GB1594831A

GB1594831A - Aquaculture system

Info

Publication number: GB1594831A
Application number: GB8058/77A
Authority: GB
Original assignee: Goodson M G
Current assignee: Goodson M G
Priority date: 1978-02-23
Filing date: 1978-02-23
Publication date: 1981-08-05

Description

(54) IMPROVED AQUACULTURE SYSTEM (71) I, MALCOLM GRAHAM GOODSON, of 52 Eastgate, Deeping St. James, Peterborough, of British Nationality, do hereby declare the invention, for which I pray that a Patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an aquaculture system for rearing aquatic creatures in the form of fish (in particular, but not exclusively, salmonids, centrarchids, clupeids, cyprinids and perciforms), crustacea and shellfish, and to a method of operating the system. For the sake of simplicity in the ensuing description and claims, these aquatic creatures will be referred to as fish.

All intensive fish rearing systems have one very important basic requirement, namely a reliable supply of good quality water in large quantity. For example, to maintain a stock of 100 tonnes of trout, about 30 million gallons of oxygen saturated water would be required each day. Clearly there are few locations where such large quantities of water can be reliably obtained, and naturally these sites attract a high premium. As a result, aquaculture systems have been developed in which the water is repeatedly re-cycled through the tank holding the fish.

An aquaculture system in which the water is re-cycled for repeated use has the following main advantages compared with a system to which fresh water is supplied continuously: (a) It does not require large quantities of water, although the water supply must be of good quality.

(b) Virtual elimination of transmitted disease. Disease can be totally eliminated if the system includes means for ozonisation or ultra-violet sterilization, which is economical with high re-use systems.

(c) There are no problems regarding turbidity or the screening of raw water.

(d) Total control over the environment, particularly the temperature, can be obtained, resulting in shorter growing times and consequently maximum conversion of food into fish growth.

(e) There is a low quantity of good quality effluent. Solids are usually removed separately and disposed of to the land as sludge. Normally the effluent may be discharged to a watercourse without further treatment, although in some cases removal of nitrates may be required.

(f) Production cycles are more predictable and more easily controllable.

The disadvantages of the system are that it involves higher capital and operating costs, although this is offset to some extent by improved food conversion and increased turnover, its reliance on a power supply for pumping, aeration, etc., necessitating the provision of standby power plant, and, owing to its sophistication, the need for a high standard of technical and scientific control.

Water passing through the fish-holding tank of an aquaculture system undergoes two main changes, namely a loss of oxygen due to the respiratory activity of the fish, and an addition of excretory products and unconsumed food particles which, if allowed to accumulate, become toxic to the fish. The main objectives in all aquaculture systems in which the water is re-cycled are to replace the oxygen and to remove the toxic waste products. The effectiveness of these two activities determines the number of times that the feed water can be re-cycled.

Re-cycling aquaculture systems being developed at present usually consist of five basic items, namely a fish-holding tank, means for removal of fish faeces, means for removal of toxic ammonia, means for removal of fine suspended solids and means for re-oxygenating the water.

Fish faeces can contribute up to 70 per cent by weight of the pollutional load on the system and their efficient removal is therefore essential. This is normally accomplished in a separate settlement tank, the faeces being removed from the bottom of this tank as a sludge. For a system employing a fish-holding tank of volume V, the volume of the faeces settlement tank is usually from 0.3 to 1.0V.

Ammonia is produced by fish in the faeces as urea, but is also generated as a respiratory by-product. Ammonia is extremely toxic to fish and is therefore the most important excretory product requiring treatment. Several alternative methods of ammonia removal are available, but the currently preferred methods are biological systems using microorganisms to degrade the ammonia first to nitrite and then to nitrate, this activity commonly being referred to as nitrification. Several treatment systems rely on this method, namely submerged and cascade gravel filters, rotating disc filters, activated sludge systems and fine gravel filters. If a re-cycling ratio in excess of 500:1 is required, or when a low concentration of nitrate is necessary in the final effluent, additional biological treatment is provided involving the conversion by microorganisms of nitrate to gaseous nitrogen. This is an anoxic process and is normally referred to as de-nitrification. Most of the above treatment systems are capable of de-nitrification, either by the design of the system or by the way in which it is operated. All these systems require a large treatment volume, for example a volume of from ].5 to 3.()V for a fish-holding tank of volume V.

With the exception of fine gravel filters, all the biological ammonia removal systems must be supplemented with an arrangement for the elimination of fine solids. This is commonly achieved using sand filters, having a volume of from 0.1 to 0.3V for a fish-holding tank of volume V.

Rc-oxygenation of the water can be achieved in a variety of different ways, for example injection of pure oxygen, mechanical surface aeration and/or bubble aeration, but most systems require a separate re-oxygenation chamber having a volume of about 0.2V for a fish-holding tank of volume V.

Therefore, a typical aquaculture system in which water is re-cycled for repeated use and consisting of a fish-holding tank (of volume V), a settlement tank (of volume V), an ammonia removal system (of volume 3V), a fine solids removal system (of volume 0.3V) and a re-oxygenation chamber (of volume 0.2V), can have a total volume of up to 5.5V. In such a system the water can usually be re-cycled up to about 200 times before complete replacement of the water with fresh water is necessary.

The present invention aims to provide an improved method of operating an aquaculture system of the kind having a fish-holding tank through which water is re-cycled for repeated use, and to provide an aquaculture system of this kind which is considerably smaller than hitherto known systems having a fish-holding tank of comparable size.

According to one aspect of the invention a method of operating an aquaculture system of the kind having a fish-holding tank and means for circulating effluent water from the tank through a treatment plant and back to the tank, comprises the steps of separating fish faeces from the water, passing substantially faeces-free effluent water upwardly through a bed of particulate material to effect fluidization of the bed, the bed material being of a nature to effect microbiological conversion of ammonia in the water to nitrate and adjustment of the alkalinity of the water to a value within a predetermined range, and re-oxygenating the water prior to its return to the tank.

The nature of the effluent water from most aquaculture systems is such that pollutant carbonaceous material in the water will also be degraded by the microorganisms which effect the conversion of ammonia into nitrate.

Preferably, the effluent water is re-oxygenated prior to its passage through the fluidized bed. By appropriate periodic reduction of the oxygen content of the effluent water entering the fluidized bed, de-nitrification of the effluent water can be encouraged as well as the breakdown of the ammonia and carbonaceous material. Alternatively, de-nitrification may be accomplished by by-passing a proportion of the effluent water from the fish-holding tank, prior to re-oxygenation, through an anoxic fluidized bed.

According to a further aspect of the invention, an aquaculture system comprises a fish-holding tank having means for removing fish faeces from the tank water, means for circulating effluent water from the tank through a treatment plant and back to the tank, said treatment plant comprising a biological purification plant in which substantially faeces-free effluent water passes upwardly through a bed of particulate material and creates a fluidized bed of the material in which microbiological conversion of ammonia in the water to nitrate and adjustment of the alkalinity of the water to a value within a predetermined range occur.

and means for re-oxygenating the water prior to its return to the tank.

Preferably, the re-oxygenating means is incorporated in a common vessel with the biological purification plant, so that the treatment plant consists of a single vessel through which the effluent water from the fish-holding tank is circulated and in which the fluidized bed is created.

In some cases the system may require in addition a filter for the removal of'fine solids, and this may be arranged in the fish-holding tank, in a separate chamber externally of the tank, or in the water-circulating means.

In one embodiment of the aquaculture system in accordance with the invention, the fish-holding tank is a shallow circular tank having a diameter to depth ratio of about 10:1 and a floor which slopes from the peripheral wall down to a central hopper with a gradient of from 1:15 to 1:50. Water is introduced into the tank in a circumferential direction at one or more points at or adjacent to the peripheral wall, and water flows from the tank through one or more outlets spaced above the central hopper. With this arrangement of water inlet(s) and outlets(s) a spiral flow path of water is induced in the tank from the peripheral wall towards the centre of the tank. Fish faeces and other process solids fall to the floor of the tank and are swept along spiral paths to the central hopper from which they are periodically removed as a sludge for treatment elsewhere. A fish-holding tank of this nature is described and claimed in my Specification No. 1594832 (Application No. 7234/78).

Substantially faeces-free effluent water discharged from the water outlets(s) of the tank just described passes to a flow distribution chamber whence it is pumped to the biological purification plant. A filtration screen for filtering fine solids from the effluent water may be arranged in the flow distribution chamber, the screen preferably being removable for periodic cleaning.

In the biological purification plant the effluent water from the fish-holding tank is passed upwardly through a bed of sand at a velocity greater than the settling velocity of the sand such that fluidization of the sand occurs. in a new system, microorganisms gradually become established on the surface of the sand grains and are responsible for the removal of the pollutants in the water. This "seeding" operation can, however, be accelerated by the addition of sand from an established (mature) bed, or by addition of suitable biological sludges containing in particular populations of the nitrifying bacteria Nitrosomonas and Nitrobacter, viz. activated/humus sludges from a nitrifying waste-water treatment plant.

Because of the low level of pollutants in the effluent water, nitrification and carbonaceous oxidation proceed simultaneously. Various sizes of sand particles may be employed in the fluidized bed, depending on the required upward velocity of the effluent water. Grain sizes of from 100 to 600 microns are suitable for an upward velocity of the water of from 6 to 40m/hour.

The nitrification reaction is the determining factor for the residence time of the effluent water in the fluidized bed, the removal of ammonia being a function of time (t) in accordance with the equation 2.303.log,,, Ci K = where t is the time in minutes required in the fluidized bed to remove Cj - CO mg/l of NH3-N, Ci being the NH3-N concentration in the water entering the fluidized bed and CO being the NH3-N concentration in the water leaving the fluidized bed. K is a reaction rate constant the value of which is from about 0.75/min to about 1.00/min depending on the dissolved oxygen content of the water. The lower the dissolved oxygen content, the lower the value of K.

The oxidation of ammonia is an aerobic process which theoretically requires 4.5 mg of oxygen of NH3-N oxidised. However, in practice it is found that, allowing for the carbonaceous oxidation requirement, a total of from 7.0 to 9.0 mg of oxygen is required per mg of NH3-N removed. De-nitrification can also be made to occur in the fluidized bed if the dissolved oxygen of the water leaving the bed can be maintained below 0.5 mg/l. In this way a nitrate level of from 50 to 150 mg/l is naturally maintained in the system.

The head loss hF, measured in m, through the fluidized bed can be calculated from the equation hF = Fs (PF - Pw) H, where Fs is the concentration of solids in the fluidized bed, expressed as a proportion of a sample volume of the bed after it has been allowed to settle for five minutes in a cylinder of 11 volume, PF is the specific gravity of the bed solids, Pw is the specific gravity of the water and H is the height, in m, of the fluidized bed. In practice it has been found that a suitable figure forp is about 1.8 and that the optimum value of Fs is from 0.35 to 0.40. There is no improvement in pollutant removal efficiency for values of Fs greater than 0.40.

Carbon dioxide is produced in the fluidized bed as a result of the nitrification reaction, leading to a loss of alkalinity from the water. Conversely, the de-nitrification step, if provided, returns alkalinity to the water. There is a nitrification loss of approximately 7 mg of CaCO3 alkalinity per mg of NH3-N oxidised, and a de-nitrification gain of about 4 mg of CaCO3 alkalinity per mg of NO3-N reduced to gaseous nitrogen, resulting in a nett loss of 3 mg of CaCO3 alkalinity per mg of NH3-N oxidised.

In order to prevent the establishment of acidic water conditions, which would inhibit the nitrification reaction, it is necessary to maintain the pH in the fluidized bed at a value of from 7 to 8.5 by the addition of alkalinity to the system. The preferred way of achieving this is to arrange a bed of reconstituted dolomitic material under the sand bed in the biological purification plant and to cause the effluent water from the fish-holding tank to pass upwardly through the bed of dolomitic material immediately before it flows upwardly through the sand bed. The dolomitic material automatically maintains the CO2/alkalinity ratio of the effluent water, replacing alkalinity as it is lost.

A presently preferred embodiment of the biological purification plant described in general terms above comprises a vessel of circular cross-section in a horizontal plane having a closed bottom and a false floor of mesh material spaced above the bottom. The effluent water is supplied to the vessel through a vertical pipe which extends axially down through the vessel, passes through said false floor and open into the space between the false floor and the bottom of the vessel. This supply pipe has a frusto-conical portion extending through a major part of the height of the vessel, the larger diameter end of this frusto-conical portion being the lower end of the pipe which terminates in the space between the false floor and the bottom of the vessel. The annular space between the wall of the vessel and the effluent water supply pipe receives the material which is to form the fluidized bed when water passes from the lower end of the supply pipe and flows upwardly through the vessel. In one particular plant the bed material consisted of five layers, the lowermost layer, which rested on the false floor of mesh material, consisting of glass spheres having a diameter of 10 mm. On top of this first layer there was a second layer of glass spheres having a diameter of 5 mm, and above this there were two layers of the reconstituted dolomitic material known under the Trade Mark "Akdolit", the particles in the lower layer having a mean size of about 1.7 mm and the particles in the upper layer having a mean size of about 0.8 mm. On top of the "Akdolit" layers there was a layer of sand having a particle size range from 300 to 500 microns.

Re-oxygenation of the effluent water is preferably effected in the above described vessel, it being preferred to introduce pure oxygen from a compressed source of the gas into the vertical effluent supply pipe in the region of the upper end of its frusto-conical portion. The pressure and rate of oxygen supply and the velocites of the effluent water entering the upper end of the frusto-conical portion of the water supply pipe and leaving the lower end of the latter are chosen so that the buoyant velocity of the oxygen bubbles is less than the velocity of the effluent water at the upper end of the frusto-conical portion of the supply pipe but greater than the velocity of the effluent water as it leaves the lower end of the frusto-conical portion to pass upwardly into the fluidized bed part of the vessel.

Consequently, the oxygen bubbles are trapped in the frusto-conical of the effluent water supply pipe, thus providing a long contact time between the bubbles and the water.

Purified and re-oxygenated water flows from the upper part of the vessel back to the fish-holding tank. Preferably, the pump which supplies the effluent water from the aforesaid flow distribution chamber to the biologigal purification plant is arranged to create a head of water in the fluidized bed vessel such that the purified water leaving the latter flows by gravity back to the fish-holding tank. If desired this return flow may take place via a separate compartment of the flow distribution chamber.

On embodiment of an aquaculture system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure I is a schematic plan of the system Figure 2 is a plan of one of the fish-holding tanks of the system of Figure 1, Figure 3 is a sectional view taken on the line III - III of Figure 2, and Figure 4 is a sectional side view of the biological purification plant of the system of Figure 1.

The system shown in Figure 1 comprises two fish-holding tanks T, a flow distribution chamber C, a biological purification plant B and a sump S for the reception of faeces from the tanks T. Effluent water from each of the tanks T passes through a pipe 11 to a compartment C" of the chamber C, whence it is pumped by means of a pump P to the plant B. After purification and re-oxygenation in the plant B, the water flows into a compartment Ch of the chamber C, and from this compartment it returns via pipes 9 to the tanks T. Each tank is connected by a sludge extraction pipe 14 to the sump S.

The two tanks T are of substantially the same construction and one of them will now be described with reference to Figures 2 and 3. The tank T shown in. these Figures comprises a vessel, generally designated by the numeral 1, which has a circular cylindrical wall 2 and a frusto-conical floor 3 which slopes downwardly towards the centre of the vessel 1. In the centre of the floor 3 there is a circular aperture 4 which defines the upwardly-open end of a frusto-conical hopper 5 secured to, and depending downwardly from, the floor 3. Secured to, and projecting upwardly from, the floor 3 around the aperture 4 are a number of vertical supports 6 to which a circular cylindrical screen 7 is secured. The screen 7, which is made of a metallic, for example steel, or plastics mesh material, has its longitudinal axis aligned with the longitudinal axis of the hopper 5 and the vertical axis of symmetry of the vessel 1. The screen 7 extends upwardly from the flour 3 to the level of the upper edge of the wall 2. The screen 7 has a coarse mesh, its purpose being to prevent fish, which are being reared in the tank, from gaining access to the interior of the screen and hopper 5.

The water return pipe 9, which returns purified and re-oxygenated water to the tank from the compartment Cb of the chamber C (see Figure 1), is provided with one or more nozzles 10 for directing the water into the tank in a generally circumferential direction. The water outlet pipe 11, which conducts effluent water from the tank to the compartment Ca of the chamber C (see Figure 1), has a vertically disposed limb 12 within the screen 7, the limb 12 having its upper end open and its lower end connected to a horizontal limb 13 of the pipe 11. The limb 13 is radially disposed in the vessel 1 and passes through the screen 7 and through the wall 2. One of the supports 6 is provided with a clamp 8 for supporting the limb 13.

The sludge extraction pipe 14, which is connected to the sump S (see Figure 1), has a downwardly-open vertical limb 15 within the hopper 5 and a horizontal limb 16 connected to the upper end of the limb 15 in the hopper 5 and extending radially outwards through the wall of the hopper and underneath the floor 3 of the vessel 1.

When the above described tank is in use in the system of Figure 1, purified and re-oxygenated water is supplied continuously to the vessel 1 from the chamber compartment Cb through the pipe 9 and nozzles 10. With the surface of the water at the level 18 (see Figure 3), the outlet pipe 11 is submerged and water flows out through the pipe 11 to the chamber compartment Ca for onward transmission to the plant B (see Figure 1), where it is re-purified, re-oxygenated and returned to the vessel 1 via the chamber compartment Cb and the pipe 9. The orientation of the nozzles 10 and the position of the outlet pipe limb 12 causes the water in the tank to adopt a spiral flow path towards the outlet pipe limb 12, as indicated by the arrows A in Figure 2. When the tank is stocked with fish, the faeces produced and other solid particles, for example unconsumed food particles, fall to the floor 3 and are swept towards the aperture 4 by the spiral flow of the water in the vessel 1, pass through the screen 7 and collect in the hopper 5. Periodically, the sludge of faeces and other solid material is removed from the hopper 5 to the sump S (Figure 1) via the pipe 14 by opening a valve 24 in this pipe.

The vessel 1 and the hopper 5 of the tanks T may be made of glass reinforced plastics material, and the pipes 9, 11 and 14 may be made of plastics material. The wall and floor of the vessel 1 may be thermally insulated, for example with layers of foamed plastics material (not shown). A sheet 20 of transparent plastics material (shown only in Figure 3) may be removably mounted over the top of the vessel 1, to reduce the rate of heat loss from the water in the vessel, to reduce evaporation loss and to prevent foreign matter falling into the vessel. The sheet 20 may be supported on a light framework (shown only in Figure 3) consisting of central supporting members 21 and flexible cords 22, for example of nylon, extending from the upper end of the supporting members 21 to the upper edge of the wall 2.

Each tank T may be partly embedded in the ground, as shown in Figure 3 where the numeral 23 designates the surface of the ground. It should, of course be appreciated that the tanks need not be partly embedded in the ground, but may be provided with supports enabling them to stand on any suitable base.

The effluent water from each tank T is delivered to the compartment C, of chamber C, see Figures 1 and 4. The chamber C is a vessel of rectangular cross-section having side walls 26, end walls 28, a floor 30 and a removable cover 32. A partition 34 divides the chamber into the two compartments Ca and Ch. Within the compartment C(, two removable fine mesh screens 36 define a sub-compartment 38 into which depends a pipe 40 connected to the suction side of the pump P. Effluent water flowing into the compartment Cb, via the pipes 11 from the tanks T must pass through the screens 36 to enter the sub-compartment 38. These screens 36 remove fine solid material which enters the pipe limbs 12 in the tanks T.

The pump P draws effluent water from the sub-compartment 38 of chamber C and passes it via a pipe 42 to the biological purification plant B, see Figure 4. The plant B comprises a vessel 44 having a right-circular cylindrical upper wall portion 46, a right-circular cylindrical lower wall portion 48 and an intermediate frusto-conical wall portion 50 connecting the wall portions 46 and 48. The vessel has a closed bottom 52 and a removable cover 54. Above the bottom 52 there is a false floor 56 made of mesh material.

The pipe 42 is connected to a vertically disposed pipe, generally designated by the numeral 58, which is axially disposed in the vessel 44. The pipe 58 comprises an upper right-circular cylindrical portion 60, a lower right-circular cylindrical portion 62 of larger diameter than the portion 60 and an intermediate frusto-conical portion 64 connecting the portions 60 and 62. The portion 62 of the pipe 58 passes through the false floor 56 and opens into the space between the latter and the bottom 52.

A right-circular cylindrical hood 66 surrounds the pipe portion 60 and the upper part of the pipe portion 64. Six tubes 68 extend radially from the hood 66 into the annular space between the hood and the vessel wall portion 46. These tubes are spaced apart equi-angularly around the vertical axis of the vessel 44. The radially outer end of each tube 68 is closed and at its radially inner end it opens into the hood 66. Each tube 68 has a plurality of holes 70 drilled through its wall.

Below the tubes 68, a pipe 72 extends radially from the hood 66 and passes out through the wall portion 46 of the plant B. The pipe 72 is joined by a length of flexible pipe 74 to the compartment Ch of the chamber C.

The fluidizible bed material of the plant B consists of five different layers of material in the annular space between the pipe 58 and the wall portions 46, 48 and 50. The upper surface of each layer, in the fluidized state of the bed, is indicated by the chain lines 76, 78, 80, 82 and 84. ln the particular embodiment being described, the lowermost layer, which rests on the false floor 56 and extends up to the line 76, consists of glass balls having a diameter of 1 cm. The layer between the lines 76 and 78 consists of glass balls having a diameter of 0.5 cm, the layer between the lines 78 and 80 consists of grade 1 "Akdolit" particles (having a mean particle size of about 1.7 mm), the layer between the lines 80 and 82 consists of grade 0 "Akdolit" particles (having a mean particle size of about 0.8 mm) and the layer between the lines 82 and 84 consists of sand having a particle size in the range of from 300 to 500 microns.

In use of the above described purification plant B, effluent water, pumped along the pipe 42 from the compartment C" of chamber C, passes down through the pipe 58 into the space between the false floor 56 and the bottom 52 and then rises through the bed material causing fluidization of the sand and "Akdolit" layers. In passing though the "Akdolit" layers, automatic correction of the alkalinity of the water takes place, and in passing through the sand layer the previously described nitrification process takes place as well as oxidation of carbonaceous material, and, depending on the dissolved oxygen concentration, de-nitrification also takes place. The purified water rising from the sand layer enters the holes 70 in the tubes 68, passes into the hood 66 and flows out through the pipe 72 to the compartment Cz, of chamber C. From the compartment CS, the purified water flows back to the tanks T via the pipes 9.

The plant B also has provision for re-oxygenating the effluent water passing therethrough. To this end, a pipe 84 enters the pipe 58 near the lower end of the portion 60 of the latter. The pipe 84 leads to a source (not shown) of compressed oxygenating gas, for example air or pure oxygen. When the plant is in use, oxygenating gas is supplied through the pipe 84 at such a rate that the gas bubbles are forced downwardly into the frusto-conical portion 64 of the pipe 58 by the flow of water through the pipe portion 60. As the water descends through the pipe portion 64, its downward velocity will decrease and the dimensions of the pipe portion 64 are chosen so that the water has its velocity reduced to such an extent that the gas bubbles are not forced down below the lower end of the pipe portion 64. With this arrangement the bubbles of oxygenating gas are in contact with the water for a considerable time so that there is a high take-up of the oxygenating gas into the water. Since the presence of bubbles of free oxygenating gas in the fluidized bed would inhibit the microbiological purification processes taking pla to the drawings, the tanks T had a total water volume of approximatley 100 rn, the plant B had a volume of approximately 16 m3 and the chamber C had a volume of approximately 2 m3. This means that the total system had a volume of approximately 118 m , that is to say approximately one and one fifth times the fish-holding volume. The system therefore had a considerably smaller volume than the hitherto known re-cycling aquaculture systems discussed earlier in this specification.

In the system described above with reference to the drawings, the main purposes of the fluidized bed in the plant B are to effect the nitrification process, oxidation of the carbonaceous material and adjustment of the alkalinity of the water. As a result of the nitrification process there is a gradual build-up of nitrate in the water circulating through the system and if nothing is done about this the nitrate concentration will rise to a level at which it becomes harmful to the fish. In practice, it is found that if a small quantity of the water is periodically removed from the system and replaced by fresh water, the nitrate concentration can be kept at an acceptable level if an amount of water equal to the total amount in the system is replaced with fresh water during the time the water takes to be re-cycled through the system up to 500 times. This is referred to as a re-cycling ratio of 500:1, and for many aquaculture systems this is perfectly acceptable. It is, however possible to achieve higher re-cycling ratios, and ways of achieving this will now be described.

De-nitrification of the nitrate (into gaseous nitrogen) is an anoxic process, i.e. it only takes place in the virtual absence of oxygen. Owing to the re-oxygenating facility in the plant B, there is only a negligible possibility of de-nitrification occurring in the fluidized bed. However, since the microorganisms responsible for de-nitrification are facultative, i.e.

they can exist either aerobically or anoxically, they will normally be present in the fluidized bed, where they act aerobically on the carbonaceous material. If the oxygen supply via pipe 84 is cut off periodically, then the only oxygen available to the microorganisms in the fluidized bed will be that provided in the effluent water from the fish-holding tank and this oxygen will be exhausted quickly in the lower part of the fluidized bed. The remainder of the fluidized bed then becomes anoxic, with the microorganisms therein obtaining oxygen from that contained in the nitrate, leaving gaseous nitrogen which is evolved in the vessel 44. The disadvantage of causing de-nitrification in this way, however, is that the fish rely on a continuous supply of oxygen and therefore the re-oxygenating supply in the plant B can only be switched off for short periods, for example no longer than 5 minutes per hour. By operating the plant B in this way, it is possible to achieve a re-cycling ratio significantly higher than 500:1.

As an alternative to creating short periods of de-nitrification in the vessel 44 as described above, some of the effluent water from the tanks T can be by-passed from the pipe 42 immediately downstream of the pump P and fed into a separate and much smaller auxiliary vessel (not shown in the drawings) in which a fluidized bed of sand is created by upward flow of the by-passed water. The purpose of the auxiliary vessel is to effect de-nitrification in the by-passed effluent water. Obviously, there would be some oxygen in the by-passed water, and in the lower region of the fluidized bed in the auxiliary vessel there would be nitrification and carbonaceous oxidation until the oxygen became exhausted. By by-passing approximately 5 to 10 per cent by volume of the effluent water through such an auxiliary vessel it is possible to achieve a re-cycling ratio of at least 2000:1.

The invention is not, of course, limited to the particular system illustrated in the drawings. For example, there may be a single tank T associated with the plant B, or there may be more than two tanks T associated with a common plant B. Again, instead of arranging removable screens 36 in the chamber C, the filtration of fine solids from the effluent water may be effected in the tank(s) T by arranging a filtration screen around the pipe limb 12 inside the screen 7. Alternatively, the pump P may be provided with means for filtering fine solids from the effluent water.

WHAT I CLAIM IS: 1. A method of operating an aquaculture system of the kind having a fish (as hereinbefore defined)-holding tank and means for circulating effluent water from the tank through a treatment plant and back to the tank, comrprising the steps of separating fish faeces from the water, passing substantially faeces-free effluent water upwardly through a bed of particulate material to effect fluidization of the bed, the bed material being of a nature to effect microbiological conversion of ammonia in the water to nitrate and adjustment of the alkalinity of the water to a value within a predetermined range, and re-oxygenating the water prior to its return to the tank.

2. A method according to claim 1, in which the bed material also effects microbiological breakdown of carbonaceous matter in the effluent water.

3. A method according to claim 1 or 2, in which the effluent water is re-oxygenated prior to its passage through the fluidized bed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. to the drawings, the tanks T had a total water volume of approximatley 100 rn, the plant B had a volume of approximately 16 m3 and the chamber C had a volume of approximately 2 m3. This means that the total system had a volume of approximately 118 m , that is to say approximately one and one fifth times the fish-holding volume. The system therefore had a considerably smaller volume than the hitherto known re-cycling aquaculture systems discussed earlier in this specification. In the system described above with reference to the drawings, the main purposes of the fluidized bed in the plant B are to effect the nitrification process, oxidation of the carbonaceous material and adjustment of the alkalinity of the water. As a result of the nitrification process there is a gradual build-up of nitrate in the water circulating through the system and if nothing is done about this the nitrate concentration will rise to a level at which it becomes harmful to the fish. In practice, it is found that if a small quantity of the water is periodically removed from the system and replaced by fresh water, the nitrate concentration can be kept at an acceptable level if an amount of water equal to the total amount in the system is replaced with fresh water during the time the water takes to be re-cycled through the system up to 500 times. This is referred to as a re-cycling ratio of 500:1, and for many aquaculture systems this is perfectly acceptable. It is, however possible to achieve higher re-cycling ratios, and ways of achieving this will now be described. De-nitrification of the nitrate (into gaseous nitrogen) is an anoxic process, i.e. it only takes place in the virtual absence of oxygen. Owing to the re-oxygenating facility in the plant B, there is only a negligible possibility of de-nitrification occurring in the fluidized bed. However, since the microorganisms responsible for de-nitrification are facultative, i.e. they can exist either aerobically or anoxically, they will normally be present in the fluidized bed, where they act aerobically on the carbonaceous material. If the oxygen supply via pipe 84 is cut off periodically, then the only oxygen available to the microorganisms in the fluidized bed will be that provided in the effluent water from the fish-holding tank and this oxygen will be exhausted quickly in the lower part of the fluidized bed. The remainder of the fluidized bed then becomes anoxic, with the microorganisms therein obtaining oxygen from that contained in the nitrate, leaving gaseous nitrogen which is evolved in the vessel 44. The disadvantage of causing de-nitrification in this way, however, is that the fish rely on a continuous supply of oxygen and therefore the re-oxygenating supply in the plant B can only be switched off for short periods, for example no longer than 5 minutes per hour. By operating the plant B in this way, it is possible to achieve a re-cycling ratio significantly higher than 500:1. As an alternative to creating short periods of de-nitrification in the vessel 44 as described above, some of the effluent water from the tanks T can be by-passed from the pipe 42 immediately downstream of the pump P and fed into a separate and much smaller auxiliary vessel (not shown in the drawings) in which a fluidized bed of sand is created by upward flow of the by-passed water. The purpose of the auxiliary vessel is to effect de-nitrification in the by-passed effluent water. Obviously, there would be some oxygen in the by-passed water, and in the lower region of the fluidized bed in the auxiliary vessel there would be nitrification and carbonaceous oxidation until the oxygen became exhausted. By by-passing approximately 5 to 10 per cent by volume of the effluent water through such an auxiliary vessel it is possible to achieve a re-cycling ratio of at least 2000:1. The invention is not, of course, limited to the particular system illustrated in the drawings. For example, there may be a single tank T associated with the plant B, or there may be more than two tanks T associated with a common plant B. Again, instead of arranging removable screens 36 in the chamber C, the filtration of fine solids from the effluent water may be effected in the tank(s) T by arranging a filtration screen around the pipe limb 12 inside the screen 7. Alternatively, the pump P may be provided with means for filtering fine solids from the effluent water. WHAT I CLAIM IS:

1. A method of operating an aquaculture system of the kind having a fish (as hereinbefore defined)-holding tank and means for circulating effluent water from the tank through a treatment plant and back to the tank, comrprising the steps of separating fish faeces from the water, passing substantially faeces-free effluent water upwardly through a bed of particulate material to effect fluidization of the bed, the bed material being of a nature to effect microbiological conversion of ammonia in the water to nitrate and adjustment of the alkalinity of the water to a value within a predetermined range, and re-oxygenating the water prior to its return to the tank.

4. A method according to claim 3, in which the re-oxygenation is periodically

interrupted to allow microbiological conversion of nitrates to nitrogen in the effluent water passing through the fluidized bed.

5. A method according to any of claims 1 to 3, comprising the step of by-passing a proportion of the water from passing through said treatment plant and causing it instead to flow upwardly through a second bed of particulate material to effect fluidization of the second bed, the bed material of this second bed being of a nature to effect microbiological conversion of nitrate in the by-passed water to gaseous nitrogen.

6. An aquaculture system comprising a fish (as hereinbefore defined)- holding tank having means for removing fish faeces from the tank water, means for circulating effluent water from the tank through a treatment plant and back to the tank, said treatment plant comprising a biological purification plant in which substantially faeces-free effluent water passes upwardly through a bed of particulate material and creates a fluidized bed of the material in which microbiological conversion of ammonia in the water to nitrate and adjustment of the alkalinity of the water to a value within a predetermined range occur, and means for re-oxygenating the water prior to its return to the tank.

7. A system according to claim 6, in which microbiological breakdown of carbonaceous material in the effluent water also occurs in said fluidized bed of material.

8. A system according to claim 6 or 7, in which the re-oxygenating means is incorporated in a common vessel with the biological purification plant.

9. A system according to claim 8, comprising means for by-passing a proportion of the water from passing through said treatment plant and causing it instead to flow upwardly through a second bed of particulate material to effect fluidization of the second bed, the bed material of this second bed being of a nature to effect microbiological conversion of nitrate in the by-passed water to gaseous nitrogen.

10. A system according to any of claims 6 to 9, comprising a filter for the removal of fine solids from the effluent water, said filter being arranged in the fish-holding tank, in a separate chamber externally of the tank, or in the water circulating means.

11. A system according to any of the claims 6 to 10, in which said first mentioned bed of material comprises a layer of reconstituted dolomitic material surmounted by a layer of sand.

12. An aquaculture system constructed and arranged substantially as herein described with reference to, and as illustrated in, the accompanying drawings.