Initially, EPA required local governments with POTWs to construct and implement "secondary treatment," which is defined as the best available control technology. This step was costly, but with significant levels of federal grant funding, it resulted in a major nationwide improvement in wastewater treatment and water quality.

Over the past decade, regulatory requirements expanded from secondary sewage treatment, to compliance with water quality based treatment requirements at sewage treatment plants; control of CSOs and then SSOs; implementation of BMPs and other controls for stormwater systems to comply with NPDES permits established for stormwater facilities under the 1987 CWA amendments. 

Reproduced courtesy from the Association of Metropolitan Sewerage Agencies (AMSA) - www.amsa-cleanwater.org. The Cost of Clean is also available on AMSA's web site or by contacting AMSA's National Office at (202) 833-AMSA.