Device for lake restoration by oxygen-enriching of the water
This is a continuation, of application Ser. No. 258,546 filed June 1, 1972, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and a device for restoration of lakes by oxygenation of the water. More particularly the invention intends to solve the problem of air-treatment of lakes the depth of which exceeds 8 to 10 meters without disturbing of the thermal stratification of the water. 
     During the summer, the water of such lakes is stratified into two different layers, one upper warmer layer, epilimnion, and one lower colder layer, hypolimnion. The border between the epilimnion and the hypolimnion layer is normally situated at a depth of 8 to 10 meters. In very large lakes the hypolimnion may be located at a depth ranging between 30 and 50 meters. The surface layer, epilimnion, has intimate contact with the atmosphere and is therefore able to take up some oxygen. In this upper layer, plankton algae produce organic matter with oxygen as a by-product. When nutrient concentrations in the upper layer are high by nature, or by action of man, a rapid production of organic matter takes place. A large proportion of this matter settles into the lower colder layer where it is broken down by bacteria into its inorganic components. However, these processes will only occur if there is oxygen in the hypolimnion water. 
     If there is insufficient oxygen for this organic break-down, nutrient salts will diffuse into the water from the sediment layer at the bottom of the lake during the stagnant summer period. In the course of the following spring circulation, these nutrient salts are distributed throughout the bulk of water and thus become available for further organic production. This continuous increase in production of organic matter makes the oxygen balance progressively worse and the lake cannot avoid this development without outside help. 
     One way to help the lake out of this situation is to supply oxygen to the water. The oxygen must be supplied to the lower, oxygen-consuming and oxygen-deficient layer, hypolimnion. In this connection it becomes important not to mix the hypolimnion water with the oxygen-rich surface water because a total oxygen deficiency in the lake could be the result. 
     According to one previously proposed method for oxygenation of lake hypolimnion, water is transported to the surface by means of an air-lift pump and, after having been in contact with the atmosphere, it is returned to its original depth. 
     This known method has the disadvantage that it requires very bulky equipment which is difficult to handle. The air-lift pump has to be at least 10 meters high to penetrate the epilimnion layer. 
     Another disadvantage inherent in this method is that the water is brought into contact with air under atmospheric pressure with consequent relatively low oxygen solubility. 
     According to this invention these problems are solved by air-treatment of the water within the hypolimnion itself, which means that oxygen is introduced into the water at the pressure existing in the hypolimnion. If, for instance, the water is oxygenated at a depth of twenty meters, the amount of oxygen soluble in water is about three times higher than the corresponding amount at the surface. It also means that the device required is much smaller than the previously described air-lift pump. A device for air-treatment of water within the hypolimnion is included in the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view illustrating the thermal stratification of the water in a deep lake during summer. The upper layer, epilimnion, has a temperature differential from about 20° C at the surface to about 4° to 6° at a depth of about 10 meters, whereas the lower layer, hypolimnion, has a constant temperature from the ten meter level downwards. 
     FIG. 2 is a schematic view illustrating the water restoration method according to the invention. 
     FIG. 3 shows a vertical section of a device according to the invention. FIG. 4 shows a horizontal section taken along line IV--IV to FIG. 3. 
     The vertical section shown in FIG. 3 is taken along line III--III in FIG. 4. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     According to the method illustrated in FIG. 2, air is introduced in the hypolimnion through a conduit 10. This conduit terminates at the lower end of a vertically arranged tube 11 which is open at both ends. As air is blown into the water through conduit 10, bubbles rise up through the tube 11 establishing an air-lift pump. The upper end of the tube 11 is located in a substantially bell-shaped housing 12 the purpose of which is to collect the rising air bubbles. Thus, at the uppermost part of the housing 12, an air cushion 13 is formed, the size of which is controlled by a pressure-sensitive restricting valve 14. Excess air is evacuated to the atmosphere through the valve 14 and a conduit 15. The water is oxygenated by contact with the rising air bubbles in the tube 11 as well as within the air cushion 13 in the housing. 
     According to the new method, the water enters the tube 11, rises with the air bubbles through the tube 11, passes through the air cushion 13 and leaves the housing. It is important to prevent air bubbles from escaping the housing together with the leaving water because the freely-rising air bubbles would cause an undesirable upward flow of water outside of the apparatus which in turn would disturb the thermal stratification of the water. 
     Referring to FIGS. 3 and 4, a device for oxygenating water according to the invention will hereinafter be described. The device comprises a housing 21 consisting of an upper part 22, an intermediate part 23 and a lower part 24. The housing 21 has a circular cross section and the upper part 22 and the intermediate part 23 are conical and frusto-conical respectively. The lower part 24 is cylindrical and provided with a plurality preferably three radially arranged outlet openings 25a-c. The lower part 24 is provided with a bottom 26 having a central, circular opening. Moreover, the device comprises a tube 27 which is arranged vertically with its upper end inside the housing 21 level with the intermediate part 23 of the housing and extends downwards through the circular opening. 
     At the lower end of the tube 27 there is mounted an annular nozzle 28 (see FIG. 4) which is connected to a source of pressurized air (not shown) via a hose 29. 
     The tube 27 is attached to three symmetrically disposed brackets 30a-c on the lower part 24 of the housing and is radially supported by three symmetrically disposed guide frames 31a-c mounted on the inside of the housing 21. The attachment of the tube 27 makes it possible to displace the tube 27 axially relative to the housing. For that purpose the tube 27 is provided with axially directed mounting wings 32a-c which are provided with a number of holes for fixing of the tube 27 in different axial positions by means of screw joints. This arrangement makes it possible to vary the annular clearance between the upper edge of the tube and the conical inner wall of the housing 21, whereby the velocity of the downwardly directed outlet flow of water can be varied in accordance with the amount of supplied air. This velocity must be kept below the speed of the rising air bubbles in order to prevent air bubbles from being drawn down with the stream. 
     The upper part 22 of the housing 21 is formed as an air collecting bell provided with an air outlet 33 which communicates with the atmosphere through a conduit 34 and an adjustable flow control valve 35. 
     To the outlet openings 25a-c in the lower part 24 of the housing are connected distributing ducts 39a-c which are provided with air collecting chambers 36a-c each consisting of a tube section having the same diameter as the outlet openings 25a-c and being provided with bell-shaped bulges. The air collecting chambers 36a-c communicate with the atmosphere through conduits 37a-c which are provided with flow control valves 38a-c. The distributing ducts 39a-c may consist of thin-walled plastic tubes which are advantageous by reason of being cheap, light and easy to handle. 
     The device according to the invention also includes stabilizing means consisting of floats 40 and 41 connected to the upper part 22 of the housing 21 and of radially extending arms 42a-c on the housing 21. These arms 42a-c are connected to an anchor block 44 at the bottom of the lake by means of wires 43a-c or the like. 
     The device shown in the FIGS. 3 and 4 also includes a safety device comprising valve scuttles 45 disposed in the upper part 22 of the housing 21. By means of a wire 46, these valve scuttles 45 are connected to a dead-weight 47 resting on the bottom of the lake. 
     The oxygenation device according to the invention is operated from a platform carried by a floating vessel (not shown) on which the compressed air source is located. On the floating platform there is also arranged a hoist by means of which the entire device can be lifted up or lowered down at the installation. The operator&#39;s platform does not comprise part of the invention and for that reason it is not shown in the drawings. 
     The above described device operates as follows: 
     In order to oxygenate water by air-treatment, the device according to the invention is lowered into the hypolimnion which is situated at a depth exceeding 8 to 10 meters. The desired depth is obtained by lowering the device until the anchor block 44 and the dead weight 47 rest upon the lake bottom. The floats 40 and 41 are so dimensioned as to carry the main part of the weight of the device besides the anchor block 44 and the dead weight 47. That means that the floats 40, 41 keep the device in an upright position. The lengths of the wires 43a-c and 46 that connect the device to the anchor block 44 and the dead weight 47 are adapted so as to keep the lower edge of the tube 27 at a distance from the lake bottom of about 2 to 3 meters. In deep lakes it may be advantageous to increase the distance substantially. 
     The device starts working as oxygen-containing gas is supplied to the nozzle 28 from the compressed air source via the conduit 29. The air leaves the nozzle as bubbles which rise through the tube 27 and brings the surrounding water with it. This air-lift pump transports the water up through the tube 27 and into the housing 21. 
     As the air bubbles leave the vertical tube 27 they go on rising and are finally collected and united into an air volume in the upper-most part of the housing 21. Owing to the pressure of the air volume in the upper part 22 of the housing 21 the water has to change direction of flow and go downwards through the annular space between the housing and the vertical tube 27 and further on out through the outlet openings 25a-c. Then, the oxygen-enriched water passes through the air collecting chambers 36a-c in which air bubbles which might have been drawn down with the stream are allowed to rise and to be collected in the bell-shaped bulges. Then, the treated water is distributed through the ducts 39a-c over the lake. 
     The remaining gas which is collected in the upper part 22 of the housing 21 is successively conducted to the atmosphere through the air outlet 33, the conduit 34 and the valve 35. In order to avoid the air outlet conduit acting as an air-lift pump, the air flow has to be restricted. That is accomplished by the flow control valve 35. This valve 35 is adjusted so as to make the air volume in the housing as small as possible and to keep it constant in size. This flow control is important, because if the back-pressure in the conduit 34 were too low, water would enter the conduit 34 and a second air-lift pump would be established. On the other hand, if the back-pressure were too high, the air volume inside the housing would grow and finally reach the point where the water circulation in the housing would stop and, may be, the entire device would rise to the surface. 
     To avoid rising of the entire device the valve scuttles 45 in the upper part 22 of the housing 21 will be opened immediately as the entire weight of dead-weight 47 is acting upon the wire 46. As the safety scuttles 45 are opened, a large volume of air rapidly escapes from the housing 21, whereby the air volume inside the housing as well as the lifting force acting on the device rapidly decreases. 
     In the air collecting chambers 36a-c a secondary collection of air bubbles takes place. This arrangement insures an efficient separation of air bubbles from the water since those air bubbles which have not been collected in the housing 21 are collected later in the air collecting chambers 36a-c. Thus, the circulation rate in the housing 21 can be relatively high without risking air bubbles escaping into the hypolimnion. The collected air is evacuated from the collecting chambers 36a-c through restricting valves 38a-c in the same manner as air is evacuated from the housing 21. 
     In the described embodiment of the invention the water levels within the housing 21 and the air collecting chambers 36a-c are controlled by restricting valves which are designed to be adjusted manually so as to produce desirable air volumes. These valves may of course be made to function automatically, for instance by using floats as sensing means for regulating the air flow in accordance with the water levels. 
     According to another embodiment of the invention the device has no air collecting chambers in the outlet ducts but is provided with a wider space between the tube 27 and the housing which makes it possible to increase the circulation rate without increasing the velocity of the water flow through the housing. Furthermore, it even makes it possible to decrease the water flow velocity and simultaneously increase the circulation rate. By decreasing the water flow velocity in the housing, substantially all of the air may be collected in the housing, thus making a secondary air collection unnecessary. 
     The size of the air volume within the housing may be controlled by a means which is sensitive to the weight of the device in the water. In other words, if the air volume within the housing is too large, the lift force acting on the device balances the weight of the device. In such a case the control valve has to increase the air flow in the outlet conduit so as to diminish the air volume within the housing. 
     The invention is not limited to the shown embodiments but can be freely varied within the scope of the claims.