A system and method for prevention of fouling in a forward osmosis unit for dialysis fluid

Abstract

A system (1) for preventing fouling in forward osmosis units for the preparation of dialysis fluids The system (1) comprises a fluid path (2) including a forward osmosis-,FO-, arrangement (60) comprising one or more FO-units (6) each including a draw side (6a) and a feed side (6b) separated by a FO-membrane (6c), the FO-unit (6) fluidly connected to the fluid path (2), wherein the FO-unit (6) is configured to receive one or more dialysis concentrate fluids (4a) at the draw side (6a) and to receive a feed fluid at the feed side (6b), wherein water is transported from the feed fluid to the one or more dialysis concentrate fluids (4a) through the FO-membrane (6c) via an osmotic pressure gradient between the draw side (6a) and the feed side (6b), thereby diluting the one or more dialysis concentrate fluids (4a) into a diluted dialysis concentrate fluid. The FO-membrane (6c) comprises a hollow fiber membrane including at least one hollow fiber, each of the at least one hollow fibers comprising a lumen side (61) and a shell side (62). Each of the FO-units (6) of the FO-arrangement (60) comprises a diameter (D) and wherein the diameter (D) of each FO-unit (6) in the FO-arrangement (60) has a decreasing diameter (D) from a feed side (6b) inlet to a feed side (6b) outlet which causes each FO-unit (6) in the FO-arrangement (60) to have a decreasing cross-sectional area from the feed side (6b) inlet to the feed side (6b) outlet, and wherein the decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane (6c).

Description

[0001] A system and method for prevention of fouling in a forward osmosis unit for dialysis fluid

[0002] Technical field

[0003] The present invention relates to the field of dialysis, and to systems and methods for the prevention of fouling in forward osmosis units used to prepare dialysis fluid.

[0004] Background

[0005] Peritoneal dialysis (PD) is a method for treating patients suffering from renal failure. During PD, the peritoneal cavity of a patient is filled with fresh PD fluid, and waste and fluid is transported from the blood of the patient, via the patient’s peritoneal membrane, to the fresh PD fluid. The used PD fluid (also referred to as “effluent”) is thereafter drained from the patient. There are several kinds of PD. In automated peritoneal dialysis (APD) a machine or cycler is used to fill the peritoneal cavity with fresh PD fluid, and after a specified dwell period, automatically drains the used PD solution from the body. This procedure is repeated several times, typically during overnight. In continuous flow peritoneal dialysis (CFPD) the machine is used to provide a continuous flow of fresh PD fluid to the peritoneal cavity of the patient, and a continuous flow of used PD fluid from the patient.

[0006] Hemodialysis (HD) is another type of dialysis method for treating patients suffering from renal failure. During HD, blood is removed from the patient to a cycler and passed through a dialyzer. In the dialyzer, the removed blood moves along one side of a membrane and dialysis fluid moves in an opposite direction along the other side of the membrane. The dialyzer membrane, which is semipermeable, causes waste and excess water in the blood to be removed to the side of the dialysis fluid. The filtered blood is then returned to the patient.

[0007] Some dialysis cyclers utilize a solution generation device including a forward osmosis unit to dilute concentrated sources of dialysis fluid. The forward osmosis unit promotes the flow of water from a low concentrate source into the concentrated dialysis fluid to prepare the dialysis fluid for use in dialysis treatment. Such a system is described in PCT Publication No. WO2021 / 156429, published August 12, 2021 , which is hereby incorporated by reference in its entirety. The forward osmosis unit contains a membrane used to separate the low concentrate source from the concentrated dialysis fluid. This membrane can become fouled or obstructed from chemical build-up of carbonates or phosphates from the patient present in the effluent (referred to as scaling or inorganic scaling), or from surface adherence of biomolecules such as proteins, lipids, fibrine, and others (referred to as fouling). Both mechanisms may be present at once or in combination as well. For example, the inorganic scaling may form a matrix that promotes the build-up of fouling.

[0008] Certain risk factors exist in conventional dialysis solution generation devices with forward osmosis units that can enhance the probably that a forward osmosis unit will become fouled. These risk factors include: a high concentration of the fouling agents in the solutions on either side of the forward osmosis membrane; prolonged contact time between the fouling / scaling agents and the forward osmosis unit components; and a lack or diminished shear forces near the forward osmosis membrane (potentially due to low flow velocity in the forward osmosis unit).

[0009] Obstruction of the forward osmosis membrane can cause inaccuracies or difficulty in forming a dialysis fluid with the correct concentration levels for treatment. Obstruction of the forward osmosis membrane can also cause early obsolescence of a given dialysis solution generation device and inefficient use of feed water resources, amongst other drawbacks. There is accordingly a need to reduce the negative consequences and mitigate the risk factors that cause a forward osmosis membrane to become fouled or otherwise obstructed.

[0010] It is an objective of the disclosure to alleviate at least some of the drawbacks with known dialysis fluid preparation machines. It is a further objective to provide a cost-efficient solution for the prevention of fouling in forward osmosis units (especially the membrane(s) of the forward osmosis units used to prepare dialysis fluids). It is a still further objective to provide an efficient solution for producing fluid for dialysis at the point of care, which can consume significant amounts of water.

[0011] These objectives and others are at least partially achieved by the system and method according to the independent claims, and by the embodiments according to the dependent claims.

[0012] According to one aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the disclosure relates to a system for the prevention of fouling of a forward osmosis unit of a dialysis fluid preparation device. The system comprises a fluid path having one or more concentrate connectors each configured to be connected to a source of concentrate fluid, and an inlet connector configured to be connected to a fluid line arranged for transportation of feed fluid from a source of feed fluid. The system further comprises a forward osmosis (FO-) arrangement comprising one or more FO- units each including a draw side and a feed side separated by a FO-membrane. The FO-unit is fluidly connected to the fluid path. The FO-unit is configured to receive the one or more concentrate fluids at the draw side and to receive the feed fluid at the feed side, wherein water from the feed fluid is transported to the one or more concentrate fluids through the FO-membrane by means of an osmotic pressure gradient between the draw side and the feed side. The one or more dialysis concentrate fluids are thereby diluted into a diluted dialysis concentrate fluid. The FO-membrane comprises a hollow fiber membrane including at least one hollow fiber, each of the at least one hollow fibers comprising a lumen side and a shell side. Each of the FO-units of the FO- arrangement comprises a diameter, and the diameter of each FO-unit in the FO- arrangement has a decreasing diameter from a feed side inlet to a feed side outlet which causes each FO-unit in the FO-arrangement which causes each FO-unit in the FO-arrangement to have a decreasing cross-sectional area from the feed side inlet than to the feed side outlet. The decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane. The proposed system can at least partially reduce the incidence of foulant and scaling build up before it occurs (i.e. decreasing the propensity of an FO- membrane to become fouled), or by at least partially removing foulant and scaling build up after it has occurred within an FO-arrangement. This increases the efficiency of the osmotic process, reducing any waste of novel pure water or effluent used as the feed fluid. It also works to increase the longevity of the FO- unit(s) such that the FO-unit(s) and even the entire dialysis fluid preparation device is not discarded prematurely, creating undue waste and cost.

[0013] According to another aspect of the present disclosure, on the feed side, the differing cross-sectional area is achieved by having a decreasing inner diameter of the lumen of each of the at least one hollow fibers along the intended flow direction in at least one of the one or more FO-units.

[0014] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the decreasing inner diameter is arranged to cause an increase in flow velocity of the feed fluid as it flows through the feed side of the at least one of the one or more FO-units relative to the flow velocity at a constant diameter.

[0015] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the decreasing inner diameter is arranged to cause an increase in flow velocity of the feed fluid as it flows through the feed side of the at least one of the one or more FO-units.

[0016] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, a corresponding increasing inner diameter on the draw side (6a) is arranged to cause the flow velocity of fluid on the draw side to remain substantially constant relative to the FO-membrane as it flows through the draw side (6a) of the at least one of the one or more FO-units. According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the shell side has an increasing cross- sectional area along the intended flow direction.

[0017] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the variable cross-sectional area is configured to mitigate a combination of scaling and fouling of the FO-membrane.

[0018] According to another aspect of the present disclosure, the FO-arrangement comprises a plurality of FO-units.

[0019] According to another aspect of the present disclosure, the plurality of FO-units comprises three FO-units.

[0020] According to another aspect of the present disclosure, the plurality of FO-units is connected in series in the flow path.

[0021] According to another aspect of the present disclosure, each of the plurality of FO- units comprises a diameter and wherein the diameter of each FO-unit of the plurality of FO-units has a different, smaller diameter than a FO-unit preceding that FO-unit in the flow direction of the feed side of the plurality of FO-units which causes each FO-unit of the plurality of FO-units to have a decreasing cross- sectional area in direction of flow of the feed side, and wherein the decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane.

[0022] According to another aspect of the present disclosure, decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit as it flows through the feed side of each of the FO- units relative to the flow velocity at a constant diameter.

[0023] According to another aspect of the present disclosure, decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit as it flows through the feed side of each of the FO- units.

[0024] According to another aspect of the present disclosure, wherein on the draw side the diameter of each FO-unit along the intended flow direction of the draw side increases.

[0025] According to another aspect of the present disclosure, decreasing a diameter of each FO-unit comprises having a decreasing inner diameter of the lumen of each of the at least one hollow fibers along the intended flow direction of the feed side between each FO-unit of the FO-arrangement.

[0026] According to one aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the disclosure relates to a system for the prevention of fouling of forward osmosis units of a dialysis fluid preparation device. The system comprises a fluid path having one or more concentrate connectors each configured to be connected to a source of concentrate fluid, and an inlet connector configured to be connected to a fluid line arranged for transportation of feed fluid from a source of feed fluid. The system further comprises a forward osmosis (FO-) arrangement comprising a plurality of FO- units each including a draw side and a feed side separated by a FO-membrane. The FO-unit is fluidly connected to the fluid path. The FO-unit is configured to receive the one or more concentrate fluids at the draw side and to receive the feed fluid at the feed side, wherein water from the feed fluid is transported to the one or more concentrate fluids through the FO-membrane by means of an osmotic pressure gradient between the draw side and the feed side. The one or more dialysis concentrate fluids are thereby diluted into a diluted dialysis concentrate fluid. The plurality of FO-units are connected in series in the flow path. Each of the FO-units of the FO-arrangement comprises a diameter, and the diameter of each FO-unit in the FO-arrangement has a different, smaller diameter than a FO-unit preceding that FO-unit in the flow direction of the feed side of the FO-arrangement to have a decreasing cross-sectional area in direction of flow of the feed side. The decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane.

[0027] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit as it flows through the feed side of each of the FO-units relative to the flow velocity at a constant diameter.

[0028] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit as it flows through the feed side of each of the FO-units.

[0029] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, on the draw side the diameter of each FO-unit along the intended flow direction of the draw side increases.

[0030] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the increasing diameter on the draw side causes the flow velocity of fluid on the draw side to remain substantially constant relative to the FO-membrane as it flows through the draw side of each FO-unit.

[0031] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, decreasing a diameter of each FO-unit comprises having a decreasing inner diameter of the lumen of each of the at least one hollow fibers along the intended flow direction of the feed side between each FO-unit of the FO-arrangement.

[0032] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, having a decreasing inner diameter of the lumen of each of the at least one hollow fibers causes an increase in flow velocity of the fluid through the lumen side. According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, decreasing a diameter of each FO-unit comprises having a decreasing number of hollow fibers along the intended flow direction of the feed side of each FO-unit.

[0033] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, having a decreasing number of hollow fibers causes an increase in flow velocity of the fluid through the lumen side.

[0034] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, decreasing a diameter of each FO-unit comprises both having a decreasing number of hollow fibers and decreasing an inner diameter of the lumen of each of the at least one hollow fibers along the intended flow direction of the feed side of each FO-unit.

[0035] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the plurality of FO-units comprises three FO-units.

[0036] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, released sealant and foulant is disposed through a drain line.

[0037] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, each FO-unit has a variable cross- sectional area along at least a portion of a length of the FO-unit.

[0038] According to another aspect of the present disclosure, which may be combined with any other aspect or portion thereof, the disclosure relates to a system for the prevention of fouling of a forward osmosis unit of a dialysis fluid preparation device. The system comprises a fluid path having one or more concentrate connectors each configured to be connected to a source of concentrate fluid, and an inlet connector configured to be connected to a fluid line arranged for transportation of feed fluid from a source of feed fluid. The system further comprises a forward osmosis (FO-) arrangement comprising one or more FO- units each including a draw side and a feed side separated by a FO-membrane. Each of the one or more FO-unit is fluidly connected to the fluid path. Each FO- unit is configured to receive the one or more concentrate fluids at the draw side and to receive the feed fluid at the feed side, wherein water from the feed fluid is transported to the one or more concentrate fluids through the FO-membrane by means of an osmotic pressure gradient between the draw side and the feed side. The one or more dialysis concentrate fluids are thereby diluted into a diluted dialysis concentrate fluid. The FO-arrangement has a varying cross-sectional area that is configured to mitigate at least one of scaling and fouling of the FO- membrane.

[0039] Brief description of the drawings

[0040] Fig. 1 illustrates a prior art dialysis fluid preparation system employing a forward osmosis unit.

[0041] Fig. 2 illustrates a system for preventing fouling in a forward osmosis unit for the preparation of dialysis fluid according to some embodiments.

[0042] Fig. 3A illustrates a close-up view of an FO-unit of the embodiment of Fig. 2.

[0043] Fig. 3B illustrates a diagram of fluid flow in the FO-unit of Fig. 3A.

[0044] Fig. 4 illustrates a different embodiment of a system for preventing fouling in a forward osmosis unit for the preparation of dialysis fluid according to some embodiments.

[0045] Fig. 5 illustrates a close-up view of the FO-unit arrangement of the embodiment of Fig. 4.

[0046] Detailed description

[0047] In the following disclosure, several embodiments of systems and methods for the prevention of fouling in a forward osmosis (FO-) unit for preparing dialysis fluids will be described. The embodiments may each make use of forward osmosis to dilute one or more dialysis concentrates using water transported over a FO- membrane from used dialysis fluid or “effluent”. Water from the effluent is reused for mixing with dialysis concentrates to provide new dialysis fluid. The solution may be used for different variants of automated dialysis, including on-line mixing of dialysis fluid and batch-wise mixing of dialysis fluid.

[0048] A plurality of systems for producing fluids for dialysis are described herein, with reference to Figs. 1 , 2 and 4. References that are the same throughout the figures may not be textually described in each embodiment but include all of the structure, functionality and alternatives that are described in the other embodiments.

[0049] FO Preparation of Dialysis Fluids Generally

[0050] Fig. 1 illustrates a conventional system 1 according to some prior art of a dialysis fluid preparation machine employing a FO-unit to produce dialysis fluid. The system 1 includes a fluid path 2, a plurality of connectors, a forward osmosis- (FO-) unit 6 and a control unit 40. The fluid path 2 may be enclosed inside an enclosure (not shown in Fig. 1 ). The fluid path 2 may be part of an apparatus or a disposable system. The fluid path 2 includes a plurality of fluid lines. The fluid path 2 may include some or all of the fluid lines as described herein. The connectors include one or more dialysis concentrate connectors 3a. Each dialysis concentrate connector is configured to be connected to a source of dialysis concentrate fluid 4a. As described herein, then, dialysis concentrate fluid 4a may be considered a first concentrate fluid, while a second dialysis concentrate fluid (not illustrated here) may also be incorporated into the system. A source of dialysis concentrate fluid is typically a bag with dialysis concentrate. Each dialysis concentrate connector is then configured to be connected to a corresponding connector provided with the dialysis concentrate fluid bag. The connectors also include an inlet connector 5a. The inlet connector 5a is typically configured to be connected to a fluid line 5 arranged for transportation of effluent fluid from a patient. The inlet connector 5a may be configured to be connected to any source of feed fluid in alternative embodiments.

[0051] The FO-unit 6 comprises a draw side 6a and a feed side 6b separated with a FO- membrane 6c. As used herein, feed side refers to the portion of an FO-unit through which the low-concentrate fluid flows during normal operation of the FO- unit, while draw side refers to the portion of the FO-unit through which the high- concentrate fluid flows during normal operation of the FO-unit. Normal operation of the FO-unit is when the FO-unit is used in such a way that a positive osmotic pressure gradient is established from the feed side to the draw side. Reference is also made in the description to a lumen side and a shell side. As used herein, lumen side refers to, in a hollow-fiber FO-membrane, the interior portion of the hollow-fiber in the hollow-fiber membrane, while shell side refers to the exterior portion of the hollow-fiber in the hollow-fiber membrane. During normal operation of the FO-unit, the lumen side is in fluid alignment with the feed side and the shell side is in fluid alignment with the draw side (i.e. low concentrate fluid flows through the lumen side of the hollow-fiber and high concentrate fluid flows through the shell side of the hollow-fiber). More detailed reference to these definitions may be had in the description of Figs. 3A and 3B later on.

[0052] Returning to Fig. 1 , the FO-unit 6 is fluidly connected to the fluid path 2. The FO- unit 6 is configured to receive the one or more dialysis concentrate fluids 4a at the draw side 6a and to receive the effluent at the feed side 6b to transport water from the effluent or other feed fluid to the one or more dialysis concentrate fluids through the FO-membrane 6c by means of an osmotic pressure gradient between the draw side 6a and the feed side 6b. The one or more dialysis concentrate fluids are therefore diluted into a diluted dialysis concentrate fluid. The FO-membrane 6c is a water permeable membrane. It separates the effluent (feed side) and the dialysis concentrate (draw side). The different sides may also be referred to as compartments. The fluids in these sides typically flow in countercurrent flow, but may alternatively flow in co-current flows. In some embodiments, the fluids flow single pass, wherein the fluid passes through the FO-unit 6 only once.

[0053] The water in the effluent is transported over the FO-membrane by means of the driving force created by the osmotic pressure gradient between the effluent (feed solution) and the one or more dialysis concentrate fluids (draw solution). This means that the effluent, which initially has about the same osmolarity as final dialysis fluid, will become more concentrated throughout the FO process. The one or more dialysis concentrates will on the other hand be more diluted throughout the FO process. The geometry of the membrane may be flat-sheet, tubular or hollow fiber.

[0054] In more detail, a first dialysis concentrate bag 4a is connected via a first bag connector to a first dialysis concentrate connector 3a. A first fluid line 21 is fluidly connected between the first dialysis concentrate connector 3a and an inlet port of the draw side 6a. The first fluid line 21 thus connects the first dialysis concentrate connector 3a and the draw side 6a.

[0055] A second fluid line 22 is fluidly connected between an outlet of the draw side 6a and an outlet connector 1 1 a. The second fluid line 22 thus fluidly connects the draw side 6a and the outlet connector 1 1 a. The outlet connector 1 1 a is configured to be connected to a corresponding connector of a fluid line 1 1 , which is configured to transport final dialysis fluid directly to a catheter of patient, to a cycler for pumping the fluid to a patient, or to a batch container. A third fluid line 25 is connected between the inlet connector 5a and an inlet of the feed side 6b. The third fluid line 25 thus fluidly connects the inlet connector 5a and the feed side 6b. The effluent may optionally be collected in an effluent container 15 before the effluent is fed into the feed side 6b. Thus, the effluent container 15 may be arranged to collect effluent fluid received from the patient P before it is transported into the feed side 6b. The effluent container 15 is fluidly connected to the third fluid line 25. Thus, the effluent container 15 is fluidly connected to the fluid path 2 and the inlet connector 5a.

[0056] An effluent pump 44 is arranged in Fig.1 to control the flow rate of effluent fluid into the feed side 6b. Further, the effluent pump 44 is arranged to control the flow rate of effluent from the effluent container 15 into the feed side 6b. Here, the effluent pump 44 is arranged to control the flow rate of effluent fluid in the third fluid line 25. Effluent container 15 includes effluent container connector 3e, which is here an inlet connector, and an additional effluent container connector 3f, allowing effluent to flow into and out of effluent container 15. An additional drain pump 49 under control of control unit 40 allows effluent incoming from line 5 to be pumped to effluent container 15 through effluent container line 83, while effluent pump 44 independently pumps effluent into the feed side of FO-unit 6. First pump 41 may at the same or different time pump first dialysis concentrate from container 4a into the draw side of FO-unit 6. Second dialysis concentrate pump 42, system pump 43 and concentrate pump 81 operate as described below to add and mix a second dialysis concentrate to form a final dialysis solution.

[0057] A fourth fluid line 26 is connected between an outlet of the feed side 6b to the drain 75. Thus, the fourth fluid line 26 connects the feed side 6b and the drain 75. The drain is also configured to remove the effluent after use.

[0058] The system 1 further comprises a container 9 fluidly connected or connectable to the fluid path 2. The container 9 is arranged to receive the diluted dialysis concentrate fluid. A fifth fluid line 18 is connected between a first port 9a of the container 9 and the second fluid line 22. The fifth fluid line 18 thus connects the first port 9a of the container 9 and the second fluid line 22. A sixth fluid line 27 is connected between a second port 9b of the container 9 and the second fluid line 22. The sixth fluid line 27 thus connects the second port 9b of the container 9 and the second fluid line 22. The fifth fluid line 18 connects to the second fluid line 22 at a first point of the second fluid line 22. The sixth fluid line 22 connects to the second fluid line 22 at a second point of the second fluid line 22. A first pump 41 is arranged to pump fluid in the second fluid line 22. The first pump 41 is arranged in Fig. 1 upstream of the first point and the second point of the second fluid line 22. The first pump 41 is also configured to control the flow rate of the one or more dialysis concentrate fluids into the draw side 6a.

[0059] System 1 includes multiple valves each under control of control unit 40 (e.g., electrically actuated, energized open solenoid pinch valves). The valves include valve 37 located along and allowing or disallowing flow through effluent leg line 5; valve 32 located along and allowing or disallowing flow through dialysis fluid line 22; valve 30 located along and allowing or disallowing flow through fresh leg line 11 ; bypass valve 39 located along and allowing or disallowing a bypass flow of fresh dialysis fluid to drain 75; reject valve 55 located along and allowing or disallowing flow through reject fluid line 26 to drain 75; vent valve 38 located along and allowing or disallowing vent flow from a top of mixing chamber 46 to drain 75; inlet and outlet valves 33 and 80, respectively, allowing or disallowing flow through inlet and outlet lines 27 and 18, respectively, into and out of diluted dialysis concentrate (e.g., buffer) container 9, and wherein valve 80 is disposed on draw side outlet line 84; and valve 45 allowing or disallowing filtered forward osmosis water from the draw side FO-unit 6 to dialysis fluid line 22.

[0060] System 1 includes mixing chamber 46 for mixing a final dialysis fluid having a prescribed concentration of the two or more dialysis concentrate fluids (e.g., buffer and glucose). Mixing chamber 46 also serves as a gas trap that allows gas (e.g., air) to migrate out of the final dialysis fluid into the top of the gas trap. Mixing chamber 46 in the illustrated embodiment is made to operate with level sensors 46a and 46b, which output to control unit 40, wherein control unit 40 uses the signals from sensors 46a and 46b to operate system pump 43 so as to maintain a desired final dialysis fluid level between the sensors.

[0061] Control unit 40 causes effluent pump 44 to remove effluent from effluent container 15 to the feed side of FO-unit 6 at a flow rate that is desirable for FO extraction, wherein the portion of effluent that is not extracted is delivered to drain 75 from the FO-unit via valve 55. At the same or different time, control unit 40 causes diluting pump 41 to pump first dialysis concentrate 4a through the draw side of FO-unit 6 via valves 45 and 80 at a desired flow rate to deliver a required amount of diluted dialysis concentrate to container 9. At the same or different time, control unit 40 causes first dialysis concentrate pump 81 to remove diluted first dialysis concentrate from diluted dialysis concentrate container 9 and deliver same to second fluid line 22. Dialysis concentrate pump 81 may be controlled so that the proper conductivity at sensor 13 is reached (wherein the first dialysis concentrate pump 41 and effluent pump 44 are controlled to follow the flow rate of dialysis concentrate system pump 43 in one embodiment). Control unit 40 also causes system pump 43 to pull a desired amount of purified water from purified water container 7 via valve 82. Control unit 40 also causes second dialysis concentrate pump 42 and system pump 43 to run at fixed flow rates. System pump 43 pumps final dialysis fluid to patient P, or to a dialysis cycler or batch bag depending on the arrangement of the dialysis fluid preparation machine, via valve 30.

[0062] A first concentration (e.g., conductivity) sensor 8 is arranged to sense a concentration of the fluid in the second fluid line 22 between the first point and the second point of the second fluid line 22. The fluid path 2 includes a circulation fluid path, which includes the container 9, the fifth fluid line 18, the sixth fluid line 27, and the second fluid line 22. The fluid path 2 further comprises a water source connector 7a configured to be connected to a source of pure water 7. An eighth fluid line 28 is connected between the water source connector 7a and the second fluid line 22. The eighth fluid line 28 thus fluidly connects the water source connector 7a and the second fluid line 22. The fluid path 2 also comprises an osmotic agent connector 3b configured to be connected to a source of osmotic agent 4b. A ninth fluid line 29 is connected between the osmotic agent connector 3b and the second fluid line 22. The ninth fluid line 29 thus fluidly connects the osmotic agent connector 3b and the second fluid line 22.

[0063] A second pump 42 is configured to control a flow rate of the osmotic agent from the source of osmotic agent 4b to the fluid path 2. The second pump 42 is configured to control the flow rate of the osmotic agent in the ninth fluid line 29. A heater 14 is configured to heat the fluid in the fluid path 2. The heater 14 is in Fig. 1 configured to heat the fluid in the second fluid line 22 downstream the water source 7, but upstream the osmotic agent source 4b. Alternatively, the heater 14 may be arranged to heat the fluid at any other place along the second fluid line 22. A system pump 43 is configured to control a flow rate of the final dialysis fluid in the second fluid line 22. A second concentration sensor 13 is configured to sense the concentration of the final dialysis fluid in the second fluid line 22.

[0064] Fig. 1 illustrates an optional line 47 and valve 48 under control of control unit 40. Optional line 47 and valve 48 may be considered to be a bypass valve. Optional line 47 and valve 48 allow for any fluid sent to drain 75 to be delivered instead to effluent container 15 for reprocessing at FO-unit 6. For example, the volume of mixed fluid that does not yet have the correct concentration, which may normally be sent to drain during online mixing, may here instead be pumped with valve 48 open and valves 30, 38 and 39 closed to effluent container 15 for later extraction at FO-unit 6.

[0065] The system 1 may also comprise various sensors such as a temperature sensor 51 and / or a plurality of pressure sensors 52, 53. A UV lamp 58 may also be included for sterilization of effluent drawn from effluent line 5. System 1 may include a coarse or pre-filter 70 located along effluent line 5. In system 1 , coarse filter 70 may be located along effluent leg line 5, while sterilizing grade filters 69a and 69b are located along fresh leg line 11 . Coarse filter 70 may for example have larger pore sizes that are selected to remove fibrin or other patient materials, preventing the same from reaching FO-unit 6. Coarse filter 70 may also help to reduce fouling at FO-unit 6.

[0066] The system 1 further comprises a control arrangement configured to control a flow rate of effluent fluid into the feed side 6b and configured to be carried out by the control unit 40. Effluent fluid control is for example made by means of the effluent pump 44. In some embodiments, the control arrangement is further configured to control a flow rate of the one or more dialysis concentrate fluids into the draw side 6a to a flow rate that matches a certain production rate of a final dialysis fluid with a prescribed concentration of the one or more dialysis concentrate fluids. The final dialysis fluid has a composition of dialysis concentrates and water that is prescribed or predetermined beforehand. It is thus also known, that is prescribed, which concentration of the one or more dialysis concentrates the final dialysis fluid should have. The final dialysis fluid is dialysis fluid that is ready to be delivered to the peritoneal cavity of a patient. The production rate may be batch-wise, that is, a certain or prescribed volume of final dialysis fluid is produced. The certain volume constitutes a batch. Alternatively, the production mode includes a continuous or continual flow rate of final dialysis fluid, as embodied in Fig. 1 .

[0067] The control arrangement is configured to add correct amounts of the one or more dialysis concentrates to the fluid path 2, to achieve a prescribed final composition of the final dialysis fluid. During the forward osmosis process in the FO-unit 6, water from the effluent is transported over to the one or more dialysis concentrates. The one or more dialysis concentrates then becomes diluted into a diluted dialysis concentrate in the FO-unit 6 and is outputted as diluted dialysis concentrate fluid from the FO-unit 6 into the second fluid line 22. The control arrangement is configured to control the flow rate of the one or more dialysis concentrate fluids into the draw side 6a based on the sensed concentration of the diluted dialysis concentrate fluid.

[0068] It is desired to withdraw as much water as needed from the effluent to produce final dialysis fluid. In order to achieve such withdrawal in some embodiments, the flow rate of the effluent into the feed side 6b is matched with the flow rate of the one or more dialysis concentrates into the draw side 6b. The flow rates are initially set to approximate flow rates based on the desired composition of the dialysis fluid, the concentration of the dialysis concentrate and the amount of effluent at hand and the time available for the FO session. The amount of dialysis concentrates in the desired composition dialysis fluid is known or predetermined beforehand, as well as the concentration of the dialysis concentrate(s). Thus, for each batch of final dialysis fluid, the amount of dialysis concentrate(s) to be supplied into the draw side 6a is known. The amount of effluent is also known from how much effluent the effluent pump 44 has been pumping into the effluent bag 15, or from weighting the effluent bag 15. However, a build up of foulant or scaling will reduce the efficiency of the osmotic process, which may mean that more fluid must be supplied to the feed side 6b in order to achieve the same final concentration of the dialysis fluid. Preventing build up of scaling or foulant would ensure proper efficiency and functioning of the FO-unit 6 and thereby of the system as a whole.

[0069] One of the one or more dialysis concentrate sources 4a comprises a fluid including one or more of ions and / or salts, such as, lactate, acetate, citrate, bicarbonate, KCI, MgCI2, CaCI2, NaCI. For example, the one dialysis concentrate source may include a fluid containing buffer agents, e.g., one or more of lactate, citrate, acetate and bicarbonate. This fluid, when eventually diluted with water and possibly other dialysis concentrates becomes the final dialysis fluid which has a pH applicable for dialysis treatments, and one or more of KCI, MgCI2, CaCI2 and NaCL Final dialysis fluids may be formed having standard glucose levels, such as 1 .36% or 2.27% glucose.

[0070] The system 1 is further configured to sense a concentration of the final dialysis fluid with the second concentration sensor 13. The control arrangement is configured to control the flow rate of the diluted one or more dialysis concentrate fluids to achieve a prescribed composition of the final dialysis fluid based on the sensed concentration of the final dialysis fluid. Thus, the sensed concentration may be used to fine-tune the flow rate of the first pump 41 , to achieve the final composition, and thus a final concentration for the final dialysis fluid. Alternatively, the flow rate delivered with the second pump 42 may be fine-adjusted to achieve the final composition, and thus a final concentration for the final dialysis fluid. The control arrangement, as discussed previously, also comprises a control unit 40 including a processor and a memory. The memory typically stores a program that when executed by the processor controls the system 1 as described herein. The control unit 40 may also comprise a communication interface enabling the control unit 40 to communicate data and signals to and from the components of the system 1 , for example, send control signals to valves and pumps, and receive sensed data from concentration sensors and feedback signals from the valves and pumps. In particular, control unit 40 of system 1 in Fig. 1 is in one embodiment programmed to run in a CFPD mode in which effluent is removed slowly from patient P to effluent container 15 via drain pump 49 and valve 37. Control unit 40 at the same time fills patient P at the same rate (or may be a different flow rate than the drain flow rate, e.g., to account for ultrafiltration) via system pump 43 and valve 30. If the fill and drain flow rates are the same, the net volume within the patient does not change other than the volume of fluid absorbed as ultrafiltration (“UF”). Control unit 40 of system 1 of Fig. 1 may alternatively or additionally be programmed to run a batch-type APD treatment. In Fig. 1 , with all valves closed except valves 45, 55 and 80, which are open, system 1 may perform a dwell phase in which final dialysis fluid dwells within the peritoneal cavity of patient P for a specified period of time. Control unit 40 at the same time may run effluent pump 44 and diluting pump 41 to perform FO and create and deliver new diluted first dialysis concentrate to diluted dialysis concentrate container 9. In either CFPD and APD modes, control unit 40 may be programmed to simultaneously run an FO session and drain patient P. Here, in addition to valves 45, 55 and 80 being open and pumps 44 and 41 running, valve 37 is open and drain pump 49 is operated to pull effluent from patient P to effluent container 15. Drain valve 55 is open to allow drain fluid to flow to drain 75. In either CFPD and APD modes, control unit 40 may be programmed to simultaneously run an FO session and fill patient P with fresh, heated final dialysis fluid. Here, when water is needed for further dilution, in addition to valves 45, 55 and 80 being open (valve 37 optionally being open) and pumps 44 and 41 running, valves 30 and 82 (water valve) are open (valve 32 is closed) and pumps 81 , 42 and 43 are operated. If the diluted first dialysis concentrate is instead too diluted, or if diluted dialysis concentrate container 9 is empty, valve 32 may be opened to allow pure first dialysis concentrate from container 4a to be delivered to second fluid line 22 via diluting pump 41 . A system that prevents fouling in the FO-unit 6 would reduce the amount of corrections needed to be made by the system (i.e. extra dilution cycles or additional concentrate) because the resulting fluid would have a more predictable and consistent concentration.

[0071] Although certain components of prior art system 1 aid in the prevention of fouling by removing certain compounds from the fluid before reaching the FO-unit 6, some amount of these compounds still necessarily make their way to the FO-unit and cause fouling. Those skilled in the art will also recognize from the foregoing that the velocity of fluid leaving the pumps is necessarily determined by the required amount of dilution from the FO-unit 6. Therefore, although some problems recognized in the art could be solved by increasing the rate of flow from the pumps through the FO-unit 6, this approach would be impractical as it would affect the dilution of the resulting fluid. Thus, different systems as discussed below to address the issues with the prior art systems are necessary.

[0072] The embodiments below of system 1 from Figs. 2-5 are arranged to allow for the prevention of fouling of the FO-unit 6, and especially the FO-membrane 6c. The membrane 6c may become fouled or obstructed during use. Scaling or fouling can cause clogging of the pores in the membrane 6c, which in turn may affect the rate of transmission of water from the feed side 6b to the draw side 6a, or may reduce the longevity of the FO-unit 6 and therefore the system 1 generally. The described embodiments may be implemented on systems such as those described in Fig. 1 or any similar system. The system of Fig. 1 has been described as an example of systems which may employ the embodiments of Figs. 2-5, and not as a limitation of the scope of the disclosed embodiments. Figs. 2-5 describe two system 1 arrangements which allow for two different methods of membrane fouling prevention: (i) conically-shaped hollow-fiber membranes; and (ii) serial, decreasing cross-sectional area FO-units. A combination of these systems is also contemplated, as will be appreciated by those skilled in the art.

[0073] Conically-Shaped Hollow-Fiber Membranes

[0074] Referring to Figs. 2, 3A, and 3B, a system 1 which operates in a substantially similar manner to the system 1 of Fig. 1 is illustrated, except that the FO-unit 6 differs in that it is designed with conically-shaped hollow-fibers in a hollow-fiber membrane version of the FO-unit 6’.

[0075] As may be appreciated from Fig. 2, the hollow-fiber embodiment of FO-unit 6’ comprises the same set of inlets and outlets on the feed side and the draw side, such that the hollow-fiber FO-unit 6’ can be readily incorporated into any embodiment of system 1 .

[0076] Fig. 3A illustrates a detailed view of the FO-unit 6’ of Fig. 2. Multiple hollow-fibers are implemented to create a plurality of lumens 61 and a shell 62 (the shell 62 being a single compartment that surrounds all the lumens 61 ), which interface at a FO-membrane 6c. During normal operation of the FO-unit 6’, effluent is sent through the lumens 61 from an inlet at the first end 8 of the FO-unit 6’ toward an outlet at a second end 10. Dialysis concentrate, on the other hand, is sent through the shell 62 from an inlet disposed near the second end 10 toward an outlet disposed near the first end 8. A positive osmotic pressure gradient is thus established between the lumen side to the shell side across the FO-membrane 6c. The FO-unit 6’ comprises a diameter D. Each of the lumens 61 comprises a substantially conical shape, in particular a truncated cone shape, such that the diameter of the lumen decreases from the first end 8 to the second end 10. Likewise, the shell 62 may decrease in diameter from the first end 8 to the second end 10 (which correlates to an increase in diameter from the second end 10 to the first end 8). The diameter D of FO-unit 6’ decreases from the first end 8 to the second end 10 based on the decrease of the diameters of the lumens 61 and / or shell 62 from the first end 8 to the second end 10.

[0077] Fig. 3B shows a flow diagram of lumen 61 and shell 62 of a typical (non-conical) FO-unit 6 and a conical hollow-fiber FO-unit 6’ in line with each other to illustrate the differences. As illustrated in Fig. 3B, the (non-conical) FO-unit 6 lumen 61 has a constant cross-sectional area, while the conical FO-unit 6’ lumen 61 is a truncated cone-shaped hollow-fiber lumen 61 which decreases in cross-sectional area from the first end 8 to the second end 10, as the flow of effluent Qf moves downstream. In a typical FO-unit 6, according to the top embodiment of Fig. 3B (with a constant, non-conical cross sectional area) as the flow of effluent Qf reaches the second end 10, the velocity of the fluid v(Qf) is slowed because of water transport to the draw side 6a and the concentration of the effluent is increased, which have the following effects: the high concentration near the second end will cause the highest levels of fouling to occur near the feed side outlet of the FO-unit 6; the residence time (contact time) of effluent will increase throughout the FO-unit 6 and be highest near the feed side outlet, causing fouling to accumulate; and the shear forces from the flow of effluent Qf will decrease throughout the FO-unit 6 and be lowest near the feed side outlet due to the decrease in velocity of the fluid v(Qf), causing fouling to accumulate.

[0078] Starting from the principle that flow rate is equal to the velocity of a fluid multiplied by the cross-sectional area of the flow (i.e. Q=Av) and recognizing that the volume of fluid on the feed side 6b decreases (and the volume of fluid on the draw side 6a increases) as water permeates from the feed side 6b to the draw side 6a, the conical shape of the conical hollow fiber FO-unit 6’ lumens 61 will cause the velocity of effluent v(Qf) in the lumens 61 to increase the closer the effluent is to the second end 10 (because the cross sectional area has decreased but the flow rate Qf remains constant from the pumps). The term “increase” here is used relative to a non-conical lumen. The increased velocity allows the used effluent to move over the surface of the FO-membrane 6c more quickly, reducing the residence time and increasing the shear forces present, in order to combat the build-up of fouling on the FO-membrane 6c. The lumen 61 may be configured to maintain a constant velocity v(Qf) throughout the lumen or to increase the velocity v(Qf). An increasing velocity v(Qf) may be desirable because of the increasing concentration in the effluent as it passes through the FO-unit 6, 6’. However, the velocity v(Qf) will have a maximum desirable value so that enough water is still able to permeate from the feed side 6b to the draw side 6a.

[0079] As far as the draw side 6a is concerned, an opposite effect has been observed to the feed side 6b in terms of fouling risk factors. For instance, the highest risk of fouling is present near the inlet of the draw side 6a (closer to the second end 10). This is due to the concentration of the dialysis concentrate 4a being highest at the inlet of the draw side 6a. Therefore, an increasing cross-sectional area is desired from the second end 10 to the first end 8 (or more generally from the inlet of the draw side 6a to the outlet). The increasing cross-sectional area along with the increasing flow rate of the fluid being diluted Qd (thus increased fluid volume from osmosis) leads to a decreasing velocity v(Qd) from the draw side 6a inlet to the outlet. The term “decrease” here is used relative to a non-conical shell. Although a decreased velocity v(Qd) is desirable compared to non-conical shells 62, the velocity v(Qd) will have a minimum desirable value so that the fluid does not become overly diluted from prolonged exposure to the FO-unit 6’.

[0080] In alternative embodiments of the system 1 , the dialysis concentrate may flow co- currently with the effluent in the FO-unit (i.e. the draw side inlet may be disposed toward the first end 8 and the draw side outlet may be disposed toward the second end 10). In such a case, the shells 62 of the hollow-fiber FO-units 6’ will instead have an increasing cross-sectional area from the first end 8 to the second end 10 to achieve the same desired effect as above. For example, the shells 62 may have an inverse conical shape to the lumens 61 . This may result in the FO- unit 6’ having a substantially cylindrical or bowtie-shaped overall profile, but comprised of inversely positioned lumens 61 and shell 62 in the interior of the FO- unit 6’.

[0081] Serial, Decreasing Cross-sectional Area FO-units

[0082] Referring now to Figs. 4 and 5, a system 1 which operates in a substantially similar manner to the system 1 of Fig. 1 is illustrated, except that the FO-unit 6 differs in that it is replaced by a FO-arrangement 60 comprising a plurality of FO- units 6 in series and each with a different diameter than the other FO-units 6.

[0083] As described above, it is desirable for the cross-sectional area of the feed side 6b to decrease from the first end 8 (feed side inlet) to the second end 10 (feed side outlet), while it is desirable for the cross-sectional area of the draw side 6a to increase from the second end 10 (draw side inlet) to the first end 8 (draw side outlet). The system 1 of Figs. 4 and 5 accomplishes this by connecting FO-units 6 in series from the first end 8 to the second end 10 to form a FO-arrangement 60, with each consecutive FO-unit 6 having a smaller diameter D (and thereby cross- sectional area of both lumen side and shell side) than the previous FO-unit 6. Each feed side 6b outlet is connected by a fluid line to the next consecutive FO- unit 6 feed side 6b inlet, and each draw side 6a outlet is connected by a fluid line to the next consecutive FO-unit 6 draw side 6a inlet in order to create the serial connection of the plural FO-units 6.

[0084] As effluent is pumped at a constant rate by pump 44 from the first end 8 to the second end 10, it will both lose volume due to the osmotic process moving water from the feed side 6b to the draw side 6a, as well as increase concentration due to the same loss of water. As the dialysis concentrate is pumped at a constant rate by pump 41 from the second end 10 to the first end 8 to be diluted, it will have the highest concentration toward the second end 10 and the volume of the fluid will increase due to the osmotic process. In the system 1 of Figs. 4 and 5, the varying diameter D of the FO-units 6 creates regions of decreased cross-sectional area as the effluent moves through the feed side 6b and regions of increased cross-sectional area as the dialysis concentrate moves through the draw side 6a. As discussed previously, this results in an increased velocity v(Qf) of the effluent compared to a single FO-unit 6 system with a constant diameter D (or substantially constant velocity v(Qf) relative to the FO-membrane 6c), as well as a decreased velocity v(Qd) of the dialysis concentrate compared to a single FO-unit 6 system with a constant diameter D (or substantially constant velocity relative to the FO-membrane 6c). On the feed side 6b, this ensures that the residence time of the higher concentrate effluent is minimized while the shear forces of the fluid are maximized at the locations of highest risk for fouling. On the draw side 6a, this ensures that the shear forces near the second end 10 are sufficient for the highest concentration level in that location while ensuring that the dialysis concentrate is able to be sufficiently diluted by the FO-process.

[0085] The FO-units 6 in the FO-arrangement 60 of Figs. 4 and 5 may be of any type. For instance, the FO-membrane 6c may be a single sheet membrane separating a single feed side 6b region from a single draw side 6a region. In such a case, the diameter D is determined by the desired singular cross-sectional areas of the feed side 6b and draw side 6a regions. More preferably, the FO-units 6 have hollowfiber FO-membranes 6c, which are more efficient at accomplishing the osmotic process. In this case, the diameter D may be varied by either reducing the diameter of each lumen 61 and shell 62 in the FO-unit or by decreasing the number of lumens 61 and the total cross section area of the shell 62 in each FO- unit, or by a combination of the two methods. For instance, the FO-unit 6 closest to the first end 8 may have twelve lumens and the largest cross section shell, while the next FO-unit 6 in the series has nine lumens and a smaller cross section shell, and the final FO-unit 6 in the series has three lumens and the smallest cross section shell.

[0086] Although the embodiment of Figs. 4 and 5 are depicted with a series of three FO- units 6, any plural number of FO-units of varying cross-sectional areas is contemplated as comprising the FO-arrangement 60. Furthermore, the FO-units can have any appropriate cross-sectional profile, such as cylindrical, rectangular, other polygonal (including asymmetrical polygons), or otherwise. Furthermore, it is expressly contemplated that these systems 1 as embodied in Figs. 2 and 4 could be combined to create a system 1 with the functionality of any combination of the embodiments. A system 1 could be implemented where one of the FO-units is a conically-shaped FO-unit 6’ or a plurality of conically-shaped FO- units 6’ of varying ranges of diameter are arranged in series to ensure that the velocity of each fluid relative to the FO-membranes 6c stays as close to constant as possible (i.e. the FO-arrangement 60 of Figs. 4 and 5 has at least one FO-unit 6’ of Figs. 2, 3A, and 3B in the series of FO-units). While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims

26Claims1 . A system (1 ) for preventing fouling in a forward osmosis unit for the preparation of dialysis fluids, the system (1 ) comprising: a fluid path (2) including a forward osmosis- (FO-) arrangement (60) comprising one or more FO- units (6) each including a draw side (6a) and a feed side (6b) separated by a FO- membrane (6c), the FO-unit (6) fluidly connected to the fluid path (2), wherein the FO-unit (6) is configured to receive one or more dialysis concentrate fluids (4a) at the draw side (6a) and to receive a feed fluid at the feed side (6b), wherein water is transported from the feed fluid to the one or more dialysis concentrate fluids (4a) through the FO-membrane (6c) via an osmotic pressure gradient between the draw side (6a) and the feed side (6b), thereby diluting the one or more dialysis concentrate fluids (4a) into a diluted dialysis concentrate fluid, wherein the FO-membrane (6c) comprises a hollow fiber membrane including at least one hollow fiber, each of the at least one hollow fibers comprising a lumen side (61 ) and a shell side (62), wherein each of the FO-units (6) of the FO-arrangement (60) comprises a diameter (D) and wherein the diameter (D) of each FO-unit (6) in the FO- arrangement (60) has a decreasing diameter (D) from a feed side (6b) inlet to a feed side (6b) outlet which causes each FO-unit (6) in the FO-arrangement (60) to have a decreasing cross-sectional area from the feed side (6b) inlet to the feed side (6b) outlet, wherein the decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane (6c).

2. The system of claim 1 , wherein on the feed side (6b) the differing cross- sectional area is achieved by having a decreasing inner diameter of the lumen of each of the at least one hollow fibers (62) along the intended flow direction in at least one of the one or more FO-units (6).

3. The system of claim 2, wherein the decreasing inner diameter is arranged to cause an increase in flow velocity of the feed fluid as it flows through the feed side(6b) of the at least one of the one or more FO-units (6) relative to the flow velocity at a constant diameter.

4. The system of any one of claims 1 to 3, wherein the decreasing inner diameter is arranged to cause an increase in flow velocity of the feed fluid as it flows through the feed side (6b) of the at least one of the one or more FO-units (6).

5. The system of claim 3 or 4, wherein a corresponding increasing inner diameter on the draw side (6a) is arranged to cause the flow velocity of fluid on the draw side (6a) to remain substantially constant relative to the FO-membrane (6c) as it flows through the draw side (6a) of the at least one of the one or more FO-units.

6. The system of any one of claims 1 to 5, wherein the shell side has an increasing cross-sectional area along the intended flow direction.

7. The system of any one of claims 1 to 6, wherein the variable cross-sectional area is configured to mitigate a combination of scaling and fouling of the FO- membrane (6c).

8. The system of any one of claims 1 to 7, wherein the FO-arrangement (60) comprises a plurality of FO-units (6).

9. The system of claim 8, wherein the plurality of FO-units (6) comprises three FO-units (6).

10. The system of claim 8 or 9, wherein the plurality of FO-units (6) is connected in series in the flow path.11 . The system of any one of claims 8 to 10, wherein each of the plurality of FO- units (6) comprises a diameter (D) and wherein the diameter (D) of each FO-unit (6) of the plurality of FO-units (6) has a different, smaller diameter (D) than a FO- unit (6) preceding that FO-unit (6) in the flow direction of the feed side (6b) of the plurality of FO-units (6) which causes each FO-unit (6) of the plurality of FO-units(6) to have a decreasing cross-sectional area in direction of flow of the feed side (6b), and wherein the decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane (6c).

12. The system of claim 11 , wherein decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit (6) as it flows through the feed side (6b) of each of the FO-units (6) relative to the flow velocity at a constant diameter.

13. The system of claim 11 or 12, wherein decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit (6) as it flows through the feed side (6b) of each of the FO-units (6).

14. The system of any one of claims 11 to 13, wherein on the draw side (6a) the diameter of each FO-unit (6) along the intended flow direction of the draw side (6a) increases.

15. The system of any one of claims 11 to 14, wherein decreasing a diameter of each FO-unit (6) comprises having a decreasing inner diameter of the lumen of each of the at least one hollow fibers (62) along the intended flow direction of the feed side (6b) between each FO-unit (6) of the FO-arrangement (60).

16. A system (1 ) for preventing fouling in forward osmosis units for the preparation of dialysis fluids, the system (1 ) comprising: a fluid path (2) including a forward osmosis- (FO-) arrangement (60) comprising a plurality of FO- units (6) each including a draw side (6a) and a feed side (6b) separated by a FO- membrane (6c), the FO-unit (6) fluidly connected to the fluid path (2), wherein the FO-unit (6) is configured to receive one or more dialysis concentrate fluids (4a) at the draw side (6a) and to receive a feed fluid at the feed side (6b), wherein water is transported from the feed fluid to the one or more dialysis concentrate fluids (4a) through the FO-membrane (6c) via an osmotic pressure gradient between the29 draw side (6a) and the feed side (6b), thereby diluting the one or more dialysis concentrate fluids (4a) into a diluted dialysis concentrate fluid, wherein the plurality of FO-units are connected in series in the flow path, wherein each of the FO-units (6) of the FO-arrangement (60) comprises a diameter (D) and wherein the diameter (D) of each FO-unit (6) in the FO- arrangement (60) has a different, smaller diameter (D) than a FO-unit (6) preceding that FO-unit (6) in the flow direction of the feed side (6b) of the FO- arrangement (60) which causes each FO-unit (6) in the FO-arrangement (60) to have a decreasing cross-sectional area in direction of flow of the feed side (6b), wherein the decrease in cross-sectional area is configured to mitigate at least one of scaling and fouling of the FO-membrane (6c).

17. The system of claim 16, wherein decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit (6) as it flows through the feed side (6b) of each of the FO-units (6) relative to the flow velocity at a constant diameter.

18. The system of claim 16 or 17, wherein decreasing the diameter causes an increase in flow velocity of the feed fluid relative to the flow velocity through the previous FO-unit (6) as it flows through the feed side (6b) of each of the FO-units (6).

19. The system of any one of claims 16 to 18, wherein on the draw side (6a) the diameter of each FO-unit (6) along the intended flow direction of the draw side (6a) increases.

20. The system of claim 19, wherein the increasing diameter on the draw side (6a) causes the flow velocity of fluid on the draw side (6a) to remain substantially constant relative to the FO-membrane (6c) as it flows through the draw side (6a) of each FO-unit.21 . The system of any one of claims 16 to 20, wherein decreasing a diameter of each FO-unit (6) comprises having a decreasing inner diameter of the lumen of30 each of the at least one hollow fibers (62) along the intended flow direction of the feed side (6b) between each FO-unit (6) of the FO-arrangement (60).

22. The system of claim 21 , wherein having a decreasing inner diameter of the lumen of each of the at least one hollow fibers (62) causes an increase in flow velocity of the fluid through the lumen side.

23. The system of any one of claims 16 to 22, wherein decreasing a diameter of each FO-unit (6) comprises having a decreasing number of hollow fibers (62) along the intended flow direction of the feed side (6b) of each FO-unit (6).

24. The system of claim 23, wherein having a decreasing number of hollow fibers (62) causes an increase in flow velocity of the fluid through the lumen side.

25. The system of any one of claims 16 to 24, wherein decreasing a diameter of each FO-unit (6) comprises both having a decreasing number of hollow fibers (62) and decreasing an inner diameter of the lumen of each of the at least one hollow fibers (62) along the intended flow direction of the feed side (6b) of each FO-unit (6).

26. The system of any one of claims 16 to 25, wherein the plurality of FO-units (6) comprises three FO-units (6).

27. The system of any one of claims 16 to 26, wherein released sealant and foulant is disposed through a drain line (12).

28. The system of any one of claims 16 to 27, wherein each FO-unit (6) has a variable cross-sectional area along at least a portion of a length of the FO-unit (6).

Images