Field of invention

The present invention relates to a device for dewatering waste such as sunken particulate waste in a closed fish cage, wherein the device preferably is arranged connected to an outlet of water. The present invention also relates to a use of said device, and a fish cage comprising such a device.

Background of the invention

The annual production of salmonids in Norway was about 1 .3 millions tons in 2012, 99 % being Atlantic salmon and trout. The production of salmon is at present mainly based on cage aquaculture in the sea, where salmon are cultivated in large net pen units. The intensive production entails different forms of waste produced during the production process such as faeces, unconsumed feed etc. Even though this is often regarded as waste and a potential environmental threat, the discharges from sea farms also represent a potentially valuable source for nutrients.

The waste includes particulate organic nitrogen and phosphorus, typically originate from feed pellets, small particles from feed and faeces as well as other particles from fouling on the equipment of the cage. It is said that about 3 % of the feed is not consumed by the fish and unconsumed feed is discharged as waste from the farms. When collected or accumulated at the seabed the unconsumed feed, faeces, and other organic waste and sunken particles appear as sludge. During intensive production in net cages, the waste/sludge is not only affecting the environment in vicinity to fish farms, but it also represent a significant loss of biomass and nutrients which may be used for other purposes.

In order to reduce problems with diseases and parasites, in particular salmon lice, as well as escapes from net cage farms, different types of closed sea-based cultivation units have been developed and tested. These closed systems or closed fish cages are typically more or less water tight cultivation units, e.g. fish cages made of tarpaulin, concrete or the like, instead of nets. Water is pumped into these units to supply the fish with fresh water and oxygen. Particulate sunken waste such as faeces and feed losses are typically removed at the bottom of the closed cultivation unit together with the effluent water.

Today, combined waste collection and suction systems for sunken waste such as for dead fish (also referred to as “morts”), and larger particles (unconsumed feed pellets and faeces) are in use for fish farm cages. These systems are often referred to as “mort collection systems” by the industry as their main purpose is to remove dead fish from the cage. A well known system, being in use in many salmon farms, is a collection head where dead fish are collected in a funnel formed head at the bottom of the cage and sucked up to the surface intermittently using an air lifting pump system (also called mammoth pump system). GB2563996 discloses such a type of collection head typically used in salmon aquaculture, also having a separate system for removal of sludge. WO2021/225450 and WO2014/123427 describe other systems designed for closed fish cages, having an outlet for effluent water as well as outlets for dead fish and particulate waste. The main disadvantage of these known systems is that the waste contains a large amount of water, and therefore pipes and pumps must be oversized to handle the amount of waste and water.

Thus, there is a need for a device and a system for handling wet waste in particular particulate organic waste such as faeces and feed losses from intensive sea-based aquaculture production such as from salmon farming, without the disadvantages stated above. Further, the devices should be easily maintained without dissembling, and not require shutdown. Yet another object is that the device and method should be easy to scale to larger or smaller systems. Finally, the device and system should comprise materials and parts withstanding the harsh environments below the surface of the sea.

The invention

The objects are met by a device, a fish cage and a use according to the independent claims. Further features are stated in the corresponding dependent claims. The present invention relates to a device for dewatering waste collected in a fish cage. The device comprises an inlet for wet waste, an outlet for water and an outlet for dewatered waste. The device further comprises a mesh having a retentate side connected to the inlet for wet waste and outlet for dewatered waste and a filtrate side connected to the outlet for water, a solid wall surrounding the mesh and means arranged to create a suction on the filtrate side of the mesh.

As most of the water flows from the inlet for wet waste to the outlet for water, an upstream end of the device will be close to the inlet, and a downstream end will be close to the outlet, and a downstream direction will be from the inlet for wet waste to the outlet for water.

By "wet waste" it is herein meant waste as described above, that is unconsumed feed, faeces, and other organic waste and sunken particles, sedimented and collected in the fish cage. Dewatered waste or sludge is the feed, faeces and particles remaining on the retentate side of the mesh, and as the water passes through the mesh, the content of water of the waste is reduced.

The mesh to be used in the present invention may be a grid, screen, sieve or any other convenient type of filter having openings defining the size of the particles to be retained. The coarseness of the mesh will depend on the location of the fish cage, as all particles flowing through the mesh will be released to the environments of the cage. The acceptable size of particles to be released to the environment, will be known to a skilled person. The mesh may be made of a number of metal grids arranged in layers and rotated in relation to each other. The coarseness of the mesh as a whole may be adjusted by the number of layers, or by the size of the holes in each grid. The grids may be equal or different.

Means are arranged to create suction on the filtrate side of the mesh, this will ensure efficiency of the filter, and increase the dewatering of the waste as more water will be drawn through the mesh. The means may be a venturi nozzle, rotating baffles or any other convenient means, including a pump. The device may be attached to a closed cage, and effluent water flowing out of the closed cage may be arranged to drive the means creating the suction. In such cases, no additional powered equipment may be needed to create the suction.

By "suction" it is herein meant a pressure being lower than the surroundings which will draw as much fluid as possible through the mesh.

The solid wall is surrounding the mesh, meaning that the mesh is arranged inside the solid wall at least in a direction perpendicular or angled to the flow direction of the water. That is, if the water is flowing vertically, the solid wall will surround the mesh in horizontal direction. The solid wall and mesh may be attached to each other, for instance by spacers or bars, keeping the mesh arranged centrally in the solid wall.

Both the mesh and the solid wall may be hollow cylinders, wherein the solid wall surrounds the mesh in radial direction, and an annular space is created between them. The annular space is also referred to as "inner annular". Spacers or bars as mentioned above may be used to keep the mesh central in the wall and the annular space having equal radius in all directions. The wall may preferably be longer than the mesh, and protrude beyond the mesh at the downstream end.

By “cylinder” it should in the context of this application be understood that the cross section may be a circle, an oval or even a polygon having more than 5 sides. A circular cross section is preferred.

When the mesh and solid wall are hollow cylinders, a floor may be arranged between the mesh and the wall, creating an annular space closed in one end. The inlet for wet waste is arranged between the solid wall and the mesh, upstream of the floor, and the filtrate side of the mesh is on the inside of the mesh cylinder. The water will then flow from the inlet, into the annular space, through the mesh and out through the water outlet. The floor may be solid or a mesh. The outlet for dewatered sludge is arranged at or close to the floor. The floor may be at a given distance from the inlet, or slanting towards the outlet for dewatered sludge in order to assist the removal of dewatered sludge. When the floor is slanting, it will appear as a helix, having the lower end close to the outlet.

Alternatively, when the mesh and solid wall are hollow cylinders, a floor may be arranged inside the mesh cylinder, and the inlet of wet waste is arranged inside the mesh, upstream of the floor, and the filtrate side of the mesh is outside the mesh cylinder, in the annular space between the mesh and solid wall. The water will then flow from the inlet, into the mesh, through the mesh, into the annular space, and out through the water outlet. The floor may be solid or a mesh. The outlet for dewatered sludge is arranged at or close to the floor. The floor may be at flat or slanting towards the outlet for dewatered sludge in order to assist the removal of dewatered sludge. The floor may also be concave if the outlet is arranged at the lowest point in the centre.

When the device is provided with a floor as described above, the floor is preferably arranged at the downstream end of the mesh, because no filtration will occur downstream of the floor and thus there is no reason to prolong the mesh beyond the floor.

The device according to the invention may further comprise elements for attaching it to the lower end of a fish cage. If the fish cage is provided with an outlet or drain, the device may be attached to the outlet. The elements may be any convenient elements, such as a flange, pipe collar, bayonet/claw coupling or the similar.

The means for creating a suction on the filtrate side of the mesh may be a venturi nozzle. The venturi nozzle may be created by arranging an outer casing around and beyond the solid wall in a downstream direction, meaning that the outer casing surrounds the solid wall and creates a passage between an inside of the outer casing and an outside of the solid wall, and further that the outer casing protrudes beyond a downstream end of the solid wall. The radius of the outer casing should be decreasing towards a downstream end of the solid wall, and thereby the passage will be narrowing. The passage may also be referred to as "outer annulus". An inlet for water may be connected to an upstream end of the passage and a fluid communication may be arranged between the filtrate side of the mesh and a downstream end of the passage.

The relationship between speed and pressure is well known from Bernoulli, and as the passage becomes more narrow, the speed of the water will increase. This will create a suction at the lower end of the solid wall. By "a fluid communication " it means that the device is arranged in such a way that any liquid passing through the mesh from the retentate side to the filtrate side, will be feeding into the same outlet as any fluid flowing through the passage between the outer casing and the solid wall. In this way, by flowing liquid in the passage, the speed will increase towards the more narrow end, and a suction will be created at the filtrate side of the filter, improving the efficiency of the mesh.

The outer casing is protruding beyond the solid wall in a downstream direction, and the protruding part may be shaped as a cylinder having constant radius. The outer casing will then have two parts, an upstream part having constantly decreasing radius in a radial direction, corresponding a cone, which part is surrounding the solid wall, and a downstream part shaped as hollow cylinder having a constant radius. This will improve the venturi effect as the suction will be directed towards the centre of the casing, and thus the filtrate side of the mesh.

The inlet for water connected to the an upstream end of the passage, may be an inlet for effluent water flowing out of a fish cage. If the device according to the invention is attached to a closed fish cage, then the effluent water flowing out of the fish cage may be used to create suction on the filtrate side of the mesh, improving the dewatering of the waste.

The device may comprise elements for attachment to a lower end of a fish cage, such as a basin on a closed fish cage. The elements may be arranged at the upper end of the solid wall and/or mesh, or at the upper end of the outer casing. When a device as described above is attached to a fish cage, it may be used in a method for dewatering waste for the fish cage. The waste collected in the fish cage will flow into the inlet for wet waste, the effluent water of the fish cage will flow into the passage between the outer casing and the solid wall and create a suction on the filtrate side of the mesh. Water of the wet waste will flow through the mesh and be mixed with the effluent water while the waste retained on the retentate side will be dewatered and removed.

The solid wall surrounding the mesh and the outer casing surrounding the solid wall, may both be made of a watertight material, such as plastic or metal, more preferred stainless steel.

The dewatered waste may be collected in any convenient way, for instance may a pump be connected to the outlet for dewatered sludge at the retentate side of the filter. The system for backflushing will also contribute to flushing the dewatered waste towards the outlet, and the pump may remove the dewatered waste continuously or intermittently. The dewatered waste contains much less water than the wet waste flowing out of the fish cage, and thereby the device according to the invention reduces the need for pumping, the amount needed to be pumped, and possibly also the operating time of the pumps. The dewatered waste may be further processed and the nutrients of the waste may be recycled or recovered.

When using a device according to the invention, wet waste will be flowing into the device through the inlet, water will be sucked through the mesh by the means for generating suction, and flow out of the device through the outlet for water. The dewatered waste will be accumulated on the retentate side of the mesh and removed through the outlet for dewatered waste.

In order to avoid shut down of the system, or periods where the mesh is not functioning properly, the device may further comprise a system for backflushing the mesh, arranged on the filtrate side of the mesh. If the filtrate side of the mesh is inside the mesh, then the system for backflushing should be arranged on the inside of the mesh, and correspondingly, if the filtrate side of the mesh is in the annular space between the solid wall and the outer casing, the system for backflushing should be arranged in the annular space.

The system for backflushing may comprise a number of nozzles arranged to flush perpendicularly onto the mesh, and thus any waste fastened to the mesh will be flushed off. The nozzles are preferably arranged in a row on a pipe in an upstreamdownstream direction of the mesh. Further, the mesh may be a hollow cylinder as described above, and arranged to rotate when the backflushing is activated. As the mesh rotates the whole mesh will be cleaned, even if the nozzles are constantly flushing in the same direction.

The system for backflushing may also comprise baffles arranged to rotate the mesh, and a nozzle arranged to flush water onto the baffles. The nozzle arranged to flush water onto the baffles may preferably be arranged in the same row as the nozzles to flush the mesh.

The backflushing system reduces the risk of clogging of the the mesh, and reduces the need for maintenance. Further it keeps the mesh and device operative even during times with higher amounts of wet waste than normal. The backflushing system may be running at intervals, continuously or as a response to signals from sensors or detectors. In order to keep the system fully operative and the openings of the mesh unclogged, it is advantageous to keep the backflushing system running continuously.

The invention will in the following be described by way of exemplary embodiments and accompanying drawings. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a closed or semiclosed fish farm typically deployed in the sea. However, it should be appreciated that device is also applicable and suitable for use in respect to any other type of aquaculture systems, requiring handling of water and sludge. Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment.

In the description relative terms such as front, top, centre, bottom, side, lower, upper, downward, upward, outward, sideward, vertical, and horizontal etc. are all related to the device when in upright position i.e. when mounted in a fish cage ready for use.

In the context of the present invention an aquaculture cage refers to a pen or cage, most commonly an aquaculture farm arranged in a body of water, typically a seabased fish cage. The term also includes closed systems or semi-closed systems where the farmed organisms are kept in a more or less watertight container, effectively stopping a direct water exchange with the surrounding water and having an external water supply by one or more pumps. The system can be deployed in water e.g. in the sea or a lake or as part of a land-based aquaculture farm. It may also be part of a recirculation aquaculture systems (RAS) both sea and land based.

Examples

Embodiments of the present invention will now be described, with reference to the following schematic figures wherein:

Fig. 1 shows a device according to the present invention, arranged on a fish cage, the fish cage and device is shown in cross section, Fig. 2 shows an axial cross section of the device, and

Fig. 3 shows a partial axial cross section of the device in a perspective from above

The same reference numbers in different drawings identify the same or similar elements. The figures are for illustration purposes only, and the different parts may necessarily not be in scale to each other. In Figure 1 it is shown a closed fish cage 1, having a floating collar 2 at the surface of the sea, and a fish bag 3 of tarpaulin or the similar, keeping the water inside separate from the water outside. The fish cage is provided with a system for replacement of water (not shown), wherein water is added at the upper part, and effluent water and waste flows out through different parts of an outlet basin 4 at the lower end of the bag. A device 5 according to the invention, for dewatering waste collected in the fish cage, is arranged below the outlet basin 4.

The device 5 is shown in detail in Fig. 2 and 3, and has an inlet 10 for wet waste, an outlet 11 for water and an outlet 12 for dewatered waste. The device comprises a mesh 13 having a retentate side 14 and a filtrate side 15, and a solid wall 16 surrounding the mesh. The retentate side 14 of the mesh 13 is connected to the inlet 10 for wet waste and the outlet 12 for dewatered waste. The filtrate side 15 of the mesh is connected to the outlet 11 for water. The device according to the invention also comprises means to arranged suction on the filtrate side of the mesh.

In the shown embodiment, the mesh 13 and solid wall 16 are hollow cylinders having a circular cross-section, wherein the solid wall 16 surrounds the mesh 13 in a radial direction and creates an inner annular space 14 between them. The inlet 10 is connected to the annular space 14, and thereby the retentate side of the mesh is within the inner annular space 14, and the filtrate side is inside 15 the mesh cylinder being connected to the outlet 11 of water.

A floor 17 is arranged between the mesh 13 and the solid wall 16, giving the annular space a solid bottom. In Figure 2 the floor 17 is shown higher at the right side of the annular space than on the left side, because the floor is slanting towards the outlet 12 for dewatered sludge, as a helix.

An upstream end of the mesh is provided with a lid 18, preventing wet waste to enter the inside 15 of the mesh cylinder. The inlet 10 for wet waste is arranged upstream of the mesh, and thus wet waste will flow onto the lid 18 of the mesh 13 and into the annular space 14. In order to avoid any waste being collected in the system, the lid 18 is designed as a cone, having the tip upstream and the wider part attached to the mesh.

When the means for arranging suction on the filtrate side of the mesh are activated, water in the wet waste will flow out of the annular space 14, through the mesh 13 and into the outlet 11 for water. In this way the waste will be dewatered.

In the shown embodiment, the means for arranging suction on the filtrate side of the mesh is a venturi nozzle, comprising an outer casing 19 arranged around the solid wall 16 in radial direction. The outer casing 19 comprises two parts, an upstream part 19A surrounding the solid wall and creating a passage 20 between the outer casing and solid wall, and a downstream part protruding beyond the solid wall. The radius of the upstream part 19A of the outer casing is decreasing downstream, while the radius of the downstream part is shown to be constant. The passage 20 between the upper part 19A and the solid wall 15, is thus an annular space having larger cross section upstream than downstream.

The means for creating suction further comprises an inlet 21 for water, connected to an upstream end of the passage 20, and an outlet for water at the downstream end of the outer casing. Further, it is arranged a fluid communication between the filtrate side of the mesh 13 the lower part 19B of the outer casing. In the shown embodiment this communication is arranged by letting any fluid entering the filtrate side of the mesh flow directly into the lower part of the outer casing 19B, and that the outlet 11 for water passing through the mesh 13 is the same outlet as the water entering the passage 20 between the solid wall 16 and the outer casing 19.

In the shown embodiment, partition walls 22 are arranged between the outer casing 19 and the solid wall 16 to keep the solid wall 16 arranged in center of the casing 19.

The inlet 21 for water into the passage 20, is preferably connected to the fish cage, and effluent water from the fish cage may be used to create suction on the filtrate side of the mesh 16. The particles of the wet waste entering the device through the inlet 10 will be retained by the mesh 13 and accumulate in the inner annular space 14 between the mesh 13 and the solid wall 16. The outlet 12 for dewatered waste is arranged at the bottom of the inner annular 14, close to the floor 17. A pump (not shown) is connected to the outlet, to remove dewatered waste when needed. The interval for removal may be depending on time or amount.

In order to avoid that the mesh 13 is clogged by waste, a system for backflushing is arranged on the filtrate side 15 the mesh 13, in the shown embodiment this is on the inside of the mesh cylinder 13. The shown backflush system comprises a pipe 23 with a number of nozzles 24 arranged to flush water perpendicularly onto the mesh 13. The pipe 23 is running axially along the mesh, and is connected to a supply-pipe 25 for supply of water, possibly pressurized water.

The mesh 13 is arranged to rotate inside the solid wall 16, for instance by being pivotally supported by an axle 26. When the backflushing system is activated, the mesh rotates, and thus the whole mesh will be cleaned. In the shown embodiment, a lower part of mesh 13 is provided with angular baffles 27 in such a way that when water is flushed onto the baffles 27, the mesh 13 will rotate. The use of angular baffles to cause rotation is well know to a skilled person. The pipe 23 having a number of nozzles 24 further comprises a nozzle (not shown) arranged to flush water onto the baffles 27, and thus rotation of the mesh will automatically be generated once the backflushing system is activated.

When using the device shown in the Figures, fastened to a fish cage as shown in fig. 1 , wet waste will be flowing from the fish bag 3 of the fish cage 1 , through the outlet basin 4 and into the device 5 through the inlet 10. Then the wet waste will flow along the lid 18 of the mesh 13 and into the inner annular space 14. Effluent water from the fish cage 1 will flow through the passage I outer annular space 20 between the outer casing 19 and the solid wall 16, and as the radius of the upper part 19A of the outer casing is decreasing, the velocity of the water will be increasing, and a suction will be generated on the filtrate side 15 of the mesh. The suction will be transmitted through the device according to the invention, and into the outlet basin where it will contribute to the removal of waste from the cage.

Water in the wet waste will thus be sucked through the mesh 13 and flow into the lower part 19B of the outer casing, and out of the device through outlet 11. The waste not passing through the mesh 13 will accumulate in the inner annular space 14 and will be removed through the outlet 12 for dewatered waste. Clean water will be supplied to pipe 25 and flow into pipe 23 and out of the nozzles 24 perpendicularly onto the mesh, flushing the mesh to keep the openings of the mesh open. The clean water will flow through the mesh and into the inner annular space 14, to be mixed with the wet waste. The pipe 23 also comprises a nozzle flushing water onto angular baffles of the mesh, causing rotation of the mesh.

Even if the invention is described above with reference a shown embodiment, the invention also comprises modifications and variants within the scope of the enclosed claims.