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
 Rota virus
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.
     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 has flexible dosing control.        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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          Waste water
            effluent
          Flow
         meter
     Chlorine
      diffuser


     Chlorine
      contact
       basin

       Sulfur
      dioxide
      diffuser
Control signal
                   Chlorinator
1lniector Chlorine gas
J-*



Chlorine
residual
analyzer

-,

1

           Drain
                                                 Chlorine
                                                   gas
                                               Control
                                               signal
                            Sulfur
                          dioxide gas
                                                          Evaporator
                                                        o
                                                             Compressed chlorine
                                                                 gas storage
       Evaporator
                                  o-
                          Sulfonator
         Sulfur dioxide gas
           Injector
            Liquefied sulfur
            dioxide storage
                         Dechlorinated
                           effluent
                 Dechlorinated
                    effluent
              Waste water
               effluent
             Flow
            meter
                                         (a)
       Control signal

       Vacuum line
                             Chlorine gas
        Chlorine
       induction/
       mixing unit
   (see Fig. 12-42)

        Chlorine
         contact
           basin

           Sulfur
         dioxide
        induction
       mixing unit
   (see Fig. 12-42)
                                               Chlorinator
              Chlorine
              residual
              analyzer
                                                           Vacuum line
                                                           Chlorine gas
Control
 signal
                                                                Compressed chlorine
                                                                    gas storage
               Drain
              Sulfur dioxide gas

               Vacuum line
                                     Vacuum line
    Sulfur dioxide gas

  Sulfonator
               Liquefied sulfur
               dioxide storage
                             Dechlorinated
                               effluent
                    Dechlorinated
                       effluent
                                           (b)
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
ADWF
1
10
100
1
10
100
1
10
100
(mgd)
PWWF
2.25
20.00
175.00
2.25
20.00
175.00
2.25
20.00
175.00
Estimated Capital Costs ($)
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.I 30th 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

          IMTB
          Excellence in compliance through optimal technical solutions
          MUNICIPAL  TECHNOLOGY

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