Method for producing purified water and method for reducing silica scale risk

Abstract

The present invention provides: a method for producing purified water that can reduce operating costs as much as possible while suppressing silica scale risk; and a method for reducing silica scale risk that uses the method for producing purified water. The present invention relates to a method for producing purified water that includes a membrane separation step in which: feed water 17 that includes soluble silica at a concentration of CSi (ppm) is passed through a UF membrane 11 and an NF membrane 21; and water 18 that has passed through the UF membrane and water 28 that has passed through the NF membrane are mixed at a mixing ratio Rmix (—) that satisfies formula (1). Formula (1): 50 ≤ [CSi / (1 + Rmix)] × [((100 − RNF / 100) × Rmix + 1] < 80. (In the formula, CSi represents the concentration (ppm) of the soluble silica, RNF represents the soluble silica removal rate (%) of the NF membrane, and Rmix represents the mixing ratio (FNF / FUF) (—) of the mixing quantity FNF of the water that has passed through the NF membrane to the mixing quantity FUF of the water that has passed through the UF membrane.)

Description

Method for producing purified water and method for reducing the risk of silica scale

[0001] The present invention relates to a method for producing purified water, which includes a membrane separation step of separating feed water containing soluble silica using a UF (ultrafiltration) membrane and an NF (nanofiltration) membrane, and a method for reducing the risk of silica scale formation using such a membrane separation method.

[0002] Separation techniques using separation membranes are widely used in the fields of water purification and wastewater treatment because of their low energy load, and methods that use a combination of separation membranes with different filtration performance are also widely adopted (e.g., Patent Document 1). In these cases, the quality of the raw water to be treated varies, and the raw water may contain hardness components such as soluble silica and calcium, and there is a growing demand for removing these to reduce the risk of scale formation and to soften the water.

[0003] As an apparatus for membrane separation of feed water containing such soluble silica, for example, Patent Document 2 proposes a water quality modification apparatus that includes a reverse osmosis membrane that separates feed water into permeate and concentrate, and an NF membrane that further separates the concentrate, which is used as feed water, into permeate and concentrate, the reverse osmosis membrane having a total dissolved solids removal rate of 90% or more and a silicon dioxide removal rate of 90% or more, and the nanofiltration membrane having a total dissolved solids removal rate of 40 to 60% and a silicon dioxide removal rate of 1 to 10%, and mixing the permeate separated by the reverse osmosis membrane and the nanofiltration membrane.

[0004] This water quality improvement device reduces the TDS content to 2000 mg / L and the SiO ２ The mixed permeate contained 265 mg / L of TDS and 60 mg / L of SiO ２ A production water amount of 15.6 mg / L was obtained as shown in the example.

[0005] JP 61-200810 A JP 2009-172462 A

[0006] However, the water quality modification device of Patent Document 2 uses a nanofiltration membrane with a silicon dioxide removal rate of 10% or less, so it is necessary to use a reverse osmosis membrane with a higher removal rate in combination, which poses a problem of unavoidable increase in operating costs due to the use of a reverse osmosis membrane. ２ It was also discovered that excessive reductions in volume were resulting in additional operating costs.

[0007] Therefore, an object of the present invention is to provide a method for producing purified water that can reduce operating costs as much as possible while suppressing the risk of silica scale formation. Another object of the present invention is to provide a method for reducing the risk of silica scale formation by utilizing such a membrane separation method.

[0008] The present inventors have conducted extensive research to solve the above problems, and have found that selective separation performance for solutes and removal rate of soluble silica differ depending on the type of NF membrane. This led to the idea of ​​solving the above problems by setting the mixing ratio of permeate water according to such separation performance, and has completed the present invention. That is, the present invention includes the following aspects.

[0009] [1] Dissolved silica at a concentration of C Ｓｉ The feed water containing 1,000 ppm of ammonium hydroxide is passed through a UF membrane and an NF membrane, and the UF membrane permeate and the NF membrane permeate are mixed at a mixing ratio R ｍｉｘ A method for producing purified water, comprising a membrane separation step of mixing 50≦[C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1]<80...(1) (wherein, C Ｓｉ is the concentration of soluble silica (ppm), R ＮＦ is the soluble silica removal rate (%) of the NF membrane, R ｍｉｘ is the amount of UF membrane permeate mixed F ＵＦ Mixing amount of NF membrane permeate water F ＮＦ Mixing ratio (F ＮＦ / F ＵＦ ) (-) is represented.

[0010] According to the method for producing purified water of the present invention, as described below, the soluble silica concentration of the permeate after mixing can be set to less than 80 ppm based on the soluble silica concentration of the feed water and the soluble silica removal rate of the NF membrane, thereby suppressing the risk of silica scale formation. Furthermore, the soluble silica concentration of the permeate after mixing can be set to 50 ppm or more, thereby avoiding excessive reduction in the soluble silica concentration and reducing operating costs as much as possible within an appropriate concentration range. As a result, a method for producing purified water can be provided that can reduce operating costs as much as possible while suppressing the risk of silica scale formation.

[0011] [2] The method for producing purified water according to [1], wherein the supply water has a pH of 6 to 8 and contains soluble silica at a concentration of 80 to 110 ppm, and the NF membrane has a soluble silica removal rate of 5% or more.

[0012] Even when using feed water that may pose a risk of silica scaling, the risk of silica scaling can be suppressed while reducing operating costs as much as possible by using an NF membrane with a soluble silica removal rate of 5% or more and adjusting the mixing ratio.

[0013] [3] The NF membrane is made of MgSO with a concentration of 2000 mg / L. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ The method for producing purified water according to [1] or [2], wherein the rejection rate is 90% or more.

[0014] It has been confirmed that such NF membranes have a soluble silica removal rate of 5% or more, and also have the ability to selectively separate and remove divalent or higher ions, so they can increase the permeation flow rate while also removing Mg, a hard water component. ２＋ and Ca ２＋ By selectively removing these, it is possible to soften purified water while saving operating energy.

[0015] [4] The method for producing purified water according to [1] or [2], wherein the NF membrane has a rejection rate of NaCl of 90% or more when treating an aqueous NaCl solution with a concentration of 500 mg / L at an operating pressure of 0.48 MPa at 25°C.

[0016] It has been confirmed that such NF membranes have a soluble silica removal rate of 80% or more, and because the concentration of soluble silica in the permeate is low, the mixing ratio Rmix can be reduced. As a result, the amount of permeate through the NF membrane, which has a higher energy load, can be reduced, thereby saving overall operating energy.

[0017] [5] A method for reducing the risk of silica scale, comprising: a membrane separation step of producing purified water by the method for producing purified water according to any one of [1] to [4]; and a step of supplying the produced purified water to a place of use via piping, or supplying the purified water to a reverse osmosis membrane to perform membrane separation.

[0018] When purified water is supplied via piping or supplied to a reverse osmosis membrane for membrane separation, there is a risk of silica scale forming depending on the concentration of soluble silica. However, by using purified water produced according to the present invention, it is possible to reduce the risk of silica scale formation while minimizing operating costs.

[0019] According to the method for producing purified water of the present invention, it is possible to provide a method for producing purified water that can reduce operating costs as much as possible while suppressing the risk of silica scale formation. According to the method for reducing the risk of silica scale of the present invention, it is possible to provide a method for reducing the risk of silica scale formation as much as possible while suppressing the risk of silica scale formation.

[0020] Fig. 1 is a schematic diagram showing an example of a method for producing purified water. Fig. 2 is a schematic diagram showing another example of a method for producing purified water. Fig. 3 is a schematic diagram showing an example of a method for reducing the risk of silica scale.

[0021] Hereinafter, an embodiment of the present invention will be described.

[0022] [Method for Producing Purified Water] The method for producing purified water of the present invention is characterized by comprising a membrane separation step in which feed water containing soluble silica is permeated through a UF membrane and an NF membrane, and the UF membrane permeate and the NF membrane permeate are mixed at a predetermined mixing ratio. Ｓｉ(ppm) of the feed water, the mixing ratio R satisfies the following formula (1): ｍｉｘ (-) is adopted. 50≦[C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1]<80...(1) (wherein, C Ｓｉ is the concentration of soluble silica (ppm), R ＮＦ is the soluble silica removal rate (%) of the NF membrane, R ｍｉｘ is the amount of UF membrane permeate mixed F ＵＦ Mixing amount of NF membrane permeate water F ＮＦ Mixing ratio (F ＮＦ / F ＵＦ ) (-) is represented.

[0023] Here, when UF membrane permeate water and NF membrane permeate water are mixed in a continuous process, R ｍｉｘ is the mixed flow rate of UF membrane permeate water F ＵＦ Mixed flow rate F of NF membrane permeate water (L / min) ＮＦ (L / min) mixing ratio (F ＮＦ / F ＵＦ ) (-). When UF membrane permeate water and NF membrane permeate water are mixed in a batch process, R ｍｉｘ is the total mixed amount of UF membrane permeate water F ＵＦ Total mixed amount of NF membrane permeate water to (L) F ＮＦ (L) mixing ratio (F ＮＦ / F ＵＦ ) (-) can be calculated.

[0024] In principle, the concentration of soluble silica in the UF membrane permeate after membrane separation is C because soluble silica is hardly removed by the UF membrane. Ｓｉ Since this is fed to the NF membrane, the concentration of soluble silica in the NF membrane permeate will be C Ｓｉ × (100-R ＮＦ ) / 100.

[0025] For example, when UF membrane permeate water and NF membrane permeate water are mixed in a continuous process, the mixing ratio R ｍｉｘ When (-) is adopted, the mixed flow rate of the UF membrane permeate water F ＵＦ(L / min) × Permeate concentration (C Ｓｉ ) and the mixed flow rate of the NF membrane permeate F ＮＦ (L / min) × Permeate concentration (C Ｓｉ × (100-R ＮＦ The concentration of soluble silica in the permeate after mixing is determined by the ratio of F to 100. ＵＦ / (F ＵＦ +F ＮＦ ) and the flow rate fraction of the NF membrane permeate is F ＮＦ / (F ＵＦ +F ＮＦ )

[0026] As a result, based on the mass balance, [C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1], the concentration of soluble silica in the permeate after mixing can be determined.

[0027] On the other hand, when UF membrane permeate water and NF membrane permeate water are mixed in a batch process, the mixing ratio R ｍｉｘ When (-) is adopted, the total mixed amount of UF membrane permeate water F ＵＦ (L) Mixture fraction × Permeate concentration (C Ｓｉ ) and the total mixed amount of NF membrane permeate water F ＮＦ (L) Mixture fraction × Permeate concentration (C Ｓｉ × (100-R ＮＦ The concentration of soluble silica in the permeate after mixing is determined by [C ) / 100)). The mixing fraction in a batch process can be calculated in the same way as the flow fraction in a continuous process, so based on the material balance, Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1], the concentration of soluble silica in the permeate after mixing can be determined.

[0028] Tables 1 to 4 show the results of R ＮＦ 10-95%, R ｍｉｘThe results of determining the concentration of soluble silica in the permeate after mixing in the range of 0.1 to 5.0 are shown. The gray cells indicate cases where the concentration of soluble silica in the permeate after mixing is 80 ppm or more or less than 50 ppm.

[0029]

[0030]

[0031]

[0032]

[0033] On the other hand, when purified water is supplied via a pipe or supplied to a reverse osmosis membrane for membrane separation, there is a high risk of silica scale formation if the soluble silica concentration exceeds 80 ppm. Furthermore, according to the studies of the present inventors, it has been found that the selective separation performance for solutes and the removal rate of soluble silica differ depending on the type of NF membrane.

[0034] Therefore, from the results of Tables 1 to 4, it can be seen that regardless of the concentration of soluble silica in the feed water, the mixing ratio R ｍｉｘ It can be seen that it is desirable to set (-). Furthermore, in the present invention, based on the soluble silica concentration of the feed water and the soluble silica removal rate of the NF membrane, the soluble silica concentration of the permeated water after mixing can be made less than 80 ppm, thereby suppressing the risk of silica scale. Furthermore, because the soluble silica concentration of the permeated water after mixing can be made 50 ppm or more, excessive reduction of the soluble silica concentration can be avoided, and operating costs can be reduced as much as possible within an appropriate concentration range. As a result, a method for producing purified water can be provided that can reduce operating costs as much as possible while suppressing the risk of silica scale.

[0035] From this point of view, the mixture ratio R ｍｉｘ Regarding (-), it is preferable that the following formula (1A) is satisfied, and it is more preferable that the following formula (1B) is satisfied: 60≦[C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x Rｍｉｘ +1]<78...(1A) 65≦[C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1] < 75 ... (1B) In this specification, "soluble silica" means SiO ２ However, due to the influence of other components and pH, Si(OH) ４ It refers to silica components that can dissolve in water due to the occurrence of dimerization, ionization, etc. Also, sodium silicate (Na ２ SiO ３ ) and other water-soluble salts are dissolved in water, resulting in the formation of ionic components (SiO ３ ２－ ) etc.

[0036] The concentration of soluble silica can be measured in accordance with JIS K0101:1998 "Industrial Water Testing Methods" 44.1.2 "Molybdenum Blue Absorption Spectrophotometric Method." This method involves coloring only the silica present as a monomer in an aqueous silica solution blue, and measuring the concentration of the monomeric silica using an ultraviolet-visible spectrophotometer. The concentration of soluble silica, such as ionic components, can be measured using an ion chromatograph (ICS6000 manufactured by Thermo Fisher Scientific) based on a calibration curve prepared from standard samples of known concentrations.

[0037] The removal rate (%) of soluble silica by the NF membrane is also higher than that of sodium silicate and amorphous SiO ２ The removal rate (%) of soluble silica can be measured by the following method using feed water in which the above-mentioned compound is dissolved (soluble silica concentration: 6.5 ppm). The removal rate (%) of soluble silica may vary slightly during membrane separation, but in the present invention, the value measured by this method is used when calculating formula (1) and the like.

[0038] That is, the flat composite semipermeable membrane was set in a cell of a cross-flow test system for flat membrane evaluation, and the permeation flux was 25 LMH (Lm －２ h －１The operating pressure is adjusted so that the soluble silica removal rate is 0.05%. The feed water is allowed to permeate at a temperature of 25°C for 30 minutes, and then the concentrations of the feed water and the permeated water are measured. The soluble silica removal rate can be determined by calculating the concentration of the soluble silica removal rate from the measurement results using the following formula.

[0039] Removal rate of soluble silica (%) = (1 - (soluble silica concentration of permeate water / soluble silica concentration of feed water)) x 100. In the present invention, when producing purified water, the mixing ratio R satisfies the formula (1) from start to finish. ｍｉｘ (-), and at least initially, the mixture ratio R satisfies the formula (1). ｍｉｘ In addition, the mixture ratio does not need to be constant at all times, and even if the mixture ratio fluctuates, the average mixture ratio is R that satisfies the formula (1). ｍｉｘ It is preferably (-).

[0040] When producing purified water, the mixing ratio may not be controlled, but it is preferable to control the mixing ratio to be constant. In a continuous process, the mixing flow rate F of the UF membrane permeate water is controlled to be constant. ＵＦ (L / min) and the mixed flow rate of the NF membrane permeate F ＮＦ (L / min) according to the flow rate. To adjust the flow rate, for example, a valve whose opening can be adjusted according to the flow rate or pressure can be used.

[0041] (Membrane Separation Apparatus) The method for producing purified water of the present invention can be carried out using a membrane separation apparatus such as that shown in FIG. 1 or FIG.

[0042] For example, the membrane separation apparatus shown in FIG. 1 includes a membrane module M1 having a separation membrane 11 which is a UF membrane, a supply section for feed water 17, a discharge section for permeated water 18, and a discharge section for concentrated water 19, and a membrane module M2 having a separation membrane 21 which is an NF membrane, a supply section to which the permeated water 18 is supplied as feed water 27 for the separation membrane 21, a discharge section for permeated water 28, and a discharge section for concentrated water 29.

[0043] This membrane separation device also includes a path for mixing a portion of the UF membrane permeate 18 that has been permeated through the UF membrane 11 and the NF membrane 21 with the NF membrane permeate 28, and a mixed flow rate F ＵＦMixed flow rate F of NF membrane permeate water (L / min) ＮＦ (L / min) mixing ratio (F ＮＦ / F ＵＦ ) (-) is the mixture ratio R ｍｉｘ It is set to be.

[0044] In the illustrated example, the mixture ratio R ｍｉｘ The permeated water mixed in step 3 is temporarily stored in a mixing tank 31 and is used as purified water via a pipe 32. At this time, the soluble silica in the purified water is reduced to a concentration C mix (ppm) can be 50 ppm or more and less than 80 ppm. The concentrated water 19 discharged from the membrane module M1 can be recycled as feed water or discarded. The concentrated water 29 discharged from the membrane module M2 can be used as feed water for further membrane treatment or discarded.

[0045] 2 includes a membrane module M1 having a separation membrane 11, which is a UF membrane, a supply section for feed water 17, a discharge section for permeated water 18, and a discharge section for concentrated water 19, and a membrane module M2 having a separation membrane 21, which is an NF membrane, a supply section to which permeated water 18 is supplied as feed water 27 for the separation membrane 21, a discharge section for permeated water 28, and a discharge section for concentrated water 29. The membrane separation apparatus also includes a UF permeated water tank 33 that can temporarily store a portion of the UF membrane permeated water 18, an NF permeated water tank 35 that can temporarily store the NF membrane permeated water 28, and a mixing tank 31 that mixes the permeated waters discharged from these tanks. When the permeated waters are mixed in the mixing tank 31, the total mixed amount F of the UF membrane permeated water from the UF permeated water tank 33 is ＵＦ Total mixed amount of NF membrane permeate water to (L) F ＮＦ (L) mixing ratio (F ＮＦ / F ＵＦ ) (-) is the mixture ratio R ｍｉｘ The mixture is mixed so that

[0046] In the illustrated example, the mixture ratio R ｍｉｘ The permeated water mixed in step 3 is used as purified water via pipe 32. At this time, the soluble silica in the purified water is reduced to a concentration C mix (ppm) can be 50 ppm or more and less than 80 ppm.

[0047] Such membrane separation devices are equipped with other devices such as pumps, sensors, tanks, control valves, and control devices as needed, and are configured so as to be able to operate under desired conditions.

[0048] (Feed Water Containing Soluble Silica) The concentration of soluble silica in the feed water supplied to the UF membrane is preferably 110 ppm or less, more preferably 100 ppm or less, from the viewpoint of reducing the operating cost of membrane separation using an NF membrane.

[0049] From the viewpoint of durability of the UF membrane and NF membrane and stable membrane operation, the pH of the feed water supplied to the UF membrane is preferably 5 to 9. Therefore, feed water having a pH of 6 to 8 and containing soluble silica at a concentration of 80 to 110 ppm is preferred because the present invention can effectively suppress the risk of silica scale formation.

[0050] The feed water may contain hardness components such as calcium and magnesium, and the hardness components can be effectively removed by using an NF membrane that can selectively remove divalent ions. The total concentration of calcium and magnesium in the feed water is, for example, 5 to 2000 ppm, and may be 10 to 1000 ppm. The concentration of these hardness components can be measured using titration or ion chromatography.

[0051] The feed water supplied to the UF membrane may have the concentrations of soluble silica and hardness components reduced by a pretreatment process as described below. The water to be treated that is the subject of pretreatment is not particularly limited, and may be river water, lake water, groundwater, wastewater from a factory, or the like. A pretreatment process can be carried out depending on the type and quality of the water to be treated, and the pretreated water can be used as the feed water to be supplied to the UF membrane.

[0052] (UF membrane) Membrane separation using a UF membrane (ultrafiltration membrane) is carried out to remove insoluble components from the water to be treated, such as inorganic fine particles, organic fine particles, viruses and other suspended solids, as well as soluble components, such as high molecular weight components, endotoxins, proteins, emulsions, etc. Here, a UF membrane refers to a membrane with an average pore size of approximately 0.001 μm to 0.01 μm.

[0053] The material of the UF membrane is not particularly limited, and examples thereof include polymeric materials such as cellulose ester polymers such as cellulose acetate, polyethylene, polypropylene, polysulfone, polyvinylidene fluoride, polyethersulfone, etc. From the viewpoints of durability and washability, polyvinylidene fluoride and polyethersulfone are preferred.

[0054] The shape of the UF membrane is not particularly limited, and can be selected from flat membrane, hollow fiber membrane, pleated membrane, tubular membrane, etc. In particular, a so-called spiral membrane element, which is made by processing a flat membrane into an envelope shape and winding the membrane together with a support such as a net into a spiral shape, is preferred because it allows for a large membrane area.

[0055] (NF membrane) Membrane separation using an NF membrane (nanofiltration membrane) is carried out to reduce the concentration of soluble silica and to remove hardness components, water-soluble low-molecular-weight organic substances, coloring components, odor components, ionic components, etc. Here, an NF membrane generally refers to a semipermeable membrane with lower rejection performance than an RO membrane. In this specification, an NF membrane refers to a membrane with a sodium chloride rejection rate of 5% or more but less than 99% when filtering a test solution with a sodium chloride concentration of 500 to 2,000 mg / L at an operating pressure of 0.3 to 1.5 MPa.

[0056] In the present invention, from the viewpoint of reducing operating costs while suppressing the risk of silica scale, it is preferable to use an NF membrane having a soluble silica removal rate of 5% or more, more preferably 10% or more, and particularly preferably 80% or more. Also, from the viewpoint of permeability and operating pressure, it is preferable to use an NF membrane having a soluble silica removal rate of 99% or less, and particularly preferably 97% or less.

[0057] The material of the NF membrane is not particularly limited, and examples thereof include polymeric materials such as cellulose ester polymers such as cellulose acetate, polyamide, polyester, polyimide, vinyl polymer, polyethersulfone, sulfonated polyethersulfone, and polyamide. Multiple materials may also be used. In particular, polyamide, polyethersulfone, polyvinyl alcohol, and mixtures thereof are preferably used for the NF membrane.

[0058] The shape of the NF membrane is not particularly limited, and can be selected from flat membrane, hollow fiber membrane, pleated membrane, tubular membrane, etc. In particular, a so-called spiral membrane element, which is made by processing a flat membrane into an envelope shape and winding the membrane together with a support such as a net into a spiral shape, is preferred because it allows for a large membrane area.

[0059] A spiral membrane element for NF membranes comprises, for example, a perforated central tube and a wound body containing a separation membrane wound around the central tube. More specifically, the element comprises a plurality of membrane leaves with a permeate-side channel material interposed between opposing separation membranes, a feed-side channel material interposed between the membrane leaves, a perforated central tube around which the membrane leaves and the feed-side channel material are wound, and a sealing portion that prevents mixing of the feed-side channel and the permeate-side channel.

[0060] NF membranes are broadly classified into selective separation type NF membranes and partial desalination loose type NF membranes, and it has been found that the removal rate of soluble silica varies in the range of 5 to 95%, including those with intermediate properties.

[0061] The selective separation type NF membrane is a membrane that can separate monovalent ions (Cl － etc.) and multivalent ions (SO ４ ２－ The NF membrane has the property of selectively separating salts (usually about 60% rejection). As a selective separation type NF membrane, a MgSO 2 with a concentration of 2000 mg / L is used. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ In this case, a substance having a rejection rate of 90% or more can be used.

[0062] A partial desalination loose-type NF membrane is an NF membrane (usually with a salt rejection rate of about 90%) that has a separation functional layer with a looser chemical structure than an RO membrane. A partial desalination loose-type NF membrane that can be used is one that has a NaCl rejection rate of 90% or more when treated with a 500 mg / L NaCl aqueous solution at an operating pressure of 0.48 MPa at 25°C.

[0063] The selective separation type NF membrane can be used more effectively than the partial desalination loose type NF membrane when membrane separating feed water having a relatively high concentration of polyvalent ions such as sulfate ions.

[0064] Commercially available NF membranes can be used as the NF membrane. For example, as a selective separation type NF membrane, the NANO-SW series manufactured by Nitto Denko Corporation (Hydranautics), the DK and DL series manufactured by Suez, and the NF270 manufactured by DuPont can be used.

[0065] As the partial desalination loose-type NF membrane, the ESNA series and HYDRACoRe series manufactured by Nitto Denko Corporation (Hydranautics), NF90 manufactured by DuPont, and the like can be used.

[0066] The method for producing purified water of the present invention includes a membrane separation step using a UF membrane and an NF membrane, and may further include a pretreatment step for removing at least one of soluble silica and hardness components, or a pretreatment step for removing relatively large insoluble components.

[0067] For example, as a pretreatment step for reducing the concentration of soluble silica, a method can be adopted in which a magnesium salt is added to the water to be treated to cause a reaction, thereby insolubilizing the soluble silica, a flocculant is added to the insolubilized water to cause flocculation, and the flocculated silica is subjected to solid-liquid separation.

[0068] For example, a pretreatment step for removing hardness components can involve adding an alkaline agent to the water to be treated to cause a reaction that insolubilizes the hardness components, adding a flocculant to the water to be treated after the reaction as needed to cause flocculation, and then subjecting the flocculates to solid-liquid separation.Hardness components can also be removed by a resin softening method, for example, by performing an ion exchange treatment using an ion exchange resin or the like to adsorb and remove the hardness components.

[0069] As a pretreatment step for removing relatively large insoluble components, membrane separation using an MF membrane can be performed. An MF membrane has an average pore size of about 0.01 μm to several μm, and can separate fine particles and microorganisms with a size of about 0.05 to 10 μm from a liquid.

[0070] (Uses) The method for producing purified water of the present invention can be used to produce drinking water, which has been in increasing demand overseas in recent years, and is particularly useful as a method for producing purified water with reduced amounts of organic compounds such as herbicides and odorous components.

[0071] The purified water obtained by the method for producing purified water of the present invention can be supplied directly to the place of use via piping, but it is also possible to further supply the purified water to a reverse osmosis membrane (RO membrane) for membrane separation.

[0072] Membrane separation using an RO membrane can be carried out to further reduce salts, ionic components, and other low molecular weight components that cannot be sufficiently removed by an NF membrane.

[0073] [Method for reducing the risk of silica scale formation] The method for reducing the risk of silica scale formation of the present invention includes a membrane separation step of producing purified water by the method for producing purified water of the present invention, and a step of supplying the produced purified water to a place of use via piping, or supplying the purified water to a reverse osmosis membrane and performing membrane separation.

[0074] When purified water is supplied via pipes or supplied to a reverse osmosis membrane for membrane separation, there is a risk of silica scale forming depending on the concentration of soluble silica. However, by using purified water produced according to the present invention, it is possible to reduce operating costs as much as possible while suppressing the risk of silica scale formation.

[0075] When the method further includes a step of supplying purified water to a reverse osmosis membrane to perform membrane separation, this can be performed using, for example, a membrane separation apparatus as shown in Fig. 3. For example, the membrane separation apparatus shown in Fig. 3 includes a membrane module M1 including a UF membrane, a membrane module M2 including an NF membrane, and a mixing tank 31, which are similar to those shown in Fig. 1. Furthermore, the membrane separation apparatus shown in Fig. 3 includes a separation membrane 41 which is an RO membrane, and a membrane module M3 which includes a supply section for feed water 47, a discharge section for permeate water 48, and a discharge section for concentrated water 49.

[0076] The purified water discharged from the mixing tank 31 is supplied to the RO membrane as feed water 47 for the membrane module M3, and permeate water 48 with reduced concentrations of salts and the like is obtained. The concentrated water 49 discharged from the membrane module M3 can be used as feed water for further membrane treatment or can be discarded.

[0077] (RO Membrane) An RO (reverse osmosis) membrane is a membrane that has a sodium chloride rejection rate of, for example, 93% or more when filtering a test solution having a sodium chloride concentration of 500 to 2,000 mg / L at an operating pressure of 0.5 to 3.0 MPa. In the present invention, a sodium chloride rejection rate of 96% or more is preferred, and a sodium chloride rejection rate of 99% or more is more preferred.

[0078] The material of the RO membrane is not particularly limited, and may be a polymer material such as a cellulose ester polymer such as cellulose acetate, polyamide, polyester, polyimide, vinyl polymer, polyethersulfone, sulfonated polyethersulfone, or polyamide. Multiple materials may also be used. Among these, polyamide is preferably used for the RO membrane because of its proven high rejection performance.

[0079] A preferred RO membrane using polyamide is a composite semipermeable membrane comprising a porous support having a porous resin layer and a separation functional layer formed of a polyamide resin on the porous resin layer. The polyamide resin forming the separation functional layer contains, for example, components derived from a divalent polyfunctional amine and a trivalent or higher polyfunctional acid halide.

[0080] The separation functional layer can be formed by interfacial polymerization of a polyamide resin, and is particularly preferably a separation functional layer containing a polyamide resin obtained by polymerizing a polyfunctional amine component and a polyfunctional acid halogen component.

[0081] The polyfunctional amine component is a polyfunctional amine having two or more reactive amino groups, and examples thereof include aromatic, aliphatic, and alicyclic polyfunctional amines, but it is preferable to include an aromatic diamine. The polyfunctional acid halide component is a polyfunctional acid halide having two or more reactive carbonyl groups, and examples thereof include aromatic, aliphatic, and alicyclic polyfunctional acid halides, but it is preferable to include a trivalent or higher aromatic acid halide.

[0082] The shape of the RO membrane is not particularly limited, and can be selected from flat membrane, hollow fiber membrane, pleated membrane, tubular membrane, etc. In particular, a so-called spiral membrane element, which is made by processing a flat membrane into an envelope shape and winding the membrane together with a support such as a net into a spiral shape, is preferred because it can increase the membrane area.

[0083] As the RO membrane, in addition to ultra-low pressure reverse osmosis membranes and low pressure reverse osmosis membranes used for applications such as pure water production and wastewater recovery, medium pressure reverse osmosis membranes and high pressure reverse osmosis membranes used for applications such as seawater desalination are commercially available, and these can be used. For example, examples of ultra-low pressure reverse osmosis membranes and low pressure reverse osmosis membranes include ES15 (manufactured by Nitto Denko), TM720D (manufactured by Toray), BW30HRLE (manufactured by Dow Chemical), and LFC3-LD (manufactured by Hydranautics). Examples of high pressure reverse osmosis membranes include SWC5-LD (manufactured by Hydranautics), TM820V (manufactured by Toray), and XUS180808 (manufactured by Dow Chemical).

[0084] In the membrane separation process using an RO membrane, chemicals such as a pH adjuster, a scale dispersant that suppresses scaling of inorganic salts in the system, and a bactericide that suppresses the growth of microorganisms in the system may be added.

[0085] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the examples, physical properties were measured or evaluated by the following methods. The physical properties in the present invention are specifically values ​​measured by the following methods.

[0086] (1) Preparation of Feed Water Sodium silicate (Fujifilm Wako Pure Chemical Industries, Ltd.) was used as soluble silica. It was dissolved at 25°C with stirring in 20 L of RO water adjusted to pH 7 with NaOH. Finally, the pH was adjusted to 7 with NaOH to prepare feed water in which soluble silica was dissolved. At this time, the concentration of soluble silica in the feed water was adjusted by adjusting the amount of sodium silicate added.

[0087] (2) Measurement of Soluble Silica Concentration Using ion chromatography (ICS6000 manufactured by Thermo Fisher Scientific), the soluble silica concentration was measured based on a calibration curve prepared from standard samples of known concentrations.

[0088] (3) Removal rate of soluble silica A commercially available NF membrane element was disassembled to remove the composite semipermeable membrane. The flat composite semipermeable membrane was cut into a predetermined shape and size and placed in a cell (effective membrane surface area: 44.2 cm) of a cross-flow test system for flat membrane evaluation. ２ ) and the permeation flux was set to 25 LMH (Lm －２ h －１ The operating pressure was adjusted so that the above feed water (soluble silica concentration: 6.5 ppm) was allowed to permeate through the composite semipermeable membrane at a temperature of 25°C for 30 minutes, and the soluble silica removal rate was then measured. The soluble silica removal rate was calculated from the measurement results obtained by measuring the concentrations of the feed water and the permeated water using the following formula: Soluble silica removal rate (%) = (1 - (soluble silica concentration in permeated water / soluble silica concentration in feed water)) x 100

[0089] (Experimental Example 1) A commercially available selective separation type NF membrane, NF270 manufactured by DuPont, was used to measure the removal rate (%) of soluble silica from the feed water prepared in (1) above. As a result, the removal rate of soluble silica was 10%. The NF membrane used was a 2000 mg / L MgSO 4 membrane. ４SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ The inhibition rate was 99.5%.

[0090] (Experimental Example 2) A commercially available partial desalination loose-type NF membrane, ESNA1-K1 manufactured by Nitto Denko Corporation, was used to measure the soluble silica removal rate (%) for the feed water prepared in (1) above. As a result, the soluble silica removal rate was 90%. The NF membrane used had a NaCl rejection rate of 98% when treating a 500 mg / L NaCl aqueous solution at 25°C under an operating pressure of 0.48 MPa.

[0091] (Experimental Example 3) Using a commercially available NF membrane element (NF90, manufactured by DuPont), the removal rate (%) of soluble silica from the feed water prepared in (1) above was measured. As a result, the removal rate of soluble silica was 92%.

[0092] (Examples 1 and 2, Comparative Example 1) Assuming operation using the membrane separation apparatus shown in Figure 1, the soluble silica concentration of the permeate after mixing was determined from the soluble silica concentration of the feed water and the mixing ratio of the permeate shown in Table 5, based on the soluble silica removal rates of the NF membranes obtained in Experimental Examples 1 to 3. The operating energy cost at that time was calculated as follows.

[0093] When the flow rate of the feedwater is constant, the flow rate of the UF membrane permeate is also constant, and the relative throughput of the NF membrane is determined from the mixing ratio of the NF membrane permeate. Meanwhile, the operating pressure of the NF membrane is roughly determined by the permeation performance corresponding to the removal performance of the NF membrane, and the operating energy cost can be calculated by multiplying the operating pressure by the relative throughput of the NF membrane. Therefore, the relative cost ratio was calculated by setting Example 1 as 1.

[0094] The results are shown in Table 5, along with the reference values ​​for the calculations.

[0095]

[0096] As shown in the results of Table 5, in Examples 1 and 2, the mixing ratio R satisfying the formula (1) was determined based on the concentration of soluble silica in the feed water and the removal rate of soluble silica by the NF membrane. ｍｉｘBecause the mixture is mixed at (-), the soluble silica concentration in the permeate after mixing can be kept below 80 ppm, which reduces the risk of silica scaling. In addition, excessive reduction of the soluble silica concentration can be avoided, reducing operating costs.

[0097] In contrast, in Comparative Example 1, the mixture ratio R ｍｉｘ Since the permeate was not mixed in (-), the concentration of soluble silica in the permeate after mixing was excessively reduced, resulting in increased operating costs.

[0098] According to the present invention, it is possible to provide a method for producing purified water that can reduce operating costs as much as possible while suppressing the risk of silica scale formation, and a method for reducing the risk of silica scale formation using such a membrane separation method.

[0099] 11 Separation membrane (UF membrane) 17 Feed water 18 UF membrane permeate 21 Separation membrane (NF membrane) 27 Feed water 28 NF membrane permeate 31 Mixing tank 41 Separation membrane (RO membrane) C Ｓｉ Concentration of soluble silica in feed water F ＵＦ Mixing amount of UF membrane permeate water F ＮＦ Mixing amount of NF membrane permeate water C mix Concentration of soluble silica in permeate after mixing

Claims

1. Add soluble silica to a solution of concentration C Ｓｉ The feed water containing 1,000 ppm of ammonium hydroxide is passed through a UF membrane and an NF membrane, and the UF membrane permeate and the NF membrane permeate are mixed at a mixing ratio R ｍｉｘ A method for producing purified water, comprising a membrane separation step of mixing 50≦[C Ｓｉ / (1+R ｍｉｘ )]×[((100-R ＮＦ ) / 100) x R ｍｉｘ +1]<80...(1) (wherein, C Ｓｉ is the concentration of soluble silica (ppm), R ＮＦ is the soluble silica removal rate (%) of the NF membrane, R ｍｉｘ is the amount of UF membrane permeate mixed F ＵＦ Mixing amount of NF membrane permeate water F ＮＦ Mixing ratio (F ＮＦ / F ＵＦ ) (-) is represented.

2. A method for producing purified water according to claim 1, wherein the feed water has a pH of 6 to 8 and contains soluble silica at a concentration of 80 to 110 ppm, and the NF membrane has a soluble silica removal rate of 5% or more.

3. The NF membrane is treated with MgSO at a concentration of 2000 mg / L. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ The method for producing purified water according to claim 1, wherein the rejection rate of 90% or more is 90% or more.

4. The method for producing purified water according to claim 1, wherein the NF membrane has a NaCl rejection rate of 90% or more when treating an aqueous NaCl solution with a concentration of 500 mg / L at an operating pressure of 0.48 MPa at 25°C.

5. A method for reducing the risk of silica scale, comprising: a membrane separation step of producing purified water by the method for producing purified water according to any one of claims 1 to 4; and a step of supplying the produced purified water to a place of use via piping, or supplying the purified water to a reverse osmosis membrane for membrane separation.

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