The nitrate produced by the one-stage partial nitritation-anammox (PN/A) process can be removed through partial denitrification (PD) by adding carbon source. In this study, a 1D multi-population biofilm model was developed to evaluate the contribution of partial denitrification on the one-stage autotrophic nitrogen removal process at influent NH4+ = 100 mg N/L. The dynamic simulation that was carried out to investigate the effect of nitrite-oxidizing bacteria (NOB) revealed that PD contributed to the reactor to obtain total nitrogen removal efficiency (TNR) of above 90% and the effluent nitrate was significantly decreased with the absence of NOB. However, PD decreased TNR of the one-stage PN/A process with the presence of NOB. Increased influent chemical oxygen demand (COD) widened the dissolved oxygen (DO) range required for high TNR whether NOB were present or not. The steady-state simulation results showed that NOB were always absent in the granules at high DO and COD levels and the optimum DO > 0.5 mg/L when influent COD was over 50 mg/L. Besides, higher influent COD/NH4+ (C/N) and larger granule diameter (diameter > 1600 µm) were contributed to widening the range of DO required for high TNR. The nitrogen removal contribution of anammox bacteria (AMX) was significantly higher than denitrification in the reactor.

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