Metagenomics Based Study On Anaerobic Digestion Process Of Municipal Organic Waste

With the rapid development of the urbanization in China,the municipal organic waste produced by human settlements has posed a serious threat to the ecological environment.At present,the annual output of municipal organic waste around world reached 1.3 billion tons(1.2 kg/day per capita).There is an urgent need for proper treatment and disposal of municipal organic waste.Anaerobic digestion technology can convert the organic matter in the waste to methane through the decomposition of functional microorganisms.Anaerobic digestion technology meets the concept of sustainable development in China due to the low cost and high benefits,which is very promising.However,the current research also shows that:(1)there are some technical difficulties in the anaerobic digestion process of municipal organic waste,such as poor system stability or low methane production efficiency,(2)the microecological system of the key functional flora in the anaerobic digester still unclear,and(3)the effects of emerging antibiotic pollution from municipal organic waste on anaerobic digestion process remains unknown.Taking the representative municipal organic waste,including toilet waste,dewatered sewage sludge and food waste,as examples,this study combined with field investigation and laboratory simulation,based on the traditional anaerobic digestion technology.The amplicon sequencing,metagenomic methods and bioinformatics analysis were applied to study the following three aspects:(1)The effects of anaerobic digestion process parameters on the operation stability and methane production were investigated,including organic load rate,material ratio,hydraulic retention time,temperature,ammonia nitrogen and other factors.It was found that the enrichment of hydrogenotrophic methanogens relieved the ammonia inhibition.The addition of nano-zero-valent iron accelerated the hydrolysisacidification process of substrates.The co-digestion of sludge and food waste improved the nutrient balance of feeding materials.The rapid start-up strategy shortened the establishment time of thermophilic anaerobic digesters.In this paper,the removal rate of organic matter and the yield and purity of methane were significantly improved by optimizing the multiple process parameters and operating conditions.(2)Based on high-throughput sequencing technique,the relationship between different anaerobic digestion conditions and the dynamic changes of functional microbial communities was revealed.The three-levels response mechanism among environmental factors,microbial community structure,and reactor operation performance was established.It was found that the relative abundance and diversity of dominant flora(such as Firmicutes,Actinobacteria,Euryarchaeota,Proteobacteria),the relative abundance of functional genes,and the metabolic pathways of microorganisms(such as hydrogenotrophic and acetoclastic methanogenesis pathway)could be used to indicate the operation state.A wide range of low abundance microorganisms(such as Hyphommicium,Gordonia,Methanospirillum)are likely to be the key drivers of anaerobic digestion process.Hydraulic retention time and organic load rate exert different impacts on the microbial co-occurrence network.The network module with high connectivity plays an important role in the identification of "core microbiome" and the design of artificial microbial co-cultures in anaerobic system.(3)Taking tetracycline and sulfonamides as examples,the effects of long-term antibiotic selective pressure on anaerobic digestion process were studied by the metagenomic method.It was found that the presence of antibiotics could have a strong effect on the archaeal community structure,hydrogenotrophic and acetoclastic methanogenesis pathway and carbohydrate/fatty acid metabolism,which led to the accumulation of volatile fatty acids and the decrease of methane production in the system.There was a significant positive correlation between the concentration of specific antibiotics and the abundance of corresponding resistance genes and potential host microorganisms.The antibiotic selective pressure increased the diversity and abundance of antibiotic resistance gene subtypes and enhanced the chemotaxis mechanism of bacteria.It also promoted the gene exchange between drug-resistant bacteria and indigenous environmental microorganisms.Mesophilic anaerobic digestion technology has a limited effect on the removal of antibiotic resistance genes.High temperature conditions are beneficial to reduce the abundance of antibiotic resistance genes by removing the potiential hosts and class 1 integron-integrase gene.In this paper,study on metagenomics based on anaerobic digestion of municipal organic waste revealed the microbial ecology of anaerobic functional groups from three levels: operation performance,keystone taxa,and functional gene.The results will put forward new ideas for the development of new anaerobic digestion enhancement process,understanding the anaerobic biological metabolism pathway of organic matter,exploring the key flora in complex system,controlling the risk of emerging antibiotic resistance transmission.