Analysis of the impact of reflux ratio on coupled partial nitrification–anammox for co-treatment of mature landfill leachate and domestic wastewater

•A UASB-A/O-ANAOR combined system was used to treat a mix of landfill leachate and domestic sewage.•Biological processing alone efficiently removed organic matter and nitrogen.•Advanced nitrogen removal from leachate with low C/N ratio was achieved without addition of carbon sources.•Inhibition of bacteria by free ammonia is key to partial nitrification and anammox.•An optimum reflux ratios can improve processing and enrich anammox bacteria.

A combined system of anaerobic–aerobic techniques was used for deep co-treatment of mature landfill leachate and domestic sewage, mixed at a ratio of 1:5, through partial nitrification–anammox. While ensuring synchronous removal of organic matter and nitrogen, we investigated changes in several factors under different reflux ratios (0%, 100% and 300%). High nitrification efficiency and a relatively high accumulation rate of NO2−-N were achieved, the latter through selective inhibition of bacteria by free ammonia. The results indicated that maintaining the reflux ratio within an optimum range contributes to the enrichment of anammox bacteria. In this study, the optimum ratio, which permitted the highest removal rates of chemical oxygen demand (COD), NH4+-N, and total nitrogen, was 300%. The COD level in the anammox reactor (ANAOR) decreased to lower than 70 mg/L, resulting in no inhibitory effect on anammox and maximum enrichment of bacteria.