A CEI Report: The 9 Things Every Farmer Should Know About Biogas Digesters

A biogas digester is a facility designed to accelerate the decomposition of organic matter and extract biogas.  Biogas, is similar to natural gas and is typically composed of 60% methane and 40% CO2. It is a clean and renewable energy that may be used for cooking, providing heat, or generating electricity. 

Organic waste is put into a sealed tank called a digester (or bioreactor) where it is heated and agitated. In the absence of oxygen anaerobic bacteria consume the organic matter to multiply and produce biogas.

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A variety of organic material may be used as feedstock for the digester, including food scraps, crops and animal waste.  In farming environments, livestock manure is commonly the primary digestate given its abundant availability. Farm digesters are generally engineered to process certain types and volumes of manure depending on the species and count of livestock on site.  

No. While dairy cows are an excellent source of digester feedstock, other livestock such as hogs and poultry are often equally suitable.

A biogas digester, properly designed, constructed and maintained, can provide multiple streams of additional revenue for a commercial farm.  Most significantly, biogas production generates valuable carbon-offset credits under environmental regulatory programs in the State of California and under US federal law.  These credits, known as Renewable Identification Numbers (RINs) and Low Carbon Fuel Standard (LCFS) credits, are sold in a marketplace to transportation companies and manufacturing plants required to offset their greenhouse gas emissions.  

Additionally, the biogas produced by the farm digester is injected into commercial gas pipelines and sold into the gas power grid. 

Solid and liquid effluents may also be sold as fertilizers or fertilizer additives.  Solid effluents may also be sold for use in soil compost and livestock bedding, as well as for the manufacturing of biodegradable planters and construction materials like medium density fiberboards and wood-plastic composites.  

The natural decomposition of manure and other organic waste produces methane and CO2.  In the atmosphere, these greenhouse gasses absorb heat energy from the sun and increase in temperature. Biogas digesters capture the methane and CO2 gas that would otherwise escape into the environment. Additionally, once the gas is processed and cleaned, it may be distributed in pipelines and used as energy for heating, cooking, transportation and a variety of other purposes. Since the source of the gas, organic waste, is abundant and renewable, captured biogas is considered a renewable, green energy. Use of biogas helps reduce dependency on nonrenewable natural resources such as oil and coal. 

A farm's CI Score (or "Carbon Intensity" Score) is an essential factor in determining the value of tax credits available under California's Low Carbon Fuel Standard, and in turn the overall revenue potential for a biogas digester project. The CI Score is determined by a representative of the California Air Resource Board in consideration of several factors relevant to the farm's operations, herd management and manure handling practices. 

Renewable Identification Numbers (RINs)  are a form of carbon-offset credit recognized by the US Environmental Protection Agency under its Renewable Fuel Standard. Similarly, Low Carbon Fuel Standard (LCFS) credits are a carbon-offset credit established by the California Air Resources Board. The federal Renewable Fuel Standard and California's Low Carbon Fuel Standard are programs designed to encourage the use of cleaner low-carbon fuels, promote the production of those fuels, and in turn reduce greenhouse gas emissions and decrease petroleum dependence. 

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Companies regulated under these programs (such as transportation companies) are required to utilize certain quantities of renewable fuel. In lieu of purchasing and using that fuel directly, these companies may instead purchase RINs and LCFS credits from credit generators, such as farms that operate biogas digesters. 

Cleaned biogas may be sold into commercial gas pipelines, providing an added revenue stream. An Offtake Agreement is a contract negotiated between the biogas producer and a gas retailer. Under this contract, the gas retailer agrees to purchase some or all of the biogas delivered to the pipeline.

Digesters are available in a variety of sizes and can support any size farm. The smallest of these are "home" digesters, which are easily installed, inexpensive, and capable of processing light quantities of food and animal waste. These digesters are available from several agriculture supply retailers.

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At the other end of the scale, industrial digesters are specially engineered and constructed to process manure from large, commercial farm operations. Design and construction may require up to two years and substantial capital investment.  If a farm is too small to substantiate such investment, they may be able to participate in the operation of a nearby digester by selling its manure to the digester operator.  

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If your are considering a biogas digester for your commercial farm but are unsure about the type of digester you need or its revenue potential, CEI's complementary Digester Feasibility Consulting service is the right place to start.