NO345941B1

NO345941B1 - A method and an arrangement to provide oxygen rich water into an upper part of a fish pen

Info

Publication number: NO345941B1
Application number: NO20200150A
Authority: NO
Inventors: Nils Almvik; Rune Stangvik
Original assignee: Vard Aqua Sunndal As
Priority date: 2020-02-05
Filing date: 2020-02-05
Publication date: 2021-11-08
Also published as: NO20200150A1; WO2021158122A1

Description

A METHOD AND AN ARRANGEMENT TO PROVIDE OXYGEN RICH WATER INTO AN UPPER

PART OF A FISH PEN

Technical Field

[0001] The present invention relates to a method and an arrangement to provide oxygen rich water into an upper part of a fish pen.

Background Art

[0002] In aquaculture and especially in fish breeding it is of utmost importance to provide oxygen rich water for the species. If the oxygen concentration falls below a certain saturation, which for salmon is in the magnitude of 40-50 %, the appetite of the fish will be reduced, the risk of diseases will increase, and the mortality will increase.

[0003] The oxygen concentration will vary across the seasons and is highly depending on the water temperature. The warmer the water is, the less oxygen the water is capable of dissolving. Unfortunately, when the temperature is at its highest, the activity of parasites, such as salmon louse (Lepeophtheirus salmonis) and algae, is at its highest.

[0004] A particular challenge is therefore to keep the oxygen concentration up during louse prevention. When there is a high number of lice in the water, it is desirable to prevent the lice from entering the pen. A common method of achieving this is to close off the upper few meters of the pen by a nonpermeable of party permeable skirt. This is efficient in preventing the lice, which lives in the upper layer of the sea, from entering the pen. However, the skirt will also prevent the natural exchange of water between the inside and the outside of the pen. Hence, the oxygen concentration in the water of the pen will sink, especially in the upper part of the pen, where, incidentally, the salmon also naturally will be found.

[0005] A measure to counteract a low oxygen concentration in the pen is to pump water from a certain depth below the pen up and into the upper part of the pen. This more oxygen rich water mixes with the oxygen poor water in the upper part of the pen. However, this pumping requires high power pumps and a substantial amount of energy. Moreover, the colder pumped up water, if not mixed properly with the warmer upper water, will have a tendency to sink back down to a lower level.

[0006] In land based closed pens for smolt it is common to provide oxygen through hoses with small apertures arranged at the bottom of the pen. This method has, however, not proven to be efficient in open sea-based pens.

[0007] NO342827 shows an arrangement where a ring, having holes for air to escape, is arranged in a pen at a depth of more than 20 meters below the surface. Air is forced through the holes in the ring and forms a bubble curtain that rises to the surface.

[0008] NO343181 shows an arrangement where air under pressure is supplied to the lower part of the pen; in the vicinity of the bottom ring. The supply of air is through one or more perforated tubes, hoses or mats.

[0009] JP H0787919 shows an arrangement where oxygen is supplied to the bottom of a pen. The oxygen is supposed to create an upwelling in the pen.

[0010] WO 2016/050374 shows an arrangement where both oxygen and air are supplied to the same ejector. The ejector is arranged within a tube in the middle of the pen.

[0011] Although, the above prior art solutions may increase the oxygen concentration in a t least some of the water in the pen, there is a continuous desire to do this more efficiently, and to increase the oxygen level both to a higher concentration and more evenly throughout the upper part of the pen.

Summary of invention

[0012] The present invention has as a main objective to increase the oxygen concentration in the upper part of the pen without requiring pumping of a large amount of water into the pen. This is achieved by the method defined in the subsequent claim 1 as well as the arrangement defined in the subsequent claim 5.

[0013] Preferable embodiments of the method are set out in the dependent claims 2-4, and preferable embodiments of the arrangement are set out in the dependent claims 6-8.

Brief description of drawings

[0014] The invention will now be described in further detail, referring to the exemplary embodiment shown in the drawings, in which:

Figure 1 shows a principle sketch of the arrangement of the invention,

Figure 2 shows a detail of the arrangement according to the invention,

Figure 3 shows the arrangement of figure 1 arranged in a conventional fish pen, and

Figure 4 shows a detail whith the oxygen diffusor omitted.

Detailed description of the invention

[0015] The invention has been described in connection with a fish pen, but it may of course also be used for providing oxygen rich water to other types of sea species that primarily lives in the upper water layers.

[0016] The fish pen may be an open pen, but the invention may also be used in a closed fish pen.

[0017] Figure 1 shows a principle sketch of the arrangement of the invention in a cross-section elevation view. The figure shows schematically a fish pen 5. In the fish pen 5 are arranged an oxygen diffusor 2 and an air diffusor 1. The oxygen diffusor 2 is coupled to an oxygen hose 7 and the air diffusor 1 is coupled to an air hose 6.

[0018] Air to the system is supplied from an air compressor 8, which draws air directly from the atmosphere. The outlet of the air compressor 8 is coupled to the air hose 6 via a first valve 9. The outlet of the air compressor 8 is also coupled to an oxygen generator 10 via a second valve 11.

[0019] A controller 12 is set up to control the compressor 8 and valves 9, 11. By controlling the output from the air compressor 8 and the position of the valves 9, 11, the flow of air and oxygen to the diffusors 1, 2 can be controlled.

[0020] Figure 4 shows a view in which the oxygen diffusor 2 has been omitted. Consequently, it comprises an air diffusor 1. The air diffusor 1 is conveniently made up by a set of rings, which preferably are concentrically arranged. The rings are arranged at a depth of about 15 meters below the surface. Conveniently, the diffusor 2 is arranged centrally in the fish pen 5 (see figure 2). The rings are equipped with apertures (not visible) to let air from the inside of the rings out and into the sea.

[0021] Pressurized air at a pressure of around 5 – 6 bar is supplied to the inside of the rings, preferably at a rate of 25 - 30 m<3 >per hour. The air escapes from the rings through the apertures. The apertures have a size that produces air bubbles (13) with a diameter, at the moment the bubbles leave the diffusor 2, in the range of 1 - 10 mm, preferably 1 - 5 mm.

[0022] The air bubbles (13) migrate at a high speed upwards in the pen and will set the surrounding water into an upward movement. The upward movement creates an upwelling in the pen 5, where water in the central part is flowing upwards until it is close to the water surface. As the water flowing upwards has a lower temperature than the water at the surface, it will flow outwards and downwards, as shown by the arrows 4 in figure 3. When bubbles are continuously introduced into the water, this flow will become a steady upwelling. Hence the water in the pen 5 will be subjected to an ongoing agitation that will supply the upper part of the pen 5 with oxygen rich water from a depth of 15 – 20 meters.

[0023] At a depth of 15 – 20 meters the water is usually free of both lice and algae.

[0024] Figure 1 shows an enhanced embodiment of the invention, where an oxygen diffusor 2, which, in a similar way as the air diffusor 1, comprises a set of rings having small apertures (too small to be visible in the drawing) that are substantially smaller than the apertures of the air diffusor 1 defined above. The oxygen diffusor 2 is arranged above the air diffusor 1; preferably 2 – 5 meters, more preferably around 3 meters, above.

[0025] Oxygen is supplied under pressure to the inside of the rings of the diffusor 1 at a rate of 8 - 12 kg/hour. The oxygen escapes through the small apertures as tiny bubbles 3 (see figure 3) of a size that is smaller than the size of the air bubbles, i.e. a diameter when the bubble escapes the diffusor in the range of 0,1 - 3 mm, preferably 0,1 – 1 mm.

[0026] Tests have shown that water 1,5 – 2 meters above the air rings 2 are subjected to a lift. Hence, the supply of oxygen at a distance of about 3 meters above the air diffusor 1 will ensure that the water that is being lifted is enriched with further oxygen. The small oxygen bubbles 3 will dissolve more quickly into the water than the larger air bubbles (13).

[0027] The supply of oxygen into water that is already rising upwards ensures that the oxygen is not simply carried out of the pen by side currents but is transported to the surface within the pen.

[0028] An air compressor 8 is a part of standard equipment at a modern fish farming site. Likewise, oxygen tanks or an oxygen generator 10 are also standard equipment. Hence, the invention can be realized simply by installing the diffusor 1 and hoses 6, 7 and connecting these to the compressor 8 and oxygen generator 10. The controller 12 can be used to control the ratio between air and oxygen as well as the amount of air and oxygen. The controller can be coupled to sensors measuring the oxygen level in the pen 5. If the oxygen level drops below a certain threshold, the supply of air or air and oxygen can be automatically increased by the controller 12.

[0029] Although, concentric rings are preferred both for the oxygen diffusor 2 and the air diffusor 1, it is also possible to use other known diffusors, as is known per se by the person of skill.

Claims

1. A method of providing oxygen rich water into an upper part of a fish pen, where air bubbles (13) are introduced into the water in the pen (5), at a rate that provides a lift of the water to create a continuous upwelling of water from said depth to the surface of said pen (5), thereby bringing oxygen rich water from said depth to the upper part of the pen (5), characterised in that the air bubbles (13) are introduced at a depth of between 10 and 20 meters below the water surface, and that oxygen bubbles (3) of a smaller size than the air bubbles (13) are introduced into the water at a distance above the depth at which the air bubbles (13) are introduced.

2. The method of claim 1, characterised in that the rate of air bubbles (13) is 25 – 30 m<3 >per hour with a bubble size at the moment the bubbles (13) enter the water between 1 and 10 mm, preferably between 1 and 5 mm.

3. The method of claim 1 or 2, characterised in that the air bubbles (13) are introduced into the water in concentric circles.

4. The method of claim 1, 2 or 3, characterised in that the oxygen bubbles (3) at the moment the bubbles enter the water are of a diameter between 0,1 and 3 mm, preferably between 0,1 and 1 mm.

5. An arrangement to provide oxygen rich water into an upper part of a fish pen, comprising an air diffusor (1) arranged inside a fish pen (5) and an air compressor (8) coupled to the air diffusor (1), characterised in that the air diffusor (1) is arranged at a depth of 10 – 20 meters, and that the arrangement further comprises an oxygen diffusor (2) arranged about 3 meters above the air diffusor (1).

6. The arrangement of claim 5, characterised in that the air diffusor (1) comprises a set of perforated pipes or hoses arranged in hollow concentric circles.

7. The arrangement of claim 5 or 6, characterised in that the diffusor (2) comprises perforated pipes or hoses arranged in hollow concentric circles.

8. The arrangement of any of the claims 5-7, characterised in that it comprises the air compressor (8), said air compressor (8) supplying air to an oxygen generator (10) and to the air diffusor (1), and further oxygen to the oxygen diffusor (2), a set of valves (9, 11), controlling the split between supply of air to the air diffusor (1) and to the oxygen generator (10).