Actual seawater using a reverse osmosis (RO) desalination process. Two types of RO membrane elements were used, cellulose acetate hollow fiber membrane element, and aromatic polyamide spiral wound membrane element. Under a recovery rate of 40%, the salt rejection was 99.7% for the cellulose acetate membrane, and 99.5% for the aromatic polyamide membrane. The amount of THMs found in RO feed water ... [Show full abstract] during the chlorination process was 15~31 μg/L mainly constituted bromoform. The concentration of THMs in the permeate became greater in case of using the cellulose acetate membrane, a 1.2- to 1.5-fold increase comparing the same to that in the feed water, indicating that there was no removal of THMs by RO membrane. In contrast, the concentration of THMs in the permeate for the case of the aromatic polyamide membrane decreased by approximately 10% to 20%. There was almost no formation of THMs found during the chloramine process.