Anaerobic Digestion Of Amino Acids:Effect Of Structure And Configuration And Promotion Of Refractory Ones

Anaerobic digestion technology which can realize pollutant treatment and resource recovery,is widely used in the biological treatment of organic waste.Protein is an important component of organic waste.After hydrolysis,protein can produce various amino acids which not only have a variety of structures,but also have different configurations(namely L-and D-configuration).Therefore,it is necessary to investigate the influence and mechanism of amino acids strucutrue and configuration on their anaerobic digestion,and emplore the methods to promote the anaerobic digestion of refrectory ones.Firstly,the effect of L-amino acid structure on their anaerobic digestion was studied.The results showed that the methane yield of polar amino acids(162.3-308.1m L/g CODadd)was higher than that of non-polar amino acids(10.7-140.5m L/g CODadd),except glycine.The increased methane production in polar amino acids according to the carbon content was as followed: 4-C<6-C< 3-C<5-C.In the non-polar amino acids,the methane production of long-chain amino acids(carbon number 5～11-C)was lower than that of short-chain amino acids(glycine and alanine,2～3-C).The methane production of long-chain non-polar amino acids was affected by their functional groups,and the decreased methane production was as followed:proline>amino acid with alkyl groups> amino acids with benzene ring,among them,the amino acids with alkyl groups and benzene ring showed relatively poor degradation performance,which were considered as refractory ones in this study.The mechanism study showed that the structure of L-amino acid influenced the cell surface hydrophobicity(CSH)of microorganism,the bacterial membrane permeability,activity of key enzymes of amino acid metabolism(glutamate dehydrogenase,amino acid deaminase,enzymes for VFAs production)and the composition of microbial community in the reactor.All these factors resulted in the different yield and composition of VFAs from amino acid metabolism,and ultimately affected the efficiency of methane production.Then the effect of amino acid configurations on their anaerobic digestion were investigated.The results showed that in the acidification stage,the VFAs production rate of L-amino acid was higher than that of the corresponding D-amino acid.In the degradation of D-amino acids,D-amino acids should be first converted into L-amino acids by racemization,and the higher the racemization capacity,the closer the degradation rate and VFAs production of D-amino acids were to the corresponding Lamino acids.In the methanogenic stages,the configuration of different kinds of amino acids has different effects on methane production.Among them,the configuration of D-amino acids with strong racemization has no significant effect on methane production when D-amino acids showed strong racemization,while the methane production of D-amino acids(D-threonine,valine,leucine and methionine)with weak racemization was significantly lower than that of the corresponding L-configuration(these D-amino acids were called refractory D-amino acdis in this paper).Further methanism investigation through metagenomics and metaproteomics found that in acidification stage,the key bacteria involved in amino acids metabolism included Clostridium,Pepclostridium and Sedimentabacteria,and they contained kinds of proteins(chemotaxis and quorum sensing)participated in the regulation of microbial behavior,and the expression of these protein in L-amino acid was higher than that of corresponding D-amino acid.Bacterial behavior could affect the composition of microbial community which further influenced the expression of metabolic protein,such as elongation factor,substrate transportation,energy metabolism,amino acids metabolism and acidogenic proteins,so that the expression of most of these protein expressions in D-amino acid were down-regulated than that of corresponding L-amino acids,which resulted in the acidification rate of L-amino acids was higher than D-form.Because D-amino acids would transformed into L-forms,the expression of racemase in D-amino acid reactor was higher than the corresponding L-amino acid,which was consistent with the conversion of D-amino acids to L-forms.However,the racemization ability of different D-amino acids was significantly different.In the methanogenic stage,Methanobacteriaceae and Methanosarcinaceae were the main methanogenic archaea with L-amino acid as substrate,which were higher than D-amino acid,resulting in the methane yield of D-amino acid was lower than L-amino acid.According to the above results,the paper finally studied the methods to promote the biotransformation of refractory L-amino acids and D-amino acids.For refractory Lamino acids,the biotransformation and methane yield of L-amino acids were significantly improved by utilizing surfactant to regulating the hydrophobicity and membrane permeability of microorganisms.The CSH of microorganisms in reactors was decreased,and the membrane permeability and activity of glutamate dehydrogenase were increased after adding 50 mg/L rhamnolipid,the VFAs production was increased significantly,and methane production was improved by 40.9～139.8%.For refractory D-amino acids,by adding L-alanine to promote the biotansformation of D-amino acids,the effect of D-amino acid conversion to VFAs was increased by2.1～5.2 times and the yield of methane was increased by 40～176%.In this paper,the information of the effect of amino acid structure and configuration on anaerobic digestion was obtained for the first time,and the mechanism of microbes was revealed,which was helpful to understand anaerobic digestion more deeply and comprehensively.A new method was proposed to promote the methane production from refractory L-and D-amino acids,which provided a new idea for improving the anaerobic digestion effect of proteinaceous wastes.