Disinfection of municipal wastewater is necessary for safe potable water supplies and for healthy rivers and streams. Microorganisms are present in large numbers in sewage treatment plant effluents and waterborne disease outbreaks have been associated with sewage-contaminated water supplies or recreational waters.(1-11)

Chlorination is by far the most common method of wastewater disinfection and is used worldwide for the disinfection of pathogens before discharge into receiving streams, rivers or oceans.(11-14) Chlorine is known to be effective in destroying a variety of bacteria, viruses and protozoa, including Salmonella, Shigella and Vibrio cholera.

Wastewater chlorination was initially applied in 1910 in Philadelphia, PA, and was soon implemented in many other cities in the United States based on this early success.(11)

Today, wastewater chlorination is widely practiced to reduce microbial contamination and potential disease risks to exposed populations.

There is a water use cycle in which drinking water is treated, then consumed and discharged as wastewater. Following additional treatment, wastewater is discharged and may enter source waters used for drinking and recreation. Then the treatment-use-discharge process begins again, continuing the water use cycle.

Risks From Wastewater Contamination

Wastewater can be discharged from a treatment plant into the environment where human exposure may occur through the potable (drinking) water supply, recreation (swimming, snorkeling, etc.) or eating shellfish.

Pathogens commonly found in wastewater effluents are E. coli, Streptococcus, Salmonella, Shigella, mycobacteria, Pseudomonas aeroginosa, Giardia lamblia and enteroviruses. Tacnia, Ascaris and hookworm ova may be present in raw sewage. All of these microorganisms can make people sick.

The US Centers for Disease Control and Prevention has recorded a number of cases of shigellosis outbreaks caused by the consumption of freshwater shellfish harvested from waters contaminated by wastewater effluent.(15) Natural decomposition processes would normally reduce these pathogens due to decay, predation and dilution. However, increasing human populations and discharge of effluent into receiving waters have limited the natural capability of self purification, making it necessary to disinfect the effluents before they are discharged.

Disinfection to remove or inactivate microorganisms is the most important step in wastewater treatment to prevent downstream users from contracting waterborne infectious diseases caused by microbes traditionally present in wastewaters. For example, wastewater disinfection helps prevent the accumulation of toxic microorganisms in fish, shellfish and other aquatic organisms. And, where sources of drinking water supplies may be contaminated by wastewater effluent, the importance of applying disinfection both upstream and downstream cannot be overstated.

The application of chlorine or any chemical disinfectant to wastewater results in the formation of by-products. The nature of biological treatment (prior to chlorination) and the presence of ammonia may have a substantial impact on the extent of by-product formation. Dechlorination of excess chlorine normally is performed prior to discharge to help prevent harm to aquatic life and also to reduce the formation and impact of disinfection by-products.

Chlorine—The Effective Disinfectant

Chlorination plays a key role in the wastewater treatment process by removing pathogens and other physical and chemical impurities. Chlorine's important benefits to wastewater treatment are listed below:

• Disinfection
• Controlling odor and preventing septicity
• Aiding scum and grease removal
• Controlling activated sludge bulking
• Controlling foaming and filter flies
• Stabilizing waste activated sludge prior to disposal
• Foul air scrubbing
• Destroying cyanides and phenols
• Ammonia removal

Safe Handling and Use

Chlorine can be used in wastewater disinfection as either elemental chlorine (gas) or as a chlorinated compound such as liquid sodium hypochlorite solution or solid calcium hypochlorite. Elemental chlorine is generally the most cost-effective option, but other factors must also be considered. In weakly buffered waters, for example, elemental chlorine depresses the pH, while the hypochlorites slightly raise the pH. The speed and efficacy of chlorine disinfection against pathogens may be affected by the pH of the water being treated.

As with all chemicals, the use of chlorine requires safe handling. Several publications are available that outline handling precautions, recommendations and safety practices. These include The Chlorine Institute's "The Chlorine Manual," "Chlorine Safety for Water and Wastewater Operators" and the American Water Works Association's "Safety Practices for Water Utilities."

Chlorination of Drinking Water

Chlorine's effectiveness as a disinfectant also has been demonstrated by its use in drinking water treatment for a century. The beneficial effects of drinking water chlorination to reduce and remove microorganisms were soon applied to wastewater disinfection. Considered one of the most significant public health advances in the 20th century, drinking water chlorination has virtually eliminated waterborne diseases such as cholera, typhoid and dysentery in the United States. (16)

Chlorine is the water treatment of choice because it is efficient, economical and easy to use. Moreover, drinking water chlorination ensures the presence of a residual disinfectant throughout the water distribution system from the treatment plant to the consumer's tap.

References

1. WPCF Task Force on Wastewater Disinfection (1986). Wastewater Disinfection Manual of Practice No. FD-I 0. Alexandria, VA, Water Pollution Control Federation.
2. Blostein, J. (1991). Shigellosis from Swimming in a Park Pond in Michigan. Public Health Reports 106 (3): 317-21.
3. Drenchen, A. and Bert, M. (1994). A Gastroenteritis Illness Outbreak Associated with Swimming in a Campground Lake. Journal of Environmental Health 57 (2): 7-10.
4. Haas, C.N. (1986). Wastewater Disinfection and Infectious Disease Risks. CRC Critical Reviews in Environmental Control 17 (1): 1-20.
5. Herwaldt, B.L., Craun, G.F., Stokes, S.L. and Juranek, D.D. (1991). Waterborne Disease Outbreaks, 1989-1990. Morbidity and Mortality Weekly Report 40 (SS-3): 1-21.
6. Herwaldt, B.L., Craun, G.F., Stokes, S.I.. and Juranek, D.D. (1992). Outbreaks of Waterborne Disease in the United States: 1989-90. Journal of the American Water Works Association (4): 129-135.
7. Levine, W.C., Stephenson, W.T. and Craun, G.F. (1990). Waterborne Disease Outbreaks, 1986-1988. MMWR CDC Surveill Summ 39 (1): 1-13.
8. Reeve, G.D.L., Martin, J., Pappas, R.E., Thompson and Greene, K.D. (1989). An Outbreak of Shigellosis Associated with the Consumption of Raw Oysters. New England Journal of Medicine 321 (4): 224-7.
9. Rosenberg, M. and al., e. (1980). The Risk of Acquiring Hepatitis from Sewage Contaminated Water. American Journal of Epidemiology 1l2: 17.
10. Rosenberg, M.L., Hazlet, K.K., Schaefer, J. Wells, J.G. and Pruneda, R. C. (1976). Shigellosis from Swimming Journal of the American Medical Association 236 (16): 1849-52.
11. Stover, E.L., Haas, C.N., Rakness, K.L. and Scheible, O.K. (1986). Design Manual: Municipal Wastewater Disinfection. Cincinnati, OH, US Environmental Protection Agency.
12. White, G.C. (1978). Handbook of Chlorination. New York, Van Nostrand Reinhold Company.
13. Haas, C.N. and et al. (1987). Assessing the Need for Wastewater Disinfection. Journal of the Water Pollution Control Federation 59: 856-64.
14. Water Pollution Control Federation: Disinfection Committee (1984). Wastewater Disinfection: A State-of the-Art Report. Alexandria, VA, Water Pollution Conrtol Federation.
15. Delzell, E., Giesy, J., Munro, I., Doull, J., Mackay, D. and Williams, G. (1994). Interpretive Review of the Potential Adverse Effects of Chlorinated Organic Chemicals on Human Health and the Environment. Regulatory Toxicology and Pharmacology 20 (1, Part 2 of parts): S1-S1056.
16. World Health Organization (1993). Guidelines for Drinking-Water Quality. 2nd Ed. Vol. I Recommendations.