Sewage Treatment Plant Points, Tutuila AS, 2009, US EPA Region 9
Description
Across the United States, municipal wastewater treatment plants receive and treat sewage and other wastewater collected from homes, businesses, and industries. These plants are designed to treat wastewater prior to reuse or discharge to streams, oceans, or the ground. There are two basic stages in the treatment of municipal wastewater: primary and secondary treatment, although more advanced treatment (known collectively as ‘tertiary’) is becoming increasingly common. Primary treatment screens out large objects (such as rags), removes grit (such as cinders, sand and small stones), and allows the wastewater to settle (where objects that float, such as sticks, are skimmed off the surface, and materials that sink are removed from the bottom). When secondary treatment is used, wastewater receives primary treatment and is then exposed to microorganisms (such as bacteria). Different biological treatment techniques allow the microorganisms to consume most of the waste’s organic matter. The microorganisms are then removed prior to discharge. In 1972, Congress passed the Federal Water Pollution Control Act amendments, which required Publicly Owned Treatment Works (POTWs) to achieve secondary treatment by 1977. In 1977, Congress added section 301(h) to the Clean Water Act (CWA), which allowed the USEPA, on a case-by-case basis, to grant variances from secondary treatment requirements to POTWs discharging into marine waters.