The degradation of (RS)-MCPP was investigated in an anaerobic membrane bioreactor (AnMBR) using nitrate as an available electron acceptor under different COD/NO3--N ratios. Results showed high soluble COD removal efficiency (80-93%) when the reactor was operated at high COD/NO3--N ratios. However, the COD removal started to decline (average 15%) at high nitrate concentrations coinciding with a drop in nitrate removal efficiency to 37%, suggesting that the denitrification activity dropped and affected the AnMBR performance when nitrate was the predominant electron acceptor. Additionally, the removal efficiency of (RS)-MCPP increased from 2% to 47% with reducing COD/NO3--N ratios, whilst the (RS)-MCPP specific utilisation rate (SUR) was inversely proportional to the COD/NO3- - N ratio, suggesting that a lower COD/NO3- - N ratios had a positive influence on the (RS)-MCPP SUR. Although nitrate had a major impact on methane production rates, the methane composition was stable (approximately 80%) for COD/NO3- - N ratios of 23 or more. (C) 2012 Elsevier Ltd. All rights reserved.