Due to the degradation processes the mass of active bacteria in the activated sludge system is growing. Further accumulation occurs due to entrainment of inert suspended solids with the raw waste water and due to chemical reactions like phosphorus removal by ferrous solution as well. Moreover, also inorganic residuals from biological degradation processes increase the mass of sludge.

At least the mass of sludge in the activated sludge system is growing. Thus, it is not possible to return all the sludge settling in the secondary clarifier as return sludge back to the aeration tank. This would disturb the equilibrium envisaged in the system. The share of discharged sludge is called surplus sludge. It is directly taken from the sludge that is drained from the secondary clarifier. When calculating the sludge balance also the floating sludge taken from the secondary clarifier and the residual (low) content of fine suspended solids in the clarified outlet of the WWTP has to be taken into account. If there is installed a final fine filtration, it would retain most of that fines as filter cake.

The mass of discharged surplus sludge is one of the parameters to determine the solids retention time SRT in the activated sludge system


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