effluent

Effluent is wastewater or other liquid waste discharged into a receiving environment from a source such as a municipal sewer, an industrial facility, or a stormwater system. It is the outflow after some level of treatment or, in some cases, untreated discharge. It is distinct from influent, the incoming wastewater that enters a treatment facility.

Common sources include municipal wastewater treatment plants, industrial processes (textiles, metal finishing, pulp and paper), agricultural drainage, and urban runoff. Effluent may contain organic matter, suspended solids, nutrients (nitrogen and phosphorus), salts, metals, chemicals, pathogens, and emerging contaminants. In untreated or inadequately treated discharges, effluent can deplete dissolved oxygen and contribute eutrophication and toxicity in receiving waters.

Treatment aims to reduce pollutants before discharge. Municipal facilities may use primary settling, secondary biological treatment (activated sludge, trickling filters), and tertiary processes (filtration, disinfection). Industrial effluent may require specialty treatment to address specific contaminants. Effluent quality is regulated by permits that specify discharge limits for metrics such as BOD, COD, total suspended solids, nutrients, salinity, pH, metals, and sometimes micro-contaminants. Compliance is monitored by sampling and reporting.

In many regions, effluent is discharged into rivers, lakes, or coastal waters, or used for beneficial reuse such as irrigation or groundwater recharge, subject to quality standards. Ongoing concerns include emerging contaminants, antibiotic resistance genes, and climate-related changes in dilution and temperature that affect treatment and ecosystem response.