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Sedimentation (Secondary Sedimentation)
The oxidised sewage from the trickling filters or aeration chamber is led into the secondary sedimentation tank (SST) where it is detained for 2-3 hours. The sludge that collects in the SST is called aerated sludge or activated sludge because it is fully aerated. It differs from the sludge in the primary sedimentation tank in that it is practically inoffensive and is rich in bacteria, nitrogen and phosphates. It is a valuable manure, if dehydrated. Part of activated sludge is pumped back into the aeration tanks in the activated sludge process and the rest is pumped into the sludge digestion tanks for treatment and disposal.