In the United States, wastewater discharges to surface water are regulated through the National Pollutant Discharge Elimination System (NPDES) permit program. NPDES permits are issued and enforced either by the United States Environmental Protection Agency (EPA) or by state agencies under authority delegated by EPA. NPDES permits generally include technology-based effluent limitations; more stringent limits are applied on a case-by-case basis if necessary to protect receiving water quality. Water-quality-based NPDES permit limits are set so that the fully diluted effluent will not exceed ambient water quality criteria. However, EPA and many states recognize that receiving water can be protected without requiring an effluent to meet water quality criteria at the point of discharge. Water-quality-based permits often include mixing zone allowances to account for the dilution that takes place around an outfall.
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