Factor Analysis And Process Optimization Of Various Substrates In Anaerobic Co-digestion

Developing knowledge in anaerobic co-digestion of various substrates is an importantway to transform agricultural residues into resources and to reduce environmental pollution.Anaerobic co-digestion, for one thing, can replace the digestion of single substrate with thedrawbacks of low efficiency, long retention time and floating, and for another, improve theproduction efficiency and the economic benefit of organic waste treatment process during theindustrialization of methane production. Therefore, based on the industrial value of thistechnology and as the key scientific problem, conducting the research in anaerobicco-digestion is of great significance. This research analyzed the influences of feedingcomposition, C/N, inital substrate loadingand inoculum to substrate ratio （ISR） on theefficiency of anaerobic co-digestion, and based on these factors, optimized this process. Themain results were as followed:（1） The methane yield in anaerobic co-digestion was increased compared with thedigestion of single substrate. Anaerobic co-digestion of three raw substrates were better thanthat of two raw substrates. With the changes of feeding composition, the biogas yield andmethane potential increased first and then decreased. The feeding composition was differentfor different mixtures in the highest biogas yield and methane potential, with DM （Dairymanure）/CM （Chicken manure）50:50in the mixture of DM, CM and wheat straw （WS）,DM/SM （Swine manure）75:25in the mixture of DM, SM and WS, and CM/SM75:25in themixture of CM, SM and WS. The high efficiency in suitable feeding ratios lied in theappropriate pH value and total ammonia and free NH3contents. There were synergetic effectsamong various substrates, with the best effects in the mixture of CM, SM and WS, followedby the mixture of DM, SM and RS, and the least in the mixture of DM, CM and RS,suggesting that SM was the best substrate for anaerobic co-digestion.（2） C/N had significant effects on the effects of anaerobic co-digestion. For differentmixtures, with the increase of C/N, the biogas yield and methane potential increased first andthen decreased. For the mixture of DM, CM and RS, the biogas yield and methane potentialwere589.6mL g^-1VS and262.5mL g^-1VS, respectively; for the mixture of DM, SM and WS,the biogas yield and methane potential were614.0mL g^-1VS and286.5mL g^-1VS,respectively; for the mixture of CM, SM and RS, the biogas yield and methane potential were 543.5mL g^-1VS and259.7mL g^-1VS, respectively. Lower C/N led to the accumulation ofammonia and then reduced the biogas yield, whereas, higher C/N led the decrease of pH value,inhibiting the activities of methanobacteria and reducing the efficiency.（3） The results in mixture design （MD） suggested that the methane potential of three rawsubstrates was higher than those of single or two substrates. The synergetic effects amongsubstrates improved the methane potential and the contribution of manure on the improvedmethane potential was higher than straw, but no significant difference was found amongdifferent manures or straws. The results in central composite design （CCD） suggested that thefeeding composition, C/N and their interaction affecting the methane potential, indicating thatthe synergetic effects were not only due to the balanced C/N, but also the synergy ofmicroorganisms and nutritional balance. Optimum feeding compositions were obtainedthrough MD and CCD, but no significant difference was found between these two methods.However, CCD was more accurate and had wider range of applicability.（4） The initial substrate loading had significant effects on biogas production. When thesubstrate concentration was below20gVS L^-1, the biogas yield and methane potential linearlyincreased with the increase of substrate concentration, however, higher substrateconcentration easily led to the acidification of liquid and reduce fermentation efficiency.Under the same substrate concentration, the mixture of DM, SM and RS had the highestfermentation efficiency, followed by the mixture of DM, CM and RS, and the mixture of CM,SM and RS is the worst. ISR had a significant impact on biogas production. When the ISRwas less than2.4, with the increase of ISR, the cumulative biogas production and methanepotential linearly increased. The lower the ISR, the more easily the acid accumulated, withhigher inhibition on methanobacteria. When the ISR was higher than2.4, increased ISR wasunable to further improve the efficiency. Under the same ISR, the biogas yield and methanepotential were the same with the treatment under the same substrate concentration.（5） Temperature significantly influenced the biogas production. With increasedtemperature, pH in liquid, total ammonium and free NH3contents all increased significantly.Under20°C, accumulated liquid acids reduced the efficiency. Under mesophilic andthermophilic temperatures, methanobacteria had higher tolerance on free NH3and the higherfree NH3contents did not inhibit the fermentation process. Under different temperature, theeffects of C/N on anaerobic co-digestion were different. Under35°C, ammonia inhibitionoccurred under C/N15:1and20:1, whereas, under55°C, the same inhibition also occurredunder C/N25:1. Under35°C, the optimum C/N was25:1, with the methane potential of272mL g^-1VS, and under55°C, the optimum C/N was30:1, with the methane potential of286mLg^-1VS. Whether the ammonia inhibition occurred or not depended both on the C/N and temperature.（6） Response surface methodology can be used for optimizing the process of anaerobicco-digestion of DM, DM and RS. C/N, DM/CM, initial substrate loading and ISR all hadsignificant effects on methane potential and interctions between C/N and DM/CM, C/N andinitial substrate loading were significant on methane potential, initial substrate loading andISR. With the increase of C/N and DM/CM, the methane potential increased first and thendecreased, but the effects of C/N were higher than DM/CM. With the increase of C/N andsubstrate concentrations, the methane potential increased first and then decreased, but theeffects of C/N were higher than substrate concentrations. With the increase of substrateconcentrations and ISR, the methane potential increased first and then decreased, but theeffects of ISR were higher than substrate concentrations. When the initial substrate loadingand ISR were low, methane potential was also low. The highest methane of394mL g^-1VSwas predicted under optimum conditions with C/N of26.31, DM/CM of42.96:57.04, aninitial substrate loading of15.90g VS L^-1and ISR of2.34.