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Sewage treatment involves physical, chemical, and biological processes to remove contaminants from wastewater and produce an effluent that is safe to discharge back into the environment. It generally involves three stages - primary treatment to separate solids, secondary treatment using microorganisms to break down organic matter, and tertiary treatment using additional processes like filtration, nutrient removal, and disinfection to further polish the water before discharge. The goal is to protect water quality by removing harmful pathogens, excess nutrients, and other pollutants from residential, commercial, and industrial wastewater before returning the treated water to nature.