JP2003326259A - Fresh water generating method and fresh water generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fresh water generating method and a fresh water generator by which fresh water can be generated at a low running cost by using an NF membrane and/or an RO membrane and in which long-term stable operation is made possible by preventing a pipeline from being corroded. <P>SOLUTION: Raw water containing at least silica is separated into permeated water and concentrated water by using the NF membrane and the RO membrane. When S≥2t-0.012 (wherein t (°C) is the temperature of the raw water; S (mg/liter) is the silica concentration of the concentrated water; Z (μS/cm) is the electric conductivity of the concentrated water) is satisfied, the pH of the raw water should be adjusted to ≤6. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention uses an NF membrane (nanofiltration membrane) and / or RO membrane (reverse osmosis membrane) suitable for obtaining drinking water from fresh water such as river water and lake water. The present invention relates to a fresh water producing method and a fresh water producing apparatus.

[0002]

2. Description of the Related Art In recent years, RO membranes and NF membranes have been used in various fields of desalination such as production of drinking water and industrial water. R
Since the O film has a high removal rate of monovalent ions, it is often used for desalination of seawater or brackish water, production of ultrapure water, or the like.
On the other hand, the NF membrane is said to be suitable for obtaining drinking water from fresh water such as river water and lake water because the removal rate of monovalent ions is lower than that of the RO membrane.

Now, when using these films, in general,
A cross-flow method is often adopted in which the raw water flows along the membrane surface and the permeated water flows in a direction orthogonal to the flow direction of the raw water. In this method, a part of the supplied raw water gives a shearing force to the membrane surface, so that it is possible to suppress adhesion and deposition of fouling substances such as suspended substances and colloidal substances in the raw water on the membrane surface. it can. On the other hand, the higher the recovery rate, of course, the better, but if the recovery rate is to be increased, the concentration ratio must be increased and the load on the membrane becomes large. In particular, fouling caused by microorganisms and humic acid, and scales caused by calcium and magnesium depositing as calcium carbonate, calcium sulfate hydrate, and magnesium hydroxide on the film surface, and silica attaching to the film surface. If a scale is generated due to, the amount of treated water will decrease and the differential pressure will increase, making stable operation difficult.

Further, when producing drinking water using, for example, an NF membrane, the removal rate of silica is preferably low from the viewpoint of preventing the formation of silica scale. However, even with an NF film having a low silica removal rate, when the raw water has a low water temperature and polyvalent metal ions such as aluminum ions, calcium ions, magnesium ions, and iron ions are present in the raw water, the silica scale The progress will be faster. In order to prevent this, the pH of the raw water is always 6 or less, preferably 4
It is said that it is recommended to lower the pH of the raw water to about 5.5, but if the pH of the raw water is lowered, the pH of the permeated water is also lowered. Therefore, the pH of the raw water is 5.8 to 8.6 neutral. It may not meet the tap water standard that it must be. In that case, the pH of the permeated water must be returned to neutral by using a chemical, which increases the running cost. Further, even when the alkalinity of the raw water is high, the chemicals are used to lower the pH to 6 or less, so that the running cost is similarly increased. Further, if the pH of the raw water is constantly kept at 6 or less, corrosion of metal pipes and the like progresses and iron elutes, which adheres to the membrane surface and fouling progresses. Will happen.

Therefore, the pH of the raw water can be adjusted only when the quality of the concentrated water is such that silica scale is produced.
However, it is very difficult to find out the conditions for forming silica scale because the solubility of silica greatly changes depending on the concentration of impurities in the raw water and the water temperature.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to obtain an NF membrane or an NF membrane from the temperature of raw water, the concentration of silica in concentrated water, and the electric conductivity of concentrated water. A desalination method and a desalination method that reduces the running cost by keeping the pH of the raw water at 6 or less according to the prediction of the production of silica scale of the RO membrane and prevents the corrosion of the pipes and the like to enable a long-term stable operation. To provide the equipment.

[0007]

In order to achieve the above object, the present invention separates raw water containing at least silica into permeated water and concentrated water by using an NF membrane and / or RO membrane, and t (° C.), silica concentration S (mg / liter) in concentrated water, and electric conductivity Z (μ
S / cm) is an inequality, and when S ≧ 2t−0.01Z, the pH of the raw water is adjusted to 6 or less. The pH is preferably adjusted so as to fall within the range of 4 to 5.5. Also, NF
It is also preferable that the concentrated water from the membrane and / or RO membrane is further separated into permeated water and concentrated water by using an NF membrane and / or RO membrane. Furthermore, it is also preferable to supply the raw water to the NF membrane and / or the RO membrane after solid-liquid separation. For solid-liquid separation, it is preferable to use an MF membrane and / or a UF membrane. It is also preferable to add a flocculant to the raw water before solid-liquid separation. Furthermore, solid-liquid separation may be performed so that the FI value is 5 or less, preferably 3 or less.

Further, in order to achieve the above object, the present invention provides an NF membrane module and / or a UF membrane module for separating raw water containing at least silica into permeated water and concentrated water, and a temperature t (° C.) of the raw water. Temperature detecting means for detecting the concentration, silica concentration detecting means for detecting the silica concentration S (mg / liter) in the concentrated water, and electric conductivity Z (μ of the concentrated water
S / cm) for detecting electric conductivity, pH detecting means for detecting pH of concentrated water, and temperature t (° C.) of raw water
When the silica concentration S (mg / liter) in the concentrated water and the electric conductivity Z (μS / cm) of the concentrated water satisfy the inequality, S ≧ 2t−0.01Z, the pH of the raw water is 6 Adjust to p
A H-adjusting means is provided. The pH adjusting means has a pH of the raw water of 4 to 5.5.
It is preferable that it is a means for adjusting so as to fall within the range. It is also preferable to include an NF membrane module and / or RO membrane module that further separates the concentrated water from the NF membrane module and / or the RO membrane module into permeated water and concentrated water. Furthermore, it is also preferable to provide a solid-liquid separation means for supplying raw water to the NF membrane and / or RO membrane after solid-liquid separation. The solid-liquid separation means is preferably a means using an MF membrane and / or a UF membrane. It is also preferable to provide a coagulant adding means for adding a coagulant to the raw water before solid-liquid separation.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic flow chart showing a fresh water generator according to one embodiment of the present invention. In the following, the case where an NF film is mainly used will be described as an example.
The same applies when a film is used. Depending on the type of raw water and the desired water quality, an NF membrane, an RO membrane, or both may be used together.

In the apparatus shown in FIG. 1, raw water such as river water and lake water is temporarily stored in a raw water tank 1 and then sent by a raw water pump 2 to a solid-liquid separation device 3 for solid-liquid separation. Device 3 removes suspended matter.

The solid-liquid separation device 3 is, for example, a sand filtration device or a membrane separation device. Among them, MF membranes (microfiltration membranes) or UFs having a pore size of 1 μm or less that prevent microorganisms from entering therein. A membrane separator using a membrane (ultrafiltration membrane) is preferable. In many cases, such a membrane separation device can reduce the FI value (Fouling Index) of the treated water to 2 or less, and has high turbidity, which enables more stable operation of the NF membrane. A membrane separation device using a hollow fiber membrane made of polysulfone, polyacrylonitrile, polyvinylidene fluoride, or the like is preferable because the filling rate of the membrane is high and the effective membrane area can be made large. The membrane separator is preferably of a constant flow rate filtration operation system because a constant amount of treated water can be obtained and the control of the entire process is facilitated.

When a membrane separator is used as the solid-liquid separator, a flocculant such as aluminum sulfate, polyaluminum chloride or ferric chloride may be added to the raw water. By adding a coagulant, it becomes possible to increase the permeation flux and suppress the rise in filtration differential pressure, and it is possible to increase the amount of treated water, improve the stability of operation, extend the life, etc. Become. The coagulant is preferably used in a small amount so that microflock can be formed so that the drug cost and running cost do not increase.

The treated water from the solid-liquid separation device 3 is temporarily stored in the treated water tank 4 and then, by the treated water pump 5, two NF membrane modules 6 and 6 connected in parallel.
Sent to. A temperature sensor 12 is attached to the treated water tank 4 so that the temperature of the treated water (raw water of the NF membrane module) sent to the NF membrane modules 6 can be measured. In addition, in this form,
Although two NF membrane modules are provided in parallel, one may be provided, or three or more NF membrane modules may be provided in parallel. The number of NF membrane modules may be appropriately selected according to the amount of treated water desired to be obtained.

The NF membrane module is obtained by modularizing a flat membrane of polyamide type, polypiperazine type, polyesteramide type, etc. into a spiral type, a pleated type or the like, or by bundling hollow fiber membranes in a U shape or an I shape. It's like modularization. It is preferable that these modules can be operated at low pressure from the viewpoint of suppressing running cost. In addition, a module using a composite membrane formed by forming an NF membrane on a base material is preferable because a large amount of treated water can be obtained even in low-pressure operation. Among them, a polyamide-based or polypiperazine-based composite film is preferable because it has excellent chemical resistance. Further, from the viewpoint of preventing the formation of scale, it is preferable that the inhibition rate of silica, calcium and magnesium is low.

The RO membrane module is different from the NF membrane module only in the membrane material used. As a membrane material,
Although there are cellulose acetate type, polyamide type, etc., an aromatic polyamide composite membrane having high separation performance and high water permeability is preferable.

The permeated water of the NF membrane modules 6 and 6 is taken out as it is, but the concentrated water is sent to another NF membrane module 7 arranged on the downstream side of the NF membrane modules 6 and 6, and the permeated water is discharged. And concentrated water.
A silica concentration detector 13 and an electric conductivity meter 14 are provided in the conduits of the concentrated water from the NF membrane modules 6 and 6 to measure the silica concentration in the concentrated water and the electric conductivity of the concentrated water. You can do it. Similarly, the silica concentration detector 13 is also provided in the concentrated water pipeline of the NF membrane module 7.
And an electric conductivity meter 14 are provided. Further, a pH sensor 15 is provided in this conduit so that the pH of the concentrated water by the NF membrane module 7 can be measured. It should be noted that measurement of the temperature of treated water with a temperature sensor, measurement of silica concentration with a silica concentration detector, and measurement of electric conductivity with an electric conductivity meter rarely cause rapid changes, so It only needs to be done once every few days. As a silica concentration detector, sensitivity,
Inductively coupled plasma optical emission spectroscopy (IC
P method: Inductivity Coupled Plasma) is preferable.

In this embodiment, as described above, N
The F membrane modules are installed in multiple stages, and the concentrated water from the NF membrane module in the previous stage is further processed in the NF membrane module in the subsequent stage. The recovery rate can be improved by performing the treatment in multiple stages, but this is not essential, and treatment by only one stage of the NF membrane module may be performed. In case of multi-stage, NF of the latter stage
The concentration of silica, calcium, magnesium, etc. in the concentrated water by the membrane module should not exceed their solubility.

The operating pressure (filtration pressure) of the NF membrane modules 6, 6 and 7 is about 0.5 to 3 MPa, although it varies depending on the type of raw water, the operating method and the like. Since fresh water such as river water and lake water has a low osmotic pressure, it can be filtered at a relatively low pressure. Further, the treated water by these NF membrane modules is one in which trihalomethane precursor substances, pesticides, heavy metal ions, etc. have been removed, so that it can be used as it is as drinking water.

The chemical tank 8 has an NF membrane module 6,
6 of raw water and / or p of raw water of NF membrane module 7
A chemical solution for adjusting H is stored. The chemical solution is added by the chemical solution pump 9 to the raw water of the NF membrane modules 6 and 6 via the valve 10 and to the raw water of the NF membrane module 7 via the valve 11. The chemical solution may be acidic, but the organic acid is preferably an inorganic acid such as hydrochloric acid or sulfuric acid because it increases the COD value of concentrated water and requires treatment of waste water. The chemical liquid pump 9 has the above-mentioned pH.
The pH sensor 15 is electrically connected to the sensor 15.
It is possible to control the amount of chemical solution delivered by the chemical solution pump 9 according to the pH value of the concentrated water measured in step.

Now, the temperature t (° C.) of the raw water measured by the temperature sensor 12 and the silica concentration S (mg / liter) in the concentrated water measured by the silica concentration detector 13,
When the electric conductivity Z (μS / cm) of the concentrated water measured by the electric conductivity meter 14 satisfies the following inequality, S ≧ 2t−0.01Z, the concentrated water is neutral and impurities in the concentrated water are When the hardness component, aluminum, and the like are contained in addition to silica, it indicates that the conditions are such that the possibility of progress of the silica scale becomes extremely high. It should be noted that, although this is usually impossible, the conditions under which the possibility of progress of the silica scale is extremely high when impurities other than silica are not included are S and S.
≧ 3 to 4t.

When the condition of S ≧ 2t-0.01Z is satisfied, the chemical solution pump 9 is operated to supply the chemical solution in the chemical solution tank 8 to the raw water of the NF membrane module 6, 6 or 7, and the raw water is supplied. pH is 6 or less, preferably 4 to
Adjust so that it is within the range of 5.5. That is, NF
When the raw water of the membrane modules 6 and 6 satisfies the condition of S ≧ 2t−0.01Z, the chemical liquid pump 9 is operated, the valve 10 is opened, and the chemical liquid in the chemical liquid tank 8 is changed to the raw water of the NF membrane modules 6 and 6. Supply. Similarly, the NF membrane module 7
When the condition of the raw water of S satisfies the condition of S ≧ 2t−0.01Z, the chemical liquid pump 9 is operated and the valve 11 is opened to supply the chemical liquid in the chemical liquid tank 8 to the raw water of the NF membrane module 7. As a result, the pH of the raw water of the NF membrane module 6, 6 or 7 is adjusted to 6 or less, preferably in the range of 4 to 5.5, and it is possible to prevent inconveniences such as an increase in running cost and corrosion of piping and the like. As it becomes
The NF membrane module can be stably operated over a long period of time.

[0022]

[Examples and Comparative Examples] (Examples) The fresh water generator shown in FIG. 1 was used. River water was used as raw water for the NF membrane modules 6 and 6. Further, as the solid-liquid separation device 3, an external pressure type polyacrylonitrile-based hollow fiber membrane type UF membrane module was used. The operation was a constant flow rate operation, and the permeation flux was 0.8 m / day. In addition, the NF membrane modules 6, 6 and 7 include
A spiral type module having a polyamide composite membrane with a desalination rate of 55% was used. The operation is performed with a permeation flux of 0.5 m /
The constant flow rate operation was set as a day, and the recovery rate was set to 80%.
Further, the temperature of the treated water is measured by the temperature sensor 12, and the silica concentration is measured by the silica concentration detectors 13, 13.
The electric conductivity measured by the electric conductivity meters 14 and 14 is 1
Times / day Furthermore, 1 mol for the chemical solution for pH supply
/ Liter of sulfuric acid was used.

When the raw water of the NF membrane module 6, 6 or 7 satisfies the condition of S ≧ 2t-0.01Z, the chemical liquid pump 9 is operated to move the chemical liquid in the chemical liquid tank 8 to the NF membrane module 6, 6 or 7 PH of raw water is 4
I adjusted it to be. The operating pressure of the NF membrane module was 0.35 MPa at the start of operation,
By repeating the above-mentioned operation, it was stable at 0.42 MPa even after 3,000 hours had elapsed. The amount of sulfuric acid used was 35 liters after the start of operation for 3,000 hours. No corrosion of the piping was observed. (Comparative Example 1) The water producing apparatus shown in FIG. 1 was used. However, the silica concentration detectors 13, 13, the electric conductivity meters 14, 14,
The pH meter 15 was not activated.

The chemical liquid pump 9 was constantly operated to supply the chemical liquid in the chemical liquid tank 8 to the raw water of the NF membrane modules 6, 6 and 7, and the pH of the raw water was adjusted to 4. The operating pressure of the NF membrane module is 0.3 at the start of operation.
5 MPa, 0.4 until 2,700 hours have passed
It was stable at 5 MPa, but then rose sharply,
It reached 0.9 MPa after 3,000 hours. The amount of sulfuric acid used was 65 liters after the start of operation for 3,000 hours. Furthermore, slight corrosion was observed on some of the piping. (Comparative Example 2) The water producing apparatus shown in FIG. 1 was used. However, the silica concentration detectors 13 and 13 and the electric conductivity meters 14 and 14 were not operated. Also, the chemical pump 9 is not operated,
The valves 10 and 11 were also kept closed.

The operating pressure of the NF membrane module was 0.35 MPa at the start of the operation, but after 2,000 hours had passed, it increased significantly to 1.7 MPa, and the filtration performance of the NF membrane module was recovered by chemical cleaning. The situation is unavoidable.

[0026]

According to the present invention, as is clear from the comparison between the example and the comparative example, the NF membrane and the RO can be determined from the temperature of the raw water, the concentration of silica in the concentrated water, and the electric conductivity of the concentrated water. The pH of the raw water was adjusted to 6 according to the prediction of the formation of silica scale in the membrane.
Not only can running costs be reduced by the following, but corrosion of the pipes and the like can be prevented, and stable operation can be performed over a long period of time.

[Brief description of drawings]

FIG. 1 is a schematic flow chart showing a water production device according to one embodiment of the present invention.

[Explanation of symbols]

1: Raw water tank 2: Raw water pump 3: Solid-liquid separator 4: Treated water tank 5: Treated water pump 6: NF membrane module 7: NF membrane module 8: Chemical tank 9: Chemical pump 10: valve 11: valve 12: Temperature sensor 13: Silica concentration detector 14: Electric conductivity meter 15: pH sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C02F 1/52 C02F 1/52 ZF term (reference) 4D006 GA03 GA04 GA06 GA07 HA01 HA41 HA61 HA71 KA02 KA03 KA33 KA52 KA53 KA54 KA55 KA57 KA67 KB13 KB14 KD08 KD11 KD12 KE11P KE15P KE15Q KE15R KE16P KE19P KE23Q MA01 MB11 MC18 MC29 MC39X MC51 MC54 MC62 PA01 PB04 FA37 FA02 FA17 FA17 FA02 FA17 FA02 FA23 BB05 DA08 FA14 BB05 DA08 FA14 BB05 DA08 FA14 BB04

Claims (14)

[Claims] 1. Raw water containing at least silica is separated into permeated water and concentrated water using an NF membrane and / or an RO membrane, and the temperature t (° C.) of the raw water and the silica concentration S (mg / mg / Liter) and electric conductivity Z (μ
S / cm) is an inequality, S ≧ 2t−0.01Z, the pH of the raw water is adjusted to 6 or less. 2. The method for producing water according to claim 1, wherein the pH is adjusted to be in the range of 4 to 5.5. 3. The method for producing water according to claim 1, wherein the concentrated water from the NF membrane and / or RO membrane is further separated into permeated water and concentrated water by using the NF membrane and / or RO membrane. 4. The method for producing water according to claim 1, wherein the raw water is subjected to solid-liquid separation and then supplied to the NF membrane and / or the RO membrane. 5. The method for producing water according to claim 4, wherein solid-liquid separation is performed using an MF membrane and / or a UF membrane. 6. The method for producing water according to claim 4, wherein a flocculant is added to the raw water before solid-liquid separation. 7. The method for producing water according to claim 4, wherein solid-liquid separation is performed so that the FI value becomes 5 or less. 8. The method for producing water according to claim 7, wherein solid-liquid separation is performed so that the FI value is 3 or less. 9. An NF membrane module and / or UF for separating raw water containing at least silica into permeated water and concentrated water.
Membrane module, temperature detecting means for detecting temperature t (° C) of raw water, and silica concentration S (mg / liter) in concentrated water
Silica concentration detecting means for detecting the concentration, electric conductivity detecting means for detecting the electric conductivity Z (μS / cm) of the concentrated water, pH detecting means for detecting the pH of the concentrated water, and temperature t of the raw water.
(° C) and silica concentration S in concentrated water (mg / liter)
And the electric conductivity Z (μS / cm) of the concentrated water satisfy the inequality, S ≧ 2t−0.01Z, the pH of the raw water is adjusted to 6 or less p
A H-adjusting means, and a desalination apparatus. 10. The pH adjusting means adjusts the pH of the raw water to 4-5.
10. A means for adjusting to fall within the range of 5.
Desalination apparatus according to. 11. The method according to claim 9, further comprising an NF membrane module and / or an RO membrane module for separating the concentrated water by the NF membrane module and / or the RO membrane module into permeated water and concentrated water. Desalination equipment. 12. An NF membrane and / or after solid-liquid separation of raw water.
Alternatively, the desalination apparatus according to any one of claims 9 to 11, further comprising solid-liquid separation means for supplying the RO membrane. 13. The solid-liquid separation means is an MF membrane and / or U.
The desalination apparatus according to claim 12, which is a means using an F membrane. 14. A coagulant addition means for adding a coagulant to raw water before solid-liquid separation is provided.
The water producing apparatus according to item 3.