The U.S. Environmental Protection Agency (EPA) has developed a comprehensive risk-based rule to protect the public health and the environment from reasonably anticipated adverse effects of pollutants that may be present in biosolids (sewage sludge).  Commonly known as the Part 503 rule, the regulation (40 CFR 503) was published in the Federal Register on February 19, 1993.  Much of the rule was based on the results of risk assessments that were scientifically conducted to identify what, if any, risks were associated with the use or disposal of biosolids.  This guide provides an understanding of the risk assessment process that was conducted as a basis for the Part 503 biosolids rule.

The risk assessment for the Federal Part 503 rule that governs the land application of biosolids took nearly ten years to complete and had extensive rigorous review and comment.  The risk assessment did evaluate and establish limits for a number of pollutants. These limits may be found in chapter four of the EPA publication A Guide to the Biosolids Risk Assessments for the EPA Part 503 Rule.  In the process of establishing regulatory limits, EPA compared the relevant toxic's exposure data, which was obtained from a cross section of representative studies, to the appropriate oral reference dose (RfD),  human cancer potency (Q1*) values, and relevant toxicity endpoints for terrestrial and aquatic organisms (i.e., the allowable dose of each pollutant).  These exposures were evaluated via 14 pathways of exposure with the most limiting pathway being chosen as the limit.  In spite of all the safety factors and uncertainty factors built into the process, environmental and human health risks were found to be low.

• A Guide to the Biosolids Risk Assessments for the EPA Part 503 Rule (PDF)(158 pp, 21 MB, September 1995, EPA 832-B-93-005)