Research on determination of calcium oxide (CaO) optimum concentration as pH regulator on water treatment system has been done. Water sample with pH 4.3, conductivity 530 pS/cm, TDS 400 ppm was treated with alum coagulant with concentration of 40 ppm and calcium oxide with various concentration of 40 ppm, 80 ppm, 120 ppm, 160 ppm, 200 ppm, 240 ppm to find the optimum concentration of calcium ox…