Enhancing simultaneous nitritation and anammox in recirculating biofilters: Effects of unsaturated zone depth and alkalinity dissolution of packing materials

► Three recirculating biofilters were used to enhance simultaneous nitritation and anammox. ► Furnace slag was packed in unsaturated zone and marble chips in saturated zone. ► Unsaturated zone renews O2 and CO2 for nitritation-anammox in saturated zone. ► Biofilters with 40-60 cm unsaturated zones had higher N removal rates vs. 25 cm. ► Electric arc furnace slag is low-cost, effective packing material to amend alkalinity.