JP3489315B2 - Water treatment method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating purified water, and more particularly, in a method for obtaining purified water from water by using a nanofiltration membrane composed of a low-pressure reverse osmosis membrane, a hardness component required as a umami component of purified water The present invention relates to a method for obtaining water having good water quality as purified water, by removing organic substances such as trihalomethane precursors to a high degree as well as removing water.

[0002]

In order to obtain purified water from fresh water and other raw water, contaminants such as impurities and trihalomethane precursors are highly removed, while hardness components (mineral components) that are umami components are It is important to remove water moderately (so that the required amount of hardness component remains) and obtain savory water.

Conventionally, purified water is produced by subjecting water from a water intake source to coagulation and sedimentation and sand filtration, followed by sterilization with chlorine. Further, the water obtained by such treatment may be subjected to ozone treatment, activated carbon adsorption treatment or biological treatment as a further advanced treatment.

The treatment mainly adopted as the advanced treatment is activated carbon adsorption treatment, but in the activated carbon adsorption treatment,
When activated carbon breaks through, there is a problem that the regeneration operation is required.

On the other hand, in the case of the membrane filtration method, efficient treatment can be performed without the need for regeneration treatment.

[0006] In particular, a nanofiltration method is mentioned as a membrane filtration method which is considered to be effectively applied to water purification treatment.

The nanofiltration method is 1 nanometer (1 nm)
It is a membrane filtration method that removes molecules with front and rear sizes, and is located between the ultrafiltration method and the reverse osmosis method, and the substance to be removed is a low molecular weight organic substance with a molecular weight of up to several hundreds (trihalomethane precursor, Agricultural chemicals, odorants, synthetic detergents, etc., hardness components such as calcium and magnesium, and evaporation residues.

[0008] In this nanofiltration method, a nanofiltration membrane composed of a low-pressure reverse osmosis membrane is used. Usually, in order to increase the water recovery rate, the membrane filtration is carried out at a high operating pressure as much as possible. ing.

[0009]

However, when nanofiltration is performed using a nanofiltration membrane composed of a reverse osmosis membrane,
Since the reverse osmosis membrane blocks the passage of hardness components, the resulting water has a small amount of mineral components and the taste is impaired. For example, most commercially available reverse osmosis membranes having a membrane performance with a trihalomethane precursor removal rate of 90% or more also have a hardness component removal rate of 80% or more.
When soft water is treated as raw water, the hardness component concentration is remarkably reduced and umami-free water is obtained.

On the other hand, commercially available ultrafiltration membranes do not prevent the passage of hardness components, but the removal rate of trihalomethane precursors is about 30 to 50%, which is unsuitable for advanced treatment of purified water.

The present invention has been made in view of the above-mentioned conventional circumstances, and appropriately removes a hardness component necessary as an umami component of purified water and highly removes an organic substance such as a trihalomethane precursor, It is an object of the present invention to provide a water purification method for obtaining water having good water quality as purified water.

[0012]

The water purification treatment method of the present invention is a water purification treatment method in which raw water is subjected to membrane filtration with a nanofiltration membrane, and the operating pressure of the membrane filtration is such that the removal rate of the hardness component by the membrane is pressure-dependent. Is a pressure range in which the removal rate of low molecular weight organic substances does not show pressure dependence.

The operating pressure is the transmembrane pressure, that is, the difference between the pressure on the raw water side of the membrane and the pressure on the permeate (filtered water) side.

In the present invention, a pressure range showing a pressure dependency with respect to the hardness component, that is, a pressure range in which the removal rate of the hardness component changes according to the pressure, is a low molecular weight organic substance such as a trihalomethane precursor. (Alternatively, the T
Membrane filtration is performed in a pressure range that does not show pressure dependency with respect to OC), that is, in a pressure range in which the removal rate is high and is maintained substantially constant even if the pressure changes.

By performing membrane filtration in such a pressure range, 90% or more of low molecular weight organic substances such as trihalomethane precursors are removed, while the removal rate of hardness components is suppressed to 80% or less. It is possible to obtain a purified water with a good taste by appropriately leaving the hardness component.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the water purification treatment method of the present invention will be described in detail below.

The nanofiltration membrane used in the method of the present invention has a trihalomethane precursor removal rate of 85 to 95% and a Ca ²⁺ ion removal rate of 50 to 80 at an operating pressure of 2 kgf / cm ^2. % Separation membranes are preferred.

In the water purification treatment method of the present invention, the operating pressure of the nanofiltration is within the pressure range in which the removal rate of the hardness component by the membrane shows the pressure dependence, and the removal rate of the low molecular weight organic matter has the pressure dependence. The pressure range is not shown. That is, in such a pressure range, the removal rate of the hardness component by the membrane is small, for example, 70% or less, and the removal rate of the low-molecular weight organic matter is 90% or more. .

The operating pressure adopted in the present invention varies depending on the performance of the membrane to be used, etc., but normally it is 1 to 3 kgf.
A low pressure of about / cm ² is preferable. By operating under such a low pressure condition, the contamination of the membrane is reduced, and it becomes possible to continue stable operation for a long period of time as compared with the case of operating at a high pressure.

In the water purification treatment method of the present invention, the operating pressure for nanofiltration is thus within the pressure range in which the removal rate of the hardness component by the membrane shows pressure dependency, and the removal rate of the low-molecular organic matter is the pressure. It can be carried out according to a conventional method, except that the pressure range is not dependent.

The nanofiltration according to the present invention is generally carried out after the sand filtration in the usual water purification treatment. That is, a vessel loaded with a nanofiltration element via a pressure pump is provided in the subsequent stage of the flocculation sedimentation tank and the sand filtration tank, and the permeated water is sterilized by chlorine to obtain purified water. in this case,
Further, advanced treatment such as ozone treatment or activated carbon adsorption treatment may be performed, and this advanced treatment may be omitted. Further, the nanofiltration according to the present invention may be performed on purified water obtained by a normal water purification treatment.

The pressure pump has a head of 30 m.
It is possible to use general centrifugal pumps of up to about H ₂ 0.

As the form of the membrane module, the spiral type is preferable, but other forms may be used.

[0024]

EXAMPLES The present invention will be described in more detail with reference to the following examples.

Example 1 A low pressure reverse osmosis membrane element having the following specifications was used to perform membrane filtration using Sagami Lake water having the water quality shown in Table 1 as raw water.

Low Pressure Reverse Osmosis Membrane Element Spiral low pressure reverse osmosis membrane element "TFC4921S" manufactured by Fluidsystems, USA Direct diameter: 4 inches Length: 1 m Effective pressure: 24 kgf / cm ² or less Salt desalination rate: Operating pressure 5.5 kgf / removal rate of trihalomethane precursors in the case of cm ² in 85% operating pressure 2 kgf / cm ^2: Ca ²⁺ ion removal rate under a condition of 95% operating pressure 2 kgf / cm ^2: 70% trihalomethanes removal rate of precursor pressure Pressure range that does not show dependence: 1.0 kgf / cm ² or more Pressure range where removal rate of Ca ²⁺ shows pressure dependence: 7 kgf
/ Cm ² or less Average operating pressure is 2.5 kgf / cm ² and water recovery rate is 7
Membrane filtration was performed at 5%. The average amount of permeated water is 2.5
It was m ³ / book / day. Table 1 shows the permeated water quality, the initial permeated water amount, and the permeated water amount after one month has elapsed.

Examples 2 and 3, Comparative Examples 1 to 3 Example 1 except that the average operating pressure was as shown in Table 1.
Membrane filtration was performed in the same manner as in Sagami Lake water. The permeated water quality and the amount of permeated water are shown in Table 1.

[0028]

[Table 1]

Example 4, Comparative Example 4 Tap water of Atsugi City was passed through a spiral ultrafiltration membrane having a cut-off molecular weight of 50,000, and then sodium bisulfite was continuously injected to reduce residual chlorine in the tap water. Let it be raw water. This raw water was passed through a pressure vessel loaded with a spiral low pressure reverse osmosis membrane "Element TFC4921S" similar to that used in Example 1 while changing the operating pressure.

The results are shown in Table 2.

[0031]

[Table 2]

As shown in Table 2, the operating pressure is 2.5 kgf / c.
The condition set for m ^2, permeate flow rate is 2.5 m ³ / present /
It was stable for one month a day, but the operating pressure was 8 kgf / cm.
^Under the condition set to ² , the permeated water amount was initially 8.0 m ³ / line / day, but decreased with the passage of water,
After 1 month, it was remarkably decreased to 2.8 m ³ / piece / day.

As is apparent from Tables 1 and 2, according to the present invention, it is possible to stably obtain water containing a hardness component in an appropriate amount and having an extremely low organic substance concentration for a long period of time.

[0034]

As described in detail above, according to the water purification treatment method of the present invention, the hardness component necessary as the umami component of the purified water is appropriately removed by the nanofiltration method, and the organic substances such as trihalomethane precursors are highly removed. Water which has good water quality can be obtained as purified water.

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Claims (2)

(57) [Claims] 1. A water purification treatment method for membrane filtration of raw water through a nanofiltration membrane, wherein an operating pressure of the membrane filtration is a pressure range in which a removal rate of a hardness component by the membrane shows pressure dependency,
Moreover, the water purification method is characterized in that the removal rate of the low-molecular weight organic matter is within a pressure range that does not show pressure dependence. 2. The method according to claim 1, wherein the operating pressure is 1
The water purification method is characterized in that it is set to 3 kgf / cm ² .