Talk of natural climate solutions typically conjures up images of lush forests or pristine wetlands. But in King County, Washington, one important natural solution comes from a less Instagram-worthy source: the toilets of Seattle.

After human waste gets flushed out of sight, it’s routed to a wastewater treatment plant (or, in many rural areas, to a septic tank ). At the publicly owned treatment plants, it’s put through a series of mechanical, biological, and/or chemical processes that separate water from the solid matter suspended within. The water then is released back into the environment. The solids left behind can be dealt with in one of three ways: incineration, burial in a landfill, or application on the land, typically in the form of agricultural fertilizer.

The first two options are extremely expensive in many parts of the country. They also have a number of drawbacks from a climate perspective: landfilled solids release methane, while incineration releases another powerful greenhouse gas, nitrous oxide. Land application, however, tends to be less of a burden on public finances, and has a number of mitigation and adaptation benefits.

In King County, treated human waste, also known as biosolids, plays an important part in the county’s efforts to combat global warming. “Every year we provide the equivalent of taking about 8,000 cars off the road,” said Cat Gowan, a biosolids project manager in the county’s wastewater treatment division.

Robust communications and partnerships

King County is far from alone in sending its biosolids to farms; about 55% of all treated human waste in the U.S. is added to soils. But the Seattle-area program, which began in the 1970s, is one of the nation’s most ambitious. It includes research partnerships with several universities, along with robust communications efforts: the county branded its biosolids as Loop and created a standalone website that provides in-depth information about the program.

Each day, a dozen 31-ton Loop-branded trucks deliver biosolids to farmers throughout the region. According to Doug Poole, who uses it on his crop fields, Loop costs about one-half to one-third less than chemical fertilizer.

Consisting of about 80% water, Loop is applied in a thin layer with a manure spreader. Fields typically receive applications once every few years.

Poole took over his farm from his father, who was one of the first growers to use King County biosolids; the program’s longest-running research plots are located on the family’s land. Poole credits Loop for raising his crop yields between 10 and 40%.

Andy Bary, a soil scientist at Washington State University, has been studying Poole’s farm since 1994. His research demonstrates that yields on plots that use biosolids are consistently equal to or higher than those using synthetic fertilizers.

The reason? Bary says biosolids provide a broader range of plant-beneficial nutrients than farmers can typically provide through synthetic fertilizers alone. “When you’re using biosolids, you’re not just putting down nitrogen, which is pretty typical,” he said. “You get a broad range of nutrients, the full meal deal – you’re getting nitrogen, you get phosphorus, you get sulfur, you get a whole raft of micronutrients.” Replicating this cocktail with chemical inputs is prohibitively expensive for most farmers, he said.

Mitigation, adaptation benefits

Biosolids-based yield gains have important ramifications for climate mitigation and for adaptation. Producing and applying chemical fertilizers releases carbon dioxide, methane, and nitrous oxide. Preventing these emissions without jeopardizing food supplies represents a significant achievement.

Biosolids also help sequester carbon in the soil, something that’s increasingly seen as a promising way to reduce atmospheric greenhouse gas levels. In a 20-year study on Poole’s farm, Bary found a significant increase in soil organic carbon in fields where Loop had been applied compared to those where commercial fertilizer, or none at all, was used.

Soils treated with biosolids also retain more moisture, which, on the mitigation side, can reduce the need for energy-intensive irrigation and pesticides. It’s another important adaptation benefit, helping farms become more resistant to drought.

Biosolids currently are applied to only about 1% of America’s agricultural lands. According to Ned Beecher, a longtime biosolids advocate at the North East Biosolids & Residuals Association, there’s simply not enough raw material available to dramatically scale up use. Nonetheless, he estimates that if all solids produced in U.S. wastewater treatment plants were applied to land rather than burned or sent to landfills, approximately 7 million tons of greenhouse gas emissions would be prevented each year. In addition, he wrote in an email, “recycling to soils benefits local economies, improves soils, reduces irrigation needs, etc. – and puts to use a material that has to be managed somehow (biosolids are not optional).”

Not all are fans of biosolids on farmlands

Still, it’s no surprise that not everyone is a fan of using treated human waste on farms. The USDA’s organic standards prohibit the practice, as does Switzerland. Anti-biosolids activists cite several reports of health impacts to humans and livestock, accusing EPA and other organizations of cover-ups, conspiracies, and lax oversight. Detractors often describe foul odors. Some scientific studies raise concerns that land application of biosolids may spread contaminants.

In response, proponents point to what they see as decades of successful use on farms like Poole’s, and to a substantial volume of studies concluding that biosolids pose little threat. “Early on, there was a lot of concern about pathogens and metal contents,” said Andy Bary. But now, “through research from various universities across the nation, there’s a huge amount of research showing that if you meet the pathogen reduction guidelines and the trace element reduction numbers, biosolids are very safe, if not beneficial.”

In a recent podcast produced by the nonprofit Water Environment Federation, Sally Brown, a soil scientist at the University of Washington, said that it’s important to consider questions of origin and scale when discussing biosolids contamination. The chemicals most often cited as concerns – e.g., PFAS “forever chemicals” – enter the sewer from homes, where they’re present in much higher concentrations than are found in biosolids, she said.

There’s also a big difference between identifying a contaminant in treated waste and finding enough of it to cause harm. In one recent study, Brown’s team sought to understand how much exposure to biosolids an individual would need in order to absorb one dose of pain relief medication. The result: “You would basically have to eat about 30 wet tons of biosolids to get the equivalent of two tablets,” she said.

Willing to engage with those skeptical …

According to Cat Gowan, a willingness to engage with people who are uneasy about biosolids has helped the Loop program flourish. “Our agricultural project manager, if somebody has a concern about biosolids being applied near their home, he calls them and talks them through it, and they work out a solution,” she said.

But Gowan and her colleagues put most of their outreach efforts into educating the general public, most of whom have never heard of using treated sewage as fertilizer, let alone formed strong opinions on the subject. Local gardeners who use the county’s biosolids compost product, which receives an extra layer of treatment to make it safe for use in backyards and community gardens, have become particularly enthusiastic brand ambassadors.

“People are not as against it as you’d think once you explain to them how it’s made,” said Ashley Mihle, another King County biosolids project manager. “It’s a natural process with microorganisms. It produces this thing that’s so great for the soil and helps your plants grow. And if they use it, then they’re sold forever.”

Sarah Wesseler is a Brooklyn-based writer focusing on cities, culture, and climate change.