Heat disinfection of systems for generating product water for medical use

Abstract

A system is arranged to process input water into product water for medical use. The system comprises a tank (200), which is arranged in a main flow path (1) to receive the input water, a first sub-system (300), which arranged in the main flow path to reduce water hardness, and a recirculation arrangement (9), which extends to the tank from the main flow path downstream of the first sub-system. The system is operable to perform a heat disinfection procedure, which comprises: generating, from the input water and by use of the first sub-system and the recirculation arrangement, softened water with reduced water hardness compared to the input water and accumulating the softened water in the tank; operating the system to heat the softened water; and operating the system to distribute the thus-heated softened water in at least part of the system.

Description

[0001] HEAT DISINFECTION OF SYSTEMS FOR GENERATING PRODUCT WATER FOR MEDICAL USE

[0002] Technical Field

[0003] The present disclosure relates to systems for generation of product water for medical use and, in particular, to techniques for heat disinfection of such systems.

[0004] Background Art

[0005] Many medical applications require access to purified water or "product water". For example, the product water may be used for cleaning various equipment and to produce medical fluids that are brought into contact with the body of a patient for medical treatment. Product water for medical use must meet strict requirements set by standards or guidelines, in terms of both sterility and content of potentially harmful substances. The product water may be produced from tap water by a water preparation system with purification equipment that operates, for example, by reverse osmosis and / or ion exchange.

[0006] One example of a medical application is dialysis therapy. Dialysis involves removal of water from the blood of the patient suffering from kidney failure, as well as exchange of solutes with the blood. One type of dialysis therapy is peritoneal dialysis (PD), in which a medical fluid is infused into the peritoneal cavity of the patient to interface with the blood of the patient through the peritoneal membrane. Another type of dialysis therapy is extracorporeal (EC) blood therapy, in which blood is circulated outside of the patient and interfaced with one or more medical fluids. Modalities of EC blood therapy include hemodialysis (HD), hemofiltration (HF) and hemodiafiltration (HDF).

[0007] Medical fluids used in PD and HD are commonly known as dialysis fluids. In HF, the medical fluid is known as replacement fluid, since it is infused into the blood of the patient to replace fluid removed during therapy. In HDF, both dialysis fluid and replacement fluid are used.

[0008] Over time, dialysis therapy consumes large quantities of medical fluid. It is therefore desirable to produce the required medical fluid(s) on demand at the point of care, by mixing product water with one or more concentrates. Such systems exist for some forms of dialysis therapy and it is envisioned that on-demand production will be expanded to all types of dialysis therapy in the future. This will increase the need for simple and cost-effective systems for generation of product water for medical use. For cost-effectiveness, a system for generation of product water for medical use may be configured with re-usable or permanent components to the largest extent possible. The provision of re-usable or permanent components results in a need to implement a technique of disinfecting the system, for example by heat disinfection. The heat disinfection should be performed so as to not have a major negative impact on the operative life of the system, for example in terms of the time between replacement of permanent components in the system.

[0009] Summary

[0010] It is an objective to at least partly overcome one or more limitations of the prior art.

[0011] One objective is to provide a technique for disinfecting a system for generating product water for medical use.

[0012] Another objective is to provide such a technique that has little negative impact on the life of the system.

[0013] One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by a system for generating product water for medical use, a computer-implemented method, and a computer- readable medium according to the independent claims, embodiments thereof being defined by the dependent claims.

[0014] The present disclosure proposes a technique for heat disinfection of a system that is configured to generate product water for medical use. The technique starts out from the realization that it might be beneficial to ensure that the water that is heated during the heat disinfection has a low content of solvents that contribute to water hardness. Hard water is known to cause scaling, which is the result of crystallization of solid salts, oxides, and hydroxides in the hard water. Scaling forms a solid layer on surfaces that are exposed to the hard water. The risk for scaling increases significantly with increasing temperature of the water. If uncontrolled scaling occurs during heat disinfection of a system for generation of product water for medical use, the operative life of the system is likely to be shortened significantly. The technique proposed herein capitalizes on the fact that the system includes a sub-system that is configured to reduce water hardness. This sub-system is thus utilized for preparing softened water, which is accumulated in a tank in the system in advance of heat disinfection, which is then performed by use of the softened water. The proposed technique involves clever use of existing equipment in the system to enable heat disinfection with minimal adverse consequences for the system's operative life. Still other objectives, aspects, embodiments and technical effects, as well as features and advantages may appear from the following detailed description, from the attached claims as well as from the drawings. It may be noted that any embodiment of the first aspect, as found herein, may be adapted and implemented as an embodiment of the second and third aspects.

[0015] Brief Description of the Drawings

[0016] FIG. 1 is a schematic diagram of an example water purification apparatus (WPA).

[0017] FIGS 2A-2C are block diagrams of sub-systems included in an example WPA, and FIG. 2D is a view of an example reserve osmosis (RO) unit for use in a WPA.

[0018] FIG. 3A is a block diagram of an example WPA, and FIG. 3B is a flow chart of an example method of operating the WPA in FIG. 3A.

[0019] FIG. 4A is a block diagram of an example WPA, and FIGS 4B-4D are flow charts of example methods performed in advance of and during heat disinfection of the WPA in FIG. 4A.

[0020] FIG. 5A illustrate fluid flow in a WPA during an initial phase for generation of softened water for subsequent use in heat disinfection, FIG. 5B is a graph of inlet and drain flow rates during the initial phase, and FIGS 5C-5E illustrate fluid flow during subsequent phases.

[0021] FIG. 6 illustrates fluid flow in a WPA in advance of heat disinfection.

[0022] FIGS 7A-7C are graphs of simulated data for the WPA in FIG. 6.

[0023] FIG. 8 is a flow chart of an example method performed in advance of heat disinfection of the WPA in FIG. 6.

[0024] FIG. 9 is a graph of an example drain flow rate during the method of FIG. 8.

[0025] FIGS 10A-10B are block diagrams of WPAs in accordance with alternative examples and illustrate fluid flow during a flushing phase.

[0026] FIG. 11A shows an alternative configuration of a WPA in advance of heat disinfection, and FIG. 1 IB is a flow chart of an example method performed in advance of heat disinfection of the WPA in FIG. 11 A.

[0027] FIGS 12A-12B illustrate fluid flow in the sub-systems of FIGS 2B-2C during heat disinfection.

[0028] Detailed Description of Example Embodiments

[0029] Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments are shown. Indeed, the subject of the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure may satisfy applicable legal requirements.

[0030] Also, it will be understood that, where possible, any of the advantages, features, functions, devices, and / or operational aspects of any of the embodiments described and / or contemplated herein may be included in any of the other embodiments described and / or contemplated herein, and / or vice versa. In addition, where possible, any terms expressed in the singular form herein are meant to also include the plural form and / or vice versa, unless explicitly stated otherwise. As used herein, "at least one" shall mean "one or more" and these phrases are intended to be interchangeable. Accordingly, the terms "a" and / or "an" shall mean "at least one" or "one or more", even though the phrase "one or more" or "at least one" is also used herein. As used herein, except where the context requires otherwise owing to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments.

[0031] As used herein, the terms "multiple", "plural" and "plurality" are intended to imply provision of two or more elements. The term "and / or" includes any and all combinations of one or more of the associated listed elements.

[0032] It will furthermore be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing the scope of the present disclosure.

[0033] Like reference signs refer to like elements throughout.

[0034] Well-known functions or constructions may not be described in detail for brevity and / or clarity. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0035] As used herein, "dialysis therapy" or "dialysis" refers to any therapy that replaces or supplements the renal function of a patient by use of a treatment fluid. Dialysis therapy includes, without limitation, extracorporeal (EC) blood therapy and peritoneal dialysis (PD) therapy.

[0036] As used herein, "heat disinfection" refers to a technique of deactivating bacteria and viruses by subjecting them to a fluid at a required temperature for a required time period. The fluid may be water, optionally in combination with a cleaning agent ("disinfectant"). As used herein, "AO concept" refers to an established technique of quantifying the effect of heat disinfection on deactivation of microorganisms. An AO value may be calculated according to: AO = 1Q(7’-80) / Z■ At , where T is the fluid temperature (in °C), z is a bacteria coefficient, and At is the exposure time at the fluid temperature. By use of this equation, a value of the microbial deactivation may be calculated, by summation or integration, during heat-up, holding, and cooling of any moist heat disinfection process. For example, a condition for sufficient heat disinfection may be that the AO value is at least 500, 600, 700, 800 or 900. Generally, the fluid temperature needs to exceed 65°C to be effective for heat disinfection.

[0037] As used herein, "product water" typically refers to water that has a purity suitable for medical use. In some embodiments, the product water meets criteria of so-called "water for dialysis" "water for injection", or "ultrapure water". For example, criteria for "water for dialysis" or "dialysis water" may be defined in accordance with ANSI / AAMI / ISO 23500-3:2019.

[0038] FIG. 1 is a schematic overview of a system for generation of product water in accordance with some embodiments. The system is denoted water purification apparatus, WPA, in the following. The WPA is configured to receive source water, SWA, from a water source 10. The source water may be of any quality. In some embodiments, the source water is tap water (drinking water) or some form of pre- processed tap water.

[0039] The WPA is operable to process the source water, SWA, into product water, PWA. In some embodiments, PWA is of sufficient quality for use in an intended medical treatment. In other embodiments, further downstream purification / preparation of the PWA is performed to attain water of sufficient quality for use in the medical treatment. The WPA is configured to perform a purification processing of SWA to produce PWA. As used herein, "purification" refers to removal of impurities such as undesirable chemicals, biological contaminants, suspended solids, and gases. It does not imply that PWA is sterile or substantially free of microbial activity, although this may be the case. In some embodiments, PWA (or any medical fluid that is produced from PWA) is subjected to additional processing downstream of the WPA for removal of microorganisms, endotoxins, etc.

[0040] The WPA comprises an inlet 1' for receipt of "input water", IW, which may be SWA or a pre-processed version thereof. As used herein, IW may be any type of water that is input to a reservoir or tank 200 in the WPA. In the illustrated example, the WPA comprises a pre-processing sub-system 100, which is arranged upstream of the tank 200 to receive and process SWA into IW. The pre-processing system 100, which is optional, may be configured to process SWA for removal of particles. In some embodiments, the sub- system 100 is configured to remove particles with a nominal size of 1 pm or more, i.e., to pass particles with a nominal size below 1 pm. In some embodiments, the subsystem 100 is additionally configured to reduce one or more contaminants such as volatile organic compounds (VOCs), and trihalomethanes (THMs) that may be present in SWA. In some embodiments, the system 100 is further configured to effectively remove chlorine from SWA. An example of the pre-processing sub-system 100 will be given below with reference to FIG. 2A.

[0041] The WPA has a main flow path 1 ("main path") that extends from the inlet 1' to an outlet 1" for output of PWA. The water outlet 1" may comprise a connector for engagement with a corresponding connector associated with a receiving device (not shown). The receiving device may be any device configured to generate a medical fluid by mixing the purified water with one or more concentrates. In some embodiments, the receiving device is a dialysis machine or a stand-alone apparatus for generation of medical fluid for use in dialysis therapy. In some embodiments, the WPA is integrated with the receiving device. Water purification equipment 2 is arranged in the main path 1 to process IW into PWA. In the illustrated example, the water purification equipment 2 comprises the tank 200, a first purification sub-system 300 ("first sub-system"), a second purification sub-system 400 ("second sub-system"), and a recirculation arrangement 9.

[0042] The tank 200 is arranged in the main path 1 to receive the input water, IW. The provision of the tank 200 serves to decouple the operation of the sub-systems 300, 400 from the supply of IW, and from the operation of the sub-system 100, if present. This may facilitate the design and / or operation of the sub-systems 300, 400. Further technical advantages of the tank 200 are discussed below with reference to FIG. 2A.

[0043] The first sub-system 300 is arranged in the main path 1 downstream of the tank 200 to process the water entering the first sub-system 300, so as to at least reduce the water hardness of this water. This process is commonly denoted "water softening" and involves removal of at least calcium and magnesium. One function of the first subsystem 300 is to reduce water hardness so as to mitigate or even prevent scaling of downstream components in the WPA, such as the second sub-system 400. In the following, the water generated by the first sub-system 300 is sometimes referred to as "softened water".

[0044] Water hardness is the amount of polyvalent cations and is usually attributed to the sum of dissolved calcium plus magnesium in the water. Hardness may be classified as carbonate or non-carbonate hardness, depending upon the presence of counter anions in the water. Hardness classification may be given as equivalence units of mgCaCOs / L. Apart from water softening, the first sub-system 300 may also be configured to reduce minerals and / or TDS (Total Dissolved Solids) of the water that enters the first subsystem 300. In some embodiments, the first sub-system 300 is configured to generate softened water with a water hardness of less than 60 ppm (mgCaCC / L), and preferably less than 30, 20 or 10 ppm (mgCaCC / L). In some embodiments, the first sub-system 300 is configured to reduce the water hardness of its incoming water by at least 90% and preferably by at least 95%, in terms of equivalence units of mgCaCOs / L.

[0045] The second sub-system 400 is arranged in the main path 1 downstream of the first sub-system 300 and configured to perform a final processing of the softened water to generate the purified water, PWA. The second sub-system 400 may thus be configured to remove remaining impurities not removed by the first sub-system 300 so that PWA has a required purity. Alternatively or additionally, the second sub-system 400 may be configured to reduce the microbial load of the softened water from the first sub-system 300.

[0046] The sub-systems 300, 400 may implement any suitable purification technique, for example membrane filtration, ion exchange, electrodeionization (EDI), or any combination thereof. The membrane filtration may involve reverse osmosis (RO). As known in the art, ion exchange is a technique of removing ionic impurities from water by replacing the respective ionic impurity with another ionic substance. Specifically, negative ions are replaced by OH- and positive ions are replaced by H+, with OH- and H+ being joined to form a water molecule. Typical ion exchangers are ion-exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, or soil humus.

[0047] In some embodiments, the first sub-system 300 is configured to process its incoming water by reverse osmosis (RO) to reduce water hardness. This means that the sub- system 300 comprises at least one RO unit. The RO unit comprises an RO filter or membrane which is arranged to separate ions, molecules and particles from the incoming water. Specifically, the RO unit is configured to perform cross-flow filtration (also known as tangential flow filtration) where the majority of the incoming water ("feed water") travels tangentially across the surface of the RO filter, rather than through the RO filter. The filtered water that passes the RO filter is denoted "permeate" or "permeate fluid", and the non-filtered water is denoted "retentate", "reject water", "reject fluid", or "drain water". An example of an RO unit will be described below with reference to FIG. 2D. The skilled person understands that RO is a non-selective technique and that processing by RO will not only reduce water hardness but also remove impurities that are unrelated to water hardness. Such impurities may include various inorganic and organic compounds. In some embodiments, the second sub-system 400 comprises at least one RO unit for use in processing the softened water from the first sub-system 300. In some embodiments, the sub-system 400 comprises at least one ion exchanger (IEX), instead of or in addition to the RO unit(s).

[0048] The recirculation arrangement 9 is only schematically indicated in FIG. 1 but generally includes one of more fluid lines. It may also include flow controlling components, such as valves, pumps, flow restrictors, etc. The recirculation arrangement 9 is generally configured to enable recirculation of fluid back to the tank 200 from one or more locations downstream of the first sub-system 300, such as from a location on the main path 1 intermediate the first and second sub-systems 300, 400, or a location on the main path 1 intermediate the second sub-system 400 and the outlet 1". As will be shown with reference to a detailed example in FIGS 2B-2C, the recirculation arrangement 9 may also enable recirculation of drain water from the second sub-system 400 to the tank 200.

[0049] The operation of the WPA is controlled through control signals, which are generated and output by a control arrangement 500. The control signals are collectively represented as [CS] in FIG. 1 and are supplied to operable components (not shown) in the WPA, such as pumps, valves, heaters, flow restrictors, etc. The control arrangement 500 is configured to generate the control signals based on measurement signals, collectively represented as [MS]. The measurement signals represent current values of various status parameters of the WPA, such a pressure, temperature, flow rate, chemical composition, conductivity, etc. At least some of the measurement signals may be given by sensors (not shown) in the WPA. The control arrangement 500 comprises a combination of processing circuitry 501 and memory 502. The memory 502 may store program instructions for execution by the processing circuitry 501 to implement the operation of the control arrangement 500. The program instructions may be supplied to the control arrangement 500 on a computer-readable medium, which may be a tangible (non-transitory) product (for example, magnetic medium, optical disk, read-only memory, flash memory, etc.) or a propagating signal. The processing circuitry 501 may comprise a generic processor, for example a microprocessor, microcontroller, CPU, DSP (digital signal processor), GPU (graphics processing unit), etc., or a specialized processor, such as an ASIC (application specific integrated circuit) or an FPGA (field programmable gate array), or any combination thereof. The memory 502 may include volatile and / or non-volatile memory such as read only memory (ROM), random access memory (RAM) or flash memory.

[0050] As shown, the control arrangement 500 may be connected to a human-machine interface (HMI) device 503. The term HMI device is intended to include any and all devices that are capable of performing guided human-machine interaction comprising presentation of information or instructions and receipt of input data. For example, the HMI device 503 may comprise a combination of a display device and data entry hardware. The data entry hardware may include one or more of a keyboard, keypad, computer mouse, control buttons, touch panel, microphone and voice control functionality, camera and gesture control functionality, etc. In one implementation, the HMI device 503 is or comprises a touch-sensitive display, also known as touch screen. Alternatively or additionally, the HMI device 503 may comprise a speaker or a projector to provide information or instructions to a user.

[0051] In some embodiments, the second sub-system 400 mainly serves the function of reducing the microbial load of the processed water from the first sub-system 300. Thus, in some embodiments, a main part of the chemical substances ("impurities") present in IW is removed by the first sub-system 300, whereas the second sub-system 400 has the main function of reducing the microbial load of the softened water from the first subsystem 300. As noted above, when the first sub-system 300 involves a non-selective purification technique such as RO, most chemical compounds will be affected by the purification, not only water hardness. This separation of functionality between the first and second sub- systems 300, 400 is advantageous in that it enables separate optimization of the respective sub-system 300, 400.

[0052] By connecting the sub-systems 300, 400 in series, the first sub-system 300 is more exposed to impurities than the second sub-system 400. This will extend the operative life of the second sub-system 400 and may also allow the second sub-system 400 to include more advanced / sensitive purification equipment, if desired. The first sub-system 300, on the other hand, may need additional consideration to ensure proper operation of the WPA over time. As noted above, the first sub-system 300 may comprise one or more RO units. In some embodiments, the WPA is operable to perform automated cleaning of the RO filter in the respective RO unit in the first sub-system. The cleaning may, for example, involve flushing the RO filter with water, which may or may not be heated, for example up to about 45°C. In some embodiments, at least one RO unit in the first sub-system 300 is configured as a sacrificial or disposable component, which is removably installed in the first sub-system 300. Thereby, the RO unit may be replaced as needed, for example when the RO filter is deemed to be clogged, for example as a result of scaling, and / or at regular service intervals. The use of a disposable RO unit may appear to increase the operating cost of the WPA. However, by using a disposable RO unit as proposed herein, a conventional water softener is not required in the WPA. Conventional water softeners operate by lime softening, ion-exchange resins or addition of anti-scalant such as polyphosphate. Many conventional water softeners are bulky and costly, and may require supervision and periodic maintenance. Thus, in practice, the proposed technique of using a disposable RO unit results in cost savings. Further, by the provision of the second sub-system 400, the RO unit(s) in the first sub-system 300 need not be high-end products, i.e., highly qualified products with high purification performance. Instead, relatively simple and cheap low-end RO units may be used, for example commercially available under- the- sink RO units. At present, the cost ratio between high-end and low-end RO units on the market is at least a factor of 5-10. Thus, the replacement of RO units may have a marginal impact on the operating cost of the WPA. Further, the replacement of RO unit(s) in the first sub-system 300 may lower the demands on the purification performance of the second sub-system 400, enabling reductions in the production cost of the WPA.

[0053] FIGS 2A-2C depict a WPA in accordance with a detailed implementation example. The WPA is separated into subsets in FIGS 2A-2C, where the fluid connections between the different subsets are indicated by encircled letters A-G.

[0054] FIG. 2A is a block diagram of an upstream portion of the WPA, including a preprocessing sub-system 100 and a tank 200. The source water SWA is received at A, and the main path 1 extends from A to C. It is assumed that SWA is sufficiently pressurized to flow along the main path 1 through the sub-system 100, resulting in input water IW entering the tank 200. In the flow direction along the main path 1, the sub-system 100 comprises a one-way valve 101, an inlet valve 102, a filtration device 103, a flow restrictor 104, a temperature sensor 105, and a composition sensor 106. The filtration device 103 is configured to perform both removal of particles and dechlorination. In one embodiment, the filtration device comprises activated carbon (AC) filters 103A, 103B, which are connected in series. Each AC filter 103 A, 103B is formed by a container that holds activated carbon, also known as active carbon or activated charcoal. The activated carbon has been processed (activated) to have small, low-volume pores that increase the surface area available for adsorption and / or chemical reactions. The activated carbon forms a bed inside the container. The bed of activated carbon is arranged to remove chemical compounds of chlorine from the incoming water, and to absorb toxic substances and pesticides. In an example embodiment, the bed of activated carbon is arranged to remove free and combined forms of chlorine. In a further example embodiment, the bed of activated carbon is also arranged to reduce organic compounds (TOC, total organic carbon) including pesticides of the incoming water. The bed of activated carbon also inherently filters out particles from the incoming water.

[0055] One reason for including the flow restrictor 104 is to create a backing pressure in the main path 1 so as to limit the flow rate of water through the filtration device 103. Another reason is to maintain an approximately constant flow rate of IW into the tank 200 even if the inlet pressure of SWA fluctuates. The inlet valve 102 may be operable via a control signal VC1 to selectively open and close the main path 1 to control admission of SWA into the WPA. Alternatively, the inlet valve 102 may be a flow control valve which is operable to admit SWA into the main path 1 at variable flow rate. Alternatively, the inlet valve 102 may be supplemented by such a flow control valve, which is arranged in series with the inlet valve 102 in the main path 1.

[0056] The one-way valve 101 is included to prevent flow reversal in the main path 1 and thus backflow through the filtration device 103. The temperature sensor 105 is configured to provide a measurement signal T1 indicative of the temperature of SWA. The composition sensor 106 is configured to provide signal Cl. As used herein, a composition sensor is configured to provide a measurement signal indicative of the amount of at least one substance in the water. For example, the composition sensor may be a conductivity sensor (or equivalently, a resistivity sensor), or a concentration sensor, for example an ion-selective sensor. The signals Tl, Cl may be used by the control arrangement 500 in controlling the operation of the WPA.

[0057] The tank 200 is arranged to receive and collect input water, IW, produced by the sub-system 100. The tank 200 comprises a level sensor 201, which is configured to indicate at least one fluid level in the tank 200, via a measurement signal LI. The level sensor 201 may thus signal when the water reaches one or more discrete levels, or indicate the fluid level along a continuous scale. The measurement signal LI allows the control arrangement 500 to control the level of water in the tank 200. The tank 200 comprises a plurality of ports 202-206. A first inlet port 202 for IW is defined in the top portion of the tank 200. With reference to FIG. 1, the port 202 corresponds to the inlet 1'. An outlet port 203 is defined in the bottom portion of the tank 200 and connected to the main path 1. The tank 200 is thus fluidly connected to the first sub-system 300 through the outlet port 203. A second inlet port 204 is defined in the top portion and connected to a first return line 5, which is fluidly connected to receive water that has been processed by the first sub-system 300 and the second sub-system 400, respectively (cf. FIGS 2B-2C). A third inlet port 205 is defined in the top portion of the tank 200 and connected to a second return line 2B, which is fluidly connected to receive retentate ("reject water") from the second sub-system 400. In some embodiments, at least one of the inlet ports 202, 204, 205 is connected to a spray nozzle (not shown) inside the tank 200 to disperse the incoming water. The inlet ports 204, 205 may be merged into a common port. A gas outlet or vent 206 is defined in the top portion of the tank 200. In the illustrated example, the vent 206 is defined in the side portion of the top portion of the tank 200. The vent 206 allows gases to escape from the tank 200, as indicated by a dashed arrow, and also allows air to enter the tank as needed. The water is thus held in the tank 200 at atmospheric pressure.

[0058] In some embodiments, the amount of water in the tank 200 is controlled to define an air gap 207 at the top of the tank 200, by ensuring that the fluid level in the tank 200 does not rise above a predefined maximum level in the tank 200. The control arrangement 500 may be configured to control the fluid level in the tank by controlling the inlet valve 102. Depending on the type of level sensor 201, the fluid level in the tank 200 may or may not be allowed to vary during operation of the WPA. Arranging the first inlet port 202 above the predefined maximum level will minimize the risk that water flows back from the tank 200 towards the sub-system 100, which might disrupt the operation of the filtration device 103. Likewise, the air gap 207 prevents backflow into the inlet ports 204, 205 if these are installed above the predefined maximum level. The air gap 207 also serves as a barrier to microorganisms in relation to the inlet ports 204 and 205. Activated carbon beds are known to act as a nutrient-rich environment for microorganisms and thus the pre-processing sub-system 100 may be a key point of ingress for microorganisms into the WPA. The air gap 207 will mitigate microbial contamination of the return lines 5, 2B. Another technical advantage of the tank 200 is its ability to be used as a flow rate watchdog, via the signal LI. Depending on implementation, the control arrangement 500 may be configured to detect if the fluid level cannot be maintained at the predefined maximum level in the tank 200, if the fluid level is unable to reach a predefined operative level in the tank, or if the fluid level reaches a predefined lower level in the tank 200. This may indicate a malfunction of the sub-system 100 or the water source 10 and may cause the control arrangement 500 to stop the operation of the WPA. Another technical advantage of the tank 200 is an ability to allow for thermal expansion of the water in the WPA. This may be particularly relevant during heat disinfection of the WPA. The provision of the vent 206, and thus atmospheric pressure in the tank 200, is advantageous for several reasons. It will allow for removal of air and other gases that may be released from the water in the tank 200. Further, since water is recirculated back to the tank 200 from the first and second subsystems 300, 400, it allows for continuous removal of gases in the entire hydraulic circuit of the WPA, including the RO units in the sub-systems 300, 400 (below). Further, during start-up of the WPA, gases may be present in the hydraulic circuit and need to be vented. Further, removal of gases from the hydraulic circuit is relevant whenever the hydraulic circuit has been opened during service and maintenance, for example when an RO unit is replaced with a new one. Another advantage of having atmospheric pressure in the tank 200 is that it facilitates the recirculation of water from the first and second sub-systems 300, 400 through the return lines 5, 2B. In the illustrated example of FIG. 2A, the vent 206 is arranged in the side wall of the tank 200, at a location spaced from the top of the tank 200, to define an overflow or spillway for water. The vent 206 is thus configured to not only allow gases to escape the tank 200 but also water. Thereby, the vent 206 is arranged to define a maximum attainable fluid level in the tank 200, ensuring the air gap at the top of the tank 200. Should the control of the water level in the tank 200 fail for some reason, excess water will flow through the vent 206 and the air gap 207 will be maintained.

[0059] FIG. 2B is a block diagram of the first sub-system 300 in the WPA in accordance with an example. In FIG. 2B, the main path 1 extends from C to E. In the flow direction along the main path 1, the first sub-system 300 comprises a UV irradiation device 301, a feed pump 302, a pressure sensor 303, an RO unit 304, a temperature sensor 310, a composition sensor 311, a heater 312, a flow switch 313, and a temperature sensor 314. It is to be understood that while each of these components serve a specific function in the context of the illustrated example, all components need not be included as shown. For example, any one of components 301-303, 310-314 may be omitted or placed elsewhere in other examples of the first sub-system 300.

[0060] The pressure sensor 303 is configured to provide a measurement signal Pl representing hydraulic pressure (water pressure). The sensors 310, 311 provide a respective measurement signal T2, C2 representing water temperature and water composition, respectively. The signals Pl, T2, C2 may be used by the control arrangement 500 in controlling the operation of the WPA.

[0061] The heater 312 is operable to heat the water. The heater 9 may be an electrical (resistive) heater or any other type of heating device. In some embodiments, the heater 312 is a flow-through heater that is capable of heating a passing fluid. In some embodiments, the control arrangement 500 operates the heater 312 by feedforward control based on the signal T1 and / or the signal T2 to achieve a target temperature of the water entering the second sub-system 400. In other embodiments, the control arrangement 500 operates the heater 312 by feedback control based on a measurement signal T3 from a temperature sensor 314 in the first sub-system 300 and / or a measurement signal T4 from a temperature sensor 413 in the second sub-system 400 shown in FIG. 2C.

[0062] The flow switch 313 is included as a safety measure to prevent that the heater 312 is activated when the flow rate of water through the heater 312 is too low, since this may cause irreparable damage to the heater 312. Thus, the flow switch 313 is configured to autonomously disable the heater 312 when the flow rate of water through the flow switch 313 is below a preset threshold value. The UV irradiation device 301 is included to reduce microbial activity in the passing water. By installing the device 301 upstream of the RO unit 304, the microbial load entering the RO unit 304 and other downstream components will be reduced. This will extend the life of the first sub-system 300. The device 301 may comprise a processing chamber for receiving incoming water, and at least one UV source in the processing chamber. The UV source is operable to generate UV radiation so as to irradiate at least part of the processing chamber. The UV radiation thereby interacts with the fluid within the processing chamber. In some embodiments, the UV irradiation device 301 is configured to additionally remove chlorine from the passing water. This may be achieved by increasing the irradiating power of the UV source and / or by confining the emitted UV radiation to wavelengths of high absorptivity for chlorine in the water, such as dichloramine and monochloramine. The UV irradiation device 301 may thus be installed to supplement the sub-system 100, or even replace the dechlorination functionality of the sub-system 100. Generally, a UV irradiation device 301 configured to perform dechlorination may be installed anywhere in the main path 1 upstream of the first RO unit 304.

[0063] The RO unit 304 is of conventional structure and comprises a semi-permeable filter or membrane 304'. The RO membrane 304' is commonly comprised of a thin-film, cross-linked composite polymer, and is able to withstand relatively high fluid pressure. For example, the RO membrane 304' may be a spirally wound membrane or flat sheet membrane. An enlarged and schematic view of the RO unit 304 is shown in FIG. 2D. The membrane 304' separates the body of the RO unit 304 into a feed side or chamber 304" and a permeate side or chamber 304'". The RO unit 304 comprises one or more inlet ports 304A' (one shown) and one or more outlet ports 304C' (one shown) on the feed side 304", and one or more outlet ports 304B' on the permeate side 304'". It is to be understood that the specific arrangement of the RO membrane 304' and the ports 304A', 304B', 304C' in FIG. 2D is merely given as a non-limiting example. As noted above, the RO unit 304 operates by cross-flow filtration. Such filtration is achieved by causing the water to flow along the membrane 304'. In FIG. 2D, this is achieved by proper location of inlet and outlet ports 304A', 304C' on the feed side 304". Permeate passes through the membrane 304', while reject water exits the RO unit 304 through the outlet port(s) 304C'. The fluid pressure on the feed side 304" needs to be sufficient to overcome the osmotic pressure created by solutes dissolved in the water. Thereby, filtered water is forced across the membrane 304' to form a permeate stream through the outlet port(s) 304B', while dissolved solutes are excluded and discharged with the reject water in a more highly concentrated state. The feed-side fluid pressure may be in the range of 5-50 bar, and is typically 5-15 bar. On a general level, as indicated by dot-dashed arrows in FIG. 2D, the RO unit 304 may be seen to define a first flow path FP1, which extends from the inlet port(s) 304A' through the membrane 304' to the outlet port(s) 304B', and a second flow path FP2, which extends on the feed side 304" from the inlet port(s) 304A' to the outlet port(s) 304C along the membrane 304'. It is realized that FP1 is part of the main path 1 through the WPA.

[0064] Reverting to FIG. 2B, the fluid pressure on the feed side of the RO unit 304 is defined by the feed pump 302, also known as booster pump. The pumping rate or speed of the feed pump 302 is set by a control signal PCI. The fluid pressure on the feed side may be monitored via the measurement signal Pl of the pressure sensor 303, which is arranged intermediate the feed pump 302 and the RO unit 304. A drain line (return line, reject line) 2A is connected to receive the reject water from the RO unit 304. The drain line 2A extends from the outlet port(s) on the feed side of the RO unit 304 to a drain 309. As used herein, a drain may be a fixed plumbing (floor drain, toilet, sink, etc.), or a receptacle (bag, container, etc.). A flow restriction device ("flow restrictor") 306 is arranged in the drain line 2A to maintain a desired fluid pressure on the feed side of the RO unit 304. The flow restrictor 306 may be fixed or adjustable. For example, the flow restrictor 306 may comprise a small orifice, a needle valve or other variable orifice valve. In the illustrated example, a bypass line 4A is connected to extend between locations on the drain line 2A upstream and downstream of the flow restrictor 306. The bypass line 4A comprises a drain valve 307, which is operable to selectively close and open the bypass line 4A subject to a control signal VC2. Downstream of the flow restrictor 306 and the bypass line 4, a further drain valve 308 is arranged in the drain line 2A. The further drain valve 308 is operable to selectively close and open the drain line 2A subject to a control signal VC3. It may be desirable to flush the reject water on the feed side 304" of the RO unit 304 to the drain 309. The drain valve 307 is provided to speed up such flushing by opening the drain valve 307 so that the flow restrictor 306 is bypassed. By opening the drain valves 307, 308, the entire volume of reject water is allowed to flow into the drain line 2A and out of the WPA into the drain 309. This bypass state may be advantageous in purging reject water upon initial use of the RO unit 304, for example when the RO unit 304 has been replaced for a new one. Additionally, the bypass state may facilitate a draining operation in which most or all of the water in the WPA is expelled from the WPA into the drain 309. In some embodiments, the flow restrictor 306 is motorized and capable of adjusting its flow resistance in response to a control signal. With such a motorized flow restrictor 306, valves 307, 308 and bypass line 4A may be omitted.

[0065] In the example of FIG. 2B, a connecting line 3A is arranged to fluidly connect the drain line 2A, at a location upstream of the flow restrictor 306, to the main path 1, at a location intermediate the feed pump 302 and the RO unit 304. Thereby, a water recirculation path or loop is defined between the outlet and inlet ports on the feed side of the RO unit 304. The recirculation path may be used to maintain a high rate of water flow on the feed side of RO unit 304, thereby reducing the amount of water that otherwise would be discarded to drain. An auxiliary pump ("recirculation pump") 305 is arranged within the recirculation path to increase the fluid flow velocity or flow rate along the RO membrane 304' sufficiently to inhibit a locally increased concentration close to the membrane surface. Such locally increased concentration may result in fouling of the RO membrane 304', depending on the hardness of the incoming water to the RO unit 304. The term "fouling" includes the build-up of all kinds of layers on the surface of the RO membrane 304', including biofouling and scaling. The pumping rate or speed of the recirculation pump 305 is set by a control signal PC2.

[0066] In a variant, the recirculation pump 305 and the connecting line 3A are omitted. In another variant, the recirculation pump 305 is omitted, and the connecting line 3A is arranged to fluidly connect the drain line 2A to the main path 1, at a location upstream of the feed pump 302, to define a recirculation path. Thereby, the suction created by the operating feed pump 302 will cause recirculation of water through the connecting line 3A. A flow restrictor may be arranged in the connecting line 3A for adjustment of the recirculated flow.

[0067] While it is possible to omit the recirculation pump 305 and the connecting line 3A, it is believed that the combination of the feed pump 302 and the recirculation pump 305 results in increased performance, less noise and lower power consumption. If only the feed pump 305 is present in the RO unit 304, this pump needs to have a greater capacity to create both the desired pressure and recirculation flow. If the feed pump 302 is supplemented by the recirculation pump 305, both pumps can have lower capacity, meaning less noise and lower cost in total as two lower capacity pumps are cheaper than one higher capacity pump. It may also be noted that the two pumps 302, 305 serve different purposes, which are difficult to accommodate by a single pump, namely boosting pressure versus boosting flow velocity. In the illustrated example, the recirculation pump 305 is located in the connecting line 3A, which is believed to be beneficial compared to having the recirculation pump 305 in the main path 1. The pressure sensor 303 may be located outside the recirculation path, as shown, or within the recirculation path.

[0068] In the illustrated example, the RO unit 304 is a disposable component, which is removably installed in the WPA. To this end, the main path 1 is provided with terminal connectors 304A, 304B for releasable attachment to the inlet and outlet ports 304A', 304B' of the RO unit 304, and the drain line 2A is provided with a terminal connector 304C for releasable attachment to the outlet port 304C of the RO unit 304. A return line 5 is fluidly connected to the main path 1 downstream of the heater 312. As indicated by B, the return line 5 extends to the inlet port 204 on the tank 200 and thus serves to recirculate softened water to the tank 200. A control valve 315 is arranged in the return line 5. The control valve 315 is operable to selectively close and open the return line 5 subject to a control signal VC4. Even if the fluid pressure is significantly lower on the permeate side compared to the feed side, the water that leaves the first sub-system 300 is likely pressurized above atmospheric pressure. As shown, a flow restrictor 316 may be arranged in the return line 5, to cause a pressure drop in the return line 5 and enable a controlled flow of water into the tank 200 from the first sub-system 300. As indicated by F in FIG. 2B, a further return line 7 from the second sub-system 400 is fluidly connected to or merged with the return line 5. Thus, the flow restrictor 316 also enables a controlled flow of water into the tank 200 from the second sub-system 400. The flow restrictor 316 may be fixed or adjustable.

[0069] In the example of FIG. 2B, a bypass line 6 is connected to the main flow path 1 upstream and downstream of the RO unit 304. A control valve 317 is arranged in the main path 1 upstream of the RO unit 304, intermediate the RO unit 304 and the junction of the bypass line 6 to the main path 1. A control valve 318 is arranged in the bypass line 6. A control valve 319 is arranged in the main path 1 downstream of the RO unit 304, intermediate the RO unit 304 and the junction of the bypass line 6 to the main path 1. The valves 317, 318, 319 are on / off valves and are operable by control signals VC 10, VC11, VC12. The bypass line 6 and the valves 317-319 define a bypass arrangement. When the WPA is operated to produce PWA, valve 318 is closed and valves 317, 319 are open. When the WPA is heat disinfected, as described further below, valves 317, 319 are closed and valve 318 is open to direct heated fluid through the bypass line 6 instead of through the RO unit 304. Both valves 317, 319 are closed to seal off the RO unit 304 from the heated fluid. One reason for the bypass line 6 is that when using a disposable RO unit, which is replaced when consumed, it is possible to refrain from heat disinfecting the RO unit 304. Thereby, the RO unit 304 need not be configured to withstand the high temperatures used in heat disinfection, which is likely to lower the cost of the RO unit 304.

[0070] FIG. 2C is a block diagram of the second sub-system 400 in the WPA in accordance with an example. In FIG. 2C, the main path 1 extends from E to G. In the flow direction along the main path 1, the second sub-system 400 comprises an inlet valve 401, a pressure sensor 402, a pump 403, a pressure sensor 408, an RO unit 404, a flow meter 409, an on / off valve 410, a UV irradiation device 411, a composition sensor 412, and a temperature sensor 413. It is to be understood that while each of these components serve a specific function in the context of the illustrated example, all components need not be included as shown. For example, any one of components 401-

[0071] 403, 408-413 may be omitted or placed elsewhere in other examples of the second subsystem 400.

[0072] The inlet valve 401 is operable via a control signal VC5 to selectively open and close the main path 1 to control admission of water into the second sub-system 400. The control valve 410 is operable via a control signal VC6 to selectively open and close the main path 1.

[0073] The sensors 402, 408, 409, 412 and 413 provide measurement signals P2, P3, Fl, C3, T4 representing water pressure, water pressure, water flow rate, water composition and water temperature, respectively. The signals P2, P3, Fl, C3, T4 may be used by the control arrangement 500 in controlling the operation of the WPA.

[0074] The RO unit 404 is configured to perform water filtration by reverse osmosis, similar to the RO unit 304. In the illustrated example, the RO unit 404 is permanently installed in the second sub-system 404. In other words, it is not installed to facilitate replacement by a user of the WPA. As noted above, the RO unit 404 may differ from the RO unit 304 in terms of performance, cost, etc. However, principally, the RO unit 404 has the configuration shown in FIG. 2D, comprising an RO membrane 404', a feed side 404", a permeate side 404"', inlet port(s) 404A' and outlet port(s) 404C on the feed side 404", and outlet port(s) 404B' on the permeate side 404'". The foregoing description of FIG. 2D with reference to the RO unit 304 is equally applicable to the RO unit 404. Similar to the first sub-system 300, a feed pump 403 ("booster pump") is arranged in the main path 1 to define the fluid pressure on the feed side of the RO unit 404. The pumping rate or speed of the feed pump 403 is set by a control signal PC3. A drain line (return line, reject line) 2B is connected to receive the reject water from the RO unit

[0075] 404. In contrast to the first sub-system 300, the drain line 2B does not extend to drain, but to the inlet port 205 on the tank 200. Thus, reject water from the RO unit 404 is recirculated back to the tank 200. A flow restriction device ("flow restrictor") 406 is arranged in the drain line 2B to maintain a desired fluid pressure on the feed side of the RO unit 404. The flow restrictor 406 may be fixed or adjustable. In the illustrated example, a bypass line 4B is connected to extend between locations on the drain line 2B upstream and downstream of the flow restrictor 406. The bypass line 4B comprises a control valve 407, which is operable to selectively close and open the bypass line 4B subject to a control signal VC7. Like the flow restrictor 306 in FIG. 2B, the flow restrictor 406 may be motorized. With such a motorized flow restriction mechanism, valve 407 and bypass line 4B may be omitted. A connecting line 3B is arranged to fluidly connect the drain line 2B, at a location upstream of the flow restrictor 406, to the main line 1, at a location intermediate the feed pump 403 and the RO unit 404. Thereby, like in the first sub-system 300, a water recirculation path or loop is defined between the outlet and inlet ports on the feed side of the RO unit 404. An auxiliary pump ("recirculation pump") 405 is arranged within the recirculation path. The pumping rate or speed of the auxiliary pump 405 is set by a control signal PC4.

[0076] The UV irradiation device 411 may be similar to the UV irradiation device 301 in the first sub-system 300. The UV irradiation device 411 is installed to further mitigate microbial activity. It is realized that the device 411 need not be configured to remove chlorine in the incoming water, which should be effectively free of chlorine at this location in the WPA. By placing the UV irradiation device 411 in the main path 1 downstream of the RO unit 404, the device 411 is arranged to operate on water with the lowest content of suspended solids within the WPA. This will improve the efficiency of the device 411, since suspended solids are known to be a limitation parameter for water treatment by UV irradiation due to the absorption of the light by the solids and potential shielding of pathogens from the light.

[0077] The return line 7 is fluidly connected to the main path 1 downstream of the RO unit 404. The return line 7 is arranged to enable continuous production of product water, PWA, during operation of the WPA, by allowing excess PWA to be recirculated back to the tank 200 through the return lines 7, 5. As shown, a one-way valve 414 may be arranged in the return line 7 to prevent processed water from flowing from the first subsystem 300 along the return line 7 to the outlet of the second sub- system 400 to potentially mix with the purified water PWA. In an alternative example, the return line 7 is separate from the return line 5 and extend to the tank 200. In such an alternative example, a flow restrictor may be arranged in the return line 7, similar to the flow restrictor 316 in FIG. 2B, to generate a net positive pressure in the main path 1 at the outlet from the second sub-system 400. In the specific example of FIG. 2C, the return line 7 is fluidly connected to the main path 1 downstream of the control valve 410 and the sensors 409, 412, 413.

[0078] In the example of FIGS 2A-2C, the recirculation arrangement 9 of FIG. 1 includes the return line 5, the control valve 315, the flow restrictor 316, the return line 7, the oneway valve 414, the drain line 2B, the bypass line 4B, the flow restrictor 406, and the control valve 407.

[0079] FIG. 3A is a block diagram of a simplified representation of the WPA of FIGS 2A-2C. All components in FIG. 3A have already been described with reference to FIG. 1 and FIGS 2A-2C. Many components have been omitted in FIG. 3A to facilitate the following description. FIG. 3A will be used for describing an example technique of operating the WPA for production of PWA, as implemented by the control arrangement 500.

[0080] In the illustrated example, the control arrangement 500 comprises a first controller 511, which is configured to generate a first control signal PCI for the first feed pump 302 based on the sensor signal Fl, and a second controller 512, which is configured to generate a second control signal PC3 for the second feed pump 403 based on the sensor signal P2. The first and second controllers 511, 512 may be implemented by dedicated hardware in the processor circuitry 501 (FIG. 1) and / or software executed by the processor circuitry 501. The provision of the first and second controllers 511, 512 has been found to enable simple and stable control of the WPA, by providing separate control of the first feed pump 302 and the second feed pump 403. In other words, the first and second controllers 511, 512 operate independently of each other, in the sense that they operate on different input variables and control different pumps.

[0081] The first feed pump 302 is controlled by the first controller 511 to achieve a well- defined flow rate of PWA from the second sub-system 400. The second controller 512 is arranged to achieve an adequate inlet pressure of the second feed pump 403 to ensure proper operation of the second feed pump 403. There is a risk for cavitation in the feed pump 403 if its inlet pressure is too low. As the inlet pressure of the second feed pump 403 is increased, the power consumption of the WPA is rapidly increased through increased power consumption of the first sub-system 300 to achieve the increased inlet pressure of the second feed pump 403. Further, an excessive inlet pressure of the second feed pump 403 may cause uncontrolled pressure pulsations in the hydraulic system of the WPA.

[0082] In some embodiments, as indicated in FIG. 3A, the first controller 511 is configured to generate the first control signal PCI to achieve a target flow rate, TF1, of PWA at the flow meter 409. In some embodiments, TF1 is in the range of 100-2000 ml / min, for example in the range of 200-1500 ml / min.

[0083] In some embodiments, as indicated in FIG. 3A, the second controller 512 is configured to generate the second control signal PC3 to achieve a target inlet pressure, TP2, of the processed water at the second feed pump 403. The Applicant has found that TP2 > 0 bar (atmospheric pressure) mitigates the risk of cavitation. The Applicant has further found that TP2 < 1 bar results in an acceptable power consumption and mitigates occurrence of uncontrolled pressure pulsations. It is currently believed that preferable performance is achieved with TP2 in the range of 0.1-0.9 bar.

[0084] In the example of FIG. 3A, the flow meter 409 is arranged in the main path 1 intermediate the RO unit 404 and the outlet 1". The flow meter 409 may be based on any conventional measurement technique, such as mechanical, pressure based, variable area based, optical, electromagnetic, ultrasonic, Coriolis based, thermal mass based, or vortex based.

[0085] In the example of FIG. 3A, the pressure sensor 402 is arranged in the main path 1 upstream of the second feed pump 403. The pressure sensor may be of conventional construction to operate, for example, by resistive, capacitive, inductive, magnetic or optical sensing, and using one or more diaphragms, bellows, Bourdon tubes, piezoelectrical components, semiconductor components, strain gauges, resonant wires, accelerometers, etc.

[0086] The Applicant has found that the performance of the control is improved if the hydraulic system between the feed pump 302 and the flow meter 409 is "stiff" when the WPA is operated to generate PWA. In this context, "stiff" implies that that there is no compliance in this hydraulic system. In other words, the hydraulic system is configured so that a predefined change in flow rate through the hydraulic system between the feed pump 302 and the flow meter 409 causes a deterministic change in flow rate at the flow meter 409, assuming that the flow resistance through the RO unit 304 remains the same. Such a hydraulic system is provided in the example of FIG. 3A, assuming that all fluid lines are non-yielding at relevant operation conditions. The skilled person understands that even if reject water is diverted from the RO units 304, 404 on drain lines 2A, 2B, the hydraulic system is still "stiff" under steady-state operation of the WPA.

[0087] As mentioned above, and to be further described below, the WPA may be switched between directing the water from the first sub-system 300 into the second subsystem 400 along the main line 1 (FIG. 3A), and directing the water from the first subsystem 300 into the return line 5 (FIG. 2B) and back to the tank 200. If the WPA is controlled in accordance with FIG. 3A to achieve a target inlet pressure P2 when the water is directed into and through the second sub-system 400, it may be desirable to minimize the change in pressure ("pressure swing") that occurs in the main line 1 intermediate the sub-systems 300, 400 when the WPA is switched to direct the water into the return line 5, to ensure smooth operation of the WPA. For example, the pressure swing may be maintained below 0.1-0.2 bar. This may be achieved by configuring the return line 5 with a suitable pressure drop relative to the tank 200 and / or by setting the target inlet pressure P2 in relation to this pressure drop. By providing the flow restrictor 316 in the return line 5 (FIG. 2B), the pressure drop along the return line 5 is well- defined, which facilitates minimization of the pressure swing.

[0088] The flow restrictor 316 may also be relevant if the WPA is configured with the return line 5 but without the further return line 7 and is arranged to be configured to deliver PWA to a downstream tank (not shown) held at atmospheric pressure. In such a configuration, the flow restrictor 316 may be configured to provide a sufficient pressure drop to secure that the water flows to the receiving tank instead of into the return line 5 and back to the tank 200.

[0089] FIG. 3B is a flow chart of an example method Ml of controlling the WPA in FIG. 3A to generate PWA. In the example of FIG. 3A, SWA passes the pre-processing subsystem 100 (optional) and IW enters the tank 200. IW is pumped from the tank through the first sub-system 300, which outputs softened water on the main line 1, and reject water RW1. As described with reference to FIG. 2B, RW1 may be passed to drain. The softened water from the first sub-system 300 is pumped through the second sub-system 400, which outputs PWA on the main line 1' and through the outlet 1". The second subsystem also outputs reject water RW2, which may be recirculated to the tank 200, as described with reference to FIG. 2C.

[0090] The method Ml may be performed by the control arrangement 500 in FIG. 1. In step SI 1, a target flow rate, TF1, is obtained. In step S12, a target pressure, TP2, is obtained. The target values TF1, TP2 may be retrieved from memory (502 in FIG. 1) and / or entered by an operator on an input interface of the WPA, for example the HMI device 503 in FIG. 1. The method Ml starts two control routines, which are performed independently of each other. In one routine, steps S13 and S14 are repeatedly performed. In step S13, a measured value of the flow rate of PWA from the second subsystem 400 is obtained from the measurement signal Fl. In step S14, a control signal (PCI in FIG. 3 A) for the first feed pump 302 is generated such that the measured signal Fl matches the target value, TF1. Step S14 may be based on any suitable control algorithm, including feedback control using P, PI or PID control, or feed forward control. In the other routine, steps S15 and S16 are repeatedly performed. In step S15, a measured value of the inlet pressure to the second feed pump 403 is obtained from the measurement signal P2. In step S15, a control signal (PC3 in FIG. 3 A) for the second feed pump 403 is generated such that the measured signal F2 matches the target value, TP2. Step S15 may be based on any suitable control algorithm, including feedback control using P, PI or PID control, or feed forward control.

[0091] It is realized that the WPA may be operable, by the control arrangement 500, in other phases not described herein. One such phase may be a priming phase, in which the first sub-system 300 is filled with water. The priming phase may be performed whenever first sub-system 300 is empty of water or may contain pockets of air, for example when a new RO unit 304 has been installed. Another phase may be a start-up phase, in which the first sub-system 300 is first started to produce softened water of suitable quality before the second sub-system 400 is started and allowed to receive and process the softened water from the first sub-system 300. These phases are not material to the present disclosure and will not be described in further detail. The WPA in FIGS 2A-2C and its operation in FIGS 3A-3B are given as a detailed example and are not intended to be limiting. FIG. 4A is a conceptual view of the WPA as described in the foregoing. The WPA comprises a main path 1, which extends from an inlet 1' for input water to an outlet 1" for product water. Water purification equipment 2 is arranged in the main path 1 to process the input water into product water. The water purification equipment 2 comprises a tank 200, which is arranged in the main flow path 1 to receive the input water, a first sub-system 300, which is arranged in the main path 1 downstream of the tank 200 and configured to reduce water hardness of the water that enters the first sub-system 300, and a recirculation arrangement 9, which extends to the tank 200 from the main path 1 downstream of the first sub-system 300. A drain line 2A is arranged from the first sub-system 300 to drain. In some embodiments, the first sub-system 300 is operable to generate the product water. In other embodiments, for example as shown in FIG. 4A, the water purification equipment 2 further comprises a second sub-system 400, which is arranged in the main path 1 to generate the product water by performing a final processing of the water that is produced by the first sub-system 300.

[0092] In FIG. 4A, the recirculation arrangement 9 comprises a first return path 5 and a second return path 7. The first return path 5 extends to the tank 200 from a location intermediate the first and second sub-systems 300, 400. An on / off valve 315 is arranged in the first return path 5. The second return path 7 extends to the tank 200 from a location intermediate the second sub-system 400 and the outlet 1". A heater 312 is arranged in the main path 1. The heater 312 may be an electrical heater or any other type of device capable of heating a passing fluid. In FIG. 4A, the heater 312 is arranged intermediate the first and second sub-systems 300, 400, upstream of the junction of the return line 5. Alternatively, the heater 312 may be part of the first sub-system 300 (as in FIG. 2B) or the second sub-system 400. It is conceivable that the WPA includes more than one heater.

[0093] Generally, the first and second sub-systems 300, 400 may be configured to process incoming water by any available water purification technique. Such techniques include membrane filtration and ion exchange. In the example of FIGS 2B-2C, both sub- systems 300, 400 are based on membrane filtration and includes a respective RO unit 304, 404. Either of the sub-systems 300, 400 may include plural RO units, which may be connected in series and / or in parallel.

[0094] To produce PWA, a control arrangement (cf. 500 in FIG. 1) operates the WPA in FIG. 4A to pump input water from the tank 200 into the first sub-system 300, which is operated to produce softened water, which enters the second sub- system 400 for further processing into PWA, which is supplied to the outlet 1". In a variant, the WPA is operated to circulate PWA back to the tank 200 on the return line 7, to allow a receiving device (not shown) to obtain PWA as needed from the circulating flow via the outlet 1".

[0095] It is understood that most of the WPA is made up of permanent components, even if one or more disposable RO units may be provided in the first sub-system 300. The WPA thereby needs to be intermittently disinfected. In the present disclosure, this involves a heat disinfection. FIG. 4B is a flow chart of an example method M2 of operating the WPA in FIG. 4 A to perform a heat disinfection procedure. The method M2 may be performed by the control arrangement 500 in FIG. 1.

[0096] In step S21, the WPA is operated to generate, from IW and by use of the first subsystem 300 and the recirculation arrangement 9, softened water with reduced water hardness compared to IW, and to accumulate the softened water in the tank 200. In the example of FIG. 4A, step S21 may involve circulating water from the tank 200 through the first sub-system 300 and back to the tank 200 via the return line 5, while operating the first sub-system 300 to process the passing water. Thus, the second- sub -system 400 is bypassed while water is circulated from the tank 200 through the first sub-system 300 and back to the tank 200. In FIG. 4A, this presumes that valve 315 is open and that the second sub-system 400 is operated to close the main path 1 to prevent water from entering the second sub-system 400. In FIG. 2C, this is done by closing the valve 401.

[0097] The Applicant has found that sufficiently softened water can be produced in step S21 by use of only the first sub-system 300. By bypassing the second sub-system 400 during step S21, the operative life of the second sub-system 400 can be extended. It is also likely that the duration and complexity of step S21 is reduced when only the first sub- system 300 is used for producing the softened water that is accumulated in the tank 200. Still further, if the second sub-system is electrically powered, the required energy to generate the softened water may be reduced since components of the second subsystem 400 need not perform their intended function during step S21. In the example of FIG. 2C, the pumps 403, 405 may be stopped during step S21. Likewise, the UV irradiation device 411 need not be powered during step S21.

[0098] In a variant, the second sub-system 400 is also involved in producing the softened water in step S21. In such an embodiment, the valve 315 is closed and water is circulated from the tank 200 through the first and second sub-systems 300, 400 and back to the tank 200 via the return line 7, while the first and second sub-systems 300, 400 are operated to process the passing water. It is to be noted that also PWA is softened water. One reason for including the second sub-system 400 in step S21 is to ensure that the second sub-system 400 contains sufficiently softened water when step S21 is terminated. At least part of the softened water in the second sub-system 400 may then be used in the heat disinfection, as described further below. It is also conceivable that the second sub-system 400 is only intermittently involved in producing the softened water in step S21. Thus, the water produced by the first sub-system 300 may be directed into the second sub-system 400 at one or more instances during step S21, for example to ensure that the second sub-system 400 contains sufficiently softened water when step S21 is terminated. During such an instance, in FIG. 4A, the valve 315 is closed and water is circulated from the tank 200 through the first and second sub-systems 300, 400 and back to the tank 200 via the return line 7, while the first and second sub-systems 300, 400 are operated to process the passing water.

[0099] In the detailed example of the WPA in FIGS 2A-2C, all of the incoming fluid to the second sub-system 400 is directed back to the tank 200 during operation of the second-sub-system 400. Specifically, the permeate produced by the second sub-system 400 is directed back to the tank 200 via the return line 7, and the retentate produced by the second sub-system 400 is directed back to the tank 200 via the drain line 2B. Since all incoming fluid is recirculated, the second sub-system 400 will not contribute to the softening of the water in the tank 200. Thus, in this specific example, there is no point in continuously directing water through the second sub-system 400 during step S21.

[0100] The reason for step S21 is that there is a risk for scaling if IW were to be heated and distributed in the WPA for heat disinfection. Scaling may cause components in the WPA to malfunction, such as pumps, valves, RO units, ion exchangers, etc. Sensors may also give erroneous readings through accretion of deposits caused by scaling. The risk for scaling depends on several aspects of the water, including hardness, pH, alkalinity, and temperature. In an RO unit, the risk for scaling also depends on the up- concentration of the reject water during operation of the RO unit. The risk for scaling increases with increasing temperature and is thus elevated during heat disinfection. Unless proper counter-measures are taken, heat disinfection may reduce the operative life of the WPA. Typically, the WPA as described herein does not includes a conventional water softener upstream of the tank 200. Thus, the WPA is configured to process input water IW of any hardness into product water PW. Correspondingly, the WPA is configured to form the disinfection fluid for use in the heat disinfection from input water IW of any hardness. In other words, the WPA is configured to use input water of any hardness, including input water that has a water hardness of more than 100 ppm (mgCaCO3 / L), Step S21 serves to reduce the risk of scaling during heat disinfection by reducing the concentration of solutes that contribute to water hardness, so as to provide a disinfection fluid of low water hardness for use in the heat disinfection. In some embodiments, S21 may further involve adding an acid to the water in the tank 200, before, during or after the generation of the softened water, to further reduce the hardness of the water. The acid may also act as a disinfectant and / or descaling agent during the heat disinfection. Thus, the disinfection fluid may include the acid in addition to softened water. Non-limiting examples include acetic acid, citric acid, or peracetic acid.

[0101] The duration of step S21 may be predetermined so as to result in a desired hardness of the water in the tank 200. Alternatively, the hardness of the water in the tank 200 may be estimated or measured by use of one or more water hardness sensors within the WPA, for example in the tank 200 or the recirculation arrangement 9. Any suitable type of sensor may be used to estimate the water hardness, including a dedicated water hardness sensor, a TDS sensor, a conductivity sensor, etc. Thus, step S21 may be terminated after a predefined time from the start of step S21, or when an estimated water hardness of the softened water is below a predefined limit, for example given by a sensor in the WPA.

[0102] It is to be understood that step S21 is performed without heating of the circulating water, or at least not by heating the circulating water to a temperature that is effective for heat disinfection.

[0103] In step S22, which is performed upon completion of step S21, the softened water (optionally with added acid) in the WPA is heated by the heater 312 to a temperature that is effective for heat disinfection ("heat disinfection temperature"). In the example of FIG. 4A, this is achieved by circulating the softened water from the tank through the heater 312 and back to the tank 200, while the heater 312 is operated to transfer energy to the passing fluid. It may be desirable to minimize the extent of the circulation path to limit heat losses. In FIG. 4A, this is achieved by directing the softened water into the return path 5. Thus, during step S22, the softened water may bypass the second subsystem 400.

[0104] In step S23, the heated fluid from step S22 is distributed throughout the part of the WPA that is to be heat disinfected. This part may or may not be the entire WPA and is denoted "relevant part" in the following. In FIG. 4A, the heated fluid may be circulated from the tank 200 along the main path 1 through the first sub-system 300 and back to the tank 200 via the return line 5 and along the main line 1 through the second subsystem 400 and back to the tank 200 via the return line 7. The heated fluid may also be passed to drain via the drain line 2A, so as to heat disinfect the drain line 2A. Step S23 may be continued until a heat disinfection criterion is fulfilled. In some embodiments, the heat disinfection criterion is based on the A0 concept, in which an A0 value is calculated and accumulated based on measured or estimated fluid temperature in the WPA over time. The fluid temperature may be given for the coldest location within the relevant part of the WPA. When, the A0 value reaches a target value, the heat disinfection criterion is fulfilled. In a variant, the heat disinfection criterion is fulfilled when the relevant part of the WPA has been exposed to heated fluid with a required fluid temperature for a predefined time period.

[0105] It is to be understood that part of step S23 may be performed during step S22, while the fluid temperature is gradually increased.

[0106] In a variant, not shown, the tank 200 has a heater for heating the fluid in the tank 200. The heater may be arranged for direct or indirect heating. During heating in the tank 200, the fluid in the tank may be circulated on a dedicated circulation path, which may be defined inside or outside of the tank 200.

[0107] It is currently believed to be beneficial, to mitigate the risk for scaling in step S23, for the water that is heated in step S22 to have a water hardness of less than 45 ppm (mgCaC03 / L), and preferably less than 35 ppm (mgCaC03 / L), when the water has a pH of 7.5 or less. However, this is a preliminary finding, and beneficial results may be achieved with water of other water hardness, for example depending on the pH of the water, the alkalinity of the water, presence of additives, etc.

[0108] FIG. 4C is a flow chart of an example method M3 that may be performed as part of the method M2 in FIG. 4B. Step S31 is applicable when the first sub-system 300 comprises one or more RO units. By step S31, the WPA is configured to define a bypass path to bypass the RO unit(s) of the first sub-system 300. In the example of FIG. 2B, this means that valves 317, 319 are closed, and that valve 318 is open. Step S31 may be performed in advance of step S22 or during step S22, until the fluid temperature reaches a critical value for the RO unit. In step S32, after step S31, the heater is operated to supply energy while the softened water is circulated through the relevant part of the WPA.

[0109] FIG. 4D is a flow chart of an example method M4, which may be performed in step S21 of FIG. 4B to generate and accumulate the softened water in the tank 200 in advance of the heat disinfection. The method M4 will be described with reference to FIGS 5A-5E, which show part of a WPA during these steps. In FIGS 5A-5E, the WPA is depicted to include components that are used in the method of FIG. 4D, with elements having the same reference numerals as in FIGS 2A-2C. The description of these elements will not be repeated. It is to be understood that the WPA may include additional elements, for example among the elements shown in FIGS 2A-2C. The second sub-system is not shown in FIGS 5A-5E, other than a control valve 401 in the main path 1, and a one-way valve 414 in the return path 7. The second sub-system may or may not be used in the method M4.

[0110] For clarity of presentation, operating fluid pumps and open valve portions are indicated by filled triangles. Any valve may be either normally open or normally closed. Fluid lines through which fluid is flowing are indicated by thicker lines. The flow direction is indicated by solid arrows.

[0111] In FIGS 5A-5E, the water hardness in the tank 200 is designated Ct and may be given in equivalence units of mgCaCO3 / L. Correspondingly, the water hardness in the drain flow is designated Cd, and the drain flow rate is designated Qd. The water hardness of IW is designated Cin, and the flow rate of IW into the tank 200 is designated Qin.

[0112] The method M4 comprises initial steps S41-S43, which are not explicitly shown in FIGS 5A-5E. In step S41, an initial amount of IW is admitted into the tank 200 on the main path 1, by a water inlet valve being opened (cf. valve 102 in FIG. 2A). By step S41, the tank 200 is filled to a predefined level, based on the signal LI from the level sensor 201. Step S41 will be omitted if the tank 200 already contains water at the predefined level. In step S42, the WPA is operated to define a primary circulation path that includes the tank 200 and at least the first sub-system 300. In some embodiments, the primary circulation path is defined to bypass the second sub-system 400, by valve 315 being open and valve 401 being closed. In other embodiments, the second subsystem 400 is included in the primary circulation path, by valve 401 being open and valve 315 being either closed or open. By step S42, valve 319 is also open.

[0113] In step S43, the WPA is operated to set the first sub-system 300 in a reject mode ("first mode"), in which the first sub-system 300 will produce reject water which is directed to drain. In the WPA of FIGS 5A-5E, the reject mode is attained by opening the drain valve 308.

[0114] In step S44, the WPA is operated to circulate water along the primary circulation path defined by step S42. Step S44 is shown by the top portion in FIG. 5A. In the illustrated example, the feed pump 302 is operated to pump water from the tank 200 along the main path 1 into the RO unit 304, and the recirculation pump 305 is operated to pump a first part of the reject water from the drain line 2A on the connecting line 3A back to the inlet of the RO unit 304, whereas a second part of the reject water from a drain flow that is passed to drain on the drain line 2A. The second part of the reject water will contain substances that contribute to water hardness, whereas softened water passes the RO membrane 304' and is driven from the main path 1 into the return path 5 and back to the tank 200. By step S44, the hardness of the water in the tank 200 is gradually decreased.

[0115] As indicated by a dashed arrow in the top portion of FIG. 5A, the water level in the tank 200 will decrease during step S44 as a result of the drain flow. It is not unlikely that the volume of softened water in the tank 200 at completion of step S44 is insufficient for the heat disinfection. Therefore, the method M4 may include step S45, in which further IW is admitted into the tank 200 to at least partly compensate for the water lost to the drain flow. Step S45 is shown in the bottom portion of FIG. 5A, and results in the water level in the tank 200 being increased, for example to a predefined level indicated by a dashed line. At the same time, the hardness of the softened water in the tank 200 will increase by the admission of IW. Therefore, the circulation by step S44 is continued during and after step S45 to further reduce hardness of the water in the tank 200. As indicated by block arrows in FIG. 5A, the WPA may perform step S45 multiple times during step S44, with the water level being reciprocated between a predefined first and second levels in the tank 200. FIG. 5B shows an example of the input water flow Qi and the drain flow Qd during step S44, as seven refill operations are performed by step S45. Step S45 may be performed at any suitable time interval, for example depending on how much the water level in the tank 200 is allowed to vary during step S44. At completion of step S44, the tank 200 contains an accumulated amount of softened water.

[0116] There is a limit to how far the hardness of the water in the tank 200 can be reduced by steps S44-S45. To further reduce the hardness, the method M4 may involve a step S47, in which the WPA is operated to circulate water along the primary circulation path while the first sub-system 300 is set in a non-reject mode ("second mode"), in which the first sub-system 300 passes all of its incoming water along the main path 1. In other words, the first sub-system 300 is operated to stop the drain flow. Step S47 is shown in FIG. 5C, where the non-reject mode is attained by closing the drain valve 308. By step S47, the hardness of the water in the tank 200 may be reduced further. At the same time, the hardness of the water will increase on the feed side of the RO unit 304, as well as in the recirculation pump 305. It may be undesirable to let the water hardness near the RO membrane 304' become excessive, since this may lead to scaling and permanently reduced performance of the RO unit 304. Also, the recirculation pump 305 may get stuck and / or subject to excessive wear if it is left to contain water with elevated hardness. Thus, step S47 may be performed for a limited time that ensures that the water hardness on the feed side is below a hardness limit. It is currently believed that step S47 is most effective in lowering the hardness of the water in the tank 200 if the circulation (step S44) is terminated after step S47. However, it is conceivable that step S47 is performed intermittently during step S44, in addition to or instead of being performed after step S44.

[0117] The method M4 may include a step S48, in which the WPA is operated to supply a portion of the softened water in the tank 200 to the first sub-system 300 so as to flush at least part of the water that is held in the first sub-system 300 to drain. The flushing step S48 may be particularly relevant when step S47 has been performed and aims at removing water of elevated hardness from the first sub-system 300 or part thereof. In some embodiments, step S48 is performed to at least flush the parts of the first subsystem 30 that will not be heat disinfected in step S23 (FIG. 4B). In absence of the flushing step S48, the water with elevated hardness will be left in these non-disinfected parts and may result in scaling. It may be noted that the temperature in the nondisinfected parts may be increased significantly during the heat disinfection by indirect heating, increasing the risk for scaling. The flushing step S48 is exemplified in FIG. 5D. In the illustrated example, the drain valve 308 is opened and valve 319 is closed. The feed pump 302 is operated to pump softened water along the main path 1 from the tank 200 into and through the feed side of the RO unit 304 and into the drain line 2A. In FIG. 5D, it is presumed that the recirculation pump 305 is open when not active, so that softened water also flows into and through the connecting line 3A into the drain line 2A. As indicated by a dashed arrow in FIG. 5D, step S48 comes with the cost of spending some of the softened water in the tank 200. In a variant, to reduce the consumption of softened water, the flow through the feed side of the RO unit 304 may be prevented by closing an appropriately arranged on / off valve (not shown) in the main line 1 or the drain line 2A. In a further variant, not shown, step S48 may use IW instead of softened water, provided that the WPA is configured with appropriate fluid lines and valves for this.

[0118] As shown in FIG. 4D, the method M4 may further include a step S49, in which product water contained in the second sub-system 400 is conveyed into the tank 200. Step S49 is exemplified in FIG. 5E, in which water is circulated from the tank 200 through the first and second sub-systems 300, 400 and back to the tank 200. Step S49 is based on the insight that the water in the second sub-system 400 has a lower hardness than the softened water in the tank 200. In practice, much of the water in the second sub-system 400 will be product water. In the example of FIG. 2C, there will be product water downstream of the RO membrane 404'. As noted with reference to step S42, the primary circulation path may be defined to include the second sub-system 400 to make sure that the water in the second sub-system 400 has a desired water hardness. Even if the second sub-system 400 is not included in the primary circulation path, there may still be product water of desired hardness in the second sub-system 400, for example as a result of it being used for producing product water in a preceding operating stage (cf. FIG. 3A). By step S49, the softened water in the tank 200 is further diluted, resulting in a further reduction of hardness. The Applicant has found that it is beneficial to bypass the RO unit 304 in the first sub-system 300 during step S49, to avoid increasing the hardness of the water on the feed side of the RO unit 304. This may be achieved by use of the bypass arrangement, which was described with reference to FIG. 2B and includes a bypass line 6 and valves 317-319. In the example of FIG. 5E, the WPA is operated in step S49 to define, by opening valves 317, 318 and closing valve 319, a secondary circulation path that bypasses the RO unit 304. In the example of FIG. 5E, step S49 further involves closing valve 315, opening valve 401, and operating the feed pump 302. Thereby, water will be pumped from the tank 200 along the main path 1, while bypassing the RO unit 304, into and through the second sub-system 400 and back to the tank 200 via the return line 7 and, in this example, via the return line 5. As indicated in FIG. 5E, reject water from the second sub-system 400 may also be recirculated to the tank 200 via the return line 2B.

[0119] In a variant of step S49, not shown, water is pumped from the tank 200 through both of the first and second sub-systems 300, 400 and back to the tank 200, with the first sub-system 300 being set in the non-reject mode.

[0120] The duration of the respective step S44, S47 and S48 may be predetermined so as to result in a desired hardness of the water in the tank 200. Alternatively, the hardness of the water in the tank 200 may be estimated or measured by use of one or more water hardness sensors within the WPA, for example in the tank 200 or the primary circulation path.

[0121] FIG. 6 will be used to explain an alternative technique of operating the WPA to generate and accumulate softened water in the tank 200. FIG. 6 corresponds to FIG. 5A, but differs in that the drain flow rate (Qd) is set by an adjustable flow restrictor 306 in the drain path 2A. FIG. 7 shows results from two simulations for the WPA in FIG. 6. The data for the first simulation is represented by solid lines, and the data for the second simulation is represented by lines marked with circles.

[0122] The first simulation assumes that the drain flow rate (Qd) is maintained equal to the flow rate of IW into the tank (Qin) at 200 mL / min during the full simulation (300 s), as shown in FIG. 7(A). FIG. 7(B) shows the corresponding change in water hardness (Ct) in the tank. As seen, Ct decreases over time from a start value of 1 towards an asymptotic value of 0.2, which is reached at approximately t=100 s in this example. All hardness values in the simulation are given in arbitrary units and are intended to show relative change. FIG. 7(C) shows the corresponding change in water hardness (Cd) in the drain flow. As seen, there is a significant initial increase in Cd followed by a slow decline towards the water hardness of IW entering the tank 200 (Cin, here equal to 1).

[0123] The second simulation also assumes that Qd is maintained equal to Qin, and that the permeate flow rate (Qp) through the membrane 304' is constant. The permeate flow rate Qp is indicated in FIG. 6. A constant Qp may be achieved by maintaining the fluid pressure on the feed side of the RO unit 304' at a constant value, for example by feedback control. During an initial time period (0-100 s), Qd is 500 mL / min. During a subsequent time period (100-300 s), Qd is decreased to 200 mL / min. As seen in FIG. 7(B), Ct decreases to a first asymptotic value of 0.4 during the initial time period, and then from the first asymptotic value to a second asymptotic value of 0.2 during the subsequent time period. The end result in terms of Ct is thus the same for the first and second simulations, but it takes longer to reach Ct=0.2 in the second simulation. The second simulation also shows that the asymptotic value of Ct decreases with decreasing Qd. It may thus be desirable to set Qd as low as possible to achieve a low Ct. Looking at FIG. 7(C), the second simulation results in a smaller Cd during both time periods. The simulations in FIG. 7 thus indicate that it is possible to avoid excessive water hardness in the drain flow, and thus also on the feed side of the RO unit 304 and in the recirculation pump 305, by gradually decreasing the drain flow rate towards a target flow rate that results in a desired water hardness in the tank 200.

[0124] FIG. 8 is a flow chart of an example method M5 for operating the WPA in FIG. 6 to generate and accumulate softened water in the tank 200. The method M5 is based on insights from the simulation data in FIG. 7. The method M5 comprises a step S41 of admitting an initial amount of IW into the tank 200, and a step S42 of operating the WPA to define a primary circulation path, which may or may not include the second sub-system 400. Steps S41, S42 may be similar to the corresponding steps in FIG. 4D. In step S42, the pumps 302, 305 may be started. Step S42 is followed by a group of steps S46a-S46e, which replace steps S44-S45 in FIG. 4D. In step S46a, the drain flow rate (Qd) is set to an initial value, by adjustment of the flow restrictor 306. In step S46b, the flow rate of IW into the tank (Qin) is set to balance the drain flow rate. In the example of FIG. 2A, step S46b may be performed by repeatedly switching ("toggling") the valve 102 on and off to achieve an average flow rate that matches the drain flow rate. Alternatively, the WPA may include an adjustable flow restrictor in the main path 1 upstream of the tank 200 which is adjusted appropriately. In step S46c, water is circulated in the primary circulation path to gradually reduce water hardness, as shown in FIG. 6. Step S46c may be performed for a predefined time period or until a target hardness is measured by a sensor. The method M5 is predefined to decrease the water hardness in a number of steps. An example is shown in FIG. 9, where Qd is decreased from ql to q4 in three steps at time points tl, t2 and t3. When it is time to change Qd, step S46d proceeds to step S46e, in which the flow restrictor 306 is adjusted to decrease Qd to a predefined value. The method M5 then returns to step S46b, in which Qin is adjusted accordingly. When step S46c has been performed for the final Qd (q4 in FIG. 9), step S46d may proceed to step S48 and / or step S49, which may be performed as described with reference to FIG. 4D. It is to be noted that the drain flow rate (Qd) may be decreased in any number of steps in the method M5, including a single step. In a variant, Qd is continuously decreased during step S46c, and Qin is continuously adjusted to balance Qd (step S46b). It is also conceivable to perform a combination of stepwise and continuous decrease of Qd during step S46c. Preferably, the Qin is non-increasing throughout during steps S46a-S46e.

[0125] As an alternative to using an adjustable flow restrictor 306, the drain flow rate may be set by an on / off valve in the drain line 2A, such as the control valve 308 in FIGS 5A-5E. The skilled person understands that such a valve may be toggled to achieve an average drain flow rate in the drain line 2A. In the method M5, such an average drain flow rate would correspond to Qd in the method M5.

[0126] It is realized that step S46b results in a stable water level in tank 200 throughout steps S46a-S46e. This may facilitate control of the WPA. For example, step S46b may be performed based on the signal LI from the level sensor 201. However, it is conceivable that the water level is allowed to vary in tank 200 during steps S46a-S46e, for example between two predefined levels as shown in FIG. 5A.

[0127] As understood from FIG. 7, the final Qd during steps S46a-S46e (q4 in FIG. 9) determines Ct at completion of steps S46a-S46e. Thus, the final Qd may be predefined in view of the desired target hardness of the softened water to be used in the heat disinfection.

[0128] Reverting to the flushing step S48 in FIG. 4D, it may be desirable to economize with the softened water in the tank 200 that is used to flush part of the first sub-system 300. This may be achieved by not flushing the feed side of the RO unit 304, which may be quite large. It is actually conceivable that the feed side of the RO unit 304 has a larger volume than the tank 200. FIG. 10A shows an example WPA, which is a modified variant of the WPA in FIG. 5A to enable flushing of the recirculation pump 305. First and second 3-way valves 320, 321 are arranged in the connecting line 3 A downstream and upstream, respectively, of the pump 305. A first intermediate line 322 extends from the first valve 320 and to a junction on the drain line 2A, downstream of the control valve 308. A second intermediate line 323 extends from the second valve 321 to a junction on the connecting line 3 A, between the first valve 320 and main path 1. It is realized that the WPA in FIG. 10A may be operated to define the primary circulation path shown in FIG. 5A and FIG. 6. However, FIG. 10A shows the WPA during the flushing step S48. Here, the valves 320, 321 are set to open a flushing path from the main path 1 to drain 309 via the recirculation pump 305, while blocking flow through the feed side of the RO unit 304 to drain. Thereby, the recirculation pump 305 is flushed with water from the tank 200 when the pumps 302, 305 are operated. In the example of FIG. 10A, the WPA also includes the bypass arrangement, which was described with reference to FIG. 2B and includes a bypass line 6 and valves 317-319. In the illustrated example, the WPA is operated in step S48 to define, by opening valves 317, 318 and closing valve 319, a secondary circulation path that bypasses the RO unit 304 while opening fluid communication to the flushing path. This configuration avoids a need to balance the flow rates of the pumps 302, 305 in step S48.

[0129] FIG. 10B shows an example WPA, which is a modified variant of the WPA in FIG. 10A to enable flushing of the recirculation pump 305. Compared to FIG. 10A, the first 3-way valve 320 and the intermediate line 322 have been removed. Like in FIG. 10A, the 3-way valve 321 is arranged in the connecting line 3A between its junction to the drain line 2A and the recirculation pump 305. A further 3-way valve 324 is arranged in the bypass line 6 upstream of the valve 318. The intermediate line 323 extends between the 3-way valves 321, 324. Again, it is realized that the WPA in FIG. 10B may be operated to define the primary circulation path shown in FIG. 5A and FIG. 6. In step S48, as shown in FIG. 10B, valve 308 is open to fluidly connect the feed side of the RO unit 304 to drain 309. Flow through the membrane 304' is prevented by valve 319 being closed. Further, valves 321, 324 are operated to open a flushing path from the bypass line 6 through the intermediate line 323 into the connecting line 3A, through the recirculation pump 305 and into the main path 1. Thereby, the recirculation pump 305 is flushed with water from the tank 200 when the pumps 302, 305 are operated. The valve 317 is closed so that the flow through the flushing path is controlled by the pump 305 independent of the pump 302. It may be noted that to flush the recirculation pump 305, the WPA need only be operated in the configuration of FIG. 10B until water from the tank 200 has passed the pump 305.

[0130] FIG. 11 A shows an alternative configuration of a WPA when operated to generate and accumulate softened water in the tank 200 (step S21 in FIG. 4B). Compared to FIGS 5A-5E, the WPA includes a 3-way valve 325 in the main path 1 intermediate the tank 200 and the feed pump 302. An inlet line 326 extends to the valve 325. The inlet line 326 may be connected to the source 10 or the pre-processing sub-system 100 (if present) and is thus configured to supply IW.

[0131] FIG. 1 IB is a flow chart of an example method M6 for operating the WPA in FIG. 11A to generate and accumulate softened water in the tank 200. In step S41 ', the amount of water in the tank 200 is minimized. Step S41' may be performed by operating the WPA in accordance with FIG. 5D or the top portion of FIG. 5A so as to dispose of preexisting fluid in the tank 200. Alternatively, the tank 200 may opened to drain 309 on a dedicated flow path (not shown). Step 41' may result in a predefined minimum fluid level in the tank 200. In step S42', the WPA is operated to define a one-way flow path from the inlet line 326 to the tank 200 via the first sub-system 300. In the example of FIG. 11 A, step S43' involves operating the 3-way valve 325 to close the main path 1 towards the tank 200 and open fluid communication between the inlet line 326 and the main path 1 towards the feed pump 302. The one-way path may or may not include the second sub-system 400. In step S43', the first sub-system 300 is set in the reject mode, by opening valve 308. In step S44', the WPA is operated to pump IW on the one-way path while the first sub-system 300 (and optionally the second sub-system 400) is operated to decrease the hardness of the passing fluid. Step S44' is continued until a desired amount of softened water is collected in the tank 200. As shown, the method M5 may proceed to step S48 and / or step S49, which may be performed as described with reference to FIG. 4D.

[0132] FIGS 12A-12B illustrate flow paths through the WPA of FIGS 2B-2C during heat disinfection. Principles for the operating state in FIGS 12A-12B are readily transferred to other configurations of the WPA shown herein by the skilled person. The tank 200 contains softened water and the WPA is operated to prevent inflow of IW. The WPA is operated to define a heat disinfection path and pump the softened water on the heat disinfection path while the heater 312 is operated to heat the passing water. The heat disinfection path is a circulation path that starts and ends at the tank 200. Depending on the structure of the WPA, more than one heat disinfection path may be defined during the heat disinfection. In the illustrated example, since the first RO unit 304 is a disposable component, the heat disinfection path does not extend through the first RO unit 304. This is achieved by use of the bypass arrangement, in which valve 308 in the bypass line 6 is open, and valves 317, 319 are closed. The heat disinfection path includes the return line 5, and valve 315 is thus open. The second sub-system 400 and the return path 7 are also included in the heat disinfection path, and valves 401, 410 are thus open. Also the drain line 2B, the connecting line 3B and the bypass line 4B are included, and valve 407 is open. To circulate the softened water through all parts of the heat disinfection path, the feed pumps 302, 403 and the recirculation pump 405 are operating. Since the RO unit 304 is fluidly separated from the heat disinfection path, the recirculation pump 305 is disabled.

[0133] The WPA may be operated to perform the heat disinfection in accordance with the A0 concept. This means that the heat disinfection is performed to achieve a predefined A0 value. As noted above, the A0 value is a function of fluid temperature and exposure time. The fluid temperature is given at the coldest location in the heat disinfection path. In the example of FIGS 12A-12B, the coldest location is likely to be immediately upstream of the heater 312. Thus, the A0 valve may be calculated based on the signal T2 from the temperature sensor 310. It is realized that the volume of fluid in the tank 200 will expand as a result of the heating. It is thus preferable for the level of the softened water to be located below the top of the tank 200 before the softened water is heated. At the same time, it may be desirable for the water in the tank 200 to cover as much as possible of the inner walls in the tank 200 during heat disinfection to fully disinfect the tank 200. It may thus be desirable for the tank 200 to hold as much softened water as possible before the heat disinfection is started, while still accommodating for the subsequent thermal expansion of the softened water. In some embodiments, the vent 206 is closed during at least part of the heat disinfection to allow wall portions not in contact with the heated water to be subjected to heated steam.

[0134] While the subject of the present disclosure has been described in connection with what is presently considered to be the most practical embodiments, it is to be understood that the subject of the present disclosure is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0135] Further, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.

[0136] In the following, clauses are recited to summarize some aspects and embodiments as disclosed in the foregoing.

[0137] Cl. A system for generating product water for medical use, said system comprising: a main flow path (1) extending from an inlet (1') for input water to an outlet (1") for product water; and water purification equipment (2) arranged in the main flow path (1) to process the input water into the product water, wherein the water purification equipment (2) comprises: a tank (200) arranged in the main flow path (1) to receive the input water; a first sub-system (300), which is arranged in the main flow path (1) downstream of the tank (200) and configured to reduce water hardness of water entering the first sub-system (300); a recirculation arrangement (9) extending to the tank (200) from the main flow path (1) downstream of the first sub-system (300); and a control arrangement (500) for operating the system, wherein the control arrangement (500) is configured to operate the system to perform a heat disinfection procedure, which comprises: generating (S21), from the input water and by use of the first sub-system (300) and the recirculation arrangement (9), softened water with reduced water hardness compared to the input water and accumulating the softened water in the tank (200); operating (S22) the system, by use of a heater (312) in the system, to heat the softened water; and operating (S23) the system to distribute the thus-heated softened water in at least part of the system.

[0138] C2. The system of Cl, wherein said generating (S21) softened water comprises: admitting (S41), into the tank (200), an initial amount of the input water and operating (S44) the system to perform a circulation of water held in the tank (200) through the first sub-system (300) and back to the tank (200) via the recirculation arrangement (9), so as to gradually reduce the water hardness of the water held in the tank (200), wherein at least part of the softened water is located in the tank (200) at completion of the circulation.

[0139] C3. The system of C2, wherein the first sub-system (1) is configured for fluid communication with a drain path (2A) that extends to a drain (309), and wherein the first sub-system (300), to reduce the water hardness of the water entering the first subsystem (300), is operable in a first mode, in which the water entering the first subsystem (300) is processed for reduction of water hardness such that a first part of the water entering the first sub-system (300) is passed through the first sub-system (300) along the main flow path (1) and a second part of the water entering the first sub-system (300) is output on the drain path (2A), and a second mode, in which the water entering the first sub-system (300) is processed for reduction of water hardness and passed through the first sub-system (300) along the main flow path (1), wherein the control arrangement (500) is configured to operate (S43) the first sub-system (300) in the first mode during at least part of the circulation.

[0140] C4. The system of C3, wherein said generating (S21) softened water comprises: admitting (S45) further input water into the tank (200) during the circulation to at least partly compensate for the second part output on the drain path (2A).

[0141] C5. The system of C4, wherein the control arrangement (500) is configured to admit the further input water into the tank (200) to achieve a stable water level in the tank (200) during the circulation.

[0142] C6. The system of any one of C3-C5, wherein a flow control element (306, 308) is arranged in the drain path (2A) and operable to define a flow rate of the second part, and wherein the control arrangement (500) is configured to, during the circulation, operate the flow control element (306, 308) to alter (S46e) the flow rate of the second part.

[0143] C7. The system of C6, wherein the control arrangement (500) is configured to decrease the flow rate of the second part in one or more steps and / or continuously during the circulation. C8. The system of C3 or C4, wherein the control arrangement (500) is configured to switch (S47) the first sub-system (300) from the first mode to the second mode during a final portion of the circulation.

[0144] C9. The system of any one of C2-C8, wherein the control arrangement (500) is further configured to, after completion of the circulation and before said operating (S22) the system to heat the softened water, operate (S48) the system to flush contained water within the first sub-system (300) to drain (309).

[0145] CIO. The system of C9, wherein the control arrangement (500), to flush the contained water, is configured to operate the system to supply a portion of the softened water from the tank (200) to the first sub-system (300), or to admit a portion of the input water into the first sub-system (300).

[0146] Cl 1. The system of C9 or CIO, wherein at least part of the contained water is located outside of said at least part of the system, in which the thus-heated softened water is to be distributed.

[0147] C12. The system of Cl, wherein said generating (S21) softened water comprises: operating (S44') the system to admit the input water into the main flow path (1) at a location downstream of the tank (200) and upstream of the first sub-system (300), and to pump the thus-admitted input water through the first sub-system (300) into the tank (200) while operating the first sub-system (300) to reduce the water hardness of the thus-admitted input water.

[0148] C13. The system of C12, wherein the control arrangement (500) is configured to, before operating the system to admit the input water into the main flow path (1), open (S4T) the tank (200) to a drain (309) so as to dispose of pre-existing fluid in the tank (200).

[0149] C14. The system of any preceding clause, which further comprises a second subsystem (400), which is arranged in the main flow path (1) intermediate the first subsystem (300) and the outlet (1"), wherein the second sub-system (400) is configured to process output water from the first sub-system (300) into said product water.

[0150] C15. The system of C14, wherein the control arrangement (500) is configured to, during at least part of said generating (S21) the softened water, operate the system to bypass the second sub-system (400) so that the softened water is generated without use of the second sub-system (400).

[0151] C16. The system of C14 or C15, wherein the control arrangement (500) is configured to, during said generating the softened water, operate (S49) the system to convey product water contained in the second sub- system (400) back and into the tank (200) via the recirculation arrangement (9) to thereby dilute the softened water contained in the tank (200). C17. The system of any one of C14-C16, wherein the recirculation arrangement (9) comprises a first return path (5), which extends to the tank (200) from the main flow path (1) intermediate the first and second sub-systems (300, 400).

[0152] C18. The system of C17, wherein the control arrangement (500) is configured to, during at least part of said generating (S21) the softened water, operate the system to direct the output water of the first sub-system (300) into the first return path (5).

[0153] Cl 9. The system of any one of C14-C18, wherein the second- sub- system (400) is configured to direct all of the output water of the first sub-system (300) back to the tank (200).

[0154] C20. The system of any one of C14-C19, wherein the recirculation arrangement (9) comprises a second return path (7), which extends to the tank (200) from the main flow path (1) downstream of the second sub-system (400).

[0155] C21. The system of C20, wherein the second sub-system (400) comprises a reverse osmosis, RO, unit (404), which is arranged in the main flow path (1) to receive the output water of the first sub-system (300) and generate permeate fluid and retentate fluid, wherein a reject line (2A) is arranged to direct the retentate fluid from the RO unit (404) to the tank (200), and wherein the second return path (7) is arranged to direct the permeate fluid from the RO unit (404) to the tank (200).

[0156] C22. The system of any one of C14-C20 in combination with any one of C2-C11, wherein the control arrangement (500) is configured to operate (S42) the system to, during the circulation of the water held in the tank (200) and by use of the recirculation arrangement (9), direct the water held in the tank (200) through the first sub-system (300), from the first sub-system (300) through the second sub-system (400), and from the second sub-system (400) back to the tank (200).

[0157] C23. The system of any preceding clause, wherein said operating (S22) the system to heat the softened water comprises: circulating the softened water from the tank (200) through the heater (312) and back to the tank (200) via the recirculation arrangement (9).

[0158] C24. The system of C23, wherein the control arrangement (500) is configured to, while operating the system to circulate the softened water through the heater (312), distribute the softened water in said at least part of the system so that said at least part of the system is exposed to the thus-heated softened water.

[0159] C25. The system of C23 or C24 in combination with any one of C14-C22, wherein the heater (312) is located in the main flow path (1) to heat the output water from the first sub-system (300).

[0160] C26. The system of any preceding clause, wherein the first sub-system (300) comprises a reverse osmosis, RO, unit (304) C27. The system of C26, wherein the RO unit (304) is arranged in the main flow path (1) to receive water from the tank (200) via a water inlet (304A') on a feed side (304") of the RO unit (304), and to provide output water via a permeate outlet (304B') on a permeate side (304"') of the RO unit (300), wherein a drain line (2A) is arranged to extend from a retentate outlet (304C) on the feed side (304") of the RO unit (304) to said drain (309), a connecting line (3A) is arranged in fluid communication with the drain line (2A) and the main flow path (1) to define a recirculation path from the retentate outlet (304C) to the water inlet (304A'), a feed pump (302) is arranged in the main flow path (1) upstream of the water inlet (304A'), and an auxiliary pump (305) is arranged in the recirculation path.

[0161] C28. The system of C26 or C27, wherein the control arrangement (500) is further configured to operate (S31) the system to bypass the RO unit (304), by fluidly connecting a bypass path (6) to the main flow path (1) upstream and downstream of the RO unit (304), while the system is operated to distribute the thus-heated softened water in said at least part of the system to be heat disinfected.

[0162] C29. The system of any one of C26-C28, wherein the RO unit (304) is a sacrificial component which is removably installed in the first sub-system (300).

[0163] C30. The system of any one of C26-C29 in combination with any one of C14- C20, wherein the second sub-system (400) comprises a further RO unit (404), which is arranged in the main flow path (1) to receive the output water from the first sub-system (300) via a water inlet (404A') on a feed side (404") of the further RO unit (404), and to provide the product water via a permeate outlet (404B') on a permeate side (404"') of the further RO unit (400), wherein a reject line (2B) is arranged to extend from a retentate outlet (404C) on the feed side (404") of the further RO unit (404) to the tank (200).

[0164] C31. The system of any preceding clause, wherein the softened water that is heated during the heat disinfection procedure has a hardness of less than 45 ppm (mgCaCO3 / L), and preferably less than 35 ppm (mgCaCO3 / L), when the softened water has a pH of 7.5 or less.

[0165] C32. The system of any preceding claim, wherein the control arrangement (500) is configured to terminate said generation (S21) of the softened water after a predefined time period or when an estimated water hardness of the softened water is below a predefined limit.

[0166] C33. A computer-implemented method of operating a system according to any one of C1-C32 to perform the heat disinfection procedure, said method comprising: operating (S21) the system to generate, from the input water and by use of the first subsystem (300) and the recirculation arrangement (9), softened water with reduced water hardness compared to the input water and accumulate the softened water in the tank (200); operating (S22) the heater (312) in the system to heat (S22) the softened water; and operating (S23) the system to distribute the thus-heated softened water in the at least part of the system.

[0167] C34. A computer-readable medium comprising instructions which, when executed by processor circuitry (501), causes the processor circuitry (501) to perform the method C32.

Claims

42CLAIMS1. A system for generating product water for medical use, said system comprising: a main flow path (1) extending from an inlet (1') for input water to an outlet (1") for product water, and water purification equipment (2) arranged in the main flow path (1) to process the input water into the product water, wherein the water purification equipment (2) comprises: a tank (200) arranged in the main flow path (1) to receive the input water, a first sub-system (300), which is arranged in the main flow path (1) downstream of the tank (200) and configured to reduce water hardness of water entering the first sub- system (300), a recirculation arrangement (9) extending to the tank (200) from the main flow path (1) downstream of the first sub-system (300), and a control arrangement (500) for operating the system, wherein the control arrangement (500) is configured to operate the system to perform a heat disinfection procedure, which comprises: generating (S21), from the input water and by use of the first sub-system (300) and the recirculation arrangement (9), softened water with reduced water hardness compared to the input water and accumulating the softened water in the tank (200), operating (S22) the system, after completion of said generating (S21) the softened water and by use of a heater (312) in the system, to heat the softened water, and operating (S23) the system to distribute the thus-heated softened water in at least part of the system.

2. The system of claim 1, wherein said generating (S21) softened water comprises: admitting (S41), into the tank (200), an initial amount of the input water and operating (S44) the system to perform a circulation of water held in the tank (200) through the first sub-system (300) and back to the tank (200) via the recirculation arrangement (9), so as to gradually reduce the water hardness of the water held in the tank (200), wherein at least part of the softened water is located in the tank (200) at completion of the circulation.

3. The system of claim 2, wherein the first sub-system (1) is configured for fluid communication with a drain path (2A) that extends to a drain (309), and wherein the first sub-system (300), to reduce the water hardness of the water entering the first sub-43 system (300), is operable in a first mode, in which the water entering the first subsystem (300) is processed for reduction of water hardness such that a first part of the water entering the first sub-system (300) is passed through the first sub-system (300) along the main flow path (1) and a second part of the water entering the first sub-system (300) is output on the drain path (2A), and a second mode, in which the water entering the first sub-system (300) is processed for reduction of water hardness and passed through the first sub-system (300) along the main flow path (1), wherein the control arrangement (500) is configured to operate (S43) the first sub-system (300) in the first mode during at least part of the circulation.

4. The system of claim 3, wherein said generating (S21) softened water comprises: admitting (S45) further input water into the tank (200) during the circulation to at least partly compensate for the second part output on the drain path (2A).

5. The system of claim 4, wherein the control arrangement (500) is configured to admit the further input water into the tank (200) to achieve a stable water level in the tank (200) during the circulation.

6. The system of any one of claims 3-5, wherein a flow control element (306, 308) is arranged in the drain path (2A) and operable to define a flow rate of the second part, and wherein the control arrangement (500) is configured to, during the circulation, operate the flow control element (306, 308) to alter (S46e) the flow rate of the second part.

7. The system of claim 6, wherein the control arrangement (500) is configured to decrease the flow rate of the second part in one or more steps and / or continuously during the circulation.

8. The system of claim 3 or 4, wherein the control arrangement (500) is configured to switch (S47) the first sub-system (300) from the first mode to the second mode during a final portion of the circulation.

9. The system of any one of claims 2-8, wherein the control arrangement (500) is further configured to, after completion of the circulation and before said operating (S22) the system to heat the softened water, operate (S48) the system to flush contained water within the first sub-system (300) to drain (309).4410. The system of claim 9, wherein the control arrangement (500), to flush the contained water, is configured to operate the system to supply a portion of the softened water from the tank (200) to the first sub-system (300), or to admit a portion of the input water into the first sub-system (300).

11. The system of claim 9 or 10, wherein at least part of the contained water is located outside of said at least part of the system, in which the thus-heated softened water is to be distributed.

12. The system of claim 1, wherein said generating (S21) softened water comprises: operating (S44') the system to admit the input water into the main flow path (1) at a location downstream of the tank (200) and upstream of the first sub-system (300), and to pump the thus-admitted input water through the first sub-system (300) into the tank (200) while operating the first sub-system (300) to reduce the water hardness of the thus-admitted input water.

13. The system of claim 12, wherein the control arrangement (500) is configured to, before operating the system to admit the input water into the main flow path (1), open (S4T) the tank (200) to a drain (309) so as to dispose of pre-existing fluid in the tank (200).

14. The system of any preceding claim, which further comprises a second subsystem (400), which is arranged in the main flow path (1) intermediate the first subsystem (300) and the outlet (1"), wherein the second sub-system (400) is configured to process output water from the first sub-system (300) into said product water.

15. The system of claim 14, wherein the control arrangement (500) is configured to, during at least part of said generating (S21) the softened water, operate the system to bypass the second sub-system (400) so that the softened water is generated without use of the second sub-system (400).

16. The system of claim 14 or 15, wherein the control arrangement (500) is configured to, during said generating the softened water, operate (S49) the system to convey product water contained in the second sub- system (400) back and into the tank (200) via the recirculation arrangement (9) to thereby dilute the softened water contained in the tank (200).

17. The system of any one of claims 14-16, wherein the recirculation arrangement (9) comprises a first return path (5), which extends to the tank (200) from the main flow path (1) intermediate the first and second sub-systems (300, 400).

18. The system of claim 17, wherein the control arrangement (500) is configured to, during at least part of said generating (S21) the softened water, operate the system to direct the output water of the first sub-system (300) into the first return path (5).

19. The system of any one of claims 14-18, wherein the second- sub -system (400) is configured to direct all of the output water of the first sub-system (300) back to the tank (200).

20. The system of any one of claims 14-19, wherein the recirculation arrangement (9) comprises a second return path (7), which extends to the tank (200) from the main flow path (1) downstream of the second sub-system (400).

21. The system of claim 20, wherein the second sub-system (400) comprises a reverse osmosis, RO, unit (404), which is arranged in the main flow path (1) to receive the output water of the first sub-system (300) and generate permeate fluid and retentate fluid, wherein a reject line (2A) is arranged to direct the retentate fluid from the RO unit (404) to the tank (200), and wherein the second return path (7) is arranged to direct the permeate fluid from the RO unit (404) to the tank (200).

22. The system of any one of claims 14-20 in combination with any one of claims 2-11, wherein the control arrangement (500) is configured to operate (S42) the system to, during the circulation of the water held in the tank (200) and by use of the recirculation arrangement (9), direct the water held in the tank (200) through the first sub-system (300), from the first sub-system (300) through the second sub-system (400), and from the second sub-system (400) back to the tank (200).

23. The system of any preceding claim, wherein said operating (S22) the system to heat the softened water comprises: circulating the softened water from the tank (200) through the heater (312) and back to the tank (200) via the recirculation arrangement (9).

24. The system of claim 23, wherein the control arrangement (500) is configured to, while operating the system to circulate the softened water through the heater (312),distribute the softened water in said at least part of the system so that said at least part of the system is exposed to the thus-heated softened water.

25. The system of claim 23 or 24 in combination with any one of claims 14-22, wherein the heater (312) is located in the main flow path (1) to heat the output water from the first sub-system (300).

26. The system of any preceding claim, wherein the first sub-system (300) comprises a reverse osmosis, RO, unit (304)27. The system of claim 26, wherein the RO unit (304) is arranged in the main flow path (1) to receive water from the tank (200) via a water inlet (304A') on a feed side (304") of the RO unit (304), and to provide output water via a permeate outlet (304B') on a permeate side (304"') of the RO unit (300), wherein a drain line (2A) is arranged to extend from a retentate outlet (304C) on the feed side (304") of the RO unit (304) to said drain (309), a connecting line (3A) is arranged in fluid communication with the drain line (2A) and the main flow path (1) to define a recirculation path from the retentate outlet (304C) to the water inlet (304A'), a feed pump (302) is arranged in the main flow path (1) upstream of the water inlet (304A'), and an auxiliary pump (305) is arranged in the recirculation path.

28. The system of claim 26 or 27, wherein the control arrangement (500) is further configured to operate (S31) the system to bypass the RO unit (304), by fluidly connecting a bypass path (6) to the main flow path (1) upstream and downstream of the RO unit (304), while the system is operated to distribute the thus-heated softened water in said at least part of the system to be heat disinfected.

29. The system of any one of claims 26-28, wherein the RO unit (304) is a sacrificial component which is removably installed in the first sub-system (300).

30. The system of any one of claims 26-29 in combination with any one of claims 14-20, wherein the second sub-system (400) comprises a further RO unit (404), which is arranged in the main flow path (1) to receive the output water from the first sub-system (300) via a water inlet (404A') on a feed side (404") of the further RO unit (404), and to provide the product water via a permeate outlet (404B') on a permeate side (404"') of the further RO unit (400), wherein a reject line (2B) is arranged to extend from a retentate outlet (404C) on the feed side (404") of the further RO unit (404) to the tank (200).4731. The system of any preceding claim, wherein the softened water that is heated during the heat disinfection procedure has a hardness of less than 45 ppm (mgCaC03 / L), and preferably less than 35 ppm (mgCaC03 / L), when the softened water has a pH of 7.5 or less.

32. The system of any preceding claim, wherein the control arrangement (500) is configured to terminate said generation (S21) of the softened water after a predefined time period or when an estimated water hardness of the softened water is below a predefined limit.

33. A computer-implemented method of operating a system according to any one of claims 1-32 to perform the heat disinfection procedure, said method comprising: operating (S21) the system to generate, from the input water and by use of the first sub-system (300) and the recirculation arrangement (9), softened water with reduced water hardness compared to the input water and accumulate the softened water in the tank (200), operating (S22) the heater (312) in the system to heat (S22) the softened water, and operating (S23) the system to distribute the thus-heated softened water in the at least part of the system.

34. A computer-readable medium comprising instructions which, when executed by processor circuitry (501), causes the processor circuitry (501) to perform the method claim 33.

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