The biological sludge which is used during aerobic treatment needs to be separated from the wastewater. This is typically done in secondary clarifiers. The sludge settles in small flocs and is pumped back to the start of the aeration tank.

Excess sludge which is formed during the aerobic treatment is pumped to the sludge holding tank.

The main design parameters are:
– surface loading rate
– suspended solids concentration
– sludge volume index

Secondary clarifiers with fixed bridge

Secondary clarifier with moving bridge