The shear sensitivity of activated sludge: Relations to filterability, rheology and surface chemistry

Many solid/liquid separation properties are dependent upon the number of fine (micron sized) particles present in the suspension, which in turn is a consequence of the flocculation/break-up balance of the sludge. The present study revealed strong correlations between the dispersed mass concentration, resistance to filtering (capillary suction time) and rheological parameters for activated sludge suspensions of varying solids content. It seems likely that the restriction of flow caused by development of inter-particle networks causes increased surface erosion of primary particles. The increased dispersed mass concentration in turn causes deterioration of filterability. The shear sensitivity parameter quantifies the degree of dispersion in response to turbulent shear and is thus expected to be of prime importance for solid/liquid separation processes. The parameter reflects changes of the surface chemistry, and is therefore suitable as a means of assessing variations of surface chemistry for full scale activated sludge plants. Differences in interaction energies of different sludges may be estimated from shear sensitivity estimates. The observed changes in interaction energy upon addition of sulphide or detergent, changed pH or lowering of the ionic strength were in qualitatively agreement with DLVO-theory.