Degradation And Detoxication Of Lipids In Food Wastes To Promote Directionally Microbial Conversion Of Methane Production

The clean disposal of food waste and anaerobic digestion to produce biogas for energy utilization is of great significance for promoting energy conservation,environmental protection and the development of new energy.However,the lipid content in the food waste is usually high,and a large amount of lipids still enters the anaerobic digestion system after degreasing.The lipid degradation is slow and a large number of microorganisms are coated,which easily causes the inhibition of anaerobic digestion.In this article,a variety of free radicals generated from hydrothermal and ozonation pretreatments are used to pretreat typical waste lipids from food waste,revealing the mechanism of oxidation and degradation of lipids to promote fermentative methane production.Sodium percarbonate/ultraviolet oxidation method is used to degrade and remove the fermentation inhibitors such as capsaicin in the waste lipids,and the efficiency of fermentative methane production from waste lipids is significantly improved through the microbial electrochemistry method.The common component glycerol trioleate(C57H104O6)in food waste lipids was utilized as the feedstock and hydrothermal alkali pretreatment was utilized to generate free radicals as ·OH and O2·— to promote lipid degradation and accelerate ? oxidation process in lipid anaerobic digestion,which reduced the coating of microorganisms by lipids and improved the energy conversion efficiency of methane generation.Oleic acids and hexadecanoic acids derived from degraded glycerol trioleate increased(from 43.29% to 58.22%,and from 1.06% to 8.25%,respectively)when the pretreatment temperature increased from 160°C to 220°C.The pretreatment at 220°C effectively degraded 87.56% of glycerol trioleate.The methane yield and the peak production rate of glycerol trioleate also increased(from 636.85 to 877.47 mL CH4/gTVS,and from 32.60 to 51.22 mL CH4/g-TVS/d,respectively),which led to an increased energy conversion efficiency from 48.05% to 66.21%.The mechanism of ozonation pretreatment on glycerol trioleate degradation to promote anaerobic digestion was explored.When the ozone concentration increased from 0.02 to 0.8 gO3/g-TVS,free radicals as O2·— induced the acceleration of ? oxidation reaction rate to promote the degradation reaction,and the proportion of hexadecanoic acids derived from degraded glycerol trioleate increased from 12.64% to 30.23%,while tetradecanoic acids derived from degraded glycerol trioleate increased from 0 to 43.84%,and the overall lipid degradation rate increased from 13.4% to 78.6%.The coating of methanogens by lipids during the anaerobic digestion process was substantially relieved after ozonation pretreatment.The methane yield from glycerol trioleate increased by 81.9% to 946.5 mL/g-TVS.The effects of ultrasonic and microwave pretreatments of lipids in promoting methane production by anaerobic digestion were compared and investigated.Ultrasonic pretreatment demonstrated the thermal effect due to cavitation,and its mechanical shearing force and free radical oxidation ability were stronger than the thermomagnetic effect of microwave pretreatment.After ultrasonic pretreatment,the methane yield of waste lipids increased by 43.3% to 927.97 mL/g-TVS(energy conversion efficiency of 69.89%),which was higher than the microwave pretreatment of 738.63 mL/g-TVS(energy conversion efficiency of 58.98%).The inhibitory mechanism of capsaicin,which is easily soluble in lipid components in food waste,on anaerobic digestion and metabolic processes was studied.The methane yield during anaerobic digestion of glycerol trioleate decreased by 84.0% from 780.21 to 142.10 mL/g-TVS with capsaicin(40 wt%).Capsaicin structurally damaged microorganisms via cell membrane breakage,which impaired their function.Capsaicin degradation products were combined with enzyme molecules,inhibiting the activity of biological enzymes and the anaerobic digestion of lipids.Capsaicin resulted in a negative effect on archaeal community,and the proportion of Methanosarcina decreased from 30% to 6%.The electron transfer rate kapp in lipid anaerobic digestion decreased by 99% from 4.67×10-2 to 5.66×10-4 s-1 in response to capsaicin(40 wt%),therefore obviously inhibiting the process of lipid anaerobic digestion.The mechanism of the capsaicin degradation by the sodium percarbonate/ultraviolet advanced oxidation process was studied.Increasing the sodium percarbonate concentration and the ultraviolet intensity can enhance the degradation of capsaicin.Experiments proved that the free radicals ·OH,O2·— and CO3·— generated from sodium percarbonate/ultraviolet advanced oxidation process worked together to degrade capsaicin.Capsaicin likely had four degradation pathways via conversion into the intermediate product of benzoquinone,and finally into carbon dioxide and water.After sodium percarbonate/ultraviolet degradation,the methane yield of capsaicin-containing waste lipids recovered from 27.20 to 311.23 mL/g-TVS,cell oxidative damage hardly occurred,and the anaerobic digestion performance of the waste lipids was significantly alleviated.The mechanism of promoting methane production by introducing external electric field into the lipid anaerobic digestion system was studied.When the external electric field voltage of the lipid anaerobic digestion reactor increased from 0 V to 0.8 V,the methane yield of anaerobic digestion increased from 705.71 to 791.60 mL/g-TVS,and the time to reach the peak methane production rate was shortened from 33 to 24 d.The electrochemistry method promoted lipid degradation and ? oxidation in anaerobic digestion,a large number of methanogens were attached to the electrode,and the electron transfer was promoted to increase the methane yield.