GB2242140A

GB2242140A - Tank for fish farming

Info

Publication number: GB2242140A
Application number: GB9103068A
Authority: GB
Inventors: Jeremy Robert Goddin
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-02-13
Filing date: 1991-02-13
Publication date: 1991-09-25
Anticipated expiration: 2011-02-13
Also published as: GB9103068D0; GB2241857A; GB2242140B; GB9003183D0

Abstract

A tank for fish farming has a central first outlet 34 through which water is continuously discharged from inside mesh sleeve 37 to an overflow corresponding to water level 18. Debris settling into a zone below line X-X is periodically sucked out through an annular array of gaps between a plate 41 and the tank floor and through a second, tangential outlet by opening valve 50, leading to foul drain 51. Opening 52 prevents siphoning and allows the discharge to be observed. Some floating debris is discharged by outlet 34, sleeve 37 merely excluding the fish. <IMAGE>

Description

1 R Q:> -1. Q c) 1 TANK FOR SEPARATING SOLIDS FROM WATER The present

invention relates to a tank, for separating solids from water. More particularly, but not exclusively, the present Invention relates to a tank for raising fish or other aquatic animals. Various forms of tank are in use in the fish farming industry and these include cylindrical tanks having a water inlet over the tank side and a control drain whereby water can pass through the tank at a rate adjusted by a central valve regulating the water inlet. The rate of water flow through the tank must, of course, be adequate to supply the oxygen requirements of the fish and to dilute the urinary secretions of the fish to a tolerable level. The depth of water in the tank is usually regulated by a depth con' trol device, or launder, positioned outside the tank. The water inlet is usually arranged so that the kinetic energy of the inflowing water can be vectored to control the rotational speed of the water in the tank to a value suitable for the size of the fish.

The fish will be fed a carefully calculated weight of feed each day usually in the form of dried pellets, the daily ration being divided into a series of smaller feeds at intervals during the day. When farming fish, it is common practice to provide a slight surplus of feed to ensure that there will be sufficient for the smaller fish in the tank and this inevitably means that excess feed accumulates on the tank bottom. Fish, such as salmon, can be put off their feed by diverse ambient factors such as falling water temperature, falling levels of oxygen saturation (typically during the early afternoon), windspeed and temperature, and random shadows such as a bird flying over the tank.

For this reasonsomefeeds are not taken and further excess food pellets gather on the tank bottom. Salmon and Trout prefer to eat their food as it floats past them and any pellets reaching the tank bottom are usually ignored.

As the feed pellets rest on the tank bottom they slowly soften into much smaller particles which tend to remain in the boundary layer at the bottom of the tank. The rotational speed of the water and the central drain tend to move these smaller particles towards the centre of the tank where they tend to lie in a ring around the central drain.

In addition to the surplus feed, some of the fish faeces also sink to the bottom to join the excess food. As the fish faeces are less dense than the feed pellets, a proportion of them are swept into the central drain before they can fall into the tank bottom.

As a consequence of the above, the fish farmer finds himself between two conflicting problems as, on the one hand, he cannot tolerate an accumulation of faeces and excess food in the tank and, on the other hand, he would prefer not to discharge the accumulated faeces and excess food back into the water supply. At present the fish farmer has no choice but to keep his fish healthy and it is common practice for such tanks to be "purged" at least once (usually twice) each day. Such purging involves rapidly draining water from the tank, typically by opening the depth control device, and this causes a forced vortex at the centre of the tank which stirs up all of the accumulated excess food and faeces so that it will be flushed out of the tank. This process involves discharging typically 20%-50% of the water in the tank and results in a large volume of 1 i i i 1 filthy water being discharged, usually into the water-course or into a settlement tank.

Additionally the fish are subjected to the shock of a sudden increase in the water speed which can pull smaller fish onto the screen over the centre drain, and the experience of swimming through the suspended particles which can stick in their gills and cause health problems.

According to the invention a tank, for separating particulate solids from water, comprises afirstoutlet through the tankfloorin a position remotefrom the tanksidewall and leading to afirst drainage system incorporating a depth control device for regulating the depth of water in the tank, a second outlet through the tank floor defining a suction means which is disposed around the first outlet and points outwardly along thop tank bottom to suck particles from the bottom of the tank into a second drainage system. Te side wall is preferably cylindrical, and the first outlet is positioned at the central axis of the tank.

The second drainage system may function all the time (provided the flow of water used is less than the flow of water into the tank) to clean the bottom of the tank continuously. This feature is useful when the discharge of particles into a watercourse is not a potential problem. However the second drainage system is preferably connected through a control valve to a foul drain. In this manner the control valve may normally be kept closed, but can be opened at intervals to suck accumulated particles from the tank bottom into the foul drain.

Preferably the connection between the second drainage system and the foul drain incorporates an anti-syphoning device. Preferably the connection between the second drainage system and the foul drain is arranged so that it can only drain the tank to a predetermined safe level if the control valve is left open.

Preferably the suction means is an annular orifice. Alternatively the suction means may be an annular array of orifices.

Preferably a ringshaped chamber may be provided in the tank bottom around the first drainage system to connect the suction means to the remainder of the second drainage system. The latter preferably includes a drainage duct connected tangentially to the ring-shaped chamber whereby water and particles sucked through the suction means will swirl around the ringshaped chamber into this drainage duct. Preferably the drainage duct is connected to the ring-shaped chamber so that the direction of the swirl will be the same as the direction of water rotation in the tank. Preferably the cross sectional area of the drainage duct is similar to the cross-sectional area of the suction means.

When the tank is for raising fish or other aquatic animals, a tubular exclusion screen for the fish or other aquatic animals is preferably arranged vertically about the first outlet and is connected by a tubular section to the remainder of the first drainage system. The tubular section is preferably arranged to fit into a socket connection which is positioned in the centre of the ring-shaped chamber but is connected to the remainder of the first drainage system.

The tubular section is preferably of such a length that its upper end is spaced above the bottom of the tank to leave a shallow dead zone over the tank bottom to facilitate settling of the particles. In the event that there is no exclusion screen with a tubular extension, the first outlet is preferably spaced above the bottom of the tank to leave a shallow dead zone over the tank bottom to facilitate settling of the particles. Preferably the bottom of the tank is formed so that it slopes gently downwards towards the position of the suction means.

The first and second drainage systems may be defined by a unit having a flange compatible with existing tanks which may thereby be modified to provide the features i j 1 j 1 of the present invention.

The invention is now described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a vertical cross-sectional view of a known type of cylindrical tank used for fish farming; Figure 2 is a plan view of Figure 1; Figure 3 is a vertical cross-section (to a larger scale) illustrating how the tank of Figures 1 and 2 can be modified in accordance with the present invention; Figure 4 is a plain view of Figure 3 but with the plate and centre screen omitted, and Figure 5 is a vertical cross-section similar to Figure 1 but showing part of a much larger tank constructed in accordance with the present invention.

With reference to Figures 1 and 2 a conventional glass-reinforced plastic cylindrical tank comprises two semi-cylindrical mouldings 10, a screen plate 11 and a centre moulding 12. The semi-cylindrical mouldings 10 have unshown flanges by which they are bolted together using an intervening sealing strip, and define a depending cylindrical flange 13 to the centre of the tank bottom 14. The centre moulding 12 is trapped, during assembly of the semi-cylindrical mouldings 10, inside the cylindrical flange 13 with intervening sealing strips, and is bolted in the correct position whereby the screen plate 11 can be located above, as shown, and sealed in position using mastic.

The screen plate 11 is formed with four accurate apertures over which perforated aluminium screens 15 are rivetted as shown in Figure 2. At its centre, the screen plate 11 has a cylindrical flanged opening which receives a cylindrical centre screen 16 formed from a rolled perforated aluminium sheet with a rolled flange 17 to limit its penetration into the centre moulding 12. In this manner the top of the centre screen 16 is positioned just above the water level 18 but below the tank rim.

The centre moulding 12 is formed integral with a length of polyvinylchloride piping 19 leading to a depth control device which is indicated generally by arrow 20.

The depth control device 20 comprises inner and outer concentric tubes 21 and 22 respectively, and an 0-ring seal 23 engaged by the lower end of the inner tube 21. In this manner water discharged through the pipe 19 is constrained by the seal 23 to flow upwards between the tubes 21 and 22 until it flows over the upper end of the inner tube 21 and falls, as indicated by arrow 24, into a tubular sump 25 set in concrete. From the sump 25, the discharge water flows into a drainage channel 26 which leads back to the river or lake serving as the water supply either directly or via a very large settlement tank.

The size of theperforations in the screens 15 and the tubular centre screen 16 chosen to exclude the passage of fish into the central drain defined by the moulding 12.

The water supply is from a distribution pipe 27 and a stand pipe 28, through a control valve 29, to a water inlet 30 which extends over the tank rim. The angle of the water inlet 30 can be adjusted to alter the horizontal vector of the driving force exerted by the inlet water on the tank contents.

Excess food pellets sink to the tank bottom 14 where they slowly soften the much smaller particles which tend to remain in the boundary layer and the bottom of the tank.

Due to the large flow area through the screens 15 and the centre screen 16, compared with that of the piping 19,the velocity of flow through them is generally small and is insufficient to suck particles from the bottom of the tank. Indeed particles tend to form a ring around the centre screen 16 and to block the perforations in the screens 15. If left for several hours, particles soon start to block the perforations in the lower part of the centre screen 16 as well, due to the very low pressure gradient across it. Indeed the pressure gradient is only the very tiny difference between the water level 18 and the level 4 i 1 i i 1 -7 of the water flowing over the upper end of the tube 21.

To purge the tank of these accumulated particles, the inner tube 21 of the depth control device 20 is pulled upwards by its handle 31 until its lower end disengages the seal 23, thereby suddenly applying the full head between the water level 18 wW the bottom of the depth control device 20 across the screens 15 and the centre screen l& ll& generates a very strong vertical forced vortex which stirs up the whole contents of the tank and, with a three metre diameter tank, about 50% of the water content must be discharged to evacuate the particles which are stirred up. Also, due to the very low velocities through the centre screen, water tends to be drawn from all levels of the tank, and a proportion of the fish faeces are drawn out of the tank before they can reach the bottom - this helps to keep the tank cleaner than otherwise but makes subset separation of these faeces difficult.

With reference to Figures 3 and 4, the centre moulding 12 shown in Fig 1 h replaced by a unit 32 having a flange 33 which is compatible with the flange 13 so existing tanks of the type shown in Figures 1 and 2 can be modified by. the simple insertion of the unit 32.

Unit 32 is formed from a standard polyvinylchloride pipe bend 34 incorporated into a glass reinforced plastics moulding which defines the flange 33, a ring-Amped chamber 35 and a tangential drainage duct 36.

The centre screen 16 shown in Figure 1 and 2 is replaced by a new design of fisbr excluding screen comprising a roll of welded stainless steel wire mesh 37 of which ffle longitudinal edges are over-lapped and clipped together. The ends of this tube of stainless steel wire mesh are fitted over lower and upper sections of piping 38 and39 (the upper section can be seen in Figure 5) and are held in position using contractile cups 40L The centre screen 16 is thus a simple push fit into thepipebend 34 of which the associated X --8- sealing ring secures the centre screen 16 in position. However, from trials I have carried out, the sealingring canbe omitted to leave the centre screen 16 merely sitting in the pipe bend.

An annular plate 41 fits around the lower pipe 38 of the centre screen 16 and rests on the tank bottom 14. An annular series of narrow ridges 42 hold the lower surface of the plate 41 dear of the tank bottom and define an annular array of orifices constituting a suction means pointing outwardly along the tank bottom. The plate 41 is preferably made of steel with the ridges 42 formed of welded steel strip, the whole construction being either galvanised or powder coated. The cross-sectional flow area of the annular array of orifices is approximately the same as the cross-sectional flow area of the tangential drainage duct 36. If desired, the annular plate 41 may be fixed to the tank bottom using adhesive or screws through the ridges 42. The cross-sections of each of the orifices is chosen so that none of the fish to be placed in the tank can escape through them.

In operation the unit 32 provides two separate drainage systems from the tank. The first drainage system takes water through the centre screen 37,38, 39 and the pipe bend 34 into a conventional depth control device such as that described with reference to Figures 1 and 2. The second drainage system sucks primarily along the tank bottom 14 as indicated by arrow 43, the water and suspended particles passing into thering-shaped chamber 35 and then swirling through the tangential duct 36 into a foul drain as will be shortly described with reference to Figure 5. It should be noted that water is also sucked through the slight clearance between the annular plate 41 and the lower pipe 38, as indicated by arrows 44, and this keeps the top of the annular plate 41 clear of particles.

Also it should be noted from Figure 3 that the upper end of the pipe 38 is spaced above the tankbottom 14 to leave a dead zonebelow the lineXX In this manner the water leaving the tank through the first drainage system 37,34 is drawn primarily from above -1 i the dead zone XX so that; all particles failing below the line XX tend to gather on the tank bottom 14.

Figure 5 illustrates a much larger tank which has been built to test the ffinciples of the present invention. The tank is of about eight metres diameter and is constructed from glass-coated steel sheets 45 with upper and lower reinforcing rings all tied toareinffirced concrete base 48 defining the tank floor 14 so that it slopes gently towards the centre screen 37.

The tank of Figure 5 operates in the same manner as the arrangement shown In Figures 3 and 4 but the unit 32 is replaced by a galvanised steel fabrication 49 which is cast into the concrete floor to define the ring-shaped chamber 35 and a true top surface for the annular plate 41 to bear against.

The tangential drainage duct 36 leads through a control valve 50 to a foul drain 51, the pipework downstream of the control valve 50 having an opening 52 cut in its upper surface to constitute an anti-syphoning device. In operation, the control valve 50 is normally kept closed but is opened twice a day to suck theparticles thathave accumulated on the tank floor 14 into the foul drain 51. When the control valve 50 is opened, the head between the water level and the line YY is applied across the array of orifices under the annular plate 41 to scour particles from the tank bottom 14. With the tank shown m Figure 5, 1 have found that the floor is completely purged by opening the control valve for less than one minute, twice a day. Indeed, when the valve 50 is opened, the nature of the discharge can be viewed through the opening 52 and comprises the dean water lying in the pipe 36 and the ring-shaped chamber 35, followed by a sliQrt but very concentrated slug of filthy water, and then by clean water showing that the tank has been fully purged.

Thus, the tank construction taught by the present invention enables tanks to be thoroughly purged with only a tiny percentage of the tank contents (less than 1%), collects a larger proportion of the faeces by utilising a dead zone at the tank bottom, and discharges the faeces and excess food into a separate drainage system. Hereby facilitating its subsequent treatment. This of course means that the water discharged through the level control is much cleaner than hitherto and obviates any need for large settlement tanks. Due to the small volumes discharged for very small periods into the foul drain 51, the processing of the effluent is facilitated as it can be collected in a smaller settling tank, similar to a septic tank, the sludge being recovered from time to time by conventional farm manure-spreading equipment for use as fertiliser.

Fish farms use various medications to treat their fish, for instance malachite green, formalin and chloramine-T, and these are usually discharged into the water course. With the equipment described, the valve 50 can be left open during treatment hereby diverting the medicated water into the foul drain 51.

If provision is made for diverting water from the valve 50 to the normal drainage channel 26, or other water course than the foul drain 51, the second drainage system can be used to hold the tank water level at YY whilst the fish are being caught.

Although the invention has been described with reference to cylindrical fish tanks, it may of course be applied to tanks of other configuration or requiring the separation of particulate solids for other purposes. When applied to non-cylindrical tanks, the primary water outlet should be positioned in the tank floor remote from the tank side wall, the suction means being disposed around the primary water outlet, and the tank floor being preferably profiled to give a gentle slope downwardly towards the suction means. In this manner very large tanks may be provided with multiple water outlets, each with its associated suction means for cleaning the neighbouring portion of the tank floor.

1 1 i I -1 1_

Claims

1. A tank, for separating particulate solids from water, comprising a first outlet through the tank floor in a position remote from the tank side wall and leading to a first drainage system incorporating a depth control device for regulating the depth of water in the tank, a second outlet through the tank floor defining a SU00n means which is disposed around the first outlet and points outwardly along the tank bottom to suck particles from the bottom of the tank into a second drainage system.

2. A tank, according to Claim 1, of which the side wall is substantially cylindrical, and the first outlet is positioned at the central axis of the tank.

A tank, according to Claim 1 or 2, in which the second drainage system is arranged to function all the time to clean the bottom of the tank continuously.

A tank, according to Claim 1 or 2, in which the second drainage system is connected through a control valve to a foul drain.

A tank, according to Claim 4, in which the connection between the second drainage system and the foul drain incorporates an anti-syphoning device.

6. A tank, according to Claim 4 or 5, in which the connection between the no 3.

4.

5.

7.

9.

drainage system and the foul drain is arranged so that it can only drain the tank to a predetermined level if the control valve is left open.

A tank, according to any preceding claim, in which the suction means is an annular orifice.

8. A tank, according to any of Claims 1 to 6, in which the suction means is an annular array of orifices.

A tank, according to any preceding claim, including a ring-shaped chamber provided in the tank bottom around the first drainage system to connect thesuction ' '. 1 111 A means to the remainder of the second drainage system.

10. A tank, according to Claim 9, in which the second drainage system also includes a drainage duct connected tangentially to the ring-shaped chamber whereby water and particles sucked through the suction means will swirl around the ring-shaped chamber into the drainage duct.

11. A tank, according to Claim 10, in which the drainage duct is connected to the ring- i i i 1 1 j i shaped chamber so that the direction of swirl will be the same as the direction of water rotation in the tank.

12. A tank, according to Claim 10 or 11, in which the cross-sectional area of the drainage duct is similar to the cross-sectional area of the suction means.

13. A tank for raising fish or other aquatic animals, according to any preceding claim, in which a tubular exclusion screen for the fish or other aquatic animals is arranged vertically about the first outlet and is connected by a tubular section to the remainder of the first drainage system.

14. A tank, according to Claim 13, in which the tubular section is arranged to fit into a socket connection which is positioned in the centre of the ring-shaped chamber but is connected to the remainder of the first drainage system.

15. A tank, according to Claim 13 or 14, in which the tubular section is of such a length that its upper end is spaced above the bottom of the tank to leave a shallow dead zone over the tank bottom to facilitate settling of the particles.

16. A tank, according to any of Claims 1 to 12, in which the first outlet is spaced above the bottom of the tank to leave a shallow dead zone over the tankbottom to facilitate settling of the particles.

17. A tank, according to any preceding claim, in which the bottom of the tank slopes gently downward towards the position of the suction means.

1 1 4 18. A tank, substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings, but modified by the substitution of a unit substantially as described herein with reference to Figures 3 and 4 of the accompanying drawings.

A tank incorporating a unit for separating particulate solids into a second drainage system substantially as described herein with reference to Figures 3 and 4 of the accompanying drawings.

20. A tank, substantially as described herein with reference to Figure 5 of the accompanying drawings.

21. A unit for incorporation into the floor of a tank, in accordance with any of the preceding claims, and defining a first outlet leading to a first drainage system, and a second outlet in the form of a suction means which is disposed around the first outlet and points outwardly away from the axis of the first outlet.

19.

Published 1991 at The Patent Office. Concept House. Cardiff Road. Neuport. Gwent NP9 I RH. Further copies maybe obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmfel.nfach. Cross Keys. Newport. NPI 7HZ. Printed by, Multiplex techniques ltd. St Marv Cray. Kent.