IMPACT OF EXCESSIVE AERATION ON BIOLOGICAL PHOSPHORUS REMOVAL FROM WASTE-WATER

It has been reported that deterioration of biological phosphorus remov al (BPR) efficiency at some wastewater treatment plants (WWTPs) regula rly occurred after heavy rainfall or weekends, The deterioration has b een attributed to low plant loading that took place during such events . However, it is hypothesized in this study that the cause of such det erioration may have been the excessive aeration that Look place at som e 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-se ttling sequencing hatch reactor (SBR). It was clearly demonstrated tha t the phosphorus uptake stops due to a gradual depletion of poly-hydro xy-butyrate (PHB) in an over-aerated process. If organic substrate is introduced to the system, phosphorus release is immediately at its max imal rate. However, the released phosphorus cannot be taken-up fully a gain because the PHB content limits the uptake rate. Consequently, inc omplete phosphorus uptake leads to temporary. reduction of BPR efficie ncy. This causal effect can explain the deterioration of BPR efficienc y after heavy rainfall or weekends. Since excessive aeration clearly n egatively affects the BPR processes, the aeration should be properly c ontrolled at sewage treatment plants. Some other findings of this stud y deserve to be mentioned. It was confirmed that the presence of aceta te under aerobic conditions provokes phosphorus release. This may also contribute to deterioration of the BPR efficiency, The aerobic phosph ate uptake was found to depend not only on the PHB but also on polypho sphate (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 observ ed in the enriched sludge during excessive aeration experiments. It wa s shown that, under aerobic starvation conditions, glycogen can not re place PHB for phosphate uptake and is only used for maintenance. Durin g this period, no oxygen consumption due to decay processes has been o bserved. (C) 1998 Elsevier Science Ltd. All rights reserved.