UV-advanced oxidation process without additives in liquid phase : process characterization and validation

Abstract

Human activities are gradually generating a strong impact on the environment and the quality of natural water resources. Traces of hazardous substances are detected not only in wastewater but also already in natural water bodies (surface and ground water) around the globe. The source of these trace substances is broad, starting with chemicals from industrial production like polycyclic aromatic hydrocarbons, pesticides, food ingredients, pharmaceuticals, to components of personal care, just to mention examples. Because of their constantly growing presence in water resources, and the fact that some of those are seriously affecting natural biological processes, there is an increasing need for finding ways to impede those substances to reach water bodies. The European community has adopted the water pollution problematic as one of their main concerns. As a result, on the 23 October 2000, the Directive 2000/60/EC of the European Commission was adopted. The main purpose of the directive is the protection of European natural water resources by enhancing the status of the water bodies as well as preventing further contamination. In particular, the requirement of complying with European environmental quality standards (EQS) for priority substances (PD) became compulsory. In order to meet these requirements and directives, industry and the scientific community working in the fields of water treatment, do approach new treatment processes with the purpose of upgrading current treatment plants to destruct or reduce trace substances. One alternative are the so called Advanced Oxidation Processes (AOPs), which are able to treat persistent and hardly biodegradable pollutants by oxidation processes. Between the varieties of potential AOPs the photo-induced advanced oxidation, centred on photolysis of water by radiation below 200 nm has received special attention. The key feature of the photo induced AOP is the facts of the efficient production of hydroxyl radicals as the highest possible oxidant and the singularity of no need for aux-iliary oxidants like hydrogen peroxide, ozone, peroxydisulfate or peroxymonosulfate, needed in other AOPs. One of the most important steps to bring the photo induced AOP to be used in the real field, is to prove that the process can be operated under efficient conditions, controlling the generation of hydroxyl radicals and utilizing them efficiently for the goal of treatment. An important action to push the technology to the path of product development is therefore, the integration of efficient UV sources to a photo reactor, taking into account the peculiarities of the process. Overall, the research presented in this thesis focuses on the characterization of the photoreaction zone, the light penetration and absorption process as well as the hydroxyl radical generation and provides the understanding and database to enable different approaches for the engineering of photo induced AOP. Concretely, this thesis is structured in five parts. Part I includes the literature search and its analysis; different calculations and simulations were performed with the aim of describing the heterogeneity of the reaction zone, the theoretical penetration of radiation in water and understanding the theoretical spatial generation of hydroxyl radicals. Part II includes the reactor characterization. The photo induced AOP was first tested following the degradation of Methylene Blue (MB). Subsequently it was studied the impact of the channel dimen-sions in the oxidation process under different operational parameters and the characterization of the photo reactor in terms of radiation intensity using three different methods. Part III includes the process characterization where the investigations were focused on the penetration of radiation into water and the generation of hydroxyl radicals by means of two methodologies: the transmission measurements in thin water films and the degradation of methanol as reference substance. Part IV includes the tailor made reactor validation. In this part, the focus was directed to verify the numeric models and gain data enabling the design, construction and validation of a reactor system using flat lamps. Validation of the photo induced AOP was performed by means of degradation of the pharmaceuticals Sulfaquinoxaline (SQX) and Carbamazepin (CBZ), as well as the degradation of an organic substance coming from the chemical industry Bisphenol A (BPA) and a food ingredient Caffeine (CAF). During the validation phase the concentration of the substances, the Total Organic Carbon (TOC) and the cytotoxicity using VERO and COS cell lines for each experiments were followed. Part V summarizes the results. In general, it was found that the process shows attractive potential and advantages in comparison with other oxidation technologies. For example: the steady and local production of hydroxyl radical without the addition of supplementary oxidation agents; the not strong selectivity and the capacity of mineralization of the process; the pH independency of the oxidation process; the fact that cytotoxicity of the byproducts does not overpass the initial toxicity of the tested substances and the Dose requirements for oxidation is comparable with existing process without using any additive.