Study On The Effect Of Salt On Acidogenic Fermentation Of Food Waste

Environmental pollution and energy shortage are two major problems for the world today.Food waste has high contents of organics and water,which is very suitable for anaerobic fermentation.The intermediate product of anaerobic fermentation,volatile organic acids(VFAs),is becoming more widespread used.Therefore,research on the production of VFAs by anaerobic fermentation of food waste has gradually emerged.Salt(mainly NaCl)is an indispensable flavoring agent in food processing,and it is inevitably accumulated in the food waste.Generally,the amount of NaCl contained in food waste is as high as 20-50 g/L,and high concentration of NaCl inhibits the anaerobic fermentation process,thereby affecting the production of VFAs.Therefore,how to produce anaerobic fermentation at high salt concentration and obtain higher yield of VFAs have become a hot issue for researchers.Based on this,in this paper the effect of different NaCl concentrations on VFA production from food waste by anaerobic fermentation was explored,and the changes of microbial community structure under different NaCl concentrations were analyzed.Secondly,by comparing the results of fermentation with acclimated salt-tolerant sludge and anaerobic granular sludge,the method for increasing acid production under high NaCl conditions was discussed.Finally,the metabolomics method was used to analyze the mechanism of the change in VFA production type under high salt conditions.The main contents and results of the thesis are as follows:1.In this paper the effect of different NaCl concentrations on the production of VFAs by anaerobic fermentation of food waste was explored.The production and composition of VFAs and the changes in microbial community in acidogenic fermentation were investigated at five different NaCl concentrations(0,10,30,50 and 70 g/L).At 10 g/L NaCl,the highest VFAs yield was achieved 0.542 g/g(dry weight of the food waste)and was reduced by about 19% at 70 g/L NaCl but still high.Interestingly,as the NaCl concentration increased,the residence time of lactic acid in the reactor and the maximum yield increased,and the type of acidogenic fermentation also changed from butyric acid fermentation to propionic acid fermentation.Microbial community analysis indicated that a large amount of Propionibacterium was present at the end of the fermentation,indicating that they were highly resistant to NaCl.2.Generally,the way to solve the NaCl problem is to use the acclimated sludge under high salt conditions.In this study,acclimated sludge grown under 30 g/L NaCl and non-acclimated anaerobic granular sludge were simultaneously subjected to high NaCl concentration(30 and 70 g/L)for the acidogenic fermentation of food waste.Food waste without added NaCl was used as a control with the non-acclimated sludge.The production of volatile fatty acids(VFAs)and the changes of microbial community during the acid fermentation of food waste were studied with acclimated sludge under high salt conditions.The results showed that compared with the non-acclimated group,acclimated sludge was not conducive to the hydrolysis of food waste due to the lack of microorganisms Chloroflexi and Firmicutes,which contribute to the hydrolysis of food waste.The acclimated sludge under high salt concentration in this study could not increase the yield of VFAs,but it shortened the time to reach the maximal VFA production.This may be because the acclimation process could strengthen the tolerance of microorganisms to high NaCl concentration,but the acidogenic activity of acclimated sludge is not improved.We also found that Nitrospira,Betaproteobacteria,Ochrobactrum,are more abundant in the acclimated sludge,but these microorganisms might use simple organic substrates such as acetate,pyruvate,etc.during the fermentation,resulting in the decrease in the production of VFAs.3.In order to investigate the mechanism of the change in fermentation type,we used ultraperformance liquid chromatography tandem time-of-flight mass spectrometry(UPLC-Triple TOF)to detect the acid production of food waste with 70g/L NaCl and food waste with 0g/L NaCl(control).It was found that in the process of acid fermentation,there are pre-adapted stage of fermentation substrate and adaptation to salt concentration under high salt conditions,so acid-producing fermentation also had an acclimated process.Through metabolite analysis,it was found that high salt promoted the conversion pathway of pyruvate to valine,isoleucine and leucine,and promoted the formation of propionic acid.At the low salt concentration,3-indoleacetic acid was much higher than that at the high salt concentration,which can promote the formation of acetic acid.Under high salt conditions,arginine and proline were converted to ? alanine,and ? alanine can be converted to propionic acid,which may promote the conversion of butyric acid fermentation to propionic acid fermentation.In addition,ectoine was subjected to osmotic protection of microorganisms under high salt conditions.High salt concentration can damage the cell membrane,resulting in a decrease in the level of long-chain LysoPC with high saturation.