Per- and polyfluoroalkyl substances (PFAS) are a large family of synthetic chemicals developed since the 1940s, engineered for their exceptional resistance to heat, water, and oil. This stability comes from a strong carbon-fluorine bond, making them highly persistent in the environment and earning them the nickname “forever chemicals.” Biosolids are nutrient-rich organic materials produced after treating domestic sewage sludge at wastewater treatment plants (WWTPs). They are commonly applied to agricultural land as a soil amendment and fertilizer. The widespread use of biosolids as a sustainable resource creates an environmental concern when these persistent chemicals become concentrated in a product intended for broad environmental application.
Sources of PFAS Contamination in Biosolids
PFAS contamination begins with the continuous discharge of these chemicals into municipal sewer systems. Industrial facilities that manufacture or use PFAS—such as those involved in metal plating, textiles, or paper making—release high concentrations in their wastewater effluent. These industrial discharges represent a significant input into the public sewer infrastructure.
A substantial amount of PFAS also comes from residential and commercial “down-the-drain” sources. Consumer products, including cleaning agents, personal care products, and water-repellent clothing, contain PFAS that wash into the wastewater stream. This diffuse input ensures that nearly every wastewater treatment plant receives a constant influx of these substances.
WWTPs are designed to remove biological contaminants and solids, but conventional processes cannot break down the carbon-fluorine bond in PFAS. Because the chemicals resist degradation, they tend to partition to the solid organic matter during treatment. This concentrates the PFAS within the sewage sludge, which is then treated and repurposed as biosolids.
Environmental Fate and Agricultural Movement
Once contaminated biosolids are spread onto agricultural fields, persistent PFAS are introduced directly into the soil matrix, where they can remain for decades. The movement of these chemicals is heavily influenced by the length of their carbon chain. Longer-chain PFAS, such such as perfluorooctane sulfonic acid (PFOS), adsorb more strongly to organic matter and soil particles, accumulating in the upper soil layers.
Conversely, shorter-chain PFAS compounds, like perfluorobutane sulfonic acid (PFBS), are more hydrophilic and dissolve easily in water. This solubility makes short-chain compounds highly mobile, allowing them to leach downward through the soil profile. Studies show these mobile short-chain PFAS can migrate to depths greater than 1.8 meters, posing a direct threat to groundwater aquifers and drinking water sources.
The most concerning pathway for human exposure is plant uptake from the soil. Plants absorb PFAS primarily through their roots, driven by the water-transport system. Uptake efficiency varies depending on the specific PFAS compound, soil type, and plant species.
Generally, shorter-chain PFAS are more easily taken up by roots and translocated into edible above-ground parts, such as stems, leaves, and fruits. Plants with highly fibrous root systems are particularly efficient at drawing PFAS from the soil. This process facilitates the entry of the chemicals into the food chain, affecting crops and the livestock that consume them.
Human Health Implications of Exposure
The application of PFAS-containing biosolids creates several pathways for human exposure, including consuming contaminated food, inhaling airborne dust, and direct contact for farm workers. Eating crops or livestock, such as beef and dairy products, that have accumulated PFAS represents a primary route of exposure. This process bypasses water filtration systems and introduces the chemicals directly into the food supply.
Once in the body, certain PFAS compounds, particularly the legacy chemicals perfluorooctanoic acid (PFOA) and PFOS, bioaccumulate, with half-lives in human blood ranging from several years to over a decade. The accumulation of these chemicals is associated with adverse health outcomes, often involving endocrine and immune system disruption.
PFAS interfere with hormone systems, potentially leading to issues like thyroid disease and high cholesterol. They also impact the immune system, with exposure linked to immunosuppression and a reduced antibody response to childhood vaccinations. Furthermore, long-term exposure has been linked to increased risks for kidney and testicular cancer.
The EPA’s draft risk assessment for PFOA and PFOS in sewage sludge indicated that, under certain modeled scenarios, health risks could exceed acceptable thresholds. These scenarios often involve exposure through consuming contaminated groundwater or agricultural products near application sites.
Policy and Technological Mitigation
The lack of a unified federal standard for PFAS levels in biosolids remains a central challenge, although regulatory action is progressing. The EPA recently released a draft risk assessment focusing on PFOA and PFOS in sewage sludge. This assessment is intended to inform future federal regulations under the Clean Water Act, such as setting numerical limitations or best management practices, but it is not yet a legally enforceable standard.
In the absence of federal mandates, several states have implemented rigorous measures. Maine, for instance, enacted a near-total ban on the land application of all biosolids and sludge-derived products. Other states, including Michigan and Massachusetts, focus on mandatory testing and monitoring requirements for PFAS in biosolids before land use.
Technological mitigation efforts focus on two main strategies: source control and chemical destruction. Source control involves identifying and stopping the highest-volume industrial discharges of PFAS upstream of the WWTP. Implementing pretreatment requirements for these dischargers is considered the most cost-effective solution, significantly lowering the concentration of PFAS entering the sewer system.
For biosolids that already contain PFAS, emerging technologies aim to break the carbon-fluorine bond. Thermal destruction methods, such as high-temperature incineration and pyrolysis, are being investigated. Complete destruction often requires temperatures exceeding 1,000 degrees Celsius to prevent harmful byproducts. Non-thermal options, including supercritical water oxidation and electrochemical oxidation, offer alternatives by using extreme pressure or electrical currents to break down the chemicals in the sludge.