To study the possibility of ozone application for water supply disinfection including the effect of organic substance and turbidity on the efficiency of inactivation of coliform with ozone. The research procedures were divided into 2 steps, first, using synthetic water for reverse osmosis at the original coliform quantity of 3.5x10[superscript 7] CFU/100ml by adding different concentration of both humic acid indicating organic substance and kaolin indicating turbidity, and the second, using both non-filtered and filtered water from water supply treatment process by adding the same dose of coliform added in synthetic water. The first result was that efficiency of inactivation of coliform was more than 5 log units. The reverse osmosis water had Ct value of 2.28 (mg/l)xs which was the least, this also took a shortest time in inactivation of coliform. The reverse osmosis water added kaolin with turbidity 5, 10, 15, 20 NTU had approximately Ct value of 2.33, 2.31, 2.31, 2.29 (mg/l)xs respectively while the reverse- osmosis water added humic acid with concentration 1, 3, 5, 7 mg/l had increasing Ct value of 4.63, 14.40, 30.27, 53.44 (mg/l)xs respectively. The second result was found that the non-filtered and filtered water from water supply treatment process had approximately Ct value of 8.10 and 8.13 (mg/l)xs respectively. This showed that turbidity had significantly no effect on inactivation of coliform with ozone. For the effect of organic substance, the UV 254 value increased as Ct value was increased also. The UV 254 value of water reduced more than 90% when the water had ozonation time of 300 seconds. It showed that ozone could decrease the organic substance with UV 254, still remaining TOC value constant. For the residual chlorine in the water having ozonation was more than that in the water having no ozonation when they are given the same amount of chlorine. Ozone application for water supply disinfection is an alternative way that is interesting according to the result, the efficiency of inactivation of coliform with ozone is more than 5 log units and it takes approximately Ct value of 8.1. Moreover ozone can reduce the organic substance in the front of UV 254 and the possibility of Trihalomethanes formation.
