By installing biogas recovery systems, farms all over America can produce their own electricity from animal feedlot operations and reduce the environmental problems caused by animal waste. Biogas is produced when bacteria decompose manure anaerobically -- without the presence of oxygen -- into a gas mixture composed of about 60 to 70 percent methane. The resulting biogas can be used to generate heat, hot water or electricity, any of which can be used to run farm operations. Digested manure can be used as fertilizer, bedding, mulch and potting soil. Collecting manure has the added advantages of reducing water contamination, odor pollution and global warming emissions.

In a study published in the EPA’s Biodigester Market report in 2006, the AgSTAR program found that about 7,000 U.S. farms could use biodigesters in a cost-effective way, providing about 700 megawatts of energy and reducing emissions of global warming pollution by about 1.3 million metric tons of methane -- the equivalent of taking 4.7 million cars off the road.

Trends in Energy Production by Anaerobic Digesters

How It Works

Biogas recovery systems have a few main components: an anaerobic digester that optimizes the manure for methane production, a biogas handling system that pipes the resulting gas to the gas-use device (such as a generator) and a storage tank for the remaining effluent. Biogas is most commonly used to produce electricity for on-farm use. Excess electricity can often be sold to the electricity grid, or biogas can be used as boiler fuel for space or water heating.

• Covered anaerobic lagoons are pond-like basins, often earthen, that are covered to retain the biogas. Lagoons are the simplest and therefore most popular biodigesters, but they are only viable in warmer climates, as colder temperatures can suppress methane production.
• Plug-flow digesters are long, narrow, heated tanks, often installed partially underground to retain the heat. These units work only with dairy manure.
• Complete-mix digesters are heated tanks made of reinforced concrete or steel with a mechanical, hydraulic or gas-powered mixing system. They generally require a diluted manure mix, such as manure mixed with process water.

Where It’s Used

Large dairy and swine operations have the greatest potential to produce large amounts of methane at the lowest cost because, unlike other livestock industries, they can collect and store sufficient manure as a liquid, slurry or semi-solid.

AgSTAR, a voluntary program administered jointly by the U.S. Environmental Protection Agency, Department of Agriculture and Department of Energy, promotes methane recovery systems at concentrated animal feeding operations (CAFOs) across the country.

How Much It Costs

The profitability of a biogas digester depends on the size of the operation, the method of manure management and local energy costs. According to AgSTAR, biogas recovery can be profitable in operations of at least 500 cows or 2,000 swine. The economic and environmental benefits of biodigesters suggest that new biodigestion projects are most effective at existing feedlot operations. Manure should be collected frequently (at least once a week) in a liquid, slurry or semi-solid state. Any electricity that is not used on-site can usually be sold to the local utility.

Because methane is one of the most potent greenhouse gases -- 21 times as powerful as carbon dioxide in trapping heat -- farmers who use a biodigestion system can arrange to be paid by a carbon offset company. These firms have emerged in recent years in response to the growing demand from companies and individuals who want to reduce their environmental impact by "offsetting" the emissions they produce in their everyday activities. Carbon offset companies sell "carbon credits" to interested parties, and use the proceeds to pay others to reduce their global-warming pollution.

Farm-Scale Anaerobic Digestion Technology Characteristics
Performance
Typical Duty Cycle	Baseload
Net Plant Capacity (MW)	0.150
Net Plant Heat Rate (Btu/kWh)	13,000
Capacity Factor (%)	80
Economics (2007$)
Total Project Cost ($/kW)	$4,000-6,000
Variable O&M ($/kWh)	$.017
Fuel Cost ($/MBtu)	1-3
Levelized Cost of Energy (¢/kWh)	10.0-16.8 cents
Applicable Incentives	$10/MWh PTC (>150kW only)
Technology Status
Commercial Status	Commercial
Installed Worldwide Capacity (MW)	185
Source: Black and Veatch, Renewable Energy Transmission Initiative, Phase 1A, table 5-3

Advantages

• Odor control improves significantly because the biodigester reduces the volatile organic compounds (VOCs) in manure to odorless methane and carbon dioxide. Hydrogen sulfide, that "rotten egg" odor, is captured in the biogas and destroyed during combustion.
• Cleaner water is an important benefit: biodigesters significantly reduce bacteria levels in animal waste, which means that any runoff to surface waters will be less harmful. Digesters also reduce biochemical oxygen demand (BOD), a measure of the ability of organic wastes to remove oxygen from water. Aquatic species depend on dissolved oxygen in water for survival, so farms that reduce BOD help protect aquatic ecosystems.
• Greenhouse gas reduction is critical in mitigating climate change. Seven percent of methane emissions in the United States come from livestock and poultry manure, most of which in turn comes from swine and dairy operations. Biodigesters eliminate nearly all methane emissions, and as a renewable source of energy they reduce our reliance on fossil fuels.

What's around the Corner

• Biodigesters are increasingly popular for a number of reasons, including farmers' interest in selling carbon credits and utilities' interest in meeting the renewable portfolio standards that 30 states have adopted. A renewable portfolio standard is a government mandate, usually at the state level, for electricity supply companies to produce a specified fraction of their electricity from renewable energy sources. Certified renewable energy generators earn certificates (known as renewable energy credits) for every unit of electricity they produce, and can sell these along with their electricity to supply companies.
• The USDA Rural Business-Cooperative Service (RBS) is accepting applications for grants and loan guarantees from agricultural producers and rural small businesses that want to purchase renewable energy systems and/or improve their energy efficiency. Eligible systems include anaerobic digesters that use animal waste and other substrates to produce thermal or electrical energy.
• New digesters with gas-fired engines will be subject to emission limits for nitrogen oxides, carbon monoxide, and volatile organic compounds (VOCs). For more information, visit the EPA's AgSTAR newsroom.
• Congress should limit greenhouse gas emissions(GHG) from large manure management facilities in confined animal feeding operations, or CAFOs. According to the EPA, greenhouse gas emissions from livestock in large CAFOs account for a significant portion of agricultural GHG emissions in the United States.1

Renewable Energy Meets Wildland and Wildlife Conservation

Certain sensitive lands -- such as parks, monuments and wildlife conservation areas -- and ecologically sensitive marine areas are not appropriate for energy development. In some of these places, energy development is prohibited or limited by law or policy, and in others it would be highly controversial. NRDC does not endorse locating energy facilities or transmission lines in such areas. Siting decisions must always be made extremely carefully, with impacts mitigated and operations conducted in an environmentally responsible manner.

For more information on the intersection between clean energy development and wildland and wildlife conservation in the American West, including locations of parks, wildlife refuges and other conservation areas, see this Google Earth-based feature.