Change of the Oxidation-Reduction Potential of Model and Natural Waters in the Ozone Disinfection Process

The successful use of ozonation for water disinfection and purification requires the determination of optimal treatment conditions, which is associated with a large amount of experiments. At the same time, traditional methods for controlling the process of inactivation of microorganisms, being quite laborious and time consuming, allow one to obtain data on the degree of water disinfection only after 18–48 h after the experiment and sampling. Given that ozone is an oxidizing reagent and its presence in the treated water causes a change in the oxidation-reduction potential (ORP) of the solution, this study is aimed at changing the ORP during ozone disinfection of water and at investigating the possibility of express control of this process. As objects of study, we have used model solutions containing 104–107 CFU/cm3 of Escherichia coli and various concentrations (0–20 mg/dm3) of humic acids (HA) in a 0.05 M phosphate buffer at pH 6.8, as well as real natural waters of the Dnieper and Desna Rivers. The ozone concentration in the ozone–air mixture varies in the range of 0.5–2.0 mg/dm3, depending on the particular object of study. It is shown that changes in the ORP of a solution reflect changes in the microbiological and chemical composition of water during ozonation, owing to which measurements of this indicator can serve as a preliminary control method for the process of disinfection, as well as water decoloration. The inflection point on the curve of dependence of ORP on ozonation time practically coincides with the inflection point on the curve of dependence of the degree of water disinfection on the treatment time. In the case of treatment of model waters that are free of organic impurities, a sufficient level of disinfection of the solution is evidenced by an increase in the ORP of the solution to about 440–550 mV. In case of treating natural surface waters (with a high content of natural organic impurities), a sufficient level of disinfection and decoloration of the solution is detected by a decrease in the ORP of the solution by about 30–40 mV.