AN IMPROVED MEMBRANE WATER DESALINATION PROCESS

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to water desalination processes and, more particularly, to improved membrane water desalination processes.

Desalination is a process in which dissolved salt impurities are removed from saline water to obtain two parts - the treated water or product water, which has a low concentration of salts, and the residual water, having a much higher concentration than the original feed water, and which is usually referred to as "brine", "brine concentrate" or "concentrate". The two major types of technologies that are used around the world for desalination can be broadly classified as either thermal or membrane technologies.

Thermal technologies involve the heating of saline water and collecting the condensed vapor (distillate) to produce pure water. Thermal technologies are mostly used for seawater desalination, and are only rarely used for brackish water desalination. Thermal desalination technologies can be sub-divided into three groups: Multi-Stage Flash Distillation (MSF), Multi-Effect Distillation (MED), and Vapor Compression Distillation (VCD) [I].

Multi-Stage Flash Distillation (MSF) is the most common thermal desalination method. It involves the use of distillation through several (multi-stage) chambers whereas the feed water is first heated under high pressure, and is led into the first "flash chamber", where the pressure is released, causing the water to boil rapidly and resulting in sudden evaporation or "flashing". This "flashing" of a portion of the feed continues in each successive stage, because the pressure at each stage is lower than in the previous stage. The vapor generated by the flashing is converted into fresh water by being condensed on heat exchanger tubing that run through each stage. The tubes are cooled by the incoming cooler feed water.

Generally, only a small percentage of the feed water in thermal processes is converted into vapor and condensed, and therefore thermal processes are known for relatively low process efficiencies. However, since they are mainly used for the

desalination of large volumes of seawater, the low efficiency is not a problem. Furthermore, in many cases the low efficiency of the process is even an advantage, since the concentrate, which is re-introduced into the seawater, must contain salt concentrations that are as close as possible to the original seawater composition, to avoid damaging the marine life forms.

MSF plants are subject to corrosion, erosion and impingement attack, and the use of antiscalants (otherwise known as "antiscaling agents") which prevent the deposition of salts upon concentration of the brine, is often needed [I]. Typically, antiscalants comprise proprietary polymers, among them polyphosphonates, polymaleic acid and poly acrylates [1, 2].

Thermal processes often use iron ions as coagulants for the treatment of organic matter, in particular during the pre-treatment stage. Iron ions are also formed therein as corrosion by-products formed during MSF. One study has shown that the presence of these ions (for example Fe ⁺³ , Fe(OH) ₂ , Fe(OH) ₃ , Fe ₂ O ₃ but also Cu ⁺² , Ni ⁺² and rust) in MSF processes, can decrease the efficiency of the antiscaling agents used during pre-treatment [3]. However, in another study, it has been demonstrated that the effect of Fe ⁺³ concentration on the prevention of CaCO ₃ and BaSO ₄ precipitation by a variety of antiscalants, is largely unknown and is not obvious [4].

Membrane technologies are generally preferred over thermal technologies of desalination, since they require less energy, have higher efficiencies, and are simpler to use and maintain. These techniques can be subdivided into two broad categories: Electrodialyis (ED or EDR), and Reverse Osmosis (RO). Membrane desalination, in particular RO, is currently the most widely used desalination process in the world, and the preferred method for the treatment of brackish water. The RO process uses pressure as the driving force to push saline water through a semi-permeable membrane into a product water stream and leaving behind a concentrated brine stream. This process is mainly used for removal of sodium and chloride from water, as well as most of the other ions, microbes and some organic molecules. Nanofiltration (NF) is a similar membrane process that is used only for removal of divalent salt ions such as calcium, magnesium, and sulphate.

During the membrane desalination process, the concentration of sparingly soluble salts, which are soluble in the raw water, increases and upon passing the

membrane(s) exceeds the saturation value. In order to prevent the scaling of these sparingly soluble salts on the membrane, pretreatment is used, for example by filtration, pH and temperature modification, and by the addition of antiscalants. The antiscalants raise the saturation limit of the solution, rendering the concentrate supersaturated by the sparingly soluble salts and preventing the precipitation of these salts. The antiscaling agents do not pass the membrane and are deposited along with the brine.

The efficiency of membrane desalination processes varies, mainly depending on water quality, but in industrial processes is generally about 80%, very rarely reaching an efficiency of 90%. Thus, at least 10%, but generally even 20%, of the volume of raw water, is obtained as waste and must be deposited of.

Unfortunately, the safe disposal of membrane desalination concentrates, containing extremely high, and thus environmentally unacceptable, salt levels, presents ecological, as well as economical problems, especially for inland plants which treat brackish water [5, 6]. Mostly, the large volumes of brine are first placed in large evaporation ponds, the cost of which can reach from 15$ to 20$ per square meter. Hence, the costs associated with the disposal of reject brines could range from 5% to 33% of the total costs of desalination [5].

Quite understandably then, decreasing the volume of the reject brines and approaching Zero Liquid Discharge (ZLD) is of major concern in membrane desalination processes [6, 7]. In order to obtain lower volumes of reject brines (concentrates) it is needed to achieve high levels of supersaturation, equivalent to higher concentration factors (CF) and higher volume concentration factors (VCF). The practical VCF value in desalination processes, and hence the lowering of concentrate volume, is limited by the maximum supersaturation level of the sparingly soluble salts, that can be achieved and maintained by the antiscalants.

Since the concentrate stream contains high concentrations, in fact supersaturated concentrations, of dissolved sparingly-soluble salts, the concentrate may not be further concentrated, and thus the VCF cannot be increased beyond the maximum supersaturation level, unless the level of supersaturation is significantly reduced, for example by precipitating the salts out of the concentrate. In that case, the presence of the antiscalants in the concentrate becomes problematic since their

presence hinders salt precipitation. In fact, although some level of natural salt precipitation does occur over time in the presence of antiscalants, the precipitation is very slow, and the need arises to find a way of effectively overcoming the antiscalants' undesirable retarding effect on the de-supersaturation of concentrates. In other words, the need arises to force the precipitation of the supersaturated salts thereby "stretching" the composition of the concentrate beyond the performance limits of the antiscalants [8].

Several techniques have been suggested to overcome the effect of the antiscalants, including massive seeding, increasing pH, addition of carbonate, electrolytic precipitation, addition of surfactants and advanced oxidation [8, 9, 10a, 10b, H]. Unfortunately, the application of these methods is often complicated, expensive and does not always comply with the requirements for secondary desalination. For example, it may be necessary to readjust the pH or to remove some reagents, such as oxidizing agents, before the brine can pass through the membranes once again. Hence, further treatment of the concentrate is needed, resulting in the consumption of relatively high amounts of chemicals, as well as in the handling and disposal of solid waste much in excess of the precipitated salts (reaching even 10 times the original amount of solids).

There is thus a widely recognized need for, and it would be highly advantageous to have, a novel membrane desalination process which will include a fast and controllable de-supersaturation of the desalination concentrates, while not affecting the composition of the concentrate and producing no excessive solid waste.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a water treatment process, comprising: a) providing a water stream containing one or more soluble species capable of forming one or more sparingly soluble salts; b) adding an effective concentration of at least one antiscaling agent to the water stream to thereby obtain water containing one or more soluble species and at least one antiscaling agent;

c) passing the water through one or more desalination membranes, to obtain a desalinated water permeate and further to obtain a concentrate supersaturated by one or more sparingly soluble salts and containing at least one antiscaling agent; d) adding an effective concentration of a ferric (Fe ⁺³ ) ion into the supersaturated concentrate containing at least one antiscaling agent, thereby precipitating one or more sparingly soluble salts out of the concentrate; and e) separating the one or more precipitated salts from the concentrate to obtain a de-supersaturated water effluent.

According to further features in preferred embodiments of the invention described below, the water stream is obtained from a water source selected from natural water sources and artificial water sources.

According to still further features in the described preferred embodiments, the water source is selected from the group comprising of process water, recirculating cooling water, desalination water, and crude petroleum recovery systems.

According to still further features in the described preferred embodiments, the water source is selected from the group comprising of seawater, saline water, desalination water, waste water and brackish water. Preferably, the water source is selected from the group consisting of seawater, brackish water and desalination water. According to still further features in the described preferred embodiments, the one or more sparingly soluble salts is selected from the group comprising of: CaSO ₄ , CaCO ₃ and CaPO ₄ .

According to still further features in the described preferred embodiments, the antiscaling agent is selected from the group comprising of phosphonates, polyphosphonates, phosphates, phosphonic acid, polymaleic acid, polycarboxylates and polyacrylates. Preferably, the antiscaling agent is an organic phosphonate, a neutralized phosphonic acid or a phosphate.

According to still further features in the described preferred embodiments, the concentration of the antiscaling agent within the supersaturated concentrate ranges from about 5 mg/liter to about 50 mg/liter. Preferably, the concentration ranges from about 5 mg/liter to about 30 mg/liter.

According to still further features in the described preferred embodiments, a weight ratio between the antiscaling agent and the ferric ion ranges from about 2.5: 1 to about 0.5:1. Preferably, the weight ratio between the antiscaling agent and the ferric ion is about 1:1. According to still further features in the described preferred embodiments, a source of the ferric ion is selected from the group comprising of ferric chloride, ferric nitrate, ferric citrate, ferric acetate, ferric oxalate, ferric sulfate, ferric bromide, ferric bichromate, ferric formate, ferric stearate, ferric myristate, ferric palmitate, ferric behenate, and mixtures thereof, ferric naphthenate and ferric phosphate. Preferably, the source of the ferric ion is selected from the group comprising of: FeCl ₃ and Fe ₂ (SO ₄ )-?.

According to still further features in the described preferred embodiments, the concentration of the ferric ion within the supersaturated concentrate ranges from about 3 mg/liter to about 50 mg/liter. According to still further features in the described preferred embodiments, the process described hereinabove is used in a reverse osmosis process.

According to still further features in the described preferred embodiments, the process described hereinabove is used in a nanofiltration process.

According to still further features in the described preferred embodiments, the process described herein, further comprises treating the de-supersaturated effluent.

According to still further features in the described preferred embodiments, the treating is conducted until a concentrate volume concentration factor value of at least 2 is obtained. Preferably, the concentrate volume concentration factor value is at least 2.5. According to still further features in the described preferred embodiments, the process described herein, further comprises recycling the de-supersaturated effluent.

According to still further features in the described preferred embodiments, the recycling comprises providing the de-supersaturated effluent as a water stream, and repeating the process described hereinabove. According to still further features in the described preferred embodiments, the recycling is conducted until a concentrate volume concentration factor value of at

least 2 is obtained. Preferably, the concentrate volume concentration factor value is at least 2.5.

According to still further features in the described preferred embodiments, there is provided a process as described herein, having an efficiency higher than 90%. Preferably, the process has an efficiency higher than 95%, more preferably higher than 99%.

According to still further features in the described preferred embodiments, there is provided a process as described herein, having a a concentrate volume concentration factor (VCF) ranging from about 1 to about 3. Preferably, the concentrate volume concentration factor ranges from about 1.5 to about 3, more preferably concentrate volume concentration factor ranges from about 2 to about 3.

The present invention successfully addresses the shortcomings of the presently known configurations by providing a membrane desalination process which includes an easy and controllable method of de-supersaturation of desalination concentrates, thus enabling recycling and/or successive desalination of the concentrate stream, while overcoming the effect of antiscalants present therein.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention 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 drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: FIG. 1 is a schematic flow block diagram of a general scheme of implementing the present embodiments; and

FIG. 2 is a flow block diagram of the actual scheme and procedures of an apparatus for treating a brackish water stream, according to a representative example of implementing the present embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of an improved membrane desalination process.

Specifically, the present invention can be used to increase the efficiency of membrane desalination process by successfully de-supersaturating a concentrate containing antiscalants through the addition of a ferric ion source.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Membrane desalination technologies, in particular Reverse Osmosis (RO) and nanofiltration (NF), are widely used worldwide, and present the preferred method for the treatment of brackish water.

As detailed hereinabove, in order to overcome scale building up on membranes during membrane desalination processes, various measures are taken, including the addition of antiscalants, which act by increasing the supersaturation levels of the concentrate, as a pretreatment of the saline water source.

As also explained in the background section hereinabove, the volume of the concentrate is to be lowered as much as possible, since it present an economical and

environmental burden. This requires increasing the efficiency of the membrane desalination process by subjecting the concentrate to additional desalination runs.

However, as further explained in the background section hereinabove, since the concentrates are already at supersaturation, before any additional supersaturation can take place, some of the dissolved sparingly-soluble salts must first be deposited and removed, a process which is very slow in the presence of antiscalants, and cannot be counted upon in industrial processes.

As yet further explained in the background section hereinabove, over the years there have been many attempts to interfere with the action of the antiscalants as a means to induce salt precipitation (massive seeding, increasing pH, addition of carbonate, electrolytic precipitation, addition of surfactants and advanced oxidation) but none has been found suitable for industrial membrane desalination systems.

The present inventors have now found that the addition of very small amounts of ferric (Fe ⁺³ ) ion, to the supersaturated concentrate obtained in a membrane desalination process and which contains antiscaling agents therein, successfully overcomes the supersaturation effect of the antiscaling agents, thereby effectively de- supersaturating the concentrate, and rendering it possible to re-use it, thereby increasing the efficiency of the entire membrane desalination process (obtaining higher CF and higher VCF values) and decreasing the total concentrate (waste) volume.

The treated concentrate may be re-used for example by repeating the membrane desalination process (recycling), or by subjecting it to additional, secondary, treatment (desalination) processes.

While further exploring the process of membrane desalination, the present inventors have now designed and successfully practiced a process for treating water, which is based on the addition of ferric ion into a supersaturated concentrate obtained in a conventional membrane desalination process and containing at least one antiscaling agent, to consequently induce precipitation of one or more sparingly soluble salts out of the supersaturated concentrate, thereby effectively de- supersaturating it, and then separating the precipitate from the concentrate to obtain a de-supersaturated effluent, which can be recycled and/or re-processed to increase the efficiency of the overall process, as depicted for example in Figures 1 and 2.

As demonstrated in the Examples section below (see for example, Table 2) the induction time for precipitation of CaCO ₃ from supersaturated solutions simulating real desalination brine, an indication of supersaturation, was significantly reduced by the addition OfFe ⁺³ ions to the brine. For example, a "Granot 1" solution (containing no antiscalant) had an induction time of 32.9 minutes. This time was shortened by a ten-fold factor to 3.4 minutes by the addition of 10 mg/liter OfFe ⁺³ .

As further demonstrated in the Examples section below, this effect remained valid even in the presence of antiscalants. Thus, a "Granot 1" solution containing 8 mg/liter of a Permatreat PC-191 (PC) antiscalant had an induction time of 43.3 minutes. This time was shortened by a ten-fold factor to 4.9 minutes by the addition of 10 mg/liter Of Fe ⁺³ . This effect was repeated in the more concentrated "Granot 2" solution, whereas a solution containing 24 mg/liter of a PC antiscalant had an induction time of 98 minutes. This time was shortened by almost a 20-fold factor to 5.1 minutes by the addition of 10 mg/liter OfFe ⁺³ (see for example, Table 2). The significance of the effect of the ferric ion addition is further highlighted when noting the net effect of the antiscaling agent, which significantly increases the induction time. For example, as is demonstrated in Table 2 in the Examples section below, a "Granot 1" solution containing no antiscalant had an induction time of 32.9 minutes, but this time was actually increased to 95 minutes (a 3 -fold increase) by the addition of 8 mg/liter of a Genesis LF (GE) antiscalant.

As is yet further demonstrated in the Examples section below, even increasing the concentration of the antiscalant (to 16 mg/liter and 25 mg/liter in Table 2) did not prevent the precipitation, and the effect of the ferric ion was still evident.

As is further demonstrated in the Examples section below, the same effects were observed using "real" desalination concentrates (see Tables 4 and 5), not only for an "aged" antiscalant solution ("Batch 1", Table 4), in which natural precipitation may have started, but also on freshly prepared solutions ("Batch 2", Table 5) where no natural precipitation has yet occurred. Thus, for example an "aged" concentrate containing about 25 ppm of a GE antiscalant had a CaSO ₄ induction time of 80 minutes. This time was shortened to 12 minutes upon the addition of 31.3 mg/liter of Fe ⁺³ (Table 4). In another example, a "fresh" concentrate containing about 10 ppm of

a Calgon antiscalant had a CaSO ₄ induction, time of 45 minutes. This time was shortened to 19 minutes upon the addition of 15 mg/liter OfFe ⁺³ (Table 5).

The concentrate volume concentration factor of the process described herein is at least 2, equivalent to an ability to decrease the concentrate output by half, for example starting from 20% volume output to obtain a 10% concentrate volume output, or starting from 10% volume output to obtain a 5% concentrate volume output, etc.

However, it is expected that using additional cycles, the concentrate volume concentration factor may reach 2.5 and even 3. After adding the appropriate amount of antiscalant, the supersaturation level can be increased again. A typical concentrate VCF is around 2 (based on analysis of the de-supersaturated solution). This means a 50% reduction of concentrate volume.

These results show the efficiency of the present embodiments for a variety of antiscaling agents, at a variety of antiscaling agent concentrations, and in a variety of salt compositions and concentrations, manifesting its broad applicability in membrane desalination systems.

The present embodiments therefore successfully address the shortcomings of the presently known configurations by providing a process of treating water by membrane desalinating them, without producing large volumes of concentrate waste, further increasing the overall efficiency of the process.

Furthermore, the improved process can be easily applied on any conventional membrane desalination method, even in an industrial scale, thereby improving the efficiencies of existing processes by considerable amounts.

Before providing a further detailed description of the process, as delineated hereinabove and in accordance with the present embodiments, attention will be given to the advantages and potential applications offered thereby: a) The process is a membrane desalination process, which is generally preferred over thermal desalination processes; b) The process has a lower environmental impact, due to the decrease of the salt concentration in the waste brine; c) The process is of low-cost;

d) The process produces comparably lower volumes of effluent waste, thereby approaching a Zero Liquid Discharge (ZLD); e) The process is applicable to a large variety of membrane desalination systems, of varying antiscalant content; f) The process is applicable to a large variety of water sources which have varying salt contents and concentrations; g) The process advantageously prevents deposition of scale on the membrane(s). h) the process produce solid waste in amounts that are close to the stoichiometric amounts (no significant amounts of solids are added)

Thus, according to the present embodiments there is provided a water treatment process. This process is effected by first providing a water stream containing one or more soluble species capable of forming one or more sparingly soluble salts.

The term "water treatment", as used herein, refers to a method or process for cleaning a water source. The term "water treatment" more preferably refers to the process of producing fresh water from saline water, otherwise known as "desalination". It should be noted that the terms "saline water" otherwise known as "salt water" or "brine" are used herein in a broad sense to denote the entire range of salt- fluid combinations including, but not limited to, sodium chloride-containing solutions, aqueous solutions of dissolved mineral salts, for example, halides, carbonates/bicarbonates and sulfates of sodium, potassium, lithium, calcium, magnesium, bromine, zinc and copper, solutions of other salts, and solutions of combinations or mixtures of salts, and combinations or mixtures of fluids and salts and materials whether or not dissolved.

The term "water stream", as used herein, refers to a stream comprising essentially water. The water stream may also contain a substantial amount of solvent, salt, dissolved hydrocarbons, acids, and other contaminants.

According to a preferred embodiment of the present invention, the source of the water stream may be both natural and artificial water sources.

Examples of natural water sources include, but are not limited to, springs, streams, rivers, lakes, seas, oceans and other accumulations of water, above the ground or underground.

Examples of artificial water sources include, but are not limited to, drainage water, sewage, process water, recirculating cooling water, desalination water, and crude petroleum recovery systems process water.

The term "desalination water" includes desalination products as well as byproducts, such as a desalination concentrate or a de-supersaturated brine.

Preferably the water source is selected from the group comprising of seawater, saline water, desalination water, waste water, and brackish water.

The waste water can be, for example, agricultural drainage water. "Brackish water" generally means water having more than 500 ppm of salt but less salt than seawater. "Sea water" as used herein means water having more than 30,000 ppm of salt. As is demonstrated in the Examples which follow, brackish water of varying compositions and concentrations, treated by the process of the present invention, were successfully desalinated (see for example Tables 2, 4 and 5).

As detailed in the Background section hereinabove, membrane desalination systems are especially suitable for the treatment of brackish water and saline water, and in some countries, are the method of choice for the treatment of these water sources. Thus, more preferably, the water source is selected from the group consisting of seawater and brackish water.

The term "soluble species", as used herein, refers to ions capable of forming sparingly soluble salts. The present invention encompasses water treatment processes which therefore include (a) separation of components of aqueous mixtures, (b) removal of materials from aqueous mixtures, and (c) separation of components from each other in aqueous mixtures.

The term "sparingly soluble salts", as used herein, refers to a saturation solubility of less than 5 grams/liter. Calcium salts, magnesium salts, phosphate salts, aluminum salts, iron salts and manganese salts are the most important examples of sparingly soluble inorganic impurities present as cations in water solutions.

As can be seen in the Examples section which follows (see Tables 1 and 3), several salts, such as calcium salts, magnesium salts, sodium salts were present in the brackish water, but many other salts may be present in other samples of other water sources. In most brackish water desalination systems, the main salts which are at levels high enough to induce precipitation are gypsum (CaSO ₄ ) and calcium carbonate (CaCO ₃ ) and to some extent calcium phosphate (CaPO ₄ ). Thus, according to a preferred embodiment of the present invention, the one or more sparingly soluble salts are selected from the group comprising OfCaSO ₄ , CaCO ₃ and CaPO ₄ .

Once the water stream is provided, the process comprises adding an effective amount, equivalent at a given volume to an effective concentration, of at least one antiscaling agent to thereby obtain water containing one or more soluble species and said at least one antiscaling agent.

The term "antiscaling agent", also used interchangeably as "antiscalant", as used herein, refers to a compound capable preventing, lowering or eliminating the appearance of scales in a water system.

Since the present process provides a way of overcoming the influence of antiscalants, this process is expected to be suitable for inducing the precipitating of any salt whose precipitation was slowed down by using an antiscalant agent.

Examples of antiscaling agents include, but are not limited to, phosphonates, polyphosphonates, phosphonic acid, polymaleic acid, polycarboxylates and polyacrylates.

As can be seen in the Examples section which follows the present invention was tested using various phosphonate antiscalants and phosphonic acid antiscalants.

Therefore, according to a preferred embodiment of the present invention, the antiscaling agent is an organic phosphonate, such as Permatreat PC-191, neutralized phosphonic acids, such as Genesis LF, and phosphates, such as CALGON.

However, it is expected that during the life of this patent many relevant antiscalants will be developed and the scope of the term "antiscalant" is intended to include all such new technologies a priori. Since these new antiscalants may be suitable to halt the deposition of salts other than those currently influenced by antiscalants, the present invention encompasses the process of inducing precipitation

of such salts as well, since by effecting the activity of the antiscalants, any salt whose precipitation has been halted by the antiscalant, could be now deposited and separated. The term "effective concentration", as referring to the antiscaling agent, denotes a concentration sufficient to prevent, lower or eliminate the appearance of scales in a water system, such that the one or more soluble species capable of forming one or more sparingly soluble salts described hereinabove do not precipitate.

Concentrations of antiscalants may vary according to the use and water conditions, but are generally in the range of from about 2 mg/liter to about 10 mg/liter in the raw water. As appears in the Examples section which follows, the present invention was successfully used in the presence of antiscalants at concentrations of from about 5 mg/liter to about 50 mg/liter, more preferably from about 5 mg/liter to about 30 mg/liter.

However, as noted hereinabove, it is expected that during the life of this patent following the development of additional antiscalants, the effective concentrations of these antiscalants will vary, and the present invention is intended to encompass the entire range of effective antiscalant concentration a priori.

The water treatment process according to preferred embodiments of the present invention is especially suitable for use in membrane desalination systems. Thus, following the addition of the antiscaling agent, the saturated water passes through one or more desalination membranes, to obtain desalinated water permeate and to further obtain a concentrate supersaturated by the one or more sparingly soluble species and containing said at least one antiscaling agent.

The term "membranes", as used herein, refers to a functional filtering unit, and may include one or more semi-permeable layers and one or more support layers.

Depending on the fineness of the membrane employed, reverse osmosis can remove particles varying in size from the macro-molecular to the microscopic, and modern reverse osmosis units are capable of removing particles, bacteria, spores, viruses and even ions such as Cl ^" or Ca ⁺² . Thus, the term "desalination membranes", as used herein, refers to membranes which are part of a desalination system, capable of lowering the salt content of saline water, as detailed hereinabove.

The most common membrane desalination systems are reverse osmosis (RO) and nanofiltration, as largely described in the Background section hereinabove.

Therefore, according to a preferred embodiment of the present invention, the process described hereinabove is used in a reverse osmosis process. According to another preferred embodiment of the present invention, the process described hereinabove is used in a nanofiltration process.

As used herein, the term "permeate" refers to that stream passing through the membrane surface, while the term "concentrate" defines that portion of the stream exiting the filter containing retained, non-permeating species. Once the permeate has passed through the membrane filter, the remaining concentrate has a very high, or supersaturated levels of the one or more soluble species capable of forming one or more sparingly soluble salts.

The term "supersaturated", as used herein, refers to a solution that goes beyond saturation, for example, after passing through a RO membrane, or in the presence of antiscaling agents, such that the amount of the salts present in the solution is above that which it would be at regular conditions, before passing through a RO membrane, or in the absence of an antiscaling agent.

The term "saturated", or "saturation" as used herein, refers to a solution that contains as much of a salt as it can, based on its solubility in water, at a given temperature and pressure.

A measure of the saturation level can be seen in the induction time, as appearing in Tables 2, 4 and 5, and is also manifested in saturation %, and in the Langelier Saturation Index (LSI).

Following passing the water through the desalination membrane(s) as described herein, an effective amount of ferric (Fe ⁺³ ) ion (equivalent at a given volume to an effective concentration) is added into the supersaturated concentrate containing the at least one antiscaling agent, thereby precipitating one or more sparingly soluble salts out of the concentrate. This is followed by separation of the precipitated salt(s) to obtain a de-supersaturated effluent. The term "ferric" refers to the iron form Fe ⁺³ .

The ferric ion is added as a "ferric ion source" or ferric salts and includes, but is not limited to ferric chloride, ferric nitrate, ferric citrate, ferric acetate, ferric

oxalate, ferric sulfate, ferric bromide, ferric bichromate, ferric formate, ferric stearate, ferric myristate, ferric palmitate, ferric behenate, and mixtures thereof, ferric naphthenate and ferric phosphate.

According to a preferred embodiment of the present invention, the source of the ferric ion is FeCl3 and Fe ₂ (SO ₄ ) _S .

The term "effective concentration", as referring to the ferric ion concentration as used herein (equivalent for a given volume to an effective amount) refers to such a concentration found effective in de-supersaturating the supersaturated water concentrate. This value may change depending on the quality of the water, its composition, the amount or concentration of the antiscaling agent, the temperature, pH and additional factors known to a person skilled in the art.

As can be seen in the examples section which follows, the addition of even very small amounts of the ferric ion, approximately equivalent to the concentration of the antiscalant, to the supersaturated concentrate, proved to be effective in lowering either the CaCO ₃ or CaSO ₄ induction times (see for example in Tables 2, 4 and 5).

Thus, according to a preferred embodiment of the present invention, the weight ratio between the antiscaling agent and the ferric ion ranges from about 2.5: 1 to about 0.5: 1, more preferably this ratio is about 1:1.

Thus, considering the amounts of antiscalants generally present according to a preferred embodiment of the present invention, the effective concentration of the ferric ion within the supersaturated concentrate ranges from about 3 mg/liter to about 50 mg/liter.

Again, given that during the life of this patent, additional antiscalants may be developed, the weight ratio between the antiscaling agent and the ferric ion, as well as the effective concemtration of the ferric ion, are intended to encompass a wider range of values a priori.

The present inventors have surprisingly found that adding ferric ion to a supersaturated concentrate obtained in a membrane desalination process, is able to reverse the action of any antiscaling agents present within. This of course, enables the separation of the precipitating salt(s), and a subsequent continued purification or desalination of the de-supersaturated concentrate.

Without being bound to any specific theory, it is thought that ferric compounds which are formed within the supersaturated solution remove the antiscalants from the solution, probably by adsorption on a ferric hydroxide species. As demonstrated in the Examples section which follows (Example 4), by monitoring phosphorus levels, which in the tested systems could only have originated from the antiscalants, this assumption has been supported, as the levels of the phosphorus decreased upon addition of the ferric ion.

By adding the ferric ion, as detailed herein, the one or more sparingly soluble salts are easily precipitated out of the supersaturated concentrate. The term "a precipitate", or "precipitating", as used herein, refers to any sparingly soluble salt, as defined hereinabove, which has become insoluble and has salted out of the solution. The use of the term "precipitate" does not imply any specific mechanism. Alternatively, the term "deposit" or "deposited" may be used, and have substantially the same meaning as the terms "a precipitate" and "precipitating".

Upon separation of these precipitates, a "de-supersaturated" concentrate is thereby formed. The term "de-supersaturated concentrate" refers to a concentrate which was supersaturated, but which, upon addition of ferric ion, according to a preferred embodiment of the present invention, has become de-saturated enough to enable the addition of another effective amount of an antiscaling agent.

The de-supersaturation effect can be monitored as described above for determining saturation levels.

For example, as demonstrated in the Examples section which follows (Examples 3, 5 and 6), the induction times for the precipitation of CaSO ₄ and CaCO ₃ dramatically decreased upon the addition of the ferric ion to the brine, in some cases reaching even a 10-fold effect.

Once that the brine (supersaturated concentrate) is successfully de- supersaturated, the deposited salts are easily separated by any conventional method, such as filtration methods, sedimentation, centrifugation, ultra or micro filtration etc., and the de-supersaturated clear effluent can be recycled by repeating the process , described hereinabove using the de-supersaturated concentrate as a water stream, or can be run in a different and separate secondary desalination process, thereby,

obtaining a secondary concentrate and a secondary purified water stream. An exemplary process and apparatus to better clarify this process as outlined hereinabove are depicted in Figures 1 and 2.

Conventional purification methods may be combined with the membrane desalination conducted as a first desalination stage.

Thus, the number of desalination cycles can be significantly increased, thereby decreasing the volume of the waste brine, and increasing the concentrate volume concentration factor (VCF). Thus, according to a preferred embodiment of the present invention, the desalination process has a concentrate VCF value of at least 2, but even 2.5 or 3.

Furthermore, being able to successfully decrease the volume of the concentrate in the desalination process inherently increases the efficiency of the overall process, reaching efficiencies of at least 90 %, and even above 95%, above

99% and almost 100%, much higher than efficiencies of existing industrial membrane based desalination processes.

The process outlined hereinabove can be conducted either in a batch mode or in a continuous mode, but continuous processes are preferred over batch processes in commercial desalination plants.

The term "continuous mode" or "continuous process", as used herein, refers to a process that is effected without the need to be intermittently stopped or slowed.

Reference is now made to Figure 2 which is a schematic illustration of an apparatus 10 for treating a brackish water stream 11. Apparatus 10 can be used for executing selected steps of the process described hereinabove. In various exemplary embodiments of the invention apparatus 10 comprises a first pretreatment unit 12, into which enters a brackish water stream 11, containing one or more soluble species capable of forming one or more sparingly soluble salts. In the pretreatment unit 12 are included the addition of at least one antiscaling agent, but are optionally further included the addition of other active or non-active adjuvants, as well as a variety of other processes common in the field of desalination, which are chosen depending on the quality of the brackish water stream 11 and on the requirements of the desalination process and products. The pretreatment may thus optionally include pH adjustment, temperature adjustment, removal of organic matter, etc. Apparatus 10 further

comprises a membrane-based desalination unit 13, as further detailed hereinabove. The characteristics of the membrane-based desalination unit 13, for example the size, type and number of the membranes, the flow rates etc., vary and are generally described in the art, for example in "The NALCO Water Handbook" by Kemmer, Frank N. (editor in chief), (McGraw-Hill Book Company, 1979). The brackish water stream 11, which has been pretreated in unit 12 as described hereinabove, passes through the first membrane-based desalination unit 13, to produce 14, a stream of clear desalinated water (permeate), and 15, a stream of concentrate, which includes compounds that have not passed the membranes, including the at least one antiscaling agent and one or more sparingly soluble salts. Apparatus 10 further comprises a mixing unit 16, which may be a mixing tank or an on-line mixing device, into which enter both the concentrate 15, and a source of ferric ion 17. The residence time of the resulting solution within the mixing unit 16 is pre-determined to exceed the induction time needed for the precipitation process of the one or more sparingly soluble salts to occur, and is generally approximately 10 minutes. Apparatus 10 further comprises a pump 18 which transfers the mixture of the mixing unit 16, into a solid/liquid separation unit 19. The solid/liquid separation unit 19 can be, for example, a press filter, a sand filter, a centrifuge, a micro-filter, an ultra-filter, a decanter etc. From the solid/liquid separation unit 19, a solid phase 20 is separated for disposal, and a clear liquid phase of a de-supersaturated concentrate 21 is collected for further treatment.

The de-supersaturated concentrate 21 is transferred via a second pump 22, which is preferably a high-pressure pump, into a secondary pretreatment unit 23, as described hereinabove, which then enters a secondary desalination unit 24, to produce 25, a stream of clear secondary desalinated water (secondary permeate), and 26, a stream of secondary concentrate, which may be either further treated or disposed of. The secondary desalination unit 24 can be any desalination unit known in the art, including both thermal desalination units and membrane desalination units.

Alternatively, marked by a dotted line in Figure 2, the de-supersaturated concentrate 21 is transferred via a second pump 22, which is preferably a high- pressure pump, into the first pretreatment unit 12, to be recycled and reprocessed in Apparatus 10, as described hereinabove.

Additional objects, advantages, and novel features of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following examples, which are not intended to be limiting. Additionally, each of the various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below finds experimental support in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with the above descriptions, illustrate the invention in a non limiting fashion.

MATERIALS AND ANALYTICAL METHODS

All common reagents (CaCl ₂ , MgCl ₂ , NaHCO ₃ , Na ₂ SO ₄ and NaCl) were obtained from various vendors.

Permatreat PC-191, a proprietary mixture of organic phosphonates antiscalants, was obtained from Mekorot, Israel.

Genesis LF, an aqueous solution of neutralized phosphonic acid antiscalant, was obtained from Mekorot, Israel. A hexa-meta sodium phosphate antiscalant was obtained from Calgon.

FeCl ₃ was obtained from ANALAR. Hydrochloric acid (33%) was obtained from ANALAR . Desupersaturation experiments were carried out in 20 ml vials, suitable for turbidity measurements. Water turbidity was measured using a Hach turbidity meter every 2 minutes in order to determine the induction time.

Induction time for salt precipitation was measured from the time of attainment of salt supersaturation.

The Langelier Saturation Index (LSI, also termed "Langelier Stability Index"), which is commonly applied as an indicator for the tendency of the solution to precipitate calcium carbonate, is expressed as the difference between the actual system pH and the saturation pH (pHs). It was calculated accordingly using actual pH

value, the total dissolved solids value (TDS, in mg/liter), the temperature of the water (°C), the calcium hardness value (in mg/liter Ca ⁺² as CaCO ₃ ), and the alkalinity value (mg/liter as CaCOs), as follows:

LSI = pH - pH _s whereas pH _s = (9.3 + A + B) - (C + D)

A = (Log ₁₀ [TDS] - iyiθ, B = -13.12 x Log ₁₀ (°C + 273) + 34.55, C = Log ₁₀ [Ca ⁺² as CaCO ₃ ]- 0.4, and D = Log ₁₀ [alkalinity as CaCO ₃ ].

The weight saturation (%) was calculated for a certain salt by dividing the actual concentration of the salt by its solubility product (measured at saturation).

The Volumetric Concentration Factor (VCF) was calculated as the ratio between the total starting feed volume and the current concentrate volume.

EXAMPLE! Preparation of Stock Solutions

An aqueous stock solution (hereinafter designated "Solution A") was prepared by mixing predetermined quantities of CaCl ₂ and MgCl ₂ with water.

Another aqueous stock solution (hereinafter designated "Solution B") was prepared by mixing predetermined quantities of NaHCO ₃ , Na ₂ SO ₄ and NaCl with water.

Stock solutions of two commercial antiscalants, Permatreat PC-191 and Genesis LF were prepared to contain 800 ppm of antiscalant.

An aqueous stock solution OfFeCl ₃ was prepared to contain 1000 ppm OfFe ⁺³ .

EXAMPLE 2

Preparation of Feed Solutions "Granot 1" and "Granot2"

A solution which simulates the composition of the concentrate of the Granot brackish water desalination plant in Israel (hereinafter designated "Granot 1") was prepared by mixing equal volumes of stock solution A and stock solution B, as prepared according to Example 1.

The pH of the resulting "Granot 1" solution was about 8, its calculated Langelier Saturation index (LSI) was 1.8-1.9 and the calculated weight percent of saturation Of CaSO ₄ was about 12 %.

As a comparison, a second solution (hereinafter designated "Granot 2"), having double the ion concentrations, was prepared in a similar manner,.

The ion composition of solutions "Granot 1" and "Granot 2" is presented in Table 1 below.

Table 1

EXAMPLE 3 De-supersaturation : General Procedure

Reference is now made to Figure 1 which is a flowchart diagram of a process suitable for treating water, in particular for the membrane-based desalination of a saline water stream, according to various exemplary embodiments of the present invention. It is to be understood that, unless otherwise defined, the process steps described hereinbelow can be executed either contemporaneously or sequentially in many combinations or orders of execution. Specifically, the ordering of the flowchart diagrams is not to be considered as limiting. For example, two or more method steps, appearing in the following description or in the flowchart diagrams in a particular order, can be executed in a different order (e.g., a reverse order) or substantially

1

24

contemporaneously. Additionally, several method steps described below are optional and may not be executed.

The method begins at step 1 in which a saline water stream is provided. The water stream 1 then undergoes a pretreatment stage 2, which includes the addition of at least one antiscalant, and is then fed into a one or more desalination membrane(s) 3 to obtain a desalinated water stream 3a (permeate) and a supersaturated concentrated effluent (also known as "concentrate" or "brine") 3b, which is withdrawn for further treatment. Ferric (Fe ) ion is then added in step 4 to the withdrawn concentrate 3b to induce the precipitation 5 of one or more sparingly soluble salt(s). The precipitated salt(s) are then separated in step 6 from the liquid phase of the concentrate to obtain a de-supersaturated effluent 6a and a solid waste 6b which is disposed of. The de- supersaturated effluent 6a may be further desalinated by a secondary desalination process 7, thereby obtaining secondary desalinated water stream 7a and secondary waste solids 7b. Alternatively (marked by a dotted line), the supersaturated effluent 6a may be recycled according to the process described hereinabove, repeating the pretreatment (antiscalant addition), membrane desalination and de-supersaturation cycle, described hereinabove, as needed.

EXAMPLE 4 Desupersaturation of "Granot 1" and "Granot 2 " Feed Solutions

Aliquots of a PC or GE antiscalant solutions, and/or a FeCl ₃ solution, prepared according to the processes of Example 1, were added to "Granot 1" and "Granot 2" feed solutions, freshly prepared before each experiment according to the process of Example 2, as presented in Table 2 below.

Table 2

As seen in Table 2, the induction time for the precipitation of CaCO ₃ from the supersaturated "Granot 1" and "Granot 2" solutions (simulating real desalination brine) was significantly reduced by the addition of Fe ⁺³ ions to the brine. This effect was observed even in the presence of antiscalants. It was further observed that an additional increase of the antiscalant concentration did not interfere with the precipitation.

The phosphorus level was monitored, as an indicator of antiscalant concentration, since it can only originate from the addition of the antiscalants (mainly phosphates and/or phosphonates). Thus, it has been found that prior to the

T/IL2007/000591

26

precipitation, the concentration of the phosphorus (total P) was around 2.7 mg/liter, equivalent to about 20 mg/liter of antiscalant. After de-supersaturation, its concentration dropped to about 0.1-0.3 mg/liter.

EXAMPLE 5

Preparation of Desalination Pilot Plant Feed Concentrate Solutions ("Batch 1" and "Batch 2")

Experiments were further conducted on brackish water from Mashabei Sadeh wells, at a desalination pilot plant facility at Sde Boker using a commercial sized (2540) Hydranautics ESPA-I membrane, having a VCF of about 10.

Prior to desalination, the water was acidified with hydrochloric acid (33%) to pH=6 in order to avoid precipitation of calcium carbonate within the desalination membrane.

A GE antiscalant solution was added to "Batch 1" aged concentrate solution, to reach a 2 ppm concentration in the raw water, calculated to be equivalent to about 25 ppm in the "Batch 1" concentrate.

A Calgon antiscalant solution was added to "Batch 2" fresh concentrate solution, to reach a 1 ppm concentration in the raw water, calculated to be equivalent to about 10 ppm in the "Batch 2" concentrate. Analysis of the major components of the raw water, as well as of the aged and the fresh concentrate ("Batch 1" and "Batch 2", respectively) of this process, are summarized in Table 3 below:

Table 3

EXAMPLE 6 Desupersaturation of "Batch 1" and "Batch 2" Feed Solutions

Because of the relatively low concentrations of calcium and sulfate in the brackish feed water of example 4, and hence low concentrations thereof in the "Batch 1" and "Batch 2" concentrates, precipitation of calcium sulfate hardly occurs therein (the VCF limiting factor being silica). Thus, in order to get measurable induction times for the precipitation of calcium sulfate in the ensuing desupersaturation experiments, the concentrations of calcium and sulfate were increased by adding these ions, as well as a bicarbonate ion (HCO ₃ ^' ) to the "Batch 1" and "Batch 2" concentrates, as appears in respective Tables 4 and 5 below.

A FeCl ₃ solution, prepared according to the process of Example 1 was also added to the "Batch 1" and "Batch 2" concentrates.

Table 4

* - The initial concentrations for each experiment are the sum of the data in Table 3 and the "added ions .

Table 5

* - The initial concentrations for each experiment are the sum of the data in Table 3 and the "added ions".

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application

shall not be construed as an admission that such reference is available as prior art to the present invention.

References

1. Amjad Z. "Scale Inhibition in Desalination Applications: An Overview", CORROSION 96, Paper No. NACE-96-230, (NACE International, 1996).

2. Darton, E.G., "Scale Inhibition Techniques Used in Membrane Systems", Desalination, 113, (1997), 227-229.

3. DaM A. G. L, Mohammad N.M.K, Al-Sultani S., Sahul K, Al-Rashid R., "Role of chemical constituents in recycle brine on the performance of scale control additives in MSF plants", Desalination, 129, (2000), 173-186.

4. Zhou Bai-qing; Li Yan-li; Xia Huan-huan; Li Qin, "Effect of Fe ³⁺ and Al ³⁺ on Scale Inhibition". Water Quality Eng. Dep., Wuhari Univ., Wuhan, Peop. Rep. China Shuichuli Jishu (2004), 30(2), 85-87 (Journal written in Chinese).

5. Mohamed A.M.O., Maraqa M., Al-Handhaly J., "Impact of Land Disposal of Reject Brine from Desalination Plants on Soil and Groundwater", Desalination, 182, (2005), 411- 433.

6. Ahmed M., Shaya W., Hoey D. Al-Handhaly J.K., "Brine Disposal from Inland Desalination Plants: Research Needs Assessment", Water Int., 27(2), (2002), 194-201.

7. Mickley M., "Costs of Concentrate Management", Presented at the International Conference on Desalination Costing, MEDRC, 6-8 December, 2004, Limassol, Cyprus.

8. Williams M., Evangelista R., Cohen Y., "Non-Thermal Process for Recovering Reverse Osmosis Concentrates: Process Chemistry and Kinetics", Presented at the 2002 Water Quality Technology Conference, AWWA, November 10-14, 2002, Seattle, Washington.

9. Melin, T, Hasson, D., et al., "Mechanism of Antiscalant Action in Brackish Water Desalination Processes", Presented at "Water Is Life" Conference, December 7-8, 2005, Jerusalem, Israel.

10. a) Bremere L, et al., "Controlling Scaling in Membrane Filtration Systems Using a Desupersaturation Unit", Desalination, 124, (1999), 51-62. b) Bremere I. et al., "Optimizing Dose of Antiscalant in Membrane Filtration Systems Using a Desupersaturation Unit", Water Supply 3(5), (2003), 147-153.

11. Yang Q., Ma Z., Hasson D., Semiat R., "Destruction of Anti-Sealants in RO concentrates by Electrochemical Oxidation", Huangong Xuebau (Chinese Edition), 55(2), (2004), 339-340.

12. Kemmer, Frank N. (editor in chief), "The NALCO Water Handbook", McGraw-Hill Book Company, (1979).