Effect of influent nitrogen concentration on feasibility of short-cut nitrification during wastewater treatment in activated sludge systems

The inhibitory effect of free ammonia and free nitrous acid on nitrite-oxidising bacteria (NOB) was studied in a laboratory-scale sequencing batch reactor with a suspended microbial culture. The reactor was operated at 15°C, with a dissolved oxygen concentration in excess of 5 mg L−1 and a nitrogen-loading rate of 0.2 kg m−3 d−1. Diluted reject water with varying total ammonia nitrogen (TAN) concentrations was used as influent. N-NO2− represented more than 90 % of all of the oxidised nitrogen, with influent TAN concentrations of 600 mg L−1, 300 mg L−1 and 150 mg L−1, respectively. With a TAN concentration of 75 mg L−1, a gradual increase in N-NO3− concentration was detected, indicating the threshold value enabling short-cut nitrification (SN) to be between 150 mg L−1 and 75 mg L−1 under the pertaining conditions. Next, the influent concentration of TAN was gradually increased from 75 mg L−1 to 1000 mg L−1 but the nitrite accumulation was not restored. This indicates that once NOB are established in the suspended microbial culture, even high TAN concentrations are not sufficient for NOB inhibition.