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Vertical skein of hollow fiber membranes and method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate |
| RE39294 |
Vertical skein of hollow fiber membranes and method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate
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| Patent Drawings: | |
| Inventor: |
Mahendran, et al. |
| Date Issued: |
September 19, 2006 |
| Application: |
09/975,220 |
| Filed: |
October 11, 2001 |
| Inventors: |
Mahendran; Mailvaganam (Hamilton, CA)
Rodrigues; Carlos Fernando F. (Brampton, CA)
Pedersen; Steven Kristian (Burlington, CA)
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| Assignee: |
Zenon Environmental Inc. (Oakville, CA) |
| Primary Examiner: |
Kim; John |
| Assistant Examiner: |
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| Attorney Or Agent: |
Bereskin & Parr |
| U.S. Class: |
210/321.69; 210/321.8; 210/356; 210/500.23; 210/636; 264/258 |
| Field Of Search: |
210/220; 210/257.2; 210/321.69; 210/321.79; 210/321.8; 210/321.85; 210/321.89; 210/355; 210/356; 210/500.23; 210/636; 210/641; 210/650; 210/251; 210/252; 210/258; 210/416.1; 210/410; 210/411; 264/258 |
| International Class: |
B01D 65/02 |
| U.S Patent Documents: |
3704223; 3708071; 3730959; 4540490; 4647377; 5141031; 5248424; 5484528; 5783083; 5910250; 5932099; 5944997; 6045698; 6193890; 6214231; 6303035; 6319411; 6325928; 6402955; 6555005 |
| Foreign Patent Documents: |
10045227; 1213048; 61-293504; 61-263605; 61-291007; 62-144712; 62-250908; 61-167407; 04-250898; 05-285348; 06-218238; 06-218361; 06-285496; 07-136471; 07-275665; 07-289860; 08-010585; 61-242607; 09-141063; WO 93/02779; WO 94/11094; WO 97/06880 |
| Other References: |
English language abstract of JP 07-289860. cited by other.
English language abstract of JP 07-136471. cited by other.
English language abstract of JP 61-291007. cited by other.
English language abstract of JP 61-293504. cited by other.
English language abstract of JP 61-167407. cited by other.
English language abstract of JP 61-242607. cited by other.
English language abstract of JP 07-275665. cited by other.
English language abstract of JP 08-010585. cited by other.
English language abstract of JP 09-141063. cited by other.
Declaration for Reissue Application executed Jun. 14, 1999 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Amendment dated Jan. 8, 1998 in file history of U.S. Pat. No. 5,783,083, incorporated by reference in Declaration for Reissue Application executed Jun. 14, 1999 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Declaration Under 37 CFR 1.132 of Steven Pedersen executed Jan. 5, 1998 in file history of U.S. Pat. No. 5,783,083, incorporated by reference in Declaration for Reissue Application executed Jun. 14, 1999 in File history of U.S. Pat. No. Re. 37,549.cited by other.
Declaration Under 37 CFR 1.132 of Kenneth Goodboy executed Dec. 27, 1997 in file history of U.S. Pat. No. 5,783,083, incorporated by reference in Declaration for Reissue Application executed Jun. 14, 1999 in File history of U.S. Pat. No. Re. 37,549.cited by other.
Office Action mailed Nov. 5, 1999 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Supplemental Declaration for Reissue Application executed Mar. 3, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Amendment dated Mar. 4, 2000in File history of U.S. Pat. No. Re. 37,549. cited by other.
Supplement Amendment dated Mar. 20, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Office Action mailed Mar. 31, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Amendment dated May 9, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Third Supplement Declaration for Reissue Application executed May 8, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Notice of Allowability dated Jun. 1, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Supplemental Declaration for Reissue Application executed Nov. 7, 2000 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Preliminary Amendment dated Nov. 6, 2000 in File history of of U.S. Pat. No. Re. 37,549. cited by other.
Office Action mailed Feb. 6, 2001 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Amendment dated May 4, 2001 in File history of U.S. Pat. No. Re. 37,549. cited by other.
Notice of Allowability dated May 29, 2001 in File history of U.S. Pat. No. Re. 37,549. cited by other.
English language abstract of JP62-144712. cited by other.
English language abstract of JP62-250908. cited by other.
English language abstract of JP 61-263605. cited by other.
English language abstract of JP 06-285496. cited by other.
English language abstract of JP 05-285348. cited by other.
English language abstract of JP 06-218361. cited by other.
English language abstract of JP 06-218238. cited by other.
Kaiya et al., "Water Purification Using Hollow Fiber Microfiltration Membranes", 6th World Filtration Congress, Nagoya, 1993, pp. 813-816. cit- ed by other. |
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| Abstract: |
A vertical skein of "fibers", opposed terminal portions of which are held in headers unconfined in a modular shell, is aerated with a gas-distribution means which produces a mass of bubbles serving the function of a scrub-brash for the outer surfaces of the fibers. The membrane device is surprisingly effective with relatively little cleansing gas, the specific flux through the membranes reaching an essentially constant relatively high value because the vertical deployment of fibers allows bubbles to rise upwards along the outer surfaces of the fibers. Further, bubbles flowing along the outer surfaces of the fibers make the fibers surprisingly resistant to being fouled by build-up of deposits of inanimate particles or microorganisms in the substrate provided that the length of each fiber is only slightly greater than the direct center-to-center distance between opposed faces of the headers, preferably in the range from at least 0.1% to about 5% greater. For use in a larger reservoir, a bank of skeins is used with a gas distributor means and each skein has fibers preferably >0.5 meter long, which together provide a surface area >10 m.sup.2. The terminal end portions of fibers in each header are kept free from fiber-to-fiber contact with a novel method of potting fibers. |
| Claim: |
We claim:
1. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multicomponent liquid substrate while increasing the concentration of particulatematerial therein, said membrane device including: a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m.sup.2, said fibers being swayable in said substrate, said fibersbeing subject to a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each fiber having a length >0.5 meter; a first header and a second header disposed in transversely spaced-apartrelationship with said second header within said substrate; said first header and said second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face ofat least one header; permeate collection means to collect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and, means to withdraw said permeate; the improvement comprising, saidfibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in curedresin; said first header being upper and disposed in vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance; each of said fibers having substantially the same length, said length being from 0.1% to lessthan 5% greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber.
2. The membrane device of claim 1 wherein each said header is a mass of synthetic resinous material in which said terminal end portions are potted and said fibers are formed from an organic resinous material or a ceramic.
3. The membrane device of claim 2 wherein each said hollow fiber has an outside diameter in the range from about 20 .mu.m to about 3 mm, a wall thickness in the range from about 5 .mu.m to about 2 mm, and, said fiber is formed from a materialselected from the group consisting of natural and synthetic polymers, and pore size in the range from 1000 .ANG. to 10000 .ANG., and, said displacement is in the lateral or horizontal direction.
4. The membrane device of claim 3 wherein said transmembrane pressure differential is in the range from 3.5 kPa (0.5 psi) to about 175 kPa (25 psi), said fibers are in the range from 0.5 m to 5 m long, and said terminal end portions of saidfibers are potted within said mass of thermosetting synthetic resinous material to a depth in the range from about 1 cm to about 5 cm.
5. The membrane device of claim 3 wherein said substrate is maintained at a pressure in the range from about 1-10 atm, said fibers extend as a skein upwardly from a fiber-supporting face of each of said headers, each header is a rectangularprism having substantially the same dimensions, said fibers extend downwardly through the permeate-discharging face of said headers, and said permeate is discharged upwardly relative to said upper header.
6. The membrane device of claim 4 wherein said terminal end portions of said fibers are potted within a mass of thermosetting synthetic resinous material to a depth in the range from about 1 cm to about 5 cm and protrude through apermeate-discharging face of each said header in a range from about 0.1 mm to about 1 cm.
7. The membrane device of claim 6 wherein said open ends of fibers are bounded by a geometrically regular peripheral boundary around the outermost peripheries of the outermost fibers in the boundary, and the length of a fiber is essentiallyindependent of the strength of said fiber, or its diameter.
8. The membrane device of claim 7 wherein said fibers together have a surface area in the range from 10 to 10.sup.3 m.sup.2.
9. The membrane device of claim 8 wherein said first and second headers are each a rectangular parallelpiped and said first header is disposed parallel to said second header.
10. In a gas-scrubbed assembly comprising, a microfiltration membrane device in combination with a gas-distribution means to minimize build-up of particulate deposits on the surfaces of hollow fiber membranes ("fibers") in said device, and torecover permeate from a multicomponent liquid substrate while leaving particulate matter therein, said membrane device comprising: a multiplicity of fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m.sup.2, saidfibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each having a length >0.5 meter; a first and second header disposedin spaced-apart relationship within said substrate; said first header and said second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of atleast one header; permeate collection means to collect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and, means for withdrawing said permeate; and, said gas-distribution meansis located within a zone near the base of said skein, having through-passages therein adapted to have sufficient gas flowed therethrough to generate enough bubbles flowing in a column of rising bubbles through and around said skein fibers, to keepsurfaces of said fibers awash in bubbles; the improvement comprising, said fibers, said headers and said permeate collection means together forming a skein wherein said fibers are essentially vertically disposed and terminal end portions of individualfibers are potted in proximately spaced-apart relationship in cured resin; said first header being upper and disposed in vertically spaced-apart relationship above said second header at a fixed distance; each of said fibers having substantially thesame length, said length being from at least 0.1% greater, to less than 5% greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber; and, said gasdistribution means having through-passages therein to discharge a cleansing gas in an amount in the range from 0.47-14 cm.sup.3/sec per fiber (0.001 scfm/fiber to about 0.03 scfm/fiber) in a column of bubbles which rise vertically substantially parallelto, and in contact with said fibers, movement of which is restricted within said column; whereby said permeate is essentially continuously withdrawn while concentration of said particulate matter in said substrate is increased.
11. The gas-scrubbed assembly of claim 10 wherein said fixed distance is adjustable, said gas-distribution means includes at least two distribution means disposed, one on each side of said skein, said gas-distribution means generate bubbleshaving an average diameter in the range from about 0.1 mm to about 25 mm which bubbles contact said fibers, maintain their buoyancy, and maintain said fibers' outer surfaces essentially free from build-up of deposits of said particulate matter.
12. The gas-scrubbed assembly of claim 11 wherein said through-passages in said gas-distribution means generate bubbles in the size range from 1 mm to 25 mm in relatively close proximity, in the range from 1 cm to about 50 cm, to saidthrough-passages.
13. The gas-scrubbed assembly of claim 10 wherein said fibers have pores in the size range from about 1000 .ANG. to 10000 .ANG., each said header is a rectangular prism having substantially the same dimensions, said gas is an oxygen-containinggas, and said particulate matter comprises biologically active microorganisms growing in said substrate.
14. The gas-scrubbed assembly of claim 10 wherein said particulate matter comprises finely divided inorganic particles.
15. In a process for maintaining the outer surfaces of hollow fiber membranes essentially free from a build-up of deposits of particulate material while separating a permeate from a multicomponent liquid substrate in a reservoir, said processcomprising, submerging skein fibers within said substrate unconfined in a modular shell, said fibers being securely head in laterally opposed, spaced-apart first and second headers, said fibers having a transmembrane pressure differential in the rangefrom about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), a total surface area >1 m.sup.2, and a length sufficiently greater than the direct distance between opposed faces of said first and second headers, so as to present said skein in a swayableconfiguration above a horizontal plane through the horizontal centerline of a header; mounting said headers in fluid-tight open communication with collection means to collect said permeate; flowing a fiber-cleansing gas through a gas-distribution meansproximately disposed relative to said skein, within a zone near the base of said skein, and contacting surfaces of said fibers with sufficient physical impact of bubbles of said gas to maintain essentially the entire length of each fiber in said skeinawash with bubbles and essentially free from said build-up; maintaining an essentially constant flux through said fibers substantially the same as an equilibrium flux initially obtained after commencing operation of said process; collecting saidpermeate in said collection means; and, withdrawing said permeate, the improvement comprising: introducing a cleansing gas in an amount in the range from 0.47-14 cm.sup.3/sec per fiber (0.001 scfm/fiber to about 0.03 scfm/fiber) to generate a column ofsaid bubbles alongside and in contact with outer surfaces of said fibers; deploying said skein fibers within said column in an essentially vertical configuration, with said headers in fixed spaced apart relationship at a fixed distance, said skeinhaving fibers of substantially the same length and from at least 0.1% greater, to about 5% greater than said fixed distance, said fibers being independently swayable from side-to-side within a vertical zone of movement with terminal end portions ofindividual fibers potted in proximately spaced-apart relationship in cured resin; restricting movement of said fibers to said vertical zone defined by lateral movement of outer fibers in said skein; vertically gas-scrubbing said fibers' outsidesurfaces with bubbles which flow upward in contact with said surfaces; maintaining said surfaces substantially free from said deposits of particulate matter during a period when specific flux through said fibers has attained equilibrium; and,simultaneously, essentially continuously, withdrawing said permeate while increasing the concentration of said particulate material in said substrate.
16. The process of claim 15 wherein each said hollow fiber has an outside diameter in the range from about 20 .mu.m to about 3 mm, and a wall thickness in the range from about 5 .mu.m to about 1 mm; each said header is formed from a mass ofthermosetting or thermoplastic synthetic resinous material; terminal end portions of said fibers are potted within said resinous material to a depth in the range from about 1 cm to about 5 cm; said particulate matter is selected from the groupconsisting of microorganisms and finely divided inorganic particles; and, said gas-distribution means generates bubbles having an average diameter in the range from about 1 mm to about 25 mm.
17. A method of forming a header for a skein of a multiplicity of fibers, comprising, forming a stack of at least two superimposed essentially coplanar and similar arrays, each array comprising a chosen number of fibers supported on a supportmeans having a thickness corresponding to a desired lateral spacing between adjacent arrays; holding the stack in a first liquid with terminal portions of the fibers submerged, until the liquid solidifies into a first shaped lamina, provided that thefirst liquid is unreactive with material of the fibers; pouring a second liquid over the first shaped lamina to embed the fibers to a desired depth, and solidifying the second liquid to form a fixing lamina upon the first shaped lamina, the secondliquid also being substantially unreactive with either the material of the fibers or that of the first shaped lamina; forming a composite header in which terminal portions of the fibers are potted, the composite header comprising a laminate of afugitive lamina of fugitive material and a contiguous finished header of fixing lamina; and, removing the first shaped lamina without removing a portion of the fixing lamina so as to leave the ends of the fibers open and protruding from the aft face ofthe header, whereby the open ends having a circular cross-section are exposed without cutting the fibers.
18. The method of claim 17 wherein said second light upon solidification forms a thermosetting or thermoplastic synthetic resin, and said first liquid upon solidification forms a solid which has a melting point or glass transition temperaturelower than the melting point or glass transition temperature of said synthetic resin.
19. The method of claim 18 wherein said first liquid upon solidification is flowable at a temperature at which said second liquid upon solidification remains solid.
20. The method of claim 18 wherein said first liquid upon solidification is soluble in a chosen solvent, and said second liquid upon solidification is insoluble in said solvent.
21. A header in which a multiplicity of hollow fiber membranes or "fibers" is potted, said header comprising, a molded body of arbitrary shape striated in a fixing lamina and a fugitive lamina, said fugitive lamina formed from a fugitivepotting material and said fixing lamina formed from a fixing material; said fibers having terminal portions thereof potted in said fugitive potting material which when solidified plugs ends of said fibers, plugged ends having an essentially circularcross-section, said fugitive lamina maintaining said ends in closely spaced-apart substantially parallel relationship; said fugitive lamina having an aft face towards which said plugged ends protrude, and a fore face through which said fibers extendvertically; said fugitive lamina having said fixing lamina adhered thereto, said fixing lamina having a thickness sufficient to maintain said fibers in substantially the same spaced-apart relationship relative to one and another as the spaced-apartrelationship in said lower portion.
22. The header of claim 21 wherein said fixing lamina has a cushioning lamina embedding said fibers and coextensively adhered to said fixing lamina, said fixing lamina has a hardness in the range from about Shore D 50 to Rockwell R 110, andsaid cushioning layer has a hardness in the range from Shore A 30 to Shore D 45.
23. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi-component liquid substrate while increasing the concentration of particulate material therein, said membrane device including: amultiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m.sup.2, said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressuredifferential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each fiber having length >0.5 meter; a first header and a second header disposed in transversely spaced-apart relationship with said second header within saidsubstrate; a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header; permeate-collection means tocollect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and, means to withdraw said permeate; the improvement comprising, said fibers, said headers and said permeate collectionmeans together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in cured resin; said first header being upper and disposedin vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance; each of said fibers having substantially the same length, said length being from between 0.1% to less than 5% greater than said fixed distance soas to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber; and, a gas distribution system having through-passages adapted to discharge bubbles near to rise through or around the skeinof fibers, the gas distribution system including one or more gas tubes which space the first and second headers apart and which also carry air to the through-passages.
24. The device of claim 23 wherein the upper and lower headers are cylindrical and the one or more gas tubes are a single gas tube located in about the center of the headers.
25. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi-component liquid substrate while increasing the concentration of particulate material therein, said membrane device including: amultiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m.sup.2, said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressuredifferential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each fiber having length >0.5 meter; a first header and a second header disposed in transversely spaced-apart relationship with said second header within saidsubstrate; a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header; permeate-collection means tocollect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and, means to withdraw said permeate; the improvement comprising, said fibers, said headers and said permeate collectionmeans together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in cured resin; said first header being upper and disposedin vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance; each of said fibers having substantially the same length, said length being from between 0.1% to less than 5% greater than said fixed distance soas to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber, wherein the headers are rectangular in plan view and the skein has about 30 or less arrays of fibers.
26. A device for withdrawing permeate from a multicomponent liquid substrate comprising, (a) a reservoir under essentially ambient pressure having a feed zone for containing a substrate; (b) a microfiltration membrane device, for withdrawingpermeate essentially continuously from the multi-component liquid substrate while increasing the concentration of particulate material therein, said membrane device including: a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell ofa module, said fibers together having a surface area >1 m.sup.2, said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), andeach fiber having length >0.5 meter; a first header and a second header disposed in transversely spaced-apart relationship with said second header within said substrate; a first header and a second header having opposed terminal end portions of eachfiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header; permeate-collection means to collect said permeate, sealingly connected in open fluid communication with apermeate-discharging face of each of said headers; and, means to withdraw said permeate; said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminalend portions of individual fibers are potted in proximately spaced-apart relationship in cured resin; said first header being upper and disposed in vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance; each of said fibers having substantially the same length, said length being from between 0.1% to less than 5% greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movementof another fiber, the outside of the membranes in fluid communication with the feed zone of the reservoir; (c) a pump in fluid communication with the insides of the membranes through the permeate collection means, the pump operable to supply a suctionto the lumens of the hollow fiber membranes to draw permeate through the membranes; and, (d) a gas distribution means including a plurality of through-passages for discharging bubbles which rise and contact fibers.
27. The device of claim 26 wherein at least some of the through-passages have outlets located within the skein.
28. The device of claim 26 wherein the headers are rectangular in plan view and have about 30 arrays or less of fibers and the outlets of the through-passages are located to the side of the headers.
29. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi-component liquid substrate while increasing the concentration of particulate material therein, said membrane device including: amultiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m.sup.2, said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressuredifferential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each fiber having length >0.5 meter; a first header and a second header disposed in transversely spaced-apart relationship with said second header within saidsubstrate; a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header; permeate-collection means tocollect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and, means to withdraw said permeate; the improvement comprising, said fibers, said headers and said permeate collectionmeans together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in cured resin; said first header being upper and disposedin vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance; each of said fibers having substantially the same length, said length being from between 0.1% to less than 5% greater than said fixed distance soas to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber; walls extending downwards from a lower header of the first and second header, the walls being adapted to retain a gas belowthe lower header; and, through-passages for gas to pass through the lower header from an area below the lower header bordered by the walls.
30. The device of claim 29 wherein the through passages are located such that gas flowing from the area below the lower header bordered by the walls, exits between fibers.
31. The membrane device of claim 1 wherein said fibers, said headers and said permeate collection means are all submersible below the surface of the substrate.
32. The membrane device of claim 31 further comprising a manifold for withdrawing permeate, the manifold extending from the permeate collection means to a point above the surface of the substrate.
33. The membrane device of claim 31 wherein the permeate collection means further comprises a pan or header enclosure covering each permeate discharging face.
34. A membrane filtration system comprising: (a) a tank for holding a substrate at ambient pressure during filtration; (b) a membrane device according to claim 1 immersed below the surface of the substrate; (c) an aeration system forproducing bubbles in the substrate which contact the fibers; and, (d) a source of suction in fluid communication with the membrane filtration device.
35. The membrane filtration system of claim 34 further comprising a backwashing system for backwashing the membrane filtration device with a liquid. |
| Description: |
BACKGROUND OF THE INVENTION
This invention relates to a membrane device which is an improvement on a frameless array of hollow fiber membranes and a method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate, which is also the subject ofU.S. Pat. No. 5,248,424; and, to a method of forming a header for a skein of fibers. The term "vertical skein" in the title (hereinafter "skein" for brevity), specifically refers to an integrated combination of structural elements including (i) amultiplicity of vertical fibers of substantially equal length; (ii) a pair of headers in each of which are potted the opposed terminal portions of the fibers so as to leave their ends open; and, (iii) permeate collection means held peripherally influid-tight engagement with each header so as to collect permeate from the ends of the fibers.
The term "fibers" is used for brevity, to refer to "hollow fiber membranes" of porous or semipermeable material in the form of a capillary tube or hollow fiber. The term "substrate" refers to a multicomponent liquid feed. A "multicomponentliquid feed" in this art refers, for example, to fruit juices to be clarified or concentrated; wastewater or water containing particulate matter; proteinaceous liquid dairy products such as cheese whey, and the like. The term "particulate matter" isused to refer to micron-sized (from 1 to about 44 .mu.m) and sub-micron sized (from about 0.1 .mu.m to 1 .mu.m) filtrable matter which includes not only particulate inorganic matter, but also dead and live biologically active microorganisms, colloidaldispersions, solutions of large organic molecules such as fulvic acid and humic acid, and oil emulsions.
The term "header" is used to specify a solid body in which one of the terminal end portions of each one of a multiplicity of fibers in the skein, is sealingly secured to preclude substrate from contaminating the permeate in the lumens of thefibers. Typically, a header is a continuous, generally rectangular parallelpiped of solid resin (thermoplastic or thermosetting) of arbitrary dimensions formed from a natural or synthetic resinous material. In the novel method described hereinbelow,the end portions of individual fibers are potted in spaced-apart relationship in cured resin, most preferably by "potting" the end portions sequentially in at least two steps, using first and second potting materials. The second potting material(referred to as "fixing material") is solidified or cured after it is deposited upon a "fugitive header" (so termed because it is removable) formed by solidifying the first liquid. Upon removing the fugitive header, what is left is the "finished" or"final" header formed by the second potting material. Of course, less preferably, any prior art method may be used for forming finished headers in which opposed terminal end portions or fibers in a stack of arrays are secured in proximately spaced-apartrelationship with each other.
The '424 patent required potting the opposed ends of a frameless array of fibers and dispensed with the shell of a module; it was an improvement on two proceeding configurations disclosed in U.S. Pat. Nos. 5,182,019, and 5,104,535, each ofwhich used frameless arrays and avoided potting the fibers. The efficiency of gas-scrubbing a '424 array was believed to be due, at least in large part, to a substantial portion of the fibers of the fibers in the array lying in transverse relationshipto a mass of rising bubbles, referred to herein as a "column of rising bubbles", so as to intercept the bubbles. Specific examples are illustrated in FIGS. 9, 9A, 10 and 11 of the '424 patent.
A '424 "array" referred to a bundle of arcuate fibers the geometry of which array was defined by the position of a pair of transversely spaced headers in which the fibers were potted. In the '424 array, as in the array of this invention, eachfiber is free to move independently of the others, but the degree of movement in the '424 is unspecified and arbitrary, while in the vertical skein of this invention, movement is critically restricted by the defined length of the fibers between opposedheaders. Except for their opposed ends being potted, there is no physical restraint on the fibers of a skein. To avoid confusion with the term "array" as used for the '424 bundle of arcuate fibers, the term "skein fibers" is used herein to refer toplural arrays. An "array" in this invention refers to plural, essentially vertical fibers of substantially equal lengths, the one ends of each of which fibers are closely spaced-apart, either linearly in the transverse (y-axis herein) direction toprovide at least one row, and typically plural rows of equidistantly spaced apart fibers. Less preferably, a multiplicity of fibers may be spaced in a random pattern. Typically, plural arrays are potted in a header and enter its face in a generally x-yplane (see FIG. 5). The width of a rectangular parallelpiped header is measured along the x-axis, and is the relatively shorter dimension of the rectangular upper surface of the header; and, the header's length, which is its relatively longer dimension,is measured along the y-axis.
This invention is particularly directed to relatively large systems for the microfiltration of liquids, and capitalizes on the simplicity and effectiveness of a configuration which dispenses with forming a module in which the fibers are confined. As in the '424 patent, the novel configuration efficiently uses a cleansing gas, typically air, discharged near the base of a skein to produce bubbles in a specified size range, and in an amount large enough to scrub the fibers, and to cause the fibersto scrub themselves against one another. Unlike in the '424 system, the fibers in a skein are vertical and do not present an arcuate configuration above a horizontal plane through the horizontal center-line of a header. As a result, the path of therising bubbles is generally parallel to the fibers and is not crossed by the fibers of a vertical skein. Yet the bubbles scrub the fibers. The restrictedly swayable fibers, because of their defined length, do not get entangled, and do not abrade eachother excessively, as is likely in the '424 array. The defined length of the fibers herein minimizes (i) shearing forces where the upper fibers are held in the upper header, (ii) excessive rotation of the upper portion of the fibers, as well as (iii)excessive abrasion between fibers. The fibers of this invention are confined so as to sway in a "zone of confinement" (or "bubble zone") through which bubbles rise along the outer surfaces of the fibers. The side-to-side displacement of an intermediateportion of each fiber within the bubble zone is restricted by the fiber's length. The bubble zone, in turn, is determined by one or more columns of vertically rising gas bubbles, preferably of air, generated near the base of a skein.
Since there is no module in the conventional sense, the main physical considerations which affect the operation of a vertical skein in a reservoir of substrate relate to intrinsic considerations, namely, (a) the fiber chosen, (b) the amount ofair used, and (c) the substrate to be filtered. Such considerations include the permeability and rejection properties of the fiber, the process flow conditions of substrate such as pressure, rate of flow across the fibers, temperature, etc., thephysical and chemical properties of the substrate and its components, the relative directions of flow of the substrate (if it is flowing) and permeate, the thoroughness of contact of the substrate with the outer surfaces of the fibers, and still otherparameters, each of which has a direct effect on the efficiency of the skein. The goal is to filter a slow moving or captive substrate in a large container under ambient or elevated pressure, but preferably under essentially ambient pressure, and tomaximize the efficiency of a skein which does so (filters) practically and economically.
In the skein of this invention, all fibers in the plural rows of fibers, staggered or not, rise generally vertically while fixedly held near their opposed terminal portions in a pair of opposed, substantially identical headers to form the skeinof substantially parallel, vertical fibers. This skein typically includes a multiplicity of fibers, the opposed ends of which are potted in closely-spaced-apart profusion and bound by potting resin, assuring a fluid-tight circumferential seal aroundeach fiber in the header and presenting a peripheral boundary around the outermost peripheries of the outermost fibers. The position of one fiber relative to another in a skein is not critical, so long as all fibers are substantially codirectionalthrough one face of each header, open ends of the fibers emerge from the opposed other face of each header, and substantially no terminal end portions of fibers are in fiber-to-fiber contact. We found that the skein of fibers, deployed to berestrictedly swayable, were as ruggedly durable as they were reliable in operation.
The fibers are stated to be "restrictedly swayable", because the extent to which they may sway is determined by the free length of the fibers relative to the fixedly spaced-apart headers, and the turbulence of the substrate. When a large numberof fibers is used in a skein, as is typically the case herein, the movement of a fiber adjacent to others may be modulated by the movement of the others, but the movement of fibers within a skein is constricted. This system is therefore limited to theuse of a skein of fibers having a critically defined length relative to the vertical distance between headers of the skein. The defined length limits the side-to-side movement of the fibers in the substrate in which they are deployed, except near theheaders where there is negligible movement.
In the prior art, a vertical skein of fibers in a substrate is typically avoided due to expected problems relating to channelling of the feed. However, because the fibers are restrictedly swayable in a "bubble zone" as described herebelow, thefibers are substantially evenly contacted over their individual surfaces with substrate and provide filtration performance based on a maximized surface which is substantially the sum of the surface areas of all fibers in contact with the substrate. Moreover, because of the ease with which the substrate coats the surfaces of the vertical fibers in a skein, and the accessibility of those surfaces by air bubbles, the fibers may be densely arranged in a header to provide a large membrane surface of upto 1000 m.sup.2 and more.
One header of a skein is displaceable in any direction relative to the other, either longitudinally (x-axis) or transversely (y-axis), only prior to the headers being vertically fixed in spaced apart parallel relationship within a reservoir, forexample, by mounting one header above another, against a vertical wall of the reservoir which functions as a spacer mess. This is a also true prior to spacing one header above another with other spacer means such as bars, rods, struts, I-beams,channels, and the like, to assemble plural skeins into a "bank of skeins" ("bank" for brevity), in which bank a row of lower headers is directly beneath a row of upper headers. After assembly into a bank, a segment intermediate the potted ends of eachindividual fiber is displaceable along either the x- or the y-axis, because the fibers are loosely held in the skein. There is essentially no tension on each fiber because the opposed faces of the headers are spaced apart at a distance less than thelength of an individual fiber.
By operating at ambient pressure, mounting the headers of the skein within a reservoir of substrate, and by allowing the fibers restricted movement within the bubble zone in a substrate, we minimize damage to the fibers. Because, a headersecures at least 10, preferably from 50 to 50,000 fibers, each generally at least 0.5 m long, in a skein, it provides a high surface area for filtration of the substrate.
The fibers divide a reservoir into a "feed zone" and a withdrawal zone referred to as a "permeate zone". The feed of substrate is introduced externally (referred to as "outside-in" flow) of the fibers, and resolved into "permeate" and"concentrate" streams. The skein, or a bank of skeins of this invention is most preferably used for microfiltration with "outside-in" flow. Typically a bank is used in a relatively large reservoir having a volume in excess of 10 L (liters), preferablyin excess of 1000 L, such as a flowing stream, more typically a reservoir (pond or tank). Most typically, a bank or plural banks with collection means for the permeate, are mounted in a tank under atmospheric pressure, and permeate is withdrawn from thebank.
Where a bank or plural banks of skeins are placed within a tank or bioreactor, and no liquid other than the permeate is removed the tank is referred to as a "dead end tank". Alternatively, a bank or plural banks may be placed within abioreactor, permeate removed, and sludge disposed of; or, in a tank or clarifier used in conjunction with a bioreactor, permeate removed, and sludge disposed of.
Operation of the system relies upon positioning at least one skein, preferably a bank, close to a source of sufficient air or gas to maintain a desirable flux, and, to enable permeate to be collected from at least one header. A desirable flux isobtained, and provides the appropriate transmembrane pressure differential of the fibers under operating process conditions. "Transmembrane pressure differential" refers to the pressure difference across a membrane wall, resulting from the processconditions under which the membrane is operating.
The relationship of flux to permeability and transmembrane pressure differential is set forth by the equation: J=k .DELTA.P wherein, J=flux; k=permeability constant; .DELTA.P=transmembrane pressure differential; and k=1/.mu.Rm where.mu.=viscosity of water, and Rm=membrane resistance.
The transmembrane pressure differential is preferably generated with a conventional non-vacuum pump if the transmembrane pressure differential is sufficiently low in the range from 0.7 kPa (0.1 psi) to 101 kPa (1 bar), provided the pump generatesthe requisite suction. The term "non-vacuum pump" refers to a pump which generates a net suction side pressure difference, or, net positive suction head (NPSH), adequate to provide the transmembrane pressure differential generated under the operatingconditions. By "vacuum pump" we refer to one capable of generating a suction of at least 75 cm of Hg. A pump which generates minimal suction may be used if an adequate "liquid head" is provided between the surface of the substrate and the point atwhich permeate is withdrawn; or, by using a pump, not a vacuum pump. A non-vacuum pump may be a centrifugal, rotary, crossflow, flow-through, or other type. Moreover, as explained in greater detail below, once the permeate flow is induced by a pump,the pump may not be necessary, the permeate continuing to flow under a "siphoning effect". Clearly, operating with fibers subjected to a transmembrane pressure differential in the range up to 101 kPa (14.7 psi), a non-vacuum pump will provide adequateservice in a reservoir which is not pressurized; and, in the range from 101 kPa to about 345 kPa (50 psi), by superatmospheric pressure generated by a high liquid head, or, by a pressurized reservoir.
The fibers are not required to be subjected to a narrowly critical transmembrane pressure differential though fibers which operate under a small transmembrane pressure differential are preferred. A fiber which operates under a smalltransmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 70 kPa (10 psi) may produce permeate under gravity alone, if appropriately positioned relative to the location where the permeate is withdrawn. In the range from 3.5kPa (0.5 psi) to about 206 kPa (30 psi) a relatively high liquid head may be provided with a pressurized vessel. The longer the fiber, which greater the area and the more the permeate.
In the specific instance where a bank is used in combination with a source of cleansing gas such as air, both to scrub the fibers and to oxygenate a mixed liquor substrate, most, if not all of the air required, is introduced either continuouslyor intermittently,. near the base of the fibers near the lower header. The perforations through which the gas is discharged near the header are located close enough to the fibers so as to provide columns of relatively large bubbles, preferably largerthan about 1 mm in nominal diameter, which codirectionally contact the fibers and flow vertically along their outer surfaces, scrubbing them. The outer periphery of the columns of bubbles define the zone of confinement in which the scrubbing forceexerted by the bubbles on the fibers, keeps their surfaces sufficiently free of attached microorganisms and deposits of inanimate particles to provide a relatively high and stable flow of permeate over many weeks, if not months of operation. Thesignificance of this improvement will be better appreciated when it is realized that the surfaces of fibers in conventional modules are cleaned nearly every day, and sometimes more often.
Because this system, like the '424 system, does away with using a shell, there is no void space within a shell to be packed with fibers; and, because of gas being introduced proximately to, and near the base of skein fibers, there is no need tomaintain a high substrate velocity across the surface of the fibers to keep the surfaces of the fibers clean. As a result, there is virtually no limit to the number of restrictedly swayable fibers which may be used in a skein, the practical limit beingset by (i) the ability to pot the ends of the fibers reliably; (ii) the ability to provide sufficient air to the surfaces of essentially all the fibers, and (iii) the number of banks which may be deployed in a tank, pond or lake, the number to bedetermined by the size of the body of water, the rate at which permeate is to be withdrawn, and, the cost of doing so.
Typically, a relatively large number of long fibers, at least 100, is used in a skein of restrictedly swayable fibers, the fibers operate under a relatively low transmembrane pressure differential, and permeate is withdrawn with a nonvacuum pump. If the liquid head, measured at the vertical distance between the level of substrate and the level from which permeate is to be withdrawn, is greater than the transmembrane pressure differential under which the fiber operates, the permeate will beseparated from the remaining substrate, due to gravity.
Irrespective of whether a non-vacuum pump, vacuum pump, or other type of pump is used, or permeate is withdrawn with a siphoning effect, it is essential that the fibers in a skein be positioned in a generally vertical attitude, rising above thelower header. As understanding of how a vertical skein operator will make it apparent that, since fibers in a skein are anchored at the base of the skein by the lower header, the specific gravity of the fibers relative to that of the substrate isimmaterial and will not affect their vertical disposition.
The unique method of forming a header disclosed herein allows one to position a large number of fibers, in closely-spaced apart relationship, randomly relative to one another, or, in a chosen geometric pattern, within each header of syntheticresinous material. It is preferred to position the fibers in arrays before they are potted to ensure that the fibers are spaced apart from each other precisely, and, to avoid wasting space on the face of a header; it is essential, for greatestreliability, that the fibers not be contiguous. By sequentially potting the terminal portions of fibers in stages as described herein, the fibers may be cut to length in an array, either after, or prior to being potted. The use of a razor-sharp knife,or scissors, or other cutting means to do so, does not decrease the open cross-sectional area of the fibers' bores ("lumens"). The solid resin forms a circumferential seal around the exterior terminal portions of each of the fibers, open ends of whichprotrude through the permeate-discharging face of each header, referred to as the "aft" face.
Further, one does not have to cope with the geometry of a frame, the specific function of which is to hold fibers in a particular arrangement within the frame. In a skein, the sole function of the header spacing means is to maintain a fixedvertical distance between headers which are not otherwise spaced apart. In a skein of this invention, there is no frame.
The skein of this invention is most preferably used to treat wastewater in combination with a source of an oxygen-containing gas which is bubbled within the substrate, near the base of a lower header, either within a skein or between adjacentskeins in a bank, for the specific purpose of scrubbing the fibers and oxygenating the mixed liquor in activated sludge, such as is generated in the bioremediation of wastewater. It was found that, as long as enough air is introduced near the base ofeach lower header to keep the fibers awash in bubbles, and the fibers are restrictedly swayable in the activated sludge, a build-up of growth of microbes on the surfaces of the fibers is inhibited while permeate is directly withdrawn from activatedsludge, and excellent flow of permeate is maintained over a long period. Because essentially all surface portions of the fibers are contacted by successive bubbles as they rise, whether the air is supplied continuously or intermittently, the fibers aresaid to be "awash in bubbles."
The use of an array of fibers in the direction treatment of activated sludge in a bioreactor, is described in an article titled "Direct Solid-Liquid Separation Using Hollow Fiber Membrane in an Activated Sludge Aeration Tank" by Kanzuo Yamamotoet al in Wat. Sci. Tech. Vol. 21, Brighton pp 43-54, 1989, and discussed in the '424 patent, the disclosure of which is incorporated by reference thereto as if fully set forth herein. The relatively poor performance obtained by Yamamoto et al wasmainly due to the fact that they did not realize the critical importance of maintaining flux by aerating a skein of fibers from within and beneath the skein. They did not realize the necessity of thoroughly scrubbing substantially the entire surfaces ofthe fibers by flowing bubbles through the skein to keep the fibers awash in bubbles. This requirement becomes more pronounced as the number of fibers in the skein increases.
As will presently be evident, since most substrates are Contaminated with micron and submicron size particulate material, both organic and inorganic, the surfaces of the fibers in any practical membrane device must be maintained in a cleancondition to obtain a desirable specific flux. To do this, the most preferred use of the skein as a membrane device is in a bank, in combination with a gas-distribution means, which is typically used to distribute air, or oxygen-enriched air between thefibers, from within the skein, or between adjacent skeins, at the bases thereof.
Tests using the device of Yamamoto et al indicate that when the air is provided outside the skein the flux decreases much faster over a period of as little as 50 hr, confirming the results obtained by them. This is evident in FIG. 1 described ingreater detail below, in which the graphs show results obtained by Yamamoto et al, and the '424 array, as well as those with the vertical skein, all three assemblies using essentially identical fibers, under essentially identical conditions.
The investigation of Yamamoto et al with downwardly suspended fibers was continued and recent developments were reported in an article titled "Organic Stabilization and Nitrogen Removal in Membrane Separation Bioreactor for Domestic WastewaterTreatment" by C. Chiemchaisri et al delivered in a talk to the Conference on Membrane Technology in Wastewater Management, in Cape Town, South Africa, Mar. 2-5, 1992, also discussed in the '424 patent. The fibers were suspended downwardly and highlyturbulent flow of water in alternate directions, was essential.
It is evident that the disclosure in either the Yamamoto et al or the Chiemchaisri et al reference indicated that the flow of air across the surfaces of the suspended fibers did little or nothing to inhibit the attachment of microorganisms fromthe substrate.
SUMMARY OF THE INVENTION
It has been discovered that bubbles of a fiber-cleansing gas ("scrubbing gas") flowing parallel to fibers in a vertical skein are more effective than bubbles which are intercepted by arcuate fibers crossing the path of the rising bubbles. Bubbles of an oxygen-containing gas to promote growth of microbes unexpectedly fails to build-up growth of microbes on the surfaces of the fibers because the surfaces are "vertically air-scrubbed". Deposits of animate and/or inanimate particles upon thesurfaces of fibers are minimized when the restrictedly swayable fibers are kept awash in codirectionally rising bubbles which rise with sufficient velocity to exert a physical scrubbing force (momentum provides the energy) to keep the fiberssubstantially free of deleterious deposits. Thus, an unexpectedly high flux is maintained over a long period during which permeate is produced by outside-in flow through the fibers.
It has also been discovered that permeate may be effidently withdrawn from a substrate for a surprisingly long period, in a single stage, essentially continuous filtration process, by mounting a pair of headers in vertically spaced apartrelationship, one above another, within the substrate which directly contacts a multiplicity of long vertical fibers in a "gas-scrubbed assembly" comprising a skein and a gas-distribution means. The skein has a surface area which is at least >1m.sup.2, and opposed spaced-apart ends of the fibers are secured in spaced-apart headers, so that the fibers, when deployed in the substrate, acquire a generally vertical profile therewithin and sway within the bubble zone defined by at least one columnof bubbles. The length of fibers between opposed surfaces of headers from which they extend, is in a critical range from at least 0.1% (per cent) longer than the distance separating those opposed faces, but less than 5% longer. Usually the length offibers is less than 2% longer, and most typically, less than 1% longer, so that sway of the fibers is confined within a vertical zone of movement, the periphery of which zone is defined by side-to-side movement of outer fibers in the skein; and, themajority of the fibers near the periphery move in a slightly larger zone than one defined by the projected area of one header upon the other. Though the distance between headers is fixed during operation, the distance is preferably adjustable to providean optimum length of fibers, within the aforesaid ranges, between the headers. It has been found that for no known reason, fibers which are more than 5% but less than 10% longer than the fixed distance between the opposed faces of the headers of askein, tend to shear off at the face; and those 10% longer tend to clump up in the bubble zone.
The terminal end portions of the fibers are secured non-contiguously in each header, that is, the surface of each fiber is sealingly separated from that of another adjacent fiber with cured potting resin. Preferably, for maximum utilization ofspace on a header, the fibers are deliberately set in a geometrically regular pattern. Typically permeate is withdrawn from the open ends of fibers which protrude from the permeate-discharging aft (upper) face of a header. The overall geometry ofpotted fibers is determined by a `fiber-setting form` used to set individual fibers in an array. The skein operates in a substrate held in a reservoir at a pressure in the range from 1 atm to an elevated pressure up to about 10 atm in a pressurizedvessel, without being confined within the shell of a module.
It is therefore a general object of this invention to provide a novel, economical and surprisingly trouble-free membrane device, for providing an alternative to both, a conventional module having plural individual arrays therewithin, and also toa frameless array of arcuate fibers; the novel device includes, (i) a vertical skein of a multiplicity of restrictedly swayable fibers, together having a surface area in the range from 1 m.sup.2 to 1000 m.sup.2, preferably from 10 m.sup.2 to 100 m.sup.2,secured only in spaced-apart headers; and (ii) a gas-scrubbing means which produces at least one column of bubbles engulfing the skein. A skein includes permeate pans disposed, preferably non-removably, within a substrate held in a reservoir ofarbitrary proportions, the reservoir typically having a volume in excess of 100 L (liters), generally in excess of 1000 L. A fluid component is to be selectively removed from the substrate.
It is a specific object of this invention to provide a membrane device having hollow fibers for removing permeate from a substrate, comprising, a skein of a multiplicity of fibers restrictedly swayable in the substrate, the opposed terminal endportions of which fibers in spaced-apart relationship, are potted in a pair of headers, one upper and one lower, each adapted to be mounted in vertically spaced apart generally parallel relationship, one above the other, within the substrate; essentiallyall the ends of fibers in both headers are open so as to pass permeate through the headers; the fibers in a skein have a length in the range from at least 0.1% greater, but less than 5% greater than the direct distance between opposed faces of the upperand lower headers, so as to present the fibers, when they are deployed, in an essentially vertical configuration; permeate is collected in a collection means, such as a permeate pan; and, permeate is withdrawn through a ducting means including one ormore conduits and appropriate valves.
It has also been discovered that skein fibers are maintained sufficiently free from particulate deposits with surprisingly little cleansing gas, so that the specific flux at equilibrium is maintained over a long period, typically from 50 hr to1500 hr, because the skein is immersed so as to present a generally vertical profile, and, the skein is maintained awash in bubbles either continuously or intermittently generated by a gas-distribution means ("air-manifold"). The air-manifold isdisposed adjacent the skein's lower header to generate a column of rising bubbles within which column the fibers are awash in bubbles. A bank of skeins is "gas-scrubbed" with plural air-tubes disposed between the lower headers of adjacent skeins, mostpreferably, also adjacent the outermost array of the first and last skeins, so that for "n" headers there are "n+1" air-manifolds. Each header is preferably in the shape of a rectangular parallelpiped, the upper and lower headers having the sametransverse (y-axis) dimension, so that plural headers are longitudinally stackable (along the x-axis). Common longitudinally positioned linear air-tubes, or, individual, longitudinally spaced apart vertically rising air-tubes, service the bank, and oneor more permeate tubes withdraw permeate.
It is therefore a general object of this invention to provide a gas-scrubbed assembly of fibers for liquid filtration, the assembly comprising, (a) bank of gas-scrubbed skeins of fibers which separate a desired permeate from a large body ofmulticomponent substrate having finely divided particulate matter in the range from 0.1 .mu.m-44 .mu.m dispersed therein, (b) each skein comprising at least 20 fibers having upper and lower terminal portions potted spaced-apart, in upper and lowerheaders, respectively, the fibers being restrictedly swayable in a bubble zone, and (c) a shaped gas-distribution means adapted to provide a profusion of vertically ascending bubbles near the lower header, the length of the fibers being from at least0.1% but less than 5% greater than the distance between the opposed faces of the headers. The gas-distribution means has through-passages therein through which gas is flowed at a flow rate which is proportional to the number of fibers. The flow rate isgenerally in the range from 0.47-14 cm.sup.3/sec per fiber (0.001-0.03 scfm/fiber) (standard ft.sup.3 per minute per fiber), typically in the range from 1.4-4.2 cm.sup.3/sec/fiber (0.003-0.009 scfm/fiber). The surface area of the fibers is not used todefine the amount of air used because the air travels substantially vertically along the length of each fiber. The gas generates bubbles having an average diameter in the range from about 0.1 mm to about 25 mm, or even larger.
It is a specific object of this invention to provide the aforesaid novel gas-scrubbed assembly comprising, a bank of vertical skeins and a shaped gas-distribution means for use with the bank, in a substrate in which microorganisms grow, theassembly being used in combination with vertically adjustable spacer means for mounting the headers in vertically spaced apart relationship, and in open fluid communication with collection means for collecting the permeate; means for withdrawing thepermeate; and, sufficient air is flowed through the shaped gas-distribution means to generate enough bubbles flowing upwardly through the skein, between and parallel to the fibers so as to keep the surfaces of the fibers substantially free from depositsof live microorganisms as well as small inanimate particles which may be present in the substrate.
It has still further discovered that a system utilizing a bank of vertical skeins of fibers potted in headers vertically spaced-apart by spacer means, and deployed in a substrate containing particulate material, in combination with a proximatelydisposed gas-distribution means to minimize fouling of the membranes, may be operated to withdraw permeate under gravity alone, so that the cost of any pump to withdraw permeate is avoided, provided the net positive suction head corresponding to thevertical height between the level of substrate, and the location of withdrawal of permeate, provides the transmembrane pressure differential under which the fibers function in the skein.
It is therefore a general object of this invention to provide the foregoing system in which opposed terminal end portions of skein fibers are essentially free from fiber-to-fiber contact after being potted in upper and lower headers keptvertically spaced-apart with spacer means, the skein being unconfined in a shell of a module and deployed in the substrate without the fibers being supported during operation except by the spacer means which support only the headers; the headers beingmounted so that the fibers present a generally vertical profile yet are restrictedly swayable in a zone of confinement defined by rising bubbles; means for mounting each header in open fluid communication with collection means for collecting permeate,and, means for withdrawing the permeate; and, shaped gas-distribution means adapted to generate bubbles from micron-size to 25 mm in nominal diameter, most preferably in the size range from 1 mm to 20 mm, the bubbles flowing upwardly through and parallelto the fibers at a flow rate chosen from the range specified hereabove; whereby the fibers are scrubbed with bubbles and resist the attachment of growing microorganisms and any other particulate matter to the surfaces of the fibers, so as to maintain adesirable specific flux during operation.
Still further, a low cost process has been discovered for treating a multicomponent substrate under pressure ranging from 1-10 atm in a pressurizable vessel, particularly for example, an aqueous stream containing finely divided inorganic mattersuch as silica, silicic acid, or, activated sludge, when the substrate is confined in a large tank or pond, by using a bank of vertical skeins each comprising restrictedly swayable unsupported fibers potted in headers in open fluid communication with ameans for withdrawing permeate, in combination with a source of air which generates bubbles near the lower header.
It is therefore a general object of this invention to provide a process for maintaining relatively clean fiber surfaces in an array of a membrane device while separating a permeate from a substrate, the process comprising, submerging a skein ofrestrictedly swayable substantially vertical fibers within the substrate so that upper and lower headers of the skein are mounted one above the other with a multiplicity of fibers secured between said headers, the fibers having their opposed terminalportions in open fluid communication with permeate collecting means in fluid-tight connection with said headers; the fibers operating under a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345-kPa (50 psi), and alength from at least 0.1% to about 2% greater than the direct distance between the opposed faces of upper and lower headers, so as to present, when the fibers are deployed, a generally vertical skein of fibers; maintaining an essentially constant fluxsubstantially the same as the equilibrium flux initially obtained, indicating that the surfaces of the fibers are substantially free from further build-up of deposits once the equilibrium flux is attained; collecting the permeate; and, withdrawing thepermeate.
It has still further been discovered that the foregoing process may be used in the operation of an anaerobic or aerobic biological reactor which has been retrofitted with the membrane device of this invention. The anaerobic reactor is a closedvessel and the scrubbing gas is a molecular oxygen-free gas, such as nitrogen.
It is therefore a general object of this invention to provide an aerobic biological reactor retrofitted with at least one gas-scrubbed bank of vertical skeins, each skein made with from 500 to 5000 fibers in the range from 1 m to 3 m long, incombination with a permeate collection means, and to provide a process for the reactor's operation without being encumbered by the numerous restrictions and limitations imposed by a secondary clarification system.
A novel composite header is provided for a bundle of hollow fiber membranes or "fibers", the composite header comprising a molded, laminated body of arbitrary shape, having an upper lamina formed from a "fixing" (potting) material which islaminated to a lower lamina formed from a "fugitive" potting material. The terminal portions of the fibers are potted in the fugitive potting material when it is liquid, preferably forming a generally rectangular parallelpiped in which the open ends ofthe fibers (until potted) are embedded and plugged, keeping the fibers in closely spaced-apart substantially parallel relationship. The plugged ends of the fibers fail to protrude through the lower (aft) face of the lower lamina, while the remaininglengths of the fibers extend through the upper face of the lower lamina. The upper lamina extends for a height along the length of the fibers sufficient to maintain the fibers in the same spaced-apart relationship relative to one and another as theirspaced-apart relationship in the lower portion. If desired, the composite header may include additional laminae, for example, a "cushioning" lamina overlying the fixing lamina, to cushion, each fiber around its embedded outer circumference; and, a"gasketing" lamina to provide a suitable gasketing material against which the permeate collection means may be mounted.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects and advantages of the invention will best be understood by reference to the following detailed description, accompanied by schematic illustrations of preferred embodiments of the invention, in whichillustrations like reference numerals refer to like elements, and in which:
FIG. 1 is a graph in which the variation of flux is plotted as a function of time, showing three curves for three runs made with three different arrays, in each case, using the same amount of air, the identical membranes and the same membranesurface area. The results obtained by Yamamoto et al are plotted as curve 2 (under conditions modified to give them the benefit of doubt as to the experimental procedure employed, as explained below); the flux obtained using the gas-scrubbed assembly ofthe '424 patent is shown as curve 1; and the flux obtained using the gas-scrubbed assembly of this invention is shown as curve 3.
FIG. 2 is a perspective exploded view schematically illustrating a membrane device comprising a skein of fibers, unsupported during operation of the device, with the ends of the fibers potted in a lower header, along with a permeate collectionpan, and a permeate withdrawal conduit. By "unsupported" is meant `not supposed except for spacer means to space the headers`.
FIG. 2A is an enlarged detail side elevational view of a side wall of a collection pan showing the profile of a header-retaining step atop the periphery of the pan.
FIG. 2B is a bottom plan view of the header showing a random pattern of open ends protruding from the aft face of a header when fibers are potted after they are stacked in rows and glued together before being potted.
FIG. 3 is a perspective view of a single array, schematically illustrated, of a row of substantially coplanarly disposed parallel fibers secured near their opposed terminal ends between spaced apart cards. Typically, multiple arrays areassembled before being sequentially potted.
FIG. 4 illustrates a side elevational view of a stack of arrays near one end where it is clamped together, showing that the individual fibers (only the last fiber of each linear array is visible, the remaining fibers in the array being directlybehind the last fiber) of each array are separated by the thickness of a strip with adhesive on it, as the stack is held vertically in potting liquid.
FIG. 5 is a perspective view schematically illustrating a skein with its integral finished header, its permeate collection pan, and twin air-tubes feeding an integral air distribution manifold potted in the header along an outer edge of the skeinfibers.
FIG. 6 is a side elevational view of an integral finished header showing details of a permeate pan submerged in substrate, the walls of the header resting on the bottom of a reservoir, and multiple air-tubes feeding integral air distributionmanifolds potted in the header along each outer edge of the skein fibers.
FIG. 7A is a perspective view schematically illustrating an air-manifold from which vertical air-tubes rise.
FIG. 7B is a perspective view schematically illustrating a tubular air-manifold having a transverse perforated portion, positioned by opposed terminal portions.
FIG. 8 is a perspective view of an integral finished header having plural skeins potted in a common header molded in an integral permeate collection means with air-tubes rising vertically through the header between adjacent skeins, and along theouter peripheries of the outer skeins.
FIG. 9 is a detail, not to scale, illustratively showing a gas distribution means discharging gas between arrays in a header, and optionally along the sides of the lower header.
FIG. 10 is a perspective view schematically illustrating a pair of skeins in a bank in which the upper headers are mounted by their ends on the vertical wall of a tank. The skeins in combination with a gas-distribution means form a"gas-scrubbing assembly" deployed within a substrate, with the fibers suspended essentially vertically in the substrate. Positioning the gas-distribution means between the lower headers (and optionally, on the outside of skein fibers) generate masses(or "columns") of bubbles which rise vertically, codirectionally with the fibers, yet the bubbles scrub the outer surfaces of the fibers.
FIG. 11 is a perspective view of another embodiment of the scrubbing-assembly showing plural skeins (only a pair is shown) connected in a bank with gas-distribution means disposed between successive skeins, and, optionally, with additionalgas-distribution means fore and aft the first and last skeins, respectively.
FIG. 12 is an elevational view schematically illustrating a bank of skeins mounted against the wall of a bioreactor, showing the convenience of having all piping connections outside the liquid.
FIG. 13 is a plan view of the bioreactor shown in FIG. 12 showing how multiple banks of skeins may be positioned around the circumference of the bioreactor to form a large permeate extraction zone while a clarification zone is formed in thecentral portion with the help of baffles.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The skein of this invention may be used in a liquid--liquid separation process of choice, and more generally, in various separation processes. The skein is specifically adapted for use in microfiltration processes used to remove large organicmolecules, emulsified organic liquids and colloidal or suspended solids, usually from water. Typical applications are (i) in a membrane bioreactor, to produce permeate as purified water and recycle biomass; for (ii) tertiary filtration of wastewater toremove suspended solids and pathogenic bacteria; (iii) clarification of aqueous streams including filtration of surface water to produce drinking water (removal of colloids, long chain carboxylic acids and pathogens); (iv) separation of a permeableliquid component in biotechnology broths; (v) dewatering of metal hydroxide sludges; and, (vi) filtration of oily wastewater, inter alia.
The problem with using a conventional membrane module to selectively separate one fluid from another, particularly using the module in combination with a bioreactor, and the attendant costs of operating such a system, have been avoided. In thoseinstances where an under-developed country or distressed community lacks the resources to provide membrane modules, the most preferred embodiment of this invention is adapted for use without any pumps. In those instances where a pump is convenientlyused, a vacuum pump is unnecessary, adequate driving force being provided by a simple centrifugal pump incapable of inducing a vacuum of 75 cm Hg on the suction side.
The fibers used to form the skein may be formed of any conventional membrane material provided the fibers are flexible and have an average pore cross sectional diameter for microfiltration, namely in the range from about 1000 .ANG. to 10000.ANG.. Preferred fibers operate with a transmembrane pressure differential in the range from 7 kPa (1 psi)-69 kPa (10 psi) and are used under ambient pressure with the permeate withdrawn under gravity. The fibers are chosen with a view to perform theirdesired function, and the dimensions of the skein are determined by the geometry of the headers and length of the fibers. It is unnecessary to confine a skein in a modular shell, and a skein is not.
Preferred fibers are made of organic polymers and ceramics, whether isotropic, or anisotropic, with a thin layer or "skin" on the outside surface of the fibers. Some fibers may be made from braided cotton covered with a porous natural rubberlatex or a water-insoluble cellulosic polymeric material. Preferred organic polymers for fibers are polysulfones, poly(styrenes), including styrene-containing copolymers such as acrylonitrile-styrene, butadiene-styrene and styrene-vinylbenzylhalidecopolymers, polycarbonates, cellulosic polymers, polypropylene, poly(vinyl chloride), poly(ethylene terephthalate), and the like disclosed in U.S. Pat. No. 4,230,463 the disclosure of which is incorporated by reference thereto as if fully set forthherein. Preferred ceramic fibers are made from alumina, by E.I. dupont deNemours Co. and disclosed in U.S. Pat. No. 4,069,157.
Typically, there is no cross flow of substrate across the surface of the fibers in a "dead end" tank. If there is any flow of substrate through the skein in a dead end tank, the flow is due to aeration provided beneath the skein, or to suchmechanical mixing as may be employed to maintain the solids in suspension. There is more flow through the skein in a tank into which substrate is being continuously flowed, but the velocity of fluid across the fibers is generally too insignificant todeter growing microorganisms from attaching themselves, or suspended particles, e.g. microscopic siliceous particles, from being deposited on the surfaces of the fibers.
For hollow fiber membranes, the outside diameter of a fiber is at least 20 .mu.m and may be as large as about 3 mm, typically being in the range from about 0.1 mm to 2 mm. The larger the outside diameter the less desirable the ratio of surfacearea per unit volume of fiber. The wall thickness of a fiber is at least 5 .mu.m and may be as much as 1.2 mm, typically being in the range from about 15% to about 60% of the outside diameter of the fiber, most preferably from 0.5 mm to 12 mm.
As in a '424 array, but unlike in a conventional module, the length of a fiber in a skein is essentially independent of the strength of the fiber, or its diameter, because the skein is buoyed both by bubbles and the substrate in which it isdeployed. The length of fibers in the skein is preferably determined by the conditions under which the skein is to operate. Typically fibers range from 1 m to about 5 m long, depending upon the dimensions of the body of substrate (depth and width) inwhich the skein is deployed.
The fixing material to fix the fibers in a finished header is most preferably either a thermosetting or thermoplastic synthetic resinous material, optionally reinforced with glass fibers, boron or graphite fibers and the like. Thermoplasticmaterials may be crystalline, such as polyolefins, polyamides (nylon), polycarbonates and the like, semi-crystalline such as polyetherether ketone (PEEK), or substantially amorphous, such as poly(vinyl chloride) (PVC), polyurethane and the like. Thermosetting resins commonly include polyesters, polyacetals, polyethers, cast acrylates, thermosetting polyurethanes and epoxy resins. Most preferred as a "fixing" material (so termed because it fixes the locations of the fibers relative to eachother) is one which when cured is substantially rigid in a thickness of about 2 cm, and referred to generically as a "plastic" because of its hardness. Such a plastic has a hardness in the range from about Shore D 50 to Rockwell R 110 and is selectedfrom the group consisting of epoxy resins, phenolics, acrylics, polycarbonate, nylon, polystyrene, polypropylene and ultra-high molecular weight polyethylene (UHMW PE). Polyurethane such as is commercially available under the brand names Adiprene.RTM. from Uniroyal Chemical Company and Airthane.RTM. from Air Products, and commercially available epoxy resins such as Epon 828 are excellent fixing materials.
The number of fibers in an array is arbitrary, typically being in the range from about 1000 to about 10000 for commercial applications, and the preferred surface area for a skein is in the range from 10 m.sup.2 to 100 m.sup.2.
The particular method of securing the fibers in each of the headers is not narrowly critical, the choice depending upon the materials of the header and the fiber, and the cost of using a method other than potting. However, it is essential thateach of the fibers be secured in fluid-tight relationship within each header to avoid contamination of permeate. This is effected by potting the fibers essentially vertically, in closely-spaced relationship, either linearly in plural equally spacedapart rows across the face of a header in the x-y plane; or alternatively, randomly in non-linear plural rows. In the latter, the fibers are displaced relative to one another in the lateral direction.
FIG. 1 presents the results of a comparison of three runs made, one using the teachings of Yamamoto in his '89 publication (curve 2), but using an aerator which introduced air from the side and directed it radially inwards, as is shown inChiemchaisri et al. A second run (curve 1) uses the gas-scrubbed assembly of the '424 patent, and the third run (curve 3) uses the gas-scrubbed skein of this invention. The specific flux obtained with an assembly of an inverted parabolic array with anair distributor means (Yamamoto et al), as disclosed in Wat. Sci. Tech. Vol. 21, Brighton pp 43-54, 1989, and, the parabolic array by Cote et al in the '424 patent, are compared to the specific flu obtained with the vertical skein of this invention.
The comparison is for the three assemblies having fibers with nominal pore size 0.2 .mu.m with essentially identical bores and surface area in 80 L tanks filled with the same activated sludge substrate. The differences between the statedexperiment of Yamamoto et al, and that of the '424 patent are of record in the '424 patent, and the conditions of the comparison are incorporated by reference thereto as if fully set forth herein. The vertical skein used herein differs from the '424skein only in the vertical configuration of the 280 fibers each of which was about 1% longer than the distance between the spaced apart headers during operation. The flow rate of air for the vertical skein is 1.4 m.sup.3/hr/m.sup.2 using a coarse bubblediffuser.
It will be evident from FIG. 1 in which the specific flux, liters/meter.sup.2 hr/kPa (conventionally written as (lmh/k.Pa), is plotted as a function of operating time for the three assemblies, that the curve, identified as reference numeral 3 forthe flux for the vertical skein, provides about the same specific flu as the parabolic skein, identified as reference numeral 1. As can be seen, each specific flu reaches an equilibrium condition within less than 50 hr, but after about 250 hr, it isseen that the specific flux for the inverted parabolic array keeps declining but the other two assemblies reach an equilibrium.
Referring to FIG. 2 there is illustrated, in exploded view a portion of a membrane device referred to as a "vertical skein" 10, comprising a lower header 11 of a pair of headers, the other upper header (not shown) being substantially identical; acollection pan 20 to collect the permeate; and, a permeate withdrawal conduit 30. The header shown is a rectangular prism since this is the most convenient shape to make. If one is going to pot fibers 12 in a potting resin such as a polyurethane or anepoxy. Though the fibers 12 are not shown as close together as they would normally be, it is essential that the fibers are not in contact with each other but that they be spaced apart by the cured resin between them.
As illustrated, the open ends of the terminal portions 12' of the fibers are in the same plane as the lower face of the header 11 because the fibers are conventionally potted and the header sectioned to expose the open ends. A specific pottingprocedure in which the trough of a U-shaped bundle of fibers is potted, results in forming two headers. This procedure is described in the '424 patent (col 17, lines 44-61); however, even cutting the potted fibers with a thin, high-speed diamond blade,tends to damage the fibers and initiate the collapse of the circumferential wall. In another conventional method of potting fibers, described in U.S. Pat. No. 5,202,023, bundled fibers have their ends dipped in resin or paint to prevent potting resinpenetration into the bores of the fibers during the potting process. The ends of the bundle are then placed in molds and uncured resin added to saturate the ends of the fiber bundle and fill the spaces between the individual fibers in the bundle and theflexible tubing in which the bundle is held. The cured molded ends are removed from the molds and the molded ends cut off (see, bridging cols 11 and 12). In each prior art method, sectioning the mold damages the embedded fibers.
Therefore a novel method is used to form a header 11 in the form of a rectangular prism. The method requires forming a composite header with two liquids. A first liquid fugitive material, when solidified (cured), forms a "fugitive lamina" ofthe composite header; a second liquid of non-fugitive fixing material forms a "fixing lamina". By a "fugitive material" we refer to a material which is either (i) soluble in a medium in which the fibers and fixing material are not soluble, or (ii)fluidizable by virtue of having a melting point (if the material is crystalline) below that which might damage the fibers or fixing material; or, the material has a glass transition temperature Tg (if the material is non-crystalline), below that whichmight damage the fibers or material(s) forming the non-fugitive header; or (iii) both soluble and fluidizable.
The first liquid is poured around terminal portions of fibers, allowed to cool and solidify into a fugitive lamina; the fibers in the fugitive lamina are then again potted, this time by pouring the second liquid over the solid fugitive lamina.
In greater detail, the method for forming a finished header for skein fibers comprises, forming a stack of at least two superimposed essentially coplanar and similar arrays, each array comprising a chosen number of fibers supported on a supportmeans having a thickness corresponding to a desired lateral spacing between adjacent arrays; holding the stack in a first liquid with terminal portions of the fibers submerged, until the liquid solidifies into a first shaped lamina, provided that thefirst liquid is unreactive with material of the fibers; pouring a second liquid over the first shaped lamina to embed the fibers to a desired depth, and solidifying the second liquid to form a fixing lamina upon the first shaped lamina, the second liquidalso being substantially unreactive with either the material of the fibers or that of the first shaped lamina; whereby a composite header is formed in which terminal portions of the fibers are potted, preferably in a geometrically regular pattern, thecomposite header comprising a laminate of a fugitive lamina of fugitive material and a contiguous finished header of fixing lamina; and thereafter, removing the first shaped lamina without removing a portion of the fixing lamina so as to leave the endsof the fibers open and protruding from the aft face of the header, the open ends having a circular cross-section.
The step-wise procedure for forming an array "A" with the novel header is described with respect to an array illustrated in FIG. 3, as follows:
A desired number of fibers 12 are each cut to about the same length with a sharp blade so as to leave both opposed ends of each fiber with an essentially circular cross-section. The fibers are coplanarly disposed side-by-side in a linear arrayon a planar support means such as strips or cards 15 and 16. Preferably the strips are coated with an adhesive, e.g. a commercially available polyethylene hot-melt adhesive, so that the fibers are glued to the strips and opposed terminal portions 12''respectively of the fibers, extend beyond the strips. Intermediate portions 12' of the fibers are thus secured on the strips. Alternatively, the strips may be grooved with parallel spaced-apart grooves which snugly accommodate the fibers. The stripsmay be flexible or rigid. If flexible, strips with fibers adhered thereto, are in turn, also adhered to each other successively so as to form a progressively stiffer stack for a header having a desired geometry of potted fibers. To avoid gluing thestrips, a regular pattern of linear rows may be obtained by securing multiple arrays on rigid strips in a stack, with rubber bands 18 or other clamping means. The terminal portions 12'' are thus held in spaced-apart relationship, with the center tocenter distance of adjacent fibers preferably in the range from 1.2 (1.2 d) to about 5 times (5 d) the outside diameter meter `d` of a fiber. Spacing the fibers further apart wastes space and spacing them closer increase the risk of fiber-to-fibercontact near the terminal end portions when the ends are potted. Preferred center-to-center spacing is from about 1.5 d to 2 d. The thickness of a strip and/or adhesive is sufficient to ensure that the fibers are kept spaced apart. Preferably, thethickness is about the same as, or relatively smaller than the outside diameter of a fiber, preferably from about 0.5 d to 1 d thick, which becomes the spacing between adjacent outside surfaces of fibers in successive linear arrays.
Having formed a first array, a second array (not shown because it would appear essentially identical to the first) is prepared in a manner analogous to the first, strip 15 of the second array is overlaid upon the intermediate portions 12' onstrip 15 of the first array, the strip 15 of the second array resting on the upper surfaces of the fibers secured in strip 15 of the first array. Similarly, strip 16 of the second array is overlaid upon the intermediate portions 12' on strip 16 of thefirst array.
A third array (essentially identical to the first and second) is prepared in a manner analogous to the first, and then overlaid upon the second, with the strips of the third array resting on the upper surfaces of the fibers of the second array.
Additional arrays are overlaid until the desired number of arrays are stacked in rows forming a stack of arrays with the adhesive-coated strips forming the spacing means between successive rows of fibers. The stack of arrays on strips is thenheld vertically to present the lower portion of the stack to be potted first.
Referring to FIG. 4, there is schematically illustrated a rectangular potting pan 17 the length and width dimensions of which correspond substantially to the longitudinal (x-axis) and transverse (y-axis) dimensions respectively, of the desiredheader. The lower stack is submerged in a first liquid which rises to a level indicated by L1, in the pan 17. Most preferred is a liquid wax, preferably a water-soluble wax having a melting point lower than 75.degree. C., such as a polyethylene glycol(PEG) wax.
The depth to which the first liquid is poured will depend upon whether the strips 15 are to be removed from, or left in the finished header. A. First illustrated is the potting of skein fibers in upper and lower headers from which the stripswill be removed.
(1) A first shaped lamina having a thickness L1 (corresponding to the depth to which the first liquid was poured) is formed to provide a fugitive lamina from about 5-10 cm thick. The depth of the first liquid is sufficient to ensure that boththe intermediate portions 12' on the strips and terminal portions 12'' will be held spaced apart when the first liquid solidifies and plugs all the fibers.
(2) The second liquid, a curable, water-insoluble liquid potting resin, or reactive components thereof, is poured over the surface of the fugitive lamina to surround the fibers, until the second liquid rises to a level L2. It is solidified toform the fixing lamina (which will be the finished header) having a thickness measured from the level L1 to the level L2 (the thickness is written "L1-L2"). The thickness L1-L2 of the fixing lamina, typically from about 1 cm to about 5 cm, is sufficientto maintain the relative positions of the vertical fibers. A first composite header is thus formed having the combined thicknesses of the fugitive and fixing laminae.
(3) In a manner analogous to that described immediately hereinabove, a stack is potted in a second composite header.
(4) The composite headers are demolded from their potting pans and hot air blown over them to melt the fugitive laminae, leaving only the finished headers, each having a thickness L1-L2. The fugitive material such as the PEG wax, is then reused. Alternatively, a water-soluble fugitive material may be placed in hot water to dissolve the wax, and the material recovered from its water solution.
(5) The adhered strips and terminal portions of the fibers which were embedded within the fugitive lamina are left protruding from the permeate-discharging aft faces of the headers with the ends of the fibers being not only open, but essentiallycircular in cross section. The fibers may now be cut above the strips to discard them and the terminal portions of the fibers adhered to them, yet maintaining the circular open ends. The packing density of fibers, that is, the number of fibers per unitarea of header preferably ranges from 4 to 50 fibers/cm.sup.2 depending upon the diameters of the fibers. B. Illustrated second is the potting of skein fibers in upper and lower headers from which the strips will not be removed, to avoid the step ofcutting the fibers.
(1) The first liquid is poured to a level L1' below the cards, to a depth in the range from about 1-2.5 cm, and solidified, forming fugitive lamina L1'.
(2) The second liquid is then poured over the fugitive lamina to depth L2 and solidified, forming a composite header with a fixing lamina having a thickness L1'-L2.
(3) The composite header is demolded and the fugitive lamina removed, leaving the terminal portions 12'' protruding from the aft face of the finished header, which aft face is formed at what had been the level L1'. The finished header having athickness L1'-L2 embeds the strips 15 (along with the rubber bands, 18, if used). C. Illustrated third is the potting of skein fibers to form a finished headers with a cushioning lamina embedding the fibers on the opposed (fore) faces of the headersfrom which the strips will be removed.
The restricted swayability of the fibers generates some intermittent `snapping` motion of the fibers. This motion has been found to break the potted fibers around their circumferences, at the interface of the fore face and substrate. Thehardness of the fixing material which forms a "fixing lamina" was found to initiate excessive shearing forces at the circumference of the fiber. The deleterious effects of such forces is minimized by providing a cushioning lamina of material softer thanthe fixing lamina. Such a cushioning lamina is formed integrally with the fixing lamina, by pouring cushioning liquid (so termed for its function when cured) over the fixing lamina to a depth L3 as shown in FIG. 4, which depth is sufficient to provideenough `give` around the circumferences of the fibers to minimize the risk of shearing. Such cushioning liquid, when cured is rubbery, having a hardness in the range from about Shore A 30 to Shore D 45, and is preferably a polyurethane or silicone orother rubbery material which will adhere to the fixing lamina. Upon removal of the fugitive lamina, the finished header thus formed has the combined thicknesses of the fixing lamina and the cushioning lain/ha, namely L1-L3 when the strips 15 are cutaway. D. Illustrated fourth is the formation a finished header with a gasketing lamina embedding the fibers on the header's aft face, and a cushioning lamina embedding the fibers on the header's fore face; the strips are to be removed.
Whichever finished header is made, it is preferably fitted into a permeate pan 20 as illustrated in FIG. 2 with a peripheral gasket. It has been found that it is easier to seal the pan against a gasketing lamina, than against a peripheral narrowgasket. A relatively soft gasketing material having a hardness in the range from Shore A 40 to Shore D 45, is desirable to form a gasketing lamina integrally with the aft face of the finished header. In the embodiment in which the strips are cut away,the fugitive lamina is formed as before, and a gasketing liquid (so termed because it forms the gasket when cured) is poured over the surface of the fugitive lamina to a depth L4. The gasketing liquid is then cured. Upon removal of the fugitive lamina,when the strips 15 are cut away, the finished header thus formed has the combined thicknesses of the gasketing lamina (L1-L4), the fixing lamina (L4-L2) and the cushioning lamina (L2-L3), namely an overall L1-L3.
In another embodiment, to avoid securing the pan to the header with a gasketing means, and, to avoid positioning one or more gas-distribution manifolds in an optimum location near the base of the skein fibers after a skein is made, the manifoldsare formed integrally with a header. Referring to FIG. 5 there is illustrated in perspective view an "integral single skein" referred to generally by reference numeral 100. The integral single skein is so termed because it includes an integral finishedheader 101 and permeate pan 102. The pan 102 is provided with a permeate withdrawal nipple 106, and fitted with vertical air-tubes 103 which are to be embedded in the finished header. The air-tubes are preferably manifolded on either side of the skeinfibers, to feeder air-tubes 104 and 105 which are snugly inserted through grommets in the walls of the pan. The permeate nipple 106 is then plugged, and a stack of arrays is held vertically in the pan in which a fugitive lamina is formed embedding boththe ends of the fibers and the lower portion of the vertical air-tubes 103. A fixing lamina is then formed over the fugitive lamina, embedding the fibers to form a fixing lamina through which protrude the open ends of the air-tubes 103. The fugitivelamina is then melted and withdrawn through the nipple 106. In operation, permeate collects in the permeate pan and is withdrawn through nipple 106.
FIG. 6 illustrates a cross-section of an integral single skein 110 with another integral finished header 101 having a thickness L1-L2, but without a cushioning lamina, formed in a procedure similar to that described hereinabove. A permeate pan120 with outwardly flared sides 120' and transversely spaced-apart through-apertures therein, is pre-fabricated between the side walls 111 and 112 so the pan is spaced above the bottom of the reservoir.
A pair of air-manifolds 107 such as shown in FIGS. 7A or 7B, is positioned and held in mirror-image relationship with each other adjacent the permeate pan 120, with the vertical air-tubes 103 protruding through the apertures in sides 120', andthe ends 104 and 105 protrude from through-passages in the vertical walls on either side of the permeate pan. Permeate withdrawal nipple 106 (FIG. 6) is first temporarily plugged. The stack of strips 15 is positioned between air-tubes 103, verticallyin the pan 120 which is filled to level L1 to form a fugitive lamina, the level being just beneath the lower edges of the strips 15 which will not be removed. When solidified, the fugitive lamina embeds the terminal portions of the fibers 12 and alsofills permeate tube 106. Then the second liquid is poured over the upper surface of the fugitive lamina until the liquid covers the strips 15 but leaves the upper ends of the air-tubes 103 open. The second liquid is then cured to form the fixing laminaof the composite header which is then heated to remove the fugitive material through the permeate nozzle 106 after it is unplugged.
FIG. 7A schematically shows in perspective view, an air-manifold 107 having vertical air-tubes 103 rising from a transverse header-tube which has longitudinally projecting feeder air-tubes 104 and 105. The bore of the air-tubs which may beeither "fine bubble diffusers", or "coarse bubble diffusers", or "aerators", is chosen to provide bubbles of the desired diameter under operating conditions, the bore typically being in the range from 0.1 mm to 5 mm. Bubbles of smaller diameter arepreferably provided with a perforated transverse tube 103' of an air-manifold 107' having feeder air-tubes 104' and 105', illustrated in FIG. 7B. In each case, the bubbles function as a mechanical brush.
The skein fibers for the upper header of the skein are potted in a manner analogous to that described above in a similar permeate pan to form a finished header, except that no air manifolds are inserted.
Referring to FIG. 8 there is schematically illustrated, in a cross-sectional perspective view, an embodiment in which a bank of two skeins is potted in a single integral finished header enclosure, referred to generally by reference numeral 120b. The term "header enclosure" is used because its side walls 121 and 122, and end walls (not shown) enclose a plenum in which air is introduced. Instead of a permeate pan, permeate is collected from a permeate manifold which serves both skeins. Anothersimilar upper enclosure 120u (not shown), except that it is a flat-bottomed channel-shaped pan (inverted for use as the upper header) with no air-tubes molded in it, has the opposed terminal portions of all the skein fibers potted in the pan. Foroperation, both the lower and upper enclosures 120b and 120u, with their skein fibers are lowered into a reservoir of the substrate to be filtered. The side walls 121 and 122 need not rest on the bottom of the reservoir, but may be mounted on a sidewall of the reservoir.
The side walls 121 and 122 and end walls are part of an integrally molded assembly having a platform 123 connecting the walls, and there are aligned multiple risers 124 molded into the platform. The risers resemble an inverted test-tube, thediameter of which need only be large enough to have an air-tube 127 inserted through the top 125 of the inverted test-tube. As illustrated, it is preferred to have "n+1" rows of air-tubes for "n" stacks of arrays to be potted. Crenelated platform 123includes risers 124 between which lie channels 128 and 129. Channels 128 and 129 are each wide enough to accept a stack of arrays of fibers 12, and the risers are wide enough to have air-tubes 127 of sufficient length inserted therethrough so that theupper open ends 133 of the air-tubes protrude from the upper surface of the fixing material 101. The lower ends 134 of the air-tubes are sectioned at an angle to minimize plugging, and positioned above the surface S of the substrate. The channel 129 isformed so as to provide a permeate withdrawal tube 126 integrally formed with the platform 123. Side wall 122 is provided with an air-nipple 130 through which air is introduced into the plenum formed by the walls of the enclosure 120b, and the surface Sof substrate under the platform 123. Each stack is potted as described in relation to FIG. 6 above, most preferably by forming a composite header of fugitive PEG wax and epoxy resin around the stacks of arrays positioned between the rows of risers 124,making sure the open ends of the air-tubes are above the epoxy fixing material, and melting out the wax through the permeate withdrawal tube 126. When air is introduced into the enclosure the air will be distributed through the air-tubes between andaround the skeins.
Referring to FIG. 9 there is shown a schematic illustration of a skein having upper and lower headers 41u and 41b respectively, and in each, the protruding upper and lower ends 12u'' and 12b'' are evidence that the face of the header was not cutto expose the fibers. The height of the contiguous intermediate portions 12u' and 12b' respectively, corresponds to the cured depth of the fixing material.
It will now be evident that the essential feature of the foregoing potting method is that a fugitive lamina is formed which embeds the openings of the terminal portions of the fibers before their contiguous intermediate portions 12u' and 12u''and 12b' and 12b'' are fixed respectively in a fixing lamina of the header. An alternative choice of materials in the use of a fugitive potting compound which is soluble in a non-aqueous liquid in which the fixing material is not soluble. Still anotherchoice is to use a water-insoluble fugitive material which is also insoluble in non-aqueous liquids typically used as solvents, but which fugitive material has a lower melting point than the final potting material which may or may not be water-soluble.
The fugitive material is inert relative to both, the material of the fibers as well as the final potting material to be cast, and the fugitive material and fixing material are mutually insoluble. Preferably the fugitive material forms asubstantially smooth-surfaced solid, but it is critical that the fugitive material be at least partially cured, sufficiently to maintain the shape of the header, and remain a solid above a temperature at which the fixing material is introduced into theheader mold. The fugitive lamina is essentially inert and insoluble in the final potting material, so that the fugitive lamina is removably adhered to the fixing lamina.
The demolded header is either heated or solvent extracted to remove the fugitive lamina. Typically, the fixing material is cured to a firm solid mass at a first curing temperature no higher than the melting point or Tg of the fugitive lamina,and preferably at a temperature lower than about 60.degree. C.; the firm solid is then post-cured at a temperature high enough to melt the fugitive material but not high enough to adversely affect the curing of the fixing material or the properties ofthe fibers. The fugitive material is removed as described hereinafter, the method of removal depending upon the fugitive material and the curing temperature of the final potting material used.
Since, during operation, a high flux is normally maintained If cleansing air contacts substantially all the fibers, it will be evident that when it is desirable to have a skein having a cross-section which is other than generally rectangular, forexample elliptical or circular, or having a geometrically irregular periphery, and it is desired to have a large number of skein fibers, it will be evident that the procedure for stacking consecutive peripheral arrays described above will be modified. Further, the transverse central air-tube 52 (see FIG. 9) is found to be less effective in skeins of non-rectangular cross-section than a vertical air-tube which discharges air radially along its vertical length and which vertical air-tube concurrentlyserves as the spacing means. Such skeins with a generally circular or elliptical cross-section with vertical air-tubes are less preferred to form a bank, but provide a more efficient use of available space in a reservoir than a rectangular skein.
Referring further to FIG. 2, the header 11 has front and rear walls defined by vertical (z-axis) edges 11' and longitudinal (x-axis) edges 13'; side walls defined by edges 11' and transverse (y-axis) edges 13''; and a base 13 defined by edges 13'and 13''.
The collection pan 20 is sized to snugly accommodate the base 13 above a permeate collection zone within the pan. This is conveniently done by forming a rectangular pan having a base 23 of substantially the same length and width dimensions asthe base 13. The periphery of the pan 20 is provided with a peripheral step as shown in FIG. 2A, in which the wall 20' of the pan terminates in a step section 22, having a substantially horizontal shoulder 22'' and a vertical retaining wall 22'.
FIG. 2B is a bottom plan view of the lower face of header 13 showing the open ends of the fibers 12' prevented from touching each other by potting resin. The geometrical distribution of fibers provides a regular peripheral boundary 14 (shown indotted outline) which bounds the peripheries of the open ends of the outermost fibers.
Permeate flows from the open ends of the fibers onto the base 23 of the pan 20, and flows out of the collection zone through a permeate withdrawal conduit 30 which may be placed in the bottom of the pan is open flow communication with the innerportion of the pan. When the skein is backwashed, backwashing fluid flows through the fibers and into the substrate. If desired, the withdrawal conduit may be positioned in the side of the pan as illustrated by conduit 30'. Whether operating undergravity alone, or with a pump to provide additional suction, it will be apparent that a fluid-tight seal is necessary between the periphery of the header 11 and the peripheral step 22 of the pan 20. Such a seal is obtained by using any conventionalmeans such as a suitable sealing gasket or sealing compound, typically a polyurethane or silicone resin, between the lower periphery of the header 11 and the step 22. As illustrated in FIG. 2, permeate drains downward, but it could also be withdrawnfrom upper permeate port 45u in the upper permeate pan 43u (see FIG. 9).
It will now be evident that a header with a circular periphery may be constructed, if desired. Headers with geometries having still other peripheries (for example, an ellipse) may be constructed in an analogous manner. If desired, butrectangular headers are most preferred for ease of construction with multiple linear arrays.
Referring to FIGS. 9 and 2A, six rows of fibers 12 are shown on either side of a gas distribution line 52 which traverses the length of the rows along the base of the fibers. The potted terminal end portions 12b'' open into permeate pan 43b. Because portions 12u' and 12b' of individual fibers 12 are potted, and the fibers 12 are preferably from 1% to 2% longer than the fixed distance between upper and lower headers 41u and 41b, the fibers between opposed headers are generally parallel to oneanother, but are particularly parallel near each header. Also held parallel are the terminal end portions 12u'' and 12b'' of the fibers which protrude from the headers with their open ends exposed. The fibers protrude below the lower face of the bottomheader 41b, and above the upper face of the upper header 41u. The choice of fiber spacing in the header will determine packing density of the fibers near the headers, but fiber spacing is not a substantial consideration because spacing does notsubstantially affect specific flux during operation. It will be evident however, that the more fibers, the more tightly packed they will be, giving more surface area.
Since the length of fibers tends to change while in service, the extent of the change depending upon the particular composition of the fibers, and the spacing between the upper and lower headers is critical, it is desirable to mount the headersso that one is adjustable in the vertical direction relative to the other, as indicated by the arrow V. This is conveniently done by attaching the pan 43u to a plate 19 having vertically spaced apart through-passages 34 through which a threaded stud 35is inserted and secured with a nut 36. Threaded stud 35 is in a fixed mounting block 37.
The density of fibers in a header is preferably chosen to provide the maximum membrane surface area per unit volume of substrate without adversely affecting the circulation of substrate through the skein. A gas-distribution means 52 such as aperforated air-tube, provides air within the skein so that bubbles of gas (air) rise upwards while clinging to the outer surfaces of the fibers, thus efficiently scrubbing them. If desired, additional air-tubes 52' may be placed on either side of theskein near the lower header 41b, as illustrated in phantom outline, to provide additional air-scrubbing power. Whether the permeate is withdrawn from the upper header through port 45u or the lower header through port 45, or both, depends upon theparticular application, but in all instances, the fibers have a substantially vertical orientation.
The vertical skein is deployed in a substrate to present a generally vertical profile, but has no structural shape. Such shape as it does have changes continuously, the degree of change depending upon the flexibility of the fibers, theirlengths, the overall dimensions of the skein, and the degree of movement imparted to the fibers by the substrate and also by the oxygen-containing gas from the gas-distribution means.
Referring to FIG. 10 there is illustrated a typical assembly referred to as a "wall-mounted bank" which includes at least two side-by-side skeins, indicated generally by reference numerals 40 and 40' with their fibers 42 and 42'; fibers 42 arepotted in upper and lower headers 41u and 41b respectively; and fibers 42' in headers 41u' and 41b'; headers 41u and 41b are fitted with permeate collecting means 46u and 46b respectively; headers 41u' | | | |
