One-stage and multistage anoxic/oxic systems based on continuous plug-flow anammox process for mature landfill leachate treatment

As a promising biotechnology, the anammox process has been increasingly investigated for landfill leachate treatment. However, its application has been limited by the long generation time of anammox bacteria (AnAOB) and insufficient nitrite supply in actual projects. To overcome these obstacles, this study innovatively established one-stage and multistage anoxic/oxic systems (S1 and S2) based on the continuous plug-flow anammox process to treat mature landfill leachate. Ultra-efficient nitrogen removal was achieved through the partial nitrification coupled anammox (PNA) and the partial denitrification coupled anammox (PDA) pathways. Over 97.0% of nitrogen was removed from the leachate (1800-2000 mg NH4+-N/L). Stable isotope tracing tests demonstrated that 89.8%-92.4% and 90.3%-95.0% of the nitrogen loss were contributed by the anammox pathway in S1 and S2, respectively. 16S rRNA sequencing revealed that the oxic zone, especially the oxic biofilm was the hotspot for AnAOB enrichment. Candidatus_Kuenenia with higher nitrite affinity and stronger tolerance to free ammonia (FA) and salinity stress outcompeted Candidatus_Brocadia and became the dominant anammox genus. Multiple bacteria capable of hydrolysis and acidogenesis were enriched under the in-situ FA anoxic treatment, facilitating complex organic matters degradation and volatile fatty acids production, which could be used as the carbon source during the PDA process.