Patent Application
20250025836
This disclosure relates to a solution treatment method utilizing membrane separation by which method, from a solution containing at least an electrolyte in an amount of 1000 mg/L or more and containing a low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less, the electrolyte and water are recovered while the low-molecular-weight nonelectrolyte molecule is separated.
In recent years, there is an increasing need for selective separation techniques for extracting a useful substance from a solution in which a plurality of components are dissolved or for separating impurities. For example, valuable substances are taken out from salt lake water in which various elements are dissolved, impurities are taken out from a solution containing impurities, and the like are demanded. More specifically, for example, JP H09-290275 A describes a method of removing boron dissolved in fresh water obtained by sea water desalination, raw water used for tap water, or the like using a reverse osmosis membrane and a loose RO membrane. In addition, JP 2003-088863 A describes a method of removing salts while leaving mineral components from sea water or deep ocean water using a reverse osmosis membrane and a nanofiltration (NF) membrane.
Under needs for various selective separation techniques, in recent years, the number of patients on artificial dialysis treatment is increasing worldwide along with the improvement of living standards, and a technique for removing impurities from a waste dialysate discharged via treatment and regenerating the waste dialysate as a dialysate is attracting attention. Common artificial dialysis treatment requires the use of a large quantity of dialysate as large as 90 L to 150 L in a single treatment. Since a dialysate used for artificial dialysis treatment contains waste products such as urea transferred from the blood, the dialysate is basically discarded via a single use. Therefore, there is a problem that a large amount of waste dialysate is generated.
As a method of preparing a dialysate, a method of adding a necessary electrolyte, glucose, or the like to pure water obtained by treating tap water with a reverse osmosis (RO) membrane or the like to produce a dialysate is common. However, in a water shortage area where tap water is supplied intermittently, it may be difficult to secure a necessary amount of tap water, and a reduction in water usage is required. Furthermore, even in an area where water is supplied sufficiently, if water supply is stopped continuously at the time of disaster, dialysate cannot be made from tap water. Therefore, there is an increasing demand for reuse of a waste dialysate by removing waste products such as urea from the waste dialysate and reusing the treated waste dialysate as a dialysate, and several proposals of waste dialysate reuse technique have been made.
For example, it has been proposed to remove impurities, waste products, and electrolytes from a used waste dialysate with an adsorbent. However, depending on the dialysis treatment to be performed, several kilograms of adsorbent is needed for regeneration of a waste dialysate, and a system that suppresses weight and cost as much as possible is desired.
As a method for reducing the amount of an adsorbent, JP 2014-204958 A proposes a system using an adsorbent cartridge in two stages.
JP 2014-530643 A proposes a urea removal system using urease and an ion exchange resin or an inorganic adsorbent.
WO 2020/218571 A proposes a system that removes impurities using a reverse osmosis membrane after removing electrolytes to some extent with an adsorbent or by electrodialysis.
The techniques described in JP 2014-204958 A and JP 2014-530643 A are techniques for separating urea from a used dialysate by decomposing urea into ammonia with urease and further capturing ammonia. Therefore, a plurality of adsorbents is required to completely capture urea and ammonia which is a decomposition product of the urea. Because of a concern that ammonia that has not been captured may remain if the amount of the adsorbent is small, a large amount of the adsorbent is required, which has caused a problem that the cost increases or the weight of a waste dialysate recycling device increases.
The technique described in WO 2020/218571 A is a technique for separating waste products such as urea using a reverse osmosis membrane having a pore diameter of 7.0 Å or less and regenerating a waste dialysate. In this technique, particularly when water is treated with high recovery, it is necessary to remove electrolytes in a waste dialysate as pretreatment to reduce an osmotic pressure difference in a reverse osmosis membrane, and an ion exchange resin or an electrodialyzer is used as the pretreatment. However, when an ion exchange resin is used, there is a problem that the waste of the ion exchange resin used in a single dialysis treatment is discharged. In addition, when an electrodialyzer is used, this is unfavorable in terms of safety because a high voltage is required for desalination and there is a concern of chlorine gas generation.
In view of the above, it could therefore be helpful to provide a solution treatment method in which a solution containing a separation target substance such as a neutral molecule is treated with a combination of a plurality of types of membranes, and a component and water desired to be left as a purified liquid are recovered while the unnecessary separation target substance is separated. More specifically, it could be helpful to provide a solution treatment method which is to be applied to a solution containing at least an electrolyte in an amount of 1000 mg/L or more and containing a low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less, and by which the electrolyte and water are recovered while the low-molecular-weight nonelectrolyte molecule is removed, with combination of a plurality of membranes.
We thus provide:
(1) A solution treatment method includes treating a raw liquid with a first separation membrane 1 and a second separation membrane 2 which each separate a feed liquid into a permeate liquid and a concentrated liquid, and separating the raw liquid into a purified liquid containing water and an electrolyte and a discharge liquid containing a low-molecular-weight nonelectrolyte molecule, wherein
It is noted that the “raw liquid” refers to a solution fed to the most upstream, and a solution fed to each separation membrane is referred to as a “feed liquid” based on the separation membrane.
By performing a specific membrane separation treatment on a solution containing a separation target substance, it is possible to recover a component desired to be recovered while separating the separation target substance. In addition, for example, when the membrane separation treatment is performed on a waste dialysate after being subjected to artificial dialysis treatment, an electrolyte such as a salt and water can be recovered while urea is separated. Therefore, it is possible to reuse the waste dialysate, and it becomes possible to stabilize the supply of dialysate or control the dialysis medical cost.
Hereinafter, embodiments will be described in detail with reference to the drawings, but this disclosure is not limited thereto at all.
The raw liquid is a solution containing at least an electrolyte in an amount of 1000 mg/L or more and containing a low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less (hereinafter sometimes simply referred to as “low-molecular-weight nonelectrolyte molecule”), and examples of such a solution include a waste dialysate containing urea as a low-molecular-weight nonelectrolyte molecule, and sea water or brackish water containing boron as a low-molecular-weight nonelectrolyte molecule.
Examples of the electrolyte include sodium ion, potassium ion, calcium ion, magnesium ion, chloride ion, sulfate ion, and nitrate ion, and examples of the low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less include urea, boron, methanol, and ethanol. These can be confirmed by analyzing components by such a method as ion chromatography or liquid chromatography. As described later, for the first separation membrane 1 to exhibit sufficient selective separation performance, the molecular weight of the low-molecular-weight nonelectrolyte molecule contained in the raw liquid needs to be 70 or less. The concentration of the low-molecular-weight nonelectrolyte molecule in the raw liquid is preferably 10 mg/L or more, more preferably 100 mg/L or more, and still more preferably 500 mg/L or more to sufficiently exhibit the selective separation performance.
In the following, dialysis and waste dialysate will now be described in detail.
Publicly-known methods are applied for dialysis. A dialyzer has a dialysis membrane that permeates toxins such as urea and does not permeate plasma components. While a dialysate is fed to one side of the dialysis membrane, blood is fed to the other side. The composition of the dialysate is known, and includes sodium ion, potassium ion, calcium ion, magnesium ion, glucose, and so on. Urea in the blood is diffused into the dialysate through the dialysis membrane, whereby urea is removed from the blood. Besides urea, potassium ion is one of the substances that need to be removed by dialysis. Since about 90% of potassium ion are excreted from the kidneys, dialysis patients are likely to accumulate potassium ion in the body. Hyperpotassemia can cause cardiac arrest, and controlling blood potassium ion concentration is also an important role in dialysis. As a dialysis system, it is necessary to remove blood potassium ion in a range of 26% to 35% in every dialysis and to control the blood potassium ion concentration.
As the waste dialysate that has passed through the dialyzer, for example, one containing 10000 mg/L of electrolyte including 100 mg/L of potassium ion, and about 600 mg/L of urea, which is a low-molecular-weight nonelectrolyte molecule, is assumed. It is preferable to treat such a waste dialysate as a raw liquid. Then, a solution containing a small amount of an electrolyte or the like is added to the purified liquid to yield a regenerated dialysate. The regenerated dialysate preferably has a urea concentration reduced by 65% or more and potassium ion reduced by 2% or more with respect to the waste dialysate, and to reduce the amount of the electrolyte to be added, the purified liquid preferably has recovered 80% or more of the electrolyte in the waste dialysate.
The raw liquid is treated using at least two types of separation membranes. Each of the two types of separation membranes may be a single layer, or may be a composite membrane including a separation functional layer and a substrate. In addition, in the composite membrane, there may be a porous support layer between the separation functional layer and the substrate.
The two types of membranes, that is, the first separation membrane 1 and the second separation membrane 2 are disposed such that the permeate liquid from the first separation membrane 1 is fed to the second separation membrane 2. To ensure a required electrolyte removal rate (described later), the first separation membrane 1 preferably has a pore diameter of 7 Å or more as measured using a positron annihilation lifetime spectroscopy. On the other hand, to ensure a required electrolyte removal rate (described later), the second separation membrane 2 preferably has a pore diameter of 7 Å or less as measured using a positron annihilation lifetime spectroscopy. When the pore diameter of the second separation membrane 2 is 7 Å or less, the second separation membrane 2 can highly remove such components as urea and electrolytes.
The pore diameter is measured using a positron annihilation lifetime spectroscopy. The positron annihilation lifetime spectroscopy is a method of measuring a time period (on an order of several hundreds of picoseconds to several tens of nanoseconds) from the incidence of a positron on a sample to the annihilation of the positron, and nondestructively evaluating information such as a size of a pore of 0.1 to 10 nm, a number density thereof, and distribution of the size thereof, on the basis of the annihilation lifetime. This measurement method is described in “Experimental Chemistry Lecture 4th Edition”, volume 14, page 485, edited by the Chemical Society of Japan, Maruzen Co., Ltd. (1992).
When the separation membrane is a composite membrane, the average pore radius R of the separation functional layer is determined from the following formula (1) on the basis of a positron annihilation lifetime T described above. Formula (1) shows a relationship in a case where it is assumed that o-Ps (ortho-positronium) is present in a pore having a radius R in an electron layer having a thickness ΔR, and ΔR is empirically determined to be 0.166 nm (details thereof are described in Nakanishi, etc., Journal of Polymer Science, Part B: Polymer Physics, Vol. 27, p. 1419, John Wiley & Sons, Inc. (1989)).
It is necessary that the separation membranes 1 and 2 have the salt removal rate and the urea removal rate specified below, respectively.
First, the salt removal rate and the urea removal rate of the first separation membrane 1 are each defined by the reduction rate of the concentration of the target component in the permeated solution relative to the concentration of the target component in the fed solution when a solution 1 with pH 7 in which 10000 mg/L of sodium chloride (NaCl) and 250 mg/L of urea are mixed is fed at a temperature of 36° C. and a pressure of 1.2 MPa.
The salt removal rate and the urea removal rate of the second separation membrane 2 are each defined by the reduction rate of the concentration of the target component in the permeated solution relative to the concentration of the target component in the fed solution when a solution 2 with pH 7 in which 1000 mg/L of sodium chloride (NaCl) and 700 mg/L of urea are mixed is fed at a temperature of 36° C. and a pressure of 1.8 MPa.
Under the evaluation conditions described above, a flow rate on membrane surface with which the recovery rate, which is the ratio of the amount of the permeate liquid to the amount of the feed liquid to be fed, can be approximated to 0, namely, a sufficient flow rate on membrane surface is required.
The concentration of urea is measured using the urease GLDH method. The urease GLDH method is a method for measuring urea nitrogen (BUN) by performing the following first reaction and second reaction and measuring the change in the amount of a coenzyme (NADPH).
In reaction formula (II), the endogenous ammonia generated via reaction formula (I) is eliminated by the action of α-ketoglutaric acid, reduced nicotinamide adenine dinucleotide phosphate (NADPH), and glutamic acid dehydrogenase (GLDH), and the oxidized nicotinamide adenine dinucleotide phosphate (NADP) generated at this time is reduced by the action of L-isocitrate dehydrogenase (ICDH) via the reaction formula (III) to change to NADPH.
After eliminating the endogenous ammonia via the first reaction, urea is decomposed into ammonia and carbon dioxide by the action of urease. The ammonia and the α-ketoglutaric acid (α-KG) are converted to glutamic acid by the action of GLDH, while NADPH is converted to NADP. NADPH has an absorption maximum at 340 nm, and the rate of decrease in absorbance is measured to determine a urea nitrogen value. Incidentally, at this time, the reaction formula (III) of the first reaction is stopped by the action of a chelating agent added to a second reagent.
The sodium chloride removal rate of the first separation membrane 1 measured under the evaluation conditions of the first separation membrane 1 is 90% or more. In addition, to preferentially remove 65% or more of urea as a whole system, the urea removal rate of the second separation membrane 2 measured under the evaluation conditions of the second separation membrane 2 is 85% or more, and the difference between the urea removal rate of the second separation membrane 2 measured under the evaluation conditions of the second separation membrane 2 and the urea removal rate of the first separation membrane 1 measured under the evaluation conditions of the first separation membrane 1 is 40% point or more (preferably 60% or more). By satisfying these removal rates, sufficient performance can be exhibited as a whole process.
When the process is combined with a dialyzer as shown in
The type of the first separation membrane 1 is not particularly limited as long as the conditions described above are satisfied, but in general, a nanofiltration membrane or a low-pressure RO membrane is suitably used.
The type of the second separation membrane 2 is not particularly limited as long as the conditions described above are satisfied, but a dense RO membrane to be used for sea water desalination or the like is suitably used.
The separation functional layer may be a layer having both a separation function and a support function, or may have only a separation function. The “separation functional layer” refers to a layer having at least a separation function.
When the separation functional layer has both a separation function and a support function, a layer containing a polymer selected from the group consisting of cellulose, polyvinylidene fluoride, polyethersulfone, and polysulfone as a main component is suitably applied as the separation functional layer.
On the other hand, only to exhibit a separation function, a crosslinked polymer is preferably used because this is easy to control the pore diameter and is superior in durability. In particular, a polyamide separation functional layer obtained via polycondensation of a polyfunctional amine and a polyfunctional acid halide, an organic-inorganic hybrid functional layer, etc. are suitably used from the viewpoint that they are superior in performance to separate components in a feed liquid. These separation functional layers can be formed via polycondensation of monomers on a porous support layer separately prepared. For example, the polyamide separation functional layer is obtained by applying an aqueous polyfunctional amine solution to a porous support layer, removing an excess aqueous polyfunctional amine solution with an air knife or the like, and then applying an organic solvent solution containing a polyfunctional acid halide to cause interfacial polycondensation.
The porous support layer is a layer that supports the separation functional layer, and can also be referred to as a porous resin layer when the material of the layer is a resin.
The material to be used for the porous support layer and the form of the layer are not particularly limited, but, for example, the layer may be formed of a porous resin on a substrate. The composition of the porous support layer is not particularly limited, but is preferably formed of a thermoplastic resin. The thermoplastic resin refers to a resin that is made of a chain polymer substance and exhibits a property of being deformed or flowed by an external force when heated. Examples of the thermoplastic resin include homopolymers or copolymers such as polysulfone, polyether sulfone, polyamide, polyester, cellulose-based polymer, vinyl polymer, polyphenylene sulfide, polyphenylene sulfide sulfone, polyphenylene sulfone, and polyphenylene oxide, and these may be used either singly or as a blend. As the cellulose-based polymer, cellulose acetate, cellulose nitrate, etc. can be used, and as the vinyl polymer, polyethylene, polypropylene, polyvinyl chloride, polyacrylonitrile, an acrylonitrile-styrene copolymer, etc. can be used. Among them, it is preferable to use polysulfone, which has high chemical, mechanical, and thermal stability and whose pore diameter is easily controlled.
The porous support layer can be produced, for example, by casting a solution of the polysulfone in N,N-dimethylformamide in a certain thickness onto a substrate (for example, a densely woven polyester nonwoven fabric) to be described later, and then wet-solidifying the solution in water.
In terms of the strength, dimensional stability, and so on of a separation membrane, the separation membrane may have a substrate. As the substrate, a fibrous substrate is preferably used in terms of strength and fluid permeability. In particular, a continuous fiber nonwoven fabric and a short fiber nonwoven fabric are preferable.
The first separation membrane 1 and the second separation membrane 2 can be configured in the form of a spiral-type element in which a flat membrane-like membrane is wound around a perforated central tube, a plate-and-frame-type element in which flat membranes stretched on both sides of a plate-type support plate are laminated at regular intervals with a spacer interposed therebetween, a tubular-type element using a tubular membrane, or a hollow fiber membrane element in which hollow fiber membranes are bundled and housed in a case. Further, one or more of these elements are connected in series in a pressure-resistant container and stored to form a separation membrane module. The form of the element may be any form, but it is preferable to use a spiral-type separation membrane element as illustrated in
The flat membrane-like separation membrane has a permeate-side face and a feed-side face. A combination of a separation membrane leaf arranged with its permeate-side face facing itself with a permeate-side channel material interposed and a feed-side channel material disposed on one side of the feed-side face of the separation membrane leaf is call a separation membrane body. A permeate-side channel is formed such that a permeated fluid flows into a perforated central tube, and a space between the permeate-side faces of the separation membrane is opened at only one side lying inside in a winding direction and sealed at other three sides. The feed-side channel is formed between the feed-side faces of the separation membrane, and separation membrane elements of various types, such as I-type, L-type, reverse L-type, and U-turn-type, are formed as described later by a method of sealing a feed-side channel material.
In a separation membrane element (201) called I-type, as illustrated in
In addition, the feed-side channel material preferably has a form that makes the flow of the feed liquid turbulent to inhibit concentration polarization of the feed liquid. Specifically, the feed-side channel material may be a member having a continuous shape such as a net or a perforated film provided with protrusions, or may be a plurality of discontinuous members directly disposed on the separation membrane. The discontinuous members are provided such that their projected area ratio with respect to the separation membrane is more than 0 and less than 1. The feed-side channel material may be separable from the separation membrane or may be fixed to the separation membrane.
The material of the feed-side channel material is not particularly limited, and may be either the same as or different from the material of the separation membrane.
When the thickness of the feed-side channel material is large, the pressure loss decreases, but when an element is formed including such a feed-side channel material, the membrane area that can be packed in a pressure vessel decreases. When the thickness thereof is small, the pressure loss in the channel increases, and the separation performance or the permeation performance deteriorates. Considering the balance among performances or the operation cost, the thickness of the feed-side channel material is preferably 200 to 1000 μm, and more preferably 300 to 900 μm. The thickness of the feed-side channel material can be directly measured with a commercially available thickness measuring instrument, or can be measured by analyzing an image taken using a microscope.
When the feed-side channel material is a net, the net is formed of a plurality of yarns. The plurality of yarns intersect with each other at intersection, and the thickness is the largest at the intersection portion. The diameters of the yarns constituting the net may be constant in a length direction of the yarns, or may be uniformly increased or decreased in the length direction, or may be a form in which the increase and decrease in diameter are repeated. When the diameters of the yarns are repeatedly increased and decreased in the length direction, it is preferable that a plurality of yarns intersect with each other at a position where the diameter of the yarn is the largest. By intersecting a plurality of yarns at the point where the diameter of the yarn is the largest, the pressure loss in the feed-side channel can be reduced.
The diameters of the plurality of intersecting yarns may be either the same or different. When the diameters of the plurality of intersecting yarns are different, if the thickness is constant, yarns with a small diameter have a large effect of reducing the pressure loss, and yarns with a large diameter have a large turbulence effect of disturbing the flow.
From the balance between the pressure loss and the turbulence effect, as for the diameter of the cross section of the yarn constituting the net, the ratio of (the diameter of the smallest part)/(the diameter of the largest part) is preferably 0.1 or more and 0.7 or less, and more preferably 0.3 or more and 0.6 or less.
If the inclination angle of the yarns constituting the net is parallel to the flowing direction of the feed liquid, the pressure loss can be lowered, but the concentration polarization reduction effect decreases. On the other hand, if the inclination angle is near a direction perpendicular to the flowing direction, the pressure loss increases, but the concentration polarization reduction effect can be increased. From the balance between the pressure loss and the concentration polarization reduction effect, the inclination angle of the yarns is preferably −60° or more and 60° or less with respect to an average flow angle of the feed liquid. The average flow angle is an average value of flow angles in one separation membrane body.
The larger the interval between the intersections at which plurality of yarns intersect with each other, the smaller the pressure loss; whereas the smaller the interval, the larger the pressure loss. In terms of their balance, the intersection interval is preferably from 1.0 mm or more and 10 mm or less, more preferably 1.1 mm or more and 8 mm or less, and still more preferably 1.2 mm or more and 5 mm or less.
The cross-sectional form of the yarns constituting the net is not particularly limited, and an ellipse, a circle, a triangle, a square, an irregular shape, or the like can be employed. It is preferable that the area of a portion where the net and the separating membrane surface are in contact is small because deterioration of the separation membrane performance due to rubbing between the separating membrane surface and the net can be inhibited, a flow dead zone can be reduced, and the concentration polarization can be inhibited. The projected area ratio of the portion where the net and the separating membrane surface are in contact with respect to the separation membrane is preferably 0.01 or more and 0.25 or less, and more preferably 0.02 or more and 0.2 or less.
The material of the yarns constituting the net is not particularly limited as long as it can maintain the rigidity as the feed-side channel material and does not damage the surface of the separation membrane. The material may be either the same as or be different from that of the separation membrane, and polyethylene, polypropylene, polylactic acid, an ethylene-vinyl acetate copolymer, polyester, polyurethane, a thermosetting elastomer, etc. can be suitably used.
The permeate-side channel material is disposed such that this material is sandwiched between the permeate-side faces of two facing separation membranes, and plays a role of forming a permeate-side channel that guides a permeate liquid that has permeated the separation membranes to the holes of the perforated central tube.
The permeate-side channel material preferably has a cross-sectional area ratio of 0.30 to 0.75, and more preferably 0.40 to 0.60 from the viewpoint of reducing the flow resistance of the permeate-side channel, inhibiting sinking of the separation membrane into the permeated liquid channel even under pressure filtration, and stably forming the channel. The type of the permeate-side channel material is not limited, and a weft knitted fabric such as tricot, a sheet in which protrusions are arranged on a porous sheet such as a nonwoven fabric, an irregularity-processed sheet obtained by subjecting a film or a nonwoven fabric to irregularity processing, or the like can be used.
The cross-sectional area ratio is defined, in a cross section obtained by cutting the permeate-side channel material such that the cutting line passes through a protrusion of the permeate-side channel material along the direction parallel to the longitudinal direction of the perforated central tube of the separation membrane element, by the ratio of the cross-sectional area of the permeate-side channel material occupied between the center of a certain protrusion and the center of an adjacent protrusion to the product of the distance between the center of the certain protrusion and the center of the adjacent protrusion and the height (thickness) of the permeate-side channel material. The cross-sectional area ratio can be calculated as an average value at 30 arbitrary positions using, for example, a high-precision shape analysis system KS-1100 manufactured by KEYENCE Corp.
When the thickness of the permeate-side channel material is large, the pressure loss can be reduced, but the membrane area that can be packed in the vessel of the separation membrane element decreases. When the thickness of the permeate-side channel material is small, the membrane area that can be packed in the separation membrane element increases, but the pressure loss increases. In terms of their balance, the thickness of the permeate-side channel material is preferably 0.1 mm to 0.5 mm, and more preferably 0.2 mm to 0.4 mm.
The thickness of the permeate-side channel material can be directly measured with a commercially available thickness measuring instrument.
The material of the permeate-side channel material may be any material as long as it can be easily wound around the perforated central tube, and the compressive elasticity modulus of the permeate-side channel is preferably 0.1 to 5 GPa. When the compressive elasticity modulus is within this range, the permeate-side channel material can be easily wound around the perforated central tube. Specifically, polyester, polyethylene, polypropylene, and the like are suitably used.
The compressive elasticity modulus of the permeate-side channel material can be measured by performing a compression test using a precision universal testing machine and preparing a stress-strain curve.
The perforated central tube is just required to be configured such that a permeate liquid flows therein, and the material and form thereof are not particularly limited. When the diameter of the perforated central tube is large, the membrane area that can be packed in the separation membrane element is reduced, whereas when the diameter is small, the flow resistance when a permeate liquid flows inside the perforated central tube increases. The diameter of the perforated central tube is appropriately designed according to the flow rate of the permeate liquid, and is preferably 10 to 50 mm, and more preferably 15 to 40 mm. As the perforated central tube, for example, a cylindrical member having a side face provided with a plurality of holes is used.
As illustrated in
As illustrated in
A separation membrane element (222) called reverse L-type is a separation membrane element having a structure in which a liquid flow is inverted from that of the L-type as illustrated in
The L-type or reverse L-type separation membrane element that performs an operation in which the recovery rate, which is the ratio of the permeate liquid to the feed liquid, is constant has the following advantages. That is, in the separation membrane element in which the ratio L/W of the length W of the separation membrane body in the longitudinal direction of the perforated central tube to the length L in the direction perpendicular to the longitudinal direction of the perforated central tube is 2.5 or more, the flow rate of the feed liquid passing through the feed-side channel in the separation membrane element is increased in comparison with the I-type separation membrane element with the same ratio L/W. Therefore, the L-type and reverse L-type separation membrane elements having an L/W of 2.5 or more can inhibit concentration polarization of a membrane surface even in a high recovery rate operation, and can be operated more stably. The channel materials to be used for the L-type and reverse L-type separation membrane elements may be the same as the channel material to be used for the I-type separation membrane element.
To further increase the flow rate of the feed liquid, a U-turn-type separation membrane element, which is a separation membrane element having two types of separation membrane bodies in one separation membrane element, may be used. One example of the U-turn-type separation membrane element is an I-type-reverse L-type separation membrane element (224) in which an I-type separation membrane body and a reverse L-type separation membrane body are combined as illustrated in
The L-type, reverse L-type, and U-turn-type separation membrane elements may be collectively called “high flow rate type separation membrane elements”.
Next, a separation membrane module (216) in which an I-type separation membrane element (201) and a reverse L-type separation membrane element (222) are sealed in series in a pressure vessel (220) is illustrated in
First, the recovery rate of an electrolyte such as a salt, the urea removal rate, and the potassium ion removal rate in the entire process are determined from the following formulas.
The numbers in parentheses in the above formulas mean the reference signs in
As in the formulas given above, the electrolyte recovery rate, the urea removal rate, and the potassium ion removal rate as the entire process are defined not by a removal rate defined by a concentration but by the amount of substance including a flow rate ratio. This is because, for example, when the present membrane process is used for a regeneration treatment of a waste dialysate, the amount of water and the electrolyte concentration are adjusted by adding an adjustment solution in the subsequent stage to form a regenerated dialysate, so that the final concentration of the regenerated dialysate differs from the final concentration of the purified liquid.
In the solution treatment method, a raw liquid is separated into a purified liquid containing water and an electrolyte and a discharge liquid containing a low-molecular-weight nonelectrolyte molecule using separation membranes 1 and 2 having the characteristics described above. At this time, the permeate liquid from the first separation membrane 1 is fed to the second separation membrane 2 and subjected to a separation treatment, and (i) a part of the permeate liquid from the second separation membrane 2 is mixed with a raw liquid and then fed to the first separation membrane 1, and the whole or a part of the remaining permeate liquid from the second separation membrane 2 is mixed with the concentrated liquid from the first separation membrane 1 to afford a purified liquid, or (ii) the whole of the permeate liquid from the second separation membrane 2 is mixed with the raw liquid and then fed to the first separation membrane 1, and the concentrated liquid from the first separation membrane 1 is obtained as a purified liquid.
That is, a solution obtained by mixing the permeate liquid from the second separation membrane 2 and the raw liquid is pressurized and treated with the first separation membrane 1 having a sodium chloride removal rate of 90% or more and having a urea removal rate lower than that of the second separation membrane 2 by 40% point or more, thereby affording the permeate liquid from the first separation membrane 1 and the concentrated liquid from the first separation membrane 1. At this time, the recovery rate, which is the ratio of the amount of the solution fed to the first separation membrane 1 and the amount of the solution having permeated the first separation membrane 1, is preferably 75% or more. By adopting such a recovery rate, the selective separation performance of the first separation membrane 1 can be further exhibited. Since the first separation membrane 1 has removal rates as described above, the permeate liquid from the first separation membrane 1 contains a large amount of low-molecular-weight nonelectrolyte molecule such as urea, and does not contain much electrolyte. On the other hand, the concentrated liquid from the first separation membrane 1 contains both an electrolyte and a low-molecular-weight nonelectrolyte molecule, but since the first separation membrane 1 has a higher electrolyte removal rate than the urea removal rate, the concentrated liquid contains the electrolyte more than the low-molecular-weight nonelectrolyte molecule.
Further, the permeate liquid from the first separation membrane 1 is treated with the second separation membrane 2 having a urea removal rate higher than that of the first separation membrane 1 by 40% point or more to afford a concentrated liquid from the second separation membrane 2 and a permeate liquid from the second separation membrane 2. At this time, the recovery rate, which is the ratio of the amount of the solution fed to the second separation membrane 2 and the amount of the solution having permeated the first separation membrane 1, is set according to the water recovery rate of the entire process, but since the permeate liquid from the second separation membrane 2 is utilized for circulation to the feed liquid to the first separation membrane 1, the recovery rate of the second separation membrane 2 is preferably 50% or more. Since the second separation membrane 2 exhibits the removal rates as described above, the permeate liquid from the second separation membrane 2 contains an electrolyte and a low-molecular-weight nonelectrolyte molecule such as urea both only in a trace amount, and the concentrated liquid from the second separation membrane 2 contains a low-molecular-weight nonelectrolyte molecule such as urea in a large amount.
Finally, the concentrated liquid from the first separation membrane 1 is taken out while being mixed with the permeate liquid from the second separation membrane 2, as necessary, to afford a purified liquid containing only a low-molecular-weight nonelectrolyte molecule such as urea selectively separated.
Embodiments of the solution treatment method are shown in
In the solution treatment method shown in
The solution treatment method shown in
To perform a waste dialysate treatment, it is effective to incorporate a solution treatment unit in which lines are constituted in a flow as described in (4) above and valves and pumps are arranged at appropriate positions as a waste dialysate treatment unit into a dialysis system together with a dialyzer. That is, it is effective to combine a waste dialysate treatment unit (301) with a dialyzer (302) as shown in
In addition, it is also possible to combine the waste dialysate treatment unit of the present application with a tank, treat a waste dialysate offline, thereby adjust the electrolyte concentration, and then subject the waste dialysate to dialysis treatment.
Our method will be described in more detail with reference to Examples, but is not limited by the Examples at all.
For PA membrane A, PA membrane B, PA membrane C, PA membrane F, which are polyamide membranes, and CA membrane, which is a cellulose acetate membrane, the NaCl removal rate and the urea removal rate were measured at a temperature of 36° C. and a pressure of 1.2 MPa for a solution 1 at pH 7 in which 10000 mg/L of sodium chloride (NaCl) and 250 mg/L of urea were mixed.
For PA membrane D, PA membrane E, and PA membrane G, which are polyamide membranes, the NaCl removal rate and the urea removal rate were measured at a temperature of 36° C. and a pressure of 1.8 MPa for a solution 2 at pH 7 in which 1000 mg/L of sodium chloride (NaCl) and 700 mg/L of urea were mixed.
It is noted that the removal rates referred to here are each defined by a reduction rate of the component concentration in the permeated solution relative to the component concentration in the feed liquid for each membrane.
Positron annihilation lifetime measurement of a separation functional layer in each example was performed using a positron beam method as follows. Specifically, the separation functional layer was dried at room temperature under reduced pressure, and cut into a 1.5 cm×1.5 cm square, and thus an inspection sample was prepared. In a positron annihilation lifetime measurement device for thin membrane equipped with a positron beam generator (this device is described in detail in, for example, Radiation Physics and Chemistry, 58, 603, Pergamon (2000)), the inspection sample was measured at a total count of 5,000,000 by a scintillation counter made of barium difluoride through using a photomultiplier tube at a beam intensity of 1 keV at room temperature in vacuum, and analyzed by POSI TRONFIT. An average pore diameter was calculated on the basis of an average positron annihilation lifetime T of a fourth component obtained by the analysis.
A 16.0% by weight DMF (dimethylformamide) solution of polysulfone (PSf) was cast at room temperature (25° C.) on a polyester nonwoven fabric (air permeability: 2.0 cc/cm2/sec) to a thickness of 200 μm, and immediately immersed in pure water and allowed to stand for 5 minutes, and thus a support membrane was prepared.
A 1.5% by weight aqueous solution of m-phenylenediamine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excess aqueous solution from a surface of the support membrane, then applied with a 25° C. decane solution containing 0.065% by weight of trimesoyl chloride (TMC) in a booth maintained at 25° C. such that the surface was completely wetted, and was allowed to stand for 60 seconds. Thus, a reverse osmosis membrane was obtained. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 97% and a urea removal rate of 18%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 7.2 Å.
A 1.8% by weight aqueous solution of m-phenylenediamine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excess aqueous solution from a surface of the support membrane, then applied with a 25° C. decane solution containing 0.06% by weight of trimesoyl chloride (TMC) in a booth maintained at 25° C. such that the surface was completely wetted, and was allowed to stand for 60 seconds. Thus, a reverse osmosis membrane was obtained. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 98% and a urea removal rate of 25%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 7.0 Å.
A 0.2% by weight aqueous solution of piperazine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excess aqueous solution from a surface of the support membrane, then applied with a 45° C. decane solution containing 0.17% by weight of trimesoyl chloride (TMC) in a booth maintained at 45° C. such that the surface was completely wetted, and was allowed to stand for 1 minute. Next, to remove excess solution from the membrane, the membrane was held vertically for 1 minute to perform liquid draining, and then the membrane was dried by blowing a gas at 25° C. using a blower. After the drying, the membrane was immediately washed with water and stored at room temperature, affording a nanofiltration membrane. The performance of the obtained nanofiltration membrane was an NaCl removal rate of 93.7% and a urea removal rate of 16%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 9.0 Å.
A 6.0% by weight aqueous solution of m-phenylenediamine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excessive aqueous solution from a surface of the support membrane, then applied with a 45° C. decane solution containing 0.17% by weight of trimesoyl chloride (TMC) in a booth maintained at 45° C. such that the surface was completely wetted, and was allowed to stand for 10 seconds. The support membrane was placed in an oven at 140° C. and heated for 30 seconds while water vapor at 100° C. was supplied from a nozzle provided on a back surface side of the membrane, affording a reverse osmosis membrane. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 99.6% and a urea removal rate of 90%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 5.1 Å.
A 5.5% by weight aqueous solution of m-phenylenediamine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excessive aqueous solution from a surface of the support membrane, then applied with a 45° C. decane solution containing 0.15% by weight of trimesoyl chloride (TMC) in a booth maintained at 45° C. such that the surface was completely wetted, and was allowed to stand for 10 seconds. The support membrane was placed in an oven at 140° C. and heated for 30 seconds while water vapor at 100° C. was supplied from a nozzle provided on a back surface side of the membrane, affording a reverse osmosis membrane. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 99.1% and a urea removal rate of 85%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 6.0 Å.
A 2.0% by weight aqueous solution of m-phenylenediamine was prepared. The support membrane obtained by the above operation was immersed in the above aqueous solution for 2 minutes, the support membrane was slowly pulled up in a vertical direction, blown with nitrogen from an air nozzle to remove excessive aqueous solution from a surface of the support membrane, then applied with a 25° C. decane solution containing 0.12% by weight of trimesoyl chloride (TMC) in a booth maintained at 25° C. such that the surface was completely wetted, and was allowed to stand for 40 seconds. The support membrane was placed in an oven at 140° C. and heated for 30 seconds while water vapor at 100° C. was supplied from a nozzle provided on a back surface side of the membrane, affording a reverse osmosis membrane. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 99.0% and a urea removal rate of 58%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 6.8 Å.
An aqueous solution containing 1.8% by weight of m-phenylenediamine and 4.5% by weight of F-caprolactam was prepared. The aqueous solution was applied to the support membrane obtained by the above operation, and then blown with nitrogen from an air nozzle to remove excessive aqueous solution from a surface of the support membrane. Then, a 25° C. n-decane solution containing 0.06% by weight of trimesoyl chloride was applied to the support membrane such that the surface was completely wetted. Thereafter, excess solution was removed from the membrane by air blowing, the membrane was washed with hot water at 80° C., and then subjected to draining by air blowing, affording a reverse osmosis membrane. The performance of the obtained reverse osmosis membrane was an NaCl removal rate of 98.5% and a urea removal rate of 75%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 7.2 Å.
A cast solution prepared by mixing 25% by weight of cellulose acetate, 45% by weight of acetone, and 30% by weight of formamide was cast on the support membrane obtained by the above operation, and was evaporated for 2 minutes, and then the support membrane was immersed in iced water. Next, the membrane was immersed in hot water at 90° C., affording a nanofiltration membrane. The performance of the obtained nanofiltration membrane was an NaCl removal rate of 93.0% and a urea removal rate of 15%. The pore diameter measured using a positron annihilation lifetime spectroscopy was 10 Å.
A separation membrane was cut into six pieces, and each piece was folded with the feed side facing inward such that the inner peripheral end was a fold. Then, a net (thickness: 0.8 mm, pitch of fibers constituting the net (hereinafter, briefly referred to as pitch): 5 mm×5 mm, fiber diameter: 380 μm, projected area ratio of a portion where the net and the separation membrane surface were in contact with respect to the separation membrane (hereinafter, briefly referred to as projected area ratio): 0.15) was used as a feed-side channel material, and was disposed such that the inclination angle of yarns constituting the net was 450 with respect to the winding direction. As a permeate-side channel material, tricot having a uniform thickness (thickness: 280 μm) was prepared, and cut into six pieces. From these pieces, six separation membrane bodies having a length of 850 mm and a width of 930 mm, 465 mm, or 230 mm were prepared. The permeate-side channel material was disposed on the permeate-side face of the separation membrane, an adhesive was applied to the permeate-side channel material such that the inner peripheral end was opened, and the channel material was spirally wound around a perforated central tube (length: 1020 mm, 510 mm, or 280 mm, diameter: 30 mm, number of holes: 40 or 20 holes×1 linear row) made of ABS (acrylonitrile-butadiene-styrene). After winding, a film was wound around the outer periphery and fixed with a tape, followed by edge cutting, attachment of an end plate, and filament winding, and thus an I-type separation membrane element having an effective membrane area of 8 m2, 4 m2, or 2 m2 was prepared.
A separation membrane was cut into six pieces, and each piece was folded with the feed side facing inward such that the inner peripheral end was a fold. Then, a net (thickness: 0.8 mm, pitch: 5 mm×5 mm, fiber diameter: 380 μm, projected area ratio: 0.15) was used as a feed-side channel material, and was disposed such that the inclination angle of yarns constituting the net was 45° with respect to the winding direction. As a permeate-side channel material, tricot having a uniform thickness (thickness: 280 μm) was prepared, and cut into six pieces. From these pieces, six separation membrane bodies having a length of 850 mm and a width of 230 mm were prepared. The ratio L/W of the length W of each of the separation membrane bodies in the longitudinal direction of the perforated central tube to the length L in a direction perpendicular to the longitudinal direction of the perforated central tube is 3.7. The permeate-side channel material was disposed on the permeate-side face of the separation membrane, an adhesive was applied to the permeate-side channel material such that the inner peripheral end was opened, and the channel material was spirally wound around a perforated central tube (length: 280 mm, diameter: 30 mm, number of holes: 12 holes×1 linear row) made of ABS (acrylonitrile-butadiene-styrene). After the winding, a film with holes was wound around the outer periphery and fixed with a tape, and then edge cutting was performed. Thereafter, an adhesive was applied to one end face in the longitudinal direction of the perforated central tube to perform sealing. Further, an adhesive was applied such that 20% of the inner peripheral portion of the end face opposite from the sealed side was opened, and thus L-type and reverse L-type separation membrane elements having an effective membrane area of 2 m2 were prepared. In the L-type and reverse L-type separation membrane elements, since the above-described ratio L/W is 3.7, the length L is ¼ of the length of the I-type separation membrane element having an effective membrane area of 8 m2, and accordingly, the effective membrane area is also ¼ of that of the I-type separation membrane element having an effective membrane area of 8 m2. In addition, this separation membrane element serves as an L-type separation membrane element when a feed liquid is fed from the outer peripheral part, and serves as a reverse L-type separation membrane element when a feed liquid is fed from one end face of the separation membrane element.
A separation membrane was cut into six pieces, and each piece was folded with the feed side facing inward such that the inner peripheral end was a fold. Then, a net (thickness: 0.8 mm, pitch: 5 mm×5 mm, fiber diameter: 380 μm, projected area ratio: 0.15) was used as a feed-side channel material, and was disposed such that the inclination angle of yarns constituting the net was 450 with respect to the winding direction. Among these separation membranes sandwiching the feed-side channel material, the outer peripheral ends of the feed-side channel materials of three separation membranes were bonded such that an I-type separation membrane body illustrated in
In Examples 1 to 10 and Comparative Examples 1 to 3, an artificial waste dialysate was used as a raw liquid. The components in the raw liquid were electrolyte: approximately 10000 mg/L (including 100 mg/L of potassium ion), and urea (molecular weight 60.06): 630 mg/L.
In Example 11, brackish water having a high boron concentration was used as a raw liquid. The components in the raw liquid were electrolyte: 1500 mg/L, and boron (in the form of boric acid) (molecular weight: 61.83): 1.5 mg/L.
The electrolyte recovery rate, the urea removal rate, and the boron removal rate in the entire process were determined from the following formulas.
The numbers in parentheses in the above formulas mean the reference signs in
Furthermore, the maximum operation pressure and the maximum pump flow rate in each module were also measured with a flow meter and a pressure gauge.
A waste dialysate was used as the raw liquid. The PA membrane A obtained by the method as described above was use as the first separation membrane 1, and the PA membrane D obtained by the method as described above was used as the second separation membrane 2. As each of both separation membrane elements, one I-type separation membrane element having a width of 930 mm was prepared by the method described above.
To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. One separation membrane module (1) and one separation membrane module (2) were prepared in the same manner as in Example 1 except that the width of the I-type separation membrane elements was 465 mm. Using these, pumps and valves were prepared, and piping was connected as shown in
A waste dialysate was used as the raw liquid. Separation membrane modules were prepared in the same manner as in Example 2 except that the separation membrane used for the separation membrane module (1) was the CA membrane. To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. Separation membrane modules were prepared in the same manner as in Example 1 except that the separation membrane used for the separation membrane module (2) was the PA membrane E. To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. Separation membrane modules were prepared in the same manner as in Example 1 except that the separation membrane used for the separation membrane module (1) was the PA membrane B. To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. Separation membrane modules were prepared in the same manner as in Example 2 except that the separation membrane used for the separation membrane module (1) was the PA membrane C. To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. Separation membrane modules the same as those in Example 2 were prepared. To each of the separation membrane elements prepared were attached an end plate and a brine seal, and the separation membrane elements were each placed in a pressure vessel, affording one separation membrane module (1) and one separation membrane module (2). As shown in
A waste dialysate was used as the raw liquid. Using the same separation membranes as those in Example 1, one I-type separation membrane element and one reverse L-type element each having a width of 230 mm were prepared as the separation membrane modules (1) and (2). To each of the separation membrane elements were attached an end plate and a brine seal, and the separation membrane elements were enclosed in a pressure vessel such that the reverse L-type separation membrane element was positioned at the subsequent stage as illustrated in
A waste dialysate was used as the raw liquid. Using the same separation membranes as those in Example 1, one I-type separation membrane element and one L-type element each having a width of 230 mm were prepared as the separation membrane modules (1) and (2). To each of the separation membrane elements were attached an end plate and a brine seal, and the separation membrane elements were enclosed in a pressure vessel such that the L-type separation membrane element was positioned at the subsequent stage as illustrated in
A waste dialysate was used as the raw liquid. Using the same separation membranes as those in Example 1, one I-type separation membrane element and one U-turn-type element each having a width of 230 mm were prepared as the separation membrane modules (1) and (2). To each of the separation membrane elements were attached an end plate and a brine seal, and the separation membrane elements were enclosed in a pressure vessel such that the U-turn-type separation membrane element was positioned at the subsequent stage as illustrated in
Brackish water having a high boron concentration was used as the raw liquid. One separation membrane module (1) and one separation membrane module (2) were prepared in the same manner as in Example 2. Using these, pumps and valves were prepared, and piping was connected as shown in
Separation membrane modules the same as those in Example 2 were prepared. As shown in
Separation membrane modules were prepared in the same manner as in Example 1 except that the separation membrane used for the separation membrane module (1) was the PA membrane F. As shown in
Separation membrane modules were prepared in the same manner as in Example 1 except that the separation membrane used for the separation membrane module (2) was the PA membrane G. As shown in
This disclosure is suitably used for regeneration treatment of waste dialysate, removal of boron from brackish water, and so on.
This application is a US national stage filing under 35 U.S.C. § 371 of International Application No. PCT/JP2021/043741, filed Nov. 30, 2021, which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/043741 | 11/30/2021 | WO |
