EPA to Release Biosolids Screening Tool for Risk Assessment

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October 2, 2023

Environmental Protection Agency has developed a standardized framework to assess the risks of PFAS and other chemicals in biosolids

On Sept. 21, 2023, the full Environmental Protection Agency Science Advisory Board (SAB) voted to approve, with some revisions, the Standardized Framework for Sewage Sludge Chemical Risk Assessment developed by EPA to assess the risks of per- and polyfluoroalkyl substances (PFAS) and other chemicals in biosolids during land application or landfill disposal.

EPA, along with scientific experts and stakeholders, worked to design a framework that can efficiently identify, assess, and manage the risks associated with chemicals in biosolids. The framework involves a three-step process intended to:

  1. Evaluate the risks associated with chemicals — e.g., perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), benzo(a)pyrene, trichloroethylene (TCE), 2,4-dichlorophenol — present in biosolids
  2. Assess the potential hazards posed by these chemicals when biosolids are applied to land or disposed of in various ways
  3. Ensure that the assessment process is comprehensive and efficient

A three-step process for sewage sludge chemical risk assessment

The Standardized Framework for Sewage Sludge Chemical Risk Assessment involves the following three steps:

  • Step 1: Prioritization using a public information curation and synthesis process

The first step involves identifying and prioritizing the more than 700 chemicals that have been found in biosolids for further risk assessment. The public information curation and synthesis (PICS) process is used to gather publicly available information about these chemicals, considering factors such as persistence, toxicity, and exposure potential. This process helps establish a preliminary list of chemicals that may pose risks to human health and the environment. This step is crucial for focusing resources on chemicals that are most likely to require risk management actions.

  • Step 2: Deterministic screening-level risk assessment using biosolids screening tool

In this step, the chemicals identified in Step 1 undergo a deterministic screening-level risk assessment using the biosolids screening tool (BST), a computational model that estimates potential human and ecological hazards associated with exposure to these chemicals when biosolids are applied to land or disposed of in various ways, such as in landfills. This step helps identify and prioritize which chemicals of concern have the potential to cause adverse effects and whether further action is necessary.

  • Step 3: Refined risk assessment

Chemicals that exceed predefined levels of concern in Step 2 move on to the refined risk assessment. This step involves a more detailed and comprehensive evaluation of the risks associated with these chemicals. The refined risk assessment uses advanced models and methodologies to consider exposure scenarios in greater detail. It helps EPA determine whether numerical standards, management practices, or other measures are needed to mitigate the risks posed by these chemicals in biosolids. This is the final step in the assessment process and guides decision-making about risk management actions.

Concerns with the BST and other risk assessments

The SAB biosolids panel held public meetings on April 5, May 2 and 3, and July 5, 2023. Panelists expressed several concerns and made recommendations about the BST in Step 2 of the framework. Concerns with the BST primarily focused on the lack of consideration of the biosolids matrix and the use of simplistic fate and transport models that could overestimate risks. The main concerns highlighted by the panelists included:

  • Conservatism in prioritization and screening steps: Panelists were concerned that the prioritization and deterministic screening steps of the framework might include overly conservative elements. They emphasized the importance of ensuring that the approach does not overestimate risks through the unnecessary compounding of risk. The panelists believed the framework could be more aligned with actual exposure scenarios and the ways in which biosolids are used, especially considering that only a small fraction of biosolids is applied to land used for human food crops.
  • Dermal exposure scenario: Panelists highlighted the need to broaden exposure scenarios and include dermal exposure pathways in the assessment process. They recommended incorporating dermal exposure in the screening-level risk assessment and the refined risk assessment. This would provide a more comprehensive understanding of the potential risks associated with chemicals in biosolids and how they could affect human health and the environment.
  • Biosolids matrix and chemical concentrations: The panel emphasized the importance of considering the biosolids matrix in addition to chemical concentrations. They noted that chemicals behave differently in the biosolids matrix than in solution. Therefore, the panel recommended that the framework should account for this distinction to improve the accuracy of risk assessments.
  • Refined risk assessment: Panelists raised concerns about the refinement of risk assessments and how this step determines numerical standards, management practices, and disposal limits for chemicals exceeding concern levels. They wanted this step to be more inclusive and thorough, incorporating sophisticated models and methodologies to ensure a comprehensive evaluation of risks.

Potential implications of the approved framework

According to EPA's 2021 Biosolids Annual Program Report, nearly 4,500,000 dry metric tons of biosolids were used or disposed of that year using multiple methods, including land application (e.g., agriculture, reclamation), landfilling, incineration, and other management practices (e.g., deep well injection, syngas) across the country. Because of the widespread use of biosolids in the U.S., the approved framework may have multiple implications for entities that rely on the benefits of biosolids, or on biosolids disposal techniques, to make their business feasible. These implications may include:

  • Biosolids to biochar: It is currently unknown whether the framework will impact the production of biochar, a carbonaceous material made from the pyrolysis of organic waste material, such as biosolids, with properties similar to activated carbon. While the framework could possibly limit biochar manufacturing if contaminant levels are above the assessment's threshold, the conversion of biosolids into biochar could be a convenient route to reduce chemical concentrations in residual solids due to high temperatures and volatilization of certain constituents during manufacturing.
  • Consumer products disposal: The presence of many chemical contaminants, like PFAS, in consumer products is well known. However, it is unknown whether the disposal of such products (both by end users and by manufacturers) to landfills may be limited due to the framework.
  • New permits to discharge into wastewater treatment plants: The approved framework will likely limit how much biosolid material can be disposed of per area, which will in turn likely create financial and logistical struggles for wastewater treatment plants (WWTPs). To address this hurdle, WWTPs may consider creating specific permits to limit the load of various chemicals, such as PFAS, being discharged from industrial or domestic dischargers. While industrial discharger facilities commonly have facility-specific permits, domestic permits are uncommon as households are usually not a source of emerging or hazardous contaminants. However, due to the usage of various chemicals discharged into the WWTP collection system, domestic wastewater may be further regulated with specific permits or pretreatment programs.
  • Biosolids in reclamation sites: Reclamation sites are areas that have been highly disturbed and possibly contaminated in the past. Such sites may benefit from biosolids to restore their ecosystem due to the biosolids' ability to establish vegetation, reduce bioavailability of toxic substances, control soil erosion, and regenerate soil layers. Thus, the disposal of biosolids can be beneficial despite the presence of certain contaminants at low levels in the biosolids. Still, the approved framework may limit the quantity of biosolids that can be applied to reclamation sites.


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