Effect Of Inoculum Mixing Ratio And Oil-fat On Dry Anaerobic Digestion Of Kitchen Waste

With the continuous development of economy and the increasing number of population,the amount of kitchen waste generation shows a growing trend.As kitchen waste has the characteristics of high water content,high organic matter content,high oil content and easy to rot and deteriorate,if the kitchen waste is not properly treated and disposed,it will cause waste of resources and secondary pollution of the environment,and even endanger human health.Anaerobic digestion using microbial action can make the decomposition of organic matter in kitchen waste into clean energy such as methane,and simultaneously realize the harmless and resourceful treatment of kitchen waste.Therefore,anaerobic digestion has become an important treatment technology for the resource utilization of kitchen waste.Compared with wet anaerobic digestion(solids content rate less than 15%),dry digestion(solids content rate between 15% and 40%)has the advantages of small footprint,low methane yield and impact resistance,thus dry anaerobic digestion of kitchen waste has attracted much attention from researchers and engineering practitioners.The inoculation mixing ratio is a key parameter affecting the performance of anaerobic digestion of kitchen waste,although there have been studies on the effect of inoculum mixing ratio on anaerobic fermentation of kitchen waste,the studies on inoculum mixing ratio of kitchen waste mainly focus on the ratio between inoculum and fermentation substrate in wet anaerobic fermentation,but there are few studies on the effect of the ratio between mixed inoculum on dry anaerobic digestion of kitchen waste.In addition,kitchen waste contains a large amount of oil and fat,on the one hand,anaerobic digestion of fat has a high gas production potential,on the other hand,the high concentration of oil and fat will inhibit the microbial metabolism of the anaerobic digestion system of kitchen waste and reduce the accumulation of biogas Previous studies on the utilization of oil and fat on anaerobic digestion of kitchen waste have mostly focused on wet anaerobic digestion.However,the effect of fat and oil on dry anaerobic digestion of kitchen waste and the mechanism have been little investigated so far.Therefore,in this work,kitchen waste was used as the digestion substrate,and different ratios of inoculum(sludge and digestate mixture)were added to the kitchen waste firstly,and the effect of inoculum mixing ratio on the dry anaerobic digestion performance of kitchen waste was investigated.Then,the effect of oil and fat on the dry digestion performance of kitchen waste was investigated by comparing different oil and fat contents,and the mechanism of oil and fat on the dry digestion of kitchen waste was revealed by analyzing the gas production performance,organic matter biotransformation pattern,microbial diversity and community structure.Firstly,the effect of inoculation mixing ratio on dry anaerobic digestion of kitchen waste was investigated in this work,and the results showed that the inoculation ratio affected the gas production efficiency of dry anaerobic digestion of kitchen waste.The cumulative gas production was 187.65 m L/g VS in the experimental group with sludge as inoculum alone,and 300.41 m L/g VS,281.39 m L/g VS,280.16 m L/g VS,and 225.50 m L/g VS in the experimental groups with inoculated sludge to inoculated digestate mass ratios of 2:1,1:1,1:2,and 0:2,respectively,225.50 m L/g VS,the gas production was increased by 59.22%,48.93%,48.46% and 20.98%,respectively,compared with the experimental group using only sludge as inoculum.Compared to inoculated sludge,digestate addition helps to improve the efficiency of kitchen waste anaerobic digestion gas production.The low percentage of inoculated digestate addition makes it easy to accumulate volatile fatty acids(VFA)excessively during anaerobic digestion,leading to a sudden drop in p H,resulting in a p H plunge to 3.48,which causes excessive acidification of the system,In the group in which the ratio of inoculated sludge to inoculated digestate was 0:2,the high ratio of inoculated digestate addition resulted in high accumulation of ammonia nitrogen and VFA in the reaction system,and the accumulation of ammonia nitrogen and VFA in the reaction system increased to 866.86 mg/L and 9.40 g/L,respectively,which inhibited the anaerobic process to some extent and reduced the gas production compared to the group with low ratio of added digestate,but compared to the experimental group inoculated with sludge only,it still promoted the overall gas production.Subsequently,this work investigated the effect of oil and fat content on the dry anaerobic digestion of kitchen waste.The results showed that the addition of oil and fat concentration could promote the gas production of dry anaerobic digestion of kitchen waste.When the addition of oil and fat was in the range of 0% to 4%,the effect of oil and fat addition on the enhancement of kitchen waste anaerobic digestion gas production gradually increased,and the kitchen waste anaerobic digestion had the highest cumulative gas production of 269.81 m L/g VS at 4% oil content,which was177.9% higher than that of the blank group.In the group with 4% oil and fat content,the dissolved sugar content was 47.19 g/L,which was 75.2% higher than that of the blank group,and the VFA concentration was 8.02 g/L,which was significantly higher than that of the blank group at 7.25 g/L.When the addition of oil and fat concentration gradually increased from 4% to 8%,the cumulative gas production in the reactor gradually decreased,and when the oil and fat concentration was When the oil and fat concentration was 8%,the cumulative gas production was 161.53 m L/g VS,which was still 66.4% higher than that of the blank group,and with the increase of grease content,although the experimental group with higher mass fraction of added oil and fat had higher VFA content,its system contained more digestive bacteria that can break down fatty acids more quickly,which made the p H in the reactor stabilize earlier.This indicates that the addition of oil and fat can decompose more VFA to provide digestion substrate for digestive bacteria and promote their gas production,in addition,when the oil and fat content is from 0% to 6%,the reduction rate of kitchen waste increased with the increase of oil and fat content,and there was a maximum reduction rate of 7.83% when the oil and fat content was 6%,but the reduction rate gradually decreased when the oil and fat content further increased.Finally,this work analyzed the effect of oil and grease on the microbial diversity and structural characteristics in the dry anaerobic digestion of kitchen waste using high-throughput sequencing.The results showed that the oil and fat content could influence the microbial community characteristics within the dry digestion of kitchen waste,and there were significant differences in community richness among the blank group,the three groups with 4% oil and grease content,and the three groups with 8%oil and fat content.At the bacterial phylum level,increase in oil and fat concentration decreased the relative abundance of Firmicutes,but it increased the relative abundance of Proteobacteria.At the level of bacterial genera,the relative abundance of Lactobacillus was the highest in all three groups of samples,because Lactobacillus is suitable for growth in an acidic environment.Moreover,Leuconostoc,which catabolizes dissolved sugars,was mainly present in the blank group,and in the two groups with 8% oil content,which may lead to the ability of dissolved sugars in these two groups to be rapidly degraded during the reaction.This work investigated the effect of inoculum mixing ratio and grease on dry anaerobic digestion of kitchen waste,and analyzes its mechanism of action to enrich the related research,and the results provide some theoretical basis and technical reference for the efficient resource utilization of kitchen waste.