United States Environmental Protection Agency Office ofWater Washington, D.C. EPA 832-F-99-062 September 1999 

# &EPA Wastewater Technology Fact Sheet 

# Chlorine Disinfection

DESCRIPTION The impact ofuntreated domestic wastewater on communityreservoirshas raised severalhealthand 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 withthem. Inresponseto these concerns, disinfection has become one of the primary mechanisms for the inactivation/destruction ofpathogenic 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 organismsby oxidizing cellularmaterial. 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 atreatment 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. Absenceof toxicresiduals andmutagenicor 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 Rotavirus 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 ofthis product.
Advantages


### •

• Chlorination IS a well-established technology. Presently, chlorine is more cost-effective thaneitherUV orozone disinfection (except whendechlorinationisrequiredandfire code requirements must be met).

• The chlorine residual that remains in the wastewatereffluentcanprolong disinfection even after initial treatment and can be measured to evaluate the effectiveness.


### •

•

• Chlorinedisinfectionisreliableandeffective against a wide spectrum of pathogenic organisms. Chlorine is effective in oxidizing certain organic and inorganic compounds. Chlorination has flexible dosing control.

• Chlorinecaneliminatecertainnoxiousodors 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 certaintypes oforganicmatter in wastewater, creating more hazardous compounds (e.g., trihalomethanes [THMs]).

• The level oftotal 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 shownresistance to low doses of chlorine, including oocysts of Cryptosporidiumparvum, cysts, ofEndamoeba 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) alsoreferredto asfree availablechlorine. 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 enhancedisinfectionbyinitiatingareactionbetween thefree chlorineinthechlorine solutionstreamwith 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

Vacuum line Chlorine

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 oftime. 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 andtheirimpact 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 ofits effect on aquatic species. To minimize the effect, chlorinated wastewater must often be dechlorinated. Dechlorination is the process ofremoving the free and combinedchlorineresidualsto reduceresidual 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 installedan ORP systemto monitorandregulatethe amount ofchlorine presentin solution. The control systemmeasuredthe chlorinedemandandregulated the amount of chlorine needed to achieve and maintain the ORP setpointparameters. The system was calibratedto maintainthetotal chlorine control limit between 0.2 and 0.1 mg/L.Anelectrode, placed about 300 feet upstreamfrom the injectionpoint, measuredthe 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 changingchlorinedemandinthe system. A second electrode was used onthe discharge fallout line to monitor the accuracy of the chlorine control system. The treatmentplantwas then ableto meetthe 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. Atthetime ofthe study, itwas estimatedthat 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, optimizingthe dechlorination system wasacriticalpartinmeetingtheNationalPollution Discharge Elimination System permit limit ofno 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,feedpump,metering system, controlvalve, and injection device. The SBS system was integrated into the overall dechlorination operation by control set points on the sulfurdioxide (S02 ) residual analyzerand setto 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 offthe 

## 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 usagewiththe continuedincreaseinchemicalcosts. The original chlorine dose was 15 mg/L, where 5to 6 mg/L was consumed with 9 to 10 mg/L as a residual. The residual chlorine was then gradually lowered from 9to 10 mg/L to 3to 5mg/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 treatmentplantwas ableto reduce its chemicalcosts 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 ofthe system, such as. meters and floats, once every six months.

• Iron andmanganese deposits shouldberemoved with, for example, muriatic acid.

• Boosterpumps 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.

• Anemergencyresponseplanforonsite storageof gaseous chlorine must be developed. Whenusing chlorine itis very importantto properly and safely store all chemical disinfectants. The storage of chlorine is strongly dependent on the compoundphase. Forfurther details onthe safe use

### and storageofchlorinereferto chemical'sMaterial 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 inrubber-lined steel orfiberglass 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 ofthe plant, and the characteristics ofthe 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, inorder to avoidtransporting chlorine through populated areas. In addition to the costs incurred by the chlorination process, some municipalities will also have to considerthe costs of introducing the dechlorination process. The total cost ofchlorination 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 Federationfor secondaryeffluentsfromdisinfection facilities at average dry weather flow rates ofI, 10, and 100 mgd (2.25, 20, and 175 mgd peak wet weatherflow, respectively). The annual O&Mcosts 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 ofUV 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 ofthe 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 StringentResidual ChlorineLimit." 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. WastewaterDisinfection. Manualof 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 Bruce Adams Operations Foreman City ofCortland 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 Avenue Bridgewater, MA 02324 Thementionoftradenamesorcommercialproducts doesnotconstituteendorsementorrecommendation 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

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