INT–dehydrogenase test for activated sludge process control

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Abstract
Dehydrogenase activity assay of activated sludge using the redox dye 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT) was investigated. INT-dehydrogenase activity (INT–DHA) was directly proportional to INT dosage and inversely proportional to bio-mass concentration over limited ranges. INT dosages exceeding 2.5m/M were toxic to dilute activated sludge suspensions. INT–DHA was greatest near pH 9, whereas the peak oxygen uptake rate (OUR) occurred at pH 8. Both INT–DHA and OUR varied inversely with sludge age, but INT–DHA was the more sensitive of the two parameters to this variable. Consistently good and highly significant correlations between INT–DHA and OUR of chlorine stressed activated sludge were found at sludge ages ranging 2.2–7.0 days.