DE10004863A1 - Separation of solids and liquid from sludge suspension using high efficiency, low cost agitated filter, passes flow of suspended air bubbles tangentially and turbulently past loosely-held filter membrane - Google Patents

Abstract

The filter membrane has a free downstream end. A stream of air bubbles driving flow, passes tangentially over the membrane surface. An Independent claim is included for corresponding equipment. The aerator (10) introduces air (9) into the suspension (2). Rising air bubbles produced, drive the flow (11). Permeate (21) passing through the filter membrane is taken via the drain line (17) to a collection tank. Preferred features: The downstream end of the membrane, opposite the free end, is secured. It may be held at two opposite ends, transversely to the flow direction. Air bubble-driven flow is superimposed on an independent vertical flow of the suspension. Permeate is withdrawn at intervals, varying the intensity of the flow driven by bubbles. They produce turbulent flow at the membrane surface. Permeate withdrawal is driven by reduced pressure, 0.1-0.6 bar, in the permeate collection tank. Variants of the withdrawal method are described. The vertical flow is recirculated.

Description

The invention relates to a method for separating solids and liquid from a solid Liquid suspension using at least one filter membrane bran, which is an immersion in the suspension and of a vertically ascending bubble buoyancy flow out into the Solid-liquid suspension introduced air flowed outer membrane surface and one of the outer membrane opposite surface, a permeate collection space for the inner liquid delimiting as permeate Has membrane surface, as well as a to carry out device suitable for this method. With the solid Liquid suspension can in particular be a  the biological wastewater treatment biomass-what act in a mixed mix.

In such a known method (EP 0 510 328 B1) are all on rigid, plate-shaped membrane units surface-mounted filter membranes or double-sided clamped tubular filter membranes used. An on Number of these membrane units is one filter module summarized that contains the biomass-water mixture Aeration tank is immersed and vertically from the Flow of the biomass-water mixture and the air bubbles buoyancy flow is flowed through. This requires a ver relatively high air consumption. Furthermore, pigtailed ending wastewater ingredients such as fibers and hair around the mo the roof structure and the membrane units. This ver impurities cannot be cleaned without a business interruption remove. Finally, the retrofitting is existing aeration tanks with these filter modules difficult.

In another known method (US 5 248 424 A and US 5,403,479 A) are air-loaded hollow fiber membranes NEN, from which permeate is withdrawn on both sides at the membrane end is used. The type of inflow provided there however, it is difficult to rinse the membranes from plait-forming membranes fibrous substances, such as hair, fibers and the like. It enables also no decoupling of permeate discharge and air bubbles drive current.

The invention has for its object a method of the type mentioned to further develop that the air supplied is used more effectively and at the same time the necessary to carry out the procedure directional effort is reduced.

According to the invention, this task with respect to the Ver driving solved in that a flexible filter membrane  Membrane flag with a free one pointing in the direction of flow End is used, the flowed membrane surface itself aligns tangentially to the bubble lift flow.

In the method according to the invention, the the solid-liquid suspension immersed movable Membrane flag of the bubble buoyancy flow and the like caused flow of the solid-liquid suspension follows tangentially and experiences through the tangentia len flow attack to generate the membrane for required shear stress on the membrane surface, so the wall shear stress of the membrane, required force Handle. At the same time, the air bubble buoyancy causes a loop-like circulation of the solid-liquid liquid pension in the reactor used for the separation stem, for example an activated sludge pool. In practice A large number of them are used regularly in regular intervals stood arranged membrane flags applied to each other. This can be produced inexpensively over a large area and also after sluggishly in existing reactor systems, e.g. B. Animated sludge pool to be installed. The on the membrane flags interfering flows of the solid-liquid suspension and the air bubbles effectively ensure that the specified flowed continuously flushed outer membrane surfaces and thus impair the filter effect is avoided by solids that build up. In particular are fibrous substances such as hair, fibers and the like Braids tend to be transported to the membrane end where they are peel off. The tangential flow of the solid-liquid speed suspension and the bubble flow effect on the one hand a flushing of the membrane surfaces with both Air as well as water while holding the membrane fibers in motion, making an effective filtration operation is guaranteed.

An advantageous embodiment of the invention The procedure is that the bubble buoyancy flow  an independent vertical flow of the suspension is stored. The tangential flow of the solid-liquid Speed suspension and the bubble buoyancy flow are there by decoupled from each other and separated in their strength adjustable. Through this decoupling of the solids flows liquid suspension and the tangential flow Air bubble buoyancy flow can be flexible to the process different requirements, for example increase or reduction in the permeate output.

There is an expedient way of carrying out the process in that the permeate is subtracted and the The intensity of the bubble buoyancy flow is varied. There the result is an air purge of the membrane flags ranges through which the membrane flags of one in the barrel the top layer of ingredients that starts to operate the suspension are freed. This cleaning process will especially by increasing the intensity at intervals the bubble buoyancy flow favors.

The method is also expediently carried out in this way led that the membranes on the permeate side with permeate and / or permeate mixed with cleaning agents or another Cleaning liquid can be flushed back.

The method is expediently carried out in such a way that due to the air bubble buoyancy flow Membrane surface creates a turbulent flow becomes. Due to the prevailing turbulence, the membrane becomes upper surface particularly effective and kept clean.

Further advantageous procedural measures and settings result from the remaining subclaims.

In terms of device, that of the invention underlying task in a device for separation of solid and liquid from a solid-liquid  suspension with the introduction of air into the Aeration device serving solid-liquid suspension device for generating a vertical bubble buoyancy flow tion and a filter membrane assembly that at least one in the solid-liquid suspension immersed filter membrane bran with a flow from the air bubble upflow th outer membrane surface, one of the outer membrane upper opposite, a permeate collection space for the inner membrane delimiting liquid as permeate branch surface and one connected to the permeate collection space has its connection area of a permeate discharge, since solved by that the filter membrane as a flexible membrane flag is formed, which extends from one end of the membrane flag defining connection area extends and a in Strö direction of the free end.

The device according to the invention enables the through implementation of the method according to the invention with simple teln. The multitude of movable ones envisaged in practice Membrane flags can be easily placed in the flow Place away from the solid-liquid suspension. In a preferred embodiment is additionally a drive for Support the vertical flow of the solid-liquid speed suspension arranged. The one for generating the vertical Flow serving the solid-liquid suspension can drive, for example, a propeller drive, a pump or an agitator. The solid Reactor containing liquid suspension, for example an aeration tank, designed such that the tangential flow passes through it like a loop. This can be done by the outer shape of the reactor, for example a slim one Execution of the reactor, or by currents arranged therein guide walls are brought about.

With regard to the design of the filter membranes provided in the context of the invention that the membrane flag a Number of hollow fiber membranes, the one in the An  closing area of the permeate discharge and fixed there put open end and an opposite end to the open end tes, free, closed end. With these Hohlfa The capillary cavity forms the water or capillary membranes Permeate collecting space, which is only on the at the connection area the permeate discharge end of the hollow fiber or Ka pillare is open and there the permeate in the permeate tion transferred.

In a further embodiment of this embodiment the arrangement preferably made such that the Hohlfa membrane membranes combined into at least one tuft and the open ends of the hollow fiber membranes of the tuft in a common connection opening of the connection area are arranged.

Another alternative is that the membrane flag as a flat membrane pocket with two under each other was arranged, the permeate collection space between them bordering membrane surfaces and one in a connection opening the outlet opening opening area is.

The band-shaped rectangular flat membrane pocket is at for example 0.5 to 1.5 m high and 1 to 50 m long and will large area of two membrane doors forming the membrane surfaces Chern manufactured that equipped with a spacer enclose the permeate collecting space and on all four edges are welded or glued liquid-tight. The mem branfahne, for example, will have more than 2 to 100 deflection tubes guided differently and has in the area of the deflection pipes via outlet openings to the permeate line.

The deflection pipes are as permeate discharge pipes with an closing openings executed with the outlet openings of the flat membrane pockets are aligned.  

A mounting frame to which the deflection tubes are fixed, is arranged in the flow so that the deflection pipes are in the direction of flow and the solid-liquid suspension tangential through that of the flat membrane pocket formed open flow channels flows through.

A particularly important application of the fiction moderate device is that the membrane flags incoming tangential flow loop in a reactor circulates. In this case the device is in the reak gate installed.

The device according to the invention can advantageously be of this type be designed so that the ventilation device at least one with the solid-liquid suspension as Driving jet liquid and air on the other hand has injector. The injector, also as a two-substance mixture known nozzle, the solid-liquid suspension with means of a pump as propellant fluid, which the supplied carried air entrained, abandoned. In this way, the Injector both contribute to the flow of solid Liquid suspension as well as the desired air bubbles drive current. The required flow of solids Liquid suspension can only by this effect of Injector can be brought about. Alternatively, this effect of the injector to amplify one of an additional Drive, such as a propeller drive Flow of the solid-liquid suspension used become.

In the following description, the invention is described in Reference to the drawing explained in more detail. Show it:

Fig. 1 is a vertical section through a container tion provided with an embodiment of the invention reac,

Fig. 2 is a Fig. 1 corresponding vertical section of a modified embodiment,

Fig. 3 is a vertical section of another exporting approximate shape of the device according to the invention,

Fig. 4 is a schematic representation of sermembranen from Hohlfa composite membrane tufts

FIG. 4a a cross section through the attachment of the hollow fiber membrane bundle at a connection area of a permeate discharge line,

. 5a is a schematic representation of a modular construction as Fig frame with flat membrane pockets in a mounting,

Fig. 5b is a horizontal section through the mounting frame of Fig. 5a,

Fig. 5c is a view of a flat membrane bag,

Fig. 5d a view of a return tube,

Fig. 5e is a view of a tension spring,

Fig. 5f an axial section through a deflection tube and the mounting frame,

Fig. 5g shows a radial detail section through an order steering tube.

In Fig. 1 and 2 are cross sectional views of two From EMBODIMENTS a reaction vessel 1 is shown, in which a device for the separation of solids and liquid from a present in the reaction vessel 1 solid-liquid suspension 2 is arranged. The reaction vessel ter 1 has in both embodiments a flat horizon tal pelvic floor 3 and an outgoing vertical Be tenwandung 4 . Overall, the reaction container 1 has a round or rectangular outline shape. Parallel to the sides wall 4 extends at a distance from the pool floor 3 is a vertical flow guide 5, which defines with the basin bottom 3 and the side wall 4 an endlessly circulating Strömungska nal 6 as a loop reactor. In the round version, the flow guide wall 5 is designed as a guide tube open at the top and bottom.

In the flow channel 6 , a propeller drive 7 is arranged, which puts the solid-liquid suspension 2 in a flow 8 in the flow channel 6 rotating vertically. Air distributors 10 , which are supplied by an external compressed air supply 9 , are arranged on the pool floor 3 and can be designed, for example, as pillow, surface, plate, tube aerators or injectors ( 100 , FIG. 3). The ejected from the Luftver 10 air bubbles form in the circulating solid-liquid suspension 2 an Luftbla senauftriebsströmung 11 , the direction of which on the one hand in Fig. 1 in the interior of the flow guide wall 5 and in Fig. 2 in that between the flow guide wall 5 and the side wall 4th limited outer space of the flow guide wall 5 vertically on the flow of the solid-liquid suspension sion 2 following and on the other hand the buoyancy of the air blowing from the pool floor 3 is directed upwards.

In the vertically upward flow of the solid-liquid suspension 2 , a plurality of membrane flags 12 is immersed, the possible structure of which is illustrated in FIGS. 4 and 4a. Thereafter, each membrane neck 12 consists of a plurality of bundles of elongated hollow fiber or capillary membranes 13 , the inner membrane surface of which in each case delimits a permeate collection space for the water passing through the membrane from the solid-liquid suspension 2 and which is open at one of its two ends 15 are. At this open end 15 , the hollow fiber membranes 13 of a tuft open into a connection opening 16 , which is formed in a connection area 18 of a meat discharge line 17 . The open end 15 opposite end 14 of the hollow fiber membranes 13 is upstream of the connection area 18 freely in the flow and is closed, so that the liquid entering the permeate collection space can only escape at the open end 15 and thus into the connection area 18 of the permeate discharge line 17 reached. In this way, the connection area 18 of each permeate discharge line 17 with the membrane membranes 12 connected thereto forms a membrane module.

In the embodiments shown in FIGS. 1 and 2, a plurality of the connection openings 16 having connection regions 18 of the permeate discharge line 17 can be arranged in a plane directed transversely to the vertical flow 8 at a mutual distance. Also, several of these membrane modules can be arranged one above the other in superimposed levels. Of these terminal portions 18 of the Permeatab 17 lines from the membrane lugs 12 extend freely into the solid-liquid suspension 2 and can be aligned by the prevailing flow field. In the illustrated embodiments, a permeate pump 20 is connected to the permeate derivatives 17 , which generates a permeate suction vacuum, for example from 0.1 to 0.6 bar. In this way, the purified liquid which has passed from the solid-liquid suspension 2 into the permeate collecting spaces of the membrane flags 12 is withdrawn as permeate 21 from the separation process. To a corresponding extent, contaminated wastewater is fed to the reaction vessel 1 , as in FIGS. 1 and 2.

In Fig. 1 are two and in Fig. 4, a total of four by the arranged in one plane each connecting areas 18 of the permeate discharge lines 17 and membrane membrane attached thereto Mem 12 formed membrane modules are provided in addition to the associated air distributors 10 . The number of these Mem branmodule can also be reduced depending on the desired filtration performance or z. B. by ver different side by side and stacked membrane levels can be increased.

Fig. 3 shows a compared to the corresponding Fig. 1 and 2 modified embodiment in that instead of the air distributor 10 in Fig. 1 and 2 near the pool bottom 3 injectors or two-substance mixing nozzles 100 are arranged. On the one hand, this is supplied by a propulsion jet pump 101, the solid-liquid suspension 2 as a propulsion jet liquid, and on the other hand, air is supplied from the external compressed air supply 9 . The driving jet of liquid breaks in the in jector 100 with the supplied air and thereby generates the sweeping away across the membrane lugs 12 Luftblasenauf propulsion flow. 11 At the same time, the propellant jet also contributes to the propeller drive 7 to the tangential flow 8 circulating in the flow channel 6 . Otherwise, this embodiment corresponds to the embodiment shown in FIG. 1, so that reference can be made to the above description thereof, the same reference numerals being used for the corresponding parts.

The embodiment of membrane modules shown in FIGS. 5a to 5g differs from the embodiment shown in FIGS. 4 and 4a with regard to the design of the membrane lugs 12 . Incidentally 1, the same reference numerals as in Fig are for the Fig. 4a Meaning to like elements throughout. 1 to 4 is used. In this respect, reference is made to the description there. Instead of hollow fiber membranes 13 are in the embodiment of Fig. 5a to 5g rectangular band-shaped Flachmem brantaschen 25 are provided, which are arranged in the current flow field tangential to the direction of flow. The flat membrane pockets 25 are equipped with an inner spacer 28 and are guided in a meandering shape 27 around deflection pipes 19 , which form the permeate discharge lines 17 .

As is apparent from Fig. 5a in detail, the order steering tubes 19 are fixed with their axial ends in frame parts 30 of a mounting frame. The frame members 30 extend rectangularly in two perpendicular to the flow 11 planes, the 19 having, downstream upper frame parts 30 collector as permeate 31 are formed for the supplied from the Umlenkrohren 19 permeate 21, the upper ends of the change pipes. The change pipes 19 supporting frame parts 30 face each other across the flow direction 11 opposite to and support the meandering guided around Flachmem Brant ash 25 from where their transverse to the flow direction 11 edges are exposed in the flow 11 and the flat membrane bag is flown tangentially 25th

The horizontal section shown in Fig. 5b through the mounting frame shows the meandering guidance of the Flachmem brantasche 25 particularly clearly. The latter has two membrane surfaces 33 which limit the permeate collecting space between and are held by a spacer 28 arranged in the permeate collecting space at a small distance from the surface area of the membrane surfaces 33 . Fig. 5b further makes the connection openings 16 in the permeate discharge line 17 serving as Umlenkrohren 19 and thus aligned discharge openings 34 in the membrane surfaces 33 he know (see Fig. 5d). The permeate collecting in the permeate collection space is discharged through the outlet openings 34 and the connection openings 16 into the deflection pipes 19 and from there into the permeate collector 31 . Otherwise, the two membrane surfaces 33 are tightly connected to one another at the edge 32 of the band-shaped flat membrane pocket 25 (see FIG. 5c).

Fig. 5g illustrates the meander-shaped arrangement of the flat membrane pocket 25 shown in Fig. 5b in more detail. Thereafter, the flat membrane pocket 25 is guided in a C-shape by more than 180 ° around each deflection tube 19 . Between the radially outer surface of the deflecting tube 19 and the radially inner diaphragm side 33 , a seal 29 is arranged. A cross-sectionally C-shaped tension spring 23 engages over the area of the flat membrane pocket 25 wrapped around the deflection tube 19 . Between the radially outer membrane side 33 and the radially inner surface of the tension spring 23 , a further seal 29 is arranged. The tension spring 23 ensures a tight fit of the flat membrane pocket 25 on the deflection tubes 19 , while the seals 29 ensure that the permeate through the outlet opening 34 and the connection opening 16 is introduced into the deflection tube 19 and no permeate can escape to the outside. An overall view of the tension spring 23 is shown in Fig. 5e.

Fig. 5f illustrates once again the shape of the order pipes 19 and fixing them to the frame parts 30th In the tubular casing, the slot-shaped openings 16 are recognizable. The vertically lower end of the deflecting tube 19 is plugged onto a bearing journal of the lower frame part 30 and is closed in a liquid-tight manner by a sealing ring 35 arranged in an annular groove of the bearing journal. Since against includes a similar bearing journal of the permeate manifold 31 constructed as the upper frame member 30 in a through-bore through which the interior of the circulation path 19 communicates with the interior of the frame part 30 in connection and thus the processing of the permeate Ablei 21 allows. Otherwise, similarly as at the lower end of the bearing journal of the upper frame part 30, it is also provided with a sealing ring 35 for the purpose of sealing. As can be seen from the top right of FIG. 5f, a further sealing ring 35 can additionally be provided between the free end of the deflection tube 19 and the outside of the frame part 30 .

List of reference symbols

1

Reaction vessel

2

Solid-liquid suspension

3rd

pelvic floor

4

Side wall

5

Flow guide

6

Flow channel

7

Propeller drive

8th

Vertical flow

9

Compressed air supply

10th

Air distributor / ventilation device

11

Bubble buoyancy flow

12th

Membrane flags

13

Hollow fiber membrane

14

closed end

15

open end

16

Connection opening

17th

Permeate discharge

18th

Connection area

19th

Deflection pipes

20th

Permeate pump

21

Permeate

23

Tension spring

25th

Flat membrane pockets

27

meander

28

Spacers

29

poetry

30th

Frame parts

31

Permeate collector

32

edge

33

membrane

34

Outlet openings

35

Sealing ring

100

Injectors

101

Jet pump

110

Backwashing device

Claims (28)

1. A method for separating solid and liquid from a solid-liquid suspension by means of at least one filter membrane which flows into an air immersed in the suspension and from a vertically rising air bubble flow from the air introduced into the solid-liquid suspension and an outer membrane surface opposite the outer membrane surface , A permeate collecting space for the liquid separated as a permeate has an inner membrane surface, characterized in that a flexible membrane flag with a free end pointing in the direction of flow is used as filter membrane, the flowed membrane surface of which is oriented tangentially to the air bubble buoyancy flow. 2. The method according to claim 1, characterized in that that the membrane flag at its free end downstream opposite end is held. 3. The method according to claim 1, characterized in that the membrane plume on two are transverse to the direction of flow opposite ends is held. 4. The method according to any one of claims 1 to 3, characterized characterized in that the air bubble buoyancy flow there overlaid by independent vertical flow of the suspension becomes. 5. The method according to any one of claims 1 to 4, characterized characterized in that the permeate is withdrawn at intervals and the intensity of the bubble buoyancy flow varies becomes. 6. The method according to any one of claims 1 to 5, characterized characterized in that by the air bubble buoyancy flow  a turbulent flow on the membrane surface history is generated. 7. The method according to any one of claims 1 to 6, characterized characterized in that the withdrawal of the permeate with one in the Permeate collecting space prevailing negative pressure takes place. 8. The method according to claim 7, characterized in that that the vacuum is between 0.1 and 0.6 bar. 9. The method according to any one of claims 1 to 6, characterized characterized in that the withdrawal of the permeate by a the solid-liquid suspension exerted pressure follows. 10. The method according to any one of claims 1 to 6, characterized characterized in that the deduction by hydrostatic overpressure the solid-liquid suspension takes place. 11. The method according to claim 9 or 10, characterized records that the overpressure is between 0.1 and 3 bar. 12. The method according to any one of claims 1 to 11, characterized characterized in that the vertical flow as loop flows mung is performed. 13. Apparatus for separating solid and liquid speed from a solid-liquid suspension with an aeration device ( 10 ) serving to introduce air ( 9 ) into the solid-liquid suspension ( 2 ) for generating a vertical air bubble buoyancy flow ( 11 ) and a filter membrane arrangement, the at least one filter membrane ( 12 ) immersed in the solid-liquid suspension with an outer membrane surface against which the air bubble buoyancy flow ( 11 ) flows, a surface opposite the outer membrane surface, a permeate collection space for the liquid delimited as permeate ( 21 ) and an inner membrane surface delimiting the permeate is closed terminal portion (18) of a permeate discharge line (17), characterized in that the filter membrane is formed as a flexible membrane lug (12) extending from said fixing one end of the membrane flag terminal region (18) a us extends and has a free end pointing in the direction of flow. 14. The apparatus according to claim 13, characterized in that the connection region ( 18 ) is arranged downstream of the free end of the membrane flag ( 12 ). 15. The apparatus according to claim 13, characterized in that two transversely to the flow direction opposite lying connection areas are provided, between which the membrane flag ( 12 ) extends with opposite ends in the flow direction. 16. The apparatus according to claim 13 or 14, characterized in that the membrane flag ( 12 ) has a number of hollow fiber membranes ( 13 ) which opens into the connection region ( 18 ) of the permeate discharge ( 17 ) and is fixed there open end ( 15 ) and the open end opposite, free, closed end ( 14 ). 17. The apparatus according to claim 16, characterized in that the hollow fiber membranes ( 13 ) are combined into at least one Bü tuft and the open ends ( 15 ) of the Hohlfa sermembranen ( 13 ) of the tuft in a common connection opening ( 16 ) of the connection area ( 18th ) are arranged. 18. The apparatus according to claim 13 or 15, characterized in that the membrane lug ( 12 ) as Flachmembranta cal ( 25 ) with two spaced apart, the permeate collecting space between the limiting membrane surfaces ( 33 ) and one in a connection opening ( 16 ) At the closing area ( 18 ) opening outlet ( 34 ) is ausgebil det. 19. The apparatus according to claim 18, characterized in that a spacer ( 28 ) is arranged between the two membrane surfaces ( 33 ) of the flat membrane pocket ( 25 ). 20. The apparatus of claim 18 or 19, characterized in that the membrane surfaces ( 33 ) of the Flachmembranta cal ( 25 ) are rectangular and liquid-tightly connected to one another at the rectangular edge, the outlet opening ( 18 ) being arranged in the membrane surfaces ( 33 ) . 21. Device according to one of claims 18 to 20, characterized in that the flat membrane pocket ( 25 ) is band-shaped and meandering ( 27 ) over deflection pipes ( 19 ) which serve as connection areas ( 18 ) and with which in each have outlet openings ( 34 ) which are formed on the diaphragm surface ( 33 ) and which are in contact with the deflecting tubes ( 19 ) and have aligned connection openings ( 16 ). 22. The apparatus according to claim 21, characterized in that the flat membrane pocket ( 25 ) on the deflecting tubes ( 19 ) with one of the deflecting tubes ( 19 ) adjacent area of the flat membrane pocket ( 25 ) C-shaped clamping spring ( 23 ) is clamped. 23. The apparatus according to claim 22, characterized in that the deflecting tubes ( 19 ) are fixed in a mounting frame having a frame part ( 31 ) designed as a permeate collector. 24. Device according to one of claims 13 to 23, characterized in that a drive is provided for generating a flow of the solid-liquid suspension ( 2 ). 25. The device according to any one of claims 13 to 24, characterized in that the ventilation device has at least one on the one hand with the solid-liquid suspension ( 2 ) as propellant jet fluid and on the other hand with air-injected injector ( 100 ). 26. The apparatus according to claim 24, characterized in that the drive used to generate a flow has a propeller agitator ( 7 ) and / or a circulating pump. 27. The device according to one of claims 13 to 26, characterized in that the permeate discharge line is connected to a per meat pump ( 20 ) with which the permeate is pumped continuously or intermittently from the permeate discharge line. 28. Device according to one of claims 13 to 27, characterized in that the permeate drainage has a backwashing device ( 110 ) for intermittent permeate-side backwashing of the membrane flags.