JPH10225682A - Method of removing boron in reverse osmosis seawater desalination - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of removing boron in a reverse osmosis seawater desalination device in which method a filter membrane separation device compact and easy to maintain is used as a pretreatment of a reverse osmosis membrane device to highly and effectively remove boron and clean water for finish is not needed in a final process. SOLUTION: Raw seawater to which a pH modifier and a coagulant for forming a boron containing insoluble deposit are added is fed into a circulating water tank. A supernatant in the circulating water tank is filtered by a microfilter membrane or an ultrafilter membrane and unfiltered component concentrated water of the microfilter membrane or the ultrafilter membrane is circulated to the circulating water tank and filtered water is permeated through a reverse osmosis membrane, allowing fresh water which seawater gets rid of boron to form to be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reverse osmosis seawater desalination method capable of highly and efficiently removing boron in seawater.

[0002]

2. Description of the Related Art Various technologies for desalination of seawater using reverse osmosis membranes have been developed, and most of them are those that clarify raw seawater by filtering it with a filter medium such as sand (revised Chemical Industry Handbook). Fourth Edition, p. 938, Maruzen, 1978, Science and Technology in Japan, Vol. 26, No. 235, p. 85, 1985).

[0003] This conventional reverse osmosis seawater desalination has been performed using, for example, an apparatus as shown in FIG. As shown in FIG.
In the process of being supplied to the sand filtration device 105, a disinfectant for disinfecting microorganisms, bacteria and the like contained in the raw seawater and a flocculant for flocculating suspended substances and colloidal substances contained in the raw seawater ( Ferric chloride is mainly used) is added in advance through the pipes 103 and 104, respectively. Next, the treated water 106 from which suspended matter has been removed by the sand filtration device 105 is introduced into the storage tank 107. Further, the treated water 106 is supplied to the
It is pressurized to kg / cm ² and supplied to the reverse osmosis membrane device 109 for desalination. Fresh water is collected from pipe 110,
The concentrated water is collected from the pipe 111. Further, in order to make the concentration of boron in fresh water 0.2 mg / l or less, which is a water quality monitoring item value according to the tap water method, it is suitable as drinking water after blending with tap water supplied from the pipe 113 in the production water tank 112. The production water is obtained.

[0004]

In the above prior art,
The area where fresh water suitable for drinking can be obtained by the reverse osmosis method is limited to an area where tap water necessary for blending in the final step can be sufficiently supplied. For this reason, it has been impossible to use the reverse osmosis seawater desalination apparatus in an area suffering from chronic water shortage or for emergency drought and disaster countermeasures.

The present invention has been made to overcome the above-mentioned drawbacks of the prior art, and uses a compact and easy-to-maintain filtration membrane separation apparatus as a pretreatment for a reverse osmosis membrane apparatus. An object of the present invention is to provide a method for removing boron in a reverse osmosis seawater desalination apparatus which can efficiently remove boron and does not require clean water for finishing in the final step.

[0006]

In order to solve the above-mentioned problems, a method for removing boron in a reverse osmosis seawater desalination according to the present invention is directed to a method for forming a pH-adjusting agent and a boron-containing insoluble precipitate in raw seawater. An agent is added and supplied to a circulating water tank, and the supernatant of the circulating water tank is filtered through a microfiltration membrane or an ultrafiltration membrane. It is configured to circulate and permeate filtered water through a reverse osmosis membrane.

In the above method, the pH of the raw seawater is adjusted to 9 or more, a calcium compound or an aluminum compound is used as a flocculant, or the washing wastewater of a microfiltration membrane or an ultrafiltration membrane is supplied to a circulating water tank. The above object is more preferably achieved.

In the raw seawater targeted by the present invention, about 4 m
g / l of boron, usually present in the form of boric acid or borate.

In the present invention, the rate of boron deposition due to coprecipitation and the like is increased by returning non-filtration component concentrated water of the microfiltration membrane or ultrafiltration membrane to the circulating water tank. The amount can be less than 20% of the raw seawater. And about 75-85% can be further removed in the reverse osmosis membrane device. Therefore,
The boron content in the final production water can be reduced to 0.2 mg / l or less, which is the monitoring value of water quality by the Water Supply Law, and can be used as drinking water without using clean tap water in the final stage. Suitable production water can be obtained.

In this method, the pH of the raw seawater is adjusted to 9 or more, a calcium compound or an aluminum compound is used as a coagulant, or the cleaning wastewater of a microfiltration membrane or an ultrafiltration membrane is supplied to a circulation water tank. The removal rate of boron can be further increased.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The coagulant used in the present invention is capable of reacting with borate ions contained in seawater to form a boron-containing insoluble precipitate. For example, a calcium compound or an aluminum compound is used. Can be Examples of the calcium compound include calcium hydroxide. Examples of the aluminum compound include aluminum sulfate, polyaluminum chloride (PAC), and the like.
The addition amount (total amount when the Al compound and the Ca compound are used together) is about 1 to 1000 mg / l with respect to the raw seawater,
Preferably, the amount is about 5 to 100 mg / l. However, it goes without saying that it can be increased or decreased according to the quality of the raw seawater.

As the pH adjuster, for example, sodium hydroxide or the like can be used and calcium hydroxide or the like can also be used. The pH adjuster makes the pH of the raw seawater 7 or more, preferably 8 or more, and more preferably 9 or more. On the other hand, the upper limit of the pH is about 11, preferably about 10.

In the raw seawater, a disinfectant, if necessary,
Other agents can be added. As a disinfectant, chlorine, sodium hypochlorite, or the like is used. The fungicide is added in an amount of about 1 to 10 mg / l, usually about 1 to 5 mg / l.

The coagulant may be added in the form of an aqueous solution, and the concentration may be about 5 to 50 mg / l. The temperature of the raw seawater to which the flocculant is added does not need to be particularly adjusted and may be left as it is.
It is preferred to adjust to ° C.

The addition of the pH adjuster and the coagulant may be carried out through a pipe for supplying the raw seawater to the circulating water tank, or may be directly introduced into the circulating water tank.

The circulating water tank forms a circulation line with a device equipped with a microfiltration membrane or an ultrafiltration membrane, and returns the non-filtration component concentrated water of the microfiltration membrane or the ultrafiltration membrane to the water tank. It has a structure to make it. The raw seawater causes the boron-containing insoluble precipitate to coagulate and settle in this tank. The volume of the circulating water tank is designed so that the residence time (passage time) of the raw seawater is about 1 to 10 hours, preferably about 2 to 5 hours.

A microfiltration membrane or an ultrafiltration membrane is mounted on the filtration device at the preceding stage.

The ultrafiltration membrane has a pore size of 0.01 to 0.1 μm.
m, the molecular weight cut-off is 1000 or more, usually 1,000 to 10
It is about ten thousand. Preferred in the present invention are those having a molecular weight cut off of 1
It is about 100,000 to 100,000. The material is cellulose acetate, polyacrylonitrile, polyamide-imide,
There are polyamide-based, polysulfone-based, polyvinylidene fluoride-based, and polyolefin-based ones, and any of them may be used. Preferred are polyacrylonitrile-based and polyolefin-based ones.

The microfiltration membrane has a pore size of about 0.1 to 1 μm. The material is cellulose acetate, nitrocellulose, polyvinyl chloride, polysulfone, polyamide polycarbonate, polypropylene, polytetrafluoroethylene, etc., and any of them may be used. Polypropylene,
Polysulfone type and the like. The shape of the module of the microfiltration membrane or the ultrafiltration membrane may be either a tube type or a immersion tank type.

Further, a plurality of devices equipped with these filtration membranes may be provided in series or in parallel.

The former filtration membrane device mainly removes suspended substances (including colloids, microorganisms, etc.) in the raw seawater, and the boron-containing insoluble precipitate flowing out of the circulating water tank is also used here. Is captured by

The filtration membrane is backwashed to recover the decrease in filtration ability due to clogging. By introducing the backwash wastewater into the circulation water tank, the concentration of the boron-containing insoluble precipitate in the circulation water tank is reduced. To promote the precipitation and precipitation of boron from raw seawater. Raw seawater, membrane filtered water or the like may be used as the backwash water, or pressurized air or the like may be fed into the backwash means.

The reverse osmosis membrane is formed by adhering a polyamide, polyethyleneimine, polyethylene oxide, cellulose acetate, polysulfone or polyfuran membrane to the inner surface of a membrane support made of synthetic paper or the like. is there. Preferred are cellulose acetates, polyamides and the like. As the shape of the membrane, there are various types such as a flat membrane type, a tubular type, a spiral type, and a hollow fiber type. A plurality of devices equipped with reverse osmosis membranes may be provided in series or in parallel.

In the latter membrane apparatus, fresh water is obtained mainly by separating dissolved inorganic salts and the like.

[0025]

【Example】

(Example 1) The apparatus shown in FIG. 1 was used. This apparatus comprises a line mixer 6, a circulating water tank 8, a filtration membrane separation device 12, a backwash means 14, a storage tank 18, a reverse osmosis membrane device 20, and a production water tank 23. A raw seawater supply pipe 1 is connected to an inlet side of the line mixer 6 via a pump 2.
In the middle of the pipe 1, a disinfectant supply pipe 3, a pH adjusting agent supply pipe 4, and a coagulant supply pipe 5 are connected. The outlet side of the line mixer 6 is connected to a circulating water tank 8 by a pipe 7. A precipitate discharge pipe 9 is connected to the bottom of the circulating water tank 8, and the pipe 9 is opened and closed by a valve 10. The outlet side of the circulating water tank 8 is connected to the inlet of the filtration membrane separator 12 via a pump 11, and the non-filtrate side outlet of the filtration membrane separator 12 is connected to the non-filtration component concentrated water return pipe 1.
3 is connected to the circulating water tank 8 to form a circulating line as a whole. On the other hand, a backwash water pipe 15 is connected to the filtrate side, and the backwash drainage is supplied to the circulating water tank 8 through the pipe 16. The outlet pipe 17 of the filtration membrane separation device 12 is connected to a storage tank 18, and the storage tank 18 is connected to a reverse osmosis membrane device 20 via a high-pressure pump 19. A concentrated water pipe 22 and a fresh water pipe 21 are connected to an outlet of the reverse osmosis membrane device 20. Fresh water pipe 21 is used for production tank 2
3 from which fresh water is withdrawn as production water.

Using this apparatus, fresh water from which boron was removed from seawater was produced as follows. First, boron 4.
In a pipe for supplying raw seawater containing 1 mg / l from a pipe 1 to a circulating water tank 8 by a pump 2, calcium hydroxide 70 m
g / l and 10 mg / l of aluminum sulfate were added to adjust the pH of the raw seawater to 9.1. This was sufficiently stirred in a pipe by a line mixer 6 to generate an insoluble precipitate, and then charged into a circulating water tank 8. The liquid was sent from the circulating water tank 8 to the filtration membrane separation device 12 by the pump 11, and the non-filtration side liquid was circulated to the circulating water tank 8 by the concentrated water return pipe 13. The filtrate is put into a storage tank 18 through a pipe 17, and is pressurized to about 60 kg / cm ² by a pump 19, and
0. The concentrated water of the reverse osmosis membrane device 20 was taken out from the pipe 22, and the fresh water was put into the birth tank 23 from the pipe 21. The backwash wastewater from the filtration membrane separator was not charged into the circulating water tank.

The specifications and operating conditions of each membrane device are as follows.

Microfiltration membrane device Membrane Material: Polypropylene Membrane Shape: Hollow fiber type Nominal pore size: 0.2 μm Filtration method: External pressure type total filtration Membrane flux: 1.5 m / day Backwash interval: 20 minutes filtration / Backwashing reverse osmosis membrane device for 3 minutes Material: Polyamide synthetic composite membrane Shape: Spiral type Salt rejection: 99.5% Operating pressure: 60 kg / cm ²

A water flow experiment was conducted under the above experimental conditions.
The result as shown in FIG.

Example 2 Backwash wastewater in Example 1 was charged into a circulating water tank, and the results shown in Table 1 were obtained.

Example 3 The same treatment as in Example 2 was carried out except that the amount of calcium hydroxide added was 50 mg / l and the pH of the raw seawater was 8.0, and the results shown in Table 1 were obtained.

(Comparative Example 1) The same treatment as in Example 2 was carried out except that aluminum sulfate and calcium hydroxide were not added to the raw seawater, and the results shown in Table 1 were obtained.

[0033]

[Table 1]

Example 4 Using the apparatus shown in FIG. 1, 5 mg / l aluminum sulfate and 60 mg / l calcium hydroxide were added to raw seawater containing 4.15 mg / l boron to adjust the pH to 9.0. And processed using an ultrafiltration membrane device and a reverse osmosis membrane device having the following specifications. The backwash wastewater from the filtration membrane separator was not charged into the circulating water tank.

Microfiltration membrane device Membrane material: Polyacrylonitrile Membrane Shape: Hollow fiber type Molecular weight cut off: 13000 Filtration method: External pressure type cross flow filtration Membrane flux: 0.7 m / day Backwash interval: 20 minutes filtration / 20 seconds backwashing Circulating water volume: 1.5 times the amount of raw seawater Reverse osmosis membrane device Membrane Material: Polyamide-based synthetic composite membrane Membrane shape: Spiral type Salt rejection rate: 99.5% Operating pressure: 60 kg / cm ²

A water flow experiment was conducted under the above experimental conditions.
The result as shown in FIG.

(Example 5) The same treatment as in Example 4 was carried out except that the physical cleaning washing wastewater of the former membrane separation apparatus was returned to the circulating water tank, and the results shown in Table 2 were obtained.

Example 6 The same treatment as in Example 5 was performed except that the pH of the raw seawater was adjusted to 8.2, and the results shown in Table 2 were obtained.

Comparative Example 2 The same treatment as in Example 5 was performed except that aluminum sulfate and calcium hydroxide were not added to the raw seawater, and the results shown in Table 2 were obtained.

[0040]

[Table 2]

[0041]

As described above, according to the present invention, the boron-containing concentration in the final stage fresh water can be reduced to 0.2 mg / l or less in addition to the water quality monitoring item value. Moreover,
By adopting a membrane separation device such as a microfiltration membrane or ultrafiltration membrane in front of the reverse osmosis membrane device, the device becomes more compact, the operation of the whole process is simplified, and operation requires skill. And has an effect that unmanned driving becomes possible.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a reverse osmosis seawater desalination apparatus used in a boron removal method according to one embodiment of the present invention.

FIG. 2 is an overall configuration diagram of an example of a conventional reverse osmosis seawater desalination apparatus.

[Explanation of symbols]

 6 ... line mixer 8 ... circulating water tank 10 ... membrane separation device 12 ... membrane permeated water 14 ... backwashing means 18 ... storage tank 20 ... reverse osmosis membrane device 23 ... production water tank

Claims (4)

[Claims] 1. A pH adjusting agent and a flocculant capable of forming a boron-containing insoluble precipitate are added to raw seawater and supplied to a circulating water tank, and the supernatant of the circulating water tank is filtered through a microfiltration membrane or an ultrafiltration membrane. The non-filtration component concentrated water of the microfiltration membrane or the ultrafiltration membrane is circulated to the circulating water tank, and the filtered water is permeated by a reverse osmosis membrane to produce fresh water from which boron is removed from seawater. how to 2. The method according to claim 1, wherein the pH of the raw seawater is adjusted to 9 or more. 3. The method according to claim 1, wherein the coagulant is a calcium compound and / or an aluminum compound. 4. The method according to claim 1, wherein the cleaning wastewater of the microfiltration membrane or the ultrafiltration membrane is supplied to a circulating water tank.