Phosphate uptake under anoxic conditions and fixed-film nitrification in nutrient removal activated sludge system

Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system. The aim of the experiments was to find whether poly-P bacteria are capable of taking up phosphate under anoxic conditions by utilising nitrate as an electron acceptor. The phosphate uptake in anoxic conditions was compared to that in aerobic environment in batch tests. The results of the long-term operation of continuous-flow lab-scale system as well as the results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after preceding 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 enabled to reduce the losses of organic carbon by aerobic oxidation and to stabilise the slow-growing population of nitrifiers in the system.