Dewatering of Activated Sludge

Summary

Activated sludge is often conditioned with the aid of polyelectrolyte to a highly flocculent state to facilitate dewatering by filtration. Activated sludges in this condition have excellent drainage characteristics when subject to low solids stress, as in gravity drainage at low filter loadings. When subject to large hydraulic gradients, however, as in pressure filtration, the solids are compressed by flow-generated drag forces and resistance to filtration increases. At constant applied pressure, the average specific resistance to filtration varies with time and filtrate volume discharged. The widely used simple model of cake-filtration, which implies a linear correlation of reciprocal filtration rate and filtrate volume is not valid in such circumstances. Thus, the specific resistance to filtration, as measured by the Buchner funnel method is of limited value as a measure of filterability. Using a scaled-up version of the Buchner funnel system, employing positive pressure in place of vacuum, the influences of applied pressure and solids loading on the filterability of polyelectrolyte - conditioned activated sludge were experimentally investigated. The results illustrate the dominant effect of compressibility in pressure-filtration and the need for a better understanding of compressible filtration mechanics.