Anaerobic digesters can be installed for many reasons and at many different facility types. First and foremost, it’s important to conduct a technical feasibility assessment to determine if anaerobic digestion is a good choice for your facility.
Determine feedstock type. Even if a feedstock is known to be difficult to digest, it’s possible to add other feedstock types for codigestion, helping to stimulate the reaction. Additional feedstock types can be collected from nearby sources, such as food waste from restaurants or cafeterias, fats and oils from restaurant grease traps, and crop residues from farms. Fats and oils are known to be some of the best feedstocks for yielding the most biogas per volume of feedstock. Many dairy farms will add local food scraps to their manure to increase the amount of biogas produced (Figure 2). AgSTAR, a national partner organization of the US Environmental Protection Agency (EPA), provides assistance and resources for implementing biogas recovery across the US. Its informational sheet on Increasing Anaerobic Digestion Performance with Codigestion describes how to increase performance with codigestion.
Figure 2: Benefits of codigestion
Manure supplemented with feedstocks suitable for codigestion can produce much more methane than manure alone.
Determine size. Sizing requirements vary by application and digester type and depend on feedstock volume and frequency. Sending feedstock to a centralized anaerobic digestion facility can be more economical if close enough to feedstock production.
Ensure feedstock quality. Make sure your organic waste is separated from undesirables such as rocks, straw, and debris. Biogas is a good choice for farms that regularly collect manure. Whether it’s manure or other organic material, waste needs to be fresh because the older it is, the less energy it will contain upon digestion. Energy content varies widely across different kinds of organic waste feedstocks (Table 1).
Table 1: Feedstock energy content
A major benefit of anaerobic digestion is the wide range of feedstocks a digester can receive and process. Fats in particular are known to provide the most energy per feedstock ton.
Assess temperature requirements. In cold climates, facilities may need to provide supplemental heat to drive the reaction. In warm climates, having digester covers exposed to sunlight can be a great option, although keep in mind specific temperature ranges should be maintained for the various digestion types.
Choose how to use the gas
Heating. Biogas can be used in boilers to produce hot water or steam, burned directly for process heat, and used in furnaces for space heating. Boilers are generally able to burn gas without cleaning the gas beforehand.
Electricity via cogeneration. Biogas recovery systems that generate electricity can provide an additional revenue stream for facility owners. In 2016, the EPA published Interconnection Guidelines for connecting biogas generation systems to the grid for the sale of electricity. The heat provided via cogeneration can also be fed back into the digesters to maintain process temperatures. Figure 3 depicts the cogeneration process, where both heat and electricity are recovered from the biogas. See also the Distributed Generation Technologies page, Microturbines, Fuel Cells, and Stirling Engines.
Figure 3: Cogeneration process
Cogeneration involves adding water to a hot gas turbine that is generating electricity. The water heats up from interfacing with the engine and then can be delivered to the facility as water or steam for heating or cooling. The electricity can be used on-site or sold to the local utility.
Sell refined or unrefined. When biogas is conditioned or upgraded into biomethane, it can potentially sell for a higher price than natural gas. This is because biomethane has a higher direct heating value.
Conduct a financial assessment
There are many ways to recover biogas and just as many financial incentives to help encourage adoption. The AgSTAR National Mapping Tool displays updated information on existing digestion projects and state policies and incentives across the US. One of the biggest hurdles for implementing anaerobic digestion systems is the up-front capital cost, but it’s important to take into account all outputs produced by the digestion process. In addition to electricity, separated solids can be sold as animal bedding and compost. In the EPA report, Funding On-Farm Anaerobic Digestion (PDF), Tollenaar Holsteins Dairy notes that these sales generated more revenue than electricity. The same report mentions that, although pricier, codigestion can double electricity sales, with one facility (Butler Farms) estimating $130,600 per year in revenue. For comprehensive planning, AgSTAR hosts a resource hub on Financing Anaerobic Digestion Projects.
Legal requirements and permits
Anaerobic digesters are subject to local, state, and federal permits, which vary considerably from place to place and change frequently. The first step is to contact your local government to determine rules concerning construction, zoning, and storm water management. Then contact your state environmental agency to assess current regulations before starting a digester project. The AgSTAR Guidelines and Permitting for Livestock Anaerobic Digesters offers comprehensive resources on anaerobic digestion, codigestion, and permitting guidelines.
State environmental permits for digesters may be required for air, solid waste, and water. An air permit may be required if the combustion engines are emitting more than federal standards allow. Solid waste processing facilities must meet the federal Resource Conservation and Recovery Act Subtitle D (nonhazardous solid wastes) and 40 CFR Part 258 (landfills) requirements, which are administered through state agencies. Additional solid waste permits may be required if the facility is planning to use codigestion or receive off-site waste. If the anaerobic digesters only process manure, often these are permit-exempt. Finally, a National Pollutant Discharge Elimination System permit (administered through state agencies) is required if the anaerobic digester directly discharges into US waters.
Due to the various types of digesters and feedstocks, it’s imperative to discuss these options with system designers who will know what best suits your facility’s needs. Digester system designers typically take their design cues according to the findings from laboratories that conduct ongoing research for various feedstock types and digester systems.