NO347319B1

NO347319B1 - Sludge separation tank

Info

Publication number: NO347319B1
Application number: NO20220013A
Authority: NO
Inventors: Knut Solberg; Bjørn Myrseth
Original assignee: Solinova As; Vitamar AS
Priority date: 2022-01-05
Filing date: 2022-01-05
Publication date: 2023-09-18
Also published as: NO20220013A1

Description

SLUDGE SEPARATION TANK

The present invention relates to fish farming and in particular to treatment of sludge from fish pens, fish tanks and the like.

Background Art

It is recognized that excessive discharge of faeces along with uneaten feed along the sea bottom in and around the fish farming cages / fish pens into the sea causes environmental problems, in particular in shallow and confined waters, for instance in fjords. It is known to collect sludge, along with feed residuals at a bottom outlet of fish pens having a bottom part configured for accumulation of the sludge. The coarse sludge particles will sink down through the water, collect at the said outlet, and can thus easily be taken care of by pumping it out from the pen through a sludge outlet.

However, the remaining sludge, made up of finer particles, will typically be removed from the pen along with the water that exits the pen through the water outlet. The water outlet is typically, however not always, arranged at some vertical distance above the sludge outlet, so that the water exiting contains less impurities. This part of the sludge hence ends up in the environment, unless taken care of.

One way of handling this part of the water exiting the pen, is to filter it. However, filtering large amounts of water is costly.

Publication FR2577757A1 presents a solution for purifying water from an onshore fish tank using a settling tank with a floating filter made of granulated material.

Furthermore, publication JPS62125900 presents a sedimentation tank to collect sludge from the bottom of a body of water in which fish is bred. The sludge is pumped directly from the bottom through a pipe extended into the sludge.

International application publication WO19198063 presents a land-based, saltwater flow-through pool, wherein water is pumped into the pool from a nearby ocean. Wastewater is collected at the bottom of the pool and transferred to a settling tank. After settling, water is cleaned with a filter and then returned to the ocean. Additional and cleaner water exits the pool through a water outlet, arranged at some vertical distance above the wastewater outlet. The additional, cleaner water is led directly back to the ocean without cleaning.

Moreover, publication US2005211607 discloses a sedimentation pool where sludge is separated from water. The pool has a centrally located inlet pipe and a height-adjustable inlet that guides water into the tank. Water flows horizontally in the pool. Cleaned water is guided out through an outlet in the upper portion of the tank, while sludge is collected at the bottom.

An object of the present invention may be to provide a sludge cleaning arrangement configured to clean fine sludge particles out of vast amounts of pumped water.

Another object may be to provide a sludge cleaning arrangement, which in a cost-effective and environmentally friendly manner provides a high degree of removal of fine-particulate sludge.

These and further objects are met with the present invention.

Summary of invention

According to the present invention, there is provided a sludge separation tank. The sludge separation tank comprises an upper wall portion and a lower wall portion, which enclose a volume of water inside the sludge separation tank. The upper wall portion is water permeable. The sludge separation tank further comprises a tank water inlet and a tank water outlet. Furthermore, there is a horizontal flow path between the tank water inlet and the tank water outlet. The sludge separation tank also has a tank sludge outlet. According to the invention, the sludge separation tank floats in the sea and is connected, with a water transfer line, to a fish farming pen or fish farming tank, i.e., an enclosure inside which fish is farmed.

With the term horizontal flow path is meant that the water that exits from the tank water inlet, into the sludge separation tank, will flow at least a horizontal distance to reach the tank water outlet. The flow path may also have a vertical component, in embodiments where the tank water inlet and tank water outlet are located at different vertical levels. However, it is considered advantageous to have the tank water inlet and tank water outlet at the same or at substantially the same vertical level, such that the water can flow substantially horizontally.

When stating that the upper and lower wall portions enclose the said volume of water, it shall be clear that in some embodiments, there may be additional wall portions that also contributes to the enclosure.

By stating that the sludge separation tank is connected, with a water transfer line, to a fish farming pen or fish farming tank, it is meant that the sludge separation tank is connected such that it may receive water from a fish farming pen, tank, cage, or similar fish farming enclosure. It will be understood that the said connection may be an indirect connection. Hence, the received water may be transported via one or more intermediate arrangements.

Preferably, the lower wall portion can be water impermeable. In this manner, the water will only exit through the tank water outlet (except for some water exiting along with removed sludge through the tank sludge outlet).

In some embodiments, the upper wall portion can have a circular configuration, wherein the tank water inlet is arranged inside the circular configuration and with a horizontal distance from the upper wall portion. The tank water outlet can be distributed along the perimeter of the circular configuration. The sludge separation tank can further comprise a water inlet duct, which is fluidly connected to the water transfer line, and wherein the water inlet duct extends to the tank water inlet.

In such embodiments, the water that exits the tank water inlet will slow down its speed as it approaches the tank water outlet. In this way, one enables the water to flow slowly. This increases the amount of sludge that separates (sinks) before the water flows out of the sludge separation tank through the tank water outlet along its periphery.

Preferably, the tank water inlet can be arranged centrally with respect to the circular configuration.

Moreover, herein the term circular configuration may also relate to oval or elliptic configurations. Hence, a geometrically "perfect" circle (with a constant radius) is not required for carrying out such embodiments of the invention.

The lower wall portion extends down from the upper wall portion and can exhibit a downwardly narrowing shape. Moreover, the water inlet duct can extend vertically downwards from the tank water inlet to a narrowed portion of the lower wall portion.

The upper wall portion may comprise a net and a water-permeable filter sheet, wherein the net and the filter sheet are parts of the tank water outlet. Typically, the net will provide structural strength while the filter sheet will filter out particles that should be prevented from escaping into the surrounding sea.

The filter sheet may extend from a position above the water surface of the sea, typically down to a connection point, wherein the connection point is arranged below the sea surface and at the upper end of or further down on the lower wall portion.

The filter sheet can preferably be configured to filter out lice larvae, salmon lice, and other living organisms that may have been transported from the fish pen and that should be prevented from reaching the sea outside.

The vertical extension of the upper wall portion, through which the water exits the sludge separation tank, can for instance between 0,5 and 3 meters, or between 0,5 and 2 meters, below the sea surface.

In some embodiments, the horizontal flow path can extend across a confined flow area, wherein at least 10%, 30 %, 70 % or even 90 % of the confined flow area is confined by said tank water inlet and/or tank water outlet. In some embodiments, which will be discussed in more detail further below, the tank water outlet may substantially entirely confine the confined flow area.

The tank water inlet and the tank water outlet must of course have some vertical extension, such that an area through which the water can flow is provided. To obtain the sludge separation process, however, wherein sludge sinks down while the water flows in a horizontal direction, it is advantageous to use a horizontally broad water tank inlet and/or water tank outlet. The tank water outlet can typically be in the form of a filter, sieve, or a plurality of distinctive apertures.

In some embodiments, the sludge separation tank can comprise a bottom assembly that has an attachment flange connected to the lower wall portion, and further comprises the tank sludge outlet.

The bottom assembly can further comprise a coupling pipe with a connector which is connected to the water transfer line, wherein the coupling pipe is connected to the water inlet duct.

In some embodiments, the bottom assembly can also comprise a guiding means. The guiding means has a pointed configuration and is open for flowthrough of water. The guiding means facilitates installation of the sludge separation tank.

The sludge separation tank can comprise a float that extends along a perimeter of the sludge separation tank, and a plurality of beams extending between the perimeter of the sludge separation tank and the water inlet duct, wherein the water inlet duct is centrally located and vertically oriented. The number of beams can typically be two, three or four. The said perimeter of the sludge separation tank can advantageously be circular but can also have other configurations such rectangular or other polygonal shapes.

In some embodiments the sludge separation tank may further comprises a sludge flushing hose connected to a lower part of the lower wall portion and configured to dissolve accumulated sludge by application of pressurized fluid. The fluid is preferably a pressurized liquid. The fluid is inserted into the accumulated sludge such that the sludge dissolves and is possible to pump out through the sludge transfer line.

In some embodiments, a flocculation agent supply arrangement can be arranged, configured to supply a flocculation agent to the water that is transported to the sludge separation tank. The flocculation agent supply arrangement can for instance be arranged in association with the water transfer line, such that the flocculation agent is supplied to and mixed with the water before exiting the duct water exit.

Detailed description of the invention

While various features of the invention have been presented in general terms above, a more detailed and non-limiting example of embodiment will be given in the following with reference to the drawings, in which

Fig. 1 is a side-view showing an overview of a fish pen and a sludge separation tank;

Fig. 2 is an enlarged side-view illustrating the perimeter of the sludge separation tank, in particular the tank water outlet;

Fig. 3 is a top view of a circular sludge separation tank, illustrating the water flow from the tank water inlet towards the tank water outlet;

Fig. 4 is an enlarged cross section side-view showing the tank water inlet, arranged in a water inlet duct;

Fig. 5 is an enlarged cross section side-view of a bottom assembly, where separated sludge accumulates for transport to a handling facility;

Fig. 6 is a top view showing an alternative embodiment of the invention;

Fig. 7 shows an overview of another embodiment, wherein the sludge separation tank is connected to two fish pens;

Fig. 8 is a side-view of an alternative embodiment of the invention;

Fig. 9 is a top view of the embodiment shown in Fig.8;

Fig. 10 is an enlarged top view of the embodiment shown in Fig.8 and Fig.9;

Fig. 11 is an enlarged side-view of the embodiment shown in Fig.8 to Fig.10;

and

Fig. 12 depicts an alternative embodiment, wherein the sludge separation tank receives water from a land-based facility.

Fig. 1 illustrates an overview of an embodiment of the present invention. A fish pen 1 is floating in the sea 3, such as in a fjord. The fish pen 1 can typically be a closed pen having water-impermeable walls and bottom. Other types of fish farming cages can also be used, where water comprising sludge exits through a water outlet, even land-based facilities. In embodiments where salmonoids are produced, fresh seawater is typically pumped into the pen from a certain depth to avoid salmon lice. As known in the art, the fish pen can have circular shaped sidewalls 5, and the fresh water can be supplied at the sidewalls 5 with a direction to provide a circulating water flow inside the pen.

The fish pen 1 has a pen sludge outlet (not shown) arranged to the bottom of the fish pen 1. Sludge particles, typically the particles above a certain size, will sink down to the bottom and accumulate at the center of the pen bottom, where the pen sludge outlet is located. This sludge is pumped out from the fish pen 1 and to a sludge treatment arrangement 7, through a not shown pen sludge line.

While fresh seawater is pumped into the fish pen 1 at its periphery, the fish pen 1 has a pen water outlet 9 at an elevated part of the fish pen 1. The pen water outlet 9 is, in the shown example, arranged centrally with respect to the circular configuration of the sidewalls 5. Moreover, the pen water outlet 9 is arranged at the water surface.

As shown in Fig.1, the pen water outlet 9 connects to a water transfer line 11. The water transfer line 11 connects to a sludge separation tank 13. In particular, the water transfer line 11 connects to the lower end of a water inlet duct 15 of the sludge separation tank 13. The water inlet duct 15 is preferably vertically arranged.

The sludge separation tank 13 floats in the sea 3, typically adjacent several fish pens 1 in a fish farming facility.

The sludge separation tank 13 has an upper wall portion 17 and a lower wall portion 19. In the shown embodiment, the upper wall portion 17 is vertically oriented. Moreover, the upper wall portion 17 has a circular configuration. The water inlet duct 15 is centrally arranged with respect to the upper wall portion 17.

Down from the upper wall portion 17 extends the lower wall portion 19. The lower wall portion has a conical configuration that narrows in the downward direction, towards a narrowed portion 22. At the narrowed portion 22 of the lower wall portion 19, the sludge separation tank 13 has a tank sludge outlet 21. This will be discussed in more detail below.

At an upper portion of the water inlet duct 15, the sludge separation tank 13 comprises a tank water inlet, which in the shown embodiment is in form of a duct water exit 23. The duct water exit 23 is centrally arranged at the water surface or immediately below the water surface. Moreover, as will be discussed further below, the upper wall portion 17 is water permeable. Hence, water exiting out from the duct water exit 23 will flow with continually reduced speed towards and through the circular upper wall portion 17. The permeable upper wall portion thus constitutes a tank water outlet 18. During the travel of the water from the center of the sludge separation tank 13 towards the waterpermeable upper wall portion 17, sludge will leave the water flow and sink down towards the bottom of the sludge separation tank 13.

Between the tank water inlet, i.e., the duct water exit 23 of the present embodiment, and the tank water outlet 18, the water flows along a horizontal flow path 20. By having the tank water inlet and the tank water outlet 18 at substantially the same vertical level, the water will flow substantially only in a horizontal direction. One may, however, arrange the tank water inlet and tank water outlet at different vertical levels. The water must then also flow a vertical distance to exit the sludge separation tank 13. Notably, the said horizontal flow path 20 should always comprise a horizontal component, such that particles are enabled to sink with a direction (i.e. downwards) that does not coincide with the direction of the water flow.

The lower wall portion 19 is water impermeable, such that sludge sinking down will be collected at the tank sludge outlet 21.

To the tank sludge outlet 21 there is connected a sludge transfer line 25. In this embodiment, the sludge transfer line 25 extends to the fish pen 1 and connects to the sludge treatment arrangement 7. It will be appreciated that the sludge transfer line 25 could transport the collected sludge to another location for handling.

The lower wall portion 19, which in the shown embodiment has a conical configuration but which in other embodiments can have another shape, is water impermeable, as laid out above. It can be made of a variety of different materials with different characteristics. For instance, it can be made of a flexible tarpaulin or fabric, a rigid composite material such as GRP (glass fiberreinforced plastic), steel, or even concrete. Advantageously, it should exhibit an inner surface which can be easily cleaned, and which preferably enables in letting the sludge move downwards along the inner surface towards the tank sludge outlet 21.

Reference is now made to Fig.2, which is an enlarged cross section view showing inter alia the tank water outlet 18 of the upper wall portion 17. The sludge separation tank 13 comprises two ring-shaped floats 27. The floats 27 extends around the upper wall portion 17 and are provided for buoyancy to the sludge separation tank 13. Resting on top of and attached to the floats 17 is a work platform 29, configured to support personnel (not shown). A fence 31 is also arranged.

At the tank water outlet 18, the upper wall portion 17 comprises a net 17a. The net 17a allows water to flow through its mesh while providing structural strength to the upper wall portion 17. Furthermore, the upper wall portion 17 comprises a filter sheet 17b. The filter sheet 17b is water permeable. However, it prevents small particles, such as salmon lice, louse larvae, as well as other organisms and particles above a certain size, from exiting the sludge separation tank 13. The filter sheet 17b can typically be in form of a cloth.

Advantageously, the filter sheet 17b is flexible. However, one may consider a rigid sheet, provided it is water permeable and allows filtering of particles that should not exit the sludge separation tank 13 through the tank water outlet 18.

The filter sheet 17b is connected at an upper connection point 33, at a position above the water surface, such as to the fence 31. From this upper position, it extends substantially vertically down to a lower connection point 35, which is arranged some distance below the sea surface, for instance between 0,5 and 3 meters below the sea surface. Preferably, the filter sheet 17b can at the lower connection point 35 connect to the lower wall portion 19 with a hook-and-loop arrangement (often referred to as Velcro). It shall be understood that although the term connection point is used herein, the connection point will typically extend along the perimeter of the circular shape of the sludge separation tank 13. An alternative term could thus be a connection stripe or a connection edge.

The net 17a is also attached above the sea surface, such as to the fence 31. It extends downwards and connects to the lower wall portion 19 (shown in Fig.1).

Fig. 3 shows a top view of the sludge separation tank 13. The arrows indicate the water flowing from the duct water exit 23 towards the peripherally arranged tank water outlet 18. Notably, the tank water outlet 18 is preferably evenly distributed about the perimeter of the sludge separation tank 13, to obtain an even and relatively slow flow of water from the duct water exit 23 towards the tank water outlet 18. The slow speed of the water flow enables the sludge to sink dawn towards the lower wall portion 19 and the tank sludge outlet 21.

As a means to maintain the position of the water inlet duct 15, four beams 37 extend between the centrally arranged upper portion of the water inlet duct 15 and the perimeter of the sludge separation tank 13. The beams 37 can for instance connect to the work platform 29 that extends in a circular fashion along the perimeter. Advantageously, the beams 37 can also be configured as working platforms or walkways, to enable access for personnel to the duct water exit 23 at the upper portion of the water inlet duct 15. Preferably, the beams 37 are arranged above the water surface.

Instead of the shown beams 37, other components, such as wires, ropes, rods, or struts, can be used for fixation of the water inlet duct 15.

Fig. 4 depicts the central and upper portion of the sludge separation tank 13 with a cross section side view. The duct water exit 23 can be in the form of a plurality of water apertures 24 in the water inlet duct 15. In the shown embodiment, the water inlet duct 15 is a rigid pipe, such as a plastic pipe, and extends a distance above the sea surface and above the duct water exit 23. The horizontal arrows illustrate the water flow out from the water inlet duct 15. The vertical arrows indicate sinking particles, i.e., sludge, which will be collected at the bottom.

In other embodiments, the water inlet duct 15 may instead be of a flexible material, such as a channel made of a flexible sheet or cloth.

To the water inlet duct 15, at the position of the duct water exit 23, there may be attached a float (not shown) to maintain the duct water exit 23 close to or at the position of the water surface.

Fig. 5 is a side view of the bottom portion of the sludge separation tank 13. An amount of sludge 39 has sunk down to the bottom and has accumulated. The lower wall portion 19 connects to a bottom assembly 41. The bottom assembly 41 comprises a conical attachment flange 43, to which the lower wall portion 19 is connected. Furthermore, the bottom assembly 41 comprises a cylindrical portion 45 that encompasses the water inlet duct 15, such that there exists an annulus 47 between the outer part of the bottom assembly 41 and the water inlet duct 15. The sludge 39 accumulates in this annulus 47.

Furthermore, the bottom assembly 41 comprises an inclined base plate 51, which together with the cylindrical portion 45 provides a downwardly pointing apex 53. The tank sludge outlet 21 is located at this apex 53. Consequently, the sludge transfer line 25 connects to the bottom assembly 41 at the apex 53.

Still referring to Fig.5, the water transfer line 11 connects to the water inlet duct 15 over a coupling pipe 55. The coupling pipe 55 has a connector 57 for connection to the water transfer line 11. Furthermore, a bolt 59 connects the water inlet duct 15 to the coupling pipe 55.

The coupling pipe 55 is part of the bottom assembly 41. To the lower end of the water inlet duct 15, there is arranged a guiding means 15a. The guiding means 15a has a pointed configuration and open for flow-through of water. It can typically be made of flanges or ribs. The guiding means 15a contributes to guiding the water inlet duct 15 into the coupling pipe 55 when the water inlet duct 15 is lowered down, such as with a crane (not shown). The lower part of the water inlet duct 15 can then be attached to the coupling pipe 55 by attachment of the bolt 59, such as with a diver or an ROV (remotely operated vehicle).

Fig. 6 presents, with a top view, an alternative configuration of the sludge separation tank 13. In this embodiment, instead of the circular configuration of the previous example, the sludge separation tank 13 has a rectangular configuration. Instead of introducing the water with a duct water exit 23, as shown above, water from the fish pen 1 is with this embodiment introduced with a water distribution pipe 23a. Hence, in this embodiment the sludge separation tank 13 comprises a tank water inlet that is in form of a water distribution pipe 23a. The water distribution pipe 23a connects to the water transfer line 11. The water distribution pipe 23a extends parallel to one of the sides of the rectangular configuration of the sludge separation tank 13. Moreover, the water distribution pipe 23a comprises a plurality of water apertures 24 distributed along its length, such that water from the fish pen 1 is evenly distributed at the water surface inside the sludge separation tank 13.

The tank water outlet 18 is arranged at the opposite side of the rectangular shape. Advantageously, the tank water outlet 18 can have the same or a similar configuration as the net 17a and filter sheet 17b discussed above. The sides that extend between the water distribution pipe 23a and the water tank outlet 18, comprises an upper wall portion 17 which is water impermeable.

Consequently, water exiting the water distribution pipe 23a flows across the rectangular area while sludge will deposit at the bottom of the sludge separation tank 13.

Common for both of the embodiments discussed above, is that the horizontal flow path 20 is confined to a confined flow area 26. I.e., the flow path is not in direct communication with the water outside the sludge separation tank 13, except through the tank water outlet 18. In the embodiment shown in Fig.1 to Fig. 5, since the tank water outlet 18 extends substantially along the entire periphery of the circular shape, the horizontal flow path 20 is substantially fully confined by the tank water outlet 18.

Moreover, in the embodiment shown in Fig.6, the horizontal flow path 20 is confined by the straight water distribution pipe 23a and the straight tank water outlet 18, along with the non-permeable sides of the rectangular shape.

Notably, also in this embodiment, the tank water inlet and the tank water outlet 18 constitute a significant portion of the confinement. With, for example, such rectangularly shaped sludge separation tanks 13, typically at least 30 % or at least 40 % of the confined flow area 26 is constituted by the tank water inlet and the tank water outlet 18.

Fig. 7 is an overview illustration of a further embodiment, wherein two distinct fish pens 1 are connected to one sludge separation tank 13. In other embodiments, three or even more fish pens 1 can be connected to one sludge separation tank 13 with water transfer lines 11.

The sludge separation tank 13 can also receive water, through a water transfer line 11, from one or more fish pens 1 or fish farming tanks arranged on land. In such embodiments, the fish farming tanks could typically be located close to the sea, using seawater or freshwater in the land-based fish tanks.

Fig. 8 and Fig.9 is a side view and a top view, respectively, of an embodiment wherein the water inlet duct 15 has a horizontal extension. Instead of connecting to the bottom of the sludge separation tank 13, as shown in the embodiments discussed above, the water transfer line 11 connects to the sludge separation tank 13 at the position of the upper wall portion 17.

The water inlet duct 15 is supported by one of the beams 37 and extends towards the center of the sludge separation tank 13.

In this embodiment, the bottom assembly 41 is not connected to the water transfer line 11. The bottom assembly 41 still comprises the conical attachment flange 43 (not shown in Fig.8) which connects to the lower wall portion 19. Moreover, the bottom assembly 41 comprises the tank sludge outlet 21, for removal of accumulated sludge.

Deposited sludge that accumulates at the tank sludge outlet 21 may in some cases harden, such that it is difficult to pump it out through the sludge transfer line 25. To solve this problem, a sludge flushing hose 28 is arranged in association with the tank sludge outlet 21, such that a flushing fluid, preferably a liquid, can be pumped into the accumulated sludge to dissolve it. In this manner, the accumulated sludge will be possible to pump. The sludge flushing hose 28 can preferably extend in parallel with the sludge transfer line 25. In Fig. 8, the sludge flushing hose is shown with a dotted line and extends to a control housing on arranged in association with the fish pen 1. Hence, if the personnel notices that the sludge is not pumpable, the sludge is dissolved by applying pressurized fluid (liquid) through the sludge flushing hose 28. A sludge flushing pump (not shown) will typically be arranged to pressurize the fluid.

Fig. 10 is an enlarged top view of the tank water inlet of the embodiment shown in Fig.8 and Fig.9. Reference is also made to Fig.11, illustrating the tank water inlet with a side view. In this embodiment, the tank water inlet is a water permeable ring. In particular, in this embodiment it is a grated ring 23b. The grated ring 23b is shaped like a cylinder with a bottom plate 23c. The bottom plate 23c is flat and water impermeable. Water exits the water inlet duct 15 above the bottom plate 23c, as shown in Fig.10. Since the grated ring 23b is permeable, water can flow horizontally out of the grated ring 23b, towards the tank water outlet 18 at the periphery. In some embodiments, a flow obstruction 23d can be arranged such that the water flow is distributed along the perimeter of the grated ring 23b.

Fig. 12 depicts another application of a sludge separation tank 13, wherein the water transfer line 11 extends from a land-based facility 1a. The sludge transfer line 25 extends from the tank sludge outlet 21 and back to land for handling of the separated sludge. With such an embodiment, no pump is needed for transport of the water through the water transfer line 11, since the sludge separation tank 13 is arranged vertically below the land-based facility 1a.

In the embodiment shown in Fig.12, there is also provided a flocculation agent supply arrangement 50. To the flocculation agent supply arrangement 50, there is connected a flocculation agent supply line 51. The flocculation arrangement supply line 51 delivers a flocculation agent to the water flowing through the water transfer line 11, upstream of the duct water exit 23. The flocculation agent contributes to flocculation of the particles in the supplied water, and thus increases the degree of separation.

It will be appreciated that the flocculation agent supply arrangement 50 also could be applied in association with any other embodiment of the sludge separation tank 13, such as the embodiment discussed above.

Herein is also presented a sludge separation tank connected to a water transfer line configured to supply water to the sludge separation tank, for separating impurities, such as sludge, from the supplied water. The sludge separation tank comprises an upper wall portion and a lower wall portion, which enclose a volume of water inside the sludge separation tank. The upper wall portion is water permeable. The sludge separation tank further comprises a tank water inlet and a tank water outlet. Furthermore, there is a horizontal flow path between the tank water inlet and the tank water outlet. The sludge separation tank also has a tank sludge outlet. According to the invention, the sludge separation tank floats in the sea. Embodiments of the sludge separation tank may be according to any one of the dependent claims, except their dependency on claim 1.

Thus, such a sludge separation tank may be identical to the sludge separation tank discussed above, however without being connected to a fish farming cage or the like. For instance, such a sludge separation tank may be used to treat spill water from an industrial process.

Claims

1. A sludge separation tank (13) comprising

- an upper wall portion (17) and a lower wall portion (19), which enclose a volume of water inside the sludge separation tank (13), wherein the upper wall portion (17) is water permeable;

- a tank water inlet (23, 23a, 23b) and a tank water outlet (18), and a horizontal flow path (20) between the tank water inlet and the tank water outlet;

- a tank sludge outlet (21);

characterized in that the sludge separation tank (13) is connected with a water transfer line (11) to a fish farming pen (1) or fish farming tank, and floats in the sea (3).

2. A sludge separation tank (13) according to claim 1, characterized in that the upper wall portion (17) has a circular configuration, wherein the tank water inlet (23) is arranged inside the circular configuration and with a horizontal distance from the upper wall portion (17), and wherein the tank water outlet (18) is distributed along the perimeter of the circular configuration, wherein the sludge separation tank (13) further comprises a water inlet duct (15), which is fluidly connected to the water transfer line (11), wherein the water inlet duct (15) extends to the tank water inlet (23).

3. A sludge separation tank (13) according to claim 2, characterized in that the lower wall portion (19) extends down from the upper wall portion (17) and exhibits a downwardly narrowing shape, and that the water inlet duct (15) extends vertically downwards from the tank water inlet (23) to a narrowed portion (22) of the lower wall portion (19).

4. A sludge separation tank (13) according to one of the preceding claims, characterized in that the upper wall portion (17) comprises a net (17a) and a water-permeable filter sheet (17b), wherein the net (17a) and the filter sheet (17b) are parts of the tank water outlet (18).

5. A sludge separation tank (13) according to claim 4, characterized in that the filter sheet (17b) extends from a position above the water surface of the sea (3), down to a connection point (35), wherein the connection point (35) is arranged below the sea surface and at the upper end of or further down on the lower wall portion (19).

6. A sludge separation tank (13) according to one of the preceding claims, characterized in that the horizontal flow path (20) extends across a confined flow area (26), wherein at least 30 % of the confined flow area is confined by said tank water inlet (23, 23a) and/or tank water outlet (18).

7. A sludge separation tank (13) according to one of the preceding claims, characterized in that it further comprises a bottom assembly (41), wherein the bottom assembly (41) comprises an attachment flange (43) connected to the lower wall portion (19) and the tank sludge outlet (21).

8. A sludge separation tank (13) according to claim 2 and claim 7, characterized in that the bottom assembly (41) further comprises a coupling pipe (55) with a connector (57) connected to the water transfer line (11), wherein the coupling pipe (55) is connected to the water inlet duct (15).

9. A sludge separation tank (13) according to claim 7 or claim 8, characterized in that the bottom assembly (41) further comprises a guiding means (15a), wherein the guiding means (15a) has a pointed configuration and is open for flow-through of water.

10. A sludge separation tank (13) according to claim 2, or according to claim 2 and any other preceding claim, characterized in that it further comprises a float (27) extending along a circular perimeter of the sludge separation tank (13), a plurality of beams (37) extending between the perimeter of the sludge separation tank (13) and the water inlet duct (15), wherein the water inlet duct (15) is centrally located and vertically oriented.

11. A sludge separation tank (13) according to one of the preceding claims, characterized in that it further comprises a sludge flushing hose (28) connected to a lower part of the lower wall portion (19) and configured to dissolve accumulated sludge (39) by application of pressurized fluid.