Effects Of Thermal Pretreatment On Organics Transformation And Anaerobic Digestion Performance Of Food Waste

Food waste?FW?generated in China characterizes a typical three-phase medium system?oil,liquid,and solid phases?.The characteristics of high organic content and particle-based organic matter result in a long retention time,low biogas yield,and unstable operation during the anaerobic digestion.The study aims to investigate the effects of thermal pretreatment on the organics transformation and the anaerobic digestion performance of FW and reveal the corresponding mechanisms.In addition,the demonstration project verification and economic analysis were carried out using optimized parameters corresponding to thermal pretreatment.The outcomes obtained from this study can provide important information on how biomethane production can be enhanced and optimized.The optimum composition ratios of carbohydrate,protein and lipid for maintaining high methane yield and process stability were identified.The carbohydrate-protein-lipid ratio could be served as a critical parameter correlating with digestion performance parameters,thus affecting methane yield and final reduction of organics.Controlling the carbohydrate higher than 8.3%but lower than10.1%,protein lower than 5.0%but higher than 4.3%and lipid lower than 5.6%but higher than 4.7%could maintain stable digestion with higher methane production,shorter digestion retention,and a methane bioconversion efficiency of organics in terms of chemical oxygen demand.The effects of thermal pretreatment on the variation of FW composition were investigated.The results show that the hydrolysis of organics and the leaching of floating oil were promoted by thermal pretreatment.According to the hydrolysis kinetics study of lipid and protein during the thermal pretreatment process,the activation energy during the hydrolysis of lipid to saturated fatty acids,the hydrolysis of unsaturated fatty acids to monounsaturated fatty acids,and the hydrolysis of proteins to essential amino acids were higher.The Maillard reaction occured when the thermal pretreatment temperature ranges from 140 to 160°C with the treatment exceeding 50 min.The hydrolysis order of the organic components during the thermal pretreatment process was-CH->amide band I>fat>-CH₂->carbohydrate,while the conformational change of the protein mainly occured in the amide I band.The results of digestion performance of FW with thermal pretreatment revealed that the thermal pretreatment could enhance the organic reduction,and the highest values were achieved when FW was pretreated at 55-90°C and 120-160°C for 70and 50 minutes,respectively.The hydrolysis stage posed a great influence on the biomethane production efficiency,and the main factor affecting the hydrolysis stage is the volatile solid?VS?removal rate during the thermal pretreatment stage.Basing on the variation characterstics of processing stability parameters during digestion process,the degree of hydrolysis during thermal pretreatment can be divided into three categories:low?VS removal rate lower than 6%?,moderate?6.8%-11%?,and slightly high?12%-38%?.Response surface methodology was used to describe the relationship between the responses for characterizing the digestion performance and independent variables for representing the effects of thermal pretreatment.The results show that variations in the organic molecules of FW induced by thermal pretreatment are the main factors affecting the digestion performance parameters,ie,biomethane production and methane yield rate,which are most related to the content of polyunsaturated fatty acids,non-essential amino acids and total amino acids.Based on the improvement of oil recovery and biogas production,the appropriate thermal pretreatment temperature and time were 90°C and 70 min,respectively.The verification results of the optimized thermal pretreatment parameters in the national FW demonstration project with treatment capacity of 250 tons per day,located in Jiaxing,Zhejiang province showed that the stable digestion with higher methane production,shorter digestion retention,and low processing cost?82 yuan/t?could be achieved,while the recovery rates of oil and biogas from FW were increased by 34%and 22%,respectively.