Preliminary Treatment – STP
Pretreatment removes all materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers
A bar screen is a mechanical filter used to remove large particles from wastewater, such as rags and plastics.
Pretreatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand, grit, stones, and broken glass.
Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions. Equalization basins provide a place to temporarily hold incoming sewage during plant maintenance and regular flow before it pumped to the treatment. Cleaning may be easier if the basin is downstream of screening and grit removal
In larger plants, Oil and grease are removed by passing the sewage through a small tank where skimmers collect the oil floating on the surface.
