| The main objective of this research was to investigate the feasibility of using reactor off-gas N2O as a aerobic-phase nitrification monitoring and aeration system control parameter in a sequencing batch reactor (SBR) wastewater treatment system. This was accomplished by subjecting an anoxic-aerobic SBR system, operating under oxygen-limited conditions, to wastewater (i.e. ammonia, readily degradable carbon, slowly degradable carbon (SDC)) and aeration rate perturbations, outside the normal baseline operating conditions, and monitoring system response. Specific experiments were also conducted to confirm the source of generated N2O, investigate the effects of DO concentration and slowly degradable carbon utilization rate on aerobic-phase heterotrophic N2O reduction-consumption, examine the influence of nitrite and nitrous acid levels on N2O generation, and evaluate the reactor gas mass transfer characteristics.; The data support the hypothesis that AOB were responsible for N 2O generation, with aerobic-phase heterotrophic denitrification generating little, if any, N2O. For a given SBR cycle, reactor oxygen supply rate/DO concentration, nitrite concentration, and pH level-nitrous acid concentration were shown to impact the aerobic-phase N2O generation rate. In addition, the availability of biologically utilizable carbon (i.e. SDC), under suitable DO conditions, could provide significant aerobic-phase heterotrophic N2O reduction-consumption rates, and thus affect the observed (i.e. net) N2O generation rate. The N2O reduction rate was sensitive to oxygen availability/DO concentration, likely related to the oxygen sensitivity of the heterotrophs N2O reductase enzyme.; From a process monitoring perspective, it was shown that off-gas N 2O information can be used to identify a change in the oxygen-competition dynamic between AOB and NOB that is induced by a change in aeration rate, as well as a change in wastewater characteristics. This phenomenon affects the relative difference in the ammonia and nitrite oxidation rates, induces subtle differences in transient nitrite levels that impacts N2O generation, and ultimately provides an indication of how the ammonia oxidation rate has changed due to the altered oxygen availability, via off-gas N 2O data. Alternately, for many combinations of aeration rate perturbations and wastewater slowly degradable carbon (SDC) utilization rates, DO and pH data alone could not be used to provide an indication of the effect that a change in SBR operating condition had on the ammonia oxidation rate. Furthermore, the ammonia oxidation rate was observed to decrease later in the aerobic-phase with decreasing mixed liquor ammonia concentration. A reduction in N 2O generation rate was coincidental with the reduction in ammonia oxidation rate, and was clearly resolvable in the off-gas N2O data, providing advanced indication of the timing of completed nitrification. The DO and pH data could not be used to identify the reduction in ammonia oxidation rate. (Abstract shortened by UMI.)... |
