Biogas from biomass
Biogas is produced from biomass through the process of anaerobic decomposition. Anaerobic bacteria—bacteria that live without the presence of free oxygen—occur naturally in soils, in water bodies such as swamps and lakes, and in the digestive tracts of humans and animals. These bacteria eat and break down, or digest, biomass and produce biogas. Biogas is composed mostly of methane and carbon dioxide. Methane (CH4) is the same energy-rich compound found in natural gas. The composition of biogas varies from 40%–60% methane to 60%–40% carbon dioxide (CO2), with small amounts of water vapor and other gases.
Biogas forms in, and can be collected from, municipal-solid-waste landfills and livestock manure holding ponds. Biogas can also be produced under controlled conditions in special tanks called anaerobic digesters. Biogas can be treated to remove CO2 and other gases, and it can be used as a fuel just like natural gas. The material that is left after anaerobic digestion is complete is called digestate, which is rich in nutrients and can be used as a fertilizer.
Collecting and using biogas from landfills
Landfills for municipal solid waste are a source of biogas. Biogas is produced naturally by anaerobic bacteria in municipal-solid-waste landfills and is called landfill gas. Landfill gas with a high methane content can be dangerous to people and the environment because methane is flammable. Methane is also a strong greenhouse gas. Biogas contains small amounts of hydrogen sulfide, a noxious and potentially toxic compound when in high concentrations.
In the United States, regulations under the Clean Air Act require municipal-solid-waste landfills of a certain size to install and operate a landfill gas collection and control system. Some landfills reduce landfill gas emissions by capturing and burning—or flaring—the landfill gas. Burning the methane in landfill gas produces CO2, but CO2 is not as strong a greenhouse gas as methane. Many landfills collect landfill gas, treat it to remove CO2, water vapor, and hydrogen sulfide, and then sell the methane. Some landfills use the methane gas to generate electricity.
The U.S. Energy Information Administration (EIA) estimates that in 2017 about 282 billion cubic feet (Bcf) of landfill gas was collected at about 370 U.S. landfills and burned to generate about 11.5 billion kilowatthours (kWh) of electricity, or about 0.3% of total U.S. utility-scale electricity generation in 2017.
Biogas from sewage and industrial wastewater treatment
Many municipal sewage treatment plants and manufacturers such as paper mills and food processors use anaerobic digesters as part of their waste treatment processes. Some sewage treatment and industrial facilities collect and use the biogas produced in anaerobic digesters to heat the digesters, which enhances the anaerobic digestion process and destroys pathogens, and some use it to generate electricity to use at the facility or to sell. EIA estimates that in 2017, about 107 of these types of waste treatment facilities in the United States produced a total of about 1.2 billion kWh of electricity.
Using biogas from animal waste
Some dairy farms and livestock operations use anaerobic digesters to produce biogas from manure and used bedding material from their barns. Some livestock farmers cover their manure holding ponds (also called manure lagoons) to capture biogas that forms in the lagoons. The methane in the biogas can be burned to heat water and buildings and as fuel in diesel-engine generators to generate electricity for the farm. EIA estimates that in 2017, nine large dairies in the United States produced a total of about 116 million kWh of electricity from biogas.