A research study to monitor the micropollutant levels present in the Han River, a major drinking water source for the Seoul Metropolitan district in Korea, was performed over a five-year period from 2000 to 2004, inclusive. Of the detected micropollutants, phthalates were found to be the major contaminants. In this study, the estrogenic activities of the detected phthalates and raw water samples contaminated with the pollutants were assessed by the E-screen assay using the MCF-7 breast cancer cell line. Of the phthalates, diethyl phthalate (DEP) and di(2-ethylhexyl) phthalate (DEHP) showed relatively high cell proliferation. Using DEP as a phthalate probe, three candidate processes, ozone alone, UV alone, and the ozone/UV combined, were evaluated for their efficiency in removing DEP and reducing its estrogenic activity. The ozone/UV process was shown to have the highest efficiency for the elimination of DEP and its by-products, leading to the complete mineralization of DEP. This study also found that the ozone/UV process is the best candidate to reduce the estrogenicity induced by DEP and its by-products.