[en] The US Nuclear Regulatory Commission (NRC), in accordance with 10 CFR 20, and agreement states, in accordance with state regulations, regulates the discharge of radioactive materials into sanitary sewer systems. A one-year study was conducted by Pacific Northwest Laboratory (PNL) for the NRC to assess whether radioactive materials that are dischared to sanitary sewer systems undergo significant reconcentration within the wastewater treatment plants (WWTP) and to determine the physical and/or chemical processes that may result in radionuclide reconcentration within the WWTPs. The study objectives were addressed by collecting information and data on wastewater treatment, relevant geochemical processes, and individual radionuclide behavior in WWTPs from the open literature, NRC reports, EPA surveys, and interviews with NRC licensees and staff of WWTPs that may be impacted by these discharges. Radionuclide mass balance and removal efficiencies were calculated for WWTPs at Oak Ridge, TN; and Erwin, TN, but were not shown to be reliable since the licensee release data generally underestimated the mass of radionuclide that was ultimately found in the sludge. This disparity may be due, in part, to the fact that data available for use in this study were collected to address regulatory concerns and not to perform mass balance calculations. A limited modeling study showed some promise for predicting radionuclide behavior in WWTPS, however, the general applicability of using these empirical models remains uncertain. With the data and models currently available, it is not possible to quantitatively determine the physical and chemical processes that cause reconcentration or to calculate, a priori, reconcentration factors for specific WWTP unit processes or WWTPs in general