Biogas From Manure
Revised April 2022.
What is Biogas?
Biogas is produced from the process of anaerobic digestion. The process produces biogas that is composed of methane (~60%), carbon dioxide (~40%), and other trace gases. The biogas is usually either combusted to generate electricity and heat or can be further processed and refined into renewable natural gas or other fuels. Farm digesters rely on animal manure as the primary feedstock used to create biogas. In addition to creating renewable energy that can be used on-farm or sold as an additional revenue source, biogas digesters treat animal waste, reduce odors, and produce helpful co-products that can benefit the farming operation.
How is it produced?
Biogas is produced in an anaerobic digester, which is a large, air-tight tank. The system contains microorganisms that biologically digest and break down organic material anaerobically, or in the absence of oxygen, to produce biogas. While the primary feedstock for anaerobic digestion in the United States is manure, other biomass feedstocks can be added to the digester to boost biogas production. Common feedstocks to codigest with manure are food waste or wastewater, but there are other possible herbaceous feedstocks or energy crops that could be added as a possible feedstock for biogas production.
Anaerobic digesters can be installed for many reasons, including odor reduction, manure management, revenue generation and improving the farm’s public image. While simple in concept, there are many sizes, styles and applications of digesters. Anaerobic digesters are not inexpensive, and projects require detailed technical and financial planning to be successful. There are a number of tools available to help farmers investigate whether anaerobic digestion is feasible for farm operation (see links below).
|Biogas digesters on a farm in Iowa (Photo courtesy of the C-CHANGE project).|
How is it used?
There are two primary products of the digestion process: biogas and digestate. Biogas can be combusted immediately for energy or it can be upgraded into biomethane, also known as renewable natural gas (RNG). If combusted immediately, biogas is often used to power a combined heat and power plant (CHP), where biogas is ignited to turn a generator and create power that can be added to the electrical grid. Waste heat from the generator is used to keep the digester warm and to provide for other on farm heating needs. Upgrading biogas into pure methane (by removing the CO2, water vapor and other impurities) is an expensive process but allows the RNG to be integrated into the natural gas grid or compressed and transported for alternative use.
Digestate is a nutrient-rich co-product of the anaerobic digestion process. The nutrient-rich digestate can be used directly as a fertilizer or soil amendment. It is also possible to separate digestate into liquid and solid fractions. The liquid can be used as a fertilizer, while the solid fraction may be utilized as animal bedding.
Anaerobic digestion (AD) has environmental benefits that might also be considered. Anaerobic digestion is touted for its ability to minimize and contain odors. Furthermore, the digestion process reduces the odor producing compounds in the manure. AD also provides environmental sustainability benefits. Using digestate can reduce the need of chemical fertilizers by converting nutrients in manure to a more accessible form for plants to use. Digesters provide a waste-to-energy system that provides renewable energy in the form of biogas that can be used to create electricity and heat to sustain the anaerobic digestion process. Furthermore, well managed digesters reduce methane emissions by capturing and combusting or upgrading methane for off-farm use that would otherwise escape into the atmosphere.
Energy is the primary product from a biogas digester and can take three different forms. Biogas can combusted in an engine-generator to produce electricity for use on-farm or sold to the electric grid. In this process, waste heat can be captured in co-generation systems and then recycled to heat the digester and maintain the process and/or to meet water and space heating needs on the farm. Upgrading biogas to renewable natural gas may be particularly profitable. For example, several states (most notably California) have a Low-Carbon Fuel Standard incentive program that offers incentives for greener fuels, such as RNG. This gives greater value to RNG compared to conventional natural gas. Energy production from anaerobic digestion may also be eligible for renewable energy credits, depending on the state in which it is produced and the manner in which it is produced and utilized.
An important source of revenue can come from tipping fees from accepting organic waste from off-farm, such as food waste from grocery stores or food processing plants. Farm digesters fed by food waste can generate revenue from municipal waste disposal fees or tipping fees. Rather than send food waste to a landfill, anaerobic digesters could accept the waste for a fee. This has the potential to significantly increase the profitability of the digester. In addition to the direct revenue from tipping fees, co-digestion with these waste streams can boost biogas production.
Digestate can be used to create a variety of byproducts including fertilizer, livestock bedding, and soil amendments that can be used on-farm or sold to a neighboring farm. This can maximize the value of manure and diversify the revenue stream for the producer.
Links to Relevant Resources
- Planning AD Projects, United States Environmental Protection Agency – Landing page for resources for AD projects from planning to project completion.
- Anaerobic Digestion Screening Tool, Global Methane Initiative – This is an Excel-based tool, which allows stakeholders to understand and evaluate the feasibility of AD projects. There is a tutorial and user manual available for users.
- On-Farm Biogas Development Handbook, Canadian Agricultural Partnership – While this handbook is directed to farmers in British Columbia, it provides a comprehensive guide to on-farm biogas projects that are applicable to U.S. farmers. This includes introductory information, economics, development stages, and self-assessment tools.
- Market Opportunities for Biogas Recovery Systems at U.S. Livestock Facilities, United States Environmental Protection Agency – This guide provides an overview of the environmental and economic benefits of biogas systems and information for creating more profitable systems.
- Renewable Natural Gas from Agricultural-Based AD/Biogas Systems, United States Environmental Protection Agency, AgSTAR – This article provides introductory information for the potential of upgrading biogas into renewable natural gas projects.