A high frequency electrolysis method was used to reduce the calcium in circulating water by precipitating CaCO 3 on the electrodes. The oxidation reduction potential, pH, and chemical composition were measured during the precipitation of the CaCO 3 . The calcite precipitated on the electrode cover. The concentration of calcium ions in the solution decreased under reducing conditions, and the precipitation rate of calcite depended on the pH. For initial Ca and HCO 3 concentrations of 200 ppm, the maximum precipitation rate of CaCO 3 was 2.4 mg/m 2 /s at pH = 6.8, and the precipitation rate decreased linearly as the pH decreased to pH = 4.6. The precipitation rate depended on the initial concentrations of calcium and HCO 3 , rather than being regulated by the initial pH using citric acid.