Abstract: A wastewater treatment system, comprising a tank and an elongate draft tube. The tank comprises a bottom and at least one partition wall extending short of a tank curved turning wall to form at least a pair of channels for movement of a volume of a liquid. The elongate draft tube is at least partially submerged beneath the liquid and is rotated about its longitudinal axis for orbitally moving the liquid through the at least a pair of channels of the tank in a fixed direction. A process for treating wastewater is also provided.

Abstract: There is provided a microbubble generating method including: supplying a gas to a rotator 2 while rotating the rotator 2 in a liquid; injecting the gas into the liquid from bubble injection holes formed in the surface of the rotator 2; and applying a shear force, produced by the relative movement between the liquid and the rotator, to gas bubbles present on and in the vicinity of the surface of the rotator 2 to generate microbubbles. The microbubble generating method can easily generate a large amount of microbubbles without forcibly creating a swirling flow of fluid.

Abstract: The invention relates to an agitating device, in which a hyperboloid agitating body (6) is connected to a drive mechanism (2, 3) by means of a shaft (4). Several ribs (7) running from the circumferential boundary (Ur) in the direction of the shaft (4) are provided on the upper side (O) of the agitating body (6). According to the invention, the efficiency of the agitating device may be improved, whereby a height (h1) of the ribs (7) at least incrementally increases from the circumferential boundary (Ur) of the agitating body (6) in the direction of the shaft (4).

Abstract: An irrigation system for supplying aerated water to plants includes a diffuser. Oxygen is diffused into water by the diffuser to generate aerated water. The aerated water is supplied to an irrigation device which supplies the aerated water to the plants.

Abstract: A multi-direction submersible floating aerator has a gas-ring compressor (4), a base (6), a gas inlet tube (9), a seat (11), multiple submersible hollow shaft motors (13), and multiple propellers (14). The base (6) is disposed between the gas-ring compressor (4) and the gas inlet tube (9). The propeller (14) is coaxially connected to the submersible hollow shaft motor (13). The seat (11) is disposed between the gas inlet tube (9) and the submersible hollow shaft motor (13). The seat (11) is hollow. A first opening is disposed at the top of the seat (11), and multiple second openings are disposed on one side of the seat (11). The seat is (11) used for evenly distributing compressed air. Thus, the aeration is more even and stable, and aeration efficiency is greatly improved.

Abstract: A diffuser which includes a cylinder open at one end and closed at the opposite end, the open end of the cylinder being securable to air supply; the wall of the diffuser being formed with a plurality of holes therethrough, each hole having a larger diameter on the exterior of the wall of the diffuser than on the interior of the wall of the diffuser, the longitudinal axis of each hole being inclined at an acute angle to both the radius and the longitudinal axis of the diffuser, and the orientation of each hole with respect to the longitudinal axis of the diffuser being such that when the diffuser is rotated about it longitudinal axis in use, the leading edge of each hole is at a higher elevation than the trailing edge of each hole.

Abstract: Provided is a submersible floating aerator, including a submersible hollow shaft motor (1) including a hollow shaft (2), a bottom shaft extension (4), a first motor flange (15), a second motor flange (16), a first flange (24) and a second flange (25), a gas-ring compressor (17), a base (21) having a gas inlet hole (22), a gas inlet tube (23), a dome (26), a propeller assembly (60) and an air outlet opening (18); wherein the base (21) is fixed on the top of the gas inlet tube (23), the gas-ring compressor (17) is fixed on the base (21) via a bracket (19); the air outlet opening (18) is connected to the gas-inlet hole (22) via a pipe (20); the gas inlet tube (23) is fixed to the first motor flange (15) via a first flange (24); the second motor flange (16) is fixed to the dome (26) via the second flange (25); the bottom shaft extension (4) is coaxially connected to the propeller assembly (60); and the hollow shaft (2) rotates along with the propeller assembly (60).

Abstract: A swing-type submersible floating aerator, including a gas-ring compressor, a base, a pipe, a support tube, a swing device and a driving device, wherein the base is disposed between the gas-ring compressor and the support tube, the base is disposed at the center of the swing device, and the driving device drives the swing device to rotate. The swing-type submersible floating aerator is capable of rotating 360 degrees in a clockwise or a counterclockwise direction, and push-flow and aeration can be performed in any direction.

Abstract: A wastewater treatment system, comprising a tank and an elongate draft tube. The tank comprises a bottom and at least one partition wall extending short of a tank curved turning wall to form at least a pair of channels for movement of a volume of a liquid. The elongate draft tube is at least partially submerged beneath the liquid and is rotated about its longitudinal axis for orbitally moving the liquid through the at least a pair of channels of the tank in a fixed direction. A process for treating wastewater is also provided.

Abstract: A composite hollow aspirator shaft is formed of fiberglass-reinforced phenolic resin that resists abrasion and reduces the amount of vibration that is transferred to the motor rotating the shaft, and the motor's bearings to extend motor and bearing life. An aspirator tip is adhesively attached to the composite aspirator shaft and includes an access hole for cleaning inside the hollow aspirator shaft. The hole is in axial alignment with the hollow aspirator shaft and a selectively-removable plug is used to seal the hole during aspirator use. When the aspirator tip is rotated in a liquid, fluidic forces are transferred to the shaft where they are attenuated. A splined hub is used to attach a foam restrictor near the other end of the shaft. The composite shaft material has a flexural modulus, or ratio, within the elastic limit of any applied stress.

Abstract: A wastewater treatment system, including a tank and an elongate draft tube. The tank includes a bottom and at least one partition wall extending short of a tank curved turning wall to form at least a pair of channels for movement of a volume of a liquid. The elongate draft tube is at least partially submerged beneath the liquid and is rotated about its longitudinal axis for orbitally moving the liquid through the at least a pair of channels of the tank in a fixed direction. A process for treating wastewater is also provided.

Abstract: An apparatus for introducing gas into a large body of liquid, including a horizontal frame on ballast adjustable floats, a pressurized gas source, a vertical shaft rotatable about its axis, and a plurality of blades submerged in the liquid and extending radially from a hub on the lower end of the shaft. The blades each have an elastomeric membrane around a longitudinal member, where the longitudinal member is hollow with a closed end and in communication with the pressurized gas source through the shaft on the other end, with openings through its lower side, and the elastomeric membrane has perforations which are spaced from the longitudinal member openings. A drive on the platform rotates the shaft via a ring gear around the shaft with a key connection thereto allowing axial movement therethrough, and an inwardly facing surface supported on bearings. A selectively driven and smaller pinion gear directly engages the ring gear.

Abstract: An apparatus for stirring and injecting a gas into a liquid. A drive device, located above a container containing a liquid, is connected to a vertical output shaft. The shaft has a self-suction turbine at its. The turbine is made of two stacked disks where the surface area of the lower disk is less than the surface area of the upper disk.

Abstract: A method and apparatus of sanitizing drinking water to be dispensed from a water dispenser having a reservoir includes the steps of providing the ozone gas generator that generates an ozone gas stream, transmitting the ozone gas stream from the generator to the water dispenser reservoir, mechanically breaking up the ozone gas stream inside the reservoir to produce ozone gas bubbles, and using the ozone gas bubbles to disinfect water in the reservoir. The ozone gas stream can be mechanically broken up using a pump such as, for example, an impeller type pump.

Abstract: An aspirator includes a plurality aspirator tubes, each preferably extending at approximately 70 degrees from a central axis and including a fin extending a limited distance below the aspirator tube outlet. The design improves mixing and gas contact, yet has reduced power requirements, by using a plurality of debris-shedding, finned aspirator tubes to inject approximately 35% more air than the best prior art designs at the same rotational speed. Further, the aspirator of the present invention improves motor life by reducing the average current draw on the motor. The aspirator is useful in many applications, including sewage and wastewater treatment, oxygenation, fermentation, aeration, hydrogenation, oxidation, and oil-sand separation.

Abstract: An apparatus for mixing gas and liquid is disclosed. The apparatus includes a draft tube having a gas inlet, a liquid inlet, a gas-liquid outlet, and an impeller rotatably mounted within the draft tube. The gas can be entrained into the liquid by rotation of an impeller having a low pitch ratio, such as less than 1:1. The impeller can have a diameter than is greater than the axial length of the impeller and includes at least one blade extending at least 30° around an axis of rotation of the impeller. Liquid turning vanes can also be positioned external to the draft tube to rotate liquid entering the draft tube in a direction opposite the direction of rotation of the impeller. The impeller can be constructed to create a reduced pressure zone, which directs gas axially downward within the draft tube upon rotation of the impeller.

Abstract: A composite hollow aspirator shaft is formed of fiberglass-reinforced phenolic resin that reduces the amount of vibration that is transferred to the motor rotating the shaft, and the motor's bearings. This extends motor and bearing life. When the aspirator tip is rotated in a liquid, fluidic forces are transferred to the shaft. Rather than transmitting those forces into the motor and its bearings, the composite aspirator shaft attenuates the forces to prevent most of them from reaching the motor and its bearings. The composite shaft material has a flexural modulus, or ratio, within the elastic limit of any applied stress, and is sufficiently low to allow the transfer of the flexural or oscillatory energy into heat within the shaft instead of conveying the energy in the form of vibration/oscillations along the axial length of the shaft. As an added advantage, the fiberglass resists abrasion better in wastewater than stainless steel used in the prior art.

Abstract: The invention describes a stirring device made from a gassing stirrer and a liquid mixer or two liquid mixers, which are arranged on a shaft and each have a feed and at least one exit opening, wherein the exit openings of the gassing stirrer and of the liquid mixer or the liquid mixers are at a distance from one another, the ratio, a/d, of the distance, a, between the exit openings to the diameter, d, of the gassing stirrer or liquid mixer being 0.02 to 0.5 and the ratio, b/d, of the distance, b, between the outer edges to the diameter, d, of the gassing stirrer or liquid mixer being 0.01 to 0.4.

Abstract: An apparatus for mixing gas and liquid is disclosed. The apparatus may have a plurality of blades rotated about a common shaft. The blades may have a forward portion, which cuts through a liquid, and a trailing portion, which forms the leading part of a gas cavity. The gas cavity is in communication with a gas inlet and the bulk fluid. As the blades pass through a liquid, a reduced pressure zone is formed immediately behind the blade. The reduced pressure zone allows air to be aspirated from the gas inlet into the gas cavity adjacent to the blade. Gas in the gas cavity is sheared into discrete bubbles and transferred to the liquid by the action of the trailing blade wake currents.

Abstract: An apparatus for use in fluid agitation which may include a hollow drive shaft having a first end and a second end wherein the first end is coupled to a selectively rotatable power source, a first propeller, a second propeller and an atomizer coupled to the drive shaft, and a submergible vortex control plate disposed above the first propeller, and may also include a forced air source in fluid connection with the drive shaft, and methods of use thereof.

Abstract: The water resistant aerator design of the present invention includes a combination of known waterproofing techniques with new techniques for wastewater aerators comprising: use of a threaded joint and thread sealing compound between the lifting handle and the top endbell to provide a watertight connection between the parts; use of a single lip seal for the top lip seal that has over 18 degrees of wear motion before pressure is lost; use of a grease seal between the top lip seal and the top bearing to impede moisture from entering the motor; use of a grease seal between the bottom lip seal and the bottom bearing to impede moisture from entering the motor; use of a multi-surface silicone sealant between the bottom endbell and the air seal enclosure to provide a watertight joint that prevents water from entering the air seal enclosure; and use of an O-ring seal between the endbells and the stator shell.

Abstract: The design of an air seal enclosure of the system and method of the present invention includes a combination of use of a multisurface silicone sealant between the bottom endbell and the air seal enclosure to provide a watertight joint that prevents water from entering the air seal enclosure. The air seal enclosure is formed from a suitable flexible material and is attached to a bottom endbell with bolts through holes in a periphery thereof. When tank liquid rises during flooding, the air seal enclosure provides a pocket of trapped air like a diving bell that prevents the tank liquid from ever reaching the bottom of the aerator.

Abstract: An irrigation system for supplying aerated water to plants includes a diffuser. Oxygen is diffused into water by the diffuser to generate aerated water. The aerated water is supplied to an irrigation device which supplies the aerated water to the plants.

Abstract: The present invention is an aerator for inducing air flow below the surface of a liquid. The aerator includes a motor having a drive shaft. A propeller is operably connected to the drive shaft of the motor and a blower is operably connected to the motor. The aerator further includes an air flow path that has an inlet and an outlet, with the inlet connected to the blower and the outlet located near the propeller. The blower and the propeller rotate at different speeds.

Abstract: An aerator for mixing an ambient gas with a liquid and agitating the liquid incorporates at least three bearings that are rigidly connected to an aerator housing. A propeller is driven by a central shaft that is rotatably mounted in the aerator housing using the bearings. Two bearings are located near the ends of the central shaft. At least one additional bearing is located between the ends, for example, in a intermediate position. This additional bearing may absorb some of the force that would otherwise be transferred to the bearing near the propeller end of the central shaft. The bearings are thus subjected to lower stress and may exhibit a prolonged useful lifespan.

Abstract: The invention relates to a flotation machine, particularly to a flotation machine rotor, that is used for dispersing air, supplied via the rotor axis, to the surrounding slurry, and in which rotor there are formed alternating air ducts and slurry grooves, so that the ends of both the air ducts and the slurry grooves, projecting outwardly from the rotor, form the outer surface of the rotor According to the invention the air ducts (3,15) are arranged in the rotor at essentially equal distances, starting radially from the rotor outer surface, so that the air ducts (3,15) form in the center part of the rotor a space (6,17) for the slurry, in which space the slurry surrounding the rotor is to be made to flow along the slurry grooves (5, 16) provided in between the air ducts (3,15).

Abstract: An agitator drive apparatus for agitating and injecting gas into a liquid in a mixing vessel. The apparatus includes a first and second housing coupled to one another along with a shaft having a first and second end that is rotatably coupled to the first housing and extends from the first housing into the second housing. The shaft includes an inner bore that extend from the second end at least partially to the first end. The shaft also includes a plurality of air ports in communication with the inner bore.

Abstract: An aerator/mixer which includes a rotatable hollow drive shaft one or more air intake ports adjacent one end, and a propeller and an air permeable diffuser mounted adjacent to the other end; the diffuser is mounted below the propeller; and is in communication with the interior of the hollow drive shaft such that air supplied to the air intake port can exit only through the diffuser, the aerator/mixer further includes a device for rotating the drive shaft and the propeller and a device for supplying air at above atmospheric pressure to the or each air intake port.

Abstract: In accordance with the present invention, a system is provided for circulating fluids, both gases and liquids, in a body of liquid. The system comprises a rotatable shaft having a tubular segment immersed in the body of liquid. The tubular segment of the shaft is in fluid communication with the fluid to be circulated. Means are provided for rotating the shaft. One or more tubular arms extend radially outwardly from the tubular shaft segment, and are in fluid communication with the tubular shaft segment. Each tubular arm comprises an extension spring which, under static conditions, is substantially straight but which upon rotation of the shaft is believed to be arcuately deflected by hydrodynamic force so as to impel fluid into the body of liquid. The system may be utilized for simply circulating the liquid, or for injecting a gas into the body of liquid.

Abstract: An gas diffusion device for introducing gas into a large body of liquid having at least one diffuser blade that engages over a mounting tube having a spar affixed to its exterior. A passage in the diffuser blade is dimensioned to cooperatively engage over the spar and mounting tube which holds the diffuser blade top surface at a determined angle once so engaged thereover. Changing the position of the spar on the exterior of the mounting tube in relation to the center axis of the mounting tube changes the resulting angle of the diffuser blade top surface. The diffuser blade can be attached to a hub mounted on a mainshaft that automatically cantilevers out of the fluid should compressed gas supplied to the diffuser blade through the mainshaft cease to avoid damage to the diffuser blade and diffuser pad on the blade from particulate in the liquid.

Abstract: In a flotation machine rotor, a body portion defines an interior area that is adapted to communicate with a gas source. At least one rotor blade projects radially outwardly from the body portion with at least one aperture being defined thereby and in gaseous communication with the interior area. The aperture is positioned adjacent the rotor blade and is defined in part by a lower peripheral edge. At least one lip projects radially outwardly from the lower peripheral edge so that during operation of the rotor, solids traveling along the body portion are projected radially away from the rotor by the lip and become approximately aligned with the aperture so that gas flowing from the aperture creates bubbles in the slurry that entrain at least a portion of the solid particles. The bubbles and solid particles form a froth on the surface of the slurry which can then be removed from the flotation machine.

Abstract: The invention relates to a flotation mechanism (4) comprising a directional element (25) and vertical vanes (26) located in a flotation cell (2). The directional element is symmetrical and is fixed at the center to the lower section of the hollow shaft of the mechanism. According to the corresponding method, due to the directional element (25), which is cylindrically inclined downwards from the outer edge the flotation mechanism directs the gas-slurry suspension that is formed in a downward slanting direction towards the side wall of the cell. The mineral suspension rises upwards from the side wall towards the center of the cell, from where the flow is diverted to the edges of the cell and the froth generated is removed from the cell. Using this flotation mechanism enables a powerful agitation, extending throughout the entire mixing zone (I).

Abstract: The invention relates to a device for a flotation machine in which a drive shaft for mixing slurry and transporting air is driven by a drive arrangement and the drive shaft is provided with a bearing assembly and the drive shaft is at least partly hollow in the interior for transferring air to a rotor. According to the invention the drive shaft is provided with at least one bearing in a housing separated from at least one element forming an air transfer chamber around the drive shaft. Further, the drive shaft has at least one opening in order to create an essentially continuous connection between the air transfer chamber and the interior of the drive shaft.

Abstract: The invention relates to a rotor used in the flotation mechanism in the concentration of ores. According to the invention, upper part of the rotor chambers under the rotor cover are formed to be downward inclined from the outer edge of the chambers into the core so that they form an angle between 5 to 70 degrees with the horizontal plane. In addition the upper surface of the rotor cover can be inclined upwards raising from around the rotor shaft towards the outer edge at an angle between 5-70 degrees. With the means inside and above the rotor the slurry is directed upwards through the stator of the flotation mechanism. The developed rotor improves the suspension of coarse particles with high specific gravity within a flotation machine. The invention relates also to a method for inverting the material flow disharging from the rotor.

Abstract: An agitator drive apparatus for agitating and injecting gas into a liquid in a mixing vessel. The apparatus includes a first and second housing coupled to one another along with a shaft having a first and second end that is rotatably coupled to the first housing and extends from the first housing into the second housing. The shaft includes an inner bore that extend from the second end at least partially to the first end. The shaft also includes a plurality of air ports in communication with the inner bore.

Abstract: The invention relates to an apparatus for the treatment of contaminated media, comprising a rotatable housing having at least one inlet (28) for gas emerging in the stirrer housing, and at least one suction opening (10; 32) for the medium to be treated. The housing is provided with a plurality of outlet openings (25; 27) arranged along its periphery. A plurality of vanes (22; 23) are arranged in the outlet openings (25; 27). A rotation transferring means (24) is provided to transfer a rotational movement to the stirrer. A method according to the invention comprises generating a vortex in the medium in such a way that an under-pressure occurs in the center of the vortex, and that a nitrogen containing gas is introduced in the center of the vortex.

Abstract: One or more aerators move below an area of an assembly of immersed filtering membranes to be aerated. The movement can take various forms including the sweep of a pendulum. Positions in the assembly of membranes may be alternately aerated for a period of time and then not aerated for a period of time.

Abstract: An aerator/mixer which includes a rotatable hollow drive shaft one or more air intake ports adjacent one end, and a propeller and an air permeable diffuser mounted adjacent to the other end; the diffuser is mounted below the propeller; and is in communication with the interior of the hollow drive shaft such that air supplied to the air intake port can exit only through the diffuser, the aerator/mixer further includes means for rotating the drive shaft and the propeller and means for supplying air at above atmospheric pressure to the or each air intake port.

Abstract: A collapsible sparger for insertion into a vessel through a flanged opening is disclosed having a central tubular conduit having a plurality of rotatable members along its length. The rotatable members may be rotated and secured into position after insertion.

Abstract: The invention relates to a device that sets liquid or solid substances in motion and to a method for the implementation of said device. The invention more specifically relates to a device that sets liquid or solid substances in motion by means of gas bubbles, comprising a receptacle for said substances, at least one moveable gas emitting nozzle arranged in the lower part of said receptacle, and a system for conveying the gas to said nozzle.

Abstract: An apparatus for dispersing a gas in a liquid or slurry has a reaction vessel with an inner zone for the downward flow of liquid, an outer zone for the upward flow of liquid, a plurality of aerators in the outer zone and a propeller to induce downward flow in the inner zone and promote mixing and circulation of the liquid/slurry. The apparatus is particularly suitable for use in the bacterial decomposition of organic waste matter, for efficiently aerating large volumes of waste/water slurries.

Abstract: This invention is a magnetically coupled microorganism culture apparatus used in the expansion culture of microorganisms, especially in the expansion culture conducted for test tube microorganisms. In one example, the culture apparatus is powered by a micro motor, rotation of the micro motor is transferred to the air impeller by virtue of a magnetically coupled device, causing the air impeller to rotate at a high speed. Under the effect of centrifugal force, the air in the air impeller is sprayed into the culture medium liquid in the container for the growth of microorganisms. This invention is substantially different from the shaker being used today, thus achieving the purpose of reducing cost and saving energy.

Abstract: A system for providing improved bulk liquid mixing and effective gas-liquid contacting for mass transfer of the gas to the liquid, especially a non-Newtonian liquid, the viscosity of which decreases when under shearing conditions (shear thinning), in an upright tank. An upright draft tube is mounted within the tank and has a lower end spaced from the tank bottom and an upper end spaced below the surface of the liquid in the tank. A plurality of mixing impellers in the draft tube are sufficiently close to each other to establish a field or pattern of agitation to cause shear thinning and upflow throughout the draft tube and which produces significant turbulence at the liquid surface. A plurality of radially inwardly projecting, circumferentially spaced baffles extend from the draft tube and are proximate the mixing impellers to prevent swirling of the liquid within the draft tube.

Abstract: An agitator for agitating liquids in an agitator vessel for suspending solids and dispersing gas in a liquid, in particular in an absorber of a flue gas desulfurizing plant, includes an agitator vessel, an agitator shaft extending into the agitator vessel, and an agitating propeller mounted to the agitator shaft within the agitator vessel. The agitator shaft is hollow to define an interior space for conducting gas into the agitator vessel and injecting gas on the pressure side of the agitating propeller into the liquid.

Abstract: An apparatus for dispersing a gas in a liquid or slurry has a reaction vessel with an inner zone for the downward flow of liquid, an outer zone for the upward flow of liquid, a plurality of aerators in the outer zone and a propeller to induce downward flow in the inner zone and promote mixing and circulation of the liquid/slurry. The apparatus is particularly suitable for use in the bacterial decomposition of organic waste matter, for efficiently aerating large volumes of waste/water slurries.

Abstract: In a method of and apparatus for producing sub-micron bubbles in liquids, slurries, and sludges, gas is maintained on the interior of the gas permeable partition at predetermined pressure. Relative movement between the gas permeable partition and the surrounding material forms sub-micron sized bubbles in the liquid, slurry, or sludge.

Abstract: An auto-aspirating, rotational dispersion device for gases and liquids with a rotating hollow shaft employed for gas suction and gas channels communicating with said hollow shaft to openings in the gas channels that are positioned at intervals over the circumference of the device, at which openings the gas and liquid are mixed. The device is distinguished by the fact that the gas being dispersed flows in a manner separate from the liquid, from the hollow shaft through the gas channels and is mixed with the liquid outside of the device.

Abstract: This invention is a magnetically coupled microorganism culture apparatus used in the expansion culture of microorganisms, especially in the expansion culture conducted for test tube microorganisms. In one example, the culture apparatus is powered by a micro motor, rotation of the micro motor is transferred to the air impeller by virtue of magnetically coupled means, causing the air impeller to rotate in a high speed. Under the effect of centrifugal force, the air in the air impeller is sprayed into the culture medium liquid in the container for the growth of microorganisms.

Abstract: The aspirator is utilized to introduce air, oxygen, or other gases into a liquid. The advantages of the present design include the ability to inject approximately 30% more air than a conventional current design at 50% of the rotational speed. This reduces the power requirements. Additionally, the 45 degree angle of the aspirator tubes helps to shed debris that might otherwise collect on the aspirator. The aspirator design includes a fin along the aspirator tube, which is formed at a 45 degree angle relative to the aspirator shaft. Superior mixing is provided by the larger profile of the aspirator. The fins along the bottom of the aspirator tubes help to create larger voids in the liquid. This increases the aspirator's capacity to inject air, while minimizing the material requirements of the part.

Abstract: An impeller for use with apparatus to enhance mixing of a gas and a liquid, as used for example in froth flotation apparatus, having a substantially upright portion (11) substantially parallel to the axis of rotation (17), and a plurality of vanes (12) extending outwardly therefrom, at least some of the vanes (12) having an upper part (19) adapted to induce liquid flow generally downwardly towards the impeller and a lower part (20) adapted to induce liquid flow generally upwardly towards the impeller, wherein the impeller further comprises barrier means (14) at least partially dividing the upper and lower parts, and a gas input situated on the same side of the barrier means (14) as the lower part (20). The barrier means (14) may comprise a flange like member in a plane generally perpendicular to the axis of rotation (17) and may intersect, or be integral with at least part of the vanes.