NL2032759B1

NL2032759B1 - Method and apparatus for reducing sodium content in cultivation water for plants and cultivation system comprising said apparatus

Info

Publication number: NL2032759B1
Application number: NL2032759A
Authority: NL
Inventors: Jan Van Den Berg Bart
Original assignee: Van Dijk Heating B V
Priority date: 2022-08-16
Filing date: 2022-08-16
Publication date: 2024-02-21

Abstract

A method is presented for reducing sodium content of cultivation water for plants. This method comprising the steps of: contacting cultivation water with a nanofiltration membrane configured to soften water, resulting in a retentate E and a filtrate F which respectively have an increased and a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water; contacting the filtrate F with a cation exchange resin configured to capture at least sodium ions, resulting in an effluent F*; contacting an aqueous solution 8 with the cation exchange resin to remove sodium ions from the cation exchange resin, resulting in an effluent 8* comprising sodium ions, the aqueous solution 8 comprising cations not being sodium ions and anions soluble in water with the cations; discharging the effluent 8*; contacting an aqueous solution R with the cation exchange resin to regenerate the cation exchange resin, resulting in an effluent R*, the aqueous solution R comprising an acid; and recirculating at least one of the retentate E, the effluent F* and the effluent R* for use for plants. Further, an apparatus suitable for implementing said method is presented as well as a cultivation system for plants comprising said apparatus.

Description

Method and apparatus for reducing sodium content in cultivation water for plants and cultivation system comprising said apparatus

[1] The present invention concerns the reduction of sodium content in cultivation water for plants. The invention relates to a method for reducing sodium content in cultivation water for plants and to an apparatus enabling said method as well as a cultivation system in which said apparatus is incorporated.

[2] In cultivation systems for plants, careful balancing is required of the composition of cultivation water that is fed to the plants. One of the components generally occurring in cultivation water is sodium, present in the form of dissolved sodium ions. This sodium may originate from a water source used to produce the cultivation water, such as rainwater, surface water or groundwater. Many plants do not require much sodium compared to other components of the cultivation water. Moreover, too high a sodium content in the cultivation water is disadvantageous for development of plants and/or their products, such as fruits, vegetables and flowers. However, high sodium content is known to occur regularly, especially when cultivation water is recirculated for reuse and / or exposed to evaporation. Sodium content may then build up in the cultivation water to levels unacceptable for use for plants.

[3] Known solutions to this problem involve diluting the cultivation water containing too high a sodium content with fresh water containing lower sodium content. This solution is often combined with discharging part of the cultivation water containing too much sodium, including all its other components such as pesticides and nutrients, to sewage, surface water or other waste stream. This is disadvantageous because of environmental pollution and loss of valuable cultivation water, especially with increasing water shortages in many parts of the world.

[4] It is an aim of the present invention to avoid or at least reduce disadvantages of known technologies such as the above.

[5] This aim is achieved by a method for reducing sodium content of cultivation water for plants, the method comprising the steps of: - contacting cultivation water with a nanofiltration membrane configured to soften water, resulting in a retentate E and a filtrate F which respectively have an increased and a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water; - contacting the filtrate F with a cation exchange resin configured to capture at least sodium ions, resulting in an effluent F*; - contacting an aqueous solution S with the cation exchange resin to remove sodium ions from the cation exchange resin, resulting in an effluent S* comprising sodium ions, the aqueous solution S comprising cations not being sodium ions and anions soluble in water with the cations; - discharging the effluent S*; - contacting an aqueous solution R with the cation exchange resin to regenerate the cation exchange resin, resulting in an effluent R*, the aqueous solution R comprising an acid; and - recirculating at least one of the retentate E, the effluent F* and the effluent R* for use for plants.

[6] The sodium content of cultivation water is reduced because sodium ions are captured by the cation exchange resin and then removed from the cation exchange resin by exchange with cations of the aqueous solution S. The effluent S* resulting from this step comprises sodium ions previously captured from the cultivation water by the cation exchange resin. This effluent S* is discharged, for example as a waste stream. In contrast, the retentate E having an increased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water and / or the effluent F*, resulting from the step of contacting cultivation water with the cation exchange resin, and / or the effluent R*, resulting from the step of contacting the aqueous solution R with the cation exchange resin to regenerate the cation exchange resin, are recirculated for use for plants.

[7] The identifiers E and F are used for conciseness and are respectively associated with the retentate and the filtrate obtained by nanofiltration of the cultivation water. The identifiers F/F*, S/S* and R/R* are used for conciseness and are respectively associated with filtrate loading, sodium exchange and regeneration of the cation exchange resin, wherein the * indicates the liquid emerging as effluent after contact with the cation exchanger resin. Further, the at least one of the retentate E, the effluent F* and the effluent R* may also be termed the recirculated effluent for conciseness.

[8] The step of contacting cultivation water with a nanofiltration membrane configured to soften water results a retentate E and a filtrate F. The retentate E has an increased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water that was brought into contact with the nanofiltration membrane, while the filtrate F has a decreased concentration ratio of divalent cations versus monovalent cations, again compared to the cultivation water that was brought into contact with the nanofiltration membrane.

[9] Rather than contacting the cultivation water directly with the cation exchange resin, it is particularly favourable to first contact cultivation water with the nanofiltration membrane configured to soften water, resulting in a retentate E and a filtrate F which respectively have an increased and a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water. The filtrate F is then contacted with the cation exchange resin configured to capture at least sodium ions, so that sodium ions are captured from the filtrate F obtained from nanofiltration of the cultivation water rather than from the cultivation water directly. Because the filtrate F exhibits a decreased concentration ratio a divalent cations versus monovalent cations compared to the cultivation water, this advantageously improves the efficiency of the available cation exchange resin for capturing sodium ions. In other words, the filtrate F has an increased concentration ratio of monovalent cations (including sodium ions) versus divalent cations compared to the cultivation water. It is advantageous to feed the filtrate F to the cation exchange resin as this requires a smaller amount of cation exchange resin and less aqueous solution S per mole of removed sodium ions.

Conversely, the same amount of cation exchange resin can be used to increase the sodium removal capacity when compared to a cation exchange sodium removal system without a preceding nanofiltration step. As a further advantage, the regeneration solution

R for removing sodium ions from the cation exchange resin can be used more efficiently since less strongly bound divalent cations {or multivalent cations in general) have to be removed from the cation exchange resin for its regeneration. Moreover, an upstream nanofiltration step reduces contamination of the downstream cation exchange resin, such as pesticides, in particular cationic species.

[10] The pore size of the nanofiltration membrane is such that small monovalent ions, like sodium ions and potassium ions but also chlorine and nitrate, can pass through the membrane as filtrate (also termed permeate), while larger divalent ions (or multivalent ions in general), such as calcium ions and magnesium ions, are at least partially retained by the membrane. The nanofiltration membrane may be of any known suitable type to soften water, i.e. configured to at least partially separate divalent cations, such as calcium ions and magnesium ions, from monovalent cations, such as potassium ions and sodium ions, when an aqueous solution such a cultivation water is made to contact said membrane. The increase in the concentration ratio of divalent to monovalent cations of the retentate E, compared to the initial cultivation water before nanofiltration, may be at least a factor of 1.5, preferably at least a factor of 2. For example, the nanofiltration membrane may be of a spiral-wound type, a hollow fibre type, or a ceramic type.

Nanofiltration membranes as conventionally used for water softening applications can readily be employed in the present invention.

[11] The term cation exchange resin is conventionally used for materials with cation exchanging properties and should therefore not be understood to be limited to synthetic resins, though these are preferred. The term cation exchange resin therefore comprises any cation exchange material capable of capturing at least sodium ions.

[12] The term cultivation water here comprises any water suitable for use for plant cultivation. It may be newly produced cultivation water, even straight from a water source without purification or addition of components, recycled cultivation water such as drainage water and mixtures thereof. The method is thus not limited to reusing cultivation water, but can equally be applied to freshly sourced cultivation water of which sodium content is to be reduced before using said cultivation water for plants. Either or both of the effluents F* and R* may be recirculated for use as cultivation water for plants, for example as reusable, recycled or purified cultivation water in a circular watering system. If either of the effluents F* or R* is not recirculated for use for plants, the not circulated effluent may be discharged combined with or separate from the effluent S*. However, it may often be preferred to recirculate both effluents F* and R* to retain their components for use in cultivation water for plants as much as possible. The retentate E may also be recirculated for use for plants and already exhibits a reduced sodium content because of its increased concentration ratio of divalent ions versus monovalent ions, including sodium ions, while also including most of the larger ions and molecules with nutritional or other function purpose. Discharging is distinct from recirculating and may be understood to mean discharging/removing from circulation, or at least preventing recirculation for use for plants for which the cultivation water is intended. The retentate E, the effluent F* and the effluent R*, in particular in combination, can be used for cultivation of plants, for example as reusable, recycled or purified cultivation water having a reduced sodium content in a circular watering system. The sodium content of such cultivation water can in particular be reduced in a selective manner by the present invention.

[13] The retentate E from the nanofiltration membrane may comprise larger neutral 5 molecules and the majority, or at least a significant portion, of the divalent ions and may still contain some monovalent ions while the filtrate F from the nanofiltration membrane may comprises the majority, or at least significant portion, of the monovalent ions and smaller neutral molecules including water. The filtrate may comprise a reduced concentration of monovalent ions compared to the initial cultivation water prior to contacting the nanofiltration membrane. The retentate E comprises an increased concentration of divalent ions compared to the incoming cultivation water as well as compared to the filtrate F. The retentate E may also comprise pesticides or other larger molecules which cannot penetrate the nanofiltration membrane. The filtrate F (also termed permeate) from the nanofiltration membrane may still comprise a small fraction of divalent ions but in any case a high concentration of monovalent ions relative to this divalent ion concentration. The filtrate F comprises a reduced amount of pesticides and other larger molecules, if any.

[14] The flow of cultivation water over the nanofiltration membrane can be controlled to favour a higher or lower concentration ratio of divalent ions versus monovalent ions in the retentate E or the filtrate F. Such control can be used to optimise sodium removal from the cultivation water, amount

[15] The effluent F* comprises neutral molecules and anions of the cultivation water since these are not captured by the cation exchange resin. Further, the effluent comprises any removed or exchanged cations from the cation exchange resin that are replaced by captured cations from the cultivation water (depending on affinity and concentration as described below). Usually, the effluent F* comprises protons removed from the cation exchange resin, especially when the cation exchange resin has been regenerated before to the step of contacting the cultivation water with the cation exchange resin. In any case, the cation exchange resin is at least capable of capturing sodium ions and is thus configured for this purpose, for example by complete or partial regeneration.

[16] The aqueous solution S comprises cations at a concentration sufficient to remove sodium ions from the cation exchange resin, as the step of contacting the aqueous solution S with the cation exchange resin results in an effluent S* comprising sodium ions that were previously captured by the cation exchange resin. The cation concentration in the aqueous solution S depends on the affinity of the cations for the particular cation exchange resin used in the method compared to the affinity of sodium ions for the same cation exchange resin. A lower affinity of the cations requires a higher concentration, while a lower concentration suffices with a higher affinity of the cations. Cations with a higher affinity than sodium ions are preferred, though this is not necessarily the case.

When using cations with a lower affinity, their concentration should be increased correspondingly. Note that, from the perspective of the cation exchange resin, affinity is also termed selectivity.

[17] In addition to sodium ions, the effluent S* comprises any other cations removed from the cation exchange resin by exchange with the cations of the aqueous solution S, possibly as a cascade of exchanging cations based on their affinities and concentrations.

Further, the effluent S* comprises neutral and anionic species present in the aqueous solution S.

[18] The aqueous solution R comprises acid at a concentration sufficient to regenerate the cation exchange resin. In case of a strong acid, a lower concentration is sufficient while a higher concentration may be required for weak acids to regenerate, preferably completely, the cation exchange resin. The pH value or proton molarity of the aqueous solution R is preferably such that the cation exchange resin is completely regenerated.

This pH value or proton molarity is primarily determined by the type of cation exchange resin. For example, with a strong acid cation exchange resin, the aqueous solution R preferably has a pH of 0 or a proton molarity of 1 mol/L.

[19] The effluent R* comprises cations that are removed from the cation exchange resin by protons from the aqueous solution R, neutral and anionic species of the aqueous solution R as well as excess acid of the aqueous solution R.

[20] Because the effluent S* is discharged, sodium ions are removed from the cultivation water. This reduces the sodium content of cultivation water in an absolute sense. However, the reduction of sodium content is also at least partially selective in that the sodium content is reduced relative to other components of the cultivation water, including cations other than sodium and / or anions, because the effluent F*, the effluent

R* or both are recirculated.

[21] Preferably, the cations of the aqueous solution S and / or a conjugated base associated with the acid of the aqueous solution R are suitable as plant nutrient. In contrast, the anions of the aqueous solution S need not be suitable as plant nutrient, or can in fact be selected specifically to be unsuitable as plant nutrient, as these anions will not be recirculated but discharged as part of the effluent S*.

[22] Preferably, the cation exchange resin comprises a strong acid cation exchange resin and the acid of the aqueous solution R comprises a strong acid. Alternatively, a weak acid cation exchange resin can be employed, in which case the acid of the aqueous solution R may comprise a weak acid.

[23] The cation exchange resin may be selected from a group of resins based on sulfonic acid, said group comprising poly(2-acrylamido-2-methylpropanesulfonic acid), polystyrene sulfonates and sulfonated poly(styrene-co-divinylbenzene).

[24] The cations of the aqueous solution S may comprise at least one of calcium ions, magnesium ions, potassium ions, ammonium ions and protons, preferably calcium ions and / or potassium ions, more preferably potassium ions.

[25] The anions of the aqueous solution S may comprise at least one of chloride, sulfate, nitrate, carbonate and hydrogen carbonate, preferably hydrogen carbonate.

[26] Itis preferred that the aqueous solution S comprises calcium hydrogen carbonate, potassium carbonate and / or potassium hydrogen carbonate, more preferably potassium hydrogen carbonate. In other words, preferred combinations of cations and anions in the aqueous solution S are calcium hydrogen carbonate, potassium carbonate and / or potassium hydrogen carbonate.

[27] The aqueous solution R may comprise one or more than one acid selected from a group containing hydrogen chloride, sulfuric acid, nitric acid, phosphoric acid, carbonic acid, hydrogen carbonate, acetic acid and oxalic acid, preferably sulfuric acid, nitric acid and phosphoric acid, more preferably nitric acid.

[28] Advantageously, the method may further comprise a step of contacting carbon dioxide, preferably gaseous carbon dioxide, with an aqueous alkali solution comprising the cations of the aqueous solution S and hydroxide to produce the aqueous solution S.

[29] The method may further comprise a step of contacting the at least one of the effluent F* and the effluent R* with the aqueous alkali solution or a further aqueous alkali solution to neutralise the at least one of the effluent F* and the effluent R*. Though a solution is preferred, a solid can also be used, e.g. calcium carbonate.

[30] The method may further comprise a step of determining a sodium saturation level of the cation exchange resin, preferably a maximum sodium saturation level of the cation exchange resin, wherein the step of contacting the aqueous solution S with the cation exchange resin is executed based on the determined sodium saturation level of the cation exchange resin.

[31] Further, the step of determining the sodium saturation level of the cation exchange resin may comprise determining pH value of effluent coming from the cation exchange resin. Preferably, the maximum sodium saturation level is determined based on an increase in pH value by a predetermined amount, for example by about one pH, relative to a minimum pH value determined during the step of contacting cultivation water with the cation exchange resin.

[32] Additionally or alternatively, the step of determining the sodium saturation level of the cation exchange resin may comprise determining electrical conductivity of effluent coming from the cation exchange resin. Preferably, the maximum sodium saturation level is determined based on a decrease in electrical conductivity by a predetermined amount, for example about one-tenth, relative to a maximum electrical conductivity determined during the step of contacting cultivation water with the cation exchange resin.

[33] In any of the embodiments of the method, the step of recirculating the at least one of the effluent F* and the effluent R* may comprise: - feeding the at least one of the effluent F* and the effluent R* to an intermediate supply for cultivation water; and - feeding content of the intermediate supply to a usage supply of cultivation water; preferably further comprising at least one of the steps of: - homogenising the intermediate supply; and - rinsing conduits with fresh water to the usage supply.

[34] The filtrate F may be temporarily stored in a tank before being fed onto the cation exchange resin. The continuous process of nanofiltration may then be made compatible with the batch-wise process of ion exchange. Further, a relatively small nanofiltration unit can be employed to provide a small yet steady stream of filtrate F which is temporarily stored in a filtrate F storage tank before it is loaded batch-wise onto the cation exchange resin, i.e. the filtrate F is intermittently loaded from this storage tank. In general terms, the method may comprise accumulating the filtrate F continuously from the nanofiltration unit and guiding the filtrate F in batches to the cation exchanger.

[35] As explained above, the feed stream for the method can be freshly sourced cultivation water, which is used for preparation of finished cultivation water. In many current cultivation systems with circulating cultivation water, sodium ions mainly originate from such freshly sourced cultivation water. The method according to the invention can remove sodium ions from freshly sourced cultivation water with nanofiltration followed by cation exchange as previously described. The retentate E from the nanofiltration membrane as well as effluent F* and effluent R* of the cation exchange unit can be reused in the cultivation water system.

[36] Optionally and preferably, the filtrate F from the nanofiltration membrane is first concentrated using reverse osmosis prior to contacting the cation exchange resin.

Reverse osmosis results in two streams: a concentrate and a permeate. The concentrate carries most ions while the permeate is reduced in ion concentration. The remaining reverse osmosis permeate stream can directly be used as make-up water for the cultivation system, i.e. it can be recirculated for use for plants with at least one of the retentate E, the effluent F* and the effluent R*.

[37] Reverse osmosis and nanofiltration both rely on membrane technology. The difference may reside in membrane pore size, with nanofiltration membranes having a larger pore size than reverse osmosis membranes. For example, nanofiltration membranes may have pore sizes ranging from 1 — 10 nm while reverse osmosis membranes may have pore sizes up to 1 nm, or up to only 0.1 nm, thereby even retaining monovalent ions.

[38] The invention further relates to an apparatus for reducing sodium content of cultivation water for plants, the apparatus comprising a nanofiltration unit in fluid connection with a cultivation water feed and a cation exchanger in fluid connection with:

-the nanofiltration unit; - a removal agent facility; - a regeneration agent facility; - a discharge; and -arecirculation facility, wherein: - the nanofiltration unit comprises a nanofiltration membrane configured to soften water; - the apparatus is configured to guide the filtrate F to the cation exchanger and thereby provide an effluent F*; - the cation exchanger comprises a cation exchange resin which is configurable to capture at least sodium ions; - the cultivation water feed is configured to guide cultivation water to the nanofiltration unit and thereby provide a retentate E and a filtrate F which respectively have an increased and a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water; - the apparatus is configured to guide the filtrate F to the cation exchanger and thereby provide an effluent F*; - the removal agent facility is configured to guide an aqueous solution S to the cation exchanger to remove sodium ions from the cation exchange resin and thereby provide an effluent S* comprising sodium ions, the aqueous solution S comprising cations not being sodium ions and anions soluble in water with the cations; - the discharge is configured to discharge the effluent S*; - the regeneration agent facility is configured to guide an aqueous solution R to the cation exchanger to regenerate the cation exchange resin and thereby provide an effluent R*, the aqueous solution R comprising an acid; and - the recirculation facility is configured to recirculate at least one of the retentate E, the effluent F* and the effluent R* for use for plants.

[39] The apparatus may further comprise a filtrate storage arranged in fluid connection between the nanofiltration unit and the cation exchanger, wherein the filtrate storage is preferably configured to receive the filtrate F continuously from the nanofiltration unit and guide the filtrate F in batches to the cation exchanger.

[40] The filtrate F from the nanofiltration membrane is preferably guided to a nanofiltration filtrate storage, e.g. in the form of a tank, arranged in fluid connection between the nanofiltration unit and the cation exchanger for temporary storage before being fed to the cation exchanger. This allows accumulation of filtrate F in the filtrate storage while the cation exchanger processes the filtrate F in batches. The filtrate storage may be a tank equipped with a filling level sensor and a valve for emptying the filtrate storage tank towards the cation exchanger once a predetermined filling level is reached and/or the cation exchanger is ready to receive a new batch of the filtrate F.

[41] The apparatus may further comprise a reverse osmosis unit arranged in fluid connection between the nanofiltration unit and the cation exchanger. The filtrate F can be guided from the nanofiltration unit to the reverse osmosis unit to be concentrated into a concentrate filtrate F while a permeate can be recirculated for use for plants from the reverse osmosis unit for example by means of the recirculation facility.

[42] When the apparatus comprises both the filtrate storage and the reverse osmosis unit, it is preferred that the reverse osmosis unit is arranged in fluid connection between the nandfiltration unit and the filtrate storage. In that case, the filtrate storage receives the concentrate filtrate F from the reverse osmosis unit and subsequently guides the concentrated filtrate F to the cation exchanger.

[43] The recirculation facility is preferably configured to recirculate at least the effluent

R* for use for plants, more preferably to recirculate both the effluent F* and the effluent R* for use for plants.

[44] Preferably, the removal agent facility is further configured to provide the aqueous solution S and / or the regeneration agent facility is further configured to provide the aqueous solution R. The removal agent facility may provide the aqueous solution S from a storage facility, preferably with an associated pump or other dosing unit, and / or by combining components to provide the aqueous solution S by production, for example by dosing one or more than one salt forms of the cations and the anions of the aqueous solution S into water, such as fresh water and / or cultivation water.

[45] Additionally or alternatively, the removal agent facility comprises a first feed and a second feed in fluid connection with the first feed and the removal agent facility is further configured to produce the aqueous solution S by contacting carbon dioxide, preferably gaseous carbon dioxide, supplied via the first feed with an aqueous alkali solution supplied via the second feed, the aqueous alkali solution comprising hydroxide and the cations of the aqueous solution S. With this arrangement, the aqueous solution S can be produced in-line, which is beneficial for automation of the apparatus.

[46] The first feed preferably comprises an adjustable pressure valve for gaseous carbon dioxide. The second feed preferably comprises an adjustable pump for the aqueous alkali solution. Providing gaseous carbon dioxide via an adjustable pressure valve allows pressure control over carbon dioxide infeed into the aqueous solution S, thus controlling pressure in conduits and in the cation exchanger. When the aqueous solution

S encounters any protons, carbon dioxide may again form, leading to an increase in pressure. This may be a particular risk when the aqueous solution S contacts a partly regenerated cation exchange resin still comprising protons, especially when this cation exchange resin is a strong acid cation exchange resin. The adjustable pressure valve may thus mitigate risk of overpressure when the equilibrium of dissolved carbon dioxide shifts towards gaseous form. For example, when dosing of carbon dioxide is controlled by the adjustable pressure valve in dependence on (e.g. via a fixed ratio) a water infeed and alkali solution infeed, the maximum concentration of carbon dioxide in the produced aqueous solution S is fixed. Hence, the maximum pressure of this solution when the equilibrium shifts to carbonic acids is fixed as well. Additionally or alternatively, a pressure relief valve may be employed.

[47] Preferably, the aqueous solution S is produced from fresh water as the associated effluent S* is to be discharged. Though cultivation water or effluent F* can be used to produce the aqueous solution S, in addition or alternatively to fresh water, these include plant nutrients and anions that are better retained and recirculated for use for plants rather than discharging these into the environment. Whichever water source is employed, it is preferred to remove calcium before using it to produce the aqueous solution S in order to avoid precipitation of calcium carbonate. Calcium can advantageously be removed by guiding any water used to produce the aqueous solution S over the cation exchanger already present in the apparatus of the invention. No separate water softener is required. Further, the calcium ions thus captured, are again released as part of the effluent R* and available for use for plants as a nutrient in recirculated cultivation water.

[48] Further, the removal agent facility preferably comprises a tank and the removal agent facility is preferably further configured to guide the aqueous solution S via the tank to the cation exchanger. Advantages of this arrangement include that the aqueous solution S is readily available, whereas fresh production may require a delay in order to establish an equilibrium composition of the aqueous solution S.

[49] Preferably, the second feed of the removal agent facility also serves as a neutralisation facility to neutralise the effluent F* and / or the effluent R*. The aqueous alkali solution may then be fed via the second feed to the effluent F* and / or the effluent

R* in order to provide hydroxide for neutralising protons present in the effluent F* and / or the effluent R*.

[50] Advantageously, the apparatus may further comprises a sodium saturation level sensor configured to determine a sodium saturation level of the cation exchange resin, preferably a maximum sodium saturation level of the cation exchange resin.

[51] Further, the removal agent facility may be controllable based on the sodium saturation level of the cation exchange resin determined by the sodium saturation level sensor. This enables optimisation of the timing at which the removal agent facility ensures contact of the aqueous solution S with the cation exchange resin to remove previously captured sodium ions from the cation exchange resin.

[52] The sodium saturation level sensor may comprise a pH sensor and / or an electrical conductivity sensor arranged near an effluent output of the cation exchanger.

[53] The apparatus preferably further comprises an intermediate supply and a usage supply, wherein the recirculation facility is configured to recirculate the at least one of the effluent F* and the effluent R* to the usage supply via the intermediate supply. The intermediate supply improves mixing of the recirculated effluent(s) and reduces sudden compositional changes of said effluent, such as increased proton concentration or increased cation or anion (e.g. as nutrients) concentration, that may negatively affect the plants it is intended to be used for.

[54] The apparatus preferably further comprises a fresh water feed configured to guide fresh water to the cation exchanger. The fresh water feed may be configured to rinse conduits and / or the cation exchanger, to perform a backwash of the cation exchanger, to dilute fluids such as cultivation water, to provide fresh water for providing or producing the aqueous solution S and / or the aqueous solution R.

[55] The apparatus preferably further comprises a pH sensor and / or an electrical conductivity sensor at an input side of the cation exchanger configured to monitor the composition of fluids flowing towards the cation exchanger. Such fluids include cultivation water, fresh water, the aqueous solution S and the aqueous solution R. When said pH sensor and / or electrical conductivity sensor register values outside a predetermined range, additional fresh water and / or components may be added to the fluid flowing towards the cation exchanger. For example, when pH value and / or electrical conductivity of the cultivation water are above a threshold value signifying a cation concentration that is too high, fresh water can be admixed into the cultivation water to reduce said cation concentration.

[56] A preferred embodiment of the apparatus comprises a controller configured to at least one of: - control nanofiltration filtrate and retentate streams based on divalent and monovalent ion concentrations and / or fouling of the nanofiltration membrane; - control the cultivation water feed based on filling level of the filtrate storage and / or batch cycle stage of the cation exchanger; - control guiding of the filtrate F towards the cation exchange resin based on the sodium saturation level of the cation exchange resin determined by the sodium saturation level sensor; - control the removal agent facility based on the sodium saturation level of the cation exchange resin determined by the sodium saturation level sensor; - control the regeneration agent facility and / or the fresh water feed to regenerate the cation exchange resin; - control the discharge and / or the recirculation facility based on activity of at least one of the cultivation water feed, the removal agent facility and the regeneration agent facility; - control the fresh water feed to rinse conduits and / or the cation exchanger; - control the fresh water feed to perform a backwash of the cation exchanger; and - control the fresh water feed to dilute fluids flowing to the cation exchanger, such as cultivation water, based on the composition thereof monitored by the pH sensor and / or the electrical conductivity sensor at the input side of the cation exchanger.

[57] The controller allows further automation of the sodium ion reduction process performed by the apparatus. For example, the controller closes the cultivation water feed and starts the removal agent facility when the sodium saturation level sensor registers an optimal sodium saturation level of the cation exchange resin.

[58] The discharge in any embodiment of the apparatus preferably comprises at least one active carbon bed configured to purify the effluent S*. An active carbon bed may filter the effluent S* and remove unwanted components such as pesticides from the effluent S* before it is discharged. However, nanofiltration may prevent such components from reaching the cation exchanger in the first place, so that the effluent S* may not comprise pesticides and other components that should not be discharged. Nanofiltration prior to cation exchange thus has a further advantage in that filtrations steps after cation exchange may nt be required.

[59] The apparatus may further comprise an anion exchanger configured to capture anions from at least one of the effluent F*, the effluent R* and the effluent S*. This allows further purification, for example to retain anionic components of the otherwise discharged effluent S* or to remove anions from the (possibly combined) effluents R* and S*.

Alternatively or additionally, the apparatus may be configured to capture and recirculate most if not all cations and anions by means of the cation exchanger and the anion exchanger while discharging the remainder of the cultivation water, including neutral components. This enables discharging of unwanted, possibly contaminated cultivation water, while retaining for recirculation most if not all of the useful cations and anions of the cultivation water.

[80] The apparatus is preferably configured to execute the method of the invention or at least one of the preferred embodiments of said method.

[61] Finally, the invention relates to a cultivation system for plants comprising: - a cultivation space; - a usage supply of cultivation water; - an apparatus according to the invention; - feed conduits configured to guide cultivation water from the usage supply to the cultivation space; - return conduits configured to guide drainage water from the cultivation space to the apparatus; and - recirculation conduits configured to recirculate cultivation water with a reduced sodium content from the apparatus to the usage supply.

[62] The cultivation system enables efficient recirculation of cultivation water for plants by reducing sodium content of the cultivation water. This reduces water consumption from external sources and prevents environmental contamination or eutrophication by reducing discharge of cultivation water and / or components thereof compared to conventional systems.

[63] Preferably, the cultivation system further comprises a disinfection device configured to disinfect cultivation water.

[64] Additionally or alternatively, the cultivation system may further comprise a nutrient dosing device configured to add plant nutrients to cultivation water.

[65] Additionally or alternatively, the cultivation system may further comprise a fresh water feed configured to add fresh water to cultivation water.

[66] Additionally or alternatively, the filtrate F from the nanofiltration membrane is concentrated by reverse osmosis prior to contacting it with the cation exchange resin. In other words, the method may include the step of concentrating the filtrate F by reverse osmosis and then contacting the concentrated filtrate F with the cation exchange resin.

The reverse osmosis permeate may be recirculated for use for plants, preferably together with any of the retentate E, the effluent F* and / or the effluent R*. This reduces the volume that has to pass over the cation exchange resin and increases the recirculated volume.

[67] The usage supply of the cultivation system preferably comprises a first tank for drainage water, a second tank for purified cultivation water and a third tank for finished cultivation water.

[68] The invention is further clarified by example embodiments illustrated in the following figures:

FIG. 1 shows a flow diagram of a method for reducing sodium content of cultivation water for plants according to an example embodiment of the invention;

FIG. 2 shows data for determining sodium saturation level of cation exchange resin;

FIG. 3 shows an apparatus for reducing sodium content of cultivation water for plants according to an example embodiment of the invention;

FIG. 4 shows a modification of the apparatus of FIG. 3;

FIG. 5 shows a cultivation system for plants according to an example embodiment of the invention;

FIG. 6 shows a modification of the cultivation system for plants of FIG. 5; and

FIG. 7 shows a further modification of the cultivation system for plants of FIG. 6.

[69] The following reference signs are used:

SO contacting cultivation water with a nanofiltration membrane

S1 contacting filtrate F with cation exchange resin

S2 contacting aqueous solution S with cation exchange resin

S83 discharging effluent S*

S4 contacting aqueous solution R with cation exchange resin

S5 recirculating at least one of retentate E, effluent F* and effluent R*

S6 contacting carbon dioxide with aqueous alkali solution

S7 contacting at least one of the effluent F* and the effluent R* with aqueous alkali solution

S8 determining sodium saturation level of cation exchange resin

S9 rinsing cation exchange resin with water

S10 backwashing cation exchange resin

S11 feeding at least one of effluent F* and effluent R* to intermediate supply

S12 homogenising intermediate supply

S13 feeding content of intermediate supply to usage supply

S14 rinsing conduits with water to usage supply t1 end time of performing step S1 t2 start time of performing step S2 13 end time of performing step S2 1 apparatus for reducing sodium content of cultivation water for plants 2 cation exchanger 3 cation exchange resin 4 cultivation water feed 5 removal agent facility 6 regeneration agent facility 7 discharge 8 recirculation facility 9 upper inlet 10 intermediate inlet 11 lower inlet

12 filling level of cation exchange resin 13 filter 14 pump 15 controller 16 tank 17 active carbon bed 18 usage supply 19 intermediate supply 20 sodium saturation level sensor 21 pH sensor 22 electrical conductivity sensor 23 fresh water feed 24 valve 25 first feed 26 second feed 27 gas cylinder 28 mixer 29 return conduit 30 tank 40 cultivation system 41 cultivation space 42 feed conduits 43 return conduits 44 recirculation conduits 45 disinfection device 46 nutrient dosing device 47 first tank 48 second tank 49 third tank 50 nanofiltration unit 51 filtrate conduit 52 retentate conduit 53 filtrate storage 54 reverse osmosis unit permeate conduit 56 concentrate conduit

57 nanofiltration feed pump 58 reverse osmosis feed pump 59 by-pass 60 water to usage supply conduit

[70] FIG. 1 illustrates a method for reducing sodium content in cultivation water according to an example embodiment of the invention comprising essential as well as optional features.

[71] Ina step SO, cultivation water is contacted with a nanofiltration membrane configured to soften water. This results in a retentate E with an increased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water and in a filtrate F with a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water. Conversely, the filtrate F has a higher concentration ratio of monovalent cations versus divalent cations compared to the cultivation water initially brought in contact with the nanofiltration membrane. The filtrate F thus carries relatively more of the sodium ions from the original cultivation water. In other words, nanofiltration separates incoming cultivation water into the retentate E (relatively rich in divalent ions such as calcium and magnesium) and the filtrate F {relatively rich in monovalent ions such as sodium and potassium).

[72] The illustrated method further comprises a step S1 of contacting the filtrate F with a cation exchange resin configured to capture at least sodium ions. The sodium content of the cultivation water is to be reduced by capturing sodium ions from the filtrate F with the cation exchange resin. To this end, the cation exchange resin is configured to capture at least sodium ions, for example by complete or partial regeneration with protons and / or cations with an affinity for the cation exchange resin that is lower than that of sodium ions.

[73] Step S1 results in an effluent F*. As the filtrate F is brought into contact with the cation exchange resin during step S1, the effluent F* flows from the cation exchange resin. The effluent F* thus comprises any cations, including protons, removed from the cation exchange resin by the sodium ions by cation exchange. Generally, the process may be started with a regenerated cation exchange resin, i.e. loaded with protons, so that mainly protons will be removed therefrom when the filtrate F is loaded thereon and the effluent F* will then comprise these protons. Further, the effluent F* comprises components of the filtrate F not captured by the cation exchange resin.

[74] In other words, the step S1 may be seen as loading the cation exchange resin with sodium ions from the cultivation water present in the filtrate F. The effluent F* may also be termed filtrate loading effluent.

[75] The illustrated method further comprises a step S2 of contacting an aqueous solution S with the cation exchange resin in order to remove sodium ions from the cation exchange resin. The aqueous solution S comprises cations, not being sodium ions, and anions that are soluble in water with the cations. The step S2 results in an effluent S* comprising sodium ions that were previously captured by the cation exchange resin from the filtrate F. In other words, the aqueous solution S may be termed a sodium removal solution or sodium exchanging solution, while the step S2 may be seen as exchanging sodium ions for the cations of said solution. The effluent S* may be termed sodium containing effluent. Preferably, the aqueous solution S comprises no sodium ions at all, or atleast not to a level at which these sodium ions materially interfere with the object of the step S2, being removal of sodium ions from the cation exchange resin by directly or indirectly exchanging these with non-sodium cations of the aqueous solution S.

[76] In a step S3 of discharging the effluent S*, the sodium ions are removed from cultivation water circulation.

[77] Atthis stage, the sodium content of the cultivation water has been reduced.

However, components of the cultivation water are yet captured by the cation exchange resin. This is especially the case for cations of the cultivation water that have a higher affinity for the cation exchange resin than sodium ions and than the cations of the aqueous solution S. For example, when the captured sodium ions are exchanged with potassium ions, any magnesium ions or calcium ions that were yet present in the filtrate F may yet remain captured by the cation exchange resin. As explained above, loading the cation exchanger with the filtrate F rather than with cultivation water directly, here has the advantage that less divalent ions are captured by the cation exchange resin and more of its capacity is available for sodium ions.

[78] Itis therefore favoured to remove these components from the cation exchange resin for use for plants with a step S4 of contacting an aqueous solution R with the cation exchange resin in order to regenerate the cation exchange resin. For this purpose, the aqueous solution R comprises an acid, which provides a sufficient amount of protons to remove all or some of the cations from the cation exchange resin. The step S4 results in an effluent R* which comprises said removed cations as well as any surplus components of the aqueous solution R.

[79] In other words, the step S4 may be seen as regenerating the cation exchange resin with protons. The aqueous solution R may be termed regeneration solution and the effluent R* may be termed regeneration effluent.

[80] Ina step S5, at least one of the retentate E, the effluent F* and the effluent R* for use for plants is recirculated. This enables recycling of parts of the cultivation water brought in contact with the nanofiltration membrane in step SO and / or the cation exchange resin in step S1. The retentate E comprises most divalent ions, including valuable plant nutrients such as calcium ions and magnesium ions. Since anionic and neutral components of the cultivation water are generally not captured by cation exchange resins, these are comprised in effluent F*. The effluent R* contains cations of the cultivation water in the filtrate F that were captured in the step S1 and again removed in the step S3. Thus, anionic and neutral as well as cations components of the cultivation water may be recirculated for use for plants. When all of the retentate E, the effluent F* and the effluent R* are recirculated, reuse of the cultivation water is maximized.

[81] Preferably, the cation exchange resin comprises a strong acid cation exchange resin and the acid of the aqueous solution R comprises a strong acid. Alternatively, a weak acid cation exchange resin can be employed, in which case the acid of the aqueous solution R may comprise a weak acid.

[82] Examples of the cation exchange resin may be selected from a group of resins based on sulfonic acid comprising poly(2-acrylamido-2-methylpropanesulfonic acid), polystyrene sulfonates and sulfonated poly(styrene-co-divinylbenzene). A cation exchange resin suitable for applying the present invention is commercially available under the name Amberlite™ HPR1200H.

[83] The cations of the aqueous solution S and / or a conjugated base associate with the acid of the aqueous solution R may advantageously be suitable as plant nutrient. In this way, said cation and / or said base are useful in performing the method and in enriching the effluent F* and / or the effluent R* with plant nutrients, thus serving a double purpose. This reduces material consumption. Examples of such cations and bases are given below. The cations of the aqueous solution S that exchange the sodium ions in the cation exchange resin are in turn exchanged for protons of the aqueous solution R in the regeneration step S4. The conjugate base associated with the acid of the aqueous solution R is also introduced in said step. The effluent R* thus comprises cations of the aqueous solution S and conjugate base of the acid of the aqueous solution R. The effluent R* can be recirculated for use for plants as enriched cultivation water.

[84] The cations of the aqueous solution S may comprise at least one of calcium ions, magnesium ions, potassium ions, ammonium ions and protons, preferably calcium ions and/or potassium ions, more preferably potassium ions. At least calcium ions, magnesium ions, potassium ions and ammonium ions are known to be plant nutrients.

[85] Type and concentration of the cations employed in the aqueous solution S may be selected depending on affinity of the cation for the selected cation exchange resin in comparison to that affinity of sodium ions. The concentrations of the cations in the aqueous solution S depend on the affinity of the cation for the cation exchange resin relative to the affinity of sodium ions for the same cation exchange resin. A lower concentration suffices for cations with an affinity that is higher than that of sodium ions, while a higher concentration is required for cations with an affinity that is lower than that of sodium ions. The relative affinities and sufficient or required concentrations can be determined by ordinary testing protocols.

[86] For example, when the affinity of the cation exchange resin is in the order of

Ca?" > Mg?" > K* > NH4* > Na* > H* alower concentration of potassium ions suffices to remove an equivalent of sodium ions compared to a higher concentration of ammonium ions. An even lower concentration of calcium ions is required because of its higher affinity as well as the divalent cationic charge of calcium ions compared to the monovalent potassium and ammonium ions. In this example however, potassium is most preferred because it is less likely to remove calcium ions and magnesium ions from the cation exchange resin, thus avoiding discharging these plant nutrients with the effluent S*. Further, potassium ions are an important plant nutrient. Though ammonium ions and even protons are also possible in this example, these would require higher concentrations to remove sodium ions from the cation exchange resin leading to waste in effluent S*. Furthermore, the chosen concentration of the cations in the aqueous solution S is also related to the volume of effluent S*. A higher concentration is favourable because it reduces the amount of water that is discharged (e.g. to the sewer).

[87] In practice, a concentration of potassium ions in the aqueous solution S in the range of 5 — 100 mmol/L, preferably 10 — 50 mmol/L and more preferably 20 — 30 mmol/L, has proven effective in removing sodium ions from generally available cation exchange resins including Amberlite™ HPR1200H. Equivalent concentrations can be employed for other cations, taking into account their charge and their (relative) affinity for the cation exchange resin being used.

[88] The anions of the aqueous solution S may comprise at least one of chloride, sulfate, nitrate, carbonate and hydrogen carbonate, preferably hydrogen carbonate.

Because anions are generally not captured by the cation exchange resin, these emerge as part of the effluent S*, which is discharged in the step S3. The anions listed here are advantageous for high solubility in water and, especially for carbonate and hydrogen carbonate, of low toxicity and low environmental impact. Plant nutrients such as sulfate and nitrate are possible though these may pose a risk of environmental eutrophication when discharged via the effluent S* while this disadvantage does not occur with carbonate and hydrogen carbonate.

[89] Various combinations of cations and anions may be made for the aqueous solution

S. For example, the aqueous solution S comprises at least one of calcium chloride, calcium sulfate, calcium nitrate, calcium hydrogen carbonate, magnesium chloride, magnesium sulfate, magnesium nitrate, magnesium carbonate, magnesium hydrogen carbonate, potassium chloride, potassium sulfate, potassium nitrate, potassium carbonate, potassium hydrogen carbonate, ammonium chloride, ammonium sulfate, ammonium nitrate, ammonium carbonate, ammonium hydrogen carbonate, hydrogen chloride, sulfuric acid, nitric acid and carbonic acid. In particular advantageous embodiments, the aqueous solution S comprises calcium hydrogen carbonate, potassium carbonate and / or potassium hydrogen carbonate, preferably potassium hydrogen carbonate. These compounds are highly soluble in water, thus allowing high concentrations thereof for efficient removal or exchange of sodium ions from the cation exchange resin. Potassium hydrogen carbonate is particularly preferred because potassium serves as a plant nutrient of which even high concentrations are generally acceptable and even desirable for plants while hydrogen carbonate avoids polluting the effluent S*. It is noted that the combination of calcium and carbonate in the aqueous solution S is not preferred because of insolubility of calcium carbonate in water.

[80] The aqueous solution S may be prepared by dissolving a salt comprising the cations and anions in water to obtain the aqueous solution S with a desired concentration of its cations. Advantageous alternatives for the production of the aqueous solution S are described below.

[91] The aqueous solution R of step S4 may comprise one or more than one acid selected from a group containing hydrogen chloride, sulfuric acid, nitric acid, phosphoric acid, carbonic acid, hydrogen carbonate, acetic acid and oxalic acid. At least the conjugate bases of sulfuric acid, nitric acid and phosphoric acid — that is sulfate, nitrate and phosphate — are known to be plant nutrients. Nitric acid is most preferred because it is a strong acid allowing a high proton concentration for efficient regeneration of the cation exchange resin. Further, nitric acid introduces nitrate into the effluent R*. Nitrate is a ubiquitous plant nutrient of which high concentrations can be tolerated or are even desired in cultivation water. In an advantageous combination with potassium as the cation or one of the cations in the aqueous solution S, which is captured by the cation exchange resin in the step S2 and released in the step S4 as part of the effluent R*, the method provides potassium nitrate in effluent R*, which is a valuable plant nutrient. In such case and other cases, it may thus be preferred that at least the effluent R* is recirculated for use for plants in the step S5.

[92] Inthe illustrated example embodiment, the method further comprises an optional step S6 of contacting carbon dioxide with an aqueous alkali solution to produce the aqueous solution S. To this end, the aqueous alkali solution comprises the cations of the aqueous solution S and hydroxide. The carbon dioxide is preferably in gaseous form. The aqueous solution S thus comprises hydrogen carbonate by dissolution of carbon dioxide in the aqueous alkali solution as well as the cations of the aqueous solution S already present in the aqueous alkali solution. The step S6 allows in-line production of the aqueous solution S, enabling continuous production in an automated and adjustable fashion while reducing risks such as overpressure associated with overdosing hydrogen carbonate in the aqueous solution S.

[83] The step S6 may thus be seen as providing the aqueous solution S for the step S2 in an advantageous manner. By using carbon dioxide and hydroxide, the anions of the aqueous solution S comprise carbonic acid, hydrogen carbonate and / or carbonate depending on pH value achieved in combination with the aqueous alkali solution.

Preferably, the pH value is adjusted to 7 + 0.5 to favour hydrogen carbonate. More generally, the pH can be adjusted to a value of at least 6 to favour hydrogen carbonate and/or carbonate.

[94] A step S7 of contacting the at least one of the effluent F* and the effluent R* with an aqueous alkali solution to adjust pH is further illustrated, though this step is optional.

Preferably, this pH adjustment leads to neutralisation of the at least one of the effluent F* and the effluent R* which are recirculated for use for plants. If both the step S6 and the step S7 are performed, it is preferred that the aqueous alkali solution for pH adjustment of the recirculated effluent in the step S7 is the same as the aqueous alkali solution for producing the aqueous solution S in the step S6. In FIG. 1, the step S7 is illustrated as following on the step S6, however, these may occur simultaneously or in reversed order.

[95] The aqueous alkali solution employed in the step S6 of producing the aqueous solution S may thus be the same as or differ from the aqueous alkali solution employed in the step S7 of neutralising the at least one of the effluent F* and the effluent R*.

Generally, the effluent F* of the step S1 is acidic because of protons removed from the cation exchange resin via exchange by cations captured by the cation exchange resin, while the effluent R* of the step S4 is acidic because of use of an excess of protons in the aqueous solution R in order to regenerate the cation exchange resin by removing captured cations from the cation exchange resin and replacing these with protons.

[96] The aqueous alkali solution in the step S6 and / or the step S7 can comprise any of the cations listed for the aqueous solution S employed in the step S2.

[97] Though neutralisation to a pH value of about 7 is often preferred, alternatively, an adjustment or increase of pH value of the recirculated effluent (that is, the at least one of the effluent F* and the effluent R*) may be performed without achieving complete neutralisation of said effluent. This may be advantageous for acid loving plants that thrive on cultivation water with a pH value below 7. By adding an excess of aqueous alkali solution to the recirculated effluent, it is even possible to increase pH value to above 7.

[98] The method as illustrated further comprises an optional step S8 of determining a sodium saturation level of the cation exchange resin, wherein the step of contacting the aqueous solution S with the cation exchange resin is executed based on the determined sodium saturation level of the cation exchange resin. Preferably a maximum sodium saturation level of the cation exchange resin is determined to ensure removal of a maximum amount of sodium anions from the filtrate F of the cultivation water in each iteration of the method. However, optimisation with respect to time-efficiency may lead to a predetermined sodium saturation level that is different from, in particular below, the maximum sodium saturation level of the cation exchange resin. As alternative to the sodium saturation level of the cation exchange resin, sodium ion breakthrough may be employed.

[99] The step S8 of determining the sodium saturation level of the cation exchange resin may comprise determining pH value and / or electrical conductivity of effluent coming from the cation exchange resin. The maximum sodium saturation level or the sodium ion breakthrough moment may be determined based on an increase in pH value by a predetermined amount relative to a minimum pH value determined during the step

S1 of contacting the filtrate F with the cation exchange resin. Alternatively or additionally, a decrease in the electrical conductivity may be used. The step S8 is further explained in connection with FIG. 2 below.

[100] The embodiment of FIG. 1 further shows three optional yet advantageous rinsing steps S9 (individually labelled $9.1, $9.2 and $9.3) that may be implemented jointly or separately as follows. - The step S9.1 of rinsing the cation exchange resin with water may be implemented after the step S1. The step $9.1 leads to a first rinsing effluent coming from the cation exchange resin upon rinsing it with water. The first rinsing effluent is preferably recirculated for use for plants. - The step $9.2 of rinsing the cation exchange resin with water may be implemented after the step S2. The step S9.2 leads to a second rinsing effluent coming from the cation exchange resin upon rinsing it with water. The second rinsing effluent is preferably discharged. - The step S9.3 of rinsing the cation exchange resin with water may be implemented after the step S4. The step S9.3 leads to a third rinsing effluent coming from the cation exchange resin upon rinsing it with water. The third rinsing effluent is preferably recirculated for use for plants.

[101] Another optional step is a step S10 of performing a backwash of the cation exchange resin. The backwash step S10 comprises flushing the cation exchange resin with pressurised water in a direction with a component against gravity. This enables resettlement of the cation exchange resin for improved performance in a next iteration of the method. The backwash step S10 need not be implemented with each iteration of the method.

[102] The rinsing steps S9.1-9.3 and the backwash step S10 are preferably performed with fresh water to which no or hardly any plant nutrients have been added, such as ground water, rain water, drinking water, softened water or reverse osmosis water.

Though rinsing and backwashing are preferred for optimal performance, the method can be executed without these steps and still achieve substantial reduction of sodium content of cultivation water.

[103] In the illustrated embodiment, the step S5 of recirculating the at least one of the effluent F* and the effluent R* comprises one or more than one of the following four optional steps. - A step S11 of feeding the at least one of the effluent F* and the effluent R* to an intermediate supply for cultivation water. This allows passive homogenisation of the recirculated effluent in the intermediate supply without directly exposing cultivated plants to changing composition of the recirculated effluent. - A step S12 of homogenising the intermediate supply, that is, actively homogenising contents of the intermediate supply, for example by pumping or stirring.

Steps S11 and S12 are also beneficial when combined with the pH-adjustment step S7. - A step S13 of feeding content of the intermediate supply to a usage supply of cultivation water. This step serves to reduce changes in composition of the usage supply of cultivation water from which plants are to be fed. - A step S14 of rinsing conduits with water to the usage supply. That is, conduits leading towards the usage supply. This step ensures that any plant nutrients produced by the method are transported to the usage supply.

[104] In FIG. 1, the method is illustrated as a batch process, which is restarted after the step S10. Alternatively, the step S1 may again be performed after the step S4 or the step

S9.3. The step SO of nanofiltration can be performed continuously. To match the continuous production of the filtrate F to the batch process starting with the step S1, an intermediate filtrate storage is envisaged in which the filtrate F can accumulate until the batch process can again start with the step S1. Alternatively, the step SO of nanofiltration can be performed when the step S1 of loading the cation exchange resin with the filtrate F is performed. The step SO may then be performed intermittently and simultaneously with the step S1.

[105] Fig. 2 shows three graphs with data for determining sodium saturation level of the cation exchange resin in the step S8 from a lab-scale experiment. The horizontal axes of the graphs indicate effluent volume coming from the cation exchange resin. Because of the constant flow rate of 12.7 L / hour, this axis can also be seen as an indication of time.

The vertical axes indicate concentration of sodium ions and potassium ions for the top graph and electrical conductivity (EC) or pH for the remaining graphs. Data points were collected by taking effluent samples, though automated in-line measurements are preferred in implementing the invention. The cation exchange resin was Amberlite™

HPR1200H, having a cation selectivity in the order:

Ca?" > Mg?" > K* > NH4* > Na* > H*

[108] In all experiments presented here, a representative composition of cultivation water was used, comprising the following ions: potassium (6 mmol/L), sodium (6 mmol/L), magnesium (4 mmol/L), calcium (7 mmol/L), nitrate (13 mmol/L), chloride (6 mmol/L), sulfate (7 mmol/L). In these experiments, the cultivation water was contacted with the cation exchange resin directly without first undergoing nanofiltration.

[107] The step S8 of determining the sodium saturation level of the cation exchange resin may comprise determining pH value of effluent coming from the cation exchange resin. The step S2 of contacting the aqueous solution S with the cation exchange resin is then executed based on the determined sodium saturation level of the cation exchange resin. The maximum sodium saturation level may be determined based on an increase in pH value by a predetermined amount, for example by at least 0.2 or by about one pH, relative to a minimum pH value determined during the step of contacting cultivation water with the cation exchange resin.

[108] Additionally or alternatively, the step of determining the sodium saturation level of the cation exchange resin may comprise determining electrical conductivity of effluent coming from the cation exchange resin. Preferably, the maximum sodium saturation level is determined based on a decrease in electrical conductivity by a predetermined amount, for example in a range of 0.1 — 1 mS or a decrease by one-tenth, relative to a maximum electrical conductivity determined during the step S1 of contacting cultivation water with the cation exchange resin.

[109] pH value and / or electrical conductivity of effluent coming from the cation exchange resin provide measures of concentration of sodium ions in said effluent. In turn, this concentration is an indicator of sodium saturation level of the cation exchange resin.

For example, increasing pH value and / or decreasing electrical conductivity during loading of the cation exchange resin with cations and removal of protons (such as by contacting cultivation water with the cation exchange resin in the step S1) are indicative of increasing cation concentration and reducing proton concentration in the effluent.

Especially when sodium ions have a low affinity for the cation exchange resin (compared to other cations), increase in pH value and decrease in electrical conductivity each, and preferably in combination, indicate sodium ions passing through the cation exchange resin without being captured by it and thus provide a measure of sodium ion breakthrough. Since sodium saturation level of the cation exchange resin is linked to sodium ion breakthrough moment, the latter can serve as an indirect measure of the former.

[110] The cation exchange resin is loaded with cultivation water in the step S1, which is performed at the start of the graphs of FIG. 2 (effluent volume of about 21 L) up to about 48 L, as indicated by time t1. The time t1 corresponds to the moment of switching from the step S1 of loading the cation exchange resin with cultivation water to the step S9.1 of rinsing the cation exchange resin with water.

[111] Up to time t1, the cation exchange resin is being loaded with cations from the cultivation water and protons are removed from it, giving rise to a low pH for the associated effluent. From an effluent volume of about 40 L, the concentration of sodium ions in the effluent increases, indicating breakthrough of sodium ions. The pH of the effluent therefore increases, as seen in the bottom graph. Simultaneously, the electric conductivity decreases, as seen in the middle graph, because protons have a higher electrical conductivity than the sodium ions and any other cations now present in the effluent. In this case, sodium ions break through first because of the low selectivity of the cation exchange resin for sodium ions compared to other cations.

[112] Atthe time t1, performing of the step S1 ends and the step $9.1 is performed.

During the step S9.1, the cation exchange resin is rinsed with fresh water, removing remaining cultivation water and any unbound cations, including protons removed from the cation exchange resin and now in solution. The effluent arising from this step has a further decreased electrical conductivity and a further increased pH. Though the step S9.1 is here performed from the time t1 to a later time t2, it is possible to omit the step $9.1 and switch directly from the step S1 to the step S2.

[113] At the time t2, the process switches from the step S9.1 to the step S2 of loading the cation exchange resin with the aqueous solution S to remove captured sodium ions by exchange with selected cations. In the present example, the aqueous solution S includes potassium hydrogen carbonate. As the step S2 is performed, the concentration of sodium ions in the effluent increases before reaching a maximum at about 58 L and decreasing thereafter. Potassium ions of the aqueous solution S now load the cation exchange resin while removing therefrom the sodium ions previously captured by the cation exchange resin from the cultivation water. The sodium ions are thus exchanged by potassium ions which have a higher affinity for the cation exchange resin. Potassium ions start to appear in the effluent from about 56 L, indicating break through of these ions present in the aqueous solution S. The step S2 is continued until the sodium concentration in the effluent is (again) below a predetermined threshold. Electrical conductivity and / or pH value are indicative of this threshold.

[114] Attime t3, performing the step S2 ends. The process may here switch to performing the step S4 by regenerating the cation exchange resin with aqueous solution

R. Alternatively, the step S9.2 of rinsing the cation exchange resin with water may be performed before continuing with the step S4.

[115] The times t1, t2 and / or t3 may be automatically determined via monitoring of the electrical conductivity and / or the pH of effluent flowing from the cation exchange resin.

That is, the step S1 may be terminated upon detection of a predetermined pH value and / or electrical conductivity. Likewise, execution of the step $9.1 and / or the step S2 may be performed based on pH value and / or electrical conductivity of the effluent. In addition to the timing of the method steps, flow rate, volume and / or composition of the aqueous solution S may be controlled based on the determined sodium saturation level of the cation exchange resin, preferably through the pH value and / or electrical conductivity of the effluent F*. This can be implemented by electronic control mechanisms.

[116] In the experiments presented here, pH value and electrical conductivity are shown as indicators of sodium ion concentration, which in turn is a measure of sodium saturation level of the cation exchange resin when measured in the effluent coming from the cation exchange resin. However, a direct measurement of the concentration of sodium ions is also possible with manual or automated sampling or by dedicated ion-specific sensors. It is also noted that when a constant concentration of sodium ions in the effluent coming from the cation exchange resin is detected, a maximum amount of sodium ions is captured by the cation exchange resin. This indicates a dynamic equilibrium where the incoming concentration of sodium ions is equal to the outgoing concentration of sodium ions. However, a maximum captured amount of sodium ions may not be the most efficient embodiment of the method to remove as large an amount of sodium ions from cultivation water in a particular amount of time. In the test of FIG. 2, a trade-off is presented to remove a substantial amount of sodium ions in a relatively short time.

[117] Further test results are given in table 1 and 2 below, each table presenting data at a different flow rate of fluid over the cation exchanger resin. BV is bed volume, the effluent volume compared to the volume of the cation exchange resin bed, and EC is electrical conductivity. ea eee ed

[118] From the above data, absolute or relative changes in pH and / or electrical conductivity (EC) prove indications of sodium ion breakthrough in the step S1 and therefore indirectly of sodium saturation level of the cation exchange resin. A reduction in electrical conductivity of 0.1 mS is sufficient to indicate that sodium ions break through the cation exchange resin. A threshold of 0.2 mS, 0.5 mS or 1 mS is also suitable.

Corresponding relative decreases may also be employed. Further, an increase in pH value of at least 0.2 can be employed as alternative indicator or in addition to the electrical conductivity. For example, a combined threshold for pH increase and electrical conductivity decrease during the step S1 can be set with predetermined values to optimise sodium ion capture by the cation exchange resin.

[119] FIG. 3 shows an apparatus 1 for reducing sodium content of cultivation water for plants. The apparatus 1 comprises a nanofiltration unit 50 and a cation exchanger 2 with a cation exchange resin 3 configured to capture at least sodium ions. The cation exchanger 2 is in fluid connection with a cultivation water feed 4 via the nanofiltration unit 50, a removal agent facility 5, a regeneration agent facility 6, a discharge 7 and a recirculation facility 8. The nanofiltration unit 50 is arranged between the cultivation water feed 4 and the cation exchanger 2. The apparatus is configured to guide the filtrate F from the nanofiltration unit 50 to the cation exchanger 2, for example via a filtrate conduit 51, while the retentate E from the nanofiltration unit 50 may be recirculated for use for plants, for example via a retentate conduit 52. Intended flow directions are indicated.

[120] The nanofiltration unit 50 is in fluid connection with the cultivation water feed 4 and the cation exchanger 2 and comprises a nanofiltration membrane configured to soften water, i.e. to separate divalent ions such as calcium from an incoming aqueous solution, here cultivation water. The cultivation water feed 4 is configured to guide cultivation water to the nanofiltration unit 50 and thereby provide the retentate E and the filtrate F which respectively have an increased and a decreased concentration ratio of divalent cations versus monovalent cations compared to the cultivation water.

[121] A description of the retentate E, the filtrate F, the aqueous solutions S, R and the effluents F*, S*, R* has been given above in relation to the method and equally applies for the apparatus 1, which is configured to be compatible with the selected compounds.

Moreover, the apparatus 1 may be configured to execute at least one embodiment of the method.

[122] The apparatus 1 preferably further comprises a filtrate storage 53 arranged in fluid connection between the nanofiltration unit 50 and the cation exchanger 2. The filtrate storage 53 may be configured to receive the filtrate F continuously from the nanofiltration unit 50, for example via the filtrate conduit 51, and guide the filtrate F in batches to the cation exchanger 2.

[123] In the illustrated embodiment of the apparatus 1, the cation exchanger 2 comprises an upper inlet 9, an intermediate inlet 10 and a lower inlet 11, where the indications upper, intermediate and lower are with respect to gravity. Preferably, a filling level 12 of the cation exchange resin 3 in the cation exchanger 2, at least in an unexpanded state of the cation exchange resin 3, is such that the upper inlet 9 is above the filling level 12 while the intermediate inlet 10 is just below the filling level 12 and the lower inlet 11 is at or near the bottom of the cation exchanger 2. Though three inlets are illustrated here to achieve efficient operation of the apparatus 1, only one inlet/outlet is required to enable the fluid connection between the cation exchanger 2 with the nanofiltration unit 50, the removal agent facility 5, the regeneration agent facility 6, the discharge 7 and the recirculation facility 8, for example via a manifold. Each of the inlets 9, 10, 11 may also serve as outlet, thus forming an inlet/outlet.

[124] The cultivation water feed 4 is configured to guide cultivation water to the nanofiltration unit 50, resulting in the filtrate F which flows out of the nanofiltration unit 50.

The apparatus is configured to guide the filtrate F to the cation exchanger 2. Preferably, the filtrate F is guided to the filtrate storage 53 before being guided towards the cation exchanger 2. Preferably, the filtrate F enters the cation exchanger 2 via the upper inlet © to contact the cation exchange resin 3 while the effluent F* emerges from the lower inlet 11, now serving as outlet. The cultivation water feed 4 may further comprise a filter 13 that is different from a nanofiltration unit, e.g. a coarse particulate filter, to prevent downstream contamination and / or a pump 57 to feed cultivation water into the nanofiltration unit 50. The pump 57 may be adjustable and controllable by a wired or wireless controller 15. The cultivation water feed 4 may be connected to or is connectable to a supply of cultivation water from which sodium content is to be reduced, such as drainage water, recycled cultivation water, freshly produced cultivation water and water sources suitable for use for plants. Other pumps 14 can be arranged in the apparatus 1 as illustrated.

[125] The removal agent facility 5 is configured to guide the aqueous solution S to the cation exchanger 2 in order to remove sodium ions from the cation exchange resin 3, which results in the effluent S* comprising sodium ions. Preferably, the aqueous solution

S is fed into the cation exchanger 2 via the upper inlet 9 while the effluent S* emerges from the lower inlet 11, now serving as outlet. Alternatively, the aqueous solution S may be fed into the cation exchanger 2 via the intermediate inlet 10 and / or the lower inlet 11 and / or an additional inlet (not shown in fig. 3) arranged between the intermediate inlet 10 and the lower inlet 11, in which case cations of relatively weak affinity for the cation exchange resin 3 (such as sodium ions, which tend to be captured in lower parts of the cation exchanger 2 when loading from top to bottom) are even more selectively removed from the cation exchanger 2 compared to cations with higher affinity (such as calcium and magnesium, which tend to be captured in higher parts of the cation exchanger 2). The aqueous solution S may thus be fed into the cation exchanger 2 via at least one of the upper inlet 9, the intermediate inlet 10, the lower inlet 11 and the additional inlet between inlets 10 and 11, while the effluent S* may be collected from at least one of the upper inlet 9, the intermediate inlet 10 and the lower inlet 11, preferably different from the inlet used for the aqueous solution S.

[126] Preferably, the removal agent facility 5 is further configured to provide the aqueous solution S. Various ways of implementing this feature are described below.

[127] The regeneration agent facility 6 is configured to guide the aqueous solution R to the cation exchanger 2 in order to regenerate the cation exchange resin 3, which results in an effluent R*. In a simple embodiment, the regeneration agent facility 6 is connectable to a supply of the aqueous solution R. Further, the regeneration agent facility 6 may be configured to provide the aqueous solution R, for example by means of a pump 14 preferably coupled to a tank 16 comprising the aqueous solution R or a concentrate thereof, which may be combined with water from fresh water feed 23 for dilution to a desired concentration. Preferably, the aqueous solution R is fed into the cation exchanger 2 via the lower inlet 11 while the effluent R* emerges from the intermediate inlet 10, now serving as outlet. It is further preferred that fresh water is pumped into the upper inlet 9 during regeneration of the cation exchange resin 3, which also emerges from the intermediate inlet 10, to stimulate regeneration by compacting the cation exchange resin 3 towards the bottom of the cation exchanger 2.

[128] The discharge 7 is configured to discharge the effluent S*. The discharge 7 ensures that the effluent S*, containing concentrated sodium, is removed from the cultivation water and not recirculated for use for plants. Preferably, the discharge 7 is in fluid connection with at least the inlet 11 of the cation exchanger 2, here serving as outlet.

The discharge 7 may lead to various disposal pathways, such as a treatment plant, a sewer system, a well and surface water. Preferably, the discharge 7 comprises at least one active carbon bed 17, which is configured to purify or filter the effluent S*, for example by filtering out pesticides.

[129] The recirculation facility 8 is configured to recirculate at least one of the retentate

E, the effluent F* and the effluent R* for use for plants. The recirculation facility 8 may be configured to guide effluent directly from the cation exchanger 2 to said plants or to a usage supply 18 of cultivation water for plants. It is preferred that the recirculation facility 8 is configured to recirculate the at least one of the effluent F* and the effluent R* to the usage supply 18 via an intermediate supply 19 for cultivation water. By using the intermediate supply 19 for the recirculated effluent, potentially strong fluctuations in the composition of cultivation water are reduced and thus negative effects on the cultivated plants prevented or at least mitigated. For example, the effluent F* and R* may have a pH value or nutrient concentration which varies strongly in time but which is averaged out in the intermediate supply 19, preferably by homogenising the contents of the intermediate supply 19. By mixing said contents, for example by stirring and / or circulating through the apparatus 1, fluctuations in cultivation water composition can be further reduced. The intermediate supply 19 further allows controlled feeding of recirculated effluent to plants directly or into the usage supply 18 depending on desired composition of the cultivation water to be used for said plants. The recirculation facility may be configured to guide the retentate E from the nanofiltration unit 50 to the usage supply 18, for example via the retentate conduit 52 as illustrated. However, though less preferred, the retentate E may also be guided to the intermediate supply 19 just like the effluents F* and R*.

[130] Inthe embodiment of FIG. 3, the recirculation facility 8 allows recirculation of effluent to the intermediate supply 19, from which contents may be circulated through the apparatus 1, both by means of the pump 14 which is also associated with feeding the filtrate F to the cation exchanger 2. Further, the recirculation facility 8 allows recirculation of effluent directly into the usage supply 18 or via the intermediate supply 19. An additional conduit may be arranged between the intermediate supply 19 and the usage supply 18 to provide a direct fluid connection between the two, preferably with an additional pump 14 to allow operation independent from other components of the apparatus 1. When circulating the effluents F* and/or R* back through the apparatus 1 from the intermediate supply 19, for example to mix the fluids or adjust pH thereof, itis preferred that the retentate E with its increased concentration ratio of divalent cations verses monovalent cations, is guided directly to the usage supply 18 without being mixed into intermediate supply 19 or with circulated effluent so that the divalent cations from the retentate are prevented from contacting the cation exchanger resin 3.

[131] The apparatus 1 may further comprise a sodium saturation level sensor 20 configured to determine a sodium saturation level of the cation exchange resin 3, such as a maximum sodium saturation level of the cation exchange resin 3. Preferably, the removal agent facility 5 is controllable based on the sodium saturation level of the cation exchange resin 3 determined by the sodium saturation level sensor 20. For example, the controller 15 may be communicatively coupled to the sodium saturation level sensor 20 and the removal agent facility 5 to control the removal agent facility 5 based on the sodium saturation level of the cation exchange resin 3 determined by the sodium saturation level sensor 20.

[132] The sodium saturation level sensor 20 may comprise a pH sensor 21 and / or an electrical conductivity sensor 22 arranged near an effluent outlet of the cation exchanger 2, such as downstream from the lower inlet 11.

[133] The apparatus may further comprise a fresh water feed 23 configured to guide fresh water to the cation exchanger 2. In the illustrated embodiment, fresh water is guided from the fresh water feed 23 to the cation exchanger 2 by means of the pump 14 associated with feeding filtrate F from the filtrate storage 53 to the cation exchanger 2. A separate pump may also be employed. The fresh water feed 23 is connected to (or at least connectable to) a source of water suitable for use for plants, such as a tap or drinking water pipeline, a ground water well, a softened water source or reverse osmosis water installation. This may in fact involve an additional apparatus 1, as described in relation to FIG. 4.

[134] Further, the apparatus 1 may comprise a pH sensor 21 and / or an electrical conductivity sensor 22 at an input side of the cation exchanger 2 to monitor composition of fluids flowing towards the cation exchanger 2. Preferably, said pH sensor 21 and / or electrical conductivity sensor 22 are arranged upstream from the upper inlet 9 of the cation exchanger 2. Advantageous but non-limited use of these sensors is as follows. The pH sensor 21 can be employed to monitor acidity of the aqueous solution S. The electrical conductivity sensor 22 can be employed to monitor cation content of the filtrate F fed into the cation exchanger 2. Even when such sensor is configured to measure ion content more generally (e.g. cations as well as anions), its readings can at least be correlated to cation content. If said cation content is above a predetermined threshold value, for example based on capacity of the cation exchange resin, fresh water may be mixed into the filtrate F or the cultivation water, for example from fresh water feed 23, to reduce cation concentration. The controller 15 may be configured to perform this in an automated fashion.

[135] The controller 15 of the apparatus 1 may be configured to perform at least one of the following functions, for which it is communicatively coupled to the relevant components of the apparatus 1, including valves 24, pumps 14, 57, 58 and sensors 20, 21, 22. The controller 15 may: - control nanofiltration filtrate and retentate streams, e.g. through filtrate conduit 51 and retentate conduit 52 and control of the pump 57, based on divalent and monovalent ion concentrations and / or fouling of the nancfiltration membrane; - control the cultivation water feed 4 based on filling level of the filtrate storage and / or batch cycle stage of the cation exchanger 2; - control guiding of the filtrate F towards the cation exchange resin 3 and / or control the removal agent facility 5, each preferably based on the sodium saturation level of the cation exchange resin 3 determined by the sodium saturation level sensor 20; - control the regeneration agent facility 6 and / or the fresh water feed 23 to regenerate the cation exchange resin 3; - control the discharge 7 and / or the recirculation facility 8 based on activity of at least one of the cultivation water feed 4, the removal agent facility 5 and the regeneration agent facility 6 (for example, when the controller 15 causes cultivation water to be fed to the cation exchanger 2 from the cultivation water feed 4, it causes the discharge 7 to be closed and the recirculation facility 9 to be opened); - control the fresh water feed 23 to rinse conduits and / or the cation exchanger 2; - control the fresh water feed 23 to perform a backwash of the cation exchanger 2; and - control the fresh water feed 23 to dilute fluids flowing to the cation exchanger 2, such as cultivation water, preferably based on the composition thereof monitored by the pH sensor 21 and/ or the electrical conductivity sensor 22 at the input side of the cation exchanger 2.

[136] In summary, the controller 15 may be configured to cause execution of any of the steps of the method of the invention in the apparatus 1.

[137] Various embodiments of the removal agent facility 5 are now described. In a simple embodiment, the removal agent facility 5 is connected or connectable to a supply of the aqueous solution S, such as a vessel or tank. However, the removal agent facility 5 may also be configured to produce the aqueous solution S.

[138] The removal agent facility 5 may be configured to combine the cations and the anions of the aqueous solution S with water to produce the aqueous solution S at the desired concentration. Any of the cations and the anions mentioned above in relation to the method may be employed with the apparatus 1. Said cations and anions may be in solid form, such as powder, or in the form of a stock solution that is to be diluted to the desired concentration.

[139] In a preferred embodiment, the removal agent facility 5 comprises a first feed 25 and a second feed 26 in fluid connection with the first feed 25. The first feed 25 is configured to provide the anions of the aqueous solution S while the second feed 26 is configured to provide the cations of the aqueous solution S. At least one of the first feed and the second feed 26 may be configured to provide water, or the fresh water feed 23 may be coupled to the removal agent facility 5.

[140] The removal agent facility 5 may further be configured to produce the aqueous 25 solution S by contacting carbon dioxide, preferably gaseous carbon dioxide, which is supplied via the first feed 25 with an aqueous alkali solution which is supplied via the second feed 26, wherein the aqueous alkali solution comprises hydroxide and the cations of the aqueous solution S. Thus, the removal agent facility 5 can produce the aqueous solution S from (gaseous) carbon dioxide and said aqueous alkali solution. In this case, the anions of the aqueous solution S are hydrogen carbonate and / or carbonate.

[141] In FIG. 3, the second feed 26 takes the aqueous alkali solution from a tank 16 via a pump 14, preferably an adjustable pump, into a conduit leading to the cation exchanger 2, in particular to the upper inlet 9 thereof. Fresh water may also be provided at this stage to adjust hydroxide concentration. The first feed 25 provides carbon dioxide into the same conduit. Gaseous carbon dioxide is provided from a gas cylinder 27 at a pressure adjusted by a valve 24, such as a pressure regulator, and mixed into the stream of the aqueous alkali solution. The carbon dioxide may be mixed into solution by a sparger or an injector. Mixers 28 are provided to ensure adequate mixing of fluids, both gas and liquid.

[142] When carbon dioxide and an aqueous alkali solution are employed to produce the aqueous solution S, it is preferred to monitor the pH value of the aqueous solution S. To this end, the pH sensor 21 upstream from the upper inlet 9 of the cation exchanger 2 is preferably arranged downstream from the removal agent facility 5. The controller 15 may be configured to control the first feed 25 and / or the second feed 26, and optionally also the fresh water feed 23, to adjust the pH of the aqueous solution S. Preferably, this pH value is at least 6 or in the range of 6 — 8, more preferably 7.0 — 7.5. This is advantageous because higher concentrations of hydrogen carbonate can be achieved in this range.

[143] Although gaseous carbon dioxide is preferred to produce the aqueous solution S, liquid or solid carbon dioxide are also possible.

[144] The removal agent facility 5 may further comprise a tank 30 for storing the produced aqueous solution S or a produced intermediate thereof, such as a hydrogen carbonate solution, and may further be configured to guide the aqueous solution S via said tank 30 to the cation exchanger 2. This allows production of the aqueous solution S in parallel to execution of other steps, reducing delays caused by the time needed to dissolve carbon dioxide in water.

[145] This last example may be implemented in the following way to produce an aqueous solution S comprising potassium hydrogen carbonate. Though the example of potassium is used here, any of the cations listed above may in principle be used. Further, the quantities are given as an illustration of an advantageously working example only.

[146] A 1500 L/hr fresh water stream is taken from the fresh water feed 23 and is pumped to a conduit or duct (7 m long, 14 mm internal diameter). At the beginning of this duct, a sparger (31.2 mm length, 8.05 diameter and 2 um pore size) of the first feed 25 is arranged inside the duct parallel to the flow direction. Upstream from the sparger, the second feed 26 is arranged to feed an aqueous alkali solution of 50% potassium hydroxide into the water stream at 2.76 L/min with a positive displacement membrane pump 14 in order to obtain a potassium hydroxide solution of 25 mmol/L and pH 12.4.

Carbon dioxide gas is injected via the sparger of the first feed 25. Most of this carbon dioxide dissolves into the aqueous stream along the 7 m long duct, reducing the pH to roughly 10. The aqueous solution leaving the duct is collected in a tank 30 for potassium hydrogen carbonate solution. Once a sufficient amount of the potassium hydrogen carbonate solution is produced, dosing of potassium hydroxide via the second feed 26 is stopped. The suction side of the pump 14 is then switched from the fresh water feed 23 to the potassium hydrogen carbonate tank 30 and circulated through the tank 30 and the duct. More gaseous carbon dioxide is added to the potassium hydrogen carbonate solution while circulating it until a target pH value is reached. The pH value may be monitored with the pH sensor 21. The target pH is at least 6 or in the range of 6 — 12, preferable 7 — 9, more preferable 7 — 7.5. When the target pH has been reached, the aqueous solution S is prepared for use and may remain in the tank 16 until sodium ions are to be removed from the cation exchange resin 3.

[147] Itis preferred that a fresh water stream is employed from which any calcium ions are removed in order to avoid calcium carbonate precipitation. This can readily be achieved in the apparatus 1 of the invention by guiding water from the fresh water feed 23 over the cation exchange resin 3 to remove calcium ions and then guiding the softened effluent to the removal agent facility 5 to produce the aqueous solution S. As illustrated in

FIG. 3, this can be implemented in the apparatus 1 by means of a return conduit 29 in fluid connection with the outlet 11 of the cation exchanger 2 and the removal agent facility 5, optionally including a valve 24.

[148] The filtrate F and / or the effluent F* can also be used to produce the aqueous solution S in addition or as alternative to fresh water as the filtrate F but even more so the effluent F* has already been softened. However, use of filtrate F and / or effluent F* is less preferred because it ultimately results in discharging nutrients that are present in the effluent F* once the aqueous solution S has been guided over the cation exchanger 2 resulting in the effluent S* that is to discharged.

[149] When the removal agent facility 5 of the apparatus 1 comprises the second feed 26 for the aqueous alkali solution, the second feed 5 may also serves as a neutralisation facility to neutralise the effluent F* and / or the effluent R*. Alternatively or additionally, a separate neutralisation facility may be employed.

[150] Finally, the apparatus may further comprise an anion exchanger configured to capture anions from at least one of the effluent F*, the effluent R* and the effluent S*. The anion exchanger may be arranged downstream the cation exchanger 2. By capturing anions from the effluent S*, such anions may be prevented from being discharged via the effluent S* and thus retained for use for plants. Further, when combining an anion exchanger with the cation exchanger 2, protons in the effluent F* and / or the effluent R* may be reused in the anion exchanger to neutralise hydroxide anions released from an anion exchange resin in the anion exchanger. Further, the hydroxide anions may serve as components of the aqueous alkali solution used for neutralising the at least one of the effluent F* and the effluent R*. Advantageously, the aqueous alkali solution (used for producing the aqueous solution S) may also be employed to regenerate the anion exchange resin. Moreover, conduits, valves and pumps may be used for both the cation exchanger 2 and the anion exchanger in an efficient way. An apparatus 1 comprising both the cation exchanger 2 as well as the anion exchanger allows removal of not only sodium ions but also unwanted anionic species from cultivation water. Alternatively or additionally, the combination of the cation exchanger 2 and the anion exchanger may be configured to remove unwanted neutral species by recovering cationic and anionic nutrients from the cultivation water.

[151] FIG. 4 shows a modification of the apparatus 1 of FIG. 3 in which the nanofiltration unit 50 is arranged differently. In the embodiment of FIG. 4, the apparatus 1 is particularly suited to treat cultivation water not yet circulated through a cultivation system, here fresh water introduced into the apparatus 1 by means of fresh water feed 23. The arrangement of FIG. 4 can be implemented as an alternative to that of FIG. 3, while it is also envisaged to use these arrangements together. That is, having a first apparatus 1 to treat recirculating cultivation water and a second apparatus 1 to treat fresh water (e.g. as illustrated in FIG. 6 and FIG. 7).

[152] The apparatus 1 of FIG. 4 will now be described in so far as it differs from that of

FIG. 3. The fresh water feed 23 guides fresh water to the nanofiltration unit 50. A by-pass 59 is arranged over the nanofiltration unit 50 so that fresh water may also be guided directly to the cation exchanger 2, for example via a valve 24 and a pump 14. From the nanofiltration unit 50, the retentate 52 may be guided towards the cation exchanger 2 and / or to the usage supply 18, for example by water to usage supply conduit 60. The filtrate conduit 51 is configured to guide filtrate from the nanofiltration unit 50 to a reverse osmosis unit 54, for example via a pump 58. The reverse osmosis unit 54 may be arranged between the nanofiltration unit 50 and the filtrate storage 53, if the latter is present in the apparatus 1.

[153] The reverse osmosis unit 54 may be arranged in fluid connection between the nanofiltration unit 50 and the cation exchanger 2. The apparatus 1 is further configured to guide the filtrate F from the nanofiltration unit 50 to the reverse osmosis unit 54 and thereby provide a concentrated filtrate F, e.g. via a concentrate conduit 56, and a permeate, e.g. via a permeate conduit 55. The apparatus 1 may be configured to guide the concentrate filtrate F to the cation exchanger 2, preferably via the filtrate storage 53 if present. The permeate is preferably recirculated for use for plant, for example by means of the recirculation facility 8. The water to usage supply conduit 60 can be employed for this purpose.

[154] The reverse osmosis unit 54 and it associated conduits can also be employed in the apparatus 1 described in relation to FIG. 3. The by-pass 29 can also be implemented there independently from any reverse osmosis unit 54.

[155] Alternatively or additionally to the reverse osmosis unit 54 arranged to concentrate the filtrate F, a reverse osmosis unit 54 can be arranged at various other positions in the apparatus 1. For example, a reverse osmosis unit 54 can be arranged between the cation exchanger 2 and the discharge 7 to receive the effluent S* from the cation exchanger 2 and to concentrate the effluent S* before it is discharged from the apparatus 1 via the discharge. In this example, the concentrated effluent S* is discharged while the permeate can be recirculated for use for plants. In this way, loss of water can be reduced.

[156] It is also conceived to use one and the same reverse osmosis unit 54 for providing both the concentrated filtrate F as well as the concentrated effluent S*. This can be implemented efficiently in parallel to the batch process of cation exchange, for example during sodium removal, regeneration and / or washing steps. Further, the concentrated filtrate F may be kept in the filtrate storage 53 while the effluent S* undergoes reverse osmosis.

[157] FIG. 5 shows a cultivation system 40 for plants in a preferred embodiment. The cultivation system 40 for plants comprises a cultivation space 41, here shown as a green house, though uncovered or open cultivation spaces are also possible. The cultivation system 40 further comprises a usage supply 18 of cultivation water, an apparatus 1 according to the invention, feed conduits 42 configured to guide cultivation water from the usage supply 18 to the cultivation space 41, return conduits 43 configured to guide drainage water from the cultivation space 41 to the apparatus 1 and recirculation conduits 44 configured to recirculate cultivation water with a reduced sodium content from the apparatus 1 to the usage supply 18. In the illustrated example, an apparatus 1 of FIG. 3 is preferably employed, though any of the described modifications can be used.

[158] The cultivation system 40 enables efficient recirculation of cultivation water for plants by reducing sodium content of the cultivation water. A disadvantageous build-up of sodium ions is prevented or at least mitigated. This reduces water consumption from external sources and prevents environmental contamination or eutrophication by reducing discharge of cultivation water and / or components thereof compared to conventional systems.

[159] Preferably, the cultivation system 40 further comprises a disinfection device 45 configured to disinfect cultivation water. This reduces risk of germs such as parasites, viruses and bacteria that harm the plants that are to be cultivated. The disinfection device 45 may comprise a UV source, sonication device, ozonation and / or heater to perform disinfection.

[160] Additionally or alternatively, the cultivation system 40 may further comprise a nutrient dosing device 46 configured to add plant nutrients to cultivation water. A sensor, such as a pH sensor 21 and / or an electrical conductivity sensor 22, may be placed downstream from the nutrient dosing device 46 to monitor nutrient content. Some of the plant nutrients in the cultivation water may have been added via the apparatus 1 via the aqueous solution S and / or the aqueous solution R.

[161] Additionally or alternatively, the cultivation system 40 may further comprise a fresh water feed 23 configured to add fresh water to cultivation water. In the illustrated embodiment, the fresh water feed 23 of the apparatus 1 is shared with the cultivation system 40 though these may be separate.

[162] When the nutrient dosing device 46 and the fresh water feed 23 are both used, it is preferred to arrange the pH sensor 21 and / or the electrical conductivity sensor 22 downstream from the nutrient dosing device 46 in order to monitor composition of the cultivation water and control the fresh water feed 23 to adjust the inflow of fresh water from the fresh water feed 23 to obtain a desired composition of the cultivation water.

[163] The usage supply 18 of the cultivation system 40 preferably comprises a first tank 47 for drainage water, a second tank 48 for purified cultivation water and a third tank 49 for finished cultivation water. In this case, as illustrated in FIG. 5 — 7, the disinfection device 45 is preferably arranged between the first tank 47 and the second tank 48, the nutrient dosing device 46 and / or the fresh water feed 23 are preferably arranged between the second tank 48 and the third tank 49. As shown in FIG. 5 — 7, the apparatus 1 of the cultivation system 40 may recirculate effluent to the first tank 47 of the usage supply 18, preferably via the intermediate storage 19.

[184] FIG. 6 shows a modification of the cultivation system 40 for plants of FIG. 5 in which a further apparatus 1 is provided to in particular treat fresh water from the fresh water feed 23 in particular. The apparatus 1 corresponding to that of FIG. 5 may be termed the first apparatus 1, while the further apparatus 1 may be termed the second apparatus 1.

[165] The cultivation system 40 of FIG. 6 is now described in so far as it differs from that of FIG. 5. The fresh water infeed 23 provides fresh water to the second apparatus 1, in particular to the nanofiltration unit 50 thereof. The retentate E is guided in the cultivation water usage supply 18, in particular between the second tank 48 and third tank 49 thereof, via a retentate conduit 52. The retentate E may be recirculated for use for plants, preferably avoiding the cation exchange resin 3, though it may also be discharged. The cation exchanger 2 receives the filtrate F, preferably via the filtrate storage 53, resulting in the effluent F*. The effluent F*, as well as the effluent R* obtained after regeneration of the cation exchange resin 3, can be recirculated for use for plants, for example via the return conduits 43. However, the effluent F* and / or the effluent R* may also be provided to the first apparatus 1, for example as purified water for rinsing steps S9, backwashing step S10 and the like.

[166] FIG. 7 shows a further modification of the cultivation system 40 for plants of FIG. 6 in that the second apparatus 1 here comprises the reverse osmosis unit 54. In this example, both the permeate from the reverse osmosis unit 54 (via the permeate conduit 56) as well as the retentate E from the nanofiltration unit 50 (via the retentate conduit 52) are guided towards the cation exchanger 2 of the first and / or second apparatus 1. The combination of the retentate E and the permeate provides a stream from which sodium ions are more selectively removed and in which water is retained. This is particularly useful when treating fresh water, because this involves large quantities of liquid with a relatively low concentration of cations compared to recirculating cultivation water.

[167] The apparatus 1 with the reverse osmosis unit 54 finds particular advantageous use when processing fresh water. Fresh water generally has a lower concentration of cations, nutrients, pesticides and other components that are present at higher concentrations in cultivation water that has already been used for plants. Such recirculating cultivation water also usually has a much higher concentration of sodium ions. In order to remove the same amount of sodium ions from fresh water, a much larger volume of fresh water must therefore be processed compared to recirculating cultivation water. This necessitates a much larger cation exchanger, much higher flow velocity, or both, when processing fresh water compared to recirculating cultivation water. For an efficient processing of fresh water, it is therefore advantageous to increase the sodium concentration, for example by means of the reverse osmosis unit 54. Even more so because nanofiltration in many cases results in a filtrate F having a reduced sodium ion concentration compared to the incoming liquid, more water often passing through the nanofiltration membrane than being retained in the retentate E. Using nanofiltration followed by reverse osmasis results in an increased, or at least a less reduced, concentration of sodium ions in the concentrated filtrate F. By increasing the sodium ion concentration prior to contacting the cation exchange resin 3, a lower flow velocity can be used to contact the filtrate F with the cation exchange resin 3 in order to achieve more efficient sodium capture. Further, a neater separation of cations over the cation exchange resin 3 can be achieved, for example between sodium ions and potassium ions. This enables an even more selective separation of sodium ions from the cultivation water.

[168] The apparatus 1 with the reverse osmosis unit 54 may also be positioned at the location of the first apparatus 1 of FIG. 6 or FIG. 7. Moreover, one apparatus 1 can be arranged in fluid connection with both the cultivation water feed 4 and the fresh water feed 23 to switch between these two feeds, for example by means of a valve 24. In such a case, a single apparatus 1 can be employed to reduce sodium content of cultivation water by, on the one hand, providing fresh water to the cultivation system 40 after reducing the sodium content of said fresh water and, on the other hand, by reducing sodium content of recirculating cultivation water.

Claims

Conclusions

1. Method for reducing the sodium content of plant cultivation water, the method comprising the steps of: - (S0) contacting cultivation water with a nanofiltration membrane configured to soften water, resulting in a retentate E and a filtrate F which have an increased and a decreased concentration ratio of divalent cations to monovalent cations, respectively, compared to the cultivation water; - (S1) contacting the filtrate F with a cation exchange resin configured to capture at least sodium ions, resulting in an effluent F*; - (82) contacting an aqueous solution S with the cation exchange resin to remove sodium ions from the cation exchange resin, resulting in an effluent S* comprising sodium ions, the aqueous solution S comprising cations other than sodium ions and anions containing the cations be water soluble; - (S3) the discharge of the effluent S*; - (S4) contacting an aqueous solution R with the cation exchange resin to regenerate the cation exchange resin, resulting in an effluent R*, the aqueous solution R comprising an acid; and - (S5) recycling at least one of the retentate E, the effluent F* and the effluent R* for use for plants.

The method of claim 1, further comprising contacting the filtrate F with a reverse osmosis membrane resulting in a concentrated filtrate F and a permeate, the concentrated filtrate F being used in the contacting step (S1) with the cation exchange resin in the step (S1) and the permeate is preferably recycled for plant use.

Method according to claim 1 or 2, wherein the step (S5) comprises recycling the retentate E for use for plants.

Method according to any preceding claim, wherein the step (S5) comprises recirculating at least one, preferably both, of the effluent F* and the effluent R* for use for plants.

Method according to any preceding claim, wherein the cations of the aqueous solution S and/or an acid residue associated with the acid of the aqueous solution R are suitable as a plant nutrient.

A method according to any preceding claim, wherein the anions of the aqueous solution S include at least one of carbonate and hydrogen carbonate.

Method according to any preceding claim, further comprising adjusting the pH value of the aqueous solution S to at least 8, preferably in the range 6 - 12, more preferably 6 - 8, most preferably 7 + 0.5.

Method according to any preceding claim, wherein the cations of the aqueous solution S comprise at least one of calcium ions, magnesium ions, potassium ions, ammonium ions and protons, preferably potassium ions.

9. Method according to any preceding claim, wherein the aqueous solution R comprises one or more acid selected from a group consisting of hydrogen chloride, sulfuric acid, hydrogen nitrate, phosphoric acid, dihydrogen carbonate, hydrogen carbonate, acetic acid and oxalic acid, preferably sulfuric acid, hydrogen nitrate and phosphoric acid , more preferably hydrogen nitrate.

A method according to any preceding claim, further comprising a step (S6) of contacting carbon dioxide, preferably gaseous carbon dioxide, with an aqueous caustic solution comprising the cations of the aqueous solution S and hydroxide to form the aqueous solution S produce.

A method according to the preceding claim, further comprising a step (S7) of bringing at least one of the effluent F* and the effluent R* into contact with the same aqueous caustic solution to adjust the pH value of the at least one of the effluent F*and adjust the effluent R*.

Method according to any preceding claim, further comprising a step (S8) of determining a degree of sodium saturation of the cation exchange resin, preferably a maximum degree of sodium saturation of the cation exchange resin, wherein the step (S4) of contacting the aqueous solution S with the cation exchange resin is carried out based on the determined degree of sodium saturation of the cation exchange resin.

Method according to claim 12, wherein the step (S8) of determining the sodium saturation of the cation exchange resin comprises determining the pH value of effluent originating from the cation exchange resin, wherein the sodium saturation degree is preferably determined on the basis of an increase of the pH value by at least 0.2, more preferably in the range 0.2 - 1, compared to a minimum pH value determined during the step (S1) of bringing the cultivation water into contact with the cation exchange resin has been determined.

Method according to claim 12 or 13, wherein the step (S8) of determining the sodium saturation of the cation exchange resin comprises determining electrical conductivity of effluent originating from the cation exchange resin, wherein the degree of sodium saturation is preferably determined on the basis of a decrease in electrical conductivity of at least 0.1 mS, more preferably in the range of 0.1 - 1 mS, compared to a maximum electrical conductivity observed during the step (S1) of bringing the cultivation water into contact with the cation exchange resin has been determined.

15. Device for reducing the sodium content of cultivation water for plants, the device comprising a nanofiltration unit in fluid connection with a cultivation water supply and a cation exchanger in fluid connection with: - the nanofiltration unit; - an expulsion device; - a regenerant facility; - a drain; and - a recirculation facility, wherein: - the nanofiltration unit comprises a nanofiltration membrane configured to soften water; - the device is configured to guide the filtrate F to the cation exchanger and thereby provide an effluent F*; - the cation exchanger comprises a cation exchange resin configurable to capture at least sodium ions; - the cultivation water supply is configured to direct cultivation water to the nanofiltration unit and thereby provide a retentate E and a filtrate F that have respectively an increased and a decreased concentration ratio of divalent cations to monovalent cations compared to the cultivation water; - the device is configured to guide the filtrate F to the cation exchanger and thereby provide an effluent F*; - the expelling agent facility is configured to direct an aqueous solution S to the cation exchanger to remove sodium ions from the cation exchange resin and thereby provide an effluent S* comprising sodium ions, the aqueous solution S comprising cations other than sodium ions and anions associated with the cations be water soluble; - the regenerant facility is configured to direct an aqueous solution R to the cation exchanger to regenerate the cation exchange resin and thereby provide an effluent R*, the aqueous solution R comprising an acid; - the drain is configured to discharge the effluent S*; and - the recirculation facility is configured to recirculate at least one of the retentate E, the effluent F* and the effluent R* for use for plants.

An apparatus according to claim 15, wherein the apparatus further comprises a filtrate storage arranged in fluid communication between the nanofiltration unit and the cation exchanger, the filtrate storage preferably being configured to receive the filtrate F from the nanofiltration unit in a continuous manner and to guide the filtrate in batches to the cation exchanger.

An apparatus according to claim 15 or 16, further comprising a reverse osmosis unit arranged in fluid communication between the nanofiltration unit and the cation exchanger, wherein: - the apparatus is further configured to convey the filtrate F to the reverse osmosis unit to conduct and thereby provide a concentrated filtrate F and a permeate; - the device is configured to guide the concentrated filtrate F to the cation exchanger, preferably via the filtrate supply if present; and - wherein the recirculation facility is preferably further configured to recirculate the permeate for use for plants.

An apparatus according to any one of claims 15 to 17, wherein the recirculation facility is configured to recirculate the retentate E for use for plants.

An apparatus according to any one of claims 15 to 18, wherein the recirculation facility is configured to recirculate at least one, preferably both, of the effluent F* and the effluent R* for use for plants.

A device according to any one of claims 15 to 19, wherein: - the propellant means is further configured to produce the aqueous solution S, wherein the anions of the aqueous solution S comprise at least one of carbonate and hydrogen carbonate, wherein the aqueous solution S preferably has a pH value of at least 6 or in the range of 6 - 12, more preferably 6 - 8, most preferably 7 + 0.5.

21. Device as claimed in any of the claims 15 - 20, wherein: - the expulsion means provision comprises a first supply and a second supply in fluid connection with the first supply; and - the propellant supply is further configured to produce the aqueous solution S by contacting carbon dioxide, preferably gaseous carbon dioxide, supplied via the first supply with an aqueous caustic solution supplied via the second supply, the aqueous caustic solution comprising hydroxide and the cations of the aqueous solution S.

22. Device according to claim 21, wherein: - the first supply comprises an adjustable pressure valve for gaseous carbon dioxide; and - the second supply comprises an adjustable pump for the aqueous lye solution.

An apparatus according to any one of claims 15 to 22, further comprising a return line configured to direct softened effluent from the cation exchanger to the expellant supply for producing the aqueous solution S.

An apparatus according to any one of claims 15 to 24, wherein: - the expulsion means comprises a tank configured to store the produced aqueous solution S or a produced intermediate product thereof; and - the expellant supply is further configured to conduct the aqueous solution S via the tank to the cation exchanger.

25. Device according to any of the claims 15 - 24, wherein the second supply of the expelling agent provision also serves as a neutralization facility to at least partially neutralize the effluent F* and/or the effluent R*.

26. Device according to any one of claims 15 to 25, further comprising a sodium saturation degree sensor configured to determine a sodium saturation degree of the cation exchange resin, preferably a maximum sodium saturation degree of the cation exchange resin, wherein the sodium saturation degree sensor is preferably a pH sensor and/or an electrical conductivity sensor arranged near an effluent outlet of the cation exchanger.

27. Device according to claim 26, wherein the expelling agent supply can be controlled on the basis of the sodium saturation degree of the cation exchange resin determined by the sodium saturation sensor.

An apparatus according to any one of claims 15 to 27, further comprising a fresh water supply configured to direct fresh water to the nanofiltration unit and/or the cation exchanger.

29. Device according to any of claims 15 - 28, further comprising: - a pH sensor and/or an electrical conductivity sensor on an inlet side of the cation exchanger configured to monitor the composition of liquids flowing to the cation exchanger.

A device according to any one of claims 15 to 29, further comprising a control configured to control at least one of: - flows of nanofiltration filtrate and retentate based on divalent and monovalent ion concentrations and/or contamination of the nanofiltration membrane; - control the cultivation water supply based on a filling level of the filtrate storage and/or batch cycle phase of the cation exchanger; - control the conduction of the filtrate F to the cation exchanger and/or control the expelling agent supply, on the basis of the sodium saturation degree of the cation exchange resin determined by the sodium saturation sensor;

- control the regenerant supply and/or the fresh water supply to regenerate the cation exchange resin; - control the discharge and/or recirculation facility based on the activity of at least one of the cultivation water supply, the expellant supply and the regenerant supply; - control the fresh water supply to rinse pipes and/or the cation exchanger; - control the fresh water supply to perform a backwash of the cation exchanger; and - control the fresh water supply to dilute liquids flowing to the cation exchanger, such as cultivation water, based on their composition monitored by the pH sensor and/or the electrical conductivity sensor on the inlet side of the cation exchanger.

The device of any one of claims 15 to 30, further comprising an anion exchanger configured to capture anions from at least one of the effluent F*, the effluent R* and the effluent S*.

An apparatus according to any one of claims 15 - 31, which is configured to perform a method according to any one of claims 1 - 14.

33. Cultivation system for plants comprising: - a cultivation area; - a usable supply of cultivation water, preferably comprising a first tank for drainage water, a second tank for purified cultivation water and a third tank for ready cultivation water; - a first device according to any one of claims 15 - 32; - supply pipes that are configured to conduct cultivation water from the user supply to the cultivation area; - return pipes configured to conduct drainage water from the cultivation area to the facility; and - recirculation lines configured to recirculate cultivation water with a reduced sodium content from the device to the use supply, and preferably further comprising at least one of: - a disinfection device configured to disinfect cultivation water;

- a nutrient dosing device configured to add plant nutrients to cultivation water; and - a fresh water supply configured to add fresh water to the cultivation water.

34. Cultivation system according to claim 33, further comprising a second device according to any one of claims 15 to 32 which is arranged in fluid communication between a fresh water supply, which is configured to add fresh water to the cultivation water, and the return pipes.