This study shows that an MBR pilot plant with UCT configuration is able to obtain high nutrient removal efficiency already during start-up. The biological nutrient removal (BNR) efficiencies significantly increased towards the end of the experimental run, achieving a COD removal efficiency exceeding 94% and N removal efficiency in the range of 89 to 93%. P removal efficiencies in the range of 80 to 92% have been obtained. During the experimental period (4 months) the evolution of the activity of polyphosphate-accumulating organisms, obtained from P-release and P-uptake rates, showed a small increase in the activity of polyphosphate-accumulating organisms (PAOs) and denitrifying polyphosphate-accumulating organisms (DPAOs). The specific phosphate accumulation at the end of the experimental run amounted to 8.0 mg P g(-1)VSS h(-1) and 3.29 mg P g(-1)VSS h(-1), for the PAOs and DPAOs respectively. Moreover, the DPAOs activity increased faster than PAOs activity, i.e. from 0.36 to 0.41 of phosphate uptake rate (PUR) ratio. (C) 2009 Elsevier B.V. All rights reserved.