Rejection increases significantly when operating RO (reverse osmosis) systems at pH values above 10.0. Such a high pH operation is impractical in seawater RO plants, resulting in typical permeate Boron concentrations in the range of 1.0 to 2.0 mg/1.

These concentrations exceed by far the Boron concentration values desired for irrigation of most plants.

Attempts have been made to reduce Boron content by the use of 2 pass designs in which the pH of the permeate of the Sea Water Pass Reverse Osmosis unit (s) is increased by adding alkali. The elevated pH permeate is fed to a Second Reverse Osmosis Pass so as to obtain low Boron final product water. If the permeate of such second pass meets the specified Boron concentration demands, it can be used for irrigation and for human consumption. If not, then additional Boron removal techniques such as a Third Permeate Pass Reverse Osmosis or Boron specific ion exchange"polishers"are required. The second pass brine can either be recycled backwards to the seawater pass feed for repeated desalination, or it can be further treated downstream to yield a product that may be blended into the plant mainstream product water output (namely, the second pass permeate).

The pH elevation, required for the achievement of the desired Boron rejection in the second pass, is limited by the presence of minor, yet critical amounts of Magnesium in the first pass permeate. Such pH elevation induces the deposition of the sparingly soluble base, Magnesium Hydroxide-Mg (OH) 2 that impairs the performance of the second pass RO unit (s).

The precipitation potential of Mg (OH) 2 is significantly increased as feed water pH is raised above 10.0. Other factors that influence the deposition potential are the first pass Magnesium ion passage, a temperature dependent factor by itself, and the recovery ratio of the second RO pass. In the absence of an anti-scalant, the permissible Magnesium concentration for operating the second pass at a feed pH of 10.2 is around 1.0 PPM at 90% recovery, and around 11.0 PPM at 75% recovery.

However, typical Magnesium concentrations in seawater permeate are 4 to 12 PPM, depending on temperature, system design and membranes'operating conditions.

Treatment of the Second Pass Low Salinity Brine (SPLSB) for Boron rejection by a Third Pass RO unit poses an even more complex task as its Boron and Magnesium feed water contents are 3 to 4 times higher in respect to the Boron content in the second pass feed water, and 5 times in respect to the Magnesium content. In order to extract from this stream a low Boron permeate at high recovery, it must be operated at a feed water pH of 10.5 or higher. This can not be performed unless the Magnesium content of the SPLSB is first reduced to less than 0.1 PPM.

It is therefore desirable to provide a softener RO-brine unit to be implemented in a Reverse Osmosis system for low Boron desalted water production from sea water that enables sufficient reduction of the Magnesium content in the downstream brine, such that the pH of the said downstream brine may be raised whereby improved Boron rejection is achieved.

It is further desirable that high salinity Magnesium-free brine produced by the Reverse Osmosis system be recycled and used for regenerating the Magnesium reduction means.

The present application provides an inventive Magnesium Kidney System and process that enables the practical implementation of such downstream brine treatment.

Summary of the Invention The present invention proposes an inventive softener-RO brine unit (Magnesium Kidney System) to be applied in a Reverse Osmosis system for producing Low Boron water from sea water. The inventive softener-RO brine unit comprises an ion exchange softener for entrapping the Magnesium ions from the downstream brine, and an RO unit. In accordance with another aspect of the invention a method is proposed for increasing the efficiency of a Reverse Osmosis system for producing Low Boron water from sea water by driving the downstream brine through an ion exchange softener to entrap the Magnesium ions, adding Sodium hydroxide to the Magnesium-free downstream brine to elevate the pH, treating the resulting brine in a reverse osmosis membrane unit and using the high salinity Magnesium-free brine output of the reverse osmosis membrane unit to regenerate the said ion exchange softener.

Brief Description of the Drawings Fig. 1 is a schematic illustration of a typical sea water RO unit in which the inventive softener-RO brine unit is applied Fig. 2 is a general schematic illustration of a reverse osmosis membrane unit that may be applied in the softener-RO brine unit of the invention Fig. 3 is a general schematic illustration of an ion exchange softener that may be applied in the softener-RO brine unit of the invention Detailed Description of the Invention The present invention was conceived in the course of a search for a cost effective yet safe process for producing low Boron water from sea water by reverse osmosis. The invention proposes a high-efficiency low-cost method of treating the second pass Low-Salinity brine (SPLSB) by an ion exchange softener that is regenerated by the third Pass High-Salinity Brine (TPHSB). The TPHSB possesses excellent regenerant properties where low magnesium leakage is desired. The process of the invention enables the ion exchange softener to entrap the Magnesium ions and reduce their concentration in the effluent to less than 0.1 PPM. Thus, pH values beyond 10.5 in the feed water of the third RO pass are safely applicable and the production of usable permeate for blending can be achieved. The softener RO-brine unit acts like a kidney that extracts undesirable Magnesium to protect the 3rd pass brine RO unit while the 3rd pass brine RO unit reciprocates by providing the"kidney"with excellent resources (pure brine) for its own self-cleaning regeneration process.

The overall recovery of the Boron removal process that is based on the Magnesium Kidney ion exchange brine RO team process chain is in excess of 95%. (end user combined permeate to sea water pass permeate, end user combined permeate being the mixture of the 2nd pass and 3rd pass permeates) when the 2nd pass RO operates at only 80% recovery, the above taking into account ion exchange losses and brine RO losses. This also means a safer operation and a better Boron rejection in the main stream second pass, while the sea water RO unit is reduced.

It will be understood by those versed in the art that while the invention is described herein with respect to a unit that reduces the Magnesium content in the second pass brine and that is regenerated by the third pass brine, the invention is applicable in Reverse Osmosis systems with any number of passes.

It will be further understood that the Magnesium content reduction means may be introduced at any stage of the desalination process, and that high salinity brine that is produced downstream in the desalination process may be used to regenerate the Magnesium reduction means.

The inventive softener-RO brine unit is cost saving, while improving the performance of existing systems. Thus SPLSB can be treated for Magnesium reduction by using filtered seawater as a regenerant for the ion exchange softener, however the efficiency of such system will be much lower than that of the invention and unless high quality artificial brine is also provided, the Magnesium leakage will be much higher and will not allow operation of the third pass at high pH values.

The invention will be described hereinbelow in respect of a preferred embodiment. It will be understood however that many other variations and modifications of the inventive system and process may be made that still remain within the scope of the invention and the claims.

Fig. 1 schematically illustrates a typical sea water Reverse Osmosis system in which the softener-RO brine unit may be implemented. The sea water Reverse Osmosis system of Fig. 1 may be operated at any practical recovery, with any applicable pretreatment, with or without an energy recovery system and at any membranes array, all of which are irrelevant for the invention subject matter of the application.

The Reverse Osmosis system comprises three RO units respectively designated 1D 1C and 1B, the RO unit 1B being comprised in the softener RO-brine unit of the invention.

The RO unit 1D is a seawater desalination unit, consisting of an array of reverse osmosis membrane elements, housed in pressure vessels. In the Reverse Osmosis system of Fig. 1, 1D is the"first pass"in the desalination process. Seawater is fed to RO unit ID by high pressure pump lDa, which is in conjunction to an energy recovery device, lDb. The brine of unit 1D is disposed from the system as shown at 1 Dc while the permeate is further delivered to the subsequent RO unit, commonly known as"second pass", schematically illustrated in Fig. 1 and designated 1 C.

To enhance Boron rejection, pH elevation is carried out by dosing Caustic Soda to the "second pass"feed, at ICa. The required pressure for desalination in this pass is supplied by high pressure pump lCb. Permeate from this unit is further delivered to product post treatment units (not shown in Fig. 1) while brine is delivered to the softener-RO brine unit or"Magnesium Kidney"that comprises the RO unit I B.

The brine leaving the"second pass"RO unit 1C has a low content of salt as indicated by its denomination SPLSB (Second Pass Low Salt Brine). The amounts of Magnesium that do exist in the SPLSB stream, are readily captured in an ion exchange softener unit 1A that in return releases to the SPLSB solution sodium ions This softening process results in a softened solution that is the feed solution to the "third pass"RO, designated as 1B in Fig. 1. Now, in the absence of Magnesium ions, pH elevation is carried out safely and to a higher pH level by the introduction of Sodium hydroxide at the dosing point lBa. The required pressure for desalination in this pass is supplied by a high pressure pump lBb. The low-Boron permeate from this unit is further delivered to product post treatment units (not shown in Fig. 1) while the high-salinity brine, also denominated TPHSB (Third Pass High Salt Brine) is delivered back to the ion exchange softener unit 1A and serves as its regenerant.

Spent regenerant is disposed out from the system as seen at lAa, while the pass 2 and pass 3 permeates leave the system as the combined low salt, low Boron output of the desalination process.

The softener-RO brine unit or"Magnesium kidney"of the invention, enclosed in sqare frame F, is comprised of the ion exchanger 1A and the reverse osmosis (RO) unit designated as IB.

It will be understood by those versed in the art that the"Magnesium kidney"unit may be installed in a preexisting Reverse Osmosis Desalination system.

It will be further understood that the preexisiting Reverse Osmosis Desalination system may comprise an RO unit such as 1B that is suitable for integration in the said softener RO brine unit or"Magnesium kidney". In this case only the ion exchange softener 1A needs to be installed in the said preexisiting Reverse Osmosis Desalination system in order to implement the inventive process.

The inventive softener-RO brine unit need not be implemented in a three pass Reverse Osmosis Desalination system and it need not be implemented at the third pass as shown in Fig. 1. The invention may be used in a Reverse Osmosis Desalination System of any configuration and at any phase of the desalination process.

A schematic drawing of an RO unit is shown in Fig. 2, the said RO unit comprising an array of membrane elements of which only a pair of elements RI and R2 are shown, housed in a pressure vessel R4. The RO unit further comprises an inner collecting pipe R3. Feed enters the RO unit of Fig. 1 and flows from element RI to element R2 and onwards becoming concentrate brine while permeate is collected in the inner collecting pipe R3.

It will be understood that the RO unit of Fig. 2 is only shown by way of example and the invention is not limited to RO units of the type shown in Fig. 2.

A general schematic drawing of a ion exchange softener unit that may be applied in the softener-brine unit of the invention is shown in Fig. 3.

As seen in Fig. 3, the ion exchange softener unit comprises a pressure vessel unit with an upper port and a lower port and a strong acid resin core. Hard water enters the upper port of the pressure vessel and flows down through the said strong acid resin that captures the Magnesium ions and releases Sodium ions in return. Softened water leaves the vessel from the port located in the bottom of the vessel. In regeneration, the regenerant usually enters the vessel from the lower port and leaves through the upper port. In the regeneration process, the resin Sodium ions are restored and the Magnesium ions previously absorbed in the resin are released.

It will be understood that the ion exchange softener unit of Fig. 2 is only shown by way of example and the invention is not limited to ion exchange softener units of the type shown in Fig. 2.