Creep effects in activated sludge filter cakes

Constant-pressure and stepped-pressure filtrations have been used to study the creep behaviour of filter cakes formed of activated sludge, anatase, or poly(styrene-co-acrylic acid) particles, respectively. No creep is observed in the case of the anatase filter cake, whereas the poly(styrene-co-acrylic acid) and activated sludge filter cakes exhibit pronounced creep behaviour. The consolidation stage for the poly(styrene-co-acrylic acid) filter cake can be divided into a primary consolidation stage controlled by hydrodynamic effects and a secondary consolidation stage controlled by creep effects. Up to 70% of the consolidation stage was secondary consolidation. Furthermore, the retardation time due to creep deformation was found to be much longer than the filtration time, which suggests that creep does not influence the filtration stage. The creep behaviour observed in activated sludge is pronounced, however no clear transition between primary and secondary consolidation was found. The retardation time is of the same magnitude as the filtration time. This suggests that activated sludge filtration is strongly influenced by creep, and may explain the difficulties in modelling activated sludge filtration and dewatering.