Minimizing Backwash Volume From Coagulation/Filtration for

Transcription

Arsenic Water Technology Partnership
Minimizing Backwash Volume
From Coagulation/Filtration
for Arsenic Removal
Subject Area: Water Quality
Arsenic Water Technology Partnership
for Arsenic Removal
© 2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
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for Arsenic Removal
Prepared by:
Issam Najm, Nancy Patania Brown, and Karl Gramith
Water Quality & Treatment Solutions, Inc., 21018 Osborne Street
Suite 1, Canoga Park, CA 91304
Jointly sponsored by:
Water Research Foundation
6666 West Quincy Avenue, Denver, CO 80235-3098
and
U.S. Department of Energy
Washington, DC 20585-1290
Published by:
WERC, a Consortium for
Environmental Education and
Technology Development at
New Mexico State University
DISCLAIMER
This study was jointly funded by the Water Research Foundation (Foundation)
and Sandia National Laboratories (SNL) under Agreement No. FI061030711
through the Arsenic Water Technology Partnership. The comments and views
detailed herein may not necessarily reflect the views of the Foundation, its officers,
directors, affiliates or agents, or the views of SNL and the Arsenic Water Technology
Partnership. The mention of trade names for commercial products does not
represent or imply the approval or endorsement of the Foundation or SNL. This
report is presented solely for informational purposes.
Copyright © 2010
by Water Research Foundation and Arsenic Water Technology Partnership
ALL RIGHTS RESERVED.
No part of this publication may be copied, reproduced
or otherwise utilized without permission.
ISBN 978-1-60573-126-1
Printed in the U.S.A.
CONTENTS
LIST OF TABLES�� vii
LIST OF FIGURES�� xi
FOREWORD�� xv
ACKNOWLEDGMENTS�� xvii
EXECUTIVE SUMMARY�� xix
CHAPTER 1: INTRODUCTION�� 1
Project Objectives�� 1
Project Approach�� 2
CHAPTER 2: BACKGROUND�� 3
Coagulation/Filtration for Arsenic Removal From Groundwater�� 3
Anthracite-Sand vs. Ultra-Light Filter Media�� 6
Filter Backwashing�� 7
CHAPTER 3: METHODS AND MATERIALS�� 9
Ultra-Light Filter Media�� 9
Polystyrene Media�� 9
PFC100E Media�� 9
Pilot Coagulation/Filtration Unit�� 9
Filter Run Termination Criteria�� 10
Pilot Testing Sites�� 12
Investigative Field Testing�� 12
Field Demonstration Testing�� 12
Steady-State Operation�� 12
Analytical Methods�� 12
Zeta Potential�� 12
Water Quality Monitoring�� 13
Arsenic�� 13
On-Site Analytical Methods�� 13
General Physical, Mineral, and Metals Sampling�� 14
CHAPTER 4: RESULTS AND DISCUSSION�� 17
Characterization of Polystyrene Filter Media�� 17
Surface Charge�� 17
Media Size and Density�� 17
v
vi | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
Source Water Quality�� 19
Preliminary Determination of Backwash Requirements�� 19
Initial Evaluation of Polystyrene Media Performance for Removal of
Ferric Floc�� 21
Repeated Filter Runs to Demonstrate Backwash Effectiveness�� 23
Initial Start-Up�� 28
Field Demonstration Operating Conditions�� 29
Performance of Pilot C/F Unit With Polystyrene Filter Media�� 29
Water Quality�� 37
Feedwater Recovery�� 39
Long-Term Steady-State Operation�� 40
Long-Term Steady-State Operating Conditions�� 40
Need for Air Scour�� 42
Performance of Pilot C/F Unit With Polystyrene Filter Media�� 45
Additional Analysis of Polystyrene Media�� 59
Steady-State Operation With Alternative Ultra-Light Media�� 64
Performance of Pilot C/F Unit With PFC100E Filter Media�� 66
CHAPTER 5: SUMMARY AND CONCLUSIONS�� 79
Characterization of Polystyrene Filter Media�� 79
Long-Term Steady-State Operation With Polystyrene Media�� 81
Steady-State Operation With Alternative Ultra-Light Media�� 83
Overall Study Conclusions�� 84
CHAPTER 6: RECOMMENDATIONS TO UTILITIES�� 87
CHAPTER 7: FURTHER RESEARCH NEEDS�� 89
APPENDIX A: QUALITY ASSURANCE/QUALITY CONTROL RESULTS�� 91
REFERENCES�� 95
ABBREVIATIONS�� 97
TABLES
ES.1 Backwash regime for long-term steady-state testing of ultra-light media�� xxi
ES.2 Comparison of backwash volumes for typical anthracite-sand and ultra-light
media filter�� xxii
2.1
Conventional anthracite-sand media design in a C/F process used to remove arsenic
from a Nevada groundwater�� 4
2.2
Backwash conditions for anthracite-sand media in C/F process for arsenic removal
from a Nevada groundwater�� 4
3.1
Routine water quality monitoring during pilot testing�� 13
3.2
Laboratory analytical methods�� 15
4.1
Polystyrene media size characteristics�� 18
4.2 Source water quality during investigative field testing�� 19
4.3 Backwash conditions for polystyrene media identified during preliminary
evaluation of backwash requirements�� 20
4.4
Filter run time (minutes) required to achieve filtered water turbidity/iron goals
during initial evaluation�� 22
4.5
Operating conditions during repeated filter runs to demonstrate backwash
effectiveness�� 25
4.6
Filter run time and total headloss for repeated filter runs�� 25
4.7
Source water quality during field demonstration testing�� 28
4.8 Operating conditions during field demonstration testing�� 29
4.9
Summary of filter run results from field demonstration testing�� 31
4.10 Arsenic, iron, and turbidity levels near run termination of three filter runs performed
during field demonstration test�� 36
4.11 Filtered water quality during field demonstration testing�� 37
vii
viii | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
4.12 General physical, mineral, and metals during field demonstration testing�� 38
4.13
Source water quality during steady-state operation�� 41
4.14 Operating conditions during long-term steady-state operation�� 41
4.15
Backwash regime for initial testing of polystyrene media without air scour................... 42
4.16 Backwash regime for initial testing of polystyrene media with air scour�� 44
4.17 Comparison of results from initial testing of polystyrene media with and without
air scour�� 45
4.18 Backwash regime for long-term steady-state testing of polystyrene media�� 45
4.19 Summary statistics for full filter runs in long-term steady-state operation with
polystyrene media�� 46
4.20
Backwash volume measurement for polystyrene media�� 54
4.21
Raw water arsenic and filtered water quality during long-term steady-state
operation with polystyrene media�� 54
4.22
General physical, mineral, and metals analysis during long-term steady-state
operation of polystyrene media�� 58
4.23
Volatile organic compounds in chlorinated raw water and polystyrene media
rinse water�� 60
4.24
Physical characteristics of Purofine PFC100E�� 65
4.25
Summary statistics for full filter runs in steady-state operation with PFC100E media�� 66
4.26
Raw water arsenic and filtered water quality during steady-state operation
with PFC100E media�� 73
4.27
General physical, mineral, and metals analysis during steady-state operation
of PFC100E media�� 77
5.1
Optimized backwash conditions for polystyrene media�� 80
5.2
Backwash regime for long-term steady-state testing of polystyrene media�� 81
6.1
Comparison of backwash volumes for typical anthracite-sand and ultra-light
media filter�� 88
Tables | ix
A.1
Results of duplicate arsenic measurements during long-term steady-state testing
with polystyrene media and long-term testing with PFC100E media�� 92
A.2
Results of iron method verification during long-term steady-state testing with
polystyrene media and long-term testing with PFC100E media�� 93
A.3
Results of duplicate iron analyses during field demonstration testing�� 93
A.4
Results of duplicate iron measurements during long-term steady-state testing with
polystyrene media and long-term testing with PFC100E media�� 94
FIGURES
2.1
Profiles of iron and arsenic breakthrough in one filter run conducted during the
Nevada pilot study.............................................................................................................. 4
2.2
Summary of the impact of iron dose and filtration rate on arsenic removal during
the Nevada pilot study........................................................................................................ 5
2.3
Summary of the impact of filtration rate and iron dose on filter run length in the
Nevada pilot study.............................................................................................................. 5
2.4
Turbidity measurement as an indicator of iron breakthrough in the Nevada pilot
study.................................................................................................................................... 6
3.1
Schematic diagram of the pilot coagulation/filtration unit............................................... 10
3.2
Photograph of the pilot coagulation/filtration unit............................................................ 11
4.1
Zeta potential of polystyrene media particles and silica sand particles in a
groundwater sample.......................................................................................................... 18
4.2
Diagram of backwash screen holder................................................................................. 21
4.3
Filter headloss and filtered water turbidity and iron for runs conducted at 4 gpm/ft2
during initial evaluation.................................................................................................... 24
4.4
Turbidity and headloss profiles for repeated filter runs conducted at ferric chloride
dose of 4 mg/L as Fe and filtration rate of 4 gpm/ft2........................................................ 26
4.5
Iron, turbidity, and headloss profile for the sixth repeated filter run (Run 528)
conducted at ferric chloride dose of 4 mg/L as Fe and filtration rate of 4 gpm/ft2........... 26
4.6
Iron, turbidity, and headloss profile for first three hours of filter operation during
the sixth repeated filter run (Run 528).............................................................................. 27
4.7
Steady-state performance of the pilot C/F unit during field demonstration testing
conducted at a target ferric chloride dose of 0.95 mg/L as Fe and filtration rate
of 4 gpm/ft2 ...................................................................................................................... 30
4.8
Filter run time and total headloss for 24 full filter runs performed during field
demonstration testing conducted at a target ferric chloride dose of 0.95 mg/L as
Fe and filtration rate of 4 gpm/ft2...................................................................................... 32
xi
xii | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
4.9
Relationship between measured influent iron concentration and filter run length
during field demonstration testing.................................................................................... 33
4.10
Filter turbidity and headloss profile from Run 566 conducted at an average Fe dose
of 0.9 mg/L and a filtration rate of 4 gpm/ft2.................................................................... 33
4.11
4.12
4.13
Turbidity, iron, and arsenic profiles during filter maturation from Run 583 conducted
at an average ferric chloride dose of 1.2 mg/L as Fe and filtration rate of 4 gpm/ft2....... 35
4.14
Wells in service during long-term steady-state testing at Elsinore Valley Municipal
Water District, CA............................................................................................................. 42
4.15
Turbidity and headloss profile for example filter run (Run 738) using polystyrene
media without air scour..................................................................................................... 43
4.16
Turbidity and headloss profile for example filter run (Run 774) using polystyrene
media with air scour.......................................................................................................... 44
4.17
Turbidity and headloss profile for filter run (Run 822) from third week of long-term
steady-state operation with polystyrene media................................................................. 47
4.18
Turbidity and headloss profile for filter run (Run 929) from ninth week of long-term
4.19
Turbidity and headloss profile for filter run (Run 1004) from 14th week of long-term
4.20
Turbidity, arsenic, and iron profiles from filter maturation during 15th week of pilot
plant operation with polystyrene media (Run 1010)......................................................... 49
4.21
Turbidity, arsenic, and iron profiles from turbidity breakthrough during 17th week
of pilot plant operation with polystyrene media (Run 1046)............................................ 50
4.22
Clean media headloss from full filter runs during long-term steady-state testing
with polystyrene media..................................................................................................... 51
4.23
Headloss from full filter runs during long-term steady-state testing with polystyrene
media................................................................................................................................. 52
Figures | xiii
4.24
Filter run time from full filter runs during long-term steady-state testing with
polystyrene media............................................................................................................. 52
4.25
Filter effluent turbidity during stable operating portion of full filter runs during
long-term steady-state testing with polystyrene media..................................................... 53
4.26
Raw water and filter effluent arsenic concentrations during long-term steady-state
testing with polystyrene media......................................................................................... 55
4.27
Filter influent and effluent iron concentrations during long-term steady-state testing
with polystyrene media..................................................................................................... 56
4.28
Filter effluent iron concentrations during long-term steady-state testing with
4.29
Raw water and filter effluent pH during long-term steady-state testing with
4.30a Scanning electron microscopy photograph of new polystyrene filter media.................... 62
4.30b Scanning electron microscopy photograph of new polystyrene filter media
(close-up).......................................................................................................................... 62
4.30c Scanning electron microscopy photograph of new polystyrene bead filter media........... 63
4.31a Scanning electron microscopy photograph of used polystyrene bead filter media........... 63
4.31b Scanning electron microscopy photograph of used polystyrene bead filter media
(close-up).......................................................................................................................... 63
4.31c Scanning electron microscopy photograph of used polystyrene bead filter media
(close-up surface).............................................................................................................. 64
4.32
Expansion of polystyrene and PFC100E media as a function of backwash rate................65
4.33
Turbidity and headloss profile for filter run from third week of steady-state operation
with PFC100E media (Run 1084)..................................................................................... 67
4.34
Turbidity and headloss profile for filter run from sixth week of steady-state operation
with PFC100E media (Run 1124)..................................................................................... 68
4.35
Turbidity and headloss profile for filter run from eighth week of steady-state
operation with PFC100E media (Run 1345)..................................................................... 69
4.36
Turbidity, arsenic, and iron profiles from filter maturation during seventh week
of pilot plant operation with PFC100E media (Run 1332)............................................... 70
xiv | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
4.37
Clean media headloss from full filter runs during steady-state testing with
PFC100E media................................................................................................................ 71
4.38
Headloss from full filter runs during steady-state testing with PFC100E media.............. 71
4.39
Filter run time from full filter runs during steady-state testing with PFC100E media..... 72
4.40
Filter effluent turbidity during stable operating portion of full filter runs during
steady-state testing with PFC100E media......................................................................... 72
4.41
Raw water and filter effluent arsenic concentrations during steady-state testing with
PFC100E media................................................................................................................ 74
4.42
Filter influent and effluent iron concentrations during steady-state testing with
PFC100E media................................................................................................................ 75
4.43
Filter effluent iron concentrations during steady-state testing with PFC100E media...... 75
4.44
pH of chlorinated raw water and filter effluent................................................................. 76
FOREWORD
The Water Research Foundation (Foundation) is a nonprofit corporation that is dedicated to
the implementation of a research effort to help utilities respond to regulatory requirements and
traditional high-priority concerns of the drinking water community.
The Arsenic Water Technology Partnership (AWTP) program is a partnership between the
Foundation, Sandia National Laboratories (SNL) and WERC, a Consortium for Environmental
Education and Technology Development at New Mexico State University that is funded by the
Department of Energy and the Foundation. The goal of the program is to provide drinking water
utilities, particularly those serving small and rural communities, with cost-effective solutions for
complying with the new 10 ppb arsenic MCL. This goal is being met by accomplishing three tasks:
(1) bench-scale research to minimize operating, energy and waste disposal costs; (2) demonstration of technologies in a range of water chemistries, geographic locales, and system sizes; and (3)
cost effectiveness evaluations of these technologies and education, training, and technology
transfer.
The AWTP program is designed to bring new and innovative technologies developed at the
laboratory and bench-scale to full-scale implementation and to provide performance and economic
information under actual operating conditions. Technology transfer of research and demonstration
results will provide stakeholders with the information necessary to make sound decisions on costeffective arsenic treatment.
The Foundation participates in the overall management of the program, helps to facilitate
the program’s oversight committees, and administer the laboratory/bench-scale studies. SNL conducts the pilot-scale demonstrations and WERC oversees the education, training, economic analysis, and outreach activities associated with this program.
Roy L. Wolfe, Ph.D.
Chair, Board of Trustees Water Research Foundation
Robert C. Renner, P.E.
Executive Director
xv
ACKNOWLEDGMENTS
This report is the product of a collaborative effort between the members of the Arsenic
Water Technology Partnership and was made possible by funds from Congress and the drinking
water community. A special thanks to U.S. Senator Pete Domenici for his support and assistance
in helping to bring low-cost, energy efficient solutions for the removal of arsenic from drinking
water.
The authors of this report thank the Arsenic Water Technology Partnership for funding this
project. The authors thank the Water Research Foundation (Foundation) for its technical and
administrative assistance with this project. Specifically, the authors thank the Project Manager,
Dr. Hsiao-Wen Chen, and the Project Advisory Committee members: Dr. David Hand, Dr. Mark
Waer, and Dr. Ganesh Ghurye for their technical comments and guidance during the course of the
project. The authors thank Dr. David Hand and his staff at the Michigan Technological University
for their generous contribution of the scanning electron microscopy conducted for the study.
Further, the authors thank:
Mr. Steve Seffer, Operations Supervisor at the City of Azusa, California, Canyon Filtration
Plant, where the initial investigative field testing was conducted.
Mr. Joe Mathein and Mr. Steve Samaras of the San Bernardino County Special Districts
Department, Water & Sanitation Division, for providing the test site for the field demonstration
testing and for assisting in the set-up of the pilot unit.
Mr. Steve Samaras and the staff of the San Bernardino County Special Districts Department
for the operation of the arsenic treatment system during the six-week field demonstration period.
Mr. Ronald Young, General Manager, and Mr. Julius Ma, Water Production Manager, of
the Elsinore Valley Municipal Water District for their support and assistance during the 6 months
of steady-state operation at the Back Basin Groundwater Treatment Plant, as well as William
Leonard, Frank Engstrom and Chris Owens for pilot plant operation.
xvii
EXECUTIVE SUMMARY
OBJECTIVES
The objective of this project was to evaluate performance of ultra-light filter media in a
coagulation/filtration (C/F) process for removal of arsenic from groundwater. Specifically, pilot
testing was conducted to achieve the following objectives:
1. Evaluate the ability of the ultra-light filter media for the removal of ferric-arsenic floc
formed with coagulation
2. Evaluate the ability of the ultra-light filter media to be adequately and consistently
backwashed at the much lower backwash rates compared to those of conventional
sand-anthracite media
3. Demonstrate the long-term operational stability and performance of the ultra-light
media in a C/F process for arsenic removal from groundwater.
BACKGROUND
Coagulation/filtration with conventional anthracite-sand media in pressure vessels is a
demonstrated treatment technology for the removal of arsenic from groundwater. The process utilizes ferric coagulant addition to adsorb arsenic from water. Applying a C/F process for arsenic
removal from groundwater is less costly than the other available technologies such as adsorption
on granular ferric oxide media or ion exchange. However, one drawback of the C/F process is the
production of a relatively high volume of waste backwash water compared to other adsorptive
technologies. The backwash volume is a function of the backwash velocity required to fluidize the
media. Due to the high density of sand and anthracite, the backwash velocity needs to be between
15 and 20 gpm/ft2 (37 to 49 m/hr). If a 5-ft (1.5-m) pressure vessel is backwashed at this rate for
10 min, the volume of waste backwash water could be 3,000 gal (11 m3) or more. The disposal of
this volume of backwash water is highly problematic for very small systems because many dispose
their waste to an on-site septic system. Even if a small water system had access to a sanitary sewer,
the rate of backwash water production is too high for direct discharge into the sewer, and would
require construction of an equalization basin.
This project investigated the replacement of the conventional anthracite-sand filter media
in a C/F process for arsenic removal from groundwater with an ultra-light filter media having a
specific gravity as low as 1.05, which greatly reduces the required backwash velocity. The same
5-ft (1.5-m) vessel described above would produce as little as 600 gal (2 m3) of water during backwashing. If this process modification were proven viable, it would overcome the primary drawback of the C/F process, and make the application of this technology highly desirable compared to
other more costly technologies.
APPROACH
The project was performed by Water Quality & Treatment Solutions, Inc. (WQTS) of
Canoga Park, California. The ultra-light filter media material used for the majority of the project
xix
xx | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
was polystyrene beads obtained from Glen Mills, Inc. (Clifton, NJ). The typical use for the spherical polystyrene beads is for surface polishing and cleaning of engineered plastic parts in manufacturing applications. Use as a filter media was a novel application for the polystyrene beads;
therefore, this material was not NSF 61 certified for contact with drinking water at the time of the
study.
The project was conducted in three phases:
1. Investigative field testing
2. Field demonstration testing
3. Steady-state operation
The first phase, investigative field testing, was conducted at the Canyon Filtration Plant in
the City of Azusa, CA, using chlorinated groundwater as the pilot plant feedwater. The polystyrene
beads were used as the filter media in the pilot filter column. The objectives of investigative field
testing were to:
1. Determine if the polystyrene media could effectively remove ferric floc and produce
a high-quality filtered water over a range of coagulant doses and filtration rates
2. Determine if the polystyrene media could be effectively cleaned at the low rates
required to fluidize the media.
After completion of the investigative field testing, the pilot C/F unit was moved to a well
site in Helendale, CA, for field demonstration testing. The groundwater well contained an arsenic
concentration of approximately 20 μg/L. The objective of the field demonstration testing was to
demonstrate the ability of the polystyrene media to consistently produce high quality water, with
arsenic concentrations less than 10 μg/L, over an extended period of operation.
The third phase of the project was steady-state operation. The pilot C/F unit was located at
the Elsinore Valley Municipal Water District in Lake Elsinore, CA. The steady-state testing was
performed to demonstrate the stability and reliability of the ultra-light filter media in a C/F process
for arsenic removal from groundwater. Long-term steady-state operation of the pilot C/F unit with
the ultra-light polystyrene media was performed for 4 months. Additionally, an alternative ultralight filter media, Purofine PFC100E ion exchange resin (The Purolite Company, Bala Cynwyd,
PA), was tested for 8 weeks. The NSF 61 certified PFC100E material is a strong acid cation exchange
media but was employed as a filter media rather than as an ion-exchange media in this study.
RESULTS AND CONCLUSIONS
While the ultra-light media was adequately fluidized at a backwash rate of 2 gpm/ft2
(4.9 m/hr), a backwash rate of at least 3 gpm/ft2 (7.3 m/hr) was found to be necessary to remove
the accumulated ferric floc material based on visual observation of the ultra-light media in the
clear PVC pilot filter column.
Air scour of approximately 2 scfm/ft2 (36.6 m/hr) for 1 min was required to break up flocmedia agglomerates into small particles, allowing them to be removed by backwashing. A rest
period of 3 to 5 min with no flow was required for the media to settle completely and return to its
pre-backwash bed depth. The optimized backwash conditions used for the long-term steady-state
testing are shown in Table ES.1.
Executive Summary | xxi
Table ES.1
Backwash regime for long-term steady-state testing of ultra-light media
Backwash procedure
Stage 1
Stage 2
Rest
Air scour (during Stage 1)
Flow velocity
2.0 gpm/ft2 (4.9 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
2.0 scfm/ft ft2 (37 m/hr)
Duration
3 min
10 min
5 min
2 min
The key conclusions from this study are as follows:
1. Ultra-light polystyrene beads served as an effective filter media for removing ferric
floc (including adsorbed arsenic) from groundwater in a pressure C/F system at three
different testing locations with three different raw water qualities.
2. Seven months of pilot testing with two different raw water qualities using two different ultra-light filter media materials indicated that the C/F process with ultra-light
media was effective for arsenic removal from groundwater. In a total of 96 sample sets
collected during the study, raw water arsenic concentrations averaged between 20 and
30 μg/L and filter effluent arsenic concentrations averaged approximately 4 to 5 μg/L.
Only two of the 96 filter effluent arsenic samples had an arsenic concentration that
exceeded the maximum contaminant level (MCL) of 10 μg/L.
3. The maximum backwash rate of the ultra-light filter media (3.3 gpm/ft2 or 8.1 m/hr)
was less than 20 percent of that typically used for conventional sand/anthracite media
(18 to 20 gpm/ft2 or 44 to 49 m/hr).
4. Both types of ultra-light filter media materials tested (polystyrene beads and PFC100E
cationic exchange resin) were found to be effective filter media for the C/F system for
arsenic removal from groundwater.
5. The backwash regime for the ultra-light media included a maximum backwash rate of
3.3 gpm/ft2 (8.1 m/hr) and air scour. All of the study’s criteria for a properly cleaned
filter indicated that the backwash regime provided effective cleaning of the ultra-light
media throughout the study.
6. Air scour was critical for effective backwashing of the ultra-light media. Without air
scour, the maximum backwash velocity of 3.3 gpm/ft2 (8.1 m/hr) provided insufficient
energy for effective backwashing. Based on visual observation, air scour provided the
energy necessary to break up floc/media agglomerates and small mudballs. Floc particles could then be removed from the filter media at the low backwash rate used.
7. Feedwater recovery with the ultra-light filter media was approximately 99 percent.
8. The unit backwash volume during the study was approximately 30 to 35 gal/ft2 (1.2 to
1.4 m3/m2). For a very small system, this low unit backwash volume could likely be
accommodated by an on-site septic system or leach field. Further, the backwash flow
rate of 3.3 gpm/ft2 (8.1 m/hr) is sufficiently low that the backwash flow could be
routed directly to a sewer with no need for an equalization basin.
9. Considering that a typical filtration rate is on the order of 5 gpm/ft2 (12 m/hr), the
backwash flow rate of 3.3 gpm/ft2 (8.1 m/hr) could be provided by the production rate
of a parallel filter, thereby eliminating the need for a dedicated backwash pumping
xxii | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
Table ES.2
Comparison of backwash volumes for typical anthracite-sand and ultra-light media filter
Flow velocity (gpm/ft2)
AnthraciteUltra-light
Backwash stage
filter media
sand
1.0
Stage 1
6
3.3
Stage 2
19
1.5
Stage 3
9
Total backwash volume (gal)
Duration (min)
2
8
4
Backwash volume (gal)
sand
filter media
236
39
2,984
518
707
117
3,927
674
system. This cannot be accommodated in a conventional sand/anthracite filter system
due to the high backwash rate required.
APPLICATIONS AND RECOMMENDATIONS
This study demonstrated that ultra-light filter media could effectively and consistently
remove arsenic from groundwater in a C/F process. Due to the very low specific gravity of the
ultra-light media tested (1.05 to 1.27) compared to that of sand media (2.63) typically used in C/F
processes, the backwash requirements of the ultra-light media were demonstrated to be much
lower than those of conventional sand or anthracite media. The low backwash requirements have
significant implications for small systems with arsenic-contaminated groundwater or those with
constraints on disposal of waste backwash water.
Conventional C/F systems are an attractive arsenic removal option for small systems
because the technology is not proprietary and is less costly than many other arsenic removal technologies. However, use of conventional anthracite-sand filter media has relatively high backwash
water requirements. Many small systems do not have a sanitary sewer connection at well sites.
Rather, waste must be disposed in an on-site septic system. The high backwash water requirements
are incompatible with disposal to an on-site septic system. Other utilities may have constraints on
the flow rate or volume of waste backwash water that can be disposed of in a sanitary sewer.
As an example, assume a 5-foot (1.5-m) diameter filter in a C/F system for arsenic removal
from groundwater. The area of this filter is 19.6 ft2 (1.8 m2). A typical conventional filter may consist of a 12-inch (30.5 cm) sand layer under 24 inches (61 cm) of anthracite. A typical effective size
(ES) for the sand would be 0.55 mm, with a uniformity coefficient (UC) of 1.4 and a specific gravity of 2.63. The anthracite on top of the sand would have an ES of about 1.0 mm, a UC of 1.4, and
a specific gravity of 1.7. The calculated backwash rate for this filter is on the order of 19 gpm/ft2
(47 m/hr). The backwash rate of the ultra-light media was established in the study as 3.3 gpm/ft2
(8.1 m/hr). Thus, for a filter area of 19.6 ft2 (1.8 m2), the backwash rate for the anthracite-sand filter
would be 373 gpm (85 m3/hr) and the backwash rate for the ultra-light media filter media would be
65 gpm (15 m3/hr). The required backwash rate for the ultra-light media filter would be 17% of that
required for the anthracite-sand filter.
Taking the example a step further, the total backwash volume can be calculated for each
filter by scaling up an example backwash procedure and applying it to both types of media. The
results are shown in Table ES.2.
Table ES.2 shows that the total backwash volume produced by the ultra-light media filter
was 674 gal (2.6 m3), while the total backwash volume produced by the anthracite-sand filter was
Executive Summary | xxiii
3,927 gal (15 m3). The backwash volume produced by the ultra-light media filter was only 17 percent of that produced by the anthracite-sand filter.
The above example illustrates that both the backwash rate (373 gpm, 85 m3/hr) and the
total backwash volume (3,927 gal, 15 m3) produced by a conventional anthracite-sand media in a
C/F process are incompatible with the waste disposal limitations faced by many small systems. At
a minimum, an equalization basin would be required to capture the high flow rate of the backwash
water. With the ultra-light filter media, both the backwash rate (65 gpm, 15 m3/hr) and total backwash volume (674 gal, 2.6 m3) could be accommodated by a sewer connection or on-site septic
system without equalization. This greatly simplifies the design and operation of the C/F process.
Use of the ultra-light filter media offers another key advantage for a small system over
conventional filter media. The filtration rate used during the pilot testing was 4 gpm/ft2 (9.8 m/hr).
The maximum backwash flow rate was 3.3 gpm/ft2 (8.1 m/hr). Thus, one pressure filtration vessel
produces more than enough flow to completely backwash another vessel without interrupting filter
operation. The system could be designed such that two pressure vessels would be operational;
water produced by one vessel would be used to backwash the other vessel. This is a standard
design in many industrial applications, such as ion exchange systems. On the other hand, the maximum backwash flow rate for the conventional anthracite-sand media was 19 gpm/ft2 (47 m/hr).
Even if the filtration rate was 6 gpm/ft2 (15 m/hr) rather than the 4 gpm/ft2 (9.8 m/hr) used in this
study, the flow of three pressure vessels would be required to produce water for backwashing only
one vessel.
This study demonstrated that the use of ultra-light filter media in a C/F process for arsenic
removal from groundwater offered many advantages with respect to the very low backwash
requirements compared to those of conventional anthracite-sand media. While promising results
were obtained in this study, a large-scale demonstration study is necessary to fully assess the use
of ultra-light media as a replacement for conventional anthracite-sand media in a C/F process. All
pilot testing for this study was performed using a 6-inch (15-cm) filter column, and a source water
flow rate on the order of 1 gpm (4 L/min). The next step toward full-scale implementation would
be a demonstration-scale evaluation on the order of 50 gpm (190 L/min), typical of very small
systems. Ideally, the demonstration study would evaluate a full-scale C/F system using ultra-light
media to treat water from an arsenic-contaminated well under field conditions. A scale on the order
of 50 gpm (190 L/min) would allow the use of a full-size pressure vessel. This type of demonstration study would provide valuable information for proceeding with full-scale implementation of
the C/F process using ultra-light media for arsenic removal from groundwater.
CHAPTER 1
INTRODUCTION
Coagulation/filtration (C/F) with conventional anthracite-sand media in pressure vessels is
a demonstrated treatment technology for the removal of arsenic from groundwater (Jekel and Seith
2000, Karcher et al. 1999). The process utilizes ferric salt addition (e.g., ferric chloride) to adsorb
the arsenic from the water. The water is then passed through granular filter media to remove the
ferric-arsenic flocs. The filter is periodically backwashed to remove the accumulated floc material
before it is put back in service. Because this process uses non-proprietary components, a C/F process is substantially less costly to construct and operate than other technologies being implemented
for arsenic removal from groundwater.
One drawback of the C/F process is the production of a relatively high volume of waste
backwash water compared to other arsenic-removal technologies. Due to the high density of sand
and anthracite, the required backwash velocity is typically between 15 and 20 gpm/ft2 (37 to
49 m/hr) to effectively fluidize the media. If a 5-ft (1.5-m) vessel is backwashed at this rate for
10 minutes, the volume of waste backwash water could be between 2,950 to 3,930 gallons (11 to
15 m3). The disposal of this amount of backwash water is highly problematic for very small systems because many dispose their waste to an on-site septic system. Even if a system has access
to a sanitary sewer, the rate of backwash water production is too high for direct discharge into the
sewer, requiring an equalization tank.
To address these constraints, this project investigated the replacement of the anthracitesand media in a C/F process for arsenic removal from groundwater with ultra-light filter media.
The two types of ultra-light filter media tested in this study had specific gravities less than 1.3,
which greatly reduces the required backwash velocity. The estimated backwash velocity required
for commercially available ultra-light filter media materials is on the order of 3 gpm/ft2 (4.9 m/hr).
This is only 15 to 20 percent of that required for backwashing sand or anthracite grains of similar
size, which translates into an equal reduction in the volume of backwash water production from
a C/F process. If the use of ultra-light media in a C/F process is proven viable, it would make the
application of the C/F process highly desirable compared to other more costly technologies for
arsenic removal from groundwater. The same 5-ft (1.5-m) vessel described above would produce
as little as 600 gallons (2 m3) of water during backwashing.
PROJECT OBJECTIVES
The objective of this project was to evaluate performance of ultra-light filter media in a
C/F process for removal of arsenic from groundwater. Specifically, pilot testing was conducted to
achieve the following objectives:
1. Evaluate the ability of the ultra-light filter media for the removal of ferric-arsenic floc
formed with coagulation
2. Evaluate the ability of the ultra-light filter media to be adequately and consistently
backwashed at the much lower backwash rates compared to those of conventional
sand-anthracite media
3. Demonstrate the long-term operational stability and performance of the ultra-light
media in a C/F process for arsenic removal from groundwater.
1
2 | Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
PROJECT APPROACH
The project was performed by Water Quality & Treatment Solutions, Inc. (WQTS) of
Canoga Park, California. The project was conducted in three phases:
1. Investigative field testing
2. Field demonstration testing
3. Steady-state operation
The first phase, investigative field testing, was conducted at the Canyon Filtration Plant
in the City of Azusa, CA, using chlorinated groundwater as the pilot plant feedwater. Ultra-light
polystyrene beads were used as the filter media in the pilot filter column. The objectives of investigative field testing were to:
1. Determine if the polystyrene media could effectively remove ferric floc and produce a
high-quality filtered water over a range of coagulant doses and filtration rates
2. Determine if the polystyrene media could be effectively cleaned at the low rates
required to fluidize the media.
After completion of the investigative field testing, the pilot C/F unit was moved to a well
site in the high desert community of Helendale, CA, for field demonstration testing. The community had a groundwater well containing an arsenic concentration of approximately 20 μg/L. The
objective of the field demonstration testing was to demonstrate the ability of the polystyrene media
to consistently produce high quality water, with arsenic concentrations less than 10 μg/L, over an
extended period of operation. During field demonstration testing, the staff of the San Bernardino
County Special Districts Department, who operated the well, took over day-to-day operation of
the fully-automated pilot C/F unit for six weeks, collecting daily arsenic samples from the raw
and treated water, preparing ferric chloride feed solutions, and monitoring the coagulant dose and
various water quality parameters.
The third phase of the project was steady-state operation. The pilot C/F unit was located
at the Elsinore Valley Water District in Lake Elsinore, CA. Long-term steady-state operation of
the pilot C/F unit with ultra-light polystyrene media was performed for 4 months. Additionally,
an alternative ultra-light filter media, Purofine PFC100E, was tested for 8 weeks. The steady-state
testing was performed to demonstrate the stability and reliability of the ultra-light filter media in a
C/F process for arsenic removal from groundwater.
CHAPTER 2
BACKGROUND
Treatment systems for removal of arsenic from groundwater are often small well-head
systems operated by small water providers. C/F is a widely-used, reliable and cost-effective treatment strategy for arsenic removal from groundwater, particularly for small systems with limited
resources. Typically, these types of systems are comprised of coagulation and flocculation via
in-line coagulant chemical addition, and pressure filtration using conventional granular media such
as anthracite-sand.
COAGULATION/FILTRATION FOR ARSENIC REMOVAL FROM GROUNDWATER
Extensive pilot-scale evaluation and full-scale operation of the C/F process has been performed for the removal of arsenic from groundwater using conventional sand-anthracite media.
This section presents some of the results obtained from previous studies conducted by WQTS
using the same automated pilot C/F unit used in this study.
In a study conducted by WQTS for the Virgin Valley Water District (VVWD) in Mesquite,
NV (WQTS 2004), the use of C/F technology was evaluated for the removal of arsenic from two
groundwater wells. The water from the first well contained between 35 and 40 µg/L of arsenic
and 25 mg/L of silica, while the other contained approximately 80 µg/L of arsenic and 45 mg/L
of silica. The media size and depth used in the study are shown in Table 2.1, while the backwash
conditions employed are listed in Table 2.2 (WQTS 2004).
Figure 2.1 shows the iron and arsenic concentration profiles in the filter effluent at a filtration rate of 5 gpm/ft2 (12.2 m/hr) while adding ferric chloride at a dose of 2 mg/L as Fe. The
results show that the influent arsenic level was reduced to an average of 5 µg/L under the conditions tested. The iron concentration in the filtered water was well below 0.05 mg/L for the entire
filter run, which lasted for approximately 55 hours before iron breakthrough occurred. As shown
in Figure 2.1, arsenic breakthrough did not occur before iron breakthrough, which was confirmed
by all the other runs. Therefore, terminating the filter run on iron breakthrough ensured two important water quality factors: first, no elevated arsenic concentration was present in the filtered water
throughout the run, and second, no elevated iron concentrations were discharged into the distribution system. Time to iron breakthrough can be determined in several ways, but the simplest and
most practical is on-line turbidity measurement. This testing indicated that turbidity measurement
was an ideal tool for detecting iron breakthrough and terminating the filter run.
The Nevada study also evaluated the removal of arsenic at multiple ferric chloride doses
and two filtration rates. A summary of the results is shown in Figure 2.2. At an iron dose of 1 mg/L
and a filtration rate of 7.5 gpm/ft2 (18.3 m/hr), the arsenic concentration was reduced from approximately 37 µg/L to 13 µg/L. Increasing the dose to 2 mg/L and then to 4 mg/L further reduced the
arsenic to 6.9 µg/L and 4.8 µg/L, respectively. With an iron dose of 2 mg/L, reducing the filtration
rate from 7.5 gpm/ft2 (18.3 m/hr) to 5 gpm/ft2 (12.2 m/hr), improved arsenic removal. However,
no such improvement was observed at an iron dose of 4 mg/L.
Figure 2.3 shows the impact of iron dose and filtration rate on the filter run length experienced in the Nevada pilot study. As expected, increasing the filtration rate or iron dose correspondingly reduced the filter run length. At an iron dose of 2 mg/L, reducing the filtration rate
3
Table 2.1
Conventional anthracite-sand media design in a C/F process used to remove arsenic
from a Nevada groundwater (WQTS 2004)
Parameter
Depth, inches
Effective size, mm
Uniformity coefficient
Sand
24
0.5
1.4
Anthracite
24
0.9
1.4
Table 2.2
Backwash conditions for anthracite-sand media in C/F process for arsenic removal
from a Nevada groundwater (WQTS 2004)
Flow velocity
10 gpm/ft2 (24.4 m/hr)
20 gpm/ft2 (48.8 m/hr)
10 gpm/ft2 (24.4 m/hr)
0 gpm/ft2 (0 m/hr)
Duration
2 min
10 min
2 min
1 min
2.0
50
1.8
45
1.6
40
1.4
35
Effluent Iron
Influent Arsenic
Effluent Arsenic
1.2
1.0
0.8
30
25
20
Conditions:
FeCl3 Dose = 2 mg/L as Fe
0.6
15
2
Filtration Rate = 5 gpm/ft
0.4
10
0.2
5
0.0
Arsenic Concentration (ug/L)
Backwash stage
Stage 1
Stage 2
Stage 3
Rest
Iron Concentration (mg/L)
0
0
10
20
30
40
50
60
Run Time (hrs)
Figure 2.1 Profiles of iron and arsenic breakthrough in one filter run conducted during the
Nevada pilot study (WQTS 2004)
Chapter 2: Background | 5
14
13
Filt. Rate = 7.5 gpm/ft2
Effluent Arsenic (ug/L)
12
Influent arseninc = 35 to 40 ug/L
10
8
6.9
6
4.8
4.2
4.6
4
2
0
1
2
4
Iron Dose (mg/L)
Figure 2.2 Summary of the impact of iron dose and filtration rate on arsenic removal during
the Nevada pilot study (WQTS 2004)
80
Time Between Backwashes (hrs)
70
60
60
55
50
40
30
25
24
20
14
12
10
0
1
2
4
6
Iron Dose (mg/L)
Figure 2.3 Summary of the impact of filtration rate and iron dose on filter run length in the
Nevada pilot study (WQTS 2004)
Turbiditiy (NTU) or Fe Concentration (mg/L)
0.40
Turbidity (NTU)
0.35
Iron (mg/L)
0.30
Conditions:
FeCl 3 Doe = 6 mg/L as Fe
2
Filtration Rate = 5 gpm/ft
0.25
0.20
0.15
0.10
0.05
0.00
11
12
13
14
15
16
Run Time (hrs)
Figure 2.4 Turbidity measurement as an indicator of iron breakthrough in the Nevada pilot
study (WQTS, 2004)
from 7.5 gpm/ft2 (18.3 m/hr) to 5 gpm/ft2 (12.2 m/hr) increased the run length from 25 hrs to 55
hrs. Even at a dose of 6 mg/L iron (approximately 18 mg/L FeCl3) and a filtration rate of 5 gpm/ft2
(12.2 m/hr), the filter run length was 14 hrs, which is well suited for an automatic filtration process
in which the filter is backwashed and put back in service by a programmable logic controller (PLC)
system without the need for an operator intervention. This is similar to a membrane backwashing
process which takes place ever 30 to 45 minutes and is fully conducted by the PLC.
One of the perceived concerns over the use of C/F for arsenic removal is knowing when
to terminate the filter run. In surface water treatment, an on-line turbidimeter is used to indicate
breakthrough of particulate material originating from the influent water to the filter. However, in
the application of C/F for arsenic removal from groundwater, the groundwater contains little to no
measurable suspended solids. Nevertheless, turbidity of the filtered water was found to correlate
very well with the iron concentration in the filtered water. Figure 2.4 shows such correlation during
one of the filter runs conducted during the Nevada study (WQTS, 2004). As the iron began to break
through the filter effluent, the filtered water turbidity correspondingly increased. The increase in
turbidity was sufficient to be detected by standard on-line turbidimeters. The Nevada study also
showed that the arsenic breakthrough occurs with the iron breakthrough because the arsenic is
bound to the iron floc formed. These results suggest that turbidity is a good indicator of iron breakthrough in a C/F process used for arsenic removal from groundwater.
ANTHRACITE-SAND VS. ULTRA-LIGHT FILTER MEDIA
The arsenic treatment system investigated in this project is a unique modification to the
C/F process, which is an existing and proven arsenic treatment technology. The water industry
Chapter 2: Background | 7
has developed extensive experience in the application of the C/F process for arsenic removal from
groundwater. The process utilizes conventional media (sand and/or anthracite) in pressure vessels.
For systems where a sewer discharge option is available, the C/F process has been found to be
more economical than other proven arsenic removal technologies.
The primary drawback to the C/F process is the production of a significant volume of
backwash water at a high flow rate. The most common disposal options for waste backwash water
include: (1) discharge into the local sewer, and (2) on-site drying in sludge lagoons followed by
off-site disposal of the dry solids. The latter is not a viable option for very small systems because
it requires significant operator attention and land use. The sewer-disposal option is problematic
for very small systems for two reasons. First, most small systems do not have access to a sanitary
sewer, but rather discharge their domestic wastewater to an on-site septic system. The disposal of
large volumes of backwash water into the septic system is not a viable option. Second, even in the
event that a sanitary sewer connection is available, the flow rate generated during backwashing
of a typical vessel is too high for direct discharge into a common sewer line. Therefore, the water
would have to be collected into an equalization tank before it can be metered slowly into the sewer.
Such a tank is costly and requires additional footprint.
The size and density of the filter media dictate the high backwash rate and the resulting high
volume of backwash water produced. The size of the media is set by the filter performance needs,
and therefore cannot be changed. However, the density of the filter media is completely irrelevant
to the filter performance. The density of silica sand is typically about 2.63 g/cm3 while that of
anthracite is approximately 1.65 g/cm3. The density of the two types of ultra-light filter media used
in this study was approximately 1.0 g/cm3. As a result, while the backwash rate required for typical
sand and anthracite media is between 15 and 20 gpm/ft2 (37 to 49 m/hr), the backwash velocity
required for ultra-light filter media could be as low as 2 to 3 gpm/ft2 (4.9 to 7.4 m/hr). If the two
media are backwashed for the same period of time, the volume of water produced from backwashing ultra-light filter media may only be 15 to 20 percent of that produced from backwashing sand
and anthracite media. This could be a tremendous advantage to small systems, especially if the
backwash water can only be discharged into the local septic system. Even if the backwash water
can be discharged to a local sanitary sewer, the flow rate used for backwashing polystyrene media
is also only 15 to 20 percent of that used to backwash sand and anthracite media. Therefore, the
backwash water flow is sufficiently low that it can be discharged directly into the sewer without
the need for equalization.
Polystyrene media is commercially available, and non-proprietary, although at the time of
this study it was not NSF 61 certified for contact with drinking water. The alternative ultra-light
filter media, Purofine PFC100E, is commercially available and NSF 61 certified. Either of these
two types of ultra-light filter media is certainly more costly than sand or anthracite. However, the
media are durable and can resist abrasion. The main concern over the use of ultra-light filter media
is the possibility that their very low backwash velocity (as low as 2 gpm/ft2 or 4.9 m/hr) may not
be sufficient to adequately clean the media. This issue was a key variable investigated during the
study.
FILTER BACKWASHING
Equation 2.1 expresses the optimum backwash rate, VB, for granular media (Kawamura,
1975), and is based on the empirical determination that the optimum backwash velocity is 10% that
of the settling velocity of the granular media in water. This velocity is higher than the minimum
fluidization velocity of the media.
1
^ρ − ρ h2 2 3
VB = 1.47 # =1.8 # 10 # m
g G # UC # d10 ρμ
−5
(2.1)
where VB = backwash rate, gpm/ft2
rm = density of the filter media, g/cm3
r = density of water, g/cm3
µ = viscosity of water, g/cm-sec
g = gravitational acceleration = 981 cm/sec2
UC = filter media uniformity coefficient
d10 = filter media effective size, mm
For example, if a filter contains sand with an effective size of 0.5 mm, a uniformity coefficient of 1.4, and density of 2.65 g/cm3, then the backwash velocity required at 20°C is calculated
at 17.2 gpm/ft2 (42 m/hr) as follows:
3
]
g2
VB =1.47 # ;1.8 # 10 −5 # 2.65 − 1 # 981 2E # 1.4 # 0.5 = 17.2 gpm/ft 2
1 # 0.01009
1
On the other hand, if same-size polystyrene media were used (0.5 mm), with a density
of 1.05 g/cm3, the backwash velocity required is calculated to be only 1.7 gpm/ft2 (4.2 m/hr) as
follows:
3
]
g2
VB = 1.47 # ;1.8 # 10 −5 # 1.05 − 1 # 981 2E # 1.4 # 0.5 = 1.7 gpm/ft 2
1 # 0.01009
1
While a backwash rate of 1.7 gpm/ft2 is very low, it is understandable considering the very
light polystyrene material used. The use of lighter-than-sand material as filter media is not new.
Granular activated carbon (GAC) has long been used successfully as filter media. The density
of GAC in water is approximately 1.35 g/cm3. As in the example above, the required backwash
velocity for a 0.5-mm diameter GAC media with a uniformity coefficient of 1.4 is calculated using
Equation 2.1 to be only 6 gpm/ft2 (15 m/hr).
The very low backwash velocity of 1.7 gpm/ft2 (4.2 m/hr) translates into a low shear force
between the water and the particles of media. The shear force between the water and the particles
may be insufficient to adequately clean the filter media. This issue was a key variable investigated
during the study.
CHAPTER 3
METHODS AND MATERIALS
ULTRA-LIGHT FILTER MEDIA
Two different ultra-light filter media materials were utilized during the study. The majority
of testing was performed using polystyrene beads as the ultra-light filter media. Limited testing
was also performed using a cation exchange resin as an ultra-light filter media.
Polystyrene Media
Polystyrene beads were the primary ultra-light filter media used in the project. The beads
had a size range of 0.61 to 0.99 mm and a specific gravity of 1.05. The polystyrene beads were
obtained from Glen Mills, Inc. (Clifton, NJ). The typical use for the spherical polystyrene beads is
for surface polishing and cleaning of engineered plastic parts in manufacturing applications. Use
as a filter media was a novel application for the polystyrene beads; therefore, this material was not
NSF 61 certified for contact with drinking water at the time of the study.
PFC100E Media
Purofine PFC100E strong acid cation exchange resin (The Purolite Company, Bala Cynwyd,
PA) was used as an ultra-light filter media for an 8-week period during the steady-state operation
phase of the project. PFC100E is a gel-type media with a polymer structure consisting of gel polystyrene cross-linked with divinylbenzene. The media was reported by the manufacturer to have a
mean size of 0.52 to 0.62 mm, specific gravity of 1.27, and maximum uniformity coefficient of 1.2.
Purolite PFC100E media is NSF 61 certified for use in potable water applications.
PILOT COAGULATION/FILTRATION UNIT
Figure 3.1 is a schematic of the pilot C/F unit used for all pilot testing performed for this
study. The pilot C/F unit consisted of two separate chemical feed systems for adding sodium hypochlorite and coagulant, a 6-inch (15.2-cm) filter column with media support, and a fully automated
control system. The pilot C/F unit required a pressurized groundwater source. The water entering
the unit flowed through a backflow prevention valve and a pressure reducing valve, which reduced
the pressure to approximately 25 psi (172 kPa). A magnetic flow meter measured the flow rate.
The water then flowed through two static mixers. Chlorine, when not already present in the source
water, could be added before the first static mixer. Since all of the raw waters used in this study
already contained residual free chlorine, no chlorine addition was used during the study. The coagulant chemical, ferric chloride, was added before the second static mixer. After coagulant addition
and mixing through the second static mixer, the water entered the top of the 6-inch (15.2‑cm)
clear polyvinyl chloride (PVC) filter column and flowed through the filter media. The media was
supported in the column with a Leopold Integrated Media Support (IMS) cap (F.B. Leopold,
Zelienople, PA). After exiting the bottom of the filter column, the treated water passed through
an effluent flow-modulating valve that was automatically adjusted by the system to maintain the
9
Direction of Water Flow During Filtration
Direction of Water Flow During Backwashing
5- gal
Ferric
Chloride
Solution
Sampling Tap
[ Filter Influent /
Backwash Water]
Pump
5- gal
Chlorine
Solution
6 in
Static
Mixer
Static
Mixer
Drain
Pump
PDE
Sampling Tap
[ Raw Water ]
M
Drain
Pressurized
Raw Water
Source
Drain
55-gal
Clearwell
Tank
Sampling Tap
[ Filtered Water ]
Backwash
Pump
Note: Modifications shown with dashed line were made before the Long-Term Operation phase.
Figure 3.1 Schematic diagram of the pilot coagulation/filtration unit
desired filtration rate, based on the flow signal from the magnetic flow meter. Sampling taps were
available for collecting samples of the raw water, filter influent water (after chemical addition) and
filter effluent water.
The pilot C/F unit was equipped with a programmable logic controller (PLC) (Rockwell
Automation/Allen-Bradley, Milwaukee, WI) that automatically monitored and controlled system
operation. The PLC maintained the filtration rate as well as the chemical feed rates.
Filter Run Termination Criteria
The PLC could be programmed by the user to automatically stop the filter run and initiate
a backwash cycle if any of the following criteria was reached:
1. Filter effluent turbidity exceeded the user-set breakthrough limit
2. Filter headloss through the unit reached the user-set terminal headloss
3. Filter run time exceeded the user-set maximum value
The pilot C/F unit used untreated chlorinated groundwater to backwash the filter during
the investigative field testing and field demonstration testing phases of the project. The unit was
modified before the steady-state operation phase of the project to use filtered water from the pilot
Chapter 3: Methods and Materials | 11
Figure 3.2 Photograph of the pilot coagulation/filtration unit
plant as the backwash feed water. The modifications included the addition of a 55-gal polyethylene
clearwell tank to store the filtered water, and a small centrifugal backwash pump. These modifications are shown in dashed lines on Figure 3.1. During backwash, the PLC repositioned the three
3-way valves on the pilot C/F unit to backwash the filter. At the end of the programmed backwash
cycle, the PLC terminated the backwash and put the filter back into service for the next filter run.
The pilot filter utilized a three-stage backwash and a rest period as described in Chapter 4. Both
the backwash flow rate and the duration of each stage of backwash were set by the user from the
system PLC. In addition the filter unit included a compressor that was used for air scour, if desired.
Both the duration and flow rate of air scour were user adjustable.
The PLC monitored and recorded the flow rate of filter effluent, the filter headloss and the
filter effluent turbidity during operation. Filter effluent turbidity was monitored continuously using
a Hach 1720E online turbidimeter (Hach Company, Loveland, CO), and filter headloss was monitored with an Omega Model PX771A differential pressure transmitter (Omega Engineering Inc.,
Stamford, CT). The PLC touch screen allowed the user to set the time interval between recordings,
with more frequent monitoring during filter maturation if desired. A photograph of the filter unit is
presented in Figure 3.2.
PILOT TESTING SITES
The three phases of the project were conducted at different locations. The investigative
field testing was conducted using potable water as the feedwater source for the pilot C/F unit, while
the field demonstration testing and steady-state operation phases of the project were conducted at
well sites with groundwater having arsenic levels greater than the current MCL of 10 µg/L.
Investigative Field Testing
Investigative field testing was conducted at the Canyon Filtration Plant, a surface water
treatment plant owned and operated by the City of Azusa, CA approximately 30 miles (48 km)
northeast of Los Angeles. The pilot C/F unit and on-site laboratory equipment were set up in a
building over the full-scale plant sedimentation basin. Potable water, which was a combination of
treated groundwater and surface water, was used as the feedwater to the pilot C/F unit.
Field Demonstration Testing
The field demonstration testing phase of the project was conducted at a well site in
Helendale, CA, located approximately 120 miles (190 km) northeast of Los Angeles. The well was
operated by the County of San Bernardino Special Districts Department. The arsenic concentration
from the 300-gpm (68 m3/hr) well was typically near 20 µg/L. The pilot C/F unit and on-site laboratory equipment were set up in an emergency generator building near the well site. Chlorinated
well water was supplied to the pilot C/F unit and waste flows were discharged to the municipal
sewer system.
Steady-State Operation
The final phase of the project was conducted at the Elsinore Valley Municipal Water
District’s Back Basin Groundwater Treatment Plant (BBGWTP) located in the City of Lake
Elsinore, CA, approximately 70 miles (113 km) southeast of Los Angeles. The BBGWTP is a
4-mgd (15-ML/d) C/F plant that is supplied with chlorinated groundwater from two wells. The
two wells, Cereal Well 3 and Cereal Well 4, have typical flows of 1,400 gpm (320 m3/hr) and
1,600 gpm (360 m3/hr), respectively, and typical arsenic concentrations of 35 µg/L and 20 µg/L,
respectively. The BBGWTP has six 14-foot (4.3-m) diameter pressure filters, with 48-in (122-cm)
deep anthracite/sand dual media filters that are automatically backwashed based on filter run time.
The pilot C/F unit and on-site laboratory equipment were set up in the BBGWTP building near the
full-scale plant sampling station. Chlorinated raw well water was routed to the pilot C/F unit from
the full-scale plant’s raw water sample pipe. Waste flows from the pilot plant were discharged to
the municipal sewer system.
ANALYTICAL METHODS
Zeta Potential
Samples of the polystyrene media and conventional silica sand media were sent to
Brookhaven Instruments Corporation (Holtsville, NY) along with a sample of Southern California
Table 3.1
Routine water quality monitoring during pilot testing
Parameter
pH
Temperature
Iron
Turbidity*
Free Chlorine
Arsenic†
Chlorinated raw water
ü
ü
ü
ü
ü
ü
Filter influent
ü
ü
Filter effluent
ü
ü
ü
ü
ü
ü
*Turbidity was measured in the chlorinated raw water and filter influent during investigative field testing only.
†Arsenic samples were collected during field demonstration testing and steady-state operation phases only; samples
were analyzed at an off-site laboratory.
groundwater for analysis of zeta potential. The zeta potential of the media particles when immersed
in the groundwater sample was determined using the BI-EKA electro kinetic analyzer (Brookhaven
Instruments, Holtsville, New York).
Water Quality Monitoring
A number of water quality parameters were routinely measured during the project, including pH, temperature, iron, turbidity and free chlorine. These parameters were measured in the
chlorinated raw water, the filter influent and the filter effluent during all three phases of the study.
The water quality parameters monitored and the locations sampled are shown in Table 3.1. With
the exception of arsenic, which was measured at an off-site commercial laboratory, all parameters
shown in Table 3.1 were analyzed at each pilot testing site by the pilot plant operations staff.
Arsenic
All arsenic samples were analyzed at MWH Laboratories (Monrovia, CA) using EPA
Method 200.8, inductively coupled plasma–mass spectrometry (ICP/MS). The method reporting
limit (MRL) was 1 μg/L.
On-Site Analytical Methods
Iron
The Hach ferrozine method (Method 8147, Hach Company, Loveland, CO) was used along
with a Hach DR890 portable colorimeter to measure total iron in the raw water, filter influent and
filter effluent. The method had an upper limit of 1.3 mg/L. All samples with iron concentrations
exceeding 1.3 mg/L were diluted with distilled water before analysis. The method detection limit
was 0.01 mg/L. Liquid ferrozine reagent was purchased from the manufacturer.
Free Chlorine
The Hach DPD colorimetric method (Method 8021, Hach Company, Loveland, CO) was
used along with a Hach DR890 colorimeter to analyze for free chlorine in the chlorinated source
water and filter effluent. The method had an upper limit of 2.00 mg/L. All samples with free chlorine concentrations above 2.00 mg/L were diluted with distilled water before analysis. The estimated method detection limit was 0.02 mg/L. The USEPA-compliant free chlorine reagent powder
was purchased as a 100-test dispenser (USABluebook, Waukegan, IL). Individual DPD powder
packets (product #2105569) were purchased (Hach Company, Loveland, CO) and used during the
Steady-State Operation phase of the project.
Turbidity
Filter effluent turbidity was continuously monitored using a Hach 1720E online turbidimeter (Hach Company, Loveland, CO) with the flow rate through the turbidimeter adjusted to 150–
200 mL/min. In addition, grab samples from the chlorinated source water and the filter influent
were analyzed for turbidity during investigative field testing using a Hach 2100P portable turbidimeter (Hach Company, Loveland, CO).
pH and Temperature
A Hanna Model pHep 4 pH meter (Hanna Instruments, Woonsocket, RI) was used to measure pH and temperature of the chlorinated source water and filter effluent.
General Physical, Mineral, and Metals Sampling
Single samples were collected from the chlorinated source water, filter effluent and backwash water and submitted to MWH Laboratories (Monrovia, CA) for analysis of general physical
parameters, minerals and metals during field demonstration testing and steady-state operation.
Backwash samples collected during field demonstration testing were produced by collecting four routine backwashes from Runs 568 through 571. Each backwash consisted of approximately 6 gal (23 L). The backwash water was collected into a 25-gallon (95-L) polyethylene tank.
Three different samples of the backwash water were prepared and collected for analysis from this
tank. First, a composite backwash sample was collected by placing the tank on a large magnetic
stir plate and using a stir bar to mix the contents until homogenous. The sample bottles were filled
from the tank using a peristaltic pump while the tank was being mixed. Second, a decant backwash
sample was prepared by turning off the stir plate and allowing solids in the tank to settle for 1 hour.
The decant sample bottles were filled using the peristaltic pump. Finally, the intake tubing from the
peristaltic pump was placed into the layer of sludge at the bottom of the tank and sludge sample
bottles were filled.
During steady-state operation, only a mixed backwash sample was collected. The mixed
backwash sample was prepared by first collecting all of the backwash water from a single backwash into a 10-gallon (38-L) polyethylene tank. The backwash volume was approximately 7 gal
(27 L). The tank had a sample tap added at approximately one-third of the tank height with a PVC
Table 3.2
Laboratory analytical methods
Parameter
Apparent Color
Odor
Turbidity
TSS
Alkalinity
Calcium, Total
Chloride
Copper, Total
Fluoride
Hardness, Total
Iron, Total
Langelier Index
Magnesium, Total
Manganese, Total
Nitrate (as N)
Nitrite (as N)
pH, Lab pH
Potassium, Total
Silica
Sodium, Total
Specific Conductance
Sulfate
Analytical method*
ML/SM 2120B
ML/SM 2150B
ML/EPA 180.1
ML/EPA 160.2
SM 2320B/EPA 310.1
ML/EPA 200.7
ML/EPA 300.0
ML/EPA 200.8
SM 4500F-C
ML/SM 2340B
ML/EPA 200.7
ML/SM 2330B
ML/EPA 200.7
ML/EPA 200.8
ML/EPA 300.0
ML/EPA 300.0
SM 4500H+/EPA 150
ML/EPA 200.7
ML/EPA 200.7
ML/EPA 200.7
SM 2510B/SW 9050
ML/EPA 300.0
Parameter
Surfactants
TDS
Total Phosphorus-P
Aluminum, Total
Antimony, Total
Arsenic, Total
Barium, Total
Beryllium, Total
Cadmium, Total
Chromium, Total
Lead, Total
Mercury
Nickel, Total
Selenium, Total
Silver, Total
Thallium, Total
Uranium
Vanadium, Total
Zinc, Total
BOD
COD
Analytical method*
SM 5540C/EPA 425.1
SM 2540C
ML/EPA 365.1
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 245.1
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
ML/EPA 200.8
SM 5210B/EPA 405.1
ML/EPA 410.4
*ML: MWH Laboratories; SM: Standard Methods; SW: EPA Publication SW-846; Test Methods for Evaluating
Solid Waste, Physical/Chemical Methods.
pipe threaded from the sample tap to the approximate center of the tank. After collecting the backwash water, the tank was placed on a large stir plate and a stir bar was added. Mixing speed was
adjusted to provide sufficient energy to uniformly suspend the floc before collecting the mixed
backwash sample from the sample tap.
The parameters analyzed in these samples and the corresponding analytical methods are
shown in Table 3.2. BOD and COD samples were not collected during steady-state operation.
CHAPTER 4
RESULTS AND DISCUSSION
The primary objective of the study was to demonstrate the ability of ultra-light filter media
in a C/F process for arsenic removal from groundwater, and particularly to demonstrate the ability of low-rate backwash to effectively and reliably clean the ultra-light media. The first step to
accomplish these objectives was to characterize the polystyrene beads used as an ultra-light filter
media for the majority of the study. The second step was to perform investigative field testing to
determine the ability of the polystyrene media to remove ferric flocs and to establish backwash
procedures for the polystyrene media. The third task was to conduct field demonstration testing
with the polystyrene filter media in a fully-automated C/F system. Finally, steady-state operation
of the C/F process with the polystyrene media was performed for 4 months, and with an alternative
ultra-light media for an additional 8 weeks, to demonstrate the long-term operational reliability
and stability of the C/F process with ultra-light filter media for arsenic removal from groundwater.
CHARACTERIZATION OF POLYSTYRENE FILTER MEDIA
Surface Charge
The performance of media filtration depends to a large extent on the chemical and physical
properties of the media. One important chemical property is the media’s surface charge, which is
known to impact the attachment of floc particles to the media. The surface charge of the polystyrene media was quantified by measuring its zeta potential when immersed in a sample of Southern
California groundwater. The zeta potential of a sample of sand filter media was also subjected
to the same testing for comparison. The results of the zeta potential analysis are presented in
Figure 4.1. Despite small differences, the results show that the zeta potential of the polystyrene
media particles in the narrow pH range of 8.5 to 8.9 was fairly similar to that of the sand particles
(approximately –3.5 to –4.5 mV). These results indicate that the chemical interaction between the
ferric-arsenic floc and the polystyrene media should be similar to that between the floc and a silica
sand media typical of C/F applications for arsenic removal from groundwater.
Media Size and Density
The physical characteristics of the polystyrene media are significant to its performance as
filter media. Both its effective size and uniformity coefficient are important for filter media design
and backwashing. The density of the media is important for selecting the appropriate backwash
rate. A sample of the polystyrene media was sent to MWH Laboratories (Monrovia, CA) for determination of media size distribution and specific gravity. The results are presented in Table 4.1. The
effective size was 0.66 mm. The uniformity coefficient was 1.3, indicating that the media was quite
uniform in size. The media had a very low specific gravity of 1.05. Using the values in Table 4.1, the
backwash rate calculated for the polystyrene media using Equation 2.1 was 2.1 gpm/ft2 (5.1 m/hr).
17
-3.0
Polystyrene Beads
Silica Sand
-3.5
Zeta Potential (mV)
-4.0
-4.5
-5.0
-5.5
8.5
8.6
8.7
8.8
8.9
pH
Figure 4.1 Zeta potential of polystyrene media particles and silica sand particles in a
groundwater sample
Table 4.1
Polystyrene media size characteristics
Parameter
Effective size
Uniformity coefficient
Specific gravity
Result
0.66 mm
1.3
1.05
INVESTIGATIVE FIELD TESTING
The investigative field testing was conducted using the pilot-scale C/F unit at the Canyon
Filtration Plant in Azusa, CA. The testing was performed using finished water drawn from a distribution system reservoir as the source water to the pilot C/F system. No arsenic sampling was
performed during this phase of the study. The objectives of this testing were limited to establishing
the performance of the polystyrene media for removing ferric floc and identifying the backwash
procedures necessary to clean the media effectively between consecutive filter runs. The 45-inch
(114-cm) pilot filter column was filled with polystyrene beads to a depth of 24 inches (61 cm).
Chapter 4: Results and Discussion | 19
Table 4.2
Source water quality during investigative field testing
Parameter
Temperature
Turbidity
pH
Free chlorine
Iron
Alkalinity (as CaCO3)
Total dissolved solids
Unit
°C
NTU
mg/L
mg/L
mg/L
mg/L
Count
19
18
19
14
13
1
1
Average
20.0
0.19
7.4
0.32
0.05
130
250
Range
17.9–22.6
0.12–0.26
7.2–7.6
0.21–0.59
0.02–0.09
N/A
N/A
N/A = not applicable
Source Water Quality
The source water for the pilot C/F unit during the investigative field testing was drawn from
the distribution system. According to City operations staff, this water is predominantly chlorinated
groundwater. The pH, temperature, iron, turbidity and free chlorine were routinely measured in
the source water using portable equipment. In addition, one alkalinity and one TDS sample were
obtained from the source water. The source water quality during the investigative field testing is
summarized in Table 4.2.
Preliminary Determination of Backwash Requirements
The first task of the investigative field testing was to define the backwash conditions that
would effectively clean the polystyrene media without leading to excessive media loss. The initial
conditions for this task were established as follows:
1. A three-stage backwash regime was assumed with the Stage 1 and Stage 3 rates set at
one-half the Stage 2 rate
2. The Stage 2 backwash rate was set equal to that calculated using Equation 2.1 for the
polystyrene media
3. The duration for each stage was initially set equal to that implemented in previous
studies conducted using conventional anthracite-sand media.
During this test, the ferric chloride dose was held constant at approximately 4 mg/L as Fe,
and the filtration rate was held constant at 4 gpm/ft2 (9.8 m/hr). The backwash rates and duration
were then refined based on visual observation of backwashing in the clear PVC filter column. The
findings were as follows:
• Upon initiation of filter runs at the beginning of the pilot testing, the white polystyrene
media placed into the clear PVC filter column rapidly became stained a dark orange/
red color by the ferric chloride coagulant. However, a distinction could be made
between the clean yet stained media and the dark reddish color of the media bed with
accumulated ferric floc material. Further, accumulated dark-red floc particles were
Table 4.3
Backwash conditions for polystyrene media identified during preliminary
evaluation of backwash requirements
Backwash stage
Stage 1
Stage 2
Stage 3
Rest
•
•
•
•
•
Flow velocity
1 gpm/ft2 (2.4 m/hr)
2 scfm/ft2 (37 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
1.5 gpm/ft2 (3.7 m/hr)
0 gpm/ft2 (0 m/hr)
Duration
2 min
1 min
8 min
2 to 4 min
3 to 5 min
visible in the media bed when the filter needed backwashing or had been inadequately
backwashed.
Based on visual observation, a backwash rate of 2 gpm/ft2 (4.9 m/hr) was adequate to
fully fluidize the bed of polystyrene media, but insufficient to remove ferric floc from
the media. After backwashing at a rate of 2 gpm/ft2 (4.9 m/hr), the polystyrene media
in the clear PVC column remained a dark red color due to remaining ferric floc, and
accumulated floc particles were visible in the media bed.
While the polystyrene media was adequately fluidized at 2 gpm/ft2 (4.9 m/hr), a backwash rate of 3 gpm/ft2 (7.3 m/hr) was required to flush the ferric floc out of the filter. At this higher backwash rate, the media expansion was measured at 80 percent.
After backwashing at a rate of 3 gpm/ft2 (7.3 m/hr), no dark-red ferric floc particles
remained visible on the media.
Air scour of approximately 2 scfm/ft2 (37 m/hr) for a 1-minute period was required to
break up floc-media agglomerates into particles sufficiently small to be lifted by the
backwash flow.
A backwash screen was needed at the top of the filter to minimize media loss which
occurred during air scour. Without the backwash screen, large quantities of the polystyrene media were washed out the top of the filter during air scour.
A rest period of at least 3 minutes with no flow was required for the media to settle
completely and return to its pre-backwash bed depth.
From visual observation, it was apparent that air scour was needed to break up floc-media
agglomerates that formed during filtration. In general, these agglomerates had a diameter of 0.25 to
0.5 in (0.6 to 1.3 cm) at the beginning of backwash and, although they were reduced in size during
water-only backwashes, they were not completely removed and tended to remain near the bottom
of the fluidized media bed. An initial air scour during the first backwash stage, at a flow rate from
1 to 2 scfm/ft2 (18 to 37 m/hr) for 1 to 2 min, combined with a minimal backwash rate of 1 gpm/ft2
(2.4 m/hr), was found to be effective in breaking up the floc-media agglomerates. The backwash
conditions identified during this task are shown in Table 4.3.
It became apparent through observing repeated backwashes that the amount of free space
between the top of the fully-fluidized media and the top of the filter column was a factor in determining the volume of backwash water required. Minimizing the free space between the top of the
fully-fluidized media and the top of the filter column minimized any additional backwash water
needed to move the floc material out the top of the filter column. For purposes of this project, the
filter depth was set to the expanded depth of the media during backwashing (80% above resting
TOP VIEW
First and Second Outer Rings: 5.25"OD x
4.75"ID
Support Ring #1: 3.46"OD x 3.20"ID
Support Ring #2: 2.08"OD x 1.83"ID
Stainless steel mesh screen, USA 30 mesh,
0.6 mm opening
SIDE VIEW
Second Ring: 5.25"OD x 4.75"ID
from 3/8” thick P V C
Stainless steel mesh screen, USA 30 mesh
First Ring: 5.25"OD x 4.75"ID
from 3/8” thick P V C
Disc: 5.25" diameter x 3/8” thick P V C
Threaded hole (½” NT P ) located in center of disc
Figure 4.2 Diagram of backwash screen holder
depth of media) such that little to no space remained between the top of the expanded media and
the filter top.
A backwash screen was needed at the top of the filter column to prevent loss of the polystyrene media that occurred with air scour. A diagram of the backwash screen holder that was
constructed during this testing is presented in Figure 4.2. The screen holder was cut from a sheet
of 3⁄8-inch (0.7-cm) thick PVC. The size of the screen used to retain the media (USA #30 mesh,
0.6 mm opening) was selected based on the media size distribution shown above in Table 4.1,
in which 99 percent of the media was larger than 0.6 mm. The screen holder was screwed into a
0.5‑inch (1.3-cm) threaded hole in the filter column cover plate and effectively eliminated media
loss for the remainder of field testing. The mesh size was large enough to allow the ferric floc to
pass through the screen openings during backwash but small enough to retain the polystyrene
media during air scour.
Initial Evaluation of Polystyrene Media Performance for Removal of Ferric Floc
Further testing was conducted to determine the ability of the polystyrene media to remove
ferric floc over a range of filtration rates and coagulant doses. No evaluation of arsenic removal
was performed during this testing. The objectives of the testing were limited to determining if the
ultra-light polystyrene media could effectively remove ferric floc formed over a variety of coagulant doses at a variety of filtration rates. The operational conditions tested were:
• Coagulant doses of approximately 2, 4 and 6 mg/L as Fe (6, 12 and 18 mg/L as ferric
chloride)
• Filtration rates of 2, 4 and 8 gpm/ft2 (4.9, 9.8 and 19.6 m/hr)
Table 4.4
Filter run time (minutes) required to achieve filtered water
turbidity/iron goals during initial evaluation
Ferric chloride dose
(mg/L as Fe)
2
4
6
2 gpm/ft2
10/15
11/11
0/12
Filtration rate
4 gpm/ft2
0/20
4/15
8/12
8 gpm/ft2
10/15
8/12
0/10
Note: Minutes to turbidity goal of 0.1 NTU are displayed on the left and printed in bold italic.
The filtered water quality goals were established solely to evaluate the ability of the polystyrene filter media to remove ferric floc. For this reason, filtered water quality goals were limited
to evaluating the ability of the polystyrene media to remove turbidity and iron. The filtered water
quality goals for this testing were:
• Turbidity ≤ 0.1 NTU
• Iron ≤ 0.06 mg/L
The backwash conditions used were generally those determined from the previous testing
to determine backwash requirements and were shown above in Table 4.3. During some runs, the
Stage 3 backwash was extended by 1 minute based on the pilot operator’s visual evaluation of the
cleanliness of the media.
During this testing the filter effluent iron concentration and turbidity, and filter headloss,
were monitored only during the first 2 to 3 hr of the filter run conducted at each of nine conditions
simply to determine if the media was capable of achieving effective removal of ferric floc. If the
filtered water quality goals were not met during the first 2 to 3 hr of the filter run, then the filter run
was designated as unsuccessful. If the filtered water quality goals were met during the first 2 to 3 hr
of the filter run, then the testing objectives had been achieved (the polystyrene media would have
effectively removed the ferric floc) and there was no need to continue the filter run at that condition. Full filter runs evaluating headloss accumulation and breakthrough behavior were scheduled
for subsequent testing periods.
In all nine of the conditions tested, filtered water quality goals for both turbidity and iron
were met well before 2 hr of filter operation. The time, in minutes, required to reach the filtered
water turbidity and iron goals at each condition tested are shown in Table 4.4.
Table 4.4 shows the filtered water turbidity goal (0.1 NTU) was attained in 11 min or less at
all conditions tested. The filtered water iron goal (0.06 mg/L) was achieved in 20 min or less at all
conditions tested. These results demonstrate that the polystyrene media was capable of effectively
removing ferric floc.
An examination of the data in Table 4.4 shows a general trend of decreasing time to achieve
the iron goal with increasing ferric chloride dose at each of the filtration rates tested. Under several
of the test conditions, filtered water iron concentrations were below the method detection limit
of 0.01 mg/L within the 2-hr filter runs. Further, turbidity reached levels below 0.03 NTU during
the 2-hr filter runs. Overall, the polystyrene media was able to achieve filtered water goals over
the range of coagulant doses and filtration rates tested and the objectives of this set of tests were
achieved.
Profiles of the filter headloss, and filtered water turbidity and iron for the runs conducted
at a filtration rate of 4 gpm/ft2 (9.8 m/hr) are presented in Figure 4.3 as an example of the trends
observed at each test condition. Results from the filter run at a coagulant dose of 2 mg/L as Fe are
presented in the upper plot, with results from filter runs at coagulant doses of 4 mg/L and 6 mg/L
as Fe presented in the middle and lower plots, respectively.
Figure 4.3 illustrates the rapid decrease in filtered water iron concentrations from highs
between 0.2 and 0.5 mg/L, which occurred after approximately 2 min of run time, down to below
the detection limit of the method (0.01 mg/L Fe). Results below the detection limit for iron were
plotted at the detection limit of 0.01 mg/L. The headloss profiles in the figures illustrate the increase
in the rate of headloss accumulation with increasing coagulant dose (from 2 mg/L to 6 mg/L as Fe).
Repeated Filter Runs to Demonstrate Backwash Effectiveness
The next step in the evaluation of the backwash regime was to conduct repeated filter runs
at the same filtration rate and coagulant dose with the pilot C/F unit in fully automatic mode. In
fully automatic mode, the pilot C/F unit automatically terminated a filter run on one of the three
set points: maximum run time, terminal headloss, or turbidity breakthrough. The pre-programmed
backwash sequence was performed by the pilot C/F unit and then the unit would start a new filter run. The effectiveness of the backwash was assessed based on demonstrating repeated filter
runs with consistent run time and total headloss, indicating that floc material was not accumulating in the filter bed due to ineffective backwashing. The water quality objectives for this testing
included a filtered water turbidity goal of 0.07 NTU and filtered water iron concentration less than
0.06 mg/L after the initial filter maturation period.
The run termination, filtration and backwash conditions used during the repeated filter runs
are presented in Table 4.5. The maximum run time and maximum headloss criteria for run termination were set at arbitrarily high levels to ensure that the maximum turbidity criterion of 0.07 NTU
would be reached before the run time or headloss criteria. In this way, the filter runs could be
assessed for reproducibility without terminating based on arbitrary run time or headloss set points.
A total of six filter runs were conducted under these conditions, with all filter runs terminating due to turbidity breakthrough at 0.07 NTU. The filter run time and total headloss at the time of
turbidity breakthrough for the six filter runs are presented in Table 4.6. The filter run lengths and
total headloss were very consistent. Filter run time averaged 23.6 hours and ranged from 21.9 to
24.5 hours. Total headloss accumulation averaged 12.4 ft of H2O (37 kPa) and ranged from 11.7
to 12.8 ft of H2O (35 to 38 kPa). The consistency of the run lengths indicate that the ferric floc did
not accumulate in the filter media from run to run; therefore, the backwash conditions employed in
the testing were effective for cleaning the polystyrene media.
The filter effluent turbidity and headloss profiles for the six repeated filter runs illustrate the
consistent filter performance during repeated runs at the same operating conditions (Figure 4.4).
The headloss profiles of the six filter runs are almost on top of each other, with nearly identical
slopes. The upward trend in turbidity started within the same approximately 2- to 3-hr period in
all six filter runs.
As an example of filter performance during the entire filter run, the turbidity and headloss
profiles for the sixth repeated filter run (Run 528) are shown in Figure 4.5. After the first hour of
the filter run, turbidity was stable at 0.01 to 0.02 NTU for approximately 20 hr. Turbidity began to
climb above 0.02 NTU at a run time of approximately 21 hr and reached the breakthrough criterion
of 0.07 NTU at approximately 24 hr. Headloss accumulation had a generally linear trend over the
24-hr filter run.
Headloss (ft of H2O)
Run 510
FeCl 3 = 2 mg/L as Fe
Headloss
Effluent Turbidity
Effluent Iron
4
0.1
2
0
Iron (mg/L), Turbidity (NTU)
1
6
0.01
0.0
0.5
1.0
1.5
2.0
Run Time (hrs)
1
Run 508
FeCl 3 = 4 mg/L as Fe
Headloss
Effluent Turbidity
Effluent Iron
4
0.1
2
0
Iron (mg/L), Turbidity, (NTU)
6
0.01
0.0
0.5
1.0
1.5
2.0
Run Time (hrs)
Run 515
FeCl3 = 6 mg/L as Fe
Headloss
Effluent Turbidity
Effluent Iron
4
0.1
2
0
0.0
0.5
1.0
Run Time (hrs)
1
6
0.01
1.5
2.0
Figure 4.3 Filter headloss and filtered water turbidity and iron for runs conducted at
4 gpm/ft2 during initial evaluation
Table 4.5
Operating conditions during repeated filter runs to demonstrate backwash effectiveness
Filtration conditions
Filtration rate
Value
2 mg/L as Fe
Run termination criteria
Maximum time
Maximum headloss
Maximum turbidity
Value
48 hours
20 ft H2O (60 kPa)
0.07 NTU
Backwash procedure
Stage 1
Stage 2
Stage 3
Rest
Flow velocity
1.0 gpm/ft2 (2.4 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
1.5 gpm/ft2 (3.7 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
Duration
2 min
8 min
4 min
4 min
1 min
Table 4.6
Filter run time and total headloss for repeated filter runs
Run #
520
521
522
523
526
528
Average
Standard deviation
Run time
(hours)
23.0
24.5
21.9
23.6
24.1
24.3
23.6
1.0
Total headloss
(ft of H2O)
12.5
11.9
12.5
12.6
12.8
12.2
12.4
0.3
The first three hours of the same filter run are presented in Figure 4.6. Iron samples were
collected during the first 1.5 hr of the filter run. The filtered water iron goal of 0.06 mg/L was
met in the sample collected at 30 min. The filtered water turbidity was less than 0.1 NTU after
approximately 12 min. The results indicate that the polystyrene filter media was capable of meeting filtered water quality goals after repeated runs at the operating conditions listed in Table 4.5.
Feedwater Recovery During Repeated Filter Runs
Feedwater recovery is calculated as follows:
Feedwater Recovery = 100 # e
Vproduct − VBW
o
Vproduct
(4.1)
where Vproduct = volume of product water from a single filter run
VBW = volume of water used during a single backwash
10
20
Headloss
Effluent Turbidity
1
10
0.1
Turbidity (NTU)
15
5
0
0.01
0
6
12
18
24
30
Run Time (hrs)
Figure 4.4 Turbidity and headloss profiles for repeated filter runs conducted at ferric chloride
dose of 4 mg/L as Fe and filtration rate of 4 gpm/ft2
Headloss
Effluent Turbidity
Effluent Iron
14
12
1
10
8
6
0.1
4
10
16
2
0
0.01
0
6
12
18
24
Run Time (hrs)
Figure 4.5 Iron, turbidity, and headloss profile for the sixth repeated filter run (Run 528)
conducted at ferric chloride dose of 4 mg/L as Fe and filtration rate of 4 gpm/ft2
Headloss
Effluent Turbidity
Effluent Iron
14
12
1
10
8
6
0.1
4
10
16
2
0
0.01
0
1
2
3
Run Time (hrs)
Figure 4.6 Iron, turbidity, and headloss profile for first three hours of filter operation during
the sixth repeated filter run (Run 528)
Using the average run time of 23.6 hr from Table 4.6, the filtration rate of 4 gpm/ft2
(9.8 m/hr) and the total volume calculated from the backwash regime defined in Table 4.5, the
volume of water produced during a filter run averaged 5,664 gal/ft2 (230 m3/m2) and the backwash
volume was 34.4 gal/ft2 (1.4 m3/m2). Using these values, the average feedwater recovery during
the repeated filter runs was calculated to be 99.4 percent. The backwash volume of 34.4 gal/ft2
(1.4 m3/m2) is approximately 17 percent of a typical backwash volume of 200 gal/ft2 (8.1 m3/m2)
required for backwashing conventional anthracite-sand media. Thus, in addition to meeting the
filter effluent water quality goals, the polystyrene media required significantly less backwash water
to effectively clean the media.
FIELD DEMONSTRATION TESTING
Once the investigative field testing had identified effective backwash conditions for the
polystyrene filter media and demonstrated its ability to remove ferric floc effectively from groundwater, the next step of the study was to demonstrate the performance of the pilot C/F unit and the
polystyrene media under real-world operating conditions. For field demonstration testing, the pilot
C/F unit was moved to Well #3 in the high desert community of Helendale, CA. The arsenic concentration in this well water was historically approximately 21 μg/L. The San Bernardino County
Special Districts Department operates the well and had expressed interest in evaluating the C/F
process to reduce the arsenic concentration to below the new maximum contaminant level (MCL)
of 10 μg/L.
Table 4.7
Source water quality during field demonstration testing
Parameter
pH
Free chlorine
Iron
Temperature
Arsenic
Unit
mg/L
mg/L
°C
μg/L
Count
40
39
2
40
25
Average
7.7
0.89
0.012
23.0
20.7
Range
7.2–7.9
0.55–1.16
0.011–0.013
18.9–27.5
17.0–25.0
The objectives of this task were to validate the ability of the filter to remove arsenic to
below the MCL of 10 µg/L, and to demonstrate the reliable and consistent operation of the fullyautomated pilot C/F unit with polystyrene filter media over an extended period under field operating conditions. Thus, the filter unit was placed in fully automatic mode using chlorinated water
from Well #3 as the source water and was allowed to operate uninterrupted for approximately six
weeks using County operations staff.
Initial Start-Up
The filter unit was placed in a utility shed adjacent to the well and hydraulically connected.
The well water was chlorinated before being routed to the pilot C/F unit. The waste filtered water
was directed to the sanitary sewer and the waste backwash water was collected in a 200-gallon
(760-L) polyethylene tank. The polystyrene filter media used in the field demonstration testing was
the same as that used in the investigative field testing.
The first task was to determine the ferric chloride dose required to remove arsenic to a
concentration less than 10 μg/L. The filter was backwashed at the same conditions used during the
investigative field testing (Table 4.5), and a test was conducted to estimate the required coagulant
dose. The filter was started at a ferric chloride dose of 2 mg/L as Fe and was operated for 2 hr at
4 gpm/ft2 (9.8 m/hr). Raw and filtered water arsenic samples were collected, and then the ferric
chloride dose was reduced to 1.0 mg/L as Fe for 30 min, and then finally increased to 1.6 mg/L as
Fe for 30 min. Raw and filter effluent arsenic samples were collected at each of these coagulant
doses. The three raw water samples had arsenic concentrations of 22, 23, and 23 μg/L. The filter
effluent arsenic concentrations were 2.7 μg/L at a ferric chloride dose of 2 mg/L as Fe, 1.8 μg/L at
a dose of 1.0 mg/L as Fe, and 1.0 μg/L at a ferric chloride dose of 1.6 mg/L as Fe. Since all three of
the filter effluent arsenic concentrations were well below the arsenic MCL of 10 μg/L, a ferric chloride dose just below 1 mg/L as Fe was selected as the set point for the field demonstration testing.
The quality of the chlorinated groundwater used as the source water to the pilot C/F unit
during the field demonstration testing is shown in Table 4.7. Samples were collected and analyzed daily for pH, free chlorine and temperature. Arsenic samples were collected 3 times a week.
Arsenic levels in the source water ranged from 17 to 25 μg/L in the 25 samples collected. Iron
levels in the two samples collected were just above the method detection limit of 0.01 mg/L. Free
chlorine concentrations ranged from 0.5 to 1.2 mg/L during the field demonstration test period.
Table 4.8
Operating conditions during field demonstration testing
Filtration rate
Maximum time
Maximum headloss
Maximum turbidity
Backwash procedure
Stage 1
Stage 2
Stage 3
Rest
Value
0.95 mg/L as Fe
Value
48 hours
20 ft H2O (60 kPa)
0.07 NTU
Flow velocity
1.0 gpm/ft2 (2.4 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
1.5 gpm/ft2 (3.7 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
Duration
2 min
8 min
4 min
5 min
1 min
Field demonstration testing was performed during May and June, 2006; and the source water temperature increased over the duration of the testing period.
Field Demonstration Operating Conditions
The operating conditions during the six-week field demonstration testing period are shown
in Table 4.8. The backwash conditions were the same as those used during the previous investigative field testing with the exception that the duration of the rest period was increased to 5 min.
The maximum run time of 48 hr and maximum headloss of 20 ft of H2O (60 kPa) were set to
these relatively high levels so the filter runs would terminate at the filter effluent turbidity set
point of 0.07 NTU. The filter column height of 45 in (114 cm) and polystyrene media depth of
24 in (61 cm) were also the same as during the investigative field testing. The ferric chloride dose
of 0.95 mg/L as Fe was based on testing performed during system start-up that showed a dose of
1.0 mg/L as Fe was more than sufficient to produce filtered water arsenic concentrations below the
Federal MCL of 10 μg/L.
Performance of Pilot C/F Unit With Polystyrene Filter Media
The goal of the field demonstration testing was to demonstrate that the pilot C/F unit could
reliably and effectively remove arsenic using the polystyrene media under real-world conditions.
The pilot C/F unit was operated in fully automatic mode.
Steady-State Performance of Pilot Filtration Unit With Ultra-Light Polystyrene Media
The steady-state performance of the pilot filtration unit is summarized in Figure 4.7. The
figure illustrates source water and filter effluent arsenic concentrations, and filter effluent turbidity
and iron levels during the six-week field demonstration period.
1.5
30
Effluent Turbidity
Effluent Iron
Effluent Arsenic
Influent Arsenic
1.0
20
Arsenic MCL
0.5
10
0.0
Arsenic (ug/L)
Turbidity (NTU), Iron (mg/L)
0
0
200
400
600
800
1000
Cumulative Operating Time (hrs)
Figure 4.7 Steady-state performance of the pilot C/F unit during field demonstration testing
conducted at a target ferric chloride dose of 0.95 mg/L as Fe and filtration rate of 4 gpm/ft2
The average iron concentration in the filter effluent was approximately 0.07 mg/L, and filter effluent turbidity averaged approximately 0.05 NTU. These results indicate that the polystyrene
media reliably and effectively removed the floc material created by ferric chloride coagulation.
The source water arsenic concentration fluctuated between 17 and 25 µg/L, while the arsenic concentration in the filter effluent averaged approximately 6 µg/L. No arsenic sample had a
concentration exceeding the MCL of 10 µg/L. The results in Figure 4.7 demonstrate that the filter
consistently removed arsenic over a 6-week period while operating in a fully-automated mode.
Filter Operation
A total of 25 filter runs were completed during the 43-day field demonstration testing
period. The filter run start time, clean media headloss, run length, filter influent iron concentration, maximum headloss and filter effluent turbidity at the end of each run during the field demonstration testing are presented in Table 4.9. The average clean media headloss was 0.8 ft of H2O
(2.4 kPa), with a standard deviation of 0.2 ft of H2O (0.7 kPa). Twenty-four of the 25 filter runs
terminated upon reaching the maximum effluent turbidity set point of 0.07 NTU. One run was
manually terminated early for reasons discussed below. The average run length for the 25 filter
runs was 38.8 hours, with a standard deviation of 6.7 hours. The average headloss at the time each
filter run terminated due to turbidity breakthrough was 8.6 ft of H2O (26 kPa), with standard deviation of 1.1 ft of H2O (3.3 kPa).
Figure 4.8 illustrates the filter run length and total headloss for each of the 24 filter runs
performed during field demonstration testing that terminated due to turbidity breakthrough. The
Table 4.9
Summary of filter run results from field demonstration testing
Run
565
566
567
568
569
570
571
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
Clean media
Run start date
headloss
and start time
(ft of H2O)
0.3
5/11/2006 13:30
5/13/2006 7:12
0.5
5/15/2006 1:35
0.9
5/16/2006 23:55
0.8
5/18/2006 19:37
1.0
5/20/2006 15:13
0.8
5/22/2006 6:24
0.7
5/24/2006 18:16
0.9
5/26/2006 2:01
0.8
5/27/2006 21:12
0.7
5/29/2006 14:58
1.1
5/31/2006 15:19
0.9
6/2/2006 9:37
0.9
6/4/2006 2:21
0.7
6/5/2006 19:11
1.2
6/7/2006 6:35
0.9
6/8/2006 14:12
1.1
6/9/2006 23:04
0.7
6/11/2006 7:07
0.8
6/12/2006 15:19
1.0
6/13/2006 11:39
0.6
6/15/2006 4:18
1.0
6/16/2006 21:50
0.7
6/18/2006 19:49
0.8
6/20/2006 18:34
1.3
Average†
0.84
Standard deviation†
0.22
Run length
(hr)
41.3
42.0
46.0
43.3
43.2
38.8
43.5
31.3
42.8
41.3
48.0
41.8
40.3
40.5
35.0
31.2
32.5
31.7
31.8
20.0
40.2
41.2
45.5
46.3
40.8
38.8
6.7
Average
Headloss at
influent iron end of run
(mg/L)*
(ft of H2O)
0.97 (3)
9.3
0.94 (2)
9.0
0.91 (2)
10.6
0.96 (2)
9.3
0.85 (2)
9.1
0.93 (1)
9.1
0.94 (2)
8.9
0.98 (2)
9.3
0.97 (2)
9.0
0.88 (2)
9.1
0.92 (1)
9.8
0.87 (2)
8.8
0.96 (2)
9.4
0.95 (2)
9.5
1.03 (2)
8.6
1.19 (1)
7.5
No data
8.1
1.08 (1)
7.5
1.08 (2)
7.1
No data
5.1
No data
7.3
0.89 (2)
7.5
0.74 (1)
8.7
0.76 (2)
8.8
0.85 (2)
8.1
8.6
1.1
Turbidity at
end of run
(NTU)
0.07
0.06
0.07
0.07
0.06
0.06
0.07
0.07
0.07
0.06
0.07
0.07
0.07
0.07
0.07
0.05
0.07
0.07
0.07
0.03
0.07
0.07
0.07
0.07
0.06
*Number next to average influent iron values is the count of measurements during the filter run.
†Excludes Run 587, which was terminated manually before turbidity breakthrough.
figure indicates that there was no trend of decreasing filter run length or increasing headloss at
the time of turbidity breakthrough that would suggest inadequate backwashing of the polystyrene
media during the 24 filter runs.
Run 587 was manually terminated at 20 hours of run time to investigate potential reasons
for shorter run lengths observed during Runs 582 through 586. The length of previous filter runs
had reached or exceeded 40 hours fairly consistently. It was found that the shorter run lengths were
due to a slightly increased coagulant dose. The ferric chloride dose during Runs 582 through 586
had increased to the range 1.1 mg/L to 1.2 mg/L as Fe from the previous level of 0.85 mg/L to
1.0 mg/L as Fe. The higher ferric chloride dose was believed to be due to preparation of a slightly
stronger batch of ferric chloride feed solution by the pilot plant operator. When the iron dose was
adjusted back to previous levels by making an adjustment to the system PLC, filter run lengths
once again exceeded 40 hours.
Run Length
Headloss
50
20
15
40
Elevated Iron
30
10
20
60
Run Length (hrs)
5
10
0
565
566
567
568
569
570
571
575
576
577
578
579
580
581
582
583
584
585
586
588
589
590
591
592
0
Filter Run Number
Figure 4.8 Filter run time and total headloss for 24 full filter runs performed during field
demonstration testing conducted at a target ferric chloride dose of 0.95 mg/L as Fe and
filtration rate of 4 gpm/ft2
There was some correlation between the length of each filter run and the average iron concentration measured in the filter influent. Figure 4.9 shows the relationship between the measured
filter influent iron concentration and filter run length, using the data presented in Table 4.9. A relationship is apparent between the two parameters. This relationship was expected because the filter
runs were allowed to terminate on turbidity breakthrough. A higher iron dose led to the formation
of more floc material being applied to the filter. The higher solids loading caused the solids retention capacity of the filter to be reached more rapidly, thereby leading to a shorter time to turbidity
breakthrough and a shorter filter run. However, there was no correlation (R2 = 0.14) between the
filter influent iron concentration and the total headloss measured at the time the run terminated due
to turbidity breakthrough.
Filter Turbidity and Headloss Profiles
The filter effluent turbidity and filter headloss profiles for three of the 25 filter runs conducted during the field demonstration testing (Runs 566, 583 and 591) are presented in Figures 4.10
through 4.12. These data were collected automatically by the system PLC every 3 minutes for the
first 30 minutes of each filter run and every 10 minutes thereafter. The results shown in the figures
were selected from filter runs performed during the beginning, middle and near the end of the field
demonstration testing period, and represent the range of coagulant doses added.
Each of the three figures shows turbidity decreasing rapidly from an initial peak between
0.2 to 0.3 NTU to less than 0.1 NTU within 20 min of run time. The turbidity during the stable
60
Run Length (hr)
50
40
30
Run Length (hrs) = -38.5 * Influent Fe (mg/L) + 76.5
2
20
R = 0.5939
10
0
0.5
0.7
0.9
1.1
1.3
1.5
Filter Influent Iron (mg/L)
Figure 4.9 Relationship between measured influent iron concentration and filter run length
during field demonstration testing
1
15
Headloss
10
0.1
5
0
Turbidity (NTU)
Effluent Turbidity
0.01
0
6
12
18
24
30
36
42
48
Run Time (hr)
Figure 4.10 Filter turbidity and headloss profile from Run 566 conducted at an average Fe
dose of 0.9 mg/L and a filtration rate of 4 gpm/ft2
15
1
Headloss
10
0.1
5
0
Turbidity (NTU)
Effluent Turbidity
0.01
0
6
12
18
24
30
36
42
48
Run Time (hrs)
15
1
Headloss
10
0.1
5
0
Turbidity (NTU)
Effluent Turbidity
0.01
0
6
12
18
24
30
36
42
48
Run Time (hr)
1
Effluent Arsenic
Effluent Turbidity
Effluent Iron
Arsenic (µg/L)
25
20
15
0.1
10
30
5
0
0.01
0
1
2
3
Run Time (hrs)
Figure 4.13 Turbidity, iron, and arsenic profiles during filter maturation from Run 583
conducted at an average ferric chloride dose of 1.2 mg/L as Fe and filtration rate of 4 gpm/ft2
portion of the filter runs was consistently 0.02 or 0.03 NTU. The turbidity began to increase a number of hours before finally reaching the turbidity breakthrough criterion of 0.07 NTU.
Filter headloss consistently increased over the course of each of the three filter runs from
a clean bed headloss of 0.7 ft of H2O (2 kPa) at the beginning of the filter run to between 7.5 and
9.0 ft of H2O (22 to 27 kPa) at the time the run terminated due to turbidity breakthrough. In general, headloss accumulation followed a roughly linear trend.
Filter Maturation and Breakthrough
Filter maturation refers to the period of time at the beginning of a filter run when the
residual turbidity remaining in the filter media at the end of backwashing is released into the filter
effluent. This typically translates into a spike in filter effluent turbidity lasting for 10 to 30 min. In
surface water treatment, this phenomenon is of concern because the turbidity spike could include
pathogenic microorganisms. However, for groundwater treatment containing no microorganisms
of concern, and since the contaminant being removed involves chronic, not acute, public health
concern, filter maturation is not of concern. Nevertheless, for the interest of this study, filter maturation was monitored in detail during one of the field demonstration runs. During Run 583 (the
16th of 26 filter runs), iron and arsenic were monitored in the filter effluent for the initial 3-hr
period of the filter run. Turbidity was monitored by on-line turbidimeter every 3 min during the
first 30 min, and then every 10 min thereafter. The results are presented in Figure 4.13. Arsenic
concentration is shown on the left vertical axis of the chart and iron and turbidity are shown on
the right vertical axis. Turbidity, dissolved iron, and arsenic all peaked at approximately 4 minutes
Table 4.10
Arsenic, iron, and turbidity levels near run termination of three filter runs
performed during field demonstration test
Run
581
591
592
Filter effluent
turbidity
(NTU)
0.06
0.05
0.06
Filter effluent
iron
(mg/L)
0.20
0.11
0.06
Filter effluent
arsenic
(μg/L)
9.8
9.2
8.3
Run time at sample
collection
(hr)
39.6
40.5
40.3
Run
length
(hr)
40.5
46.3
40.8
after filter run initiation. Filter effluent arsenic rapidly decreased from a high of 20 μg/L, encountered at 4 min into the filter run, to less than 10 μg/L at approximately 10 min into the filter run.
Since the filter was backwashed with chlorinated untreated well water, the highest filter effluent
arsenic concentration of 20 μg/L corresponded well with the concentration of arsenic in the raw
water, which was measured at 18 μg/L on that day. Filter effluent iron reached its maximum concentration of 0.77 mg/L at 4 min, which is the same time that the arsenic peak occurred. After
the initial peak, iron concentrations also rapidly decreased, reaching 0.3 mg/L after 8 min and
0.1 mg/L after 20 min.
During field demonstration testing, the filter was intentionally set to backwash at a turbidity breakthrough of only 0.07 NTU, which did not allow for detailed tracking of arsenic
breakthrough. Nevertheless, data from samples collected during routine monitoring suggest that
turbidity breakthrough was accompanied by slight breakthrough of iron and arsenic. Results are
shown in Table 4.10, which show filter effluent turbidity, iron and arsenic data for three routine
monitoring samples collected near the end of Runs 581, 591 and 592. All three of these filter runs
terminated on turbidity breakthrough after run times exceeding 40 hours. It should be noted that
pilot plant operations staff visited the pilot plant once a day to collect samples and check on pilot
plant operation; their visits to the pilot plant site did not correspond to any specific time relative to the filter run times. The data shown in Table 4.10 were collected during these daily visits,
when the visits happened to coincide with times near the end of the three filter runs. Filter effluent
turbidity of the three samples all exceeded 0.05 NTU, indicating that the run was approaching
termination. Iron concentrations in the three samples ranged from 0.06 mg/L to 0.20 mg/L, which
are elevated compared to the iron levels near the method detection limit of 0.01 mg/L reached
after the first 3 hours of filter operation (see Figure 4.13). Filter effluent arsenic concentrations
ranged from 8.3 μg/L to 9.8 μg/L in the three samples. The samples for Runs 581 and 592 were
collected less than 1 hour before run termination. The sample for Run 591 was collected within 6
hours of run termination. The data in Table 4.10 suggest that turbidity breakthrough corresponded
to breakthrough of iron and arsenic. This result is expected since all three parameters (turbidity,
iron and arsenic) are incorporated into the floc material collected in the pilot plant filter. When
the filter’s solids retention capacity was approached, breakthrough of turbidity, iron and arsenic
would be expected to occur simultaneously. These results also support the findings reported by
WQTS (2004) from the study discussed in Section 2.
Table 4.11
Filtered water quality during field demonstration testing
Parameter
pH
Turbidity (on-line)
Free chlorine
Iron
Arsenic
Unit
NTU
mg/L
mg/L
μg/L
Count
38
41
39
42
26
Average
7.6
0.03
0.81
0.05
5.7
Range
7.2–7.8
0.02–0.13
0.45–1.1
<0.01–0.35
3.9–9.8
Water Quality
Filtered Water Quality
A number of water quality parameters were monitored on a daily basis to ensure proper
operation of the C/F process. The results for water quality parameters measured on site as well
as the results for filter effluent arsenic samples analyzed at a commercial laboratory are presented
in Table 4.11. Iron averaged 0.05 mg/L in the filter effluent and ranged from <0.01 mg/L up to
0.35 mg/L. The iron result of 0.35 mg/L was measured from a sample collected approximately
5 minutes before the end of a filter run with a filter effluent turbidity of 0.06 NTU. The highest filter
effluent turbidity during routine monitoring (0.13 NTU) was observed during filter maturation and
occurred 4 minutes after the start of the first filter run.
Arsenic in the raw water averaged 20.7 μg/L and ranged from 17 μg/L to 25 μg/L (see
Table 4.7). Arsenic in the filter effluent averaged 5.7 μg/L and ranged from 3.9 μg/L to 9.8 μg/L.
The highest filter effluent arsenic result of 9.8 μg/L was obtained 39 hours and 40 minutes into a
filter run, when the filter effluent turbidity was 0.06 NTU. This filter run terminated 50 minutes
later at the turbidity breakthrough set point of 0.07 NTU.
In addition to daily arsenic sampling from the source water and filter effluent, single samples were collected from the raw source water, filter effluent and backwash water and submitted
to MWH Laboratories (Monrovia, CA) for analysis of general physical parameters, minerals and
metals.
The backwash samples were produced by collecting four routine backwashes in a 25gallon (95-L) polyethylene tank. Three different samples of the backwash water were prepared
and collected for analysis from this tank. First, a composite backwash sample was collected by
placing the tank on a large magnetic stir plate and using a stir bar to mix the contents. The sample
bottles were filled from the tank using a peristaltic pump while the tank was being mixed. Second,
a decant backwash sample was prepared by turning off the stir plate and allowing solids in the tank
to settle for 1 hour. The decant sample bottles were filled using the peristaltic pump. Finally, the
intake tubing from the peristaltic pump was placed into the layer of sludge at the bottom of the tank
and sludge sample bottles were filled. The results from these samples are presented in Table 4.12.
The composite backwash sample represents the type of water that would be discharged
to an on-site septic system for most very small systems that are potential users of the C/F process with polystyrene filter media. The expected arsenic concentration in the composite backwash
Table 4.12
General physical, mineral, and metals during field demonstration testing
Unit
Raw
water
Filter
effluent
General Physical Analyses
Apparent Color
Odor
Turbidity
TSS
PCU
TON
NTU
mg/L
<3
2
0.15
<10
<3
1
0.05
<10
Mineral Analyses
Alkalinity
Calcium, Total
Chloride
Copper, Total
Fluoride
Hardness, Total
Iron, Total
Langelier Index
Magnesium, Total
Manganese, Total
Nitrate (as N)
Nitrite (as N)
pH, Lab pH
Potassium, Total
Silica
Sodium, Total
Specific Conductance
Sulfate
Surfactants
Total Dissolved Solids
Tot. Phosphorus-P
mg/L
mg/L
mg/L
μg/L
mg/L
mg/L
mg/L
None
mg/L
μg/L
mg/L
mg/L
Unit
mg/L
mg/L
mg/L
µmhos/cm
mg/L
mg/L
mg/L
mg/L
127
64
120
2.6
1.3
173
<0.020
0.70
3.2
<2.0
2.3
<0.50
8.1
3.2
36
180
1,190
240
<0.50
748
<0.010
138
64
120
2.5
1.4
173
0.067
0.70
3.3
<2.0
2.3
<0.50
8.1
3.2
36
180
1,200
240
<0.050
748
<0.010
Metals Analyses
Aluminum, Total
Antimony, Total
Arsenic, Total
Barium, Total
Beryllium, Total
Cadmium, Total
Chromium, Total
Lead, Total
Mercury
Nickel, Total
Selenium, Total
Silver, Total
Thallium, Total
Uranium
Vanadium, Total
Zinc, Total
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
<25
<1.0
21
29
<1.0
<0.50
5.7
<0.50
0.207
<5.0
<5.0
<0.50
<1.0
11
16
<5.0
<25
<1.0
6.5
28
<1.0
<0.50
5.9
<0.50
<0.20
<5.0
<5.0
<0.50
<1.0
11
7.9
<5.0
Special Analyses
BOD
COD
mg/L
mg/L
Backwash
composite
1,410
Backwash
decant
10
1
0.55
<10
1.3
144
69
130
13
2.1
187
0.83
0.60
3.6
2.9
2.4
<0.50
8.0
3.5
29
200
1,240
250
<0.050
756
<0.010
250
<10
4,100
290
<10
<5
75
100
3.9
<50
<50
<5.0
<10
26
2,300
1,100
<25
<1.0
24
26
<1.0
<0.50
7.7
0.69
<0.20
<5.0
<5.0
<0.50
1.6
12
13
5
2,600
270
940
180
Backwash
sludge
1,940
1,219
1,300
4,600
400
0.17
1,100
<10
22,000
1,300
<10
<5
300
460
18
<500
<50
<5.0
<10
67
1,107
5,400
<3
<100
sample can be calculated based on the volume of water filtered, the volume of backwash water
produced in one backwash, and the arsenic concentrations in the raw water and filter effluent.
Assuming a 40-hour filter run time at a filtration rate of 4 gpm/ft2 (9.8 m/hr), the volume of water
filtered was 9,600 gal/ft2 (390 m3/m2). The volume of backwash water was calculated at 34.4 gal/
ft2 (1.4 m3/m2) based on the backwash procedure used during the field demonstration testing (see
Table 4.8). The volume filtered during a filter run divided by the backwash volume results in a
concentration factor of 280. The raw water and filter effluent arsenic concentrations indicate that
14.5 μg/L of arsenic was removed by the C/F process. A concentration of 14.5 μg/L multiplied by
the concentration factor of 280 results in a calculated composite backwash arsenic concentration
of 4,060 μg/L. This calculated arsenic concentration compares well with the measured composite
backwash arsenic concentration of 4,100 μg/L.
The elevated levels of constituents measured in the composite backwash water have two
possible sources: they were either removed from the raw water, or they were present in the ferric
chloride solution and concentrated onto the filter media along with the iron. The constituents that
were clearly removed from the raw water and concentrated in the backwash water include arsenic,
vanadium, and barium. This was determined by recognizing that their levels in the backwash water
are approximately 280 times the difference between the raw water and filtered water levels. The
constituents that clearly originated from the ferric chloride solution include iron and manganese. It
is well known that manganese is an impurity in commercial ferric chloride solutions. However, it
is not clear whether several constituents with elevated levels in the backwash water were removed
from the raw water or originated from the ferric chloride solution since their levels could not be
clearly accounted for by their reduction in the raw water levels. These include copper, silica, phosphate, aluminum, chromium, lead, mercury, zinc, and uranium.
Table 4.12 also includes the composition of the backwash decant sample. The decant sample represents the backwash waste water after most of the ferric sludge has settled. The table
shows that the decant water had a composition very similar to that of the raw water. The similarity between the raw water and the decant water offers the possibility of recovering and retreating
the backwash decant water, trading additional system complexity against minimizing waste flows
from the treatment system.
Feedwater Recovery
The feedwater recovery of the process was calculated using the average filter run length
of 38.8 hr during the field demonstration testing (see Table 4.9), the filtration rate of 4 gpm/ft2
(9.8 m/hr) and the measured backwash volume. The average filter run length was calculated
excluding results from Run 587, which was terminated prematurely as discussed above. The backwash volume used in this calculation was determined by collecting the backwash water from
one complete backwash in buckets and measuring the volume using a graduated cylinder. The
measured unit backwash volume was 30.8 gal/ft2 (1.3 m3/m2). This volume is lower than the
backwash volume calculated based on the backwash flow rates and times as defined in Table 4.7
(34.4 gal/ft2 or 1.4 m3/m2). This discrepancy is due to the fact that the valve required about 30 seconds to fully open before the first stage flow rate was reached, and had some delay before the flow
rate changed from one stage to another. Therefore, the actual backwash water volume was less
than the calculated volume, which assumed instantaneous change in flow rate between stages.
Based on these values, the feedwater recovery was calculated using Equation 4.1 as follows:
• Average unit volume of filtered water produced = 9,312 gal/ft2 (379 m3/m2)
• Unit backwash volume = 30.8 gal/ft2 (1.3 m3/m2)
• Feedwater recovery = 100 × (9,312 – 30.8)/9,312 = 99.7%
LONG-TERM STEADY-STATE OPERATION
After completion of the field demonstration testing, the next phase of the project involved
steady-state operation of the pilot C/F unit with ultra-light media for a period of 4 months. The
pilot C/F unit was located at the Back Basin Groundwater Treatment Plant (BBGWTP) of the
Elsinore Valley Municipal Water District (District). The BBGWTP is a full-scale treatment plant
employing the C/F process with conventional granular media (sand/anthracite) pressure filters to
treat two wells (Cereal Wells 3 and 4) with a combined capacity of 2,800 gpm (635 m3/hr) containing between 20 and 40 μg/L of arsenic.
The primary objective of the long-term steady-state operation was to demonstrate the
successful operation of the fully-automated C/F process with ultra-light filter media for arsenic
removal from groundwater for a period of 4 months to validate the effective and reliable long-term
performance of the media and the ease of operation of the fully-automated C/F process. Additional
objectives included refining the backwash regime for the ultra-light media by determining the
necessity for air scour, and evaluating the use of treated water for backwashing. The 4 months of
steady-state operation was performed between May and September 2009 using new polystyrene
bead media from the same batch as that used for the field demonstration testing.
The quality of the chlorinated groundwater used as the source water to the pilot C/F unit is
summarized in Table 4.13 for the water quality parameters measured routinely at the pilot plant.
While the quality of Wells 3 and 4 was similar with respect to temperature and the added free chlorine, the pH and arsenic concentration of Well 3 were both higher than that of Well 4. pH averaged
9.4 in Well 3 and 9.1 in Well 4 during the pilot testing. Arsenic concentrations averaged 37 µg/L in
Well 3 and 25 µg/L in Well 4. When the two wells were both operating, the arsenic concentration
of the resulting source water averaged 30 µg/L during the pilot testing.
Long-Term Steady-State Operating Conditions
The pilot C/F unit was operated with 30 inches (76 cm) of polystyrene media in the filter
column that was 60 inches (152 cm) deep. Operating conditions for the long-term steady-state testing are summarized in Table 4.14.
Coagulant dosing in the pilot C/F unit initially was based on that of the full-scale BBGWTP.
The dose of ferric chloride depended on which of the two wells was in service. Figure 4.14 shows
that both Wells 3 and 4 were in operation during the majority of the pilot testing period; however,
there were brief intermittent periods of time during which either Well 3 or Well 4 was operated
individually. When Well 3 was in service, either individually or with Well 4 also in service, the
Table 4.13
Source water quality during steady-state operation (May through November 2009)
Parameter
Well 3
pH
Temperature
Free Chlorine
Arsenic
Well 4
pH
Temperature
Free Chlorine
Arsenic
Wells 3 and 4
pH
Temperature
Free Chlorine
Arsenic
Iron
Unit
Count
Average
Standard
deviation
Minimum
Maximum
°C
mg/L
µg/L
15
15
15
16
9.4
27.2
2.52
37
0.1
0.7
0.21
2
9.0
25.6
2.04
33
9.6
28.2
2.88
39
°C
mg/L
µg/L
11
11
11
11
9.1
26.6
2.47
25
0.2
0.7
0.23
7
8.9
24.9
2.10
12
9.3
27.3
2.86
32
°C
mg/L
µg/L
mg/L
43
43
43
43
2
9.3
27.4
2.53
30
<0.02
0.1
0.5
0.28
2
0
9.1
26.4
1.62
25
<0.02
9.6
28.9
3.14
34
<0.02
Table 4.14
Operating conditions during long-term steady-state operation
Filtration rate
Maximum time
Maximum headloss
Maximum turbidity
Value
4.1–6.9 mg/L as Fe
Value
48 hr
20 ft H2O (60 kPa)
0.1 NTU
ferric chloride dose used in the full-scale BBGWTP was 20 mg/L (6.9 mg/L as Fe). When Well 3
was not in service, the ferric chloride dose used in the full-scale plant was 12 mg/L (4.1 mg/L as
Fe). For the first 6 weeks of steady-state pilot plant operation, the pilot plant was operated using
the same coagulant dosing approach as the full-scale plant. After 6 weeks of steady-state pilot plant
operation, the pilot plant was operated with a stable ferric chloride dose of 15 mg/L (5.2 mg/L as
Fe) regardless of which of the two wells was in service.
Since the goal of long-term steady-state testing of the ultra-light media was to determine
its performance over time, the criteria for a properly-cleaned filter for purposes of this study were
defined as:
1. No trend of increasing clean media headloss in successive filter runs
2. No trend of increasing headloss accumulation rate in successive filter runs
3. No trend of increasing filter effluent turbidity during the stable operating portion of
successive filter runs.
5000
4500
4000
Flow Rate (gpm)
3500
Wells 3&4 (2800 gpm)
3000
2500
2000
Well 4 (1600 gpm)
1500
Well 3 (1370 gpm)
1000
500
0
1-May
31-May
30-Jun
30-Jul
29-Aug
28-Sep
28-Oct
27-Nov
Date
Figure 4.14 Wells in service during long-term steady-state testing at Elsinore Valley
Municipal Water District, CA
Table 4.15
Backwash regime for initial testing of polystyrene media without air scour
Backwash procedure
Stage 1
Stage 2
Stage 3
Rest
Air scour
Flow velocity
2.0 gpm/ft2 (4.9 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
1.0 gpm/ft2 (2.4 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
None
Duration
2 min
10 min
1 min
5 min
Need for Air Scour
Before initiating long-term steady-state operation, an evaluation of the need for air scour
was conducted in the pilot C/F unit for the polystyrene media. Initial backwash conditions for the
operation of the polystyrene media without air scour are shown in Table 4.15.
The fully-automated pilot C/F unit was operated at the conditions listed in Table 4.15 for
one week. During this time, 22 filter runs were conducted. An example turbidity and headloss profile for one of the 22 filter runs is shown in Figure 4.15.
The results illustrated in Figure 4.15 indicate that there was a large increase in headloss
from 1.7 to 8.6 ft of H2O (5.1 to 26 kPa) within minutes of run initiation, and terminal headloss
of 20 ft of H2O (60 kPa) was reached after only 2.2 hr of run time. At that time, the filter effluent
25
1.00
Headloss
15
0.10
Turbidity
10
Turbidity (NTU)
20
Immediate headloss increase
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.15 Turbidity and headloss profile for example filter run (Run 738) using polystyrene
media without air scour
turbidity was 0.06 NTU, suggesting imminent turbidity breakthrough as well. Thus, the filter
performance was entirely unacceptable during this filter run without air scour.
For the 22 filter runs conducted without air scour, the average filter run length was only
4.1 hr, with several of the filter runs lasting less than 3 hr. The average filter effluent turbidity
during the stable operating portion of the 22 filter runs was 0.08 NTU and the average headloss
at run termination was 17.7 ft H2O (53 kPa). These results indicate that the polystyrene media
produced unacceptably short filter runs when backwashed without air scour.
A filter backwash cycle was observed for one of the 22 filter runs. During the high-rate
(Stage 2) portion of the backwash cycle, it was observed that the media bed moved upward in
the pilot filter column as a plug to the top of the column, where it remained during the rest of the
backwash cycle, including during the rest period. Once filtration was resumed after completion
of the backwash cycle, the media plug moved down to the bottom of the filter column after
approximately 5 min. Following this backwash cycle, the clean media headloss exceeded 7 ft H2O
(21 kPa).
After 1 week of operation without air scour, the polystyrene media in the pilot filter column was backwashed with vigorous air scour to break up the mudballs that had formed during
operation without air scour. New backwash conditions were implemented that included air scour
as shown in Table 4.16.
During one week of operation using the polystyrene media with air scour, a total of 11 filter runs were conducted by the fully-automated pilot C/F unit. An example turbidity and headloss
profile for one of the 11 filter runs is shown in Figure 4.16. This filter run (Run 774) terminated
due to turbidity breakthrough at a filter run length of 12.7 hr. At that time, the headloss was 18.6 ft
Table 4.16
Backwash regime for initial testing of polystyrene media with air scour
Backwash procedure
Stage 1
Stage 2
Stage 3
Rest
Flow velocity
2.0 gpm/ft2 (4.9 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
None
0.0 gpm/ft2 (0.0 m/hr)
2.0 scfm/ft2 (37 m/hr)
Duration
3 min
10 min
5 min
2 min
25
1.00
15
0.10
Headloss
10
Turbidity (NTU)
20
Turbidity
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.16 Turbidity and headloss profile for example filter run (Run 774) using polystyrene
media with air scour
of H2O (56 kPa), indicating that turbidity breakthrough was occurring as the terminal headloss of
20 ft of H2O (60 kPa) was approached.
For the 11 runs performed using the polystyrene media with air scour, the average filter run
length was 11.8 hr, the average filter effluent turbidity was 0.10 NTU at run termination, and the
average headloss at run termination was 16.6 ft of H2O (50 kPa). A summary of results comparing
filter performance with and without air scour are shown in Table 4.17.
The results shown in Table 4.17 indicate that when the backwash regime included air scour,
the polystyrene media produced consistent and acceptable filter performance, with filter runs averaging approximately 12 hr and terminating due to turbidity breakthrough at a headloss of approximately 17 ft of H2O (51 kPa). The turbidity during the stable operating portion of the filter runs
was 0.03 NTU when the backwash regime included air scour. Without air scour, the results of the
initial testing indicate that the very low backwash velocities necessary to fluidize the ultra-light
Table 4.17
Comparison of results from initial testing of polystyrene media
with and without air scour
Filter performance characteristic
Filter runs performed in 1 week
Average filter run length
Average filter effluent turbidity during
stable portion of run
Average turbidity at run termination
Headloss at run termination
Without air scour
22
4.1 hr
0.06 NTU
With air scour
11
11.8 hr
0.03 NTU
0.08 NTU
17.7 ft of H2O
0.10 NTU
16.6 ft of H2O
Table 4.18
Backwash regime for long-term steady-state testing of polystyrene media
Backwash procedure
Stage 1
Stage 2
Rest
Flow velocity
2.0 gpm/ft2 (4.9 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
Duration
3 min
10 min
5 min
2 min
polystyrene media provided insufficient energy to properly clean the accumulated ferric floc material from the media. Air scour was found to be necessary for cleaning the media to the extent that
filter performance was acceptable in the subsequent filter run. Therefore, it was concluded that the
long-term steady-state testing should be performed with air scour included in the backwash regime
for the polystyrene filter media.
Performance of Pilot C/F Unit With Polystyrene Filter Media
The four months of long-term steady-state testing was performed from May 20 to
September 14, 2009. During this time, the pilot C/F unit was operated using the conditions shown
above in Table 4.14. Based on results from the two weeks of initial testing to determine the need
for air scour, the backwash regime shown in Table 4.18 was identified for long-term steady-state
testing of the polystyrene media.
As experienced in the operation of any water treatment facility, operating conditions were
not constant during the four months of operation for long-term steady-state testing. Both Wells 3
and 4 were in service most of the time, but one or the other well alone was in service for intermittent periods of time throughout the testing period. Further, occasional shut-downs of the BBGWTP
caused concurrent shut-downs of the pilot plant. Ferric chloride dosing varied between 12 and
20 mg/L (as ferric chloride) for the first 6 weeks of operation, and then was changed to 15 mg/L
regardless of which of the two wells was in operation. Acidified ferric chloride solution was used
during the fourth week of long-term steady-state operation instead of the regular ferric chloride
solution used during the rest of the testing period. However, these minor changes in operating
conditions are a typical part of full-scale plant operation. Results in this section are presented to
demonstrate the performance of the pilot C/F unit with ultra-light media throughout the testing
period regardless of minor operational changes.
Table 4.19
Summary statistics for full filter runs in long-term steady-state
operation with polystyrene media
Parameter
Clean media headloss
Stable filter effluent turbidity
Filter run time
Unit filter run volume
Turbidity at run termination
Unit
ft of H2O
NTU
hr
gal/ft2
NTU
ft of H2O
Count
168
168
168
168
168
168
Average
1.2
0.03
11.9
2,856
0.08
15.0
Standard
deviation
0.6
0.01
1.1
264
0.02
2.0
Minimum
0.4
0.02
8.2
1,968
0.03
10.0
Maximum
5.5
0.07
14.2
3,408
0.10
20.1
During the four months of long-term steady-state operation, a total of 168 full filter runs
were performed by the full-automated pilot C/F unit. “Full filter runs” are those that terminated
due to turbidity breakthrough or terminal headloss. Filter runs that were terminated by the pilot
plant PLC due to an operational glitch or that were terminated by the pilot plant operators for operational reasons unrelated to treatment performance were not included in the total of 168 filter runs
since these runs were not indicative of the performance of the polystyrene filter media. Table 4.19
provides a summary of relevant statistics for the 168 filter runs.
The results in Table 4.19 indicate that the clean media headloss for the polystyrene media
was quite low, at 1.2 ft of H2O (3.6 kPa). Turbidity during the stable operating portion of the filter
runs with polystyrene media was quite low, averaging 0.03 NTU, with a standard deviation of only
0.01 NTU.
Filter run length averaged 11.9 hr for the 168 full filter runs. The minimum filter run length
was 8.2 hr and the maximum was 14.2 hr. The average unit filter run volume was approximately
2,900 gal/ft2 (116 m3/m2), with a standard deviation of approximately 260 gal/ft2 (11 m3/m2). The
minimum UFRV was approximately 2,000 gal/ft2 (80 m3/m2) and the maximum was approximately 3,400 gal/ft2 (139 m3/m2). Because the filtration rate setpoint was a constant 4 gpm/ft2
(9.8 m/hr), UFRV was solely a function of filter run length for the long-term steady-state testing.
The average headloss at filter run termination was 15 ft of H2O (45 kPa), with a range of
10 to 20 ft of H2O (30 to 60 kPa). Since the maximum headloss was 20 ft of H2O (60 kPa), these
results indicate that while some filter runs terminated due to terminal headloss, on average the filter
runs did not reach terminal headloss. The average turbidity at filter run termination was 0.08 NTU,
with a maximum of 0.10 NTU, indicating that in general filter runs tended to terminate due to
turbidity breakthrough or imminent turbidity breakthrough rather than to terminal headloss. The
results also suggest that when filter runs terminated due to turbidity breakthrough, the headloss
tended to be well below the terminal headloss setpoint.
Example turbidity and headloss profiles from filter runs throughout the 4 months of testing are shown in Figures 4.17 through 4.19. Figure 4.17 is a turbidity and headloss profile from
Week 3 of pilot plant operation. This filter run (Run 822) terminated due to turbidity breakthrough
at a run time of 11.7 hr. At that time, the headloss was 14.0 ft of H2O (42 kPa). A linear trend of
headloss accumulation was observed, with slight fluctuations. A slight turbidity peak to approximately 0.3 NTU occurred immediately after the filter was placed into service but filter maturation
occurred rapidly and the turbidity reached a stable minimum within 1 hr of run time. The filter
effluent turbidity during the stable operating portion of the filter run was 0.03 to 0.04 NTU. The
25
1.00
15
Headloss
0.10
10
Turbidity (NTU)
20
Turbidity
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.17 Turbidity and headloss profile for filter run (Run 822) from third week of longterm steady-state operation with polystyrene media (ferric chloride dose = 20 mg/L)
onset of turbidity breakthrough was observed after approximately 11 hr of run time, and the PLC
of the fully-automated pilot C/F unit terminated the filter run when the turbidity reached 0.08 NTU.
Similar trends were observed in the example filter run from the ninth week of long-term
steady-state testing (Run 929), as shown in Figure 4.18. This filter run terminated due to turbidity
breakthrough at 11.7 hr of run time, when the headloss was 15.5 ft of H2O (46 kPa). A brief turbidity peak to less than 0.4 NTU occurred upon placing the filter in service after backwash, with turbidity returning to less than 0.1 NTU within 20 min and reaching a stable minimum of 0.03 NTU
until the onset of breakthrough at approximately 11 hr. The headloss accumulation was linear with
minor fluctuations.
Filter performance was similar in the 14th week of pilot plant operation. The turbidity
and headloss profile for an example Week 14 filter run (Run 1004) is shown in Figure 4.19. This
filter run terminated due to turbidity at 15.9 hr of run time when the headloss was 15.9 ft of H2O
(47 kPa). As in the previous two example filter runs, the brief maturation peak was less than 0.4
NTU, the stable filter effluent turbidity was 0.03 NTU and the onset of breakthrough occurred at
approximately 13 hr. Headloss accumulation was linear.
These three figures indicate that acceptable filtration performance with respect to turbidity,
headloss and filter run length was achieved in the pilot C/F unit operated with polystyrene media
when air scour was used in the backwash regime, and that the filter performance was consistent
during the 4 months of pilot plant operation with the polystyrene media.
25
1.00
Headloss
15
0.10
10
Turbidity (NTU)
20
Turbidity
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.18 Turbidity and headloss profile for filter run (Run 929) from ninth week of longterm steady-state operation with polystyrene media (ferric chloride dose = 15 mg/L)
25
1.00
15
Headloss
0.10
10
Turbidity (NTU)
20
Turbidity
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.19 Turbidity and headloss profile for filter run (Run 1004) from 14th week of longterm steady-state operation with polystyrene media (ferric chloride dose = 15 mg/L)
35
30
Arsenic (µg/L)
25
20
0.1
15
Turbidity
10
5
1
Arsenic
Iron
0
0
1
0.01
2
3
Run Time (hrs)
Figure 4.20 Turbidity, arsenic, and iron profiles from filter maturation during 15th week of
pilot plant operation with polystyrene media (Run 1010)
Filter Maturation and Breakthrough
Sampling for iron and arsenic was performed during the first hour of one filter run to assess
filter maturation performance of the polystyrene media. Nine iron and arsenic samples were collected during the first hour of Run 1010 on August 24, 2009 during Week 15 of the pilot testing
period. Results are illustrated in Figure 4.20.
The results show that turbidity was 0.24 NTU at 1 min after the filter was placed into service after completion of the backwash cycle. Turbidity then reached a peak of 0.8 NTU at approximately 10 min of run time and decreased below the operating criterion of 0.1 NTU within 20 min.
A stable minimum turbidity level of 0.02 NTU was reached after approximately 1.5 hr of run time.
The iron data follow the same trend as turbidity, with a concentration of 0.04 mg/L at 1 min, a peak
of 0.36 mg/L reached at a run time of 5 min, and a rapid decrease to a minimum concentration of
0.01 mg/L after approximately 45 min of run time. Despite the fact that the filter was backwashed
with filtered water produced by the previous pilot plant filter run, the highest arsenic concentration
of 31 μg/L was measured at a run time of 1 min, when turbidity and iron concentrations had not yet
reached their peak. After the initial peak, arsenic concentrations declined rapidly, reaching 10 μg/L
at 10 min of run time and reaching 5 μg/L at a run time between 15 and 20 min. After 30 min, the
arsenic concentration reached a minimum level below 3 μg/L.
Limited sampling was performed during one filter run after the onset of turbidity breakthrough. The filter run was Run 1046 on September 14, 2009, just before the completion of pilot
testing for the long-term steady-state operation with polystyrene media. The turbidity, iron and
arsenic profiles are illustrated in Figure 4.21 for the end of the filter run.
6
1
Arsenic
Iron
Turbidity
4
3
2
0.1
Turbidity
Ir on (mg/L), Tur bidity (NTU)
5
Ar senic (µg/L)
1
0
10.0
0.01
10.5
11.0
11.5
12.0
12.5
13.0
Run Time (hrs)
Figure 4.21 Turbidity, arsenic, and iron profiles from turbidity breakthrough during 17th
week of pilot plant operation with polystyrene media (Run 1046)
The figure indicates that there was a clear point at approximately 12 hr of run time at
which turbidity began to increase from the stable minimum level of 0.03 NTU. Turbidity breakthrough was relatively rapid after that time, with the turbidity increasing from 0.03 NTU to the
breakthrough criterion of 0.1 NTU over a period of 1 hr. The filter run terminated due to turbidity breakthrough at a run time of 13.0 hr. Iron and arsenic sampling was initiated at a run time of
12.7 hr. The arsenic concentration at that time was 4.3 μg/L. In 6 samples collected during the
next approximately 20 min of run time, as filter effluent turbidity increased from 0.07 NTU to
0.10 NTU, the filter effluent arsenic concentration increased from 4.3 μg/L to 5.0 μg/L and the
filter effluent iron concentration increased from 0.22 to 0.47 mg/L. These results indicate that as
turbidity breakthrough occurred, arsenic and iron breakthrough was also occurring. The results
also suggest that when turbidity reached the maximum concentration of 0.1 NTU, at which the
pilot plant’s PLC terminated the filter run, the filter effluent arsenic concentration was well below
the primary MCL of 10 μg/L. However, at that point in the filter run, the filter effluent iron concentration exceeded the secondary MCL of 0.3 mg/L.
Criteria for a Properly Cleaned Filter
At the beginning of long-term steady-state testing, criteria were defined for demonstration
of effective backwash to produce a properly-cleaned filter. These criteria were defined as:
9.0
Clean Media Headloss (ft of H2O)
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.22 Clean media headloss from full filter runs during long-term steady-state testing
with polystyrene media
Figures 4.22 through 4.25 illustrate the trends in these parameters for the full filter runs
performed during the 4-month testing period with the polystyrene media. Figure 4.22 illustrates
the clean media headloss for all full filter runs performed during the 4 months of long-term steadystate operation using the polystyrene media. In general, the clean media headloss ranged from 0.8
to 1.5 ft of H2O (2.4 to 4 kPa) throughout the testing period. There was no trend of increasing clean
media headloss over the testing period, indicating that the backwash regime provided effective
cleaning of the polystyrene media.
Figure 4.23 shows the headloss from all full filter runs performed during the 4 months of
long-term steady-state testing with the polystyrene media. The figure shows that few of the 168
filter runs terminated due to maximum headloss. In general, the headloss at run termination was
approximately 17 ft of H2O (51 kPa) or below. There is no trend of increasing headloss accumulation rate or increasing headloss at run termination over the testing period, which is another indication that the backwash regime provided effective cleaning of the polystyrene media.
Figure 4.24 shows filter run time for all full filter runs performed during long-term steadystate testing. With a few exceptions, filter run times were generally 11 to 14 hr for the 168 filter
runs. There is no trend of shorter filter runs over time.
Figure 4.25 shows filter effluent turbidity during the stable operating portion of each of
the 168 full filter runs. With a few exceptions, the stable operating turbidity was between 0.02 and
0.04 NTU, and was 0.03 NTU during the majority of the filter runs. There is no trend of increasing
30
25
20
15
10
5
0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.23 Headloss from full filter runs during long-term steady-state testing with
polystyrene media
16.0
14.0
Filter Run Time (hr)
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.24 Filter run time from full filter runs during long-term steady-state testing with
polystyrene media
Stable (Minimum) Turbidity (NTU)
0.2
0.1
0.0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.25 Filter effluent turbidity during stable operating portion of full filter runs during
long-term steady-state testing with polystyrene media
filter effluent turbidity over time during the testing period, which is another indication that the
backwash regime provided effective cleaning of the polystyrene media.
Backwash Volume
The complete volume of a backwash was collected at the pilot plant six times during the
long-term steady-state testing with polystyrene media. Results are shown in Table 4.20.
The results in Table 4.20 indicate that the backwash regime used for long-term steady-state
operation with the polystyrene media (as defined in Table 4.18) utilized a unit backwash volume
(UBWV) of approximately 35 gal/ft2 (1.4 m3/m2). The backwash regime defined in Table 4.18
was 2 gpm/ft2 (4.9 m/hr) for 3 min, followed by 3.3 gpm/ft2 (8.1 m/hr) for 10 min, producing a
total calculated backwash volume of 7.66 gal (29.0 L). For the pilot filter with a diameter 0.5 ft
(15.2 cm), the calculated UBWV was 39 gal/ft2 (1.6 m3/m2). The difference between measured
and calculated backwash volumes was likely due to the time required for valves to open and close
during the backwash, which would result in the measured backwash volume being lower than the
calculated backwash volume.
Feedwater Recovery
The feedwater recovery of the C/F process using the ultra-light polystyrene filter media was
calculated using the average filter run length of 11.9 hr as reported in Table 4.19, the filtration rate
Table 4.20
Backwash volume measurement for polystyrene media
Measured volume
(gal)
6.54
7.47
7.09
6.67
6.73
6.99
6.91
0.34
Day of operation
39
61
86
96
100
111
Average
Standard Deviation
Measured unit backwash volume
(gal/ft2)
33.3
38.0
36.1
34.0
34.3
35.6
35.2
1.7
Table 4.21
Raw water arsenic and filtered water quality during long-term steady-state operation with
polystyrene media
Parameter
Raw water
Arsenic
Filter effluent
pH
Temperature
Free chlorine
Turbidity
Iron
Arsenic
Units
Count
Average
Standard
deviation
Minimum
Maximum
µg/L
47
30
4.9
12
37
°C
mg/L
NTU
mg/L
µg/L
46
46
46
37
47
47
7.9
27.3
2.5
0.04
0.03
3.9
0.3
0.8
0.34
0.02
0.03
3.1
7.2
24.8
1.7
0.02
<0.01
<1
8.6
30.2
3.4
0.15
0.14
17
of 4 gpm/ft2 (9.8 m/hr) and the average measured backwash volume of 6.9 gal (26.2 L) as reported
in Table 4.20. Based on these values, the feedwater recovery was calculated using Equation 4.1 as:
• Average UFRV = 2,856 gal/ft2 (116 m3/m2)
• Average UBWV = 35.2 gal/ft2 (1.4 m3/m2)
• Feedwater recovery = 100 × (2,856 – 35.2) / 2,856 = 98.8%
Water Quality
A number of water quality parameters were monitored routinely at the pilot plant. Results
for water quality parameters measured on site as well as arsenic results analyzed at a commercial
laboratory are summarized in Table 4.21.
The table shows that pH averaged 7.9 in the filter effluent during the testing period. Turbidity
averaged 0.04 NTU with a standard deviation of 0.02 NTU. Free chlorine levels occurring in the
filter effluent from chlorination of the well water upstream of the pilot plant averaged 2.5 mg/L.
Iron concentrations in the filter effluent averaged 0.03 mg/L, with a standard deviation of 0.03
and range of <0.01 to 0.14 mg/L. The maximum iron concentration of 0.14 mg/L is less than 50%
50
Raw Arsenic
Effluent Arsenic
45
40
35
30
25
20
15
MCL
10
½ MCL
5
0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.26 Raw water and filter effluent arsenic concentrations during long-term steadystate testing with polystyrene media
of the secondary MCL for iron of 0.3 mg/L. The filter effluent arsenic concentration averaged
3.9 μg/L, with standard deviation of 3.1 μg/L and a range of <1 to 17 μg/L. The average arsenic
concentration of 4.1 μg/L is less than 50% of the MCL of 10 μg/L, when the raw water arsenic
concentration averaged 30 μg/L during long term steady-state testing with polystyrene media. The
temperature of the pilot plant effluent averaged 27.3°C, a relatively warm temperature indicative
of pilot plant operation in southern California from May to September.
Figure 4.26 shows arsenic concentrations in the chlorinated raw water and the filter effluent
measured at the pilot plant during long-term steady-state operation. For the 47 raw water arsenic
measurements, the average was 30 μg/L, with a standard deviation of 5 μg/L and a range of 12 to
37 μg/L. Two of the 47 filter effluent arsenic measurements (4% of the total) exceeded the MCL
of 10 μg/L. Nine of the 70 filter effluent arsenic measurements (19% of the total) exceeded 50%
of the MCL.
Figure 4.27 shows the filter influent and effluent iron concentrations measured at the pilot
plant during long-term steady-state operation. Filter influent iron concentrations were directly
related to the dose of ferric chloride applied to the water upstream of the filter influent sample
point. Initially, a ferric chloride dose of 12 mg/L (4.1 mg/L as Fe) was used in the pilot plant when
Well 4 was in service without Well 3. A ferric chloride dose of 20 mg/L (6.9 mg/L as Fe) was used
in the pilot plant when Well 3 was in service, either individually or with Well 4. After 6 weeks of
long-term steady-state testing, the pilot plant was operated with a stable ferric chloride dose of
15 mg/L (5.2 mg/L as Fe) regardless of which of the two wells was in service. Figure 4.27 indicates
that the filter effluent iron concentration was stable regardless of the ferric chloride dose.
10
Influent Iron
Effluent Iron
9
8
FeCl3 = 20 mg/L
7
6
5
FeCl3 = 15 mg/L
4
FeCl3 = 12 mg/L
3
2
1
0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.27 Filter influent and effluent iron concentrations during long-term steady-state
testing with polystyrene media
Figure 4.28 shows the filter effluent iron concentrations relative to the secondary MCL for
iron of 0.3 mg/L. The figure shows that all routine samples collected during the long-term steadystate testing, with the exception of the samples collected during filter maturation and turbidity
breakthrough as described above, had iron concentrations less than 50% of the MCL of 0.3 mg/L.
Figure 4.29 shows the pH of the chlorinated raw water and the filter effluent during the four
months of long-term steady-state operation. The chlorinated raw water had pH levels that were
typically between 9.0 and 9.5. The filter effluent pH levels were typically between 7.5 and 8.3. The
decrease in pH was due to ferric chloride addition in the pilot plant.
In addition to the water quality sampling discussed above, single samples were collected
from the chlorinated raw water, the filter effluent and the backwash water for general physical,
mineral and metals analyses. These samples were of particular importance for the polystyrene
media since the polystyrene beads had not been used in a drinking water treatment application
to date. Results are presented in Table 4.22. The filter effluent data in Table 4.22 indicate that the
ultra-light polystyrene media performed effectively for constituents in addition to iron and arsenic
when used with ferric chloride coagulation. The filter effluent produced by the pilot C/F unit with
polystyrene media met all primary and secondary MCLs for the parameters listed in Table 4.22.
To collect the backwash water sample, the volume of water from one full backwash cycle
was collected in a tank. The backwash water was mixed and samples were collected from the
mixed backwash water in the tank. The expected arsenic concentration in the mixed backwash
0.5
0.4
SMCL
0.3
0.2
½ SMCL
0.1
0.0
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.28 Filter effluent iron concentrations during long-term steady-state testing with
polystyrene media
12
Raw pH
Effluent pH
11
pH
10
9
8
7
6
0
20
40
60
80
100
120
140
Days of Operation
Figure 4.29 Raw water and filter effluent pH during long-term steady-state testing with
polystyrene media
Table 4.22
General physical, mineral, and metals analysis during long-term
steady-state operation of polystyrene media
Parameter
General physical
Agressiveness index-calculated
Alkalinity in CaCO3 units
Apparent color
Langelier index - 25 degree
Odor at 60°C (TON)
pH
Specific conductance, 25°C
Total dissolved solids
Total hardness as CaCO3
Total suspended solids
Turbidity
Mineral
Bromide
Chloride
Fluoride
Nitrate as nitrogen
Nitrite nitrogen
Sulfate
Surfactants
Metals
Aluminum
Antimony
Arsenic
Barium
Beryllium
Cadmium
Calcium
Chromium
Copper
Iron
Lead
Magnesium
Manganese
Mercury
Nickel
Potassium
Selenium
Silver
Sodium
Thallium
Vanadium
Zinc
Unit
Raw water
Filter effluent Mixed backwash
—
mg/L
ACU
None
TON
—
µmho/cm
mg/L
mg/L
mg/L
NTU
13
88
<3
0.55
2
9.1
700
380
22
<10
0.074
11
74
<3
–0.71
1
7.9
710
380
23
<10
0.057
13
77
300
0.95
2
8.9
700
350
99
1,000
110
µg/L
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
100
110
0.78
<0.2
<0.1
66
<0.05
No data
120
0.79
<0.2
<0.1
66
<0.05
No data
110
0.42
<0.2
<0.1
66
<0.05
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
mg/L
µg/L
µg/L
mg/L
µg/L
mg/L
µg/L
µg/L
µg/L
mg/L
µg/L
µg/L
mg/L
µg/L
µg/L
µg/L
35
<1
31
27
<1
<0.5
8.2
<1
<2
<0.02
<0.5
0.32
<2
<0.2
<5
<1
<5
<0.5
130
<1
150
<20
<20
<1
2.7
21
<1
<0.5
8.6
<1
<2
<0.02
<0.5
0.34
<2
<0.2
<5
<1
<5
<0.5
130
<1
18
<20
2,700
<1
2,200
660
<1
<0.5
38
82
110
380
1.2
0.82
1,200
<0.2
43
<1
<5
<0.5
130
<1
12,600
74
sample can be calculated based on the volume of water filtered, the volume of backwash water
produced in one backwash, and the arsenic concentrations in the raw water and filter effluent. Using the average filter run length of 11.9 hr (Table 4.19) at a filtration rate of 4.0 gpm/ft2
(9.8 m/hr), the volume of water filtered during the average filter run was 560 gal (2.1 m3). From
Table 4.20, the average backwash volume from the backwash regime used during the long-term
steady-state testing with polystyrene media was 6.9 gal (26.2 L). The filtered water volume
divided by the backwash volume results in a concentration factor of approximately 80. The raw
water and filter effluent data shown in Table 4.22 indicate that an average of 28 μg/L of arsenic
was removed by the C/F process. A concentration of 28 μg/L multiplied by the concentration
factor of 80 results in a calculated mixed backwash arsenic concentration of 2,240 μg/L. This
arsenic concentration compares well with the measured arsenic concentration in the mixed backwash water sample of 2,200 μg/L.
As discussed above with respect to the field demonstration testing, the elevated levels of
constituents measured in the mixed backwash water have two possible sources. The constituents
could have been removed from the raw water, or the constituents could have been present in the
ferric chloride solution and concentrated onto the filter media along with the iron. Based on a concentration factor of 80, results in Table 4.22 indicate that in addition to arsenic, the constituents
that may have been removed from the raw water and concentrated in the backwash water include
chromium and copper. The results further indicate that in addition to iron the constituents that may
have originated in the ferric chloride solution were calcium, iron and manganese. In fact, an analysis of the ferric chloride dosing solution used at the pilot plant indicated that the ferric solution
had a total iron concentration of 12,000 mg/L and a total manganese concentration of 32 mg/L. It
is unclear based on the results in Table 4.22 if aluminum, barium, nickel, vanadium and zinc were
removed from the raw water, contributed by the ferric chloride solution, or both.
Additional Analysis of Polystyrene Media
Because the polystyrene beads used as the ultra-light filter media during the long-term
steady-state testing had not been used to date in a drinking water treatment application and were
not NSF-61 certified for contact with drinking water, two additional types of analyses were performed to further assess their potential suitability as a filter media. First, an evaluation of volatile
organic chemicals (VOCs) was performed to determine if the polystyrene media may contribute
VOCs to the water passing through the filter. Second, samples of polystyrene media were analyzed
by scanning electron microscopy (SEM).
To evaluate the potential contribution of VOCs to the water passing through a bed of polystyrene bead filter media, a sample of new polystyrene beads was rinsed with chlorinated raw water
that was the feedwater to the pilot plant. VOC samples were collected from the chlorinated raw
water, raw water that had been passed through a polyester mesh screen (“screen rinse”), and raw
water that had been shaken in a jar with 100 mL of new polystyrene beads and then separated from
the beads using the polyester mesh screen material. VOC samples were collected from the first
500-mL sample of chlorinated raw water (“first media rinse”), the rinsing process was repeated
9 more times using 500 mL of chlorinated raw water for each rinse, and then VOC samples were
collected from the 10th rinse (“tenth media rinse”). Results are shown in Table 4.23.
The results in Table 4.23 indicate that the only VOC compounds measured in any of the
four samples were the four total trihalomethane (THM) compounds (bromodichloromethane, bromoform, dibromochloromethane and chloroform) and styrene. The highest of the THM compounds
Table 4.23
Volatile organic compounds in chlorinated raw water and polystyrene media rinse water
Parameter
1,1,1,2-Tetrachloroethane
1,1,1-Trichloroethane
1,1,2,2-Tetrachloroethane
1,1,2-Trichloroethane
1,1-Dichloroethane
1,1-Dichloroethylene
1,1-Dichloropropene
1,2,3-Trichlorobenzene
1,2,3-Trichloropropane
1,2,4-Trichlorobenzene
1,2,4-Trimethylbenzene
1,2-Dichloroethane
1,2-Dichloropropane
1,3,5-Trimethylbenzene
1,3-Dichloropropane
2,2-Dichloropropane
2-Butanone (MEK)
4-Methyl-2-Pentanone (MIBK)
Benzene
Bromobenzene
Bromochloromethane
Bromodichloromethane
Bromoethane
Bromoform
Bromomethane (Methyl Bromide)
Carbon Tetrachloride
Chlorobenzene
Chlorodibromomethane
Chloroethane
Chloroform (Trichloromethane)
Chloromethane(Methyl Chloride)
cis-1,2-Dichloroethylene
cis-1,3-Dichloropropene
Di-isopropyl ether
Dibromomethane
Dichlorodifluoromethane
Dichloromethane
Ethyl benzene
Hexachlorobutadiene
Isopropylbenzene
m,p-Xylenes
m-Dichlorobenzene (1,3-DCB)
Methyl Tert-butyl ether (MTBE)
n-Butylbenzene
Unit
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
First media
Raw water Screen rinse
rinse
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<5
<5
<5
<5
<5
<5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
1.7
1.6
1.4
<0.5
<0.5
<0.5
17
17
14
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
6
5.7
5.2
<0.5
<0.5
<0.5
0.56
0.51
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<3
<3
<3
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
Tenth
media
rinse
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<5
<5
<0.5
<0.5
<0.5
1.7
<0.5
17
<0.5
<0.5
<0.5
6.2
<0.5
0.51
<0.5
<0.5
<0.5
<3
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
(continued)
Table 4.23 (Continued)
Parameter
n-Propylbenzene
Naphthalene
o-Chlorotoluene
o-Dichlorobenzene (1,2-DCB)
o-Xylene
p-Chlorotoluene
p-Dichlorobenzene (1,4-DCB)
p-Isopropyltoluene
sec-Butylbenzene
Styrene
tert-amyl Methyl Ether
tert-Butyl Ethyl Ether
tert-Butylbenzene
Tetrachloroethylene (PCE)
Toluene
Total 1,3-Dichloropropene
Total THM
Total xylenes
trans-1,2-Dichloroethylene
trans-1,3-Dichloropropene
Trichloroethylene (TCE)
Trichlorofluoromethane
Trichlorotrifluoroethane(Freon 113)
Vinyl chloride (VC)
Unit
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
First media
Raw water Screen rinse
rinse
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
17
<3
<3
<3
<3
<3
<3
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
25.0
24.0
20.0
<1
<1
<1
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.3
<0.3
<0.3
Tenth
media
rinse
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
6.5
<3
<3
<0.5
<0.5
<0.5
<0.5
26.0
<1
<0.5
<0.5
<0.5
<0.5
<0.5
<0.3
was bromoform which was measured at 14 to 17 μg/L in all four samples. The bromide concentration in the chlorinated raw water reported in Table 4.22 was 100 μg/L, indicating that formation
of brominated THM compounds upon addition of free chlorine was not unexpected. The presence
of THM compounds in the chlorinated raw water and screen rinse in addition to the first and 10th
media rinse samples indicates that the THM compounds did not originate in the polystyrene media.
The only VOC compound that was present in the media rinse samples but was not present in the
raw water was styrene. Styrene was below detection in the raw and screen rinse samples, but was
detected at a concentration of 17 μg/L in the first media rinse and at 6.5 μg/L in the tenth media
rinse. These results indicate that the polystyrene media contributed styrene to the chlorinated raw
water that came into contact with it under circumstances simulating that of water passing through
a bed of filter media. After the polystyrene media was subjected to 10 rinses, the styrene concentration decreased from 17 to 6.5 μg/L, suggesting that the styrene contribution was being washed
out of the media as it was used. The primary MCL for styrene is 100 μg/L, as is the MCLG. Thus,
the styrene concentrations in both the first and the tenth rinse samples were well below the MCL.
Upon completion of the four months of long-term steady-state testing, a sample of the
polystyrene media that had been in place for the four months of long-term steady-state testing
was removed from the pilot filter column and was analyzed by SEM at Michigan Technological
University (Houghton, MI), along with a sample of new polystyrene media. The used media
Figure 4.30a Scanning electron microscopy photograph of new polystyrene filter media
Figure 4.30b Scanning electron microscopy photograph of new polystyrene filter media
(close up)
removed from the filter was shipped in a damp condition and was dried at the laboratory before
analysis. Upon visual examination, the used media had an orange tint, as did the interior of the
clear PVC filter column, from the ferric chloride used at the pilot plant. SEM photos are included
in Figures 4.30 and 4.31.
Figures 4.30a through 4.30c show the new polystyrene bead media, which are uniform,
smooth spheres. The SEM photograph in Figure 4.30a clearly shows the uniformity of the polystyrene beads, which were reported in Table 4.1 to have a uniformity coefficient of 1.3. Figure 4.30b
is a close-up of the new polystyrene beads from the area highlighted in Figure 4.30a, showing the
uniform spheres with smooth surfaces. Figure 4.30c is a further close-up of the media surface,
demonstrating more clearly that the surface is smooth and clean.
Figures 4.31a through 4.31c show the polystyrene bead media that had been removed from
the filter after 4 months of long-term steady-state operation. As observed for the new media in
Figure 4.30a, the used media also appears uniform and smooth. A few of the polystyrene beads
appear to be slightly irregular in shape and/or agglomerated together. A close-up of the highlighted
area is shown in Figure 4.31b. This SEM photograph shows two particles stuck together with what
is likely ferric floc material that was not completely removed by the backwash regime. Further, the
Figure 4.30c Scanning electron microscopy photograph of new polystyrene bead filter media
(close-up of surface)
Figure 4.31a Scanning electron microscopy photograph of used polystyrene bead filter media
Figure 4.31b Scanning electron microscopy photograph of used polystyrene bead filter
media (close up)
Figure 4.31c Scanning electron microscopy photograph of used polystyrene bead filter media
(close-up of surface)
lower of the two spheres appears to be irregularly shaped. Figure 4.31c is a further close-up of the
media surface from the highlighted area of Figure 4.31b. In contrast to the clean media, the used
media has a thin, non-uniform layer of flaky material on the surface of the sphere. This material is
likely the accumulated iron floc material that gave the used media the orange tint that was apparent by visual examination. In general, the used media was very uniform and smooth and showed
little sign of degradation, either from damage to the polystyrene spheres or from accumulation of
foulants, from the more than 168 backwash cycles performed during the four months of long-term
steady-state operation.
Additional analysis using a higher accelerating voltage (20 KeV) did not indicate the presence of arsenic in the material accumulated on the surface of the used media. A spectrum collected
for 200 sec did not produce a peak for arsenic. The coating on the used media surface was found
to be rich in iron, as expected due to the use of an iron-based coagulant.
STEADY-STATE OPERATION WITH ALTERNATIVE ULTRA-LIGHT MEDIA
After completion of the long-term steady-state testing with polystyrene bead media, a further 8 weeks of steady-state testing was performed with an alternative ultra-light media, from
September 23 to November 16, 2009. The alternative media was Purofine PFC100E (The Purolite
Corporation, Bala Cynwyd, PA), a gel-type strong acid cationic exchange resin. The PFC100E was
used as an ultra-light filter media in the pilot C/F unit rather than as an ion exchange resin. Because
it was not regenerated, the ion exchange capacity of the PFC100E was likely exhausted early
on during the 8 weeks of steady-state operation, and it functioned as an ultra-light filter media.
The PFC100E media is NSF-61 certified for contact with drinking water. A sieve analysis of the
PFC100E media could not be performed because it was supplied by the manufacturer in a moist
state. Details of the PFC100E media are shown in Table 4.24.
The pilot filter column was filled with PFC100E media to a depth of 30 inches (76 cm),
which is the same depth used in the long-term steady-state testing with the polystyrene media. The
specific gravity of the PFC100E media was higher than that of the polystyrene beads (1.27 compared to 1.05) and the uniformity coefficient was lower than that of the polystyrene beads (1.2 compared to 1.3). The reported size range of 0.52 to 0.62 mm was smaller than the measured effective
size of the polystyrene beads (0.66 mm).
Table 4.24
Physical characteristics of Purofine PFC100E
Parameter
Structural composition
Ionic form as shipped
Functional group
Appearance
Density
Size range
Specific gravity (Na+ form)
Uniformity coefficient (maximum)
Value
Gel polystyrene crosslinked with divinylbenzene
Na+
Sulphonic acid
Brown spherical beads
50 lb/ft3 (0.8 g/cm3)
0.52 to 0.62 mm
1.27
1.2
100%
Polystyrene Media
90%
PFC 100E Media
Media Expansion (%)
80%
Polystyrene Media, 23.4 °C
70%
60%
50%
40%
30%
PFC 100E Media, 24.4°C
20%
10%
0%
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Backwash Rate (gpm/sf)
Figure 4.32 Expansion of polystyrene and PFC100E media as a function of backwash rate
For a given backwash rate, expansion of the polystyrene media was greater than that of the
PFC100E media, as shown in Figure 4.32. This result was expected based on the higher specific
gravity of the PFC100E media.
Despite the lower expansion of the PFC100E media, the backwash regime for the PFC100E
media was the same as that used for the polystyrene media, as defined in Table 4.18. The optimum
high rate backwash velocity of the PFC100E media at 20°C was 4.0 gpm/ft2 (9.8 m/hr) using the
empirical approach of Kawamura (1975). However, with the use of air scour and concerns about
potential loss of media out the top of the filter column, the high rate backwash (Stage 2 of the backwash regime) was maintained at 3.3 gpm/ft2 (8.1 m/hr). The screen at the top of the filter column
Table 4.25
Summary statistics for full filter runs in steady-state operation with PFC100E media
Parameter
Clean media headloss
Stable filter effluent
turbidity
Filter run time
Unit filter run volume
Turbidity at run termination
Unit
ft of H2O
NTU
Count
81
81
Average
6.0
0.02
Standard
deviation
1.2
0.01
Minimum
1.9
0.02
Maximum
8.5
0.04
hr
gal/ft2
NTU
ft of H2O
81
81
81
81
12.4
2,976
0.06
23.4
1.6
384
0.03
1.7
9.2
2,208
0.02
18.7
15.7
3,768
0.10
25.0
was replaced with a smaller-mesh screen to prevent loss of the smaller-diameter PFC100E media.
The new screen was USA 45 mesh with 355-μm openings.
Operating conditions for the steady-state testing with PFC100E media were the same as
those used for the long-term steady-state testing with polystyrene media, including the ferric chloride dose of 15 mg/L, the filtration rate of 4.0 gpm/ft2 (9.8 m/hr), and criteria for a properly cleaned
filter. The filter run termination criteria were the same as for the long-term steady-state testing with
polystyrene media, with the exception that terminal headloss was set at 25 ft of H2O (75 kPa) for
the PFC100E media, rather than the 20 ft of H2O (60 kPa) used for the polystyrene media. The
higher level of terminal headloss was used due to the higher clean media headloss of the PFC100E
media compared to the polystyrene media.
Performance of Pilot C/F Unit With PFC100E Filter Media
During the 8 weeks of steady-state operation with the PFC100E media, a total of 81 full
filter runs were performed by the full-automated pilot C/F unit. As discussed above with respect
to the polystyrene filter media, “full filter runs” are those that terminated due to turbidity breakthrough or terminal headloss. Filter runs that were terminated by the pilot plant PLC due to an
operational glitch or that were terminated by the pilot plant operators for operational reasons unrelated to treatment performance were not included in the total of 81 filter runs since these runs were
not indicative of the performance of the PFC100E filter media. Table 4.25 provides a summary of
relevant statistics for the 81 full filter runs.
The results in Table 4.25 indicate that filter run length averaged 12.4 hr for the 81 full filter
runs. The minimum filter run length was 9.2 hr and the maximum was 15.7 hr, with a standard
deviation of 1.6 hr. The average unit filter run volume was approximately 3,000 gal/ft2 (122 m3/
m2), with a standard deviation of approximately 400 gal/ft2 (16 m3/m2). The minimum UFRV
was approximately 1,500 gal/ft2 (61 m3/m2) and the maximum was approximately 3,800 gal/ft2
(155 m3/m2). Because the filtration rate setpoint was a constant 4 gpm/ft2 (9.8 m/hr), UFRV was
solely a function of filter run length for the steady-state testing with the PFC100E media.
Clean media headloss for the PFC100E media averaged 6.0 ft of H2O (18 kPa), with a standard deviation of 1.2 ft of H2O (3.6 kPa). The average headloss at filter run termination was 23.4 ft
of H2O (70 kPa), with a maximum of 25 ft of H2O (75 kPa). Since the terminal headloss was 25 ft
of H2O (75 kPa), these results indicate that the filter runs tended to terminate due to headloss. The
average turbidity at filter run termination was 0.06 NTU, with a standard deviation of 0.03 NTU, a
25
1.00
Headloss
15
0.10
10
Turbidity (NTU)
20
5
Turbidity
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.33 Turbidity and headloss profile for filter run from third week of steady-state
operation with PFC100E media (Run 1084)
minimum of 0.02 NTU and maximum of 0.10 NTU. The headloss and turbidity statistics indicate
that in general filter runs with the PFC100E media tended to terminate due to headloss rather than
to turbidity breakthrough. The results also suggest that when filter runs terminated due to headloss,
the turbidity tended to be approaching breakthrough.
Example turbidity and headloss profiles from filter runs throughout the 8 weeks of testing
with the PFC100E media are shown in Figures 4.33 through 4.35. Figure 4.33 is a turbidity and
headloss profile from Week 3 of pilot plant operation with the PFC100E media. This filter run (Run
1084) terminated due to turbidity breakthrough at a run time of 13.0 hr. At that time, the headloss
was approaching terminal headloss at 23.7 ft of H2O (71 kPa). The figure shows that headloss
accumulation was nonlinear. Clean media headloss was 4.1 ft of H2O (12.2 kPa). During the first
few minutes of the run, a brief period of very rapid headloss accumulation occurred, from 4.1 to
5.4 ft of H2O (12.2 to 16.1 kPa) in the first 5 minutes of the run. This very rapid but brief increase
was followed by a moderate headloss accumulation rate during the first approximately 4 hr of
run time, and then a slightly lower headloss accumulation rate after 4 hr. A low turbidity peak to
0.16 NTU occurred immediately after the filter was placed into service. Filter maturation occurred
rapidly, with turbidity decreasing below 0.1 NTU after less than 15 min and reaching a stable minimum of ≤0.03 NTU within approximately 20 min of run time.
An example filter run from the 6th week of steady-state testing with the PFC100E media
(Run 1124) is shown in Figure 4.34. This filter run terminated due to headloss at 12.7 hr of run
time, when the turbidity was still at the stable minimum of 0.04 NTU. A brief turbidity peak to
0.34 NTU occurred upon placing the filter in service after backwash, with turbidity returning
to less than 0.1 NTU within approximately 20 min and reaching a stable minimum of 0.03 to
25
1.00
Headloss
15
0.10
10
Turbidity (NTU)
20
Turbidity
5
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.34 Turbidity and headloss profile for filter run from sixth week of steady-state
0.05 NTU in less than 30 min of run time. The trend of nonlinear headloss accumulation discussed
for Figure 4.33 was also observed in Figure 4.34.
Filter performance was similar in the 8th and final week of pilot plant operation with the
PFC100E media. The turbidity and headloss profile for an example filter run (Run 1345) is shown
in Figure 4.35. This filter run terminated due to terminal headloss at 12.0 hr of run time when the
turbidity was at the stable minimum of 0.03 NTU. As in the previous two example filter runs, the
brief maturation peak was less than 0.4 NTU, the turbidity fell below 0.1 NTU in less than 20 min,
and the stable filter effluent turbidity of 0.03 NTU was reached in approximately 40 min. Headloss
accumulation was non-linear, with the headloss accumulation rate decreasing after approximately
4 hr of run time.
These three figures indicate that acceptable filtration performance with respect to turbidity,
headloss and filter run length was achieved in the pilot C/F unit operated with the PFC100E media
when air scour was used in the backwash regime, and that the filter performance was consistent
during the 8 weeks of pilot plant operation with the PFC100E media.
Filter Maturation
Sampling for iron and arsenic was performed during the first hour of one filter run to
assess filter maturation performance of the PFC100E media. Nine iron and arsenic samples were
collected during the first hour of Run 1332 on November 9, 2009 during Week 7 of the PFC100E
testing period. Results are illustrated in Figure 4.36.
25
1.00
Headloss
15
0.10
10
Turbidity (NTU)
20
5
Turbidity
0
0.01
0
2
4
6
8
10
12
14
Run Time (hrs)
Figure 4.35 Turbidity and headloss profile for filter run from eighth week of steady-state
The results show that turbidity was 0.02 NTU at 1 min after the filter was placed into
service after completion of the backwash cycle. Turbidity then reached a peak of 0.16 NTU at
approximately 15 min of run time and decreased below the operating criterion of 0.1 NTU within
20 min. A stable minimum turbidity level of 0.02 to 0.03 NTU was reached after approximately
45 min of run time. The iron data follow the same trend as turbidity, with a concentration of
0.01 mg/L at 1 min, a peak of 0.53 mg/L reached at a run time of 5 min, and a rapid decrease to a
minimum concentration of 0.01 mg/L after approximately 25 min of run time. Despite the fact that
the filter was backwashed with filtered water produced by the previous pilot plant filter run, the
highest arsenic concentration of 36 μg/L was measured at a run time of 1 min, when turbidity and
iron concentrations had not yet reached their peak. After the initial peak, arsenic concentrations
declined rapidly, reaching 10 μg/L at a run time between 15 and 20 min. The arsenic concentration
in the final sample at 60 min was 5.5 μg/L.
Criteria for a Properly Cleaned Filter
The criteria for a properly-cleaned filter were the same for the PFC100E media as for the
polystyrene media, specifically:
40
1
30
25
20
0.1
15
Turbidity
10
35
Arsenic (µg/L)
Arsenic
5
Iron
0
0
1
0.01
2
3
Run Time (hrs)
Figure 4.36 Turbidity, arsenic, and iron profiles from filter maturation during seventh week
of pilot plant operation with PFC100E media (Run 1332)
Figures 4.37 through 4.40 illustrate the trends in these parameters for the 8-week testing
period with the PFC100E media. Figure 4.37 illustrates the clean media headloss for all full filter
runs performed during the steady-state operation using the PFC100E media. In general, the clean
media headloss ranged from 5 to 8 ft of H2O (15 to 24 kPa) throughout the testing period. The
figure underscores the rationale for increasing the terminal headloss criterion from 20 ft of H2O
(60 kPa) for the polystyrene media to 25 ft of H2O (75 kPa) for the PFC100E media. There was
no trend of increasing clean media headloss over the testing period, indicating that the backwash
regime provided effective cleaning of the PFC100E media.
Figure 4.38 shows the headloss from all full filter runs performed during the 8 weeks of
steady-state testing with the PFC100E media. The figure shows that many of the PFC100E filter
runs throughout the testing period terminated due to maximum headloss of 25 ft of H2O (75 kPa).
There is no indication in the figure of any trend of higher headloss accumulation rate over time,
thereby suggesting that the backwash regime provided effective cleaning of the PFC100E media.
Figure 4.39 shows filter run time for all full filter runs performed during the steady-state
testing with PFC100E media. With a few exceptions, filter run times were generally 11 to 15 hr for
the 81 full filter runs. There is no trend of shorter filter runs over time.
Figure 4.40 shows filter effluent turbidity during the stable operating portion of each of
the 81 full filter runs with PFC100E media. With a few exceptions the stable operating turbidity
was between 0.02 and 0.04 NTU, and was 0.02 to 0.03 NTU during the majority of the filter runs.
There was no trend of increasing filter effluent turbidity over time during the testing period, which
is another indication that the backwash regime provided effective cleaning of the PFC100E media.
Clean Media Headloss (ft of H2O)
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0
10
20
30
40
50
60
Days of Operation
Figure 4.37 Clean media headloss from full filter runs during steady-state testing with
PFC100E media
30
25
20
15
10
5
0
0
10
20
30
40
50
60
Days of Operation
Figure 4.38 Headloss from full filter runs during steady-state testing with PFC100E media
18.0
16.0
Filter Run Time (hr)
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
10
20
30
40
50
60
Days of Operation
Figure 4.39 Filter run time from full filter runs during steady-state testing with PFC100E
media
1.0
Stable (Minimum) Turbidity (NTU)
0.1
0.0
0
10
20
30
40
50
60
Days of Operation
Figure 4.40 Filter effluent turbidity during stable operating portion of full filter runs during
steady-state testing with PFC100E media
Table 4.26
Raw water arsenic and filtered water quality during steady-state
operation with PFC100E media
Parameter
Raw water
Arsenic
Filter effluent
pH
Temperature
Free chlorine
Turbidity
Iron
Arsenic
Units
Count
Average
Standard
deviation
Minimum
Maximum
µg/L
23
34
3.4
30
39
°C
mg/L
NTU
mg/L
µg/L
22
22
22
19
23
23
8.0
27.1
2.2
0.04
0.03
4.7
0.3
0.5
0.51
0.02
0.03
2.1
7.5
25.8
0.34
0.02
<0.01
2.1
8.5
27.8
2.9
0.08
0.12
8.2
Feedwater Recovery
The feedwater recovery of the C/F process using the PFC100E filter media was calculated
using the average unit filter run volume of 2,976 gal/ft2 (122 m3/m2) as reported in Table 4.25,
the filtration rate of 4 gpm/ft2 (9.8 m/hr) and the average measured backwash volume of 6.9 gal
(26.2 L) as reported in Table 4.20 for the polystyrene media. Based on these values, the feedwater
recovery for the PFC100E media was calculated using Equation 4.1 as:
• Average UFRV = 2,976 gal/ft2 (122 m3/m2)
• Average UBWV = 35.2 gal/ft2 (1.4 m3/m2)
• Feedwater recovery = 100 × (2,976 – 35.2) / 2,976 = 98.8%
Water Quality
Results for water quality parameters measured on site as well as arsenic results analyzed
at a commercial laboratory during the 8 weeks of testing with PFC100E media are summarized in
Table 4.26.
The table shows that pH averaged 8.0 in the filter effluent during the testing period.
Turbidity averaged 0.04 NTU with a standard deviation of 0.02 NTU. Free chlorine levels occurring in the filter effluent from chlorination of the well water upstream of the pilot plant averaged
2.2 mg/L. Iron concentrations in the filter effluent averaged 0.03 mg/L, with a standard deviation
of 0.03 and range of <0.01 to 0.12 mg/L. The maximum iron concentration of 0.12 mg/L is less
than 50% of the secondary MCL for iron of 0.3 mg/L. The raw water arsenic concentration averaged 34 μg/L. The filter effluent arsenic concentration averaged 4.7 μg/L, with standard deviation
of 2.1 μg/L and a range of 2.1 to 8.2 μg/L. The average arsenic concentration of 4.7 μg/L is less
than 50% of the MCL of 10 μg/L. The temperature of the pilot plant effluent averaged 27.1°C,
which is a relatively warm temperature indicative of pilot plant operation in southern California
from September to November.
Figure 4.41 shows arsenic concentrations in the chlorinated raw water and the filter effluent
measured at the pilot plant during steady-state operation with PFC100E media. The figure shows
50
Raw Arsenic
Effluent Arsenic
45
40
35
30
25
20
15
MCL
10
½ MCL
5
0
0
20
40
60
80
Days of Operation
Figure 4.41 Raw water and filter effluent arsenic concentrations during steady-state testing
with PFC100E media
that none of the filter effluent arsenic measurements exceeded the MCL of 10 μg/L. Nine of the
23 measurements (approximately 40% of the total) exceeded 5 μg/L, which is 50% of the MCL.
Figure 4.42 shows the filter influent and effluent iron concentrations measured at the pilot
plant during steady-state operation with PFC100E media. Filter influent iron concentrations reflect
the 15 mg/L ferric chloride dose (5.2 mg/L as Fe) applied to the water upstream of the filter influent sample point. The figure shows indicates that filter effluent iron concentrations were stable
throughout the 8 weeks of steady-state testing with PFC100E media.
Figure 4.43 is a detail of the filter effluent iron concentrations with the PFC100E media.
The figure shows that all filter effluent iron measurements were below 0.15 mg/L, or 50% of the
secondary MCL of 0.3 mg/L. All but one of the 23 iron measurements were at or below 0.1 mg/L.
Figure 4.44 shows the pH of the chlorinated raw water and the filter effluent during the
8 weeks of pilot testing with the PFC100E media. The pH of the chlorinated raw water was
relatively stable between pH 9.0 and 9.6. The pH of the filter effluent was generally between 7.5
and 8.2.
In addition to the water quality sampling discussed above, single samples were collected
from the chlorinated raw water, the PFC100E media filter effluent, and the backwash water for
general physical, mineral and metals analyses. Results are shown in Table 4.27. The filter effluent
data in Table 4.27 indicate that the PFC100E media performed effectively for constituents in addition to iron and arsenic when used with ferric chloride coagulation. The filter effluent produced by
10
Influent Iron
Effluent Iron
9
8
7
6
5
FeCl3 = 15 mg/L
4
3
2
1
0
0
20
40
60
80
Days of Operation
Figure 4.42 Filter influent and effluent iron concentrations during steady-state testing with
PFC100E media
0.5
0.4
SMCL
0.3
0.2
½ SMCL
0.1
0.0
0
20
40
60
80
Days of Operation
Figure 4.43 Filter effluent iron concentrations during steady-state testing with PFC100E
media
12
Raw pH
Effluent pH
11
10
pH
9
8
7
6
0
20
40
60
80
Days of Operation
Figure 4.44 pH of chlorinated raw water and filter effluent
the pilot C/F unit with PFC100E media met all primary and secondary MCLs for the parameters
listed in Table 4.27.
The backwash water sample was collected from the complete volume of one filter backwash cycle that was mixed at the time of sample collection. The expected arsenic concentration in
the mixed backwash sample can be calculated based on the volume of water filtered, the volume
of backwash water produced in one backwash, and the arsenic concentrations in the raw water and
filter effluent. Using the average filter run length of 12.4 hr (Table 4.25) at a filtration rate of 4.0
gpm/ft2 (9.8 m/hr), the volume of water filtered during the average filter run was 580 gal (2.2 m3).
The average backwash volume from the backwash regime used during the steady-state testing
with PFC100E media was the same as that used during long-term steady-state testing with polystyrene media, since the backwash regimes for testing of both types of media were identical. From
Table 4.20, the measured average backwash volume was 6.9 gal (26.2 L). The average filtered
water volume divided by the average backwash volume results in a concentration factor of 85. The
raw water and filter effluent data shown in Table 4.27 indicate that 30 μg/L of arsenic was removed
by the C/F process. A concentration of 30 μg/L multiplied by the concentration factor of 85 results
in a calculated mixed backwash arsenic concentration of 2,550 μg/L. This arsenic concentration
compares well with the measured arsenic concentration in the mixed backwash water sample of
1,800 μg/L, with a difference of 30% between the calculated and measured arsenic concentrations.
As discussed above with respect to the field demonstration testing and the long-term
steady-state testing with polystyrene media, the elevated levels of constituents measured in the
mixed backwash water have two possible sources. The constituents could have been removed from
the raw water, or the constituents could have been present in the ferric chloride solution and concentrated onto the filter media along with the iron. Based on a concentration factor of 85, results
Table 4.27
General physical, mineral, and metals analysis during steady-state
operation of PFC100E media
Parameter
General Physical
Agressiveness Index-Calculated
Alkalinity in CaCO3 units
Apparent Color
Langelier Index - 25°C
Odor at 60°C (TON)
pH
Specific Conductance, 25°C
Total Dissolved Solids (TDS)
Total Hardness as CaCO3
Total Suspended Solids (TSS)
Turbidity
Mineral
Bromide
Chloride
Fluoride
Nitrate as Nitrogen
Nitrite Nitrogen
Sulfate
Surfactants
Metals
Aluminum
Antimony
Arsenic
Barium
Beryllium
Cadmium
Calcium
Chromium
Copper
Iron
Lead
Magnesium
Manganese
Mercury
Nickel
Potassium
Selenium
Silver
Sodium
Thallium
Vanadium
Zinc
Unit
Raw
water
Filter
effluent
Backwash
waste
None
mg/L
ACU
None
TON
Units
µmho/cm
mg/L
mg/L
mg/L
NTU
12
76
<3
0.390
1
9.2
550
300
14
<10
0.09
11
63
<3
–0.720
1
8.2
560
290
13
<10
0.11
13
77
3500
1.10
1
9.0
540
300
87
920
62
µg/L
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
42
80
1.1
0.20
<0.1
55
<0.05
No data
90
1.1
0.20
<0.1
56
<0.05
No data
79
0.34
<0.2
<0.1
55
<0.05
µg/L
µg/L
µg/L
µg/L
µg/L
µg/L
mg/L
µg/L
µg/L
mg/L
µg/L
mg/L
µg/L
µg/L
µg/L
mg/L
µg/L
µg/L
mg/L
µg/L
µg/L
µg/L
37
<1
37
13
<1
<0.5
4.9
2.1
<2
<0.02
<0.5
0.31
<2
<0.2
<5
<1
<5
<0.5
110
<1
160
<20
<20
<1
7.0
7.0
<1
<0.5
4.7
2.4
<2
<0.02
<0.5
0.28
<2
<0.2
<5
<1
<5
<0.5
110
<1
31
<20
2600
<1
1800
1000
<1
<0.5
33
75
130
400
1.6
1.5
810
<0.2
77
<1
<5
<0.5
110
<1
10000
110
in Table 4.27 indicate that the constituents removed from the raw water and concentrated in the
backwash water included arsenic and vanadium. The results further indicate that the constituents
that may have originated in the ferric chloride solution were chromium, copper, iron and manganese. As discussed with respect to the backwash results from long-term steady-state testing with
polystyrene media, an analysis of the ferric chloride solution used at the pilot plant indicated that
the ferric solution had a total iron concentration of 12,000 mg/L and a total manganese concentration of 32 mg/L. It is unclear based on the results in Table 4.27 if aluminum, barium, calcium, lead,
nickel and zinc were removed from the raw water, contributed by the ferric chloride solution, or
both.
CHAPTER 5
SUMMARY AND CONCLUSIONS
Coagulation/filtration (C/F) with conventional anthracite-sand media in pressure vessels
is a demonstrated treatment technology for the removal of arsenic from groundwater. The process
utilizes ferric coagulant addition to adsorb arsenic from water. Applying a C/F process for arsenic
removal from groundwater is less costly than that of any of the other available technologies such
as adsorption on granular ferric oxide media or ion exchange. However, one drawback of the C/F
process is the production of a relatively high volume of waste backwash water compared to other
adsorptive technologies. The backwash volume is a function of the backwash velocity required to
fluidize the media. Due to the high density of sand and anthracite, the backwash velocity needs to
be between 15 and 20 gpm/ft2 (37 to 49 m/hr). If a 5-ft (1.5-m) pressure vessel is backwashed at
this rate for 10 minutes, the volume of waste backwash water could be 3,000 gallons (11 m3) or
more. The disposal of this volume of backwash water is highly problematic for very small systems
because many dispose their waste to an on-site septic system. Even if a small water system had
access to a sanitary sewer, the rate of backwash water production is too high for direct discharge
into the sewer, and would require construction of an equalization basin.
This project investigated the replacement of the conventional anthracite-sand filter media
in a C/F process for arsenic removal from groundwater with an ultra-light filter media with a specific gravity as low as 1.05, which greatly reduces the required backwash velocity. The same 5-ft
(1.5-m) vessel described above would produce as little as 600 gallons (2 m3) of water during backwashing. If this process modification were proven viable, it would overcome the primary drawback of the C/F process, and make the application of this technology highly desirable compared to
other more costly technologies.
CHARACTERIZATION OF POLYSTYRENE FILTER MEDIA
The zeta potentials of polystyrene media beads and sand media particles immersed in
groundwater were very similar within the pH range of 8.5 to 8.9, suggesting that the chemical
interaction between the ferric-arsenic floc material and the polystyrene media should be similar
to that between the floc material and a silica sand media typical of C/F applications for arsenic
removal from groundwater.
The polystyrene beads used as the ultra-light filter media for the majority of this study had
an effective size of 0.66 mm, a uniformity coefficient of 1.3, and a specific gravity of 1.05. For a
media with this specific gravity, the required backwash rate was calculated to be 2.1 gpm/ft2
(5.1 m hr).
INVESTIGATIVE FIELD TESTING
The objective of investigative field testing was to establish the performance of the polystyrene media for removing ferric floc and to identify the backwash procedures necessary to clean
the media effectively. While the polystyrene media was adequately fluidized at a backwash rate of
2 gpm/ft2 (4.9 m/hr), a backwash rate of 3 gpm/ft2 (7.3 m/hr) was found to be necessary to remove
79
Table 5.1
Optimized backwash conditions for polystyrene media
Backwash stage
Stage 1
Air scour
Stage 2
Stage 3
Rest
Flow velocity
2 scfm/ft2 (36.6 m/hr)
3.3 gpm/ft2 (8.0 m/hr)
1.5 gpm/ft2 (3.7 m/hr)
0 gpm/ft2 (0 m/hr)
Duration
2 min
1 min (during Stage 1)
8 min
2 to 4 min
3 to 5 min
the accumulated ferric floc material based on visual observation of the polystyrene media in the
clear PVC filter column.
Air scour of approximately 2 scfm/ft2 (36.6 m/hr) for 1 min was required to break up flocmedia agglomerates into small particles, allowing them to be removed by backwashing. A rest
period of 3 to 5 min with no flow was required for the media to settle completely and return to its
pre-backwash bed depth. The optimized backwash conditions determined during this testing are
summarized in Table 5.1.
A backwash screen was installed on the filter inlet pipe (which is the backwash water outlet
pipe) to minimize media loss during backwashing. Without the backwash screen, large quantities
of the polystyrene media were washed out the top of the filter during air scour.
Initial testing evaluated the filter effluent turbidity and iron concentrations produced by
the ultra-light polystyrene media in the pilot C/F unit. The polystyrene media was found to be
capable of meeting filter effluent turbidity and iron goals. Additional testing provided a preliminary
assessment of backwash effectiveness by determining filter run length and headloss accumulation
rates over 6 repeated filter runs. Filter run length and headloss at the time of turbidity breakthrough
were very consistent between the 6 repeated filter runs, indicating that the backwash regime defined
in Table 5.1 was effective for the ultra-light polystyrene media.
FIELD DEMONSTRATION TESTING
After investigative field testing, the next step of the study was to demonstrate the performance of the pilot C/F unit and the polystyrene filter media under real-world operating conditions.
The pilot C/F unit was moved to a groundwater well in Helendale, California, for field demonstration testing. Historically, the well had an arsenic concentration of approximately 21 µg/L. The pilot
C/F unit was placed in full automatic mode treating chlorinated well water as the source water, and
was allowed to operate uninterrupted for six weeks. The target ferric chloride dose was 0.95 mg/L
as Fe, and ranged from 0.7 to 1.2 mg/L as Fe during the 6 weeks of testing. The filtration rate was
held constant by the system PLC at 4 gpm/ft2 (9.8 m/hr). The turbidity breakthrough set point was
0.07 NTU. Backwash conditions were identical to those shown in Table 5.1, with the exception
that the duration of Stage 3 was 4 min instead of 2 min.
Twenty-five filter runs were performed during the six-week testing period. The average iron
concentration in the filter effluent was approximately 0.07 mg/L, and the average filter effluent
turbidity was 0.05 NTU. These results indicate that the polystyrene filter media reliably and effectively removed the floc material created by ferric chloride coagulation. The arsenic concentration
in the chlorinated source water varied between 17 and 25 µg/L, while the arsenic concentration in
Chapter 5: Summary and Conclusions | 81
Table 5.2
Backwash regime for long-term steady-state testing of polystyrene media
Backwash procedure
Stage 1
Stage 2
Rest
Flow velocity
2.0 gpm/ft2 (4.9 m/hr)
3.3 gpm/ft2 (8.1 m/hr)
0.0 gpm/ft2 (0.0 m/hr)
Duration
3 min
10 min
5 min
2 min
the filter effluent averaged approximately 6 µg/L. The fully-automated C/F unit steadily removed
arsenic over a six-week period.
Twenty-four of the 25 filter runs terminated on turbidity breakthrough at 0.07 NTU. One
filter run was terminated manually as discussed below. The average run length for the 25 runs was
38.8 hours. The average headloss at the time of turbidity breakthrough was 8.6 feet of H2O (26 kPa).
There was no trend of decreasing filter run length or increasing headloss at the time of turbidity
breakthrough that would suggest inadequate backwashing of the polystyrene media during the 25
During filter maturation, turbidity dropped rapidly from an initial peak of 0.2 to 0.3 NTU
to less than 0.1 NTU within 20 minutes of run time. Turbidity during the stable portion of the filter
run was 0.02 to 0.03 NTU. Headloss increased consistently during each filter run in a roughly linear trend, from a clean-bed headloss of approximately 0.7 feet of H2O (2 kPa) to between 7.3 and
10.6 feet of H2O (22 to 32 kPa) when the runs terminated on turbidity breakthrough.
The UFRV of the filter during the 25 filter runs was calculated at 9,312 gal/ft2 (379 m3/m2),
and the UFBV was calculated at 30.8 gal/ft2 (1.3 m3/m2). The backwash volume was measured by
collecting the backwash water from one complete backwash in buckets and measuring the volume
using a 1-L graduated cylinder. Based on the UFRV and UFBV calculated, the feed water recovery
was estimated at 99.7 percent.
LONG-TERM STEADY-STATE OPERATION WITH POLYSTYRENE MEDIA
Long-term steady-state operation was performed at the Elsinore Valley Municipal Water
District in Lake Elsinore, CA at a water treatment plant using the C/F process with conventional
sand/anthracite filter media to remove arsenic from groundwater. The polystyrene media depth
was 30 in (76 cm). The ferric chloride dose was 4.1 to 6.9 mg/L as Fe and the filtration rate was
4 gpm/ft2 (9.8 m/hr). The turbidity breakthrough criterion was 0.1 NTU and terminal headloss was
20 ft of H2O (60 kPa).
A 1-week evaluation of the need for air scour was performed before long-term steady-state
testing began. Without air scour included in the backwash regime, filter run length averaged only
4.1 hr in 21 filter runs. Observation of one backwash cycle indicated that the media bed moved
upward in the filter as a plug, and did not move to the bottom of the filter column until filtration
was resumed in the fully-automated pilot C/F unit. Based on these results, air scour was used for
all long-term steady-state testing. The backwash regime is shown in Table 5.2.
During the 4 months of long-term steady-state testing with polystyrene media, 168 full
filter runs were performed by the fully-automated pilot C/F unit. Filter run length averaged 11.9 hr
and the unit filter run volume (UFRV) averaged 2,856 gal/ft2 (116 m3/m2). In general, filter runs
terminated due to turbidity breakthrough. For the 168 filter runs, the headloss at run termination
averaged 15 ft of H2O (45 kPa). In general, headloss accumulation tended to be linear, and the
clean-bed headloss averaged 1.2 ft of H2O (3.6 kPa) for the 168 filter runs. The filter effluent turbidity during the stable operating portion of the 168 filter runs averaged 0.03 NTU, indicating
effective filtration performance by the polystyrene media.
No changes to the backwash regime were implemented during the 4 months of long-term
steady-state testing. The criteria for a properly-cleaned filter were defined for the study as:
In general, the clean media headloss ranged from 0.8 to 1.5 ft of H2O (2.4 to 4 kPa) for the
168 filter runs and there was no trend of increasing clean media headloss over time during the longterm steady-state testing. Few of the 168 filter runs terminated due to headloss, and there was no
trend of increasing headloss accumulation rate during the 4 months of testing. Further, there was
no trend of shorter filter run lengths, with filter run times generally between 11 and 13 hr during
the 4 months of testing. Filter effluent turbidity during the stable operating portion of the filter runs
was generally between 0.02 and 0.04 NTU, and there was no trend of increasing stable filter effluent turbidity over the 4 months of testing. All of these criteria indicate that the backwash regime
provided effective cleaning of the polystyrene media throughout the long-term steady-state testing.
The total volume of backwash water was collected during 7 filter runs throughout the
4 months of testing. For the 7 backwashes, the volume averaged 6.9 gal (26 L). The calculated
backwash volume based on the flow rates and durations shown in Table 5.2 was 7.7 gal (29 L). The
difference between measured and calculated backwash volumes is likely due to the time required
for valves to open and close during the backwash.
The feedwater recovery of the C/F process using the polystyrene media was calculated
using the average filter run length of 11.9 hr, the filtration rate of 4 gpm/ft2 (9.8 m/hr) and the average measured backwash volume of 6.9 gal (26 L). Based on these values, the feedwater recovery
was calculated at 98.8%.
During the 4 months of testing, a total of 47 arsenic samples were collected. Arsenic averaged 30 μg/L in the raw water and 3.9 μg/L in the filter effluent. Filter effluent arsenic concentrations ranged from <1 to 17 μg/L. Two of the 47 filter effluent arsenic samples exceeded the primary
MCL of 10 μg/L. Filter effluent iron concentrations averaged 0.03 mg/L, with a range of <0.01 to
0.14 mg/L; all filter effluent iron concentrations were less than 50% of the secondary MCL of
0.3 mg/L.
One filter run was sampled during filter maturation. Turbidity and iron reached peaks of
0.8 NTU and 0.36 mg/L, respectively, at 5 to 10 min of run time. Despite the fact that the filter was
backwashed with filtered water produced by the previous pilot plant filter run, the arsenic peak of
31 μg/L occurred at a run time of 1 min. Turbidity, iron and arsenic concentrations declined rapidly
after the initial peaks. Turbidity decreased below 0.1 NTU within 20 min, iron decreased below
0.1 mg/L within 15 min, and arsenic decreased below 10 μg/L within 10 min.
One filter run was sampled during turbidity breakthrough. There was a clear point at
approximately 12 hr of run time when filter effluent turbidity began to increase from the stable
minimum of 0.3 NTU. Turbidity increased from 0.03 NTU to the breakthrough criterion of 0.1 NTU
over a period of 1 hr. Filter effluent iron and arsenic samples were collected between 12.7 and 13.0
hr, as the turbidity increased from 0.07 to 0.10 NTU. During this time, filter effluent iron concentrations increased from 0.22 to 0.47 mg/L and filter effluent arsenic concentrations increased from
4.3 to 5.0 μg/L. These results indicate that as turbidity breakthrough occurred, breakthrough of
iron and arsenic was also occurring.
The polystyrene media was evaluated for the potential contribution of volatile organic
chemicals (VOCs) to the filtered water. New polystyrene beads were rinsed with chlorinated raw
water 10 times. Styrene was the only VOC compound not present in the chlorinated raw water but
present in the rinse water. Styrene found at 17 μg/L in the water from the first rinse and 6.5 μg/L in
the water from the tenth rinse, suggesting that the styrene was being washed out of the media with
use. It should be noted that the primary MCL for styrene is 100 μg/L, so the styrene concentrations
in both the first and tenth rinse samples were well below the MCL.
Scanning electron microscopy photographs of new polystyrene beads and a sample of
polystyrene beads removed from the pilot filter column after 4 months of operation indicated very
little wear on degradation. Both the new and used polystyrene beads were uniform, smooth spheres.
The used media had an irregular and thin coating of iron-rich material, which was likely residue
from the ferric chloride coagulant fed at the pilot plant.
STEADY-STATE OPERATION WITH ALTERNATIVE ULTRA-LIGHT MEDIA
An alternative ultra-light media was tested for 8 weeks at the same location as the long-term
steady-state testing with polystyrene media. The alternative media was Purofine PFC100E (The
Purolite Company, Bala Cynwyd, PA), a gel-type strong acid cationic exchange resin. Because it
was not regenerated, the ion exchange capacity of the PFC100E resin was likely exhausted early on
during the 8 weeks of testing and it functioned as an ultra-light filter media. The PFC100E media
had a density of 0.8 g/cm3, a specific gravity of 1.27, a uniformity coefficient of 1.2, and a size
range of 0.52 to 0.62 mm. The PFC100E media depth was 30 in (76 cm). The backwash regime
used for the PFC100E media was the same as that used for the polystyrene media, as defined in
Table 5.2. Operating conditions were the same for the PFC100E media as for the polystyrene
media, with the exception that terminal headloss was 25 ft of H2O (75 kPa) due to the higher clean
media headloss of the PFC100E media.
During the 8 weeks of steady-state operation with the PFC100E media, 81 full filter runs
were performed by the fully-automated pilot C/F unit. Filter run length averaged 12.4 hr and the
unit filter run volume (UFRV) averaged 2,976 gal/ft2 (122 m3/m2). Filter runs typically terminated
due to headloss. For the 81 filter runs, the headloss at run termination averaged 23.4 ft of H2O
(70 kPa). In general, headloss accumulation tended to be non-linear, with a more rapid headloss
accumulation rate during the first 4 hr of the filter run and then a slightly lower headloss accumulation rate thereafter. Clean-bed headloss averaged 6.0 ft of H2O (18 kPa) for the 81 filter runs. The
filter effluent turbidity during the stable operating portion of the 81filter runs averaged 0.02 NTU,
indicating effective filtration performance by the polystyrene media.
In general, the clean media headloss ranged from 5 to 8 ft of H2O (15 to 24 kPa) for the
81 filter runs and there was no trend of increasing clean media headloss over time during the
8 weeks of steady-state testing. There was no trend of increasing headloss accumulation rate in
successive filter runs during the 8 weeks of testing. Further, there was no trend of shorter filter run
lengths, with filter run times generally between 11 and 15 hr during the 8 weeks of testing. Filter
effluent turbidity during the stable operating portion of the filter runs was generally between
0.02 and 0.04 NTU, and there was no trend of increasing stable filter effluent turbidity over the
8 weeks of testing. All of these criteria indicate that the backwash regime provided effective cleaning of the PFC100E media throughout the 8 weeks of steady-state testing.
The feedwater recovery of the C/F process using the PFC100E media was calculated using
the average filter run length of 12.4 hr, the filtration rate of 4 gpm/ft2 (9.8 m/hr) and the average
measured backwash volume of 6.9 gal (26 L). Based on these values, the feedwater recovery was
calculated at 98.8%.
During the 8 weeks of testing, a total of 23 arsenic samples were collected. Arsenic averaged 34 μg/L in the raw water and 4.7 μg/L in the filter effluent. Filter effluent arsenic concentrations ranged from 2.1 to 8.2 μg/L. None of the 23 filter effluent arsenic samples exceeded the
primary MCL of 10 μg/L. Filter effluent iron concentrations averaged 0.03 mg/L, with a range of
<0.01 to 0.12 mg/L; all filter effluent iron concentrations were less than 50% of the secondary
MCL of 0.3 mg/L.
One filter run was sampled during filter maturation. Turbidity and iron reached peaks of
0.16 NTU and 0.53 mg/L, respectively, at 5 to 15 min of run time. As observed in the maturation
results from the polystyrene media, the arsenic peak of 36 μg/L occurred at a run time of 1 min,
which was earlier than the turbidity and iron peaks. Turbidity, iron and arsenic concentrations
declined rapidly after the initial peaks. Turbidity decreased below 0.1 NTU within 20 min, iron
decreased below 0.1 mg/L within 20 min, and arsenic decreased below 10 μg/L within 15 to
20 min.
OVERALL STUDY CONCLUSIONS
The key conclusions from this study are as follows:
1. Ultra-light polystyrene beads served as an effective filter media for removing ferric
floc (including adsorbed arsenic) from groundwater in a pressure C/F system at three
different testing locations with three different raw water qualities.
2. Seven months of pilot testing with two different raw water qualities using two different ultra-light filter media materials indicated that the C/F process with ultra-light
media was effective for arsenic removal from groundwater. In a total of 96 sample sets
collected during the study, raw water arsenic concentrations averaged between 20 and
30 μg/L and filter effluent arsenic concentrations averaged approximately 4 to 5 μg/L.
Only two of the 96 filter effluent arsenic samples had an arsenic concentration that
exceeded the maximum contaminant level (MCL) of 10 μg/L.
3. The maximum backwash rate of the ultra-light filter media (3.3 gpm/ft2 or 8.1 m/hr)
was less than 20 percent of that typically used for conventional sand/anthracite media
(18 to 20 gpm/ft2 or 44 to 49 m/hr).
4. Both types of ultra-light filter media materials tested (polystyrene beads and PFC100E
cationic exchange resin) were found to be effective filter media for the C/F system for
arsenic removal from groundwater.
5. The backwash regime for the ultra-light media included a maximum backwash rate of
3.3 gpm/ft2 (8.1 m/hr) and air scour. All of the study’s criteria for a properly cleaned
filter indicated that the backwash regime provided effective cleaning of the ultra-light
media throughout the study.
6. Air scour was critical for effective backwashing of the ultra-light media. Without air
scour, the maximum backwash velocity of 3.3 gpm/ft2 (8.1 m/hr) provided insufficient
energy for effective backwashing. Based on visual observation, air scour provided the
energy necessary to break up floc/media agglomerates and small mudballs. Floc particles could then be removed from the filter media at the low backwash rate used.
7. Feedwater recovery with the ultra-light filter media was approximately 99 percent.
8. The unit backwash volume during the study was approximately 30 to 35 gal/ft2 (1.2 to
1.4 m3/m2). For a very small system, this low unit backwash volume could likely be
accommodated by an on-site septic system or leach field. Further, the backwash flow
rate of 3.3 gpm/ft2 (8.1 m/hr) is sufficiently low that the backwash flow could be
routed directly to a sewer with no need for an equalization basin.
9. Considering that a typical filtration rate is on the order of 5 gpm/ft2 (12 m/hr), the
backwash flow rate of 3.3 gpm/ft2 (8.1 m/hr) could be provided by the production rate
of a parallel filter, thereby eliminating the need for a dedicated backwash pumping
system. This cannot be accommodated in a conventional sand/anthracite filter system
due to the high backwash rate required.
CHAPTER 6
RECOMMENDATIONS TO UTILITIES
This study demonstrated that ultra-light filter media could effectively and consistently
remove arsenic from groundwater in a C/F process. Due to the very low specific gravity of the
ultra-light media tested (1.05 to 1.27) compared to that of sand media (2.63) typically used in
C/F processes, the backwash requirements of the ultra-light media were demonstrated to be much
lower than those of conventional sand or anthracite media. The low backwash requirements have
significant implications for small systems with arsenic-contaminated groundwater or those with
constraints on disposal of waste backwash water.
There are numerous adsorptive technologies available for removing arsenic from groundwater in a well-head treatment system. However, these technologies are generally proprietary and
may be cost-prohibitive for very small systems. Further, adsorptive technologies present their own
set of challenges with respect to residuals handling and disposal. Conventional C/F systems are an
attractive option because the technology is not proprietary and is less costly than other technologies. However, use of conventional anthracite-sand filter media has relatively high backwash water
requirements. Many small systems do not have a sanitary sewer connection at well sites. Rather,
waste must be disposed in an on-site septic system. The high backwash water requirements are
incompatible with disposal to an on-site septic system. Other utilities may have constraints on the
flow rate or volume of waste backwash water that can be disposed of in a sanitary sewer.
As an example, assume a 5-foot (1.5-m) diameter filter in a C/F system for arsenic removal
from groundwater. The area of this filter is 19.6 ft2 (1.8 m2). A typical conventional filter may consist of a 12-inch (30.5 cm) sand layer under 24 inches (61 cm) of anthracite. A typical effective size
(ES) for the sand would be 0.55 mm, with a uniformity coefficient (UC) of 1.4 and a specific gravity of 2.63. The anthracite on top of the sand would have an ES of about 1.0 mm, a UC of 1.4, and a
specific gravity of 1.7. Using Equation 2.1, the backwash rate for this filter is estimated at 19 gpm/
ft2 (47 m/hr). The backwash rate of the ultra-light media was established in the study as 3.3 gpm/
ft2 (8.1 m/hr). Thus, for a filter area of 19.6 ft2 (1.8 m2), the backwash rate for the anthracite-sand
filter would be 373 gpm (85 m3/hr) and the backwash rate for the ultra-light filter media would be
65 gpm (15 m3/hr). The required backwash rate for the ultra-light media filter would be 17% of that
required for the anthracite-sand filter.
Taking the example a step further, the total backwash volume can be calculated for each
filter by scaling up an example backwash procedure and applying it to both types of media. The
results are shown in Table 6.1.
Table 6.1 shows that the total backwash volume produced by the ultra-light media filter
was 674 gal (2.6 m3), while the total backwash volume produced by the anthracite-sand filter was
3,927 gal (15 m3). The backwash volume produced by the ultra-light media filter was only 17 percent of that produced by the anthracite-sand filter.
The above example illustrates that both the backwash rate (373 gpm, 85 m3/hr) and the
total backwash volume (3,927 gal, 15 m3) produced by a conventional anthracite-sand media in
a C/F process are incompatible with the waste disposal limitations faced by many small systems.
At a minimum, an equalization basin would be required to capture the high flow rate of the backwash water. With the ultra-light filter media, both the backwash rate (65 gpm, 15 m3/hr) and total
backwash volume (674 gal, 2.6 m3) could be accommodated by a sewer connection or on-site
87
Table 6.1
Comparison of backwash volumes for typical anthracite-sand and ultra-light media filter
Flow velocity (gpm/ft2)
Backwash stage
sand
filter media
6
1.0
Stage 1
Stage 2
19
3.3
9
1.5
Stage 3
Total backwash volume (gal)
Duration
(min)
2
8
4
Backwash volume (gal)
sand
filter media
236
39
2,984
518
707
117
3,927
674
septic system without equalization. This greatly simplifies the design and operation of the C/F
process.
Another consideration is the source of backwash water. The C/F system used in the field
demonstration testing phase of the study utilized untreated chlorinated well water as backwash
water. The benefit of using the untreated well water is that it is already under pressure, so no pumping is required. The use of filtered water as the source of backwash water would require a filtered
water storage tank and a backwash pump; these components add complexity to the system and
increase its footprint. Alternatively, pressurized water from the distribution system could be used
for backwashing; however, the necessary piping and backflow prevention would also add complexity to the system. The trade-off for using untreated well water as the source of backwash water
is the arsenic spike that was observed during filter maturation as the backwash water was displaced
from the filter. As discussed in Chapter 4, the short-duration arsenic peak during filter maturation
does not suggest the need for filter-to-waste since the average arsenic concentration over the duration of the filter run was well below the 10-µg/L MCL.
If filtered water was used as the source of backwash water, as implemented during the
steady-state testing phase of the study, the ultra-light filter media still offers substantial advantages
for a small system over conventional filter media. The filtration rate used during this study was
4 gpm/ft2 (9.8 m/hr). The maximum backwash flow rate was 3.3 gpm/ft2 (8.1 m/hr). Thus, one
pressure filtration vessel produces more than enough flow to completely backwash another vessel
without interrupting filter operation. The system could be designed such that two pressure vessels
would be operational; water produced by one vessel would be used to backwash the other vessel. This is a standard design in many industrial applications, such as ion exchange systems. On
the other hand, the maximum backwash flow rate for the conventional anthracite-sand media was
19 gpm/ft2 (47 m/hr). Even if the filtration rate was 6 gpm/ft2 (15 m/hr) rather than the 4 gpm/ft2
(9.8 m/hr) used in this study, the flow of three pressure vessels would be required to produce water
for backwashing only one vessel.
This study demonstrated that the use of ultra-light filter media in a C/F process for arsenic removal from groundwater offered many advantages with respect to the very low backwash
requirements compared to those of conventional anthracite-sand media.
CHAPTER 7
FURTHER RESEARCH NEEDS
While promising results were obtained in this study for using ultra-light filter media in a
C/F process for arsenic removal from groundwater, a large-scale demonstration study is necessary
to fully assess the use of ultra-light media as a replacement for conventional anthracite-sand media
in a C/F process. All pilot testing for this study was performed using a 6-inch (15-cm) filter column,
and a source water flow rate on the order of 1 gpm (4 L/min). The next step toward full-scale implementation would be a demonstration-scale evaluation on the order of 50 gpm (190 L/min), typical
of very small systems. Ideally, the demonstration study would evaluate a full-scale C/F system
using ultra-light media to treat water from an arsenic-contaminated well under field conditions. A
scale on the order of 50 gpm (190 L/min) would allow the use of a full-size pressure vessel. This
type of demonstration study would provide valuable information for proceeding with full-scale
implementation of the C/F process using ultra-light media for arsenic removal from groundwater.
89
APPENDIX A
QUALITY ASSURANCE/QUALITY CONTROL RESULTS
Quality assurance/quality control (QA/QC) results for the key water quality parameters
arsenic and iron are provided herein to demonstrate the precision and accuracy by presenting
results from duplicate samples analyzed at the on-site and commercial water quality laboratories
employed in the study, as well as results of standard curves for the iron samples analyzed on site.
Duplicate arsenic and iron samples were collected from the raw water (arsenic only), filter
influent (iron only), and filter effluent during the pilot study. The relative percent difference (RPD)
of the duplicate samples was calculated using Equation A.1. A relative percent difference of ±25
percent or less was considered acceptable.
RPD = 100 #
y1 − y2
ty
(A.1)
where y1 = result of sample 1
y2 = result of duplicate sample 2
ŷ = average of y1 and y2
ARSENIC ANALYSES
Duplicate arsenic samples were collected at the pilot plant and analyzed by MWH
Laboratories (Monrovia, CA) approximately weekly during the long-term steady-state testing with
polystyrene media and the steady-state testing with PFC100E media. In general, duplicate samples
were collected of the raw water and filter effluent on alternating weeks. Results are shown in
Table A.1, along with the calculated RPD . The table indicates that all RPDs were well below 10%,
and the RPD was 0% for the majority of duplicate sample sets.
IRON ANALYSES
Hach Ferrozine Iron Method Calibration Check
The accuracy of the Hach ferrozine iron method and the Hach DR890 colorimeter (Hach
Company, Loveland, CO) were verified according to the procedure outlined in the Hach DR890
manual at the beginning of investigative field testing and throughout the steady-state testing with
polystyrene media and PFC100E media.
For the investigative field testing, the verification procedure involved preparation of a
0.4 mg/L iron standard followed by standard additions of 0.1 mg/L iron. A volume of 1.62 mL of
a 25 mg/L iron standard was pipetted and diluted to 100 mL in a volumetric flask, resulting in a
calculated iron concentration of 0.427 mg/L for the solution. A volume of 25 mL of this solution
was placed in a sample cell and the Hach DR890 colorimeter was zeroed. A volume of 0.5 mL of
ferrozine reagent was added to this solution and allowed to react for 5 minutes. The measured iron
concentration was 0.435 mg/L, an error of +1.9 percent. Three standard additions of 0.1 mg/L iron
were performed by pipetting 0.1 mL of the Hach 25 mg/L iron standard into the 25-mL sample vial
91
Table A.1
Results of duplicate arsenic measurements during long-term steady-state
testing with polystyrene media and long-term testing with PFC100E media
Date
5/26/2009
6/1/2009
6/8/2009
6/15/2009
6/22/2009
6/29/2009
7/6/2009
7/13/2009
7/20/2009
7/27/2009
8/3/2009
8/24/2009
9/8/2009
9/14/2009
9/28/2009
10/5/2009
10/12/09
10/19/2009
10/26/2009
11/2/2009
11/9/2009
11/16/2009
Sample location
Raw
Effluent
Raw
Effluent
Raw
Effluent
Raw
Effluent
Raw
Effluent
Effluent
Raw
Effluent
Raw
Effluent
Raw
Effluent
Raw
Effluent
Raw
Effluent
Raw
Arsenic (µg/L)
Original result
Duplicate
12
12
6.1
5.7
30
29
4.1
4.0
37
37
2.2
2.2
37
37
2.8
2.9
33
33
1.1
1.1
2.2
2.2
31
31
3.9
3.9
29
29
2.5
2.5
33
33
2.8
2.7
32
33
2.2
2.2
38
38
5.5
5.5
37
37
RPD
(%)
0.0
6.8
3.4
2.5
0.0
0.0
0.0
–3.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
3.6
–3.1
0.0
0.0
0.0
0.0
and allowing an additional 5-minute reaction time before measurement. The resulting measured
iron concentrations were 0.539 mg/L, 0.642 mg/L and 0.744 mg/L, representing errors of +0.75%,
+1.10% and +1.22%, respectively.
For the steady-state testing with polystyrene media and PFC100E media, a similar procedure was used approximately monthly during pilot plant operation, except that a 0.5 mg/L iron
standard was prepared by dilution of a 25 mg/L standard solution obtained from Hach. Standard
additions of 0.1 mg/L were then performed. Results are shown in Table A.2. The table indicates
that all method verifications at all concentrations produced errors less than 5%.
Duplicate Iron Measurements
Duplicate iron samples were collected and analyzed at the pilot plant approximately weekly
during the field demonstration testing. Results are shown in Table A.3.
All of the duplicate filter influent iron measurements had an RPD of 2.5 percent or lower.
Two of the four duplicate filter effluent iron measurements had an RPD less than 4%. The two
RPDs that exceeded 4% were from duplicate filter effluent samples which had iron concentrations near the method detection limit of 0.01 mg/L. The single set of duplicate samples outside
Appendix A: Quality Assurance/Quality Control Results | 93
Table A.2
Results of iron method verification during long-term steady-state testing with
polystyrene media and long-term testing with PFC100E media
0.5 mg/L
0.524
–4.8%
0.496
0.8%
0.506
–1.2%
0.5
0.0%
0.523
–4.6%
0.521
–4.2%
Measured
% Difference
Measured
% Difference
Measured
% Difference
Measured
% Difference
Measured
% Difference
Measured
% Difference
Results for Iron Standard
0.6 mg/L
0.7 mg/L
0.622
0.723
–3.7%
–3.3%
0.6
0.697
0.0%
0.4%
0.611
0.714
–1.8%
–2.0%
0.602
0.701
–0.3%
–0.1%
0.623
0.715
–3.8%
–2.1%
0.622
0.721
–3.7%
–3.0%
0.8 mg/L
0.821
–2.6%
0.793
0.9%
0.818
–2.2%
0.802
–0.3%
0.812
–1.5%
0.818
–2.2%
Date
4/2/2009
7/1/2009
8/19/2009
9/10/2009
10/5/2009
11/6/2009
Table A.3
Results of duplicate iron analyses during field demonstration testing
Run #
524
520
512
510
507
505
523
513
509
508
Iron (mg/L)
Original result
Duplicate
Filter influent samples
2.23
2.21
2.05
2.07
1.70
1.70
2.00
2.05
3.84
3.87
4.51
4.52
Filter effluent samples
0.020
0.020
0.024
0.020
0.026
0.025
0.015
0.008
Relative percent difference
(%)
0.9
1.0
0.0
2.5
0.8
0.2
0.0
18
3.9
61
the acceptance criterion of ±25 percent (RPD = 61 percent) had one measurement just above the
method detection limit and one measurement below the method detection limit.
Duplicate iron samples were collected at the pilot plant filter influent and effluent approximately weekly during the steady-state testing with polystyrene media and PFC100E media.
Samples were analyzed at the pilot plant using the Hach method. Results are shown in Table A.4.
Data outside the 25% acceptance criteria are indicated in bold font in the table. Of the 23 weekly
sample sets, the RPD of 6 sample sets are outside the 25% acceptance criteria. All six of these
sample sets are from the filter influent sampling location, and all have iron concentrations at or
near the manufacturer’s reported method detection limit of 0.01 mg/L.
Table A.4
Results of duplicate iron measurements during long-term steady-state testing with
polystyrene media and long-term testing with PFC100E media
Iron (mg/L)
Date
6/8/09
6/15/09
6/22/09
6/29/09
7/6/09
7/13/09
7/20/09
7/27/09
8/3/09
8/10/09
8/17/09
8/31/09
9/8/09
9/14/09
9/21/09
9/28/09
10/5/09
10/12/09
10/19/09
10/26/09
11/2/09
11/9/09
11/16/09
Sample location
Influent
Effluent
Influent
Influent
Effluent
Influent
Effluent
Influent
Effluent
Effluent
Effluent
Influent
Effluent
Effluent
Influent
Effluent
Influent
Effluent
Influent
Effluent
Influent
Influent
Effluent
Original
result
4.34
0.003
4.33
7.31
0.018
5.22
0.107
4.94
0.012
0.105
0.038
5.26
0.014
0.016
5.25
0.020
5.05
0.011
5.25
0.017
5.28
5.37
0.004
Duplicate
4.40
0.000
4.35
7.37
0.009
5.15
0.127
4.98
0.016
0.128
0.042
5.18
0.020
0.018
5.33
0.025
5.20
0.008
5.23
0.014
4.96
5.25
0.002
Relative
percent
difference
1.4
200
0.5
0.8
67
1.4
17
0.8
29
20
10
1.5
35
12
1.5
22
2.9
32
0.4
19
6.3
2.3
67
REFERENCES
Jekel, M., and R. Seith. 2000. Comparison of Conventional and New Techniques for the Removal
of Arsenic in a Full Scale Water Treatment Plant. Water Supply, 18(1/2):628–631.
Karcher, S., L. Caceres, M. Jekel, and R. Contreras. 1999. Arsenic Removal From Water Supplies
in Northern Chile Using Ferric Chloride Coagulation. Jour. Chart. Inst. Water Environ.
Manage., 13(3):164–169.
Kawamura, S. 1975. Design and Operation of High-Rate Filters—Part 2. Jour. AWWA,
67(11):653–662.
WQTS. 2004. Pilot Testing of Coagulation-Filtration for Arsenic Removal. Final Report submitted
to Bowen Collins & Associates, and the Virgin Valley Water District, Mesquite, Nevada.
95
ABBREVIATIONS
ASME
American Society of Mechanical Engineers
BBGWTP
BOD
Back Basin Groundwater Treatment Plant
biochemical oxygen demand
°C
C/F
cm
COD
degrees Celsius
coagulation/filtration
centimeter
chemical oxygen demand
ES
EPA
EVMWD
effective size
Environmental Protection Agency
Elsinore Valley Municipal Water District
ft
ft2
foot
square foot
gal
gal/ft2
g/cm3
gpm
gpm/ft2
gallon
gallons per square foot
grams per cubic centimeter
gallons per minute
gallons per minute per square foot
hr
hour
ID
IMS
inside diameter
integrated media support
kPa
kilopascal
L
L/d
liter
depth-to-diameter ratio
m
MCL
MCLG
mg/L
m/hr
m3
m3/hr
m3/m2
min
meter
maximum contaminant level
maximum contaminant level goal
milligrams per liter
meters per hour
cubic meter
cubic meters per hour
cubic meters per square meter
minute
97
ML
mm
mV
µg/L
Montgomery Laboratories
millimeter
millivolt
micrograms per liter
NSF
NTU
NSF International
nephelometric turbidity unit
OD
outside diameter
PCU
PLC
psi
PVC
platinum-cobalt color unit
programmable logic controller
pounds per square inch
polyvinyl chloride
RPD
relative percent difference
scfm/ft2
SEM
SM
standard cubic feet per minute per square foot
scanning electron microscopy
Standard Methods
TDS
THM
TON
TSS
total dissolved solids
trihalomethane
threshold odor number
total suspended solids
UC
UFBV
UFRV
USEPA
uniformity coefficient
unit filter backwash volume
unit filter run volume
United States Environmental Protection Agency
VOC
VVWD
volatile organic chemical
Virgin Valley Water District
WQTS
Water Quality & Treatment Solutions, Inc.
Minimizing Backwash Volume From Coagulation/Filtration for Arsenic Removal
6666 West Quincy Avenue, Denver, CO 80235-3098 USA
P 303.347.6100 • F 303.734.0196 • www.WaterResearchFoundation.org
3164
1P-3C-3164-11/10-FP

Minimizing Backwash Volume From Coagulation/Filtration for

Transcription

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