Intensification of industrial wastewater treatment using hydrodynamic cavitation combined with advanced oxidation at operating capacity of 70 L

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Treatment of Coking Wastewater Using Hydrodynamic Cavitation Coupled with Fenton Oxidation Process

Effective and economical processes for the advanced treatment of coking wastewater were urgently needed to reduce the persistent organic pollutants of external drainage. In the present work, we investigated the degradation of organic pollutants in coking wastewater through IHC/FO (imping stream hydrodynamic cavitation (IHC) coupled with the Fenton oxidation (FO) process) and IHC alone for their feasibility in the advanced treatment of coking wastewater. To select the optimum parameters, attention was paid to the effects of main operation conditions including inlet fluid pressure, medium temperature, initial pH, reaction time, and initial Fe(II) and initial H2O2 concentrations. The results showed that the effects of conditions that need energy to be maintained (such as initial pH and inlet pressure) on the organic pollutant removal efficiency through IHC/FO were less pronounced than those through IHC alone. Moreover, the application of IHC/FO could remove more organic pollutants from coking wastewater than IHC even at an energy-efficient condition. For example, the highest COD removal efficiency of 12.5% was achieved in the IHC treatment at 0.4 MPa, pH 3, and 60 min for the reaction time. In the case of IHC/FO, the maximum COD removal of 33.2% was obtained at pH 7, 0.1 MPa, 12 mmol/L H2O2, and 3 mmol/L Fe2+ after reacting for 15 min. The ultraviolet and visible spectrophotometry (UV-Vis) absorption spectra and gas chromatography and mass spectrometry (GC-MS) analysis further revealed that the kinds and amounts of pollutants (especially those that had benzenes) remaining in water treated through IHC/FO were much fewer and smaller than in water treated through IHC alone. The better performances of IHC/FO than IHC alone were likely related to the more hydroxyl radicals produced through IHC/FO. Taken together, our findings indicate that IHC/FO has great application potential in the advanced treatment of coking wastewater.

Integrated physicochemical processes to tackle high-COD wastewater from pharmaceutical industry

Wastewater decontamination in pharmaceuticals is crucial to prevent environmental and health risks from API residues and other contaminants. Advanced oxidation processes (AOPs) combined with cavitational treatments offer effective solutions. Challenges include designing reactors on a large scale and monitoring the effectiveness and synergies of the hybrid technology. In the present work, pilot-scale treatment of a real high COD (485 g/L) pharmaceutical wastewater (PW) was investigated using hydrodynamic cavitation (HC) operated individually at 330 L/h or in combination with oxidants and electrical discharge (ED) with cold plasma (15 kV and 48 kHz). The first approach consisted of PW cavitational treatment alone of 7 L of 1:100 diluted PW at a HC-induced pressure of 60 bar and a flow rate of 330 L/h. However, this strategy did not provide satisfactory results for COD (∼15% less), and only when HC treatment was extended to more than 30 min in a recirculation mode, encouraging results were obtained (∼45% COD reduction). Consequently, a hybrid approach combining HC with ED-cold plasma was chosen to treat this high-COD PW. Aiming to establish an efficient flow-through hybrid process, after optimising all cavitation and electrical discharge parameters (45 bar HC pressure and 10 kHz ED frequency), the best COD abatement of ∼50 % was recorded with a 1:50 diluted PW. However, a subsequent adsorption step over activated carbon was required to achieve an almost quantitative COD reduction (95%+). Our integrated physicochemical process proved to be extremely efficient in treating high-COD industrial wastewater and resulted in a remarkable reduction of the COD value. In addition, the residual surfactants content in the PW were also drastically reduced (98%+) when a small amount of oxidants was added in the hybrid HC/ED treatment.

Simultaneous hydrodynamic cavitation and nanosecond pulse discharge plasma enhanced by oxygen injection

A novel Hydrodynamic Cavitation-Assisted Oxygen Plasma (HCAOP) process, which employs a venturi tube and oxygen injection, has been developed for enhancing the production and utilization of hydroxyl radicals (·OH) in the degradation of organic pollutants. This study has systematically investigated the fluid characteristics and discharge properties of the gas-liquid two-phase body in the venturi tube. The hydraulic cavitation two-phase body discharge is initiated by the bridging of the cavitation cloud between the electrodes. The discharge mode transitions from diffuse to spark to corona as the oxygen flow rate increases. The spark discharge has the highest current and discharge energy. Excessive oxygen results in the change of the flow from bubbly to annular and a subsequent decrease in discharge energy. The effects of cavitation intensity, oxygen flow rate, and power polarity on discharge characteristics and ·OH production were evaluated using terephthalic acid as a fluorescent probe. It was found that injecting 3 standard liter per minute (SLPM) of oxygen increased the ·OH yield by 6 times with only 1.2 times increase in power, whereas<0.5 SLPM of oxygen did not improve the ·OH yield due to lower breakdown voltage. Negative polarity voltage increased the breakdown voltage and ·OH yield due to asymmetric density and pressure distribution in the throat tube. This polarity effect was explained by numerical simulation. Using indigo carmine (E132) as a model pollutant, the HCAOP process degraded 20 mg/L of dye in 5 L water within 2 min following a first-order reaction. The lowest electric energy per order (EEO) was 0.26 (kWh/m3/order). The HCAOP process is a highly efficient flow-type advanced oxidation process with potential industrial applications.

Detergent Dissolution Intensification via Energy-Efficient Hydrodynamic Cavitation Reactors

In this study, we explored the potential of hydrodynamic cavitation (HC) for use in dissolution of liquid and powder detergents. For this, microfluidic and polyether ether ketone (PEEK) tube HC reactors with different configurations were employed, and the results from the reactors were compared with a magnetic stirrer, as well as a tergotometer. According to our results PEEK tube HC reactors present the best performance for dissolution of liquid and powder detergents. In the case of liquid detergent, for the same level of initial concentration and comparable final dissolution, the PEEK tube consumed 16.7 and 70% of the energy and time of a tergotometer and 16.7 and 14.8% of that of a magnetic stirrer, respectively. In the case of powder detergent, the PEEK tube used 12% less power than a tergotometer and 81.2% less power than a magnetic stirrer. Additionally, the time required to dissolve the detergent was reduced significantly from 1200 s in the tergotometer and 1800 s in the magnetic stirrer to just 50 s in the PEEK tube. These results suggest that HC could significantly improve the dissolution rate of liquid and powder detergents and energy consumption in washing machines.

Pretreatment of herbal waste using sonication

The effect of hydrodynamic cavitation (HC) and the manner it affects the biodegradability of herbal waste suspended on municipal wastewater subjected to mechanical pre-treatment was examined in this paper. The HC was performed at an optimal inlet pressure equal to 3.5 bar and with the cavitation number of 0.11; the number of recirculation passes through the cavitation zone amounted to 30.5. The BOD₅/COD ratio was enhanced by more than 70% between the 5th and 10th minute of the process, indicating the enhanced biodegradability of herbal waste shortly. Fiber component analysis, FT-IR/ATR, TGA and SEM analysis were conducted to check the findings and to demonstrate changes in the chemical and morphological structure of herbal waste. It confirmed that hydrodynamic cavitation visibly influenced the herbal composition and their structural morphology, decreased hemicellulose, cellulose and lignin content, but did not form the by-products affecting the subsequent biological treatment of herbal waste.

Hydrodynamic cavitation coupled with zero-valent iron produces radical sulfate radicals by sulfite activation to degrade direct red 83

In the present research, hydrodynamic cavitation (HC) and zero-valent iron (ZVI) were used to generate sulfate radicals through sulfite activation as a new source of sulfate for the efficient degradation of Direct Red 83 (DR83). A systematic analysis was carried out to examine the effects of operational parameters, including the pH of the solution, the doses of ZVI and sulfite salts, and the composition of the mixed media. Based on the results, the degradation efficiency of HC/ZVI/sulfite is highly dependent upon the pH of the solution and the dosage of both ZVI and sulfite. Degradation efficiency decreased significantly with increasing solution pH due to a lower corrosion rate for ZVI at high pH. The corrosion rate of ZVI can be accelerated by releasing Fe2+ ions in an acid medium, reducing the concentration of radicals generated even though ZVI is solid/originally non-soluble in water. The degradation efficiency of the HC/ZVI/sulfite process (95.54 % + 2.87%) was found to be significantly higher under optimal conditions than either of the individual processes (<6% for ZVI and sulfite and 68.21±3.41% for HC). Based on the first-order kinetic model, the HC/ZVI/sulfite process has the highest degradation constant of 0.035±0.002 min-1. The contribution of radicals to the degradation of DR83 by the HC/ZVI/sulfite process was 78.92%, while the contribution of SO4•- and •OH radicals was 51.57% and 48.43%, respectively. In the presence of HCO3- and CO32- ions, DR83 degradation is retarded, whereas SO42- and Cl- ions promote degradation. To summarise, the HC/ZVI/sulfite treatment can be viewed as an innovative and promising method of treating recalcitrant textile wastewater.

A Review on Rotary Generators of Hydrodynamic Cavitation for Wastewater Treatment and Enhancement of Anaerobic Digestion Process

The issue of ever-increasing amounts of waste activated sludge (WAS) produced from biological wastewater treatment plants (WWTPs) is pointed out. WAS can be effectively reduced in the anaerobic digestion (AD) process, where methanogens break down organic matter and simultaneously produce biogas in the absence of oxygen, mainly methane and CO2. Biomethane can then be effectively used in gas turbines to produce electricity and power a part of WWTPs. Hydrodynamic cavitation (HC) has been identified as a potential technique that can improve the AD process and enhance biogas yield. Rotary generators of hydrodynamic cavitation (RGHCs) that have gained considerable popularity due to their promising results and scalability are presented. Operation, their underlying mechanisms, parameters for performance evaluation, and their division based on geometry of cavitation generation units (CGUs) are presented. Their current use in the field of wastewater treatment is presented, with the focus on WAS pre/treatment. In addition, comparison of achieved results with RGHCs relevant to the enhancement of AD process is presented.

Modeling of Hydrodynamic Cavitation Reactors: Reflections on Present Status and Path Forward

Hydrodynamic cavitation (HC) is finding ever increasing applications in water, energy, chemicals, and materials sectors. HC generates intense shear, localized hot spots, and hydroxyl radicals, which are harnessed for realizing desired physicochemical transformations. Despite identification of HC as one of the most promising technology platforms, its potential is not yet adequately translated in practice. Lack of appropriate models for design, optimization, and scale-up of HC reactors is one of the primary reasons for this. In this work, the current status of modeling of HC reactors is presented. Various prevailing approaches covering empirical, phenomenological, and multiscale models are critically reviewed in light of personal experience of their application. Use of these approaches for different applications such as biomass pretreatment and wastewater treatment is briefly discussed. Some comments on extending these models for other applications like emulsions and crystallization are included. The presented models and discussion will be useful for practicing engineers and scientists interested in applying HC for a variety of applications. Some thoughts on further advances in modeling of HC reactors and outlook are shared, which may stimulate further research on improving the fidelity of computational models of HC reactors.

The advanced treatment of textile printing and dyeing wastewater by hydrodynamic cavitation and ozone: Degradation, mechanism, and transformation of dissolved organic matter

The emission standards for textile printing and dyeing wastewater are stricter due to serious environmental issues. A novel technology, hydrodynamic cavitation combined with ozone (HC + O₃), has attracted wide attention in wastewater advanced treatment, whereas the contaminants removal mechanism and transformation of dissolved organic matter (DOM) were rarely reported. This study investigated the removal efficiency and mechanism of HC + O₃. The maximum removal rates of UV₂₅₄, chrominance, COD_Cr, and TOC were 64.99%, 91.90%, 32.30%, and 36.67% in 60 min, respectively, at the inlet pressure of 0.15 MPa and O₃ dosage of 6.25 mmol/L. The synergetic coefficient of HC + O₃ was 2.77. The removal of contaminants was the synergy of ¹O₂, ·OH and ·O₂^-, and high molecular weight and strong aromaticity organic matters were degraded effectively. The main components in DOM were tryptophan-like and tyrosine-like, which were effectively removed after HC + O₃. Meanwhile, most DOM had decreased to low apparent relative molecular weight (LARMW) compounds. Additionally, the HC + O₃ effluent can reach the emission standard in 60 min for 8.07 USD/m³. It can be concluded that HC + O₃ is an effective technology for the advanced treatment of industrial wastewater. This study will provide suggestions for the engineering application of HC + O₃.

Contemporary application of microbubble technology in water treatment

Microbubble (MB) technology constitutes a suite of promising low-cost technologies with potential applications in various sectors. Microbubbles (MBs) are tiny gas bubbles with diameters in the micrometre range of 10-100 μm. Along with their small size, they share special characteristics like slow buoyancy, large gas-liquid interfacial area and high mass-transfer efficiency. Initially, the review examines the key dissimilarities among the different types of microbubble generators (MBG) towards economic large-scale production of MBs. The applications of MBs to explore their effectiveness at different stages of wastewater treatment extending from aeration, separation/ flotation, ozonation, disinfection and other processes are investigated. A summary of the recent advances of MBs in real and synthetic wastewater treatment, existing research gaps, and limitations in upscaling of the technology, conclusion and future recommendations is detailed. A critical analysis of the energetics and treatment cost of combined approaches of MB technology with other advanced oxidation processes (AOPs) is carried out highlighting the potential applicability of hybrid technology in large-scale wastewater treatment.

Degradation of pefloxacin by hybrid hydrodynamic cavitation with H2O2 and O3

The degradation of toxic chemicals, antibiotics and other residues in organic wastewater has attracted much attention. Among various degradation technologies, hydrodynamic cavitation (HC) reactors have the advantage of being simple to operate. Through the combination of HC and other oxidants, the removal efficiency and energy efficiency of organic matter can be greatly improved, and the consumption of chemicals and the processing costs can be reduced. In this work, HC technology combined with oxidants was used to degrade pefloxacin (PEF), and the effect of different operating conditions on PEF degradation was investigated. The results indicated that the removal efficiency of PEF treated with HC alone was 84.9% under the optimal HC conditions of pH 3.3 and 120 min, which is much higher than that (35.5%) of pH 5.3. When co-treating the PEF solution with HC and H₂O₂ at 0.3 MPa and pH 5.3, the optimal molar ratio of PEF to H₂O₂ was 1:5, the highest PEF removal efficiency was 69.7%, and the synergy index (SI) was 4.4. When combining HC with O₃, the PEF removal efficiency gradually elevated with increasing ozone addition. When the addition amount of ozone was 0.675 g/h, the removal efficiency of PEF was the highest, which was 91.5% after treatment of 20 min. The intermediate products in the reaction process were analyzed based on UV-Vis spectroscopy and LC-MS, and the mechanism and reaction pathways of PEF were proposed.

Printing ink wastewater treatment using combined hydrodynamic cavitation and pH fixation

Printing ink wastewater (PIW) carries a heavy load of pollutants, the composition of which makes treatment difficult, especially when trying to minimize the pollution load. According to the latter, the present study aims to investigate PIW treatment with different various methods and to determine the maximum color, COD (chemical oxygen demand) and TSS (total suspended solids) removal. First, hydrodynamic cavitation (HC) was tested and the effect of hydrogen peroxide dosage (0-10 g L^-1), and pH (3, 5, 8, 10) was examined concerning the removal of PIW initial COD concentrations 4000 and 8000 mg L^-1. Removal was high (more than 81%) only at pH 5 in HC reactor. The second method involved treatment with separate pH fixation of the undiluted PIW (COD 17000 mg L^-1, actual pH 8 ± 0.2). This treatment, maximized removals, reaching reduction of the initial values more than 91%, at pH 5. Finally, PIW was treated with a combination of the above methods, leading to 93-97% removals for 8000 mg L^-1 PIW treatment and 97-99% for 17000 mg L^-1 PIW respectively. Process cost calculations showed that the latter method is an effective and affordable treatment method for PIW streams, while toxicity tests of the treated PIW showed substantial toxicity reduction.

Advanced oxidation processes: Performance, advantages, and scale-up of emerging technologies

Advanced oxidation processes (AOPs) are promising technologies for partial or complete mineralization of contaminants of emerging concern by highly reactive hydroxyl, hydroperoxyl, superoxide, and sulphate radicals. Detailed investigations and reviews have been reported for conventional AOP systems that have been installed in full-scale wastewater treatment plants. However, recent efforts have focused on the peroxymonosulphate, persulphate, catalytic ozonation, ultrasonication and hydrodynamic cavitation, gamma radiation, electrochemical oxidation, modified Fenton, and plasma-assisted AOPs. This critical review presents the detailed mechanisms of emerging AOP technologies, their performance for treatment of contaminants of emerging concern, the relative advantages and disadvantages of each technology, and the remaining challenges to scale-up and implementation. Among the evaluated technologies, the modified electrochemical oxidation, gamma radiation, and plasma-assisted systems demonstrated the greatest potential for successful and sustainable implementation in wastewater treatment due to their environmental safety, compatibility, and efficient transformation of contaminants of emerging concern by a variety of reactive species. The other emerging AOP systems were also promising, but additional scale-up trials and a deeper understanding of their reaction kinetics in complex wastewater matrices are necessary to determine the technical and economic feasibility of full-scale processes.

Degradation of acid red 73 wastewater by hydrodynamic cavitation combined with ozone and its mechanism

Many azo dyes are consumed in the textile and dyeing industry, which makes the wastewater recalcitrant and toxic to the aquatic environment. Dye degradation by the combination of hydrodynamic cavitation and ozone (HC + O₃) has caused extensive interest. The degradation mechanism of the hybrid system needs further investigation. This study investigated the degradation of acid red 73 (AR73) by HC + O₃. Meanwhile, the degradation pathways and mechanisms were present. The optimal operation parameters were: inlet pressure of 0.15 MPa, O₃ dosage of 45 mg/min, initial dye concentration of 10 mg/L, and initial pH at 7.5. As a result, the decolorization rate, removal of UV₂₅₄ and NH₃-N were 100%, 71.28%, and 87.36% in 30 min, respectively. Humic acid and most of the co-existing anions (HCO₃^-, SO₄^2-, Cl^-, PO₄^3-, NO₃^-) played a positive role in the degradation of AR73, while NO₂^- restrained. The reactive species of singlet oxygen (¹O₂), hydroxyl radicals (·OH) and super oxygen radicals (·O₂^-) showed synergism in the hybrid system, and the decolorization was attributed to the fracture of azo bonds by ¹O₂. Meanwhile, aromatic amines were generated and further degraded into small molecule compounds. The research certificated that the HC + O₃ can be an effective technology for azo dye degradation.

Continuous Cultivation of Microalgae in Cattle Slaughterhouse Wastewater Treated with Hydrodynamic Cavitation

Cattle slaughtering produce large amounts of wastewater containing high concentrations of organic matter and nutrients and requires significant treatment before disposal or reutilization. However, the nutrients contained can be valued as a medium for microalgal biomass generation. In this work, hydrodynamic cavitation (HC) followed by membrane filtration or biological (microalgae cultivation) treatment in continuous mode were performed. From cattle slaughterhouse wastewater (CSW), by the effect of HC treatment with air injection in batch mode, more than 20% of the chemical oxygen demand (COD) was removed. In a continuous HC process, the COD content in output was 324 mg O2/L, which is 68% lower than the supplied CSW. After that, 76% of residual COD was removed by filtration through a tubular alumina membrane (600 nm). Finally, 85% of residual COD after HC treatment in 24 h in a batch mode was removed by microalgae. On the other hand, the COD concentration in the output was around 59 mg O2/L in continuous mode, which represents 85–93% COD removal. The process involving HC and microalgae growing looks promising since in addition to water treatment, the microalgae produced could be valued in a biorefinery concept.

Rapid Degradation of Chlortetracycline Using Hydrodynamic Cavitation with Hydrogen Peroxide

Chlortetracycline (CTC), which has been frequently detected in surface water, is generated primarily by the discharge of high-concentration CTC wastewater from pharmaceutical and livestock plants. The development of effective CTC degradation technology is critical. In this study, the extent of CTC degradation at 80 mg/L was investigated by combining hydrodynamic cavitation (HC) and hydrogen peroxide (H₂O₂). The results indicate degradation ratios of 88.7% and 93.8% at 5 and 30 min, respectively. Furthermore, the possible mechanisms of CTC degradation were determined via HPLC-MS. The CTC degradation pathways include ring openings, C–N bond cleavage, demethylation, dehydroxylation, and desaturation in the sole system of HC, and a series of additional reactions, such as glycine conjugation and the cleavage of C–C double bonds, occurs in the binary system of HC + H₂O₂. Nevertheless, the treated water poses ecological risks and cannot be directly discharged into the environment. Therefore, HC + H₂O₂ treatment may be a rapid and effective primary method for the degradation of high-concentration CTC in pharmaceutical factories.

Landfill leachate treatment using hydrodynamic cavitation: exploratory evaluation

Hydrodynamic cavitation is a new technology used for the treatment of wastewater. Landfill leachates contain a large variety of organic pollutants and inorganic matter, with recalcitrant and bio-refractory compounds. The present study was designed to evaluate the effect of hydrodynamic cavitation on landfill leachate quality indices. Three experimental designs were proposed. First, the influence of collection climate on leachate quality characteristics was analyzed. Second, the best cavitation time was chosen, which promoted the greatest reduction in the effluent pollutant load. Finally, the hydrogen peroxide (H₂O₂) concentration was evaluated as an adjuvant in the cavitation process. A model TEKMASH TEK-1SL equipment was used. This cavitation unit operated with a flow rate of 30 m³ h⁻¹, a temperature of 75 °C, and an inlet pressure of 3 bar. The cavitation chamber was of the annular flow type. The statistical analyses were run through ANOVA and Tukey's test, with significance α = 0.05. The response variables for the factors were biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), total organic carbon (TOC) and total suspended solids (TSS). An influence of the climatic condition on the leachate quality parameters was found, and the difference was marked in COD. In all cases, both for the cavitation process and for the cavitation-oxidant scheme, there was a reduction of 23%–51% BOD₅, 30%–53% COD, 12%–21% TOC and 100% removal in TSS. In a 30-minute treatment, the highest COD removal percentage was reached, corresponding to 53.20%. Furthermore, a 200 ppm concentration of hydrogen peroxide enhanced the reduction of BOD₅ and COD with proportions of 51.55% and 38.21%, respectively. Hydrodynamic cavitation offers advantages in the treatment of wastewater and can be used as an independent technique or as a hybrid method.

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Hydrodynamic cavitation is a useful and efficient technology for treating leachate.

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The annular flow in the reactor (geometry) shows good cavitation performance.

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Hybrid treatment with cavitation and H₂O₂, improves the reduction of BOD₅ and COD.

Cavitation based treatment of industrial wastewater: A critical review focusing on mechanisms, design aspects, operating conditions and application to real effluents

Acoustic cavitation (AC) and hydrodynamic cavitation (HC) coupled with advanced oxidation processes (AOPs) are prominent techniques used for industrial wastewater treatment though most studies have focused on simulated effluents. The present review mainly focuses on the analysis of studies related to real industrial effluent treatment using acoustic and hydrodynamic cavitation operated individually and coupled with H₂O₂, ozone, ultraviolet, Fenton, persulfate and peroxymonosulfate, and other emerging AOPs. The necessity of using optimum loadings of oxidants in the various AOPs for obtaining maximum COD reduction of industrial effluent have been demonstrated. The review also presents critical analysis of designs of various HCRs that have been or can be used for the treatment of industrial effluents. The impact of operating conditions such as dilution, inlet pressure, ultrasonic power, pH, and operating temperature have been also discussed. The economic aspects of the industrial effluent treatment have been analyzed. HC can be considered as cost-efficient approach compared to AC on the basis of the lower operating costs and better transfer efficiencies. Overall, HC combined with AOPs appears to be an effective treatment strategy that can be successfully implemented at industrial-scale of operation.

Can thermal intensification be considered a sustainable way for greening Fenton processes?

Life cycle assessment and kinetic modeling were used to elucidate the impact of thermal intensification (TI) on resource consumption and the techno-economic feasibility of a Fenton process at laboratory scale. Increasing temperature from 25 to 55 °C lowers treatment time (96.5%) and electricity use (67.8%) due to the positive impact of temperature on the reaction rate; however, beyond 50 °C no significant diminution in energy use, emissions, and operating cost was observed. The environmental footprint of the process is linked with energy use, operating pH, and the electricity share of the country; nevertheless, the impact of transport and infrastructure materials was lower. At 55 °C and pH of 2.8, emissions of precursors of freshwater and marine eutrophication, particulate matter formation, and ionizing radiation were reduced more than half; besides, in most of the midpoint categories, pondered by the ReCiPe-2016 method, emissions were lowered ca. 43.3%. The endpoint categories human health, ecosystem quality, and resource availability had a significant decline in disability-adjusted life years (46.0%), time-integrated species loss (42.0%), and surplus cost (33.1%). Harnessing the energy present in the wastewater itself decreased 41.9% global warming potential (GWP), but the use of steam for heating raised it 718.8%. In countries where electricity generation is dependent on fossil fuels, GWP could increase (2.0-20.0%) whereas GWP would decrease (8.8-9.4%) when renewable energy sources dominate. Operating at 55 °C and pH of 5.5 rose the reaction time (1835.5%), GWP (29.3%), particulate matter formation (44.3%), terrestrial acidification (21.8%), marine (48.9%), and freshwater eutrophication (66.7%). TI of Fenton processes could increase their treatment capacity with a small reduction in the quality of the effluent; furthermore, they can be made affordable for low-to-medium scale industries in emerging economies due to decreased resources consumption and emissions, leading to a lower treatment cost (US$0.49/m³).

Making precious metals cheap: A sonoelectrochemical - Hydrodynamic cavitation method to recycle platinum group metals from spent automotive catalysts

Platinum group metals, such as Pd and Pt, found in three-way catalyst converters were recycled in a two-step method: hydrodynamic cavitation followed by sonoelectrochemical dissolution. High shear forces were obtained by using a convergent nozzle with a throat diameter of 0.2 mm, feeded by a plunger pump at a pressure of 60 MPa. Cavitating submerged jets acted locally on the water dispersed waste catalyst. As-obtained samples were analyzed by scanning electron microscopy and transmission electron microscopy. Electron microscopy on the initial sample showed that round shaped Pd and Pt nanoparticles were randomly distributed on the Al₂O₃ matrix. Cavitated samples show two zones in which Pt and Pd were partially and completely separated from the cordierite. The hydrodynamic cavitation separates the Pd and Pt from the cordierite leading to an apparent increase in Pd and Pt concentrations of 9% and 34% respectively. Conventional electrochemistry showed a dissolution of 20% in 1 h. To further accelerate the dissolution, a sonotrode operating at 20 kHz and 75 W was placed inside an electrochemical cell in order to increase the mass transport and obtain high dissolution rates. Indeed, the results showed that 40% of the available Pd and Pt can be recycled in just 1 h. In the absence of hydrodynamic cavitation and using conventional electrochemistry less than 10% of the available Pt and Pd is recovered in 1 h. The cost analysis showed that Pd and Pt can be recovered at less than 10 EUR per g which is 5 times smaller than their current market price.

Comparison of acoustic and hydrodynamic cavitation based hybrid AOPs for COD reduction of commercial effluent from CETP

The present work investigates the treatment of commercial effluent obtained from Common Effluent Treatment Plants (CETP) using acoustic cavitation (AC) and hydrodynamic cavitation (HC) based hybrid AOPs. Comparison of different hybrid AOPs viz. H₂O₂, Fe²⁺/H₂O₂, Fe²⁺/H₂O₂/Air, Fe²⁺/H₂O₂/S₂O₈^2- and Fe²⁺/H₂O₂/S₂O₈^2-/Air in combination with both AC and HC has been performed in terms of extent of chemical oxygen demand (COD) reduction and kinetic rate constants. The best results of COD reduction as 95.2% and 97.28% were obtained for AC/Fe²⁺/H₂O₂/Air and HC/Fe²⁺/H₂O₂/Air systems respectively at Fe²⁺/H₂O₂ ratio of 0.1 and pH of 2 within 60 min of treatment under conditions of ultrasonic power dissipation as 150 W, inlet pressure for HC as 4 bar (as applicable depending on process) and temperature of 30 ± 2 °C. Slightly lower efficacy was established for the combination approach involving AC or HC coupled with Fe²⁺-activated S₂O₈^2- and H₂O₂ yielding COD reduction of 82.9% and 86.93% for the AC/Fe²⁺/H₂O₂/S₂O₈^2-/Air and HC/Fe²⁺/H₂O₂/S₂O₈^2-/Air systems respectively at Fe²⁺/H₂O₂/S₂O₈^2- ratio of 1:40:17.5. Cost estimation on the basis of cavitational yield performed on the AC and HC based treatment systems revealed economical nature of HC based treatment. Kinetic studies were also performed by fitting the experimental data with pseudo first order kinetic model (PFOKM), generalized kinetic model (GKM) and Behnajady-Modirshahla-Ghanbery kinetic model (BMGKM). It was demonstrated that GKM provided best fitting for all the experiments whereas BMGKM was most suitable for Fenton based reactions. It was clearly established that complex CETP effluent can be effectively treated using the combined approaches based on HC with potential for larger scale operation.

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.

Sonochemical synthesis of amphoteric Cu0-Nanoparticles using Hibiscus rosa-sinensis extract and their applications for degradation of 5-fluorouracil and lovastatin drugs

Recent studies reported the detection of numerous emerging and active pharmaceutical constituents in the ground and surface water. To address these issues, the present study reported the ultrasound-assisted synthesis of zero-valent copper (Cu⁰) nanoparticles using Hibiscus rosa-sinensis extract as reducing and stabilizing agent. The catalyst was characterized using XRD, SEM, EDX, PSA, BET, etc., and the results revealed that sonochemical synthesis technique influenced the crystallinity with controlled growth of Cu⁰. While the hard ligand hydroxyl group (-OH) reduces the Cu²⁺ to Cu⁰ and soft ligand carbonyl group (CO) present in the oxidized polyphenols helps in capping and stabilizing the Cu⁰-nanoparticles. During the ultrasound application, continuous release of Cu⁺ from Cu⁰ promoted the degradation by producing OH and O₂^•- radicals. Approx. 91.3 % and 93.2 % degradation efficiencies were achieved for 5-fluorouracil and lovastatin. The results showed that Cu⁰ nanoparticles were amphoteric in nature and the synergy calculation revealed that ultrasound has a direct influence on degradation of drugs which are difficult to degrade/mineralize using conventional techniques. Based on the results, a possible degradation mechanism of drug molecules in the presence of oxidants, zero-valent copper and ultrasound has been proposed.

The removal of bisphenols and other contaminants of emerging concern by hydrodynamic cavitation: From lab-scale to pilot-scale

The rapid growth in the variety and quantity of contaminants of emerging concern (CEC) in wastewater indicates the necessity for developing efficient and environmentally friendly methods for their removal. This study investigates the removal efficiency of 46 CEC, including 12 bisphenols, from wastewater using a lab and pilot-scale hydrodynamic cavitation generator alone and in combination with UV illumination (pilot-scale). During lab-scale cavitation, the highest removal efficiencies of bisphenols (15-63%) for this specific design of cavitator were obtained at a rotational frequency (v_cav) = 9500 rpm and time (t_cav) = 10 min. Temperature and the physicochemical properties (e.g. K_ow) of the studied compounds also had a significant effect on removal efficiency. At the pilot-scale, 11 CECs were quantifiable in the wastewater influent, and the generator operated at ν_cav = 2290 and 2700 rpm. The highest removal efficiencies (15-90%) were obtained at a lower ν_cav = 2290 rpm while neither an increase in ν_cav, t_cav or the presence of UV-C light increased the removal efficiency. A lower ν_cav also reduced the hydrodynamic power of the cavitator from 477 W to 377 W, resulting in reduced energy consumption. Overall, the results show the potential of hydrodynamic cavitation for a large-scale application as a pre-treatment technology and pave the way for future improvements in the design of cavitation reactors.

A review on hydrodynamic cavitation disinfection: The current state of knowledge

Disinfection, which aims to eliminate pathogenic microorganisms, is an essential step of water treatment. Hydrodynamic cavitation (HC) has emerged as a promising technology for large-scale disinfection without introducing new chemicals. HC, which can effectively induce sonochemistry by mechanical means, creates extraordinary conditions of pressures of ~1000 bar, local hotspots with ~5000 K, and high oxidation (hydroxyl radicals) in room environment. These conditions can produce highly destructive effects on microorganisms in water. In addition, the enhancements of chemical reactions and mass transfers by HC produce the synergism between HC and disinfectants or other physical treatment methods. HC is generated by hydrodynamic cavitation reactors (HCRs), therefore, their performance basically determines the effectiveness, economical efficiency, and applicability of HC disinfection. Therefore, developing high-performance HCRs and revealing the corresponding disinfection mechanisms are the most crucial issues today. In this review, we summarize the fundamental principles of HC and HCRs and recent development in HC disinfection. The energy release from cavitation phenomenon and corresponding mechanisms are elaborated. The performance (effectiveness, treatment ratio, and cost) of various HCRs, effects of treatment conditions on performance, and applicability of HC disinfection are evaluated and discussed. Finally, recommendations are provided for the future progress based on the analysis of previous studies.

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 H₂O₂and O₃with 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 within60 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 + O₃surpassed by far the performances of HC + H₂O₂and HC + O₃.

Hydrodynamic Cavitation: A Promising Technology for Industrial-Scale Synthesis of Nanomaterials

One of the most challenging issues for the large-scale application of nanomaterials, especially nanocarbons, is the lack of industrial synthetic methods. Sonochemistry, which creates an extreme condition of high pressure and temperature, has been thereby applied for synthesizing a wide variety of unusual nanostructured materials. Hydrodynamic cavitation (HC), characterized by high effectiveness, good scalability, and synergistic effect with other physical and chemical methods, has emerged as the promising sonochemistry technology for industrial-scale applications. Recently, it was reported that HC can not only significantly enhance the performance of biochar, but also preserve or improve the respective chemical composition. Moreover, the economic efficiency was found to be at least one order of magnitude higher than that of conventional methods. Due to the great potential of HC in the industrial-scale synthesis of nanomaterials, the present perspective focuses on the mechanism of sonochemistry, advances in HC applications, and development of hydrodynamic cavitation reactors, which is supposed to contribute to the fundamental understanding of this novel technology.

Ultrasound-Assisted Treatment of Landfill Leachate in a Sequencing Batch Reactor

Purification of leachates is currently a big challenge due to their high variability in composition and amount. The complexity of the medium, namely leachates, makes new solutions highly sought after and finds the existing ones in need of optimization. The effects of ultrasound pretreatment (20 kHz, 12 µm) on biological treatment of landfill leachates in the form of processes carried out in two sequencing batch reactors were investigated. The experiment was divided into two stages. In the first stage, leachate was treated by an ultrasonic field at different sonication times (0.5, 1, 3, 5, 10 and 15 min). Next, leachates with and without conditioning were combined with municipal wastewater in the following ratios: 5, 10, 15 and 25% v/v. For optimal processing time (3 min), 16% removal of COD was achieved. In turn, the BOD5/COD ratio was 0.3, which is higher by approximately 270% than that of the non-conditioned sample. Further elongation of sonication time did not significantly affect both parameters. Also, pretreatment of leachate resulted in a maximum increase noted in the study of specific oxygen uptake rate and dehydrogenase activity of approximately 21 and 2 times compared to the non-conditioned sample. The implementation of a pretreatment step prior to the biological treatment was shown to result in higher pollutant removal efficiency. Depending on the share of leachates in the mixture, the removal enhancements of BOD, COD, and ammonium nitrogen for conditioned samples ranged from 6–48.5%, 4–48% and 11–42%, respectively. Furthermore, pretreatment of leachate allows for an increased (by up to 20%) share of leachate volume in the influent stream entering the reactor, while maintaining the quality of effluents in accordance with national regulation requirements. However, in scenarios without pretreatment, the leachate ratio cannot exceed 5% of the total wastewater due to poor quality of the effluents. The operational cost of ultrasound pretreatment of leachate was 22.58 €/(m3·g removed COD).