Source: http://www.google.com/patents/US6537456?dq=7,468,661
Timestamp: 2014-03-09 06:57:25
Document Index: 559103784

Matched Legal Cases: ['Application No. 02235899', 'Application No. 02235899', 'Application No. 61138486', 'Application No. 61138486', 'Application No. 6117235', 'Application No. 6117235']

Patent US6537456 - Method and apparatus for high efficiency reverse osmosis operation - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA process for treatment of water via membrane separation to remove hardness and non-hydroxide alkalinity by simultaneous removal in a weak acid cation exchange resin. The process includes ionization of sparingly ionizable components, such as silica, by adjusting the pH up to about 10.5 or higher. Their...http://www.google.com/patents/US6537456?utm_source=gb-gplus-sharePatent US6537456 - Method and apparatus for high efficiency reverse osmosis operationAdvanced Patent SearchPublication numberUS6537456 B2Publication typeGrantApplication numberUS 09/242,249PCT numberPCT/US1997/014239Publication dateMar 25, 2003Filing dateAug 12, 1997Priority dateAug 12, 1996Fee statusPaidAlso published asUS20020125191Publication number09242249, 242249, PCT/1997/14239, PCT/US/1997/014239, PCT/US/1997/14239, PCT/US/97/014239, PCT/US/97/14239, PCT/US1997/014239, PCT/US1997/14239, PCT/US1997014239, PCT/US199714239, PCT/US97/014239, PCT/US97/14239, PCT/US97014239, PCT/US9714239, US 6537456 B2, US 6537456B2, US-B2-6537456, US6537456 B2, US6537456B2InventorsDebasish MukhopadhyayOriginal AssigneeDebasish MukhopadhyayExport CitationBiBTeX, EndNote, RefManPatent Citations (63), Non-Patent Citations (96), Referenced by (49), Classifications (44), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetMethod and apparatus for high efficiency reverse osmosis operationUS 6537456 B2Abstract A process for treatment of water via membrane separation to remove hardness and non-hydroxide alkalinity by simultaneous removal in a weak acid cation exchange resin. The process includes ionization of sparingly ionizable components, such as silica, by adjusting the pH up to about 10.5 or higher. Their separation by the membrane is significantly increased. The passage of boron, silica and TOC is reduced by a factor of ten or more. Recovery of 90% or higher is achievable with most brackish feedwaters, while substantial reduction in cleaning frequency is simultaneously achieved. The apparatus used for the water treatment process includes reverse osmosis membrane(s) (30), mixed bed ion exchange unit (44, 46), micron filter (48), ultraviolet sterilization unit (49), decarbonation unit (20), and electrodeionization unit (150).
TECHNICAL FIELD My invention relates to a method for the treatment of water in membrane based water treatment, purification, and concentration systems, and to apparatus for carrying out the method. In one embodiment, my invention relates to methods for feedwater pretreatment and for operation of reverse osmosis (�RO�) equipment, which achieve increased solute rejection, thereby producing very high purity (low solute containing) product water, while significantly increasing on the on-stream availability of the water treatment equipment.
BACKGROUND A continuing demand exists for a simple, efficient and inexpensive process which can reliably provide water of a desired purity, in equipment which requires a minimum of maintenance. In particular, it would be desirable to improve efficiency of feed water usage, and lower both operating costs and capital costs for high purity water systems, as is required in various industries, such as semiconductors, pharmaceuticals, biotechnology, steam-electric power plants, and nuclear power plant operations.
In most water treatment systems for the aforementioned industries, the plant design and operational parameters generally are tied to final concentrations (usually expressed as total dissolved solids, or �TDS�) which are tolerable in selected equipment with respect to the solubility limits of the sparingly soluble species present. In particular, silica, calcium sulfate, and barium sulfate often limit final concentrations achievable. In many cases, including many nuclear power plants and many ultrapure water plant operations, boron or other compounds of similarly acting ampholytes have a relatively low rejection across membranes in conventionally operated RO systems, and may dictate design or operating limitations. More commonly, the presence of such compounds result in sufficiently poor reverse osmosis product water, known as permeate, that additional post RO treatment is required to produce an acceptably pure water. In any event, to avoid scale formation and resulting decreases in membrane thruput, as well as potential deleterious effects on membrane life, the design and operation of a membrane based water treatment plant must recognize the possibility of silica and other types of scale formation, and must limit water recovery rates and operational practices accordingly. In fact, typical RO plant experience has been that declines in permeate flow rates, or deterioration of permeate quality, or increasing pressure drop across the membrane, require chemical cleaning of the membrane at regular intervals. Such cleaning has been historically required because of membrane scaling, particulate fouling, or biofouling, or some combination thereof. Because of the cost, inconvenience, and production losses resulting from such membrane cleaning cycles, it would be advantageous to lengthen the time between required chemical cleaning events as long as possible, while nevertheless efficiently rejecting undesirable ionic species and reliably achieving production of high purity permeate.
From a typical end user's point of view, several areas of improvement in RO technology�chlorine tolerance being one of them�are still sought. Thin film composite membranes, at least partly due to their surface charge and characteristics, are relatively prone to biological and particulate fouling. With certain feedwaters, particularly from surface water sources, membrane fouling and the frequent cleaning required to combat fouling can present some arduous, costly, and time-consuming operational challenges.
It is known that rejection of weakly ionized species, such as total organic carbon (�TOC�), silica, boron, and the like, is significantly lower than rejections for strongly ionized species as sodium, chloride, etc. Since the efficiency of post-RO ion exchange is largely determined by the level of the, weak anions present in the RO permeate, it would be advantageous to remove (reject) as many weak anions as possible in the RO unit operation. In other words, by removing (rejecting) more silica (and boron) in the RO step, a higher throughput is achievable in the ion-exchange unit operation that follows the RO unit.
Also, a method used in high purity water applications is disclosed in Japanese KOKAI No. Sho 58-112890, Published Jun. 29, 2984 by Yokoyama, et al., for a METHOD OF DESALINATION WITH A REVERSE OSMOSIS MEMBRANE UNIT. His examples show reverse osmosis units utilizing a pretreatment process of strong acid cation exchange resin (�SAC�) for softening in one example, and without softening in the other example. While his process will work for certain feedwaters, it does not teach how operation at higher pH levels may be employed while still avoiding scaling of RO membranes.
In a given RO reject water, in order to avoid carbonate scaling, it most preferable to keep the LSI negative, i.e. in a condition so that CaCO3 will dissolve. However, in the field, it has been found that under some conditions, with use of certain types of anti-scalant additives, an LSI of up to about +1.5 can be tolerated, without CaCO3 scale formation resulting. In any event, at the pH of any given RO reject, pHs must be minimized in order to avoid undesirable scale formation. To put this into perspective, consider that in any RO pretreatment operation, it can be anticipated that there will always be at least some leakage of calcium from the softening step. Thus, depending upon the raw feedwater hardness and the pretreatment process scheme practiced, a lower limit on the achievable value of the pCa term, due to the concentration of the Ca++ ion present in the treated RO feedwater, can be anticipated. Furthermore, in all events, the value of C is fixed by the total ionic strength and by the temperature. Thus, to keep the LSI in an acceptable range�in order to provide scale free RO operation�the leakage of calcium (as well as other hardness such as magnesium) becomes a critical factor. The Tao et al. patent, identified above, approaches this problem by providing various types of softeners in series. Specifically, he simply accepts the inevitably high capital and operating costs associated therewith. Yokoyama, on the other hand, evidently decided to limit RO operation to a pH which is consistent with the degree of calcium removal. When he operates with RO reject at a pH of 9, assuming 0.1 ppm of Ca++ leakage from the ion exchange train disclosed, and a concentration factor of 5 (�5X�) in the RO, his RO operation may be expected to provide an RO reject with an LSI of about −0.5. That LSI is acceptable for non-scaling operation, with or without scale inhibitors. However, if the pH in Yokoyama's example were increased to 11, for example, given the same pretreatment method, an LSI of about +2.4 might be expected. In such a case, the Langelier Saturation Index of the reject water would be well above the level where current anti-scalants have the ability to provide scale free RO operation.
SUMMARY I have now invented a novel water treatment method based on aggressive hardness and alkalinity removal, followed by membrane separation at high pH, to produce a high quality permeate with extremely low silica concentration.
OBJECTS, ADVANTAGES, AND FEATURES From the foregoing, it will be apparent that one important and primary object of the present invention resides in the provision of a novel method for treatment of water to reliably and continuously produce over long operational cycles a water product stream of a pre-selected extremely high purity quality standard.
BRIEF DESCRIPTION OF THE DRAWING In the drawing, identical features shown in the several figures will be referred to by identical reference numerals without further mention.
DETAILED DESCRIPTION I have developed a new method for process design and operation of RO systems. This new method for process design and operation of RO systems has been thoroughly tested. The process has shown that it is capable of achieving important improvements in RO operational objectives.
(2) Very high achievable recovery�ninety percent (90%) or higher recovery can be achieved.
Feedwaters utilized for production of high purity water, as well as those encountered in wastewater treatment, include the presence of silicon dioxide (also known as silica or SiO2) in one form or another, depending upon pH and the other species present in the water. For membrane separation systems, and in particular for RO type membrane separation systems, scaling of the membrane due to silica is to be religiously avoided. This is because (a) silica forms relatively hard scale that reduces productivity of the membrane, (b) is usually rather difficult to remove, (c) the scale removal process produces undesirable quantities of spent cleaning chemicals, and (d) cleaning cycles result in undesirable and unproductive off-line periods for the equipment. Therefore, regardless of the level of silica in the incoming raw feedwater, operation of conventional membrane separation processes generally involves concentration of SiO2 in the high total dissolved solids (�TDS�) stream to a level not appreciably in excess of 150 ppm of SiO2 (as SiO2). Typically, RO systems are operated at lowered recovery rates, where necessary, to prevent silica concentration in the reject stream from exceeding roughly 150 ppm.
Reject 32 from membrane separation unit 30 may be sewered or sent to further treatment, as appropriate in particular site circumstances. Permeate 34 from membrane separation unit 30 may utilized �as is� or may be further purified to remove residual contamination, for example, for high purity water users such as semiconductor manufacturing, where 18.2 meg ohm purity water is desired. A conventional post-RO treatment train for production of high purity water 38 in the semiconductor industry includes a cation exchanger 40, followed by an anion exchanger 42, with primary 44 and secondary 46 mixed bed polisher ion exchange units. Somewhat different post RO treatment trains may be utilized to meet the particularized needs of a given site, raw water chemistry, and end use, without departing from the advantages and benefits which may be gained by the RO process method disclosed herein. For example, it may be desirable in some circumstances to omit the cation 40 and anion 42 ion-exchangers, and bypass the RO permeate via line 47 to directly reach the primary mixed bed 44 and polish mixed bed 46 ion-exchange units. Finally, in many ultrapure water plants, the product from the polishing mixed bed ion-exchange units 46 is currently further treated in final filtration units 48 and ultraviolet irradiation units 49 to eliminate particulates and biofouling, respectively. Additional treatment operations may added as appropriate to meet the needs of a particular end user.
Another distinct and unique advantage of my method of RO system operation is that it may be possible, under various raw feedwater chemistry and operating conditions, to operate the entire post-RO ion exchange train (i.e., ion-exchange, units 40, 42, 44, and 46) without regeneration. Depending upon chemistry, it may be possible to simply replace the cation 40 and anion 42 exchangers. In the more usual case, the secondary or polishing mixed bed unit 46 may be replaced with new resin, and the old polishing resin moved to the primary bed 44 position. This is possible, particularly in ultrapure and boiler feed type water treatment systems, because the polishing mixed bed unit 46 is controlled by ending operation when the silica, boron, or other ion leakage reaches a predetermined value. When the predetermined ion leakage value is reached, the then polishing mixed bed unit 46 is substituted for, and placed into the position of, the primary mixed bed ion-exchange unit 44. When the change over of mixed bed ion-exchange units is made, the �old� primary mixed bed unit 44 resin is taken out, and either discarded or sold to other less demanding resin users. New resin is then put into the �old� primary mixed bed ion-exchange unit 44, whereupon it becomes the �new� polishing mixed bed ion exchange unit 46.
The weak acid cation (�WAC�) ion-exchange resins used in the first step of the preferred embodiment of my method, as illustrated in FIG. 2, are quite efficient in the removal of hardness associated with alkalinity. Such a reaction proceeds as follows:
EXAMPLE�PILOT TEST A pilot water treatment system was set up to test the efficacy of the method disclosed disclosed. The pilot water treatment system was designed for treating an incoming raw city water supply to provide high purity product water for potential future use in a semi-conductor manufacturing plant. The objectives were (a) to increase recovery, so as to minimize water usage, (b) to increase the purity of treated water, and (c) to increase the average time between membrane cleanings. The pilot system performed a series of tests. In each of the tests, the system was started up with 450 ppm or higher silica level in the RO reject. The pilot plant system was operated continuously until either (a) a ten percent (10%) decline in normalized RO permeate water flow was experienced, or (b) a fifteen percent (156) increase in axial differential pressure across the RO membrane was reached. The pilot test was performed with a membrane separation unit including a Dow/Filmtec RO Membrane Model FT30/BW4040, which was operated at pressures from about 130 psig to about 185 psig, with feedwater temperatures ranging from about 200� C. to about 25� C., and at feedwater rates of up to about 8 US gallons per minute (30 liters per minute) maximum. As seen in FIG. 6, long term normalized permeate flows of slightly more than 5 US gallons per minute (about 20 liters per minute) were tested. The pilot test apparatus included a pair of weak acid cation ion exchange beds operated in parallel, utilizing Rohm and Haas Company (Philadelphia, Pa.) weak acid cation resin product number IRC-86, followed by a forced air decarbonator, sodium hydroxide injection, separation of the treated feedwater by the RO membrane into a reject stream and a permeate stream.
The improved rejection of total organic carbon (�TOC�) in my process also provides a significant benefit to RO plant operators. It is normal for waters of natural origin to contain detectable quantities of high molecular weight organic acids and their derivatives, particularly humic, fulvic, and tannic acids. These compounds result from decay of vegetative materials, and are usually related to condensation products of phenol-like compounds. Broadly, humic acids include the fraction of humic substances which are soluble in water at alkaline pH, but which precipitate at acidic pH. Fulvic acids include the fraction of humic substances which are water soluble at alkaline and acidic pH. These acids, and their decomposition products, can be carried around in the feedwater stream and form undesirable deposits on selected substrates, particularly anion selective substances. Also, they tend to contribute to fouling in conventional RO systems. Therefore, it is desirable to minimize the effect of such molecules on or through the reverse osmosis membrane, so that adverse consequences of their presence can be avoided, particularly at the anion ion-exchange unit. As can be seen by reference to Table 1, the TOC content of the permeate 34 is substantially lower in comparison to TOC from a conventional RO process with identical TOC in the raw feedwater. Specifically, there is rejection of ninety nine point sixty six percent (99.66%) of TOC in the pilot plant RO system, compared to only ninety to ninety five percent (90 to 95%) recovery in conventional RO systems. As in the cases of silica and boron, increased ionization of TOC at the elevated pH of my new process attributes to this important result. Thus, taking advantage of the ionization range of ionizable organic carbon species enables effective TOC reductions when operating RO systems according to the method set forth herein.
Biological fouling of thin film composite membranes has heretofore tended to be a common problem, and, with certain specific feedwater sources, has been virtually insurmountable. Although it was anticipated that control of biological fouling would be improved due to operation at relatively high pH levels, the degree of biological fouling control actually achieved far exceeded expectations, with bacteria levels being virtually non-detectable during autopsy of RO membrane elements. This means that instead of accumulating living and dead bacteria against the membrane surface, as is common in conventional RO systems, in my unique method, incoming bacteria are killed and dissolved away from the membrane surface. Thus, this method of RO pretreatment and operation may become useful for treating problematic water sources. This is effective because high pH solutions cause disinfection by cell lysing or rupture of the cell wall. This is a quite potent and quick acting method of anti-bacterial activity, when compared, for example, with chlorination which acts by the much slower method of diffusion through the cell wall to cause death by inactivation of the microorganism's enzymes. Also in contrast to chlorine sanitized systems, at the high pH operation preferred in the present method, viruses and endotoxins (lipopolysaccharide fragments derived from cell walls of Gram-negative bacteria) are effectively destroyed by lysis, thus enabling the present method to be employable for the production of pyrogen free or sterile water. In essence, the present method, when operated at a pH in excess of about 10, provides sanitization (3 log reduction in bacteria and destruction of vegetative matter), and may also prove to essentially provide true sterilization (12 log reduction in bacteria and the elimination of biofilm and spores) of the process equipment, as test results showed a zero (0) bacteria count in the permeate. Also, it should be noted that the increased pH of permeate in this method of operation enables similar, helpful results in the post RO treatment equipment. Such a method of operation should be of particular benefit in the production of high purity water for pharmaceutical applications, where the requirements for United States Pharmacopeia 23 (�USP 23�) standards, as supplemented, must ultimately be met by the final product water. In this regard, the avoidance of use of raw water polymers, antiscalants, and other proprietary chemicals in RO pretreatment, as described herein with respect to a preferred embodiment, can eliminate undesirable additives to pharmaceutical grade water, and reduce costs by reducing the necessary tests on RO product water. More concisely, the selection of a pH for RO operating conditions which does not support bacteria growth, and carrying out of hardness and alkalinity removal to a level which avoid use of additives, is a superior method for production of high purity water.
High flux, or permeate production, is also achievable due to the unique operating conditions of my method for operating an RO system. Several factors contribute to this result. Flux, expressed as gallons of water passed through one square foot of membrane in one day, generally termed �GFD�, is anticipated at about 15 GFD, for conventional RO systems. In pilot testing, the noted thin film composite type FILMTEC BW membrane was operated at 24 GFD, and potential for up to 30 GFD was favorably evaluated. While the latter flux rate is believed to be the approximate current hydraulic limit of conventional RO module design, based on spacer configurations, it is anticipated that even increased flux can be achieved in this method of operation (up to 50 GFD or so) when membrane modules become available that can support such increased flux. This is a most advantageous result for RO system operators, since, for example, if the normal flux is doubled by use of this method, then the total square feet of membrane surface required is reduced by a factor of two. Corresponding decreases in capital cost (specifically, for membranes and pressure vessels) and floor space requirements are therefore achieved. Operating cost, already significantly lowered by other benefits of the instant method, are further decreased by lowered membrane replacement costs. The one hundred fifty percent (150%) plus flux increase demonstrated in testing over the design basis for conventional RO systems provides an immediate benefit.
COMPARISON OF HERO � RO VS. CONVENTIONAL RO
Most commonly occurring microbial species are completely lysed (physically destroyed) at the high operating pH. In fact, even virus, spores, and endotoxins are either destroyed or rendered incapable of reproduction/proliferation at very high pH levels. Saponification of lipids (fat) is expected to play a role in the process as well since fatty acids and their corresponding glycerides will form soluble �soaps� at the high operating pH.
To ensure scale-free operation at 90 percent recovery, one or more of the following must be achieved �residual calcium content must be less than 0.1 mg/l, or the RO operating conditions must be changed. While calcium carbonate scale inhibitors are known to generally allow a high Ksp, I am not aware of any such formulation which would efficiently and cost effectively allow continuous high pH operation of RO. Important, it should be noted that during the long-term testing of the HERO system, no scale inhibitors were used whatsoever.
The method and apparatus for processing water via membrane separation equipment, and in particular, via the HERO brand reverse osmosis (�RO�) process design as described herein, provides a revolutionary, paradoxical result, namely, simultaneous increase in levels of silica in the RO reject, but with lower levels of silica in the purified RO permeate. This method of operating membrane separation systems, and in particular, for operating reverse osmosis systems, represents a significant option for reducing water use while simultaneously reducing capital and operating costs of the water treatment system. Water recovery, that is, the ratio of the quantity of the permeate product stream produced to the quantity of the feedwater stream provided is clearly in excess of about 50%, and easily will be up to about 85% or more, and often, will be up to about 95%, and, at times, will reach levels of about 99%. Further, given the efficiencies, dramatically less usage of chemical reagents, either for ion exchange regenerant or for RO cleaning, will be consumed per gallon of pure water produced.
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210/661, 210/650, 210/639, 210/638, 210/651International ClassificationC02F1/42, B01D61/04, C02F9/02, C02F1/32, C02F9/00, C02F1/66, B01D61/02, B01D61/58, C02F5/00, C02F1/44, B01D65/08Cooperative ClassificationY10S210/90, C02F1/66, B01D2321/164, C02F1/001, B01D61/48, C02F2001/427, B01D65/08, B01D2311/04, C02F2001/425, C02F1/32, B01D61/025, B01D61/58, C02F1/441, C02F5/00, C02F2103/04, B01D61/027, C02F2001/422, B01D61/04, B01D61/022, C02F9/00European ClassificationB01D61/58, B01D61/02B, B01D61/04, B01D65/08, C02F9/00, C02F1/44BLegal EventsDateCodeEventDescriptionAug 11, 2010FPAYFee paymentYear of fee payment: 8Sep 18, 2006FPAYFee paymentYear of fee payment: 4RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google