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The secondary process treats the residual organic material, both suspended and dissolved in the sewage, by reacting it with aerobic bacteria in an aerated system, so that the bacteria digest the organics and oxygen, growing in the process. This is a timerelated process, with a more complete digestion occurring the longer the reaction is allowed to proceed. It is, however, brought to an end after an optimum reaction time dictated by the effluent quality needs of the treated sewage to be discharged. The generated bacteria are then separated from the treated liquid by sedimentation, leaving an effluent capable of being discharged to all but the most sensitive waters.
The first stage of the secondary treatment process is intended to bring the settled sewage into contact with active bacteria and oxygen as efficiently as possible to allow the digestion of the organic content. In older and smaller works this is done in a
‘ trickle filter’, which is not a filter at all, but a packed bed of inert material over which the settled sewage is caused to flow and trickle down from one piece to another of the packing, mixing with air as it does so. Very quickly a biological film grows on the packing and the contact with the bacteria starts the digestion of the sewage organics. The established film continues in use for a considerable time. As the film grows by means of the digestion process, surplus bacteria break way as solids to be separated.
Most large works now use the activated sludge process, in which a mass of aerobic bacteria is suspended in a tank full of settled sewage, which is agitated by streams of air bubbles. The residence time in the tank is quite long, normally several days, during which time the organic content is largely removed. The tank is effectively a sedimentation vessel, but with the system parameters such that the biomass is kept in suspension.
The suspension coming from the underdrains of the trickling filter, or the overflow from the activated sludge tank, is settled in a secondary sedimentation tank, the clear effluent being discharged if clean enough, and the separated sludge being
sent for disposal (although a fraction of the activated sludge is recycled to the head of the activated sludge process to maintain the active bacteria stock in the reactor).