Achieving partial nitrification in a continuous-flow bioreactor by free ammonia treatment combined with low dissolved oxygen

Abstract

Partial nitrification (PN) is still difficult to achieve in continuous-flow bioreactors. In this study, free ammonia (FA) treatment combined with low dissolved oxygen (DO) were applied to achieve PN in an A/O bioreactor. Batch tests showed 200 mg/L FA with 12 h treatment time could effectively expand the activity difference between ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), with an 24.86% falling of AOB activity and an 54.34% decrease of NOB activity. Furthermore, AOB activity can be recovered in a shorter time after once FA treatment than NOB. FA treatment combined with low DO could effectively establish AOB advantage and achieved PN with the nitrite accumulation ratio (NAR) with 86.95% and the ammonia removal efficiency (ARE) with 93.11% in our A/O reactor. FA treatment caused a decrease of extracellular polymeric substance (EPS) secretion, among which the protein reduction may be a key factor that prevented NOB activity recovered. Meanwhile, Nitrospira abundance with FA treatment once a day decreased by 79.1%, while Nitrosomonas abundance dropped by 16.4% only. Amo and hao abundance increased while nxr abundance reduced by 71.8%. The decreased abundance of Nitrospira and nxr was the key to achieve PN. This study demonstrated critical regulating factors for PN achievement in continuous-flow bioreactors using FA and DO, and may provide a new insight for PN in engineering applications.