Investigation on N2O production and emission as a byproduct of different deammonification systems

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The purpose of the overall study is to investigate the formation of nitrous oxide (N2O) as a byproduct of the different nitrogen conversion steps connected with the deammonification process (nitritation, nitrite oxidation, denitrification and anaerobic ammonium oxidation) (1st and 2nd year) and finally to develop measures for the reduction of emissions (3rd year). During the first two years, the research focus was to investigate the background of biological formation of nitrous oxide under the typical boundary conditions of these processes. Specifically relevant formation pathways and their influencing factors were considered separately for aerobic and anoxic processes, conversion kinetics were described and the overall relevance of nitrous oxide formation was evaluated. Accumulation of nitrite and limitation or variable availability of specific substrates like oxygen (nitrification) and organic carbon sources (denitrification) could be corroborated as crucial factors for nitrous oxide formation also in high-loaded processes. The results indicate furthermore that even with operational conditions that are regarded favorable for the process of nitritation N2O formation can be limited but not avoided. Following the investigations on the influence of operational conditions on N2O formation, the study will be oriented towards the possibilities to reduce the emission of N2O in the 3rd year. Operational strategies for N2O minimization will be developed, tested and examined specifically for deammonification systems (combination of partial nitritation and anaerobic ammonia oxidation for high-strength wastewater). For the following technical and procedural changes in process design, which result from the preceding investigations, the potential reduction of N2O emissions will be determined and the practicability tested:a) Minimization of the gas transfer by avoiding the stripping effect of common aeration systems making use of diffusion-based systems for oxygen supply;b) Integration of N2O degradation (denitrification to N2) into the deammonification process, hereby reduction of N2O concentration in the off-gas;c) Minimization of the N2O formation potential by adapted load control for the first stage (nitritation)The investigation will be realized in a continuously fed two-step aerobic/anoxic lab-scale plant. The continuous operation of differently operated lab-scale reactors for a longer period (10 months) enables differentiation of short- and long-term effects and inclusion of parameters that cannot be tested in batch experiments. Application of the methods for determination of the N2O formation potential (not identical with the final N2O emission) developed in the first two years of the project will be continued. The 3rd year will be concluded with an estimation of the potential reduction of N2O emissions by application of the respective measures in full-scale plants.