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Partial nitrification (nitritation) has been coupled with anaerobic ammonium oxidation (anammox) in recent years as a cost-effective alternative to the nitrification-denitrification process. This study investigated the effects of unsaturated zone depth and alkalinity on nitrogen removal in three recirculating biofilters, which had different depths of an unsaturated zone and the same depth (35 cm) of a saturated zone. Electric arc furnace slag was mixed with rubber mulch in the unsaturated zones to raise pH. Marble chips were packed in the saturated zone to supplement bicarbonate and buffer pH. The unsaturated zone may maintain low concentrations of dissolved oxygen and enhance entrapment of carbon dioxide for co-occurrence of aerobic ammonia oxidation and anammox in the saturated zones. The biofilters were operated by a batch mode with a cycle length of 2 or 3 weeks. The biofilter with a 60-cm unsaturated zone had the highest ammonium and total inorganic nitrogen removal rates (16.5-32.7 g N/m3d and 3.1-16.3 g N/m3d respectively) compared with the biofilters with 25 and 40 cm unsaturated zones. Marble chips start to release alkalinity when pH is below 7.3, which is sufficient to support ammonium removal up to 1520 g N/m3d at pH between 6.2 and 7.3 using the nitritation-anammox process. This study demonstrated that recirculating biofilters with an unsaturated zone were effective single-stage ecologically engineered treatment systems for nitrogen removal using nitritation-anammox.