WO2004065312A1

WO2004065312A1 - Method and device for increasing oxygen content of hypolimnion and for removing harmful gases in hypolimnion

Info

Publication number: WO2004065312A1
Application number: PCT/FI2004/000033
Authority: WO
Inventors: Matti Lappalainen
Original assignee: Vesi-Eko Oy Water-Eco Ltd
Priority date: 2003-01-23
Filing date: 2004-01-22
Publication date: 2004-08-05
Also published as: EP1594808A1; FI20030105A0

Abstract

The present invention relates to a method for increasing oxygen content of hypolimnion (1; 20) and for removing harmful gases in hypolimnion, in which method the hypolimnetic water is pumped with a pumping device (3; 22) through a riser tube (2; 21) reaching from the hypolimnion up near the surface to an upper basin (9) on the water surface, where the hypolimnetic water is aerated and harmful gases are removed, and from which upper basin (9) the oxygenated water is transferred through a downcomer (11) back to the hypolimnion. The present invention also relates to a device for increasing the oxygen content of the hypolimnion (1; 20) and for removing harmful gases in the hypolimnion, which device comprises an upper basin (9), a riser tube (2; 21) reaching from the hypolimnion up near the water surface, a pumping device (3; 22) for pumping the hypolimnetic water in a riser tube and a downcomer (11) conducted back from the upper basin to the hypolimnion for conducting the oxygenated hypolimnetic water back to the hypolimnion. Characteristic to the method in accordance with the invention is the fact, that the hypolimnetic water (1; 20) is conducted through a riser tube (2; 21) to an accelerating device (4; 25), where the flow velocity of the hypolimnetic water is increased and that the hypolimnetic water (1; 20) is conducted to a basic guide (5; 24) located at least partly over the surface of the water, by means of which the hypolimnetic water is directed to a distance above the water surface and back towards the water surface at suitable velocity and at a suitable angle of attack. Characteristic to the device in accordance with the invention is the fact that the device comprises an accelerating device (4; 25) for increasing the flow velocity of the hypolimnetic water conducted through the riser tube (2; 21) and a basic guide (5; 24) reaching at least partly above the water level for directing the hypolimnetic water (1; 20) coming through the riser tube a distance above the water level and back towards the water surface at suitable velocity and at a suitable angle of attack.

Description

METHOD AND DEVICE FOR INCREASING OXYGEN CONTENT OF HYPOLIMNION AND FOR REMOVING HARMFUL GASES IN HYPOLIMNION

The present invention relates to a method for increasing oxygen content of hypolimnion and for removing harmful gases in hypolimnion, in which method hypolimnetic water is pumped through a riser tube reaching from hypolimnion up near the surface to an upper basin on the water surface, where the hypolimnetic water is aerated and harmful gases are removed, and from which upper basin the oxygenated water is conducted through a downcomer back to the hypolimnion. The present invention also relates to a device for increasing the oxygen content of the hypolimnion and for removing harmful gases in the hypolimnion, which device comprises an upper basin, a riser tube reaching from the hypolimnion up near the water surface, a pumping device for pumping the hypolimnetic water in a riser tube and a downcomer conducted back from the upper basin to the hypolimnion for conducting the oxygenated hypolimnetic water back to the hypolimnion.

The importance of aeration and oxygenation of lakes due to increasing of loads in climate and surface water directed to inland water bodies has significantly increased during the past decades. Numerous aeration and oxygenation devices and methods being used in stratified, deep or middle deep lakes both in summer and in wintertime have been developed starting mainly since 1960's. Known methods are based on a few main principles. The Bernhard aeration method (Full Lift Aeration), invented already in the 1960's and commonly recognized through the world, is one of them. Fall aeration, known in river aeration, is another method related to the field of application of the invention.

The most essential characteristics of the Bernhard method are: 1) compressed air released to the lower part of the vertical pipe placed in the hypolimnion induces water flow (a mammoth pump phenomenon), where oxygen is dissolved, 2) water is discharged to an upper basin, where harmful gases are removed into atmosphere, 3) oxygenated water is forced to discharge through a downcomer from the upper basin back to the hypolimnion, in which case the temperature of the hypolimnion as well as the stratification of the lake does not change, because the change in temperature of the water after visiting in the upper basin is extremely small. In principle, there are several applications of device, aeration devices with double pipes, for example, in which the downcomer surrounds the riser tube. Fall aeration has been developed by studying water masses rushing into a tail water basin over river weirs. It has been found out in these studies that while water falls down into a tail basin under a weir (so called tail basin) quite a great amount of oxygen is dissolved into water. It is essential in terms of efficient oxygenation result that a fall has a certain optimal falling height, which appears to be app. 0,7 - 1,2 m, according to studies. Thus, by employing the formula v = (2-g-h) we get for water 3,71 - 4,85 m/s as the optimal collision velocity to the surface of water. In addition, at the end, water needs to fall as a rather compact but suitably rough on the surface jet, to obtain the ability to take as much as possible air bubbles to the depth of 0,4 - 0,8 m. Minimum depth of the tail basin must equal about 0,6 times the falling height. By employing fall aeration already with falling heights of 0,7 - 1 m E values (E= oxygen increase / oxygen deficit) 0,3 - 0,6 of transferring efficiency are reached, which correspond to increasing of oxygen saturation rate for 30 - 60 %. Therefore, fall aeration is a rather efficient and extremely simple method. In practical applications of the method, which are employed in waste water basins of waste water treatment plants the necessary fall and motion energy have been created by lifting water by pumping to the height corresponding to overrushing. Applications for lakes of fall aeration are not commonly known at the moment.

All method versions of the Bernhard method include following disadvantages among others:

1. Discharging compressed air to the lower part of the hypolimnion, to the depth of 10 m for example, means 1 bar overpressure, which demands a lot of energy. Although, on the other hand, overpressure facilitates solubility of gases, the solution of oxygen into water from air bubbles rising freely is slow, because air bubbles are not broken (to have a good transferring efficiency continuous "disturbing" of the bubble-water interface is needed). In practice, the specific transferring capability ranges 0,4 - 0,8 kg 0₂/kWh depending on the size of orifices of diffusers and the depth of use, which, as being somewhat poor, causes big sizes of devices and applying costs. A notable reason why the specific transferring capability usually stays poor, in practice, is the fact that fine-pore diffusers (pore size 0,1 - 0,2 mm of size) which are more efficient than coarse-pore diffusers get blocked-up due to chemical and biological processes rather quickly demanding usually a lot of cleaning work by divers.

2. Due to poor specific transferring capacity the device needs to be of big size to reach sufficient transfer of oxygen. It is, however, difficult to realize the control of big devices in lake conditions, while exposed to the influence of wind, waves, freezing phenomena and breaking up of ice.

Correspondingly, disadvantages appearing in lake applications of fall aeration are:

1. Lifting of water to the optimal falling height (0,7 - 1,2 m) and the controlled distribution to even jets requires quite a large and especially stable upper basin. This may be realized on sheltered waste water reservoirs, but it is difficult to be realized in lake conditions while exposed to the influence of wind, waves, freezing phenomena and breaking up of ice. In addition, sustaining water of a large upper basin needs a large float.

2. Keeping in desired form of separate, thin, 0,7 - 1,2 m high partial jets, based merely on falling acceleration is, in windy and rocking lake conditions, almost impossible. Building of wind shelters and stabilizing constructions would bring remarkable additional costs and would increase the size of already big devices.

3. Based on several researches it is known that dividing of the total flow into small partial jets improves the oxygenation results in windless and stable laboratory conditions. Based on those following target practice may clearly be conducted: For reaching the optimal result the flow of partial jets needs to be on the rage of 1 - 4 I/s. It is known, as well, that increasing the height of fall (that is the collision velocity) improves the oxygenation result. However, it has been noticed already in laboratory conditions, that while decreasing the flow velocity of a partial jet falling from high the head of the jet breaks into drops, and the oxygenation result gets remarkable worse. This is based on the combination of the gravity of the earth accelerating and thus stretching and narrowing the jet in a high fall and, in addition, the air resistance is trying to break it. In a high fall the jet needs to be thick for the head to hold together. Therefore, a high fall may not be divided into small partial jets as, on the other hand, should. These disadvantages are being emphasized in windy and rocking lake conditions, which may explain why the method has not become general.

The object of the invention is to provide a method, by employing of which the techno-economical disadvantages and problems included in the earlier mentioned original methods may remarkable be decreased. Especially, the object of the invention is to provide a method, by employing of which increasing of the oxygen content of the hypolimnion of lakes and removing of harmful gases are managed to realize reliably, simply and economically by operating and purchasing costs. In addition, the object of the invention is to provide a device in accordance with the method, which is well suitable for lake conditions, small in size, efficient and economical by costs.

The object of the invention is accomplished by a method and device, the characteristics of which are presented in the claims.

Characteristic to the method in accordance with the invention is the fact, that hypolimnetic water is conducted through a riser tube to an accelerating device, where the flow velocity of the hypolimnetic water is increased and that the hypolimnetic water is conducted through a riser tube to a basic guide located at least partly over the surface of the water, by means of which the hypolimnetic water is directed to a distance above the water level and back towards the water surface at suitable speed and at a suitable angle of attack. By means of this kind of a method the kinetic energy bound in the total flow velocity of pumped hypolimnetic water may be directed in such shape and roughness of the surface which maintains the motion energy as well as possible towards the upper basin and collided against the surface of water such, that while the motion energy is absorbed the transfer of oxygen to water and the transfer of harmful gases from water to the air is realized more effective per removed amount of water than in today known fall aeration. In addition, problems caused by wind and rocking conditions (waves) typical to fall aeration are avoided, because the falling height is essentially lower than in fall aeration. Therefore, by employing this kind of a method increasing of oxygen content of hypolimnion and removing of harmful gases succeeds reliably, simply as well as with less costs than earlier.

In an advantageous application of the method in accordance with the invention, the total flow rate of hypolimnetic water is divided in the basic guide with jet guides into partial jets suitable in size by their flow velocity, and it is roughened with surface roughening guides. It has been stated in many researches that the dividing of the total flow rate into small partial jets improves the oxygenation result. Therefore, by dividing the total flow rate in this way as efficiently as possible into partial jets, exact by their shape and surface roughness and efficiently collecting oxygen to the hypolimnetic water, it is possible to improve the oxygenation result attained by the method.

In the second advantageous application of the method in accordance with the invention the hypolimnetic water is pumped through the riser tube to the basic guide so, that the collision velocity of the hypolimnetic water to the water surface gains app. 3,5 - 5 m/s. It has been stated in researches that with this kind of velocity it is possible to gain the best possible exchange of gases, that is oxygenation of the hypolimnetic water and removing of harmful gases.

In the third advantageous application of the method in accordance with the invention the hypolimnetic water is guided with a basic guide such, that the angle of attack of water against the water surface is at 60 - 90° angle at the moment of collision. By employing this kind of angle of attack, dividing up of water at the moment of hitting the water surface is prevented. This improves the oxygenation result.

In the fourth advantageous application of the method in accordance with the invention, the hypolimnetic water is pumped with a propeller pump located in a riser tube. A propeller pump is a highly profitable pumping device and the efficiency of a propeller pump is good. By the construction, a propeller pump is well applicable to device applications of the method, because it is symmetric in revolution, and therefore suitable for mounting inside a tube-like or square frame. Due to this, a device based on a propeller pump is possible to create as small, well- balanced and simple by construction.

In the fifth advantageous application of the method in accordance with the invention, the flow velocity is increased with a narrowing part located inside the rise tube. In this way, the flow velocity of the hypolimnetic water conducted to a basic guide may be increased in order to create suitable collision velocity simply and advantageously. The nan-owing part is possible to shape to join the basic guide as evenly as possible such, that the flow velocity accelerates evenly and without loss as far as possible.

In the sixth advantageous application of the method in accordance with the invention, the flow velocity of the hypolimnetic water is decreased with a enlargening part located after the pumping device and increased with jet nozzles accelerating the flow velocity located in the basic guide. In this way, it is possible to decrease the energy losses due to flow resistance taking place between the pumping device and the jet nozzle to be as small as possible. This improves the efficiency of the device and decreases the energy consumption of the device. In addition, while employing this kind of method it is possible to create the basic guide very simple by construction and economical by manufacturing costs. Characteristic to the device in accordance with the invention is the fact that the device comprises a basic guide reaching through a riser tube at least partly above the water surface for guiding the hypolimnetic water coming through a riser tube a distance above the water surface and back towards the water surface at suitable speed and at a suitable angle. This kind of device is possible to construct compact by construction and size, notably lower by parts located above the water surface than in cases of traditional fall aeration heights, capable for distributing the total flow rate into numerous partial jets improving efficiency and about 2 - 3 times more effective than bubble aeration by specific transferring capability. In addition, this kind of device is nearly free from breaking influence of wind of water jets, enduring motions of waves and rocking as well as moving under ice while being pushed by an ice float. Therefore, it is decisively techno-economically more advantageous than presently known devices used for increasing oxygen content of the hypolimnion.

Next, the invention will be explained in more detail with reference to the accompanying drawings, in which,

Figure 1 illustrates a principle drawing of a device in accordance with the invention,

Figure 2 illustrates a basic guide included in the device in accordance with figure 1 viewed from side, Figure 3 illustrates a basic guide included in the device in accordance with figure 1 viewed from above, and

Figure 4 illustrates a principle drawing of another device in accordance with the invention.

The device in accordance with figures 1-3 comprises an upper basin located on the water surface, a riser tube 2 conducted from near the water surface to the hypolimnion 1, a propeller pump 3 mounted inside the riser tube by means of a motor housing acting as the basic float 13, a basic guide 5 and a jet guide 6 and a surface roughening guide 7 located in the basic guide as well as a downcomer 11 conducted from the upper basin 9 to the hypolimnion 1. In addition, the application in accordance with figures 1-3 comprises, among other things, a flow guide 12, attached to the riser tube 2 under the downcomer, with which flow guide, in case of low hypolimnion, the access of removed water back to the riser tube is prevented, wires 18 and a bottom weight 19, with which the device has been anchored slightly to float freely along with rising and lowening of the surface. Furthermore, the device comprises, among other things, devices for current supply and controlling of the pumping device. The upper basin 9 is an object, cylindrical in the upper part, the edge construction

14 of which forms the float carrying the upper basin. A cylindrical part forms the lower part of the upper basin, which cylindrical part has been connected with the upper part of the downcomer 11 as illustrated in figure 1. The walls of the upper basin are made of suitable plate like material, which is preferably of material well isolating heat, such as some suitable plastic. The edge construction 14 of the upper part of the upper basin is made, in this case, of two layer plastic tube with air between the layers, which is a good heat isolation and which, in wintertime, prevents the heat of warm water (2-4 degrees) from transferring through the wall of the tube outside the basin. The depth of the upper basin 9 needs to equal at least the depth where air bubbles reach while diving. In the application in accordance with figures 1-3 the depth of the upper basin is about 0,5 - 1 m. The diameter of the upper basin corresponds to the horizontal area, what is needed for bubbles to rise on the surface of water. The diameter of the upper basin is, in this case, about 2 m.

The riser tube 2 is a round by cross-section plastic tube with a curved narrowing part 4 in the upper part. The length of the riser tube has been chosen such that the tube reaches from the surface of water to the lower part of the hypolimnion just a suitable distance from the bottom of the lake. The magnitude of the narrowing of the narrowing part 4 has been defined such that with a defined flow velocity of the hypolimnetic water suitable collision velocity of the hypolimnetic water against the water surface in the upper basin may be attained. The downcomer 11 is a plastic tube like the riser tube reaching from the lower part of the upper basin to the upper part of the hypolimnion. The downcomer is bigger by diameter than the riser tube placed inside the downcomer such, that there is a space between the inner part of the downcomer and outer part of the riser rube where oxygenated water may flow downwards to the upper part of the hypolimnetic stratum. At a suitable distance from the lower part of the downcomer there is a flow guide 12, the purpose of which is to secure that oxygenated water coming from the downcomer spreads as evenly as possible to the surrounding hypolimnion and does not flow back to the downcomer even in case of low hypolimnion. The flow guide 12 is, in this case, a flange type object attached round the riser tube.

The pumping device 3 is a low speed (about 150 - 400 rpm) submerged propeller pump, which is driven by an electric motor with reduction gear. The motor and gear of the pump have been encased such, that the case of the pump forms a high basic float 13 inside the downcomer 2. The basic float 13 has been attached with a wire

15 to the edge construction 14 of the upper basin 9. The regulation of the height of the basic guide is partly carried out by changing the size of the basic float 13 but mainly the regulation is carried out by regulating the height and the floating ability of the edge construction 14 of the jet basin functioning as an additional float and by regulating the carrying wires 15.

The basic guide 5 is a round surface (torus) curved to the direction of the edges of the upper basin, on the lower surface of which the hypolimnetic water coming through the riser tube stays due to the centrifugal force. On the inner surface of the basic guide there are several jet guides 6 illustrated in figures 2 and 3, by means of which the hypolimnetic water flow, otherwise shaping round and lowening, is narrowed such, that flow velocity and motion energy hold as well as possible and that the total flow is divided into desired series of partial jets. The basic guide 5 comprises also surface roughening guides 7 located after jet guides 6, with which partial jets are roughened in order to increase the forming of air bubbles. In this case, a metal basic guide 5 has been attached to the upper part of the basic float 13 such that the heat of the motor of the pumping device is transferred to those parts of the basic guide, which are in danger of gathering ice coat in wintertime.

The device has been anchored slightly with two stay wires 18 and a bottom weight 19 to float freely along with rising and lowening of the surface. The floating characteristics of the device are defined to be rather heavy, in order to avoid damages due to moving of ice. Stay wires are mounted to rise rather steeply, for example at 60 - 70 degree angle in respect of the horizontal plane. In this case, while an ice float pushes the device sideways, a component of force drawing the device downwards is created such that the device easily finds its way below the ice.

In order to compensate the torsion moment due to torsion of the propeller of the propeller pump functioning as a pumping device 3, directing plates are mounted above the propeller which function in a corresponding way as the directing parts of a turbine. In addition, additional weights are mounted while needed to stay wires in order to create counter moment preventing circling.

While employing a device in accordance with figures 1-3 the hypolimnetic water 1 with a low oxygen rate is pumped through a riser tube 2 with a propeller pump 3 into the narrowing part 4 of the riser tube, where the water velocity accelerates to 3,5 - 4,5 m/s and further from there to a basic guide 5. In this way a suitable hypolimnetic water flow velocity in the basic guide is achieved, which is about 1-4 liters per second. In the basic guide 5 about 0,1 - 0,3 m above the water surface vertical flow is turned radial, in this case, with a round basic guide back against the surface of the upper basin such that the motion energy and the motion rate hold as well as possible, which is best realized when the flow velocity is kept constant. Jet guides 6 and surface roughening guides 7 divide the total water flow rate into several partial jets 8 suitable rough by their surface, which collide at a great angle of attack against the surface of the basin 9. Water jets with lots of kinetic energy created in this way take plenty of air bubbles 10 with them, from which oxygen dissolves in injected water and to water near the jet due to strong turbulence. At the same time possible harmful gases in injected water are removed into atmosphere. Dived air bubbles discharge from the surface of water basin into the air and water which has no bubbles but is oxygenated flows under small hydrostatic pressure to the downcomer 11 and the hypolimnetic water returns to the upper part of the hypolimnetic stratum, from where it slowly flows deeper to the depth corresponding to its density.

The method and the device in accordance with the invention are not limited to the presented application in figures 1-3 but it may be realized in many parts differing from that. Various parts of the device may be built differently while needed. For example, the depth and the diameter of the upper basin depend on the velocity of the water colliding against the water in the upper basin as well as the thickness of used jets and the diameter depends on the area needed for upcoming bubbles which area depends on earlier mentioned factors. The length and the diameter of the riser tube may be varied. The cross-section is preferably round but also other shapes are possible. The cross-sectional area of the riser tube may also vary. The cross- sectional area depends on the needed volume flow rate. Increasing the volume flow rate results in increase in flow velocity in which case also flow resistances increase. Therefore, the flow velocity is kept low before the pumping device, typically about 0,5 - 1,5 m/s. In addition, due to this it is advantageous that the inner surface of the riser tube is as smooth as possible.

A basic guide attached to a high basic float in accordance with figures 1-3 as well as jet guides and surface roughening guides attached to the basic guide may be varied in order to optimize the oxygenation rate and, on the other hand, to emphasize aesthetic values in many ways. A basic guide may be a series of curved pipes, of round or square shape, the total cross-sectional area of which equals the cross- sectional area of the acceleration point of the riser tube. There are no jet guides but suitable surface roughening guides are employed at the end of the tube. In one application the basic guide is a series of curved flumes, opening downwards and square by cross-section, into which the rising water is directed and where water is kept by created centrifugal force. In this case, there are no jet guides but there are suitable surface roughening guides at the end of the flume. In another application the basic guide is a plane attached to the basic float, the edges of which plane are curved torus-like downwards for example about 80 degrees. The riser tube is, in this case, bigger and the flow velocity correspondingly smaller. In the riser tube there is only a torus edge rounded outwards, at the point of the outer edge of which the edge of the basic guide starts to turn downwards. The difference in height of the torus edge of the riser tube and the basic guide is regulated to be such small that the flow velocity of water, taking into account the accelerating effect of jet guides mounted between them, amounts only at the arc curving downwards the desired 3,5 - 4,5 m/s. Suitable surface roughening guides are attached to jet guides and/or basic guides, which surface roughening guides may be, for example, in accordance with the application in figures 1-3. Further, in an application the basic guide is a vessel supported above the water surface to the basic float, into which vessel the hypolimnetic water is conducted through a riser tube and in which there are orifices at suitable points, which orifices form jet guides. Surface rougheners for water may be formed into orifices, for example, by shaping the edges of the orifices.

An application in accordance with the invention has been illustrated in figure 4. In there the basic guide 24 has been formed of a plate-like vessel, into which the hypolimnetic water 20 is pumped with a propeller pump 22 in accordance with the application in figures 1-3 through a riser tube 21. This application does no comprise a narrowing part after the riser tube 21 and the pumping device 22 but, on the contrary, unlike in the application in figures 1-3 in this application the flow velocity of the hypolimnetic water is slowed down with a enlargening part 23 placed at the point of the narrowing part and the increasing of the flow velocity is not realized until in the jet nozzles 25 of the basic guide 24 (there are 57 jet nozzles in this application), which inject the hypolimnetic water pumped to the basic guide to the upper basin 27. In this way losses occurring due to flow resistances on the area of riser tube and the basic guide are tried to be minimized. The application in accordance with figure 4 comprises also a mixing flow tube 26. It is a tube or a hose made of suitable tube material such as a plastic tube, with a suitably thick diameter, which has been conducted from the basic guide 24 below the riser tube 21 near the bottom. Water flow from the end of the mixing flow tube 26 causes turbulence near the bottom, which mixes and rises the heaviest hypolimnetic water near the bottom to the strata near the riser tube and to the suction flow of the riser tube. Therefore, due to the mixing flow tube 26 also the hypolimnetic water near the bottom and often with the poorest oxygen content appears in the riser tube 21. A mixer of the hypolimnetic water near the bottom functioning as does a mixing flow tube 26 may be realized by employing a propeller mixer, which is run by a turbine propeller placed in the riser tube, or with a suction flow guide, which causes suitable mixing turbulence in the hypolimnion.

Also many other parts included in the device may be realized in a different way. For example, as a pumping device some other types of pumps than a propeller pump run by an electric motor may be used. Many high productive pump types are possible. In addition, thinking about easy-to move and temporary use of a pump the power to the pump may be taken, for example, from a combustion engine placed above the whole device.

For emphazising aesthetic values the height of the device may, in some applications, be increased by shaping and lengthening the basic guide. In this case flow resistances increase a bit, which needs to be taken into account in the power of the pumping device. Tubes and flumes may be made of transparent material and they may be shaped compromising over hydraulics. In case using open flumes the height and the rising speed are taken into account such, that the speed is enough for making the water stay in the flume. A basic guide may be formed of solid tubes, the cross-section of which is round or square. In this case the rising height may be remarkable big, depending on a pump even 2 - 4 m, but in case the lower end of a tube is brought optimizing also the aeration purpose siphon-like to the normal 0,1 - 0,3 m discharge height, the needed pumping energy does not significantly increase. In these cases, also spot lighting may be used.

For optimizing aesthetic and ecological values the device may also be built to be high and sprinkling impressively water at desired times of the day, but optimized to aeration purpose at the other times of the day. Changes according to the function may be carried out remote-controlled or automatically.

The invention is not limited to the presented advantageous application but it can vary within the frames of the idea of the invention formed in the claims.

Claims

1. Method for increasing oxygen content of hypolimnion (1; 20) and for removing harmful gases in water, in which method hypolimnetic water is pumped with a pumping device (3; 22) through a rise tube (2; 21) reaching from the hypolimnion up near the surface to an upper basin (9;27) on the water surface, where the hypolimnetic water is aerated and harmful gases are removed, and from which upper basin (9; 27) the oxygenated hypolimnetic water is conducted through a downcomer (11) back to the hypolimnion, c harac teriz e d in that hypolimnetic water (1; 20) is conducted through a riser tube (2; 21) to an accelerating device (4; 25) where the flow velocity of the hypolimnetic water is increased and that the hypolimnetic water (1; 20) is conducted to a basic guide (5; 24) reaching at least partly above the water surface, by means of which the hypolimnetic water is directed to a distance above the water surface and back towards the water surface at suitable velocity and at a suitable angle.

2. Method in accordance with claim 1, charac te ri z e d in that in the basic guide (5; 24) the total flow rate of the hypolimnetic water (1; 20) is divided with jet guides (6; 25) into partial jets suitable in size by their flow rate and roughened with surface roughening guides (7).

3. Method in accordance with claim 1 or 2, ch ara cte riz e d in that hypolimnetic water (1; 20) is pumped through a riser tube (2; 21) to a basic guide

(5; 24) so, that the collision velocity of the hypolimnetic water (1; 20) against the water surface is about 3,5-5 m/s.

4. Method in accordance with any of claims 1-3, ch ar act eriz e d in that hypolimnetic water (1; 20) is directed with a basic guide (5; 24) so that the angle of attack of the water at the time of collision is at 60 - 90° angle relatively to water.

5. Method in accordance with any of claims 1-4, ch ar act e riz ed in that hypolimnetic water (1; 20) is pumped with a propeller pump (3; 22) located in a riser tube (2; 21).

6. Method in accordance with any of claims 1-5, c h ar ac t eriz e d in that the water flow velocity is increased with a narrowing part (4) located in a riser tube

(2).

7. Method in accordance with any of claims 1-5, ch aracteri z e d in that the hypolimnetic water (20) flow velocity is decreased with a enlargening part (23) located after a pumping device (22) and increased with the flow velocity accelerating jet nozzles (25) located in the basic guide (24).

8. Method in accordance with any of claims 1-7, characteriz e d in that the hypolimnetic water (20) near the bottom is mixed with a mixer (26) reaching below the riser tube (21).

9. Device for increasing oxygen content of hypolimnion (1; 20) and for removing harmful gases in water, which device comprises an upper basin (9; 27), a riser tube (2; 21) reaching from the hypolimnion up near the surface, a pumping device (3; 22) for pumping the hypolimnetic water through the riser tube and a downcomer (11) conducted from the upper basin back to the hypolimnion for pumping the oxygenated hypolimnetic water back to the hypolimnion, char act eri ze d in that the device comprises an accelerating device (4; 25) for increasing the hypolimnetic water flow velocity conducted through the riser tube (2; 21) and a basic guide (5; 24) reaching at least partly above the water level for directing the hypolimnetic water (1; 20) coming through the riser tube a distance above the water level and back towards the water surface at suitable velocity and at a suitable angle of attack.

10. Device in accordance with claim 9, c har act eriz e d in that the basic guide comprises several jet guides (6; 25) for dividing the total flow rate into partial jets, and that the jet guides (6; 25) comprise surface roughening guides (7) for roughening the partial jets.

11. Device in accordance with claim 9 or 10, chara ct eri z e d in that a pumping device (3; 22) is a propeller pump mounted inside the riser tube (2; 21).

12. Device in accordance with any of claims 9- 11, ch aracteri z e d in that in a riser tube (2) there is a narrowing part (4) for increasing the flow velocity of the hypolimnetic water (1) before the basic guide (5).

13. Device in accordance with any of claims 9- 11, characteriz e d in that the device comprises a spreading part (23) after the pumping device (22) for decreasing the flow velocity of the hypolimnetic water (20), and that the basic guide (24) comprises flow velocity accelerating jet nozzles (25) for increasing the flow velocity.