Torque requirement for high-solids centrifugal sludge dewatering

Abstract

In dewatering of compactible sludges to high dryness, the solid-bowl decanter centrifuge is operated with the annular volume between the conveyor hub and the bowl wall packed almost completely with cake solids. The torque required to maintain the conveyor rotating at differential speed with respect to the bowl can be determined for a given type of sludge from the “effective frictional coefficient”, which for municipal and industrial sludges, is typically in the range between 0.5 and 1.5. The back-drive torque, which sustains the conveyor under a small differential speed with respect to the rotating bowl, is used to convey the cake. Concurrently applying a churning action on the cake allows the moisture in the interstitial voids of the cake to be expressed out to the cake surface away from the bowl wall. Higher torque is a consequence of either, or a combination of, higher operating centrifugal gravity, higher solids throughput, thicker cake from lower differential speed, longer transport distance from a large length-to-diameter decanter, and drier cake. The decanter can be operated either under torque control, differential-speed control, or a hybrid algorithm involving both variables. A high-solids decanter mimics a rheometer with continuous inflow and outflow of test fluid material wherein the rheological properties are determined from the torque-speed measurements.