Conventional schemes of biological nitrogen removal can be combined with phosphorus removal schemes. One of the common technology schemes for biological nitrogen removal is the aeration zone and the anoxic zone in one tank. The nitrification and denitrification are carried out during the aeration switching on and off. The anaerobic zone is equipped behind the nitrification/denitrification tank for biological phosphorus removal. Exchange of the anaerobic and aerobic conditions is necessary for biological phosphorus removal. The concentration of oxygen must be sufficient in the aerobic zone for microorganisms that have accumulated phosphorus for complete or near complete nitrification.

The aim of the work was to evaluate the effect of biological nitrogen removal from wastewater on biological phosphorus removal. The evaluation of ammonia nitrogen quantity in the aeration zone was carried out before complete nitrification and the moment of phosphates split from microorganisms to the wastewater, and the time when the phosphates split is complete, if the aerobic phase is extended and the mixture temperature varies.

Dependence between partial nitrification and phosphate release in the aerobic zone and the impact of the aerobic phase prolongation on phosphate release in the aerobic zone was estimated in warm and cold seasons.

Keywords: total nitrogen (TN), total phosphorus (TP), biological active potential (BPA), ratio of biochemical oxygen demand and total phosphorus (BOD7/TP), phosphates, nitrates