Treatment performance and N 2 O 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.