METHOD FOR DETERMINING THE HARDNESS OF WATER AND DEVICE FOR MEASURING THE HARDNESS OF WATER

Description

[0001] The invention relates to a method for determining the hardness of water, a device for detecting the hardness of water, and the use of the device in a water treatment plant.

[0002] The sum of the concentrations of all dissolved alkaline earth metals (which can exist as carbonates or hydrogen 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). The majority of the total hardness in drinking water is usually 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 typically present only as trace elements in water 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 dissolved carbon dioxide (according to the pH-dependent calcium carbonate equilibrium).

[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.Water softening or desalination is achieved through a combination of cation and anion exchangers or through reverse osmosis. For the operation and control of water softening or desalination plants, knowledge of the water hardness of the incoming raw water and / or the softened or desalinated water, or the hardness of blended water produced by mixing hard raw water with softened or desalinated water, is important, for example, to achieve a desired target hardness of the water supplied by a water softening plant or to prevent hardness breakthrough when the ion exchangers become exhausted.

[0004] 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 generate high costs due to the use of the titrant. Therefore, they are rarely used in automated measurement procedures.

[0005] 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.

[0006] 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 measure the absolute conductivity of a water sample, are only of limited use for determining the total hardness of the water sample.

[0007] However, differential measurements of the electrical conductivity of water samples or the detection of changes in the electrical conductivity of the water sample after treatment are frequently used to determine water hardness. EP 2 169 392 B1 discloses a method for determining water hardness in which at least two values ​​of the electrical conductivity of a water sample are measured under different conditions, and the hardness of the water sample is determined from these electrical conductivity measurements. The water sample is divided into at least two fractions by means of a first distribution station, and at least one of the fractions is subjected to a treatment that can influence the electrical conductivity of the water. After the treatment of at least one fraction, which, for example,While the determination of electrical conductivity can be achieved by heating or electrochemically, the different fractions of the water sample are alternately directed to a conductivity measuring device via a second distribution station to measure the electrical conductivity of at least two of the fractions. Since all ions dissolved in water contribute to electrical conductivity, this method allows for a more precise determination of those components of the water, particularly the concentrations of magnesium and calcium ions, which are primarily responsible for water hardness, by measuring two electrical conductivity values ​​under different conditions. This enables continuous measurement of the electrical conductivity of the individual fractions and therefore a continuous in-situ determination of the water hardness.However, the device used to carry out this procedure for determining the total hardness of water is complex, as a first and a second distribution station are required to separate the water sample into at least two fractions and to alternately direct the fractions to the conductivity measuring device.

[0008] DE 10 2016 100 195 A1 discloses a method for determining the hardness of water in water heating devices, in which the conductivity of the water used is recorded at the beginning of a heating process and at a later time, and the difference in conductivity values ​​is used as a measure of the hardness of the water.

[0009] Based on this, the invention aims to provide a method and a device for determining the hardness of a water sample. This method, with its simple and cost-effective design, enables the most accurate possible determination of the total hardness of the water sample without the use of chemicals, at low operating costs, while requiring minimal maintenance, long-term stability, and no calibration. The accuracy of the determination of the total hardness of the 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 method should, in particular, ensure in-situ determination of the total hardness of a water flow, for example, to...to enable continuous determination of the hardness of raw water supplied to the water treatment plant and / or the hardness of water treated in the water treatment plant, especially softened or desalinated water, in water treatment plants, particularly water softening plants or desalination plants.

[0010] These problems are solved by the method according to claim 1 and the device with the features of claim 10. Preferred embodiments of the method and the device are set forth in the dependent claims.

[0011] In the inventive method for determining the hardness of water from a water sample containing alkaline earth ions, in particular calcium and / or magnesium ions, as well as hydrogen carbonate ions in excess of the alkaline earth ions, the following steps are carried out successively: Determination of the original electrical conductivity (Lf 1 ) of the water sample, precipitation of the alkaline earth ions by electrolysis of the water sample in an electrolysis cell, determination of the conductivity (Lf 2 ) of the water sample after complete or at least largely complete precipitation of the alkaline earth ions and determination of the amount of the change in conductivity (ΔLf = |Lf 2 - Lf 1 |) of the water sample caused by the precipitation of the alkaline earth ions, determination of the total hardness of the water of the water sample from the determined change in conductivity (ΔLf) and a given conversion factor (F) or a given characteristic curve of the course of the conductivity change of water as a function of the hardness change of the water during precipitation of the carbonate hardness.

[0012] The method according to the invention assumes that in a water sample in which there is an excess of hydrogen carbonate ions compared to alkaline earth ions, which is naturally the case in some drinking waters, the total hardness of the water in the water sample can be determined from the change in conductivity (ΔLf = Lf 2 - Lf 1) caused by the precipitation of the alkaline earth ions, in particular from the decrease in conductivity, after at least largely complete electrolytic precipitation of the alkaline earth ions in the form of carbonates, because the difference in the conductivity of the water sample before and after the precipitation of the alkaline earth ions allows a determination of the total concentration of the alkaline earth ions in the water, the sum of which defines the total hardness of the water.In the case of an excess of hydrogen carbonate ions over alkaline earth ions, which is assumed in the process according to the invention, the carbonate hardness of the water sample during electrolysis corresponds to the total hardness of the water in the electrolysis cell. 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 the hardness-causing alkaline earth ions in the water sample. Consequently, the total hardness of the water in the sample can be determined from the measured change in conductivity (ΔLf) using a predetermined conversion factor (F) or a predetermined characteristic curve that characterizes the change in conductivity of water as a function of the change in water hardness during carbonate hardness precipitation.The change in conductivity (ΔLf) is determined in the inventive method by first measuring the original electrical conductivity (Lf 1 ) of the water sample and, after at least substantially complete precipitation of the alkaline earth ions in the electrolysis cell, measuring the electrical conductivity (Lf 2) of the water sample again and determining the amount of the difference (ΔLf = |Lf 2 - Lf 1 |) from the two measured values.

[0013] 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 carried out using a cost-effective conductivity sensor. The original electrical conductivity (Lf1) of the water sample is preferably measured immediately before the start of the electrolytic precipitation, but can also be determined at or shortly after the start of precipitation, since the precipitation reaction and the resulting change in conductivity begin slowly.

[0014] In the process according to the invention, the electrolysis of the water sample in the electrolysis cell is carried out for the purpose of precipitation of the alkaline earth ions. The further effects of the electrolysis of the water sample, in particular the decomposition of the water into hydrogen and oxygen, are not utilized.

[0015] In order to deduce the total hardness of the water sample from the measured change in conductivity (ΔLf) resulting from the at least largely complete precipitation of alkaline earth ions in the electrolysis cell, it is necessary that the total hardness of the water sample undergoes complete conversion in the electrolysis cell up to the equivalence point of hardness precipitation through at least largely complete precipitation of the alkaline earth ions in the form of carbonates. This requires that an excess of bicarbonate ions (HCO₃⁻) be present in the water sample.This is automatically the case with blended water, which is obtained by mixing raw water with softened water (obtained by softening the raw water), at least under typical mixing ratios, because the concentration of bicarbonate is maintained in the blended water, and compared to the raw water, bicarbonate ions are present in excess of alkaline earth ions. To reliably maintain this condition even with a water sample not produced by mixing raw water with softened water, such as raw water from a different source, it can be advantageous to condition the water sample before measuring its conductivity so that bicarbonate ions (HCO₃⁻) are present in excess of alkaline earth ions. This can be achieved, for example, by...The determination of the total hardness of the water sample can be carried out by mixing the water sample with softened water obtained by softening the water sample itself, and / or by adding bicarbonate ions and / or carbonate ions to the water sample. If the inventive measuring method is to be used to determine the total hardness of blended water produced in a water softening plant by mixing raw water with softened raw water, the condition of an excess of bicarbonate ions (HCO₃⁻) over the alkaline earth ions is automatically fulfilled at least when the hardness of the blended water is less than 3°dH. Therefore, the inventive method is particularly suitable for determining the total hardness of blended water from water softening plants and / or for determining the total hardness of the raw water used for this purpose.In this process, the water sample is produced as blended water by mixing raw water with softened raw water in a predetermined mixing ratio, whereby the total hardness of the blended water can be determined from the measured change in conductivity (ΔLf) and, taking into account the mixing ratio, the total hardness of the raw water can also be determined.

[0016] The determination of the total hardness of the water sample from the measured change in conductivity (ΔLf) of the water sample is carried out in the method according to the invention using a conversion factor (F), which is in particular a proportionality factor of a linear relationship between the change in conductivity of water and the change in total hardness or carbonate hardness of the water during electrolytic precipitation of the carbonate hardness. The conversion factor (F) can also be taken from a characteristic curve that generally describes a linear progression of electrical conductivity as a function of the carbonate hardness of the water.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 linearly proportional to the precipitated concentration of alkaline earth ions, with the conversion factor (F) being the constant of proportionality for this linear relationship.

[0017] The conversion factor (F) or 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 titrating 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. The total hardness is determined titrimetrically when determining the conversion factor or characteristic curve, e.g.,...by EDTA titration (according to DIN 38406-3, Group E- Part 3).

[0018] 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.

[0019] Assuming that the precipitation of at least largely complete alkaline earth ions in the electrolysis cell in the measuring method according to the invention precipitates all hardness-causing substances contained in the water sample, the measured change in conductivity (ΔLf) represents a measure of the total hardness of the water sample. This can be converted to a standard hardness scale using the conversion factor (F) or the characteristic curve and thus expressed as the total hardness of the water. The (original) total hardness of the water sample is then derived from the precipitated (temporary) hardness via mass balance calculations. If the boundary condition of the measuring method is met, namely that the hydrogen carbonate ions are present in excess of the alkaline earth ions in the water sample whose hardness is to be determined, the total hardness corresponds to the carbonate hardness of the water sample.

[0020] To ensure at least a largely complete precipitation of the alkaline earth ions in the electrolysis cell and thus a high accuracy in determining the total hardness of the water, the conductivity of the water sample is preferably measured in the inventive method during the electrolytic precipitation of the alkaline earth ions up to a transition point (U), at which the measured conductivity has a minimum value (Lf 2 ) due to complete precipitation of the alkaline earth ions. With further electrolysis of the water sample beyond the transition point (U), an increase in the measured conductivity of the water sample can be observed, which can be abrupt or gradual.This increase in conductivity allows for a clear determination of the minimum value (Lf 2 ) 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.

[0021] In order to achieve sufficient accuracy in determining the total hardness of the water in the water sample, in particular in accordance with the requirements of the standard DIN 19636-100, the conductivity of the water sample 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.

[0022] To precipitate the alkaline earth ions, the water sample is preferably introduced into an electrolysis cell containing at least two electrolysis electrodes, which are supplied with direct current 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 continuously passed through the electrolysis cell between the two electrolysis electrodes in a flow-through operation, or introduced in a batch operation between the two electrolysis electrodes into the then-closed electrolysis cell.

[0023] To determine the conductivity of the water sample before, during, and / or after the precipitation of alkaline earth ions, the electrolysis cell advantageously includes at least one conductivity sensor, which is preferably integrated into the electrolysis cell. This allows for a compact design. The conductivity sensor integrated into the electrolysis cell preferably comprises two measuring electrodes, which are subjected to an alternating voltage to determine the conductivity of the water sample.

[0024] To avoid interference, it is advantageous if the electrolysis cell alternates between electrolytic precipitation of alkaline earth ions and conductivity measurement of the water sample in a single 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.

[0025] In a preferred embodiment of the method according to the invention, a specific volume of a water sample, 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 initial conductivity Lf 1 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 integrated into the electrolysis cell is switched off during this 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 water and to record the conductivity of the water sample at specific points or quasi-continuously during precipitation 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.

[0026] In measurement mode, a DC voltage with a first polarity is applied to the electrolysis electrodes during electrolysis cycles to electrolyze the water sample in the electrolysis cell and thereby precipitate the alkaline earth ions. During the electrolysis cycles, the precipitated carbonates, particularly calcium and magnesium carbonate, are deposited on the electrolysis electrodes (especially the cathode), which can distort the measurement results if the deposit thickness becomes too great. Therefore, preferably in a regeneration mode, which is initiated particularly after a certain operating time of the device and / or a certain number of measurement cycles or at specific times, a DC voltage with a polarity opposite to the first polarity is applied to the electrolysis electrodes to remove the carbonate deposits on the cathode.

[0027] To account for the influence of temperature on the conductivity of the water sample, the temperature of the water sample is preferably measured in measurement mode, and especially during measurement cycles. When calculating the total hardness of the water sample from the measured change in conductivity (|ΔLf|), a temperature correction is preferably applied using a temperature correction factor or a temperature characteristic curve that represents the dependence of water's conductivity on temperature. The electrolysis cell preferably includes a temperature sensor for measuring the temperature of the water sample. The temperature sensor can also advantageously be integrated into the conductivity sensor.

[0028] The device according to the invention for determining the hardness of water from a water sample is particularly suitable for carrying out the method described above and comprises an electrolysis cell with at least two electrolysis electrodes and at least one conductivity sensor, which is preferably integrated into the electrolysis cell, as well as a control and evaluation unit, which is configured to first detect the original electrical conductivity (Lf 1 ) of the water sample, then apply a DC voltage to the electrolysis electrodes (A, K) with a first polarity in order to precipitate the alkaline earth ions of the water sample at least partially and preferably completely, and finally detect the conductivity (Lf 2 ) of the water sample during and / or after the precipitation of the alkaline earth ions and determine the difference between the original conductivity and the conductivity measured after the precipitation of the alkaline earth ions (ΔLf = |Lf 2 - Lf 1 |).

[0029] To determine the hardness of the water sample, the control and evaluation unit expediently includes a data storage device in which the conversion factor (F) or the characteristic curve of the conductivity of water as a function of the carbonate hardness of the water is stored. The control and evaluation unit accesses the conversion factor (F) or the characteristic curve and is preferably configured to calculate the total hardness of the water sample from the detected change in conductivity (ΔLf) and the conversion factor (F) or the characteristic curve, using a hardness scale assigned to the conversion factor (F) or the characteristic curve.

[0030] 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 and are advantageously designed as flat electrodes. This allows for a sufficient volume of the electrolysis cell to accommodate the water sample while maintaining a compact device design. 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, titanium, or graphite foil. Particularly preferred is the anode and the cathode made of the same material, especially platinized titanium.

[0031] To determine the conductivity of the water sample before and / or during and after the precipitation of alkaline earth ions, the device includes at least one conductivity sensor. Preferably, at least one conductivity sensor is integrated into the electrolysis cell, 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 one or more conductivity sensors into the electrolysis cell allows for a compact and space-saving design of the device. However, it is also possible for at least one conductivity sensor and a second conductivity sensor to be arranged outside or even inside the electrolysis cell. For example,a first conductivity sensor may be placed at an input, in particular upstream of the input of the electrolysis cell, and a second conductivity sensor may be placed at an output, in particular downstream of the output of the electrolysis cell.

[0032] Other conductivity sensors, such as high-frequency titration sensors, can also be used to measure electrical conductivity.

[0033] In a preferred embodiment of the device, which is particularly suitable for carrying out the above-described process in batch operation, the device comprises a single conductivity sensor integrated into the electrolysis cell, with two measuring electrodes arranged between the opposing electrolysis electrodes of the electrolysis cell and, in particular, designed as rod electrodes. This enables cost-effective manufacturing of the device in a compact design.

[0034] In one embodiment of the device according to the invention, the water sample can flow continuously through the electrolysis cell as a flow-through stream during measurement mode. The water flows, in particular, between the two opposing electrolysis electrodes from an inlet to an outlet through the electrolysis cell. The initial conductivity Lf 1 of the water sample is measured upstream of the electrolysis cell or at an inlet of the electrolysis cell using a first conductivity sensor. This can be done continuously, particularly with a first conductivity sensor located outside the electrolysis cell or at the inlet of the electrolysis cell, or in predetermined measurement cycles, particularly with a first conductivity sensor located inside the electrolysis cell. As the water stream passes from the inlet of the electrolysis cell to the outlet, the alkaline earth ions are precipitated, at least partially, and preferably completely.A second conductivity sensor is arranged at the outlet of the electrolysis cell or downstream of the electrolysis cell, with which the conductivity Lf₂ of the water flow is measured. The change in conductivity ΔLf = |Lf₂ - Lf₁| caused by the precipitation of alkaline earth ions is determined from the difference between the conductivities measured at the inlet and outlet of the electrolysis cell. If the change in conductivity ΔLf thus determined exceeds a predetermined limit, it can be concluded that hardness-causing alkaline earth ions are present in the water flowing into the electrolysis cell at the inlet, which have been at least partially precipitated during the flow through the electrolysis cell, resulting in the detected change in conductivity ΔLf. With this embodiment of the device according to the invention, for example,In water softening systems, detect hardness breakthroughs that can occur when the ion exchanger(s) become exhausted.

[0035] To accelerate the precipitation of alkaline earth ions during measurement cycles, 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, 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 alkaline earth ions during measurement cycles and therefore reduces the measurement time, i.e., the time until the alkaline earth ions are completely precipitated.

[0036] The device according to the invention is particularly suitable for use in a water treatment plant, such as a water softening plant or a desalination plant, and especially in a reverse osmosis (RO) plant, a membrane capacitive deionization (MCDI) plant, or a nanofiltration (NF) plant, to determine the hardness of raw water supplied to the water treatment plant and / or the hardness of water treated in the water treatment plant, in particular softened or desalinated water. The device can also be used in water softening plants or desalination plants to detect hardness breakthroughs, which can occur, for example, when ion exchangers become exhausted or when a (filter or osmosis) membrane becomes clogged.The device according to the invention can be integrated into the water treatment plant to perform an in-situ determination of the hardness of the raw water and / or the treated water during operation of the water treatment plant. However, it is also possible to design the device as a mobile measuring device, which can be used, for example, to adjust the operating parameters during the initial commissioning of a water treatment plant by determining the hardness of the raw water and / or the treated water. Further applications of the method and the device include, for example, determining the hardness of water supplied to a washing machine or dishwasher in order to adjust or automate the dosage of a detergent or cleaning agent and / or a descaling agent based on the determined hardness of the supplied water.

[0037] With the method and device according to the invention, the total hardness of a water sample whose total hardness corresponds to the carbonate hardness (temporary hardness) can be determined with sufficient accuracy for the applications mentioned above. Unlike, for example, thermal precipitation, the electrolytic precipitation of the alkaline earth ions utilizes the entire deposition capacity of the alkaline earth ions, thereby enabling the determination of the total carbonate hardness and thus, at least approximately (and at least for most drinking water qualities), the total hardness of the water in the sample. The method and device enable in-situ detection of the change in conductivity caused by the precipitation of the alkaline earth ions and, based on this, an in-situ determination of the water hardness in the sample in an automated process without the use of chemicals.

[0038] These and other advantages and applications as well as preferred features of the method and device according to the invention will become apparent from the exemplary embodiments of the invention described below with reference to the drawings.

[0039] This shows: Fig. 1: a schematic representation of a first embodiment of a device according to the invention for determining the hardness of water from a water sample; Fig. 2: a schematic representation of a second embodiment of a device according to the invention; Fig. 3: a schematic representation of a third embodiment of a device according to the invention; Fig. 4: a schematic representation of an application example for the inventive method and the inventive device for determining the hardness of water which has been produced as blended water in a water softening plant; Fig. 5:a schematic representation of a further application example of the method and device according to the invention for determining the hardness of water treated in a water treatment plant; Fig. 6: a diagram of a typical time course of the electrical conductivity of a water sample during the execution of the method according to the invention; Fig. 7: a diagram of the time course of the electrical conductivity of a water sample recorded in an application of the method according to the invention, as well as the hardness of the water sample determined using the method according to the invention.

[0040] In Figure 1 A schematic representation of a first embodiment of a device according to the invention for determining the hardness of a water sample is shown. The device of Figure 1The electrolysis cell 1 comprises an electrolysis cell with a container 5, which has an access point (not shown) for introducing the water sample and for draining the water sample after hardness determination. The access point is closable, so that the water is contained in a sealed volume after the water sample has been introduced into the container 5. In this embodiment, the electrolysis cell 1 is used to determine the hardness of the water sample in batch operation, in which the hardness of a predetermined volume of water sample is determined.

[0041] 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 forming the anode (A) and the other the cathode (K). The two electrolysis electrodes A and K are connected to a direct current source (DC), as shown in Figure 1 As indicated. The two electrolysis electrodes A, K are supplied with direct current via the DC source. By supplying the two electrolysis electrodes A, K with direct current, the water of the water sample located in container 5 of the electrolysis cell 1 is electrolyzed, whereby the alkaline earth ions present in the water are deposited as carbonates at the cathode (K) and are thereby precipitated from the water.

[0042] A conductivity sensor 2 with 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 alternating voltage with a predetermined frequency via the AC voltage source. The electrode pair 3 acts as a conductivity sensor 2, with which the conductivity of the water in the container 5 can be measured. The two measuring electrodes of the electrode pair 3 are located in the Figure 1 The example shown is designed as rod electrodes, the longitudinal axis of which runs parallel to the plane of the two electrolysis electrodes A, K designed as flat electrodes.

[0043] The in Figure 1The electrolysis cell 1 shown, with its two electrolysis electrodes A and K and the conductivity sensor 2 integrated within it, is coupled to a control unit (not shown in the drawing). This control unit regulates the supply of direct current to the electrolysis electrodes A and K during predefined electrolysis cycles and measures the electrical conductivity of the water in the container 5 using the conductivity sensor 2 by applying an alternating voltage to the electrode pair 3 of the conductivity sensor 2 at defined measurement intervals.Preferably, the control and regulating device is configured such that measurement cycles with a predetermined measurement duration and electrolysis cycles with a predetermined electrolysis duration are carried out alternately, wherein preferably an alternating electrical voltage is applied to the electrode pair 3 of the conductivity sensor 2 during a measurement cycle, 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.

[0044] The control unit 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 carbonate hardness on a hardness scale. The data storage device can also contain several such characteristic curves, with each curve representing, for example, the electrical conductivity of water as a function of its carbonate hardness on different hardness scales, such as the German hardness scale (°dH) or the French hardness scale (°fH).Since the characteristic curve exhibits a linear progression of electrical conductivity as a function of the carbonate hardness of the water, the proportionality factor, which indicates the slope of this linear progression, can also be stored in the memory of the control unit as a conversion factor F, either in addition to or instead of the characteristic curve. A method for determining the characteristic curve and the conversion factor F will be explained below.

[0045] To determine the hardness of the water sample in container 5 of electrolysis cell 1, an alternating voltage at a predetermined frequency is first applied to the electrode pair 3 of the conductivity sensor 2 while electrolysis electrodes A and K are de-energized. The current flowing through the measuring electrodes of electrode pair 3 is then measured to determine the initial conductivity Lf1 of the water. This measurement constitutes the first measurement cycle. The alternating voltage supply to 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 direct current voltage to the two electrolysis electrodes A and K, the water in container 5 is electrolyzed, and simultaneously the alkaline earth ions present in the water begin to precipitate as carbonates, particularly calcium carbonate and magnesium carbonate. 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 alternating current 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.

[0046] The resulting time course of the electrical conductivity of the water in container 5 is illustrated by an example in Figure 6 shown. In the time course of the electrical conductivity of the Figure 6 The individual measurement cycles M, in which the conductivity of the water does not change, are visible. 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. Figure 6The transition point marked U indicates the point at which the conductivity of the water, as measured by conductivity sensor 2, increases abruptly or gradually. The increase in electrical conductivity at the transition point U is presumably due to a sudden rise in pH and a resulting excess of anions (CO₃²⁻ and OH⁻) with higher equivalent conductivity compared to the hydrogen carbonate ions after completion of cathodic precipitation.

[0047] 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 = |Lf 2 - Lf 1| caused by the precipitation of the alkaline earth ions can be calculated from the difference between the conductivity measured before the start of electrolysis (Lf 1) and the conductivity measured after complete precipitation of the alkaline earth ions (Lf 2). 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 container 5 can be determined.For this purpose, the control and regulating device is programmed to calculate the difference between the measured values ​​of the original conductivity Lf 1 and the conductivity of the water after complete precipitation of the alkaline earth ions Lf 2, ΔLf = |Lf 2 - Lf 1 |, and to determine the water hardness 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 and regulating device as follows: H = ΔLf / F , where H is the total 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 conductivity of water as a function of the carbonate hardness and is in particular the proportionality factor of a linear course of this characteristic curve.

[0048] The conversion 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 in the figure. Figure 6 This is evident. 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.

[0049] The conversion factor F can be determined experimentally on a drinking water sample with a sufficiently high water hardness of preferably more than 3°dH and an excess of hydrogen carbonate ions compared to the alkaline earth ions using the following steps: 1. Measurement of the original conductivity (conductance Lf A ) and temperature (T) of the drinking water sample and determination of the conductivity Lf A (T 0 ) of the drinking water sample at a standard temperature T 0 of e.g. 15°C using a temperature correction factor; 2. Performance of a water analysis to determine the total hardness H of the drinking water sample on a specified hardness scale, e.g. the German hardness [°dH], by means of complexometric titration, e.g. with ethylenediaminetetraacetate (EDTA) according to DIN 38406-3, Part E3; 3. Electrolysis of the drinking water sample with the initial conductance Lf A (T 0 ) under defined boundary conditions, in particular the temperature, and recording of the conductivity of the drinking water sample during electrolysis to determine a conductance curve over time, whereby the conductance decreases linearly with time and at a certain time t U a minimum of the conductivity (conductance minimum Lf min ) is reached at a transition point U (see Figure 6), which is recorded and converted to the conductivity at the standard temperature T 0 using the temperature correction factor in order to obtain the conductance minimum Lf min (T 0 ) at the standard temperature T 0; 4. Optionally, further electrolysis under the same boundary conditions at the time t U of the transition point U, now known from step 3, to verify the curve and improve the accuracy of the determined conductance minimum Lf min, and termination of the electrolysis after reaching the transition point U; 5.Taking a partial volume of the drinking water sample after reaching the transition point U and performing a titrimetric water analysis of the taken sample to determine the total hardness of the drinking water sample after electrolysis, wherein the titrimetric water analysis is preferably carried out using the same method as in step 2 and a hardness of 0 is recorded if, in step 3, at least largely complete precipitation of the alkaline earth ions has occurred (after reaching the transition point U); 6. Calculation of the conversion factor F from the difference in conductivity. Leitwertdifferenz ΔLf T 0 = Lf A T 0 − Lf min T 0 mit der Formel F = ΔLf T 0 / H .

[0050] The boundary condition that the drinking water sample contains an excess of bicarbonate ions compared to alkaline earth ions can be met by conditioning the drinking water sample before determining the conversion factor, for example by adding bicarbonate ions to the drinking water sample or by partially softening the drinking water sample. This ensures that the total hardness of the conditioned drinking water sample corresponds to the carbonate hardness.As an alternative to conditioning the drinking water sample, a drinking water sample that is not specifically preconditioned to have an excess of hydrogen carbonate ions compared to alkaline earth ions can also be used if, in steps 2 and 5, the change in carbonate hardness ΔH C of the drinking water sample is determined, which results from the precipitation of the carbonates, and the conversion factor is determined from the change in conductivity (ΔLf) and the change in carbonate hardness (ΔH C) caused by the precipitation using the formula F = ΔLf (T 0 ) / ΔH C.

[0051] Using this method, an average value for the conversion factor F was obtained from empirical measurements of a large number of drinking water samples from Germany of different origins and compositions at a temperature of T 0 = 15°C, for example. F = 30 μS / cm ° dH = 0 , 030 mS / cm ° dH determined.

[0052] From the in Figure 6Based on the time course of the electrical conductivity of a water sample shown, the hardness of the water in this sample can therefore be calculated as follows: H = ΔLf / F = 0 , 70 − 0 , 59 mS / cm 0 , 030 mS / cm ° dH = 3 , 6 ° dH

[0053] In the Figures 2 and 3 Two further embodiments of a device according to the invention with an electrolysis cell 1 are shown, wherein the electrolysis cell 1 contains two separate conductivity sensors 2a, 2b. A first conductivity sensor 2a is arranged at a closable input 15a of the electrolysis cell 1 and a second conductivity sensor 2b is arranged at a closable output 15b of the electrolysis cell 1. Each conductivity sensor 2a, 2b comprises a pair of electrodes 3a, 3b with two measuring electrodes. In the exemplary embodiment of Figure 2 the two electrolysis electrodes A, K are arranged, which, as in the exemplary embodiment of Figure 1, designed as flat electrodes and arranged opposite each other at a predetermined distance. The in Figure 2The illustrated embodiment of the electrolysis cell 1 can be used to determine the hardness of a water sample that flows continuously through the electrolysis cell 1 from inlet 15a to outlet 15b in a flow-through operation, with measurement cycles and electrolysis cycles being performed continuously and simultaneously during the flow of the water sample. As the water sample flows through the electrolysis cell 1, the alkaline earth ions in the water sample are at least partially precipitated.During the flow of the water sample through the electrolysis cell 1, the conductivity of the water stream at inlet 15a and outlet 15b is continuously measured by the two conductivity sensors 2a and 2b. The first conductivity sensor 2a measures the initial conductivity Lf1 of the water sample at inlet 15a (before the electrolytic precipitation of the alkaline earth ions), while the second conductivity sensor 2b measures the electrical conductivity Lf2 of the water stream at outlet 15b of the electrolysis cell 1 (after precipitation of the alkaline earth ions). The control and evaluation unit 14 calculates the difference ΔLf = |Lf2 - Lf1| from the measured values ​​Lf1 and Lf2, thus determining the change in conductivity caused by the precipitation of the alkaline earth ions.The calculated difference in electrical conductivity ΔLf of the water sample before and after precipitation of the alkaline earth ions is compared with a predefined limit value in the control and evaluation unit 14. If the measured difference in electrical conductivity ΔLf exceeds this limit value, it can be concluded that a certain quantity of hardness-causing alkaline earth ions is present in the water.

[0054] To determine the hardness of the water in the water stream, the device of the second embodiment can be used according to Figure 2 The system is switched from continuous flow operation to batch operation by closing inlet 15a and outlet 15b of the electrolysis cell. The water hardness is then determined according to the embodiment of the Figure 1 described measurement mode.

[0055] In the Figure 3In the illustrated embodiment, several electrolysis electrodes A, K are arranged in the electrolysis cell 1. In particular, the electrolysis cell 1 of the embodiment contains Figure 3 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 Figure 3As can be seen, a cation exchange membrane KAT is arranged between each corresponding electrolysis electrode A, K. The multiple electrolysis electrodes A, K, and 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 / or allows the flow rate of the water sample through electrolysis cell 1 to be increased.

[0056] In Figure 4Figure 1 shows an application example of the method and device according to the invention, wherein a device according to the invention with an electrolysis cell 1, which has a container 5 with an inlet 15a and an outlet 15b, is integrated into a water softening device 10. The Figure 4The water softening device 10, shown schematically, comprises, in addition to the device according to the invention, a softening unit 12 containing a regenerable ion exchanger 13. The ion exchanger 13 is arranged in an ion exchanger tank 8, which has an inlet 11a and an outlet 11b. For regenerating the ion exchanger 13 of the softening device 12, the water softening device 10 includes a regeneration unit 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 into and through the ion exchange tank 8 in a regeneration mode, whereby the ion exchanger 13 is replaced by sodium ions by the exchange of the calcium and magnesium ions bound therein during the softening of the raw water R.After the regeneration mode has ended, the used regeneration solution is directed into a channel 19 via a discharge line 23.

[0057] The water softening unit 10 further comprises a raw water inlet 11 for supplying raw water R, wherein the raw water inlet 11 is connected to the inlet 11a of the softening device 12 via a control head 24. A conductivity sensor 16 is arranged in the raw water inlet 11 to detect the conductivity of the supplied raw water R. An adjustable blending valve v1 is provided for generating blended water V, which can be controlled by the control unit 14. The control head 24 serves to control the flow rates 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 also controlled by the control unit 14 for this purpose.The control head 24 together with the blending valve v1 forms a blending device 24, v1, in which blending 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 specified by the position of the controllable valve v1.

[0058] 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 unit 24, v1, the softened water W is mixed with raw water R in a defined mixing ratio to produce blending water V. The blending water V should have a predetermined target hardness, for example, H = 5 °dH. The hardness of the blending 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 blending water V produced in the blending unit 24, v1 is conveyed in the blending water line 20 and directed to a consumer line 22 via a consumer line 21 connected to the blending water line 20.Consumer 22 could, for example, be the drinking water installation of a household or a water appliance to which the blended water is directly supplied.

[0059] To determine and adjust a suitable mixing ratio, selected so that the blended water V has a hardness that corresponds as closely as possible to the specified target hardness, e.g., 3°dH or less, the water softening unit has a control and evaluation unit 14, which is coupled to the blending unit 24, v1. This unit allows the mixing ratio to be adjusted by setting controllable valves of the blending unit 24, v1 so that the hardness of the blended water V corresponds to the specified target hardness. To set an initial mixing ratio at which the hardness of the blended water V corresponds at least approximately to the target hardness, the control and evaluation 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, the control and evaluation unit 14 calculates an initial mixing ratio by referring to a characteristic curve that describes the course of the electrical conductivity of water as a function of the water hardness.

[0060] To fine-tune the mixing ratio, the exact hardness of the blended water V is determined using the water hardness measuring device integrated into the water softener. For this purpose, the blended water V is routed via the blended water line 20 through an adjustable valve V2 to the inlet 15a of the electrolysis cell 1. In the electrolysis cell 1, the electrical conductivity of the blended water V is measured using the conductivity sensor 2 integrated into the electrolysis cell 1. Subsequently, with the electrolysis cell 1's container 5 closed, the blended water V is electrolyzed by the electrolysis electrodes A and K by applying a direct current voltage to these electrodes. This causes the alkaline earth ions contained in the blended water V to precipitate out of 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 the electrolysis cell 1 is measured with the integrated conductivity sensor 2 to detect the transition point U, which indicates a complete precipitation of the alkaline earth ions, as shown above using the apparatus of . Figure 1As described above, 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 caused by the precipitation of the alkaline earth ions is determined by calculating the difference with the original conductivity of the blended water V. For this purpose, the control and evaluation unit 14 is coupled to the conductivity sensor 2 to obtain the measured values ​​of the conductivity before, during, and after the precipitation of the alkaline earth ions. The control and evaluation unit 14 calculates the exact hardness of the blended water V from the measured difference in electrical conductivity ΔLf, using the conversion factor F stored in a memory of the control and evaluation unit 14, as described above.The blended water V electrolyzed in the electrolysis cell 1 can be drained from the container 5 into a channel 19 via an outlet 15b after the measurement has been completed.

[0061] From the determined hardness of the blended water V, the total hardness of the raw water can also be calculated using a hardness balance calculation, provided the mixing ratio of raw water R and softened raw water, or the volume fraction of raw water R in the blended water V, is known. This calculation assumes that the blended water is produced by mixing raw water and fully softened raw water (soft water), which has a total hardness of zero.

[0062] Based on the precise hardness of the blended water V, determined by the control and evaluation unit 14, the control and evaluation unit 14 controls the blending unit 24, v1 to adjust the mixing ratio of the raw water with the softened water W so that the blended water V has a hardness corresponding to the specified target hardness. This readjustment of the mixing ratio preferably occurs iteratively in a feedback process to establish an optimal mixing ratio where the hardness of the blended water V corresponds as closely as possible to the specified target hardness. Changes in the hardness of the raw water R, detected by the conductivity sensor 16, which can occur, for example, when the water quality of the drinking water supplied by the public water supply changes, can also be taken into account by adjusting the mixing ratio to the changed hardness of the raw water R.Furthermore, the timing of regeneration of the water treatment plant can be controlled based on the specific hardness R of the raw water.

[0063] For example, if raw water R with a total hardness of HR = 15°dH is completely softened by a water softening system, and the softened raw water (soft water with 0°dH total hardness) is mixed with the raw water R to form a blended water V with a raw water content of 1 / 3, a blended water V with a total hardness of HV = 15°dH · 1 / 3 = 5°dH is produced. Therefore, if the total hardness HV of a blended water V with a raw water content of 1 / n is determined using the method according to the invention, the total hardness of the raw water HR = n · HV can be determined from this.

[0064] In Figure 7 An example is 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 softening device of the Figure 4The diagram shows the changing hardness of the raw water R supplied via the raw water inlet 10, as well as the time course of the hardness HV of the blended water V (determined from the conductivity of the blended water V) and the hardness HR of the raw water R. From the start of the measurement at time t = 0, raw water R with a specific raw water hardness HR was supplied to the water softening device 12. From time t = 1 until time t = 3, raw water R' with a changed composition and a higher hardness HR' was supplied to the water softening device 12, and from time t = 3 onwards, the original raw water R was supplied again. This allows for the detection of sudden changes in the composition or water quality, and especially the hardness, of tap water supplied from the public drinking water network, which frequently occur in practice.Throughout the entire measurement period between t = 0 and t ≅ 9 h, the conductivity Lf R (t) of the raw water R or R' was continuously recorded using the conductivity sensor 16. At time t 1, an increase in the conductivity Lf R (t) of the raw water was observed due to the higher hardness of the modified raw water R'. From time t 0 onwards, a measurement mode was implemented to determine the hardness HV of the blended water V and, from this, the hardness HR of the raw water R. The mixing ratio of raw water R with softened water W was then adjusted so that the hardness HV of the blended water V corresponds to a predetermined target hardness of approximately 3°dH.Based on a recorded change in the conductivity Lf R (t) of the raw water at times t 1 and t 3, a measurement of the conductivity of the blended water was initiated in batch operation at times t 2 and t 4 in electrolysis cell 1, followed by precipitation of the alkaline earth ions by electrolysis of the blended water in the electrolysis cell, and, as described above, the conductivity Lf V of the (electrolyzed) blended water and the change in conductivity ΔLf during electrolysis were recorded, and from this the hardness of the blended water HV and the hardness HR' of the changed raw water R' were determined.

[0065] In Figure 5A further application example for the device according to the invention in a water treatment plant 10 is schematically illustrated, wherein the water treatment plant 10 is a membrane capacitive deionization module (MCDI) 12' which produces partially demineralized water TE from the raw water R supplied via the raw water inlet 11 by means of membrane capacitive deionization, which is conveyed via a consumer line 21 to a consumer 22. The hardness of the partially demineralized water TE is thereby determined as in the application example of the Figure 4 The temperature is determined by means of a device according to the invention, to which the partially demineralized water TE is supplied via line 20. Otherwise, the construction and, in particular, the design and function of the electrolysis cell 1 correspond to the embodiment of the Figure 4 The deionization module (MCDI) 12' can also be used – like the water softener of the Figure 4- have a blending device for producing partially demineralized blending water V by mixing fully demineralized raw water with raw water, wherein the control for adjusting the hardness of the blending water V to a predetermined target hardness corresponds to the embodiment of the Figure 4 corresponds.

[0066] In the following Table 1 Preferred ranges and particularly preferred values ​​for the parameters of the method according to the invention are specified: Table 1 parameter Preferred areas Particularly preferred parameters Electrolysis cell volume 5-200 ml 18 ml Electrode surface of the electrolysis electrodes 0.3 cm² (rod electrodes) 10 cm² up to 110 cm² (flat electrodes) Electrode geometry of the electrolysis electrodes Flat electrodes, rod electrodes, expanded metal grids Flat electrodes Number of electrolysis electrodes 2 One anode and one cathode each Height x width x thickness of the electrolysis electrodes 15 x 67 x 1 mm Electrolysis electrode material: Anode oxidation-stable materials Platinum-plated titanium (1.5µm) Material of the electrolysis electrodes: Cathode alkali-resistant materials Graphite foil 1 mm with PTFE content of 15% (PV15) Platinum-plated titanium (1.5µm) Distance between the electrolysis electrodes 1 mm - 30 mm 18 mm Electrical parameters of the The electrolysis electrode Type of current Direct current with constant voltage (CV) or constant current (CC) CV Tension 2 - 15 V CV: 5-14.5 V current 5 - 400 mA 20 - 100 mA Electrical parameters of the measuring electrodes of the conductivity sensor Type of current Alternating current Voltage alternating current 5 - 20 V 15 V Frequency alternating current 1 - 20 kHz 2 kHz

Claims

1. Method for determining the hardness of water in a water sample containing alkaline earth ions, in particular calcium and / or magnesium ions, as well as hydrogen carbonate ions in excess of the alkaline earth ions, comprising the following steps: - measuring the original electrical conductivity (Lf1) of the water sample, - precipitating the alkaline earth ions by electrolysis of the water sample in an electrolysis cell, - detecting the conductivity (Lf2) of the water sample after a complete or at least largely complete precipitation of the alkaline earth ions and detecting the change in conductivity (ΔLf = |Lf2 - Lf1|) of the water sample caused by the precipitation of the alkaline earth ions, - determination of the total hardness of the water in the water sample from the measured change in conductivity (ΔLf) and a predetermined conversion factor (F) or a predetermined characteristic curve of the change in conductivity of water as a function of the change in hardness of the water upon precipitation of the carbonate hardness.

2. Method according to claim 1, characterised in that the water sample is treated to adjust the excess of hydrogen carbonate ions relative to the alkaline earth ions before the precipitation of the alkaline earth ions by mixing with softened water and / or by adding hydrogen carbonate ions and / or carbonate ions.

3. Method according to any one of the preceding claims, characterised in that the conversion factor (F) is a proportionality factor for a linear relationship between the conductivity of water and the absolute total hardness or the absolute carbonate hardness of the water and, in particular, characterises the linear course of the change in the electrical conductivity of a water sample as a function of the change in hardness of the water sample on a hardness scale, in particular the German water hardness, in the event of precipitation of the carbonate hardness.

4. Method according to any one of the preceding claims, characterised in that the water sample is produced by mixing raw water (R) with softened raw water (W) in a predetermined mixing ratio, whereby the total hardness of the raw water (R) is determined from the recorded change in conductivity (ΔLf) of the water sample, taking into account the mixing ratio.

5. Method according to one of the preceding claims, characterised in that the conductivity of the water sample is measured during the electrolytic precipitation of the alkaline earth ions up to an inflection point (U) at which the measured conductivity exhibits a minimum value (Lf2) due to an at least largely complete precipitation of the alkaline earth ions, and optionally thereafter, during further electrolysis of the water sample, an increase in the detected conductivity of the water sample is observed, wherein the conductivity of the water sample is detected during the electrolytic precipitation of the alkaline earth ions, in particular until at least 90% of the alkaline earth ions, preferably more than 95%, have been precipitated by the electrolysis.

6. Method according to any one of the preceding claims, characterised in that the water sample is introduced into an electrolysis cell (1) for the precipitation of the alkaline earth ions, wherein the electrolysis cell (1) contains at least two electrolysis electrodes (A, K), which are supplied with direct current for the electrolytic precipitation of the alkaline earth ions, and at least one conductivity sensor (2), wherein the at least one conductivity sensor (2) is integrated into the electrolysis cell (1) or a first conductivity sensor (2a) is arranged at an inlet (15a) and a second conductivity sensor (2b) is arranged at an outlet (15b) of the electrolysis cell (1).

7. Method according to claim 6, characterised in that the water sample is introduced into the electrolysis cell (1) in a batch mode for the precipitation of the alkaline earth ions, wherein an electrolytic precipitation of the alkaline earth ions and a measurement of the conductivity of the water sample are performed alternately in the electrolysis cell (1).

8. Method according to one of claims 6 or 7, wherein in a measurement mode, a direct voltage is applied to the electrolysis electrodes (A, K) with a first polarity and, in a regeneration mode, which is initiated in particular after a certain operating time of the device and / or a certain number of measurement cycles performed or at certain times, a direct voltage is applied to the electrolysis electrodes (A, K) with a polarity opposite to the first polarity.

9. Method according to any one of the preceding claims, characterised in that during the electrolytic precipitation of the alkaline earth ions, the temperature of the water sample is measured and, when determining the total hardness of the water sample, the detected change in conductivity (ΔLf) is subjected to a temperature correction via a temperature correction factor or a temperature characteristic curve.

10. Device for measuring the hardness of water in a water sample containing alkaline earth ions, in particular calcium and / or magnesium ions, as well as hydrogen carbonate ions in excess of the alkaline earth ions, wherein the device comprises an electrolysis cell (1) with at least two electrolysis electrodes (A, K), at least one conductivity sensor (2), and a control and evaluation device, wherein the control and evaluation device is designed such that it first detects the original electrical conductivity (Lf1) of the water sample, then at least partially and preferably completely or at least largely completely precipitates the alkaline earth ions of the water sample by applying a direct voltage with a first polarity to the electrolysis electrodes (A, K), and finally detects the conductivity (Lf2) of the water sample during and / or after the precipitation of the alkaline earth ions and determines the amount of the difference between the original conductivity and the conductivity measured after the precipitation of the alkaline earth ions (ΔLf = |Lf2 - Lf1|).

11. Device according to claim 10, characterised 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) are connected or can be connected to a direct current source and the measuring electrodes are preferably arranged between the opposing electrolysis electrodes (A, K) of the electrolysis cell (1) and are preferably designed as rod electrodes.

12. Device according to claim 10 or 11, characterised in that the at least one conductivity sensor (2) is integrated into the electrolysis cell (1) or a first conductivity sensor (2a) is arranged at an inlet (15a) and a second conductivity sensor (2b) is arranged at an outlet (15b) of the electrolysis cell (1), wherein the conductivity sensor (2) integrated in the electrolysis cell (1) or each conductivity sensor (2a, 2b) preferably comprises at least one electrode pair (3) with two measuring electrodes, wherein the measuring electrodes are connected or connectable to an AC voltage source.

13. Device according to any one of claims 10 to 12, characterised in that a cation exchange membrane (KAT) is arranged between two opposing electrolysis electrodes (A, K), wherein the cation exchange membrane (KAT) runs in particular parallel to the electrolysis electrodes (A, K) designed as flat electrodes.

14. Device according to any one of claims 10 to 13, characterised in that the control and evaluation device contains a data memory in which a conversion factor (F) and / or a characteristic curve of the conductivity of water as a function of the total hardness or carbonate hardness of the water is stored, wherein the control and evaluation device is set up such that, in a measurement mode, accesses the stored conversion factor (F) stored therein and calculates the total hardness of the water on a hardness scale assigned to the conversion factor (F) or the characteristic curve from the detected change in conductivity (ΔLf) and the conversion factor (F) or the characteristic curve, and the control and evaluation device is preferably configured such that, in a regeneration mode which is initiated in particular after a certain operating period of the device and / or a certain number of measurement cycles performed or at certain times, applies a direct voltage to the electrolysis electrodes (A, K) with a polarity opposite to the first polarity.

15. Use of the device according to any one of claims 10 to 14 in a water treatment plant, in particular in a water softening plant or a desalination plant and in particular in a reverse osmosis plant (RO) or a membrane capacitive deionisation plant (MCDI) or a nanofiltration plant (NF), for determining the hardness of raw water supplied to the water treatment plant and / or for determining the hardness of water treated in the water treatment plant, in particular softened or desalinated water.