WO2021205072A1

WO2021205072A1 - Water purification system

Info

Publication number: WO2021205072A1
Application number: PCT/FI2021/050251
Authority: WO
Inventors: Esko Aulanko; Jorma Mustalahti
Original assignee: Emp-Innovations Oy
Priority date: 2020-04-06
Filing date: 2021-04-06
Publication date: 2021-10-14
Also published as: EP4132688A4; EP4132688A1

Abstract

This invention relates to a water purification system with an inlet (1) for the raw water, an outlet (3) for the purified water, a membrane filter unit (2) comprising an inlet (2a) for the raw water and an outlet (2b) for the purified water, and a pressure tank (11) with a first compartment (11a) arranged to receive purified water from the outlet (2b) of the membrane filter unit (2). For flushing the membrane filter the first compartment (11a) is arranged to deliver purified water to the membrane filter unit (2) through the outlet (2b) of the membrane filter unit (2).

Description

WATER PURIFICATION SYSTEM

The object of the invention is a water purification system according to claim 1.

Large areas in the world suffer from poor or non-usable drink ing water. Also, in many areas of the world tap water is not suitable for drinking or cooking. The reason is that the water contains too high levels of impurities, such as contaminants, microbes, salt, iron, heavy metals, fluoride, arsenic, carcino gens, and other chemicals and substances that are unwanted and/or detrimental to health. The most problematic areas are densely populated areas, such as China, India, Indonesia, Afri ca but the problem is very common also in Southern Europe, America and in the rest of Asia. Growing world population, increasing urbanization, pollution and global warming make the problem even bigger.

Main reasons for the poor or non-potable tap water are contami nated water supplies, overloaded and old municipal water piping systems and infrastructure and the ever-increasing demand. To compensate the contaminants of the water supply and old over loaded pipe systems, municipal waterworks must overdose chlo rine or chloramines to keep microbial quality acceptable, but this kind of overdosing creates toxic and/or carcinogenic sub stances in the water.

Alternatives for the consumers to cope with the problem are to use bottled water or to install domestic, residential-specific small water purification devices to purify tap water.

Bottled water is costly and also ecologically harmful, because it generates huge amounts of plastic waste and transport emis sions. In addition, it is heavy to carry around and it needs a lot of room in shops and stores. Small domestic water purification devices offer a better alter native. Most often they are low cost Point-Of-Use (POU) sys tems, which have limited capacity and purify mainly drinking and cooking water. Typically, they are "under-the-sink" or "counter-top" systems producing 30-300 litres purified water daily. However, these small water purification devices also have disadvantages, e.g. investment costs and maintenance costs. Depending on the technology, they also may not remove all contaminants and they may have poor water economy. This means that they waste too much water. In addition, they may need electric power and thus they consume energy.

Usually, one purification device may comprise more than one purification method. For instance, in addition to a reverse osmosis (RO) stage, RO systems almost always comprise also sediment filters and activated carbon filters.

Typical purification methods in POU systems for domestic tap water are a filtration, activated carbon filtering, UV disin fection and reverse osmosis (RO) systems.

The filtration method is effective for suspended solids with particle size above 1-5 ym but it is ineffective against dis solved substances, toxins and microbes.

Activated carbon filtering is effective for suspended solids with particle size above 5 ym, removing big organic molecules and gases, and it improves taste and smell. However, it is not effective to dissolved substances and microbes and it does not remove most toxins.

UV disinfection is effective for microbes, but it does not remove any suspended or dissolved solids or any toxins or car cinogens. In addition, it needs electric power, the light sources used have a limited lifetime and they contain mercury.

Reverse osmosis (RO) systems are effective for suspended and dissolved solids, microbes, toxins and carcinogens. However, RO systems of prior art are big as their effective volume is small. In spite of their disadvantages, RO systems are the most common POU systems, as they are most effective, removing prac tically all the contaminants from the water.

POU water purifiers are traditionally equipped with a small pressure tank, where purified water is collected, and from where the water is delivered to a small faucet. The pressure to the faucet is made by the membrane / air pressure inside the pressure tank: the tank is divided into two parts by a flexible membrane; water is on one side of the membrane while on the opposite side of the membrane is pressurized air. When the tank is empty from water, air pressure is typically 0,3 - 2 bar. When the tank is filled with water, air compresses and air pressure increases. As the maximum output pressure from the purifying system to the tank is quite limited (typically 1-3 bar), maximum water content in the tank is typically 50% or less of the tank total volume.

Thus, the pressure tank volumetric efficiency is quite poor. POU devices are usually placed in very limited spaces, and external dimensions of the system is one important sales argu ment.

Another disadvantage with traditional pressure tanks is that the output pressure varies from the maximum to the membrane pressure. Thus, output flow from the faucet varies accordingly, and is typically quite low.

Point Of Entry (POE) water purifiers are traditionally equipped with a rather large purified water container, which works as a buffer between purifying system and house water system. POE systems have a delivery pump to pump the purified water from the buffer tank to the house piping. The pump is quite costly and needs energy, typically 500-1500W. It is also a wearing component. The tank must be equipped with level switches to stop purifying when the tank is full, start purifying after some consumption, and stop the delivery pump when the tank is near empty, allow ing it to operate again after water level has increased enough.

As water supply is stopped when the tank is near empty the tank must be dimensioned quite big to avoid this taking place too often. A big tank needs a big space and is costly.

One of the biggest problems with traditional membrane pressure tanks is that air is leaking slowly out, which is why the air pressure has to be checked every 6 months and increased if necessary. This requires a pressure gauge and a pump that most households do not have. Inspection and maintenance may also be simply neglected. As a result, the pressure vessel will cease operation within a couple of years at the latest.

Purpose of this invention is to solve a.m. problems in both POU and POE or similar water purifying systems.

The water purification system according to according to the invention is characterized by what is presented in the charac terization part of claim 1. Other embodiments of the invention are characterized by what is presented in the other claims.

An aspect of the invention is to provide a water purification system which comprises an inlet for the raw water, an outlet for the purified water, a membrane filter unit, a pump to drive the raw water through the membrane filter unit and a buffer tank for the purified water. Advantageously, the purified water taken from the buffer tank is directly pressurized by the pres sure of the inputted raw water.

An important advantage to be achieved with the invention is that the water supply is inexhaustible. Another significant advantage is an efficiency of space utilization of the buffer tank. In the system according to the invention substantially the total volume of the buffer tank can be utilized whereas in the systems of the prior art the maximum of half, commonly only one third of the volume can be utilized. This means that the buffer tank according to the invention can be smaller in size than conventional buffer tanks in corresponding systems. Thus, the whole system or apparatus can be smaller in its size. Yet another advantage is a constant pressure at the outlet of the purified water. The purified water from the outlet is always disposable at the same pressure as the raw water to be purified in the inlet. In the systems of the prior art the outlet pres sure depends directly on the degree of fullness and is at its maximum when the buffer tank is full and its minimum when the buffer tank is empty, et a further advantage is the mainte nance-free structure of the buffer tank. The buffer tank ac cording to the invention does not leak air that should be added time to time. Further one significant advantage is an automatic flushing of the membrane in the membrane filter unit. This gives longer operating life for the membrane filter unit and gives more pure water from the outlet as soon the outlet tap is opened.

The primary intended use of the invention is the application of it in the reverse osmosis separation. Many effects or technical solutions are very similar in other membrane separation tech niques. Within the scope of applicability, the inventive solu tions can therefore be extended to be used in other membrane separation procedures also, although the description and expla nations of the technical solutions are presented here in this context most often in a reverse osmosis device environment.

In the following the invention will be described in more detail by the aid of examples of its embodiments with reference to the attached simplified drawings, wherein

Fig. 1 shows a principal structure of a RO water purifying system according to the invention with a high-pressure pump, in a situation where purification is in progress, Fig. 2 shows the RO water purifying system according to Fig. 1 in a situation where flushing of the membrane filter unit of the system is in progress,

Fig. 3 shows a principal structure of a RO water purifying system according to the invention without a high- pressure pump, and

Fig. 4 shows the RO water purifying system according to Fig. 3 in a situation where flushing of the membrane filter unit of the system is in progress.

A water purification system according to the invention compris es mainly an inlet 1 for the raw water, an outlet 3 for the purified water, a membrane filter unit 2, such as a reverse osmosis unit with an inlet 2a and outlet 2b, a pump 8, prefera bly a high-pressure pump, to drive the raw water through the membrane filter unit 2 and a buffer tank 4, such as a pressure tank for storing the purified water coming from the membrane filter unit 2.

The system comprises an inlet channel la from the raw water inlet 1 to the membrane filter unit 2. The inlet channel la extends as an end section lb from the pump 8 to the inlet 2a of the membrane filter unit 2. The end section lb are later also called the feed channel lb. The system further comprises an outlet channel 3a for the purified water from outlet 2b of the membrane filter unit 2 to the outlet 3 for the purified water.

The raw water flow is indicated with thin black arrows 16 and the purified water or permeate flow is indicated with thicker white arrows 17.

The buffer tank 4 comprises a first compartment 4a for the raw water and the second compartment 4b for the purified water. The buffer tank 4 is divided into said two compartments 4a, 4b by an element preventing contents of the separated compartments 4a, 4b from mixing. The dividing element is such that it essen- tially equalizes pressure in both the compartments 4a, 4b. The flexible element dividing the buffer tank 4 is a flexible mem brane or a flexible bag.

The first compartment 4a of the buffer tank 4 is connected to the inlet channel la and the second compartment 4b of the buff er tank 4 is connected to the outlet channel 3a. In this way, the force to drive the purified water from the second compart ment 4b to the outlet 3 is achieved by the pressure of the raw water in the first compartment 4a of the buffer tank 4.

The system comprises a filter arrangement 5 in the inlet chan nel la placed between the connection of the buffer tank 4 and the pump 8. The filter arrangement 5 comprises preferably a, for example, a sediment filter 5a and an activated carbon fil ter 5b. The location of the filters 5a, 5b in the inlet channel la after the connection of the buffer tank 4 gives an advantage that the filters do not limit the raw water flow to the first compartment 4a of the buffer tank 4.

The water purification system according to the invention also comprises a sensor means to control the pump 8. The sensor means comprises a pressure switch 7 connected to the outlet channel 3a, and a solenoid valve 6 connected to the inlet chan nel la for the raw water. Pressure switch 7 is arranged to control both the motor of the pump 8 and the solenoid valve 6. The pressure switch 7 is arranged to sense the fill status of the second compartment 4b by measuring the pressure in the outlet channel 3a. When the pressure decreases it means that the amount of purified water in the second compartment 4b is decreasing. In other words, the purified water is taken from the outlet 3. After the purified water has been taken from the second compartment 4b so much that a certain predetermined limit is reached the pressure switch 7 is arranged to make a connection in order to start the pump 8 to begin the purifying process. Correspondingly, when the second compartment 4b is full of purified water the pressure switch 7 is arranged to disconnect in order to stop the pump 8. The system comprises also a check valve 9 in the outlet channel 3a to prevent the purified water from returning to the membrane filter unit 2, especially from the second compartment 4b. Pref erably, the check valve 9 is between the outlet 2b of the mem brane filter unit 2 and the connection of the second compart ment 4b with the outlet channel 3a.

The solenoid valve 6 is arranged to switch off when the pres sure in the outlet channel 3a exceeds the pressure in the inlet channel la by a predetermined margin. That stops the pump 8 and closes the raw water flow 16 to the pump 8. Correspondingly, the solenoid valve 6 is arranged to switch on when the pressure in the outlet channel 3a goes below the pressure in the inlet channel la by a margin. That starts the pump 8 and opens the raw water flow to the pump 8.

Further the water purification system according to the inven tion comprises a pressure tank 11 for flushing the membrane filter unit 2 with purified water or permeate. Preferably, the pressure tank 11 is connected to a channel 10 that is connected between the feed channel lb and the outlet channel 3a. Prefera bly, the channel 10 is connected to the feed channel lb between the pump 8 and the inlet 2a of the membrane filter unit 2. The channel 10 comprises a check valve 12 placed between the pres sure tank 11 and the feed channel lb. The check valve 12 is arranged to prevent the raw water flow 16 from the feed channel lb to the pressure tank 11.

The pressure tank 11 comprises a first compartment 11a for the purified water or permeate. The compartment 11a is connected via the channel 10 to the outlet channel 3a and to the feed channel lb in order to empty the contents of the first compart ment 11a for flushing the membrane filter unit 2 with the puri fied water after the pump 8 is stopped. Preferably, the com partment 11a of the pressure tank 11 is connected via the chan nels 10 and 3a directly to the outlet 2b of the membrane filter unit 2, and via the channels 10 and lb through the check valve 12 to the inlet 2a of the membrane filter unit 2.

The pressure tank 11 comprises a second compartment lib sepa rated from the first compartment 11a by a flexible element preventing the contents of the separated compartments from mixing. The second compartment lib may be filled with pressur ized gas, such as air. The second compartment lib may also be filled with the raw water from the inlet channel la. In that case the second compartment lib is connected either before the pump 8 with the inlet channel la or after the pump 8 with the feed channel lb that acts as an extension to the inlet channel la.

In the system according to the invention the raw water, for example tap water, is inputted to the inlet 1 of purifier sys tem at e.g. 2-4 bar pressure. The raw water flow 16 is connect ed to the first compartment 4a of the buffer tank 4, which is a tank with two compartments 4a and 4b separated by a flexible membrane.

After the connection between the inlet channel la and the first compartment 4a of the buffer tank 4 the raw water passes through prefilters of the filter arrangement 5, for example the sediment filter 5a and the activated carbon filter 5b, which remove excessive undissolved particles, chlorine etc. substanc es harmful to the purifying system.

The solenoid valve 6 is controlled by the pressure switch 7, which also powers the pump 8, which drives the raw water to the membrane filter unit 2. From the outlet 2b of the membrane filter unit 2 the purified water 17 exits as a permeate and from the drain exit 2c of the membrane filter unit 2 the reject concentrate is drained. Permeate or purified water exiting from the outlet 2b flows to the second compartment 4b for the puri fied water. If the purified water is not let out from the outlet 3, the second compartment 4b for the purified water fills and at the same time drives the raw water out of the first compartment 4a of the buffer tank 4. The raw water is pushed out until the buffer tank 4 is full of the purified water. As the pump 8 still runs, the pressure on the second compartment 4b increases until the pressure switch 7 cuts the power from the pump 8 and the solenoid valve 6 closes the raw water flow 16 in the inlet channel la. In this situation the check valve 9 prevents the purified water from returning to the membrane filter unit 2.

When water is used, raw water pressure drives the purified water out from the buffer tank 4. If the water output flow or volume is high enough, the pressure on the pressure switch 7 drops and the switch closes, starting again the pump 8 and the purification.

As the input flow 16 of the raw water is the driving force for the output of the purified water flow 17, the output flow and pressure are essentially similar as when taking the water di rectly from the raw water source, and the output pressure is essentially constant. No delivery pump is needed, saving costs, space and energy.

As the full volume of the buffer tank 4 is in use for the puri fied water, the space taken by the buffer tank 4 is only about a half of that with conventional buffer tanks.

Advantageously, the system may be equipped with a flushing arrangement for flushing the feed water side through inlet 2a and the opposite side of the membrane through outlet 2b in the membrane filter unit 2 with the permeate or purified water. Preferably, the flushing arrangement comprises the pressure tank 11, the check valve 12 and the channel 10 connected to the first compartment 11a of the pressure tank 11, to the feed channel lb and to the outlet channel 3a. Through the channel 10 the first compartment 11a is preferably in continuous and di rect contact with the outlet 2b of the membrane filter unit 2. Thanks to the direct contact the first compartment 11a is ar ranged to receive purified water from the outlet 2b automati cally always when the purification is in operation. In addi tion, the first compartment 11a is arranged to automatically deliver purified water to the outlet 2b always when the pump 8 is not running and/or the pressure in the first compartment 11a is greater that the pressure in the outlet 2b. The continuous and direct contact here means that there are no limiting compo nents, such as valves or alike, in the channel line between the first compartment 11a and the outlet 2b of the membrane filter unit 2.

Figure 2 shows the RO water purifying system according to Fig. 1 in a situation where flushing of the membrane filter unit 2 of the system is in progress.

The first compartment 11a of the pressure tank 11 has been filled with the purified water 17 to the same pressure as the cut-off pressure of the pressure switch 7. Depending on the pressure differences, when the pressure switch 7 cuts off the pump 8 stops, the pressure on the pump output drops and there fore also the pressure in the inlet 2a and outlet 2b drops, and thanks to the higher pressure in the first compartment 11a of the pressure tank 11 the purified water 17 begins to flow from the first compartment 11a of the pressure tank 11 to the chan nel 10 where it flows into two opposite directions.

The first direction of the purified water flow 17 in the flushing phase is to the outlet channel 3a and further to the outlet 2b of the membrane filter unit 2. From the outlet 2b the purified water flows through the membrane filter unit 2 in the opposite direction to the normal purifying direction, at the same time flushing the membrane from the second side of the membrane.

At the same time when the pressure switch 7 cuts off, the puri fied water 17 also begins to flow from the first compartment feed channel lb and further to the inlet 2a of the membrane filter unit 2, and through the membrane filter unit 2 at the same time flushing the membrane from the feed side or first side of the membrane.

The purified water or permeate used for flushing discharges from the membrane filter unit 2 through the drain exit 2c.

Preferably, an apparatus according to the water purification system of the invention is arranged to operate as follows:

• The apparatus is in sleep mode; the second compartment 4b of the buffer tank 4 is full of purified water or perme ate; the pressure in the second compartment 4b is in the cut-off area of the pressure switch 7; the pressure switch 7 is disconnected; the pump 8 is not running.

• Purified water is taken from the outlet 3; as a result, the pressure in the second compartment 4b and in the out let channel 3a drops to the pressure level in the inlet channel la, or even below because of flow losses; as a result, the pressure switch 7 closes which causes the opening of the solenoid valve 6 and starting the pump 8.

• The apparatus purifies raw water using reverse osmosis; purified water or permeate flows from outlet 2b; puri fied water is taken from outlet 3; pressure in the feed channel lb and in the inlet 2a is greater than the pres sure in the outlet 2b and in the outlet channel 3a; as a result, permeate cannot flow through the channel 10 from the outlet channel 3a to the end section lb; the check valve 12 prevents the raw water flow 16 from the end section lb through the channel 10 to the outlet channel 3a; the pressure is essentially the same in the outlet 2b, outlet channel 3a, first compartment 11a of the pres sure tank 11, first compartment 4a of the buffer tank 4, and second compartment 4b of the buffer tank 4.

The use of purified water from outlet 3 is cut off by. • The purification of the raw water continues automatical ly until the second compartment 4b of the buffer tank 4 is full; at that stage, the pressure in the second com partment 4b is essentially the same as the pressure of in coming raw water in the inlet 1; as a result, the pressure in the compartments 4b and 11a increases until the pres sure reaches the cut-off limit of the pressure switch 7; at this point the pump 8 is stopped and the solenoid valve 6 is closed.

• As a result, the flushing phase automatically begins.

• When the reject discharges from the drain exit 2c the pressure in the membrane decreases until the pressure lev el in the membrane reaches the pressure level of the first compartment 11a of the pressure tank 11; at that time the purified water or permeate begins to flow from the first compartment 11a to the inlet 2a and outlet 2b; the osmotic pressure over the membrane assists the permeate flow 17 through the outlet 2b into the membrane filter unit 2; the pressure in the inlet 2a and outlet 2b is essentially the same because the check valve 12 allows the permeate 17 to flow from the first compartment 11a to the feed channel lb and further to the inlet 2a; the check valve 9 prevents the pressure from discharging from the second compartment 4b of the buffer tank 4.

• The pressure in the first compartment 11a discharges as permeate flow 17 through the membrane to the drain exit 2c flushing the membrane effectively.

• After the first compartment 11a has become empty the flushing operation ceases and the apparatus enters to sleep mode to wait for the next use of the purified water from the outlet 3.

Figure 3 presents a second embodiment of the invention. This structure comprises neither the high-pressure pump 8 nor the buffer tank 4 but the rest of the structure is substantially similar to the structure described in connection with figures 1 and 2.

The water purification system according to the second embodi ment of the invention comprises mainly an inlet 1 for the raw water, an outlet 3 for the purified water and a membrane filter unit 2, such as a reverse osmosis unit, with an inlet 2a and outlet 2b. The system also comprises an inlet channel la with an end section lb from the raw water inlet 1 to the membrane filter unit 2 and an outlet channel 3a for the purified water from the membrane filter unit 2 to the outlet 3 for the puri fied water. The end section lb of the inlet channel la is also called the feed channel lb.

Further the system of the second embodiment comprises a filter arrangement 5 and a shut-off valve 14 in the inlet channel la and a check-valve 9 and a shut-off valve 15 in the outlet chan nel 3a. The shut-off valve 14 is arranged to close the inlet channel la when the pressure in the outlet channel 3a increases to the same level with the pressure in the inlet channel la. The check valve 9 in the outlet channel 3a is arranged to pre vent the purified water from returning to the membrane filter unit 2. Preferably, the check valve 9 is between the outlet 2b of the membrane filter unit 2 and the shut-off valve 15.

The water purification system according to the second embodi ment of the invention also comprises a pressure tank 11 for flushing the membrane filter unit 2 with purified water. The pressure tank 11 comprises a first compartment 11a for the purified water. Preferably, the pressure tank 11 is connected with a channel 10 between the feed channel lb and the outlet channel 3a. The channel 10 comprises a check valve 12 placed between the pressure tank 11 and the feed channel lb. The check valve 12 is arranged to prevent the raw water flow 16 from the feed channel lb to the first compartment 11a of the pressure tank 11. The first compartment 11a of the pressure tank 11 is connected via the channel 10 to the feed channel lb in order to empty the first compartment 11a for flushing the membrane filter unit 2 with the purified water after the shut-off valve 15 is closed. Preferably, the pressure tank 11 is connected to the outlet channel 3a between the shut-off valve 15 and the outlet 2b of the membrane filter unit 2.

The pressure tank 11 comprises a second compartment lib sepa rated from the first compartment 11a by a flexible element preventing the contents of the separated compartments from mixing. The second compartment lib is filled with pressurized gas, such as air. The second compartment lib may also be filled with the raw water from the inlet channel la. In that case the second compartment lib is connected with the inlet channel la.

Since the membrane filter unit 2 is a reverse osmosis unit the flushing of the membrane filter unit 2 in both of the embodi ments described above is arranged to be done partially by the help of osmosis. In that case, during the flushing phase, the greater amount of purified water flow 17 runs through the out let 2b into the membrane filter unit 2 because the osmotic pressure over the membrane enhances the purified water flow 17 through the outlet 2b. Thus, during the flushing the purified water flow 17 into the membrane filter unit 2 is greater through outlet 2b than through the inlet 2a. That makes the flushing much more effective compared to the flushing systems of the prior art.

It is obvious to the person skilled in the art that the in vention is not limited to the examples described above, but that it may be varied within the scope of the claims present ed below as well as of the description and the drawing pre sented.

A generally used membrane filtration method for liquids is a so-called cross-flow filtration which is in use in many differ ent types of membrane filtering methods, e.g. in reverse osmo- sis filtration, nanofiltration, ultrafiltration and microfil tration.

It is obvious to the person skilled in the art that the inven- tion is not limited to the reverse osmosis filtration only but also other types of membrane filtering methods can be used in the system according to the invention. Thus, for example, the membrane filter units used in the system according to the invention may also be nanofiltration, ultrafiltration and mi- crofiltration units.

Claims

1. Water purification system with an inlet (1) for the raw water, an outlet (3) for the purified water, a membrane filter unit (2) comprising an inlet (2a) for the raw water and an outlet (2b) for the purified water, and a pressure tank (11) with a first compartment (11a) arranged to receive purified water from the outlet (2b) of the membrane filter unit (2), characterized in that for flushing the membrane filter the first compartment (11a) is arranged to deliver purified water to the membrane filter unit (2) through the outlet (2b) of the membrane filter unit (2).

2. Water purification system according to claim 1, characterized in that for flushing the membrane filter the first com partment (11a) is connected both to the outlet (2b) of the membrane filter unit (2) and to the inlet (2a) of the membrane filter unit (2).

3. Water purification system according to claim 2, characterized in that during flushing the purified water flow (17) into the membrane filter unit (2) is greater through outlet (2b) than through inlet (2a).

4. Water purification system according to claim 1, 2 or 3, characterized in that the system comprises a pump (8) to drive the raw water through the membrane filter unit (2) and a buffer tank (4) for the purified water, and that the purified water taken from the buffer tank (4) is directly pressurized by the pressure of the inputted raw water.

5. Water purification system according to claim 4, characterized in that the system comprises an inlet channel (la) from the raw water inlet (1) to the membrane filter unit (2) and an outlet channel (3a) for the purified water from the membrane filter unit (2) to the outlet (3) for the purified water, and that the buffer tank (4) comprises a first compartment (4a) for the raw water and the second compartment (4b) for the purified water.

6. Water purification system according to claim 5, characterized in that the first compartment (4a) is connected to the inlet channel (la) and the second compartment (4b) is connected to the outlet channel (3a).

7. Water purification system according to claim 4, 5 or 6, characterized in that the pump (8) is controlled by a sensor means sensing the fill status of the second compartment (4b) containing purified water, and arranged to start the pump (8) when the purified water is consumed enough and arranged to stop the pump (8) when the second compartment (4b) is full.

8. Water purification system according to claim 7, characterized in that the sensor means is arranged to switch off when the pressure in the outlet channel (3a) exceeds the pressure in the inlet channel (la) by a margin, and to switch on when the pressure in the outlet channel (3a) goes below the pressure in the inlet channel (la) by a margin.

9. Water purification system according to any of the preceding claims, characterized in that the pressure tank (11) is con nected with a channel (10) between the end section (lb) of the inlet channel (la) and the outlet channel (3a).

10. Water purification system according to claim 9, characterized in that the pressure tank (11) is connected through a check valve (12) to the end section (lb) of the inlet channel (la) between the pump (8) and the inlet (2a) of the membrane filter unit (2).

11. Water purification system according to any of the claims 4- 10, characterized in that the first compartment (11a) for the purified water is connected via the channel (10) to the outlet channel (3a) and to the end section (lb) of the inlet channel (la), and that the pressure tank (11) is arranged to empty the first compartment (11a) after the pump (8) is stopped for flushing the membrane filter unit (2) with the purified water.

12. Water purification system according to any of the preceding claims 1-11, characterized in that the pressure tank (11) com prises a second compartment separated from the first compart ment by a flexible element preventing contents of the separated compartments from mixing, which second compartment is filled with pressurized gas.

13. Water purification system according to any of the preceding claims 1-11, characterized in that the pressure tank (11) com prises a second compartment separated from the first compart ment by a flexible element preventing contents of the separated compartments from mixing, which second compartment is connected to the inlet channel (la) for filling the second compartment with the raw water.

14. Water purifying system according to any of the preceding claims 1-13, characterized in that the membrane filter unit (2) is a reverse osmosis unit and the flushing of the membrane filter unit (2) is enhanced by osmosis.