A FILTER, APPARATUS INCLUDING THE FILTER AND A METHOD OF USE OF THE APPARATUS

Abstract: A filter (10) is provided comprising a first filter layer (12) made of TiO2 or Ti4O7 particles deposited on a second filter layer (14) by stainless steel particles. The first layer (12) has a smaller pore size than the second layer (14). The construction of the filter (10), the relative thicknesses of the layers (12, 14) and the resistance thereof to movement therethrough of electrolytically generated gas located between the two layers (12, 14) when the filter (10) is permeated with liquid being such that such gas reaches the upstream side of the filter before the downstream side. Thus in one embodiment stainless steel particles are sintered and rolled with a textured roller before ceramic particles are applied and rolled. An intermediate layer of metal particle peaks and ceramic filled troughs is thus provided which has high tortuosity. The resistance to flow at the metal/ceramic interface can be increased by electrodeposition of metal there.

Description: A FILTER, AFPARATUS INCLUDING THE FILTER AND A METHOD OF USE OF THE APPARATUS The invention relates to a filter, apparatus including the filter and a method of use of the apparatus. Filters are known for filtration at different particle size levels. Filtration at a very fine particle size level is required for example in the treatment of beer for clarification, in the treatment of chemical effluent and in particular in the treatment of water to remove, for example, bacteria. Many conventional membrane filtration processes for fine particle size filtration of liquids rely on high velocity cross flow to induce turbulence in the feed and reduce the thickness of the laminar flow boundary layer along the membrane surface. In this way, the rate at which the "filter cake" builds up is reduced, and the transmembrane flux may be kept higher than would otherwise be the case. Nevertheless, the typical transmembrane flux to be expected for any form of membrane when used for real feeds is ~100 l/hr/m'/Bar, compared with pure water flux of a typical polymer or organic icrofiltration membrane in the range ~1000-10000 1/hr/πr/Bar. Therefore, not only are ail types of microfi ltration membranes used at < 10% of their theoretical efficiency, a lot of power input is wasted in doing it. The direct membrane cleaning process developed by AEA Technology, Harwell, and described in GB 2160545 relies on the generation of hydrogen bubbles at the surface of an electrically-conducting, cathodically- charged, porous membrane to break up the filter cake. In this way, it is possible to maintain transmembrane fluxes approaching the value to be expected for pure water. Inorganic membranes typically cost at least five times as much as organic membranes. If the transmembrane flux for an inorganic membrane can be increased significantly by use of this process, not only does it become more cost effective, because the filtration area will be much smaller, but the running costs may also be less because high cross flow veloci ies may not be required. The process of GB 2160545 uses sintered metal membranes. These have pore sizes in the range of 1 micrometre, so c_o not reject bacteria and such like micro-organisms and cannot be used for many critical processes of commercial interest. Carbon membranes can be obtained with a wide range of pore sizes. However, as most are in the form of tubes, they would need either a coaxial wire anode, or an external tube anode to be used in the electrolytic process. These membranes tend to be flexible and sσ would have to be short to maintain the correct anode-cathode distance. Also, it would be much more difficult to develop good enough electrical contact. These factors would increase considerably the engineering cost of such a filter module. One known membrane which is made by the Cltram Company comprises a flat, sintered-metai substrate with a pore size of approximately one micrometer and, with a smaller pore size, ceramic, final filtration layer. The latter is usually kept as thin as possible because the materials from which it is made are expensive. Nevertheless, the small pore sizes are required for the membrane to reject bacteria. It is known to clean this filter by the use of back pressure. According to one aspect of the invention there is provided a filter comprising a first, less electrically conductive filter layer and a seconα, more electrically conductive filter layer downstream thereof, the first layer having a smaller pore size than the second layer, the construction of the filter, the relative thicknesses of the layers and the resistances thereof to movement therethrough of electrolyticaliy generated gas located between the two layers when the filter is permeated wi h liquid being such that such gas reaches the upstream side of the filter before the downstream side. In this way, filters of the general construction of the known Ultram membranes described above can be electrolytically cleaned, as at least some gas electrolytical ly generated at the surface of the conductive layer will pass through the non-conductive layer to the upstream surface of the filter to clean away the filter cake. The pore size of the less electrically conductive .ayβr may u. no more tnan ι . s, or no more ___an __.._um, preferably no more than Iμm, most preferably no more than O.Su . Preferably, the less conductive layer has a pore size su∑riciεnt to re ect bacteria. The filter may be constructed with a further layer such that gas passes to the upstream side of the filter before the downstream side. In a preferred embodiment the filter includes an intermediate layer between the first and second layers, the intermediate layer being more electrically conductive than the first layer and with resistance to flow similar in magnitude to the first layer or greater than the first layer. In this way, gas can be electrolytically produced at the junction of the intermediate layer and the first layer and may experience lower resistance to flow by passing through the first layer. The thickness of the intermediate layer and its resistance to flow may not be so high as to have a large effect on flow through the filter in 10 The intermediate layer may comprise a composite of a more Sisctri άiιy conductive material ana a isss electrically conductive material. The porosity of the intermediate layer may be 0.1 to 50% lower than the porosity of the first layer. The resistance to flow 15 is effective at 0.1% increased porosity while at 50% increased porosity the reduction in maximum flow rate through the filter is beginning to affect performance. The volume fraction of the more electrically conductive material in the intermediate layer may be 20 highest adjacent the second layer. Conveniently, the more exectricax_,y conductive material or tne intermediate layer may be the same material as that of the second layer and the less electrically conductive material of the intermediate layer may be the same _. _. .. eSS e i SC i i i . α n y UUU U . I V S ιιιe_ ι_ C- _ α - α.5 ...a -, uϊ .l e first layer. This may assist in bonding of the three layers and reduce relative thermal expansion stresses. The intermediate layer may comprise a composite of larger particles and smaller particles. Where the first layer is made of small particles of a uniform size, the use of small and large particles in the intermediate layer will provide the desired increased flow resistance. The larger particles may be made of a more electrically conductive material and the smaller particles may be made of less electrically conductive material. The more conductive particles may be present in the form of isolated particles or as clusters of particles in the range 50 to 2500 particles or clusters per square millimetre of the intermediate layer in plan, preferably 100 to 2500mm , -_, -2- most preferably 250 to 2500mm . From 250mm and above a significant improvement in performance is seen. The intermediate layer may alternatively comprise particles in a porous matrix. The particles may be made of a less electrically conductive material and the matrix may be made of a more electrically conductive material. The first layer may be made from substantially non¬ conducting material. The first layer may be made of ceramic such as an oxide of titanium e.g. titanium dioxide. The ceramic may be a ceramic with high electrical conductivity such as Ti^O^. The second layer may be made of a material which is a good conductor and may be made o metal such as stainless steel . In another embodiment, the first and second layers may 5 be provided directly on one another, the thickness of the second layer in relation to the first layer ensuring that electrolytically generated gas reaches the upstream side of the filter first. 10 In a further embodiment, the filter may comprise a further layer of increased flow resistance downstream of the second layer. The further layer may be made from the same material as the first layer. i _ι According _o ano_.__εr aspect ox tne invention tners is p ύ iύcύ apparatus comprising a i-.-βr according _o t__e xirs- aspect ox _..e invention ana an opposite electrode to enable electrolysis with the filter ac.ing as a turtuεr electrode. n rx __bl The apparatus thus comprises the filter and the means to c^ean it. _,ι_e cleaning may u6 coπα ctδu in tne same way as dεsc iαeα in GB __IUU_I---J. 25 Preferably, the apparatus further includes puise means to provide pulsed power to the electrodes. The use of a sufficient voltage to produce a current density causing a good level of gas to be generated is needed. This power will be lower than in the apparatus of GB 2160545 and also need be applied for a shorter time, resulting in a more economical filtration process. The electrode may take any suitable form and may comprise a plate or mesh to be arranged parallel to the filter surface. Preferably the opposite electrode is arranged to apply a potential difference to parts of tne i x .εr sequenti x xy. xnαeeα according to a further aspect of the invention there is provided a ii i .sr apparatus comprising a i ;J .εr naving at .east one conductive layer and an opposite electrode, the opposi .s £ι8C>.rθuc 3Ci g arrangεα _o aρριy a ρo<_βr.-isx -iixxεrεnce to parts ox tne concuctive ayer Ci tne x x x .er acqucu.xαxxy . ___c AIU.U OX _ac u^uOSx -e_ δlεctrouc may ι_>e susstantιax xy sma xer t__ar_ tne xi ter surface and the apparatus may further include means for moving the opposite electrode over the filter surxace. in anotxxεr emoodiment, tne opposite electrode may comprise a xiexibiε sneet and means to itisnipuiά-c _ s sneet to oring eacxx part ox tne shee. sequεn_.iax y close to ana away xrom .πε iii .sr surxace. The manipula ing means may comprise means for sending a wave across the sheet. n tne seconα aspect ox tne inven ion apparatus may comprise a plurality c±. xixter oαules and a plurality of electrode modules, each filter module including the said filter with the more conductive layer being connected to an electrical supply, each elεctrode module comprising the said opposite electrode, each module being bounded by a frame, the filter and electrode modules being arranged alternately side by sidε and thε frames secured toge.xxer in a suuS.an iaiiy xiqui_. _ig ι manner. Indeed, according to a further aspεct of the invention tixere is provide., a xi ter apparatus comprising a piiiraxity ox xiitεr ou ies and a plurality ox εicc rOuc moiαUiεs, eaci ∑i_.er iύOuuiε comprising a fi x .εr leaving a conductive layer ccnnεc.εo .o a power suppl , eacxx εiεctrodε o uxe comprising at iεast onε εiectrode eiεment, εac__ moαuie oeing ounoεα oy a frame, the filter and electrode modules being arrangeα alternately side by side and the frames being secured toge^xxer in a Sux.stan i i liquid tignt manner. The filter apparatus can thus be provided in a very compact fashion and the modular also form simplifies manufacture. Preferably, the xrames are of common plan outer Each electrode module prexerably comprises a plurality of electrode plates arranged in a substantially co- planar fashion. Each electrode plate is preferably 250 to 400 cm ; in area. This enables European mains power supply to be used. Each electrode moαuie prexerably comprises at least least one eiec.rooε eiemεnt to tne o^ osi _ε xacε ox _11C X X C.1HC.. in.o an<_. out oi ___e oαui ana iso re___cεs cost ~γ εnabx ing tπinner elements to be used. The eiectrode elements to εacn xacε ox tnε xramε may e connected txxroug a connecting emoεr etween the eiectrode elements thε connεctmg member being connected to an cie .π άi supply. Preferably, a leak path is provided between two module frames from internally of the frames on the input side of the filter to externally of the xrames. The leak α .n cua urea .i . x x α. x ______ auυu i u_ uι x OAU cue input to the output side ox the filter, leakage will take place preferen i lly along the leak path provided between the two module frames as that leak path leads to atmosphere whereas any leak to the output side of the filter would be to a pressure above atmospheric pressure αuε to ___s oac prεssurε xrom resistance further through the filter apparatus. The leak path may conveniently comprise a groove or rebate in at least one of two side by side frames. The membrane periphery in the region of the seais wiii itself be sealed to prevent leakage through thε membrane. The filter of each filter module is preferably a filter according to the first aspect of thε invention. Ξacxi xi er moαuxe referably comprises a xliter membrane covering each open side of the frame. Each xixter mεmorane is preferaoxy provided on a pεrxorated support Wxiien may _.e a gauzε. ne saiα ρerx ate<__ support is prefεrabiy electrically conducting. The said per∑ora.eα support may DE ρrcvi__eα on a second support whicn dexines xewεr, larger perxorations. __ιβ sβccύu support is preisrάu.y εxectricaiiy conducting, trexeraoxy, a resiiien. memoer is provided oεtween tne filter membranes. The resilient member may be corrugated. The resilient member may be electricalxy conducting or may be made xrom plastics. Onε eiectrode module suitably includes an input for xiniεo in paraiiεx or in series. in tnis way, __ε apparatus can oe uεsu MI _.ι a constant tnroiigxxxiow ox. _.r a1ternativeiv can oε ssα as a series dead end system. In thε dead end, series arrangement, the solid material in the fluid to be filtered will tend to accumulate at the dead end and preferably the last eiectrode module in the series includes a valve to allow continuous or intermittent dumping Oi accumuxateα soiiα material. According to another aspect of the invention there is provided a method ox cleaning a filter using apparatus according to the second, third or fourth aspects of t.ie invεntion, tnε metncα comprising applying potential across tne electrodes. According to a further aspect of the invention there is provided a method of making a filter membrane comprising -.epositing matεriax at an in.erxace in a body comprising a xirst .ess εiεctricaiiy conductive filter layer and a second orε electrically conductive filter layer the intεrfacε being defined between the layers, the xirst xayer naving a smaller pore size .x.an tne secon.. iayεr, tnε matεri x oεing more eiεctricaiiy conductive tnan tnε xirst iayεr. Thε dεposition is conveniently carried out by eiectrodeposi ion, hydrometai iurgy or elεctroiess pi .i ix ) . According to another aspect of the invention there is provided a method of making a filter membrane comprising passing a current across two eiectroαes in an eiectrode catn, a xirst exectrodε comprising a oody comprismg a xirst xess s_-.c ri___.x_y conductive ii..sr layer and a second more electrically conductive filter layer, the first layer having a smaller pore size than the second layer and the method being such that e1ectrodeposition takεs pxacε at t____ intεrxacε oεtween -lie xayers. xie second eiectrode may oe ne anode. xxxε excC. ouc os1 ion at _nc interxace oetweεn tne xayers wixx cause t__ε xormation ox a region ox rεducεd porosity. r-rεxεraoxy, tnε εtnod is a mεtnoo ox m ing a xix .sr according to tnε xirst aspect ox tnε invention. Tx.us, .ne εxectro_.eρosition results in tne axoresai-t intermediate xayer -.eing xormed. The electrolyte may be a simple electrolyte, or may be a bright electroplating electrolyte. An advantage of electroplating at the interface is in binding the two xixter xayers togetnεr oy means O cue electrodeposited material. The said second layer of the body may comprise a layer of particles, e.g. rolled particles, onto which the first layer is deposited. The ' method is particularly useful in this situation where the powder rolled second layer has a flat surface onto which the first layer may not adhere well without such treatment. Alternatively, the body may comprise a second layer of particles, e.g. powder rolled particles with further similar particles fixed to the surface onto which the first layer is subsequently deposited. The body may comprise the titanium dioxide/stainless steel filter currently made by the Ultram Company. According to a further aspect of the invention there is provided a method of making a filter membrane comprising providing a second filter layer, texturing the surface of the layer and forming a first filter layer onto the textured surface. According to a further aspect of the invention there is provided a filter comprising an upstream surface made from Ti^O^.. Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is an elevation in cross-section of a fi . _ is a e iled viεw ox tnε εlevation ox Fig. i snowing hydrogen initiation points: Fig. 3 is a graph of pressure against time during u.u.xiu. i i c - £ -ϋ-. _r—iBα- _.. i. O __il . m. : _ .. i_.i l: _u x. x -.. e -. £ x x : ~x *i —c _ v __»>___. T L- : y _ . 1 i ., Fig. 4 is an elevation in cross-section of a filtεr in a second embodiment of the invention; _ ■ ig. ~> is an Sisvά.ioπ in cross—section ox a t ig. <_> is a grapn ox tortuosi y and ρorε sizε xirougn _xxe oεptn ox tnε -. i itεr ox ri . _» ; Fig. 7 is a view ox Fig. 5 snowing -.uooie U __l: —S—.—l X; •_..U.__..X:W——X1., Fig. 8 is a partiaxiy scnematic view partly in cross—section ox apparatus including tnε x liter ox x xg• -* , Fig. 3 is a perspective view ox apparatus in a fourth embodiment of the invention; Fig 10. is an εlevation in cross-section ox an intermediate stage in manufacture of a filter according _o a xixtn emcoαiment ox tne invention; Fig. 11 is a side view in cross-section of two eiεctrodε oduiεs and a xiitεr moduiε ox apparatus in a further embodiment ox tnε invention; _*ιg. i _. is a pεrspεctivε view ex an siεctrooε moduiε of Fig. 1 ; _*'i . iJ is a sidε viεw in cross—sεction ox a connecting member of the eiectrodε moduiε of Fig. 11; fig. i-_ is a pian view x an εiec rocε od iε ox ly . ι ι , Fig. 1 mi is a detail side view in cross-section ox one end of the filter module of Fig. 11; Fig. 16 is a detail side view in cross-section of anotner en_t ox .us liter moouie o_. Fig. 1 ι ; and, _" ig. i i is a perspec ive view ex apparatus in a The filter membrane 10 of Fig. 1 comprises a first x yer ι__ ox itanium -iioxioε ceramic particies provided on a tiά., second, substrate layer >-_ xorming upstream and downstream sides of thε filter membrane i XJ rSapCL . i v e l y . x lie ac uau l αy ci i -i x Ξ iiiα-ie u i sintered particies , o ox stainiess stsε.. xnβ ceramic particiεs >3 arε xiner tnan tne sintered metai particles >D to provide a smaller pore size. in normal use, _nβ iiqui-i o _o sε xiitered iows tnrougn the filter membrane 10 from the ceramic layer 12 to _nβ metai sintεrεo iayεr \ — and xii .εr ca_ε _._J ouiids p On tnS Cβrά iC lάVci" filter 10, the sintered metal layer 14 is connεctεd to a power supply (not show > which is also connected to an elongate elεctrodε 20 which is shown in transverse cross-section in Fig. X The power supply provides pulses of eiεctricai current between the filter 10 and the electrode 20, the filter 10 being the cathode. The liquid 8 may be an aqueous solution. Hydrogen will thus be generated at the boundary between the sintered metal substrate 14 and the ceramic layer 12. The sintered metal layer 14 is a good electrical conductor and, when it is made the cathode of an electrochemical cell, hydrogen will be generated on the surface of the metal particles 16 nearest the anode 20. It does not matter that the hydrogen is generated below the membrane surface when it has a superficial layer of non-conducti g particles 18, only that sufficient hydrogen emerges from the upstream surface. Three factors are important: (1) Where the gas bubbles are nucleated. (2) The pressure in the gas bubbles needed to force fluid to flow through the pores. (3) The side of the membrane 10 from which the bubbles of hydrogen emerge. The pressure difference, Δ , of the gas inside a spherical gas bubble in a fluid is given by Young's equation: - Δp = 20 /τ (1) ^ is the surface tension of the fluid with respect to the gas, r is the radius of the bubb1e When the radius is very low, A p becomes very high. If the gas nucleates in a cavity, there is less gas/fluid surface. The pressure needed to form the bubble nucleus is then less than it would be for a hemispherical cap on a flat surface. Therefore, the most likely place where bubbles of hydrogen 9 will form is where the small ceramic particles 18 touch the larger metal particles 16, see Fig. 2. This is at the interface between the two layers 12,14 and is also the position nearest the anode 20, therefore with lowest solution resistance for passage of ions across the anode-cathode gap. The hydrogen 9 will preferentially fill the part of the membrane 10 with least capillary pressure. This, from (1), will be that with largest pore size, the substrate 14. If the hydrogen 9 is produced at a low enough rate such that its pressure is always below the capillary pressure of the small pore size layer 12, the hydrogen 9 will all emerge from the downstream surface of the membrane 10 and will serve no useful purpose. Until the first bubble emerges from the v ownstrεam surxacε, tnε pressure witnin tne suostratε 14 will bε bεtween the capillary pressure of the substrate 14 plus the downstream prεssurε and thε capillary pressure of the first layer 12 plus the upstream pressure. When uusu εs are produced tne pressurε it in m SUJ SL~C1.6 <*. in oscixxatε wi miπ this rangε, see Fig. 3. x tnε nyorogεn is .o oε used to xorce ne 111.εr c r.ε xrom .uc surxace ox _ne s a x pore size xayεr _, its prεssurε must oε incrεasεd ~ s. xεast tne caρι ± xary prεssurε o_. tnat x yεr ι __ pxiis tne pressure in tne upstream cnam e . nis can ε cone i . ICU mooixying cue membrane ι. in two ways. Tne irst is to Cx sε oxx .he Cιamι_>εr be inc _he xix.er with e.g. a soxεnoid v i vc. xiic. nyu uijcn <_jc___> μ_x cΞ_.ux . r-3 t-iic __ncxuιx_ l -_x . iiici . me IUU uιaαuv ιι.α<je5. inc. _. i x__ ι_ xΞ 1.1101. _ιιc xi xtεr em rane is x.i—axi x y stressεo, a may xaix ecn nicax xy. xu£ sεcono is mia * _ it may x ix _. ox tne supporting scructure an_x o e exectricax contact κι m it. me tnird is tnat onxy a s ax proportion ox .ue ydrogεn wi x emerge xrom tne upstream surxace, giving rise .0 xow εxectricax exxiciency. mε xourtn is v.na>. .ιe catnOi e xi .εr may oow so xar as to toncn tnε anode and cause a snc . circui . e second way * _o oo tnis is to increase _πε πyorogen production rate IIX II .-.ux x d t. . u x 5cr , 5ϋ un ti. thε back pressure genεratεd by thε resistance to flow of the permeate fluid through the substrate 1-J to the downstream surface ox the membrane 10 is high enough to force some of the hydrogen to the upstream erabran. surface. However, before bubbles emergε from the upstream surface, it is necεssary first to displace the fluid in thε small pores which wiii also contribute to removal of filter cake. Furthermore, this fluid must be dispiacεd bεxorε thε fluid is displaced through thε substrate 14. Two factors arε important hεrε; the pore radii and thε porε iεngths. Thε Hagen-Poisseui1lε εquation givεs thε volumetric flow-rate through a capillary of length 1, radius r, tortuosity t, of a fluid with viscosity ' ■ under a 6 pressure drop .^ p:- 5.) tl Assuming constant pressure and neglecting kinetic energy, the instantaneous velocity of the hydrogen- fluid interfacε in thε capillary is obtainεd by dividing thε flow-rate by the arεa of thε capillary Integration of the equation shows that the tube wii become empty at a time given by: - _Λpr- ι _^ in tfiiΞ C sε is ϊπε QΪ IIS CΠ C jjε n€επ πε i n_εrr_Qi pressure or πε nycrogen S,Π IU6 cd i ±iary pressure pius 5t-sιic pressure, ΞO tne nεc pressure uTi i g x xUx G LurOUya tne Suϊ . x pOTc SlZc Icyi GH 1 Ξ — ,..-,*--. " ___,-._— — — — ι__ - — —__.,..___ -__ ■ .._..._ ___. * .'__.._, _...__. x ^ _ Ucu x e-_a_-_> idli nαc ci i v iii-^ xx xc e nx e_.y n m SuuSirάi 1*3. ne_r xore , n ensure tnat c e nyGrogen breaks through the s al 1 pores first the thickness of LU SΪT-CLX x ρorε si e r gio ι — neeos to uε cecreaseG or »ui_.e * xnx_ c-_,.r.-_..i—e-_.5-.5— u-x- xuve_-. *ι -ιx n„ccu:a_ —u uι e inc ea d to pp op i t rexative sizes. xne IOΓIΠSΓ x . — 5. _ i_ilO._l. * .c _~ c _jx c x c n L.a _ * n _ x cue -_ ..» α -. — c x ce -.x ___ . r ilrO _-.W-,c — - v. -.-x___, _ --3. x.. n_.e_e_--, ... ιu_.e •__. COSt Ox luc cαi pO ucI ϊ iDIc IS 3 iuHi I i άπ" pαT" Ox Lu ovcrάj. uuSz ά c proG cc co , "His approacn may Dc xess α' rά "iVc Iroϊπ l c oi t or view ex cost. ιi H mem-crane is IΓIGQII ISQ in sucn a way tne icxxcwing aGvan ges in accrue.. ±ewer stress, etter eicC ricάi contact, nign exectricai eixici ncy, o it is not necessary * to Cx an x or -_ne ri tεr at once * In the embodiment of Fig. 4, the membrane is symmetrlcax , anci cnεre is a smaxi pore-size layer m ψ on x-io h siucs or a sinter€ci at i i i yεr in. ne ups ream ayer ■ x, u i g e nnεr tnan tne Go nscream iayεr 22. In this casε, when the hydrogen is generated, the .hole of the interior 14 of the membrane 10 is pressu ised at least to the capillary prεssurε ox the upstream small pore-sized layer 12 plus the upstream prεssurε. Kydrogεn will rεach thε upstream surface 25 of the membrane 10 first, as the upstream layεr .2 is sufficiently thinner than thε downstrεam layεr 22. Fig. b snows a tnir -o r~ _ε_mm"or~..o—.dr-.i ■ nmaεn. r'v. .inch also has three layers. The first layer 12 is a filtration layer. This is made from titanium dioxide ceramic powder although other suitable ceramic powdεrs could be used. Thε iayεr 12 may be up to ICC microns thick herε being 40μm, although in anothεr ε bodimεnt it may be up to ΞOμ thick, preferably being .5μm thick. The pore size may be C.2μm. There is then an intermedi te layer 24 which is a multipurpose layer, cne purpose ox which is to provide the εxtra rεsistancε to gas flow. It is adε from a mixturε of the powder of thε first layer 12 and stainless stεei powder. The thickness ox this layer may be up to about 50μm, here being 3Cμm and in another embodiment is IΞμ . Thε porε size is about The final layer 14 is a sintered stainless steel substrate, typically gas-atomised powder or chopped fibre of the same type used in the intεrmediate layer 24. It may bε up to about 250μm thick, here being OOμm. • T"h-.-es —p..o-.τr-=e s: ∑e is about - 2μm. Tne particxes ox xayer .4 are powoεr roiiea and .ayer 1■ intermediate layer 24 are then applied and roiled and sintered. Finally the ceramic particies of the layer 12 are applied as a slurry and roiled. The intermediate layer 24 performs three roles Tne xirst is to act as a conαing xayer. Tne surface of the second layer ' .4 is likely to be quite smooth because it is usually made by a form of pressing or roiling process. If thε first iayεr 12 were to be applied to it directly, forces produced by differential thermal contraction on cooling from the igh temperature sintering treatment would be concentrated into a small volume of material, and fracture would be likely along this interxace. Thε intermediate iayεr 24 will have a coefficient of thermal expansion __TE. i termediate between the other two iayεrs 1 , 1 , so the maximum differential thermal contraction stress will bε rεducεd, and the strained region will have a grεater volume reducing the chance of fracture. This is particularly important for cεramics (other than zirconia or ceria, which have unusually high CTE for ceramics;. In this way the distancε between the surface of metai particles at the top of the intermediate layer and the top of the second iayεr 14 is grεater than the distance between the surface of the metal particles at the top of the intermediate layer 24 and the top of the first layer. interxace oetwεεn tne intermediate xayer _4 and tne first layer 1 will act as thε sitεs for thε generation of hydrogen relatively near the surface of the first layer 12. A typical metal particle 15 may be ~15-2Cμm in size. If it protrudes by a third ox its diameter from the mean of the top face of the intermediate layer 24, it will penetrate significantly into the first layer 12. It is advantageous for the hydrogen to be generated from a number of such sites with controlled spacing. It is also advantageous if a controlled addition of larger rnetai particies is added to create hydrogen sources nearer the surface of the first layer 12. Thε third roiε is that it will act as a barπεr to flow of thε hydrogen to the downstream surface of the membrane 10 by reducing the volume fraction of voids compared with the first layer 12 and increasing thε tortuosity. From the arguments given above it is easy to understand that if the permeation rate for fluid and hydrogen through thε intεrmεdiate layer 24 is greatεr, but not by too much, than that through thε first layεr 12, more of the hydrogen will emerge from the upstream surxace, .here it is wanted. Fig. 5 shows a graph of pore size P and tortuosity τ against distance through the filter membrane 13 in parallel with Fig. Ξ. The pore size in the first layer 12 and intermediate layer 24 is about the same out increases markedly in the second layer 14. The tortuosity is at its highest in the intermediate layer 24. The fact that the tortuosi y increases from the first layer 12 intermediate layer 24 while the po. electroly ical ly gεnεratεd hydrogen through the intermediate iayεr 24 towards the downstream sidε . ' of thε filter membrane 10. Fig. 7 shows a typical distribution of bubbles of hydrogen in the filter membrane 10 during cleaning. The primary purpose o_ x ay i as oarr .r to :α, x. x n ox :nε nyαrog. generated at the metal particles 15 within it to the second layer 14. To some extent, it wii pεrmeation rate of fluid as well. However, since without thε εiεctroiytic cleaning process the permeation rate xor xiuid is very xow anyway, tms loss is insignificant compared with the gam eisεwhεrε. Factors important in dεtεrmining thε pεrmεation ratε through thε intermediate layer 24 will be; volume fraction and size of metai particles 15, volume fraction and size of the ceramic precursor particies 15, and colIcid-forming/dispersi e additives to the ceramic slurry. Fig. o snows an emoodiment ox tne apparatus ox tne . ion i i qu i α - __ . .llt-< _α oy o- J-Z -IIUJ-. . ϋ x n i-o a . ai x ~ . u ~ In the tank S is provided plurality of parallel equi -spaced anode plates 30. Interleaved with the anode plates 30 is a plurality of filter units 32. Each filter unit 32 comprises two fiat filtεr membranes 10 mounted parallel to the adjacent anode plates 30 by a surrounding framework 34 such that liquid which has been pumped into the tank can only enter a filter unit 32 through a filtεr membrane 10. Each filter unit 32 includes an output pipe 35 and the output pipes 35 cf the filter units 32 combine in a main output pipe 35. A pulse genera or 40 is connected to each of the anode plates 30 and also to thε filtεr membranes 10. The spacing of each anode plate 30 from the adjacent filter membrane 10 is about 3mm and a pulsed current density of about O.lA/cm * is developed by the puisε gεnεrator 40. The tank 28 includes an outlet 42 for recycling of liquid. In use, liquid is pumpεd by thε pump 25 into thε tank 28. At least some of thε liquid is filtered through the filter units 32 and passes out through the pipes 35,33. When a filter cake has built up on the filter ϋembranes 10 , thε pulse generator .pera.cϋ -o cause hydrogen to be g. l x mix n Fig. 11 shows a further embodiment of the apparatus of the invention, this time in modular form. The pump 25 and generator 40 are not shown. The apparatus consists ox Eacn eiεctrooε odule 5__ consists ox a oooy oo navmg a square frame 54 made from square section material such as polypropylene. The material should be a good insulator, chemically inert for the process application, and thermally stable for the process tεmpεraturε. In another embodiment it could be FVC, for examplε. The body 53 is recessed each side providing recesses 55 and includes twelve circular apεrturεs 55 drilled througn the body 53 at each of two opposite inner sidε εdgεs 55 of the frames 54. Thε circular apεrturεs 35 xntersect bores 57 through two sides of the frame 54. Anode plates 30 are mounted in each recess 55 between the inner side edges mode plates 30 arε provided substantially co-planar fashion, each anodε platε 3C extending from one inner side edge 53 to the opposite inner side edge 55 so that the pxates 30 arε edge to edge. A further three plates 30 are arranged in thε same way in the other recess 55 bεnina thε first three plates 30 and parallel to them to form aligned pairs. A connecting member 51 is provided between each pair of plates 30 in an aperture _n the oody 53 and is resistance welded 54 to each plate 33. Each connec ing mεmoεr 5' _Ξ a. the form of a stee. disc and defines a radial, threaded bore 53 to receive a ■ ough a bore in the body 53 to the sidε 70 of thε frame 54 and through the sidε 70 of the frame 54 where it is connected to a bus bar. A sεai may oε provided where the rod 55 leaves the booy 53. The ends of the bores 57 form an _niet and an outiεt for the module 52 at eacn corner of the frame 54. 50 axso compr _s :ram« n. narrower tnan tne :C.rϋut * ame * .•_. t is ox tnε samε outer dimensions in pxan. Tne frame 72 includes nine drilled apertϋrεs 74 in an inner side εdgε 75. Thε apεrturεs 74 intersect a bore 75 through the frame 72 the end of which at a corner of thε frame 72 acts as an outlet 99. Two shoulders 78,80 are formed in each side face of the frame 72 to form rεbates 32,84. Thε first rebate 52 receives the filter membrane 10 of one of the previous embodiments. The ceramic particle layer 1 faces outwards. The which has relatively large circular perforations 35. Between the steel sheet 35 and the filtεr εmbranε 10 is provided a steεl gauze 90 in the rebate 34. In contact with and extending between the two stεεi pεrforatεd sheets 35 of each module 30 is a corrugated sheet ;not shown; in which the corrugations run towards the apertures 74 to permit filtered liquid flow in that direction. To the opposite side of the frame 72 from the apεrturεs 74, a cathode rod 92 extends through thε frame "2 from the outer edgε 94 to the inner εdgε 75. A U-shaped connector 96 is bolted contact with the perforated sheets 55. The U-shaped connector 95 lies in a recεss in thε frame 7 ~ ' The cathode rod 92 is seaiεd on the outer edgε 94 and connected to a further bus bar. A shallow groove 98 is provided on each side of the frame 72 from the rebate 32 to the outer edge 94 of the frame 72. In use, a plurality of the filter modules 50 and electrode modules 52 are arrangεd alternately side by side with an elεctrode module at each end of the row. A clamping plate is placed over εitner end and the clamping plates are connected by tie rods 'not shown;. Thε frames 52,72 are thus held togεthεr in a substantially fluid tight manner. In a series mode for thε system thε outiεts 99 from thε filter modules 72 are connected together to a common take-off line ;not shown; and the outlets 59 from the eiectrode module 52 at one end of thε apparatus is connected to the inlet 57 to the next module 52 and so on to the x na x e x C ι_ x ooe iiiodu i - _ nu i il iC x U es show i at its outlet 59. In ail casεs liquid to be filtered is pumped through the inlet 57 to the first elεctrode module 52 and passes through thε frame 54 into thε arεa adjacent the filter membrane 10. Filtered liquid passes through the openings in the gauze 90 and through the perforated aperturεs 38 in thε sheet 85 into the folds of the corrugated sheet in thε central opening ox the filter module 72. Thε fiitεrεd liquid passes through the aperturεs 74 in the xram. '2 and out through the outlet 99. If a leak should arise on either side of the filter membrane 10, rather than passing along the floor of the rebate 52 and into the central opening of thε filtεr module 72, the leak wiii flow prefεrentiai ly along the shallow channel 95 to atmosphere as this will reprεsεn grεatεr pressure drop. The use of a plurality of relatively thin εiεctrode plates has εnabiεd the use of a relatively low current to evolve hydrogen in the filter membrane 10 to clean filter cake from it. Indeed, with each anode piate being say 10cm by 30cm, thε mains supply can be used for each plate to produce a suitable current density. In a dead end series mode for the system, liquid to be filtered enters the inlet 57 of the first anode module 52. Liquid from its outlet 39 then enters the inlet 59 of the second module and so on up to the last module. In use, the particulates that form thε fiitεr cake arε progressing! * / concentrates by fluid flow into the last anode module 52. By use of a solenoid valve [not shown- it is possible periodically or continuousiy to discharge this highly concentrated material from thε outlet 59 of the last anode module 52. sificient than systems wnεrε tne entir. apparatus must be washed through to remove filter cakε, as a concentrated accum lation of filter cake X X -^ U X o . Thε apparatus described when used in this dead end mode can be used for filtration of a suspension of disrupted bacteria which have bεen genetically manipulated to producε a required material such as a drug. The drug will pass through the filter, while the cεll bodiεs of the bacteria will not pass through thε fiitεr membranes 10. This biotechnoic icai technique is expensive and it is important to minimise the loss of the drug by minimising the loss of liquid in the filtering process. The apparatus can also be re-arranged for use in cross the anode modules 32 may be connected to a common inlet manifold »not shown,. Also ail of the outlets 59 of the anode modules may be connected to a common outlet manifold ,no. shown, . Part of the fluid from the common outlet manifold may be drained through a control valve ( n3. shown, , the rest being passed through the re-circuiation pump. It is also possible to operate tne system in cross x.ow mooe yet witn sets outlet 59 of the first such module is connected to thε inlet 57 of the second module and perhaps repeated for a plurality of other anode modules, yet the output 59 of thε final anodε module in thε sεt shall be connected along with the final output of other module sets for a common manifold for splitting into drain and recycle. The velocity of the liquid entεπng thε eiectrode modules 52 is controlled to provide either laminar flow, which will minimise power consumption, or turbulεnt flow, which may rεsuit in somε cleaning of thε filtεr by the turbulent liquid movement. In a further embodiment, a bi odal particle size •__x-a.<-x x-->u_. on is usεd _n the εtai particles 15 to be used in thε intermediate layer 24. A controlled number of larger particles 15 of diameter aoout 5μm arε providεd and will penetrate wel ,hϊϋ nc x xx S x. i yεr . _ to εnsurε tnat tne nyorogen is generated as near to the surface ox that layer as possible, and with an optimum spacing to clear the filter cake as quickly as possible with minimum power consumption and consequent reduced heating effect. This will ensure that the resistance to fluid/gas flow through thε intermediate layer 24 can be kept low enough that i will not reduce the performance of the overall system significantly As an alternative to the εxongate exectrcαe _0 and tne anode plates 32, a flexible conductive sheet 44 may be used in parallel with a filter me orane 10 as shown in Fig. 9. A motor 45 pushes a rod 45 across the rεsiiient sheet to create a wave. The anode cathodε gap is about 3mm and thε wavε amplitude is about 1.5mm. In this case a constant potential is appiiεd waves transmitted across the sheet 44 provide the cleaning effect serially across thε membrane 10. Fig. '7 shows a further alternative arrangement. Here as the anode. Opposite edges 112 of thε sneet 110 are connected along their length to two rods 114. Each rod 114 is partly rotatable by a motor (not shown,. Partial rotation of the rods 114 clockwise and anti¬ clockwise will cause a wave to be transmitted across the relatively stiff sheet. The trough 115 of the wave will bε close to tnε layer 14 constituting tne cathodε of thε system and thε sεparation between the a good rate ox nyαrogen production rεsuxts oppositε the trougn 115 in the sneet 110, wnich little or no hydrogen production takes place opposite the remainder of thε sheet 110. In this way lower power can be used and the fiitεr 10 can bε cleaned in a sweeping motion of the trough 116/wave across the sheet 110. :- __!._.. »x y .c cleaning is taking p άcε, x_ac_ prεssurε ox x ic may xso __>ε usεo x tne Known manner. n addition tne i .sr may oε ioratεd, xor exampxe, oy a assist in Cxεaning. Tne x.ac__ pressure may oε m pnase HItu _ne appxica ion ox powεr puxses wnerε usεu. xxie x x xtex iilciuxii a lie i ϋ ΞixOWn xn r x ^ . i oan oe .icαicu _o prcviuc an in-,εrmει__ιatε xayer xn a numoer ox ways, inc ε εcL~O yv_ε useo xor nyorogεn εvoxution at .ne interxace may u£ repxacec oy a suitaoxε eiectrox' / te xor cue dεposition ox a conductive a eriax en tnε Ξ. i ncSΞ steex partxCxes . G at tne xnterxace. .ne exectroxy_.ε may oε an aquεous nιc_cεx— .in suxpnate aO x u i- x oil , nex e oe x nχ_j uuoi a_aιιnOu5 auxpiiat-e -Sii-Jo, — l . , U,.S-. £ u r uxy mm ' i ' — ii x . o _-r:-__--_ x. a —.u.x" _p_n_. _. -i.e_. x ι-. x. _t-arox ( . - _i T il- o rx _ p x' . U.a__ ■ l. Λ-m_ " / / • x concentrate ^ suxpnuric acio wnicn wix exectropxate a _iii.c_i._-_.i. -_m._i. a_ux -i.a_o•_x ■ e_ ma_ie_.x.x. a_x•S_ _c.o_ ■o o__e__o__.x. xi.e-i: couxd be a πiCiC i alone, tin alone, ccuάi , gox_x or _ p-.i1α-.xt-n. -.i,U,—,. τ xx v_. x ..a. -_U_._U»-V.C__I_U.C__U__I_. x ..O _ e _. x e_-C_ ^ _ _..-r_ O___f * _, x'i a-.xC— _ X:. X. o_._m_ a — —n aqueous solution which rεquirεs that thε metal to bε exεctroρxa_.e_x must oe uepcsited prexerence to nyorogε . xθ deposit ic-εx axone i-ne aoove procεss aU x fu< .. • •i i xje ucuuS _l. -cu : — tin lύ.ciiύε.ά nn αnu : cer.aiii c i r cu s _. ncεa structure penεtratmg into thε first iayεr of cεramic particiεs 13. The dendri ic structure will bond thε layεrs 12,14 togεthεr more ciosεiy and will providε an intεrmεdiatε layer of eiεctrodεposited metai dendπtes and ceramic particles 15. This serves two purposes. The first is to gεnεrate hydrogen nearer the upstream surface. The sεcond xS to provide a layer of xncreasεd rεsistancε to flow of evoivεd hydrogεn to thε downstrεa surfacε so that evolved hydrogen will reach the upstream surface 25 of the membrane 10 bεforε the downstream surface. If hydrogen gas is genεratεd electrochemically, its pressurε, hεnce chemical activity can bε high enough dirεctly to reduce some metal salts to metai, as is well known in hydro etal iurgy. This procεss may also bε usεd in prεcipitate rπetai i the first ceramic layer 12. Other eiεctroiess plating mεthods may also be use . The layer structure of tne initial material of the embodiment of Fig. 1 is formed by roiling a stainiεss stεεl powder and partially sintering it to provide a flat metal layεr in grεen state. The titanium dioxidε layεr is thεn appliεd as a particle slurry to one surface. As discussed, thε powder roiled surface of thε sintεrεd atεπai provides a flat interface which icαύs to tne concentration ox tnermai contraction strεsses which can lead to delamina ion. The structure may thus be fragile and liable to delaminatε under its desirεd usage. The electrodeposited material not only provides the requirεd intεriayεr of increased resistance to ensure that the membrane 10 can be cleaned by evolution of gas, but may also operate to improve the resistancε to mechanical failure of the filter. The ductile electroplated metai may incrεasε thε work of fracture of the ceramic body. The εiectrodepositεd material may be added such as to rεcucε the porosity of the first layer at the interface by 0. i to 50%. Fig. 10 shows a further membrane structure 100. In this case, to avoid the thermal contraction strεsses dela inating the layers 12,14 steel powder layer 14 has been rolled and partially sintered, a slurry of stainless steel particles IS is applied to its surface and heated to providε an irregular and less closely packed adherence of stainless steel particles to the surfacε of thε flat, roiiεo initial iayεr. includes peaks 102 and troughs 104 on the microscaie The titanium dioxide particies 1 slurry :rougn Thε tendency to άc i αui uiά . i ϋri d ue c .ncriϊiα i oon t x aotion stresses is reducεd for this structure 100. If the method dεscπbεd for the creation of an intεrmεdiate iayεr in thε filter of Fig. 1 is used, then thε eiεctrodeposi ion of the tin-nickel intεrmetai 1ic will take place on the pεaks 102 of thε stainless steel particle layer 14 prefεrεntiai ly to the troughs 104. Any gaps in coverage by the cεramic particles due to faults in manufacture will be a εlioratεd by the deposition which will tend to fill xn thε gaps. Thε gaps may be caused by skin particles or hairs, for example. If the distance between peaks is too great it is possible that some areas of the embranε might not bε cleaned effectively. Good results in cleaning are obtained with isolated particles or particle clusters i.e. pea * _s 102 provided at a density of 50 to 2500mm . At over 150mm the pea_cs 102 arε ciosε .nougn that during clean .ιixnn<-j xe . ox axea ox memoranε are not cxεanεo oy h evoxution. Alternatively or in addition an aqueous bright electroplating electrolyte containing, for example jCgl ' potassium stannate plus Cml/l concεntratεd sulphuric acid can bε usεd to causε more even εiεctroplating. Thε concentration gradient of metal ions which results in a dendritic growth is reduced by this method and so a more even eiectropiating upon the interfacε between the layers 12,14 is achieved. An alternative way to provide the desired textured surface is to roll the sintered metal particies using a textured roller before thε ceramic particles are added. Suitable laser scribed rollers are know which can texture on a micrometre scale. This gives very good control of the texture so that no two "peaks" are too far apart which can occur with thε appliεd slurry method. The rolling could also take place during sintering to further reduce manufacturing time. 0 The TiOj particles could be replaced by Ti,0, particles or reduced to that composition. Ti,0, has a conductivity of about 3C00S_' , cm which is close to that of ne etaxS. Tnis wixx raise cne surxace o 5 evolution of the hydrogen away from the intεrface oetween ne ceramic and metai particxe i yers and can be used to ensure that thε hydrogen reaches the upstream surface of thε membrane first. TiO, particies can be reduced by heating to 1200°C in hydrogen. To 0 improve production the final sintering of the filter could takε place in a hydrogεn εnricheo atmospnerε to combine the sintering sπu reduction processes. Alternatively very small metal particies could bε used 25 to raise conductivity, but the only metal likely to be sufficiently chemically stable are thε prεcious metals gold and platinum which arε expensive. Another higher conduct!vely ceramic such as 3.0 aCr j j ^.O j could be used (B.C.H. Steεle; kendall et al ; Proceedings of the Conference "Advanced Ceramics in Chemical Process Engineering" Imperial Coiiεgε τio_—nc __xO___ii . i O 3 — _ 1_ Λc> — o.e-_C_- 1 i Q 3 uO θO — p —j f - . . i (D- J • : - i i i -. _ fc-.u. o_. x. x : a_ 'n_. e—o__ o..y. x c- 1n- _e_ Ins i c cε ox oeramicε .

Claims: i . n Xix er co prising a xirst, iess eiectricaxx * / conductive xixter xayεr an_χ a secono, more εiεccricalxy conduc ive xiitεr layer downstream tnereox , tnε xirst xayεr naving a smaxier pore size nan tnε secono xayer, ue construction o_. cue xix er, the rexativε tnic nεssεs ox tnε xayεrs ancx thε resistances thereof to roovemεnt thεrethrough of exεctroxy ica y generated gas when tne xixter is permεatεd i n ± iquio oεing sucn tnat sucn gas rεacnεs cne upstream sι_e ox ne i liter oεxorε tnε downstream cx im * 111 -. x ax pore sizε .ess e x eo ex . oa _, x y i ouocx e . άVi - ι -: ". . _._._. _ _. _ l _ : ___. j . _ _. : _ . _ i ..'__„_ . _ _ _. _ _J . n xx x cex aa oxaxmec in oxaxiu ι , /.nexexn cue pore sizε ox cnε xess eiectricaxx- / conductive x yer is nxj mox e _ cn_.a-—n _ "_ . m ci uiu . £ : . x. _ n x x x cer as claimed in Cxaim 1 , wnεr in tne j^orε size o * no mu__r. e _ c.n_.a_._n. 1 i . n x . el,—m . ρorε size of thε less eiεctπcally conductivε iayεr is n i. x x tβr S o x a 1 iue xn oxaxiu i , nnexcxn ne less conductive layεr has a ρorε size sufficiεnt to reject oaccεria. i. /. xix εr as cxaimεo in an * / prεcedin^ Cxaim, wnerein cue xirs and sεcono xayεrs arε provioεo v_ι x x eo c x y cn aliO cnex xayεr in rexation cθ tnε irsc xayεr εnsuring e xeo cxox y cxoax gεnerateo gas reacnes upstream A ilter as cl im * any wuc ciπ cue xi x cer x5 oonacx oceci v. x en a xul ciicx xayex iuc'i cnat gas passes to ue upstre __a =m rr. s _-i i .- * - Oϊ τι r . l x _ __ _ _ _ -. _ . ι _ _ ■ _ _ ~ ._■ ._ _. — n xx x ccx aS oxax iiie x xn oxaxiu u , .nerexii cue xirsc άiiu second xayers, tne intermediate xayer oeing orε εxεctricaxx * / conouctivε tnan ue xirst .s εr and ni .i * resistance to Xxow simixar in magnitude .o tne xirst xayεr or grεacer tnan tne xirst xayer. ,- _ : _ __• _ .__, n .._. . i a x IIICU in oxaxiϊi _> , «n- in εr ic ent ayer nas a porosit wnicn is xower tnan nε porosity ox cne xirst xayer oy ., ι . ox orε. .πerein tne incεrmit ent xayer nas a porosity wnicn is or less of porosi' irst xayer. i x.. n xx xter as oxaxmccx xn c i α i Hi _> , , x or ι ι , wnerein mc intermexiate xayεr comprises a composite O a more εiectricaxxy conductivε matεriax an_χ a ιεss c. i ά xy * conductive materi l . τ< _r : i x _ _,- _, . ____ _ __ . _ . _ - - .._.___ . _ _ ι_ . Λ x x x cex aa oxaxmccx xn oxax i -. , W ne x e x i.- cne VO x Ufi more e ec cr i a x x * / conduct ivi x ii cex iiicu x a ce . ay ex nignεst c * iine S -c-o —oi c ~i ~ xa --/.cx Wnεrεin tnε morε exεctricax * / conouctive atεriax ox ne ui.eriiicuiolc xayex xa cne Same inaccx xax S cnac Ox the second 1 ayer . n τι _ x:x : 1i xce _._r a .a_. o _.x. a_, x. _iue._,c__x x : i_x o __x; a_,x. _iu 11---1 , 11 ~ o __x 11 •,_1 cne xiicer ecxxace xayex xa cue aame xcaa ex€ocx xCax xy conductive matεriax as tnat ox enε xirst xayεr. ID. A xixcεr as cxaimεd in any o cxai s ι __ cθ 15, wherein the intermediate layer comprisεs a composicβ ox xargεr particies and smaxxεr particies. i i . n x x cei aa oxax lϊicC- n c i α i ill xargεr particxes are made ox a morε eiεccricax x * / conductive material and thε smaller particies arε made Ox a ιεss exectric x xy conductivε materia . 13. wherein cue more conductive particies are presεnt cue xox in ox xaoxacco -ax c xoxea ox oxUΞccrΞ ox •oa an je _c.nu .5_/u xaox a ccci .ciεs and clusters per squarε illimetrε of ιyer 'I n, ι_). ή xiiter as claimed in Cx im >o or ciai , , , wnerεin nε morε conciuctivε particxes are present in x axio e ι uu co __ _j u u xaoxacecx ax xoxea αuu x S a per Sc± ,Uaxe _ iiix; x. x. : itiecx e o __x: _ c • ue x . i exmcc :x : ace - xayex i . n_ _ c ι x an . wnerεin mε morε concuccive particies arε prεsεnt in tnε xorm ox isolated particies or Clusters in .ne range 250 to ___.0G isoiateo particies an_ι clusters per squarε millimεtre of ne intεrmεcxiate xayer in ριan. -.1 . n x x x eel aa oxax liici 111 any o o x a x ItiS 1 _. I-., Wnerein tne intεrmεdiate xayer comprises particies in a porous matrix. - J ..x J1 ... n Tl x £x - *x ic.ε m.-r. aa o _.x. a-. i,m-_.e_.o__ xn m. t _»i.α_.i.i_i_l - "-. I> , r W,L.ε A T-«e_. x. _ n. X iπe * . particiεs are made of a less electrically conductive mate ial Ti cue iϊi eriX iS ma e Ox a iuOrS eCt iC xiy o __o_..n-.__x.u .o _-_c-x ..v.c-_ — niα --..e-.r_-.α -i 1 . _ -.1_ "... τι _r : 1 1. __x_ a _a _- c _ ιiα_ .m_cu : x . _n o _x 1 a _x . _m c _> , . π.i_.ic_i_ ci .i_i _ cu_.e. Λ xx x ce ixter mcxUdes a xurcnεr xayεr ox incrεaseo xiow resiacance oownscrεam ox cue second layer. -._-. n 1 ii .cl aa iάiiϊlcu in any' precedi g C. wnerein cne xirsc xayεr is aue ox ceramic. -ι_ .,_. _.__, . _ 1 X a 1 HΪCU im-i , _ιιcι6iιι cn_.. irst layεr mad * ox x canxum UIUΛIUC x -1. — / . A __ _rx ; lx xce _ _r_ -a _ o _.x. a_. x.m__.__x 1. _11 o __ x a_ x. i_ll _ ~_ c_. , . rt,l'-lc__r_S-. x._ 11 — cn ceramic is a ceramic witn nign eiectricai conductivity. £ ■ ι. - - -. :eram; claimed in any prεcεoing claim. wner* x ayex mater: wnicn .s a good conductor. wn. second l ayεr .3 made ox metai A filter as claimed in claim 30, wher in th. ϊaαe ox stamiεsε steex .2. A filter substantiai ly lith reference to the accompanying drawings, .3. Apparatus comprising a filter as ciaimεd in in* / prεcεding claim and an opposite electrodε to .nabiε electrolysis with the filter acting as a .urthεr eiectrode. _ι . Apparatus as ciai ec in claim JO, wnerein the apparatus further includes pulse means to provide pulsed power to the electrodes. 35. Apparatus as claimed in claim 33 or claim 34, wherein the electrodε comprises a plate or mesh tc arranged parallel Ξurxace, 3δ. Apparatus as claimed in claim 33, 34 or 35, wherein the opposite εlectrode is arranged to apply a potential differεncε to parts of thε filter sequential ly. 37. A filtεr apparatus comprising a fiitεr having at lεast onε conductive layer, and an oppositε electrode, the opposite eiectrode being arranged to apply a potential diffεrεnce to parts of the conductive layer of the filter sεquεntiai ly. 35. Apparatus as ciaimεd in claim 35 or 37, wherein the width of the opposite eiectrode is substantially smaller than the filter surface and the apparatus further includes means for moving the opposite electrode over the filter surface. 39. Apparatus as ciaimεd in claim 35 or 37, wherein thε opposite electrode comprises a flexible sheet and means to manipulate the sheet to bring each part of the sheet sequentially close to and away from 40. Apparatus as ciaimεd in claim 39, wherein the manipul ting means comprises means for sending a 41. Apparatus as ciaimεd in any of claims 33 to 40, wherein the filtεr apparatus comprises a plurality of filter odulεs and a plurality of εiεctrodε modules, each filtεr moduiε including thε said fiitεr with the more conductive layer being connεctεd to an εiectricai supply, each eiectrode module comprising thε said opposite eiectrodε, εacn module being boundεd by a framε, thε filter and eiectrode modules being arranged aiternatεly sidε by sidε and thε framεs secured togεthεr in a substantially liquid tight manner. 42. A filter apparatus comprising a plurality of filtεr modules and a plurality of electrode modules, each filter module comprising a filter having a conductive layer to be connected to a power supply, each εlectrode module comprising at least one elεctrodε element, each module being boundεd by a frame, the filter and eiectrodε modulεs bεmg arranged alternatεly side by side and the framεs being securεd togεthεr in a substantially liquid tight manner. 13. Apparatus as claimed in claim 4. or claim .herein thε frames are 44. Apparatus as ciaimεd in claim 41, 42 cr 43, wherein each εiectrode module comprises a plurality o eiectrode plates. Apparatus as ciai eo eacn e. trooe pj 46. Apparatus as claimεd in claim 4- 45, whεrεin in each electrode module a plurality of tectrodε plaatεs is arrang. in a substantia ly planar fashion. Apparatus as claimed in any of claims 41 to OΪΪC — X CO 1 east one eiεctrodε element to the opposite face of ■_o. A pa a s as ciaimec in ciai -_ , , wnerein the electrode eiε εnt or elements to each face of the frame are connεctεd through a connecting member between the eiεctrodε elements the connecting mεmber being connected to an eiεctrical supply. 49. Apparatus as claimed in any of claims 1 to 45, wherein a leak path is provided between two module frames from internally of the frames on the input side to to Lit LΠ 5C .nt iε oεr is __rov icu oec* _εn tnε filtεr εmbranes. 57. Apparatus as claimed in claim 55, wherein the resilient εmbεr is corrugatεd. 58. Apparatus as claimεd in any of claims 4. to 57, wherein the modules arε in thε dead end, series arrangement and the last electrode module in thε series includes a valve to allow conti uous cr intermittent dumping of accum lated solid material. 59. Apparatus substantially as described herein with reference to the accompanying drawings. 50. A method of cleaning a filter using apparatus according to any of claims 33 to 59, the method comprising applying potεntiai across the eiεctrodεs. 51. A method of making a filter membrane comprising depositing material at an interface in a body comprising a first less electrical ly conductive filtεr layεr and a second more electrically conductive filtεr layer the interface being defined between the layers, thε first iayεr having a smaller pore size than the second layer, the material being more t > i in CD LΠ en wherein the surxace of tx.ε second i.i.εr xayer is textured by rolling with a texturεd roilεr. uu. -i iiic ncjci aS c i α i iiicu xn any υx c axliiS u i c ¬ ll / , wherein cne me oc is a method or maKinc * a iii cεr mentor ane as cxaimeo x Cxaim 32 whεn dεpendent on claim 9. 59 n ulc.iiOu _U. . -S — caxx c x . a. x1 1 x. y. a__ u -c -Ξ r ci x - X -J εo nεreii reierencε ~o m ε **; accoiiipax'y * i ng drawings. i n n £ - —-.— — _— . _ . — — . — __. _. — £ — _.-. ___. t x - i-l x x x ccx cc_ιιif x x--x xcj an cιc_S c x cam aui xac_e uiacx - 7 i . A filter as c_aimed in iάim ' C , wnεrem cne upstream surface comprises a xayer or Tι,u. particies. -xaimεi in any ox cxaiπs , .o o , wherein tne __iiιerencs in cons ctivity oεtween cne xayεrs is sun i icπ.iy s axx .nά * _ne siirracε oz cicc.roi .ic gas evoxu ion is ii i a ~nε iirs. ayεr.

Inventors: BRAY DAVID JAMES, DAVIDSON ALEXANDER PHILIP, BRAY, DAVID, JAMES, DAVIDSON, ALEXANDER, PHILIP
Applicants: BRAY DAVID JAMES, DAVIDSON ALEXANDER PHILIP, IMAS UK LTD, BRAY, DAVID, JAMES, DAVIDSON, ALEXANDER, PHILIP, IMAS (UK) LIMITED
Application year: 1995
Main classification: B01D71
IPC codes: B01D24, B01D35, B01D39, B01D61, B01D65, B01D71