| Simultaneous nitrification and denitrification(SND) is one of techonogy that could achieve nitrification and denitrification in a single reactor. Compared with traditional biological nitrogen removal, SND have some advantage such as simple operating process, less carbon source and alkalinity demand, which attract people’s great attention. However, the metabolic processes of microorganisms during biological nitrogen removal would inevitably generate an important greenhouse gas---N2 O, which global warming potential is 310 times higher than that of carbon dioxide(CO2). Moreover, the reaction with atomic oxygen contributes to deplete the stratosphere ozone. Therefore, it is meaningful to study the N2 O emission process and then reduce N2 O emissions during the process of simultaneous nitrification and denitrification.The study emphasise the effect of different operation mode on the nitrogen removal and N2 O emission charateristics during the SND. Two SNDs were successfully achieved by adjusting the fully aerobic mode and the anoxic-aerobic mode. And high-throughput sequencing analysis revealed the relationship between N2 O emission and microbial species, which also provided some theoretical principle for regulating the N2 O emissions during SND. In addition, this article also investigated the effect of influent organic loading rates on the nitrogen removal and N2 O emission in the two SNDs.After two months domestication, aerobic granule sludge was formed with steady and efficient nitrogen removal efficiency in the two operation modes. The total nitrogen(TN) removal rate and SND efficiency of the fully aerobic SND reached 64.8% and 82.3%, respectively. And the average N2 O release amount of the fully aerobic SND was 7.34 mg/Cycle, which occupied 21.9±7.1% of the removed TN. In the anoxic- aerobic SND, 82.6% and 93.2% of TN removal rate and SND efficiency were found, and the average N2 O emission quantity of 2.31 mg/Cycle was monitored, equal to 7.0±1.5% of removal nitrogen. In a word, the anoxic-aerobic SND performed better than the fully aerobic SND, with TN removal and SND efficiencies being 17.8% and 10.1% higher, respectively. Whereas, the N2 O release amount had reduced 66.7% compared with the fully aerobic SND.The results of nitrification inhibition experiments indicated that denitrification by nitrifiers were the main source of N2 O emission in both systems. 55.4% of N2 O yield was due to nitrifier denitrification in the fully aerobic SND, while nitrifier denitrification represented 64.2% of N2 O production in the anoxic-aerobic SND. The contribution of heterotrophic denitrification decreased with the addition of pre-anoxic phase.High-throughput pyrosequencing analysis revealed that there is a huge difference between the microbial communites of two SNDs, indicating that the abundance of nitrifier in the fully aerobic SND was higher than that of the anoxic-aerobic SND.The enrichment of Nitrosomonas – like bacteria in the aerobic granule sludge of the fully aerobic SND was the main reason to explain the high N2 O emission. What’more, a greater abundance of Mesorhizobium sp. in the fully aerobic SND than the anoxic-aerobic SND also closely associated with N2 O emission.Unfavorable performances were obtained in the aspect of nitrogen removal and SND efficiency on the condition of higher or lower influent organic loading rate, and more adverse effects were found under low organic loading rate. The TN removal rate was 47.5% on the condition of high influent organic loading rates and only 30.7% under low influent organic loading rate when operated in fully-aerobic mode, meaning decreasing by 26.7and 52.6% compared with normal organic loading rate. Whereas enhanced nitrogen removal efficiency was achieved in the anoxic-aerobic SND when operated in higer influent organic loading rate and the TN removal rate could be reached 88.2%, increased by 6.8. Low influent organic loading rate led to decreasement of denitrification efficiency and thus made nitrogen removal dropped to 40.7%, decreaed by 50.7%. It was because high influent organic loading rate had strong inhibited influence on nitrification in the fully aerobic SND, while the adverse condition could be avoided for the existence of pre-denitrification phase.High influent organic loading rate conduce to N2 O reduction in both SNDs. The average N2 O release amount of the fully aerobic SND and the anoxic-aerobic SND was 1.30 mg/Cycle and 1.41 mg/Cycle, respectively, which meanings reducted by 76.9% and 56.1% compared with normal organic loading rate. The fully aerobic SND system was ascribed to the inhibition of nitrification and less ammonia nitrogen was inverted into NO2-, the denitrification rate was greater than the nitrification rate. Whereas less N2 O generated during the anoxic-aerobic SND was attributed to sufficient carbon source and less accumulation of NO2-.10% higher of nitrogen removal was achieved under low influent OLR in the anoxic-aerobic SND than that in the fully aerobic SND, while 3.5 times greater of N2 O emission than that in fully aerobic SND was emitted. Denitrification by AOB and heterotrophic bacteria was the main source of N2 O emission. |
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