Sometimes it is necessary to mix chemicals into the activated sludge before it enters the secondary sedimentation tank. For example, chemicals may be used to perform phosphorous removal on the effluent from the biological process. Many plants use chemicals to improve sludge thickening just prior to the sludge buffer tank.

The sludge buffer tank can be used for a variety of purposes. The most common application is the blending and homogenization of highly concentrated primary, secondary or digested sludge. The solution used to mix the sludge itself, or to mix the sludge with the dosed thickening chemicals, depends on the design and volume of the sludge buffer tank. Submersible mixer types ABS XRW and RW, as well as the agitator type Scaba, can be used to provide highly efficient mixing. Positive displacement, progressing cavity transfer pumps and Muffin Monster™ grinders are used for solids reduction and sludge transfer from sludge feed to sludge dewatering process.