DE102009039290A1 - Method for oxidation anodic treatment of electrically conductive, natural water and/or aqueous solution, comprises pressing a perforated structure in a cell housing on a cation exchanger membrane and a porous cathode plate - Google Patents

Abstract

The method for oxidation anodic treatment of electrically conductive, natural water and/or aqueous solution for its disinfection by anodically generated ozone and/or for oxidatively decomposing of organic and/or inorganic components under use of an electrolysis cell divided as solid electrolyte by cation exchanging membrane suited for anodic ozone generation, where the electrolysis cell having anode plate made of conductive material coated with doped diamond, with perforated structure, comprises pressing the structure in a cell housing on cation exchanger membrane and porous cathode plate. The method for oxidation anodic treatment of electrically conductive, natural water and/or aqueous solution for the purpose of its disinfection by anodically generated ozone and/or for oxidatively decomposing of organic and/or inorganic components under use of an electrolysis cell divided as solid electrolyte by cation exchanging membrane suited for anodic ozone generation, where the electrolysis cell having an anode plate made of a conductive material coated with doped diamond, with a perforated structure, comprises pressing the structure in a cell housing on a cation exchanger membrane and a porous cathode plate that flows through by formed and/or supplied gas from top towards bottom. The anode plate has a strength of 0.5-3 mm, is disposed over a gap volume of 30-80% and is flowed in longitudinal direction of the aqueous solution to be treated with a flow velocity of 0.02-1.0 m/s, related to the gap volume of the anode plate. A diamond-coated expanded grid made of niobium is introduced as anode plate with perforated structure. The aqueous solution to be treated has an electrical conductivity of 10 mS/cm. The porous cathode consists of foam, knitted fabrics or nonwoven made of carbon, stainless steel or from nickel and/or its alloy. The aqueous solution to be treated is conveyed in circulation through the electrode plate. The anolyte circuit is flowed through in the sense of a reactor cascade one after another from the aqueous solution to be treated. The aqueous solution is conveyed without circulation guidance in unique conduit by the anode plate. Ozone formed in unique flow per gram is conveyed to 100% of the solution to be treated through the anode plate. A diluted acid (5) is dosed in the porous cathode plate in such a quantity in the existence of hardeners in the aqueous solution to be treated so that a weak acid with a pH-value of = 3 exists at lower discharge. An independent claim is included for an electrolysis cell for oxidation anodic treatment of electrically conductive, natural water and/or aqueous solution.

Description

The invention relates to a method and a device for the anodic treatment of electrically conductive natural waters and / or aqueous solutions for the purpose of their disinfection by means of anodically generated ozone and or for the oxidative degradation of organic and inorganic ingredients using an electrochemical ozone generator. The ozone generator used is an electrolytic cell divided by cation exchange membranes with diamond-coated anodes and cathodes having a porous structure, pressed against the solid-electrolyte cation exchange membrane on both sides.

The process is suitable for generating ozone in conductive drinking, utility and waste water for the purpose of disinfecting it and / or oxidatively degrading inorganic or organic constituents, in particular pollutants, contained in them. The process enables the discoloration of colored wastewater, the oxidative degradation of complexing agents and other pollutants with simultaneous disinfection by the excess ozone. It is of particular advantage that the drinking or service water can be used without prior softening.

The current state of the art in the field of electrochemical ozone generators is characterized by electrolysis cells with cation exchange membranes as solid electrolyte, so-called Membrel cells (according to S. Stucki: "Reaction and Process Technology of Membrel Water Electrolysis" DECHEMA monograph, Verlag Chemie 94 (1983) 211 ) in which are used with β-lead dioxide or doped diamond coated anodes with a high oxygen surges application. The initially exclusively used lead-dioxide-coated anodes had the disadvantage of too low durability and it was necessary to maintain a protection potential at standstill of the electrolysis. The toxidicity of the lead also prevented the use of these cells for the disinfection of drinking water. Increasingly, therefore, the lead dioxide-coated anodes are replaced by those of diamond-coated niobium (US Pat. DE 198 42 396 ) or silicon ( DE 103 16 45 ) replaced. Such made by doping conductive diamond layers are characterized by high anode potentials and a significantly higher durability and can also be used for the disinfection of drinking water. However, they have the drawback that the porous structures having a large surface area accessible to lead dioxide, adjacent to the solid electrolyte membrane, are not currently available. Therefore, in order to form the three-phase boundary important for ozone formation, anode-solid electrolyte-water anode plates having a broken structure are used, necessitating adjustment of cell construction. The anode plates must be pressed firmly against the solid electrolyte membrane and again against the cathode. As anodes with a broken structure, preferably diamond-coated expanded metal electrodes are used. According to electrode arrangements DE 10 2004 015 680 , consisting of expanded metal anode and cathode plates, pressed against the cation exchange membrane, are already widely used for the disinfection of waters with a low electrical conductivity.

In the case of the ozone cells equipped therewith, however, the current yield of ozone formation drops sharply with increasing electrical conductivity of the electrolyte solution. For example, with a conductivity of the natural drinking water of about 0.6 to 0.8 mS / cms only 2 to 3% current efficiency achieved, while using deionized with a conductivity <0.02 mS / cm current yields above 20% are possible (sz BA Kraft: "Doped Diamond Electrodes, New Trends and Developments" Galvanotechnik (2008) H. 11, S 2808 ). Therefore, it makes little economic sense to use such conductive waters directly in ozone cells for the purpose of their disinfection and enrich with ozone. In addition, such natural waters contain hardeners, which in any case preclude their use in ozone cells with solid electrolyte cation exchange membranes, since it comes after a short period of electrolysis to increase the cell voltage by lime deposition in the membrane and on the cathode. Continuation of the electrolysis finally leads to irreversible damage to the membrane and thus to total failure of the ozone cell.

For these reasons, the prior art corresponding ozone cells are operated in particular with a circulating through the cell promoted deionized, from which the ozone formed emerges in gaseous form together with the oxygen also formed anodically. In the case of an undivided cell, the cathodically developed hydrogen additionally enters the gas mixture, which further reduces the ozone concentration. In order that the thus formed ozone gas mixture can be used to disinfect electrically conductive and hardness-containing natural waters and oxidative degradation contained pollutants contained in these waters, it must be dissolved in a Qzon cell downstream reactor therein and reacted. In addition to the additional equipment required for this, losses of ozone are usually unavoidable, since the critical for the solubility of the ozone partial pressure is greatly reduced by the excess of oxygen and optionally hydrogen.

The object of the present invention is therefore to significantly improve the achievable current efficiency of ozone formation in natural waters with higher electrical conductivity and thereby prevent lime precipitation by present hardeners.

According to the invention, this object is achieved by the electrolysis process illustrated in claims 1 to 9. For this purpose, the anode compartment of the divided by a cation exchange membrane ozone generator by means of a pressed to this 0.5 to 3 mm thick diamond-coated electrode plate with openwork structure and a void volume of 30 to 80% is formed, which rests on the back over the entire surface on a base plate so that they from being supplied below the anode plate to be treated natural water in the longitudinal direction to the above-arranged outlet is positively flowed through. The use of a diamond-coated expanded metal layer as an anode plate with a perforated structure proved to be particularly advantageous.

Preferably, the forced flow takes place at flow velocities of 0.02 to 1.0 m / s, based on the free cross section of the anode plate formed by the void volume. Surprisingly, it has been found that it is possible even at relatively low flow velocities within the anode plate from 0.02 m / s, with natural waters having an electrical conductivity between 0.6 and 1.0 mS / cm, relatively high current yields of ozone formation from 17 to 15% to realize. According to the invention, waters with electrical conductivities up to a maximum of 10 mS / cm can be used, while current yields between 15 and 11% can still be achieved.

Within the anode plate with a broken structure, the forced flow obviously causes strong microturbulences, which greatly accelerate the mass transport to and from the three-phase boundary solid electrolyte-anode surface electrolyte which is important for ozone formation. In contrast, the expanded metal anodes diminish in anode arrangements DE 10 2004 015 680 from the water to be disinfected laterally flowed from the outside. In this case, the water preferably flows through the intermediate spaces between the individual expanded metal-membrane combinations or between them and the housing walls. It is to be assumed due to the much larger flow resistance within the expanded metal layers that only a small part of the water actually flows through the expanded metal and that immediately adjacent to the three-phase boundary anode-electrolyte membrane can hardly form a directional flow. Therefore, the mass transfer to and from the three-phase boundary is preferably by diffusion. As a result, the saturation limit for the ozone-oxygen mixture in this range is also rapidly exceeded, so that gas bubbles form which, together with the water, emerge from the electrode region as a two-phase flow. It is therefore necessary for the complete dissolution of the ozone in the amount of water flowing through the cell housing a downstream device for dispersing and dissolving the gas fraction in the main amount of water flowing past the expanded metal anode.

In the method according to the invention, the aqueous solution to be treated can be circulated through the anode plate of the ozone cell. Such an electrolyte circuit can, for. B. operated as a batch reactor until the required degree of conversion in the oxidative degradation of ingredients is reached. If necessary, several of these Anolytkreisläufe of the metered water can be continuously flowed through successively in the sense of a reactor cascade.

But it is also possible to promote the aqueous solution to be treated, bypassing a circuit only once through the anode plate. For this purpose, it is necessary according to a further feature of the present invention to increase the throughput through the anode plate in a single pass without circulation so strong that all the ozone-oxygen mixture formed is dissolved in the water flowing through, so that no gas phase is formed.

As a result, no downstream reactors for dispersing and dissolving the ozone are needed. According to the invention, flow velocities in the gap volume of the anode plate through which flow is forced can be realized up to 1 m / s with a pressure drop in the anode of up to 4 bar. To achieve such complete dissolution, the amount of water to be delivered through the anode plate must be increased to at least 100 liters per gram of ozone produced and not consumed for the oxidative degradation of water constituents.

In the presence of oxidizable water constituents, a part of the ozone formed is already consumed for its oxidation or it is not even formed by a direct oxidation proceeding parallel thereto at the electrode surface. Then can be electrolyzed with a correspondingly adapted lower water flow rate to completely dissolve the excess ozone-oxygen mixture.

Ideally, there is an oxidative degradation of oxidizable water constituents and also to the formation of a sufficient amount of ozone for disinfection. This is z. B. for applications for water purification and disinfection in fish breeding and fish breeding systems of great importance. Thus it could be proven experimentally, that the fish poison ammonia in addition to other metabolic products of the fish can be anodized and oxidized in the ozone cells during the forced passage through the anode plate.

To avoid limescale deposits in the membrane and on the cathode of the ozone cell by the hardeners contained in the natural waters on the patent application AZ. DE 10 2009 005 011.6 and the technical teaching contained therein are adapted to the present problem solution. This includes a method of preventing scale deposits in water disinfecting cells separated by separators in the form of cation exchange membranes or microporous diaphragms in cathode and anode spaces. According to the technical teaching of this patent application, a porous cathode is used, to which the separator is pressed and which is flowed through by the cathodically formed or a gas supplied as a continuous phase from top to bottom. Above the cathode, a dilute acid is metered in such an amount and concentration that it emerges as a discontinuous phase together with the gas phase below the porous cathode having a weakly acidic pH ≦ 3. Due to the forming acidic trickle film in the region of the membrane lime deposition in the membrane and the adjacent cathode areas is reliably prevented.

The preferred construction of an electrolytic cell according to claim 10 for carrying out the method according to claims 1 to 9 is in the 1 shown. It shows an example of a construction variant of the ozone generator according to the invention in longitudinal section. It consists of the anodic edge plate 1 with the entry and exit of the anolyte solution 2 . 3 , Equally contains the cathodic edge plate 4 above the entrance for gas and for the diluted acid 5 , which at the lower exit 6 leaves the electrolysis cell again. Separated by the anodic and cathodic sealing plates 7 are the anode and cathode base plates 8th . 9 arranged with the power supply lines. Adjacent to it are the anode plate with a broken structure 10 and the porous cathode plate 11 , Both electrode plates are outwardly through the anode and cathode sealing frames 12 . 13 limited and sealed. In between is the solid electrolyte membrane 14 clamped ..

applications

example 1

It was a divided electrolysis cell for ozone generation according to 1 used with a forced-through diamond-coated niobium-expanded metal anode. The cation exchange membrane used was a NAFION N350 membrane. The porous cathode was formed by an approximately 2 mm thick nickel foam, which was traversed from top to bottom of air (so-called "empty cathode space"). The anode area (projection of the expanded metal anode) was 70 cm ² . An expanded metal with the following technical data was used. Width of the expanded metal 70 mm Length of the expanded metal 100 mm mesh length 12.5 mm mesh width 7.5 mm web width 1.5 mm material thickness 1.0 mm Expanded metal thickness 2.1 mm grammage 6.0 kg / m ² void volume 67.0% Surface magnification factor for surface projection 3.1 cm ² / cm ²

The two-sided coating of boron-doped diamond was about 5 microns thick. The meshes were arranged transversely to the flow direction. The flow cross-section results from this to 70 mm × 2.1 mm × 0.67 = 98.5 mm ² .

Electrolysis was carried out with a current of 20 A (0.29 A / cm ² , based on the expanded metal surface). As anolyte, deionized water and softened drinking water were alternately pumped through the expanded metal anode in a temperature-controlled circuit with different flow rates. The deionized had a conductivity of about 20 μS / cm, the softened drinking water of about 780 μS / cm. After saturation of the anolyte volume with the anodically generated ozone-oxygen mixture, the escaping gas mixture was absorbed in a wash bottle with KJ solution. The ozone content was determined by titration with thiosulfate and from this the current yield of ozone formation was calculated. The results obtained (average values of several measurements) are summarized in Table 1 with the calculated mean flow velocities, based on the free flow cross-section. Table 1: Dependence of the current yields on the flow velocity anolyte Throughput l / h Temperature ° C Flow velocity m / s Current efficiency% deionized water 5.5 35 0.016 18.1 deionized water 7.5 35 0,021 16.8 deionized water 12.0 32 0.034 18.9 deionized water 15.0 30 0.043 21.0 deionized water 15.0 33 0.043 20.1 deionized water 26.0 27 0.074 22.1 Drinking water 6.5 35 0,018 15.2 Drinking water 9.5 33 0027 15.2 Drinking water 15.5 27 0,044 16.7 Drinking water 24.0 26 0,068 17.7

It becomes clear that with the forced-flow anode even at relatively low mean flow velocities of around 0.02 m / s comparatively high current yields of around 17% for deionized water and still around 15% for drinking water are achieved. In the range of 0.05 to 0.07 m / s, the current yields approach a threshold value, that of the deionate. 21%, while drinking water is 17%. The differences are thus surprisingly low despite an approximately 40 times higher conductivity of the softened drinking water compared to the deionized.

Example 2

With the same electrolysis cell and experimental setup as in Example 1, the dependence of the current efficiency and the cell voltage on the conductance of the circulating anolyte was determined at a circulation rate in the limit range of about 25 l / h (about 0.07 m / s). In a first series of experiments was based on deionized, to which sulfuric acid, hydrochloric acid or sodium sulfate was added to increase the conductivity. In this case, as in Example 1 with "empty" from top to bottom air flowed through the cathode compartment was electrolyzed. In a second series of experiments it was assumed that a non-softened drinking water with a hardness of 22 ° dH, to which further increasing the conductivity also varying amounts of sulfuric acid, hydrochloric acid and sodium sulfate were added. To avoid Kalkabscheidungen in the cation exchange membrane and on the cathode, in addition to the air flow, an amount of dilute hydrochloric acid (about 10 g / l) was metered into the porous cathode, that at the lower outlet still a pH ≤ 3 was established. In both series of experiments, the stationary cell voltages were measured and the resulting specific DC consumption was calculated. The results obtained are summarized in Table 2. Table 2: Dependencies of the performance data of conductance with and without acid feed into the porous cathode Anolyte consisting of Feed in cathode conductance Zellspa. O ₃ -Ab. E _spec mS / cm V % kWh / g Deionate alone Air alone 0.02 13.2 23.3 0,190 Deionate with HCl additive Air alone 1.25 12.0 13.5 0,298 Deionate with H ₂ SO _{4 added} Air alone 0.80 11.8 15.1 0,262 Deionate with H ₂ SO _{4 added} Air alone 1.25 12.5 14.6 0.287 Deionate with Na ₂ SO _{4 added} Air alone 3.50 12.0 12.8 0,314 Drinking water alone Air + dil. HCl 0.70 10.2 17.8 0.192 Drinking water with HCl additive Air + dil. HCl 1.60 9.8 14.8 0.222 Drinking water with H ₂ SO ₄ -Zus. Air + dil. HCl 1.50 8.8 13.2 0.223 Drinking water with H ₂ SO ₄ -Zus .. Air + dil. HCl 2.10 8.5 13.0 0.219 Drinking water with Na ₂ SO ₄ -Zus. Air + dil. HCl 1.25 8.5 16.5 0.173 Drinking water with Na ₂ SO ₄ -Zus. Air + dil. HCl 2.10 8.3 12.9 0,216 Drinking water with Na ₂ SO ₄ -Zus. Air + dil. HCl 3.00 8.5 12.8 0.222 Drinking water with Na ₂ SO ₄ -Zus. Air + dil. HCl 5.00 8.4 11.4 0.247 Drinking water with Na ₂ SO ₄ -Zus. Air + dil. HCl 8.00 8.4 11.0 0.256

It becomes clear that even at high conductance values in the range of ≥ 5 mS / cm current efficiencies of around 11% can be achieved. In the case of natural waters with hardness formers, acid feeding into the porous cathode reduces the cell voltage by 2 to 3 V. As a result, the increase in the specific direct current consumption is at least partially compensated by the lower cell voltages due to the decreasing current efficiency. It can also be seen that it is irrelevant in the yield reduction by increasing the conductance, whether it is caused by the addition of acids or neutral salts.

Example 3

The same divided ozone cell as in Examples 1 and 2 was used. Instead of the anolyte circuit, the cell was connected via a flow meter directly to the drinking water network. It was electrolysed at a maximum pressure difference between inlet and outlet of 3.5 bar again with 20 A. The ozone content in the exiting drinking water was measured and used to calculate the current yield of ozone formation. The flow rate at which the ozone-oxygen mixture formed had already completely dissolved during the passage through the expanded metal anode was determined. In addition, the current efficiency was determined at the maximum achievable flow rate. Diluted hydrochloric acid (0.3 l / h with about 10 g / l HCl) was metered into the porous cathode as in example 2 with the air stream in order to prevent calcification in the cathode area. Already with a run of approx. 100 l / h drinking water no gas bubble formation was to be observed. This results in a specific minimum throughput of 5 l / A required for complete dissolution. An ozone content of 7.1 mg / l was determined in the drinking water outlet, corresponding to a current efficiency of 11.9%. The maximum flow through the expanded metal anode was determined to be 175 l / h, corresponding to an average flow velocity, based on the free cross-section of the forced-flow expanded metal anode, of approximately 0.5 m / s. The ozone content was determined to be 4.0 mg / l, corresponding to a current efficiency of 11.7%. The cell voltage was in both cases between 8.5 and 9.0 V. The achievable during the passage of these large amounts of drinking water power yields were significantly lower than in Example 2 with drinking water circulation. This is probably due to a higher self-consumption of ozone for the oxidation of ingredients contained in drinking water.

Example 4

With the same experimental arrangement as in Example 3, an electrolytic cell modified in the following manner was used to increase the flow rate. In addition to the previously used expanded metal anode with transverse flow meshes, an additional expanded metal plate of the same type but with longitudinally flown meshes was arranged. The anode plate with transverse flowed mesh was applied to the cation exchange membrane as before, the additional expanded metal mesh plate with longitudinally flown mesh lay on the anodic contact plate and was used to supply power from the anode contact plate to the expanded mesh anode. With the longitudinal flow of the additional expanded metal plate, a larger maximum flow rate should be achieved at the same pressure difference and meshing of the same meshes are prevented. It has reached a maximum flow rate of 690 l / h with an ozone content in the drinking water outlet of about 1.1 mg / l. This corresponds to a current efficiency of 11.6%.

Example 5

The degradation of water constituents parallel to the formation of ozone was investigated by the example of the decolorization of a strongly colored water using the same electrolysis cell as in Examples 1 to 3. The model water used was an aqueous solution of 1 g / l of the dye Brilliant Red 5b. The electrical conductivity of the dye solution was 0.66 mS / cm (solution A). Addition of sodium sulfate increased the conductivity to 1.5 mS / cm (solution B) and 2.3 mS / cm (solution C) to clarify the dependence of the decolorization on the conductivity. The electrolysis was carried out with recirculation of 1 l of the anolyte each with a circulation rate of about 20 l / h and a current of 20 A. As a scale for the decolorization, the relative decolorization after the electrolysis in% of initial color was determined. The color depth used was the weighted color code FSZ, which results from the extinction measurements at the three wavelengths E ₁ (436 nm), E ₂ (525 nm) and E ₃ (620 nm) according to the formula: FSZ = (E ₁ ² + E ₂ ² + E ₃ ² ) / (E ₁ + E ₂ + E ₃ )

The relative decolorization was determined as a function of the electrolysis time and the resulting specific current input in Ah / l and compiled in Table 3. Table 3: Relative discoloration depending on spec. power entry Electrolysis time in min Spec. Current input in Ah / l Relative discoloration in% Solution A Solution B Solution C 4.5 1.5 73 67 62 6.0 2.0 82 77 72 7.5 2.5 90 84 80 9.0 3.0 98 91 88

An approximately 90% relative decolorization was achieved with the solution A after 7.5 min, with the solution B after 8.4 min and with the solution C after 9.6 min. It also results in pollutant degradation with increasing electrical conductivity, a decrease. However, this is lower than expected due to the decrease in the current efficiency of ozone formation. It can be concluded that the oxidative degradation takes place not only via the ozone formed, but also by direct anodic oxidation at the diamond electrode. In all cases sufficiently high ozone concentrations were obtained for effective disinfection.

Example 6

A drinking water was added about 30 mg / l of ammonium in the form of ammonium sulfate. One liter of this solution was electrolyzed in the same electrolytic cell in the circuit with a current of 20 A. Dilute hydrochloric acid was again metered into the cathode compartment to prevent limescale removal. After 60 minutes of electrolysis, the ammonium was completely degraded. At the same time, about 0.9 g of ozone was formed, corresponding to a current efficiency of ozone formation of about 15%. The example illustrates that it is possible with the method according to the invention of ozone production to sufficiently disinfect the water circulating in fish farms (aquaculture facilities) and at the same time oxidatively degrade the fish poison ammonia excreted as the metabolic product of the fish.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19842396 [0003]
• DE 1031645 [0003]
• DE 102004015680 [0003, 0009]
• DE 102009005011 [0015]

Cited non-patent literature

• S. Stucki: "Reaction and Process Technology of Membrel Water Electrolysis" DECHEMA Monograph, Verlag Chemie 94 (1983) 211 [0003]
• BA Kraft: "Doped Diamond Electrodes, New Trends and Developments" Electroplating (2008) H. 11, S 2808 [0004]

Claims (10)

Process for the oxidative anodic treatment of electrically conductive, natural waters or aqueous solutions for the purpose of their disinfection by means of anodically generated ozone and / or for the oxidative degradation of organic and / or inorganic ingredients using an electrolysis cell suitable for anodic ozone generation, divided by means of cation exchange membranes as a solid electrolyte consisting of at least one anode plate of a doped diamond-coated conductive material having an openwork structure pressed in a cell housing against the cation exchange membrane and a porous cathode plate through which the gas generated or flows from top to bottom, characterized by that the anode plate is 0.5 to 3 mm thick, has a void volume of 30 to 80%, and longitudinally of the aqueous solution to be treated at a flow rate of 0.02 to 1.0 m / s , with respect to the void volume of the anode plate, is forced through. A method according to claim 1, characterized in that a diamond-coated expanded mesh made of niobium is used as the anode plate with a broken structure. Process according to claims 1 and 2, characterized in that the aqueous solution to be treated has an electrical conductivity of at most 10 mS / cm. Process according to claims 1 to 3, characterized in that the porous cathode consists of foams, knits or fleeces of carbon, stainless steel or of nickel or its alloys. Process according to claims 1 to 4, characterized in that the aqueous solution to be treated is circulated through the positively flowed through electrode plate. Process according to claims 1 to 5, characterized in that several such anolyte circuits in the sense of a reactor cascade are successively flowed through by the aqueous solution to be treated. Process according to claims 1 to 4, characterized in that the aqueous solution to be treated without recycling is conveyed in a single pass through the anode plate. A method according to claim 7, characterized in that the single flow per gram of ozone formed at least 100 l of the aqueous solution to be treated are conveyed through the anode plate. Process according to claims 1 to 8, characterized in that in the presence of hardness formers in the aqueous solution to be treated from above a dilute acid is metered into the porous cathode plate in such an amount that at the lower outlet still a weakly acidic pH of ≤ 3 is present .. Electrolysis cell for carrying out the method according to claims 1 to 10, consisting of a separated by a solid electrolyte membrane in anode and cathode compartment cell housing containing a diamond-coated anode plate of broken structure and a porous cathode plate, both of which are pressed by means of anode and cathode base plates to the solid electrolyte membrane characterized in that the diamond coated anode plate having a broken structure is fitted in the anode sealing frame between the anode base plate and the solid electrolyte such that the aqueous solution dosed below the anode plate is forced to forcefully flow through the anode plate in the longitudinal direction.

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