United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-00-022

September 2000

Wastewater

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. III, et al., 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 waterborn organic materials.
Disadvantages

• 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 (HOCl) and hypochlorite

ions (OClG) which, taken together, are referred to as

“free chlorine.” (Free, uncombined chlorine, Cl2, 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 and Nweke, 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):

(1) SO3

-2 + HOCl ÿ SO4-2 + Cl-

 + H+

(2) SO3

-2 + NH2Cl + H20 ÿ SO4-2 + Cl-

 + NH4

+

Since free chlorine and inorganic chloramines react

rapidly with S(IV) (Helz, 1998), a short contact

time of one to five minutes is considered to be

sufficient; nevertheless, complete blending at the

point of application is essential for effective

dechlorination (WEF and ASCE, 1992).

Proper dosage is critical to produce a nondetectable chlorine residual. On a mass basis, 0.9

parts sulfur dioxide (or 1.46 parts NaHSO3 or 1.34

parts Na2S2O5) is required to dechlorinate 1.0 part

residual chlorine. In practice, approximately a oneto-one ratio is used (WEF, 1996). Dosing in excess

must be avoided because excess sulfite can react

with dissolved oxygen (four parts sulfite to one part

oxygen) in the wastewater to produce sulfates,

potentially leading to reduced dissolved oxygen

concentrations and low pH levels in the finished

effluent for high levels of overdose (WEF, 1996).

Careful process control will help prevent

overdosing.

Equipment for gaseous sulfonation

Equipment required for gaseous sulfonation using

SO2 is similar in design to that used for

chlorination, except that the materials are chosen

for their application-specific chemical resistance

(WEF, 1996). The four basic components of the

system include: sufficient gas supply with

automatic switch-over between cylinders; a

metering system, usually consisting of a vacuum

regulator and a rotameter for feed rate control; one

or more injectors with check valves; and a residual

analyzer to measure and transmit a continuous

signal proportional to the chlorine residual in the

sample stream.

In small concentrations, exposure to SO2 can cause

eye and throat irritation. In high concentrations,

exposure can produce a suffocating effect caused by

irritation to the upper respiratory tract. Therefore,

facility design should include features for safe

storage, handling, and use of sulfur dioxide. The

chlorine and sulfur dioxide cylinders should be

located in separate rooms and stored in a well-
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 lb/hr for a 150 lb container, or 25 lb/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 “zeroshifted” 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.
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

1.5 mg/L.

Calculate: Maximum dosage of sulfur dioxide

needed per day. Assume a 1:1 ratio

of SO2 to Cl2.

Capacity = (flow rate) x (Cl2 residual) x

(dosage ratio) = (20 mgd) x (1.5

mg/L Cl 2) x (8.34

lb/Mgal.mg/L)x(1 mg/L SO2 per

mg/L Cl2) = 250 lb 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 al., 1982;

Rein et al., 1992). Furthermore, one estimation of

chlorine removal efficiency is from 87 to 98 percent

(Helz and Nweke, 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

may be 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

and Nweke, 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 sulfinators are composed of different polymeric

materials (sulfinators 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

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. Sitespecific 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

one-third reduction in chlorine use—resulting from

the purchase of a new automatic process control

system (Bossard et al., 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.

TABLE 1 COST OF A 2,800 PPD GAS

SULFUR DIOXIDE SYSTEM IN 1994

Component Cost ($)

Sulfonator and ancillary

mechanical equipment 100,000

Gas Scrubber 150,000

Ventilation System 45,000

Miscellaneous piping and valves 30,000

Building & Architectural items 250,000

Electrical 75,000

Instumentation 30,000

Subtotal 680,000

Contractor Overhead (@6.5%) 44,200

Total (In 1993 dollars) 724,200

Source: Parsons Engineering Science, Inc., 2000.
2. Bossard, G.; Briggs, G.; and Stow, T., 1995.

Optimizing Chlorination/Dechlorination at

a Wastewater Treatment Plant. Public

Works. 126 (January): 33-5.

3. Brungs, W.A., 1973. Effects of Residual

Chlorine on Aquatic Life. Journal of the

Water Pollution Control Federation. 45:

2180.

4. Finger, R.E.; Harrington, D.; and Paxton,

L.A. Development of an On-Line Zero

Chlorine Residual Measurement and

Control System. Journal of the Water

Pollution Control Federation. 57: 1068-

1073.

5. Hall, L.W., Jr.; Burton, D.R.; Graves, W.C.;

and Margery, S.L., 1992. Environmental

Science and Technology. 15: 573-78.

6. Helz, G.R., 1998. Maryland Water

Resources Research Center Annual Report.

Prepared for the U.S. Department of the

Interior Geological Survey by the

University of Maryland Water Resources

Research Center, College Park, Maryland. 

7. Helz, G.R. and Nweke, A.C., 1995.

Incompleteness of Wastewater

Dechlorination. Environmental Science and

Technology. 29: 1018-1022.

8. Lind, C., 1995. Wastewater Dechlorination

Options. Public Works. 126 (September):

69-70.

9. Mattice, J.D. and Zittel, H.E., 1976. SiteSpecific Evaluation of Power Plant

Chlorination. Journal of the Water

Pollution Control Federation. 48: 2284.

10. Nagel, W.H., 1994. Controlled

Dechlorination Achievable with New

Systems. Water Engineering and

Management. February: 24-25.

11. 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/

cfr-table-search.html, accessed June 1999.

12. Parsons Engineering Science, Inc., 2000. 

Personal Communication.

13. Plant, L., 1994. Hydrogen Peroxide

Applications for Dechlorination of

Wastewater. National Environmental

Journal. 4: 26-29.

14. Rein, D.A.; Jamesson, G.M.; and Monteith,

R.A., 1992. In Water Environment

Federation 65th Annual Conference and

Exposition, pp 461-71. Alexandria, VA:

Water Environment Federation.

15. U.S. EPA, 1986. Design Manual:

Municipal Wastewater Disinfection.

EPA/625/1-86/021, U.S. EPA, Cincinnati,

OH.

16. Water Environment Federation, 1996.

Wastewater Disinfection: Manual of

Practice No. FD-10. Alexandria, VA:

Water Environment Federation.

17. Water Environment Federation, 1996.

Operation of Municipal Wastewater

Treatment Plants, 5

th edition, MOP 11.

Alexandria, VA: Water Environment

Federation.

18. Water Environment Federation and the

American Society of Civil Engineers, 1991.

Design of Municipal Wastewater Treatment

Plants. 2 vols. WEF Manual of Practice

No. 8. Alexandria, VA: Water

Environment Federation.
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

1200 Pennsylvania Avenue, NW

Washington, D.C. 20460

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.