| With the continuous growth of energy demand,fuel ethanol produced from biomass,as a renewable fuel that is expected to replace fossil fuels,is receiving more and more attention.However,a large amount of wastewater will be generated in fuel ethanol production process,which has a high concentration of chemical oxygen demand(COD),deep color,low pH value,and will pollute the environment if directly discharged,so it has become a major problem to limit the production of fuel ethanol.Anaerobic digestion is a good choice for the treatment of poplar fuel ethanol wastewater due to its advantage of treating high loads of organic matter and high methane yield.However,the anaerobic digestion process is affected by many factors,such as inoculum source,temperature and pH value.Therefore,in order to improve the performance of methane production,the effects of inoculum selection and temperature on the anaerobic fermentation process were explored,acid-tolerant sludge was acclaimed by acetic acid gradient addition,and the effect of inoculation ratio of acid-tolerant sludge on the fermentation characteristics of poplar fuel ethanol wastewater was investigated and the microbial community of different anaerobic fermentation was analyzed.The main results and conclusions were as follows:The digestion characteristics of anaerobic fermentation of poplar fuel ethanol wastewater using pond sludge and sewage plant sludge as inoculations were explored.Compared the treatment effects of different inoculants on wastewater by analyzing the changes in biogas production,methane production,concentration and composition of volatile fatty acids(VFAs),pH and other indicators during the fermentation process.The results showed that the cumulative methane yield of the sewage plant sludge group reached 525.70 ±61.57 mL/gVS,which was 169.51%higher than that of the pond sludge group.The results of volatile fatty acids(VFAs)detection in the fermentation broth during the fermentation process showed that the degradation efficiency of the sludge group of the sewage plant was faster.Fourier transform infrared spectroscopy and three-dimensional fluorescence spectroscopy showed the sugars and long-chain substances in the fermentation broth were effectively degraded and the content of humic acids was increased.Microbial community analysis showed that Firmicutes,Proteobacteria,Chloroflexi,Actinobacteriota and Bacteroidota were the five most abundant bacterial phyla.At the end of anaerobic fermentation,the relative abundance of Methanosarcina was the highest in the archaea community in the pond sludge group,while the sewage plant sludge group was mainly Methanosarcina,Methanobacterium and Methanosaeta,indicating that the methanogenic bacteria of sewage plant sludge were more diverse and the role between archaea was closer,which may be the reason for the high methane production.Using sewage plant sludge as inoculum,the effect of temperature on the anaerobic fermentation process of poplar fuel ethanol wastewater was explored.The cumulative methane production was the highest at 55℃,906.40±50.67 mL/gVS,which was 81.06%,72.42%,and 13.33%higher than that at 25,35,and 45℃,respectively.The results of VFAs detection showed that during the anaerobic fermentation process,when the fermentation temperature was 55℃,the concentration of VFAs was much higher than that of 25℃,35℃ and 45℃,and the concentration of VFAs was higher in the later stage of anaerobic fermentation,but its biogas production was decreased.This was attributed to the mass concentration of propionic acid:acetic acid being greater than 1.4 at this time,so it inhibited the fermentation.Microbial community diversity showed that the microbiota at 25℃ and 35℃ were similar in structure and metabolic pathways.Firmicutes and Proteobacteria were the main flora covering the 25-55℃-based phylum or below it.The relative abundance of Methanosaeta was the highest when fermentation temperatures were 25 and 35℃,while the relative abundance of Methanosarcina and even hy drogenotroph i c methanogens such as Methanoculleus and Methanothermobacter were dominant when fermentation temperatures were 45 and 55℃,which indicated that the metabolic pathway changed from acetoclastic methanogenesis to hydrogenotrophic methanogenesis.In view of the high acidity of poplar fuel ethanol wastewater,the pH must be adjusted to close to neutral before anaerobic fermentation,which consumed a large amount of alkali,furthermore too low methane yield of anaerobic fermentation if unneutralized poplar fuel ethanol wastewater was directly treated by sewage plant sludge.The acetic acid gradient method was used to acclaim acid-tolerant sewage plant sludge.The amount of acetic acid added gradually increases with the biogas production performance during the acclimation process,and the pH value of the anaerobic digestion system was gradually reduced by the addition of acetic acid,the methane content and acetic acid conversion rate were high when the concentration of acetic acid reached 22 g/L,the lowest pH value was 5.34,indicating that the acid-tolerant sludge was successfully acclaimed.The Shannon and Chao indices of microbial communities both showed a decreasing trend,indicating that with the increase of acetic acid concentration,the diversity and richness of microbial communities decreased.The relative abundance of Methanosarcina in the sludge after acclamation increased from 0.86%to 25.25%.Anaerobic digestion experiments with the inoculation ratio of acid-tolerant sludge showed that the experimental group with 25%acid-tolerant sludge inoculation ratio had the best anaerobic fermentation performance,and its cumulative methane yield reached 104.39±5.78 mL/gVS,which was 181.22%,141.76%,101.64%and 100.06%higher than that of the 0%,50%,75%and 100%acid-tolerant sludge groups,respectively. |
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