Wastewater treatment: a novel energy efficient hydrodynamic cavitational technique
Introduction
Treatment of industrial wastewaters, particularly textile wastewaters having used dye solution, has attracted many researchers in recent years. The problem of colour in textile wastewaters and the importance of research in this area have been well discussed by Neill et al. [1]. Generally, aerobic treatment provides effective colour removal, in some instances, in addition to reducing the sludge volume. In many cases, textile mills are being asked to treat on site, highly coloured wastewaters before discharging into municipal sewers or surface waters. Of the myriad chemical, physicochemical and biological treatment methods available for treating the textile wastewaters, biological methods generally provide the most cost-effective solution. Anaerobic reactors are particularly effective for decolourization of wastewaters containing azo dyes. However, the aromatic amine products are usually not degraded under anaerobic conditions. These circumstances lead to the need to develop some other alternative technology for the decolourization of wastewaters as each and every technique discussed above has its own disadvantages.
Hydrodynamic cavitation occurs when a liquid undergoes a dynamic pressure reduction due to constriction devices like venturi, orifice plates, etc., while operating under constant temperature. Phenomenon of the hydrodynamic cavitation results in the formation of cavities filled with a vapour–gas mixture inside the liquid flow or at the boundary of the constriction devices due to a local pressure drop caused by the movement of the liquid. Mixing, emulsification, homogenization and dispersion are some of the commonly studied areas using this cavitation as well as acoustic cavitation. These effects are due to a substantial plurality of force effects acting on the treated mixture of components due to the collapse of the cavitation bubbles. The collapse of the cavitation bubbles near the boundary of “liquid–solid particles” phases results in the dispersion of these particles in the fluid and in the formation of the suspension, while in the “liquid–liquid” system, one fluid is dispersed into the other fluid and it results in the formation of the emulsion. In both cases, the boundary between the phases is destroyed or eroded and a dispersion of the phases is formed.
The collapse of the cavitation bubbles also initiates physico-chemical effects, in addition to the above mechanical effects, resulting in the intensification of physical dispersion processes. Here, the physical effects include the production of shear forces and shock waves whereas the chemical effects result into the generation of radicals. But, the application of this hydrodynamic cavitation method for getting the desired chemical effects requires rigorous optimisation taking into consideration various internal and external parameters as they are interrelated. For example, increase in viscosity, decrease in surface tension and density of the fluid, as well as an increase in the vapour content reduces the efficiency of the cavitation effect.
Application of ultrasound to wastewater treatment is not new to the researchers [2], [3], [4], [5], [6], [7], [8]. Violent collapse of the cavities in hydrodynamic cavitation systems results in the formation of reactive hydrogen atoms and hydroxyl radicals which recombine to form hydrogen peroxide as similar to acoustic cavitation. However there are not many reports indicating the applications of hydrodynamic cavitation reactors for wastewater treatment scheme. Kalumuck and Chahine [9] have studied destruction of p-nitrophenol in recirculating flow loops using a variety of cavitating jet configurations and operating conditions and shown that, indeed, hydrodynamic cavitation degraded p-nitrophenol. Submerged cavitating liquid jets were found to generate a two order of magnitude increase in energy efficiency compared to the ultrasonic means. Jyoti and Pandit [10] have studied disinfection of water using different techniques and reported that hydrodynamic cavitation is an economically attractive alternative compared to techniques such as ozonation and heat sterilisation for reducing bacterial counts.
Botha [11] and Botha and Buckley [12] studied combination of hydrodynamic cavitation and UV irradiation and reported higher cavitational yields. It has been found that such a combination increases the destructive efficiency while maintaining the cost effectiveness.
Also, there exists a commercial process, known as CAV-OX process, developed by Magnum Water Technology Inc., California. This is a hybrid system involving hydrodynamic cavitation, UV irradiation and oxidation with hydrogen peroxide. Several contaminants of concern such as pentachlorophenol (PCP), benzene, toluene, ethyl benzene, xylenes, cyanide, phenol, atrazine have been successfully degraded to a significant extent. Case studies at pilot plant scale showed that the process is effective for a wide variety of effluents obtained from various chemical industries.
In addition to the above, few other papers have also discussed the application of this method [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. The present work shows an attempt in utilising this technique using multiple hole orifice plates for waste water treatment as used by Vichare et al. [23] for the KI decomposition studies. They have used KI decomposition as a model reaction and confirmed the occurrence as well as the intensity and thereby the efficiency of the multiple hole orifice plates in generating the cavitation. But, KI decomposition studies have no industrial relevance. Thus, in terms of making this technique practically applicable the present work concentrates on applying cavitation using the same constriction device, multiple hole orifice plates, to an industrially relevant reaction. For this purpose we have selected the degradation of rhodamine B, a textile dye effluent. This is due to the fact that similar rhodamine B studies have already been carried out for optimising cavitation energy but using sonochemical equipments. Also, the importance of the treatment of this dye has already been mentioned [26]. Thus, it becomes more appropriate in doing the same reaction by hydrodynamic cavitation in order to have a comparison among various cavitating equipments (acoustic and hydrodynamic). Also, the present study aims to check whether the trends observed with the model reaction of KI are valid for other industrial reactions. The effect of the cavitating device geometry for a different reaction also needs confirmation. With this objective in mind, the experiments have been carried out with rhodamine B degradation.
Section snippets
Materials
Rhodamine B (tetraethylrhodamine) was a laboratory reagent grade product and was used without further purification. In order to simulate the conditions of this dye solution from industry, tap water was used to dissolve and to get the required strength of rhodamine B solution (typically 5–6 μg/ml of 50 l solution).
All the experiments were carried out within the temperature range of 35–40 °C. Degradation of the dye was followed by checking its absorbance value at 553 nm using UV–VIS
Hydraulic characteristics
In the earlier work [23], the hydraulic characteristics of the orifice plates have been discussed in detail. The characteristics have been found to be much similar to that of a conventional centrifugal pump.
Cavitation number has been defined to characterise the hydrodynamic as well as the cavitating conditions existing downstream of the orifice as follows:where, p2 is the fully recovered downstream pressure, pv is the vapour pressure of the liquid and v0 is the liquid velocity
Conclusions
Present work is a significant contribution to the area of application of the wastewater treatment scheme as there are only very few reports where hydrodynamic cavitation has been used for the degradation of complex chemical moieties. In hydrodynamic cavitation, altering the flow geometry and hence the turbulent pressure fluctuation frequency (fT) could enhance the cavitational yield. Optimum frequency of turbulence can be achieved by manipulating the flow conditions and geometry of the
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.
References (30)
- et al.
Chem. Eng. Sci.
(1993) - et al.
- et al.
Chem. Eng. Res. Des.
(1997) - et al.
Chem. Eng. Sci.
(2000) - et al.
J. Chem. Technol. Biotechnol.
(1999) - et al.
Env. Sci. Technol.
(1991) - et al.
J. Phys. Chem.
(1991) - et al.
Env. Sci. Technol.
(1992) - et al.
Env. Sci. Technol.
(1992) - et al.
J. Phys. Chem.
(1994)
J. Phy. Chem. A.
Env. Sci. Technol.
FED (Am. Soc. Mech. Eng.)
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