United States
                       Environmental Protection
                       Office of Water
                       Washington, D.C.
EPA 832-F-00-022
September 2000
Waste water
Technology  Fact  Sheet

Dechlorination is the process of removing residual
chlorine from  disinfected  wastewater  prior to
discharge into the environment.  Sulfur dioxide is
most commonly used for dechlorination and is the
major focus of this fact sheet. Some dechlorination
alternatives include carbon adsorption,  sodium
metabisulfite,  sodium  bisulfite, and  hydrogen
peroxide.   Sodium  metabisulfite  and  sodium
bisulfite are mainly used in small facilities because
these  materials are  more  difficult  to  control
compared to sulfur dioxide. Hydrogen peroxide is
not frequently  used because it is  dangerous to
handle (WEF, 1996).


Chlorination has been  used widely to  disinfect
wastewater prior to discharge since passage of the
1972  Federal  Water  Pollution   Control  Act
(WPCA), (Finger et al., 1985).  In the first years
following the WPCA, disinfected wastewater with
significant levels of residual chlorine was routinely
discharged into the receiving waters.  It became
clear, however, that residual chlorine is toxic to
many  kinds of aquatic life  (see,  for example,
Mattice  and Zittel, 1976, and Brungs,  1973).
Moreover, the  reaction of chlorine with organic
materials   in  the  water  formed  carcinogenic
trihalomethanes and organochlorines  (WEF  and
ASCE, 1991).  As  a  result, dechlorination  was
instituted  to  remove  residual chlorine  from
wastewater prior to discharge into sensitive aquatic

Dechlorination minimizes the effect of potentially
toxic disinfection byproducts by removing the free
                      or total combined chlorine residual remaining after
                      chlorination.    Typically,  dechlorination  is
                      accomplished by adding sulfur dioxide or sulfite
                      salts (i.e.,  sodium sulfite, sodium  bisulfite, or
                      sodium metabisulfite). Carbon adsorption is also an
                      effective dechlorination method, but is expensive
                      compared to other methods. Carbon adsorption is
                      usually implemented when total dechlorination is

                      Specific   design   criteria  and   monitoring
                      requirements for a particular region are determined
                      by the state regulatory agency.   Typically, the
                      treatment plant's National Pollutant Discharge
                      Elimination System (NPDES) permit limits effluent
                      chlorine residual and toxicity.   Currently, many
                      permits require very low or "non-detect" chlorine
                      residuals, making dechlorination critical.

                      One important  alternative to dechlorination is to
                      achieve disinfection without the use of chlorine.
                      Other means of disinfection, such  as ozone or
                      ultraviolet  disinfection,   have  also  become
                      increasingly prevalent (U.S. EPA,  1986; Blatchley,
                      E.R. Ill, etal, 1996).



                      •      Protects aquatic life from toxic effects of
                             residual chlorine.

                      •      Prevents formation of harmful chlorinated
                             compounds in  drinking  water  through
                             reaction of residual chlorine with water-
                             born organic materials.

•      Chemical dechlorination can be difficult to
       control when near zero levels  of residual
       chlorine are required.

•      Significant overdosing of sulfite can lead to
       sulfate formation,  suppressed  dissolved
       oxygen  content,  and  lower  pH  of the
       finished effluent.


Chemistry of Dechlorination by Sulfonation

Sulfur dioxide (SO2) is a corrosive, nonflammable
gas  with a  characteristic  pungent  odor.    At
atmospheric  temperature and  pressure, it is a
colorless vapor.  When compressed and cooled, it
forms a colorless liquid. Sulfur dioxide is supplied
as liquefied gas under pressure in 100 or 150 pound
containers and one-ton cylinders. As an alternative
to sulfur dioxide gas,  various dry  chemicals  are
available which  form sulfur  dioxide  in solution.
These  include sodium sulfite (Na2SO3), sodium
metabisulfite  (Na2S2O5),   sodium  bisulfite
(NaHSO3), a 38 percent aqueous solution of sodium
metabisulfite, and  sodium  thiosulfate  (Na2S2O3),
among others (Lind,  1995).

When dissolved  in water, chlorine  hydrolyzes to
form hypochlorous acid (HOC1) and hypochlorite
ions (OC1* ) which, taken together, are referred to as
"free chlorine." (Free, uncombined chlorine, C12, is
rarely found in wastewater since the conditions of
formation are relatively extreme  [Lind,  1995]).
Once formed, the free chlorine reacts with natural
organic matter in  water  and wastewater to form
chlorinated organic compounds. The free chlorine
also combines with ammonia to form mono-, di-,
and trichloramines in quantities dependent on the
ratio of chlorine to  ammonia nitrogen (Lind,  1995).

When  either sulfur  dioxide  or sulfite salts  are
dissolved in water, aqueous sulfur compounds in
the +4 oxidation state are produced, often notated
S(IV) (Helz andNweke, 1995). The S(IV) species,
such as the sulfite ion (SO3"2), reacts with both free
and  combined forms of chlorine, as illustrated in
equations (1) and (2) (WEF, 1996):
(2) SO3'2 + NH2C1 + H20 •   S
ventilated, temperature-controlled area so that their
temperature never drops below 18 or exceeds 70
degrees Celsius. Gas leak detectors are necessary in
the storage area and  the  sulfonator area.  An
emergency  eyewash  shower  and self-contained
breathing apparatus should also be provided. All
personnel   should receive  emergency  response
training. Facilities with more than 1,000 pounds of
SO2 stored on-site must abide by  the Process
Management  Safety   Standard in  the  OSHA
regulations (OSHA, 1998).

Effect of Temperature on Gas Withdrawal Rate

The room  temperature where the gas  supply is
located should be maintained around  70 degrees F
to  ensure  optimal  gas withdrawal  rates (WEF,
1996).  At this temperature, the maximum safe
sulfur dioxide gas withdrawal rate is approximately
2 Ib/hr for a 150 Ib container, or 25 Ib/hr for a ton
container.   Higher temperatures are required to
achieve higher continuous  gas withdrawal  rates.
Strip heaters or liquid  baths may be  used for this

Injector Selection

Proper selection of the injector is critical for proper
system operation. The  injector produces a vacuum
that  draws  sulfur  dioxide  gas   through the
sulfonator.  It then mixes the gas with dilution
water supply  and  injects  the  solution  into the
wastewater. To properly size the injector, the back
pressure on the injector at the point of application
and the water supply pressure required  at the
injector must  be determined.  The  injector can
either be installed in a pipe or an open channel.  As
an alternative to the typical vacuum regulator with
injector system, a chemical induction system may
be used to introduce the sulfur dioxide directly as a


At present, few options exist for reliable long-term
measurement of sulfite salts or close-to-zero levels
of residual  chlorine in the finished effluent (ASCE
and WEF, 1991).  In recent practice, the only viable
method   for   continuous   residual  chlorine
measurement has been the amperometric technique,
but this suffers from loss of accurate calibration at
low concentrations (Finger et al., 1985).  Though
some sources  claim  to  have developed  process
control methods employing oxidation reduction
potential (ORP) as  an effective stand-in for direct
chlorine measurement (Bossard et al., 1995), other
sources  assert  that  ORP  is  an   inappropriate
technique for this purpose (WEF, 1996). For these
reasons, control of  dechlorination—particularly
dechlorination  to   zero  residual—has  been
problematic. Treatment plant operators have had to
work around this limitation using various control

One commonly used strategy is the use of a "zero-
shifted" or "biased" analyzer (WEF, 1996; Nagel,
1994).  In this scheme, a residual chlorine analyzer
is used and a known concentration, X, of chlorine
is added to the effluent sample to be analyzed.  In
this technique, the  "zero" point is  shifted by the
value of X, and residual chlorine or sulfur dioxide
can be inferred from the result of sample analysis.

Two types of control systems are often used for
dechlorination  (WEF,  1996).   A  "feed-back"
control  system can be used at treatment plants that
are not required to dechlorinate their effluents to
zero levels. With feed-back control, an analyzer
measures   the  chlorine  residual   at  a  point
downstream of the sulfur dioxide addition. This
signal, along with flow rate data, is relayed back to
the sulfonator and  the  dosage is   automatically
adjusted accordingly.  Though there is a lag time
between the injection point and the  sample point,
the lag is deemed  to   be  minimal  since  the
dechlorination   reaction   occurs   almost

For treatment  plants that must discharge a zero
concentration or undetectable residual chlorine but
are not equipped with biased or direct  reading
analyzers, feed-back control is typically not feasible
(WEF,  1996).   Therefore,  such  dechlorination
systems often  use  a  "feed-forward" control that
measures the chlorine residual after disinfection but
prior to the addition of sulfur dioxide.  A mass flow
signal  is sent to the sulfonator from the in-line
analyzer and  the sulfur  dioxide delivery  rate is
automatically calculated and  adjusted to the ratio
required for proper dechlorination of the effluent.

Instrumentation  combining feed forward  control
with biased  analysis may provide  an effective
method to dechlorinate the effluent to low-level
residuals.  Another useful design feature involves
using an  automated chlorine flush of the  sample
line (with  sulfonator response temporarily locked).
This procedure prevents the buildup of slime and
algae in the sample line, thus eliminating chlorine
demand in the line  which can suppress residual
chlorine analyzer results (Nagel, 1994).

Sample Calculation

Given:        Peak  flow  =  20 mgd.  Measured
              chlorine  residual is approximately

Calculate:     Maximum dosage of sulfur dioxide
              needed per day.  Assume a 1:1 ratio

Capacity =    (flow  rate)  x  (C12 residual) x
              (dosage ratio)  = (20 mgd) x  (1.5
              mg/L   C12)    x      (8.34
              lb/Mgal.mg/L)x(l mg/L SO2  per
              mg/L  C12) = 250 Ib SO2 per  day

Using Sulfite Salts

Upon dissolution,  sulfite salts produce the same
sulfite  ion as sulfur dioxide gas  (WEF,  1996).
While  the gas  has  the  highest  dechlorinating
efficiency per net pound of the product added, many
smaller facilities choose to use one of the sulfite
salts because of the storage, handling, feeding, and
safety  problems associated  with  using gaseous
sulfur dioxide on a large scale.   Of all the sulfite
salts available, sodium metabisulfite has the lowest
addition rate required for dechlorination (Lind,


Sulfonation has been widely considered effective
for removal of chlorine compounds in disinfected
wastewater and  reduction of toxicity for aquatic
life.  Nevertheless,  two studies have suggested that
disinfected/sulfonated wastewater poses a hazard to
some sensitive aquatic species (Hall et a/., 1982;
Rein et a/., 1992).  Furthermore, one estimation of
chlorine removal efficiency is from 87 to 98 percent
(Helz andNweke, 1995), leaving the actual residual
chlorine  following  sulfonation    above  most
regulatory limits.

Chloramines  tend to be  longer  lived and  less
reactive  than   other   chlorinated  species   in
wastewater  (Lind,  1995).   While hydrophilic
organic  chloramines  have been  thought  of as
generally nontoxic, Helz and Nweke have  found
that the S(IV) fraction resistant to dechlorination
maybe composed of hydrophobic secondary amines
and  peptides,  including chloramines,  suggesting
possible toxicity for aquatic organisms in receiving
streams.   The authors note that  this fraction of
S(IV)-resistant   chlorine  has been overlooked
because the dechlorinating agent interferes with
standard analytical methods for total chlorine (Helz
andNweke, 1995).  Continued testing is underway
to further characterize the dechlorination-resistant
fraction and its effects on aquatic organisms (Helz,


Components of the pressure manifold—especially
flexible  connectors, valves, and the injector  and
solution system—are the most likely to need repair
(WEF, 1996).  In view of this, these components
should be  inspected at least every  six  months.
Additionally,  diaphragms  and injector gaskets
should be replaced every two years. The gasket
should be replaced each time the joint is broken in
a gasketed pressure connection.  Asbestos fiber
gaskets are not recommended because they  often
do not seal properly.  Used gaskets should never be
re-used.  Springs should be replaced according to
the manufacturer's instructions.

Spare parts—especially  parts  for the pressure
manifold—and standby equipment, should be kept
on hand  to prevent significant down time in the
event of equipment problems.  Because sulfonators
are configured  with the  same  components as
chlorinators, some  plant engineers tend to treat
sulfonators as standby chlorinators (WEF, 1996),
but this  practice should be avoided.  Chlorinators
and sulfmators are composed of different polymeric
materials  (sulfmators  typically  of  PVC   and

chlorinators of  ABS  plastic), each  chosen  for
application-specific chemical resistance.   Use of
non-chemical resistant materials with chlorine or
sulfur dioxide gases can lead to equipment failure.
Moreover,  equipment misuse leading to accidental
mixing of  chlorine and sulfur  dioxide gases can
lead to  an exothermic chemical  reaction  and
equipment failure.

Water  can  be used  to clean most component
surfaces. For buildup of impurities or for stains, a
dilute hydrochloric (muriatic) acid solution may be
necessary (WEF, 1996).  Following cleaning, the
components must be thoroughly dried before they
are reassembled.   Drying  is best  done using
compressed dry air or nitrogen.


Costs for  a dechlorination  system  vary widely
depending  on particular site conditions.   Detailed
estimates are not included here because they may be
misleading.   One of the  largest overall  cost
variables is whether an existing facility  is being
upgraded   to  accommodate  dechlorination  or
whether the chlorination/ dechlorination system is
a component  of new plant construction.   Site-
specific construction costs can vary by hundreds of
thousands  of dollars,  depending  on  the type of

Table 1 gives  a cost example  of a 2,800 ppd gas
sulfur dioxide  system installed in  1994 in Florida.
Cost considerations must also include the cost of
equipment, installation, labor, and operation and
maintenance. The type of dechlorination agent to
be used will affect both the equipment and chemical
costs.   For large plants, sulfur  dioxide gas is
typically  the  agent  of choice  because of  its
dechlorinating efficiency on a per pound basis.
However, as previously mentioned,  many smaller
plants may find that storage, handling, and safety
issues offset whatever gains in efficiency  can be
obtained by using compressed gas.

Effective process control will help prevent chemical
overdosing and allow for  chemical cost savings.
One 40-mgd facility has reported  a savings of
$7,000 per month—a greater than  1,000 pound per
day reduction in SO2 use (>50%) and a greater than
Cost ($)
 Sulfonator and ancillary
 mechanical equipment

 Gas Scrubber

 Ventilation System

 Miscellaneous piping and valves

 Building & Architectural items




 Contractor Overhead (@6.5%)

 Total (In 1993 dollars)	









 Source: Parsons Engineering Science, Inc., 2000.

one-third reduction in chlorine use—resulting from
the purchase of  a new automatic process control
system  (Bossard et  a/.,  1995).   The  savings
represented a three  month payback period on the
new equipment.


Other Fact Sheets

Chlorine Disinfection
EPA 832-X-99-062
September 1999

Other  EPA  Fact  Sheets can  be  found at the
following web address:

1.      Blatchley, E.R.,  III;  Bastian,  K.C.;  and
       Duggirala,   R.K.,  1996.     Ultraviolet
       Irraditation and Chlorination/Dechlorination
       for Municipal  Wastewater Disinfection:
       Assessment  of Performance Limitations.
       Water Environment Research. 68:194-204.

      Bossard,G.;Briggs,G.; and Stow, T., 1995.    11.
      Optimizing Chlorination/Dechlorination at
      a  Wastewater Treatment Plant.  Public
      Works.  126 (January): 33-5.

      Brungs, W.A., 1973.  Effects of Residual
      Chlorine on Aquatic Life.  Journal of the
      Water Pollution  Control Federation.  45:
      2180.                                     12.

      Finger, R.E.; Harrington,  D.; and Paxton,
      L.A.  Development of an On-Line Zero    13.
      Chlorine  Residual   Measurement  and
      Control  System.   Journal  of the  Water
      Pollution Control Federation.   57: 1068-
      Hall, L.W., Jr.; Burton, D.R.; Graves, W.C.;
      and Margery, S.L., 1992.  Environmental
      Science and Technology.  15:573-78.

      Helz,  G.R.,  1998.    Maryland  Water
      Resources Research Center Annual Report.    15.
      Prepared  for the U.S. Department  of the
      Interior   Geological   Survey  by  the
      University of Maryland Water Resources
      Research Center, College Park, Maryland.
Helz,  GR.  and  Nweke,  A.C.,  1995.
Incompleteness   of   Wastewater
Dechlorination. Environmental Science and
Technology. 29: 1018-1022.

Lind, C., 1995. Wastewater Dechlorination
Options.  Public Works. 126 (September):
Mattice, J.D. and Zittel, H.E., 1976.  Site-
Specific  Evaluation  of  Power  Plant    18.
Chlorination.    Journal  of  the   Water
Pollution Control Federation.  48: 2284.

Nagel,   W.H.,   1994.      Controlled
Dechlorination  Achievable  with   New
Systems.     Water   Engineering  and
Management. February: 24-25.
OSHA (Occupational  Safety  and Health
Administration, U. S. Department of Labor),
1998.   Process Safety Management of
Highly Hazardous Chemicals.  29  CFR
1910.119.      Internet   site   at
cfir-table-search.html, accessed June 1999.

Parsons Engineering Science, Inc., 2000.
Personal Communication.

Plant,  L.,  1994.   Hydrogen  Peroxide
Applications   for  Dechlorination  of
Wastewater.    National  Environmental
Journal. 4: 26-29.

Rein, D.A.; Jamesson, G.M.; andMonteith,
R.A.,  1992.    In  Water Environment
Federation 65th Annual Conference  and
Exposition, pp 461-71.  Alexandria, VA:
Water Environment Federation.

U.S.  EPA,  1986.    Design  Manual:
Municipal  Wastewater   Disinfection.
EPA/625/1-86/021, U.S. EPA, Cincinnati,

Water  Environment  Federation,  1996.
Wastewater Disinfection:    Manual  of
Practice  No.  FD-10.   Alexandria,  VA:
Water Environment Federation.

Water  Environment  Federation,  1996.
Operation  of  Municipal  Wastewater
Treatment  Plants,  5th  edition., MOP 11.
Alexandria, VA:  Water  Environment

Water  Environment Federation  and the
American Society of Civil Engineers, 1991.
Design ofMunicipal Wastewater Treatment
Plants.  2 vols.  WEF  Manual of Practice
No.   8.     Alexandria,   VA:   Water
Environment Federation.


Water Environment Federation
601 Wythe Street
Alexandria, Virginia 22314-1994

American Society of Civil Engineers
World Headquarters
1801 Alexander Bell Drive
Reston, Virginia 20191-4400

Water and Wastewater Equipment Manufacturers
Association, Inc.
Dawn Kristof, President
P.O. Box 17402
Washington, D.C. 20041

The  mention  of trade  names  or  commercial
products  does  not constitute  endorsement  or
recommendation for use by the U.S. Environmental
Protection Agency.
                                                         For more information contact:

                                                         Municipal Technology Branch
                                                         U.S. EPA
                                                         Mail Code 4204
                                                         1200 Pennsylvania Avenue, NW
                                                         Washington, D.C. 20460
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