This project evaluates a method for the removal of trihalomethane (THM) precursors from surface water sources. A pilot plant was operated for 80 weeks to test the combination of ozone and granular activated carbon (GAC). This combination is sometimes referred to as biological activated carbon (BAC). An important objective was to investigate the possibility of microbiological degradation of THM precursors in the GAC columns and the effect of preozonation on this process. Analysis of the pilot plant data shows microbiological activity to be a significant contributor to the removal of total organic carbon (TOC) and trihalomethane formation potential (THMFP) in GAC columns under the conditions tested. Comparison of costs associated with the addition of GAC and BAC to traditional water treatment plants of 100-, 10-, and 1-mgd capacities shows that, for the conditions of this study, the addition of ozone was not cost effective in extending the time between reactivations of the GAC.