WO2017115378A1

WO2017115378A1 - Method and system for reverse osmosis with high recovery

Info

Publication number: WO2017115378A1
Application number: PCT/IL2016/051404
Authority: WO
Inventors: Ora Kedem
Original assignee: Ora Kedem
Priority date: 2015-12-30
Filing date: 2016-12-29
Publication date: 2017-07-06

Abstract

There is provided herein a desalination system, including a first reverse osmosis (RO) system and a crystallizer comprising a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein a brine outlet of the first RO system is fluidly connected with the first compartment of the crystallizer, wherein the filter comprises calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from the first compartment to the second compartment of the crystallizer, thereby to produce a filtrate of the brine in the second compartment of the crystallizer.

Description

METHOD AND SYSTEM FOR REVERSE OSMOSIS WITH HIGH RECOVERY

TECHNICAL FIELD

The invention relates to systems and processes for desalination of brackish water. BACKGROUND An important factor limiting product recovery in desalination by membrane processes is the formation of precipitates on the membranes. Precipitation of organic salts leads to formation of scale; precipitation and adsorption of organic compounds may create layers on the membrane, encouraging growth of bacteria. These interrelated phenomena, fouling and scaling, may cause partial clogging of the membranes and even of some entrance/exit passages. The appearance of precipitates can be avoided or shifted to higher concentrations of the brine, by pretreatment or by addition of various chemicals, such as acids and anti-scalants.

Brackish feed water from natural sources nearly always contains calcium bicarbonate, often referred to as alkali containing water. Formation of calcium carbonate precipitate depends on the concentration of calcium bicarbonate, the pH of the solution, temperature, and possible presence of nucleating particles. In conventional reverse osmosis (RO) of brackish water, precipitation of calcium carbonate on the membranes can be prevented by eliminating scale with a suitable pretreatment or by acidification of the feed.

Lime (calcium hydroxide) can be added to the alkali containing water, sometimes followed by soda ash (sodium carbonate) in order to precipitate calcium and/or magnesium as carbonate salts, thereby rendering the water "soft". These carbonate salts, often referred to as "lime sludge", are then physically removed. Alternatively, sodium hydroxide may be added as pretreatment. The latter pretreatment requires acceleration of precipitation. Seeding was carried out in pellet reactors [A. Graveland, J. AWWA 1983]. A different method of seeding, by cake filtration was reported [O. Kedem US

5,125,904]. All these processes require addition of chemicals. Other methods, such as the use of nanofiltration to reduce the concentrations of calcium, magnesium, and sulfate [Hassan et al., Desalination 1998 118(1— 3):35— 51]; ion exchange for the reduction of sulfates [De Maio et al., Desalination 1983 45(2): 197-207]; and partial softening combined with nanofiltration (Al-Rawajfeh et al., Heat Transfer Eng 2011 33(3):272-279), have been used on a limited scale.

Precipitation of calcium carbonate by air stripping of carbon dioxide (C0₂) was recently studied and developed as a softening process to be used for volume reduction of RO brine [D. Hasson et al. Desalination 283 (2011) 80; R. Segev et al. Desalination 281(2011) 75]. Removal of CO2 from brine by a stream of air leads to slow precipitation of carbonate and is cumbersome, requiring nonconventional equipment and a series of large vessels.

Both substantial volumes of waste and accumulation of chemicals pose an increasing threat to the environment. There is an urgent need for a simple water treatment process which would lead to high product recovery and thus low waste volume, with minimal need for the addition of chemicals.

Concentration of feed water containing calcium and carbonates by RO would lead to precipitation of calcium carbonate on the membranes. To avoid this, without adding acid as pretreatment, anti-scalants may be added to the feed, inhibiting the precipitation of calcium carbonate. This allows substantial volume reduction of the feed without precipitation of carbonate in the first RO membrane elements. The retentate then includes an alkali-containing brine. The alkali-containing brine can include a mixture of ions including chloride, sulfate, bicarbonate, sodium and calcium. The product recovery achieved by anti-scalants may be smaller than that achieved by acidification, but reduces the addition of chemicals to the environment. Current technologies for zero liquid disposal (ZLD), which include, for example, evaporation and deep well injection, are very expensive. A hybrid with electrodialysis (ED) following RO was suggested and studied in detail [Y. Oren et al., Desalination,2010]. The slow precipitation of calcium sulfate during ED requires careful filtration such as ultrafiltration and very large crystallizers/settlers. These, and possible precipitation of colloidal silicic acid, turn the first RO/ED hybrid into a difficult process.

There is a need for saving costs and increasing recovery of water treatment processes such as RO. In addition, there is a need for a process which avoids or reduces use of chemicals and achieves high recovery in RO desalination.

SUMMARY OF THE INVENTION

The present invention provides, in accordance with some embodiments, desalination systems and methods, which can increase the recovery ratio of brackish water, treated by RO or Nanofiltration (NF) and reduce the volume of wastewater. The present system includes, according to some embodiments, a RO system and a crystallizer. The present system includes, according to additional/alternative embodiments, a NF system and a crystallizer.

Advantageously, in the crystallizer the carbonate containing brine produced by RO may be treated without adding chemicals or with adding a reduced amount of chemicals compared to other methods. In some embodiments, the treated brine exiting the crystallizer may be added to the first RO feed, thereby reducing the volume of waste without adding chemicals. Alternatively, in other embodiments, the treated brine exiting the crystallizer may be further desalinated by a second RO system producing an additional volume of product water and thereby reducing the volume of waste. The treatment of brackish water containing carbonates may produce brine, including a supersaturated solution of carbonates stabilized by anti-scalants.

In some embodiments, the brine is transferred directly to the crystallizer. In other embodiments, the system may include an auxiliary vessel into which the brine is transferred to inactivate any anti-scalant contained in the brine before entering the crystallizer. In other embodiments, the inactivating agent may be added to the brine without a vessel added for this purpose, for example, an inactivating agent may be inserted to the flow between the first RO and the crystallizer. The crystallizer may include a vessel divided into two compartments, a first compartment and a second compartment, separated by a filter, where the supersaturated brine is transferred from the first RO system into the first compartment and passed by applied pressure through the filter into the second compartment. According to some embodiments, the crystallizer includes a vessel. According to further embodiments, the filter is housed within the vessel. According to some embodiments, the vessel is cylindrical. According to some embodiments, the vessel includes a first compartment and a second compartment.

The terms "first compartment" and "second compartment" may refer to the volumes to be filled with the supersaturated solution and the filtrate, respectively. The compartments are separated by one or more filters and may have a variety of shapes.

The filter vessel and the filters therein may serve as crystallizer of calcium carbonate, other carbonates and other species, which may be co-precipitated. The filtering system may contain one or more pumps. Some of the pumps may be pressure pumps. The crystallizer may further include an outlet configured to allow removal of the precipitated calcium carbonate. In some embodiments, the outlet is located in the first compartment of the crystallizer vessel

The filter, according to some embodiments, is configured to separate suspended particles from the brine solution. According to further embodiments, the suspended particles include precipitated calcium carbonate and possibly other co-precipitated species. According to some embodiments, the filter holds a calcium carbonate cake. The filter may be pre-coated by a calcium carbonate cake.

According to some embodiments, the filtering system is configured to allow separation of any pre-coated cake and the collected precipitate and, optionally, other particles from the filter screen. The separation of the particles can be performed by backwashing of the filter or by other processes provided by the configuration of the filtering systems. The particles/cake may be thrown into the first compartment by the backwash. Following the backwash, some of the particles/cake may be used to rebuild a carbonate cake on the filter. Each filter may go through three phases: filtering, backwash and rebuilding of the cake. This cycle is automatically controlled in the filtering system.

According to some embodiments, the filter may consist of two elements: the supporting element and a screen. According to some embodiments, the filter may include at least one porous screen. In some embodiments, a screen is supported on a porous body. In other embodiments, the screen is supported on a suitably formed body allowing flow of solution through the screen and along the supporting body. The supporting element may be made of plastic, metal or various strong nets. The screen may be made of woven nets, nonwoven nets or microfiber. In certain embodiments, the filter vessel contains a plurality of filters, which can carry a carbonate cake, where filtration and/or removal of the cake/particles from each of the screens can be separately controlled. In some embodiments, the phases of filtering and/or removing the cake/particles may alternate between groups of screens in the same vessel. In some embodiments, more than one filter can operate in parallel, and the filters contained in the vessels may be controlled to alternate the phases of operation. This cycle includes re-constructing the cake from the precipitate removed from the filters.

In another embodiment, the vessel contains a plurality of filters and provides a plurality of inlets and outlets, and the filters go successively through the cycle.

In a particular embodiment, the porous screen holds the carbonate cake. In further embodiments, the filter may include a mesh screen. The mesh size of the screen may be in the range of 2-200 micron, for example, between 2-10 micron, 5-50 micron or 10-100 micron.

According to some embodiments, the first compartment includes a supersaturated solution of carbonates. According to further embodiments, the first compartment includes a calcium carbonate suspension. According to further embodiments, the first compartment includes a supersaturated solution of carbonates and a calcium carbonate suspension. According to some embodiments, the second compartment includes a filtrate of the brine solution. The concentration of calcium ions in the filtrate is lower than in the brine solution created by RO. A pH gradient may exist in the crystallizer vessel, between the first compartment and the second compartment.

According to further embodiments, the desalination system includes at least one pump. According to some embodiments, the pump is configured to induce flow of the brine solution from the first RO system to the crystallizer. Optionally, the desalination system can include a plurality of pumps. According to some embodiments, the pumps are pressure pumps. The pressure pump can be a low-pressure pump.

The precipitant can be removed from the first compartment of the crystallizer vessel through an outlet in the first compartment. According to some embodiments, the filter includes at least one porous screen. The porosity of the filter may be in the range of 2 micron up to 200 micron, for example, between 2-10 micron, 5-50 micron or 10-100 micron.

In a particular embodiment, the porous screen holds the carbonate cake. In further embodiments, the porous screen may include a mesh screen. The mesh size of the filter may be in the range of 2-200 micron, for example, between 2-10 micron, 5-50 micron or 10-100 micron. Filters may include different materials and/or different porosities.

According to some embodiments, the operation of a hybrid of ED following RO can be substantially improved by the filter containing cake filtration described herein. In a hybrid RO- filter- ED the filter serves as pretreatment for the ED. According to such embodiments, the treated RO brine will contain much less calcium and bicarbonate and need not be acidified. In the treatment of feed with high silica content, a part of it may be co-precipitated in the filter. Following the general mode of operation of ED it will suffice to acidify the brine by hydrochloric acid to keep pH > 8 (pH higher than 8). In these condition silicic acid will not precipitate. According to some embodiments, there is provided herein a desalination system, including: a first reverse osmosis (RO) system; and a crystallizer including a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein a brine outlet of the first RO system is fluidly connected with the first compartment of the crystallizer, wherein the filter includes calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from the first compartment to the second compartment of the crystallizer, thereby to produce a filtrate of the brine in the second compartment of the crystallizer. The second compartment of the crystallizer may be fluidly connected to a feed of the first RO system.

According to some embodiments, there is provided herein a filtering system for use in conjugation with a reverse osmosis (RO) system, the desalination system including: a crystallizer including a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein the first compartment is configured to receive a brine from an outlet of the first RO system and wherein the filter includes calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from the first compartment to the second compartment, thereby to produce a filtrate of the brine in the second compartment of the crystallizer. The second compartment of the crystallizer may be configured to fluidly connect to a feed of the first RO system. The second compartment of the crystallizer may be configured to fluidly connect with a feed of a second RO system.

According to some embodiments, there is further provided herein a use of a filtering system in conjugation with a reverse osmosis (RO) system, wherein the desalination system includes: a crystallizer including a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein the first compartment is configured to receive a brine and gas from an outlet of the first RO system, and wherein the filter includes calcium carbonate and is configured to allow passage of the brine and gas and to prevent passage of precipitated material from the first compartment to the second compartment. According to some embodiments, there is further provided herein a desalination system including: a crystallizer including a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein the first compartment is configured to receive a brine and wherein the filter includes calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from the first compartment to the second compartment, thereby to produce a filtrate of the brine in the second compartment of the crystallizer; and a reverse osmosis (RO) system (may also be called a second RO system), wherein the second compartment is configured to fluidly connect with a feed of the first RO system. The first compartment may be configured to receive a brine from an outlet of a first RO system.

According to some embodiments, there is further provided herein a desalination system including: a crystallizer including a first compartment, a second compartment and a filter separating between the first and the second compartments, wherein the first compartment is configured to receive a brine and wherein the filter includes calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from the first compartment to the second compartment, thereby to produce a filtrate of the brine in the second compartment of the crystallizer; and an electrodialysis (ED) system, wherein the second compartment is configured to fluidly connect with a feed of the ED system. The first compartment may be configured to receive a brine from an outlet of a first RO system.

According to some embodiments, the filter may further be configured to allow passage of carbon dioxide carried along by the brine.

According to some embodiments, the desalination system may further include a controller configured to interrupt filtration at a predetermined pressure across the filter for constant flow or at predetermined fluid flow for constant pressure in the crystallizer.

According to some embodiments, the desalination system may further include an auxiliary vessel configured to inactivate antiscalant material. The auxiliary vessel may include an inlet for introduction of an agent suitable for antiscalant inactivation. The auxiliary vessel may be configured to receive the brine from the first RO system and to inactivate antiscalant material prior to the brine entering the first compartment of the crystallizer.

According to some embodiments, the desalination system may further include a conduit configured to pass the filtrate of the brine to a feed of the first RO system. According to some embodiments, the filter may include calcium carbonate particles, calcium carbonate cake or both. The filter may be configured to retain calcium carbonate particles, calcium carbonate cake or both. The filter may be configured to separate precipitated particles of calcium carbonate from the brine. The filter may be configured to remove a calcium carbonate cake and/or calcium carbonate particles retained on the filter by a backwash into the first compartment. The filter may include at least one porous screen. The filter may include a plurality of screens. The filter may include a plurality of screens including different materials and/or having different porosities. The porous screen/s may be made of a material including plastic, metal, fabric, microfibers or any combination thereof. The filter may include mesh fabric, woven, non-woven, microfibers and/or screens made of various plastic fibers. The crystallizer may include a vessel and a plurality of filters housed within the vessel. The desalination system may include a plurality of crystallizers. The plurality of crystallizers may be configured to operate in parallel to each other. The plurality of crystallizers, each crystallizer or each group of crystallizers may be separately controllable and may be configured to operate independent of other crystallizers or groups of crystallizers. The crystallizer may include an outlet configured to allow removal of precipitated particles. The crystallizer may be configured to induce precoating of the filter with precipitated calcium carbonate particles/cake prior to passing the brine through the crystallizer and/or following a backwash.

According to some embodiments, the desalination system may further include at least one pump configured to induce flow of the brine through the filter from the first compartment to the second compartment of the crystallizer. The pump may be a pressure pump. According to some embodiments, the desalination system may further include a second RO system. The second compartment of the crystallizer may be in a fluid flow connection with a feed of the second RO system.

According to some embodiments, the desalination system may further include an electrodialysis (ED) system. The second compartment of the crystallizer may be in a fluid flow connection with a feed of the ED system. The permeate/brine of the second RO system may be in a fluid flow connection with a feed of an ED system.

According to some embodiments, there is provided herein a desalination method including: applying a first reverse osmosis (RO) process on a feed of brackish water to produce a product and a brine; and passing the brine to a crystallizer from a first compartment to a second compartment of the crystallizer through a filter including calcium carbonate, separating between the first and the second compartments, wherein the filter allows passage of the brine therethrough and prevents passage of precipitated material from the first compartment to the second compartment of the crystallizer, thereby producing filtrate of the first RO brine in the second compartment of the crystallizer. The method may further include adding the produced filtrate to a feed of the first RO. The filter may further allow passage of carbon dioxide carried along by the brine. The brine in the first compartment of the crystallizer may include a supersaturated solution of calcium carbonate. The first compartment of the crystallizer may include a calcium carbonate suspension. The method may further include inactivating antiscalant material in the brine produced by the first RO prior to passing it through the filter, for example by adding an antiscalant inactivating agent to the flow.

According to some embodiments, the method includes transferring the first RO brine solution to a vessel in which the anti-scalant is deactivated and then is transferred from this vessel into the first compartment of the filter vessel. In yet further embodiments, the method includes transferring the brine directly to the filter vessel.

According to some embodiments, the brine is passed by a pump from the first compartment of the crystallizer vessel to the second compartment of the crystallizer vessel through a filter. According to some embodiments, the method may further include a step of loading the filter with precipitated calcium carbonate. The step of loading the filter may be performed prior to passing the fluid through the desalination system. Optionally, the step of loading the filter is performed prior to passing the fluid through the crystallizer. In other embodiments, the precipitated calcium carbonate may spontaneously be formed on and/or in the filter in the course of passing the fluid through the desalination system, and particularly through the crystallizer.

In some embodiments, the method may further include a step of backwashing the filter. The backwashing may be performed by exerting pressure on the filter in a direction opposite to the flow of the brine solution through the filter. The pressure may be exerted by fluid, compressed gas or a combination thereof. In some embodiments, the pressure may be exerted by at least one pump. In further embodiments, the method may include a step of removing the precipitated calcium carbonate from the crystallizer by other methods, such as centrifugation. According to some embodiments, the filter may include a calcium carbonate cake prior to passing therethrough the brine produced by the first RO. According to additional/alternative embodiments, the filter may retain a calcium carbonate cake during the passing of the brine produced by the first RO therethrough. According to some embodiments, the filter separates precipitated particles of calcium carbonate from the brine. According to some embodiments, the method further includes removing a calcium carbonate cake and/or calcium carbonate particles retained on or in the filter by applying a backwash into the first compartment.

According to some embodiments, the method includes passing the brine to a plurality of filters and/or a plurality of crystallizers. According to some embodiments, the method may further include operating the plurality of filters and/or the plurality of crystallizers in parallel to each other. According to some embodiments, the method may further include operating the plurality of filters and/or the plurality of crystallizers independently.

According to some embodiments, the method may further include removing precipitated particles from the first compartment of the crystallizers.

According to some embodiments, the method may further include precoating the filter with precipitated calcium carbonate particles/cake prior to passing the brine through the crystallizer and/or following a backwash. According to some embodiments, the method may further include passing the filtrate of the first RO brine through a second RO process.

According to some embodiments, the method may further include passing the filtrate of the first RO brine through an ED process. According to some embodiments, the method may further include passing the brine produced in the second RO process through an ED process.

BRIEF DESCRIPTION OF THE FIGURES

Examples illustrative of embodiments of the invention are described below with reference to figures attached hereto. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below.

Figure 1 schematically illustrates a desalination system, according to some embodiments;

Figure 2 schematically illustrates a desalination system, according other embodiments; and

Figure 3 schematically illustrates a desalination system, according to other embodiments. DESCRIPTION OF THE INVENTION

The present invention provides, according to some embodiments, a desalination system and a method for softening the brine, treating it and increasing the recovery of the desalination system. The desalination system, according to the principles of some embodiments, includes an RO system and a crystallizer, configured to precipitate carbonate, such as calcium carbonate and/or other precipitate or co-precipitate with the carbonate (e.g., calcium carbonate) from the brine produced by the first RO. According to some embodiments the crystallizer includes a filter system suitable for particle filtration, wherein the filter may carry a particle cake, for example, consisting largely of calcium carbonate. According to some embodiments the filtering system provides backwash and removal of the particles. According to some embodiments, the filtration and/or the backwash are automatically controlled. The cycles may be controlled to retain some particles removed from a filter and return these to a backwashed filter forming a new calcium carbonate cake. The filtrate from the crystallizer may be returned to the feed or alternatively treated by second RO. Without wishing to be bound by theory or mechanism of action, high concentration of bicarbonate created by the volume reduction in RO leads to carbonate which, in the presence of calcium ions, yields calcium carbonate precipitate, which results in scaling of the membranes. Bicarbonate ions are in equilibrium with both carbonate and with carbonic acid. The two simultaneous equilibria are: (Equilibrium a) Ca(HC0₃)2 CaC0 +C0₂ +H₂0

(Equilibrium b) HC0 + H+ H₂C0 H₂0 + C0₂

High concentration of bicarbonate and/or calcium lead to super saturation and eventually to precipitation of calcium carbonate.

One of the currently available solutions to the phenomenon of membrane scaling due to calcium carbonate precipitation is the addition of acid. When acid is added to the recycling brine, the concentration of carbon dioxide (C0₂) is increased (equilibrium b), pushing the first equilibrium backwards, thus decreasing the formation of carbonate. The addition of sulfuric acid leads to formation of calcium sulfate. Though the calcium sulfate is much more soluble than the calcium carbonate, it will precipitate at high volume reduction and thus limit the recovery.

Alternatively dosing of antiscalants allows the formation of supersaturated solutions in the first RO brine. Though the recovery achieved by antiscalants for calcium carbonate may be smaller than that achieved by addition of acid, the former may be preferable for both economic and environmental operations.

Without wishing to be bound by theory or mechanism of action, precipitation takes place on and in carbonate cake when solutions are supersaturated. A major difficulty in water softening is the slow precipitation of CaC0₃ from its supersaturated solutions. According to some embodiments, passage of the supersaturated solution through the calcium carbonate cake may provide effective seeding and precipitation. In other words, the passage of CaC0₃ through a cake may relieve supersaturation in a fast process. The new crystals formed in and on the cake may be calcite and/or aragonite or vaterite, Fast precipitation may lead to small footprint.

The desalination process (method) disclosed herein, in accordance with some embodiments, provides water softening without adding chemicals (such as acids, NaOH etc.), which are both expensive and environmentally (and hence regulatorily) problematic. Without wishing to be bound by theory or mechanism of action, contrast to, for example, NaOH softening with particle filtration, the supersaturation according to some embodiments of the present invention, is created by increased concentration produced by making use of the first RO process itself and not by addition of chemicals. This is possible by making use of the high concentration of both Ca and HC0₃ created by the first RO process. Thus the process disclosed herein, in accordance with some embodiments, recovers some of the energy of RO. More energy recovery may be possible by using the pressure of the first RO brine in order to induce flow through the filter. According to some embodiments, this pressure may even be used instead of a pump.

Without wishing to be bound by theory or mechanism of action, in terms of the Langeley approximation for the saturation index of CaC0₃: S = pH -pHs where pHs is the equilibrium pH where CaC03 neither dissolves nor precipitates. According to some embodiments, instead of raising the pH by added alkali (such as NaOH) the supersaturation is obtained by lower pHs, decreased by higher concentration

In addition, natural water often contains silica. In acid pH, colloidal silicic acid precipitates and causes difficulty in membrane processes such as ED as well as RO, for example, due to clogging the membranes and entrance/exit ports. Advantageously, and in accordance with some embodiments, in the present processes and systems using the filter, for precipitation of carbonate, acidification is not required. Colloid silica will not be formed and the silicate is partly removed by the filter. In accordance with some embodiments of the present processes and systems, the first RO feed is not acidified and the silica, as sodium salt, does not precipitate. Further, it was shown that in the filtration/cake precipitation of CaC03, a part of the silicate is co-precipitated, therefore, advantageously, in accordance of some embodiments, the brine of the first RO treated by the crystallizer and serving as feed for the second RO, contains less silica than it would contain iftreated otherwise. In some embodiments, the supersaturated brine containing the antiscalant may be passed through a filter (for example, filter carrying calcium carbonate cake). In another embodiment, the brine may be passed first through an intermediate vessel where the antiscalant is inactivated. In these embodiments supersaturation can be relieved and calcium carbonate precipitated on or in the cake. The newly created crystals may have calcite or aragonite morphology. Some calcium sulfate and/or any other precipitates may co-precipitate with the carbonate. A controlled fraction of the carbonate precipitate dispersed by the backwash solution may be returned to the next brine solution to create a new calcium carbonate cake. Depending on the composition of the filtrate and the parameters of the first RO process, the filtrate may be recycled to the feed of the first RO, treated by a second RO and/or further treated by an electrodialysis (ED) system. As indicated above, with precipitation of calcium carbon dioxide (CO2) is evolved.

Ca(HC0₃)₂ —► CaC0₃ + CO2 + H₂0

Without wishing to be bound by theory or mechanism of action, the efficacy of relieve of supersaturation by filtering the solution through a carbonate cake appears to be due to the intimate contact between solution and crystals created passage through the filter. Further, at least part of the CO2 may be carried away by the filtered solution. The fraction of CO2 carried away by the flow of the brine solution will not interfere with further precipitation of calcium carbonate from the solution in the first compartment. This means that the composition of the brine will be less effected by the precipitation on the cake than in conventional seeding, while the filtrate will contain less calcium and more CO2. These changes may enable in some cases the return of the filtrate to the feed. Another factor enhancing the effectivity of the filtering through a calcium carbonate cake may the adsorption of some antiscalants by the precipitated crystals. According to some embodiments, precipitation of calcium carbonate in the crystallizer is enhanced by the filter housed in the crystallizer vessel. The filters useful in the desalination systems and methods in accordance with some embodiments, should be capable of separating precipitated particles from the brine solution, and preventing the flow of particles through the filter. The filters should further be capable of allowing periodical removal of the particles from the filter. The removal of the particles may be performed by backwashing of the filter. A wide variety of filter materials and configurations may be used in the filters, according to the principles of embodiments of the present invention. Non- limiting examples of materials suitable for use in the filter include plastic, metal, and/or fabric. Various filters for filtration of particles from water and easy backwash may be used in the desalination systems of embodiments of the present invention. Industrial filters for particle filtration and cake filtration are available in a variety of designs. Some are constructed in cylindrical vessels, such as the filters produced e.g. by the Adams company, and can be used as crystallizer vessels having filters housed therewith. In another commercially available configuration, a plurality of mesh filters ("Cassettes") are contained in a single vessel, for example, as produced by the Amiad company.

In the desalination system, in accordance with some embodiments, the filter carries a calcium carbonate cake. Thus, the filter should further be capable of accommodating the calcium carbonate cake, while allowing flow of the brine solution through the filter. In some embodiments, the filter is pre-coated by the calcium carbonate cake prior to passing the fluid through the crystallizer. In other embodiments, the calcium carbonate cake is formed by spontaneous precipitation of calcium carbonate particles when the brine is passed through the crystallizer. According to some embodiments, the method of the present invention includes returning the treated brine solution, which passed through the crystallizer, to the first RO feed. In the course of calcium carbonate precipitation reaction, carbon dioxide (CO2) evolves. The evolved carbon dioxide (CO2) can be at least partially dissolved in the brine filtrate. Since the brine filtrate may be returned to the feed, or serve as feed of second RO, at least a portion of the evolved carbon dioxide (CO2) can be passed into the first RO system. This changed composition of the brine solution, which passes through the crystallizer, may help to suppress scale formation in RO, whether added to the feed or separately processed.

It is also known that ED works at high concentrations and yields high recovery. It may thus serve for concentration of RO brine up to 97% recovery. This is close to the target of no liquid disposal (ZLD). Hybrid RO-ED for ZLD was already studied, and the first RO brine was acidified by sulfuric acid, either for the first RO itself or only for the ED. The slow precipitation of the CaS04 requires very large settlers and careful filtering, even ultrafiltration. There is thus provided herein, according to some embodiments, a use of hybrid systems and methods incorporating a use of electrodialysis (ED), for example, RO- crystallizer-ED or ROl- crystallizer -R02-ED, wherein ROl represents a first RO and R02 represents a second RO. In other words, according to some embodiments, there is provided a system and method for treating brackish water with RO, the brine thereof is then filtered using the crystallizer, the filtrate is then treated by ED. According to other embodiments, there is provided a system and method for treating brackish water with a first RO, the brine thereof is then filtered using the crystallizer, the filtrate is then treated by a second RO and the brine produced by the second RO is then treated by ED. With the hybrid systems and methods provided herein, according to some embodiments, the permeation of ED may be possible with less equipment and smaller footprint.

The improved RO (RO followed by specified cake filtration, which is also termed a crystallizer) and hybrid methods and systems provided herein, according to some embodiments, substantially increases the recovery from groundwater, tap water or any feed solution containing carbonate.

Without wishing to be bound by theory or mechanism of action, according to embodiments of the invention, the potential savings for the user by the crystallizer (filter) per 100 m³/d of pretreated feed are as follows:

In the system including a first RO followed by filtration and followed by a second RO: the first RO system is represented by Ri and the second system following filtration is represented by R₂. Accordingly, there are two recovery ratios: n and r₂.

The input to R₂ is the brine of Ri. The overall recovery, r, is given by:

r₂ includes any loss of brine from Ri with the backwashed solid.

The saving by the process is calculated as r-ro where ro is taken as 80%, as might be obtained by acidification and not actual n which may be lower.

Possible recovery to be obtained with the filter (crystallizer): assuming n is 75%. If in the filtrate feed quality (concentration of scaling elements) was not achieved, smaller recovery can be assumed, for example 50%- 60%.

If r₂=50%. Then: r = 0.75+0.125 = 0.875 if r₂= 60%. Then: r = 0.75 + 0.15 = 0.90 The overall recovery of a system, including a first RO followed by filtration and followed by a second RO, according to some embodiments, may thus be at least between 0.80-0.95, for example, between 0.875-0.90.

The overall recovery of a system, including a first RO followed by filtration and followed by a ED, according to some embodiments, may thus be at least between 0.90- 0.99, for example, between 0.97-0.99.

In some embodiments, the method of the present invention does not require pre- coating of the filter by the calcium carbonate cake. If the brine without antiscalants contains a suspension of carbonate, the new cake may be created by spontaneous precipitation on the filter during the operation of the crystallizer. The newly created crystals may have calcite or aragonite morphology.

Reference is made to Figure 1, which schematically illustrates a desalination system, in accordance with some embodiments. Desalination system 100 includes a first RO system 11 and a crystallizer 44. first RO system 11, includes a feed compartment la and a product compartment lb. first RO system 11 further includes a feed stream inlet 1 and a product stream outlet 2. A conduit 3 serves as brine outlet and enables fluid flow (optionally by a pump 13) between first RO system 11 and crystallizer 44. Crystallizer 44 includes a filter 5 optionally carrying a carbonate cake (not shown), a first compartment 4a, a second compartment 4b, optionally a layer 6a created periodically by backwash of filter 5 and an outlet 6b. Filter 5 may be pre-coated with calcium carbonate prior to passing the brine solution through crystallizer 44. Alternatively, calcium carbonate cake can be formed on filter 5 while passing the brine solution through crystallizer 44, if a suspension of carbonate is present in the brine solution. Filter 5 is configured to allow passage of brine solution and gas, such as carbon dioxide gas (CO2) and to prevent passage of precipitates such as precipitated calcium carbonate and any co-precipitates that may be formed therewith. Filter 5 may schematically represent a plurality of filter units (not shown). Newly-precipitated calcium carbonate is collected on the carbonate cake of filter 5. Filter 5 divides crystallizer 44 vessel into first compartment 4a (as mentioned hereinabove), also termed "precipitant compartment" and second compartment 4b (as mentioned hereinabove) also termed "filtrate compartment".

The brine solution formed in first RO system 11 can enter first compartment (precipitant compartment) 4a through conduit 3 (optionally by pump 13) and pass through filter 5 carrying carbonate cake. Calcium carbonate may be deposited in and on the cake and/or in filter 5. The newly formed crystals of calcium carbonate may have calcite or aragonite morphology. Precipitant first compartment 4a contains supersaturated solution of calcium carbonate and may further include a suspension of calcium carbonate. First compartment (precipitant compartment) 4a can include the preformed carbonate cake and/or calcium carbonate, which precipitates on or in filter 5 during the cycling of the brine solution. First compartment (precipitant compartment) 4a further includes outlet 6b, which allows discarding of the calcium carbonate removed from filter 5, for example, by backwash (represented by arrow 10a). The filtrate passing through filter 5 (as represented by arrow 10b) leaving compartment 4b may be added to the first RO feed through a conduit 7.

At least a fraction of carbon dioxide (C0₂) liberated during precipitation of calcium carbonate on the carbonate cake is passed into the filtrate and added to feed 1 through conduit 7.

Reference is now made to Figure 2, which schematically illustrates a desalination system, in accordance with some embodiments. In figure 2 desalination system 200 includes a first RO system 11 ', an auxiliary vessel 33, a crystallizer 442 and a second RO system 22. first RO system 11 ', includes a feed compartment la' and a product compartment lb', first RO system 1 Γ further includes a feed stream inlet and a product stream outlet 2'. The retentate, also termed brine, is transferred through a conduit 3a into auxiliary vessel 33, where antiscalant is inactivated by a suitable agent added through inlet 3b. From vessel 33 the brine is passed through conduit 3' and optionally by a pump 13' into crystallizer 442. Crystallizer 442 includes a filter 5' optionally carrying a carbonate cake (not shown), a first compartment 4a', a second compartment 4b', optionally a layer 6a' created periodically by backwash of filter 5' and an outlet indicated by 6b'. Filter 5' may be pre-coated with calcium carbonate prior to passing the brine solution through crystalhzer 442. Alternatively, calcium carbonate cake can be formed, while passing the brine solution through crystalhzer 442, if a suspension of carbonate is present in the crystalhzer. Filter 5' is configured to allow passage of brine solution and gas, such as carbon dioxide gas (C0₂) and to prevent passage of precipitated calcium carbonate. Filter 5' can schematically represent a plurality of filter units (not shown). Newly-precipitated calcium carbonate is collected on the carbonate cake of filter 5'. Filter 5' divides crystalhzer 442 vessel into first compartment 4a' (as mentioned hereinabove), also termed "precipitant compartment", and second compartment 4b (as mentioned hereinabove) also termed "filtrate compartment".

The brine solution enters precipitant compartment 4a' and passes through filter 5 ' carrying the carbonate cake, and calcium carbonate is deposited on the cake. The newly formed crystals of calcium carbonate may have calcite or aragonite morphology. First (precipitant) compartment 4a' contains supersaturated solution of calcium carbonate and may further include a suspension of calcium carbonate. First (precipitant) compartment 4a' can include the preformed carbonate cake and/or calcium carbonate, which precipitates on filter 5' during the cycling of the brine solution. First (precipitant) compartment 4a' further includes outlet 6b', which allows removal of the calcium carbonate cake from filter 5' and possibly sedimented (6a'), by, for example, backwash (represented by an arrow 10a). The filtrate passing through filter 5' (as represented by an arrow 10b') leaving second compartment 4b' is transferred to a compartment 2a of second RO system 22 through a conduit 8.

At least a fraction of carbon dioxide (CO2) liberated during precipitation of calcium carbonate on the carbonate cake is passed into the filtrate and added to the feed of second RO system 22 producing a product 221 (through compartment 2b) and brine 222.

According to some embodiments, crystalhzer 442, second RO system 22 and optionally any conduits or passages therebetween, and optionally a controller, may be considered as a separate unit. Reference is made to Figure 3, which schematically illustrates a desalination system, in accordance with some embodiments. Desalination system 300 includes a first RO system 11", a crystallizer 443 and an electrodialysis (ED) system 55. first RO system 11", includes a feed compartment la" and a product compartment lb", first RO system 11" further includes a feed stream inlet 1 " and a product stream outlet 2". A conduit 3" serves as brine outlet and enables fluid flow (optionally by a pump 13") between first RO system 11" and crystallizer 443. Crystallizer 443 includes a filter 5" optionally carrying a carbonate cake (not shown), a first compartment 4a", a second compartment 4b", optionally a layer 6a" created periodically by backwash of filter 5", and an outlet 6b". Filter 5" may be pre-coated with calcium carbonate prior to passing the brine solution through crystallizer 443. Alternatively, calcium carbonate cake can be formed on filter 5" while passing the brine solution through crystallizer 443, if a suspension of carbonate is present in the brine solution. Filter 5" is configured to allow passage of brine solution and gas, such as carbon dioxide gas (C0₂) and to prevent passage of precipitates such as precipitated calcium carbonate and any co-precipitates that may be formed therewith. Filter 5 may schematically represent a plurality of filter units (not shown). Newly-precipitated calcium carbonate is collected on the carbonate cake of filter 5". Filter 5" divides crystallizer 443 vessel into first compartment 4a' ' (as mentioned hereinabove), also termed "precipitant compartment" and second compartment 4b" (as mentioned hereinabove) also termed "filtrate compartment".

The brine solution formed in first RO system 11" can enter first compartment (precipitant compartment) 4a" through conduit 3" (optionally by pump 13") and pass through filter 5" carrying carbonate cake. Calcium carbonate can be deposited in and on the cake and/or in filter 5". The newly formed crystals of calcium carbonate may have calcite or aragonite morphology. Precipitant first compartment 4a' ' contains supersaturated solution of calcium carbonate and may further include a suspension of calcium carbonate. First compartment (precipitant compartment) 4a" can include the preformed carbonate cake and/or calcium carbonate, which precipitates on or in filter 5" during the cyclic phases "filtration/backwash/rebuild-of-cake" automatically controlled in the filtration system by a controller (not shown). First compartment (precipitant compartment) 4a" further includes outlet 6b", which allows discarding of the calcium carbonate removed from filter 5", for example, by backwash. The filtrate passing through filter 5" leaving compartment 4b" may be added to a feed 55a of ED system 55 through a conduit 9 for further treatment. The product (diluent) of ED system 55 (which may be between about 95%-98%, for example about 97%, of the volume entered to ED system 55) may be transferred to feed 1 " of first RO 11" by a conduit 65. An outlet 60 can allow exit of waste material.

According to some embodiments, crystallizer 443, ED system 55, optionally any conduits or passages therebetween, and optionally a controller, may be considered as a separate unit.

It is noted that each of crystallizer 44, crystallizer 442 and crystallizer 443 represents a separate embodiment, with or without a controller unit, with or without a pump, with or without any conduits leading thereto or therefrom.

It is noted that crystallizers 44/442/443 and auxiliary vessel 33 represent together a separate embodiment, with or without a controller unit, with or without a pump, with or without any conduits leading thereto or therefrom.

The terms "carbon dioxide", "C0₂" and carbon dioxide gas are interchangeably used.

The exemplary modes of operation described are not intended to be limiting in any form or manner. It may be evident to a person skilled in the art that other modes of operation are possible, which may include variations in the steps performed including the sequence in which they are performed.

According to some embodiments, the first RO may be replaced with

Nanofiltration (NF). In the description and claims of embodiments of the present invention, each of the words, "comprise" "include" and "have", and forms thereof, are not necessarily limited to members in a list with which the words may be associated. The invention has been described using various detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments may comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described and embodiments of the invention including different combinations of features noted in the described embodiments will occur to persons with skill in the art.

Claims

CLAIMS What we claim is:

1. A desalination system, comprising: a first reverse osmosis (RO) system; and a crystallizer comprising a first compartment, a second compartment and a filter separating between said first and said second compartments, wherein a brine outlet of said RO system is fluidly connected with said first compartment of said crystallizer, wherein said filter comprises calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from said first compartment to said second compartment of said crystallizer, thereby to produce a filtrate of the brine in the second compartment of the crystallizer.

2. The desalination system of claim 1, wherein said second compartment of said crystallizer is fluidly connected to a feed of said first RO system.

3. The desalination system of claim 1, further comprising a second RO system.

4. The desalination system of claim 3, wherein said second compartment of said crystallizer is in a fluid flow connection with a feed of said second RO system.

5. The desalination system of claim 1, wherein said filter is further configured to allow passage of carbon dioxide carried along by the brine.

6. The desalination system of claim 1, further comprising a controller configured to interrupt filtration at a predetermined pressure across the filter for constant flow or at predetermined fluid flow for constant pressure in said crystallizer.

7. The desalination system of claim 1, further comprising an auxiliary vessel configured to inactivate antiscalant material.

8. The desalination system of claim 7, wherein said auxiliary vessel comprises an inlet for introduction of an agent suitable for antiscalant inactivation.

9. The desalination system of claim 7, wherein said auxiliary vessel is configured to receive the brine from said RO system and to inactivate antiscalant material prior to the brine entering said first compartment of said crystallizer.

10. The desalination system of claim 1, further comprising a conduit configured to pass the filtrate of the brine to a feed of the first RO system.

11. The desalination system of claim 1 , wherein said filter comprises calcium carbonate particles, calcium carbonate cake or both.

12. The desalination system of claim 1, wherein said filter is configured to retain calcium carbonate particles, calcium carbonate cake or both.

13. The desalination system of claim 1, wherein said filter is configured to separate precipitated particles of calcium carbonate from the brine.

14. The desalination system of claim 1, wherein said filter is configured to remove a calcium carbonate cake and/or calcium carbonate particles retained on or in said filter by a backwash into said first compartment.

15. The desalination system of claim 1, wherein said filter comprises at least one porous screen.

16. The desalination system of claim 1, wherein said filter comprises a plurality of screens.

17. The desalination system of claim 1, wherein said filter comprises a plurality of screens comprising different materials and/or having different porosities.

18. The desalination system of claim 15, wherein said porous screen is made of a material comprising plastic, metal, fabric, microfibers or any combination thereof.

19. The desalination system of claim 1, wherein said filter comprises mesh fabric, woven, non-woven, microfibers and/or screens made of various plastic fibers.

20. The desalination system of claim 1, wherein said crystallizer comprises a vessel and a plurality of filters housed within said vessel.

21. The desalination system of claim 1, comprising a plurality of crystallizers.

22. The desalination system of claim 1, comprising a plurality of crystallizers configured to operate in parallel to each other.

23. The desalination system of claim 1, comprising a plurality of crystallizers, each crystallizer or each group of crystallizers are separately controllable and are configured to operate independent of other crystallizers or groups of crystallizers.

24. The desalination system of claim 1, wherein said crystallizer comprises an outlet configured to allow removal of precipitated particles.

25. The desalination system of claim 1, wherein said crystallizer is configured to induce precoating of said filter with precipitated calcium carbonate particles/cake prior to passing the brine through the crystallizer and/or following a backwash.

26. The desalination system of claim 1, further comprising at least one pump configured to induce flow of the brine through said filter from said first compartment to said second compartment of said crystallizer.

27. The desalination system of claim 26, wherein said pump is a pressure pump.

28. The desalination system of claim 1, further comprising an electrodialysis (ED) system.

29. The desalination system of claim 28, wherein said second compartment of said crystallizer is in a fluid flow connection with a feed of said ED system.

30. The desalination system of claim 3, further comprising an electrodialysis (ED) system.

31. The desalination system of claim 29, wherein a permeate of said second RO system is in a fluid flow connection with a feed of said ED system.

32. A filtering system for use in conjugation with a first reverse osmosis (RO) system, the desalination system comprising: a crystallizer comprising a first compartment, a second compartment and a filter separating between said first and said second compartments, wherein said first compartment is configured to receive a brine from an outlet of the first RO system and wherein said filter comprises calcium carbonate and is configured to allow passage of the brine and to prevent passage of precipitated material from said first compartment to said second compartment, thereby to produce a filtrate of the brine in the second compartment of the crystallizer.

33. The filtering system of claim 32, wherein said second compartment of said crystallizer is configured to fluidly connect to a feed of the first RO system.

34. The filtering system of claim 32, wherein said second compartment of said crystallizer is configured to fluidly connect with a feed of a second RO system.

35. The filtering system of claim 32, wherein said second compartment of said crystallizer is configured to fluidly connect with a feed of an electrodialysis (ED) system.

36. The filtering system of claim 34, wherein a permeate of said second RO system is configured to fluidly connect with a feed of an ED system.

37. A use of a filtering system in conjugation with a first reverse osmosis (RO) system, wherein the desalination system comprising: a crystallizer comprising a first compartment, a second compartment and a filter separating between said first and said second compartments, wherein said first compartment is configured to receive a brine and gas from an outlet of the first RO system and wherein said filter comprises calcium carbonate and is configured to allow passage of the brine and gas and to prevent passage of precipitated material from said first compartment to said second compartment.

38. A desalination method comprising: applying a first reverse osmosis (RO) process on a feed of brackish water to produce a product and a brine; and passing the brine to a crystallizer from a first compartment to a second compartment of the crystallizer through a filter comprising calcium carbonate, separating between the first and the second compartments, wherein the filter allows passage of the brine therethrough and prevents passage of precipitated material from the first compartment to the second compartment of the crystallizer, thereby producing filtrate of the first RO brine in the second compartment of the crystallizer.

39. The method of claim 38, further comprising adding the produced filtrate to a feed of the first RO.

40. The method of claim 38, further comprising passing the filtrate of the first RO brine through a second RO process.

41. The method of claim 38, wherein the filter further allows passage of carbon dioxide carried along by the brine.

42. The method of claim 38, wherein the brine in the first compartment of the crystallizer comprises a supersaturated solution of calcium carbonate.

43. The method of claim 38, wherein the first compartment of the crystallizer comprises a calcium carbonate suspension.

44. The method of claim 38, further comprising inactivating antiscalant material in the brine produced by the first RO prior to passing it through the filter.

45. The method of claim 38, wherein the filter comprises a calcium carbonate cake prior to passing therethrough the brine produced by the first RO.

46. The method of claim 38, wherein the filter retains a calcium carbonate cake during the passing of the brine produced by the first RO therethrough.

47. The method of claim 38, wherein the filter is separating precipitated particles of calcium carbonate from the brine.

48. The method of claim 38, further comprising removing a calcium carbonate cake and/or calcium carbonate particles retained on or in the filter by applying a backwash into the first compartment.

49. The method of claim 38, passing the brine to a plurality of filters and/or a plurality of crystallizers.

50. The method of claim 38, further comprising operating the plurality of filters and/or the plurality of crystallizers in parallel to each other.

51. The method of claim 38, further comprising operating the plurality of filters and/or the plurality of crystallizers independently.

52. The method of claim 38, further comprising removing precipitated particles from the first compartment of the crystallizers.

53. The method of claim 38, further comprising precoating the filter with precipitated calcium carbonate particles/cake prior to passing the brine through the crystallizer and/or following a backwash.

54. The method of claim 38, further comprising passing the filtrate of the first RO brine through an ED process.

55. The method of claim 40, further comprising passing the brine produced in the second RO process through an ED process.