Description

The invention relates to a filtration system for liquid, particularly raw water, comprising a filtration module for filtering the liquid, an inlet pipe for feeding liquid to the filtration module, an outlet pipe for discharging filtrate from the filtration module and a backwash tank connected to the outlet pipe. The invention also relates to a method for backwashing a filtration system for liquid, particularly raw water.

Water treatment is one of the most vital applications of filtration processes which thus experience a strong interest not only due to global water scarcity, particularly in draught-prone and environmentally polluted areas, but also due to the continuous need for drinking water supplies and for treatment of municipal or industrial waste water. Typically, water treatment relies on a combination of different methods and technologies, which depend on the intended purpose of the cleaned water as well as on the quality and degree of the contaminated or raw water. Conventionally, water treatment is based on treatment steps such as flocculation, sedimentation and multi-media filtration. In recent years, however, membrane technologies such as micro- filtration, ultrafiltration, nanofiltration and reverse osmosis have emerged, providing more efficient and reliable filtration processes. Membrane-based processes, such as microfiltration or ultrafiltration, remove turbidity caused by suspended solids and microorganisms such as pathogens like bacteria, germs and viruses from raw water. Further, significant advantages of membrane based processes are that considerably less chemical and no temperature treatment is required.

Common membranes for filtration are either flat-shaped membranes or tubular membranes with one or more capillaries. Typically, such membranes are semi-permeable and mechanically separate permeate or filtrate and the retentate from raw water. Thus, the microfiltration and ultrafiltration membranes allow permeate, such as water, to pass and hold back suspended particles or microorganisms as retentate. In this context, vital membrane parameters are, amongst others, the selectivity, the resistance to fouling and the mechanical stability. The selectivity is mainly determined by the pore size usually specified in terms of the exclusion limit given by the nominal molecular weight cut-off (NMWC) in Dalton (Da). The NMWC is usually defined as the minimum molecular weight of a globular molecule retained by the membrane to 90%. For example in ultrafiltration, the nominal pore size lies between 50 nm and 5 nm and the NMWC lies between 5 kDa and 200 kDa. In nanofiltration, the pore size lies between 2 nm and 1 nm and the NMWC lies between 0.1 kDa and 5 kDa. Thus, while ultrafiltration already filters bacteria, viruses and macromolecules, leading to water with drinking quality, nanofiltration leads to partially demineralized water. In reverse osmosis, the nominal pore size shrinks even further, below 1 nm and the NMWC shrinks below 100 Da. Reverse osmosis is thus suitable for filtering even smaller entities, such as salts or small organic molecules. In combining the different filtration technologies, a wide variety of filtration actions can be obtained which may be adapted to a specific intended purpose.

Membranes are usually embedded within a filtration system which allows to feed the raw water and to discharge permeate as well as concentrate. For this purpose, filtration systems encompass an inlet as raw feed and outlets to discharge both permeate and concentrate. For tubular-shaped membranes, different designs of filtration systems exist.

WO 2006/012920 A1 discloses a filtration system for tubular membranes. The tubular membrane includes a number of capillaries which are embedded in a porous substrate. The liquid to be filtered flows from or to at least one long inner channel of the capillaries for transporting the liquids to be filtered or filtered liquid. The tubular membrane is disposed in a tubular housing with an inlet and outlets for discharging permeate and concentrate. In particular, permeate is discharged through an outlet opening located centrally along the long axis of the tubular housing.

EP 0 937 492 A2 discloses a capillary filtration membrane module comprising a filter housing with an inlet, an outlet and a membrane compartment. To discharge permeate, the membrane compartment further comprises discharge lamellae, which guide permeate to a centrally located discharge compartment.

DE 197 18 028 C1 discloses a filtration system including an apparatus housing with membrane modules connected parallel to each other. The filtration apparatus further comprises a back flush component which allows back flushing one of the membrane modules while the others remain in filtration operation.

WO 2001/23076 A1 is related to an apparatus for purifying feed water which is fed to bundles of hollow fiber membranes arranged within the apparatus. The feed water is introduced at the top of the apparatus into a perforated tube which leads the feed water into the membranes. Filtrate is collected at the bottom and is partially stored in a diaphragm tank that is used for backwash operation. WO 2003/013706 A1 describes a membrane module assembly with a hollow fiber membrane that is located in a vessel. The ends of the membranes open into respective collection headers. Feeds are located on the side of the vessel applying feed to the side walls of the membrane fibers and withdrawing permeate through the fiber lumens. Filtrate is removed from the headers and waste is discharged through discharge ports located on the side of the vessel opposite the feed ports.

WO 2006/047814 A1 discloses a membrane module having a plurality of hollow fiber membranes extending between upper and lower headers. The fibers in the upper header open into a permeate collection chamber. The lower header has a plurality of aeration openings for feeding gas and/or liquid into the membrane module.

DE 10 2005 032 286 A1 discloses a filtration system including several filtration modules. Each filtration module has an inlet pipe connected to an inlet compartment for the liquid to be filtered and an outlet pipe connected to an outlet compartment for the filtrate. In filtration operation, the liquid, particularly raw water is fed through the inlet pipe to the inlet compartment. The filtrate permeates a membrane and reaches the outlet compartment, while the retentate remains within the inlet compartment. The retentate is eliminated from the inlet compartment by backwash operation. For backwash operation, pure filtrate is used.

EP 2 008 704 A1 discloses a filtration system including several filtration modules. The filtration modules are connected to an inlet pipe and to an outlet pipe. For backwash operation pressurized air is fed to the outlet pipe whereat filtrate is pressed from the outlet pipe to the filtration modules.

Therefore, it is an object of the invention to provide a filtration system that allows backwash operation for cleaning the filtration module from retentate included in the liquid with improved effectivity. A further object of the invention is to provide a method for backwashing a filtration system for cleaning the filtration module from retentate included in the liquid with improved effectivity.

These objects are achieved according to the present invention by a filtration system for liquid, particularly raw water. The filtration system comprises at least one filtration module for filtering the liquid in filtration operation, at least one inlet pipe for feeding liquid to the filtration module in filtration operation and at least one outlet pipe for discharging filtrate from the filtration module in filtration operation. The filtration system also comprises a backwash tank connected to the outlet pipe which is used for backwash operation. The backwash tank is designed and connected to the outlet pipe such that in filtration operation filtrate discharged from the filtration module flows through the outlet pipe and bypasses the backwash tank.

According to the invention, a pressurized air device is connected to the backwash tank, such that when applying pressurized air to the backwash tank in backwash operation, pressure in the backwash tank increases, for example up to 6.0 bar. During backwash operation, pressure in the backwash tank presses filtrate which is contained in the backwash tank into the outlet pipe. Pressure further presses filtrate from the outlet pipe into the filtration module and through a filtration membrane of the filtration module. Hence, backwash operation with increased pressure is possible whereby cleaning effectivity of the filtration module, in particular cleaning effectivity of the filtration membrane, is improved.

Advantageously, at least one concentrate valve is arranged in the at least one inlet pipe. When the concentrate valve is closed, increased pressure extends from the backwash tank and the outlet pipe into the filtration module up to the concentrate valve in the inlet pipe. By opening the concentrate valve, pressure decreases abruptly and liquid flows abruptly out of the filtration module through the inlet pipe with relatively high speed. Thus, in filtration operation the inlet pipe serves for feeding liquid to the filtration module, and in backwash operation, the inlet pipe serves for discharging liquid from the filtration module.

According to an advantageous embodiment of the invention, at least one drain pipe for discharging retentate from the filtration module in backwash operation is provided, and at least one drain valve is arranged in the at least one drain pipe. The drain pipe allows liquid with retentate to flow out of the filtration module into a sluice or a sewer that is arranged remote from the filtration module.

According to a further development of the invention, an expansion tank is connected to the drain pipe, in particular via a collecting pipe. In filtration operation, the expansion tank contains air at relatively low pressure which is marginally greater than ambient pressure, for example 1 .5 bar. In backwash operation, when liquid flows through the drain pipe, liquid can enter the expansion tank whereat pressure in the drain pipe decreases temporary. Hence, in backwash operation, the expansion tank reduces pressure in the drain pipe and allows the liquid to flow steadily and at relatively high speed. The drain pipe may be connected directly to the filtration module. Hence, in backwash operation liquid flows directly out of the filtration module into the drain pipe and thus bypasses the inlet pipe. Preferably, the drain pipe is connected to the inlet pipe, in particular via a collecting pipe. Hence, a separate connection point for the drain pipe at the filtration module is not necessary. Thus, in filtration operation the inlet pipe serves for feeding liquid to the filtration module, and in backwash operation, the inlet pipe serves for discharging liquid and retentate from the filtration module.

According to an advantageous embodiment of the invention, the backwash tank is connected to an aeration device for deaerating, in particular via a tank aeration valve. By means of the aeration device, air can escape the backwash tank whereby filtrate from the outlet pipe can flow into the backwash tank.

The objects of the invention are further achieved by a method for backwashing a filtration system for liquid, particularly raw water, whereat the filtration system comprises at least one filtration module for filtering the liquid, at least one inlet pipe for feeding liquid to the filtration module and at least one outlet pipe for discharging filtrate from the filtration module. The filtration system also comprises a backwash tank that is connected to the outlet pipe and that contains filtrate. The backwash tank is designed and connected to the outlet pipe such that in filtration operation filtrate discharged from the filtration module flows through the outlet pipe and bypasses the backwash tank.

According to the invention, in backwash operation pressurized air is applied to the backwash tank, such that pressure in the backwash tank is increased, for example up to 6.0 bar. Hence, filtrate which is contained in the backwash tank is pressed from the backwash tank into the outlet pipe. Furthermore, filtrate from the outlet pipe is pressed into the filtration module and through a filtration membrane of the filtration module. Hence, backwash operation with increased pressure is executed whereby cleaning effectivity of the filtration module, in particular cleaning effectivity of the filtration membrane, is improved.

Advantageously, a concentrate valve that is arranged in the inlet pipe is closed before pressurized air is applied to the backwash tank, such that pressure in the filtration module is increased.

Further advantageously, a concentrate valve arranged in the at least one inlet pipe is opened after pressurized air is applied to the backwash tank, such that liquid is pressed from the filtration module into the inlet pipe. By opening the concentrate valve, pressure is decreased abruptly and liquid is discharged abruptly out of the filtration module through the inlet pipe with relatively high speed.

According to a further development of the invention, a first concentrate valve arranged in a first inlet pipe is opened after pressurized air is applied to the backwash tank, a second concentrate valve arranged in a second inlet pipe is opened after the first concentrate valve is completely open, and the first concentrate valve is closed when the second concentrate valve is at least partially open. That means, the second concentrate valve is opened a certain time after the first concentrate valve has been opened, such that a certain amount of liquid flows through the first inlet pipe. Then, the first concentrate valve is closed when the second concentrate valve has reached a minimum opening angle or when the second concentrate valve is open for a relatively short time.

Preferably, the second concentrate valve is opened when a switching charging level of filtrate in the backwash tank is reached. That means, the instant of time for the second concentrate valve to open depends on the charging level of filtrate in the backwash tank.

According to an advantageous embodiment of the invention, applying of pressurized air to the backwash tank is stopped when a lower charging level of filtrate in the backwash tank is reached. Thereby, pressure in the filtration module is decreased smoothly. Advantageously, the lower charging level of filtrate in the backwash tank is determined such that when the pressurized air remaining in the backwash tank expands, remaining filtrate is pressed into the outlet pipe, and pressure in the backwash tank is decreased, whereat air escaping into the outlet pipe is avoided. Thereby, a waste of energy for executing the backwash is avoided.

Preferably, a drain valve arranged in a drain pipe for discharging retentate from the filtration module is opened before the concentrate valve arranged in the at least one inlet pipe is opened. Hence, liquid with retentate can be discharged out of the filtration module into a sluice or a sewer that is arranged remote from the filtration module.

According to a further development of the invention, an expansion tank that is connected to the at least one drain pipe for discharging retentate from the filtration module, in particular via a collecting pipe, is dewatered, at least partially, before or after backwash operation. Hence, after dewatering, the expansion tank contains air at relatively low pressure which is marginally greater than ambient pressure, for example 1.5 bar. In backwash operation, when liquid is discharged through the drain pipe, liquid is pressed into the expansion tank, whereat pressure in the drain pipe is decreased temporarily. Hence, in backwash operation, pressure in the drain pipe is reduced and liquid flows steadily and at relatively high speed through the drain pipe.

Preferably, the drain pipe for discharging retentate from the filtration module is connected to the inlet pipe, in particular via a collecting pipe, such that liquid is pressed from the filtration module into the drain pipe via the inlet pipe.

Advantageously, before or after backwash operation, the backwash tank is deaerated at least partially. Hence, after deaerating, the backwash tank contains filtrate, and eventually also air, at ambient pressure.

Brief description of the drawings

For a better understanding of the afore-mentioned embodiments of the invention as well as additional embodiments thereof, reference is made to the description of embodiments below in conjunction with the appended drawings showing

Figure 1 a schematically given single filtration module of a filtration system with connections to further elements of the filtration system and Figure 2 a schematically given filtration system with a plurality of filtration modules.

Hereinafter, preferred embodiments of the present invention will be described as reference to the drawings. The drawings only provide schematic views of the invention. Like reference numerals refer to corresponding parts, elements or components throughout the figures unless indicated otherwise.

Description of Embodiments

In figure 1 , a filtration module 20 for a liquid, particularly for raw water, is shown schematically with connections to further elements. The filtration module 20 comprises a filtration membrane 25, which separates an inlet compartment 24 from an outlet compartment 28 of the filtration module 20. A first inlet pipe 21 and a second inlet pipe 22 are connected to the inlet compart- ment 24 of the filtration module 20. An outlet pipe 26 is connected to the outlet compartment 28 of the filtration module 20.

In filtration operation, liquid is pressed through the first inlet pipe 21 or through the second inlet pipe 22 into the inlet compartment 24 of the filtration module 20. The liquid contains water and impurities like particles of dirt. The filtration membrane 25 of the filtration module 20 is constructed to be permeated by the water, but to retain the impurities. In the following, the water that permeates the filtration membrane 25 of the filtration module 20 is called filtrate or permeate, and the impurities that are retained by the filtration membrane 25 of the filtration module 20 are called concentrate or retentate.

In filtration operation, the filtrate which has permeated the filtration membrane 25 of the filtration module 20 is pressed through the outlet pipe 26 out of the outlet compartment 28 of the filtration module 20. Hence, in filtration operation, the filtrate is flowing into a first flow direction 51 , as shown by an arrow in figure 1 , from the inlet compartment 24 through the filtration membrane 25 to the outlet compartment 28 of the filtration module 20. Then, the filtrate flows further into the outlet pipe 26.

A first concentrate valve 31 is arranged in the first inlet pipe 21 , and a second concentrate valve 32 is arranged in the second inlet pipe 22. The first inlet pipe 21 and the second inlet pipe 22 are connected to a collecting pipe 30. The collecting pipe 30 is connected to a feed pipe 42, in which a feed valve 44 is arranged.

When the feed valve 44 is open and one of the concentrate valves 31 , 32 is open, liquid can pass through the feed pipe 42 and one of the inlet pipes 21 , 22 into the inlet compartment 24 of the filtration module 20. The feed valve 44 and the concentrate valves 31 , 32 are operated automatically, in particular electrically, pneumatically or hydraulically.

An outlet valve 36 is arranged in the outlet pipe 26. When the outlet valve 36 is open, the filtrate can pass through the outlet pipe 26 and the outlet valve 36 out of the outlet compartment 28 of the filtration module 20. When the outlet valve 36 is closed, the filtrate cannot pass through the outlet pipe 26 and the outlet valve 36 out of the outlet compartment 28 of the filtration module 20. The outlet valve 36 is operated automatically, in particular electrically, pneumatically or hydraulically.

The second inlet pipe 22 is connected to an inlet aeration valve 75. By opening the inlet aeration valve 75, the inlet pipe 22 can be deaerated. The outlet pipe 26 is connected to an outlet aeration valve 76. By opening the outlet aeration valve 76, the outlet pipe 26 can be deaerated.

A drain pipe 46 is connected to the collecting pipe 30. In backwash operation, filtrate is pressed from the outlet pipe 26 back into the outlet compartment 28 of the filtration module 20. The filtrate then permeates the filtration membrane 25 in a second flow direction 52 and enters the inlet compartment 24. The second flow direction 52 which is shown by an arrow in figure 1 is contrariwise to the first flow direction 51 . Then, the filtrate, the liquid and the retentate are pressed out of the inlet compartment 24 of the filtration module 20 through the inlet pipes 21 , 22 to the collecting pipe 30 and to the drain pipe 46.

A drain valve 48 is arranged in the drain pipe 46. When the drain valve 48 is open, the liquid and the retentate can pass through the drain pipe 46 and the drain valve 48 out of the inlet compartment 24 of the filtration module 20. When the drain valve 48 is closed, the liquid and the retentate cannot pass through the drain pipe 46 and the drain valve 48 out of the inlet compartment 24 of the filtration module 20. The drain valve 48 is operated automatically, in particular electrically, pneumatically or hydraulically.

In figure 2, a filtration system 10 for a liquid, particularly for raw water, is shown schematically. The filtration system 10 comprises several filtration modules 20 that are connected to other elements as shown in figure 1. The filtration modules 20 are arranged in parallel. A first inlet pipe 21 is connected to the inlet compartments 24 of the filtration modules 20 via adaption members that are not shown here in figure 2. A second inlet pipe 22 is also connected to the inlet compartments 24 of the filtration modules 20 via adaption members that are not shown here in figure 2. An outlet pipe 26 is connected to the outlet compartments 28 of the filtration modules 20 via outlet adaption members 27.

An expansion tank 40 is connected to the collecting pipe 30. In filtration operation, the expansion tank 40 contains air at pressure which is marginally greater than ambient pressure, for example 1 .5 bar. The expansion tank 40 is connected to a pressurized air device 60 via an expansion valve 62. When the expansion valve 62 is open, the pressurized air device 60 can supply pressurized air to the expansion tank 40. The pressurized air device 60 contains air at relatively high pressure, for example 6.0 bar. The pressurized air device 60 is presently a tank, but could also be a pump. The expansion valve 62 is operated automatically, in particular electrically, pneumatically or hydraulically.

A backwash tank 50 is connected to the outlet pipe 26 in an area between the outlet valve 36 and the outlet adaption members 27 of the filtration modules 20. An intake tube 54 extends from a top area into the backwash tank 50 almost until a bottom area. In filtration operation, the backwash tank 50 contains filtrate and air at ambient pressure, whereat the intake tube 54 extends through the contained air into the filtrate.

The backwash tank 50 is connected to the pressurized air device 60 via a backwash valve 64. When the backwash valve 64 is open, then the pressurized air device 60 can supply

pressurized air to the backwash tank 50. The backwash tank 50 is also connected to an aeration device 70 via a tank aeration valve 72. When the tank aeration valve 72 is open, then pressurized air that is present in the backwash tank 50 can escape through the tank aeration valve 72 and the aeration device 70. The backwash valve 64 and the tank aeration valve 72 are operated automatically, in particular electrically, pneumatically or hydraulically. A cleaning branch 80 is arranged between the first inlet pipe 21 and the second inlet pipe 22. The cleaning branch 80 contains a circulation pump 84 and a cleaning valve 86 that are arranged in series. When the cleaning valve 86 is open, then the circulation pump 84 can pump liquid from the second inlet pipe 22 to the first inlet pipe 21. In that case, liquid is circulated through the cleaning branch 80, the first inlet pipe 21 , the filtration modules 20 and the second inlet pipe 22. The cleaning valve 86 is operated automatically, in particular electrically, pneumatically or hydraulically. A first dosing feeder 81 , a second dosing feeder 82 and a third dosing feeder 83 are connected to the cleaning branch 80. The dosing feeders 81 , 82, 83 allow to add cleaning chemicals into the cleaning branch 80 for a chemically enhanced backwash operation. For instance, an alkaline cleaning agent can be added via the first dosing feeder 81 , an acid cleaning agent can be added via the second dosing feeder 82, and a chlorine cleaning agent can be added via the third dosing feeder 83. When a cleaning agent is added into the cleaning branch 80, then the cleaning agent can be circulated through the cleaning branch 80, the first inlet pipe 21 , the filtration modules 20 and the second inlet pipe 22 by means of the circulation pump 84, as described above. A control unit, which is not shown here, is connected electrically to the first concentrate valve 31 , the second concentrate valve 32, the outlet valve 36, the feed valve 44, the drain valve 48, the expansion valve 62, the backwash valve 64, the tank aeration valve 72 and the cleaning valve 86. By means of said control unit, said valves can be opened or closed. The circulation pump 84 and the dosing feeders 81 , 82, 83 are also connected electrically to the control unit and can be started or stopped by means of said control unit.

When the filtration system 10 is in filtration operation, the feed valve 44 is open, the drain valve 48 is closed, the first concentrate valve 31 is open, the second concentrate valve 32 is closed, and the outlet valve 36 is open. Alternatively, the first concentrate valve 31 is closed, and the second concentrate valve 32 is open, or both concentrate valves 31 , 32 are open. Furthermore, the cleaning valve 86 is closed, the expansion valve 62 is closed, the backwash valve 64 is closed, and the tank aeration valve 72 is closed.

In filtration operation, liquid is pressed through the feed pipe 42, the collecting pipe 30, the first inlet pipe 21 or the second inlet pipe 22 into the filtration modules 20. Filtrate is pressed out of the filtration modules 20 through the outlet adapter members 27 and the outlet pipe 26.

Retentate is retained by the filtration membranes 25 of the filtration modules 20 and remains in the inlet compartments 24 of the filtration modules 20. In filtration operation, the expansion tank 40 contains air at relatively low pressure which is marginally greater than ambient pressure, for example 1 .5 bar. The backwash tank 50 contains filtrate, and eventually also air, at ambient pressure. The backwash tank 50 is designed and connected to the outlet pipe 26 such that in filtration operation, filtrate that is discharged from the filtration modules 20 flows straight through the outlet pipe 26 and bypasses the backwash tank 50.

Preparing backwash operation, initially the feed valve 44 is closed, and the outlet valve 36 is closed. Then, the first concentrate valve 31 and the second concentrate valve 32 are closed, respectively remain closed. Subsequently, the drain valve 48 is opened. The cleaning valve 86, the expansion valve 62 and the tank aeration valve 72 remain closed.

To start backwash operation, the backwash valve 64 is opened. Thus, pressurized air from the pressurized air device 60 is applied to the backwash tank 50. Thereby, pressure in the backwash tank 50, in the outlet pipe 26, in the filtration modules 20 and in the inlet pipes 21 , 22 is increased.

Subsequently, the first concentrate valve 31 or the second concentrate valve 32 is opened. Thus, pressure in the first inlet pipe 21 or in the second inlet pipe 22 is decreased, and liquid and permeate contained in the filtration modules 20 are pressed abruptly out of the first inlet pipe 21 or out of the second inlet pipe 22 into the collecting pipe 30 and further into the drain pipe 46. Thus, pressure in the collecting pipe 30 is increased, and liquid is also pressed into the expansion tank 40. Filtrate is pressed from the backwash tank 50 into the outlet pipe 26, and filtrate is pressed from the outlet pipe 26 into the filtration modules 20. Within the filtration modules 20, filtrate is pressed from the outlet compartment 28 through the filtration membrane 25 into the inlet compartment 24, in second flow direction 52. Thereby, the filtration membrane 25 is cleaned. Optionally, the first concentrate valve 31 is opened for a start. Then, the second concentrate valve 32 is opened a certain time after the first concentrate valve 31 is completely open, and a certain amount of liquid has already flown through the first inlet pipe 21. Then, the first concentrate valve 31 is closed relatively promptly, in particular while the second concentrate valve 32 is still opening and is only partially open, or when the second concentrate valve 32 is open for only a relatively short time.

The instant of time for the second concentrate valve 32 to open depends on the charging level of filtrate in the backwash tank 50. Hence, the second concentrate valve 32 is opened when a defined switching charging level of filtrate in the backwash tank 50 is reached.

Thereby, the amount of filtrate in the backwash tank 50 is decreased, and the charging level of filtrate in the backwash tank 50 drops. When a determined lower charging level of filtrate in the backwash tank 50 is reached, the backwash valve 64 is closed. Hence, applying of further pressurized air to the backwash tank 50 is stopped. The air remaining in the backwash tank 50 is still under pressure and therefore expands further. While expanding, the air that remains in the backwash tank 50 presses further filtrate out of the backwash tank 50, until pressure of the air in the backwash tank 50 is decreased sufficiently. Said lower charging level of filtrate in the backwash tank 50 is determined such that when the pressurized air that remains in the backwash tank 50 expands until the pressure of said air is decreased sufficiently, no air escapes into the outlet pipe 26. Hence, air escaping out of the backwash tank 50 into the outlet pipe 26 is avoided. Thereby, filtrate contained in the backwash tank 50 is discharged almost completely into the outlet pipe 26 until the backwash tank 50 contains almost only air.

Thereby, pressure in the outlet pipe 26, in the filtration modules 20, in the inlet pipes 21 , 22 and in the collecting pipe 30 is decreased smoothly. The liquid that has flown into the expansion tank 40 is discharged out of the expansion tank 40 into the collecting pipe 30 and further into the drain pipe 46. When pressure in the backwash tank 50, in the outlet pipe 26, in the filtration modules 20, in the inlet pipes 21 , 22 and in the collecting pipe 30 is decreased sufficiently, and the backwash tank 50 contains almost only air, backwash operation is complete. After backwash operation, if there is still some liquid remaining in the expansion tank 40, the expansion valve 62 is opened such that pressurized air from the pressurized air device 60 is pressed into the expansion tank 40 and liquid remaining in the expansion tank 40 is discharged in to the collecting pipe 30 and further into the drain pipe 46. Thus, the expansion tank 40 is dewatered. When the expansion tank 40 is dewatered, the expansion valve 62 is closed.

Alternatively, the expansion tank 40 can be dewatered before backwash operation.

To return to filtration operation, the drain valve 48 is closed, the outlet valve 36 is opened, and the feed valve 44 is opened. Eventually, one of the concentrate valves 31 , 32 is closed or both concentrate valves 31 , 32 remain open. Hence, liquid is fed from the feed pipe 42 via the collecting pipe 30 and at least one of the inlet pipes 21 , 22 into the filtration modules 20.

Permeate is discharged from the filtration modules 20 into the outlet pipe 26.

After backwash operation, the tank aeration valve 72 is opened such that air remaining in the backwash tank 50 can escape through the aeration device 70. Filtrate is flowing from the outlet pipe 26 into the backwash tank 50 until the backwash tank 50 is filled, at least almost completely, with filtrate. Thus, the backwash tank 50 is deaerated. When the backwash tank 50 is deaerated, the tank aeration valve 72 is closed. Alternatively, the backwash tank 50 can be deaerated before backwash operation. The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings and those encompassed by the attached claims. The embodiments were chosen and described in order to explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. List of Reference Numerals

10 Filtration system

20 Filtration module

21 First inlet pipe

22 Second inlet pipe

24 Inlet compartment

25 Filtration membrane

26 Outlet pipe

27 Outlet adaption member

28 Outlet compartment

30 Collecting pipe

31 First concentrate valve

32 Second concentrate valve

36 Outlet valve

42 Feed pipe

40 Expansion tank

44 Feed valve

46 Drain pipe

48 Drain valve

50 Backwash tank

51 First flow direction

52 Second flow direction

54 Intake tube

60 Pressurized air device

62 Expansion valve

64 Backwash valve

70 Aeration device

72 Tank Aeration valve

75 Inlet Aeration valve

76 Outlet Aeration valve

80 Cleaning branch

81 First dosing feeder

82 Second dosing feeder

83 Third dosing feeder

84 Circulation pump

86 Cleaning valve