GB2249307A

GB2249307A - Process for purifying water by means of a combination of electrodialysis and reverse osmosis

Info

Publication number: GB2249307A
Application number: GB9122914A
Authority: GB
Inventors: Franco Ciallie; Giuseppe Campolo
Original assignee: IONICS ITALBA SpA
Current assignee: IONICS ITALBA SpA
Priority date: 1990-10-30
Filing date: 1991-10-29
Publication date: 1992-05-06
Also published as: IT9021920A1; FR2668469A1; GB9122914D0; IT1243991B; DE4135166A1; IT9021920A0

Abstract

Desalinated or ultrapure water is obtained using a combination of at least two membrane units/processes, i.e. electrodialysis (4) (ED, including the variation with reversal EDR and the variation with filled cells EDI) followed by reverse osmosis (5) (RO), including nanofiltration, arranged in a series configuration wherein each membrane unit progressively purifies the water until the required degree of purity is ensured. At least part of the waste brine of the reverse osmosis is recycled to the electrodialysis unit (4). This waste current is used in order to save water in the overall process. <IMAGE>

Description

k 1 22493')7 PROCESS FOR PURIFYING WATER BY MEANS OF A COMBINATION OF

MEMBRANE SEPARATION UNITS AND REIATED APPARATUS The present invention generally relates to the treatment of water and more particularly to the production of pure or ultra-pure desalinated water by using a combination of processes based on the use of membranes, i.e., in the simplest combination, of at least one electrodialysis process and of a reverse osmosis process.

The invention furthermore relates to a purifying method according to which the waste current in output from the reverse osmosis unit is exploited by recycling it in the 10 electrodialysis unit instead of discharging it into the sewage system as normally provided for by conceptually old methods.

An apparatus for carrying out said process is also within the scope of the invention.

The latest technological progress has made the availability of pure or ultra-pure water practically indispensable for numerous industrial and scientific applications, as well as in the field of energy production.

Although the previously conceived conditioning systems are 20 already capable of supplying water with characteristics which comply with the current statutory provisions to a considerable extent, such methods are subjected to numerous disadvantages. In particular, a known method which entails the use of filtration means and/or cartridges, followed by a reverse osmosis (R.O.) process and by an ion exchange treatment with double or mixed bed regenerated on-site, entails the following negative aspects: (a) the need for the frequent countercurrent washing of the filtration means or 2 for the frequent replacement of the filtration cartridges, (b) bacterial proliferation in the filtration means; (c) the need for the chemical treatment and disposal of waste consequent to the on-site regeneration of the ion-exchange resins; and, even more important, (d) the need to introduce chemical products in order to acidify and/or pre-treat the water in another manner prior to subjecting it to reverse osmosis. A previous water treatment method, using mobile units which exclusively contain ion-exchange columns, is 10 described in US patents no. 4,280,912, 3,766,060, 4,188,291, 4,332,685 and in others. Another known treatment method entails the use of electrodialysis prior to ion exchange, with final purification of the resulting water by means of hollow-fiber ultrafiltration units (see Zmolek, C.R., 15 IfUltrapure water for Integrated Circuits Processing", in Industrial Water Engineering, December 1977). However, none of the above mentioned methods relates to the treatment method described herein, which is composed of an electrodialysis followed by reverse osmosis, in combination with a specific recovery system suitable for ensuring an unprecedented improvement to the overall process. In the new configuration, the saline waste current which flows out of the reverse osmosis unit is stored and returned into the salt enrichment chambers, in the dilution chambers and/or the electrode chambers which compose the electrodialysis unit, instead of discharging it into the sewage system.

The aim of the present invention is therefore to provide a water treatment system which can economically provide purified water for long periods without said process entailing frequent countercurrent washes, filter r, 3 replacements, addition of reactive agents, excessive water losses at the discharge or other maintenance operations.

Another object of the invention is to provide a process which can separate the organic substances and obtain a biologically pure water characterized by a specific electric resistance in the range of 17-18 megohm/cm at 250 C.

A further object of the present invention is to provide a water purifying apparatus which comprises a combination or a series of individual elements which are connected to a 10 water pipeline or to another water supply system in order to treat the water so as to purify it while reducing the losses due to discharge into the sewage system. According to the invention, said apparatus consists of at least two membranebased treatment systems, i.e. electrodialysis (ED, even in the reverse variant IIEDRII and in the filled-cell variant IIEDIII) and reverse osmosis (RO), even in the form of nanofiltration (IINFII), arranged in series with means which allow to use the saline waste current of the RO as feed current for the electrodialysis unit (it should be noted 20 that RO is sometimes called hyperfiltration or, abbreviated, HF). The combination can be integrated with other elements in order to provide the required characteristics of the final water -- for example: an ultrafiltration system, a cross-flow filtration system, ion exchange beds, ozone generators and/or ultraviolet-ray (UV) sources for destroying micro- organisms when the product is used. The purifying apparatus described above can, if required, be easily mounted in a container or in a semitrailer so that it can be easily transferred to the user's location. once 30 connected to the outlets for electric power, feed water, 4 waste water and purified water, the apparatus can deliver treated water which complies with the requirements of any customer or user.

The advantages of the invention can be summarized as follows:

a) the ultrafiltration or cross-flow filtration unit or element ensures a more effective pre-treatment with respect to ordinary filters with loose or cartridge-enclosed filtration means. In view of the minimum size of the pores Of UF membranes (from 0.002 to 0.02 microns), ultrafiltration is certainly preferable and not only separates the suspended particles but also reduces the bacterial content of the treated water. These are the specific advantages which are obtained when, during pretreatment upstream of the membrane processes, UF or cross-flow filtration is used instead of filters with loose and/or cartridge-enclosed filtration means. Loose filtration means can migrate, disintegrate or facilitate an abundant proliferation of microbiological species. on the other hand, 20 filters with an enclosed filtration element can discharge due to the effect of momentary overpressures, be bypassed due to assembly errors or be rendere d ineffective due to the passage of solid matter released during the operations for the replacement of the filtration elements.

b) The electrodialysis unit or element, besides reducing the total dissolved solids (TDS) content, can also reduce the pH of the water, i.e. increase its acidity (a pH range comprised between 4 and 6.8 is considered preferential) and thus eliminate the need for external acid additions prior to sending the water to the reverse osmosis I treatment. In the purifying apparatus, the electrodialysis unit acts as primary demineralization component.

During the desalination treatment performed by electrodialysis, "polarization films" can form against the inner surfaces of the membranes which delimit the demineralization cells, in the presence of current densities which are relatively high in relation to the salinity of the water to be demineralized. This phenomenon usually occurs when the water being treated has a low content of total 10 dissolved solids and the current density applied is such as to deplete the ions contained in the water layers which are in direct contact with the surfaces of the membranes; in this manner, the current which is subsequently applied is carried by the hydrogen and hydroxyl ions which have formed in the depleted layer following the decomposition of the water molecules. This water separation is usually much more intense at the anionic membranes, and therefore the hydroxyl ions deposited thereon, which have a negative charge, easily pass through said membranes to reach the adjacent 20 concentration cells. on the other hand, the hydrogen ions thus generated, which have a positive charge, cannot pass through the anionic membrane and thus tend to accumulate in the demineralization cell, thus causing the acidification of the downstream water current. This "natural" acidification is exploited in an original manner by the present invention, in that the addition of any external acid is no longer provided for prior to the subsequent treatment of reverse osmosis of the partially demineralized product. The effects of polarization are examined in depth in an article entitled 30 "Limiting Current in Membrane Cells", published in 6 Industrial & Engineering Chemistry, vol. 49, p. 780, April 1957.

The advantages of ED as a pre-treatment system for RO are the following.

1) the water current which flows into the concentration cells, the electrode cells and/or the demineralization cells of the ED unit can be obtained at least partially from the saline waste current of the unit (RO). By using the waste current in this original manner, instead of discharging it 10 into the sewage system, the water is conserved in the process together with the hydrogen ions.

2) The bicarbonate and the hardness can be eliminated without having to resort to chemical additions. The Langelier index of the water flowing out of an ED unit is generally lower than that of the inflowing water and often has a negative sign. Therefore the water produced by the ED unit can almost always be fed to the RO unit without the conventional chemical additions.

3) The inclusion of a degassing unit, with the related 20 need f or repumping and the risk of contamination, is made superfluous by'the ED pretreatment upstream of the RO.

4) The efficiency of an ED unit is not appreciably affected by the low temperature, although the demineralization percentage may be reduced by it. RO generally reacts in an opposite manner to a temperature decrease, i.e. there is a lower hydraulic efficiency, whereas the waste content remains substantially unchanged. Therefore, a combination of the two processes ensures a better overall maintenance of the qualitative and quantitative terms in case of temperature variations.

7 Besides, reverse osmosis (RO) reduces the mineral content (TDS) of the water, and if required it can be performed with a final treatment by means of ion exchange, in order to obtain extrapure water. Since the resulting water (permeate) which derives from the RO treatment can have an extremely low content of dissolved salts (ions), a further treatment with ion exchange resins can be continued for very long times before the resins must be regenerated.

This allows the use of portable ion-exchange tanks or 10 cartridges instead of the ion exchangers with on-site regeneration entailed by the prior solutions. The need for difficult and expensive chemical treatments on-site is thus eliminated. The problems related to the disposal of the concentrated waste current at the output of the RO unit are furthermore reduced or eliminated by using said current as a feed for the concentration, demineralization and/or electrode cells in the ED units. For a moredetailed description of the new combination of electrodialysis/reverse osmosis apparatuses with the use of a duct to cause the concentrated waste current of the RO to reflow to the'ED unit, reference is made to the following paragraph and to figure 2.

Further characteristics and advantages of the present invention will become apparent from the following description, given with reference to the accompanying drawings, which are given only by way of non-limitative example., and wherein:

figure 1 is a flowchart of the combination of elements which comprises a preferential purification apparatus 30 according to the invention, and 8 figure 2 is a detailed schematic representation, in transverse crosssection, of the combination of electrodialysis and reverse osmosis in the preferential purification apparatus of the preceding figure.

In figure 1, a water source 1 is shown to be connected to an optional pre-filtration device 2, selected among a cartridge filter or a filter with dissolved filtration means of the conventional type with a charge of activated carbon, silver-treated active carbon, large-lattice anionic exchange resins for the absorption of organic substances, and "scavenger" anionic exchange resins (such as Ambersorb resins by Rohm & Haas) which have the purpose of trapping any relatively large suspended particles. It should be noted that the prefilter (like some other elements of the system) can be modified according to the requirements or can be eliminated completely. If prefilters are provided, they may be conveniently used in pairs and connected so as to allow the circulation of water in series, in parallel or in single units. The various optional rigid and flexible pipes (for 20 example for series or parallel circulation, recycling, feed and discharge,' etc.) which can be installed in order to increase the versatility of the system are naturally neither illustrated nor described herein, since they constitute solutions which are already well-known to the specialists in the field. For the same reason, the various components commonly used in water treatment systems, such as buffer tanks, electrical conductivity measurement devices, measurement instruments, flow-rate indicators, recorders, pumps, valves, measurement devices, etc. are also not 30 illustrated. Subsequently, the water subjected to optional 9 pre-filtration is conveyed under pressure to the ultrafiltration unit 3, which has membranes provided with pores having a diameter advantageously comprised between 0.002 and 0.02 microns, for separating the colloidal particles and the residual organic substances, or to a cross-flow filtration unit (FCF). The constitution and the operation of the UF and FCF units are well-known in the field. A type of UF with spiral coiling can be easily obtained by contacting Osmonics Inc. in Minnetonka, 10 Minnesota, USA. Herein and hereinafter, the term 11ultrafiltration" or 11UF11 is understood to also include the concept of "cross-flow filtration" or 11FCF11. The subsequent step in the treatment consists of primary demineralization by means of one or more electrodialysis units 4 which remove the main part of the electrolytes having a low relative molecular mass from the feed current 15 which is fed into the demineralization cells of the electrodialysis unit or units (an ED or EDI system with adequate characteristics, marketed under the trade-name Aquamite, can be supplied by 20 Ionics Inc. in Watertown, Mass., USA). The system uses batteries or packs of membranes, alternately formed by "anionic" membranes and "cationic" membranes which delimit the cells in which the liquid circulates: the ED/EDI units separate the dissolved ionized impurities. The EDR unit with reversal can maintain a constant product quality by reversing the polarity of the electric current flowing through the battery, as described in detail in the US patent no. 4,381,232 (D. Brown). The water 12 which flows out of the ED/EDR batteries not only has a considerably reduced 30 mineral content, but due to a near-polarizing electric current it also shows a sharp increase in acidity. Said acidity (preferably in the pH range between 4 and 6.8) constitutes a favorable characteristic for water which contains calcareous hardness which must be treated by RO (5). The use of electrodialysis furthermore eliminates the need to provide a mineral acid infeed from an external source into the feed water current. The acidified water prevents or attenuates the soiling of the RO membranes, so as to allow continuous operation of the system. Treatment 10 with RO furthermore allows separation of the residuals of colloids, bacteria and electrolytes, as well as certain organic substances dissolved in the water thus treated.

Suitable RO filters are available on a commercial scale from numerous suppliers. The saline waste current 13 flowing out of the RO unit 5 is caused to reflow, as feed solution, to the concentration, demineralization and/or electrode cells in the electrodlialysis unit or units through the duct 13. The permeate (product) 14 resulting from the RO process can be subjected to further purification by means of a double20 or mixed-bed ion-exchange resin (IE) 6 which separates the other dissolved minerals. The mixed bed can be formed by an anionic resin, such as Amberlite 410, and by a cationic resin, such as Amberlite 1R-120 (both manufactured by Rohm & Haas), but it is also possible to use other mixed-bed resins successfully. The ion-exchange bed or beds eliminate the unwanted ions still left in the water treated with RO, with a process fully known to the specialists in the field. The ion-exchange resins are preferably not regenerated on-site, but are replaced with elements having fresh resin loads.

The water, depleted of practically all its dissolved- 11 solids content, is subsequently caused to flow within a source of ultraviolet rays or ozone (03) 7 in order to kill all or almost all living micro-organisms. A wavelength of approximately 2537 Angstrom will effectively destroy almost all the living organisms which may have remained in the water. A UV disinfection system manufactured by U.V.

Technology Inc. (California) has been found to be perfectly suitable for the purpose. Naturally, as will be easily understood by the specialists in the field, lo amount the required of ultraviolet radiation and/or ozone depends substantially on the flow-rate of the system and on other factors. At this point the water is already pure enough for most applications. However, if required, purification can be increased further by means of the optional use of one or more mixed ultra-purification resin beds 8 which contain an additional adsorbing resin and a final microporous cartridge filter 9 with extrafine pores. The water thus treated should then be conveniently caused to flow through an electrical conductivity measurement device 10 which indicates the 20 electrical resistance of the treated water, so as to ensure the purity thereof required at the point of use 11.

A bed of activated carbon or, better still, of silver- treated activated carbon, can be used to separate the organic carbon instead of, or in addition to, the mixed bed (8). Said activated carbon can advantageously be integrated by the RO module according to the US patent no. 4,735,717.

In addition to the electrical conductivity measurement device it is possible to use total carbon analyzers or total organic carbon analyzers and/or particle or bacteria counters.

12 Figure 2 is a schematic view of a source of water 1 which is fed as infeed 3 into the demineralization cells 15 of an electrodialysis unit (ED) 4 of figure 1, possibly in the form of a reversal electrodialysis unit (EDR) or of a filled-cell electrodialysis unit (EDI).

An electrodialysis unit or battery 4 (figure 2) composed of electrode chambers 6 which are arranged at the two ends of the battery and respectively contain the electrodes 7 and 8. In the intermediate part between the two 10 electrodes there are various cells, alternately for demineralization. (dilution) 15 and for concentration (9), which are delimited by membranes which are alternately permeable to cations and to anions. Said membranes delimit the circulation cells (none of which is shown in full in the above-mentioned figure).

In order to mutually separate the membranes so as to form alternating demineralization and concentration cells, it is possible to use convoluted-path spacers of the type described in the US patents 2, 708,657 and 2,891,889, or mesh spacers. The combination formed by an anionic exchange membrane, a cationic exchange membrane, a demineralization cell and a concentration cell constitutes a pair of cells.

Any number of cell pairs can be grouped between a pair of electrodes, so as to form a demineralization battery which, in a typical configuration, comprises 100 or more cell pairs. Systems of this type are described in greater detail in the US patents no. 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 the properties of the selective membranes of 30 the type used in electrodialysis systems are discussed in is 13 depth in the US patents no. Re. 24,865, 2,730,768, 2,702,272, 2,731,411 and in many others. Due to the electric potential which is transmitted through the battery, the cations of sodium, calcium, magnesium and others (with a positive charge) migrate through the cationic membranes and into the waste current or in the concentrate current (brine) 10. In the same manner, the negatively charged particles of chloride, sulphate, nitrate, bicarbonate and other anions end up in the discharge current 10 by passing through the 10 anionic membranes. Although the above mentioned ions generally constitute the main group of unwanted salts, other ionic substances with a low relative molecular mass are, or can be, separated in a similar manner as well. Furthermore, in the operation of the battery, a current of electrolytes 22 circulates in contact with the electrode 7, and a similar current circulates in contact with the electrode 6. In the EDR process (electrodialysis with reversal), the electric polarity is periodically reversed with a method which is well-known among the experts in the field. This produces a 20 reversal of the direction of motion of the ions, with an effect of %lectric washing" on the encrusted ions and on other charged particles which can deposit on the surfaces of the membranes. It is also possible to significantly fill the demineralization cells of the ED battery 4, or both the demineralization cells and the concentration cells, with balls, fibers, fabrics, foamed sheets etc. for ion exchange, as already well-known in the field. In the present description, said filled ED batteries, with the related systems and processes, are termed respectively EDR 30 batteries, systems and processes. Where, here and in the 14 claims, mention is made of "electrodialysis" or "ED", reference is also meant to the variations constituted by reversal electrodialysis or EDR and filled-cell electrodialysis or EDI. At this stage, the partially demineralized water which is collected within the effluent 13 is fed at high pressure by the pump 16 as a feed current into a reverse osmosis unit 5. Said unit is composed of a reverse osmosis membrane 17, an effluent duct 18 for the permeate and an effluent duct 19 for the waste brine or 10 waste solution. It is possible to include a pressure reduction valve 20 in the line 19 in order to reduce the pressure of the current of concentrated solution leaving the RO unit. At least part of the waste brine current 19 (but preferably all of it) is conveyed, by means of the duct 27, to the brine recirculation loop 21, in order to cause it to reflow, by means of a recirculation pump 24, into the electrode cells 6, through an inlet duct 22, or into the concentration cells 9, through the inlet duct 23. Part of the recirculating brine 21 can be discharged into the duct 20 29 by means of the drain valve 25. Alternatively, or additionally, At least part of the waste brine current 19 is conveyed, by means of the duct 28, into the demineralization cells 15 by means of the inlet duct 3. The selected methods of use of the brine current 19 depend on the details of the operation of the system. For example, if the source of water I to be treated contains X ppm of TDS and the brine current 19 has a concentration which is significantly higher than X, it is convenient to add the current 19 to the current 21. on the other hand, if the current 19 has a concentration which 30 is significantly lower than X, it may be convenient to add 1 it to the current 3.

Where mention is made, herein and in the claims, of "reverse osmosis" or 11ROU, reference is also meant to the hyperfiltration (IF) and nanofiltration (NF) variations.

An extremely important part of the present invention is constituted by the utilization of the waste brine current from the reverse osmosis 19 as a feed current for the recirculation loop of the concentrated saline current 21 of electrodialysis, instead of the direct discharge into the sewage system by means of the duct 26. This leads to a more efficient and reliable demineralization system which is characterized by an increase efficiency in terms of purified water production.

The system illustrated in figure 2, once included in the system the flowchart whereof is illustrated in figure 1, produces ultrapure water (over 17 megohm/cm) starting from a slightly saline current such as pipeline water (see examples), but it can also be used, in the form schematically illustrated in figure 2, in order to process 20 salt water or sea water so as to obtain water with a reduced salinity content.

The process and the system according to the invention are illustrated even better by the following contrasting examples. Example 1 illustrates a water treatment system wherein the waste brine current from reverse osmosis is discharged into the sewage system, as normally occurs with previously conceived methods. Example 2 illustrates, in contrast, the improved process and system, wherein, according to the present invention, the waste brine current 30 from reverse osmosis is not lost but is stored and made to 16 reflow into the electrodialysis unit/battery. Example 1 The pipeline water to be treated is fed into the purification system by flowing through a cutoff valve and a pressure adjustment valve. The water has a pH of 8.1, a temperature of 17. 5 0 C and an electrical conductivity of 500 microsiemens/cm; these values are equivalent to approximately 300 ppm of dissolved salts. The water is pressurized by means of a pump and is made to flow into an 10 ultrafiltration unit of the type with a spiral coiling, which includes polysulphonic membranes which have a cutoff level of approximately 50,000 dalton in terms of relative molecular mass. The permeate of the ultrafiltration -system (UF), which is equal to approximately 31.9 m 3 /h, acts as feed' f or an Aquamite X reversal electrodialysis unit (manufactured by Ionics, Inc., Watertown, Massachusetts, USA); of this amount, approximately 28.4 m 3 /h flow into the demineralization (desalination) cells and, as regards the remainder, 3.4 m 3 /h flow into the concentration cells as 20 brine refill and 0.34 m 3 /h flow into the electrode cells.

The Aquamite 'X apparatus is composed of three membrane batteries arranged in series, uses ion-exchange membranes measuring 1811 x 4011 x 0.02011, and each battery is composed of 500 pairs of demineralization and concentration cells. On the total of 27.2 m 3/h of purified water flowing out of the EDR unit, 0.6 m 3 /h are discharged into the sewage system (or poured into the brine and into the electrode cells), since they do not comply with the statutory provisions. Thus, the net efficiency of the unit is 26.7 m 3 /h of product for 30 subsequent treatment by RO. The effluent of the EDR unit has 17 an electrical conductivity of approximately 60 microsiemens/cm (equivalent to approximately 335 ppm of dissolved salts) and an increased acidity (pH 5.5-5.8). The effluent of the concentration cells is recycled as feed current for the cells themselves; simultaneously, 4.3 m 3 /h of brine or concentrated solution are drained from the brine recirculation loop for discharge into the sewage system. The same is done with 0.34 M3 /h of effluent from the electrode cells. During the electric operation of the EDR 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 demineralization cells to the concentration cells. The acidified demineralized product current obtained from the EDR treatment is then sent under pressure into the reverse osmosis unit, which is of the hollow-fiber type with cellulose triacetate membranes. The reverse osmosis units had membranes with approximately 0.0005 micron pores and were supplied by Dow Chemical Co., Midland, Michigan, USA. The permeate of the RO unit corresponds to approximately 22.7 M 3 /h, with an electrical conductivity equal to approximately -2.0 microsiemens/cm (approximately 1 ppm of TDS) and a temperature of 19.4 OC. The waste current of RO (3.97 M3 /h) has an electrical conductivity of approximately 65 AS/cm. and is discharged into the sewage system. The resulting water is subjected to a further treatmentby passing through two banks of ion exchange cylinders arranged in series; each bank is composed of eight cylinders with parallel feeds. Each cylinder contains 100 liters of mixed ion exchange resins, and precisely 60 liters of A 101D 3o anionic resins in the OH form and 40 liters of C 20 H is cationic resin in the]+ form. Said resins are supplied by Rohm & Haas Co. The total effluent (22.7 m 3 /h) of the ion exchange treatment has a practically neutral pH and a final resistance of 17.8 megohm/cm. Preferably, it is subjected to ultraviolet radiation in order to destroy almost all living organisms prior to discharge and final use on the part of the customer. The UV treatment apparatus is readily available; a type which can be supplied by Aquafine Corp., Valencia, California, USA, proves itself undoubtedly 10 suitable for the requirements of this application.

All this in summary. As can be seen, starting from 31.9 m 3 /h of UF permeate, one obtains 22.7 m 3 1h of purified water as final product, with a relatively low water yield equal to 71.2%.

Example 2

In this second example, the aspect of the invention which consists in using the waste brine current of the RO as feed current for the concentration cells and for the electrode cells in the EDR units has been exploited, 20 obtaining a significant improvement in the water yield (80.6%, with -a one-third reduction of loss due to direct discharge). Specifically, 28.1 m 3 /h of UF permeate was used as starting point as-a feed current for the demineralization cells. The UF permeate was not fed directly into the concentration cells or into the electrode cells, as in the preceding case. The 3.97 m 3 /h of waste solution at 65 gS/cm from RO, which in Example 1 were discharged into the sewage system, are used here: 3.63 m 3 /h are used as feed current for the brine recirculation loop, and 0.34 m 3 /h are used as 30 feed for the electrode cells. The final product which leaves 19 the RO unit was 22.7 m 3 /h, a flow-rate corresponding to an 80.6% water recovery with respect to the 28.1 m 3 /h of initial volume of treated water.

or, alternatively, the waste of the RO is fed into the demineralization cells of the battery ED, in replacement of 3.47 in 3 /h of feed. The product which is not within the tolerances was accumulated and drained in the brine/electrode recirculation loop in replacement of 0.57 m 3 /h of feed water. The purified water yield was 10 approximately 81.4% and the energy consumption in the ED batteries was approximately 3.5% lower.

To conclude, the preferred process and apparatus according to the present invention are based on a combination of the following treatment steps, arranged in series:

(a) ultrafiltration or cross-flow filtration for the pre- treatment of the feed water, in order to make it suitable for the subsequent treatments; (b) electrodialysis of the pre-filtered water in order to reduce its salinity 20 considerably and increase its acidity (for example from pH 4 to pH 6.8) so as to prepare it for the RO step; (c) reverse osmosis, in order to further reduce the salinity, simultaneously using the waste current of the brine to feed it back to the ED units, preferably followed by the following additional treatment steps; (d) ion exchange (preferably with portable units) in order to further decrease the mineral impurity content; (e) ultraviolet or ozone disinfection treatment in order to destroy the bacteria.

Taken individually, the individual treatment steps included in the present invention (ultrafiltration/crossflow filtration, electrodialysis and reverse osmosis) are known to the specialists in the field. However, it has been observed that as a consequence of the combination of ED + RO (integrated by the original solution consisting in recycling the current of concentrated solution leaving the RO unit, as described above) a synergy is produced, i.e. the benefits of the global process increase unexpectedly and thus said process becomes extremely useful (especially as a water 10 treatment system installed on mobile units) when it is necessary to obtain desalinated and/or ultrapure water without having to add external chemical substances to the water prior to the reverse osmosis step; precious feed water is furthermore saved and the volume of the discharge to be disposed is reduced.

The above description is intended to illustrate the representative and preferential embodiments of the present invention. In the appended claims, wherein the various elements of the method and of the apparatus are referenced 20 generically, it should be understood that said references relate to the-above described corresponding elements, with the related equivalents.

21 j

Claims

1 1. Process for purifying water, characterized in that 2 it comprises the steps of providing an electrodialysis unit 3 in combination with a reverse osmosis unit, said 4 electrodialysis unit having one or more pairs of demineralization and concentration cells delimited by 6 membranes which are permeable to anions and to cations 7 respectively, said cells being arranged between a pair of 8 electrode cells which are in turn arranged at the ends, feed 9 water being made to flow in the demineralization cells while 10 a direct electric current is applied between said pair of 11 electrodes, so as to reduce the saline content of the feed 12 solution, causing the passage of salt from the 13 demineralization cells to the concentration cells, followed 14 by the separation of the partially desalinated effluent from the electrodialysis unit, and-by the flow of said effluent,

16 under pressure, to the inlet of a membrane unit which uses 17 the reverse osmosis principle, a permeated liquid being 18 drawn from an outlet of said unit, said permeated liquid 19 being already considerably desalinated and having passed 20 through the reverse osmosis membrane, a concentrated waste 21 liquid being drawn from another outlet of said membrane 22 unit, said waste liquid not having passed through the 23 reverse osmosis membrane, at least part of this last liquid 24 being then recycled as feed current for said electrodialysis unit.

2. Process according to claim 1, characterized in that 2 the saline waste liquid is recycled continuously in a loop

3 system which comprises the concentration cells of the 1 22 1

4 electrodialysis unit, with the simultaneous drawing or

5 drainage, from the electrodialysis unit, of part of said

6 recirculating brine.

1 3. Process according to claim 2, characterized in that 2 said recirculating brine is conveyed as feed to at least one 3 of the electrode cells.

1 4. Process according to claim 1, characterized in that 2 the water is subjected to ultrafiltration or cross-flow 3 filtration before passing to the electrodialysis step.

1 5. Process according to claim 1, characterized in that 2 the electrodialysis treatment occurs in practically 3 polarizing conditions in order to acidify the partially 4 desalinated product flowing out of the demineralization 5 cells of the electrodialysis unit.

1 6. Process according to claim 5, characterized in that 2 the partially desalinated product is acidified by 3 electrodialysis to a pH comprised between approximately 4 4 and approximately 6.8.

1

7. Process according to claim 1, characterized in that 2 after the reverse osmosis step the permeate is subjected to 3 further treatment by means of ion exchange in order to 4 separate therefrom all or almost all the residual dissolved ions.

8. Process according to claim 7, characterized in that 2 the water produced by means of the ion exchange treatment is 3 sterilized by exposing it to ozone at an adequate 4 concentration or to ultraviolet rays of adequate intensity in order to destroy its biological contaminants.

1

9. Process according to claim 7, characterized in that 2 the water produced by means of the ion-exchange treatment is 23 1 1 3 subjected to a final ultrapurification treatment in order to 4 obtain, as a product, water having an electrical resistance in excess of approximately 17 negohm/cm at the temperature 6 of 25 0 C.

1

10. Process according to claims 1 to 9, characterized 2 in that it is performed within a transportable container.

1

11. Process according to claim 1, characterized in that 2 the polarity of the direct current is reversed at regular 3 intervals, with the simultaneous switching of the 4 circulation of solutions toward the demineralization cells and the concentration cells.

1

12. Apparatus for separating dissolved salts from an 2 aqueous solution, characterized in that it comprises a multiple-cell electrodialysis unit in combination with a reverse osmosis unit, said electrodialysis unit comprising a plurality of cells, the two end cells being the electrode cells, demineralization cells and concentration cells being arranged alternately between said electrode cells and being 8 equally alternately delimited by cation-permeable membranes 9 and by anion-permeable membranes; feed devices for feeding the feed solution into the demineralization, concentration 11 and electrode cells, emission devices for drawing an 12 effluent solution from said cells, and devices for the flow 13 of a direct current through said membranes and cells, and 14 devices for pressurizing at least part of the solution leaving the demineralization cells being furthermore 16 provided; said reverse osmosis unit having feed devices for 17 receiving the effluent thus pressurized of said 18 demineralization cells, emission devices for drawing the 19 permeate of the reverse osmosis membrane and emission 24 devices for separating the pressurized waste current, as 21 well as further devices for recycling at least part of said 22 waste current as part of the feed current of said 23 electrodialysis unit.

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13. Apparatus according to claim 12, characterized in 2 that there are devices for conveying at least part of the 3 waste current to the electrode cells in the electrodialysis 4 unit as feed influent.

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14. Apparatus according to claim 12. characterized in 2 that there are devices for recirculating the solution 3 through the concentration cells as well as devices for 4 drawing part of said solution for discharge into the sewage system.

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15. Apparatus according to claim 12, characterized in 2 that said aqueous solution is treated with ultrafiltration 3 devices arranged upstream of the electrodialysis unit.

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16. Apparatus according to claim 12, characterized in 2 that there are devices suitable for operating the 3 electrodialysis unit in practically polarizing conditions.

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17. Apparatus according to claim 12, characterized in 2 that a mixed-b6d ion exchange system is installed downstream 3 of the reverse osmosis unit and in series with respect to 4 the related emission devices for drawing the permeate.

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18. Apparatus according to claim 17, characterized in 2 that a sterilization system is installed downstream of the 3 reverse osmosis unit and in series with respect to the 4 related emission devices for drawing the permeate.

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19. Apparatus according to claim 12, characterized in 2 that it is arranged in a transportable container.

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20. Apparatus according to claim 12, characterized in 1 2 that there are devices suitable for reversing the polarity 3 of the direct current at regular intervals.

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21. Process for purifying water by means of a 2 combination of membrane separation units and related 3 apparatus substantially as described with reference to the 4 accompanying drawings