In the aerated period ammonium and phosphorus uptake occurred, while nitrate was produced. Once ammonium reached its desired set-point the oxygen concentration in the bulk was lowered to allow for maximum denitrification potential. Denitrification is taking place inside the granule. Since the diffusion of oxygen into the granule is slower than the conversion rate, an anoxic volume is created inside the granules (de Kreuk et al., 2005; Osman et al.,2001). Storage polymers of the GAOs and PAOs are delivering the reducing power for the conversion of nitrate to dinitrogen gas. Once both nitrogen and phosphorus have reached the required effluent quality the reactor is ready to receive new influent.