Revealing the Role of Partial Denitrification in the Mainstream Single-Stage Partial Nitrification/Anammox Process: Experimental Evidence and Kinetic Modeling Insights.

The application of the single-stage partial nitrification/anammox (PN/A) process remains challenging under mainstream conditions. A continuous-flow single-stage PN/A reactor treating low-strength ammonium (NH4 +) wastewater without influent chemical oxygen demand (COD) was investigated, and a modified kinetic model was developed to clarify the role of partial denitrification (PD) in the presence of influent COD. Long-term operation showed that oxygen control alone could not substantially improve reactor performance. At dissolved oxygen (DO) levels of 0.20, 0.10, and 0.15 mg O2/L, the NH4 + removal efficiency was 97.97%, 92.56%, and 97.63%, whereas the total nitrogen removal efficiency was only 78.77%, 62.79%, and 80.01%, respectively. Lowering oxygen supply reduced both ammonium-oxidizing bacteria and NOB activities, whereas increasing oxygen supply restored NH4 + oxidation but again promoted nitrate (NO3 -) accumulation. Model analysis indicated that a small amount of influent COD (60 mg COD/L) enhanced nitrogen removal and process stability under decreasing temperatures and influent NH4 + fluctuations. This effect was mainly attributed to PD, which converted excess NO3 - to nitrite (NO2 -) and supplied an additional substrate for anammox bacteria (AMX). Meanwhile, heterotrophic growth increased oxygen consumption, lowered bulk DO levels, enhanced NOB suppression, and alleviated oxygen inhibition on AMX. A favorable influent COD/NH4 + range of approximately 0.5-1.25 was identified under the simulated conditions, which suggests that PD can play an important role in stabilizing mainstream single-stage PN/A.