What goes down your toilet can end up on farm fields across the United States.

Biosolids, or sewage sludge, are the solid byproducts of the wastewater treatment process. Rich in nutrients, they can be used as fertilizer on agricultural fields or compost on lawns.

But the process for producing these materials can vary greatly, and some unwanted things can end up in those biosolids. A recent study released by the U.S. Environmental Protection Agency suggested that human health risks associated with toxic per- and polyfluoroalkyl substances (PFAS) were elevated in some places where sludge was applied to farm fields.

The amount of these “forever chemicals” that could end up in biosolids depends on how much were in the water coming into the plant.

“They are very widespread,” said Carsten Prasse, an assistant professor at Johns Hopkins University who studies biosolids. But they’re not in all biosolids and sometimes it’s only in small amounts, he added.

Here’s how they’re made:

At most treatment plants, wastewater is pumped in through large pipes and large debris removal begins immediately. At a facility just outside Columbus, Ohio, a machine screens out large debris such as toilet paper and trash. That debris is collected, compacted and taken to a landfill for disposal.

After large debris is removed, the wastewater flows into a sedimentation tank, a large, open-air and cone-shaped tank where gravity pulls heavier solids to the bottom. A long mechanical arm corrals floating solids into a drain on the top of the tank. A similar arm at the bottom of the tank gathers up the solids there.

The liquid is moved from the sedimentation tanks to an aeration tank where air encourages the growth of microbes that further break down organic particles. The wastewater then gets moved back to another sedimentation tank where more solids are removed.

The solids that are removed at each step of the process are put through thickening centrifuges, then go into these tanks for anaerobic digestion, where microbes continue breaking down organic matter. The tanks are heated to a prescribed temperature for a certain number of days to kill off most pathogens.

After another thickening step, the solids are ready to apply to farm fields based on the U.S. Environmental Protection Agency’s regulations for land-applied biosolids.

“There are values in using biosolids in agriculture because of their high nutrient concentrations,” including nitrogen and phosphorus, Prasse said. They can also help strengthen soil and help with irrigation, he added.

Some treatment facilities further dry out sludge, turning it into a dirt-like material that can emit steam as microbes do their work in decomposition.

The waste that make up biosolids can vary widely depending on what’s in the water. In addition to human waste, biosolids might contain all kinds of everyday household chemicals like beauty products and pharmaceuticals, some of which may contain forever chemicals, Prasse added. At the Columbus facility, the majority of their waste comes from households and businesses.

The Columbus facility tests the final product for metals and nutrients such as phosphorus and nitrogen every month even when they are being stored before land application.

In Columbus, the city takes half of these dirt-like biosolids to a composting facility nearby, where they are mixed with other organic material like trees and leaves and turned into compost after an additional round of heating and aeration to further kill off pathogens. These biosolids are less regulated and can be used on land such as parks or a residential yard.

The Columbus facility, like most such operations, doesn’t test the incoming material for PFAS, an expensive proposition. Dixon said any future testing or treatment of wastewater for PFAS at his facility would depend on future federal regulations.

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