HomeMy WebLinkAboutDisinfection Byproduct Compliance Evaluation (PDF)TABLE OF CONTENTS
TABLE OF CONTENTS.....................................................................................i
LIST OF TABLES........................................................................................... iii
LIST OF FIGURES ......................................................................................... iii
EXECUTIVE SUMMARY .................................................................................iv
1.0 BACKGROUND....................................................................................................................1-1
1.1 General ........................................................................................................................1-1
2.0 REGULATIONS....................................................................................................................2-1
2.1 Introduction.................................................................................................................2-1
2.2 Stage 1 D/DBPR..........................................................................................................2-3
2.3 Proposed Stage 2 D/DBPR..........................................................................................2-5
2.3.1 Initial Distribution System Evaluations (IDSE)..............................................2-6
2.3.2 IDSE – Standard Monitoring Program (SMP)................................................2-6
2.3.3 Minimum Requirements for an SMP IDSE Report.........................................2-9
2.3.4 IDSE System-Specific Study (SSS)................................................................2-9
2.3.5 Methods to Conduct an SSS............................................................................2-9
2.4 Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR)...................2-10
2.5 Proposed Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR)...2-12
3.0 SUMMARY OF COHASSET’S DBP’S................................................................................3-1
3.1 Total Trihalomethanes (TTHMs)................................................................................3-1
3.2 Haloacetic Acids (HAA5s)..........................................................................................3-4
3.3 Total Organic Carbon (TOC)......................................................................................3-5
4.0 TREATMENT OPTIONS......................................................................................................4-1
4.1 General ........................................................................................................................4-1
4.2 Relocation of the Pre-Chlorination Injection Point.....................................................4-1
4.3 Groundwater Supply....................................................................................................4-3
4.4 Source Water Quality..................................................................................................4-3
4.5 Enhanced Coagulation.................................................................................................4-4
4.6 Powdered Activated Carbon........................................................................................4-6
4.7 Filter Media Replacement...........................................................................................4-6
4.8 Clearwell Modification/Expansion..............................................................................4-7
4.9 Disinfectants and Oxidants..........................................................................................4-9
4.9.1 General ............................................................................................................4-9
4.9.2 Chlorine...........................................................................................................4-9
4.9.3 Chloramine....................................................................................................4-11
4.9.4 Potassium Permanganate...............................................................................4-12
4.9.5 Ozone.............................................................................................................4-12
4.9.6 Chlorine Dioxide...........................................................................................4-14
Final i Weston & Sampson
4.9.7 Mixed Oxidants.............................................................................................4-14
5.0 RECOMMENDATIONS AND ESTIMATED COSTS.........................................................5-1
5.1 General ........................................................................................................................5-1
5.2 Phase 1.........................................................................................................................5-1
5.2.1 General ............................................................................................................5-1
5.2.2 Enhanced Coagulation/Polyaluminum Chloride.............................................5-2
5.2.3 Filter Media Replacement...............................................................................5-2
5.2.4 Clearwell Modification....................................................................................5-3
5.2.5 Potassium Permanganate.................................................................................5-3
5.3 Phase 2.........................................................................................................................5-3
5.3.1 Chloramines.....................................................................................................5-4
5.3.2 Ozone...............................................................................................................5-4
5.3.3 Chlorine Dioxide.............................................................................................5-4
5.3.4 Mixed Oxidant Process ...................................................................................5-5
5.4 Cost Summary.............................................................................................................5-5
Final ii Weston & Sampson
LIST OF TABLES
TABLE 2-1:SUMMARY OF STAGE 1 D/DBPR REQUIREMENTS.....................................2-4
TABLE 2-2:REQUIRED PERCENT REMOVAL OF TOC ................................................2-4
TABLE 2-3:SUMMARY OF SMP SAMPLE SITE REQUIREMENTS ....................................2-7
TABLE 2-4:SMP SAMPLING REQUIREMENTS ........................................................2-8
TABLE 2-5:SUMMARY OF LT1ESWTR REQUIREMENTS ......................................... 2-10
TABLE 2-6:BIN CLASSIFICATION SUMMARY ....................................................... 2-13
TABLE 2-7: POTENTIAL TREATMENT OPTIONS UNDER LT2ESWTR ........................... 2-14
TABLE 3-1:BATES LANE TTHM SUMMARY .........................................................3-1
TABLE 3-2:TTHM SUMMARY .........................................................................3-2
TABLE 3-3: BATES LANE HAA5 SUMMARY .........................................................3-4
TABLE 3-4:RAW WATER TOC SUMMARY ...........................................................3-5
TABLE 3-5:TOC SUMMARY............................................................................3-5
TABLE 3-6:SUVA SUMMARY .........................................................................3-6
TABLE 4-1:IMPACT OF MOVING THE POINT OF CHLORINATION ON DBP FORMATION .........4-2
TABLE 4-2:CT CALCULATIONS FOR A PH OF 6.5, 0.5 LOG INACTIVATION .......................4-8
TABLE 4-3:ADVANTAGES AND DISADVANTAGES OF DISINFECTANTS AND OXIDANTS ....... 4-10
TABLE 5-1:SUMMARY OF ESTIMATED COSTS ........................................................5-6
LIST OF FIGURES
FIGURE 3-1:HISTORICAL TRIHALOMETHANE RESULTS ..............................................3-3
Final iii Weston & Sampson
EXECUTIVE SUMMARY
Weston & Sampson Engineers, Inc. (WSE) was hired by the Town of Cohasset to evaluate
disinfection byproducts. The evaluation included a review of current and future drinking water
regulations including the Stage 1 and 2 Disinfectants/Disinfection Byproduct Rules, the Long
Term 1 Enhanced Surface Water Treatment Rule and the Long Term 2 Enhanced Surface Water
Treatment Rule. A regulatory compliance determination was made based on available water
quality data and recommended improvements were developed to reduce total organic carbon
(TOC) and disinfectant byproducts to meet current and future regulatory requirements.
The following tables summarize the major aspects of the aforementioned drinking water
regulations and compliance dates for the Town of Cohasset. The contaminants summarized in
these tables are those that are applicable to the Town of Cohasset at this point in time. A full
summary of these rules is included in Chapter 2.0 of this report. Any items that Cohasset are not
in compliance with or are close to the regulatory limit that could be determined at the time of this
report are listed in the tables in bold italicized text.
MAJOR REQUIREMENTS FOR COHASSET
STAGE 1 DISINFECTANTS/DISINFECTION BYPRODUCT RULE (D/DBPR)
Contaminant Regulatory
Limit
Monitoring Frequency Compliance Compliance
Date
Maximum Contaminant Levels
Total Trihalomethanes
(TTHM)
80 Pg/L
Five Haloacetic Acids
(HAA5)
60 Pg/L
1 sample per WTP per
quarter
RAA January 2004
Maximum Residual Disinfectant Levels
Chlorine 4 mg/L Same as TCR RAA
DBP Precursors 35-50% TOC
Removal
Monthly for TOC and
Alkalinity
RAA
January 2004
RAA – Running Annual Average
Final iv Weston & Sampson
MAJOR REQUIREMENTS FOR COHASSET
STAGE 2 DISINFECTANTS/DISINFECTION BYPRODUCT RULE (D/DBPR)
Contaminant Regulatory Limit Monitoring
Frequency
Compliance Date1
Stage 2A
TTHM 120 Pg/L
HAA5 100 Pg/L
LRAA January 2008
Stage 2B
TTHM 80 Pg/L
HAA5 60 Pg/l
LRAA xJanuary 2011 (if water is sold to Hingham)
xJune 2012 (if no Cryptosporidium sampling
required)
xJune 2013 (if Cryptosporidium sampling is
required)
Recommended Standard Monitoring Program (SMP)
Start Date
xJune 2005 (if water is sold to Hingham)
xJune 2007 (if water is not sold to Hingham)
Submit Initial Distribution System Evaluation
(IDSE) Report
xJanuary 2007 (if water is sold to Hingham)
xJanuary 2009 (if water is not sold to Hingham)
LRAA – Locational Running Annual Average
1Compliance dates are based on January 2005 promulgation
MAJOR REQUIREMENTS FOR COHASSET
LONG TERM 1 ENHANCED SURFACE WATER TREATMENT RULE (LT1ESWTR)
Contaminant Regulatory Limit Monitoring
Frequency
Compliance Date
0 oocysts/L MCLGCryptosporidium
99% removal (2-log)
N/A
Turbidity
Combined Filter
Effluent
0.3 NTU in at least 95% of monthly
samples; Maximum of 1 NTU
Samples collected
every 4 hours
<0.5NTU for the first four hours or a
filter profile is required
Samples collected
every 15 minutes
<1 NTU in 3 months; or a self-
assessment of the filter is required
Samples collected
every 15 minutes
Turbidity
Individual Filter
Effluent
<2 NTU in 2 months; or a
comprehensive performance evaluation
is required
Samples collected
every 15 minutes
January 2005
Disinfection Profiling
TTHM
HAA5
64 Pg/L TTHM
48 Pg/L HAA5
Running Annual
Average
Start: July 2003
Finish: June 2004
Conduct Sanitary Survey every three years January 2005
Final v Weston & Sampson
MAJOR REQUIREMENTS FOR COHASSET
LONG TERM 2 ENHANCED SURFACE WATER TREATMENT RULE (LT2ESWTR)
Contaminant Regulatory Limit Compliance Criteria Compliance
Date1
E. Coli 10 E. Coli/100 mL for lake/reservoirs
50 E. Coli/100 mL for flowing streams
Annual mean
concentration; if limit is
exceeded,
Cryptosporidium sampling
is required.
Compliance
dates are
summarized
below
Bin 1: <0.075 Crypto oocysts/L No treatment required
Bin 2: 0.075< Crypto <1.0 oocysts/L 1 log removal
Bin 3: 1.0 < Crypto <3.0 oocysts/L 2 log removal
Cryptosporidium
(required if
E.Coli exceeded
Bin 4: Crypto >3.0 oocysts/L 2.5 log removal
Compliance
dates are
summarized
below
E. Coli Sampling
Sampling schedule submittal for E. coli monitoring April 2007
Start 12-month sampling of E.coli. June 2007
Begin disinfection profiling (if not already required under LT1ESWTR) June 2008
Disinfection profile completed and on file June 2009
Cryptosporidium Sampling (sampling required if E.coli trigger is exceeded)
If E.coli trigger exceeded, sampling schedule submittal for Cryptosporidium
monitoring.
September 2008
Start 12-month sampling for Cryptosporidium. January 2009
Begin disinfection profiling (if not already required under LT1ESWTR). June 2009
Submit initial Bin classification June 2010
Disinfection profile completed and on file June 2013
Install treatment technique June 2013
1Compliance dates are based on January 2005 promulgation
SUMMARY OF THE RECOMMENDED IMPROVEMENTS
The capital and operations and maintenance cost for each recommended improvement is
summarized below. The improvements are summarized into two phases. Not all improvements
are required to reduce DBPs to below 80 percent of regulatory limits. The estimated costs for
each improvement includes construction costs, engineering costs and contingencies. These costs
were updated to an Engineering News Record (ENR) Boston index 7976 for December 2003.
Other sources include the 1998 Means “Building Construction Cost Data” and manufacturers’
quotations.
These costs are preliminary and will need to be further evaluated and /or pilot tested prior to the
implementation of an improvement. As previously mentioned, it may not be necessary to
institute all the recommended improvements to achieve compliance with the DBP rules.
Final vi Weston & Sampson
SUMMARY OF ESTIMATED COSTS
Phase Improvement Estimated
Construction
Costs
Estimated
O&M
Costs
20-year
Present
Worth Cost
Phase 1
Enhanced Coagulation or use of Polyaluminum
Chloride NA $11,000 $310,000
A
Construction of baffles in the clearwell. $85,000 NA $85,000
B Reactivate potassium permanganate chemical
feed system.$0 $39,000 $1,101,000
Source water quality improvements Costs being developed or included in
other reports
C
Re-activation of the groundwater sources Costs already incurred
Install granular activated carbon in existing
filter beds OR $230,000 NA $1,073,000
1D
Replacement of the filter media $151,000 NA $353,000
1
Phase 2
A Convert final disinfection from chlorine to
chloramines $196,000 $8,000 $422,000
Convert pre-chlorination to ozone OR $1,500,000 $17,000 $1,980,000
Convert pre-chlorination to chlorine dioxide
OR $196,000 $41,000 $1,353,000
B
Convert pre-chlorination to a mixed oxidant
system $412,000 $5,000 $553,000
1Assumes GAC filter media replacement is required every 4 years and sand/anthracite filter media replacement is
required every 10 years.
Final vii Weston & Sampson
1.0 BACKGROUND
1.1 General
The town of Cohasset water system delivers an average flow rate of about 0.75 million gallons
per day (MGD) of potable water to its estimated 7,100 customers. Surface water is drawn from
Lily Pond and Aaron River and is treated by the Lily Pond water treatment plant (WTP) prior to
distribution. Additional water sources for the Town of Cohasset include the Elms Meadow and
Sohier well fields. These groundwater sources have been inactive since or before 1979 when the
Lily Pond WTP went on-line. The wellfields are currently being rehabilitated. Water from these
groundwater sources does not require filtration and upon activation, will pump directly into the
distribution system.
The Lily Pond WTP is a conventional surface water treatment plant which operates eight to
twelve hours per day, seven days per week on average.The treatment process sequence consists
of four unit operations: coagulation, flocculation, sedimentation and filtration.The WTP
utilizes aluminum sulfate (alum) as a primary coagulant, polymer as a filtration aid, chlorine gas
for chlorination and disinfection, lime for corrosion control and sodium fluoride as a source of
fluoride to prevent tooth decay. As with most surface water supplies, the raw water quality
fluctuates with the seasons. During the summer months, June through August, the raw water
quality has higher levels of turbidity and color. To compensate for these water quality changes,
Cohasset adjusts the alum dosage from 40 parts per million (ppm) to 50 ppm when higher levels
of turbidity and color are detected in the raw water.
The Town of Cohasset’s water system consists of the Lily Pond WTP, the Lily Pond and Aaron
River reservoirs, over 40 miles of 6- to 14-inch piping, two inactive well fields and two water
storage tanks. The two water storage tanks, the Bear Hill and Scituate Hill tanks, set the
hydraulic gradeline for the single pressure zone distribution system. Both tanks are typically
operated with a 10-foot fluctuation in water level within each tank.
G:\WATER\COHASSET\203151 DBP\REPORT\REPORT_FINAL.DOC
Final 1-1 Weston & Sampson
2.0 REGULATIONS
2.1 Introduction
Chlorination has made public water supplies safe from illness-producing bacteria, viruses, and
parasites and has almost completely eliminated the risk of waterborne diseases such as typhoid
fever, cholera, and dysentery. However, using chlorine as a disinfectant introduces its own
health risks because of the by-products produced during the disinfection process.
Disinfection byproducts (DBPs) from chlorine disinfection form when chlorine reacts with
naturally occurring organic matter (NOM) and naturally occurring inorganic compounds in
water. It has been found that the resulting DBPs pose significant health threats to humans when
consumed over long periods of time. DBPs are currently summarized into two groups: total
trihalomethanes (TTHMs) and haloacetic acids (HAA5s). TTHMs are a group of chemicals
consisting of chloroform, bromodichloromethane, dibromodichloromethane, and bromoform.
HAA5s are a group of haloacetic acids including monochloroacetic acid, dichloroacetic acid,
trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid
TTHMs and HAA5s have been associated with an increased risk of cancer and liver, kidney, and
central nervous system problems. Epidemiological studies strongly suggest that DBPs are likely
to be reproductive toxins under appropriate exposure conditions and can cause such reproductive
health effects as miscarriage, stillbirth, birth defects, and low birth weight. This is further
supported by toxicology studies that show that, in addition to adverse reproductive effects, DBPs
may also be hazardous to developmental health.
In 1983, the Environmental Protection Agency (EPA) identified the best technologies and
treatment techniques that would enable public water systems to comply with the regulated levels
of DBPs. These include
xMoving the point of chlorination downstream in the water treatment plant
xOptimizing the coagulation process to enhance the removal of DBP precursors (also referred
to as enhanced coagulation)
Final 2-1 Weston & Sampson
xThe use of chloramines to supplement or replace the use of free chlorine as a post-
disinfectant, and
xThe replacement of pre-chlorination by pre-oxidation with an alternate disinfectant that
produces less DBPs.
Each of these available technologies, as they apply to Cohasset, will be further discussed later in
this report.
To address DBPs, EPA promulgated the Stage 1 Disinfectants/Disinfection Byproducts Rule
(D/DBPR) to minimize the health concerns associated with disinfection byproducts (DBPs). The
D/DBPR specifically targets TTHM and HAA5 levels because of their known health effects and
further limits the amount of allowable byproducts created when drinking water is disinfected.
The Stage 2 D/DBPR, which was proposed on August 18, 2003, will further reduce DBPs.
Additional regulations that will effect the Town of Cohasset include the Long Term 1 Enhanced
Surface Water Treatment Rule (LT1ESWTR) and the Long Term 2 Enhanced Surface Water
Treatment Rule (LT2ESWTR). The LT2ESWTR was proposed on August 11, 2003. These
rules aim to improve the control of microbial pathogens, specifically the protozoan
Cryptosporidium in drinking water by requiring water systems to meet strengthened filtration
requirements. These rules also require systems to calculate levels of microbial inactivation to
ensure that microbial protection is not jeopardized by systems making changes to comply with
requirements of the Stage 1 D/DBPR.
The Stage 2 D/DBPR and the LT2ESWTR are being promulgated at the same time to try to
address some technological conflicts that may arise when a water system is trying to comply
with both rules at the same time. The conflict arises in that the D/DBPR decreases disinfectant
levels to minimize the formation of DBP and the LT2ESWTR increases protection against
microbial contaminants. The requirements of the Stage 1 D/DBPR, Stage 2 D/DBPR,
LT1ESWTR and LT2ESWTR are summarized below.
Final 2-2 Weston & Sampson
In December 1998, the Environmental Protection Agency (EPA) promulgated the Stage 1
D/DBPR to minimize the health concerns associated with DBPs. This rule further limits the
amount of allowable byproducts created when drinking water is disinfected. The Stage 2
D/DBPR was proposed on August 18, 2003; a final rule is expected in early 2005.
2.2 Stage 1 D/DBPR
The Stage 1 D/DBPR lowers the previous TTHM maximum contaminant level (MCL) allowed
for a drinking water supply from 100 micrograms per liter (Pg/L) to 80 Pg/L and establishes a
new DBP level for HAA5 with a MCL of 60 Pg/L. Compliance for both TTHM and HAA5 is
determined by a running annual average (RAA) of samples taken quarterly.
Under the Stage 1 D/DBPR for the Cohasset water system, one sample is required per water
treatment plant per quarter. The sample location should represent the maximum residence time
of water within the distribution system. If a water system elects to sample more frequently than
the minimum required, at least 25 percent of all samples collected each quarter (including those
taken in excess of the required frequency) must be taken at locations that represent the maximum
residence time of the water in the distribution system. The remaining samples must be taken at
locations representative of at least average residence time in the distribution system.
The rule also establishes DBP maximum contaminant level goals (MCLG) and maximum
residual disinfection levels (MRDLS). The new regulatory limits, compliance dates, and
monitoring requirements for the Stage 1 rule are listed below in Table 2-1. Small water systems
(serving <10,000 population), such as Cohasset, must comply with the Stage 1 rule by January 1,
2004.
Naturally occurring organic matter (NOM) is considered to be the precursor to the formation of
DBPs. Total organic carbon (TOC) has been determined to be a good indicator for NOM in
water. As a result, the Stage 1 D/DBPR has also established a minimum total organic carbon
(TOC) removal treatment technique for surface water systems that use conventional filtration.
Final 2-3 Weston & Sampson
TABLE 2-1
SUMMARY OF STAGE 1 D/DBPR REQUIREMENTS
FOR SURFACE WATER SUPPLIES SERVING <10,000
Substance MCL MCLG Compliance Sampling Frequency
Regulated Contaminants
TTHM 80 Pg/L N/A
HAA5 60 Pg/L N/A
Running
Annual Average
One sample per
quarter
Chlorite 1 mg/L 0.8 mg/L Daily (At Chlorine
Dioxide Plants)
Daily at WTP;
Monthly in system
Bromate 10 Pg/L 0 Pg/L Running Annual
Average
(At Ozone Plants)
Monthly
Chloroform N/A 0 Pg/L
Dibromochloromethane N/A 60 Pg/L
Bromodichloromethane N/A 0 Pg/L
Bromoform N/A 0 Pg/L
Dichloroacetic Acid N/A 0 Pg/L
Trichloroacetic Acid N/A 0 Pg/L
Running Annual
Average
One sample per
quarter
Regulated Disinfectant Residuals
Chlorine Dioxide
(as ClO2)
0.8 mg/L 0.8 mg/L Daily (At Chlorine
Dioxide Plants)
Daily at WTP;
Monthly in system
Chloramines (as Cl2) 4 mg/L 4 mg/L
Chlorine (as Cl2) 4 mg/L 4 mg/L
Running Annual
Average
Same location and
frequency as TCR
sampling
The specific percentage of TOC removal required is dependent on the source water TOC and
alkalinity levels, as are summarized in Table 2-2. Compliance is based on the RAA of monthly
raw water alkalinity and percent removals. The required percent removal can change per quarter
based on raw water alkalinity and TOC concentration. The percent TOC removal is measured
between the raw water sampling point and the combined filter effluent turbidity monitoring
location.
TABLE 2-2
REQUIRED PERCENT REMOVAL OF TOC
Source Water Alkalinity (mg/L as CaCO3)Source Water
TOC (mg/L)0-60 > 60-120 >120
> 2 to 4 35% 25% 15%
> 4 to 8 45% 35% 25%
> 8 50% 40% 30%
Final 2-4 Weston & Sampson
As an alternative to sampling for TOC, a water system can also monitor for specific ultraviolet
absorption (SUVA). SUVA is the ratio of ultraviolet (UV) absorption at a wavelength of 254
nanometers (nm) to the dissolved organic carbon (DOC) content of the water. SUVA is an
indicator of the humic content of water. Waters with low SUVA values contain primarily non-
humic organic matter and are not amenable to removal by enhanced coagulation. Waters with
high SUVA values are generally amenable to removal of TOC by enhanced coagulation.
Enhanced coagulation is a treatment technique designed to optimize the coagulant dose to
increase the removal of TOC by conventional treatment.
Alternative compliance criteria have also been developed to allow WTPs flexibility for
establishing compliance with the TOC removal requirements. The alternative criteria were
developed to address the difference in source waters and the fact that certain waters are less
amenable to effective removal of TOC by coagulation. A WTP can therefore establish
compliance with the coagulation TOC removal if any one of the following alternative
compliance criteria is met. These criteria are met if the required concentration is met and the
calculated quarterly RAA. These criteria can also be used on a monthly basis (i.e. for individual
months in which the criteria is met).
xSource water TOC < 2.0 mg/L
xTreated water TOC < 2.0 mg/L
xRaw water SUVA d 2.0 L/mg-m
xTreated water SUVA d 2.0 L/mg-m
xRaw water TOC < 4.0 mg/L, Raw water alkalinity > 60 mg/L as CaCO3, TTHM < 40
Pg/L and HAA5 < 30 Pg/L
xTTHM < 40 Pg/L and HAA5 < 30 Pg/L with chlorine as the only disinfectant
2.3 Proposed Stage 2 D/DBPR
The Stage 2 Rule proposes a two-phased approach for DBPs. Stage 2A requires that all water
supply systems meet “locational” running annual averages (LRAA) of 120 Pg/L and 100 Pg/L
for TTHM and HAA5, respectively, within three (3) years of the rule promulgation. Rule
promulgation is expected to occur in early 2005 making compliance required in early 2008. In
addition, water systems must continue to maintain Stage 1 D/DBPR RAA of 80 Pg/L and 60
Pg/L for TTHM and HAA5, respectively.
Final 2-5 Weston & Sampson
Stage 2B further lowers the LRAA to 80 Pg/L and 60 Pg/L for TTHM and HAA5, respectively.
Small systems must comply within 7.5 years after rule promulgation. Rule promulgation is
expected to occur in early 2005 making compliance required in mid-2012. Stage 2 also requires
that water system suppliers conduct Initial Distribution System Evaluations (IDSEs) to select
new Stage 1 compliance monitoring locations that more accurately represent peak disinfection
byproduct concentrations in the distribution system.
2.3.1 Initial Distribution System Evaluations (IDSE)
IDSE requirements apply to all surface water and groundwater systems that use a primary
disinfectant other than UV, or that add a residual to their water. Any system whose historical
individual TTHM/HAA5 Stage 1 D/DBPR compliance data are < 40/30 Pg/L for TTHM/HAA5
respectively for a specified period may be exempt from performing an IDSE.
An IDSE can be conducted by a Standard Monitoring Program (SMP) or by a System-Specific
Study (SSS). Under an SMP, a supplier’s distribution system is monitored for one year,
collecting sampling data from select sites. The SSS does not require sampling, but relies on
historical data and system modeling to obtain representative system data.
2.3.2 IDSE – Standard Monitoring Program (SMP)
The SMP is a one-year monitoring program designed to generate data that is subsequently used
to select Stage 2B D/DBPR compliance monitoring sites. These new monitoring sites should
better capture representative high TTHM and HAA5 concentrations. Each water supplier’s
program will be based on a schedule and plan determined by system size, source water type,
number of plants, and disinfection method (chlorine or chloramine).
Because so many system factors influence the formation of TTHMs and HAA5s, the locations of
the TTHM and HAA5 sites are system-specific. All high TTHM and HAA5 SMP sampling
results should represent the maximum DBP concentrations in the finished water that is delivered
to customers.
Final 2-6 Weston & Sampson
High TTHM sites are typically characterized by long residence times, low or no disinfectant
residual, high heterotrophic plate count or positive coliform, areas with historical data showing
high TTHM levels, and are located in remote areas with few customers and after storage
facilities. Large, dead-end water mains with only a few customers, especially if they are located
in remote areas of the distribution system, are likely to have high TTHM concentrations. Dead-
end mains with no service, however, should not be used for a sampling site because a customer
never consumes this water. Although there is limited data for HAA5s, high HAA5 sites are
typically characterized by varying residence times and a low, but positive, disinfectant residual.
An IDSE for systems serving <10,000 people requires two types of quarterly sampling locations
(Note that the SMP sampling locations are in addition to Stage 1 D/DBPR compliance
monitoring sites). Table 2-3 summarizes the number and type of sample sites required for a
small system. Large systems (serving >10,000 people) are required to sample at the same sites
as small systems as well as at the entry point to the distribution system and at locations with the
system that are representative of average residence times.
TABLE 2-3
SUMMARY OF SMP SAMPLE SITE REQUIREMENTS
System Characteristics SMP Sample Sites per Plant
High TTHM High HAA5 Total Sites Surface Water Systems
(<10,000)112
It is important to note that the samples taken at each site are “dual samples”, meaning two
samples must be collected at the same time at each site. One sample is analyzed for TTHM and
the other for HAA5. For this example, where Cohasset is required to collect samples from 2
sites per plant, a total of 4 samples (2 per site) will be required for each quarter.
The monitoring schedule is determined from historical DBP data and is system specific. The
month with the highest TTHM/HAA5 level or maximum water temperature is referred to as the
controlling month. At least one set of “dual” samples must be collected during the controlling
month. The other rounds of sampling must be scheduled at three month intervals based on the
date selected for sample collection in the controlling month. The sampling dates for the entire
Final 2-7 Weston & Sampson
year must be scheduled in advance. If the highest TTHM and HAA5 levels do not occur in the
same month, the controlling month should be based on the month with the highest TTHM
concentrations. If there is no historical DBP data, the controlling month shall be determined
based on the month having the maximum annual water temperature.
Table 2-4 summarizes SMP sampling requirements for Cohasset for both small and large
systems. The sampling requirements described herein do not consider special circumstances
such as wholesaling water systems and systems with temporary sources (less than 90 days). If
Cohasset were to sell water to Hingham, a large system (serving >10,000), Cohasset would need
to meet the deadline requirements for a large system. Otherwise, the deadline for a small system
would need to be met. For either case, the sampling frequency would not change. Water
suppliers should refer to the proposed Stage 2 D/DBPR for complete explanations of all special
conditions.
TABLE 2-4
SMP SAMPLING REQUIREMENTS
System Type
(Population
Served)
IDSE Report
Due Date
Recommended
SMP Sampling
Start Date
Sampling
Frequency
Sampling Control
Month
Surface water
systems
(< 10,000)
4 years after
final Stage 2
rule
promulgation
2 years and 9
months after
final Stage 2
rule
promulgation
Every 90 days (3
months) for one
year
1.Month with highest
TTHM/HAA5 level
OR
2. Month with
maximum
temperature
Surface water
systems
(> 10,000)
2 years after
final Stage 2
rule
promulgation
9 months after
final Stage 2
rule
promulgation
Every 90 days (3
months) for one
year
1. Month with highest
TTHM/HAA5 level
OR
2. Month with
maximum
temperature
The success of an SMP, and ultimately an IDSE, depends upon the selection of accurate,
representative sample sites. Many water suppliers possess various types of information helpful
in identifying preliminary SMP sites. The following list contains several data sources and tools
for identifying preliminary SMP sites:
Final 2-8 Weston & Sampson
xMaps
xDistribution system water quality data
xSimulated distribution system laboratory tests
xDistribution system hydraulic model
xTracer studies
xSystem operating data
xGeographic Information System (GIS)
2.3.3 Minimum Requirements for an SMP IDSE Report
Once an SMP is completed, systems must report the results, along with recommended Stage 2
D/DBPR sampling sites, within the appropriate timeframe. At a minimum, reports must contain
the following items:
xOriginal SMP monitoring plan and an explanation of any deviations from that plan.
xAll SMP TTHM and HAA5 analytical results.
xAll TTHM and HAA5 analytical results from Stage 1 D/DBPR compliance monitoring
collected during the period of the IDSE.
xA schematic of the system’s distribution system with results, locations, and dates of all IDSE
SMP. All compliance samples should be noted.
xRecommendations for locating Stage 2B compliance monitoring sites. The typical
recommendation for monitoring sites is the two sites having the highest TTHM and HAA5
LRAAs from analytical results from the SMP monitoring plan.
xJustification for selection of Stage 2B compliance monitoring sites.
2.3.4 IDSE System-Specific Study (SSS)
An SSS allows systems with extensive data, previous pertinent studies, or detailed knowledge of
their operations to use these resources to choose new monitoring sites, thus avoiding the expense
of an SMP. However, the Stage 2 D/DBPR requires that an SSS provide equivalent or superior
selection of new Stage 2 D/DBPR compliance monitoring sites targeting high DBP levels as
compared to the sites selected from an SMP.
2.3.5 Methods to Conduct an SSS
Although historical monitoring data, new monitoring data, water distribution modeling, tracer
study results, and other distribution system analyses can be used as part of an SSS, there are two
“pre-defined” methods that are expected to provide equivalent or superior selection of
compliance sites as compared to an SMP.
Final 2-9 Weston & Sampson
1. The use of historical DBP data that equals or exceeds the IDSE monitoring requirements.
2. The use of a calibrated water distribution system hydraulic model and one new round of
sampling during the month of peak DBP levels (or peak water temperature if DBP data is
not available).
Water suppliers should refer to the proposed Stage 2 D/DBPR for complete explanations of all
requirements of these two methods.
2.4 Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR)
The Interim Enhanced Surface Water Treatment Rule (IESWTR) is an amendment to the SWTR
and applies to surface water systems and systems having groundwater under the influence of
surface water (GWUDI) that serve at least 10,000 people. The Long Term 1 Enhanced Surface
Water Treatment Rule applies similar requirements to surface water systems that serve less than
10,000 people. The LT1ESWTR was designed to optimize treatment reliability and to enhance
physical removal efficiencies to minimize Cryptosporidium levels in finished water. The rule
also includes disinfection benchmark provisions to assure continued levels of microbial
protection while facilities take the necessary steps to comply with new DBP standards. Table 2-
5 summarizes the LT1ESWTR requirements for small systems.
TABLE 2-5
SUMMARY OF LT1ESWTR REQUIREMENTS
Contaminant Regulatory Limit Monitoring Frequency
0 oocysts/L MCLG Cryptosporidium
99% removal (2-log)
N/A
Turbidity Combined
Filter Effluent
d0.3 NTU in at least 95% of monthly
samples; Maximum of 1 NTU
Samples collected every
4 hours
<0.5 NTU for the first four hours of
operation or a filter profile is required
Samples collected every
15 minutes
<1 NTU in 3 months; or a self-assessment
of the filter is required
Samples collected every
15 minutes
Turbidity Individual
Filter Effluent
<2 NTU in 2 months; or a comprehensive
performance evaluation is required
Samples collected every
15 minutes
The LT1ESWTR also requires that disinfection profiling be conducted by any system whose
TTHM or HAA5 RAA are greater than or equal to 80 percent of the MCLs. The 80 percent
thresholds are 64 Pg/L and 48 Pg/L for TTHMs and HAA5s, respectively. Disinfection profiling
Final 2-10 Weston & Sampson
is designed to evaluate impacts on microbial risk before a significant change in disinfection
practices is made to ensure that adequate microbial protection is maintained. Given the historical
TTHM and HAA5 RAAs for Cohasset, disinfection profiling is required. Systems required to
develop a disinfection profile must consult the state prior to making a significant change in their
disinfection practices. Significant changes may include moving the point of disinfection
application, changing the type of disinfectant used and changing the disinfection process (i.e.
changing the contact basin geometry).
Disinfection profiling for Giardia inactivation (3-log removal) is required for all systems that
exceed the 80 percent trigger TTHM and HAA5 concentrations, such as Cohasset. A
disinfection profile is a graphical representation of a system’s level of Giardia or virus
inactivation measured during the course of a year. The EPA website
(http://www.epa.gov/ogwdw000/mdbp/lt1eswtr.html) contains an excel spreadsheet that can be used to
develop a disinfection profile. Disinfection profiling for virus inactivation (4-log removal) is
required for any system that proposes to add or switch to ozone or chloramines; it is
recommended for any system that proposes to add or switch to chlorine dioxide. The
inactivation calculation is a function of the peak hourly flow, chlorine residual concentration,
contact time, temperature and pH over each disinfection segment within the WTP. A
disinfection segment is defined as the section of the treatment system beginning at one
disinfectant injection or monitoring point and ending at the next disinfectant injection or
monitoring point.
If there is less than one year of data available, the disinfection benchmark is calculated as the
average of the monthly average inactivations. If there is one year of data available, the
disinfection benchmark is the lowest monthly average of inactivation. The disinfection
benchmark will be used as a monitor to assess microbial protection when significant disinfection
modifications are made to the plant such that microbial protection is not compromised for lower
DBPs.
The LT1ESWTR was promulgated in January 2002. By June 30, 2003, systems serving less than
10,000 people, such as Cohasset, must indicate that their system has started their disinfection
Final 2-11 Weston & Sampson
profiling. The disinfection profile must be completed by June 30, 2004. Compliance with the
rule (i.e. turbidity standards, individual filter monitoring) is required by January 14, 2005.
Under the rule, sanitary surveys must be conducted every three years by the state for all surface
waters and GWUDI systems regardless of size.
2.5 Proposed Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR)
The LT2ESWTR was proposed on August 11, 2003 and is expected to be finalized with the
Stage 2 D/DBPR in early 2005. The intent of the rule is to provide additional public health
protection, if needed, from Cryptosporidium and other pathogenic microorganisms in drinking
water.
Under the LT2ESWTR, systems will initially conduct source water monitoring for
Cryptosporidium to determine their treatment requirements.Cryptosporidium monitoring by
large systems will begin six months after the LT2ESWTR is finalized and will last for a duration
of two years. Small systems, such as Cohasset, are on a delayed schedule and will start
monitoring when the large system monitoring period has been completed. To reduce monitoring
costs, small filtered systems will initially be required to conduct one year of monitoring for E.
coli. Cryptosporidium monitoring will be required of small systems only if their E. Coli results
exceed specified trigger concentrations.
It has been proposed that the annual mean source water E. coli concentration of 10 and 50 per
100 ml are appropriate values to trigger the requirement for Cryptosporidium monitoring in
lakes/reservoirs and flowing streams, respectively. Both E. coli and Cryptosporidium monitoring
will include one sample collected twice a month over a one-year period. A second round of
monitoring will be required six years after the initial bin classification. It is recommended that
Cohasset begin to collect source water samples for E. coli twice a month such that the
requirements of the LT2ESWTR will be known well in advance of the compliance dates.
The analytical results of the Cryptosporidium monitoring, if applicable, will classify systems into
different categories or “bins”. Additional treatment will be required based on which bin a system
Final 2-12 Weston & Sampson
is assigned. For small systems ( 10,000 population) bin classification will be based on the one
year mean based on Cryptosporidium samples collected twice per month for 12 months. If
Cryptosporidium monitoring is not required, a system is automatically placed into Bin 1
requiring no additional treatment. A system is exempt from monitoring for the purposes of the
bin classification if they can provide 2.5 log treatment for Cryptosporidium in addition to
conventional treatment requirements.
The bin classification will trigger the water supplier into meeting certain levels of
Cryptosporidium treatment, ranging from no treatment for Bin 1 classification to 2.5 log
removal/inactivation for a Bin 4 classification. The bins are defined below in Table 2-6.
TABLE 2-6
BIN CLASSIFICATION SUMMARY
Bin Cryptosporidium Concentration Required Additional Treatment
1 Cryptosporidium <0.075/L No additional treatment requirement for a
system using conventional treatment
2 0.075/L Cryptosporidium <1.0/L 1-log treatment will be required.
3 1.0/L Cryptosporidium <3.0/L 2-log treatment will be required.
4 Cryptosporidium 3.0/L 2.5 log treatment will be required.
In preparation for the LT2ESWTR, the EPA has also conducted investigations to determine what
bin classification systems would fall into. Based on their studies they estimate that most of the
water systems will fall into a bin assignment of Bin 1 or Bin 2.
The rule employs a “microbial toolbox” approach where properly designed and implemented
“tools” will receive a corresponding Cryptosporidium log credit reduction. Examples of “tools”
include watershed controls, the development of alternative sources, the addition of pretreatment
and the improvement of existing treatment and disinfection processes
The proposed LT2ESWTR presented various treatment options and the removal/inactivation
for Cryptosporidium that each will receive. Table 2-7 lists potentially applicable treatment
alternatives.
Final 2-13 Weston & Sampson
Compliance with the rule will be required three years after initial bin classification. A second
round of monitoring will be required six years after the initial bin classification.
TABLE 2-7
POTENTIAL TREATMENT OPTIONS UNDER LT2ESWTR
Treatment Approach Potential Log Credit
Watershed Control Program 0.5 log removal/inactivation credit
Combined Filter Performance 0.5 log removal/inactivation credit for combined filter
effluent turbidity 0.15 NTU in 95% of samples each
month at a monitoring frequency of every four hours.
Individual Filter Performance 1.0 log removal/inactivation credit for individual filter
effluent turbidity 0.15 NTU in 95% of the samples each
month and turbidity <0.30 NTU in two consecutive
samples taken at 15 minute intervals.
Chlorine dioxide Up to 1.0 removal/inactivation log credit based on
demonstration of compliance with CT tables
Ozone Up to 2.0 removal/inactivation log credit based on
demonstration of compliance with CT tables
UV Up to 2.5 log credit for Cryptosporidium, Giardia and
viruses based on the delivered UV dose provide in
relation to the UV dose tables.
Membrane Filtration Up to 2.5 log credit for Cryptosporidium removal based
on demonstration testing and daily membrane integrity
testing during operation
Second Stage Filtration 0.5 log credit for Cryptosporidium removal for a second
stage filtration stage (i.e. rapid sand, dual media, GAC or
other fine grain media).
G:\WATER\COHASSET\203151 DBP\REPORT\REPORT_080603.DOC
Final 2-14 Weston & Sampson
3.0 SUMMARY OF COHASSET’S DBP’S
3.1 Total Trihalomethanes (TTHMs)
Based on recent data from February 2002 to the present, the RAA for the Bates Lane TTHMs
sample site ranged from 55.0 to 80.0 Pg/L indicating that Cohasset is barely in compliance with
the new Stage 1 TTHM MCL of 80 Pg/L. Table 3-1 summarizes the Bates Lane TTHM sample
results from May 2000 to the present.
TABLE 3-1
BATES LANE TTHM SUMMARY
Date Sampled TTHM (Pg/L) RAA (Pg/L)
05/03/00 167.5 --
06/07/00 98.8 --
06/21/00 96.1 --
10/18/00 130.5 --
11/27/00 49.7 --
06/26/01 44.6 --
09/19/01 41.4 --
02/20/02 82.7 --
05/21/02 65.6 --
09/25/02 66.1 --
12/18/02 60.4 68.7
01/29/03 44.3 59.1
05/28/03 49.2 55.0
09/17/03 126.5 70.1
11/17/03 100.1 80.0
12/31/031 74.9 73.7
1Assumed to be taken in the first quarter for calculation purposes
The Stage 1 D/DBPR requires that the sampling site be located at a point representing the
maximum residence time in the distribution system. High TTHMs are typically characterized by
long residence times within the distribution system. A hydraulic model of the distribution
system was used to identify a sample location having a maximum distribution system residence
time. Beginning in December 2003, the TTHM sample location was relocated from Bates Lane
to Whitehead Road, at the eastern extent of the distribution system. Samples collected from this
location had TTHM concentrations of 78.9 and 81.7 Pg/L for samples collected on December 24,
Final 3-1 Weston & Sampson
2004 and December 31, 2004, respectively. Based on samples collected on December 31, 2004,
the TTHM concentration at Whitehead Road was almost 13 percent higher than the
concentration at Bates Lane.
Figure 3-1 and Table 3-2 summarizes historical TTHM sampling data collected from 1994
through 2003 throughout the distribution system. Based on the TTHM summary in Table 3-2,
the RAA based on a “mock” quarterly average of all available data from each quarter is 88.5
Pg/L, which is greater than the current regulatory level of 80 Pg/L. By summarizing the data by
quarter, the variation of TTHM concentrations between seasons can be evaluated.
TABLE 3-2
TTHM SUMMARY
TTHM Concentration (Pg/L)
Quarter Minimum Maximum Average Standard Deviation
Number of
Samples
1 44.3 138.7 75.8 37.9 5
2 44.6 175.0 110.3 47.5 15
3 41.4 126.5 88.4 27.4 7
4 49.7 130.5 79.4 24.8 9
The town will need to refine their TOC removal and evaluate current disinfection practices to
reduce DBP formation to ensure that the D/DBPR Stage 1 RAA and Stage 2 LRAA TTHM
levels are met consistently. As a first step, the pre-chlorination injection point has been relocated
from the rapid mix basins to post-sedimentation/pre-filtration.
Between September 1995 and October 2000, seven samples were collected from the WTP
effluent and were analyzed for TTHM. TTHM concentrations leaving the plant ranged from
non-detect to 66.3 Pg/L with an average concentration of 31.8 Pg/L TTHM. Because high
TTHM sites are typically characterized by long residence times and low or no disinfectant
residual, the TTHM concentration leaving the plant will increase as it moves to the extent of the
distribution system. Measures should be taken to reduce the amount of DBPs formed within the
WTP.
Final 3-2 Weston & Sampson
Figure 3-1Historical Trihalomethane Results54.5104.693.279.958.6104.055.5138.768.961.191.237.39.0167.5149.3130.50.00.50.00.026.327.241.496.582.765.666.144.666.356.549.792.896.198.8147.5175.0172.3160.974.9126.5100.184.578.90204060801001201401601802003/2/939/1/933/3/949/2/943/4/959/3/953/4/969/3/963/5/979/4/973/6/989/5/983/7/999/6/993/7/009/6/003/8/019/7/013/9/029/8/023/10/039/9/03DateConcentration (mg/L)TTHM ConcentrationStage 1 RAA/Stage 2B LRAAStage 2A LRAA
3.2 Haloacetic Acids (HAA5s)
Based on data from September 2002 to the present for Bates Lane, the RAA for HAA5s ranged
from 63.0 to 89.6 Pg/L. The HAA5 MCL was exceeded in all RAAs calculated for the given
timeframe. Between May 2001 and the present, ten samples have been collected from this
sample location and analyzed for HAA5s. The results are summarized in Table 3-3. The RAA
MCL was exceeded in the last three quarters indicating that treatment modifications are required
to meet the requirements of the D/DBPR Stage 1 RAA and Stage 2 LRAA.
TABLE 3-3
BATES LANE HAA5 SUMMARY
Date Sampled Total HAA5
(Pg/L)
RAA
(Pg/L)
05/21/01 60.6 --
09/19/01 63.7 --
02/27/02 60.0 --
09/25/02 37.4 --
12/18/02 35.4 --
03/31/03 108.7 --
05/28/03 70.6 63.0
09/17/03 80.9 73.9
11/17/03 51.4 77.9
12/31/031 98.0 89.6
1Assumed to be taken in first quarter for calculation purposes
Beginning in December 2003, the HAA5 sample location was moved from Bates Lane to
Whitehead Road, at the eastern extent of the distribution system. Samples collected from this
location had an HAA5 concentration of 1.3 Pg/L and was not detected in the second sample for
samples collected on December 31, 2004 and December 24, 2004, respectively. As has been
previously mentioned, high HAA5 concentrations are typically characterized by average
residence times within the distribution system. Based on the sampling results from the new
sampling location, HAA5 concentrations are greatly reduced from what was previously seen at
the Bates Lane sampling location. After the IDSE is completed under the Stage 2 D/DBPR, this
sample location may need to be relocated.
Final 3-4 Weston & Sampson
3.3 Total Organic Carbon (TOC)
Average raw water alkalinity from samples collected from Lily Pond range from 5.5 to 5.9 mg/L
as calcium carbonate, as was reported in ENSR’s Lily Pond Limnology and Water’s Edge Study,
dated February 2003. Typical raw water TOC samples collected from the same study ranged
from 5.0 to 12.0 mg/L at different locations in the Pond. The TOC sample results are
summarized in Table 3-4. The higher TOC concentration in Brass Kettle Brook is attributed to
its heavily vegetated wetland system.
TABLE 3-4
RAW WATER TOC SUMMARY
Sample Location TOC Concentration (mg/L)
Shoreline near intake wingwall 5.0
Peppermint Brook 6.2
Along eastern shoreline 9.8
Brass Kettle Brook 12.0
Beginning in August 2003, monthly raw water and finished water samples were collected and
analyzed for TOC to provide a preliminary evaluation of compliance with the Stage 1 D/DBPR.
Table 3-5 summarizes the TOC analytical results. Based on the results, the WTP was in
compliance with the required TOC removal percentage.
TABLE 3-5
TOC SUMMARY
TOC (mg/L)Sample Date
Raw Water Finished Water
Percent
Removal
Percent Removal
Required
Compliance
Met?
8/14/2003 12.0 4.3 64.2 50 YES
9/17/2003 13.0 5.3 59.2 50 YES
10/14/2003 11.0 4.6 58.2 50 YES
11/17/2003 11.0 5.3 51.8 50 YES
12/28/2003 7.8 3.2 59.0 45 YES
On August 14, 2004, raw water and finished water samples were collected and were also
analyzed for DOC and SUVA. Table 3-6 summarizes the analytical results. If compliance isn’t
met under the TOC removal criteria, compliance can be met by the alternate SUVA compliance
criteria where either the raw water or finished water SUVA is less than 2.0 L/mg-m. The
alternate SUVA compliance criteria was not met for the raw water and finished water samples,
Final 3-5 Weston & Sampson
but because the TOC removal criteria was met, the WTP is in compliance. The finished water
SUVA was 2.4 L/mg-m. It is possible that the finished water SUVA may be below 2.0 L/mg-m
at other times of the year.
TABLE 3-6
SUVA SUMMARY
Sample Type DOC UV254 SUVA
(mg/L) (cm-1) (L/mg-m)
Raw Water 12 0.69 5.8
Finished Water 4.2 0.101 2.4
Based on the analytical results of the monthly samples collected between August and December
2003, it appears that the WTP will be in compliance with the Stage 1 D/DBPR. However, since
compliance with TOC removals is based on a running annual average of monthly data, there is
insufficient TOC and alkalinity data available to make a determination as to whether the WTP
will be able to remove the required TOC the remainder of the year. Additional TOC, DOC and
SUVA raw water and surface water samples will need to be collected before a final compliance
determination can be made.
G:\WATER\COHASSET\203151 DBP\REPORT\REPORT_FINAL.DOC
Final 3-6 Weston & Sampson
4.0 TREATMENT OPTIONS
4.1 General
An initial review of best available technologies for removal of TTHMs and HAA5s for the Lily
Pond WTP include relocation of the pre-chlorination injection point, enhanced coagulation,
powdered activated carbon (PAC) addition, replacement of the filter media with sand/anthracite
or granular activated carbon (GAC), modifying the clearwell and changing/eliminating oxidants
and disinfectants. In addition, the reactivation of the groundwater wellfields and the
improvement of source water quality will also contribute to DBP reduction. These technologies
are further discussed in the following sections of this chapter.
An additional available treatment technology is membrane filtration. The installation of
membranes will result in significantly higher capital costs than previously mentioned
technologies. For this reason, this technology was not considered as other more cost-effective
technologies for TTHM/HAA5 removal are available.
Any modification to the treatment process will likely require a pilot study to document process
reliability and performance to the Massachusetts Department of Environmental Protection
(DEP). This requirement is triggered in part by the LT1ESWTR since the running annual
average of TTHMs or HAA5 levels in Cohasset are greater than or equal to 80 percent of the
MCLs of 64 Pg/L and 48 Pg/L for TTHMs and HAA5s, respectively.
4.2 Relocation of the Pre-Chlorination Injection Point
As has been described, the key to reducing TTHMs and HAA5s is to remove any organics prior
to chlorine addition to avoid creating disinfection byproducts. Until December 1, 2003, chlorine
gas was added at the rapid mix basins and post-filtration at concentrations of 3.0 and 2.0 mg/L,
respectively. Based on these typical chlorine dosages from April 2002 to April 2003, free
chlorine residuals throughout the distribution system ranged from 0.03 to 0.55 mg/L with an
average concentration of 0.23 mg/L. Samples collected from the extents of the system
(Jerusalem Road, South Main Street) had residual concentrations ranging from 0.03 to 0.10 mg/L
with an average concentration of 0.05 mg/L.
Final 4-1 Weston & Sampson
Based on a review of data from April 2002 through April 2003, pre-chlorine dosages were
decreased from 3.0 mg/L to 2.4 mg/L during the summer months. This is a good WTP practice
as the effectiveness of chlorine increases with higher temperature, thereby requiring a lower
dosage in the summer months. The reduction/optimization of the chlorine dose will helps to
counteract the formation of DBP at the time of year when DBP formation is highest.
At the Lily Pond WTP, one of the easiest and simplest ways to reduce DBPs in the water is to
eliminate or relocate the pre-chlorination injection point. The elimination or relocation of the
pre-chlorine injection point will decrease the formation of DBPs as the amount of total organic
carbon (TOC) available to react with the chlorine at the application point will be reduced through
settling and filtration. Chlorination can not be totally eliminated as a required amount of
chlorine contact time (CT) and a disinfectant residual in the distribution system is necessary to
meet regulatory requirements
Table 4-1 shows the percent reduction of DBPs as the pre-chlorination injection point is moved
throughout a conventional WTP. The table also shows the reductions for enhanced coagulation,
which is further described below. The Environmental Protection Agency (EPA) has not
provided information regarding potential DBP reduction for a pre-chlorine injection point at
post-filtration.
TABLE 4-1
IMPACT OF MOVING THE POINT OF CHLORINATION ON DBP FORMATION
Median Reduction in DBP Formation (%)
TTHM HAA5
Point of Application Baseline
Coagulation
Enhanced
Coagulation
Baseline
Coagulation
Enhanced
Coagulation
Pre Rapid Mix -- 17 -- 4.7
Post Rapid Mix 1.6 21 5.3 21
Mid-Flocculation 8.7 36 14 36
Post-Sedimentation 21 48 35 61
Source: USEPA, Enhanced Coagulation and Enhanced Precipitative Softening Guidance Manual, May 1999
Final 4-2 Weston & Sampson
Based on tracer studies conducted at the plant in November 2003, the pre-chlorination injection
point was relocated from the rapid mix basins to post-sedimentation/pre-filtration in December
2003. This change should reduce the amount of TOC available to react with the chlorine from
pre-chlorination and reduce the formation of DBPs. The relocation of the injection point may
reduce required chlorine gas dosages by 10 to 15 percent.
4.3 Groundwater Supply
The Elms Meadow and Sohier groundwater wellfields are currently being renovated and are
expected to be placed back on-line within the next year. The wells will pump directly in to the
distribution system. Assuming that these wells are consistently operated and have low TOC
levels, the addition of this groundwater supply to the distribution system will help to reduce the
system DBPs. Groundwater sources are characterized as having lower TOC and therefore a
lower potential to form DBPs. DBPs in the distribution system will be reduced proportional to
the amount of groundwater that is pumped from these wellfields.
4.4 Source Water Quality
The watershed of Lily Pond is approximately 1,603 acres. The high watershed area to lake area
ratio suggests that in-lake water quality is highly dependent on the water quality of the
watershed. Based on an evaluation of physical, chemical, and biological characteristics
conducted by ENSR in February 2003, Lily Pond was classified as eutrophic (overly productive).
The classification was based on nutrient loadings and plant densities in the lake, both of which
contribute to a deterioration of water quality. This problem is compounded during the summer
months when the bar racks and microscreens on the Lily Pond intake structure become clogged
with weeds, causing operating problems.
Lily Pond receives water from seasonal releases from the Aaron River Reservoir, which is
located upstream of the Lily Pond. The Bound Brook control structure, located at the
intersection of Bound Brook Lane and Beechwood Street, controls water released by the Aaron
River Reservoir. Released water from Aaron Reservoir either pools in the wetlands located to
the south of Lily Pond or flows to Lily Pond. During drier periods when the surface elevation of
Final 4-3 Weston & Sampson
Lily Pond falls below the controlling elevation at the Bound Brook control structure and/or when
water is withdrawn from Lily Pond, water is drawn into Lily Pond.
Studies being conducted by Weston and Sampson and by others have suggested alternatives to
improve the source water quality of Lily Pond and to minimize clogging of the intake structure.
These alternatives include construction of a raw water transmission main from Aaron Pond
directly to the Lily Pond WTP and the installation of a floating silt curtain around the Lily Pond
intake structure to prevent clogging and improve water quality. For additional details concerning
the methodologies to improve source water quality, refer to these reports.
4.5 Enhanced Coagulation
Enhanced coagulation is defined as the process of obtaining improved removal of DBP
precursors (TOCs/SUVA) through conventional treatment. The process includes increasing the
alum (coagulant) dose to a point where the TOC removal is optimized at a pH of 5.5 to 6.0. The
primary goal of enhanced coagulation is to achieve significant TOC reduction without the
addition of unreasonable amounts of coagulant. Jar testing using enhanced coagulation is
currently being conducted by Tom Sawyer, an independent consultant. The jar testing program
also includes testing an alterative coagulant, polyaluminum chloride (PACl). Results of this
testing will indicate if TOC removals can be increased. If removals are greater than the current
treatment process, Cohasset should consider changing their coagulant. Advantages of PACl
include reducing the coagulant dosage, providing better sludge dewatering and better particulate
removal for some surface waters.
TOC removal compliance, as it pertains to enhanced coagulation, is a two-step process. The first
step involves the collection of monthly source water alkalinity and source water and finished
water TOC data to determine compliance. Once a full year of data has been collected, the RAA
for the year can be calculated and it can be determined whether a water system is in compliance
with the required TOC removals. The percent TOC removal is measured between the raw water
sampling point and the combined filter effluent turbidity monitoring location.
Final 4-4 Weston & Sampson
If a system is not in compliance with the TOC removals previously listed in Table 2-2, jar testing
and/or pilot testing can be conducted to identify an alternative TOC removal requirement that is
system specific. The alternate TOC removal requirement was included in the Stage 1 D/DBPR
rule to ensure that treatment requirements could be met for all source water types and qualities.
Some source waters will be easy to treat, while others will be difficult to treat. This process
allows a system to define the TOC removal that is realistic to their water system and source
waters.
To determine an alterative TOC removal requirement during jar tests, the alum dose is increased
in 10 mg/L increments to determine the TOC removal for each dose. Increased dosages must be
added until the target pH (for waters having 0 to 60 mg/L as CaCO3 of alkalinity) of 5.5 is
reached.
The Stage 1 D/DBPR has identified a point of diminishing return (PODR) for determining the
optimal alum dose. This point is defined as the point where less than 0.3 mg/L TOC is removed
per every additional 10 mg/L alum dose. The intention of the PODR is to limit the dose to a
point beyond which the TOC removal is insignificant. The water system is required to submit
the results of the jar testing within three months of failure to comply. Once approved by DEP, a
strategy for full-scale implementation should be developed.
In addition to establishing a new coagulant dose, the jar tests are intended to determine if there
are any secondary effects as a result of the increased coagulant dosage. Secondary effects will
vary depending on the source water and may include the following:
xIncreased inorganic levels. Enhanced coagulation reduces the pH of the water. The lower
pH slows the rate of oxidation and may decrease the oxidation/removal of some
manganese. Other potential impacts include increased iron, aluminum, sulfate, chloride
and sodium concentrations. Based on current WTP practices and raw water quality, it is
unlikely that significant increases in concentrations of these inorganics will be encountered.
xCorrosion control. The increased coagulant dosage results in a lower pH. The alkalinity of
the water will also be reduced as each milligram of alum added to the water consumes
Final 4-5 Weston & Sampson
approximately 0.5 mg of alkalinity as calcium carbonate. Studies have also shown that the
removal of NOM from water increases the potential for lead corrosion. The finished water
lime dosage will need to be adjusted to increase the pH and alkalinity prior to distribution.
Based on lead and copper sampling rounds conducted between June 1994 and August
1998, it is not expected that Cohasset will have a problem continuing to meet the Lead and
Copper Rule, if enhanced coagulation is practiced.
xParticulate and pathogen removal. The lower pH and higher coagulant dosage may result
in the restabilization of particles and an increase in the settled water turbidity. This has not
been shown to occur in all cases. It is recommended that settling performance be studied
with jar tests prior to implementation.
xIncreased residuals production. The increased TOC removal as a result of the increased
coagulant dosage will increase the volume of residuals that is produced. The sludge
characteristics and its dewaterability may also be altered.
4.6 Powdered Activated Carbon
Powdered activated carbon (PAC) has been used in water treatment for taste and odor control
and has been shown to remove some TOC. Typically, PAC is added at the rapid mix stage and is
settled out in the sedimentation stage. Because PAC is added at the same time that coagulants
are added, the growth of floc around PAC particles can block adsorption onto the PAC particles.
Many studies have indicated that NOM removal by PAC increases with contact time up to seven
days or longer. For this reason, the contact time in conventional settling basins, such as those at
the Lily Pony WTP, is not long enough for the effective removal of NOM by PAC. Therefore,
PAC is not recommended for DBP reduction.
4.7 Filter Media Replacement
The existing filter media is a dual media filter consisting of anthracite coal and silica sand. Filter
media typically requires replacement every 5 to 10 years. The existing media was last replaced
in 1995 and is scheduled for replacement within the next several years. The media replacement
will improve the efficiency of the treatment process and will decrease the TOC and turbidity
Final 4-6 Weston & Sampson
levels leaving the filters. The reduced TOC levels will decrease potential disinfection. It is
assumed that filter media replacement will be required every ten years.
As an alternative to anthracite and sand as filter media, granular activated carbon (GAC) can also
be used for the removal of TOC precursors including humic and fulvic acids that react with
disinfectants to produce disinfection byproducts. GAC is also used to remove organic
compounds that cause taste and odor problems and can be installed in place of sand and/or
anthracite in an open filter or can be installed as a capping layer over other media. If the media
in the existing filters at the Lily WTP were replaced with GAC, the required bed expansion
during a backwash would increase from 30 to 50 percent based on the DEP Guidelines and
Policies for Public Water Systems. Based on the 40 inch depth of media in the existing filters
and assuming a similar GAC media depth, there is sufficient space available for the required 50
percent bed expansion required.
The effective life of GAC for this type of application has been shown to range between a few
months to 4 years. Therefore, before GAC is installed in a filter, a pilot study should be
conducted to determine how long the adsorptive qualities of the GAC will last given the organic
loading on the filter.
4.8 Clearwell Modification/Expansion
Pre-chlorination could be totally eliminated if additional chlorine contact time was provided in
the clearwell. The existing clearwell size limits the residence time of the water. If additional
space is provided in the clearwell, the expanded clearwell would be sized to increase the chlorine
contact time through the plant to comply with the Surface Water Treatment Rule (SWTR) such
that pre-chlorination is no longer required. To maintain the current gravity feed within the plant,
a clearwell expansion should be continuous with the existing clearwell and would need to be
constructed with baffles.
Available space adjacent to the existing clearwell is limited to the south side of the WTP
between the WTP building and Lily Pond. The costs associated with this improvement would
exceed other improvement options included herein and would not offer significant additional
Final 4-7 Weston & Sampson
compliance with the regulations. The use of an alternate disinfectant or oxidant will reduce DBP
levels and meet future regulatory levels at a lower cost.
Instead of expanding the clearwell, baffles could be constructed within the existing clearwell to
increase the contact time. The contact time will be based on a minimum detection time of 90
percent of the measured flow through the clearwell (T10). Based on the tracer study conducted in
November 2003, the T10 was determined to be 52 minutes between the filters and the clearwell.
Assuming that 90 percent of the T10 applies to the filters, gullet and manifold before the
clearwell, the portion of the T10 applicable to the clearwell would be 5.2 minutes.
For a well-baffled clearwell, the T10 can be estimated as 60 percent of the theoretical contact
time. To be conservative, it was assumed that the estimated T10 is 50 percent of the theoretical
contract time, resulting in an estimated clearwell T10 of 17.3 minutes for a well baffled clearwell.
Hypothetically, the construction of baffles in the clearwell would increase the estimated existing
contact time from 5.2 minutes to 17.3 minutes, allowing for a lower chlorine dose from the post-
sedimentation/pre-filtration injection point.
Based on the estimated T10 of 17.3 minutes, the re-location of the pre-chlorination injection point
from pre-filtration/post-sedimentation to before the clearwell can be evaluated. Table 4-2
summarizes the required chlorine dosages to meet CT values at a pH of 6.5, 0.5 log activation
given an estimated T10 of 17.3 minutes.
Table 4-2
CT Calculations for a pH of 6.5, 0.5 log inactivation
Temperature
(oC)
CT Required
(mg/L-min)
Minimum Residual Chlorine
Concentration
(mg/L)
525 1.44
25 6 0.35
Based on these results, it appears that the construction of baffles in the clearwell would enable
the pre-chlorination injection point to be moved further downstream to a point before the
clearwell. A tracer study of the clearwell alone should be conducted to verify the assumptions
contained herein.
Final 4-8 Weston & Sampson
4.9 Disinfectants and Oxidants
4.9.1 General
Raw water oxidants/disinfectants are added to oxidize iron, manganese and other organics,
remove tastes and odors, improve coagulation/filtration efficiency and to prevent microbiological
growth in a WTP prior to disinfection. There are a variety of disinfectants and oxidants that may
be used in water treatment for this purpose. The following sections review the use of chlorine,
chloramine, potassium permanganate, ozone, chlorine dioxide and a mixed-oxidant process. Of
the disinfectants and oxidants listed, typically all but chloramines are used for pre-treatment.
Chloramines and chorine are typically used for post-treatment, because they can maintain a
residual in the distribution system. Table 4-3 summarizes the advantages and disadvantages of
the previously listed disinfectants and oxidants.
4.9.2 Chlorine
Chlorine is the most widely used chemical in the water treatment industry. It is primarily used
for water disinfection, however its oxidizing power also makes it popular for other purposes in
water treatment. Chlorine used for disinfection is typically in the form of chlorine gas, sodium
hypochlorite, or calcium hypochlorite.
Because chlorine reacts with so many naturally occurring organic and inorganic compounds in
water, it readily produces undesirable DBPs. It is becoming popular to combine chlorine with a
second disinfectant to prevent the formation of DBPs while still achieving effective disinfection.
Chlorine has many attractive features that contribute to its wide use in the industry. The four key
advantages of chlorine are that it 1) effectively inactivates a wide range of pathogens commonly
found in water, 2) leaves a residual in the water that is easily measured and controlled, 3) is
economical, and 4) has an extensive track record of successful use in improving water treatment
operations. Other advantages include control of nuisance organisms, taste and odor control,
maintenance of filter media, removal of iron and manganese, destruction of hydrogen sulfide,
maintenance of distribution system water quality by controlling slime growth, and bleaching of
certain organic colors.
Final 4-9 Weston & Sampson
TABLE 4-3
ADVANTAGES AND DISADVANTAGES OF DISINFECTANTS AND OXIDANTS
Oxidant Advantages Disadvantages
Chlorine
xStrong oxidant
xEasy to feed
xPersistent residual
xLong history of use
xDisinfection by-products (DBPs)
xTaste and odor problems possible
xpH influences effectiveness
Chloramines
xNo trihalomethane (THM) formation
xReduces taste and odor
xReduces biofilm
xPersistent residual
xEasy to feed
xLong history of use
xWeak oxidant
xSome total organic halide formation
xpH influences effectiveness
xTaste, odor, and growth problems
possible
xMore difficult to operate than
chlorine
xIncreases corrosivity in water
xPotential need of break point
chlorination if nitrification occurs in
system
xHydrant flushing concerns
xAmmonia handling concerns
Potassium
Permanganate
xModerately strong oxidant
xEasy to feed
xNo THM formation
xReduces taste and odor
xPink water
xBy-products are unknown
xNo disinfectant properties
xLimited TOC removal capabilities
Ozone
xStrong oxidant
xUsually no THM or total organic
halide formation
xNo taste or odor problems
xSome by-products are biodegradable
xLittle pH effect
xCoagulant aid
xShort half-life
xOnsite generation required
xEnergy intensive
xComplex generation and feeding
xCorrosive
Chlorine
Dioxide
xStrong oxidant
xRelatively persistent residual
xNo THM formation
xNo pH effect
xChlorite and chlorate by-products
xChlorite must be monitored daily
xOnsite generation required
xHydrocarbon odors possible
Mixed
Oxidant
Process
xStrong oxidants
xMultiple oxidants can be more
effective over a wider range of
conditions
xLow THM formation
xRelative amounts of oxidants are
difficult to characterize
xEffectiveness of disinfectant is difficult
to gauge.
The major disadvantage of using chlorine as a disinfectant is the formation of DBPs that result
from free chlorine’s reaction with naturally occurring matter in the water. Hazards associated
with using chlorine, specifically chlorine gas, that require special treatment and response
Final 4-10 Weston & Sampson
programs, and taste and odor problems resulting from high chlorine doses are two other
significant disadvantages. Other disadvantages of using chlorine include the potential
deterioration in the coagulation and filtration of dissolved organic substances, storage issues with
sodium and calcium hypochlorites, and the loss of disinfection efficiency at high pH.
4.9.3 Chloramine
When chlorine and ammonia are both present in water, they react to form chloramines. As
opposed to free chlorine, the chloramines are referred to as “combined chlorine.” The inorganic
chloramines consist of three species: monochloramine (NH2Cl), dichloramine (NHCl2) and
trichloramine or nitrogen trichloride (NCl3). The principal reactions, which occur in a stepwise
fashion, are:
NH4+ + HOCl o NH2Cl + H2O (Monochloramine)
NH2Cl + HOCl o NHCl2+ H2O (Dichloramine)
NHCl2+ HOCl o NCl3+ H2O (Trichloramine)
The disinfection method of chloramination uses ammonia and chlorine to generate chloramines,
which maintain a disinfectant residual in the distribution system. Chloramination disinfection
usually works best at system pHs between 8.0 and 8.5. The optimum ratio of chlorine to
ammonia-nitrogen used for generating chloramines for drinking water is between 3:1 and 5:1.
Higher ratios begin to generate di- and tri-chloramines which can cause taste and odor problems.
Lower ratios allow free ammonia into the distribution system and can cause nitrification in the
distribution system. There has been some research that shows that biofilms are more sensitive
to inactivation by chloramination than chlorination. However, if nitrification occurs in a
distribution system, raising the chloramine residual will not prevent nitrification. Typically, the
use of a free chlorine residual or flushing of the distribution system is the only way to eliminate
nitrification, once it has begun.
The major advantage of chloramine disinfection is the reduction of DBP concentrations and the
fact that no detectable trichloroacetic acids are produced as byproducts. Other benefits include
reduced taste and odor, potential inactivation of biofilms in the distribution system, and greater
resistance to decay than chlorine. Disadvantages of chloramines are potential nitrification in the
Final 4-11 Weston & Sampson
distribution system, which requires breakpoint chlorination, and more complicated operation and
higher costs than chlorine. Another disadvantage to chloramines is that when flushing the
distribution system, any flushed water that may run-off into surface waters must be handled and
neutralized prior to being discharged as chloramines are extremely toxic to fish.
4.9.4 Potassium Permanganate
Potassium permanganate (KMnO4) is a strong oxidizing agent used primarily to control taste and
odors, remove color, control biological growth in treatment plants, and remove nuisance
organisms, iron, and manganese. Because of this, potassium permanganate is often used in
conjunction with chlorine for water treatment. The use of KMnO4 has not been shown to result
in significant reductions in TOC.
Although potassium permanganate is an excellent oxidizer, it is a poor disinfectant whose
effectiveness against bacteria and viruses is questionable, except at extremely high doses. It is
currently not registered with the EPA as a disinfectant.
To be most effective, potassium permanganate requires a long detention time. When added to
the water, the water will turn straw colored. If it is overdosed, the water will turn pink. Hence, it
is important not to overdose potassium permanganate.
Currently, jar testing using potassium permanganate is being conducted by Mr. Tom Sawyer, an
independent consultant. Tests are being conducted to evaluate the effectiveness of potassium
permanganate in controlling tastes and odors and in TOC removal. If the testing proves
successful, Cohasset should consider the addition of potassium permanganate.
4.9.5 Ozone
The use of ozone (O3) is quickly gaining popularity in the United States because it is both a
strong oxidant and an effective disinfectant. Ozone very effectively removes tastes, odors, and
color in water. It also oxidizes organic substances, which reduces the formation of DBPs. Other
benefits of ozone include improved coagulation, which lowers the costs of coagulant chemicals
and sludge disposal, reduction of chlorine demand, and removal of iron, manganese, and sulfide.
Final 4-12 Weston & Sampson
Ozone’s oxidizing strength comes from its spontaneous decomposition during water treatment to
form hydroxyl free radicals, which are among the most reactive oxidizing agents in water.
Because ozone is an unstable molecule, it is generated at the point of application for use in water
treatment. Ozone can be generated on-site by means of a generator that converts oxygen gas into
ozone through a uniformly charged air space with electrical energy. As a result, chemical
storage and delivery are not needed if air is used as the source of oxygen. Liquid oxygen can
also be used as the oxygen source for this application. Considering the size of the Cohasset
water system, it is suggested that ozone be generated utilizing the liquid oxygen method.
However, should Cohasset decide to use ozone for water treatment purposes, a more detailed
investigation of the source of oxygen should be conducted. Ozone generation requires
considerable energy input and can significantly raise electrical costs.
Ozone is a bluish, toxic gas with a pungent odor that is hazardous to health at relatively low
concentrations in air: the threshold odor level is 0.05 ppm and the 8-hour OSHA standard is 0.1
ppm. Therefore, destruction of the off-gas from ozone is necessary for safety reasons. Once the
ozone has been produced, it is diffused through the raw water in a contact chamber. Any excess
ozone gas that has passed through the raw water without being consumed is captured and
converted back to oxygen gas before being released to the atmosphere. Three methods that may
be used to eliminate off-gas are thermal destruction, thermal/catalytic destruction and catalytic
destruction. The use of ozone as an alternative disinfectant produces bromate, a DBP also
having adverse health effects. Monitoring for bromate would be required.
Ozone is chemically unstable and leaves no residual. As a result, it is necessary to use chlorine
or chloramines to maintain a disinfectant residual in the distribution system.
Nonhalogenated DBPs can form with the use of ozone as an oxidant. Ozone can oxidize
organics and form simple organic compounds that commonly appear as biodegradable dissolved
organic compounds (BDOC) or assimilated organic carbon (AOC). These compounds can
contribute to the growth of biofilms in the distribution system. Typically, this is not a problem if
some form of filtration follows the ozone process.
Final 4-13 Weston & Sampson
4.9.6 Chlorine Dioxide
Chlorine dioxide (ClO2) is another strong oxidizing agent that must be prepared onsite. It is
most often used as an alternative disinfectant to chlorine for reducing DBPs. Chlorine dioxide
disinfects by oxidation, but does not chlorinate. Its most popular use is as an initial oxidant for
treating raw water whose DBP precursors are high, which often helps obtain CT disinfection
credits. It also controls iron and manganese; removes color, and organic and sulfuric
compounds; and controls tastes and odors. Because chlorine dioxide can add a specific taste and
odor to water, its maximum residual should be 0.4-0.5 mg/L as ClO2.
Chlorine dioxide is a relatively small, volatile, highly energetic molecule that is a free radical
even while in dilute aqueous solutions. It is very soluble in water (approximately 10 times more
soluble than chlorine) and can easily be removed from dilute aqueous solutions with minimum
aeration. Chloride dioxide cannot be compressed or stored commercially as a gas because it is
explosive under pressure. As a result, chlorine dioxide is commonly generated on-site and is
applied as aqueous solutions between 0.1 and 0.5 percent for potable water treatment.
One disadvantage in using chlorine dioxide as an alternative disinfectant is that it produces
chlorite and chlorate, DBPs also having adverse health effects. Chlorate and chlorite are
inorganic compounds that form during the generation of chlorine dioxide. Approximately 50 to
70 percent of the chlorine dioxide consumed by oxidation reactions is converted to chlorite under
typical conditions in water treatment. Chlorite is relatively stable in the presence of organic
material, but can be oxidized to chlorate by free chlorine. Therefore, chlorate is most often
produced through the reaction of residual chlorite and free chlorine during secondary
disinfection. Chlorite and chlorate can also form under highly alkaline conditions (pH>9). Like
TTHMs and HAA5s, the health effects due to chlorate and chlorite include potential risks of
cancer and adverse reproductive outcomes in human populations. Chlorate is also known to
cause abdominal pain, diarrhea, and comas.
4.9.7 Mixed Oxidants
A mixed oxidant process produces a liquid stream of oxidants from salt, water and electricity.
The process generates a combination of ozone, chlorine dioxide, hypochlorite ion, hypochlorous
Final 4-14 Weston & Sampson
acid and elemental chlorine from the passage of an electric current through a continuous-flow
brine (salt) solution.
Compared to the use of a single oxidant, the use of mixed oxidants can be more effective. The
combination of oxidants can allow for better results over a wider range of conditions. Similarly,
the residuals from this process will be more effective over a wider range of conditions and over a
greater diversity of microorganisms. The increased effectiveness as compared to chlorination is
due to the combined use of ozone and chlorine dioxide, both of which are more effective in the
inactivation of microorganisms than ozone or chlorine alone.
The combination of oxidants results in shorter required contact time since the reaction rate of
mixed-oxidants is shorter as compared to chlorination due to the combined action of ozone,
chlorine dioxide and chlorine. However, since there is not sufficient information regarding the
exact components of the product, the more conservative CT tables for chlorine will need to be
used until such time that the CT values for mixed oxidant processes have been determined and
EPA approved.
A disadvantage of a mixed oxidant process is that it is difficult to characterize the relative
proportion of each oxidant generated. As a result, it is difficult to evaluate the effectiveness of
the mixed-oxidant. The use of chlorine dioxide as one of the oxidants produces chlorite and
chlorate, DBPs also having adverse health effects and would require monitoring. Mixed
oxidants have not proven to be reliable for removing turbidity and are only able to provide slight
reductions in color.
G:\WATER\COHASSET\203151 DBP\REPORT\REPORT_FINAL.DOC
Final 4-15 Weston & Sampson
5.0 RECOMMENDATIONS AND ESTIMATED COSTS
5.1 General
The following summarizes the recommended sequence of improvements that Cohasset should
undertake to reduce DBPs in their system. After each sequential improvement is completed,
water quality monitoring should be conducted to quantify the effectiveness of the improvements
to reduce DBPs.It may not be necessary to institute all the recommended improvements to
achieve compliance with the DBP rules.The Commission has determined that the objective of
these improvements should be to reduce the DBPs to a level that is 80 percent of the maximum
contaminant levels (MCLs) for total trihalomethanes (TTHMs) and haloacetic acids (HAA5s).
The corresponding target concentrations are 64 Pg/L and 48 Pg/L for TTHMs and HAA5s,
respectively.
Recommended improvements are broken down into two phases. The following sections
summarize each improvement by phase including a general description of the construction and/or
equipment requirements. Costs for each improvement is included in Section 5.4 of this chapter.
At this point in time, there is insufficient TOC data to fully evaluate a treatment approach to
lower DBPs in the Lily Pond WTP and distribution system. Once additional TOC data have
been collected, preferably during the warmer summer months and at least a full year of monthly
data is available, the following phased improvements can be further defined. It is recommended
that this report be updated once additional data has been obtained.
5.2 Phase 1
5.2.1 General
Phase 1 improvements to the WTP include the following, which are further discussed in the
following sections:
xEnhanced coagulation or the use of polyaluminum chloride
xConstruction of baffles within the clearwell
xFilter media replacement with sand/anthracite or GAC
Final 5-1 Weston & Sampson
xInstallation of a potassium permanganate chemical feed system to reduce taste and
odor complaints and to reduce TOC concentrations
It is estimated that the Phase 1 improvements may reduce TTHMs levels by 25 to 30 percent and
that HAA5 levels may be reduced by 40 to 50 percent. Based on the running annual averages
(RAAs) calculated for the last two quarters of 2003, TTHM levels will be reduced by up to 17.5
to 20.0 micrograms per liter (Pg/L), resulting in TTHM levels between 66 and 75 percent of the
MCL. The RAA for HAA5s over the same time frame will be reduced by up to 29.5 to 39.0
Pg/L, reducing the RAA to levels between 65 and 74 percent of the MCL. Once a phase
improvement has been completed, water quality monitoring should be conducted to quantify the
effectiveness of the improvements to reduce DBPs and increase TOC removal.
5.2.2 Enhanced Coagulation/Polyaluminum Chloride
Based on the results of the jar testing program, Cohasset may decide to practice enhanced
coagulation with alum or use PACl. Enhanced coagulation obtains improved removal of DBP
precursors (TOCs/SUVA) by conventional treatment. The improved TOC removal is
accomplished by increasing the coagulant dose to a point where TOC removal is optimized at a
pH of 5.5 to 6.0. The current average alum dosage is 40 mg/L. The optimization of the alum
dosage may result in an increase of the existing dosage by up to 50 percent. The increased
dosage of 20 mg/L has been incorporated into the operation and maintenance costs included
herein.
The use of PACl can also improve TOC removal. PACl is typically more expensive than alum.
Therefore, we have assumed that the increased operational costs for PACl would be the same as
enhanced coagulation. These costs should be re-estimated once actual dosages have been
determined.
5.2.3 Filter Media Replacement
Filter media typically requires replacement every 5 to 10 years. The existing sand and anthracite
filter media was last replaced in 1995 and is scheduled for replacement within the next several
years. The media replacement will improve the efficiency of the treatment process and will
Final 5-2 Weston & Sampson
decrease the TOC and turbidity levels leaving the filters. The reduced TOC levels will decrease
potential disinfection. It is assumed that filter media replacement will be required every ten
years.
Instead of replacing the existing filter media in-kind, GAC could be installed to increase TOC
removal within the Lily WTP. Before GAC is installed in a filter, a pilot study should be
conducted to determine how long the adsorptive qualities of the GAC will last given the organic
loading on the filter and the level of TOC removal. To develop costs for this option, it is
assumed that GAC filter media replacement will be required every four years.
5.2.4 Clearwell Modification
The current clearwell is not baffled. The installation of plastic curtain baffle walls would
increase the contact time within the clearwell, would increase the T10 value and may lower the
required pre-chlorination dosage to limit DBP formation.
5.2.5 Potassium Permanganate
The Lily Pond WTP was designed with a potassium permanganate system to treat manganese
from the groundwater wellfields. Since the groundwater sources have been off line for a number
of years, the potassium permanganate system has not been used. The existing potassium
permanganate system will need to be evaluated to determine whether it needs to be repaired,
rebuilt or replaced. We have assumed that the existing system is operable. The effectiveness of
potassium permanganate as a pre-oxidant to control taste and odor problems and to remove TOC
is currently being evaluated. Chemical addition costs have been included assuming that TOC
will be reduced.
5.3 Phase 2
Phase 2 improvements include changing either the pre- or post- disinfection chemical feed
systems. As was previously discussed in Chapter 4.0 of this report, there are a variety of
disinfectants and oxidants that may be used in water treatment. If it is necessary to further
reduce DBPs after Phase 1 improvements, it is recommended that the next step be to either
convert the post-chlorination system to a chloramine system or to change the pre-chlorination
Final 5-3 Weston & Sampson
system to ozone, chlorine dioxide or MIOX. A pilot study of each new chemical feed system
will likely be required by DEP and should be conducted prior to selecting an alternate pre-or
post-disinfectant.
5.3.1 Chloramines
To create chloramines, chlorine is added to the water prior to entering the clearwell. Ammonia is
then added to the water in the effluent pipe as it is pumped from the WTP to the distribution
system. The existing chlorine dosage of 2 mg/L will likely be increased to produce a typical
chloramine dosage of 3 mg/L. The optimum pH range for chloramines formation is between 8.0
and 8.5. Currently, the finished water pH leaving the WTP is 7.5. The finished water pH may
need to be increased to optimize chloramine formation. The ammonia chemical feed system
required to convert chlorine gas to chloramines includes an ammonia storage tank, vaporizer,
control valves, piping and miscellaneous improvements.
The Town of Hingham currently uses chlorine as a disinfectant. Should Cohasset begin to
chloraminate the water, the water quality between the systems will need to be evaluated to verify
that the water quality of each system is compatible. Competing water quality is not anticipated
to be a problem as several communities in Massachusetts, including Wilmington, Peabody and
other Massachusetts Water Resource Authority (MWRA) communities currently mix chlorinated
water with chloraminated water without detrimental effects.
5.3.2 Ozone
The ozone chemical feed system includes ozone generators, off-gas destruct units, injection
equipment, a baffled contact tank located adjacent to the WTP building and miscellaneous
improvements and site work. The cost savings from the elimination of the pre-chlorination
dosage has been incorporated into the operation and maintenance costs presented herein.
5.3.3 Chlorine Dioxide
The chlorine dioxide system includes chlorine dioxide generators, storage tanks, injection
equipment and miscellaneous improvements and site work. The use of chlorine dioxide may
Final 5-4 Weston & Sampson
reduce the existing pre-chlorination dosage by up to 50 to 60 percent. The reduced dosage and
annual cost has been incorporated into the operation and maintenance costs included herein.
5.3.4 Mixed Oxidant Process
The mixed oxidant (MIOX¥) chemical feed system includes mixed oxidant generators, brine
and oxidant storage tanks, injection equipment and miscellaneous valves and piping. The cost
savings from the elimination of the pre-chlorination dosage has been incorporated into the
operation and maintenance costs presented herein.
5.4 Cost Summary
The estimated costs include construction costs, engineering costs, and contingencies.
Engineering design, construction administration costs and contingencies were estimated at a
graduated scale of 40 to 50 percent of construction costs. The costs were developed in part by
using recent construction costs for similar work in the northeast. These costs were updated to an
estimated Engineering News Record (ENR) Boston index of 7976 for December 2003. Other
sources include recent project bid data, the 2003 Means “Building Construction Costs Data,” and
manufacturers’ quotations.
Operation and maintenance (O&M) costs include chemical, labor, energy/power, and general
maintenance costs. Maintenance costs include costs for equipment repair/replacement, etc.
Electricity costs for ozonation were based on a rate of $0.11 per kilowatt-hour (kw-hr). A
present worth analysis over a 20-year period was used to compare costs assuming a 3-percent
inflation rate.
The installation of a new chemical feed system (i.e. chloramines, potassium permanganate,
ozone, chlorine dioxide or mixed oxidants) assumes that the equipment is sized to provide 100
percent redundancy. Standby power will need to be provided to these systems. The costs
included herein do not include costs for a standby generator for the case that the existing
generator does not have sufficient capacity to provide power to these systems. According to the
Massachusetts Guidelines and Policies for Public Water Systems, ammonia, ozone and chlorine
dioxide chemical feed systems must be housed in a separate room with an inspection window,
Final 5-5 Weston & Sampson
similar to the requirements for chlorine gas. Construction of a separate room or building will be
required to house these chemical feed systems. Capital and O&M costs are based on a plant
flowrate of 3.0 million gallons per day (mgd).
These costs are preliminary and will need to be further evaluated and /or pilot tested prior to the
implementation of an improvement. As previously mentioned, it may not be necessary to
institute all the recommended improvements to achieve compliance with the DBP rules.
TABLE 5-1
SUMMARY OF ESTIMATED COSTS
Phase Improvement Estimated
Construction
Costs
Estimated
O&M
Costs
20-year
Present
Worth Cost
Phase 1
Enhanced Coagulation or use of Polyaluminum
Chloride NA $11,000 $310,000
A
Construction of baffles in the clearwell. $85,000 NA $85,000
B Reactivate potassium permanganate chemical
feed system.$0 $39,000 $1,101,000
Source water quality improvements Costs being developed or included in
other reports
C
Re-activation of the groundwater sources Costs already incurred
Install granular activated carbon in existing
filter beds OR $230,000 NA $1,073,000
1D
Replacement of the filter media $151,000 NA $353,000
1
Phase 2
A Convert final disinfection from chlorine to
chloramines $196,000 $8,000 $422,000
Convert pre-chlorination to ozone OR $1,500,000 $17,000 $1,980,000
Convert pre-chlorination to chlorine dioxide
OR $196,000 $41,000 $1,353,000
B
Convert pre-chlorination to a mixed oxidant
system $412,000 $5,000 $553,000
1Assumes GAC filter media replacement is required every 4 years and sand/anthracite filter media replacement is
required every 10 years.
G:\WATER\COHASSET\203151 DBP\REPORT\REPORT_FINAL.DOC
Final 5-6 Weston & Sampson