The Surface Water Treatment Rule (SWTR) will require filtration of most surface waters, as the EPA recognizes filtration and disinfection as methods to meet specified goals for the removal and/or inactivation of a variety of microorganisms of health concern, including viruses and Giardia cysts. These biological 'particles' vary greatly in size (i.e., 0.05 μm versus 10 μm), as well as other characteristics. This, and the fact that many previously unfiltered supplies of high quality may be treated by direct (coagulation, flocculation, filtration) or contact (coagulation, filtration) filtration, suggests the need for an improved understanding of the role of suspended particle size and flocculation in filtration performance. The objective of this study was to investigate the effects of chemical addition and flocculation duration on filtration performance as measured by particle removal and head loss development.