N₂O Emission Characteristics And Mechanisms During Simultaneous Nitrification And Denitrification Process

Simultaneous nitrification and denitrification (SND) technology has emerged as a promising process, due to its high nutrient removal efficiency and low energy consumption. However, a significant amount of N2O may be produced during biological nitrogen removal via SND process. N2O is an important greenhouse gas. It can also destroy the ozone layer and promote the formation of acid rain. Therefore, it is very important to study the mechanisms of bilogical nitrogen removal and characteristics of N2O emission during low-oxygen SND process.In this study, the SND process was achieved in anaerobic-aerobic SBR biological nitrogen removal system by controling the dissolved oxygen concentration. By comparing with sequencing nitrification and denitrification (SQND) process, the contaminant transformation and N2O emission characteristics during SND process were systemically studied. The dominant N2O emission source during low-oxygen SND process was determined, and the changes of microbial community structure related to N2O emission during low-oxygen SND process were successfully tracked. The contributions of different microorganisms on N2O emission were figured out through the comparative analysis of changes between microbial community structure and system feature. Furthermore, the impacts of influent phosphorus load and metal ions (Cu2+and Fe3+) on N2O emission cahracteristics during SND process were evaluated to optimize the influent parameters of low-oxygen SND process. The main research conclusions are as follows:(1) The SND process enhanced the nitrogen removal greatly but also stimulate the N2O emission. By comparing with the SQND process, the contaminant removal performance and N2O emission characteristics during low-oxygen SND process was analysis and found that the N2O emission amount during SND process was about four-flod of that during tranditional nitrification-denitrification process, and the emission mainly occurred during aerobic stage. The N2O emission had significant correlation with oxygen uptake rate (OUR) of microbes, and the OUR could reflects the N2O emission trend more exactly than the DO concentration. In addition, some denitrifiers could used intracellular PHA as carbon source for denitrification under low C/N condition, causing the competion of denitrifying enzyme for electron and accumulation of N2O.(2) Denitrification of AOB was the dominant pathway of N2O emission during SND process, and the microbial soucre of N2O was mainly the AOB of JV. europaea and Nitrosomonas-liike. By using chemical inhibition method, the dominant source of N2O emission during low-oxygen SND process was determined. Meanwhile the microbial source was also analyzed through PCR-DGGE technique. The results showed that the N2O yield was about2times higher than that of heterotrophic denitrification. The community structure of AOB was significantly affected by the low-oxygen condition, and some AOB capable of denitrification (i.e. N. europaea and Nitrosomonas-liike) was enriched, resulting in the enhancement of nitrifier denitrification. The microbial community of denitrifiers was affected insignificantly by the low DO.(3) The impacts of influent phosphorus load on N2O emission was studied by analyzing the process of contaminant transformation and N2O emission under different phosphorus load. Results showed that:with the increasing of influent phosphorus load, the removal of TN and TP during low-oxygen SND process was enhanced. Meanhile the N2O yield was also decreased. The control of N2O emission under high phosporus load was due to the decrease of N2O produced bt heterotrophic denitrification. Compared with the low phosphorus load, high influent phosporus load could synthesize more PHB, thus easing the competion of denitrifying enzemys for electron donor. In addition, the enrichment of PAOs under high phosphorus load was favored to the N2O control.(4) The impacts of Cu2+and Fe3+on contaminant removal performance and N2O emission characteristics were studied. Results showed that:the addtion of certain amount of Cu2+could enhance the removal of TN and decrease the N2O yield. The addtion of Fe3+enhanced the removal of TP greatly. To some extent, a small amount of Fe3+could enhance the removal of TN and the impact on N2O emission was insignificant. However, high concentration of Cu2+and Fe3+could inhibit the activity of microbes, leading to low nutirent removal and high N2O yield.