The National Pollutant Discharge Elimination System (NPDES) is a regulatory program for direct discharges of pollutants from point sources to surface waters that fall under the jurisdiction of the Clean Water Act. NPDES permits cover many point sources of pollution, including industrial and municipal discharges, stormwater discharges, runoff from construction sites, mining operations, and animal feedlots.
NPDES permits include a specific, numeric, measurable set of limits on the amount of various pollutants that can be contained in the discharged effluent. Those limits can be either technology-based or water quality-based; regardless, these limits are performance standards, and the permittee is free to use any process necessary to meet them.
NPDES permits can also require the use of certain BMPs.
An NPDES permit can be given to individual facilities, or group NPDES permits can cover a large number of similar facilties with a single permit document. An NPDES permit is issued for no more than 5 years.
For more information, go here: http://cfpub.epa.gov/npdes/home.cfm?program_id=45