Phosphorus removal by chemical precipitation is a well-established and widely used technique in municipal waste water treatment plants (WWTPs). Very low effluent concentrations can be achieved in order to comply with standards for discharge in sensitive areas, in force in the EU; nevertheless, failures of this system are occasionally recorded. In this work, a 330,000 people equivalent (p.e.) WWTP was studied, where co-precipitation was not effective to guarantee an effluent concentration stably below 1 mg P/L, despite the great expenditure for chemicals (around 260,000 (sic)/y) and additional sludge disposal (around 160,000 (sic)/y). Based on results of laboratory tests and mathematical simulations, it was shown that bio-P release occurs in final clarifiers under special conditions, related to Sludge Retention Time (SRT) in the settling tanks, Dissolved Oxygen (DO) in nitrification basins and nitrates concentration in the effluent. Therefore, complying with effluent standards should require keeping process conditions as follows: DO > 1 mg/L, N-NO3- > 5 mg/L and SRT < 3 h. As additional measure, a post-precipitation (required dosage: 4-5 mg Al/mg P) could be applied. (C) 2013 Elsevier B.V. All rights reserved.