The operation of nitrifying biomass as biofilms or granules allows the treatment of higher ammonia loads compared to the conventional activated sludge systems, due to the large amounts of biomass accumulated inside the reactors. In the present work, a sequencing batch reactor (SBR) containing nitrifying granules with an average diameter of 1.9-2.9 mm was operated for 880 d at room temperature (18-24 degrees C). The obtained granules presented high settling velocities around 100 m h(-1) and low sludge volumetric indexes of 30 mL(g VSS)(-1) which indicated their good settling properties. When nitrogen loading rate (NLR) of 0.4g NH4+-N L-1 d(-1) was fed, complete nitrification to nitrate was reached. The increase of the NLR to 0.8 g NH4+-N L-1 d(-1) caused a stable accumulation of nitrite with only 20% of nitrate production even by keeping the dissolved oxygen (DO) concentration in the bulk liquid at 8 mg O-2 L-1. Batch assays performed in a range from 2 to 30 mg O-2 L-1 of DO showed that partial nitrification was reached by fixing the DO concentration in the range from 2.0 to 3.5 mg O-2 L-1 in the bulk liquid. The total surface of the granules to bulk liquid ratio was low and ranged from 58 to 216m(2) m(-3) being the oxygen mass transfer through the interface bulk liquid-granule surface the limiting step. This oxygen transfer limitation allowed reaching stable partial nitrification at room temperature. For more than one year, an effluent with a mean NO2-/NH4+ molar ratio close to 1.0, suitable to feed a subsequent Anammox reactor, was obtained. A maximal nitrite production of 1 g NO2--N L-1 d(-1) was reached without nitrite oxidation to nitrate. The bacteria populations in the granules belonged mainly to the genus Nitrosomonas and were placed in the external layer of the granules of 100 mu m. (C) 2010 Elsevier B.V. All rights reserved.