Oxygen inhibition of activated sludge denitrification

The effect of dissolved oxygen (DO) on denitrification by activated sludge in a bench-scale sequencing batch reactor (SBR) was investigated over a range of 0.09 to 5.6 mg/l DO. Mixed liquor DO as low as 0.09 mg/l was found to significantly inhibit denitrification, resulting in a rate decrease of 35%. On the other hand, some denitrification activity was observed with DO levels as high as 5.6 mg/l, although at only 4% of the anoxic rate. A one-parameter non-linear model was found to fit results of significant inhibition of denitrification at low DO and persistent denitrification at higher DO. A more complex two-parameter model predicted that denitrification would finally cease at a maximum liquid DO concentration when the activated sludge floc volume is completely aerobic. The rate of denitrification in the SBR activated sludge flocs was reduced by 85% when the mixed liquor DO was 2 mg/l (28% saturation), indicating that aerobic denitrification is more persistent in activated sludge than has been reported in dispersed cultures of bacteria in water and soil, probably due to limited oxygen diffusion into the flocs.