Background High P/COD loading has been extensively applied to obtain a highly enriched culture of phosphate-accumulating organisms (PAOs) in an anaerobic-oxic (A/O) system. However, phosphorus rich influent was also observed to deteriorate the phosphorus metabolism. This study therefore operated acetate- and propionate-fed A/O reactors to examine the long-term effect of high P/COD loading on the enhancement of PAOs in both reactors. Results Long-term cultivating with 40/400 mg P (mg COD)-1 influent suppressed the proliferation of PAOs in the acetate-fed reactor, and more importantly provided glycogen-accumulating organisms (GAOs) with a selective advantage over PAOs. GAOs eventually dominate the A/O system even when the influent phosphorus was reduced back to 15 mg L-1. Conversely, PAOs was encouraged, and then stably sustained in the propionate-fed reactor with the same phosphorus influent (40/400 mg P (mg COD)-1). Obviously, acquiring a stablePAOs-enriched system via high phosphorus loading relied considerably on the choice of carbon source. Conclusions Propionate was more beneficial than acetate for supporting PAOs to suffer a high phosphorus loading in an enhanced biological phosphorus removal system. (c) 2012 Society of Chemical Industry