Reuse of process water in the flotation of sulfide ores has crucial importance for environmental sustainability and improved process economy. However, the chemistry of process water may be a critical subject for the flotation efficiency as the dissolved ion concentration increases with water reuse. In this study, the effects of water reuse on the flotation efficiency of a Cu-Zn sulfide ore were investigated. The flotation flowsheet consists of a pre-flotation section to remove the naturally floatable talc particles, and sequential copper and zinc flotation sections. Calcium, sulfate, thiosulfate, and xanthate were found as major contaminating ionic species in the process water discharged from flotation circuits. The flotation tests with recirculated water from the zinc rougher tailing revealed that the presence of residual xanthate caused unintentional activation of copper minerals in the pre-float section. Copper recovery increased in the pre-flotation section and resulted in the loss of copper to the pre-float concentrate, which is considered as a tailing stream in the current flowsheet. Various types of activated carbon samples were tested to remove the residual xanthate from the tailing water. The carbon samples could be regenerated by heat treatment and reused for water treatment. Performance of the activated carbon samples was directly related to the pore size and surface area. Carbon-treated tailing water could be re-used in flotation without affecting the flotation performance.