Abstract: The present invention relates to an apparatus and a process for disinfecting polluted water to supply water suitable for drinking, swimming water, or industrial use. The apparatus and process according to the invention disinfect water by using ClO.sub.2 as a disinfectant. The ClO.sub.2 is generated by an electrolytic generator comprising an anodic cell and a cathodic cell having an anode and a neutral electrode in the anodic cell and a cathode in the cathodic cell. A sensing element is arranged in the reactor to detect the ClO.sub.2 content in the disinfected water. A monitor connected to the sensing element and a regulator for the electric power source adjusts the voltage and current supplied to the anode of the electrolytic generator in response to the signal from the sensing element so as to continuously provide a suitable amount of ClO.sub.2 to effectively disinfect the polluted water. The apparatus and process of the present invention reduces electric power consumption.