During the decant phase, operating under a flow-paced batch operation, no more than onethird of the volume contained in the basin (i.e., the tank contents) should be decanted each time in order to prevent disturbance of the sludge blanket. The decant phase should not interfere with the settled sludge, and decanters should avoid vortexing and taking in floatables. The problem with decanting more than one-third is that it increases the chance that solids will be decanted into the effluent, thereby impairing the effluent quality. For the plant to run optimally, it is important that the decant volume is the same as the volume added during the fill phase. The length of the decant weir can have an impact that is very similar to that of the over-flow weir found in a clarifier. The flux (upward forces) caused by the discharge of the decant creates an upward force that may pull poorly settled solids up and out the discharge.

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