Coupled advanced oxidation and biological processes for wastewater treatment

Abstract

The incapability of conventional biological wastewater treatment to remove effectively biorecalcitrant and/or toxic pollutants, as well as the shortage of world water resources have promoted the research of more efficient and ecologically y friendly water treatment technologies. This thesis contributes to the development of a new hybrid technology combining advanced oxidation processes (AOP) and biological processes for the treatment of wastewater containing biorecalcitrant and toxic pollutants. In the proposed coupled system, AOP is applied exclusively as pre-treatment with the aim to modify the chemical structure of the pollutants to transform them into biodegradable intermediates. During this step partial mineralization of pollutant take place, and the subsequent biological treatment is applied to complete mineralization. This approach is viable from the economic and environmental point of view, and appears as an alternative to the drastic and/or inefficient single-step processes actually applied for the treatment of biorecalcitrant wastewater. This thesis is organized in 6 chapters and focuses on the degradation of a model biorecalcitrant pollutant: 5-amino-6-methyl-2-benzimidazolone (AMBI) an important precursor in the industrial production of dyes. In the first chapter, AOP and the concept of coupling AOP-biological process are introduced. An overview of studies which used a combination of photoassisted and biological degradation of organic contaminants in water was performed. Chapter 2 focuses on an exploratory study with some of the most representative AOP Thus sonochemical, electrochemical and photochemical oxidation processes were applied to degrade AMBI. The comparison of these AOP revealed that the iron photo-assisted processes are the most advantageous, and have an application potential in sunny regions. Chapter 3 focuses on the degradation of AMBI by means of the hν/Fe(III)/O2(air) and hν/ Fe(III)/H2O2 systems using an artificial irradiation source. The transformation of AMBI photoinduced by the Fe(III) in presence of both O2(from air) and H2O2 electron acceptors is studied. The effect of AMBI, Fe3+ and H2O2 concentration for the degradation of AMBI wastewater in the photo-Fenton process was discussed and optimal conditions were found. Chapter 4 focuses on coupling iron photoassisted process with a biological system at lab scale. Here a general strategy to develop combined photochemical and biological systems for biorecalcitrant wastewater treatment is proposed. Following this strategy, two kinds of combined systems were developed and tested using for the photocatalytic pretreatment hν/Fe3+/O2(air) or hν/Fe3+/H2O2 and in both cases fixed bed with activated sludge culture for the biological step. To replace relatively expensive artificial irradiation in photoassisted processes, the solar irradiation was applied. Chapter 5 illustrates the development and optimization at pilot scale, of a coupled solar-biological system for water treatment. The following points were taking into account: (i) the choice of the most appropriate solar collector and the most efficient photocatalytic system, (ii) the optimization of the photocatalytic system, (iii) the monitoring of the chemical and biological characteristics of photo-treated solution and (iv) the evaluation of the performances of the coupled solar-biological system for the treatment of real industrial wastewater containing AMBI. Results indicate that coupling solar-biological processes at pilot scale is an effective method to the treatment of non-biodegradable industrial pollutants such as AMBI. To overcome the problem of electricity supply for pumps used for the recirculation of wastewater in a coupled water detoxification process, chapter 6 proposes a new Hybrid Photocatalytic-Photovoltaic System (HPPS). HPPS is a device which allows simultaneously decontaminate water and convert solar energy into electricity. This ecological equipment (which is actually following a patent procedure at the EPFL) was designed, installed, and tested. The results show that the HPPS represents an autonomous and environmentally friendly method for this strategy of polluted water remediation.