Surface-water treatment plants are required by the Surface Water Treatment Rule to remove viruses and Giardia lamblia cysts or to inactivate them to specific levels. Particle removal data can be used to demonstrate the level of Giardia removal provided by the treatment system. This article presents the results of particle removal by two clarification-filtration treatment trains and investigates the effect of preoxidation on process performance. The enhanced removal of particles during filtration, when ozone was used as the preoxidant, was a significant advantage of ozone over chlorine, chlorine dioxide, and potassium permanganate. Includes 24 references, tables, figures.