The energy we use and the garbage we throw away are usually part of a linear system that leads to outsized environmental footprints. But connecting the two by using waste to create energy creates a much more sustainable circular system. A lot of developing technologies attempt to convert waste into energy. But one of the most straightforward and practical is renewable natural gas.
Natural gas is made up mostly of methane. It is the largest source of electricity in the United States, accounting for 35% of our electric power. Natural gas forms, like coal, from the decomposition of biological matter under pressure and heat in a process that takes centuries. Among fossil fuels, natural gas burns the cleanest. The only pollutant it releases is carbon dioxide, and it releases less carbon dioxide than either coal or oil.
But natural gas is neither renewable nor sustainable. Carbon dioxide is the most significant greenhouse gas contributing to climate change. And the environmental impacts of natural gas are not limited to emissions from burning. Extracting the gas is itself a fossil-fuel intensive process that generates significant emissions. Extraction methods like fracking can impact local hydrology and produce toxic and sometimes radioactive wastewater. It can even generate earthquakes and has been linked to health impacts in neighboring communities.
Compared to “the dump” of the 20th century, a modern sanitary landfill is a complex system that cleanly and safely contains garbage. But sanitary landfills still have an environmental impact. The most significant component of their carbon footprint is the generation of landfill gas (LFG). Landfill gas is formed when organic materials break down inside the landfill. Equal parts methane and carbon dioxide, LFG is a powerful greenhouse gas. By law, all landfills extract the odorous and highly flammable gas. Most of them burn the collected gas at flare stations, making landfills the third-largest source of human-related methane emissions in the United States.
Renewable Natural Gas
Because it takes centuries to form, true natural gas is not renewable. So the term “renewable natural gas” would seem to be an oxymoron. Landfill gas is chemically similar to natural gas. And it is renewable in the sense that it is created by the ongoing human activity of burying organic waste. With only a little processing, LFG can be purified to pipeline-quality gas that is fully interchangeable with conventional natural gas. It can be used in natural gas vehicles or distributed through the same pipeline system that powers homes. Today, about 69% of currently operational LFG energy projects in the United States are used to generate electricity.
Landfill gas is a major source of RNG, but it is not the only source. A biogas is any gas that forms from the anaerobic decomposition of organic matter. Biogas contains 45%-65% methane and can be processed into renewable natural gas for use in place of conventional natural gas. In addition to landfills, RNG is generated at wastewater treatment plants, livestock farms, food production facilities, and commercial composting operations. To date, agriculture-based RNG projects outnumber landfill-based projects.
Benefits of RNG
Replacing conventional natural gas with renewable natural gas reduces greenhouse gas emissions from landfill operations, as well as from natural gas extraction. But it has other benefits, too. Hazardous air pollutants and VOCs that are present at low concentrations in LFG are destroyed, rather than released into the atmosphere. LFG energy projects generate revenue for the utility while often providing electricity to the customer at lower cost. Renewable natural gas is usually consumed locally, rather than transported across long distances in pipelines or trucks. The expenses from a project producing RNG also mostly benefit local economies in the form of jobs associated with the project.
Better than RNG
Although renewable natural gas is preferable to conventional natural gas, the Zero Waste hierarchy dictates that we don’t purposely toss compostable materials in the garbage. Where possible, organic matter should be composted and used to improve soil health. Where possible, electric energy should come from renewable sources that are cleaner than gas. But as long as there are landfills, LFG will be generated. Commercial composting and industrial agriculture also produce some gases that can be refined for use as RNG. So there is still a place for RNG in the energy mix. Some utilities, like Puget Sound Energy, offer RNG as part of a green power portfolio program. Check to see if your local utility is one of them.