In this study, the effect of various factors such as C:N ratio, carbon source, percentage P content in the sludge influencing the simultaneous denitrification and enhanced biological phosphorus removal was investigated in batch tests on bean and tomato waste sludge from an upflow anaerobic sludge blanket reactor-anoxic/aerobic system and municipal sludge from a circulating fluidized bed bioreactor. A correlation between the change in redox potential and rate of P release was developed. Interestingly, maximum P release was observed at positive redox potential in some of the batch tests. Simultaneous denitrification and P release under anoxic conditions was observed during all the batch tests. Sludge acclimatization improved the efficiency of the sludge and proved independency of maximum specific denitrification rate and P content of sludges. The contribution of denitrifying PAOs to anoxic P uptake was determined through the denitrification control test at an initial level Of PO4-P of 100-120 mg dm(-3). (c) 2006 Society of Chemical Industry.