MODELIZATION OF MICROPOLLUTANT REMOVAL IN DRINKING-WATER TREATMENT BY OZONATION OR ADVANCED OXIDATION PROCESSES (O-3/H2O2)

A simulation program is described, tested and used, to predict micropollutant removal in an ozonation bubble tower with or without hydrogen peroxide addition. To compute the removal efficiency, we need to know the chemical reactivity between organic compounds and oxidant species (molecular ozone and hydroxyl radicals), the ozone mass transfer from the gaseous phase to the liquid phase (k(L)a) and the hydrodynamic model describing the reactor. In this case, we divide the reactor into three parts (water arrival, air arrival and intermediate zones). Each part is modelled using completely stirred tank reactors in series (CSTR). In each CSTR, the calculation of oxidant concentrations (O-3, H2O2) is made through mass balance equations and a semi-empirical formula which gives hydroxyl radical concentrations as a function both of ozone concentration and the main characteristics of the water to be treated (pH, TOC, alkalinity). Another semi-empirical formula links ozone consumption to the same characteristics. A comparison between the computed data with information taken from three publications on atrazine pesticide removal shows a good correlation between experimental and computed results (Figures 2-3). Therefore, this program can be used to simulate the influence of the following main parameters of the process on micropollutant elimination: - contact time and k(L)a (Table 2), - treatment rate and H2O2/O-3 ratio (Figure 4), - flow pattern and point of H2O2 injection (Table 3), - pH (Figure 5), - alkalinity (Figure 6), - type of micropollutant (Figure 7). The conditions providing the best results occur for an H2O2/O-3 ratio between 0.2 and 0.35 g/g (according to residual hydrogen peroxide levels), and a pH between 7.7 and 8.3. Bicarbonate and carbonate ions also play a very important role and raw waters from chalk and limestone areas are unfavorable. Finally, the variation of the elimination of organic products is due to differing kinetic constants between micropollutant and hydroxyl radicals.