INT‐Reduction Assays and Control of Sludge Bulking

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Abstract
Tetrazolium reduction assays, based on the reduction of 2‐(p‐iodophenyl)‐3‐(p‐nitrophenyl)‐5‐phenyl tetrazolium chloride (INT) to INT‐formazan by active bacterial electron transport systems, were used to differentiate between the physiological activities of filamentous and floc‐forming microorganisms in activated sludge following applications of chlorine. With proper dosage of chlorine, filaments (predominantly Sphaerotilus natans) were almost completely inactivated while floc‐forming microorganisms were only partially inhibited. Trends in the specific activity of S. atans anticipated similar trends in sludge volume index. Tetrazolium reduction assays were used in conjunction with a short term, static biassay to predict the chlorine dosage which controlled Sphaerotilus bulking in a bench scale, activated sludge system.