This study presents a comparison of filter effluent particle counts and turbidity, total organic carbon (TOC) removal, and headloss development between full-scale biological and conventional filters, treating the same source water. Two water treatment plants in the San Francisco Bay Area were selected for this study. Both plants treat the same source water: Sacramento River via Cache Slough and the North Bay Aqueduct. The Fleming Hill Water Treatment Plant, however, has pre and intermediate ozonation with granular activated carbon/sand filters. The intermediate ozone application follows sedimentation and results in biologically active filters. The Benicia Water Treatment Plant, on the other hand, is a conventional (non-biological) plant, and a chlorine residual is maintained through the treatment process. Includes tables, figures.