Nitrogen Removal Pathways And Kinetic Models Of Soluble Inorganic Sulfur Autotrophic Denitrification

Sewage treatment is a high energy-consuming industry.The development of new high-efficiency and low-carbon biological denitrification technology is an important measure to implement the major national strategic needs such as"carbon peak,carbon neutralization"and the construction of ecological civilization.The insufficient carbon to nitrogen ratio of sewage in the denitrification stage is an objective factor restricting the improvement of biological nitrogen removal efficiency.Aeration and carbon source dosing cause high-energy consumption in sewage treatment.There are many kinds of soluble inorganic sulfur in sulfide-laden wastewater,which can replace the traditional organic carbon source and become the electron donor of sulfur autotrophic denitrification.This can realize simultaneous denitrification and desulfurization,and"treat waste with waste".Sulfur autotrophic partial denitrification can be further coupled with anaerobic ammonia oxidation（anammox）process to realize low-carbon sewage treatment.Sulfide is the main form of soluble inorganic sulfur in sulfide-laden wastewater.It is very easy to convert into thiosulfate in a micro-oxygen environment.Therefore,sulfide and thiosulfate are the actual electron donors involved in sulfur autotrophic denitrification.Due to the complex pathways of soluble inorganic metabolism and the intermediate accumulation,the nitrogen removal pathways and reaction kinetics of sulfide-or thiosulfate-based denitrification are still unclear.Herein,a sulfide autotrophic denitrification biofilm system and a thiosulfate autotrophic partial denitrification-anammox system were established to investigate their nitrogen removal pathways,nitrogen removal microenvironment,microbial community characteristics and reaction kinetic models.The main research results are as follows:（1）The denitrification pathways and reaction microenvironment characteristics of sulfide-based autotrophic denitrification biofilm system under alkaline conditions were explored,and the promotion mechanism of different extracellular polymers on the structural stability of biofilm was revealed.Under high S/N conditions,sulfur oxidizing bacteria oxidize sulfide to S⁰,and nitrate is directly reduced to nitrogen.Under low S/N conditions,Nir cannot use the electrons provided by S⁰ oxidation,resulting in nitrite accumulation.The biofilm had high roughness and weak structural stability.e DNA and extracellular protein were the main and secondary components to maintain the cohesion and adhesion of the biofilm,and polysaccharide was the cohesive component to maintain the stability of the biofilm.It was speculated that the structural stability of biofilm in alkaline environment was closely related to the interaction mediated by e DNA and metal bridging.The double helix structure of e DNA molecule can provide special binding sites and form strong coordination bonds with magnesium ions.（2）The kinetic model of sulfide-based autotrophic denitrification biofilm system with polysulfide as the intermediate was established,and the responses to sulfide-based autotrophic denitrification efficiency under different biofilm thickness,influent concentration and influent p H were clarified.The mass transfer resistance increases with the increase of biofilm thickness.The effective biomass of biofilms with different thickness is determined by the mass transfer distance of reactants in the biofilm.The treatment capacity of the biofilm system increases with the increase of influent concentration,because the increase of influent concentration increases the concentrations of NO₃^-,NO₂^-and S_n^2-in the biofilm,thus increasing the reaction rate.The intermediate product S⁰ can react with sulfide to produce S_n^2-,and the alkaline environment is conducive to the conversion of S⁰ to S_n^2-,but the strong alkaline environment will inhibit the reaction activity of sulfur oxidizing bacteria,and the optimal p H of the reaction should be maintained at about 9.0.（3）A new process of thiosulfate autotrophic partial denitrification coupled with anammox was developed,the coupling nitrogen removal mechanism of the process was revealed,the symbiotic relationship between sulfur-oxidizing bacteria（SOB）and anammox bacteria was clarified.The denitrification metabolism pathways of the coupled system was divided into three steps:the first step was that SOB reduced nitrate to nitrite,oxidized most of thiosulfate to sulfate,and produced a small amount of S⁰ at the same time;in the second step,anammox bacteria converted the accumulated nitrite and ammonium into nitrogen and produced a small amount of nitrate as by-products;in the third step,SOB used S⁰ as electron donor to further reduce the remaining nitrate in the system.The molar ratio of NH₄⁺/NO₃^-and S₂O₃^2-/NO₃^-were two key factors affecting the denitrification performance of the coupled system.NH₄⁺/NO₃^-affects the removal efficiency of total nitrogen by competing with nitrite,while S₂O₃^2-/NO₃^-molar ratio affects the removal efficiency of total nitrogen by affecting the reduction of nitrate.The functional bacteria involved in sulfur metabolism in the coupling system include Thiobacillus,Thiotrix,Bacillus and Thermomonas;The functional bacteria involved in anaerobic ammonia oxidation reaction is Candidatus brocadia.（4）The kinetic model of thiosulfate-based autotrophic denitrification based on electron transfer intermediates was established,the affinity ranking of various reductases to electrons was clarified,and the electron competition mechanism of thiosulfate-based autotrophic denitrification and the substrate competition mechanism of the coupled process were revealed.The competitive ability of each reductase for intracellular electrons was:sulfur oxidation complex enzyme（Sox）>nitrate reductase（Nar）>nitrite reductase（Nir）.The electron consumption rate of Sox is controlled by the concentration of thiosulfate,and the electron consumption rate of Nar and Nir is controlled by the concentration of reduced electron carrier(M_red).The strategy of continuous adding low concentration thiosulfate can alleviate the internal electronic competition,reduce the accumulation of S⁰ and improve the denitrification efficiency.The kinetic basis for the successful coupled system was that the formation of S⁰consumes part of the electrons produced by thiosulfate oxidation,and the very low oxidation rate of S⁰ weakens the competitiveness of SOB for nitrite.The research results improve the denitrification theory of low-carbon wastewater treatment with sulfur autotrophic denitrification as the key process,and developed the kinetic model of sulfide-and thiosulfate-based autotrophic denitrification,which provides a theoretical basis and technical support for the co-treatment of low C/N sewage and sulfide-laden wastewater.