A study on advanced drinking water treatment was conducted in a pilot scale plant taking raw water from Minaga Reservoir, Japan. Ozonation-biological activated carbon process (O3-BAC), BAC process (BAC) and chlorination-granular activated carbon adsorption process (Cl2-GAC) were evaluated based on the following parameters; dissolved organic carbon (DOC), adsorbable DOC (ADOC) and biodegradable DOC (BDOC). The raw water used was pre-treated by biofiltration for suspended solid removal and biological treatment. Contact time for ozonation and chlorination was 24 min. Empty bed contact time of activated carbon column was 15 min. Ozone dosage was 3 mg • 1−1. Chlorination was carried out to maintain a residual chlorine concentration of 0.2 mg • 1−1. Effluent DOC from biofiltration consisted of mainly ADOC (90%) and BDOC (10%). Therefore, DOC was mainly removed by adsorption in all processes. However, biodegradable DOC increased by 20% after ozonation in O3-BAC and was removed effectively by the attached bacteria on the activated carbon. Significant differences in DOC removal were not noted among the three processes during 8 months of operation. After the saturation of activated carbon, the effluent DOC from the O3-BAC was lower than that from BAC and Cl2-GAC. Ozonation improved biodegradability of organic substances and effluent water quality by the following biodegradation of biodegradable DOC by biological activated carbon.