A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions
Introduction
New developments in the variety of fields to meet the ever-increasing requirements of human beings have also led to the presence of new compounds in the effluent streams of processing plants, which are not readily degraded by the conventional effluent treatment methods (Bauer and Fallmann, 1997, Mantzavinos et al., 1997, Otal et al., 1997, Feigelson et al., 2000). The focus on waste minimization and water conservation in recent years has also resulted in the production of concentrated or toxic residues. It is of utmost importance to dispose off these residues in a proper manner as well as to keep the concentration of chemicals in the effluent stream to a certain minimum level in order to comply with the environmental laws, which are becoming more stringent these days. Thus, research into new or more efficient waste water treatment technologies so as to degrade the complex refractory molecules into simpler molecules is vital to combat the deteriorating water quality. It should be noted that some of these newly developed technologies, e.g. cavitation may be more efficient on the laboratory scale and the knowledge required for the scale-up of the same and efficient large-scale operation is lacking (Adewuyi, 2001, Gogate, 2002, Mason, 2000). Hence, it may happen that the new technologies have to be used only as a pretreatment stage followed by the conventional biological oxidation techniques. In this series of articles on the imperative technologies for the wastewater treatment, we have tried to concentrate on the oxidation technologies (operating at ambient conditions) operating both individually as well as in synergism with each other generally described as the hybrid technologies. In the present work, the oxidation technologies operating at ambient conditions individually will be discussed while in the next article hybrid techniques will be discussed. It should also be noted that there are other oxidation technologies as well, such as hydrothermal oxidation processes (further classified as wet air oxidation, sub-critical, critical and super-critical water oxidation processes) applied to variety of model pollutants as well as actual wastewaters, but are not discussed in the present work as these technologies are viable for highly concentrated effluents (COD load>40 000 ppm) to think in terms of return (energy recovery) or investment (high pressure equipments) and it is imperative to develop oxidation technologies operating at ambient conditions, which can also be easily monitored without sophisticated instrumentation for high temperature/high pressure operation. The oxidation technologies discussed in the present work can be classified as advanced oxidation processes (Cavitation, Photocatalytic oxidation and Fenton's chemistry) and chemical oxidation (use of ozone and hydrogen peroxide) and these processes have the potential to degrade the new toxic chemicals, bio-refractory compounds, pesticides, etc. either partially or fully, but most importantly under ambient conditions.
Advanced oxidation processes are defined as the processes that generate hydroxyl radicals in sufficient quantities to be able to oxidize majority of the complex chemicals present in the effluent water. These processes include cavitation (generated either by means of ultrasonic irradiation or using constrictions such as valves, orifice, venturi, etc. in the hydraulic devices (Adewuyi, 2001, Gogate, 2002, Gogate and Pandit, 2001, Gonze et al., 1999, Keil and Swamy, 1999, Pandit and Moholkar, 1996, Moholkar et al., 1999a, Senthilkumar and Pandit, 1999), photocatalytic oxidation (using ultraviolet radiation/near UV light/Sun light in the presence of semiconductor catalyst (Bhatkhande et al., 2002, Blake, 1997, Herrmann, 1999, Yawalkar et al., 2001) and Fenton chemistry (using reaction between Fe ions and hydrogen peroxide, i.e. Fenton's reagent (Venkatadri and Peters, 1993, Bigda, 1995, Bigda, 1996, Nesheiwat and Swanson, 2000). Hydroxyl radicals are powerful oxidizing reagents with an oxidation potential of 2.33 V and exhibits faster rates of oxidation reactions as compared to that using conventional oxidants like hydrogen peroxide or KMnO4 (Gogate et al., 2002a). Hydroxyl radicals react with most organic and many inorganic solutes with high rate constants (Glaze et al., 1992, von Sonntag, 1996, Hoigne, 1997).
Chemical oxidation technologies constitute the use of oxidizing agents such as ozone and hydrogen peroxide, but exhibit lower rates of degradation as compared to the processes based on the free radicals (Echigo et al., 1996, Weavers et al., 1998, Freese et al., 1999, Fung et al., 2000a, Zwinter and Krimmel, 2000, Arslan and Balcioglu, 2001a, Gogate et al., 2002a). Moreover, additional mass transfer resistances between the pollutant and the oxidizing agents generally hamper the overall efficacy of the process especially for the ozonation process. Free radicals are generated when ozone is used in combination with hydrogen peroxide or action of ozone or hydrogen peroxide is supplemented by other energy dissipating components such as use of UV/sun light or ultrasound and these hybrid techniques have been found to result in lower treatment times as compared to any of the individual techniques (Weavers et al., 2000, Fung et al., 2000a, Gogate et al., 2002a) though the cost/energy efficiency will be dependent on the operating conditions and the type of the effluent. The discussion about the hybrid techniques will be presented in details in the next article.
Majority of these oxidation technologies, however, fail to degrade the complex compounds completely, especially in the case of real wastewaters and moreover, cannot be used for processing the large volumes of waste generated with the present level of knowledge about these reactors (e.g. Commenges et al. (2000) have shown that cavitation has failed to give substantial degradation in the case of real industrial effluent, whereas similar results have been reported by Beltran et al. (1997) for the case of photocatalytic oxidation being applied to distillery and tomato wastewaters). Hence, these can be used to degrade the complex residue up to a certain level of toxicity beyond which the conventional methods can be successfully used for further degradation (Rachwal et al., 1992, Beltran et al., 1999a, Beltran et al., 1999b, Engwall et al., 1999, Kitis et al., 1999, Mastin et al., 2001). It should also be noted that the efficacy of conventional methods would also depend on the level of toxicity reached in the pretreatment stages, using the oxidation techniques. Thus, it is important to select proper pretreatment technique to improve the overall efficiency of the wastewater treatment unit. In this work, the above-mentioned five oxidation techniques have been discussed, also highlighting the work required for transferring the efficient laboratory scale technique to large-scale operations.
Section snippets
Cavitation
Cavitation is defined as the phenomena of the formation, growth and subsequent collapse of microbubbles or cavities occurring in extremely small interval of time (milliseconds), releasing large magnitudes of energy (Lorimer and Mason, 1987, Mason and Lorimer, 1988, Suslick, 1990, Shah et al., 1999). It should also be noted that though the release of energy is over very small pocket, cavitation events occur at multiple locations in the reactor simultaneously (some indication about the number of
Photocatalysis
The photocatalytic or photochemical degradation processes are gaining importance in the area of wastewater treatment, since these processes result in complete mineralization with operation at mild conditions of temperature and pressure. There are good reviews available on this subject by Fox and Duley, 1993, Legrini et al., 1993, Kamat, 1993, Hoffmann et al., 1995 covering the analysis of the studies prior to 1995 and depicting basics of the processes including the mechanism of oxidation of
Fenton chemistry
The oxidation system based on the Fenton's reagent (hydrogen peroxide in the presence of a ferrous salt) has been used for the treatment of both organic and inorganic substances under laboratory conditions as well as real effluents from different resources like chemical manufacturers, refinery and fuel terminals, engine and metal cleaning etc. (Bigda, 1996). The process is based on the formation of reactive oxidizing species, able to efficiently degrade the pollutants of the wastewater stream
Oxidation systems with direct attack of oxidants
Ozonation and addition of hydrogen peroxide belong to separate class of oxidation systems as compared to the above three methods, but their combination, i.e. ozone/hydrogen peroxide or presence of additional energy dissipation in terms of UV/near-UV/Sunlight or ultrasonic irradiation results in the generation of free hydroxyl radicals and these combination methods again belong to the class of advanced oxidation processes. These combination methods will be discussed in detail in the future work
Use of hydrogen peroxide
Hydrogen peroxide is another strong oxidant readily applied to wastewater treatment in the past. For wastewater applications 50% hydrogen peroxide solution is normally recommended while 35% solution gives lower rates though higher safety and 70% may produce detonable mixtures with many of the organic compounds. Hydrogen peroxide has been found to be effective in degradation of compounds or treatment of real wastewaters requiring less stringent oxidation conditions (Ayling and Castrantas, 1981)
Conclusions and recommendations
Advanced oxidation processes viz. cavitation, photocatalytic oxidation and Fenton chemistry work on the principle of generation of free radicals and subsequent attack of the same on the contaminant molecules whereas ozonation and use of hydrogen peroxide work on the direct attack of the oxidants or via formation of free radicals if another energy dissipating mode such as ultrasound or UV irradiation is present. The efficacy of process depends strongly on the rate of generation of the free
Acknowledgements
Authors would like to acknowledge the funding of the Indo-French Centre for Promotion of Advanced Research (Centre Franco-Indien Pour La Promotion de La Recherche Avancee), New Delhi, India for the collaborative research work.
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