Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system (DEPHANOX). The aim of the experiments was to find whether it can be technologically convenient to use the capability of poly-P bacteria of taking up phosphate under anoxic condition by utilizing nitrate as electron acceptor. An experimental comparison of removal efficiencies with a more conventional treatment flow scheme (JHB), using low influent TKN/COD ratio, was also carried out. Phosphate uptake in anoxic condition was compared to that in oxic environment in batch tests. PHB anoxic and oxic degradation was evaluated by respirometric tests. Results of the long-term operation of continuous-flow lab-scale system as well as results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after a previous anaerobic substrate uptake could significantly reduce the extent of competition for organic substrate between poly-P bacteria and denitrifiers. A side-stream nitrification in fixed-film reactor reduced the losses of organic carbon by oxidation and stabilized the slow-growing population of nitrifiers in the system.

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