Abstract:
A desalination system is provided. The desalination system comprises a desalination apparatus. The desalination apparatus comprises first and second electrodes, and a first group of paired ion exchange membranes disposed between the first and second electrodes to form a first group of alternating first and second channels. The first channels are configured to receive a first stream for desalination and the second channels are configured to receive a second stream to carry away ions removed from the first stream, respectively. The desalination apparatus further comprises a plurality of spacers disposed between each pair of the adjacent ion exchange membranes and between the first and second electrodes and the respective ion exchange membranes. Wherein each of the ion exchange membranes in the first group is a cation exchange membrane. A desalination system and a method for removing ions from an aqueous stream area also presented.

Description:
BACKGROUND OF THE DISCLOSURE 
     The invention relates generally to desalination systems and methods for pure water production. More particularly, this invention relates to desalination systems and methods using ion exchange membranes for removal of hardness ions for high purity water production. 
     Water with high purity are widely used in in many industrial processes, such as in high-pressure boilers in power plants, production of semiconductor elements, production of printed circuit boards, production of integrated circuit, and production of injection water for medical purpose. 
     Typically, due to continuous operation, and stable and relatively higher quality of product water, electrodeionization (EDI) apparatuses have been employed for processing such liquid steams, for example for production of pure water. Generally, the EDI apparatuses use conventional electrodialysis systems with ion exchange resin filled therein to process the liquid streams. However, during operation, because polyvalent cations with higher concentration may cause scaling tendency, the EDI apparatuses have a hardness tolerance for the liquid streams to be processed. For example, the hardness tolerance of the EDI apparatuses is about less than 1 ppm, which may need rigorous pretreatment of the liquid streams to decrease the hardness therein before the liquid streams are introduced into the EDI apparatus. 
     There have been attempts to pretreat the liquid streams to decrease the hardness therein. For example, reversal osmosis (RO) apparatuses are employed. However, the processing efficiency of the reversal osmosis apparatuses may be relatively lower for decreasing the hardness in the liquid streams to a certain level suitable for the EDI apparatuses. As a result, the processing cost may be increased accordingly. 
     Therefore, there is a need for new and improved desalination system and method for removal of hardness ions for high purity water production. 
     BRIEF DESCRIPTION OF THE DISCLOSURE 
     A desalination system is provided in accordance with one embodiment of the invention. The desalination system comprises a desalination apparatus. The desalination apparatus comprises first and second electrodes, and a first group of paired ion exchange membranes disposed between the first and second electrodes to form a first group of alternating first and second channels. The first channels are configured to receive a first stream for desalination and the second channels are configured to receive a second stream to carry away ions removed from the first stream, respectively. The desalination apparatus further comprises a plurality of spacers disposed between each pair of the adjacent ion exchange membranes and between the first and second electrodes and the respective ion exchange membranes. Wherein each of the ion exchange membranes in the first group is a cation exchange membrane. 
     A desalination system is provided in accordance with another embodiment of the invention. The desalination system comprises a desalination apparatus configured to remove hardness ions from an aqueous stream. The desalination apparatus comprises first and second electrodes, and a first group of paired ion exchange membranes disposed between the first and second electrodes to form a first group of alternating first and second channels to receive a first feed stream for removal of the hardness ions and a second feed stream to carry away the hardness ions removed from the first feed stream respectively. The desalination apparatus further comprises a second group of the paired ion exchange membranes disposed between the first group of the ion exchange membranes and at least one of the first and second electrodes to form a second group of the alternating first and second channels, and a plurality of spacers disposed between each pair of the adjacent ion exchange membranes and between the first and second electrodes and the respective ion exchange membranes. Wherein each of the ion exchange membranes in the first group of the paired ion exchange membranes is a cation exchange membrane, and wherein the second group of the paired ion exchange membranes comprises a plurality of alternating cation and anion exchange membranes. 
     Embodiment of the invention further provides a method for removing ions from an aqueous stream. The method comprises passing a first feed stream through first channels of a first group of alternating first and second channels defined by a first group of paired cation exchange membranes of a desalination apparatus for removing ions to produce a first output stream, and passing a second feed stream through the second channels of the first group of the alternating first and second channels defined by the first group of the paired cation exchange membranes of the desalination apparatus to carry away ions removed from the first feed stream. 
     These and other advantages and features will be better understood from the following detailed description of embodiments of the invention that is provided in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a desalination system in accordance with one embodiment of the invention; 
         FIG. 2  is schematic diagram of a desalination apparatus in accordance with one embodiment of the invention; 
         FIG. 3  is an experimental graph illustrating hardness ion removal efficiency of the desalination apparatus in accordance with one embodiment of the invention; and 
         FIG. 4  is a schematic diagram of the desalination system in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the disclosure in unnecessary detail. 
       FIG. 1  is a schematic diagram of a desalination system  10  in accordance with one embodiment of the invention. As illustrated in  FIG. 1 , the desalination system  10  comprises a desalination apparatus  11  and an electrodeionization (EDI) apparatus  12  in fluid communication with the desalination apparatus  11 . 
     In some embodiments, the desalination apparatus  11  is configured to receive a first feed stream  13  having salts or other impurities from a first liquid source (not shown) for desalination and to receive a second feed stream  14  from a second liquid source (not shown) during or after desalination of the first feed stream  13  so as to carry charged species or target ions removed from the first feed stream  13  out of the desalination apparatus  11 . 
     In non-limiting examples, the salts in the first feed stream  13  may include charged ions, such as sodium (Na + ), chlorine (Cl − ), hardness ions including magnesium (Mg 2+ ) and/or calcium (Ca 2+ ), and/or other ions. In one example, the charged ions in the first stream  13  at least include target ions, such as a portion of the hardness ions. 
     Thus, due to desalination of the desalination apparatus  11 , at least a portion of the charged or target ions including the hardness ions in the first feed stream  13  may be removed. As a result, a first output stream  15  is produced and then may be introduced into the EDI apparatus  12  for further processing, which may be a dilute liquid and have a lower concentration of the charged species, such as the hardness ions as compared to the first feed stream  13 . Meanwhile, a second output stream (a concentrated stream)  16  is also produced, which has a higher concentration of charged species, such as the hardness ions than the second feed stream  14 . 
     In some applications, the first output stream  15  may be circulated into the desalination apparatus  11  or introduced into any other suitable desalination apparatuses, such as reversal osmosis apparatuses for further processing to remove the charged ions therein before introduced into the EDI apparatus  12 . In certain applications, the EDI apparatus may or may not be employed based on different applications. 
     As used herein, the term “EDI” means an electrochemical purification process using ion exchange membranes and ion exchange resin to remove target ions or charged species from water or other fluids so as to produce higher quality water, for example, pure or ultrapure water. 
     In non-limiting examples, the EDI apparatus  12  comprises a pair of electrodes configured to act as an anode and a cathode, respectively. A plurality of alternating anion and cation exchange membranes are disposed between the anode and the cathode to form a plurality of alternating first and second channels therebetween, which are also referred to as dilute and concentrate channels under operating conditions. A plurality of spacers are disposed between each pair of the membranes, and between the electrodes and the respective adjacent membranes, which may be similar to the structure of an electrodialysis (ED) apparatus. Similarly, the anion exchange membrane(s) are configured to be passable for anions. The cation exchange membrane(s) are configured to be passable for cations. 
     In addition, based on different applications, the ion exchange resin may be filled into the first channels or the second channels for facilitation of transportation of ions, enhancing the conductivity between the adjacent ion exchange membranes, and electrochemical splitting of water. In non-limiting examples, the ion exchange resin may be filled into the first (dilute) channels. 
     For some arrangements, the electrodes of the EDI apparatus  12  may be in the form of plates that are disposed parallel to each other to form a stacked structure. Alternatively, the electrodes may be arranged in varying configurations. For example, the electrodes may be disposed concentrically with a spiral and continuous space therebetween. In some applications, the electrodes may include electrically conductive materials. The spacers may comprise any ion-permeable, electronically nonconductive material, including membranes and porous and nonporous materials. 
     In non-limiting examples, the cathode may include stainless steel. The anode may include iridium oxide or platinum coated titanium. The anion exchange membrane may comprise a polymeric material that includes quaternary amine groups. The cation exchange membrane may comprise a polymeric material that includes sulfonic acid groups and/or carboxylic acid groups. The ion exchange resin may include crosslinked polystyrene or other suitable materials. 
     Accordingly, during operation, an electrical current is applied to the EDI apparatus  12 . The first output stream  15  from the desalination apparatus  11  is introduced into the dilute channels filled with the ion exchange resin for further removal of the target ions, such as the hardness ions therein so as to produce a product fluid  17  with a higher quality. A third stream  18  from a liquid source (not shown) is introduced into the concentrate channels to carry the removed target ions from the respective dilute channels out of the EDI apparatus  12  so as to produce a concentrate fluid  19 . In certain applications, the product fluid  17  may be circulated into the EDI apparatus  12  for further processing. 
     Meanwhile, water splitting reactions occur in the ion exchange resin in the first channels to produce H +  and OH −  for regeneration of the ion exchange resin for facilitation of continuous operation. An electrolyte stream (not shown) may pass through surfaces of the electrodes to remove gases, such as hydrogen and chlorine generated during the operation to protect the electrodes. 
     Generally, the EDI apparatus  12  has a hardness tolerance for a liquid to be processed therein. For example, the hardness tolerance of the EDI apparatuses may be about less than 1 ppm. In order to decrease the hardness in the liquid to a suitable level so as to alleviate or avoid scaling or fouling tendency in the EDI apparatus  12  during processing, as depicted in  FIG. 1 , the desalination apparatus  11  is employed to pretreat the first feed stream  13  so as to produce the first output stream  15  having a suitable level of the target ions including, but not limited to the hardness ions. 
       FIG. 2  illustrates a schematic diagram of the desalination apparatus  11  in accordance with one embodiment of the invention. As illustrated in  FIG. 2 , the desalination apparatus  11  comprises a first electrode  20 , a second electrode  21 , a plurality of cation exchange membranes  22  and a plurality of spacers  23 . In the illustrated example, the first and second electrodes  20 ,  21  are connected to positive and negative terminals of a power source (not shown) so as to act as an anode and a cathode, respectively. Alternatively, the polarity of the first and second electrodes  20 ,  21  may be reversed. 
     In some examples, the first and second electrodes  20 ,  21  may include metal materials with different shapes, such as titanium plates or platinum coated titanium plates. In other examples, the first and second electrodes  20 ,  21  may include electrically conductive materials, which may or may not be thermally conductive, and may have particles with smaller sizes and large surface areas. In some examples, the electrically conductive material may include one or more carbon materials. Non-limiting examples of the carbon materials include activated carbon particles, porous carbon particles, carbon fibers, carbon aerogels, porous mesocarbon microbeads, or combinations thereof. In other examples, the electrically conductive materials may include a conductive composite, such as oxides of manganese, or iron, or both, or carbides of titanium, zirconium, vanadium, tungsten, or combinations thereof. 
     In the illustrated example, the first and second electrodes  20 ,  21  are in the form of plates that are disposed parallel to each other to form a stacked structure. In other examples, the first and second electrodes  20 ,  21  may have varied shapes, such as a sheet, a block, or a cylinder. In addition, the first and second electrodes  20 ,  21  may be arranged in varying configurations. For example, the first and second electrodes  20 ,  21  may be disposed concentrically with a spiral and continuous space therebetween. 
     The cation exchange membranes  22  are configured to be passable for cations and are disposed between the first and second electrodes  20 ,  21  so as to form a plurality of alternating first and second channels  24 ,  25  therebetween, which are also referred to as dilute and concentrate channels under operating conditions, respectively. In the illustrated example, four cation exchange membranes  22  are employed to form one first channel  24  and two second channels  25 , which are disposed alternatingly. Alternatively, at least three cation exchange membranes  22  may be employed so as to form one or more first channels and one or more second channels between the first and second electrodes  20 ,  21 . 
     In some applications, the cation exchange membranes  22  may comprise normal cation exchange membranes configured to be passable for not only the monovalent anions but also polyvalent anions. In certain applications, based on different applications, for example, for removal of monovalent cations, the desalination apparatus  11  may comprise one or more monovalent cation exchange membranes. Thus, the normal cation exchange membranes and the monovalent cation exchange membranes may be disposed alternately to facilitate removal of the hardness ions. Non-limiting examples of suitable materials for use in the normal cation exchange membranes include a polymeric material that includes sulfonic acid groups and/or carboxylic acid groups, for transmission of the cations. 
     The spacers  23  are disposed between each pair of two adjacent ion exchange membranes  22 , and between the first and second electrodes  20 ,  21  and the respective adjacent membranes  22 . In some embodiments, the spacers  23  may comprise any ion-permeable, electronically nonconductive material, including membranes and porous and nonporous materials. 
     Accordingly, during operation, when the desalination apparatus  11  is at a normal polarity state, while an electrical current is applied to the desalination apparatus  11 , liquids, such as the first and second streams  13 ,  14  are introduced into the first channel  24  and the second channels  25 , respectively. In certain applications, the first and second stream  13 ,  14  may or may not be introduced into desalination apparatus  11  simultaneously. 
     During the first and second stream  13 ,  14  pass through the respective dilute and concentrate channels  24 ,  25 , due to presence of the cation exchange membranes  22 , in the dilute channel  24 , at least a portion of the target ions, such as Mg 2+  and Ca 2+ , and other cations, such as Na +  in the first feed stream  13  may migrate through the respective cation exchange membranes  22  towards the anode  20  to enter into the concentrate channels  25 . Anions, such as Cl −  in the first feed stream  13  may not migrate through the respective anion exchange membrane and remain in the dilute channel  24 . 
     In the concentrate channels  25 , anions, such as Cl −  in the second feed stream  14  may not migrate through the anion exchange membrane  22  and remain therein. In certain applications, a portion of the removed target cations, such as Mg 2+  and/or Ca 2+  migrated into the concentrate channels  25  from the first feed stream  13  in the dilute channel  24  and other cations, such as Na+ may further migrate through the cation exchange membranes  22  to enter into the dilute channel(s)  24  from the respective adjacent concentrate channels  25  during operation. 
     In non-limiting examples, in order to prevent at least a portion of the removed target ions migrated into the concentrate channels  25  from the dilute channels  24  from entering into the dilute channels  24  so as to enter into the first feed stream  13  again, the second feed stream  14  may include active monovalent cations, such as Na +  (which is referred to be as Na + -rich stream), which may carry at least a larger portion of the ionic current than the target ions migrated into the concentrate channels  25  from the dilute channel  24  when the cations migrate from the concentrate channels  25  to the respective dilute channels  24  during operation. 
     In non-limiting examples, a concentration of the active monovalent cations may be greater than a concentration of the removed target ions in the respective concentrate channels  25 . In some examples, an ionic mobility of the active monovalent ions may be greater than the ionic mobility of the removed target ions in the respective concentrate channels  25  when migrated from the concentrate channels  25  to the respective dilute channels  24 . In other examples, amounts of the active monovalent cations in the second feed stream  14  may be greater than amounts of the removed target ions in the concentrate channels  25  when migrated from the concentrate channels  25  to the respective dilute channels  24 . 
     As a result, at least a larger portion of the active monovalent cations in the second feed stream  14  in the concentrate channels  25  may migrate through the cation exchange membrane  22  to enter into the adjacent dilute channel  24 . Accordingly, during operation, since the active monovalent cations in the second feed stream  14  may carry at least a larger portion of the ionic current than the removed target ions in the concentrate channels  25  when continuing to migrate from the concentrate channels  25  to the respective dilute channels  24  during operation, at least a larger portion of the removed target ions migrated into the concentrate channels  25  from the dilute channels  24  may not migrate through the cation exchange membranes  22  to further enter into the dilute channels  24  to remain in the respective concentrate channels  25  so as to increase the efficiency of the target ions removed from the first feed stream  13 . 
     For some arrangements, in order to increase the ionic current carried by the active monovalent cations in the second feed stream  14  when migrated into the dilute channels  24  from the concentrate channel  25 , as illustrated in  FIG. 1 , the desalination system  10  further comprises an ion adjustment unit  26  in fluid communication with the second feed stream  14  so as to facilitate that the ionic current carried by the active monovalent anions in the second feed stream  14  are greater than the ionic current carried by the target ions in the concentrate channels  25  when migrated from the concentrate channels  25  to the respective dilute channels  24 . In non-limiting examples, the active monovalent cations may include Na + , K +  or H + . In one example, the ion adjustment unit  26  introduces sodium chloride solution into the second feed stream  14  to increase the concentration of the active monovalent ions, such as sodium ions (Na + ). In certain applications, the ion adjustment unit  26  may or may not be employed. 
     Accordingly, as depicted in  FIG. 2 , during operation, the second feed stream  14  passes through the concentrate channels  25  to carry at least a portion of the removed target anions, such as the hardness ions migrated from the dilute channels  24  out of the desalination apparatus  11 , so that the first output stream  15  having a suitable level of the hardness ions is then introduced into the EDI apparatus  12  for further processing. 
     In some examples, the polarity of the first and second electrodes  20 ,  21  of the desalination apparatus  11  may be reversed. In the reversed polarity state, the dilute channels  24  from the normal polarity state may act as the concentrate channels to receive the second feed stream  14 , and the concentrate channels  25  from the normal polarity state may function as the dilute channels to receive the first feed stream  13  for desalination, for example, for removal of the hardness ions in the first feed stream  13  and alleviation of the fouling tendency of the anions and cations in the desalination apparatus  11 . 
     It should be noted that the arrangements in  FIG. 2  is merely illustrative. In some applications, the desalination apparatus  11  may be employed to remove different target ions using different active cations. In the illustrated example, the target ions to be removed at least include the hardness ions, and the active ions in the second feed stream  14  include Na + . In other examples, the active ions may include, but not limited to K +  and H + . 
       FIG. 3  is an experimental graph illustrating hardness ion removal efficiency of the desalination apparatus  11  in accordance with one embodiment of the invention. In this experimental example, the desalination apparatus  11  comprises nine cation exchange membranes  22 . The DC voltage on the desalination apparatus  11  is about 5 volts. As illustrated in  FIG. 3 , during continuous processing for about 50 minutes in the desalination apparatus  11 , the first feed stream  13  having a hardness of about 13 ppm is processed to produce the first output stream (product stream)  15  having a hardness of about 3 ppm. 
     Thus, about 77% of the hardness ions may be removed from the first feed stream and the hardness of the product stream  15  remains around 3 ppm during operation, which may indicate the desalination apparatus  11  has a relatively higher and stable remove efficiency of the hardness ions. 
     In the illustrated example in  FIG. 1 , the desalination apparatus  11  and the EDI apparatus  12  are disposed separately. In other examples, as illustrated in  FIG. 4 , the desalination apparatus  11  and the EDI apparatus  12  of the desalination system  10  may be disposed unitarily by using a common cathode and a common anode to act as a desalination apparatus  30 . The same numerals in  FIGS. 1-2 and 4  may indicate similar elements. For easy illustration, some elements, for example the ion exchange resin are not illustrated in the arrangement in  FIG. 4 . 
     As depicted in  FIG. 4 , the desalination apparatus  30  comprises a first electrode  31 , a second electrode  32 , a plurality of ion exchange membranes  38 ,  39 ,  40 , and a plurality of spacers  33 . In the illustrated example, the first and second electrodes  31 ,  32  are connected to positive and negative terminals of a power source (not shown) so as to act as an anode and a cathode, respectively. 
     In some applications, the first and second electrodes  31 ,  32  may include electrically conductive materials. In non-limiting examples, the cathode may include stainless steel. The anode may include iridium oxide or platinum coated titanium. For some arrangements, the first and second electrodes  31 ,  32  may be in the form of plates that are disposed parallel to each other to form a stacked structure. Alternatively, the electrodes may be arranged in varying configurations. For example, the first and second electrodes  31 ,  32  may be disposed concentrically with a spiral and continuous space therebetween. 
     In the illustrated example, the ion exchange membranes are divided into first and second groups  36 ,  37  disposed between the first and second electrodes  31 ,  32  to form first groups of alternating first and second channels  34 - 35 , and second groups of alternating first and second channels  34 ′- 35 ′ therebetween, which are also referred to as first and second groups of dilute and concentrate channels under operating conditions, respectively. In non-limiting examples, each of the first and second groups of the alternating first and second channels may comprises a plurality of the alternating first and second channels. 
     The second group  37  is disposed between the first group  36  and the first electrode  31  so that the first channels  34 ′ of the second group of the first and second channels  34 ′- 35 ′ are configured to receive a liquid from the first channel  34  of the first group of the first and second channels  34 - 35  in the first group  36  from further processing. One concentrate channel  35  is formed between the first and second groups  36 ,  37 . Alternatively, the second group  37  may be disposed between the first group  36  and the second electrode  32 . Each of the ion exchange membranes of the first group  36  comprises a cation ion exchange membrane  38 . The second group  37  comprises a plurality of alternating cation and anion exchange membranes  39 ,  40  so as to form the second group of the alternating dilute and concentrate channels  34 ′,  35 ′. 
     In some examples, each of the cation ion exchange membranes  38  of the first group  36  may comprise similar materials to the materials of the cation exchange membrane  22  in  FIG. 2 . Non-liming examples of the cation and anion exchange membranes  39 ,  40  of the second group  37  include similar materials to the respective cation and anion exchange membranes in the electrodeionization (EDI) apparatus  12 . 
     In certain applications, during operation, in order to protect the ion exchange membrane of the second group  37 , for example, the anion exchange membrane  40 , which is disposed adjacent to the first group  36  to endure a higher pressure difference, the thickness of the anion exchange membrane  40  adjacent to the first group  36  may be thicker than the thickness of other ion exchange membranes, which are not adjacent to the respective electrodes  31 ,  32 , in the first and second groups  36 ,  37 . In non-limiting examples, the thickness of the anion exchange membrane  40  adjacent to the first group  36  may be in a range of from 1 mm to 3 mm, for example, 2 mm. In other examples, the membranes adjacent to the respective first and second electrodes  31 ,  32  may also have a higher thickness, for example in a range of from 1 mm to 3 mm, such as 2 mm. 
     The spacers  33  are disposed between each pair of the adjacent ion exchange membranes, and between the first and second electrodes  31 ,  32  and the respective adjacent membranes  38 ,  39 . In some embodiments, the spacers  33  may also comprise any ion-permeable, electronically nonconductive material, including membranes and porous and nonporous materials. 
     Accordingly, similar to the arrangements in  FIG. 2 , during operation, while an electrical current is applied to the desalination apparatus  30 , liquids, such as first feed stream  13  is introduced into the first (dilute) channel  34 . The second feed stream  14  is introduced into the second (concentrated) channels  35  in the first group  36  and between the first and second groups  36 ,  37  respectively. As a result, at least a portion of target ions including, but not limited to hardness ions may be removed from the first feed stream  13  to produce a first output stream (a dilute stream)  15 . The second feed stream  14  carries at least a portion of the target ions removed from the first feed stream  13  out of the desalination device  30  during or after desalination of the first feed stream  13  to produce a second output stream (a concentrate stream)  16 . 
     Subsequently, similar to the EDI apparatus  12  shown in  FIG. 1 , the first output stream  15  from the first group  36  is introduced into the dilute channels  34 ′ filled with the ion exchange resin (not shown) of the second group  37  for further removal of the target ions, such as the hardness ions so as to produce a product fluid  17  with a higher quality. Meanwhile, water splitting reactions occur in the ion exchange resin in the dilute channels  34 ′ for regeneration of the ion exchange resin. A third stream  18  from a liquid source (not shown) is introduced into the concentrated channels  35 ′ of the second group  37  to carry the removed ions from the respective dilute channels  34 ′ out of the desalination device  30  so as to produce a concentrate fluid  19 . 
     It should be noted that the arrangement in  FIG. 4  is merely illustrative. In the illustrated example, the EDI apparatus  12  is integrated with the desalination apparatus  11  shown in  FIG. 1 . Alternatively, other desalination apparatuses, such as electrodialysis (ED) apparatuses or electrodialysis reversal (EDR) apparatuses may also be disposed unitarily with the desalination apparatus  11  by using a common cathode and a common anode. Similarly, other target ions, including, but not limited to the hardness ions may also be removed. In some examples, the polarity of the first and second electrodes  31 ,  32  may be reversed. 
     In embodiments of the invention, the desalination apparatus  11  employs the cation ion exchange membranes to process a liquid for removal of the target ions, such as the hardness ions therein, which has stable and relatively higher removal efficiency. As a result, when the liquid having a suitable concentration level of the target ions is introduced into the EDI apparatus for further processing, the scaling or fouling issues may be avoided or alleviated so that the EDI apparatus sustains a continuous and stable operation. In addition, the desalination apparatus  11  may be disposed separately from or unitary with the EDI apparatus or other desalination apparatuses, which improves the system flexibility for processing of a liquid. 
     While the disclosure has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown, since various modifications and substitutions can be made without departing in any way from the spirit of the present disclosure. As such, further modifications and equivalents of the disclosure herein disclosed may occur to persons skilled in the art using no more than routine experimentation, and all such modifications and equivalents are believed to be within the spirit and scope of the disclosure as defined by the following claims.