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During pre‑treatment, readily removable solids—such as grit, oils, fats, greases, and large debris—are screened out of the incoming flow.
In the primary clarifier, the wastewater is held long enough for solids to settle into a sludge layer while oils and greases rise to the surface for skimming. The separated solids and liquids are then handled separately for further processing.
Secondary treatment employs aerobic biological processes. Microorganisms such as protozoa and bacteria digest biodegradable organic matter—human waste, food residues, and soaps—reducing the biochemical oxygen demand (BOD).
To achieve final polish, tertiary processes disinfect and further purify the effluent. Common methods include chlorination, ultraviolet (UV) irradiation, ozone oxidation, and advanced filtration. Chlorine is economical but can generate disinfection by‑products; UV is environmentally friendly but requires clear water; ozone is highly reactive and effective against a broad range of contaminants. In many plants, a combination of these methods is used to meet regulatory standards and prepare water for reuse or safe return to natural water bodies.
For more detailed guidance, see the U.S. EPA’s overview of wastewater treatment.
