The interaction between enhanced biological phosphorus removal (EPBR) and biological nitrogen removal may result in EBPR failure in full-scale wastewater treatment plants (WWTPs). This work studies one of the common causes of this failure: the presence of nitrate in the anaerobic phase, which may act as an inhibitor for polyphosphate accumulating organisms (PAO) activity or may activate the competition between PAO and denitrifying bacteria for the carbon source. Several batch experiments were performed with different carbon sources (acetic acid, propionic acid and sucrose) at different nitrate concentrations using PAO-enriched sludge from two different pilot plants: an anaerobic/aerobic sequential batch reactor (SBR) and an anaerobic/anoxic/aerobic (A2/O) continuous plant. The results imply that the operational conditions of the A2/O pilot plant selected a PAO population capable of i) coexisting with nitrate without an inhibitory effect and ii) outcompeting denitrifying bacteria for the carbon source, in contrast to the SBR pilot plant where nitrate had an inhibitory effect on EBPR. Copyright (c) 2012 Society of Chemical Industry