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Why is wastewater disinfected before it is released into surface water?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Wastewater is disinfected before being released into surface water primarily to protect public health by destroying remaining pathogenic bacteria and other disease-causing microorganisms.12

Because domestic sewage is a primary source of pathogens excreted in feces, it poses a direct threat to human health if discharged without adequate treatment. While primary and secondary treatment processes significantly reduce the levels of organic matter and suspended solids, they do not eliminate all pathogens. Consequently, disinfection serves as the final, essential step in the treatment process to ensure that the effluent is safe before it enters bodies of water that may be used for recreation, irrigation, or as a downstream water supply.213

Common methods for this process include the use of chlorine or ultraviolet radiation, both of which effectively neutralize pathogens to prevent the spread of waterborne diseases.41