Nitrifying bacteria are characterized by low biomass yields and slow activity rates, the enhancement of nitrifying biomass retention by promoting the formation of granules would allow improving nitrification process. The use of a Pulsing sequencing batch reactor (SBR) was tested to produce nitrifying granules minimizing the air consumption. After 400 days of operation, 1.07 g VSS/L of granular biomass with a mean diameter of 0.9 mm was accumulated in the system. The maximal nitrate production rate reached a value of 0.3 kg N/(m(3) d). Sporadic nitrite accumulations were registered and attributed to oxygen diffusion limitations which were solved improving the granule-liquid mass transfer with a liquid recirculation system. FISH technique was used in order to identify the main populations in the nitrifying granules: Nitrosomonas spp. and Nitrospiro spp. were identified as the main ammonia and nitrite oxidizing population, respectively. With the introduction of liquid recirculation in the reactor the ammonia oxidation efficiencies improved and the proportion of Nitrospira spp. increased from 1% to 15%. (C) 2009 Elsevier Ltd. All rights reserved.