Wastewater Treatment Plant Effluent

Wastewater treatment plant effluent is the treated discharge from a sewage works, the dominant point source of nutrients and faecal indicator bacteria to many coastal waters. Treatment runs in stages: primary settling removes solids, secondary biological treatment cuts biochemical oxygen demand and suspended solids, and tertiary steps add nutrient removal and disinfection. The EU Urban Waste Water Treatment Directive 91/271/EEC requires secondary treatment for agglomerations above 2,000 population equivalent and stricter nutrient limits in sensitive, eutrophication-prone areas. Even compliant effluent carries residual nitrogen, phosphorus, and trace pharmaceuticals into the receiving water. In the United States, plant discharge is permitted and limited under the NPDES program.