Understanding what limits biological phosphorus removal at Uster’s wastewater treatment plant

This study aims at better understanding what limits the biological phosphate removal at Uster’s wastewater treatment plant (WWTP). Two hypotheses were proposed to explain the limited biological phosphate removal observed at Uster’s WWTP: (i) A heterogeneous injection of the wastewater that prevented an optimal contact between the sludge and the raw water during the anaerobic feeding or (ii) a limited availability of organic matter in the raw water that prevented the development of polyphosphate accumulating organisms. Three approaches were applied to address the hypotheses. An on-site pilot-reactor was installed to test how a homogenous injection of the wastewater at the reactor bottom influences the biological phosphate removal (Approach 1). Batch experiments were performed to investigate the influence of the wastewater composition on the biological phosphate removal (Approach 2). Finally, sequencing batch reactors were simulated with different influent compositions and sequence of operation, in order to identify the optimum operation of the reactor to obtain biological phosphorus removal (Approach 3). The main finding of this master project is that the organic matter content of the raw water limits the denitrification, and ultimately the growth of polyphosphate accumulating organisms. Full denitrification is indeed necessary to maintain anaerobic conditions, which are necessary for the phosphate release and then aerobic uptake. Thus, the COD concentration limits the phosphorus removal, by limiting the denitrification. Both the batch experiments and the modeling validated this conclusion: In the batch tests (Approach 2), when the COD concentration of the wastewater was higher (with medium strength WW, addition of acetate and Uster’s WW pre-clarifier), full denitrification was assured and consequently phosphate removal. According to the model predictions (Approach 3) the length of the anaerobic period in the operation of the reactor was found to have an important impact on the achievement of biological phosphorus removal from Uster’s wastewater. Increasing the length of the anaerobic period helps to achieve full denitrification. According to the model predictions, biological phosphorus removal from Uster’s WW was possible only with an extended anaerobic period of 2h30, instead of 1h30 initially. The operation of the pilot-scale reactor was on the other hand hampered by several technical problems. Stable operating conditions were thus not achieved. Nevertheless, the homogeneous injection of the wastewater and the operation of the reactor at constant volume should theoretically have a positive impact on biological phosphate removal efficiency.