Modelling Ozonation Processes for Disinfection By-Product Control in Potable Water Treatment: From Laboratory to Industrial Units

__________________________________________________________________________ Given the stringent regulations on Disinfection By-Product (DBP) formation levels (10 μg.L-1 in the case of bromate ions), process modellers are seeking an in-depth understanding of all chemical phenomena occurring during ozonation. This remains an uneasy task since ozone may, directly via molecular reactions involving ozone or indirectly via reactions involving radicals formed during ozone decomposition, interact with a variety of species naturally present in water. Facing this complexity, two approaches were considered. On one hand, numerous authors have proposed integrated approaches, for which the main influent experimental factors were characterised and reduced to empirical laws with one or more adjustable parameter(s). The results of empirical laws are restricted to the calibration domain of the model, which tends to be very narrow for natural waters. On the other hand, the possibilities offered by modern experimental techniques, such as pulse radiolysis, fostered the advances in fundamental kinetics for all reactions in which ozone may be involved. Hence, several mechanistic models are available in literature for the following phenomena: ozone self-decomposition, reactions with alkaline species, reactions for the formation of bromate ions, reactions with micropollutants. Despite the fact that many phenomena have been understood and modelled, ozone chemistry remains for a large part unknown when dealing with natural waters. The fractionation techniques used to individually study the interactions of ozone – and the main radical species – with classes of natural substances (humic, fulvic, tanic acids...) gave interesting results with artificially recreated waters. However, no conclusive evidence of a possibility to extrapolate such results to natural waters could be given. Moreover, fractionation techniques remain difficult to systematically carry out in order to characterise water samples. Authors have therefore proposed empirical or semi-empirical models for the role of Natural Organic Matter (NOM) based on rough chemical considerations (division of the NOM into several fractions with distinct roles). As a consequence, mechanistic models available in the literature can be chosen for the following phenomena: ozone self-decomposition: a Staehelin, Bühler and Hoigné-based (SBH-based) mechanism shall be chosen; reactions with inorganic carbon: a classical pathway combining reactions widely used in different studies will be considered; reactions for bromate formation: all the reactions present in the literature shall not be considered, but a selection shall be done based on kinetics. Considering the influence of NOM, a semi-empirical model shall be proposed for modelling needs. The model shall be based on the classical categorisation: consumers, initiators, promoters and inhibitors. The number of adjustable parameters may vary.