Water treatment system using alkaline water electrolysis device and alkaline fuel cell

Abstract

Provided is a water treatment system using an alkaline water electrolytic device and an alkaline fuel cell in which for continuing an electrolytic treatment, a hydrogen gas and an oxygen gas required in an alkaline water electrolytic device and an alkaline fuel cell, an amount of water corresponding to raw water lost through the electrolytic treatment, and an electrolytic solution are efficiently circulated and used in a water treatment system to considerably reduce electric power consumption. The water treatment system is a water treatment system using an alkaline water electrolytic device and an alkaline fuel cell in which an alkaline water electrolytic device and an alkaline fuel cell are connected to each other, the volume of raw water is reduced, an oxygen gas and a hydrogen gas that are generated from the alkaline water electrolytic device are fed to the alkaline fuel cell, the oxygen gas and hydrogen gas are used to generate electric power by the alkaline fuel cell, electric energy and water are collected, and the collected electric energy is fed to the alkaline water electrolytic device as an electric power source thereof.

Claims

1. A water treatment system using an alkaline water electrolytic device and an alkaline fuel cell, configured to have following features, (1) the alkaline water electrolytic device and the alkaline fuel cell are connected to each other, (2) an electrolytic solution obtained by mixing raw water, which is water to be treated by the alkaline water electrolytic device, and an alkaline aqueous solution so as to form a mixture and adjusting the mixture to have a desired alkali concentration, an amount of water corresponding to an amount of the raw water lost through an electrolytic treatment are fed to the alkaline water electrolytic device, and the electrolytic treatment is performed continuously while an alkali concentration is maintained at the initial desired concentration, and the electrolytic solution is circulated, a volume of the raw water is reduced, an oxygen gas is generated from an anode chamber of the alkaline water electrolytic device, and a hydrogen gas is generated from a cathode chamber of the alkaline water electrolytic device, (3) the electrolytic solution formed of the alkaline aqueous solution adjusted to have the desired alkali concentration, the oxygen gas and the hydrogen gas generated by the alkaline water electrolytic device are fed to the alkaline fuel cell, at least part of the oxygen gas and the hydrogen gas is used to generate electric power by the alkaline fuel cell, and electric energy and water are collected, and (4) the collected electric energy is fed to the alkaline water electrolytic device as an electric power source thereof, part or all of the collected water is fed to a circulation line of the electrolytic solution in the alkaline water electrolytic device, and the electrolytic treatment is continued, whereby most or part of electric power energy required in each of the alkaline water electrolytic device and the alkaline fuel cell, the hydrogen gas and the oxygen gas serving as raw materials for generating the electric power energy, and the amount of the water corresponding to the raw water lost through the electrolytic treatment is recycled in the water treatment system.

2. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 1, wherein 2nd, 3rd, . . . and nth alkaline water electrolytic devices and 2nd, 3rd, . . . and nth alkaline fuel cells are connected to the alkaline water electrolytic device and the alkaline fuel cell in a cascade mode, an electrolytic solution electrolytically treated and concentrated by the alkaline water electrolytic device is sequentially fed to the 2nd, 3rd, . . . and nth alkaline water electrolytic devices, an electrolytic treatment is performed in the same manner as in the electrolytic treatment using the alkaline water electrolytic device, an oxygen gas and a hydrogen gas generated are fed to at least one of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells, at least part of the oxygen gas and hydrogen gas is used to generate electric power by the at least one of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells, electric energy is collected, and water is generated, the collected electric energy is fed to at least one of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices, and the electrolytic treatment is continued, water generated in the electric power generation by the at least one of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells is discarded or fed to a circulation line of the electrolytic solution in at least one of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices, and the volume of the raw water subjected to volume reduction by the alkaline water electrolytic device is further reduced by the 2nd, 3rd, . . . and nth alkaline water electrolytic devices.

3. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 1, wherein pure water is used as the raw water.

4. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 1, wherein raw water containing tritiated water is used as the raw water.

5. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 1, wherein raw water containing tritiated water containing impurities including chloride ions is used as the raw water.

6. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 2, wherein each of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices comprises: an anode; a cathode; and a diaphragm, and each of the anode and the cathode comprises an electrode formed of a Ni or iron base material, or an electrode obtained by subjecting a surface of the base material to Raney nickel coating, Ni-based dispersion plating, or noble metal-based pyrolytic coating.

7. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 2, wherein each of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells comprises: a positive electrode; a negative electrode; and an anion exchange membrane, and each of the positive electrode and the negative electrode is formed of an electrode material with a platinum catalyst or a ruthenium-platinum alloy catalyst carried on a carbon black carrier.

8. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 1, wherein raw water containing tritiated water that contains impurities including chloride ions is used as the raw water, distilling of the raw water for removing the impurities is performed prior to the electrolytic treatment by the alkaline water electrolytic device, and in the distilling, the raw water containing the impurities including chloride ions, which is supplied as a salt slurry, is distilled, and the raw water containing tritiated water after the impurities are removed is supplied to the alkaline water electrolytic device.

9. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 8, wherein in the distilling, the salt slurry is concentrated, and a solid is separated and collected.

10. The water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to claim 2, wherein the electrolytic solution, which is used for the electrolytic treatment by the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices and used in the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells, comprises the alkaline aqueous solution in an amount from 5 to 60 mass %.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a conceptual view illustrating one embodiment of a water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to the present invention.

(2) FIG. 2 is a flow chart illustrating one example of an alkaline water treatment device to be used in one embodiment of the water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to the present invention.

(3) FIG. 3 is a flow chart illustrating one example of an alkaline fuel cell to be used in one embodiment of the water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to the present invention.

(4) FIG. 4 is a flow chart illustrating, as another embodiment of the present invention, one example of the water treatment system using an alkaline water electrolytic device and an alkaline fuel cell according to the present invention.

DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

(6) FIG. 1 is a conceptual view illustrating one embodiment of a water treatment system according to the present invention.

(7) As illustrated in FIG. 1, in the first embodiment of the present invention,

(8) (1) an alkaline water electrolytic device 1 and an alkaline fuel cell 2 are connected to each other;

(9) (2) an electrolytic solution 3 formed of an alkaline aqueous solution adjusted to a desired concentration, raw water 4, and an amount of water 5 corresponding to raw water lost through the electrolytic treatment are fed to the alkaline water electrolytic device 1, the volume of the raw water 4 is reduced by performing an electrolytic treatment while circulating the electrolytic solution 3 with the alkali concentration kept at an initial concentration in the alkaline water electrolytic device 1, and an oxygen gas 6 and a hydrogen gas 7 are generated by the alkaline water electrolytic device 1.

(10) (3) the oxygen gas 6 and the hydrogen gas 7 generated by the alkaline water electrolytic device 1, and the electrolytic solution 3 formed of an alkaline aqueous solution adjusted to a desired concentration are fed to the alkaline fuel cell 2, electric power is generated by the alkaline fuel cell 2, and electric energy 9 and water 10 are collected.

(11) (4) the collected electric energy 9 is fed to the alkaline water electrolytic device 1 as an electric power source thereof, and the water 10 collected from the alkaline fuel cell 2 is fed to a circulation line of the electrolytic solution formed of the alkaline aqueous solution, and can be used as water for adjusting the alkali concentration or water to be fed to an alkaline water electrolytic bath in the next cascade, or can be discarded when containing impurities.

(12) In this manner, the alkaline water electrolytic device 1 and the alkaline fuel cell 2 (AFC) are combined with each other, whereby the electric energy 9 required in the alkaline water electrolytic device 1 and the alkaline fuel cell 2, the hydrogen gas 7 and the oxygen gas 6 serving as raw materials for the electric power, the electrolytic solution 3 formed of the alkaline aqueous solution, and a partial amount of water 10 corresponding to raw water lost through the electrolytic treatment are circulated and used in the water treatment system. Thereby, the raw material components and the intermediate products are circulated and used, so that the alkaline water electrolytic device 1 and the alkaline fuel cell 2 can be efficiently operated, electric power costs can be considerably reduced, the volume of the raw water can be efficiently reduced, both the hydrogen gas and the oxygen gas generated by the alkaline water electrolytic device 1 can be used as a pure fuel for the alkaline fuel cell without being discarded, and thus a non-wasteful efficient water treatment system can be provided.

(13) In the present invention, when pure water is used as raw water or when raw water containing a very small amount of impurities is used, an electrolytic treatment can be performed using a hydrogen gas and an oxygen gas which are generated by the alkaline water electrolytic device 1, for most of a fuel for the alkaline fuel cell 2 that is used as an electric power source for the alkaline water electrolytic device 1. Therefore, it is unnecessary to supply large-capacity energy from the outside. Electrolysis is performed while an electrolytic solution is circulated, and thus the hydrogen gas and the oxygen gas serving as a fuel for the alkaline fuel cell can be continuously generated, and continuously fed to the alkaline fuel cell to continue the operation of the alkaline water electrolytic device, so that a water treatment can be performed inexpensively and efficiently.

(14) The utilization rate of a hydrogen gas and an oxygen gas which are generated by the alkaline water electrolytic device is around 60% in one-time cell inside passage reaction depending on the contact state of the gas with a catalyst in the alkaline fuel cell. The utilization rate of the gas generated from the 2nd, 3rd . . . nth alkaline water electrolytic devices is similarly around 60% in one-time cell inside passage reaction.

(15) Further, in the alkaline water electrolytic device, raw water or pure water is supplied from outside the system during electrolysis for keeping the alkali concentration at an initial concentration. According to the present invention, water 10 that is generated by the alkaline fuel cell 2 can be used as the water to be supplied, and therefore the introduction amount of water to be newly supplied from the outside can be reduced. The utilization rate of water in this case is comparable to the utilization rate of the gas. When the utilization rate of the generated gas is 60%, the utilization rate of water is about 60%.

(16) Further, in the alkaline water electrolytic device and the alkaline fuel cell, it is necessary to use an electrolytic solution formed of an alkaline aqueous solution. According to the present invention, the concentration of alkaline aqueous solution to be used is 5 to 60% by mass in both the devices, and the alkaline aqueous solution can be shared by both the devices.

(17) As described above, the water treatment system according to the present invention is one in which an alkaline fuel cell and an alkaline water electrolytic device are connected to each other. First, the alkaline water electrolytic device in the present invention will be described.

(18) The present invention is particularly advantageous when tritiated water-containing contaminated water is treated as the raw water. Hereinafter, a case will be described where raw water including contaminated water containing tritium is used. The same applies to a case where pure water is used as the raw water.

(19) (1) Alkaline Water Electrolytic Device

(20) FIG. 2 illustrates one embodiment of the alkaline water electrolytic device to be used in the water treatment system of the present invention, which can be applied to raw water containing tritiated water that does not contains impurities such as chloride ions or contains the impurities to a degree of not hindering the operation of the electrolytic system. In this case, an alkaline water electrolytic treatment is continuously performed while the alkali concentration of the raw water containing tritiated water is kept constant without providing a pre-process for removing impurities.

(21) FIG. 2 illustrates the alkaline water electrolytic device. The alkaline water electrolytic device includes a raw water storage tank 11, a raw water treatment bath 12, an alkaline water electrolytic bath 13, a circulation tank 14, an electrolytic solution circulation pipe 19 and a feeding pump 18. The alkaline water electrolytic bath 13 includes an anode chamber 15 for storing an anode, a cathode chamber 16 for storing a cathode, and a diaphragm 17 for dividing the anode chamber 15 and the cathode chamber 16 from each other. The diaphragm 17 is preferably a neutral diaphragm, but a cation exchange membrane may also be used.

(22) In this embodiment, the later-described distillation process for removing impurities such as chloride ions contained in the raw water containing tritiated water is not necessary, and the raw water containing tritiated water may be fed directly to the circulation tank 14 of the alkaline water electrolytic device. At this time, for example, the raw water may be fed from the raw water storage tank 11 for storage to the circulation tank 14 through the raw water treatment bath 12 to which a part of the raw water is transferred as an object to be treated as illustrated in FIG. 2.

(23) Raw water containing tritiated water that does not contain impurities such as chloride ions can be treated by the alkaline water electrolytic device illustrated in FIG. 2.

(24) Even in the case where raw water containing tritiated water containing impurities such as chloride ions is used, the raw water containing tritiated water can be treated in this embodiment when the treatment amount is small or the treatment time is short; the amount of impurities is small; or the system is configured to remove impurities during continuous electrolysis.

(25) Hereinafter, a case will be described where as raw water containing tritiated water, 800,000 m.sup.3 of raw water containing only a small amount of impurities such as chloride ions is treated by an alkaline water electrolytic system (I).

(26) (a) In this embodiment, the object to be treated in the alkaline water electrolytic device is 800,000 m.sup.3 of raw water containing tritiated water which is stored in the raw water storage tank 11. As a part of the raw water, 400 m.sup.3/day of raw water is fed from the raw water storage tank 11 through the raw water treatment bath 12 to the circulation tank 14 by the pump 18. In parallel, alkaline aqueous solution is fed to the circulation tank 14 (not illustrated).

(27) It is preferred that all of the raw water in the raw water storage tank 11 is fed to the circulation tank 14 through the raw water treatment bath 12, and electrically treated. However, when the amount of the raw water in the raw water storage tank 11 is large, it is preferred that the raw water is sent in a plurality of parts to the raw water treatment bath 12 to continuously treat the raw water in the raw water treatment bath 12. The same applies to embodiments and examples below.

(28) The anodes and cathodes of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices are preferably electrodes formed of a Ni or iron base material, or electrodes with the surface of the base material plated with nickel, or coated with various kinds of materials such as Raney nickel and noble metals for reducing an electrode overvoltage.

(29) (b) An electrolytic solution obtained by mixing the raw water and alkaline aqueous solution in the circulation tank 14 and adjusting the mixture to a desired alkali concentration in the circulation tank 14 is then fed to the alkaline water electrolytic bath 13 to perform an electrolytic treatment.

(30) (c) The concentration of the alkaline aqueous solution in the electrolytic solution is preferably high. The concentration is preferably 5 to 60% by mass, more preferably not less than 15% by mass and not more than 60% by mass, further preferably not less than 20% by mass and not more than 60% by mass. The alkali to be used is preferably KOH or NaOH.

(31) The volume of the electrolytic solution in the alkaline water electrolytic bath 13 is 400 m.sup.3. The volume of the electrolytic solution in the circulation tank 14, a pipe and so on is 400 m.sup.3. Thus, the total electrolytic process volume is 800 m.sup.3.

(32) (d) The electrolytic solution mixed in the circulation tank 14 and controlled to a desired alkali concentration is fed to the anode chamber 15 of the alkaline water electrolytic bath 13 through the electrolytic solution circulation pile 19 by a circulation pump 18, fed to the cathode chamber 16 of the alkaline water electrolytic bath 13 through the electrolytic solution circulation pipe 19 by the circulation pump 18, and electrolyzed. The electrolytic solution is electrolyzed through the diaphragm 17. As a result of the electrolysis, in the anode chamber 15, an oxygen gas is generated, the generated oxygen gas and the electrolytic solution are separated from each other. The separated electrolytic solution is circulated to the circulation tank 14 by the electrolytic solution circulation pipe 19.

(33) At the same time, in the cathode chamber 16, a hydrogen gas is generated. The generated hydrogen gas and the electrolytic solution are separated from each other. The separated electrolytic solution is circulated to the circulation tank 14 by the electrolytic solution circulation pipe 19. When the current density at this time is a high current density, the time required for the electrolytic treatment can be reduced. While the range of the operation current density depends on the performance of the electrolytic bath, particularly the structures of main elements thereof including an anode, a cathode, a diaphragm and an electrolytic bath, etc., the current density is preferably not less than 5 A/dm.sup.2 and not more than 80 A/dm.sup.2. The current density is further preferably not less than 5 A/dm.sup.2 and not more than 60 A/dm.sup.2. Particularly, when the amount of gas formation in electrolysis of water is set to be small, the process amount inevitably decreases, and when large-volume electrolysis is attempted, the process amount generally increases.

(34) According to the studies by the present inventors, for alkaline water electrolysis, even an electrolytic solution having an alkali concentration of 32% by mass can be electrolyzed. However, it is not advantageous that the alkali concentration is higher than 32% by mass because the viscosity of the electrolytic solution increases, the generated gas is no longer quickly discharged to outside the system, and the cell voltage becomes high, leading to an increase in energy consumption.

(35) Where the electrolytic treatment amount is 400 m.sup.3/day in the above-described method, the whole of 800,000 m.sup.3 of raw water containing tritiated water will be treated in 5.5 years (800,000 m.sup.3 400 m3/day 365 days=5.5 years).

(36) Since the circulation liquid amount of the electrolytic solution at this time is 800 m.sup.3, the amount of tritiated water-containing water will be reduced from 800,000 m.sup.3 to 800 m.sup.3 in 5.5 years.

(37) (e) In the above-described long-term treatment, an amount of raw water corresponding to raw water lost through the electrolytic treatment is continuously fed from the inside of the storage tank 11 to the circulation tank 14, and the alkali concentration of the electrolytic solution is kept at an initial concentration. The electrolysis continued while the electrolytic solution is circulated, so that the whole of a large amount of raw water stored in the storage tank 11 is electrolytically treated.

(38) (f) As a result of the treatment in the alkaline water electrolytic device, the raw water containing tritiated water (HTO) is formed into a gas, and converted into a tritium gas (HT)-containing hydrogen gas and an oxygen gas. The concentration of tritium in the tritium gas (HT)-containing hydrogen gas is reduced to 1/1,244 as compared to that in the tritiated water, and the volume of the raw water is reduced from 800,000 m.sup.3 to 800 m.sup.3.

(39) In the above-described continuous electrolytic method, tritiated water corresponding to water decomposed and lost in electrolysis is continuously fed to the process, and physical operating environments including the liquid amount in the electrolytic bath and the circulation pump discharge amount within the process are always kept unchanged. At this time, the amount of tritiated water fed to the process corresponds to the concentration in the raw water.

(40) In the case where water is continuously fed, operations are carried out in such a manner as to keep the tritium concentration in the process at the concentration in the raw water, so that the concentration in the electrolytic bath does not increase. In this continuous operating condition, the gas generated in electrolysis is converted at a ratio corresponding to the concentration ratio of light water and tritiated water.

(41) Examples of main specifications and performance in the alkaline water electrolytic device described above are as follows.

(42) [Specifications]

(43) 1) Raw water including tritium-contaminated water: 800,000 m.sup.3

(44) 2) Electrolytic treatment volume: treatment amount of 400 m.sup.3/day

(45) 3) Alkali: caustic soda, alkali concentration: 20% by mass

(46) 4) Discharged tritium concentration: 1.35010.sup.3 Bq/L

(47) 5) Alkaline water electrolytic bath: 48 baths (75 elements per bath)

(48) 6) Current density: 40 A/dm.sup.2

(49) 7) Electrolytic process: circulating electrolytic process+continuous feeding of raw water to electrolytic process

(50) [Performance]

(51) The conversion ratio of tritium in raw water, depending mainly on the concentration of tritium in general, is 1.0 to 0.6 (where tritium is usually fractionated as a tritium molecular gas).

(52) Where the concentration of tritium contained in the raw water is 4.210.sup.6 Bq/L, the concentration of tritium contained in a raw material fluid after the treatment in the electrolytic system is as follows.
4.210.sup.60.4/1.244 Bq/L=1.35010.sup.3 Bq/L

(53) Here, the concentration limit in the exhaust or air is 710.sup.4 Bq/L or less, and the tritiated water effluent standard is 610.sup.4 Bq/L or less.

(54) When raw water containing tritiated water contains a large amount of impurities such as chloride ions, it is necessary to remove the impurities before the raw water is fed to the alkaline water electrolytic device.

(55) The alkaline fuel cell that collects electric power energy and water to be fed to the alkaline water electrolytic device will now be described.

(56) (2) Alkaline Fuel Cell

(57) A hydrogen gas and an oxygen gas generated by the alkaline water electrolytic device have been heretofore released into the air to be discarded. However, in the present invention, the gases are sent to the alkaline fuel cell, and electric power energy is collected, sent to the alkaline water electrolytic device, and used as an electric power source for the alkaline water electrolytic device. The utilization rate of the generated gas was about 60%.

(58) FIG. 3 illustrates one embodiment of the alkaline fuel cell for use in the present invention. In the alkaline fuel cell, an anion exchange membrane (electrolyte layer) 21 is provided between separators 20 and 20, a positive electrode catalyst layer 22 is provided on the oxygen electrode side of the anion exchange membrane (electrolyte layer) 21, and a gas diffusion layer 23 is provided on the outside of the positive electrode catalyst layer 22. A negative electrode catalyst layer 24 is provided on the fuel electrode side of the anion exchange membrane (electrolyte layer) 21, and a gas diffusion layer 25 is provided on the outside of the negative electrode catalyst layer 24.

(59) The anion exchange membrane 21 is impregnated with an electrolytic solution formed of an alkaline aqueous solution with a desired concentration. For the electrolytic solution formed of an alkaline aqueous solution, an electrolytic solution with a concentration almost equal to that of the electrolytic solution to be used in the alkaline water electrolytic device is used. The concentration of the alkaline aqueous solution of the electrolytic solution is preferably high, specifically 5 to 60% by mass. The alkali to be used is preferably KOH or NaOH.

(60) The hydrogen gas and the oxygen gas generated by the alkaline water electrolytic device pass through a buffer tank before being fed to the alkaline fuel cell, so that the hydrogen gas and the oxygen gas are fed to the alkaline fuel cell without stagnation.

(61) In the present invention, the anion exchange membrane is impregnated with the electrolytic solution formed of an alkaline aqueous solution, the oxygen gas generated by the alkaline water electrolytic device is then fed into the alkaline fuel cell through a channel 26 provided on the positive electrode catalyst layer 22 side, and the hydrogen gas generated by the alkaline water electrolytic device is fed to the alkaline fuel cell through a channel 27 provided on the negative electrode catalyst layer side.

(62) The oxygen gas fed to the alkaline fuel cell through the channel 26 provided on the positive electrode catalyst layer 22 side passes through the gas diffusion layer 23. The oxygen reacts with water under the positive electrode catalyst layer 22 to generate hydroxyl ions (OH.sup.), and the hydroxyl ions (OH.sup.) move to the negative electrode catalyst layer side by passing through the anion exchange membrane 21, and reacts with the hydrogen gas to generate water.

(63) The reaction formulae in the alkaline fuel cell are as follows.

(64) Whole 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O

(65) Fuel electrode (negative electrode) 2H.sub.2+4OH.sup..fwdarw.4H.sub.2O+4e.sup.

(66) Oxygen electrode (positive electrode) O.sub.2+2H.sub.2O+4e.sup.4OH.sup.

(67) Preferably, the positive electrodes and negative electrodes of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells are made of an electrode material with a platinum catalyst or a ruthenium-platinum alloy catalyst carried on a carbon black carrier.

(68) Since an alkaline fuel cell is used as a fuel cell, for the alkaline aqueous solution to be used in the fuel cell, the alkaline aqueous solution used for the electrolytic solution in the alkaline water electrolytic device can also be used as an electrolytic solution in the fuel cell.

(69) For the hydrogen gas and the oxygen gas that are used in the alkaline fuel cell for use in the present invention, pure hydrogen and oxygen that do not contain a carbonaceous substance can be used, so that efficiency is extremely improved.

(70) When air is used as an oxidant, the electrolytic solution absorbs carbon dioxide to be degraded. However, as in the present invention, an oxygen gas generated by alkaline water electrolysis is a high-purity oxygen gas, so that the problem of degradation of the electrolytic solution does not occur. Since the electrolyte is an aqueous solution, equipment is inexpensive.

(71) In contrast, for example, automobile alkaline fuel cells which have been previously developed and used cannot be used efficiently because air is used as an oxygen gas, and thus an electrolytic solution formed of an alkaline aqueous solution may be degraded by carbon dioxide etc. contained in the air when circulated and used.

(72) Next, as another embodiment of the water treatment system according to the present invention, 2nd, 3rd, . . . and nth alkaline water electrolytic devices and 2nd, 3rd, . . . and nth alkaline fuel cells are connected to an alkaline water electrolytic device 1 and an alkaline fuel cell 2 in a cascade mode, respectively. An electrolytic solution electrolytically treated by the alkaline water electrolytic device is fed to at least one of the 2nd, 3rd, . . . and nth alkaline water electrolytic devices, and an electrolytic treatment is performed in the same manner as in the alkaline water electrolytic device 1. An oxygen gas and a hydrogen gas generated are fed to at least one of the alkaline fuel cell 2 and the 2nd, 3rd, . . . and nth alkaline fuel cells. At least a part of the oxygen gas and hydrogen gas is used to generate electric power by at least one of the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells, electric energy is collected, and water is generated. The collected electric energy is fed to at least one of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices to continue the electrolytic treatment. Water generated in the electric power generation by the alkaline fuel cell and the 2nd, 3rd, . . . and nth alkaline fuel cells is used as supplementary water for raw water lost through the electrolytic treatment by at least one of the alkaline water electrolytic device and the 2nd, 3rd, . . . and nth alkaline water electrolytic devices. The volume of the raw water subjected to volume reduction by the alkaline water electrolytic device is further reduced by the 2nd, 3rd, . . . and nth alkaline water electrolytic devices.

(73) FIG. 4 illustrates one example where 2nd, 3rd, . . . and nth alkaline water electrolytic devices and 2nd, 3rd, . . . and nth alkaline fuel cells are connected to the alkaline water electrolytic device 1 and the alkaline fuel cell 2 in a cascade mode.

(74) Contaminated water is used as raw water. First, the alkaline water electrolytic device is started by external electric power, and the resulting hydrogen gas and oxygen gas are sent to the alkaline fuel cell. Electric energy and water are collected by the alkaline fuel cell. Electrolysis is continued using the obtained electric energy. The obtained water is used as supplementary water for the alkaline water electrolytic device, alkaline water electrolysis is continued, and the contaminated water as raw water is concentrated. The concentrated contaminated water is sent to the second alkaline electrolytic device, electric energy and water are collected by the second fuel cell in the same manner as described above, and second alkaline water electrolysis is continued.

(75) Electrolysis is continued by the third and fourth alkaline water electrolytic devices in the same manner, so that the concentrated contaminated water is further concentrated.

(76) While the combination of the alkaline water electrolytic device and the alkaline fuel cell can be changed according to the treatment amount. Usually the treatment is suitably done in four stages as illustrated in FIG. 4.

(77) One of the alkaline fuel cell and the 2nd, 3rd, . . . nth alkaline fuel cells may be provided for one of the alkaline water electrolytic device and the 2nd, 3rd, . . . nth alkaline fuel cells, or may be provided for two or more of the alkaline water electrolytic devices.

(78) Further, water collected from the alkaline fuel cell and the 2nd, 3rd, . . . nth alkaline fuel cells may be supplied to make up for water lost through electrolysis in the alkaline water electrolytic device, or may be fed to raw water to be concentrated.

EXAMPLES

(79) Examples of the present invention will now be described, but the present invention is not limited to these examples.

Example 1

(80) A simulated liquid of raw water containing tritiated water that does not contain impurities (hereinafter, also referred to as a simulated liquid), a simulated liquid with the following components was used.

(81) Simulated Liquid: 180 L

(82) Initial concentration of tritium in simulated liquid: 4.210.sup.6 Bq/L

(83) As illustrated in FIG. 2, a raw water storage tank 11 containing 180 L of the simulated liquid was provided. In this test, the simulated liquid was fed from the raw material storage tank 11 to a circulation tank 14 through a treatment bath 12. Specifically, 9.67 L/day of the simulated liquid was fed from the raw material storage tank 11 to the circulation tank 14 through the treatment bath 12 by a pump 18. In this test, the simulated liquid was continuously fed in an alkaline water electrolytic device.

(84) To the circulation tank 14, 9.60 L/day of the simulated liquid is fed by the pump 18, and also alkaline aqueous solution is fed. In the circulation tank 14, the simulated liquid and the alkaline aqueous solution are mixed, and adjusted to an electrolytic solution having an alkali concentration of 20% by mass. Continuous electrolysis is performed while 9.67 L/day of the electrolytic solution is circulated.

(85) The volume of the electrolytic solution in an alkaline water electrolytic bath 13 is 30 L (two 15 dm.sup.2 cells (each 15 L)), the volume of the electrolytic solution in the circulation tank 14, a pipe and so on is 12 L. Thus, the total electrolytic process volume is 42 L. The electrolytic solution obtained by mixing with the alkali in the circulation tank 14 and controlled to an alkali concentration of 20% by mass was fed to an anode chamber 15 of an alkaline water electrolytic bath 13 through the electrolytic solution circulation pipe 19 by a circulation pump 18, and fed to a cathode chamber 16 of the alkaline water electrolytic bath 13 through the electrolytic solution circulation pipe 19 by the circulation pump 18. In the anode chamber 15, an oxygen gas was generated, and the generated oxygen gas and the electrolytic solution are separated each from each other. The separated electrolytic solution was circulated to the circulation tank 14 through the electrolytic solution circulation pipe 19. At the same time, in the cathode chamber 16, a hydrogen gas was generated, gas-liquid separation occurred to separate the generated hydrogen gas and the electrolytic solution from each other. The separated electrolytic solution was circulated to the circulation tank 14 through the electrolytic solution circulation pipe 19.

(86) As described above, in this embodiment, the electrolytic solution formed of simulated liquid (raw water) and alkaline aqueous solution was electrolyzed by an alkaline water electrolytic method as illustrated in FIG. 2 to decompose the raw water into oxygen and hydrogen. Tritium existing as water molecules in the raw water was fractionated from the raw water as tritium molecules. The water was decomposed into only hydrogen and oxygen gases by electrolysis. Therefore, after adjustment of the initial alkali concentration, electrolysis was performed while an amount of raw water (simulated liquid) corresponding to water lost through electrolysis was fed to the circulated electrolytic solution. If necessary, distilled water or pure water may be additionally fed in addition to raw water (simulated liquid) for keeping the alkali concentration at an initial concentration.

(87) In this example, continuous alkaline electrolysis in the alkaline water electrolytic device was performed under the following conditions.

(88) Electrolytic cell: two 15 dm.sup.2 cell (each 15 L, total 30 L) were used.

(89) Operation current density: 40 A/dm.sup.2.

(90) Concentration of caustic soda: NaOH, 20% by mass.

(91) Membrane: diaphragm.

(92) Anode/cathode: Ni base material+active coating.

(93) Circulation: external circulation system.

(94) Water sealing: water sealing system for control of gas pressure.

(95) 50 to 100 mmH.sub.2O cathode pressure.

(96) Electrolytic solution volume: 42 L (electrolytic cell: 30 L, circulation pipe etc.: 12 L).

(97) The electrolytic current was 600 A (15 dm.sup.240 A/dm.sup.2).

(98) In the continuous electrolytic method, an amount of simulated liquid (raw water) corresponding to water decomposed and lost in electrolysis is continuously fed to the process as described above, and physical operating environments including the liquid amount in the electrolytic bath and the circulation pump discharge amount within the process are always kept unchanged. In the case where simulated liquid (raw water) was continuously fed, operations were carried out in such a manner as to keep the tritium concentration in the process at the concentration in the simulated liquid, so that the concentration in the electrolytic bath did not increase. Therefore, in this continuous operating condition, the gas generated in electrolysis is converted at a ratio corresponding to the concentration ratio of light water and tritiated water.

(99) The circulation liquid amount of the electrolytic solution at this time was 42 L, and thus the amount of tritiated water-containing water was reduced from 180 L to 42 L in 15.2 days (365 hours).

(100) The alkaline water electrolytic device started electrolysis by means of a usual driving electric power source at the start of electrolysis. When the amount of the oxygen gas generated by electrolysis reached 1.044 L and the amount of the hydrogen gas generated by electrolysis reached 2.088 L, the gases were fed to the alkaline fuel cell illustrated in FIG. 3. In the alkaline fuel cell, an anion exchange membrane including a quaternary ammonium group is impregnated with the electrolytic solution formed of an alkaline aqueous solution with a 20% by mass of NaOH, the oxygen gas generated by the alkaline water electrolytic device is then fed into the alkaline fuel cell through a channel 26 provided on the positive electrode catalyst layer 22 side, and the hydrogen gas generated by the alkaline water electrolytic device is fed to the alkaline fuel cell through a channel 27 provided on the negative electrode catalyst layer side.

(101) The oxygen gas fed to the alkaline fuel cell through the channel 26 provided on the positive electrode catalyst layer 22 side passes through the gas diffusion layer 23, and reacts with water under the positive electrode catalyst layer 22 to generate hydroxyl ions (OH.sup.). The hydroxyl ions (OH.sup.) move to the negative electrode catalyst layer side by passing through the anion exchange membrane 21, and reacts with the hydrogen gas to generate water. 60% of the hydrogen gas and the oxygen gas sent to the alkaline fuel cell contributed to the reaction to obtain electric energy and water. The hydrogen gas and oxygen gas which did not contribute to the reaction were released to outside.

(102) The reaction formulae in the alkaline fuel cell are as follows.

(103) Whole 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O

(104) Fuel electrode (negative electrode) 2H.sub.2+4OH.sup..fwdarw.4H.sub.2O+4e.sup.

(105) Oxygen electrode (positive electrode) O.sub.2+2H.sub.2O+4e.sup..fwdarw.4OH

(106) As a positive electrode material and a negative electrode material, an electrode material with a platinum catalyst or a ruthenium-platinum alloy catalyst carried on a carbon black carrier was used.

(107) The obtained electric energy was sent to the alkaline water electrolytic device, and used as an electric power source thereof. The water was sent to the alkaline water electrolytic device as supplementary water for the alkaline water electrolytic device.

(108) Thermodynamically, 60% of the introduced gas was recovered as electric energy.

Example 2

(109) Except that the raw water used in Example 1 was replaced by pure water, just the same procedure as in Example 1 was carried out to obtain the same results as in Example 1.

Example 3

(110) As illustrated in FIG. 4, 2nd, 3rd and 4th alkaline water electrolytic devices and 2nd and 3rd alkaline fuel cells were connected to the alkaline water electrolytic device 1 and the alkaline fuel cell 2 in a cascade mode, respectively.

(111) 180 L of the simulated water used in Example 1 is used as raw water (contaminated water), and the electrolytic solution (alkali concentration of 20% by mass) formed of the contaminated water and the alkaline aqueous solution is used. First, the alkaline water electrolytic device is started by external electric power, and the hydrogen gas and oxygen gas obtained by the alkaline electrolysis are sent to the alkaline fuel cell. Electric energy and water are collected by the alkaline fuel cell. Electrolysis is continued using the obtained electric energy, the obtained water is used as supplementary water for the alkaline water electrolytic device, alkaline water electrolysis is continued, and the contaminated water is concentrated. The concentrated contaminated water is sent to the second alkaline electrolytic device. Electric energy and water are collected by the second fuel cell in the same manner as described above, and second alkaline water electrolysis is continued.

(112) Electrolysis is continued by the third and fourth alkaline water electrolytic devices in the same manner, so that the concentrated contaminated water is further concentrated.

(113) As a result, by the electrolytic treatment using the alkaline water electrolytic device, the contaminated water was concentrated to 3.2 times in one cascade treatment, and concentrated to not less than 100 times as high as the concentration in the raw water by performing a fourth-stage cascade treatment. The use of the heavy water concentration function and generated electric power of the fuel cell made it possible to concentrate contaminated water and considerably reduce treatment energy.

INDUSTRIAL APPLICABILITY

(114) According to the present invention, an alkaline water electrolytic device and an alkaline fuel cell (AFC) are combined with each other, whereby each of electric power required in the alkaline water electrolytic device and the alkaline fuel cell, a hydrogen gas and an oxygen gas serving as raw materials for the electric power, an electrolytic solution formed of the alkaline aqueous solution, and an amount of water corresponding to raw water lost through the electrolytic treatment is circulated and used in the water treatment system. Thereby, raw material components and intermediate products are all effectively used, so that the alkaline water electrolytic device and the alkaline fuel cell can be efficiently operated. Thus, electric power costs can be considerably reduced, and the oxygen gas generated by alkaline water electrolysis is a high-purity oxygen gas, so that the problem of degradation of the electrolytic solution does not occur. Since the electrolyte is an aqueous solution, the equipment is inexpensive, and the water treatment system can be applied to a wide range of use purposes.

REFERENCE SIGNS LIST

(115) 1: Alkaline water electrolytic device 2: Alkaline fuel cell 3: Electrolytic solution 4: Raw water 5: Water 6: Oxygen gas 7: Hydrogen gas 9: Electric power energy 10: Water 11: Raw water storage tank 12: Raw water treatment bath 13: Alkaline water electrolytic bath 14: Circulation tank 15: Anode chamber 16: Cathode chamber 17: Diaphragm 18: Pump 19: Electrolytic solution circulation pipe 20: Separator 21: Anion exchange membrane 22: Positive electrode catalyst layer 23: Gas diffusion layer 24: Negative electrode catalyst layer 25: Gas diffusion layer 26: Channel for oxygen gas 27: Channel for hydrogen gas