GB2145709A - Membrane system for water purification - Google Patents

Abstract

A system for obtaining ultra pure water employing a novel combination of three membrane processes i.e., ultrafiltration, electrodialysis and reverse osmosis arranged in a series configuration with each membrane unit progressively purifying the water until the desired product purity is reached.

Description

SPECIFICATION Membrane system for water purification This invention relates to water treatment; and more particularly to the production of ultra pure water by the use of a combination of membrane processes.

The invention further relates to the production of ultra pure water using a mobile water treatment facility.

Recent advances in technology have made the availability of ultra pure water a necessity in many industrial and scientific applications. Research, cosmetic and pharmaceutical manufacture and the electronic industry have a need for water having purity levels set forth by various groups such as the American Society for Testing Materials (ASTM) and others.

Although prior art water purifying units will furnish water sufficient to meet established specifications, such methods contain many disadvantages. A prior method of employing media and/or cartridge filtration, followed by reverse osmosis (RO), and on site regenerated ion-exchange treatment has the following drawbacks: (a) frequent backwashing of the media or replacement of filter cartridges; (b) bacterial growth in the media; (c) chemical handling and waste disposal resulting fron on-site ion-exchange regeneration and most important; (d) the necessity of a chemical feed to acidify the water prior to reverse osmosis treatment.

A prior art treatment of water using mobile units containing only ion-exchange columns is disclosed for example in U.S. Patents Nos. 4,049,548 and 4,383,920. The use of ion-exchange with filtration and/or reverse osmosis is disclosed in U.S Patents Nos. 4,280,912, 3,766,060,4,188,291, 4,332,685 and others. Another prior art treatment involves the use of electrodialysis prior to ion-exchange treatment with final polishing of the resulting water using hollow-fibre ultra filtration units. (Zmolek, C.R., "Ultrapure Water for Integrated Circuits Proces sing," Industrial Water Engineering (December, 1977).

The present invention seeks to provide a water treatment system which will economically provide purified water over long time periods without requiring frequent backwashes, filter changes, chemical additions or other maintenance operations and that will remove organic matter and make a water which is biologically pure and preferably having a specific electrical resistance of about 17 to 18 megohms/cm.

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The invention employs a novel combination of three separate membrane processes arranged in a specific configuration to give unexpected improvements in the purification of water.

From one aspect the invention provides a process for treating water comprising subjecting said water to ultrafiltration to remove suspended particles therefrom subjecting said treated water to electrodialysis in a manner to remove a substantial portion of dissolved minerals therefrom and to acidify said product and treating the productfrom said electrodialysis treatment by reverse osmosis to further reduce dissolved minerals.

From another aspect the invention consists of an apparatus for carrying out a process for purifying water comprising a combination or series of indi vidual elements connected to a source of tap water or other suitable water supply for the purpose of treating into purified water, preferably water having a final resistance of 17-18 megohms/cm. The system comprises at a minimum three membrane processes, ultrafiltration (U.F.), electrodialysis (E.D.) and reverse osmosis (R.O.) connected in series, preferably with a final treatment using mixed bed ionexchangers. Other elements may be added into the system depending on the desired specifications of the product water, such as for example an ultraviolet (U.V.) radiation source for the destruction of micro organisms at the point of product use.

The above described water purification system can be mounted in a mobile trailer which can be transported to a customer's facility. The system after being hooked up to electrical, feed water, waste water and product water connections can deliver a treated water supply according to a customer's specifications.

The advantages of the present invention are as follows: a) The ultrafiltration unit or element provides improved pretreatment filtration over media or cartridge filters because of the very small pore sizes of the UF membranes (0.002 to 0.02 microns) and thus will not only remove suspended articles but will also reduce the bacterial content of the water.

There are advantages to the use of UF as a pretreatment for membrane processes instead of media and/or cartridge filters. Media in filters can migrate, disintegrate, or promote luxuriant growths of microbiological species. Wound cartride filters can unload on pressures surges, can be by-passed through misassembly or through the passing of sloughed-off dirt after filter changes.

b) The electrodialysis unit or element in addition to reducing the total dissolved solids (TDS) content will also reduce the pH of the water, i.e., it increases the acidity of the water (preferably a pH range of about 4 - 6.8) thus eliminating the need for adding an outside source of acid to the water prior to treatment by the reverse osmosis unit. The ED unit functions as the primary demineralizer in the water treatment system.

c) The reverse osmosis (R.O.) unit further reduces the mineral content (TDS) of the water; preferably followed by polishing with ion-exchange treatment to obtain extra pure water. Since the water resulting from the R.O. treatment is so low in dissolved salts (ions), further treatment with ion-exchange resins can be employed for long periods of time before the resin becomes exhausted. This makes feasible the use of portable ion-exchange cartridges or tanks in place of the prior art on-site regeneration ionexchange tanks. The use of a portable ion-exchange system thus eliminates the difficult and expensive on-site chemical handling and waste disposal problem.

During the desalting treatment by electrodialysis; so called polarization films will occur adjacent to the inner surfaces of those membranes defining the desalting chambers. This phenomenon normally occurs where the water under treatment becomes low in total dissolved solids and the current density employed depletes the ions contained in the film of water which is in immediate contact with the membrane surface; thus further current is transferred by the hydrogen and hydroxyl ions thereby formed at the depleted film from the breakdown of water molecules. Normally the anion membrane will polarize first so that the negatively charged hydroxyl ions formed thereon will readily pass through said anion membrane into the adjacent salt concentrating chamber.However, the positively charged hydrogen ions so generated cannot pass through the anion membrane and thus will tend to accumulate in the desalting chamber resulting in the acidification of the product water stream. This naturally occurring acidification is taken advantage of in a novel manner whereby in the product streams further treatment by reverse osmosis, the need for the addition thereto from an outside source of acid prior to R.O. treatment is no longer required. Polarization is thoroughly discusssed in a publication "Limiting Current in Membrane Cells", Industrial & Engineering Chemistry, volume 49, page 780, April 1957.

The advantage of ED as a pretreatment unit for RO comes about because of the following ED characteristics: 1. Bicarbonate and hardness are removed almost always without need for chemical addition. The Langelierlndexofa product water from an ED unit is always lower than that of the feed and is often negative hence the productwaterfrom an ED unit can almost always be fed to an RO unit without any chemical feeds.

2. Use of an ED pretreatment avoids the necessity for a degasifierwith the resulting requirement for re-pumping and the chance of contamination.

3. The product output of an ED unit is not diminished by low temperature, although its demineralization percentage is reduced at lower temperatures. RO has the opposite reactions to lower temperatures, namely reduced hydraulic output but rejection is maintained. Hence, a combination of the two processes has a better over-all maintenance of output at lower temperatures.

The invention will now be further described with reference to the accompanying drawings.

The drawing demonstrates the preferred embodiment of the novel combination of elements of the water treating system in schematic fashion. As illustrated, the drawing shows a potable water source(1 ) such as city tap water being passed to prefilter means(2) of the conventional cartridge type or media type for the purpose of removing when present relatively large size suspended particles. It must be stated that the prefilter (and certain other elements of the system) may be modified as required or totally eliminated. Where prefilters are employed, it is preferred that they be used in pairs and piped upto allowfor water flow in series, parallel or singly.It should be understood that the various hose and piping options (i.e. series or parallel flow, recycle, feed and bleed etc.) which may be employed to permit flexibility of the system is not herein illustrated or described since such options are well known to those versed in the art. Additionally the various items employed in the water treatment system, such as holding tanks, conductivity cells, gauges, flow indicators, recorders, pumps, valves, meters, etc, are not illustrated since their use is also well known in the art. Next, the prefiltered water is passed under pressure into an ultrafiltration unit(3) having membranes with pore sizes between 0.002 0.02 microns, which removes colloidal particles, bacteria and residual organics.The construction and operation of UF apparatus is well known in the art and one spiral wound type is readily available from Osmonics Inc. of Mannetonka, Minn. The next step in the treatment is primary demineralization by electrodialysis unit(s) (4) which removes a major portion of the dissolved minerals. Satisfactory ED apparatus identified by the trademark Aquamite is commercially available for lonics Incorporated of Watertown, MA. Such apparatus employs membrane stacks comprised of alternating anion selective and cation selective membranes defining fluid flow chambers. The ED units will remove the ionized impurities in solution and will maintain consistent output quality by the automatic reversal of current polarity across the stack as is fully disclosed in U.S.

Patnet No. 4,381,232 (D.Brown). The product water from the ED stacks not only will be substantially reduced in its mineral content but will also show an increase in acidity. This acidity (preferably pH of 4- 6.8) is a desirable characteristic for water that is to be treated by R.O. (5) and the use of ED eliminates the necessity of adding a mineral acid to the feed water stream from an outside source. The acidic water is required to prevent or reduce the fouling of the R.O.

membranes to allow for continuous operation.

Treatment by R.O. will further remove residual colloids, bacteria and dissolved minerals from the product water. Suitable R.O. filters are commercially available from Dow and are known by the name R.O.

- 20K-Permeator. The permeate (product) resulting from the R.O. treatment is then further treated with a mixed bed ion-exchange resin (6) where any remaining dissolved minerals are removed. The mixed bed may be composed of an anion resin of the Rohm & BR< Haas type Amberlite (R.T.M.) 410 and cation resin Amberlite 1 R-120 although other mixed bed resins may be successfully used. The mixed ion-exchange bed will remove undesirable ions remaining in the R.O. treated water in a manner which is conventionally understood by those skilled in the water treatment art. Preferably the mixed bed exchanger is not regenerated on site but is replaced with units having a new resin charge.

Next the water which has now substantially achieved total removal of dissolved solids is passed into a source of ultraviolet radiation (7) to kill all living micro-organisms. A wave length range of about 2537 angstroms along with a calculated producted water flow rate past the U.V. source will effectively destroy all living organisms in the water.

A U.V. disinfection system manufactured by U.V.

Technology Inc. of California was found sufficient for this purpose as well as others. At this point the water is sufficiently purified for most uses. However if so required further polishing treatment of the water may be employed by the alternate use of a mixed bed resin polishing bed(s) (8) containing an additional absorbent resin, or a micro porous cartridge type polishing filter (9) having extra fine pore sizes. The thus treated water then passes to a conductivity meter (10) which indicates the electrical resistance of the final product water to insure 17 - 18 megohms/ cm. purity at the point of use (11).

The process according to the invention is best illustrated by the following example.

Example A raw feed water enters the treatment system through an isolating valve and pressure regulating valve at a flow rate of 126 gpm. The raw water has a pH of 8.1 a temperature of 17.5 C and a conductivity of 83 micro Siemens/cm. which is equivalent to about 45 ppm of dissolved salts. The water is pressurized by a pump and passed through the ultrafiltration unit of the spiral wound type employing polysulfone membranes having a molecular weight cut off of about 50,000. The permeate product from the ultrafiltration (U.F.) system which amounts to about 88 gpm is passed into an electrodialysis (ED) feed tank and the U.F. reject portion is recycled back to the pressurized feed pump and/or passed to waste in a preset proportion. The water collecting in the ED feed tank still has it's original pH of about 8.1 and conductivity of about 83 micro Siemens/cm.

This water is then passed into an Aquamite sX electrodialysis unit at a total input flow of about 88 gpm; about 75 gpm being passed into the salt diluting cells and the remainder into the salt concentrating cells. The Aquamite X unit is comprised of two membrane stacks using 18"x40" size ionexchange membranes with each stack containing 500 cellspairs. The product effluent (71 gpm) from the ED treatment has a conductivity of about 10 micro Siemens/cm. (about 6 ppm of dissolved salts) and an increased acidity (pH of 5.5 - 5.8). The effluent from the salt concentrating cells is passed to waste and/or recycled back as a feed stream into the salt concentrating cells.The demineralized acidic product stream resulting from the ED treatment is then passed under pressure into the reverse osmosis unit of the hollow fiber type employing cellulose triacetate membranes. Reverse osmosis units having membrane pore sizes in the range of about 0.0005 to 0.005 microns are readily available from Dow Che mical C. of Midland, Michigan. The product water exits the R.O. unit at about 50 gpm and has a conductivity of about 0.5 micro Siemens/cm. (less than 1 ppm of TDS) and a temperature of 19.4 C. This product water is further treated by passing in series through two banks of ion-exchange tanks; each bank comprising eight tanks or cylinders fed in parallel flow.Each cylinder contains 3.5 cubic ft. of ionexchange resins comprised of 2.1 cubic ft. of A 101 D anion resins in the OH form and 1.4 cubic ft. of C 20 H cation resin in the H+ form. These resins are obtainable from Diamond Shamrock Co., The total effluent (50 gpm) from the ion-exchange treatment which has a pH of about neutral and a final resistance of 17.8 megohms/cm. or better is passed through an ultraviolet light unit to destroy living organisms prior to discharge and final usage by the customer. Ultraviolet light equipment is readily available and one type obtainable from Aquafine corporation of Valencia, California is sufficient for this purpose.

In summary, the novel apparatus and process of the present invention comprises the combination of the following treatment steps in series: a) ultrafiltration to pretreat feed water to make it suitable as a feed to the later reverse osmosis step, b) electrodialysis of the pretreated U.F. water to substantially reduce the salt content and increase the acidiity (pH 4 to 6) to prepare it as a feed to the R.O. step.

c) reverse osmosis to further reduce the salt content prior to ion-exchange treatment and preferable the following additional treatment steps of, d) ion-exchange (preferably portable) to even further reduce mineral content and, e) ultrafiolet disinfection system to destroy bacteria.

The individual water treatment steps employed in the present invention, i.e., ultrafiltration, electrodialysis and reverse osmosis are known per se.

However, it has thus been shown that the novel combination and flow arrangement of the various treatment steps described herein produces a synergism, i.e. it increases the usefulness of each step in an unexpected manner and makes them extremely useful especially as a water treatment system mounted in a mobile trailer for obtaining ultra pure water without the necessity of adding an outside chemical source to the water prior to reverse osmosis treatment.

Claims (12)

1. A process for treating water comprising subjecting said water to ultrafiltration to remove suspended particles therefrom, subjecting said treated water to electrodialysis in a manner to remove a substantial portion of dissolved minerals therefrom and to acidify said product and treating the product from said electrodialysis treatment by reverse osmosis to further reduce dissolved minerals.

2. The process of claim 1 wherein subsequent to the reverse osmosis treatment said water is further treated with ion-exchangers to remove substantially any remaining dissolved minerals therefrom.

3. The process of claim 2 wherein the water product from said ion-exchange treatment is subjected to sterilization by exposing said water to ultraviolet light of sufficient intensity to destroy biological contamination.

4. The process of claim 2 wherein the water produced from said ion-exchange treatment is further subjected to a final polishing treatment resulting in a product water having an electrical resistance of 17-18 megohms/cm. purity,

5. The process of any one of claims 1 - 4 which is performed in a mobile vehicle.

6. The process of claim 1 wherein said product is acidified to a pH of between 4 to 6.8.

7. A process for treating water substantially as herein described with reference to the example.

8. Apparatus for treating water comprising a water supply, pumping means for directing said water in series through successive elements of a water treatment system including in sequence, ultrafiltration means, electrodialysis means and reverse osmosis means.

9. Apparatus of claim 8 wherein subsequentto said reverse osmosis means and in series therewith is located ion-exchange means.

10. Apparatus of claim 9 wherein sterilization means are located after said ion-exchange means.

11. Apparatus of any one of claims 8 - 10 which is located and positioned in a mobile vehicle.

12. Apparatus for the purification of water sub- stantially as described with reference to the drawing.