Parag R. Gogate

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.

Synthesis of biodiesel from waste cooking oil using sonochemical reactors

Investigation into newer routes of biodiesel synthesis is a key research area especially due to the fluctuations in the conventional fuel prices and the environmental advantages of biodiesel. The present work illustrates the use of sonochemical reactors for the synthesis of biodiesel from waste cooking oil. Transesterification of used frying oil with methanol, in the presence of potassium hydroxide as a catalyst has been investigated using low frequency ultrasonic reactor (20 kHz). Effect of different operating parameters such as alcohol-oil molar ratio, catalyst concentration, temperature, power, pulse and horn position on the extent of conversion of oil have been investigated. The optimum conditions for the transesterification process have been obtained as molar ratio of alcohol to oil as 6:1, catalyst concentration of 1 wt.%, temperature as 45 degrees C and ultrasound power as 200 W with an irradiation time of 40 min. The efficacy of using ultrasound has been compared with the conventional stirring approach based on the use of a six blade turbine with diameter of 1.5 cm operating at 1000 rpm. Also the purification aspects of the final product have been investigated.

Cavitation: an auxiliary technique in wastewater treatment schemes

Abstract New techniques are being added to wastewater treatment schemes for meeting the high standards of environmental regulations. The present work highlights the use of one such technique, cavitation, for wastewater treatment applications. Two types of cavitation phenomena depending on the type of generation have been discussed and the optimum operating and geometric parameters have been presented for maximum efficiency. Experimental results have been given for sonochemical reactors for the degradation of formic acid to supplement the discussion about operating parameters. Available literature for acoustic and hydrodynamic cavitation have been critically assessed and recommendations have been made for enhanced energy efficient applications of cavitation based technologies. Overall it appears that a lot of work is required in designing the cavitational reactors for large scale operations and a few recommendations for theoretical and experimental studies have been made.

Some aspects of the design of sonochemical reactors

The magnitudes of collapse pressures and temperatures as well as the number of free radicals generated at the end of cavitation events are strongly dependent on the operating parameters of the equipment namely, intensity and frequency of irradiation along with the geometrical arrangement of the transducers and the liquid phase physicochemical properties, which affect the initial size of the nuclei and the nucleation process. In the present work, the effect of these parameters on the collapse pressure generated and the maximum size of the cavity during the cavitation phenomena have been studied using the bubble dynamics equation, which considers the compressibility of the medium and a single bubble in isolation. The different liquid phase properties considered include, liquid vapor pressure, viscosity, bulk liquid temperature, surface tension and nature of dissolved gases (polytropic constant of the gas). The theoretical predictions have been also compared with the experimental results observed in the literature qualitatively and some recommendations have been made for the selection of the operating parameters so as to achieve maximum benefits. The work presented here is novel in sense that no earlier studies have considered the compressibility of the liquid medium and tried to evaluate the effect of all the operating parameters on the cavitational activity.

Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst.

The degradation of trichlorophenol has been studied at different intensities of irradiation using ultrasonic horn by changing the power input to the system. Effect of presence of catalyst TiO2 and concentration of catalyst on the degradation rates has also been investigated. The rates of degradation are found to be higher at higher intensities in the absence of catalyst but reverse trend is observed in the presence of catalyst. Adsorption and desorption characteristics of trichlorophenol on TiO2 catalyst have been examined. The catalyst enhances the rates of degradation but it also adsorbs some amount of TCP during the degradation process protecting it from ultrasonic degradation. Thus it is essential to consider the adsorption and desorption kinetics to get proper estimates of the degradation rates when the rates of degradation of TCP are calculated by analyzing the residual concentration of the compound in the liquid. The degradation of the pollutant seems to take place in the liquid only and that too only after desorption of the same from the solid particles. Solid particles seem to play a physical role in the overall degradation scheme, providing additional surface cavitation.

Degradation of dichlorvos containing wastewaters using sonochemical reactors

The present work deals with application of sonochemical reactors for the degradation of dichlorvos containing wastewaters. The sonochemical reactor used in the work is a simple ultrasonic horn type operating at 20 kHz with a power rating of 270 W. The effect of different operating parameters such as operating pH, temperature and power density on the extent of degradation has been investigated initially followed by intensification studies using additives such as hydrogen peroxide, Fentons reagent and CCl(4). It has been observed that low frequency sonochemical reactors can be effectively used for treatment of pesticide wastewaters and acidic conditions and optimum values of temperature and power dissipation favors the degradation of dichlorvos. The efficacy of sonochemical reactors can be further enhanced by using different additives at optimized loadings. Complete removal of the pesticide at the given loading has been obtained using an optimized combination of ultrasound and Fentons chemistry. The controlling mechanism for the sonochemical degradation has been confirmed to be the free radical attack based on the studies involving radical scavengers. The novelty of the present work is clearly established as there have been no earlier studies dealing with degradation of dichlorvos pesticide using sonochemical reactors operating at low frequency which offers distinct advantage in terms of cost and the stability of the reactor.

Sonochemical reactors for waste water treatment: comparison using formic acid degradation as a model reaction

Sonochemical reactors find a promising future in the area of wastewater treatment as one of the advanced oxidation methods. The degradation of formic acid has been studied using different cavitation equipments such as ultrasonic horn, ultrasonic bath, dual frequency flow cell and triple frequency flow cell. The effect of several operating parameters such as time of destruction, initial concentration of the pollutant, intensity and frequency of irradiation, introduction of air, and liquid level in the case of ultrasonic bath on the extent of degradation has been investigated. Comparison of these types of equipment has been made using the two characteristic parameters, i.e. energy efficiency and the cavitational yield. The triple frequency hexagonal flow cell has been found to be more efficient in the destruction of formic acid. The present work clearly illustrates that the multiple frequency multiple transducer reactors can be used with higher energy efficiency and cavitational yield at a larger scale of operation. A new design in terms of hexagonal flow cell and also combination of bath and horn has been used for the first time with improved results.

Removal of Brilliant Green from wastewater using conventional and ultrasonically prepared poly(acrylic acid) hydrogel loaded with kaolin clay: A comparative study

The present work deals with the removal of Brilliant Green dye from wastewater using a poly(acrylic acid) hydrogel composite (PAA-K hydrogel) prepared by incorporation of kaoline clay. The composite has been synthesized using ultrasound assisted polymerization process as well as the conventional process, with an objective of showing the better effectiveness of ultrasound assisted synthesis. It has been observed that the hydrogel prepared by ultrasound assisted polymerization process showed better results. The optimum conditions for the removal of dye are pH of 7, temperature of 35°C, initial dye concentration of 30mg/L and hydrogel loading of 1g. The extent of removal of dye increased with an increase in the contact time and initial dye concentration. A pseudo-second-order kinetic model has been developed to explain the adsorption kinetics of dye on the PAA-K hydrogel. Thermodynamic and kinetic parameters indicate that the adsorption process is spontaneous in nature and the PAA-K hydrogel prepared by ultrasound process is a promising adsorbent compared to conventional process. The obtained adsorption data has also been fitted into commonly used adsorption isotherms and it has been found that Freundlich as well as Langmuir adsorption isotherm models fits well to the experimental results.

Degradation of dichlorvos using hydrodynamic cavitation based treatment strategies.

The degradation of an aqueous solution of dichlorvos, a commonly used pesticide in India, has been systematically investigated using hydrodynamic cavitation reactor. All the experiments have been carried out using a 20 ppm solution of commercially available dichlorvos. The effect of important operating parameters such as inlet pressure (over a range 3-6 bar), temperature (31 °C, 36 °C and 39 °C) and pH (natural pH = 5.7 and acidic pH = 3) on the extent of degradation has been investigated initially. It has been observed that an optimum value of pressure gives maximum degradation whereas low temperature and pH of 3 are favorable. Intensification studies have been carried out using different additives such as hydrogen peroxide, carbon tetrachloride, and Fentons reagent. Use of hydrogen peroxide and carbon tetrachloride resulted in the enhancement of the extent of degradation at optimized conditions but significant enhancement was obtained with the combined use of hydrodynamic cavitation and Fentons chemistry. The maximum extent of degradation as obtained by using a combination of hydrodynamic cavitation and Fentons chemistry was 91.5% in 1h of treatment time. The present work has conclusively established that hydrodynamic cavitation in combination with Fentons chemistry can be effectively used for the degradation of dichlorvos.