This paper presents results of a plant-scale study examining the impact of biofiltration following chlorination for the control of haloacetic acids (HAA) concentrations in finished drinking water. At the beginning of this study, the raw water was pre-chlorinated, coagulated, and settled before being filtered either through sand filters or granular activated carbon (GAC)/sand filters. A free chlorine residual was maintained across the sand filters, but the applied chlorine to the GAC filters dissipated across the filter bed. Early in the study, the water treatment plant switched to pre-chloramination of its raw water to control the extent of DBP production during pre-treatment. Again, a chlorine residual (this time a combined residual) was maintained across the sand filters but not always across the GAC bed. The filtered water from each set of filters was subsequently blended, then treated with additional chlorine and ammonia before being pumped into the distribution system. HAAs and THMs were formed as a result of pre-chlorination and are still formed, albeit to a lesser degree, as a result of pre-chloramination. Includes 8 references, tables.