Optimization Of Acidogenic Fermentation From Food Waste And Fermentative Characteristics Of Protein-rich Substrates

Volatile fatty acids(VFAs)are valuable substrates for several applications.Generally,VFAs are produced commercially by chemical routes,which need a large amount of non-renewable petrochemicals as the raw material.Biological route—anaerobic fermentation can provide an opportunity to recycle organics while producing the value-added chemicals VFAs,thus attracting the wide interest.Food waste(FW)is the most explored solid waste commonly utilized for VFA production,with its dominate proportion in municipal solid waste and high total chemical oxygen demand(COD).To enhance VFA production from FW is of great importance.Some thorny questions still exist during the process of acidogenesis from FW,such as the optimization of pretreatment,the controllment of anaerobic condition,the conversion of FW composition and so on.Therefore,the aims of this study were:firstly,to evaluate the effects of prefermentation(PF)and hydrothermal pretreatments(HT)on substrate hydrolysis and acidogenesis;then,to study the influence of redox potential(ORP)level on FW acidogenic fermentation mainly from the aspect of microbial community structure and key enzyme activity;lastly,to explore the acidogenic fermentation mechanism of protein-rich waste.Main conclusions were drawn as following:(1)Prefermentation(PF)and prefermentation+hydrothermal(PF-HT)pretreatmentssignificantly promoted the dissolution and hydrolysis of organic materials.PF mainly enhancedthe hydrolysis of carbohydrates,which contributed to the saccharification of polysaccharides(i.e.,starch)in an acidic environment caused by Lactobacillus in FW.However,to some complex substrates(i.e.,cellulose and protein),PF-HT pretreatment seemed to be a better strategy for their solubilization.VFA production rate increased noticeably at the beginning of anaerobic fermentation using pretreated FW.However,because of the high contents of cellulose and lipids in FW,the yield of VFAs would be limited,even after PF and HT pretreatments.The acidogenic pathway of fermentation system varied after different pretreatments.After PF pretreatment,the substrates were first converted to lactate then produced VFAs;while VFAs were produced directly from the substrates after the PF-HT pretreatment.Economic analysis indicated anaerobic fermentation with appropriate PF and HT preteatmentswas an earning-effective approach for FW to bio-energy,such as VFAs.(2)Anaerobic condition(ORP-200 to-300 mV)was not suitable for VFA production at pH 6.0.The optimal ORP level for VFA production was between-100 and-200 mV,which could be obtained by limited aeration during acidogenic fermentation.Limited aeration condition,with anaerobic sludge as an inoculum(LA+AnS),promoted the degradation of substrates(especially proteins),and thus enhanced VFA yield.Higher VFA yield appeared to depend on the greater abundance of Firmicutes and more diverse microbial community under LA+AnS conditions.BES addition in the anaerobic acidogenesis process would improve VFA yield mainly by increasing the abundance of some acidogens.(3)Significant difference in VFA production and composition were observed between two kinds of protein-rich substrates with the same content of TCOD.Specially,HT pretreatment was not favorable to the biodegradation of egg white.The most prevalent VFA was acetate in tofu,but all kinds of VFAs accounted for similarly by egg white fermentation.In addition,stoichiometric analysis of amino acids showed that the Stickland reaction was the main pathway for VFA accumulation from tofu.However,lactate and VFAs were the main products from egg white,which was not strictly consistent with the Stickland reaction.VFA yield of all reactors mainly was influenced by the difference in microbial community structure:Sporanaerobacter and Aminobacterium were the dominant genus in tofu(21.0%)and egg white(10.9?19.9%)groups,respectively.More lactate-producing bacteria(Leuconostoc,Lactobacillus)were observed during egg white fermentation.According to the nitrogen balance analysis,the conversion efficiency of proteins had a good agreement with the VFA accumulation for all reactors.Some amount of soluble protein(30?40%of the total protein)was not converted at the end of fermentation for all conditions.By the separation of protein,we found that up to 50%of the unconverted soluble protein in the HT groups was protease.