Aniruddha B. Pandit

A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions

Nowadays, due to the increasing presence of molecules, refractory to the microorganisms in the wastewater streams, the conventional biological methods cannot be used for complete treatment of the effluent and hence, introduction of newer technologies to degrade these refractory molecules into smaller molecules, which can be further oxidized by biological methods, has become imperative. The present work aims at highlighting five different oxidation processes operating at ambient conditions viz. cavitation, photocatalytic oxidation, Fentons chemistry (belonging to the class of advanced oxidation processes) and ozonation, use of hydrogen peroxide (belonging to the class of chemical oxidation technologies). The work highlights the basics of these individual processes including the optimum operating parameters and the reactor design aspects with a complete overview of the various applications to wastewater treatment in the recent years. In the next article of this two article series on imperative technologies, hybrid methods (basically combination of the oxidation processes) will be discussed and the current work forms a useful foundation for the work focusing on hybrid technologies.

A review of imperative technologies for wastewater treatment II: Hybrid methods

Abstract In the first part of this two article series on the imperative technologies for wastewater treatment, a review of oxidation processes operating at ambient conditions was presented. It has been observed that none of the methods can be used individually in wastewater treatment applications with good economics and high degree of energy efficiency. Moreover, the knowledge required for the large-scale design and application is perhaps lacking. In the present work, an overview of hybrid methods (the majority are a combination of advanced oxidation processes) has been presented. Hybrid methods viz Ultrasound/H2O2 or ozone, UV/H2O2 or ozone, Ozone/H2O2, Sono-photochemical oxidation, Photo–Fenton processes, catalytic advanced oxidation processes, use of advanced oxidation processes in conjunction with biological oxidation, SONIWO (sonochemical degradation followed by wet air oxidation), and CAV-OX have been discussed with specific reference to the principles behind the expected synergism, different reactor configurations used and optimum considerations for the operating and geometric parameters. An overview of different chemicals degraded has been presented. Some of the important works evaluating the application of these processes to real effluents have been described in detail. Some guidelines for the future work required to facilitate efficient large-scale operation have been given. A model effluent treatment scheme based on the various techniques discussed in the present work has been presented.

Mechanically agitated gas-liquid reactors

The hydrodynamic, heat and mass transfer characteristics of mechanically agitated contactors have been critically reviewed. The mixing time (θmix) can be correlated well by a model based on circulation path and average circulation velocity. Flow patterns in mechanically agitated contactors (single phase) provided with disk turbine, pitched blade turbine and propeller can be characterised. The minimum impeller speed required for the gas induction (Ncr) in hollow shaft impellers can be predicted; the minimum impeller speed required for surface aeration (Ns) can also be predicted. Different strategies of operating mechanically agitated contactors have been examined. The advantages of multistage contactors over single contactors have been stressed. Recommendations have been made for correlations which can be used for design purpose; lacunae in the available literature have been delineated and recommendations for further work have been made.

Ultrasound enhanced degradation of Rhodamine B: optimization with power density.

Degradation of Rhodamine B, a waste water dye effluent was studied under the influence of ultrasound. In the present study, optimization of this degradation was carried out with power parameters, namely, power density (W/ml) and power intensity (W/cm2) using different ultrasonic equipments. From the studies, it has been observed that the degradation does not increase indefinitely with an increase in the power parameters, but instead, it reaches an optimum value and then decreases with a further increase in the power parameters. Also, these optima were not the same in all the ultrasonic equipments in which these studies were carried out. Difference in the type of acoustic field generated may be responsible for the different power optima observed with different ultrasonic equipments. The comparative study of the degradation of Rhodamine B using various ultrasonic equipments establishes a relationship between the area-specific parameter (intensity) and the volume-specific parameter (density) of reactivity.

Cavitation reaction engineering

1. Sources and Types of Cavitation.- 1.1. Introduction.- 1.2. Hydrodynamic Cavitation.- 1.2a. Cavitation Number.- 1.3. Acoustic Cavitation.- 1.4. Optic and Particle Cavitation.- 2. Cavitation Bubble Dynamics.- 2.1. Introduction.- 2.2. Bubble Dynamics.- 2.2a. Bubble Nuclei: Blake Threshold.- 2.2b. Dynamic Equations of a Spherical Bubble: Analysis of an Empty Bubble.- 2.2c. Dynamics of a Gas Bubble.- 2.2d. Equation Involving Compressibility of a Liquid.- 2.2e. Rayleigh Analysis of a Cavity and Its Extensions.- 2.2f. Adiabatic Collapse of a Gas-Filled Cavity.- 2.2g. Damping of Stable Bubbles.- 2.2h. Modifications for Hydrodynamic Cavitation.- 2.3. Cluster Dynamics.- 2.3a. Model Equations for Cluster Dynamics.- 2.4. Heat and Mass Transfer Effects in Cavitation.- 2.4a. Rectified Diffusion.- 2.4b. Rectified Heat Transfer in Bubble Oscillations.- 2.4c. Effect of Simultaneous Diffusion and Evaporation on Bubble Dynamics.- 2.5. Concluding Remarks.- 3. Factors Affecting Cavitation Behavior.- 3.1. Introduction.- 3.2. Factors Affecting Cavity Behavior in Hydrodynamic Cavitation.- 3.2a. Recovered Discharge Pressure and Time of Pressure Recovery.- 3.2b. Downstream Pipe Size.- 3.2c. Orifice-to-Pipe Diameter Ratio.- 3.2d. Initial Bubble Radius and the Noncondensable Gas Fraction in Cavitating Liquids.- 3.3. Factors Affecting Cavity Behavior in Acoustic Cavitation.- 3.3a. Acoustic Frequency.- 3.3b. Acoustic Intensity.- 3.3c. External Pressure.- 3.3d. Nature of the Dissolved Gas.- 3.3e. Physical Properties of the Cavitating Medium.- 3.3f. Pretreatment of the Liquid.- 3.3g. Bulk Liquid Temperature.- 3.3h. Initial Bubble Radius.- 3.4. Factors Affecting Optical Cavitation.- 3.5. Factors Affecting Cavity Cluster Behavior in Hydrodynamic Cavitation.- 3.5a. Effect of Recovery Pressure.- 3.5b. Effect of Time of Pressure Recovery.- 3.5c. Effect of Initial Cluster Radius.- 3.5d. Effect of Bubble Volume Fraction.- 3.6. Factors Affecting Cavity Cluster Behavior in Acoustic Cavitation.- 3.7. Concluding Remarks.- 4. Gas-Liquid Cavitation Chemistry.- 4.1. Introduction.- 4.2. Mechanisms for Cavitation Reaction.- 4.3. Factors Affecting Cavitation Chemistry.- 4.3a. Acoustic Frequency.- 4.3b. Acoustic Intensity.- 4.3c. External Pressure.- 4.3d. Gas Solubility.- 4.3e. Nature of the Gas.- 4.3f. Liquid Properties.- 4.3g. Bulk Temperature.- 4.4. Inorganic and Organic Cavitation Reactions.- 4.4a. Water.- 4.4b. Effect of Other Dissolved Gases.- 4.4c. Inorganic Reactions.- 4.4d. Organic Reactions.- 4.4e. Solute Hydrophobicity and Reactivity.- 4.5. Depolymerization and Repolymerization Reactions.- 4.6. Ultrasound and Homogeneous Oxidation.- 4.7. Ultrasound and Liquid-Liquid Phase-Transfer Reactions.- 5. Gas-Liquid-Solid Cavitation Chemistry.- 5.1. Introduction.- 5.2. General Effects of Ultrasound on Gas-Liquid-Solid Reactions.- 5.2a. Surface Cleaning.- 5.2b. Morphological Changes in Metal Catalysts.- 5.2c. Cavitation Erosion.- 5.2d. Shape, Size, and Specific Area of Particle.- 5.2e. Improved Mass Transport.- 5.2f. Mechanisms for Gas-Liquid-Solid Cavitation Reaction.- 5.3. Specific Role of Ultrasound on Gas-Liquid-Solid Reactions.- 5.3a. Catalyst and Reagent Preparation.- 5.3b. Effects of Ultrasound on Catalyst-Reagent Activation.- 5.3c. Catalyst Induction Period.- 5.3d. Reactions with Continuous Ultrasound.- 5.3e. Effects of Ultrasound on Catalyst Regeneration.- 5.4. Case Studies.- 5.4a. Cavitation Effect on Heterogeneous Catalytic Oxidation.- 5.4b. Cavitation Effect on Liquid-Solid Phase-Transfer Reactions.- 5.4c. Cavitation Effect on Gas-Liquid-Solid Biological Reactions.- 5.4d. Cavitation Effect on Photo-oxidation Reactions.- 5.4e. Cavitation-Induced Microfusion.- 6. Cavitation Reactors.- 6.1. Introduction.- 6.2. Hydrodynamic Cavitation Reactors.- 6.2a. High-Pressure Homogenizer.- 6.3. Acoustic Cavitation Reactors.- 6.3a. Transducers and Horns.- 6.3b. Measurements of Acoustic Power.- 6.3c. Methods for Measuring Amplitude.- 6.3d. Hydrophones.- 6.3e. Sonochemical Reactor Geometries.- 6.3f. Qualitative Considerations for Reactor Choice, Scaleup, and Optimization.- 6.4. Laser Cavitation Reactors.- 6.5. Some Additional Considerations for Flow Reactors.- 6.6. Health and Safety Aspects of Laboratory Reactors.- 6.7. Integration of Cavitation into Existing Scaled-Up Processes.- 6.8. Concluding Remarks.- 7. Models for Cavitation Reactors.- 7.1. Introduction.- 7.2. General Considerations for a Gas-Liquid Cavitation Reactor Model.- 7.2a. Bubble Dynamics.- 7.2b. Pyrolysis Reactions in the Bubble.- 7.2c. Free Radical Reactions in the Liquid Film.- 7.3. Modeling a Batch Gas-Liquid Acoustic Reactor.- 7.3a. Physical Description.- 7.3b. Model Equations and Analysis.- 7.3c. Further Improvements in the Model.- 7.4. Characterization of the Reaction Zone.- 7.4a. Physical Description.- 7.4b. Reaction Zone based on Probability Density function.- 7.5. Reactor Design and Scaleup based on the Concept of Cavitation Yield.- 7.6. Memory Effect in a Loop Cavitation Reactor.- 7.7. Concluding Remarks.- 8. Energy Efficiency and the Economics of the Cavitation Conversion Process.- 8.1. Introduction.- 8.2. Efficiency of Energy Transformation.- 8.2a. Steps for Energy Transformation.- 8.2b. Equipment Efficiency.- 8.2c. Energy Efficiency for the Cavity Implosion.- 8.2d. Cavitation Yield Model.- 8.2e. G-Method for Energy Efficiency.- 8.2f. Case Studies.- 8.3. Economics of Cavitation Conversion Processes.- 8.3a. Case Study 1.- 8.3b. Case Study 2.- 8.3c. Sonochemistry vs. Photochemistry.- 8.4. Concluding Remarks.- 9. CAV-OX Process.- 9.1. Introduction.- 9.2. Description of Process.- 9.3. Process Economics.- 9.4. Case Studies.- Case 1. Superfund Site for Wood-Treatment, Pensacola, Florida.- Case 2. Chevron Service Station, Long Beach, California.- Case 3. Presidio Army Base, San Francisco, California.- Case 4. Chemical Plant, East Coast, United States.- Case 5. Mannesmann Anlagenbau, Salzburg, Austria.- Case 6. Steel Mill, South Korea.- Case 7. Perdue Farms, Bridgewater, Virginia.- Case 8. Southern California Edison, Los Angeles, California.- Case 9. Corporacion Mexicana de Investigacion en Materials, S.A. de C.V. (CMIMSA).- Case 10. University of Natal, Durban, South Africa.- Nomenclature.- References.

Wastewater treatment: a novel energy efficient hydrodynamic cavitational technique

A novel method of treating a dye solution has been studied by hydrodynamic cavitation using multiple hole orifice plates. The present work deals with the effect of geometry of the multiple hole orifice plates on the degradation of a cationic dye rhodamine B (rhB) solution. The efficiency of this technique has been compared with the cavitation generated by ultrasound and it has been found that there is substantial enhancement in the extent of degradation of this dye solution using hydrodynamic cavitation. Large-scale operation coupled with better energy efficiency makes this technique a viable alternative for conventional cavitational reactors.

Experimental quantification of chemical effects of hydrodynamic cavitation

Abstract The classical ultrasonically induced reaction of the decomposition of aqueous KI Solution has been studied on a 501 hydrodynamic cavitation set-up. This reaction, which has been previously studied using acoustic cavitation, has been shown to be influenced by hydrodynamic cavitation as well. Methodology has been suggested to enhance the reaction rates, when using hydrodynamic cavitation set-up. Manipulation of the throttling device (orifice plates) and the operating conditions have increased the iodine liberation rates, three times more than acoustic cavitation at equivalent power dissipation rates. The scale-up possibilities of hydrodynamic cavitation as a means of an alternative to acoustic cavitation have been discussed.

Kinetics of p-nitrophenol degradation: effect of reaction conditions and cavitational parameters for a multiple frequency system

In order to assess the ultrasound dual frequency effects, sonochemical degradation of p-nitrophenol (p-NP) in an aqueous solution has been carried out with ultrasound at three operating frequencies, i.e., at 25, 40 kHz each independently, and the combination of two frequencies (25+40 kHz) simultaneously. Based on the rates of degradation, a kinetic study has been performed which leads to the evaluation of apparent kinetic rate constants for the degradation of p-NP. The influence of various parameters including initial solution pH, bulk solution temperature, on the degradation of p-NP was studied for these three frequency modes (25, 40 and 25+40 kHz) in order to investigate the temporal behaviour of this reaction, especially when it was operated in combined mode (25+40 kHz). The energy efficiency in the case of dual frequency mode is much better than single frequency modes. Modelling and cavity dynamics simulations have also been carried out to explain the observed effects. During combined mode operation, an improvement in the rate of degradation has been observed. The variation in the rate constants has been explained based on the difference in the acoustic pressure field in different systems including ultrasonic bath and dual frequency processor.

Correlations to predict droplet size in ultrasonic atomisation

In conventional two fluid nozzles, the high velocity air imparts its energy to the liquid and disrupts the liquid sheet into droplets. If the energy for liquid sheet fragmentation can be supplied by the use of ultrasonic energy, finer droplets with high sphericity and uniform size distribution can be achieved. The other advantage of ultrasound induced atomisation process is the lower momentum associated with ejected droplets compared to the momentum carried by the droplets formed using conventional nozzles. This has advantage in coating and granulation processes. An ultrasonic probe sonicator was designed with a facility for liquid feed arrangement and was used to atomise the liquid into droplets. An ingenious method of droplet measurement was attempted by capturing the droplets on a filter paper (size variation with regard to wicking was uniform in all cases) and these are subjected to image analysis to obtain the droplet sizes. This procedure was evaluated by high-speed photography of droplets ejected at one particular experimental condition and these were image analysed. The correlations proposed in the literature to predict droplet sizes using ultrasound do not take into account all the relevant parameters. In this work, a truly universal correlation is proposed which accounts for the effects of physico-chemical properties of the liquid (flow rate, viscosity, density and surface tension), and ultrasonic properties like amplitude, frequency and the area of vibrating surface. The significant contribution of this work is to define dimensionless numbers incorporating ultrasonic parameters, taking cue from the conventional numbers that define the significance of different forces involved in droplet formation. The universal correlations proposed are robust and can be used for designing ultrasonic atomisers for different applications. Among the correlations proposed here, those ones that are based on the dimensionless numbers and Davies approach predict droplet sizes within acceptable limits of deviation. Also, an empirical correlation from experimental data has been proposed in this work.

Mixing in Mechanically Agitated Gas-Liquid Contactors, Bubble Columns and Modified Bubble Columns

Abstract Mixing time measurements were made in 300 and 1000 mm i.d. mechanically agitated contactors with different types of impellers, located at different heights from the bottom of the vessel. Mixing time measurements were also made in 150, 200, 385 and 1000 mm i.d. bubble columns with varying liquid heights. Transient pH measurement and conductivity measurement were used to measure the mixing times. Impeller speed was varied in the range of 3.33–20 r/sec in the case of mechanically agitated contactors and gas superficial velocity was varied in the range of 10–250 mm/sec in bubble columns. Effect of physical properties of the fluid (surface tension, ionic strength, liquid viscosity) and that of the non-Newtonian behavior on mixing time was studied. Mixing time in the presence of drag reducing agents was also investigated. In the range of variables covered in this work mixing time in mechanically agitated contactors and bubble columns was found to be in the range of 4–6 Mixing time predictions based on the longest loop length and circulation velocity are made in the presence and absence of a gas for mechanically agitat A procedure is given for the prediction of the critical impeller speed for gas phase dispersion in mechanically agitated contactors.