HomeMy WebLinkAboutFinal Report THM
Town of Cohasset
Trihalomethanes Removal Evaluation
Final Report
Tt No.: 200-121837-18001
July 3, 2019
Town of Cohasset
Trihalomethanes Removal Evaluation
Final Report
PRESENTED TO PREPARED BY
Town of Cohasset
41 Highland Ave.
Cohasset, MA 02025
Tetra Tech
160 Federal St., 3rd Floor
Boston, MA 02110
P +1-617-443-7500
F +1-407-839-3790
www.tetratech.com
Tt #200-121837-18001
July 3, 2019
Trihalomethanes Removal Evaluation Final Report
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TABLE OF CONTENTS
1.0 INTRODUCTION ..................................................................................................................................................1
1.1 Background ....................................................................................................................................................1
1.2 Purpose ..........................................................................................................................................................2
1.3 Scope .............................................................................................................................................................2
2.0 OVERVIEW OF THE COHASSET WATER SYSTEM .........................................................................................3
2.1 Water Supply ..................................................................................................................................................3
2.2 Water Treatment Facilities .............................................................................................................................6
2.2.1 Lily Pond Water Treatment Plant..........................................................................................................6
2.2.2 Ellms Meadow Pumping Station ...........................................................................................................7
2.3 Distribution .....................................................................................................................................................7
2.4 Water Production ...........................................................................................................................................7
2.5 Water Quality .................................................................................................................................................9
2.6 Prior Studies ................................................................................................................................................ 14
2.6.1 Disinfection Byproduct Compliance Evaluation Report ..................................................................... 14
2.6.2 Pilot Test Report Polyaluminum Chloride .......................................................................................... 15
2.6.3 Pilot Test Report MIEX DOC Resin Treatment ................................................................................. 16
2.6.4 Pilot Test Report Ozone/BAC Treatment .......................................................................................... 16
3.0 DISINFECTION BYPRODUCTS REVIEW AND ANALYSIS ........................................................................... 19
3.1 DBP Formation and Control ........................................................................................................................ 19
3.2 DBP Regulatory Perspective ...................................................................................................................... 20
3.3 Analysis of Historical DBP Data .................................................................................................................. 20
3.4 Water Quality goals and objectives ............................................................................................................. 27
3.4.1 Development of DBP Water Quality and Treatment Goals ............................................................... 28
3.4.2 Development of TOC Water Quality and Treatment Goals ............................................................... 28
4.0 DISINFECTION BYPRODUCT CONTROL ALTERNATIVES .......................................................................... 31
4.1 Aeration Treatment ..................................................................................................................................... 31
4.1.1 Treatment Process and Technology Overview ................................................................................. 31
4.1.2 Floating Spray Nozzle THM Removal System .................................................................................. 32
4.1.3 Integration of Spray Aeration within the Cohasset System ............................................................... 33
4.2 GAC/BAC Treatment ................................................................................................................................... 34
4.2.1 Calgon Carbon GAC Media and Pressurized Vessel System ........................................................... 36
4.2.2 Integration of GAC Adsorption at the Lily Pond WTP ....................................................................... 37
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4.3 Biologically Activated Carbon (BAC) Treatment ......................................................................................... 38
4.3.1 Ozonation .......................................................................................................................................... 40
4.3.2 Integration of BAC Treatment at the Lily Pond WTP ......................................................................... 42
4.4 Ion Exchange Treatment ............................................................................................................................. 42
4.4.1 Tonka Water OrganixTM System ........................................................................................................ 43
4.4.2 Integration of IX Treatment at the Lily Pond WTP ............................................................................. 44
4.5 Convert to Monochloramine Disinfectant Residual ..................................................................................... 45
4.5.1 Integration of Chloramination within the Cohasset System ............................................................... 46
5.0 CONCEPTUAL EVALUATION OF DBP CONTROL ALTERNATIVES ........................................................... 47
5.1 Advantages and Disadvantages ................................................................................................................. 47
5.2 Treatment Performance .............................................................................................................................. 48
5.3 Conceptual Cost Analysis ........................................................................................................................... 48
5.4 Conclusions and Recommendations .......................................................................................................... 49
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LIST OF TABLES
Table 2-1. Lily Pond and Aaron River Reservoir ..................................................................................................5
Table 2-2. Ellms Meadow Wellfield ........................................................................................................................5
Table 2-3. 2017 Water Supply Withdrawal .............................................................................................................6
Table 2-4. Finished Water Production ...................................................................................................................8
Table 2-5. Raw and Finished Water Quality ....................................................................................................... 10
Table 2-6. 2017 Microbial Compliance Sampling Results ................................................................................ 12
Table 2-7. 2017 Inorganic Compliance Sampling Results ................................................................................ 12
Table 2-8. 2017 Disinfectant/Disinfectant Byproducts Compliance Sampling Results ................................. 13
Table 2-9. 2017 Radiological Compliance Sampling Results ........................................................................... 13
Table 2-10. 2017 Lead and Copper Compliance Sampling Results ................................................................. 13
Table 2-11. 2017 Filter Performance Compliance Sampling Results .............................................................. 14
Table 2-12. 2017 Secondary Contaminant Compliance Sampling Results..................................................... 14
Table 3-1. Regulated DBP Species ...................................................................................................................... 20
Table 3-2. Water Quality and Treatment Goals .................................................................................................. 28
Table 3-3. Estimated TOC Removal Requirements ........................................................................................... 30
Table 4-1. FILTRASORB® 400 Technical Specifications ................................................................................... 36
Table 4-2. Model 12-40 System Dimensions and Operating Conditions .......................................................... 37
Table 4-3. OrganixTM System Dimensions and Operating Conditions ............................................................. 43
Table 5-1. Advantages and Disadvantages ........................................................................................................ 47
Table 5-2. Treatment Performance Conceptual Evaluation ............................................................................. 48
Table 5-3. Conceptual Cost Opinions ................................................................................................................. 49
LIST OF FIGURES
Figure 1-1. Cohasset Water System Structure .....................................................................................................1
Figure 2-1. Town of Cohasset Water System Map ................................................................................................4
Figure 2-2. Lily Pond WTP Raw and Finished Water TOC ................................................................................ 11
Figure 2-3. Lily Pond WTP Raw and Finished Water Turbidity ........................................................................ 11
Figure 3-1. Cohasset Water Distribution System TTHMs ................................................................................. 21
Figure 3-2. Cohasset Water Distribution System HAA5 .................................................................................... 21
Figure 3-3. Deer High School Location TTHMs ................................................................................................. 22
Figure 3-4. Police Station Location TTHMs ....................................................................................................... 23
Figure 3-5. 2 Whitehead Location TTHMs .......................................................................................................... 23
Figure 3-6. 4 Beechwood Location TTHMs ........................................................................................................ 24
Figure 3-7. Average Distribution System TTHM Speciation by Location ....................................................... 24
Figure 3-8. Average Distribution System TTHM Speciation by Location ....................................................... 25
Figure 3-9. Lily Pond WTP Filtered Water - Simulated TTHM Formation Potential ........................................ 26
Figure 3-10. Lily Pond WTP Filtered Water - Simulated HAA5 Formation Potential ....................................... 26
Figure 3-11. Lily Pond WTP Filtered Water - DBP Formation Potential Curves ............................................. 27
Figure 3-12. DBP Formation versus Filtered Water TOC (MIEX Pilot Test Report) ....................................... 29
Figure 3-13. DBP Formation versus Filtered Water TOC (Full-Scale Data) .................................................... 29
Figure 3-14. TOC Removal Percent Distribution ............................................................................................... 30
Figure 4-1. Spray Aeration Treatment Schematic ............................................................................................. 34
Figure 4-2. GAC/BAC Treatment Schematic ...................................................................................................... 38
Figure 4-3. Ozone-BAC Treatment Schematic ................................................................................................... 42
Figure 4-4. Ion Exchange Treatment Schematic ............................................................................................... 44
Figure 4-5. Breakpoint Chlorination Curve ........................................................................................................ 45
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AC RONYMS/ABBREVIATIONS
Acronyms/Abbreviations Definition
µg/L Microgram per Liter
Ave. Avenue
AWWA American Water Works Association
BAC Biologically Active Carbon
bls below land surface
BV Bed Volume
CaCO3 Calcium Carbonate
CUR Carbon Usage Rate
D/DBP Disinfectants and Disinfection Byproducts
DBP(s) Disinfection Byproduct(s)
DOC Dissolved Organic Carbon
Dr. Drive
EBCT Empty Bed Contact Time
ft Feet
g/cm3 Grams per Centimiter Cubed
GAC Granular Activated Carbon
gal Gallon(s)
gpd Gallons per Day
gpm Gallons per Minute
H2S Hydrogen Sulfide
HAA5 Haloacetic Acids (Five Regulated Species)
HAAs Haloacetic Acids
hp Horsepower
Hwy. Highway
IX Ion Exchange
lbs Pounds
LRAA Locational Running Annual Average
MCL(s) Maximum Contaminant Level(s)
mg/L Miligram per Liter
MGD Million Gallons per Day
MIEX® Magnetic Ion Exchange
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Acronyms/Abbreviations Definition
min Minute(s)
mm Milimiter
MOR(s) Monthly Operating Report(s)
NF Nanofiltration
NPDOC Non-Purgeable Dissolved Organic Carbon
ORP Oxidation Reduction Potential
PDR Preliminary Design Report
POE Point of Entry
psi Pounds per Square Inch
psig Pounds per Square Inch Gauge
PWS Public Water System
Rd. Road
RO Reverse Osmosis
St. Street
TDS Total Dissolved Solids
THMs Trihalomethanes
TOC Total Organic Carbon
TTHMs Total Trihalomethanes
USEPA United States Environmental Protection Agency
UV 254 Ultraviolet Adsorption at 254 nanometer wavelength
WTP Water Treatment Plant
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1.0 INTRODUCTION
1.1 BACKGROUND
The Town of Cohasset (Town) delivers an average 0.81 million gallons per day (MGD) of potable water to its
estimated 7,200 residents and the Linden Pond complex from a combination of treated surface water and
groundwater sources. Surface water is supplied by Lily Pond and Aaron River Reservoir. Lily Pond served as the
Town’s original source of water supply, which was then supplemented by the construction of the Aaron River
Reservoir in 1978. The raw surface water supply is treated by a conventional coagulation-clarification-filtration
treatment process with free chlorine disinfection at the Lily Pond Water Treatment Plant (WTP). The City’s
groundwater source is supplied from the Ellms Meadow Wellfield. The wellfield pumps surficial groundwater from a
shallow glacial valley aquifer located along James Brook. The groundwater is chlorinated and stabilized for
corrosion control prior to being pumped directly into the distribution system via the Ellms Meadow pumping station.
The City targets an approximate 88% to 12% split between the surface and groundwater sources, respectively. A
generalized schematic of City’s water supply structure is illustrated in Figure 1-1.
The natural organic matter present in the Lily Pond and Aaron River Reservoir source waters react with the chlorine
disinfectant to form regulated disinfection byproducts (DBPs). The level of organic matter, measured as total organic
carbon (TOC), varies with season and with the source composition within the reservoir. Because of this, TOC levels
can range from approximately 4.6 mg/L to upwards of 16 mg/L in the raw water and from approximately 1.6 mg/L
to 7.0 mg/L in the combined filtered water. The range in filtered water TOC creates a potential for regulated DBPs
to approach and at times go over the regulated limit.
With the onset of EPA’s Stage 2 Disinfectants and Disinfection Byproducts (D/DBP) Rule, the Town was proactive
in the early 2000s to investigate and implement process modifications to enhance treatment efficiency and reduce
the potential for formation of regulated DBPs. As part of these efforts, the Town completed a DBP compliance
evaluation in 2004 and a series of bench- and pilot-scale studies in 2005 and 2006 that examined the performance
of alternative organic removal treatment processes. The organic removal technologies that were investigated
included magnetic ion exchange (MIEX®) resin, ozonation with biologically active carbon (BAC) filtration, and
polyalum inum chloride use. Based on the results from the DBP compliance evaluation, the Town implemented a
series of recommendations, including the optimized use of polyaluminum chloride and potassium permanganate in
the coagulation process, re-activation of groundwater sources, and optimization of chlorine dosing. These
improvements have allowed the Town to remain in compliance with the Stage 2 D/DBP Rule.
Figure 1-1. Cohasset Water System Structure
Supply•Lily Pond•Aaron River Reservoir•Ellms Meadow Wellfield
Treatment•Lily Pond WTP•Ellms Meadow Pump Station
Distribution•Cohasset Water System Service Area
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1.2 PURPOSE
Although the Cohasset water system is currently in compliance with the Stage 2 D/DBP rule, the potential for
exceedance still exists particularly of total trihalomethanes (TTHMs) during the summer season when higher TOC
levels and temperatures can drive their formation. Therefore, the Town has determined that a reduction in TTHMs
in the finished water is necessary for enhancing distribution system water quality and provide ongoing compliance
with the Stage 2 D/DBP rule. To this end, the Town retained Tetra Tech to review historical water quality data and
previous study results and provide recommendations on to help the Town select a TTHM reduction strategy. The
purpose of this report is to summarize the Town’s historical DBP water quality and previous studies; develop and
evaluate alternative DBP control strategies; and provide recommendations for an effective DBP control strategy.
1.3 SCOPE
The Town contracted with Tetra Tech to perform a conceptual review and evaluation to assist in selecting a DBP
reduction alternative for reducing TTHM levels in their drinking water system. Tetra Tech’s scope of services
includes the following major tasks:
1. Conduct a review of existing and historical system data, including raw and treated water quality (TOC,
TTHMs, HAA5, and chlorine residual as available), finished water production, treatment schemes and
operation, and previous studies. The reviewed existing information will be summarized and analyzed to
identify seasonal patterns, DBP speciation, treatment efficiencies, and locational variation. The previous
study reports to be reviewed include:
• Disinfection Byproduct Compliance Evaluation – March, 2004
• Pilot Test Report Ozone/BAC Treatment – November, 2006
• Pilot Test Report MIEX DOC Resin Treatment – November, 2006
• Pilot Test Report Polyaluminum Chloride – November, 2006
2. Develop water quality goals and objectives for TTHM concentrations in the distribution system and
treatment requirements relative to TOC removal and TTHM reduction.
3. Develop TTHM control alternatives and perform a desktop comparison evaluation based upon capacity
requirements, budget cost for implementation including design and construction cost, significant
advantages and disadvantages, relative ease of operation and TOC and/or TTHM reduction potential.
4. Prepare a summary report that compiles the information developed in the study, including the system
data, comparison evaluation findings, and recommendations.
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2.0 OVERVIEW OF THE COHASSET WATER SYSTEM
The Town of Cohasset Water System (PWS ID #: 4065000) is a community system that serves a population of
approximately 7,200 people through 2,637 service connections throughout their water distribution system. The
Town’s distribution system consists of over 40-miles of 6-inch to 14-inch diameter piping and is served by two water
treatment facilities—the Lily Pond WTP and the Ellms Meadow Pump Station. Surface water from the Lily Pond and
Aaron River Reservoir supplies raw water to the Lily Pond WTP; and groundwater from the Ellms Meadow Wellfield
supplies raw water to the Ellms Meadow Pump Station. Additionally, the Town’s water system is equipped with two
(2) emergency interconnects with the Hingham water system and the Scituate water system, respectively. In the
event of a water emergency, these interconnects could be opened to transfer water from one system to another.
An overview map of the Cohasset Water System is included in Figure 2-1.
2.1 WATER SUPPLY
The Town’s current water supplies consist of two surface water reservoirs (Lilly Pond and Aaron River Reservoir),
one active groundwater wellfield (Ellms Meadow Wellfield), and one inactive groundwater wellfield (Sohier Street
Wellfield). These surface and groundwater supplies are located within the South Coastal watershed basin. The
watershed for Lily Pond and the Aaron River Reservoir covers an area of about 7.5 square miles, of which about
40% falls within the Town boundary. Lily Pond has a maximum storage capacity of about 80 million gallons (MG)
and is about 52 acres in size. The Aaron River Reservoir can store up to about 480 MG across 150 acres of surface
area. The safe yield for both the Lily Pond and Aaron River Reservoir is approximately 5.2 million gallons per day
(MGD), and the permitted maximum day withdrawal is 3.0 MGD. The Ellms Meadow Wellfield, located at 24 James
Lane, consists of one supply well that can supply up to 141,000 gallons per day (gpd). A summary of the Town’s
surface and groundwater supply features is presented in Table 2-1 and Table 2-2. The raw water withdrawals from
each source for 2017 are summarized in Table 2-3.
Since glacial times, Lily Pond has obtained recharge water primarily from Brass Kettle Brook, Peppermint Brook,
and overland flow. Brass Kettle Brook has a watershed that is located to the northwest of the Pond and enters in
the southwestern quadrant of the lake. Peppermint Brook, which has a watershed to the north and northeast of Lily
Pond, enters at the top end of the waterbody. Herring Brook, which normally serves as the pond’s outlet, can serve
as a tributary to Lily Pond when water is impounded at the Bound Brook control structure. The Bound Brook control
structure, located on Beachwood Street, regulates discharge to meet fish passage requirements under the Town’s
Water Management Act permit and is tied to water restrictions.
Lily Pond can receive hydrologic inputs from seasonal releases from the Aaron River Reservoir, which is located
upstream of Lily Pond. Aaron River Reservoir receives recharge from the Aaron River and the Aaron River
Watershed, mostly located in large wetland systems in Norwell. Due to the control elevation at the Bound Brook
control structure, water released by the Aaron River Reservoir may be pooled in the wetlands located to the south
of Lily Pond. During periods when the surface elevation of Lily Pond falls below the controlling elevation at the
Bound Brook control structure or during times of high water demands and withdrawals from Lily Pond, there is a
potential for water to be drawn into Lily Pond from the Aaron River Reservoir via Herring Brook. Any unneeded
water from the Aaron River Reservoir is discharged through the Bound Brook control structure. The flow of water
from Aaron River Reservoir to Lily Pond is not metered; thus, raw water withdrawals are measured at the Lily Pond
WTP intake.
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Overall, the organic color (brown-colored water mostly from tannins) in the tributary brooks to Lily Pond is relatively
low. However, observations of water leaving Bound Brook in Norwell have indicated that the water can have high
tannin levels before it reaches Aaron River, except during peak flow periods. Post glacial soils and vegetation were
not completely removed from the Aaron River Reservoir area when constructed. Furthermore, wetland areas
supplying the Aaron River Reservoir may have longer retention times than the relatively small brooks that supply
Lily Pond. Of the brooks supplying Lily Pond, the total organic carbon (TOC) levels in the Brass Kettle Brook (12.0
mg/L) have been reported to be higher than in the Peppermint Brook (6.2 mg/L) [ENSR’s Lily Pond Limnology and
Water’s Edge Study report]. The higher TOC level in Brass Kettle Brook was suggested to result from the heavily
vegetated wetland system that feeds the brook. These different raw water inputs into Lily Pond have the potential
to influence and vary TOC levels in the Lily Pond raw water supply.
Table 2-1. Lily Pond and Aaron River Reservoir
Parameter Lily Pond Aaron River Reservoir
DEP ID Number 4065000-02S 4065000-01S
Watershed South Coastal Basin South Coastal Basin
Watershed Area 2.53 square miles 4.9 square miles
Surface Area 52 acres 150 acres
Maximum Storage Capacity 79 MG 479 MG
Pumping Station Capacity 2,100 gpm N.A.
Number of Transfer Pumps 3 N.A.
Pump Motor Horsepower 15 hp N.A.
Total Safe Yield 5.2 MGD
Max Withdrawal 3.0 MGD
Table 2-2. Ellms Meadow Wellfield
Parameter Ellms Meadow Wellfield
DEP ID Number 4065000-02G
Watershed South Coastal Basin
Well Depth 20 ft
Well Casing Height 20 ft
Well Casing Depth 15 ft
Maximum Withdrawal 141,000 gpd
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Table 2-3. 2017 Water Supply Withdrawal
Month-Year Lily Pond Raw Supply
(gpd)
Ellms Meadow Wellfiled
Supply (gpd)
Total Raw Water Supply
(gpd)
January 2017 500,768 89,063 589,830
February 2017 414,841 134,287 549,128
March 2017 440,667 121,201 561,868
April 2017 499,448 100,330 599,777
May 2017 676,690 131,914 808,604
June 2017 875,604 126,966 1,002,570
July 2017 1,042,980 128,930 1,171,910
August 2017 1,065,519 129,658 1,195,177
September 2017 932,725 98,940 1,031,665
October 2017 850,167 0 850,167
November 2017 504,788 104,046 608,833
December 2017 456,726 134,232 590,958
Total 2017 690,497 128,933 798,589
Average 500,768 89,063 589,830
Maximum1 1,501,188 135,310 1,501,188
2.2 WATER TREATMENT FACILITIES
2.2.1 Lily Pond Water Treatment Plant
The Town owns and operates the Lily Pond WTP (ID# 4065000-01T) located at 339 King Street, Cohasset, MA
02025. The Lily Pond WTP has a permitted capacity of 3.0 MGD and operates on average eight to twelve hours
per day, seven days per week. The facility employs a conventional surface water treatment scheme, consisting of
coagulation, flocculation, sedimentation, filtration, disinfection, and stabilization. Raw surface water is withdrawn
from Lily Pond and pre-filtered through course and fine screens at the raw water intake building. An option for dosing
gaseous chlorine for pre-oxidation is also available. Following pre-screening, the raw water is conveyed to a rapid
mixing tank at the treatment facility, where ferric chloride, sodium hydroxide, sodium permanganate, and
polyaluminum chloride are added and mixed with the raw water. The ferric chloride and polyaluminum chloride are
coagulant agents that serve to destabilize the fine particulate, colloidal, and dissolved inorganic and organic matter
naturally present in the Lily Pond supply and form small solid particles. The sodium hydroxide addition helps to
maintain the pH and alkalinity at optimum levels during the coagulation process as the addition of ferric chloride
tends to depress the pH and alkalinity of the water. Sodium permanganate is added in the process for iron and
manganese removal.
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The rapid mixing process is followed by gentle mixing, along with the addition of a slight dosage of polyacrylamide
polymer to promote the aggregation of the destabilized particles into larger particles or “flocs” and make these flocs
heavy enough to settle from the water in the sedimentation process. After settling, the coagulated-flocculated-settled
water is filtered through layers of fine coal and silicate sand to filter any fine flocs or particles and reduce turbidity.
Following filtration, sodium hydroxide, blended ortho/poly phosphates, and hexametaphosphate are added for pH
adjustment, stabilization, and corrosion control. Gaseous free chlorine is applied for disinfection, and sodium
fluoride is added to promote dental health. The finished water is stored in an 0.13 MG underground concrete clear
well. The finished water is pumped into the distribution system by the high service pump station, which consists of
two pumps with a firm, maximum capacity of 2,100 gpm.
2.2.2 Ellms Meadow Pumping Station
The Town also owns and operates the Ellms Meadow Pumping Station (ID# 4065000-03T) located at 39 James
Lane, Cohasset MA, 02025. Because of the high raw groundwater quality, the treatment processes include
disinfection, fluoridation, stabilization, and corrosion control. The groundwater is withdrawn from the surficial aquifer
and disinfected with sodium hypochlorite to achieve 4-log virus removal. Sodium fluoride is also added to promote
dental health. Corrosion control and finished water stabilization is achieved by adding sodium hydroxide and
hexametaphosphate corrosion inhibitor. The finished water is pumped directly into the distribution system by one
(1) 115-gpm capacity pump. The permitted maximum day withdrawal and distribution is 141,000 gpd.
2.3 DISTRIBUTION
Treated drinking water from the Lily Pond WTP and Elms Meadow pumping station is delivered to the Town’s
service area customers via 2,637 service connections in their water distribution system. The distribution system
consists of over 40 miles of 6-inch to 14-inch transmission piping, two (2) water storage tanks, and a pumping
station. The distribution system is also connected to both the Hingham water system and the Scituate water system.
These interconnects enhance the system’s resiliency by allowing the transfer of water from one system to the other
in the event of a water emergency.
The two (2) system water storage tanks, the Bear Hill and Scituate Hill tanks, set the hydraulic grade line for the
single pressure zone distribution system. The Bear Hill tank is a steel ground storage tank that can store up to 2
MG of finished water. The Scituate Hill tank is also a steel ground storage tank with a storage capacity of 1.8 MG.
Both tanks are typically operated with a 10-foot fluctuation in water level within each tank. The Whitney Xing
Interconnect Pumping Station, located at 16 Chief Justice Cushing Highway, has a pumping capacity of 750 gpm
and is equipped with three (3) booster pumps. Based on information included in the 2006 MIEX® DOC Resin
Treatment Report, it is assumed that previous hydraulic modeling of the distribution system has been performed
and that the expected longest detention time in the distribution system is 77 hours.
2.4 WATER PRODUCTION
The Town’s Public Water Supply Annual Statistical Reports for 2016 and 2017 and Monthly Operating Report
(MOR) for January 2019 were reviewed to understand the system water demands. Table 2-4 presents a summary
of the total monthly average finished water production from the Lily Pond WTP and the Ellms Meadow Wellfield
Pumping Station. The yearly average and maximum day flow for each year are also included in Table 2-4. The
maximum day flow for 2018 was not available from the MOR. The total finished water production supplies drinking
water to most of the Town and the Linden Pond complex in Hingham. Based on the historical water production
results, the current average system demand is nearly 0.81 MGD with a maximum day demand of approximately 1.5
MGD to 1.7 MGD. The system water demands are higher in the summer months (June through August) as
compared to the winter and early spring months (December through March). The total water production is slightly
less than the raw water quantities presented in Table 2-3 due in part to a slight loss in water for filter backwashing.
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Table 2-4. Finished Water Production
Month-Year Finished Water Production (gpd)
January-16 562,728
February-16 549,920
March-16 558,181
April-16 570,673
May-16 813,046
June-16 1,204,112
July-16 1,425,367
August-16 1,171,040
September-16 696,526
October-16 607,470
November-16 545,526
December-16 592,397
2016 Annual Average 778,328
2016 Maximum Day 1,700,817
January-17 555,166
February-17 525,093
March-17 539,314
April-17 566,749
May-17 771,902
June-17 958,050
July-17 1,106,202
August-17 1,120,927
September-17 961,627
October-17 793,098
November-17 579,936
December-17 575,094
2017 Annual Average 756,182
2017 Maximum Day 1,461,400
January-18 606,742
February-18 572,607
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Month-Year Finished Water Production (gpd)
March-18 548,290
April-18 565,567
May-18 758,839
June-18 1,120,667
July-18 1,263,226
August-18 1,197,548
September-18 1,014,900
October-18 732,968
November-18 666,733
December-18 648,258
2018 Annual Average 809,592
2.5 WATER QUALITY
A review of the historical raw and finished water quality was performed based on data from the Town’s previous
studies [MIEX® DOC Resin Treatment and Ozone/BAC Treatment Pilot Test Reports (Weston & Sampson, 2006)],
the MassDEP Drinking Water Program’s Water Quality Database from 1/1/2000 to 9/13/2018, and the Town’s 2017
Water Quality Report. A summary of the available data reviewed from the studies and MassDEP database is
presented in Table 2-5. As demonstrated by the Ellms Meadow Wellfield finished water quality data, this source is
of high quality since mainly disinfection and stabilization for corrosion control is applied. For the Lily Pond WTP raw
and finished waters, a corresponding time-series graph of the TOC levels from August 2003 to January 2018 is
presented in Figure 2-2. The time series graph of the Lily Pond WTP raw and finished water turbidity from August
2003 to August 2006 is shown in Figure 2-3.
Overall, the Lily Pond surface water supply is a low alkalinity, organic-laden water with neutral pH, moderate TDS,
and low hardness based on finished water quality. The TOC levels ranged from 4.6 mg/L to upwards of 16 mg/L
with an average of 9.4 mg/L. Turbidity is generally moderate with an average of 2.3 NTU with the potential for the
occasional spike in raw water turbidity as shown in Figure 2-3. Based on the time series representation of the TOC
data, a general seasonal variation can be observed, where the TOC tends to be more elevated during the summer
months (June through August) and lower during the winter and springs months (April). TOC levels during fall months
appeared to fluctuate from relatively low to high. During the summer months, the raw water quality also tends to
have higher levels of turbidity and color.
To compensate for these water quality changes, the Town’s operations team adjusts the coagulants and polymer
dosages when higher levels of turbidity, color, and TOC are detected in the raw water. As supported by the data,
operations adjustments serves to effectively treat the raw water turbidity by achieving a finished water turbidity of
0.35 NTU on average, thus removing approximately 85% of the turbidity on an average basis. Additionally, the
enhanced coagulation process achieves about 67% removal of the TOC on average. The Lily Pond WTP also
achieves effective iron and manganese removal as demonstrated by the low to non detect finished water values.
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Table 2-5. Raw and Finished Water Quality
Parameter Unit Lily Pond WTP Raw
(Range)1
Lily Pond WTP
Finished2
Ellms Meadow
Wellfield Finished3
Temperature deg-C 15.1 (4 – 27.5) 23.5 -
pH - 7.4 (6.7 – 9.1) 6.3 6.0
Alkalinity mg/L as CaCO3 - 11.0 50.5
Turbidity NTU 2.3 (0.8 – 18.8) 0.35 0.40
Total Dissolved
Solids
mg/L
- 195 375
Hardness mg/L as CaCO3 - 45 111
Apparent Color PCU 236 (159 – 375) 1.7 -
True Color PCU 159 (88 – 324) 1.6 -
UVA cm-1 0.4 (0.223 – 0.863) 0.06 -
TOC mg/L 9.4 (4.6 – 16) 3.1 -
DOC mg/L - 2.3 -
Calcium mg/L - 13.5 26.3
Iron mg/L 0.3 (0.02 – 0.82) ND ND
Copper mg/L - 0.01 0.05
Potassium mg/L - 0.60 1.80
Sodium mg/L - 38 60
Magnesium mg/L - 2.7 11.1
Manganese mg/L 0.54 (0.176 – 0.9) 0.05 ND
Chloride mg/L 43 (30 – 60) 83 97
Fluoride mg/L - 0.9 0.8
Sulfate mg/L 9.5 (6.4 – 15.3) 15.2 21.0
Nitrate
- 0.09 3.37
Odor TON - 2.0 1.0
1. Data (except TOC) represents average and range of water quality data reported in the MIEX DOC Resin Treatment and
Ozone/BAC Treatment Pilot Test Reports (Weston & Sampson, 2006). TOC data represents average and range of data
reported in Pilot Test Reports and available data from the MassDEP Drinking Water Program’s Water Quality Database
from 1/1/2000 to 9/13/2018.
2. Temperature, apparent and true color, UVA, TOC, and DOC data represents average of water quality data reported in the
MIEX DOC Resin Treatment and Ozone/BAC Treatment Pilot Test Reports (Weston & Sampson, 2006). Remaining data
represents average of available data from the MassDEP Drinking Water Program’s Water Quality Database from 1/1/2000
to 9/13/2018.
3. Data represents average of available data from the MassDEP Drinking Water Program’s Water Quality Database from
1/1/2000 to 9/13/2018.
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Figure 2-2. Lily Pond WTP Raw and Finished Water TOC
Figure 2-3. Lily Pond WTP Raw and Finished Water Turbidity
0
2
4
6
8
10
12
14
16
18
4/19/01 1/14/04 10/10/06 7/6/09 4/1/12 12/27/14 9/22/17 6/18/20TOC (mg/L)Lily Pond Raw Lily Pond WTP Filtered
0
2
4
6
8
10
12
14
16
18
20
12/10/02 6/28/03 1/14/04 8/1/04 2/17/05 9/5/05 3/24/06 10/10/06 4/28/07Turbidity (NTU)Lily Pond WTP Filtered Lily Pond Raw+'Raw & Treated WQ'!$A$2:$A$94
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A summary of the Town’s compliance summary data from the 2017 Water Quality Report is provided in Table 2-6
through Table 2-12. The data demonstrates that the Town is in compliance with drinking water standards. The
treatment processes achieve good microbial inactivation, particulate control, and an effective corrosion control
strategy. Additionally, the Town is in compliance with regulated disinfection byproducts. Although the plant is
meeting DBP regulations, the seasonal variability in raw and finished water TOC can cause the potential to produce
varying levels of TTHMs approaching the TTHM limit. Due to the potential for DBP formation of the organic laden
Lily Pond water supply, the Town has conducted various DBP control evaluation studies to identify options for
optimizing the existing process and identify potential effectiveness of further organic removal treatment. A summary
of these studies is provided in the following section.
Table 2-6. 2017 Microbial Compliance Sampling Results
Microbial
Contaminants
Date
Sampled
MCL MCLG Highest
Level
Detected
Range of
Detection
Violation?
(Y/N)
Coliform
Bacteria
Monthly
2017
One positive
monthly
sample for
systems that
collect less
than 40
samples
0 1 0-1 N
Table 2-7. 2017 Inorganic Compliance Sampling Results
Inorganic
Contaminants
Date
Sampled
MCL MCLG Highest
Level
Detected
Range of
Detection
Violation?
(Y/N)
Fluoride (ppm) Monthly
2017
4(1) 4 0.7 ND-0.7 N
Nitrate (ppm) 2017 10 0 3.8 0.05-3.8 N
Barium (ppm) 6/6/17 2 2 0.015 0.015 N
Perchlorate
(ppb)
2017 2 N/A 0.21 0.05-.21 N
1. Fluoride also has an optimal level of 0.7 ppm and a secondary MCL of 2 ppm.
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Table 2-8. 2017 Disinfectant/Disinfectant Byproducts Compliance Sampling Results
Disinfectant
and
Disinfection
Byproducts
Date
Sampled
Highest
Running
Average
Range
Detected
MCL or
MRDL
MCLG or
MRDLG
Violation?
(Y/N)
Total
Trihalomethanes
(TTHMs) (ppb)
Quarterly,
2017
63 43-63 80 ND-0.7 N
Haloacetic Acids
(HAA5) (ppb)
Quarterly,
2017
33 9.8-33 60 0.05-3.8 N
Chlorine (ppm)
(free)
8 times per
Month
0.29(1) 0.11-0.57 4 4 N
1. Highest monthly average.
Table 2-9. 2017 Radiological Compliance Sampling Results
Radioactive
Contaminants
Date
Sampled
Highest
Level
Detected
Range
Detected
MCL or
MRDL
MCLG or
MRDLG
Violation?
(Y/N)
Gross Alpha
(pCi/l)
9/12/16 ND 15 0 N
Radium 226 &
228 (pCi/l)
9/12/16 0.76 ND-0.76 5 0 N
Table 2-10. 2017 Lead and Copper Compliance Sampling Results
Lead &
Copper(1)
Date
Sampled
90th
Percentile
Action
Level
MCLG # sites
above AL
# sites
sampled
Lead (ppb) Jul – Sep
2016
5 15 0 0 20
Copper (ppm) Jul – Sep
2016
0.167 1.3 1.3 0 20
1. Reduction in frequency for Lead & Copper sampling to every 3 years.
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Table 2-11. 2017 Filter Performance Compliance Sampling Results
Turbidity TT Lowest Monthly
% of Samples
Highest Monthly
Value
Violation? (Y/N)
Monthly Maximum
(NTU)
1.0 NTU(1) --- 0.40 No
Monthly Compliance
(NTU)
0.3 NTU(2) 97% --- No
1. Maximum turbidity limit that the system may not exceed at any time during the month.
2. Monthly turbidity compliance is related to a specific treatment technique (TT). The system filters the water so that at least
95% of our samples each month must be less than or equal to 0.3 NTU.
Table 2-12. 2017 Secondary Contaminant Compliance Sampling Results
Contaminants Date
Sampled
Highest
Level
Detected
Range of
Detection
Average
Detected
SMCL Health
Advisory
ORSG
Chloroform
(ppb)
2017 5.0 ND-5.0 - - - -
Sodium (ppm) 2017 47 42-47 - - - -
Manganese
(ppb)
2017 82 10-82 - 50 300
2.6 PRIOR STUDIES
In an effort to optimize and investigate options for enhancing the organic removal performance of the Lily Pond
WTP and manage DBPs, the Town has completed the following DBP evaluation and control studies:
• Disinfection Byproduct Compliance Evaluation – March, 2004
• Pilot Test Report Polyaluminum Chloride – November, 2006
• Pilot Test Report MIEX® DOC Resin Treatment – November, 2006
• Pilot Test Report Ozone/BAC Treatment – November, 2006
A review of these previous studies was conducted to understand the water chemistry and help to identify options
for further reduction of distribution system DBPs.
2.6.1 Disinfection Byproduct Compliance Evaluation Report
The Town authorized a study by Weston and Sampson which was completed in 2004 entitled “Disinfection
Byproduct Compliance Evaluation.” The study summarized requirements of the Stage 2 Disinfection/Disinfection
Byproducts Rule which include the completion of activities to identify the locations in the distribution system
expected to have the highest levels of total trihalomethanes and total haloacetic acids and switching the method of
determining compliance with the MCL’s from an annual average of all sampling locations to a locational running
annual average which became effective for the Town January 2004. Under the Stage 1 D/DBP rule, all of the
quarterly sampling results were averaged together to determine compliance. This could mean that areas having
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consistently high concentrations of disinfection byproducts would not cause a violation when averaged with the
results from other sampling locations. The LRAA requires that the data be looked at individually by sampling location
taking the most current quarterly sampling result and averaging it with the previous three quarterly sampling results
from that location to determine compliance. The intent is to prevent customers from receiving consistently high
concentrations of disinfection byproducts in areas of the distribution system.
The report also reviewed the requirements of the Long Term 1 Enhanced Surface Water Treatment Rule which was
promulgated to require enhanced treatment of water from surface water sources to enhance removal of
Cryptosporidium through more stringent filter performance requirements. The rule also requires disinfection
profiling to be performed by systems whose DBP levels exceed 80% of the MCL, which applied to the Town at the
time of the study. Disinfection profiling was required to ensure that systems were meeting the log inactivation
requirements for viruses and Giardia.
The report used historical DBP sampling results at the Bates Lane sampling location to calculate a LRAA for TTHM’s
which ranged from 55.0 to 80.0 ug/L which indicated that this location would just be in compliance and ranged from
63.0 to 89.6 ug/L for HAA5 which all exceeded the MCL of 60 ug/L. The report also summarized the historical
TTHM sampling results from 1994 to 2003 for all sampling locations and concentrations ranged from 41.4 to 175.0
ug/L. A hydraulic analysis of the distribution system identified a sample location that represented the maximum
residence time within the distribution system and the sampling location was moved from Bates Avenue to Whitehead
Road in December 2003.
The study proposed system improvements be implemented to reduce the DBP’s in the distribution system to below
80 percent of the regulatory limit. The study reviewed several treatment techniques/improvements to achieve this
goal which included enhanced coagulation/PACl, PAC addition, GAC filter media, ozonation, moving the
chlorination injection location, use of groundwater, source water improvements, clear well modifications, potassium
permanganate feed, mixed oxidants, chlorine dioxide and chloramines. A two phase improvement program was
recommended consisting of enhanced coagulation, reactivation of potassium permanganate feed, reintroduction of
the groundwater source and GAC filter bed or media replacement in phase one followed by switching the secondary
disinfectant to chloramines and using either ozone, chlorine dioxide or a mixed oxidant in the place of pre-
chlorination for oxidation of organics to enhance their removal in the coagulation process and prevent the formation
of DBPs earlier in the treatment process.
2.6.2 Pilot Test Report Polyaluminum Chloride
The Town authorized a follow up full-scale pilot testing study to test the effectiveness of polyaluminum chloride as
the primary coagulant performed by Westin and Sampson which is summarized in the report, “Pilot Test Report
Polyaluminum Chloride”, November 2006. The pilot study looked at changing the primary coagulant from alum to
polyaluminum chloride which began in December 2004 and then adding ferric chloride as a secondary coagulant
which began in September 2005. The pilot testing and sample collection was continued through August 2006. The
pilot testing was primarily looking at the effectiveness of the new coagulants to remove total organic carbon from
the raw water which was monitored by paired sampling for TOC and UV254 absorbance as a surrogate for TOC.
DBP samples were also collected from the distribution system during the pilot study.
The pilot study used different doses of PACl to determine the TOC removal by measuring UV254 as a surrogate.
The study concluded that a dose >90 mg/L should be used going forward. This is a very high dose for PACl which
is a pre-polymerized aluminum coagulant that is often used in low turbidity, low TOC surface waters to reduce the
coagulant dose compared to alum and ferric salts alone. The use of PACl and PACl with ferric chloride was not
reported to have any adverse impact to the overall water quality, with the exception of a couple instances where
the raw water manganese was elevated and the finished water exceeded the secondary MCL. The report states
that TOC removal increased when the raw water TOC was greater than 9.0 when feeding PACl and ferric chloride
and that the average removal efficiency was 67%. Primarily during the winter months when the TOC is less than
9.0 mg/L the TOC removal efficiency was not increased and averaged 62% excluding the months of March and
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April 2005 for which the removal efficiency was less than 50% when a lower PACl dose was fed and the filter media
was changed out.
2.6.3 Pilot Test Report MIEX DOC Resin Treatment
A second pilot study was performed at the site by Weston and Sampson using equipment and technical assistance
from Orica to test MIEX to treat the raw water to remove TOC. The results of the study were presented in the report
entitled, “Pilot Test Report MIEX DOC Resin Treatment,” November 2006. Raw water for the pilot study was taken
from the raw water port prior to coagulant addition, but after the addition of lime and potassium permanganate. The
pilot treatment system consisted of a MIEX pilot unit followed by a conventional treatment pilot unit. Treated water
from the pilot system taken after the MIEX process, after settling and after filtration were compared to finished water
samples taken from the existing treatment plant after filtration.
The MIEX process consists of a contact tank in which the raw water is mixed with continuously recycled and
regenerated magnetic ion exchange resin and a settling tank in which the resin with TOC attached is separated
from the treated water. The resin is a proprietary product of Orica that can be applied to the raw water. There are
other commercially available TOC removal resins supplied in pressure vessel systems that can only be applied to
a filtered water to prevent clogging of the media. A separate loop removes a portion of the resin from the settling
tank underflow and passes it through a tank mixing it with brine to release the TOC and regenerate the resin. New
virgin resin is also added to this stream to replace resin that has broken down in the process. One common
complaint from clients who are using this system is that the amount of makeup resin is higher than expected. The
process does produce a liquid waste stream which is a concentrated brine with TOC that must be disposed of.
The study reported that the pilot system achieved higher TOC and UV254 removal than the conventional process
using significantly less coagulant dose, 30 mg/L as compared to 140 mg/L and 20 mg/L as compared to 90 mg/L,
in the summer and winter, respectively. The reduction in coagulant usage is not unexpected because natural long
chain organics in the water tend to act as scale inhibitors which retard the formation of inorganic precipitates. The
coagulation process works by adding a coagulant chemical which forms an insoluble precipitate which has positive
charge sites which attract the negatively charged sites on the fulvic and humic acids allowing them to be removed
in the sedimentation and filtration processes. The organics present can inhibit the formation of larger floc particles
which can reduce the effectiveness of the coagulation and flocculation processes requiring higher coagulant doses
to achieve satisfactory performance.
The TOC concentration was reduced by 67% by the conventional process of the existing treatment plant and
removals of 80 and 81 percent were achieved by the MIEX process in the summer and winter, respectively.
Samples were also collected to determine the SDS (77 hour) DBP concentrations of the finished water from the
existing water treatment plant and the pilot system. The results showed that in the summer that MIEX pretreatment
reduced the THM and HAA concentrations by 43% and 72%, respectively, as compared to samples collected from
the existing water treatment plant. The results for the winter period showed that the MIEX pretreatment reduced
the THM and HAA concentrations by 56% and 76%, respectively, as compared to samples collected from the
existing water treatment plant. The average measured SDS (77 hour) DBP concentrations were 56.0 and 28.5 µg/L
for TTHM and 18.9 and 7.9 µg/L for HAA5 in the winter and summer, respectively, which are well below the MCLs
of 80 and 60 µg/L.
2.6.4 Pilot Test Report Ozone/BAC Treatment
A third pilot study was performed by Westin and Sampson with equipment and technical support provided by
Blueleaf, Inc. to test the use of ozone and biological filtration for the removal of TOC. The results of the study were
presented in a report entitled, “Pilot Study Report Ozone/BAC Treatment,” November 2006. The pilot study tested
the use of ozone and BAC at two points in the treatment process. One test used water from the existing plant after
filtration and applied ozone to the water which was then applied to a pilot GAC column. This option tested using
ozone and BAC as a polishing stage after filtration. The advantage of this arrangement is that the additional
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treatment is applied to the finished water after much of the TOC has already been removed to potentially provide a
lower overall finished water TOC. The second test used settled water from the existing treatment plant prior to
filtration to which ozone was applied and the ozonated water was applied to a column with GAC media on top of
sand to provide biological treatment and filtration. This would simulate that case where the anthracite on top of the
existing filters was removed and replaced with GAC. Ozone would be applied to the settled water and chlorine
would not be applied until after filtration so that the filters were operating in a biologically active mode. This
alternative would represent a potential capital cost savings by using the existing filters as the GAC contactors
instead of constructing new contactors. However, the ozonation facilities and contact chamber would be new
construction.
The pilot study was run beginning in the summer of 2005 and continuing through the summer of 2006. The pilot
study was started as a post filtration process only and was changed over the course of the pilot testing period so
that in the summer of 2006 it was an intermediate treatment process only so that two seasons of operation were
conducted for both modes of operation. Over the course of the testing the ozone dose was varied between 0 and
8 mg/L and ozone contact time was either 7.5 or 15 minutes. A total of 6 trials were performed during the pilot
testing. Media for the pilot test was obtained from other operating facilities and the absorptive capacity of the media
was considered to be exhausted so that the primary TOC removal mechanism would be biological. Sodium bisulfite
was added to the ozonated water after the contact tank to remove any residual oxidant.
The first trial was run using finished water after filtration with no ozone dosed to the feed. The detention time thru
the column was not reported. The columns achieved a UV254 removal of 15-22%. Samples were not collected to
determine the removal of TOC or the reduction in disinfection byproducts from the biologically active carbon when
ozone was not being applied to the water fed to the columns. The second trial was run using finished water after
filtration dosed with 8 mg/L of ozone and allowed to react for a 15 minute contact time before the residual ozone
was quenched before the water flowed into the carbon columns. The pilot system provided a 54 to 58% median
reduction in UV254 absorbance, an 8 to 12% reduction in TOC, 27 to 35 % reduction in TTHM formation and 6-
17% reduction in HAA5 formation. The third trial reduced the ozone dose from 8 to 4 mg/L and the contact time
from 15 minutes to 7.5 minutes. The pilot system in the third trial did not provide any reduction in UV254 absorbance
after ozonation and a 6 to 13% reduction in UV254 absorbance after running thru the biologically active GAC
columns. The authors concluded that this did not provide significant treatment for UV254 reduction as compared
to the first trial in which no ozone was fed. The summary does not report removals for TOC or DBP formation during
this trial. The fourth trial was the last trial run using the treated water from the existing plant after filtration and was
conducted primarily during the winter months. In this trial the ozone dose was increased up to 8 mg/L and the
ozone contact time was maintained at 7.5 minutes. The results indicated that there was an average UV 254
absorbance reduction of 26 to 29%. Two of the columns showed a 14 to 19 % reduction in TOC, but one column
showed a small increase in TOC. The sampling in the fourth trial showed a 14 to 19 % reduction in TTHM formation
and a 7 to 15% reduction in HAA5 formation. It should be noted that after the 14 to 19% reduction in TTHM formation
the sample formation potential concentrations ranged from 93 to 99 µg/L which is approximately 17 % over the
MCL.
The fifth trial was run using settled water from the existing water treatment plant that was obtained prior to filtration.
The trial was begun in March and ran through the beginning of summer. This trial was run with an ozone dose of
8 mg/L and an ozone contact time of 7.5 minutes. The UV 254 absorbance reduction was 45% versus a 16%
removal provided by the full scale filtration process. The pilot column provided a 18% removal of TOC versus the
full scale filtration process of 9%, a 31% reduction in THM formation versus a 22% reduction by the full filtration
process and a 57% reduction of the HAA5 formation versus 45% for the full scale filtration process. Trial six was
performed during the summer of 2006 from July through September treating water from the settling basins prior to
filtration. The trial was run with an ozone dose of 8 mg/L and an ozone contact time of 7.5 minutes the same as
trial 5. This trial achieved a 36% reduction of the UV 254 absorbance and no measurement of the plant filtration
process was recorded. The pilot column provided a 39% removal of TOC versus the full scale filtration process of
9%, a 48% reduction in THM formation versus a 21% reduction by the full filtration process and a 64% reduction of
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the HAA5 formation versus 42% for the full scale filtration process. The trial 6 results show a significantly higher
TOC removal which is most likely due to the warmer temperatures which promote greater biological activity. It
should be noted that even though the removals were higher in trial 6 the TTHM formation potential concentration
after biological treatment was 161 µg/L or double the MCL.
There are some reasons to question the results of this study which are summarized below.
• The Blueleaf, Inc. report in the appendix to the study states the trials were performed with an empty bed
contact time (EBCT) for the biologically active carbon columns of 10 minutes. However, the Blueleaf report
later states that “During the first season of piloting, the three pilot columns were set up to test the effect of
different empty bed contact times and media type on BAC performance.” In the design of carbon contactors
one of the key design parameters is the EBCT. It is not clear from the report what the impact upon
performance of the BAC system was from different EBCT or media type.
• The report seemed to imply that UV 254 absorbance reduction could be used as a surrogate for TOC
removal and reduction in TTHM and HAA5 formation potential, but did not use the data to verify if there
was a strong correlation between these parameters. In trials 1 and 3 no other parameters were measured,
so a direct comparison between the performance of these trials and the others for the parameters of interest,
TOC, TTHM and HAA5, could not be made.
• The study indicates that a 20 mg/L sodium bisulfite feed was used to quench the ozone residual after the
ozone contact tank. The report indicates that the 20 mg/L dose was used because of variances in the
ozone residual to ensure that no ozone residual remained. This suggests that there were periods when a
low ozone residual after the contact tank may have resulted in a significant sodium bisulfite dose being
applied to the carbon columns. Sodium bisulfite is a mild biocide used in wine and canning as a preservative
to prevent the growth of mold, fungus and bacteria and is used as a deoxygenating agent. Dissolved
oxygen in the feed to the columns and the effluent from the columns was not measured so there is a
question of whether the microbial growth required to reduce the organic carbon in the water was uninhibited.
• It was also noted in the laboratory results for several sample sets, including those taken on February 10,
2006, March 16, 2006 and March 20, 2006, measured chlorine residuals from the columns greater than 5
mg/L. If there was a measurable chlorine residual in the column effluent, this may have also inhibited
microbial growth within the columns. There is some evidence to support this when looking at the results of
the heterotrophic plate counts performed on the column effluent. The first set of samples collected on
October 20, 2005 had heterotrophic plate counts >5,700 indicated a higher level of microbial activity within
the columns. Subsequent samples taken throughout the rest of the study had heterotrophic plate counts
ranging from 2 to 1,100, the majority of the readings being less than 100.
• DBP formation potential samples were collected throughout the study, but DBP SDS samples were only
collected during the latter portion of the test when the column was operating off of the settled water from
the existing plant. Therefore, a direct comparison to the performance of the system treating water from the
filters cannot be made.
It is difficult to draw strong conclusions from the ozone and BAC pilot study for the reasons described above.
However, looking at the TTHM formation potential sampling results, it is clear that the BAC system operating on
filtered water consistently produced lower TTHM concentrations than when operating off of the settled water.
Therefore, based upon these results the system would be more effective if applied to the finished water after filtration
rather than before filtration. The TTHM SDS results were all lower than the MCL when treating the settled water as
compared to the formation potential results which were approximately twice the MCL. If this result can be
extrapolated to the earlier trials in which the system was treating the filtered water, it would imply that the system
could provide levels in the distribution system that were below the MCL based upon the SDS detention times.
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3.0 DISINFECTION BYPRODUCTS REVIEW AND ANALYSIS
3.1 DBP FORMATION AND CONTROL
DBPs are formed as an unintended consequence of disinfection through reactions of disinfectants or oxidants (i.e.
chlorine) with natural organic matter and reduced inorganic substances present in the source water. Formation of
the regulated TTHMs and HAAs depends primarily on organic content, disinfectant type and dosing, contact time,
pH, temperature, and the presence of bromide ions. The highest DBP formations tend to occur when free chlorine
is applied as the disinfectant. Typically, DBP levels increase with increasing organic precursor material, chlorine
dose and residual, and temperature. While TTHMs generally increase with pH levels and water age, HAAs are
favored under more acidic pH values and are less predictable with water age due to biodegradation at low chlorine
residuals. The presence of bromide in the source water results in a higher fraction of brominated DBPs.
Faced with balancing the tradeoff between protecting the public against
exposure to pathogens and exposure to DBPs, the water industry has
recognized the following techniques for minimizing regulated DBPs in
drinking water:
• Removing organic precursors
• Reducing the chlorine dosage and/or contact time
• Using alternate primary and/or secondary disinfectants
• Removing TTHMs and/or HAAs after their formation
Water utilities may implement disinfection and distribution system operation management approaches to reduce
the chlorine dose, residual and system water age. Additionally, chloramines can be used in place of free chlorine in
secondary disinfection to lower quantities of regulated TTHMs and HAA5. Although applying chloramines as an
alternate disinfectant helps to reduce the amount of regulated TTHMs and HAA5, chloramines contribute to the
formation of non-regulated DBPs that could be regulated in the future.
The majority of recent DBP control studies have focused on organic precursor removal. Common treatment
methods for the removal of organic precursors include coagulation and clarification, adsorption, ion exchange,
biological filtration, ozone oxidation, reverse osmosis (RO), and nanofiltration (NF). The organic precursor removal
efficiency is typically gauged according to the reduction in total organic carbon (TOC), dissolved organic carbon
(DOC), 254 nm ultraviolet absorption (UV 254), and TTHM and HAA5 formation potentials. Removal of the regulated
DBPs may also be a viable option. Treatment methods for the removal of regulated DBPs include granular activated
carbon (THMs only), air stripping (THMs only), biological filtration (HAAs only), and RO (both THMs and HAAs).
Due to the high variability in source water characteristics, the applicability of a DBP control strategy is site specific
and should be carefully evaluated in terms of operational requirements, water quality, ease of integration into an
existing system, and costs. Of the available DBP control techniques, the Town has implemented coagulation
process optimization and chlorine dosage optimization strategies; and has preliminarily investigated alternative
organic removal technologies. Therefore, this conceptual desktop study focuses on assessing the applicability of
organic precursor removal, post-formation TTHM removal, and alternative distribution management approaches for
further reducing regulated DBPs.
In this section, an analysis of the existing system’s historical DBP data was performed to establish distribution
system DBP goals and develop corresponding TTHM reduction and TOC removal requirements. The treatment
requirements developed in this section will provide the basis for identifying and evaluating DBP reduction
alternatives.
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3.2 DBP REGULATORY PERSPECTIVE
The USEPA’s Stage 2 D/DBP Rule establishes maximum contaminant levels (MCLs) for two (2) sets of regulated
disinfection byproducts (DBPs), referred to as Total Trihalomethanes (TTHMs) and Haloacetic Acids (HAAs). The
MCLs for TTHMs and HAAs are currently 0.08 mg/L (80 µg/L) and 0.06 mg/L (60 µg/L), respectively. The four
species of THMs and five species of HAAs or HAA5 that are regulated by the D/DBP Rule are presented in Table
3-1. Compliance with the Stage 2 DBP MCLs is based on a locational running annual average (LRAA), where the
annual average is determined for each sampling location in the distribution system. The LRAA is intended to ensure
that customers, including those at the hydraulic extremes of the system, receive water that meets the regulatory
requirements for DBPs. In concept, LRAA locations are selected to represent the highest TTHM and HAA5 levels
in the system. The highest TTHM and HAA5 formation do not necessarily occur at the same locations. Thus,
regulated DBP formation at maximum detention times in the system should be considered.
Table 3-1. Regulated DBP Species
Regulated THMs (80 µg/L) Regulated HAAs (60 µg/L)
Chloroform Chloroacetic Acid
Bromoform Dichloroacetic Acid
Bromodichloromethane Trichloroacetic Acid
Dibromochloromethane Bromoacetic Acid
Dibromoacetic Acid
3.3 ANALYSIS OF HISTORICAL DBP DATA
A review of historical TTHM and HAA5 data was performed based on data from the MassDEP Drinking Water
Program’s Water Quality Database. The TTHM and HAA5 data was collected in accordance with the Stage 2 D/DBP
Rule from November 2013 to August 2018 at the following system compliance locations:
• 10202 – Deer Hill School, 208 Sohier Street, Cohasset, MA 02025
• 10204 – Police Station 62 Elm St., Cohasset, MA 02025
• 10205 – 2 Whitehead Road, Cohasset, MA 02025
• 10206 – 4 Beechwood Street, Cohasset, MA 02025
Times series representations of the TTHM and HAA5 results are provided in Figure 3-1 and Figure 3-2,
respectively. As illustrated by the TTHM data in Figure 3-1, the TTHM results at the compliance locations within
the Cohasset distribution system have ranged from approximately 12 µg/L to 110 µg/L over the last five years.
Based on these historical TTHM levels, single sample TTHM levels have been found to rise above the regulated
MCL at three of the four sampling locations during the summer and spring quarterly samplings. These TTHM results
reveal an overall seasonal variation in TTHM formation, where peaks in TTHM levels are typically observed during
the summer sampling events.
As shown in Figure 3-2, the corresponding HAA5 at the compliance locations have fluctuated from approximately
2 µg/L to 48.5 µg/L over the last five years. The highest HAA5 level (48.5 µg/L) was measured at the 4 Beechwood
Street location and is approximately 81% of the 60 µg/L MCL. Unlike the TTHMs, the HAA5 data does not exhibit a
distinct seasonal pattern, which suggest that the other mechanisms are influencing HAA5 levels in the distribution
system. One of these mechanisms, could be chlorine residual management to maintain adequate but generally
lower residuals and an accompanying biodegradation of HAAs within biofilms. Overall based on the full-scale data,
the HAA5 formation appears to be effectively controlled by the existing treatment and operation processes.
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Figure 3-1. Cohasset Water Distribution System TTHMs
Figure 3-2. Cohasset Water Distribution System HAA5
0
20
40
60
80
100
120
10/1/2013 4/19/2014 11/5/2014 5/24/2015 12/10/2015 6/27/2016 1/13/2017 8/1/2017 2/17/2018Total Trihalomethanes (µg/L)10202-Deer Hill School 10204-Police Station 10205-2 Whitehead 10206-4 Beechwood TTHM MCL
0
10
20
30
40
50
60
70
10/1/2013 4/19/2014 11/5/2014 5/24/2015 12/10/2015 6/27/2016 1/13/2017 8/1/2017 2/17/2018Total Trihalomethanes (µg/L)10202-Deer Hill School 10204-Police Station 10205-2 Whitehead 10206-4 Beechwood HAA5 MCL
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A closer review of the locational TTHM data was performed to compare the LRAA to the MCL and understand the
composition of the TTHM species. TTHM LRAA results for each compliance location are presented in Figure 3-3
through Figure 3-6. In each figure, the TTHMs results are represented as columns and the corresponding LRAA is
shown as a connected line. The average TTHM speciation for each compliance location is shown in Figures 3-7
and 3-8. For the Deer Hill School, Police Station, and 2 Whitehead Road locations, individual TTHM results fell
above the regulated MCL on three to four occasions. However, the corresponding LRAA, which is the basis for
compliance, remained below the regulatory MCL due to the lower seasonal TTHM levels. At the 4 Beechwood
Street location, individual TTHM and LRAA results remained below the regulatory MCL. As emphasized in Figures
3-7 and 3-8, chloroform is the predominant species, comprising approximately 60% to 65% of the TTHMs on
average. The chloroform fraction is followed by bromodichloromethane, comprising approximately 30% of the
TTHMs, on average.
In addition to the historical monitoring DBP data, the Town recently performed DBP formation potential testing on
the Lily Pond WTP combined filter effluent in December 2018. Prior to dosing with free chlorine for the DBP
formation potential test, the combined filter effluent was pH adjusted to match current plant operations (6.8 to 7.2
pH units). The pH adjusted water was then dosed with increasing doses of free chlorine to maintain a chlorine
residual of about 2.0 mg/L at varying detention times (2, 6, 24, 48, and 169 hours). The TOC of the combined filter
water sample was 2.84 mg/L. The dosed chlorine samples were incubated at 25 oC; and the TTHM and HAA5 levels
were measured at each time interval. A summary of the TTHM and HAA5 experimental results are presented in
Figure 3-9 and Figure 3-10, respectively. In both figures, the speciated TTHMs and HAA5 results are represented
as stacked columns. The TTHM and HAA5 formation potential curves are shown in Figure 3-11.
Figure 3-3. Deer High School Location TTHMs
0
20
40
60
80
100
120
10/1/201312/1/20132/1/20144/1/20146/1/20148/1/201410/1/201412/1/20142/1/20154/1/20156/1/20158/1/201510/1/201512/1/20152/1/20164/1/20166/1/20168/1/201610/1/201612/1/20162/1/20174/1/20176/1/20178/1/201710/1/201712/1/20172/1/20184/1/20186/1/20188/1/2018Total Trihalomethanes (µg/L)10202-Deer Hill School LRAA MCL
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Figure 3-4. Police Station Location TTHMs
Figure 3-5. 2 Whitehead Location TTHMs
0
20
40
60
80
100
120
10/1/201312/1/20132/1/20144/1/20146/1/20148/1/201410/1/201412/1/20142/1/20154/1/20156/1/20158/1/201510/1/201512/1/20152/1/20164/1/20166/1/20168/1/201610/1/201612/1/20162/1/20174/1/20176/1/20178/1/201710/1/201712/1/20172/1/20184/1/20186/1/20188/1/2018Total Trihalomethanes (µg/L)10205-2 Whitehead LRAA MCL
0
20
40
60
80
100
120
10/1/201312/1/20132/1/20144/1/20146/1/20148/1/201410/1/201412/1/20142/1/20154/1/20156/1/20158/1/201510/1/201512/1/20152/1/20164/1/20166/1/20168/1/201610/1/201612/1/20162/1/20174/1/20176/1/20178/1/201710/1/201712/1/20172/1/20184/1/20186/1/20188/1/2018Total Trihalomethanes (µg/L)10204-Police Station LRAA MCL
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Figure 3-6. 4 Beechwood Location TTHMs
Figure 3-7. Average Distribution System TTHM Speciation by Location
0
10
20
30
40
50
60
70
80
90
10/1/201312/1/20132/1/20144/1/20146/1/20148/1/201410/1/201412/1/20142/1/20154/1/20156/1/20158/1/201510/1/201512/1/20152/1/20164/1/20166/1/20168/1/201610/1/201612/1/20162/1/20174/1/20176/1/20178/1/201710/1/201712/1/20172/1/20184/1/20186/1/20188/1/2018Total Trihalomethanes (µg/L)10206-4 Beechwood LRAA MCL
65%28%
7%0%Deer Hill School TTHM Speciation
Chloroform BromodichloromethaneDibromochloromethaneBromoform
60%27%
12%2%Police Station TTHM Speciation
Chloroform BromodichloromethaneDibromochloromethaneBromoform
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Figure 3-8. Average Distribution System TTHM Speciation by Location
In the TTHM formation potential graph, the increase in TTHM concentration with contact time followed a typical
logarithmic shape. Initially, chlorine reacted rapidly with TOC to form TTHMs, then the reaction kinetics slowed after
about 1 day of contact time. The rapid formation resulted in TTHM levels that exceeded the MCL after about a day
of contact time. Additionally, the speciation for this sample was about 70% to 80% chloroform. The HAA5 formation
potential curve showed an exponential growth in HAAs with time; and HAA5 levels exceeded the MCL after about
1 day. The DBP formation potential results represent highly conservative values because the target chlorine residual
was maintained at 2 mg/L throughout the testing. In the full-scale system, the chlorine residual decays as demand
is exerted throughout the distribution system. Although the chlorine residual is required to remain above 0.2 mg/L,
the normal decay in chlorine residual within the system generally reduces the DBP formation driving force.
The experimental HAA data does not appear to be representative of the observed full-scale data most likely due to
the higher chlorine residual (about 2 mg/L) measured in the testing as compared to a lower residual (0.2 mg/L to
0.8 mg/L) typically carried in the system. The biodegradation of HAAs by the biofilms present within distribution
system piping can occur at lower chlorine residuals. Although the HAA5 experimental data may not be representative
of full-scale conditions, the experimental TTHM data offers a conservative basis for determining the TTHM reduction
needed in a “worst-case” scenario. Using the empirical logarithmic equation presented in Figure 3-10, a
conservative estimate of the TTHM formation after 77-hours of contact time is approximately 130 µg/L. About 40%
to 75% of this 130 µg/L concentration is formed within the first 6 to 24 hours.
57%28%
13%2%2 Whitehead TTHM Speciation
Chloroform BromodichloromethaneDibromochloromethaneBromoform
63%
30%
7%0%4 Beechwood TTHM Speciation
Chloroform BromodichloromethaneDibromochloromethaneBromoform
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Figure 3-9. Lily Pond WTP Filtered Water - Simulated TTHM Formation Potential
Figure 3-10. Lily Pond WTP Filtered Water - Simulated HAA5 Formation Potential
0
20
40
60
80
100
120
140
160
0.5 2 6 24 48 168Total Trihalomethanes (µg/L)Chlorine Contact Time (hours)
Chloroform (µg/L)Bromodichloromethane (µg/L)Dibromochloromethane (µg/L)Bromoform (µg/L)
Cl2 Dose = 3.5 mg/L
Cl2 Residual = 2.1 mg/L
Cl2 Dose = 3 mg/L
Cl2 Residual = 1.5 mg/L
Cl2 Dose = 4 mg/L
Cl2 Residual = 1.9 mg/L
Cl2 Dose = 4.8 mg/L
Cl2 Residual = 1.8 mg/L
Cl2 Dose = 6.1 mg/L
Cl2 Residual = 2.7 mg/L
Cl2 Dose = 7.1 mg/L
Cl2 Residual = 1.9 mg/L
0
20
40
60
80
100
120
140
0.5 2 6 24 48 168DBP Concentration (µg/L)Chlorine Contact Time (hours)
Monochloroacetic Acid (µg/L)Dichloroacetic Acid (µg/L)Trichloroacetic Acid (µg/L)
Dibromoacetic Acid (µg/L)Monobromoacetic Acid (µg/L)
Cl2 Dose = 3.5 mg/L
Cl2 Residual = 2.1 mg/L
Cl2 Dose = 3 mg/L
Cl2 Residual = 1.5 mg/L
Cl2 Dose = 4 mg/L
Cl2 Residual = 1.9 mg/L
Cl2 Dose = 4.8 mg/L
Cl2 Residual = 1.8 mg/L
Cl2 Dose = 6.1 mg/L
Cl2 Residual = 2.7 mg/L
Cl2 Dose = 7.1 mg/L
Cl2 Residual = 1.9 mg/L
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Figure 3-11. Lily Pond WTP Filtered Water - DBP Formation Potential Curves
3.4 WATER QUALITY GOALS AND OBJECTIVES
Based on the results from the historical water quality data and previous studies reviewed, water quality goals and
corresponding treatment requirements were established for TTHMs, HAA5, and TOC levels. The following
assumptions were considered in developing the water quality goals and treatment requirements:
• The expected longest detention time in the distribution system is 77-hours.
• The typical chlorine residual levels are 0.2 to 0.8 mg/L at the hydraulic extremes of the system.
• The compliance monitoring locations are representative of locations, where the highest DBP formation is
expected to occur.
• The December 2018 experimental TTHM formation potential data represents a conservative estimate of
the TTHM formation in the distribution system.
• The TTHM and HAA5 concentration goals at the compliance monitoring locations are 80% of the MCL based
on the 2004 DBP Evaluation Compliance Report.
• The seasonal fluctuation of TOC levels has a significant impact on DBP formation and can be used to
estimate TOC level goals and treatment requirements.
• System demand and temperatures also influence DBP formation. During the peak summer months, system
demands are higher, which would contribute to a shorter detention time and lower DBP levels. However,
corresponding temperatures are also higher, which drive DBP formation. Due to the various counteracting
factors that contribute to DBP formation, the water quality goals are intended to be conservative.
• Facility operations will continue to optimize coagulation and disinfection operations for maximizing DBP
control effectiveness of current processes.
The water quality goals and treatment requirements for TTHMs, HAA5, and TOC are summarized in Table 3-2. The
methodology used to develop these goals and treatment requirements is outlined in the following subsections.
y = 25.06ln(x) + 17.48
R² = 0.98
y = 20.052x0.359
R² = 0.997
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100 120 140 160 180DBP Concentration (µg/L)Contact Time (hours)
TTHM HAA5
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Table 3-2. Water Quality and Treatment Goals
Constituent Goal Basis Concentration Goal Treatment Requirement
TTHMs 80% of MCL 64 µg/L 40% to 50%
HAA5 80% of MCL 48 µg/L Goal Currently Met
TOC Meet DBP Goals ≤ 2.0 mg/L 50% to 65%
3.4.1 Development of DBP Water Quality and Treatment Goals
The DBP water quality goal for each of the compliance locations is established at 80% of the MCL based on the
2004 DBP Evaluation Compliance Report. The historical DBP data and recent DBP formation potential testing
results were used to estimate the reduction in TTHM and HAA5 levels that would be required to achieve this goal at
the LRAA compliance locations. Over the past five years, the highest HAA5 level measured at the LRAA compliance
locations has been 48.5 µg/L, which is approximately 81% of the 60 µg/L MCL. Therefore, no further treatment is
anticipated to be required for HAA5 control.
On the other hand, TTHM levels rose up to 110 µg/L at the LRAA compliance locations over the past five years.
Reducing this maximum level to meet the DBP goal would require a 42% reduction in TTHMs. Additionally, the
recent TTHM formation potential testing showed that TTHM levels could increase to about 130 µg/L after 77-hours
of detention time with a robust 2.0 mg/L chlorine residual and moderate TOC level of 2.84 mg/L. Based on these
experimental results, a 51% reduction in TTHMs would be required to meet the DBP goal. Using the historical and
experimental data, the level of TTHM treatment necessary to meet the DBP goals can be bracketed between
approximately 40% and 50%.
3.4.2 Development of TOC Water Quality and Treatment Goals
The target TOC in the combined filtered water was estimated based on a scatter plot analysis of paired TTHM
versus filtered water TOC data from the 2006 MIEX® DOC Resin Treatment Report and from the full-scale Cohasset
historical data (2006 to 2017). The TTHM vs TOC scatter plots developed from the pilot and full-scale data are
illustrated in Figure 3-12 and Figure 3-13, respectively. The scatter plots reveal a linear positive correlation
between the TOC concentration in the filter effluent and the corresponding TTHM level. The pilot TTHM vs TOC
data has a strong linear correlation with an R-squared value of 0.9. The pilot TTHM levels were simulated in a
controlled environment by dosing chlorine based upon maintaining a residual of 0.2 to 0.8 mg/L at the end of 77
hours of incubation. Based on the linear regression equation shown in Figure 3-12, a TOC level of 2.0 mg/L in the
filter effluent would correspond with a TTHM formation of 64 µg/L after 77-hours of contact time. Although the full-
scale TTHM vs TOC scatter-plot (Figure 3-13) has a weaker linear correlation due to other system factors that
influence DBP formation, the full-scale data generally supports a TOC target of 2.0 mg/L to achieve effective TTHM
control in the distribution system.
Based on meeting a 2.0 mg/L TOC target, the required removal effectiveness was estimated using historical full-
scale data collected from the combined filter effluent from August 2003 through January 2018. Due to the observed
seasonal variation in water quality, the data was separated into winter/spring (December through May) and
summer/fall (June through November). The summary statistics for the TOC removal requirements in winter/spring
and summer/fall are included in Table 3-3. On average, reducing the current filtered water TOC levels to the 2.0
mg/L target would require 25% removal in the winter/spring and 41% removal in the summer/fall months.
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Figure 3-12. DBP Formation versus Filtered Water TOC (MIEX Pilot Test Report)
Figure 3-13. DBP Formation versus Filtered Water TOC (Full-Scale Data)
y = 30.98x + 2.15
R² = 0.90
y = 26.53x -20.42
R² = 0.93
0
20
40
60
80
100
120
140
0 0.5 1 1.5 2 2.5 3 3.5 4 4.577-hour DBP Concentration (µg/L)TOC (mg/L)
77-hr TTHM 77-hr HAA5 TTHM Goal
Basis for TOC Target
y = 17.57x + 7.56
R² = 0.21
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6TTHM Concentration (µg/L)TOC (mg/L)
TTHM vs TOC TTHM Goal
Basis for TOC Target
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To determine the design maximum TOC removal requirement, an evaluation of the percentile distribution of TOC
removal data was performed. The percentile distribution graph of the TOC removal data is presented in Figure 3-
14. The percentile distribution graph shows that 95% of the historical data from August 2003 through January 2018
required at most 53% removal in the winter/spring and 63% in the summer/fall months. Since DBP compliance is
based on quarterly DBP monitoring, basing the TOC removal target on the 95th percentile instead of the absolute
maximum value provides an acceptable level of confidence for identifying and sizing treatment equipment. Thus,
the TOC treatment goal will be based on achieving 50% to 65% removal of TOC in the combined filtered water.
Table 3-3. Estimated TOC Removal Requirements
Summary Statistics Winter/Spring TOC Removal Summer/Fall TOC Removal
Mean 25% 41%
Median 27% 41%
Minimum 0% 5%
Maximum 70% 71%
Figure 3-14. TOC Removal Percent Distribution
0%
20%
40%
60%
80%
100%
120%
0%10%20%30%40%50%60%70%80%Percent DistributionTOC Removal (%)
Summer-Fall Winter-Spring
63% TOC Removal53% TOC Removal
95% Percentile
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4.0 DISINFECTION BYPRODUCT CONTROL ALTERNATIVES
Based on the review of historical system DBP data, previous studies, and established water quality goals, the
following alternative treatment and management strategies were identified as options for reducing regulated DBP
levels in the Cohasset water system.
• Alternative 1 – Aeration Treatment
• Alternative 2 – Granular Activated Carbon (GAC) in Adsorption and Biologically Active Treatment Modes
• Alternative 3 – Biologically Active Carbon (BAC) with Ozonation Treatment
• Alternative 4 – Ion Exchange Treatment
• Alternative 5 –Monochloramine Disinfectant Conversion
These identified alternatives include post-formation TTHM removal (aeration treatment), TOC precursor removal
(GAC, BAC, and ion exchange treatment), and secondary disinfection conversion (monochloramine). A process
description for each alternative DBP reduction strategy is provided in the following subsections.
4.1 AERATION TREATMENT
4.1.1 Treatment Process and Technology Overview
Aeration or air stripping is a treatment process that removes volatile contaminants from water by contacting the
water with air and promoting the transfer of the contaminants into the air stream. The efficiency of aeration for
volatile contaminant removal is principally dependent on the air-to-water ratio, described in Equation 1. As the air-
to-water ratio increases, so does the removal efficiency until the air-to-water ratio reaches equilibrium conditions.
The equilibrium between air and water can be described by Henry’s Law. The equilibrium or Henry’s Law constant
is used to gauge compound volatility. Generally, compounds with Henry’s Law constants greater than 0.05 are
considered volatile. Chloroform, which is the dominant THM in the Cohasset distribution system, is the most volatile
THM having a Henry’s Law constant of 0.13 at 20ºC. Bromodichloromethane is the second most volatile THM
having a Henry’s Law constant of 0.08 at 20ºC. The dibromochloromethane and bromoform species are less volatile
with Henry’s Law constants of 0.04 and 0.02, respectively, at 20ºC.
�AW�=QaQw =Qa tVw Equation 1
Where:
Qa = Air flow rate
Qw = Water flow rate
t = Aeration time
Vw=Volume of water sample
Because of the volatility of these THMs, aeration technologies have been used successfully to remove volatile
THMs from drinking water. While aeration can be effective in reducing THM levels, aeration treatment does not
impact or reduce HAA formation because HAAs are not volatile species, and hence not amenable to removal by
aeration. Additionally, THMs re-form in the distribution system after their removal by aeration. Based on these
general concepts, water systems that are good candidates for this technology are characterized by the following:
• Chloroform comprises the majority (ideally greater than 75%) of the total THMs.
• The majority of the THM formation potential has been exerted prior to aeration treatment being applied
(higher concentration differential driving force).
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• The THM formation potential does not continue to rapidly increase after aeration (i.e. THM formation
potential curve is modeled by a logarithmic relationship).
• HAA formation is low or controlled by other methods.
The typical aeration technologies include packed tower, cascade, tray, diffusion, surface, and spray aeration. Of
these, spray aeration within water storage tanks is most commonly applied for TTHM removal due to the ability to
achieve effective air to water ratios and to utilize existing system infrastructure. The Medora Corporation and PAX
Water Technologies manufacture and supply spray aeration systems for achieving volatile TTHM removal within
storage tanks. These proprietary in-tank spray aeration systems are typically more economically favorable for utility
applications with existing water storage facilities. Spray aeration systems can also be engineered by a third-party
consultant and do not have to be procured from a proprietary supplier. For purposes of this DBP reduction
evaluation, however, these proprietary spray systems were used for comparison with the other identified treatment
alternatives.
Spray aeration involves the pumping of water through a spray nozzle which in turn produces many small water
droplets. The TTHMs evaporate from the water droplets. A literature review indicated that spray aeration systems
are typically more energy efficient than typical diffused aeration systems (air introduced at the bottom of the tank).
In addition, spray aeration systems have been shown to remove chloroform as well as the brominated species of
TTHMs. Two manufacturers of these types of spray aeration systems were identified and summarized as part of
this evaluation.
4.1.2 Floating Spray Nozzle THM Removal System
The Medora Corporation’s GridBee® Floating
Spray Nozzle THM Removal System is designed
for installation within clear wells and storage
tanks for removal of TTHMs by spray aeration.
This system has two main components: the
floating frame with intake hose and a one-piece
pump and spray nozzle assembly. Four
standard pre-manufactured sizes range from 3
to 15 horsepower or 0.2 to 1.1 MGD. The
patented spray nozzle produces millions of
droplets per second, uses low pressure, has a
long life, and generates high air movement. An
air blower mounted on the top of the tank pushes
air into the tank. A ventilation system allows for
the TTHMs that evaporate from the water to be
removed from the tank. This system does require a minimum of 2.0 feet to 2.5 feet of head space. The models fit
through a 24-inch by 24-inch or larger hatch opening. For this system, no infrastructure piping is required, and no
modifications to the tank other than small access openings for electrical cable and the headspace air ventilation are
required. Factory installation is available with the exception of the electrical connections. A local electrical
contractor is required to complete installation. Additionally, the manufacturer has a BeeKeeper program that
typically costs approximately 6% of the equipment costs which includes preventative maintenance with no additional
charges to keep the equipment in “like new” condition.
Trihalomethane Removal System (TRS)
PAX Water's TTHM removal technology is also applied within water storage tanks and clear wells. PAX Water
representatives work with municipalities and engineers to develop a custom-design in-tank aeration system. The
in-tank aeration system consist of a mixing device to circulate the water up towards the top of the tank and spray
aeration nozzles. The energy-efficient spray aeration creates a droplet distribution pattern that maximizes THM
GridBee® In-Tank/In-Clear well Floating Spray Nozzle Design
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removal while minimizing cost. Active ventilation removes THMs from the headspace of the tank and reintroduces
fresh air.
The Trihalomethane Removal System (TRS) is a custom-
designed turnkey design-build aeration system to reduce
THM levels in drinking water storage tanks and
reservoirs. TRS is the product of a partnership between
PAX Water Technologies and Utility Service Company.
The TRS includes sprayers, ventilators, and mixers. PAX
Water Technologies has developed a proprietary design
and performance modeling software called the
NEPTUNE Toolbox™. The TRS is customized for each tank and its operating
conditions to maxim ize effectiveness while minimizing cost. A pump is used to
pump water from the bottom of the tank to the sprayer located at the top of the
tank. A minimum of five feet of head space is recommended. An active
ventilation system removes THMs which evaporate from the water during
spraying. In some applications, a mixer is added at the bottom of the tank to
enhance performance of the spray aeration system. The spray system is
permanent, but the mixer could be removed and placed in another tank.
The NEPTUNE Toolbox™ has been shown to predict fairly accurate results in a
number of case studies. PAX Water Technologies offers a warranty on the
removal rate, backed by a money back guarantee. Additionally, they are able to
offer financing over several years at a zero percent interest rate.
4.1.3 Integration of Spray Aeration within the Cohasset System
Spray aeration systems as described above can be installed in the clear well at the water treatment plant and in
one or both of the existing system storage tanks. Spray aeration can typically remove 20 up to 40% of the volatile
TTHMs (chloroform and bromodichloromethane) present. The removal percentage is dependent upon the
concentration of TTHMs, recirculation rate, and the ventilation of the head space.
The historical DBP data and recent DBP formation potential determination reveals that although the average
chloroform fraction (60% 70%) is not ideal, the remaining THM fraction is comprised of bromodichloromethane,
which is also volatile and effectively removed by air stripping treatment. Additionally, based on the recent DBP
formation potential results (Figure 3-10), TTHM formation appears to be characterized by a logarithmic relationship
in which the majority (40% to 75%) of the 77-hour TTHM formation occurs within the first 6 to 24 hours. This rapid
initial formation and slowed formation rate of TTHMs over time supports the feasibility of applying aeration to reduce
TTHM levels in the Cohasset water system. The HAA levels, which would not be reduced by aeration, appear to be
effectively controlled by the current coagulation-clarification-filtration processes at the Lily Pond WTP.
Spray aeration treatment could be integrated within the existing clear well to reduce the levels of TTHM entering
the distribution system. The Lily Pond WTP is operated on average eight to twelve hours per day. Therefore, during
the overnight hours water that is stored within the plant clear well will have a longer time to react with the chlorine
and form increased levels of TTHM. The formation potential curve in Figure 3-11 demonstrates that up to 79 µg/L
could form during a 12-hour detention time at a higher water temperature. A spray aeration system installed in the
clear well could remove a fraction of those TTHMs before entering distribution to the system. If a minimum removal
efficiency of 20% was achieved that would equal a TTHM reduction of up to 15 µg/L from the water remaining in
the clear well overnight. While aeration does not stop the formation, it could help lower the overall levels entering
the system.
Since the TTHM levels will continue to form because the reactants (TOC and chlorine) are still present, additional
aeration treatment could be provided in the existing distribution system ground storage tanks—Bear Hill and
TRS50 Series
TRS40 Series
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Scituate Hill water tanks. Engineered spray aeration system installed within tanks which are well designed and well
mixed can provide up to 40% removal of the volatile TTHMs present. If the detention in the system and detention
time within the tank is at least 12 hours, then up to 80 µg/L of TTHMs could have formed and assuming that up to
75% of them are volatile, the system could provide a TTHM reduction up to 24 µg/L. Aeration within these storage
tanks would help reduce peaks in seasonal and locational TTHM levels. Of course, these reductions in TTHMs
would only occur in the portions of the system which are primarily served by the tanks. A conceptual process
schematic of the integration of spray aeration system at the Lily Pond WTP and distribution system is included in
Figure 4-1.
Figure 4-1. Spray Aeration Treatment Schematic
4.2 GAC/BAC TREATMENT
GAC adsorption is used to remove targeted dissolved contaminants, including taste and odor causing compounds,
synthetic organic chemicals, color forming organics, and DBP precursors. GAC media is manufactured from natural,
carbonaceous materials, such as coal, peat, and coconuts by various high temperature processes. GAC is typically
configured in a down-flow, contactor mode. As the water flows through the GAC media, dissolved species are
adsorbed onto active carbon sites by chemical reactions and physical attractions. This results in the removal of the
dissolved species from the treated water.
The extent of adsorption primarily depends upon the strength of the affinity of the GAC for the dissolved organic
species. This relationship is quantified by adsorption isotherm models, which describe the amount of dissolved
species (i.e. organics) that can be adsorbed onto the activated carbon at equilibrium. A widely utilized isotherm
model is the Freundlich isotherm, which is described by the mathematical expression presented in Equation 2. 𝑞𝑞𝑒𝑒=𝐾𝐾𝐶𝐶𝑒𝑒1 𝑛𝑛� Equation 2
Where:
qe = Loading or equilibrium adsorbent-phase concentration of organics (mg dissolved organics/g GAC)
K = Freundlich adsorption capacity parameter (mg/g)(L/mg)1/n
1/n = Freundlich adsorption intensity parameter (dimensionless)
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Ce = Equilibrium concentration of dissolved organics in solution (mg/L)
This isotherm relationship is typically developed through laboratory experiments and can be used to conservatively
estimate the carbon usage rate (CUR) according to Equation 3. The carbon usage rate, which relates the mass of
GAC used to the total volume of water treated, depends on the unique chemical and physical interactions between
the source water quality and media used. While isotherm data provides a preliminary estimate of GAC usage rate,
pilot testing using the site-specific source water provides a more accurate estimate of carbon usage due to the
variability in TOC speciation. In the absence of isotherm or pilot data, computer modeling data from carbon
manufacturers can be used as an initial estimate of the carbon usage rate. Additionally, performance data from
similar applications could be used to gauge the carbon usage rate. CUR =CinfK(Cinf)1 n�=𝑀𝑀𝐺𝐺𝐺𝐺𝐺𝐺𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 Equation 3
Where:
CUR = Carbon usage rate (g/L)
Cinf = Influent concentration of dissolved organics (g/L)
MGAC = Mass of GAC at selected EBCT (g)
Vtreated = Volume of water treated (L)
In conjunction with the carbon usage rate, the EBCT is the key operational parameter that controls the trade-off
between capital costs associated with GAC contactor size requirements and the operational costs related to carbon
change-out frequencies. The mathematical relationships that relate the EBCT, GAC mass needed, and GAC bed
life (replacement frequency) are presented in Equations 4 through 6. For TOC removal applications, an EBCT
between 10 to 15 minutes is typically applied to balance this trade-off between capital and operational costs.
EBCT =VQ Equation 4 MGAC =EBCT × Q × ρF Equation 5 Bed Life = VtreatedQ Equation 6
Where:
EBCT = Empty bed contact time (min)
Q = Flow rate to GAC contactor (L/min)
V = Volume of GAC in contactor (L)
MGAC = Mass of GAC at selected EBCT (g)
ρF = Density of GAC media (g/L)
Vtreated = Volume of water treated (L)
GAC adsorption treatment is commonly applied as pressurized contactor vessels. Some major manufacturers of
GAC media and contactor systems include Calgon Carbon, Evoqua, and Norit. These manufacturers supply similar
media products and pre-manufactured adsorption vessel systems for DBP precursor removal. A summary of
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Calgon’s GAC system for dissolved organic rem oval is provided in the following subsection to illustrate the typical
equipment in a pressure vessel system.
4.2.1 Calgon Carbon GAC Media and Pressurized Vessel System
Calgon’s recommended GAC media product for dissolved organic carbon removal is Filtrasorb® 400. According to
the manufacturer’s data sheet, this activated carbon is made from bituminous coal through a process known as re-
agglomeration to produce a high activity, durable, granular product.
Filtrasorb® 400 is formulated to comply with applicable provisions of the AWWA Standard for Granular Activated
Carbon (B604) and the requirements of ANSI/NSF Standard 61 for use in drinking water treatment facilities. A
summary of the Filtrasorb® 400 specifications is provided in Table 4-1.
Table 4-1. FILTRASORB® 400 Technical Specifications
Parameter Value
Media Identification FILTRASORB® 400
Carbon Type Bituminous Coal
Iodine Number 1000 mg/g (min)
Moisture by Weight 2% (max)
Effective Media Size 0.55 – 0.75 mm
Uniformity Coefficient 1.9 (max)
Abrasion Number 75 (min)
Media Apparent Density 0.54 g/cm3
Calgon’s recommended GAC contactor system for dissolved organic carbon removal is their Model 12-40. The
vessels are sized to hold 40,000 pounds of GAC. This
additional mass of GAC provides additional contact
time for the removal of trace or poorly absorbing
compounds. One vessel can treat a 1 MGD flow with
an EBCT of 15 minutes. System sizing is based upon
the flow rate, surface loading rate and EBCT. Smaller
vessels are offered to treat smaller flows. The Model
12-40 contactor is supplied as a standard two-vessel
system (Model 12-40 System) or as a single vessel
concept (Model 12-40 Single). The Model 12-40
System is delivered as two vessels, a centrally
located valve manifold and interconnecting piping that
allows for the vessels to operate in series or in
parallel. The Model 12-40 Single is delivered as a
single vessel with process piping. The single vessel
is typically provided for systems consisting of multiple units operated in parallel. Common features of the Model 12-
40 vessels include:
• Internal cone under-drain that provides treated water collection, backwash water distribution, and complete
discharge of spent carbon.
• In-bed sample ports for monitoring process of the adsorbent as it flows through the bed.
• One (1) GAC fill line and two (2) GAC discharge lines.
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• Vessel is sized to contain 40,000 lbs of GAC and to allow for backwash expansion of about 25%.
The Model 12-40 system is designed for use with Calgon Carbon’s closed loop carbon exchange service. Carbon
transport trailers are used to remove the spent carbon from the vessel via a pressurized carbon-water slurry. The
vessel is then replenished with fresh carbon through the same process. Calgon Carbon also manages the disposal
or reuse of spent carbon, which is typically returned to Calgon Carbon for reactivation. A summary of system
dimensions and operating conditions is provided in Table 4-2.
Table 4-2. Model 12-40 System Dimensions and Operating Conditions
Parameter Value
Vessel Diameter 12 ft
Overall Width 13-1/3 ft
Overall Length (Two Vessels) 31-1/3 ft
Carbon per Vessel 40,000 lbs
Pressure Rating 125 psig @ 140 oC
Pressure Relief Graphite rupture disk (125 psig)
Temperature Rating 140 oF maximum
Clean Bed Pressure Drop 3 psi/ft depth
Backwash Rate Typical 1,100 gpm (25% expansion)
Carbon Transfer Air pressure slurry transfer
4.2.2 Integration of GAC Adsorption at the Lily Pond WTP
Total organic carbon and other compounds are absorbed by the GAC as the flow passes through the treatment
vessel. The highest removal is provided when the carbon is fresh and decreases as the absorption capacity of the
carbon is used up. When the minimum design removal efficiency occurs the GAC must be removed and replaced
with fresh carbon of the carbon removed must be regenerated. The longest bed life can be achieved when the
lowest TOC concentration water is fed to the GAC absorption vessel. Therefore, the most cost-effective application
point at the existing water treatment plant is after filtration where the maximum TOC removal has been achieved by
the conventional treatment process. Figure 4-2 presents a process schematic of the integration of GAC treatment.
The maximum TOC absorption that can be achieved by new, fresh carbon is approximately 70 to 80%. The removal
efficiency will decrease over time to approximately 20 to 30% as the absorption capacity is exhausted and biological
removal takes over as the primary removal mechanism. Use of multiple vessels with staggered carbon change out
allows for maintenance of higher average removal efficiencies. Maintaining the highest removal efficiencies, 60 to
70%, requires the most frequent carbon change out which can be as frequent as every two weeks. Lower removal
efficiencies can be maintained by multiple vessels with less frequent carbon change outs. In the Cohasset system
a GAC system could be operated by allowing it to operate in the biological mode during the winter, spring and part
of the fall months and in the absorption mode during the summer and a portion of the fall months with more frequent
carbon change outs.
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Figure 4-2. GAC/BAC Treatment Schematic
4.3 BIOLOGICALLY ACTIVATED CARBON (BAC) TREATMENT
Biofiltration applications consist of operating granular media filters to promote the attachment and growth of a
biofilm. Biofilms can support a wide variety of microbial groups, but generally form in the same sequential manner:
deposition and adsorption of organic molecules to form a “conditioning film”, adsorption of microorganisms on the
film, secretion of extracellular polymeric substances (EPS) to fortify the film, and growth of microorganisms. The
primary difference between a biofilter or a biologically active GAC filter and a regular dual media filter or GAC
contactor is that a disinfectant is not fed to the water upstream to allow the microbes to populate the media.
Biofiltration can be applied to remove biodegradable natural organic matter, ammonia, nitrate, iron, manganese,
and taste and odor caused by geosmin and methylisoborneol (MIB). Additionally, biological filters remove natural
organic matter by using it as a substrate, and in turn, can produce biologically stable water with lower DBP formation
potential.
Biofiltration can be applied as conventional open, gravity filters or within pressure vessels, such as the same applied
for GAC adsorption. The process water is filtered through biologically active granular media in a down-flow
configuration. Similar to GAC systems, the biofiltration vessels or tanks are equipped with a media support system
that allows the media bed to be backwashed. Piping and valves are provided for the isolation of the process units,
backwash supply, and spent backwash collection. Support facilities include a source of clean water for backwash,
backwash pumps and controls, and facilities to handle and recycle or dispose of the spent backwash water.
Pressurized biofiltration equipment is the same as the vessel-type contactors used for GAC adsorption and is
supplied by the same manufacturers, including Calgon Carbon Corporation, Evoqua, Norit, Tonka Water, and
WesTech. These manufacturers supply the process equipment for biofiltration, but do not typically market
equipment as a biofiltration system. The anthracite and GAC media is supplied by several entities including Calgon
Carbon Corporation, Ingevity, Cabot Corporation, Evoqua, and Northern Filter Media.
Biofiltration design considerations include the EBCT, hydraulic loading rate, granular media type, and availability of
nutrients in the source water. The EBCT, defined previously in Equation 4, is a chief design parameter that sets
the retention time of the water in contact with the biologically active media and dictates the size of the filter bed.
Generally, increasing the EBCT increases the organic matter removal efficiency at the expense of larger filter bed
requirements. A balance between this trade-off can typically be reached with an EBCT of about 10 to 15 minutes;
however, pilot testing is necessary to gauge and confirm treatment effectiveness. The ozone and BAC pilot study
that was previously performed for the City used an EBCT of 10 minutes. The study indicated that different EBCT
were utilized during the initial trial, but comparative performance data were not provided. Increasing the EBCT can
be accomplished by increasing the filter depth and is related to the hydraulic loading rate. The hydraulic loading
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rate describes the water filtration velocity across the depth of the biofilter. Traditionally, biofilters have been operated
at slow filtration rates (0.5 to 2 gpm/ft2). More rapid filtration rates (2 to 6 gpm/ft2) can be applied to reduce footprint
requirements, but still maintain effective DOC removals.
Biofiltration can be employed on adsorptive media, such as GAC, or on non-adsorptive media, such as sand and
anthracite. While sand and anthracite biofilters have been demonstrated to achieve similar biodegradable DOC
removal rates as biologically active carbon (BAC) filters, BAC tends to achieve more favorable DOC removals under
colder or less favorable conditions.
The major water quality parameters that influence biofiltration performance include nutrients (nitrogen and
phosphorus), biodegradable natural organic matter, temperature, pH, and alkalinity. Therefore, “engineered
biofiltration” strategies can be applied to enhance biofilter performance. These engineered biofiltration strategies
include: supplementing nutrient availability by feeding orthophosphorus or a nitrogen source; applying ozone as a
pre-oxidant (ozonation) to increase the fraction of biodegradable organic matter and improve the overall organic
removal and corresponding DBP control. Depending on whether water quality adjustment is necessary for
enhancing biofilter performance, the following chemicals may be required: orthophosphate, ammonia (nitrogen
source), methanol or acetic acid (carbon source), sodium hydroxide, sulfuric acid, and soda ash (sodium carbonate).
The DBP precursor removal effectiveness of biofiltration depends on many site-specific factors. Therefore, on-site
pilot testing is necessary to gauge DBP control effectiveness. TOC removal by biofiltration can range from about
5% to 60% with 20-25% being a typical average removal efficiency for EBCTs of 10 minutes or greater. The
corresponding reduction in DBP formation potential can range from 8% to 50% for TTHMs and 10% to 70% for
HAAs. Applying pre-ozonation increases the biodegradability of natural organic matter and can enhance TTHM and
HAA formation potential reductions to possibly greater than 50% and 75%, respectively. In the pilot study performed
for the Town, TOC removal efficiencies were not determined for operation without ozone and 8-14% TOC removal
was achieved with ozone from the filtered water. There is some question as to whether the pilot study performed
would be representative of the results that could be achieved with a properly operated system as noted in the
discussion of this study in Section 2.
Biofiltration processes require minimal operation requirements. Backwashing is necessary to avoid high pressure
drops caused by deposited particles and excess bio-growth (biofouling). Backwashing frequencies can vary from
once a day to every other week. Backwashing with water containing a disinfectant residual (0.5 to 1.0 mg/L as Cl2)
may help biofilters operate at the longer biweekly run times. Backwashing strategies include bed fluidization,
collapse pulsing, and air scour with negligible to 50% bed expansions. The previous pilot study performed for the
Town indicated that the filters could be run for 60 hours between backwashes. Biofiltration is a simple, robust
process that does not require intensive mechanical equipment. Coupled with ozone biofiltration can be considered
a reliable process relative to treatment performance. The treatment process should include at least two (2) biofilter
beds (largest out of service) to prevent interruptions in service due to backwashing.
For conventional open, gravity filters, media would need to be replaced to account for media loss or excessively
broken-down media. Eventual non-adsorptive media replacement will be necessary depending on biofilter
performance (possibly up to 20 year media life). Operating GAC media in biological mode extends the bed life of
the carbon as compared to operating in GAC adsorptive mode. The GAC bed life could be extended by up to year,
but depends on monitored system performance. Residuals generated by the process include spent BAC media
and spent backwash water that require disposal. Some GAC companies offer a service to remove and regenerate
the GAC media.
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4.3.1 Ozonation
Ozone is often applied ahead of BAC systems to breakdown the naturally occurring
larger chain organic molecules in the raw water to render them more readily
biodegradable. Ozone oxidation (ozonation) is typically accomplished by diffusing on-
site generated ozone (O3) into the water to obtain a desired ozone dose. In water,
ozone oxidation occurs through two major mechanisms—direct oxidation by ozone and
indirect oxidation by hydroxyl free radicals formed by the reaction between ozone and
hydroxide ions. The decay of these hydroxyl radicals is pH dependent; therefore, pH is
a key parameter in determining the concentration of ozone and hydroxyl radicals and
the oxidations rates. Additionally, hydroxyl radicals react with target compounds or are
consumed by bicarbonate and carbonate alkalinity.
The major process components of ozone systems include a gas feed system, an ozone
generator, cooling water supply, an ozone contactor, and an off-gas destruction
system. The gas feed system provides a clean, dry source of oxygen to the generator.
Gas feed systems typically consists of either liquid oxygen-based systems or air based
systems. The liquid oxygen feed system components include a storage tank, an evaporator to convert the liquid to
a gas, filters to remove impurities, and pressure regulators to limit the gas pressure to the ozone generators. Air
based systems, such as pressure or vacuum swing adsorption, are more complicated requiring air pretreatment
equipment to prevent damage to the ozone generator. Air processing systems consist of compressors, after coolers
(optional), refrigerant dryers, desiccant dryers, filter, pressure regulators, and adsorbent bed of zeolite. As
compared to the air-based systems, liquid oxygen feed systems are relatively simple and less capital and
operationally intensive.
Ozone generators convert the oxygen gas feed to ozone gas using electrodes through concentric tube cylinders or
parallel plates. Cooling water is supplied to maintain generator efficiency. The generated ozone rich gas is applied
to the process water to achieve adequate contact and reaction time for oxidation. Ozone application systems include
fine bubble diffusers, injectors/static mixers and turbine mixers. Off gas destruction is required as ozone is toxic in
the concentrations that may be present in the off-gas. Some systems also include a quench chamber to remove
ozone residual in the ozonated water.
The common functions of ozonation are to provide primary disinfection, gain additional Cryptosporidium log
removal, destroy color, taste, and odor causing constituents (organic micro-pollutants, algae, sulfide, iron, and
manganese), enhance coagulation, increase biodegradability of organic compounds, reduce regulated DBP
formation potential by direct oxidation of DBP precursors, and reduce chlorine demand. Because ozonation tends
to increase the fraction of biodegradable natural organic matter, applying subsequent GAC or BAC treatment can
remove this biodegradable organic fraction and improve organic precursor removal and corresponding DBP
formation potential.
Recognized suppliers of ozone generation and dosing equipment include WEDECO, Ozonia, and Pinnacle Ozone.
Process design considerations for ozone systems include the water flow rate, desired ozone dose, and contact time
as dictated by disinfection CT requirements. The process flow and ozone dose are used to estimate the required
ozone production in pounds per day (ppd). This required ozone production provides the basis for equipment sizing.
The required ozone dose depends on the treatment application and site specific conditions. When combined with
biofiltration, the ozone dose required typically ranges from 1.5 to 4 mg/L. For direct organics oxidation and DBP
control, an ozone dose of about 3 mg/L or greater is typically required. While this estimated ozone dose is based
on targeting an overall reduction in the DBP formation potential, the effectiveness of ozone oxidation is influenced
by site-specific water quality, including temperature, pH, alkalinity, and concentration of reduced species. The pilot
study performed for the Town determined that an ozone dose of 8 mg/L and an ozone contact time of 7.5 minutes
provided the best performance based upon the conditions tested.
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Ozonation prior to chlorination may reduce the overall THM and HAA formation potentials by reacting with and
destroying chlorine precursor sites. For neutral pH and moderate levels of alkalinity, THMFP level reductions of 3
to 20 percent have been shown at doses ranging from 0.2 to 1.6 mg ozone per mg carbon. However, applying
ozone within this range has also been shown to increase the DBP formation potential. Filtering the ozonated water
through a biological filter may further decrease THM and HAA formation potentials by possibly up to 40 percent.
Site specific, bench or pilot scale testing is necessary to evaluate DBP reduction effectiveness, particularly for
source waters having higher pH and alkalinity levels. In the pilot study performed for the Town in the tests performed
on the filtered water reductions of TTHM formation potential ranged from a low of around 15% up to a high of
approximately 50%.
Although ozonation may decrease overall THM and HAA formation potentials, ozonation may increase the formation
of some HAA species and unregulated DBPs, such as chloropicrin and chloral hydrate. Ozonation may also result
in a shift towards brominated DBPs and produce a significant quantity of regulated bromate (0.010 mg/L MCL) in
waters with bromide concentrations greater than 50 µg/L. Bromate formation was monitored during the pilot study
performed for the Town and did not exceed the 10 µg/L MCL in the effluent from the pilot columns, but bromate
concentrations measured in the settled and filtered water did exceed the MCL.
Bromate formation can generally be reduced by limiting ozone dose, lessening contact time, lowering pH, and
adding ammonia. Providing tight ozone dose and contact control along with pH control is usually sufficient for low
to medium raw water bromide concentrations; however, advanced oxidation pre-treatment may be required for
higher bromide and ozone doses. While reducing the applied ozone dose and contact time is more favorable for
limiting bromate formation, it may not be favorable for lowering the THM and HAA formation potential.
Although ozone systems are complex, the process is highly automated and typically reliable. Back-up generator
units are usually installed to help maintain reliability. However, there are many electro-mechanical components that
must be regularly inspected to maintain system performance and reliability. Ozone systems require a source of
oxygen supply, cooling water, and significant electrical feed. A modest degree of operator skill and time is needed
to operate an ozone system. Typical operation and maintenance activities associated with ozone systems include:
• Monitoring the ozone process flow meters and process control instruments.
• Performing a daily check of generators.
• Periodic pressure testing of liquid oxygen tanks.
• Inspecting piping and injection components for leaks and signs of corrosion.
• Monitoring gas phase ozone at the ozone generator output, contactor off gas, ozone destruct off gas, and
ambient air in ozone process areas.
• Monitoring system for bromate with samples collected at the entrance to the distribution system.
Maintenance on generators typically requires skilled technicians. If trained maintenance staff are not available at
the plant, generator maintenance can be performed by the equipment manufacturer.
The ozonation process has high power requirements, which depend on the required ozone output and system size.
Generally, approximately 8 to 16 kWh per pound of ozone is required for the oxygen supply system, ozone
generation, generator cooling, and ozone injection. Assuming an 8 mg/L ozone dose, the power consumption would
be approximately 500 to 1,100 kWh/MGD. A 480 VAC power supply and low frequency (50 or 60 Hz) to medium
frequency (60 to 1,000 Hz) generators are most commonly used in the water industry. Other than replacement
parts for maintenance, ozone systems do not require expendable materials. The ozone generation process requires
a clean, dry source of oxygen (conditioned air or liquid oxygen). The ozonation process does not generate residuals
that require disposal.
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4.3.2 Integration of BAC Treatment at the Lily Pond WTP
The pilot study performed for the Town looked at applying the ozone/BAC treatment after the sedimentation basins
and after filtration. The results from the Ozone/BAC pilot study showed that the percentage reduction of TTHM
formation potential was much greater for the intermediate system as compared to treating the filtered water.
However, the resulting formation potentials after treatment were generally higher for the intermediate system trials
because the formation potentials after sedimentation were much higher than following filtration. The advantage of
the intermediate system is the ability to remove the anthracite from the existing filters and use them as biologically
active filters as opposed to constructing separate contactors. However, a new ozone contact basin would need to
be constructed to apply the ozone prior to the filters. Placing the ozone/BAC system after filtration as shown in
Figure 4-3 using it as a polishing step may be more advantageous from an efficiency standpoint to control the DBPs
although this will require additional capital cost to construct the GAC contactors.
Figure 4-3. Ozone-BAC Treatment Schematic
4.4 ION EXCHANGE TREATMENT
Natural organic matter tends to be highly ionized in water and anionic in nature. Because of this, anion exchange
(IX) may be applied as a DBP precursor removal technique. Conventional IX technology consists of a packed-bed
of synthetic resin beads, which are positively charged and range in size from 0.3 to 1.5 mm. As water passes
through the packed bed of IX beads, negatively charged organic matter exchanges with chloride (or hydroxide) ions
present on the surface of the resin.
The extent of organic matter removal is a function of water quality and resin-specific parameters. Typically, IX
technologies can reduce about 30% to 60% of the TOC with run times ranging between about 500 to 5,000 bed
volumes. Bed volumes refer to the treated water throughput divided by the resin bed volume (BV). As organic matter
is removed, the active exchange sites on the resin are exhausted and the resin needs to be regenerated. During
regeneration, a concentrated sodium chloride (or sodium hydroxide) solution is contacted with the resin to substitute
chloride (or hydroxide) ions for the dissolved organic matter. The dissolved organic matter is released from the resin
into the concentrated brine solution, which requires disposal. In many IX applications, the brine stream is disposed
to sanitary sewer and treated at the wastewater treatment facility.
In IX applications, bench-scale and pilot-scale studies are necessary to determine the resin’s treatment
effectiveness and optimum operating parameters. The main criteria that is typically developed from bench and pilot
studies include the resin run time, treatment flow rate, regenerant dose and concentration, backwash flow rate, and
brine waste stream generation.
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Conventional IX technologies are typically configured as a packed-bed of resin beads in pressurized contactor
vessels. Some major manufacturers of these IX systems include Tonka Water, Hungerford & Terry, and Calgon
Carbon. An alternative to conventional IX systems is the MIEX® resin system. The MIEX® resin was developed by
Orica Watercare, Inc. to remove dissolved organic carbon by exchanging with a chloride ion on the magnetized
resin surface. The magnetized iron oxide component allows the resin to be applied in a fluidized bed process as
opposed to the fixed-bed operation of conventional IX systems. While the MIEX® technology has been shown to
effectively remove organic matter and reduce DBP formation, the MIEX® system requires more frequent resin
replenishment and maintenance troubleshooting. The MIEX® system uses a fluidized bed for contacting the resin
with the water receiving treatment and therefore can be used on the raw water. However, treating the filtered water
after the maximum TOC has been removed by the conventional treatment process will allow longer run times and
lower finished water TOC concentrations. A fixed bed system is more cost effective for application to the filtered
water.
For this study, Tonka Water’s conventional IX system was used to illustrate the resin and equipment for a fixed bed
IX system for DBP precursor removal and control. The following subsection presents a summary of Tonka Water’s
IX system for removing dissolved organic compounds.
4.4.1 Tonka Water OrganixTM System
Tonka Water’s IX system for removing dissolved organics is their
OrganixTM system. The OrganixTM system uses organic selective ion
exchange resin configured as a packed bed to exchange dissolved
organic matter for chloride ions. The resin is housed in a pressurized,
down-flow vessel. The vessels are typically designed to target a
hydraulic loading rate of 8.0 gpm/ft2 and a resin capacity of 15,000
gal/ft3. A summary of typical dimensions and operating conditions for the
OrganixTM system is provided in Table 4-3.
Table 4-3. OrganixTM System Dimensions and Operating Conditions
Parameter Value
Typical Hydraulic Loading Rate 8 gpm/ft2
Typical Resin Capacity 15,000 gal/ft3
Working Pressure 100 psi
Clean Bed Pressure Drop 1.2 psi
Test Pressure 130 psi
Typical Regeneration Frequency1 4 – 5 days
1. Assumes operation at the typical hydraulic loading rate or full design flow.
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Regeneration of the OrganixTM resin is necessary to maintain the organic removal efficiency. The treatment run time
in between regenerations depends on site-specific water quality and is estimated using pilot testing. Typical full-
scale regeneration frequencies for this system is every 4 to 5 days. However, a longer time between regenerations
is anticipated for this application as the typical flow rates are less than the design flow rates. The regeneration
procedure consists of four consecutive steps:
• Up-flow Backwash for 10 minutes
• Brine Contact for 45 minutes
• Slow Rinse for 60 minutes
• Fast Rinse for 10 minutes
In addition to routine regeneration, the manufacturer recommends a yearly caustic cleaning or “squeeze” to be
performed by contacting the resin with a mixture of 2% caustic solution and 10% brine solution. The purpose of the
caustic cleaning is to release organics from the resin not released during normal regeneration. After about 8 to 10
years of operation, the resin’s organic removal efficiency is no longer recovered with regeneration and the system
requires resin bed replacement.
4.4.2 Integration of IX Treatment at the Lily Pond WTP
A fixed bed ion exchange system would be designed to treat all or a portion of the treated water from the existing
plant after filtration and prior to chlorination for primary disinfection as illustrated below in Figure 4-4. At a maximum
TOC removal efficiency of 60% the entire flow would need to be treated during the summer and fall months and a
portion of the flow could be bypassed around the IX process during the rest of the year to maintain a minimum level
of removal. This would maintain low DBP concentrations in the distribution system and decrease the operating cost
during the rest of the year.
Figure 4-4. Ion Exchange Treatment Schematic
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4.5 CONVERT TO MONOCHLORAMINE DISINFECTANT RESIDUAL
Many drinking water utilities apply combined chlorine (chloramines) as a secondary disinfectant to lower the
formation potential of regulated TTHMs and HAAs. In addition to effective regulated DBP control, chloramines tend
to carry a more stable residual in the distribution system as compared to free chlorine. While chloramines generally
form trace levels of TTHMs and HAAs, they essentially halt the formation of any additional DBPs after the ammonia
is added to the chlorinated water. However, they can contribute to the formation of other, non-regulated DBPs.
Chloramines form from reactions of ammonia with free chlorine. As demonstrated in Equation 7, free chlorine
reacts with ammonia to form monochloramine. Subsequent reactions of monochloramine with free chlorine form
dichloramine and nitrogen trichloride (trichloramine) (Equations 8 and 9). These equations are also demonstrated
by the typical breakpoint chlorination curve shown in Figure 4-5.
𝑁𝑁𝐻𝐻3 +𝐻𝐻𝐻𝐻𝐶𝐶𝐻𝐻→𝑁𝑁𝐻𝐻2 𝐶𝐶𝐻𝐻 (𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝐻𝐻𝑚𝑚𝑙𝑙𝑙𝑙𝑚𝑚𝑙𝑙𝑚𝑚𝑙𝑙)+𝐻𝐻2 𝐻𝐻=QaQw =QatVw Equation 7
𝑁𝑁𝐻𝐻2 𝐶𝐶𝐻𝐻+𝐻𝐻𝐻𝐻𝐶𝐶𝐻𝐻→𝑁𝑁𝐻𝐻𝐶𝐶𝐻𝐻2 (𝑑𝑑𝑙𝑙𝑚𝑚ℎ𝐻𝐻𝑚𝑚𝑙𝑙𝑙𝑙𝑚𝑚𝑙𝑙𝑚𝑚𝑙𝑙)+𝐻𝐻2 𝐻𝐻 Equation 8
𝑁𝑁𝐻𝐻𝐶𝐶𝐻𝐻2 +𝐻𝐻𝐻𝐻𝐶𝐶𝐻𝐻→𝑁𝑁𝐶𝐶𝐻𝐻3 (𝑚𝑚𝑙𝑙𝑛𝑛𝑙𝑙𝑚𝑚𝑛𝑛𝑙𝑙𝑚𝑚 𝑛𝑛𝑙𝑙𝑙𝑙𝑚𝑚ℎ𝐻𝐻𝑚𝑚𝑙𝑙𝑙𝑙𝑑𝑑𝑙𝑙)+𝐻𝐻2 𝐻𝐻 Equation 9
To achieve effective disinfection and prevent odor issues associated with the di- and trichloramines, chloramine
systems are typically operated to target monochloramine as the dominant species. As shown in the breakpoint
curve, a Cl2:N weight ratio between 3 and 5 is typically used for chloramination. This ammonia dosage must be
carefully controlled to prevent excess free ammonia, which increases the potential for nitrification in the distribution
system. Furthermore, chloramine systems typically require a routine flushing program as preventive maintenance
against nitrification episodes and often require periodic free chlorine “burns” to remove built up biofilms for the
distribution system piping.
Figure 4-5. Breakpoint Chlorination Curve
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4.5.1 Integration of Chloramination within the Cohasset System
Free chlorine is currently used as the primary disinfectant to achieve compliance with the regulations governing the
inactivation of Giardia and viruses. Although chloramines can be used to achieve the required disinfection credits,
the detention time is significantly longer than with free chlorine and would require the construction of additional
contact volume. For the Cohasset system, ammonia would be added to the water following the required contact
time to meet disinfection requirements to convert the free chlorine to a monochloramine residual. Ammonia feed
facilities would be needed at both the existing Lily Pond WTP and the Ellms Meadow Pumping Station. DBPs that
formed during the free chlorine contact period would not be affected by the conversion to chloramines, but additional
DBPs would not be formed in the distribution system as long as the chloramine residual was present. A high level
of DBP control could be achieved by the use of chloramines as the secondary disinfectant.
Water carrying a monochloramine residual when mixed with a water containing a free chlorine residual can increase
the chlorine to ammonia ratio which can lead to the formation of dichloramine and trichloramine which impart color
and a strong chlorine smell to the water. If the chlorine to ammonia ratio is increased to a high level as illustrated
in Figure 4-3 above, the chlorine residual will begin to drop and may disappear. This could occur as a result of
mixing the chloraminated water with water from the other interconnected systems using a free chlorine residual.
For this reason, chloramines have not been recommended for implementation unless the other interconnected
systems were also willing to convert to a chloramine residual as the secondary disinfectant.
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5.0 CONCEPTUAL EVALUATION OF DBP CONTROL ALTERNATIVES
The identified treatment strategies were conceptually evaluated according to advantages and disadvantages,
treatment performance, and conceptual opinions of probable construction cost. The conceptual evaluations
considered site-specific information from the existing system, manufacturer system data and recommendations,
and experience with analogous systems. The following subsections present the results and comparison for each of
the treatment alternatives and recommendations for implementation.
5.1 ADVANTAGES AND DISADVANTAGES
The advantages and disadvantages of the identified alternatives are outlined in Table 5-1.
Table 5-1. Advantages and Disadvantages
Alternative Advantages Disadvantages
Spray Aeration • Over 85% of the TTHMs are comprised of
strippable chloroform and
bromodichloromethane
• Reduce TTHM levels at the WTP and within
the distribution system
• Existing storage tanks facilitate integration
within the existing system
• Lower capital and operation costs (compared
to other treatment options)
• Not effective in controlling HAA levels
• Requires formation of THMs prior to
removal
• Potential for re-formation of THMs
• May not achieve TTHM goal of 80% of
MCL at every location
• Possibility for ventilation system noise
(particularly if located within
residential area)
GAC/BAC
Filtration
• Reduce dissolved organic precursors and
meet TOC removal goal
• Meet TOC removal and DBP goal of 80% of
MCL
• Operating the GAC in biological mode during
winter/spring seasons would extend GAC bed
life and reduce media replacement cost due to
biological removal of TOC
• Capital cost
• Potentially high GAC replacement
frequency and costs if operated
continuously in adsorption mode
• Require a fraction of the water to be
treated by GAC year around to keep
the GAC media active.
Ozone-BAC • Reduce dissolved organic precursors
• Meet TOC removal and DBP goal of 80% of
MCL during winter/spring months
• Meet DBP compliance year around
• Lower GAC media replacement cost by
extending GAC bed life due to biological
removal of TOC
• Ozone increases the fraction of
biodegradable DOC available for bacterial
consumption and promotes biological
carbon removal
• Produces biologically stable water that in turn
reduces chlorine demand in the system.
• Additional capital cost of ozonation
equipment
• Higher energy and operating cost
• May not achieve TOC target during
seasonally high summer/fall months.
• May not achieve TTHM goal of 80% of
MCL during seasonally high
summer/fall months.
Ion Exchange • Reduce dissolved organic precursors
• Meet TOC removal and DBP goal of 80% of
MCL
• Could be placed out of service when TOC
removal is not necessary
• Capital cost
• Routine resin regeneration
requirement
• Import of salt required for resin
regeneration
• Brine waste disposal
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5.2 TREATMENT PERFORMANCE
A water treatment evaluation was performed by conceptually assessing the DBP precursor (TOC) and TTHM
removal effectiveness for each alternative. A summary of typical TOC and TTHM treatment efficiencies for each
alternative is presented in Table 5-2. Of the treatment alternatives, GAC adsorption and anion exchange are
anticipated to be the most effective with respect to organic carbon and DBP control with typical removal percentages
ranging from 40% to 70% by GAC and 30% to 60% by ion exhange. On average, BAC is anticipated to achieve
approximately 25% removal of TOC with similar reduction in TTHMs. The application of ozonation ahead of BAC
filtration would likely improve TTHM removal to a higher 15% to 50% range. When chloroform comprises the majority
of the TTHMs, spray aeration technologies can achieve approximately 20% to 40% reduction in TTHMs depending
on the initial concentration of TTHMs and the rate of re-formation of TTHMs after aeration.
Table 5-2. Treatment Performance Conceptual Evaluation
Water Quality →
Treatment↓
DBP Precursor
(i.e. TOC)
TTHMs
Spray Aeration Does not Remove TOC 20% to 40%
GAC/BAC Filtration
Adsorption
Biological
30 to 70%
25% Typical (5% to 60% Range)
30 to 70%
8% to 50%
Ozone-BAC 25% Typical (5% to 60% Range) 15% to 50%
Ion Exchange 30% to 60% 30% to 60%
5.3 CONCEPTUAL COST ANALYSIS
Conceptual cost opinions for the identified treatment alternatives were developed based upon cost curves, vendor
based estimates and experience with analogous systems. The conceptual cost opinions are intended to be used
for comparison purposes only, as they do not represent design-based engineering estimates. Based on the current
system maximum day demand of 1.5 MGD, the cost opinions were based on providing an initial treatment capacity
of 2.0 MGD. A contingency of 30% was added to the estimated capital costs. Capital costs for each alternative are
presented in Table 5-3.
Based on the capital cost opinions, applying in-tank spray aeration within the clear well is the lowest cost treatment
option ($770,000) followed by tray aeration within the Bear Hill and Scituate Hill tanks ($1,600,000), GAC/BAC
treatment ($2,000,000), anion exchange treatment ($3,900,000), and ozone-BAC ($5,800,000).
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Table 5-3. Conceptual Cost Opinions
Alternative at 2 MGD Capacity Construction Cost Opinion
($)1
Unit Construction Cost Opinion
($/gal)2
Spray Aeration System within
Clear W ell
$770,000 $0.40/gal
Spray Aeration System within
Bear Hill and Scituate Hill tanks
$1,600,000 $0.80/gal
GAC/BAC Filtration $2,000,000 $1.00/gal
Ion Exchange $3,900,000 $1.95/gal
Ozone-BAC $5,800,000 $2.90/gal
1. Construction cost opinions include contractor overhead and profit (15%), general requirements, mobilization, and bonding
(7%). Cost opinions do not include engineering.
2. Unit construction cost opinion is based on a 2.0 MGD treatment capacity.
5.4 CONCLUSIONS AND RECOMMENDATIONS
Based on the analysis of the historical Cohasset water system data and conceptual comparison of the alternative
treatment options, the following observational conclusions are offered.
Historical Water System Data Analysis:
• The LRAAs for TTHMs and HAA5 at the compliance sampling locations meet the regulatory MCLs and are
in compliance with the Stage 2 D/DBP Rule.
• Current HAA5 levels meet the HAA5 treatment goal of 48 µg/L (80% of the MCL) at each of the compliance
locations sampled quarterly throughout the year. Therefore, the HAA5 formation is effectively controlled by
current treatment processes and operations at the Lily Pond WTP; and additional treatment does not appear
to be required for HAA5 control.
• Current individual TTHM levels exceed the regulatory MCL at three of the four sampling locations,
particularly during the summer sampling events. Additional treatment for the reduction of TTHMs would be
required to reduce TTHMs below the MCL and meet the TTHM treatment goal of 64 µg/L (80% of the MCL)
at each of the compliance locations.
• The majority (over 85%) of the TTHMs is comprised of chloroform and bromodichloromethane, which are
the most strippable of the regulated TTHM species.
• To meet the TTHM goal at the assumed longest system detention time of 77 hours, an overall 40% to
50% reduction in TTHMs is estimated to be required.
• According to a scatter-plot analysis of TTHM formation versus TOC, organic carbon removal treatment
would need to target a 2.0 mg/L of TOC prior to disinfection to meet the TTHM goal throughout the
distribution system. Based on this TOC target, removals of up to 50% would be required in the
winter/spring seasons and of up to 65% would be required in the summer/fall seasons.
Alternative Treatment Options Evaluation:
• Based on the volatility of the TTHMs and identified TOC removal target, four (4) treatment options were
identified for reducing TTHMs: spray aeration, GAC/BAC filtration, ozone-BAC, and ion exchange.
• GAC in the adsorption mode and ion exchange treatment have the highest expected treatment efficiency
relative to organic removal and TTHM control. This would also further reduce HAA5 concentrations.
• Running the GAC in biological mode during the seasonally low winter and spring months has the potential
to extend the media bed life and meet the TOC removal and TTHM reduction goals.
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• The integration of ozonation ahead of biofiltration would likely enhance the BAC treatment efficiency. The
ozone-BAC treatment system would likely be effective at meeting the TTHM goal during the winter/spring
months, but could fall short of meeting the TTHM goal during the summer/fall months.
• While aeration does not remove organic carbon, post-formation TTHM removal by aeration may provide a
sufficient reduction in TTHMs to be at or below the MCL at the compliance locations.
• On a capital construction basis, spray aeration was the lowest cost option, followed by GAC/BAC
adsorption, ion exchange, and ozone-BAC.
In consideration of these observational conclusions and our understanding of the Cohasset water system, the
following two-phase approach to TTHM control is recommended.
Phase I – Investigation and Implementation of Spray Aeration:
• Considering the relatively lower cost and potential removal effectiveness of aeration, it is recommended
that the Town perform a bench-top evaluation of spray aeration to confirm the TTHM removal effectiveness.
• The Lily Pond WTP is operated on average eight to twelve hours per day. Consequently, the overnight
downtime leads to higher chlorine contact time in the clear well and higher formation of TTHMs prior to
distribution when the plant in placed online. If the bench-top evaluation results confirm an adequate removal
efficiency of TTHM by spray aeration, it is recommended that the Town consider the installation of a spray
aeration system within the clear well to provide aeration of the clear well during the overnight downtime
hours. Spray aeration within the clear well would be operated overnight while the plant is off-line and would
provide a reduction of the TTHMs formed overnight before sending the finished water to the distribution
system when the plant is placed back online. While TTHMs will continue to rise in the distribution system
after aeration, the aeration treatment within the clear well would decrease the overall TTHM levels being
introduced to the system at start-up.
• After installation of spray aeration within the Lily Pond WTP clear well, it is recommended that the Town
monitor full-scale performance of the distribution system TTHM levels. It is anticipated that the aeration
system within the clear well would be operated during the overnight down-time hours. When the plant is
operating, the hydraulic detention time within the clear well may not be long enough for sufficient pre-
formation of the TTHMs prior to their removal by spray aeration. Therefore, if further reduction in TTHMs is
desired, then the Town can consider providing additional spray aeration treatment at the Bear Hill and
Scituate Hill tanks or provide organic removal treatment at the Lily Pond WTP.
Phase II – Investigation and Implementation of Organic Removal Treatment:
• If further TTHM reduction is needed to meet the TTHM goal at compliance locations or spray aeration is
proven to be ineffective, it is recommended that the Town consider the installation of an organic removal
treatment technology capable of removing 50% to 65% of the combined filtered water TOC levels.
• The Lily Pond WTP is permitted for 3.0 MGD, but the current maximum day demand is approximately 1.5
MGD. Based on this, it is recommended that the treatment system be sized for a smaller capacity of 1.5
to 2.0 MGD that is expandable to 3.0 MGD as system demands grow.
• Either GAC adsorption or ion exchange technologies could be integrated to treat the filtered water and
achieve the TOC reduction goals throughout the year.
• GAC could be allowed to convert into biological mode during the higher quality winter/spring months and
will likely still achieve the necessary TOC reduction goals to lower TTHMs level to at or below 80% of the
MCL.
• If GAC/BAC filtration is selected as the treatment process, the design could be phased to allow the
subsequent installation of ozonation, if required, at a later phase. This would save on initial capital and
operation costs and allow the Town to gauge the TOC treatment effectiveness of the GAC filtration in
adsorption and biological modes; and determine whether ozonation is required for enhancing treatment
performance.
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