The activated sludge solids form a sludge-level interface which progressively falls toward the floor of the basin. The flocs adhere together and the mass settles as a blanket leaving a clear supernatant. At the end of the aeration period, the sludge is at a uniform concentration. During the initial settling period, the sludge undergoes internal flocculation due to the residual mixing energy within the basin. As this energy dissipates the sludge interface forms and settles as a blanket.
Dense solids fall through the formed mass to settle on the basin floor.
There is an initial slow settling velocity which increases and then gradually falls off due to the compressive accumulation of solids on the basin floor. . Zone settling velocity is a function of the initial solids concentration, basin depth, total area of the basin and nature of the biological solids. The weir trough of the decanter is situated above top water level for both aeration and settling phases to prevent the accidental discharge of mixed liquor suspended solids. When operated during the decant phase of the cycle, the decanter travels down at an initial fast speed. Interaction with the liquid level is detected by a level indicator float switch which causes the skimmer to proceed at its design rate of travel producing a constant rate of discharge of treated effluent from the basin.