INFLUENCE OF OZONATION CONDITIONS ON THE TREATABILITY OF SECONDARY EFFLUENTS (REPRINTED FROM THE ROLE OF OZONE IN WASTE-WATER TREATMENT, 1997)

With the adoption of European Community Directives regarding bathing w ater quality and the potential extension of bathing waters standards t o all recreational waters, the disinfection of wastewater effluents ha s become an option of increasing interest. Ozone is a very powerful ox idant, capable of fast and effective disinfection. Due to its oxidizin g potential, its application to final municipal effluents could have a beneficial effect on the effluent quality, although some concerns exi st over the formation of potentially harmful by-products such as organ ic 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 eff ect of ozone on the concentration of organic halides and effluent toxi city. The treatability and the performance of the system for different methods of ozone application also is evaluated. The effluent was trea ted with ozone generated from air and from oxygen. Using oxygen as a f eed 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 ozo ne affected the physico-chemical properties of the effluent by reducin g the GOD, BOD, and color concentrations, and by increasing the dissol ved oxygen concentration of the effluent. In the ozone dose range like ly to be applied for disinfection, the amount of halogenated organic s pecies increased. This increase, however, was not associated with chan ges in toxicity. At higher ozone doses, the organic halide concentrati on 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 ozonatio n conditions. However, an increased ozone concentration in the feed ga s increased the mass transfer rate of ozone to the liquid, resulting i n a faster effluent treatment.