Degradation of 2,4-dinitrophenol using a combination of hydrodynamic cavitation,chemical and advanced oxidation processes

In the present work, degradation of 2,4-dinitrophenol (DNP), a persistent organic contaminant with high toxicity and very low biodegradability has been investigated using combination of hydrodynamic cavitation (HC) and chemical/advanced oxidation. The cavitating conditions have been generated using orifice plate as a cavitating device. Initially, the optimization of basic operating parameters have been done by performing experiments over varying inlet pressure (over the range of 3–6 bar), temperature (30 °C, 35 °C and 40 °C) and solution pH (over the range of 3–11). Subsequently, combined treatment strategies have been investigated for process intensification of the degradation process. The effect of HC combined with chemical oxidation processes such as hydrogen peroxide (HC/H₂O₂), ferrous activated persulfate (HC/Na₂S₂O₈/FeSO₄) and HC coupled with advanced oxidation processes such as conventional Fenton (HC/FeSO₄/H₂O₂), advanced Fenton (HC/Fe/H₂O₂) and Fenton-like process (HC/CuO/H₂O₂) on the extent of degradation of DNP have also been investigated at optimized conditions of pH 4, temperature of 35 °C and inlet pressure of 4 bar. Kinetic study revealed that degradation of DNP fitted first order kinetics for all the approaches under investigation. Complete degradation with maximum rate of DNP degradation has been observed for the combined HC/Fenton process. The energy consumption analysis for hydrodynamic cavitation based process has been done on the basis of cavitational yield. Degradation intermediates have also been identified and quantified in the current work. The synergistic index calculated for all the combined processes indicates HC/Fenton process is more feasible than the combination of HC with other Fenton like processes.     

Degradation of benzene present in wastewater using hydrodynamic cavitation in combination with air

The degradation of benzene present in wastewater using hydrodynamic cavitation (HC) alone as well as in combination with air has been studied using nozzles as cavitating device of HC reactor. Initially, the energy efficiency of the HC reactor operated at different inlet pressures was determined using the calorimetric studies. Maximum energy efficiency of 53.4% was obtained at an inlet pressure of 3.9 bar. The treatment processes were compared under adiabatic as well as isothermal conditions and it was observed that under the adiabatic condition, the extent of degradation is higher as compared to isothermal condition. Studies related to the understanding the effect of inlet pressure (range of 1.8–3.9 bar) revealed that the maximum degradation as 98.9% was obtained at 2.4 bar pressure using the individual operation of HC under adiabatic conditions and in 70 min of treatment. The combination of HC with air was investigated at different air flow rates with best results for maximum degradation of benzene achieved at air flow rate of 60 mL/sec. A novel approach of using cavitation for a limited fraction of total treatment time was also demonstrated to be beneficial in terms of the extent of degradation as well as energy requirements and cost of operation. Based on the cavitational intensity, the resonant radius of aggregates of cavitation bubbles was also determined for distilled water as well as for aqueous solution of benzene. Overall, significant benefits of using HC combined with air have been demonstrated for degradation of benzene along with fundamental understanding into cavitation effects.     

Intensification of biogas production using pretreatment based on hydrodynamic cavitation

The present work investigates the application of hydrodynamic cavitation (HC) for the pretreatment of wheat straw with an objective of enhancing the biogas production. The hydrodynamic cavitation reactor is based on a stator and rotor assembly. The effect of three different speeds of rotor (2300, 2500, 2700 rpm), wheat straw to water ratios (0.5%, 1% and 1.5% wt/wt) and also treatment times as 2, 4 and 6 min have been investigated in the work using the design of experiments (DOE) approach. It was observed that the methane yield of 31.8 ml was obtained with untreated wheat straw whereas 77.9 ml was obtained with HC pre-treated wheat straw confirming the favourable changes during the pre-treatment. The combined pre-treatment using KOH and HC gave maximum yield of biogas as 172.3 ml. Overall, it has been established that significant enhancement in the biogas production can be obtained due to the pretreatment using HC which can also be further intensified by combination with chemical treatment.     

Degradation of diclofenac sodium using combined processes based on hydrodynamic cavitation and heterogeneous photocatalysis

Diclofenac sodium, a widely detected pharmaceutical drug in wastewater samples, has been selected as a model pollutant for degradation using novel combined approach of hydrodynamic cavitation and heterogeneous photocatalysis. A slit venturi has been used as cavitating device in the hydrodynamic cavitation reactor. The effect of various operating parameters such as inlet fluid pressure (2–4 bar) and initial pH of the solution (4–7.5) on the extent of degradation have been studied. The maximum extent of degradation of diclofenac sodium was obtained at inlet fluid pressure of 3 bar and initial pH as 4 using hydrodynamic cavitation alone. The loadings of TiO₂ and H₂O₂ have been optimised to maximise the extent of degradation of diclofenac sodium. Kinetic study revealed that the degradation of diclofenac sodium fitted first order kinetics over the selected range of operating protocols. It has been observed that combination of hydrodynamic cavitation with UV, UV/TiO₂ and UV/TiO₂/H₂O₂ results in enhanced extents of degradation as compared to the individual schemes. The maximum extent of degradation as 95% with 76% reduction in TOC has been observed using hydrodynamic cavitation in conjunction with UV/TiO₂/H₂O₂ under the optimised operating conditions. The diclofenac sodium degradation byproducts have been identified using LC/MS analysis.     

Wastewater treatment using hybrid treatment schemes based on cavitation and Fenton chemistry: A review

Advanced oxidation processes such as cavitation and Fenton chemistry have shown considerable promise for wastewater treatment applications due to the ease of operation and simple reactor design. In this review, hybrid methods based on cavitation coupled with Fenton process for the treatment of wastewater have been discussed. The basics of individual processes (Acoustic cavitation, Hydrodynamic cavitation, Fenton chemistry) have been discussed initially highlighting the need for combined processes. The different types of reactors used for the combined processes have been discussed with some recommendations for large scale operation. The effects of important operating parameters such as solution temperature, initial pH, initial pollutant concentration and Fenton’s reagent dosage have been discussed with guidelines for selection of optimum parameters. The optimization of power density is necessary for ultrasonic processes (US) and combined processes (US/Fenton) whereas the inlet pressure needs to be optimized in the case of Hydrodynamic cavitation (HC) based processes. An overview of different pollutants degraded under optimized conditions using HC/Fenton and US/Fenton process with comparison with individual processes have been presented. It has been observed that the main mechanism for the synergy of the combined process depends on the generation of additional hydroxyl radicals and its proper utilization for the degradation of the pollutant, which is strongly dependent on the loading of hydrogen peroxide. Overall, efficient wastewater treatment with high degree of energy efficiency can be achieved using combined process operating under optimized conditions, as compared to the individual process.     

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 Fenton’s 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 Fenton’s chemistry. The maximum extent of degradation as obtained by using a combination of hydrodynamic cavitation and Fenton’s chemistry was 91.5% in 1 h of treatment time. The present work has conclusively established that hydrodynamic cavitation in combination with Fenton’s chemistry can be effectively used for the degradation of dichlorvos.     

Degradation of organic wastewater by hydrodynamic cavitation combined with acoustic cavitation

In this paper, the decomposition of Rhodamine B (RhB) by hydrodynamic cavitation (HC), acoustic cavitation (AC) and the combination of these individual methods (HAC) have been investigated. The degradation of 20 L RhB aqueous solution was carried out in a self-designed HAC reactor, where hydrodynamic cavitation and acoustic cavitation could take place in the same space simultaneously. The effects of initial concentration, inlet pressure, solution temperature and ultrasonic power were studied and discussed. Obvious synergies were found in the HAC process. The combined method achieved the best conversion, and the synergistic effect in HAC was even up to 119% with the ultrasonic power of 220 W in a treatment time of 30 min. The time-independent synergistic factor based on rate constant was introduced and the maximum value reached 40% in the HAC system. Besides, the hybrid HAC method showed great superiority in energy efficiency at lower ultrasonic power (88–176 W). Therefore, HAC technology can be visualized as a promising method for wastewater treatment with good scale-up possibilities.     

Intensification of Red-G dye degradation used in the dyeing of alpaca wool by advanced oxidation processes assisted by hydrodynamic cavitation

Red-G dye is one of the main dyes used in the textile industry to dye alpaca wool. Therefore, considering the large volume of processed wool in Perú, the development of efficient technologies for its removal is a present scientific issue. In this study, an integrated system based on hydrodynamic cavitation (HC) and photo-Fenton process was evaluated to remove the Red-G dye. Using a hybrid cavitation device (venturi + orifice plate), the effect of pH was evaluated, achieving 21 % of removal at pH 2 which was more than 80 % higher compared to pH 4 and 6. The effect of temperature was also evaluated in HC-system at pH 2, where percentage of dye degradation increased at lower temperatures (around 20 °C). Then, 50.7 % of dye was removed under optimized condition of HC-assisted Fenton process (FeSO₄:H₂O₂ of 1:30), that value was improved strongly by UV-light incorporation in the HC-system, increasing to 99 % removal efficiency with respect to HC-assisted Fenton process and reducing the time to 15 min. Finally, the developed cavitation device in combination with photo-Fenton process removed efficiently the dye and thus could be considered an interesting option for application to real wastewater.     

Biodiesel production process intensification using a rotor-stator type generator of hydrodynamic cavitation

Triglyceride transesterification for biodiesel production is a model reaction which is used to compare the conversion efficiency, yield, reaction time, energy consumption, scalability and cost estimation of different reactor technology and energy source. This work describes an efficient, fast and cost-effective procedure for biodiesel preparation using a rotating generator of hydrodynamic cavitation (HC). The base-catalyzed transesterification (methanol/sodium hydroxide) has been carried out using refined and bleached palm oil and waste vegetable cooking oil. The novel HC unit is a continuous rotor-stator type reactor in which reagents are directly fed into the controlled cavitation chamber. The high-speed rotation of the reactor creates micron-sized droplets of the immiscible reacting mixture leading to outstanding mass and heat transfer and enhancing the kinetics of the transesterification reaction which completes much more quickly than traditional methods. All the biodiesel samples obtained respect the ASTM standard and present fatty acid methyl ester contents of >99% m/m in both feedstocks. The electrical energy consumption of the HC reactor is 0.030 kW h per L of produced crude biodiesel, making this innovative technology really quite competitive. The reactor can be easily scaled-up, from producing a few hundred to thousands of liters of biodiesel per hour while avoiding the risk of orifices clogging with oil impurities, which may occur in conventional HC reactors. Furthermore it requires minimal installation space due to its compact design, which enhances overall security.     

Performance and energetic analysis of hydrodynamic cavitation and potential integration with existing advanced oxidation processes: A case study for real life greywater treatment

The current work is a “first of a kind” report on the feasibility and efficacy of hydrodynamic cavitation integrated Advanced Oxidation Processes (AOP’s) towards treatment of a real life greywater stream in form of kitchen wastewater. The work has been carried out in a sequential manner starting with geometry optimization of orifice plate (cavitating device) followed by studying the effects of inlet pressure, pH, effluent dilution ratio on degradation of TOC and COD. Under optimized conditions of pH 3, 4 bar pressure, TOC and COD reduction of 18.23 and 25% were obtained using HC for a period of 120 min. To improve the performance of HC, further studies were carried out by integrating H₂O₂ and O₃ with HC. Using 5 g/h optimum dosage of H₂O₂, 87.5% reduction in COD was obtained beyond which it started decreasing. Moreover, integrating O₃ (57.5% reduction in COD) increased the treatment cost. However, a hybrid process (HC + H₂O₂ + O₃) yielded 76.26 and 98.25% reductions in TOC and COD within 60 min. The energetics of all the processes and the treatment costs were studied in detail and it was concluded that combined process of HC + H₂O₂ + O₃ surpassed by far the performances of HC + H₂O₂ and HC + O₃.     

Multi-objective optimization of the cavitation generation unit structure of an advanced rotational hydrodynamic cavitation reactor

Hydrodynamic cavitation (HC) has been widely considered a promising technique for industrial-scale process intensifications. The effectiveness of HC is determined by the performance of hydrodynamic cavitation reactors (HCRs). The advanced rotational HCRs (ARHCRs) proposed recently have shown superior performance in various applications, while the research on the structural optimization is still absent. The present study, for the first time, identifies optimal structures of the cavitation generation units of a representative ARHCR by combining genetic algorithm (GA) and computational fluid dynamics, with the objectives of maximizing the total vapor volume, $V_{\mathit{vapor}}$ , and minimizing the total torque of the rotor wall, ${\overrightarrow{M}}_z$ . Four important geometrical factors, namely, diameter (D), interaction distance (s), height (h), and inclination angle (θ), were specified as the design variables. Two high-performance fitness functions for $V_{\mathit{vapor}}$ and ${\overrightarrow{M}}_z$ were established from a central composite design with 25 cases. After performing 10,001 simulations of GA, a Pareto front with 1630 non-dominated points was obtained. The results reveal that the values of s and θ of the Pareto front concentrated on their lower (i.e., 1.5 mm) and upper limits (i.e., 18.75°), respectively, while the values of D and h were scattered in their variation regions. In comparison to the original model, a representative global optimal point increased the $V_{\mathit{vapor}}$ by 156% and decreased the ${\overrightarrow{M}}_z$ by 14%. The corresponding improved mechanism was revealed by analyzing the flow field. The findings of this work can strongly support the fundamental understanding, design, and application of ARHCRs for process intensifications.     

Decolorization of azo dyes Orange G using hydrodynamic cavitation coupled with heterogeneous Fenton process

The present work demonstrates the application of the combination of hydrodynamic cavitation (HC) and the heterogeneous Fenton process (HF, Fe⁰/H₂O₂) for the decolorization of azo dye Orange G (OG). The effects of main affecting operation conditions such as the inlet fluid pressure, initial concentration of OG, H₂O₂ and zero valent iron (ZVI), the fixed position of ZVI, and medium pH on decolorization efficiency were discussed with guidelines for selection of optimum parameters. The results revealed that the acidic conditions are preferred for OG decolorizaiton. The decolorization rate increased with increasing H₂O₂ and ZVI concentration and decreased with increasing OG initial concentration. Besides, the decolorization rate was strongly dependent on the fixed position of ZVI. The analysis results of degradation products using liquid chromatography–ESI–TOF mass spectrometry revealed that the degradation mechanism of OG proceeds mainly via reductive cleavage of the azo linkage due to the attack of hydroxyl radical. The present work has conclusively established that the combination of HC and HF can be more energy efficient and gives higher decolorization rate of OG as compared with HC and HF alone.     

Degradation of reactive orange 4 dye using hydrodynamic cavitation based hybrid techniques

In the present work, degradation of reactive orange 4 dye (RO4) has been investigated using hydrodynamic cavitation (HC) and in combination with other AOP’s. In the hybrid techniques, combination of hydrodynamic cavitation and other oxidizing agents such as H₂O₂ and ozone have been used to get the enhanced degradation efficiency through HC device. The hydrodynamic cavitation was first optimized in terms of different operating parameters such as operating inlet pressure, cavitation number and pH of the operating medium to get the maximum degradation of RO4. Following the optimization of HC parameters, the degradation of RO4 was carried out using the combination of HC with H₂O₂ and ozone. It has been found that the efficiency of the HC can be improved significantly by combining it with H₂O₂ and ozone. The mineralization rate of RO4 increases considerably with 14.67% mineralization taking place using HC alone increases to 31.90% by combining it with H₂O₂ and further increases to 76.25% through the combination of HC and ozone. The synergetic coefficient of greater than one for the hybrid processes of HC + H₂O₂ and HC + Ozone has suggested that the combination of HC with other oxidizing agents is better than the individual processes for the degradation of dye effluent containing RO4. The combination of HC with ozone proves to be the most energy efficient method for the degradation of RO4 as compared to HC alone and the hybrid process of HC and H₂O₂.     

Quantifying OH radical generation in hydrodynamic cavitation via coumarin dosimetry: Influence of operating parameters and cavitation devices

Hydrodynamic cavitation (HC) has been extensively investigated for effluent treatment applications. Performance of HC devices or processes is often reported in terms of degradation of organic pollutants rather than quantification of hydroxyl (OH) radicals. In this study, generation of OH radicals in vortex based cavitation device using coumarin dosimetry was quantified. Coumarin was used as the chemical probe with an initial concentration of 100 µM (15 ppm). Generation of OH radicals was quantified by analysing generated single hydroxylated products. The influence of operating parameters such as pH and type of acid used to adjust pH, dissolved oxygen, and inlet and outlet pressures was investigated. Acidic pH was found to be more conducive for generating OH radicals and therefore subsequent experiments were performed at pH of 3. Sulphuric acid was found to be more than three times effective than hydrochloric acid in generating OH radicals. Effect of initial levels of dissolved oxygen was found to influence OH radical generation. Performance of vortex based cavitation device was then compared with other commonly used cavitation devices based on orifice and venturi. The vortex based cavitation device was found to outperform the orifice and venturi based devices in terms of initial per-pass factor. Influence of device scale (nominal flow rate through the device) on performance was then evaluated. The results presented for these devices unambiguously quantifies their cavitational performance. The presented results will be useful for evaluating computational models and stimulate further development of predictive computational models in this challenging area.     

Intensified depolymerization of aqueous polyacrylamide solution using combined processes based on hydrodynamic cavitation,ozone, ultraviolet light and hydrogen peroxide

The present work deals with intensification of depolymerization of polyacrylamide (PAM) solution using hydrodynamic cavitation (HC) reactors based on a combination with hydrogen peroxide (H₂O₂), ozone (O₃) and ultraviolet (UV) irradiation. Effect of inlet pressure in hydrodynamic cavitation reactor and power dissipation in the case of UV irradiation on the extent of viscosity reduction has been investigated. The combined approaches such as HC + UV, HC + O₃, HC + H₂O₂, UV + H₂O₂ and UV + O₃ have been subsequently investigated and found to be more efficient as compared to individual approaches. For the approach based on HC + UV + H₂O₂, the extent of viscosity reduction under the optimized conditions of HC (3 bar inlet pressure) + UV (8 W power) + H₂O₂ (0.2% loading) was 97.27% in 180 min whereas individual operations of HC (3 bar inlet pressure) and UV (8 W power) resulted in about 35.38% and 40.83% intrinsic viscosity reduction in 180 min respectively. In the case of HC (3 bar inlet pressure) + UV (8 W power) + ozone (400 mg/h flow rate) approach, the extent of viscosity reduction was 89.06% whereas individual processes of only ozone (400 mg/h flow rate), ozone (400 mg/h flow rate) + HC (3 bar inlet pressure) and ozone (400 mg/h flow rate) + UV (8 W power) resulted in lower extent of viscosity reduction as 50.34%, 60.65% and 75.31% respectively. The chemical structure of the treated PAM by all approaches was also characterized using FTIR (Fourier transform infrared) spectra and it was established that no significant chemical structure changes were obtained during the treatment. Overall, it can be said that the combination of HC + UV + H₂O₂ is an efficient approach for the depolymerization of PAM solution.     

ANN based modelling of hydrodynamic cavitation processes: Biomass pre-treatment and wastewater treatment

We have developed artificial neural network (ANN) based models for simulating two application examples of hydrodynamic cavitation (HC) namely, biomass pre-treatment to enhance biogas and degradation of organic pollutants in water. The first case reports data on influence of number of passes through HC reactor on bio-methane generation from bagasse. The second case reports data on influence of HC reactor scale on degradation of dichloroaniline (DCA). Similar to most of the HC based applications, the availability of experimental data for these two applications is rather limited. In this work a systematic methodology for developing ANN model is presented. The models were shown to describe the experimental data very well. The ANN models were then evaluated for their ability to interpolate and extrapolate. Despite the limited data, the ANN models were able to simulate and interpolate the data for two very different and complex HC applications very well. The extrapolated results of biomethane generation in terms of number of passes were consistent with the intuitive understanding. The extrapolated results in terms of elapsed time were however not consistent with the intuitive understanding. The ANN model was able to generate intuitively consistent extrapolated results for degradation of DCA in terms of number of passes as well as scale of HC reactor. The results will be useful for developing quantitative models of complex HC applications.     

Scale-up of vortex based hydrodynamic cavitation devices: A case of degradation of di-chloro aniline in water

Hydrodynamic cavitation (HC) is being increasingly used in a wide range of applications. Unlike ultrasonic cavitation, HC is scalable and has been used at large scale industrial applications. However, no information about influence of scale on performance of HC is available in the open literature. In this work, we present for the first time, experimental data on use of HC for degradation of complex organic pollutants in water on four different scales (~200 times scale-up in terms of capacity). Vortex based HC devices offer various advantages like early inception, high cavitational yield and significantly lower propensity to clogging and erosion. We have used vortex based HC devices in this work. 2,4 dichloroaniline (DCA) – an aromatic compound with multiple functional groups was considered as a model pollutant. Degradation of DCA in water was performed using vortex-based HC devices with characteristic throat dimension, d_t as 3, 6, 12 and 38 mm with scale-up of almost 200 time based on the flow rates (1.3 to 247 LPM). Considering the experimental constraints on operating the largest scale HC device, the experimental data is presented here at only one value of pressure drop across HC device (280 kPa). A previously used per-pass degradation model was extended to describe the experimental data for the pollutant used in this study and a generalised form is presented. The degradation performance was found to decrease with increase in the scale and then plateaus. Appropriate correlation was developed based on the experimental data. The developed approach and presented results provide a sound basis and a data set for further development of comprehensive multi-scale modelling of HC devices.     

Intensifying extraction of biomolecules from macroalgae using vortex based hydrodynamic cavitation device

Macroalgae have a tremendous potential to become an important renewable resource for valuable biomolecules and chemicals. New and improved ways of cell disruption and of enhancing rate as well as yield of extraction of valuable products from macroalgae are needed to fully realise this potential. In this work, hydrodynamic cavitation (HC) was used for intensifying rate and yield of extraction of phycoerythrin, proteins and carbohydrates from marine macroalgae Palmaria palmata. We use vortex-based HC devices which do not use small restrictions like orifice-based HC devices or moving parts like rotor–stator based HC devices. A bench scale setup with a nominal slurry flow rate of 20 LPM was established. Dried and powdered macroalgae was used. Influence of key operating parameters like pressure drop and number of passes on extraction performance (the rate and yield) was measured. A simple, yet effective model was developed and used for interpreting and describing experimental data. The results indicate that there exists an optimum pressure drop across the device at which extraction performance is maximum. The extraction performance with HC was found to be significantly better than the stirred vessels. HC has resulted in 2 to 20 times improvement in the rate of extraction of phycoerythrin (R-PE), proteins and carbohydrates. Based on the results obtained in this work, pressure drop of 200 kPa and number of passes through the HC devices of about 100 were found to be most effective for HC-assisted intensified extraction from macroalgae. The presented results and model will be useful for harnessing vortex-based HC devices for intensifying the extraction of valuable products from macroalgae.     

Hybrid hydrodynamic cavitation (HC) technique for the treatment and disinfection of lake water

Water reclamation from lakes needs to be accomplished efficiently and affordably to ensure the availability of clean, disinfected water for society. Previous treatment techniques, such as coagulation, adsorption, photolysis, ultraviolet light, and ozonation, are not economically feasible on a large scale. This study investigated the effectiveness of standalone HC and hybrid HC + H₂O₂ treatment techniques for treating lake water. The effect of pH (3 to 9), inlet pressure (4 to 6 bar), and H₂O₂ loading (1 to 5 g/L) were examined. At pH = 3, inlet pressure of 5 bar and H₂O₂ loadings of 3 g/L, maximum COD and BOD removal were achieved·H₂O₂ was observed to significantly improve the performance of the HC when used as a chemical oxidant. In an optimal operating condition, a COD removal of 54.5 % and a BOD removal of 51.5 % using HC alone for 1 h is observed. HC combined with H₂O₂ removed 64 % of both COD and BOD. The hybrid HC + H₂O₂ treatment technique resulted in a nearly 100% removal of pathogens. The results of this study indicate that the HC-based technique is an effective method for removing contaminants and disinfection of the lake water.