The effect of alternating aerobic and anoxic conditions on denitrification by activated sludge was investigated in bench-scale sequencing batch reactors with a total of 10.5 hours of reaction time. After 2 to 5 hours of anoxic denitrification, mixed liquor was aerated for 36, 94, or
174 minutes and then returned to anoxic mixing. Acetate was added during both the denitrification and aerated reaction periods, except for one experiment with aeration without substrate. The specific denitrification rate was reduced by 26% from the totally anoxic rate of 0.023 mg nitrate/mg
mixed liquor volatile suspended solids•h when the activatcd sludge received 36 minutes aeration and by 61% for both 94 and 174 minutes of aeration. However, aeration did not seem to cause the reduction directly. Rather, the addition of acetate with oxygen caused aerobic growth of bacteria,
indicated by the significant increase in the specific oxygen uptake rate of the activated sludge consuming substrate under aerobic conditions.
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