AU2010246959A1

AU2010246959A1 - Separation of Mg2+ ions from sea-and brackish water for the purpose of re-mineralization of water and wastewater

Info

Publication number: AU2010246959A1
Application number: AU2010246959A
Authority: AU
Inventors: Liat Birnhack; Ori Lahav
Original assignee: RENEWED WATER MINERALS Ltd
Current assignee: RENEWED WATER MINERALS Ltd
Priority date: 2009-05-13
Filing date: 2010-05-13
Publication date: 2012-01-12
Also published as: WO2010131251A2; WO2010131251A3; SG176061A1

Abstract

System comprising: DU (desalination unit); separation unit (SU) providing divalent ions enriched brine, and monovalent ions enriched permeate, and configured to: split salty water between DU and SU, add permeate to DU, and add brine to desalinated water from DU.

Description

WO 2010/131251 PCT/IL2010/000383 1 SEPARATION OF MG2+ IONS FROM SEA- AND BRACKISH WATER FOR THE PURPOSE OF RE-MINERALIZATION OF WATER AND WASTEWATER FIELD OF THE INVENTION The present invention relates to addition of Mg 2 * from the reject of nanofiltration (NF) or low salt-rejection reverse osmosis (RO) membranes to desalinated water and wastewater. BACKGROUND The WHO recommends in its recent publications the inclusion of magnesium and calcium ions in desalinated and naturally soft waters (WHO, 2008; WHO, 2009) because of their acknowledged public health beneficial effects. Both ions are also welcome in desalinated water designated for irrigation. From the human health perspective, magnesium ions are recommended by the WHO at a concentration higher than 10 mg Mg/I and calcium ions at a concentration higher than 20-30 mg/I (WHO, 2005; WHO, 2009). In most desalination (and soft water) plants, calcium ions are added as part of the treatment since calcium (and bicarbonate) ions are also required for "chemical stability" purposes, i.e. to reduce the aggressiveness of soft waters to water distribution systems. Calcium ions are added to desalinated waters in the "post treatment" step, typically through the dissolution of solid CaCO 3 (quarry limestone), which releases to the water both Ca 2 + ions and carbonate alkalinity. However, to date, in most desalination plants, Mg 2 + ions are not added. The addition of Mg2+ ions to desalinated water can potentially be carried out using one of the following processes: (1) Direct dosage of magnesium salts, such as MgCl 2 or MgSO 4 , to the water; (2) dissolution of quarry dolomite (MgCa(C0 3

)

2 ) rocks (Birnhack et al., 2009); or (3) extraction of Mg 2 + from seawater by a cation exchange resin and subsequent release of the Mg 2 + to the desalinated water in exchange for Ca 2 *, which originates from CaCO 3 dissolution (PCT Patent application no. PCT/lL2007/001261 by Lahav et al.).

WO 2010/131251 PCT/IL2010/000383 2 Another option that has been proposed in seawater desalination operations is to blend the desalinated water with -1% seawater (volume to volume basis) to attain Mg 2 * concentration of -13 mg/. This option suffers from a major disadvantage which is that in addition to Mg 2 + and other welcome elements, the water is enriched with a very high concentration of unwanted Na 4 and C ions. Summary of the invention According to one aspect, a system for production of divalent ion supplemented water is provided, the system comprising: a system inlet line; a system outlet line; a DU (desalination unit), a DU inlet line fluidly connected to the system inlet line, a DU outlet line fluidly connected to the system outlet line, a first and a second junction on the DU line, a third junction on the DU outlet line, the second junction positioned between the first junction and the DU, the DU capable of: desalinating DU salty water fed to the DU via the DU inlet line, and releasing desalinated water from the DU into the DU outlet line; at least one divalent ion separation unit (SU) inlet line, at least one SU outlet line and at least one SU reject line, and at least one SU, wherein at least one SU is capable of: receiving SU salty water fed from a SU inlet line, rejecting brine to a SU reject line, wherein the brine comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than in said received SU salty water, and releasing permeate to an SU outlet line, wherein the permeate comprises a higher concentration of monovalent ions and a lower concentration of monovalent ions than in said received SU salty water, wherein the system is configured to: split at the first junction salty water fed via the system inlet line such that part of the fed salty water is fed to the DU and a another part of the fed salty water is fed to at least one SU via at least one SU inlet line; WO 2010/131251 PCT/IL2010/000383 3 add at the second junction permeate from at least one SU outlet to the salty water fed via the first junction, thereby producing the DU salty water fed to the DU, add at the third junction brine from at least one SU reject line to the desalinated water, thereby producing the divalent ion supplemented water. Most preferably, said divalent ions comprise magnesium ions. Said divalent ions preferably further comprise calcium ions. Said monovalent ions preferably comprise sodium ions and chloride. In some embodiments the at least one SU reject line is a SU inlet line, such that brine from a first SU is SU salty water fed to a second SU, whereby brine from the second SU comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than the brine from the first SU. The system may further comprise means for evaluating the pH of the divalent ion supplemented water and for adjusting the pH level of the supplemented water. In some embodiments the DU unit is a RO unit and the SU units are selected from one or more of the group comprising: RO, UF, ion exchange resin and ion exchange column. In some embodiments the system further comprises a pretreatment unit capable of making the DU salty water essentially free of bacteria. The system may further comprise a post-treatment unit capable of supplementing the divalent ion supplemented water with calcium ions. In some embodiments, the SU units comprise an ion exchange unit and at least one UF unit, wherein the ion exchange unit SU inlet line is the SU reject line of a first UF unit, and the first UF unit is capable of rejecting Mg 2 ' and calcium ions at over 70% and C at under 30%.

WO 2010/131251 PCT/IL2010/000383 4 In other embodiments, the first UF unit is capable of rejecting Mg 2 + at approximately 98%, calcium ions at approximately 92%, sulfate at approximately 99%, Cl~ at approximately 24% and HCO3~ at approximately 44%. first UF unit may comprises an NF array capable of rejecting Mg 2 * at approximately 95%, calcium ions at approximately 82%, sulfate at approximately 99%, Cr at approximately 17% and HC0 3 ~ at approximately 45%. the SU units of the system may comprise a ion exchange unit and two UF units, the ion exchange unit SU characterized by a high affinity towards S0 4 2 ions and a low (but not very low) affinity towards CI ions, the system being configured so that: at a load step the SU inlet line of the ion exchange unit comprises the SU reject line of a first UF unit, and the eluate of the ion exchange unit and the permeate of first UF unit are fed to the DU, and at a CF absorbance step the first UF unit is bypassed so that the ion exchange unit is fed with salty water fed via the first junction, and the eluate of the ion exchange unit is fed to the SU inlet line of the second UF unit, the brine of the second UF unit being fed to the third junction and the permeate of the second UF unit being fed to the second junction, whereby the divalent ion supplemented water has reduced chloride ion concentration. The ion exchange unit may be for example Purolite A-850 or Amberlite IRA-67. According to another aspect of the invention, a method for production of divalent ion supplemented water is provided, the method comprising: providing: a system inlet line, a system outlet line; a DU (desalination unit), a DU inlet line fluidly connected to the system inlet line, a DU outlet line fluidly connected to the system outlet line, a first and a second junction on the DU line, a third junction on the DU outlet line, the second junction positioned between the first junction WO 2010/131251 PCT/IL2010/000383 5 and the DU; at least one divalent ion separation unit (SU) inlet line, at least one SU outlet line and at least one SU reject line, and at least one SU; desalinating DU salty water fed to the DU via the DU inlet line; releasing desalinated water from the DU into the DU outlet line; receiving SU salty water fed from a SU inlet line; rejecting brine to a SU reject line, wherein the brine comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than in said received SU salty water; releasing permeate to an SU outlet line, wherein the permeate comprises a higher concentration of monovalent ions and a lower concentration of monovalent ions than in said received SU salty water, splitting at the first junction salty water fed via the system inlet line such that part of the fed salty water is fed to the DU and a another part of the fed salty water is fed to at least one SU via at least one SU inlet line; adding at the second junction permeate from at least one SU outlet to the salty water fed via the first junction, thereby producing the DU salty water fed to the DU, and adding at the third junction brine from at least one SU reject line to the desalinated water, thereby producing the divalent ion supplemented water. In particular, a method of intentional struvite precipitation for phosphate recovery from wastewater is provided, the method comprising adding brine produced from a SU according to claim 14 to the wastewater. BRIEF DESCRIPTION OF THE FIGURES For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings. With specific reference now to the drawing in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of WO 2010/131251 PCT/IL2010/000383 6 embodiments only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawing making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: Figure 1 illustrates a schematic drawing of an embodiment for addition of Mg 2 * from the reject of nanofiltration (NF) or low salt-rejection reverse osmosis (RO) membranes to desalinated water. Figure 2 shows a schematic drawing of another embodiment for addition of Mg 2 + from the reject of nanofiltration (NF) or low salt-rejection reverse osmosis (RO) membranes to desalinated water, wherein the reject is further treated before the addition. Figure 3 is graph of chloride rejection vs. sulfate levels in feed to NF membranes Dow, 200B 2540 and CSM, NE 2540-70. Figure 4 shows an ion exchange-aided novel configuration used for increasing the So4~-:(Mg2* + Ca 2 +) ratio in the NF feed; Figure 5 schematically illustrates recovery of phosphorus from excess sludge of wastewater; Figure 6 shows in schematic form a system embodiment for production of a struvite precipitation factor; Figure 7 shows a collection of data from use of a membrane that is provided in the embodiment described in Example 5, and Figure shows shows a collection of data from use of a membrane that is provided in the embodiment described in Example 6.

WO 2010/131251 PCT/IL2010/000383 7 DETAILED DESCRIPTION OF THE SELECTED EMBODIMENTS A new method is presented to add magnesium ions to desalinated water and wastewater streams in a simple and cost effective way. Brine that contains a high Mg 2 + concentration (along with proportional Ca 2 + and So42 concentrations) can be used to enrich the product water of a desalination plant with Mg 2 * ions. Two options are suggested for the use of the brine: The first option is to blend the Mg-rich brine with the product water of the desalination plant. The blending ratio is designed to result in the required magnesium concentration in the water. The byproduct of this action is that the water in also enriched by other ions present in the brine. While Ca 2 + and S042- are typically welcome, C and Na* are usually not. Thus, a membrane would be selected so that the rejection of Cl and Na* will result in minimum addition of these ions due to the blending action. A second option may be based on the first option, further including reducing the C and Na* content of the brine by for example passing the brine through further NF membranes with identical characteristics ( 2 nd pass, 3 rd pass etc., as required or loading specific ion exchange resins with Mg 2 + and thereafter releasing the magnesium ions held in the resin to the desalinated water as described in PCT Patent application no. PCT/IL2007/001261. This option allows enriching the product water with magnesium ions without the concurrent addition of other ions present in the brine. Magnesium ions can be largely separated from solution by passing pretreated (UF) inlet water (seawater or brackish water) through a membrane characterized by a high rejection (>70%) toward divalent ions (Mg 2 +, Ca2+, S042-, etc) and a low rejection (typically <30%) toward monovalent ions (Cr, Na*, K*, HCO~, etc). Such membranes can be defined as nanofiltration (NF) membranes or low salt-rejection reverse osmosis (RO) membranes. The brine of such operation will be rich in Mg 2 4, Ca 2 + and S042- and relatively poor in Na* and C[.

WO 2010/131251 PCT/IL2010/000383 8 Moreover, a membrane can be chosen so that the ratio between Mg 2 + and Ca 2 + in the brine is higher than the original ratio in the inlet water (seawater or brine, see examples below). Another alternative to reduce the C and Na* concentrations in the brine is to pass the brine through further membranes with identical characteristics ( 2 nd pass, 3 rd pass etc., as required in order to further reduce the concentrations of the monovalent ions). Such action will only slightly further reduce the divalent ion concentrations, but will significantly reduce the Na' and C concentrations. The Mg 2 + rich brine can be utilized in two ways: (1) If water quality criteria of the desalination plant do not allow the addition of Na* and C[ to the product water, the brine can be used to load a specific ion exchange resin with Mg 2 * and thereafter release it to the product water using the process described in PCT Patent application no. PCT/1L2007/001261 to Technion - Research & Development Foundation Ltd and (2) In case it is acceptable to increase the C and Na* concentrations in the product water, the brine can be blended with the permeate of the desalination plant (after this has been subjected to the post treatment step, i.e. blending of the brine will be done with water that already contains the required Ca 2 + and alkalinity concentrations) to attain a required Mg 2 + concentration. After the blending, the pH may be elevated using concentrated NaOH to attain a required stability index (LSI or CCPP). Clearly, the membrane should be selected due to its specific rejection properties to both Mg 2 + and C and Na* to result in minimum addition of the unwanted ions due to the blending action. Local economic considerations will determine whether it is cost effective to reduce the monovalent ion concentrations by passing the brine of the 1 st pass through further membrane separations. Process Train Out of the two options to use the brine, the one that is based on loading ion exchange resins with magnesium is further explained in detail in PCT Patent application no. PCT/IL2007/001261, incorporated in its entirety by reference. The blending option is described herein: The process and system 100 in accordance with a preferred embodiment is depicted schematically in Fig. 1.

WO 2010/131251 PCT/IL2010/000383 9 The system 100 for production of divalent ion supplemented water includes: a system inlet line 11 2A; a system outlet line 112B; a DU (desalination unit) 126, a DU inlet line 122A fluidly connected to the system inlet line 11 2A, a DU outlet line 122B fluidly connected to the system outlet line 112B, a first 11 4A and a second 11 4B junction on the DU line 122A, a third junction 114C on the DU outlet line 122B, the second junction 11 4B positioned between the first junction 11 4A and the DU 126, the DU 126 capable of: desalinating DU salty water fed to the DU 126 via the DU inlet line 122A, and releasing desalinated water from the DU 126 into the DU outlet line 122B; a divalent ion separation unit (SU) inlet line 132A, a SU outlet line 132B and a SU reject line 132C, and a SU 136, the SU 136 being capable of: receiving SU salty water fed from the SU inlet line 132A, rejecting brine to SU reject line 132C, wherein the brine comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than in said received SU salty water, and releasing permeate to SU outlet line 132B, wherein the permeate comprises a higher concentration of monovalent ions and a lower concentration of monovalent ions than in said received SU salty water, System 100 is thus configured to: split at the first junction 11 4A salty water fed via the system inlet line 11 2A such that part of the fed salty water is fed to the DU 126 and a another part of the fed salty water is fed to the SU 136 via the SU inlet line 132A; add at the second junction 11 4B permeate from SU outlet 132B to the salty water fed via the first junction 1 14B, thereby producing the DU salty water fed to the DU 126, add at the third junction 114C brine from the SU reject line 132C to the desalinated water, thereby producing the divalent ion supplemented water. In other embodiments there are multiple SU units. Figure 2 shows one such embodiment, a system 200 in which one SU reject line 232C is a SU inlet line 242A, WO 2010/131251 PCT/IL2010/000383 10 such that brine from the first SU 236 is SU salty water fed to a second SU 246, whereby brine from the second SU 246 comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than the brine from the first SU 236. Preferably, the systems, as shown in the system 200, further include means 250 for evaluating the pH of the divalent ion supplemented water and for adjusting the pH level of the supplemented water. The systems, such as shown in Figure 2, may further include a pretreatment unit 260 capable of making the DU salty water essentially free of bacteria and larger microorganisms. The systems, as shown for example in the system 200, may further include a post treatment unit 270 capable of supplementing the divalent ion supplemented water with calcium ions. Note that wherever the word NF appears it stands for both NF membranes and low salt-rejection RO membranes alike. Also note that passing the brine of the first membrane pass through further passes is optional and will be carried out when C- and Na* concentrations are to be reduced prior to blending with the product water. In general, the desalination plant is provided with an adjunct system that comprises a separation unit. Pretreated seawater (typically 1.0% to 2.0% of the overall flow rate of the plant) is first passed through UF filtration units to remove microorganisms. From there, the water flows to the magnesium separation unit (NF or low rejection RO membranes). The permeate of the magnesium separation unit flows back to the entrance of the desalination RO whereas the brine is blended with the post-treated RO permeate to attain the required Mg 2 + concentration. Finally, the pH is raised to adjust CaCO 3 stability indices by the addition of concentrated NaOH solution or by aeration for C02 stripping. The option of loading ion exchange resins with the brine is not covered in Fig. 1. Seawater and brackish inlet water application: the brine blending process can be implemented in both brackish water and seawater desalination plants. The major differences will be (1) in the blending ratio of brine to the salty water fed via the first junction, that will typically be lower in brackish water operations because of the lower WO 2010/131251 PCT/IL2010/000383 11 Mg 2 * concentration in the brackish water and (2) the ratio of Ca 2 + to Mg 2 + and alkalinity to Mg 2 + in the brine may be higher in brackish water, a fact that should be taken into consideration in the design of the post treatment processes of the desalination plant, and may reduce the cost of adding Ca 2 + and alkalinity to the water in the post treatment stage, and (3) In seawater the ratio between the combined concentration of Ca 2 + + Mg 2 + (in meq/I) and the S0 4 2 concentration is approximately constant. In brackish water, in contrast, it may vary significantly. In case this ratio is low, chloride rejection may have to be relatively high, in order to maintain electro neutrality in the reject solution. As described below, in some embodiments treatment of the water involves additional equipment and actions. In particular, embodiments such as the systems 100 and 200 shown in Figures 1 and 2 may be modified by additional treatment of salty water supplied to US 136, 236, respectively, at units 180 and 190 respectively, for example. Option for a further treatment step aimed at reducing the addition of Cl~ ions to the final water product In certain places a restriction is posed on the Cl and Na* concentrations in the final desalination water product. In Israel, for example, the new large seawater desalination plants (Ashkelon, Hadera, Soreq, etc.) are required to supply water with Cr and Na* concentrations lower than 20 mg/I and 30 mg/. In such a case there is a need for a further step to ensure that the S0 4 2 to C concentration ratio in the water that is fed to the NF membranes is higher than the typical ratio in seawater. Theoretical explanation: In seawater, the combined concentration of Ca 2 + and Mg 2 * (i.e. 400/20 + 1280/12.15 = 125 meq/) is higher than the So 4 2 concentration (2800/49 = 58.3 meq/l). Assuming a close to complete rejection of the three species, a certain (significant) CI concentration has to be rejected by the membrane, simply in order to attain electro-neutrality in the brine solution (Wilf, 2007). A further C concentration will be rejected to compensate for the rejection of Na* ions, but according to the literature (Abdullatef, 2007) this is a relatively small concentration. In other words, the passage of ions through NF membranes during pressure-driven processes is essentially a coupled transport process (Schaep, 1998; Mukherjee and WO 2010/131251 PCT/IL2010/000383 12 SenGupta, 2006). Thus, coupling the divalent cations (i.e. Ca and Mg) with a divalent anion (i.e. sulfate) instead of with chloride in the feed, would minimize the passage of chlorides through the NF membrane. Two preliminary examinations of the above mentioned hypothesis were carried out. In the first experiment natural seawater and seawater with elevated S02^ concentration were passed through a NF membrane (Dow, 200B 2540), while in the second experiment seawater and modified solutions simulating the effluent of the IX step (i.e. seawater with elevated So 4 2 - concentration and correspondingly lower Cl concentration) were passed through a different NF membrane (CSM, NE 2540-70). Figure 3 shows the results of these tests. Chloride rejection of NF 2540 (circles) and NE 2540-70 (triangles) is shown vs. So 4 2 -:(Mg 2 + + Ca 2 +) in seawater (the So 4 2 - concentration in seawater was elevated by dissolution of weighed Na 2 SO4). It can be seen that the rejection of CI- is reduced as the ratio between SO42 and the sum of Ca 2 + + Mg 2 + increases. Based on the above, it is logical that if the S0 4 2 concentration in the water fed to the NF membrane is increased up to a point in which the ratio SO42 to (Mg 2 + + Ca2+) is close to 1 (meq : meq) the CI- (and Na*) concentration in the brine can be minimized The system 300 used for increasing the S0 4 2 -:(Mg 2 + + Ca2+) ratio in the NF feed is depicted in Fig. 4. The process combines the use of two NF units 336A, 336B and an ion exchanger (IX) 346, and thus denoted NF - IX - NF. To increase the S042: (Mg 2 + + Ca 2 *) ratio a specific anion exchange resin is used. The resin is characterized by a high affinity towards S042 ions and a low (but not very low) affinity towards C ions. The resin used in this step may be Purolite A-850 or Amberlite IRA-67 or equivalent (Purolite A-850 was shown to have a separation factor (asosc) of 0.54, at 400 meq/ electrolyte concentration, Sarkar and SenGupta, 2008), Such resins can exchange S0 4 2 with Cl-. In the 1 st step the resin is "loaded" with S0 4 2 - by passing through it brine generated when seawater is passed through a NF membrane.

WO 2010/131251 PCT/IL2010/000383 13 The Cr: So 4 2 - ratio in the NF brine in this step may be approximately 3 to 1 (equivalent to equivalent) up to 5 to 1 depending on the membrane used, but in any event the ratio is much lower than the typical -10 to 1 ratio in seawater. At the end of this load step, the mass of S042 absorbed to the resin (out of its total capacity) might be between 70% and 40% depending on the affinity of the resin towards the relevant anions, and the S0 4 2 : C- ratio in the NF brine, which is used as the load solution. In the second step UF filtered seawater (seawater from which bacteria and larger microorganisms are removed) is passed through the S0 4 2 loaded resin. The seawater contacted with the ion exchange column at this step can be regarded as untreated seawater since its chemical composition is not changed by the UF pretreatment. Since in untreated seawater the Clr:S042 ratio is high (~10: 1 on equivalent basis), C ions will be absorbed in this step on the resin and S042- ions will be released from it. As a result, the S042-: Cr ratio in the water leaving the ion exchange column will significantly increase (relative to the ratio in seawater), and more importantly, the ratio between So42 concentration and the combined concentration of Ca 2 + and Mg 2 + can be increased (up to 1:1 and higher), allowing for full rejection of Mg2+ and Ca 2 + with minimal rejection of C required for attaining electro-neutrality in the brine. It is possible to add another intermediate step between the load (first) step and the following exchange step, in which the loading solution that remains in the bed after the load step is removed from the bed pores by either washing the bed or draining the solution by applying pressurized air. Application of the proposed process in near-shore Brackish Water (BW) desalination plants In places where the proposed process is used in the context of BW and seawater is available (e.g. Ma'agan Michael, Israel), a slightly modified NF - IX - NF process can be used: At the start up of the process the resin is loaded with sulfate using seawater (the seawater leaving the IX column flows back to the sea). The absorbed sulfate is recycled over and over again (see Sarkar and SenGupta, 2008).

WO 2010/131251 PCT/IL2010/000383 14 The process comprises of the following steps: first, sulfate is unloaded from the resin into the BW and thus the BW's sulfate concentration is increased and its CI~ concentration decreased. In the next step the chemically modified BW is passed through a NF membrane. The brine of this NF operation is then used for two purposes: a fraction of it is contacted with the resin, in order to reload it with sulfate; the remaining reject is blended with the post treated desalinated water, to enrich it with Mg 2 +. Theoretically, only a small amount of seawater is required for this application since the sulfate (initially separated from seawater) is released from the resin to the BW, and then rejected by the membrane and reloaded into the resin. Thus, it is hypothesized that only a small volume of seawater is required in this process to compensate for the portion of sulfate that leaves the resin during the load step. Utilizing a NF brine rich in Mg 2 + and Ca2+ NF brine rich in Mg 2 + can be used for improving the precipitation of solids from water and wastewater streams, for example, to precipitate and remove struvite (MgNH 4

PO

4 -6H 2 0) from wastewater, as explained herein. Struvite, a white crystalline compound consisting of Mg 2 +, NH 4 * and P04- at equal molar concentrations (MgNH 4

PO

4 e6H 2 0), is known to precipitate and clog pipes and pumps, causing operational difficulties and increased maintenance costs in wastewater treatment plants (WWTP) around the world. While struvite is a recognized operational problem in WWTP, it has been also shown that a significant percentage of the dissolved phosphate in the wastewater can be recovered from anaerobic digester supernatants through struvite crystallization, if controlled precipitation is applied (Battistoni et al., 2001; Munch and Barr, 2001; Ueno and Fuji, 2001;Wu and Bishop 2004). This is important because natural phosphorus resources are dwindling worldwide and according to current estimations stocks will reach a critically low point by the end of the century (Doyle and Parsons, 2002). Owing to its low (but not very low) solubility in neutral pH solutions (Bridger et al., 1961), struvite may be used separately as a cheap replacement to slow-release fertilizers or as a component in other commercial fertilizers (Hu et al., 1996; Gaterell et al., 2000; Battistoni et al., 2002).

WO 2010/131251 PCT/IL2010/000383 15 Struvite precipitation occurs only at relatively high concentrations of magnesium (Mg 2 +), ammonium (NH 4 +) and phosphate (P04-). Such concentration combination is encountered in WWTP only in the anaerobic sludge treatment line. Since the Mg 2 + concentration is typically around one order of magnitude smaller than that of ammonium and phosphate, the dosage of external magnesium salts (MgCl 2 , MgSO 4 , Mg(OH) 2 ) is typically required in order to precipitate a significant mass of struvite solids (Munch and Barr, 2001; Lee et al., 2003; Chimenos et al., 2003; Nelson et al., 2003; van Rensburg et al., 2003 To date, the high cost of magnesium salts prohibits the application of this process, which, in the near future, will become essential for maintaining appropriate phosphorus supply for food production, all over the world. To conclude, intentional struvite precipitation is currently the most promising method for phosphate recovery from wastewater. However, the Mg 2 + concentration present in municipal wastewater is a limiting factor. As a result, addition of Mg salts is necessary. Dosage of Mg rich brine (for example, the brine detailed in example 7), could replace the dosage of costly magnesium salts. Figure 5 shows in schematic form an embodiment 400 in which brine from the NF-IX-NF Mg2+ separation process is used to recover struvite from dewatering supernatant from excess sludge of wastewater. Figure 6 illustrates an embodiment 500 for provision of Mg2+ enriched brine as a struvite precipitation factor. EXAMPLES Salt rejection properties of NF and/or low salt-rejection RO membranes vary considerably. To exemplify the process for seawater and brackish water we make use of published salt-rejection results from two different membranes. In the following examples only one separation step is assumed; however, it is stressed that additional steps are optional. EXAMPLE 1 Hassan (2002, quoted in Eriksson et al., 2005), incorporated in its entirety by reference, reported on a NF membrane with the following salt rejection WO 2010/131251 PCT/IL2010/000383 16 characteristics for seawater application: Mg 2 + was rejected by this membrane at 98%, calcium at 92% and sulfate at 99%. With regard to monovalent ions, Cr was rejected at 23.8% and HCO3 at 44% (Na* rejection was assumed equivalent to Cr rejection). Thus, assuming a typical concentration of 1280 mg/L and a recovery rate of 65% (Eriksson et al., 2005), incorporated in its entirety by reference, the concentration of Mg 2 + in the reject of such operation would be approximately 3600 mg/l; similarly, Ca 2 * will be present at app. 1083 mg/l; S0 4 2 at 7948 mg/, Na* at 15862 mg/l, HC03- at -265 mg/I and C at 27398 mg/l. The addition of HC03- (bicarbonate, or alkalinity) due to the blending action is thus negligible. The mass ratio Mg 2 + to Ca 2 * in the brine is 3.32 to 1 relative to 3.2 in seawater. This fact, along with the high Mg 2 + concentration, makes the brine ideal for loading of the ion exchange resin that allows essentially complete separation of Mg 2 + from the unwanted C and Na*, as explained in PCT Patent application no. PCT/IlL2007/001261 to the applicant. The second option is to blend the brine directly with the post treated RO permeate (assuming that the addition of C and Na* is acceptable): Assuming that a Mg 2 + concentration of 10 mg/I is required in the product water, the dilution ratio between brine and permeate needs to be approximately 360 to 1. Thus, the additional concentration of Ca 2 *, S0 4 2 Na* and C in the water, as a result of the blending, would be 3.0, 22.0, 43.9 and 75.9 mg/, respectively. Note that the recovery ratio in the membrane application is of low importance. If a recovery ratio lower than 65% is used, this will result in requiring a different blending ratio to obtain the same magnesium ion concentration, however the product water composition will be identical and water will not be lost, since both streams (permeate and reject) will be further used in the process to make up the product water. EXAMPLE 2 Pontie et al. (2004), incorporated in its entirety by reference, reported on results obtained from the use of an old generation polyamide NF membrane (denoted NF70) with salty water.

WO 2010/131251 PCT/IL2010/000383 17 The concentrations of the ions present in the NF70 concentrate were approximately as follows: [Mg 2 +] = 2470 mg/l; [Ca 2 4] = 1090 mg/l; [Na*] = 19870 mg/I and [CI] at 30790 mg/I (no details were given on S0 4 2 - but it may be assumed that it was rejected at >95%). The mass ratio of Mg 2 * to Ca 2 + in this brine is therefore 2.3 to 1, i.e. lower than the typical ratio in seawater (3.2) Therefore, there is no logic in loading ion exchange resins with it, as explained in the 1 st example. However it is feasible to blend the brine directly with the post treated RO permeate (assuming that the additional concentration of CI- and Na* to the water is acceptable), as described herein: Assuming that Mg 2 + concentration of 10 mg/I is required in the product water the dilution ratio between brine and desalination permeate needs to be approximately 247 to 1. Thus, the additional concentration of Ca 2 +, Na*, and Cl~ in the water, as a result of the blending, would be 4.4, 80.4 and 124.6 mg/I, respectively. EXAMPLE 3 Using the membrane described in the 1 st example and the following brackish water composition (Maagan Michael, Israel): Mg 2 + = 146 mg/I, Ca 2 + = 195 mg/I, Na* = 1014 mg/, Cr = 1970 mg/, So 4 2 - = 251 mg/I and HCO 3 - = 372 mg/l, and assuming a recovery rate of 70%, the ionic composition of the brine is expected to be as follows: Mg2+ = 479.8 mg/I, Ca 2 + = 613.6 mg/I, Na* = 1577 mg/l, C = 3064 mg/I, So 4 2 - = 830 mg/I and HC0 3 ~ = 753 mg/. This brine may be used for loading resins with magnesium since the ratio of Mg2+ to Ca 2 + is 1.29 to 1 (equivalent per equivalent), in case a strict restriction is posed on the addition of CE, Na*, or both, or, alternatively, it can be blended directly with the post-treated permeate. In the case of direct blending, the blending ratio for attaining Mg 2 + concentration of 10 mg Mg/I is app. 48 to 1. Note that the additional Ca 2 + and bicarbonate concentrations in the product water in the direct blending option are 12.8 and 15.0 mg/I respectively. Thus, in the post treatment stage less Ca2+ and HCO3~ need to be supplied to satisfy water quality requirements with respect to Ca 2 * and alkalinity.

WO 2010/131251 PCT/IL2010/000383 18 EXAMPLE 4 We make use of the data reported by Abdullatef (2007), which describes an extensive research carried out for the design of a cost-effective pre-treatment system for seawater. The paper concentrates on different treatment arrangements, and, among other things, reports on the rejection of the various ions, as a function of such parameters as temperature, recovery rate, NF array, etc. However, Abdullatef (2007), incorporated in its entirety by reference, did not optimize the process for the goals of the application suggested herein but rather for goals relevant to pretreatment of seawater prior to desalination. However, he gives data on several sets of NF membrane arrangements and associated ion rejections; one of these described below: The following ion rejections were recorded when a 4-element NF array was operated (single stage operation): Mg 2 * was rejected by this membrane (specific details on the membrane make were not given) at 94.8%, calcium at 82.3% and sulfate at >99.3% (the exact rejection was not specified, thus in the following calculation it was assumed to be 99.3%). With regard to monovalent ions, Cr was rejected at 16.7% and HC0 3 - at 45.1% (Na* rejection was not reported). In order to calculate the ion concentrations in the reject (brine) the following Gulf seawater ion composition (Hassen, 2000, incorporated in its entirety by reference) was considered: Mg 2 + = 1610 mg/l; Ca 2 + 480 mg/I; S0 4 2 - = 3200 mg/, Cr = 22500 mg/l; Na* = 12630 mg/I and HCOs~ = 128 mg/. As in the previous examples, the brine concentrations were calculated using the following equation: C 1 ,RwRait Brine ~ Feed 1-R, Where C' represents concentration of an ion i, Rw represents the recovery and R'sait represents the rejection of ion i. Considering the reported recovery rate of 50%, the Mg 2 * concentration in the reject was approximated at 1610(1+0.5*0.948/(1-0.5)) = 3136 mg/l; similarly, Ca 2 * will be present at app. 875 mg/I; S0 4 2 at 6,378 mg/l, Na* at 13,000 mg/I and Cl- at 26,257 mg/l. The mass ratio of Mg 2 * to Ca 2 + in the brine is 3.6 to 1 relative to 3.2 in seawater.

WO 2010/131251 PCT/IL2010/000383 19 This fact, along with the high Mg 2 + concentration, makes the brine suitable for loading of the ion exchange resin that allows essentially complete separation of Mg 2 + from the unwanted CI and Na*, as explained in PCT Patent application no. PCT/1L2007/001261 to Technion - Research & Development Foundation Ltd. A second option is to blend the brine directly with the post treated RO permeate (assuming that the addition of Cl and Na* to the product water is acceptable): Assuming that Mg 2 * concentration of 10 mg/I is required in the product water, the dilution ratio between brine and permeate needs to be app. 313 to 1. Thus, the additional concentration of Ca 2 +, S042 Na* and C in the water, as a result of the blending, would be 2.8, 20.3, 41.4 and 83.7 mg/I, respectively. Note that the recovery ratio in the membrane application is of low importance. If a recovery ratio lower than 50% is used, this will result in a different blending ratio, but water will not be lost, since both streams (permeate and reject) are used in the process to make up the final product water. A higher recovery ratio is however not recommended since it will result in a need to add antiscalants to the water fed to the NF system. Addition of antiscalant should be avoided in the suggested process, since the brine of the membrane is directly blended with the product water. EXAMPLE 5 Results from the application of a membrane manufactured by Hydranautics ltd. are used (see data characterizing the membrane in Figure 7.Filtration of brackish water (from San Pasqual, TDS = 1790 mg/I) was tested using 18 NF elements (one stage operation). The ion concentrations of the feed water were: Mg 2 * = 91 mg/l; Ca2+ = 198 mg/l; S0 4 2 = 713 mg/I, Cl = 375 mg/l; Na* = 257 mg/I and HC0 3 = 303 mg/l. The brine composition was reported as follows: [Mg 2 *] = 142 mg/l; [Ca 2 +] = 294 mg/l; [Na*] = 19870 mg/I and [CL] at 30790 mg/I (no details were given on the S0 4 2 concentration but it can be assumed that it was rejected at >95%). The ratio between Mg 2 + and Ca 2 + in this brine is therefore 0.5 to 1 (mass per mass); therefore there is no logic in loading ion exchange resins with it, as explained in the 1 *t example.

WO 2010/131251 PCT/IL2010/000383 20 However it is feasible to blend the brine directly with the post treated RO permeate (assuming that the resulting additional concentration of C and Na* to the product water is acceptable), as described herein: Assuming that Mg 2 + concentration of 10 mg/I is required in the product water the dilution ratio between brine and desalination permeate needs to be app. 14.2 to 1. Thus, the additional concentration of Ca 2 +, Na*, and C in the water, as a result of the blending, would be 20.7, 33.3 and 31.7 mg/I, respectively. Note that the additional Ca 2 ' and bicarbonate concentrations in the blend option are 20.7 and 35.6 mg/I respectively. Thus, in the post treatment stage significantly less Ca 2 + and HC0 3 need to be supplied to satisfy water quality requirements with respect to Ca 2 * and alkalinity. EXAMPLE 6 Using another data set produced by Hydranautics (see data characterizing the membrane in Figure 8) and the following seawater composition: Mg 2 + = 1330 mg/I, Ca 2 + = 400 mg/I, Na* = 11300 mg/l, C = 20500 mg/I, S0 4 2 = 2530 mg/I and HCO 3 131 mg/I, the ionic composition of the brine is expected to be as follows: Mg 2 + = 4209 mg/, Ca 2 * = 897 mg/I, Na* = 15270 mg/I, CE = 34167 mg/I, S0 4 2 = 10645 mg/I and HCO3- = 336 mg/. This brine may be used for loading resins with Mg 2 + since the ratio of Mg 2 + to Ca 2 * is 4.7 to 1 (mass per mass, i.e. 7.7 equivalent per equivalent), in case a strict restriction is posed on the addition of CE, Na*, or both, or, alternatively, it can be blended directly with the post-treated permeate. In the latter case, the blending ratio for attaining Mg 2 + concentration of 10 mg Mg/I is app. 421 to 1. Thus, the additional concentration of Ca 2 +, Na*, and CE in the water, as a result of the blending, would be approximately 2.1, 36.2 and 81.1 mg/I, respectively. EXAMPLE 7 The Dow NF45 membrane is used in the oil industry for pre-treating seawater used for re-injection. High chloride concentration in the permeate is typically considered to be problematic, but on the contrary, in this invention a higher chloride concentration in WO 2010/131251 PCT/IL2010/000383 21 the permeate is considered advantageous since it results in lower required pressure. Thus, a membrane was chosen having low rejection towards chloride. As a result of the filtration, the following brine was generated: Mg 2 + = 5010 mg/I, Ca 2 + = 673 mg/I, Na* = 12,736 mg/l, CE = 22,000 mg/I, S0 4 2 = 10,456 mg/I and

HCO

3 ~ = 500 mg/l. The blending ratio for attaining Mg 2 * concentration of 10 mg / is app. 501 to 1. Thus, the additional concentration of Ca 2 *, Na*, and CE in the water, as a result of the blending, would be 1.34, 25.4 and 43.9 mg/I, respectively.

WO 2010/131251 PCT/IL2010/000383 22 References cited Abdullatef A.Z., Farooque A.M., AI-Otaibi G.F., Kither N.M. and Al Khames S.I. (2007) Optimum Nanofiltration Membrane Arrangements in Seawater Pretreatment: Part-1 IDA World Congress-Maspalomas Gran Canaria - Spain October 21-26, 2007 Battistoni, P., Boccadoro, R., Pavan, P., Cecchi, F. (2001) Struvite crystallization in sludge dewatering supernatant using air stripping: the new full scale plant at Treviso (Italy) sewage works. In: Proceedings of the 2 nd International Conference on phosphorus Recovery for Recycling from Sewage and Animal Wastes, Birnhack L. and Lahav 0. (2007) A new post treatment process for attaining Ca 2 +, Mg2+, S42 and alkalinity criteria in desalinated water. Water Research, 41(17), 3989-3997. Birnhack L., Fridman N., and Lahav 0. (2009) Potential applications of quarry dolomite for post treatment of desalinated water. Desalination and Water Treatment, 1, 58-67. Doyle, J.D., Parsons, S.A., (2002) Struvite formation, control and recovery. Water Research 36, 3925-3940. Hassan A.M., Farooque A.M., Jamaluddin A.T.M., Al-Amoudi A.S., Al-Sofi M. AK., A1-Rubaian A.F., Kither N.M., Al-Tisan I.A.R. and Rowaili A. (2000) A demonstration plant based on the new NF-SWRO process. Desalination, 131 157 171 Munch, E., Barr, K. (2001) Controlled struvite crystallization for removing phosphorus from anaerobic digester sidestreams. Water Research 35, 151-159. Sarkar S. and SenGupta A.K. (2008) A new hybrid ion exchange-nanofiltration (HIX NF) separation process for energy-efficient desalination: Process concept and laboratory evaluation Journal of Membrane Science 324 76-84 Schaep J., Van der Bruggen B., Vandecasteele C. and Wilms D., (1998) Influence of Ion size and charge in nanofiltration, Sep. Purif. Technol. 14 155-162. Lee, N., Nielsen, P.H., Aspegren, H., Henze, M., Schleifer, K.H., Jansen, J.L. (2003) Long-term population dynamics and in situ physiology in activated sludge system with enhanced biological phosphorus removal operated with and without nitrogen removal. Systems Applied Microbiology, 26, 211-227.

WO 2010/131251 PCT/IL2010/000383 23 Mukherjee P. and SenGupta A.K., (2006) Some observations about electrolyte permeation mechanism through reverse osmosis and nanofiltration membranes, J. Membr. Sci. 278 301-307. Nelson, N.O., Mikkelsen, R.L., Hesterberg, D.L. (2003) Struvite precipitation in anaerobic swine lagoon liquid: effect of pH and Mg:P ratio and determination of rate constant. Bioresource Technology 89, 229-236. Ueno, Y., Fujii, M. (2001) 3 years operating experience selling recovered struvite from full scale plant. In: Proceedings of the 2 nd International Conference on phosphorus Recovery for Recycling from Sewage and Animal Wastes, Noordwijkerhout, Holland, March 12-14. van Resburg, P., Musvoto, E.V., Wentzel, M.C., Ekama, G.A. 2003. Modelling multiple mineral precipitation in anaerobic digester liquor. Water Research, 37, 3087-3097. Wilf M. (2007) Membrane desalination technology. Balaban Desalination Publications (Chapter 16: Nanofiltration technology and applications). World Health Organization (2005) Minerals in drinking water. Water, Sanitation and Health Protection and the Human Environment World Health Organization, Geneva. 186 pp. World Health Organization, Guidelines for Drinking -Water Quality, 2 nd addendum to 3 rd Ed. (2008) Volume 1. 5 pp. http://www.who.int/water sanitation health/dwq/secondadden-dum20081119.pdf World Health Organization (2009) Calcium and magnesium in drinking water: Public health significance. 180 pp. Wu Q.Z., Bishop P.L. (2004) Enhancing struvite crystallization from anaerobic supernatant. Journal of Environmental Engineering Science, 3. 21-29

Claims

1. A system for production of divalent ion supplemented water, the system comprising: a system inlet line; a system outlet line; a DU (desalination unit), a DU inlet line fluidly connected to the system inlet line, a DU outlet line fluidly connected to the system outlet line, a first and a second junction on the DU line, a third junction on the DU outlet line, the second junction positioned between the first junction and the DU, the DU capable of: desalinating DU salty water fed to the DU via the DU inlet line, and releasing desalinated water from the DU into the DU outlet line; at least one divalent ion separation unit (SU) inlet line, at least one SU outlet line and at least one SU reject line, and at least one SU, wherein at least one SU is capable of: receiving SU salty water fed from a SU inlet line, rejecting brine to a SU reject line, wherein the brine comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than in said received SU salty water, and releasing permeate to an SU outlet line, wherein the permeate comprises a higher concentration of monovalent ions and a lower concentration of monovalent ions than in said received SU salty water, wherein the system is configured to: split at the first junction salty water fed via the system inlet line such that part of the fed salty water is fed to the DU and a another part of the fed salty water is fed to at least one SU via at least one SU inlet line; add at the second junction permeate from at least one SU outlet to the salty water fed via the first junction, thereby producing the DU salty water fed to the DU, add at the third junction brine from at least one SU reject line to the desalinated water, thereby producing the divalent ion supplemented water.

2. The system as claimed in Claim 1, wherein said divalent ions comprise magnesium ions. WO 2010/131251 PCT/IL2010/000383 25

3. The system of claim 2, wherein said divalent ions further comprise calcium ions.

4. The system of any one of claims 1 to 3, wherein said monovalent ions comprise sodium ions and chloride,

5. The system of any one of claims 1 to 4, wherein at least one SU reject line is a SU inlet line, such that brine from a first SU is SU salty water fed to a second SU, whereby brine from the second SU comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than the brine from the first SU.

6. The system of any one of claims 1 to 5, further comprising means for evaluating the pH of the divalent ion supplemented water and for adjusting the pH level of the supplemented water.

7. The system of any one of claims 1 to 6, wherein the DU unit is a RO unit and the SU units are selected from one or more of the group comprising: RO, UF, ion exchange resin and ion exchange column.

8. The system of any one of claims 1 to 7, further comprising a pretreatment unit capable of making the DU salty water essentially free of bacteria. The system of any one of claims 1 to 8, further comprising a post-treatment unit capable of supplementing the divalent ion supplemented water with calcium ions.

9. The system of claim 7, the SU units comprising an ion exchange unit and at least one UF unit, wherein the ion exchange unit SU inlet line is the SU reject line of a first UF unit, and the first UF unit is capable of rejecting Mg 2 + and calcium ions at over 70% and CI at under 30%.

10. The system of claim 9, wherein the first UF unit is capable of rejecting Mg 2 + at approximately 98%, calcium ions at approximately 92%, sulfate at approximately 99%, CF at approximately 24% and HCO 3 at approximately 44%.

11. The system of claim 1, wherein the first UF unit comprises an NF array capable of rejecting Mg 2 * at approximately 95%, calcium ions at approximately 82%, sulfate at approximately 99%, CF at approximately 17% and HC0 3 ~ at approximately 45%.

12. The system of claim 7, the SU units comprising a ion exchange unit and two UF units, the ion exchange unit SU characterized by a high affinity towards S0 4 2 ions and a low (but not very low) affinity towards CE ions, the system being configured so that: WO 2010/131251 PCT/IL2010/000383 26 at a load step the SU inlet line of the ion exchange unit comprises the SU reject line of a first UF unit, and the eluate of the ion exchange unit and the permeate of first UF unit are fed to the DU, and at a Cr absorbance step the first UF unit is bypassed so that the ion exchange unit is fed with salty water fed via the first junction, and the eluate of the ion exchange unit is fed to the SU inlet line of the second UF unit, the brine of the second UF unit being fed to the third junction and the permeate of the second UF unit being fed to the second junction, whereby the divalent ion supplemented water has reduced chloride ion concentration.

13. The system of claim 12, wherein the ion exchange unit is selected from the group comprising: Purolite A-850 and Amberlite IRA-67.

14. A method for production of divalent ion supplemented water, the method comprising: providing: a system inlet line, a system outlet line; a DU (desalination unit), a DU inlet line fluidly connected to the system inlet line, a DU outlet line fluidly connected to the system outlet line, a first and a second junction on the DU line, a third junction on the DU outlet line, the second junction positioned between the first junction and the DU; at least one divalent ion separation unit (SU) inlet line, at least one SU outlet line and at least one SU reject line, and at least one SU; desalinating DU salty water fed to the DU via the DU inlet line; releasing desalinated water from the DU into the DU outlet line; receiving SU salty water fed from a SU inlet line; rejecting brine to a SU reject line, wherein the brine comprises a higher concentration of divalent ions and a lower concentration of monovalent ions than in said received SU salty water; releasing permeate to an SU outlet line, wherein WO 2010/131251 PCT/IL2010/000383 27 the permeate comprises a higher concentration of monovalent ions and a lower concentration of monovalent ions than in said received SU salty water, splitting at the first junction salty water fed via the system inlet line such that part of the fed salty water is fed to the DU and a another part of the fed salty water is fed to at least one SU via at least one SU inlet line; adding at the second junction permeate from at least one SU outlet to the salty water fed via the first junction, thereby producing the DU salty water fed to the DU, and adding at the third junction brine from at least one SU reject line to the desalinated water, thereby producing the divalent ion supplemented water.

15. A method of intentional struvite precipitation for phosphate recovery from wastewater, the method comprising adding brine produced from a SU according to claim 14 to the wastewater.