Disinfection Byproduct
Risk in Drinking Water treated with a Booster Chlorination Scheme
By: Stacia
Thompson
Advisor: Dr. R. Scott Summers
Graduate Students: Eric Dickenson, Lori Work
Center for Drinking Water Optimization
REU Environmental Engineering Program
Summer 2002
Research Proposal
Microbial safe drinking water is a major public health issue
in the
Drinking water
disinfection has been a major issue in the
It
is difficult to determine the chlorine dose for a WTP because of system
requirements.�� The chlorine dose must be
high enough to produce microbial inactivation (primary disinfection) as well as
residual maintenance throughout the distribution system (secondary
disinfection).� It also must be low
enough that the formed DBPs conform to the DBP Rule,
taste thresholds, and odor thresholds.�
The Stage I Disinfection By-Products Rule (DBPR) established maximum
contaminant levels for four trihalomethanes and five haloacetic acids (USEPA, 1998).�� Residual maintenance refers to a trace
amount of chlorine in the distribution system that protects the water from
pathogen infiltration due to leaking pipes, low pressure, or wall pipe reactions.�� In conventional disinfection, the initial
chlorine dose must meet both primary and secondary disinfection
requirements.�� In another approach
termed booster chlorination, primary disinfection requirement is separated from
the residual concentration requirement.��
The initial chlorine dose at the WTP inactivates the pathogens while the
booster disinfectant dose sustains the chlorine residual within the
distribution system.� This decoupling
decreases the amount of DBPs that are formed from the
initial disinfection dose.
The Center for Drinking Water Optimization research group
explores short-term chlorine decay; short-term formation rate of disinfection
by-products, treatments including ozonation-biofiltration,
granulated activated carbon, and membrane filtration; chloramine
disinfection, and pre-oxidant analyses.��
Conducting experiments on the major components in a full-scale water
treatment plant will dictate the optimal treatment process.�� An optimal water treatment
process being defined as one that produces sufficient microbial disinfection,
adequate disinfectant residual throughout the distribution system, and
compliance with DBP maximum contaminant levels.
It is important that
the water treatment not only provides disinfection, but also maintains a residual
that will inactivate pathogens that enter the water distribution system through
pipe leaks and NOM reactions on pipe walls.�
Most water treatment facilities have sought to meet these needs by
applying a disinfection dose that is large enough to maintain chlorine residual
for the consumers on the distribution system periphery.�� Recently, increasing concern with
disinfection by-products and the epidemiological and toxicological studies
linking these compounds with spontaneous abortion and cancer has caused further
scientific exploration of the connection between chlorine disinfection of
disease-causing pathogens and the formation of harmful and potentially
carcinogenic DBPs.��
This booster chlorination system design separates the
microbial disinfection from the formation of DBPs.� If waterborne pathogens are such a tremendous
public health risk, why is there such concern with THMs
and HAAs?� A
study on the health effects of disinfectants and disinfection by-products
established these chemicals as suspected carcinogens (Uber
et al., 2001).�� There have also been
toxicological studies linking DBPs to spontaneous
abortion (Uber et al., 2001).� Because of its threat to public health, it is
important that the DBP concentrations are reduced as much as possible without
sacrificing the safety of the drinking water.
Utilizing a booster chlorination scheme effectively
maintains distribution system residuals while decreasing the concentration of
total trihalomethanes (TTHMs)
(Uber et al., 2001).�
For example, a conventional system (Figure 1) might add 3 mg/L Cl2
at the WTP.� This high disinfectant dose
must be enough to provide a sufficient residual for the consumers on the system
periphery.� Usually the minimum residual
(0.2 mg/L) occurs at the location with the maximum residence time (5 days), so
the disinfectant dose must be enough to last until the periphery
residences.� The TTHM formation for the
conventional system, where the maximum residence time is 5 days and the initial
dose is 3 mg/L is shown along with chlorine decay in Figure 2.�� The portion of the population that consumes
water with an �age� of 1-2 days is exposed to high concentrations of DBPs (60-75 mg/L).��
In a booster scenario, assuming
the mass additions and flow rates for both systems are equivalent, 2 mg/L is added
at the plant and an additional 1 mg/L after the water has aged 2 days, so the
population receiving 1-2 days old water is exposed to decreased THM
concentrations (35-60 mg/L).�� Figure 4 shows the chlorine decay rate and
the TTHM formation rate for the booster chlorination system.� The initial chlorine concentration is 2 mg/L
at its entrance to the water distribution system.� The disinfectant quickly decays to a small,
yet sufficient residual (0.25 mg/L) in 2 days.��
The 2-day-old water receives more chlorine through a booster dose that
immediately increases the concentration to a little over 1 mg/L.�� The chlorine decays to a minimum residual
level of approximately 0.2 mg/L at the maximum residence time (5 days).�� Figure 4 indicates the TTHM concentration
rate for two different conventional systems: one that produces a final TTHM
concentration of 100 mg/L and one with
a maximum concentration of 80 mg/L as well as a
booster chlorination system (dotted line).���
The TTHM concentration for the booster system initially demonstrates 80 mg/L curve behavior, but finishes
at a concentration of 100 mg/L.�� Although the maximum concentration was the
same value, the TTHM concentrations from 1-4 days were much lower than the TTHM
concentrations on the 100 mg/L curve.
Figure 4: Chlorine Decay and TTHM for
Booster Chlorination System
This research project falls under the auspices of the Center
for Drinking Water Optimization.� This
EPA funded research initiative, directed by R. Scott Summers, conducts research
on the chemical and physical processes involved in water treatment plants (WTPs).�� This
research project will include both laboratory experiments and mathematical
modeling.� Using bench scale treatment,
water samples will be treated with booster chlorination techniques.� The water samples will receive an initial
concentration of chlorine to neutralize the microorganisms and an additional
dose after 48 hours have passed.��
Increasing the chlorine dose has been shown to increase the chlorine
decay rate (Summers et al. 21).� It has
also been observed that the formation rate of THMs increases
with an increase in chlorine dose.�
Through chlorine residual measurements and gas chromatography, the
chlorine decay rate and the formation rate of disinfection by-products can be
determined for various times following the water�s release from the water
treatment plant or various locations within the water distribution
system`.�� Using several experimental
samples should yield a balance between the required chlorine residual and an
allowable concentration of DBPs.�� Mathematical models will be used to
calculate the risk associated with these DBP concentrations, such as a log
normal distribution function for water age within the distribution system.�� These risk factors can be applied to
distribution systems with various population densities in order to determine
DBP health risk to drinking water consumers.
It is hypothesized that booster chlorination will lower the
DBP exposure risk to the human population. A higher chlorine dose produces more disinfection
byproducts, so it is concluded that the booster chlorination method will yield
lower DBP concentrations at average times in the water distribution
system.� On average, the drinking water
population will be exposed to lower concentrations of DBPs,
although the maximum concentration may be equivalent to the high disinfectant
dose situation.� The concentrations of
three THMs, dichlorobromomethane,
chlorodibromomethane, and bromoform
can be related to human health risk through toxicological studies in order to
quantify the health risk levels produced by the booster chlorination method.
For this study, water will be collected from the City of
In order to determine the chlorine dose for each
experimental run, chlorine demand study will be performed.� A demand study entails the addition of five
different chlorine doses to five bottles containing enhanced coagulated
water.�� Once the chlorine dose is added,
the bottle will be capped, inverted several times, filled the rest of the way
with water, and capped headspace free.�
The chlorine will be allowed to decay for one day.� The chlorine residual concentration remaining
after one day will be measured for each of the five bottles.� The chlorine doses will be plotted against
the chlorine residuals to obtain a linear relationship.�� The slope of this line can be utilized to
determine the exact chlorine dose required for a chlorine residual of 0.3 mg/L
at 1 day.�� This same technique will be
applied for the 2 day and 10 day chlorine demand tests.
For each chlorine decay and TTHM study, a one-half gallon
amber bottle (the exact volume known) will be filled three-quarters full of
water.�� The chlorine dosing solution
will then be added to the bottle to obtain a specific initial chlorine
concentration using a 1.00 to 5.00 mL pipette.� The cap will be placed on the bottle and it
will be inverted twice to mix the chlorine dosing solution uniformly in the
water.� The bottle will then be filled
the rest of the way with water and capped headspace free.�� For the chlorine decay and TTHM studies of
the high dose (chlorine dose that will result in a 0.3 mg/L residual at 10
days), the water will be poured into 4 oz. bottles, headspace free.�� The chlorine residual measurement from a 4
oz bottle will be taken at each of the following reaction times: 1 hour, 2, 4,
8, 24, 120, and 240 hours.� The TTHM
measurements will be quenched with ammonia chloride (65 mg/40 mL) at the same reaction times.�� The same procedure will be performed for the
1 day chlorine residual dose (dose that will result in a 0.3 mg/L residual at 1
day) and the 2 day chlorine residual dose (dose that will result in a 0.3 mg/L
residual at 2 days).�
For the 1-day booster chlorination measurements, the initial
chlorine dose will be added as specified previously, but the water will not be
immediately poured into smaller bottles.�
The one-half gallon amber bottle will remain capped for 24 hours.�� At that time a booster chlorine dose will be
added, then the water will be poured into individual
bottles for chlorine decay and TTHM formation analysis.� The chlorine measurements will be taken at 1
hour, 2, 4, 8, 24, 96, and 216 hours.��
The TTHM measurements will be quenched at the same times.�� The same procedure will be used for the
2-day booster chlorination measurements except that the water will remain in
the one-half amber bottle for 48 hours before it will be dosed and poured into
individual bottles for chlorine and TTHM measurements.� The chlorine measurements and TTHM quenched
measurements will occur at the reaction times of 1 hour, 2, 4, 8, 24, 72, and
192 hours.���
The chlorine decay and TTHM studies will be conducted using
the following base conditions, which model operation conditions: temperature of
20�C and pH of 8.0 (buffered).� The
chlorine dosing solution and the water must be buffered to pH of 8.0.�� The chlorine dosing solution will be made
from concentrated 4-6% sodium hypochlorite and pH 6.7 borate buffer
in laboratory clean water to achieve 4 to 7 mg/L Cl2, buffered at pH
8.0.�� The chlorine dosing solution
strength will be measured by the iodometry method, SM
4500-CIB.� The chlorine residual will be
measured using the DPD-FAS SM 4500-CID and TTHM will be measured using U.S. EPA
Method 551.1.
This booster chlorination distribution system may not
decrease the maximum level of DBPs in drinking water,
but it may prove to substantially lower the health risk for consumers within the
water treatment plant�s vicinity.��
Epidemiological and toxicological studies link disinfection by-products
with cancer and pregnancy loss.�
Decreased DBP exposure for a population and consequently decreased
environmental health risk would be an invaluable practice.� In addition to lowering the DBP exposure for
the population segment preceding the booster chlorination site and maintaining an adequate chlorine residual, this booster disinfection
water distribution system may decrease chemical costs for specific water
distribution systems with significant suburban or urban sprawl.�
Summers, R. S.,
Dickenson, E., and Work, L.�� �Short-Term Chlorine Decay and Disinfection By-Product Formation,�
Department of Civil, Environmental, and Architectural Engineering, University
of
Uber,
J.G., Summers, R.S., Boccelli, D.L., and Tryby, M.E. Maintaining
Distribution System Residuals Through Booster Chlorination , Awwa Research Foundation and American Water Works
Association, (2001).
USEPA.
National Primary Drinking Water Regulations: Disinfectant/Disinfection
Byproduct Final Rule. Federal
Register, (1998).
Water Quality and Treatment. 5 th edition. Edited by Raymond
D. Letterman. (1999).
�
Low Bromide Water: Species composed of one or more bromine atoms were only found in
trace amounts.� For both experimental
scenarios, High Dose and Boost Dose, the water was dominated by chloroform,
which contains no bromine atoms, and small concentrations of dichlorobromomethane, which contains one bromine atom.� There were only trace amounts of chlorodibromomethane and bromoform.� It is concluded that the Boost Dose scenario
is equivalent to the High Dose scenario with respect to THM exposure risk.
High Bromide Water: Dichlorobromomethane was the dominant THM
species followed closely by chlorodibromomethane and
chloroform.
It
is concluded that the Boost Dose scenario is equivalent to the High Dose
scenario with respect to chlorodibromomethane
concentration.� The Boost Dose scenario
yields a greater risk for exposure to bromoform and a
decreased risk for exposure to dichlorobromomethane.
Risk Figures
Additional Websites on Related Topics:
Environmental
Protection Agency
Integrated Risk
Information System
Agency for Toxic
Substances and Disease Registry