A WATER PURIFICATION APPARATUS AND A METHOD FOR CONTROLLING AT LEAST ONE PROPERTY OF THE FLUID IN A WATER PURIFICATION APPARATUS

Abstract

This disclosure relates to a water purification apparatus comprising a reverse osmosis (RO) device that produces a flow of purified water and a corresponding method. The proposed method comprises detecting at least one fluid property of purified water in the purified water path and regulating a water flow rate in the recirculation path to meet one or more predetermined criteria for the purified water in the purified water path, based on the at least one detected fluid property. This disclosure also relates to a computer program and a computer program product that implements the method.

Description

A WATER PURIFICATION APPARATUS AND A METHOD FOR CONTROLLING AT LEAST ONE FLUID PROPERTY IN A WATER PURIFICATION APPARATUS anennn / cznz / B / vi TECHNICAL FIELD This disclosure relates to a water purification apparatus and the corresponding methods for controlling at least one fluid property in a water purification apparatus. This disclosure also relates to a computer program and a computer program product that implements the method. BACKGROUND Dialysis therapy is used to treat patients with acute or chronic kidney failure. Three general categories of dialysis therapy are hemodialysis (HD), peritoneal dialysis (EP), and continuous renal replacement therapy (CRRT). In hemodialysis, the patient's blood is cleaned by passing it through an artificial kidney in an extracorporeal membrane system (ECMS), which is part of a dialysis machine. The blood treatment involves extracorporeal circulation through an exchanger with a semipermeable membrane (dialyzer). The patient's blood circulates on one side of the membrane, while a dialysis fluid, containing the main blood electrolytes in concentrations close to those of a healthy individual's blood, circulates on the other. A pressure difference is created between the two dialyzer compartments, separated by the semipermeable membrane, causing a fraction of the plasma to pass through the membrane via ultrafiltration into the compartment containing the dialysis fluid. CRRT is used as an alternative therapy for patients who are too ill or unstable for standard hemodialysis. It is similar to hemodialysis and uses a semipermeable membrane for diffusion and, to some extent, convection. However, it is a slower form of blood treatment than hemodialysis and can last from a few hours to several days. In peritoneal dialysis, dialysis fluid is infused into the patient's peritoneal cavity. This cavity is lined by the highly vascularized peritoneal membrane. Metabolites are removed from the patient's blood by diffusion across the peritoneal membrane into the dialysis fluid. Excess fluid, i.e., water, is also removed by osmosis induced by a hypertonic dialysis fluid. Through these two processes—diffusion and osmotic ultrafiltration—appropriate amounts of solute metabolites and fluid must be removed to maintain the patient's body fluid volumes and composition within appropriate limits. There are several types of peritoneal dialysis therapies, including continuous ambulatory peritoneal dialysis (CAPD), automated peritoneal dialysis (APD), including tidal flow APD, and continuous flow peritoneal dialysis (CFPD). CAPD is a manual dialysis treatment. The patient manually connects an implanted catheter to a drain, allowing spent dialysate to drain from the peritoneal cavity. The patient then connects the catheter to a bag of fresh dialysate, infusing the new dialysate through the catheter and into the patient. The patient then disconnects the catheter from the bag of fresh dialysate, allowing the dialysate to remain in the peritoneal cavity, where the transfer of waste products, toxins, and excess water takes place. Automated peritoneal dialysis (APD) is similar to continuous active dialysis (CAPD) in that dialysis treatment includes cycles of draining, filling, and dwelling. APD machines, however, perform these cycles automatically, usually while the patient sleeps. APD machines free patients from having to manually perform treatment cycles and from having to transport supplies during the day. APD machines connect seamlessly to an implanted catheter, a source or bag of fresh dialysis fluid, and a fluid drain. APD machines pump fresh dialysis fluid from the dialysis fluid source, through the catheter, into the patient's peritoneal cavity, allowing the dialysis fluid to remain within the cavity and enabling the transfer of waste products, toxins, and excess water. APD machines then pump spent dialysate from the peritoneal cavity, through the catheter, to the drain.As with the manual procedure, several cycles of drainage, filling, and dwelling occur during APD. A final filling often occurs at the end of CAPD and APD, which remains in the patient's peritoneal cavity until the next treatment. Both CAPD and APD are batch systems that send spent dialysis fluid to a drain. Tidal flow systems are modified batch systems. With tidal flow, instead of removing all the fluid from the patient over a longer period, a portion of the fluid is removed and replaced after smaller time increments. Continuous flow dialysis (CFPD) systems clean or regenerate spent dialysate instead of discarding it. CFPD systems are typically more complex than batch systems. CAPD, APD (including tidal flow), and CFPD systems may employ a pump cassette. The pump cassette typically includes a flexible membrane that moves mechanically to push and draw dialysis fluid out of and into the cassette, respectively. In one form of peritoneal dialysis, an automated cycler is used to infuse and drain the dialysis fluid. This form of treatment can be performed automatically overnight while the patient sleeps. The cycler measures the amount of fluid infused and the amount withdrawn to calculate the net fluid removal. The treatment sequence typically begins with an initial drainage cycle to empty the peritoneal cavity of spent dialysate. The cycler performs a series of fill, dwell, and drain cycles, usually ending with a fill cycle. Peritoneal dialysis typically requires large volumes of dialysis fluid. Generally, with each application or exchange, a given patient will have 2 to 3 liters of dialysis fluid infused into the peritoneal cavity. The dialysis fluid is allowed to remain for approximately 1 to 3 hours, at which point it is drained and replaced with fresh dialysis fluid. Typically, four of these exchanges are performed daily. Therefore, approximately 8 to 20 liters of dialysis fluid are required per day, 7 days a week, 365 days a year for each patient. Dialysis fluids, for use in the treatments mentioned above, have traditionally been provided in a sealed, ready-to-use container bag. For example, peritoneal dialysis is typically performed using bags with three different dextrose concentrations. The bags are shipped to the patient's home in sizes ranging from 1 to 6 liters, each with a different dextrose concentration. A normal daily consumption is around 8 to 20 liters of PD dialysis fluid. The fluid is supplied in sterile bags up to six liters in size, which are packaged in boxes and delivered, for example, monthly, for use at the patient's home. The fluid boxes can be cumbersome and heavy for patients with peritoneal dialysis (PD) to handle and take up considerable space in a room of their homes. The bags and boxes also produce a relatively large amount of waste that is disposed of weekly or monthly. In light of the above, several problems become apparent. Shipping and storing the large volume of fluids required consumes a lot of space. Furthermore, the use of multiple pre-filled bags generates waste material in the form of empty containers and packaging. Therefore, subsystems are needed for a general peritoneal dialysis (PD) system that creates a dialysis solution at the point of use, e.g., in the PD machine. The PD dialysis fluid is administered directly into the patient's peritoneal cavity. Therefore, the PD fluid must be sterilized to a suitable level for introduction into the patient's peritoneum. The PD dialysis fluid is premixed and sterilized accordingly, usually before delivery at the point of use, typically the patient's home. In addition, in hemodialysis and CRRT, systems are needed that create a dialysis solution at the point of use, for example, on the hemodialysis machine or the CRRT machine. A comprehensive hemodialysis system, PD or CRRT in some modalities, includes three main components: a dialysis machine, a water purifier, and a disposable assembly that works with both the dialysis machine and the water purifier. The dialysis machine is, for example, a PD cycler, a hemodialysis machine, or a CRRT machine. The dialysis machine prepares dialysis fluid from purified water from the water purifier and concentrates. The water purifier produces purified water from, for example, tap water, at the point of use of the purified water. BRIEF DESCRIPTION OF THE INVENTION Under certain circumstances, it is desirable to deliver a specific product water flow rate. For example, to ensure the timely delivery of a certain quantity of purified water, or to overcome a pressure drop caused by filters located downstream of the water purification unit. However, the water purification unit's hardware and filters can deteriorate over time. For instance, sterilizing-grade filters can become clogged with bacteria, endotoxins, and possibly other matter. This can affect the product water flow rate of the water purification unit. Consequently, the output at a constant pressure will decrease over time. Therefore, the quantity of purified water produced by the water purification unit may become uncertain.Therefore, one objective of disclosure is to control the properties of the product water flow to, for example, maintain a constant (or nearly constant) flow rate or pressure. Another objective is to maintain the operating point (e.g., pressure, temperature, or flow rate) of the components in the water purification apparatus within certain ranges. These and other objects are achieved at least in part by the apparatus and methods according to the independent claims, and by the modalities of the dependent claims. According to the first aspect, the description refers to a water purification apparatus for producing purified water. The water purification apparatus comprises a reverse osmosis (RO) device, an RO pump, a recirculation path, a purified water path, a control device, at least one detector, and a control unit. The reverse osmosis (RO) device is arranged to produce a flow of purified water. The RO device comprises a feed inlet arranged to receive feed water and a purified water outlet, and the RO pump is arranged to pump feed water to the feed inlet.Furthermore, the recirculation path is arranged to recirculate a portion of the purified water flow from a first point downstream of the RO device to a second point upstream of the RO device, and the purified water path is arranged to transport purified water from the purified water outlet to a product water port. The purified water path comprises a product water path arranged downstream of the recirculation path to transport product water to the product water port. The control unit is configured to control the control device to regulate a flow rate of purified water in the recirculation path, based on a fluid property detected by at least one detector. At least one detector is arranged to detect a product fluid property of the product water in the product water path.The control unit is also configured to control the control device for regulating a product fluid property of the product water in the product water path to meet one or more predetermined product water criteria, based on the product fluid property detected by at least one detector. The at least one detector comprises a flow sensor, and the product fluid property detected by the flow sensor is the flow rate of the product water in the product water path. Furthermore, the one or more predetermined product water criteria stipulate that the product water flow rate in the product water path corresponds to a predetermined flow rate. Therefore, one or more fluid properties can be controlled in the purified water path of the water purification unit. More specifically, one or more product fluid properties can be controlled within the dialysis machine, ensuring that the desired product fluid properties are maintained throughout production and also, for example, during start-up and shutdown. This allows for a consistent flow rate of the product water over time. According to some embodiments, where at least one detector comprises a pressure sensor, the product fluid property detected by the pressure sensor is the fluid pressure in the product water path, and the predetermined product water criterion comprises one or more conditions: the product water in the product water path remains below a predetermined upper pressure level and / or the product water pressure in the product water path corresponds to a predetermined pressure. Therefore, the product water pressure in the product water path can be controlled to remain within a range that is desirable for optimal operation. This prevents components from breaking or deteriorating due to excessively high pressure in the product water path. According to some embodiments, at least one filter is arranged to filter the product water flowing through the product water path and wherein the predetermined upper pressure level corresponds to a pressure tolerance level of the at least one filter or any other component arranged in the product water path. As water is pumped through the filters, bacteria, endotoxins, and possibly other matter can reduce the permeability of the filters arranged in connection with the product water path. This means that the performance, for a given pressure, will decrease over time. Using the proposed technique, the product water pressure in the product water path can be increased, up to the maximum permissible level, to compensate for this behavior. In some configurations, the control unit is set to activate an alarm function in response to a change in at least one product fluid property detected by at least one detector. Therefore, the operator or patient can be alerted if a suspected error is detected. In some configurations, the control unit is set to control the control device to deliver a predetermined flow rate through the product water port for a predetermined period of time, in order to produce a predetermined quantity of water. This allows the dialysis machine to produce the quantity of product requested. The requested quantity is typically between 0.5 and 400 liters, for example, 1, 2, 5, 10, 20, 50, 70, 90, 150, 200, or 300 liters. In some models, the water purification system includes a heater designed to heat the product water flowing in the product water line. This allows the production of product water at the temperature required by the dialysis machine. The heater can also be used to control the temperature of an RO membrane in the RO device. In some embodiments, the water purification unit includes a temperature sensor designed to measure the water temperature downstream of the heater. According to these embodiments, the control unit is configured to regulate the temperature of the water flowing through the RO membrane of the RO device, based on the temperature detected by the sensor. This allows the RO membrane temperature to remain relatively constant, which is desirable for operation. According to some models, the water purification apparatus comprises a tank arranged to receive water from an external water source and to supply water to the feed inlet. In some embodiments, the water purification apparatus comprises a polishing device arranged downstream of the recirculation circuit in the purified water path. The polishing device, for example, comprises an electrodeionization (EDI) device. According to some embodiments, the water purification apparatus comprises a permeate water path arranged to transport purified water from the purified water outlet of the RO device to an inlet of the polishing device. anRnnn / eznz / R / Yi According to some modalities, the product water path is arranged to transport purified water from an outlet of the polishing device to the product water port. According to a second aspect, the description refers to a corresponding method for controlling at least one fluid property in a water purification apparatus that produces purified water. The water purification apparatus comprises a reverse osmosis device (RO device) that produces a flow of purified water, and a recirculation path arranged to recirculate a proportion of the purified water flow from a point downstream of the RO device to a point upstream of the RO device.The method comprises detecting at least one fluid property of purified water in a purified water path, including detecting at least one product fluid property of product water in a product water path of the purified water path, wherein the product water path is arranged downstream of the recirculation, and regulating a flow rate of water in the recirculation path to meet one or more predetermined criteria of the purified water in the purified water path, based on at least one detected fluid property, including regulating a flow rate of water in the recirculation path to meet one or more predetermined criteria of the product water of the product water in the product water path, based on the at least one detected product fluid property.The at least one product fluid property comprises a product water flow rate in the product water path and wherein the predetermined product water criterion comprises that the water flow rate in the product water path corresponds to a predetermined flow rate. Therefore, as described above, the properties of the product fluid can be controlled to meet certain criteria, such as those defined by the manufacturer or user. This allows for more efficient water production and safer dialysis treatment. The method also enables smaller and faster adjustments to the product water flow rate compared to simply adjusting the pumping frequency used to feed the RO device. According to some variations, the method involves estimating the amount of product water produced during a production period based on the duration of that period and the corresponding flow rate of purified water detected during that time. The ability to control the pressure makes it possible to avoid high pressure in the product water flow path, which in the worst-case scenario could cause a failure. According to some models, the method involves triggering a predetermined action when the quantity reaches a predefined production volume. For example, an alert signal or action (e.g., a message sent to the dialysis machine) can be activated when a requested volume has been produced. According to some embodiments, at least one product fluid property comprises pressure in the product water path, and wherein one or more predetermined product water criteria comprises that the product water pressure in the product water path remains below a predetermined upper pressure level. According to some embodiments, the method involves measuring the water temperature in the purified water path downstream of a heater located in the purified water path. According to these embodiments, the control then involves regulating the water flow rate in the recirculation path so that the temperature of the water flowing through an RO membrane of the RO device meets a predetermined temperature criterion, based on the temperature detected by the temperature sensor. In this way, the temperature range of the water entering the RO membrane will depend less on the incoming water temperature and the ambient temperature, since a return flow of heated purified water can be used to raise the water temperature in the tank. Consequently, the membrane's filtration performance will be more stable. According to some modalities, the method involves continuously performing detection and regulation while the water purification apparatus produces purified water. According to some versions, the method involves activating an alarm function in response to a change in at least one detected product fluid property. Therefore, the proposed method, according to these versions, also allows the water purifier to detect sudden pressure changes, such as a filter rupture, which results in a lower pressure drop and, consequently, lower pressure and increased flow in the product water path. Alternatively, a leak between the water purifier and the filters will also cause a pressure drop, which should trigger the alarm. According to some modalities, the predetermined upper pressure level corresponds to a pressure tolerance level of at least one filter arranged to filter product water downstream of the product water path or of any other component arranged in, or within a predetermined distance of, the product water path. According to some models, the control system involves regulating the flow rate of the product water to achieve a predetermined flow rate over a predetermined period of time, in order to produce a predetermined quantity of water. Furthermore, the water purifier can continue to deliver the required volume to the dialysis machine even if communication with the machine is lost. The predetermined quantity is typically between 0.5 and 400 liters. According to some modalities, the method involves controlling the temperature of the product water flowing in the product water path. According to some embodiments, a polishing device is arranged downstream of the recirculation circuit in the purified water flow, and then the product water path is arranged to transport the product water from an outlet of the polishing device to the product water port. According to a third aspect, disclosure refers to a computer program comprising instructions that, when the program is executed by a computer, cause the computer to carry out the method described above and below. According to a fourth aspect, disclosure refers to a computer-readable medium comprising instructions, which, when executed by a computer, causes the computer to carry out the method described above and below. BRIEF DESCRIPTION OF THE FIGURES The embodiments of the invention are described in more detail with reference to the accompanying drawings illustrating examples of embodiments of the invention in which: Fig. 1 is a front elevation view of a modality of a PD dialysis system that has a care dialysis fluid production point that uses purified water from a water purification apparatus. Fig. 2 is an elevation view of one type of disposable assembly used with the system illustrated in Fig. 1. Fig. 3 is a schematic of some functional parts of the water purification apparatus. Fig. 4a illustrates a first exemplary embodiment of a water purification apparatus 300 comprising an RO device 301. Fig. 4b illustrates the functionality of a control unit of the 300 water purification apparatus. Fig. 5 illustrates a flow diagram of a method for use in a dialysis machine. Fig. 6 illustrates an example of a water purification apparatus in great detail. DETAILED DESCRIPTION When using a water purification system, for example, at the point of care, it may be desirable to be able to control the flow rate of the purified water, i.e., the product water. If the product water flow rate is constant, or at least known, it is possible to predict the amount of water produced during a certain production time. In general, it is desirable to produce a desired quantity of product water as quickly as possible. However, if the product water flow rate is too high, the water pressure in the water purification unit may become excessively high, which can damage the fluid system and other equipment in or connected to the water purification unit. Furthermore, if the product water flow rate or pressure is too high, the filters in a dedicated line supplying product water, for example, to a dialysis machine, could rupture, potentially exposing the patient to bacteria and endotoxins. Therefore, the proposed technique offers a method for controlling the flow rate of product water from a water purification unit based on one or more properties or parameters of the product fluid, such as flow rate, pressure, or temperature of the product water in the product water path. The control is implemented, for example, using an electrically controlled proportional valve in a recirculation path of the water purification unit. The electrically controllable valve can also be used to control other properties of the purified water fluid, such as pressure or temperature. For a better understanding of the proposed technique, a water purification unit, where the proposed technique can be implemented, is explained below as a component of a peritoneal dialysis system. However, the proposed technique can also be implemented in a water purification unit used to produce purified water for other types of dialysis systems, such as hemodialysis or CRRT systems, for use in producing dialysis fluids for point-of-care hemodialysis or CRRT treatments. With reference now to the drawings, and in particular to Fig. 1, System 10a illustrates a peritoneal dialysis system having a point-of-use dialysis fluid production unit. System 10a includes a cycler 20 and a water purification unit 300. Suitable cyclers for cycler 20 include, for example, the Amia® or HomeChoice® cycler marketed by Baxter International Inc., with the understanding that these cyclers require updated programming to perform and use point-of-use dialysis fluid produced in accordance with System 10a. For this purpose, cycler 20 includes a control unit 22 having at least one processor and at least one memory. The control unit 22 further includes a wired or wireless transceiver for sending and receiving information from a water purification unit 300.The water purification unit 300 also includes a control unit 112 having at least one processor and at least one memory. The control unit 112 further includes a wired or wireless transceiver for sending and receiving information from the cycler's control unit 22. Wired communication can be via an Ethernet connection, for example. Wireless communication can be via any of the following protocols: Bluetooth™, WiFi™, Zigbee®, Z-Wave®, wireless universal serial bus (USB), or infrared, or any other suitable wireless communication technology. The control unit 22 comprises a computer program containing instructions that, when executed by the control unit 22, cause the control unit 22 and the water purification unit to carry out one or more of the methods and programs according to any of the modalities described herein.The instructions can be stored on a computer-readable medium, such as a portable memory device, for example a USB flash drive, a laptop computer, or similar, and loaded into control unit 22. The cycler 20 includes a housing 24, which contains equipment programmed via control unit 22 to prepare a fresh dialysis solution at the point of use, pump the freshly prepared dialysis fluid into patient P, allow the dialysis fluid to remain within patient P, and then pump the used dialysis fluid to a drain. In Fig. 1, the water purification apparatus 300 includes a first drain path 384, leading to a drain 339, which may be a housing drain or a drain container. The equipment programmed via control unit 22 to prepare a fresh dialysis solution at the point of use may include equipment for a pneumatic pumping system, which includes, among others, (i) one or more positive pressure reservoirs, (ii) one or more negative pressure reservoirs, and (iii) a compressor and a vacuum pump, each under the control of control unit 22.or a single pump that creates positive and negative pressure under the control of control unit 22, to provide positive and negative pressure to be stored in one or more positive and negative pressure reservoirs, (iv) plural pneumatic valve chambers to supply positive and negative pressure to plural fluid valve chambers, (v) several pneumatic pump chambers to supply positive and negative pressure to several fluid pump chambers, (vi) several electrically actuated pneumatic solenoid on / off valves under the control of control unit 22 located between the plural pneumatic valve chambers and the plural pneumatic fluid valve chambers, (vii) several electrically actuated pneumatic variable orifice valves under the control of control unit 22 located between the plural pneumatic pump chambers and the plural fluid pump chambers,(viii) a heater under the control of control unit 22 for heating the dialysis fluid as it is being mixed in one mode, and (viii) an occluder 26 under the control of control unit 22 for shutting off the patient and draining the lines in alarm and other situations. In one embodiment, the plural pneumatic valve chambers and the plural pneumatic pump chambers are located on a front face or surface of the cycler housing 24 20. anBnnn / cznz / B / vi The heater is located inside the housing 24 and, in some versions, includes heating coils that contact a heating tray, which is located at the top of the housing 24, under a heating cover (not visible in Fig. 1). The cycler 20 in Fig. 1 also includes a user interface 30. The control unit 22 in one configuration includes a video controller, which may have its own processing and memory to interface with the primary control processing and memory of the control unit 22. The user interface 30 includes a video monitor 32, which may operate with a touchscreen overlay placed on the video monitor 32 to input commands through the user interface 30 to control the unit 22. The user interface 30 may also include one or more electromechanical input devices, such as a membrane switch or other button. The water purification unit 300 in Fig. 1 also includes a user interface 120. The control unit 112 of the water purification unit 300 may include a video controller, which may have its own processing and memory for interacting with the primary control processing and memory of the control unit 112. The user interface 120 includes a video monitor 122, which may also operate with a touchscreen overlay placed on the video monitor 122 for entering commands into the control unit 112. The user interface 120 may also include one or more electromechanical input devices, such as a membrane switch or other button. The control unit 112 may further include an audio controller for playing sound files, such as alarm or alert sounds, through one or more speakers 124 of the water purification unit 300. With further reference to Fig. 2, a disposable assembly 40 is illustrated. The disposable assembly 40 is also illustrated in Fig. 1, coupled to the cycler 20 to move fluid within the disposable assembly 40, for example, to mix dialysis fluid as discussed herein. The disposable assembly 40 in the illustrated example includes a disposable cassette 42, which may include a flat, rigid plastic piece covered on one or both sides by a flexible membrane. The membrane, pressed against the housing 24 of the cycler 20, forms a pumping membrane and valves. Fig. 2 illustrates that the disposable cassette 42 includes fluid pump chambers 44 that operate with the pneumatic pump chambers located in the housing 24 of the cycler 20 and fluid valve chambers 46 that operate with the pneumatic valve chambers located in the housing 24 of the cycler 20. Figures 1 and 2 illustrate that the disposable assembly 40 includes a patient line 50 extending from a patient line port on cassette 42 and terminating in a patient line connector 52. Figure 1 illustrates that the patient line connector 52 connects to a patient transfer assembly 54, which in turn connects to a permanent catheter located in the patient's peritoneal cavity. For example, the disposable assembly 40 includes a drainage line 56 extending from a cassette 42 drainage line port and terminating in a drainage line connector 58. Figure 1 illustrates that the drainage line connector 58 is detachably connected to a drainage port 118 on the water purification apparatus 300 to receive the spent dialysis fluid from the cycler 20. Figures 1 and 2 further illustrate that the disposable assembly 40 includes a heater / mixing line 60 that extends from a heater / mixing line port of the cassette 42 and terminates in a heater / mixing bag 62 discussed in more detail below. The disposable assembly 40 includes an upstream pipe segment 64a that extends to a water inlet of the water accumulator 66. A downstream pipe segment 64b extends from a water outlet 66b of the water accumulator 66 to the cassette 42. In the illustrated examples, the upstream pipe segment 64a begins at a pipe connector 68 and is located upstream of the water accumulator 66. Figure 1 illustrates that the pipe connector 68 is detachably connected to a product water port 128 of the water purifier 110. The 300 water purification unit produces purified water and water suitable for, for example, peritoneal dialysis (WFPD). WFPD is water suitable for making dialysis fluid for delivery to the patient's peritoneal cavity; WFPD is, for example, dialysis water or water for injection. In one embodiment, a sterile sterilization-grade filter 70a is installed upstream of a sterile sterilization-grade filter 70b downstream. Filters 70a and 70b may be installed in pipe segment 64a upstream of water accumulator 66. The sterile sterilization-grade filters 70a and 70b may be through-filters that do not have a reject line. The pore sizes for the sterilization filter may, for example, be less than one micron, such as 0.1 or 0.2 microns. Suitable sterile sterilization filters 70a and 70b may, for example, be Pall IV-5 or GVS Speedflow filters, or filters supplied by the assignee of this disclosure. In alternative embodiments, only one or more of two sterilization-grade filters are installed in pipe segment 64a upstream of water accumulator 66.One or more sterile, sterilizing-grade filters may be arranged near the water accumulator 66, making it easier to fold the disposable assembly 40. In additional alternative embodiments, there are no sterile, sterilizing-grade filters in the pipe segment 64a. The sterile, sterilizing-grade filters may, for example, be replaced by one or more ultrafilters located in the product water path of the water purification unit 300. Figure 2 further illustrates that a final sample bag or line 72 can be provided, extending from a final sample bag or port of cassette 42. The final sample bag or line 72 terminates in a connector 74, which can be connected to a coupling connector of a final pre-filled dialysis fluid bag or to a sample bag or other sample collection container. The final sample bag or line 72 and connector 74 can alternatively be used for a third type of concentrate if desired. Figures 1 and 2 illustrate that the disposable assembly 40 includes a first concentrate line 76 extending from a first cassette concentrate port 42 and terminating at a first cassette concentrate connector 80a. A second concentrate line 78 extends from a second cassette concentrate port 42 and terminates at a second cassette concentrate connector 82a. Fig. 1 illustrates that a first concentrate container 84a contains a first concentrate, for example, glucose, which is pumped from container 84a through a container line 86 to a first container concentrate connector 80b, which is coupled to the first cassette concentrate connector 80a. A second concentrate container 84b contains a second, for example, buffer, concentrate, which is pumped from container 84b through a container line 88 to a second container concentrate connector 82b, which is coupled to the second cassette concentrate connector 82a. To begin treatment, patient P typically loads cassette 42 into the cycler and, in a random or designated order, (i) places the heater / mixing bag 62 into the cycler 20, (ii) connects the upstream tubing segment 64a to the product water port 128 of the water purification apparatus 300, (ii) connects the drain line 56 to the drain port 118 of the water purification apparatus 300, (iv) connects the first cassette concentrate connector 80a to the first container concentrate connector 80b, and (v) connects the second cassette concentrate connector 82a to the second container concentrate connector 82b. At this point, the patient connector 52 is still capped. Once fresh dialysis fluid is prepared and verified, patient line 50 is primed with fresh dialysis fluid, after which patient P can connect patient line connector 52 to transfer equipment 54 for treatment.Each of the above steps can be illustrated graphically on the video monitor 32 and / or provided through voice guidance from the speakers 34. The 300 water purification device will now be described in more detail. Figure 3 shows a schematic of the functional parts of the water purification unit 300, which includes a pretreatment module 160, a reverse osmosis (RO) module 170, and a post-treatment module 180. The water purification unit 300 comprises an inlet port 399 for supplying water from a water source 398, for example, a water tap, into the unit for water purification. The incoming water from the water source is fed through the inlet port 399 to the pretreatment module 160. Pretreatment module The pretreatment module 160 treats the incoming water with a particle filter and an activated carbon bed. The particle filter is designed to remove particles such as clay, silt, and silica from the incoming water. The particle filter is designed to prevent particles the size of a millimeter, and optionally larger endotoxin molecules, from entering the water. The activated carbon bed is designed to remove chlorine and chlorinated compounds from the incoming water and to absorb toxic substances and pesticides. In one example, the activated carbon bed is designed to remove one or more of hypochlorite, chloramine, and chlorine. In another example, the activated carbon bed is also designed to reduce total organic carbon (TOC), including pesticides, from the incoming water. In some models, the particle filter and activated carbon bed are integrated into a single consumable part. This consumable part is replaced, for example, at a predefined interval that depends on the quality of the incoming water. The quality of the incoming water is, for example, tested and determined by qualified personnel before the first use of the 300 water purification unit at a point of care. Optionally, the pretreatment module 160 comprises an ion exchange device for the protection of downstream devices such as a reverse osmosis, RO, membrane and polisher. The pretreatment module 160 thus filters the incoming water and delivers pretreated water to a downstream RO module 170. RO Module The RO 170 module removes impurities from filtered water, such as microorganisms, pyrogens, and ionic material, from pretreated water using reverse osmosis. The pretreated water is pressurized by a pump and forced through the RO membrane to overcome osmotic pressure. The RO membrane is, for example, a semipermeable membrane. The pretreated water stream, called the feed water, is then divided into a reject water stream and a permeate water stream. In one example, the reject water can be passed through one or both of a first and a second rejection path. The first rejection path recirculates the reject water back to the RO pump's feed water path to re-feed it into the RO device.Recirculated reject water increases the feed flow to the RO device, to obtain sufficient flow beyond the reject side of the membrane anennn / cznz / B / vi. RO to minimize scaling and fouling of the RO membrane. The second reject path directs the reject water to the drain. This keeps the concentration level on the reject side low enough to achieve the appropriate and required permeate concentration. If the feed water has a low solute content, some of the drain flow can also be directed to the inlet side of the RO membrane, thus increasing the efficiency of the water purification unit 300. The RO 170 module thus treats the pre-treated water and delivers permeate water to a post-treatment module 180 located downstream. Post-treatment module The 180 post-treatment module polishes the permeate water to remove more ions. The permeate water is polished using a polishing device such as an electrodeionization (EDI) unit or a mixed-bed filter. The EDI device uses electrodeionization to remove ions from the permeate water, such as aluminum, lead, cadmium, chromium, sodium, and / or potassium, that have penetrated the RO membrane. The EDI device uses electricity, ion-exchange membranes, and resin to deionize the permeate water and separate the dissolved ions, i.e., impurities. The EDI device produces polished water, polished to a higher purity level than the permeate water. The EDI device has an antibacterial effect on the product water and can reduce the amount of bacteria and endotoxins in the water, due in part to the electric field generated by the device. In one mode, the EDI device has a production capacity of 70–210 ml / min. This capacity limits the flow rate of the produced water. The mixed bed filter device comprises a column, or vessel, with a mixed bed ion exchange material. The polished water, also referred to in this document as product water, is ready for delivery from a product water port 128 of the water purification unit 300 to a point of use. The product water is suitable for dialysis, i.e., dialysis water. In one embodiment, the product water is water for injection. In one example embodiment, a disposable assembly 40, including a pipe 56, is arranged in the water purification unit 300 to convey the product water to a point of use. Optionally, the water purification unit 300 comprises a drain port 118. The drain port 118 is used in one example embodiment to receive used fluid, for example, from a PD patient, via a drain line 64, for subsequent conveyance through a first drain path 384 within the water purification unit 300 to a drain 339 of the water purification unit 300. anBnnn / cznz / B / vi As a further option, the drain port 118 receives a sample of the mixed solution to transport further to a conductivity sensor arranged in the water purification apparatus 300, for example, in the first drain path 384. The disposable assembly 40 is arranged here with sterile filters 70a, 70b, to filter the product water from the water purification apparatus 300 to ensure product water quality as for injection water. Therefore, the product water collected in the accumulator bag 66 has passed through one or more sterilized sterilization filters from the disposable assembly 40 for the removal of bacteria and endotoxins, i.e., to produce sterile product water. According to one modality, the sterilizing-grade sterile filters are redundant. By collecting the sterile product water in the accumulator bag 66, the water purification apparatus 300 and the cycler 20 are decoupled in terms of pressure, so that the high pressure needed to push the water through the sterilized sterilization filters does not affect the cycler 20. The control unit 112 of the water purification unit 300 is configured to place the unit in various operating states, such as STANDBY, ON, INACTIVE, RUNNING, and MAINTENANCE. The water purification unit 300 is configured to operate according to the commands of cycler 20. The 300 water purification device, when not in use but switched on, goes into standby mode. In STANDBY mode, the 300 water purification unit awaits the CONNECT or MAINTENANCE command. The main steps for the different states are explained. Steps taken to mitigate risks, such as comparing flow sensors, checking that the flow path is leak-free, etc., are omitted. CONNECT status During the CONNECT state, the system tests the sensors and checks the EDI device to see if the system is ready when the command to enter the INACTIVE state is received. The CONNECT state may also include the flushing of certain components, for example, in the 160 pretreatment module. The patient is also usually asked to take a sample of the incoming water, at a sampling port located after pretreatment module 160. What is checked in this sample is that the chlorine level, including hypochlorite, chloramine and chlorine, is below the permitted levels. When all the steps of the CONNECT state have been completed, the system is ready to operate. INACTIVE status In this state, the water purification apparatus 300 is waiting for a conductivity measurement of the return fluid (when a freshly prepared dialysis fluid is to be tested) or a new request for product water supply from cycler 20. In this state, the water purification unit 300 can be prepared to supply product water. The water purification unit 300 then begins producing product water, but instead of delivering the product water outside the product port 128, the produced product water is recirculated to tank 350 until the product water reaches a stable conductivity level, and the RO device is operating at a desired operating point for the RO device 301. The 300 water purification unit occasionally recirculates the water path to minimize start-up time for the water production phase. The INACTIVE state may also include washing certain components, for example, in the pretreatment module 160. Status: IN PROGRESS In the RUNNING state, the water purification apparatus 300 supplies product water (for example, a volume requested by the cycler 20) to the accumulator bag of the disposable assembly 66. The proposed technique will now be described in more detail by referring to Fig. 4a, Fig. 4b and Fig. 5. Figure 4a illustrates a water purification apparatus 300 comprising an RO device 301. It should be noted that Figure 4a is only a conceptual drawing and only illustrates parts of the water purification apparatus 300 that are relevant to the proposed technique. A more detailed illustration of an exemplary water purification apparatus 300 and its operation is provided in relation to Figure 6. The water purification apparatus 300 of Fig. 4a comprises an RO device 301, a tank 350, an RO pump 450, a feed water path 390, a recirculation path 375, a purified water path 371, a control device 305a, a temperature sensor 303, a pressure sensor 308, a flow sensor 309, a heater 302, a flow sensor 380, a product water port 128, and a control unit 112. The RO 301 device is arranged to produce a purified water flow and a reject flow. In more detail, the RO 301 device comprises an RO 324 membrane, a feed inlet 301a, a purified water outlet 301b, and a reject outlet 301c. The RO 324 membrane separates the feed inlet 301a and the reject outlet 301c from the purified water outlet 301b. The reject flow is directed to a first reject path 385b and / or to a drain 339 of the water purification apparatus 300. The first reject path 385b is seamlessly connected to the reject outlet 301c and the feed water path 390. The feed water path 390 is arranged to carry feed water to the feed inlet 301a. The feed water path 390 is seamlessly connected to the feed inlet 301a. Tank 350 is positioned along the feed water path 390 to collect water. More specifically, tank 350 is designed to receive water from an external water source and to supply water to feed inlet 301a. In some configurations, tank 350 is optional, as indicated by dashed lines in Fig. 4a. Pump RO 450 is located in the feed water path 390 to pump feed water to feed inlet 301a. Pump RO 450 is located downstream of tank 350 (when present). Pump RO 450 is set to operate at a specific pumping rate corresponding to a specific permeate flow rate. As the permeability of membrane RO 324 increases with increasing feed water temperature, the relationship between pumping rate and flow rate depends on the feed water temperature at feed inlet 301a and, therefore, on the temperature of membrane RO 324. Product water port 128 is arranged to supply product water, for example, to a dialysis machine, via a dedicated set of lines. Sterilization-grade filters (not shown) are typically located in the line established outside the water purification unit 300, downstream of product water port 128. The recirculation path 375 is arranged to recirculate a portion of the purified water flow from a first point downstream of device RO 301 to a second point upstream of device RO 301. More specifically, the recirculation path 375 is arranged to circulate heated purified water from a point downstream of device RO 301 to the feed water path 390 within the water purification unit 300. In the example shown in Fig. 4a, the purified water is recirculated to tank 350 and fed back into the feed inlet 301a of device RO 301. However, the purified water can alternatively be recirculated directly to the pipe upstream of pump RO-450. The purified water path 371 is fluidly connected from the purified water outlet 301b to the product water port 128. The purified water path 371 is configured to transport purified water from the purified water outlet 301b to the product water port 128. The purified water path 371 comprises the permeate water path 371a and a product water path 371c. The product water path herein refers to the portion of the purified water path 371 closest to the product water port 128, where the fluid properties, such as pressure and flow rate, are the same (or similar) as at the product water port 128. Heater 302 is arranged to heat the product water flowing in the product water path 371c. Heater 302 is, for example, a heater arranged to heat the purified water produced by device RO 301. Furthermore, in the example of Fig. 4a, the purified water leaving device RO 301 also passes through flow sensor 410 and temperature sensor 303, which are included in permeate water path 371a. The purified water path 371 comprises a polishing device 306, for example, an electrodeionization (EDI) device. Alternatively, the polishing device 306 is a mixed-bed filter device. The polishing device 306 is arranged downstream of the recirculation circuit 374 in the purified water path 371. Therefore, the polishing device 306 is arranged in the purified water path 371 downstream of the point where the recirculation path 375 connects to the purified water path. The polishing device 306 is seamlessly connected to the permeate water path 371a and the product water path 371c.In other words, according to some embodiments, the permeate water path 371a is arranged to transport purified water from the purified water outlet 301b of the RO device 301 to an inlet of the polishing device 306 and the product water path 371c is arranged to transport purified water from an outlet of the polishing device 306 to the product water port 128. This disclosure is based on the idea that a fluid property, such as a product water pressure or flow rate in the product water path 371c, can be controlled by controlling the portion of the permeate flow produced by the RO device that is recirculated to the feed inlet 301a. The control device 305a, such as an electrically controllable valve, is arranged to permit such control. In other words, the control device 305a is arranged to regulate a flow rate of the purified water in the recirculation path 375. In one embodiment, the control device 305a is configured to receive control data and regulate the proportion of the permeate flow that is recirculated based on that control data. The control data may be an electrical signal (analog or digital). The control device 305a is typically a flow control device such as a proportional valve.The proportional valve is typically electrically controlled. However, a mechanical proportional valve can also be used. In one embodiment, the control device 305a is a pump, for example, a positive displacement pump such as a volumetric pump or a piston pump. As described above, the proposed technique allows the control of at least one fluid property, such as the flow rate or pressure in the product water path 371c, when the water purification apparatus is operating. According to some embodiments, the proposed technique allows the control of other properties, for example, permeate fluid properties, such as the RO membrane temperature 324 or the operating point of the RO device. To enable such control, the relevant fluid property(ies) must be measured or at least detected or estimated in some way. Therefore, at least one detector is arranged to detect a fluid property of the purified water in the purified water path 371. According to some embodiments, at least one detector is arranged to detect a product fluid property of the product water in the product water path 371c. The at least one detector can be implemented in a plurality of ways. According to some embodiments, the at least one detector is configured to provide product fluid property data, defining at least one product fluid property. According to some embodiments, the control is based on other properties, such as permeate fluid properties, for example, the temperature of the purified water flowing in the permeate water path 371a. In Fig. 4a, the at least one detector is the flow sensor 309 and the pressure sensor 308. The product fluid property measured by the flow sensor 309 is the flow rate of the product water in the product water path 371c. The product fluid property detected by the pressure sensor 308 is the pressure in the product water path 371c. Additionally, the temperature sensor 303 is arranged to measure the temperature of the purified water in the permeate water path 371a, downstream of the heater 302. The 112 control unit typically comprises one or more 1122 microprocessors and / or one or more circuits, such as an application-specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), and the like. The control unit 112 may also comprise at least one memory 1123, such as a non-transient memory unit (e.g., a hard disk, flash memory, optical disk, etc.) and / or volatile storage devices (e.g., dynamic random access memory (DRAM)). The control unit 112 further comprises an interface 1121 configured to allow communication with (e.g., transmitting control data and receiving sensor data) the other components of the water purification apparatus 300, and in particular with the control device 305a and at least one detector, e.g., pressure sensor 308 and / or flow sensor 309. The control unit 112 is configured to perform functions of the water purification apparatus 300. In particular, the control unit 112 is configured to implement all modes of the proposed art described herein, including the method described in relation to Fig. 6. To achieve this, the control unit 112 is configured to receive fluid property data from at least one sensor and to send control data to the control device 305a. More specifically, the control unit 112 is configured to control the control device 305a to regulate a flow rate of purified water in the recirculation path 375, based on the fluid property detected by the at least one sensor, for example, to meet one or more predetermined criteria of the purified water in the purified water path 371. The fluid property is, for example, measured by any sensor in the purified water path 371. According to some embodiments, control unit 112 is configured to control control device 305a to control a product fluid property of the product water in the product water path 371c to meet one or more predetermined product water criteria, based on the fluid property detected by at least one detector, for example, pressure sensor 308 and / or flow sensor 309. In other words, control unit 112 is configured to control the water flow in the recirculation path 375 to meet one or more criteria, such as obtaining certain fluid properties, for example, a certain pressure or flow rate, in the product water flow. As explained earlier, different properties of the product fluid can be controlled. Therefore, product water criteria can comprise one or more regulating conditions. Some examples will now be given. It should be understood that these could be used alone or in combination. In its simplest form, the at least one product water criterion comprises only a single condition. In a first example, the control goal is to achieve a constant product water flow rate. The control criterion would be to maintain a constant flow of product water through product water port 128. The flow rate through product water port 128 is typically the same (or at least approximately the same) as throughout the product water path 371c. Therefore, according to some modalities, the predetermined criteria include that the product water flow rate in product water path 371c corresponds to a predetermined flow rate, for example, 150 ml / min or 250 ml / min. If a constant product water flow rate can be achieved, it is easy to estimate how long it will take to produce a certain quantity of product water. For example, the water purification unit 300 can be controlled to produce product water at a certain constant flow rate for a predetermined period. In other words, depending on the configuration, the control circuit is set to control the control device 305a to deliver a predetermined flow rate through the product water port 128 for a predetermined period, in order to produce a predetermined quantity of water. The predetermined quantity is, for example, between 0.5 and 400 liters. This quantity can correspond to the amount required for one or more dialysis treatments. For example, the water purification unit 300 can be controlled to produce 0.5, 1, 2.5, 10, 20, 50, 70, 90, 150, 200, 250, 300, or 400 liters of purified water. In one example, the control objective is to achieve a constant product water flow rate. The product water pressure in product water path 371c should not normally exceed a maximum permissible level. This maximum permissible level would, for example, ensure that hardware, such as internal filters or those connected to the water purification unit or polishing device 306, is not damaged. In other words, according to some embodiments, the predetermined upper pressure level corresponds to a pressure tolerance level for at least one filter (e.g., sterilization-grade filters) or any other component located in product water path 371c. Therefore, according to some embodiments, the predetermined criteria include that the product water pressure in product water path 371c remains below a predetermined upper pressure level. A typical implementation of the predetermined criteria might, for example, involve controlling control device 305a to attempt to obtain a predetermined flow rate of product water in product water path 371c, provided that the product water pressure in product water path 371c remains below a predetermined upper pressure level. If the pressure reaches the predetermined upper pressure level, then control device 305a will instead control the control device to maintain the pressure at that level, even if the product water flow rate in the product water path falls below the predetermined flow rate. As discussed earlier, the throughput, for a given product water pressure, will decrease over time. By controlling the amount of permeate recirculated in the recirculation path 375, the product water pressure can be successively increased to compensate for this behavior. In other words, according to some embodiments, the predetermined product water criterion comprises the product water pressure in the product water path 371c corresponding to a pressure level. The pressure level in the product water path 371c may, for example, correspond to an expected throughput through the product water port 128 and, therefore, may vary (generally increase) over time. In a third example, the objective of the control is to maintain a certain operating point for one or more of the hardware components of the water purification apparatus 300, such as a hardware component in the permeate water path 371a or the polisher water path 371b (Fig. 6), for example, the RO device 301 (which is considered to be at least partially included in the permeate water path 371a) or the polisher device 306. The operating point is, for example, a certain pressure, a certain flow rate, or a certain temperature. An operating point criterion is generally formulated to maintain the operating point within a certain range. For example, the flow rate or water pressure in the permeate water path 371a directly downstream of the RO device 306 is measured (or estimated) using the flow sensor 410. In principle, any detector can be used in the permeate water path 371a or the polisher water path 371b. Therefore, a permeate fluid property, such as a pressure in the RO device (in particular a transmembrane pressure of the RO membrane) or a flow rate through the polishing device 306, can be controlled using the control device 305a. In other words, according to some embodiments, the control unit 112 is configured to control the control device 305a to control a permeate fluid property (e.g., to meet a working point criterion of the RO membrane 324 or polishing device 306) of the permeate water in the permeate water path 371a to meet one or more predetermined permeate water criteria, based on the permeate fluid property detected by at least one detector, e.g., temperature sensor 302 or flow sensor 410. In a fourth example, the objective is to maintain the operating temperature of the RO 324 membrane of the water purification unit 300 at a constant temperature, regardless of, for example, the temperature of the inlet water fed through the inlet port 399 (Fig. 3) or the ambient temperature. A constant temperature is generally desirable, as the operating properties, such as performance and purification properties, of the RO 324 membrane typically depend on its temperature. A constant operating temperature of the RO membrane can be achieved by maintaining a constant temperature of the water passing through the RO 324 membrane. The temperature t_ro of the water passing through the RO 324 membrane is (at least essentially) the same as the temperature of the purified water in the permeate water path 371a directly downstream of the RO 301 device, i.e., upstream of the heater 302.This temperature depends on several factors, such as the temperature of the inlet water fed to the inlet port 399 (Fig. 3), the proportion of heated water that is recirculated in the recirculation path 375 and the temperature of the recirculated water, i.e., the temperature 7'2 of the purified water after the heater. The ratio of the temperature T_R0 of the purified water before heater 302 and the temperature Γ2 of the purified water after heater 302 can be calculated using thermodynamics and the formula: P = Q x cp x ΔT T_RO = T2- P / (Q x cp} (Equation 1) In the formula, P represents the power (watts) of heater 302, Q is the flow rate through heater 302 [l / s] (which is the same as the flow rate through the RO 324 membrane), T2 is the temperature of the purified water downstream of heater 302, and t_ro is the temperature upstream of heater 302 (i.e., the temperature of the water flowing through the RO 324 membrane). Therefore, ΔT is the temperature difference between the water upstream of heater 302 and the water downstream of heater 302, i.e., ΔT = T2 - T_RO. Additionally, cp is the specific heat capacity of water. Specific heat capacity, or thermal capacity, is a measurable physical quantity equal to the ratio of heat added to (or removed from) an object to the resulting temperature change. The specific heat capacity of water is 4.19 kJ / K. For example, if the flow rate Q through the RO 324 membrane is 210 ml / min (i.e., 0.0035 l / s) and the temperature of the purified water in the permeate water path T2 is 85 °C and the heating power P is 200 W, then the resulting temperature of the RO membrane would be estimated to be:. TR0= 85 - 200 / (0.0035x4190) = 85-13.6°C = 71.4°C (Equation 2) The temperature T2 of the purified water in the permeate water path 371a can be measured using the temperature sensor 303. Therefore, the temperature of the RO membrane 324, or rather the temperature of the water flowing through the RO membrane 324, can be estimated from the measured temperature T2 of the purified water in the permeate water path, as the power of the heater 302 and the flow rate Q through the RO membrane 324 are known. For example, if a change in the temperature T2 of the purified water in the permeate water path 371a is detected, while the heater power 302 and the flow rate Q through the heater are kept constant, it is an indication that the temperature T_RO of the feed water passing through the RO membrane 324 has changed due, for example, to a change in the temperature of the inlet water or the surroundings. One way to achieve the goal of maintaining a constant T_RO is to adjust the power P supplied by the heater (i.e., control the temperature of the recirculated water) or to change the flow rate Q through the heater 302 in response to a measured change in the temperature T2 of the purified water in the permeate water path 371a. The flow rate Q of water flowing through the heater 302 (and the RO membrane 324) can be controlled by changing the pumping frequency of the RO pump 450. However, in some configurations, it is desirable to use a single pump frequency for each batch of water. Another way to achieve the goal of maintaining a constant t_ro is to change the amount of heated water recirculated in the recirculation path 375. For example, if more heated water is recirculated, the water temperature in tank 350 will increase. This, in turn, would increase the temperature of the feed water entering through the feed inlet 301a and, consequently, the temperature T_RO of the water passing through the RO membrane 324. From the above, it follows that the temperature t_ro of the water passing through the RO 324 membrane can be estimated from the measured temperature T2 of the purified water in the permeate water path 371a, using Equation 1. The temperature T_RO of the water passing through the RO 324 membrane can be kept constant by controlling the control device 305a to regulate the proportion of recirculated permeate flow in the recirculation path, based on the estimate. For example, the proportion of recirculated permeate flow in the recirculation path can be continuously adjusted so that the estimated temperature T_RO of the water passing through the RO 324 membrane remains constant. In other words, according to some configurations, control unit 112 is set to control control device 305a to regulate the temperature T_RO of the water flowing through the RO membrane 324, based on the temperature detected by temperature sensor 303. Typically, control unit 112 is configured to control control device 305a to regulate the temperature T_RO so that a predefined temperature criterion is met. This criterion, for example, requires that the temperature T_RO of the water flowing through the RO membrane 324 be maintained within a predefined range. Therefore, the 305a control device can be controlled to maintain the water temperature after the RO 324 membrane at a predetermined temperature or within a predetermined temperature range. Examples three and four can be used in combination with the preceding methods and the corresponding product water criteria, which are intended to control a product fluid property of product water in the product water path 371c. Then, the different criteria relating to pressure, flow, and temperature must be combined (e.g., prioritized and weighted) for optimal control. In an alternative mode, these modes (third and fourth) are independent of the modes described above. Therefore, control unit 112 may not be configured to (at least not simultaneously) control control device 305a to control the product fluid property of the product water in the product water path 371c to meet one or more predetermined product water criteria, but instead, for example, control control device 305a to control the temperature of the water flowing through the RO membrane 324, based on the temperature detected by temperature sensor 303. According to some configurations, the control unit is set to activate an alarm function in response to a change in at least one product fluid property detected by at least one detector, for example, pressure sensor 308 and / or flow sensor 309. For example, to minimize the risk of exceeding the predetermined upper pressure level, the control unit 112 can be set to activate an alarm if the pressure measured by the pressure sensor exceeds the predetermined upper pressure level. The alarm can also be triggered in response to a significant or sudden pressure drop, or similar event, which would indicate a fault. For example, an upgrade in a filter, such as sterilization-grade filters, can cause a pressure drop and, consequently, a decrease in pressure and an increase in the flow rate of product water in the 371c product water path. Since these events do not coincide, the 112 control unit may trigger an alarm in such a situation. In another example, a system leak between the water purification unit 300 and the sterilization-grade filters 70a and 70b will also result in a drop in product water pressure in the product water path 371c. A leak would also be a serious fault that should trigger an alarm. In other words, according to some modalities, the control unit 112 is configured to activate an alarm function in response to a pressure change measured by the pressure sensor 308 and / or a flow rate change measured by the flow sensor 309. Figure 4b illustrates the functionality of a control unit 112 of the water purification apparatus 300 according to an example implementation. In this example, the control unit comprises a cascade control arrangement, comprising a flow controller 112a and a pressure controller 112b. In a cascade control arrangement, there are two (or more) controllers, the output of one of which controls the setpoint of another controller. In this example, flow controller 112a is driving the setpoint of pressure controller 112b to obtain a predetermined flow rate of product water in product water path 371c. In other words, flow controller 112a generates the initial control data for pressure controller 112b based on the product water flow rate in product water path 371c, as measured by flow sensor 309, and a reference flow rate, for example, 200 ml / min. anRnnn / eznz / R / vi Pressure controller 112b, in turn, actuates control device 305a to match the flow rate to the setpoint requested by flow controller 112a, provided the pressure does not exceed a predetermined pressure level, for example, 300 kPa. In other words, pressure controller 112b generates second control data d2 based on the product water pressure in the product water path 301c, as measured by pressure sensor 308, and the first control data aΔ. Pressure controller 112b then controls control device 305a using the second control data d2. The controller that controls the setpoint (the flow controller 112a in the previous example) is called the primary, external, or master controller. The controller that receives the setpoint (the pressure controller 112b in the example) is called the secondary, internal, or slave controller. The control loop frequency of the internal loop is typically higher than that of the external loop. For example, the control loop frequency of the pressure controller 112b is 10 Hz. A corresponding method for controlling at least one fluid property in a 300 water purification apparatus that produces purified water will now be described, with reference to the flow diagram in Fig. 5, and the exemplary modalities in the other figures. The method is typically performed in control unit 112 of the water purification unit 300. The method can be implemented as program code and stored in memory 1123 in control unit 112. Therefore, the steps of the method can be defined in a computer program, which comprises instructions that, when executed by a computer (e.g., control unit 112), cause the computer to perform the method. Alternatively, the steps of the method can also be defined on a computer-readable medium, such as removable memory like a USB flash drive. The computer-readable medium then comprises instructions that, when executed by a computer, cause the computer to perform the method. In a typical scenario, the method is performed when the water purification unit is in the RUNNING state and is supplying product water to, for example, a dialysis machine. However, it should be noted that the proposed method can also be performed in the CONNECTED or INACTIVE state, when product water is not being supplied but is instead recirculated in the additional recirculation path 381 as described in Fig. 6. The method comprises detecting S1 at least one purified water fluid property in a purified water path 371. According to some embodiments, detection S1 comprises detecting at least one product fluid property of product water in a product water path 371c of the purified water path 371. As described above (Fig. 4a), the product water path 371c is arranged downstream of the recirculation path 375. This step involves measuring product fluid properties such as pressure and flow rate of the product water in the product water path. Typically, the corresponding sensors 308, 309 produce sensor data that is supplied to the control unit 112 that performs the method. The method further comprises regulating S2 a flow rate of water in the recirculation path 375 to meet one or more predetermined criteria of the purified water in the purified water path 371, based on at least one detected fluid property. According to some embodiments, regulation S2 involves regulating a water flow rate in recirculation path 375 to meet one or more predetermined product water criteria in product water path 371c, based on at least one detected product fluid property. In other words, the water flow rate in recirculation path 375 is adjusted to control certain product fluid properties. Alternatively, regulation S2 comprises regulating a water flow rate in the recirculation path 375 to meet one or more predetermined permeate water criteria of the permeate water in the permeate water path 371a, based on at least one detected fluid property of the product. An example of a permeate water criterion is that the permeate water has a certain pressure or temperature. For example, the water flow rate in the recirculation path 375 is adjusted so that the product water flow rate in the product water path 371c is constant or within a predetermined range. The at least one product fluid property comprises a product water flow rate in the product water path, and wherein the predetermined product water criterion comprises that the water flow rate in the product water path 371c corresponds to a predetermined flow rate. In another example, the water flow rate in recirculation path 375 is adjusted so that the product water pressure in product water path 371c does not exceed a threshold. In other words, according to some embodiments, the default product water criterion stipulates that the product water pressure in product water path 371c corresponds to a specific pressure level. Therefore, the default product water criterion stipulates that the product water pressure in product water path 371c remains below a predetermined upper pressure level. Detection S1 and regulation S2 are typically performed continuously in the RUNNING state. Therefore, each change detected by at least one detector, for example, pressure sensor 308 and flow sensor 309, can trigger regulation S2. anRnnn / eznz / R / vi Stated differently, the method comprises continuously performing detection and regulation while the water purification apparatus produces purified water. The predetermined upper pressure level, for example, corresponds to a pressure tolerance level of at least one filter arranged to filter the product water flow from the product water path 371c or from any other component arranged in, or within a predetermined distance from, the product water path 371c. According to some embodiments, the method involves activating an alarm function (S3) in response to a change in at least one product fluid property. In other words, if detection reveals a certain change, such as a sudden increase or decrease in pressure, this could be considered an indication of a potential error, as exemplified earlier in relation to Fig. 4a. In such a situation, an alarm function can be activated to alert the user to the possible error. The alarm can be a sound, a flashing light, or a text message sent or displayed to the user. In some situations, it may be desirable to produce product water at a specific temperature. This temperature is, for example, required by the dialysis machine, to which the water purification unit 300 is requested to deliver purified water. The product water temperature can be controlled accordingly. Therefore, according to some embodiments, the method comprises controlling the temperature of the product water flowing in the product water line 371c. This could be done by heating it using the heater 302. The temperature can be set to virtually anything, but the range may be limited to 20 to 35 °C. If the product water flow rate is continuously monitored, it is also possible to calculate how much water has passed through the product water path 371c, as the quantity would correspond to the integral of the flow rate. According to some embodiments, the control involves estimating S6 the quantity of product water produced during a production time period based on the duration of the production time period and a corresponding flow rate of purified water detected during that time period. The production time period typically corresponds to the time from the start of production until its completion, or until the current moment if production is ongoing, i.e., production has not yet finished. In the second scenario, i.e., when production is underway, a predetermined action, such as an alarm or notification, can be triggered when the desired quantity of product water has been produced. The desired quantity can, for example, be specified by the user and entered through the user interface. In other words, according to some modalities, the method involves triggering a predetermined action (S6) when the quantity reaches a certain production volume. The action could be to stop production, notify a connected dialysis machine, or activate an alarm. According to some models, the control involves controlling the purified water flow through product water port 128 to achieve a predetermined flow rate over a predetermined period of time, in order to produce a predetermined quantity of water. The predetermined quantity is, for example, between 0.5 and 400 liters. This quantity could correspond, for instance, to the volume required for one or more dialysis treatments. As described above, it may also be desirable to maintain the operating temperature of the RO membrane 324 fairly constant. Therefore, according to some embodiments, the method comprises measuring the water temperature in the purified water path 371 downstream of a heater 302 arranged in the purified water path. The regulation S2 then comprises regulating a water flow rate in the recirculation path so that the temperature T_RO of the water flowing through an RO membrane 324 meets a predetermined temperature criterion, based on the temperature detected by the temperature sensor 303. As discussed above, the regulation can be carried out in some embodiments in this way, either in combination with or independently of the other embodiments described herein. Figure 6 illustrates an example of the implementation of the 300 water purification unit according to some embodiments in more detail. In other embodiments, the 300 water purification unit may include fewer or more components or modules. The water purification unit 300 in Fig. 6 receives water from a water source 398 (Fig. 3), such as a continuous supply of potable or drinking water from a patient's home. In various configurations, the water purification unit 300 can be installed in a room that has access to the water source 398 to provide WFPD to the cycler 20 as described herein. The water is optionally filtered using a particle pre-filter 334 to remove dirt and sediment before being delivered to the water purification unit 300. The water enters the water purification unit 300 through the water inlet port 333. As previously described, the water purification unit 300 includes a pretreatment module 160, an RO module 170, and a post-treatment module 180.The pretreatment module 160 includes a particulate filter and an activated carbon filter, i.e., an activated carbon bed, to remove further contaminants and impurities. The particulate filter and the activated carbon filter are incorporated into filter pack 331. Filter pack 331 is a disposable pack. The pretreatment module 160 includes an inlet valve 332 and a constant flow device 330 upstream of filter pack anBnnn / cznz / B / vi. 331. The inlet valve 332 controls the feed water inlet under the control of the control unit 112. The constant flow device 330 provides a constant flow to the tank 350 as long as the water pressure is above a minimum pressure for the inlet valve 332. Furthermore, the pretreatment module 160 comprises a sampling valve 329 with a sampling port outlet 329a, a tank valve 328, a pretreatment conductivity sensor 327, and a feedwater temperature sensor 326 downstream of the filter pack 331. The sampling port outlet 329a allows for sampling the feedwater, for example, to test the chlorine level. The tank valve 328 controls the flow of filtered feedwater to tank 350. The pretreatment conductivity sensor 327 monitors the conductivity of the filtered feedwater, and the feedwater temperature sensor 326 monitors the temperature of the filtered feedwater. The temperature of the filtered feedwater is necessary, for example, to calibrate the filtered feedwater conductivity measurement. The described components are included in the feedwater path 390.The feed water line 390 is connected to the water inlet port 333 and terminates in the tank 350. The inlet valve 332 and the tank valve 328 are configured to be controlled by the control unit 112 of the water purification unit 300. Water softening in the pretreatment module 160 can be achieved alternatively or additionally by using lime softening, ion exchange resins, or an antiscalant such as polyphosphate, as is commonly known. It should be noted that the filter pack 331 is not required in some configurations and may not be present. As explained previously, the RO 170 module comprises a tank 350, a pump RO 450, and a device RO 301. Device RO 301 has already been described in detail with reference to Fig. 4a, and that description is referenced for further explanation. Filtered (or unfiltered) feed water enters tank 350, for example, from the top of the tank. The feed water accumulates in tank 350, and pump RO450 pumps it to the feed inlet 301a (see Figs. 5-7) of device RO 301. Empty, low-level, and high-level switches 350a, 350b, and 350c are provided in tank 350 to detect its water level. A computer program running on control unit 112 of the water purification unit 300 is configured to control the opening and closing of the inlet valve 332 and the tank valve 328. The inlet valve 332 opens during the filling of tank 350 and closes when the water level in tank 350 triggers its high-level switch 350c, which is connected to control unit 112. The inlet valve 332 opens again when the water level drops below the low-level switch 350b in tank 350, thereby triggering the low-level switch connected to control unit 112.If the water level in tank 350 rises too high, the excess water is drained through an air vent line from tank 325 and the air vent of tank 335 (overflow connection), for example, into a tray 420 or drain 339. The air vent of tank 335 is accessible from outside the water purification unit 300. The air vent of tank 335 can be closed, for example, during transport of the water purification unit 300, so that any water in tank 350 is prevented from flowing into tray 420 and causing water to leak from the water purification unit 300. Control unit 112 is configured to stop pump RO 450 if the empty level switch 350a in tank 350 detects air or a critically low water level. Pump RO 450 is configured to provide the water flow and pressure required for the reverse osmosis process taking place in device RO 301. As previously described, for example, with reference to Fig. 4a, device RO 301 filters the water to provide purified water at its permeate outlet 301b. The water leaving device RO 301 is rejected at a reject outlet 301c (it can be fed back into pump RO 450 to conserve water consumption or, alternatively, pumped out to drain 339). The purified water exiting device RO 301 is conveyed in a purified water path 371 within the water purification apparatus 300 before exiting through a product water port 128. The purified water path 371 comprises (as in Fig. 4a) the permeate water path 371a, the polishing water path 371b, and the product water path 371c. The polishing device 306 can be bypassed via the bypass path 371d. The bypass path 371d is connected to the water path upstream of the polishing device 306, herein an EDI device, and to the water path downstream of the EDI device. The purified water exiting device RO 301 passes a flow sensor 410, a heater 302, and a permeate temperature sensor 303, all included in the permeate water path 371a. The 410 flow sensor monitors the flow of purified water coming out of the RO 301 device.Heater 302, under the control of control unit 112, heats the purified water exiting device RO 301. Permeate temperature sensor 303 monitors the temperature of the purified water exiting device RO 301 directly downstream of heater 302. An additional conductivity sensor 304 monitors the conductivity of the purified water exiting device RO 301. Downstream of the heater 302, the permeate temperature sensor 303, and the conductivity sensor 304, the purified water enters the post-treatment module 180 through the polisher water path 371b. The post-treatment module 180 comprises the polisher device 306. The three-way valve 305c is arranged to be controlled by the control unit 112 to selectively direct the flow of purified water to the polisher device 306 or to the bypass path 371d to bypass the polisher device 306. The polisher device 306 is configured to produce product water. A product channel valve 307 regulates the flow of product water in the product water path 371c from the polishing device 306. The concentrated water path 377c is arranged to pass fluid from the polishing device 306 back to the tank 350. The product water is passed to the product water port 128, and then to a pipe connected to the same 64 (64a, 64b) of the disposable assembly 40 for transport to the point of care. The disposable assembly 40 comprises two sterile sterilization filters 70a, 70b. The sterile sterilization filters 70a, 70b filter the product water, leaving the outlet of the product water port 128 as sterile product water, which is suitable for injection. According to some alternative embodiments, these filters are omitted, or the number of filters is less than or greater than two. A drain port 118 defines a first drain path 384 to drain 339. A drain line 56 from the disposable assembly 40 is connected to drain port 118 to pass water, such as the used PD fluid, from drain port 118 to drain 339. The first drain path 384 here incorporates the portion of a cycler drain path that is present within the water purification apparatus 300. The first drain path 384 comprises a conductivity sensor 336, a drain path temperature sensor 315, and a drain line valve 341. The conductivity sensor 336 is configured to measure the conductivity of the water in the drain path. The temperature sensor 315 is arranged to measure the temperature of the water in the first drain path 384.The drain line valve 341, under the control of the control unit 112, is arranged to regulate the flow in the first drain path 384 through the conductivity sensor 336. The first drain path 384 further comprises a bypass path 384a arranged to bypass the conductivity sensor 336, the drain path temperature sensor 315, and the drain line valve 341. The bypass path 384a comprises a valve 340. The valve 340 is arranged to regulate the flow through the bypass path 384a. As in Fig. 4a, a control device 305a is configured to control the flow rate of purified water in the recirculation path 375 arranged from a point downstream of the heater 302, the permeate temperature sensor 303, and the additional conductivity sensor 304, and back to the tank 350. A product water pressure sensor 308 is arranged to monitor the product water pressure in the product water path 301c downstream of the polishing device 306. As in Fig. 4a, a flow sensor 309 is arranged to monitor the product water flow rate downstream of the polishing device 306. The product water pressure and flow rate are fed to the control unit 112. The control unit 112 is configured to control the operation of the control device 305a.More specifically, the control unit is configured to regulate the water flow rate in recirculation path 375 based on the product water pressure and flow rate, in order to control the product water flow rate to a desired level and the product water pressure to a desired level. The control device 305a is, for example, a motorized flow control valve configured to finely regulate the water flow rate in recirculation path 375. A product water valve 305d is arranged, under the control of the control unit 112, to control the flow of produced product to the product water port 128 or to the tank 350 via an additional recirculation path 381. A drain valve 396 is arranged to control the water flow in the additional recirculation path 381. The additional recirculation path 381 is seamlessly connected to the product water path 371c via an air trap chamber 319. A product water conductivity sensor 312 is arranged to monitor the conductivity of the product water upstream of the air trap chamber 319. A product water temperature sensor 313 is configured to monitor the temperature of the product water upstream of the air trap chamber 319. In operation, a portion of the rejected water leaving device RO 301 through a fluid path 385a passes through a constant-flow auxiliary device 318, which provides a constant flow of rejected water to a three-way valve 305b (e.g., a three-way solenoid valve) under the control of control unit 112. A remaining portion of the rejected water returns to pump RO-450 through a valve 320 (e.g., a manual needle valve) in a first reject path 385b. The three-way valve 305b is configured to selectively divert the rejected water to drain 339 or return to tank 350 through a second drain path 388 or return to tank 350 through a second reject path 389. A bypass path 385f is provided to circumvent the constant-flow auxiliary device 318.A flow control device 321 is arranged to control the flow in the bypass path 3851 by controlling the control device 112. When a treatment is complete, the water purification unit 300 prepares for disconnection (for example, in response to a message received by the cycler 20) from the disposable line assembly 40 and closes a cover (not shown) that covers the product water port 128 and the drain port 118 from the outside and at the same time connects the product water port 128 and the drain port 118 by a path 401a, so that the heated fluid can flow from the product water port 128 and into the drain port. 118 and beyond drain 339 through the first drainage path 384. All the meters and sensors described in connection with the water purification apparatus 300 in Fig. 6 are configured in some modes to send their corresponding signals to the control unit 112. To protect the components of the 300 water purification unit as much as possible, for greater reliability and to prevent bacterial growth, the 300 water purification unit provides hardware and programs for cleaning. The water purification apparatus 300 also comprises a container 392 that holds a microbiological growth inhibitor. The microbiological growth inhibitor is used to prepare a cleaning solution, such as citric acid, which is introduced into the water path in some embodiments. As illustrated, the container 392 is in fluid communication with an inlet 392a of the water purification apparatus 300. In Fig. 6, line 382 connects the container 392 to the water path of the water purification apparatus 300. Alternatively, the container 392 can be connected via a line (not illustrated) leading directly to the disposable cassette 42 operated by the cycler 20, or it can be connected to pipe 64, or to drain line 56. The agent inhibiting microbiological growth in vessel 392 may be a suitable physiologically safe acid, such as citric acid, citrate, lactic acid, acetic acid, or hydrochloric acid (or a combination thereof). In one embodiment, vessel 392 contains citric acid, citrate, or a derivative thereof. Note that vessel 392 may also include additives supplied with the acid (such as with citric acid). A chemical inlet 392a is located, for example, at the front of the water purification unit 300. A presence sensor (not shown, for example, an optical sensor) is arranged to detect when vessel 392 is connected to chemical inlet 392a. A three-way valve 317, under the control of control unit 112, at chemical inlet 392a is arranged to open to a second pump, which is a chemical intake pump 316, and the tank 350.The chemical inlet pump 316 is arranged to feed the disinfectant solution to tank 350. An optical sensor is arranged to detect whether the cleaning or disinfecting solution source is connected or disconnected. If / when the container 392 is removed or the optical sensor does not detect it, the chemical inlet pump 316 stops or does not activate, and the three-way valve 317 closes to the chemical inlet 392a. The three-way valve 317, under the control of the control unit 112, can also be used to recirculate water and disinfectant to and from tank 350 during the chemical disinfection, cleaning, and / or rinsing phases. The chemical inlet pump 316 and a valve 310 are arranged in a path 379 that seamlessly connects the three-way valve 317 and the product water path anennn / cznz / B / vi. 371c. Valve 310 is arranged to control flow in path 379. In a more detailed example of the disinfection phase, when chemical disinfection begins, the level in tank 350 is adjusted to just above the low-level switch 350b. The control unit 112 causes the RO pump 450 to start and run until the empty-level switch 350a indicates the presence of air. The RO pump 450 then stops, and the inlet valve 332 opens. The inlet valve 332 remains open until the empty-level switch 350a indicates water. The chemical inlet pump 316 runs until a predetermined amount of chemical solution is introduced into tank 350. When the level in tank 350 reaches a predetermined level, the three-way valve 317 opens to drain 339. The RO pump 450 circulates water in the flow path during the chemical inlet phase and can operate in both directions to create turbulent flow and increase disinfection time and contact time.At the end of the admission phase, the reject bypass valve 321 opens and the three-way valve 305b is activated to open the second drain path 388 to drain 339 and drain the water level in the tank 350 to its low level at the low level switch 350b. The described pretreatment module 160, RO module 170, and posttreatment module 180 are enclosed within a single water purification cabinet 110a, with the exception of the filter pack 331, which is detachably mounted, for example, on the outside of the individual water purification cabinet 110a. The filter pack 331 can then be replaced when it is depleted. Alternatively, the modules can be arranged in separate units. As mentioned previously, the purified water is supplied from the water purification unit 300 to the disposable assembly 40 via pipe 64. With reference to Fig. 1, pipe 64 supplies purified water to a water port 282 of cassette 42 in the disposable assembly 40.The tubing 64 is, in one embodiment, a flexible tubing with one end connected to the product water port 128 of the water purification unit 300 and a second end connected to the water port 282 of the cycler 20. The tubing 64 can be at least 2 meters long and, in one embodiment, more than 4 meters. The tubing 64 allows the water purification unit 300 to be installed in a room with an available water source, while the cycler 20 is located in a different room where the patient resides, for example, while sleeping. Consequently, the tubing 64 can be as long as necessary to connect the water purification unit 300 to the cycler 20. Figure 6 also illustrates that the disposable assembly 40 includes a drain line configuration 56 arranged to convey water, such as used dialysis fluid, to the drain 339 of the water purification apparatus 300. The drain line 56 is, for example, a tube having a first end connected to the cassette 42 of the cycler 20 and a second end that includes a drain line connector 58 (Fig. 1) connected to a drain port 118 of the water purification apparatus 300. The drain line 56 can alternatively be a flexible tube, which can be more than 2 meters long and in some embodiments more than 4 meters. Drain line 56 can be as long as necessary to connect between water purification unit 300 and cycler 20. Pipe 64 and drain line 56 in the illuminated configuration run parallel using double-lumen pipes.It is also possible that the water purification unit 300 and the cycler 20 are positioned together, so that the same two-line water path, including pipe 64 and drain line 56, may be, for example, less than 0.5 meters. Furthermore, while a double-span pipe 64 and drain line 56 are being polished, pipe 64 and drain line 56 may be separated. A water tray 420 is placed under the water purification appliance 300. A liquid sensor 370 is arranged at the bottom of the water tray 420 to detect any leaks from the water purification appliance 300. This disclosure is not limited to the preferred embodiments described above. Various alternatives, modifications, and equivalents may be used. Therefore, the foregoing embodiments should not be considered as limiting the scope of the disclosure, which is defined in the appended claims.

Claims

1. A water purification apparatus (300) characterized in that it comprises: a reverse osmosis device (301) configured to produce purified water, the reverse osmosis device (301) including a feed inlet (301a) arranged to receive feed water and a purified water outlet (301b); a reverse osmosis pump (450) configured to pump feed water to the feed inlet (301a); a recirculation path (375) configured to recirculate a proportion of the purified water from a first point located downstream of the reverse osmosis device (301) to a second point located upstream of the reverse osmosis device (301);a purified water path (371) configured to transport purified water from the purified water outlet (301b) to a destination, wherein the purified water path (371) includes (i) a permeate water path (371a) located upstream of the recirculation path (375) and (ii) a product water path (371c) located downstream of the recirculation path (375) to transport product water to the destination, wherein the permeate water path (371a) splits into the recirculation path (375) and the product water path (371c) at the first point; a flow sensor (410) configured to detect a purified water flow rate in the permeate water path (371a); and a control unit (112) configured to control the reverse osmosis pump (450) at a certain pumping rate corresponding to a certain flow rate of purified water through the permeate water path (371a).; 2. The water purification apparatus (300) according to claim 1, characterized in that the flow sensor (410) is a first flow sensor, and wherein the water purification apparatus (300) includes a second flow sensor (309) configured to detect a product water flow in the product water path (371c).

3. The water purification apparatus (300) according to claim 1 or 2, characterized in that it further comprises a heater (302), wherein the heater (302) is located downstream of the reverse osmosis device (301) for heating the purified water flowing in the purified water path (371).

4. The water purification apparatus (300) according to claim 3, characterized in that it includes a temperature sensor (303) positioned to measure a temperature of the purified water downstream of the heater (302), and wherein the control unit (112) is configured to control the temperature of the feed water flowing through a reverse osmosis membrane (324) of the reverse osmosis apparatus (301) based on the temperature detected by the temperature sensor (303).

5. The water purification apparatus (300) according to any of claims 1 to 4, characterized in that it includes at least one pressure sensor (308) located and arranged to detect a purified water pressure in the purified water path (371), and wherein the control unit (112) is configured to use the detected pressure to control the purified water pressure so that (i) it remains below a predetermined upper pressure level or (ii) it attempts to reach a predetermined pressure.

6. The water purification apparatus (300) according to any of claims 1 to 5, characterized in that it further comprises a tank (350) configured to receive water from an external water source and to provide feed water to the feed inlet (301 a) of the reverse osmosis device (301).

7. The water purification apparatus (300) according to claim 6, characterized in that the water from the external water source flows through a filter pack (331) before reaching the tank (350).

8. The water purification apparatus (300) according to claim 6, characterized in that the second upstream point of the reverse osmosis device (301) for the recirculation path (375) is provided in the tank (350).

9. The water purification apparatus (300) according to claim 6, characterized in that it includes an air vent line (325) extending from an upper portion of the tank (350).

10. The water purification apparatus (300) according to any of claims 1 to 9, characterized in that it further comprises a polishing device (306) positioned downstream of the reverse osmosis device (301) in the purified water path (371).

11. The water purification apparatus (300) according to claim 10, characterized in that the permeate water path (371a) is positioned to transport purified water from the purified water outlet (301b) of the reverse osmosis device (301) to an inlet of the polishing device (306).

12. The water purification apparatus (300) according to claim 10 or 11, characterized in that the product water path (371c) is positioned and arranged to transport purified water from an outlet of the polishing device (306) to the destination. anRnnn / eznz / R / Yi 13. The water purification apparatus (300) according to any of claims 1 to 9, characterized in that it includes a flow control device (305a) positioned along the recirculation path (375).

14. The water purification apparatus (300) according to any of claims 1 to 13, characterized in that the destination includes a product water port (128).

15. The water purification apparatus (300) according to any of claims 1 to 14, characterized in that it includes a detector (308, 309, 313) configured to detect a fluid property in the product water path (371c), and a flow control device (305a) located along the recirculation path (375), wherein the control unit (112) is configured to control the flow control device (305a) based on the fluid property detected by the detector (308, 309).

16. The water purification apparatus (300) according to claim 15, characterized in that the fluid property includes fluid pressure, fluid flow rate or fluid temperature.

17. A peritoneal dialysis system characterized in that it comprises: a water purification apparatus comprising: a reverse osmosis device (301) configured to produce purified water, the reverse osmosis device (301) including a feed inlet (301a) positioned to receive feed water and a purified water outlet (301b), a reverse osmosis pump (450) configured to pump feed water to the feed inlet (301a), a recirculation path (375) configured to recirculate a proportion of the purified water from a first point located downstream of the reverse osmosis device (301) to a second point located upstream of the reverse osmosis device (301), a purified water path (371) configured to transport purified water from the purified water outlet (301b) to a destination,wherein the purified water path (371) includes (i) a permeate water path (371a) located upstream of the recirculation path (375) and (ii) a product water path (371c) located downstream of the recirculation path (375) for transporting product water to the destination, wherein the permeate water path (371a) is divided into the recirculation path (375) and the product water path (371c) at the first point, a flow sensor (410) configured to detect a flow rate of purified water in the permeate water path (371a), and a control unit (112) configured to control the reverse osmosis pump (450) at a certain pumping rate corresponding to a certain flow rate of purified water through the permeate water path (371a); and a peritoneal dialysis (PD) cycler positioned and disposed of to use PD fluid during PD treatment,The PD fluid mixed using product water from the water purification apparatus (300)., 18. The peritoneal dialysis system according to claim 17, characterized in that the destination includes a product water port (128).

19. A method for controlling at least one fluid property in a water purification apparatus, the water purification apparatus (300) comprising a reverse osmosis device (301) configured to produce purified water, and a recirculation path (375) arranged to recirculate a proportion of the purified water from a first point downstream of the reverse osmosis device (301) to a second point upstream of the reverse osmosis device (301), the method characterized in that it comprises: arranging a purified water path (371) to include (i) a permeate water path (371a) located upstream of the recirculation path (375) and (ii) a product water path (371c) located downstream of the recirculation path (375); dividing the permeate water path (371a) into the recirculation path (375) and the product water path (371c) at the first point;and controlling a reverse osmosis pump (450) at a certain pumping rate corresponding to a certain flow rate of purified water through the permeate water path (371a).; 20. The method according to claim 19, characterized in that it further comprises estimating a quantity of product water produced during a production time period based on a duration of the production time period and a corresponding flow rate of purified water detected during the production time period.

21. The method according to claim 19 or 20, characterized in that it further comprises triggering a predetermined action when the quantity reaches a predefined production volume.

22. The method according to any of claims 19 to 21, characterized in that it includes monitoring at least one pressure sensor (308) located and arranged to detect a purified water pressure in the purified water path (371), and using the detected pressure to control the purified water pressure so that (i) it remains below a predetermined upper pressure level or (ii) it attempts to reach a predetermined pressure.

23. The method according to any of claims 19 to 22, characterized in that it includes monitoring a temperature sensor (303) positioned to detect a temperature of the purified water, and controlling the temperature of the water flowing through a reverse osmosis membrane (324) of the reverse osmosis device (301) based on the temperature detected by the temperature sensor (303).