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Title:
DESALINATION OF WATER CONTAINING HIGH SILICA CONTENT
Document Type and Number:
WIPO Patent Application WO/2009/102442
Kind Code:
A1
Abstract:
A method for improved productivity for desalination by reverse osmosis (RO). The method includes an integrated membrane and chemical treatment process for increasing recovery based on permeate, defined as the ratio of permeate flow to feed flow, by reducing the effects of silica deposition or fouling. Saline feed water that requires RO desalination but that is high in silica concentration is first filtered with a particle removing filter, such as a microporous or ultrafiltration membrane filter, to remove suspended solids that may otherwise foul the RO membranes. A silica dispersant is introduced prior to the RO step. Some or all of the primary RO concentrate is preferably filtered by a microporous or ultrafiltration membrane to reduce dispersed silica concentration and the filtrate is further treated with RO. This microporous or ultrafiltration membrane can be either the same as used for pretreatment ahead of the RO or a separate MF or UF dedicated to only receive RO concentrate as its feed.

Inventors:
ZUBACK, Joseph, Edward (172 Cottage Grove Avenue, Camarillo, California, 93012, US)
Application Number:
US2009/000872
Publication Date:
August 20, 2009
Filing Date:
February 11, 2009
Export Citation:
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Assignee:
SIEMENS WATER TECHNOLOGIES CORP. (181 Thorn Hill Rd, Warrendale, Pennsylvania, 15086, US)
ZUBACK, Joseph, Edward (172 Cottage Grove Avenue, Camarillo, California, 93012, US)
International Classes:
B01D61/02
Foreign References:
US20030127391A12003-07-10
US5858240A1999-01-12
US20070186574A12007-08-16
US20070181496A12007-08-09
Attorney, Agent or Firm:
MUSACCHIO, Pasquale et al. (Siemens Corporation, Intellectual Property Dept.170 Wood Avenue Sout, Iselin NJ, 08830, US)
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Claims:

What Is Claimed Is:

1. A method for treating water with an ionic solute concentrating membrane, comprising the steps of: a. providing a feed stream containing the ionic solutes; b. filtering the feed stream with a membrane; c. adding at least a silica dispersing chemical to the filtered feed stream; d. separating the filtered stream into an ionic solute reduced permeate and a concentrated ionic solute retentate stream with an ionic solute concentrating membrane, wherein dispersed silica is concentrated in the retentate stream; e. passing the retentate stream through a dispersed silica removal membrane; and f. separating the filtered retentate stream into an ionic solute reduced permeate and a concentrated ionic solute retentate stream with an ionic solute concentrating membrane, which may be the membrane of step d or a separate ionic solute concentrating membrane.

2. The method of Claim 1 wherein each ionic concentrating membrane comprises a reverse osmosis membrane.

3. The method of Claim 1 wherein the membrane of step b comprises a dispersed silica removing membrane.

4. The method of Claim 3 wherein the membrane of step b comprises a microporous membrane.

5. The method of Claim 3 wherein the membrane of step b comprises an ultrafiltration membrane.

6. The method of Claim 4 wherein the membrane of step b additionally comprises a backwash system.

7. The method of Claim 5 wherein the membrane of step b additionally comprises a backwash system.

8. The method of Claim 1 wherein step c comprises adding at least one anti- scaling chemical.

9. A method for treating water with an reverse osmosis membrane, comprising the steps of: a. providing a feed stream containing the ionic solutes; b. passing said feed stream through a back-wash enabled membrane apparatus; c. adding at least a silica dispersing chemical to the filtered feed stream; d. imposing the feed stream onto the reverse osmosis membrane to produce a ionic solute reduced permeate and a concentrated retentate stream, wherein dispersed silica is concentrated in the retentate stream; e. passing the retentate through a back-wash enabled dispersed silica removing membrane having a dispersed silica removal capability greater than the membrane filter of step b; and f. separating the filtered retentate stream into an ionic solute reduced permeate and a concentrated ionic solute retentate stream with a reverse osmosis membrane, which can be the membrane of step d or a separate reverse osmosis membrane.

10. The method of Claim 9 wherein the membrane filter of step b is a microporous membrane.

11. The method of Claim 9 wherein the membrane filter of step b is an ultrafiltration membrane.

12. The method of Claim 9 wherein step c comprises adding at least a silica dispersing chemical and an anti-sealant material.

13. A method for treating water with a reject staged reverse osmosis membrane system, comprising the steps of: a. providing a feed stream containing the ionic solutes;

b. passing said feed stream through a first back-wash enabled membrane apparatus; c. adding at least a silica dispersing chemical to the filtered feed stream; d. imposing the feed stream onto a first reverse osmosis membrane to produce a ionic solute reduced permeate and a concentrated retentate stream; e. wherein dispersed silica is concentrated in the retentate stream; f. passing the retentate through a dispersed silica removing membrane; g. adding at least a silica dispersing chemical to the filtered feed stream; and h. imposing the filtered retentate from step e. onto a second reverse osmosis membrane to produce a second ionic solute reduced permeate and a further concentrated second retentate stream.

14. The method of Claim 13 wherein the membrane of step b is capable of removing dispersed silica.

15. The method of Claim 14 wherein the membrane filter of step b is a microporous membrane.

16. The method of Claim 14 wherein the membrane filter of step b is an ultrafiltration membrane.

17. The method of Claim 13 wherein in either or both of steps c. and f. at least one anti-scaling chemical is additionally added.

18. The method of Claim 13 wherein in step g the second retentate stream is fluidly joined with the feed stream and filtered through the membrane of step b.

19. A method for concentrating at least a component in a water feed stream with a reverse osmosis membrane, comprising the steps of: a. providing water containing the ionic solutes to a feed volume holding apparatus; b. providing a feed stream from the apparatus;

c. filtering the feed stream with a membrane; d. adding at least a silica dispersing chemical to the filtered feed stream; e. separating the filtered stream into an ionic solute reduced permeate and a concentrated ionic solute retentate stream with an reverse osmosis membrane, wherein dispersed silica is concentrated in the retentate stream; f. passing the retentate stream through a dispersed silica removal membrane; and g. providing the filtered retentate stream to the holding apparatus.

20. The method of Claim 19 wherein the membrane of step b comprises a dispersed silica removing membrane.

21. The method of Claim 20 wherein the membrane of step b comprises a microporous membrane.

22. The method of Claim 20 wherein the membrane of step b comprises an ultrafiltration membrane.

23. The method of Claim 21 wherein the membrane of step b additionally comprises a backwash system.

24. The method of Claim 22 wherein the membrane of step b additionally comprises a backwash system.

25. The method of Claim 19 wherein step c comprises additionally adding at least one anti-scaling chemical.

Description:

DESALINATION OF WATER CONTAINING HIGH SILICA CONTENT

Cross Reference to Related Application and Priority Claim

This application claims the benefit under 35 U. S. C. § 119(e) of copending U.S. Provisional Application Serial No. 61/027,520, entitled DESALINATION OF HIGH SILICA WATER WITH INTEGRATED MEMBRANE PROCESS filed on February 11 , 2008, which is hereby incorporated by reference in its entirety.

Field of the Invention

The invention relates to water desalination, and more particularly, to a water desalination process comprising an integrated membrane and chemical treatment process which provides for improved pure water productivity.

Background of the Invention

Silica solubility limits water use in applications such as cooling, boiler, and reverse osmosis (RO), and geothermal applications. Silica concentrations above about 150 to 180 mg/L at ambient temperatures will cause accelerated fouling due to limited silica solubility. At these concentrations, and especially above about 180 mg/L, reactive silica polymerizes to form colloidal silica which will foul membranes and may even plug the feed spacer in membrane modules.

Silica in water is in the reactive or unreactive form. The reactive form refers to monomeric SiO 4 . The polymerized form results when the silica concentration exceeds the saturation limit at the use conditions. Unreactive silica consists of polymerized silica as well as colloidal and granular silica.

Reverse osmosis filtration is used to desalinate water using a semipermeable membrane at elevated pressure. Reverse osmosis is operated at pressures above the osmotic pressure of the feed, which is determined by the type and concentration of salts in the feed stream. The driving force for permeation through the membrane is related directly to the difference between the feed stream pressure and the osmotic pressure. The larger the difference (Feed minus osmotic) the higher the permeation rate. Recovery (discussed below) limits the permeation rate. Purified water, the permeate, passes from the higher pressure side and dissolved entities, such as

ions, are retained on the high pressure side of the membrane, denoted variously as the concentrate, retentate or reject.

Reverse osmosis membranes can be supplied in a variety of properties. So- called seawater membranes are used to desalinate seawater (equivalent to approximately 35,000 ppm NaCI) at pressure of 800 - 1500 psi. This type of membrane will retain over 99% of incident salt. So-called brackish water membranes operate at lower pressures in waters of lower ionic strength. They will have relatively lower inherent retention of salt ions, but have a higher permeability and when properly engineered, will operate economically. Nanofiltration membranes are so-called "loose" reverse osmosis membranes which retain species of greater than about 400 molecular weight. They have relatively higher permeability than the previously described membranes. For simplicity, reverse osmosis (RO) as used herein will refer to all the previous mentioned membranes, brackish, seawater and nanofiltration.

Operators of reverse osmosis plants use a variety of methods to prevent silica fouling. Pretreatment methods to remove or reduce colloidal and larger particulates from gaining access to the RO membrane include, for example, slow sand filtration, particularly dual media sand filtration. This method uses a layer of anthracite over a layer of fine sand. Other methods may be used singularly or in combination. These include, but are not limited to, mixed media filtration, non-woven fabric cartridge filtration, and membrane filtration. Ultrafiltration and microporous membrane filtration, while more expensive, has become more popular because these technologies remove colloidal species more effectively than traditional clarification and filtration methods.

Flocculation, coagulation and precipitation may also be used. However, these methods generate large quantities of sludge. Also, aluminum residuals from alum coagulation may cause colloidal fouling of RO membranes by formation of aluminum silicates. Moreover, polyvalent metal ions, such as used in lime or other precipitation methods, (i.e., iron, aluminum, calcium, magnesium, etc.) can cause silica absorption or complexes and catalyze silica polymerization.

Much research and development has gone into silica control technology in aqueous systems. The approaches that have been primarily used include inhibiting silica polymerization, increasing the silica solubility as it forms and dispersion of precipitated silica and silicate compound using polymeric dispersants

Magnesium silicate is commonly encountered in RO systems. Magnesium silicate precipitation depends on solution pH and temperature. Above pH 9, magnesium hydroxide and silicate ions are prone to form magnesium silicate. Hydroxide salts such as calcium, strontium, and sodium, may also react with silicate ion, but produce more soluble products and have less fouling potential.

The use of boric acid and/or its water soluble salts to control silica based deposits in cooling water systems operating at 250 to 300 mg/L silica has been reported. However, boric acid is poorly rejected by RO and may lead to problems downstream (effluent discharge limitations on boron).

Solution pH governs silica polymerization. At high silica concentrations, higher pH generates the problem of magnesium silicate scale. Reducing pH simply changes the problem from magnesium silicate to silica.

Chemical methods are also used. Silica inhibitors retard polymerization of monomeric silica. Dispersants place a repelling charge on the silica particle surfaces, which prevent combining and enhance silica particle dispersion in to the water. This subject has been much studied and many chemical and polymer systems have been reported. Examples of polymeric silica dispersants are polyacrylamide-based treatment programs, phosphonate and a copolymer of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid and hydroxyphosphonoacetic acid and a copolymer of acrylic acid and allyl hydroxy propyl sulfonate. Many of the developed products and formulations are proprietary.

Reference articles such as UltraPure Water, Vol. 16, No. 2, Feb-99, Tall Oaks Publishing, 1999, and Desalination, Volume 167, 15 August 2004, Pages 257-272 describe such systems. Commercial chemical systems generally combine inhibitors, dispersants and anti-sealants in various combinations for different waters. Examples

are PermaTreat® PC-510 (Ondeo Nalco, Naperville, IL 60563) and Carbosperse™ K-XP212 Copolymer (Lubrizol Corp Wickliffe, Ohio 44092).

Desalination practitioners commonly use once-through flow in reverse osmosis operations. Practitioners may also use concentrate recirculation, where the concentrate is returned to the feed storage tank. In relatively small applications, such as waste water, where intermittent or non-continuous discharge is used, a batch or semi-batch method is common. A batch operation is one in which the feed is collected and stored in a tank or other reservoir, and periodically treated. In semi- batch mode, the feed tank is refilled with the feed stream during operation.

The RO system may have single or multiple stages. In a single stage system, the feed passes through one or more pressure vessel arranged in parallel. Each pressure vessel will have one or more membrane modules in series. The number of stages in a multiple staged system is defined as the number of single stages the feed passes through before exiting the system. Permeate staged systems use permeate from the first stage as feed for the second stage, and if multiple stages are used, permeate from a stage just prior is used as feed for the following stage. In reject staged systems, the reject stream of a stage is sent to become the feed stream of a subsequent, usually the next, stage. Reject, concentrate and retentate and similar terms have synonymous meanings in RO processing

RO systems can be engineered in a variety of conformations, depending on the amount of water to be processed, the feed concentrations and the required output. Reverse osmosis system design is the topic of several books, such as The Guidebook to Membrane Desalination Technology: Reverse Osmosis, Nanofiltration and Hybrid Systems Process, Design, Applications and Economics (WiIf, M., et al; Desalination Publications).

Silica is sometimes called the recovery limiting component of a water being desalinated. In RO operation, the high pressure feed stream is imposed on and flows across one side of the membrane and purified water is removed from the other side. The purified flow is the permeate or product stream. The feed side retains the majority of dissolved species, and usually less than 5%, and more usually, less than about 1% of salts and other species pass through the membrane. This causes species concentration in the feed side to increase. The stream being removed is the

recovery stream, which is what remains of the feed steam minus the permeate. The recovery stream is not to be confused with process recovery, which will be called permeate recovery herein. The recovery stream carries away the concentrate or rejected species. Permeate recovery, or simply recovery is defined mathematically as the ratio of permeate flow to feed flow, P/F, limits RO operation in two ways. As the concentration in the feed side increases, osmotic pressure increases, which reduces the driving force for permeation. If the solubility limit of a species is reached, precipitation will occur on the membrane surfaces, and the resulting membrane fouling will decrease permeation, reducing productivity, or require an increase in pressure, increasing energy costs.

Permeate recovery for lower salt containing brackish waters typically are in the range of about 70% to 80%. For seawater, with about 35,000mg/L salt, recovery can be about 35%. Concentrate flow is a major cost factor as it is high pressure waste. Practitioners seek to increase the percentage of permeate flow, i.e., product recovery, and decrease retentate or concentrate flow. Decreasing concentrate flow is limited by silica precipitation due to the increase in solute concentration. Therefore methods for increasing product recovery, or reducing concentrate flow will be beneficial to the desalination industry.

Summary of the Invention

The inventive method uses a particle removal filter, preferably a microfiltration or ultrafiltration membrane filter, more preferably, a backwashable microfiltration or ultrafiltration membrane filter to remove the dispersed fraction of silica from reverse osmosis reject or concentrate. This allows the dissolved salts in the reject to be further concentrated without silica fouling by feeding additional silica dispersant, if and as needed, to the particle removal filter filtrate and feeding the reject to an RO step.

In an embodiment, raw water feed is filtered with a particle removal filter and mixed with a silica dispersant before reaching a reverse osmosis membrane process. The concentrate flow from the RO process is returned to the feed side of the particle removal filter to be filtered and combined with raw water to make up the RO membrane feed after filtration and dispersant addition. In an alternative to this

embodiment, the concentrate is returned to a point after the particle removal filter and combined with filtered raw water feed.

In an embodiment, raw water feed is filtered with a particle removal filter and mixed with a silica dispersant before reaching a reverse osmosis membrane process. The concentrate flow from the RO process is filtered in a separate filtration apparatus, preferably with a filter capable of removing smaller entities than the particle removal filter. The filtered concentrate flow is returned to the feed side of the particle removal filter and combined with raw water to make up the RO membrane feed after particle removal filtration and dispersant addition. In an alternative to this embodiment, the filtered concentrate is returned to a point after the particle removal filter and combined with filtered raw water feed.

In an embodiment, a reject staged reverse osmosis system is used. In this embodiment, raw water feed is filtered with a particle removal filter and mixed with a silica dispersant before reaching a reverse osmosis membrane process. The concentrate flow from the RO process is filtered in a separate filtration apparatus and used as feed to a second RO process.

In an embodiment, feed and concentrate stream flow are mixed in a feed tank to concentrate the dissolved species in the feed.

Brief Description of the Drawings

Figure 1 shows a process wherein the raw water feed is combined with concentrate and filtered through a particle removal filter before silica dispersant addition prior to the RO step.

Figure 2 shows a process wherein the concentrate is filtered separately before being combined with the raw water feed and filtered through a particle removal filter before silica dispersant addition prior to the RO step.

Figure 3 shows the process used in a reject staged RO process.

Figure 4 shows a process with a recycle feed tank for a concentration process.

Detailed Description of the Invention

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of FIGURES 1 -4.

The method described herein provides for improved productivity for desalination by reverse osmosis (RO). The method is an integrated membrane and chemical treatment process, with a special emphasis on increasing recovery based on permeate, defined as the ratio of permeate flow to feed flow, by reducing the effects of silica deposition or fouling.

Saline feed water that requires RO desalination but that is high in silica concentration is first filtered with a particle removing filter, preferably a microporous (MF) or ultrafiltration (UF) membrane filter, more preferably, a backwashable microfiltration or ultrafiltration membrane filter to remove suspended solids that could otherwise foul the RO membranes. A silica dispersant is introduced prior to the RO step and the water recovery in the RO is controlled to the per cent recovery point where the silica concentration in the RO reject does not exceed the dispersant manufacturer's recommendation.

Some or all of the primary RO concentrate is preferably filtered by a microporous or ultrafiltration membrane, more preferably, a backwashable microfiltration or ultrafiltration membrane filter and the filtrate is further treated with

RO. This microporous or ultrafiltration membrane can be either the same as used for pretreatment ahead of the RO or a separate MF or UF dedicated to only receive RO concentrate as its feed. In the former case (Figure 1), the feed to the MF will be a blend of recycled RO concentrate and feed water. In the latter case, the MF or UF filtrate can be either introduced to the feed of the primary RO, or alternatively introduced to a separate or secondary RO for further treatment and salt concentration.

Backwashing physically removes solids accumulated in a membrane module during filtration. Gas, usually air (gas backwash) or pumped filtrate (liquid backwash) is forced through the membrane filter from the permeate side to the feed side. Backwashing is done periodically or as needed to maintain permeation rate, either automated or manually.

Ultrafiltration membranes are filters having pore size rating of from about 10 nanometers to about 100 nanometers. Microporous membrane filters have pore size rating of from about 0.1 micron to about 10 microns.

The technique could potentially be applied to remove other dispersed species or dispersed colloidal solids from RO concentrate such as but not limited to calcium fluoride, sulfate, phosphate, etc. where the dispersed colloidal particles are filterable with MF or UF, either alone or in conjunction with a coagulant chemical added to the feed prior to the MF or UF. The technique could be extended to other concentration processes where chemicals must be used to prevent precipitation of silica and salts that exceed their solubility limit in the concentrate. Examples include evaporators and cooling towers.

The method removes silica by a combination of a chemical dispersant and a membrane filter to removed dispersed silica. Other colloidal and particulate entities will also be removed concurrently. Concentrate recovery plays an important role in this process, so it is important that the silica be brought to a state which will allow removal from the stream.

Figure 1 illustrates an embodiment of the current invention. Feed water from the source is introduced to the process in raw feed flow stream 1. Recovery stream 7 joins the raw water stream to form the membrane system feed flow stream 3. This

is filtered by particle removal filter 2, which is preferably a microporous or ultrafiltration membrane filter, more preferably a filter having a back-wash capability. Particle removal filter 2 may be a microporous membrane or an ultrafiltration membrane, depending on the condition of the dispersed silica and the raw feed water. Silica dispersant 4 is metered or otherwise mixed into stream 3 to disperse silica into a form that will reduce prevent or greatly reduce silica precipitation, fouling or deposition onto or into the membrane surface. The dispersant may be added directly to feed stream 3 by a variety of methods, examples being, directly before high pressure pumping system 5, or through a static mixer, such as a Kenics mixer (Chemineer, Dayton OH 45414), or into a stirred tank to insure homogeneity. Stream 3 with added dispersant is pumped to the RO membrane system 6 and separated into the purified water permeate stream 8 and stream 7, the recovery or concentrate stream.

Stream 7 is returned to particle removal filter 2, which is sized to be capable of removing dispersed silica and combined with raw feed water. By reusing concentrate stream 7, an overall higher permeate recovery is attained.

For the process shown in Figure 1 , a mass balance around the membrane for silica gives;

1. F-CsiF= R-CsiR -i- P-csiP

Where F =Feed Flow rate after the particle removal membrane

2. R = Recovery flow rate 3. P = Permeate flow rate

Csi = Silica concentration in the various flow; indicated by F, R, P

Assuming that greater part of the silica is retained by the RO membrane,

And defining RR as the recovery flow based recovery, =R/F

5. CSJF/ CsiR= RR

Equation 5 shows that if the silica concentration in the feed is reduced, R R decreases to maintain the same recovery concentration, which is the concentration being imposed on the membrane. Since the permeation recovery, R P , which is desired to be maximized is R P = P/F = (1 -RR), decreasing RR will increase R P . That is, higher productivity will result.

If the recovery flow is returned to the feed side of the particle removing membrane, the Feed flow to the membrane will be P + R. The silica concentration will be CsiM = (P- C S JF +R- c SiR )/(P+R) = CSIF/O +R/P).

Here, CSIM is the silica concentration at the membrane. For illustration, it is assumed that the membrane removes all or most of the siica associated with the recovery stream. It can be seen that the silica concentration will always be lower than the raw feed concentration by the factor 1/(1 +R/P).

In Figure 2, an intermediate concentrate stream filter 29 is added before the particle removal filter 22. In this embodiment, feed water from the source is introduced to the process in raw feed flow stream 21. Recovery stream 27 joins the raw water stream to form the membrane system feed flow stream 23. This is filtered by particle removal filter 22, which is preferably a microporous or ultrafiltration membrane filter, more preferably a filter having a back-wash capability. Particle removal filter 22 may be a microporous membrane or an ultrafiltration membrane, depending on the condition of the dispersed silica and the raw feed water. Silica dispersant 24 is metered or otherwise mixed into stream 23 to disperse silica into a form that will prevent or greatly reduce silica precipitation, fouling or deposition onto or into the membrane surface. The dispersant may be added directly to feed stream 23 before high pressure pumping system 25, or through a static mixer, such as a Kenics mixer (Chemineer, Dayton OH 45414), or into a stirred tank to insure homogeneity. Stream 23 with added dispersant is pumped to the RO membrane system 26 and separated into the purified water permeate stream 28 and recovery or concentrate stream 27.

Filter 29 is particularly useful if the dispersed silica colloids or particles are too small to be effectively filtered by particle removal filter 22. Filter 29 may be an ultrafiltration membrane or a microporous membrane of smaller pore size rating than filter 22. Filter 29 may be a nanofiltration membrane. Filter 29 may preferably be a

backwash capable filter. The use of a more retentive membrane in this position in the process also has the benefit of allowing a less retentive filter to be used for filter 22. Since filter 22 will have a higher flow through it, this will reduce energy requirements.

Figure 3 illustrates how the process could be used in a reject staged RO process. An optional intermediate recovery stream filter 39 is added before the particle removal filter 32. In this embodiment, feed water from the source is introduced to the process in raw feed flow stream 31. Recovery stream 37 joins the raw water stream to form the membrane system feed flow stream 33. This is filtered by particle removal filter 32, which is preferably a microporous or ultrafiltration membrane filter, more preferably a filter having a back-wash capability. Particle removal filter 32 may be a microporous membrane or an ultrafiltration membrane, depending on the condition of the dispersed silica and the raw feed water. Silica dispersant 34 is metered or otherwise mixed into stream 33 to disperse silica into a form that will prevent or greatly reduce silica precipitation, fouling or deposition onto or into the membrane surface. The dispersant may be added directly to feed stream 33 before high pressure pumping system 35, or through a static mixer, such as a Kenics mixer (Chemineer, Dayton OH 45414), or into a stirred tank to insure homogeneity. Stream 33 with added dispersant is pumped to the RO membrane system 36 and separated into the purified water permeate stream 38 and recovery or retentate stream 39. Retentate stream 39 is used as the feed stream for a second RO system 40. Stream 39 may be fed directly to the RO system 40, or through a dispersed silica filter. Since the solute concentration in this stream is higher than that filtered by the first RO system, a RO system having a higher intrinsic rejection capability may be used. For example, by way of illustration, the first RO system may be a brackish water system, while the second RO system may be a sea water RO system. Additional silica dispersant may also be added. RO 40 separates stream 39 into a permeate 41 and a recovery stream 37. Stream 37 may flow directly to raw water feed 31 , or through a concentrate stream filter as described in the discussion for Figure 2.

Figure 4 shows a basic arrangement wherein the process may be used to concentrate an aqueous feed having high silica content. This case would be useful where the concentrate contains a value species, or conversely, where the

concentrate contains a species that must be disposed for environmental or safety reasons. Feed water from the source is introduced to a concentrate holding tank 60 by raw feed flow stream 51. Recovery stream 57 joins the raw water stream in tank 60. Feed from the tank 61 is filtered by particle removal filter 52, which is preferably a microporous or ultrafiltration membrane filter, more preferably a filter having a back-wash capability. Particle removal filter 52 may be a microporous membrane or an ultrafiltration membrane, depending on the condition of the dispersed silica and the raw feed water. Silica dispersant 54 is metered or otherwise mixed into stream 53 to disperse silica into a form that will prevent or greatly reduce silica precipitation, fouling or deposition onto or into the membrane surface. The dispersant may be added directly to feed stream 3 by a variety of methods, examples being, directly before high pressure pumping system 5, or through a static mixer, such as a Kenics mixer (Chemineer, Dayton OH 45414), or into a stirred tank to insure homogeneity. Stream 3 with added dispersant is pumped to the RO membrane system 6 and separated into the purified water permeate stream 8 and stream 7, the recovery or concentrate stream.

Concentrate Stream 57 is returned to the feed tank, optionally being filtered by a dispersed silica removal membrane.

The proper use of a dispersant is required for this method as exemplified by these examples. If the silica forms colloids or granules that are too large to be dispersed, or to be carried away with the recovery flow, membrane fouling will probably result. If the silica is dispersed in a too small form, the membrane will not effectively remove them.

Moreover, reject stream filtration has to be sized to effectively remove dispersed silica and reduce silica content to allow reject stream to be further concentrated.

Practitioners of water treatment by membrane systems will realize that the variations of the inventive method described herein offer benefits in terms of improved productivity with reduced chemical use, reduced sludge compared to flocculation and precipitation methods, simplified operation due to possible reduction in the number of RO units required. Concentration of solutes is enhanced with the present method.

Practitioners skilled in the art will recognize that RO desalination plant design and operation will vary depending on site conditions. Each location will have its own combination of feed - types and concentrations of salts, organic solutes, and foulants - and ambient conditions. The examples described herein are meant to be representative, and are not to be limiting in any way, but to be used plan and implement the novel process, modified for the conditions and requirements of a specific case.