<i>In Situ</i> Anammox Self-Enrichment Usinga Pilot-Scale Hybrid Membrane-Aerated Biofilm Reactor System for MunicipalWastewater Treatment with a Low Carbon to Nitrogen Ratio

This study pioneers the demonstration of <i>in situ</i> Anammox self-enrichment using a pilot-scale hybrid membrane-aerated biofilm reactor (MABR) system. This system was inoculated with ordinary nitrifying–denitrifying sludge to treat municipal wastewater with a low sCOD/TN ratio of 1.5 to 1.9. At a hydraulic retention time of 6 h and a sludge reflux ratio of 50%, the hybrid MABR achieved an average volumetric total nitrogen removal rate of 192.1 g/m³·d in the anoxic unit, outperforming the Modified Ludzack–Ettinger (MLE) system by 3.9 times. <i>Candidatus Brocadia</i>, representing 6.91%, was the most dominant bacterial genus in the MABR biofilm, providing vital evidence of Anammox bacterial enrichment. Additionally, quantitative PCR was used to quantify specific functional genes in three different sections (inlet, middle, and outlet) of the MABR biofilm, revealing spatial heterogeneity with a longitudinal O₂ level shift. As a result, the abundances of <i>narG</i> (positively correlated with partial denitrification, PD) and <i>nxr</i> (negatively correlated with partial nitritation, PN) were both high in the inlet section and synchronously decreased toward the outlet. This result is promising in light of the hypothesis that the main supporting reaction for Anammox near the inlet is highly potentially PD, then gradually shifting to PN as the O₂ level diminishes toward the outlet, enabling the coupling of PN/A-PD/A in MABR. These findings offer valuable insights into the mechanisms underlying Anammox application in MABR, advancing the treatment of municipal wastewater with a carbon-to-nitrogen ratio.