A full-scale study at Pihlajaniemi BNR plant in Savonlinna Finland investigated in detail the conditions in which phosphorus release in the secondary clarifier can occur. For this purpose nutrient and ORP profiles were taken in different process conditions from the sludge blanket in the secondary clarifiers. The results show that in anaerobic conditions secondary phosphorus release can take place in the secondary clarifiers without any addition of exogenous COD. Long sludge retention time in the secondary clarifier was a prerequisite of anaerobic conditions in the sludge blanket, but retention time required varied with seasons and process conditions. Some indications of simultaneous phosphorus release and denitrification without COD input was also found. The re-absorption of the released phosphorus was observed in those parts of the sludge blanket, where nitrates were present. Even with very long sludge retention times the released phosphorus stayed inside the sludge blanket and it did not impair the effluent quality. An 18 month follow-up of the BNR process revealed, however, that occasionally dissolved phosphorus escaped from the sludge blanket. It was suggested, that this was the consequence of the missing nitrate containing layer at the upper part of the sludge blanket.