Field of the Invention

The present invention relates to an apparatus for supplying gas to a liquid within a receptacle and to the use of such an apparatus. The present invention further relates to a method for supplying gas to a liquid within a receptacle.

Technical Background

In waste water treatment systems or other similar systems, for example biological aquatic systems such as fish farming, an aerobic process may be desired. For this process oxygen is required.

It is known to ad air into the waste water so that the water comprises a surplus of oxygen. This can be done in several ways.

One way is to use surface aerators for example paddle wheels or rotating brushes, which are partially submerged into the water and

mechanically mixes the air into the water. This causes turbulence in the water so that the oxygen can diffuse.

Another method is blowing air into the water. The air may be finely distributed through a perforated tube or pipe. This method has been further developed by using membranes so that the air which shall be mixed with the water is supplied into the water as small bubbles. The bubbles are preferable as small as possible.

A third method is using a rotating propeller, which is submerged into the water, to create a zone with low pressure behind the propeller. Air is then supplied through a tube or pipe to this zone. The air is then mixed into the water with the aid of the propeller.

In all these methods the efficiency of the oxygenation of the water is low, since a large part of the air that is forced down into the water goes back to the atmosphere. This, because the generated air bubbles are so large that the contact surface between the air and the water is too small to enable that all oxygen in the air bubble dissolve with the water during the time the air bubble raises towards the water surface. Further, in all these systems aerosols are created. Since the water to be treated with an aerobic

bioprocess often is waste water from humans and animals, they may comprise infectious matter, which may spread with the aerosols. Thus, there are restrictions where these systems may be arranged and precautions have to be taken regarding ventilation, disinfection of the air coming from the systems etc.

Other ways of adding air to the waste water which do not have the drawback of aerosols and which are more efficient regarding the oxygenation of the water are disclosed in the following documents:

US4874509 discloses an apparatus and method for dissolving a gas in a liquid comprising a long tube or conduit having an inlet connected to a source of gas and liquid mixed together under pressure and an outlet connected to a back pressure valve that maintains the pressure in the tube above the partial gas pressure necessary to ensure that the liquid is saturated with the dissolved gas.

US3662890A disclose a system and a method of treating contaminated liquid, such as domestic waste liquid in a septic tank, by progressively withdrawing part of the liquid into a closed chamber and aerating it under pressure to permeate it extensively with clean ambient air, and discharging the saturated liquid into the septic tank.

US4290979A discloses an apparatus for supplying air into a liquid which can be used as an aeration apparatus for a biochemical waste water treatment system.

Even if the efficiency of the oxygenation of the water in these systems is higher than the systems described in the beginning, there is still a need to improve the efficiency even more in regard of the oxygenation and of the whole waste water treatment system. Summary of the Invention

The object of the present invention is to provide an apparatus that has a high efficiency of liquid oxygenation and a method to maintain a desired oxygen level or increasing the oxygen level in a liquid in a receptacle.

The invention is based on the insight that if the oxygen is dissolved in a liquid, the efficiency of the liquid oxygenation is increased.

The invention relates to an apparatus for supplying gas to a liquid within a receptacle, comprising a liquid circulating ducting means having an inlet in said receptacle for receiving said liquid and an outlet returning said liquid to said receptacle, said apparatus further comprises a first pump arranged downstream of said inlet in said liquid circulating ducting means adapted to withdraw said liquid from said receptacle through said inlet, wherein said gas is supplied into said liquid downstream of said first pump. Said apparatus further comprises a second pump, which is connected downstream of said supplied gas in order to increase the pressure of said withdrawn liquid and mix said gas with said liquid. When supplying gas to a liquid it is desired that the gas dissolves in the liquid. Preferably, the liquid becomes saturated and even supersaturated by the gas. The dissolving of the gas in the liquid is dependent on the pressure inside the apparatus. By applying more pressure by using the second pump the possible saturation degree of the liquid increases. The second pump also mixes the gas efficiently with the liquid so that the gas is finely dispersed in the liquid. The whole liquid volume in the apparatus may, thus be provided with gas. This has the effect that the supplied gas can be dissolved in the liquid instead of becoming gas bubbles. Gas bubbles have the effect that when they are supplied into the receptacle they will raise towards the liquid surface. This is because of the density differences between the water and the gas. Thus, the supplied gas would then go back to the atmosphere instead of being dissolved. If more gas is supplied into the liquid than what can be solved, the gas which cannot dissolve after the liquid is saturated will become

microscopic bubbles when the liquid is supersaturated. By increasing the pressure and distributing the gas finely in the liquid more gas can dissolve in the liquid and the efficiency of the liquid oxygenation is increased. The small microscopic bubbles will dissolve in the liquid in the receptacle before they reach the liquid surface in the receptacle.

According to at least one exemplary embodiment said apparatus is for aerating biochemical waste water and said liquid is said biochemical waste water. An apparatus for distributing gas into a liquid within a receptacle would be advantageously in a biochemical waste water treatment system where an aerobic process often is desired.

According to at least one exemplary embodiment said receptacle is a tank having atmospheric pressure.

According to at least one exemplary embodiment said receptacle is open and in contact with air.

According to at least one exemplary embodiment said outlet is controllable. By having a controllable outlet, the outlet can control how much liquid comprising the gas shall be let out into the tank. By reducing the amount of liquid and the gas which is coming out of the apparatus, and depending on the volume of the liquid circulating ducting means, comprising the liquid, and the pressure from the pumps it is possible to control the pressure inside the apparatus and how long time the liquid and the gas shall be in the apparatus. The time spent in the apparatus and the pressure inside the apparatus are factors which influence how much gas dissolves in the liquid and if the liquid becomes saturated or even supersaturated. Depending on the amount of liquid which is pumped through the apparatus, the time in the apparatus will vary. Normally, the pump flow will be constant.

According to at least one exemplary embodiment said outlet is controlled by a liquid jet nozzle. It has an advantage that the energy from the liquid comprising the gas when flowing out of the outlet can be used when mixing it with the liquid in the receptacle, so that a desired dilution and mixture is created to get an effective transmittal of the gas with the liquid in the receptacle.

According to at least one exemplary embodiment said outlet is controlled by a valve. According to at least one exemplary embodiment said valve is a throttle valve. An advantage by using a throttle valve is that a throttle valve of the shelf can be used.

According to at least one exemplary embodiment said gas is supplied by gas providing device.

According to at least one exemplary embodiment said gas providing device is an air compressor or an oxygen generator. Depending on what kind of gas is supplied into the apparatus, i.e. air or oxygen, a suitable gas providing device shall be used. The gas providing device has to be

dimensioned to be able to produce a sufficient amount of compressed air with a correct pressure to keep the process running.

According to at least one exemplary embodiment said gas is air. Air is all around us, so it is always available.

According to at least one exemplary embodiment said gas is oxygen gas. By using oxygen it is easier to make fine adjustments. Thus, the accuracy of the amount of gas which shall be supplied may be higher when supplying oxygen instead of air into the system.

According to at least one exemplary embodiment a pressure vessel is arranged downstream of said second pump. By having a pressure vessel, where the water and the liquid can be hold for a certain amount of time, secure that contact between the liquid and the gas in the whole volume of liquid is established, so the liquid may become saturated or even

supersaturated. The pressure vessel may be a receptacle or a tank.

According to at least one exemplary embodiment said liquid circulating ducting means comprises an outlet means downstream of said second pump and upstream of said outlet which acts as a pressure vessel. This will also have the same effect as the pressure vessel described above, however without the need of a pressure vessel. An advantage of having a pressure vessel is that a shorter outlet means can be used.

According to at least one exemplary embodiment at said outlet a mixing device is arranged to blend said fluid comprising said gas with said fluid in said receptacle. By having a mixing device the liquid comprising the gas can be distributed over the whole tank and not only in one area of the tank. This makes the aerobic process more effective. Further it gives a desired dilution and mixture in order to get an effective transmittal of the gas to the liquid in the receptacle.

According to at least one exemplary embodiment said mixing device is at least one liquid ejector. A liquid ejector has the advantage that the energy from the liquid comprising the gas when flowing out of the outlet can be used when mixing it with the liquid in the receptacle, so that a desired dilution and mixture is created to get an effective transmittal of the gas to the liquid in the receptacle. Other possible mixing devices are propeller stirrer or mixing pump.

According to at least one exemplary embodiment an oxygen measuring device is arranged in said tank. The oxygen measuring device measures the amount of oxygen in the liquid so that the amount of added gas can be adjusted for example through a control system. The control system may be so designed that the oxygen concentration in the tank always exceeds an arbitrary value larger than 0, i.e. >0, for example 2 mg/litre. When the concentration is below this level more gas is supplied into the apparatus. The control system may also be so dimensioned that when the oxygen

concentration is higher than a second arbitrary value, for example 5mg/litre, the supply of gas is stopped or reduced. That is, when the oxygen degree of the liquid in the tank is decreasing, the gas supplied into the apparatus may be increased. When the oxygen degree of the liquid in the tank is increasing, the gas supplied in the apparatus may be decreased. This creates an effective and safe process. Safe in such a manner that when the amount of oxygen which corresponds to the used amount of oxygen is added, all oxygen which is provided into the system will be used by the aerobic process and no oxygen will leave the tank in the shape of air bubbles.

According to at least one exemplary embodiment a bypass pipe is arranged downstream said first pump and bypasses the gas supply and the second pump and connects to the liquid circulating ducting means

downstream the second pump and a first valve is arranged in the bypassing pipe and a second valve is arranged downstream the second pump, before the bypass pipe connects the liquid circulating ducting means . By having a bypass pipe, the gas supply and the second pump can be turned off by opening the bypass pipe using i.e. opening said first valve in the bypass pipe and closing the second valve after the second pump. The liquid will the only circulate in the liquid circulating ducting means, bypassing the gas supply and the second pump, and the circulated liquid can be used for mixing the liquid in the tank.

The invention also relates to a method for supplying gas to a liquid within a receptacle comprising the steps of withdrawing said liquid from said receptacle through an inlet with a first pump, providing said liquid with said gas downstream of said first pump, increasing the pressure of said withdrawn liquid comprising the gas further with a second pump, which is connected downstream of said supplied gas, returning said liquid comprising said gas to said receptacle through an outlet. The method has the same advantages as the apparatus described above. Further with the method it is possible to maintain a desired oxygen level or increasing the oxygen level in a liquid in a receptacle, for example a tank or a pond used in an aquatic industry such as fish farming or in a water treatment industry.

According to at least one exemplary embodiment said method further comprises the step of controlling said outlet with a valve or a liquid jet nozzle arranged close to said outlet so that pressure is further increased and that the contact time between the liquid and said gas is large enough so that said gas dissolves in said liquid before passing said valve or said liquid jet nozzle and returning said liquid comprising said gas to said receptacle. The liquid may becomes saturated or supersaturated. The controllable outlet maintains and increases the pressure together by the second pump. The outlet can also control how much liquid comprising the gas shall be let out into the tank. By reducing the amount of liquid and the gas which is coming out of the apparatus it is possible to control how long time the liquid and the gas shall be in the apparatus. The time spent in the apparatus together with the pressure inside the apparatus are factors which influences how much gas dissolves in the liquid and if the liquid becomes saturated or even

supersaturated. The valve or the nozzle may be arranged close to said outlet, upstream said outlet, for example at a distance which allow the fluid comprising the gas to flow 0-15 seconds after passing the jet liquid nozzle or the valve before entering through the outlet into the receptacle.

According to at least one exemplary embodiment said method further comprises the step feeding said liquid comprising said gas from said second pump to a pressure vessel, arranged downstream of said second pump before passing said outlet. By having a pressure vessel, where the water and the liquid can be hold for a certain amount of time, secure that contact between the liquid and the gas in the whole volume of liquid is established, so the liquid becomes saturated or even supersaturated.

According to at least one exemplary embodiment said method further comprises the step mixing said liquid comprising said gas with said liquid in said tank by using a mixing device. By having a mixing device the liquid comprising the gas can be distributed over the whole tank and not only in one area of the tank. This makes the aerobic process more effective.

All the parts mentioned in the method, such as gas, liquid, tank, inlet, outlet, pressure vessel etc. may have the same features and functions as described in regard to the apparatus above.

Said invention further relates to the use of an apparatus described above in a contaminating liquid treatment system. Such a system may for example be a biochemical waste water treatment system. As an alternative the apparatus could be used in aquatic industry such as fish farming.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, as well as from the drawings. Brief Description of the Drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Fig. 1 shows a schematic diagram of an embodiment of the present invention.

Fig. 2 shows a second embodiment of the present invention as a schematic diagram.

Detailed Description of Preferred Embodiments

An embodiment of the invention will be described in more detail in the following with reference to the accompanying drawings.

Fig. 1 shows a part of a contaminating liquid treatment system 1 , where an aerobic process is desired for treating the contaminated liquid. The liquid is here exemplified as biochemical waste water. The liquid may however be any kind of contaminated liquid which can be treated in an aerobic process. The liquid to be treated may for example be municipal waste water, or excess water from industrial processes i.e. oil industry, general industry, slaughter houses, etc.

The part of the system described is not limited to be part of a larger system, it may be a separate system. This part of the waste water treatment system 1 comprises a receptacle 4 which is exemplified as a tank 4 for liquid 3, which shall be treated. The receptacle 4 is not limited to a tank, may for example be a lake or a pond. The liquid 4 is biochemical waste water containing biological degradable substances and will be referred to hereinafter as the liquid. The tank 4 has an atmospheric pressure. It has a liquid inlet means 17 for supplying the liquid 3 into the tank 4. The liquid inlet means 17 may be a pipe, a tube or any kind of channel. It also has a liquid outlet means 18 to be able to remove the processed liquid. To enable the function of the aerobic process it is required that free oxygen is available in the water. New liquid is continuously provided into the tank 4 through the liquid inlet means 17 and processed liquid is continuously removed from the tank 4 through the liquid outlet means 18.

This part of the waste water treatment system 1 further comprises an apparatus 2 for supplying gas to the liquid 3 within the tank 4 in order to establish the free oxygen in the water in order to be able to carry out the aerobic process. The gas may either be air or oxygen.

The apparatus 2 comprises a liquid circulating ducting means 20 comprising an inlet 8 and an outlet 14. The liquid circulating ducting means 20 withdraws and circulates the liquid 3 from the tank 4 continuously (could however also be done in batches), oxygenating it by adding air or oxygen to the liquid and letting the air or oxygen dissolve in the liquid. The liquid may be saturated or supersaturated before it is returned to the tank 4. The liquid will then be distributed over the whole tank.

The apparatus 2 is arranged outside the tank 4, except for the parts of the liquid circulating ducting means 20 comprising the inlet 8 and the outlet 14. When the liquid and gas mix 3 ^' returns to the tank 4 it mixes with the rest of the liquid 3 in the tank 4 so that the oxygen is transmitted to the liquid 3 in the tank 4. The liquid 3 in the tank 4 may then be saturated with oxygen to a degree which is equal to or less than the maximum oxygen saturation degree. In the aerobic process the oxygen in the liquid 3 supplied into the tank by the apparatus 2 will be used. By providing an amount of oxygen which

corresponds to the used amount of oxygen consumed by the aerobic process, all oxygen which is provided into the system will be used by the aerobic process and no oxygen will leave the tank in the shape of air bubbles.

The apparatus 2, withdraws the liquid 3 from the tank 4 through an inlet means 7 with a first pump 6 arranged to the liquid circulating ducting means 20 downstream of the inlet 8. The inlet means 7 may be a tube or a pipe arranged into or on the tank so its inlet 8 is submerged in the liquid body in the tank 4. The inlet means 7 is arranged so that its inlet 8 is arranged at the tank 4 on one side and at half the depth of the tank 4. However, the inlet means 7 and its inlet 8 is not limited to this. The inlet 8 may be arranged into the tank at any depth and in any direction. It is further not limited to be on the side of the tank 4. The inlet means 7 may protrude into the tank 4 so that the inlet 8 is inside the tank 4 instead at one side of the tank 4. The inlet 8 can be arranged where it is most suitable, depending on the volume and shape of the receptacle.

The apparatus 2 further comprises a second pump 9, which is connected in series with said first pump 6, i.e. downstream of said fist pump 6, in order to increase the pressure of said withdrawn liquid 3. The first pump 6 may pressurize the liquid 3 with half of the desired end pressure. Half of the end pressure may for example be 2-3 bar. The liquid 3 is then fed to the second pump 9, which increases the pressure of the liquid, in this case with the second half of the desired end pressure, i.e. 2-3 bar. The end pressure will hence be 4-6 bar. The two pumps 6, 9 are connected to each other via a connection device 10, which is here exemplified as a pipe or a tube. The first pump 6 should retain a flow and sufficient pressure so that the second pump 9 has enough of liquid so that no cavitations occur. The two pumps may be of the same kind or be two different kinds. For example centrifugal pumps or displacement pumps can be used. It is also possible to use pumps with different pump curves.

Gas is supplied to the withdrawn liquid 3 between the first pump 6 and the second pump 9. That is, the gas is supplied into the connection device 10 between the two pumps. The gas, which is air or oxygen, is supplied by a gas supply device 1 1 through a pressure pipe 12. The pressure pipe 12 may comprise a pressure regulator and a valve (not shown) which together with the gas supply device 1 1 can supply the liquid 3 with a desired amount of oxygen or air and thereby oxygen. The gas pressure should be higher that the pressure created by the first pump 6. The gas supply device 1 1 may be an air compressor, an air pump or any other suitable device for supplying air. The gas supply device 1 1 then has to be dimensioned to be able to produce a sufficient amount of compressed air with a correct pressure to keep the process running. If oxygen shall be supplied into the system then an oxygen gas pressure vessel or oxygen generator may be used. The second pump 9 also has the advantage that it mixes the gas with the liquid efficiently.

After the second pump 9, i.e. downstream of the second pump 9, is a pressure vessel 16 arranged. The second pump 9 and the pressure vessel 16 are connected by a pipe or a tube 17. The pressure vessel 16 is further connected to a discharge outlet means 13, which returns said liquid

comprising said gas 3 ^' , into the tank 4 through the outlet 14. Thus, the discharge outlet means 13 is arranged downstream of the pressure vessel 16. The discharge outlet means 13 may be a tube or a pipe. As an alternative, the discharge outlet means 13 may act as the pressure vessel 16 if its volume is large enough. The discharge outlet means 13 will then be directly connected to the second pump 9, i.e. downstream of the second pump 9. At the end of the discharge outlet means 13 is a control device 15 here exemplified as jet mixing nozzle, i.e. a liquid jet nozzle 15 arranged, which restricts the liquid flow out of the apparatus. The nozzle may be arranged close to said outlet, upstream said outlet, for example at a distance which allow the fluid

comprising the gas to flow 0-15 seconds after passing the jet liquid nozzle before entering through the outlet into the receptacle. Instead of a liquid jet nozzle a valve may be used. The valve may be a throttle valve. Depending on how much of the liquid the jet mixing nozzle 15 or the valve is letting back into the tank 4, together with the volume of the apparatus comprising the liquid and the pressure of the pumps the pressure inside the apparatus and time of the liquid comprising the gas spent inside the apparatus can be controlled. By keeping the pressure in the apparatus for a certain time, for example a few seconds, the gas will dissolve in the liquid until it is saturated for that pressure. As an example, the saturation for oxygen in water at 15 degree Celsius and at l atmosphric pressure is ca. 10mg/litre and by 4 bar pressure, hold for ca 30 seconds, the saturation degree increases to 40mg/litre.

Preferably the liquid circulating ducting means 20 is so designed that its volume is large enough so that the liquid and the gas is kept in the liquid circulating ducting means 20 so long that the gas is dissolved under pressure so that the liquid may gets saturated.

To increase the efficiency even further the liquid can be supersaturated by applying an overpressure. The time spent inside the apparatus is to allow that the contact time between the gas and the liquid is enough so that all liquid 3 which passes into the apparatus 2 may become saturated or supersaturated with oxygen and that the oxygen is distributed over the whole volume. The overpressure, which may be created by the pumps 6, 9, will be released when the liquid comprising the gas 3 ^' passes the liquid jet nozzle or the valve and returns into the tank 4.

Further, an oxygen measuring device (not shown) may be arranged in the tank 4 in order to measure the amount of oxygen in the liquid 3 so that the amount of added gas can be adjusted for example through a control system. The control system may be so dimensioned that the oxygen concentration in the tank always exceeds an arbitrary value larger than 0, i.e. >0, for example 2mg/litre. When the concentration is below this level more gas is supplied into the apparatus 2. The control system may also be so dimensioned that when the oxygen concentration is higher than a second arbitrary value, for example 5mg/litre, the supply of gas is stopped or reduced. That is, when the oxygen degree of the liquid 3 in the tank 4 is decreasing, the gas supplied into the apparatus 2 may be increased. When the oxygen degree of the liquid 3 in the tank 4 is increasing, the gas supplied in the apparatus 2 may be decreased. This creates an effective and safe process. Safe in such a manner that when the amount of oxygen which corresponds to the used amount of oxygen is added, all oxygen which is provided into the system may be used by the aerobic process and no oxygen will leave the tank in the shape of air bubbles. This is however dependent on if the amount of gas which is supplied into the liquid 3 is tuned against the volume flow and the pressure within the apparatus 2. If the second arbitrary value is lower that the degree for saturation in the tank at a certain temperature, no oxygen will depart to the atmosphere, and the supplied oxygen will be distributed in the liquid 3 in the tank 4 through diffusion and be absorbed as soluble oxygen until it is consumed by the aerobic process.

A mixing device 19 is arranged near the outlet 14 on the discharge outlet means 13 in order to get an effective mixing of the liquid comprising the gas 3 ^' with the liquid 3 in the tank 4. The mixing device 19 is exemplified as a liquid ejector, and the liquid ejector is arranged upstream of the outlet 14 and down streams the pressure vessel 16. However, other mixing devices such as a propeller stirrer, mixing pump or other suitable devices may be used and arranged at the same position as the liquid ejector or close to the outlet 14. It may be arranged to the discharge outlet means 13 or arranged separately from the apparatus 2 in the tank 4. The mixing device 19 also prevents that the oxygen dissolved in the liquid, which may be saturated or oversatu rated, does not become air bubbles when the liquid comprises the gas 3 ^' returns into the tank 4. This is done by mixing the liquid comprising the gas 3 ^' at high intensity with the liquid in the tank, which has a low level of oxygen. The excess oxygen from the liquid comprising the gas 3 ^' will be used/absorbed by the liquid in the tank, since it wants to become saturated. The mixing device 19 is preferably used when a valve is used instead of the liquid jet nozzle 15 for controlling the amount of liquid returning back to the tank. The liquid jet nozzle 15, which is a liquid jet mixing nozzle, has an advantage that it alone may mix the liquid comprising the gas 3 ^' with the liquid 3 in the tank 4 and mixing device 19 then might not be necessary.

Further, the tank 4 may be provided with pipes or walls so that the liquid 3 in the tank 4 is guided to circulate through or past the mixing area created by the mixing device 19.

Thus, the apparatus 2 comprises a first pump 6 arranged downstream of said inlet 8 in said liquid circulating ducting means 20 withdrawing said liquid from said receptacle through said inlet 8, wherein gas is supplied into said liquid 3 downstream of said first pump 6, a second pump 9, which is connected downstream of said supplied gas in order to increase the pressure of said withdrawn liquid and mix said gas with said liquid 3. The liquid comprising the gas 3 ^' then is supplied to some kind of pressure vessel 16, arranged downstream of the second pump 9 where the liquid is hold for a certain amount of time to ensure saturation, if that is desired, before the liquid is returned to the tank through the outlet 14. The liquid may become saturated or even supersaturated.

The outlet 14 is arranged in the middle of the tank 4. Preferably in the lower part of the tank 4. However, it is not limited to this, the outlet 14 may be arranged where it is at most suitable. The apparatus 2 is not limited to have only one outlet 14, it may have more than two. Each outlet 14 may have their own valve 15 or liquid jet nozzle and their own mixing device 19 or a common one.

The process which occurs in the apparatus 2 is the following: the tank

4 is filled with liquid 3, here exemplified as biochemical waste water, which shall be treated. The apparatus 2 pumps up a part of the liquid 3 from the tank 4 with the aid of the first pump 6 through the inlet 8 of the inlet means 7. The first pump 6 creates hence a pressure on the liquid. To the liquid 3 gas is supplied by the gas supply device 1 1 downstream the first pump 6. The second pump 9, which is arranged downstream of the gas supply device 1 1 , increases the water pressure further, which has the effect that the gas is better mixed with the liquid 3 and the total amount of gas to be solved raises.

The pressure is retained until the liquid comprising the gas 3 ^' returns to the tank 4 by the liquid jet nozzle or valve 15. The liquid comprising the gas 3 ^' is fed to the pressure vessel 16, which is arranged downstream of the second pump 9 and to the discharge outlet means 13 or to the discharge outlet means 13 alone if it acts as a pressure vessel. By the liquid jet nozzle 15 at the end of the discharge outlet means 13 the liquid comprising the gas 3 ^' is maintained in the apparatus 2 for a sufficient amount of time so that the contact time between the gas and the liquid is large enough so that is assured, if desired, that the liquid is saturated or supersaturated by the oxygen. An overpressure of the liquid 3 is created in the apparatus 2 and it is released when passing the nozzle 15 into the tank 4. The energy created when releasing the liquid comprising the gas 3 ^' into the tank may be used to mix the liquid comprising the gas 3 ^' with the liquid 3 in the tank 4. The liquid ejectors 19 help mixing the liquid comprising the gas 3 ^' with the liquid in the tank. When the liquid comprising the gas 3 ^' is released into the liquid 3 in the tank 4 it is saturated with oxygen at the present pressure in the tank 4 and all possible supersaturating of oxygen in the liquid-gas mix 3 ^' will be released as small microscopically bubbles. When these bubbles come in contact with the liquid 3 in the tank 4 the oxygen will be taken up by this liquid 3, which oxygen saturation degree is bellow the degree of saturation at the present pressure. During the process in the tank 4 the oxygen will continuously be used, hence the oxygen saturation degree will decrease over time. By adding a degree of oxygen which corresponds to the used oxygen, all oxygen supplied into the system is used by the process. Thus, no oxygen will leave the system as air bubbles. This supply of oxygen can be controlled by the oxygen measuring device (not shown) mentioned above and it may communicate with the gas providing device 1 1 so the right amount of gas is supplied into the liquid 3 in the apparatus 2.

Fig. 2 shows the tank 4 and the same apparatus 2 as in fig. 1 .

However, a bypass pipe 21 and two valves 22, 23 are arranged to the liquid circulating ducting means 20. The bypass pipe 21 is arranged after, i.e.

downstream of, the first pump 6, and bypasses the gas supply and the second pump 9 and connects to the liquid circulating ducting means 20 downstream the second pump 9. The first valve 22 is arranged in the bypassing pipe 21 . The second valve 23 after the second pump, i.e.

downstream the second pump 9, however, before the bypass pipe 21 connects the liquid circulating ducting means 20 again. By having a bypass pipe 21 , the gas supply and the second pump 9 can be turned off by opening the bypass pipe 21 using, i.e. open said first valve 22 in the bypass pipe 21 and closing the valve 23 after the second pump 9. The liquid 3 will the only circulate in the liquid circulating ducting means 20, bypassing the gas supply and the second pump, and the circulated liquid 3 can be used for mixing the liquid 3 in the tank 4.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.