Abstract: In this work, the ozone inactivation of resistant microorganisms is studied and a method to assess the efficiency of a drinking water plant to inactivate resistant microorganisms using ozone is proposed. This method aims at computing the fraction of resistant microorganisms that are not inactivated at the exit of an ozonation step by evaluating the duration of the lag phase of the ozone inactivation of these microorganisms and the contact time distribution of these microorganisms with the ozone in the step. To evaluate the duration of the lag phase of the ozone inactivation of resistant pathogenic microorganisms, an experimental procedure is proposed and applied to Bacillus subtilis spores. The procedure aims at characterizing the ozone inactivation kinetics of B. subtilis spores for different temperature and ozone concentration conditions. From experimental data, a model of the ozone inactivation of B. subtilis spores is built. One of the parameters of this model is called the lag time and it measures the duration of the lag phase of the ozone inactivation of B. subtilis spores. This lag time is identified for different temperature and ozone concentration conditions in order to establish a correlation between this lag time and the temperature and ozone concentration conditions. To evaluate the contact time distribution between microorganisms and the ozone in a disinfection step of a drinking water plant, a computational fluid dynamics tool is used. The proposed method is applied to the ozonation channel of an existing drinking water plant located in Belgium and operated by Vivaqua. Results show that lag times and contact times are both in the same order of magnitude of a few minutes. For a large range of temperatures and ozone concentrations in the Tailfer ozonation channel and for the highest hydraulic flow rate applied, a significant fraction of resistant microorganisms similar to B. subtilis spores is not inactivated.
Abstract: This paper presents a global model of three phase flow (gas–liquid–solid) in an internal airlift reactor. The airlift is composed of four zones: a riser (on the aerated side on the internal wall), a downcomer (on the opposite side) and two turning zones above and below the internal wall. Tap water is the liquid continuous phase and the dispersed phases are air bubbles and polyethylene particles. The global modelling of the airlift involves mass and momentum equations for the three phases. The model enables phase velocities and phase volume fractions to be estimated, which can be compared to experimental data. Closure relations for the gas and solid drift velocities are based on the model proposed by Zuber and Findlay. The drift flux coefficients are derived from CFD numerical simulations of the airlift. Gas bubble and solid particle averaged slip velocities are deduced from momentum balances, including drag coefficient correlations. The link between Zuber and Findlay model and the two-fluid model is established. In the experiment as well as in the model, the gas flow rate is fixed. However, the liquid and solid flow rates are unknown. Two closure relations are needed to predict these flow rates: the first closure relation expresses that the volume of solid injected into the airlift remains constant; the second closure relation expresses a global balance between the difference of column height in the riser and the downcomer and the total pressure drop in the airlift. The main parameters of a three phase airlift reactor, like gas and solid volume fractions, are well predicted by the global model. With increasing solid filling rate (40%), the model starts to depart from the experimental values as soon as coalescence of bubbles appears.
