Activated sludge flocculation: On-line determination of floc size and the effect of shear

This paper presents a technique which enables on-line monitoring of activat ed sludge flocculation. The usefulness of this technique is demonstrated by investigating the effect of shear on steady-state floc sizes. The techniqu e is based on a method developed to investigate flocculation of inorganic p articles. Samples of activated sludge are sonicated for 3 min at 50 W to pr oduce the necessary primary particles to observe flocculation. The samples are then stirred in a batch mixing vessel and sized on-line using a Malvern Mastersizer/E. The dynamics of flocculation were found to be reproducible and follow the p rinciples of inorganic shear induced flocculation. The median flee size was found to increase until an equilibrium between the rates of aggregation an d breakage was reached. At this point, a steady-state floc size was maintai ned. The effect of shear on the steady-state flee size was quantified using the experimental technique. The change in floc size-with shear followed a power law relationship. This relationship was used to investigate the breakage m echanisms. (C) 2000 Elsevier Science Ltd. All rights reserved.