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Influence of activated sludge flocculation time on secondary clarification

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The success of gravity separation of activated sludge from a treated effluent depends on the flocculent nature of the mixed liquor entering the secondary clarifier. Despite its importance to the overall effectiveness of the activated sludge process, flocculation phenomena are not routinely considered in the design and operation of the process. Further optimization of the activated sludge process to meet higher performance demands requires that the competing reactions of floc aggregation and breakup be maximized and minimized, respectively. Accordingly, the goal of this study was to develop an improved understanding of activated sludge flocculation.

A theoretically based and easily performed batch flocculation procedure was developed. The procedure enabled the quantification of the flocculation characteristics of activated sludges. The procedure was field applied, testing 30 activated sludges obtained at 21 full-scale facilities. Results obtained during the field study indicated that the equilibrium concentration of supernatant suspended solids following batch flocculation and settling is comparable for a wide variety of activated sludges regardless of the initial aggregative state of the mixed liquors or the aeration device employed. The results indicated that flocculation of activated sludge cannot be used to reduce supernatant suspended solids below a certain limit. Moreover, the results indicated that attainment of equilibrium is rapid; the activated sludge flocculation reaction in batch reactors was 99% complete within 10 minutes for all but six of the activated sludges studied. Field-determined estimates of activated sludge flocculation characteristics can be used to predict the performance of flocculators placed either upstream of or in secondary clarifiers. These estimates also can be used to determine the impact of altering process variables on flocculation, thereby affording a procedure for optimizing an activated sludge's flocculation potential.
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Keywords: activated sludge; clarification; flocculation; model

Document Type: Research Article

Publication date: 1994-09-01

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  • Water Environment Research (WER) is published monthly, including an annual Literature Review. A subscription to WER includes access to the latest content back to 1992, as well as access to fast track articles. An individual subscription is valid for 12 months from month of purchase.

    Water Environment Research (WER) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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