Pre-anaerobic phase persists benefits for nitrogen removal in high carbon/nitrogen scenarios: Comparative study of aerobic–anoxic and anaerobic–aerobic–anoxic sequencing batch reactors

The pre-anaerobic phase in anaerobic–aerobic–anoxic (AOA) process is critical to intensifying nitrogen removal at low C/N ratios, yet its applicability to high C/N scenarios remains vague. Here, four bench-scale aerobic–anoxic sequencing batch reactors (OA-SBRs) were established at different C/N ratios (3–16.5) and compared with the parallel AOA-SBRs. The results showed that the NH4+–N and TN removal in the OA-SBRs increased by 24.7% and 112.2%, respectively, as the C/N ratio elevated from 3 to 16.5, but they remained lower than those in the AOA-SBRs till carbon leakage. Typical cycle tests indicated that the pre-anaerobic phase probably endowed the endogenous nitrification/denitrification in the AOA-SBRs with kinetic advantages over the canonical ones in the OA-SBRs, supported by the improved enzymatic activities. Further, microbial insights identified more intensive niche competition between heterotrophic nitrifiers/denitrifiers and non-functional heterotrophs in the OA-SBRs, which could explain the poorer nitrogen removal without the pre-anaerobic phase. This study underscores the implications of pre-anaerobic phase in nitrogen removal of AOA process in high C/N scenarios. [Display omitted] •Heterotrophic nitrification/denitrification dominated in OA-SBRs at C/N 12–16.5.•Heterotrophic nitrogen removal remained carbon-limited in OA-SBRs even at C/N 12.•Carbon competition impaired advantages of canonical heterotrophic nitrogen removal.•Pre-anaerobic phase can relieve niche competition from non-functional heterotrophs.•Pre-anaerobic phase can persist benefits for nitrogen removal at high C/N ratios.