| Biological denitrification is an important method for wastewater nitrogen removal.Traditional biological denitrification relies on heterotrophic denitrification process,which requires organic carbon source as electron donor to realize denitrification process.However,the carbon nitrogen ratio of sewage in China is generally low,so most sewage treatment plants still need to add additional organic carbon sources such as methanol in the sewage treatment process,which not only greatly increases the cost of sewage treatment,but also does not meet the strategic goal of "carbon neutralization and carbon emission reduction".In addition,the added organic carbon sources are difficult to control,which is easy to cause problems such as the increase of effluent COD.At the same time,the anaerobic digestion of sludge to produce biogas has gradually become an important way for the resource recovery of excess sludge in sewage treatment plants.However,the biogas produced often contains a certain concentration of hydrogen sulfide,which affects the direct utilization of biogas as an energy material.As a necessary way of biogas resource utilization,biogas desulfurization also increases the cost of biogas resource utilization.In view of the above problems,this topic has constructed a denitrification system based on mixed electron donor to couple denitrification anaerobic methane oxidation(DAMO)and autotrophic denitrification(AD)process,which uses hydrogen sulfide and methane in biogas as electron donors and nitrogen pollutants in sewage as electron acceptors,so as to realize sewage denitrification,simultaneous biogas desulfurization and in-situ utilization.At the same time,this subject deeply studied the system from the aspects of operation efficiency,influencing factors,metabolic pathway and biological mechanism,so as to provide theoretical basis and biological support for the further promotion and application of the system.The construction of the denitrification system based on mixed electron donors adopts the EGSB reactor configuration,and the electron receptors are nitrate and nitrite respectively.The stable operation lasts for 735 days.According to different operating conditions,the operation process is divided into 12 stages to explore the optimal operation efficiency and metabolic mechanism of the system.The experimental results show that the two systems can achieve high-efficiency sewage denitrification and synchronous biogas desulfurization without additional carbon source.When the hydraulic retention time is 10 days and the influent nitrogen concentration is 50～100 mgN/L,the denitrification and desulfurization efficiency can reach 100%.In addition,nitrate is more suitable as the electron acceptor of the system.The introduction of biogas not only strengthens the overall denitrification efficiency of the nitrate system,but also strengthens the DAMO process of the system.The denitrification rate contributed by DAMO increases by 2.1 times,from the original 2.9 mgN/(L·d)to 6.2 mgN/(L·d).The denitrification ratio contributed by AD and DAMO is 1.2:1.However,in the system with nitrite as the electron acceptor,the denitrification process mainly depends on the autotrophic denitrification process.The denitrification rate contributed by DAMO becomes one tenth of the original after the introduction of biogas.The denitrification ratio contributed by AD and DAMO is 21.2:1.The operation efficiency,community structure and metabolic law of the system are greatly affected by the ratio of nitrogen to sulfur(N/S)and the type of culture gas.As N/S of the system rising,the overall nitrogen removal efficiency gradually decreases,while the contribution proportion of DAMO and AD for nitrogen removal gradually increases.When N/S increases from 2.3 to 23.3,the nitrate removal rate will drop from 100%to 15.4%,but the contribution of DAMO/AD to nitrate removal will increase from 0.5 to 2.2.In the experimental group V3 with N/S=2.3 and H2S=1.5%,the denitrification efficiency is as high as 100%,while the nitrate removal efficiency contributed by DAMO and AD is 33.1%and 65.6%respectively.When the content of hydrogen sulfide in the culture gas is low(<1%),it has little effect on the microbial community structure,and the type and proportion of functional microorganisms are basically the same as that of methane as the culture gas;When the hydrogen sulfide content in the culture gas is high(>1%),the bacterial abundance of DAMO decreases to 4%,and the bacterial abundance of ad increases to more than 20%.Compared with methane as culture gas,nitrate reduction genes NapA,narG,nirK,nirS,cnorB and sulfide oxidation gene SoxB in the experimental group with high concentration hydrogen sulfide biogas as culture gas were up-regulated to>3.0×109 copies/g,while the methane oxidation genepmoA was only up-regulated to 1.0 in the experimental group with low concentration of hydrogen sulfide biogas 1.0×109 copies/g.The dominant functional microorganism of the system with nitrite as the electron acceptor is the autotrophic denitrifying bacterium Thiobacillus,while the system with nitrate as the electron acceptor can realize the coexistence of autotrophic denitrifying bacterium Thiobacillus and anaerobic methane oxidizing bacterium Candidatus methylmirabilis.In the system with nitrate as the electron acceptor,the introduction of biogas sharply increased the abundance of DAMO bacterium Candidatus methylmirabilis from 0.3%to 19.6%and stimulated the potential of this bacterium to remove nitrate without the participation of ANME archaea.At the same time,the results of metagenomic binning analysis showed that the significantly up-regulated methane furan synthesis gene mfnE only came from the DAMO bacterium Candidatus methylmirabilis.Combined with the unique phenomenon that the bacterium can independently metabolize nitrate in the system,this topic proposed that the system was mediated by the sulfur cycle,and the new anaerobic methane oxidation pathway regulated by the methane furan synthesis gene mfnE in DAMO bacteria was stimulated,The new academic view that the bacterium can carry out complete nitrate respiration independently.The denitrification system based on mixed electron donors constructed in this project can realize efficient sewage denitrification,synchronous biogas desulfurization and in-situ utilization without additional carbon source and has great engineering application potential and value.The results and findings of this study provide a new idea for sewage treatment.The new academic viewpoint proposed in this topic complements the traditional theory that DAMO bacteria need to cooperate with DAMO archaea for nitrate respiration,improves the anaerobic methane oxidation theory,and provides a new perspective for the cyclic metabolism of biomethane,nitrate and sulfide. |
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