Fenton and UV-vis based advanced oxidation processes in wastewater treatment: Degradation, mineralization and biodegradability enhancement

Abstract

[eng] Up until relatively recently, the discharging of waste in the environment was the way of eliminating them, until the auto-purifying capacity of the environment was not sufficient. The permitted levels have been vastly exceeded, causing such environmental contamination that our natural resources cannot be used for certain uses and their characteristics have been altered. The main problem stems from waste coming from industry and agriculture, despite the fact that the population also plays an important role in environmental contamination. Phenols, pesticides, fertilizers, detergents, and other chemical products are disposed of directly into the environment, without being treated, via controlled or uncontrolled discharging and without a treatment strategy. In this general context, it is very clear that the strategy to continue in the search of solutions to this problem that every day presents a sensitive growth, mainly in the developing countries, will be guided to two fundamental aspects: - The development of appropriate methods for contaminated drinking, ground, and surfaces waters, and mainly - The development of appropriate methods for wastewaters containing toxic or non-biodegradable compounds. This thesis is focused in the second of these aspects. In this sense, it has been deepened in the treatment of organic compounds in aqueous solution by means of advanced oxidation processes (AOP), in the search of their elimination or transformation into more biodegradable compounds. The experimental work has been divided into four chapters. First part (chapter 3) is focused on the kinetic study of Fenton process for what two model compounds have been chosen: phenol, as reference (model compound widely studied) and an aromatic non-biodegradable compound, nitrobenzene. The second part (chapter 4) addresses to the optimisation of some treatment processes, such as photo-Fenton, H2O2/UV and Fe3+/UV-vis using different sources of artificial light and sunlight. The experimental work of this chapter was divided into two parts. In the first one, experiments were performed at laboratory scale at the University of Barcelona. In the second one, experiments in pilot plants were carried out at the EPFL (Ecole Politechnique Federale de Lausanne, Switzerland) and at the Plataforma Solar de Almería, Spain. The experimental results showed that the photo-Fenton process was the most effective method in the mineralization of the treated solutions. It is very important to stand out that it was more effective when solar light was used as radiation source. Chapters 5 and 6 represent an application of the treatment methods used in chapter 4, in which their influence on the biodegradability of an organic chloride compound (DCDE) and of waters coming from a textile industry was studied. For this final part of the thesis, the experimental work was carried out at the University of San Diego (San Diego, USA) and at the EPFL (Lausanne, Switzerland), respectively. In the case of water solutions of DCDE, H2O2/UV process was used as pre-treatment method to obtain oxidized solutions of 25, 50, 75 and 100% DCDE degrdation. After applying different biodegradability tests, it was observed that, as the percentage of oxidation increased, it increased the biodegradability of the treated solution, thus demostrating the effectiveness of the pre-treatment. In the case of treated textile wastewaters, a strategy was put in practice to obtain a general vision when it facing the case industrial wastewaters. When applied to the textile water under study, it was found that the photo-Fenton process should be used as post-treatment step of a biological process.