Water treatment plant and methods for operating a water treatment plant

Abstract

Method for operating a water treatment plant (10), comprising: - a raw water inlet (11) for providing 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, v1) for mixing raw water (R) from the raw water inlet (11) with softened water (W) from the water softening device (12), wherein the blending device (24, v1) 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 configured for the conductometric determination of the total hardness of a water sample, characterized in that the measuring device (15) includes an electrolysis cell (1) with at least two electrolysis electrodes (A,K) and at least one conductivity sensor (2), wherein blending water (V) from the blending device (24, v1) is supplied to the measuring device (15) and an electrolytic precipitation of the alkaline earth ions takes place in the supplied blending water by means of the electrolysis cell (1), and the measuring device (15) performs a conductometric measurement before and after and / or during the electrolytic precipitation of the alkaline earth ions, wherein the electrical conductivity (Lf1) of the blending water (V) before precipitation and the electrical conductivity (Lf2) of the blending water (V) after precipitation and / or the change in conductivity (ΔLf = |Lf2- Lf1|) caused during the electrolytic precipitation of the alkaline earth ions of the blending water (V) is determined, and the measurement results are forwarded to the control device (14), which calculates the total hardness (GHV) of the blending water from the measurement results. (V) determined.

Description

[0001] The invention relates to a method for operating a water treatment plant and to a water treatment plant.

[0002] The hardness of water is determined by the concentration of dissolved alkaline earth metal cations and, in specific applications, also by the concentration of their associated anionic counterparts, particularly bicarbonate. The sum of the concentrations of all dissolved alkaline earth metals (which can exist as carbonates, sulfates, chlorides, nitrites, nitrates, and phosphates) is referred to as total hardness. The portion bound only to carbonic acid is called carbonate hardness (or temporary hardness), and the difference between total hardness and carbonate hardness is called non-carbonate hardness (or permanent hardness). In drinking water, the majority of the total hardness is typically carbonate hardness. Calcium and magnesium ions are the primary hardness-causing agents contributing to the total hardness of water. The sum of the concentrations of calcium and magnesium ions therefore closely approximates the total water hardness.The other alkaline earth metals, such as strontium and barium, are usually only present in water as trace elements and therefore contribute very little to water hardness. Carbonate hardness can be removed by eliminating calcium and magnesium carbonate from the water. The dissolved hardness-causing minerals calcium and magnesium can form sparingly soluble compounds in water, particularly as carbonates with the carbon dioxide dissolved in the water.

[0003] The formation of water-poorly soluble compounds by hardness-causing minerals leads, particularly when the water is heated, to the formation of limescale in household appliances, heating systems, and water heaters, and reduces the effectiveness of detergents and washing agents in dishwashers and washing machines. Furthermore, water hardness affects the taste of food and beverages prepared with water. To avoid or reduce these adverse effects of hard water, i.e., water with a high concentration of alkaline earth metal ions, calcium and magnesium ions are partially or completely removed from the water in water softening systems or demineralization plants. This is done, for example, by replacing them with sodium ions using cation exchangers or—in the case of demineralization—by removing them completely from the water along with all other dissolved ions.through a combination of cation and anion exchangers or through reverse osmosis.

[0004] However, fully softened or demineralized water can lead to corrosion problems. Furthermore, complete water softening in water softening systems with one or more regenerable ion exchangers requires a large quantity of regenerant, such as regenerating salt, to produce an aqueous saline solution used to regenerate the ion exchangers in the water softener when they become exhausted. In many applications, it is also necessary or desirable to use water with a predefined medium hardness level, rather than fully softened or demineralized water.Water treatment plants are known from the prior art in which, to produce partially softened or partially demineralized water with a predefinable water hardness, a supplied raw water stream is divided into two partial streams. The first partial stream is fully softened or fully demineralized, and the fully softened or fully demineralized first partial stream is mixed with a second partial stream of the raw water, which remains untreated, to produce a blended water stream. The hardness of the resulting blended water can be adjusted to a predefinable target hardness value by adjusting the mixing ratio of the first and second partial streams. Blending devices are used to mix the fully softened or fully demineralized first partial stream with the raw water-containing second partial stream. These devices utilize adjustable valves to mix the two partial streams at a predefinable ratio.

[0005] In order to be able to set a specific blended water hardness via the mixing ratio of the two partial streams, it is necessary to determine the hardness of the blended water and / or the hardness of the raw water as accurately as possible.

[0006] For the precise determination of water hardness, complexometric titration methods can be used, for example, with the disodium salt of ethylenediaminetetraacetic acid (EDTA) as the titrant. These methods can measure the concentrations of alkaline earth ions dissolved in the water and thus determine the total hardness of the water. Measuring instruments are available that photometrically detect the color change point of the titration. These instruments are characterized by good measurement accuracy, but are complex and expensive to manufacture, require regular maintenance, and, due to the use of the titrant, are difficult to integrate into automated measurement processes.

[0007] Furthermore, the total hardness of water can be determined using ion-selective electrodes (ISEs), which detect the ion 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 procedures.

[0008] Besides 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 water hardness can be inferred from the electrical conductivity of the water. However, since all other ions dissolved in the water contribute to the electrical conductivity of the water, in addition to the hardness-causing calcium and magnesium ions, this method is very non-specific, especially if the water has a high proportion of non-carbonate hardness, e.g., due to a high chloride content.The relationship between electrical conductivity and total water hardness is very weak and must be determined using characteristic curves by establishing the relationship between the titrimetrically determined hardness of various water samples and their electrical conductivity. Furthermore, the electrical conductivity of water is strongly temperature-dependent, which is why, in addition to conductivity, the temperature must also be measured, and the hardness determination via the characteristic curve must include a temperature correction. Therefore, conductivity measurements, which determine the absolute conductivity of a water sample, are only of limited use for determining the total hardness of a water sample.

[0009] From DE 10 2007 059 058 A1, a water softening system and a method for its operation are known, wherein the conductivity of the raw water is measured by means of a conductivity sensor and a total hardness of the raw water is determined from the measured conductivity, 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 based on the derived total hardness of the raw water in a feedback loop so that a blended water hardness is obtained that corresponds to a predetermined target hardness. The water hardness determined from the calibration curve corresponds to a good approximation of a medium water hardness that occurs in a large number of drinking water analyses at the measured conductivity.The actual water hardness, determined titrimetrically at a given conductivity, can deviate considerably from this average value, especially if the raw water has a relatively high non-carbonate hardness (permanent hardness). If the raw water hardness, as determined by the calibration curve, deviates from this average, the actual hardness of the blended water will not match the specified target hardness.

[0010] A public drinking water supply typically provides a limited number of different drinking water qualities from various sources, with varying composition and hardness. The introduction of these different drinking water qualities into the public drinking water network can change within a single day or even over longer periods, meaning 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 supply. Such changes in water quality, which can occur several times a day, alter the overall hardness of the blended water produced in a water treatment plant's blending unit, resulting in a water hardness that no longer meets the desired standard.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, it is necessary to quickly detect any changes in the composition or quality of the raw water during the operation of a water treatment plant.

[0011] Based on this, the invention aims to provide a method for operating a water treatment plant and a water treatment plant with a blending device that enables the most accurate possible determination of the total hardness of the raw water and / or the blended water without the use of chemicals, at low installation and operating costs, while requiring little maintenance, long-term stability, and no calibration. The accuracy of the determination of the total hardness of the raw water and / or the blended water should, in particular, meet the requirements of the standard DIN 19636-100 "Water softening systems (cation exchangers) in drinking water installations" for the accuracy of the blending.Furthermore, the procedure is intended to ensure, in particular, the most accurate possible calibration of the blending device during the initial commissioning of the water treatment plant, as well as in-situ monitoring 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 should produce blended water with a total hardness that corresponds as closely as possible to a specified target hardness, regardless of the hardness of the supplied raw water and also in the event of changes in the quality or hardness of the raw water.

[0012] These problems are solved by the method according to claim 1 and the water treatment plant with the features of claim 15. Preferred embodiments of the method and the water treatment plant are set forth in the dependent claims.

[0013] In the inventive method 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 the conductometric determination of the total hardness of a water sample, softened water is produced from the raw water in the softening device and in the blending device a 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 supplied to the measuring device and the measuring device performs a conductometric measurement, in particular before and after an electrolytic precipitation of the alkaline earth ions and transmits the measurement results to the control device, which determines the total hardness of the blended water from the measurement results.

[0014] The conductometric determination of the total hardness of the blended water allows a direct comparison of the determined total hardness of the blended water with a predefined target value for the water hardness of the blended water, as well as a readjustment of the initial mixing ratio if a deviation occurs, especially if the deviation exceeds a predefined limit.The conductometric measurement performed in the measuring device before and after electrolytic precipitation of the alkaline earth ions enables an accurate determination of the total hardness of the blended water, since all hardness-forming alkaline earth ions, especially as carbonates, are precipitated from the water by electrolytic precipitation and the change in conductivity of the blended water caused by the precipitation of the alkaline earth ions is recorded by a conductometric differential measurement and the amount of precipitated alkaline earth ions and thus the total hardness of the blended water can be determined very accurately using a conversion factor or a characteristic curve.

[0015] It is assumed that in a water sample containing an excess of hydrogen carbonate ions over alkaline earth ions, which is the case with blended water produced by mixing raw water with softened raw water, and assuming at least largely complete electrolytic precipitation of the alkaline earth ions in the form of carbonates, the total hardness of the water sample can be determined from the change in conductivity (ΔLf = Lf2 - Lf1) caused by the precipitation of the alkaline earth ions, in particular from the amount of the decrease in conductivity, because the difference in the conductivity of the water sample before and after the precipitation of the alkaline earth ions allows for the determination of the total concentration of the alkaline earth ions in the water, the sum of which defines the total hardness of the water.When hydrogen carbonate ions are more abundant than alkaline earth ions in the blended water, the carbonate hardness of the water sample obtained through electrolysis in the electrolysis cell corresponds to the total hardness of the blended water. The change in conductivity caused by the at least substantially complete precipitation of the alkaline earth ions therefore represents a measure of the concentration of hardness-causing alkaline earth ions in the blended water. Consequently, the total hardness of the blended water can be determined from the measured change in conductivity (ΔLf) using a predetermined conversion factor (F), which is specifically related to a particular hardness scale, or using a predetermined characteristic curve that illustrates the change in conductivity as a function of the water's hardness during carbonate precipitation.In the inventive method, the change in conductivity (ΔLf) is preferably determined by first measuring the original electrical conductivity (Lf1) of a water sample from the blending water and, after at least substantially complete precipitation of the alkaline earth ions in the electrolysis cell, measuring the electrical conductivity (Lf2) of the water sample again and determining the difference (ΔLf = |Lf2 - Lf1|) from the two measurements. The measurement of the original electrical conductivity (Lf1) of the water sample from the blending water and the electrical conductivity (Lf1) of the water sample after at least largely complete precipitation of the alkaline earth ions can be carried out using a cost-effective conductivity sensor.

[0016] The condition that there is an excess of bicarbonate ions compared to alkaline earth ions in the water sample is generally automatically met in blended water obtained by mixing raw water with softened water (produced by softening the raw water). This is because the bicarbonate concentration is maintained in the blended water, and compared to the raw water, bicarbonate ions are present in excess of alkaline earth ions. It has been shown that a preponderance of bicarbonate ions over alkaline earth ions is present in blended water when the total hardness of the blended water is less than 3°dH.Empirical measurements of numerous drinking water samples from Germany have shown that when blended water has a titrimetrically determined total hardness of 5°dH or less, more than 98% of the drinking water samples exhibit an excess of hydrogen carbonate ions compared to alkaline earth ions. This is especially true if the blended water does not already have an excess of hydrogen carbonate ions compared to alkaline earth ions (which can be the case, in particular, if the total hardness GH is higher). v (< 5°dH), hydrogen carbonate can be added to the blended water to comply with the measurement condition, in order to create an excess of hydrogen carbonate ions compared to the alkaline earth ions.

[0017] The control unit is expediently designed 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 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 to initiate a regeneration process for the water softener, in particular one or more ion exchangers, with a regeneration solution, e.g., a saline solution, when the water softener is exhausted.

[0018] The conductometric determination of the total hardness of the blended water can be carried out both during the commissioning of the water treatment plant and after a change in the quality, and in particular the hardness, of the incoming raw water. Changes in the total hardness of the raw water can be continuously monitored during operation of the water treatment plant by measuring the conductivity of the raw water using a conductivity sensor installed in the raw water inlet or by using a measuring device. Any changes in the hardness of the incoming raw water can then be detected from the measured conductivity.

[0019] During the initial commissioning of the water treatment plant, the hardness of the raw water can be roughly estimated from the conductivity of the raw water, which is measured with the conductivity sensor located 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 corresponds at least approximately to a predetermined target hardness of the blended water.Subsequently, during the commissioning of the water treatment plant, a precise conductometric determination of the total hardness of the blended water is carried out using the measuring device, and from the determined total hardness of the blended water, taking into account the initial mixing ratio, the total hardness of the raw water is determined and, if necessary, the mixing ratio is adjusted to set the total hardness of the blended water to the specified target value.

[0020] During operation of the water treatment plant, the conductivity of the raw water is monitored either by means of the conductivity sensor located in the raw water inlet or by means of the measuring device in order 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 of water can at best only provide a rough estimate of the water hardness using characteristic curves, a differential measurement, which detects a change in the electrical conductivity of the water, is very well suited for detecting changes in water hardness.

[0021] To continuously measure the conductivity of the raw water during operation of the water treatment plant using the measuring device, which is advantageously arranged downstream of the softening device and the blending unit for determining the total hardness of the blended water, the raw water is supplied in a preferred embodiment via a bypass line that bypasses the softening device. In this embodiment, an additional conductivity sensor in the raw water inlet is not required.

[0022] To precisely adjust the mixing ratio of the raw water and the softened water, the control unit is coupled to the blending unit, and the initial mixing ratio is adjusted by the control unit, particularly during the commissioning of the water treatment plant, after the conductometric determination of the total hardness of the blended water, depending on the measured 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.

[0023] If, during the operation of the water treatment plant, a change in conductivity is detected when measuring the conductivity of the raw water using the measuring device or an additional conductivity sensor in the raw water inlet, for example, due to a change in the quality or composition of the supplied raw water, a conductometric determination of the total hardness of the blended water is preferably initiated using the measuring device, particularly if the amount of the detected change in conductivity exceeds a predetermined limit. This is done to adjust the mixing ratio set during the commissioning of the water treatment plant to the changed hardness of the raw water so that the blended water produced in the blending device again has a total hardness that corresponds to the predetermined target hardness.

[0024] Advantageously, the conductivity measurements of the raw water, recorded during operation of the water treatment plant, are stored in a data memory of the control unit along with the determined total hardness of the blended water and / or the corresponding blending ratio. This allows the control unit to adjust the blending ratio and regeneration trigger to the changed water quality and / or hardness of the incoming raw water, based on a change in the measured conductivity of the raw water, without having to re-determine 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 typically provides a limited number of different drinking water qualities with varying composition and hardness, and that these different drinking water qualities can change within a day or even over longer periods. Therefore, the composition and hardness of the raw water supplied to the water treatment plant can change according to the quality of the drinking water supplied via the raw water inlet. After a certain operating period of the water treatment plant, it is advantageous to have all drinking water qualities available at the plant's location and their total hardness.whose conductivity has been recorded and the recorded measured values ​​of conductivity as well as the mixing ratios with softened or desalinated water and the total hardness of the blended water resulting from the corresponding mixing ratio are stored in the memory of the control unit, so that in the event of a detected change in the conductivity of the raw water, it can infer the changed water quality of the supplied raw water and, using the stored data, adjust the mixing ratio and the regeneration trigger to the changed water quality of the raw water and, in particular, its hardness.

[0025] In the inventive method, the total hardness of the blended 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 blended water supplied to the measuring device, wherein the measuring device preferably determines the electrical conductivity of the blended water before precipitation (Lf1) and the electrical conductivity of the blended water after precipitation (Lf2). From the measured values ​​of the electrical conductivity of the blended water before precipitation (Lf1) and after precipitation (Lf2), the change in conductivity caused by the electrolytic precipitation of the alkaline earth ions in the blended water (ΔLf = |Lf2 - Lf1|) can be determined in magnitude, and the total hardness of the blended water can then be determined from this using a conversion factor or a calibration curve.

[0026] 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 desalinated water to the raw water and at least with blended water with a total hardness of less than 3°dH, a change in conductivity (ΔLf = Lf2 - Lf1), in particular a decrease in conductivity, is caused by at least largely complete electrolytic precipitation of the alkaline earth ions in the form of carbonates.In a preferred embodiment of the method according to the invention, this change in conductivity is detected by the measuring device, wherein the amount of the detected change in conductivity (ΔLf) represents a measure of the concentration of the hardness-causing alkaline earth ions in the blended water and therefore, from the detected change in conductivity (ΔLf) using a predetermined conversion factor or a predetermined characteristic curve of the conductivity of water as a function of the total hardness of the water, the total hardness of the blended water can be deduced and, from the set blend, the total hardness of the raw water can be deduced.

[0027] The change in conductivity (ΔLf) is conveniently determined by first measuring the original electrical conductivity (Lf1) of a water sample from the blended water and, after at least substantially complete precipitation of the alkaline earth ions in an electrolysis cell of the measuring device, measuring the electrical conductivity (Lf2) of the water sample again and calculating the difference (ΔLf = |Lf2 - Lf1|) from the two measurements. The measurement of the original electrical conductivity (Lf1) of the water sample and the electrical conductivity (Lf2) of the water sample after at least largely complete precipitation of the alkaline earth ions can be performed with a cost-effective conductivity sensor, which is preferably integrated into the electrolysis cell of the measuring device or can also be arranged outside of 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 precipitation of the alkaline earth ions. The other effects of the electrolysis of the water sample, in particular the decomposition of the water into hydrogen and oxygen, are not utilized.

[0028] The determination of the total hardness of the blended water sample from the measured change in conductivity (ΔLf) is carried out using a conversion factor (F), which is, in particular, a proportionality factor for a linear relationship between the conductivity of water and the carbonate hardness of the water. The conversion factor can be derived from a characteristic curve that, when carbonate hardness is precipitated in a water sample, describes a linear change in electrical conductivity as a function of the change in total hardness or carbonate hardness of the water.

[0029] The conversion factor (F) is derived from the ratio between the mass of hardness-causing minerals removed during the electrolysis of water through the precipitation of alkaline earth ions (as carbonates) and the resulting reduction in the electrical conductivity of the water sample due to the decreased ion concentration in the water caused by the precipitation of the 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 constant of proportionality for this linear relationship.

[0030] The conversion factor (F) or the characteristic curve can be determined experimentally and, in particular, empirically using a large number of different water samples, especially various drinking water samples of different origins or compositions with an excess of bicarbonate ions compared to alkaline earth ions. This is done by conductometric titration of the water samples 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 graph or characteristic curve is generated showing the change in conductivity as a function of the total hardness of the water. A common hardness scale, especially the German or French water hardness scales, can be used as a measure of total hardness for this purpose.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).

[0031] The conversion factor (F) or the characteristic curve can also be determined experimentally using water samples with a sufficient concentration of alkaline earth ions without a predetermined excess of bicarbonate ions (i.e., without preconditioning the water sample). This is achieved by performing electrolytic precipitation of the carbonates on the water samples and recording both the change in electrical conductivity caused by the precipitation and the change in hardness (total hardness or carbonate hardness) of the water sample in a graph. The relationship between the change in conductivity and the change in hardness is then determined. The conversion factor F is derived as a proportionality factor from the typically linear relationship between the change in conductivity and the change in hardness.

[0032] Assuming that the precipitation of at least largely complete alkaline earth ions in the electrolysis cell of the measuring device precipitates all hardness-causing minerals contained in the blended water sample, the measured change in conductivity (ΔLf) represents a measure of the total hardness of the blended water. This can be converted to a standard hardness scale using a conversion factor or characteristic curve and thus expressed as the total hardness of the blended water. This calculation involves deriving the (original) total hardness of the blended water sample from the precipitated (temporary) hardness using mass balance calculations.This is made possible by the excess of bicarbonate ions compared to alkaline earth ions present in the blending water, because this allows the entire concentration of alkaline earth ions in the blending water to precipitate (at least largely) as carbonates in the electrolysis cell. The resulting change in conductivity is therefore a measure of the total concentration of alkaline earth ions in the blending water and thus of the total hardness of the blending water. Due to this excess of bicarbonate ions compared to alkaline earth ions, the (precipitated) carbonate hardness of the blending water sample obtained through electrolysis corresponds to the total hardness of the blending water, provided that the alkaline earth ions have been precipitated at least largely as carbonates in the electrolysis cell.

[0033] To ensure at least a largely complete precipitation of the alkaline earth ions in the electrolysis cell of the measuring device, and thus a high 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 reaches a minimum value (Lf2) due to complete precipitation of the alkaline earth ions. If the water sample is electrolyzed further beyond this transition point, a sudden or gradual increase in the measured conductivity of the water sample can be observed.This increase in conductivity allows for a clear determination of the minimum value (Lf2) of the conductivity from the time course of the conductivity of the water sample measured during electrochemical precipitation, and thus an accurate determination of the change in conductivity (ΔLf) caused by a complete precipitation of the alkaline earth ions.

[0034] In order to achieve sufficient accuracy in determining the total hardness of the water sample, in particular in accordance with the requirements 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, and particularly preferably more than 95%, have been precipitated by electrolysis.

[0035] To precipitate the alkaline earth ions, the water sample from the blended water is preferably introduced into an electrolysis cell containing at least two electrolysis electrodes, which are subjected to a direct current voltage for electrolytic precipitation of the alkaline earth ions. The electrolysis electrodes can, for example, be designed as conductive plates (flat electrodes) arranged parallel to each other and spaced apart. 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, a specific volume of a water sample from the blended water, which expediently corresponds to the internal volume of the electrolysis cell, is introduced into the electrolysis cell in batch operation during a measurement mode, which is carried out particularly during the commissioning of the water treatment plant. Immediately before the start of the electrolytic precipitation of the alkaline earth ions, the initial conductivity Lf1 of the water sample is recorded in a first measurement cycle. The electrolytic precipitation of the alkaline earth ions can then begin by applying a DC voltage to the electrolysis electrodes during a first electrolysis cycle. The conductivity sensor, preferably integrated into the electrolysis cell, is switched off during the first electrolysis cycle; that is, the measuring electrodes are not subjected to an AC voltage during the electrolytic precipitation of the alkaline earth ions.The first electrolysis cycle can then be followed alternately by further measurement cycles and further electrolysis cycles to precipitate the alkaline earth ions contained in the blended water. During precipitation, the conductivity of the blended water sample is recorded either intermittently or quasi-continuously until the alkaline earth ions have precipitated at least substantially completely. After the alkaline earth ions have precipitated at least largely completely, the electrolysis electrodes are de-energized, and in a final measurement cycle, the conductivity Lf2 of the water sample is recorded. By calculating the difference between the original conductivity Lf1 and the conductivity Lf2 measured in the last measurement cycle, the change in conductivity caused by the precipitation of the alkaline earth ions, ΔLf = |Lf2 - Lf1|, is determined. The duration of the electrolysis cycles is preferably between 2 and 10 minutes and is, for example,5 minutes and the duration of the measurement cycles is appropriately between 30 seconds and 120 seconds, for example 60 seconds.

[0037] To determine the conductivity of the water sample from the blending water before and / or during and after the precipitation of the alkaline earth ions, the electrolysis cell advantageously includes 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 determine 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 electrolysis cell, electrolytic precipitation of alkaline earth ions and conductivity measurement of the water sample are performed alternately during the measurement mode. This decouples the electrolytic precipitation of the alkaline earth ions from the conductivity measurement using the at least one conductivity sensor integrated into the electrolysis cell, thereby preventing interference during the conductivity measurement.

[0039] 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 measurement 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 measured change in conductivity (ΔLf), 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 desalinated raw water from the softening device in a predefinable 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 supplied to the conductometric measuring device and the measuring device is connected to the blending device in order to supply 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 the latter performs 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 unit is expediently designed 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 water softener can be made and / or, if the softening capacity is predicted to be exhausted soon, a regeneration process can be initiated to regenerate the water softener, in particular the ion exchangers, 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 for 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 includes a temperature sensor for detecting the temperature of the water sample during a measurement mode.This allows for temperature correction to a predetermined standard temperature via a known temperature correction factor when measuring conductivity, thus taking into account the influence of temperature on conductivity 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 each other in the electrolysis cell of the measuring device and are expediently designed as flat electrodes. This allows for a sufficient volume of the electrolysis cell to accommodate a water sample from the blended water while maintaining a compact design for 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 determine the conductivity of the water sample before, during, and / or after the precipitation of alkaline earth ions, the measuring device includes at least one conductivity sensor. Preferably, at least one conductivity sensor is integrated into the electrolysis cell of the measuring device, wherein this conductivity sensor comprises at least one electrode pair with two measuring electrodes that are connected or connectable to an AC voltage source. Integrating the one or more conductivity sensors into the electrolysis cell allows for a compact and space-saving design of the measuring device. However, it is also possible for at least one conductivity sensor and / or a second conductivity sensor to be arranged outside the electrolysis cell. For example, a first conductivity sensor can be located 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 electrode pair with two measuring electrodes, in particular designed as rod electrodes, which are connected to an alternating voltage source. Advantageously, the measuring electrodes are arranged between the opposing electrolysis electrodes of the electrolysis cell.

[0046] To continuously monitor the conductivity of the raw water during the 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] To adjust a suitable mixing ratio of the blended water, the control unit of the water treatment plant according to the invention is preferably coupled to the blending device and configured such that the mixing ratio is regulated to a predetermined setpoint of the total hardness of the blended water depending on the determined total hardness of the blended water, wherein the conductivity of the raw water detected by the measuring device or by the additional conductivity sensor in the raw water inlet is preferably supplied to the control unit continuously or at specific times and the control unit initiates a conductometric determination of the total hardness of the blended water by means of the measuring device depending on the detected conductivity of the raw water, in particular in the event of a detected change in the conductivity of the raw water.

[0048] To determine the hardness of the blended water, the control unit of the water treatment plant according to the invention expediently includes a data storage device containing a conversion factor or a characteristic curve that describes the change in conductivity of water during carbonate hardness precipitation as a function of the water's hardness change. The control unit 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, based on the detected change in conductivity (ΔLf) and the conversion factor or characteristic curve.

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

[0050] With the inventive method and water treatment plant, 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 changes in the conductivity of the supplied raw water, in order to be able to quickly adjust the mixing ratio in case 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 the capacity can be recalculated.

[0051] These and further advantages and applications as well as preferred features of the inventive method and the inventive water treatment plant will become apparent from the exemplary embodiments of the invention described below with reference to the drawings. The drawings show: Fig. 1: a schematic representation of an embodiment of a water treatment plant according to the invention; Fig. 2: Schematic representations of various embodiments of a conductometric measuring device that can be used in the water treatment plant and the method according to the invention; Fig. 3: a flowchart of a preferred embodiment of the method according to the invention; 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 during the execution of the method according to the invention; Fig. 5: a diagram of the time course of the electrical conductivity of a water sample with blended water from the water treatment plant as determined in the method according to the invention. Fig. 1 and the temporal progression of the hardness of the blending water determined by the inventive method.

[0052] In Fig. Figure 1 shows an embodiment of the water treatment plant according to the invention, wherein the water treatment plant 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 producing blended water V, a control unit 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 configured 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 unit 14.The control head 24, together with the blending valve v1, forms a blending device 24, v1, 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 v1. The measuring device 15 is arranged downstream of the blending device 24, v1.

[0053] The ion exchanger 13 of the water 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 water softening device 12, the water treatment plant 10 further comprises a regeneration unit 18, which has a regeneration tank 9 containing an aqueous regeneration solution, in particular a sodium chloride solution. When the ion exchanger 13 of the water softening device 12 is exhausted, the regeneration solution is passed through the ion exchange tank 8 in a regeneration cycle, thereby replacing the calcium and magnesium ions bound in the ion exchanger 13 during the softening of the raw water R with sodium ions. After completion of the regeneration cycle, the spent regeneration solution is discharged via a discharge line 23 into a channel 19.

[0054] The inlet 11a of the water softening device 12 is connected via the control head 24 to the raw water inlet 11, through which (hard) raw water R is supplied to the water softening device 12 for softening. A conductivity sensor 16 is arranged in the raw water inlet 11 to measure the conductivity of the supplied raw water R.

[0055] The outlet 11b of the water softener 12 is connected via an outlet line 7 to the control head 24 to direct the water softened by ion exchange in the ion exchanger 13 (soft water W) into the blending water line 20. In the blending 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 v1, to produce blending water V. The blending water V should have a predetermined target hardness GH. V soll exhibit, for example, those of GH V soll = 3 °dH.

[0056] 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 produced in the blending unit 24, v1 is discharged via the blending water line 20 and conveyed via a consumer line 21 connected to the blending water line 20 to a consumer 22. 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 directly supplied.

[0057] To determine and adjust a suitable mixing ratio, which is chosen so that the blended water V has a hardness that corresponds as closely as possible to a specified target hardness GH. V sollThe control unit 14 is coupled to the blending device 24, v1 to allow the mixing ratio to be appropriately adjusted by setting the adjustable blending valve v1 of the blending device 24, v1. To set an initial mixing ratio r:w_0, at which the hardness of the blended water V corresponds at least approximately to the target hardness, the control unit 14 is coupled to the conductivity sensor 16 located in the raw water inlet 11 and receives the measured value of the electrical conductivity of the raw water R from this sensor. Based on the measured value of the conductivity of the raw water R (conductance L) R) The control unit 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 an initial mixing ratio r:w_0 from this, which is determined in such a way that the predicted hardness of the blended water corresponds at least approximately to the target hardness.

[0058] To fine-tune the mixing ratio, the exact total hardness (GH) is used. V The volume of the blended water V is determined by means of the measuring device 15. For this purpose, the blended water V is directed via the blended water line 20 through a valve V2, which can be controlled by the control unit, to an inlet 15a of the measuring device 15 and flushes the device.

[0059] The measuring device 15 comprises an electrolysis cell 1, which has a container 5 with an inlet 15a and an outlet 15b. A water sample, the hardness of which is to be determined, can be introduced into the container 5 of the measuring device 15 via the inlet 15a. To carry out a conductometric measurement, the inlet 15a and the outlet 15b of the container 5 are closed. After completion of a measurement, the water sample can be discharged into a channel 19 by opening the outlet 15b.

[0060] In Fig. Figure 2 schematically shows different embodiments of the measuring device 15. The in Fig. Figure 2A shows a measuring device 15 comprising an electrolysis cell 1 with a container 5, which has an inlet 15a (not shown) for introducing the water sample and an outlet 15b for draining the water sample after hardness determination. 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.

[0061] Inside container 5, two flat electrodes are arranged parallel to each other and spaced apart. These two flat electrodes form corresponding electrolysis electrodes A and K, with one flat electrode serving as the anode (A) and the other as the cathode (K). The two electrolysis electrodes A and K are connected to a direct current source (DC), as shown in Fig. 2A is indicated. The two electrolysis electrodes A and K are supplied with direct current via the DC power source. This direct current electrolyzes the water sample in container 5 of electrolysis cell 1, causing the alkaline earth ions in the water to precipitate as carbonates at the cathode (K).

[0062] A conductivity sensor 2 with two measuring electrodes, forming an electrode pair 3, is arranged between the two electrolysis electrodes A and K. The electrode pair 3 is connected to an AC voltage source and can be subjected to an AC voltage of a predetermined frequency via the AC voltage source. When an AC voltage is applied, the electrode pair 3 acts as a conductivity sensor 2, which measures the current flowing through the measuring electrodes of the electrode pair 3 to determine the conductivity of the water in the container 5. The two measuring electrodes are located in the Fig. 2A shown as a rod electrode whose longitudinal axis runs parallel to the plane of the two electrolysis electrodes A, K which are designed as flat electrodes.

[0063] The measuring device 15, comprising the electrolysis cell 1 and the two electrolysis electrodes A and K arranged therein, and the conductivity sensor 2 integrated into the electrolysis cell 1, is coupled to the control unit 14. The control unit 14 regulates the supply of direct current to the electrolysis electrodes A and K during predefined electrolysis cycles. Furthermore, the control unit 14 also controls the measurement of the electrical conductivity of the water sample located in the container 5 using the conductivity sensor 2 by applying an alternating voltage to the electrode pair 3 of the conductivity sensor 2 in defined measurement cycles.Preferably, the control device 14 is configured such that measurement cycles with a predetermined measurement duration and electrolysis cycles with a predetermined electrolysis duration are carried out alternately one after the other, wherein preferably during a measurement cycle the electrode pair 3 of the conductivity sensor 2 is supplied with alternating voltage, while the two electrolysis electrodes A, K are switched off, and during an electrolysis cycle the two electrolysis electrodes A, K are supplied with direct current, while the electrode pair 3 is de-energized.

[0064] The control unit 14 comprises a data storage device and a processing unit. The data storage device contains at least one characteristic curve that represents the electrical conductivity of water as a function of its total hardness on a hardness scale. The data storage device 14 can also contain several such characteristic curves, with each curve representing, for example, the electrical conductivity of water as a function of its hardness on different hardness scales, such as the German hardness scale (°dH) or the French hardness scale (°fH). If the characteristic curve exhibits a linear relationship between electrical conductivity and water hardness, the proportionality factor, which specifies the slope of this linear relationship, can also be stored in the control unit 14's memory as a conversion factor F, either in addition to or instead of the characteristic curve.

[0065] To determine the hardness of the water sample in container 5 of electrolysis cell 1, the initial conductivity Lf1 of the water is measured in a measurement mode with electrolysis electrodes A and K de-energized. This is done by applying an alternating voltage at a predetermined frequency to the electrode pair 3 using the conductivity sensor 2. This measurement constitutes the first measurement cycle. The voltage to the electrode pair 3 of the conductivity sensor 2 is then switched off, and a first electrolysis cycle begins for a predetermined duration. For this cycle, a direct current voltage is applied to both electrolysis electrodes A and K.By applying a DC voltage to the two electrolysis electrodes A and K, the water in container 5 is electrolyzed. Simultaneously, the alkaline earth ions present in the water begin to precipitate as carbonates, particularly calcium carbonate and magnesium carbonate, and are deposited at the cathode (K). After completion of the first electrolysis cycle, the current supply to the electrolysis electrodes A and K is switched off, and a second measurement cycle is performed by applying an AC voltage to the electrode pair 3 of the conductivity sensor 2 for a predetermined measurement duration. During this cycle, the electrical conductivity of the water in container 5 is recorded. Due to the partial precipitation of the alkaline earth ions, the conductivity measured in the second cycle is lower than the original conductivity of the water, as 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.

[0066] The resulting time course of the electrical conductivity of the water in container 5 is illustrated by an example in Fig. 4 shown. In the time course of the electrical conductivity of the Fig. Figure 4 shows the individual measurement cycles M, in which the conductivity of the water does not change. Between successive measurement cycles M with constant conductivity are the electrolysis cycles E, whereby the electrical conductivity of the water decreases further in each electrolysis cycle E due to the progressive filling with alkaline earth ions, until it reaches a certain value. Fig. 4. Transition point marked U, at which the conductivity of the water, as measured by conductivity sensor 2, increases abruptly or gradually. The (possibly abrupt) increase in electrical conductivity at transition point U presumably results from a sudden rise in pH and a resulting excess of anions (CO3). 2- and OH - ) with a higher equivalent conductivity compared to the hydrogen carbonate ions after completion of cathodic precipitation. At the transition point U, the conductivity of the water detected by conductivity sensor 2 exhibits a minimum value Lf2.

[0067] 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 ΔLf = |Lf2 - Lf1| caused by the precipitation of the alkaline earth ions can be calculated from the difference between the conductivity measured before the start of electrolysis (Lf1) and the conductivity measured after complete saturation of the alkaline earth ions (Lf2). From this difference value, which corresponds to the difference between the original conductivity of the water and the conductivity measured after complete precipitation of the alkaline earth ions, 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 unit 14 is programmed to calculate the difference between the measured values ​​of the original conductivity Lf1 and the conductivity of the water after complete precipitation of the alkaline earth ions Lf2, ΔLf = |Lf2 - Lf1|, and to determine the hardness of the water on the hardness scale of the characteristic curve or the conversion factor F from the difference value, using the characteristic curve and / or the conversion factor F stored in the memory of the control unit 14 as follows: H=ΔLf / F where H is the hardness of the water, ΔLf is the difference in the measured conductivities of the water before and after the complete precipitation of the alkaline earth ions, and F is the conversion factor that results from the characteristic curve of the (linear) course of the change in conductivity of water during precipitation of the carbonate hardness of the water as a function of the change in hardness, 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.

[0068] The (empirically determinable) factor F results from the ratio between the mass of hardness-causing minerals removed by precipitation and the reduction in conductivity due to proportionally decreased ion concentrations. Up to the transition point U, the curve of an electrolytic hardness precipitation (at constant current) is almost linear, as shown by... Fig. As can be seen in Figure 4. At constant voltage (decreasing current), the conductivity is no longer linear over time, but the transition point U remains unchanged at the same conductivity value. With an excess of bicarbonate compared to alkaline earth ions, the transition point U corresponds quite closely to the total concentrations of alkaline earth ions due to the comparable equivalent conductivities of Ca and Mg. Empirical measurements of numerous drinking water samples from Germany of varying origins and compositions yield, for example, a conversion factor F of the following value at a temperature of 15°C: F=30 μS / (cm°dH)=0.030 mS / (cm°dH).

[0069] From the in Fig. Based on the time course of the electrical conductivity of a water sample shown in section 4, the following calculation results for the hardness of the water: H=ΔLf / F=(0.70−.059)mS / cm0.030 mS / (cm°dH)=3.6°dH

[0070] In the Fig. 2A and Fig. Figure 2B shows 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 at the closable outlet 15b of the electrolysis cell 1. Each conductivity sensor 2a, 2b comprises an electrode pair 3a, 3b, each with a first measuring electrode a (measuring anode) and a second measuring electrode k (measuring cathode). In the embodiment shown, a cable is connected between the inlet 15a and the outlet 15b of the electrolysis cell 1. Fig. 2B the two electrolysis electrodes A, K are arranged, which, as in the embodiment of Fig. 2A, designed as flat electrodes and arranged opposite each other at a predetermined distance.

[0071] The in Fig. The embodiment of the electrolysis cell 1 shown in Figure 2B can be used to determine the electrical conductivity of a water sample that flows continuously through the electrolysis cell 1 from inlet 15a to outlet 15b in flow-through mode. During the flow of the water sample through the electrolysis cell 1, the conductivity Lf is measured. a of the water flow at entrance 15a and the conductivity Lf b The conductivity is continuously measured at output 15b using the two conductivity sensors 2a and 2b. The control unit 14 calculates the difference ΔLf = |Lf from the measured values ​​at the two conductivity sensors 2a and 2b. b - Lf a| and compares the calculated difference in electrical conductivity ΔLf with a predefined limit value. If the measured difference in electrical conductivity ΔLf exceeds this limit value, it can be concluded that a certain amount of hardness-causing alkaline earth ions is present in the water, indicating a hardness breakthrough in the water softening device 12. Furthermore, the measuring device 15 can be used to determine the Fig. 2B in flow-through operation a change in the electrical conductivity of the blended water V is detected, which is due, for example, to a change in the water quality of the raw water R, which is supplied to the water treatment plant via the raw water inlet 11.

[0072] To determine the total hardness of the water in the water stream supplied to the measuring device 15, the measuring device 15 can be configured according to the second embodiment of the Fig. 2B is switched from continuous flow operation to batch operation by closing the inlet 15a and the 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 device as described in the embodiment of the Fig. 2A described measurement mode.

[0073] In the Fig. In the embodiment of the measuring device 15 shown in Figure 2C, several electrolysis electrodes A, K are arranged in the electrolysis cell 1. In particular, the electrolysis cell 1 of the embodiment contains Fig. 2C 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 another outer anode A, as shown in Fig. Figure 2C shows that a cation exchange membrane KAT is arranged between corresponding electrolysis electrodes A and K. The multiple electrolysis electrodes A and K, along with the cation exchange membrane KAT arranged between them, increase the efficiency of the electrolytic precipitation of alkaline earth ions by precipitation as alkaline earth carbonates at the cathodes K (particularly due to a larger electrode area). This reduces the measurement time and allows for a lower flow rate of the water sample through electrolysis cell 1.

[0074] A preferred embodiment of the method according to the invention is described below with reference to the flowchart of Fig. Section 3 explains the steps involved in setting up and initially starting up the water treatment plant 10: A target hardness GH is transmitted to the control unit 14 via a communication interface. V soll The conductivity of the raw water is specified, which is preferably between 2°dH and 15°dH, and more preferably between 3°dH and 10°dH, and most preferably between 3°dH and 6°dH (step S1). The electrical conductivity of the raw water Lf is then continuously or intermittently measured at predefinable time intervals using the conductivity sensor 16 arranged in the raw water inlet 11. R measured (step S2). The control unit 14 estimates Lf based on the measured conductivity of the raw water. R by accessing a characteristic curve stored in the data storage, the hardness H R of the raw water and positions the blending valve v1 of the blending device 24, v1 so that an initial mixing ratio r:w_0 results, at which the expected total hardness GH vThe blending water V has a low value, preferably ≤ 3°dH (step S3). The blending water V produced with this setting of the blending valve v1 has an excess of bicarbonate ions compared to alkaline earth ions, thus enabling an accurate measurement of the total hardness GH. v The blended water is measured using the method described above. Thousands of drinking water analyses have shown that, with this setting, the bicarbonate content is present in excess of the alkaline earth metals.

[0075] To determine the total hardness of the blended water V, a water sample of the produced blended water V is fed into the container 5 of the measuring device 15 and the container 5 is closed, so that it contains the water sample of the blended water V with a certain volume (step S4).

[0076] The blending valve v1 is then pre-positioned to a preliminary mixing ratio r:w_1, at which, based on the raw water hardness roughly determined in step S2, the hardness of the blended water corresponds approximately to the target hardness (step S5). The water treatment plant 10 is operated with this preliminary mixing ratio r:w_1 setting until the blended water hardness GH can be accurately determined. V and the raw water hardness H R is completed and, based on these values, a fine adjustment of the mixing ratio can be made to achieve the specified target hardness GH. V soll to achieve the blending water as accurately as possible.

[0077] For this purpose, a measuring mode is carried out simultaneously with or after step S5 in the measuring device 15, in which the total hardness GH is measured. Vof the blended water V and from this, using the known initial mixing ratio r:w_0, the hardness of the raw water R is determined (step S6)..

[0078] At the beginning of the measurement mode (step S6), the (original) electrical conductivity of the blended water V is measured in electrolysis cell 1 of the measuring device 15 using the conductivity sensor 2 integrated into electrolysis cell 1. Subsequently, the water sample of the blended water V located in the container 5 of electrolysis cell 1 is electrolyzed with the container 5 closed by means of the electrolysis electrodes A, K, by applying a direct current voltage to the electrolysis electrodes A, K. This causes the alkaline earth ions contained in the water sample of the blended water V to precipitate from the water. During the electrolytic precipitation of the alkaline earth ions, the electrical conductivity of the blended water V located in the container 5 of electrolysis cell 1 is measured using the integrated conductivity sensor 2.The conductivity sensors 2a and 2b were measured to detect the transition point U, which indicates complete precipitation of the alkaline earth ions, as shown above using the measuring devices of . Fig. 2 described. After complete precipitation of the alkaline earth ions, the electrical conductivity Lf2 of the electrolyzed blended water V is measured, and the change in conductivity ΔLf = |Lf2 - Lf1| caused by the precipitation of the alkaline earth ions is determined by calculating the difference with the original conductivity Lf1 of the blended water V. For this purpose, the control unit 14 is coupled to the conductivity sensor 2 to obtain the measured values ​​of the conductivity measurement (conductance values) before, during, and after the precipitation of the alkaline earth ions. The control unit 14 calculates the exact total hardness GH from the measured difference in electrical conductivity ΔLf, using the conversion factor F stored in a memory of the control unit 14. Vof the blending water V, as described above (step S7). The measurement time required for the complete precipitation of the alkaline earth ions depends on the hardness of the water sample and is approximately between 10 and 15 minutes / ° dH. Due to the low hardness of the blending water set via the initial mixing ratio r:w_0, this time is less than 45 minutes.

[0079] Based on the total hardness GH determined by the control unit 14 V The control unit 14 adjusts the blending unit 24, v1 for this purpose in order to adapt the preliminary mixing ratio r:w_1 of the raw water with the softened water W to a new mixing ratio r:w_2 so that the blending water V has a total hardness GH v exhibits characteristics that meet the specified target hardness GH v soll This corresponds to (step S8). The control unit 14 (in step S7) can then determine the total hardness GH from the specified value. VThe hardness H of the blended water V, taking into account the original mixing ratio, is also determined. R Calculate the hardness of the raw water (R). Calculate the hardness (H). R The raw water R can be used, for example, to predict the 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.

[0080] During the operation of the water treatment plant, changes in hardness H RChanges in the hardness of the raw water R, which can occur, for example, when the water quality of the drinking water supplied by the public water supply changes, are detected by the conductivity sensor 16 and taken into account when adjusting the blending valve v1 by adapting the mixing ratio to a changed hardness of the raw water R. For this purpose, the current conductivity readings Lf recorded by the conductivity sensor 16 are used. R (n+1) of the raw water R is fed to the control unit 14, which compares the recorded measured values ​​with the preceding measured values ​​Lf R (n) compares (step S9). If this results in a change in conductivity ΔLf R = Lf R (n+1) - Lf R (n) of the raw water R results, the amount of which exceeds a given limit δLf R If the value is not present, a further measurement mode is initiated in which the total hardness GH is measured using the measuring device 15 in the manner described above. VThe amount of blending water V is determined, and depending on this, the mixing ratio of the blending water V is adjusted if necessary. The limit value δLf R The conductivity (Lf) is appropriately in the range of 30 to 40 µS / cm and is specifically 30 µS / cm. This applies as long as there is no change in conductivity (Lf) during the operation of the water treatment plant. R of the raw water R is recorded, the amount of which exceeds the limit value δLf R If the position is unchanged, the setting of the blending valve v1 of the blending device 24, v1 remains unchanged (step S10).

[0081] The conductivity values ​​of the raw water Lf recorded by the conductivity sensor 16 R are measured together with the associated raw water hardness H RThe raw water conductivity value is stored in a memory location of the control unit 14. If a previously known raw water conductivity value is detected again, the analysis process is not started again; instead, the raw water hardness H, already stored in the control unit's memory and assigned to this conductivity value, is used. R is used as a new control parameter for adjusting the mixing ratio to achieve the target hardness GH. v soll to achieve the blending water V.

[0082] In Fig. Figure 5 is an example of the time course of a measurement of the electrical conductivity of the blended water V or the raw water R (measured conductivity values ​​Lf). V or Lf R ) in the water treatment plant of the Fig. 1 in the case of changing hardness H R of the raw water R supplied via the raw water inlet 11 and the time course of the hardness GH determined from the conductivity of the blended water V Vof the blending water V and the hardness H calculated from it, taking into account the mixing ratio R of the raw water R. The softening device 12 was supplied with raw water R with a specific raw water hardness H between the start of the measurement at time t = 0. R supplied. From time t1 until time t2, the softening device 12 was supplied with raw water R' with a changed composition and a higher hardness H. R' The raw water was added, and from time t3 onwards, the original raw water R was added again. The conductivity Lf was continuously measured throughout the entire measurement period between t = 0 and t ≅ 9 h. R (t) of the raw water R or R' is recorded with the conductivity sensor 16. At time t1, due to the higher hardness of the modified raw water R', there is an increase in the conductivity Lf. R(t) of the raw water was observed. From time t0 onwards, a measurement mode was carried out to determine the total hardness GH. v of the blending water V and from that the hardness H R to determine the raw water R and to adjust the mixing ratio of raw water R with softened water W so that the total hardness GH v of the blended water V of the specified target hardness GH V soll This corresponds to 3°dH. Based on a recorded change in conductivity Lf R(t) of the raw water at times t1 and t3, at times t2 and t4 in electrolysis cell 1, a measurement of the conductivity of the blended water was initiated in batch operation, followed by precipitation of the alkaline earth ions by electrolysis of the blended water in electrolysis cell 1, and, as described above, the conductivity Lf2 of the (electrolyzed) blended water and the change in conductivity ΔLf during electrolysis were recorded, and from this the total hardness of the blended water GH was determined. v as well as the hardness H R' of the changed raw water R` determined. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 102007059058 A1

[0009] Cited non-patent literature

[0000] DIN 19636-100

[0011]

Claims

[1] Method 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, v1) 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, v1) 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 by, that blending water (V) from the blending device (24, v1) is supplied to the measuring device (15) and that the measuring device (15) 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 (14), which calculates the total hardness (GH) from the measurement results v ) of the blending water (V) is determined. [2] Method according to claim 1, characterized by that the conductivity (Lf R ) of the raw water (R) is detected by means of a conductivity sensor (16) arranged in the raw water inlet (11) or by means of the measuring device (15) and that the detected conductivity (Lf R) of the raw water (R) the hardness (H) of the raw water (R) is roughly estimated in order to adjust an initial or provisional mixing ratio based on the estimated hardness (H) of the raw water (R) when commissioning the water treatment plant (10) such that the total hardness (GH) v ) of the blended water (V) at least approximately a specified target hardness (GHv) soll ) of the blending water (V). [3] Method according to claim 1 or 2, characterized by that the conductivity (Lf R ) 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). [4] Method according to any of the preceding claims, characterized by, 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, v1) and that the mixing ratio is adjusted according to the determination of the total hardness (GHv) of the blended water (V) depending on the measured total hardness (GH v ) of the blending water (V) is adjusted to a mixing ratio at which the total hardness (GHv) of the blending water (V) corresponds to a predetermined target hardness (GH V soll ) of the blending water (V). [5] Method according to any of the preceding claims, characterized by that the conductivity (Lf R ) of the raw water (R) is continuously or at specific time intervals recorded by means of the measuring device (15) or by means of an additional conductivity sensor (16), and that depending on the recorded conductivity (Lf R) of the raw water (R), especially in the case of a detected change in conductivity (Lf) R ) of the raw water (R), a conductometric determination of the total hardness (GHv) of the blended water (V) is initiated using the measuring device (15). [6] Method according to any one of claims 2 to 5, characterized by , that the control device (14) calculates the total hardness (GH) from the total hardness (GHv) of the blended water (V), taking into account the initial mixing ratio or the preliminary mixing ratio. R ) of the raw water (R) determined. [7] Method according to claim 6, characterized by that the calculated total hardness (GH) R ) of the raw water (R) and the measured conductivity (Lf R ) of the raw water (R) are stored in a data storage device of the control unit (14). [8] Method according to any of the preceding claims, characterized by that the total hardness (GHv) and / or a specified target hardness (GHv)soll ) 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. [9] Method according to any of the preceding claims, characterized by , 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 (Lf1) 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 (ΔLf = |Lf2 - Lf1|) caused by the electrolytic precipitation of the alkaline earth ions of the blended water (V) is determined. [10] Method according to claim 9, wherein the control device (14) detects the change in conductivity (ΔLf = |Lf2 - Lf1|) of the water sample of the blending water (V) caused by the precipitation of the alkaline earth ions and determines the total hardness (GHv) of the blending water (V) from this using a conversion factor (F) or a calibration curve (Lf(H)). [11] 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. [12] Method according to one of claims 10 or 11, characterized by, 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. [13] Method according to any of the preceding claims, characterized by , that the measuring device (15) comprises an electrolysis cell (1) which contains at least two electrolysis electrodes (A, K) which are supplied with direct current for the electrolytic precipitation of the alkaline earth ions of the supplied water sample. [14] Method according to claim 13, characterized by, that the electrolysis cell (1) contains at least one integrated conductivity sensor (2) with two measuring electrodes (a, k) which are supplied with alternating current to detect the conductivity of the supplied water sample. [15] 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, v1) 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 set up to determine the hardness of the water of a water sample supplied to the conductometric measuring device (15), characterized by , that the measuring device (15) is connected to the blending device (24, v1) in order to supply blending water (V) from the blending device (24, v1) 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) such that it performs a conductometric measurement in particular before and after an electrolytic precipitation of the alkaline earth ions and transmits the measurement results to the control device, which calculates the total hardness (GH) from the measurement results v ) of the blending water (V) is determined. [16] Water treatment plant according to claim 15, characterized by, 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), wherein the conductivity sensor (2) is preferably integrated into the electrolysis cell (1). [17] Water treatment plant according to claim 15 or 16, wherein the control device (14) is configured to first detect the original conductivity (Lf1) 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 (GH) from the difference between the original conductivity and the conductivity measured after complete precipitation of the alkaline earth ions (ΔLf = |Lf2 - Lf1|). v ) of the blended water (V) is determined on a hardness scale. [18] Water treatment plant according to claim 16 or 17, characterized by, 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. [19] Water treatment plant according to any one of claims 15 to 18, characterized by , that the conductivity sensor (2) 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 and are preferably arranged between the opposing electrolysis electrodes (A, K) of the electrolysis cell (1). [20] Water treatment plant according to any one of claims 15 to 19, characterized by , that a further conductivity sensor (16) is arranged in the raw water inlet (11). [21] Water treatment plant according to one of claims 15 to 20, characterized by, that the control device (14) for adjusting the mixing ratio of the blending water (V) is coupled to the blending device (24, v1) and the control device (14) is configured such that the mixing ratio is adjusted to a predetermined setpoint (GH) depending on the determined total hardness (GHv) of the blending water (V). V soll ) the total hardness of the blended water (V) is regulated. [22] Water treatment plant according to one of claims 15 to 21, wherein the control device (14) preferably continuously or at specific times receives the conductivity (Lf) detected in the raw water inlet (11) by means of the measuring device (15) or by means of an additional conductivity sensor (16). R ) of the raw water (R) is supplied and the control device (14) depending on the detected conductivity (Lf R ) of the raw water (R), especially in the case of a detected change in conductivity (Lf) R) of the raw water (R), a conductometric determination of the total hardness (GH) V ) of the blended water (V) using the measuring device (15) and thus the total hardness (GH) R ) of the raw water (r) is introduced.