CA1209057A

CA1209057A - Microporous filter system and process

Info

Publication number: CA1209057A
Application number: CA000427676A
Authority: CA
Inventors: David B. Pall; Abraham Krasnoff
Original assignee: Pall Corp
Current assignee: Pall Corp
Priority date: 1982-05-07
Filing date: 1983-05-06
Publication date: 1986-08-05
Also published as: JPS5926116A; JPS6362242B2; FR2526327A1; ES522148A0; PT76616A; EP0094226B1; GB8312115D0; GB2120116B; CH654758A5; GB2120116A; US4431545A; PT76616B; EP0094226A2; FR2526327B1; ES8406891A1; DE3378357D1; EP0094226A3

Abstract

Abstract:

A microporous filter system comprises two types of hydrophilic, microporous filter media operating in series. The two filter media have opposite zeta potentials with the upstream or first filter medium preferably having the positive zeta potential and the downstream or second filter having the negative zeta potential. The first filter medium typically has an absolute pore rating of from about 0.1 to about 1.0 micrometer and the second or downstream filter medium typically has an absolute pore rating of from about 0.02 to about 0.1 micrometer. The downstream or second filter has a finer absolute pore rating than the upstream or first filter. Fluids contaminated with ultrafine particles can be purified with an essentially absolute efficiency to remove 99.99 per-cent or more of the particulate matter in the con-taminated fluid. The filter system finds particular use in the preparation of ultrapure effluent water of near theoretical resistivity and in the removal of bacteria and endotoxins to form sterile fluids.

Description

5~

MICROPOROUS FILTER SYSTEM ~ND PROCESS

The present invention relates in one aspect to a process for the filtration of ~ contaminated fluid containing submicronic particulate matter and in a second aspect to a filter system, suitable in particular or carrying 5 ou~ the process.

To appreciate the significance of the subject invention, it is necessary that certain terms used herein be defined.
The.terms "ultra-filtration" and "ultrafilter" are used 10 herein to describe a filtration process and a filter respectively having the ability to remove particles as fine as about 0.001 micrometer up to abou~ lO micrometers (microns), a range of particle sizes commonly referred to as "ultrafine". Ultrafiltration media with very fine 15 pore sizes are recognised as useful for filtering ultra-fine particles from various liquid media. Unfortunately, ultrafilters in generaly have efficien~ies below 100 percent in the range of below 0.1 micrometer.

20 The term "efficien~y", as used herein, ~eans the ability of a filter medium to remove particulate con~aminant of a given type, that is, it is the percent of that particular type particulate contaminant which is unable tc pass through the filter. As use~ herein the term "essentially 25 absolute efficien~y" means the ability tv remove a parti-cular particulate contaminant at the 99.99 percent level or better. Correspondingly, the: term "substantially free" of a particular contaminant means that the level of the particular contaminant in the effluent from the filt-30 er system has been reduced by 99.99 percent of its influentconcentration and, in some cases, to substan~ially lower levels.

~2~ ii7 The function of a filter is the removal of suspended particulate m~terial ~nd the passage of the clarified fluid medium (filtra~e or effluent~. A filter can achieve fluid clarification by different mechanisms.
Particulate material can be removed through mechanical sieving, wherein all particles la~ger than the pores of the filter are removed~from the fluid, A filter may al50 remove suspended particulate material by adsorption onto the filter surfaces, that i~, the surfaces of the pores in the ~ilter. Removal of parti-culate material by this mechanism is controlled by the surface characteristics of (1) the suspended particulate matter, and (2~ the filter. Most suspended solids which are commonly subjected to removal by filtration are negatively charged in aqueous systems. This feature has long been recognized in wate~ treatment processes where cationic flocculating agents, oppositely charged ~o the suspended matter, are employed to improve settling efficiencies during water clarification.

Colloid stability theory can be used to predict the inter-actions of electrostatically charged part~clPs and filter surfaces. If the charges of the suspended particles and the filter surfaces are of like sign and with zeta pot-entials of greater than about 20 mV, mutual repulsive forces will be sufficiently strong to prevent capture by adsorption. If the zeta potentials of the suspended particles and the filter surfaces are small or, more desirably, of opposite sign, the particles will tend to adhere tv the filter surfaces with high capture efficien-cies. Most particles in the suspensions encountered in industrial practice have a negativP zeta potential. Thus~
microporous filters characterised by positi~e zeta potent-ials are capable, in a large num~er of industrial applic-ations, of rem~ving particles much smaller than the pore diameters of the filter through the mechanism of electro-static capture. As a result, the high pressure drops, reduced dirt capacity and shortened fi.lter life encount-5 ered with a filter operating strictly as a me~hanicalsieve can, to a large extent, be aYoided.
.

The problem to be solved by the present invention is to provide filter processes and,systems capable of enhanced 10 filtration efficiency over a broad pH range and with a wide variety of par~iculate contaminants, including ultra-fine particulates, particularly very fine, negatively-charged, particulates, ~ery fine, positiYely-charged, particles, and substantially neutrally or uncharged 15 particles. According to the present invention in one aspect there is proYided a process for filtration of a contaminated fluid containing submicronic matter, said process being characterised by the steps of passing said fluid sequentially through a first filter medium and then 20 through a second ilter medium, said first and second filter media having zeta potentials of opposite sign and the se~ond or downstream filter medium having a finer..
absolute pore ratin~ than the first or upstream fil~er medium, said process producing a filtrate su~stan~ially 25 free of (1) both electronegatively and electropositively charged particulate matter and (2) particulate matter greater in size than the absolute pore ~ating of said second filter medium.

30 According to the present:invention in another aspect there is provided a filter system comprising a first filter medium and a seccnd filter medium arranged in series, said media having zeta potentials of opposite sign and ~ Z~ ~3 the second medium having an absolute pore rating finer than that of the first medium.

~ process in accordance with the subject invention pro~ides 5 for the filtration of a contaminated fluid eomprising submicronic particulate matter which may include various contaminants hereinafter described,the process compris-ing:
(a) . passing the fluid through a first filter 10 medium comprised o a hydrophilic, microporous membPr having a positive zeta potential to remove electronegat-ively-charged particulate matter from said fluid; and (b) the passing ~he fluid (substantially free of electronegatively charged particulate matter2 15 through a second filter medium comprised of a hydrophilic, m~croporous member having a negative zeta potential and an absolute pore rating finer than that of the first filter medium to form a filtrate substantially free of (1) both electronegatively and electropositively charged 20 particulate matter and (2) particulate matter greater in size than the absolute pore rating of the second filter mediu~.

The order of the tw~ filter media, that is the positive 25 and negative zeta po~ential filter media, can be reversed.
However, whichever filter medium first sees or contacts the ~ontaminated fluid, ~he second filter medium in series must have the finer absolute pore rating.

30 rne first fil~er medium is preferably comprised of a sur-face-modified, hydrophilic , microporous polyamide membrane having positive zeta poten~ial and an absolute rating of from about 0.05 to ~bout 1.0, preferably rom about 0.1 ~Z~ 57 ~o about 0.5 micrometer.

The second filteF medium preferably is comprised of a hydrophilic, microporous polyamide memhrane having a 5 negative zeta poten~ial and an ahsolute pore rating of from about 0.01 to about 0.1, preferably from about 0.02 to about 0.06 micrometer.

The combination of these two preferred filter media, 10 either in the form o a composite filter sheet or as separate filter elements operating in series, provides an ultrafiltration system for the removal of positively and negatively charged particles down to essentially molecular dimensions at an essentially absolute efficiency 15 together with substantially complete removal, i.e., at the 99.g9 percent level or higher, of ~ltrafine particulate matter of substantially neutral or uncharged nature down ~o a size as small as a~out ~.01 micrometer.

20 The two-stage ultrafilter system thus provided finds particular use in the preparation of effluent water of near theoretical resistivi~y, i.e., gre~ter. than 14 megaohms/cm, after yery short onstream times.

25 As described aboYe, the subject invention is also directed to a filter system and a process fo~ using it. The , filter system is comprised of two microp~rous filter media with zeta potentials of opposite sign operating in series.
Preferably, the upst~eam filter medium, which first 30 contacts or sees the contam;nated fluid ~ontaining submic-ronic particulate suspended or dissolved material, has a positive zeta potential since the vast majority of cont-aminated fluids encountered in industrial applications ~ontain a large proportion of negatively charged partic~late 35 matter than positively ~harged. Howeyer, i~ eithe_ this preferred embodiment or the alternati~e, where the neg-~, ati~e zeta potential filter medium is ups~ream of thefilter medium ha~ing a posi~ive zeta potential, the downstream or second filter medium must have an absolu~e pore rating s~ erthan that of the upstream or fir~t 5 filter medium. In this mann~r7 ~he finer pored downstream or second filter medium has a longer filter life since it does not become clogged with the relatively large particles taken ou~ by the coarser upstream or first filter medium.
Fil~er Medium ~ith a Positi~e Zeta Potential:

To perform satisfactorily as the filter medium with a positive zeta poten~ial in the upstream positiont i.e., 15 as the first filter medium, the particular medium chosen fihould have the following characteristics:
(l) a positive zeta pote~tial under the conditions encountered during ~he filtering ope~ation;

(2) a microporous structure, typically with an 20 absolute pore rating in the range o~ from about 0.05 to about 5.0 micrometer, and

(3) be hydrophilic, i.e., readily wetted by water, which is visually observabl~ by the rapid spreading of a drop of water placed in contac~ with the filter 25 medium.

When a filter medium with a positive zeta potential is used as ~the downstream filter medium, it shoula have the same characteristics described in (l~ to (3) above except 30 that the absolute pore rating must be finer than the up-stream or first fil~er. Typically, then , the absolute pore rating will be reduced to be in the range of from about 0.01 to about 0.1 micrometer, preferably from about 0.02 to about 0.06 micrometer.

The preferre~ filter media having a positive zeta poten-tial, when used as either the first "coarser" filter medium or as the "finer" second filte~ medium are the surface modified, hydrophilic, microporous, polyamide 5 membranes which have the followi~g charac~eristics:

1. a positive zeta potential over the pH
range of from abo~t 3.to about 10;
2. an absolute pore rating of from about 0.01 to about l.0 micrometers; and 3. an essentially absolute efficiency foT
removal of negat;vely-ch~rged particulate matter down to molecular ~imensions.
These hydrophilic, surace (charge2 modified, microporous lS polyamide membranes. can be prepared as follows:

Surface (charge) modified, hydrophilic, microporous, polyamide membranes are prepared by the steps of (1) preparing a casting solution comprised of (A) a casting 20 resin system comprised of ~a) an alcohol-insoluble polyamide resin having a ratio CH2:NHC0 of me~hylene CH2 to amide NHC0 groups within the range of from about 5:1 to about 7:1, nylon 66 being a preferred polyamide resin, and (b) a membrane surface modifying polymer; and 25 (b) a solvent system in which the casting resin system is soluble, such as mixture of formic acid and water;
(2~ inducing nucleation of the casting solution by controlled addition of a nvnsolvent (such as water) for the casting resin system under controlled conditions 30 of concentration, temperature, addition ra~e and degree of agitation to obtain a visible precipitate of casting resin system particles which may or may not thereafter partially or completely redissolve, thereby forming a casting composition; .(3) preferably filtering the cast-35 ing composition to remoye visible precipitated particles;

(4) spreading the casting composition on a su~strate -- 8to form a thin film thereof on the sl1bstrate; (5) contacting ~nd diluting the film of casting composition with a liquid non-solYent system for the casting resin system comprised of a mix~ure of solvent (such as formic

5 acid) and non-solvent liquid (such as water) and con-taining a substantial proportion of the solvent liquid but less than the proportion in the casting compositisn, thereby precipitating the casting resin system from the casting composition in the form o a thin, skinless, 10 hydrophilic, surface modified, microporous membrane;
~6) washing the membrane; and (7) drying the membrane.

The resulting surface modified, microporous, alcohol-insoluble polyamide membranes are hydrophilic, i.e., 15 they are readily wetted by water. They have a~solute pore ratings of from about Q.01 up to about 10 micro-meters or more and modified zeta potentials, i.e., strongly positive zeta potentials, over the p~ range of from 3 to 10. The membranes of this type, when 20 used in this invention as the first filter medium in the upstream position, will typically have absolute pore ratings of from about 0.05 to about 1.0 micrometer, preferably from about 0.05 to about 0.2 micrometer.
When such membranes are used as the second or final 25 filter medium in the downstream position, they will typ-ically have absolute pore ratings of from about O.Ol to about 0.1 micrometer, preferably from about 0.02 to abou~ 0.06 micrometer.

30 The membrane surface modifying polymers or resins useful in preparing these membranes are the cationic, water-soluble, quaternary ammonium, ~hermosetting polymers.
Preferred polymers within this class are the epoxy-functional polyamido/polyamino-epichlorohydrin resins.
35 The epoxy-functional polyamine-epichlorohydrin resins are particularly preferred.

It is also desirable with some resins that ~n ion exchange be carried out to render the membr~ne less suscep~ible 5 to undesirable ~hifts in the pH of filtrate water flow-ing through the.treated membranes. ~dditionally, in certain applications where an ultrapure w~ter is required, such as electronics manufacture, the filter membrane may be sujbected to a water treatment. This treatment 10 comprises fl~wing very pure water through the filter membrane until the fil~rate downstream of the filter membrane has the de~ired level of purity.

Other materials may be used as the filter medium with 15 a positive zeta potential provided they meet the criter-ion set out above, namely a positive zeta potential under the conditions encountered in the filtering process, a microporous structure with appropriate absol-ute pore ratings, typically in the ran~e of from about 20 0.05 to about 1.0 micrometer when used as the upstream or first filter and from about 0.01 to about 0.1 micro~
meter in the downstream or second filter, and hydrophilic-ity. A conventional ultrafilter which is typically a ski-nned membrane is undesirable for this purpose for the 25 reasons described above.

Other filter media with a positive zeta potent;al satisf-act~ry for use as the first filter medium include hydro-philic, polymeric, microfibrous filter sheets. These 30 types of filter media can be prepared as follows:

- 1~
~%~
Method Of Preparation of Hydrophilic, Microfibrous, Polymeric Filter Sheets:

The general method of preparing hydrophilic, microfibrous, polymeric filter sheets comprises four steps:
(1) applying a first solution or dispersion of a precipitating agent to a hydrophobic web comprised of polymeric microfibers to at least partially wet the web with this first solution;
~ 2) applying a second solution of a water-soluble, non-colloidal, cationic, thermosetting binder resin or polymer eo the wetted web ~f step (1) above to form a web wetted with a mixture of the first solution or dispersion and the second solution;
(3) working the wetted web of st~p (~) above to mix the first solutisn or dispersion, thereby facili-tating the precipitation of the binder resin or poly-mer and the distribu~ion in a uniform manner of the 2~ precipitated binder resin or polymer as a coa~ing on the surfaces of the microfibers making up:the worked web; and (4~ drying the coated web of step ~3) above and curing the precipitated binder resin or~polymer coat-ing to provide a hydrophilic, microfibrous, polymeric filter sheet with a positive zeta potential and which is further characteri~ed by the surfaces of the micro-fibers therein being coated with a cured, precipitated, thermoset, cationic binder resin or polymer.
Variations in the four basic process steps outlined above, as well as certain additional proces-sing steps, may be utilized in practicing the pcocess of this inv~ntion. For example, steps (1) and (2) above can be reversed, albeit the preferred order of application is as set out above. Additionally, ie ~2~ ;7 may be desirable in certain instan~es to use prewet-ting solutions containing a wetting agent such as a surfactant, or a lower alcohol in aqueous solution to prewet the hydrophobic webs, followed by water wash-ing ~o remove at least the major portion of the wet-ting agent from the web, preferably as completely as possible, while main~aining the web in a water wetted form, and ~hen applying the first and second treating solutlons a~ descrlbed above or ln rever~e order.
(Herein, the terms "solution~ or "treating solution~
are sometimes used in describing processing under steps ~1) and (2) above.) It should be understood that when the precipitating agent containing composi-tion is being referred to it may be present as either a solution or a dispersion.
Additionally, it may be desirable, with some carboxylate precipitating agents, to convert Some of the carboxylic acid groups therein to their salt form by neutralization with inorganic bases, e.g., sodium hydroxide, or organic bases, e.g., diethanolamine or triethanol amine. This treatment improves the solu-bility of the precipitating agent and, in some in-stances, improves the wetting characteristics of the solution or dispersion of the precipitating agent in th& treatment of the hydrophobic web, thereby permit-ting the deletion, in some cases, of the prewetting steps referred to above. When the binder resin or polym~r i5 used as the first solution, i.e., when it is applied to the web first in step (1) above, simi-lar materials may be desirable for the same reasons,particularly for improving the wetting characteris-tics of the binder resin or polymer solution. Indeed, for some webs with a lesser degree of hydrophobicity, the prewetting step can be avoided by the use of diethanolamine or a similar material as a component i7 of the first solution applied.
Preferably, the hydrophobic web is fully wet-ted, i.e., saturated, in step ~1? above, i.e., with the first solution added, whether that be a solution of the precipitating agent or the solu~ion of the binder resin or polymer. Prior to the application of the second solution to the web, any excess of the first solution may be removed, e.g., by mechanical wiping using a wiper blade or the like, padding, etcetera. Preferably, prior to the application of the second solution to the web, a sufficient portion of the first solution is removed so that the web is not fully wetted, i.e., saturated, with the first solution when the second solution is applied.
After the second solution has been applied and the web preferably fully wetted with a mi~ture of the first and second solutions, it is necessary to work the wetted web ~o mix the first solution and second solution, thereby facilitating the precipitation of the binder resin or polymer and the distribution thereof as a coating on the surfaces of the micro-fibers making up the worked web. This working can be carried out by a variety of techniques, including mechanical agitation, the action of tensioned wiper blades or subjecting the web to pressure between two rollers or a roller and a flat surface.
The filter sheets have positive zeta potentials over the p~ range of from 3 to 10, and when used as the first filter medium in the sub-ject invention typically have absolute pore ratings in the range of from about 0.5 to about 1.0 micro meter or higher. Typically they have rinse up times to produce ultrapure water of resistivity greater than 14 megaohms/cm in less than 10 minutes.
Preferred base web materials for preparing the hydrophilic, microfibrous, polymeric filter sheets of this type are hydrophobic, polymeric webs comprised of microflbers of polyolefins, polyesters or polyamides, including polypropylene, polyethylene, polybutylene terephthalate, polyethylene terephthalate, nylon 66, nylon 6, nylon 610 and nylon 11. Preferred binder resins or polymers for use in preparing these microfibrous, polymeric filter sheets are the epo~y-based, water-301uble reslns, such aq the epoxy-functional polyamido/polyamino-epichlorohydrin resins.
Particularly preferred are the epoxy-functional poly-amine/epichlorohydrins containing quaternary ammonium groups. Preferred precipitating agents may be selected from a group of synthetic, water-soluble or dispersible polymers containing carboxylate groups, such as acrylic acid resins.

Filter Medium With A Negative Zeta Potential:

To perform satisfactorily as the filter medium with a negative zeta potential in the downstream position, i.e., as the second filter medium, the particular medium chosen should have the following characteristics:
(1) a negative zeta potential under the condi-tions encountered in the filtering operation;
(2) a microporous structure, typically with an absolute pore rating of from about 0.01 to about 0.1 micrometer, and in all cases finer than that of the upstream or first filter medium; and (3) be hydrophilic.
When a filter medium with a negative zeta po-tential is used as the upstream filter medium, it should have the same characteristics described in (1) to (3) above except that the absolute pore rating ~Z~3S~5~

must be less fine than ~hat of the downstream or second filter, Typically9 then, the absolute pDre rating will be increased to the range of from about 0.05 to about 1.0 micrometer, typically from ~bout 0.1 to about 0.5 micrometer.

The skinless, hydrophilic, microporous, polyamide mem-branes of U.S. Patent Specification 4,340,&79, describes a preferred class of filter media meeting the criteria set forth.above.

Basically, the hydrophilic, microporous, polyamide filter membranes disclosed in U,S. Patent Specification No., ~,34Q,479 are membranes prepared from alcohol-insoluble polyamide resins haying a methylene to amide ratio in the range of about 5:1 to about 7:1. Membranes of this group include copolymers of hexamethylene diamine and adipic acid (nylon 66), copolymers of hexamethylene diamine and sebacic acid (nylon 610~, homopolymers of poly-e-caprolactam (nylon 62 and copolymers of hexamethylene diamine ~nd azelaic acid ~nylon 69).
Nylon 66 is preferred. Hydrophilic, microporous, polyamide membranes (nylon 66) of this type having absolute pore ratlngs from about 0.02 to 8 micrometer.
or greater are aYailable from Pall Corporation under the trademark Ultipor N66. These untre~ted membranes have negatiye zeta potentials in alkaline media, that is from about pH 6.5 and up.

2~ ~ ~5 When the filter medium with a negative zeta potential is used as the first filter medium or the upstream prefi.lter, the hydrophilic polyamide membranes of U.S. Patent Specification 4,340,479 are still preferred.
5 However, membrane ~aterial with absolute pore ratings typically in the range of from about 0.05 to 1.0 micro-meter, preferably from about 0.05 to about 0.2 micrometer, are preferably used to reduce the tendency for the first filter medium to clog.
The membranes of U.S. Patent Specification 4,340,479 have a negative zeta potential at about pH 6.5 and above, making them useful as the negative zeta potential filter medium under m~st normally encountered operating 15 conditions.

A class o surface controlled microporous, hydrophilic polyamide membranes which maintains their negative zeta potential over the broad pH range of from 3 to 10 20 are prepared as follows:

Surface modified, hydrophilic, microporous polyamide membranes with negative zeta potentials DY~r the pH
range of from 3 to 10 are prepared ~y the steps of Sl) 25 preparing a casting solution comprised of (A) casting resin system comprised of (a) an alcohol-insoluble polyamide resin having a ratio CH2:NHCO of methylene CH2 ~o amide NHC0 groups within the range from about 5:1 to about 7:1, nylon 66 being a preferred polyamide resin, 30 and (b) a water-soluble, membrane sur~ace modifying polymer having functional polar groups, such as carboxyl and sulfonic, and a molècular weight of lQ,000 or greater; ~nd ~) a solvent system (such as formic acid and water) in ~2~$~5~

which the casting resin system is soluble; 12) in-ducing nucleation of the casting solution by con-trolled addition of a nonsolvent ~such as water) for the casting resin system under controlled conditions of concentratlon, temperature, addition rnte and -1~ degree of agitation to obtain a visible precipitate : of casting resin system particles which may or may not thereafter partially or completely redissolve, thereby forming a casting composition; ~3) preferably filtering the casting composition to remove visible precipitated particles; (4) spreading the casting composition on a substrate to form a thin film there-of on the substrate; ~5) contacting and diluting th~
film of casting composition with a liquid nonsolvent system comprised of a mixture of solvent (formic acid) and nonsolvent (water) liquids and containing a substantial porportion of the solvent liquid (formic acid) but less than the proportion in the casti~g composition, thereby precipitating the casting resin ~ system from the casting composition in the form of a thin, skinless, hydrophilic, surface modified, micro-porous membrane; (6) washing the membrane to remove solvent; and (7) drying the membrane.
The resulting surface modified, alcohol-insol-uble polyamide membranes are hydrophilic, have abso-30 lute pore ratings of from about 0.01 to about 10 micrometers or more, and have negative zeta poten-tials over the pH range o~ from 3 to 10.
The membrane surface modifying polymers or resins useful in preparing membranes of this type are water-soluble polymers with molecular weights of 5~
10,000 or greater, preferably 20,000 or greater, such as carboxyl-containing polymers, such as polymers of acrylic acid, and sulfonic-containing compositions, such as a homopolymer of s~yrene sulfonic acid.
When negative zeta potential membranes of this r type are used as the second ~ilter medlum in the downstream position, they will typically have abso-lute pore ratings of from about O.Dl to about 0.1 micrometer, preferably from about 0.02 to about 0.06 micrometer. When used as the first filter medium in the upstream position, they will typically have abso-lute pore ratings of from about 0.05 to about 1.0 micrometer, preferably from about 0.05 to about 0.2 micrometer.

~ . .
The filter systems of the subject invention operate in a series mode. That is, the fluid medium contaminated with submicronic particulate matter is passed through the first filter medium (the prefilter which removes larger particles by a sieve mechanism as well as either electronegatively or electroposi-tively charged particles by adsorption).The fluid from the first filter medium (which is now substan-tially free of fine particulate matter having acharge opposite to the zeta potential of the first filter medium) is then passed through the second filter medium (also referred to as the final filter) which removes the remaining electrically charged particles of opposite sign from those removed on the first filter and, by a sieve mechanism, removes un-charged or neutral particulates. The second filter medium operates as a last chance or final filter removing any particulate matter larger than the abso-~Z~$~ 5~
lute pore rating of the final filter.
The form that the serially operating filter system takes may vary. For example, a composite filter sheet comprised of a firse and second filter S medium may be formed and used as a flat, planar sheet. Alternatively, the composite sheet may be formed into a pleated or accordion form and used in a conventional element such as a cartridge. As another aiternative, the fir~t and second filter media can be formed as separate sheets which can independently be formed into elements and incorporated into separate cartridges of the type conventional in the industry and then used in a series arrangement.
As will be evident from the following examples, the filter system of the subject invention provides an economical means for enhanced removal of fine particulate contaminants from fluid media, particu-larly particulates in the ultrafine region, at essen-tially absolute efficiencies, i.e., 99.99 percent or higher, and in many instances at substantially higher levels. Additionally, the subject invention provides a novel and economic way for procesqins ultrapure `~ water approaching theoretical resistivity, i.e., free from contamination from dissolved or suspended mater-ial, such as for use in electronics manufacture and other applications requiring pure water free from particulates and ionic impurities. It should also be recognized that the filter system of this invention ran be used downstream of a coarse prefilter which removes relatively coarse particulate matter, e.g., on the order of 1 to 30 micrometers or greater.
~y removing coarse or gross particulate matter prior to contacting the contaminated fluid with the filter system of this invention, the life of the subject ~24~r~$7 filter system will be extended.

Method of Tes~ing the Filter System of the Followin~ Examples:

The proper~ies of the filter sys~ems of the following examples were evaluated by a variety of test methods as described below:

(a) Ze~a Potenti_ :

Zeta potentials are calculated from measurements of the streaming potentials generated by flow of a 0.001 weight percent solution of KCl in distilled water through several layers of the filter mem~rane secured in a filter sheet holder. Zeta potential is a measure of the net immobile electrostatic charge on a membrane surface exposed to a fluid. It is related to the streaming p~tential generated when that fluid flows through the filter sheet by the following formula (J.T. Davis et al), Interfacial Phenomena, ~cademic Press, New York, 1963):

Zeta Potential ~mV~ . E.c D P
wherein ~ is the Yiscosity of the flowing solution, D
is the dielectric cons~ant of the s~lution, ~ is its conductivity, Es is the streaming potential, and P is the pressure drop in pounds per square inch across the fil~er sheet ~uring ~he period of flow. In the follows ing examples, the quantity 4~ ~ is constant, having the value 2.052 x 10-2, or, --when converted to Kglm2 the quantity must be multiplied by the conversion factor 703.1, so that the ze.~ potential can ~e expressed:

Zeta Potential ~mV) = 14.43. E~ (Volt~ mho/cm) (b Latex Particle Removal:

Mondisperse suspensions of polystyrene latex with well-characterised particle sizes (available ~rom Do~- Dia~n-ostics Inc.) were prepar2d in approximate 0.1 percent by weight solutions in deionized water containing 0.1 10 percent Triton X-100 (an adduct of nonyl phenol with about 10 moles of ethylene oxide). Latex suspensions were pumped through the filter systems positionPd in a disc holder 47 millimeters in diameter and having an effective filtration area of 9.29 cm2 using a Sage 15 Instrument Model 341 syringe p~mp at a rate of 2 mill-iliters per minute. The effluent was passed through an optical flow cell in a light scattering photometer (Model 2000D, available from Phoenix Pre~ision Instrument Inc.). The scattering signal from a beam of 537 nm 20 light, measured at 90 degrees, was converted to latex bead concentra~ion by means of an empirically determined concPntration-scattering intensity correlation for each latex size. Late~ bead capacities were derived from measured efficiencies and total ~olume of latex bead 25 challenge by the following formula:

concentration of input_ (0.1%~
concentration o e uent % removal e~fici~ncy F ~ X 100 ~.2~57 (e) Resistivity Test:

The effluent water from the filter systems of the exam-ples was monitored for resitivity with a Model 3418 conductivity cell (Yellow Springs Instru~ent Company).
The conductivity cell was connected to ~ Model 31 conduc-tivity bridge (Yellow Springs Instrument Company) which allowed the direct measuremen~ of effluentresistivity.

EXAMPLE 1.
(A) A skinless, surface modified, hydrophilic, micro-porou~ polyamide (nylon 66~ membrane with a positive zeta potential under the conditions encountered ;n this example and an absolute pore rating of ~bout 0.1 micro-meter was conyerted to a pleated filter cartridge with a membrane area of about 0.84 ~ 2 (cartridge 1).
In like ~anner, a skinless, hydrophilic, microporous polyamide (nylon 66) membrane with a negative zeta potential under the conditions encountPred in this example and an absolute pore rating of about ~.04 micrometer was converted to a pleated filter cartridge with a mem-brane area of about 0.84 m2 (cartridge 2~, Industrial plan~ water, containing native pseudomonas-type bacteria in concentrations varying from 100 organisms per liter to greater than 1000 organisms per liter, was passed serially through cartridge 1 and then through cartridge 2 at a constant flow rate of about 7.6 litres per minute.
The filtrate water delivered by this filter system was periodically monitored for the presence of bacteria by standard microbiological procedures and found to be bacterially sterile for a period of 53 days, ~fter which time the test was discontinued. These results indicate that the filter system of Example l (A) functi~ns as an absolute bacter~ filter ~o provide a bacteria-free (sterile) filtrate water.

i~ 7 (B) A skinless, surface modified, hydroph;lic, microporous polyamide (nylon 66) membrane with a positive zeta potential under the conditions encoun-tered in this example and an absolute pore rating of about 0.1 micrometer (membrane A) and a skinless, hydrophilic, microporous polyamide ~nylon 66) mem-brane with a negative ~eta potential under the condi-tions encountered in this example and an absolute pore rating o~ abou~ 0.04 mlcrom~ter (mqmbrane a) were assembled into a composite layered membrane system and secured in a conventional membrane holder with membrane A mounted upstream of membrane B. The membrane system was then challenged with an aqueous suspension of latex spheres with a mean diameter of 0.038 micrometerA A latex removal efficiency greater than 99.99 percent was measured at a total latex sphere challenge level of 0.1 gram per 929 membrane surface.
(C) A skinless, surface modified, hydrophilic, microporous polyamide membrane with a positive zeta potential under the conditions encountered in this example and an absolute pore rating of about 0.1 micrometer was converted into a filter element with a membrane area of about 0.84 n~ . (element A). In like manner, a skinless, hydrophilic, microporous polyamide membrane with a negative ~eta potential under the conditions encountered in this example and an absolute pore rating of about 0.04 micrometer was converted into a second element with a membrane area oi about 0.84 m2 (element B). The same poly-amide membrane was used to prepare the filter car-tridge 1 of (A) above, the membrane A of (B) above and element A of (C). Similarly, the same polyamide membrane was used to prepare the filter cartridge 2 of (A) above, the membrane B of (B) above and element ~ of ~C).
The two elements were then employed as ~ filter system operating in series with element A preceding or upstream of element ~. Electronlcs grade water of resistivity greater than 14 megaohms/cm was f lowed through the filter system at a flow rate of about 1.6 litres per minute. After 7 mlnutes of onstream time, the reslstivity o~ the effluent was measured to be ~reater than 14 me~aohms per centlmeter, as re-qulred ~or electonics process appllcatlon.
(D) A sklnless, surface modifled, hydrophillc, microporous polyamide ~nylon 6S) membrane wlth a positive zeta potential under the conditions encoun-tered iD this example and an absolute pore rating ofabout O.l micrometer (membrane A) and a skinless, hydrophilic, mlcroporous polyamide (nylon 66) mem-brane with a negatlve zeta potential under the condi-tions encountered 1~ this example and an absolute 2G pore rating of about 0.04 micrometer tmembrane B~
were assembled lnto a composite layered membrane system and secured in a conventional mem`orane holder with membrane A mounted upstream of membrane ~. The membrane system was then challenged wlth an aqueous ~5 suspension of mycoplasma (acholeplasma laidlawii, ATCC 2320) to a total challenge level of 1. B x 10 organisms per g~9 cm2 membrane area. ~nalysis of the effluent from the filter system by standard microbiological procedures demonstrated that the effluent was free from mycoplasma and hence the filter system operated with a removal efficiency in excess of 99.9999999994 perce~t.
It is not uncommon for water supplies to contain 104 to 106 bacteria per liter, and for a filter cart-ridge rated at 10 liters/minute to be on stream for ~Z~ 57 lO,000 hours. Thus, such a filter may have lncidenton lt asnany as 6 x 1011 bacteria durlng its llfetime.
The efficiency of such a filter must therefore be in excess of s (1 ~ 6 x loll) X lO0 = 99.9999999~98x In order to avoid the use of so many numera~s, this same requirement can be concisely expressed by stating that the titre reduction (TR) which ls the ratio of influent to effluent concentratlon must exceed 6 X 1011, and efficiency for any given TR
can be calculated from Efficiency, X, = (l ~ Tl ) X lO0 Conventional ultrafilters operate typically in the TR range of 103 to 107, and thus a lO liter per minute ultrafilter could pass lO,00~ or more bacterla during a l~,000 hour service period.
The results as set out in the above example establish the filter systems of the sub~ect inven-tion are capable of (l~ sterilizlng filtrate water by complete removal of incident bacteria, l.e., 100 percent efficiency at high capacities, l2) capable of efficiently removing very fine pariiculate material at high efficiencies (99.99 percent) and at high load-ings (0.1 gram per ~29 ~m2 ) and (3) capable of delivering water of near theoretical resistivity, i.e., greater than 14 megaohms/cm resistivity, after short onstream time. This filter system, then, provides high purity water free from bacterial contamination, particulate and ionic contaminants and therefore is partic~larly desirable for electronlc filtration applications. When these capabilities are - ~20~¢5~

combined with high flow rates at relatively low pressure drops, e.g., 1.4 Kg/cm2 or less, ~is-a-vis convention-al skinned membranes oper~ting at pressures in the neighbourhood of 2.8 Kg/cm2 coupled with the inability 5 to provide bacterially sterile filtrates and having limited loading capacities, the desirability of the subject invention is manifest.

i7 A first filter system comprised of a composite of (1) a first or upstream skinless, hydrophilic, microporous, nylon 66 membrane having a negative zeta potential under the conditions encountered in this example and an absolute pore rating of about 0.1 micrometer and ~2) a second or downstream skinless, hydrophllic, microporous nylon 66 membrane also hav-ing a negative zeta potential under the conditionsencountered in this example but an absolute pore rating of about 0.04 micrometer was prepared (filter system I).
In like manner, a second filter system was prepared of first and second hydrophilic, micropor-ous nylon 66 membranes having the same respective pore ratings as described for th~ first and second membranes of filter sy-stem I above but with the f irst or upstream hydrophilic nylon 66 membrane being sur-face modified and having a positive zeta potential~filter sys*em II).
Filter system I and filter system II were each `~ challenged independently with a solution of 0.038 micrometer latex beads in a water suspension ~con-centration of the latex beads in the water was 0.01 weight percent).
The capacity of the two filter systems for 0.038 micrometer latex while operating at an effici-ency of 99.995 pe~cent was determined with the results set out below:
~1) filter system I had a capacity of 0.03 qrams per 92q:c~2 of filter surface when chal-lenged at a rate of 200 milliliters of the dispersion ~per 92g c~ per minute;
~2) filter system II had a capacity of 0.11 .

ji7 grams per 929 cm2 when challenged with the latex bead suspension at a rate of 200 milliliters per 929 cm~ of filter surface per minute.
These results show a nearly ~our-fold increase in capacity when operating at this high efficiency for the filter system of the subject invention com-bining a positive zeta potential ~irst filter with a downstream finer pored negative zeta potential fil-ter when compared with filter system I.

Two cartridge elements having pleated filter membranes, each with about 0.84-m~ of filter surface area and with the characteristics set out below, were mounted in series relationship. The first element contained a surface modified, hydro-philic, microporous nylon 66 membrane having a posi-tive ~eta potential and an absolute pore rating of 0.1 micrometer. The second element contained a hydrophilic, microporous, nylon 66 membrane having a negative zeta potential and an absolute pore rating of D.04 micrometer.
An influent stream of ultrapure water with a resistivity of 18 megaohms per centimeter was fil-tered through the two element fil~er system described above, flowing in series through the first element and then through the second element, at a constant flow rate of 7.6 Ii~res per minute.
Within 15 minutes the effluent water from the two stage filter system had a resistivity of about 18 megaohms per centimeter indicating that the filter was quickly purged of any contaminants and was then capable of operation at a high purity level. After about 30 minutes of onstream time, the influent water ~2~5i7 to the ~wo stage filter system was contaminated with a low level of tap water, reducing the influent water purity and lowering its resistivity to a constant level of about 12 megaohms per centimeter. Under these conditions, the resistivity of the effluent water from the two stage filter system dropped for a brief period and then recovered in less than 1 minute to 14 megaohms per centimeter and within about 5 minutes had risen to abou~ 1~ megaohm~ per centl- ¦
meter, all while the influent water resistivity re-mained at 12 megaohms. The system was run for about an additional 5 minutes before being shut down and, over that time span, the resistivity of the effluent water remained at 14 megaohms per centimeter or bet-ter.

This example demonstrates that a filter systemof this invention when operating as a last chance or final filter -has the ability to control upsets in the purity of water prepared in an ultrapure water filtra-tion system, upsets which can occur frequently due to the very low level of impurities necessary to cause `~ an upset. This ability is particularly important in systems conventionally used to prepare deioni~ed water where a mixed ionic bed of ion exchange parti-cles is used to ensure the removal of both positive and negative contaminants. In such a case, the par-ticulate matter needed to be removed in a last chance or fir.al filter may be either positive or negative.
The subject filter system removes both positive and negative particles in a very efficient manner when such an upset occurs.
Other tests of filter systems of the subject invention have demonstrated the ability to remove from aqueous solutions ~1) dextrans In the molecular weight range of from 2 x 106 to S x 106 Daltons, (2) an uncharged endotoxin molecule of molec~lar weight of about 30,000 Daltons with efficiencies greater than 99.998 percent and t3) 0.021 micrometer silica particles and 0.038 micrometer latex beads _ at efficiencies greater than 99.99 percent.
When filter systems of this invention are used to treat water for use in microel~ctronlcs manufacture and the like where a resistlvlty of greater than 14 10 megaohms/cm i9 requlred, the surface modified filter d media;used in preparing the filter systems of this invention are flushed with an aqueous ammonium hydroxide solutions, e.g., a 0.2 molar solution, to convert qUaterDary ammonium groups to the hydroxide 15 form. This can be carried out in any convenient manner:~for example, after formation into element form as was d~ne in Examples 1 tC) and 3.

~Q~

Industrial Applicability:

The essentially absolute efficien~y of the filter system o this inyention in removing ultrafine partic-5 ulates, including both electr~positively and electroneg-atively charged particles, the ab~lity to remove bact-eria at an absolute level providing a ~acterially free, sterile effluent, and the ability to pro~ide ultrapure water of near theoretical resisti~ity i.e., greater 10 than 14 megaohms/cm, after short onstream times, and the ability to deliver water with increased resistivity and hence greater purity compared with the influent water, ha~e been demonstrated. Because of these characteristics of the filter systems of this invention and coupled 15 with their ability to be both manufactured and operated in an economi~al manner, the filter systems of this invention find use in industry and the medical field to treat water supplies for critical application such as water for injection into humans, in microelectronics 20 manufac~ure, in filtration of blood serum to help achieve sterility, for filtration of parenterals and generally for any use where an ionizing liquid is to be filtered to a high degree of clarity and purity.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for filtration of a contaminated fluid containing submicronic particulate matter, said process being characterised by the steps of passing said fluid sequentially through a first filter medium and then through a second filter medium, said first and second filter media having zeta potentials of opposite sign and the second or downstream filter medium having a finer absolute pore rating than the first or upstream filter medium, said process producing a filtrate substantially free of (1) both electronegatively and electropositively charged particulate matter and (2) particulate matter greater in size than the absolute pore rating of said second filter medium.

2. A process according to claim 1 characterised in that said first filter medium comprises a surface-modified, hydrophilic, microporous, polyamide membrane and said second filter medium comprises a hydrophilic microporous, polyamide membrane.

3. A process according to claim 1 or claim 2, charact-erised in that said fluid is water and said filtrate is ultrapure water substantially free of bacteria and having a resistivity greater than 14 megaohms/cm.

4. A filter system comprising a first filter medium and a second filter medium arranged in series, said media having zeta potentials of opposite sign and the second medium having an absolute pure rating finer than that of the first medium.

5. A system according to claim 4, characterised in that the first filter medium comprises a hydrophilic, microporous member having a positive zeta potential and the second filter medium comprises a hydrophilic, microporous member having a negative zeta potential.

6. A system according to claim 4 characterised in that said first filter medium comprises a surface-modified, hydrophilic, microporous, polyamide membrane and said second filter medium comprises a hydrophilic, microporous, polyamide membrane.

7. A system according to claim 6, characterised in that said second filter medium is a surface-modified membrane.

8. A system according to claim 4 characterised in that said first filter medium comprises a first surface-modified, hydrophilic, microporous, polyamide membrane and said second filter medium comprises a second surface-modified, hydrophilic, microporous, polyamide membrane.

9. A system according to claim 6, claim 7, or claim 8, characterised in that both said first polyamide membrane and said second polyamide membrane are comprised of nylon 66.

10. A system according to claim 4, 5 or 6 characterised in that said first filter medium has an absolute pore rating of from 0.05 to 1.0 micrometer and said second filter medium has an absolute pore rating in the range of from 0.01 to 0.1 micrometer.

11. A system according to claim 4 characterised in that said first filter medium comprises a hydrophilic, microfibrous, polymeric web, the microfiber surfaces of which are coated with a cured, precipitated, thermoset, cationic binder resin or polymer.

12. A system according to claim 11, characterised in that said polymeric web is of polyethylene, polypropylene, polybutylene terephthalate, polyethylene terephthalate, nylon 66, nylon 6, nylon 610 or nylon 11.

13. A system according to claim 4 characterised in that said first filter medium comprises a surface-modified, hydrophilic, microporous member containing quaternary ammonium groups in the hydroxide form and having a positive zeta potential and said second filter medium comprises a hydrophilic, microporous member having a negative zeta potential and an absolute pore rating finer than that of said first filter medium.

14. A system according to claim 13, characterised in that said first filter medium has been contacted with an aqueous solution of ammonium hydroxide to convert said quaternary ammonium group to the hydroxide form.

15. A system according to claim 4, 5 or 6, characterised in that the filter media are in the form of a composite sheet.

16. A system for the filtration of a contaminated fluid containing submicronic particulate matter, said system comprising:

(a) a first filter medium comprising a surface-mod-ified, hydrophilic, skinless, microporous, alcohol-insoluble polyamide membrane derived from an alcohol-insoluble hydrophobic, polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range of from 5:1 to 7:1, said membrane having (i) an absolute pore rating in the range of from 0.05 to 1.0 micrometer and (ii) a positive zeta potential, to remove electronegatively charged particulate matter from said fluid;
and (b) a second filter medium comprised of a hydrophilic skinless, microporous, alcohol-insoluble polyamide mem-brance derived from an alcohol-insoluble hydrophobic, polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within a range of from 5:1 to 7:1, said membrane having (i) an absolute pore rating finer than that of said first filter membrane and in the range of from 0.01 to 0.1 micrometer and (ii) a negative zeta potential, said system serving to form a filtrate sub-stantially free of (1) both electronegatively and electro-postively charged particulate matter, (2) bacteria and endotoxins, and (3) particulate matter greater in size than the absolute pore rating of said second filter medium.

17. A process for the filtration of a contaminated fluid comprising ultrafine particulate material with particle sizes in the range of from about 0.001 to about 10 micro-meters said process comprising: (a) passing said fluid through a first filter medium comprised of a surface modi-fied, hydrophilic, microporous member, said first filter medium further characterized by (i) an absolute pore rating in the range of from about 0.05 to about 1.0 micrometer and (ii) a positive zeta potential, to remove electronegatively charged particulate matter from said fluid; and (b) then passing said fluid through a second filter medium comprised of a hydrophilic, microporous member, said second filter medium further characterized by (i) an absolute pore rating finer than that of said first filter membrane and in the range of from about 0.01 to about 0.1 micrometer and (ii) a negative zeta potential, to form a filtrate substantially free of (1) both electronegatively and electropositively charged particulate matter, (2) bacteria and endotoxins, and (3) particulate matter greater in size than the absolute pore rating of said second filter medium.

18. The process of claim 17, wherein both said first filter medium and said second filter medium are com-prised of nylon 66.

19. The process of claim 18, wherein said second filter medium has an absolute pore rating of from about 0.02 to about 0.06 micrometer.

20. The process of claim 18, wherein said contamina-ted fluid is water.

21. The process of claim 18, wherein said filtrate is comprised of ultrapure water having an effluent resis-tivity greater than 14 megaohms/cm.

22. A process for the filtration of a contaminated fluid comprising ultrafine particulate matter with particle sizes in the range of from about 0.001 to about 10 micro-meters, said process comprising: (a) passing said fluid through a first filter medium comprised of a hydrophilic, microporous member said first filter medium further charac-terized by (i) an absolute pore rating in the range of from about 0.05 to about 1.0 micrometer and (ii) a negative zeta potential, to remove electropositively charger particulate matter from said fluid; and (b) then passing said fluid through a second filter medium comprised of a surface modi-fied, hyrophilic, microporous member said second filter medium further characteized by (i) an absolute pore rating finer than that of said first filter membrane and in the range of from about 0.01 to about 0.1 micrometer and (ii) a positive zeta potential, to form a filtrate substantially free of (1) both electronegatively and electropositively charged particulate matter, (2) bacteria and endotoxins, and (3) particulate matter greater in size than the absolute pore rating of said second filter medium.

23. The process of claim 22, wherein both said first filter medium and said second filter medium are com-prised of nylon 66.

24. The process of claim 23, wherein said second filter medium has an absolute pore rating of from about 0.02 to about 0.06 micrometer.

25. The process of claim 23, wherein said con-taminated fluid is water.

26. The process of claim 23, wherein said second filtrate is comprised of ultrapure water having a effluent resistivity greater than 14 megaohms/cm.

27. A filter system comprising, in combination, a first filter medium comprised of a surface modified, hydro-philic, microporous member having a positive zeta potential and an absolute pore rating in the range of from about 0.05 to about 1.0 micrometer and a second filter medium comprised of a hydrophilic, microporous member having a negative zeta potential and an absolute pore rating in the range of from about 0.01 to about 0.1 micrometer and finer than that of said first filter medium.

28. The filter system of claim 27, wherein both said first filter medium and said second filter medium are comprised of nylon 66.

29. The filter system of claim 28, wherein said filter system is formed into a filter element.

30. The filter system of claim 28, wherein said filter system comprises a pleated filter element in cart-ridge form.

31. The filter system of claim 28, wherein said system is capable of providing a filtrate of ultrapure water having a resistivity greater than 14 megaohms/cm. after a rinse up time of less than 10 minutes.

32. A filter system comprising, in combination, a first filter medium comprised of a hydrophilic, microporous polyamide membrane having a negative zeta potential and an absolute pore rating in the range of from about 0.05 to about 1.0 micrometer and a second filter medium comprised of a surface modified, hydrophilic, microporous polyamide membrane having a positive zeta potential and an absolute pore rating in the range of from about 0.01 to about 0.1 micrometer and finer than that of said first filter medium.

33. The filter system of claim 32, wherein said first filter medium and said second filter medium are com-prised of nylon 66.

34. A filter system comprising, in combination, a first filter medium comprised of a surface modified, hydrophilic, microporous polyamide membrane containing quaternary ammonium groups in the hydroxide form and having a positive zeta potential and an absolute pore rating of from about 0.05 to about 1.0 micrometer and a second filter medium comprised of a hydrophilic, microporous polyamide membrane having a negative zeta potential and an absolute pore rating in the range of from about 0.01 to about 0.1 micrometer and finer than that of said first filter medium.

35. The filter system of claim 34, wherein said first filter medium has been contacted with an aqueous solu-tion of ammonium hydroxide to convert said quaternary ammonium group to the hydrozide form.