Regulations, Permits, Reporting, and Records
Permit framework
The Clean Water Act establishes the NPDES program; states with primacy issue permits with effluent limits, monitoring requirements, and reporting schedules. Limits take several forms - monthly average, weekly average, maximum daily, and occasionally instantaneous - and each is evaluated on its own. Secondary treatment under 40 CFR 133 is 30 mg/L BOD and 30 mg/L TSS as monthly averages with at least 85% removal. Pretreatment and local limits fall under 40 CFR 403, and biosolids under 40 CFR 503 (Class A or B, vector attraction reduction, pollutant limits).
Reporting
Discharge Monitoring Reports are submitted on schedule and signed by an authorized official; monthly operating reports summarize process and maintenance data. Noncompliance that may endanger health or the environment requires immediate (often 24-hour) notification with written follow-up, and bypass or upset claims require specific affirmative-defense conditions. Sanitary sewer overflows carry their own notification rules. Retain records at least three years unless the permit requires longer.
Traps
A single bad day can violate a maximum daily limit even when the monthly average passes. Percent removal is calculated from influent and effluent values for the same period and has a floor provision. Reporting a value below the detection limit requires the permit's specific convention, not simply zero.
Worked example
Influent BOD averages 200 mg/L and effluent averages 15 mg/L. Removal = (200 − 15) ÷ 200 × 100 = 92.5%, which satisfies the 85% requirement, but the individual monthly average limit still has to be met separately.