The sewage passes into large sedimentation tanks to provide a quiescent settlement period of about 8 hours. Most of the solids settle to the bottom of the tanks and form a watery sludge, known as ‘primary sludge’, which is removed for separate treatment. The sewage remaining after settlement has taken place is known as ‘settled sewage’.

      Secondary (biological) treatment
Settled sewage then flows to an aerobic biological treatment stage where it comes into contact with micro-organisms which remove and oxidise most of the remaining organic pollutants.

      At smaller works, the biological stage often takes the form of a packed bed of graded mineral media through which the sewage trickles and on the surfaces of which the micro-organisms grow. At most larger works, the sewage is mixed for several hours with an aerated suspension of flocs of micro-organisms (known as the activated sludge process). As well as removing most of the polluting organic matter, modern biological treatment can, where necessary, remove much of the nitrogen and phosphorus in the sewage, thus reducing the nutrient load on the receiving waters.

      Final settlement

      Following secondary (biological) treatment, the flow passes to final settlement tanks where most of the biological solids are deposited as  sludge (secondary sludge) while the clarified effluent passes to the outfall pipe for discharge to a watercourse. In the case of the activated sludge process, some of the secondary sludge is returned to the aeration tanks for further contact with the sewage. The secondary sludge from biological treatment also requires separate treatment and disposal and may be combined with the primary sludge for this purpose.

      Tertiary treatment