Coupled anammox and denitrification for advanced nitrogen removal from partial denitrification effluent of landfill...

Partial denitrification coupled with anaerobic ammonium oxidation (anammox) offers a promising pathway for advanced nitrogen removal from nitrate-rich wastewaters with limited carbon availability. In this study, an anaerobic reactor was operated using partial denitrification effluent as the primary influent, supplemented with a fraction of landfill leachate to provide ammonium and biodegradable organic carbon, enabling the coupling of anammox and denitrification. When the influent COD, NH₄⁺-N and NO₂^--N were 165 ± 12, 75 ± 11 and 90 ± 9 mg L^-1, stable nitrogen removal was achieved with the effluent total nitrogen of 24 ± 12 mg L^-1 and a nitrogen removal rate of 2.20 ± 0.11 kg m^-3 d^-1. Stoichiometric analysis confirmed the concurrent contributions of anammox and denitrification, particularly under elevated COD conditions, which mitigated nitrate accumulation typically observed in standalone anammox systems. High-throughput sequencing revealed pronounced spatial stratification within the reactor, with granular anammox bacteria affiliated with Candidatus Kuenenia were enriched in the lower zone, whereas denitrifying genera, including Thauera and Pseudomonas, predominated in the upper region. This vertical partitioning likely reduced direct substrate competition and supported functional complementarity between autotrophic and heterotrophic nitrogen pathways. The results demonstrate that partial denitrification effluent-fed anammox-denitrification coupling provides an effective strategy for advanced nitrogen removal from complex and low C/N wastewaters such as landfill leachate.