With the development of industry and the rapid growth of population, the current water treatment technologies face many challenges. Hydrodynamic cavitation as a green and efficient means of water treatment has attracted much attention. During the hydrodynamic cavitation, enormous energy could be released into the surrounding liquid which causes thermal effects (local hotspots with 4600 K), mechanical effects (pressures of 1500 bar) and chemical effects (hydroxyl radicals). These conditions can degrade bacteria and organic substance in sewage. Moreover, the combination of hydrodynamic cavitation and other water treatment methods can produce a coupling effect. In this review, we summarize the methods of hydrodynamic cavitation and the performance of water treatment for different types of sewage. The application of hydrodynamic cavitation reactors with different structures in water treatment are also evaluated and discussed. The design and optimization of high-performance hydrodynamic cavitation reactor are the most crucial issues for the application of hydrodynamic cavitation in water treatment. Finally, recommendations are provided for the future progress of hydrodynamic cavitation for water treatment.

  • The cutting-edge understanding of hydrodynamic cavitation for water treatment is reviewed.

  • Effectiveness and conditions of various hydrodynamic cavitation reactors for sewage are assessed.

  • The state-of-the-art structures of hydrodynamic cavitation reactors are dicussed.

Graphical Abstract

Graphical Abstract
Graphical Abstract

With the development of industry and the continuous growth of population, the issue of water security plays a critical role in national economy and social life. Water treatment is an important part of the whole process of water utilization, including drinking water treatment and sewage treatment (Barisci & Suri 2021). The main purpose of water treatment is to improve water quality and reach hygiene standards. Many waterborne diseases, such as typhoid, cholera, hepatitis and dysentery, are transmitted through contaminated drinking water, which contains an ocean of viruses and pathogenic bacteria. In under-developed areas, about 500,000 people die from water-borne diseases every year (Zhou et al. 2014). Sewage includes domestic wastewater and industrial wastewater, which contains a lot of heavy metals, antibiotics, arsenide, endocrine disruptors (EDCS), etc (Burzio et al. 2020). If the wastewater is treated improperly, it will cause great pollution to the human body and the water environment. Therefore, it is vital to seek advanced water treatment methods which can improve work efficiency and economic efficiency.

With hundreds of years of development, water treatment technology has matured that is different for drinking water and sewage. For drinking water treatment, chlorine is mainly used for disinfection treatment, but in recent years a large number of carcinogenic disinfection by-products (DBPS) and the enhancement of drug resistance of some pathogenic bacteria limit the further development of chlorine disinfection (Gopal et al. 2007). For sewage treatment, the sewage treatment methods such as Fenton method (Selihin & Tay 2022) and membrane separation method (Liang et al. 2019) are difficult to balance the cost and processing efficiency. For the shortage of traditional water treatment methods, a variety of new water treatment methods have been proposed. Nowadays, there are many emerging water treatment methods, including biological adsorption (Liu et al. 2012), biological repair (Gupta et al. 2019), photocatalytic technology (Chong et al. 2010), ultrasonic degradation (Qiu et al. 2019), microwave degradation (Verma & Samanta 2018), nano degradation (Yuan et al. 2018), etc. Hydrodynamic cavitation, as an economic and efficient water treatment method without disinfection by-products, has attracted widespread attention (Bui et al. 2019).

According to different modes of cavitation generation, cavitation can be divided into ultrasonic cavitation, photoinduced cavitation and hydrodynamic cavitation.

  • Ultrasonic cavitation. When high intensity ultrasonic waves act on fluid, mechanical vibration is generated inside the fluid. Under the positive pressure phase, ultrasonic waves extrude the fluid molecules, so that the molecules accumulate. However, under the negative pressure phase, the separation effect is applied to the fluid molecules and makes the molecules discrete. Therefore, in the stage of negative pressure, when ultrasonic intensity reaches a certain level, the average distance between liquid molecules will exceed the maximum distance, resulting in cavitation bubbles. If cavitation bubbles get out of negative pressure, it cannot exist. Cavitation bubbles will collapse under the pressure of fluid, resulting in ultrasonic cavitation phenomenon (Improve Reactions with Hydrodynamic Cavitation | Request PDF n.d.; Saharan et al. 2012).

  • Photoinduced cavitation. Pulse laser is used to input energy into the local area of the liquid. As the energy reaches a certain level, the optical breakdown occurs, and then dense free plasma is generated. A large number of bubbles are generated in the liquid because of absorption of laser energy. When the laser energy decreases or disappears, the bubbles collapse and photoinduced cavitation is formed (Ikai et al. 2010).

  • Hydrodynamic cavitation. As hydrodynamic cavitation occurs, the local pressure inside the fluid drops sharply, which is below the saturated vapor pressure of this state, and the gas dissolves in the water to be separated out, which leads to the generation and growth of cavitation bubbles. With the fluid flowing, the local pressure returns to normal state, and then the cavitation bubbles collapse, which results in a rapid rise in temperature and pressure (Li 2000). The development of cavitation bubble can be observed by speed cameras. Outi Supponen (Supponen et al. 2015) forced on the development of cavitation bubbles can as shown in Figure 1. When the cavitation bubble collapses, it folds on itself and is pierced by a liquid microjet during its collapse.

Figure 1

The cavitation bubble is broken by pressure field (Supponen et al. 2015).

Figure 1

The cavitation bubble is broken by pressure field (Supponen et al. 2015).

Close modal

Compared with the other two cavitation modes, hydrodynamic cavitation has higher efficiency and lower energy consumption. Besides, it is easier to realize in practical application and has absolute advantages in economy. Therefore, hydrodynamic cavitation is more suitable for water treatment industry and has been widely studied worldwide.

Since the nineteenth century, steam machinery has been widely used. When the speed of the propeller increased to a certain extent, the speed of the ship did not increase accordingly. The propeller speed also decreased in some steam engine ships. For this phenomenon, Barnaby and Parsons first put forward ‘cavitation’ (Ozonek 2012; Cvetković et al. 2015), which referred to hydrodynamic cavitation. Hydrodynamic cavitation refers to the process of cavitation bubble formation, growth, and collapse in the liquid when the local pressure of the fluid is lower than the saturated vapor pressure. The bubble collapse process can generate a sharp rise in temperature and pressure, which has influence on hydraulic machinery (Tullis 2007), such as the damage of impeller, the intensified vibration and the degraded performance of machinery (Tao et al. 2016), and water conservancy construction, such as the damage of bottom outlets of dam (Yamini et al. 2020; Yamini et al. 2021). However, various chemical and physical processes can be strengthened by utilizing the high temperature and pressure liquid environment generated by cavitation bubble collapse, which makes it possible to apply hydrodynamic cavitation to water treatment.

After decades of development, research emphasis has changed, which mainly includes cavitation suppression and cavitation utilization. In terms of cavitation suppression, people focus on how to reduce the influence of cavitation on the hydraulic machinery's life span, hydraulic performance, vibration performance by optimizing impeller, guide vane and volute. In terms of the cavitation utilization, people focus on how to use hydrodynamic cavitation equipment to apply physical and chemical conditions that are produced by hydrodynamic cavitation to the industrial production, such as water treatment (Sun et al. 2020a), impinging on rock (Improvement of penetration rate with hydraulic pulsating-cavitation jet compound drilling technology | Request PDF n.d.), emulsion preparation (Terán Hilares et al. 2019), food processing (Zhang et al. 2016), etc.

With the rapid development of modern industry, hydrodynamic cavitation has been applied in water treatment. Besides, it has been proved that it is feasible for water treatments. As an efficient and energy-saving water treatment method, hydrodynamic cavitation has been investigated widely, and research mainly includes the effects of hydrodynamic cavitation water treatment, cavitation mechanism, cavitation reactor structure optimization and so on.

In the review, we summarize the methods of hydrodynamic cavitation and the performance of water treatment to different types of sewage (microbial sewage, dye wastewater and other industrial wastewater). The influence of cavitation bubble collapsing and water treatment mechanisms are elaborated. The water treatment performance of different types of hydrodynamic cavitation reactors is evaluated for venturi tube, orifice plate and rotor hydrodynamic cavitation reactor. Finally, recommendations are provided for the future progress of hydrodynamic cavitation for water treatment.

When the cavitation bubbles collapse, a large amount of energy is released, which affects the surrounding liquid environment in the form of thermal effect, mechanical effect and chemical effect. The liquid environment generated by hydrodynamic cavitation can be used effectively for water treatment.

  • Thermal effect. When a single cavity collapses, a mass of heat would be released in the local area. As forming an extremely high temperature environment with thousands of kelvins, the hot spot effect appears. Collection of the hot spot effect of all cavitation bubbles forms the thermal effect of hydrodynamic cavitation (Ashokkumar 2011). The thermal effect of hydrodynamic cavitation includes temperature gradient and temperature rise rate (Suslick et al. 1986). The thermal effect has multiple effects on the efficiency of water treatment. First, it is beneficial to damage the structure of pathogenic bacteria and microbial cell wall/membrane in sewage. Then, high temperature is a beneficial factor to assist other effects produced by cavitation or other water treatment methods. Moreover, the heat helps to open the chemical bond between molecules, and generate strong oxidizing free radicals to degrade organic matter (Adewuyi 2001).

  • For the thermal effect of hydrodynamic cavitation, scientists in various countries quantify the strength of hydrodynamic cavitation heat effect through different methods, but the data obtained are different. Currently, the recognized temperature of cavitation heat effect is 103 orders of magnitude. In the presence of methane, Hart et al. (1990) use the ratio of ethylene, acetylene and ethane to calculate the effective temperature in the cavity, and the effective temperature obtained is 2800 Rae et al. (2005) propose a method that can quantify the thermal effect of cavitation. The temperature of cavitation thermal effect was measured by the yield of hydrocarbon produced by the thermal decomposition of alcohol. Using this method to measure the cavitation heat effect of ultrasonic cavitation, the bubble collapse temperature is between 2300 and 4600 K. Cavitation bubble temperatures as a function of surface excess values of different alcohols are shown in Figure 2. By analyzing the internal energy equation of cavitation and studying the heat transfer of photoinduced cavitation, Kwak & Yang (1995) found that the thermal effect temperature of bubble collapse can reach 7000 K—44000 K under ultrasonic condition, and the heat flux of bubble collapse can reach 47 GW/m2.

  • Mechanical effect. The mechanical effects of hydrodynamic cavitation include the strong shock wave generated when cavitation collapses, high-speed microjet and high shear stress. The intensive mechanical effect distorts and breaks the outer wall of microorganisms on the physical plane, making microorganisms inactivated, so as to achieve the purpose of water treatment. Dijkink & Ohl (2008) measured the wall shear stress of cavitation bubble near the rigid boundary. It is found that the shear stress is generated in the radial diffusion process of cavitation surface after jet impact. Lee et al. (2011) studied the implosion of a single spherical bubble by the laser forced optical breakdown and proposed a time-resolved velocity method to measure the impact of extreme peak pressure after cavitation collapse, as shown in Figure 3. The first shock wave is emitted at the focal point of the laser shedding, the second shock wave is emitted when the bubble implodes. Liu et al. (2013) studied the growth and collapse of a single cavity near the heating wall and its influence on heat transfer by numerical simulation, and then the microjet velocity generated by the collapse of the cavity is obtained.

  • Chemical effect. As a bubble collapses, the extreme conditions can break the chemical bonds of the water molecule and form strongly oxidizing hydroxyl radicals -OH and hydrogen peroxide radicals -OH2. Strong oxidizing radicals can effectively degrade organic pollutants in sewage (Vidanage et al. 2020). Meanwhile, the chemical effect of hydrodynamic cavitation is greatly influenced by the environment. The chemical effect of hydrodynamic cavitation can be effectively enhanced and the water treatment quality can be improved (Senthil Kumar et al. 2000).

Figure 2

Cavitation bubble temperatures as a function of surface excess values of different alcohols (Rae et al. 2005).

Figure 2

Cavitation bubble temperatures as a function of surface excess values of different alcohols (Rae et al. 2005).

Close modal
Figure 3

A sequence of a shock wave induced by cavitation bubble growth and collapse (Lee et al. 2011).

Figure 3

A sequence of a shock wave induced by cavitation bubble growth and collapse (Lee et al. 2011).

Close modal

Microbial sewage

Kitchen waste sewage, toilet sewage or unpasteurized drinking water in daily life contains a large number of bacteria, algae and other microorganisms (Sukhani & Chanakya 2020). Hydrodynamic cavitation releases a lot of energy, which can degrade ordinary organic matter, bacteria and algae by breaking down their cell walls (Sun et al. 2020b). In addition, it is difficult for bacteria to survive under high temperature, and sterilization can be realized by using the hot-spot temperature as high as several thousand kelvins released by hydrodynamic cavitation. Therefore, hydrodynamic cavitation has a good effect on killing bacteria and other active microorganisms.

Jain et al. (2019) used the cavitation of the hydrodynamic cavitation reactor to disinfect drinking water containing bacterium. They also studied the disinfection effect of Gram-positive bacteria (using Escherichia coli as a template) and Gram-negative bacteria (using Staphylococcus aureus as a template) under different pressure (0.5, 1, 2, 5, 10 bar) by using a new-type of vortex diode and found that vortex diode can achieve 99% removal of E. coli under low pressure drop (0.5 bar) while under the high pressure drop (10 bar), 98% removal of S. aureus can be achieved, which means the disinfection effect of vortex diode is much greater than that of the orifice plate. The vortex diode hydrodynamic cavitation reactor is easy to be used widely and it has high commercial value. Wei et al. (2019) used a rotor hydrodynamic cavitation reactor to study the bactericidal effect of E. coli. The hydrodynamic cavitation intensity was adjusted by changing the temperature (0.02, 0.04, 0.05 MPa) and inlet pressure (30, 40, 50, 55, 60, 65 °C), and the bactericidal effect was evaluated by using a microscope and the plate counting method. It was found that the cavitation strength of hydrodynamic cavitation reactor increases with the increase of inlet pressure, when the temperature is between 50 and 60 °C, the higher the temperature is, the better the bactericidal effect of E. coli is obtained. Table 1 summarizes the microbial research on hydrodynamic cavitation disinfection in recent years.

Table 1

Microbial research on hydrodynamic cavitation disinfection in recent years

Target microorganismHydrodynamic cavitation reactorVariable conditionsConclusionReference
E. coli, S. Aureus Orifice plate, eddy current diode Pressure drop (0.02, 0.04, 0.05 MPa), reactor type, microbial kinds 
  • The geometry of the reactor has a significant effect on the disinfection

  • The orifice plate requires a high pressure drop (10 bar) to efficiently kill gram-positive bacteria

  • Hydrodynamic cavitation damages the microbiological mechanism and the cell dies due to the damage of DNA

 
Jain et al. (2019)  
E. coli Rotor reactor Temperature (30, 40, 50, 55, 60, 65 °C), pressure (0.02, 0.04, 0.05 MPa) 
  • As the inlet pressure of the reactor increases, the cavitation intensity inside the reactor increases and more hydroxyl radicals are generated

  • The number of E. coli decreased linearly with the increase of cavitation time (0–240 s). When the cavitation time is more than 240 seconds, the sterilization rate reaches 100%

 
Wei et al. (2019)  
Micro algae Orifice plate Orifice parameters, time (15∼195 min), pressure (0∼10 bar) 
  • The optimum condition of microalgae destruction by hydrodynamic cavitation was cavitation device orifice, time (180 min), pressure (5 bar) and solid load (0.45% w/v)

  • Compared with ultrasonic cavitation, the energy efficiency of destruction of microalgae by using hydrodynamic cavitation is much better

 
Waghmare et al. (2019)  
E. coli Conical nozzle Pressure (5, 10 Mpa) 
  • The efficiency of cavitation jet is high, and it can produce high turbulence and eddy in the liquid in a short time, and kill the microorganism in the water more effectively

  • The best working condition of E. coli destruction by hydrodynamic cavitation was cavitation device cavitation jet, time (900 s), pressure (10 Mpa)

 
Dalfré Filho et al. (2015)  
E. coli Orifice plate  Cavitation number (0.2. 0.4. 0,65), initial concentration of strain (2e+6, 3e+4, 3e+3) 
  • The effect of cavitation number and initial concentration on disinfection efficiency was studied

  • The formation and collapse of bubbles play an important role in water treatment

 
Burzio et al. (2020
Legionella, Bacillus subtilis, E. coli Rotor-type hydrodynamic cavitation reactor  
  • The cavitation effect produced by the new rotor reactor can kill Legionella and E. coli completely, and is also effective against the Gram-positive bacteria and B. subtilis.

  • When the bacterial cell enters the cavitation zone, the pressure drops makes the cell wall/membrane rupture.

 
Šarc et al. (2018)  
E. coli, B. subtilis High pressure jet device Pipeline structure (wider pipe, narrow middle pipe, hose pipe) 
  • The high-pressure jet device (HPJD) was developed and the analysis of released intracellular compounds revealed the destruction of bacterial cell wall/membrane structure by hydrodynamic cavitation

  • Shear collision and cavitation damage E. coli and B. subtilis cells in different ways

  • Cavitation caused by high pressure jet device can damage both E. coli and B. subtilis cells, while shear collision has limited damage on B. subtilis cells.

 
Xie et al. (2017
Target microorganismHydrodynamic cavitation reactorVariable conditionsConclusionReference
E. coli, S. Aureus Orifice plate, eddy current diode Pressure drop (0.02, 0.04, 0.05 MPa), reactor type, microbial kinds 
  • The geometry of the reactor has a significant effect on the disinfection

  • The orifice plate requires a high pressure drop (10 bar) to efficiently kill gram-positive bacteria

  • Hydrodynamic cavitation damages the microbiological mechanism and the cell dies due to the damage of DNA

 
Jain et al. (2019)  
E. coli Rotor reactor Temperature (30, 40, 50, 55, 60, 65 °C), pressure (0.02, 0.04, 0.05 MPa) 
  • As the inlet pressure of the reactor increases, the cavitation intensity inside the reactor increases and more hydroxyl radicals are generated

  • The number of E. coli decreased linearly with the increase of cavitation time (0–240 s). When the cavitation time is more than 240 seconds, the sterilization rate reaches 100%

 
Wei et al. (2019)  
Micro algae Orifice plate Orifice parameters, time (15∼195 min), pressure (0∼10 bar) 
  • The optimum condition of microalgae destruction by hydrodynamic cavitation was cavitation device orifice, time (180 min), pressure (5 bar) and solid load (0.45% w/v)

  • Compared with ultrasonic cavitation, the energy efficiency of destruction of microalgae by using hydrodynamic cavitation is much better

 
Waghmare et al. (2019)  
E. coli Conical nozzle Pressure (5, 10 Mpa) 
  • The efficiency of cavitation jet is high, and it can produce high turbulence and eddy in the liquid in a short time, and kill the microorganism in the water more effectively

  • The best working condition of E. coli destruction by hydrodynamic cavitation was cavitation device cavitation jet, time (900 s), pressure (10 Mpa)

 
Dalfré Filho et al. (2015)  
E. coli Orifice plate  Cavitation number (0.2. 0.4. 0,65), initial concentration of strain (2e+6, 3e+4, 3e+3) 
  • The effect of cavitation number and initial concentration on disinfection efficiency was studied

  • The formation and collapse of bubbles play an important role in water treatment

 
Burzio et al. (2020
Legionella, Bacillus subtilis, E. coli Rotor-type hydrodynamic cavitation reactor  
  • The cavitation effect produced by the new rotor reactor can kill Legionella and E. coli completely, and is also effective against the Gram-positive bacteria and B. subtilis.

  • When the bacterial cell enters the cavitation zone, the pressure drops makes the cell wall/membrane rupture.

 
Šarc et al. (2018)  
E. coli, B. subtilis High pressure jet device Pipeline structure (wider pipe, narrow middle pipe, hose pipe) 
  • The high-pressure jet device (HPJD) was developed and the analysis of released intracellular compounds revealed the destruction of bacterial cell wall/membrane structure by hydrodynamic cavitation

  • Shear collision and cavitation damage E. coli and B. subtilis cells in different ways

  • Cavitation caused by high pressure jet device can damage both E. coli and B. subtilis cells, while shear collision has limited damage on B. subtilis cells.

 
Xie et al. (2017

Dye wastewater

The treatment of dye wastewater has always been a vital project in the world water treatment area. Dyes are widely used in cosmetics, fabrics and plastics. Due to the complex composition of dye wastewater, it is difficult to degrade, which contains a large number of macromolecules. Some dyes have high toxicity and strong carcinogenicity. And unreasonable discharge will be harmful for the environment and human body (Ghosh et al. 2020). Nowadays, the main treatment methods for dye wastewater include membrane separation method, flocculation method and adsorption method. These methods are expensive, inefficient, and difficult to meet the needs of water treatment. Therefore, hydrodynamic cavitation is an economic and efficient water treatment method for dye wastewater treatments.

It can be seen from experiments that although hydrodynamic cavitation can degrade dyes to a certain extent, it is difficult to completely kill some dyes with complex molecular structure. Therefore, research on the treatment of dye wastewater by hydrodynamic cavitation can be roughly divided into two aspects. One is to study the degradation of dyes by hydrodynamic cavitation itself. The other is to realize the efficient removal of dyes by the combination of hydrodynamic cavitation and other methods. Dyes are generally composed of organics with complex structures. Studies on the use of hydrodynamic cavitation in combination with other water treatment methods are described below. Gore et al. (2014) combined hydrodynamic cavitation with different substances, such as H2O2, ozone and other oxidants degradation of reactive orange 4 dye (RO4), and found that this method is much better than that of hydrodynamic cavitation alone (14.67%), particularly, the effect of combining hydrodynamic cavitation with ozone is the best method (76.25%). Abbas-Shiroodi et al. (2021) studied the effect of different structural parameters of orifice plate (Number of holes 1, 13, 25, 33) and venturi tube (inlet angle 20.7, 22.7, 24.7) on decolorization of Congo red by CFD combined with experiments and finally determined the best degradation conditions and decolorization effect of Congo red by using orifice plate (26.2%) and Venturi tube (38.8%). Table 2 shows the study of dye treatment by combining hydrodynamic cavitation with other water treatment methods.

Table 2

Study on the combination of hydrodynamic cavitation and other water treatment methods to treat dyes

Target dyeHydrodynamic cavitation reactorMethod of combined use and conditionsConclusionReference
Methyl orange Orifice plate, venturi tube H2O2, TiO2 
  • The removal rate of hydrodynamic cavitation combined with TiO2 or H2O2 was 50%, and the removal rate of hydrodynamic cavitation combined with TiO2 and H2O2 was 70%

  • The higher the dye concentration, the lower the venturi cavitation efficiency

 
Innocenzi et al. (2019)  
Congo red Orifice plate, venturi tube H2O2; Number of holes (1, 13, 25, 33), inlet angle (20.7, 22.7, 24.7) 
  • The optimal condition of congo destruction by hydrodynamic cavitation was cavitation device orifice plate, cavitation number (0.12), inlet pressure (5 bar)

  • The degradation rate increased when the inlet pressure was increased to 5 bar

 
Abbas-Shiroodi et al. (2021)  
AR-88 Venturi tube H2O2, Fe – TiO2; Inlet pressure (0∼8 bar), initial concentration (0∼160 M) 
  • The degradation degree of AR-88 increased with the increase of the initial concentration of dye, and the degradation rate was related to the pH of solution

  • Inputting the H2O2 and catalyst (Fe-Tio2) can enhance the degradation rate of AR-88

 
Saharan et al. (2012)  
AR-18 Orifice plate, venturi tube H2O2, TiO2; Pressure (2∼7 bar), ph (3, 5, 7, 9), initial concentration (15, 20, 25 30 ppm), hydrogen peroxide dose (1, 100, 200, 300, 400 mg/mL) 
  • Compared with ultrasonic cavitation, the degradation rate of hydrodynamic cavitation (pore plate) is higher than that of ultrasonic cavitation, regardless of whether TiO2 is used

  • The degradation of AR-18 can be improved by combining the advanced oxidation process and hydrodynamic cavitation

 
Dhanke & Wagh (2020)  
RR-120 Venturi tube H2O2; Inlet pressure (3∼8 bar), cavitation number (0.1∼0.25) 
  • The degradation of RR-120 is related to the pH of solution, and the degradation rate is higher in acidic medium

  • The degradation rate can be improved if H2O2 is used in the process

 
Saharan et al. (2011)  
Coomassie blue Oblique face plate KPS, H2O2; Pressure (4∼8 bar), H2O2 concentration (0∼1000 mg/L), PS concentration (0∼1000 mg/L) 
  • The synergistic effect of different catalysts combined with hydrodynamic cavitation (HC-KPS, HC-H2O2, KPS-H2O2, and HC-H2O2-KPS) was confirmed

 
Baradaran & Sadeghi (2019
Congo red  H2O2, FeSO4; Initial pH (3∼10), H2O2 concentration (0∼1000 mg/L), FeSO4 concentration (0∼50 mg/L) 
  • The combination of hydrodynamic cavitation with Fenton's reagent H2O2 and FeSO4 had the best effect on the degradation of Congo red

  • Residence time distribution (RTD) analysis method was used to study the flow characteristics of hydrodynamic cavitation reactor

 
Askarniya et al. (2020
RO-4 Orifice, circular venturi tube, slit venturi tube H2O2, Ozone; feed rate of ozone (1, 2, 3, 4, 5, 8 g/h) ph (2, 3.2, 5, 7.3, 10) 
  • The efficiency of hydrodynamic cavitation can be significantly improved by combing with H2O2 and ozone

  • Hydrodynamic cavitation combined with other oxidants can degrade RO-4 better than using it alone

  • Hydrodynamic cavitation combined with ozone is the most efficient method to degrade RO-4

 
Gore et al. (2014
Target dyeHydrodynamic cavitation reactorMethod of combined use and conditionsConclusionReference
Methyl orange Orifice plate, venturi tube H2O2, TiO2 
  • The removal rate of hydrodynamic cavitation combined with TiO2 or H2O2 was 50%, and the removal rate of hydrodynamic cavitation combined with TiO2 and H2O2 was 70%

  • The higher the dye concentration, the lower the venturi cavitation efficiency

 
Innocenzi et al. (2019)  
Congo red Orifice plate, venturi tube H2O2; Number of holes (1, 13, 25, 33), inlet angle (20.7, 22.7, 24.7) 
  • The optimal condition of congo destruction by hydrodynamic cavitation was cavitation device orifice plate, cavitation number (0.12), inlet pressure (5 bar)

  • The degradation rate increased when the inlet pressure was increased to 5 bar

 
Abbas-Shiroodi et al. (2021)  
AR-88 Venturi tube H2O2, Fe – TiO2; Inlet pressure (0∼8 bar), initial concentration (0∼160 M) 
  • The degradation degree of AR-88 increased with the increase of the initial concentration of dye, and the degradation rate was related to the pH of solution

  • Inputting the H2O2 and catalyst (Fe-Tio2) can enhance the degradation rate of AR-88

 
Saharan et al. (2012)  
AR-18 Orifice plate, venturi tube H2O2, TiO2; Pressure (2∼7 bar), ph (3, 5, 7, 9), initial concentration (15, 20, 25 30 ppm), hydrogen peroxide dose (1, 100, 200, 300, 400 mg/mL) 
  • Compared with ultrasonic cavitation, the degradation rate of hydrodynamic cavitation (pore plate) is higher than that of ultrasonic cavitation, regardless of whether TiO2 is used

  • The degradation of AR-18 can be improved by combining the advanced oxidation process and hydrodynamic cavitation

 
Dhanke & Wagh (2020)  
RR-120 Venturi tube H2O2; Inlet pressure (3∼8 bar), cavitation number (0.1∼0.25) 
  • The degradation of RR-120 is related to the pH of solution, and the degradation rate is higher in acidic medium

  • The degradation rate can be improved if H2O2 is used in the process

 
Saharan et al. (2011)  
Coomassie blue Oblique face plate KPS, H2O2; Pressure (4∼8 bar), H2O2 concentration (0∼1000 mg/L), PS concentration (0∼1000 mg/L) 
  • The synergistic effect of different catalysts combined with hydrodynamic cavitation (HC-KPS, HC-H2O2, KPS-H2O2, and HC-H2O2-KPS) was confirmed

 
Baradaran & Sadeghi (2019
Congo red  H2O2, FeSO4; Initial pH (3∼10), H2O2 concentration (0∼1000 mg/L), FeSO4 concentration (0∼50 mg/L) 
  • The combination of hydrodynamic cavitation with Fenton's reagent H2O2 and FeSO4 had the best effect on the degradation of Congo red

  • Residence time distribution (RTD) analysis method was used to study the flow characteristics of hydrodynamic cavitation reactor

 
Askarniya et al. (2020
RO-4 Orifice, circular venturi tube, slit venturi tube H2O2, Ozone; feed rate of ozone (1, 2, 3, 4, 5, 8 g/h) ph (2, 3.2, 5, 7.3, 10) 
  • The efficiency of hydrodynamic cavitation can be significantly improved by combing with H2O2 and ozone

  • Hydrodynamic cavitation combined with other oxidants can degrade RO-4 better than using it alone

  • Hydrodynamic cavitation combined with ozone is the most efficient method to degrade RO-4

 
Gore et al. (2014

Other industrial wastewater

Industrial wastewater refers to the sewage and waste liquid produced in the process of industrial production. There are many kinds of wastewater and components of industrial wastewater are complex. For example, the wastewater from the petroleum refining industry contains phenols. The wastewater from pesticide manufacturing industry contains various pesticides. The wastewater from heavy metal smelting industry contains plumbum, cadmium and other metallic elements. The wastewater from the electroplating industry contains cyanide, chromium and other heavy metals (Sharma & Sanghi 2012). The composition of industrial wastewater is complex and often contains a large number of harmful substances, so it must meet the discharge standards before discharge. In the study of industrial wastewater treatment by hydrodynamic cavitation, the effect of using hydrodynamic cavitation alone is not very good because industrial wastewater contains a large number of organics and macromolecules. Therefore, the combination of hydrodynamic cavitation and other methods has become the main research trend of industrial wastewater treatment. In addition, due to the complex composition of industrial wastewater, universal variables for sewage assessment are often used for quantitative research in experimental studies, such as COD (the demand of chemical oxygen), VSS (volatile suspended solids), TOC (total organic carbon), SVI (sludge volume index), kinetic rate coefficient, etc (Gągol et al. 2018; Dhanke et al. 2019).

Thanekar & Gogate (2019) combined hydrodynamic cavitation and the oxidation process, based on hydrogen peroxide (H2O2), ozone (O3) and persulfate (KPS). In order to reduce COD, scholars compared the effect of treating industrial wastewater by hydrodynamic cavitation alone or in combination with other oxidation processes. The hydrodynamic cavitation has higher energy efficiency, and the treatment cost is significantly lower than that of ultrasonic cavitation. Doltade et al. (2019) treated refining wastewater by using orifice plate and venturi tube, summarized degradation rate and energy consumption ratio, evaluated the treatment performance of hydrodynamic cavitation treatment of refining wastewater, and then optimized the structure of cavitation reactor. Table 3 shows the research on the treatment of industrial wastewater or harmful substances in industrial wastewater by hydrodynamic cavitation in recent years.

Table 3

Research on the treatment of industrial wastewater or harmful substances in industrial wastewater by hydrodynamic cavitation in recent years

Target pollutantHydrodynamic cavitation reactorResearch methods and conditionsConclusionReference
Tetracycline Venturi tube Combined with photocatalysis; Experimental study; pH (2.1, 4.2, 7.2, 8.3, 10), inorganic anions (20 M Cl, 20 M SO42−
  • The degradation of tetracycline depends on pH and is more suitable for the degradation of tetracycline under alkaline environment

  • HCO3 can promote the photocatalytic degradation of tetracycline

  • Hydrodynamic cavitation can inhibit the photocatalysis

 
Wang et al. (2017)  
DDVP Rotor reactor Combined with ozone disinfection; Experimental study; Pressure (4, 5, 6, 7 bar), pH (3, 5, 7, 9) 
  • In terms of kinetic rate constant, the combined pretreatment of hydrodynamic cavitation and ozone can effectively improve the degradation rate of DDVP

 
Thanekar et al. (2018a)  
Methomyl Venturi tube Combined with H2O2, Fenton method, ozone; Experimental study; pH (1, 2. 5, 3, 4, 6), pressure (2, 3, 4, 5, 7 bar) 
  • The combined use of hydrodynamic cavitation and ozone is the most effective way to degrade Methomyl, which has the highest synergy coefficient and energy efficiency

  • Methomyl can be degraded completely with hydrodynamic cavitation in the presence of catalysts

 
Raut-Jadhav et al. (2015)  
Industrial wastewater Venturi tube Combined with ozone, H2O2, persulfate; Experimental study 
  • In terms of COD reduction, cavitation yield calculation and treatment cost, hydrodynamic cavitation is good in energy efficiency and economy

 
Thanekar & Gogate (2019)  
Activated sludge Rotor reactor Experimental study 
  • The particle degradation rate and oxidation performance of hydrodynamic cavitation are generally better than that of ultrasonic cavitation

 
Kim et al. (2019)  
Refinery wastewater Orifice plate, venturi tube Experimental study 
  • Obtained the optimum conditions for degradation of refinery wastewater by orifice plate and venturi tube

  • By increasing the times of treatments and combining with other advanced oxidation processes, the disinfection effect can be further improved

  • Under low pressure, the germicidal effect is much better but the reduction effect of COD is inferior

 
Doltade et al. (2019)  
Ibuprofen Venturi tube Experiment method and numerical simulation method are combined; Pressure (3, 5, 7 bar) 
  • The optimal condition of Ibuprofen destruction by hydrodynamic cavitation was cavitation device Venturi tube, inlet pressure (0.35 MPa)

  • Five intermediates from different hydroxylation reactions were identified

  • PH value does not affect the degree of degradation

 
Musmarra et al. (2016)  
2,4,5 trichlorophenol Venturi tube Combined with ozone, H2O2; Experimental study ; Pressure (2, 3, 4, 5 bar), pH (3, 5, 7, 9, 11) 
  • The optimal condition of Ibuprofen destruction by hydrodynamic cavitation was cavitation device Venturi tube, inlet pressure (0.35 MPa), temperature (30 °C), initial pH (7)

  • Hydrodynamic cavitation combing with ozone and H2O2 can achieve complete removal of 2,4,5 trichlorophenol

 
Barik & Gogate (2018
Carbamazepine Slit venturi tube Combined with ozone, H2O2; Experimental study; pH (3, 4, 5, 6, 11), pressure (3, 4, 5 bar), HC and H2O2 loading (1:2∼1:5) 
  • The optimal condition of carbamazepine destruction by hydrodynamic cavitation was cavitation device slit venturi tube, inlet pressure (4 bar), initial pH (4)

  • Using the optimal cavitation reactor combined with ozone and hydrogen peroxide is the most economical and efficient method

 
Thanekar et al. (2018b)  
Kitchen wastewater Orifice plate Combined with ozone, H2O2; pH (2∼11), pressure (1.5∼5.5 bar) 
  • Combining with various methods, we can obtain different results of water treatment. Using hydrodynamic cavitation combined with H2O2 is the most economical and efficient way of water treatment

 
Mukherjee et al. (2020
Target pollutantHydrodynamic cavitation reactorResearch methods and conditionsConclusionReference
Tetracycline Venturi tube Combined with photocatalysis; Experimental study; pH (2.1, 4.2, 7.2, 8.3, 10), inorganic anions (20 M Cl, 20 M SO42−
  • The degradation of tetracycline depends on pH and is more suitable for the degradation of tetracycline under alkaline environment

  • HCO3 can promote the photocatalytic degradation of tetracycline

  • Hydrodynamic cavitation can inhibit the photocatalysis

 
Wang et al. (2017)  
DDVP Rotor reactor Combined with ozone disinfection; Experimental study; Pressure (4, 5, 6, 7 bar), pH (3, 5, 7, 9) 
  • In terms of kinetic rate constant, the combined pretreatment of hydrodynamic cavitation and ozone can effectively improve the degradation rate of DDVP

 
Thanekar et al. (2018a)  
Methomyl Venturi tube Combined with H2O2, Fenton method, ozone; Experimental study; pH (1, 2. 5, 3, 4, 6), pressure (2, 3, 4, 5, 7 bar) 
  • The combined use of hydrodynamic cavitation and ozone is the most effective way to degrade Methomyl, which has the highest synergy coefficient and energy efficiency

  • Methomyl can be degraded completely with hydrodynamic cavitation in the presence of catalysts

 
Raut-Jadhav et al. (2015)  
Industrial wastewater Venturi tube Combined with ozone, H2O2, persulfate; Experimental study 
  • In terms of COD reduction, cavitation yield calculation and treatment cost, hydrodynamic cavitation is good in energy efficiency and economy

 
Thanekar & Gogate (2019)  
Activated sludge Rotor reactor Experimental study 
  • The particle degradation rate and oxidation performance of hydrodynamic cavitation are generally better than that of ultrasonic cavitation

 
Kim et al. (2019)  
Refinery wastewater Orifice plate, venturi tube Experimental study 
  • Obtained the optimum conditions for degradation of refinery wastewater by orifice plate and venturi tube

  • By increasing the times of treatments and combining with other advanced oxidation processes, the disinfection effect can be further improved

  • Under low pressure, the germicidal effect is much better but the reduction effect of COD is inferior

 
Doltade et al. (2019)  
Ibuprofen Venturi tube Experiment method and numerical simulation method are combined; Pressure (3, 5, 7 bar) 
  • The optimal condition of Ibuprofen destruction by hydrodynamic cavitation was cavitation device Venturi tube, inlet pressure (0.35 MPa)

  • Five intermediates from different hydroxylation reactions were identified

  • PH value does not affect the degree of degradation

 
Musmarra et al. (2016)  
2,4,5 trichlorophenol Venturi tube Combined with ozone, H2O2; Experimental study ; Pressure (2, 3, 4, 5 bar), pH (3, 5, 7, 9, 11) 
  • The optimal condition of Ibuprofen destruction by hydrodynamic cavitation was cavitation device Venturi tube, inlet pressure (0.35 MPa), temperature (30 °C), initial pH (7)

  • Hydrodynamic cavitation combing with ozone and H2O2 can achieve complete removal of 2,4,5 trichlorophenol

 
Barik & Gogate (2018
Carbamazepine Slit venturi tube Combined with ozone, H2O2; Experimental study; pH (3, 4, 5, 6, 11), pressure (3, 4, 5 bar), HC and H2O2 loading (1:2∼1:5) 
  • The optimal condition of carbamazepine destruction by hydrodynamic cavitation was cavitation device slit venturi tube, inlet pressure (4 bar), initial pH (4)

  • Using the optimal cavitation reactor combined with ozone and hydrogen peroxide is the most economical and efficient method

 
Thanekar et al. (2018b)  
Kitchen wastewater Orifice plate Combined with ozone, H2O2; pH (2∼11), pressure (1.5∼5.5 bar) 
  • Combining with various methods, we can obtain different results of water treatment. Using hydrodynamic cavitation combined with H2O2 is the most economical and efficient way of water treatment

 
Mukherjee et al. (2020

For the hydrodynamic cavitation researches and application of water treatment, a hydraulic device is needed for sewage treatment and disinfection. These hydraulic devices are collectively called hydrodynamic cavitation reactors. The performance of the hydrodynamic cavitation reactor has great influence on water treatment research and application. Using an effective and economic hydrodynamic cavitation reactor can increase the quality of water treatment and the commonly used hydrodynamic cavitation reactor include the venturi tube cavitation reactor, orifice plate hydrodynamic cavitation reactor and rotor hydrodynamic cavitation reactor (Wang et al. 2021). The experimental study of the hydrodynamic cavitation reactor can not only optimize the structure of the hydrodynamic cavitation reactor and study the application of water treatment, but also provide a theoretical basis for optimizing the hydraulic performance of fluid machinery and reduce the influence of cavitation phenomenon on the comprehensive performance of fluid machinery.

Venturi tube

When the liquid flows through the throat of the venturi tube, the decrease of the cross-sectional area leads to the increase of the flow velocity, which reduces the local pressure of the liquid. When the local pressure falls below the corresponding saturated vapor pressure, cavitation bubbles are generated and grow. As the liquid continues to flow, the pressure returns to normal, cavitation bubbles collapse, and cavitation phenomenon occurs (Li et al. 2019). In practice, the cavitation performance of the venturi tube hydrodynamic cavitation reactor can be optimized by changing the length of venturi tube throat to change the duration of hydrodynamic cavitation or the inclination angle of venturi tube's inlet and outlet (Zhang 2017).

Geng & Dong (2017) studied the sterilization effect of hydrodynamic cavitation on E. coli by using the venturi tube hydrodynamic cavitation reactor. Scholars changed the diameter of tube throat, the length of tube throat and flow rate of venturi tube to optimize the performance of the hydrodynamic cavitation reactor. They finally evaluated the sterilization effect of hydrodynamic cavitation by comparing the killing rate of E. coli. Simpson & Ranade (2019) discussed the influence of venturi tube throat's length, diffuser angle, inlet pressure and other factors on the hydrodynamic cavitation of venturi tube. They also used CFD numerical simulation method to analyze the influence of various structural parameters on the hydrodynamic cavitation of venturi tube reactor.

Orifice plate

When the liquid flows through the orifice of the orifice plate hydrodynamic cavitation reactor, the sudden decrease of the cross-sectional area leads to the increase of the liquid pressure, resulting in hydrodynamic cavitation phenomenon (Ai & Ding 2010). The orifice plate hydrodynamic cavitation reactor is used commonly because of its simple structure, convenient fabrication, low price and high stability.

He & Zhang (2017) studied the cavitation characteristics of orifice and the effects of a series of parameters on the cavitation effect of the orifice hydrodynamic cavitation reactor, such as back pressure and cavitation number by means of high-speed photography and numerical simulation. By simulation and experiment, it is found that the back pressure is inversely proportional to the flow loss when the pressure difference is constant. Dhanke & Wagh (2020) used the experiments to prove that the porous plate hydrodynamic cavitation reactor was the best hydrodynamic cavitation reactor for the degradation of AR-18 dye. Bedsides, they combined catalyst and hydrodynamic cavitation degradation to improve the degradation efficiency greatly, and obtained the optimal degradation conditions of the porous plate hydrodynamic cavitation reactor.

Rotor hydrodynamic cavitation reactor

The orifice plate and venturi tube have some advantages, such as simple structure and easy operation. However, similar to baffles, orifice plates and venturi tubes cause great energy loss which limits their wide spread use in industrial production (Al-Mansori et al. 2020). Through the study and summary of cavitation mechanism, it is found that the reactor with rotating parts has higher cavitation efficiency. The relative movement of rotating parts and other structures is easy to generate shear cavitation. The rotor rotation is easy to produce pressure surface and thrust surface, which makes a low-pressure area that is conducive to the generation of cavitation. Rotor hydrodynamic cavitation reactor is a general term for cavitation reactor with rotating parts. It is generally composed of rotor and stator. The rotor and stator are provided with structures that are prone to hydrodynamic cavitation. The rotor type hydrodynamic cavitation reactor has similarities with the principle of a centrifugal pump. The reduction effects of hydrodynamic cavitation on the centrifugal pump have been the focal point. The study of rotor hydrodynamic cavitation reactor is of great significance for reducing the effects of hydrodynamic cavitation on the centrifugal pump. Meanwhile, the rotor hydrodynamic cavitation reactor also can be used as the power source of the whole device. Wang (2017) modified the centrifugal pump into the rotor hydrodynamic cavitation reactor. A rotor and stator were installed at the impeller inside the centrifugal pump. The stator and rotor formed a new cavitation area, which made it easier for cavitation bubbles to generate, grow and collapse. The consumption of free radicals in the centrifugal hydrodynamic cavitation reactor is 15 times higher than that in the traditional orifice hydrodynamic cavitation reactor, and the quality of hydrodynamic cavitation treatment is improved.

The cavitation efficiency of the rotor reactor is generally much higher than that of the conventional cavitation reactors. The design of different cavitation reactors is often based on different mechanisms. Several rotors hydrodynamic cavitation reactors based on different design ideas are introduced below, which can provide direction for the design and optimization of cavitation reactors. Petkovšek et al. (2013) proposed a new type of hydrodynamic cavitation reactor, which consists of two rotors. These two rotors are driven by electric motors, which can cause periodic pressure drop when rotors are rotating at a speed of about 2800 rpm and local speed up to 26 m/s. There are radial grooves on the two rotors, rotating in opposite directions, and the gap between the rotors is similar to venturi structure. The periodic change of pressure drop leads to the generation of cavitation in the gap of the rotors, and then the cavitation is induced into the groove and continues to develop. The mechanism of cavitation in the groove is the formation of shear cavitation, that is, two shear layers move in opposite directions to form cavitation. The reactor structure is shown in Figure 4.

Figure 4

Schematic diagram of reactor rotor and groove (Petkovšek et al. 2013).

Figure 4

Schematic diagram of reactor rotor and groove (Petkovšek et al. 2013).

Close modal

Badve et al. (2013) proposed a new cavitation reactor that consists of a rotor and a stator. The rotor is a solid cylinder with grooves on its surface. Because of the high-speed rotation of the rotor, the surface of the rotor produces extremely high surface velocity. The liquid enters the groove with high speed, and as it flows out of the groove, an area of low pressure is created near the surface of the groove, where shear cavitation occurs. At the same time, the surface velocity of the liquid is very fast, causing the pressure in the low-pressure region to be lower than the saturated vapor pressure of the liquid. Many processes lead to cavitation occurring in the groove and nearby areas. The reactor structure is shown in Figure 5.

Figure 5

Schematic diagram of reactor and groove (Badve et al. 2013).

Figure 5

Schematic diagram of reactor and groove (Badve et al. 2013).

Close modal

Sun et al. (2021) proposed a rotor-type hydrodynamic cavitation reactor, as shown in Figure 6, which is composed of a stator and a rotor. Cavitation is generated by the relative action of the stator. There are special grooves on the rotor and stator. As the rotor rotates, cavitation occurs between grooves, and the two grooves and gaps form local cavitation units. The periodic relative motion of rotor and stator leads to a periodic close and leave process between grooves, which results in periodic cavitation. In this process, cavitation is mainly caused by shear cavitation between fluid and groove and vortex cavitation in the groove during rotor rotation.

Figure 6

Schematic diagrams of reactor and groove (Sun et al. 2021).

Figure 6

Schematic diagrams of reactor and groove (Sun et al. 2021).

Close modal

The cavitation strength and efficiency of the rotor hydrodynamic cavitation reactor mentioned above are much higher than that of traditional reactors. After the study and summary of the cavitation principle, we designed a novel rotor-radial groove hydrodynamic cavitation reactor (Song et al. 2022). The reactor has a certain head of delivery, which can not only be used as a cavitation generation device, but also can provide power for pipeline circulation. The cavitation reactor consists of rotor, stator and casing. There are a number of blind holes on the stator and rotor. A gap between the stator blind hole and the rotor (1 mm) promotes the generation of shear cavitation. The fluid enters the housing axially and is thrown out radially after being rotated by the rotor. In the process of high-speed rotation of the rotor, cavitation occurs due to periodic pressure drop. Bubbles are induced to the position of the blind hole through the rotor then shear cavitation occurs near the position of the blind hole, and then the bubbles enter the blind hole. Meanwhile, vortex cavitation occurs. Then, the bubbles develop until collapse in the middle of the blind hole. Figure 7 shows the reactor geometric model and blind hole gas phase cloud image.

Figure 7

Geometrical model of reactor and blind hole gas phase cloud image (Song et al. 2022).

Figure 7

Geometrical model of reactor and blind hole gas phase cloud image (Song et al. 2022).

Close modal

Hydrodynamic cavitation has been widely shown to be an effective and clean technology for water treatment. However, there are still many challenges in the study of hydrodynamic cavitation for water treatment. Based on the shortcomings of existing research, the following expectations are proposed:

  • (1)

    The commonly used hydrodynamic cavitation reactors, such as orifice-plate hydrodynamic cavitation reactors and venturi tube cavitation reactors, are inefficient and difficult to be used widely in the industrial production. Therefore, it is vital to invent a new and functional hydrodynamic cavitation reactor. As for the application fields of hydrodynamic cavitation, it gradually tends to be micro. The volume of the cavitation reactor is gradually reduced. The micro-hydrodynamic cavitation reactor solves the shortcoming of the traditional hydrodynamic cavitation reactors and makes the hydrodynamic cavitation reactor more flexible and smaller. Therefore, the research and mechanism of micro-hydrodynamic cavitation reactors need to be strengthened.

  • (2)

    Organics are hard to be degraded completely by using hydrodynamic cavitation. Therefore, the method of combining hydrodynamic cavitation with other water treatment methods can be adopted to study the coupling degree of hydrodynamic cavitation with other water treatment methods. The research on the combination method focus on Fenton's reaction method, hydrogen peroxide method and other commonly used water treatment methods in recent years. Additionally, new water treatment methods, such as nano-fiber material method and material adsorption method, also have excellent treatment results. Therefore, the combination of hydrodynamic cavitation and new water treatment methods is worthy of study.

  • (3)

    Hydrodynamic cavitation has matured after hundreds of years of development, but the mechanism of hydrodynamic cavitation is still debated. Studies mostly focus on its results in application. Industrial engineers judge the difference of treatment results between hydrodynamic cavitation and other water treatment methods by some targets but pay little attention to its cavitation mechanism, especially the cavitation mechanism in the cavitation reactor. The undefined cavitation mechanism affected the application of hydrodynamic cavitation in all walks of life. Therefore, the mechanism of hydrodynamic cavitation is an urgent problem to be solved.

  • (4)

    As some microorganisms have tolerance to hydrodynamic cavitation, a long time is needed to ensure complete inactivation. So, it is difficult to meet the demand of industrial applications. The combined treatment method should be adopted to accelerate inactivation and improve the effect of hydrodynamic cavitation sterilization.

  • (5)

    In numerical simulations and experiments, the hydrodynamic cavitation effect is mainly based on the collapse of cavitation bubble. Therefore, it is inaccurate to evaluate cavitation effect by the total volume of the cavitation bubble. In response to this phenomenon, some quantitative indicators for evaluating the cavitation effect have been proposed such as heat production rate, cavitation number and thermal efficiency. However, it is difficult to evaluate the influence of cavitation bubble collapse. It is necessary to define proper parametric characterization that represents cavitation effect.

With the development of cavitation research, hydrodynamic cavitation technology has been applied in various fields. Especially in the field of water treatment, it has been proven to have a great potential of development. At present, the research on hydrodynamic cavitation water treatment mainly focuses on the cavitation principle, factors affecting hydrodynamic cavitation water treatment, conjunctive use of hydrodynamic cavitation and other water treatment methods, optimization of hydrodynamic cavitation reactor and so on. The present study reviews the development of hydrodynamic cavitation technology for water treatment. Hydrodynamic cavitation could inactivate bacteria, microalgae and organic substances based on thermal effect, mechanical effect and chemical effect. Hydrodynamic cavitation disinfection cannot completely inactivate organic substances such as dyes and medicines, so it can be coupled with chemical or physical methods to achieve a higher processing effect. In the past two decades, several new hydrodynamic cavitation reactors were proposed and researched, which improved hydrodynamic cavitation efficiency and application effect. In summary, hydrodynamic cavitation is an efficient technology for water treatment and more efforts are required to effectively promote the industrialization of this technology.

This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2021QE157, ZR2020QE193); Key Laboratory of Fluid and Power Machinery at Xihua University, Ministry of Education (No. LTDL2021-014); National Natural Science Foundation of China (No. 52006126).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Abbas-Shiroodi
Z.
,
Sadeghi
M. T.
&
Baradaran
S.
2021
Design and optimization of a cavitating device for Congo red decolorization: experimental investigation and CFD simulation
.
Ultrason. Sonochem.
71
,
105386
.
https://doi.org/10.1016/j.ultsonch.2020.105386
.
Adewuyi
Y. G.
2001
Sonochemistry: environmental science and engineering applications
.
Ind. Eng. Chem. Res.
40
,
4681
4715
.
https://doi.org/10.1021/IE010096 L
.
Ai
W. Z.
&
Ding
T. M.
2010
Orifice plate cavitation mechanism and its influencing factors
.
Water Sci. Eng.
3
,
321
330
.
https://doi.org/10.3882/J.ISSN.1674-2370.2010.03.008
.
Al-Mansori
N. J. H.
,
Alfatlawi
T. J. M.
,
Hashim
K. S.
&
Al-Zubaidi
L. S.
2020
The effects of different shaped baffle blocks on the energy dissipation
,
Civ. Eng. J.
6
,
961
973
.
https://doi.org/10.28991/cej-2020-03091521
.
Ashokkumar
M.
2011
The characterization of acoustic cavitation bubbles – an overview
.
Ultrason. Sonochem.
18
,
864
872
.
https://doi.org/10.1016/j.ultsonch.2010.11.016
.
Askarniya
Z.
,
Sadeghi
M. T.
&
Baradaran
S.
2020
Decolorization of Congo red via hydrodynamic cavitation in combination with Fenton's reagent
.
Chem. Eng. Process. - Process Intensif.
150
,
107874
.
https://doi.org/10.1016/j.cep.2020.107874
.
Badve
M.
,
Gogate
P.
,
Pandit
A.
&
Csoka
L.
2013
Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry
.
Sep. Purif. Technol.
106
,
15
21
.
https://doi.org/10.1016/j.seppur.2012.12.029
.
Baradaran
S.
&
Sadeghi
M. T.
2019
Coomassie Brilliant Blue (CBB) degradation using hydrodynamic cavitation, hydrogen peroxide and activated persulfate (HC-H2O2-KPS) combined process
.
Chem. Eng. Process. - Process Intensif.
145
,
107674
.
https://doi.org/10.1016/j.cep.2019.107674
.
Barik
A. J.
&
Gogate
P. R.
2018
Hybrid treatment strategies for 2,4,6-trichlorophenol degradation based on combination of hydrodynamic cavitation and AOPs
.
Ultrason. Sonochem
40
,
383
394
.
https://doi.org/10.1016/j.ultsonch.2017.07.029
.
Barisci
S.
&
Suri
R.
2021
Occurrence and removal of poly/perfluoroalkyl substances (PFAS) in municipal and industrial wastewater treatment plants
.
Water Sci. Technol.
84
,
3442
3468
.
https://doi.org/10.2166/WST.2021.484
.
Bui
X.-T.
,
Chiemchaisri
C.
,
Fujioka
T.
&
Varjani
S.
2019
Water and Wastewater Treatment Technologies
.
https://doi.org/10.1007/978-981-13-3259-3_18
.
Burzio
E.
,
Bersani
F.
,
Caridi
G. C. A.
,
Vesipa
R.
,
Ridolfi
L.
&
Manes
C.
2020
Water disinfection by orifice-induced hydrodynamic cavitation
.
Ultrason. Sonochem.
60
,
104740
.
https://doi.org/10.1016/J.ULTSONCH.2019.104740
.
Chong
M. N.
,
Jin
B.
,
Chow
C. W. K.
&
Saint
C.
2010
Recent developments in photocatalytic water treatment technology: a review
.
Water Res.
44
,
2997
3027
.
https://doi.org/10.1016/J.WATRES.2010.02.039
.
Cvetković
M.
,
Kompare
B.
&
Klemenčič
A. K.
2015
Application of hydrodynamic cavitation in ballast water treatment
.
Environ. Sci. Pollut. Res.
22
,
7422
7438
.
https://doi.org/10.1007/S11356-015-4360-7
.
Dalfré Filho
J. G.
,
Assis
M. P.
&
Genovez
A. I. B.
2015
Bacterial inactivation in artificially and naturally contaminated water using a cavitating jet apparatus
.
J. Hydro-Environment Res.
9
,
259
267
.
https://doi.org/10.1016/j.jher.2015.03.001
.
Dhanke
P. B.
&
Wagh
S. M.
2020
Intensification of the degradation of acid RED-18 using hydrodynamic cavitation
.
Emerg. Contam.
6
,
20
32
.
https://doi.org/10.1016/j.emcon.2019.12.001
.
Dhanke
P.
,
Wagh
S.
&
Patil
A.
2019
Treatment of fish processing industry wastewater using hydrodynamic cavitational reactor with biodegradability improvement
.
Water Sci. Technol.
80
,
2310
2319
.
https://doi.org/10.2166/WST.2020.049
.
Dijkink
R.
&
Ohl
C. D.
2008
Measurement of cavitation induced wall shear stress
.
Appl. Phys. Lett.
93
,
1
4
.
https://doi.org/10.1063/1.3046735
.
Doltade
S. B.
,
Dastane
G. G.
,
Jadhav
N. L.
,
Pandit
A. B.
,
Pinjari
D. V.
,
Somkuwar
N.
&
Paswan
R.
2019
Hydrodynamic cavitation as an imperative technology for the treatment of petroleum refinery effluent
.
J. Water Process Eng.
29
,
100768
.
https://doi.org/10.1016/j.jwpe.2019.02.008
.
Gągol
M.
,
Przyjazny
A.
&
Boczkaj
G.
2018
Effective method of treatment of industrial effluents under basic pH conditions using acoustic cavitation – A comprehensive comparison with hydrodynamic cavitation processes
.
Chem. Eng. Process. - Process Intensif.
128
,
103
113
.
https://doi.org/10.1016/J.CEP.2018.04.010
.
Geng
K. Z. K.
&
Dong
Z.
2017
Experimental study of Escherichia coli killed by hydrodynamic cavitation due to venturi tube
.
China Environ. Sci.
37
(
9
),
3385
3391
.
Ghosh
S. K.
,
Saha
P. D.
&
Di
M. F.
2020
Recent Trends in Waste Water Treatment and Water Resource Management
.
https://doi.org/10.1007/978-981-15-0706-9
.
Gopal
K.
,
Tripathy
S. S.
,
Bersillon
J. L.
&
Dubey
S. P.
2007
Chlorination byproducts, their toxicodynamics and removal from drinking water
.
J. Hazard. Mater.
140
,
1
6
.
https://doi.org/10.1016/J.JHAZMAT.2006.10.063
.
Gore
M. M.
,
Saharan
V. K.
,
Pinjari
D. V.
,
Chavan
P. V.
&
Pandit
A. B.
2014
Degradation of reactive orange 4 dye using hydrodynamic cavitation based hybrid techniques
.
Ultrason. Sonochem.
21
,
1075
1082
.
https://doi.org/10.1016/j.ultsonch.2013.11.015
.
Gupta
P.
,
Rani
R.
,
Chandra
A.
,
Varjani
S.
,
Kumar
V.
,
Gupta
P.
,
Rani
Á. R.
,
Chandra
Á. A.
,
Kumar
Á. V.
&
Varjani
S.
2019
The role of microbes in chromium bioremediation of tannery effluent
.
Energy, Environ. Sustain.
369
377
.
https://doi.org/10.1007/978-981-13-3259-3_17
.
Hart
E. J.
,
Fischer
C. H.
&
Henglein
A.
1990
Sonolysis of hydrocarbons in aqueous solution
.
Int. J. Radiat. Appl. Instrumentation. Part.
36
,
511
516
.
https://doi.org/10.1016/1359-0197(90)90198-Q
.
He
Y. C. Z.
&
Zhang
X.
2017
Visualization and numerical simulation of hydrodynamic cavitation in single hole orifice plate
.
J. Jiangsu Univ.
38
(
4
),
417
422
.
Ikai
H.
,
Nakamura
K.
,
Shirato
M.
,
Kanno
T.
,
Iwasawa
A.
,
Sasaki
K.
,
Niwano
Y.
&
Kohno
M.
2010
Photolysis of hydrogen peroxide, an effective disinfection system via hydroxyl radical formation
.
Antimicrob. Agents Chemother.
54
,
5086
5091
.
https://doi.org/10.1128/AAC.00751-10
.
Improvement of penetration rate with hydraulic pulsating-cavitation jet compound drilling technology | Request PDF
n.d.
.
Improve Reactions with Hydrodynamic Cavitation | Request PDF
n.d.
.
Innocenzi
V.
,
Prisciandaro
M.
&
Veglio
F.
2019
Wastewater cavitation treatment processes utilizing hydrodynamic
.
16th International Conference on Environmental Science and Technology
. Rhodes, 4–7 September.
Jain
P.
,
Bhandari
V. M.
,
Balapure
K.
,
Jena
J.
,
Ranade
V. V.
&
Killedar
D. J.
2019
Hydrodynamic cavitation using vortex diode: an efficient approach for elimination of pathogenic bacteria from water
.
J. Environ. Manage.
242
,
210
219
.
https://doi.org/10.1016/j.jenvman.2019.04.057
.
Kwak
H. Y.
&
Yang
H.
1995
An aspect of sonoluminescence from hydrodynamic theory
.
J. Phys. Soc. Japan.
64
,
1980
1992
.
https://doi.org/10.1143/JPSJ.64.1980
.
Lee
H.
,
Gojani
A. B.
,
Han
T. H.
&
Yoh
J. J.
2011
Dynamics of laser-induced bubble collapse visualized by time-resolved optical shadowgraph
.
J. Vis.
14
,
331
337
.
https://doi.org/10.1007/s12650-011-0094-x
.
Li
S. C.
2000
Cavitation of Hydraulic Machinery
, Vol.
1
.
https://doi.org/10.1142/P219
.
Li
M.
,
Bussonnière
A.
,
Bronson
M.
,
Xu
Z.
&
Liu
Q.
2019
Study of Venturi tube geometry on the hydrodynamic cavitation for the generation of microbubbles
.
Miner. Eng.
132
,
268
274
.
https://doi.org/10.1016/J.MINENG.2018.11.001
.
Liang
B.
,
He
X.
,
Hou
J.
,
Li
L.
&
Tang
Z.
2019
Membrane separation in organic liquid: technologies, achievements, and opportunities
.
Adv. Mater.
31
,
1806090
.
https://doi.org/10.1002/ADMA.201806090
.
Liu
B.
,
Cai
J.
,
Li
F.
&
Huai
X.
2013
Simulation of heat transfer with the growth and collapse of a cavitation bubble near the heated wall
.
J. Therm. Sci.
22
,
352
358
.
https://doi.org/10.1007/s11630-013-0635-9
.
Mukherjee
A.
,
Mullick
A.
,
Teja
R.
,
Vadthya
P.
,
Roy
A.
&
Moulik
S.
2020
Performance and energetic analysis of hydrodynamic cavitation and potential integration with existing advanced oxidation processes: a case study for real life greywater treatment
.
Ultrason. Sonochem.
66
,
105116
.
https://doi.org/10.1016/j.ultsonch.2020.105116
.
Musmarra
D.
,
Prisciandaro
M.
,
Capocelli
M.
,
Karatza
D.
,
Iovino
P.
,
Canzano
S.
&
Lancia
A.
2016
Degradation of ibuprofen by hydrodynamic cavitation: reaction pathways and effect of operational parameters
.
Ultrason. Sonochem.
29
,
76
83
.
https://doi.org/10.1016/j.ultsonch.2015.09.002
.
Ozonek
J.
2012
Application of hydrodynamic cavitation in environmental engineering
.
Appl. Hydrodyn. Cavitation Environ. Eng.
1
123
.
https://doi.org/10.1201/B11825
.
Petkovšek
M.
,
Zupanc
M.
,
Dular
M.
,
Kosjek
T.
,
Heath
E.
,
Kompare
B.
&
Širok
B.
2013
Rotation generator of hydrodynamic cavitation for water treatment
.
Sep. Purif. Technol.
118
,
415
423
.
https://doi.org/10.1016/J.SEPPUR.2013.07.029
.
Rae
J.
,
Ashokkumar
M.
,
Eulaerts
O.
,
Von Sonntag
C.
,
Reisse
J.
&
Grieser
F.
2005
Estimation of ultrasound induced cavitation bubble temperatures in aqueous solutions
.
Ultrason. Sonochem.
12
,
325
329
.
https://doi.org/10.1016/j.ultsonch.2004.06.007
.
Raut-Jadhav
S.
,
Saini
D.
,
Sonawane
S.
&
Pandit
A.
2015
Effect of process intensifying parameters on the hydrodynamic cavitation based degradation of commercial pesticide (methomyl) in the aqueous solution
.
Ultrason. Sonochem.
28
,
283
293
.
https://doi.org/10.1016/j.ultsonch.2015.08.004
.
Saharan
V. K.
,
Badve
M. P.
&
Pandit
A. B.
2011
Degradation of reactive red 120 dye using hydrodynamic cavitation
.
Chem. Eng. J.
178
,
100
107
.
https://doi.org/10.1016/j.cej.2011.10.018
.
Saharan
V. K.
,
Pandit
A. B.
,
Satish Kumar
P. S.
&
Anandan
S.
2012
Hydrodynamic cavitation as an advanced oxidation technique for the degradation of acid Red 88 dye
.
Ind. Eng. Chem. Res.
51
,
1981
1989
.
https://doi.org/10.1021/IE200249 K
.
Šarc
A.
,
Kosel
J.
,
Stopar
D.
,
Oder
M.
&
Dular
M.
2018
Removal of bacteria Legionella pneumophila, Escherichia coli, and Bacillus subtilis by (super)cavitation
.
Ultrason. Sonochem.
42
,
228
236
.
https://doi.org/10.1016/j.ultsonch.2017.11.004
.
Senthil Kumar
P.
,
Siva Kumar
M.
&
Pandit
A. B.
2000
Experimental quantification of chemical effects of hydrodynamic cavitation
.
Chem. Eng. Sci.
55
,
1633
1639
.
https://doi.org/10.1016/S0009-2509(99)00435-2
.
Sharma
S. K.
&
Sanghi
R.
2012
Advances in water treatment and pollution prevention
.
Adv. Water Treat. Pollut. Prev.
9789400742048
,
1
457
.
https://doi.org/10.1007/978-94-007-4204-8
.
Song
Y.
,
Hou
R.
,
Liu
Z.
,
Liu
J.
&
Zhang
W.
2022
Ultrasonics Sonochemistry Cavitation characteristics analysis of a novel rotor-radial groove hydrodynamic cavitation reactor
.
Ultrason. Sonochem.
86
,
106028
.
https://doi.org/10.1016/j.ultsonch.2022.106028
.
Sukhani
S.
&
Chanakya
H.
2020
Use of sewage to restore man-Made waterbodies – nutrient and energy flow regulation approaches to enabling sustainability
.
Recent Trends Waste Water Treat. Water Resour. Manag.
143
149
.
https://doi.org/10.1007/978-981-15-0706-9_14
.
Sun
X.
,
Liu
J.
,
Ji
L.
,
Wang
G.
,
Zhao
S.
,
Yoon
J. Y.
&
Chen
S.
2020a
A review on hydrodynamic cavitation disinfection: the current state of knowledge
.
Sci. Total Environ.
737
.
https://doi.org/10.1016/j.scitotenv.2020.139606
.
Sun
X.
,
Jia
X.
,
Liu
J.
,
Wang
G.
,
Zhao
S.
,
Ji
L.
,
Yong Yoon
J.
&
Chen
S.
2020b
Investigation on the characteristics of an advanced rotational hydrodynamic cavitation reactor for water treatment
.
Sep. Purif. Technol.
251
.
https://doi.org/10.1016/j.seppur.2020.117252
.
Sun
X.
,
Xuan
X.
,
Song
Y.
,
Jia
X.
,
Ji
L.
,
Zhao
S.
,
Yong Yoon
J.
,
Chen
S.
,
Liu
J.
&
Wang
G.
2021
Experimental and numerical studies on the cavitation in an advanced rotational hydrodynamic cavitation reactor for water treatment
.
Ultrason. Sonochem.
70
.
https://doi.org/10.1016/j.ultsonch.2020.105311
.
Supponen
O.
,
Kobel
P.
,
Obreschkow
D.
&
Farhat
M.
2015
The inner world of a collapsing bubble
.
Phys. Fluids.
27
.
https://doi.org/10.1063/1.4931098
.
Suslick
K. S.
,
Hammerton
D. A.
&
Cline
R. E.
1986
The sonochemical hot spot
.
J. Am. Chem. Soc.
108
,
5641
5642
.
https://doi.org/10.1021/JA00278A055/ASSET/JA00278A055.FP.PNG_V03
.
Tao
Y.
,
Cai
J.
,
Huai
X.
,
Liu
B.
&
Guo
Z.
2016
Application of hydrodynamic cavitation to wastewater treatment
.
Chem. Eng. Technol.
39
,
1363
1376
.
https://doi.org/10.1002/ceat.201500362
.
Terán Hilares
R.
,
dos Santos
J. G.
,
Shiguematsu
N. B.
,
Ahmed
M. A.
,
da Silva
S. S.
&
Santos
J. C.
2019
Low-pressure homogenization of tomato juice using hydrodynamic cavitation technology: effects on physical properties and stability of bioactive compounds
.
Ultrason. Sonochem.
54
,
192
197
.
https://doi.org/10.1016/J.ULTSONCH.2019.01.039
.
Thanekar
P.
&
Gogate
P. R.
2019
Combined hydrodynamic cavitation based processes as an efficient treatment option for real industrial effluent
.
Ultrason. Sonochem.
53
,
202
213
.
https://doi.org/10.1016/j.ultsonch.2019.01.007
.
Thanekar
P.
,
Murugesan
P.
&
Gogate
P. R.
2018a
Improvement in biological oxidation process for the removal of dichlorvos from aqueous solutions using pretreatment based on Hydrodynamic Cavitation
.
J. Water Process Eng.
23
,
20
26
.
https://doi.org/10.1016/j.jwpe.2018.03.004
.
Thanekar
P.
,
Panda
M.
&
Gogate
P. R.
2018b
Degradation of carbamazepine using hydrodynamic cavitation combined with advanced oxidation processes
.
Ultrason. Sonochem.
40
,
567
576
.
https://doi.org/10.1016/j.ultsonch.2017.08.001
.
Tullis
J. P.
2007
Fundamentals of cavitation
.
Hydraul. Pipelines
119
132
.
https://doi.org/10.1002/9780470172803.CH5
.
Verma
P.
&
Samanta
S. K.
2018
Microwave-enhanced advanced oxidation processes for the degradation of dyes in water
.
Environ. Chem. Lett.
163
(
16
),
969
1007
.
https://doi.org/10.1007/S10311-018-0739-2
.
Vidanage
V. V. D. N. G.
,
Karunarathna
A. K.
,
Alahakoon
A. M. Y. W.
&
Jayawardene
S. M. N.
2020
Development of an effective and efficient integrated charcoal filter constructed wetland system for wastewater treatment
.
Recent Trends Waste Water Treat. Water Resour. Manag.
47
56
.
https://doi.org/10.1007/978-981-15-0706-9_5
.
Waghmare
A.
,
Nagula
K.
,
Pandit
A.
&
Arya
S.
2019
Hydrodynamic cavitation for energy efficient and scalable process of microalgae cell disruption
.
Algal Res.
40
,
101496
.
https://doi.org/10.1016/j.algal.2019.101496
.
Wang
H. L. J.
2017
Experiment on cavitation erosion mechanism of centrifugal hydrodynamic cavitation generator
.
Trans. Chinese Soc. Agric. Eng.
33
(
14
),
49
55
.
Wang
X.
,
Jia
J.
&
Wang
Y.
2017
Combination of photocatalysis with hydrodynamic cavitation for degradation of tetracycline
.
Chem. Eng. J.
315
,
274
282
.
https://doi.org/10.1016/j.cej.2017.01.011
.
Wang
B.
,
Su
H.
&
Zhang
B.
2021
Hydrodynamic cavitation as a promising route for wastewater treatment – A review
.
Chem. Eng. J.
412
,
128685
.
https://doi.org/10.1016/J.CEJ.2021.128685
.
Wei
P. Y. D.
,
Shi
Z. M.
,
Bin
Y. H.
&
Ming
Y. L.
2019
Experimental study on the sterilization based on hydrodynamic cavitation technology
.
Energy Conserv. Technol.
37
,
285
288
. .
Xie
L.
,
Terada
A.
&
Hosomi
M.
2017
Disentangling the multiple effects of a novel high pressure jet device upon bacterial cell disruption
.
Chem. Eng. J.
323
,
105
113
.
https://doi.org/10.1016/j.cej.2017.04.067
.
Yamini
O. A.
,
Kavianpour
M. R.
&
Movahedi
A.
2020
Performance of hydrodynamics flow on flip buckets spillway for flood control in large dam reservoirs
.
J. Human, Earth, Futur.
1
,
39
47
.
https://doi.org/10.28991/hef-2020-01-01-05
.
Yamini
O. A.
,
Mousavi
S. H.
,
Kavianpour
M. R.
&
Ghaleh
R. S.
2021
Hydrodynamic performance and cavitation analysis in bottom outlets of Dam using CFD modelling
.
Adv. Civ. Eng.
2021
.
https://doi.org/10.1155/2021/5529792
.
Yuan
R.
,
Hu
L.
,
Yu
P.
,
Wang
Z.
,
Wang
H.
&
Fang
J.
2018
Co3o4 nanocrystals/3D nitrogen-doped graphene aerogel: a synergistic hybrid for peroxymonosulfate activation toward the degradation of organic pollutants
.
Chemosphere.
210
,
877
888
.
https://doi.org/10.1016/J.CHEMOSPHERE.2018.07.065
.
Zhang
J. X.
2017
Analysis on the effect of venturi tube structural parameters on fluid flow
.
AIP Adv.
7
,
065315
.
https://doi.org/10.1063/1.4991441
.
Zhang
Z.
,
Wang
G.
,
Nie
Y.
&
Ji
J.
2016
Hydrodynamic cavitation as an efficient method for the formation of sub-100 nm O/W emulsions with high stability
.
Chinese J. Chem. Eng.
24
,
1477
1480
.
https://doi.org/10.1016/J.CJCHE.2016.04.011
.
Zhou
N.
,
Liang
R.
&
Hu
A.
2014
Nanotechnology for Water Treatment and Purification
, Vol.
22
.
https://doi.org/10.1007/978-3-319-06578-6
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).