Modeling nitrogen removal with partial nitritation and anammox in one floc-based sequencing batch reactor

Full-scale application of partial nitritation and anammox in a single floc-based sequencing batch reactor (SBR) has been achieved for high-rate nitrogen (N) removal, but mechanisms resulting in reliable operation are not well understood. In this work, a mathematical model was calibrated and validated to evaluate operating conditions that lead to out-competition of nitrite oxidizers (NOB) from the SBRs and allow to maintain high anammox activity during long-term operation. The validity of the model was tested using experimental data from two independent previously reported floc-based full-scale SBRs for N-removal via partial nitritation and anammox, with different aeration strategies at aeration phase (continuous vs. intermittent aeration). The model described the SBR cycle profiles and long-term dynamic data from the two SBR plants sufficiently and provided insights into the dynamics of microbial population fractions and N-removal performance. Ammonium oxidation and anammox reaction could occur simultaneously at DO range of 0.15-0.3 mg O2 L(-1) at aeration phase under continuous aeration condition, allowing simplified process control compared to intermittent aeration. The oxygen supply beyond prompt depletion by ammonium oxidizers (AOB) would lead to the growth of NOB competing with anammox for nitrite. NOB could also be washed out of the system and high anammox fractions could be maintained by controlling sludge age higher than 40 days and DO at around 0.2 mg O2 L(-1). Furthermore, the results suggest that N-removal in SBR occurs via both alternating nitritation/anammox and simultaneous nitritation/anammox, supporting an alternative strategy to improve N-removal in this promising treatment process, i.e., different anaerobic phases can be implemented in the SBR-cycle configuration.