United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-99-062
September 1999
Waste water
Technology Fact Sheet
Chlorine Disinfection
DESCRIPTION
The impact of untreated domestic wastewater on
community reservoirs has raised several health and
safety concerns. The organisms of concern in
domestic wastewater include enteric bacteria,
viruses, and protozoan cysts. Table 1 summarizes
the most common microorganisms found in
domestic wastewater and the types of human
diseases associated with them. In response to these
concerns, disinfection has become one of the
primary mechanisms for the
inactivation/destruction of pathogenic organisms.
In order for disinfection to be effective wastewater
must be adequately treated.
APPLICABILITY
Chlorine is the most widely used disinfectant for
municipal wastewater because it destroys target
organisms by oxidizing cellular material. Chlorine
can be supplied in many forms, which include
chlorine gas, hypochlorite solutions, and other
chlorine compounds in solid or liquid form. Some
alternative disinfectants include ozonation and
ultraviolet (UV) disinfection. Choosing a suitable
disinfectant for a treatment facility is dependent on
the following criteria:
Ability to penetrate and destroy infectious
agents under normal operating conditions.
Safe and easy handling, storage, and
shipping.
Absence of toxic residuals and mutagenic or
carcinogenic compounds after disinfection.
TABLE 1 INFECTIOUS AGENTS
POTENTIALLY PRESENT IN UNTREATED
DOMESTIC WASTEWATER
Organism
Disease Caused
Bacteria
Escherichia coli
Leptospira (spp.)
Salmonella typhi
Salmonella (=2100 serotypes)
Shigella (4 spp.)
Vibrio cholerae
Protozoa
Balantidium coli
Cryptosporidium parvum
Entamoeba histolytica
Giardia lamblia
Helminths
Ascaris lumbricoides
T. solium
Trichuris trichiura
Viruses
Enteroviruses (72 types) e.g.,
polio echo and coxsackie
viruses)
Hepatitis A virus
Norwalk agent
Rotavirus
Gastroenteritis
Leptospirosis
Typhoid fever
Salmonellosis
Shigellosis (bacillary
dysentery)
Cholera
Balantidiasis
Cryptosporidiosis
Amebiasis (amoebic
dysentery)
Giardiasis
Ascariasis
Taeniasis
Trichuriasis
Gastroenteritis, heart
anomalies, meningitis
Infectious hepatitis
Gastroenteritis
Gastroenteritis
Source: Adapted from: Crites and Tchobanoglous (1998) with
permission from The McGraw-Hill Companies.
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Affordable capital and operation and
maintenance (O&M) costs.
ADVANTAGES AND DISADVANTAGES
Chlorine is a disinfectant that has certain health
and safety limitations, but at the same time, has a
long history of being an effective disinfectant.
Before deciding whether chlorine meets the
municipality's needs, it is necessary to understand
the advantages and disadvantages of this product.
Advantages
Chlorination is a well-established
technology.
Presently, chlorine is more cost-effective
than either UV or ozone disinfection (except
when dechlorination is required and fire code
requirements must be met).
The chlorine residual that remains in the
wastewater effluent can prolong disinfection
even after initial treatment and can be
measured to evaluate the effectiveness.
Chlorine disinfection is reliable and effective
against a wide spectrum of pathogenic
organisms.
Chlorine is effective in oxidizing certain
organic and inorganic compounds.
Chlorination has flexible dosing control.
Chlorine can eliminate certain noxious odors
during disinfection.
Disadvantages
The chlorine residual, even at low
concentrations, is toxic to aquatic life and
may require dechlorination.
All forms of chlorine are highly corrosive
and toxic. Thus, storage, shipping, and
handling pose a risk, requiring increased
safety regulations.
Chlorine oxidizes certain types of organic matter
in wastewater, creating more hazardous
compounds (e.g., trihalomethanes [THMs]).
The level of total dissolved solids is increased in
the treated effluent.
The chloride content of the wastewater is
increased.
Chlorine residual is unstable in the presence of
high concentrations of chlorine-demanding
materials, thus requiring higher doses to effect
adequate disinfection.
Some parasitic species have shown resistance to
low doses of chlorine, including oocysts of
Cryptosporidium parvum, cysts, of Endamoeba
histolytica and Giardia lamblia, and eggs of
parasitic worms.
Long-term effect of discharging dechlorinated
compounds into the environment are unknown.
DESIGN CRITERIA
When chlorine gas and hypochlorite salts are added
to water, hydrolysis and ionization take place to
form hypochlorous acid (HOC1) and hypochlorite
ions (OC1) also referred to as free available chlorine.
Free chlorine reacts quickly with ammonia in non-
nitrified effluents to form combined chlorine,
principally monochloramine, which actually is the
predominant chlorine species present.
Chlorination
Figure 1 shows a flow chart of the Chlorination
process using liquid and gaseous chlorine. For
optimum performance, a chlorine disinfection
system should display plug flow and be highly
turbulent for complete initial mixing in less than
one second. The goal of proper mixing is to
enhance disinfection by initiating a reaction between
the free chlorine in the chlorine solution stream with
the ammonia nitrogen. This prevents prolonged
chlorine concentrations from existing and forming
other chlorinated compounds.
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Wast
effl
Flow
meter
Chlorine /"
diffuser
Chlorine
contact
basin
Sulfur
dioxide -
diffuser
a water
uent
i Control signal
Lr
,
/
- ^_
~\
1 1njector ^^
J"
^_
1
Dechlorir
efflue
Waste water
effluent
FlOW [*l
meter
Chlorine
induction/ '
mixing unit
(see Fig. 12-42)
Chlorine
contact
basin
Sulfur
dioxide -.
induction
mixing unit
(see Fig. 12-42)
1 » ^
j gas
4
Cc
Chlorine Sj(
residual
analyzer
«-(""!
i r
U i . ^ c
1 * ' * "
fa Drain
Lr* 1 I
T\ Sulfur dioxide gas
1 ^ Injector
Dechlorinatt
effluent
lated
nt
Contro
3d
(a)
Cf
signal
T Vacuum line
-~-_
Chlorine gas
.
1 .
~O
Vx
1 Dechlo
1 effk
Dechlorinated
effluent
Chlorir|e
residual
analyzer
-Fl
» [-,
"V! --
Sulfur dioxide g
"~~ Vacuum line
rinated
lent
(b)
of
Evaporator
(-}
Chlorine v" J /^\~^\
gas ( 1 \
Compressed chlorine
Dntrol 9as storage
jnal
ioxideUgas^aP°ralor
\_) s^t^
Sulfonator f ' J
Liquefied sulfur
dioxide storage
ilorinator
Vacuum line
Chlorine gas ^4-^
( )
Compressed chlorine
gas storage
Control
signal
Vacuum line
Sulfur dioxide gas .-4^.
Sulfonator ( )
us .(V^n,
Liquefied sulfur
dioxide storage
Source: Crites and Tchobanoglous, used with permission from the McGraw-Hill Companies, 1998.
FIGURE 1 A COMPOUND-LOOP CONTROL SYSTEM FOR CHLORINATION WITH
CHLORINE AN DECHLORINATION WITH SULFUR DIOXIDE: (a) INJECTION OF
LIQUID CHLORINE AND (b) INJECTION OF CHLORINE GAS BY INDUCTION
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Another important process that contributes to
optimal disinfection is contact. The contact
chamber should be designed to have rounded
corners to prevent dead flow areas and be baffled
to minimize short-circuiting. This design allows
for adequate contact time between the
microorganisms and a minimal chlorine
concentration for a specific period of time.
The required degree of disinfection can be
achieved by varying the dose and the contact time
for any chlorine disinfection system. Chlorine
dosage will vary based on chlorine demand,
wastewater characteristics, and discharge
requirements. The dose usually ranges from 5 to
20 milligrams per liter (mg/L). Table 2 describes
some of the more common wastewater
characteristics and their impact on chlorine. There
are several other factors that ensure optimum
conditions for disinfection and they include
temperature, alkalinity, and nitrogen-content. All
key design parameters should be pilot tested prior
to full-scale operation of a chlorine disinfection
system.
Dechlorination
After disinfection, chlorine residual can persist in
the effluent for many hours. Most states will not
allow the use of chlorination alone for pristine
receiving waters because of its effect on aquatic
species. To minimize the effect, chlorinated
wastewater must often be dechlorinated.
Dechlorination is the process of removing the free
and combined chlorine residuals to reduce residual
toxicity after chlorination and before discharge.
Sulfur dioxide, sodium bisulfite, and sodium
metabisulfite are the commonly used
dechlorinating chemicals. Activated carbon has
also been used. The total chlorine residual can
usually be reduced to a level that is not toxic to
aquatic life. Chlorination/dechlorination systems
are more complex to operate and maintain than
chlorination systems. Figure 1 shows a schematic
of the chlorination/dechlorination system using
sulfur dioxide.
TABLE 2 WASTEWATER
CHARACTERISTICS AFFECTING
CHLORINATION PERFORMANCE
Wastewater
Characteristic
Effects on Chlorine
Disinfection
Ammonia
Biochemical Oxygen
Demand (BOD)
Hardness, Iron, Nitrate
Nitrite
PH
Total Suspended Solids
Forms chloramines when
combined with chlorine
The degree of
interference depends on
their functional groups
and chemical structures
Minor effect, if any
Reduces effectiveness of
chlorine and results in
THMs
Affects distribution
between hypochlorous
acid and hypochlorite ions
and among the various
chloramine species
Shielding of embedded
bacteria and chlorine
demand
Source: Darby et al., with permission from the Water
Environment Research Foundation, 1995.
PERFORMANCE
Marsh Creek Wastewater Treatment Plant in
Geneva, New York
The Marsh Creek Wastewater Treatment Plant in
Geneva, New York, met stringent state permitting
requirements for residual chlorine and fecal
coliforms by adopting a new chlorine control
strategy. The strategy was devised to monitor the
plant's changing chlorine demand and to feed the
required chlorine by measuring the oxidation
reduction potential (ORP).
After conducting a three-month study, the plant
installed an ORP system to monitor and regulate the
amount of chlorine present in solution. The control
system measured the chlorine demand and regulated
the amount of chlorine needed to achieve and
maintain the ORP setpoint parameters. The system
was calibrated to maintain the total chlorine control
limit between 0.2 and 0.1 mg/L.
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An electrode, placed about 300 feet upstream from
the injection point, measured the ORP, which was
then converted to a 4 to 20 milliampere signal.
Based on the signal, the control system drove the
chlorinator and matched the feed rate to the
changing chlorine demand in the system. A second
electrode was used on the discharge fallout line to
monitor the accuracy of the chlorine control
system.
The treatment plant was then able to meet the fecal
coliform limits and maintain an effluent chlorine
residual of less than 0.25 mg/L. In addition to
meeting the permit requirements, the plant
significantly lowered the chlorine consumption
cost. At the time of the study, it was estimated that
the ORP control system could be paid for in
approximately 30 months due to the reduction in
the chlorine consumption cost.
East Bay Municipal Utility District's
Wastewater Plant in Oakland, California.
The East Bay Municipal Utility District in
Oakland, California, owned and operated a
wastewater treatment plant with a design flow of
310 million gallons per day (mgd), where
chlorination and dechlorination were mandated
parts of the treatment process. With this
requirement, optimizing the dechlorination system
was a critical part in meeting the National Pollution
Discharge Elimination System permit limit of no
chlorine residual during dry and wet weather
operations.
A sodium bisulfite (SBS) system was added as
backup to the dechlorination operation. It
performed very well and kept the plant in
compliance. This system is similar to a typical
liquid chemical addition facility with a storage
system, feed pump, metering system, control valve,
and injection device.
The SBS system was integrated into the overall
dechlorination operation by control set points on
the sulfur dioxide (SO2) residual analyzer and set to
maintain a concentration of 3 to 4 mg/L. The SBS
system is set to kick in at a calculated SO2
concentration of 1.5 mg/L. It is also set to begin
operation when the SO2 leak detection system
automatically shuts off the SO2 feed, or during wet
weather operations when the SO2 demand may
exceed the SO2 system's capacity.
The treatment plant also had to optimize chemical
usage with the continued increase in chemical costs.
The original chlorine dose was 15 mg/L, where 5 to
6 mg/L was consumed with 9 to 10 mg/L as a
residual. The residual chlorine was then gradually
lowered from 9 to 10 mg/L to 3 to 5 mg/L, without
affecting the compliance requirements. This also
resulted in using less SO2 in addition to the
reduction in chlorine usage.
By adopting a strategy to increase the focus on
controlling costs through process optimization, the
treatment plant was able to reduce its chemical costs
by more than 30%.
OPERATION AND MAINTENANCE
A routine O&M schedule should be developed and
implemented for any chlorine disinfection system.
Regular O&M includes the following activities:
Disassembling and cleaning the various
components of the system, such as meters and
floats, once every six months.
Iron and manganese deposits should be removed
with, for example, muriatic acid.
Booster pumps should be maintained.
Valves and springs should be inspected and
cleaned annually.
All manufacturer's O&M recommendations
should be followed.
Equipment must be tested and calibrated as
recommended by the equipment manufacturer.
An emergency response plan for onsite storage of
gaseous chlorine must be developed.
When using chlorine it is very important to properly
and safely store all chemical disinfectants. The
storage of chlorine is strongly dependent on the
compound phase. For further details on the safe use
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and storage of chlorine refer to chemical's Material
Safety Data Sheets. Chlorine gas is normally
stored in steel containers (150-pound or 1-ton
cylinders) and transported in railroad cars and
tanker trucks. Sodium hypochlorite solution must
be stored in rubber-lined steel or fiberglass storage
tanks. Calcium hypochlorite is shipped in drums
or tanker trucks and stored with great care.
COSTS
The cost of chlorine disinfection systems is
dependent on the manufacturer, the site, the
capacity of the plant, and the characteristics of the
wastewater to be disinfected. Hypochlorite
compounds, for example, tend to be more
expensive than chlorine gas. On the other hand
several large cities have switched to hypochlorite,
despite the expense, in order to avoid transporting
chlorine through populated areas. In addition to
the costs incurred by the chlorination process,
some municipalities will also have to consider the
costs of introducing the dechlorination process.
The total cost of chlorination will be increased by
approximately 30 to 50% with the addition of
dechlorination.
Table 3 summarizes the results of a 1995 study
conducted by the Water Environment Research
Federation for secondary effluents from disinfection
facilities at average dry weather flow rates of 1, 10,
and 100 mgd (2.25, 20, and 175 mgd peak wet
weather flow, respectively). The annual O&M costs
for chlorine disinfection include power
consumption, cleaning chemicals and supplies,
miscellaneous equipment repairs, and personnel
costs. The costs associated with the Uniform Fire
Code requirements can be high for small facilities
(as high as 25%).
REFERENCES
1. Crites, R. and G. Tchobanoglous. 1998.
Small and Decentralized Wastewater
Management Systems. The McGraw-Hill
Companies. New York, New York.
2. Darby, J. et al. 1995. Comparison of UV
Irradiation to Chlorination: Guidance for
Achieving Optimal UV Performance. Water
Environment Research Foundation.
Alexandria, Virginia.
TABLE 3 ESTIMATED TOTAL ANNUALIZED COST FOR
CHLORINATION/DECHLORINATION
Flow (mgd)
Estimated Capital Costs ($)
ADWF
1
10
100
1
10
100
1
10
100
PWWF
2.25
20.00
175.00
2.25
20.00
175.00
2.25
20.00
175.00
CI2 Dose
(mg/L)
5
5
5
10
10
10
20
20
20
Chlorination
410,000
1,804,000
10,131,000
441,000
2,051,000
10,258,000
445,000
2,113,500
10,273,000
Dechlorination
290,000
546,000
1,031,000
370,000
664,000
1,258,000
374,000
913,500
1,273,000
UFC*
239,000
264,000
788,000
239,000
264,000
788,000
239,000
264,000
788,000
Total
1,127,000
3,137,000
14,340,000
1,260,000
3,575,000
14,765,000
1,270,000
3,949,000
14,801,000
Estimated
O&M Costs
49,300
158,200
660,000
59,200
226,700
721,800
76,600
379,100
1,311,000
*UFC = Uniform Fire Code (Costs include provisions to meet Article 80 of the 1991 UFC)
ADWF = average dry weather flow PWWF = peak wet weather flow
Source: Darby et al., with permission from the Water Environment Research Foundation, 1995.
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3. Eddington, G. June 1993. "Plant Meets
Stringent Residual Chlorine Limit." Water
Environment & Technology, p. 11-12.
4. Horenstein, B.; T. Dean; D. Anderson; and
W. Ellgas. October 3-7, 1993.
"Dechlorination at EBMUD: Innovative
and Efficient and Reliable." Proceedings
of the Water Environment Federation
Sixty-sixth Annual Conference and
Exposition. Anaheim, California.
5. Metcalf & Eddy, Inc. 1991. Wastewater
Engineering: Treatment, Disposal, and
Reuse. 3d ed. The McGraw-Hill
Companies. New York, New York.
6. Task Force on Wastewater Disinfection.
1986. Wastewater Disinfection. Manual of
Practice No. FD-10. Water Pollution
Control Federation. Alexandria, Virginia.
7. U.S. Environmental Protection Agency
(EPA). 1986. Design Manual: Municipal
Wastewater Disinfection. EPA Office of
Research and Development. Cincinnati,
Ohio. EPA/625/1-86/021.
ADDITIONAL INFORMATION
Joseph Souto
Plant/Sewer Superintendent
Bridgewater Wastewater Treatment Facility
100 Morris Avenue
Bridgewater, MA 02324
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
Bruce Adams
Operations Foreman
City of Cortland Wastewater Treatment Plant
251 Port Watson Street
Cortland, NY 13045
Jim Jutras
Plant Director
Essex Junction Wastewater Facility
2 Lincoln Street
Essex Junction, VT 05452
John O'Neil
Johnson County Wastewater Treatment Facility
7311 W. 130th Street
Overland Park, KS 66213
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
MTB
1
Excellence in compliance through optimal technical sgjutions.
MUNICIPAL TECHNOLOGY B R A"fT
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