The use of ozone and benzoic acid to enhance treatment performance of conventional slow sand filtration is presently under evaluation. Ozone at doses of 0.5 and 1.0 mg O3 consumed/mg nonpurgeable dissolved organic carbon (NPDOC) and benzoate at doses of 0.05 and 0.1 mg benzoate/mg NPDOC were evaluated during a 29 day filter run in December 1989-January 1990. Six pilot filters were operated at a filtration rate of 0.05 m/h in configurations designed to evaluate the separate and interactive effects of ozone and benzoate addition on treatment efficiency. Head loss during the run was exponential for all the filters, except the control, indicating that a cake-like filtration process was occurring in the schmutzdecke at the water column-filter media interface. Filters receiving either the high ozone dose or the high ozone and high benzoate dose provided better treatment compared to the control, high benzoate, or low ozone and low benzoate filters. Direct oxidation by ozone played a more significant role in reducing UV absorbance and THMFP than biological filtration. NPDOC removal was more influenced by biological filtration than direct oxidation. Ozonation tended to decrease the UV absorbance across the entire molecular weight (mw) spectrum (1K daltons to 100K daltons); redistribute the NPDOC into the lower mw fraction; increase carboxylic acidity and hydrophilicity; and increase the biodegradable organic carbon (BDOC) content of the raw water. Biological filtration tended to remove the more functionalized, lower molecular weight, hydrophilic material. The addition of benzoate did not significantly enhance filter performance compared to the control in this filter run. Additional pilot filter runs at the Portsmouth, New Hampshire and Andover, Massachusetts facilities are underway or planned to evaluate seasonal and source water effects.