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Why must wastewater be treated before it is discharged into rivers or lakes?

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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 must be treated before discharge into rivers or lakes to protect public health and maintain water quality. Untreated sewage poses several critical risks:12

Pathogen Control: Domestic sewage contains pathogenic bacteria and other disease-causing microorganisms excreted in feces. Treatment, particularly disinfection, is essential to destroy these pathogens and prevent the spread of waterborne diseases.12
Oxygen Depletion: Sewage contains biodegradable organic matter that consumes dissolved oxygen as it decomposes. High levels of this organic load can deplete oxygen supplies, endangering fish and other aquatic life.2
Eutrophication: Wastewater is a major source of nitrates and phosphates. These nutrients trigger rapid algal blooms, leading to eutrophication. As algae die and decompose, oxygen levels plummet, creating "dead zones" where aquatic life cannot survive.342
Toxic Contamination: Treatment is necessary to neutralize industrial wastes and toxic chemicals that can destroy aquatic organisms or persist in the environment, potentially entering the food chain.152