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Turns Out That Using Human Poop to Fertilize Crops Isn't Such a Great Idea

Is it time to reexamine regulations on sewage sludge fertilizer?

If you talk to its proponents, and there are lots of them, sewage sludge fertilizer is a great way to divert human waste from landfills and to grow crops, despite the unappealing picture it may conjure. The US Environmental Protection Agency has a nicer name for the muck that's left over after processing our shit—"biosolids"—and has encouraged its widespread use as a cheap, effective way to fertilize crops and recycle human waste. But while the EPA requires that bacteria and viruses are killed off before sludge is applied to farm fields, other contaminants, like pharmaceuticals and metals, are only minimally regulated, if at all.


New research suggests this could be a problem, as contaminants are now showing up in treated sludge—and, in lower levels, even in some animals that have fed off the plants it fertilizes.

"I don't think the present rules are even remotely adequate," Murray McBride, a soil contaminant researcher at Cornell University, told me. "There are a lot more toxic metals on the periodic table [and potentially in the soil] than what they decided to regulate."

According to him, the EPA's rules are outdated. They regulate only nine metals with known health risks—including lead, cadmium and arsenic. And metals are just the beginning. Pharmaceuticals and other organic chemicals found in biosolids are cause for even greater concern, he said. Others agree. "If you look at what [could be] potentially regulated by EPA, it's just a tiny fraction of the universe of the chemicals we live in," David L. Lewis, a former EPA scientist who is now a fierce critic of the agency, told me.

"What the EPA regulates is negligible."

Cities like San Diego turn their sewage into fertilizer. Photo: William Garrett/Flickr

EPA officials haven't said whether the rules will be revised, but agency spokesperson Robert Daguillard noted in an email that the agency plans to assess the risks posed by pharmaceuticals in sludge. While they haven't yet determined these risks, they do know what's in it. An EPA sludge survey, Daguilllard said, includes "92 pharmaceuticals, steroids, and hormones." But none of those are actually subject to enforceable limits under current rules.


For many farmers, biosolids are a cheaper alternative to synthetic fertilizers. Municipal governments like them, too: since some cities divert as much as 50 percent of sludge to farms, they can reduce the amount of waste they have to pay to landfill. San Diego, Portland and Edmonton, among many others, process their waste to produce biosolids.

It makes sense, then, that demand has been increasing. While the EPA hasn't recently estimated the size of the market, it put biosolids production at 7.2 million tons in 2004, up four percent from six years earlier. If that growth rate has continued roughly apace, production would be near 8 million tons today. Organic farmers can't use biosolids as fertilizer without risking their organic certification, but it's relatively easy for others to get, often via local organizations.

Biosolids boosters say the fertilizer is effective. "If there are any small negative impacts [from contaminants], they are overwhelmed by the positive effects" on crop yields, said Ned Beecher of Northeast Biosolids and Residuals Association, which promotes its use. He believes that the EPA rules are adequate as-is, noting that the agency has already considered the risks posed by dozens of sludge contaminants, and judged them minimal. "Just because there is no limit set, doesn't mean risk assessment hasn't been done," Beecher told me.

"What's happening in natural environments is long term exposure to low concentrations"


Before sewage sludge is used on farm fields, the EPA requires that it undergo two processes aimed at destroying pathogens: anaerobic digestion—in which bacteria break sludge down in the absence of oxygen––and high-heat sterilization. Even so, some suggest that enough bacteria may survive to contribute to the spread of antibiotic-resistant strains. That's a major concern, because when these "superbugs" spread in livestock farms and hospitals, disease can run rampant. And the bacteria can't be easily killed with penicillin or other antibiotics.

Edo McGowan, a retired environmental scientist and outspoken critic of biosolids, is concerned about research showing antibiotic-resistance genes in soils treated with biosolids. These genes, he said, are easily spread by farm equipment or wind, winding up in bacteria that can be ingested by people and animals. "Some of these bugs are resistant to pretty much anything you can throw at them," he told me.

Not all experts agree that using biosolids as fertilizer is hurting the effectiveness of antibiotics, mostly because we still don't know how much disease resistance occurs naturally in soils. "It's just speculation at this point," said Marc Habash, who researches microbes in biosolids at the University of Guelph in Ontario. "Antibiotic resistance genes are in our soil. There are a lot of natively-occurring bacteria that possess these genes."

Earthworms seem to have picked up some contaminants. Photo: schizoform/Flickr

Research has backed up some of the critics. A 2012 study led by environmental chemist Chad Kinney of Colorado State University, Pueblo, found that earthworms in soil treated with biosolids contained a variety of manmade compounds, including pharmaceuticals and personal care products, like the antibiotic drug trimethoprim (used to treat urinary tract infections and other conditions) and the disinfectant triclosan (a common ingredient in antibacterial hand soap).


Whether those synthetic compounds actually harm earthworms is unknown, and Kinney notes that the concentrations are low. But he said their presence shows that manmade contaminants in biosolids are moving up the food web. This suggests they could be reaching humans, too.

Despite his worrisome findings, Kinney said that the risks of biosolids are mostly speculative and hard to measure, while the benefits are clear: it's a way to keep sewage sludge out of landfills and return nutrients to soil. "If sludge isn't meeting the regulatory requirements [to be classified] as biosolids, it has to be disposed some way. But then you lose all that valuable organic carbon and nutrients that can be released into a soil environment."

Any health risks associated with recycling contaminants in fertilizer and sending them up the food web may not be known for generations—if ever, according to Kinney.

Part of the problem, he said, is that most studies look only for immediate damage or death, not slower, less obvious effects like reductions in fertility. "Most of the toxicology is based on acute studies of one compound in short-term exposure," Kinney told Motherboard. "What's happening in natural environments is long term exposure to low concentrations, so you're not going to see those acute effects per se. We're not going to see typical things like lethality."

"It's going to be slower, subtler effects," he said, "that have generational effects."