Various chemicals are discharged into the aquatic environment as industrial liquid wastes. Some of them are not only toxic but also partly biodegradable; therefore they are not easily removed in biological wastewater treatment plants. That is why there is a need to develop effective methods for the degradation of organic pollutants, render into less harmful compounds (non aromatic) or to their complete mineralization [1].Conventional processes to remove these pollutants involve physical, chemical, and biological methods. Nevertheless, the individual application of these techniques is generally limited and cannot degrade completely the recalcitrant organic matter [2]. Biological processes such as activated sludge and aerated lagoons are used to treat pulp mill effluents but these have limited ability to metabolize recalcitrant moieties because of their size and complex structure. Latest investigations on the degradation of organic pollutants are focused on the combination of biological and physical–chemical treatments. For example, advanced oxidation followed by biological treatment or vice versa has been studied to improve removal efficiencies and reduce process energy cost [3].Chemical precipitation, filtration, electro-deposition, ion-exchange adsorption, and membrane systems are some of the conventional methods for water treatment and have found certain practical applications [4].However, these methods may not be very effective, because they are either slow or non-destructive to some or most persistent organic pollutants. Besides, large scale implementations of these methods have some limitations, owing to the expensive equipments involved in these processes. It is therefore essential to investigate the use of efficient catalytic materials to remove highly toxic compounds from potential sources of drinking water. Semiconductor heterogeneous photocatalysis is a popular technique that has the great potential to control the organic contaminants in water or air. This process which is also known as “Advanced Oxidation Process (AOP)” is suitable for the oxidation of recalcitrant contaminants such as dyes and phenolic compounds. Heterogeneous photocatalytic oxidation, developed in the 1970s, has attracted considerable attention particularly when used under solar light [2-4]. In the past decades, numerous studies have been carried out by researchers from all over the world on the application of heterogeneous photocatalytic oxidation process to decompose and mineralize certain recalcitrant contaminants [5].Advanced oxidation processes (AOP), which involve the in situ generation of highly potential chemical oxidants such as hydroxyl radical •OH , with oxidation potential 2.8 V [6] have recently emerged as an important class of technologies for accelerating the oxidation and destruction of a wide range of organic contaminants in polluted water and air [7]. Further, when these processes are applied on a right place, offer a good opportunity to reduce the contaminants concentration from several hundreds ppm (mg\L) to less than 5 ppb (μg\L), therefore, they are called “the treatment processes of the 21st century” [1].As a response, the development of newer eco-friendly methods of destroying these pollutants became an imperative task. Ultimately, research activities centred on advanced oxidation processes (AOPs) for the destruction of synthetic organic species resistant to conventional methods. AOPs rely on in situ generation of highly reactive radical species, mainly .OH by using solar, chemical or other forms of energy. The most attractive feature of AOPs is that this highly potential and strongly oxidizing radical allows the destruction of a broad range of pollutants quickly and non selectively. Among AOPs, heterogeneous photocatalysis has proved to be of real interest as efficient tool for degrading both aquatic and atmospheric organic contaminants. Heterogeneous photocatalysis involve the acceleration of photoreaction in presence of semiconductor photocatalyst. One of the major applications of heterogeneous catalysis is photocatalytic oxidation (PCO) to promote partial or total mineralization of gas phase or liquid phase contaminants to benign substances. Even though degradation begins with a partial degradation, the term ‘photocatalytic degradation’ usually refers to complete photocatalytic oxidation or photomineralisation, essentially to CO2, H2O, NO3-, PO43− and halide ions [8].
In recent years advanced oxidation processes (AOPs) involving hydrogen peroxide, ozone and /or Fenton reagents, with or without a source of UV light have been reported to be useful for the photo-oxidation of organic pollutants in waste waters. It removes substantial amount of Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) from industrial effluents. However, these oxidation methods results in partial oxidation of organics and more often lead to the generation of potentially harmful chemicals. Total oxidation of organics by these technologies is both cost and energy intensive. Among the various AOPs, semiconductor mediated photocatalysis has been accorded great importance over the last few years due to its potential to destroy a wide range of organic and inorganic pollutants at ambient temperatures and pressures, without the production of harmful byproducts [9].For the removal of dye pollutants, traditional physical techniques (adsorption on activated carbon, ultra filtration, reverse osmosis, coagulation by chemical agents, ion exchange on synthetic adsorbent resins… etc.) can generally be used efficiently [10]. Nevertheless, they are non-destructive, since they just transfer organic compounds from water to another phase, thus causing secondary pollution. Consequently, regeneration of the adsorbent materials and post-treatment of solid-wastes, which are expensive operations, are needed. Due to the large degree of aromatics present in dye molecules and the stability of modern dyes, conventional biological treatment methods are ineffective for decolorization and degradation. Furthermore, the majority of dyes is only adsorbed on the sludge and is not degraded. Chlorination and ozonation are also being used for the removal of certain dyes but at slower rates as they have often high operating costs and limited effect on carbon content [11]. These are the reasons why advanced oxidation processes (AOPs) have been growing during the last decade since they are able to deal with the problem of dye destruction in aqueous systems. AOPs such as Fenton and photo-Fenton catalytic reactions, H2O2/UV processes and TiO2 mediated photo-catalysis have been studied under a broad range of experimental conditions in order to reduce the color and organic load of dye containing effluent waste waters [10-12].
Formation of .OH on surface of nano TiO2 [10]:
TiO2 + 
TiO2 (e- CB + h+VB )
