Use Of Static Mixer For Oxidation And Disinfection By Ozone

Ozone is used in drinking water treatment as a biocide, as an oxidant and as a pretreatment in order to improve the performance of subsequent processes. Increasing concern over the quality of drinking water has led to a number of new stringent regulations in the control of chemical and microbiological contaminants. Disinfection deals with the concept of “CT”, which is the need to maintain a certain minimum concentration for a given time. Under ideal laboratory conditions, it is 0.4 mg O3/L for 4 min. In practice, since the method for the CT determination has not been finalized by the EPA, “T” can be the minimum detention time of 90% of total flow, and “C” can be a measured ozone residual at the outlet of cells of the contactor. New standards for micropollutants in drinking water imply an optimization of the ozonation step, by improving the ozone transfer from gas to water, and the control of the detention time as well as ozone residual within the contactor. All these considerations have led us to use static mixers to transfer ozone into water. This process enables us to control the ozone concentration in water and detention time. It is a very simple system, with very low maintenance requirements due to the lack of moving parts. Civil engineering is minimized. A pilot scale study is presented here. It took place at the Méry-sur-Oise water treatment plant, on a pilot plant working at 8-12 m3/h. It is composed of a static mixer for the transfer of ozone from gas to liquid, linked to an air lift to separate gas from liquid, providing ozonated water. The optimization of transfer was achieved by studying the impact of water flow, gas flow and ozone concentration in the gas. It is possible to reach 90% of transfer in less than 15 s. Headloss (ΔP) across the mixer is a function of gas and water flows and remains economically very acceptable as 0.15 bar for 12 m3/h. Atrazine removal was studied using a static mixer, an air lift and a contact pipe 80-m long, providing an optimum contact time phase, working as a plug flow reactor. Ozone and H2O2/O3 treatments were compared. The maximum reduction of atrazine concentrations (e.g., for an infinite contact time) is a function of the amount of transferred ozone, but H2O2 influences the kinetics of the reaction. In the presence of H2O2 with a ratio of H2O2 to O3 of 0.4 w/w, maximum elimination is reached in 2 min 30 s. The effect of such treatments on environmental bacteria also was followed. A counting of total germs at 20°C showed a decrease of 1- to 3-logs 10 after 1 min 30 s of contact time for about 2 mg/L of transferred ozone. No significant difference between treatments with or without H2O2 was shown. The same conclusions were obtained from heterotrophic plate counts (37°C) and epifluorescence countings.