Sludge produced in wastewater treatment plants must be conveyed from point to point in the plant in conditions ranging from a watery sludge to a thick sludge. Sludge may also be pumped off-site for long distances for treatment. For each type of sludge and pumping application a different type of pump may be needed.

Pumps can generally be divided into centrifugal pumps (with different impellers) and displacement pumps (progressive cavity (see figure1), rotary lobe, piston). Centrifugal pumps are suitable for high flow rates and low solid contents, the problem is choosing a proper size. At any given speed, centrifugal pumps operate well only if the pumping head is within a relatively narrow range. The variable nature of sludge, however, causes pumping heads to change. The selected pump must have sufficient clearance to pass the solids without clogging. Usual centrifugal pumps can cause the break up of flocculent particles in activated sludge. Good experiences in this respect have been made with screw-shaped impellers.

Table 2:Suitable pumps for sludge

Displacement pumps can convey dewatered sludge up to 30% TS. Displacement pumps have a fixed ratio between revolutions and flow rate. Any pipeline obstruction causes damages to the pipeline or pump. Generally check valves are not necessary. For primary sludge, a grinder normally proceeds progressive cavity pumps.

Figure 1: Scheme of a progressive cavity pump