The treatment of sewage using activated sludge

Activated sludge plants

Sludge AerationActivated sludge plants provide secondary treatment of sewage effluent using the principles of extended aeration. All systems have a two staged process.

Activated sludge process

In the first stage the effluent is aerated which sets up a biochemical reaction. In the second stage of treatment separation occurs, using gravity sedimentation.

The two stages of the activated sludge system take place within interconnected tanks. This allows the mixed liquor to gravitate from the aeration tank to the settling tank and sludge to be recycled from the settling tank to the aeration tank.


Within the aeration tank the effluent is supplied with oxygen and is mixed in the fine suspension of various micro-organisms. The organic material contained within the sewage effluent is oxidized and the biomass of micro-organisms grows.

Regulating the process

The biomass gets separated from the effluent in the settling tank and the thickened sludge is returned to the inlet of the aeration chamber. The effluent is then discharged from the final tank for further treatment or directly to a water course.

A small proportion of the recycled flow is removed from the system as surplus sludge. The amount of sludge removal is an important variable within the process, determining the average concentration of solids in the aeration tank and the overall rate of treatment.

Plant variants

Different styles of activated sludge plant are available. Compact plants feature two concentric tanks with the outer ring as the aeration tank which utilizes the principles of plug-flow, feeding to the inner settlement tank.