Risk Assessment of Pollutants in Sewage Sludge
What does the EPA mean by risk?
Risk assessment is a scientific process that is used to characterize the nature and magnitude of health risks to humans (i.e., children and adults) and ecological receptors (i.e.., aquatic and terrestrial plants and wildlife) from pollutants. An environmental risk assessment considers three primary factors:
- Presence (i.e., how much of a pollutant is present in the environment);
- Exposure (i.e., how much contact humans or wildlife have with the pollutant); and
- Toxicity of the pollutant (i.e., the health effects the pollutant causes in humans or wildlife).
The concentration of pollutants found in sewage sludge varies across space and time, depending on industrial and other inputs to individual wastewater treatment plants (WWTPs). The presence of a pollutant in sewage sludge alone does not necessarily mean that there is risk to human health or the environment from its use or disposal. The U.S. Environmental Protection Agency estimates potential exposures to humans and environmental receptors by modeling the fate and transport of a pollutant through the environment, taking into account different environmental conditions and exposure scenarios, and then estimates risk by comparing those potential exposures to toxicity values.
Assessing Pollutants Found in Sewage Sludge through Risk Assessment
The top priority of the EPA’s Biosolids Program is to assess the potential human health and environmental risk posed by pollutants found in sewage sludge. After identifying pollutants in sewage sludge, the EPA has proposed a framework that includes prioritization, screening, and refined risk assessment to evaluate potential risk from chemicals found in sewage sludge.
Identifying Chemicals in Sewage Sludge
The EPA reviews sewage sludge regulations to identify chemicals that may occur in sewage sludge. To date, the EPA has found over 700 chemicals in sewage sludge. This number is based on the EPA’s nine biennial reviews of published literature covering 2004 through 2021 and three EPA national sewage sludge surveys conducted from 1988 to 2006.
- Biennial Reviews of Sewage Sludge Standards search publicly available peer-reviewed literature to identify additional pollutants that have been found in sewage sludge and collect data on those pollutants. Reports are published every two years.
- Sewage Sludge Surveys collect nationally representative occurrence data on pollutant concentrations in sewage sludge from wastewater treatment plants.
Prior to 2021, there was inconsistent reporting of chemicals identified and the EPA did not cumulatively track chemicals found in sewage sludge. In 2022, the EPA published an article describing the curation process to produce a list of the chemicals found in sewage sludge based on the eight biennial review reports and three national sewage sludge surveys published at that time. Through the efforts presented in the article, the EPA produced a list of chemicals and structure-based classes found in sewage sludge based on biennial reviews and national sewage sludge surveys.
- EPA's CompTox Chemicals Dashboard: List of Chemicals Detected in Biosolids
- Scientific Article: Curation of a list of chemicals in biosolids from EPA National Sewage Sludge Surveys & Biennial Review Reports
Prioritization and Screening of Chemicals found in Sewage Sludge
In 2021, the Biosolids Program requested that the EPA’s Science Advisory Board (SAB) form a panel and review the prioritization and screening approach as part of the proposed framework for assessing risk and provide input. The EPA proposed a transparent and reproducible approach that identified low and high priority chemicals for further risk analysis using publicly available data on concentration, toxicity, persistence, and mobility (among others). In this proposed approach, high priority chemicals would be screened for risk using a highly exposed population (i.e., a self-sufficient farm family). Chemicals that showed risk in the screening analysis (or “failed the screen”) would then advance to the refined risk assessment phase.
In this proposed approach, the results of the screening analysis would not be regulatory standards or be a substitute for a refined risk assessment. Instead, the goal of the prioritization and screening approach was to determine which chemicals do not pose any risk to even the highly exposed populations, meaning the EPA can confidently exclude them from further risk analysis and potential regulation.
The EPA is working to incorporate the SAB comments and expects to release a response to the comments and revised prioritization and screening framework in 2025.
The Biosolids Screening Tool (BST) is a multimedia, multi-pathway, multi-receptor 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'.
- Download the Biosolids Screening Tool (BST) (zip) and BST Supporting Documents (zip) .
Refined Risk Assessments of Pollutants in Sewage Sludge
In the refined risk assessment process, the EPA uses fate and transport models to estimate environmental concentrations of pollutants that are possible from common sewage sludge use and disposal practices in the US. The EPA then calculates potential risks to humans, wildlife, livestock, or crops associated with exposure to those pollutants. To date, the EPA has published two refined risk assessments for pollutants found in sewage sludge.