Effect of Free Nitrous Acid on Nitrous Oxide Production and Denitrifying Phosphorus Removal by Polyphosphorus-Accumulating Organisms in Wastewater Treatment

The inhibition of free nitrous acid (FNA) on denitrifying phosphorus removal has been widely reported for enhanced biological phosphorus removal; however, few studies focus on the nitrous oxide (N(2)O) production involved in this process. In this study, the effects of FNA on N(2)O production and anoxic phosphorus metabolism were investigated using phosphorus-accumulating organisms (PAOs) culture highly enriched (91 ± 4%) in Candidatus Accumulibacter phosphatis. Results show that the FNA concentration notably inhibited anoxic phosphorus metabolism and phosphorus uptake. Poly-β-hydroxyalkanoate (PHA) degradation was completely inhibited when the FNA concentration was approximately 0.0923 mgHNO(2)-N/L. Higher initial FNA concentrations (0.00035 to 0.0103 mgHNO(2)-N/L) led to more PHA consumption/TN (0.444 to 0.916 mmol-C/(mmol-N·gVSS)). Moreover, it was found that FNA, rather than nitrite and pH, was likely the true inhibitor of N(2)O production. The highest proportion of N(2)O to TN was 78.42% at 0.0031 mgHNO(2)-N/L (equivalent to 42.44 mgNO(2)-N/L at pH 7.5), due to the simultaneous effects of FNA on the subsequent conversion of NO(2) into N(2)O and then into N(2). The traditional nitrite knee point can only indicate the exhaustion of nitrite, instead of the complete removal of TN.