Design aspects of UV/H2O2 reactors

Robust optimization of a novel ultraviolet (UV) photoreactor for water disinfection: A neural network approach

To optimize the ultraviolet (UV) water disinfection process, it is crucial to determine the ideal geometric dimensions of a corresponding model that enhance performance while minimizing the impact of uncertain photoreactor inputs. As water treatment directly affects people's lives, it is crucial to eliminate the risks associated with the non-ideal performance of disinfection photoreactors. Input uncertainties greatly affect photoreactor performance, making it essential to develop a robust optimization algorithm in advance to mitigate these effects and minimize the physical and financial resources required for constructing the photoreactors. In the suggested algorithm, a two-objective genetic algorithm is integrated with a non-intrusive polynomial chaos expansion (PCE) technique. Additionally, the Sobol sampling method is employed to select the necessary samples for understanding the system's behavior. An artificial neural network surrogate model is trained using sufficient data points derived from computational fluid dynamics (CFD) simulations. A novel type of UV photoreactors working based on exterior reflectors is chosen to optimize the process with three uncertain input parameters, including UV lamp power, UV transmittance of water, and diffusive fraction of the reflective surface. In addition, four geometrical design variables are considered to find the optimal configuration of the photoreactor. The standard deviation (SD) and the reciprocal of log reduction value (LRV) are set as the objective functions, calculated using PCE. The optimal design provides a LRV of 3.95 with SD of 0.2. The coefficient of variation (CoV) of the model significantly declines up to 7%, indicating the decreased sensitivity of the photoreactor to the input uncertainties. Additionally, it is discovered that the robust model exhibits minimal sensitivity to changes in reflectivity in various flow rates, and its output variability aligns with the SD obtained through robust optimization.

Development and modeling of a novel type of photoreactors with exterior ultraviolet (UV) reflector for water treatment applications

Ultraviolet (UV) water disinfection method has emerged as an alternative to chemical methods of disinfection. In typical UV photoreactors for water treatment, water flows in the space between the lamp's sleeve and outer shell. The contact of water and sleeve causes fouling, which reduces the effectiveness of UV. To clean the photoreactor, the quartz sleeve must be replaced; this may lead to quartz or lamp breakage and mercury leakage into water during cleaning. In this study, a novel type of multi-lamp UV photoreactors is proposed, in which the UV lamps are placed out of the water channel and their UV irradiation is redirected into the channel using an outer cylindrical reflector. This allows for the installment of a self-cleaning mechanism for the water channel. A well-validated three-dimensional CFD model is utilized to model the performance of this photoreactor for microbial inactivation. The impacts of several geometrical and optical parameters are investigated on the inactivation of microorganisms. The results revealed that the difference in log reduction values (LRV) between fully specular and fully diffuse reflector ranges from 10 to 47% as the lamp-to-channel distance increases. For the volumetric flow rate of 25 GPM, the LRV of a photoreactor with fully diffuse reflector can be 46% higher than a fully specular one. In addition, the performance of the proposed photoreactor is compared against a classic L-shaped annular photoreactor. The results show that the new design can provide equal or better microbial performance compared to the classic photoreactor, but it removes many of their common issues such as quartz fouling, lamp overheating at low flow rates, and sleeve breakage during lamp replacement.

Computational fluid dynamics-based modeling and optimization of flow rate and radiant exitance for 1,4-dioxane degradation in a vacuum ultraviolet photoreactor

1,4-Dioxane is one of the most persistent organic micropollutants in conventional drinking-water-treatment processes. Vacuum ultraviolet (VUV) treatment is a promising means of removing micropollutants such as 1,4-dioxane from source water, but this approach has not yet been implemented in a full-scale water treatment plant, partly because the operating parameters for pilot and full-scale VUV photoreactors have not been optimized. Here, we developed a computational fluid dynamics-based method for optimizing VUV photoreactor performance through energy-based analyses that take into account the effects of two important operating parameters-flow rate and radiant exitance. First, we constructed a computational fluid dynamics model and determined the sole parameter required for the model, the pseudo-first-order rate constant for the reaction of 1,4-dioxane, by simple batch experiment. Then, we validated the model by using a pilot-scale flow-through annular photoreactor. Finally, we used the validated model to examine the effects of flow rate and radiant exitance on the efficiency of 1,4-dioxane degradation in a virtual annular photoreactor. Radiation efficiency, which was defined as the ratio of the logarithmic residual ratio of 1,4-dioxane to the theoretical minimum logarithmic residual ratio (best possible performance) under the given operating conditions, was calculated as an energy-based index of cost-effectiveness. Radiation efficiency was found to increase with increasing flow rate but decreasing radiant exitance. An electrical energy per order (EEO) analysis suggested that VUV treatment under laminar flow was most economical when low-power lamps and a high flow rate were used. In contrast, VUV treatment under turbulent flow was suggested to be most economical when high-power lamps were used at a high flow rate.

Micropollutants as internal probe compounds to assess UV fluence and hydroxyl radical exposure in UV/H2O2 treatment

Organic micropollutants (MPs) are increasingly detected in water resources, which can be a concern for human health and the aquatic environment. Ultraviolet (UV) radiation based advanced oxidation processes (AOP) such as low-pressure mercury vapor arc lamp UV/H₂O₂ can be applied to abate these MPs. During UV/H₂O₂ treatment, MPs are abated primarily by photolysis and reactions with hydroxyl radicals (^•OH), which are produced in situ from H₂O₂ photolysis. Here, a model is presented that calculates the applied UV fluence (H_calc) and the ^•OH exposure (CT_•OH,calc) from the abatement of two selected MPs, which act as internal probe compounds. Quantification of the UV fluence and hydroxyl radical exposure was generally accurate when a UV susceptible and a UV resistant probe compound were selected, and both were abated at least by 50 %, e.g., iopamidol and 5-methyl-1H-benzotriazole. Based on these key parameters a model was developed to predict the abatement of other MPs. The prediction of abatement was verified in various waters (sand filtrates of rivers Rhine and Wiese, and a tertiary wastewater effluent) and at different scales (laboratory experiments, pilot plant). The accuracy to predict the abatement of other MPs was typically within ±20 % of the respective measured abatement. The model was further assessed for its ability to estimate unknown rate constants for direct photolysis (k_UV,MP) and reactions with ^•OH (k_•OH,MP). In most cases, the estimated rate constants agreed well with published values, considering the uncertainty of kinetic data determined in laboratory experiments. A sensitivity analysis revealed that in typical water treatment applications, the precision of kinetic parameters (k_UV,MP for UV susceptible and k_•OH,MP for UV resistant probe compounds) have the strongest impact on the model's accuracy.

Development and application of relevance and reliability criteria for water treatment removal efficiencies of chemicals of emerging concern

With the growth in production and use of chemicals and the fact that many end up in the aquatic environment, there is an increasing need for advanced water treatment technologies that can remove chemicals of emerging concern (CECs) from water. The current lack of a homogenous approach for testing advanced water treatment technologies hampers the interpretation and evaluation of CEC removal efficiency data, and hinders informed decision making by stakeholders with regard to which treatment technology could satisfy their specific needs. Here a data evaluation framework is proposed to improve the use of current knowledge in the field of advanced water treatment technologies for drinking water and wastewater, consisting of a set of 9 relevance criteria and 51 reliability criteria. The two criteria sets underpin a thorough, unbiased and standardised method to select studies to evaluate and compare CEC removal efficiency of advanced water treatment technologies in a scientifically sound way. The relevance criteria set was applied to 244 papers on removal efficiency, of which only 20% fulfilled the criteria. The reliability criteria were applied to the remaining papers. In general these criteria were fulfilled with regards to information on the target compound, the water matrix and the treatment process conditions. However, there was a lack of information on data interpretation and statistics. In conclusion, a minority of the evaluated papers are suited for comparison across techniques, compounds and water matrixes. There is a clear need for more uniform reporting of water treatment studies for CEC removal. In the future this will benefit the selection of appropriate technologies.

Selecting the best piping arrangement for scaling-up an annular channel reactor: An experimental and computational fluid dynamics study

This study is focused on the selection of the best piping arrangement for a pilot scale annular channel reactor intended for the remediation of waters and wastewaters. Two annular channel reactors composed of a single UV lamp and distinct piping arrangements were considered: (i) a novel reactor with tangential inlet/outlet pipes - the FluHelik reactor, and (ii) a conventional Jets reactor. These two reactors were manufactured at lab scale and characterized in terms of residence time distribution (RTD), radiant power and ability to degrade aqueous solutions spiked with a model compound - 3-amino-5-methylisoxazole (AMI) - by H₂O₂/UVC and UVC processes. Computational fluid dynamics (CFD) simulations were used to assess the hydrodynamics, RTD and UV radiation intensity distribution of both reactors at pilot scale. In general, experimental results at lab scale revealed quite similar RTDs, radiant powers and AMI degradation rates for both reactors. On the other hand, CFD simulations at pilot scale revealed the generation of a helical motion of fluid around the UVC lamp in the FluHelik reactor, inducing: (i) a longer contact time between fluid particles and UV light, (ii) more intense dynamics of macromixing as a result of larger velocity gradients, turbulent intensities and dispersion of RTD values around the peak, and (iii) a more homogeneous UV radiation distribution. In addition, the design of the FluHelik reactor can favor the implementation of various reactors in series, promoting its application at industrial scale. The FluHelik reactor was chosen for scaling-up. A pre-pilot scale treatment unit containing this reactor was constructed and its feasibility was proven.

Origin, Fate and Control of Pharmaceuticals in the Urban Water Cycle: A Case Study

The aquatic environment and drinking water production are under increasing pressure from the presence of pharmaceuticals and their transformation products in surface waters. Demographic developments and climate change result in increasing environmental concentrations, deeming abatement measures necessary. Here, we report on an extensive case study around the river Meuse and its tributaries in the south of The Netherlands. For the first time, concentrations in the tributaries were measured and their apportionment to a drinking water intake downstream were calculated and measured. Large variations, depending on the river discharge were observed. At low discharge, total concentrations up to 40 μg/L were detected, with individual pharmaceuticals exceeding thresholds of toxicological concern and ecological water-quality standards. Several abatement options, like reorganization of wastewater treatment plants (WWTPs), and additional treatment of wastewater or drinking water were evaluated. Abatement at all WWTPs would result in a good chemical and ecological status in the rivers as required by the European Union (EU) Water Framework Directive. Considering long implementation periods and high investment costs, we recommend prioritizing additional treatment at the WWTPs with a high contribution to the environment. If drinking water quality is at risk, temporary treatment solutions in drinking water production can be considered. Pilot plant research proved that ultraviolet (UV) oxidation is a suitable solution for drinking water and wastewater treatment, the latter preferably in combination with effluent organic matter removal. In this way >95% of removal of pharmaceuticals and their transformation products can be achieved, both in drinking water and in wastewater. Application of UV/H2O2, preceded by humic acid removal by ion exchange, will cost about €0.23/m3 treated water.

UV Induced Mutagenicity in Water: Causes, Detection, Identification and Prevention

At first it seemed that UV processes for disinfection and advanced oxidation were "harmless", as they didn't involve the addition of "dangerous" chemicals nor seemed to result in the formation of toxic byproducts. However, recently it has become clear that also during UV processes mutagentic/genotoxic byproducts may be formed. It was found that these are nitrogen containing aromatic compounds, which are formed by the reaction of photolysis products of nitrate with (photolysis products of) natural organic matter. Now more has become clear on the formation process of these compounds, it is possible to limit or even prevent their formation during e.g. UV/H₂O₂ processes. Besides, it appears to be possible to remove such byproducts by means of filtration processes. Thus, UV based processes can safely be applied in water treatment.