Methane Production From Protein-rich Substrates And Associated Microbial Community Dynamics

Biomass energy has gained an increasing attention worldwide as a biofuel,due to growing energy crisis and environmental concerns.Anaerobic digestion(AD)is a widely employed technology for the conversion of biomass such as organic wastes to biomethane for renewable energy production.An important fraction of the organic wastes comprises of food waste with varying biochemical compositions.In China,rapid economic growth has fueled a dramatic change in China's dietary landscape,where,protein-rich food is widely consumed in the form of seafood,meat(pork,beef,lamb),dairy products,poultry,grains and legumes,thus contributing to a mass of protein-rich wastes produced annually as food waste,thereby causing severe environmental pollution.This protein-rich wastes can be effectively utilized as a valuable substrate for methane production via AD.During AD,proteins are initially hydrolyzed into 20 kinds of amino acids,which are then fermented into short-or branched-chain organic acids,and eventually to methane,which is the final product.However,each amino acid has a different methane production performance and biodegradability.Therefore,the digestion of protein-rich food wastes greatly depends on its amino acid composition and concentration.Hence,it is imperative to investigate the methane production performance of each of the amino acids for the effective utilization of protein-rich wastes in AD.In this study,the AD performance of twenty different amino acids and associated microbial community changes were investigated.Then,the AD performance of different kinds of food substrates with high to moderate protein content under both mesophilic and thermophilic conditions was investigated systematically along with microbial community analysis.Lastly,the effect of using biochar as additives was seen on the methane production performance of protein substrates.The results are as followsFirst,the real methane production potential of each of the twenty amino acids such as leucine,isoleucine,valine,lysine,alanine,threonine,glutamine,glutamic acid,histidine,arginine,serine,asparagine,aspartic acid,glycine,phenylalanine,tyrosine,methionine,tryptophan,cysteine,and proline was investigated along with microbial community analysis.Results revealed that as subunits of protein,each amino acid had different anaerobic digestion performance.Among non-polar amino acids such as leucine,isoleucine,valine,alanine,and glycine had a higher methane yield as compared to methionine,phenylalanine,tryptophan,and proline.Among polar amino acids such as serine,threonine,glutamine,and asparagine had higher methane yield in the range of 229.2-347.9 mL/g-VS.The basic amino acids(lysine,arginine,and histidine)had higher methane yield(281.3-402.5 mL/g-VS)as compared to acidic amino acids such as aspartic and glutamic acids,which had a methane yield of 229.2 and 310.7 mL/g-VS,respectively.Amino acids such as methionine,tryptophan,cysteine and proline have given methane yields less than 50 mL/g-VS.The microbial community analysis showed that Fastidiosipila,and VadinBC27 among bacterial genera and Methanosaeta and Methanoculleus among archaeal genera had a higher relative abundance which might have contributed to the amino acid conversion to biomethane.Second,the methane production characteristics of twelve kinds of substrates with varying protien content were investigated under mesophilic(37?)and thermophilic(55?)conditions.Among the selected substrates,fish waste,beef meat,pig blood,intestinal waste,chicken waste,casein,gelatin,and egg were protein-rich having protein as the main composition,whereas,cheese,soybean curd,quinoa and spoiled milk had a relatively moderate protein content.Results revealed that the cumulative methane yield(CMY)of protein-rich substrates ranged from 131.2-381.8 mL/g-VS during mesophilic anaerobic digestion(MPAD),which increased to 212.8-550.4 mL/g-VS during thermophilic anaerobic digestion(TPAD).The CMY of substrates with relatively moderate protein content ranged from 238.2-388.9 mL/g-VS during MPAD,which increased to 384.6-557 mL/g-VS during TP AD.The modified Gompertz model showed a high maximum methane production rate of all substrates during TP AD as compared to MPAD,indicating that methane production process enhanced at high temperatures.The microbial community analysis revealed that Advenella and Saccharofermentans were predominant bacterial genera during MPAD,whereas Clostridium sensu stricto,Defluvitigo,and Coprothermobacter were predominant bacterial genera during TPAD.Among archaeal genera Methanothrix was predominant during MPAD,whereas Methanosarcina was the most predominant methanogen during TPAD.A correlation between the biochemical components such as the amino acids composition,lipid and carbohydrate content,and the methane yields of different protein substrates with high to moderate protein content was also established,which might be helpful in predicting biogas performance of a wide range of protein wastes.Third,biochar derived from agricultural residues was used as a potential economical additive for enhancing the methane yield of six different kinds of food substrates with high to moderate protein content,which include nut residues,casein,gelatin,cheese,lentils,and quinoa.In order to better explore the possible role of biochar on the enhancement of methane production performance,two types of biochars namely mushroom stalk biochar(MSB)and Robinia pseudoacacia biochar(RPB)pyrolyzed at different temperatures of 500? and 800? were introduced into the AD system.Results showed that the improvement yield with both biochars for nut residues,casein,gelatin,cheese,lentils,and quinoa was in the range of 11.19-19.46%,2.3-28.2%,9.8-26.8%,6.1-18.9%,5.4-17.3%,and 3.17-22.8%respectively,as compared to the control group.The highest EMY of 515.3 mL/g-VS was obtained for nut residues by adding MSB500?,followed by casein(468.1 mL/g-VS),gelatin(401.4 mL/g-VS),quinoa(388.9 mL/g-VS),lentils(325.0 mL/g-VS),and cheese(309.2 mL/g-VS).Overall,among all the biochars used the highest EMY was shown by MSB500?,followed by RPB500?,MSB800? and the lowest was attained by RPB800?.These results imply that the biochars prepared at low pyrolysis temperature have enhanced the biomethane productivity of these substrates.Fourth,the economic feasibility of AD plants utilizing food waste in China was carried out in the light of the economic incentives defined by 13th five-year plan.It was found that the government has allocated higher investments and social in order to develop the AD industry at a faster pace.The results of the cost-benefit analysis showed that AD projects which utilize municipal solid waste have a good market potential,bringing an attractive internal rate of return(IRR)of 31%and a net profit margin of 12%.The industry can also attract foreign entrepreneurs,while making good profits.This study gave deep insights into the degradation characteristics of different kinds of protein-rich wastes and methane production performnace of the 20 basic units of protein,each of which has a different characteristics and participation,which ultimately influences the final AD performance of protein.In addition,it was found that anaerobic digestion carried out at thermophilic tempertature and the use of biochars as additives resulted in the enhanced methane production performance of these substrates.The experimental results provided by this study are a valuable reference for the efficient application of protien-rich wastes for bioenergy production.The economic feasibility serves as a direction for the AD systems implementation in China as well as other developing countries.