Inhibitory effect of oxygen on denitratification and denitritification in sludge from an oxidation ditch

Using return sludge in an oxidation ditch, the effect of oxygen concentration on the rates of denitratification and denitritification was investigated by the acetylene inhibition method during initial l-h incubation period after preculture under anaerobic conditions. The denitratification rate decreased as the oxygen concentration increased, and above 30 μM O 2, the reaction was suppressed completely. In contrast, denitritification occurred even under aerobic conditions. In the nitrite range of 70–7000 μM at temperature of 13°C, the denitritification rate under aerobic conditions was 5–20 μmol (g-VSS) −1h −1, which was 10–60% of that under anaerobic conditions. The inhibition of denitritification by oxygen seemed to conform to the simple competitive inhibition model.