With the adoption of European Community Directives regarding bathing water quality and the potential extension of bathing waters standards to all recreational waters, the disinfection of wastewater effluents has become an option of increasing interest. Ozone is a very powerful oxidant, capable of fast and effective disinfection. Due to its oxidizing potential, its application to final municipal effluents could have a beneficial effect on the effluent quality, although some concerns exist over the formation of potentially harmful by-products such as organic halides. This paper describes a preliminary laboratory-based study designed to investigate the changes in the physico-chemical properties of a secondary municipal effluent after ozonation, as well as the effect of ozone on the concentration of organic halides and effluent toxicity. The treatability and the performance of the system for different methods of ozone application also is evaluated. The effluent was treated with ozone generated from air and from oxygen. Using oxygen as a feed gas, three different ozone concentrations in the gas were examined, namely 1.5% w/w, 4.5% w/w and 13.5% w/w. The results showed that ozone affected the physico-chemical properties of the effluent by reducing the GOD, BOD, and color concentrations, and by increasing the dissolved oxygen concentration of the effluent. In the ozone dose range likely to be applied for disinfection, the amount of halogenated organic species increased. This increase, however, was not associated with changes in toxicity. At higher ozone doses, the organic halide concentration was found to decrease, whereas toxicity, although undetectable with the use of standard protocols, was found to increase. For a constant ozonation rate, the treatment performance was similar for all ozonation conditions. However, an increased ozone concentration in the feed gas increased the mass transfer rate of ozone to the liquid, resulting in a faster effluent treatment.
