Sewage Treatment Plant – Conventional Activated Sludge Process (ASP)


Conventional activated sludge systems commonly include an aeration tank and secondary clarifier.  Aerobic biomass reduces the biochemical oxygen demand (BOD) and ammonia concentrations in the aeration tank. Biomass then flows to the secondary clarifier, where it is separated into clarified water and thickened biomass by gravity sedimentation. The clarified treated water overflows at the top of the secondary clarifier, and the thickened biomass is recycled to the aeration tank or managed at sludge dewatering facilities.

Extended aeration is a method of sewage treatment using modified activated sludge procedures. It is preferred for relatively small waste loads, where lower operating efficiency is offset by mechanical simplicity.

An Activated Sludge Plant involves:

  • Wastewater aeration in the presence of a microbial suspension,

  • Solid-liquid separation following aeration,

  • Discharge of clarified effluent,

  • Wasting of excess biomass, and

  • Return of remaining biomass to the aeration tank.

  • activated sludge process wastewater containing organic matter is aerated in an aeration basin in which microorganisms metabolize the suspended and soluble organic matter. Part of organic matter is synthesized into new cells and part is oxidized to CO2 and water to derive energy. In activated sludge systems the new cells formed in the reaction are removed from the liquid stream in the form of a flocculent sludge in settling tanks. A part of this settled biomass, described as activated sludge is returned to the aeration tank and the remaining forms waste or excess sludge.