United States 

Environmental Protection 

Agency 

Office of Water 

Washington, D.C. 

EPA 832-F-99-062 

September 1999 

&EPA Wastewater 

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 

Bacteria 

Escherichia coli 

Leptospira (spp.) 

Salmonella typhi 

Salmonella (=2100 serotypes) 

Shigella (4 spp.) 

Vibrio cholerae 

Protozoa 

Balantidium coli 

Cryptosporidium parvum 

Entamoeba histolytica 

Giardia Iamblia 

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 

Disease Caused 

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. 
• 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 Iamblia, 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 (HOCl) and hypochlorite 

ions (OCl) also referred to as free available chlorine. 

Free chlorine reacts quickly with ammonia in nonnitrified 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. 
Wastewater 

effluent 

Chlorine 

diffuser 

Chlorine 

contact 

basin 

Sulfur 

dioxide 

diffuser 

Control signal - - - - - - - - - - - --- --. Chlorinator 

I 

Injector Chlorine gas r -

I 

Dechlorinated 

effluent 

I 

I I 

: I 

1 i Control 

Chlorine 1 1 signal residual 1 1 

analyzer : 1 ___ I ___ _! 

-, I 

I ~- 1 

Drain ---

(a) 

Wastewater 

effluent 

Chlorine 

induction/ 

mixing unit 

(see Fig. 12-42) 

Chlorinator 

Control signal 

-·---------·------------~--

Vacuum line 

Chlorine gas 

Chlorir1e 

residual 

analyzer 

:Control 

: signal 

____ I .. ____ .. I 

I I 

I I 

Compressed chlorine 

gas storage 

Liquefied sulfur 

dioxide storage 

Vacuum line 

Chlorine gas 

Compressed chlorine 

gas storage 

Chlorine Vacuum line 

contact 

basin Sulfur dioxide gas 

Sulfur 

dioxide 

induction 

mixing unit 

(see Fig. 12-42) 

Vacuum line 

Dechlorinated 

effluent 

Dechlorinated 

effluent 

(b) 

Sulfonator 

Liquefied sulfur 

dioxide storage 

Source: Crites and Tchobanoglousl used with permission from the McGraw-Hill Companies, 1998. 

FIGURE 1 A COMPOUND-LOOP CONTROL SYSTEM FOR CHLORINATION WITH 

CHLORINE AN DECHLORINATION WITH SULFUR OIOXIDE: (a) INJECTION OF 

LIQUID CHLORINE AND (b) INJECTION OF CHLORINE GAS BY INDUCTION 
Another important process that contributes to 

optimal disinfection is contact. The contact 

chamber should be designed to have rounded 

comers 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 

Ammonia 

Biochemical Oxygen 

Demand (BOD) 

Hardness, Iron, Nitrate 

Nitrite 

pH 

Total Suspended Solids 

Effects on Chlorine 

Disinfection 

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. 
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 m 

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 (S02) residual analyzer and set to 

maintain a concentration of3 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 S02 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 
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 I, 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 PWWF Cl2 Dose Chlorination Dechlorination UFC* Total Estimated 

(mg/L) O&M Costs 

2.25 5 410,000 290,000 239,000 1,127,000 49,300 

10 20.00 5 1,804,000 546,000 264,000 3,137,000 158,200 

100 175.00 5 10,131,000 1,031,000 788,000 14,340,000 660,000 

2.25 10 441,000 370,000 239,000 1,260,000 59,200 

10 20.00 10 2,051,000 664,000 264,000 3,575,000 226,700 

100 175.00 10 10,258,000 1,258,000 788,000 14,765,000 721,800 

2.25 20 445,000 374,000 239,000 1,270,000 76,600 

10 20.00 20 2,113,500 913,500 264,000 3,949,000 379,100 

100 175.00 20 10,273,000 1,273,000 788,000 14,801,000 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. 
3. 

4. 

Eddington, G. June 1993. "Plant Meets 

Stringent Residual Chlorine Limit." Water 

Environment & Technology. p. 11-12. 

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. EP A/625/1-86/021. 

ADDITIONAL INFORMATION 

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 

Joseph Souto 

Plant/Sewer Superintendent 

Bridgewater Wastewater Treatment Facility 

100 Morris A venue 

Bridgewater, MA 02324 

The mention oftrade 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 

401 M St., S.W. 

Washington, D.C., 20460 

~MTB