INFLUENCE OF ACTIVATED-SLUDGE FLOCCULATION TIME ON SECONDARY CLARIFICATION

The success of gravity separation of activated sludge from a treated e ffluent depends on the flocculent nature of the mixed liquor entering the secondary clarifier. Despite its importance to the overall effecti veness of the activated sludge process, flocculation phenomena are not routinely considered in the design and operation of the process. Furt her optimization of the activated sludge process to meet higher perfor mance demands requires that the competing reactions of flee aggregatio n and breakup be maximized and minimized, respectively. Accordingly, t he goal of this study was to develop an improved understanding of acti vated sludge flocculation. A theoretically based and easily performed batch flocculation procedure was developed. The procedure enabled the quantification of the flocculation characteristics of activated sludge s. The procedure was field applied, testing 30 activated sludges obtai ned at 21 full-scale facilities. Results obtained during the field stu dy indicated that the equilibrium concentration of supernatant suspend ed solids following batch flocculation and settling is comparable for a wide variety of activated sludges regardless of the initial aggregat ive state of the mixed liquors or the aeration device employed. The re sults indicated that flocculation of activated sludge cannot be used t o reduce supernatant suspended solids below a certain limit. Moreover, the results indicated that attainment of equilibrium is rapid; the ac tivated sludge flocculation reaction in batch reactors was 99% complet e within 10 minutes for all but six of the activated sludges studied. Field-determined estimates of activated sludge flocculation characteri stics can be used to predict the performance of flocculators placed ei ther upstream of or in secondary clarifiers. These estimates also can be used to determine the impact of altering process variables on flocc ulation, thereby affording a procedure for optimizing an activated slu dge's flocculation potential.