Insight into continuous-flow partial nitrification granular sludge system: Long-term performance, formation mechanism, and partial nitrification granular sludge/Anammox coupled system for mature landfill leachate treatment

[Display omitted] •PNGS exhibited efficient NO2–-N accumulation under 200 mg N/L free NH3.•Effects of MLL on PN performance, genes abundance, enzymes activity were explored.•EPS secretion mediated by AHLs and c-di-GMP enhanced when treating MLL.•Enhanced EPS adsorbed refractory organics and embedded SiO2 for granulation.•PNGS/Anammox system could simultaneously remove COD and TIN in MLL. A continuous-flow partial nitrification granular sludge (PNGS) coupled Anammox system was constructed for mature landfill leachate (MLL) treatment. Stable NO2–-N accumulation was achieved with NH4+-N to NO2–-N transformation ratio (NTR) of 98–100 % with influent NH4+-N ranged from 342 ± 29 to 1106 ± 20 mg/L. When treating MLL, particular acyl homoserine lactones (AHLs), cyclic dimeric guanosine monophosphate (c-di-GMP) concentration significantly increased and more extracellular polymeric substances (EPS) were secreted, which adsorbed refractory organics and embedded SiO2 derived from MLL for granulation. A strong and positive correlation was found between PNGS average diameter and EPS, indicating that AHLs and c-di-GMP may play a significant role in the formation and evolution of PNGS via regulating EPS secretion. The PNGS/Anammox system could remove COD and nitrogen simultaneously under different MLL loadings, with COD and total inorganic nitrogen removal efficiency of 28 ± 5 %-71 ± 2 % and 66 ± 2 %-89 ± 1 %, respectively.