The activated sludge system is widely used worldwide for the treatment of domestic and industrial sewage in situations where high quality industrial effluent is required and reduced area requirements. However, this type of system requires sophisticated mechanization and greater energy consumption.
It is in the aeration tank that the biochemical reactions of organic matter removal occur. The microorganisms are used of the substrate (BOD) present in the sewage to develop. In the secondary decanter the sedimentation of the solids occurs, allowing the final effluent to be clarified. The sedimented solids at the bottom of the decanter return to the sandblasting tank, increasing the biomass concentration, which is responsible for the high efficiency of the system.
The following modalities are found in an activated sludge system: conventional aeration; prolonged aeration; and batch. The choice of modality will depend on the availability of area, final effluent quality, energy expenditure, among others.