Nitrate and ammonium removal from purified swine wastewater using biogas and air was investigated in continuous reactor operation. A novel type of reactor, a semi-partitioned reactor (SPR), which enables a biological reaction using methane and oxygen in the water phase and discharges these unused gases separately, was operated with a varying gas supply rate. Successful removal of NO3- and NH4+ was observed when biogas and air of 1 L/min was supplied to an SPR of 9 L water phase with a NO2,3--N and NH4+-N removal rate of 0.10 g/L/day and 0.060 g/L/day, respectively. The original biogas contained an average of 77.2% methane, and tile discharged biogas from the SPR contained an average of 76.9% of unused methane that was useable for energy like heat or electricity production. Methane was contained in the discharged air from the SPR at all average of 2.1%. When gas supply rates were raised to 2 L/min and the nitrogen load was increased, NO3- concentration was decreased, but NO2- accumulated in the reactor and the NO2,3--N and NH4+-N removal activity declined. To recover the activity, lowering of the nitrogen load and the gas supply rate was needed. This study shows that the SPR enables nitrogen removal from purified swine wastewater using biogas under limited gas supply condition. (C) 2007 Elsevier Ltd. All rights reserved.