This study investigates variations of phosphorus and polyhydroxyalkanoates (PHAs) in a combined activated sludge - biofilm process, operating under various sludge retention times (5, 10 and 15 days) and different dissolved oxygen conditions (0.1, 0.5, 1.0 and 2.0 mg/l in aerobic stage). Experimental results indicate that phosphorus uptake closely corresponds to utilization of PHAs during anoxic and aerobic stages. Moreover, the sludge in the anoxic stage exhibits a higher PHAs utilization efficiency with respect to phosphorus uptake than sludge in the aerobic stage, when it is under low COD-SS loading conditions. The values of r(P/PHAs) representing sludge capacity on phosphorus uptake, range from 0.1-1.0 mg P/mg PHAs. In addition, analyzing the distribution of 3-hydroxybutyrate (3HB), 3-hydroxy-2-methylbutyrate (3H2MB), 3-hydroxyvalerate (3HV) and 3-hydroxy-2-methylvalerate (3H2MV) reveals that 3HB and 3HV are the major components of PHAs. The values of 3HB/PHAs and 3HV/PHAs vary with COD-SS loading of the process. When F/M ratio increases, 3HV/PHAs value increases and 3HB/PHAs value decreases simultaneously. This phenomenon implies that more bacteria accumulated 3HV as storage matter under high COD-SS loading conditions. The kind of bacteria population shift would intensify the competition of "G bacteria" with polyphosphate accumulating organisms, possibly causing process deterioration during phosphorus removal.
