The U.S. EPA's Municipal Environmental Research Laboratory is assessing the treatability and removability of toxic substances from municipal wastewater by various treatment processes. The studies feature occurrence surveys, treatability studies to assess individual toxic removal mechanisms and pilot-scale plant removal studies using wastewater spiked with selected priority pollutants (metals and organics). In the plant removal studies, the treatment plant performance on spiked wastewater is usually compared to the performance of identical treatment on the unspiked raw wastewater. The assessments employ costly analyses (GC/MS and atomic adsorption methods) for the selected toxic substances in the various process streams and sludges of treatment plants. A biomonitoring approach to assess health and ecosystem effects is also being evaluated to supplement the specific toxic substance removal data. From the studies to date, conventional primary-activated sludge treatment is generally effective in removing toxic substances, typically achieving better than 90 percent removal of organics and from 60-80 percent removal of the metals. However, a number of the toxic substances were found in the plant final effluents at concentrations which, based upon EPA recommended water quality criteria may present a possible environmental hazard.