Effect Of Oil Content On The Efficiency Of Anaerobic Membrane Bioreactor For Food Waste Treatment

In recent years,the output of food waste in our country has been increasing.Driven by the demand of energy,anaerobic digestion technology has become the most mainstream technology in the reduction,harmless and resource recovery of food waste.However,traditional anaerobic fermentation technology is prone to the loss of functional microorganisms under high organic load（OLR）.Lipids,as an important component of food waste,have higher methanogenic potential than carbohydrates and proteins.However,high concentrations of lipids will adsorb and cover the surface of microorganisms,causing more serious sludge loss problems in traditional anaerobic reactors.The anaerobic membrane bioreactor（An MBR）technology can effectively separate HRT and SRT,so as to achieve efficient interception of functional microorganisms,which has certain technical advantages for the treatment of high-lipid food waste.However,the accumulation of LCFAs and membrane pollution in the process are still bottlenecks that hinder its further promotion.At present,there are still limited studies on the application of AnMBR in the treatment of high-lipid food waste.Therefore,this study adopted external An MBR to study the methanogenic and stable properties of An MBR in the treatment of high-lipid food waste under different oil contents,and explored the influence of oil contents on the membrane pollution characteristics of An MBR and microbial community succession and metabolism.It provides theoretical support for the practical application of An MBR in the treatment of high-lipid food waste and the control range of oil content.The main conclusions are as follows:（1）During An MBR operation,as the oil content increased from 0%to 5%,the methane production rate increased from 0.290 to 0.375L-CH₄/g COD,the lipid degradation rate reached the highest 78.19%,and the COD removal rate remained at about99.24%.When the oil content further increases to 8%,the methane production rate and lipid degradation rate decrease slightly,and the COD removal rate also drops to 97.40%,indicating that the system’s ability to recover methane decreases.When the oil content is8%,the accumulated concentrations of VFAs and LCFAs reach 1967.00 and 999.68mg COD/L,respectively,the ammonia nitrogen decreases to 1100mg N/L,and the alkalinity decreases to 6200mg Ca CO₃/L.The ratio of bicarbonate alkalinity to total alkalinity gradually decreased to 0.6,and the ratio of TVFA to alkalinity rapidly increased to 0.31.The stability of the system decreased and the ability to resist acidification was weak.The four-stage analysis shows that the methanation stage is more sensitive to the increase of oil content.When the oil content is 8%,the imbalance of the four stages is intensified,resulting in the accumulation of organic acids with higher concentration.（2）The experimental results of methanation characteristics of volatile fatty acid（VFAs）showed that with the oil content increasing from 0%to 8%,the specific methanogenic activity（SMA）of microorganisms using acetic acid and propionic acid as substrate decreased by 22.58%and 26.49%,respectively,while the SMA using butyric acid as substrate increased by 47.58%,indicating that the degradation ability of acetic acid and propionic acid became weaker.Therefore,the VFAs accumulated at this time were mainly acetic acid and propionic acid.The methanation characteristics of long chain fatty acids（LCFAs）showed that the SMA of palmitic acid,stearic acid and oleic acid decreased in the 8%oil content period compared with that in the 5%oil content period,so higher concentrations of LCFAs accumulated.The results of hydrogenophilic activity showed that with the increase of oil content,the microbial activity of the hydrotrophic methanogenic path increased significantly,and reached 0.21 g CH₄-COD/g VS/d when the oil content was 8%.In addition,the calculated microbial productivity coefficient decreases from 13.61%to 8.86%with the increase of oil content,and the COD flowing to microbial growth decreases,which is conducive to the formation of methane.（3）The results of microbial community analysis showed that the complementary distribution of Porphyromonadaceae and Prolixibacteraceae,which can degrade complex organic macromolecules,made the hydrolysis rate and acidification rate maintain better under the 8%cycle.When the oil content increased from 5%to 8%,the relative abundance of Synergistaceae,which can oxidize acetate,decreased from 12.74%to 4.09%,which may be one of the reasons for the increase of acetic acid concentration in the system.At the same time,the abundance of the acetotrophic Methanothrix decreased from 54.83%to 3.97%,and the abundance of the plenum Methanosarcina increased from 1.29%to 42.71%.This may be caused by the inhibition of the acetotrophic pathway using acetyl-co A synthase（EC:6.2.1.1）in the acetotrophic pathway,promoting the use of acetokinase（EC:2.7.2.1）and phosphotransacetylase（EC:2.3.1.8）,thus maintaining the acetodegradation activity at this time.In addition,the relative abundance of the hydrotrophic Methanospirillum increased from 13.50%at 0%oil content to 51.67%at 8%oil content,indicating that the hydrogen utilization capacity of the system is enhanced with the increase of oil content,thus enhancing the hydrogen activity.（4）Monitoring TMP during operation indicated that with the increase of oil content from 0%to 8%,the membrane contamination rate increased by 10 times while the membrane resistance increased by 24.85%,and the mud cake layer resistance accounted for 84.09%.The results of three-dimensional fluorescence spectrum,Fourier infrared spectrum and CLSM showed that the main pollutants on the surface of the membrane were proteins and polysaccharides,and the pollution was deep in the pores of the membrane,while the microorganisms mainly existed in the mud cake layer on the surface of the membrane.The mechanism of oil’s influence on membrane pollution is that,due to its adhesion effect and inhibition effect on microorganisms,oil causes the concentration of TS,VS,SMP and LB-EPS to increase,which leads to the increase of sludge particle size and makes the formation of mud cake layer more dense and thick.In addition,the high protein/polysaccharide in SMP,LB-EPS and the mud cake layer on the membrane surface will further increase the risk of membrane contamination,thus aggravating the membrane pollution.