Treatment performance and N2O emission in the UASB-A/O shortcut biological nitrogen removal system for landfill leachate at different salinity

• Saline landfill leachate was treated using UASB combine with A/O by SBNR.
• The characteristics of N2O emission under different salinity were elucidated.
• The N2O conversion ratio to the influent removed NH4+-N increased by 14.7 times.
• The COD entering the A/O was increased and thus stimulated N2O emission remarkably during the aerobic zone.
• The increase of Nitrosomonas europaea was an important factor for the increase of N2O emission.

Landfill leachate was treated using the upflow anaerobic sludge blanket (UASB) combined with anoxic/aerobic reactor (A/O) for shortcut biological nitrogen removal. The treatment performance and nitrous oxide (N2O) emission were studied under different salinity. When salinity increased from 10 to 35 g/L, the removal efficiency of ammonium nitrogen and total nitrogen decreased from 99.3 to 83.9% and from 85.4 to 68.4%, respectively while the nitrite accumulation rate always remained high at more than 94.3%. The N2O conversion ratio increased by 14.7 times, from 0.3 to 4.7%. The concentration of biodegradable COD in influent of the A/O reactor increased with the increase of salinity, which lead to the decrease of dissolved oxygen and thus stimulated N2O emission remarkably during the aerobic zone. Meanwhile, the N2O emissions were likely to occur by a combination of pathways since the simultaneous nitritation and denitritation was occurred in aerobic zone. The increase of NH4+-N concentration, NO2−-N concentration and pH caused by high salinity also promoted the N2O emission. The population of Nitrosomonas europaea increased with salinity, which was important for N2O emission.