In Situ Examination of Effects of Pressure Differential on Compressible Cake Filtration

Abstract

A synchrotron X-ray absorbance technique was used to monitor suspended solids concentration changes in compressible filter cakes formed at different constant pressure differentials. Results for a kaolin slurry and an activated sludge manifested the markedly higher compressibility of the latter sludge. A model developed to describe effects of pressure differential on filtrate production behavior of an ideal compressible slurry indicated that the kaolin slurry behaved as such. However, the activated sludge was not ideal because changes in pressure differential did not cause the idealized change in filtrate production rate. A thin skin of activated sludge solids at the filter medium accounted for the non-ideal compressive behavior. Porewater pressure data acquired 0.86 mm above the filter medium during activated sludge filtration provided evidence of skin formation and its effect - virtually the total pressure drop occurred within this 0.86 mm region throughout cake formation (after which shrinkage began). Hence, significant effective stresses required to consolidate the sludge cake did not develop in most of the cake until cake formation was complete and shrinkage began.