Evaluation of the potential effects of equalization on the performance of biological phosphorus removal systems

Experimental data confirming that the phosphorus removal efficiency in biol ogical excess phosphorus removal (BEPR) systems temporarily decreases when the amount of volatile fatty acids (VFAs) added in the anaerobic phase is s uddenly increased are presented. This decrease in efficiency results from t he fact that acetate uptake is a rapid process and that the phosphate conce ntration at the end of the anaerobic phase increases rapidly. Because of th e nonlinear dependence of the phosphate uptake rate on the poly-beta -hydro xyalkanoate (PHA) content of phosphate-accumulating organisms (PAOs), the i ncrease in PAO PHA content associated with VFA uptake is not able to cause a proportional increase in the rate of phosphate uptake. This causes a temp orary imbalance between phosphate release and uptake, leading to lower phos phate removal efficiency. The VFA loading to full-scale BEPR systems is not constant throughout the day, and temporary imbalances such as the ones imp osed in the batch tests can occur in full-scale systems. The effect of diur nal variations in loading was demonstrated through simulation of the behavi or of an A/O (TM) system receiving a time-variable influent. Equalization i s proposed as a method to diminish the potential for imbalances between pho sphate release and uptake by avoiding sudden increases of VFA loading to th e plant. Significant improvements in the effluent quality from the simulate d system were achieved using equalization. The improvements were greater wh en the influent contained VFAs than when the VFAs were formed by fermentati on in the anaerobic zone. The simulations suggested that it may be possible to decrease the amount of phosphorus discharged by a factor as high as 4 t hrough use of concentration equalization. When both flow and concentration equalization were used, the total amount of phosphorus discharged was decre ased by a factor of 8. Equalization can be used, in concert with other stra tegies for preservation of the PHA content of PAOs under periods of low loa dings, to minimize the magnitude of Monday phosphate peaks.