Nitrous Oxide (N₂O) Emission Characteristics In Different Anammox-based Process Systems

Nitrous oxide(N₂O) is an important greenhouse gas with 300 times the greenhouse effect of carbon dioxide and is an unavoidable by-product of the conventional biological nitrogen removal(nitrification/denitrification)process in wastewater treatment.Anammox,as a sustainable nitrogen removal technology,has the advantages of high nitrogen removal efficiency,no organic carbon,low sludge production and almost no greenhouse gas production.However,the characteristics and formation mechanisms of N2O emissions in biological wastewater treatment systems coupled other heterotrophic/autotrophic denitrification technologies with Anammox are not clear.In this study,the nitrogen removal efficiency and N2O emission characteristics of three different Anammox processes(SAD,ASDAD and INPDA)for low-concentration nitrogen wastewater were investigated.In the SAD system,the effects of feed water carbon nitrogen ratios and temperature of the system on the nitrogen removal were investigated,and the best overall nitrogen removal performance and low N2O emission were achieved when the C/N ratio was 0.5 and the temperature was 30?.A novel anammox coupled with sulfite autotrophic denitrification process(ASDAD)was studied and developed for the first time,and the effects of different pH values were investigated.The highest total nitrogen removal rate of the system was 94.5%and the release of N2O was relatively low,and its gas release of N2O was significantly lower than that of N2O in other anammox coupled with heterotrophic denitrification processes.High-throughput sequencing results showed that anammox bacteria and sulfur autotrophic denitrifying bacteria coexisted,and the mechanisms of nitrogen metabolism and N2O production were explored;the effect of different C/N(C/N:4.0,4.5,5.0,6.0)and water temperature(30?,25?,20?,15?)on nitrogen removal and N2O emission in a novel mainstream nitrogen removal process(INPDA)system;the best overall nitrogen removal effect of 95.3%was achieved and the conversion rate of N2O was only 0.73%when the C/N was 5.0 and the temperature was 30?.High-throughput analysis confirmed that nitrogen removal was achieved by complete-nitrification,partial-denitrification and anammox pathway.Candidatus Kuenenia,as the main anammox bacteria in INPDA,had the maximum relative abundance of 1.86%under the optimal condition.The results of this study provide more insight into the mainstream Anammox system to enhance nitrogen removal and reduce the N₂O footprint.