Effect Of Long Chain Fatty Acids On Methanation Performance Of Food Waste

In recent years,with the improvement of residents’living standards,the annual output of kitchen waste in my country has also increased.Anaerobic fermentation technology is currently considered to be the best way to deal with food waste,but the oil content of food waste is high,and the long chain fatty acids（LCFAs）produced by anaerobic fermentation process have certain effects on anaerobic microorganisms.However,its accumulation characteristics and effects on anaerobic fermentation process still need to be further explored.Based on this,this study investigated the production and degradation characteristics of LCFAs during anaerobic fermentation of food waste under different organic loading rates（OLR）,and revealed the accumulation of typical LCFAs from the perspective of microbial growth kinetics.The effects of LCFAs on each stage of anaerobic fermentation（hydrolysis stage,acid production stage and methanogenesis stage）were systematically analyzed,and the inhibitory effect of LCFAs on each stage of anaerobic fermentation was clarified.The main conclusions are as follows:（1）The F/M of food waste was set as 0,0.37,0.63,1.19 and 1.58 respectively.With the increase of F/M,the accumulation concentration of LCFAs gradually increased.With the increase of F/M from 0 to 0.63,the accumulation of LCFAs increased sharply.When F/M was greater than 0.63,the increase trend of total LCFAs gradually flattened,indicating that within the range of F/M from 0 to 0.63,the impact on the accumulation of LCFAs was more significant.In addition,compared with the blank group（LCFAs accumulation 35.41mg/l）,the LCFAs concentration under each F/M condition increased by 45.8%,94.1%,102.2%and 106.2%respectively,and the main LCFAs accumulation types under each load condition were palmitic acid,stearic acid and oleic acid,of which stearic acid accounted for the highest proportion,reaching 43.02%,59.2%,61.2%and 63.3%respectively under each concentration（except the blank group）;（2）The methanogenic characteristics of typical LCFAs（palmitic acid,stearic acid and oleic acid）at different concentrations（1000 g COD/L,3000 g COD/L,5000g COD/L,8000 g COD/L and 10000 g COD/L）were investigated.The results showed that the inhibition of LCFAs on Methanogens was more obvious with the increase of concentration.The degradation rate of typical LCFAs at different concentrations showed the same trend,that is,palmitic acid>stearic acid>oleic acid.In addition,although the delay period of oleic acid is long,the maximum growth rate of microorganisms(1.28d^-1)is greater than that of stearic acid(0.35d^-1).This result is mainly because the microorganisms can quickly recover their activity after the degradation of oleic acid is lower than the inhibition concentration,which indicates that this inhibition is reversible;（3）The results of microbial community analysis show that:Clostridium＿Sensu＿Stricto is the dominant bacterium in each LCFAs group.It converts unsaturated LCFAs into saturated LCFAs during anaerobic fermentation,which is also the reason why the relative abundance of this bacterium in oleic acid（10.3%）is much higher than that of palmitic acid（3.2%）and stearic acid（3.4%）.From the perspective of archaea,Methanothrix is the main Archaea in palmitic acid and stearic acid,while Methanosarcina,Methanobacterium and Methanomassiilicoccus are the main Archaea in oleic acid,indicating that the degradation of palmitic acid and stearic acid is mainly completed by methanogens of acetic acid,The degradation of oleic acid is the process of methanation by traditional hydrogen trophic methanogens;（4）Through the study on the influence of LCFAs on each stage of the anaerobic fermentation of food waste,it was found that in the hydrolysis stage,the inhibition degree of oleic acid on this stage was greater than that of palmitic acid and stearic acid,and the degradation rates of protein and polysaccharide in the oleic acid group decreased by 76.7%and 48.3%respectively with the increase of concentration,while in the palmitic acid group and stearic acid group,protein The degradation rate of polysaccharide decreased by 7.9%and 14.6%,3.3%and 11.5%respectively with the increase of concentration;In the acid production stage,it was found that the addition of LCFAs could accelerate the production of tvfas,and there was a significant positive correlation between tvfas and the concentrations of palmitic acid（r=0.981,p<0.003）,stearic acid（r=0.998,p<0.000）and oleic acid（r=0.988,p<0.002）;In the methanogenic stage,the addition of LCFAs can inhibit the methanogenic process,and IC50 palmitic acid（7666.64mg COD/L）>IC50 stearic acid（6037.82mg COD/L）>IC50 oleic acid（4276.23mg COD/L）,which indicates that LCFAs has a stronger inhibition on the methanogenic process with the increase of carbon chain and unsaturated double bond.