Free nitrous acid and pH determine the predominant ammonia-oxidizing bacteria and amount of NO in a partial nitrifying reactor.

We investigated the effects of free ammonia (FA) and free nitrous acid (FNA) concentrations on the predominant ammonia-oxidizing bacteria (AOB) and the emission of nitrous oxide (NO) in a lab-scale sequencing batch reactor for partial nitrification. The reactor was operated with stepwise increases in the NH loading rate, which resulted in a maximum FA concentration of 29.3 mg-N/L at pH 8.3. Afterwards, FNA was increased by a gradual decrease of pH, reaching its maximum concentration of 4.1 mg-N/L at pH 6.3. Fluorescence in situ hybridization indicated that AOB remained predominant during the operation, achieving specific nitrification rates of 1.04 and 0.99 g-N/g-VSS/day at the highest accumulations of FA and FNA, respectively. These rates were in conjunction with partial nitrification efficiencies of >84%. The NO emission factor of oxidized NH was 0.90% at pH 7.0, which was higher than those at pH 8.3 (0.11%) and 6.3 (0.12%), the pHs with the maximum FA and FNA concentrations, respectively. High-throughput sequencing of 16S ribosomal RNA genes showed that increases in FNA drastically changed the predominant AOB species, although increased FA produced no significant changes. This study demonstrates that the FNA concentration and pH are the main drivers that determine the predominant AOB species and NO-emission in a partial nitrifying bioreactor.