Risk Assessment of Pollutants in Biosolids
What does the EPA mean by risk?
The EPA considers risk to be the chance of harmful effects to human health or to ecological systems resulting from exposure to an environmental stressor. A stressor is any physical (e.g., radiation), chemical (e.g., pesticides), or biological entity (e.g., microbes) that can induce an adverse response. EPA uses risk assessments to characterize the nature and magnitude of health risks to humans and ecological receptors (e.g., plants and animals) from chemical contaminants and other stressors.
Learn more about risk and EPA’s risk assessment process.
Assessing Pollutants Found in Biosolids through Risk Assessment
40 CFR Part 503 was developed based on the results of risk assessments to identify what, if any, risks were associated with the use or disposal of biosolids via land application, surface disposal, or incineration.
The top priority of the EPA’s Biosolids Program is to assess the potential human health and environmental risk posed by pollutants found in biosolids. Pollutants found in biosolids vary in space and time, depending on industrial and other inputs to individual wastewater treatment facilities. The presence of a pollutant in biosolids alone does not necessarily mean that the biosolids pose risk to human health and the environment (i.e., the pollutant has to present at a level above which toxic effects are known or anticipated to occur). Potential harm from a pollutant is determined by conducting a risk assessment. Risk assessment is a scientific process that considers two primary factors: 1) exposure (i.e., how much contact a person or ecological receptor, such as plants or fish, has with the contaminant in environmental media due to contamination of biosolids); and 2) the toxicity of the pollutant.
The Biosolids Program has developed a framework to evaluate risks from exposure to chemicals in biosolids. The framework consists of three steps: 1) prioritize chemicals for assessment, 2) screen for human health and environmental risk, and 3) perform a refined risk assessment for chemicals that fail the screen. The Biosolids Program has requested that EPA’s Science Advisory Board (SAB) review this proposed framework and provide input on the approach. The risk assessment approach was reviewed by the SAB and the final reported was published in October 2023.
- Science Advisory Board: Approach to Biosolids Chemical Risk Assessment and Biosolids Screening Tool
- Download the Biosolids Screening Tool (BST) (zip) and BST Supporting Documents (zip) . The BST is a multimedia, multipathway, multireceptor deterministic, problem formulation, and screening-level model that can estimate high-end human and ecological hazards based on potential exposures associated with land application of biosolids or placement of biosolids in a surface disposal unit. The results can be used to identify pollutants, pathways, and receptors of greatest interest and to inform decisions about the need to perform more refined modeling or to address data gaps or uncertainties. The BST supporting documents provide additional details for model evaluation and the sources for literature references. A user guide for the BST is provided in the folder labeled 'Help'.
- A Guide to the Biosolids Risk Assessments for the EPA Part 503 Rule
This guide was prepared to provide an understanding of the risk assessment process that was conducted as a basis for the Part 503 biosolids rule. The document 1) describes the risk assessment procedures used to develop Part 503 pollutant limits; 2) provides historical accounting and discussion of the steps taken during risk assessment; 3) discusses the issues that arose during the risk assessment process and their resolutions; 4) explains assumptions and policy decisions; and 5) answers commonly asked questions.