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Chlorination: An Enduring Public Health Practice
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.
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, mycobacterium, 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.
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 wastewater. 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 byproducts. The nature of biological treatment (prior to chlorination nation) and the presence of ammonia may have a substantial impact on the extent of byproduct 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 byproducts.
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
SAFE HANDLING AND USE
Chlorine can be used in wastewater disinfection as a gas, liquid sodium hypochlorite solution or solid calcium hypochlorite. Where a large amount of chlorine is needed for disinfection, chlorine gas is preferred over sodium hypochlorite due to its cost effectiveness. In wastewater chlorination, chlorine gas provides better germicidal efficiency in poorly buffered wastewater as it tends to lower the pH of treated wastewater. (16)
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" and the American Water Works Association's "Safety Practice for Water Utilities." The chlorine industry sponsors research on water disinfection with chlorine-based products. Information in this publication is based on the report from the Chlorine Institute, Benefits and Risks of Wastewater Chlorination, Edition 1, April 1997.
CHLORINATION OF DRINKING WATER
Chlorine's effectiveness as a disinfectant also has been demonstrated by its use in drinking water treatment for over 90 years. 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. (17)
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.
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