Fluid cleaning device

Abstract

A fluid cleaning device 1, includes a fluid container 6 and a nano-bubble generator 8. The nano-bubble generator comprises a bubble generating device (80, Fig. 3) and a venturi (301, Fig. 3). The nano

Description

BACKGROUND OF THE INVENTION This invention relates to a fluid cleaning device and method, for example a water cleaning device. It is known to clean fine and dissolved substances like suspended solids, inorganic compounds and contaminants, microorganisms, dissolved gases and organic compounds, such as proteins, from water using a device referred to as a protein skimmer or foam fractionator. In such a device bubbles of macro or micro size are generated, and move up through a water column. As they move upwards they collect fine and dissolved substances on their surface, at the gas / fluid interface. The bubbles rise to the top of the skimmer and form a foam which carries the collected fine and dissolved substances. This foam is separated, or skimmed, off the top of the water, leaving behind cleaned water. The present invention seeks to provide an improved method and device for fluid cleaning. SUMMARY OF THE INVENTION From a first aspect, the invention provides a fluid cleaning device, comprising: a fluid container; and a nano-bubble generator, wherein the nano-bubble generator comprises a bubble generating device and a venturi; the nano-bubble generator arranged to generate nano-bubbles and to introduce the nano-bubbles into fluid contained within the fluid container, so as to clean the fluid. From a second aspect, the invention provides a method of cleaning fluid, comprising: generating nano-bubbles using a bubble generating device and a venturi; and cleaning fluid by introducing the nano-bubbles into fluid contained within a fluid container. The Applicant has appreciated that there are distinct attributes of nano-bubbles which make them particularly advantageous for fluid purification purposes. These inherent characteristics achieve many benefits compared to known fluid cleaning methodologies, notably compared to the use of micro-bubbles for cleaning fluid. As a result of their extremely small size, nano-bubbles move through fluids with Brownian motion, which is random movement due to collisions with fluid molecules. This fast and erratic movement causes the nano-bubbles to contact more fine particles and dissolved organic matter (and other contaminants) than in the case of larger micro-bubbles, which are buoyant and rise quickly to the surface of a fluid. This interaction leads to the accumulation and attachment of finer particles and dissolved elements to the nano-bubbles in a remarkably efficient manner. Nano-bubbles have neutral buoyancy, which also contributes to them staying longer in fluid that they are filtering (rather than rising quickly to the surface). The Applicant has appreciated that nonetheless they can be used effectively for fluid cleaning because as substances are absorbed or adhered to their surface, they lose their neutral buoyancy and transition into a colloidal state akin to conventional bubbles. As a result, they begin behaving like “normal” bubbles, in terms of flow dynamics, such that they rise to the surface and form a foam layer that can be collected to extract the molecules that they have collected from the fluid, thereby cleaning it. Nano-bubbles are also very stable over time. Their smaller size means they can tolerate greater pressure. Other factors such as surface charge, high zeta potential (electro-kinetic potential), hydrophobicity and the presence of dissolved gases help prevent coalescence and dissolution of the nano-bubbles. The fact that nano-bubbles are long-lasting improves the cleaning effect they provide, since they remain in the fluid, collecting contaminants, for a longer period of time. As a result of their high zeta potential nano-bubbles inherently have a large propensity for electrostatic interactions, making them potent adsorbents. Their charged exteriors provide an environment very well suited to attracting and attaching to fine and dissolved substances. Overall, the sustained presence of nano-bubbles as a result of these properties helps to ensure a more thorough and comprehensive cleansing process. Nano-bubbles also have a high surface area-to-volume ratio, which gives them high diffusion coefficients, meaning they can diffuse quickly and therefore disperse well (i.e. very uniformly) throughout a fluid, and which also enables them to dissolve gases more readily than micro-bubbles. Their higher surface area-to-volume ratio than microbubbles (up to 400 times higher for the same volume of gas) also provides a greater surface area for collecting contaminants, thus improving cleaning. The effect of surface tension can cause nanobubbles to merge and combine into larger bubbles even before reaching a liquid's surface. Through careful manipulation, the risk of coalescence can be controlled in order to extend nanobubbles' longevity and optimize their functionality. Nano-bubbles also disrupt the fluid flow within pipes due to their ability to disrupt surface tension of the fluid. As nanobubbles intercalate themselves amidst the molecules of the fluid, they subtly disturb the cohesive forces that contribute to uneven flow profiles. This effect leads to the attenuation of flow irregularities, resulting in a more uniform speed profile within the pipe. By effectively altering the surface tension dynamics, nanobubbles foster a harmonious distribution of fluid particles, minimizing velocity variations and reducing the likelihood of turbulent eddies. This effect is particularly advantageous in the context of fluid cleaning. Usually, the profile of fluid speed is at a maximum in the centre of the fluid container, since in the centre there is no interaction between molecules of fluid and the walls of the container. This usually leads to non-uniform mixing, with better mixing in the middle, and less mixed fluid close to the walls. However, using nanobubbles disturbs the bonds between molecules and thereby creates a gap between the molecules of the fluid and the walls of the fluid container, creating a more evenly distributed well-mixed solution. In addition, using nanobubbles helps to reduce friction drag force in the interaction surface between fluid molecules and the walls. As a result of these advantages, nano-bubbles are particularly effective at reducing dissolved organic matter, small particulates and turbidity of fluid, and also at increasing the oxygen content of the fluid (where an oxygen-containing gas is contained within the nano-bubbles). Cleaning fluid with nano-bubbles provides the opportunity for achieving these advantageous cleaning effects without requiring the use of mechanical filtration, which can consume a large amount of energy and which is ineffective at removing these fine substances. The Applicant has further appreciated that by providing a venturi as part of the nanobubble generator, the bubble generator is able to produce bubbles of nano-size more reliably and consistently than with known bubble generation methods (i.e. providing an improved distribution of bubble-sizes compared to known bubble generators). This also provides a very efficient nano-bubble generator. This in turn significantly improves the removal rate of small particles and other contaminants compared to existing methods. It will be appreciated that the bubble generating device may generate bubbles of any suitable size, providing that the venturi is able to break these bubbles up so that ultimately the bubbles are output from the venturi as nano-bubbles. Thus, the bubble generating device may be arranged to generate nano-bubbles, micro-bubbles, a combination of both, or bubbles larger than micro-bubbles. The bubbles are supplied to the venturi which breaks these bubbles down to nano-bubbles, thus the overall arrangement provides a nano-bubble generator. The fluid cleaning device may be suitable to clean a wide array of fluids. For example, the fluid cleaning device may be a device for cleaning: water (including freshwater, groundwater, green water like rainfall, and surface water), beverages (beer and wine), liquid solutions (that contain additives or chemicals for targeted delivery, dispersion, or extraction purposes), cosmetics and personal care products, petrochemicals (oil, oil derivation), pharmaceuticals, industrial water (wastewater treatment, metal surface cleaning, and industrial cooling systems), and agricultural runoff and pollution. Thus, the fluid may be water. The nano-bubble generator is arranged to generate nano-bubbles. By this it will be understood that the nano-bubble generator is arranged to generate a plurality of bubbles, wherein the plurality of bubbles comprises nano-bubbles. Thus, it is arranged to generate at least nano-bubbles. In some embodiments, the plurality of bubbles may consist of nano-bubbles (i.e. the nano-bubble generator may generate exclusively nano-bubbles). However, in other embodiments, the plurality of bubbles need not comprise exclusively nano-bubbles, since this may not be needed to achieve the advantages set out above. This furthermore may be difficult to achieve in practice, since some nano-bubbles may join together and thereby create micro-bubbles during the bubble generation process. It will be understood that the nano-bubble generator is arranged to primarily (or predominantly) generate nano-bubbles. Thus, a majority (i.e. over 50%) of the plurality of bubbles generated by the nano-bubble generator may be nano-bubbles. More than 60% of the plurality of bubbles generated by the nano-bubble generator may be nanobubbles, optionally more than 75%, further optionally more than 90%, further optionally more than 99%. The bubbles produced by the nano-bubble generator may not be entirely uniform in size. Thus, the nano-bubble generator may be arranged to generate a plurality of bubbles having a distribution of diameter sizes. More than half of the distribution may be within the nano-bubble size range. In some embodiments, the mean of the distribution may be within the nano-bubble size range. Nano-bubbles, sometimes referred to as “ultrafine” bubbles, will be understood as bubbles having a diameter of less than 1000 nm. Thus, the nano-bubbles generated by the nano-bubble generator (i.e. the proportion of the plurality of bubbles referred to above) may have a diameter of less than 1000 nm, optionally of less than 500 nm, further optionally less than 250 nm, further optionally less than 200 nm, further optionally less than 150 nm, further optionally less than 100 nm. The nano-bubbles may have a diameter of approximately 200 nm. The distribution of diameter sizes of the plurality of bubbles may have a mean value of less than 1000 nm, optionally of less than 500 nm, further optionally less than 250 nm, further optionally less than 200 nm, further optionally less than 150 nm, further optionally less than 100 nm. The distribution of diameter sizes of the plurality of bubbles may have a mean value of between 50 nm and 200 nm, optionally of approximately 200 nm. The nano-bubbles may have a (mean) diameter of between 50 to 500 times smaller than micro-bubbles (i.e. smaller than the micro-bubble range). Micro-bubbles will be understood as bubbles having a diameter typically between 1 pm and 10 pm (i.e. between 1000 nm and 10,000 nm). It will be appreciated that the precise diameter of the bubbles generated by the nanobubble generator is not critical, provided that they are sufficiently small to have the advantageous properties described above. Thus, the nano-bubbles may be sized such that they move through fluid (e.g. water) with Brownian motion. The nano-bubbles may be sized such that they have neutral buoyancy in fluid (e.g. water). The nano-bubble generator includes a venturi (i.e. sometimes referred to as a venturi tube, a venturi pipe or a venturi valve). A venturi will be understood as a pipe or tube shaped such as to induce the venturi effect in fluid flowing through the pipe (i.e. shaped to contain a constriction). The venturi effect is a reduction in fluid pressure that arises as fluid flows through a constricted section of a pipe, causing an increase in fluid flow speed. The venturi may comprise a gas inlet. The gas inlet may be located within the constricted section of the venturi. The gas inlet may be arranged to introduce the gas into the bubbles within the venturi. The gas inlet may be arranged to supply, for example, air, (pure) oxygen, or ozone. The bubble generating device may be part of the venturi (e.g. it may be provided by the gas inlet) or it may be separate from the venturi, and located upstream of the venturi so as to produce bubbles that are input to the venturi (i.e. to be broken down to nano-size). Whilst the venturi is described as part of the nano-bubble generator it will be understood that all that this requires is that the venturi and the bubble generating device cooperate together to ultimately produce an output of nano-bubbles, thus forming an overall system referred to as a "nano-bubble generator’. This does not require that the venturi be integrated with, or located at or adjacent to. the bubble generating device, although in some embodiments it may be. In some embodiments, the fluid cleaning device comprises a diffuser. The diffuser may be positioned downstream of the nano-bubble generator (i.e. positioned downstream of the venturi). It will be understood that the diffuser spreads out (i.e. diffuses) the nano-bubbles. This helps to provide an improved distribution of nano-bubbles within the fluid to be cleaned, i.e. to spread the nano-bubbles more uniformly throughout the fluid. In some embodiments, the diffuser comprises one or more diffuser holes. The diffuser holes thus may be sized and / or distributed so as to spread out the nano-bubbles throughout the fluid. The one or more diffuser holes may form a membrane. The one or more diffuser holes may all be of uniform (i.e. substantially identical) size, or (some or all) may be of different sizes (i.e. to each other). The diffuser holes may be arranged (e.g. sized) to break down bubbles that pass through the holes, further reducing the size of the bubbles produced by the nano-bubble generator, and thus providing overall a greater proportion of nano-bubbles in the fluid. In some embodiments, the fluid cleaning device (e.g. the diffuser) comprises a mixer (e.g. a turbine) to induce turbulence in the fluid. This may be located downstream of the nano-bubble generator. Thus advantageously, the mixer (e.g. the diffuser) may also shear and break down bubbles into smaller bubbles using turbulence induced by flowing liquid. The mixer (e.g. the mixer portion of the diffuser) may be located upstream of the diffuser holes. This induces turbulence in fluid which is then incident upon the diffuser holes, driving the fluid and the nano-bubbles through the diffuser holes and increasing distribution of the nano-bubbles and size breakdown. In some embodiments, the nano-bubble generator generates nano-bubbles filled with air, (pure) oxygen, or ozone. This advantageously helps to increase the oxygen of the fluid as it is cleaned. However, it will be appreciated that alternative gases may be used in accordance with the requirements of a particular application of the fluid cleaning device. In some embodiments, the fluid cleaning device further comprises a bubble spreader, arranged to distribute the nano-bubbles within the fluid container. The bubble spreader may be positioned downstream (further along the flow path) of the nano-bubble generator, i.e. so that nano-bubbles produced by the nano-bubble generator flow to the bubble spreader. The bubble spreader may be positioned downstream of the diffuser and / or the mixer. In some embodiments, the bubble spreader is positioned within the fluid container. The bubble spreader may be tapered (i.e. cone-shaped). The bubble spreader may be tapered outwards away from the nano-bubble generator (i.e. away from the direction in which the nano-bubble generator is located, albeit there may be other components such as the diffuser / mixer between the nano-bubble generator and the bubble spreader). This may help distribution of the nano-bubbles within the fluid container. The bubble spreader may be positioned at a lower part of the fluid container. The fluid container may be elongate, and may be arranged such that the elongate direction is aligned upright (i.e. vertical). In some embodiments, the fluid container is cylindrical (optionally an elongate cylinder). The lower part of the fluid container may be the part that is lowest along the upright direction, and the upper part of the fluid container may be the part that is highest along the upright direction. In some embodiments, the fluid cleaning device comprises one or more reinforcing collars, arranged around the fluid container (e.g., around the circumference of the cylinder). This provides strength to the fluid container and prevents buckling. In some embodiments, the fluid container comprises a transparent portion (or may be transparent). This allows the cleaning process to be observed and supervised. In some embodiments, the fluid cleaning device further comprises a fluid inlet, arranged to supply fluid (i.e. to be cleaned) into the fluid container. The fluid inlet may also supply fluid to the nano-bubble generator. In some embodiments, the fluid cleaning device further comprises a separator, arranged to separate an inlet fluid supply into a first stream (supply, path), supplied to the fluid container, and a second stream, supplied to the nano-bubble generator. The first stream may comprise at least 50% of the inlet fluid supply, optionally at least 60%, further optionally at least 75%, further optionally at least 80%. Similarly, the second stream may comprise less than 30% of the inlet fluid supply, optionally less than 20%, further optionally less than 15%, further optionally approximately 13%. Using the same inlet fluid supply that is to be cleaned to also supply the nano-bubble generator is advantageous since it avoids altering the properties of the cleaned fluid by introducing fluid from a different source. Use of a separate fluid source for bubble generation would risk affecting, for example, the temperature, pH and / or salinity of the cleaned fluid compared to the input fluid supply (the fluid supply for cleaning). Alternatively, in other embodiments, the fluid cleaning device further comprises a first fluid inlet, arranged to supply fluid into the fluid container, and a second (separate) fluid inlet, arranged to supply fluid to the nano-bubble generator. In some embodiments, the fluid cleaning device further comprises an inlet valve, wherein the supply of fluid into the fluid container is controllable using the inlet valve. Thus, the inlet valve may allow the rate of flow of fluid into the fluid container to be controlled, and / or the inlet valve may allow the flow of fluid into the fluid container to be ceased (i.e. stopped, prevented). The inlet valve may be positioned downstream of the fluid inlet (e.g. downstream of the separator). It may be located within a fluid-container-inlet pipe. In some embodiments, the fluid cleaning device further comprises an outlet valve, wherein the flow of fluid out of the fluid container is controllable using the outlet valve. Thus, the outlet valve may allow the rate of flow of fluid out of the fluid container to be controlled, and / or the outlet valve may allow the flow of fluid out of the fluid container to be ceased (i.e. stopped, prevented). The fluid cleaning device may further comprise a fluid outlet (i.e. a fluid outlet pipe). The outlet valve may be located in a (corresponding) outlet pipe. Thus, in some embodiments, the method further comprises controlling the flow of fluid into and / or out of (i.e. through) the fluid container using the inlet valve and / or the outlet valve, and thereby controlling the cleaning time for which the fluid is cleaned. This allows the residence time of the nano-bubbles within the fluid, and therefore the interaction time of the nano-bubbles with the contaminants, to be controlled. This advantageously allows the cleaning time to be adjusted based on the fluid that is input for cleaning, i.e. how dirty the fluid is and any specific contaminants it contains, and also based on the desired level of cleanliness of the output, cleaned fluid, i.e. whether it must meet drinking standards. In some embodiments, the fluid container comprises a fluid entry point, through which fluid is delivered into the fluid container e.g. from the fluid inlet. The fluid entry point may be located at an upper part of the fluid container, i.e. such that delivered fluid for cleaning flows downwards through the fluid container from the upper part due to gravity. This advantageously improves mixing of the fluid to be cleaned with the nano bubbles without requiring the use of a pump, thus improving the energy efficiency of the fluid cleaning. Alternatively, the fluid entry point may be located at a lower part of the fluid container (i.e. in a part that is below the upper part). The fluid-container-inlet pipe may comprise the fluid entry point. In some embodiments, the outlet valve is located at the upper part of the fluid container. Alternatively, the outlet valve may be located at the lower part of the fluid container. It may be located at the bottom of the fluid container. In further embodiments, the fluid container comprises a first outlet valve, located at the upper part of the fluid container, and a second outlet valve, located at a lower part of the fluid container. This allows cleaned fluid to be extracted from either the top or the bottom of the fluid container, depending on its desired operation. For example, fluid may be input at the lower part of the fluid container, and pumped upwards for circulation, and then collected from the lower part of the fluid container. The system's design thus allows for the controlled direction of input fluid flow, either upward or downward, achieved through valve manipulation at both the top and bottom. This careful control ensures optimal impact and efficiency. In some embodiments, the fluid cleaning device further comprises a foam extraction portion. The foam extraction portion may be located at the upper part of the fluid container. This helps with the effective collection of foam since foam collects on the top surface of the fluid, at the upper part of the fluid container. In some embodiments, the foam extraction portion comprises a tapered cone, wherein the cone tapers upwards (i.e. its width or diameter decreases along the upwards direction). This pushes foam from the surface of the fluid upwards through the cone. The tapered cone may be attached to an upper edge of the fluid container. In some embodiments the foam extraction portion comprises a foam buffer tank. The foam buffer tank may be connected to an upper edge of the tapered cone. This may collect foam that has been pushed up through the tapered cone. The foam buffer tank may be connected to a foam drain, i.e. for draining foam out of the fluid container. Thus, in some embodiments, the method comprises extracting (or draining) foam out of the fluid container, e.g. off the top surface of the fluid within the fluid container. The method may further comprise drying the extracted foam, and retrieving phosphorous and / or nitrogen from the dried foam. The method may further comprise using the foam (or retrieved phosphorous and / or nitrogen) to provide fertilizer or animal feed or introducing the foam (or retrieved phosphorous and / or nitrogen) to an aquaponic system. This increases the utility of the fluid cleaning process. The fluid outlet may be located below (i.e. at a lower height than, not necessarily directly below) the foam extraction portion (e.g. than the tapered cone). This allows cleaned fluid to be extracted out of the fluid outlet below the foam level, reducing the chances of some of the foam, which contains the extracted contaminants, being drawn out with the cleaned fluid. The fluid container may comprise saltwater. Thus, in some embodiments, the fluid cleaning device is a salt-water cleaning device. Similarly, in some embodiments, the method may be a method of cleaning salt water. Effective fluid cleaning is particularly challenging in the context of marine aquaculture because marine species, unlike their freshwater counterparts, yield more fine particles and exhibit reduced urine production. Existing water cleaning systems are ineffective at filtering fine particles. Small particulates can cause bacterial growth on the walls of aquaculture tanks which can cause illness or death in fish being farmed. The present invention therefore may be particular advantageous applied to the context of marine aquaculture. In some embodiments, the fluid cleaning device is an aquaculture water cleaning device. The fluid container may be arranged to receive water from an aquaculture system for cleaning. The invention therefore extends to an aquaculture system, comprising a fluid cleaning device according to the invention. Similarly, in some embodiments, the method may be a method of cleaning fluid within an aquaculture system. Furthermore, the use of nano-bubbles provides beneficial effects in an aquaculture system as it promotes biological filtration. The nano-bubbles interact with, and carry around, beneficial bacteria. This interaction enhances the attachment of the beneficial bacteria to various surfaces within the biofilter of the aquaculture system. These bacteria play a crucial role in breaking down organic matter and maintaining water quality. By facilitating the attachment of these bacteria, the nanobubbles effectively optimize the biofilter's performance, resulting in an even more efficient removal of harmful substances and improvement in water quality. Nanobubbles offer a ground-breaking solution for the removal of microplastics from polluted water sources. The mechanism behind nanobubble-based microplastic removal is rooted in their high surface area-to-volume ratio and their strong adhesion to hydrophobic contaminants like microplastics. When introduced into dirty water, nanobubbles attach to the microplastics due to their hydrophobic nature. This attraction is further amplified by the Brownian motion of nanobubbles, enhancing collision rates with microplastics. As described above, nanobubbles adhere to microplastics, creating buoyant agglomerates that can be easily separated from the water through methods such as flotation or sedimentation. Their small size ensures minimal interference with the water matrix, preserving water quality. There are several important advantages of nanobubble-based microplastic removal. Firstly, it is highly efficient, capable of removing microplastics with sizes ranging from a few micrometers to nanometers. Secondly, it is environmentally friendly, as it does not introduce harmful chemicals or byproducts into the water. Additionally, nanobubbles can be generated using minimal energy input, making the process cost-effective and sustainable. The above-described system and method are therefore particularly advantageous in the context of removing microplastics from contaminated fluid. Thus, in some embodiments, the method may be a method of removing microplastics from contaminated fluid (e.g. water). Similarly, the invention extends to a microplastics removal system, comprising a fluid cleaning device according to the invention. Nanobubbles also have the potential to revolutionize wastewater treatment by significantly improving its efficiency and sustainability. Nanobubbles are highly effective at transferring oxygen into wastewater, which accelerates the aerobic degradation of organic pollutants and bolsters beneficial microbial growth. Additionally, nanobubbles' unique properties enable them to adsorb and agglomerate contaminants, simplifying their removal while reducing the reliance on chemical additives. This innovation leads to shorter treatment times, increased capacity, cost savings, and reduced environmental impact Furthermore, nanobubbles are versatile and can be integrated into various treatment processes, making them adaptable to diverse wastewater treatment needs. In essence, nanobubbles offer a transformative solution that optimizes wastewater treatment, enhances water quality, and contributes to the preservation of our precious water resources. The fluid cleaning device according to the present invention is versatile and can be seamlessly integrated into various wastewater treatment processes, including activated sludge systems, lagoons, and biological reactors. Its adaptability ensures it meets diverse treatment needs, making it an attractive option for wastewater treatment plants of varying scales. Thus, in some embodiments, the method may be a method of treating wastewater. Similarly, the invention extends to a wastewater treatment system (e.g. a lagoon or a biological reactor), comprising a fluid cleaning device according to the invention. Furthermore, the present invention may be advantageous applied to treatment for other water types, such as for agriculture, drinking water and freshwater. Thus the invention extends to a drinking water system comprising a fluid cleaning device according to the invention. Similarly, the invention also extends to a freshwater system, comprising a fluid cleaning device according to the invention. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap. It will furthermore be understood that features described in the context of the device may likewise be present in the corresponding method, and likewise the device may be arranged or configured to carry out any of the steps described as part of a method. BRIEF DESCRIPTION OF THE DRAWINGS Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing showing a fluid cleaning device according to an embodiment of the present invention; Figure 2 is an external, perspective view of the fluid cleaning device of Figure 1; Figure 3 is a cross-section of the nano-bubble generator of Figures 1 and 2; Figure 4 is an external view of the venturi of Figure 3; Figure 5 is a perspective view of the diffuser of the fluid cleaning device of Figures 1 and 2; and Figures 6-8 are graphs showing how various measured properties of a sample of water changed over time as the sample was cleaned using nano-bubbles, according to the present invention. DETAILED DESCRIPTION Figure 1 is a schematic drawing showing a fluid cleaning device 1 according to an embodiment of the present invention. In this particular example, water is the fluid to be cleaned. It will therefore be understood that where reference is made to “water” in this particular example, this may be equally understood as “fluid”, i.e. any suitable fluid. Water (represented by arrow 3) is received for cleaning through a water inlet pipe 2. The inlet water 3 is then separated at a separator 4, so that it travels either along a first pipe 20 towards a fluid container 6, without passing through additional other components (not considering valves) or through a second pipe 22 towards a nanobubble generator 8. The fluid container 6 is an elongate cylinder, containing water. Where the “top” and “bottom” of the fluid container 6 are referred to below, these directions refer to the orientation of the tank illustrated in Figure 1 - which is the orientation of the tank in use. Thus, the top of the tank is one end of the elongate shape (i.e. along the elongate direction), the upper end relative to the vertical direction, and the bottom is the opposite end, the lower end relative to the vertical direction. In this example, a majority of the water (e.g. over 75%, optionally approximately 87%) flows directly to the fluid container 6, through the first pipe 20. This flow of water into the fluid container 6 is shown by another arrow 5. The first pipe 20 extends into the top part of the fluid container 6, and includes (outside of the fluid container 6) an inlet valve 10, which can be used to vary the flow rate of water into the fluid container 6, and even to stop the flow altogether. The remaining portion of the inlet water that isn’t sent directly to the fluid container 6 through the first pipe 20 flows into the nano-bubble generator 8 through the second pipe 22. The nano-bubble generator 8 generates bubbles of nano size, and introduces these into the fluid container 6 along a third pipe 24 via a diffuser 12 and a bubble spreader 14. As can be seen, the bubble spreader 14 is located within the fluid container 6, towards the lower end. The bubble spreader 14 tapers outwards along the upwards direction, so as to spread the bubbles across the width direction of the fluid container 6 as they travel out of the third pipe 24. In an alternative example (not illustrated) the first pipe 20 and the second pipe 22 may be entirely separate, such that fluid is supplied separately (i.e. independently) to the fluid container 6 and to the nano-bubble generator 8. Clean fluid may be used to supply the nano-bubble generator, or the fluid to be used by the nano-bubble generator may be drawn from the same supply as the fluid that is supplied to the fluid container 6 for cleaning. Water flows into the tank out of the first pipe 20, at a water outlet 26 (i.e. a water entry point for water into the fluid container 6) which is located towards the top of the fluid container 6, as seen in Figure 1. It then flows downwards through the fluid container 6 due to gravity. This flow is seen as arrow 7. The nano-bubbles are contained within water, which flows out of the bubble spreader 14, therefore carrying the nano-bubbles with it into the fluid container 6, as indicated by arrow 9. As a result of their small size the nano-bubbles move through the water in the fluid container 6 in Brownian motion. Brownian motion refers to the random movement of particles suspended in a fluid due to collisions with the fluid molecules. The small size of nano-bubbles allows them to be influenced strongly by the thermal fluctuations of water molecules, resulting in fast and erratic motion. This fast and erratic motion causes the nano-bubbles to contact more contaminant molecules within the water. As the nano-bubbles contact contaminant molecules within the water, they collect molecules on their surface, which provides an air / water interface (or interface of another gas with water) to which the contaminant molecules (or parts of those molecules) are attracted. The relatively large surface area of the nano-bubbles, compared to their volume, enables them to collect a large amount of contaminant molecules. The nano-bubbles also absorb gases within them. Once other molecules are absorbed or attached to the surface of the nano-bubbles, they lose their neutral buoyancy, colloidize and begin behaving like “normal” bubbles, in terms of their flow behaviour, such that they rise to the surface, at the top of the fluid container 6. Ultimately, the bubbles form a foam at the top of the fluid container 6. The water cleaning device 1 includes a foam extraction portion 17, which includes a tapered cone 16, a foam buffer tank 18 and a fourth pipe 28 out of which foam is drained. The tapered cone 16 is attached to the top of the fluid container 6. The tapered shape of this cone 16 pushes foam that has risen to the top of the fluid container 6 up into the foam buffer tank 18, positioned at the top of the fluid container 6. Foam is drained out of the foam buffer tank 18 through the fourth pipe 28, which connects to an outlet pipe 30. The fourth pipe 28 contains a first drain valve 32, which is actuatable to enable or prevent draining foam out of the fourth pipe 28. This allows foam to be drained at select times, but not at others (i.e. when cleaned water is being outlet along the drain pipe 34 and the outlet pipe 30). Depending on the content of the inlet water 3 the foam produced by the filtering process may have a high content of phosphorous and nitrous matter. This may make it useful as a fertilizer. In the context of aquaculture, it may be particularly advantageous to remove the foam through the fourth pipe 28, collect it and use it to fertilize plants. It may then be possible to use the plants grown with this fertilizer to feed fish grown through aquaculture, thus recycling aquaculture by-product. Water that has been cleaned using the nano-bubbles is removed from the fluid container 6 using a drain pipe 34. Provided that the level of water within the tank is above the level of the drain pipe 34, cleaned water can be drawn off down the drain pipe 34 using gravity alone, without requiring a pump. The drain pipe 34 is positioned below the entry to the tapered cone 16, and therefore below the level at which the foam collects. This reduces the likelihood of foam being drawn out down the drain pipe 34 together with the cleaned water. The drain pipe 34 contains a second drain valve 36. This second drain valve 36 can be used to vary the flow rate at which cleaned water is drawn out of the fluid container 6, or even to stop the flow of water out of the fluid container 6 altogether. This allows the cleaning time of water to be varied and controlled. The drain pipe 34 connects to the outlet pipe 30. The drain pipe 34 includes an upright drain portion 31. This serves to separate off any foam which is drawn into the drain pipe, and also separates off an upper layer of the water which has a high foam content. This upright drain portion 31 may be drained into a part of the foam extraction portion 17, or may be drained separately. In this described example, bubbles are injected from the bottom of the fluid container 6, move through the water by Brownian motion contacting contaminant molecules within the water and collecting them on their surface, and, then rising through the water in order to clean it. The resulting foam and cleaned water are both extracted towards the top of the fluid container 6. In some cases, it may be advantageous to draw off foam and / or cleaned water from the lower end of the fluid container 6. For this reason, a third drain valve 38 and a fourth drain valve 40 are located at a lower end of the fluid container 6. The third drain valve 38 is also connected to the outlet pipe 30, whereas the fourth drain valve 40 is connected to a separate, secondary outlet pipe 42. Figure 2 is an external, perspective view of the water cleaning device 1 of Figure 1, in which the same reference numerals as used in Figure 1 have been used. In this figure reinforcing collars 44a, 44b are more clearly visible, surrounding the exterior of the fluid container 6. These help to provide strength to the long, thin fluid container 6 and reduce the risk of buckling of the cylindrical tank. They may also be used to attach top and bottom end parts onto a central cylinder, in order to provide the fluid container 6. Figure 3 shows the nano-bubble generator 8 of Figure 1 in greater detail, and also shows (schematically) the diffuser 12 to which the nano-bubble generator 8 supplies the generated nano-bubbles. According to the present invention, the nano-bubble generator 8 comprises a bubble generating device 80 and a venturi 301. The bubble generating device 80 may be a standard, commercially available bubble generator, capable of producing bubbles. The bubbles may be micro-bubbles, nanobubbles, a mixture of micro- and nano-bubbles or bubbles that are larger than micro size. The second pipe 22 brings in some of the inlet water 3 (water to be cleaned) on the left-hand side with reference to the view shown in Figure 3. The bubble generating device 80 produces bubbles 82 in this supply water and outputs the water, containing bubbles, into the venturi 301 as seen in Figure 3. As stated above, the bubbles 82 produced by the bubble generating device 80 may be nano-sized, or may be larger than nano-sized bubbles. llpon exiting the bubble generating device 80, first, the water containing the bubbles 82 reaches a converging section 300. In this section the diameter of the pipe reduces, down to a minimum diameter which defines a constricted section, referred to as a throat section 302. Due to the Venturi effect, the fluid pressure of the water within this throat section 302 is reduced relative to the pressure in the converging section 300. As a result of this reduced fluid pressure, gas (e.g. air) is drawn in through a gas inlet 304, which is located within the throat section 302, and this drawn in gas is introduced into the inlet bubbles 82, producing (relatively large) bubbles 306 that are filled (at least partially) with the selected inlet gas supplied through gas inlet 304. The gas supplied through the gas inlet 304 may be, for example, air, oxygen, or ozone. The gas supplied into the bubbles through the gas inlet 304 may therefore increase the pressure within the bubbles. Additional gas can be added into a bubble until the pressure inside the bubble is greater than the external pressure of the surrounding fluid, which is applying pressure to the surface of the bubble. Nanobubbles are highly stable and are therefore less likely to burst as a result of increased pressure inside the bubble. Although not illustrated in the embodiment of Figure 3, in an alternative embodiment, the gas inlet 304 may provide the bubble generator, such that bubbles are not supplied to the converging section, but are rather formed in the throat section 302 as a result of gas being drawn in through the gas inlet 304. Next the water, and the newly formed bubbles 306, flow into a diverging section 308 of the nano-bubble generator 8. In the diverging section 308 the fluid pressure is increased, due to the tapering outwards of the diameter of a pipe. This increase in local pressure within the water breaks the larger bubbles 306 down into smaller bubbles 310. Appropriate dimensions for the venturi nano-bubble generator 8 are chosen so that the resulting bubbles 310 (or at least a large proportion of them) are of nano-bubble size. In other words, the venturi 301 is able to break down whatever size of bubble is produced by the chosen bubble generating device 80 such that the bubbles output from the venturi contain (preferably a majority of) nano-size bubbles. The venturi 301 advantageously provides good control of bubble size, reliably and consistently producing nanobubbles, whilst doing so highly efficiently. Figure 4 shows an external view of the venturi 301, in which the gas inlet 304 and the throat section 302 are visible. As shown in Figure 3, the nano-bubble generator 8 further includes a diffuser 12, located downstream of the venturi 301. Thus, the water containing the nano-bubbles 310 produced by the venturi 301 is supplied to the diffuser 12 through third pipe 24. The diffuser 12 is shown in greater detail in the perspective view of Figure 5. The water containing nano-bubbles, as produced by the venturi 301, is supplied to an inlet 50 of the diffuser. From here it flows out of an ejector cap 52 into a chamber known as an ejection cylinder 54. Within the ejection cylinder 54 the water and bubbles are mixed, and driven upwards, by a static mixer 56, into a mixer cylinder 58. The static mixer 56 induces turbulence within the flowing water, which subjects the created bubbles to shear forces and thereby induces fragmentation, reducing the average bubble size. This helps to further break the nano-bubbles into smaller nano-bubbles. The mixer cylinder 58 includes two concentric pipes, an inner pipe 60 and an outer pipe 62. The mixed water, containing nano-bubbles, travels upwards within the inner pipe 60. The inner pipe 60 contains a plurality of diffuser holes 64, which are openings in its surface, i.e. passing all the way through the thickness of the inner pipe 60. These diffuser holes form a membrane, and act to spread the nano-bubbles more evenly throughout the fluid. They therefore diffuse the nano-bubbles. Nano-bubbles (and the surrounding water that they are contained in) pass from the inner pipe 60, through the diffuser holes 64 and out into the outer pipe 62. Bubbles are broken down as they pass out of the inner pipe 60 through the diffuser holes 64, which further helps to reduce the bubble size. The fluid from both the inner pipe 60 and the outer pipe 62 may be carried to the fluid container 6 (i.e. to the bubble spreader 14). This fluid will contain on average smaller bubbles than the fluid that exits the venturi, and it will contain a more uniform distribution of nano-bubbles, as a result of the diffuser holes 64. Alternatively, the water and bubbles remaining within the inner pipe 60 may be expelled as discharge, whereas the water and bubbles that diffuse into the outer pipe 62 may be supplied to the bubble spreader 14, from which it is passed out into the fluid container 6 through the bubble spreader 14 as described above. Retaining only the fluid from the outer pipe 62 is particularly advantageous since the fluid in the outer pipe 62 has particularly desirable nano-bubble content and distribution as a result of the action of both the mixer 56 and the diffuser holes 64. Thus the diffuser 12 further breaks down the bubbles from the venturi 301 into smaller bubbles, ensuring a greater proportion of nano-bubbles in the fluid that exits the bubble spreader 14, and it also evenly spreads the bubbles throughout the pipe that connects to the bubble spreader 14, and therefore improves the distribution of bubbles within the fluid container 6, improving the cleaning effect of the nano-bubbles. By integrating the venturi 301 with the diffuser 12, the Applicant has appreciated that the strengths of both components may be leveraged to improve the efficiency and precision of nanobubble production, and thereby improve the cleaning effect. Figures 6-8 are graphs showing how various measured properties of a sample of water changed over time as the sample was cleaned using a cleaning device comprising a nano-bubble generator comprising a venturi (solid line) and also cleaned using a cleaning device which did not include a venturi (dashed line). The sample was salt water. It was confirmed that the salinity, temperature and pH of the water remained constant (subject to any standard, expected fluctuations) over the time period of filtering. Along the x-axis 600, 700, 800 of all of Figures 6-8 there is time, in units of minutes. The later data point, illustrated at the right-hand side for each graph, was measured for a sample extracted after 20 minutes of cleaning time. Figure 6 shows, along the y-axis 602, the quantity of suspended matter in the sample, in units of mg / L. The dashed line 604 represents the value measured using a cleaning device which does not include a venturi, whilst solid line 606 represents data obtained using a cleaning device according to the present invention. It can clearly be seen that the reduction in suspended matter is significantly larger where a venturi is used to compared to when no venturi is used. Figure 7 shows, along the y-axis 702, the turbidity of the water sample in units of FNU (Formazin Nephelometric Unit). The turbidity is a measure of relative clarity of a liquid. In particular FNU is a measure of scattered light at 90 degrees from the incident beam, using an infra-red light source. As shown, the turbidity of the sample over the 20- minute period was reduced using both cleaning methods. In this particular test sample, the turbidity reduced by substantially the same amount for both cleaning methods (i.e. with and without the venturi), so only a single line 704 is visible on the graph. This sample had relatively low turbidity in the starting sample. It is expected that were the starting turbidity higher a more significant cleaning effect would be seen when using the venturi than without. Figure 8 shows, along the y-axis 802, the quantity of oxygen in the water sample, in units of mg / L. The dashed line 804 represents the value measured after using a cleaning device which does not include a venturi to clean the sample, whilst solid line 806 represents data obtained using a cleaning device according to the present invention, including a venturi. It can clearly be seen that the oxygen present in the water sample increases significantly more when using the cleaning device according to the present invention than when no venturi is used. This effect is particularly beneficial in the context of aquaculture since oxygen is one of the primary and most important needs of any fish facility. Moreover, the oxygen level was found to remain consistently at this higher level across the 20-minute sampling period. In contrast, where a cleaning device without a venturi was use, the oxygen level was found to be higher at times, but not to remain consistently high, and to instead vary over the sampling period. It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

1. A fluid cleaning device, comprising:a fluid container; anda nano-bubble generator, wherein the nano-bubble generator comprises a bubble generating device and a venturi;the nano-bubble generator arranged to generate nano-bubbles and to introduce the nano-bubbles into fluid contained within the fluid container, so as to clean the fluid.

2. The fluid cleaning device of claim 1, wherein the nano-bubbles generated by the nano-bubble generator have a diameter of less than 500 nm.

3. The fluid cleaning device of claim 1 or 2, wherein the nano-bubble generator is arranged to generate a plurality of bubbles, wherein a majority of the plurality of bubbles are nano-bubbles.

4. The fluid cleaning device of any preceding claim, wherein the nano-bubble generator is arranged to generate a plurality of bubbles having a distribution of diameter sizes, wherein the distribution of diameter sizes of the plurality of bubbles has a mean value of between 50 nm and 200 nm.

5. The fluid cleaning device of any preceding claim, wherein the venturi comprises a gas inlet.

6. The fluid cleaning device of any preceding claim, wherein the nano-bubble generator generates nano-bubbles filled with air, oxygen, or ozone.

7. The fluid cleaning device of any preceding claim, further comprising a diffuser.

8. The fluid cleaning device of claim 7, wherein the diffuser comprises one ormore diffuser holes.

9. The fluid cleaning device of claim 7 or 8, wherein the diffuser is positioned downstream of the nano-bubble generator.

10. The fluid cleaning device of any preceding claim, further comprising a mixer toinduce turbulence in the fluid.

11. The fluid cleaning device of claim 10, further comprising a diffuser, wherein the diffuser comprises the mixer.

12. The fluid cleaning device of any preceding claim, further comprising a bubble spreader arranged to distribute the nano-bubbles within the fluid container.

13. The fluid cleaning device of claim 12, further comprising a diffuser, wherein the bubble spreader is positioned downstream of the diffuser.

14. The fluid cleaning device of any preceding claim, wherein the fluid container comprises a transparent portion.

15. The fluid cleaning device of any preceding claim, further comprising an inletvalve, wherein the supply of fluid into the fluid container is controllable using the inlet valve and / or an outlet valve, wherein the flow of fluid out of the fluid container is controllable using the outlet valve.

16. The fluid cleaning device of any preceding claim, wherein the bubble generating device generates nano-bubbles and / or micro-bubbles and / or bubbles larger than micro-bubbles.

17. An aquaculture system, comprising the fluid cleaning device as claimed in any preceding claim.

18. A microplastics removal system, comprising the fluid cleaning device as claimed in any preceding claim.

19. A wastewater treatment system, comprising the fluid cleaning device as claimed in any preceding claim.

20. A drinking water system, comprising the fluid cleaning device as claimed in any preceding claim.

21. A method of cleaning fluid, comprising:generating nano-bubbles using a bubble generating device and a venturi; and cleaning fluid by introducing the nano-bubbles into fluid contained within a fluid container.

22. The method of claim 21, wherein the fluid cleaning device further comprises an inlet valve, wherein the supply of fluid into the fluid container is controllable using the inlet valve, the method further comprising controlling the flow of fluid into the fluid container using the inlet valve and thereby controlling the cleaning time for which the fluid is cleaned.

23. The method of claim 21 or 22, wherein the fluid cleaning device further comprises an outlet valve, wherein the supply of fluid out of the fluid container is controllable using the outlet valve, the method further comprising controlling the flow of fluid out of the fluid container using the outlet valve and thereby controlling the cleaning time for which the fluid is cleaned.

24. The method of any of claims 21 to 23, further comprising extracting foam out of the fluid container.

25. The method of claim 24, further comprising using the foam to provide fertilizer or animal feed.

Images