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Floc Breakup and Aggregation in Batch Activated Sludge Systems

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Mixing intensity is an important factor in determining floc structure and size in activated sludge systems and thus the settling characteristics of the floc. In batch systems, such as sequencing batch reactors (SBRs), mixing intensity could be controlled to promote flocculation either during the react period or separate flocculation period. Separation of aeration and mixing could result in lower operation cost for aerobic processes. Improved flocculation in batch systems should lower effluent suspended solids. Physical conditioning of activated sludge has been studied for continuous flow activated sludge systems (CFASSs). Physical conditioning of activated sludge flocculation has not been elucidated SBRs and thus was investigated herein.

Activated sludge solids studied were grown in a denitrifying sequencing batch reactor (G=32 sec−1). Flocculation was examined in-situ (during react cycle of SBR) and ex-situ using a jar test apparatus. Mixing intensities (Gs) used were 32, 96, 137 and 174 sec−1. Primary particle concentration was characterized by supernatant (after 30 minutes of settling) suspended solids concentration and turbidity.

Data for the SBR system studied suggests that both floc breakup and aggregation are a function of primary particle concentration (as measured by turbidity). This is contrary to previous investigations which have proposed that floc breakup is not a function of primary particle concentration. Flocculation occurs both during the react and settling periods of an SBR and the dominant mechanisms are different. Differences in hydraulic regime between SBRs and CFASSs and the measurement technique used for primary particles can account observed differences in floc breakup and aggregation between the two activated sludge systems.

Document Type: Research Article


Publication date: January 1, 2000

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