The effect of cationic polymer on the settling characteristics of activated sludge

Abstract

The physical properties of activated sludge flocculated by cationic polymer (polyethyleneimine) were investigated. It was found that the hindered settling rate, the typical settling rate of a typical aggregate and the sediment volume increased with the amount of cationic flocculant due to an increase in aggregate size. But the surplus addition of flocculant caused the deflocculation of aggregates and decreased the hindered settling rate and the sediment volume. It was confirmed by measuring the buoyant density of the individual flocculated aggregate that the flocculant made the aggregate more compact than a natural aggregate of activated sludge. The mean buoyant density of the flocculated aggregates was calculated from a hydrodynamic volume coefficient which was obtained in the hindered settling experiments. The relationship between the calculated mean buoyant density and the size of a typical aggregate which had the typical settling rate at the various concentrations of flocculant was roughly similar to the relationship between the buoyant density of an individual aggregate and the aggregate size when the concentration of flocculant was 3 p.p.m. In spite of an increase in buoyant density in comparison with a natural aggregate which had the same size as the flocculated one, the settling rate of a flocculated aggregate did not increase so much as was expected by applying the settling rate equation for a rigid body. This result was explained by the effect of permeability on the settling rate.