In most cases the use of centrifugal pumps is restricted to relatively diluted (less than 1% solids), trash-free sludge. Conventional centrifugal pumps are commonly applied for transport of return activated sludge because of the pump’s high volume and excellent efficiency. Centrifugal pumps are not recommended for pumping primary sludge, primary scum, or thickened sludge applications.

Screw-combination centrifugal pumps are a combination of screw feeder and a normal centrifugal impeller. They feature high efficiency and the corkscrew action of screw impellers can improve the handling of thicker sludge.