Risk Assessments for "Round Two" Focusing on Dioxins and PCBs in Sewage Sludge
Dioxins refers to a group of chemical compounds that share certain chemical structures and biological characteristics. Several hundred of these chemicals exist and are members of three closely related families 1) chlorinated dibenzo-p-dioxins (CDDs); 2) chlorinated dibenzofurans (CDFs); and 3) certain polychlorinated biphenyls (PCBs). CDDs and CDFs are not manufactured intentionally by industry but are produced during combustion by human activities like the backyard burning of trash and natural events like forest fires. PCBs are manufactured products, but they are no longer produced in the United States.
Some industrial activities (e.g., bleaching of paper pulp) may have deposited dioxins in water that was subsequently treated at wastewater treatment plants (WWTPs). During the treatment process the dioxins attach to sewage sludge.
During the risk assessment in support of Part 503, the EPA further identified 31 pollutants and pollutant categories to be prioritized for the second planned rule (called “Round Two”). To select pollutants for Round Two, the EPA identified pollutants for further risk assessment using the results of the 1988 National Sewage Sludge Survey and a Preliminary Hazard Identification analysis. These 31 pollutants were further narrowed using a Comprehensive Hazard identification study. As a result of this evaluation, in 1995 the EPA shared that it was considering proposing regulations for dioxins and PCBs in Round Two. In 1999 the EPA proposed pollutant limits for dioxins and PCBs in sewage sludge based on the results of the 1988 National Sewage Sludge Survey and a refined risk assessment.
- Read the 1999 proposed rule which includes an explanation of the risk assessment methodologies
After the public comment period for the proposed rule, the Agency decided to conduct an updated national sewage sludge survey (NSSS) to obtain more recent occurrence and concentration data for dioxins and PCBs in sewage sludge. Using the results of the 2001 NSSS and other updates to the refined risk assessment, the EPA published a decision not to regulate dioxins and PCBs in sewage sludge.
- The AMSA 2000/2001 Survey of Dioxin-like Compounds in Biosolids
In October 2000, the Association of Metropolitan Sewerage Agencies (AMSA) (now called National Association of Clean Water Agencies or NACWA) began a voluntary survey to determine current levels of dioxin-like compounds in sewage sludge. These data were collected to help determine whether dioxin concentrations in sewage sludge had decreased since earlier surveys. The survey was open to all publicly owned treatment works in the U.S. After this survey, the EPA conducted their own survey. - Learn more about the 2001 National Sewage Sludge Survey
- Technical materials for "Round Two" risk assessment:
- Final decisions not to regulate dioxins in sewage sludge and the supporting documents: