A review of imperative technologies for wastewater treatment II: hybrid methods
Introduction
In the previous work (Gogate and Pandit, 2003), five different oxidation technologies viz cavitation, photocatalytic oxidation, ozonation, use of hydrogen peroxide and Fenton chemistry were discussed in detail. These processes work on the principle of generation of free radicals and the subsequent attack of same or the direct attack of the oxidants on the contaminant molecules and are mainly applicable to bio-refractory molecules with an aim to either completely mineralize the contaminants or to convert it into less harmful or lower chain compounds which can be then treated biologically. The efficacy of process depends strongly on the rate of generation of the free radicals along with the extent of contact of the generated radicals with the contaminant molecules and the efficient design should aim at maximizing both these quantities.
The similarity between the mechanism of destruction in the case of different advanced oxidation techniques and some of the common optimum operating conditions point towards the synergism between these methods, and the fact that combination of these advanced oxidation processes should give better results as compared to individual techniques. Moreover, some of the drawbacks of the individual techniques can be eliminated by some characteristics of other techniques. For example, the efficiency of photocatalytic oxidation is severely hampered by two main factors viz mass transfer limitations and fouling of the solid catalyst. If photocatalytic oxidation technique is used in combination with the ultrasonic irradiation, not only the rate of generation of hydroxyl radicals will be increased (due to increased energy dissipation and generation of extreme conditions of temperature and pressure due to the cavitation phenomena), but also due to the acoustic streaming and turbulence created by ultrasonic irradiation, mass transfer resistance will be eliminated. Also, the turbulence helps in cleaning of the catalyst, which increases the efficiency of photocatalytic oxidation process. There are many other combination techniques studied extensively in the literature for a variety of contaminants. The present work aims at reviewing different combination techniques and recommendations have been made in terms of the selection of operating conditions for cost-effective operation.
Section snippets
Ultrasound/H2O2 or ultrasound/ozone processes
The main driving mechanism in the degradation of pollutants using ultrasound is the generation and subsequent attack of the free radicals though some of the reactions have been explained more suitably on the basis of hot-spot theory (localized generation of extreme conditions of temperature and pressure). In the case of reactions where the controlling mechanism is the free radical attack, the use of hydrogen peroxide or ozone should enhance the rates of degradation due to generation of
UV/H2O2 or UV/ozone processes
The principle behind the beneficial effects observed using ultraviolet light in combination with hydrogen peroxide or ozone as compared to the individual application, lies in the fact that the rate of generation of free radicals is significantly enhanced in the case of combination technique, which is very similar to Ultrasound/H2O2 or Ultrasound/O3 processes as discussed earlier; only difference being the energy required for the generation of free radicals from dissociation of ozone or hydrogen
Ozone/H2O2 processes
The capability of ozone in oxidizing various pollutants by direct attack on the different bonds (CC bond (Stowell and Jensen, 1991), aromatic rings (Andreozzi et al., 1991)) are further enhanced in the presence of hydrogen peroxide due to the generation of highly reactive OH radicals. The dissociation of hydrogen peroxide results in the formation of hydroperoxide ion, which attacks the ozone molecule resulting in the formation of hydroxyl radicals (Staehelin and Hoigne, 1982, Staehelin and
Sonophotocatalytic oxidation
In the case of photocatalytic oxidation, the most common problem associated is the reduced efficiency of photo-catalyst with continuous operation possibly due to the adsorption of contaminants at the surface and blocking of the UV activated sites, which makes them unavailable for the destruction. Thus, our aim should be in devising a technique for proper continuous cleaning of the catalyst surface during the photocatalytic operation. Ultrasonic irradiation is one such technique that can be used
Photo–Fenton processes
A combination of hydrogen peroxide and UV radiation with Fe(II) or Fe(III) oxalate ion, the so-called photo–Fenton process produces more hydroxyl radicals in comparison to the conventional Fenton method (Fe(II) with hydrogen peroxide) or the photolysis, thus promoting the rates of degradation of organic pollutants. It should be also noted here that the exact mechanism and the role of Fe(II) and Fe(III) ions; identifying the exact equilibrium concentration of the two species is very complicated
Use of different catalysts in combination with advanced oxidation processes
The use of semiconductor catalyst with ultraviolet/solar irradiations have been reported to give better results as compared to photochemical oxidation and it forms a separate class of oxidation treatment described more commonly as photocatalytic oxidation. In the earlier work (Gogate and Pandit, 2003), photocatalytic oxidation has been described in detail. Solid catalysts have also been reported to enhance the rates of other advanced oxidation processes namely ozonation (Al-Hayck et al., 1989,
Use of advanced oxidation process followed by biological oxidation
The efficiency of the biological oxidation techniques is often hampered by the presence of bio-refractory materials, though these are most conventionally used and economical treatment strategies. On the other hand, though advanced oxidation processes promise degradation of almost all the contaminants, their use is hampered by the fact that the knowledge required for the design and efficient operation of the large-scale reactors is perhaps lacking. Moreover, considering the economical aspects,
SONIWO (sonication followed by wet air oxidation)
The use of wet air oxidation is becoming popular to treat various industrial toxic and refractory wastes. Wet oxidation is a process of subcritical oxidation of organic matter in aqueous phase with oxygen (either in pure form or as air) at elevated temperatures (100–350 °C) and at pressures ranging from 0.5 to 20 MPa. Joshi et al., 1985, Mishra et al., 1995, Matatov-Meytal and Sheintuch, 1998, Imamura, 1999 have covered various aspects of wet oxidation technology in detail.
The slow rate of
Model hybrid method
Fig. 5 represents the model hybrid system based on the analysis presented above as well as in the earlier article (Gogate and Pandit, 2003). It must be noted here that the model hybrid system is solely based on the theoretical analysis and has not been used in the actual practice until now. The aim is to present a universal system for the treatment of different industrial effluents and the cost of the operation must be optimized by adjusting the operating parameters depending on the loading and
Conclusions and recommendations
The expected synergism between different hybrid methods discussed in the present work is mainly due to an identical controlling reaction mechanism, i.e. the free radical attack. Generally, combination of two or more advanced oxidation processes such as UV/ozone, UV/H2O2, Ultrasound/ozone, sonophotochemical/sonophotocatalytic oxidation etc. leads to an enhanced generation of the hydroxyl radicals, which eventually results in higher oxidation rates. The efficacy of the process and the extent of
Acknowledgements
Authors would like to acknowledge the funding of the Indo-French Center 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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