Nitrogen removal from landfill leachate by single-stage anammox and partial-nitritation process: effects of microaerobic condition on performance and microbial activities

Nitrogen levels in landfill leachate (LL) could potentially pollute water bodies and surrounding groundwater, when discharged untreated. Against this backdrop, pollutant removal from leachate generated at landfill sites is essential. However, a cost-efficient approach, which could concurrently remove various pollutants within LL via single-stage approach, and under microaerobic conditions, has not been fully explored. In this study, single-stage nitrogen removal using anammox and partial-nitritation (SNAP) process was proposed and employed to treat LL under microaerobic condition in an upflow sludge blanket. When reactor dissolved oxygen-(DO) of 0.2 mg/L was implemented along with nitrogen loading rate-(NLR) ranging 0.31-1.84 kg/m(3).d, 99.5% NH4+-N, 94.3% TN and 31.04% chemical oxygen demand (COD) removals was concurrently achieved. Conversely, when DO was elevated to 0.6 mg/L, nitrogen removal was curtailed to <20%, NO2-N accumulation reached 98.68%, indicating inhibition of anammox bacterial (AmxB) activities. 16S-rRNA-high-throughput gene sequencing revealed ammonium oxidizing bacteria-[AOB] (Nitrosomonas and Sphingomonas), AmxB (Candidatus Kuenenia), heterotrophic denitrifiers-[HDB] (Comamonas, Dechloromonas), and COD degrading bacteria (Bellilinea, Ignavibacterium) as the dominant genera which coordinated synergistic activities to establish the SNAP process and led to the single-stage pollutants removal from the LL. Sludge morphological revealed by SEM and energy-dispersive X-ray spectroscopy indicated that reactor sludge comprised of granulated sludge, and was noticeably embedded in extracellular polymeric substances (EPS), an indication of a defense mechanism initiated by microbes against heavy metals toxicity. The SNAP process demonstrated a promising cost-efficient simultaneous nitrogen and carbon removal from LL.