Fuqing Xu,Zhiwu Wang,Yebo Li 2016-06-29 04:30:33
More than 300 million dry tons of waste biomass, including animal manure, food processing waste, and crop residues, are generated every year from agricultural and food processing systems in the U.S. In addition, about 250 million tons of municipal solid waste are discarded each year, about 68% of which is biodegradable. Disposal of this huge volume of waste not only increases the risk of air, water, and soil pollution but also wastes the energy potential in these waste streams. Anaerobic digestion (AD) is a biological process that converts organic matter into methane-rich biogas as a renewable energy source. In the AD process, organic materials are first hydrolyzed into smaller molecules, including sugars, amino acids, and fatty acids, which are then converted by anaerobic microbes through several fermentation pathways into acetic acid, H2, and CO2. These compounds serve as substrates for methanogenic microbes, which produce methane. The final gaseous product, called biogas, contains about 40% to 70% methane, 30% to 60% CO2, and trace amounts of other gases. Biogas can be used directly for cooking and heating as well as for electricity generation, and it can be purified and upgraded to produce natural gas and transportation fuels. AD also removes a large portion of the carbon from organic waste, and the remaining nutrient-rich residue can be used as a fertilizer or soil amendment. Overall, AD is one of the most cost-effective and mature technologies for treating organic wastes and producing bioenergy. Liquid versus solid Traditional AD focuses on the treatment of high-strength liquid waste, such as sewage sludge, animal manure, and food processing wastewater. These AD systems usually operate at low total solids contents (<15%) and are referred to as liquid AD (L-AD) systems. Recently, more researchers have started to explore the potential of solid organic wastes and biomass, such as the organic fraction of municipal solid waste (OFMSW) and crop residues. These materials have high total solids contents, so feeding them into an L-AD process would require a large amount of dilution water. Fibrous materials, such as crop residues, are also not suitable for L-AD because the fibers float to the top of the digester and cannot be effectively degraded by anaerobic microbes. Instead, solid-state anaerobic digestion (SS-AD), which operates at a total solids content greater than 20%, is more suitable for solid organic wastes. The contents of an SS-AD system appear similar to compost and have minimum visible water. Because solid-state digesters contain much less water than liquid digesters, the total volume is 3 to 5 times smaller, and thus the energy required for heating the digester is significantly lower. Although SS-AD holds promise for energy production from a wide variety of solid feedstocks, SS-AD processes are less understood than L-AD, which has been widely studied and used throughout the world. Recently, research on SS-AD has progressed regarding inocula, operating parameters, inhibitors, and mass transfer. However, some SS-AD mechanisms, such as the mass transfer of substrates and inhibitors, are still in the hypothesis stage and are difficult to measure with current instruments. Therefore, modeling is a useful method for verifying hypotheses about SSAD mechanisms, and it provides an important tool for process prediction and optimization. Modeling the process Different SS-AD models have been developed in the past decade using theoretical, empirical, and statistical approaches. Theoretical models can provide more insight into the complex system mechanisms but require a series of inputs, including initial conditions, boundary conditions, and kinetic constants, and thus are usually too complicated for general applications. Statistical models are easier to derive but are sometimes considered to be “black boxes” that are hard to interpret. Empirical models are derived from empirical equations, and their complexity is usually between that of theoretical models and statistical models. The effect of total solids content on methane yield has become a hot topic in recent SS-AD models. Most of the theoretical SS-AD models published before 2005 focused mainly on the effect of the heterogeneous initial distribution of inoculum in the substrate and how methanogenic microbes spread throughout the digester, while more recent SS-AD models have started to focus on total solids content. It has been commonly observed that methane yield and methane production in SS-AD decrease with an increase in total solids content. Our 2014 study (see the “Further Reading” sidebar) found that the maximum methane production rate changed with total solids content, following a bell-shaped curve. Most recent SS-AD models have tried to interpret this phenomenon by assuming that one or two key kinetic rate constants in SS-AD, such as the hydrolysis rate constant, maximum microbial growth rate, half-saturation coefficient, or diffusion coefficient, are influenced by the total solids content. According to the model results, total solids should affect microbial growth kinetics, mass transfer, and the concentration of inhibitors. Although the phenomena predicted by many of these models have not been observed in experiments, these models have deepened the understanding of SS-AD and provide guidance to researchers for future studies. To date, diverse models have been proposed for SS-AD based on the different understandings of researchers about this process. Further investigation of SS-AD mechanisms is essential for rational construction of SS-AD models and to provide guidance for the improvement and commercialization of SS-AD systems. Further Reading Ge, X., Xu, F., & Li, Y. (2016). Solid-state anaerobic digestion of lignocellulosic biomass: Recent progress and perspectives. Bioresource Technology, 205, 239-249. http://dx.doi.org/10.1016/j.biortech.2016.01.050 Li, Y., Park, S. Y., & Zhu, J. (2011). Solid-state anaerobic digestion for methane production from organic waste. Renewable and Sustainable Energy Reviews, 15(1), 821- 826. http://dx.doi.org/10.1016/j.rser.2010.07.042 USEPA. (2015). Advancing sustainable materials management: 2013 fact sheet. Washington, D.C.: U.S. Environmental Protection Agency. Xu, F., Wang, Z.-W., Tang, L., & Li, Y. (2014). A mass diffusionbased interpretation of the effect of total solids content on solid-state anaerobic digestion of cellulosic biomass. Bioresource Technology, 167, 178-185. http://dx.doi.org/10.1016/j.biortech.2014.05.114 Xu, F., Wang, Z.-W., & Li, Y. (2015). Mathematical modeling of solid-state anaerobic digestion. Progress in Energy and Combustion Science, 51, 49-66. http://dx.doi.org/10.1016/j.pecs.2015.09.001 ASABE member Fuqing Xu, Research Associate, Department of Food, Agricultural, and Biological Engineering, The Ohio State University, Wooster, USA, firstname.lastname@example.org ASABE member Zhiwu (Drew) Wang, Assistant Professor, Department of Civil and Environmental Engineering, Virginia Tech, Manassas, USA, email@example.com. ASABE member Yebo Li, Professor, Department of Food, Agricultural, and Biological Engineering, The Ohio State University, Wooster, USA, firstname.lastname@example.org.
Published by ASABE. View All Articles.