v°/EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-022
September 2000
Waste water
Technology Fact Sheet
Dechlorination
DESCRIPTION
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).
APPLICABILITY
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
waters.
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
desired.
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).
ADVANTAGES AND DISADVANTAGES
Advantages
• 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.
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Disadvantages
(1)
• 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.
DESIGN CRITERIA
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
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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
purpose.
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
gas.
Control
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
strategies.
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
instantaneously.
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.
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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
1.5mg/L.
Calculate: Maximum dosage of sulfur dioxide
needed per day. Assume a 1:1 ratio
ofSO2toC!2.
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
maximum.
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,
1995).
PERFORMANCE
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,
1998).
OPERATION AND MAINTENANCE
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
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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
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
project.
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
$7,000 per month—a greater than 1,000 pound per
day reduction in SO2 use (>50%) and a greater than
TABLE 1 COST OF A 2,800 PPD GAS
SULFUR DIOXIDE SYSTEM IN 1994
Component
Cost ($)
Sulfonator and ancillary
mechanical equipment
Gas Scrubber
Ventilation System
Miscellaneous piping and valves
Building & Architectural items
Electrical
Instumentation
Subtotal
Contractor Overhead (@6.5%)
Total (In 1993 dollars)
100,000
150,000
45,000
30,000
250,000
75,000
30,000
680,000
44,200
724,200
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.
REFERENCES
Other Fact Sheets
Chlorine Disinfection
EPA 832-X-99-062
September 1999
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
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.
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9.
10.
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-
1073.
14.
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.
16.
17.
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):
69-70.
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
http://www.access.gpo.gov/nara/cfr/
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,
OH.
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
Federation.
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.
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ADDITIONAL INFORMATION
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
MTB
Excelence fh compliance through optimal technical solJttons
MUNICIPAL TECHNOLOGY BRANCH
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