The performance of an MBR pilot plant for biological nutrient removal was evaluated during 210 days of operation. The set point values for the internal recycles were determined in advance with the use of an optimisation spreadsheet based on the ASM2d model to optimise the simultaneous removal of C, N and P. The biological nutrient removal (BNR) efficiencies were high from the start of operation with COD and N removal efficiencies of 92 +/- 6% and 89 +/- 7, respectively. During the course of the experiment P removal efficiencies increased and finally a P-removal efficiency of 92% was achieved. The activity of poly-phosphate accumulating organisms (PAOs) and denitrifying poly-phosphate accumulating organisms (DPAOs) increased and the specific phosphate accumulation rates after 150 days of operation amounted to 13.6 mg P g(-1) VSS h(-1) and 5.6 mg P g(-1) VSS h(-1), for PAOs and DPAOs, respectively. (C) 2010 Elsevier Ltd. All rights reserved.