Full-scale modelling of an ozone reactor for drinking water treatment

BIOMATH, Department of Applied Mathematics, Biometrics and Process Control, Ghent University, Coupure Links 653, 9000 Gent, Belgium
Chemical Engineering Journal (Impact Factor: 4.06). 03/2010; 157(2-3):551-557. DOI: 10.1016/j.cej.2009.12.051

ABSTRACT In 2003, the Flemish Water Supply Company (VMW) extended its drinking water production site in Kluizen (near Ghent, Belgium) with a combined ozonation and biological granular activated carbon (BGAC) filtration process. Due to this upgrade, biostability increased, less chlorination was needed and drinking water quality improved significantly. The aim of this study was to describe the full-scale reactor with a limited set of equations. In order to describe the ozonation process, a model including key processes such as ozone decomposition, organic carbon removal, disinfection and bromate formation was developed. Kinetics were implemented in WEST® and simulation results were compared to real data. The predicting performance was verified with a goodness-of-fit test and key parameters were determined through a local sensitivity analysis. Parameters involving optical density (both rate constants and stoichiometric coefficients) strongly affect model output. Some parameters with respect to bromate and bacteria showed to be only, but to a large extent, sensitive to their associated concentrations. A scenario analysis was performed to study the system's behavior at different operational conditions. It was demonstrated that the model is able to describe the operation of the full-scale ozone reactor, however, further data collection for model validation is necessary.

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    ABSTRACT: Aqueous ozone decomposition proceeds through a complex chain mechanism of radical reactions. When natural organic matter (NOM) is present, the system becomes much more complex and often (semi-)empirical modelling approaches are used to describe ozonation of water and wastewater systems. Mechanistic models, however, can be of great value to gain knowledge in the chemical pathways of ozonation and advanced oxidation processes in view of engineering applications. However, the numerous model parameters and model complexity often restrict their applicability. Model simplification is then an option to cure these drawbacks. In this study, sensitivity analyses (SAs) were used to determine the most important elementary reactions from the complex kinetic model. Additionally, SAs were used to understand the reaction mechanism. It was demonstrated that only seven of the twenty-eight first and second order rate constants showed to impact ozone and HO • concentrations. Processes involving HO • scavenging by inorganic carbon were of minor importance. Mass-transfer related parameters k L a and [O 3 *] were of major importance in all cases. Hence, it is of extreme importance that these parameters are determined with high accuracy. It was shown that the aqueous ozone concentration is extremely sensitive to parameters involving NOM at very low scavenger concentrations implying that impurities should always be considered in models, even in ultrapure water systems. Uncertainty analysis showed that especially the HO • concentration is susceptible to variations in influent composition. The uncertainty regarding this species significantly reduced with increasing levels of scavengers and especially NOM. It was demonstrated that simplification of the elementary radical scheme should be considered. On the other hand, a model extension with regard to reactions involving NOM should be performed in order to improve the applicability of future wastewater ozonation models. The use of ozone in water treatment is an established technique for several decades. Ozone is known as a strong disinfectant which produces less disinfection by-products than chlorine when appropriate doses are applied. Additionally, due to its oxidative power, ozone is able to selectively attack specific moieties of micropollutants to produce harmless metabolites (Buffle et al., 2006b). These properties gave rise to numerous full-scale applications in drinking water treatment worldwide. Moreover, the application of ozone in (biological) wastewater treatment is recently gaining interest, especially as an oxidative tertiary treatment step (Zimmermann et al., 2011).

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