevention purposes. Indirect dryers may be
operated on a continuous or batch basis, depending on the manufacturer.
7.4.10
Innovative Biosolids Drying
Several innovations in drying technology are occurring, often in an attempt to reduce the
energy requirements. These technologies are largely being developed in Europe, and they
include the following:
•
Belt drying. This method includes lower temperature drying – typically using hot air at
350 to 400 degrees F. Biosolids are spread onto belts to maximize surface area
exposed to warm air. The system uses energy at almost the same level as the moreconventional heat dryers, and is geared toward producing a dry (i.e., 90 percent
solids) Class A product. The system has been geared to smaller operations, but
larger operations are possible.
•
Microwave drying. High-efficiency multi mode microwaves are used to heat the
material from within. Heated air is used to carry away the moisture. Again, smaller
plants have been targeted to date, but larger operations are possible.
The major factor that is likely to be available at the San Francisco facilities is a large hot
water stream (at perhaps 180 degrees F) from the cogeneration facilities. This waste heat
hot water stream needs to be explored for use in innovative drying applications such as the
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above, and also together with solar technology, to determine if a biosolids drying operation
could be developed with much lower purchased energy use.
7.4.11
Combined Centrifuge/Dryer
Centrifuge technology has been combined with thermal drying technology in the Centridry®
process to enhance dewatering. Dewatered cake from the centrifuge system drops into a
chamber where hot air contacts the material and quickly evaporates moisture so that the
product is typically about 60 to 70 percent solids. The heating of the biosolids is not
sufficient to provide Class A (pathogen-free) biosolids. A portion of the exhaust air is
recycled to increase thermal efficiency and reduce the volume of air that is treated
discharged. The process was pilot tested in King County, Washington several years ago
and produced a semi-dry product that had some odor emissions. The process is used at
several European plants prior to combustion of the sludge/biosolids.
7.5
Other Solids Processing Technologies
Technologies that do not fit in the previous categories are described below. These are not
stand-alone processes, but are typically used in association with previously-discussed
processes.
7.5.1
Disintegration Processes
Waste activated sludge (WAS) is primarily composed of cell tissue that is more-difficult to
digest than primary sludge. Disintegration processes are available that are designed to
break the walls of the WAS cells, or in some cases break apart large agglomerations of
cells called bioflocs, thus making them more readily digestible. The potential benefits
include higher volatile solids reduction, increased biogas production, reduced digested
solids quantities, and better dewatering performance. The disintegration processes
manufactured by Sonico and IWE Tech use ultrasonic waves to break cell walls. The
MicroSludge® process uses chemicals and pressure/heat to achieve disintegration goals.
Disintegration processes have been successfully implemented in Europe in the last 5 years.
Research continues in North America, including full-scale demonstrations. The costs to
operate disintegration processes must be carefully evaluated in conjunction with
demonstrated benefits to determine if a net financial gain will be realized.
7.5.2
Nutrient Removal Processes
Biosolids dewatering and other processes can create waste side streams containing high
concentrations of ammonia, phosphorus, and other nutrients. Under certain conditions a
chemical precipitate (scale), called struvite, can form in piping and mechanical equipment.
Struvite is difficult to remove, and can cause significant pipeline restrictions or complete
clogging. One technique for controlling struvite formation is to remove the phosphorus from
the waste side stream. The Crystalactor process, offered by DHV Water BV, is a fluidized
bed reactor designed to precipitate phosphorus from a liquid waste stream, creating crystal
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pellets. Chemicals are added to the liquid waste stream to adjust the pH and create suitable
conditions for phosphorus precipitation. The reactor contains sand or minerals to provide
suitable seed material for crystal growth. The crystals become heavier as they grow,
moving to the bottom of the reactor. The finished pellets are removed from the bottom of
the reactor, air dried, and sold as fertilizer.
7.5.3
Cannibal® Process
The Cannibal® process is a proprietary solids reduction process offered by USFilter
Corporation. The process combines fine screening and a proprietary bioreactor to
significantly reduce the mass of biological solids requiring disposal. Return activated sludge
is passed through a 250-micron screen to remove grit, inorganic trash (plastics, etc.), and
organic materials that are not readily degradable, such as hair and lint. The screenings are
sent to landfill. A portion of the screened return activated sludge is sent to a proprietary
“side-stream interchange bioreactor.” The bioreactor is a batch-fed anoxic process
designed to reduce the mass of solids through digestion by facultative bacteria. The effluent
from the bioreactor is best returned to an anoxic zone within the liquid treatment process.
The manufacturer claims sludge yields of 0.1 pounds per pound of BOD5 removed in the
liquid treatment process, plus 0.2 to 0.25 pounds of screenings per pound of BOD5
removed in the liquid treatment process. The Cannibal® process is designed for
wastewater treatment plants that do not have a primary clarification process. Existing plant
capacities range from 0.75 to 16 million gallons per day.
7.6
Biogas Processing and Use Technologies
With anaerobic digestion of wastewater solids and other organic feedstocks, an energy-rich
gas is produced which usually contains between 60 and 70 percent methane. This gas is
often called digester gas or biogas, and we refer to it as “biogas” in this report. The biogas
is increasingly valuable in today’s energy markets and is a renewable fuel which has
important implications for sustainability goals. Capturing methane and using it wisely
minimizes methane emissions to the atmosphere – a critical greenhouse gas concern.
Biogas can be used wherever natural gas is used; however, required gas characteristics
vary depending on the combustion method/device. Characteristics required for stationary
biogas applications at the treatment plant are much less stringent than required to meet
pipeline quality natural gas or vehicle fuel needs. Options for biogas use are presented and
discussed here along with technologies for biogas processing and handling.
7.6.1
On-Site Electric Power Generation and Cogeneration
Electrical power is a highly valued and flexible commodity in California and elsewhere.
Therefore, on-site power generation or cogeneration is very popular at California
wastewater treatment plants when considering options for using the biogas from anaerobic
digestion of solids. The heat byproduct from cogeneration facilities is also valuable in
heating the digesters and providing heat for buildings, and even for building cooling via
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absorption chillers. Biogas power generation via internal combustion engines is a mature
and well proven technology. Gas turbine technology is also used for electric power
production from biogas. Several major reciprocating engine and gas turbine manufacturers
have dozens of successful operating projects. Biogas to electricity projects have been more
risk-free than some of the other biogas use options, and the power that is created is fully
usable. The optional technologies for power generation and cogeneration are:
•
Internal combustion engines are the most frequently-used technology for biogas
cogeneration at wastewater plants. Heat recovery from the engine is almost always
implemented, creating a hot water system for heating purposes. Engine development
has produced greater efficiency over the years.
•
Gas turbines are also used for biogas cogeneration, particularly at larger treatment
plants. For combustion gas turbines, slightly less biogas energy goes to electric
power production and somewhat more waste heat energy is produced. Turbines are
more sensitive to gas moisture content and characteristics such as siloxane. Gas
turbines also require higher pressure biogas than most other gas use technologies –
often 150 to 250 psi. Therefore, gas pretreatment becomes more critical.
•
Fuel cells produce electric power and some waste heat with no moving parts. Fuel
cells combine hydrogen (extracted from the biogas) and oxygen (from air) in an
electrochemical reaction and represent a very clean technology from the standpoint
of air emissions. The technology is now available, but is very costly and still
undergoing additional development.
San Francisco has had engine-based cogeneration facilities at both the SEWPCP and the
OSWPCP. And, a fuel cell project is being developed by the City. Therefore, electric power
generation and cogeneration technologies are well-known to City staff.
7.6.2
On-Site Direct Engine Drives
Some plants operate biogas engines to directly drive aeration blowers, large pumps, or
other large mechanical equipment that has constant or near-constant load. Waste heat can
be recovered and used as described previously. There is likely to be limited cost-effective
opportunity for this option at the City’s treatment plants because major changes would be
required in powering this type of equipment.
7.6.3
Sell Biogas to Local User
If a local user for the biogas was available in San Francisco, the biogas could be piped
directly to the user and the biogas sold directly. Unfortunately, no known user is available
that fits this situation in San Francisco. At Sacramento and Los Angeles (Hyperion Plant),
biogas produced in the digesters is piped to immediately-adjacent electric power or
cogeneration plants owned or operated by power utilities. In both these cases, the biogas
represents a relatively small portion of the total energy used in the power/cogen plants.
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Steam from these power/cogen plants is piped back to the digestion facility for heating
purposes. Contracts between the wastewater plant utility and the power utility were
negotiated in the 1990s to handle these transfers of gas and steam. This option could arise
in the future at San Francisco, but currently there is no plan for this type of arrangement.
7.6.4
Clean the Biogas and Sell to Utility
With the price of natural gas rising in recent years in the US and worldwide, the concept of
cleaning the biogas to pipeline or utility-quality natural gas is gaining interest, however,
currently there are very few wastewater digestion plants using this approach. Various
technologies are available for biogas cleaning and scrubbing, but removing carbon dioxide
and various pollutants is a major task involving capital facilities and additional operating
cost. This may be an option to evaluate in the future if technology improves and economics
of options change.
7.6.5
Clean the Biogas and Use for Vehicle Fuel
An option being used in Europe and gaining some momentum there, is to clean the biogas
and compress it to high pressures (2000 to 3000 psi) that are required for use in
compressed natural gas (CNG) vehicles. Typically, a CNG fleet vehicle organization is
required at close proximity to the digestion plant to make this option work. Cleaning the
biogas to high quality standards is required for the high pressure compressors and to
protect vehicle engines. This option is expected to be difficult for San Francisco because of
the need to find a CNG user/organization and contract for the use of the fuel on a very
reliable basis. Also, the economics of this approach have been evaluated by others in the
US, and it does not appear favorable for San Francisco.
7.6.6
Use for Heat-Drying Biosolids
A few wastewater treatment plants, mostly in the Eastern US, have used their biogas to
provide energy to thermally dry their dewatered biosolids. Sometimes a mixture of biogas
and other fuels is used to provide the required hot air for direct dryers or provide hot
steam/oil for indirect drying systems. This option becomes more workable if the drying
operation is conducted continuously (24/7 on a reliable basis) so that the biogas can be
used continuously in the drying system. If heat drying is only conducted 6 days per week or
the drying system frequently goes down for maintenance, then the biogas would be flared
during these periods unless there was another use for the biogas during dryer down-times.
7.6.7
Boiler Use for Heating
The most common use of biogas has been, and continues to be, for heating of sludge in the
anaerobic digesters. There are several methods used for such digester heating, but the
most common method is to use biogas as fuel in a hot water boiler, and the hot water is
then used in heat exchangers to heat the sludge/biosolids slurry. Steam boilers are also
used at some plants to provide steam for heating (and, through absorption chillers, for
cooling uses in buildings). Even if the digestion plant uses cogeneration or other methods of
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biogas utilization on a normal basis, the plant will typically need to maintain a boiler system
as a backup heating method, so that digestion heating is assured even if mechanical
breakdown or other problems arise in the biogas utilization system.
7.6.8
Biogas Flares
Biogas flares (sometimes referred to as waste gas burners or emergency flares) are
required at digestion plants as a safety and air emission control measure. If biogas use in
boilers, cogeneration, or other methods is not able to handle the quantities of biogas being
produced, the flares must be used to combust excess biogas. Flare design in the last
couple of decades has achieved improved combustion of the biogas to minimize products
of incomplete combustion and NOx emissions. Typically, flares are sized to handle the peak
potential biogas production from the facility, to account for the emergency situation whereby
the biogas may not be able to be sent to any other combustion device.
7.6.9
Hydrogen Sulfide Removal
Reduction of hydrogen sulfide gas concentrations within the biogas is becoming very
common because of air emission regulatory controls. For wastewater agencies in
California, the most common approach has been to add iron salts so that the ferric or
ferrous ion precipitates the sulfide within the liquid phase sludge. Agencies in California
using this approach can often reduce hydrogen sulfide concentrations from 1000 ppm or
greater down to 100 ppm or less. However, air districts sometimes require hydrogen sulfide
levels less than 100 ppm. In this case, agencies have used iron oxide/hydroxide technology
(iron sponge). Other technologies occasionally used include water or hydroxide scrubbing,
or activated carbon treatment.
7.6.10
Moisture Removal
Biogas is saturated with water vapor. At mesophilic temperature (37 °C, or 99 °F), water
vapor is about 6 percent by volume. However, at thermophilic temperature (55 °C, or
131 °F), water vapor is about 15 percent by volume. Moisture must be removed prior to
combustion and is also removed to minimize condensation within gas piping (potential pipe
blockages), to protect instruments, and limit corrosion from low pH liquid conditions.
Moisture is removed by cooling the biogas. Condensers are common technology for this
purpose. A slight re-heating of the biogas after condensation limits chances for moisture
problems downstream.
7.6.11
Siloxane Removal
Organo-siloxanes and silicones are present in typical municipal wastewater sludge, and
these compounds are also contained in the biogas. These compounds cause tough,
whitish-colored deposits to form on biogas combustion surfaces in boilers, engines, and
other devices. Increasingly, these siloxane-caused deposits cause maintenance problems
and equipment downtime, and, therefore, there is a need to minimize the siloxane content
in the biogas. Condensation of biogas for moisture removal (cooling to about 4 °C
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temperature) removes a small portion of the siloxanes from the biogas, but greater removal
is often required. The most common current method to bring siloxane levels down to < 1
ppm, is through the use of granular activated carbon. Another method for siloxane removal
is to cool the biogas down to very low temperature (-30 °C or colder); however, this
approach has not met with high success rate to date. Other removal methods are
undergoing research work.
7.6.12
Carbon Dioxide Removal
Several technologies are available for carbon dioxide removal from biogas. However, water
scrubbing/absorption has been used more frequently than other methods in the US
because, for wastewater treatment plants, there is an abundance of water available to carry
off the carbon dioxide. Hydrogen sulfide is also largely removed in this absorption process.
The driver for CO2 removal is normally to create pipeline-quality gas or vehicular fuel.
These CO2 removal technologies can be complicated, and, therefore, wastewater plants
are less inclined to pursue this approach without clear economic advantage. Besides water
scrubbing, other methods for CO2 removal include polyethylene glycol absorption, carbon
molecular sieves (pressure swing adsorption), and membrane separation.
7.6.13
Biogas Storage
A small amount of biogas storage is normally used at sludge digestion plants as a means of
equalizing the gas pressure in the system, and providing consistent feed of biogas to
engines, boilers, and other gas use systems. The storage volume provided is often one
hour or less of biogas production; however, sometimes several hours of storage time are
constructed. Beyond this quantity of storage, the cost typically becomes hard to justify.
Most biogas storage at wastewater plants is within low pressure units such as variable
volume bladder tanks or within digester covers that can store biogas directly. Compressing
the biogas for medium or high pressure storage is more costly and is usually done only
when the biogas use system requires increased pressure.
7.7
Screening Criteria
The technologies described above are screened here to ascertain their suitability for
implementation in San Francisco. The screening criteria are described below.
7.7.1
Technology maturity
Biosolids and sludge treatment technologies typically require time to develop into viable
options. The USEPA has developed three categories to assess technology maturity
(USEPA, September 2006), and others have used similar descriptions of technology
development:
•
Embryonic – Technologies in the development stage and/or tested at laboratory or
bench scale. New technologies that have reached the demonstration stage overseas,
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but cannot yet be considered to be established there, are also considered to be
embryonic with respect to North American applications.
•
Innovative – Technologies meeting one of the following qualifications: (1) have been
tested at a full-scale demonstration site in this country; (2) have been available and
implemented in the United States (U.S). for less than 5 years; (3) have some degree
of initial use (i.e., implemented in less than 25 utilities in the U.S.; and (4) are
established technologies overseas with some degree of initial use in the U.S.
•
Established – Technologies widely used (i.e., generally more than 25 facilities
throughout the U.S.) are considered well-established.
Established technologies generally represent the lowest risk for investment of substantial
amounts of public funds. Substantial investment in innovative technologies carries greater
risk for the public agency involved. Embryonic technologies are not suitable for full-scale
application using public funds.
7.7.2
Experience at similar-size facilities
Biosolids and sludge processes often present significant materials handling challenges for
equipment manufacturers. Experience has shown that design and operational problems are
often encountered when equipment size is scaled-up to meet the needs of larger
wastewater treatment plants. For these reasons it is prudent for wastewater agencies to
carefully consider whether technologies have proven success at similar-sized facilities prior
to investment of significant quantities of public funds.
7.7.3
Area Requirements
Some technologies require significantly more land area than others. Area requirements are
a significant issue for highly-developed San Francisco.
7.7.4
Odor Risk
Processing technologies produce varying degrees and types of odors, depending on the
physical and chemical nature of the reactions involved. The likelihood/extent of odor
produced and the complexity of the odor control systems that will be required to mitigate
odor risks are significant issues for San Francisco.
7.7.5
Operations and Maintenance Complexity
Some technologies require higher skill levels to operate and maintain than others. The
operations and maintenance complexity of technologies must be considered because
qualified staff must be hired, trained, and retained throughout the life of a project.
7.7.6
Worker Health and Safety
Some technologies present greater worker health and safety challenges than others due to
chemical handling needs, high pressures, high temperatures, radiation, or equipment
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inertia. The complexity of maintaining a safe and healthy work environment must be
considered.
7.7.7
Product Marketability
Market considerations were discussed in detail in a previous section of this report. The
marketability of the products produced by technologies must be carefully considered,
including regulatory compliance, product aesthetics, and market diversification potential.
7.7.8
Implementation Risks
Some technologies present greater implementation risks (such as permitting, overcoming
negative public perceptions, etc.) than others.
7.8
Screening to Identify Viable Technologies
Table 29 presents the result of the technology screening. Each technology was considered
with respect to the screening criteria described above. The determination was then made
whether:
•
The technology has good potential for near-term application at San Francisco. These
technologies were carried forward for more-detailed evaluation.
•
The technology, with development and refinement has potential for future use at San
Francisco. The SFPUC should continue to monitor the development of these
technologies, but they are not carried forward for more-detailed evaluation at this
time.
•
The technology is not suitable for San Francisco, and is eliminated from further
consideration.
7.9
•
Recommendations on Viable Technologies
Table 30 provides a summary of the viable technologies based on the screening
review in Table 29.
7.10 Solids Processing Approach
In evaluating the Technology Recommendations Summary in Table 30, it is obvious that
there are many processes not well suited to San Francisco’s situation. The reasons for this
are presented previously in Section 5. It is also obvious that there are many processes that
should be tracked over time to determine if they develop into systems that could be
considered for the future.
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Table 29
Category
Thickening
Digestion
Stabilization
Technology Screening
Further
Evaluation
Warranted?
No
Technology
Thicken PS in clarifier
Screening Evaluation and Assessment
Better methods are available.
Gravity belt thickener
In common use in North America. Good performance, and use for co-thickening
service.
Yes
Dissolved air flotation
In common use in North America. Good performance, and can use for cothickening service.
Yes
Centrifuge
Costly operation, and thickening performance not as good as other options.
No
Gravity thickening
Odorous process. Performance not as good as other options.
No
Rotary drum thickening
Use for smaller WWTPs. Not as good a process for co-thickening.
No
Membrane thickening
Not in common use. No MBR facilities at San Francisco, at this time.
Anaerobic digestion mesophilic
Pasteurization/mesophilic
anaerobic digestion
Most common sludge stabilization technology in North America.
Yes
Used in Europe historically. Now used at a few plants in North America. Class A
product.
Yes
Anaerobic digestion –
thermophilic
Temperature phased
anaerobic digestion
Increasing use in North America, including at some large plants in California.
Yes
Increasing experience in North America – benefit of additional volatile solids
reduction. Can be Class A process with proper configuration.
Yes
Acid/gas phased digestion
(including 3-phase
digestion)
Increasing experience in North America. Can be Class A with proper
configuration.
Yes
Future
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Table 29
Category
NonDigestion
Stabilization
Further
Evaluation
Warranted?
Yes
Technology
Class A Thermophilic
Digestion – using batch or
multiple stages.
Includes several advanced digestion process options to produce pathogen-free
biosolids within the digestion process. Working at large plants in North America.
Thermal
hydrolysis/anaerobic
digestion
Aerobic digestion
Experience in Europe is increasing. Pilot tested at San Francisco in 2001. Class
A process and high-solids cake.
Yes
Common for small plants and plants with only waste-activated sludge. High
energy costs and only Class B pathogen reduction.
No
Auto-thermal thermophilic
aerobic digestion (ATAD)
Used at small plants and has had significant odor problems/concerns. Class A
process. Vertad process is similar to ATAD.
No
Dual digestion
Consider with high purity oxygen plants. Can be Class A. City of Tacoma has
had success. Odor concerns.
Yes
Anaerobic/aerobic
digestion
Alkaline stabilization
(PSRP)
Very limited experience – new research being conducted at Virginia Tech.
Rarely used at larger plants. Product use perceived as minimal in Bay Area and
Northern California. Odor concerns. Creates larger mass of biosolids for
transport and disposition, due to addition of alkaline amendments.
No
Alkaline treatment (Class
A)
Involves high pH, high temperature, and drying. Significant odor issues.
Consider as Class A option for rapid implementation if situation warrants.
Yes
Composting – unconfined
Inadequate space within San Francisco, and odors would be too high, even with
digested feedstock. Unconfined composting considered infeasible.
No
Future
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Table 29
Category
Further
Evaluation
Warranted?
Yes
Technology
Composting – confined
Space/footprint is major issue; therefore, only small-scale operation is
considered feasible. Extensive odor control would be required. Digested
biosolids required as feedstock.
Vermiculture
Lack of experience at required scale. Space requirements are significant.
Slurry-Carb® process
First facility may be built in Rialto, CA by 2008. Pressurized and heated
reactions allows high-solids dewatering for energy value. Rialto facility product
to be used in nearby cement kiln.
Future
Pyrolysis
High-temperature processes to create char product and combustible off-gas for
energy value. Public perception may be difficult to overcome.
Future
Gasification
Limited experience and odor concerns. Testing work at Philadelphia has been
troubling over the years.
Future
Sludge-to-oil technology
Very limited experience. Process has been in development for at least 20 years.
Thermal depolymerization
and thermal conversion
process
Thermal processing with
energy recovery
First plant at Carthage, MO working on turkey waste – no facilities using
biosolids. Odor problems at Carthage facility.
Destruction of organics and pathogens. Concerns from air quality perspective,
and major investment required. Ash is the final product, usually disposed.
Continues to be a successful process at approximately 50 US WWTPs. Public
perception may be difficult to overcome.
No
No
Future
Yes
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Table 29
Category
Further
Evaluation
Warranted?
No
Technology
Thermal conditioning and
heat treatment
Significant odor problems at these plants over time. Existing plants with this
technology have been, and continue to be, phased out.
Wet air oxidation in deep
well
Small footprint is advantage. Very little experience. Possible advancements in
future, but also risks from deep wells. Essentially, an ash is produced from the
process. Odor may be crucial concern.
Irradiation
Pathogen reduction process, which can produce Class A. Not a stabilization
process.
High temperature melting
and vitrification
Limited experience and odor potential. Perceived as high cost approach.
Destruction of organics and pathogens.
Future
Bio-brick production
Lack of experience at required scale. Involves high temperature processes.
Advancements in technology are possible as costs for biosolids management
increase.
Future
Belt filter press
Very common dewatering process at scale required. Low-shear process.
However, the technology has not achieved high solids content cake material,
even with newer advancements.
Yes
Centrifuge
Very common dewatering process at scale required. Achieves good cake solids
content, but can be high-shear process with odor regrowth potential.
Yes
Dewatering
and Drying
Future
No
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Table 29
Category
Further
Evaluation
Warranted?
Yes
Technology
Screw press
Relatively new process for biosolids, used at smaller plants to date. Low-speed
machine with low-shear. Space requirements may be excessive for San
Francisco. A version of this process adds steam to produce Class A cake.
Rotary press
Used at smaller plants – space requirements for San Francisco would be
excessive.
Plate and frame pressure
filter
Low-shear dewatering conducted in batches. Sludge/biosolids industry has had
few installations, and most have been phased out. Newer technology using
vacuum and heat provides Class A, but only used at smaller-scale plants to
date.
Future (for
newer form
of these
filters)
Air/solar drying – open
systems
Inadequate space in San Francisco. Even with excellent upstream stabilization,
there would be odor concerns.
No
Air/solar drying – within
structure
New, mechanical greenhouse-type systems. Odor must be highly controlled.
Not yet proven at scale required. Might be implemented for portion of City’s
biosolids production, if space is available.
Yes
Heat drying – graded
pellet product
Digested feedstock required. Very high degree of odor control needed.
Experience is increasing in North America, and considerable experience in
Europe at required scale. Safety is an issue – particularly fire/explosion. Class A
product.
Yes
Heat drying – ungraded
product
Digested feedstock required. With highly controlled systems and advances in
dust control and safety, this type of heat drying may be feasible at San
Francisco.
Yes
No
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Table 29
Category
Technology
Innovative Biosolids
Drying
Combined
Centrifuge/Drying
Disintegration processes
Other Solids
Processing
Technologies
Use of waste-heat hot water stream from cogeneration at San Francisco needs
to be explored for possible use with other innovative drying techniques using
solar energy, belt drying or other technology.
Implemented in Europe, primarily as pre-processing before incineration. Not a
Class A product.
Further
Evaluation
Warranted?
Yes
Future
Applied to TWAS, normally, to achieve greater volatile solids reduction in
digestion. Processes being researched and tested in North America. Several
facilities built in Europe and overseas in last 5 years.
Future
Nutrient removal
processes
Purposeful crystallization to remove phosphorus and perhaps ammonia from
sludge streams. Crystals used as fertilizer material. Implemented overseas
primarily.
Future
Cannibal® process
Process to minimize sludge production. Not very conducive if plants have
primary clarifiers and fairly low MCRT biological process. Has been
implemented at small plants to date.
Future
On-Site
Biogas
Power/Cogeneration
Processing
and Use
Technologies
On-Site Direct Engine
Drives
Sell Biogas to Local User
Most common form of biogas utilization at WWTPs after boiler use. Produces
electrical power (and heat from cogeneration) for use in the WWTP. Already in
use at San Francisco.
Not cost-effective, currently, to make changes to this approach in San Francisco
plants due to distributed nature of sources that could use direct drive engines.
No identified user or power plant located near the San Francisco wastewater
plants.
Yes
Future
No
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Table 29
Category
Further
Evaluation
Warranted?
Future
Technology
Clean Biogas and Sell to
Utility
Rarely done in North America, but interest is increasing. The historical example
is at the King County South Plant in Renton, Washington. Complex
technologies required.
Clean Biogas for Vehicle
Fuel
Rarely conducted in North America, but there have been some examples, and
interest is increasing due to increased fuel prices. Complex technologies
required, and CNG user fleet must be organized.
No
Use to Heat-Dry Biosolids
This is being done in a few North American plants. Must be continuous, reliable
heat drying system for continuous use of biogas.
Future
Boiler Use for Heating
Most common use for biogas produced at wastewater plants. Boilers are almost
mandatory as a backup system, if other gas utilization methods are normally
used.
Yes
Biogas Flares
Required for safety and air pollution control.
Yes
Hydrogen Sulfide
Removal
Required for almost all biogas utilization methods. Reliable systems are
available.
Yes
Moisture Removal
Required for almost all biogas utilization methods. Reliable systems are
available.
Yes
Siloxane Removal
Very likely to be required for biogas utilization. Reliable systems are available,
and research for new technologies is underway.
Yes
Carbon Dioxide Removal
Few carbon dioxide removal systems have been implemented in North America
at wastewater treatment plants. These technologies would be typically used for
options to sell the gas to the natural gas utility or to use the gas for vehicle fuel.
Future
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Table 29
Category
Technology
Biogas Storage
Biogas storage is required for safety and operating reasons, however, storage
quantity is usually limited. Reliable systems are available and used at WWTPs.
Further
Evaluation
Warranted?
Yes
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Table 30
Technology Recommendations Summary
Category
Thickening
Technologies to Carry Forward for
More-Detailed Evaluation
• Gravity belt thickener
• Dissolved air flotation
Digestion
Stabilization
•
•
•
•
•
•
•
•
Technologies with Potential for Future
Application. SFPUC Should Continue to
Monitor the Development of These
Technologies.
• Membrane thickening
Anaerobic digestion – mesophilic •
Pasteurization/mesophilic anaerobi
digestion
Anaerobic digestion – thermophilic
Temperature phased anaerobic
digestion
Acid/gas phased digestion
Class A Thermophilic Digestion
options
Thermal hydrolysis/anaerobic
digestion
Dual digestion
Anaerobic/aerobic digestion
•
•
•
•
•
•
Technologies not
Suitable for San
Francisco
Thicken PS in Clarifier
Centrifuge
Gravity Thickening
Rotary Drum Thickenin
Aerobic digestion
Auto-thermal
thermophilic aerobic
digestion
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Table 30
Category
Non-Digestion
Stabilization
Dewatering and
Drying
Other Solids
Processing
Technologies
• Alkaline treatment (Class A)
• Composting – confined
• Thermal processing with energy
recovery
•
•
•
•
•
•
•
Technologies.
• Slurry-Carb® Process
•
• Pyrolysis
•
• Gasification
• Thermal depolymerization and thermal
•
conversion process
• High temperature melting and vitrification •
•
• Bio-brick production
Belt filter press
• Plate and frame pressure filter
Centrifuge
• Combined centrifuge/dryer
Screw press
Air/solar drying – within structure
Heat drying – graded pellet product
Heat drying – ungraded product
Innovative biosolids drying
•
Disintegration processes
•
Nutrient removal processes
•
Cannibal® process
•
•
•
Technologies not
Suitable for San
Francisco
Alkaline stabilization
(PSRP)
Composting –
Unconfined
Vermiculture
Sludge-to-oil technolog
Thermal conditioning
and heat treatment
Irradiation
Lagooning
Air/solar drying – open
systems
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Table 30
Category
Biogas Processing
and Use
Technologies
• On-Site Power/Cogeneration
• Boilers for Heating
• Flares
• Hydrogen sulfide removal
• Moisture removal
• Siloxane removal
• Biogas storage
Technologies.
• On-site direct engine drives
• Use to heat-dry biosolids
• Clean biogas and sell to utility
• Carbon dioxide removal
•
•
Technologies not
Suitable for San
Francisco
Sell biogas to local use
Clean biogas for vehicl
fuel
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The stabilization processes that are available for continued planning and development at
this time in San Francisco are dominated by the anaerobic digestion category (see
Table 30). Based on work in Section 5, the only non-digestion processes to be evaluated
further are as follows:
•
Alkaline treatment (Class A). This is on the list primarily as a fast-track method to
produce a Class A biosolids product if that were required. In the long-term, alkaline
treatment is not considered a good candidate.
•
Composting – Confined. This is on the list to process a portion of the City’s biosolids,
however, the site for this is undetermined, and, therefore, substantial development of
this concept would be required.
•
Thermal processing with energy recovery. This approach is longer-term, and needs
further study and development before it could be considered ready for
implementation.
•
Heat drying. Thermal or heat drying of biosolids is an important category for follow on
evaluation by the City. However, heat drying should be evaluated only for biosolids
which has already received digestion stabilization. Therefore, heat drying is an
additional process to be considered following anaerobic digestion and dewatering.
•
Solar and innovative drying. Potential exists to dry, or partially dry, biosolids with
newer solar and methods using waste heat available from cogeneration facilities.
Therefore, the City should proceed with anaerobic digestion as its base or core stabilization
and processing approach for wastewater solids processing and biosolids management.
This will necessarily involve rebuilding the digestion and related facilities on the Bayside.
For Oceanside, this approach involves the continued use of digestion and related facilities.
Proceeding with anaerobic digestion as a base program provides a sound stabilization
method that fits well with essentially all potential add-on, advanced, Class A, and thermallyoriented biosolids alternatives that could be considered for San Francisco.
7.10.1
Solids Facilities for Bayside
The solids facilities that need to be rebuilt are primarily the anaerobic digestion facilities and
associated biogas handling and processing equipment. Many of these facilities, as
identified in other SSMP tasks, are structurally deficient, have outlived their service lives,
and are the cause of odor problems. Also, the thickening facilities need to be modified
because thicker feed to digestion has proven more cost-effective (work at the OSWPCP
over the last decade has shown this). Implementing co-thickening of sludge can produce six
percent solids on a reliable basis to feed digestion, and is, therefore more cost-efficient.
Thickening facilities will need to be rebuilt to make this change; however the City has
already invested in two large GBT machines at the SEWPCP which could be transferred to
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a new co-thickening facility. Dewatering equipment also should be changed out to newer,
more efficient machines.
For biogas processing, review of options in Section 5 indicates that on-site power or
cogeneration is the preferred approach and is already being used at the City’s two solids
processing plants. An expansion of this approach to produce even more electrical power
and heat is recommended.
Therefore, a rebuilt and mitigated Bayside solids processing system is needed as a base
program for Bayside biosolids. This rebuilt facility would have the following major elements:
•
Co-thickening of sludges for digester feed.
•
Anaerobic digestion, using mesophilic, Class B digestion for the base program
•
Biogas utilization using an on-site cogeneration system
•
Mechanical dewatering of biosolids and truck loadout facility
•
All associated facilities and equipment including odor and aesthetic control and
mitigation to assure a neighbor-friendly facility.
The construction cost estimate for this base program for Bayside biosolids facilities is
estimated to be about $500 million. This major facility will need to be well planned,
designed, and implemented and must be sited properly. Section 6 describes siting options
for this facility which is termed the Bayside Biosolids Center, or, if it is located on the
Oceanside, it is termed toe Oceanside Biosolids Center.
7.10.2
Solids Facilities for Oceanside
Mesophilic (Class B) anaerobic digestion facilities were built at the OSWPCP within the last
two decades and can provide good service for several more decades. Co-thickening of
sludge using GBTs was implemented at the Oceanside plant in the 1990s. Biogas
processing includes cogeneration, and dewatering uses belt filter presses. There will be
need for upgrades to these solids facilities over time, but the major processing elements
exist, with generally adequate capacity for the planning period.
7.11 References
Brown and Caldwell, 1982. San Francisco Wastewater Solids Facilities Project. For the City
and County of San Francisco. August 1982
Brown and Caldwell, 1998. SEWPCP Anaerobic Digestion Upgrade Project – Facilities
Planning Report. For the San Francisco Public Utilities Commission, August 1998.
Brown and Caldwell, 2000. Screening of Feasible Technologies. For the San Francisco
Public Utilities Commission. July 2000.
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Carollo, 1997. Long-Term Biosolids Management Plan. For the City and County of San
Francisco. December 1997.
CH2M-Hill, 2006. Bay Area Regional Biosolids Management Program – Initial Market
Assessment. Prepared for the Bay Area Clean Water Agencies. April 2006.
City and County of San Francisco, 2001. Preliminary Design Report – Solids Handling
Upgrade Project – Southeast Water Pollution Control Plant, December 2001.
USEPA, 1979. Process Design Manual for Sludge Treatment and Disposal. EPA 625/1-79011
USEPA, 2003. Environmental Regulations and Technology, Control of Pathogens and
Vector Attraction in Sewage Sludge. EPA/625/R-92/013. Revised July 2003.
USEPA, 2006. Emerging Technologies for Biosolids Management. EPA 832-R-06-005.
September 2006.
Water Environment Federation, 1992. Design of Municipal Wastewater Treatment Plants.
WEF Manual of Practice no. 8.
Water Environment Research Foundation, 1998. Biosolids Management: Assessment of
Innovative Processes. Project 96-REM-1.
8.0
SOLIDS PROCESSING SITES
This section considers alternative sites for wastewater solids processing. Minimum land
area requirements for solids processing have been determined within other SSMP work,
and that work is summarized here. Several different sites for solids processing have been
considered over the last decade in San Francisco, and these were reviewed, along with
new potential sites that came to the attention of the planning team. Screening and
evaluation of the sites is followed by site recommendations.
8.1
Dispersed Versus Centralized Solids Processing
The options of implementing decentralized liquid treatment and distributed liquid treatment
for water reuse are the subjects of two separate project memoranda within the SSMP.
Decentralization of wastewater treatment facilities has been considered for San Francisco
for environmental justice reasons. Facility dispersion could potentially spread the
wastewater treatment site burdens for the City among several, rather than just a few,
neighborhoods. Similarly, solids processing facilities could potentially be dispersed
throughout the City, rather than become centralized. However, centralization of solids
processing at San Francisco is preferred for the following reasons:
•
Significant economies of scale can be realized by implementing centralized solids
processing systems, particularly when solids processing moves to higher levels of
607-92
treatment. Over the last 25 years, several wastewater agencies in the US have
centralized their solids processing by piping or otherwise transporting solids/sludges
to central or regional processing centers.
•
Larger, centralized solids processing systems are less prone to upset conditions than
smaller, decentralized systems, where adequate monitoring and control are difficult
and costly.
•
Larger, centralized solids processing systems offer significantly greater opportunities
for renewable energy production than smaller, decentralized facilities.
•
Greater odor risk is presented to the community by having several solids processing
facilities, each with its own set of odor issues, control measures, and need for proper
maintenance and attention.
•
Truck traffic associated with solids processing can be channeled to more appropriate
routes with centralized facilities that are properly sited.
Therefore, the remainder of the siting evaluation assumes that all Bayside wastewater
solids are treated at a centralized location, and solids generated at the OSWPCP continue
to be processed at the OSWPCP.
8.2
Land Area Needs for Centralized Processing
An evaluation was undertaken to determine the minimum land area or footprint
requirements for a centralized solids processing facility. This work is contained within the
SSMP Project Memorandum titled: “Criteria/Footprint Requirements and Costs for the
Bayside Biosolids Center and the Oceanside Biosolids Center”, dated February 18, 2007.
For SSMP Alternatives 1, 2, and 4, this centralized facility is located in the Bayside portion
of the City and is called the Bayside Biosolids Center (BBC). For SSMP Alternative 3, this
centralized facility is located at the OSWPCP site and is called the Oceanside Biosolids
Center (OBC). The centralized facility is the same size and contains the same solids
processing systems whether it is located on the Bayside or the Oceanside. Sizing
assumptions included the following for this centralized facility:
•
Wastewater solids projections from Section 2 of this document are used, which cover
the Bayside portion of the City for the SSMP planning period, including estimated
trucked brown grease quantities.
•
Solids processing facilities are summarized in Section 5.10 of this report for a core
biosolids program which includes: co-thickening, anaerobic digestion (Class B),
biogas utilization, dewatering, and truck loadout.
•
Additional facilities area is estimated to upgrade the digestion to a Class A anaerobic
digestion facility.
607-93
•
Design and operating criteria for the facilities are defined in the Project Memorandum
reference above. These criteria are used to size the facilities.
•
Area for a future “advanced biosolids” processing facility is included. The area is
estimated at 2.0 acres, but the specific technology is undetermined at this time.
•
Area is included for all associated facilities such as chemical systems, odor control,
electrical, instrumentation, and process control.
•
A key assumption is that process buildings have three levels, either (1) a basement
plus two levels above grade; or (2) no basement and three levels above grade.
Based on these assumptions and criteria, the minimum land area required for a centralized
processing facility would be 8.6 acres. A minimal area evaluation was also completed for
the option whereby combined thickening/digestion/biogas utilization was conducted at one
site, and dewatering/advanced processing/loadout was conducted at a second site nearby.
In this case the two sites would require minimums of 5.5 and 4.2 acres, respectively, for a
total of 9.6 acres. In all cases, these minimum areas or footprints do not include
requirements for site or facility screening or mitigation in terms of buffer needs, perimeter
landscaping, berming, or other concepts. There could also be the need for internal
mitigation, depending on the specific site.
Also, these areas do not include potential area for San Francisco’s organic waste
processing. There could be advantages to a joint processing facility for wastewater solids
and food waste. Anaerobic digestion of both of these waste materials produces valuable
biogas. Combining the biogas production from digestion of both these waste materials
would allow a much larger electric power production or cogeneration facility to be
implemented, resulting in greater renewable energy production and other benefits. Further
evaluation of organic waste processing in San Francisco is underway.
8.3
Bayside Solids Site Alternatives
Siting evaluations for Bayside solids processing have been underway since the mid-1990s,
when it was determined that the anaerobic digestion facilities at the SEWPCP needed
replacement. Site options in the late 1990s focused on parcels either part of, or adjacent to,
the SEWPCP and included several variations at the existing solids processing site, as well
as options across Jerrold Avenue at the Central Plants site and at the Caltrans site to the
north of the plant. Site options were also examined to the north of Islais Creek near the
Bay, at the Hunter’s Point Shipyard area, and on a portion of the large Tuntex site in
Brisbane to the south of San Francisco.
A preliminary design of new solids processing facilities was developed in 2001 for the 6 ½
acre Caltrans site on the south side of Islais Creek, adjacent to I-210 and Evans Ave. This
preliminary design included egg-shaped digesters. Public review of this option resulted in
further discussion of facility requirements, sites, and community needs.
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Sites for the Bayside Biosolids Center evaluated for the SSMP in 2006/2007 included
primarily the sites shown on Figures 12 and 13. Within the SEWPCP boundary, the existing
solids property south of Jerrold Avenue totals about 9.5 acres. The Central Shops portion of
the site contains about 5.3 acres. These are the two portions of the SEWPCP site that
could be potentially available for solids processing. In the event that the current solids area
south of Jerrold Avenue is considered for the BBC, there may be an option of acquiring the
private property to the southeast of that site and converting it to a buffer area.
The sites evaluated for the BBC are listed in Table 31 along with their areas and other
important information. It is recognized that the existing solids site at the SEWPCP
(9.5 acres) may be large enough for the BBC, however, the Central Shops portion of the
site is only adequate for a portion of the solids facilities, such as the
thickening/digestion/gas utilization. Therefore, there is a severe limitation for the Central
Shops site.
Table 31
Sites Considered for BBC
Gross
Area
Site Designation
(acres)
Zoning(1)
Existing SEWPCP + Buffer
14.8(2)
P/M-2/R
Properties
Selby Wedge
6.6
M-1
CALTRANS + Parcels A, B,
17.4
M-2
C, and D
Circosta Metals
3.0
M-2
Pier 92/94 Backlands
35
M-2
Building Height
Limit
(feet)
65
Current
Ownership
CCSF(3)/Private
80
80/65
Private
State/Private
80
40
Private
Port of San
Francisco
CCSF/Private
Griffith Pump Station
8.8
P/M-1
40
Notes:
(1) P = Public Use; M-1 = Light Industrial; M-2 = Heavy Industrial
(2) Existing solids treatment portion = 9.5 acres Central Shops portion = 5.3 acres
(3) CCSF = City and County of San Francisco
8.4
Oceanside Solids Siting
The existing OSWPCP is located on 12 acres of a 42.7 acre site. The site is zoned for
Public Use, and is owned by the City and County of San Francisco. The solids processing
facilities for the plant are located integral to the treatment plant, and include thickening,
digestion, biogas utilization, and dewatering and truck loadout. It is assumed that this solids
processing facility remains intact to handle the Oceanside solids production, although these
solids processing facilities may need modification over time. The National Guard Armory
has a long-term lease on 27.7 acres of the remaining portion of the site.
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Figure 12
Sites Considered for Bayside Biosolids Center (1)
(see separate pdf file)
607-96
Figure 13
Sites Considered for Bayside Biosolids Center (2)
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For SSMP Alternative 3, solids processing for Bayside production would be located
adjacent to the OSWPCP. The area/footprint required for this would be the same as
described above for the BBC. The only site identified for this solids processing on the
Oceanside is adjacent to the OSWPCP. Other sites are not available on the Oceanside due
to relatively complete urbanization.
8.5
Evaluation of Bayside Site Options
The Bayside sites listed and described above are evaluated and screened here to eliminate
infeasible or unsuitable properties.
8.5.1
Siting Criteria
Table 32 summarizes criteria considered when screening the alternative BBC sites.
Table 32
BBC Siting Criteria
Criteria
Considerations
Zoning
Planned uses for property and adjacent land uses. Allowable
building height. Proximity to residences.
Acquisition time
Shortest to longest acquisition time:
Property currently owned by City and County of San Francisco
Surplus property owned by another public agency
Private property, willing seller
Private property, use of eminent domain required
Constructability
Site features that would make project more or less difficult to
construct.
Geotechnical issues
Presence of site soil or other conditions that would make
structures significantly more expensive to construct.
Contamination
Presence of hazardous wastes or contaminated soils that require
remediation.
Terrain
Natural hills can provide visual screening.
Area - footprint
Sufficient area is required for facility, including set-backs, buffers,
visual mitigation, etc.
Wind conditions
Locations are preferred where downwind odor impacts are limited.
Access
Acceptable truck routes to/from the site.
Other planned uses
Redevelopment or recreational opportunities or needs – at or
adjacent to the site
Distance from liquid
Sludge pipelines and pumping costs.
treatment facilities
8.5.2
Bayside Site Screening Evaluation
The sites listed in Table 29 were screened using the criteria listed in Table 30 to
determine suitability for the BBC. Table 33 presents the screening results.
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Table 33
BBC Site Screening Evaluation
Sufficient
Site Designation
Area?
Other Considerations
Construction phasing would be
Existing SEWPCP +
Yes (for the
Buffer Properties
existing solids difficult. Close to residential
properties. Buffer properties are
site).
privately owned. The Central Plants
No (for the
Central Plants portion of the site can only
portion of site) accommodate a portion of the BBC
facilities - a serious drawback.
Selby Wedge
No
Railroad right-of-way separates this
property from the SEWPCP, raising
major access issues. Private
ownership of site. Inadequate area for
BBC.
CALTRANS +
Yes
Parcels A, B, C, and D are privately
Parcels A, B, C, and
owned. Railroad right-of-way
D
intersects through the site. Site is
adjacent to Islais Creek, with future
recreation potential. Difficult
geotechnical conditions.
Circosta Metals
No
Site remediation is likely to be
required. Private ownership.
Inadequate footprint/area for entire
BBC.
Pier 92/94
Yes
Possibly difficult soil conditions. 40Backlands
foot allowable building height would
increase footprint needs. Increased
pipeline lengths to reach the site from
SEWPCP.
Griffith Pump
No
Inadequate area for BBC considering
Station
setback and buffer/mitigation needs
for the site. Truck access is poor
through residential area. Significant
distance from SEWPCP.
8.5.3
Potentially
Suitable?
Yes (for
the existing
solids site)
No
Yes
No
Yes
No
Suitable Bayside Sites
Only three of the Bayside sites are shown to be potentially feasible and adequate.
Advantages and disadvantages associated with each of the three sites are presented in
Table 34. The final site selection will be made after the environmental and public review
process of the SSMP.
If City organic waste processing/digestion/biogas utilization was to be considered in
addition to wastewater solids processing, the footprint/area requirements for the joint site
would need to be increased by several acres at a minimum. In this case, the existing
solids site at the SEWPCP (south of Jerrold Ave) would almost certainly not have
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sufficient acreage, and this site option, therefore, would probably be eliminated.
However, the other two site options in Table 34 would likely have sufficient area for this
enlarged operation. Truck access is also good at these two sites for the increased truck
traffic expected from an organic waste processing operation.
8.5.4
Oceanside Site
The existing OSWPCP site appears to be a suitable location for a centralized biosolids
processing facility on the oceanside of the city. .
8.6
References
Brown and Caldwell, 1998. SEWPCP Anaerobic Digestion Upgrade Project – Facilities
Planning Report. Prepared for the San Francisco Public Utilities Commission, August
1998.
Brown and Caldwell, 2007. Criteria/Footprint Requirements and Costs for the Bayside
Biosolids Center and the Oceanside Biosolids Center. February 18, 2007
City and County of San Francisco, 2001. Preliminary Design Report – Solids Handling
Upgrade Project - Southeast Water Pollution Control Plant, December 2001.
(Preliminary Design at the CALTRANS site)
City and County of San Francisco, 2007. Site and parcel information for various potential
solids processing sites.
EA Engineering, Science and Technology, 2000. Alternatives Analysis Study –
Southeast Water Pollution Control Plant (Draft Report). Prepared for the San Francisco
Public Utilities Commission. May 2000.
9.0
EVALUATION OF BIOSOLIDS MANAGEMENT
ALTERNATIVES
This section develops and evaluates several biosolids management alternatives for San
Francisco. These are developed as categories of alternatives (i.e., categorical
alternatives) rather than alternatives having specifically defined processes with tankage
and equipment sizing. Costing and economic analysis is completed on each categorical
alternative by selecting example processes and facilities within each category that have
relatively well-known costs – both capital costs and annual operating costs (and
revenues, if applicable). This approach can be useful in comparing the economics as
well as the pros and cons of each categorical alternative.
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Table 34
Suitable Bayside Sites - Advantages and Disadvantages
Site Designation
Advantages
Existing SEWPCP + Buffer
• Closest site to liquid treatment processes of
Properties (existing solids
SEWPCP.
processing site south of
• Heavy industrial and public use zoning for the
Jerrold Avenue)
process parcels.
• Process parcels are already owned by the
City and County of San Francisco.
• Reasonable truck access.
CALTRANS + Parcels A, B,
C, and D
•
•
•
•
Pier 92/94 Backlands
•
•
•
•
•
Heavy industrial zoning.
Highest building height limit.
CALTRANS parcel is owned by a public
agency.
Good truck and rail access.
Site located in port/industrial area.
Public (Port of San Francisco) ownership
reduces acquisition time.
Heavy industrial zoning.
Large site offers construction advantages and
better opportunities for site mitigation.
Good truck and rail access.
Disadvantages
1. Closest site to residential properties.
2. May require purchase of residential
properties for odor/site buffer – time for
property purchase.
3. History of odors at the existing site may
make implementation difficult.
4. Difficult to phase construction on this site
due to space limitations.
• Existing warehouse and other
commercial/industrial uses would require
relocation.
• Privately-owned parcels increase
acquisition time.
• Site adjacent to Islais Creek may require
more mitigation or buffer.
• Forty-foot building height limit.
• Possibly difficult soil conditions.
• Greater pipeline length connections to/from
liquid treatment plant.
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Five categorical alternatives have been developed and are outlined in Table 35. All of the
alternatives include, as a minimum, the mitigation/upgrading of the Bayside biosolids
processing system as described in Section 5 of this document. And, for reasons explained
in Section 5, anaerobic digestion is retained as the base stabilization process for
wastewater solids in San Francisco. The alternatives are termed B-1, B-2, etc. to stand for
Biosolids-1, Biosolids-2, etc, so they are differentiated from the SSMP’s overall wastewater
Alternatives 1 through 4, which are defined and evaluated in other SSMP documents. Each
alternative is defined and developed within a separate section below.
Table 35
9.1
Outline of Five Categorical Biosolids Alternatives
Alt. #
Title and Key Features
B-1
Retain/Upgrade Existing Class B Program - Rebuild and mitigate the
Bayside biosolids processing system and continue City’s Class B
dewatered Cake program.
B-2
Upgrade to Class A Program and Expand Uses Rebuild/mitigate/upgrade to Class A digestion and expand uses for
Class-A cake-based product program.
B-3
Create Marketable Products for ~Half of Production Rebuild/mitigate/upgrade to Class A digestion and produce marketable
Class A biosolids products for horticulture/silviculture and agricultural
markets with about half of City’s biosolids production.
B-4
Create Marketable Products for Entire Production Rebuild/mitigate/upgrade to Class A digestion and produce marketable
Class A biosolids products for horticulture/silviculture and agricultural
markets for entire City biosolids production.
B-5
Utilize Thermal Processing - Rebuild/mitigate Bayside biosolids
System to Class B digestion, and use new/evolving thermal
technologies to destroy pollutants, minimize residuals, and create
products or provide other benefits.
Alternative B-1 - Retain/Upgrade Existing Class B Program
The approach for Alternative B-1 is outlined by the following summary statements:
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•
Rebuild/mitigate the Bayside biosolids processing system (currently located at the
SEWPCP) as discussed previously in this report. Provide anaerobic digestion
stabilization of solids to the Class B pathogen level.
•
Continue the current City program of contracting with private companies to transport
and use/dispose of the Class B dewatered cake material. Over the past 10+ years,
this has included mostly land application in Northern and Central California counties,
as well as Alternative Daily Cover (ADC) at landfills and occasional landfill disposal
for the City’s biosolids.
•
Backup arrangements for this alternative, in case of failure of primary use outlets,
would be landfilling. Such landfilling is likely to be at increasingly distant landfills over
time.
9.1.1
Solids Facilities Required for Alternative B-1
For Alternative B-1 (as for all biosolids management alternatives), the Bayside solids
processing facilities would be reconstructed. This includes the thickening, anaerobic
digestion, biogas utilization, dewatering and cake loadout facilities. Anaerobic digestion
would be provided to produce Class B biosolids for this alternative. Possible sites for these
rebuilt and mitigated solids facilities are discussed in Section 6. The rebuilt facilities are
referred to as the Bayside Biosolids Center (BBC) if constructed on the Bayside of the City
(as part of SSMP Alternatives 1, 2, and 4) and are referred to as the Oceanside Biosolids
Center (OBC) if constructed on the Oceanside portion of the City (as part of SSMP
Alternative 3). In this document, this replacement facility is often referenced as being the
BBC/OBC facility, depending on the SSMP alternative selected.
No significant facility changes or modifications are anticipated for Alternative B-1 at the
Oceanside Plant (OSWPCP).
9.1.2
Disposition of Biosolids Materials/Residuals in Alternative B-1
There are two materials/residuals created from Alternative B-1: (1) dewatered cake
biosolids material; and (2) biogas. The use of biogas would be maximized through various
possible alternative systems including engines, turbines, fuel cells or other technology that
is developed to convert methane gas to electrical energy, heat, or both.
Anaerobic digestion reduces the quantity of raw sludge solids material by almost 50 percent
(in terms of dry weight of solids). The remaining digested solids would be dewatered to a
consistency of between 15 and 30 percent solids content (probably about 15 percent for
belt press dewatering and about 25 percent solids for centrifuge dewatering).
For Alternative B-1, it is envisioned that the City would continue to contract with private
companies specializing in this business to transport the dewatered cake material from both
Bayside and Oceanside to beneficial use (or disposal) locations. The availability of
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traditional land application sites is becoming more restrictive over time, and contractors will
need to find new areas/sites at further distances from San Francisco, such that these costs
are likely to increase over time. Likewise, availability of nearby landfills for using the
biosolids as ADC or availability of landfills for disposal is also becoming more limited with
time. Recent evaluation by City staff shows that there are fewer landfills in Northern
California that are willing or allowed to take biosolids materials today than there were
several years ago, and prices for such landfill disposal at remaining landfills have risen.
Figure 14 shows historical prices for truck transport and use/disposal in California for
Class B dewatered cake materials. This figure shows that prices have risen dramatically in
recent years. Prices have risen higher in Southern California than in Northern California, but
the trend is clear within the State. Recent contracts (2006 and 2007) in Southern California
have reached price levels of $60 to $70 per wet ton. The figure also shows estimates of the
likely range for prices in the future based on trends in California and in other portions of the
country.
Figure 14
Price Range for Biosolids Cake Disposition in California
As prices rise, there are greater potential areas that could be considered for land
application and landfill use/disposal. Transporting the biosolids materials 200 miles or more
is often economical at prices of about $50 per wet ton. Biosolids in Southern California are
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sometimes transported over 300 miles to sites in Arizona or Nevada. Therefore, if/when
prices rise to the range of $50 to $60 per wet ton in Northern California, additional potential
sites and use areas could become available. Transporting Class B cake by rail to Nevada
or even Utah has been suggested as potentially feasible if prices rise to levels of about $70
per wet ton. However, use/disposal at out-of-state locations is not considered to be
politically acceptable in the long-term.
If Northern California hauling and use/disposal prices rise to the range of say $60 to $80
per wet ton and higher in coming years, contractors may develop biosolids processing
centers. Sometimes these operations are called Merchant Facilities, to identify them as
private company facilities, whereby the company has contracted with one or more
municipalities to have their biosolids materials brought to these sites for processing and
final disposition. These facilities could include composting operations or could involve other
types of processing to produce marketable biosolids products for Northern California. These
types of facilities and contractor operations have risks for agencies contracting with them,
but there are also potential advantages. At this time, it is unknown if these types of private
processing centers (Merchant Facilities) would be developed, whether the products or
materials produced would be acceptable to users long-term, and whether the operations
would be profitable at competitive price ranges. Clearly, there are significant risks and
issues for Alternative B-1.
9.2
Alternative B-2 - Upgrade To Class A Program and Expand Uses
The approach for Alternative B-2 is outlined here:
•
Upgrade the solids processing systems at both Bayside and Oceanside to Class A,
advanced digestion processing for the entire solids production of the City.
•
Improve the characteristics of the dewatered cake material to minimize objections by
the public, users, and regulatory agencies.
•
Continue the practice of contracting for hauling and final use/disposition of the Class
A cake biosolids materials.
•
By creating Class A biosolids, there are reduced risks and greater acceptance of the
material by the public and users. New product uses and users are likely with Class A
biosolids (over Class B), and opportunities may exist for contractors to create
marketable products from the Class A cake.
9.2.1
A key objective of Alternative B-2 would be to improve the characteristics of the digested,
dewatered cake material as indicated here:
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•
Increase the stability of the biosolids material, so that it has reduced odor and has
very little attraction to vectors.
•
Create a pathogen-free, or Class A, biosolids material/product.
•
Improve the visual characteristics of the material/product by removing plastics, other
debris, and extraneous materials.
•
Dewater the biosolids to a higher degree to improve the cake appearance, enhance
handleability as a bulk solid material, and improve the potential for creation of more
marketable materials.
For Alternative B-2, the BBC/OBC would be reconstructed and an advanced digestion
process would be used to produce Class A biosolids. As discussed in Section 5, several
potential digestion (and perhaps pre-digestion) processes are available for this purpose.
Such advanced digestion processes also typically destroy a somewhat larger amount of
volatile solids, thus reducing the final dry weight of biosolids, usually by about 5 to 10
percent. Increased gas production and energy recovery are also achieved by destroying
more solids in the digestion process. For some processes, significant improvement in
dewatered solids content is realized. These benefits can offset much of the additional
incremental cost of an advanced, Class A digestion process. Additional upgrades to solids
processing would probably include screening for debris removal, so that the final Class A
cake material has the best chance of acceptability.
At the OSWPCP, the digestion process would be upgraded to Class A by the addition of a
supplemental process – several such processes are available as described in Section 5.
Screening to remove debris would also be a logical addition at the OSWPCP.
Energy recovery facilities would be used at both Bayside and Oceanside to make maximum
use of the digester gas, probably producing both electrical power and heat.
9.2.2
Disposition of Biosolids/Residuals for Alternative B-2
As with Alternative B-1, there are two types of products/residuals created from Alternative
B-2 facilities: (1) dewatered cake biosolids material; and (2) biogas. Since about 10 to 20
percent greater gas production is likely with the advanced digestion systems of Alternative
B-2, energy recovery would be commensurately increased over Alternative B-1 (electrical
power, heat, or both).
The dewatered cake material in Alternative B-2 has significantly improved characteristics
over that produced in Alternative B-1, with the primary feature involving Class A or
pathogen-free biosolids in Alternative B-2. This feature, along with the other improved
biosolids characteristics discussed, would be strong reasons for improved long-term
acceptability by the public, product users, and regulatory agencies responsible for safe
biosolids beneficial use practices. Some California counties have implemented ordinances
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requiring that only Class A biosolids materials be used in agricultural land application or
related practices within those counties. In response to these ordinances, several California
wastewater agencies have proceeded to upgrade/modify their solids processing system so
that they produce Class A materials. California county biosolids ordinances are continuing
to be modified over time, with some counties implementing or attempting to implement a
complete ban on biosolids land application. In addition to Class A requirements, some
ordinances require additional monitoring of the biosolids, or place other requirements and
restrictions on the land spreading operations and on the agencies or private companies
involved.
By producing Class A dewatered cake materials, the City would provide opportunities for
development of further processing arrangements to create more marketable products. One
example of this is the system developed by the City of Tacoma, Washington. At Tacoma,
the wastewater department staff has developed its own set of products under the trade
name of Tagro. The digestion system at Tacoma’s Central Wastewater Plant produces
Class A biosolids, and following dewatering, the staff mix the cake with sand and sawdust
in varying percentages to produce bulk Class A products used for landscaping and related
uses in the Tacoma area. The products are sold directly by Tacoma. Other communities are
evaluating Tacoma’s success with this approach.
City of San Francisco staff could evaluate similar post-dewatering processing and product
marketing/distribution, and determine if this has potential. The major point here is that once
Class A dewatered cake is produced, there are likely opportunities for product development
and use. The costs (and potential revenues) for such an approach need further research
and evaluation.
It is unlikely that Class A dewatered products would become revenue-generators, but
producing such Class A material could make uses available within shorter distances of San
Francisco, thus reducing the costs of final transportation.
9.3
Alternative B-3 - Create Marketable Products for ~Half of the
Production
•
Rebuild/mitigate/upgrade the solids processing system at Bayside and Oceanside to
Class A, advanced digestion.
•
Implement advanced biosolids processing to create one or more marketable biosolids
products – i.e., Class A products having much improved characteristics over
dewatered cake material. The capacity envisioned for these products is about half of
the City’s biosolids production.
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•
The remainder of the City’s biosolids production would be produced as a Class A
dewatered cake material, and it is envisioned that private contractors would continue
to transport and use the cake material in Northern and/or Central California.
•
would be landfilling of biosolids. In this case, either dewatered cake or marketable
product material could be landfilled in emergency, however, due to the value of the
marketable products, there are likely to be outlets other than landfill.
9.3.1
For Alternative B-3, reconstruction of the Bayside solids processing facilities is required,
similar to previous alternatives (i.e., thickening, digestion, biogas utilization, dewatering and
cake loadout). The digestion process would provide Class A biosolids at both the BBC/OBC
and at the OSWPCP.
A major requirement for Alternative B-3 is to provide advanced biosolids processing
facilities that would create marketable biosolids products. We define “marketable products”
as biosolids products that:
•
Are bought and sold in an organized system,
•
Have characteristics that are significantly or greatly improved over a dewatered cake
material, and
•
Meet Class A or pathogen-free requirements, as well as chemical or other pollutant
limitations for use by the public.
Examples of such marketable products are a compost product and a heat-dried and
granulated/pelletized product. Such products could also include nutrient-fortified Class A
materials, or partially dried and blended Class A materials.
A major issue in providing this further processing system within San Francisco is that the
process would need to be conducted within a relatively small footprint site. Composting is a
process that is not feasible on small sites (of a few acres) because considerable area is
required for the long detention times required for the process, as well as areas for bulking
agent storage and transfer, and land for curing piles, stockpiling, and product loadout. Even
a small capacity composting system would be difficult to site in San Francisco unless areas
of say 20 to 50 acres could be made available at the Presidio, Golden Gate Park or another
location. Even if sites this large were located, there may be considerable concern about
odor emissions and odor impacts that would be difficult to overcome. Therefore, at this
time, it does not seem reasonable to consider a composting operation unless it was
operating on a limited capacity and was fully contained with very reliable and highperformance odor control systems.
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Heat drying of biosolids (producing a granulated/pelletized product) is an example of a
Class A biosolids process that has been implemented successfully on small footprint sites
in urban areas. Area requirements for drying half of the City’s biosolids production (about
30 to 40 dry tons/day) could be as low as about 1.5 acres. Adequate odor control might be
challenging for a small-footprint heat drying operation in San Francisco, however, there are
odor control systems available that should be able to meet even the stringent odor control
requirements needed for the SSMP.
For this size range of facility anticipated, a single equipment train would be a logical
approach for maximum cost-efficiency. This approach provides for planned shutdowns of
the advanced processing facility. When the advanced processing facility is not operating,
then the only product is dewatered cake material, so that during these periods, the City
would need to provide for disposition of the entire cake production.
To evaluate Alternative B-3 and be able to compare it with other alternatives, the
assumption is made here that marketable products such as a heat-dried and
granulated/pelletized product could be produced, and such a facility could be sited as part
of the BBC/OBC. This assumption allows costing work and economic analysis to proceed
on this alternative.
Innovations are occurring in low-temperature biosolids drying, solar drying, and related
fields that could offer advantages to San Francisco if a few acres of land were available.
There is likely to be considerable excess waste heat in the form of hot water available from
the cogeneration facilities. The hot water might be used to advantage in an innovative
drying operation for a portion of the Class A cake, to achieve say 50 or 60 percent solids
(i.e., partial drying). This is an option to heat drying, or as supplemental to heat drying, to
significantly reduce the need for outside fuel usage for drying. This option needs further
research and development to determine its feasibility and to determine the type of
product(s) that could be created and their potential use or marketability.
9.3.2
For Alternative B-3, there are three types of products/residuals that are created from the
biosolids processing system: (1) dewatered cake biosolids material; (2) biogas; and (3) one
or more marketable biosolids products. Biogas would be used for its energy value – either
within a cogeneration arrangement, or as a fuel for the advanced biosolids processing
system (as a fuel for a heat drying operation, as an example).
The marketable product that is developed using about half of the City’s biosolids production
would be distributed and sold within San Francisco, the San Francisco Bay Area, and
perhaps wider distribution depending on the type of product and its market situation. Recent
biosolids product marketing work by BACWA for a good-quality heat-dried product showed
that there is sufficient demand in the Bay Area and Northern California for even larger
quantities than discussed for Alternative B-3. There are several options concerning how
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such a product is actually marketed and distributed, and often a specialized company is
used to handle this aspect of the program. For agencies with small production quantities in
rural areas, the marketing and distribution is handled more casually, but for production
quantities at San Francisco, a well-organized system is required to insure that product is
moved out of the production site on a regular basis, and any further processing for bulk or
bagged sales is probably handled at another location by the distribution and marketing firm.
For the other half of the City’s biosolids production, the City would continue to contract for
dewatered cake hauling and use/disposal at locations in Northern California. The
dewatered cake from the plants would be Class A biosolids.
9.4
Alternative B-4 - Create Marketable Products for Entire Production
•
Rebuild/mitigate/upgrade the solids processing system at the BBC/OBC to provide
Class A, advanced digestion. Upgrade the OSWPCP solids processing system to
Class A digested material.
•
Implement advanced biosolids processing to create one or more marketable products
– i.e., Class A products similar to that described in Alternative B-3. The capacity
envisioned for these products is the entire City’s biosolids production, or as nearly
close to 100 percent as possible. The production of such Class A products could be
located at each of the two plants, or a centralized facility could be implemented
•
Under this alternative, dewatered cake material would typically not be hauled out of
the City unless this cake material would become a biosolids product elsewhere.
•
would be landfilling of biosolids, use as ADC or other outlet. Either dewatered cake or
marketable product material could be landfilled in emergency
9.4.1
The major requirement for Alternative B-4 is to provide advanced biosolids processing
facilities that would create marketable biosolids products. These products would be the
same as defined previously under Alternative B-3. The difference for Alternative B-4 is that
100 percent of the City’s biosolids production would be processed in this advanced system
to produce marketable products. Therefore, to be reliable for 100 percent of the production,
the advanced processing system would need to be sized to handle the peaks in production
and also have a degree of backup and reliability that would be subject to further
assessment. Advanced biosolids processing could be located at both the OSWPCP and at
the BBC/OBC, or a centralized facility would be an option.
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A major issue in providing this further processing system within San Francisco is that the
process would probably need to be conducted within a small footprint site as discussed
above for Alternative B-3. As discussed in that section, composting does not seem feasible
to be operated with San Francisco except perhaps at small scale in a highly-odor-controlled
situation. Heat drying facilities (producing a granulated/pelletized product) represent an
example of a Class A biosolids process that has been implemented successfully on small
footprint sites in urban areas. Area requirements for drying all of the City’s biosolids
production (about 80 to 90 dry tons/day) could be as low as about 2 acres on a centralized
facility, and somewhat less than this for smaller capacities.
To evaluate Alternative B-4 and be able to compare it with other alternatives, the
assumption is made here that marketable products such as a heat-dried and
granulated/pelletized product could be produced, and such a facility would be sited as part
of the BBC/OBC and at the OSWPCP. This assumption allows costing work and economic
analysis to proceed on this alternative
As discussed for Alternative B-3, various innovative drying concepts might be explored to
reduce the extreme energy requirements for thermal drying. The disadvantage of this option
is that almost certainly more land would be required – likely several acres as a minimum.
9.4.2
For Alternative B-4, there are several types of products/residuals that could be created from
the biosolids processing system on a normal basis: (1) biogas; and (2) one or more
marketable biosolids products. Biogas would be used for its energy value – either within a
cogeneration arrangement, or as a fuel for the advanced biosolids processing system (as a
fuel for a heat drying operation, as an example).
The marketable product(s) that are developed using the entire City’s biosolids production
would be distributed and sold within San Francisco, the San Francisco Bay Area, and
perhaps wider distribution depending on the type of product and its market situation, similar
to Alternative B-3. Recent biosolids product marketing work by BACWA for a good-quality
heat-dried product showed that there is sufficient demand in the Bay Area and Northern
California for the quantities required under this alternative.
There is the opportunity to create different marketable products at each of the two treatment
plants. At the BBC/OBC, for instance, a heat-dried product could be produced for
horticulture, silviculture and even agriculture. At the OSWPCP, a different type of process
could be utilized to create a different product, thus providing more diversity to the City’s
biosolids program. Or, if the innovative drying option proved feasible, that approach may
create another different product.
On a normal basis for Alternative B-4, there would probably be little dewatered cake
material removed from the City for use or disposal, since most material would be processed
into marketable products locally. However, if the advanced biosolids processing systems
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were not working properly, or there was excessive equipment down for maintenance or
repair, dewatered cake material would need to be removed from the treatment plants for a
period of time. This material could be transported to landfill if there were no beneficial use
outlets for the material. Over time, however, landfilling is expected to become more
restrictive in Northern California, such that transport distances to acceptable landfills would
be longer.
9.5
Alternative B-5 - Utilize Thermal Processing
•
Rebuild/mitigate/upgrade the solids processing system at the BBC/OBC to Class B
digestion. Continue to dewater the Class B biosolids to cake form at the OSWPCP
and Bayside.
•
Utilize thermal processing systems to minimize the final end use/disposal of the
biosolids on land or as products for soil conditioning. These thermal processes
destroy or modify the organic material within biosolids such that fuels, construction
products, or other materials are created.
•
Backup arrangements for this alternative, in case of failure of the thermal processing
systems, or in those instances when equipment is down for maintenance and repair,
would be landfilling of dewatered cake biosolids. As indicated, landfilling over time is
likely to be at increasingly distant locations.
9.5.1
For Alternative B-5, reconstruction of the Bayside solids processing facility is required,
similar to previous biosolids alternatives. The digestion process would provide Class B
biosolids. Little, if any, changes would be required at the OSWPCP for the digestion
process.
This alternative would provide final biosolids processing within one or more thermal
processing systems. By the term, “thermal processing systems”, we mean one of several
potential types of system that exist or are being developed to destroy volatile solids and
create fuel or other types of final products. Examples are:
•
Pyrolytic processes are conducted at various pressures and temperatures, and create
a fuel char having high solids content, typically. These chars can be used as fuel in
cement kilns and biomass to energy facilities.
•
Gasification processes are driven by partial pyrolysis, normally to create a
combustible gas along with solid residues of tar or char. The gas would be
combusted to create energy that is recovered through heat-recovery systems (i.e.,
steam from a heat recovery boiler).
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•
Vitrification is the melting of biosolids at high temperatures with oxygen. The organics
are burned off and the inorganics melt to form an aggregate or similar construction
product.
•
Wet oxidation in various forms. Historically, the Zimpro process (and similar
processes) were used, but oxidation in higher temperature/pressure processes can
fully oxidize organic material, resulting in ash residues. Ash residues can have
beneficial uses in construction.
Some or many of these thermal processing systems are still in the developmental stage,
although some full-scale installations are operating in North America and overseas.
9.5.2
Disposition of Residuals for Alternative B-5
The residuals from these thermal processing systems would be used for other-than-land
application or other-than-soil fertilization approaches. The residuals would be solid fuel or
construction materials, pyrolytic gases for combustion, or other types of residuals. If a fuel
char is used in cement manufacturing, these biosolids residuals end up as part of the
cement product. If the fuel char, tar, or oil is used as part of a biomass to energy facility, the
ash material from combustion would be the final residual. Vitrification would result in
aggregate material used in construction. And, ash residuals could become part of
construction as roadway base or other use.
These methods of final residuals disposition are attractive in the long-term because the
organic material and organic pollutants would be largely or completely destroyed (i.e.,
oxidized) by the thermal processing system. And, the residual inorganic materials become
part of a product matrix – i.e., within fuels, road base, concrete, or other product. If fuel
materials are created, the final combustion product that results is often ash, which can be a
useful material, not necessarily disposed to landfill. However, if final ash residues are
disposed to landfill, they represent only a fraction of the original biosolids quantity.
9.6
Economic Comparisons
The operating and overall annual costs for the five biosolids management alternatives are
developed here and compared. Costs for each alternative are developed from costs known
for similar facilities in other locations, taking into account the differential price levels
between locations. Costs for these categorical alternatives are shown within ranges, since
these alternatives are not highly defined at this point.
9.6.1
Basis for Costs
For all five biosolids management alternatives, the current Bayside solids facilities need to
be replaced and mitigated and this represents a base program applicable to all options.
Therefore, the costs of constructing the BBC/OBC as a Class B digestion system along with
thickening, dewatering and gas utilization are not used in the alternative cost comparisons.
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All costs for facilities beyond mesophilic, Class B digestion and dewatering, are included in
this economic comparison. The major cost factors for the biosolids alternatives are
summarized here:
•
Hauling and Use/Disposition. The most significant operating cost factor for many
years (and at least for the near-term future as well), is truck hauling and
use/disposal/disposition of the dewatered cake material. A wide range of future costs
for this factor is used in this analysis to attempt to provide cost sensitivity on this
issue. For estimating near the midpoint of the planning period (2018), this becomes
speculative; however, various information used to develop Figure 14 would indicate
that for Class B cake, a range of $55 to $110 per wet ton is not unreasonable. For
Class A cake, we estimate that a 10 percent cost reduction (over Class B cake) may
be supportable in the future, to account for likely advantages in use/disposition of
Class A biosolids cake. Therefore, a range of $50 to $100 per wet ton is used for
Class A cake hauling and use/disposition.
•
Class A Digestion. Incremental costs to implement Class A digestion (over Class B
digestion) have been determined for both the BBC/OBC and for the OSWPCP. There
are several Class A digestion and pre-digestion processes that could be used for this
purpose (see Section 5), and we have selected representative processes for costing
purposes. For the BBC/OBC, we used thermophilic digestion in batch tanks as a
basis for costs, and for OSWPCP, we used thermal hydrolysis (prior to mesophilic
digestion) for the cost basis. Operating cost increases for Class A digestion (over
Class B digestion) are largely (and perhaps entirely) offset by increased gas
production and value of additional electrical power. The impact is too small to affect
overall alternative economics, and, therefore, is not considered.
•
Reduced Cake Quantities With Class A Advanced Digestion. Several different
advanced Class A digestion processes are available. Besides producing Class A
biosolids, these processes either destroy more volatile solids during digestion or
result in improved cake solids content, or both. As a general rule, these processes
typically provide about 10 percent less final weight of dewatered cake material than a
comparative Class B mesophilic digestion process. Some processes would provide
more than 10 percent, and some less than this. For the economic comparisons here,
we have chosen to use a 10 percent reduction as an average situation.
•
Advanced Biosolids Processes. As indicated, the advanced biosolids process that
might be used for Alternative B-3 and B-4 is unknown at this time. However, for
economic evaluation, we have assumed this is a thermal drying facility that would
create a Class A marketable product that is different from biosolids cake products and
is acceptable to many users. Capital and O&M costs (and potential revenues) for this
type of facility are known from other projects. Large quantities of energy are used in
such drying facilities and, typically, natural gas has been the fuel used. This use of
non-renewable fuel is a major disadvantage. Biogas from anaerobic digestion might
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be used in drying if the drying operation was conducted full-time (24 hrs/day, 7
days/week) and had the reliability of using the biogas on a continuous basis. In
reality, however, this has proven difficult at other plants and few facilities attempt this.
Furthermore, if the biogas is used for thermal drying, it would not be available for use
in cogeneration and power production.
•
9.6.2
Thermal Processing. The thermal processing category includes technologies which
are still evolving and being developed, and, therefore, this category is the most
difficult one for estimating costs. We have chosen to use costs for incineration and
similar processing at this time. Again, as for advanced biosolids processing
technologies, fuel usage from non-renewable fuels could be an issue.
Cost Development
Table 36 presents the costs that have been developed based on the previous discussion of
these categorical biosolids management alternatives. In the table, costs are presented in
$millions or in $millions/year and the size of the range is an indication of the uncertainty
associated with each major cost factor. A factor of 30 percent has been added onto
construction costs to account for “soft” costs such as engineering, administration, legal,
construction management, and similar costs. All capital costs are brought to an annual cost
basis using 30 years at 5 percent interest.
Table 36
Costs for Biosolids Management Alternatives
Alternative
Capital Cost Range
Annual O&M Cost
Range
B-1
Retain/Upgrade
Existing
Class B Program
None required beyond Class B
rebuild at BBC/OBC.
Hauling + use/disposalrange of $55 to
$110/wet ton - $6 to 12
mil/yr
B-2
Upgrade to Class
A Program and Expand
Uses
Class A Digest BBC/OBC = $50 to
55 mil
Class A Digest OSWPCP = $25 to
30 mil
Total Construction = $75 to 85 mil
+ 30 % Soft
= $22 to 25 mil
Total Capital Cost = $97 to 110
mil
Annualized Capital = $6 to $7
mil/yr
O&M of Class A
digestion offset by gas
production. Hauling +
use/disposal – range of
$50 to $100/wet ton =
$5 to $10 mil/yr.
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Table 36
Costs for Biosolids Management Alternatives
Alternative
Capital Cost Range
Annual O&M Cost
Range
B-3
Create
Marketable Products for
about Half of Production
55 mil
30 mil
Adv. Process = $30 to 40 mil
Total Construction = $105 to 125
mil
+ 30 % Soft
= $31 to 38 mil
mil
mil/yr
O&M of Class A
digestion offset by gas
production hauling +
use/disposal for 50,000
tons/yr = $3 to $5
mil/yr. Drying O&M at
$90 to $110/wet ton for
45,000 wet tons/yr = $4
to $5 mil/yr
B-4
Create
Marketable Products for
Entire Production
55 mil
30 mil
Adv. Process = $70 to 100 mil
Total Construction = $145 to 185
mil
+ 30 % Soft
= $44 to 55 mil
mil
mil/yr
Drying O&M at $90 to
$110/wet ton for
95,000 wet tons/yr = $9
to 11 mil/yr.
Hauling and
use/disposal of cake
material is small at ~
3,000 tons/yr for
<$0.3 mil/yr, so this is
not used in the
calculations.
B-5
Utilize Thermal
Processing
Thermal Processing = $180 to
220 mil
+30% Soft
= $54 to 66 mil
Total Capital
= $234 to 286 mil
Annualized Capital = $15 to 19
mil/yr
O&M for thermal
processing at $120 to
$150 per wet ton = $13
to 18 mil/yr.
Annual costs are based on quantities estimated to occur at the mid-point of the planning
period – about 2018. For instance, dewatered cake quantities for Class B cake are
estimated to total 106,000 wet tons/year at this midpoint. Dewatered cake quantities for
Class A cake are estimated to be 10 percent less (95,000 wet tons/year) at the midpoint.
Figure 15 compares the total annual costs for the five biosolids management alternatives.
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Figure 15
9.7
Biosolids Management Alternatives
Evaluation of the Alternatives
The comparison or evaluation criteria are presented and discussed here. The evaluation
and comparison is then conducted on the biosolids management alternatives.
9.7.1
Evaluation Criteria
Table 37 presents the evaluation criteria. The criteria are defined within five categories as
follows:
•
Technical and Risk Criteria
•
Customer Service and Local Criteria
•
Environmental Criteria
•
Recycling and Sustainability Criteria
•
Economic Criteria
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Table 37
Evaluation Criteria for the Biosolids Management Alternatives
Criteria
Description – Definition
Constructability/schedule
Relative complexity of the facilities to be built and,
thereby, potential lengthening of implementation
schedules.
O&M complexity
Degree of difficulty in operations and maintenance
for the facilities
Proven and Reliable
Proven through previous implementation
elsewhere at similar scale, with reliability
provisions that have been shown successful.
Flexibility to changing situations
A system that can adapt to changing input solids
quantities and characteristics or operating
conditions.
Regulatory/permits
Degree of regulatory risk and ability to receive and
operate successfully under reasonable permit
conditions.
Health and Safety
Risks of health and safety to workers and
neighbors
Space/footprint
Adequate space available to construct and
operate the necessary facilities
Odor and other local emissions
Risk of odors, dust and related local emissions
from solids processing and risks of odor from
product transport or use.
Visual/aesthetics
Compatibility with surrounding land use, visual
quality, and complexity of mitigation required.
Traffic/noise
Quantity and type of traffic and compatibility with
sites. Degree of risk from noise of processing,
trucking or related activities.
Interest group concerns including
Ability to meet the concerns of various interest
jobs
groups and ability of the project to provide local
jobs and employment.
Water quality concerns
Risk of groundwater or surface water quality
impacts
Air quality and GHG concerns
Risks of air quality impacts from processing,
transport, or end use/disposal of biosolids,
including Greenhouse Gas (GHG) issues.
Biological impacts
Impacts to biological resources during
construction, processing, or use/disposal of
biosolids
Recycling and Sustainability Criteria
Use/recycling of biosolids products Degree of use and recycling of biosolids and
development of products and markets
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Table 37
Evaluation Criteria for the Biosolids Management Alternatives
Criteria
Description – Definition
Sustainability
Production of sustainable products and ability to
meet City’s sustainability goals
Biosolids program diversification
Ability to provide a diversified product mix over
time
Product quality
Ability to provide biosolids products with quality
and characteristics that meet or exceed customer
expectations and regulatory minimums.
Resource commitments
Quantity of fuel, chemicals, power, and other
resources committed to biosolids processing or
use/disposal.
Economic Criteria
Capital costs
Minimizing capital resources for biosolids
management is important for the City wastewater
program
Total Annual Costs
Total annual costs for biosolids management are
critical for all City customers and users.
Costs/Legal Risks
Ability of the alternative to minimize legal,
regulatory or other biosolids cost risks over time
for the City.
9.7.2
Evaluation and Comparisons
The criteria presented in Table 37 have been used in comparing the five biosolids
management alternatives. This comparison and evaluation is summarized in Table 38.
There are several key comments that are recognized with this review:
•
The biosolids management alternatives encompass several different types of
processing technologies. The technologies are all proven at this time, with the
possible exception of Alternative B-5 processes, assuming newly developing
technologies are pursued. The more complex processes of Alternatives B-3, B-4, and
B-5 receive somewhat reduced ratings for space/footprint, O&M complexity, and
flexibility. Odors and visual/aesthetic issues can be controlled from all processes and
technologies under evaluation here.
•
Energy-wise and environmentally, a key issue is use of non-renewable fuel/energy for
thermal drying. If improved, innovative forms of drying were available to greatly cut
the use of outside energy requirements, Alternatives B-3 and B-4 would rate much
better in terms of resource commitments and Greenhouse Gas issues.
•
The long-term sustainability of Alternative B-1 is questionable, and has been
recognized in Section 4 in terms of the Class B cake product and its outlook for future
land application in California. This problem for Alternative B-1 may be mitigated by
the ability of private industry in the future to organize further processing of the cake
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product at other sites to create products. However, this is conjecture at the present
time, and, therefore, Alternative B-1 has high risk for the long-term.
Table 38
Ratings of Biosolids Management Alternatives
Criteria
B-1
B-2
B-3
Constructability/Schedule
Good
Good
Good
O&M Complexity
Good
Good
Fair
Proven and Reliable
Fair
Good
Good
Flexible to changing
Fair
Good
Fair
situations
Regulatory/Permits
Fair
Fair
Good
Health and Safety
Fair
Good
Good
Space/Footprint
Good
Good
Fair
Odor and Emissions
Good
Good
Good
Visual/Aesthetics
Good
Good
Good
Traffic/Noise
Fair
Fair
Good/Fair
Interest Group Concerns
Fair
Fair
Good
and Jobs
Water Quality Concerns
Good
Good
Good
Air Quality and GHG
Good
Good
Fair
Concerns
Biological Impacts
Good
Good
Good
Recycling & Sustainability Criteria
Use/Recycling of
Fair
Good
Good
Biosolids Products
Sustainability
Poor
Fair
Good
Biosolids Program
Poor
Fair
Good
Diversification
Product Quality
Poor
Fair
Good
Resource Commitments
Good
Good
Fair
Economic Criteria
Capital Costs
Good
Fair
Fair
Total Annual Costs
Good
Good
Fair
Cost/Legal Risks
Poor
Fair
Good
B-4
B-5
Good
Fair
Good
Fair
Fair
Fair
Poor
Fair
Good
Good
Fair
Fair/Poor
Fair
Fair
Good
Good
Good
Good
Good
Good
Good
Good
Good
Poor
Good
Fair
Good
Good
Good
Fair
Good
Good
Good
Poor
Good
Poor
Fair
Fair
Fair
Fair
Good
Fair
Fair
Fair
•
The long-term sustainability of Alternative B-2 (Class A cake use/disposition) is
improved over Alternative B-1, however, the Class A cake material still has
visual/odor characteristics that can be objectionable. The Class A cake material does
have advantages, however, in that it can be a feedstock to create Class A products
with improved characteristics.
•
Alternatives B-3 and B-4 are preferred in the long-term because of the creation of
biosolids products that have recognized value to consumers. However, a key
607-120
concern, as stated above, is the potentially high use of energy for thermal drying, and
associated GHG issues associated with this activity. Innovations in biosolids drying
should be evaluated further to determine if outside energy use can be reduced
substantially, and, thereby, make these alternatives more self-sustaining.
•
9.8
Alternative B-5 has long-term value because of the destruction of organic biosolids
material and yet creation of useful products. Specific technologies for this alternative
are still evolving, so that this alternative may be best associated with continued
research and development.
Recommendations
Based on the evaluation of biosolids management alternatives and assessment of
processing options for San Francisco, the following are the primary biosolids program
recommendations for the SSMP:
•
The City should proceed to rebuild and mitigate the Bayside biosolids processing
system which would include thickening, anaerobic digestion, biogas utilization,
dewatering and cake loadout facilities. Depending on the SSMP Alternative, this
facility would become either the Bayside Biosolids Center or the Oceanside Biosolids
Center.
•
The BBC/OBC should include upgrading to Class A digestion via one of several
potential technologies. Likewise, the OSWPCP solids processing system should be
upgraded to produce Class A digested biosolids. This upgrading to Class A biosolids
can be accomplished on-site at both plants at reasonable incremental cost and
impact, and would reduce health risks and improve public perception.
•
There are available sites that have the attributes necessary for the BBC and the
OBC. The options for the siting will receive further evaluation during environmental
review and decisions will then need to be made. The BBC sites that have additional
space for digesting certain City organic waste materials could be a major advantage.
•
The City should pursue innovations in biosolids drying that could significantly reduce
fuel/energy needs for drying – even if such options require additional site footprint.
•
There are several biosolids processes and use/disposal options that should continue
to be tracked to determine if they can be useful in a longer-term program for San
Francisco. These include new product development such as Class A blended
mixtures, as well as new uses such as mine/land reclamation in California.
•
A phased program could logically begin with Alternative B-2, and, over time, add an
advanced biosolids process such as biosolids drying for a portion of the production
(Alt B-3) or for the entire City production quantities (Alt B-4). Alternative B-5 may
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remain a longer-term objective that can be implemented when the technology is
improved and when public perception swings to allow such activities.
9.9
References
Brown and Caldwell, 2007. Cost Estimates for SSMP Alternatives. January 2007
Brown and Caldwell, 2002. Cost Estimates for Arlington Biosolids Master Plan. For
Arlington County, Virginia.
USEPA, 2006. Emerging Technologies for Biosolids Management. September 2006
10.0 IMPLEMENTATION PROGRAM
This section presents the biosolids implementation program based on the evaluation and
recommendations from Section 7.
10.1 Program Description
A phased biosolids program is envisioned that has the following overall elements:
•
Rebuild and mitigate the Bayside biosolids processing system, which would include
thickening, anaerobic digestion, biogas utilization, dewatering and loadout. Sites are
available on the Bayside for replacement and mitigation of the existing solids
processing facilities which are located at the SEWPCP.
•
Upgrade both the BBC/OBC and OSWPCP digestion systems to Class A digestion so
that a base program of producing Class A biosolids is developed at the outset. This
would provide Class A dewatered cake material.
•
Add advanced biosolids processing at either the BBC/OBC or OSWPCP, or both,
over time, to produce Class A biosolids products. These could be dried products or
other types of products that are viewed much more favorably by the public than
dewatered cake material. As a place-holder, a 30 dry ton/day heat drying system is
included within the BBC/OBC plan; however, this could be a different process as
planning and development proceeds.
10.1.1
Bayside Biosolids Center (BBC) and Oceanside Biosolids Center (OBC)
A simplified schematic diagram for the BBC/OBC is shown in Figure 16. For SSMP
Alternatives 1, 2, and 4, primary sludge, waste activated sludge, and scum would be
pumped to the BBC from the SEWPCP liquid processing systems in underground pipelines.
Under SSMP Alternative 3, the OBC is located in concert with the new treatment facility on
the Oceanside. Trucked brown grease would also be brought to the BBC/OBC to be
introduced into the digestion system. The dilute sludges and scum/grease would be co-
607-122
Figure 16
BBC/OBC Simplified Solids Process Schematic
thickened using gravity belt thickeners. A thickened sludge tank would provide equalization
storage prior to feeding to a Class A anaerobic digestion process. Stabilized, Class A
biosolids would be dewatered using centrifuge technology to create a Class A biosolids
cake material. The dewatered cake material would be loaded into covered trucks for
transport to beneficial use or other disposition sites. Space would be provided for future
implementation of an advanced processing facility at the BCC/OBC. Liquid removed from
the solids during the thickening and dewatering processes (and from future systems) would
be pumped back to the liquid treatment plant for processing. Biogas produced by the
anaerobic digestion process would be used to generate renewable electrical power, with
the waste heat from the cogeneration process being used to heat the digestion process.
Excess waste heat would also be available for other heating uses at the plant.
10.1.2
Oceanside WPCP
OSWPCP solids would continue to be processed at the OSWPCP site. A simplified
schematic diagram for the OSWPCP biosolids processing system is shown in Figure 17.
Primary sludge, waste activated sludge, and scum would be co-thickened using gravity belt
thickeners. Thickened sludge would be stabilized in a Class A anaerobic digestion process,
or a pre-digestion Class A process would be utilized in concert with digestion. The Class A
biosolids would be dewatered to create a Class A cake material. The dewatered cake
material would be loaded into covered trucks for transport to the end use or disposition
sites. Biogas produced by the anaerobic digestion process would be used to generate
renewable electrical power, with the waste heat from the cogeneration process being used
to heat the anaerobic digestion process and OSWPCP facility.
607-123
Figure 17
OSWPCP Simplified Solids Process Schematic
10.2 Site Selection for BBC
There are three available sites with adequate area/footprint for the Bayside Biosolids
Center as shown on Figure 18:
•
The existing solids processing site at the SEWPCP (south of Jerrold Ave)
•
The Caltrans and ABCD parcels to the north of the SEWPCP
•
Pier 92/94 backlands site about ½ mile east of the SEWPCP
These sites will need further examination and evaluation for their potential use. If organic
waste digestion will also be conducted at the same site along with wastewater solids
processing, this would be a major consideration for site selection.
10.3 Products and Markets
In the near-term the biosolids management program would focus on management of a
Class A biosolids cake material Potential outlets for Class A cake materials include bulk
agricultural or reclamation use, alternative daily cover at landfills, further processing at a
regional biosolids facility, and further processing/handling at contract facilities. The
objective would be to maintain at least two or three active biosolids outlets (via contracting
methods) to ensure a diversified biosolids program. In addition, contract provisions would
be desirable for emergency disposal of biosolids at landfills.
607-124
Figure 18
Potential Sites for Bayside Biosolids Center
607-125
In the longer-term, there is need to transition to producing products with better aesthetic
properties (such as heat-dried pellets), or focusing on the development of energy markets
that can use suitable biosolids products as a renewable fuel source. Space would be
provided at the BCC/OBC site for implementation of an advanced processing facility
(technology to be determined) to create suitable products for future market needs.
Regulatory and market issues would continue to be actively monitored to identify trends that
may lead to adjustments to the biosolids management strategies, including implementation
of advanced processing technologies.
10.4 Technology Follow-Up
Biosolids processing technologies need to be monitored over time to assess potential
suitability for implementation at San Francisco. Research and pilot testing of innovative
technologies are likely to be required – technologies that offer promise within the context of
the San Francisco biosolids management program.
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City and County of San Francisco 2030 Sewer System Master Plan

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