Bioenergy Edited by J. Wall et al. © 2008 ASM Press, Washington, DC Chapter 16 Biomethane from Biomass, Biowaste, and Biofuels ANN C. WILKIE Biofuel production from agricultural, municipal, and in- (Wilkie et al., 2004). However, methane production is dustrial wastes is efficiently accomplished through con- not limited to conversion of animal manures. Biogas version to biogas, a mixture of mostly methane (CH4) can be made from most biomass and waste materials and carbon dioxide (CO2), via anaerobic digestion. regardless of the composition and over a large range of Anaerobic digestion is a process by which a complex moisture contents, with limited feedstock preparation. mixture of symbiotic microorganisms transforms or- The feedstocks for this omnivorous process can be ganic materials under oxygen-free conditions into bio- composed of carbohydrates, lignocellulosics, proteins, gas, nutrients, and additional cell matter, leaving salts fats, or mixtures of these components. The process is and refractory organic matter. In practice, microbial suitable for conversion of liquid, slurry, and solid anaerobic conversion to methane is a process for both wastes; it can even be employed for the conversion of effective waste treatment and sustainable energy pro- gaseous combustion products (synthesis gas) from ther- duction. In waste treatment, this process can provide a mochemical gasification systems. In addition, methane source of energy while reducing the pollution and odor production can be effectively applied to improve energy potential of the substrate. Unlike fossil fuels, use of re- yields from other biofuel production processes includ- newable methane represents a closed carbon cycle and ing bioethanol, biodiesel, and biohydrogen production. thus does not contribute to increases in the atmos- Implementation of digestion technology at agricultural, pheric concentration of carbon dioxide (Wilkie, 2005). municipal, and industrial facilities allows efficient de- Microbial methane production has the potential centralized energy generation and distribution to local for reducing the demand for fossil fuels like coal, oil, markets. While traditionally applied to wastes and and natural gas that have provided the power for de- wastewaters, the anaerobic digestion of energy crops veloping and maintaining the technologically advanced can also be employed in a sustainable bioenergy system. modern world. However, fossil resources are finite, and their continued recovery and use significantly impact our environment and affect the global climate. Short- ANAEROBIC MICROBIOLOGY ages of oil and gas are predicted to occur within our lifetimes or those of our children. To prepare for a Methane is the end product of anaerobic metabo- transition to more sustainable sources of energy, viable lism—a metabolic sequence carried out by communities alternatives for conservation, supplementation, and re- of hydrolytic bacteria and fungi, acid-producing inter- placement must be explored, posthaste. mediary organisms, and finally, methanogenic Archae- Biogas production from agricultural, municipal, bacteria. Methane-producing communities are very sta- and industrial wastes can contribute to sustainable en- ble and resilient, but they are also complex and largely ergy production, especially when nutrients conserved in undefined. the process are returned to agricultural production Buswell and Sollo (1948) demonstrated the treat- (Fig. 1). Little energy is consumed in the process, and ability of a range of wastes and emphasized the concept consequently the net energy from biogas production is of an acid phase versus a methane phase, showing the high compared to other conversion technologies. The importance of volatile organic acids as intermediates in technology for methane production is scalable and has the process. They also demonstrated the applicability been applied globally to a broad range of organic of a stoichiometric equation that balanced carbon, hy- waste feedstocks, most commonly animal manures drogen, and oxygen (equation 1) to predict the amount Ann C. Wilkie • Soil and Water Science Department, University of Florida—Institute of Food and Agricultural Sciences, Gainesville, FL 32611. 195 196 WILKIE Figure 1. Biogas cycle of methane and carbon dioxide evolved from conver- teria ferment the larger acids to a combination of acetic sion of organic compounds with a known empirical acid, one-carbon compounds, hydrogen, and carbon formula. Later, 14C tracers were used to show that ac- dioxide. The homoacetogenic bacteria synthesize acetic etate was indeed cleaved to form methane and carbon acid by utilizing hydrogen/carbon dioxide or one- dioxide, suggesting that acids were important interme- carbon compounds or by hydrolyzing multicarbon com- diates in the conversion process. pounds. The methanogenic Archaebacteria uniquely catab- ϩ Ϫ Ϫ → CnHaOb (n a/4 b/2)H2O olize acetic acid and one-carbon compounds to meth- Ϫ ϩ ϩ ϩ Ϫ (n/2 a/8 b/4)CO2 (n/2 a/8 b/4)CH4 (1) ane. The methanogens are obligate anaerobes that can pick up electrons from dead-end fermentations, through Of great importance to the understanding of an- interspecies hydrogen transfer, and shuttle these elec- aerobic microbiology was the discovery of Bryant trons through a unique form of respiration which re- et al. (1967), through isolating the elusive “S-organism” sults in the reduction of carbon dioxide to methane. from Methanobacterium omelianskii, that the conver- The organisms that use hydrogen to reduce CO2 are sion of ethanol to methane was accomplished with commonly regarded as the earliest life forms due to a mixed culture. The discovery of other cocultures their chemoautotrophic abilities. quickly followed, and the number of species isolated in All morphological forms are represented among pure culture increased. With the identification of the methanogens including rods, cocci, spirals, sarci- closely coupled syntrophic cocultures of methanogens nae, and filamentous organisms. Surprisingly, this di- and other species, the earlier hypothesis of an acid phase verse group of organisms is known to metabolize a very followed by a methanogenic phase developed into a limited number of substrates including acetate, for- more descriptive scheme that embraces the importance mate, methanol, acetone, methylamines, carbon monox- of hydrogen as an intermediate in the process. ide, and H2/CO2. The substrates for methanogenesis di- First the fermentative, or hydrolytic, bacteria and vide these organisms into groups, two of which are fungi hydrolyze complex organic polymers and ferment notably important in active digesters: the aceticlastic me- them to organic acids, hydrogen (or formate), and car- thanogens, which cleave acetic acid, and the hydrogen- bon dioxide. The hydrogen-producing acetogenic bac- utilizing methanogens, which utilize hydrogen and one- CHAPTER 16 • BIOMETHANE FROM BIOMASS, BIOWASTE, AND BIOFUELS 197 carbon compounds. However, this distinction is not al- Table 1. Theoretical methane yield of biomass components ways useful since some species may metabolize both Component Methane yield (liter/g of VS) substrates. Carbohydrates . 0.35 For aceticlastic methanogens, low levels of acetate Proteins (leucine) . 0.57 (Ͻ50 mg/liter) favor the growth of more-filamentous Fats (lauric acid) . 0.95 organisms (e.g., Methanosaeta) that must rely on a larger surface-to-volume ratio in order to improve sub- strate diffusion rates. High levels of acetate favor the The natural assemblages in the mixed culture have predominance of clusters of aceticlastic methanogens evolved to form robust and stable cultures with ex- (e.g., Methanosarcina), which have lower surface-to- tremely broad substrate utilization capabilities. There is volume ratios that serve to protect them from the in- no requirement for genetically modified organisms to hibitory nature of high organic acid concentrations. extend catabolic activity, so sterilization of process re- Differences in maximum growth rate and substrate uti- siduals is not necessary. Although the free energy avail- lization affinities can be exploited to select for predom- able from anaerobic conversion of substrates to meth- inant methanogens. Organisms such as Methanosarcina ane is low, causing low microbial-growth rates, the should be favored for selection if high conversion rates activity and turnover rates of substrates are higher than of high-strength wastes are the primary goal, whereas for aerobic metabolism. Also, anaerobic respiration of Methanosaeta should be favored if low effluent bio- methanogens results in the production of a nonin- chemical oxygen demand is more important. In addi- hibitory product, methane, that moves into the gaseous tion, these attributes can be exploited together by stag- phase, which contrasts with other fermentation proc- ing an anaerobic process with the first stage favoring esses that produce inhibitory final products (e.g., etha- high conversion rates and the next stage favoring efflu- nol) that remain in solution. This gives the process a ent quality. distinct advantage for either continuous or batch con- version of substrates to energy products. Of significance for the application of anaerobic di- PROCESS gestion is the high level of energy recovery in the biogas compared to the energy content of the substrate uti- In practice, anaerobic digestion is the engineered lized. The efficiency of this conversion is directly re- methanogenic decomposition of organic matter, carried lated to the low level of free energy of reaction avail- out