PRODUCTION OF ULTRAPURE WATER BY NON-CHEMICAL TREATMENT

Abstract: A non-chemical pretreatment method for producing ultrapure water, the essential steps of which are subjecting an aqueous feed to an electromagnetic energy field to cause polarization and agglomeration of suspended particulates, followed by multimedia bed filtration for removal of the particulates.

Description: PRODUCTION OF ULTRAPURE WATER BY NON-CHEMICAL TREATMENT BACKGROUND OF THE INVENTION The demand for high quality, easily produced ultrapure water is increasing in a number of industries, including the pharmaceutical industry, in nuclear fuel processing, and in the electronic device fabrication industry. In the semiconductor industry for example, as the scale of microchip semiconductor devices continues to decrease, there is an increasing need for reduction of residual contaminants in water used in the manufacture of silicon wafers. Conventional wafer production standards for ultrapure water are deionization to a resistivity of 18 megohms, and the absence of particles greater than 0.20 micron in diameter. FIG. 1 is a simplified schematic diagram of a conventional large-scale industrial water ultrapurifica- tion system 10. Because source water 12 may come from wells, surface runoff, or municipal supplies, and may contain a variety of metals, organic compounds, silts, or other contaminants, scheme 10 employs a variety of processes and components to purify source water 12 to the desired water quality. Source water 12 generally under- goes multistage processing through a pretreatment stage 14, a reverse osmosis (RO) treatment stage 16, and a post-treatment stage 18 before reaching a usage stage 20. Pretreatment stage 14 is typically designed to remove contaminants that may have deleterious effects on some of the downstream purification processes that may contain more sensitive or expensive components. For example, metal contaminants, such as iron, copper, aluminum, manganese, sodium, or calcium, are known to cause a variety of problems throughout a water ultrapurification scheme. Common metal contaminants, iron and manganese, are generally treated by aeration, chemical oxidation, or media filtration. The presence of iron in water to be purified is especially troublesome, as hydroxides formed during oxidation for its removal tend to build up on anionic deionizing resin beds, and may slough off such beds without warning and be carried downstream to the point of use, thereby substantially interfering with the manufacturing process. The pretreatment stage 14 shown in FIG. 1 oxidizes contaminant metals in source water 12 by adding a pH modifier 22 and an oxidizing agent 24, such as chlorine. The oxidized metals precipitate out of solution and are subsequently removed by large particle filters 26. Because oxidizing agents 24 also pose problems to many downstream process components, in order to remove them from the water treatment systems, a reducing agent 28, such as sodium bisulfite, may be subsequently added to the oxidized source water 12, or the source water 12 may be passed through granular activated carbon (not shown) , before it is conveyed to the RO membranes treat- ment stage 16. A valve 30 regulates the source water flow through a heat exchanger 32 employed to increase the temperature of the source water 12 to about room temperature to facilitate its flow through the RO treatment stage 16. Source water 12 may also be subjected to a set of prefilters 34 before it is finally conveyed by a high pressure pump 36 to the RO treatment stage 16. State-of-the-art water ultrapurification schemes have recently begun to employ RO membranes 40 and 42 in double pass configurations, where each pass is typically arranged in a series array where the permeate of the prior membrane is pumped as the feed to the next set of membranes. The reject water or retentate from each later pass through an RO membrane is typically recycled back to the feed side pump, repressurized, and then to the feed side of the earlier RO membranes. The RO treatment stage 16 in FIG. 1 depicts such a double pass configuration through the two sets of RO membranes 40 and 42. Pump 36 forces pretreated feed water 44 through a set of RO membranes 40 at a pressure in the range of 300 to 500 psi, the first pass permeate 46 being at a reduced pressure relative to the feed side pressure and is then directed to the second set of RO membranes 42. The first pass retentate containing filtered impurities 52 is emptied into a sewer 54 or, after detoxification, into a National Pollution Discharge Elimination System (NPDES) -permitted outfall. A pump 56 repressurizes and directs the permeate of RO membranes 40 as feed to a second set of RO membranes 42. The second pass RO permeate 66 is directed through a vacuum degassifier (not shown) to a storage tank 68 that is blanketed with nitrogen from nitrogen source 70 to prevent the dissolution of oxygen, carbon dioxide or other gaseous contamination. The second pass RO retentate 72 is recycled to the feed side 76 of pump 36, and thence to RO membranes 40. Each pass of a conventional pre-RO-filtration scheme 10 utilizes a storage tank and a pump and tends to remove about 90% of the salts from the water; in combination with the double pass RO system shown in FIG. 1, about 99% of the salts are removable from the water, and the transition metal ions are removed to a parts-per-billion level. However, there is still a need in the production of ultrapure water for a degree of removal of such metal ion contaminants to a still lower parts-per-trillion level. Cellulose acetate (CA) is the most commonly employed RO membrane type and is used in flat sheet, spiral-wound and hollow fiber ("HF") configurations. However, polyamide and polyimide (collectively referred to as "PA") thin-film composite ("TFC") and PA HF membranes have several advantages. PA TFC membranes generally exhibit higher rejection and greater flux than most RO membranes. Unfortunately, the chemical makeup of RO membranes, including CA and PA, makes them highly susceptible to oxidizing agents used in RO pretreatment, such as chlorine, even at very low levels. For example, CA membranes can tolerate up to 1 ppm chlorine, but PA membranes typically can tolerate only up to 0.1 ppm chlorine. Therefore, any oxidizing agent 24 used upstream of the RO membranes 40 and 42 must be virtually completely neutralized by the reducing agent 28 before reaching the RO filters. The standard approach of adding sodium bisulfite to neutralize the chlorine typically requires the downstream removal of the added sodium. However, removal of chlorine upstream of the RO membranes 40 and 42 leaves them susceptible to biofouling by microorganisms. Thus, some water purification schemes 10 purposely leave 0.3 to 0.5 ppm residual chlorine in the upstream water, which is, of course, incompatible with the use of PA RO membranes. Conventional purification schemes must therefore carefully regulate the concentrations of chlorine and bisulfite during water, pretreatment because small shifts in their concentrations can degrade either RO membranes or the quality of the product water. Understandably, the constraints imposed by such delicate balancing of chemical concentrations tend to weigh against the use of the more efficient PA TFC and HF membranes. Ideally, such membranes should be used with a pretreatment system that uses no chemical additives of any kind. After RO treatment stage 16, RO product water 80 is conveyed by pump 82 to resin beds 84 in the post- treatment stage generally designated 18. Residual chlorine from RO treatment stage 16 is treated before it reaches resin beds 84, since contact with beds 84 may result in the sloughing off of organic materials from the resin, which in turn can result in unacceptably high total organic content (TOC) levels in the ultrapure product water. High TOC levels in the water used to manufacture semiconductors lead to wafer contamination and reduction of yield. Further post-treatment of ultrapure water may employ a microfilter 86 to capture microorganisms and resin particles, and an ultraviolet (UV) sterilizer 88 to kill any microorganism not removed by microfilter 86. Post-treated water 96 is then directed to a storage tank 98 that is also blanketed with nitrogen from nitrogen source 100 until water 96 is needed at usage stage 20. When post-treated water 96 is desired for use in manufacturing, it is conveyed by pump 102 through polishing resin columns 104 and another UV sterilizer 106 that is positioned between secondary microfilter 108 and tertiary submicrofilter 110 before reaching usage stage 20. Unused post-treated water 96 is returned to storage tank 98. Because pretreatment additives used in conventional ultrapurification systems often result in an increase in operating costs and an increase in the risk of deleterious effects downstream in connection with further water treatment, an alternative, less deleterious pretreatment process would greatly simplify water purification schemes. An object of the present invention is, therefore, to provide a simplified and efficient water ultrapurification scheme. Another object of the invention is to provide a system and method for removing metals and other particulates from source water. A further object of the invention is to provide such a system and method that does not employ oxidizing agents or any other chemical treatment. These objects and others are met by the present invention, which is summarized and described in detail below. BRIEF SUMMARY OF THE INVENTION The key to the present invention lies in the use of an electromagnetic field to induce charges in small particles in the water feed stream, thereby causing the same to agglomerate into larger clumps of particles, coupled with the use of multimedia filter beds to remove such agglomerated particles. Such a pretreatment scheme therefore dispenses with the prior art need to add and carefully regulate the concentration of oxidizing agents and then to neutralize them by the addition of reducing agents. Following such pretreatment, the water stream may be treated by conventional RO and post-RO "polishing" treatment steps. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS FIG. 1 is a schematic of a conventional water purification scheme employing particulate filters, an oxidizing pretreatment and double-pass RO filtration. FIG. 2 is a schematic of an embodiment of a water purification scheme of the present invention employing an electromagnetic field/multimedia filter bed pretreatment followed by RO and other post-treatment. DETAILED DESCRIPTION OF INVENTION FIG. 2 presents a preferred embodiment of the water purification scheme of the present invention. Feed water 120 from source 12 is fed to and through an electromagnetic cell 122, the workings of which are illus- trated with more particularity in FIG. 2a. In FIG. 2a there is shown particulates 121, some bearing electrical charges and some not, entering a section of feed stream conduit preferably of the same diameter as both the incoming and outgoing feed stream conduit, and comprising an electromagnetic cell 122 having helical windings of wire coil 123 attached to a DC source (not shown) of electrical energy. Such an electromagnetic cell is commercially available as a Linear Kinetic Cell from Water Dynamics, Inc. of Gresham, Oregon, and is described with greater particularity in U.S. Patent No. 4,326,954, the disclosure of which is incorporated herein by refer- ence. When electrical energy is supplied to wire coil 123, an electromagnetic energy field 124 is created wherein the lines of electromagnetic force are substantially parallel to the flow of feed stream 120. With respect to particles and ions already bearing a charge, the electromagnetic energy field tends to polarize such charges on opposite ends of the particles, while a positive/negative set of electromagnetic charges tends to be induced in uncharged diamagnetic particles, also on opposite ends of the particles. Because of this particle-charging effect, particles and ions tend to agglomerate, forming clumps and "molecular chains" 125, the net effect of which is to form colloidal-like particles which are easily removed by filtration from source water 12. After being subjected to the electromagnetic energy field, water feed stream 120 flows through a flow regulation valve 129 to multimedia bed filtration units 130. Multimedia bed filtration units 130 comprise multiple layers of relatively large granules of particu- late filtration media 135, varying in size generally from fine at the top to course at the bottom. Preferred filtration media include garnet ' or illminite, anthracite, silica sand, and gravel, the gravel varying in size from No. 1 to No. 16. Layers may vary in number from 3 to 15 and in depth from 3 to 36 inches and more, depending upon the capacity of the bed. Because of the large mass of the particulates 125 discharged from electromagnetic cell 122, multimedia bed filtration units 130 are much more easily cleaned by backflushing than is the case when no flocculation of feed stream particulates has taken place, rendering filtration units 130 much less susceptible to blinding and bacterial contamination. Filtered water 139 may be passed through another flow regulation valve (not shown) and through heat exchanger 140 before being directed to an RO treatment stage. Heat exchanger 140 increases the temperature of water 150 to about room temperature to optimize its flux (flow volume per membrane area per unit time) through first and second pass RO membranes 170 and 180, respectively. After RO treatment, the water is sent to a series of conventional post-RO-treat ent steps of deionization, the removal of organics, and additional filtration, all as outlined in connection with post- treatment stage 18 of FIG. 1 in the Background section above . A preferred purification scheme employs a double pass RO treatment. High pressure pump 160 delivers filtered water 150 at a pressure of 350 to 550 psi, preferably 450 psi (3.1 MPa) to one or more first pass RO membranes 170. First pass reject water or retentate is sent to a waste post-treatment stage while first pass product water or permeate 172 is sent directly to the feed side of one or more RO membranes 180 without an intervening pump. RO membranes 170 and 180 are preferably of the TFC type although other membrane types such as spiral- wound or HF membranes may be employed. An especially preferred RO membrane is a PA-containing TFC membrane. Because TFC membranes have greater flux and so cause smaller decreases in water pressure than most other RO membranes, first pass permeate or product water 172 may be sent directly to RO membranes 180 at between 200 and 300 psi without the expense or energy consumption of an intervening pump. Second pass retentate may be fed to the feed side of RO membranes 170, while second pass permeate or product water 182 is directed to posttreatment stage 190. The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims: CLAIMS : 1. A non-chemical method for the production of ultrapure water comprising the steps: (a) providing a water feed stream; (b) subjecting said feed stream to a substantially continuous electromagnetic energy field wherein the lines of electromagnetic force are substantially parallel to the flow of said feed stream, thereby causing agglomeration of particulates of less than 5 microns in diameter; and (c) passing said feed stream from step (b) through a multimedia filter bed capable of removing particulates having a diameter of >5 microns. 2. The method of claim 1, including the additional steps of: (d) subjecting said feed stream from step (c) to an RO treatment; and (e) subjecting said feed stream from step (d) to degassification, deionization and organics removal treatment. 3. The method of claim 1 wherein said electromagnetic energy field of step (b) is steady. 4. The method of claim 1 wherein said electromagnetic energy field of step (b) is pulsating. 5. The method of claim 1 wherein said multimedia filter bed of step (c) comprises layers of anthracite, garnet and gravel. 6. The method of claim 5 wherein said layers of gravel vary in size from No. 1 to No. 16. 7. The method of claim 2 wherein step (d) is a double-pass RO treatment utilizing a first RO membrane module and a second RO membrane module in series. 8. The method of claim 7 wherein said first and second RO membrane modules comprise thin-film composite membranes.

Inventors: JOHN JOY T, VANDENSON BRIAN A, JOHN, JOY, T., VANDENSON, BRIAN, A.
Applicants: JOHN JOY T, SEH AMERICA INC, VANDENSON BRIAN A, JOHN, JOY, T., SEH AMERICA, INC., VANDENSON, BRIAN, A.
Application year: 1998
Main classification: C02F1
IPC codes: C02F1