What Is Biogas? Is It Sustainable?

Biogas is fuel gas made from biomass, either by decomposition or chemical processes. Biogas is 50% to 75% methane, while the remaining percentage is carbon dioxide and traces of other compounds.

When it is reduced to nearly pure methane, biomethane can replace fossil fuel-based "natural gas" for electricity generation, transportation, heating, and home cooking. In the United States, nearly all biogas is produced for use in electricity generation.

How Is Biogas Made?

Natural organic matter such as crop residues, animal manure, as well as forestry and wood processing waste, are broken down in biodigesters, which use anaerobic (oxygen-free) digestion to produce biogas. Biogas can also come from recovering methane from landfills and from wastewater treatment plant sludge.

Most biogas in the United States comes from municipal solid waste (landfills), while in Europe it comes from crop waste and animal manure, and in China primarily from manure.

Environmental Benefits of Biogas

Biogas can have environmental benefits, some more obvious than others. For example, unlike wind and solar energy, biogas is able to be used on demand when other renewable resources are unavailable. Developing renewable biofuels as a backup energy supply allows for the development of carbon-free wind and solar energy while eliminating, or at least reducing, the need for non-renewable fossil fuel energy.

Reducing Landfill Emissions

Landfills contribute up to 20% of anthropogenic methane emissions and, in the United States, are the third leading source of methane emissions. Capturing landfill gas and converting it to biogas is part of many countries' efforts to reduce their greenhouse gas (GHG) emissions. But transforming those sources of landfill gas into biogas before they reach the landfill is a more efficient use of resources and reduces other pollution problems as well.

For example, turning wastewater sludge into biogas takes less energy than converting it into compost. Converting animal manure and crop wastes to biogas prevents runoff of these potential pollutants into waterways. Burning biogas from pre-treated leaves also produces fewer GHG emissions than composting them. The trade-off is a decrease in the availability of an important organic fertilizer.

Roughly 30% to 40% of food is wasted and ends up in landfills, making food waste the single largest category of landfill material. Giving wasted food an economic value decreases its presence in landfills and reduces municipal costs and methane emissions from rotting food.

Combatting Deforestation

Biogas can help reduce deforestation in areas of the world where firewood is the main source of home cooking and heating fuel. Half of all forest wood production worldwide is for fuelwood, one-third of which is harvested unsustainably. In sub-Saharan Africa, an estimated 70% of deforestation is due to fuelwood collection.

Switching to biogas can reduce fuelwood consumption by nearly half. As a result, it also reduces by up to half the amount of time spent collecting firewood, a task most frequently performed by women and school-age children, reducing the labor burden on the former and increasing the educational opportunities of the latter.

Cleaner Than Fossil Fuels

Compared to diesel, using biomethane as a vehicle fuel reduces GHG emissions and particulate matter by up to 75%. Compared to burning fossil methane for electricity generation, biomethane reduces GHG emissions by 62% to 80%. Coupling a biogas-burning power plant with carbon-capture technology can reduce GHG emissions even further (up to 87%), though carbon capture and storage technologies have yet to reach commercial viability.

Consequences of Burning Biogas

While it can be renewable and have environmental benefits, burning biogas still emits greenhouse gases and other pollutants into the atmosphere.

Renewable, but How Sustainable?

Transportation and storage of both biomass and biogas result in emissions of CO₂ and particulate matter. As with fossil fuel methane, fugitive emissions are a concern at biogas plants. In both cases, methane emissions result when biogas is incompletely burned. When the storage of anaerobic digestion tanks goes uncovered, the GHG emissions benefits of biogas over fossil methane disappear.

The carbon cycle involved in using agricultural products to produce biogas may be renewable. However, considering the entire life cycle of biogas production—including agricultural emissions, transportation, refining, and combustion—the use of biogas as a fuel source is by no means carbon neutral.

Air Pollutants

Biogas combustion can also lead to the emission of sulfur dioxide, carbon monoxide, volatile organic compounds (VOCs), and, most significantly, nitrous oxides, where biogas emissions are higher than those of natural gas combustion. Trace components of other pollutants at biogas plants, including carcinogens such as arsenic, have been found at higher levels than at natural gas plants as well.

Manure-to-biogas and other biogas projects based on industrial animal agriculture are often cited adjacent to low-income communities or communities of color, exposing them to pollutants and discharging nitrates into the local groundwater. These instances make biogas production an environmental justice concern.

Land Use Changes

As the market for biogas grows, so too do negative land use changes, where crops are grown specifically for biogas production. In Italy and Germany, biogas production expanded significantly in the first decade of the 21st century, leading to higher food and land rent prices, as land clearing for intensive agricultural production increased.

In Indonesia, the use of palm oil effluent increases the profitability of the palm oil industry, encouraging the spread of oil palm plantations into one of the world’s most important old-growth forests.

Is Biogas Carbon Neutral?

Biogas is touted as "renewable," "sustainable," and "carbon-neutral," mostly by its promoters. But calling biogas carbon neutral doesn't look at the whole life cycle of the product.

The carbon that is released when biogas is burned comes from plants and other sources that originally pulled that carbon from the atmosphere, making the mere burning of the material itself carbon-neutral. But looking at the entire life cycle of biogas, including its production and transportation, as well as all the carbon embedded in the equipment used in those processes, makes the biogas industry a net contributor of carbon to the atmosphere.