LANSING — A common practice by farmers to fertilize their fields may be spreading forever chemicals into new areas and into the food supply.

Biosolids are nutrient-rich organic materials derived from the treatment of sewage sludge from wastewater treatment plants that are applied as fertilizer.

They can be a cost-effective way for local governments to dispose of sewage sludge and for farmers who pay little or nothing to use it as fertilizer.

“Solids handling is a cost for municipalities,” said Christian Smith, the PFAS in Biosolids contact for the Department of Environment, Great Lakes and Energy, commonly referred to as EGLE.

“Whether that’s through land application, landfill or incineration, those are all typically done at a cost to the treatment plant,” he said.

Biosolids tend to be more cost-effective methods than other disposal methods for wastewater treatment discharge, said Sarah Campbell, the statewide coordinator for EGLE’s biosolids program.

To ensure that treated sewage sludge is safe to apply to fields, treatment plants remove pollutants including heavy metals such as arsenic, lead and mercury, she said.

But contamination by so-called “forever chemicals,” also known as PFAS, short for per- and polyfluoroalkyl substances, is a new kind of contamination that treatment plants aren’t well equipped to treat, according to Smith.

PFAS chemicals have been manufactured since the 1940s and are used in many consumer products, including food wrappings, clothing, carpeting and firefighting foam. Other important sources are industries such as metal plating facilities.

Most of these chemicals are not well studied. Some are associated with significant health problems, including cancer, thyroid disease, liver damage, decreased immune system response to vaccines and others, according to the Department of Health and Human Services.

Because of the potential for contamination, biosolids come with many restrictions on their use to ensure the safety of the food supply, Campbell said.

For example, crops grown on fields treated with biosolids are usually used only for animal feed or ethanol, she said. There are strict requirements for how and when they are applied, at what concentrations and what crops may be grown on them.

“While we allow food to be grown for humans, typically a lot of sources won’t buy it if it is known to be grown in biosolids,” Campbell said.

In fiscal year 2024, about a third of sewage sludge was applied to Michigan farms as biosolids. About a quarter was exported out-of-state, including to Wisconsin and Ontario.

Less than half was either landfilled or incinerated.

“Incineration actually is one of the more expensive ways,” Campbell said. “We’ve only got three incinerator stacks in Michigan, they’re all in Detroit. Because of air quality concerns, it’s not a very efficient process.”

Campbell said biosolids are often supplied free to farmers, though some municipalities may charge transportation costs.