The effect of free nitrous acid on key anaerobic processes in enhanced biological phosphorus removal systems

In this study, the effect of nitrite/FNA on the anaerobic metabolism of polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) is investigated. The results clearly show that FNA has a detrimental effect on the acetate uptake rate by both PAOs and GAOs, but this adverse effect is much stronger on PAOs than on GAOs. Also, when FNA was increased, phosphate release to acetate uptake ratio by PAOs increased substantially (250–300% compared to control), which was accompanied by decreases (40–60%) in glycogen degradation and PHA production to VFA uptake. In contrast, these ratios for GAOs remained constant or increased slightly towards the highest FNA concentration applied. These results indicate that the anaerobic metabolism of PAOs is more adversely affected than that of GAOs when FNA is present. This might provide a competitive advantage to GAOs over PAOs in enhanced biological phosphorus removal systems when nitrite is present.