Accumulating and utilizing PHAs (polyhydroxyalkanoates), i.e. a major carbon reserve of polyphosphate accumulating organisms (PAOs), is a prerequisite for phosphorus removal in an enhanced biological phosphorus removal (EBPR) system. To evaluate phosphorus removal, this study investigates the behavior of PHAs in a hybrid anaerobic-anoxic-aerobic process, operating under various sludge retention times (5, 10, 12 and 15 days) and dissolved oxygen conditions (0.5, 1.0 and 2.0 mg/l in aerobic stage). PHAs and phosphorus measurements in the pilot-scale experiments demonstrate that the PHAs content of sludge closely relates to phosphorus release and uptake behavios under anaerobic and aerobic conditions, respectively. The aerobic specific-phosphorus-uptake-rate is directly proportional to PHAs content of sludge in the anoxic stage. When the process is under a high organic loading condition, the sludge exhibits a large amount of PHAs having accumulated in the anoxic stage and a high phosphorus uptake rate in the subsequent aerobic stage. However, experimental results confirm that anoxic phosphorus release, leading to a high concentration of phosphorus flow into the aerobic stage, causes deficient phosphorus removal under a high organic loading condition. Moreover, a low PHAs content of sludge causes incomplete phosphorus removal; the phenomenon occurs when the process is under a low organic loading condition. Based on the results presented herein, we can conclude that the organic loading should be carefully controlled for phosphorus removal in the anaerobic-anoxic-aerobic process.

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