- water
- water-quality
Effluent is the treated water a facility discharges to a stream, river, or lake. Its permitted effluent limits — numeric caps on concentration and mass load for each regulated pollutant — are the operative terms of an NPDES permit, and the permittee proves compliance by reporting measured effluent quality in the monthly discharge monitoring report.
Effluent limits are set two ways, and the stricter governs: technology-based limits reflect what treatment can achieve, while water-quality-based limits protect the receiving water given its dilution at design low flow. That second path is why a WWTP discharge to a small stream can carry tighter limits than the same plant on a large river — the same assimilative capacity logic that a new consumptive cooling draw erodes by removing dilution flow.