Coupled Microbial Electrocatalytic Process And Magnetite-bearing Biochar For Enhancing Anaerobic Digestion Of Excess Sludge

Excess sludge（ES）is an important by-product in the biological treatment of sewage.It has the characteristics of large output,complex composition,and difficulty in dissolving and cracking.If it is not properly treated and disposed,it will cause serious environmental pollution and even threaten human health.ES is rich in organic matter and can be converted into methane by anaerobic digestion,but it has problems such as low hydrolysis rate,slow methane production and poor stability,which need to be solved urgently.Microbial electrocatalytic anaerobic digestion（MEC-AD）can enhance the dissolution of organic matter,induce the directional enrichment of functional microorganisms,and improve the efficiency of anaerobic digestion by introducing the electrocatalytic process into the traditional anaerobic digestion process.However,in the MEC-AD process,there are technical problems such as electron transfer limitation,unreasonable potential setting,and electrode microbial inactivation,and the induced metabolic mechanism is unclear.By coupling the lysis pretreatment process,dosing conductive materials,etc.,the organic matter metabolism process and electron transfer efficiency can be effectively promoted.Based on this,this paper takes ES as the treatment object,zero-valent iron/persulfate（ZVI/PDS）oxidation and MEC-AD coupling system as the technical core.In view of the difficulty of dissolving and cracking ES,the application limitation of MEC in sludge methane conversion,and the ambiguity of the metabolic mechanism,technical research and development work in the enhancement of ZVI/PDS oxidation lysis pretreatment by weak magnetic field（WMF）,preparation of magnetite-bearing biochar（MBB）and its coupling MEC-AD system to enhance microbial electrocatalytic sludge biotransformation were systematically carried out,and compared MBB with the efficiency of industrial magnetite（IM）.The main conclusions are as follows:（1）A technology of ZVI/PDS oxidation coupled with WMF to enhance sludge lysis pretreatment was developed.The synergistic mechanism of magnetic field flux Intensity（MFI）and ES initial p H was explored,and the internal mechanism of the coupled system to enhance lysis and deep dehydration was clarified.WMF（25 m T）enhanced ZVI/PDS oxidation（ZVI/PDS dosage was1.5/1.2 mmol/g-VS）sludge dewatering at p H 3.0,5.0,7.0 and 9.0,CST/CST₀ of sludge respectively decreased by 10.7%,13.6%,54.2%and 40.7%compared with 0 m T.Among them,the concentration of dissolved iron ions was as high as 222.0±118.0 mg/L and the Zeta potential increased to-2.8±0.2 m V at p H 7.0.This was due to the lorentz force and the magnetic field gradient force inducing Fe（II）move along the high MFI direction on ZVI surface,promoting local corrosion of ZVI and avoiding Fe（II）oxidation and iron（hydroxide）oxide formation.Fe（II）reacted with PDS to generate sulfate free radicals,which destroyed microbial cells and EPS,resulting in the release of intracellular substances and EPS bound water.What’s more,Fe（Ⅲ）reduced the electrostatic repulsion and interaction energy between sludge particles through coagulation,which was beneficial to the improvement of sludge dewatering performance.（2）A new coupling system of MBB-MEC-AD was constructed to study the effect of MBB on microbial electrocatalytic sludge methane conversion,and to reveal the response mechanism of electrode microbial community to MBB.MBB1-MEC-AD（MBB was about 0.1 g）produced the highest current and cumulative methane production,which were 7.6%and 33.4%higher than MEC-AD and MBB2-MEC-AD（MBB was about 0.8 g）,respectively,and corresponding to SCOD removal rate up to 63.0%.This demonstated that small dosage of MBB strengthened the biofilm activity,promoted the electron transfer process,and accelerated the decomposition of complex organic.In addition,the electrodes of MBB1-MEC-AD had higher diffusion current,lower biofilm resistance and charge transfer resistance,indicating that it had lower overpotential loss,promoted electron transfer and created a good conductive environment for microbial survival.The electrogenic bacteria Geobacter and acetoclastic methanogens Methanosaeta of MBB1-MEC-AD were 216.0%and 16.7%higher than those of MEC-AD,while the hydrogenotrophic methanogens Methanobacterium were 18.2%lower.This illustrated that the introduction of MBB realized the change of electron transfer from indirect electron transfer（IET）to more efficient direct electron transfer（DET）,which improved the methane production efficiency.（3）IM-MEC-AD system was further constructed to explore the regulation mechanism of magnetite on interspecies electron transfer,and analyze the stimulation mechanism of magnetite to enhance microbial electrocatalytic sludge anaerobic.The cumulative methane production of IM-MEC-AD was about 32.4%,2.8%and 9.4%higher than that of AD,IM-AD and MEC-AD,respectively.However,its average removal rates of TCOD and SCOD were lower at 10.6%and7.1%,respectively,which demonstated that magnetite might promote the electrochemical reduction of carbon dioxide to produce methane.In addition,IM-MEC-AD had larger background current and lower anode charge transfer resistance than MEC-AD,showing that the addition of magnetite further optimized conductive environment,and promoted the activity of anode microorganisms.The abundances of Geobacter,Methanosaeta and Methanobacterium in IM-MEC-AD were about 10.8,1.2 and 1.2 times that of MEC-AD,respectively,indicating that magnetite further promoted the reproduction of microorganisms,might consolidate the IET between fermentation bacteria and methanogens.As an electronic mediator,magnetite might also partially replace the c-type cytochrome in the pili of Geobacter,resulting in promoting the DET between Geobacter and Methanosaeta,which revealed the positive effect of magnetite on microbial electrocatalytic sludge bioconversion,and provided a new idea for the integrated technology of sludge treatment and bioenergy recovery.