METHOD AND APPARATUS FOR THE TREATMENT OF WATER, ESPECIALLY FOR THE OXYGEN ENRICHMENT OF WATER

Abstract: Method and installation for the treatment of liquids, especially oxygen enrichment of water is used for aqua culture purposes such as the breeding of fish. Thus, it is an object to provide a method where gas bubbles in the water reservoir are avoided and where one freely can treat fresh water and/or recirculated water for optimal temperature control and/or water renewal. The water flow is mixed with a flow of pure oxygen gas which is supplied from an oxygen tank 4 with liquid oxygen. The oxygen is led through an evaporator 16 into a flow system which forms a recirculating circuit, with pipeline, and oxygen enrichment means or reactor unit 2 where water and oxygen are mixed to form a turbulent, foaming gas/liquid flow. The mixture is subsequently led into a gas/liquid separator 3 where gas and liqid are separated. The gas mixture from the separator which now in addition to oxygen also will contain other contaminants like nitrogen and carbon dioxide, is recirculated by means of a compressor 6. To prevent the building up of great amounts of nitrogen and other contaminants, there is provided a de-aeration means in the form of a bleed 10 or in the form of a special separation unit.

Description: This invention relates to the treatment of liquids, especially oxygen enrichment of water which is utilised for aqua culture purposes such as fish breeding. It is important for such purposes to have available clean water, rich in oxygen and of high quality. According to the prior art it has been conventional to bubble air through the water to keep the oxygen content within above 80-100% saturation. It has, however, recently been proved that it is advantageous to increase the oxygen content to a super-saturation of from 120-250%. This increases the growth rate substantially and also makes it possible to obtain a higher degree of utilisation of a given breeding volume. Instead of using air or an oxygen containing gas for enrichment of water, it is also conventional to use pure oxygen as this results in less contamination and greater effect. It is also important that there is no super-saturation of nitrogen in the water. Already at 3-6% super-saturation there is a risk that the fish will suffocate. This can be critical, especially if heated water is utilised for fish breeding, as the amount of gas saturation in water will decrease with increasing temperature of the water. Thus, it is as important to provide a low nitrogen content in the water and eventually reduce this through treatment of the water, as it is to have a sufficiently high content of oxygen. From US Patent No. 4,116,164 it is known to enrich the oxygen content in water in a fish breeding reservoir by introducing gas, containing at least 25 per cent by volume of oxygen, into the water reservoir and where the gas is bubbled through and distributed into the water to maintain a total dissolved oxygen level of at least 120% super-saturation in relation to ambient pressure and temperature. This requires the use of an excess volume of gas. The amount which is not dispersed and dissolved in the water, will rise to the surface where it is collected. After purification and addition of fresh oxygen, the gas is returned to the water reservoir. Such a method has, however, a number of drawbacks. The rising gas bubbles will create a form of ejector action and this will cause contamination due to uprising of excrements from the bottom. The lower specific gravity of the water in the area where the gas bubbles are rising, gives a decreased volume for utilisation, because fish and fish fry will avoid the area where the bubbles are rising. The amount of water available is often a critical factor, especially when there are restricted amounts of water for utilisation. The present invention relates to an improved method for water treatment and oxygen enrichment without these drawbacks. Thus, it is the object of the invention to provide a method in which gas bubbles in the water reservoir are avoided and where there is freedom to treat fresh water and/or recirculated water for optimal temperature control and/or water renewal. French Patent No. 2368990 describes a process for oxygen enrichment of water, in one embodiment of which, the water to be treated is sucked up into a supply pipe in which it is mixed with pressurised liquid oxygen. The liquid oxygen is delivered at high speed and, on mixing, divides into microscopic droplets which, due to the temperature and pressure differences between the liquid oxygen and the water, vaporise to form a small bubbles. The mixture is passed into a pressure vessel and undissolved gas rises to the headspace above the liquid mixture from whence it can be collected. The gas-enriched liquid is returned into the mass of water to be treated. The advantages of the arrangement are said to be firstly, better mixing of gas and water than when gas is simply introduced into the body of water to be treated, especially when the water is stagnant, secondly, better solubility of the gas in the water due to the fact that the gas can be pressurised to above atmospheric pressure and thirdly the enriched water can be returned to any desired location in the mass of water to be treated. A method for the treatment of liquids, especially oxygen enrichment of water, in accordance with the invention in which gaseous oxygen is added to the water to give a super-saturation of between 120 to at least 250%, the water being treated with circulating gaseous oxygen, a gas mixture comprising an excess portion of the oxygen not enriched into the water and gaseous contaminants released from the water being collected after treatment of the water and being recirculated for treatment of additional water to be enriched is characterised by the steps of simultaneously pumping fresh water and/or recirculated water and an excess of gas consisting substantially or wholly of oxygen through an oxygen enrichment or reactor zone and mixing the water and oxygen therein to form a turbulent foaming gas-liquid flow and thus enriching the water with a portion of the oxygen, pumping the gas-liquid flow from the enrichment zone into a gas-liquid separation zone and therein separating the oxygen enriched water from the gas mixture including an excess portion of the oxygen and gaseous contaminants released from the water, releasing at least a portion of the gaseous contaminants from the gas mixture and recirculating the remaining gas mixture back to the enrichment zone, and, discharging the super-saturated, oxygen-enriched water from the separation zone to a position of utilisation in a breeding vessel or pond. The invention further provides an apparatus for enriching water with oxygen to increase the oxygen content of the water to a super saturation of at least 120 - 250% which comprises means for mixing a gas consisting substantially or wholly of oxygen and water to be enriched, means for collecting a gas mixture comprising excess oxygen not enriched into the water and gaseous contaminants released from the water and means for recirculating the gas mixture for mixing with additional water to be enriched, characterised in that the mixing means comprises means for simultaneously pumping water to be enriched with oxygen and an excess of oxygen into an oxygen enrichment means and therein mixing the water and oxygen to form a turbulent, foaming gas-liquid flow and thus enrich the water, gas-liquid separator means for receiving the gas-liquid flow from the mixing means and for separating the oxygen enriched water from the gas mixture, means for releasing at least a portion of the gaseous contaminants from the gas mixture and means for discharging the oxygen enriched water from the separator means to a position of utilization. The invention enables a high degree of supersaturation of the treated water, whilst at the same time, keeping the amount of nitrogen and other pollutions of the water at a low level. The ability of the water to absorb gas will increase propotionally with the increase of the water pressure. The invention allows for oxygen addition with a water pressure of several atmosphere, such that one has nearly complete freedom to choose the amount of oxygen super-saturation of the circulating water up to several hundred per cent. The invention will now be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic flow diagram illustrating a treatment unit in accordance with the invention. Figure 2 is a diagram similar to figure 1 but with a purification unit added. Figure 3 is a diagram of a central treating unit intended to supply oxygen-enriched water to a great number of smaller breeding containers or vessels, and Figure 4 illustrates a preferred dual-stage system. Referring to Figure 1 a pump 1 takes water from a fresh water source 7 and/or from a breeding pond 8. The stream of water is mixed with a stream of pure oxygen which is introduced from an oxygen tank 4 containing liquid oxygen. The liquid oxygen is passed through an evaporator 16 in a flow system forming a recirculation circuit. This consists of pipelines 9, an oxygen enrichment means 2, where water and oxygen are mixed together to form a turbulent, foaming gas/liquid flow, and a gas/liquid separator 3, where gas and liquid are separated. The gas mixture from the separator which now in addition to oxygen also may contain other contaminants like nitrogen and carbon dioxide, is recirculated by means of a compressor 6. To prevent the building up of greater amounts of nitrogen gas and other contaminants, there is provided an outlet 10, hereinafter called a 'bleed' 10, where a proportion of the contaminated gas mixture is blown off under controlled conditions. Figure 2 shows a water treatment unit corresponding to Figure 1, but with partial recovery of oxygen from the bleed gas by means of a membrane separator or a so-called pressure swing separator (PSA) where an absorbing agent is utilised to separate oxygen. In this way a greater part of that oxygen which is added from the oxygen tank 4, is utilised. If desired, the oxygen can also be prepared from air which is led to a pressure swing separator. This will replace the oxygen source 4. The pipeline 11 can then be used as air inlet. This means that a substantially greater and more expensive PSA separator has to be used. Which one of these or other solutions should be selected is, however, primarily a question of costs. Furthermore, to obtain a higher possible super-saturation of water the gas addition can be performed by elevated pressure. A valve 17 is provided to establish such higher pressure. The stream of water can conveniently be recirculated through the treatment unit. This is done by means of the pipeline connection 12, shown on the drawing of Figure 1. Figure 3 shows an arrangement for supply of oxygen-enriched water to a number of breeding vessels 18-25. This illustrates that the system is well suited for use in the form of a movable central unit which can be mounted on a movable frame or a so-called skid (not shown on the drawing) and transported to the place where it is supposed to be utilised. Water with normal oxygen content is fed under pressure into the oxygen enrichment unit 2 together with circulating oxygen gas which is provided by means of a compressor 6. The tank 4 with liquid oxygen, supplies gas to the compressor together with recirculated gas from the gas/liquid separator 3. To reduce the nitrogen content, a bleed 10 is provided in close proximity to this gas stream. Water with high content of oxygen and low content of nitrogen is taken from the bottom of the liquid/gas separator 3 and added to a number of breeding vessels by means of a pump and pipline system 13, 14. Furthermore, it is provided with a separate water supply 15 for water with normal oxygen content, in order to make it possible to control the oxygen content and temperature individually for the different vessels. Figure 4 shows a dual-stage system where the first stage is provided with a bleed 10 used to reduce enriched nitrogen. The second stage is used for the final oxygen enrichment. The drawings are assumed to be self explaining. The different components of the system are given the same reference numbers as previously in connection with the system according to Figure 1-3. As the examples below for a single-stage and a dual-stage system show, with the dual-stage system having nitrogen reduction limited to the first stage, a substantially improved utilisation of the oxygen available is obtained. The total amount of gas circulated through the compressor 6 and the enrichment means 2, does not influence the final consumption of oxygen, but is important for creating a turbulent flow in the water/gas treatment zone and thus influences the efficiency of the enrichment means. In the calculation examples below the amounts of gas which are circulated, are therefore not included. Calculation Example No 1 - One-stage System a) Water in: 1000 m³/h O₂-content : 7ppm ): 5 Nm³/h N₂-content : 18ppm ): 15 Nm³/h b) Water out: 1000 m³/h O₂-content : 25ppm ): 18 Nm³/h N₂-content : 10ppm ): 8 Nm³/h c) Gas composition in the Separator O₂ : 69% (volume) N₂ : 3l% (volume) d) "Bleed": N₂ which is removed: 15-8= 7 Nm³/h Total "bleed": 7/0,31 = 22.6 Nm³/h ): hereby O₂ (loss): 15.6 Nm³/h e) Necessary O₂-supply from tank: 18 + 15.6 - 5 = 28.6 Nm³/h f) Conclusions 1. O₂-enrichment of Water: 18-5 = 13Nm³/h ): O₂-content in Water out: 25 ppm O₂-super saturation: 260% 2. N₂-reduction i Water: 7Nm³/h ): N₂-saturation: 56% 3. Utilization of added O₂ from tank: Calculation Example No 2 - Dual-stage System a) Water in: 1000 m³/h O₂-content : 7 ppm ): 5 Nm³/h N₂-content : 18 ppm ): 15 Nm³/h b) First stage (N₂-reduction) O₂ added from tank: 8 Nm³/h Gas Composition in enrichment means: O₂: 5 + 8 = 13 Nm³/h = 46.4% (volume) N₂: 15 Nm³/h = 53.6% (volume) "Bleed": N₂ which shall be removed: 7 Nm³/h Total "bleed": 13 Nm³/h ): hereby O₂ (loss): 6 Nm³/h Necessary pressure in separator ca. 0,5 bar abs. c) Second-stage (O₂-enrichment) Water in: 1000 m³/h O₂-content : 7 Nm³/h N₂-content : 8 Nm³/h O₂ added from tank: 18-7 = 11 Nm³/h Water out: 1000 m³/h O₂-content : 18 Nm³/h N₂-content : 8 Nm³/h d) Conclusions O₂-enrichment and N₂-reduction of water as in calculation Example No. 1. Total added O₂ from tank: 8+11 = 19 Nm³/h Utilised O₂ from tank: Comment: The difference in efficiency between a dual-stage and a single-stage system is clearly demonstrated. To obtain the stipulated values for oxygen and nitrogen in water, the systems require 28.6 and 19 Nm³O2/h respectively. The practical, calculated examples which are described above, represent preferred embodiments of the utilisation of the principle of the present invention. The centralised water treatment results in great flexibility. There are no upper limits for the ration of oxygen/water due to the recirculation of the oxygen gas. Furthermore, the water flow can also be recirculated and be given repeated treatment with oxygen. Really high super-saturation values can be obtained thereby. A liquid with very high oxygen content is also very convenient for distribution into a greater volume of water. This is not so easy with a stream of gas forming bubbles in the breeding tank. Hereby the utilisation of the available breeding volume can be increased at the same time as the oxygen content can be maintained at a high level without a risk of enrichment of nitogen and other gaseous contaminants. An installation incorporating the water treatment unit is also very flexible and can be used for fresh water which is intended for addition to a pond or it can be used for water which is taken from a pond and returned to the same.

Claims: A method for the treatment of liquids, especially oxygen enrichment of water, in which gaseous oxygen is added to the water to give a super-saturation of between 120 to at least 250%, the water being treated with circulating gaseous oxygen, a gas mixture comprising an excess portion of the oxygen not enriched into the water and gaseous contaminants released from the water being collected after treatment of the water and being recirculated for treatment of additional water to be enriched is characterised by the steps of simultaneously pumping fresh water and/or recirculated water and an excess of gas consisting substantially or wholly of oxygen through an oxygen enrichment or reactor zone and mixing the water and oxygen therein to form a turbulent foaming gas-liquid flow and thus enriching the water with a portion of the oxygen, pumping the gas-liquid flow from the enrichment zone into a gas-liquid separation zone and therein separating the oxygen enriched water from a gas mixture including an excess portion of the oxygen and gaseous contaminants released from the water, releasing at least a portion of the gaseous contaminants from the gas mixture and recirculating the remaining gas mixture back to the enrichment zone, and, discharging the super-saturated, oxygen-enriched water from the separation zone to a position of utilisation in a breeding vessel or pond. A method according to Claim 1, characterised in that, after separation from the water, pressure of the gas mixture is increased whereby a higher degree of super saturation is obtained. A method as claimed in claim 1 or 2 wherein the gaseous contaminants, or a portion thereof, are released by bleeding off a portion of the gas mixture prior to recirculation thereof. A method as claimed in claim 1 or claim 2 wherein the gaseous contaminants, or a portion thereof, are released by passing the gas mixtrue into means for separating at least a portion of the gaseous contaminants from the gas mixture. A method as claimed in any preceding claim wherein the enrichment zone and separator zones are maintained at a pressure of 0.3 to 1 bar abs. and nitrogen is thus removed from the water to be enriched with oxygen. A method as claimed in any preceding claim wherein prior to discharge of the oxygen-enriched water, it is pumped simultaneously with an excess of oxygen into a second oxygen enrichment zone and the water and oxygen are mixed therein to form a second turbulent foaming gas-liquid flow to further enrich the water with oxygen, the second gas-liquid flow then being pumped from the second enrichment zone into a second gas-liquid separator zone in which the oxygen enriched water is separated from a second gas mixture including an excess portion of the oxygen not enriched into the water and gaseous contaminants released from the water, the second gas mixture being recirculated back to the second enrichment zone. A method as claimed in claim 6 wherein the pressure in the second mixing zone and the second separator zone is maintained at above 1.0 bar abs. so that nitrogen is not removed from the water during the second enrichment of the water. Apparatus for enriching water with oxygen to increase the oxygen content of the water to a super-saturation of at least 120 - 250% comprising means for mixing a gas consisting substantially or wholly of oxygen and water to be enriched, means for collecting a gas mixture comprising excess oxygen not enriched into the water and gaseous contaminants released from the water and means for recirculating the gas mixture for mixture with additional water to be enriched, characterised in that the mixing means comprises means (1) for simultaneously pumping water to be enriched with oxygen and an excess of oxygen into an oxygen enrichment means (2) and therein mixing the water and oxygen to form a turbulent, foaming gas-liquid flow and thus enrich the water, gas-liquid separator means (3) for receiving the gas-liquid flow from the mixing means and for separating the oxygen enriched water from the gas mixture, means (10) for releasing at least a portion of the gaseous contaminants from the gas mixture and means for discharging the oxygen enriched water from the separator means to a position of utilization. Apparatus as claimed in claim 8, further comprising means (17) for elevating the pressure of the gas mixture and thereby increasing the level of super saturation of oxygen into the water. Apparatus as claimed in either claim 8 or 9 wherein the releasing means (10) comprises a bleed means for bleeding off a portion of the gas mixture prior to recirculation thereof back to enrichment means (2). Apparatus as claimed in either claim 8 or 9 wherein the releasing means (10) comprises means for separating at least a portion of the gaseous contaminants from the remaining gas mixture. Apparatus as claimed in claim 11, wherein the separating means comprises a membrane separator. An apparatus as claimed in any one of claims 8 to 12 wherein the discharging means comprises pump means and pipeline(s) for forwarding the oxygen enriched water from the gas-liquid separator means to fish breeding vessel(s) or pond(s). Apparatus as claimed in any one of Claims 8 to 13 wherein the recirculating means comprises a return pipeline extending from the gas-liquid separator means (3) to the enrichment means (2) and a compressor in the pipeline for pressurising the recirculated gas mixture.

Inventors: HENRIKSEN, NOROLF
Applicants: NORSK HYDRO A/S
Application year: 1986
Main classification: A01K63
IPC codes: B01F3, A01K63, C02F1, C02F3