Water treatment facility and method for operating a water treatment facility

Abstract

The invention relates to a water treatment facility and to a method for operating same. The water treatment facility (10) has a raw water feed (11) for providing raw water (R), a softening device (12) which produces softened water (W) from the raw water (R), and a mixing device (24, vz1) for mixing raw water (R) from the raw water feed (11) with softened water (W) from the softening device (12), wherein the mixing device (24, v1) produces mixed water (V) by mixing raw water (R) with softened water (W) in an adjustable mixing ratio. The water treatment facility additionally comprises a controller (14) and at least one conductometric measuring device (15) which is designed to carry out a conductometric determination of the total hardness of the water of a water sample. The aim of the invention is to allow a detection, which is as precise as possible, of the total hardness of the raw water and / or the mixed water without using chemicals while having low installation and operating costs and while requiring little maintenance and having long-term stability without needing calibration. This is achieved in that the measuring device (15) is supplied mixed water (V) from the mixing device (24, v1), and the measuring device (15) carries out a conductometric measurement in particular prior to and after an electrolytic precipitation of alkali earth ions, wherein the measurement results are forwarded to the controller (14) in order to determine the total hardness (GHv) of the mixed water (V).

Description

[0001] Water treatment plant and method for operating a

[0002] Water treatment plant

[0003] The invention relates to a method for operating a water treatment plant and a water treatment plant.

[0004] The hardness of water is determined by the concentration of the alkaline earth metal cations dissolved in the water and, in specific applications, also by the concentration of their associated anionic partners, particularly bicarbonate. The sum of the concentrations of all dissolved alkaline earth metals (which can be present as carbonates, sulfates, chlorides, nitrites, nitrates, and phosphates) is referred to as the total hardness. The portion bound only to carbonic acid is referred to as carbonate hardness (or temporary hardness), and the difference between total hardness and carbonate hardness is referred to as non-carbonate hardness (or permanent hardness). The majority of the total hardness in drinking water is usually present as carbonate hardness. As so-called hardness formers, calcium and magnesium ions contribute primarily to the total hardness of water. The sum of the concentrations of calcium and magnesium ions therefore corresponds closely to the total water hardness.The remaining alkaline earth metals, such as strontium and barium, are usually present only as trace elements in water and therefore contribute little to water hardness. Carbonate hardness can be removed from the water by removing calcium and magnesium carbonate. The dissolved hardness-forming elements calcium and magnesium can form poorly soluble compounds in water, particularly as carbonates with the carbon dioxide dissolved in the water.

[0005] The formation of compounds that are difficult to dissolve in water by the hardness formers leads to the formation of scale in household appliances, heaters and hot water heaters, particularly when the water is heated, and reduces the effectiveness of detergents and washing machines in (dish)washers and washing machines. Water hardness also influences the taste of food and drinks prepared with water. To avoid or reduce the adverse effects of hard water, i.e. water with a high concentration of alkaline earth metal ions, the calcium and magnesium ions are partially or completely removed from the water, for example in water softening systems or in demineralisation systems. This is done by replacing them with sodium ions, for example using cation exchangers, or - in demineralisation - by completely removing them from the water together with all other dissolved ions, for example.through a combination of cation and anion exchangers or through reverse osmosis.

[0006] However, fully softened or demineralized water can lead to corrosion problems. Completely softening water in water softening systems with one or more regenerable ion exchangers also requires a large amount of regenerating agent, such as regenerating salt to produce an aqueous saline solution that can be used to regenerate the ion exchangers in a water softening system when they become depleted. Furthermore, in many applications, it is necessary or desirable to use water with a predefined average water hardness rather than fully softened or demineralized water.Water treatment plants are therefore known from the prior art in which, in order to produce partially softened or partially demineralized water with a predeterminable water hardness, an incoming raw water stream is divided into two sub-streams, a first sub-stream being fully softened or fully demineralized, and the fully softened or fully demineralized first sub-stream being mixed with a second sub-stream of the raw water, which remains untreated, to produce a blended water stream. The hardness of the blended water produced can be adjusted to a predeterminable hardness target value via the mixing ratio of the first and second sub-streams. Blending devices are used to mix the fully softened or fully demineralized first sub-stream with the second sub-stream containing raw water, in which the two sub-streams can be mixed at an adjustable mixing ratio using controllable valves.

[0007] In order to be able to set a specific blend water hardness in the blend water via the mixing ratio of the two partial streams, it is necessary to determine the hardness of the blend water and / or the hardness of the raw water as accurately as possible.

[0008] To accurately determine water hardness, complexometric titration methods, such as those using the disodium salt of ethylenediaminetetraacetic acid (EDTA) as a titrant, can be used. These titrants can measure the concentrations of alkaline earth ions dissolved in the water and thus the total hardness of the water. Measuring instruments that photometrically measure the color change point of the titration are known for this purpose. These measuring instruments are characterized by good measurement accuracy, but are complex and expensive to manufacture and require regular maintenance. Due to the use of the titrant, they are rarely suitable for automated measurement processes.

[0009] Furthermore, the total hardness of water can be determined using ion-selective electrodes (ISE), which measure the ionic activity of calcium and magnesium ions. Ion-selective sensors require regular calibration with a calibration fluid, which complicates their use in automated and low-maintenance measurement systems.

[0010] In addition to the titration method and the use of ion-selective electrodes to determine the total hardness of water, which directly measure the concentration of alkaline earth ions in the water, indirect measurement methods for determining water hardness are also known. For example, the electrical conductivity of the water can be used to determine water hardness. However, since the hardness-forming calcium and magnesium ions as well as all other ions dissolved in the water contribute to the electrical conductivity of the water, this method is very unspecific, especially when the water has a high proportion of non-carbonate hardness, e.g., due to a high chloride content.The relationship between electrical conductivity and the total hardness of water is very weak and must be determined using characteristic curves, which determine the relationship between the titrimetrically determined hardness of various water samples and their electrical conductivity. Furthermore, the electrical conductivity of water is highly dependent on temperature, which is why the temperature must be measured in addition to conductivity, and the hardness determination using the characteristic curve must be temperature-corrected. Therefore, conductivity measurements, which measure the absolute conductivity of a water sample, are only of limited use for determining the total hardness of a water sample.

[0011] DE 10 2007 059 058 A1 discloses a water softening system and a method for its operation, wherein the conductivity of the raw water is measured using a conductivity sensor, and the measured conductivity determines a total hardness of the raw water, which is used to control a blending device. The total hardness of the raw water is derived from the measured conductivity using a calibration curve, and the blending device is controlled in a feedback loop based on the derived total hardness of the raw water so that a blended water hardness is produced that corresponds to a predetermined target hardness. The water hardness determined from the calibration curve corresponds to a good approximation of an average water hardness that occurs at the measured conductivity in a variety of drinking water analyses.However, the actual water hardness at a given conductivity, which can be determined titrimetrically, can deviate considerably from this average value, especially if the raw water has a relatively high non-carbonate hardness (permanent hardness). If the water hardness of the raw water determined from the calibration curve deviates, the actual hardness of the blended water will not match the specified target hardness.

[0012] As a rule, a public drinking water supply provides a limited number of different drinking water qualities from different sources and with varying compositions and hardnesses. The discharge of these different drinking water qualities into the public drinking water supply network can vary within a single day or even over longer periods of time, so that the composition and hardness of the raw water supplied to a water treatment plant can change according to the quality of the drinking water provided by the public drinking water supply. Such changes in water quality, which can occur several times during a single day, alter the total hardness of the blended water produced in a blending device of a water treatment plant, with the result that its hardness no longer corresponds to the desired hardness.In order to be able to quickly adjust the mixing ratio in the event of a change in the water quality of the supplied raw water and to maintain the desired hardness of the blended water, rapid detection of any changes in the composition or quality of the raw water is necessary during the operation of a water treatment plant.

[0013] Based on this, the invention is based on the object of demonstrating a method for operating a water treatment plant and a water treatment plant with a blending device that enables the most accurate determination of the total hardness of the raw water and / or blended water without the use of chemicals, with low installation and operating costs, while requiring little maintenance, long-term stability, and requiring no calibration. The accuracy of determining the total hardness of the raw water and / or blended water should, in particular, meet the blending accuracy requirements of the DIN 19636-100 standard "Softening systems (cation exchangers) in drinking water installations."Furthermore, the method is intended in particular to ensure the most accurate calibration possible of the blending device during initial commissioning of the water treatment plant and an in-situ recording of changes in the hardness of the supplied raw water during operation of the water treatment plant in order to enable continuous control of the blending device, which is intended to produce blended water with a total hardness that corresponds as closely as possible to a predetermined target hardness, regardless of the hardness of the supplied raw water and even in the event of a change in the quality of the raw water or the raw water hardness.

[0014] These objects are achieved by the method according to claim 1 and the water treatment plant having the features of claim 17. Preferred embodiments of the method and the water treatment plant emerge from the dependent claims.

[0015] In the method according to the invention for operating a water treatment plant, which comprises a raw water inlet for providing raw water, a softening device, a blending device, a control device and at least one conductometric measuring device for conductometrically determining the total hardness of the water of a water sample, softened water is produced from the raw water in the softening device and in the blending device, blended water is first produced by mixing raw water from the raw water inlet with softened water from the softening device in an initial mixing ratio and fed to the measuring device and the measuring device carries out a conductometric measurement, in particular before and after an electrolytic precipitation of the alkaline earth ions, and forwards the measurement results to the control device, which determines the total hardness of the blended water from the measurement results.

[0016] The conductometric determination of the total hardness of the blended water enables a direct comparison of the determined total hardness of the blended water with a predefined target value of the water hardness of the blended water as well as a readjustment of the initial mixing ratio if a deviation occurs, in particular if the deviation exceeds a predefined limit value.The conductometric measurement carried out in the measuring device before and after an electrolytic precipitation of the alkaline earth ions enables an exact determination of the total hardness of the blended water, since all hardness-forming alkaline earth ions, in particular as carbonates, are precipitated from the water by the electrolytic precipitation and a conductometric difference measurement records the change in the conductivity of the blended water caused by the precipitation of the alkaline earth ions and the amount of precipitated alkaline earth ions and thus the total hardness of the blended water can be determined very precisely using a conversion factor or a characteristic curve.

[0017] It is assumed that in a water sample in which there is an excess of bicarbonate ions compared to alkaline earth ions, which is the case with blended water produced by mixing raw water with softened raw water, with at least largely complete electrolytic precipitation of the alkaline earth ions in the form of carbonates, the total hardness of the water in the water sample can be determined from the change in conductivity caused by the precipitation of the alkaline earth ions (ALf = Lf2 - Lfi), in particular from the amount of the reduction in conductivity, because the difference in conductivity of the water sample before and after the precipitation of the alkaline earth ions enables the total concentration of the alkaline earth ions in the water to be determined, the total concentration of which defines the total hardness of the water.If there is an excess of bicarbonate ions over alkaline earth ions in the blended water, the carbonate hardness corresponds to the total hardness of the blended water during electrolysis of the water sample in the electrolysis cell. The change in conductivity caused by the at least essentially complete precipitation of the alkaline earth ions therefore represents a measure of the concentration of hardness-forming alkaline earth ions in the blended water. Therefore, the total hardness of the blended water can be determined from the recorded change in conductivity (ALf) using a predetermined conversion factor (F), which is related in particular to a specific hardness scale, or using a predetermined characteristic curve that characterizes the course of the conductivity change as a function of the change in water hardness during precipitation of the carbonate hardness.The change in conductivity (ALf) is preferably determined in the method according to the invention by first measuring the original electrical conductivity (Lfi) of a water sample of the blended water. After at least substantially complete precipitation of the alkaline earth ions in the electrolysis cell, the electrical conductivity (Lf2) of the water sample is measured again. From the two measured values, the absolute value of the difference (ALf = |Lf2 - Lfi|) is determined. The measurement of the original electrical conductivity (Lfi) of the water sample of the blended water and the electrical conductivity (Lfi) of the water sample after at least substantially complete precipitation of the alkaline earth ions can be carried out using a cost-effective conductivity sensor.

[0018] The requirement that there be an excess of bicarbonate ions over alkaline earth ions in the water sample is generally automatically met in blended water obtained by mixing raw water with softened water, which is obtained by softening the raw water. This is because the concentration of bicarbonate is maintained in the blended water and, compared to the raw water, the bicarbonate ions are present in excess of the alkaline earth ions. It has been shown that there is an excess of bicarbonate ions over alkaline earth ions in blended water if the total hardness of the blended water is less than 3°dH.Empirical measurements on a large number of drinking water samples from Germany have shown that, in blended water with a titrimetrically determined total hardness of 5°dH or less, more than 98% of the drinking water samples exhibit an excess of bicarbonate ions over alkaline earth ions. If the blended water does not already exhibit an excess of bicarbonate ions over alkaline earth ions (which may be particularly the case if the total hardness GHv is < 5°dH), bicarbonate can be added to the blended water to create an excess of bicarbonate ions over alkaline earth ions to maintain the measurement conditions.

[0019] The control device is expediently configured to determine the total hardness of the raw water from the determined total hardness of the blended water, taking into account the initial mixing ratio or a mixing ratio adjusted during subsequent operation of the water treatment plant. The total hardness of the raw water thus determined can be used, for example, to predict the remaining capacity of the ion exchanger and to determine a suitable time for initiating a regeneration process to regenerate the softening device, in particular one or more ion exchangers with a regeneration solution, e.g., a saline solution, when the softening device is exhausted.

[0020] The conductometric determination of the total hardness of the blended water can be performed both during commissioning of the water treatment plant and after a change in the quality, and in particular the hardness, of the incoming raw water. A change in the total hardness of the raw water can be continuously recorded during operation of the water treatment plant by measuring the conductivity of the raw water using a conductivity sensor located in the raw water inlet or using the measuring device. Any changes in the hardness of the incoming raw water can be detected from the measured conductivity of the raw water.

[0021] When the water treatment plant is first put into operation, the hardness of the raw water can initially be roughly estimated from the conductivity of the raw water, which is measured with the conductivity sensor arranged in the raw water inlet or by means of the measuring device, using a characteristic curve or a conversion factor in order to adjust the initial mixing ratio based on the estimated hardness of the raw water so that the total hardness of the blended water at least approximately corresponds to a predetermined target hardness of the blended water.Then, when the water treatment plant is put into operation, an exact conductometric determination of the total hardness of the blended water is carried out using the measuring device. From the determined total hardness of the blended water, the total hardness of the raw water is determined, taking into account the initial mixing ratio. If necessary, the mixing ratio is adjusted in order to set the total hardness of the blended water to the specified target value.

[0022] During operation of the water treatment plant, the conductivity of the raw water is monitored either by a conductivity sensor located in the raw water inlet or by a measuring device to detect changes in the hardness of the incoming raw water. Although the conductivity of water only correlates to a limited extent with the total hardness of the water, and the absolute conductivity can only provide a rough estimate of water hardness using characteristic curves, a differential measurement, which detects a change in the electrical conductivity of the water, is very suitable for detecting changes in water hardness.

[0023] In order to continuously measure the conductivity of the raw water during operation of the water treatment plant using the measuring device, which is conveniently located downstream of the softening device and the blending device to determine the total hardness of the blended water, the raw water is supplied via a bypass line bypassing the softening device in a preferred embodiment. In this embodiment, an additional conductivity sensor arranged in the raw water inlet can be dispensed with.

[0024] In order to precisely adjust the mixing ratio of the raw water and the softened water, the control device is coupled to the blending device and the initial mixing ratio is set, particularly during commissioning of the water treatment plant, by the control device after the conductometric determination of the total hardness of the blended water in dependence on the recorded total hardness of the blended water to a (new) mixing ratio at which the total hardness of the blended water corresponds to the specified target hardness.

[0025] If, during operation of the water treatment plant, when recording the conductivity of the raw water by means of the measuring device or by means of an additional conductivity sensor in the raw water inlet, a change in conductivity is detected, which is caused, for example, by a changed quality or composition of the supplied raw water, a conductometric determination of the total hardness of the blended water is preferably initiated by means of the measuring device, particularly if the amount of the recorded change in conductivity exceeds a predetermined limit value, in order to adapt the mixing ratio set during commissioning of the water treatment plant to the changed hardness of the raw water in such a way that the blended water produced in the blending device again has a total hardness which corresponds to the predetermined target hardness.

[0026] Advantageously, the measured values ​​of the conductivity of the raw water recorded during operation of the water treatment plant are stored in a data memory of the control device, along with the determined total hardness of the blended water and / or the associated mixing ratio of the blended water. This allows the control device to adapt the mixing ratio and the regeneration triggering to the changed water quality if the water quality or the hardness of the supplied raw water changes and the changed water quality is detected due to a change in the measured conductivity of the raw water, without having to redetermine the total hardness of the blended water and / or the raw water using the measuring device.This takes into account the fact that a public drinking water supply usually provides a limited number of different drinking water qualities with varying compositions and hardnesses, and that these different drinking water qualities can change within a single day or even over longer periods of time. For this reason, the composition and hardness of the raw water supplied to the water treatment plant can change depending on the quality of the drinking water made available via the raw water inlet. It is advantageous to have all drinking water qualities available at the site of the water treatment plant and their total hardness orwhose conductivity has been recorded and the recorded measured values ​​of the conductivity as well as the mixing ratios with softened or demineralized water and the total hardness of the blended water resulting from the corresponding mixing ratio are stored in the memory of the control device, so that if a change in the conductivity of the raw water is detected, the control device can conclude that the water quality of the supplied raw water has changed and, using the stored data, can adapt the mixing ratio and the regeneration trigger to the changed water quality of the raw water and in particular its hardness.

[0027] In the method according to the invention, the total hardness of the blend water is preferably determined conductometrically after electrolytic precipitation of the alkaline earth ions, in particular the calcium and / or magnesium ions. For this purpose, electrolytic precipitation of the alkaline earth ions is carried out in the blend water supplied to the measuring device, wherein the measuring device preferably determines the electrical conductivity of the blend water before precipitation (Lfi) and the electrical conductivity of the blend water after precipitation (Lf2). From the measured values ​​of the electrical conductivity of the blend water before precipitation (Lfi) and after precipitation (Lf2), the change in conductivity (ALf = |Lf2 - Lfi|) caused by the electrolytic precipitation of the alkaline earth ions in the blend water can be determined in terms of amount, and the total hardness of the blend water can be determined from this using a conversion factor or a calibration curve.

[0028] It is assumed that in a water sample in which there is an excess of hydrogen carbonate ions compared to alkaline earth ions, which is usually the case with blended water due to the addition of softened or demineralized water to the raw water and at least with blended water with a total hardness of less than 3°dH, a change in conductivity (ALf = Lf2- Lfi), in particular a reduction in conductivity, is caused by at least largely complete electrolytic precipitation of the alkaline earth ions in the form of carbonates.This change in conductivity is detected in a preferred embodiment of the method according to the invention with the measuring device, wherein the amount of the detected change in conductivity (ALf) represents a measure of the concentration of the hardness-forming alkaline earth ions in the blend water and therefore the total hardness of the blend water can be deduced from the detected change in conductivity (ALf) by means of a predetermined conversion factor or a predetermined characteristic curve of the course of the conductivity of water as a function of the total hardness of the water and the total hardness of the raw water can be deduced from the set blend.

[0029] The change in conductivity (ALf) is expediently determined by first measuring the original electrical conductivity (Lfi) of a water sample of the blended water. After at least substantially complete precipitation of the alkaline earth ions in an electrolysis cell of the measuring device, the electrical conductivity (Lf2) of the water sample is measured again. From the two measured values, the absolute value of the difference (ALf = |Lf2- Lfi|) is determined. The measurement of the original electrical conductivity (Lfi) of the water sample and the electrical conductivity (Lf2) of the water sample after at least substantially complete precipitation of the alkaline earth ions can be carried out using a cost-effective conductivity sensor, which is preferably integrated into the electrolysis cell of the measuring device or can also be arranged outside the electrolysis cell.The electrolysis of the water sample in the electrolysis cell of the measuring device is carried out solely for the purpose of precipitating the alkaline earth ions. The other effects of electrolysis of the water sample, in particular the decomposition of water into hydrogen and oxygen, are not utilized.

[0030] The total hardness of the blended water sample is determined from the recorded change in conductivity (ALf) of the water sample using a conversion factor (F), which is, in particular, a proportionality factor of a linear relationship between the conductivity of the water and the carbonate hardness of the water. The conversion factor can be derived from a characteristic curve that, when the carbonate hardness of a water sample precipitates, describes a linear change in electrical conductivity as a function of the change in the total hardness or the carbonate hardness of the water.The conversion factor (F) results from the ratio between the mass of hardness-forming substances removed during electrolysis of the water through precipitation of alkaline earth ions (as carbonates) and the resulting reduction in the electrical conductivity of the water sample due to the reduced ion concentration in the water caused by the precipitation of alkaline earth ions. Typically, the reduction in the electrical conductivity of the water sample is linear to the precipitated concentration of alkaline earth ions, with the conversion factor (F) being the proportionality constant of this linear relationship.

[0031] The conversion factor (F) or the characteristic curve can be determined experimentally and, in particular, empirically on a variety of different water samples, in particular on various drinking water samples of different origins or compositions with an excess of bicarbonate ions over alkaline earth ions. Conductometric titration of the water samples is used to determine the relationship between the titrimetrically determined total hardness and the change in electrical conductivity due to complete precipitation of the alkaline earth ions. From this, a course of the conductivity change as a function of the total hardness of the water is determined in a diagram or characteristic curve. For this purpose, a common hardness scale, in particular the German water hardness or the French water hardness, can be used as a measure of the total hardness.The total hardness is determined titrimetrically when determining the conversion factor or the characteristic curve, e.g. by EDTA titration (according to DIN 38406-3, Group E - Part 3).

[0032] The conversion factor (F) or the characteristic curve can also be determined experimentally on water samples with a sufficient concentration of alkaline earth ions without a predetermined excess of bicarbonate ions (i.e., in particular, without preconditioning the water sample). This is done by subjecting the carbonates to electrolytic precipitation on the water samples. Both the change in electrical conductivity caused by the precipitation and the change in hardness (total hardness or carbonate hardness) of the water sample are recorded in a diagram using titrimetric methods, and the relationship between the change in conductivity and the change in hardness is determined from this. The conversion factor F results as a proportionality factor from the generally linear course of the change in conductivity as a function of the change in hardness.Assuming that the at least largely complete precipitation of alkaline earth ions in the electrolysis cell of the measuring device precipitates all of the hardness-forming substances contained in the blend water sample, the recorded change in conductivity (ALf) represents a measure of the total hardness of the blend water, which can be traced back to a common hardness scale using the conversion factor or the characteristic curve and thus expressed as the total hardness of the blend water. In this case, the precipitated (temporary) hardness is used to determine the (original) total hardness of the blend water sample using mass balance calculations.This is made possible by the excess of bicarbonate ions over alkaline earth ions in the blend water, because this allows the entire concentration of alkaline earth ions in the blend water to be (at least largely) precipitated as carbonates in the electrolysis cell. The resulting change in conductivity is a measure of the total concentration of alkaline earth ions in the blend water and thus of the total hardness of the blend water. Due to the excess of bicarbonate ions over alkaline earth ions in the blend water, the (precipitated) carbonate hardness of the blend water sample during electrolysis corresponds to the total hardness of the blend water if the alkaline earth ions in the electrolysis cell have at least largely been precipitated as carbonates.

[0033] To ensure at least largely complete precipitation of the alkaline earth ions in the electrolysis cell of the measuring device and thus a high degree of accuracy in determining the total hardness of the blended water, the conductivity of the blended water sample is preferably measured during the electrolytic precipitation of the alkaline earth ions up to a transition point at which the measured conductivity has a minimum value (Lf2) due to complete precipitation of the alkaline earth ions. Upon further electrolysis of the water sample beyond the transition point, a sudden or gradual increase in the measured conductivity of the water sample can be observed.This increase in conductivity enables a clear determination of the minimum value (Lf2) of the conductivity from the time course of the conductivity of the water sample measured during the electrochemical precipitation and thus a precise determination of the change in conductivity (ALf) caused by a complete precipitation of the alkaline earth ions.

[0034] In order to achieve sufficient accuracy of the determined total hardness of the water in the water sample, which in particular corresponds to the specifications of the standard DIN 19636-100, the conductivity of the water sample of the blended water is preferably recorded during the electrolytic precipitation of the alkaline earth ions at least until at least 90% of the alkaline earth ions, particularly preferably more than 95%, have been precipitated by the electrolysis.

[0035] To precipitate the alkaline earth ions, the blend water sample is preferably introduced into an electrolysis cell containing at least two electrolysis electrodes, which are subjected to a direct current for electrolytic precipitation of the alkaline earth ions. The electrolysis electrodes can, for example, be designed as conductive plates (flat electrodes) arranged at a distance from and parallel to each other. The water sample can be introduced between the two electrolysis electrodes in a batch process, and the electrolysis cell is then closed to perform the measurement.

[0036] In a preferred embodiment of the method according to the invention, in a measuring mode which is carried out in particular when commissioning the water treatment plant, a specific volume of a water sample of the blended water, which expediently corresponds to the internal volume of the electrolysis cell, is introduced into the electrolysis cell in batch operation. Immediately before the start of the electrolytic precipitation of the alkaline earth ions, the original conductivity Lfi of the water sample is recorded in a first measuring cycle. The electrolytic precipitation of the alkaline earth ions can then begin by applying a direct voltage to the electrolysis electrodes during a first electrolysis cycle. The conductivity sensor, which is preferably integrated into the electrolysis cell, is switched off during the first electrolysis cycle, i.e. the measuring electrodes are not subjected to alternating voltage during the electrolytic precipitation of the alkaline earth ions.The first electrolysis cycle can then be followed by alternating further measuring cycles and further electrolysis cycles in order to precipitate the alkaline earth ions contained in the blend water and to record the conductivity of the blend water sample selectively or quasi-continuously during the precipitation until the alkaline earth ions have at least essentially completely precipitated. After the alkaline earth ions have been at least largely completely precipitated, the electrolysis electrodes are de-energized and the conductivity Lf2 of the water sample is recorded in a final measuring cycle. By calculating the difference between the original conductivity Lfi and the conductivity Lf2 measured in the last measuring cycle, the change in conductivity ALf = |Lf2 - Lfi| caused by the precipitation of the alkaline earth ions is determined. The duration of the electrolysis cycles is preferably between 2 minutes and 10 minutes and is, for example,5 minutes and the duration of the measuring cycles is preferably between 30 seconds and 120 seconds and is, for example, 60 seconds.

[0037] To measure the conductivity of the blended water sample before and / or during and after the precipitation of alkaline earth ions, the electrolysis cell advantageously contains at least one integrated conductivity sensor. This allows for a compact design. The conductivity sensor integrated into the electrolysis cell preferably comprises two measuring electrodes, to which an alternating voltage is applied to measure the conductivity of the water sample, and the current flowing through the measuring electrodes is measured.

[0038] To avoid interference, it is advantageous if, in the measuring mode, the electrolytic precipitation of alkaline earth ions and the measurement of the conductivity of the water sample are alternately performed in the electrolysis cell. This decouples the electrolytic precipitation of alkaline earth ions and the conductivity measurement with the at least one conductivity sensor integrated into the electrolysis cell, thereby avoiding interference during the conductivity measurement.

[0039] In order to take into account the influence of temperature on the conductivity of the water sample of the blended water, the temperature of the water sample is preferably measured in the measuring mode and in particular during the electrolytic precipitation of the alkaline earth ions and when calculating the total hardness of the blended water from the recorded change in conductivity (ALf), a temperature correction is preferably carried out using a temperature correction factor or a temperature characteristic curve which represents the dependence of the conductivity of water on the temperature.

[0040] The water treatment plant according to the invention comprises a raw water inlet for providing raw water, a softening device connected to the raw water inlet, which in particular contains at least one ion exchanger or can also be designed as a desalination device, a blending device for producing blended water by mixing raw water from the raw water inlet with softened or demineralized raw water from the softening device in a predeterminable mixing ratio, a control device and at least one conductometric measuring device, wherein the control device is configured to determine the hardness of the water of a water sample fed to the conductometric measuring device and the measuring device is connected to the blending device in order to feed blended water from the blending device to the measuring device and to determine the total hardness of the blended water,wherein the control device controls the measuring device in such a way that it carries out a conductometric measurement, in particular before and after an electrolytic precipitation of the alkaline earth ions, and forwards the measurement result to the control device in order to determine the total hardness of the blended water from the measurement result.

[0041] The control device is expediently configured to determine the hardness of the raw water from the determined total hardness of the blended water, taking the mixing ratio into account. Based on the determined raw water hardness, an estimate of the softening capacity of the softening device can be made and / or, if the softening capacity is predicted to be soon exhausted, a regeneration process can be initiated to regenerate the softening device, in particular the ion exchanger, using a regeneration solution.

[0042] In a preferred embodiment of the water treatment system, the measuring device comprises an electrolysis cell with at least two electrolysis electrodes and at least one conductivity sensor, wherein the conductivity sensor is preferably integrated into the electrolysis cell. This allows a precise determination of the total hardness of the blended water by means of the measuring device after electrolytic precipitation of the alkaline earth ions, in particular the calcium and / or magnesium ions, according to the preferred embodiment of the method according to the invention. The electrolysis cell and / or the conductivity sensor expediently comprises a temperature sensor for detecting the temperature of the water sample during a measuring mode.This allows a temperature correction to a given standard temperature using a known temperature correction factor when measuring conductivity, which allows the influence of temperature on conductivity to be taken into account when determining water hardness.

[0043] The electrolysis electrodes, which are connected to a direct current source for the electrolytic precipitation of alkaline earth ions, are preferably arranged opposite one another in the electrolysis cell of the measuring device and are expediently designed as flat electrodes. This allows a sufficient volume of the electrolysis cell to accommodate a sample of the blended water while maintaining a compact design of the measuring device. The electrolysis electrodes comprise at least one anode, preferably made of platinized titanium sheet or a platinized titanium grid or a graphite film, and at least one cathode, preferably made of steel or titanium.

[0044] To measure the conductivity of the water sample before and / or during and after the precipitation of alkaline earth ions, the measuring device comprises at least one conductivity sensor. Preferably, at least one conductivity sensor is integrated into the electrolysis cell of the measuring device, with this conductivity sensor comprising at least one electrode pair with two measuring electrodes that are or can be connected to an alternating voltage source. The integration of the one conductivity sensor or the multiple conductivity sensors into the electrolysis cell enables a compact and space-saving design of the measuring device. However, it is also possible for at least a first conductivity sensor and / or a second conductivity sensor to be arranged outside the electrolysis cell. For example, a first conductivity sensor can be placed upstream of the inlet of the electrolysis cell and a second conductivity sensor downstream of the outlet of the electrolysis cell.

[0045] The conductivity sensor of the measuring device preferably comprises at least one pair of electrodes with two measuring electrodes, in particular designed as rod electrodes, which are connected to an alternating voltage source. The measuring electrodes are expediently arranged between the opposing electrolysis electrodes of the electrolysis cell.

[0046] For the continuous detection of the conductivity of the raw water during operation of the water treatment plant, a further conductivity sensor is preferably arranged in the raw water inlet of the water treatment plant according to the invention.

[0047] In order to set a suitable mixing ratio of the blended water, the control device of the water treatment plant according to the invention is preferably coupled to the blending device and set up such that the mixing ratio is regulated to a predetermined target value of the total hardness of the blended water as a function of the determined total hardness of the blended water, wherein the conductivity of the raw water detected by means of the measuring device or by means of the additional conductivity sensor in the raw water inlet is preferably fed to the control device constantly or at certain times and the control device initiates a conductometric determination of the total hardness of the blended water by means of the measuring device as a function of the detected conductivity of the raw water, in particular when a detected change in the conductivity of the raw water.

[0048] To determine the hardness of the blended water, the control device of the water treatment system according to the invention expediently contains a data memory in which a conversion factor or a characteristic curve is stored, which represents the change in water conductivity during precipitation of carbonate hardness as a function of the change in water hardness. The control device accesses the conversion factor or characteristic curve and is preferably configured to calculate the total hardness of the blended water on a hardness scale associated with the conversion factor or characteristic curve from the detected change in conductivity (ALf) and the conversion factor or characteristic curve.

[0049] To accelerate the precipitation of alkaline earth ions in the measuring mode, a cation exchange membrane can be arranged between the two opposing electrolysis electrodes of the electrolysis cell, with the cation exchange membrane running, in particular, parallel to the electrolysis electrodes designed as flat electrodes. Furthermore, several pairs of electrolysis electrodes can be arranged at a distance from one another in the electrolysis cell of the measuring device, in particular in the form of flat electrodes running parallel to one another. For example, a cascade arrangement of corresponding electrolysis electrodes can be provided in the electrolysis cell in the order anode - cathode - anode - cathode - anode. This also accelerates the precipitation of alkaline earth ions in the measuring cycles and thus reduces the measuring time.

[0050] With the method and water treatment system according to the invention, the total hardness of the blended water, which is determined by the carbonate hardness (temporary hardness), as well as the total hardness of the raw water can be determined with sufficient accuracy in an automated process without the use of chemicals. Depending on the determined total hardness of the blended water and / or the raw water, the mixing ratio or the volume fraction of the raw water in the blended water can be adjusted so that the total hardness of the blended water corresponds to a predetermined target hardness.The process and the water treatment plant also enable in-situ detection of a change in the conductivity of the supplied raw water, in order to be able to quickly adjust the mixing ratio in the event of a change in the water quality of the supplied raw water, so that the hardness of the blended water corresponds to the specified target hardness and the regeneration trigger and capacity can be recalculated.

[0051] These and other advantages and applications, as well as preferred features of the method and water treatment system according to the invention, will become apparent from the exemplary embodiments of the invention described below with reference to the drawings. In the drawings:

[0052] Fig. 1: a schematic representation of an embodiment of a water treatment plant according to the invention;

[0053] Fig. 2: schematic representations of various embodiments of a conductometric measuring device which can be used in the water treatment plant according to the invention and the method according to the invention;

[0054] Fig. 3: a flow diagram of a preferred embodiment of the inventive

[0055] procedure;

[0056] Fig. 4: a diagram of a typical time course of the electrical conductivity of a water sample with blended water from the water treatment plant of Fig. 1 when carrying out the method according to the invention;

[0057] Fig. 5: a diagram of the temporal progression of the electrical conductivity of a water sample with blended water from the water treatment system of Figure 1, as determined using the method according to the invention, as well as the temporal progression of the hardness of the blended water determined using the method according to the invention. Figure 1 shows an embodiment of the water treatment system according to the invention, wherein the water treatment system 10 comprises a raw water inlet 11 for providing raw water R, a softening device 12 connected to the raw water inlet 11, which contains at least one ion exchanger 13, a control head 24, a blending valve v1 for generating blended water V, a control device 14, and at least one conductometric measuring device 15.The control head 24 is connected to the raw water inlet 11 and to the softening device 12 and is designed to control the volume flows of the raw water R to the softening device 12 and of the water W softened in the softening device 12 into a blended water line 20 connected to the control head 24 and is controlled for this purpose by the control device 14. The control head 24, together with the blending valve vl, forms a blending device 24, vl, in which blended water V is produced by mixing raw water R from the raw water inlet 11 with softened raw water W from the softening device 12 in a mixing ratio that can be predetermined by the position of the controllable valve vl. The measuring device 15 is arranged downstream of the blending device 24, vl.

[0058] The ion exchanger 13 of the softening device 12 is contained in an ion exchange tank 8, which has an inlet 11a and an outlet 11b. For regenerating the ion exchanger 13 of the softening device 12, the water treatment system 10 further comprises a regeneration device 18, which has a regeneration tank 9 in which an aqueous regeneration solution, in particular a sodium chloride solution, is stored. When the ion exchanger 13 of the softening device 12 is exhausted, the regeneration solution is passed through the ion exchange tank 8 in a regeneration mode, whereby the calcium and magnesium ions bound in the ion exchanger 13 during the softening of the raw water R are replaced by sodium ions. After the regeneration mode has ended, the used regeneration solution is passed into a channel 19 via a discharge line 23.

[0059] The inlet 11a of the softening device 12 is connected via the control head 24 to the raw water inlet 11, via which (hard) raw water R is supplied to the softening device 12 for softening. To detect the conductivity of the supplied raw water R, a conductivity sensor 16 is arranged in the raw water inlet 11. The outlet 11b of the softening device 12 is connected via an outlet line 7 to the control head 24 in order to direct the water softened by ion exchange in the ion exchanger 13 (soft water W) into the blended water line 20. In the blended water line 20, the softened water W is mixed with raw water R in a defined mixing ratio, which depends on the position of the blending valve vl, in order to produce blended water V. The blended water V should have a predetermined target hardness GHv so11 have, for example, GHv so11 = 3 °dH.

[0060] The hardness of the blended water V depends on the hardness of the raw water R and the mixing ratio of the softened water W with the raw water R. The blended water V generated in the blending device 24, vl is discharged via the blended water line 20 and fed to a consumer 22 via a consumer line 21 connected to the blended water line 20. The consumer 22 can be, for example, the drinking water installation of a household or a water appliance to which the blended water V is fed directly.

[0061] To determine and adjust a suitable mixing ratio, which is selected so that the blend water V has a hardness that corresponds as closely as possible to a given target hardness GHv so11corresponds, the control device 14 is coupled to the blending device 24, vl in order to be able to suitably adjust the mixing ratio by adjusting the controllable blending valve vl of the blending device 24, vl. To set an initial mixing ratio r:w_0, at which the hardness of the blended water V at least approximately corresponds to the target hardness, the control device 14 is coupled to the conductivity sensor 16 arranged in the raw water inlet 11 and receives the measured value of the electrical conductivity of the raw water R detected by this sensor.On the basis of the measured value of the conductivity of the raw water R (conductivity LR), the control device 14 estimates the hardness of the raw water by referring to a characteristic curve stored in a data memory, which describes the course of the electrical conductivity of water as a function of the water hardness, and calculates therefrom an initial mixing ratio r:w_0, which is determined such that the predicted hardness of the blended water corresponds at least approximately to the target hardness.

[0062] For fine adjustment of the mixing ratio, the exact total hardness GHv of the blended water V is determined by means of the measuring device 15. For this purpose, the blended water V is passed via the blended water line 20 through a valve V2, which can be controlled by the control device, to an inlet 15a of the measuring device 15 and is flushed through.

[0063] The measuring device 15 comprises an electrolysis cell 1, which has a container 5 in which an inlet 15a and an outlet 15b are provided. A water sample, whose hardness is to be determined, can be introduced into the container 5 of the measuring device 15 via the inlet 15a. To perform a conductometric measurement, the inlet 15a and the outlet 15b of the container 5 are closed. After a measurement is completed, the water sample can be drained into a channel 19 by opening the outlet 15b.

[0064] Figure 2 schematically shows various embodiments of the measuring device 15. The measuring device 15 shown in Figure 2A comprises an electrolysis cell 1 with a container 5, which has an inlet (not shown here) at the inlet 15a for introducing the water sample and an outlet at the outlet 15b for draining the water sample after the hardness determination has been completed. The inlet and outlet are closable, so that the water is contained in a closed volume after the water sample has been introduced into the container 5.

[0065] Within the container 5, two flat electrodes are arranged at a distance from and parallel to one another. The two flat electrodes form mutually associated electrolysis electrodes A, K, with one flat electrode serving as the anode (A) and the other flat electrode serving as the cathode (K). The two electrolysis electrodes A, K are connected to a direct current source DC, as indicated in Figure 2A. The two electrolysis electrodes A, K are supplied with direct electrical current via the direct current source DC. By supplying direct electrical current to the two electrolysis electrodes A, K, the water of the water sample contained in the container 5 of the electrolysis cell 1 is electrolyzed, with the alkaline earth ions present in the water being deposited as carbonates on the cathode (K) and thereby precipitated from the water.

[0066] A conductivity sensor 2 with two measuring electrodes is arranged between the two electrolysis electrodes A, K, forming an electrode pair 3. The electrode pair 3 is connected to an alternating voltage source AC and can be supplied with an electrical alternating voltage of a predetermined frequency via the alternating voltage source AC. When an electrical alternating voltage is applied, the electrode pair 3 acts as a conductivity sensor 2, with which the conductivity of the water in the container 5 can be detected by measuring the current intensity of the current conducted through the measuring electrodes of the electrode pair 3. In the example shown in Figure 2A, the two measuring electrodes are designed as rod electrodes, the longitudinal axis of which runs parallel to the plane of the two electrolysis electrodes A, K, which are designed as flat electrodes.

[0067] The measuring device 15 with the electrolysis cell 1 and the two electrolysis electrodes A, K arranged therein, as well as the conductivity sensor 2 integrated into the electrolysis cell 1, is coupled to the control device 14. The control device 14 controls the supply of direct electrical current to the electrolysis electrodes A, K during predetermined electrolysis cycles. Furthermore, the detection of the electrical conductivity of the water sample in the container 5 by means of the conductivity sensor 2 is also controlled by the control device 14 by applying an alternating electrical voltage to the electrode pair 3 of the conductivity sensor 2 during defined measuring cycles.For this purpose, the control device 14 is preferably set up in such a way that measuring cycles with a predetermined measuring duration and electrolysis cycles with a predetermined electrolysis duration are carried out alternately one after the other, wherein preferably during a measuring cycle the electrode pair 3 of the conductivity sensor 2 is supplied with alternating voltage, while the two electrolysis electrodes A, K are de-energized and during an electrolysis cycle the two electrolysis electrodes A, K are supplied with direct electrical current, while the electrode pair 3 is de-energized.

[0068] The control device 14 comprises a data memory and a computing unit. At least one characteristic curve is stored in the data memory, which represents the course of the electrical conductivity of water as a function of the total hardness of the water on a hardness scale. The data memory of the control device 14 can also contain several such characteristic curves, wherein the individual characteristic curves represent, for example, the course of the electrical conductivity of water as a function of the hardness of the water on different hardness scales, for example the German hardness scale (°dH) or the French hardness scale (°fH). If the characteristic curve has a linear course of the electrical conductivity as a function of the water hardness, the proportionality factor, which indicates the gradient of this linear course, can also be stored as a conversion factor F in addition to or instead of the characteristic curve in the memory of the control device 14.To determine the hardness of the water sample contained in container 5 of electrolysis cell 1, the initial conductivity Lfi of the water is first measured in a measuring mode with the electrolysis electrodes A, K de-energized using the conductivity sensor 2 by applying an alternating electrical voltage to the electrode pair 3 at a predetermined frequency. This measurement represents a first measuring cycle. The voltage at the electrode pair 3 of the conductivity sensor 2 is then switched off, and a first electrolysis cycle begins for a predetermined electrolysis duration. For this purpose, a direct voltage is applied to the two electrolysis electrodes A, K.By applying a direct voltage to the two electrolysis electrodes A, K, the water in the container 5 is electrolyzed and at the same time the alkaline earth ions present in the water begin to precipitate as carbonates, in particular as calcium carbonate and magnesium carbonate, and to deposit on the cathode (K). After the end of the first electrolysis cycle, the power supply to the electrolysis electrodes A, K is switched off and a second measuring cycle is carried out by applying an alternating voltage to the electrode pair 3 of the conductivity sensor 2 for a predetermined measuring time. The electrical conductivity of the water in the container 5 is recorded. Due to the partial precipitation of the alkaline earth ions, the conductivity measured in the second measuring cycle has decreased compared to the original conductivity of the water, since the precipitated alkaline earth ions no longer contribute to the current flow in the electrolysis cell 1.Subsequently, further electrolysis cycles and measurement cycles are carried out alternately one after the other.

[0069] The resulting temporal progression of the electrical conductivity of the water in container 5 is illustrated by way of example in Figure 4. The temporal progression of the electrical conductivity in Figure 4 shows the individual measurement cycles M during which the conductivity of the water remains constant. Between successive measurement cycles M with constant conductivity are the electrolysis cycles E, whereby the electrical conductivity of the water continues to decrease in each electrolysis cycle E due to the progressive filling of the alkaline earth ions, up to a transition point marked U in Figure 4, at which the conductivity of the water, as measured by the conductivity sensor 2, increases abruptly or gradually. The (possibly abrupt) increase in electrical conductivity at transition point U presumably results from a sudden increase in the pH value and a resulting excess of anions (CCL).2 ' and OH') with higher equivalent conductivity than the bicarbonate ions after cathodic precipitation. At the transition point U, the conductivity of the water measured by conductivity sensor 2 has a minimum value Lf2.

[0070] Assuming that during the electrolysis cycles all alkaline earth ions contained in the water have been precipitated as carbonates, in particular as calcium and magnesium carbonate, the change in electrical conductivity ALf = |Lf2- Lfi| caused by the precipitation of the alkaline earth ions can be calculated from the difference between the conductivity recorded before the start of electrolysis (Lfi) and the conductivity recorded after the alkaline earth ions have been completely filled (Lf2). From this difference value, which corresponds to the difference between the original conductivity of the water and the conductivity measured after the alkaline earth ions have been completely precipitated, the total hardness or the carbonate hardness of the water in the container 5 of the measuring device 15 can be determined.For this purpose, the control device 14 is programmed in such a way that it calculates the absolute value of the difference between these measured values ​​ALf = |Lf2 - Lfi| from the recorded measured values ​​of the original conductivity Lfi and the conductivity of the water after the complete precipitation of the alkaline earth ions Lf2 and from the difference value, using the characteristic curve stored in the memory of the control device 14 and / or the conversion factor F stored there, to determine the hardness of the water on the hardness scale of the characteristic curve or the conversion factor F as follows:.

[0071] H = ALf / F, where H is the hardness of the water, ALf is the difference between the measured conductivities of the water before and after the complete precipitation of the alkaline earth ions and F is the conversion factor which results from the characteristic curve of the (linear) course of the conductivity change of water during a precipitation of the carbonate hardness of the water as a function of the hardness change, in particular the change in the total hardness of the water, and in particular is the proportionality factor of a linear course of this characteristic curve.

[0072] The (empirically determined) factor F results from the relationship between the mass of hardness-forming substances removed by precipitation and the reduction in conductivity due to proportionally reduced ion concentrations. Up to the transition point U, the curve of an electrolytic hardness precipitation (at constant current) is almost linear, as shown in Figure 4. At constant voltage (decreasing current), the temporal course of the conductivity is no longer linear, but the transition point U remains unchanged at the same conductivity value. With an excess of bicarbonate compared to the alkaline earth ions, the transition point U corresponds quite precisely to the existing total concentrations of alkaline earth ions due to the comparable equivalent conductivities of Ca and Mg.Empirical measurements on a large number of drinking water samples from Germany of different origin and composition result in a value of, for example, 0.1 for the conversion factor F at a temperature of 15°C.

[0073] F = 30 pS / (cm °dH) = 0.030 mS / (cm °dH).

[0074] The following calculation for the hardness of the water is therefore obtained from the time course of the electrical conductivity of a water sample shown in Figure 4:

[0075] (0.70 - 0.59) mS / cm

[0076] H = ALf / F = v y _ o °JU

[0077] 0.030 mS / (cm °dH)

[0078] Figures 2A and 2B show two further embodiments of a measuring device 15 with an electrolysis cell 1, wherein the electrolysis cell 1 contains two separate conductivity sensors 2a, 2b. A first conductivity sensor 2a is arranged at the closable inlet 15a of the electrolysis cell 1, and a second conductivity sensor 2b is arranged at the closable outlet 15b of the electrolysis cell 1. Each conductivity sensor 2a, 2b comprises a pair of electrodes 3a, 3b, each with a first measuring electrode a (measuring anode) and a second measuring electrode k (measuring cathode). In the embodiment of Figure 2B, the two electrolysis electrodes A, K are arranged between the inlet 15a and the outlet 15b of the electrolysis cell 1. As in the embodiment of Figure 2A, these electrodes are designed as flat electrodes and arranged opposite one another at a predetermined distance.

[0079] The embodiment of the electrolysis cell 1 shown in Figure 2B can be used to measure the electrical conductivity of a water sample which flows continuously from the inlet 15a to the outlet 15b through the electrolysis cell 1 in flow-through operation. While the water sample flows through the electrolysis cell 1, the conductivity Lf a of the water flow at the inlet 15a and the conductivity Lfi at the outlet 15b are continuously measured with the two conductivity sensors 2a, 2b. The control device 14 calculates the difference ALf = | Lft - Lf from the measured values ​​of the two conductivity sensors 2a, 2b. a| and compares the calculated difference in electrical conductivity ALf with a predefined limit value. If the detected difference in electrical conductivity ALf is above this limit value, it can be concluded that a certain amount of hardness-forming alkaline earth ions is present in the water, which indicates a hardness breakthrough in the softening device 12. Furthermore, the measuring device 15 of Figure 2B can detect a change in the electrical conductivity of the blended water V during flow operation, which is based, for example, on a change in the water quality of the raw water R supplied to the water treatment plant via the raw water inlet 11.

[0080] To determine the total hardness of the water flow supplied to the measuring device 15, the measuring device 15 can be switched from flow mode to batch mode according to the second embodiment of Figure 2B by closing the inlet 15a and outlet 15b of the container 5 of the electrolysis cell 1. The hardness of the water in the container 5 of the measuring device 15 is then determined using the measuring mode described with reference to the embodiment of Figure 2A.

[0081] In the embodiment of the measuring device 15 shown in Figure 2C, a plurality of electrolysis electrodes A, K are arranged in the electrolysis cell 1. In particular, the electrolysis cell 1 of the embodiment of Figure 2C contains a cascade of electrolysis electrodes in the sequence of an outer anode A, a first cathode K, an inner anode A, a second cathode K, and a further outer anode A, as can be seen in Figure 2C. A cation exchange membrane KAT is arranged between each corresponding electrolysis electrode A, K.The plurality of electrolysis electrodes A, K and the cation exchange membrane KAT arranged between corresponding electrolysis electrodes A, K increase the efficiency of the electrolytic precipitation of the alkaline earth ions by precipitation in the form of alkaline earth carbonates at the cathodes K (particularly due to a larger electrode surface), thereby reducing the measurement time and the flow rate at which the water sample flows through the electrolysis cell 1. A preferred embodiment of the method according to the invention is explained below using the flow diagram in Figure 3, which illustrates the steps involved in setting up and initially commissioning the water treatment system 10.

[0082] A target hardness GHv is sent to the control device 14 via a communication interface so11which is in particular between 2°dH and 15°dH and preferably between 3°dH and 10°dH and particularly preferably between 3°dH and 6°dH (step S1). The electrical conductivity of the raw water LfR is then measured continuously or selectively at predeterminable time intervals by means of the conductivity sensor 16 arranged in the raw water inlet 11 (step S2). The control device 14 roughly estimates the hardness HR of the raw water on the basis of the measured conductivity of the raw water LfR by accessing a characteristic curve stored in the data memory and positions the blending valve vl of the blending device 24, vl such that an initial mixing ratio r:w_0 results at which the expected total hardness GHv of the blended water V has a low value, which is preferably < 3°dH (step S3).The blended water V produced with this setting of the blending valve vl has an excess of bicarbonate ions compared to alkaline earth ions, which allows for a precise measurement of the total hardness GHv of the blended water using the measuring method described above. Many thousands of drinking water analyses have shown that with this setting, the bicarbonate content is in excess of the alkaline earth ions.

[0083] To determine the total hardness of the blended water V, a water sample of the produced blended water V is fed to the container 5 of the measuring device 15 and the container 5 is closed so that the water sample of the blended water V with a certain volume is located therein (step S4).

[0084] The blending valve vl is then pre-positioned to a preliminary mixing ratio r:w_l, at which, based on the raw water hardness roughly determined in step S2, the hardness of the blended water approximately corresponds to the target hardness (step S5). The water treatment system 10 is operated with this setting of the preliminary mixing ratio r:w_l until an accurate determination of the blended water hardness GHv and the raw water hardness HR is completed and, based on these values, a fine adjustment of the mixing ratio can be made in order to achieve the specified target hardness GHv. so11 of the blend water as accurately as possible.

[0085] For this purpose, a measuring mode is carried out in the measuring device 15 simultaneously with or after step S5, in which the total hardness GHv of the blended water V and, from this, the hardness of the raw water R is determined using the known initial mixing ratio r:w_0 (step S6).

[0086] At the beginning of the measuring mode (step S6), the (original) electrical conductivity of the blend water V is measured in the electrolysis cell 1 of the measuring device 15 by means of the conductivity sensor 2 integrated into the electrolysis cell 1. Thereafter, the water sample of the blend water V located in the container 5 of the electrolysis cell 1 is electrolyzed by means of the electrolysis electrodes A, K with the container 5 closed by applying an electrical direct voltage to the electrolysis electrodes A, K. This precipitates the alkaline earth ions contained in the water sample of the blend water V from the water. During the electrolytic precipitation of the alkaline earth ions, the electrical conductivity of the blend water V located in the container 5 of the electrolysis cell 1 is measured using the integrated conductivity sensor 2 orthe conductivity sensors 2a and 2b to detect the transition point U, which indicates complete precipitation of the alkaline earth ions, as described above using the measuring devices of Figure 2. After complete precipitation of the alkaline earth ions, the electrical conductivity Lf2 of the electrolyzed blend water V is measured, and by forming the difference with the original conductivity Lfi of the blend water V, the change in conductivity ALf = | Lf2 - Lfi | caused by the precipitation of the alkaline earth ions is determined. For this purpose, the control device 14 is coupled to the conductivity sensor 2 in order to obtain the measured values ​​of the conductivity measurement (conductances) before, during, and after the precipitation of the alkaline earth ions.The control device 14 calculates the exact total hardness GHv of the blended water V from the detected difference in electrical conductivity ALf, using the conversion factor F stored in a memory of the control device 14, as described above (step S7). The measurement time required for complete precipitation of the alkaline earth ions depends on the hardness of the water sample and is approximately between 10 and 15 minutes. 0dH and is less than 45 minutes due to the low hardness of the blended water set via the initial mixing ratio r:w_0. Based on the total hardness GHv of the blended water V determined by the control device 14, the control device 14 readjusts the blending device 24, vl, to adjust the preliminary mixing ratio r:w_l of the raw water with the softened water W to a new mixing ratio r:w_2 so that the blended water V has a total hardness GHv that corresponds to the specified target hardness GHv so11corresponds (step S8). In this case, the control device 14 (in step S7) can also calculate the hardness HR of the raw water R from the determined total hardness GHv of the blended water V, taking into account the original mixing ratio. The hardness HR of the raw water R can be used, for example, to predict a softening capacity of the softening device 12 and to initiate a regeneration process with the regeneration device 18 in order to carry out regeneration in good time before the ion exchanger 13 is exhausted in order to avoid a hardness breakthrough.

[0087] During operation of the water treatment plant, changes in the hardness HR of the raw water R, which can occur, for example, when the water quality of the drinking water provided by the public drinking water supply changes, are detected by the conductivity sensor 16 and taken into account when adjusting the blending valve vl by adapting the mixing ratio to a changed hardness of the raw water R. For this purpose, the measured values ​​of the conductivity LfR(n+l) of the raw water R currently detected by the conductivity sensor 16 are fed to the control device 14, which compares the detected measured values ​​with the previous measured values ​​LfR(n) (step S9).If this results in a change in the conductivity ALfR = LfR(n+l) - LfR(n) of the raw water R whose amount exceeds a predetermined limit value SLfR, a further measuring mode is initiated in which the total hardness GHv of the blended water V is determined using the measuring device 15 as described above and, if necessary, the mixing ratio of the blended water V is adjusted accordingly. The limit value SLfR is expediently in the range of 30 to 40 pS / cm and is in particular 30 pS / cm. As long as no change in the conductivity LfR of the raw water R is detected during operation of the water treatment plant whose amount exceeds the limit value SLfR, the setting of the blending valve vl of the blending device 24, vl remains unchanged (step S10).The conductivity values ​​of the raw water LfR detected by the conductivity sensor 16 are stored together with the associated raw water hardness HR in a memory of the control device 14. If a previously known raw water conductivity value is detected again, the analysis process is no longer started; instead, the raw water hardness HR already stored in the controller's memory and assigned to this conductivity value is used as the new control variable for setting the mixing ratio in order to achieve the target hardness GHv. so11 of the blend water V.

[0088] Figure 5 shows, by way of example, the time course of a measurement of the electrical conductivity of the blended water V or the raw water R (measured conductivities Lfv or LfR) in the water treatment plant of Figure 1 with a changing hardness HR of the raw water R supplied via the raw water inlet 11, as well as the time course of the hardness GHv of the blended water V determined from the conductivity of the blended water V and the hardness HR of the raw water R calculated from this, taking the mixing ratio into account. Between the start of the measurement at time t = 0, a raw water R with a specific raw water hardness HR was supplied to the softening device 12. From time t1 until time t2, a raw water R' with a changed composition and a higher hardness HR' was supplied to the softening device 12, and from time t3 the original raw water R was supplied again.During the entire measurement period between t = 0 and t = 9 h, the conductivity LfR (t) of the raw water R or R' was continuously recorded using the conductivity sensor 16. At time t1, an increase in the conductivity LfR (t) of the raw water can be observed due to the higher hardness of the modified raw water R'. From time t0, a measurement mode was carried out to determine the total hardness GHv of the blended water V and, from this, the hardness HR of the raw water R, and to adjust the mixing ratio of raw water R with softened water W so that the total hardness GHv of the blended water V corresponds to the specified target hardness GHv. so11of 3°dH. Due to a recorded change in the conductivity LfR (t) of the raw water at times t1 and t3, a measurement of the conductivity of the blend water was carried out at times t2 and t4 in the electrolysis cell 1 in batch operation, followed by precipitation of the alkaline earth ions by electrolysis of the blend water in the electrolysis cell 1. As described above, the conductivity Lf2 of the (electrolyzed) blend water and the change in conductivity ALf during the electrolysis were recorded, and from this the total hardness of the blend water GHv and the hardness HR' of the changed raw water R' were determined.

Claims

Claims process for operating a water treatment plant (10) comprising - a raw water inlet (11) for the provision of raw water (R), - a water softening device (12), which in particular includes at least one ion exchanger (13), wherein the water softening device (12) produces softened water (W) from the raw water (R), - a blending device (24, vl) for mixing raw water (R) from the raw water inlet (11) with softened water (W) from the softening device (12), wherein the blending device (24, vl) produces blended water (V) by mixing raw water (R) with softened water (W) in an adjustable mixing ratio, - a control device (14) and - at least one conductometric measuring device (15) which is set up for the conductometric determination of the total hardness of the water of a water sample, characterized in that blended water (V) from the blending device (24, vl) is supplied to the measuring device (15) and that the measuring device (15), in particular before and after an electrolytic precipitation of the alkaline earth ions, performs a conductometric measurement and forwards the measurement results to the control device (14), which determines the total hardness (GHv) of the blended water (V) from the measurement results.

2. Method according to claim 1, characterized in that the measuring device (15) comprises an electrolysis cell (1) with at least two electrolysis electrodes (A, K) and at least one conductivity sensor (2). Method according to claim 1 or 2, characterized in that the conductivity (LIR) of the raw water (R) is detected by means of a further conductivity sensor (16) arranged in the raw water inlet (11) or by means of the measuring device (15) and that the hardness (H) of the raw water (R) is roughly estimated from the detected conductivity (Lfn) of the raw water (R) in order to adjust an initial or provisional mixing ratio on the basis of the estimated hardness (H) of the raw water (R) when commissioning the water treatment plant (10) such that the total hardness (GHv) of the blended water (V) is at least approximately equal to a predetermined target hardness (GHv). so11) of the blended water (V). Method according to claim 1 or 2, characterized in that the conductivity (LI ) of the raw water (R) is detected by means of the measuring device (15) by supplying raw water (R) to the measuring device (15) via a line bypassing the softening device (12). Method according to one of the preceding claims, characterized in that the control device (14) for adjusting the mixing ratio of the raw water (R) and the softened water (W) is coupled to the blending device (24, vl) and that the mixing ratio, after determining the total hardness (GHv) of the blended water (V), is adjusted to a mixing ratio, depending on the detected total hardness (GHv) of the blended water (V), such that the total hardness (GHv) of the blended water (V) corresponds to a predetermined target hardness (GHv). so11) of the blended water (V). Method according to one of the preceding claims, characterized in that the conductivity (LIR) of the raw water (R) is continuously or at specific time intervals measured by means of the measuring device (15) or by means of an additional conductivity sensor (16), and that, depending on the measured conductivity (LIR) of the raw water (R), in particular in the case of a measured change in the conductivity (LIR) of the raw water (R), a conductometric determination of the The total hardness (GHv) of the blended water (V) is introduced by means of the measuring device (15).

7. Method according to one of claims 3 to 5, characterized in that the control device (14) determines the total hardness (GHR) of the raw water (R) from the total hardness (GHv) of the blended water (V) taking into account the initial mixing ratio or the preliminary mixing ratio.

8. Method according to claim 7, characterized in that the calculated total hardness (GHR) of the raw water (R) and the measured conductivity (LfR) of the raw water (R) are stored in a data storage device of the control unit (14).

9. Method according to one of the preceding claims, characterized in that the total hardness (GHv) and / or a predetermined target hardness (GHv) so11 ) of the blending water (V) is between 2°dH and 15°dH and preferably between 3°dH and 10°dH and preferably between 3°dH and 6°dH.

10. Method according to one of the preceding claims, characterized in that a water sample of blended water (V) is supplied to the measuring device (15) in which an electrolytic precipitation of the alkaline earth ions, in particular the calcium and / or magnesium ions, takes place, wherein the electrical conductivity (Lfi) of the blended water (V) before precipitation and the electrical conductivity (Lf2) of the blended water (V) after precipitation and / or the change in conductivity (ALf = |Lf2- Lfi |) caused by the electrolytic precipitation of the alkaline earth ions of the blended water (V) is determined.

11. Method according to claim 10, wherein the control device (14) detects the change in conductivity (ALf = |Lf2- Lfi|) of the water sample of the blended water (V) caused by the precipitation of the alkaline earth ions and uses this to determine, via a A conversion factor (F) or a calibration curve (Lf(H)) determines the total hardness (GHv) of the blended water (V).

12. Method according to one of the preceding claims, wherein the blending water (V) supplied to the measuring device (15) contains an excess of hydrogen carbonate ions compared to the alkaline earth ions.

13. Method according to one of claims 11 or 12, characterized in that the conversion factor (F) is a proportionality factor for a linear relationship between the conductivity of water and the carbonate hardness of the water on a hardness scale, in particular the German water hardness, and / or that the conversion factor (F) characterizes the course of a change in the conductivity of water as a function of the change in the hardness of the water during precipitation of the carbonate hardness.

14. Method according to one of claims 2 to 13, characterized in that the electrolysis electrodes (A, K) of the measuring device (15) are supplied with direct current for electrolytic precipitation of the alkaline earth ions of the supplied water sample.

15. Method according to one of claims 2 to 14, characterized in that the conductivity sensor (2) of the measuring device (15) is integrated into the electrolysis cell (1).

16. Method according to one of claims 2 to 15, characterized in that the conductivity sensor (2) of the measuring device (15) contains two measuring electrodes (a, k) which are supplied with alternating current to detect the conductivity of the supplied water sample.

17. Water treatment plant (10) comprising - a raw water inlet (11) for the provision of raw water (R), - a softening device (12) connected to the raw water inlet (11), which in particular includes at least one ion exchanger (13), - a blending device (24, vl) for producing blending water (V) by mixing raw water (R) from the raw water inlet (11) with softened raw water (W) from the softening device (12) in a predefinable mixing ratio, - a control device (14) and - at least one conductometric measuring device (15), - wherein the control device (14) is configured to determine the hardness of the water of a water sample supplied to the conductometric measuring device (15), characterized in that the measuring device (15) is connected to the blending device (24, vl) in order to supply blending water (V) from the blending device (24, vl) to the measuring device (15) and to determine the total hardness (GHv) of the blending water (V), wherein the control device (14) controls the measuring device (15) in such a way that the latter performs a conductometric measurement in particular before and after an electrolytic precipitation of the alkaline earth ions and forwards the measurement results to the control device, which determines the total hardness (GHv) of the blending water (V) from the measurement results.Water treatment plant according to claim 17, characterized in that the measuring device (15) comprises an electrolysis cell (1) with at least two electrolysis electrodes (A, K) and at least one conductivity sensor (2). Water treatment plant according to claim 18, characterized in that the conductivity sensor (2) of the measuring device (15) is integrated into the electrolysis cell (1).

20. Water treatment plant according to one of claims 17 to 19, wherein the control device (14) is configured to first detect the original conductivity (Lfi) of the blended water (V), then to precipitate the alkaline earth ions of the water sample at least partially, preferably at least largely completely, by applying a DC voltage to the electrolysis electrodes (A, K), and to measure the conductivity (Lf2) of the blended water (V) during and / or after the precipitation of the alkaline earth ions, and to determine the concentration of the alkaline earth ions in the blended water (V) and / or the total hardness (GHv) of the blended water (V) on a hardness scale from the difference between the original conductivity and the conductivity measured after complete precipitation of the alkaline earth ions (ALf = |Lf2 - Lfi|).

21. Water treatment plant according to one of claims 18 to 20, characterized in that the electrolysis electrodes (A, K) are designed as flat electrodes arranged opposite each other in the electrolysis cell (1), wherein the electrolysis electrodes (A, K) can be connected to or are connected to a direct current source.

22. Water treatment plant according to one of claims 18 to 21, characterized in that the conductivity sensor (2) of the measuring device (15) comprises at least one electrode pair (3) with two measuring electrodes, in particular designed as rod electrodes, wherein the measuring electrodes can be connected to or are connected to an alternating voltage source.

23. Water treatment plant according to claim 22, characterized in that the measuring electrodes of the conductivity sensor (2) of the measuring device (15) are arranged between the opposing electrolysis electrodes (A, K) of the electrolysis cell (1).

24. Water treatment plant according to one of claims 17 to 23, characterized in that a further conductivity sensor (16) is arranged in the raw water inlet (11).

25. Water treatment plant according to one of claims 17 to 24, characterized in that the control device (14) for adjusting the mixing ratio of the blended water (V) is coupled to the blending device (24, vl) and the control device (14) is configured such that the mixing ratio is adjusted to a predetermined setpoint (GHv) depending on the determined total hardness (GHv) of the blended water (V). so11 ) is regulated by the total hardness of the blended water (V).

26. Water treatment plant according to one of claims 17 to 25, wherein the control device (14) is preferably continuously or at specific times supplied with the conductivity (LI ) of the raw water (R) detected by means of the measuring device (15) or by means of an additional conductivity sensor (16) in the raw water inlet (11), and the control device (14) initiates a conductometric determination of the total hardness (GHv) of the blended water (V) by means of the measuring device (15) and thus of the total hardness (GHR) of the raw water (r) depending on the detected conductivity (Lfn) of the raw water (R), in particular in the event of a detected change in the conductivity (LIR) of the raw water (R).

Images