Waste water is introduced into an aerated tank of micro-organisms which are collectively referred to as activated sludge or mixed liquor. Aeration is achieved by the use of submerged diffused or surface mechanical aeration or combinatidns thereof, which maintain the activated sludge in suspension. Following a period of contact between the waste water and the activated sludge, the outflow is separated from the sludge in a secondary settlement tank. To maintain the desired micro-biological mass in the aeration