Types of sewage treatment plants include activated sludge plants, rotating disc systems, submerged aerated filters, suspended media filters and sequencing batch reactors. Sewage treatment plants, which are physical locations that use various processes to change the properties of waste water, can also be classified into non-electric filter and trickling filter systems.
Sewage treatment plants can be further classified into on-site or effluent systems. On-site systems use a septic tank for sewage treatment and the sewage is disposed of close to the buildings or homes. Effluent systems dispose of the treated sewage from a community in a central place called a sewage lagoon or effluent pond. Effluent systems utilize either a small septic tank located at each building, a large tank near the lagoon or in the lagoon itself.
When using on-site systems, all liquid waste is transferred through pipes to a septic tank. After treatment, effluent is disposed through drains into the surrounding soil. Some soils such as clay are not conducive to good soakage. On-site systems should not be used in areas that are prone to flooding, where water is close to the surface, where the amount of waste water is large or that are near drinking water supplies.