Contribution to Performance Characterization and Kinetic Modelling of Micropollutants Abatement in Water and Wastewater by Ozone-based Oxidation Processes

Abstract

Ozonation is nowadays a competent technology for micropollutant oxidation in wastewater effluents. The combination of ozone with hydroxyl radical formed through ozone decomposition is effective in the abatement of a number of organic compounds. However, there are still some points to be addressed for a more efficient application of ozone-based processes to wastewater treatment and reclamation. There are, for instance, several organic compounds that react very slowly with ozone. They are known as ozone-resistant micropollutants. If a high quality water is wanted to be obtained, these substances should be also removed from the effluent. In addition, modelling the abatement of micropollutants during wastewater ozonation is essential for process simulation, optimization and real-time control. To make this possible, performance characterization in terms of oxidation efficiency is required, as well as some kinetic information regarding the abatement of micropollutants. The first can be addressed by using normalizing parameters allowing the estimation, for instance, of hydroxyl radical availability as a function of the consumed ozone, or any other parameter whose measurement during the process is simple. The most accurate way of obtaining kinetic data of micropollutants oxidation is conducting individual studies on this chemicals degradation by ozone and hydroxyl radicals. Some complementary tests, in addition, may be useful to obtain valuable data regarding the mechanisms of degradation and the potential ecotoxicological effects of formed transformation products.

This thesis was divided in two parts: first, individual batch ozonation studies of selected concerning micropollutants were conducted. Particularly, the pesticides methiocarb, acetamiprid and dichlorvos were selected for that work. The second part consisted of the application of single ozonation and the combination O3/H2O2 was tested in the removal of ozone-refractory micropollutants from actual wastewater effluents. The latter was done through semi-batch ozonation experiments. The objectives were, on one hand, obtaining kinetic, mechanistic and toxicological data of some priority/emerging concern chemicals. On the other, ozonation studies with real effluent samples were performed with the aim of exploring the removal of ozone-recalcitrant chemicals and potential strategies for the modelling and real-time control of this process. In addition, improvement strategies for O3/H2O2 process application were investigated.

Different kinetics were observed for pesticides reaction with ozone, being acetamiprid the most recalcitrant compound. In addition, toxicity of some of the pesticides transformation products was revealed, especially in the case of methiocarb degradation. Regarding ozone-based processes application to actual wastewater effluents, the removal of model ozone-recalcitrant compounds was found difficult, concluding that ozone doses higher than the immediate ozone demand (IOD) value are required for a more effective abatement of these substances regardless of the process employed. In the case of the O3/H2O2 process application, dosing hydrogen peroxide simultaneously to ozone bubbling was found to potentially entail important energy savings related to oxidants use. The oxidation performance of ozone-based processes and thus the abatement of ozone-refractory compounds could be effectively modelled using kinetic parameters, the monitoring of water quality parameters and empirical relationships obtained for each effluent. Furthermore, side reactions of involved oxidants with effluent organic compounds was found to increase the content in small and oxidized organic compounds in all cases. In the case of effluents containing suspended solids, ozone application also caused a net increase in the dissolved organic load.

El proceso de ozonización constituye una de las tecnologías de tratamiento de aguas con mayor potencial para la eliminación de microcontaminantes de los efluentes de depuradoras municipales. Pese a ello, algunos aspectos como la eliminación de aquellos contaminantes resistentes al ataque del ozono y la modelización cinética de este proceso ofrecen aún amplias posibilidades de mejora. La formación de productos de transformación a partir de reacciones del ozono y el radical hidroxilo con la materia orgánica del efluente es otro aspecto escasamente considerado. Dados los problemas técnicos y ambientales que dichos subproductos pueden ocasionar, la investigación de este fenómeno es también de gran interés.

En esta tesis se estudiaron en primer lugar los fundamentos (cinética, mecanismos de reacción y efectos toxicológicos de los productos de transformación) del proceso de degradación de tres microcontaminantes orgánicos de preocupación emergente mediante el proceso de ozonización. A continuación, éstos y otros compuestos típicamente detectados en aguas residuales urbanas se emplearon como sustancias modelo en el estudio de la aplicación del ozono en efluentes reales de depuradora de calidad variada, con el fin de evaluar la eficiencia del proceso y proponer estrategias para la modelización del mismo. En este sentido, se propuso la combinación de parámetros cinéticos y la monitorización de parámetros de calidad del agua para el control a tiempo real del proceso. Las predicciones de eliminación de contaminantes llevadas a cabo mediante esta metodología mostraron una gran concordancia con los datos experimentales, tanto para el proceso de ozonización simple como para la combinación ozono-peróxido de hidrógeno, que por otro lado exhibió una gran eficiencia en la eliminación de contaminantes resistentes al ozono cuando se aplicó con dosificación simultánea de ambos oxidantes. Finalmente, la evaluación de los cambios experimentados por la materia orgánica del efluente durante la aplicación del ozono reveló una acumulación de compuestos de bajo peso molecular en todas las aguas ensayadas, además de la solubilización de materia orgánica en suspensión en efluentes con mayor carga orgánica.