NO20150946A1

NO20150946A1 - System for desalination of seawater and method for providing water of a predetermined salinity, and maintaining said salinity in an open water reservoir

Info

Publication number: NO20150946A1
Application number: NO20150946A
Authority: NO
Inventors: Helge Lunde; Torbjørn Hegdal; Eirik Dirdal; Terje Samuelsen
Original assignee: Seabox As
Priority date: 2015-07-16
Filing date: 2015-07-16
Publication date: 2017-01-17
Also published as: MA41716A1; WO2017010892A1

Description

SYSTEM FOR DESALINATION OF SEAWATER AND METHOD FOR PROVIDING WATER OF A PREDETERMINED SALINITY, AND MAINTAINING SAID SALINITY IN AN OPEN WATER RESERVOIR

The invention relates to a system for desalination of saline water from a water supply håving a surface, the system comprising an intake for feed water, such as seawater, from the water supply, and a desalination element for separating said feed water into permeate and concentrate. The desalination element requires an applied pressure in order to drive said separation, and said desalination element is provided below the surface of the water supply such that at least a portion of said required applied pressure is provided from the hydrostatic pressure resulting from the height difference between the surface and the desalination element. The invention further relates to a method for providing water of a predetermined, reduced salinity and to a method for maintenance of a predetermined reduced salinity in an open water reservoir.

In many areas of the world fresh water is a scarce resource, and one is often dependent on desalination of seawater for a sustainable supply. Desalination in large scale is typically obtained by means of various distillation or membrane processes, though other processes, such as ion exchange, are also known.

Today, membrane processes, and in particular reverse osmosis (RO), is often regarded as the preferred way of desalinating seawater due to its potential for large volume throughput and comparatively low cost and energy consumption. Still, today's RO systems face challenges such as relatively low robustness and thereby high maintenance, and even if RO may outperform some other desalination processes in terms of energy efficiency, it is still a very energy-consuming process.

In RO a significant applied pressure is required to overcome the osmotic pressure, and thus to drive the desalination process. After feeding seawater to the RO system, the outcome will be one portion of cleaned, low-salinity water and one portion of concentrated, high-salinity water. The former portion is often called permeate, while the latter portion is often called concentrate or RO reject.

In patent publication EP 0968755 A2 is described a desalination system in which the RO-unit is placed on the seabed whereas the water intake is at sea level. This makes it possible to drive the desalination process at least partly by means of the hydrostatic pressure resulting from the height difference.

Patent publication number WO 2006/006942 Al discloses a desalination plant that permits the gravitational flow of seawater to be pre-treated and subsequently to flow into a unit cell submerged below soil surface. Reverse osmosis desalination units are maintained in each unit cell. Reverse osmosis takes place due to hydrostatic pressure difference in the unit cell and the storage reservoir into which the desalinated water flows. A plurality of unit cells can be located in a large vertical shaft column. The reverse osmosis desalination units are removable for maintenance works from the unit cell by hoisting the said RO from the unit cell.

One disadvantage of the prior art is that the RO-units have to be raised above sea level for maintenance purposes. Another disadvantage is that the prior art desalination systems are rather exposed and difficult to protect against outer influence, both natural and man-made. Even another disadvantage is that the prior art systems are wet systems, requiring all components to be built for use under water, including the permeate export pump, which is an essential and vulnerable component of the system. Such systems built for subsea use and resting on the seabed have to be made such that it is possible to pull the entire system onshore, or at least up above sea level, for maintenance. Alternatively, at least some of the maintenance work could be done using remotely operated vehicles (ROVs) but this is also an expensive and complicated process.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least to provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect the invention relates more particularly to a system for desalination of saline water from a water supply håving a surface. The system comprises:

- an intake for feed water, such as seawater, from the water supply; and

- a desalination element for separating said feed water into permeate and concentrate, said desalination element requiring an applied pressure in order to drive said separa tion. Said desalination element is provided below the surface of the water supply, such that at least a portion of said required applied pressure is provided from the hydrostatic pressure resulting from the height difference between said surface and said desalination element. What characterizes the system is that the desalination element is arranged in a dry housing håving at least one dry access.

The water supply may be the sea or for instance a saline lake.

Keeping the desalination element in a dry housing with a dry access will provide a number of advantages compared to keeping the desalination element in a wet, hydrostatic pressurized environment.

One of the effects of keeping the desalination element in a dry housing is that it will be protected from the wear and tear of waves and currents, as well as from plants and animals living in the sea. This will again lead to longer lifetime of the components of the desalination element. It is also not necessary to build the equipment for use under water, which means the equipment can be easier to produce, less heavy and then consequently less expensive. Also, for instance pumps can be dry pumps instead of wet pumps. Wet pumps have the disadvantage that the engine must be protected from the surrounding water. The engine may be protected by use of a pressure container which keeps the water and the water pressure out, or the engine may be filled with a liquid, for instance water containingChemicals, for instance glycol, which will leak out to the surrounding water such that refilling is necessary. Of course, this leakage also has environmental consequences. Basically all components of the system according to the invention, i.e. valves, sensors, connections, control systems, pumps and so on, can be used as they are in a dry, one-atmospheric condition, and do not have to be built for use under water. This renders the system simpler and less expensive.

Håving the desalination elements and export pumps for permeate in the dry underground hall enables the operator to optimize the total system economy within a large span; from maximum capacity utilization of the various elements and consequently high maintenance costs, to low or medium capacity utilization and consequently low maintenance costs. The total system may consist of several desalination elements and this enables adjusting the system export capacity to desalinated water demand.

Håving at least one dry access to the dry housing makes maintenance of the equipment in the housing easier and less expensive than elevating the entire system to sea level or using ROVs for a wet system. The dry access and the dry housing thus give easy access for performing planned maintenance and easy access for unforeseen maintenance or repair. Any pumps, which may be feed pumps upstream of the desalination unit, or export pumps downstream of the desalination unit, which are prone to periodic maintenance and replacement, can also easily be accessed. Pumps in a dry environment are less expensive and much easier to maintain than pumps in a wet environment. In a large-scale desalination facility there will be several desalination elements. The dry housing allows for the various elements to be maintained one by one, such that the desalination process can continue, almost with the same capacity, despite maintenance work. Of course the system may also be designed such that it has a higher capacity than required and will then be able to produce the desired amount of desalinated water also during times of maintenance work.

The dry housing may be an underground hall. An underground hall is one possible way of housing the desalination element. An underground hall can be designed for housing large systems. Furthermore, an underground hall is suitable for providing a good environment for people to enter in order to carry out maintenance work, inspection and any other kind of necessary procedures. An underground hall is well suited for protecting the desalination element from wear and tear of waves and currents, from marine vegetation and animal life, as well as from any human interference, for instance sabotage or acts of war.

The dry housing may be a hall on the seabed. The hall may be at least partly pre-built onshore before it is lowered into the sea. The hall may be built from reinforced concrete or steel or from any other suitable material. The most important property of the building material of such a hall is the ability to withstand hydrostatic pressure. Also decommissioned fixed concrete oil platforms may serve as the subsea dry housing. A hall on the seabed is well suited for protecting the equipment from wear and tear of waves and currents, and also from biological material from marine vegetation and animals.

The dry access may be a tunnel. Håving access through a tunnel with entrance from above sea level, makes it easy to send in personnel and any necessary equipment, both for setting up the desalination system in the first place, for renewing and maintaining the system, and finally for inspecting the system. The dry access may however also be in the form of transfer of personnel and equipment from a surface vessel.

The desalination element may be provided sufficiently far below the surface for providing the required applied pressure. Preferably all, or at least a significant portion, of the required applied pressure to be provided from said height difference.

This has the effect of potentially reducing or removing the need for energy-consuming pumps normally needed to pump the seawater through the desalination element.

The desalination element may comprise a reverse osmosis membrane which is a well suitable embodiment of the invention. Said membrane may be of any type known in the art, for instance membranes produced by companies like Hydronautics, Dow and General Electric. The choice of membrane will depend on the desired salinity of the permeate, and also different membranes may work differently due to for instance the temperature of the water to be desalinated.

In case of a situation where the desalination system, for one reason or another, has to be stopped, the dry housing provides a simple solution for protection of reverse osmosis membranes. In order not to destroy the membranes, water has to be stopped from flowing in the wrong direction through the membranes. The dry housing makes it possible to drain the entire system and thus prevent damage of the membranes.

The system may further be provided with other water treatment components upstream of said desalination element. The additional water treatment components may be components known from various SWIT (seawater intake and treatment) technologies, for instance technologies according to the applicanfs granted Norwegian patents No. 333868 and No. 335691. Also components known from various microfiltration technologies, for example ceramic filter, or hollow fibre filters, may add valuable qualities to the system. By adding other water treatment elements to the system, the system may be improved by controlling also other characteristics of the water than the salinity.

Said other water treatment components may be arranged either in the dry housing (together with the desalination element) or in a further dry housing separate from the dry housing. This may increase the system robustness and lower maintenance requirements.

In a second aspect the invention relates more particularly to a method for providing water of a predetermined, reduced salinity for a water reservoir, the water being produced by means of a system according to the first aspect of the invention, from a saline feed water, such as seawater. The method comprises the steps of:

- desalinating the feed water by means of said system; and

- pumping the resulting permeate, which is the water of predetermined, reduced salinity, to the water reservoir.

Water of a predetermined salinity may be produced by adjusting the desalination ele ment in correspondence with the desired salinity. Examples of such adjustments may be changing of type of membrane, flow rate, pressure and flux rate. The predetermined, reduced salinity may be any salinity lower than the salinity of the feed water, depending on what the use of the produced water, or permeate, is supposed to be.

It must be clear that in a system with multiple desalination elements, the system may at the same time produce waters of different salinities.

The method may further comprise to desalinate the feed water to a salinity of 0-2000 ppm such that the permeate is usable for irrigation purposes.

Generally water used for irrigation should not have a salinity exceeding 2000 ppm. The effect of producing water which may be used for this purpose is that in dry areas or in periods of draught, water for irrigation may be produced from seawater and thus making food production more reliable.

The method may further comprise to desalinate the feed water to a salinity of 0-500 ppm, corresponding to fresh water. As mentioned in the introduction, fresh water is a scarce resource many places and it is clear that people have a need for water for many purposes, for example hygiene purposes, and the production of such water from seawater may provide much needed water in areas where this is not so easily accessible.

The method may further comprise to desalinate the feed water to a salinity of 0-100 ppm such that the permeate is usable as drinking water. With reference to what is already said above, it is clear that one of the most important needs for people living in dry areas is that of clean water for drinking. With the system according to the invention, drinking water may be produced on a large scale. The official limits for salinity allowed in drinking water may differ in different countries, and may be 500 ppm or even as high as 1000 ppm.

In the method the water reservoir may comprise an open water reservoir, for example an artificial lake. In addition to artificial lakes, open water reservoirs may also for instance be swimming pools, fish tanks or aquariums, or even ponds for animals to drink from. It is possible to imagine the development of an oasis surrounding an artificial pond. It must be understood that by "open water reservoir" is also included water reservoirs covered or roofed over partly or completely by for instance a dome, like a glass dome, or the like. Also, it must be understood that by "an open water reservoir" is also meant multiple water reservoirs which may be provided with water from the same desalination system.

In a third aspect the invention relates more particularly to a method for maintenance of salinity level in an open water reservoir. The method comprises the steps of: - producing a first reservoir water of a desired salinity with the system according to the first aspect of the invention; - filling the open water reservoir to a desired level with the produced first reservoir water; - resetting the system of the first aspect of the invention for producing a second reservoir water håving a lower salinity than the desired salinity of the open reservoir; - continually filling up the open water reservoir with an amount of the produced second reservoir water which exceeds the amount of water evaporated from the open water reservoir; and

- draining the open water reservoir of excess water.

For keeping the salinity at a constant level in an open water reservoir it is necessary to take into consideration that water evaporating from the water reservoir is non-saline water. If water of the desired salinity was added just to compensate for the water lost by evaporation, the salinity of the water in the water reservoir would increase, such that the salinity level would soon be higher than desired. Accordingly, as water of no salinity evaporates, also some of the remaining water of the water reservoir has to be removed to give room for adding water håving a lower salinity than the desired salinity level. The added water replaces both the evaporated and the removed water. This way the salinity can be controlled such that the desired salinity level may be kept constant or even changed to a different salinity if one changes the opinion on what is the desired salinity.

The excess water may be recycled through a local reverse osmosis plant for producing a permeate and a concentrate, and the concentrate may be returned to the saline water supply, for instance the sea. The permeate may be used for other purposes, for instance reintroduced in the open water reservoir.

Produced water of reduced salinity is pumped from the level of the desalination element, which means at least from a level below the surface of the water supply, or more precisely from the level of the desalination element. To make the most of water already pumped to the open water reservoir from the desalination element, the removed excess water (as mentioned above) could be recycled by running it through a desalination system close to the open water reservoir before returning the concentrate of high salinity to the sea and the permeate to the open water reservoir. This could reduce the volume of water needed to be pumped from the desalination system below sea level. This is of course depending on the distance and difference in altitude between the open water reservoir and the location of the desalination element, since a "local" desalination system close to the open water reservoir also requires some energy-

The concentrate may not necessarily be returned to the sea, but may be collected in any other suitable location for instance a depleted water reservoir on the ground or underground.

In a fourth aspect the invention relates more particularly to a method for providing water of a predetermined, reduced salinity for a water reservoir, the water being produced by means of a water desalination system, from a saline feed water, such as seawater. The method comprises the steps of:

- desalinating the feed water by means of the water desalination system; and

The method in accordance with the second aspect of the invention is not necessarily limited to the system in accordance with the first aspect of the invention, i.e. it may be using other water desalination systems as well. The invention covers such variants as well.

In a fifth aspect the invention relates more particularly to a method for maintenance of salinity level in an open water reservoir. The method comprises the steps of: - producing a first reservoir water of a desired salinity with a water desalination system; - filling the open water reservoir to a desired level with the produced first reservoir water; - resetting the water desalination system for producing a second reservoir water håving a lower salinity than the desired salinity of the open reservoir; - filling up the open water reservoir with an amount of the produced second reservoir water which exceeds the amount of water evaporated from the open water reservoir; and

- draining the open water reservoir of excess water.

The method in accordance with the third aspect of the invention is not necessarily limited to the system in accordance with the first aspect of the invention, i.e. it may be using other desalination systems as well. The invention covers such variants as well. In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein: Fig. 1 shows a desalination system wherein the desalination element is arranged

in an underground hall;

Fig. 2 shows a desalination system wherein the desalination element is arranged

in a housing on the seabed;

Fig. 3 is a plane view seen from above of the mutual connections between the

components in a system according to the invention; and

Fig. 4 is the system of figure 3 with an additional desalination plant.

It should be noted that the drawings are principle sketches only and they are not drawn to scale.

Equal or equivalent elements are indicated with the same reference number in the figures.

Indications of position and orientation as for instance upper, lower, above, below, vertical and horizontal refer to the position shown in the figures.

Figure 1 shows an example of a system 1 for desalination of saline water from a water supply 2, for instance seawater, for producing water of lower salinity than the seawater. The water supply 2 håving a surface 21 in the following referred to as sea level 21. An intake 3 for feed water is in this embodiment placed at sea level 21 or at least close to the sea level 21. Below sea level 21, in this case underground, a dry housing 7 is provided. The dry housing 7 houses a desalination element 6 which for instance may be one or more reverse osmosis units comprising membranes. The feed water entering the system through the intake 3 is led through for example a pipeline 31 from the intake 3 to the desalination element 6 in the housing 7. At least part of, preferably a considerable part of, the power required for the desalination process in the desalination element 6 is provided by the hydrostatic pressure resulting from the height difference between the surface 21 and the desalination element 6. The desalination element 6 is provided with an outlet 4 for permeate, which is water with a salinity lower than the salinity of the feed water. The permeate is pumped, by means of export pumps 10, through a pipeline 41 to a water reservoir 9. It must be understood that the two pumps 10 shown in the drawing are meant for illustrative purposes only, meaning that the location and number of pumps 10 may differ from what is shown.

The pumps 10 and any other components in the system 1 which require added power, other than the already mentioned hydrostatic pressure, will typically be driven by electrical motors which may get their power from any kind of known power plants like fossil-fueled plants, biomass plants, nuclear plants, hydro-electric plants, wind power plants, geothermal plants, solar power plants and so on.

Further, the desalination element 6 is provided with an outlet 5 for concentrate. The concentrate will leave the system 1 through the outlet 5 at a pressure almost enabling return to feed water intake level through a pipeline 51. However, due to friction in the upstream system and downstream system, a low-duty pump 101 is required for the concentrate to be returned to feed water intake level, by compensating for the friction loss. It must be understood that the concentrate need not necessarily be returned to the feed water intake level, but may be returned to the water supply 2, for instance the sea, at a different level than that of the feed water intake 3. The concentrate may even not be returned to the sea, but may be led to a deposit, for instance an underground deposit or a land deposit (not shown).

The water reservoir 9 may be an open water reservoir, as shown here, or a closed water reservoir (fig. 2). A dry access 71, for example a tunnel, connects the dry housing 7 to the surface and makes it possible for personnel and equipment to enter the dry housing 7.

Figure 2 shows an example of the system 1 according to the invention, where the dry housing 7 is located on the seabed and connected with the ground level through a dry access 71 in the form of a tunnel. The tunnel may be an underground tunnel as shown, or a tunnel put on the seabed, a so-called immersed tube, or similar. The dry access 71 may even be a combination of different elements, like for instance partly an underground tunnel and partly an immersed tube. Access to the housing 7 on the seabed may be from below the housing 7 if desired.

In the drawing it is shown that the system 1 is provided with other water treatment components 8 upstream of the desalination element 6. In this example the other water treatment components 8 are located in the same dry housing 7 as the desalination element 6. In other embodiments of the system 1 the other water treatment components 8 may be located in a housing separate from the housing 7 of the desalination element 6 or even in connection with the intake 3 of feed water. Such other water treatment components 8 may be components for disinfection and filtration of the feed water, or components for any other desired water treatment.

The feed water intake 3 may be located at the sea bed upstream of the dry housing 7.

Furthermore, in figure 2, the water reservoir 9 is shown as a partly buried, closed water reservoir. It must be understood that the open water reservoir 9 (fig. 1) and the closed water reservoir 9 (fig. 2) may take any suitable form. The closed water reservoir 9 may also be completely buried or placed on the ground, or wherever else it is desired. As mentioned earlier, the open water reservoir 9 may also be roofed-over or the similar.

One example of an open water reservoir 9 according to the invention could be a saline lake in a desert-like inland area. The seawater intake 3 and desalination element 5 will naturally be located at the coast. After desalination the permeate, i.e. water with reduced salinity for the lake, is pumped through suitable pipelines to the lake. The lake may be of any desired size, for example as large as 250 km<2>with a depth of 3-5 meters. The evaporation from a lake of this size may be as much as 4 mm/day, which means that the maintenance volume, i.e. the volume of water which needs to be added, may exceed 1 million m<3>/day.

In figure 3, a saline water reservoir 2, for example the sea, is at its boarder or coast

provided with a water intake 3 for feed water. Said intake 3 could also be provided on the seabed. The intake 3 may be provided with water treatment components 8 like for instance a disinfection unit, which may be an electrochlorinator wherein zodium hypochlorite is produced by electrolysis and mixed with the water entering in the water intake 3. The water may be exposed to the chlorine for a period of for instance 1-2 hours in a large still room in order to allow both particle sedimentation and accurate control of the chlorine concentration. Furthermore, the intake 3 may be provided with a unit for secondary killing of any surviving microorganisms after the chlorine treatment. Said other treatment components 8 are known from the applicanfs SWIT technology and will not be described further herein. It must be understood that the intake 3 may also be merely an intake, without any further treatment components 8.

The feed water subsequently enters the desalination element 6 in the dry housing 7 through the pipeline 31. From the desalination element 6 there is a return line 51 for concentrate, i.e. water of a higher salinity than the feed water, which runs back to the saline water reservoir 2. From the desalination element 6 there is also a line 41 representing a pipeline for transport of permeate, i.e. water of lower salinity than the feed water, said lower salinity being a predetermined salinity adjusted to the desired salinity level of an open water reservoir 9 to which the line 41 runs. Since fresh water will evaporate from the open water reservoir 9, the water reservoir 9 needs to be refilled. This is of course also true for a closed reservoir from which there is no evaporation, but from which water is used for purposes like irrigation, drinking water or other things. However, in the case of evaporation the water which is left in the reservoir 9 will be of a higher salinity than the desired, predetermined salinity. In order to be able to keep the salinity level constant some of the water with the now higher salinity has to be removed through return line 91 for instance back to the sea 21, while at the same time new water is added from the desalination element 6 through the pipeline 41. The amount of added water has to be the same as the sum of the evaporated water and the drained high salinity water. The added water will have to be water with a lower salinity level than the desired salinity level for the open water reservoir 9, except for in the rare case that all the water of the open water reservoir 9 is drained.

Normally, the process of refilling and draining the reservoir will be running continuously such that the salinity of the water in the reservoir 9 will be kept constant.

In figure 4 the other treatment components 8 are shown in the dry housing 7 together with the desalination element 6. The pipeline 31 from the intake 3 consequently runs to the treatment components 8. After treatment in the treatment components 8, the water runs through a tube, a channel or any other suitable connecting means to the desalination element 6. From the desalination element 6 a concentrate is returned, by means of a low-duty pump 101 (see fig. 1) to the sea 2 through a pipeline 51, and a permeate is pumped to the open water reservoir 9 through a pipeline 41. Similar to the system of figure 3, there is a return line 91 from the open water reservoir 9, said return line 91 håving the purpose of draining the water reservoir 9 of at least some of the water håving a salinity higher than the desired salinity, as mentioned above. Said returned water may be led to a local desalination plant 11 for recycling before the permeate from this desalination plant 11 is either returned to the open water reservoir 9 or used for other purposes. The concentrate from the desalination plant 11 will be returned to the sea through the pipeline 112. It must be understood that the local desalination plant 11 may be by-passed or only used for part of the returned water from the open water reservoir 9.

It should be noted that the above-mentioned embodiments illustrate ratherthan limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preced-

ing an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. System (1) for desalination of saline water from a water supply (2) håving a surface (21), the system comprising: - an intake (3) for feed water from the water supply (2); and - a desalination element (6) for separating said feed water into permeate and concentrate, said desalination element (6) requiring an applied pressure in order to drive said separation, and said desalination element (6) being provided below the surface (21) of the water supply (2), such that at least a portion of said required applied pressure is provided from the hydrostatic pressure resulting from the height difference between said surface (21) and said desalination element (6), characterized in thatthe desalination element (6) is arranged in a dry housing (7) håving at least one dry access (71).

2. System (1) according to claim 1, wherein the dry housing (7) is an underground hall.

3. System (1) according to claim 1, wherein the dry housing (7) is a hall on the seabed.

4. System (1) according to any one of the preceding claims, wherein the dry access (71) is a tunnel.

5. System (1) according to any one of the preceding claims, wherein said desalination element (6) is provided sufficiently far below the surface (21) for providing the required applied pressure.

6. System (1) according to any one of the preceding claims, wherein the desalination element (6) comprises a reverse osmosis membrane.

7. System (1) according to any one of the preceding claims, wherein the system (1) is further provided with other water treatment components (8) upstream of said desalination element (6).

8. System (1) according to claim 7, wherein said other water treatment components (8) are arranged either in the dry housing (7) or in a further dry housing separate from the dry housing (7).

9. Method for providing water of a predetermined, reduced salinity for a water reservoir (9), the water being produced by means of a system (1) according to any of claims 1-8, from a saline feed water, such as seawater, the method comprising the steps of: - desalinating the feed water by means of the system (1); and - pumping the resulting permeate, which is the water of predetermined, reduced salinity, to the water reservoir (9).

10. Method according to claim 9, wherein the method further comprises to desalinate the feed water to a salinity of 0-2000 ppm.

11. Method according to claim 10, wherein the method further comprises to desalinate the feed water to a salinity of 0-500 ppm.

12. Method according to claim 11, wherein the method further comprises to desalinate the feed water to a salinity of 0-100 ppm.

13. Method according to any of claims 9-12, wherein the water reservoir (9) comprises an open water reservoir.

14. Method for maintenance of salinity level in an open water reservoir (9), the method comprising the steps of: - producing a first reservoir water of a desired salinity with the system according to claim 1; - filling the open water reservoir (9) to a desired level with the produced first reservoir water; - resetting the system of claim 1 for producing a second reservoir water håving a lower salinity than the desired salinity of the open reservoir (9); - filling up the open water reservoir (9) with an amount of the produced second reservoir water which exceeds the amount of water evaporated from the open water reservoir; and - draining the open water reservoir of excess water.

15. Method according to claim 14, wherein the excess water is recycled through a local reverse osmosis plant (11) for producing a permeate and a concentrate, and the concentrate is returned to the sea.