JP7149129B2 - Silica-containing water treatment method and treatment apparatus - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method and apparatus for treating silica-containing water containing silica.

In recent years, efforts have been made to reduce the amount of wastewater discharged from factories, etc., as much as possible, and methods have been adopted to reduce the volume of wastewater by concentrating the wastewater using reverse osmosis membranes and recovering the permeated water. there is There is a tendency to increase the water recovery rate as much as possible. An increasing number of factories are even implementing ZLD (Zero Liquid Discharge) in which impurities are collected and solidified before being discharged.

As described above, if the concentration ratio in the reverse osmosis membrane device or the evaporative concentration device is increased, the risk of scaling due to silica or the like in the waste water increases accordingly. When scale is generated, the reverse osmosis membrane is clogged and the amount of permeated water is reduced, or the heat transfer surface of evaporative concentration is covered with scale and the heat transfer efficiency is lowered.

Therefore, it is desirable to reduce silica in waste water as much as possible before reverse osmosis membrane treatment. As a method for treating silica-containing wastewater, there is a method in which water is passed through a reverse osmosis membrane under alkaline conditions to increase the solubility of silica, as described in Patent Document 1.

In this method of passing water through a reverse osmosis membrane under alkaline conditions, silica-enriched concentrated water is discharged, but a treatment method for this concentrated water has not been studied. This concentrated water is usually treated as industrial waste as it is, or is introduced into an evaporative concentration apparatus, solidified, and disposed of. FIG. 2 shows a schematic configuration of a conventional silica-containing water treatment apparatus. For example, after adding an alkali to silica-containing water from a water tank 100 to be treated, reverse osmosis membrane treatment is performed in a reverse osmosis membrane treatment apparatus 102, The concentrated water is concentrated and solidified in the evaporative concentration device 104 for disposal. However, this method has the problem that the evaporation energy and the cost of industrial waste disposal are high.

Japanese Patent Application Laid-Open No. 2002-192152

An object of the present invention is to provide a method and apparatus for treating silica-containing water, which can reduce the amount of silica-concentrated water generated and reduce treatment costs in reverse osmosis membrane treatment of silica-containing water. It is in.

The present invention comprises a reverse osmosis membrane treatment step in which water to be treated containing silica is passed through a reverse osmosis membrane at a pH of 10 or higher to obtain concentrated water and permeated water, and acid is added to the obtained concentrated water in a reaction tank. After adjusting the pH to 5 to 9 by adding a second reverse osmosis membrane treatment step of obtaining a second concentrated water and a second permeated water by adjusting the pH to 10 or more by adding an alkali to the second reverse osmosis membrane, and then passing the water through the second reverse osmosis membrane; and a returning step of returning 1/2 to 9/10 of the second concentrated water to the reaction tank .

In the method for treating silica-containing water, the solid-liquid separation treatment is preferably coagulation sedimentation treatment using a strong anionic polymer flocculant.

Further, the present invention provides reverse osmosis membrane treatment means for obtaining concentrated water and permeated water by passing water to be treated containing silica at pH 10 or higher through a reverse osmosis membrane, and After adjusting the pH to 5 to 9 by adding an acid, a solid-liquid separation processing means for performing solid-liquid separation processing , and adding an acid to the solid-liquid separated water obtained by the solid-liquid separation processing means to adjust the pH is adjusted to 5 or less, or after adjusting the pH to 10 or more by adding an alkali, water is passed through the second reverse osmosis membrane to obtain the second concentrated water and the second permeated water. Second reverse osmosis membrane treatment means and a return means for returning 1/2 to 9/10 of the second concentrated water to the reaction tank .

In the apparatus for treating silica-containing water, the solid-liquid separation treatment is preferably coagulation sedimentation treatment using a strong anionic polymer flocculant.

INDUSTRIAL APPLICABILITY According to the present invention, in the reverse osmosis membrane treatment of silica-containing water, the amount of silica-concentrated water generated can be reduced, and the treatment cost can be reduced.

1 is a schematic configuration diagram showing an example of a silica-containing water treatment apparatus according to an embodiment of the present invention; FIG. 1 is a schematic configuration diagram showing a conventional silica-containing water treatment apparatus; FIG. 1 is a schematic configuration diagram showing an experimental apparatus used in Example 1. FIG. 2 is a schematic configuration diagram showing an experimental apparatus used in Comparative Example 1. FIG.

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

An outline of an example of a silica-containing water treatment apparatus according to an embodiment of the present invention is shown in FIG. 1, and the configuration thereof will be described.

The silica-containing water treatment apparatus 1 includes a reverse osmosis membrane treatment apparatus 12 as reverse osmosis membrane treatment means for obtaining concentrated water and permeated water by passing silica-containing water to be treated at pH 10 or higher through a reverse osmosis membrane. and a sedimentation tank 18 as solid-liquid separation processing means for performing solid-liquid separation processing after adding an acid to the concentrated water obtained in the reaction tank 14 . The silica-containing water treatment apparatus 1 includes a water tank to be treated 10 for storing water to be treated, a coagulation tank 16, a turbidity removal device 20, a second reverse osmosis membrane treatment device 22 as a second reverse osmosis membrane treatment means, may be provided.

In the silica-containing water treatment apparatus 1 of FIG. 1 , the outlet of the water tank 10 to be treated and the inlet of the reverse osmosis membrane treatment apparatus 12 are connected by a pipe 28 via a pump 24 . A permeated water pipe 32 is connected to the permeated water outlet of the reverse osmosis membrane treatment device 12 , and a concentrated water pipe 30 connects the concentrated water outlet and the concentrated water inlet of the reaction tank 14 . The outlet of the reaction tank 14 and the inlet of the coagulation tank 16 are connected by a pipe 34 , and the outlet of the coagulation tank 16 and the inlet of the sedimentation tank 18 are connected by a pipe 36 . A pipe 48 is connected to the sludge outlet at the bottom of the sedimentation tank 18 via a pump 26 , and a supernatant water outlet and the inlet of the clarifier 20 are connected by a pipe 38 . The outlet of the clarification device 20 and the inlet of the second reverse osmosis membrane treatment device 22 are connected by a pipe 40 . A second permeated water outlet of the second reverse osmosis membrane treatment device 22 is connected to a second permeated water pipe 42, and a return pipe 44 connects the second concentrated water outlet and the second concentrated water inlet of the reaction tank 14. ing. A pipe 46 is branched from the return pipe 44 and connected to the return pipe 44 . An alkali addition pipe 50 is connected to the pipe 28 downstream of the pump 24 as alkali addition means. An acid addition pipe 52 is connected to the reaction tank 14 as acid addition means. A coagulant addition pipe 54 is connected to the coagulant tank 16 as coagulant addition means. An acid/alkali addition pipe 56 is connected to the pipe 38 as acid or alkali addition means. The reaction tank 14 and the aggregation tank 16 are provided with stirring devices 58 and 60, which are stirring means having rotary driving means such as motors and stirring blades, respectively.

The method for treating silica-containing water and the operation of the treatment apparatus 1 according to this embodiment will be described.

The silica-containing water, which is the water to be treated, is stored in the water tank 10 to be treated as necessary, and then sent to the reverse osmosis membrane treatment device 12 through the pipe 28 by the pump 24 . Here, on the downstream side of the pump 24 in the pipe 28, alkali is added through the alkali addition pipe 50 to adjust the pH to 10 or more (alkali addition step). The alkali-added silica-containing water is passed through the reverse osmosis membrane at a pH of 10 or higher in the reverse osmosis membrane treatment device 12 to obtain concentrated water and permeated water (reverse osmosis membrane treatment step). Permeated water obtained by reverse osmosis membrane treatment (silica content is, for example, less than 1 mg/L) is discharged through permeated water pipe 32, and concentrated water is sent to reaction tank 14 through concentrated water pipe 30. .

In the reaction tank 14, an acid is added to the concentrated water through the acid addition pipe 52 while being stirred by the stirring device 58 (acid addition step). The acid-added water is sent to the aggregation tank 16 through the pipe 34 . In the flocculating tank 16, a flocculating agent is added to the acid-added water through the flocculating agent addition pipe 54 while being stirred by the stirring device 60, and flocculation treatment is performed. The flocculated water to which the flocculant has been added is sent to the sedimentation tank 18 through the pipe 36, and solid-liquid separation is performed in the sedimentation tank 18 (solid-liquid separation step). Supernatant water obtained by the solid-liquid separation process is sent to the turbidity removal device 20 through the pipe 38 . Here, in the pipe 38, acid or alkali is added through the acid/alkali addition pipe 56 to adjust the pH to, for example, 5 or less or 10 or more (acid/alkali addition step). Sludge obtained by the solid-liquid separation treatment is discharged through pipe 48 by pump 26 . After the turbidity removal process is performed in the turbidity removal apparatus 20 , the liquid is sent to the second reverse osmosis membrane treatment apparatus 22 through the pipe 40 . In the second reverse osmosis membrane treatment device 22, water is passed through the second reverse osmosis membrane at pH 5 or lower or pH 10 or higher to obtain a second concentrated water and a second permeated water (second reverse osmosis membrane treatment step). The second permeated water obtained by the second reverse osmosis membrane treatment is discharged as treated water (silica content is, for example, less than 1 mg / L) through the second permeated water pipe 42, and the second concentrated water is at least one A portion may be returned to the reaction vessel 14 through return line 44 and a portion may be discharged through return line 44 and line 46 .

In the method and apparatus 1 for treating silica-containing water according to the present embodiment, alkali is added to the water to be treated containing silica to adjust the pH to 10 or more, and then the water is passed through the reverse osmosis membrane in the reverse osmosis membrane treatment apparatus 12. In the method of obtaining concentrated water and permeated water, acid is added to the concentrated water in which silica is concentrated in the reaction tank 14 to precipitate silica, and then solid-liquid separation treatment is performed to remove silica from solid sludge. and discharged out of the system. As a result, the amount of silica-condensed water generated can be reduced, and the treatment cost can be reduced. After adding acid to concentrated silica-enriched water to deposit silica, the amount of silica-concentrated water generated is, for example, about 1/10 to 1/20 compared to the case where solid-liquid separation is not performed. can be reduced to

When solid-liquid separation treatment is performed by adding acid to concentrated water in which silica is concentrated by the method and apparatus 1 for treating silica-containing water according to the present embodiment, solid-liquid separation treatment is performed in a state where the concentration of silica is low. Compared to , it is not necessary to add metal compounds such as magnesium compounds such as inorganic salts of magnesium and aluminum compounds such as inorganic salts of aluminum, and the amount of generated sludge is reduced. Advantageously, less lift is required.

Silica-containing water to be treated is, for example, groundwater, industrial water, factory wastewater, and the like. The amount of silica in the silica-containing water is, for example, 10-400 mg/L.

The pH of the silica-containing water in the reverse osmosis membrane treatment step may be 10 or higher, and is preferably in the range of pH 10 to 11 from the viewpoint of durability of the membrane. The silica-containing water to be treated may be passed through the reverse osmosis membrane at a pH of 10 or higher in the reverse osmosis membrane treatment step. The addition step may be omitted.

Examples of the alkali used in the alkali addition step include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH) ₂ ), and the like. Of these, sodium hydroxide is preferred from the viewpoint of chemical cost and the like.

Addition of alkali in the alkali addition step may be performed in the pipe 28 as shown in FIG. may be carried out in the alkali addition tank by providing an alkali addition tank.

The temperature in the alkali addition step is not particularly limited, but is, for example, in the range of 15°C to 30°C.

When an acid is added to the concentrated water (the silica content is, for example, 200 to 2000 mg/L) that is alkaline with a pH of 10 or higher and contains a high concentration of silica in the reverse osmosis membrane treatment device 12, silica is removed. It solidifies and precipitates. The solubility of silica decreases at pH 9 or less. The pH is preferably adjusted to 5-9, more preferably 6-8, by adding acid.

Acids used in the acid addition step include, for example, hydrochloric acid and sulfuric acid. Of these, hydrochloric acid is preferred from the viewpoint of chemical cost and the like.

Addition of acid in the acid addition step may be performed in the reaction tank 14 as shown in FIG.

The temperature in the acid addition step is not particularly limited, but is, for example, in the range of 15°C to 30°C.

Silica can be removed as a solid matter by subjecting the solid matter from which silica precipitates to a solid-liquid separation treatment by a method such as coagulation sedimentation.

As a solid-liquid separation method in the solid-liquid separation step, methods such as coagulation sedimentation treatment and membrane filtration are used, but coagulation sedimentation treatment is preferable because there is no risk of clogging of the membrane.

The coagulation-sedimentation treatment includes, for example, a sedimentation step of performing solid-liquid separation by natural sedimentation in a sedimentation tank, and may include a coagulation step of adding a flocculating agent prior to the sedimentation step. Since sludge containing silica has poor flocculation and sedimentation properties, it is preferable to carry out flocculation treatment using a flocculant such as an inorganic flocculant or a polymer flocculant as necessary in the flocculation treatment. The aggregating step may include an inorganic aggregating step of aggregating using an inorganic aggregating agent and a polymer aggregating step of aggregating using a polymer aggregating agent.

Examples of the inorganic flocculant used in the flocculation step (inorganic flocculation step) include iron-based inorganic flocculants such as ferric chloride and polyferric sulfate, and aluminum-based inorganic flocculants such as aluminum sulfate and polyaluminum chloride (PAC). flocculants and the like.

The amount of inorganic flocculant added in the aggregation step (inorganic aggregation step) is preferably in the range of 20 to 200 mg/L, more preferably in the range of 50 to 100 mg/L. If the amount of the inorganic flocculant added in the flocculation step (inorganic flocculation step) is less than 20 mg/L, the flocculation reaction may not proceed sufficiently. be.

The reaction temperature in the aggregation step (inorganic aggregation step) is not particularly limited, but is, for example, in the range of 15°C to 30°C.

Polymer flocculants used in the aggregation step (polymer aggregation step) include, for example, polyacrylamide-based, 2-acryloylamino-2-methylpropanesulfonic acid (AMPS)-based strong anionic polymer flocculants, weak Anionic polymer flocculants, nonionic polymer flocculants, cationic polymer flocculants such as methacrylates and acrylates, and the like are included. Among these, a strong anionic polymer flocculant is preferable from the viewpoint of good flocculation of silica, and a combination of a strong anionic polymer flocculant and a cationic polymer flocculant may be used.

The amount of the polymer flocculant added in the aggregation step (polymer aggregation step) is preferably in the range of 0.5 to 5 mg/L, more preferably in the range of 1 to 2 mg/L. If the amount of the polymer flocculant added in the aggregation step (polymer aggregation step) is less than 0.5 mg/L, the aggregation reaction may not proceed sufficiently. may become

The reaction temperature in the aggregation step (polymer aggregation step) is not particularly limited, but is, for example, in the range of 15°C to 30°C.

The solid-liquid separated water in the solid-liquid separation process, for example, the supernatant water obtained in the coagulation sedimentation process, may be discharged if it can be discharged as it is, but if necessary, the turbidity removal process is performed by the turbidity removal device 20. After that, it is further concentrated by reverse osmosis membrane treatment (second reverse osmosis membrane treatment) by the second reverse osmosis membrane treatment device 22 to obtain highly treated permeated water (second permeated water), and the volume is reduced. It is preferred that the Compared to the case where the second reverse osmosis membrane treatment is not performed, the amount of concentrated silica water generated can be further reduced, for example, to about 1/2 to 1/10. By further recovering the solid-liquid separated water of the solid-liquid separation treatment, the water recovery rate of the system can be increased.

When performing the second reverse osmosis membrane treatment, solid-liquid separation water such as supernatant water obtained by coagulation sedimentation treatment is saturated with silica (silica content is, for example, 120 mg / L). Precipitation may cause membrane clogging. In order to suppress this, acid is added before the second reverse osmosis membrane treatment to adjust the pH to 5 or less (acid/alkali addition step), thereby reducing the silica deposition rate and suppressing silica deposition. can be concentrated by a second reverse osmosis membrane treatment. Alternatively, by adding alkali in the preceding stage of the second reverse osmosis membrane treatment to adjust the pH to 10 or more (acid/alkali addition step), the solubility of silica is increased again, silica precipitation is suppressed, and the second reverse osmosis membrane Can be concentrated by processing.

The acids and alkalis used in the acid/alkali addition step include those mentioned above.

The addition of acid or alkali in the acid/alkali addition step may be performed in the pipe 38 as shown in FIG. It may be carried out in an alkali addition tank.

The temperature in the acid/alkali adding step is not particularly limited, but is, for example, in the range of 15°C to 30°C.

Examples of the clarification device 20 include a membrane filtration device having an ultrafiltration membrane (UF membrane), a sand filtration device, a safety filter, and the like.

The second reverse osmosis membrane used in the second reverse osmosis membrane treatment is preferably a membrane with a high silica rejection rate, preferably a membrane with a silica rejection rate of 99.0% or more. A silica rejection of about 0.5% is obtained, which is more preferable.

Furthermore, part or all of the second concentrated water (silica content is, for example, 100 to 300 mg / L) for the second reverse osmosis membrane treatment is added to the preceding step of the second reverse osmosis membrane treatment, for example, the reaction tank 14 By returning, the amount of the second concentrated water generated in the second reverse osmosis membrane treatment can be further reduced. For example, when the second concentrated water generated in the second reverse osmosis membrane treatment is subjected to evaporative concentration treatment in the evaporative concentration device in the latter stage of the second reverse osmosis membrane treatment, the load of silica flowing into the evaporative concentration device is reduced, and the evaporation Concentration can be miniaturized or eliminated. Compared to the case where the second concentrated water from the second reverse osmosis membrane treatment is not returned to the previous stage, the amount of generated silica concentrated water can be further reduced, for example, to about 1/2 to 1/10.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

<Comparative Example 1>
A water flow test was conducted using the experimental equipment for the flow shown in FIG. In Comparative Example 1, in the water tank 100 to be treated, an aqueous sodium hydroxide solution was added as an alkali to the water to be treated to adjust the pH of the water to be treated to 10.7 to 11.0. - Water was passed through a reverse osmosis membrane element (Nitto Denko LFC-3) to obtain permeated water and concentrated water. Experimental conditions and results are as follows.

The amount of silica (SiO ₂ ) in water was measured by JIS K 0101 molybdenum blue spectrophotometry using an absorptiometer (Hitachi Ltd., U-2900).

[Experimental conditions/results]
・Water to be treated: Factory effluent (containing silica) SiO ₂ =80 mg/L, about 1000 L
・Water flow rate to be treated: 155L/h
・Permeate water flow rate: 140 L/h
Concentrated water flow rate: 720 L/h (of which 705 L/h is circulated to the water tank 100 to be treated, and 15 L/h is sent to the subsequent stage)
・Reverse osmosis membrane inlet flow rate: 860 L/h (including circulating water)
・Concentrated water pH: 10.8
・ Concentrated water silica concentration: 788 mg / L
・Amount of concentrated water obtained: about 100 L

<Example 1>
A water flow test was conducted using the experimental equipment for the flow shown in FIG. In Example 1, the reverse osmosis membrane-treated concentrated water prepared in Comparative Example 1 was stored in the concentrated water tank 62, and after adjusting the pH to 6 to 7 by adding hydrochloric acid as an acid in the reaction tank 14, the coagulating sedimentation apparatus was added. was watered. The supernatant water after sedimentation by the sedimentation tank 18 is stored in the supernatant tank 64, and after adjusting the pH to 10.8 by adding an aqueous sodium hydroxide solution as an alkali, the water is passed through the turbidity removal device 20 (safety filter). and passed through a 4-inch reverse osmosis membrane element (Nitto Denko LFC-3) in the second reverse osmosis membrane treatment device 22 to obtain a second permeated water and a second concentrated water. Experimental conditions and results are as follows.

[Experimental conditions/results]
・Inlet flow rate of coagulation sedimentation device (reaction tank 14): 100 L / h
・Strong anionic (polyacrylamide-based) polymer flocculant ORFLOC OA-3H (manufactured by Organo Co., Ltd.): 2 mg/L added ・Silica concentration in supernatant water after precipitation: 110 mg/L
・Raw water flow rate for second reverse osmosis membrane treatment: 186 L/h
・ Second permeate flow rate of second reverse osmosis membrane treatment: 140 L / h
・ Second concentrated water flow rate for second reverse osmosis membrane treatment: 720 L / h (674 L / h of which is circulated to the supernatant water tank 64 and 46 L / h is sent to the subsequent stage)
・Second reverse osmosis membrane inlet flow rate: 860 L/h (including circulating water)
・ Second concentrated water silica concentration: 430 mg / L
・ Obtained second concentrated water amount: about 25 L

In Example 1, silica-containing water to be treated was passed through a reverse osmosis membrane at a pH of 10 or higher, and an acid was added to the concentrated water obtained in the reaction tank, followed by solid-liquid separation treatment to remove silica. , and further concentrated by the second reverse osmosis membrane treatment. As a result, compared with Comparative Example 1, the amount of concentrated water discharged from the system was reduced from about 100 L to about 25 L, which is about 1/4.

As described above, the method and apparatus of the example were able to reduce the amount of silica-concentrated water produced in the reverse osmosis membrane treatment of silica-containing water and reduce the treatment cost.

1 silica-containing water treatment apparatus, 10,100 water tank to be treated, 12,102 reverse osmosis membrane treatment apparatus, 14 reaction tank, 16 coagulation tank, 18 sedimentation tank, 20 turbidity removal apparatus, 22 second reverse osmosis membrane treatment apparatus, 24, 26 pump, 28, 34, 36, 38, 40, 46, 48 piping, 30 concentrated water piping, 32 permeated water piping, 42 second permeated water piping, 44 return piping, 50 alkali addition piping, 52 acid addition piping , 54 coagulant addition pipe, 56 acid/alkali addition pipe, 58, 60 stirring device, 62 concentration water tank, 64 supernatant water tank, 104 evaporative concentration device.

Claims (4)

A reverse osmosis membrane treatment step of passing water to be treated containing silica through a reverse osmosis membrane at a pH of 10 or higher to obtain concentrated water and permeated water;
A solid-liquid separation treatment step of performing a solid-liquid separation treatment after adjusting the pH to 5 to 9 by adding an acid to the obtained concentrated water in a reaction tank;
Acid is added to the solid-liquid separated water obtained in the solid-liquid separation treatment step to adjust the pH to 5 or less, or alkali is added to adjust the pH to 10 or more, and then water is passed through the second reverse osmosis membrane. a second reverse osmosis membrane treatment step of obtaining a second concentrated water and a second permeated water by
A returning step of returning 1/2 to 9/10 of the second concentrated water to the reaction tank;
A method for treating silica-containing water, comprising: The method for treating silica-containing water according to claim 1 ,
A method for treating silica-containing water, wherein the solid-liquid separation treatment is a coagulation sedimentation treatment using a strong anionic polymer flocculant. a reverse osmosis membrane treatment means for passing silica-containing water to be treated at a pH of 10 or higher through a reverse osmosis membrane to obtain concentrated water and permeated water;
solid-liquid separation processing means for performing solid-liquid separation processing after adjusting the pH to 5 to 9 by adding acid to the obtained concentrated water in the reaction tank;
Acid is added to the solid-liquid separated water obtained by the solid-liquid separation treatment means to adjust the pH to 5 or less, or alkali is added to adjust the pH to 10 or more, and then the water is passed through the second reverse osmosis membrane. a second reverse osmosis membrane treatment means for obtaining a second concentrated water and a second permeated water by
return means for returning 1/2 to 9/10 of the second concentrated water to the reaction vessel;
An apparatus for treating silica-containing water, comprising: The silica-containing water treatment apparatus according to claim 3 ,
An apparatus for treating silica-containing water, wherein the solid-liquid separation treatment is coagulation sedimentation treatment using a strong anionic polymer flocculant.

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