FR2668469A1 - PROCESS FOR THE PURIFICATION OF WATER BY MEANS OF A COMBINATION OF SEPARATION UNITS WITH MEMBRANES, AND INSTALLATION FOR IMPLEMENTING SAME. - Google Patents

Abstract

The invention relates to a system for obtaining desalinated water using a combination of at least two membrane units / processes, namely the electrodialysis unit (DE), including the inversion variants. EDI and the one with full compartments, EDP), followed by the reverse osmosis unit (5), including nanofiltration, arranged in a series configuration in which each membrane unit gradually purifies the water until the desired degree of purity is obtained. The treatment is improved by using at least part of the reverse osmosis reject brine as a tributary, via (13) for the electrodialysis unit. This reject stream is used to save water in the overall process.

Description

Process for the purification of water by means of a combination of

  Membrane separation and installation for its implementation The present invention relates generally to water treatment, and more particularly to the production of pure or ultrapure desalinated water by the use of a combination of methods based on the use of membranes, that is to say, in the simplest combination, of at least one electrodialysis process and of a reverse osmosis process In addition, the invention relates to a purification technique according to which the rejection current which leaves the reverse osmosis unit is exploited by recycling it in the electrodialysis unit instead of discharging it in the sewers like that

  is normally expected in old design processes.

  The scope of the invention also relates to a

  installation for implementing said method.

  The latest technological advances have made the availability of pure or ultrapure water practically essential for numerous industrial and scientific applications, as also in the field of energy production. Even if purification systems of old design are already capable of providing water having characteristics which largely meet current standards, these methods have major drawbacks. In particular, a known method which provides for the use of filtering means and / or filter cartridges, followed by a method of reverse osmosis (RO) and an ion exchange treatment with a double or mixed bed regenerated on site, presents the following negative aspects: (a) the need for frequent washing against the current of the filtering means, or else frequent replacement of filter cartridges, (b) the proliferation of bacteria in the filter media; (c) the need for chemical treatment and disposal of the waste following the on-site regeneration of ion exchange resins; and more importantly (d) the need to use chemicals to acidify and / or pretreat water before subjecting it to reverse osmosis. A process for the prior treatment of water by the use of mobile units containing exclusively ion exchange columns is described in US Pat. Nos. 4,280,912, 3,766,060, 4,188,291, 4,332,685 and others. Another known treatment method provides for the use of electrodialysis before l ion exchange, with a final purification of the resulting water by means of a hollow fiber ultrafiltration unit (see Zmolek, CR, "Ultrapure water for Integrated Circuits Processing", published in Industrial Water Engineering from December 1977 ) In any case, none of the methods indicated above relates to the treatment method described here, based on an electrodialysis followed by reverse osmosis, in combination with a specific recovery system capable of guaranteeing deriving an unprecedented improvement in the overall process In the new configuration, the salt rejection current which leaves the reverse osmosis unit is preserved by causing it to flow back into the salt enrichment, dilution and / or electrode chambers that make up the electrodialysis unit, rather than being discharged into

sewers.

  The present invention therefore consists in the production of a water treatment system capable of economically providing purified water for long periods without the process comprising frequent backwashing, filter changes, addition of reagents, excessive water loss during discharge

  or other handling operations.

  Another object of the invention is to provide a process capable of separating organic substances and obtaining biologically pure water, characterized by a specific electrical resistance of the order of 17-18 megohms / cm at 250 C. Another aim of the present invention is to provide a water purification installation comprising a combination or a series of individual elements connected to a water pipe or to another water supply system for the purpose of treating water for To purify it, at the same time reducing the losses of discharge in the sewers. According to the invention, this installation has at least two membrane treatment systems, either an electrodialysis (DE, also according to the inverting variants "EDI" or with full "EDP" compartments) and reverse osmosis (RO), also in the form of nanofiltration ("NF"), arranged in series with means which make it possible to use the saline stream of rejection of RO in ta nt as supply current for the electrodialysis unit (remember that the 0 I is sometimes called hyperfiltration, or abbreviated as HF) The complex can be integrated with other elements to achieve the desired characteristics of the water final, for example: an ultrafiltration system, the so-called "cross-flow" filtration system), ion exchange beds, an ozone generator and / or sources of ultraviolet (UV) rays for the destruction of microorganisms

  time of use of the product The purification installation below

  if necessary, it can be easily mounted in a container or in a semi-trailer, so that it can be conveniently transferred to the user's seat. Once connected to the electrical power, water supply, waste water and purified water, the complex can provide treated water as required

  whatever customers or users.

  The advantages of the invention can be summarized as indicated below:

  a) the “cross-” ultrafiltration or filtration unit or element

  flow "(cross-flow) provides more efficient pre-treatment than

  common filters with free and / or cartridge filter media.

  Considering the minimal pore dimensions of the UF membranes (from 0.002 to 0.02 micrometers), ultrafiltration is in any case preferable and is valid not only for separating the particles in suspension, but also for reducing the bacterial level in l water These are specific advantages which can be enjoyed when using, on the pre-treatment site upstream of membrane processes, UF or cross-flow filtration as well as free and / or cartridge filter media Free filter media can migrate, disintegrate or promote an exuberant proliferation of microbiological species On the other hand, filters with a filter element can sometimes discharge under the effect of a momentary overpressure, become bypass passages as a result of assembly errors or be rendered ineffective as a result of the passage of solids released during

  replacement of filter elements.

  b) The electrodialysis unit or element, in addition to lowering the total dissolved solids (SDT) rate, can also reduce the p H of water, i.e. increase the acidity (we preferably operates in a p H range of 4 to 6.8), and thus eliminate the need to add acids from the outside before sending the water to the reverse osmosis treatment In the purification complex , the electrodialysis unit can

  have primary demineralizers.

  During desalination treatment by electrodialysis, it is possible that "polarization films" will form against the internal surface of the membranes which limit the demineralization compartments, in the presence of relatively current densities.

  high compared to the salinity of the water to be demineralized.

  Usually, this phenomenon occurs when the treated water has a reduced rate of total dissolved solids and when the density of the applied current is such that it depletes the ions contained in the strata of water brought into direct contact with the surface of the membranes. ; thus, the current applied thereafter is transported by the hydrogen and oxhydryl ions which have formed in the depleted stratum as a result of the decomposition of the water molecules. As a rule, this separation of water is much more intense in the case anionic membranes, reason why the oxhydryl ions deposited thereon, with negative charge, easily cross the membranes

  themselves to reach the adjacent concentrator compartments.

  On the other hand, the hydrogen ions thus generated, with a positive charge, cannot cross the anionic membrane and therefore tend to accumulate in the demineralizing compartment, thereby giving rise to an acidification of the water stream downstream This acidification " natural "is exploitable in an original way by the present invention in the sense that no addition of acid from an external source is planned any more before the following treatment by reverse osmosis of the partially demineralized product The effects of polarization are discussed in detail in an article titled "Limiting Current in Membrane Cells" in Industrial &

  Engineering Chemistry, vol 49, page 780, April 1957.

  The present invention therefore relates to a process for purifying water, characterized in that an electrodialysis unit is used in combination with a reverse osmosis unit, said electrodialysis unit comprising one or more pairs demineralizing and concentrating compartments delimited by membranes permeable respectively to anions and cations, which compartments are arranged between a pair of electrode compartments, themselves arranged at the ends, the passage of feed water being further provided in the demineralizing compartments while a direct electric current is applied between the aforementioned pair of electrodes, so as to reduce the salinity level of the feed water by causing the passage of salt from said demineralizing compartments to the concentrating compartments, which is followed by the output of the electrodialysis unit of the effluent partially desalted and its passage, under pressure, to the inlet of a membrane unit which operates on the principle of reverse osmosis, from an outlet from which the appreciably desalted permeate which has passed through the membrane is removed reverse osmosis, while through another outlet there is taken up a concentrated rejection liquid which has not passed through the reverse osmosis membrane, and at least part of this latter final liquid is recirculation as feed stream

  for said electrodialysis unit.

  In particular embodiments, the method of the invention may also have the following characteristics, taken individually or in combination: said discharge liquid is continuously recirculated in a ring system towards the concentrating compartments of the unit electrodialysis, with simultaneous removal from the electrodialysis unit of part of the liquid recirculated; the recirculated liquid can in particular be sent as a tributary to at least one of said electrode compartments; the water is subjected to ultrafiltration or to cross-flow filtration before passing to the electrodialysis phase; the electrodialysis treatment is carried out under practically polarizing conditions with the aim of acidifying the partially desalinated product which leaves the demineralizing compartments of the electrodialysis unit; the partially desalted product can be acidified by electrodialysis to a p H of between approximately 4 and approximately 6.8; following the reverse osmosis phase, the permeate is subjected to a subsequent treatment by ion exchange to separate all or almost all of the residual ions in solution; water produced by ion exchange treatment is subjected to ozone sterilization at an appropriate concentration or to ultraviolet rays of appropriate intensity in order to destroy biological contaminants; the water produced by means of the ion exchange treatment is subjected to a final ultra-purification treatment in order to obtain as product a water having an electrical resistance greater than about 17 megohms / cm at the temperature of 25 OC; the polarity of the direct current is reversed at regular intervals, with simultaneous switching of the circulation of the solutions towards the demineralizing compartments and the concentrating compartments. The invention also relates to an installation for the separation of dissolved salts from an aqueous solution, characterized in that it comprises an electrodialysis unit with several compartments in combination with a reverse osmosis unit, said unit d electrodialysis comprising a multiplicity of compartments, the two terminal compartments of which are the electrode compartments, demineralizing compartments and concentrating compartments being arranged alternately between said electrode compartments, said compartments being delimited in pairs alternately by cation-permeable membranes and anion-permeable membranes, with in addition inlet devices for the admission of the tributary solution into the demineralizing compartments, concentrators and electrodes, and outlet devices for the withdrawal of a solution coming from said compartments, as well as dispositi fs for the passage of a direct current through the aforementioned membranes and compartments, and devices for the pressurization of at least part of the solution which leaves the demineralizing compartments, said reverse osmosis unit having devices for inlet for receiving the pressurized effluent from said demineralizing compartments, emission devices for removing the permeate from the reverse osmosis membrane, emission devices for separating the pressurized discharge stream, and devices for recycling of at least part of this reject current as part of the supply current of

said electrodialysis unit.

  In particular embodiments, the installation according to the invention may also have one or more of the following characteristics: it includes devices for routing at least part of the rejection current to the electrode compartments of the electrodialysis unit , as a feed tributary; it includes devices for recirculating the solution through the concentrator compartments, as well as devices for withdrawing part of said solution for its discharge into the sewers; it includes ultrafiltration devices arranged upstream of the electrodialysis unit; it includes devices capable of operating the electrodialysis unit under practically polarizing conditions; downstream of the reverse osmosis unit, and in series with the emission devices for the permeate sampling, a mixed bed ion exchange system is installed; after the reverse osmosis unit, and in series with the devices for emitting the permeate sample, a sterilization system is installed; it includes devices capable of reversing the polarity and

  it is placed in a transportable container.

  The advantages of ED as a pre-treatment system for 1 OI are as follows. 1) The stream of water which reaches the concentrator, electrodes and / or demineralizer compartments of the DE unit can be obtained at least in part by the saline stream rejecting the unit (RO). original the discharge current, instead of discharging it into the sewers, we keep in the

  proceeded with the water at the same time as the hydrogen ions.

  2) Bicarbonate and hardness can be removed without the use of added chemicals The Langelier index of water exiting a DE unit is generally lower than that of incoming water and often has a sign negative Thus, the water produced by the DE unit can almost always be sent to the QI unit

  without the usual chemical additives.

  3) The DE pre-treatment upstream of the RO makes the inclusion of a degassing agent superfluous, with the need which is linked to it

  pumping and the risk of contamination.

  4) The efficiency of the DE unit is not appreciably felt by the low temperature, even if it can result in a reduction in the percentage of demineralization Generally, the RO

  reacts in the opposite direction to lowering the temperature, i.e.

  say has a lower hydraulic efficiency while the rejection rate remains substantially invariable. Therefore, a combination of the two methods guarantees a better overall maintenance in terms of quality and

  quantitative in the case of temperature variations.

  Furthermore, reverse osmosis (RO) reduces the mineral rate (CT) of the water, and if desired can be carried out with a final treatment by ion exchange, in order to obtain extrapure water. As the water thus obtained (permeate) which derives from the OI treatment can have an extremely low content of dissolved salts (ions), further treatment with ion exchange resins can be continued for fairly long periods of time before the resin needs to be regenerated This makes it possible to use cartridges or portable ion exchange tanks on the site of the ion exchangers with regeneration on the site of the previous solutions. This eliminates the need for difficult and costly chemical treatments on the site In addition, the problems associated with the flow of the concentrated rejection current at the outlet of the RO unit are reduced or eliminated by exploiting the current itself as supply for compartments

  concentrators, demineralizers and / or the electrodes of the DE unit.

  Reference is made to the following paragraph, with reference to Figure 2,

  for a more detailed description of the new combination

  electrodialysis / reverse osmosis devices by using a line allowing the current to flow back to the DE unit

concentrated rejection of OI.

  1-5 Other characteristics and advantages of the present invention

  will appear more clearly on reading the description which follows

  with reference to the accompanying drawing plates, presented for information only and in which: FIG. 1 represents a flow diagram of the combination of elements comprising a preferred purification installation forming the subject of the invention, and FIG. 2 constitutes a detailed schematic representation in cross section of the combination of electrodialysis and reverse osmosis of the preferred purification installation indicated in the figure

former.

  In FIG. 1, a water source 1 is connected to an optional pre-filtering device 2 chosen from a cartridge filter or a conventional free filtering means with a charge of active carbon, silver active carbon, macroreticular ion exchange resin for the absorption of organic substances, of anion exchange trapping resins, called "scavenger" (type of Ambersorb resins from Rohm & Haas) intended to intercept any relatively large suspended particles. It will be indicated that the prefilter ( like certain other elements of the system), can be modified as necessary or completely eliminated If provision is made for the use of prefilters, it is advantageous that they are used in pairs and connected so as to allow the circulation of water in series , in parallel or in a single unit Naturally, the various rigid or flexible tubes optional (for example for circulation in series or in pa parallel, recycling, feeding and purging, etc.) which can be installed to increase the flexibility of the system, are neither shown nor described here, since these are solutions already widely known by those skilled in the art. art For the same reason, the various components commonly used in water treatment plants, such as holding tanks, electrical conductivity measuring devices, measuring instruments, flow meters, recorders, pumps, valves, measuring devices, etc. are also not shown. Next, the water which is subjected to an optional prefiltration is sent under pressure to the ultrafiltration unit 3, with membranes having pores whose diameter is included. between 0.002 and 0.02 micrometers, for the separation of colloidal particles and residual organic substances, or to a filtration unit called "cross-flow" or (FCF) (at cr The constitution and operation of the UF and FCF units are well known in this field. A type of UF with a spiral arrangement can be easily obtained by contacting Osmonics Inc. of Minnetonka, Minnesota / USA At this stage and in what follows, the term "ultrafiltration" or UF 'is understood to also include the concept of filtration "cross-flow", that is to say "FCF" The next stage of treatment consists of primary demineralization by means of one or more electrodialysis units 4, the aim of which is to eliminate the main part of the low molecular weight electrolytes from the supply stream 15 introduced into the demineralization compartments of the electrodialysis unit (s) ( An ED or EDP system with adequate characteristics, marketed under the Aquamite brand, can be supplied by Ionics Inc of Watertown, Mass / USA The system uses batteries or bundles of membranes, formed alternately by members. "anionic" donkeys and "cationic" membranes which delimit the compartments in which the liquid circulates: the ED / EDP unit is used to separate the ionized impurities in solution The EDI unit with inversion can maintain a constant quality of the products by means the reversal of the polarity of the electric current through the battery, as described in particular in US Pat. No. 4,381,232 (D Brown) The water 12 which leaves ED / EDI batteries has not only a mineral content

  significantly reduced, but under the effect of an almost

  polarizing, even exhibits a marked increase in acidity This acidity, (which is preferably in the range of p H 4 to 6.8) constitutes a favorable characteristic for water having a calcareous hardness which must be treated by means of OI (5) The use of electrodialysis eliminates the need to add mineral acid from an external source to the supply water stream Acidified water helps prevent or reduce the formation of mud on the membrane OI, and allow the continuous functioning of the system In addition, the treatment with OI makes it possible to separate the residues of colloids, bacteria and electrolytes, as well as certain organic substances dissolved in the water thus treated. Suitable OI filters are commercially available from many suppliers The current of saline discharges which leaves the OI 5 unit is flowed back, as a supply solution, to the concentrator, demineralizer and / or tank compartments. electrodes in the electrodialysis unit (s), via the line 13 The permeate (product) 14 which results from the OI process can be subjected to another purification by means of an ion exchange resin (EI) at double or mixed bed 6, which causes the separation of other dissolved minerals The mixed bed can be formed by an anionic resin of the Amberlite 410 type and by a cationic resin of the Amberlite IR-120 type, (both being manufactured by Rohm & Haas) , but it is also possible to successfully use other mixed bed resins. The purpose of the ion exchange bed (s) is to remove the undesirable ions still remaining in the water treated with RO, with a process of which specialists in this field are widely aware Ion exchange resins are preferably not regenerated on site, but replaced by elements having charges of fresh resin. Then the water, stripped of practically all of its dissolved solids content, is passed through a source of ultraviolet rays or ozone (03) 7 to kill all or almost all of the living microorganisms A wavelength d '' approximately 253.7 nm can effectively and practically destroy all living organisms that may remain in water A UV disinfection system produced by UV Technology Inc (California) has been found perfectly suited to this purpose Naturally, as the man of the art will readily understand this, the required amount of ultraviolet radiation and / or ozone depends essentially on the flow of the system and other factors. At this point, the water is already sufficiently pure for most applications. But if the we want the purification to be further enhanced by the optional use of one or more mixed resin ultra-purification beds 8 containing an additional adsorbent resin and a final microporous filter with cartridge 9 with extra fine pores It is advantageous that the water thus treated is then sent to an electrical conductivity measuring device 10 indicating the electrical resistance of the water as well

  processed to ensure the purity required at the point of use 11.

  A bed of active carbon or better still of silver active carbon can be used for the separation of organic carbon in substitution in addition to the mixed bed 8 This active carbon can be advantageously integrated in the OI module described in US Pat. No. 4,735,717 In in addition to measuring electrical conductivity, we can use total carbon or total organic carbon analyzers

  and / or particle or bacteria counters.

  FIG. 2 schematically represents a source of water 1 admitted as a tributary via 16 into the demineralizing compartments 17 of an electrodialysis unit (DE) 4 of FIG. 1, possibly in the form of a unit of inversion electrodialysis (EDI) or

  occupied cell electrodialysis unit (EDP).

  An electrodialysis unit or battery 4 (FIG. 2) comprises electrode chambers 18 situated at the two ends of the battery and containing the electrodes 19 and 20 respectively. In the intermediate part between the two electrodes are provided various compartments, alternately demineralizing ( diluents) and concentrators 21, delimited by membranes permeable alternately with cations and anions These membranes delimit the circulation compartments (none of which is represented in its

  together in the figure in question).

  To separate the membranes from one another so as to form alternately demineralizing and concentrating compartments, it is possible to use spacers with tortuous paths of the type described in US Pat. Nos. 2,708,657 and 2,891,889, or still network spacer elements The combination formed by an anion exchange membrane, a cation exchange membrane, a demineralizing compartment and a concentrating compartment constitutes a pair of cells. Between a pair of electrodes, it is possible to group together pairs of cells according to any number, so as to form a demineralization battery comprising in a typical configuration 100 pairs of cells or more Systems of this kind are described more particularly in US Pat. Nos. 2,694,680, 2,752,306,2 848 403, 2 891 899, 3 003 940, 3 341 441 and 3 412 006 The manufacture and properties of selective membranes of ty pe used in electrodialysis electrode systems are fully discussed in US Pat. Nos. 24,865,2,730,768, 2,702,272, 2,731,411 and many others under the effect of the electrical potential which is transmitted to through the battery, the sodium, calcium, magnesium and other cations (positively charged) migrate through the cationic membranes and end up in the discharge or concentrate stream (brine) 22 Similarly, the negatively charged chloride particles , sulphate, nitrate, bicarbonate and other anions end up in the discharge stream 22 passing through the anionic membranes Although the aforementioned ions generally constitute the main group of undesirable salts, other low anionic substances

  are separated or can be similarly separated.

  In addition, during the operation of the battery, a current of electrolytes, via line 23, flows in contact with the electrode 19 and a similar current flows in contact with the electrode 20 During the EDI process (electrodialysis with inversion ), the electrical polarity is periodically reversed by a technique widely known to those skilled in the art. This results in an inversion of the direction of movement of the ions, with an "electric washing" effect on the embedded ions and other charged particles. which can be deposited on the surfaces of the membranes There is also the possibility that the demineralizing compartments of the 4 / ED battery, or even the demineralizing compartments or those which are concentrators, can be largely filled with spheres, fibers, fabrics, sheets expanded, etc., ion exchange, as is already widely

  known in this field In the present description, these DE batteries

  fulfilled, along with the associated system and process, are called EDI batteries, systems, and processes, respectively.

  is mentioned here and in the claims of an "electrodialysis", or

  "DE", we also refer to variants of electrodialysis with

  inversion or EDI and full cell electrodialysis or EDP.

  The partially demineralized water which collects in the effluent 24 is then introduced under high pressure by the pump 25 as a feed stream via 14 into a reverse osmosis unit 5 The latter comprises a reverse osmosis membrane 26 , an effluent line 27 for the permeate and an effluent line 28 for the reject brine or the discharge solution It is possible to incorporate in the line 28 a pressure reducing valve 29 to reduce the pressure of the concentrated solution at the outlet of the RO unit At least part of the discharge brine stream 28 (but preferably all of this stream) is directed via line 30 to the recirculation ring of the brine 31, to be discharged by means of a recirculation pump 32 into the electrode compartments 18 via the intake pipe 23, or into the concentrator compartments 21 via the intermediary ire of an intake pipe 33 Part of the brine recirculating via 31 can

  be discharged into line 34 through the purge valve 40.

  As a variant or in addition, at least part of the waste brine stream 28 is routed through line 41 in the demineralizing compartments 17 via the intake pipe 16 The previously chosen methods of using the waste stream brine flowing in line 28 depends on the details of the system's operation For example, if the water source to be treated I contains X ppm of CT and the brine stream 28 has a concentration substantially greater than X, it is advantageous for the stream 28 is added to stream 31 On the other hand, if stream 28 has a concentration substantially lower than X, it can be

  advantageous to add it to the current 16.

  Whenever it is mentioned here and in the claims

  reverse osmosis "or" OI ", we also refer to the variants with

  hyperfiltration (IF) and nanofiltration (NF).

  A very important part of the invention consists of the exploitation of the reverse osmosis rejection brine current 28 as a supply current for the ring for recirculating the concentrated saline current coming from the electrodialysis 31, at the direct discharge station in the sewers via line 42 This results in a more efficient and more reliable demineralization system, characterized by a better yield in terms of production

purified water.

  The system shown in Figure 2, once incorporated into the system whose flow diagram is shown in Figure 1, produces ultrapure water (more than 17 megohms / cm) from a slightly saline current like l mains water (see examples), but can also be used in the form shown in outline in Figure 2 to treat brackish water or

  seawater so as to obtain water with reduced salinity.

  The method and the system which are the subject of the invention are

  further illustrated in the following examples, by comparison.

  Example 1 shows a water treatment system in which the reverse osmosis brine stream is discharged into the sewers, as is normally done with old design methods. Example 2 shows the improved method and system in which, in accordance with the present invention, the reverse osmosis reject brine stream is not lost,

  but retained and returned to the electrodialysis unit / battery.

Example I

  The network water to be treated is sent to the purification system via an interception valve and a pressure regulation valve The water has a p H of 8.1, a temperature of 17.5 C and an electrical conductivity of 500 micro siemens / cm, these values corresponding to approximately 300 ppm of dissolved salts The water is pressurized by a pump and sent to an ultrafiltration unit of the type with spiral arrangement incorporating polysulfonic membranes having a cutoff threshold of 50,000 dalton in terms of molecular weight The permeate of the ultrafiltration system (UF), at a flow rate of approximately 31.9 m 3 / h, serves as a tributary for an electrodialysis unit with Aquamite inversion X (produced by Ionics, Inc, Watertown, Massachusetts / USA) of which approximately 28.4 m 3 / h reach the demineralizing compartments (desalinating) and as for the rest, 3.4 m 3 / h in the concentrating compartments for reintegration of brine and 0.34 m 3 / h in the s electrode compartments The Aquamite X complex includes three membrane batteries in series, uses ion exchange membranes measuring 45.72 x 101.6 x 0.05 cm (18 "x 40" x 0.020 ") and each battery comprising 500 pairs of demineralizing and concentrating compartments Out of a total of 27.2 m 3 / h of purified water leaving the EDI unit, 0.6 m 3 / h are discharged into the sewers (or poured back into brine and in the electrode compartment), insofar as they do not comply with the standard Thus, the net yield of the unit is 26.7 m 3 / h of product for the following treatment by means of OI L effluent from the EDI unit has an electrical conductivity of approximately 60 micro siemens / cm (corresponding to approximately 335 ppm of dissolved salts) and increased acidity (p H 5.5-5.8) The effluent from the concentrating compartments is recycled as a feed stream to the same compartments; simultaneously, 4.3 m 3 / h of brine or concentrated solution for discharge into the sewers are removed from the recirculation ring of brine.

  even for 0.34 m 3 / h of effluent from the electrode compartments.

  During the electrical operation of the EDI unit, part of the water (approximately 0.9 m 3 / h) is transferred (as hydration water), through the membranes together with the ions from the demineralizing compartments and concentrator compartments The stream of acidified and demineralized product which results from the EDI treatment is then sent under pressure to the reverse osmosis unit, of the hollow fiber type with cellulose triacetate membranes The reverse osmosis units included membranes with pores of about 0.0005 micrometers and were supplied by the Dow Chemical Co of Midland, Michigan / USA The permeate of the OI unit corresponds to approximately 22.7 m 3 / h, with an electrical conductivity equal to approximately 2.0 micro siemens / cm (approximately 1 ppm of CT) and a temperature of 19.40 C The rejection current of the RO (3.97 m 3 / h) has an electrical conductivity of approximately 65 g S / cm and is discharged into the sewers The resulting water is subjected to a subsequent treatment with passage through two banks of ion exchange cylinders in series, each bank comprising eight cylinders supplied in parallel Each cylinder contains 100 liters of mixed ion exchange resins, and precisely 60 liters of anion resins A 101 D (OH- form) and liters of cationic resin C 20 H (H + form) These resins are supplied by Rohm & Haas Co The total effluent (22.7 m 3 / h) of the treatment by ion exchange has a p H practically neutral and a final resistance of 17.8 megohms / cm preferably, it is subjected to ultraviolet radiation to destroy all living organisms

  before discharge and final use by the customer.

  The UV treatment facility is easy to obtain; a type which can be supplied by Aquafine Corp of Valencia, California / USA has been found

  no problems for this application.

  As can be seen, starting from 31.9 m 3 / h of UF permeate, we obtain 22.7 m 3 / h of purified water as final product, with a water yield

relatively low at 71.2%.

Example 2

  In this second example, use was made of the aspect of the invention which consists in using the brine stream for rejecting the 0 I as the feed stream for the concentrator compartments and for the electrode compartments of the EDI unit. , obtaining a significant improvement in the water yield (80.6%, with a reduction of a third of the loss by direct discharge) In this case, we started with 28.1 m 3 / h of UF permeate as supply current for the demineralizing compartments The UF permeate was not sent directly to the concentrating compartments or to the electrode compartments, as in the previous case 3.97 m 3 / h of rejection solution at 65 AS / cm of RO, which were discharged into the sewers in Example 1, are used here at the rate of 3.63 m 3 / h for the supply of the brine recirculation ring and at the rate of 0.34 m 3 / h for supplying the electrode compartments The final product q which leaves the RO unit results in a flow rate of 22.7 m 3 / h, which corresponds to water recovery of

  80.6% of the 28.1 m 3 / h of the initial volume of treated water.

  As a variant, the rejection of the OI is sent to the demineralizing compartments of the DE battery, in replacement of the 3.47 m 3 of supply. The product outside the tolerances was accumulated and purged in the brine recirculation ring. / electrodes to replace 0.57 m 3 / h of feed water The purified water yield was approximately 81.4% and the energy consumption in the

  DE batteries was about 3.5% lower.

  In conclusion, the preferred method and installation of the invention is based on a combination of the following series treatment phases: (a) ultrafiltration or “cross-flow” filtration for the pretreatment of the feed water, in the purpose of making it suitable for further processing; (b) electrodialysis of the pre-filtered water to significantly reduce its salinity and increase its acidity (for example up to a p H of approximately 4 to 6.8) so as to prepare it for the OI phase; (c) reverse osmosis to reduce the salinity while at the same time using the brine rejection stream to return it to the DE unit, preferably followed by the following stages of additional treatment; (d) ion exchange (preferably with portable units) to further decrease the level of mineral impurities; (e) disinfection treatment with ultraviolet or

  ozone to destroy bacteria.

  Each being taken as such, the individual phases of treatment provided by the present invention (ultrafiltration / "cross-flow" filtration, electrodialysis and reverse osmosis) are known to those skilled in the art. following the combination DE + OI (integrated in the original solution consisting in the recycling as described above of the stream of concentrated solution leaving the OI unit) a synergy is produced, which means that the advantages of the overall process increase by one unexpectedly and that the process itself is extremely useful (especially as a water treatment system installed on mobile units) when it comes to obtaining desalinated and / or ultrapure water without having to add substances chemicals of external water origin before the reverse osmosis phase; in addition, precious feed water is saved and the volume of

landfills to be eliminated.

  The foregoing description is intended to illustrate shapes

  representative and preferred of the present invention In the present application, whenever the various elements of the method and of the installation are defined in general, it is understood that these definitions cover elements corresponding to those which have been described, likewise that equivalent elements in relation to the

  entire scope of the subject of the invention.

Claims (17)

  1 water purification process, characterized in that an electrodialysis unit (4) is used in combination with a reverse osmosis unit (5), said electrodialysis unit comprising one or more pairs of demineralizing and concentrating compartments delimited by membranes permeable respectively to anions and cations, which compartments are arranged between a pair of electrode compartments (18), themselves arranged at the ends, the passage of feed water being in additionally provided in the demineralizing compartments (17) while a direct electric current is applied between the above-mentioned pair of electrodes, so as to reduce the salinity level of the supply water by causing the passage of salt from said demineralizing compartments towards the concentrator compartments (21), which is followed by the outlet of the electrodialysis unit of the partially desalinated effluent and so n passage, under pressure, at the inlet of a membrane unit (5) which operates according to the principle of reverse osmosis, from an outlet from which the permeate (27) which is substantially desalted which has passed is removed through the reverse osmosis membrane, while through another outlet (28) thereof is taken up a concentrated reject liquid which has not passed through the reverse osmosis membrane, and that at least one part of the latter final liquid is recirculated as a feed stream for said electrodialysis unit.  2 Method according to claim 1, characterized in that the said rejection liquid is continuously recirculated in a ring system (31) towards the concentrator compartments (21) of the electrodialysis unit, with simultaneous sampling at unity   electrodialysis of part of the recirculated liquid.   3 Method according to claim 2, characterized in that the recirculated liquid is sent as a tributary to the   at least one of said electrode compartments.   4 Method according to any one of claims   above, characterized in that the water is subjected to ultrafiltration or cross-flow filtration "before proceeding to the electrodialysis phase.   Method according to any of the claims   above, characterized in that the electrodialysis treatment is carried out under practically polarizing conditions with the aim of acidifying the partially desalinated product which leaves the demineralizing compartments of the electrodialysis unit. 6 Method according to claim 5, characterized in that the partially desalted product is acidified by electrodialysis   up to a p H of between approximately 4 and approximately 6.8.   7 Method according to any one of claims   previous, characterized in that following the reverse osmosis phase, the permeate is subjected to a subsequent treatment by ion exchange to separate all or almost all of the ions residual in solution.   8 A method according to claim 7, characterized in that the water produced by means of the ion exchange treatment is subjected to sterilization with ozone at an appropriate concentration or with ultraviolet rays of appropriate intensity for the purpose to destroy biological contaminants.   9 A method according to claim 7, characterized in that the water produced by means of the ion exchange treatment is subjected to a final ultra-purification treatment in order to obtain as product a water having an electrical resistance greater than about   17 megohms / cm at a temperature of 25 * C.   Process according to any one of Claims I to 9,   characterized by the fact that it is made in a container transportable.   11 Method according to any one of claims   above, characterized in that the polarity of the direct current is reversed at regular intervals, with simultaneous switching of the circulation of the solutions towards the   demineralizing compartments and concentrating compartments.   12 Installation for the separation of dissolved salts from an aqueous solution, characterized in that it comprises an electrodialysis unit (4) with several compartments in combination with a reverse osmosis unit (5), said unit electrodialysis comprising a multiplicity of compartments, the two terminal compartments of which are the electrode compartments (18), demineralization compartments (17) and concentrator compartments (21) being arranged alternately between said electrode compartments, said compartments being delimited by pairs alternately by cation-permeable membranes and anion-permeable membranes, with, in addition, inlet devices for the admission of the tributary solution into the demineralizing compartments, concentrators and electrodes, and outlet devices for the sampling of a solution from said compartments, as well as devices for the e passage of a direct current through the aforementioned membranes and compartments, and devices (25) for the pressurization of at least part of the solution which leaves the demineralizing compartments, said reverse osmosis unit having devices for inlet (14) for receiving the pressurized effluent from said demineralizing compartments, emission devices (27) for the removal of permeate from the reverse osmosis membrane, emission devices for separation of the pressurized discharge stream , and devices (30, 31, 41) for recycling at least part of this reject stream as part of the feed stream to said electrodialysis unit.  13 Installation according to claim 12, characterized in that it comprises devices (28, 31, 23) for routing at least part of the rejection current to the electrode compartments of   the electrodialysis unit, as a feed tributary.   14 Installation according to claim 12, characterized in that it comprises devices for recirculating the solution through the concentrator compartments, as well as devices for withdrawing part of said solution for its discharge into the sewers.   Installation according to claim 12, characterized in that it comprises ultrafiltration devices arranged in   upstream of the electrodialysis unit.   16 Installation according to claim 12, characterized in that there are provided devices capable of operating the unit   electrodialysis under practically polarizing conditions.   17 Installation according to claim 12, characterized in that downstream of the reverse osmosis unit, and in series with the emission devices for the removal of the permeate is installed a   ion exchange system with mixed bed.   18 Installation according to claim 17, characterized in that after the reverse osmosis unit, and in series with the devices for emitting the permeate sample, is installed a sterilization system.   19 Installation according to claim 12, characterized by the   that it is placed in a transportable container.   20 Installation according to claim 12, characterized in that there are provided devices capable of reversing the polarity of the   direct current at regular intervals.