Experimental and model assisted investigation of an operational strategy for the BPR under low influent concentrations

The behaviour of a pilot scale biological phosphorus removal process (BPR) of the alternating type was investigated during periods of low influent concentrations and increased hydraulic load. A process disturbance of this type result in an increase in the phosphate concentration level in the anoxic/aerobic reactors and in the plant effluent shortlyafter the influent wastewater returns to normal strength. The accumulation of phosphorus in the system was avoided by the addition of an external carbon source either to the influent or to the effluent from the anaerobic reactor in form of sodium acetate. With the help of such an addition, the internal carbon storage compounds could be maintained at a high level, which is shown by poly-hydroxy-alcanoates (PHA) measurements. Several levels of acetate addition were investigated experimentallyin order to determine a minimal amount of internallystored carbon, which could ensure the stabilization of BPR during such dynamic influent conditions. Furthermore reduction of aeration time during periods of low influent concentrations was investigated. It was observed that BPR was stabilized bycombining a reduction of aeration time with carbon source addition, which maintained the internal stored carbon at a higher level. This combined control action resulted in a desired high BPR activitywhen the normal strength of the influent wastewater was reestablished. The failure of the BPR process was sometimes observed even when comparativelyhigh concentrations of PHA could be detected and an identification of a minimal PHA level was not possible. During this investigation an extended version of the activated sludge model No. 2 (ASM2), which includes denitrification byphosphate accumulating organisms, is used for the detailed analysis of the experiments. The model predicted the phosphorus build-up after the process disturbance as well as the performance during the stabilized experiments. Assisted by the model, the investigations indicate that a PHA limitation is not the onlyfactor affecting the recovery of the BPR process during periods of low influent concentrations.