Performance Of Methane Production And Synergy Mechanism At High Solid Content Anaerobic Co-digestion Using Typical Agricultural Wastes

The modernization and intensification of agriculture considerably increases the production of agricultural wastes,such as livestock manure and crop straws,which challenges the development of sustainable agriculture in China.Anaerobic digestion is an effective technology for the treatment and reuse of these agricultural wastes in the form of biogas（a renewable energy）and digestate（organic fertilizer）.However,mono-substrate in liquid anaerobic digestion,which is commonly deployed in biogas plants in China is inefficient and instable.In this study,three typical agricultural wastes,including pig manure,corn stover and cucumber residues were co-digested.The effects of the mixing ratio of the ternary feedstocks on methane production were investigated.Moreover,biogas production of these three feedstocks at the optimum proportion were investigated under different operating conditions.Based on the results obtained,three mixing modes were performed at intermittent,low mixing intensity,and the effects of the partition formed in the reactors on stability and characteristics of microbial distribution during digestion were analyzed.Finally,the efficient and stable continuous high solid anaerobic digestion process for ternary feedstocks was achieved under different mixing modes.Results from this study could provide theoretical basis for treating various agricultural wastes around by the biogas plants.The results showed that:（1）The optimal ratio of co-digestion of pig manure,com stover and cucumber residue was 5:2:3（wet base）.Compared with mono-substrate,the co-digestion of ternary feedstocks significantly improved the methane production.When the pig manure ratio was at 40%and 30%,the proportion of cucumber residues showed negatively linear correlation with cumulative methane production.On the contrary,when the pig manure was at 50%,the cucumber residues proportion showed positively linear correlation with methane production.The proportion of cucumber residues had obvious impact on cumulative methane yields by changing composition of bacteria and archaea,especially Methanosaeta.In the environmental factor,the composition of bacteria was mostly influenced by the volatile fatty acids（VFAs）content and the composition of archaea and methane production was determined by cellulose content.This indicated the property of mixture lead to the different initial condition for anaerobic digestion and then the microbial community was influenced.Thus,the physicochemical property in the anaerobic digestion process of different mixture showed difference with each other.The microbial community interacted with physicochemical property in digestion process and finally the finally the difference in cumulative methane yields appeared.（2）In the anaerobic digestion process with the optimal mixture,the methane production decreased with the total solid（TS）increased and showed negatively linear correlation with each other（except the TS at 10%under feedstock to inoculum ratio（F/I）at 2）.Simultaneously,the Rmax and Kh value decreased with TS increased,and the value of Rmax and Kh were higher at F/I=1 than F/I=2 under the same TS indicating lower F/I could help speed up the process of digestion.The increased F/I showed more impact on TS of 10%by decreasing the pH,Rmax and Kh of the digestion process and then the methane production was influenced.The changes of physicochemical property in each reactor under different TS and F/I were basically consistent with methane production.Hence,the higher F/I was suggested used in anaerobic digestion process which TS was higher than 10%.（3）The corn stover pretreated by H2O2 and Ca（OH）2 solution used in co-digestion of ternary feedstocks at optimal ratio could obviously shorten the lag phase（λ）and improve the Kh compared with treatment without pretreatment.Moreover,the pretreatment could change the conditions in the reactors and reduce the propionic acid accumulation,indicating that pretreatment could positively balance the condition in reactor.The highest cumulative methane production was treatment with H2O2 solution pretreatment,and then treatment without pretreatment,treatment with Ca（OH）2 solution pretreatment followed.Hence,the pretreatment was suggested to use in anaerobic digestion for shortening the time needed for beginning producing biogas and start-up.（4）At intermittent low mixing intensity,the reactors with mixing only at top,middle and bottom could achieve stability with similar cumulative methane yields.The anaerobic digestion process was changed by mixing modes for the reactor with mixing at top could speed up the reaction with shortest time to produce biogas.On the contrary,the reactor with no mixing failed with low methane production.The main bacteria composition of reactor with mixing at top was Clostridium_sensu_stricto₁,and the bacteria in reactor mixing with middle and bottom was Caldicoprobacter.This was consistent with later biogas production compared with reactor with mixing at top.The main archaea in reactor with mixing at top was acetotrophic methanogen.The main archaea in reactor with mixing at middle and bottom were hydrogenophilic methanogens at upper area acetotrophic methanogen in lower area which was consistent with lower biogas production compared with reactor with mixing at top.Comprehensively,mixing only at top could change the digestion condition mostly,and produced influence on archaea,especially Methanosaeta.Combining the physicochemical property and microbial community,all the reactors mainly formed "acidification zone" at upper part and "methanation zone" at the lower part of reactor.（5）The start-up time of reactor with mixing only at top were shorter than reactor with mixing only at bottom.However,the reactor with mixing only at middle failed at day 30 with stopping to produce biogas.By analyzing the microbial community in the digestate,the bacteria community in reactor with mixing at top and bottom showed consistency with batch experiments.In specific,the main bacteria composition was Ruminiclostridium in reactor with mixing at top and Proteiniphilum in reactor with mixing at bottom,which was consistent with longer start-up time needed for reactor with mixing at bottom.The composition of bacteria in reactor with mixing at middle had big different with batch experiment with low bacteria total relative abundance.The main archaea was consistent with batch experiment.The reactor with mixing only at top was suitable for growth of Methanoculleus,and reactor with mixing only at middle had no promotion for Methanoculleus.The diversity of microbial community was lower than batch experiment,suggesting that the partition was more clearly in the continuous reactor.Overall,reactor with mixing at top and bottom maintained better stability,and reactor with mixing at top could shorten the start-up time which was more suitable for high solid state anaerobic co-digestion.