Sludge-derived Biochar Enhances Heterotrophic,Autotrophic And Mixotrophic Denitrification

Nitrogen is one of the main causes of eutrophication in water.Eutrophication can induce rapid reproduction of algae and other plankton,decrease of dissolved oxygen in water,deterioration of water quality,and death of fish and other organisms,which seriously damages the stability of the ecological environment.Denitrification of wastewater is an important method to reduce the nitrogen entering the water to control the eutrophication.Wastewater treatment plants produce a large amount of sewage sludge,and the use of heat treatment technology to treat excess sludge has attracted widespread attention,which can not only achieve carbonization and harmless treatment of organic matter and pathogenic microorganisms in excess sludge,but also resource preparation can convert excess sludge into biochar with rich pore structure.Aiming at the treatment of domestic sewage with low C/N ratio,this study is based on the idea of"treating waste with waste",and conducts research on the induction process and mechanism of biochar to promote heterotrophic,autotrophic and mixotrophic denitrification.In order to further study the feasibility of engineering application,the A/O-MBR continuous flow reactor was used to investigate denitrification efficiency of the system under different C/N and S/N operating parameters.Through material balance analysis,the carbon and nitrogen removal pathways were explored,and the effect of biochar on heterotrophic,autotrophic denitrification and anammox in the system was clarified;Based on the changes of sludge biomass,extracellular polymers and electron transport system activities,the effect of biochar addition on the changes of sludge characteristics was analyzed;Based on the changes of sludge biomass,extracellular polymers and electron transport system activities,the effect of biochar addition on the changes of sludge characteristics was analyzed;Through the characterization of biochar physicochemical and electrochemical properties,the mechanism of biochar-mediated electron transfer to enhance biological denitrification was revealed;The microbiological mechanism of biochar-enhanced denitrification was explained from the perspective of functional microorganisms and functional genes.The main conclusions of the paper are as follows:（1）Feasibility study on biochar to enhance heterotrophic and autotrophic denitrificationThe feasibility of strengthening heterotrophic and autotrophic denitrification by hydrochar and pyrochar was investigated by sequencing batch reactor.The results showed that:under the condition of C/N of 4.0,compared with the control group,the TN removal rates of the hydrothermal group and the pyrolysis group were increased by 22.23%and 31.58%.Under the condition of S/N of 1.1,compared with the control group,the TN removal rates of the hydrothermal group and the pyrolysis group were increased by 54.11%and 46.00%.The results of single batch of experiments showed that biochar can significantly enhance the effect of heterotrophic and autotrophic denitrification.（2）Enhanced nitrogen removal efficiency of AO-MBR by biochar under different nutrient conditionsThe long-term effects of hydrochar and pyrochar on the performance of A/O-MBR continuous flow reactor under different nutrient conditions were investigated.The results showed that:in the heterotrophic stage,compared with the control group,the TN removal rates of the hydrothermal group and the pyrolysis group increased by 19.18%and 7.11%,respectively;In the autotrophic stage,the TN removal rates of the hydrothermal group and the pyrolysis group increased by 39.07%and 35.96%,respectively;In the mixotrophic stage,the TN removal rates of the hydrothermal group and the pyrolysis group increased by 14.22%and 17.10%,respectively.Under different nutrient conditions,hydrochar and pyrochar acted as redox mediators,mediating and catalyzing the redox reaction,thereby enhancing biological denitrification and improving TN removal rate.The material balance analysis results of C,N and S in the system show that the carbon removal pathway is mainly through denitrification in anoxic pools（58.24%-76.16%）and oxidation in aerobic pools（3.98%-23.17%）;Nitrogen was mainly removed in the anoxic pools,and the removal ratio increased with the increase of C/N and S/N,moreover hydrochar and pyrochar enhanced the utilization rate of carbon and sulfur sources in the system.In the autotrophic stage（S/N of 0.8 and1.2）,the hydrothermal group and the pyrolysis group underwent anaerobic ammonium oxidation removing part of the ammonia nitrogen,and their denitrification contribution rates were12.64%～13.89%and 11.14%～12.24%,respectively.During the mixotrophic stage,in the control group,hydrothermal group and pyrolysis group,the contribution of autotrophic denitrification to NO₃^--N removal was 67.63%～83.33%,74.56%～90.54%and 73.25%～88.68%,respectively.The contribution of autotrophic denitrification in the biochar group was greater than that in the control group,indicating that biochar had a greater impact on the autotrophic denitrification process.（3）The effect of biochar on sludge characteristicsThe changes of sludge characteristics induced by biochar were analyzed,and the results showed that compared with the control group,biochar increased the concentration of MLSS,MLVSS and MLVSS/MLSS,improving the sludge activity of the system.Biochar increased the content of extracellular polymers and changes its composition and structure.Biochar significantly promoted protein content,promoting the reduction reaction catalyzed by biological enzymes in the system,and increased protein/polysaccharide,which is conducive to the formation of stable microbial aggregates,extracellular polymers are critical to the denitrification performance of the system.The results based on the electron transport system activity experiment showed that the activity of the microbial electron transfer system in the hydrothermal group and the pyrolysis group was 4.5 times and 4.1 times that of the control group,respectively.Biochar significantly improved the activity of the microbial electron transport system and increased the electron transfer rate.Thus,the efficiency of denitrification is improved.（4）Mechanism of biochar enhanced denitrificationThe mechanism of denitrification mediated by surface functional groups,graphitic structure and capacitance of hydrochar and pyrochar was described.The results show that the atomic ratios of H/C and O/C in hydrochar are higher than those in pyrochar,there are more oxygen-containing functional groups in hydrochar,and pyrochar has a higher degree of aromatization.FTIR showed that phenolic hydroxyl was observed at 1380 cm^-1 in hydrochar and pyrochar,and C=O groups on ketone and quinone were observed at 1630 cm^-1,and the content was more abundant in hydrochar,indicating that the hydrochar and pyrochar have redox functional groups,which are an important way to mediate electron transfer.Electrochemical analysis showed that the CV curves of both hydrochar and pyrochar showed a reduction peak at-0.4 V,and the reduction peak height of pyrochar decreased,but it had higher capacitance.Based on the characterization of biochar,it is shown that the hydrochar mainly transfers electrons through the charge-discharge process of phenolic hydroxyl and quinone functional groups,while the pyrochar mainly transfers electrons through the conductive graphitic structure and high capacitance.Moreover,the mineral components of sludge biomass enhance the redox ability of biochar.（5）Microbiological study on biochar enhanced denitrificationThe microbiological mechanism of biochar-enhanced denitrification was explained from the perspective of functional microorganisms and functional genes.In the heterotrophic stage,biochar increased the abundance of Thauera,Hyphomicrobium and Terrimonas.Compared with the control group,the total abundance of heterotrophic bacteria in the hydrothermal group and the pyrolysis group increased by 5.81%and 6.84%.In the autotrophic stage,Thiobacillus showed strong adaptability and accounted for a large proportion in the microbial community.Biochar significantly enhanced the abundance of Thiobacillus,Thiothrix and Sulfurimonas,compared with the control group,the total abundance of autotrophic bacteria in the hydrothermal group and the pyrolysis group increased by 13.51%～18.05%and 13.28%～25.07%.At the same time,in the autotrophic stage,the abundance of anammox bacteria（Candidatus＿Brocadia）in the biochar group was 1.21%～2.60%,indicating that biochar promoted anammox bacteria,cooperating denitrification bacteria increased the removal rate of TN.And biochar increased the abundance of denitrification function genes（nar G,nar A,nir K,nir S,cnor B,qnor B and nos Z）,indicating that biochar can promote the denitrification process of NO₃^--N reduction to N₂.Under different nutrient conditions,biochar significantly increased the abundance of denitrifying bacteria and denitrification function genes in the system,which was consistent with the results that the biochar group could achieve stable and efficient denitrification.