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

Experimental data confirming that the phosphorus removal efficiency in biological excess phosphorus removal (BEPR) systems temporarily decreases when the amount of volatile fatty acids (VFAs) added in the anaerobic phase is suddenly increased are presented. This decrease in efficiency results from the fact that acetate uptake is a rapid process and that the phosphate concentration at the end of the anaerobic phase increases rapidly. Because of the nonlinear dependence of the phosphate uptake rate on the poly-beta-hydroxyalkanoate (PHA) content of phosphate-accumulating organisms (PAOs), the increase 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 temporary imbalance between phosphate release and uptake, leading to lower phosphate removal efficiency. The VFA loading to full-scale BEPR systems is not constant throughout the day, and temporary imbalances such as the ones imposed in the batch tests can occur in full-scale systems. The effect of diurnal variations in loading was demonstrated through simulation of the behavior of an A/OTM system receiving a time-variable influent. Equalization is proposed as a method to diminish the potential for imbalances between phosphate release and uptake by avoiding sudden increases of VFA loading to the plant. Significant improvements in the effluent quality from the simulated system were achieved using equalization. The improvements were greater when the influent contained VFAs than when the VFAs were formed by fermentation 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 through use of concentration equalization. When both flow and concentration equalization were used, the total amount of phosphorus discharged was decreased by a factor of 8. Equalization can be used, in concert with other strategies for preservation of the PHA content of PAOs under periods of low loadings, to minimize the magnitude of Monday phosphate peaks.