Study On The Effect And Mechanism Of Recycling Iron-containing Biochar On Promoting Anaerobic Digestion And Pyrolysis Of Sludg

A large amount of sludge will be produced in the process of sewa ge treatment.According to statistics,the annual output of sludge in urban sewage treatment plants in China has exceeded 60 million tons.And with the economic development and urbanization process,the amount of sewage treatment will further increase,which will lead to further increase in sludge production.Anaerobic digestion technology is one of the important measures to reduce carbon emissions and sludge disposal.However,a large amount of biogas residue will be produced during anaerobic digestion.If the biogas residue is not treated,it will not only cause environmental pollution,but also increase carbon emissions.It has been reported that iron and biochar can effectively promote anaerobic digestion performance.Biochar is one of the by-products of pyrolysis.If anaerobic digestion and pyrolysis process are coupled,it can not only realize the maximum resource disposal of biogas residue produced in anaerobic digestion process,but also pyrolyze biogas residue into biochar to promote gas production e fficiency in anaerobic digestion process.In this study,based on the anaerobic digestion coupled with biogas residue dewatering and pyrolysis process,the solid phase product biochar in the biogas residue pyrolysis process was recycled into the sludge ana erobic digestion-pyrolysis disposal system.Therefore,in this study,two kinds of biogas residue under different anaerobic conditions were selected for pyrolysis to obtain biogas residue biochar.The anaerobic digestion without biogas residue biochar（No ne-BC）and the anaerobic digestion with biogas residue biochar（SS-BC）were used as controls to investigate the effect of iron-containing biogas residue biochar（Fe-BC）on methane generation,performance stability,anaerobic different stages and microbial community structure of sludge anaerobic digestion.The pyrolysis of biogas residue with Fe-BC was studied,and the effect of pyrolysis on pollutant release under the optimum gas production conditions was discussed by analyzing the tar and gas composition.In this regard,this study has the following main conclusions:The optimum preparation conditions of iron-containing biogas residue biochar were determined as pyrolysis temperature of 900°C,pyrolysis time of 20 min and moisture content of 0%.The optimum preparation conditions of the original sludge biogas residue biochar were pyrolysis temperature of 900°C,pyrolysis time of 10 min and moisture content of 0%.Iron-containing biogas residue biochar has a large surface area and porous structure,and the more basic metal content,it can effectively improve the efficienc y of anaerobic digestion.The cumulative methane production of SS-BC and Fe-BC reached 397 m L and 443m L,which was 32.99%and 39.95%higher than that of None-BC group（266 m L）.The gas production and methane content were significantly improved,and the lag period of methane production was shortened.During the whole anaerobic digestion process,biogas residue biochar slowed down the inhibition of TVFAs and TAN,promoted the degradation of organic matter and improved the conductivity of the system,and promoted the stability of the anaerobic digestion reactor.The iron-containing biogas residue biochar has different degrees of promotion effect on the hydrolysis,acid and methane production stages of anaerobic digestion;in terms of microbes,SS-BC increased the diversity of archaeal communities and decreased the diversity of bacterial communities.Fe-BC enriched the diversity of bacterial and archaeal communities and promoted multiple methanogenic pathways.The pyrolysis process of sludge can be roughly divided into four stages,and the sludge weight loss of four different additives has the same trend.In addition,the peak temperature on the DTG curve of fresh sludge pyrolysis（CK）was lower than that of the pyrolysis of raw sewage sludge and biogas residue（Py-RSS）,the pyrolysis of biogas residue enhanced by iron-containing biogas residue biochar（Py-Fe-BC）and the pyrolysis of biogas residue enhanced by raw sewage sludge and biogas residue biochar（Py-SS-BC）,and the weight loss curve of CK was lower than the release curve of other samples,indicating that the biogas residue after anaerobic digestion enhanced by biogas residue biochar is beneficial to pyrolysis.At 160-400°C,the activation energy of Py-Fe-BC was lower than that of CK,indicating that less energy was required for pyrolysis at this stage.Compared with CK,Py-Fe-BC reduced the release of CH₄,CO₂ and H₂O gases,and Py-Fe-BC promoted the release of CO.Moreover,Py-Fe-BC can promote the catalytic removal of tar.This study can achieve the goal of efficient anaerobic digestion capacity and efficient resource disposal of biogas residue,effectively reduce pyrolysis energy consumption and promote efficient pyrolysis capacity,reduce carbon emissions from energy sources and improve carbon compensation.This process provides a new treatment method for sludge resource disposal,and provides help for sludge disposal and production capacity improvement,which has practical application prospects and academic value.