1. A seawater desalination apparatus comprising: an intake unit which takes in seawater including impurities as a water to be treated; a flocculation unit which includes flocculant reservoirs for storing flocculants and feeds the flocculants from the flocculant reservoirs to the seawater to form floes including the impurities in the seawater; a floe removing unit which removes the floes from the seawater; and a desalination unit which desalinates the seawater, wherein: the flocculants include a metal salt in combination with a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and the flocculation unit adds the metal salt and the composite flocculant in this order to the seawater. 2 . The seawater desalination apparatus as claimed in Claim 1, wherein one of the anionic polymers contained in the composite flocculant has a weight-average molecular weight of from 100,000 to 1,000,000.

3. The seawater desalination apparatus as claimed in Claim 2, wherein the anionic polymer having a weight-average molecular weight of from 100,000 to 1,000,000 and contained in the composite flocculant is a poly(acrylic acid).

4. The seawater desalination apparatus as claimed in Claim 2, wherein another anionic polymer contained in the composite flocculant has a weight-average molecular weight of 2,000,000 or more.

5. The seawater desalination apparatus as claimed in Claim 4, wherein the anionic polymers contained in the composite flocculant have a functional group equivalent of 10 meq/g or more.

6. The seawater desalination apparatus as claimed in Claim 4, wherein a weight ratio of the low-molecular-weight anionic polymer to the high-molecular-weight anionic polymer contained in the composite flocculant is from 1 to 9.

7. A seawater desalination method comprising the steps of: taking in seawater including impurities as a water to be treated; forming floes including the impurities in the seawater by feeding flocculants from flocculant reservoirs to the seawater; removing the floes from the seawater; and desalinating the seawater, wherein: the flocculants include a metal salt in combination with a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and the metal salt and the composite flocculant are added in this order to the seawater in the step of flocculating.

8. A set of flocculants for seawater desalination, the set of flocculants comprising two different flocculants to be added to seawater containing impurities to thereby remove the impurities from the seawater, wherein: the two different flocculants are a metal salt and a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and the metal salt and the composite flocculant are to be added in this order to the seawater.

9. The set of flocculants for seawater desalination as claimed in Claim 8, wherein one of the anionic polymers contained in the composite flocculant has a weight-average molecular weight of from 100,000 to 1,000,000.

10. The set of flocculants for seawater desalination as claimed in Claim 9, wherein the anionic polymer having a weight-average molecular weight of from 100,000 to 1,000,000 and contained in the composite flocculant is a poly(acrylic acid).

11. The set of flocculants for seawater desalination as claimed in Claim 9, wherein another anionic polymer contained in the composite flocculant has a weight-average molecular weight of 2,000,000 or more.

12. The set of flocculants for seawater desalination as claimed in Claim 11, the anionic polymers contained in the composite flocculant have a functional group equivalent of 10 meq/g or more.

13. The set of flocculants for seawater desalination as claimed in Claim 11, wherein a weight ratio of the low-molecular-weight anionic polymer to the high-molecular-weight anionic polymer in the composite flocculant is from 1 to 9.

FIELD OF THE INVENTION
[0001]
The present invention relates to a seawater desalination apparatus, a seawater desalination method, and a set of flocculants for seawater desalination.

BACKGROUND OF THE INVENTION
[0002]
Water purification techniques produce drinking water and service water for other uses from river water or another natural water. The techniques have employed chemical processes such as flocculation-precipitation process; and physical processes such as sand filtration process.

[0003]
A microfiltration (MF) or ultrafiltration (UF) is also employed in the filtration so as to remove more minute impurities. In this case, a flocculant may be used in combination. Minute impurities, if undergoing no treatment, pass through the membrane filter. However, the flocculant allows the minute impurities to have larger particle sizes and thereby helps the membrane filter to trap and remove the impurities with higher performance.

[0004]
Independently, countries all over the world including those in the Middle East and Asia face a water shortage. To solve or mitigate the water shortage, a technique of desalinating seawater to produce drinking water or other service water has received attention and is on the verge of practical use. The seawater desalination has been performed by evaporation in which seawater is heated to evaporate water into steam, and the steam is cooled to give fresh water (plain water). The evaporation has been mainstream because this process is advantageously not affected by the salinity concentration (salt concentration) and organic substance concentration in the seawater.

[0005]
The evaporation, however, disadvantageously has a poor energy efficiency and suffers from a high cost. A more efficient technique is therefore demanded. Currently, reverse osmosis begins to be mainstream. The reverse osmosis desalinates seawater by membrane filtration with a reverse osmosis membrane (RO membrane) to give fresh water.

[0006]
The reverse osmosis, however, suffers from reduction in filtration performance, and the RO membrane needs replacing, because the RO membrane is polluted by impurities such as organic and inorganic substances contained in the seawater. This causes the apparatus to require a higher water desalination cost.

[0007]
To prevent the pollution of the RO membrane, an appropriate pretreatment should be performed to remove polluting substances such as suspended substances and organic substances before the seawater is allowed to pass through the RO membrane. As a pretreatment process, investigations have been made on membrane filtration using an ultrafilter membrane (UF) or membrane filter (MF) ; the use of an adsorbent such as activated carbon; and the use of a flocculant, as in the water purification treatment.

[0008]
Such a flocculant, when added, can generally increase the efficiency of the filtration treatment process in water treatment through sand filtration or membrane filtration.

[0009]
Representative flocculants for use in wastewater treatment and water purification treatment include inorganic flocculants using multivalent metal ions (cations), such as poly(aluminum chloride)s (PACs) and ferric chloride (III); and polymer flocculants using water-soluble polymers having multivalent ions (hereinafter briefly referred to as "polymer flocculant(s)"). These flocculants remove electrically-charged impurities from water by flocculation-precipitation. When sufficient effects are not obtained by a single use of either one of inorganic and organic flocculants, the combination use of inorganic and organic flocculants may give satisfactory flocculation effects.

[0010]
As is described above, the flocculants are roughly classified as inorganic and organic flocculants. Of these, inorganic flocculants are exemplified by PAC and ferric chloride (III) ; whereas organic flocculants (polymer flocculants) are exemplified by cationic, anionic, and nonionic polymers. An enormous number of several hundred types of flocculants are now commercially available for treatment of water having different qualities.

[0011]
A flocculant to be used is determined by properties of water to be treated. Either one of an inorganic flocculant and a polymer flocculant is often used. Independently, the two types of flocculants may be used in combination. It is generally believed that the combination use increases flocculation and floc-removing effects in the case where the single use of an inorganic flocculant fails to give sufficient effects.

[0012]
Japanese Unexamined Patent Application Publication (JP-A) No. 2008-264723 (Patent Document 1) discloses a method for flocculating impurities to remove the impurities from water such as seawater or river water. This method feeds an organic flocculant and an inorganic flocculant to the water simultaneously or sequentially in this order.

[0013]
JP-A No. 2008-200646 (Patent Document 2) discloses a mixing chamber including an elastic packing having a hole at the center portion thereof, through which hole water passes; and a flocculation reaction apparatus using the mixing chamber. These are provided so as to reduce scale to be deposited on piping inner walls.

[0014]
JP-A No. 2012-45522 (Patent Document 3) discloses a technique for purifying or remedying polluted water upon oil drilling or oil extraction from oil sand. This technique separately adds a poly (acrylic acid) and a salt of iron (III) with an inorganic acid to the polluted water. The poly (acrylic acid) is a water-soluble polymer compound having acidic groups; whereas the salt is a trivalent metal salt.

[0015] PRIOR ART DOCUMENTS: PATENT DOCUMENTS:
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-264723
Patent Document 2: Japanese Unexamined Patent
Application Publication No. 2008-200646
Patent Document 3: Japanese Unexamined Patent
Application Publication No. 2012-45522

SUMMARY OF THE INVENTION

[0016]
Patent Document 1 lacks the description of the removal of organic substances that cause fouling of RO membranes (fouling-causative substances).

[0017]
Patent Document 2 lacks the description of an RO membrane and fouling.

[0018]
Patent Document 3 fails to disclose a technique to be applied to seawater desalination, although it describes polluted seawater.

[0019]
It is an object of the present invention to suppress fouling of an RO membrane and to reduce the water desalination cost (running cost) in seawater desalination.

[0020]
The present invention provides a seawater desalination apparatus. The apparatus includes: an intake unit which takes in seawater including impurities as a water to be treated; a flocculation unit which includes flocculant reservoirs for storing flocculants and feeds the flocculants from the flocculant reservoirs to the seawater to form floes including the impurities

in the seawater; a floe removing unit which removes the floes from the seawater; and a desalination unit which desalinates the seawater, in which the f locculants include a metal salt in combination with a composite flocculant, the metal salt contains a trivalent metal ion, and the composite flocculant includes two or more anionic polymers having different weight-average molecular weights from each other; and the flocculation unit adds the metal salt and the composite flocculant in this order to the seawater.

[0021]
The present invention can remove organic substances from seawater, helps an RO membrane to have a longer life, and can reduce water desalination cost (running cost), because such organic substances will cause fouling of the RO membrane.

[0022]
In addition, the present invention can simplify filtration facilities and can also reduce facility cost. Typically, UF can be simplified to MF or sand filtration according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS
[0023]
Fig. 1 is a schematic block diagram illustrating a seawater desalination apparatus according to an embodiment;
Fig. 2 is a schematic view illustrating a seawater desalination apparatus according to an embodiment, in which flocculation tanks are employed after flocculant addition;
Fig. 3 is a schematic view illustrating through which process floes form; and
Fig. 4 is a schematic block diagram illustrating a modification of the seawater desalination apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0024]
The present invention relates to a flocculant for removing substances which cause fouling of an RO membrane; a floe forming method (flocculation method); and a seawater desalination apparatus using a solution of the flocculant.

[0025]
As used herein the term "fouling" refers to a phenomenon in which insoluble components, polymer solutes, colloid, and minute solids contained in untreated water deposit on a membrane and reduce the flux of the membrane.

[0026]
A flocculant (a set of flocculants) for seawater desalination according to the present invention includes a trivalent metal ion in combination with a composite flocculant, in which the composite flocculant includes two or more anionic polymers having different weight-average molecular weights, and the trivalent metal

ion and the composite flocculant are to be added in this order to the seawater.
[0027]
An apparatus, a method, and a set of flocculants for seawater desalination according to an embodiment of the present invention will be illustrated below.
[0028]
The seawater desalination apparatus includes an intake unit which takes in seawater containing impurities (fouling-causative organic substances); a flocculation unit which includes one or more flocculant reservoirs for storing flocculant solutions and feeds the flocculant solutions from the flocculant reservoirs to the seawater to form floes containing the impurities in the seawater; a floe removing unit which removes the floes from the seawater; and a desalination unit which removes salts from the seawater. The flocculant reservoirs of the flocculation unit store an aqueous solution of an inorganic flocculant and an aqueous solution of a polymer flocculant respectively or as a solution mixture. These aqueous flocculant solutions are added to the seawater. When stored respectively as aqueous solutions, the inorganic flocculant and the polymer flocculant are added in this order. The flocculation unit may be either of a system of using a flocculation tank or tanks having a stirrer, or of a system of adding the flocculants to the seawater in line without using a tank.

[0029]
In a preferred embodiment, the flocculation unit of the seawater desalination apparatus includes two or more flocculant injection units which add the flocculant solution(s) to the seawater so as to disperse the flocculants rapidly in the seawater.
[0030]
The flocculant reservoirs in the flocculation unit of the seawater desalination apparatus respectively store a solution of a flocculant containing a trivalent metal ion, and a solution of a composite flocculant including anionic polymers having different weight-average molecular weights.
[0031]
In the flocculation unit of the seawater desalination apparatus, a flocculant solution to be added through an upstream flocculant injection unit contains the trivalent metal ion; whereas a flocculant solution to be added through a downstream flocculant injection unit contains the composite flocculant.
[0032]
In another preferred embodiment, the seawater desalination apparatus further includes a pH control unit. This unit controls the hydrogen ion concentration of a mixture of the seawater and the flocculant solutions by adding a basic aqueous solution or an acidic aqueous solution to the mixture. In particular, the mixture is

preferably controlled to have a pH of from 6 to 7 . This is because the flocculant (set of flocculants) for use herein forms a salt structure with impurities and is thereby converted into floes. The flocculant probably develops maximum effects at a pH of from 6 to 7 where the salt structure is stable. The control of the seawater
(mixture) pH can also be performed by previously controlling the aqueous flocculant solutions to be acidic
(about pH 2) and reducing the pH of the seawater by the addition of the flocculants. [0033]
The floe forming method includes the steps of taking in seawater containing impurities (fouling-causative organic substances such as alginic acid); forming floes in the seawater by adding flocculant solutions from flocculant reservoirs to the seawater; and removing the formed floes from the seawater. In a preferred embodiment, the flocculant solutions are different flocculant solutions separately containing at least one of a trivalent metal ion and a composite flocculant and are added separately in the flocculation step. [0034]
In a more preferred embodiment of the floe forming method, a flocculant solution containing a trivalent metal ion, and a composite flocculant solution containing anionic polymer flocculants having different weight-average molecular weights are added in this order

to the seawater.
[0035]
The trivalent metal ion and the composite flocculant are preferably dissolved in water to form aqueous solution(s). In an embodiment, however, such solutions of the trivalent metal ion and the composite flocculant each further include a solid metal capable of forming a trivalent ion and one or more solid anionic polymer flocculants, respectively, dispersed as solids in the solutions to form solution-dispersions. In this embodiment, when the solution-dispersions are added to the seawater, the seawater acts as a solvent, and the solid components can be dissolved therein.
[0036]
The set of flocculants for seawater desalination is a set (combination) of two different flocculants to be added to seawater containing impurities and to remove the impurities from the seawater. The two different flocculants are a metal salt containing a trivalent metal ion; and a composite flocculant including two or more anionic polymers having different weight-average molecular weights. The metal salt and the composite flocculant are added in this order to the seawater.
Preferably, one of the anionic polymers contained in the composite flocculant has a weight-average molecular weight of from 100, 000 to 1,000,000. In addition, another anionic polymer contained in the composite flocculant

preferably has a weight-average molecular weight of 2,000,000 or more.
[0037]
A set of flocculants for removing fouling-causative substances; a floe forming method using the set of flocculants; and a seawater desalination apparatus using solutions of the set of flocculants will be illustrated in detail below with reference to attached drawings according to necessity.
[0038]
Fig. 1 is a block diagram schematically illustrating the structure of a seawater desalination apparatus.
[0039]
The seawater desalination apparatus 100 illustrated in Fig. 1 includes an intake unit 101 which takes in seawater 1 containing impurities; a flocculation unit 102 which adds flocculant solutions to the seawater 1; a pH control unit 103 (hydrogen ion concentration control unit) which controls a mixture including the seawater after treatment in the flocculation unit 102 to have a hydrogen ion concentration at a suitable level; and a floe removing unit 104. The seawater 1 sequentially passes through the intake unit 101, the flocculation unit 102, the pH control unit 103, and the floe removing unit 104, from which the impurities are removed during this process. The resulting liquid after the treatment in the floe removing unit 104 is further treated in a desalination unit (not

shown) to give fresh water. In a preferred embodiment for the addition of flocculants in appropriate amounts, the apparatus further includes a water-quality measurement unit (not shown) and a flocculant-addition-rate control unit (not shown). The water-quality measurement unit serves as a mechanism for measuring the fouling-causative substance concentration in the seawater. The flocculant-addition-rate control unit serves as a mechanism for adding the flocculants.
[0040]
The pH control unit 103 may be provided according to necessity.
[0041]
The seawater desalination apparatus 100 illustrated in Fig. 1 can also be regarded as a flow chart of the floe forming method. Specifically, in the floe forming method, there can be regarded the intake unit 101 as an intake step; the flocculation unit 102 as a flocculation step (floe forming step); the pH control unit 103 as a pH controlling step; the floe removing unit 104 as a floe removing step; the desalination unit as a desalination step; the water-quality measurement unit as a water-quality measurement step; and the flocculant-addition-rate control unit as a flocculant-addition-rate controlling step.
[0042]
Fig. 2 is a schematic view illustrating a seawater

desalination apparatus according to an embodiment in which flocculation tanks are provided downstream from the flocculant addition.
[0043]
Differences from the apparatus illustrated in Fig. 1 will be described below.
[0044]
With reference to Fig. 2, the apparatus further includes a water-quality measurement unit 151, a first flocculation tank 152, and a second flocculation tank 153 arranged in this order between the intake unit 101 and the floe removing unit 104; and a water-quality measurement unit 154 arranged between the floe removing unit 104 and the desalination unit 155.
[0045]
A first flocculant reservoir 161 is connected via a pump 164 to the first flocculation tank 152; whereas a second flocculant reservoir 162 is connected via a pump 165 to the second flocculation tank 153. The first flocculant reservoir 161 stores a solution containing the trivalent metal ion; whereas the second flocculant reservoir 162 stores a solution containing the composite flocculant. The first flocculation tank 152 and the second flocculation tank 153 each have a stirring mechanism which is rotatable by the action of a motor.
[0046]
The floe removing unit 104 includes one or more of

components suitably arranged. The components are exemplified by a precipitation tank (precipitation unit) , an ultrafiltration unit, a microfiltration unit, a sand filtration tank (sand filtration unit), and a multimedia filter filtration unit. The desalination unit 155 removes, for example, chloride ion and sodium ion using an RO membrane.
[0047]
The water-quality measurement units 151 and 154 are connected to the flocculant-addition-rate control unit 163. Based on information from the water-quality measurement units 151 and 154, the
flocculant-addition-rate control unit 163 controls the power of the pumps 164 and 165 and thereby controls the addition rates (feed rates; feed flow rates) of flocculants from the first flocculant reservoir 161 and the second flocculant reservoir 162.
[0048]
Each unit will be described in detail below.
[0049]
(Intake Unit)
The intake unit 101 illustrated in Fig. 1 plays a role in leading seawater into the seawater desalination apparatus . A pump to be used herein is not limited in type and is exemplified by non-positive displacement (turbo type) pumps, positive displacement pumps, and special pumps. Specifically, the pump for use herein is

exemplified by magnet pumps, plunger pumps, screw pumps, diaphragm pumps, diffuser pumps, gear pumps, Archimedean screw pumps, piston pumps, rotary pumps, peristaltic pumps (tube pumps), and submersible pumps. Other pumps than those mentioned above can also be used. [0050]
(Water Quality Measurement Unit) With reference to Fig. 2, the water-quality measurement unit 151 senses substances (fouling-causative substances) and obtains information (water quality data) on the water quality of the seawater. The fouling-causative substances cause fouling of the RO membrane and are contained in the taken seawater 1. Independently, the water-quality measurement unit 154 obtains water quality information (water quality data) of the seawater (flocculation-treated water) from which floes have been removed. [0051]
The water quality data obtained from the water-quality measurement units 151 and 154 are used in the flocculant-addition-rate control unit 163 to determine the flocculant addition rates suitable for the water quality of the taken seawater 1. This optimizes the flocculant addition rates, can prevent excessive addition of flocculants and the formation of unnecessary sludge, and can optimize the plant operating cost. Exemplary indices for the water quality to be evaluated include water

temperature, pH, electric conductivity, total organic carbon (TOC), proteins, sugars (neutral sugars and acidic sugars), and adenosine triphosphate (ATP) activity. Any index can be targeted as long as being an index regarding to any of organic and inorganic components which are contained in the seawater 1 and probably affect the fouling of the RO membrane. Typically, the present invention is applicable to seawater from sea or a canal where a red tide occurs, because red-tide-causative substances are analogous to fouling-causative substances. The flocculants are used in large amounts in this case, because large amounts of red-tide-causative substances are contained in the seawater in this case.
[0052]
In the apparatus illustrated in Fig. 2, the water-quality measurement units 151 and 154 are arranged upstream and downstream from the flocculation treatment (flocculation unit) . This arrangement is preferred for precise optimization of the flocculation treatment. For the sake of simplicity, it is possible that a water-quality measurement unit is arranged only one of upstream and downstream from the flocculation unit so as to perform water quality evaluation and flocculant-addition-rate control.
[0053]
(Flocculant-addition-rate Control Unit)
The flocculant-addition-rate control unit 163 of the

apparatus illustrated in Fig. 2 enables real-time appropriate addition of flocculants suitable for the taken seawater 1. This is performed based on the water quality data obtained from the water-quality measurement units 151 and 154; and already inputted optimal flocculant addition rate data for the fouling-causative substance concentration. Specifically, the
f locculant-addition-rate control unit 163 includes a data input unit which receives the water quality data; and a data conversion/output unit which converts the water quality data into the optimal flocculant addition rates and outputs the same.
[0054]
(Flocculation Unit)
The flocculation unit 102 illustrated in Fig. 1 employs a system as illustrated in Fig. 2. Specifically, the flocculation unit 102 has two flocculation tanks 152 and 153 which inject (add) an inorganic flocculant containing a trivalent metal ion; and an organic flocculant respectively. The organic flocculant is a composite flocculant containing two or more anionic polymers having different weight-average molecular weights from each other.
[0055]
In a preferred embodiment, stirring is performed at a high rate (quickly) in the first tank, i.e. , in the first flocculation tank 152; whereas stirring is performed at

a low rate (slowly) in the second tank, i.e., in the second flocculation tank 153. The stirring under these conditions allows the floes to have larger particle sizes and contributes to better flocculation performance. If
> there is a possibility that the anionic polymers be dissolved and dispersed insufficiently, a third flocculation tank can be arranged. In this case, stirring in the second flocculation tank is also performed at a high rate to disperse the composite flocculant
) sufficiently, and stirring in the third flocculation tank is performed at a low rate. This enables effective formation of floes. [0056] In another embodiment, there may be employed a system
i in which the flocculants are added to and mixed with the seawater in line without using flocculation tanks, as illustrated in Fig. 4. Specifically, flow-type mixing units 352 and 353 may be used instead of flocculation tanks. The other components of the embodiment illustrated in Fig.
) 4 than those mentioned above are as with the components of the embodiment illustrated in Fig. 2. [0057]
Fig. 3 is a schematic view illustrating how floes form in the flocculation unit 102 illustrated in Fig. 1. A poly(acrylic acid) is taken in Fig. 3 as an example of polymer flocculants. [0058]

With reference to Fig. 3, ferric chloride (III) (iron(III) chloride) is added to the seawater to give iron(III) ion, and the iron(III) ion is combined with impurities 201 in the seawater and forms microflocs 202. The microflocs 202 are solid or colloidal. A poly (acrylic acid) 203 serving as a water-soluble anionic polymer having carboxyl groups is then added to the seawater, combined with the microflocs 202, and forms a floe 204 (coarse floe). [0059]
The impurities 201 herein are fouling-causative organic substances and are illustrated in the embodiment of Fig. 3 as water-soluble organic substances having an acidic group (carboxyl group), which are believed to be main fouling-causative substances. The acidic group is further exemplified by sulfonic group and phosphate group, in addition to carboxyl group. An iron salt (ferric salt) is illustrated herein as the trivalent metal salt, but the trivalent metal salt is further exemplified by salts of other trivalent metals such as aluminum and neodymium.
[0060]
A divalent metal salt, if used instead of the trivalent metal salt, induces flocculation slightly, but only part of the metal trapping the organic acid through ion bonding contributes to floe formation. This is because of a low crosslinking density. Accordingly, a trivalent metal salt is preferably used to allow

flocculation of considerable quantities of the organic acid. Divalent metal ions such as Mg and Ca contained in the seawater can also be involved in flocculation and are expected to effectively remove fouling-causative substances even when one or more poly(acrylic acid) are added alone as the flocculant without using a trivalent metal salt (e.g., ferric chloride (III)). The poly (acrylic acid) s in this case, however, have about half of removal rate of fouling-causative substances in the case where ferric chloride (III) or another trivalent metal salt is used in combination.
[0061]
When a trivalent metal salt and a water-soluble polymer having acidic groups are used in combination, the addition of the trivalent metal salt prior to the water-soluble polymer having acidic groups may readily contribute to a higher removal rate of fouling-causative substances. This is probably because as follows. The water-soluble polymer having acidic groups, if added primarily, form ion bonds with Ca, Mg, and other alkaline earth metals contained in the seawater and thereby flocculate and precipitate. This may prevent sufficient bonding between fouling-causative substances and iron ions which form microfIocs . To prevent this, when the two flocculants are added to the seawater, it is preferred that the trivalent metal salt is added primarily, and the water-soluble polymer having acidic groups is added

secondarily.
[0062]
The trivalent metal salt and the water-soluble polymer having acidic groups may be added as bulks respectively to exhibit effects . However, it takes a long time to allow the bulks to spread and disperse allover the taken seawater. To prevent this, the two different flocculants are preferably added as aqueous solutions. Particularly if the trivalent metal salt is added before the water-soluble polymer having acidic groups is sufficiently dissolved, flocculation may occur only partially in the seawater, and this might cause insufficient removal of fouling-causative substances. Also to prevent this, the flocculants are preferably added as aqueous solutions.
[0063]
(pH Control Unit)
The pH control unit 103 illustrated in Fig. 1 injects an acidic or basic pH-adjusting solution to the seawater (mixture) after the addition of the flocculants and controls the hydrogen ion concentration (pH) of the seawater (mixture).
[0064]
The pH control unit 103 may further include another reservoir to store or prepare a basic solution and an acidic solution, respectively. The pH control unit 103 can also be configured to detect the pH of the seawater

and to inject an acidic solution or a basic solution in an optimal amount automatically. The pH control unit 103 controls the pH of the seawater (mixture) to be neutral. As used herein the term "neutral" refers to that the substance in question has a pH of from 6 to 8.
[0065]
In a preferred embodiment as illustrated in Fig. 1, the pH control unit 103 is arranged downstream from, and adjacent to, the flocculation unit 102 so as to perform the pH control immediately after the flocculants are added and uniformly dispersed.
[0066]
In another embodiment, the pH control unit 103 is arranged inside the flocculation unit 102. The configuration according to this embodiment can improve the pH control accuracy and enables easy control of the amounts of flocculants to suitable feeding amounts.
[0067]
In another embodiment for the pH control of the treated water, the flocculants are added as aqueous solutions, and the stored aqueous flocculant solutions are controlled to have a suitable pH (e.g., pH 2 to 3). Thus, the seawater, when having a pH of 8 or more, can be neutralized. In this embodiment, either one or both of the aqueous trivalent metal ion solution and the aqueous anionic polymer solution may be controlled to have a low pH.

[0068]
(Floe Removing Unit)
The floe removing unit 104 illustrated in Fig. 1 removes floes formed by the addition of the flocculants. The removal is mainly performed through filtration. The filtration procedure is exemplified by sand filtration, multimedia filter filtration, microfiltration, and ultrafiltration. The filtration procedure should be appropriately selected according typically to the shapes and strengths of floes.
[0069]
The present invention can give large and strong floes to improve the flocculation performance, simplify the facilities of the floe removing unit (to facilities for rougher filtration), and reduce the cost. In contrast, common seawater desalination plants employ very expensive ultrafilter membranes. The floe removing unit 104 can employ not only filtration, but also any other procedure which can remove the floes, such as supernatant collection typically through decantation; magnetic separation; or pressure-reduction floatation.

[0070]

(Desalination Unit)
The desalination unit 155 illustrated in Fig. 2 removes salts from the seawater and converts the seawater into fresh water. The desalination is generally performed with a reverse osmosis (RO) membrane . There are

designed membranes of various materials and shapes. A membrane for use in the present invention, however, is not limited on its material and shape, and any one is applicable to the desalination unit 155 illustrated in Fig. 2, as long as having the desalination function.

[0071]
(Flocculants)
In the present invention, two different flocculants may be used respectively as aqueous solutions and stored i in different reservoirs. The two different flocculants are an inorganic flocculant and a composite flocculant of two or more anionic polymer flocculants having different weight-average molecular weights. In an embodiment, two different anionic polymer flocculants, i.e., a first polymer flocculant and a second polymer flocculant, are used.

[0072]
Examples of the flocculants are as follows.

[0073]
Exemplary inorganic flocculants include alum, ferric chloride (III), ferric sulfate, aluminum chloride and aluminum sulfate.

[0074]
Exemplary anionic polymer flocculants include polyacrylamide flocculants, poly(sulfonic acid) flocculants, poly(acrylic acid) flocculants, poly(acrylic ester) flocculants, polyamine flocculants,

and poly(methacrylic acid) flocculants. Two or more polymer flocculants having two or more different average molecular weights may be employed. Among them, poly(acrylic acid)s and copoly(acrylic acid)s (poly(acrylic acid) copolymers) are preferred. This is because these polymers have high ionicity, thereby form floes also with salts (e.g., Ca2+and Mg2+) in the seawater, do not remain as dissolved in the seawater (mixture) even if added in excess, and can be removed by the floe removing unit. This effect is expected to increase with increasing functional group equivalents of the polymers. The functional group equivalent is an index for ionicity, refers to the number of functional groups per unit mass, and is indicated in meq/g (milliequivalent per gram). Although depending on addition conditions (amount and ratio with respect to ferric chloride (III)) , the polymer flocculants, if added in a functional group equivalent of less than 10 meq/g (the value is correct to two significant figures), may migrate to and remain in the treated water. [0075]

The poly(acrylic acid) has a functional group equivalent of 13.8 meq/g and less adversely affects downstream systems typified by the RO membrane. The poly(acrylic acid) copolymer, when employed, is preferably controlled typically on copolymerization ratio so as to have a functional group equivalent of 10

meq/g or more. Thus, migration of the flocculant not involved in flocculation into the treated water can be prevented. In a preferred embodiment, two (or more) poly(acrylic acid)s having different molecular weights are used as the polymer flocculants. This is preferred for the following reason. When the present invention is applied to seawater which varies in water quality on a daily basis, control of flocculant injection is required. The combination use of two (or more) poly(acrylic acid)s can probably minimize disadvantages or troubles in the above case even when it takes some time to change the injection volumes of the flocculants typically through feedback control.

[0076]
In a preferred embodiment regarding the polymers for use herein, the first polymer flocculant has a weight-average molecular weight of from about 1.0* 105 to about l.Ox 106; whereas the second polymer flocculant has a weight-average molecular weight of about 2 . Ox 106 or more. In a more preferred embodiment employing two poly (acrylic acid)s, one serving as the first polymer flocculant has a weight-average molecular weight of from about 2.5* 105 to about 4.5* 105; and the other serving as the second polymer flocculant has a weight-average molecular weight of from about 2 . Ox 106 to about 5x 106, respectively. This embodiment is preferred for reasons as follows. Specifically, the first polymer flocculant having a lower

molecular weight can incorporate the impurities (fouling-causative substances) in the seawater into floes. In contrast, the second polymer f locculant having a higher molecular weight can coarsen the floes and improve the separation performance. Polymer flocculants, if having very high molecular weights, may probably form floes with salts in the seawater immediately after addition and precipitate before they trap fouling-causative substances, as described above. Regular polymer flocculants have molecular weights of from several millions to several tens of millions. However, for the above reason, polymer flocculants to be used for seawater or another high-concentration salt water probably have appropriate molecular weights as specified above.

[0077]
The blending ratio of the first and second polymer flocculants may be controlled according to the water to be treated. In a preferred embodiment mainly expecting fouling reduction, the first polymer flocculant (low-molecular-weight anionic polymer) having a lower molecular weight is used in a weight percentage of 50% or more based on the total weight of the entire polymer flocculants . More preferably for further significant and effective fouling reduction, the first polymer flocculant is used in a weight percentage of 70% or more. However, the first polymer flocculant is preferably used in a weight percentage of 91% or less, because the second polymer flocculant (high-molecular-weight anionic polymer) is desirably used in a weight percentage of 9% or more for exhibiting satisfactory separation performance.

[0078]
Flocculants added to the seawater, when having a positive total electric charge, can incorporate large amounts of fouling-causative substances into floes. In a preferred embodiment for obtaining this effect, the added flocculants have a positive total electric charge. In a more preferred embodiment for obtaining the effect at higher level, the flocculants have a positive electric charge as much as three times or more the negative electric charge. By contrast, if the flocculants have a negative total electric charge, excessive anionic polymers might form minute floes (microflocs) and form floes before trapping fouling-causative substances by the action of positive electric charge. This might adversely affect the effect.

[0079]
Assume that ferric chloride (III) and poly (acrylic acid) s are used in combination. The ferric chloride (III) has a molecular weight (molar mass) of 160 g/mol and has a valency of three. The poly(acrylic acid) includes a constitutional repeating unit with a molar mass of 72 g/mol. The total electric charge ratio is determined according to an equation as follows.

[0080] (Total electric charge ratio) = (Positive electric charge)/(Negative electric charge)
=[(Ferric chloride (III) amount)/160x3]/[ (Poly (acrylic acid) amount)/72><1]
Based on the idea and concept as mentioned above, the present invention focuses on and specifies the types, molecular weights, and ionicity of anionic polymers. According to the present invention, therefore, not only fouling-causative substances can be effectively trapped into floes and removed from the seawater, but also added flocculants do not remain in the treated water and not adversely affect downstream systems.

[0081]
Next, the present invention will be illustrated in further detail with reference to several examples and comparative examples below.

[0082][Examples 1 to 17]
The configuration (structure) illustrated in Fig. 2 was employed in these examples.

[0083]
With reference to Fig. 2, the intake unit 101 took the seawater 1 into the seawater desalination apparatus 100. The flocculation unit 102 added an aqueous ferric chloride (III) solution and an aqueous poly (acrylic acid) solution in this order to the seawater to form floes . Next, the floe removing unit 103 removed the floes from the seawater. Finally, the desalination unit 104 removed salts from the seawater to give fresh water. The flocculants were added to the seawater as a 3.8% aqueous ferric chloride (III) solution and a 0.1% aqueous poly(acrylic acid) solution, respectively. The aqueous poly(acrylic acid) solution had a pH of about 3.7.

[0084]
The intake unit 101 employed a submersible pump. The floe removing unit 103 employed a sand filtration tank capable of removing impurities having a particle size (diameter) of about 5 urn. The desalination unit 104 employed an RO membrane supplied by Nitto Denko Corporation.

[0085]
To verify effects of the flocculation treatment, a treated water after sand filtration was sampled and evaluated on how the total organic carbon concentration (TOC) and acidic sugar concentration in the treated water and how the separation performance vary depending on the flocculant addition rates. In addition, how parameters affect the flocculation activity was evaluated. The parameters are the molecular weights and functional group equivalents of anionic polymers; ratio between ferric chloride (III) and the anionic polymers to be added; and the ratio between the first polymer flocculant and the second polymer flocculant. The seawater used as an untreated water had a TOC and an acidic sugar concentration of 0.9 ppm and 0.13 ppm, respectively.

[0086] Table 1 provides a summary of data of the examples
[0088]
(Comparative Examples 1 to 4)
These comparative examples employed an apparatus having the same structure as in Example 1, except for using an aqueous ferric chloride (III) solution alone as the flocculant. Evaluations were performed on the properties as with Example 1.
[0089]
(Comparative Example 5)
This comparative example employed an apparatus having the same structure as in Example 1, except for using poly (acrylic acid) s alone as the flocculant. Evaluations were performed on the properties as with Example 1.

[0090]
[Examples 20 to 30]
These examples employed an apparatus having the same structure as in Example 1 and added flocculants at the same addition rates as with Example 1, except for using a first polymer flocculant of a different type having a different molecular weight and/or a different functional group equivalent. Evaluations were performed on the properties as with Example 1.

[0091]
(Comparative Example 6)
This comparative example employed an apparatus having the same structure as in Example 1, added flocculants at the same rates as with Example 1, and employed a poly(acrylic acid) as the first polymer flocculant, except for using no second polymer flocculant. Evaluations were performed on the properties as with Example 1.

[0092]
Table 2 provides a summary of data obtained in the examples and the comparative examples.

We Claim:
1. A seawater desalination apparatus comprising:
an intake unit which takes in seawater including impurities as a water to be treated;
a flocculation unit which includes flocculant reservoirs for storing flocculants and feeds the flocculants from the flocculant reservoirs to the seawater to form floes including the impurities in the seawater;
a floe removing unit which removes the floes from the seawater; and
a desalination unit which desalinates the seawater, wherein:
the flocculants include a metal salt in combination with a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and
the flocculation unit adds the metal salt and the composite flocculant in this order to the seawater.

2 . The seawater desalination apparatus as claimed in Claim 1, wherein one of the anionic polymers contained in the composite flocculant has a weight-average molecular weight of from 100,000 to 1,000,000.

3. The seawater desalination apparatus as claimed in Claim 2, wherein the anionic polymer having a weight-average molecular weight of from 100,000 to 1,000,000 and contained in the composite flocculant is a poly(acrylic acid).

4. The seawater desalination apparatus as claimed in Claim 2, wherein another anionic polymer contained in the composite flocculant has a weight-average molecular weight of 2,000,000 or more.

5. The seawater desalination apparatus as claimed in Claim 4, wherein the anionic polymers contained in the composite flocculant have a functional group equivalent of 10 meq/g or more.

6. The seawater desalination apparatus as claimed in Claim 4, wherein a weight ratio of the low-molecular-weight anionic polymer to the high-molecular-weight anionic polymer contained in the composite flocculant is from 1 to 9.

7. A seawater desalination method comprising the steps of:
taking in seawater including impurities as a water to be treated;
forming floes including the impurities in the seawater by feeding flocculants from flocculant reservoirs to the seawater;
removing the floes from the seawater; and
desalinating the seawater, wherein:
the flocculants include a metal salt in combination with a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and
the metal salt and the composite flocculant are added in this order to the seawater in the step of flocculating.

8. A set of flocculants for seawater desalination, the set of flocculants comprising two different flocculants to be added to seawater containing impurities to thereby remove the impurities from the seawater, wherein:
the two different flocculants are a metal salt and a composite flocculant, the metal salt including a trivalent metal ion, and the composite flocculant including two or more anionic polymers having different weight-average molecular weights from each other; and
the metal salt and the composite flocculant are to be added in this order to the seawater.

9. The set of flocculants for seawater desalination as claimed in Claim 8, wherein one of the anionic polymers contained in the composite flocculant has a weight-average molecular weight of from 100,000 to 1,000,000.

10. The set of flocculants for seawater desalination as claimed in Claim 9, wherein the anionic polymer having a weight-average molecular weight of from 100,000 to 1,000,000 and contained in the composite flocculant is a poly(acrylic acid).

11. The set of flocculants for seawater desalination as claimed in Claim 9, wherein another anionic polymer contained in the composite flocculant has a weight-average molecular weight of 2,000,000 or more.

12. The set of flocculants for seawater desalination as claimed in Claim 11, the anionic polymers contained in the composite flocculant have a functional group equivalent of 10 meq/g or more.

13. The set of flocculants for seawater desalination as claimed in Claim 11,
wherein a weight ratio of the low-molecular-weight anionic polymer to the high-molecular-weight anionic polymer in the composite flocculant is from 1 to 9.