The issue of cavitation number value in studies of water treatment by hydrodynamic cavitation

The flow pattern effects of hydrodynamic cavitation on waste activated sludge digestibility

The disintegration of raw sludge is of importance for enhancing biogas production and facilitates the degradation of substrates for microorganisms so that the efficiency of digestion can be increased. In this study, the effect of hydrodynamic cavitation (HC) as a pretreatment approach for waste activated sludge (WAS) was investigated at two upstream pressures (0.83 and 1.72 MPa) by using a milli-scale apparatus which makes sludge pass through an orifice with a restriction at the cross section of the flow. The HC probe made of polyether ether ketone (PEEK) material was tested using potassium iodide solution and it was made sure that cavitation occurred at the selected pressures. The analysis on chemical effects of HC bubbles collapse suggested that not only cavitation occurred at low upstream pressure, i.e., 0.83 MPa, but it also had high intensity at this pressure. The pretreatment results of HC implementation on WAS were also in agreement with the chemical characterization of HC collapse. Release of soluble organics and ammonium was observed in the treated samples, which proved the efficiency of the HC pretreatment. The methane production was improved during the digestion of the treated samples compared to the control one. The digestion of treated WAS sample at lower upstream pressure (0.83 MPa) resulted in higher methane production (128.4 mL CH4/g VS) compared to the treated sample at higher upstream pressure (119.1 mL CH4/g VS) and control sample (98.3 mL CH4/g VS). Thus, these results showed that the HC pretreatment for WAS led to a significant increase in methane production (up to 30.6%), which reveals the potential of HC in full-scale applications.

Selective preparation of lithium carbonate from overhaul slag by high temperature sulfuric acid roasting - Water leaching

An efficient recycling process is developed to recover valuable materials from overhaul slag and reduce its harm to the ecological environment. The high temperature sulfuric acid roasting - water leaching technology is innovatively proposed to prepare Li2CO3 from overhaul slag. Under roasting conditions, fluorine volatilizes into the flue gas with HF, lithium is transformed into NaLi(SO4), aluminum is firstly transformed into NaAl(SO4)2, and then decomposed into Al2O3, so as to selective extraction of lithium. Under the optimal roasting - leaching conditions, the leaching rate of lithium and aluminum are 95.6% and 0.9%, respectively. Then the processes of impurity removal, and settling lithium are carried out. The Li2CO3 with recovery rate of 72.6% and purity of 98.6% could be obtained under the best settling lithium conditions. Compared with the traditional process, this work has short flow, high controllability, remarkable technical, economic, and environmental benefits. This comprehensive recycling technology is suitable for overhaul slag, and has great practical application potential for the disposal of other hazardous wastes in electrolytic aluminum industry.

Application of effective orifice jet length for treating SST effluent of STP by hydrodynamic cavitation

Advanced oxidation process in general and hydrodynamic cavitation, in particular, has emerged as a promising technology for the treatment of wastewater in the last few years as the process is energy-efficient and cost-effective. In this process, cavities are generated due to local pressure drops caused by constrictions. This work aimed to investigate the potential of hydrodynamic cavitation as a tertiary treatment to treat the secondary sedimentation tank effluent of a sewage treatment plant, with two laboratory-scale experimental setups having an orifice plate of a 1 mm diameter hole. The process dependency was estimated by optimizing inlet pressure (0.8 bar for setup I and 5 bar for setup II). Moreover, effective orifice jet length was varied to investigate the impact of fluid buoyancy force on expansion and collapse of a cavity on the chemical oxygen demand removal and disinfection potential. At L2 length (two-thirds of the original length), both setups can degrade the organic and inorganic pollutants to the maximum extent. With the optimum condition in setup II, maximum COD, TSS, and fecal coliform degradation were 80.47%, 62.83%, and 52.27%, respectively, compared to setup I.

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.

Investigation of free fatty acid reduction from mixed crude palm oil using 3D-printed rotor-stator hydrodynamic cavitation: An experimental study of geometric characteristics of the inner hole

A continuous esterification process is employed to decrease the free fatty acid (FFA) concentration of FFA-rich mixed crude palm oil. Both optimal and recommended conditions are determined for the esterification reaction conditions and the geometry of the 3D-printed rotor design in the rotor-stator hydrodynamic cavitation reactor. This study is primarily concerned with the effect of the cavitation device configuration, especially the rotor design, on FFA reduction. Instead of conventional spherical or cylindrical drilled holes, a point angle cone-shaped hole is used to create cavities over the rotor surface. These point angles are adjusted to clarify their effect on FFA reduction. The response surface methodology is applied to determine the optimal concentrations of methanol and sulfuric acid, rotor speed, hole diameter and depth, and cone point angle. The recommended conditions are 20.8 wt% methanol, 2.6 wt% sulfuric acid, 3000 rpm, 5 mm hole diameter, 5 mm hole depth, and 110°, respectively. Under this configuration, the FFA content is reduced from 12.014 wt% to around 1 wt%. A maximum yield of 97.34 vol% esterified oil is obtained through a completed phase separation step, and 93.31 vol% pure oil is collected after the cleansing step. The recommended conditions result in reduced chemical usage, cheaper FFA reduction, and lower environmental impact. This creative rotor design effectively improves our understanding of the geometry of the cavitation device, thus enhancing the cavitation effect in industrial operations.

Recent trends in the applications of sonochemical reactors as an advanced oxidation process for the remediation of microbial hazards associated with water and wastewater: A critical review

Water is one of the major sources that spread human diseases through contamination with bacteria and other pathogenic microorganisms. This review focuses on microbial hazards as they are often present in water and wastewater and cause various human diseases. Among the currently used disinfection methods, sonochemical reactors (SCRs) that produce free radicals combined with advanced oxidation processes (AOPs) have received significant attention from the scientific community. Also, this review discussed various types of cavitation reactors, such as acoustic cavitation reactors (ACRs) utilizing ultrasonic energy (UE), which had been widely employed, involving AOPs for treating contaminated waters. Besides ACRs, hydrodynamic cavitation reactors (HCRs) also effectively destroy and deactivate microorganisms to varying degrees. Cavitation is the fundamental phenomenon responsible for initiating many sonochemical reactions in liquids. Bacterial degradation occurs mainly due to the thinning of microbial membranes, local warming, and the generation of free radicals due to cavitation. Over the years, although extensive investigations have focused on the antimicrobial effects of UE (ultrasonic energy), the primary mechanism underlying the cavitation effects in the disinfection process, inactivation of microbes, and chemical reactions involved are still poorly understood. Therefore, studies under different conditions often lead to inconsistent results. This review investigates and compares other mechanisms and performances from greener and environmentally friendly sonochemical techniques to the remediation of microbial hazards associated with water and wastewater. Finally, the energy aspects, challenges, and recommendations for future perspectives have been provided.

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.

CFD-assisted modeling of the hydrodynamic cavitation reactors for wastewater treatment - A review

Hydrodynamic cavitation has been a promising method and technology in wastewater treatment, while the principles based on the design of cavitational reactors to optimize cavitation yield and performance remains lacking. Computational fluid dynamics (CFD), a supplementation of experimental optimization, has become an essential tool for this issue, owing to the merits of low investment and operating costs. Nevertheless, researchers with a non-engineering background or few CFD fundamentals used straightforward numerical strategies to treat cavitating flows, and this might result in many misinterpretations and consequently poor computations. This review paper presents the rationale behind hydrodynamic cavitation and application of cavitation modeling specific to the reactors in wastewater treatment. In particular, the mathematical models of multiphase flow simulation, including turbulence closures and cavitation models, are comprehensively described, whilst the advantages and shortcomings of each model are also identified and discussed. Examples and methods of the coupling of CFD technology, with experimental observations to investigate into the hydrodynamic behavior of cavitating devices that feature linear and swirling flows, are also critically summarized. Modeling issues, which remain unaddressed, i.e., the implementation strategies of numerical models, and the definition of cavitation numbers are identified and discussed. Finally, the advantages of CFD modeling are discussed and the future of CFD applications in this research area is also outlined. It is expected that the present paper would provide decision-making support for CFD beginners to efficiently perform CFD modeling and promote the advancement of cavitation simulation of reactors in the field of wastewater treatment.

Using CFD simulations to investigate the shear stress in hydrodynamic cavitation reactors coupled with experimental validation using colony count measurements

The current work investigates the shear stress distribution in hydrodynamic cavitation reactors with two different geometries using CFD simulations. Venturi type (positive geometry) and bore (negative geometry) were used to induce cavitation. Experimental validation of the predictions from simulations was also conducted by calculating the reduction rate in the colony count of Legionella pneumophila, a pathogenic bacterial strain. Both the numerical and experimental studies revealed the significant influence of the shape of the cavitation-inducing geometry on the flow characteristics and the distribution of shear stress. The simulation data indicated high shear stress formation in the positive geometry as a venturi, with the cavitation ranges for the two reactors being far apart from each other. The experimental study also confirmed that the flow conditions in the venturi-type reactor were more favourable compared to the bore geometry, resulting in a bacterial reduction efficiency as high as 99.98%. It was clearly demonstrated that the geometry of the cavitating device plays a crucial role in deciding the shear stress and its efficacy for the desired applications as per the predictions of the simulation model validated by the experimental results.

Characterization of cavitation under ultrasonic horn tip - Proposition of an acoustic cavitation parameter

Acoustic cavitation, generated by a piezo-driven transducer, is a commonly used technique in a variety of processes, from homogenization, emulsification, and intensification of chemical reactions to surface cleaning and wastewater treatment. An ultrasonic horn, the most commonly used acoustic cavitation device, creates unique cavitation conditions under the horn tip that depend on various parameters such as the tip diameter, the driving frequency of the horn, its amplitude, and fluid properties. Unlike for hydrodynamic cavitation, the scaling laws for acoustic cavitation are poorly understood. Empirical relationships between cavitation dynamics, ultrasonic horn operating conditions, and fluid properties were found through systematic characterization of cavitation under the tip. Experiments were conducted in distilled water with various sodium chloride salt concentrations under different horn amplitudes, tip geometries, and ambient pressures. Cavitation characteristics were monitored by high-speed (200,000 fps) imaging, and numerous relations were found between operating conditions and cavitation dynamics. The compared results are discussed along with a proposal of a novel acoustic cavitation parameter and its relationship to the size of the cavitation cloud under the horn tip. Similar to the classical hydrodynamic cavitation number, the authors propose for the first time an acoustic cavitation parameter based on experimental results.

Investigation of hydrodynamic cavitation induced reactive oxygen species production in microchannels via chemiluminescent luminol oxidation reactions

Hydrodynamic cavitation was evaluated for its reactive oxygen species production in several convergent-divergent microchannel at the transition from micro to milli scale. Channel widths and heights were systematically varied to study the influence of geometrical parameters at the transitory scale. A photomultiplier tube was used for time-resolved photon detection and monitoring of the chemiluminescent luminol oxidation reactions, allowing for a contactless and in situ quantization of reactive oxygen species production in the channels. The radical production rates at various flow parameters were evaluated, showing an optimal yield per flow rate exists in the observed geometrical range. While cavitation cloud shedding was the prevailing regime in this type of channels, the photon arrival time analysis allowed for an investigation of the cavitation structure dynamics and their contribution to the chemical yield, revealing that radical production is not linked to the synchronous cavitation cloud collapse events. Instead, individual bubble collapses occurring throughout the cloud formation were recognized to be the source of the reactive oxygen species.

Experimental and Numerical Study of Cavitation Number Limitations for Hydrodynamic Cavitation Inception Prediction

Hydrodynamic cavitation is the formation, growth and subsequent collapse of vapor bubbles in a moving liquid. It is extremely important to determine conditions of cavitation inception and when it starts damaging industrial equipment. In some cases, such as hydrodynamic cleaning it is important to understand how to improve the cavitation phenomenon in order to enhance cleaning properties. The cavitation number is a parameter used to predict cavitation and its potential effects. In this paper we discuss limitations of this parameter and demonstrate that it cannot be considered sufficient to predict cavitation inception and development in the fluid flow. The experimental setup was designed and built to study cavitation inception in various nozzles. RANS SST k–ω turbulence model was used in this study to model turbulent flow in ANSYS Fluent. CFD calculations were compared to experimental results. It was shown that cavitation inception was sensitive to change in nozzle geometry and, since geometrical parameters are not included in cavitation number formula, scenarios of cavitation inception can be different at the same cavitation number.

Cavitation Erosion Characteristics for Different Metal Surface and Influencing Factors in Water Flowing System

The impact of cavitation erosion behavior on different metals in a water flowing system was investigated experimentally. A flowing system of water was built and a transparent observation window is designed to capture the cavitation flow. Erosion tests were carried out on red copper, brass, pure aluminum, and an aluminum alloy. The cavitation behaviors are presented by the weight loss and cavitation erosion rate, and related changes in the topography of the metal surface are also discussed. The variation in the cavitation erosion on metallic specimens with increasing time could be divided into three stages: rising stage, stable stage, and attenuation stage. The pure aluminum material had the lowest yield strength, and suffered the most severe cavitation erosion while brass had the highest yield strength and good mechanical properties, which suffered the least cavitation erosion. Furthermore, the roughness of the material surface was also one of the important factors affecting the cavitation erosion rate. The weight loss of milling specimens with higher surface roughness was slightly lower than that of grinding. The high roughness of the metallic surface increased the pressure loss along the flow path and the suppressed cavitation strength. This work provides an experimental reference for the anti-cavitation ability improvement in metal materials and promotes an understanding of the related mechanism.

Recent advances in hydrodynamic cavitation-based pretreatments of lignocellulosic biomass for valorization

Recently, the hydrodynamic cavitation (HC)-based pretreatment has shown high effectiveness in laboratories and even in industrial productions for conversion of lignocellulosic biomass (LCB) into value-added products. The pretreatment capability derives from the extraordinary conditions of pressures at ∼500 bar, local hotspots with ∼5000 K, and oxidation (hydroxyl radicals) created by HC at room conditions. To promote this emerging technology, the present review summarizes the recent advances in the HC-based pretreatment of LCB. The principle of HC including the sonochemical effect and hydrodynamic cavitation reactor is introduced. The effectiveness of HC on the delignification of LCB as well as subsequent fermentation, paper production, and other applications is evaluated. Several key operational factors (i.e., reaction environment, duration, and feedstock characteristics) in HC pretreatments are discussed. The enhancement mechanism of HC including physical and chemical effects is analyzed. Finally, the perspectives on future research on the HC-based pretreatment technology are highlighted.

Hydrodynamic cavitation for micropollutant degradation in water - Correlation of bisphenol A degradation with fluid mechanical properties

The present work addresses the correlation of bisphenol A (BPA) degradation by hydrodynamic cavitation with the fluid mechanical properties of the cavitating jet in the reactor. The effects of inlet pressure and two orifices were investigated. The fluid mechanics conditions during the reaction were evaluated by optical measurements to determine the jet length, bubble volume, number of bubbles, and bubble size distribution. In addition, chemiluminescence of luminol is used to localize chemically active bubbles due to the generation of hydroxyl radicals in the reactor chamber. The correlation between the rate constants of BPA degradation and the mechanical properties of the liquid is discussed. Here, linear dependencies between the degradation of BPA and the volume expansion of the bubble volume and chemiluminescence are found, allowing prediction of the rate constants and the hydroxyl radicals generated. BPA degradation of 50% was achieved in 30 min with the 1.7 mm nozzle at 25 bar. However, the 1 mm nozzle has been demonstrated to be more energetically efficient, achieving 10% degradation with 30% less power per 100 passes. There is a tendency for the number of small bubbles in the reactor to increase with smaller nozzle and increasing pressure difference.

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.

Impact of Cryogenics on Cavitation through an Orifice: A Review

Cryogenic cavitation affects the operation of liquid propulsion systems during the first phase of a launch. Its effects within orifices or turbopumps can range from mild instabilities to catastrophic damages to the structures, jeopardizing the launch itself. Therefore, to ensure the proper designing of propulsion systems, cavitation phenomena cannot be neglected. Although hydrodynamic cavitation has been studied for decades, the impact of the nature of the fluid has been sparsely investigated. Therefore, this review, beginning from the basic concepts of cavitation, analyzes the literature dedicated to hydrodynamic cryogenic cavitation through an orifice. Our review provides a clear vision of the state-of-the-art from experimental and modeling viewpoints, identifies the knowledge gaps in the literature, and proposes a way to further investigate cryogenic cavitation in aerospace science.

Challenges of numerical simulations of cavitation reactors for water treatment - An example of flow simulation inside a cavitating microchannel

The research on the potential of cavitation exploitation is currently an extremely interesting topic. To reduce the costs and time of the cavitation reactor optimization, nowadays, experimental optimization is supplemented and even replaced using computational fluid dynamics (CFD). This is a very inviting opportunity for many developers, yet we find that all too often researchers with non-engineering background treat this "new" tool too simplistic, what leads to many misinterpretations and consequent poor engineering. The present paper serves as an example of how complex the flow features, even in the very simplest geometry, can be, and how much effort needs to be put into details of numerical simulation to set a good starting point for further optimization of cavitation reactors. Finally, it provides guidelines for the researchers, who are not experts in computational fluid dynamics, to obtain reliable and repeatable results of cavitation simulations.

Design and fabrication of a vigorous "cavitation-on-a-chip" device with a multiple microchannel configuration

Hydrodynamic cavitation is one of the major phase change phenomena and occurs with a sudden decrease in the local static pressure within a fluid. With the emergence of microelectromechanical systems (MEMS), high-speed microfluidic devices have attracted considerable attention and been implemented in many fields, including cavitation applications. In this study, a new generation of 'cavitation-on-a-chip' devices with eight parallel structured microchannels is proposed. This new device is designed with the motivation of decreasing the upstream pressure (input energy) required for facile hydrodynamic cavitation inception. Water and a poly(vinyl alcohol) (PVA) microbubble (MB) suspension are used as the working fluids. The results show that the cavitation inception upstream pressure can be reduced with the proposed device in comparison with previous studies with a single flow restrictive element. Furthermore, using PVA MBs further results in a reduction in the upstream pressure required for cavitation inception. In this new device, different cavitating flow patterns with various intensities can be observed at a constant cavitation number and fixed upstream pressure within the same device. Moreover, cavitating flows intensify faster in the proposed device for both water and the water-PVA MB suspension in comparison to previous studies. Due to these features, this next-generation 'cavitation-on-a-chip' device has a high potential for implementation in applications involving microfluidic/organ-on-a-chip devices, such as integrated drug release and tissue engineering.

Continuous acid-catalyzed esterification using a 3D printed rotor-stator hydrodynamic cavitation reactor reduces free fatty acid content in mixed crude palm oil

Free fatty acid (FFA) content in FFA-rich mixed crude palm oil (MCPO) was reduced through a continuous esterification process. The reaction conditions were optimized, the yield purified esterified oil was determined, and the average total electricity consumption of the entire process was evaluated. The key component of this study was the cost-effective, 3D-printed rotor that was installed in a continuous rotor-stator hydrodynamic reactor. The surface of the rotor was designed with spherical holes where the center-to-center distance between them was fixed. Response surface methodology (RSM) using central composite design (CCD) was employed to analyze the design of experiments (DOE) and optimize FFA-content reduction. The optimized conditions were 17.7 vol% methanol, 2.9 vol% sulfuric acid, a 3000 rpm rotor speed, and surface holes measuring 4 mm in diameter and 6 mm in depth. The experimental results showed that the FFA content in MCPO was reduced from 11.456 to 1.028 wt% upon esterification under these optimal conditions. The maximum yield of esterified oil from the phase separation step was 96.07 vol%, and that of the purified esterified oil was 91.27 vol%. The average total energy consumed by this hydrodynamic cavitation reactor to produce this esterified oil was 0.0264 kW h/L. This 3D printed rotor-stator reactor is a promising, novel reactor technology for producing biodiesel from FFA-rich oils.

Luminescence intensity of vortex cavitation in a Venturi tube changing with cavitation number

Hydrodynamic cavitation in a Venturi tube produces luminescence, and the luminescence intensity reaches a maximum at a certain cavitation number, which is defined by upstream pressure, downstream pressure, and vapor pressure. The luminescence intensity of hydrodynamic cavitation can be enhanced by optimizing the downstream pressure at a constant upstream pressure condition. However, the reason why the luminescence intensity increases and then decreases with an increase in the downstream pressure remains unclear. In the present study, to clarify the mechanism of the change in the luminescence intensity with cavitation number, the luminescence produced by the hydrodynamic cavitation in a Venturi tube was measured, and the hydrodynamic cavitation was precisely observed using high-speed photography. The sound velocity in the cavitating flow field, which affects the aggressive intensity of the cavitation, was evaluated. The collapse of vortex cavitation was found to be closely related to the luminescence intensity of the hydrodynamic cavitation. A method to estimate the luminescence intensity of the hydrodynamic cavitation considering the sound velocity was developed, and it was demonstrated that the estimated luminescence intensity agrees well with the measured luminescence intensity.

Mature Landfill Leachate as a Medium for Hydrodynamic Cavitation of Brewery Spent Grain

In this study, we evaluate the usefulness of mature landfill leachate (MLL) as a carrier allowing hydrodynamic cavitation (HD) of brewery spent grain (BSG). The HD experiments were conducted using an orifice plate with a conical concentric hole of 3/10 mm (inlet/outlet diameter) as a constriction in the cavitation device. The initial pressure was 7 bar and the number of recirculation passes through the cavitation zone reached 30. The results showed that complex organic matter was degraded and solubilized when cavitating the MLL and BSG mixture. The biochemical oxygen demand (BOD5) increased by 45% and the BOD5/total chemical oxygen demand (COD) ratio increased by 69%, whereas the COD, total solids, and nutrient concentration dropped noticeably. However, Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) revealed the generation of possibly toxic HD byproducts such as aromatic compounds. This seems to indicate that MLL could not be regarded as a suitable carrier for BSG cavitation.

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.

Experimental investigation of the dynamic cavitation behavior and wall static pressure characteristics through convergence-divergence venturis with various divergence angles

As a highly efficient and energy-saving cavitation method, Venturi cavitation is widely used in many industrial fields. This study synchronously investigated the cavity behavior and its corresponding wall static pressure characteristics in Venturi channels with various divergence angles to research the role of the divergence angle in cavity shape and the wall static pressure oscillation. Five rectangular Venturi channels with different divergence angles (4°, 6°, 8°, 10°, and 12°) were tested at the cavitation number (0.3–1.0). Based on the dynamic behaviour of gas–liquid interface, three cavity shedding types were identified: front shedding (I), central shedding (II) and tail shedding (III). A modified correlation for predicting average cavity length was proposed with the consideration of the effect of the divergence angle. Combined with the wall static pressure characteristics, as the divergence angle increased, the wall static pressure fluctuation in the Venturi became more intense. According to the wall static pressure oscillation characteristics, for the larger divergence angles (θ = 6°, 8°, 10° and 12°), the wall static pressure oscillation frequency was the same as the cavity shedding frequency and increased with the increase of the divergence angle. For smaller divergence angle (θ = 4°), no definite periodicity in pressure oscillation frequency could be observed.

Innovative Hydrodynamic Disintegrator Adjusted to Agricultural Substrates Pre-treatment Aimed at Methane Production Intensification—CFD Modelling and Batch Tests

The study objective was to adjust the hydrodynamic disintegrator dedicated to sewage sludge pre-treatment (HDS) to work with agricultural substrate. This involved the development and implementation of a mathematical model of flow via the device’s domain. An innovative disintegrator (HAD—hydrodynamic disintegrator for agriculture) was designed, built, and tested based on the obtained results. The main improvements to the HDS include the implementation of shredding knives in order to overcome clogging by crushed substrate, and the application of ribs in the recirculation zone, contributing to the development of an additional structure damage zone. The challenge of this study was also to determine the operating parameters of the HDA that would provide for an increase in methane production with positive energy balance. The testing procedures, for which maize silage was selected, involved batch disintegration tests and biochemical methane potential tests. No clogging of rotor or spontaneous shutting off of the device, in other words, problems that had occurred in the HDS, were observed. The applied pre-treatment method permitted an increase in the methane potential of maize silage by 34.4%, 27.0%, and 21.6%, respectively for samples disintegrated at energy densities of 10 kJ/L, 20 kJ/L, and 35 kJ/L with net energy profit.

Effect of hydrodynamic disintegration on the solubilisation and bioavailability of thickened excess sludge

The main objective of the study was the verification whether conducting the hydrodynamic disintegration (HD) of thickened excess sludge (TES) before the anaerobic hydrolysis (AH) can cause an increase in the efficiency of the hydrolysis process, and therefore a reduction in its duration, or allow for complete omission of the stage before the anaerobic digestion (AD). For this purpose, the HD (conducted in five levels of energy density (E_L): 140, 280, 420, 560 and 700 kJ/L) of TES was carried out, and then all sludges (before and after disintegration) were subjected to the AH. The obtained results confirmed that the process of HD can be an effective method of increasing the solubilisation and bioavailability of TES. In the process of HD, the maximum increase in ΔVFA (308-428 mg VFA/L), was reported when E_L was increased from 140 to 280 kJ/L (the solubilisation degree increased from approximately 2 to 8%). The obtained results also showed that the ΔSTN and ΔSTP were related to solubilisation degree. The most intensive increase in the ΔSTN was determined for solubilisation degree in a range of 15-20%. In the case of ΔSTP, constant intensity of release of the compounds to the sludge liquid was observed. The obtained results also confirmed that conducting the process of AH of disintegrated TES proved to change the SCOD value when contrasted with the value of this indicator at the start of the experiment (before hydrolysis): (i) the E_L equal to 140 and 280 kJ/L allowed for a higher SCOD value; (ii) at E_L higher or equal to 560 kJ/L it caused a decrease in the SCOD value.

A critical review of the current technologies in wastewater treatment plants by using hydrodynamic cavitation process: principles and applications

In the last decade, hydrodynamic cavitation (HC) was increasingly used in the field of wastewater treatment. Due to its oxidative capability, HC was applied to treat aqueous effluents polluted by organic, toxic and bio-refractory contaminants, whereas its mechanical and chemical effects have allowed to disintegrate cells of microorganisms in biological applications. Due to their geometries, HC can be detected in some reactors, in which a variation of hydraulic parameters in the fluid such as flow pressure and flow velocity is induced. HC process involves the formation, growth, implosion and subsequent collapse of cavities, occurring in a very short period of time and releasing large magnitudes of power. In this paper, the vast literature on HC is critically reviewed, focusing on the basic principles behind it, in terms of process definition and analysis of governing mechanisms of both HC generation and pollutants degradation. The influence of various parameters on HC effectiveness was assessed, considering fluid properties, construction features of HC devices and technological aspects of processes. The synergetic effect of HC combined with chemicals or other techniques was discussed. An overview of the main devices used for HC generation and different existing methods to evaluate the cavitation effectiveness was provided. Knowledge buildup and optimization for such complex systems from mathematical modeling was highlighted.

A Transport-Phenomena Approach to Model Hydrodynamic Cavitation of Organic Pollutants

Hydrodynamic cavitation (HC) has been extensively studied for the Advanced Oxidation of organic compounds in wastewaters since it physically produces an oxidative environment at ambient conditions. This process is simple and economical since it can be realized through a properly designed restriction in a pipeline, even in retrofit solutions. Several experimental works individuated similar values of the optimal operating conditions, especially with regard to the inlet pressure. Up to now, the available modeling works rely on a single-bubble dynamics (SBD) approach and do not consider the actual process configuration and pollutant transport in proximity to the oxidizing environment. This work describes different experimental results (from this research group and others) and applies a novel mathematical model based on a transport-phenomena approach, able to directly simulate the effect of HC on the pollutant degradation. The novel proposed model is able to reproduce well a large number of experimental data obtained in different conditions, with different apparatus and different molecules, and allows to interconnect both SBD, fluid-dynamics, and physio-chemical variables in order to deeply study the interaction between the transport of pollutants and the reactive environment. This paper includes collection and discussion of several experimental results with the related main process parameters, description of the novel model and validation against the cited experimental results (to explain the effect of the operating pressure), sensitivity analysis, and the performance limit of the HC with the proposed modeling approach.

Influence of Fluid Properties on Intensity of Hydrodynamic Cavitation and Deactivation of Salmonella typhimurium

In this study, three microfluidic devices with different geometries are fabricated on silicon and are bonded to glass to withstand high-pressure fluid flows in order to observe bacteria deactivation effects of micro cavitating flows. The general geometry of the devices was a micro orifice with macroscopic wall roughness elements. The width of the microchannel and geometry of the roughness elements were varied in the devices. First, the thermophysical property effect (with deionized water and phosphate-buffered saline (PBS)) on flow behavior was revealed. The results showed a better performance of the device in terms of cavitation generation and intensity with PBS due to its higher density, higher saturation vapor pressure, and lower surface tension in comparison with water. Moreover, the second and third microfluidic devices were tested with water and Salmonella typhimurium bacteria suspension in PBS. Accordingly, the presence of the bacteria intensified cavitating flows. As a result, both devices performed better in terms of the intensity of cavitating flow with the presence of bacteria. Finally, the deactivation performance was assessed. A decrease in the bacteria colonies on the agar plate was detected upon the tenth cycle of cavitating flows, while a complete deactivation was achieved after the fifteenth cycle. Thus, the proposed devices can be considered as reliable hydrodynamic cavitation reactors for “water treatment on chip” applications.

Liposome destruction by hydrodynamic cavitation in comparison to chemical, physical and mechanical treatments

Liposomes are widely applied in research, diagnostics, medicine and in industry. In this study we show for the first time the effect of hydrodynamic cavitation on liposome stability and compare it to the effect of well described chemical, physical and mechanical treatments. Fluorescein loaded giant 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles were treated with hydrodynamic cavitation as promising method in inactivation of biological samples. Hydrodynamic treatment was compared to various chemical, physical and mechanical stressors such as ionic strength and osmolarity agents (glucose, Na⁺, Ca²⁺, and Fe³⁺), free radicals, shear stresses (pipetting, vortex mixing, rotational shear stress), high pressure, electroporation, centrifugation, surface active agents (Triton X-100, ethanol), microwave irradiation, heating, freezing-thawing, ultrasound (ultrasonic bath, sonotrode). The fluorescence intensity of individual fluorescein loaded lipid vesicles was measured with confocal laser microscopy. The distribution of lipid vesicle size, vesicle fluorescence intensity, and the number of fluorescein loaded vesicles was determined before and after treatment with different stressors. The different environmental stressors were ranked in order of their relative effect on liposome fluorescein release. Of all tested chemical, physical and mechanical treatments for stability of lipid vesicles, the most detrimental effect on vesicles stability had hydrodynamic cavitation, vortex mixing with glass beads and ultrasound. Here we showed, for the first time that hydrodynamic cavitation was among the most effective physico-chemical treatments in destroying lipid vesicles. This work provides a benchmark for lipid vesicle robustness to a variety of different physico-chemical and mechanical parameters important in lipid vesicle preparation and application.

Water disinfection by orifice-induced hydrodynamic cavitation

Hydrodynamic Cavitation (HC) is considered as a promising water-disinfection technique. Due to the enormous complexity of the physical and chemical processes at play, research on HC reactors is usually carried out following an empirical approach. Surprisingly, past experimental studies have never been designed on dimensional-analysis principles, which makes it difficult to identify the key processes controlling the problem, isolate their effects and scale up the results from laboratory to full-scale scenarios. The present paper overcomes this issue and applies the principles of dimensional analysis to identify the major non-dimensional parameters controlling disinfection efficacy in classical HC reactors, namely orifice plates. On the basis of this analysis, it presents results from a new set of experiments, which were designed to isolate mainly the effects of the so-called cavitation number (σ_v). Experimental data confirm that the disinfection efficacy of orifice plates increases with decreasing σ_v. Finally, in order to discuss the significance of the results presented herein and frame the scope of future research, the present paper provides an overview of the drawbacks associated with dimensional analysis within the context of HC.

Plant and Biomass Extraction and Valorisation under Hydrodynamic Cavitation

Hydrodynamic cavitation (HC) is a green technology that has been successfully used to intensify a number of process. The cavitation phenomenon is responsible for many effects, including improvements in mass transfer rates and effective cell-wall rupture, leading to matrix disintegration. HC is a promising strategy for extraction processes and provides the fast and efficient recovery of valuable compounds from plants and biomass with high quality. It is a simple method with high energy efficiency that shows great potential for large-scale operations. This review presents a general discussion of the mechanisms of HC, its advantages, different reactor configurations, its applications in the extraction of bioactive compounds from plants, lipids from algal biomass and delignification of lignocellulosic biomass, and a case study in which the HC extraction of basil leftovers is compared with that of other extraction methods.

Effects of cavitation on different microorganisms: The current understanding of the mechanisms taking place behind the phenomenon. A review and proposals for further research

A sudden decrease in pressure triggers the formation of vapour and gas bubbles inside a liquid medium (also called cavitation). This leads to many (key) engineering problems: material loss, noise, and vibration of hydraulic machinery. On the other hand, cavitation is a potentially useful phenomenon: the extreme conditions are increasingly used for a wide variety of applications such as surface cleaning, enhanced chemistry, and wastewater treatment (bacteria eradication and virus inactivation). Despite this significant progress, a large gap persists between the understanding of the mechanisms that contribute to the effects of cavitation and its application. Although engineers are already commercializing devices that employ cavitation, we are still not able to answer the fundamental question: What precisely are the mechanisms how bubbles can clean, disinfect, kill bacteria and enhance chemical activity? The present paper is a thorough review of the recent (from 2005 onward) work done in the fields of cavitation-assisted microorganism's destruction and aims to serve as a foundation to build on in the next years.

Remoción de sulfuros presentes en el agua residual del proceso de curtido mediante cavitación hidrodinámica

El sulfuro presente en el agua residual de la industria del curtido de cueros proviene de la operación de pelambre (depilado de la piel) en concentraciones que afectan significativamente la calidad del agua del cuerpo receptor debido a la toxicidad del vertimiento produciendo la mortandad de flora y fauna, responsable de la presencia de olores desagradables en el cauce y la notable disminución del oxígeno disuelto en el agua del afluente. En la presente investigación, la remoción de sulfuro es evaluada empleando la técnica cavitación hidrodinámica (CH), un proceso de oxidación avanzada analizado desde diferentes parámetros de operación como pH inicial del agua residual y presión de entrada para un tiempo de reacción fijo de 90 minutos; parámetros de diseño fueron establecidos mediante el uso de dos prototipos, variando el número y diámetro de orificios en el punto de estrangulamiento con el objetivo de determinar las condiciones óptimas del reactor para el tratamiento de este tipo de contaminante en las aguas residuales. Se obtuvo una remoción máxima del 32,6 % de la concentración inicial de sulfuro sin aplicar algún tipo de catalizador o reactivo químico; de igual modo se realizó un análisis costobeneficio debido a la eficiencia de la tecnología aplicada en comparación con la energía requerida por el reactor empleado, donde se observa una disminución del 198 % del costo total del tratamiento actual. Finalmente, la cavitación hidrodinámica es una tecnología sustentable para la industria del curtido de cueros.

Hydrodynamic cavitation using vortex diode: An efficient approach for elimination of pathogenic bacteria from water

The present study successfully demonstrates greener methodology of hydrodynamic cavitation using rotational flows for disinfection of water. Disinfection of two model microbial strains-gram- negative (Escherichia coli) and gram-positive (Staphylococcus aureus) using vortex diode was evaluated. The removal efficacy was quantified for two different cavitation reactors. Practically complete elimination of E. coli was achieved (99%) after 1 h of cavitation at a pressure drop of only 0.5 bar. However, elimination of S. aureus using vortex diode was observed to be lower in comparison to the removal of E. coli and only 60% disinfection could be achieved under similar conditions, which can be subsequently enhanced up to 98% by increasing pressure drop. The results were compared with another cavitating device that employs linear flow for cavitation, orifice. The reactor geometry has significant impact on the disinfection process and orifice was found to require significantly higher pressure drop (10 bar) conditions for disinfection and for eliminating gram-positive bacteria with high efficiency. A plausible mechanism for disinfection was proposed to elucidate the role of cavitation in cell destruction leading to death of cells through the rupture of cell wall, oxidative damage and possible DNA denaturation. Also, a cavitation model using per pass disinfection was developed that can provide meaningful physical description of the disinfection process as against the conventional first order reaction rate model. This study would provide meaningful insight into cavitation process based on hydrodynamic cavitation for the destruction of both gram-negative and gram-positive bacteria from various water sources, including industrial wastewaters.

Affordable Production of Antioxidant Aqueous Solutions by Hydrodynamic Cavitation Processing of Silver Fir (Abies alba Mill.) Needles

Extracts from parts of coniferous trees have received increased interest due to their valuable bioactive compounds and properties, useful for plenty of experimental and consolidated applications, in fields comprising nutraceutics, cosmetics, pharmacology, food preservation, and stimulation of plant growth. However, the variability of the bioactive properties, the complexity of the extraction methods, and the use of potentially harmful synthetic chemicals, still represent an obstacle to the spreading of such valuable natural compounds. Hydrodynamic cavitation is emerging as a promising innovative technique for the extraction of precious food components and by-products from waste raw material of the agro-food production chain, which can improve processing efficiency, reduce resource consumption, and produce healthy, high-quality products. In this study, a process based on controlled hydrodynamic cavitation was applied for the first time to the production of aqueous solutions of silver fir (Abies alba Mill.) needles with enhanced antioxidant activity. The observed levels of the in vitro antioxidant activity, comparable or higher than those found for reference substances, pure extracts, and other water extracts and beverages, highlight the very good potential of the hydrodynamic cavitation (HC) process for the creation of solvent-free, aqueous solutions endowed with bioactive compounds extracted from silver fir needles.

Experimental investigation of the thermal and disinfection performances of a novel hydrodynamic cavitation reactor

In the present study, we proposed an effective, efficient, and economical approach to disinfect water using a novel, advanced, rotational hydrodynamic cavitation reactor (HCR). First, analyses of the flow field and cavitation generation mechanism in the HCR were conducted through visualization of the reactor flow field using a high-speed camera. Second, the thermal performance was tested in 20 experiments with various rotational speeds of the rotor (2700, 3000, 3300, and 3600 rpm) and pump pressure settings (0.0, 0.5, 0.7, 1.0, and 1.5 bar gauge pressure). The HCR maximally achieved a heat generation rate of 48.15 MJ/h and thermal efficiency of 82.18%. Then, the disinfection effect was evaluated using water that simulated an effluent containing Escherichia coli (E. coli) for various flow rates (8, 11, and 14 L/min), a pump pressure setting fixed at 0.5 bar, and a rotational speed of 3600 rpm. In addition, an economical assessment of the disinfection processes was performed by considering the measured electric consumption. The thermal effect generated by the HCR was the dominant factor affecting the concentration of E. coli. The HCR achieved a 100% disinfection rate with a 4.3 L/min treatment rate and a cost of US $ 3.019/m³ at the optimal flow rate. The effects of the pressure setting and rotational speed on the performance were discussed in detail. Finally, compared to the recent studies, the treatment rate of the HCR is several hundred times greater than that obtained by the HCRs utilized in those studies, and also has a reasonable cost.

Numerical Simulation of Hydraulic Characteristics in A Vortex Drop Shaft

A new type of vortex drop shaft without ventilation holes is proposed to resolve the problems associated with insufficient aeration, negative pressure (Unless otherwise specified, the pressure in this text is gauge pressure and time-averaged pressure) on the shaft wall and cavitation erosion. The height of the intake tunnel is adjusted to facilitate aeration and convert the water in the intake tunnel to a non-pressurized flow. The hydraulic characteristics, including the velocity (Unless otherwise specified, the velocity in this text is time-averaged velocity), pressure and aeration concentration, are investigated through model experiment and numerical simulation. The results revealed that the RNG k-ε turbulence model can effectively simulate the flow characteristics of the vortex drop shaft. By changing the inflow conditions, water flowed into the vertical shaft through the intake tunnel with a large amount of air to form a stable mixing cavity. Frictional shearing along the vertical shaft wall and the collisions of rotating water molecules caused the turbulence of the flow to increase; the aeration concentration was sufficient, and the energy dissipation effect was excellent. The cavitation number indicated that the possibility of cavitation erosion was small. The results of this study provide a reference for the analysis of similar spillways.

Co-treatment of spent cathode carbon in caustic and acid leaching process under ultrasonic assisted for preparation of SiC

Spent cathode carbon (SCC) from aluminum electrolysis has been treated in ultrasonic-assisted caustic leaching and acid leaching process, and purified SCC used as carbon source to synthesize silicon carbide (SiC) was investigated. Chemical and mineralogical properties have been characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and thermogravimetry and differential scanning calorimetry (TGA-DSC). Various experimental factors temperature, time, liquid-solid ratio, ultrasonic power, and initial concentration of alkali or acid affecting on SCC leaching result were studied. After co-treatment with ultrasonic-assisted caustic leaching and acid leaching, carbon content of leaching residue was 97.53%. SiC power was synthesized by carbothermal reduction at 1600 °C, as a result of yield of 76.43%, and specific surface area of 4378 cm²/g. This is the first report of using purified SCC and gangue to prepare SiC. The two industrial wastes have been used newly as secondary sources. Furthermore, ultrasonic showed significant effect in SCC leaching process.

Treatment of persistent organic pollutants in wastewater using hydrodynamic cavitation in synergy with advanced oxidation process

Persistent organic pollutants (POPs) are very tenacious wastewater contaminants. The consequences of their existence have been acknowledged for negatively affecting the ecosystem with specific impact upon endocrine disruption and hormonal diseases in humans. Their recalcitrance and circumvention of nearly all the known wastewater treatment procedures are also well documented. The reported successes of POPs treatment using various advanced technologies are not without setbacks such as low degradation efficiency, generation of toxic intermediates, massive sludge production, and high energy expenditure and operational cost. However, advanced oxidation processes (AOPs) have recently recorded successes in the treatment of POPs in wastewater. AOPs are technologies which involve the generation of OH radicals for the purpose of oxidising recalcitrant organic contaminants to their inert end products. This review provides information on the existence of POPs and their effects on humans. Besides, the merits and demerits of various advanced treatment technologies as well as the synergistic efficiency of combined AOPs in the treatment of wastewater containing POPs was reported. A concise review of recently published studies on successful treatment of POPs in wastewater using hydrodynamic cavitation technology in combination with other advanced oxidation processes is presented with the highlight of direction for future research focus.