The invention relates to a method for reducing the carbon dioxide concentration and temperature regulating water temperature in a body of water wherein fine bubbles of a stripping gas are introduced into the water. Water in this description means both freshwater (0 % salt) and seawater (up to 3,5 % salt).

The invention further relates to a recirculation aquaculture system comprising a fish tank and/or a biological filter tank wherein said fish tank and/or said biological filter are filled with water and further comprising an injection device for

introducing a stripping gas into said fish tank and/or into said biological filter.

Recirculation aquaculture systems are becoming more and more popular since they provide a predictable and constant environment for growing aquaculture such as fish.

A recirculation aquaculture system is an essentially closed system comprising one or more fish tanks and filtration and/or water treatment systems. The fish are housed within tanks and the water is cleaned continuously to guarantee optimum growing conditions. Water is cleaned through biological and mechanical filtration systems.

As a consequence of the extensive respiration of fish and bacteria in recirculation aquaculture systems, the C0 ₂ concentration in the water will increase if it is not reduced at an equal rate. An increased C0 ₂ concentration in the water has negative effects on the fish growth rate and might even increase fish mortality.

Therefore, in intensive aquaculture it is necessary to reduce the C0 ₂ concentration in the water. It is known to reduce the C0 ₂ concentration in water by forced aeration. Air is inj ected into the water in a manner which forms discrete and small bubbles and the air acts to desorbs carbon dioxide from the water which is then expelled as an off-gas. This method is known as aeration or stripping of C0 ₂ .

The effectiveness of the aeration process depends on the area of the air-water interface and the contact time. For this reason the air is normally introduced into the water such that the rising bubbles remain for a time as long as possible in the water. At a certain water depth there is an increased pressure compared to ambient pressure. Thus, the total pressure inside the air bubbles injected into the water at that depth as well as the partial pressure of the inert gases nitrogen and argon present in the air is also increased. These gases dissolve in the water body and create an inert gas over saturation. As a consequence the fish may suffer from a kind of aeroembolism, also called gas bubble disease, causing reduced fish growth or even increased mortality.

Therefore, it is an object of the invention to provide a method for reducing the carbon dioxide concentration in a body of water without increasing the inert gas concentration at the same time. It is in particular an object of the invention to provide a method for reducing the carbon dioxide concentration in a tank of a recirculation aquaculture system. The invention shall further provide an improved method for raising aquaculture in recirculation systems.

This object is achieved by a method for reducing the carbon dioxide concentration in a body of water wherein fine bubbles of a stripping gas are introduced into the water in order to desorbs carbon dioxide from the water, which is characterized in that said stripping gas is introduced into the body of water at a depth which is less than half of the water depth, preferably at a depth which is less than one third of the water depth.

The inventive recirculation aquaculture system may comprise at least one fish tank and / or a biological filter wherein said at least one fish tank and/or said biological filter are filled with water and further comprising an injection device for

introducing a stripping gas into said fish tank and/or into said biological filter is characterized in that said injection device is arranged at a depth which is less than half of the water depth, preferably at a depth which is less than one third of the water depth.

The term "water depth" shall mean the measurement of the depth of a body of water from the surface to the bottom. According to the invention the stripping gas is introduced into the water in the upper half of the water body, which is closer to the water surface than to the bottom of the water. Preferably the stripping gas is introduced in the upper third of the water body.

At a water depth of 1 m the hydrostatic pressure is about 0, 1 bar. When introducing air at that water depth the partial pressure of nitrogen or argon inside the introduced air bubbles is about 1, 1 bar. At this pressure the dissolution of nitrogen or argon in the surrounding water is only slightly increased and the risk of oversaturation of the water with these inert gases is still low. The partial pressure of nitrogen and argon which are components of air decreases when the air is introduced into the water at lower depths since the air can be introduced at low pressure values. Therefore, from an oversaturation point of view the stripping gas should be introduced into the water as close to the water surface as possible. However, the efficiency of the stripping process itself, that is the removal of C0 ₂ from the water, increases with increasing gas-water contact time which will be longer when the stripping gas is introduced at greater depths. It turned out that at a water depth of less than 1 meter, preferably between 0,4 m and 0,8 m, a good compromise between these two extremes can be achieved. Therefore, the stripping gas is preferably supplied to the water at a distance less than 1 m, preferably between 0, 4 m and 0,8 m below the water surface, more preferred between 0, 5 m and 0.7 m below the water surface.

The minimum pressure to introduce the stripping gas into the water is given by the ambient pressure plus the hydrostatic pressure of the water above the inj ection point. In order to limit the dissolution of nitrogen, argon or any other undesired gases in the water the stripping gas is preferably introduced into the water at a pressure which is only slightly above the minimum pressure as defined above. It is preferred to inject the stripping gas at a pressure which is no more than 0,005 bar, preferably no more than 0,002 bar above that minimum pressure.

The preferred stripping gas is air. By using air as stripping gas in a fish tank an additional advantage is achieved. The oxygen present in the air will be dissolved in the water and causes an oxygenation of the water which will improve the living conditions of the aquaculture. It is also possible to use another gas, for example oxygen enriched air, that is air with an oxygen content of more than 21 vol-% or a mixture of nitrogen and oxygen. In that case the oxygenation of the water is even more improved.

The invention is especially useful to reduce the carbon dioxide concentration in a water tank, especially in a water tank of a recirculation aquaculture farming system.

The term "water tank" shall mean any kind of tank, container, vessel or reservoir, in particular a man-made or artificial tank, vessel or reservoir, which is used for storing or receiving water. The invention can also be used to reduce the carbon dioxide concentration in a natural body of water, such as a pond, lake or even in a separated part of the sea. According to a preferred embodiment the invention is used for reducing the carbon dioxide concentration in tanks, especially man-made tanks, for aquaculture, especially in fish tanks, in water treatment tanks or in a biological filter tank.

The term "aquaculture" shall mean any kind of aquatic animals or aquatic species, including, but not limited to, fish, both in fresh- and seawater, crustaceans, such as crabs, shrimps or lobster and kelp.

The inventive method is in particular useful for recirculation aquaculture or fish farming systems, both for sea water and fresh water aquaculture, for on-shore fish farming and closed farming units placed in seawater. Such recirculation systems are closed systems where a substantial portion of the water is circulated and reused in operation of the system. Recirculation systems typically comprise one or more fish tanks and one or more water treatment tanks. The term "fish tank" shall mean any tank in which fish or any other aquaculture are raised. Normally the water within the fish tank is continuously cleaned to keep optimum growing conditions for the aquaculture. The water is cleaned through the water treatment tanks, for example to a mechanical filtration system and/or to a biological filter system.

A biological filter comprises a medium with a large surface area upon which bacteria will colonise. A biological filter shall mean a system for destroying or converting waste products which could be harmful to the aquaculture into non- harmful products by means of bacteria and/or microbes. The term biological filter shall in particular comprise a tank with a packed bed, a tank with disordered or ordered packing, for example a honeycomb structure or channels with honeycomb cross section, which are inhabitated by bacteria or microorganisms.

The water quality in the recirculation system, in particular in the fish tank(s) and in the biological filter(s), should be kept under optimum conditions in order to guarantee optimum living conditions and a controlled environment for the aquaculture as well as for the bacteria inhabitating the biological filter. According to a preferred embodiment the water of the fish tank(s) of a recirculation aquaculture system is treated according to the invention. The C0 ₂ concentration in the fish tank is preferably reduced by the inventive method while the aquaculture is present in the fish tank. It is not necessary to first transfer the water without the fish into an additional tank and then strip off undesired C0 ₂ from the water in the additional tank. The invention provides a system for C0 ₂ reduction integrated into the fish tank or into the biological filter. The fish can stay in the fish tank while the C0 ₂ is stripped off in the inventive manner. Accordingly, when C0 ₂ shall be removed from a biological filter the large surface medium inhabitated by bacteria can remain in the tank. For moving the water through the stripping area, only air is used for pumping the water.

A stripping gas, in particular air, is injected into the fish tank in form of fine bubbles. The air bubbles act to strip carbon dioxide from the water as they rise to the surface of the water body and allow the carbon dioxide to escape to the atmosphere or to withdraw the carbon dioxide. The stripping air is injected into the water at a depth of less than 1 meter at a pressure which is only slightly above the ambient pressure plus the hydrostatic pressure at the depth of inj ection. Thus, the air pressure and likewise the partial pressure of nitrogen and argon present in the air bubbles are as low as possible and undesirable dissolution of nitrogen and argon into the water is avoided or at least reduced to a minimum.

Instead of letting escape the stripped carbon dioxide to the atmosphere it is also possible to provide a gas collector above the area into which the stripping gas is introduced and to collect and for example recycle the carbon dioxide. By recycling the air used in stripping, and by controlling the air temperature it is possible to control the water temperature. By controlling temperature of the air, the water temperature is controlled up and down. The air can then be heated or chilled. New air (fresh), taken into the system, is preferably heat exchanged with the used air leaving the system.

In accordance with another preferred embodiment the inventive method is used to strip off carbon dioxide from the biological filter(s) of a recirculation aquaculture system. The water is recirculated between the fish tank or fish tanks and the biological filter(s). Therefore, it is desirable in addition to or instead of stripping off C0 ₂ from the water of the fish tank(s) to reduce the C0 ₂ concentration in the biological filter. This is preferably achieved in the same manner as described above with reference to stripping off C0 ₂ from the fish tank. Thereby the living conditions for aquaculture and bacteria or microbes are essentially improved. According to another embodiment the stripping gas, especially air, is introduced into the water by means of an injection device comprising pipes with a large number of stripping gas outlets, for example porous pipes, preferably arranged as a grid of pipes or arranged as several rings of pipes. The stripping gas is preferably injected into the water in such a manner that a large number of fine gas bubbles are formed in order to get the interaction surface between the water and the stripping gas as large as possible. This is preferably achieved by placing porous pipes, especially a grid of porous pipes, into the water and passing the stripping gas through these pipes into the water. Instead of pipes it is also possible to use tubes, especially perforated tubes.

The aerated space or the aerated volume shall mean the water volume into which the stripping gas is introduced. The aerated volume is between 1% and 20 %, preferably between 3% and 10 %, of the whole water body of the tank. In particular, the cross section parallel to the water surface through which stripping gas bubbles pass shall be between 5% and 85 %, preferably between 8% and 70%, of the surface of the tank where it is located, for example of the fish tank or of the biological filter. For example, when using a grid of porous pipes or perforated flexible tubes as injection device, the area which is covered by the pipes or tubes, that is the area which is delimited by the outermost pipes or tubes, is preferably within the above given limits.

In accordance with another preferred embodiment the aerated volume covers an area of regular cross section, especially of circular, rectangular, quadratic, hexagonal or octagonal cross section or cross over the tank.

The whole stripping gas is preferably introduced into the water at the same water depth and pressure, such that all stripping gas bubbles more or less rise the same distance.

The inj ection device is preferably designed to create a forced water flow in the tank. Therefore, it is preferred to provide side walls extending into the body of water defining an aerated volume between said side walls and to introduce the stripping gas into said aerated volume. The side walls thus form an outer cover around the injection device and around the aerated volume. The side walls are preferably arranged in an essentially vertical orientation such that all side walls delimit a water volume in the horizontal. Preferably the side walls extend at least to the water depth where the stripping gas is injected. In a biofilter it is preferred that the side walls extend down towards the bottom. It is advantageous to have a bottom in aerated volume of tank and biofilter. The stripping gas injected into the water within the water volume delimited by the side walls, that is within the aerated volume, rises up and causes an upward flow of water by the airlift pump principle. In this airlift pump, the velocity of the water is between 5 and 25 cm/sec, typically 10-15 cm/sec. This low velocity minimize the risk for taking particles into the stripping volume. The water is only pumped by the airlift pump, that means no rotating part in the water, and thereby no corrosion, little maintenance and a very secure and lowrisk system. The rising water will leave the water volume in a horizontal direction when having reached the water surface. Thereby, a forced water flow within the tank or in general within the body of water is achieved. That forced flow will be upwards in the open area of the bottom, in different directions inside the side walls and horizontal at the exit of the aerated volume. The water will be within the degassing volume for 10 to 60 seconds, typically 30 to 40 seconds. This residual time is very important for the efficiency of the gas stripping. When leaving the aerated volume, the water makes a flow in the direction of the water flow in the tank. The flow from the aerated volume has a speed between 10 and 100 cm/sec, typically 40-60 cm/sec, so that bubbles noth are forced to deep in the tank. This water circulation forces the primary water flow (circulation around in horisontal direction when looking down into the tank) and thereby also the secundary water flow (circulation around in vertical direction looking into the tank from the side). The secundary flow is essential for transporting particles in an efficient way to the drain in the middle of the tank.

Since the stripping gas creates a forced water circulation as described above, it is preferred to locate the gas inj ection device close to the mid of the tank, that is the surface of the aerated space covers a segment of the water surface around the mid of the tank. Preferably, in a horizontal plane the mid of the aerated space is close to the mid of the tank. Thereby, a maximum of water will be circulated within the tank, making efficient primary and secundary waterflow in the tank and this in second hand making stable and optimum conditions for the aquaculture. The side walls preferably extend to the water surface level or to a level above the water surface. The rising water within the side walls or within the aerated volume can either flow over the wall or leave the aerated volume through special openings or water outlets in the side walls as described above. According to a preferred embodiment these openings do not extend to a water depth of more than 0, 6 m. Preferably there are between two and ten such openings regularly arranged around the circumference of the side walls. For example, in case there are eight side walls defining a octangolar aerated volume, every second side wall can have one such opening.

The side walls can be provided with a base or a floor such that the side walls together with the base or floor form a cavity. In that case the floor is provided with opening to allow water to enter the cavity. The opening of the floor can be provided with a screen, for example a tarpaulin, which allows water to go through and can have an aperture width small enough that fish could not enter the water volume delimited by the side walls and the floor.

In accordance with another embodiment the side walls can be provided with a trough or channel along at least a part of its inner circumference, preferably along its whole inner circumference. That trough or channel is arranged below but close to the water surface. As described above the rising water within the aerated volume will pass the side walls in form of an horizontally directed flow close to the water surface. Thus, that water flow will also pass the trough or channel and foam which has been produced during the aeration process or particles which have been lifted up by the air bubbles are collected in that trough or channel and thereby skimmed off the water. The foam and particles can be withdrawn by a separate drain.

The present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: figure 1 schematically shows a fish tank with a C0 ₂ stripping device according to the present invention,

figure 2 a top view of the fish tank according to figure 1,

figure 3 shows a detail A of figure 1,

figure 4 schematically shows a biological filter with a C0 ₂ stripping device

according to the invention,

figure 5 shows a top view of the biological filter according to figure 5 and

Figures 1 and 2 shows a fish tank 1 of an on-shore recirculation fish farming system. The fish tank 1 has a circular cross section with a diameter L of about 10 m. The water depth D is about 5 m. The fish tank 1 is used to grow up fish, such as salmonid, cod or halibut. The fish tank 1 includes a C0 ₂ stripping device for reducing the C0 ₂ concentration in the water. The C0 ₂ stripping device comprises side walls 3 which define an octagonal water body with a diameter of for instance 5 m and a depth of for instance 0,60m the bottom of which can be closed by a screen 2. Thus, fish will not be able to enter the body of water above the screen 2. Within body of water delimited by the side walls 3 and the screen 2 a grid of porous pipes 4 immerses in the water to a depth d of for instance 0, 60 m. The porous pipes 4 are connected to an air supply line 5 for providing pressurised air. The side walls 3 immerse in the water to a depth of 0,60m. The upper ends 6 of the side walls 3 extend to a height about 0,30m above the water surface 7.

There are provided two water outlets 8 equally distributed around the circumference of the octagonal side walls 3. The water outlets 8 are arranged with its upper ends close to the water surface. The height of the water outlets 8 in a vertical direction, that is in the direction of the water depth D, is about 0,20m and their extension in the horizontal is about 2 m.

Through the air supply line 5 pressurised air at a pressure of about 1,06 bar is provided to the grid of porous pipes 4. The air is introduced into the water in the form of a large number of fine bubbles which rise up through the water. The air pressure is only slightly above the sum of atmospheric pressure plus hydrostatic pressure at a water depth of 0,60m. Thus the amount of nitrogen or argon which will be dissolved in the water is reduced to a minimum.

The rising air bubbles form an aerated volume 9. Within the aerated volume 9 the undesirable carbon dioxide is desorbed from the water and adsorbed by the rising air bubbles 10. The air bubbles together with the stripped carbon dioxide leaving the aerated volume 9, are removed via an outlet 1 1 and may be directed to a treatment unit, for example a gas scrubber, a heat exchanger or directly released to the atmosphere. A part of the used air is possible to reuse without treatment.

The air introduced into the water via the porous pipes 4 causes an upward flow 12 of water in the middle of the aeration area. The air supply is divided in two parts, one for the area in the middle and one for the rest. By controlling the air supply to the area in the middle, it is possible to control the amount of water running through the C0 ₂ stripping device and thereby the water circulation and time the water stays inside the aeration volume. The rising water leaves the aerated volume 9 via the water outlets 8. Thus, a water circulation 13 within the body of water is caused. The outlets 8 are designed with a bottom, so that bubbles in the water leaving the aerated volume 9, not are forced down in the water tank. The control of water amount running through the C0 ₂ stripping device is also essential for not forcing bubbles down in the tank after leaving the outlets.

Figure 3 shows detail A of figure 1. A trough 14 is provided along the inner circumference of the side walls 3 located directly above the water outlet 8. The air bubbles rising up in the aeration volume 9 produce foam 15 which accumulates at the surface of the water. Excess foam enters the trough 14 and can be withdrawn by means of a separate drain, not shown in the figure.

Figure 4 and 5 shows a biological filter tank 30 for recirculation in a fish farming system. The diameter L of the tank is for instance 6 meter and the water depth D for instance 5 meter.

The biological filter tank 30 includes a C0 ₂ stripping device for reducing the C0 ₂ concentration in the water and for making water circulation in the biological filter 41, 42. The C0 ₂ stripping device comprises side walls 31 which define a octagonal water body with a diameter of for instance 3 m and a depth of for instance 4,5 m. Within body of water delimited by the side walls 3 1 a grid of porous pipes 32 immerses in the water to a depth d of for instance 0,60m. The porous pipes 32 are connected to an air supply line 33 for providing pressurised air. The upper ends 34 of the side walls 31 extend to a height about 30 cm above the water surface 35.

There are provided four water outlets 36 equally distributed around the

circumference of the octagonal side walls 3 1. The water outlets 36 are arranged with its upper ends over the water surface. The height of the water outlets 36 in a vertical direction, that is in the direction of the water depth D, is for instance 0,30m and their extension in the horizontal is for instance 1,20m. The water outlets have a bottom for holding the water/biological medium near the surface when leaving the aerated volume. This is essential for not forcing air bubbles down in the tank.

Through the air supply line 33 pressurised air at a pressure of about 1,06 bar is provided to the grid of porous pipes 32. The air is introduced into the water in the form of a large number of fine bubbles which rise up through the water. The air pressure is only slightly above the sum of atmospheric pressure plus hydrostatic pressure at a water depth of 0,60m. Thus the amount of nitrogen or argon which will be dissolved in the water is reduced to a minimum.

The rising air bubbles form an aerated and rising water volume 37. Within the aerated volume 37 the undesirable carbon dioxide is desorbed from the water and adsorbed by the rising air bubbles. The air bubbles together with the stripped carbon dioxide leaving the aerated volume, are removed via an outlet 39 and may be directed to a treatment unit, for example a gas scrubber, a heat exchanger or directly released to the atmosphere. A part of the used air is possible to reuse without treatment.

The air introduced into the water via the porous pipes 32 causes an upward flow 40 of water/biological medium within the aerated volume 40. The rising water leaves the aerated volume via the water outlets 36. Thus, a water circulation 41 in the horizontal direction within the body of water is caused. The water and biological medium is also going in a circulation in vertical direction mowing the

water/biological medium down to the bottom of the biological filter, under the side wall 31 and up through the aeration volume 42.