B.A. Wols

Degradation of 40 selected pharmaceuticals by UV/H2O2

The occurrence of pharmaceuticals in source waters is increasing. Although UV advanced oxidation is known to be an effective barrier against micropollutants, degradation rates are only available for limited amounts of pharmaceuticals. Therefore, the degradation of a large group of pharmaceuticals has been studied in this research for the UV/H2O2 process under different conditions, including pharmaceuticals of which the degradation by UV/H2O2 was never reported before (e.g., metformin, paroxetine, pindolol, sotalol, venlafaxine, etc.). Monochromatic low pressure (LP) and polychromatic medium pressure (MP) lamps were used for three different water matrices. In order to have well defined hydraulic conditions, all experiments were conducted in a collimated beam apparatus. Degradation rates for the pharmaceuticals were determined. For those compounds used in this research that are also reported in literature, measured degradation results are in good agreement with literature data. Pharmaceutical degradation for only photolysis with LP lamps is small, which is increased by using a MP lamp. Most of the pharmaceuticals are well removed when applying both UV (either LP or MP) and H2O2. However, differences in degradation rates between pharmaceuticals can be large. For example, ketoprofen, prednisolone, pindolol are very well removed by UV/H2O2, whereas metformin, cyclophosphamide, ifosfamide are very little removed by UV/H2O2.

Evaluation of different disinfection calculation methods using CFD

Computational Fluid Dynamics combined with a particle tracking technique provides valuable information concerning residence times and contact times in chemical reactors. In drinking water treatment, for example an accurate estimation of the disinfection is important to predict the microbial safety. Ozone contactors are widely used for disinfection, but the complex geometry of the system causes suboptimal hydraulics and requires optimizations of the flow. This results in a lower ozone dosage, which may reduce the formation of unwanted disinfection-by-products and the consumption of energy. To that end disinfection needs to be calculated precisely, accounting for the complex hydraulics. Several calculation methods estimating the disinfection performance of ozone contactors were evaluated using Computational Fluid Dynamics. For an accurate disinfection prediction, the full distribution of ozone exposures (CT values) is needed, only a mean CT value or residence time distribution provides insufficient information for an accurate disinfection prediction. Adjustments to the geometry of the ozone contactor that reduce the short-circuit flows resulted in an increase in disinfection capacity, whereas the mean CT value remained the same. A sensitivity analysis with respect to the kinetics was conducted. The gain in disinfection capacity obtained by optimizing the hydraulics was significant for typical values used in practice.

Degradation of pharmaceuticals in UV (LP)/H2O2 reactors simulated by means of kinetic modeling and computational fluid dynamics (CFD)

UV/H2O2 treatment is a well-established technique to degrade organic micropollutants. A CFD model in combination with an advanced kinetic model is presented to predict the degradation of organic micropollutants in UV (LP)/H2O2 reactors, accounting for the hydraulics, fluence rate, complex (photo)chemical reactions in the water matrix and the interactions between these processes. The model incorporates compound degradation by means of direct UV photolysis, OH radical and carbonate radical reactions. Measurements of pharmaceutical degradations in pilot-scale UV/H2O2 reactors are presented under different operating conditions. A comparison between measured and modeled degradation for a group of 35 pharmaceuticals resulted in good model predictions for most of the compounds. The research also shows that the degradation of organic micropollutants can be dependent on temperature, which is relevant for full-scale installations that are operated at different temperatures over the year.

Residence Time Distributions in Ozone Contactors

The hydraulics in ozone systems, characterized by the residence time distribution, are investigated numerically as well as experimentally. The complex geometry of the ozone contactors requires the application of computational fluid dynamics (CFD), which in combination with experimental results gives insight in the hydraulic processes. Particle tracking provides a distribution of CT-values (dissolved ozone concentration times residence time) to estimate disinfection precisely and points out dead zones that hamper disinfection. The CFD modeling predicts that small changes in geometry reducing the strength of the recirculation zone can significantly increase the inactivation of micro-organisms.

Influence of process conditions and water quality on the formation of mutagenic byproducts in UV/H2O2 processes

UV/H2O2 processes in drinking water treatment may generate byproducts which cause an increased response in Ames fluctuation assays. As this probably involves a mixture of substances in very low concentrations, it is challenging to identify the individual byproducts. Therefore it was studied under which conditions mutagenic byproducts are formed and how this can be prevented. It was found that positive Ames fluctuation test responses only are obtained when Medium Pressure UV lamps are used, and not with Low Pressure lamps. This probably is explained by the photolysis of nitrate, which plays an important role in the formation of mutagenic byproducts. The most important parameters involved in the formation of such byproducts were demonstrated to be the nitrate concentration, the natural organic matter, the UV spectrum of the lamps, and the UV dose applied. These factors explain up to 74-87% of the Ames fluctuation test responses after UV/H2O2 drinking water treatment. By taking this into account, drinking water utilities can estimate whether UV processes applied in their case may cause the formation of mutagenic byproducts, and how to take measures to prevent it.

Comparison of CFD, Biodosimetry and Lagrangian Actinometry to Assess UV Reactor Performance

Computational fluid dynamics (CFD) is becoming increasingly popular for assessing UV reactor performance. However, due to uncertainties in flow fields, lamp output and microbial response to UV, the CFD model needs to be validated by measurements. The combination of several experimental techniques with CFD modeling allows for a thorough understanding of the dominant processes occurring in UV reactors. Therefore, CFD modeling, Lagrangian actinometry and biodosimetry experiments were performed for a cross-flow UV reactor. A good agreement was found between biodosimetry and CFD modeling. Lower inactivation levels were found for the Lagrangian actinometry method, which was explained by fluorescence decay of the microspheres.

Determination of Reaction Rate Constants in a Collimated Beam Setup: The Effect of Water Quality and Water Depth

UV/H2O2 processes can be studied using a Collimated Beam (CB) setup. In UV dose calculations, the “water factor” accounts for changes in the irradiation spectrum upon its way through the aqueous solution. Analogous to the “germicidal factor,” the “extinction factor” is introduced, correcting for the fact that resulting compounds will be subjected to an emission spectrum changing over the depth. Using the extinction factor, the fluence-based photolysis and oxidation rate constants of model compounds, as well as the quantum yield for nitrite formation, were found to be independent of the depth of the water layer applied.

Prediction of Advanced Oxidation Performance in Pilot UV/H2O2 Reactor Systems with MP- and LP-UV Lamps

Advanced oxidation processes, are becoming important barriers against organic micropollutants in water treatment. To guarantee safe drinking water, it is important to be able to adjust the process parameters to the circumstances. Modeling can play an important role in this respect. Two models, UVPerox I and UVPerox II, were developed, in which the kinetic parameters of the process are combined with computational fluid dynamics (CFD), accounting for the hydrodynamics and UV irradiation distribution of the reactors applied. Both models were applied to several pilot reactors, and good accordance was observed between predicted and experimental data.

Development and performance of a parsimonious model to estimate temperature in sewer networks

Abstract This paper presents a model (inspired by another model) to calculate water temperature in free-surface flow with two main innovations: the convective heat transfer occurs only at the wetted perimeter of pipes, and the model was integrated to commercial software used for hydraulic calculations in drainage systems. Given these innovations, we could reduce the number of modeling input data to calculate the temperature of water and soil in the radial and tangential directions along the pipes, with the advantages of using industry-standard software. To test the performance of the model, it was firstly calibrated in two sets of experiments (to calibrate the hydraulic and the thermal parameters separately), and benchmarked with a third controlled discharge against the case model. The results indicate that in unsteady-state situations the parsimonious model can be twice as accurate as the underlying model because the parsimonious model considers the hydraulic influence of sewer infrastructure.

The Use of Larger Volumes in a Collimated Beam Setup

Research into the conversion of organic micropollutants revealed that with medium pressure (MP) UV-lamps, an increase in genotoxic activity in one of two tested bacterial strains in the Ames II test may be obtained. To study the factors that affect formation of genotoxic by-products, volumes ≥ 500 mL are required. It is possible to use a collimated beam (CB) setup with water layers up to 10 cm deep. Both photolysis as well as oxidation constants of model compounds tested in a water matrix containing NOM stayed constant, independent of the depth of the water layer applied.