The present invention refers to a water softening device and to a method to operate a water softening device.

Water softening devices often comprise a filter which is filled with a strong acid cationic (SAC) ion exchange (lEX) material, regenerated under the sodium form. A raw water flows through a bed of the material, where calcium and magnesium ions (the hardness causing ions or - in short - the "hardness") are exchanged against sodium ions. The exchange reaction takes place because the lEX material exhibits a higher affinity to calcium and magnesium ions than to sodium ions. The following reaction occurs during the softening step:

2 R-SOs-Na + Ca ²⁺ -> (R-S0 ₃ ) ₂ -Ca + 2 Na ⁺

When all the lEX material is loaded with hardness, the capacity of the filter has been reached and the material bed needs to be regenerated, back to the sodium form. The regeneration exchange reaction is accomplished by exposing the material to an excess of sodium ions in order to reverse the exchange equilibrium according to the following reaction:

(R-S0 ₃ ) ₂ -Ca + 2 Na ⁺ -> 2 R-S0 ₃ -Na + Ca ²⁺

For a given amount of lEX material in the filter, the amount of brine used during the regeneration usually varies, depending inter alia on the application and on the lEX material capacity. For domestic applications a regeneration with an amount of about 90 g NaCl per liter of lEX material is typical, resulting in a raw operating lEX material capacity of about 1.1 eq/l. For industrial applications typically the amount of brine used is about 180 g of NaCl per liter of lEX material, resulting in a raw operating lEX material capacity of around 1.5 eq/l.

For a given raw water composition and a defined volume of lEX material, the capacity or volume of raw water which can be treated with the filter can be determined according to the following equation:

VRW ⁼ ClEX * VIEX/ THRW wherein

^• VRW ^' \S the volume of raw water to be treated [I],

^• CIEX ^' \S the specific capacity of the lEX material [eq.*! ¹ ]

^• VIEX ^' \S the volume of the lEX material volume in the filter [I]

^• THRW ^' \S the total hardness ([Ca ²⁺ ] + [Mg ²⁺ ]) in the raw water [meq.*! ¹ ]

As a concrete example, the following parameters are typical for a domestic application:

^• CIEX= 1.1 eq.*! ^"1 (regenerated with 90 g NaCl per liter lEX material)

^• TH _RW = 5.2 meq.*! ^"1

In this case for the capacity or volume of raw water which can be treated results:

V _RW = 1.1 *25/5.2 * 10 ³ = 52881 (about 5.3 m ³ )

This means that after a volume of about 5.3 m ³ of raw water the lEX material bed would be exhausted. When this capacity is reached, the lEX material should be regenerated.

In practice there are some additional parameters on which an lEX material capacity depends. Such parameters are, for example, temperature, flow speed though the material bed, influent sodium concentration compared to the total cationic load, content of total dissolved solids (TDS) in the raw water, concentration of the brine which is used during the regeneration step and the hardness level allowed at the end of the cycle. In order to take the influence of all those parameters into account, a safety coefficient can be applied in order to ensure that the regeneration is initiated prior to a hardness breakthrough.

To illustrate, in the previous example, the safety coefficient could be integrated into the C _tE x in the form of a safety margin of 10 %. For a regenerant level of 90 g of NaCl per liter of lEX material, the practical lEX material capacity to take into account would become 1.0 eq/l. For the capacity or volume of raw water which can be treated safely without the risk of a hardness breakthrough would result:

V _RW = 1.0 *25/5.2 * 10 ^'3 = 48081 (about 4.8 m ³ ) In this case the filter's practical operating capacity would be set at 4.8 m ³ . This volume corresponds to the volume of water that can be treated safely before the regeneration has to be initiated. Compared to the raw operating capacity of 5.3 m ³ above the difference is 0.5 m ³ .

This means that in practice the IEX material in the filter is always regenerated before its full capacity is reached and that about 10 % of the operating capacity is lost in each cycle. During each regeneration, a fixed amount of brine and water are consumed. The fact the filter is not used to its full capacity results in an increased consumption of brine and water compared to an operation mode according to which the filter would be used to its full capacity.

The present invention is based on the object to provide a technical solution to the described problem of capacity loss due to the regeneration of an IEX material before its full capacity is reached.

This object is achieved by the water softening device having the features of claim 1 and by the method to operate a water softening device having the features of claim 7. Preferred embodiments of the device are specified in dependent Claims 2 to 6. Preferred embodiments of the method are specified in dependent claims 8 to 11.

The present invention is based on the finding that by using an IEX material which is modified by applying specific regeneration conditions it is possible to identify the point when a filter is exhausted and hardness breakthrough will occur, to trigger a regeneration of the used IEX material. This allows to make use of the complete IEX material capacity, resulting in a decrease of the amount of salt and waste water consumption compared to a standard softener operation mode.

Specific regeneration conditions means, that besides the regenerant salt an additional tracer salt is used for the IEX material regeneration. This leads to an IEX material which is loaded with two different cationic ions species. The tracer salt must be chosen such that the IEX material shows an affinity to the cationic ions of the tracer salt which is lower than its affinity to hardness causing ions and higher than its affinity to the cationic ions of the regenerant salt. This means that during operation a chromatographic effect can gradually take place in the material bed. When the I EX material is exhausted, the cationic ions of the regenerant salt have been exchanged against hardness causing ions. However, prior to a hardness breakthrough the concentration of cationic ions of the tracer salt will increase in the water which exits from the filter with the modified IEX material. It has been found that this increase in concentration can be detected, in particular by monitoring electrical properties of the water. A water softening device according to the invention comprises a filter which is configured to remove hardness from a first stream of raw water to produce a second stream of softened water. The term hardness is to be understood as hardness causing ions in the raw water.

The device is characterized by the following features:

• The filter comprises an ion exchange (IEX) material and

• the IEX material is loaded with a first cationic ion species deriving from a tracer salt and

• the IEX material shows a lower affinity to the first cationic ion species than to the hardness and

• the IEX material is loaded with a second cationic ion species deriving from a regenerant salt and

• the IEX material shows a lower affinity to the second cationic ion species than to the first cationic ion species.

It is preferred that the first cationic ion species and the second cationic ion species differ in their ionic molar conductivities. This can be very advantageous. As explained above, the increase in concentration of the cationic ions of the tracer salt can be detected by monitoring changes in the electrical properties of the second stream (the softened water). If the two ion species differ in their ionic molar conductivities, the changes will tend to be higher. This can facilitate detection.

For the sake of clarity: Molar conductivity is the conductivity of an electrolyte solution (e.g. a solution of dissolved salt) divided by the molar concentration of the electrolyte (the dissolved salt). It is the conducting power of all the ions produced by dissolving one mole of an electrolyte in solution. The unit of the molar conductivity are Siemens per meter per molarity, or Siemens meter-squared per mole.

It is preferred that the IEX resin material is an IEX resin.

In preferred embodiments the water softening device comprises at least one of the following additional features:

• The IEX material is a strong acid cationic (SAC) ion exchange (IEX) material, in particular a strong acid cationic (SAC) ion exchange (IEX) resin.

• The raw water contains Ca ²⁺ and Mg ²⁺ as hardness causing ions.

• The first cationic ion species is K ⁺ or NH ₄ ⁺ .

• The second cationic ion species is Na ⁺ or LP. Particularly preferred are K ⁺ as first cationic ion species and Na ⁺ as second cationic ion species. The ionic molar conductivity of potassium is higher than the ionic molar conductivity of sodium. This will result in a conductivity peak as soon as all sodium ions have been exchanged but prior to the hardness breakthrough. By detecting this conductivity peak it is possible to trigger the regeneration right before or at the moment the breakthrough occurs.

In further preferred embodiments the water softening device comprises at least one of the following additional features:

• The device comprises or is coupled to a regenerant tank.

• The regenerant tank contains a regenerant comprising as a first salt the tracer salt and as a second salt the regenerant salt.

• The tracer salt is potassium chloride (KCl).

• The regenerant salt is sodium chloride (NaCl).

Regenerant tanks for water softening devices are commercially available. A suitable tank is described for example in EP 3103770 Al. It is possible that such a tank comprises a first salt with the first cationic ion species and a second salt with the second cationic ion species. Preferably the tank comprises an aqueous solution comprising the two salts in a dissolved state.

The amount of the first cationic ion species fixed on the lEX material compared to the amount of the second cationic ion species fixed on the lEX material will depend on the regenerant / tracer salt mix composition in the tank.

It is preferred that

• the molar ratio between the first salt and the second salt in the regenerant tank is in the range from 1 to 100, preferably from 1 to 25, and/or

• that 0.5 % to 20 %, preferably 2 % to 8 % of the capacity of the lEX material is loaded with the first cationic ion species and/or • that 80 % to 99.5 %, preferably 92 % to 98 % of the capacity of the I EX material is loaded with the second cationic ion species.

A good example would be the use of a potassium salt as tracer salt and of a sodium salt as regenerant salt. For example by adding a defined amount of potassium chloride (KCl) to brine (NaCl) in the regenerant tank, the I EX material will be regenerated under the sodium and potassium form.

In further preferred embodiments the water softening device comprises at least one of the following additional features:

• The device comprises or is coupled to a first and to a second tank.

• The first tank is a regenerant salt tank and comprises the regenerant salt (corresponding to the second salt above).

• The second tank is a tracer salt tank and comprises the tracer salt (corresponding to the first salt above).

• The first salt is potassium chloride (KCl).

• The second salt is sodium chloride (NaCl).

Theoretically it is also possible to provide the tracer salt in the form of an aqueous solution which is stored in the second tank and to mix it with an aqueous solution of the regenerant salt which is stored in the first tank, thereby providing a regenerant mixture consisting of the two aqueous solutions. However, in practice it is preferred to flush an IEX material during a regeneration first with an aqueous solution of the regenerant salt from the first tank and then, in a second step, with an aqueous solution of the tracer salt from the second tank.

In preferred embodiments the water softening device comprises at least one of the following additional features:

• The device comprises a first sensor for monitoring an electrical property of the first stream.

• The first sensor is configured to monitor an electrical conductivity and/or an electrical resistance.

• The first sensor is configured as an electrolytic cell capable of applying a current to the first stream.

• The device comprises a second sensor for monitoring an electrical property of the second stream. • The second sensor is configured to monitor an electrical conductivity and/or an electrical resistance.

• The second sensor is configured as an electrolytic cell capable of applying a current to the second stream.

Generally it is preferred that the first sensor and the second sensor are configured to measure an electrical conductivity. Sensors suitable for measuring the electrical conductivity of water, in particular electrolytic cells suitable for measuring the electrical conductivity of water, are known to persons skilled in the art and need no further explanation.

In a further preferred embodiment the device comprises a third sensor for measuring the volume of water which flows through the filter.

In further preferred embodiments the water softening device may be characterized by at least one of the following additional features:

• The device comprises an electronic control unit which is connected to the first sensor and to the second sensor.

• The electronic control is further connected to the third sensor.

• The electronic control unit comprises an internal data memory and a data processing unit.

It is preferred that the device is characterized by a combination of all of these features.

In further preferred embodiments the water softening device may be characterized by at least one of the following additional features:

• The device comprises a bypass line for mixing the second stream of softened water with raw water of the first stream.

• The bypass line comprises a valve to regulate the amount of water of the first stream mixed to the second stream.

Via the bypass line the second stream can be blended with water of the first stream. The method according to the invention is a method to operate a water softening device. Preferably the device operated according to this method is a device like the one described above. It comprises the steps of

• passing a first stream of raw water through a filter comprising an IEX material, wherein the filter, in particular the IEX material, is configured to remove hardness from the first stream of raw water, thereby producing a second stream of softened water,

• loading the IEX material with a first cationic ion species,

• loading the IEX material with a second cationic ion species which, and

• monitoring an electrical property of the second stream by a sensor.

The properties and preferred embodiments of the IEX material, the first cationic species, the second cationic species and the electric properties of the second stream of water have already been discussed in the context with the description of the device according to the invention. For example, also in connection with the method according to the invention the first cationic ion species and the second cationic are chosen such that the IEX resin shows a higher affinity to the first cationic ion species than to the second cationic ion species and the first cationic ion species and the second cationic ion species preferably differ in their ionic molar conductivities.

It is preferred that

• the monitored electrical property is the electrical conductivity or the electrical resistance of the second stream and/or

• that a change in the monitored electrical property is used as an indicator for the exhaustion state of the filter.

It is particularly preferred that the method may be characterized by at least one of the following additional steps or features:

• The device comprises or is coupled to a regenerant tank (in particular a tank as described above). • A change of the monitored electrical property triggers a regeneration process comprising flushing the filter comprising the IEX material with regenerant from the regenerant tank.

• The regenerant contains a tracer salt comprising the first cationic species.

• The device comprises or is coupled to a tracer salt tank.

• A change of the monitored electrical property triggers a regeneration process comprising flushing the filter in a first step with regenerant from the regenerant tank and in the second step with tracer salt from the tracer salt tank.

It is preferred that the method according to the invention is repeated cyclically. A complete cycle is composed of a phase of regular operation and a regeneration step. Thus, each regeneration step is followed by a phase of regular operation and - vice versa - each phase of regular operation is followed by a regeneration step. The phase of regular operation ends when the filter or - more particularly - the IEX material contained therein - is exhausted. This time point is indicated by the change in the monitored electrical property discussed above. More detailed, this time point is reached when the monitored change of the electrical property reaches a maximum or a minimum. For example, this is the case when the monitored electrical conductivity of the second stream exiting a filter containing an IEX material which is loaded with Na ⁺ as the first cationic species and K ⁺ ions as the second cationic species reaches a maximum due to the increase of K ⁺ ions in the stream.

In a preferred embodiment the method according to the invention comprises at least one of the following additional steps:

• The filter is cyclically regenerated, wherein during each regeneration a certain percentage of the capacity of the IEX material is loaded with the first cationic ion species and a certain percentage of the capacity of the IEX material is loaded with the second cationic ion species.

• After each regeneration, during the phase of regular operation, the first stream of raw water is passed through the filter and the electrical property of the second stream is monitored until the change in the electrical property of the stream which indicates that the filter is exhausted reaches a maximum ora minimum.

• The volume of water V _sw passing through the filter during the phase of regular operation is metered, preferably by the third sensor mentioned above.

In a possible next step, the raw water hardness TH _RW can be determined as a function of the volume of the metered amount of water V _sw and the specific capacity of the IEX material C^and the volume V _tE x of the ion exchange material. The following equation gives results with a relatively good accuracy, in particular if 2 % to 8 % of the capacity of the IEX material is loaded with the first cationic ion species:

In a further preferred embodiment the method according to the invention comprises at least one of the following additional steps, preferably even all of the following additional steps:

• The second stream of softened water is mixed with raw water of the first stream via a by-pass line.

• The ratio of the first stream and the second stream is kept constant during the phase of regular operation.

• The ratio of the first stream and the second stream is adjusted at the end of the phase of regular operation in dependence of the monitored change in the electrical property of the second stream.

The adjustment can be automatically controlled by the electronic control unit. More detailed description of the invention / working example

Further features and advantages of the invention can be derived from the figures and the following detailed description of preferred embodiments. The preferred embodiments described are merely for the purposes of illustration and to give a better understanding of the invention and shall not in any way constitute a restriction.

Fig. 1 illustrates schematically the general process of ion exchange during operation of a water softening device containing a filter comprising an IEX resin operated in Na ⁺ mode.

Fig. 2 illustrates schematically a preferred embodiment of a water softening device according to the invention.

Fig. 3 illustrates schematically a further preferred embodiment of a water softening device according to the invention. Fig. 4 illustrates the changes in concentrations of the cations Na ⁺ , Ca ²⁺ , Mg ²⁺ and K ⁺ in softened water exiting the filter of a water softening device according to the invention as a function of volume passing through the filter.

Fig. 5 illustrates the electrical conductivity of softened water exiting the filter of a water softening device according to the invention and the concentration of hardness therein as a function of volume passing through the filter.

Fig. 6 illustrates the hardness concentration at the conductivity peak measured in water exiting the filter of a water softening device according to the invention as a function of the loading of an IEX resin with a tracer salt.

(1) General process of ion exchange with a SAC IEX resin operated in Na ⁺ mode

For most natural water encountered, the total hardness TH corresponds to the sum of the concentrations of Ca ²⁺ and Mg ²⁺ ions in the water. Such water treated over a SAC resin bed operated under Na ⁺ form, would react as follow:

2 R-COO-Na + Ca ²⁺ -> (R-COO) ₂ -Ca + 2 Na ⁺

The Na ⁺ ions fixed on the SAC resin are exchanged against hardness (Ca ²⁺ and Mg ²⁺ ). The process is illustrated in Fig. 1.

(2) Preferred embodiment of a device according to the invention

The device 100 shown in Fig. 2 comprises a filter 101 containing a SAC IEX resin and a brine tank 108 containing brine. On the top of the filter 101 a filter head 102 is installed. This filter head 102 comprises a multiway valve or a combination of valves. The multiway valve or the combination of valves regulate all streams from and to the filter 101, including a stream of raw water to the filter 101 and a stream of softened water which exits from the filter 101. Via the valve or the combination the filter 101 is further connected to the brine tank 108.

The IEX resin in the filter 101 has to be regenerated in intervals. In the course of the regeneration saturated brine is passed from the brine tank 108 via line 109 (which includes a valve 110) and the filter head 102 into the filter 101, flushing the resin bed therein. The brine contains - besides the main re- generant salt sodium chloride - as a tracer salt dissolved potassium chloride. It is preferred to dilute the brine in the filter head 102 with water before flushing the resin bed, for example with raw water from line 103. For example, it is preferred to adjust the salt concentration in the brine to a value of about 10 % by weight. Used brine which exits from the filter 101 can be discarded via line 111. After regeneration usually from 2 % to 8 % of the capacity of the IEX resin is loaded with potassium, the rest with sodium ions. The exact value depends on the regeneration conditions and on the composition of the brine.

Raw water enters into the filter head 102 via line 103. A stream of softened water exits from the filter head 102 via line 107. There is a direct bypass line 104 which directly connects lines 103 and 107. Water which has exited from the filter 101 can be blended with raw water via the bypass line 103. The amount of raw water can be regulated via valve 105.

The device 100 comprises two sensors 112 and 114 for measuring electrical conductivity of the water flowing through the device. Sensor 112 is positioned at the outlet line 107. Sensor 114 is positioned at the inlet line 103. In addition to this, device 100 comprises sensors 113 and 119 for measuring the volume of the water which flows through the filter 101 and though the bypass line 104. The sensors 112, 113, 114 and 119 are connected to an electronic control unit 115.

(3) Further preferred embodiment of a device according to the invention

The device 200 shown in Fig. 3 comprises a filter 201 containing a SAC IEX resin, a brine tank 208 containing an aqueous solution of sodium chloride as regenerant salt and a tracer salt tank 206 comprising an aqueous solution of potassium chloride as a tracer salt. On the top of the filter 201 a filter head 202 is installed. This filter head 202 comprises a multiway valve or a combination of valves. The mul- tiway valve or the combination of valves regulate all streams from and to the filter 201, including a stream of raw water to the filter 201 and a stream of softened water which exits from the filter 201. Via the valve or the combination the filter 201 is further connected to the brine tank 208 and the tracer salt tank 206.

The IEX resin in the filter 201 has to be regenerated in intervals. In the course of the regeneration an aqueous solution of sodium chloride is first passed from the brine tank 208 via line 209 (which includes a valve 210) via the filter head 202 into the filter 201, flushing the resin bed therein. Then, in a second step, an aqueous solution of potassium chloride is passed from the tracer salt tank 206 via line 216 (which includes a valve 218) via the filter head 202 into the filter 201.

It is preferred to dilute the aqueous solution of sodium chloride and/or the aqueous solution of the tracer salt in the filter head 202 with water before flushing the resin bed, for example with raw water from line 203. For example, it is preferred to adjust the salt concentration in the brine to a value of about 10 % by weight. Used regenerant which exits from the filter 201 can be discarded via line 211. After regeneration usually from 2 % to 8 % of the capacity of the IEX resin is loaded with potassium, the rest with sodium ions. The exact value depends on the regeneration conditions and on the composition of the aqueous solution of the tracer salt.

Raw water enters into the filter head 202 via line 203. A stream of softened water exits from the filter head 202 via line 207. There is a direct bypass line 204 which directly connects lines 203 and 207. Softened water which has exited from the filter 201 can be blended with raw water via the bypass line 203. The amount of raw water can be regulated via valve 205.

The device 200 comprises two sensors 212 and 214 for measuring electrical conductivity of the water flowing through the device. Sensor 212 is positioned at the outlet line 207. Sensor 214 is positioned at the inlet line 203. In addition to this, device 200 comprises a sensor 213 for measuring the volume of the water flowing through the filter 201. The sensors 212, 213 and 214 are connected to an electronic control unit 215.

(4) Detailed description of a preferred embodiment of a method according to the invention

The method according to the invention is in particular characterized by the use of a regenerant containing a tracer salt for the IEX resin regeneration. The nature of this salt and especially the resin selectivity to this salt is of importance. In order to obtain the desired effect, the selectivity from the IEX resin to the tracer salt should be located between the selectivity for the salt used for regeneration (usually a sodium salt) and the selectivity for hardness (Mg ²⁺ and Ca ²⁺ ). For a SAC resin the affinity / selectivity order is as follow for the most common species:

Li ⁺ < H ⁺ < Na ⁺ < NH ₄ ⁺ < K ⁺ < Mg ²⁺ < Ca ²⁺ For a softener regenerated with a sodium salt (with Na ⁺ ions), a potassium salt (with K ⁺ ions) would respect the previous condition as its selectivity is located between the selectivity for sodium ions (Na ⁺ ) and hardness (Mg ²⁺ and Ca ²⁺ ). The ammonium ion (NH ₄ ⁺ ) would also respect the condition, therefore an ammonium salt could also be used as a tracer salt. In fact any salt whose selectivity is located between the hardness and the selectivity of the regenerant salt could be used as a tracer salt.

The cations of the tracer salt get fixed on the lEX resin during the regeneration and especially during the brining step. During this step, the lEX resin is flushed with brine from a brine tank. The aim at the end of the regeneration is to obtain from 0.5 to 20 % tracer salt loading, with regard to the lEX resin operating capacity.

There are two different approaches to bring the tracer salt in contact with the lEX resin. The first approach is to mix the tracer salt to the brine solution in a brine tank. This approach is preferred for domestic applications. The second approach is to install (besides a brine tank) a second tank containing the tracer salt, for example in the form of an aqueous solution. This approach is preferred for industrial applications.

For a regeneration with a sodium salt as regenerant salt under domestic conditions (~ 90 g NaCl per liter lEX resin) usually the regeneration efficiency is about 60 % for sodium. That means for 1 mol of sodium salt passed through a filter with the lEX resin, around 0.6 mol is fixed on the lEX resin. When using a regenerant containing a regenerant salt and a tracer salt the regeneration efficiency for the tracer salt is lower than for the regenerant salt. For example, in order to get a 5 % potassium loading with regard to the lEX resin capacity, the regeneration efficiency to the potassium is around 45 % (when using 90 g NaCl per liter I EX resin). Despite of the fact that the selectivity of the resin for potassium is higher, the higher brine concentration makes the selectivity for the tracer salt lower in those conditions, during the regeneration.

For a regeneration with a sodium salt as regenerant salt under industrial conditions usually brine from the brine tank is passed first through the lEX resin bed. The tracer salt is brought to the lEX resin in a second step, once the lEX resin has been regenerated with the regular regenerant salt. At the moment the tracer salt is brought to the lEX resin, there is no more a competition between both salts. In this case the lEX resin regenerated with the regular regenerant salt shows a higher affinity for the tracer salt and the regeneration efficiency to the tracer salt is close to 100 %. In order to understand what happens to an IEX resin bed (which has been regenerated with a regenerant salt and a tracer salt) during a regular operation, the following example is given:

A filter is filled with 25 liters of an SAC IEX resin. The regular regenerant for such a filter is NaCl, with a regenerant level of 90 g NaCl per liter IEX resin for a domestic application, resulting in a raw IEX resin operating capacity of 1.1 eq/l. As a tracer salt KCl was chosen. The regeneration conditions were set in order to get 5 % of the IEX resin capacity loaded with K ⁺ . The raw water contained 26°f of total hardness. The detailed raw water composition is given below:

T = 15° C

pH = 7.6

Ca ²⁺ = 2.39 mmol/l

Mg ²⁺ = 0.21 mmol/l

Na ⁺ = 0.29 mmol/l

K ⁺ = 0.06 mmol/l

N0 ₃ = 0.25 mmol/l

CI ^" = 0.25 mmol/l

SO ₄ ^2" = 0.36 mmol/l

Alkalinity = 4.88 mmol/l

Inside the filter the flow rate was configured to 630 l/h (with a velocity of 19.6 m/h). The changes in concentrations of the cations Na ⁺ , Ca ²⁺ , Mg ²⁺ and K ⁺ in the softened water exiting the filter were monitored. The results are shown in Fig. 4.

In accordance with the IEX resin selectivity of the ions Na ⁺ , Ca ²⁺ , Mg ²⁺ and K ⁺ a chromatographic effect was observed. The tracer salt (K ⁺ ) whose selectivity is located between the selectivity of the regenerant salt (Na ⁺ ) and the selectivity of the hardness (Ca ²⁺ and Mg ²⁺ ) was released prior to the hardness breakthrough but only after the Na ⁺ concentration started to decrease significantly. This resulted in a change of conductivity of the water exiting the filter, see Fig. 5.

The peak of electrical conductivity corresponds to the concentration of potassium ions in the water exiting the filter. The fact that the conductivity increases with the potassium concentration can be explained by the ionic molar conductivity of each species. Sodium ions have an ionic molar conductivity of 5.01 S.m^mol ^"1 against 7.35 S.rr^.mol ^"1 for potassium ions. As the potassium ions show a higher ionic molar conductivity, this explains why the electrical conductivity is increasing once sodium ions start to be replaced by potassium ions at the end of the filter's capacity. The potassium peak indicates that a hardness breakthrough is imminent.

Thus, it is possible to detect the hardness breakthrough by monitoring the conductivity at the outlet of the filter, in particular by detecting the conductivity peak. The present method can focus on the outlet conductivity exclusively with only one sensor at the filter's outlet, or on the conductivity ratio from outlet to inlet with a conductivity sensor on the inlet of the filter and another one installed on the outlet (compare Fig. 2: conductivity sensor 112 is positioned downstream of the filter, conductivity sensor 114 is positioned upstream of the filter). The second configuration is of particular interest in case of fluctuating raw water conditions to make sure a softened water conductivity peak linked to some raw water changes would not be interpreted by the device as a hardness breakthrough.

Industrial application

A water softening device suitable for an industrial application is shown in Fig. 3. For an industrial application the bypass 204 between the raw and softened water is either closed or absent. The required hardness level downstream of the water softening device is preferably 0°f.

For a given raw water hardness concentration, the hardness leakage concentration at the conductivity peak measured at the outlet line 207 is directly linked to the amount of tracer salt fixed on the IEX resin during the regeneration. The curve in Fig. 6 illustrates the hardness concentration at the conductivity peak measured at the outlet line 207 as a function of the loading of an IEX resin with a tracer salt.

From this curve it becomes obvious that depending on the amount of tracer salt involved the hardness leakage at the conductivity peak can be controlled for a given raw water hardness concentration. In an industrial application the usual configuration is to have a polisher softener downstream a first water softening device. Despite of the fact that the hardness level in the softened water should remain at 0°f it is therefore possible to let a limited hardness leakage at the outlet of the first water softening device. In presence of a polisher softener the leakage will be taken by the polisher.

This leads to the conclusion that once the raw water hardness is known, the amount of tracer salt involved during the regeneration can be set to the required level in order to obtain the desired hard- ness leakage at the end of each cycle when the conductivity peak occurs. For example, if raw water is characterized by a constant hardness of 26°f and the target hardness is 2°f, again at the end of each cycle when the conductivity peak occurs, a tracer salt loading (as K ⁺ ) on the IEX resin of 10 % would be appropriate. In this case, the tracer salt quantity involved during the regeneration would be set in order to obtain a 10 % loading on the IEX resin. As the regeneration efficiency to the tracer salt in this configuration is close to 100 %, the amount a tracer salt to be involved can be clearly defined to match the target leakage condition at the end of each cycle when the conductivity peak occurs.

Domestic application

The water softening device according to Fig.2 is in particular suitable for a domestic application. Inter alia it comprises a sensor 113 for measuring the amount of the water which flows through the filter 101.

Such a sensor allows to determine the volume of water V _sw passed through the filter in the time between the previous regeneration and the moment when the conductivity peak shown in Fig.5 occurs. As the regenerant level used during the previous regeneration is known, the capacity from the IEX resin is also known. From the volume of water, the IEX capacity and the volume of ion exchange resin it is then possible to estimate the raw water hardness concentration with a relatively good accuracy according to the following calculation: wherein

^• Vsw ^' \s the volume of water treated until the monitored conductivity reaches a maximum [I],

^• CIEX ^' \S the specific capacity of the IEX resin [eq.]

^• VIEX ^' \S the volume of the IEX resin volume in the filter [I]

^• THRW ^' \S the total hardness ([Ca ²⁺ ] + [Mg ²⁺ ]) in the raw water [rneq.*! ¹ ]

According to the curve in Fig.5 the conductivity peak occurs at 5.2 m ³ . In this case the raw water hardness estimation would give:

TH _RW = LI * 25/ 5200 = 5.28 meq.t ¹ * 26.4 °f For a raw water hardness of 26.0°f, the estimation at 26.4°f according to the previous method would give the raw water hardness with an accuracy of 1.5 % in this case, for a 5 % tracer salt loading on the IEX resin.

Once the raw water hardness is known after a first regeneration with a tracer salt, the bypass can be adjusted via the valve 105, for example automatically by the controller 115, in order to get the desired hardness concentration in the softened water. In a domestic application the target value is located from 8 to 12°f. The set point can be entered by the user and the bypass will adjust itself automatically in order to get the set hardness value. To do so an additional sensor to determine the volume of water passed through the bypass line 104 would be required. With such a sensor the amount of raw water mixed with the softened water can be adjusted to the required level.

It has been explained above that for a given raw water hardness and a given salt tracer loading on the IEX resin, it is possible to determine the hardness leakage concentration at the conductivity peak. At the end of each cycle, when the conductivity starts to rise, it is therefore possible to link the conductivity increase to the hardness leakage increase. Under those conditions, the bypass setting can be automatically adjusted that the valve 105 will gradually close in order to maintain the hardness level downstream the device at a constant level.

This allows to exhaust the IEX resin bed until the point the hardness level at the vessel outlet reaches the value set for the mixed water, that the maximum volume of water can be treated by the IEX resin. This point should be reached and the regeneration triggered when the bypass valve will be fully closed and the hardness leakage has reached the set value.