1.1 Introduction
The most fundamental requirement for life is water, which generally gets employed in various industrial and household activities. However, the rapid urbanization and abrupt expansion of industries, viz. textile, pharmaceuticals, fertilizers, distilleries, tanneries and mining industries, result in the formation of harmful recalcitrant wastewater consisting of various hazardous pollutants and eventually leading to a water crisis. Owing to its low biodegradability index, the recalcitrant content in wastewater becomes impervious to biological process treatment and as such is categorized by high chemical oxygen demand (COD) values. The existence of such nonbiodegradable refractory organic compounds, due to their potential carcinogenic activity in wastewater, may lead to serious health hazards. Such refractory compounds result in an increase in waterborne diseases, thereby calling for highly efficient advanced treatment techniques towards water security [1]. Although several conventional techniques extensively utilized for the elimination of toxic contaminants from wastewater have led to significantly good results viz. chemical coagulation [2], adsorption [3], biological methods [4] and membrane filtration [5], various limitations linked with each of these methods has become a matter of serious concern. For instance, the treatment time for adsorption process (pH-dependent) is very long. Also, apart from requiring a source of steam or vacuum for adsorbents regeneration, there is a gradual deterioration of the adsorbent potential with respect to an increase in the number of cycles [3]. Likewise, during chemical coagulation, there is a constant requirement for pH adjustment throughout the analysis. The process also requires the inclusion of various chemicals viz. acids and coagulants such as alum, chloride, polymeric or caustic flocculants, lime and ferric sulfate, besides generating a significant quantity of secondary contaminants and a substantial amount of sludge [2]. Moreover, in the membrane separation process, the permeate flux may significantly decline due to the occurrence of fouling on the membrane surface. The separation efficiency of the process may also be hampered due to membranes with wider pore size distribution [5]. In addition, the biological methods are generally utilized for industrial effluent treatment. Also, it requires the mandatory application of pathogenic as well as nonpathogenic microbes during the treatment [4].
However, over the last few decades, the application of various advanced oxidation processes (AOPs), viz. ozonation, Fenton oxidation, electrochemical oxidation, UV irradiation and ultrasonication, have been established as the most significant and effective techniques in removing numerous toxic and harmful pollutants present in wastewater. Glaze et al. (1987) [6] first suggested the conceptualization of AOP through the generation of hydroxyl radicals in 1987. The superoxides formed during AOPs promote the in situ production of highly reactive oxygen species (ROS) viz. singlet oxygen (1O2), ozone , superoxide radicals , hydrogen peroxide , sulfate radical and hydroxyl radicals with subsequent commencement of various oxidative reactions by reactive oxygen species present in water. Apart from the standalone AOPs, hybrid AOPs can be further employed to obtain a higher degree of degeneration for the refractory contaminants, a reduction in treatment time and an intensification of mineralization process. The hybrid AOPs discussed in this chapter may be classified as ozonation-based AOPs, Fenton-based AOPs, UV-based AOPs and ultrasound-based AOPs. However, the mechanism for the oxidation of complexing agents as well as the inactivation of pathogenic microbes by different AOPs and hybrid AOPs still lack a detailed understanding in regard to the elimination of trace organic contaminants (TrOCs), removal of heavy metal complexes, effect of different organic and inorganic compounds present, generation of various by-products, toxicity evaluation as well as consideration of economic aspects during the reduction of various wastewater contaminants.
This chapter outlines the overview of several standalone AOPs published in the literature for the removal of various complexing agents, heavy metal ions, microbes as well as TrOCs present in industrial and synthetic wastewater, with specific attention paid to hybrid AOPs as categorized previously. A brief profile for both the standalone and hybrid AOPs based on a thorough literature survey has been displayed. Moreover, detailed descriptions along with various experimental conditions associated with each of the processes, which eventually influence the performance efficiency of AOPs, are described. In addition, an economic assessment of both the standalone and hybrid AOPs is also demonstrated. This chapter also suggests future guidelines in the field of water chemistry correlation, nature of contaminants, aspects of mineralization, identification of reaction intermediates, development of rate expressions as well as identification of scale-up parameters.
