FORM 2
THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention: WATER TREATMENT SYSTEM AND WATER TREATMENT
METHOD
2. Applicant(s)

NAME NATIONALITY ADDRESS
MITSUBISHI POWER, LTD. Japanese 3-1, Minatomirai 3-Chome, Nishi-ku, Yokohama-shi, Kanagawa 2208401, Japan
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

TECHNICAL FIELD
[0001] The present disclosure relates to a water treatment system and a water treatment
method.
BACKGROUND
[0002] In a desalination facility equipped with a reverse osmosis membrane, freshwater is
produced by permeation of saltwater using a reverse osmosis membrane. The saltwater referred to here includes seawater taken from the ocean, seawater that has been used for cooling in a power plant, for example, and an aqueous solution obtained by diluting seawater with an aqueous solution other than seawater.
[0003] One of techniques for producing freshwater from saltwater is disclosed in Patent
Document 1. Patent Document 1 describes that desulfurization of flue gas is performed using seawater (seawater desulfurization), and wastewater generated by desulfurization (desulfurization wastewater) itself is used as supplementary water for a water production apparatus. Further, a technique relating to adjustment of pH of wastewater generated by desulfurization is disclosed in Patent Document 2. Patent Document 2 describes that when seawater is mixed with wastewater generated by seawater desulfurization, the amount of the seawater is such that the pH of the resulting mixed solution (saltwater) is more than 5.5.
Citation List
Patent Literature
[0004]
Patent Document 1: JPS49-110570A (especially see right column on page 3)
Patent Document 2: JP2006-55779A (especially, see paragraph [0003])
SUMMARY

Problems to be Solved
[0005] In a desalination facility equipped with a reverse osmosis membrane, in order to
suppress deterioration (for example, scaling) of the reverse osmosis membrane, agents such as an acid aqueous solution (for example, sulfuric acid) and a scale inhibitor are added to seawater supplied to the reverse osmosis membrane. However, the addition of agents increases the agent cost.
[0006] For example, in the technique described in Patent Document 1, the pH of
wastewater may have to be controlled within an appropriate range depending on the configuration of the water production apparatus. Therefore, there is agent cost for pH control. The technique of Patent Document 2 does not describe that freshwater can be produced in the first place. That is, neither Patent Document 1 nor 2 describes that saltwater having a desired pH is obtained by mixing wastewater generated by seawater desulfurization with seawater, and the obtained saltwater is supplied to a reverse osmosis membrane to produce freshwater.
[0007] The present disclosure was made in view of the above problems. An object
thereof is to provide a water treatment system and a water treatment method whereby it is possible to produce freshwater while reducing the agent cost.
Solution to the Problems
[0008] (1) A water treatment system according at least one embodiment of the present
invention comprises: a desulfurization facility including a desulfurization device for causing a sulfur oxide contained in a flue gas to be absorbed in seawater by bringing the seawater and the flue gas into contact; a desalination facility including at least one reverse osmosis membrane for producing freshwater from saltwater which is a mixture of wastewater from the desulfurization facility and seawater; at least one pH measurement device for measuring pH of water flowing through a water supply line connecting the desulfurization facility and the desalination facility; and a control device including a mixing ratio determination unit for determining a mixing ratio between the wastewater and the seawater in the saltwater, based on

pH measured by the at least one pH measurement device.
[0009] With the above configuration (1), the mixing ratio determination unit can adjust
the pH of a large amount of saltwater (mixture of desulfurization wastewater and seawater)
supplied to the reverse osmosis membrane to a pH suitable for the reverse osmosis membrane,
without using a large amount of agents for adjusting the pH to be suitable for the reverse
osmosis membrane. Thus, it is possible to produce freshwater while reducing the agent cost
for suppressing deterioration of the reverse osmosis membrane.
[0010] (2) In some embodiments, in the above configuration (1), the at least one pH
measurement device includes a first pH measurement device for measuring pH of the
saltwater which is the mixture of wastewater and seawater and is supplied to the at least one
reverse osmosis membrane. The mixing ratio determination unit is configured to determine
the mixing ratio between the wastewater and the seawater in the saltwater, based on pH
measured by the first pH measurement device.
[0011] With the above configuration (2), the pH of the saltwater can be measured by the
first pH measurement device, so that the mixing ratio such that the pH of the saltwater is in a
set range can be determined while measuring the pH of the saltwater by the first pH
measurement device. Thus, the pH of the saltwater can be controlled to suppress
deterioration of the reverse osmosis membrane.
[0012] (3) In some embodiments, in the above configuration (1) or (2), the at least one pH
measurement device includes a second pH measurement device for measuring pH of
wastewater from the desulfurization facility after absorption of the sulfur oxide. The mixing
ratio determination unit is configured to determine the mixing ratio between the wastewater
and the seawater in the saltwater, based on pH measured by the second pH measurement
device.
[0013] With the above configuration (3), since the fluctuation range of pH of seawater is
small, the mixing ratio such that the pH of the saltwater is in a set range can be determined
based on the pH of the wastewater measured by the second pH measurement device and the
pH of the seawater which is constant to some extent. Thus, the pH of the saltwater can be

controlled to suppress deterioration of the reverse osmosis membrane.
[0014] (4) In some embodiments, in any one of the above configurations (1) to (3), the
mixing ratio determination unit is configured to determine the mixing ratio between the
wastewater and the seawater so that pH of the saltwater is in a range of 4 or more.
[0015] With the above configuration (4), since the pH of the saltwater is in the above
range, the production amount of sulfurous acid (H2SO3) in the saltwater can be reduced, while
the production amount of hydrogen sulfite ion (HSO3-) and sulfite ion (SO32-) can be
increased. Sulfurous acid easily permeates the reverse osmosis membrane, but hydrogen
sulfite ion and sulfite ion do not easily permeate the reverse osmosis membrane. Thus, when
the production amount of hydrogen sulfite ion and sulfite ion is increased, sulfur derived from
sulfur oxides in the flue gas can be easily removed in the form of hydrogen sulfite ion and
sulfite ion (SO32-).
[0016] (5) In some embodiments, in any one of the above configurations (1) to (4), the
mixing ratio determination unit is configured to determine the mixing ratio between the
wastewater and the seawater so that pH of the saltwater is in a range of 7.8 or less.
[0017] With the above configuration (5), since the pH of the saltwater is in the above
range, deposition of sparingly soluble salts on the reverse osmosis membrane can be
suppressed, so that scaling can be suppressed. Thus, deterioration of the reverse osmosis
membrane can be suppressed.
[0018] (6) In some embodiments, in any one of the above configurations (1) to (5), the
mixing ratio determination unit is configured to determine the mixing ratio between the
wastewater and the seawater so that pH of the saltwater is in a range of 4 or more and 7.8 or
less.
[0019] With the above configuration (6), since the pH of the saltwater is in the above
range, sulfur derived from sulfur oxides in the flue gas can be easily removed in the form of
hydrogen sulfite ion and sulfite ion, and deterioration of the reverse osmosis membrane can be
sufficiently suppressed.
[0020] (7) In some embodiments, in any one of the above configurations (1) to (6), the

water treatment system comprises a mixing tank for obtaining the saltwater which is the
mixture by mixing seawater and wastewater from the desulfurization facility.
[0021] With the above configuration (7), the seawater and the wastewater can be
sufficiently mixed in the mixing tank. As a result, local changes in pH of the saltwater
mixture are suppressed, and the pH can be stably measured.
[0022] (8) In some embodiments, in any one of the above configurations (1) to (7), the at
least one reverse osmosis membrane includes a first reverse osmosis membrane for desalting
the saltwater and a second reverse osmosis membrane, disposed downstream of the first
reverse osmosis membrane, for removing boron in permeate produced in the first reverse
osmosis membrane. The desulfurization facility includes an aeration tank for aerating
wastewater after absorption of the sulfur oxide in the desulfurization device. The saltwater
supplied to the first reverse osmosis membrane includes the aerated wastewater.
[0023] With the above configuration (8), carbon dioxide can be released by the aeration
tank, so that the pH of the wastewater can be brought close to neutral, and the buffering effect
of carbon dioxide can be reduced. Here, in the second reverse osmosis membrane, borate
ion, which is stable in alkaline conditions, is removed as boron to be removed. Therefore,
the use of the aerated wastewater, which has a near neutral pH and a low carbon dioxide
concentration, can reduce the amount of a pH adjuster used for shifting the pH of the saltwater
to the alkaline side. Thus, it is possible to reduce the cost of using the pH adjuster.
[0024] (9) In some embodiments, in any one of the above configurations (1) to (8), the at
least one reverse osmosis membrane is composed of cellulose triacetate. The mixing ratio
determination unit is configured to determine the mixing ratio between the wastewater and the
seawater so that pH of the saltwater is in a range of 4 or more and 6.5 or less.
[0025] With the above configuration (9), since the pH of the saltwater is in the above
range, sulfur derived from sulfur oxides in the flue gas can be easily removed in the form of
hydrogen sulfite ion and sulfite ion, hydrolysis of cellulose triacetate can be suppressed, and
deterioration of the reverse osmosis membrane can be sufficiently suppressed.
[0026] (10) In some embodiments, in any one of the above configurations (1) to (9), the

water treatment system comprises an oxidation index value measurement device for
measuring an index value indicating degree of oxidation of the saltwater.
[0027] With the above configuration (10), the oxidation index value attributable to an
oxidizing agent such as sulfurous acid or dissolved oxygen can be measured. Accordingly,
the saltwater having an oxidation index value that does not allow the oxidative deterioration
of the reverse osmosis membrane to proceed as much as possible can be supplied to the
reverse osmosis membrane, and the oxidative deterioration of the reverse osmosis membrane
can be suppressed.
[0028] (11) In some embodiments, in the above configuration (10), the oxidation index
value measurement device includes at least one of an oxidation-reduction potential meter or a
residual chlorine concentration meter.
[0029] With the above configuration (11), the oxidation index value can be measured with
a simple device.
[0030] (12) In some embodiments, in the above configuration (10) or (11), the oxidation
index value measurement device is configured to measure an index value indicating degree of
oxidation of condensate produced in the at least one reverse osmosis membrane.
[0031] With the above configuration (12), since the oxidizing agent (e.g., sulfite ion) is
concentrated in the condensate, it is possible to improve the detection sensitivity.
[0032] (13) In some embodiments, in any one of the above configurations (1) to (12), the
water treatment system comprises a radical reaction inhibitor supply device for supplying a
radical reaction inhibitor for inhibiting radical reaction to water flowing upstream of the at
least one reverse osmosis membrane.
[0033] With the above configuration (13), the radical reaction inhibitor suppresses radical
reaction of sulfurous acid in the saltwater, so that the mass production of sulfurous acid due to
radical reaction can be suppressed. Thus, oxidative deterioration of the reverse osmosis
membrane due to oxidizability of sulfurous acid can be suppressed.
[0034] (14) In some embodiments, in any one of the above configurations (1) to (13), the
water treatment system comprises a pH adjuster supply device for supplying a pH adjuster for

adjusting pH of the saltwater to water flowing upstream of the at least one reverse osmosis
membrane.
[0035] With the above configuration (14), even if the pH of the saltwater supplied to the
reverse osmosis membrane cannot be controlled within a desired range with the seawater
alone, the pH of the saltwater can be easily controlled within the desired range with the pH
adjuster.
[0036] (15) In some embodiments, in any one of the above configurations (1) to (14), the
desulfurization facility includes an aeration tank for aerating wastewater after absorption of
the sulfur oxide in the desulfurization device. The water treatment system includes a
condensate supply line for supplying condensate produced in the at least one reverse osmosis
membrane to the aeration tank.
[0037] When the condensate produced in the reverse osmosis membrane contains a large
amount of sulfurous acid, the condensate cannot be discharged as it is. However, with the
above configuration (15), the condensate can be returned to the aeration tank, and sulfurous
acid contained in the condensate can be oxidized to reduce the sulfurous acid concentration,
so that it is possible to reduce the COD of water to be discharged.
[0038] (16) A water treatment method according to at least one embodiment of the present
invention comprises: a desulfurization step of causing a sulfur oxide contained in a flue gas to
be absorbed in seawater by bringing the seawater and the flue gas into contact in a
desulfurization facility; a desalination step of producing freshwater from saltwater which is a
mixture of wastewater from the desulfurization facility and seawater with at least one reverse
osmosis membrane in a desalination facility; a pH measurement step of measuring pH of
water flowing through a water supply line connecting the desulfurization facility and the
desalination facility; and a mixing ratio determination step of determining a mixing ratio
between the wastewater and the seawater in the saltwater, based on pH measured in the pH
measurement step.
[0039] With the above method (16), the mixing ratio determination step can adjust the pH
of a large amount of saltwater (mixture of desulfurization wastewater and seawater) supplied

to the reverse osmosis membrane to a pH suitable for the reverse osmosis membrane, without using a large amount of agents for adjusting the pH to be suitable for the reverse osmosis membrane. Thus, it is possible to produce freshwater while reducing the agent cost for suppressing deterioration of the reverse osmosis membrane.
Advantageous Effects
[0040] The present disclosure provides a water treatment system and a water treatment
method whereby it is possible to produce freshwater while reducing the agent cost.
BRIEF DESCRIPTION OF DRAWINGS
[0041] FIG. 1 is a system diagram of a water treatment system according to a first
embodiment of the present invention.
FIG. 2 is a block diagram of a control device.
FIG. 3 is a graph showing a correlation between the pH of saltwater and the component ratio of sulfites in saltwater.
FIG. 4 is a flowchart of a water treatment method according to a first embodiment of the present invention.
FIG. 5 is a system diagram of a water treatment system according to a second embodiment of the present invention.
FIG. 6 is a system diagram of a water treatment system according to a third embodiment of the present invention.
FIG. 7 is a system diagram of a water treatment system according to a fourth embodiment of the present invention.
FIG. 8 is a system diagram of a water treatment system according to a fifth embodiment of the present invention.
FIG. 9 is a system diagram of a water treatment system according to a sixth embodiment of the present invention.
FIG. 10 is a system diagram of a water treatment system according to a seventh

embodiment of the present invention.
DETAILED DESCRIPTION
[0042] Embodiments of the present invention will now be described in detail with
reference to the accompanying drawings. However, the following embodiments and the drawings are illustrative only, and various modifications may be applied as long as they do not depart from the object of the present invention. Further, two or more embodiments may be optionally combined in any manner. Further, in the following embodiments, similar elements will be indicated by the same reference numerals, and redundant descriptions thereof will be omitted for convenience. Further, in the second and subsequent embodiments, in order to avoid duplication of description, the description will be focused on the points different from the first embodiment.
[0043] It is intended, however, that unless particularly specified, dimensions, materials,
shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention.
For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function.
For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function.
Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved.

On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components.
[0044] FIG. 1 is a system diagram of a water treatment system 100 according to a first
embodiment of the present invention. The water treatment system 100 performs
desulfurization of flue gas using seawater, and produces freshwater using wastewater generated by the desulfurization (desulfurization wastewater). The production of freshwater is performed using saltwater obtained by mixing desulfurization wastewater having a pH of typically 7 or less (i.e., acidic desulfurization wastewater) with seawater (having a pH of about 7.5 to 9, for example).
[0045] The water treatment system 100 includes a desulfurization facility 10, a mixing
tank 2, a desalination facility 20, a first pH measurement device 41 as at least one pH measurement device, and a control device 30.
[0046] The desulfurization facility 10 includes a desulfurization device 11 for causing
sulfur oxides (e.g., sulfur dioxide) contained in flue gas to be absorbed in seawater by
bringing the seawater and the flue gas into contact. The seawater supplied to the
desulfurization device 11 is, for example, seawater taken from the ocean in the intake tank 1
and supplied through a first seawater supply line 51. The desulfurization device 11 includes,
for example, a casing (not shown) into which the flue gas is introduced, a spray nozzle (not
shown) disposed in the casing, and a packing material (grid, not shown). Inside the casing,
the seawater is sprayed from the spray nozzle to the packing material, and the seawater passes
through the packing material. Further, the flue gas introduced into the casing passes through
the packing material. The seawater and the flue gas are brought into gas-liquid contact
inside the packing material so that sulfur oxides in the flue gas are absorbed in the seawater.
The flue gas from which sulfur oxides have been removed is discharged as purified gas.
[0047] Sulfur oxides contained in the flue gas are present as, for example, sulfurous acid
(H2SO3), sulfite ion (SO32-), hydrogen sulfite ion (HSO3-) in wastewater although details will be described later with reference to FIG. 3. Hereinafter, they are collectively referred to as “sulfites”.

[0048] The desulfurization facility 10 includes an aeration tank 12 for aerating
wastewater after absorption of sulfur oxides in the desulfurization device 11 (i.e., desulfurization wastewater containing sulfites), and a discharge tank 13 for discharging the aerated wastewater to the ocean or the like. The wastewater that has absorbed sulfur oxides in the desulfurization device 11 is supplied to the aeration tank 12. The aeration tank 12 includes, for example, a diffuser tube (not shown) for diffusing the air. In the aeration tank 12, carbon dioxide in the wastewater is released to the gas phase by diffusion (bubbling) of the air into the wastewater through the diffuser tube. As a result, the pH of the wastewater can be increased. The aerated wastewater is discharged to the ocean or the like via the discharge tank 13.
[0049] In the water treatment system 100, wastewater generated in the desulfurization
device 11 is supplied to the aeration tank 12 as described above. However, the water
treatment system 100 includes a wastewater supply line 53 for supplying a part of the
wastewater generated in the desulfurization device 11 to the mixing tank 2. The wastewater
supply line 53 may be connected between the desulfurization device 11 and the aeration tank
12 as shown in FIG. 1. Alternatively, the wastewater supply line 53 may be directly
connected to the desulfurization device 11, the aeration tank 12, or the discharge tank 13, or
may be connected between the aeration tank 12 and the discharge tank 13, or may be
connected to the downstream side of the discharge tank 13, although not depicted.
[0050] The wastewater supply line 53 includes a flow rate control device 43 for
controlling the flow rate of the wastewater. The flow rate control device 43 may be, for example, a valve, a gate (water gate), or a pump controlled by an inverter. The flow rate control device 43 is connected to the control device 30 by an electrical signal line represented by the dashed arrow in FIG. 1.
[0051] The mixing tank 2 serves to mix seawater and wastewater from the desulfurization
device 11 to obtain saltwater which is a mixture. The provision of the mixing tank 2 enables sufficient mixing of seawater and wastewater in the mixing tank 2. As a result, local changes in pH of the saltwater mixture are suppressed, and the pH can be stably measured.

[0052] The mixing tank 2 is supplied with seawater through a second seawater supply line
52 connected to the intake tank 1. Therefore, the seawater supplied to the desulfurization facility 10 and the seawater supplied to the mixing tank 2 are both supplied from the intake tank 1. Thus, the intake tank 1 can be shared, and the equipment cost can be reduced. Further, the mixing tank 2 is connected to the wastewater supply line 53 as described above. In the mixing tank 2, the supplied seawater and wastewater are sufficiently agitated and mixed by an agitator blade (not shown) disposed in the mixing tank 2.
[0053] The second seawater supply line 52 connected to the mixing tank 2 includes a
flow rate control device 44 for controlling the flow rate of the seawater. The flow rate control device 44 may be, for example, a valve, a gate (water gate), or a pump controlled by an inverter. The flow rate control device 44 is connected to the control device 30 by an electrical signal line represented by the dashed arrow in FIG. 1. The function of the control device 30 will be described later with reference to FIG. 2.
[0054] The mixing tank 2 is connected to a saltwater supply line 54 (water supply line).
The saltwater supply line 54 is connected to the desalination facility 20 for producing freshwater from saltwater. Accordingly, the saltwater produced in the mixing tank 2 is supplied to the desalination facility 20 through the saltwater supply line 54.
[0055] The at least one pH measurement device includes a first pH measurement device
41, disposed on the saltwater supply line 54 (water supply line), for measuring the pH of saltwater which is the mixture of wastewater and seawater and is supplied to a first reverse osmosis membrane 22 (at least one reverse osmosis membrane). The first pH measurement device 41 is connected to the control device 30 by an electrical signal line represented by the dashed arrow in FIG. 1. Further, a mixing ratio determination unit 32 (see FIG. 2) of the control device 30 is configured to determine the mixing ratio between the wastewater and the seawater in the saltwater, based on the pH measured by the first pH measurement device 41. This point will be described with reference to FIG. 2.
[0056] When the first pH measurement device 41 is included, the pH of the saltwater can
be measured by the first pH measurement device 41, so that the mixing ratio such that the pH

of the saltwater is in a set range can be determined while measuring the pH of the saltwater by
the first pH measurement device 41. The control device 30 controls the flow rate control
device 43, 44 so as to achieve the determined mixing ratio. Thus, the pH of the saltwater can
be controlled to suppress deterioration of the reverse osmosis membrane.
[0057] The desalination facility 20 includes a pretreatment tank 21 and a first reverse
osmosis membrane 22. Of these, the pretreatment tank 21 serves to remove turbidity in the
saltwater supplied to the first reverse osmosis membrane 22. Specifically, aggregates are
generated by adding a flocculant (e.g., aluminum polychloride) to the saltwater flowing
through the saltwater supply line 54 and coagulating turbidity in the saltwater. In the
pretreatment tank 21, the generated aggregates are precipitated, and the precipitated
aggregates are removed.
[0058] The first reverse osmosis membrane 22 (at least one reverse osmosis membrane)
produces freshwater from the saltwater which is the mixture of wastewater from the
desulfurization facility 10 and seawater. The saltwater used here is saltwater from which
turbidity has been removed as described above. By using the saltwater from which turbidity
has been removed, the clogging of the first reverse osmosis membrane 22 can be sufficiently
suppressed.
[0059] The permeate obtained by permeation of the saltwater through the first reverse
osmosis membrane 22 can be used as freshwater. Thus, freshwater is produced by the
desalination facility 20. Further, the condensate that has not permeated the first reverse
osmosis membrane 22 is discharged to the outside of the system.
[0060] FIG. 2 is a block diagram of the control device 30. The control device 30
includes an information acquisition unit 31, a mixing ratio determination unit 32, and a flow
rate control unit 33. Of these, the information acquisition unit 31 is configured to measure
and acquire the pH of the saltwater (water) flowing through the saltwater supply line 54
(water supply line) with the first pH measurement device 41.
[0061] The mixing ratio determination unit 32 is configured to determine the mixing ratio
between the wastewater and the seawater in the saltwater obtained in the mixing tank 2, based

on the pH measured by the first pH measurement device 41 (at least one pH measurement device). Specifically, the mixing ratio determination unit 32 is configured to determine the mixing ratio between the wastewater and the seawater in the saltwater, based on the pH measured by the first pH measurement device 41.
[0062] With this configuration, since the fluctuation range of pH of seawater is small, the
mixing ratio such that the pH of the saltwater is in a set range can be determined based on the pH of the wastewater measured by the first pH measurement device and the pH of the seawater which is constant to some extent. Thus, the pH of the saltwater can be controlled to suppress deterioration of the first reverse osmosis membrane 22.
[0063] In the water treatment system 100, the mixing ratio determination unit 32 is
configured to determine the mixing ratio between the wastewater and the seawater so that the pH of the saltwater is in a range of 4 or more. When the pH of the saltwater is in the above range, the production amount of sulfurous acid (H2SO3) in the saltwater can be reduced, while the production amount of hydrogen sulfite ion (HSO3-) and sulfite ion (SO32-) can be increased. Sulfurous acid easily permeates the first reverse osmosis membrane 22, but hydrogen sulfite ion and sulfite ion do not easily permeate the first reverse osmosis membrane 22. Thus, when the production amount of hydrogen sulfite ion and sulfite ion is increased, sulfur (sulfites) derived from sulfur oxides in the flue gas can be easily removed in the form of hydrogen sulfite ion and sulfite ion. This point will now be described with reference to FIG. 3.
[0064] FIG. 3 is a graph showing a correlation between the pH of saltwater and the
component ratio of sulfites in saltwater. This graph shows what kind of form sulfites exist in
saltwater. The horizontal axis represents the pH of saltwater, and the vertical axis represents
the component ratio of sulfites. The dashed graph represents the component ratio of
sulfurous acid (H2SO3), the solid graph represents the component ratio of hydrogen sulfite ion
(HSO3-), and the dot-dashed graph shows the component ratio of sulfite ion (SO32-).
[0065] The component ratio of sulfurous acid (represented by the dashed line) is a
downward-sloping graph, and as the pH of saltwater gradually increases, the component ratio

of sulfurous acid decreases. The component ratio of hydrogen sulfite ion (represented by the solid line) is a convex graph, and as the pH of saltwater gradually increases, the component ratio of hydrogen sulfite ion increases. However, when the pH of saltwater reaches about 4, the component ratio of hydrogen sulfite ion starts to decrease, and the component ratio of hydrogen sulfite ion decreases. The component ratio of sulfite ion (represented by the dot-dashed line) is an upward-sloping graph, and as the pH of saltwater gradually increases from about 5, the component ratio of sulfite ion increases.
[0066] Here, sulfurous acid easily permeates the reverse osmosis membrane constituting
the first reverse osmosis membrane 22, but hydrogen sulfite ion and sulfite ion do not easily permeate the reverse osmosis membrane. That is, when hydrogen sulfite ion and sulfite ion are contained as sulfites in the saltwater supplied to the first reverse osmosis membrane 22, sulfur derived from sulfur oxides in the flue gas can be removed from the saltwater in the form of hydrogen sulfite ion and sulfite ion. Therefore, in the water treatment system 100, by setting the pH of the saltwater to 4 or more, the component ratio of sulfite ion and sulfite ion in the saltwater is increased to easily remove sulfites from the saltwater.
[0067] The mixing ratio determination unit 32 (see FIG. 2) may be configured to
determine the mixing ratio between the wastewater and the seawater so that the pH of the saltwater is in a range of 7.8 or less, preferably 7.2 or less. When the pH of the saltwater is in the above range, deposition of sparingly soluble salts on the first reverse osmosis membrane 22 is suppressed, so that scaling can be suppressed. Thus, deterioration of the first reverse osmosis membrane 22 can be suppressed.
[0068] Additionally, the mixing ratio determination unit 32 (see FIG. 2) may be
configured to determine the mixing ratio between the wastewater and the seawater so that the
pH of the saltwater is in a range of 4 or more and 7.8 or less. When the pH of the saltwater
is in the above range, sulfur derived from sulfur oxides in the flue gas can be easily removed
in the form of hydrogen sulfite ion and sulfite ion, and deterioration of the first reverse
osmosis membrane 22 can be sufficiently suppressed.
[0069] However, when the first reverse osmosis membrane 22 is composed of cellulose

triacetate (CTA), the mixing ratio determination unit 32 is preferably configured to determine
the mixing ratio between the wastewater and the seawater so that the pH of the saltwater is in
a range of 4 or more and 6.5 or less. When the pH of the saltwater is in the above range,
sulfur derived from sulfur oxides in the flue gas can be easily removed in the form of
hydrogen sulfite ion and sulfite ion, hydrolysis of cellulose triacetate can be suppressed, and
deterioration of the first reverse osmosis membrane 22 can be sufficiently suppressed.
[0070] The flow rate control unit 33 controls the flow rate control device 43, 44 so as to
achieve the mixing ratio determined by the mixing ratio determination unit 32. In the water treatment system 100, the flow rate of the saltwater supplied to the desalination facility 20 is controlled to be constant.
[0071] The control device 30 includes a CPU (Central Processing Unit), a ROM (Read
Only Memory), a RAM (Random Access Memory), and an I/F (Interface), although not depicted. The control device 30 is implemented by executing a predetermined control program stored in the ROM with the CPU.
[0072] With the water treatment system 100, the mixing ratio determination unit 32 can
adjust the pH of a large amount of saltwater (mixture of desulfurization wastewater and seawater) supplied to the first reverse osmosis membrane 22 to a pH suitable for the first reverse osmosis membrane 22, without using a large amount of agents for adjusting the pH to be suitable for the first reverse osmosis membrane 22. Thus, it is possible to produce freshwater while reducing the agent cost for suppressing deterioration of the first reverse osmosis membrane 22.
[0073] For example, when flue gas is generated by combustion of fuel (e.g., waste), the
composition of the flue gas varies depending on the fuel composition. Furthermore, when the flue gas is discharged from, for example, a thermal power generation plant, the composition of the flue gas also varies depending on fluctuations in the load of power generation. Even if the pH of the wastewater (desulfurization wastewater) changes due to the variation in the composition of the flue gas, the pH of the saltwater can be adjusted to a pH suitable for the first reverse osmosis membrane 22 by determining the mixing ratio

between the seawater and the wastewater based on the pH of the saltwater supplied to the first reverse osmosis membrane 22.
[0074] FIG. 4 is a flowchart of a water treatment method according to a first embodiment
of the present invention. This flowchart is performed with the control device 30. Therefore, FIG. 4 will be described with reference to FIGs. 1 and 2 as appropriate.
[0075] The water treatment method according to the first embodiment includes a
desulfurization step S1 performed in the desulfurization facility 10, a desalination step S2 performed in the desalination facility 20, a pH measurement step S3, a mixing ratio determination step S4, and a flow rate control step S5. During operation of the water treatment system 100, the desulfurization step S1 of causing sulfur oxides contained in flue gas to be absorbed in seawater by bringing the seawater and the flue gas into contact and the desalination step S2 of producing freshwater from saltwater, which is a mixture of wastewater from the desulfurization facility 10 and seawater, with the first reverse osmosis membrane 22 (at least one reverse osmosis membrane) are performed in parallel.
[0076] The information acquisition unit 31 measures and acquires the pH of water
flowing through the water supply line connecting the desulfurization facility 10 and the desalination facility 20 (pH measurement step S3). Specifically, in the water treatment system 100, the information acquisition unit 31 measures and acquires the pH of the saltwater, which is the mixture of wastewater and seawater and is supplied to the first reverse osmosis membrane 22 (at least one reverse osmosis membrane), with the first pH measurement device 41 disposed on the saltwater supply line 54 (water supply line).
[0077] Then, the mixing ratio determination unit 32 (see FIG. 2) determines the mixing
ratio between the wastewater and the seawater in the saltwater, based on the pH measured in the pH measurement step S3 (mixing ratio determination step S4). Specifically, for example, the mixing ratio determination unit 32 determines the mixing ratio between the wastewater and the seawater so that the pH of the saltwater is in a range of 4 or more. For example, since the pH of the seawater is higher than the pH of the wastewater (desulfurization wastewater), when the pH of the saltwater is less than the set range, the mixing ratio is

determined so as to increase the flow rate ratio of the seawater. Conversely, when the pH of the saltwater is more than the set range, the mixing ratio is determined so as to decrease the flow rate ratio of the seawater. Further, the flow rate control unit 33 (see FIG. 2) controls the opening degree of the flow rate control device 43, 44 so that the determined mixing ratio is achieved and the flow rate of the saltwater supplied to the desalination facility 20 is constant. As a result, the saltwater having a pH contained in the set range is supplied to the desalination facility 20, and freshwater is produced.
[0078] With the water treatment method according to the first embodiment, the mixing
ratio determination step S4 can adjust the pH of a large amount of the saltwater (mixture of desulfurization wastewater and seawater) supplied to the first reverse osmosis membrane 22 to a pH suitable for the first reverse osmosis membrane 22, without using a large amount of agents for adjusting the pH to be suitable for the first reverse osmosis membrane 22. Thus, it is possible to produce freshwater while reducing the agent cost for suppressing deterioration of the first reverse osmosis membrane 22.
[0079] FIG. 5 is a system diagram of a water treatment system 200 according to a second
embodiment of the present invention. The water treatment system 200 includes at least one pH measurement device for measuring the pH of water flowing through a water supply line connecting the desulfurization facility 10 and the desalination facility 20, as with the water treatment system 100. However, the water treatment system 200 includes a second pH measurement device 45 disposed on the wastewater supply line 53 (water supply line), instead of the first pH measurement device 41.
[0080] The second pH measurement device 45 included in the at least one pH
measurement device is for measuring the pH of wastewater from the desulfurization facility 10 after absorption of sulfur oxides. The second pH measurement device 45 is connected to the control device 30 by an electrical signal line represented by the dashed arrow in FIG. 5. Further, the water treatment system 200 includes a flow rate measurement device 42 for measuring the flow rate of wastewater flowing through the wastewater supply line 53. The flow rate measurement device 42 is connected to the control device 30 by an electrical signal

line represented by the dashed arrow in FIG. 5.
[0081] In the water treatment system 200, the mixing ratio determination unit 32 (see FIG.
2) is configured to determine the mixing ratio between the wastewater and the seawater in the saltwater, based on the pH measured by the second pH measurement device 45 and the flow rate of the wastewater measured by the flow rate measurement device 42. With this configuration, since the fluctuation range of pH of seawater is small, the mixing ratio such that the pH of the saltwater is in a set range can be determined based on the pH of the wastewater measured by the second pH measurement device 45 and the pH of the seawater which is constant to some extent. Thus, the pH of the saltwater can be controlled to suppress deterioration of the first reverse osmosis membrane 22.
[0082] FIG. 6 is a system diagram of a water treatment system 300 according to a third
embodiment of the present invention. In the desalination facility 20 of the water treatment
system 300, the at least one reverse osmosis membrane includes a first reverse osmosis
membrane 22 for desalting the saltwater and a second reverse osmosis membrane 23,
disposed downstream of the first reverse osmosis membrane 22, for removing boron in
permeate produced in the first reverse osmosis membrane 22. Thus, the desalination facility
20 includes the first reverse osmosis membrane 22 and the second reverse osmosis membrane
23. Unlike the first reverse osmosis membrane 22, the second reverse osmosis membrane 23
is made of a material (for example, polyamide) that is stable in an alkaline solution.
[0083] In the water treatment system 300, the wastewater supply line 53 is connected to
the aeration tank 12 of the desulfurization facility 10. Accordingly, in the mixing tank 2, the seawater and the wastewater from the aeration tank 12 are mixed, and the saltwater supplied to the first reverse osmosis membrane 22 includes the aerated wastewater.
[0084] A pH adjuster is added to permeate produced by permeation through the first
reverse osmosis membrane 22 so that the pH of the permeate is, for example, 9 or more and 10 or less. Thus, the second reverse osmosis membrane 23 is supplied with water having a pH of, for example, 9 or more and 10 or less. When the pH is alkaline, it is possible to remove boron by the second reverse osmosis membrane 23. In particular, since the pH of

water having a near neutral pH due to the removal of carbon dioxide in the aeration tank 12 is shifted to alkaline, the cost of using the pH adjuster can be reduced.
[0085] With the water treatment system 300, carbon dioxide can be released by the
aeration tank 12, so that the pH of the wastewater can be brought close to neutral, and the buffering effect of carbon dioxide can be reduced. Here, in the second reverse osmosis membrane 23, borate ion, which is stable in alkaline conditions, is removed as boron to be removed. Therefore, the use of the aerated wastewater, which has a near neutral pH and a low carbon dioxide concentration, can reduce the amount of the pH adjuster used for shifting the pH of the saltwater to the alkaline side. Thus, it is possible to reduce the cost of using the pH adjuster. In addition, it is possible to reduce the risk of precipitation of calcium carbonate scale.
[0086] FIG. 7 is a system diagram of a water treatment system 400 according to a fourth
embodiment of the present invention. The water treatment system 400 includes an oxidation index value measurement device 46 for measuring an index value indicating the degree of oxidation of the saltwater flowing through the saltwater supply line 54. When the oxidation index value measurement device 46 is provided, the oxidation index value attributable to an oxidizing agent such as sulfurous acid or dissolved oxygen can be measured. Accordingly, the saltwater having an oxidation index value that does not allow the oxidative deterioration of the first reverse osmosis membrane 22 to proceed as much as possible can be supplied to the first reverse osmosis membrane 22, and the oxidative deterioration of the first reverse osmosis membrane 22 can be suppressed.
[0087] The oxidation index value measurement device 46 includes at least one of an
oxidation-reduction potential meter (ORP meter) or a residual chlorine concentration meter, although not depicted. With this configuration, the oxidation index value can be measured with a simple device.
[0088] In the case where the oxidation index value measurement device 46 includes the
oxidation-reduction potential meter, the oxidation-reduction potential (ORP) of the saltwater is preferably, for example, 450 mV or less when a silver-silver chloride electrode is used as a

reference electrode, although it varies depending on the pH. When the ORP is in this range, the concentration of sulfurous acid, which is a starting substance of radical reaction and an oxidizing agent, can be reduced. As a result, radical reaction starting from sulfurous acid can be suppressed, and the explosive production of sulfurous acid can be suppressed. When the ORP of the saltwater is out of the above range, a radical reaction inhibitor (described later) may be supplied to the saltwater to suppress radical reaction starting from sulfurous acid. The ORP of the saltwater may be controlled by, for instance, adding an oxidation-reducing agent (e.g., air) to the saltwater.
[0089] In the case where the oxidation index value measurement device 46 includes the
residual chlorine concentration meter, the residual chlorine concentration of the saltwater is
preferably, for example, 0.01 mg/L or less. Specifically, when sulfurous acid is contained in
the saltwater, ionic chlorine (e.g., chloride ion) is oxidized to produce chlorine molecules
(gaseous chlorine). Therefore, by measuring the residual chlorine concentration and
grasping the chlorine molecule concentration in the saltwater, the presence of sulfurous acid can be indirectly evaluated. More specifically, for example, when the residual chlorine concentration is low, it can be estimated that the sulfurous acid concentration is low, and when the residual chlorine concentration is high, it can be estimated that the sulfurous acid concentration is high. When the residual chlorine concentration of the saltwater is out of the above range, a radical reaction inhibitor (described later) may be supplied to the saltwater to suppress radical reaction starting from sulfurous acid.
[0090] The water treatment system 400 includes a radical reaction inhibitor supply device
47 for supplying a radical reaction inhibitor for inhibiting radical reaction to water flowing upstream of the first reverse osmosis membrane 22 (at least one reverse osmosis membrane). Specifically, the water treatment system 400 includes a radical reaction inhibitor supply device 47 for supplying a radical reaction inhibitor to the saltwater flowing through the saltwater supply line 54. When the radical reaction inhibitor supply device 47 is provided, the radical reaction inhibitor suppresses radical reaction of sulfurous acid in the saltwater, so that the mass production of sulfurous acid due to radical reaction can be suppressed. Thus,

oxidative deterioration of the first reverse osmosis membrane 22 due to oxidizability of sulfurous acid can be suppressed.
[0091] Examples of the radical reaction inhibitor include chelating agents for capturing
heavy metal that serves as a catalyst for accelerating the reaction rate of radical reaction.
Illustrative examples thereof include ethylenediaminetetraacetic acid (EDTA). In addition,
examples of the radical reaction inhibitor include radical polymerization inhibitors for
inhibiting polymerization of radicals. Illustrative examples thereof include organic
substances such as hydroquinone. In addition, examples of the radical reaction inhibitor include radical scavengers for stopping the progress of radical reaction by capturing radicals. Illustrative examples thereof include organic substances such as mequinol.
[0092] FIG. 8 is a system diagram of a water treatment system 500 according to a fifth
embodiment of the present invention. The water treatment system 500 includes the
oxidation index value measurement device 46, as with the water treatment system 400.
However, in the water treatment system 500, the oxidation index value measurement device
46 is configured to measure an index value indicating the degree of oxidation of condensate
produced in the first reverse osmosis membrane 22 (at least one reverse osmosis membrane).
[0093] When the oxidation index value measurement device 46 is configured to measure
an index value indicating the degree of oxidation of condensate, since the oxidizing agent (e.g., sulfite ion) is concentrated in the condensate, it is possible to improve the detection sensitivity.
[0094] FIG. 9 is a system diagram of a water treatment system 600 according to a sixth
embodiment of the present invention. The water treatment system 600 includes a pH adjuster supply device 48 for supplying a pH adjuster for adjusting the pH of the saltwater to water flowing upstream of the first reverse osmosis membrane 22 (at least one reverse osmosis membrane). When the pH adjuster supply device 48 is provided, even if the pH of the saltwater supplied to the first reverse osmosis membrane 22 cannot be controlled within a desired range with the seawater alone, the pH of the saltwater can be easily controlled within the desired range with the pH adjuster.

[0095] Examples of the pH adjuster include acids such as sulfuric acid, hydrochloric acid,
and nitric acid, and alkalis such as sodium hydroxide and potassium hydroxide.
[0096] FIG. 10 is a system diagram of a water treatment system 700 according to a
seventh embodiment of the present invention. The water treatment system 700 includes a condensate supply line 55 for supplying condensate produced in the first reverse osmosis membrane 22 (at least one reverse osmosis membrane) to the aeration tank 12. Through the condensate supply line 55, condensate produced in the first reverse osmosis membrane 22 is supplied to the aeration tank 12.
[0097] When the sulfurous acid concentration in the saltwater supplied to the first reverse
osmosis membrane 22 is high, the sulfurous acid concentration in the condensate is increased. As a result, the COD of the condensate is increased, and it may be difficult to discharge the condensate into the ocean as it is. When the condensate produced in the first reverse osmosis membrane 22 contains a large amount of sulfurous acid, the condensate cannot be discharged as it is. However, when the condensate supply line 55 is provided, the condensate can be returned to the aeration tank 12, and sulfurous acid contained in the condensate can be oxidized to reduce the sulfurous acid concentration, so that it is possible to reduce the COD of water to be discharged.
Reference Signs List [0098]
1 Intake tank
2 Mixing tank

10 Desulfurization facility
11 Desulfurization device
12 Aeration tank
13 Discharge tank

20 Desalination facility
21 Pretreatment tank

22 First reverse osmosis membrane
23 Second reverse osmosis membrane

30 Control device
31 Information acquisition unit
32 Mixing ratio determination unit
33 Flow rate control unit
41 First pH measurement device
45 Second pH measurement device
42 Flow rate measurement device
43, 44 Flow rate control device
46 Oxidation index value measurement device
47 Radical reaction inhibitor supply device
48 Adjuster supply device

51 First seawater supply line
52 Second seawater supply line
53 Wastewater supply line
54 Saltwater supply line
55 Condensate supply line
100, 200, 300, 400, 500, 600, 700 Water treatment system
S1 Desulfurization step
S2 Desalination step
S3 pH measurement step
S4 Mixing ratio determination step
S5 Flow rate control step

I/We Claim:
1. A water treatment system, comprising:
a desulfurization facility including a desulfurization device for causing a sulfur oxide contained in a flue gas to be absorbed in seawater by bringing the seawater and the flue gas into contact;
a desalination facility including at least one reverse osmosis membrane for producing freshwater from saltwater which is a mixture of wastewater from the desulfurization facility and seawater;
at least one pH measurement device for measuring pH of water flowing through a water supply line connecting the desulfurization facility and the desalination facility; and
a control device including a mixing ratio determination unit for determining a mixing ratio between the wastewater and the seawater in the saltwater, based on pH measured by the at least one pH measurement device.
2. The water treatment system according to claim 1,
wherein the at least one pH measurement device includes a first pH measurement device for measuring pH of the saltwater which is the mixture of wastewater and seawater and is supplied to the at least one reverse osmosis membrane, and
wherein the mixing ratio determination unit is configured to determine the mixing ratio between the wastewater and the seawater in the saltwater, based on pH measured by the first pH measurement device.
3. The water treatment system according to claim 1 or 2,
wherein the at least one pH measurement device includes a second pH measurement device for measuring pH of wastewater from the desulfurization facility after absorption of the sulfur oxide,
wherein the mixing ratio determination unit is configured to determine the mixing ratio

between the wastewater and the seawater in the saltwater, based on pH measured by the second pH measurement device.
4. The water treatment system according to any one of claims 1 to 3,
wherein the mixing ratio determination unit is configured to determine the mixing ratio between the wastewater and the seawater so that pH of the saltwater is in a range of 4 or more.
5. The water treatment system according to any one of claims 1 to 4,
wherein the mixing ratio determination unit is configured to determine the mixing ratio between the wastewater and the seawater so that pH of the saltwater is in a range of 7.8 or less.
6. The water treatment system according to any one of claims 1 to 5,
wherein the mixing ratio determination unit is configured to determine the mixing ratio between the wastewater and the seawater so that pH of the saltwater is in a range of 4 or more and 7.8 or less.
7. The water treatment system according to any one of claims 1 to 6, comprising a mixing tank for obtaining the saltwater which is the mixture by mixing seawater and wastewater from the desulfurization facility.
8. The water treatment system according to any one of claims 1 to 7,
wherein the at least one reverse osmosis membrane includes a first reverse osmosis membrane for desalting the saltwater and a second reverse osmosis membrane for removing boron in permeate produced in the first reverse osmosis membrane, the second reverse osmosis membrane being disposed downstream of the first reverse osmosis membrane,
wherein the desulfurization facility includes an aeration tank for aerating wastewater after absorption of the sulfur oxide in the desulfurization device, and
wherein the saltwater supplied to the first reverse osmosis membrane includes the

aerated wastewater.
9. The water treatment system according to any one of claims 1 to 8,
wherein the at least one reverse osmosis membrane is composed of cellulose triacetate, and
wherein the mixing ratio determination unit is configured to determine the mixing ratio between the wastewater and the seawater so that pH of the saltwater is in a range of 4 or more and 6.5 or less.
10. The water treatment system according to any one of claims 1 to 9, comprising an oxidation index value measurement device for measuring an index value indicating degree of oxidation of the saltwater.
11. The water treatment system according to claim 10,
wherein the oxidation index value measurement device includes at least one of an oxidation-reduction potential meter or a residual chlorine concentration meter.
12. The water treatment system according to claim 10 or 11,
wherein the oxidation index value measurement device is configured to measure an index value indicating degree of oxidation of condensate produced in the at least one reverse osmosis membrane.
13. The water treatment system according to any one of claims 1 to 12, comprising a radical reaction inhibitor supply device for supplying a radical reaction inhibitor for inhibiting radical reaction to water flowing upstream of the at least one reverse osmosis membrane.
14. The water treatment system according to any one of claims 1 to 13, comprising a pH adjuster supply device for supplying a pH adjuster for adjusting pH of the saltwater to water

flowing upstream of the at least one reverse osmosis membrane.
15. The water treatment system according to any one of claims 1 to 14,
wherein the desulfurization facility includes an aeration tank for aerating wastewater after absorption of the sulfur oxide in the desulfurization device, and
wherein the water treatment system includes a condensate supply line for supplying condensate produced in the at least one reverse osmosis membrane to the aeration tank.
16. A water treatment method, comprising:
a desulfurization step of causing a sulfur oxide contained in a flue gas to be absorbed in seawater by bringing the seawater and the flue gas into contact in a desulfurization facility;
a desalination step of producing freshwater from saltwater which is a mixture of wastewater from the desulfurization facility and seawater with at least one reverse osmosis membrane in a desalination facility;
a pH measurement step of measuring pH of water flowing through a water supply line connecting the desulfurization facility and the desalination facility; and
a mixing ratio determination step of determining a mixing ratio between the wastewater and the seawater in the saltwater, based on pH measured in the pH measurement step.