designed to control and reduce pathogens and attraction of disease vectors (insects or other organisms that can transport pathogens). In this report, the term biosolids refers to sewage sludge treated to meet the land-application standards in the Part 503 rule or any other equivalent land-application standards.

The chemical and pathogen land-application standards in the Part 503 rule were developed differently. For chemicals, EPA conducted extensive risk assessments that involved identifying the chemical constituents in biosolids judged likely to pose the greatest hazard, characterizing the most likely exposure scenarios, and using scientific information and assumptions to calculate concentration limits and loading rates (amount of chemical that can be applied to a unit area of land). Nine inorganic chemicals in biosolids are currently regulated, and EPA is considering the addition of a class of organic chemicals (dioxins) to its regulation. Monitoring data on some of the regulated inorganic chemicals indicate a decrease in their concentrations over the past decade, due in part to the implementation of wastewater pretreatment programs. Thus, the chemical limits for biosolids can be achieved easily. In contrast to the chemical standards, the pathogen standards are not risk-based concentration limits for individual pathogens but are technologically based requirements aimed at reducing the presence of pathogens and potential exposures to them by treatment or a combination of treatment and use restrictions. Monitoring biosolids is required for indicator organisms (certain species of organisms believed to indicate the presence of a larger set of pathogens).

THE COMMITTEE’S TASK

In response to the Clean Water Act requirement to reassess periodically the scientific basis of the Part 503 rule and to address public-health concerns, EPA asked the National Research Council (NRC) to conduct an independent evaluation of the technical methods and approaches used to establish the chemical and pathogen standards for biosolids, focusing specifically on human health protection and not ecological or agricultural issues. The NRC convened the Committee on Toxicants and Pathogens in Biosolids Applied to Land, which prepared this report. The committee was asked to perform the following tasks:

  1. Review the risk-assessment methods and data used to establish concentration limits for chemical pollutants in biosolids to determine whether they are the most appropriate approaches. Consider the NRC’s previous (1996) review and determine whether that report’s recommendations have



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