(215t) Effluent Treatment Using Hydrodynamic Cavitation: Vortex Diode As a Cavitating Device

Water pollution because of the discharge of industrial effluents in water bodies like rivers (& seas) has increased substantially in recent years.  The specific contaminants leading to pollution in water include a wide spectrum of hazardous chemicals, heavy metal ions, pathogens, and physical changes such as elevated temperature, turbidity, discoloration, change in pH etc. The industrial wastewater effluents contain organic and inorganic substances in dissolved and suspended forms. Organic substances mainly comprise carbohydrates, fats, oils, grease, surfactants, proteins, pesticides and agricultural chemicals, volatile organic compounds, while inorganic matter includes salinity, hardness, pH, acidity, alkalinity, concentrations of ionized metals such as iron and manganese, and anionic entities such as chlorides, sulfates, sulfides, nitrates and phosphates. These result in high values of biological oxygen demand (BOD) and chemical oxygen demand (COD).

The dyestuff sector is one of the important segments of the chemical industry in India. The dyes are widely used in other industry sectors such as textiles, leather, paper, plastics, painting inks and foodstuffs. The dyestuff industries along with associated industry such as textiles are highly polluting as dyes and pigments are highly visible and easily detectable in effluent streams. It is required to minimize environmental pollution reducing COD apart from colour, even in cases where a small but visible release might be considered as toxicologically rather innocuous. Wastewater from textile processing plants is highly coloured and difficult to decolorize. In addition, the toxicity, mutagenicity and carcinogenicity of azo-dye degradation byproducts are of great environmental concern. Textile and dye wastewater exhibit low BOD to COD ratio (<0.1) indicating non-biodegradability of dyes, requiring specific designs/technologies for treatment.

The conventional technologies for effluent treatment can be divided into two main categories: biological (aerobic process, anaerobic process, anoxic process etc.) and physico-chemical (adsorption/ion exchange, coagulation, oxidation, membrane separation etc). Considering refractory nature of pollutants, most of these methodologies have limitations and no single method is generally suitable for effluent treatment for dye industry effluent.

In the present work, we have studied dye wastewater treatment on synthetic dye solution as well as on real industrial dye wastewater using hydrodynamic cavitation with Vortex Diode as a cavitating device. Hydrodynamic cavitation is relatively recent methodology that can be considered for treatment of effluents containing refractory pollutants. Design & development of new devices for newer applications is a promising area relatively unexplored so far. There are not many reports that indicate the application of hydrodynamic cavitation for wastewater treatment and most of the reported work is on conventional cavitating devices such as orifice and venturi with different configurations. Most of the studies have focused on degradation of Rhodamine B, Reactive Red, Orange G, Acid Red in low concentration ranges (upto75 mg/L) and with the use of oxidants. The results have not been satisfactory in most cases, especially under realistic wastewater conditions and at higher concentrations.

The present research work demonstrates hydrodynamic cavitation for effluent treatment using new cavitation reactor configuration-vortex diode and reports control of degradation by simple mechanical adjustment of the experimental parameters, to achieve significant decolourization/ mineralization of dyes that are difficult to degrade by other conventional means apart from appreciable COD reduction. Vortex diode relies on fluid vortex phenomena for its operation. The basic design of a vortex diode consists of a disc shaped chamber with cylindrical axial and tangential ports. The flow entering the device through the tangential port sets up a vortex, and establishes a large pressure drop across the device, generating cavities with subsequent collapse of cavities (implosion) with localized high temperature and pressure conditions, resulting into degradation of pollutants. 

In the present work Auramine O was used as a model pollutant, not reported so far in the literature. Auramine O is a Basic Yellow dye containing amino and imino groups and also combined with hydrochloric acid. Basic dyes are cationic which have positive electrical charge and classified under the class of Diarylmethane. The effect of pressure drop, effect of feed concentration and effect of flow rate on degradation rate of Auramine O was studied over wide range of concentration from 80 mg/L to 1200 mg/L. The degradation rate was studied on the basis of COD and color reduction.

As the pressure drop increases, cavitation number reduces and hence more number of cavities are formed which results into higher cavitational yield, hence higher degradation was obtained at lower cavitation number. The highest value of degradation constant and regression coefficient was found to be 1.6 x 10^-3 and 0.9943 respectively at 1.5 Kg/cm² pressure drop. The COD reduction increased from almost 15% to 26% and color reduction increased from 11% to 23% with an increase in pressure drop from 0.5 Kg/cm² to 1.5 Kg/cm².

With increase in the flow rate, i.e. an increase in the liquid velocity subsequently reduces the cavitation number which results into higher cavitational yield. Highest degradation rate constant was found to be 1.4 x 10^-3 and regression coefficient was 0.9823 at flow rate of 715 LPH. The COD reduction increased from almost 8% to 21% and color reduction increased from 7% to 17% with an increase in flow rate from 390 LPH to 715 LPH. However, the COD reduction decreased to 16% and color reduction decreased to 14% as the flow rate was increased to 900 LPH.  At increased flow rate cavities start coalescing to form a larger cavitational bubble (cavity cloud). These larger bubbles escape the liquid without collapsing or result into an incomplete and/or cushioned collapse, thus reducing the cavitational yield and thereby reducing the degradation rate after the optimum reached.

The degradation of Auramine O was investigated at different initial dye concentrations. It was observed that lower the dye concentration, shorter is the reaction period needed to degrade the dye. It was observed that the degradation rate decreases with increasing dye concentration. As the other parameters were not changed, the number of cavities and OH- radicals generated in solution approached a steady state. Considering this, the removal rate of apparent color could be decreased by increasing the initial dye concentration. Thus a reaction mixture containing more dilute solutions is expected to exhibit faster rates of degradation. Highest degradation rate was found to be 3.3 x 10^-3 and regression coefficient was 0.9445 at 400 mg/L dye concentration. The COD reduction was observed to be 75%, 27%, 16% and 17% at initial feed concentrations of 80 mg/L, 400 mg/L, 800 mg/L and 1200 mg/L respectively.

To evaluate process performance on real industrial effluent, industrial wastewater with high COD in the range 16000 mg/L was used for the treatment studies. The sample was faint green in color with pH of 6.5. Two different cavitating devices viz. orifice plate and vortex diode were used and the degradation rate was studied in terms of COD reduction. The optimization was done in terms of fluid inlet pressure to the cavitating devices. The sample was treated for 150 minutes. Significant reduction in COD was observed in initial 60 minutes. The degradation rate was higher for vortex diode than orifice plate for same energy input. The effectiveness of cavitation process greatly depends on the geometry of the element generating the cavitation phenomenon and can be characterized by the cavitation number.

The results indicate that the COD reduction efficiency was significantly affected by parameters such as the initial concentration, bulk liquid temperature, pressure drop, and flow rate. Thus, the COD removal efficiencies can be improved by optimization of operating conditions. It has been found that there is substantial enhancement in the extent of degradation of the dye solution using hydrodynamic cavitation. Large-scale operation coupled with better energy efficiency makes this technique a viable alternative for conventional effluent treatment methods. Process Integration with Adsorption/Ion exchange/Oxidation can yield COD levels well within the prescribed limits of pollution apart from drastic colour reduction. Thus, the cavitation technology not only looks promising methodology on its own, but can also be integrated effectively with other conventional methods for achieving complete techno-economically feasible solution for wastewater treatment.