The U.S. Environmental Protection Agency’s (EPA’s) plan to regulate carbon emissions is just the latest challenge facing the U.S. electric power system. Technological innovation is disrupting old ways of doing business and accelerating grid modernization. Earlier this year, AEE released Advanced Energy Technologies for Greenhouse Gas Reduction, a report detailing the use, application, and benefits of 40 specific advanced energy technologies and services. This post is one in a series drawn from the technology profiles within that report.
Anaerobic digestion (AD), is a process by which waste, such as livestock manure, food scraps, and municipal/industrial wastewater, is broken down by microorganisms in the absence of oxygen into a combination of methane and other gases, otherwise known as biogas. Unlike waste-to-energy from municipal solid waste, anaerobic digestion occurs without incineration and relies on natural breakdown of organic matter into biogas. Different digester types can be used, from covered lagoons at animal farms to above ground steel or concrete tanks. The biogas can then be burned to generate electricity on-site. Biogas can also be purified and made into a pipeline-quality substitute for natural gas, including compressed natural gas (CNG) for vehicles. Co-digestion adds fats, oils, and grease to manure or wastewater to enhance energy production. Landfill gas (LFG) is biogas produced naturally in landfills and captured for electricity generation.
In the U.S., AD is commonly used with agricultural waste or to power wastewater treatment plants. Interest in using food waste for AD is on the rise, especially as states look to reduce the amount of municipal waste going to landfills. There are over 190 anaerobic digesters operating at commercial livestock facilities, generating 244,000 megawatt-hours of electricity annually. Likewise, there are about 1,500 operating at wastewater treatment facilities. AD is often used in conjunction with CHP for efficient electricity and heat generation. The heat can be used on site by farms and wastewater treatment facilities for hot water, drying biosolids, and for heating digesters. Farms regularly utilize AD, including at Royal Farms No. 1 in Tulare, California. Biogas is generated using slurried hog manure, generating electricity and heat energy for use on the farm.
AD facilities use waste that would otherwise create methane, a more powerful greenhouse gas than CO2, when it decomposes in landfills. Many farmers who deploy such facilities are able to reduce manure odors while meeting their electricity and heating demands, avoiding the need to purchase electricity or fuels. In 2009 AD systems reduced over 1.1 million metric tons of CO2 equivalent by direct emissions reduction or avoided emissions. Municipalities that employ co-digestion biogas facilities also benefit by reducing the amount of clogging fats, oils, and grease in their waste streams, and by utilizing the residual biosolids as fertilizer. Water quality can also be improved, with disease-causing bacteria being removed from groundwater.