Oxygen effects on activated sludge denitrification in sequencing batch reactors

Dissolved oxygen inhibition of denitrification is relevant to biological nutrient removal (BNR) wastewater treatment processes because activated sludge is regularly exposed to partial aeration. Many denitrifying bacteria can use oxygen as well as nitrate as an electron acceptor, and will reduce oxygen preferentially because aerobic respiration yields more energy. In a related effect, denitrifying bacteria have been shown to reduce their synthesis of the reductase enzymes that catalyze each step of the denitrification sequence after a period of aerobic growth. On the other hand, denitrification in the presence of dissolved oxygen has been reported often; researchers have even observed simultaneous nitrification, a completely aerobic process, and denitrification.; The purpose of this research was to develop a complete understanding of the inhibitory effects of oxygen on activated sludge denitrification that included the phenomenon of denitrification during aeration. Two scenarios common to BNR processes were examined. First was activated sludge denitrification in the continuous presence of dissolved oxygen. Second was the effect of regularly alternating aerobic and anoxic periods within the same reaction cycle. Experiments for the study were carried out in bench-scale sequencing batch reactors (SBR's).; Continuous aeration to keep dissolved oxygen at 0.08 mg/L was associated with a 35% reduction in the specific denitrification rate. However, denitrification did not completely cease when dissolved oxygen was as high as 5.6 mg/L, although the rate was only 4% of the anoxic rate. A model for partial inhibition of denitrification in the presence of dissolved oxygen was developed and calibrated using experimental results.; One explanation proposed for oxygen inhibition of denitrification is competition for substrate electrons, and it was hypothesized that even completely anoxic denitrification could be disrupted by carbon limitation. When acetate was used as the carbon and energy substrate in a carbon:nitrogen (C:N) ratio {dollar}ge{dollar}2:1, anoxic denitrification was complete without the appearance of nitrite. When the C:N ratio was reduced to 1:1, while nitrate removal was almost complete, significant nitrite accumulated. Moreover, while the nitrite reduction rate decreased, the nitrate reduction rate actually increased during substrate-limited denitrification.; Inhibition of denitrification by alternating aerobic and anoxic periods during the reaction cycle was shown to depend on whether aerobic growth occurred. When substrate (acetate) was available for as little of 30 minutes of aeration during each 10.5-hour cycle, the rate of the subsequent anoxic denitrification reaction decreased by 26%. However, when no substrate was provided during longer aeration, 180 minutes, anoxic denitrification was not affected. After aerobic growth, activated sludge cultures had a significantly higher instantaneous oxygen uptake rate than denitrifying cultures, indicating a permanent shift in biomass activity to favor aerobic respiration.; Overall, it can be concluded that in activated sludge, oxygen-associated inhibition of denitrification is complex. Inhibition in the presence of oxygen is caused by competition for substrate, not direct repression of denitrification. Furthermore, substrate limitation results in unbalanced inhibition of the denitrification sequence with significant nitrite accumulation. Finally, alternating air on-off cycles decrease the rate of anoxic denitrification when biodegradable organic matter is provided with aeration, with equal suppression of nitrate and nitrite reduction.