This study was carried out to optimize a dual media BAC (biological activated carbon) process for DOC removal and DBPs (disinfection by-products) control. Pilot scale tests were coducted at the Tukdo water treatment plant in Seoul, Korea. The dual media BAC process is highly efficient in the removal of DOC and THMFP, and is more capable of sustaining microorganisms than a single layer filter. The bottom layer of the sand filter functioned as a screen for turbidity, microorganisms, and other biological material. Total DOC removal efficiency was 13 to 25%, and the corresponding THMFP (trihalomethane formation potential) removal efficiency was approximately 20 to 33%. Turbidity and DOC leakage continued for 40 min after backwashing in all reactors. Breakthrough occurred from 24-72 hours in each reactor. Ten species of microorganisms were identified in the activated carbon filled in BAC reactors. The predominant species were Clavibactor and Corynebacterium and microbial species were simple at the lower end.