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Investigation of nitrous oxide as a nitrification monitoring and aeration system control parameter in sequencing batch reactor wastewater treatment systems Shiskowski, Dean Michael

Abstract

The provision of aerobic, autotrophic nitrification in biological wastewater treatment systems requires significant mass quantities of oxygen to provide the biologically-catalyzed oxidation of ammonia to nitrite and nitrate. In addition, the relatively high oxygen half-saturation coefficient for nitrifying organisms causes the kinetic rate of these organisms to be very sensitive to oxygen availability at low bioreactor mixed liquor dissolved oxygen (DO) concentrations (i.e. < 2 mg/L). The efficiency of transferring oxygen into mixed liquor is also affected by DO concentration. However, unlike nitrification, where higher DO levels translate into higher kinetic rates (i.e. a positive impact), elevated DO levels negatively impact oxygen transfer efficiency (OTE). Thus, the dilemma in bioreactor operation, with respect to controlling the oxygen supply rate, involves balancing the required ammonia oxidation rate with OTE, such that complete nitrification can be achieved under a given set of operating conditions while maximizing OTE. This situation ultimately impacts the energy cost associated with bioreactor oxygen supply. Ammonia oxidizing bacteria (AOB), such as Nitrosomonas, can possess a nitrite reductase (NiR) enzyme. This enzyme allows the organism to use nitrite as an alternate terminal electron acceptor in oxygen-limited environments, resulting in the reduction of nitrite to gaseous nitrous oxide (N₂O). For this research, it was hypothesized that reactor off-gas N₂O data could be used to monitor the extent of AOB oxygen limitation-availability and provide an indication of the overall nitrification kinetic rate. 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 N₂O 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. Long-term SBR operation, under the oxygen-limited baseline conditions, found that the aerobic-phase generated N₂O mass to oxidized ammonia mass ratio changed over time, increasing and decreasing over a range of one order-of-magnitude. At the peak mass ratio, about 25% of the oxidized wastewater ammonia was converted to N₂O. Subtle shifts in the microbial population, with respect to the relative fractions of the AOB and nitrite oxidizing bacteria (NOB), were believed to have caused the observed phenomenon. From a process monitoring perspective, it was shown that off-gas N₂O 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 N₂O 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 N₂O 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. The research findings confirm the potential feasibility of using off-gas N₂O, collected from a covered bioreactor, as a aerobic-phase nitrification monitoring and aeration system control parameter in SBR wastewater treatment systems. To this end, a conceptual, N₂O-based aeration system control strategy was developed. The proposed strategy would utilize a pattern recognition approach, along with three key state variables: N₂O generation on-set time (OST), steady-stage N2O generation rate (SSNGR) and ammonia oxidation-N₂O generation reducing-rate time (RRT). An artificial neural network (ANN) is proposed as the implementation framework for the control strategy.

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