Impact of excessive aeration on biological phosphorus removal from wastewater

It has been reported that deterioration of biological phosphorus removal (BPR) efficiency at some wastewater treatment plants (WWTPs) regularly occurred after heavy rainfall or weekends, The deterioration has been attributed to low plant loading that took place during such events. However, it is hypothesized in this study that the cause of such deterioration may have been the excessive aeration that Look place at some of those plants due to inadequate control of aeration system during weekends and rainfall periods. In order to prove this hypothesis. the influence of excessive aeration (aeration during starvation conditions) on BPR processes was studied using a laboratory anaerobic-aerobic-settling sequencing hatch reactor (SBR). It was clearly demonstrated that the phosphorus uptake stops due to a gradual depletion of poly-hydroxy-butyrate (PHB) in an over-aerated process. If organic substrate is introduced to the system, phosphorus release is immediately at its maximal rate. However, the released phosphorus cannot be taken-up fully again because the PHB content limits the uptake rate. Consequently, incomplete phosphorus uptake leads to temporary. reduction of BPR efficiency. This causal effect can explain the deterioration of BPR efficiency after heavy rainfall or weekends. Since excessive aeration clearly negatively affects the BPR processes, the aeration should be properly controlled at sewage treatment plants. Some other findings of this study deserve to be mentioned. It was confirmed that the presence of acetate under aerobic conditions provokes phosphorus release. This may also contribute to deterioration of the BPR efficiency, The aerobic phosphate uptake was found to depend not only on the PHB but also on polyphosphate (poly-P) content of the cells. A maximal poly-P (0.18 g-P g-VSS) and minimal PHB content of the cells (2.11 mg-COD/g-VSS) were observed in the enriched sludge during excessive aeration experiments. It was shown that, under aerobic starvation conditions, glycogen can not replace PHB for phosphate uptake and is only used for maintenance. During this period, no oxygen consumption due to decay processes has been observed. (C) 1998 Elsevier Science Ltd. All rights reserved.