Application of a mechanistic UV/hydrogen peroxide model at full-scale : sensitivity analysis, calibration and performance evaluation

Numerous mechanistic models describing the UV/H2O2 process have been proposed in literature. In this study, one of them was used to predict the behavior of a full-scale reactor. The model was calibrated and validated with non-synthetic influent using different operational conditions. A local sensitivity analysis was conducted to determine the most important operational and chemical model parameters. Based on the latter, the incident UV irradiation intensity and two kinetic rate constants were selected for mathematical estimation. Hydrogen peroxide concentration, the decadic absorption coefficient at 310 nm (UVA310, as a surrogate for natural organic matter) and pH could be satisfactorily predicted during model validation using an independent data set. It was demonstrated that quick real-time calibration is an option at less controllable full-scale conditions. Parameters that determine the initiation step, i.e. photolysis of hydrogen peroxide, have a large impact on most of the variables. Some reaction rate constants were also of importance, but nine kinetic constants did show absolutely no influence to one of the variables. Parameters related to UV shielding by NOM were of main importance. Hydrogen peroxide concentration was classified as a non-sensitive variable, in contrast to the concentration of a micro pollutant which showed to be very to extremely influential to many of the parameters. UV absorption as a NOM surrogate is a promising variable to be included in future models. Model extension by splitting up the UVA310 into a soluble and a particulate fraction seemed to be a good approach to model AOP treatment of real (waste)waters containing both dissolved and particulate (suspended) material

Influence of NOM concentration on parameter sensitivity of a mechanistic ozone decomposition model

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 kLa and [O3*] 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

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

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

COD and colour removal from molasses spent wash using activated carbon produced from bagasse fly ash of Matahara sugar factory, Oromiya region, Ethiopia

The aim of this study was to investigate the removal of chemical oxygen demand (COD) and colour from a melanoidin solution using activated carbon produced from bagasse fly ash (BFA). Melanoidins are heterogeneous polymers and major contributors to the dark brown colour of molasses spent wash, which is an extensive cause of environment pollution. The surface area of the BFA was determined as 160.9 +/- 2.8m(2)/g with 90% of particle less than 156.8 mu m in size. Characterization of the BFA by Fourier transform infrared spectroscopy (FTIR) showed the presence of hydroxyl and carbonyl functional groups, whereas X-ray diffraction analysis indicated its amorphous nature. Moreover, scanning electron microscopy analysis showed a heterogeneous and irregular shape of pores. Among the adsorption isotherm models analysed, the Freundlich model fitted best to the experimental data, indicating a maximum adsorptive capacity of 124.80 mg/g. The removal of COD and colour from a melanoidin solution with this activated carbon was carried out using an experimental design taking 4 factors into account. These were adsorbent dose, contact time, pH and initial COD concentration, with removal of COD and colour as response variables. COD reduction was influenced by initial COD concentration whereas colour removal was dominated by contact time, which was in line with the findings of principal component analysis. The maximum COD removal recorded was 61.6% at the optimum condition of adsorbent dose of 4 g in 100 mL, contact time of 4 h, pH 8 and initial COD concentration 6 000 mg/L, whereas the decolourization of melanoidin solution was 64% at adsorbent dose of 4 g, contact time 4 h, pH 3 and initial COD concentration 6 000 mg/L. Hence, activated BFA is a promising option for simultaneous removal of COD and colour from molasses spent wash under the stated conditions