Study On The Production Of Volatile Fatty Acids By Anaerobic Fermentation Of Fruit Waste Liquid

In recent years,due to the rapid development of China’s fruit industry,the amount of fruit waste has increased,which may cause serious pollution if disposed improperlycan.At present,using anaerobic fermentation to produce volatile fatty acids（VFAs）is the research hotspot at both domestic and overseas.Meanwhile,the solid-liquid separation of fruit waste to obtain a liquid phase,which is dominated by soluble small-molecule organic matters,makes it possible to produce VFAs efficiently by fermentation.In this study,the effects of different factors on the production of VFAs by anaerobic fermentation were investigated by using the liquid phases of flesh-skin-connected fruit waste,flesh-skin-unconnected fruit waste and their mixure as substrates.The microbial community structure was investigated to reveal the mechanism.This study provides a theoretical basis for the efficient production of VFAs from fruit waste.The main results of this study are as follows:（1）VFAs were produced by anaerobic fermentation of watermelon waste liquid（WRL）with inoculum dosage of 5 g TS/L,initial p H 5,temperature 40°C and methane inhibitor（SBES）1 mmol.The maximum yield of VFAs reached 148.45 mg COD/g VS on the 4th day of fermentation.Inoculation improved the efficiency of VFAs production,weak acidity promoted organic matter decomposition and utilization,and medium temperature and small amount of SBES facilitated VFAs production.The maximum yield of VFAs from WRL（149.34 mg COD/g VS）was 2.20 times and 1.69 times higher than that of watermelon waste solid（WRS）and mixed phase（WRM）,respectively.Moreover,the time to reach the maximum yieldof WRL4^th day was separately 1 day and 3 days less than that of WRM and WRS.The SEM results showed a greater microbial density,which indicated that WRL fermentation could increase the yield of VFAs and shorten the fermentation time.Reflux inoculum fermentation(3^rd day)had a maximum yield of VFAs（194.98 mg COD/g VS）that was 30.01%higher than that of the original inoculum fermentation(4^th day),with acetate as the major product（64.76%）,propionate 22.43%,isobutyrate 4.24%and butyrate 8.57%.The reflux inoculum could improve the efficiency of VFAs production and change the VFAs constituents.The dominant phylum was Firmicutes,which arrived at 97.19%at the end of fermentation,and was positively correlated with VFAs（0.894）.At the end of fermentation,Lactobacillus accounted for 91.37%to become the dominant genus,which was positively correlated with VFAs（0.926）.During fermentation,the C cycle was enhanced and eventually reached 96.23%,whereas the N cycle was weakened and eventually decresed to0.31%.（2）VFAs were produced from banana waste liquid（BWL）by anaerobic fermentation.With the extension of stacking time,the p H and TS of BWL decreased to 5.18 and 4.96%,respectively,and the soluble sugars and SCOD first increased and then decreased.The fermentation with BWL stacked for 4 days obtained the largest yield of VFAs（182.41mg COD/g VS）,with 23.77%acetate,28.98%propionate,40.65%iso-butyrate,6.61 butyrate.Proper stacking promoted structural change of BW and release of organic matter,which facilitated the increase of VFAs yield.The VFAs reached a maximum yield of 227.34mg COD/g VS with propionate and iso-butyrate as the main products（41.09%and 34.17%）,acetate 11.17%,butyrate 7.02%and valerate 6.54%at an initial p H of 9,inoculum amount of3 g TS/L,temperature of 35°C and SBES of 0.5 mmol on the 4^th day.Inoculation improved the efficiency of VFAs production,weak alkalinity promoted the decomposition and utilization of organic matters,and medium temperature（35°C）and small amount of SBES（0.5 mmol）facilitated the production of VFAs.The maximum VFAs yield of BWL（224.46mg COD/g VS）was 2.51 times and 1.73 times higher than that of banana waste solid phase（BWS）and their mixed phase（BWM）,respectively.Moreover,the time to reach the maximum yield of BWL（day 4）was separately 4 days and 1 day less than that of BWS and BWM.The SEM results showed that the microbial density was greater,which indicated that BWL fermentation could increase the yield of VFAs and shorten the fermentation time.At the phylum level,Proteobacteria（65.80%）dominated at fermentation initiation.On day 4,Bacteroidetes grew to 61.93%and Firmicutes to 21.46%,both were positively correlated with VFAs（0.94 and 0.93）.At the genus level,Enterobacter was higher on the first day（37.77%）and Bacteroides reached 54.21%on the 4^th day,which was positively correlated with VFAs（0.95）.During fermentation,chemoheterotrophy and fermentation functions dominated the process.（3）When applying the mixed fruit waste liquid（FWL）as the fermentation substrate to produce,the p H and TS of FWL decreased to 4.71 and 3.70%with increasing stacking time,respectively,whereas the soluble protein and SCOD first increased and then decreased.The FWL fermentation with 3 days of stacking showed the maximum yield of VFAs（196.41mg COD/g VS）,with 48.13%of acetate,22.59%of propionate,6.70%of iso-butyrate,13.73%of butyrate,and 7.39%of valerate.Proper stacking promoted structural change of FW and release of organic matter,which facilitated the increase of VFAs yield.The VFAs reached a maximum yield of 196.17 mg COD/g VS with acetate as the main product（51.92%）and propionate（18.03%）,iso-butyrate（16.71%）,butyrate（8.56%）and valerate（4.78%）at an initial p H of 7,inoculum volume of 3 g TS/L,temperature of 35°C,and SBES of 0.5 mmol for 4 days of fermentation.Inoculation improved the efficiency of VFAs production,and neutral p H,medium temperature（35°C）and small amount of SBES（0.5 mmol）facilitated the production of VFAs.The maximum yield of VFAs（196.49 mg COD/g VS）of FWL was2.43 times and 1.61 times higher than that of mixed fruit waste solid phase（FWS）and their solid and liquid mixture（FWM）,respectively.The time to reach the maximum yield of FWL（day 4）was 2 and 3 days less than that of FWM and FWS,respectively.FWL fermentation could improve the yield of VFAs and shorten the fermentation time.At the phylum level,Proteobacteria（46.99%）dominated at the beginning of fermentation,increasing to 89.64%on day 1,and Firmicutes increased to 33.76%on day 9,both of which were positively correlated with VFAs（0.49 and 0.58）.At the genus level,Enterobacter reached 39.26%on day 1 and Lactobacillus 31.07%on day 9,both of which were positively correlated with VFAs（both 0.65）.The abundance of chemoheterotrophy and fermentation functions increased during fermentation.