Comparative ozone decolouration in bubble column and in ejector mixer units

Ozone treatments are very effective to reduce the residual coloration in wastewater from wet textile processes. The oxidative treatment is very flexible as a result of the easy and prompt regulation of the ozone generator, together with the high solubility of the reactant. Two equipment configurations were considered to evaluate the 03 mass transfer to wastewater. The two schemes are competitive since the ejector mixer prevails in terms of local mass transfer coefficient, while the bubble column has a more favourable hydrodynamics as far as the liquid phase is concerned, since a partial plug flow between the liquid and the gas phase has been demonstrated. The two units, which appear below, were compared in laboratory and in the field tests by operating at strictly controlled conditions. In the laboratory synthetic exhausted dye-liquors were prepared and fed in parallel to the two units by setting a residence time of approximately 30 min. When the experimentation was carried out in the field, real wastewater from a dye house was treated at the conditions suggested by the laboratory tests. Oxygen was fed to the generator at a rate of 200 NL/h; ozone was produced to have a dosage of 30 to 80 g/m3 of treated water. The industrial experimentation has revealed almost identical performances in terms of colour removal (> 98% in all trials), COD reduction (in the range from 15 to 45%), surfactants reduction (from 70 to 95% of the influent) in the two units. A certain performance difference was evident depending on the class of dyes considered. As a fundamental study, the bubble unit was investigated to draw the ozone concentration profile along the column height and to measure the off-gas ozone concentration. A technological conclusion suggests that bubble systems are favoured due to their simple structure and to the overall energy requirement.