Biofiltration is a treatment technique in which a granular media filter is allowed to be biologically active for the purpose of removing biodegradable constituents such as Total Organic Carbon (TOC). Biofiltration will occur in any filter that does not have a free chlorine residual or other inhibitor of natural biological growth. Consequently, Granular Activated Carbon (GAC)filters typically are biofilters since GAC dechlorinates water. Biological Activated Carbon (BAC) refers to the use of GAC, operating in both biological and adsorption modes. In this manner, dissolved organics are removed from solution by adsorption and biodegradable organic material is mineralized, ultimately to carbon dioxide and water. When the biological mode is optimized, the useful life of the GAC is extended greatly (by factors of 2 to 5) before reactivation is required, resulting inoperational cost savings. Biofiltration can be enhanced by chemical oxidation prior to filtration. By using ozonation, notonly are some dissolved organics converted to simpler, readily biodegradable organics, but primary disinfection credit, oxidation of iron and manganese, color removal and taste and odor oxidation/removal can be accomplished in varying degrees at the same time. By removing total organic carbon, biofiltration assists in compliance with the Stage 1 and 2 Disinfectant/Disinfection Byproduct Regulations. Ozone use was implemented for different reasons at the four facilities discussed in this paper. The full-scale performance of the ozonation at these facilities and the process of biofiltration are discussed. Includes 11 references, figures.