Impact of nitrite on partial nitrification in aerobic sewage treatment reactors under mainstream conditions

Stable partial nitrification is still difficult to achieve under mainstream conditions. High and low nitrite concentrations were established in two identical, completely mixed, continuous aerobic reactors in parallel to determine the effect of nitrite on partial nitrification under mainstream conditions. Influent ammonium (NH4+-N) concentrations of 50 +/- 3 and 10 +/- 2 mg/L were used to create high and low nitrite concentrations. Partial nitrification in the two reactors was established through an influent NH4+-N loading of 0.12 kg/(m(3).d) and dissolved oxygen (DO) levels of 0.1-0.3 mg/L. In a low nitrite-type reactor, the specific activity of nitriteoxidizing bacteria (NOB) was 0.061 +/- 0.005 kg/(kg volatile suspended solids WSSbd), which was lower than that in the high nitrite reactor (0.085 +/- 0.010 kg/(kg VSS.d)). Real-time quantitative PCR showed a significant correlation between the percentage of NOB in the bacterial communities and nitrite levels in the reactor. Compared to the NOB percentage in the high nitrite reactor (0.074 %), a lower percentage (0.049 %) occurred in the low nitrite reactor. A supplementary mathematical simulation indicated that a low nitrite concentration was critical for NOB repression at DO concentrations of 0.2-0.3 mg/L. Finally, the feasibility of the low nitrite strategy was demonstrated in a PN-ANAMMOX reactor with effluent recycling. Ammonium conversion efficiencies > 40.6 % and nitrite accumulation efficiencies > 43.1 % could be achieved at recycling ratios of 250-300 %, which created low nitrite concentrations (below 1.5 mg/L) in the aerobic zone.