Characteristics And Optimization Of Anaerobic Digestion Of Complex Organic Substrates For Efficient Biogas Production

China is one of the largest populous and agricultural countries in the world with0.7billion tons of stalks,3.97billion tons of animal manure,150million tons of municipal solid wastes, and43.7billion tons of organic wastewater being generated annually. Therefore, China is especially suitable for development of biomass energy industry. Anaerobic digestion (AD) is an effective method that can treat a wide range of organic substrates to reduce the organic pollutants and produce renewable energy source in the form of biogas (a mixture mainly of methane and carbon dioxide). Before utilization in large scale AD systems, organic substrates should be characterized to determine their biogas production potential by using a biochemical methane potential (BMP) assay that would help in determining the economics of these systems. However, until now, there is no standard protocol on defining the BMP assay. Consequently, biogas production potential of substrates assessed by different researchers is usually not comparable. Therefore, there is an urgent need to measure and compare the differences among various organic substrates by using a common and simple method. Besides, corn stover (CS) and chicken manure (CM) are two typical organic solid wastes and they are widespread through China. But the relative low methane yields limit their application in anaerobic digestion industry. Thus, we proposed a simple BMP measurement method, and develop two processes to treat crop straws and aminal manures. We also discussed the machnism of enhancing biogas production, which could provide useful information for improving the performance of co-digestion of crop straws and aminal manures.Firstly, we investigate the effect of inoculum sources and pre-incubation of inculum on methane production potential of different organic substrates. Results showed that inoculum source could significantly affect the bio-methane potential (BMP) and biodegradability of organic substrates (CM and CS). The inoculum from an anaerobic digester treating municipal wastewater sludge (DSMW) was better than the inoculum from an anaerobic digester treating chicken manure (DSCM) in digesting CM or CS. A minimal of21days’ pre-incubation was needed to minimize the biogas production from inoculum. For a certain inoculum, pre-incubation did not affect the BMP and biodegradability of substrates as compared to non-incubation. Yet, pre-incubation could improve the precision of calculating net methane yields of organic substrates and kinetic parameters in anaerobic digestion process.Secondly, the methane production potential, biodegradability, and kinetics of a wide range of organic substrates were evaluated by using a simple BMP measurement method, which could provide useful reference for defining the standard BMP measurement protocol. Results showed that wastes that contained high energy density (such as lipids) and easily degradable substrates (such as fruit and vegetable waste) exhibited higher BMP and biodegradability; while lignocellulosic biomass with high content of fiber presented lower BMP and biodegradability. The first-order rate constant of the tested samples ranged from0.053to0.242d-1. A negative linear correlation between lignin content and BMP (or biodegradability) was found for lignocellulosic and manure wastes. The findings also clarified that both of elemental and organic composition analysis methods could be used to calculate the theoretical methane yield (TMY) of substrates. This study validated that lignin fraction could be used to predict BMP and biodegradability of lignocellulosic and manure wastes.Methane production at different CS to CM ratios and process stability under wet (W-AD), hemi-solid state (HSS-AD) and solid state (SS-AD) conditions were evaluated under mesophilic conditions. BMP tests showed that synergistic effects were found when mixing two substrates at CS:CM ratios of3:1and1:1(on volatile solid basis). The highest methane yield of218.8mL gvs-1was achieved in W-AD at CS:CM ratio of3:1. In SS-AD, the highest volumetric methane productivity of14.2Lmethane Lreactor volume-1was found at CS:CM of1:1. VFA/TA value of0.4was regarded more important than ammonia-nitrogen and free ammonia concentrations in evaluating of the process stability during AD. Mesophilic SS-AD of CS and CM could be an attractive way to produce biogas.Solid-state NaOH pretreatment with dose of5%at room temperature for1day could partly dissolve lignin and hemicelluloses, and significantly increase the internal surface area of CS, which could make it more readily biodegradable. At a substrate to inoculum (S/I) ratio of3, mesophilic SS-AD of CS failed because of the accumulation of organic acids. However, under thermophilic SS-AD conditions, biogas and methane yields of pretreated CS were found to be386.3and194.8mL gvs-1, respectively, which were29.4%and40.1%higher than those of untreated CS. Thermophilic solid-state co-digestion of pretreated CS with CM showed no beneficial effects for enhancing the biogas and methane yields, due to the accumulation of volatile fatty acids and ammonia. Methane production could well be explained by the Cone and modified Gompertz models compared to first order model, since higher R2were obtained from these two models. Based on the results of Cone model, the first-order rate constant (k) of thermophilic SS-AD of CS and CM decreased from0.132d-1to0.039d-1with the increasing portion of CM. Through the modified Gompertz model, higher lag phase time (λ) and lower maximal methane production rate (μm) were found with the content of CM in mixtures increasing from0%to100%under thermophilic solid state co-digestion conditions. Thermophilic SS-AD of pretreated CS could be a promising way to produce biogas in the future.Co-digestion of CS and CM could enhance the richness and diversity of microbial community in mesophilic SS-AD system as compared to mono-digestion of CS or CM. Overall, Firmicutes, Euryarchaeota, and Bacteroidetes were dominant in solid-state co-digestion digester.Continuous reactor was carried out with feeding concentration of12%total solids and C/N ratio of20at organic loading rates (OLRs) of1-4gvs L-1d-1Results showed that at OLR of4gvs L-1d-1, stable and preferable methane yield of223±7mL gvs-1was found, which was equal to energy yield (EY) of8.0±0.3MJ kgvs-1. Post-digestion of digestate gave extra EY of1.5-2.6MJ kgvs-1. Pyrolysis of digestate provided additional EY of6.1MJ kgvs-1. Pyrolysis can be a promising technique to reduce biogas residues and to produce valuable gas products simultaneously.