pH as a key factor in the competition between glycogen-accumulating organisms and phosphorus-accumulating organisms

The effects of pH on the anaerobic metabolism of glycogen-accumulating orga nisms (GAOs) and phosphorus-accumulating organisms (PAOs) were compared usi ng models for the kinetics of acetate uptake. The comparison revealed that GAOs take up acetate faster than PAOs when the pH of the anaerobic zone is less than 7.25, but that PAOs remove acetate faster than GAOs at pHs greate r than 7.5. It was also found that the growth efficiencies of the two organ isms are similar. Furthermore, the amount of polyhydroxy-alkanoates availab le after replenishment of the polymers used during acetate uptake under ana erobic conditions is similar for the two organisms, making GAOs highly comp etitive in nutrient removal systems. The effects of pH on the competition b etween the two organisms were demonstrated during the operation of a labora tory-scale sequencing batch reactor. When the overall pH of the system was low, poor phosphate removal was observed. When the pH of the system was all owed to increase to a maximum of 7.5, phosphate removal improved, but was s till incomplete. Total removal was only achieved when the pH of the system was never allowed to drop lower than 7.25. After the minimum pH in the syst em was increased, total removal of phosphate was achieved in 14 days. The r esults showed that pH control is a promising strategy for minimizing the ac cumulation of GAOs and increasing the reliability of biological excess phos phorus removal systems.