Dealing With Struvite

Se
eptember 13, 2011
SANTA ANA RIVER BASIN SECTION
ANA RIVER BASIN SECTION
BIOSOLIDS SEMINAR
SEPTEMBER 13, 2011
CALIFORNIA WEA
Se
eptember 13, 2011
DEALING WITH STRUVITE
DEALING WITH STRUVITE
DAN BUHRMASTER
KEN ABRAHAM
KEN ABRAHAM
CALIFORNIA WEA
OUTLINE
• Struvite formation basics
Struvite formation basics
• Locations for struvite formation
Potential impacts and maintenance requirements
• Potential impacts and maintenance requirements
• Monitoring for struvite
• Design techniques to minimize struvite impacts
Design techniques to minimize struvite impacts
3
WHAT IS STRUVITE?
WHAT IS STRUVITE?
• Crystalline substance
Crystalline substance
• MgNH4PO4.6H2O ammonium phosphate hexahydrate
• Magnesium
Magnesium ammonium phosphate hexahydrate
4
WHERE DOES STRUVITE FORM?
WHERE DOES STRUVITE FORM?
• Anaerobic digesters
• Digested sludge piping, valves, pumps, and mixers
Di t d l d
i i
l
d i
• Dewatering equipment
t
d
t t i i
l
d
• Filt
Filtrate and centrate piping, valves and pumps
• Rough surfaces preferred
5
CONDITIONS THAT PROMOTE STRUVITE CONDITIONS
THAT PROMOTE STRUVITE
FORMATION
• Alkaline and higher pH conditions
Alkaline and higher pH conditions
• Minimum solubility is approximately pH = 10.3
Elevated temperatures
• Elevated temperatures
• Availability of constituents (magnesium, ammonia, and phosphorus)
Ksp of Struvite
Struvite Solubility Product with pH
10
9
8
7
6
5
4
3
2
1
0
Solubility decreases with increasing pH
3
4
5
6
7
8
9
pH
10 11 12 13 14
6
WHAT IS VIVIANTE?
WHAT IS VIVIANTE?
• Crystalline substance
Crystalline substance
• Fe3(PO4)2.8(H2O)
Ferric phosphate
• Ferric phosphate
• Color is typically blue, green, or gray‐black.
7
CONDITIONS THAT LEAD TO VIVIANTE CONDITIONS
THAT LEAD TO VIVIANTE
FORMATION?
• Excess iron concentrations
Excess iron concentrations
• Acidic conditions (lower pH) Elevated temperatures
• Elevated temperatures
Vivianite Solubility Product
y
Ksp off Vivianite
36.50
36.30
Solubility
Decreases
As Temp Increases
36.10
35 90
35.90
35.70
35.50
0
20
40
60
80
100
120
T
Temperature (Celsius)
t
(C l i )
8
WHERE DOES VIVIANTE FORM?
DOES VIVIANTE FORM?
• Heat exchanger sludge piping
Heat exchanger sludge piping
• Sludge conveyance systems with excess iron concentrations and lower pH
9
IMPACTS OF STRUVITE (OR VIVIANITE) IMPACTS
OF STRUVITE (OR VIVIANITE)
FORMATION
•
Increased O&M costs.
•
Increased energy costs.
•
Equipment downtime.
q p
•
Need for redundant equipment and piping.
•
Impacts to process control.
p
p
10
STRUVITE CLEANING
STRUVITE CLEANING
•
Chemical cleaning (acid or g(
dispersant)
•
High pressure flushing
•
Pigging
•
Removal and replacement of conveyance system components
conveyance system components
11
MONITORING FOR STRUVITE WITH THERMAL MONITORING
FOR STRUVITE WITH THERMAL
IMAGING
•
•
•
•
•
•
Rapid and nondestructive approach to assess the extent of scaling in piping
assess the extent of scaling in piping systems
Struvite decreases heat transfer and temperature variations can be detected on the surface of piping systems
th
f
f i i
t
Thermal imagery used to map the temperature profile for the piping systems – colder locations indicate struvite formation
i f
i
Avoids the need to remove and inspect extensive conveyance systems
M
May allow preemptive maintenance ll
ti
i t
before process problems occur
Dallas Water Utilities and Carollo have proven the utility of this method at their S h id WWTP
Southside WWTP
12
METHOD FOR MONITORING VIVIANITE & STRUVITE FORMATION AT HEAT EXCHANGERS
STRUVITE FORMATION AT HEAT EXCHANGERS
• Struvite and vivianite reduce heat transfer.
• Portable Infra‐red probe for monitoring surface Portable Infra red probe for monitoring surface
temperatures of HX and associated piping.
• On‐line heat transfer coefficient measurement and trending using temperatures and flow of hot stream and cold streams with alarms.
• Algorithm initiates acid cleaning alarm.
Algorithm initiates acid cleaning alarm
• Proven beneficial at the Truckee Meadows WWTP.
Sludge Spiral HX
Heat Transfer Q = U x Area x LMTD
Hot Water
In
U = Heat transfer Coefficient (varies with scaling)
Area = Surface Area of HX
LMTD = Log Mean Temperature Difference
LMTD = {(TE1 – TE4) –
LMTD = {(TE1 –
TE4) – (TE2 –
(TE2 – TE3)}/LN{(TE1 –
TE3)}/LN{(TE1 – TE4) –
TE4) – (TE2 –
(TE2 –
TE3)}
TE1
FE
TE2
TE3
FE
Sludge
In
Hot Water
Out
TE 4
Sludge
Out
13
CALCULATING POTENTIAL FOR STRUVITE
CALCULATING POTENTIAL FOR STRUVITE
•
Sacramento State – Office of Water Programs
g
•
Software tool for calculating struvite precipitation potential
•
Conduct sampling and laboratory testing, then input water quality parameters into software
•
Input parameters can be changed to capture various Input
parameters can be changed to capture various
operating conditions
•
CD‐ROM can be ordered for $75
•
http://www.owp.csus.edu/courses/additional/struv
ite‐precipitation‐potential‐calculation‐tool.php
14
CONVEYANCE DESIGN TO MINIMIZE STRUVITE
CONVEYANCE DESIGN TO MINIMIZE STRUVITE
•
Pipe materials – glass‐lined DIP, PVDF, or other smooth lined pipe
smooth lined pipe
•
Pipe sizing for optimal velocity
•
Valve selection to minimize turbulence
Valve selection to minimize turbulence
•
Short piping runs
•
Dual piping system to allow for one system to be Dual
piping system to allow for one system to be
taken out of service
•
Access to piping for inspection and cleaning
15
PROCESS & MECHANICAL DESIGN TO MINIMIZE PROCESS
& MECHANICAL DESIGN TO MINIMIZE
STRUVITE
•
Potential for dilution to reduce formation conditions (
(example –
l combine thickening centrate and dewatering bi thi k i
t t
dd
t i
centrate)
• Chemical addition
• Ferric or aluminum salt addition (precipitation and pH Ferric or aluminum salt addition (precipitation and pH
reduction)
• Dispersing agents
• pH adjustment (carbon dioxide)
p
j
(
)
• Heat exchangers
• Lower the temperature delta and oversize the heat exchanger
• Add ability to acid clean sludge lines – requires redundant unit
• Phosphorus recovery (OSTARA)
16
CHEMICAL PHOSPHORUS PRECIPITATION TO CHEMICAL
PHOSPHORUS PRECIPITATION TO
PREVENT STRUVITE FORMATION
•
Goal: chemically bind soluble phosphorus for removal as sludge or cake solids
•
Ferric chloride is the most common chemical used
h i l
d
•
Adding iron forms vivianite, iron hydroxide, and iron sulfate
•
Iron preferentially reacts with hydrogen sulfide (H2S); therefore, must add sufficient iron to account f
for multiple reactions
lti l
ti
•
Actual iron dosage should be determined by bench‐scale or full‐
scale testing
scale testing
17
FERRIC CHLORIDE ADDITION TO PREVENT FERRIC
CHLORIDE ADDITION TO PREVENT
STRUVITE FORMATION OR ACCUMULATION
•
•
Typical ferric chloride feed points
yp
p
• Primary clarifiers
• Upstream of anaerobic digesters
• Upstream of dewatering equipment

Cost Impacts
 Additional chemical costs
 Additional Additional
solids handling
Typical dose rates
• Prevention of struvite accumulations 10 35 pounds accumulations = 10 –
35 pounds
iron per dry tons solids (lb/dt)
• Prevention of struvite formation = up to 75 lb/dt
18
PH ADJUSTMENT
PH ADJUSTMENT
Carbon dioxide has been used to lower pH to a level that prevents struvite formation
• MWRDGC ‐ Stickney WRP (Chicago)
• Feed CO2 to digested sludge upstream of dewatering centrifuges
if
• Target pH
 Centrifuge feed = 6.5
g
 Centrate = 7.2
• Reported lower operating costs than iron addition
than iron addition
Struvite Solubility Product 10
with pH
with pH
Ksp off Struvite
•
9
8
7
6
5
4
3
2
1
0
3
4
5
6
7
8
9 10 11 12 13 14
pH
19
USE OF DISPERSING AGENTS
USE OF DISPERSING AGENTS
• Proprietary products have been developed. • Prevents struvite deposition by interfering with Prevents struvite deposition by interfering with
crystal formation.
• Multiple vendors offer products. p
p
Product Name
Product Dose per p
L of Sludge Flow
Flosperse
20 – 60 mg/L
PolyGone Lines
62 mg/L
Antiprex®
Vendor
Link
http://www.snf‐
SNF INC. (previously group.com/IMG/pdf/Dispersants‐
Polydyne)
anti_Struvite.pdf
Schaners
Schaners Wastewater http://www.struvite.com/products.h
Products
tml#polygone_lines
http://worldaccount.pu.basf.eu/wa/
BASF NAFTA~en_US/Catalog/WaterSolutio
Chemicals
ns/pi/BASF
20
USE OF DISPERSING AGENTS (CONT )
USE OF DISPERSING AGENTS (CONT.)
• Flosperse dose = 0.3 gallons per million g
gallons of wastewater treated. • Typical feed points are dewatering feed or centrate/filtrate. • Can be used alone or in combination with ferric chloride to prevent struvite formation.
• Costs are typically similar to ferric chloride Costs are typically similar to ferric chloride
addition ($9 to $14 per ton dry solids)
• Use of chemical can also dissolve or soften existing deposits. • Can be used effectively in pipelines along with high pressure flushing to clean out
with high pressure flushing to clean out deposits.
21
HEAT EXCHANGER DESIGN
HEAT EXCHANGER DESIGN
• Limit hot water temperature at heat exchanger to approximately 150 degrees F Thi l
F. This also prevents sludge “baking” t l d “b ki ”
onto the piping.
• Delta temperature between water and sludge is then limited to help minimize l d i th li it d t h l
i i i
struvite or vivianite formation.
• Increase the size of the heat exchanger to accommodate the smaller delta d t th
ll d lt
temperature.
• Acid phase digester heat exchangers are especially prone to vivianite
i ll
t i i it formation f
ti
due to low pH and high temperature conditions.
• C
Consider redundant heat exchanger to id
d d th t
h
t
allow for regular inspection and cleaning.
22
PHOSPHORUS RECOVERY ALTERNATIVES FOR PHOSPHORUS
RECOVERY ALTERNATIVES FOR
STRUVITE CONTROL
• pH control and/or magnesium addition.
pH control and/or magnesium addition.
• Proprietary fluidized bed processes have been developed in Japan – not available in the USA.
• OSTARA is a patented phosphorus harvesting process for centrate, using magnesium chloride addition and pH adjustment to create struvite in a
addition and pH adjustment to create struvite in a pellet form (pearls).
23
OSTARA PROCESS DESCRIPTION • Equalized centrate laden with Mg, P and NH3‐N pumped into reactor
• MgCl2 is added in stoichiometric excess
• pH is adjusted to slightly alkaline
• Struvite crystals grow in crystallizer on previously formed crystal stock
on previously formed crystal stock
• Crystals are harvested and screened for size. Large product crushed and recycled, small product returned for more growing
Crystals are dried and bagged
• Crystals are dried and bagged.
• Centrate recycled to plant influent
24
OSTARA PROCESS SCHEMATIC
Courtesy M. Kuzma, Ostara
25
SUMMARY
• Bad news:
• Struvite (and vivianite) formation potential exists Struvite (and vivianite) formation potential exists
at many wastewater treatment facilities.
• This causes increases to capital and O&M costs.
• Process conditions can be impacted.
d
b
d
• Good news:
• There has been significant progress in developing There has been significant progress in developing
monitoring techniques.
• Chemical feed or phosphorus recovery systems can minimize struvite formation.
i i i
i f
i
• Proper design of conveyance systems and heat exchanger facilities can lower O&M costs.
g
26
CONTACT INFORMATION:
CONTACT INFORMATION:
Dan Buhrmaster
Black & Veatch Corporation
Black & Veatch Corporation
15615 Alton Parkway, Suite 300
Irvine, CA 92618
[email protected]
27
28