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.