For demineralization, the reverse osmosis (RU) permeate of geothermal water was further treated with an electrodeionization (EDI) system where the ion exchange resins were filled in the mixed bed configuration into the central compartment. The effect of the process parameters such as applied voltage and membrane type on the quality of the product water was examined. The results show that the conductivity of the product water increased when a high potential is applied to the system. When a voltage of 25 V was applied to the system, the conductivity of feed water decreased from 15 mu S/cm to 0.6 mu S/cm. At 40 V of applied voltage, conductivity of feed water increased from 15 mu S/cm to 28 mu S/cm. The thickness of the membrane was also an important parameter for the quality of product water obtained. When the thin membranes (Neosepta-AMX and Neosepta-CMX) were used, the conductivity of the product water increased from15 mu 5/cm to 24 mu S/cm. (C) 2013 Elsevier B.V. All rights reserved.