Mitigation Of N₂O Emissions In Aerobic Nitrifying Granular Systems

Nitrous oxide (N2O) is an important greenhouse gas with 300 times higher greenhouse effectthan carbon dioxide, which threatens ozone by photodecomposition product of nitrogen oxides. Current research indicates that nitrogen removal process of wastewater contributes significantly to artificially generated N2O, as a result, it is necessary to study the mitigation strategy of N2O emission from wastewater treatment process. Aerobic granule with advantages of excellent settleability, high biomass and diverse microorganisms is a potential wastewater treatment technology. This study has researched the methods to reduce N2O release in sequencing batch reactor(SBR), the main results are as follows:(1) So far, experiments were usually investigated with one-factor-at-a-time method, making it very difficult to compare the specific effects of each factor. In this study, the Plackett-Burman (PB) multifactorial experimental design was applied to screen the significant factors for N2O emission from bioreactors. A total of seven parameters (temperature, C/N ratio, feeding strategy, pH, aeration rate, copper concentration, and aeration mode) were investigated in parallel. Five of them (other than copper concentration and chemical oxygen demand/nitrogen (C/N) ratio) were selected as significantly influential parameters.(2) Response surface methodology was utilized to analyze the operational conditions for the minimization of N2O emission in this laboratory environment. The investigation on temperature, pH, and aeration rate showed they interacted in the process of N2O release, and found the minimum emission value of N2O under the temperature of 22.3℃, aeration rate of 0.20 m3/h and pH of 7.1. A validation experiment has verified the predicted result. The microbial structure and activities would be changed under long-term operation of different A/O value, leading to varied performances of bioreactors. The bioactivities of microorganisms decreased with the shortened aeration phase, however the bioactivities of nitrifiers increased when the A/O value was further increased. The concentration of dissolved oxygen remained high in aeration phase with limited N2O emission, and N2O generated in anoxic phase was associated with the concentration of nitrite and the activities of nitrifiers. In every cycle, the emission peak of N2O could be found at the commencement of aeration.