Nitrogen Removal Pathways Of Anammox Coupling Thiosulfate Autotrophic Denitrification In Wastewater Treatment

Nowadays,a large number of traditional urban sewage treatment processes still use aeration and additional carbon source treatment processes,which can not meet the increasingly stringent sewage discharge standards,and run counter to the vision of green and low-carbon development.The anammox process has attracted increasing attention because it reduces the demand for carbon sources and energy.Usually short-cut nitrification technology is used to meet the demand for NO₂^--N in the anaerobic ammonia oxidation reaction,but short-cut nitrification process is difficult to control and the effluent from the anaerobic ammonia oxidation process still contains NO₃^--N.Thiosulfate was used as an efficient electron donor for autotrophic denitrification to provide NO₂^--N for the anaerobic ammonia oxidation reaction and reduce the subsequent effluent NO₃^--N.The anaerobic ammonia oxidation coupled sulfur autotrophic denitrification process（A/SAD）has broad prospects.However,there were few researches related to the coupling process at present,the rules and pathways were not clear,and the coupling mechanism and interaction relationship of functional bacteria needed to be further studied.In response to the above problems,this study constructed an anaerobic ammonia oxidation coupled with sulfur autotrophic denitrification system.The morphology,physical and chemical properties and pollutant removal efficiency of the sludge during the 410-day operation were studied;the periodic rules and the factors limiting denitrification were initially explored;the changes in the microbial community structure during the cultivation process were analyzed,and the anaerobic ammonia oxidation was explored.Based on the correlation and the interaction between functional bacteria,the changes in the module abundance of the system’s functional unit and the abundance of nitrogen metabolism-related enzymes were analyzed.The main conclusions of the paper were as follows:（1）Two forms of anaerobic ammonia oxidation coupled with sulfur autotrophic denitrification sludge,flocs and granules,were cultivated.The cultivated granular sludge has superior settling performance,SV value was 8%,SVI₃₀ was 85 m L/g.The average particle size was 0.5176mm,which belongs to the particle size range of granular sludge with the best performance.After 410 days of continuous operation,both the granular sludge and floc sludge reactors could stably maintain the effluent ammonia nitrogen in the reactor below 4 mg/L,the amount of nitrite nitrogen was<0.15 mg/L,and the nitrate nitrogen was<2.5 mg/L,total nitrogen removal rate was above 85%.（2）This research discussed the multi-stage reaction in the coupling system.In the coupled system,the first stage（0-1h）mainly carried out the first step of sulfur-driven autotrophic denitrification,which is the process of nitrate to nitrite,and the second stage（1-5h）mainly carried out rapid anammox reaction,the third stage（5-11.5h）is the slow anammox and sulfur autotrophic denitrification reaction.The lack of NO₂^--N in the R_freactor was the main reason for limiting the anammox reaction,but it was not in the R_greactor.Regardless of whether it was nitrate or nitrite type sulfur autotrophic denitrification,the reaction could be divided into two stages.There was a phenomenon that the reaction rate was first fast and then slow.In the sulfur autotrophic denitrification driven by nitrite and thiosulfate,the SO_X-S product in the second stage was relatively single,and only SO₄^2-might be generated.（3）This research clarified the changes in the structure of the microbial com munity during the cultivation process,explored the correlation between anaerobic ammonia oxidation and sulfur autotrophic denitrification species and the interacti on between functional bacteria,and verified the changes in the systematic denitri fication laws and denitrification pathways.Three genera of anammox bacteria app eared during the whole time,and g＿Candidatus＿Brocadia gradually became dom inant in subsequent cultivation.There was a total of 18 genera of bacteria with sulfur autotrophic denitrification function in the whole time,and the distribution was relatively even in the early culture process,and g＿Annwoodia and g＿Thio bacillus dominated in the later period.The different forms of sludge formed und er the same culture had a great common part in the denitrification function bacte ria.The same OTUs of the functional bacteria accounted for 49%of the total n umber of OTUs in the floc sludge R_f＿D380 and 75%of the total number of O TUs of the granular sludge R_g＿D380..There were significant differences in the s tructure of functional bacteria between R_f＿D380 and R_g＿D380,including g＿Ann woodia,g＿Thiobacillus,g＿Sulfurifustis and g＿Candidatus＿Brocadia.g＿Sulfurif ustis played a very important role in the coupling system.Different genera of an ammox bacteria had different correlations with different sulfur autotrophic denitrif ying bacteria.The highest correlations with g＿Candidatus＿Brocadia were g＿Ign avibacterium and g＿Sulfurifustis.At the same time,the changes in the abundanc e of denitrification functional unit modules and the abundance of nitrogen metab olism-related enzymes showed that the overall denitrification function of the com munity was enhanced after inoculation.The ratio of denitrification-related nitrate reductase nar increased in floc sludge and granular sludge after cultivation,and t he ratio of nitrite reductase nir decreased.During the cultivation process,sulfur autotrophic denitrification was used as the source of NO₂^--N instead of competiti on for NO₂^--N for anammox.The research results of anammox coupling sulfur autotrophic denitrification system nitrogen removal efficiency and pathways will enrich the theory of autotrophic nitrogen removal and provide technical support for its efficiency improvement in practical engineering applications.