FIELD OF THE INVENTION

The present invention is related to the field of fluid membrane filtration processes, such as with flat sheet or hollow fiber membranes. In particular, the present invention is related to the field of in situ fouling control in said filtration processes by magnetically induced membrane vibration to enhance the shear-rate of the feed.

BACKGROUND OF THE INVENTION

The application of membrane technology is strongly limited by membrane fouling, especially for filtration of solutions with high solid or colloid concentration. While in operation, foulants are built-up on the membrane surface, reducing the membrane permeability. In common membrane processes, periodic ^' removal of foulants is required in order to sustain the operation. Therefore, membrane industry is continuously seeking for better techniques to tackle this problem. The general remediation actions currently applied include relaxation, maintenance cleaning, intensive chemical cleaning and backwashing, are generally inefficient. The relaxation and backwashing are performed by respectively intermittently stopping the filtration process or pumping back part of the permeate. Chemical or biochemical cleaning of membranes, is costly, not environmentally friendly and can seriously decrease the membrane lifetime. The application of these techniques generally decreases the membrane throughput and increases the down time. They thus increase costs (labor, (bio)chemicals, increased required membrane area due to downtime,...) and energy consumption.

The enhancement of shear-rate at the feed side is the most efficient way to control fouling. It increases the back-transport of retained compounds and reduces the concentration polarization and the cake build-up (Jaffrin, 2008). Shear-enhanced filtrations are currently applied in several newly developed filtration technologies. It includes rotating cylindrical membranes, rotating disk systems and vibrating systems (Jaffrin, 2008). U.S. Pat. Nos. 4,872,988; 4,952,317; 5,014,564; 5,985,160; 6,322,698; 6,596,164 and 6,872,301 apply relatively violent reciprocating and torsional vibrations. It includes the vibration of enclosing vessels, stacked filter leaves or plate frame filters along with associated plumbing, - connecting devices and the contained process fluid. A relatively high construction costs is required to build structures and sealings that can withstand these conditions. High amounts of energy are also often required to generate such motions. Moreover, it requires a high complexity and safety level to operate those techniques. Therefore, those techniques have a low applicability in low-cost processes, such as wastewater treatment process, and are limited to certain niche applications.

The shear enhancement techniques have been applied in lab-scale submerged membrane bioreactor (MBR) systems. Typical examples are the dynamic microfiltration system (D S) (Beier et al. 2006) and the vibrating MBR (VMBR) system (Genkin et al. 2006). Both systems achieve shear enhancement by applying membrane vibration. To vibrate, the membrane module/element is attached to a sliding rod that is connected to an engine. The engine produces axial oscillations to induce the vibration (Beier at al. 2006). The DMS improves the critical flux (CF) of baker's yeast suspensions three times at its maximum frequency and amplitude compared to its static condition. The DMS withstands longer filtration at a very low cross-flow velocity with a minimum fouling when operated below the CF. However, DMS is limited to a maximum frequency of 30Hz and an amplitude of 1.175 mm. The VMBR system doubles the CF when it operates at maximum frequency and amplitude for filtration of an activated sludge model solution. However, this system is limited to a maximum frequency of 10Hz and an amplitude of 4 cm. For both systems, because the membrane is separated from the engine, it requires a relatively high vibration energy and the mechanical energy that is transferred into shear-rate is reduced by friction in each mechanical contact point. Both systems also operate in a single continuous vibration mode without the ability to change the both vibration parameters (frequency and amplitude) during operation.

Another method includes directing the air or other pressurized gaseous bubbles to create a scouring effect near the membrane surface. U.S. Pat. No. 6,287,467 describes the use of the air bubbling technique. The air bubbles provide direct shear to induce secondary flow of liquid and move the membranes (in the case of hollow fiber) (Cui et al. 2003). This approach has several drawbacks: relatively weak shear forces are experienced by the membrane; increasing the bubble flow reaches a "plateau" in terms of flux improvement and it is difficult to achieve an effective bubble distribution (Genkin et al. 2006).

U.S. Pat. 7,422,689 describes the use of an air bubbling technique in addition of membrane cleaning particles. The particles have a typical diameter of 100-2,500 μιη. This technique is limited to the drag force provided from the movement of air bubbles. High bubble velocities require high pumping energy. The maximum linear velocity of the particles limits this technique. The particles have to be moved under a certain limit to prevent scratching of membrane surface, and should be prepared from soft materials. The combination of air bubbles and suspended moving particles is also limited by the difficulty in achieving homogeneous shear on the full membrane surface. The present invention provides a method and filtration system that applies magnetically induced vibration of membranes to provide shear enhancement on the membrane surface. In this system, the vibration parameters (frequency and amplitude) can be adjusted, programmed or automatically controlled to combine minimal energy consumption with an effective cleaning. The number of mechanical connecting points, hence friction and energy losses, is limited. Membrane movement can be even induced contactless through walls, e.g. when needed to keep a feed liquid pressurized. The membrane moves based on electromagnetic attraction and repulsion in a 'push and pull' mode, in a direction parallel to the membrane surface. Also, the method and system of the present invention are particularly suited for low-cost, large-scale filtration processes, such as e.g. in wastewater treatment applications.

SUMMARY OF THE INVENTION

The present invention provides methods for controlling membrane fouling in a membrane filtration process as well as an improved membrane filtration apparatus or system including means for controlling membrane fouling wherein said fouling control includes magnetically inducing a tunable membrane vibration during filtration in a direction parallel to the surface of the membrane element. The kinetic energy in the form of said magnetically induced vibration is used to create and/or enhance the shear-rate at the feed side and increase or enhance back-transport of retained compounds, thus reducing or eliminating the accumulation of these retentate compounds, which causes plugging of the membranes. The method and system according to the present invention works in situ without interrupting the filtration process.

A first object of the present invention thus provides a method for controlling membrane fouling in a membrane filtration process, preferably a large scale membrane filtration process, comprising inducing the vibration of the filter membrane during the filtration process by a tunable variable magnetic flux, wherein the direction of the membrane motion is parallel to the surface of the membrane element. Preferably, the direction of said membrane motion is along the longitudinal or latitudinal axis of the membrane element or membrane. More preferably, the direction of said membrane motion is along the longitudinal or latitudinal axis of the membrane element and perpendicular to the direction of gravity.

Preferably, said tunable variable magnetic flux is generated by a magnetic field generator, such as an electromagnet, according to a vibration signal provided by a vibration driver. Said magnetic field generator is thus operated by the vibration driver who provides a signal to said magnetic field generator to control the vibration parameters such as frequency, amplitude, on/off cycle, and waveform. These vibration parameters can be constant during filtration or may be varied. It is understood that said vibration signal is tunable, meaning that it can be adapted or changed by said vibration driver as a function of time or depending on the conditions or quality (e.g. foulant load) of the feed solution, the desired permeate quality and the membrane flux and characteristics of the particular membrane filter application (e.g. the desired moving pattern of the membrane). Generally, the vibration signal is chosen and adapted to achieve optimum cleaning with minimum energy consumption.

Another preferred embodiment of the present invention provides a method for controlling membrane fouling in a membrane filtration process, comprising inducing by a tunable variable magnetic flux and according to a tunable vibration signal the vibration of the filter membrane parallel to the surface of the membrane element during the filtration process and wherein said vibration signal is automatically controlled and adjusted by a controlling device based on real-time collected information on parameters describing or predictive for the filtration performance.

In another embodiment, said tunable variable magnetic flux acts directly on said membrane element, which comprises a membrane, membrane frame or membrane element that is at least partially made up by a material having magnetic or paramagnetic properties. In another embodiment, said tunable variable magnetic flux acts on a magnetic or paramagnetic vibration rod, which is tightly linked to said membrane element or membrane module.

In yet another embodiment the method for fouling control of the present invention may also comprise other non-magnetic methods of fouling control known in the art, such as by creating a scouring effect near the membrane surface by gas bubbles, shear enhancer particulates or water jets. Said non-magnetic means for fouling control are typically directed parallel to the membrane surface. Said other non-magnetic means for fouling control may act simultaneously or intermittently with the magnetically induced membrane vibration.

Another object of the present invention provides an improved membrane filtration system or apparatus comprising (i) one or more filter membrane elements or modules; (ii) means for supplying the filtration feed solution; (iii) means for collecting the permeate; and (iv) means for magnetically induced fouling control, or stated differently, means for magnetically inducing the tunable vibration of the filter membrane element in a direction parallel to the surface of the membrane element during the filtration process. Particularly, said means for magnetically induced fouling control comprises a magnetic field generator capable of generating a (tunable) variable magnetic flux and wherein said variable magnetic flux acts on a membrane, membrane frame, membrane element or membrane module, which comprises at least partially a magnetic or paramagnetic material, in such a way as to induce membrane vibration in a direction parallel to the membrane surface.

Preferably, said means for magnetically induced fouling control comprises said magnetic field generator and a vibration driver wherein said vibration driver generates a tunable vibration signal to operate said magnetic field generator. Said vibration driver thus controls the variable magnetic flux generated by said magnetic field generator and hence, the vibration parameters, including vibration frequency, vibration amplitude or vibration cycle. Preferably, said magnetic field generator is an electromagnetic device.

In a preferred embodiment, said improved membrane filtration system or apparatus of the present invention further comprises a controlling device for automated control and steering of the vibration driver and/or the magnetic field generator. Said controlling device collects in real-time information on parameters describing or predictive for the actual filtration performance, such as trans-membrane pressure, permeate flow or membrane flux, and uses this information to recalculate and adjust the vibration signal generated by said vibration driver.

In yet another embodiment, said improved membrane filtration system comprising means for fouling control of the present invention comprises at least one electromagnetic field generator capable of generating a variable magnetic flux wherein said variable magnetic flux acts on a magnetic or paramagnetic vibration rod and wherein said vibrating rod is tightly linked to the membrane frame, possible by means of a connector.

Said improved membrane filtration system of the present invention may also comprise other, non-magnetical means for fouling control known in the art, such as a gas (air, pressurized gasses) bubble system, shear enhancer particulates or a water jet system.

In another embodiment of the method or system of the present invention the membrane filtration process is a continuous filtration process. In another preferred embodiment of the method or system of the present invention, the membrane filtration process is a MBR-type filtration process, such as used in waste water treatment. Said membrane filtration system comprises one or more hollow fiber membranes or flat sheet membranes, that are typically present in the form of one or more membrane elements or membrane modules.

DETAILED DESCRIPTION

Legends of the figures

Figure 1 presents a schematic drawing of a MBR-type filtration system having at least one flat sheet membrane element (or array thereof). Figure 2 presents a schematic drawing of (A) flat sheet membrane and (B) a flat sheet membrane subjected to vibration in a longitudinal (horizontal) and (C) a latitudinal (vertical) orientations. The full line represents the central position and the dotted lines represented the end positions.

Figure 3 shows a schematic drawing of (A) a filtration system comprising a magnetic field generator (magnetic vibration engine) and a means for generating a vibration signal (vibration driver) and (B) said filtration system when subjected to vibration. The full line represents the central position and the dotted lines represented the end positions.

Figure 4 shows a schematic drawing of a filtration system comprising a membrane module consisting of a series of flat sheet elements in a parallel (side) view.

Figure 5 presents a schematic drawing of a magnetic vibration filtration system in (A) combination with air bubble diffuser and (B) air bubble diffuser and shear enhancer particulates.

Figure 6 presents a schematic drawing of a magnetic vibration filtration system comprising at least one membrane module having at least one array of hollow fiber membrane (A) with horizontal movement and (B) with vertical movement. It is understood that the air bubble diffuser and shear enhancer particulates are optional.

Figure 7 shows a schematic drawing of a magnetic vibration filtration system comprising an automated control system to adjust the vibration signal in response to actual performance parameters of the filtration system.

Figure 8 presents a schematic diagram of the magnetic vibrating membrane system in lab- scale submerged MBR.

Figure 9 shows the performance of filtration in three different modes. Mode 1 (diamond) filtration in the non-aerated zone of the bioreactor tank; Mode 2 (Square) filtration in the aerated zone of the bioreactor tank; and Mode 3 (triangle) filtration in the non-aerated zone with a magnetic vibrating system.

Figure 10 shows the profile of the filtration resistance of a fouled membrane placed in a magnetic vibrating system.

Figure 1 shows the effect of vibration power on the resistance to fouling.

Figure 12 shows the effect of intermittent (square) or continuous (triangle) vibration on the fouling of the membrane (diamond = no vibration).

Figure 13 shows the effect of vibration-to-idle ratio per cycle for filtration performed under intermittent vibration. Figure 14 shows the effect of operational flux on membrane fouling during magnetically induced vibration.

Figure 15 shows the CFs under different vibrating power. The index of the numbers in the figures:

I- feed pipe 2-reactor fluid/feed solution

3-flat sheet membrane 4-membrane spacer

5-membrane support 6-permeate pipe

7-pressure sensor 8-(vacuum) pump

9-feed stream 10-permeate stream

I I- bioreactor tank 12-frame/membrane frame

13-permeate line 14-Head pipe

15-longitudinal/horizontal axis 16-latitudinal/vertical axis

17- vibration direction

17a,b-direction along the longitudinal/horizontal or latitudinal/vertical axis

8- Magnetic field generator (or vibrating engine)

19- vibrating rod 20-connector/frame extension

20c-upper vibrating connector 20d-lower vibrating connector

21 -means for generating the vibration signal (or vibration driver)

22-electric wire 23-energy meter

24-membrane element 25-inter-element space

26-flat sheet module 27-permeate collector

28-permeate collector line direction 29-pressured gas source

30-gas bubble system 31 -gas bubble line

32-gas diffuser and distributor 33-gas bubbles

34-hollow fiber membrane elements 35-hollow fiber module

36-hollow fiber arrangement 37-module head

37c-upper module head 37d-lower module head 38-shear enhancer particulates 39-control system

40-logic control device 41 -flow rate sensor

Description

The term "membrane" in this application means any material, particularly porous material, acting as a semi-permeable separation layer. The term "flat sheet membrane" is used to describe any membrane in flat form that is mostly attached to a support layer. Typically, the active separation layer is on the outer surface facing the filtered fluid. The term "hollow fiber membrane" may be used to describe any membrane that generally forms hollow elongated tubes. Typically, the active separation layer is at the outer surface of the hollow tubes.

The clogging or fouling materials that may consist of particulates, solids, cakes, colloidal matters and biological films (biofilm) may be referred to as "foulants", and the process of the foulant build-up is referred to as "fouling". Any process for preventing, limiting, inhibiting, removing and/or cleaning the foulant build up on the membrane may be referred to as "fouling control".

The filtration feed is defined as the fluid stream containing liquid together with foulants. This feed is separated into the desired product stream which is the permeate and into the concentrated fluid which is the retentate. The term "fluid" may be used to include liquids, gases or a combination of liquids and gases.

The term "frame" may be used to describe any cassette, cartridge, supporting part, frame assembly or any other structure suitable for holding a membrane or an array of membranes, together with any modifications of said frame for use in a particular filtration system, such as the filtration system of the present invention. Each flat sheet membrane or a bundle of hollow tubes, preferably arranged in a sheetlike manner, may be referred to as "membrane element", and may include the frame of said membrane. The membranes are usually arranged in form of multiple flat sheets or multiple bundles of hollow fibers (the latter preferably arranged in a sheetlike manner, to achieve sufficient surface area. Such an array or cluster of membrane elements may be referred to as "(membrane) module".

The term "vibration" is used synonymously with the term "oscillation".

The inventors found that magnetically induced vibration of membranes, particularly when the vibration is parallel to the surface of the membrane sheets or elements, is an efficient, flexible, tunable and easily scalable method to control fouling in membrane filtration systems. When the movement occurs in the plane of the membrane sheet, bumping of the fluid solution onto the membrane is prevented and the energy associated with the vibration is minimized. Advantageously, specific vibration patterns (signal waveform) or vibration cycles can be applied and the vibration parameters can easily be adjusted or changed by changing the frequency, amplitude, on/off cycle or waveform of the vibration signal. Also, the magnetically induced vibration according to the present invention gives homogeneous movement over the whole membrane surface. Advantageously, such method for fouling control is suited for large scale and/or continuous membrane filtration systems. In addition, such method for fouling control can be fully automated by implementing a control device that collect information on the actual performance of the filtration system (e.g. by measuring changes in permeate flow, trans-membrane pressure, ...) and that uses this information to recalculate and adjust the vibration signal or vibration parameters to improve the anti-fouling action or to minimize the energy consumption. Fouling control by magnetically induced vibration of the present invention, particularly when combined with automated readjustment of the vibration parameters based on the actual filtration performance, generally controls the fouling by preventing, limiting, inhibiting and removing the foulant build-up, thus increasing membrane fluxes, reducing the down time of operation, reducing the cleaning frequency of the membranes and, consequently, enhancing membrane lifetime and overall decreasing costs of the filtration process, which is of particular importance in large scale and/or low cost application, such as waste water treatment. This, together with less energy loss in mechanical friction, also lowers the energy consumption of the present invention.

Without being bound by theory, there are several mechanisms how the present invention leads to a more sustained filtration process:

(i) The movement of the vibrating membrane element creates a maximal shear/friction directly at the liquid-membrane surface. Most of the mechanical energy is transformed into shear-rate, resulting in lifting, detaching or removing foulants from the membrane surface. (ii) In order to change the moving directions (a and b), an acceleration and a deceleration of the membrane element occurs. This results in the release of the momentum from the moving part onto the foulant that leads to fouling control.

(iii) The movement of the vibrating membrane element results in the movement of the fluid surrounding it, creating a turbulent flow of fluid in the vicinity of the membrane.

(iv) Alternating intermittent vibration can result in scraping or scouring to provide the cleaning effect, especially when short-term 'on and off' vibrations are applied.

(v) A homogeneous anti-fouling effect occurs on the full membrane. This ensures that the full effective filtration surface of the membrane is cleaned evenly. Thus, a first object of the present invention provides a method for controlling membrane fouling in a membrane filtration process, comprising inducing the vibration of a filter membrane during the filtration process by a tunable variable magnetic flux, wherein the direction of the membrane motion is parallel to the surface of the membrane element. Preferably, the direction of said membrane motion is along the longitudinal or latitudinal axis of the membrane element.

The direction of the membrane vibration is schematically shown in Figure 2 and 3. Arrows 17 with a direction of a (left or up) and b (right or down) indicate the general direction of vibration, in casu along the longitudinal or latitudinal axis of the membrane or membrane element. As shown in Figure 2B and Figure 2C, the system vibrates respectively along the longitudinal or latitudinal directions 17. Stated differently, the movement orientation of the vibrating membrane element or membrane module faces the narrow face of the membrane element 24 or module 26 as shown in Figure 4. The membrane vibration thus occurs in the plane of the membrane sheet. This a-b movement is repeated after a particular period that creates one vibration. The number of vibrations performed per second is defined as the vibration frequency and the distance of a«→b is defined as the vibration amplitude.

Preferably, said tunable variable magnetic flux is generated by a magnetic field generator, such as an electromagnet, according to a vibration signal provided by a vibration driver. Said vibration signal may, optionally, be amplified by a signal amplifier. Said magnetic field generator is thus operated by the vibration driver who provides a vibration signal to said magnetic field generator to control the vibration parameters such as frequency, amplitude, on/off cycle, time and waveform (moving patterns such as sine, square, triangle, saw tooth or impulse). These vibration parameters can be constant during filtration or may be varied, or occurring in an intermittent or continuous operation. Typical vibration frequencies range from about 1 Hz to about 1000Hz, more in particular range from 20Hz to 200Hz, or from 30Hz to 180Hz, or from 40Hz to 150Hz. Vibration amplitude will typically depend on the particular filtration system and application, but already rather small displacements, such as between 0.5 mm and 5 mm, are suited in the context of the present invention.

Said vibration driver typically comprises computer software such as sonic generator software, to generate a vibration signal. The signal may be magnified in a signal amplifier to obtain sufficient vibrating power to create the vibration of the one or more membrane elements or modules. Optionally, the electric energy that is consumed during the vibration can be measured with an energy meter 23.

It is understood that said vibration signal, in particular its amplitude, frequency or waveform, is tunable, meaning that it can be adapted or changed by said vibration driver as a function of time or depending on the characteristics of a particular membrane filtration process, such as the conditions or quality (e.g. foulant load) of the feed solution, the desired permeate quality and membrane flux, and membrane properties.

Another preferred embodiment of the present invention provides a method for controlling membrane fouling in a membrane filtration process, comprising inducing by a tunable variable magnetic flux and according to a tunable vibration signal the vibration of the filter membrane parallel to the surface of the membrane element during the filtration process and wherein said vibration signal is automatically controlled and adjusted by a controlling device based on real-time collected information on parameters describing or predictive for the filtration performance.

Said controlling device or control system typically includes one or more sensors at the permeate side to monitor and collect information on the actual filtration performance. Suitable sensors include a flow rate sensor 41 (located in permeate outlet 10) to measure the flow rate of the permeate, a pressure sensor 7 to measure the trans-membrane pressure or a sensor assessing permeate quality or purity. The information provided by said one or more sensors is collected by a logic-control device 40. The logic-control device processes the information and computes the appropriate vibration parameters in respond to the actual filtration conditions. These new vibration parameter values are then communicated to the vibrating driver 21, which then re-adjusts the vibration signal to the new-desired values, and the variable magnetic flux generated by the magnetic field generator is changed accordingly. The new vibration parameters are expected to have better fouling control, which can be confirmed by said control system. Indeed, the sensors and the logic control device detect the change on the parameters monitored by the appropriate sensors (e.g. flow rate, TMP, permeate purity) when the new vibration parameters are in effect. When the new vibration parameters achieve a better fouling control, the control system may then steer the vibration driver 21 to change the vibration parameters to reduce energy consumption. It is understood that such automated changes in vibration parameters may be made continuously or at selected time intervals.

Another object of the present invention provides an improved membrane filtration system or apparatus comprising (i) one or more filter membrane elements; (ii) means for supplying the filtration feed solution; (iii) means for collecting the permeate; and (iv) means for magnetically induced fouling control, or stated differently, means for magnetically inducing the tunable vibration of the filter membrane element in a direction parallel to the surface of the membrane element during the filtration process. Particularly, said means for magnetically induced fouling control (also referred to as the vibration system) comprises a magnetic field generator capable of generating a (tunable) variable magnetic flux and a vibration driver wherein said vibration driver generates a tunable vibration signal to operate said magnetic field generator. Said vibration driver thus controls the variable magnetic flux generated by said magnetic field generator and hence, the vibration parameters, including vibration frequency, vibration amplitude or vibration cycle. When more than one magnetic field generator is present, they may have similar or different moving patterns or cycles. For instance, adjacent membrane elements or membrane modules may be operated by different magnetic field generators at cycles which are out of phase with each other (e.g. one membrane element or module moving to a and the adjacent membrane element or module moving to b) so as to maximize the anti-fouling effect.

In a preferred embodiment, said improved membrane filtration system or apparatus of the present invention further comprises a controlling device (as described above) for automated monitoring of the filtration performance and automated control and steering of the vibration driver and/or the magnetic field generator. Said controlling device collects, preferably in realtime, information on parameters describing or predictive for the actual filtration performance and uses this information to recalculate and adjust the vibration signal generated by said vibration driver.

In the context of the present invention, said (tunable) variable magnetic flux may act on a magnetic or paramagnetic membrane element, or part thereof or on a magnetic or paramagnetic vibrating rod 19, that is tightly linked with said membrane element or membrane frame. This way, attraction/repulsion forces in a "push and pull" mode are created on said membrane element or vibrating rod 19. Also, magnetostrictive materials are not required in the present invention.

Thus, in another embodiment, said tunable variable magnetic flux acts directly on said membrane element, which comprises a membrane, membrane frame or membrane element that is at least partially made up by a material having magnetic or paramagnetic properties. Alternatively, said tunable variable magnetic flux acts on a magnetic or paramagnetic vibration rod, which is tightly linked to said membrane element or membrane module, particularly to a membrane frame, optionally by a connector.

The system with fouling control according to the present invention, particularly combined with an automated controlling device, is a fully scalable technology. This feature is important in that it allows the filtration system to be sized to the volume, throughput, and composition of the filtration feed and permeate to be processed by the filtration system. Preferably, this technology is implemented in large scale (at least liter scale) filtrations, such as is the case in membrane filtration processes applied in e.g. wastewater treatment or in the food industry.

In the different embodiments of the present invention said membrane filtration system comprises one or more hollow fiber membranes or flat sheet membranes, that are typically present in the form of one or more membrane elements or membrane modules. It is understood by the person skilled in the art that the dimensions and configurations of a particular filtration system and the different elements used therein, as well as the types and properties of the membrane used in the filtration system are strongly dependent on the particular application. When multiple modules or membrane elements are present, the modules or membranes may be connected to a shared connector 20 or to a separate vibrating connector (not expressly shown). The latter allows different movements for different modules.

It is understood that said method and means for fouling control according to the present invention can be applied not only for a membrane element 24 but also for one or more modules 26, comprising one or more membrane elements. The schematic drawing of a filtration system having a means for fouling control of the present invention and comprising a module (in a parallel (side) view) is shown in Figure 4 and shows a number of flat sheet membrane elements 24 arranged in parallel to become a module 26. The number of membrane elements 24 in one module 26 may vary depending on the particular system and application. Lower and upper extended vibrating connectors 20c and 20d may be used to bundle the membrane elements. The permeate flows into the permeate collector 27 through permeate pipe 6. The permeate collector connects all permeate lines 13 from membrane elements 24 to the main permeate pipe. The permeate pipes transfer all permeates from the membrane module to the permeate stream 10. In the permeate collector 27, the permeate flows according to permeate collector line direction 28.

A flat sheet membrane 3, including the membrane support 5, is typically double-folded and glued to form an envelope. It consists of two surfaces acting as the active separation layer. The active separation layer of the membrane is normally located at the outer surface of the envelope and is mostly coated on a membrane support layer 5. The membrane spacer 4 is usually put in between the two membranes to prevent squeezing of the envelope under the applied vacuum or reduced pressure condition. During the filtration, the feed fluid is sucked into the envelope. To enhance the mechanical strength and to give sealing effect, the edges of membrane are glued and fixed to the frame 12, except for the outlet part 13 that connects the permeate pipe 6 to the inside of the membrane. Each flat sheet membrane or membrane element 3 has a generally rectangular configuration defined in part by the vertical (latitudinal) axis 16 and horizontal (longitudinal) axis 15 (as depicted in Figure 2A).

The schematic drawing of a vibration system that is used for hollow fiber membranes is shown in Figure 6. The vibration can be set into latitudinal (horizontal) (Figure 6A) or longitudinal (vertical) directions (Figure 6B). The filtration system in Figure 6 includes hollow fiber membrane elements 34 potted into a hollow fiber module 35. It is bundled in its upper and lower heads 37c, 37d and attached via its head to the connector 20. The number of hollow fibers attached to the vibrating connector is at least one element or one module. The hollow fiber elements 34 or modules 35 are placed into the connector in the form of a hollow fiber arrangement 36. The permeate is collected via the head pipe 14. The head pipe acts as permeate line and connects the module to the permeate collector 27. The applied vibration on the system with hollow fiber is similar to the one explained for the flat sheet membrane, i.e. according to the longitudinal axis of the membrane or, when arranged in a sheetlike element or module, according to the longitudinal or latitudinal axis of the element or module.

In yet another embodiment of the methods and systems of the present invention, the magnetic induced fouling control of the present invention may be combined with other means for fouling control known in the art, including but not limited to physical interference or physical scouring systems. The physical cleaning systems that can be applied in combination with the magnetic induced fouling control of the present invention include a gas bubble system 30, shear enhancer particulates 38 and/or a water jet system (not expressly shown). These additional systems increase the shear-rate and back-transport resulting in a better anti-fouling effect. Said other means for fouling control can be applied simultaneously with the magnetic vibration or alternating with the magnetic vibration.

A particular embodiment is schematically shown in Figure 5. For some applications, fouling control may thus be further enhanced by using gas bubbles (Figure 5A). The gas bubbles flow in the inter-element spaces to create shear and turbulence. Gas bubble systems include a source of pressured gas 29. The gases are transferred to a gas diffuser and gas distributor 32 through the gas bubble line 31. Generally, the gas distributor is located underneath the vibrating part to maximize the cleaning effect. The pressured gas source 29 provides nitrogen, air or any other suitable gases for the particular application. To obtain the maximum cleaning effect, the optimum module arrangement such as inter-element space 25 and the distributor position/orientation can be adjusted.

In yet another embodiment, the removal of foulant may be further enhanced by using shear enhancer particulates 38. The combination of the present invention with a shear enhancer particulates system is shown in Figure 5B. The application of shear enhancer particulates is done together with gas bubbles or a water jet systems 30. The gas bubbles or water jets 33 provide the drag force to the shear enhancer particulates. Therefore, it flows in the inter- element spaces to create better shear and turbulence.

In yet another embodiment, the fouling control may be further enhanced by using water jets or other suitable pumps to direct fluid flow generally parallel to the exterior portions of the vibrating part. Such fluid flow may be used either intermittently or continuously. In addition, such fluid flow may be used in combination with bubbles 33 together with shear enhancer particulates 38. As a result, the flow of the process fluid may be used to assist the transport of any foulant from the membrane surface.

One particular embodiment of the present invention refers to a method for fouling control of a MBR-type filtration system or a MBR-type filtration system, preferably a submerged MBR- type filtration system, such as used in waste water treatment systems, having a means for fouling control according to the present invention.

A typical MBR-type filtration system comprises at least a single flat sheet membrane and comprises the following elements (as schematically shown in Figure 1):

(i) a feed stream 9 that flows through a feed pipe 1 and a permeate stream 10 that flows through the permeate pipe 6,

(ii) a membrane element, comprising a membrane 3 (on a membrane support 5) and fixed to a frame 12, and an outlet 13) is submerged in a bioreactor tank 11 containing a reactor fluid 2. Multiple membrane elements may be arranged in a membrane module.

Preferably, filtration in a MBR-type filtration system is performed in a dead-end mode, derived from the negative pressure at the permeate side induced by e.g. a vacuum or rotation pump 8. The filtration pressure may be measured with a pressure sensor 7. The driving force of the filtration is the pressure difference between the membrane surface, in principle equal to the pressure in the reactor fluid 2, and the pressure in the permeate lines 6, which is referred to as "trans-membrane pressure (TMP)".

Magnetically induced membrane vibration in the system and method for fouling control of a MBR-type filtration system, preferably submerged MBR-type filtration system, is schematically presented in Figure 3 and may be executed by a means for inducing magnetic vibration (or a magnetic vibration system) as described above. Figure 3 also shows how the whole membrane element vibrates according to the longitudinal or latitudinal axis a and b. Particularly, said means for inducing magnetic vibration or said magnetic vibration system includes a vibration driver 21 , electric wire 22 (to transfer the vibration signal to the magnetic field generator (vibration engine), and vibration engine 18. A particular MBR-type filtration system with fouling control according to the present invention comprise at least one magnetic vibration system, in particular at least one vibrating engine and at least one vibration driver, e.g. depending on the particular application. When more than one vibrating engine is present, it may have similar or different moving patterns, on/off cycles, vibration frequency or vibration amplitude. The vibrating engine 18 is a magnetic field generator, such as an electromagnet, that generates a variable magnetic flux according to the vibration signal that will induce a magnetic vibration of an element comprising magnetic or paramagnetic material subjected to said variable magnetic flux.

In a particular embodiment the membrane frame comprise an element comprising magnetic or paramagnetic material that is subjected to said variable magnetic flux. Alternatively, the membrane frame is tightly linked (optionally via a connector 20) to a (magnetic or paramagnetic) vibrating rod 19. Upon induction of said variable magnetic flux, the vibrating rod 19 transfers the vibration (via the connector 20) to the membrane frame and membrane element. The whole membrane together with the vibrating rod 19 and the connector 20 may be defined together as the vibrating part. Advantageously, the tight connection between vibrating rod 19 and the connector and/or membrane frame results in that the upper and lower parts of the membrane oscillate at similar speed, and, consequently, a homogenous anti-fouling effect occurs on the full membrane and an even cleaning of the full effective filtration surface of the membrane is ensured.

In another embodiment of the present invention, the (variable) electromagnetic field is created around the feed tank, inducing the membrane movement through the tank walls. This way, the presented approach can be introduced easily in pressurized membrane processes as well. It also facilitates retrofitting of the magnetic induction into existing filtration systems, e.g. MBRs treating waste water. The above described vibration parameters and properties are valid for the present embodiment as well.

The following examples are provided to illustrate the invention. The specific details given in each example are not to be construed as limiting the scope of invention.

Experimental setup for the different examples.

The tests were performed in a lab-scale MBR as illustrated in Figure 8. A submerged MBR was fed with a synthetic wastewater. The feed was prepared from the dilution of concentrated molasses. The MBR was operated in fed-batch mode during the test. The concentrated molasses was kept in a fridge (4°C), and fresh feed was fed to the MBR every day. The sludge seed was taken from a pilot-scale MBR treating molasses wastewater.

The membranes were prepared from polysulfone (PSF) (BASF-Ultrason)/N-Methyl-2- pyrrolidone (NMP) (Acros-Organics) by phase inversion from 10 wt% polymer solution. The polymer solutions were cast on a polypropylene support (Novatexx 2471 , kindly supplied by Freudenberg, Germany). This lab-made membrane was used for experiments in Examples 1 , 2 and 3. The membrane used for experiments in examples 4-7 was the commercial Polyvinylidene Fluoride (PVDF _T ) membrane obtained from Toray. The flat sheet membrane was potted by gluing the edge to form a small envelope with both sides separated with a spacer. The element has a filtration area of 0.016 m ² . The bioreactor has an 18.6 liter working volume. The filtration was performed using a multichannel peristaltic pump (205U, Watson Marlow). An aeration system provides fine air bubbles for biological aeration and coarse bubbles for membrane aeration. These aerations also mix the mixed liquor suspended solids. The reactor was operated at a mixed liquor suspended solid (MLSS) concentration of 10.2-1 1 .1 g/l which was maintained by partially withdrawing part of the sludge from the bioreactor. TMP evolution during the filtration tests was monitored from the pressure sensor. The flux was measured from the volumetric flow of the permeate. The permeate flow was calibrated to pump speed before the test so the change of fluxes was performed by changing the pump speeds. The vibration power was calculated by multiplying the current and the voltage that are measured on the electric wire.

The membrane permeability was calculated based on eq.1. The TMP was calculated from the absolute pressures, read from the pressure sensor. The permeate flow was measured periodically to check the imposed flux (J).

L = J / TMP (l/m ² .h.bar) (1 )

The hydraulic resistance (R) was calculated based on Darcy's law (eq. 2). The R _M was obtained from the resistance to clean water filtration. The total resistance (R _T ) was obtained from the final TMP measurement on the activated sludge filtration.

R _T = TMP / ? ^J , R _T = R _m + Rf (1/m) (2)

The Critical Flux (CF) was measured using the stepwise method proposed by Le-Clech et al. (2003). The applied initial flux, step height, and step duration were 2 l/m ² .h, 2 l/m ² .h and 15 min, respectively. The choice of method, step height and duration based on technical simplicity, desired precision and time constraints. The final TMP values were plotted against the permeate flux. Below the CF, a linear relationship exists between the TMP increment and the imposed flux. The CF value was determined to be the flux at which the linear relationship between TMP and flux ceased to exist (Furquharson and Zhou, 2010).

Example 1. Fouling prevention effect

The effect of magnetic vibration on the filtration performance was tested by performing three different filtration conditions: (1 ) filtration in the non-aerated zone on the bioreactor tank, (2) filtration in the aerated zone and (3) filtration in the non-aerated zone with a magnetic vibrating system. The vibration was operated at a frequency of 50 Hz and amplitude of 2 mm. The filtration was performed in a fixed flux of 20 l/m ² .h for 30 min and was evaluated from the filtration resistance. Figure 9 shows the results of the filtrations in three different modes. When the magnetic vibration was applied, no resistance build-up occurs. This is due to the complete absence of fouling on the membrane surface. The shear enhanced by the vibration removes the foulants before they can accumulate. Results confirm that the magnetic vibration has a fouling prevention effect on the membrane in association with present invention.

Example 2. The cleaning effect of magnetic vibration on a prefouledmembrane

After the filtration of Example 1 , the fouled membrane from the filtration mode-1 was used to perform the filtration in the magnetic vibration system. The profile of the resistance during the filtration is shows in Figure 10. It shows that the corresponding resistance due to fouling immediately disappeared: Within 15 min, the magnetic vibration fully restored the filtration performance. This result confirms the ability of present invention to clean the fouled membrane.

Example 3. Effect of magnetic vibration power

To investigate the effect of the magnetic vibration power, the filtration of activated sludge in a submerged MBR system as explained in Example 1 was performed at varying vibration power. The tests were carried out in a power input range of 0 - 20 W. Here, vibrating power means the real power that is used by the presented system.

Figure 11 shows that the vibration power significantly affects the performance. The higher the vibration energy, the better the filtration performance, as represented by the lower fouling resistance (R _f ). The results clearly demonstrate that the vibration system transfers the vibration power to the action for fouling prevention. The critical values of vibration power is between 3,6 and 7,4 W for the presented activated sludge/membrane system. The critical power is defined as the power that produces the back-transport that is equal to the fouling rate. Below this value, fouling occurs and above this value the fouling can be prevented. The critical power obtained in this test does not represent an optimum specific power requirement of the large-scale system. The optimum power consumption can be calculated when a system operates at bigger scale with an optimum membrane loading and vibrating parameters.

Example 4. Effect of intermittent vibration

To conserve the energy consumption, the feasibility of intermittent vibration mode was examined. The experiments were carried out in a sub-critical vibrating power of 8 W and a fixed flux of 22 l/m ² .h for an experimental duration of 300 min. These parameters were chosen to clearly see the trend on the resistance behavior. If the experiments are conducted above the critical power or below the CF, there would be no significant affect observed. The tests were performed in three different modes: filtration without vibration (no-vibration mode), filtration with vibrations (vibration mode), and filtration with intermittent vibrations (intermittent mode). Vibration cycle is defined as the duration of vibration-and-idle in an intermittent mode. The vibration cycle of 30/30, means the vibration cycle consisted of 30 min vibration and 30 min idle phases, giving the cycle time of 60 min. These sets of values were chosen in this experiment.

The effect of intermittent vibration on the filtration resistance is shown in Figure 12. The vibration mode gives the best performance closely followed by the intermittent and much further by the no-vibration mode. A sharp decrease in permeability was found for the no- vibration mode. By applying an intermittent vibration, both antifouling mechanisms, namely preventing and cleaning the membrane, occur. During the idle-phase, the solutes start to attach and build up the foulant layer onto the membrane. However, they are immediately washed out when the vibrating phase starts, facilitating the cleaning effect. Although a rise of resistance is still found for the vibration and intermittent modes, it occurs in a much slower rate. For instance, total rise of resistance of filtration after 300 min of filtration with vibration, intermittent and no-vibration modes are 13, 43 and 84%, respectively. These results confirm that intermittent vibration is an effective way to conserve energy consumption in association with present invention.

Example 5. Effect of intermittent cycle

To further optimize the performance, we performed several tests by varying the intermittent cycles. A cycle time of 60 min with intermittent fraction of 50% means that the cycle consists of 30 min vibration and 30 min idle. In this experiment, the intermittent fraction was set to a constant value of 50%, while the cycle times were varied. The cycle times of 120, 24 and 4 min were applied for this test. The vibration power and the filtration flux were set to 8 W and 22 l/m ² .h, respectively.

The effect of intermittent fraction to the filtration performance is shown in Figure 13. A sharp rise of resistance occurs during the idle phase, but this is partially recovered during the vibration phase. The permeability lost for cycle times of 120, 24 and 4 min are 55, 43 and 43%, respectively. The cycle time of 120 min gives the worst performance followed by 24 and 4 min that give similar final permeability losses. At shorter cycle duration, the idle phase is too short to create an aggressive fouling. This kind of foulant is easy to release during the vibration phase. For all tested cycles, it is clear that the vibration creates a cleaning effect. However, the cleaning effect was unable to recover the permeability totally, so the gradual rise of resistance seems inevitable at longer filtration duration.

Example 6. Effect of operational flux

The filtration at different fluxes was tested in the magnetic vibrating system. Filtration was performed under following parameters: intermittent mode, cycle time of 4 min, vibration power of 8.00 W, intermittent fraction of 50% and filtration duration of 300 min. To ensure the efficacy of the tests, the fluxes were varied from 14 to 30 l/m ² .h.

The effect of applied fluxes is shown in Figure 14. The higher rate of fouling was found at higher fluxes. A sharp rise of resistance occurred at a flux of 30 l/m ² .h and an almost absence of resistance in association to fouling occurred at a flux of 14 and 18 l/m ² .h. Flux selection provides the most significant factor in determining fouling rate. At high flux, reversible and irreversible fouling take places more rapidly. On the other hand, at low flux, a lower rate of adsorption of materials from the mixed liquor supernatant is expected. Applying the vibration allows the filtration to be operated at higher fluxes.

Example 7. Effect of vibration power on critical flux (CF)

The CF is widely used as guideline for the flux at which an acceptable rate of fouling in a given period is observed. The fouling is normally managed by operating the system below the CF. To clearly investigate the effect of vibration power on CF, the CFs were measured under different vibration powers of 0, 5.0, 10.5, 15.4 and 27,2 W. The system was operated in full vibration mode.

The results from Figure 15 show a clear correlation between CF and vibrating power. In general, the higher the vibrating energy, the higher the CF. These results indicate that filtration can be performed at higher fluxes according to applied vibration power. A CF of up to more than 100 l/m ² .h can be attained at vibration power of 27.2 W. The correct CF could not be determined due to pump limitations. This value can be even increased further when a higher vibration power is applied. The result implies that the filtration can be performed at higher flux when the vibration in association with the present invention is applied.

Example 8. Long-term filtration, multiple membranes and energy consumption

The applicability of the present invention to control membrane fouling in somewhat long-term filtration is assessed in this example. The activated sludge filtration was addressed from two different aspects: examining the long-term filtration resistance profile and investigating the effect of multiple membranes in the system that developed based on the present invention (further referred to as a vibrated system), to allow evaluation of the energy consumption.

Long-term filtration

The MBR membrane was purchased from Kubota. In order to better represent the full-scale operation, the filtration was performed with a relaxation time included in the intermittent filtration. The filtration was operated in a 5 min cycle that consisted of 4.5 min of filtration and 0.5 min of relaxation. The experiment was performed in two sequential runs. Initially, 5 membranes were run in parallel. Two membranes were operated in the aerated zone, and 3 membranes were operated with vibrated system in the non aerated zone. The distance between the membranes in the vibrated system was about 5 mm. The flux, vibration power and vibration cycle were set at 16 L/m2 h, 6.4 W and 2.5/2.5, respectively. The applied flux was selected as the flux generally applied for the particular membrane in full-scale applications. Vibration power was set to be low enough to reduce energy consumption, but high enough to provide an acceptable fouling control.

After seven days, this setup was changed: the vibrating membranes 1-3 were situated so near to each other that the liquid between the membranes moved in-phase and almost became stagnant, moving together with the membrane and the effect of vibration on the fouling was (partially) lost. After seven days, membrane 2 was removed and filtration was continued. The remaining membranes were chemically cleaned prior to the filtration re-start. The two vibrating membranes (now with a distance of 10 mm in between) performed better in terms of fouling control than the aerated ones throughout the 15 days of operation. These results not only confirm the efficacy of the vibrated system in a long-term filtration process, but also suggest the importance of adequate design and arrangement of the membranes in one module.

Multiple membrane operation and energy consumption

The reduction of energy consumption associated with fouling control is currently one of the main objectives in membrane filtration, particularly MBR, research. The energy consumption of submerged MBRs is somewhat still higher than that of conventional activated sludge processes (Cornel et al., 2003), and it mainly comes from the energy associated with the coarse bubble aeration for fouling control (Gander et al., 2000). The use of vibrated system based on present invention might offer a promising alternative as a new approach to control fouling in the MBRs. In most shear-enhanced filtration systems, the energy consumption (ED, kWh/m3) is dominated by the energy that is consumed by the vibration engine. Therefore, the energy consumption associated with the vibrated system was monitored during this particular test. However, since the ED is calculated based on the volume of permeate, the scale of the plant becomes very significant, favoring large scale applications. To evaluate the ED of the vibrated system, the filtration with multiple membranes was conducted. The vibrating system was loaded with up to 6 membranes, to check if there were any changes in filtration performance when the number of membranes in the module increased.

Six filtration runs with activated sludge were performed with the vibrated system, with each time a different numbers of membranes attached. One additional filtration with six membranes in one module was also performed without vibration for comparison. The filtration parameters were set to the similar values used in the long-term test. The results (not shown) clearly show that the lab-scale vibrated system could operate with at least six membranes (each has 0.016 m2 effective area). The addition of up to six membranes did not significantly affect the filtration performance. The addition of more membranes to the vibrated system was not feasible in the testing set-up, due to the limited space available inside the lab-scale reactor tank.

The available information on energy consumption of full- or pilot-scale MBRs in scientific literature is scarce. Table 1 contains some related data of selected publications from the last 5 years and furthermore includes ED data from the lab-scale vibrated system. The ED associated with the vibrated system was calculated by using similar operational parameters to the ones used in the long-term test.

Table 8.1. Energy demand of lab-scale MBR and comparison with literature data

(a) Calculated from vibration power with one vibrating membrane in the reactor;

(b) Calculated from vibration power with six vibrating modules in the reactor;

(c) Fenu et al., 2010) (d) Fatone et al., 2007

(e) Verrecht et al., 2010 (g) Gil et al, 2010

(f) theoretical set-up for cost sensitivity analysis Table 1 also confirms that the ED associated with coarse bubble aeration is strongly affected by plant scale. The ED of the pilot-scale MBR (Fenu et al., 2010) is almost ten times the one of the full-scale ones. This should be considered when analyzing the lab-scale of present invention system data. Calculating the ED using six membranes (2.03 kWh/m3) gives a 6 times smaller value than with one membrane (12.12 kWh/m3). Nevertheless, this value is much lower than the ED of a pilot-scale MBR that operates with a ±160 time larger membrane area, suggesting a rather economic design of the system, despite being far from optimized yet. It is worth noting that the ED of the lab-scale vibrated system is about 3.5 times higher than the ED of the best performing full-scale MBR listed in Table 1. However, direct comparison of these data is not entirely reliable, since the ED of a lab-scale set-up is not of economical scale and the feed and sludge characteristics of an MBR have a serious influence on the filtration performance. From these comparisons, it can thus be expected that an optimised system (Frequency, amplitude, vibration cycle, ....) developed from present invention may lead to a significant cost reduction for fouling control in MBRs. With even membrane or membrane modules moving in one direction and odd membrane or membrane modules in the other, more compact, but still efficient modules could possibly be realized.

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