Sludge Wasting | Activated Sludge Process | Wastewater Treatment Plant

Sludge Wasting

Sludge wasting is an essential step in the treatment of activated sludge. There are two sources of solids in waste activated sludge (WAS). The growth of new bacterial cells in the aeration tank is the primary source of WAS. Simply put, bacteria grow and reproduce in properly functioning aeration tanks by utilizing available energy, carbon, and nutrient sources. The second source is organic and inorganic solids in raw wastewater that pass through primary clarifiers.

Why Is Sludge Wasted?

• To maintain the desired mass of microorganisms in the aeration tank, sludge is wasted. It is typically thrown away when the actual MCRT or sludge age exceeds the target value.

How Is Sludge Wasted?

• In activated sludge plants, secondary clarifiers typically thicken the activated sludge to three to four times its concentration in the aeration tank. This can result in MLSS concentrations ranging from 2,000 to 10,000 mg/l in WAS (and return activated sludge, RAS) (0.2 to 1.0 percent).
• It is critical not to make abrupt changes to the WAS rate because biological systems respond better to gradual changes. The preferred method is to waste sludge continuously, varying the WAS rate by no more than 10% to 15% from one day to the next.

 

Primary Clarifier

• The water content of the sludge produced in the bottom of primary clarifiers is lower than that of the WAS produced in the bottom of secondary clarifiers. The discharge of WAS to the primary clarifier is thus a method of reducing the water content of the sludge prior to further processing in a digester. If WAS is to be wasted in this manner, the primary clarifiers must have the solids loading capacity to accept it.

Solids Thickeners

• Another way to waste activated sludge is to send it to a solids thickener. The thickener's purpose is to reduce the water content of the sludge before it is processed further in a digester. Gravity thickeners, belt thickeners, and flotation thickeners are all used for this purpose. The type of thickener used is usually determined by the design engineer's familiarity with and/or preference for it.