Evaluation of Activated Sludge Thickening Theories

Abstract

Prevailing theories for determining the required area of gravity thickeners are reviewed and are shown to be based on the assumption that the settling velocity of a suspension depends only on the local concentration of particles. It is shown that this assumption is not valid for activated sludge. Thus, the prevailing theories are not strictly applicable to the design of final settling tanks or separate thickeners for activated sludge. Two fundamental measures of the settling properties of suspensions, the retardation factor and the ultimate settling velocity, are described.