The objective of this paper is to identify deflocculation under short-term disturbances of dissolved oxygen (DO) in a continuous system, and to explore the strategies for minimizing the carry-over of biosolids. Sludge deflocculation was examined in four parallel sequencing batch reactors (SBRs) fed with primary treated effluents from a bleached kraft pulp and paper mill. Results show that the DO transients caused a reduction of soluble COD (SCOD) removal efficiency by 50–70%, increases in suspended solid (SS) concentration and turbidity by more than 100%. Under the DO disturbances, the ratio of bulk Ca2 + /K+ decreased by 10% in the treated effluents. All these changes were reversible within 24 hrs after the DO concentrations were restored up to 4 mg/L, indicating a physicochemical response of microorganisms to the DO transients. The sludge deflocculation is correlated with the decreasing ratio of Ca2 +  to K+ in the extracellular environment. Addition of calcium chloride, tetraethylammonium chloride or glibenclamide promotes the formation of bioflocs bigger than 12.5 μm, but the deflocculation of biosolids under the DO transients wasn't completely prevented.

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