The role of suspension characteristics in continuous gravity thickening

Sludges resulting from water and wastewater treatment processes consist of agglomerated individual particles producing aggregates of various sizes and densities. Particle size, particle density, aggregate size and aggregate density are characteristics which determine the amenability of a suspension to continuous gravity thickening. To investigate the effects of these suspension characteristics on compression zone behavior, precipitation softening water treatment sludges and diatomaceous earth suspensions were gravity thickened in a pilot-scale continuous thickener. The effects of raking on thickening performance in the compression zone and the effects of suspension characteristics on channeling were also investigated.; The behavior of the sludges in the compression zone of the continuous thickener was characterized using sludge concentration profiles and permeability and compressibility relationships. Suspension aggregate size and density were evaluated using a velocity-voidage correlation for hindered settling of the suspensions.; When judged on the basis of the sludge concentration profiles and resulting underflow concentrations from the continuous thickener, the thickening performance of the sludges examined in this investigation was significantly affected by the sludge aggregate characteristics. Correlations between aggregate characteristics and permeability and compressibility explained the differences in thickening behavior.; The occurrence of channeling in the compression zone was a function of aggregate characteristics for the diatomaceous earth suspensions, whereas compression zone channeling occurred for all softening sludges examined in this investigation, regardless of aggregate properties.; Raking the compression zone obliterated channeling, causing poorer thickening performance of those suspensions which channeled in the compression zone.