JP2019076814A - Water treatment system, and electrode corrosion inhibition method and electrode corrosion inhibition apparatus of water treatment system - Google Patents

Abstract

To provide a water treatment system which can prevent pH decrease caused in an electrolytic cell in electrolytic treatment and inhibit progression of corrosion of an electrode by an acid.SOLUTION: A water treatment system 100 provided with: a circulation adjustment tank 20 into which wastewater flows; an electrolytic cell 22 which performs electrolytic treatment of the wastewater with at least one pair of electrodes; and circulation pipes 26,28 through which the wastewater is circulated between the circulation adjustment tank 20 and the electrolytic cell 22 comprises: an alkali mixing tank 36 which mixes the wastewater transferred thereto flowing from the circulation adjustment tank 20 with an alkali between an outlet 20d of the circulation adjustment tank 20 and an inlet 22c of the electrolytic cell 22.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a water treatment system for electrolytically treating wastewater containing ammonia nitrogen and organic matter.

  For wastewater treatment, methods using microorganisms are widely known. However, treatment with microorganisms requires a water tank at each step and is difficult to miniaturize, and in cold regions, there is a reduction in processing capacity, and maintenance and management of microorganisms, such as maintenance and management of microorganisms. There are challenges in terms of

  Therefore, attention has been focused on the electrolytic treatment of wastewater. The water treatment system by the electrolytic treatment is composed of a raw water tank for temporarily storing the waste water, a circulation control tank, an electrolytic tank, and an electrolytic treatment mechanism constituted of circulation pipes connecting them.

  In Patent Document 1, an ammoniacal nitrogen-containing wastewater is electrolytically treated by a pair of electrodes in the presence of chlorine ions while circulating an electrolytic cell, and hypochlorous acid generated by electrolysis is reacted with ammoniacal nitrogen to obtain nitrogen In the electrolytic treatment method of the ammonia nitrogen containing wastewater to be decomposed into gas, the wastewater is electrolytically treated in the range of pH 5 or more and less than pH 8 in the electrolytic cell, and is separate from the pair of electrodes and formed of metal material It is described that the catalyst member thus disposed is disposed in the electrolytic cell, and the waste water is brought into contact with the catalyst member so as to promote the decomposition reaction of ammoniacal nitrogen.

Patent No. 4671743

  According to Patent Document 1, in a system for electrolytically treating waste water in an electrolytic cell, a metal catalyst is disposed in the electrolytic cell to prevent a decrease in current efficiency and to further improve the treatment efficiency, and an auxiliary agent is added to the circulation control tank. An acid or an alkali is added as (pH adjuster). However, no measure is taken against corrosion of the electrode in the electrolytic cell due to the acid generated when the wastewater is electrolytically treated in the electrolytic cell. In systems that treat wastewater by electrolytic treatment, rare metals such as platinum are often used for the electrodes, and the corrosion thereof can lead to a decrease in treatment performance and an increase in operation costs.

  The present invention is an invention for solving the above-mentioned problems, and is a water treatment system capable of suppressing the progress of corrosion of an electrode, a method for suppressing electrode corrosion in a water treatment system, and an electrolytic cell in electrolytic treatment of wastewater. An object of the present invention is to provide an electrode corrosion inhibitor which suppresses the pH drop that occurs.

  In order to achieve the above object, the water treatment system of the present invention comprises a control tank (for example, circulation control tank 20) into which waste water flows and an electrolytic tank (for example, electrolytic tank 22) for electrolytically treating waste water with at least a pair of electrodes. And a circulation pipe (for example, circulation pipes 26 and 28) for circulating waste water between the adjustment tank and the electrolytic cell, the adjustment tank being between the outlet of the adjustment tank and the inlet of the electrolytic cell It has a feature that it has an alkaline mixing tank which mixes the wastewater and the alkaline that are transferred from the Further, according to the electrode corrosion suppressing apparatus of the present invention, the alkali is added to the alkali mixing tank disposed between the outlet of the adjusting tank and the inlet of the electrolytic tank, and the wastewater and the advection flowing from the adjusting tank are mixed. Suppress corrosion of electrodes in the electrolytic cell. Other aspects of the invention are described in the embodiments described below.

  ADVANTAGE OF THE INVENTION According to this invention, the pH fall which arises in an electrolytic cell at the time of the electrolytic treatment of a water treatment system can be suppressed, and advancing of corrosion of the electrode by an acid can be suppressed.

It is a block diagram which shows the water treatment system which concerns on embodiment. It is a block diagram which shows the detail of an alkali mixing tank. Is a graph showing the potential -pH of Pt-H ₂ O-Cl. It is a flowchart which shows the alkali supply process of an electrode corrosion suppression apparatus. It is a flow chart which shows supply amount adjustment processing of an electrode corrosion control device. It is explanatory drawing which shows the pH adjustment condition by an electrode corrosion suppression apparatus.

  Hereinafter, modes for carrying out the present invention (hereinafter, referred to as “embodiments”) will be described in detail with reference to the drawings as appropriate. In each of the drawings, the same reference numerals are given to the common parts, and the duplicated explanation is omitted.

  FIG. 1: is a block diagram which shows the water treatment system which concerns on embodiment. The water treatment system 100 of the present embodiment has a circulation control tank 20 (control tank) into which wastewater flows in from the raw water tank 10 through the introduction path 12 and a pair of electrodes (first electrode 22a and second electrode 22b). An electrolytic power supply 25 for applying an electric current to the electrodes, a circulation path 27 for circulating the circulation adjustment water between the circulation control tank 20 and the electrolysis tank 22, and a circulation path 27. The circulation pump 21 disposed on the circulation pipe 26 on the upper side and supplying the circulation adjustment water from the circulation adjustment tank 20 to the electrolytic cell 22 and the control for controlling the pumps and valves when operating the water treatment system 100 The apparatus 50 is provided, and the wastewater is electrolyzed continuously. The circulation control tank 20 has an overflow port 29, and a flow rate equivalent to the flow rate sent from the supply pump 11 of the introduction path 12 is discharged from the overflow port 29. The polarity of the electrodes is, for example, the anode of the first electrode 22a and the cathode of the second electrode 22b.

  The water treatment system 100 includes a pH sensor 39 for measuring the pH of the treated water electrolyzed in the electrolytic cell 22 in the circulation pipe 28 (advection pipe) from the outlet 22 d of the electrolytic cell 22 to the circulation adjustment tank 20, and Between the outlet 20d of the tank 20 and the inlet 22c of the electrolytic cell 22, there is provided an alkali mixing tank 36 for mixing the wastewater and the alkali transferred from the circulation adjusting tank 20, and adding alkali to the alkali mixing tank 36 to adjust circulation. And an electrode corrosion suppression device 30 for mixing the wastewater transferred from the tank 20 and the alkali and suppressing the corrosion of the electrode in the electrolytic cell 22.

  In the present embodiment, the position of the alkali mixing tank 36 is downstream of the circulation adjustment tank 20 and upstream of the electrolytic tank 22. That is, an alkali is added between the circulation adjustment tank 20 and the electrolytic tank 22. For that purpose, an alkali mixing tank 36 is disposed between the circulation control tank 20 and the electrolytic tank 22, and in the alkali mixing tank 36, alkali is added to and mixed with the wastewater. Thus, the total amount (100%) of the added alkali is used for the purpose of suppressing the progress of the corrosion of the electrode.

  As described in Patent Document 1 (see FIG. 1), when alkali is added in the circulation adjustment tank 20, when treated water is discharged, the alkali is also discharged, and the total amount of the added alkali is not used. , Cause a decrease in efficiency. In addition, waste water is always transferred from the raw water tank 10 to the circulation adjustment tank 20, which affects the concentration of the added alkali.

  Further, the position where the pH (hydrogen ion concentration index) in this embodiment is measured is from the outlet 22 d of the electrolytic cell 22 to the time when the treated water subjected to the electrolytic treatment is transferred to the circulation adjustment tank 20, ie, the treated water PH is not affected by others.

  The pH sensor 39 uses, for example, a glass electrode sensor. The glass electrode method is a method of measuring the pH of a certain solution by using two electrodes of a pH glass electrode and a reference electrode and knowing the voltage (potential difference) generated between the two electrodes. When a solution of different pH exists on the inside and outside of the thin film of glass, an electromotive force proportional to the difference of pH is generated in the thin film portion. This thin film is referred to as a "pH glass response film", and theoretically, when the solution is at 25 ° C, if the difference in pH between the two solutions is one difference, an electromotive force of about 59 mV is generated. In general, since a solution of pH 7 is used as the internal solution of the pH glass electrode, the pH value of the sample can be determined by measuring the electromotive force generated in the pH glass response film.

  The electrode corrosion suppression device 30 measures the pH of the treated water electrolytically treated in the electrolytic cell 22 with the pH sensor 39, and adds alkali to the alkali mixing tank 36 so as to maintain the pH in the range of 6 to 8 inclusive. Control the quantity. The details will be described later with reference to FIGS.

  Specifically, from the pH measurement unit 31 that measures the pH of the treated water that has been electrolytically treated in the electrolytic cell 22, the alkaline storage tank 33 that stores an alkaline solution, and the circulation adjustment tank 20, the electrode corrosion suppression device 30 While the wastewater flows in, the alkaline solution flows in from the alkaline storage tank 33 through the alkaline supply pipe 35, and the above-mentioned alkaline mixing tank 36 which mixes the wastewater and the alkaline solution, and the pH measured by the pH measuring unit 31 The controller 32 determines the inflow of the alkaline solution from the alkaline storage tank 33 to the alkaline mixing tank 36 in accordance with the amount of increase or decrease per unit time. The control unit 32 issues a rotational speed command to the supply pump 34 disposed in the alkali supply pipe 35 in accordance with the determined inflow.

  As the alkaline solution, for example, 10% sodium hydroxide (NaOH) can be used. Sodium hydroxide is a chemical of a strong base (alkali).

  The water treatment system 100 according to the present embodiment is a system for reducing (removing) ammonia and organic substances contained in the wastewater to be treated by electrolytic treatment and discharging it as treated water.

  The water treatment system 100 according to the present embodiment can reduce the ammoniacal nitrogen concentration in the treated water by electrolytically treating the ammonia in the waste water. By the way, nitrogen (N) contained in wastewater includes ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, etc., but when the main component is ammonia nitrogen, it is possible to reduce ammonia nitrogen; It is effective in reducing nitrogen (N) in the medium to below the target value (denitrifying).

  The raw water tank 10 is a tank in which the wastewater to be treated is stored. An introduction passage 12 in which the waste water flows from the raw water tank 10 toward the circulation adjustment tank 20 is provided, and water is supplied to the introduction passage 12 via a supply pump 11.

  The circulation adjustment tank 20 is a tank in which the waste water from the raw water tank 10 (and the electrolyte aqueous solution from the electrolyte tank (not shown)) and the circulation adjusted water processed in the electrolytic tank 22 through the circulation path 27 are stored. . The circulation path 27 is a path that returns from the circulation adjustment tank 20 to the circulation adjustment tank 20 via the circulation pump 21, the alkali mixing tank 36, and the electrolytic tank 22 via the circulation pipe 26.

  In the electrolytic cell 22, a first electrode 22a and a second electrode 22b are provided on the flow path, and a current flows between the first electrode 22a and the second electrode 22b by the electrode power supply device 25. The electrode power supply device 25 is controlled by the control device 50.

The electrolytic cell 22 is configured to be capable of decomposing organic substances and ammonia in the waste water by the electrolytic treatment. Here, when sodium chloride (NaCl) is used as the electrolyte, reaction formulas of electrolysis are as shown in the following reaction formulas (1) to (3).
Anode _{^{reaction: 2Cl - → Cl 2 + 2e}} - ... (1)
Cathodic _{^{reaction: 2Na + + 2H 2 O +}} 2e - → 2NaOH + H 2 ... (2)
Cl ₂ +2 NaOH → NaClO + NaCl + H ₂ O (3)
Thus, electrolysis produces sodium hypochlorite (NaClO). In addition, since sodium hypochlorite produced | generated is in aqueous solution, this production | generation is that hypochlorous acid (HClO) and hypochlorite ion (ClO < ^- >) generate | occur | produce.

Then, sodium hypochlorite produced by electrolysis (NaClO) by reacting with ammonia (NH _3), so that the reaction formula (4) to (6).
NH ₃ + NaClO → NH ₂ Cl + NaOH (4)
NH ₂ Cl + NaClO → NHCl ₂ + NaOH (5)
NH ₂ Cl + NHCl ₂ → N ₂ + 3 HCl (6)
Thus, sodium hypochlorite denitrifies ammonia nitrogen (N) derived from ammonia (NH ₃ ) in waste water as nitrogen gas (N ₂ ).

In addition, although the reaction formula is omitted, the organic substance is water (by the action of oxidizing hypochlorous acid (HClO) and hypochlorite ion (ClO ⁻ ) using sodium hypochlorite (NaClO) generated). Decomposition into H ₂ O) and carbon dioxide (CO ₂ ). Thus, the electrolytic cell 22 reduces ammoniacal nitrogen and organic substances in the waste water by the electrolysis process.

  FIG. 2 is a block diagram showing the details of the alkali mixing tank. The alkali mixing tank 36 has a fluid mixing mechanism in its inside, and the waste water transferred from the circulation control tank 20 and the alkali solution supplied from the alkali storage tank 33 are efficiently mixed. The fluid mixing mechanism is a static fluid mixing mechanism that is advantageous in cost and has sufficient mixing performance.

  FIG. 2 shows an example of a Kenics mixer. The waste liquid is supplied from one of the T-shaped tubes 36a, and the alkaline solution is supplied from the other. Then, the waste fluid and the alkaline solution are effectively mixed via the static fluid mixer 36b. The static fluid mixer 36b effectively alternates the fluid by the dividing action, the rotating action, and the reversing action by alternately arranging the 180 ° right and left twist spiral blade members in series by shifting them by 90 °. Mix.

  In the present embodiment, as described above, the wastewater contains a large amount of ammonia nitrogen and organic components, and by performing electrolytic treatment, the pH (hydrogen ion concentration index) in the electrolytically treated water is lowered by the electrolytic treatment, and the electrolytic treatment is performed. Acidify the water. As a result, the corrosion of the electrode of the electrolytic cell proceeds, so it is necessary to extend the maintenance period of the electrode and the like. For this reason, the electrode corrosion suppression device 30 shown in FIG. 1 is applied. Platinum group metals are mainly used for electrodes for electrolytic processing.

FIG. 3 is a graph showing the potential-pH of Pt-H ₂ O-Cl. FIG. 3 is a view shown in FIG. 10 of JP-A-2011-252217. The horizontal axis represents the value of pH, and the vertical axis represents the voltage based on the hydrogen electrode. From the potential-pH diagram, platinum (Pt) becomes a corrosion area when the pH is at least 6 or less in the use environment of the electrode.

  Moreover, as described above, it is preferable to control the amount of alkali added to the alkali mixing tank 36 so as to maintain the pH in the range of 6 or more and 8 or less based on the experimental results of the inventors.

Next, processing of control part 32 of electrode corrosion control device 30 is explained.
FIG. 4 is a flow chart showing processing of the electrode corrosion control device. The control unit 32 acquires pH from the pH measurement unit 31 every predetermined time (step S1). The control unit 32 determines whether the pH is less than the threshold pHT1 (step S2), and if the pH is less than the threshold pHT1 (step S2, Yes), starts the supply of alkali (step S3), It returns to step S1. On the other hand, if the pH is not less than the threshold pHT1, that is, if the pH is not less than the threshold pHT1 (step S2, No), the control unit 32 proceeds to step S4.

  In step S4, the control unit 32 determines whether or not alkali is being supplied. If alkali is not being supplied (Step S4, No), the process returns to Step S1. If alkali is being supplied (Step S4, Yes), the controller 32 adjusts the supply amount (Step S5), and Step S6. Go to

  In step S6, the control unit 32 determines whether the pH exceeds the threshold pHT2. If the pH exceeds the threshold pHT2 (here, pHT2> pHT1) (step S6, Yes), the supply of alkali is stopped (step S7), and the process returns to step S1. On the other hand, if the pH does not exceed the threshold pHT2, that is, if the pH is equal to or lower than the threshold pHT2 (step S6, No), the control unit 32 returns to step S1.

  FIG. 5 is a flowchart showing the supply amount adjustment processing of the electrode corrosion suppression device. FIG. 5 is a process flow of step S5 of FIG. The control unit 32 determines whether the temporal change in pH (ΔpH / Δt) is equal to or less than the threshold ΔpHT3 (step S51). If the time change of pH is equal to or less than the threshold value ΔpHT3 (step S51, Yes), the control unit 32 increases the supply amount of alkali (step S52). On the other hand, if the time change of pH is not less than the threshold ΔpHT3 (step S51, No), it is determined whether the time change of pH is more than the threshold ΔpHT4 (step S53). If the time change of pH is equal to or higher than the threshold value ΔpHT4 (step S53, Yes), the control unit 32 reduces the supply amount of alkali (step S54). On the other hand, if the time change of pH is not more than threshold value ΔpHT4 (Step S53, No), the supply amount adjustment process is ended.

  FIG. 6 is an explanatory view showing a state of pH adjustment by the electrode corrosion suppression device. Description will be made with reference to FIGS. 1, 4 and 5 as appropriate. Here, in FIG. 4, pHT1 = 6 and pHT2 = 8. When the treatment of electrolysis of the water treatment system 100 starts, the pH of the circulation regulation water in the circulation regulation tank 20 decreases. Until the time t1, the pH is 6 or more, and the alkali is not supplied.

  At time t1, when the pH becomes less than 6, the control unit 32 starts the supply pump 34 to start supply of alkali. At time t2, when the pH exceeds 8, the control unit 32 stops the supply pump 34 and stops the supply of alkali. After that, similarly, when the pH becomes less than 6 at time t3, the control unit 32 starts the supply pump 34 to start supply of alkali. At time t4, when the pH exceeds 8, the control unit 32 stops the supply pump 34 to stop the supply of alkali. At time t5, when the pH is less than 6, the control unit 32 starts the supply pump 34 to start supply of alkali. At time t6, when the pH exceeds 8, the control unit 32 stops the supply pump 34 and stops the supply of alkali.

  By this pH adjustment, the treated water from the outlet 22 d of the electrolytic cell 22 can be adjusted to a pH of 6 or more and 8 or less. Thereby, it is possible to suppress the progress of the corrosion of the circulation pipe 28 of the circulation path 27 and the electrodes (the first electrode 22a and the second electrode 22b) of the electrolytic cell 22 due to the acid.

  According to the apparatus for suppressing corrosion of electrodes in the electrolytic treatment of waste water of the present embodiment, when the waste water is transferred from the circulation adjustment tank 20 to the electrolytic cell 22, the alkali is sufficiently added and the electrolytic treatment is performed in the electrolytic cell 22. Even if the acid is generated, the degree of influence on the electrode can be suppressed. For that purpose, an alkali mixing tank 36 is disposed between the circulation adjusting tank 20 and the electrolytic tank 22, and in the alkali mixing tank 36, the alkali is added to and mixed with the wastewater transferred from the circulation adjusting tank 20, and it is electrolyzed 22. The electrolytic solution is subjected to electrolytic transfer, and the pH of the treated water is maintained in the range of 6 or more and 8 or less at the outlet 22 d of the electrolytic cell.

10 Raw water tank 11 Supply pump 12 Introduction passage 20 Circulation adjustment tank (adjustment tank)
Reference Signs List 21 circulation pump 22 electrolyzer 22a first electrode 22b second electrode 25 electrode power supply device 26 circulation piping 27 circulation path 28 circulation piping (advection pipe)
29 Overflow port 30 Electrode corrosion suppression device 31 pH measurement unit 32 Control unit 33 Alkaline storage tank 34 Supply pump 35 Alkaline supply piping 36 Alkaline mixing tank 39 pH sensor 50 control device 100 Water treatment system pHT1, pHT2 threshold ΔPHT3, ΔPHT4 threshold

In order to achieve the above object, the water treatment system of the present invention comprises a control tank (for example, circulation control tank 20) into which waste water flows and an electrolytic tank (for example, electrolytic tank 22) for electrolytically treating waste water with at least a pair of electrodes. And a circulation pipe (for example, circulation pipes 26 and 28) for circulating waste water between the adjustment tank and the electrolytic cell, the adjustment tank being between the outlet of the adjustment tank and the inlet of the electrolytic cell In the alkaline mixing tank which mixes the wastewater and the alkali which are transferred from the flow, the pH measuring unit which measures the pH of the treated water in the advection pipe between the outlet of the electrolytic cell and the inlet of the adjusting tank, and the measured pH And a control unit that controls the addition of alkali to the alkali mixing tank . Further, according to the electrode corrosion suppressing apparatus of the present invention, the alkali is added to the alkali mixing tank disposed between the outlet of the adjusting tank and the inlet of the electrolytic tank, and the wastewater and the advection flowing from the adjusting tank are mixed. Suppress corrosion of electrodes in the electrolytic cell. Other aspects of the invention are described in the embodiments described below.

Claims (9)

A water treatment system comprising: a control tank into which waste water flows, an electrolytic cell for electrolytically treating the waste water with at least a pair of electrodes, and a circulation pipe for circulating the waste water between the control tank and the electrolytic cell
A water treatment system characterized by comprising an alkali mixing tank for mixing the wastewater and the alkali transferred from the adjustment tank between the outlet of the adjustment tank and the inlet of the electrolytic cell. In the water treatment system according to claim 1,
In the advection tube between the outlet of the electrolytic cell and the inlet of the adjustment tank, a pH measurement unit for measuring the pH of treated water, and an alkali mixing tank so as to maintain the pH in the range of 6 or more and 8 or less And a control unit that controls the addition of alkali. In the water treatment system according to claim 1,
A pH measurement unit that measures the pH of the treated water electrolytically treated in the electrolytic cell;
An alkaline storage tank for storing an alkaline solution,
An alkali supply pipe for causing the alkali solution to flow from the alkali storage tank to the alkali mixing tank;
And a controller configured to determine the inflow of the alkaline solution from the alkaline storage tank to the alkaline mixing tank according to the amount of increase or decrease per unit time of the pH measured by the pH measuring section. Water treatment system. In the water treatment system according to claim 3,
A supply pump disposed in the alkali supply pipe;
The said control part controls the said supply pump, and makes the alkaline solution of the determined inflow flow in into the said alkali mixing tank from the said alkaline storage tank. The water treatment system characterized by the above-mentioned. The water treatment system according to any one of claims 1 to 4, wherein the alkali is sodium hydroxide. A method of inhibiting corrosion of an electrode for electrolytic treatment of a water treatment system in which waste water is circulated in a control tank and an electrolytic tank for electrolytic treatment
The pH of the treated water electrolyzed in the electrolyzer is measured by the advection tube between the outlet of the electrolyzer and the adjustment tank, and between the outlet of the adjustment tank and the inlet of the electrolyzer A method of inhibiting electrode corrosion in a water treatment system, comprising adding an alkali and maintaining the pH in the range of 6 or more and 8 or less. It is juxtaposed to a water treatment system provided with a control tank into which the wastewater flows, an electrolytic tank which electrolytically treats the waste water with at least a pair of electrodes, and a circulation pipe which circulates the wastewater between the control tank and the electrolytic tank. Electrode corrosion control device,
An electrode corrosion inhibiting device, characterized in that an alkaline mixing tank is provided between the outlet of the adjusting tank and the inlet of the electrolytic cell to mix the wastewater and the alkali transferred from the adjusting tank. In the electrode corrosion inhibitor according to claim 7,
In the advection tube from the outlet of the electrolytic cell to the inlet of the adjustment tank, a pH measurement unit that measures the pH of treated water, an alkaline storage tank that stores an alkaline solution, and the pH of 6 to 8 And a control unit configured to control the amount of alkali added to the alkali mixing tank so as to be held in the range of (4). In the electrode corrosion inhibitor according to claim 8,
And an alkali supply pipe for causing the alkali solution to flow from the alkali storage tank into the alkali mixing tank, and a supply pump provided in the alkali supply pipe.
The control unit determines the inflow of the alkaline solution from the alkaline storage tank to the alkaline mixing tank according to the amount of increase or decrease per unit time of the pH measured by the pH measurement unit, and the supply pump An electrode corrosion inhibiting device characterized in that an alkaline solution having a controlled inflow amount determined is made to flow from the alkaline storage tank into the alkaline mixing tank.