GB2332210A

GB2332210A - Processing waste water

Info

Publication number: GB2332210A
Application number: GB9826506A
Authority: GB
Inventors: Kenji Fujihata; Shuji Seki; Shinichi Terada
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1997-12-10
Filing date: 1998-12-02
Publication date: 1999-06-16
Anticipated expiration: 2018-12-02
Also published as: GB9826506D0; CN101643258A; DE19856840A1; ID21431A; GB2332210B; CN1220242A; SG78327A1; MY118517A

Abstract

In a processing method of waste water, electrolysis is carried out by feeding the waste water containing nitrogen compounds into an anode room 4 of an electrolytic bath in which a diaphragm 3 having selective ion-permeability is disposed between electrodes, thereby reduced nitrogen compounds such as hydrazine or ammonium ion in the waste water is oxidized by oxygen generated at an anode to a nitrogen gas and is removed from the waste water. Further, after being oxidized like this, the second electrolysis is carried out by feeding only the oxidized liquid into a cathode room 5, thereby nitrogen oxides such as a nitrate ion or a nitrite ion in the liquid are reduced to a nitrogen gas and can be removed from the waste water. Thus, without generating the secondary waste even under normal temperature and normal pressure, the nitrogen compounds can be efficiently removed from the waste water generated from a thermal power plant.

Description

1 PROCESSING METHOD OF WASTE WATER AND PROCESSING APPARATUS THEREOF

2332210 The present invention relates to a processing method of waste water and a processing apparatus thereof, further in more detail, relates to a method of removing nitrogen compounds as a nitrogen gas, by oxidizing or reducing electrochemically the nitrogen compounds, from the waste water containing the nitrogen compounds such as reduced nitrogen compounds (nitrogen hydrogen compounds) such as hydrazine and ammonium ion, and nitrogen oxides such as a nitrate ion and a nitrite ion. Incidentally, in this specification, the nitrogen compounds, reduced nitrogen compounds and nitrogen oxides also include ionic states of these compounds.

In general, the processing of deoxidation (prevention of oxidation) and corrosion protection at feedwater systems or condensation systems of thermal power plants is carried out by feeding a reducing volatile chemical such as hydrazine or ammonia. Therefore, in the waste water generated at such plants, the added hydrazine or ammonia (ammonium ion) remains, accordingly removal through application of any processing on 1 2 such remains is demanded.

So far, there are the following methods of removing the nitrogen compounds such as hydrazine or ammonium ion.

1) a method of oxidizing by use of oxidizing agent such 5 as hypochlorous acid, hydrogen peroxide, oxygen or the like, 2) a method of oxidizing under a condition of high temperature and high pressure in the presence of catalyst such as copper, lead or the like, 3) a method of filtrating under pressure by use of a 10 reverse osmosis membrane, 4) electrodialysis, by disposing many ion exchange membranes between electrodes, of separating ions by electrophoresis, 5) a method of decomposing by oxidation by use of 15 microbes which consume nitrogen component.

However, in these methods, there were such problems as that there was a difficulty in control of reaction, or the secondary waste or a byproduct were generated anew by addition of an oxidant or a catalyst, as shown in the 20 following.

That is, 1) in the oxidation processing due to an oxidant, not only there is a difficulty in handling of the oxidant, but also there is a problem such as occurrence of a byproduct due to an over reaction, 2) in the oxidation method employing a catalyst, in addition to a likelihood of the catalyst itself becoming a hazardous secondary waste, there is a problem that control of reaction is difficult. Further, 3) in the method of filtrating under pressure by use of the 3 reverse osmosis membrane, in addition to only a slight amount being processed, concentrated waste water is required to be further processed, 4) in the method due to electrodialysis, other than charged ions can not be processed, but there is a problem that the concentrated ion component is further required to be processed anew.

Therefore, under the present circumstance, 5) a method due to decomposition by oxidation by use of microbes is considered to be most effective. However, according to this method, there are problems such as that the space of equipment becomes huge, the highly-trained skill is required to breed the microbes, a huge amount of organic feed is required, an water tank is difficult in control of the conditions, and, once the decomposition reaction is disturbed, it takes a long time for the conditions to recover.

The present invention was carried out to solve such problems. That is, an object of the present invention is, in order to solve such problems, to provide a processing method of waste water which, under normal temperature and pressure, without generating the secondary waste, removes nitrogen compounds such as hydrazine, an ammonium ion, a nitrate ion, a nitrite ion, or the like as a nitrogen gas from the waste water, and a processing apparatus thereof.

A processing method of waste water of the first invention is characterized in that electrolysis is carried out by feeding the waste water containing nitrogen compounds 4 to at least one of an anode room and a cathode room of an electrolytic bath in which a diaphragm having selective ionpermeability is disposed between an anode and a cathode, thereby oxidizing or reducing the nitrogen compounds in the waste water to a nitrogen gas.

A processing method of waste water of the second invention is characterized in comprising a first electrolysis step where the first electrolysis is carried out by feeding the waste water containing nitrogen compounds to an anode room of an electrolytic bath in which a diaphragm having selective ion- permeability is disposed between an anode and a cathode, thereby oxidizing the nitrogen compounds in the waste water to a nitrogen gas, and a second electrolysis step where the second electrolysis is carried out by feeding the waste water oxidized in the first electrolysis step to the cathode room of the electrolytic bath, and thereby reducing nitrogen oxides in the waste water to a nitrogen gas.

A processing apparatus of waste water of the present invention comprises an electrolytic bath in which a diaphragm having anode and a selective ion-permeability is disposed between an and a cathode, and which is divided into an anode room cathode room by the diaphragm, a direct-current power source inputting a direct-current voltage between the anode and the cathode, the first liquid feeding means for feeding an electrolytic solution to the anode room of the electrolytic bath, a first liquid exhausting means for exhausting the electrolytic solution from the anode room of the electrolytic bath, the second liquid feeding means for feeding an electrolytic solution to the cathode room of the electrolytic bath, the second liquid exhausting means for exhausting the electrolytic solution from the cathode room of the electrolytic bath, and a gas exhausting means for exhausting the gas generated by the electrolysis from gaseous phase of the anode room and/or cathode room of the electrolytic bath.

In the present invention, as a diaphragm dividing an electrolytic bath into an anode room and a cathode room, an anion-exchange membrane, a cation-exchange membrane, a composite ion- exchange membrane in which an anion-exchange membrane and a cation- exchange membrane are compounded can be employed. These ion-exchange membranes are constituted of the solid electrolyte, posses selective permeability for ionic species, and interrupt movement of a particular ion between the anode room and the cathode room. Further, membranes of common solid electrolytes such as silver iodide ((x-Agl), alumina (P-Al.,03) or stabilized zirconia can be employed.

Further, in the present invention, the shapes of the anode and the cathode, other than bar or plate of which the inside is filled, can be formed into a reticular structure or porous structure. Further, such anode and cathode, other than being capable of being disposed separately each other appropriate places in the anode room and the cathode room separated by the diaphragm, can be disposed such that interpose therebetween the diaphragm making an intimate contact. In particular, in the case of electrolysis being 6 carried out by use of an electrolytic bath in which an anode and a cathode of reticular structure or porous structure are disposed interposing therebetween a diaphragm in close proximity, since an area of a contact interface between the electrodes and the electrolytic conductor is large and the distance between the electrodes is short, the electrochemical reaction at the surface of the electrode tends to occur. Therefore, the efficiency of the oxidation or reduction processing of the nitrogen compounds in the waste water becomes high.

The nitrogen compounds being processed by electrolysis in the present invention, are reduced nitrogen compounds (nitrogen hydrogen compounds) such as hydrazine or ammonium ion, and nitrogen oxides such as nitrate ion or nitrite ion, the reduced nitrogen compounds being oxidized in the anode room of the electrolytic bath, the nitrogen oxides being reduced in the cathode room of the electrolytic bath. Further, as the nitrogen compounds being contained in the waste water, hydroxyl amine, amines, di-amines, amides, nitroamides, tetrazine, nitrite acid, nitrogen monoxide, dinitrogen monoxide, nitrogen dioxide, dinitrogen tetraoxide, dinitrogen pentaoxide, nitrides, azides, diazo-compounds, cyanides, nitosyl salts, and nitroyl salts can be cited. These nitrogen compounds are also oxidized or reduced to nitrogen gases in the anode room or cathode room due to electrolysis, and can be removed as the nitrogen gas.

In the first invention, water in the waste water fed to the anode room or the cathode room of the electrolytic 7 bath is oxidized or reduced at the anode or the cathode as shown in the following formulas.

at the anode: 2H20 - 4e- - O + 4H+ at the cathode: 2H20 + 2e- - H2 + 20HThen, the nitrogen compounds such as hydrazine, an ammonium ion, a nitrate ion and, a nitrite ion contained in the waste water react with oxygen or hydrogen generated by the anodic oxidation or cathodic reduction in the anode room or cathode room of the electrolytic bath, thereby generating a nitrogen gas and water.

Now, the reduced nitrogen compounds (hydrazine or ammonium ion for instance) in the waste water react in the anode room with oxygen generated at the anode as shown in the following formulas, thus generating a nitrogen gas and water.

WH4 + 0, - N- T + 2H20 4NH,' + 30, - 2N 1 + 4H' + 6H20 Further, nitrogen oxides (nitrate ion or nitrite ion for instance) react in the cathode room with the hydrogen generated at the cathode as shown in the following formulas, thus generating a nitrogen gas and water.

2NO,- + 5H, - N- + 20H- + 4H-0 2NO-- + 3H- - N- + 20H_ + 2H-0 8 The generated nitrogen gas is moved from a liquid phase to a gaseous phase of the electrolytic bath, and is further exhausted from the gaseous phase to remove.

In the second invention, the waste water containing the nitrogen compounds is electrolyzed (the first electrolysis step) first in the anode room of the electrolytic bath, thereby the reduced nitrogen compounds in the waste water are oxidized to form a nitrogen gas and water. Then, the waste water oxidized in the first electrolysis step is fed to the cathode room of the electrolytic bath to be electrolyzed(the second electrolysis step), thereby the nitrogen oxides in the waste water are reduced to form a nitrogen gas and water. Thus, the reduced nitrogen compounds and nitrogen oxides in the waste water are respectively electrolyzed to form a nitrogen gas, and are removed from the liquid phase. Further, on electrolyzing at the cathode room, since, together with reduction of the nitrogen oxides in the waste water, oxygen or the like dissolved in the liquid is also reduced to water, the amount of the dissolved oxygen can be decreaced.

Fig. I is a diagram showing schematically a configuration of the first embodiment of the present invention, Fig. 2 is a graph showing a change of concentrations of hydrazine and ammonium ion in the waste water in the anode room, in the first embodiment, Fig. 3 is a diagram showing schematically a 9 configuration of the second embodiment of the present invention, Fig. 4A and Fig. 4B are a perspective view and an exploded perspective view, respectively, showing a processing apparatus of waste water being employed in the second embodiment of the present invention, Fig. 5 is a graph showing a change of concentrations of hydrazine and ammonium ion in the waste water in the anode room in the second embodiment, Fig. 6 is a graph showing a change of pH of the liquid in the cathode room, in the second embodiment, Fig. 7 is a graph showing a change of a current in the case of pure water being electrolyzed in an electrolytic bath having an anode and cathode of reticular structure, Fig. 8 is a diagram showing schematically a configuration of the third embodiment of the present invention, Fig. 9A and Fig. 9B are a perspective view and an exploded perspective view, respectively, showing a processing apparatus of waste water being employed in the third embodiment of the present invention, Fig. 10 is a graph showing a change of concentrations of nitrate ion and nitrite ion in the waste water in the cathode room, in the third embodiment, Fig. 11 is a diagram showing a configuration of the fourth embodiment of the present invention, Fig. 12 is a graph showing a change of a total nitrogen concentration in the waste water generated at a thermal power plant, in the fourth embodiment, Fig. 13 is a graph showing a change of concentrations of nitrate ion and nitrite ion in the waste water in the cathode room in the fifth embodiment of the present invention, 5 Fig. 14 is a graph showing a change of a total nitrogen concentration in the waste water, in the sixth embodiment of the present invention, Fig. 15 is a diagram showing schematically a configuration of the seventh embodiment of the present invention, Fig. 16A and Fig. 16B are a perspective view and an exploded perspective view, respectively, showing a processing apparatus of waste water being employed in the seventh embodiment of the present invention, Fig. 17 is a graph showing a change of a total nitrogen concentration in the waste water, in the seventh embodiment, Fig. 18 is a graph showing a change of a total nitrogen concentration in the waste water generated at a thermal power plant, in the seventh embodiment, Fig. 19 is a graph showing a change of pH of the liquids in the anode and cathode rooms in the case of electrolysis of an aqueous solution of sodium sulfate being carried out in an electrolytic bath having a composite ion-exchange membrane, Fig. 20 is a graph showing a change of the amount of hydrogen gas to be generated according to electrochemical calculation, in the eighth embodiment of the present invention, Fig. 21 is a graph showing a change of a metal ion 11 concentration in the liquid of the cathode room, in the ninth embodiment of the present invention.

In the following, the preferred embodiments of present invention will be described. Embodiment 1 Fig. 1 is a diagram showing schematically a configuration for explaining the first embodiment of the processing method of waste water of the present invention.

In the first embodiment, as shown in Fig. 1, between plates of an anode I and a cathode 2, a diaphragm 3 consisting of alumina which is a general ly-used solid electrolyte is disposed, and, in an electrolytic bath 6 separated by this diaphragm 3 into an anode room 4 and a cathode room 5, electrolysis is carried out by feeding waste water (waste aqueous solution) 7 containing at least one of hydrazine and ammonium ion to the anode room 4 and flowing pure water 8 to the cathode room 5. Here, as the waste water 7 containing at least one of hydrazine and ammonium ion, the waste water generated from a thermal power plant for instance can be employed.

Water in the waste water 7 fed to the anode room 4 is oxidized at the anode 1 as shown in the following reaction formula to generate oxygen.

2H-0 - 4e- - 0- + 4H' 12 And, the reduced nitrogen compounds (hydrazine and/or ammonium ion) are respectively oxidized as shown in the following formulas by the oxygen generated at the anode 1 due to the aforementioned reaction formula to a nitrogen gas, and the generated nitrogen gas moves from the liquid phase to the gaseous phase to be removed from the waste water 7.

N,H4 + 02 - N2 T + 2H20 4NH4' + 302 - 2N2 T + 4H + 6H20 Incidentally, reference numeral 9 in the figure denotes the oxidized liquid to be taken out from the anode room 4.

Fig. 2 is a graph showing measured results of a change of concentrations of hydrazine and ammonium ion in the waste water 7 of the anode room 4, when, in the first embodiment, electrolysis is carried out under the condition of an electrolysis area of 0.75dm2, an electric current density of to 7AMM2, a liquid volume of 500m1, and liquid temperatures of 25 to 35'C.

From this figure, it is recognized that, according to the first embodiment, hydrazine and ammonium ion in the waste water can be effectively removed and the concentrations thereof can be made below the detection limits, respectively.

Embodiment 2 Fig. 3 is a diagram showing schematically a configuration for explaining the second embodiment of the present invention.

13 In the second embodiment, as shown in Fig. 3, an anion exchange membrane 3a is disposed between an anode 1 and a cathode 2, in the electrolytic bath 6 divided into an anode room 4 and a cathode room 5 by this anion exchange membrane 3a, electrolysis is carried out while waste water 7 containing reduced nitrogen compounds such as hydrazine and ammonium ion is fed to the anode room 4 and pure water 8 is being flowed in the cathode room 5. Here, the anode I has a reticular structure in which platinum is plated on the surface of a titanium base, and the cathode 2 is made of SUS and has the reticular structure same as the anode 1. Further, these electrodes interpose an anion-exchange membrane (strongly basic anion-exchange membrane) 3a and are disposed in close proximity of the anion-exchange membrane.

In this second embodiment, the reduced nitrogen compounds in the waste water 7 which is fed to the anode room 4 are oxidized by the oxygen generated at the anode 1 to be a nitrogen gas, the generated nitrogen gas moves to the gaseous phase from the liquid phase to be removed from the waste water Fig. 4A and Fig. 4B are a perspective view and an exploded perspective view, respectively, showing structures of a processing apparatus of waste water being employed in the second embodiment. This processing apparatus, as shown in the figures, comprises an electrolytic bath 6 in which an anion- exchange membrane 3a is disposed in close proximity between the anode I and cathode 2 both of reticular structure, and which is divided into the anode room and the cathode room 14 by this anion-exchange membrane 3a, an external direct current power source 10 which inputs a direct current voltage between the anode 1 and the cathode 2, the first liquid feeding pipe lla feeding the waste water 7 containing the reduced nitrogen compounds into the anode room of the electrolytic bath 6, the first liquid exhausting pipe 12a exhausting the oxidized liquid 9 from the anode room, the second liquid feeding pipe llb feeding the liquid such as pure water 8 into the cathode room, and the second liquid exhausting pipe 12b exhausting the liquid from the cathode room 5. And, the first liquid feeding pipe 11a and the first liquid exhausting pipe 12a penetrate respectively an anode room forming plate 13 and are attached inside the anode room with openings, and the second liquid feeding pipe 11b and the second liquid exhausting pipe 12b penetrate respectively a cathode forming plate 14 and are attached inside the cathode room with openings. Further, the first liquid feeding pipe lla and the first liquid exhausting pipe 12a are connected through the first liquid reserving tank 15a and the first liquid circulating pump 16a, the second liquid feeding pipe llb and the second liquid exhausting pipe 12b are connected through the second liquid reserving tank 15b and the second liquid circulating pump 16b. Further, on the side of the anode room of the electrolytic bath 6, a gas exhausting means (not shown in the figure) for exhausting the nitrogen gas generated by the electrolysis is attached.

Incidentally, reference numeral 17 in the figure denotes an anode supporting body forming a bottom side portion of the anode room 4, reference numeral 18 denotes a cathode supporting body forming a bottom side portion of the cathode room.

In the second embodiment like this, electrolysis was carried out under conditions of an electrolysis area of 0.75dm', a current density of 5 to 7A/dm', a liquid volume of 500m1, and liquid temperatures of 25 to 35'C. Measured results of the change of concentrations of hydrazine and ammonium ion in the waste water 7 in the anode room 4 are shown in Fig. 5.

From this figure, it can be understood that, according to the second embodiment, the concentrations of hydrazine and ammonium ion in the waste water 7 can be reduced below the detection limits, respectively. Further, in the second embodiment, since the anion exchange membrane 3a employed as the diaphragm permeates selectively only anions in the liquid but not permeates cations, movement of the reduced nitrogen compounds to the cathode room 5 from the anode room 4 is interrupted. Therefore, oxidation reaction of the reduced nitrogen compounds in the anode room 4 can be carried out effectively.

Further, due to the effect of the fixed ions in the anion-exchange membrane, OH- moves from the cathode room 5 to the anode room 4, accordingly the increase of pH (increase of OW) accompanying the electrolysis in the cathode room 5 can be suppressed and lowering of the current efficiency can be suppressed. Further, since the permeating/moving OH- carries an electric charge, loss due to electric resistance of the liquid becomes very small.

16 In the second embodiment, electrolysis was carried out under conditions of an electrolysis area of 0.75dM2, a current density of 2 to 3A/dm2, a liquid volume of 500ml, and liquid temperatures of 25 to 35'C. Measured results of the change of pH of the liquid in the cathode room 5 are shown in Fig. 6.

From this figure, it is understood that, in the case of an anion-exchange membrane 3a being used as the diaphragm, the increase of pH in the liquid in the cathode room 5 accompanying electrolysis is small. Incidentally, in Fig. 6, for comparison purpose, the change of pH in the case of the electrolysis being carried out identically as the second embodiment by employing a diaphragm consisting of a generally-used solid electrolyte, alumina, is also shown.

Further, in the second embodiment, since an anode 1 and a cathode 2 both having a reticular structure are employed, compared with the case of the electrode of filled bar or plate being used as the electrode, the area of contact interface between the electrode and the electrolytic solution is remarkably increased. Further, since the anode 1 and the cathode 2 of reticular structure like this are disposed in close proximity interposing an anion-exchange membrane 3a therebetween, the distance between electrodes becomes also short. Therefore, the electrode reaction is liable to occur, thereby oxidation of the reduced nitrogen compounds such as hydrazine and ammonium ion in the waste water 7 is carried out effectively in short time.

The electrolysis was carried out in the electrolytic 17 bath 6, in which an anode 1 and a cathode 2 both of reticular structure are disposed in close proximity interposing an anion exchange membrane 3a therebetween, while feeding pure water to the anode room 4 and the cathode room 5. The measured results of the change of the current are shown in Fig. 7. Incidentally, the electrolysis was carried out under the condition of an electrolysis area of 0.75dm2, an input voltage of 1OV(constant), and water temperatures of 25 to 350C.

From this figure, it is understood that, due to use of an anode 1 and a cathode 2 both of reticular structure, even in the pure water, enough electrolysis can be carried out. Embodiment 3 Fig. 8 is a diagram showing schematically a configuration for explaining the third embodiment of the present invention.

In the third embodiment, as shown in Fig. 8, a cationexchange membrane 3b is disposed between an anode 1 and a cathode 2. In an electrolytic bath 6 which is divided into an anode room 4 and a cathode room 5 by this cation-exchange membrane 3b, the electrolysis is carried out by feeding waste water 7 containing nitrogen oxides such as nitrate ion or nitrite ion to the cathode room 5 and by flowing pure water 8 in the anode room 4. Here, the anode 1 has a reticular structure in which platinum plating is given on the surface of titanium base, the cathode 2 is made of SUS and has a reticular structure identical as the anode 1. Further, these electrodes interpose a cation- exchange membrane (strongly acidic cation-exchange membrane) 3b therebetween and are disposed in close proximity of this cation-exchange membrane 3b.

Water in the waste water 7 fed to the cathode room 5 is reduced at the cathode 2 as shown in the following reaction formula to generate hydrogen.

2H20 + 2e- - H, + 20H- Then, the nitrogen oxides (nitrate ion and/or ion) in the waste water 7 are respectively reduced in the following formulas by the hydrogen generated cathode 2 by the aforementioned reaction to generatE nitrite as shown at the a nitrogen gas, the generated nitrogen gas moves from the liquid phase to the gaseous phase and is removed from the waste water 7.

2NO3- + 5H- - N T + 20H_ + 4H,0 2NO-- + 3H, - N- T + 20H_ + 2H,0 Thus, the nitrogen oxides in the waste water 7 are reduced in the cathode room 5 to form a nitrogen gas to be removed. Incidentally, reference numeral 19 in the figure denotes the reduced liquid to be taken out of the cathode room 5.

Fig. 9A and Fig. 9B are a perspective view and an exploded perspective view, respectively, showing structure of a processing apparatus of waste water being employed in the 19 third embodiment. This processing apparatus, as shown in the figures, comprises an electrolytic bath 6 in which an cationexchange membrane 3b is disposed in close proximity between an anode 1 and a cathode 2 both of reticular structure and which is divided into an anode room and a cathode room by this cation- exchange membrane 3b, an external direct current power source 10 inputting a direct current voltage between the anode 1 and the cathode 2, the first liquid feeding pipe l1a feeding the liquid such as pure water 8 into the anode room of the electrolytic bath 6, the first liquid exhausting pipe 12a exhausting liquid from the anode room, the second liquid feeding pipe Ilb feeding the waste water 7 containing nitrogen oxides to the cathode room, and the second liquid exhausting pipe 12b exhausting the reduced liquid 19 from the cathode room 5. And, the first liquid feeding pipe 11a and the first liquid exhausting pipe 12a, respectively, penetrate an anode room forming plate 13, and are attached inside the anode room with openings, and the second liquid feeding pipe 1lb and the second liquid exhausting pipe 12b, respectively, penetrate a cathode forming plate 14, and are attached inside the cathode room with openings. Further, the first liquid feeding pipe lla and the first liquid exhausting pipe 12a are connected through the first liquid reserving tank 15a and the first liquid circulating pump 16a, and the second liquid feeding pipe llb and the second liquid exhausting pipe 12b are connected through the second liquid reserving tank 15b and the second liquid circulating pump 16b. Further, on the side of the cathode room of the electrolytic bath 6, a gas exhausting means (omitted in the figures) for exhausting the nitrogen gas generated by electrolysis is attached. Incidentally, reference numeral 17 in the figure denotes an anode supporting body forming a bottom side portion of the anode room, reference numeral 18 denoted a cathode supporting body forming a bottom side portion of the cathode room.

In the third embodiment like this, electrolysis was carried out under conditions of an electrolysis area of 0.75dmi, current densities of 5 to 7A/dm2, a liquid amount of 500ml, and liquid temperatures of 25 to 35C. The measured results of change of the concentrations of nitrate ion and nitrite ion in the waste water 7 in the cathode room 5 are shown in Fig. 10.

From this figure, it is understood that, according to the third embodiment, the nitrate ion and the nitrite ion in the waste water 7 are removed effectively, respectively, and the concentrations of these ions can be reduced below the detection limits thereof. Embodiment 4 20 Fig. 11 is a diagram showing schematically a configuration for explaining the fourth embodiment of the present invention. In the fourth embodiment, as shown in Fig. 11, a diaphragm 3 consisting of alumina is disposed between an anode 1 and a cathode 2 composed of a plate each, in an electrolytic bath 6 separated into an anode room 4 and a cathode room 5 by thisdiaphragm 3, while the waste water 7 containing at least one of hydrazine and ammonium ion is fed 21 to the anode room 4 and pure water is flowed to the cathode room 5, the first electrolysis is carried out. Thereby, the reduced nitrogen compounds (hydrazine and/or ammonium ion) in the waste water 7 are oxidized by the oxygen generated at the anode I to a nitrogen gas. Then, by feeding the oxidized liquid 9 to the cathode room 5 and by flowing pure water to the anode room, or by feeding the waste water 7 anew, the second electrolysis is carried out, thereby the oxidizing substances in the waste water 7 are reduced. Here, as the waste water 7 containing at least one of hydrazine and ammonium ion, waste water generated at a thermal power plant can be used for instance. Further, the electrolysis of the waste water 7 is batch processed, after completion of the first electrolysis, the total amount of the oxidized liquid 9 in the anode room 4 may be fed /fed into the cathode room 5 to carry out the second electrolysis, however, the continuous processing also can be carried out. That is, while feeding continuously the oxidized liquid 9 from the anode room 4 to the cathode room 5, and exhausting continuously the reduced liquid 19 from the cathode room 5, the second electrolysis can be carried out.

In this embodiment, after the reduced nitrogen compounds (hydrazine and/or ammonium ion) in the waste water 7 are oxidized electrochemically in the anode room 4 and are removed from the waste water 7 as nitrogen gas, thereafter the oxidizing substances in the waste water 7 are reduced electrochemically in the cathode room 5. And, in the case of the nitrogen oxides such as nitrate ion and nitrite ion being 22 contained in the waste water 7, the nitrogen oxides are reduced to be a nitrogen gas and are removed from the waste water 7. Further, due to reduction in the cathode room 5, the dissolved oxygen or the like in the waste water 7 is simultaneously removed.

In the fourth embodiment as described above, the electrolysis of the waste water 7 containing respectively hydrazine and ammonium ion generated at a thermal power plant was carried out continuously under conditions of an electrolysis area of 0.75dm, current densities of 5 to 7AMM2, a liquid amount of 500m1, and liquid temperatures of 25 to 35'C. The measured results of the change of the total nitrogen concentration in the waste water 7 are shown in Fig. 12. Incidentally, the measurement of the total nitrogen concentration was carried out on the liquid exhausted continuously from the cathode room 5. From this figure, it is recognized that, according to the fourth embodiment, the nitrogen component in the waste water 7 generated at a thermal power plant can be removed effectively.

Embodiment 5 In the fifth embodiment, with the use of the identical electrolytic bath 6 as the fourth embodiment, by feeding as identical as the fourth embodiment the waste water 7 containing at least one of hydrazine and ammonium ion, and at least one of nitrate ion and nitrite ion, respectively, to the anode room 4, the first electrolysis is carried out, after hydrazine and/or ammonium ion in the waste water -7 is oxidized by the oxygen generated at the anode 1, by feeding 23 the oxidized liquid 9 to the cathode room 5, the second electrolysis was carried out, thereby the nitrate ion and/or the nitrite ion in the waste water 7 is reduced.

In this embodiment, after hydrazine and/or ammonium ion (the reduced nitrogen compound) in the waste water 7, is oxidized electrochemically at the anode room 4 and is removed as nitrogen gas from the waste water 7, the nitrate ion and/or the nitrite ion (the nitrogen oxide), is reduced electrochemically at the cathode room 5 and is removed as a 10 nitrogen gas from the waste water 7.

In the fifth embodiment as described above, the electrolysis was carried out under conditions of an electrolysis area of 0.75dM2, current densities of 5 to 7A/dM2, a liquid amount of 500ml, and liquid temperatures of 25 to 350C. The measured results of the change of the concentrations of the nitrate ion and the nitrite ion in the waste water 7 of the cathode room 5 are shown in Fig. 13.

From this figure, it is recognized that, according to the fifth embodiment, the nitrate ion and the nitrite ion in the waste water 7 can be removed effectively. Embodiment 6 in the sixth embodiment, with the use of the identical electrolytic bath 6 as the fourth embodiment, by feeding the waste water 7 containing at least one of hydrazine and ammonium ion, and at least one of nitrate ion and nitrite ion, and containing further the nitrogen compounds other than those, respectively, to the anode room 4 as identical as the fourth embodiment, the first electrolysis is carried out, 24 thereafter by feeding the oxidized liquid 9 to the cathode room 5, the second electrolysis is carried out, thereby the nitrogen compounds in the waste water 7 are oxidized and reduced, in this order.

In this embodiment, after hydrazine and/or ammonium ion and other reduced nitrogen compounds in the waste water 7 are oxidized in the anode room 4, the nitrate ion and/or nitrite ion and other nitrogen oxides are reduced in the cathode room 5, all of them can be removed from the waste water 7 as a nitrogen gas and water.

In the sixth embodiment as described above, the continuous electrolysis of the waste water 7 containing the nitrogen compounds was carried out under the conditions of an electrolysis area of 0.75dm current densities of 5 to 7A/dm-, a liquid amount of 500ml, and liquid temperatures of 25 to 350C. The measured results of the change of the total nitrogen concentration in the waste water 7 are shown in Fig. 14.

From this figure, it is recognized that, according to the sixth embodiment, the various kinds of nitrogen compounds in the waste water 7 can be removed effectively. Embodiment 7 In the seventh embodiment, as shown in Fig. 15, between an anode 1 and a cathode 2, a composite ion-exchange membrane 3c composed of an anion- exchange membrane and a cationexchange membrane is disposed. In an electrolytic bath 6 separated to an anode room 4 and a cathode room 5 by this composite ion-exchange membrane 3c, the first electrolysis is carried out by feeding waste water 7 containing nitrogen compounds to the anode room 4 and by flowing pure water to the cathode room first, reduced nitrogen compounds in the waste water 7 are oxidized by the oxygen generated at the anode 1 to a nitrogen gas. Then, the second electrolysis is carried out by feeding the oxidized liquid 9 to the cathode room 5, thereby the oxidizing substances in the waste water 7 are reduced. Here, the anode 1 has a reticular structure given a platinum plating on a surface of a titanium base, and the cathode 2 is made of SUS and has the identical reticular structure as the anode 1. Further, these electrodes interpose the composite ion- exchange membrane 3c therebetween and are disposed in close proximity of this ion-exchange membrane.

According to this embodiment, after the reduced nitrogen compounds such as hydrazine and ammonium ion in the waste water 7 are oxidized electrochemically in the anode room 4 and are removed from the waste water 7 as a nitrogen gas, the oxidizing substances in the waste water 7 are reduced electrochemically in the cathode room 5. And, in the case of the nitrogen oxides such as nitrate ion and nitrite ion being contained in the waste water 7, the nitrogen oxides are reduced to a nitrogen gas and are removed from the waste water 7.

Fig. 16A and Fig. 16B are a perspective view and an exploded perspective view, respectively, showing a structure of an waste water processing apparatus employed in the seventh embodiment. This processing apparatus, as shown in 26 these figures, comprises an electrolytic bath 6 in which a composite ion- exchange membrane 3c is disposed in close proximity between an anode 1 and a cathode 2 of reticular structure each, and which is divided into the anode room 1 and the cathode room 2 by this composite ion-exchange membrane 3c, an external direct current power source 10 inputting a direct current voltage between the anode I and the cathode 2, the first liquid feeding pipe lla feeding waste water 7 containing the nitrogen compounds in the anode room of the electrolytic bath 6, the first liquid exhausting pipe 12a exhausting the oxidized liquid 9 from the anode room, the second liquid feeding pipe llb feeding the waste water 7 containing the nitrogen compounds to the cathode room 5, and the second liquid exhausting pipe 12b exhausting the reduced liquid 19 from the cathode room S. The first liquid feeding pipe lla and the first liquid exhausting pipe 12a, respectively, penetrate an anode room forming plate 13, and are attached inside the anode room with openings, the second liquid feeding pipe llb and the second liquid exhausting pipe 12b, respectively, penetrate a cathode room forming plate 14 and are attached inside the cathode room with openings. Further, the second liquid exhausting pipe 12b and the first liquid feeding pipe Ila are connected through the third liquid reserving tank 15c and the third liquid circulating pump 16c, the first liquid exhausting pipe 12a and the second liquid feeding pipe llb are connected through the fourth liquid circulating pump 16d. The first electrolysis processing at the anode room and the second electrolysis 27 processing at the cathode room are carried out continuously. Still further, on the sides of the anode room and cathode room of the electrolytic bath 6, gas exhausting means (omitted in the figure) for exhausting nitrogen gas generated by the electrolysis are respectively attached. Incidentally, reference numeral 17 in the figure denotes an anode supporting body forming a bottom side portion of the anode room, reference numeral 18 denotes a cathode supporting body forming a bottom side portion of the cathode room.

In the seventh embodiment as described above, the electrolysis of the waste water 7 containing hydrazine and/or ammonium ion was carried out continuously under conditions of an electrolysis area of 0.75dM2, current densities of 5 to A/dm2, a liquid amount of 500ml, and liquid temperatures of 25 to 35'C. The measured results of the change of the total nitrogen concentration in the waste water 7 are shown in Fig. 17. Incidentally, the measurement of the total nitrogen concentration was carried out on the liquid exhausted continuously from the cathode room 5. 20 From this figure, it is recognized that, according to the seventh embodiment, the nitrogen compounds such as hydrazine and ammonium ion i-1- i the waste water 7 can be removed effectively. Fig. 18 is a graph, in the seventh embodiment, showing the measured results of the change of the total nitrogen concentration in the waste water 7 in the case of the waste water 7 containing the nitrogen compounds generated from a thermal power plant being processed in the same way as the 28 above.

From this figure, the various kinds of nitrogen compounds in the waste water 7 generated from a thermal power plant also, according to the seventh embodiment, can be removed effectively, thereby the concentration of the nitrogen component can be decreased remarkably.

Further, in the seventh embodiment, a composite ionexchange membrane 3c composed of an anion-exchange membrane and a cation-exchange membrane is disposed as a diaphragm.

The cation-exchange membrane constituting this composite ionexchange membrane 3c permits only cation to transmit selectively and prevents anion from transmitting/moving, and the anion exchange membrane allow only anion to transmit selectively and prevent cation from transmitting/moving, accordingly the oxidizing reaction at the anode room 4 and the reducing reaction at the cathode room 5 can be carried out effectively respectively. Further, in both the anode room 4 and the cathode room 5, an increase of H and OH_ which deteriorate the current efficiency is suppressed, and variation of pH of the liquid is suppressed.

In order to make sure of such an effect, with an electrolytic bath having a composite ion-exchange membrane composed of a cation-exchange membrane and an anion-exchange membrane, electrolysis of an aqueous solution of sodium sulfate was carried out (the electrolysis area: 0.75dm-, the current density: 2 to 3A/dm-, the liquid amount: 500ml, the liquid temperature: 25 to 35'C).

Fig. 19 is a graph showing the measured results of 29 change of pH of the anode room and the cathode room, respectively, in such an electrolysis of an aqueous solution of sodium sulfate.

From this figure, it is understood that, since the composite ion-exchange membrane prevents both ions of the cation and the anion from transmitting/moving, in the both rooms of the anode room and the cathode room, the change of pH is small. Embodiment 8 In the eighth embodiment, with use of the identical electrolytic bath 6 as the first embodiment, the electrolysis is carried out by feeding the waste water 7 containing respectively hydrazine and ammonium ion to the anode room 4 and by flowing pure water 8 to the cathode room 5, thereby hydrazine and ammonium ion in the waste water 7 are oxidized in the anode room 4 and removed from the waste water 7 as a nitrogen gas. At the same time, the hydrogen gas generated from the cathode 2 is recovered.

At the cathode 2, water is electrolyzed (reduction decomposition) as shown in the following reaction formula to generate hydrogen.

2H-0 + 2e- - H- + 20W Thus generated hydrogen gas is recovered and the recovered hydrogen gas is reused.

Fig. 20 is a graph showing, in the eighth embodiment, one example of the change of the amount of hydrogen gas to be generated according to electrochemical calculation. From this figure, it is recognized that, since the hydrogen gas is generated steadily, by recovering this, the reusable hydrogen gas can be obtained steadily. Embodiment 9 In the ninth embodiment, with use of the identical electrolytic bath 6 as the fourth embodiment, by feeding first the waste water 7 containing respectively metal ion as well as hydrazine and ammonium ion to the anode room 4, the 10 first electrolysis is carried out, after thereby oxidizing hydrazine and ammonium ion to a nitrogen gas, the oxidized liquid 9 is fed to the cathode room 5 to carry out the second electrolysis.

The metal ion (metal ion of lower standard electrode potential than hydrogen) in the waste water 7 is reduced to metal at the cathode 2 as shown in the following reaction formula, and precipitates on the surface of the cathode 2. By recovering the precipitated metal, the metal ion in the waste water 7 can be removed.

M- + e- - M (metal) Fig. 21 is a graph showing the measured results of the change of the concentration of the metal ion in the liquid in the cathode room 5 in this ninth embodiment. From this figure, it is understood that, according to the ninth embodiment, the metal ion in the waste water 7 can be precipitated at the cathode 2 to be recovered.

31 As described above, according to the present invention, by carrying out an electrolysis, under the normal temperature and normal pressure, by disposing a diaphragm having selective ion-permeability such as ionexchange membrane between an anode and a cathode, the nitrogen compounds in the waste water generated from a thermal power plant or the like can be reduced to a nitrogen gas without generating the secondary waste, thereby can be removed efficiently.

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Claims

What is claimed is:

1. A processing method of waste water, characterized in that electrolysis is carried out by feeding the waste water containing nitrogen compounds to at least one of an anode room and a cathode room of an electrolytic bath in which a diaphragm having selective ion-permeability is disposed between an anode and a cathode, thereby oxidizing or reducing the nitrogen compounds in the waste water to a nitrogen gas.

2. The processing method of the waste water as set forth in claim 1: wherein the first electrolysis is carried out by feeding the waste water containing reduced nitrogen compounds to the anode room of the electrolytic bath in which the diaphragm having selective ionpermeability is disposed between the anode and the cathode, thereby oxidizing the reduced nitrogen compounds in the waste water to a nitrogen gas.

3. The processing method of the waste water as set forth in claim 1:

wherein the second electrolysis is carried out by feeding the waste water containing nitrogen oxides to the cathode room of the electrolytic bath in which the diaphragm having selective ion-permeability is disposed between the anode and the cathode, thereby reducing the nitrogen oxides in the waste water to a nitrogen gas.

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4. A processing method of waste water, comprising: a first electrolysis step of carrying out the first electrolysis by feeding the waste water containing nitrogen compounds to an anode room of an electrolytic bath in which a diaphragm having selective ion-permeability is disposed between an anode and a cathode, thereby oxidizing the nitrogen compounds in the waste water to a nitrogen gas; and a second electrolysis step of carrying out the second electrolysis by feeding the waste water oxidized in the first electrolysis step to the cathode room of the electrolytic bath, thereby reducing the nitrogen oxides in the waste water to a nitrogen gas.

5. The processing method of the waste water as set forth in claim 2 or claim 4: wherein the waste water contains at least one of hydrazine and ammonium ion as the reduced nitrogen compound.

6. The processing method of the waste water as set forth in claim 3 or claim 4: wherein the waste water contains at least one of nitrate ion and nitrite ion as the nitrogen oxide.

7. The processing method of the waste water as set forth in claim 1 or claim 4: wherein the waste water is that is generated at a thermal power plant.

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8. The processing method of the waste water as set forth in claim 1 or claim 4: wherein at least one of the anode and the cathode has a reticular or porous structure, and the anode and the cathode are disposed in close proximity of the diaphragm interposing the same therebetween.

9. The processing method of the waste water as set forth in claim 1 or claim 4:

wherein the diaphragm is an ion-exchange membrane.

10. The processing method of the waste water as set forth in claim 9: wherein the diaphragm is an anion-exchange membrane.

11. The processing method of the waste water as set forth in claim 9:

wherein the diaphragm is a composite ion-exchange membrane which combines a cation-exchange membrane and an anion-exchange membrane.

12. A processing apparatus of waste water, comprises: an electrolytic bath in which a diaphragm having selective ion-permeability is disposed between an anode and a cathode, and which is divided by the diaphragm into an anode room and a cathode room; a direct current power source inputting a direct current voltage between the anode and the cathode; a first liquid feeding means for feeding an electrolytic solution to the anode room of the electrolytic bath; a first liquid exhausting means for exhausting the electrolytic solution from the anode room of the electrolytic bath; a second liquid feeding means for feeding an electrolytic solution to the cathode room of the electrolytic bath; a second liquid exhausting means for exhausting the electrolytic solution from the cathode room of the electrolytic bath; and a gas exhausting means for exhausting the gas generated by electrolysis from gaseous phase of the anode room and/or the cathode room of the electrolytic bath.

13. The processing apparatus of the waste water as set forth in claim 12, comprises: an electrolytic bath in which a diaphragm having selective ionpermeability is disposed between an anode and a cathode, and which is divided by the diaphragm into an anode room and a cathode room; a direct current power source inputting a direct current voltage between the anode and the cathode; a first liquid feeding means for feeding waste water containing nitrogen compounds to the anode room of the electrolytic bath; a first liquid exhausting means for exhausting the oxidized waste water from the anode room of the electrolytic a 36 bath; a second liquid feeding means for feeding the waste water containing the nitrogen compounds to the cathode room of the electrolytic bath; 5 a second liquid exhausting means for exhausting the reduced waste water from the cathode room of the electrolytic bath; and a nitrogen gas exhausting means for exhausting the nitrogen gas generated by the electrolysis from gaseous phase of the anode room and/or the cathode room of the electrolytic bath.

14. The processing apparatus of the waste water as set forth in claim 12: wherein the first liquid exhausting means and the second liquid feeding means are connected.

15. The processing apparatus of the waste water as set forth in claim 12: wherein at least one of the anode and the cathode has a reticular or porous structure, and the anode and the cathode are disposed in close proximity of the diaphragm interposing the same therebetween.

16. The processing apparatus of the waste water as set forth in claim 12: wherein the diaphragm is an ion-exchange membrane.

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17. The processing apparatus of the waste water as set forth in claim 16: wherein the diaphragm is an anion-exchange membrane.

18. The processing apparatus of the waste water as set forth in claim 16: wherein the diaphragm is a composite ion-exchange membrane which combines a cation-exchange membrane and an anionexchange membrane.