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The disinfection efficiencies of a commercial mixture of peracetic acid against Escherichia coli were studied in laboratory scale experiments. The joint and separate action of two disinfectant agents, hydrogen peroxide and peracetic acid, were evaluated in order to observe synergistic effects. A kinetic model for each component of the mixture and for the commercial mixture was proposed. Through simple mathematical equations, the model describes different stages of attack by disinfectants during the inactivation process. Based on the experiments and the kinetic parameters obtained, it could be established that the efficiency of hydrogen peroxide was much lower than that of peracetic acid alone. However, the contribution of hydrogen peroxide was very important in the commercial mixture. It should be noted that this improvement occurred only after peracetic acid had initiated the attack on the cell. This synergistic effect was successfully explained by the proposed scheme and was verified by experimental results. Besides providing a clearer mechanistic understanding of water disinfection, such models may improve our ability to design reactors.

This work analyzes the kinetic modelling of the photocatalytic inactivation of E. coli in water using different types of kinetic models; from an empirical equation to an intrinsic kinetic model including explicit radiation absorption effects. Simple empirical equations lead to lower fitting errors, but require a total of 12 parameters to reproduce the results of four inactivation curves when the catalyst concentration was increased. Moreover, these parameters have no physical meaning and cannot be extrapolated to different experimental conditions. The use of a pseudomechanistic model based on a simplified reaction mechanism reduces the number of required kinetic parameters to 6, being the kinetic constant the only parameter that depends on the catalyst concentration. Finally, a simple modification of a kinetic model based on the intrinsic mechanism of photocatalytic reactions including explicit radiation absorption effects achieved the fitting of all the experiments with only three parameters. The main advantage of this approach is that the kinetic parameters estimated for the model become independent of the irradiation form, as well as the reactor size and its geometrical configuration, providing the necessary information for scaling-up and design of commercial-scale photoreactors for water disinfection. [ABSTRACT FROM AUTHOR]

Abstract: The inactivation (death) of Escherichia coli bacteria in water employing hydrogen peroxide has been studied and a five log decrease in CFUcm−3 was achieved. The reaction kinetics was modeled as a series of biochemical steps represented by pseudo-homogeneous reactions between hydroxyl radicals and the components of the cellular walls. Afterwards, the lysate was supposed to undergo a group of parallel reactions leading to the oxidation of the chemical components of the cell. It was assumed that the initiation step of hydrogen peroxide dissociation is promoted by the presence of iron or iron-superoxide compounds. In addition the model takes into account that the reaction forming the lysate as well as the ones that follow the destruction of the bacterium wall, compete for the available oxidizing radicals with the steps that involve the attack on active and injured bacteria. A four parameter representation shows good agreement for the whole range of employed hydrogen peroxide concentrations. The results are valid for any form and size of the employed reactor as long as the described operating conditions (pH and concentrations) are maintained. This development constitutes a very general model that is capable to describe inactivation processes whose graphical representation also shows the presence of shoulders at the beginning and tailings in the end of the operation. [Copyright &y& Elsevier]

A fixed-bed, annular photoreactor is studied for the photocatalytic disinfection of Escherichia coli aqueous suspensions. Experiments were carried out under UV radiation employing P25 titanium dioxide immobilized onto glass Raschig rings. The photoreactor performance has been simulated following a predicted procedure that takes into account explicitly the radiation absorption step and a series-event disinfection mechanism. The modeling of the radiation field was carried out by Monte Carlo simulation. The reactor model has been compared with experimental data, being able to reproduce the evolution of the concentration of viable bacteria under different bacteria initial concentrations and irradiation levels. [ABSTRACT FROM AUTHOR]

A kinetic model of the photocatalytic inactivation of Escherichia coli in an annular wall reactor is presented. The model is based on a reaction scheme that involves a series of events in which bacteria are progressively damaged and eventually led to cell lysis. The model explicitly takes into account radiation absorption effects. Photocatalytic inactivation experiments were carried out in a photoreactor operated in a closed recirculating circuit with a reservoir tank and irradiated with a 6 W black light lamp situated in the axis of the reactor. Immobilization of TiO 2 Aeroxide P25 has been carried out by the dip-coating procedure onto the inner-tube wall of the annular reactor. Experimental results for different TiO 2 layer thicknesses were used to estimate the kinetic parameters of the model. Good agreement between model predictions and inactivation experiments was achieved in the whole range of TiO 2 thicknesses studied. [ABSTRACT FROM AUTHOR]

Abstract: A model of a bench-scale, annular, continuous flow reactor for the photocatalytic disinfection of water, operated under conditions of no perfect mixing is presented. The performance of the photoreactor has been simulated following a predictive procedure, with no adjustable parameters, based on the intrinsic kinetics and the information about the geometry, irradiation source and operation conditions (catalysts concentration and its properties as well as the initial concentration of bacteria). The only experimental information required to be determined at laboratory scale are the kinetic parameters of a mechanistic model that takes into account the explicit dependence of the reaction rate with respect to the local volumetric rate of photon absorption (LVRPA) and the optical properties of the catalyst. The proposed model predicts an optimal catalyst concentration in the range 0.1–0.2×10−3 gcm−3, with a significant decrease in the disinfection efficiency for higher catalyst loadings. Important resistances to the mass transport are detected at high TiO2 concentrations, as a result of the low diffusion coefficient of the employed microorganism (Escherichia coli). Operating under the optimal catalyst concentration, model predictions show satisfactory agreement with experimental results extracted from the custom-built bench scale reactor. [Copyright &y& Elsevier]

Abstract: This study is focused on the kinetic modelling of the photocatalytic inactivation of bacteria with suspended TiO2. A rigorous model based on a proposed reaction mechanism and accounting explicitly for the rate of photon absorption has been developed. The application of the general kinetic expression to limiting cases suggests that the interaction bacteria–catalyst can be considered to be weak. In contrast, a complex dependence on the radiation absorption rate must be taken into account, as very different radiation conditions may coexist inside the photoreactor, with high absorption rates in the region near to the radiation entrance window and much lower values on the opposite side of the photoreactor. The model has been successfully validated by experimental data, being able to reproduce the evolution of the concentration of viable bacteria in a wide range of values of TiO2 concentration, irradiation power and initial concentration of bacteria with a normalized root mean square logarithmic error of 5.3%. The values of the kinetic parameters are independent of the specific reactor setup or the operating conditions and therefore, the model can be used in a predictive way for photoreactor design and scaling-up, as well as for the optimization of other reactor configurations. [Copyright &y& Elsevier]

Abstract: Microbiologically contaminated water, in this case artificially infected with Escherichia coli was treated with low wavelength (253.7nm) radiation in a laboratory reactor where all the significant operating variables were carefully measured and controlled. A modification of the series-event model was used to interpret the experimental data which were collected employing four different levels of the incident radiation arriving at the reactor. The developed model is based on a rather complex dependence with respect to the E. coli concentration and to the radiation that is effectively absorbed by the bacteria which was precisely quantified. The mathematical description of the kinetics has three parameters: (i) the threshold limit of bacteria damage (n =2), (ii) the kinetic constant [k =9.03±0.36s−1(cm3 W−1) m ] and (iii) the reaction order with respect to the bacteria photonic absorption rate (m =0.205±0.015). About 99.99% plus inactivation was reached in all cases for rather short effective contact times and predictions from the model agree very well with experimental data in the whole range of investigated variables. [Copyright &y& Elsevier]

Abstract: A modified dose concept for microorganism inactivation employing almost monochromatic, low wavelength radiation has been studied using disinfection data obtained with Escherichia coli bacteria and germicidal UV lamps. This first contribution has been applied to experiments performed in a well mixed reactor and describes the dose exploring two new concepts: (i) the use of the spatial distribution of the radiation absorption rates by the bacteria and (ii) the consideration that not necessarily the inactivation rate is of first order with respect to the radiation energy absorption rate. The proposed description agrees very well with the obtained experimental data for almost transparent water and for a medium having a concentrated culture with significant radiation absorption. [Copyright &y& Elsevier]