WO2025187237A1 - Method for producing purified water and method for reducing silica scale risk - Google Patents

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 C_Si (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 R_mix (—) that satisfies formula (1). Formula (1): 50 ≤ [C_Si / (1 + R_mix)] × [((100 − R_NF / 100) × R_mix + 1] < 80. (In the formula, C_Si represents the concentration (ppm) of the soluble silica, R_NF represents the soluble silica removal rate (%) of the NF membrane, and R_mix represents the mixing ratio (F_NF / F_UF) (—) of the mixing quantity F_NF of the water that has passed through the NF membrane to the mixing quantity F_UF 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

The present invention relates to a method for producing purified water, which includes a membrane separation process in which feed water containing soluble silica is separated using a UF (ultrafiltration) membrane and an NF (nanofiltration) membrane, and a method for reducing the risk of silica scaling using such a membrane separation method.

Separation technology using separation membranes is widely used in the fields of water purification and wastewater treatment due to its low energy load, and methods that combine separation membranes with different filtration performance are also widely adopted (for example, Patent Document 1). In such 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. There is a growing demand for removing these to reduce the risk of scale and to soften the water.

As an example of an apparatus for membrane separation of feed water containing such soluble silica, Patent Document 2 proposes a water quality modification device that includes a reverse osmosis membrane that separates the feed water into permeate and concentrate, and an NF membrane that further separates the concentrate, which serves as the feed water, into permeate and concentrate. The reverse osmosis membrane has a total dissolved solids removal rate of 90% or more and a silicon dioxide removal rate of 90% or more, and the nanofiltration membrane has a total dissolved solids removal rate of 40-60% and a silicon dioxide removal rate of 1-10%, and mixes the permeate separated by the reverse osmosis membrane and nanofiltration membrane.

In the examples, this water quality modification device was used to obtain product water containing 265 mg/L of TDS and 15.6 mg/L of _SiO2 as mixed permeate from feed water containing 2000 mg/L of TDS and 60 mg/L of _SiO2 .

Japanese Unexamined Patent Publication No. 61-200810 JP 2009-172462 A

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, which poses a problem of unavoidable increase in operating costs. Furthermore, according to the study by the present inventors, it was found that excessive reduction in the amount of _SiO2 also results in extra operating costs.

Therefore, an object of the present invention is to provide a method for producing purified water that minimizes operating costs while suppressing the risk of silica scaling. It is also an object of the present invention to provide a method for reducing the risk of silica scaling using such a membrane separation method.

As a result of extensive research into resolving the above-mentioned problems, the inventors discovered that selective separation performance for solutes and the removal rate of soluble silica differ depending on the type of NF membrane. They came up with the idea of solving the above-mentioned problems by setting the permeate water mixing ratio according to such separation performance, and thus completed the present invention. Specifically, the present invention includes the following aspects.

[1] A method for producing purified water, comprising a membrane separation step of permeating feed water containing soluble silica at a concentration C _Si (ppm) through a UF membrane and an NF membrane, and mixing the UF membrane permeate and the NF membrane permeate at a mixing ratio R _mix (-) that satisfies the following formula (1):
50≦[C _Si /(1+R _mix )]×[((100-R _NF )/100)×R _mix +1]<80
...(1)
(In the formula, C _Si represents the concentration of soluble silica (ppm), R _NF represents the soluble silica removal rate (%) of the NF membrane, and R _mix represents the mixing ratio (F _NF /F _UF )(-) of the amount of NF membrane permeate water mixed F _NF to the amount of UF membrane permeate water mixed F _UF .)

According to the purified water production method of the present invention, as described below, the soluble silica concentration in the permeate water after mixing can be kept below 80 ppm based on the soluble silica concentration in the feed water and the soluble silica removal rate of the NF membrane, thereby suppressing the risk of silica scaling. Furthermore, because the soluble silica concentration in the permeate water after mixing can be kept at 50 ppm or more, excessive reduction in 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 purified water production method can be provided that minimizes operating costs while suppressing the risk of silica scaling.

[2] The method for producing purified water described in [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.

Even when using feed water that may pose a risk of silica scaling, by using an NF membrane with a soluble silica removal rate of 5% or more and adjusting the mixing ratio, it is possible to minimize the risk of silica scaling while also reducing operating costs as much as possible.

[3] The method for producing purified water according to [1] or [2], wherein the NF membrane has a rejection rate of SO ₄ ^2- of 90% or more when treating a 2000 mg/L MgSO ₄ aqueous solution at an operating pressure of 0.76 MPa and 25 ° C.

Such NF membranes have been confirmed to have a soluble silica removal rate of 5% or more, and also have the ability to selectively separate and remove divalent or higher ions. Therefore, by selectively removing hard water components such as Mg ²⁺ and Ca ²⁺ while increasing the permeation flow rate, the purified water can be softened while saving operating energy.

[4] A method for producing purified water according to [1] or [2], 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.

Such NF membranes have been confirmed to 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 greater energy load, can be reduced, resulting in overall operating energy savings.

[5] A membrane separation step of producing purified water by the method for producing purified water according to any one of [1] to [4];
and 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.

When purified water is supplied via pipes or fed 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 while minimizing operating costs.

The method for producing purified water of the present invention can provide a method for producing purified water that can minimize operating costs while suppressing the risk of silica scale.The method for reducing the risk of silica scale of the present invention can provide a method for reducing the risk of silica scale that can minimize operating costs while suppressing the risk of silica scale.

1 is a schematic diagram showing an example of a method for producing purified water. FIG. 10 is a schematic diagram showing another example of a method for producing purified water. FIG. 1 is a schematic diagram showing an example of a method for reducing the risk of silica scale.

The following describes an embodiment of the present invention.

[Method of producing purified water]
The method for producing purified water of the present invention is characterized by comprising a membrane separation step of permeating feed water containing soluble silica through a UF membrane and an NF membrane, and mixing the UF membrane permeate and the NF membrane permeate at a predetermined mixing ratio, wherein a mixing ratio R _mix (-) that satisfies the following formula (1) is adopted for feed water containing soluble silica at a concentration C _Si (ppm):
50≦[C _Si /(1+R _mix )]×[((100-R _NF )/100)×R _mix +1]<80
...(1)
(In the formula, C _Si represents the concentration of soluble silica (ppm), R _NF represents the soluble silica removal rate (%) of the NF membrane, and R _mix represents the mixing ratio (F _NF /F _UF )(-) of the amount of NF membrane permeate water mixed F _NF to the amount of UF membrane permeate water mixed F _UF .)

When UF membrane permeate water and NF membrane permeate water are mixed in a continuous process, R _mix can be calculated as the mixing ratio (F _NF /F _UF )(-) of the mixed flow rate F _NF (L/min) of NF membrane permeate water to the mixed flow rate F _UF (L/min) of UF membrane permeate water. When UF membrane permeate water and NF membrane permeate water are mixed in a batch process, R _mix can be calculated as the mixing ratio (F _NF /F _UF )(-) of the total mixed amount F NF membrane permeate water to the total mixed amount F _UF (L) _of UF membrane permeate water.

In principle, the concentration of soluble silica in the UF membrane permeate after membrane separation is almost the same as C _Si because the UF membrane hardly removes soluble silica. Since this is supplied to the NF membrane, the concentration of soluble silica in the NF membrane permeate is C _Si × (100 - R _NF )/100, depending on the removal rate of soluble silica.

For example, when UF membrane permeate and NF membrane permeate are mixed in a continuous process, if a mixing ratio R _mix (-) is adopted, the concentration of soluble silica in the mixed permeates is determined by the flow rate fraction of the mixed UF membrane permeate flow rate _{F UF} (L/min) × the permeate concentration (C _Si ) and the flow rate fraction of the mixed NF membrane permeate flow rate F NF (L/min) × the permeate concentration (C _Si × (100 - R _NF )/100). Here, the flow rate fraction of the UF membrane permeate is F _UF /(F _UF +F _NF ), and the flow rate fraction of the NF membrane permeate is F _NF /(F _UF +F _NF ).

As a result, based on the mass balance, the concentration of soluble silica in the permeate after mixing can be determined by [C _Si /(1+R _mix )]×[((100−R _NF )/100)×R _mix +1].

On the other hand, when UF membrane permeate water and NF membrane permeate water are mixed in a batch process, if a mixing ratio R _mix (-) is used, the concentration of soluble silica in the permeate water after mixing is determined by the mixing fraction of the total mixed amount of UF membrane permeate water F _UF (L) × the permeate concentration (C _Si ) and the mixing fraction of the total _mixed amount of NF membrane permeate water F NF (L) × the permeate concentration (C _Si × (100 - R _NF )/100)). Since the mixing fraction in a batch process can be calculated in the same way as the flow rate fraction in a continuous process, the concentration of soluble silica in the permeate water after mixing can be calculated based on material balance by [C _Si / (1 + R _mix )] × [((100 - R _NF )/100) × R _mix + 1].

Tables 1 to 4 show the results of determining the soluble silica concentration in the permeate after mixing when the soluble silica concentration in the feed water was 85 ppm, 100 ppm, 110 ppm, and 120 ppm, and when R _NF was in the range of 10 to 95% and R _mix was in the range of 0.1 to 5.0. The gray cells indicate cases where the soluble silica concentration in the permeate after mixing was 80 ppm or more or less than 50 ppm.

On the other hand, when purified water is supplied via pipes or fed to a reverse osmosis membrane for membrane separation, there is a high risk of silica scaling occurring if the soluble silica concentration exceeds 80 ppm. Furthermore, research by the inventors has revealed that selective separation performance for solutes and the soluble silica removal rate vary depending on the type of NF membrane.

Therefore, from the results of Tables 1 to 4, it can be seen that regardless of the soluble silica concentration of the feed water, it is desirable to set the mixing ratio R _mix (-) according to the soluble silica removal rate of the NF membrane. 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 formation. 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 formation.

From this viewpoint, it is preferable that the mixing ratio R _mix (−) satisfies the following formula (1A), and it is more preferable that the mixing ratio R mix (−) satisfies the following formula (1B).
60≦[C _Si /(1+R _mix )]×[((100-R _NF )/100)×R _mix +1]<78
...(1A)
65≦[C _Si /(1+R _mix )]×[((100-R _NF )/100)×R _mix +1]<75
...(1B)
In this specification, "soluble silica" refers to a silica component that can dissolve in water due to the influence of other components or pH, causing _SiO2 to undergo Si(OH) ₄ , dimerization, ionization, etc. It also refers to an ion component ( _SiO32- ) that is generated when water-soluble salts such ^as sodium _silicate ( _Na2SiO3 ) are dissolved in water.

The concentration of soluble silica can be measured in accordance with 44.1.2 "Molybdenum blue absorptiometry" of JIS K0101:1998 "Testing methods for industrial water." This method involves causing only the silica present as a monomer in an aqueous silica solution to develop a blue color, and measuring the concentration of monomeric silica using an ultraviolet-visible spectrophotometer.
The concentration of soluble silica such as ionic components can be measured using ion chromatography (ICS6000 manufactured by Thermo Fisher Scientific) based on a calibration curve prepared from standard samples of known concentrations.

The soluble silica removal rate (%) of the NF membrane can be measured by the following method using feed water (soluble silica concentration 6.5 ppm) in which sodium silicate or amorphous _SiO2 has been dissolved. The soluble silica removal rate (%) 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.

That is, the flat composite semipermeable membrane is set in a cell of a cross-flow test system for flat membrane evaluation, and the operating pressure is adjusted so that the permeation flux becomes 25 LMH (Lm ⁻² h ⁻¹ ). After the feed water is allowed to permeate for 30 minutes at a temperature of 25°C, the concentrations of the feed water and the permeated water are measured, and the removal rate of soluble silica can be calculated from the measurement results using the following formula.

Soluble silica removal rate (%) = (1 - (soluble silica concentration in permeate water/soluble silica concentration in feed water)) x 100
In the present invention, when producing purified water, the mixing ratio does not need to be R _mix (-) that satisfies formula (1) from start to finish, but it is sufficient that the mixing ratio is R _mix (-) that satisfies formula (1) at least initially. Furthermore, the mixing ratio does not need to be constant all the time, and even if the mixing ratio fluctuates, it is preferable that the average mixing ratio is R _mix (-) that satisfies formula (1).

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

(membrane separation equipment)
The method for producing purified water of the present invention can be carried out using, for example, a membrane separation apparatus as shown in FIG. 1 or FIG.

For example, the membrane separation device shown in Figure 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 permeate water 18, and a discharge section for concentrate water 19, and a membrane module M2 having a separation membrane 21, which is an NF membrane, a supply section to which permeate water 18 is supplied as feed water 27 for the separation membrane 21, a discharge section for permeate water 28, and a discharge section for concentrate water 29.

In addition, this membrane separation device is provided with a path for mixing a portion of the UF membrane permeate 18 that has permeated the UF membrane 11 and the NF membrane 21 with the NF membrane permeate 28, and the mixing ratio (F _NF /F UF )(-) of the mixed flow rate F _NF (L/min) of the NF membrane permeate to the mixed flow rate F _UF (L/min) of the _{UF membrane} permeate is set to be the mixing ratio R _mix .

In the illustrated example, the permeated water mixed at a mixing ratio R _mix is temporarily stored in a mixing tank 31 and then passed through a pipe 32 to be used as purified water. At this time, the concentration C _mix (ppm) of soluble silica in the purified water can be set to 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.

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. This membrane separation apparatus also includes a UF permeated water tank 33 which can temporarily store a portion of the UF membrane permeated water 18, an NF permeated water tank 35 which can temporarily store the NF membrane permeated water 28, and a mixing tank 31 which mixes the permeated waters discharged from these tanks. When the permeated water is mixed in the mixing tank 31, the mixing ratio (F NF /F _UF )(-) of the total mixed amount F _NF (L) of the UF membrane permeated water from the UF permeated water tank 33 to the total mixed amount F _UF ( _L ) of the NF membrane permeated water is mixed so as to be R _mix .

In the illustrated example, the permeated water mixed at the mixing ratio R _mix is used as purified water via a pipe 32. At this time, the concentration C _mix (ppm) of soluble silica in the purified water can be set to 50 ppm or more and less than 80 ppm.

Such membrane separation equipment is equipped with other equipment such as pumps, sensors, tanks, control valves, and control devices as needed, and is configured to operate under desired conditions.

(Feedwater 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 the NF membrane.

Furthermore, from the viewpoint of durability of the UF and NF membranes and stable membrane operation, the pH of the feed water supplied to the UF membrane is preferably 5 to 9. Therefore, feed water with a pH of 6 to 8 and containing soluble silica at a concentration of 80 to 110 ppm is preferred, as this invention can effectively suppress the risk of silica scaling.

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

The feed water supplied to the UF membrane may have had its concentration of soluble silica and hardness components reduced by a pretreatment process, as described below. Furthermore, the water to be treated that is the subject of pretreatment is not particularly limited and may include river water, lake water, groundwater, wastewater from factories, etc. It is also possible to carry out a pretreatment process 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.

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

The material of the UF membrane is not particularly limited, and polymeric materials such as cellulose ester polymers such as cellulose acetate, polyethylene, polypropylene, polysulfone, polyvinylidene fluoride, and polyethersulfone can be used. From the standpoint of durability and washability, polyvinylidene fluoride and polyethersulfone are preferred.

The shape of the UF membrane is not particularly limited, and can be selected from flat membrane, hollow fiber membrane, pleated membrane, and tubular membrane shapes. In particular, so-called spiral membrane elements, which are made by processing a flat membrane into an envelope shape and then winding the membrane into a spiral shape together with a support such as a net, are preferred because they allow for a larger membrane area.

(NF membrane)
Membrane separation using NF membranes (nanofiltration membranes) 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.

In the present invention, from the perspective of reducing operating costs while suppressing the risk of silica scaling, it is preferable to use an NF membrane with a soluble silica removal rate of 5% or more, with a removal rate of 10% or more being more preferable, and 80% or more being particularly preferable. Furthermore, from the perspective of permeability and operating pressure, it is preferable to use an NF membrane with a soluble silica removal rate of 99% or less, with a removal rate of 97% or less being particularly preferable.

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

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, so-called spiral membrane elements, which are 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, are preferred because they allow for a larger membrane area.

An NF membrane spiral membrane element, for example, comprises a perforated central tube and a wound body containing a separation membrane wound around the central tube. More specifically, it comprises multiple 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 feed-side channel material are wound, and a sealing section that prevents mixing of the feed-side channel and the permeate-side channel.

NF membranes are broadly divided into selective separation type NF membranes and partial desalination loose type NF membranes, but it was found that the soluble silica removal rate varies between 5 and 95%, including those with intermediate properties.

A selective separation type NF membrane is an NF membrane (usually with a salt rejection rate of about 60%) that has the property of selectively separating monovalent ions (such as ^Cl- ) from polyvalent ions (such as _SO4 ^2- ). As a selective separation type NF membrane, one that has a rejection rate of SO4 _2- of 90% or more when treated with an aqueous _MgSO4 solution with a concentration of 2000 mg/ ^L at an operating pressure of 0.76 MPa at 25°C can be used.

A partial desalination loose-type NF membrane is an NF membrane (typically with a salt rejection rate of around 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.

Selective separation NF membranes can be used more effectively than partial desalination loose-type NF membranes when membrane separating feed water with a relatively high concentration of multivalent ions such as sulfate ions.

Commercially available NF membranes can be used. For example, selective separation NF membranes such as 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.

Furthermore, partial desalination loose-type NF membranes that can be used include the ESNA series and HYDRACoRe series manufactured by Nitto Denko Corporation (Hydranautics), and the NF90 manufactured by DuPont.

(Pretreatment process)
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.

For example, a pretreatment process for reducing the concentration of soluble silica can involve adding magnesium salt to the water to be treated and causing a reaction to insolubilize the soluble silica, then adding a flocculant to cause flocculation, and then subjecting the flocculates to solid-liquid separation.

For example, a pretreatment process 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, then 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 using a resin softening method, for example, by performing ion exchange treatment using ion exchange resins or the like to adsorb and remove the hardness components.

Membrane separation using an MF membrane can be performed as a pretreatment step to remove relatively large insoluble components. MF membranes have an average pore size of approximately 0.01 μm to several μm, and can separate fine particles and microorganisms approximately 0.05 to 10 μm in size from liquids.

(Application)
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.

The purified water obtained by the purified water production method of the present invention can be supplied directly to the point of use via piping, but it can also be supplied to a reverse osmosis membrane (RO membrane) for membrane separation.

Membrane separation using RO membranes can be performed to further reduce salts, ionic components, and other low molecular weight components that cannot be sufficiently removed using NF membranes.

[Methods for reducing the risk of silica scale]
The method for reducing the risk of silica scale 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.

When purified water is supplied via pipes or fed 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 minimize operating costs while suppressing the risk of silica scale formation.

If the process further includes a step of supplying purified water to a reverse osmosis membrane to perform membrane separation, this can be carried out using a membrane separation device such as the one shown in Figure 3. For example, the membrane separation device shown in Figure 3 includes a membrane module M1 containing a UF membrane, a membrane module M2 containing an NF membrane, and a mixing tank 31, which are the same as those shown in Figure 1. Furthermore, the membrane separation device shown in Figure 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.

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 a reduced concentration of salts and other contaminants is obtained. The concentrated water 49 discharged from the membrane module M3 can be used as feed water for further membrane treatment or discarded.

(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 with 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.

The material of the RO membrane is not particularly limited, and polymeric materials such as cellulose ester polymers such as cellulose acetate, polyamide, polyester, polyimide, vinyl polymer, polyethersulfone, sulfonated polyethersulfone, and polyamide can be used. Multiple materials may also be used. Among these, polyamide is preferably used for RO membranes due to its proven high rejection performance.

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-based resin on the porous resin layer. The polyamide-based resin forming the separation functional layer contains components derived from, for example, a divalent polyfunctional amine and a trivalent or higher polyfunctional acid halide.

The separation functional layer can be formed by interfacial polymerization of a polyamide-based resin, and it is particularly preferable that the separation functional layer contains a polyamide-based resin obtained by polymerizing a polyfunctional amine component and a polyfunctional acid halogen component.

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 that it contains 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 that it contains a trivalent or higher aromatic acid halide.

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, so-called spiral membrane elements, which are 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, are preferred because they allow for a larger membrane area.

As RO membranes, 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 can be used. 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).

In the membrane separation process using an RO membrane, chemicals such as pH adjusters, scale dispersants to prevent scaling of inorganic salts within the system, and disinfectants to prevent the growth of microorganisms within the system may be added.

The present invention will be explained below using examples, but the present invention is not limited to these examples in any way. In the examples, physical properties were measured or evaluated using the following methods. The physical properties in the present invention are specifically values measured using the following methods.

(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.

(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.

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

(Experimental Example 1)
A commercially available selective separation NF membrane, NF270 manufactured by DuPont, 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 10%. The NF membrane used had a rejection rate of 99.5% for SO _2- when a 2000 mg/L MgSO ₄ ^aqueous solution was treated at an operating pressure of 0.76 MPa and 25°C.

(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. The result showed a soluble silica removal rate of 90%. The NF membrane used had a NaCl rejection rate of 98% when treated with a 500 mg/L NaCl aqueous solution at 25°C under an operating pressure of 0.48 MPa.

(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%.

(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:

When the flow rate of the feed water 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, which corresponds 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. For this reason, the relative cost ratio was calculated by setting Example 1 as 1.

The results, along with the base values for the calculations, are shown in Table 5.

As shown in the results in Table 5, in Examples 1 and 2, the soluble silica concentration in the feed water and the soluble silica removal rate of the NF membrane were mixed at a mixing ratio R _mix (-) that satisfied formula (1), so the soluble silica concentration in the permeate after mixing could be kept below 80 ppm, thereby suppressing the risk of silica scale formation. In addition, excessive reduction of the soluble silica concentration could be avoided, thereby reducing operating costs.

In contrast, in Comparative Example 1, the permeated water was not mixed at a mixing ratio R _mix (-) that satisfied the formula (1), so the concentration of soluble silica in the permeated water after mixing was excessively reduced, resulting in increased operating costs.

The present invention provides a method for producing purified water that minimizes the risk of silica scale while minimizing operating costs, as well as a method for reducing the risk of silica scale using such a membrane separation method.

11 Separation membrane (UF membrane)
17 Supply water 18 UF membrane permeated water 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: Amount of UF membrane permeate mixed F _NF: Amount of NF membrane permeate mixed C _Mix : Concentration of soluble silica in permeate after mixing

Claims (5)

A method for producing purified water includes a membrane separation step of permeating feed water containing soluble silica at a concentration C _Si (ppm) through a UF membrane and an NF membrane, and mixing the UF membrane permeate and the NF membrane permeate at a mixing ratio R _mix (-) that satisfies the following formula (1):
50≦[C _Si /(1+R _mix )]×[((100-R _NF )/100)×R _mix +1]<80
...(1)
(In the formula, C _Si represents the concentration of soluble silica (ppm), R _NF represents the soluble silica removal rate (%) of the NF membrane, and R _mix represents the mixing ratio (F _NF /F _UF )(-) of the amount of NF membrane permeate water mixed F _NF to the amount of UF membrane permeate water mixed F _UF .) The method for producing purified water according to claim 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. The method for producing purified water according to claim 1, wherein the NF membrane has a rejection rate of SO ₄ ^2- of 90% or more when treating an aqueous MgSO ₄ solution with a concentration of 2000 mg/L at an operating pressure of 0.76 MPa at 25°C. The method for producing purified water described in 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 and 25°C. A membrane separation step of producing purified water by the purified water production method according to any one of claims 1 to 4;
and 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.