Free nitrous acid inhibition on the aerobic metabolism of poly-phosphate accumulating organisms

In full-scale wastewater treatment systems, phosphorus removal typically occurs together with nitrogen removal. Nitrite, an intermediate of both the nitrification and denitrification processes, can accumulate in the reactor. The inhibitory effect of nitrite/free nitrous acid (FNA) on the aerobic metabolism of poly-phosphate accumulating organisms (PAOs) is investigated. A culture highly enriched (90 ± 5%) in Candidatus “Accummulibacter phosphatis”, a well-known PAO, was used to perform a series of batch experiments at various nitrite and pH levels. FNA was found to inhibit all key aerobic metabolic processes performed by PAOs, namely PHA oxidation, phosphate uptake, glycogen replenishment and growth. The inhibitory effect on the anabolic processes (growth, phosphate uptake and glycogen production) was much stronger than that on the catabolic processes (PHA oxidation). 50% inhibition on all anabolic processes occurred at FNA concentrations of approximately 0.5 × 10^?3 mg HNO₂–N/L (equivalent to 2.0 mg NO₂^?–N/L at pH 7.0), while full inhibition occurred at FNA concentrations of approximately 6.0 × 10^?3 mg HNO₂–N. These concentrations could be found in full-scale wastewater treatment systems that achieve nitrogen removal via the nitrite pathway. In comparison, PHA oxidation remained at 40–50% of the highest rate at FNA concentrations in the range 2.0 × 10^?3–10.0 × 10^?3 mg HNO₂–N/L. Interestingly, PAOs were able to reduce nitrite under aerobic conditions (DO ≈ 3 mg/L), with the rate increasing substantially with the FNA concentration. The inhibition on phosphate uptake was found to be reversible.