Effect Of Biochar On The Efficiency Of Anaerobic Membrane Bioreactor For Food Waste Treatment

Driven by the energy strategic deployment of"carbon peaking and carbon neutrality"and the major national demand of strengthening the construction of biogas,anaerobic digestion has become the mainstream technology for the resource utilization of food waste,but the traditional continuous stirred tank reactor（CSTR）can easily cause excessive loss of functional microorganisms under low hydraulic retention time（HRT）,thus affecting the efficiency of energy recovery.Although anaerobic membrane bioreactor（AnMBR）,which combines membrane separation technology with anaerobic digestion,has attracted much attention in the field of wastewater and waste treatment because of its excellent microbial interception capacity.Membrane fouling is always the key to restrict its application.Compared with other measures to alleviate membrane fouling,such as increasing aeration intensity,biochar has the advantages of simple preparation,low cost,and can alleviate the adhesion of pollutants on the membrane surface by mechanical scouring.In addition,its electrochemical properties can induce direct interspecific electron transfer（DIET）to enhance the methane conversion rate.Therefore,in this study,AnMBR system was constructed to study the effect of biochar on the efficiency of food waste treatment by AnMBR under the OLR of 3.75 to35.00g COD/L/d.Combined with the analysis of the evolution of microbial community and the mechanism of membrane fouling,the mechanism of increasing methane production and alleviating membrane fouling by biochar was found out,and the optimal operation scheme to AnMBR was put forward.（1）The methane production rate of AnMBR decreases with the increase of OLR,and the removal rates of COD,protein,and polysaccharides are higher than 99.0%,99.5%,and 98.0%,respectively.Both reactors can continue to operate stably at an OLR of 32.25g COD/L/d,far higher than similar studies.The addition of biochar can enhance the methanogenic efficiency and stability of the system,and improve the removal rate of organic matter by AnMBR.The conversion ratio of COD to methane is 72%-87%in the biochar group,71%-84%in the control group,and the proportion of COD loss with sludge discharge in the control group is even higher.When OLR is 25.00g COD/L/d,the effect of biochar is the most obvious,and the methane production rate of the biochar group was 0.31L/L/d more than that of the control group.（2）The analysis of microbial activity and microbial community under different OLR showed that the specific methanogenic activity（SMA）with acetic acid,propionic acid and butyric acid as substrate increased with the increase of OLR,and finally reached the plateau stage,while the hydrophilic activity with H₂/CO₂ as substrate increased continuously with the increase of OLR.The dominant bacteria in the two groups of reactors were mainly Chloroflexi,Firmicutes,Bacteroidetes and Proteobacteria,and the dominant genera of archaea were Methanothrix and Methanobacterium.The maximum SMA values of the four substrates in the carbon addition group were 0.53,0.26,0.44 and 0.27g COD-CH₄/g VS/d,respectively.The acetic acid SMA and hydrophilic activity in the carbon addition group were higher than those in the control group,and the relative abundance of acetic acid trophic archaea Methanothrix in the anaerobic system of the biochar group was higher than that in the control group.It indicate that the biochar could accelerate the utilization of acetic acid and increase the reaction rate in the methanogenic stage,reducing the risk of VFAs accumulation caused by the reaction imbalance between acidogenesis and methanogenesis.（3）The sludge concentration increased from 22.50g/L to 39.38g/L by adjusting the HRT/SRT above 1/4,and the membrane flux of AnMBR decrased from 0.80LMH to0.48LMH in this process.As the sludge concentration increases from 22.50g/L to39.38g/L,the membrane flux of AnMBR decreases from 0.80LMH to 0.48LMH.The process of membrane fouling is divided into two stages,namely the stable growth period of transmembrane pressure difference（TMP）and the rapid rise period of TMP.Biochar can reduce the membrane fouling rate during the stable growth period of TMP.The addition of biochar will reduce the potential of sludge mixture to cause membrane pollution by inhibiting the secretion of EPS and increasing the particle size of sludge.At the same time,the physical erosion of biochar will delay the formation of the cake layer on the membrane surface,and reduce the resistance values of the cake layer,thus delay membrane fouling.（4）When AnMBR operates at low HRT,it is necessary to control SRT.If SRT is too high,it will cause high sludge concentration and exacerbate membrane pollution.However,if SRT is too low,it will lead to the loss of organic matter and key microorganisms,resulting in a decrease in methane conversion rate.Therefore,optimizing operating parameters SRT is of great significance for AnMBR to treat food waste.Adding biochar to AnMBR can not only delay membrane fouling,but also improve the methanogenic efficiency of the system to a certain extent,which is a feasible method to optimize the performance of AnMBR.