Mechanisms of Compressible Sludge Cake Shrinkage

Abstract

Shrinkage can effect significant consolidation of saturated filter cakes, and results in production of filtrate that must be omitted from specific resistance calculations. Shrinkage results when curved air/liquid interfaces bounded by solids form at filter cake surfaces following the cake formation stage. The resultant decrease in porewater pressure is described by the Young‐Laplace equation, which was used to express mathematically the effective stress causing shrinkage. In situ porewater pressure measurements corroborated the onset of shrinkage of a kaolin filter cake. Synchrotron x‐rays were used to monitor temporal and spatial cake suspended solids concentration changes, and significant shrinkage of an activated sludge filter cake was observed. Cake‐suspended solids concentration data were combined with filtrate production data to obtain singular values of the cake‐suspended solids mass deposited per unit volume of filtrate produced by a kaolin slurry and an activated sludge. Values of this parameter were determined by two other means to illustrate how errors in specific resistance calculations can result when the onset of shrinkage is not ascertained.