Effect Of Vegetable Waste Composition On Acid Production By Mixed Bacteria Anaerobic Fermentation

Volatile fatty acid(VFA)are intermediates of anaerobic fermentation with broad and promising application perspectives.VFA production from vegetable wastes(VW)is an effective way to dispose of wastes and recover resources.In this study,effects of the types and the mixing ratio of vegetable wastes on production and composition of anaerobic fermentation products were investigated.The main research contents and conclusions are as follows:(1)The effects of vegetable waste types on the characteristics of anaerobic fermentation were studied at 37? with potato peel,carrot,celery and Chinese cabbage used as fermentation substrates and anaerobic sludge used as inoculation.The results showed that the VFA yield of potato peel was 452 mg COD/g VSfeed(chemical oxygen demand,COD),which was 40.1%,21.5%and 124.9%higher than that of carrots,celery and Chinese cabbage,respectively.The rapid acidification of carrots caused a sharp decline in pH and led to inhibition of VFA production.The acidification of celery was slow,and the yield of hexanoic acid increased rapidly in the later stage of fermentation.The VFA yield of Chinese cabbage was inhibited due to the low initial pH,but the ethanol concentration reached 7577.04 mg COD/L.According to the VFA profile,the fermentation of potato peel,carrots,celery,and Chinese cabbage can be classified as propionate-type,butyrate-type,mixed-acid type,and ethanol-acetate type metabolic pathway,respectively.(2)Potato peel,carrot,celery and Chinese cabbage with different mixing ratios were used as fermentation materials for anaerobic fermentation.The effects of vegetable waste composition on the characteristics of anaerobic fermentation were studied.The results showed that the VFA yield and acidification rate of 50%potato peel(S2)(carrot:potato peel:celery:cabbage=16.7:50:16.7:16.7)were better than other groups.The maximum VFA concentration was 14178.92 mg COD/L,and the VFA/SCOD reached 86.60%.At the start of fermentation,the rapid acidification of 50%carrot(S1)caused a rapid decrease in pH,which affected the growth and metabolic activities of acid-producing microorganisms,and further affected the VFA production.The pH of 50%carrot and 50%cabbage(S4)was low at the start of fermentation,acetic acid and ethanol accounted for a large proportion in S1 and S4,and the fermentation could be classified as ethanol-acetate type fermentation.The main product of S2 and 50%celery(S3)was butyric acid,and the fermentation type was butyric acid type fermentation.(3)The effects of substrate concentration on the characteristics of anaerobic fermentation and microbial community structure were studied with mixed vegetable waste(carrot:potato peel:celery:cabbage=16.7:50:16.7:16.7)as fermentation substrate.The results showed that the maximum VFA concentration,SCOD(soluble chemical oxygen demand,SCOD)concentration and the acidification rate of anaerobic fermentation of the reactors with substrate concentration of 4%were 11132.99 mg COD/L,15889.33 mg/L and 90.15%,respectively,which were better than other groups.The main VFA of 2%and 3%group was acetic acid,while that of 4%and 5%group was butyric acid.Substrate concentrations had little effect on the diversity of bacteria communities at phylum level,but there were obvious differences at the genus level.The dominant microorganism in 4%group were Clostridium,Unclassified Lachnospiraceae and Unclassified Enterobacteriaceae.Clostridium,Dialister and Acinetobacter were the dominant microorganism in 2%group.(4)The mixed vegetable waste(carrot:potato peel:celery:cabbage=16.7:50:16.7:16.7)was used as raw material to study the effect of inoculation dose on the characteristics of anaerobic fermentation and the change of microbial community structure.The results showed that the yield of VFA,the concentration of SCOD and the acidification rate all increased with the increase of inoculation dose.When the inoculation dose was 5%,the main VFA was acetic acid and ethanol.In the reactors with inoculation dose of 10%and 15%,butyric acid accounted for 63.77%and 57.16%,respectively.The high butyric acid production in the 10%reactor were attributed to the relatively abundant Lactobacillus and Clostridium.(5)The effects of temperature on the characteristics of anaerobic fermentation and microbial community structure were studied using the mixed vegetable waste(carrot:potato peel:celery:cabbage=16.7:50:16.7:16.7).The results showed that the VFA yield,SCOD concentration and acidification rate of the fermentation system at 55? were significantly lower than those at 37?.At 37?,the main product was butyric acid,while the main product was acetic acid at 55?.Temperature has a great effect on microorganisms.At 37?,the dominant bacteria were Lactobacillus and Bifidobacterium,while at 55?,the dominant bacteria were Bacillus,Turicibacter and Clostridium.