What you flush down your toilet could end up on farm fields across the United States. This is because of a substance called biosolids, which are the solid leftovers from the wastewater treatment process. These materials are rich in nutrients and can be used as fertilizer for crops or compost for gardens.
However, the way biosolids are made can differ significantly from one treatment plant to another. This means some unwanted elements can end up in these materials. A recent study by the U.S. Environmental Protection Agency (EPA) raised concerns about the health risks linked to toxic chemicals known as PFAS, often referred to as "forever chemicals." These substances can remain in the environment for a long time and may pose health risks in areas where biosolids are applied to fields.
The amount of PFAS in biosolids largely depends on what enters the wastewater treatment facility. Carsten Prasse, a researcher at Johns Hopkins University, noted that while PFAS are widespread, they are not found in every batch of biosolids, and sometimes only in small amounts.
The process of making biosolids begins when wastewater is brought into treatment plants through large pipes. At these facilities, large debris like toilet paper and trash is removed right away. For example, at a plant near Columbus, Ohio, machines screen out this debris, which is then sent to a landfill.
Once the large debris is out, the wastewater moves into a sedimentation tank. Here, gravity helps pull heavier solids to the bottom. Floating solids are collected, and more solids are gathered from the bottom. The liquid then goes to an aeration tank, where air helps microbes break down organic matter further. This process is repeated in another sedimentation tank to remove even more solids.
The solids collected throughout these steps undergo thickening and are then sent to anaerobic digestion tanks. In these tanks, microbes continue to break down the organic material. They are heated for several days to kill off most pathogens. After another thickening step, the solids are ready to be applied to farmland, following EPA regulations.
Using biosolids in agriculture has benefits, as they contain high levels of nutrients like nitrogen and phosphorus, which can enhance soil health and improve irrigation. Some facilities even dry out the sludge, turning it into a dirt-like material that can produce steam as microbes decompose the waste.
The composition of biosolids can vary widely, as they include not just human waste but also household chemicals from beauty products and pharmaceuticals. At the Columbus facility, most of the incoming waste comes from homes and businesses. The facility conducts monthly tests on the final biosolid product for metals and nutrients.
In Columbus, half of the processed biosolids are taken to a nearby composting facility. There, they are mixed with organic materials like leaves and trees, undergoing additional heating and aeration to kill off any remaining pathogens. This compost is less regulated and can be used in parks or residential yards.
Currently, the Columbus facility does not test incoming wastewater for PFAS due to the high costs involved. Any future testing or treatment for these chemicals will depend on new federal regulations. As concerns about the safety of biosolids grow, the conversation around wastewater treatment and its impact on agriculture is becoming increasingly important.