GB2315428A

GB2315428A - Membrane Filter

Info

Publication number: GB2315428A
Application number: GB9709663A
Authority: GB
Inventors: Richard John Wakeman; Michael Bernard Coupla Quigley
Original assignee: ATKINS FULFORD Ltd
Current assignee: ATKINS FULFORD Ltd
Priority date: 1996-07-18
Filing date: 1997-05-13
Publication date: 1998-02-04
Also published as: GB9709663D0; GB9615073D0; GB2315426A

Abstract

A membrane filter for suspensions etc., which membrane 15 can be a flat sheet or tubular (Fig 5), has a stabilised, low velocity flow with flow straighteners (1, Fig 1) in a retentate chamber 3 and uses ultrasonics 19 and/or an electric field formed between electrodes 16, 17 to improve separation of the solids from the solution and migration of contaminants from the permeable membrane 15. Preferably one electrode is in the retentate chamber 3 and the other(s) behind the membrane in the permeate chamber 7. The electric field can be AC, DC, AC with a DC bias or pulsed. As an alternative, electrolysis gas products can be separated from the retentate or permeate by using separate anolyte and catholyte chambers (23, Fig 4) containing electrolyte fluid and ion permeable but liquid impermeable membranes. The filter can be a single or multiple cell, in parallel or series. Preferably the cross-flow rate is less than or equal to 1 metre per second. The solutions to be filtered can be municipal, industrial, sewage or biological broths, sludges or effluents.

Description

2315428 MEMBRANE FILTER WITH ELECTRICAL AND/OR ACOUSTIC ENHANCEMENT

We Atkins Fulford Limited a British Body Corporate of Edgworth Road, Sudbury, Suffolk C010 6TG, do hereby declare the invention for which we pray that a patent be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement.

FIELD OF THE INVENTION

This invention relates to tubular and flat sheet membrane filters, more particularly to those filters whose ability to separate particulate or molecular or ionic contaminants from slurries and suspensions is enhanced by the addition of one or more electric fields and/or ultrasonic fields to the filter. Such slurries and suspensions occur in, for example, water and sewage treatment, industrial and municipal waste treatment, ore processing, biological broths, food and chemicals manufacture and in the production of ceramics. The invention is the design of an electrically and/or ultrasonically augmented, tubular or flat sheet, membrane filter that utilises methods which provide a means to obtain a higher production rate of processed liquid with a higher quality than is possible with existing designs of membrane filters.

BACKGROUND OF THE INVENTION

Membrane filters are devices designed to remove particulate or dissolved materials from liquids and have been available for a number of years. A common arrangement for these is to form the membrane, which may be a polymer film or a ceramic or a sintered metal or alloy, into tubes or flat sheets or log modules having one or a multiplicity of channels. The module is incorporated into a liquid circulation system such that the liquid to be filtered passes from a pump via a pipe to the membrane where it is flowed tangentially to the surface of the membrane.

When a membrane filter is in use, contaminants from the feed liquid accumulate at the membrane surface to form a polarised layer, which is often referred to as a gel or a cake, depending on its origins. These fouling layers inhibit liquid flow through the membrane. It is common to use high velocities of the feed and turlulent flows over the membrane surface to induce a shear force at the surface of the contaminant layer to limit the growth of its thickness.

I Tlese crossflow velocities are commonly in the range of 1 to 8 ms-', leading feed/retentate flows in the inertial or turbulent flow regimes. At lower values of the crossflow velocity the thickness of the deposit tends to be greater, causing a lower rate of production of cleaned liquid product (the permeate) from the filter. Furthermore, to obtain economically viable permeate rates a pressure of up to about 40OkPa is used in microffiters and of up to about 100OkPa in ultrafilters.

The effectiveness of the membranelmodule assembly as a filter is frequently assessed by one or more of three factors. These are, firstly, the ability to remove contaminant from the liquid, secondly, the ability to produce permeate at an acceptably high rate and, thirdly, the consumption of energy in achieving either or both of the previous objectives. It is clear that the first two factors need to be as high as possible whilst the third needs to be as low as possible. The liquid production rate and energy consumption rate can be conveniently combined and expressed as the mass of liquid produced per unit of energy consumed. This factor should maximised for any separation process. The ability to remove contaminant from the liquid is commonly measured by the rejection which may be expressed as a fraction or as a percent, and is defined by Re jection = 1 - contaminant concentration in the permeate contaminant concentration in the feed to the filter Contaminants in a liquid, particularly in aqueous solutions, become charged and when subjected to an electrical field they will migrate toward one or another of the electrodes. llie magnitude and sign of the charge will depend on the type of suspension or slurry. If the electodes are located either side of a permeable membrane or barrier, which itself may be an electrical conductor or non-conductor, and the proper electrode polarity is selected, the contaminant will be caused to migrate by electrophoresis away from the membrane or barrier. Under appropriate conditions the contaminant accumulates on and around the electrode whose polarity is opposite to that of the charge on the contaminant. For example, a contaminant that acquires a negative charge will require that the positive electrode be placed in a position upstream of the membrane or barrier and the negative electrode be placed downstream. The contaminant will then collect on and around the upstream electrode.

2 Electric fields cause phenomena to occur other than the migration described above. These fields can agglomerate particles by neutralizing charges, or cause electroosmosis. Electroosmosis is the term used to describe the movement of the liquid phase through a stationary, charged, porous body when the motion is caused by an electrical field applied across the body. This phenomenon has been used as a technique to dewater slurries. Illustrative of the use of an electrical field in the dewatering of coal washery slimes is the article by N.C.Lockhart and R.E. Strickland, 1984, Dewatering of coal washery tailings ponds by electroosmosis, Powder Technology, Vol.40, pp.215-221. The deposit thickness formed in a membrane filter is so thin that any effects from the electroosmotic phenomenon are negligible.

It is further possible to separate the fine particle suspension solids from liquids by the use of a combination of DC electric and ultrasonic fields. Illustrative of this is the patent by H.Muralidhara, B.Parekh and N.Senapati, 1985, Solid-liquid separation process for fine particle suspensions by an electric and ultrasonic field, US Patent 4 561 953, in which is described a method of dewatering an aqueous suspension by concurrently subjecting it to ultrasonic (or sonic) and DC electric fields. The ultrasonic field is then applied to the suspension at a frequency and amplitude adapted to cause water to separate from the suspension particles. The electric field causes the particles to migrate towards a permeable electrode.

It is an object of this invention to describe a device that incorporates a combination of a controlled hydrodynamic field, DC, AC or pulsed electric fields and an ultrasonic field. It is proposed to use these in order to alleviate the drawbacks of operating membrane filters that are caused by the formation of fouling layers that are described above.

If the AC field has a DC bias then the solids which will effectively have been shaken free from the membrane will migrate towards the electrode as described earlier. The frequency of the AC field will be determined by experiments for particular combinations of solids and liquids in order to produce the maximum effect. One specific embodiment would be to use similar frequencies for the electric and ultrasonic fields in order to obtain some synergy or even resonance effects. A further embodiment would be to use pulsed electric fields instead of alternating fields. The pulses will impose large electric fields on the particles close to the membrane thus giving them significant acceleration away from the membrane.

3 Use of electrophoretic migration effects increases the production capacity from a given size of membrane filter, and enables greater unit mass of product to be produced from a unit input of energy. The inventor has discovered that a concurrent use of an electrical field (electrophoresis) or simultaneously applied electrical and ultrasonic fields and a flow stabilised, low velocity, laminar hydrodynamic field enables much higher permeate fluxes to be obtained than is possible using a high velocity, hydrodynamic field alone. Furthermore, rejections of contaminant are unexpectedly improved over when the hydrodynamic field alone is used.

DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a means of simultaneously increasing the permeate flux and the rejection. This technique of filtration may be used to replace or to upgrade existing membrane filters. The filter utilises the concurrent application of a low velocity, flow stabilised, hydrodynamic field andlor an electrical field andlor an ultrasonic field. 'Die electrical field may be an alternating, direct or pulsed current, and either may be applied at a continuous potential or with a time varying potential. The energy required for an incremental amount of liquid to be removed from the slurry or suspension is less with the combination of an electrical field (or electrical and acoustic fields) with a flow stabilised, low velocity hydrodynamic field than for higher velocity fields alone. Flow stabilisers are used to ensure that most fluid streamlines pass from one end of the filter to the other without deviating. Thi sis aided by the use of low velocities that cause flow to be in the laminar flow regime on the feedside of the membrane.

The flat sheet membrane version of the filter with assistance from electrical andlor ultrasonic forces is composed of an assembly of retentate plates and permeate plates. When the filter is assisted by electrical force fields it is referred to as an electroffiter, when it is assisted by ultrasonic forces it is referred to as an acoustic filter, and when it is assisted by both electrical and ultrasonic forces it is referred to as an electroacoustic filter. The design of an electrofilter is similar to that of an electroacoustic filter except that the ultrasonic source is omitted, therefore possible designs of the electroacoustic filters will be described in the following. Similarly, the design of an acoustic filter is like that of an electroacoustic filter but with means for providing the electrical field omitted.

4 One possible design of a retentate plate is shown in Figure 1, and one possible design of a permeate plate is shown in Figure 2. Referring to Figure 1, the retentate flow in the retentate chamber 3 is controlled by a series of flow straighteners I and there is provision to remove permeate through a permeate channel 2. Recesses 4 in the plate support the electrode. The permeate plate in Figure 2 shows provision for flow of the feed suspension 5 and for removal of retentate from the filter 8. Permeate is removed from the permeate chamber 7 in a direction normal to the feed and retentate flows, hence providing the shortest flow route to the permeate channel 2 which conducts the permeate towards the filter outlet porting. A membrane 6 is stretched across the surface of the permeate plate. A filter is formed by arranging these plates in a defined sequence.

In Figure 3, one possible assembly of the retentate and permeate plates is shown to form an electroacoustic filter. The filter is made up from a sequence of permeate plates 10 and retentate plates 11, enclosed by a feed plate 13 and an end plate 14.

This sequencing of plates may be repeated many times in a large scale filter to provide a large area for separation. The retentate and permeate plates may be assembled in a parallel arrangement to facilitate treatment of higher feed suspension flow rates, or the retentate and permeate plates may be assembled in a series arrangement that can be used to enable greater liquid removal from the feed suspension or so that a higher concentrating factor may be obtained. The plates may also be assembled in a series-parallel arrangement to give greater liquid removal from high feed flow volumes. Fluid flow control plates are used to control the directions of fluid flow at various points in the filter.

Figure 3 is drawn utilising the sections through A-A of Figures I and 2. Feed suspension enters through a feed port 12 and is directed into the retentate chamber 3 of the retentate plate 11. An ultrasonic transmitting source 19 is located in the feed port chamber. One wall of the retentate chamber 3 is formed by an electrode 17, which may be either porous or nonporous, and the other wall is formed by a porous filtration membrane 15. A second electrode 16 which is porous is located on the permeate chamber side of the membrane. The electrodes are constructed from an electrically conductive solid, typically carbon, stainless steel or noble metals such as gold, platinum or silver. Alternatively the electrodes may be constructed from any other material which may be electrically conductive or nonconductive but which are coated by an electrically conductive material. T'he porous filtration membrane is typically made from polymers, ceramics or sintered materials and is usually an electrical nonconductor. If as well as being liquid permeable the membrane is an electrical conductor, a separate electrode on the permeate side of the membrane is not required. Such membranes may be made from a sintered metal or a porous carbon.

Within the retentate chamber 3 lateral mixing of the suspension is prevented by flow straighteners I which serve also to stabilise the flow. Suspension mean velocities tangential to the membrane surface of less than lms-1 are used in this invention, preferably in the range from 0. 000ims-1 to 0.5ms-1. Whilst passing through the retentate chamber, the suspension is concurrently subjected to an electrical potential difference imposed across the two electrodes 16 and 17. Liquid that has been filtered by the membrane passes into the permeate chamber 7, from where it is drained away in a direction at right angles to the feed flow through appropriate permeate channels 2. The retentate, or concentrate as it is sometimes known, is led away from the filter through the retenate channel 8 to the retentate exit port 18 in the feed plate 13.

The electrical potential differtrice may be supplied as a DC or as an AC voltage or as a pulsed voltage, preferably between I volt and 400 volts, to cause migration of charged particles in the liquid away from the liquid permeable membrane and electrode and toward the electrode located in the retentate chamber. The applied DC voltage or root mean square AC voltage or pulsed voltage may be at a constant level or it may be allowed to vary in a controlled way. For example, the voltage may follow sinusoidal or pulsed wave forms, or it may be switched periodically between two voltage levels, or it may be ramped up and down at the same or at different rates. The small crossflow velocity in the retentate chamber prevents accumulation of suspended particles in the chamber and around the retentate-side electrode. 'llie applied voltage may be any voltage that will cause a sufficient current to flow so as to cause a migration of the charged particles or molecules so as to increase the rate of filtration and the rejection over when the hydrodynamic field is used separately. These electric fields can also be applied in conjunction with the ultrasonic and/or hydrodynamic fields. One particular form may be to have the frequencies of the electric and ultrasonic fields identical or as harmonics to each other in order to induce a resonance type effect.

An alternative form of the electrofilter is shown in Figure 4. It is appropriate to use this form when electrolysis gases evolved at the electrodes have a detrimental effect on either the liquid 6 or the particles or molecules being processed in the filter. This electrofilter also makes concurrent use of a low velocity, flow stabilised, laminar hydrodynamic field and electrical and/or ultrasonic fields. Feed suspension enters through a feed port 12 and is directed to the retentate chamber 3 of the retentate plate 20. The feed passes tangentially across the membrane surfaces and into retentate channels from which it passes out of the filter cell at the exit port 18. One wall of the retentate chamber 3 comprises an ion permeable membrane 22, typically made from cellulose or cellulose derivatives, which allows passage of the electric current but not flow of the liquid. The other wall of the retentate chamber comprises a liquid permeable membrane 17, which acts as the barrier between the feed suspension in the retentate chamber and the cleaned liquid (permeate) in the permeate chamber 7. The liquid from the suspension feed passes through the liquid permeable membrane and into the permeate chamber 3 in the permeate plate 10. One wall of the permeate chamber 7 is formed by the liquid permeable membrane 17 and the other is formed by an ion permeable membrane 22. Tlie ion permeable membrane separates an electrolyte solution, known as either the anolyte or catholyte depending on which electrode (the anode or the cathode) the solution is contacting, in the electrolyte chamber 23 from the retentate chamber 3 or the permeate chamber 7. The electrolyte solution flows into the filter through a feed port 24 from where it flows tangentially across and between the ion permeable membrane 22 and electrode 16 surfaces, and flushes any electrolysis gases formed by the electrode chemical reactions out of the filter through exit ports 25. The electrolyte solutions also carry electric current from the electrodes to the liquid in the permeate chamber and to the suspension in the retentate chamber.

The configuration of the electrofilter or electroacoustic filter is not limited to a flat sheet format. Alternative forms are for the membrane surface to be tubular, when it may form an electrically conducting tube or it may be deposited on the inner or outer surface of a support tube, or it may be deposited on the surface or surfaces of cylindrical channel or channels passing through a porous body.

As an example, one possible design of a tubular electroacoustic filter is shown in Figure 5. In this example feed suspension enters the filter through entry port 35, it is guided by flow straighteners 1, and thence the feed flows into the retentate chamber 32 inside the filter tube 29. The inner surface of the filter tube forms the membrane 30. The retentate or concentrate passes along the tube and out of the filter at exit port 26. 'Me feed is subjected to an electrical 7 force field applied across the inner electrode 33 and the outer electrode 31. Ilese electrodes may be porous or non-porous. The electrodes are shown to be concentric with the membrane, but electrodes eccentric with the membrane can be used as an alternative. The retentate is also subjected to an ultrasonic force applied through the transmitter 34. The permeate flows through the membrane 30 and filter tube 29 into the permeate chamber 28 from where it is discharged through exit port 27. Additionally, partial shrouding of the electrodes can be used to reduce the current drawn from the electrical supply without affecting the performance of the filter.

A single filter tube is shown in Figure 5. The techniques can be similarly applied to multiple tubes arranged in a tubesheet or to channels in a log module.

The dewatering rate of the suspension through the liquid permeable membrane and/or electrode may be increased by reducing the pressure in the permeate chamber, by increasing the pressure in the retentate chamber, or by a combination of these. The equipment and process of the invention is operable without these additional means of augmentation, so long as a small pressure difference exists between the retentate and permeate chambers. If pressure augmentation is used the means for reducing or increasing the pressure are those conventionally used for reducing or increasing pressure.

Liquid removal from the suspension may be further augmented by addition of small quantities of surfactants such as polyacrylamide or other additive chemicals. The uses these of surfactants or other chemical pretreatments of the feed suspension are tested using conventional methods, and the surfactants or other chemicals are to be added prior to feeding the suspension to the electrofilter.

The following examples serve to illustrate and not limit the present invention.

Examule 1 This comparative example illustrates the increased filtration rate and the reduced power consumption (based on the volume reduction rate of permeate) that is obtainable using the invention. The data were obtained using a filter cell with hydrodynamic flow stabilisation and 8 low velocities, with and without the use of constant DC electric fields, by filtering a 0.3% by volume pigment suspension under the conditions shown in Table 1. The rejection was 100% in each experiment.

TABLE 1

Filtration pressure Crossfiow velocity Constant voltage Permeate flux NPF (kPa) (ms-') (V) (M3m-2h-1) 69 1.0 0 0.042 1.0 69 1.0 50 2.9 0.034 207 1.0 0 0.17 1.0 207 1.0 so 3.29 0.073 241 0.8 0 0.064 1.0 241 0.8 50 3.18 0.028 In Table 1 the Normalised Power Factor (NPF) is defined by PF = Power consumed by the entire filter flow circuit kWh m-3 Steady state volumetric flow rate of permeate NP17 = PF when electrical force field is used PF without electrical force field

When no added electrical field is used, the only power consumption is by the feed delivery pump and NPF=1. When NPF<l, there is an overall productivity gain. For example, comparing the first two rows of data in Table 1, adding the electric field to the filter increased the permeate flux by 69 times, and showed a 29 times reduction in the total power consumed by the filter system.

ExamMe This comparative example indicates how much the filtration flux is increased by using a low velocity, flow stabilised, feed in an electrofilter when the DC electric field is pulsed on and off every 60 seconds and when the field is on and constant. The data in Table 2 were collected during filtration of a 0.33% by volume titanium dioxide suspension at pH 3.90 using a

9 transmembrane pressure of 241kPa and a crossflow velocity of 0.8ms-1. T'he rejection of titanium dioxide was 100% in each experiment.

TABLE2

Potential difference (volts) and type Filtration flux (M3 m-2h-1) . 0 no electric field applied 0.26 (D.C.) - pulsed every 60 seconds 1.5 (D.C.) - constant voltage 2.5 ------------- - - -------- - --- ---------------------------- Examide The following comparative example illustrates the use of a flow stabilised, low velocity, feed concurrently with a DC electric field to filter proteinaceous suspensions. The data illustrate how both permeate flux and rejection are increased when these two force fields are used concurrently.

TABLE3

Feed solute Concentration Voltage Permeate flux Rejection (gi-1) (V) (M3M-2 h") (%) ....... 1................................................................. ...........................

Denatured insoluble lactalbumin 15 0 0.12 100 is 100 2.50 100 Ovalbumin 0.1 0 0.15 44.3 0.1 75 2.40 88.7 Bovine serum albumin 1.0 0 0.10 1.5 1.0 100 2.90 98.9 ---- ---------- ------- ExamMe 4 The following comparative example illustrates the combination of a flow stabilised hydrodynamic field together with a DC electric field to filter aqueous dioctadecyldimethylarnmonium chloride surfactant dispersions at WC. The data in Table 4 illustrate how the flux and rejection both increase with applied voltage.

TABLE 4

Feed Concentration Voltage Permeate flux Rejection (gr) (V) (M3r,2S-1) (%) . 0.1 0 2.1 90 0.1 20 2.6 92 0.1 50 4.9 95 0.1 80 10.2 96 0 0.5 99 20 1.3 99.9 50 3.6 99.97 80 5.4 99.98 Example 5

This illustrative example shows the use of an AC field to increase the filtration rate of a sodium lauryl sulphate dispersion at a feed concentration of 2.5 g/litre using a 100,000 molecular weight cut-off membrane filter. The data shown in Table 5 indicate an increasing flux when a voltage is applied, dependent on the frequency of the electrical supply.

TABLE5 r.m.s. voltage Frequency Permeate flux Rejection (V) (Hz) (M3M-2S-1) (%) 0 0 0.24 48 20 0.24 50 500 0.28 50 500 0.25 so 1500 0.31 55

Claims

What is claimed is:

1. A device for filtering particles or colloids in a well dispersed state or in a floccul;ated or otherwise aggregated state from a suspension or dispersion, or for filtering molecules or macromolecules in any state of aggregation from a dispersion or solution, comprising:

(a) a means for flowing the suspension into the filtering zone in the retentate chamber; (b) a means for providing a stabilised, laminar, hydrodynamic flow field using flow straighteners inside the retentate chamber of the filter cell to give controlled flow conditions across the membrane separator surfae; (c) a means for subjecting the suspension to either DC, AC or pulsed electric fields concurrently with (b) to cause migration of particles in a direction away from the membrane surface; (d) a means of pulsing the DC signal and for varying the pulse height, duration and repitition frequency of the electrical signal for production of the electric fields; (e) a means for varying the frequency and amplitude of the AC signal and for giving it a DC bias; (f) a means for subjecting the suspension to an ultrasonic field close to the membrane surface concurrently with (b) and/or (c) to limit foulant accumulation at the membrane surface; (g) a means for varying the amplitude and frequency of the ultrasonic signal; (h) means for removing filtered liquid and concentrated retentate from the filter continuously and separately.

2. An adaptation of the filter of claim 1 that has a means of preventing electrolysis gas products from mixing with either the feed suspension or the permeate liquid, comprising:

(a) anolyte and catholyte chambers separated from the retentate and permeate chambers by ion permeable but liquid impermeable membranes; and (b) electrodes located in the anolyte and catholyte chambers and contacting electrolyte solutions that transmit electrical current to the retentate and permeate chambers.

3. A method of filtering whereby a suspension is concurrently subjected to a stabilised, low velocity, flow field and to a DC or an AC electric field.

4. A method of filtering whereby a suspension is concurrently subjected to a stabilised, low velocity, flow field and to an ultrasonic field.

12 I

5. A method of filtering whereby a suspension is concurrently subjected to a stabilised, low velocity, flow field, to DC, or AC or pulsed electric fields and to an ultrasonic field.

6. Use of the filters in claims I and 2 either singly or in multiple cells to form a parallel filter arrangement or a series filter arrangement or a series-parallel filter arrangement or any combination of these.

7. The filters in claims I and 2 and methods in claims 3, 4 and 5 employing an amount of energy for separating a unit of liquid from the suspension, which amount is less than would be required by use of an hydrodynamic field alone to separate the unit of liquid.

8. The filters in claims I and 2 and methods in claims 3, 4 and 5 wherein the mean crossflow velocity in the retentate chamber is between 0.0001 ms1 and I ms'.

9. The filters in claims I and 2 and methods in claims 3, 4 and 5 wherein the DC electrical potential difference or the root mean square A.C. electrical potential difference is between 400 volts and +400 volts.

10. The filters in claims 1 and 2 including adding a surface modifier to the suspension before or as the suspension is flowing into the retentate chamber.

10. The filters in claims I and 2 and methods in claims 3, 4 and 5 wherein the electrical potential is varied in a controlled way to give a potential that varies with the time of application of the potential.

11. The filters in claims I and 2 and methods in claims 3, 4 and 5 wherein the ultrasonic frequency is between 0.5kHz and 5MHz.

12. The filters in claims I and 2 and methods in claims 3, 4 and 5 wherein the pressure in the retentate chamber is raised to above 10 1 kPa absolute or the pressure in the permeate chamber is lowered to below 10 1 kPa absolute.

13. The filters in claims I and 2 and methods in claims 3, 4 and 5 including adding a surface modifier to the suspension before or as the suspension is flowing into the retentate chamber.

14. The methods of claims 3, 4 and 5 employing municipal, industrial or sewage sludges and effluents as the suspension.

15. The methods of claims 3, 4 and 5 employing biological broths or sludges as the suspension.

13 I L4- - Amendments to the claims have been filed as follows 1. A device for filtering particles or colloids in a well dispersed state or in a flocculated or otherwise aggregated state from a suspension or dispersion, or for filtering molecules or macromolecules in any state of aggregation from a dispersion or solution, comprising:

(a) a means for flowing the suspension into the filtering zone in the retentate chamber; (b) a means for providing a stabilised, laminar, hydrodynamic flow field using flow straighteners inside the retentate chamber of the filter cell to give controlled flow conditions across the membrane separator surface; (c) a means for subjecting the suspension to either DC, AC or pulsed electric fields concurrently with (b) to cause migration of particles in a direction away from the membrane surface; (d) a means of pulsing the DC signal and for varying the pulse height, duration and repitition. frequency of the electrical signal for production of the electric fields; (e) a means for varying the frequency and amplitude of the AC signal and for giving it a DC bias; (f) a means for subjecting the suspension to an ultrasonic field close to the membrane surface concurrently with (b) and/or (c) and/or (d) and/or (e) to limit foulant accumulation at the membrane surface; (g) a means for varying the amplitude and frequency of the ultrasonic signal; (h) means for removing filtered liquid and concentrated retentate from the filter continuously and separately.

3. Use of the filters in claims I and 2 either singly or in multiple cells to form a parallel filter arrangement or a series filter arrangement or a series-parallel filter arrangement or any combination of these.

4. The filters in claims 1 and 2 employing an amount of energy for separating a unit of liquid from the suspension, which amount is less than would be required by use of an hydrodynamic field alone to separate the unit of liquid.

5. The filters in claims 1 and 2 wherein the mean crossflow velocity in the retentate chamber is between 0.0001 ms-I and 1 ms-1.

6. The filters in claims 1 and 2 wherein the DC electrical potential difference or the root mean square A.C. electrical potential difference is between -400 volts and +400 volts.

7. The filters in claims I and 2 wherein the electrical potential is varied in a controlled way to give a potential that varies with the time of application of the potential.

8. The filters in claims I and 2 wherein the ultrasonic frequency is between 0.5kHz and 5MHz.

9. The filters in claims 1 and 2 wherein the pressure in the retentate chamber is raised to above l0lkPa absolute or the pressure in the permeate chamber is lowered to below 101kPa absolute.