Study On The Mechanism Of Adding Conductive Carbon Materials To Alleviate The Calcification Of Granular Sludge From Recycled Paper Wastewater

Papermaking from waste paper has the advantages of saving wood resources and low pollution,and conforms to the concept and requirements of clean production,so it has attracted more and more attention.However,in the process of waste paper recycling,inorganic salts continue to accumulate in the system,resulting in excessively high Ca²⁺content in the water,which easily causes calcification of granular sludge during the anaerobic treatment of wastewater,which seriously affects the efficiency of wastewater treatment.The problem of improving the efficiency of anaerobic treatment has been extensively studied.In recent years,it has been discovered that conductive carbon materials are a kind of additives that can effectively improve the methane production performance of sludge.However,the mechanism of calcification affecting sludge activity and adding conductive carbon materials to improve sludge activity is very vague at the micro level,and whether carbon materials can restore the activity of calcified sludge has not been widely studied.Therefore,this study used nano-CT technology and high-throughput sequencing technology to explore how calcification affects the anaerobic activity of sludge by affecting the microstructure and microbial community structure.On this basis,three conductive carbon materials:multi-walled carbon nanotubes,physical single-layer graphene and conductive carbon black were used as additives to enhance the anaerobic reaction,and the mechanism of action was also explained from a microscopic perspective.The main conclusions are as follows:(1)The calcified and uncalcified anaerobic granular sludge were taken to compare the anaerobic activity,microstructure and microbial community structure.The results showed that compared with uncalcified sludge,the density of calcified sludge increased slightly,and the proportion of organic matter and the content of extracellular polymer decreased by24.68%and 27.11%,respectively.In calcified sludge,some inorganic crystals were deposited on the surface of the sludge to block the pores and limited the mass transfer rate;another part of the inorganic crystals were deposited in the sludge in the form of calcite,which squeezed the biomass space and reduced the density of biomass.The abundance of the main hydrolytic fermentation bacteria in the calcified sludge decreased slightly,and the abundance of Methanosaeta decreased by 34.02%.In addition,calcification also reduced the methane production potential of the sludge.These factors reduced the cumulative methane production and specific methane production activity of calcified sludge by 26.82%and 16.23%,respectively.(2)Different carbon materials were added to the reactor of uncalcified anaerobic granular sludge for short-term cultivation.The results showed that the addition of multi-walled carbon nanotubes increased cumulative methane production and specific methane production activity by 10.23%and 17.53%,respectively,reduced calcium deposits in uncalcified sludge by 5.32%,and increased porosity by 7.52%.The addition of physical single-layer graphene increased the daily methane production and the proportion of organic matter in the sludge by 9.89%and 6.26%,respectively.The addition of conductive carbon black did not significantly promote the specific methanogenic activity of the sludge.The abundance of Choloflexi was reduced by 26.56%compared with the control group,but it had the most significant effect on the abundance of KEGG(Kyoto Encyclopedia of Genes and Genomes)gene pathway and methane-related enzyme pathway.(3)Different carbon materials were added to the reactor of calcified anaerobic granular sludge for short-term cultivation.The results showed that the addition of multi-walled carbon nanotubes increased cumulative methane production and specific methane production activity by 20.53%and 29.41%,respectively,reduced calcium deposits in calcified sludge by32.81%,and increased the ratio of pores and biomass by 9.88%and22.93%,respectively.The addition of multi-walled carbon nanotubes and conductive carbon black changed the dominant flora from TA06 to Choloflexi,and the addition of physical monolayer graphene changed the dominant flora into Actinobacteriota.Conductive carbon black had the most obvious effect on enhancing the abundance of KEGG gene pathway and methane-related enzyme pathway.