The most common suspended growth process used for municipal wastewater treatment is the activated sludge process as shown in figure :
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.
In activated sludge process wastewater containing organic matter is aerated in an aeration basin in which micro-organisms 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.