Anaerobic digestion (AD) is a natural process in which organic materials are degraded by groups of microorganisms under oxygen-free conditions. This phenomenon is commonly observed at landfills where waste is buried under a thin layer of soil and greenhouse gasses such as carbon dioxide (CO₂) and methane (CH₄) are emitted. Methane is a global warming potential (GWP) gas that is estimated to be 20 times more effective in trapping heat in the atmosphere than carbon dioxide. With a solid state anaerobic digestion system, methane from organic solid waste can be captured for energy uses as an alternative option to land fill and composting. The objective of this fact sheet is to provide a review of the design and operation parameters of solid state anaerobic digestion systems.
Europe, specifically Germany, is leading the development and applications of anaerobic digestion technology. In recent years, due to increased interest in renewable energy and concerns about greenhouse gas emissions, more anaerobic digestion systems are being built in the United States.
Anaerobic digestion technology not only fulfills the goal of reducing the use of landfills for waste disposal, but also produces biogas for electricity. Solid-state anaerobic digestion is normally chosen over liquid anaerobic digestion because the digestate can be easily composted for use as fertilizer or soil conditioner. In recent years, construction of solid-state anaerobic digesters has increased and accounted for 63% of the new installations in Europe over the past five years (Luc and Bruno, 2010). There is no commercial scale solid-state anaerobic digester operating in the United States (as of February 2011), but several are under construction.
This process is comprised of three main reactions: hydrolysis, acidogenesis/acetogenesis, and methanogenesis. Organic components such as proteins, fats, and carbohydrates are broken down to sugars and amino acids during the hydrolysis process. Both the acidogenesis and acetogenesis processes involve the conversion of the end products of hydrolysis to volatile fatty acids such as acetic acid, propionic acid, and butyric acids. Methane and CO₂ are produced from the volatile fatty acids during the methanogenesis process.
According to the total solids content of the feedstock, digesters are categorized as liquid anaerobic digestion/wet fermentation, or solid-state anaerobic digestion/dry fermentation. Currently no official standard is available for the cut-off between wet and dry fermentation but wet fermentation generally refers to substrates with total solids lower than 15%, whereas dry fermentation denotes substrates with total solids higher than 15% (Luc and Bruno, 2010; Li et al., 2011).
Solid-state AD Systems
Solid-state anaerobic digesters installed to handle high total solids content feedstocks vary in design from company to company. There are no moving parts, such as shafts or impellers, in the solid-state anaerobic digester thus reducing the maintenance of mechanical devices.
Most solid-state anaerobic digestion systems use simpler technology such as single stage and batch processes. The following solid-state anaerobic digestion systems are widely used in Europe.
DRANCO in Belgium uses a vertical plug-flow vessel with no mixing. Feedstock enters from the top and exits at the bottom of the vessel where movement of feedstock is by gravitational force (Figure 1). Digestate is dewatered and the liquid is recirculated to inoculate the fresh feedstock. The digestion is operated under thermophilic condition (55°C) and the total solids concentration is about 25–40%.
The Valorga (France) process uses vertical steel tanks with a central baffle that extends two-thirds of the way through the center of the tank (Figure 2). The material is forced to flow around the baffle from the inlet to reach the outlet port on the opposite side, creating a plug-flow in the reactor. These tanks can operate at between 25% and 35% total solids. A biogas mixing system is used to create localized mixing in the tank.
KOMPOGAS from Switzerland uses a horizontal plug flow vessel with a mixing shaft to handle feedstock with approximately 23% of total solids (Figure 3). Recycled digestate is mixed with the feedstocks to inoculate the material, and process water may be added to reduce the solids content.
BEKON from Germany has a garage type design digester that uses a liquid recycle system to distribute liquids. The building is made of concrete with an airtight sliding door (Figure 4). The floor is perforated with a drainage system to allow leachate to be recirculated and heated prior to spraying onto the feedstock in the digester. Because the methane rich biogas generated could possibly be explosive when mixed with incoming air containing oxygen, additional safety features are required for the garage type digester.
Most of the solid-state anaerobic digesters currently installed in Europe are targeted at municipal solid waste (MSW), biowaste and organic waste from kitchen households. Pre-treatment such as source separation, mechanical sorting, sieving, and grinding are generally required prior to digestion for this type of feedstock. Other promising types of feedstock are agriculture residues and energy crops such as corn silage, wheat straw, and sugar beet residue.
Solid-state anaerobic digesters that operated under mesophilic conditions (37–42°C) exhibited a poor startup performance. So, thermophilic (55–65°C) digesters were developed and have been established as a reliable and acceptable option for solid-state anaerobic digestion. Operating solid-state anaerobic digestion systems at thermophilic conditions can accelerate the anaerobic digestion process and provide the added benefit of increased pathogen kill-off during the anaerobic phase. The increased amount of heat required for thermophilic operation can be offset by the higher gas production yields and rates. Most of the solid-state digesters are operated with a solids content between 20% and 40% and an operating C/N ratio of 20 to 30, with an optimal ratio of 25 (Li et al., 2011).
The typical digesters installed by Valorga and DRANCO range from 450 to 4,500 cubic meters (m³), which are able to handle biowaste or MSW ranging from 3,000 to 240,000 tons/year. Other than the biogas required for heating the feedstock, the biogas is used for electricity generation at a rate of approximately 220 to 440 kWh/ton of feedstock. The average gas production rate is 100 to 200 m³/ton of feedstock. The gas production process lasts for 3 to 4 weeks.
Factors To Be Considered
Prior to installation of an solid-state anaerobic digestion system, factors that should be considered include waste transportation or origin, water or liquid sludge supply, location of heat or electricity users, markets for compost or fertilizer generated, and proper odor control.
Co-locating a solid-state anaerobic digestion facility at an existing landfill, transfer station, liquid anaerobic digester facility, or composting facility can maximize the co-benefits such as reduced transportation of feedstocks and sharing of equipment.
Beck, R.W. (2004). Anaerobic Digestion Feasibility Study. Bluestem Solid Waste Agency and Iowa Department of Natural Resources.
De Baere, L., & Mattheeuws, B. (2010). Anaerobic Digestion of MSW in Europe. Biocycle, 51(2): 24–26.
Li, Y., Park, S. Y., and Zhu, J. (2011). Solid-state anaerobic digestion for methane production from organic waste. Renewable and Sustainable Energy Reviews, 15(2): 821–826.
Rapport, J., Zhang, R., Jenkins, B.M., & Williams, R.B. (2008). Current anaerobic digestion technologies used for treatment of municipal organic solid waste. California Environmental Protection Agency, Integrated Waste Management Board.
Reviewed by Dr. Jay Martin, Dr. Lingying Zhao, and Randall Reeder.