Effects of solids concentration on activated sludge deflocculation, conditioning and dewatering

Optimum conditioning of activated sludge in terms of minimum CST was shown to correspond to the complete removal of turbidity, and the increase in tur bidity with shear due to e.g. pumping is therefore expected to affect condi tioning. The optimum polymer dosage was directly related to the turbidity o f activated sludge after two minutes shear, and was considerably lower than the dosage required for charge neutralisation. The turbidity produced by s hear increased more than is proportional with solids concentration and was directly related to the apparent viscosity. It is suggested that increasing solids concentration causes increased surface erosion when network structu res are broken, and this causes increases in turbidity and required polymer dosage per solids mass. For Aby activated sludge, optimum polymer dosage p er solids mass increased by 52% when the solids concentration was increased from 8.2 to 13.7 g SS/I. Modelling of the effect of solids concentration p redicts even higher increases in required polymer dosage for higher solids concentrations. This means that reduced thickening prior to pumping and con ditioning may be desirable when the hydraulic capacity of the dewatering de vice is sufficient. Similar trends were observed for an anaerobically diges ted sludge. For this sludge, reduction of turbidity with FeCl3 reduced the polymer demand.