JP6723057B2 - Water treatment method and water treatment system - Google Patents

Description

The present invention relates to a water treatment method and a water treatment system for water to be treated containing fluoride ions and magnesium ions.

As the flue gas desulfurization method used in coal-fired power plants and coke plants, the wet lime-gypsum method is the mainstream, but this method requires the disposal of a large amount of gypsum, so small-scale equipment is required. There is a problem that it is not suitable. For such a problem, a method of treating exhaust gas by using magnesium hydroxide instead of lime has been implemented. This method captures the sulfur content in the exhaust gas as magnesium sulfate, which has a large solubility in water, not as a solid substance such as gypsum, and the magnesium sulfate produced is in a dissolved state together with the wastewater. It can be released.

On the other hand, since fluoride ions are contained in the waste water from the flue gas desulfurization apparatus as mentioned above, the treatment thereof becomes a problem when discharged. As a method for removing fluoride ions in waste water, a method of adding calcium ions to waste water in a neutral pH range and precipitating and removing fluoride ions as calcium fluoride is generally used (Patent Document 1). ..

However, in this method, if magnesium ions and sulfate ions are present in the wastewater, such as the wastewater from the flue gas desulfurization apparatus that uses magnesium hydroxide, the removal rate of fluoride ions by the calcium method There was a problem that it decreased. This is considered to be because, in the case of such wastewater, a large amount of magnesium ions and fluoride ions are dissolved as a complex in the neutral pH range, and as a result, calcium fluoride is not produced.

To address this problem, when calcium ions are added to wastewater containing fluoride ions and magnesium ions to remove the fluoride ions as a precipitate, in Patent Document 2, the pH of the wastewater is 9.4 to 9.8. It is proposed that the pH of the waste water is adjusted to 8 to 10 in Patent Document 3, respectively.

Japanese Patent Publication No. 58-013230 JP, 08-057486, A JP 2000-301165 A

In the conventional methods as disclosed in Patent Documents 2 and 3 described above, when calcium ions are added to the wastewater, calcium fluoride does not precipitate and the pH of the wastewater is 9.4 to 9.8 or 8. It is considered that the magnesium ion dissolved as a complex in the waste water is precipitated and precipitated as magnesium hydroxide by adjusting the range of 10 to 10. Then, the fluoride ions dissolved as a complex in the wastewater are taken in and precipitated by the precipitation of magnesium hydroxide, and the existing calcium ions react with the free fluoride ions to give calcium fluoride. It is thought to precipitate as. Thus, it is considered that the removal rate of fluoride ions from the wastewater can be improved. In this regard, in the alkaline region, the present inventors have found that fluoride ions are "incorporated into precipitated magnesium hydroxide rather than "complex of fluorine and magnesium" in relation to coexisting magnesium ions. We believe that the presence of “precipitate” is a more stable state, and we recognize that this is a point to be noted when considering the fluorine removal treatment.

In the conventional method, magnesium ions are precipitated as magnesium hydroxide by increasing the pH of the waste water in addition to the precipitation of calcium fluoride generated by the addition of calcium ions to the waste water. Therefore, the amount of sludge generated increases, which may increase the sludge treatment cost in practical use. The present inventors have come to recognize that it is necessary to perform better and economical treatment by improving this point.

Further, the method disclosed in Patent Documents 2 and 3 is more effective than the method disclosed in Patent Document 1 in treating waste water containing magnesium ions and the like in addition to fluoride ions. The removal rate can be improved. However, as a result of further studies, the present inventors considered that there is room for further improving the removal rate of fluoride ions.

Therefore, the present invention can reduce the amount of final sludge and can achieve higher treatment efficiency when removing fluoride ions from water to be treated containing fluoride ions and magnesium ions. It seeks to provide water treatment technology.

The present invention includes an alkali addition step of adding an alkaline agent to water to be treated containing fluoride ions and magnesium ions to generate a suspended substance in the water to be treated, and the suspension in which the fluoride ions are incorporated. A solid-liquid separation step of solid-liquid separation of turbid substances, an acid addition step of adding an acid to the solid-liquid separated sludge derived from the suspended matter, and an aging step of stirring the sludge for a predetermined time after adding the acid And a water treatment method including the steps of:

According to the present invention, when removing fluoride ions from water to be treated containing fluoride ions and magnesium ions, the amount of final sludge can be reduced and higher treatment efficiency can be realized. Water treatment technology can be provided.

It is a schematic flow chart showing the water treatment method of one embodiment of the present invention. It is a schematic flow figure showing the water treatment method of another one Embodiment of this invention. It is a schematic flow figure showing the water treatment method of another one Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

With respect to the water to be treated containing the fluoride ion (F ⁻ ) and the magnesium ion (Mg ²⁺ ), the above-mentioned conventional method adjusts the pH to 8.0 to 10.0 to oxidize the magnesium ion. The precipitation rate of fluoride ions is increased by precipitating it as magnesium and performing solid-liquid separation. Therefore, in the conventional method, the amount of sludge generated increases. Therefore, the inventors of the present invention firstly need to process a large amount of sludge if the amount of sludge that needs final disposal (hereinafter, also referred to as “final sludge” in this specification) can be reduced. We realized that the cost could be reduced and it was extremely useful in practice.

Based on the above recognition, the present inventors have found that the fluoride ion is in a stable state when it is present as “a precipitate incorporated in the precipitated magnesium hydroxide” rather than as a “complex of fluorine and magnesium”. I paid attention to a certain point. Then, the step of adding an alkaline agent to the water to be treated to generate a suspended substance in the water to be treated (alkali addition step) is performed. By this step, magnesium ions in the water to be treated are sufficiently precipitated as magnesium hydroxide, and a suspended substance in which fluoride ions are incorporated into the precipitated magnesium hydroxide is positively generated.

Then, a step (solid-liquid separation step) of solid-liquid separation of a suspended substance in which fluoride ions are incorporated is performed, and a step of adding an acid to the sludge derived from the solid-liquid separated suspension material (acid addition step) is performed. To do. This acid addition step dissolves magnesium hydroxide, which is the main constituent of the sludge, and reduces the final amount of sludge. The fluoride ions taken in the sludge precipitate as magnesium fluoride (MgF ₂ ), and the MgF ₂ remains in the final sludge at a high concentration. Therefore, the removal rate of fluoride ions can be improved while reducing the final amount of sludge.

However, when the test (hereinafter referred to as “continuous test”) is conducted in a continuous process that is suitable for industrial use in the actual field, the final sludge after addition of the acid is Indicates that the fluorine concentration in the solid-liquid separated supernatant liquid is higher than the fluorine concentration in the supernatant liquid when the batch process test (hereinafter referred to as "batch test") is performed. There was found. As will be described later, since the supernatant contains fluorine, it is preferable to return the supernatant to the alkali addition step and treat again with the water to be treated, but the concentration of fluorine in the supernatant is When it is high, the efficiency of removing fluorine may be low, or the amount of alkali may be increased. Therefore, it is desirable that the concentration of fluorine in the supernatant is low.

As a result of further studies by the present inventors, in the continuous test, even if the amount of acid added to the sludge in the acid addition step was increased, and even if the time for which the acid was added was extended, It was found that it was difficult to obtain a clear effect of reducing the fluorine concentration. Therefore, the present inventors diligently studied the cause of the high fluorine concentration in the supernatant in the continuous test, focusing on the difference between the continuous test and the batch test. Specifically, in the acid addition process to the sludge, in the continuous test, the acid tends to be added at a substantially constant rate, whereas in the batch test, most of the acid is added in a short time. Is added, and there is a tendency that almost no acid is added during the remaining time. From this difference, the present inventors believe that the time when no acid is added to the sludge is important in order to further reduce the fluorine concentration in the above-mentioned supernatant liquid, and after adding the acid to the sludge, The step of stirring the sludge for a predetermined time (sludge aging step) was performed. As a result, it was found that the concentration of fluorine in the supernatant liquid obtained by solid-liquid separation from the final sludge can be reduced.

By skillfully utilizing each of the above-mentioned means, the present inventors can reduce the final amount of sludge and can further efficiently perform the removal treatment of fluoride ions in the water to be treated. Heading out, the present invention has been completed. That is, the water treatment method of one embodiment of the present invention includes an alkali addition step of adding an alkaline agent to water to be treated containing fluoride ions and magnesium ions to generate a suspended substance in the water to be treated. Then, this water treatment method includes a solid-liquid separation step of performing solid-liquid separation on sludge derived from a suspended substance in which fluoride ions are incorporated. Furthermore, this water treatment method is characterized by comprising an acid addition step of adding an acid to the sludge derived from the solid-liquid separated suspended matter, and an aging step of stirring the sludge for a predetermined time after adding the acid. To do.

Further, the water treatment method according to the embodiment of the present invention can be executed by, for example, the water treatment system according to the embodiment of the present invention. The water treatment system includes a reaction tank in which an alkaline agent is added to water to be treated containing fluoride ions and magnesium ions to generate a suspended substance in the water to be treated, and fluoride ions are incorporated in the reaction tank. A solid-liquid separation tank for solid-liquid separation of sludge derived from suspended solids, an acid addition tank for adding acid to the sludge derived from suspended solids separated in the solid-liquid separation tank, and a sludge after adding acid And a maturing tank for stirring for a predetermined time.

Hereinafter, each step in the water treatment method of one embodiment of the present invention will be specifically described with reference to the drawings. It should be noted that, in the drawings, parts common to the drawings are denoted by the same reference numerals, and description thereof may be omitted.

FIG. 1 is a schematic flow chart showing a water treatment method according to an embodiment of the present invention. As shown in FIG. 1, in the water treatment method of this embodiment, a step (alkali addition step) S11 of adding an alkaline agent to water to be treated (raw water) containing fluoride ions and magnesium ions is performed. In this step S11, a suspended substance is generated in the water to be treated. Specifically, by adding an alkaline agent to the water to be treated, magnesium ions in the water to be treated are precipitated as magnesium hydroxide, and a suspended substance in which fluoride ions are incorporated into the precipitated magnesium hydroxide (preferably Precipitate) is generated more aggressively. In this way, fluoride ions can be removed from the water to be treated by removing the suspended solids by solid-liquid separation described below.

Suitable alkali agents added to the water to be treated in the alkali addition step S11 include alkali metal hydroxides and carbonates, and alkaline earth metal hydroxides and carbonates. As the alkaline agent, one kind or two or more kinds can be used. From the viewpoint of easily precipitating magnesium ions in the water to be treated as magnesium hydroxide, the alkali agent is preferably a hydroxide of an alkali metal or an alkaline earth metal, such as sodium hydroxide (caustic soda) and potassium hydroxide (caustic potassium hydroxide). ), and calcium hydroxide (slaked lime) are more preferable. When a caustic alkali such as sodium hydroxide is used as the alkaline agent, the fluorine content in the final sludge can be further increased. On the other hand, when a calcium salt such as calcium hydroxide is used as the alkaline agent, the dewaterability of the final sludge can be enhanced.

From the viewpoint of sufficiently generating a suspended substance in which fluoride ions are taken into the water to be treated, in the alkali addition step S11, the pH of the water to be treated is adjusted to 8.5 by adding an alkaline agent to the water to be treated. It is preferable to adjust it within the range of 10.5. In the present specification, the pH of the water to be treated, the sludge described later, and the like are values at 25° C. or converted values at 25° C. For example, when the temperature of the water to be treated is higher than 25° C., the pH of the water to be treated in the alkali addition step of 8.5 to 10.5 shifts to a value lower than that range in the actual measurement value. .. More specifically, for example, when the temperature of the water to be treated is 50° C., adjusting the pH of the water to be treated to 8.5 to 10.5 in the alkali adding step is performed at 50° C. of the water to be treated. The pH value is about 8.0 to 10.0.

The amount of the alkaline agent added in the alkaline addition step S11 is not particularly limited. It is preferable to appropriately adjust the pH of the water to be treated within the range of 8.5 to 10.5 depending on the quality of the water to be treated and the like. The alkali addition step S11 is preferably performed in the reaction tank 11 as a tank separate from the solid-liquid separation step and the like described later. Further, it is more preferable that the reaction tank 11 is provided with a raw water supply section for supplying the water to be treated (raw water) to the reaction tank 11 and an alkaline agent supply section for adding the alkaline agent. .. The raw water supply unit can be configured by, for example, a supply pipe that sends the raw water from the raw water storage tank to the reaction tank 11 and a pump. The alkaline agent supply unit can be configured by, for example, a supply pipe for sending the alkaline agent from the reservoir for the alkaline agent to the reaction tank 11, a pump, and the like.

As shown in FIG. 1, in the water treatment method of the present embodiment, a step of solid-liquid separation of the suspended substance (suspended substance in which fluoride ions are incorporated in the water to be treated) generated in the alkali addition step S11 ( Solid-liquid separation step) S21 is performed. This solid-liquid separation step S21 is preferably performed in a tank (solid-liquid separation tank) 21 different from the tank (the above-mentioned reaction tank) 11 for adding the alkaline agent to the water to be treated. As the solid-liquid separation treatment, any of aggregation/precipitation treatment, membrane separation/filtration treatment, and flotation treatment can be used. Of these, it is preferable to employ a coagulation/precipitation treatment from the viewpoint that solid-liquid separation can be performed by using a suspended substance in which fluoride ions are taken as a precipitate. In this case, using a precipitation tank such as a thickener, It is preferable to perform the solid-liquid separation step S21.

Further, when the suspension substance is aggregated/precipitated, a flocculant may be used in order to accelerate the aggregation/precipitation of the suspended substance. As the aggregating agent, known inorganic aggregating agents such as polyaluminum chloride, aluminum sulfate, and iron salt aggregating agents, polyacrylic acid ester aggregating agents, polymethacrylate ester aggregating agents, and polyacrylamide aggregating agents. The well-known polymer flocculant can be used.

By the solid-liquid separation step S21, sludge derived from suspended matter is obtained. At this time, in the preferred water treatment method according to the present embodiment, the sludge obtained by solid-liquid separation of the suspended substance is composed of a mineral phase containing magnesium hydroxide as a main component and has a fluorine content of 2 It is possible to obtain about 10% by mass of sludge.

In the solid-liquid separation step S21, the sludge derived from the suspended matter and the supernatant water are separated. First, the sludge treatment method will be described below.

As shown in FIG. 1, in the water treatment method of the present embodiment, a step (acid addition step) S31 of adding an acid to sludge derived from suspended solids that has been subjected to solid-liquid separation is performed. As described above, this step S31 dissolves magnesium hydroxide, which is the main component of the sludge, and as a result, the final amount of sludge can be reduced. Further, the fluorine content taken into the sludge is precipitated as magnesium fluoride (MgF ₂ ), and the fluorine content can remain in the reduced sludge at a high concentration. The precipitated magnesium fluoride can be separated from the final sludge, and the separated magnesium fluoride can be expected to be reused as an industrial raw material. It is also useful from the viewpoint of effective use.

From the viewpoint of further reducing the final amount of sludge and increasing the amount of fluorine remaining in the sludge, in the acid addition step S31, the pH of the sludge is preferably adjusted to 3. by adding an acid to the sludge. It is adjusted within the range of 0 to 8.5, more preferably within the range of 4.0 to 8.0, and further preferably within the range of 5.0 to 7.5. In the acid addition step S31, if the pH of the sludge is set to an acid side of 3.0, the magnesium hydroxide in the sludge is of course dissolved, but the fluorine content in the sludge is dissolved at a high concentration as hydrofluoric acid. In some cases, it becomes difficult for the fluorine content to remain in the sludge. On the other hand, when the pH of the sludge is set to be closer to the alkaline side than 8.5, the fluorine content is hardly dissolved, but in this case, it is difficult to dissolve magnesium hydroxide and it is difficult to reduce the final sludge amount. There is.

In the acid addition step S31, when the pH of the sludge is adjusted and the F ⁻ concentration of the sludge is higher than the solubility of magnesium fluoride in the water to be treated, fluorine exceeding the solubility is precipitated as magnesium fluoride and the solubility is increased. The following fluorine will be present in water. Regarding the solubility, the inventors of the present invention conducted an experiment to find out that, like wastewater discharged from a flue gas desulfurization apparatus suitable as water to be treated, in addition to F ⁻ , Mg ²⁺ and sulfate ion (SO ₄ ²⁻ It was found that the fluorine content was dissolved in the liquid in the range of about 200 to 600 mg/L when the pH of the sludge was in the range of 3.0 to 8.5 when the water to be treated containing )) in a high concentration was used. .. From this, in the acid addition step S31, the fluorine content is dissolved in the liquid within the range of 200 to 600 mg/L together with magnesium hydroxide which is the main component of the sludge, whereby the amount of sludge can be further reduced. Thus, it is considered that the fluorine content is precipitated as magnesium fluoride in the reduced sludge, which allows the fluorine content to remain at a high concentration. As the water to be treated containing F ⁻ , Mg ²⁺ , and SO ₄ ²⁻ at high concentrations, specifically, F concentration is 30 to 300 mg/L, Mg concentration is 2000 to 20000 mg/L, SO ₄ Water to be treated having a concentration of 8000 to 80000 mg/L can be mentioned.

The solid-liquid separated sludge contains magnesium hydroxide as a main component, magnesium fluoride (MgF ₂ ) in which fluoride ions are incorporated into precipitated magnesium hydroxide, magnesium hydroxide fluoride (MgFOH), and the like. It may contain a fluorine compound. Most of the fluorine content contained in the sludge is considered to be MgF ₂ or MgFOH. However, from the viewpoint of the above-mentioned solubility of magnesium fluoride, when the pH of the sludge is adjusted to any pH value within the range of 3.0 to 8.5 in the acid addition step S31, the solubility corresponding to this pH value is obtained. It is considered that the fluorine content is dissolved from MgFOH by the amount corresponding to the above, and most of the other fluorine content is precipitated as magnesium fluoride. As a result, it is considered that a high concentration of magnesium fluoride will remain in the final sludge obtained by solid-liquid separation of the sludge after acid addition in the acid addition step S31.

The acid used is not particularly limited, and one capable of adjusting the pH of the sludge within the range of 3.0 to 8.5 can be preferably used. Suitable acids include, for example, hydrochloric acid, sulfuric acid, nitric acid and the like. The amount of the acid added to the sludge is not particularly limited, and can be appropriately adjusted according to the SS concentration. For example, in the case of a 75 mass% sulfuric acid aqueous solution, the acid can be added to the sludge in an amount of about 10 to 100 g/L.

The acid addition step S31 is preferably performed in the acid addition tank 31 as a tank separate from the aging step S41 described later and the like. Although not shown, it is preferable that the acid addition tank 31 is provided with a sludge supply unit for supplying solid-liquid separated sludge to the acid addition tank 31 and an acid supply unit for adding acid. .. The sludge supply unit can be configured by, for example, a pipe through which sludge passes, a pump, and the like. The acid supply unit can be configured by, for example, a supply pipe that sends the acid from the acid storage tank to the acid addition tank 31, a pump, and the like.

As shown in FIG. 1, in the water treatment method of the present embodiment, after the acid is added, a step (aging step) S41 of stirring the sludge after addition of the acid for a predetermined time is performed. In this step S41, the sludge after addition of the acid is stirred for a predetermined time while the addition of the acid to the sludge is released. A mechanical stirrer, an air diffuser, or the like can be used for stirring, and it is preferable to use a mechanical stirrer equipped with stirring blades. By the aging step S41, the reduced sludge is then subjected to solid-liquid separation, whereby the fluorine concentration in the supernatant separated from the final sludge can be reduced. It is considered that the growth of crystals in the fluorine compound proceeds in the state where no acid is added, and the fluorine concentration in the supernatant decreases. In addition, in the aging step S41, the sludge is stirred for a predetermined time, but the stirring operation may be performed also in the alkali addition step S11, the solid-liquid separation step S21, the acid addition step S31, and the like.

From the viewpoint of good treatment efficiency, the water treatment method of the present embodiment is preferably carried out by a continuous test in line with industrial use in the actual field. In the continuous test, since the above-mentioned acid addition step S31 is normally performed in the acid addition tank 31 to which acid is continuously supplied, the aging step S41 is different from the above-mentioned acid addition step S31. It is preferable to carry out in an aging tank) 41. That is, in the water treatment method of the present embodiment, the above-mentioned acid addition step S31 is performed in the acid addition tank 31, and the sludge (reduced sludge) after addition of the acid is transferred from the acid addition tank 31 to the aging tank 41. It is preferable to transfer and perform the aging step S41. By performing the aging step S41 in the aging tank 41 that is separate from the acid addition tank 31, it is possible to further reduce the fluorine concentration in the supernatant liquid separated from the final sludge, and at the same time, The total residence time of sludge after the liquid separation step S21 can be shortened.

The time (stirring time) in the aging step S41 is preferably 10 to 300 minutes, and preferably 30 to 180 minutes, from the viewpoint of effectively lowering the fluorine concentration in the supernatant separated from the final sludge. More preferably, it is 60 minutes to 120 minutes. Further, if a plurality of aging tanks 41 that stir the sludge after addition of the acid is used and the aging step S41 is continuously performed for each aging tank 41, the fluorine concentration in the supernatant liquid can be further reduced. And the total time in the aging step S41 can be further shortened. From these viewpoints, the aging step S41 is preferably performed in 1 to 5 aging tanks 41, and is preferably performed in 2 to 3 aging tanks 41.

As described above, by the aging step S41, it becomes possible to subsequently reduce the fluorine concentration in the supernatant separated from the final sludge, and as a result, the supernatant is returned to the alkali addition step S11. In the case of treatment, the addition amount of the alkaline agent can be reduced, which can contribute to the reduction of running cost. Further, if the total residence time of the sludge after solid-liquid separation can be shortened by the aging step S41, the volume of the tank to be used can be reduced, and as a result, the installation area of the tank will be reduced, thus reducing the site area of the facility. Can contribute. Furthermore, if the aging step S41 is performed in a tank (aging tank) 41 that is separate from the acid addition step S31, the residence time of sludge in the acid addition tank 31 can be shortened. This can lead to a smaller amount of addition and contribute to a reduction in running cost.

As shown in FIG. 1, the water treatment method of the present embodiment preferably further includes a step S51 of solid-liquid separating the sludge after addition of the acid after the aging step S41. In this step S51, the sludge (reduced sludge) after the addition of the acid is solid-liquid separated to obtain a final sludge and a supernatant liquid separated from the sludge. At this time, the supernatant contains, together with the dissolved magnesium hydroxide, fluorine derived from the magnesium hydroxide fluoride dissolved at the solubility corresponding to the pH value adjusted in the acid addition step S31. Therefore, it is preferable that the supernatant liquid separated from the final sludge is returned to perform the above-mentioned alkali addition step S11 and is again treated with the water to be treated (see long broken lines in FIGS. 1 to 3). ). As shown in FIG. 2, part of the final sludge (sludge) obtained in the solid-liquid separation step S51 of sludge may be returned to the aging step S41 (aging tank 41) (in FIG. 2). See short dashed line). By returning the sludge generated in the solid-liquid separation step S51 of sludge to the aging step S41 to accelerate the aging of magnesium fluoride and the like contained in the sludge, the fluorine concentration in the supernatant can be reduced. There are cases. The solid-liquid separation step S51 of sludge is preferably performed in the solid-liquid separation tank (precipitation tank) 51.

The water treatment method of the present embodiment preferably includes a step (dehydration step) S61 of performing a dehydration treatment on the final sludge obtained by solid-liquid separation of the sludge after addition of the acid. By this dehydration step, the final sludge can be treated as a dehydrated cake. Further, it is preferable that the dehydrated filtrate produced in the dehydrating step S61 is returned to the alkali adding step S11 and treated again with the water to be treated. The dehydrator 61 used for the dehydration treatment may be either a filtration type or a centrifugal separation type, but a filtration type is preferable. Suitable dehydrators 61 include a filter press type dehydrator and a vacuum dehydrator, and it is more preferable to use a filter press type dehydrator. According to the study by the present inventors, it was found that when a calcium salt such as calcium hydroxide is used as the alkaline agent in the alkali adding step S11, the final sludge dewatering property is improved. That is, in this case, the water content of the final sludge is reduced, so that the dehydrated cake amount is reduced, which can contribute to downsizing of the dehydrator and shortening of the operating time of the dehydrator. During the dehydration treatment, pretreatment such as conditioning treatment and concentration treatment may be performed so that water contained in sludge (sludge) can be easily separated.

So far, the treatment of sludge derived from suspended solids separated in the solid-liquid separation step S21 has been described, but next, the treatment of the supernatant water separated in the solid-liquid separation step S21 will be described.

When the fluorine concentration in the supernatant water obtained in the solid-liquid separation step S21 of the suspended substance satisfies the emission standard, the supernatant water can be discharged as treated water after adjusting the pH as necessary (Fig. (See S7 and S8 in 1). When the concentration of fluorine in the supernatant water after dilution satisfies the emission standard when the supernatant water is diluted with another waste water, the diluted supernatant water is treated with treated water by adjusting the pH as necessary. Can be released as.

In addition, for example, when the concentration of fluorine in the water to be treated is high, the fluorine concentration in the supernatant water cannot be satisfied as it is, or when it is diluted with another wastewater, the emission standard that can be discharged cannot be satisfied in one-stage treatment. When such a concern is concerned, it is preferable to carry out the treatment in two stages for the purpose of more reliable fluorine removal treatment. FIG. 3 shows a schematic flow chart showing the water treatment method of one embodiment of the present invention in the case of treating in two stages.

In the water treatment method shown in FIG. 3, the above-mentioned alkali addition step and the solid-liquid separation step S21 for performing solid-liquid separation of the suspended substance containing fluoride ions generated in the alkali addition step are each performed twice. Go and process in two stages. That is, in this water treatment method, the supernatant water separated from the suspended solids in the solid-liquid separation step (first step) S21 of the suspended solids generated in the above-mentioned alkali addition step (first step) S11 is treated with an alkali. The method further includes a second-step alkali addition step S12 in which the agent is added to generate a suspended substance in the supernatant water. The water treatment method further includes a second solid-liquid separation step S22 of performing solid-liquid separation on the suspended substance in which the fluoride ions are incorporated in the supernatant water by the second step alkali addition step S12. .. The second stage alkali addition step S12 is preferably performed in the second reaction tank 12, and the second solid-liquid separation step S22 is preferably performed in the second precipitation tank 22. The second-stage alkali addition step S12 can be performed in the same manner as the first-stage alkali addition step S11, and the second-stage solid-liquid separation step S22 can be performed in the same manner as the first-stage solid-liquid separation step S21. be able to.

When the treatment is performed in two stages, the subsequent treatment of the sludge obtained in each stage of the solid-liquid separation steps S21 and S22 is preferably carried out together with the above-mentioned acid addition step S31 as shown in FIG. You may perform an acid addition process separately. After the acid addition step, the aging step S41 is performed as in the method described with reference to FIG. After the aging step S41, it is preferable to carry out a step S51 of solid-liquid separating the sludge (reduced sludge) after the addition of the acid. Also in this case, although not shown, even if a part of the final sludge (sludge) obtained in the sludge solid-liquid separation step S51 is returned to the aging step S41 (aging tank 41) as described above. good.

In the first-stage solid-liquid separation step S21 for solid-liquid separating the suspended substance generated in the first-stage alkali addition step S11, the supernatant water separated from the suspended substance is added to the first-stage alkali addition step. A method of returning to S11 is also preferable. That is, it is preferable to introduce the supernatant water obtained in the first-stage solid-liquid separation step S21 into the first- or second-stage alkali addition steps S11, S12 (reaction tanks 11, 12). .. It should be noted that whether to perform the processing in the first stage or the second stage may be determined according to the situation of the processing site.

The water treatment method of the present embodiment is suitable for waste water discharged from a flue gas desulfurization apparatus as water to be treated containing fluoride ions and magnesium ions, and removes sulfur in exhaust gas using magnesium hydroxide. It is more preferable to use the waste water discharged from the flue gas desulfurization apparatus of
Specifically, the water treatment method of this embodiment is more suitable as a treatment for water to be treated that contains sulfate ions in addition to fluoride ions and magnesium ions. Examples of such water to be treated include waste water discharged from a flue gas desulfurization apparatus by a flue gas desulfurization method implemented in a coal-fired power plant or a coke plant. Since a large amount of wastewater after flue gas desulfurization from coal-fired power plants and coke plants is discharged, the water treatment method of the present embodiment finally reduces the amount of sludge that requires dehydration treatment, and Incorporating a fluorine content in the final sludge at a high concentration, which cannot be achieved by the conventional technique, has an extremely great effect in practical use.

Since the water treatment method of the present embodiment described in detail above includes the above-described alkali addition step S11, acid addition step S31, and aging step S41, it is possible to reduce the final amount of sludge and to treat water to be treated. It is possible to remove the fluoride ion from the mixture with higher efficiency. More specifically, in the water treatment method of the present embodiment, the alkali addition step S11 positively produces a suspended substance in which fluoride ions are incorporated in magnesium hydroxide precipitated in the water to be treated, and By solid-liquid separating the turbid substance, fluoride ions can be removed from the water to be treated. Then, by adding an acid to the solid-liquid separated sludge, the final amount of the sludge can be reduced, and further, by the aging step S41, the final sludge is solid-liquid separated by the solid-liquid separation. The fluorine concentration in the supernatant separated from the sludge can be reduced. Therefore, a continuous process in which the supernatant liquid separated from the final sludge is returned to the alkali addition step S11 and the treatment is performed again together with the water to be treated can be performed more favorably, and higher treatment efficiency can be realized. it can.

As described above, the water treatment method of this embodiment can have the following configurations.
[1] An alkali addition step of adding an alkaline agent to water to be treated containing fluoride ions and magnesium ions to generate a suspended substance in the water to be treated, and the suspension in which the fluoride ions are incorporated A solid-liquid separation step of solid-liquid separation of a substance, an acid addition step of adding an acid to the sludge derived from the suspension material that has been solid-liquid separated, and an aging step of stirring the sludge for a predetermined time after adding the acid And a water treatment method including.
[2] In the alkali adding step, the pH of the water to be treated is adjusted within the range of 8.5 to 10.5 by adding the alkaline agent to the water to be treated. Water treatment method.
[3] The water according to [1] or [2], wherein in the acid addition step, the pH of the sludge is adjusted within the range of 3.0 to 8.5 by adding the acid to the sludge. Processing method.
[4] The acid addition step is performed in an acid addition tank, the sludge after the acid addition is transferred from the acid addition tank to an aging tank, and the aging step is performed [1] to [3]. The water treatment method according to any one of 1.
[5] The water treatment method according to [4], wherein a plurality of the aging tanks are used and the aging step is continuously performed for each aging tank.
[6] The water treatment method according to any one of [1] to [5], further including a step of solid-liquid separating the sludge after the acid is added after the aging step.
[7] In the step of solid-liquid separating the sludge after the addition of the acid, the supernatant separated from the sludge is returned for the alkali addition step, and is again treated with the water to be treated. The water treatment method as described in [6] above.
[8] The part of the final sludge obtained in the step of solid-liquid separating the sludge after the addition of the acid is returned to perform the aging step. Water treatment method.
[9] An alkaline agent is added to the supernatant water separated from the sludge derived from the suspended material in which the fluoride ions are incorporated in the solid-liquid separation step of the suspended material, and suspended in the supernatant water. The second stage alkali addition step for producing a substance, and the second stage solid-liquid separation for solid-liquid separation of the suspended substance in which the fluoride ions in the supernatant water are taken in by the second stage alkali addition step. The water treatment method according to any one of [1] to [8], further including a separation step.
[10] The water treatment method according to any of [1] to [9], wherein the water to be treated is waste water discharged from a flue gas desulfurization device.

The water treatment method according to any of the above [1] to [10] can also be carried out by the following water treatment system.
[11] A reaction tank in which an alkaline agent is added to water to be treated containing fluoride ions and magnesium ions to generate a suspended substance in the water to be treated, and the fluoride ions are taken in the reaction tank. A solid-liquid separation tank for performing solid-liquid separation of the suspended matter, an acid addition tank for adding an acid to the sludge derived from the suspended matter separated in the solid-liquid separation tank, and the sludge after adding the acid And a maturing tank for stirring for a predetermined time.

In the above water treatment system, each step (procedure) in the above-mentioned water treatment method includes a control unit including a CPU of a device (for example, a personal computer) for managing water quality such as pH and fluorine concentration of the water to be treated. It can also be realized by. Further, in the above-mentioned water treatment system, a program capable of executing each step (procedure) in the above-mentioned water treatment method is stored in various storage media or on a network, etc., and the control unit reads and executes the program. It is also possible to realize the above water treatment method.

Hereinafter, the present invention will be described more specifically with reference to test examples, but the present invention is not limited to the following test examples.

(Preparation of artificial wastewater)
In this test example, artificial wastewater was prepared as the water to be treated. Specifically, assuming wastewater from a flue gas desulfurization system that removes sulfur in exhaust gas using magnesium hydroxide, the fluorine concentration is 150 mg/L, magnesium sulfate is 40,000 mg/L, and pH is 8. An artificial wastewater of 2 was prepared.

(Test Example 1: Sludge formation method and sludge properties)
The prepared artificial wastewater is transferred to a reaction tank, and calcium hydroxide (Ca(OH) ₂ ) is added to the artificial wastewater in the reaction tank to raise the pH of the artificial wastewater to 9.7, Suspended material was produced. The artificial wastewater that generated suspended substances is transferred to a settling tank that performs coagulation/precipitation processing, and the suspended substances are precipitated in the settling tank for solid-liquid separation, and separated into supernatant water and sludge. An analysis was conducted to examine its properties. As a result, the fluorine concentration in the separated supernatant water (treated water) was 7 mg/L. The sludge concentration of the separated sludge was 50,000 mg/L. The total fluorine concentration in the solid-liquid separated sludge was 1430 mg/L, and the pH of the sludge was 9.7. The emission standard for fluorine into rivers is 8 mg/L in terms of fluorine concentration.

(Test Example 2)
In Test Example 2, sulfuric acid (75 mass% sulfuric acid aqueous solution) was added to the separated sludge produced in the same manner as in Test Example 1 in a batch test using a water tank without water passage. The pH of the sludge was adjusted to 7.0. This acid addition step was performed over 60 minutes (residence time: 60 minutes). At this time, the addition amount of the sulfuric acid aqueous solution was 34 g/L. After that, the aging step was not performed, and the sludge after the addition of the acid was subjected to solid-liquid separation in a settling tank to obtain a final sludge and a supernatant liquid separated from the sludge.

(Test Example 3)
In Test Example 3, the sludge produced in the same manner as in Test Example 1 was transferred to an acid addition tank, and a continuous test was performed in the acid addition tank to confirm that the sulfuric acid was added to the sludge (75% by mass sulfuric acid aqueous solution) ) Was added to adjust the pH of the sludge to 7.0. At this time, the addition amount of the sulfuric acid aqueous solution was 34 g/L. Moreover, this acid addition process was performed over 60 minutes (retention time: 60 minutes). Then, the sludge having a pH of 7.0 after addition of the acid was subjected to solid-liquid separation in a settling tank. As described above, in Test Example 3, after the acid was added to continuously pass the sludge through water, the sludge was continuously subjected to solid-liquid separation without performing the aging step, and the final sludge and its A supernatant liquid separated from the sludge was obtained.

(Test Example 4)
In Test Example 4, the place where the acid addition step was performed for 60 minutes (residence time: 60 minutes) in Test Example 3 was changed to the case where the acid addition step was performed for 120 minutes (residence time: 120 minutes). A test was performed in the same manner as in Test Example 3 except for above, to obtain a final sludge and a supernatant liquid separated from the sludge. The addition amount of the sulfuric acid aqueous solution in the acid addition step was 39 g/L.

(Test Example 5)
In Test Example 5, the place where the acid addition step was performed for 60 minutes (residence time: 60 minutes) in Test Example 3 was changed to the case where the acid addition step was performed for 180 minutes (residence time: 180 minutes). A test was performed in the same manner as in Test Example 3 except for above, to obtain a final sludge and a supernatant liquid separated from the sludge. The amount of the sulfuric acid aqueous solution added in the acid addition step was 42 g/L.

(Test Example 6)
In Test Example 6, in the continuous test (continuous water flow test), the acid addition step was performed on the sludge, and after the aging step, solid-liquid separation was performed to obtain the final sludge. , A supernatant separated from the sludge was obtained. Specifically, the sludge produced by the same method as in Test Example 1 was passed through an acid addition tank having a residence time of 60 minutes, and sulfuric acid (75% by mass) was added to the sludge in the acid addition tank. (Sulfuric acid aqueous solution) was added to adjust the pH of the sludge to 7.0. Then, the sludge having a pH of 7.0 is passed through an aging tank having a residence time of 120 minutes and stirred, and then the sludge is transferred to a settling tank for solid-liquid separation, and the final sludge and the sludge are separated from each other. A separated supernatant was obtained. In Test Example 6, the total residence time from the transfer of the sludge to the acid addition tank to the transfer to the precipitation tank for obtaining the final sludge was set to 180 minutes.

(Test Example 7)
In Test Example 7, the number of aging tanks was increased to 4 (retention time of each aging tank: 30 minutes), the same test as in Test Example 6 was performed, and the final sludge and the sludge were separated. The obtained supernatant was obtained. In Test Example 7, the total residence time from transfer of the sludge to the acid addition tank to transfer to the fourth aging tank for obtaining the final sludge was 180 minutes.

(Test Example 8)
In Test Example 8, as in Test Example 6, except that the step of returning the sludge generated in the settling tank for obtaining the final sludge to the aging tank (see the short dashed line in FIG. 2) was performed. The same test was conducted to obtain a final sludge and a supernatant liquid separated from the sludge.

(Test Example 9)
In Test Example 9, the aging step was performed by using one aging tank in Test Example 6 under the condition that the residence time in the aging tank was 120 minutes. The test was performed in the same manner as in Test Example 6 except that the conditions were changed to minutes. Specifically, the sludge produced by the same method as in Test Example 1 was passed through an acid addition tank having a residence time of 60 minutes, and sulfuric acid (75% by mass) was added to the sludge in the acid addition tank. (Sulfuric acid aqueous solution) was added to adjust the pH of the sludge to 7.0. Then, sludge having a pH of 7.0 is passed through a first-stage aging tank with a retention time of 20 minutes and stirred, and then transferred to a second-stage aging tank where the retention time is 20 minutes. Water was passed through and stirred. Then, the sludge was transferred to a settling tank for solid-liquid separation, and a final sludge and a supernatant liquid separated from the sludge were obtained. In Test Example 9, the total residence time from the transfer of the sludge to the acid addition tank to the transfer to the precipitation tank for obtaining the final sludge was set to 100 minutes.

(Test Example 10: Sludge formation method and sludge properties)
In Test Example 10, the same test as in Test Example 1 was performed except that the calcium hydroxide used as the alkaline agent in Test Example 1 was changed to sodium hydroxide. Specifically, the prepared artificial wastewater is transferred to a reaction tank, and sodium hydroxide (NaOH) is added to the artificial wastewater in the reaction tank to raise the pH of the artificial wastewater to 9.7. Suspended material was formed therein. The artificial wastewater that generated suspended substances is transferred to a settling tank that performs coagulation/precipitation processing, and the suspended substances are precipitated in the settling tank for solid-liquid separation, and separated into supernatant water and sludge. An analysis was conducted to examine its properties. As a result, the fluorine concentration in the separated supernatant water (treated water) was 6 mg/L. The sludge concentration of the separated sludge was 40,000 mg/L. The total fluorine concentration in the solid-liquid separated sludge was 1440 mg/L, and the pH of the sludge was 9.7.

(Test Example 11)
In Test Example 11, an acid addition step was performed on the separated sludge generated in the same manner as in Test Example 10 in a continuous test (continuous water flow test), and then an aging step was performed. From the above, solid-liquid separation was performed to obtain a final sludge and a supernatant liquid separated from the sludge. Specifically, the sludge produced in the same manner as in Test Example 10 was passed through an acid addition tank having a residence time of 60 minutes, and sulfuric acid (75% by mass) was added to the sludge in the acid addition tank. (Sulfuric acid aqueous solution) was added to adjust the pH of the sludge to 7.0. Then, the sludge having a pH of 7.0 is passed through an aging tank having a residence time of 120 minutes and stirred, and then the sludge is transferred to a settling tank for solid-liquid separation, and the final sludge and the sludge are separated from each other. A separated supernatant was obtained. In Test Example 11, the total residence time from the transfer of the sludge to the acid addition tank to the transfer to the precipitation tank for obtaining the final sludge was set to 180 minutes. In Test Example 11, the sludge generated in the settling tank for obtaining the final sludge was returned to the aging tank.

With respect to the supernatants obtained in Test Examples 2 to 9 and 11, the total fluorine concentration in the supernatant was measured. The results are shown in Table 1 together with the properties of the sludge obtained in each test example. Table 1 also shows the properties of the sludges obtained in Test Examples 1 and 10 as blanks. The total fluorine concentration (F concentration) was measured by a spectrophotometer according to the absorptiometric method specified in JIS K0102.

From the results of Test Example 2 and Test Example 3, in the continuous test along with industrial use, which has an advantage in treatment efficiency, compared with the batch test, the final sludge after addition of the acid is solid-liquid. It was found that the separated supernatant had a high fluorine concentration. Further, from this result and the results of Test Examples 4 and 5, in the continuous test, even if the time for adding the acid in the acid addition step was extended or the amount of the acid added was increased, the supernatant was obtained. It was found that it was difficult to obtain the effect of significantly reducing the fluorine concentration in the liquid. On the other hand, in Test Example 6 in which the aging step was performed after the acid addition step, the final sludge concentration (amount) can be reduced, and the fluorine content in the sludge can be increased. It was confirmed that the fluorine concentration in the clear liquid could be significantly reduced, and it was found that high treatment efficiency can be realized.

Further, from the results of Test Example 7, when the number of aging tanks used in the aging step was increased, the fluorine concentration in the supernatant was further decreased even if the total residence time from the acid addition step to the aging step was the same. It was confirmed that it can be done. In addition, it was confirmed from the results of Test Example 6 and Test Example 9 that the time of the aging step could be shortened by increasing the number of aging tanks. Furthermore, from the results of Test Example 6 and Test Example 8, it was confirmed that the introduction of the sludge returning process reduces the fluorine concentration in the supernatant. Therefore, as a result of the water treatment method combining the acid addition step and the aging step, it is possible to reduce the fluorine concentration in the supernatant, resulting in a reduction in the addition amount of the alkali agent and acid, and the total retention of sludge. This can contribute to reduction of running cost and shortening of time and accompanying reduction of equipment site area. From the results of Test Example 8 and Test Example 11, the use of NaOH as the alkaline agent can increase the fluorine content in the final sludge more than the use of Ca(OH) _2. Was confirmed. On the other hand, when the final sludge was dehydrated and the water content of the resulting dehydrated cake was measured, Ca(OH) ₂ was used as the alkaline agent rather than NaOH. It was confirmed that the rate was low and the dehydration property could be enhanced.

S11, S12 Alkali addition step S21, S22 Solid-liquid separation step S31 Acid addition step S41 Aging step S51 Solid-liquid separation step S61 Dehydration step 11, 12 Reaction tank 21, 22 Solid-liquid separation tank 31 Acid addition tank 41 Aging tank 51 Solid liquid Separation tank 61 dehydrator

Claims (13)

By adding an alkali agent to the water to be treated containing fluoride ions and magnesium ions, wherein adjusting the pH of the water to be treated within the scope of 8.5 to 10.5, the fluoride ions the water to be treated A step of adding an alkali to generate a suspended substance in which
A first solid-liquid separation step of performing solid-liquid separation on the suspended substance in which the fluoride ions are incorporated;
An acid is added to the sludge derived from the suspended solids which has been subjected to the solid-liquid separation in the first solid-liquid separation step , and the pH of the sludge is adjusted to be in the range of 3.0 to 8.5. Acid addition step to obtain sludge of
After adding the acid, an aging step of stirring the sludge after the acid addition for a predetermined time,
After the aging step, the sludge after the acid addition is subjected to solid-liquid separation to obtain a final sludge and a second solid-liquid separation step of obtaining a final supernatant and a supernatant separated from the final sludge. seen including,
A water treatment method , wherein the supernatant liquid is returned to perform the alkali addition step, and is again treated together with the water to be treated . By adding an alkali agent to the water to be treated containing fluoride ions and magnesium ions, wherein adjusting the pH of the water to be treated within the scope of 8.5 to 10.5, the fluoride ions the water to be treated A step of adding an alkali to generate a suspended substance in which
A first solid-liquid separation step of performing solid-liquid separation on the suspended substance in which the fluoride ions are incorporated;
An acid is added to the sludge derived from the suspended solids which has been subjected to the solid-liquid separation in the first solid-liquid separation step , and the pH of the sludge is adjusted to be in the range of 3.0 to 8.5. Acid addition step to obtain sludge of
After adding the acid, an aging step of stirring the sludge after the acid addition for a predetermined time,
After the aging step, the sludge after the acid addition is subjected to solid-liquid separation to obtain a final sludge and a second solid-liquid separation step of obtaining a final supernatant and a supernatant separated from the final sludge. seen including,
A water treatment method, in which a part of the final sludge is returned to perform the aging step . The pH of the water to be treated is adjusted within the range of 8.5 to 10.5 by adding an alkaline agent to the water to be treated which contains fluoride ions and magnesium ions and has a Mg concentration of 2000 to 20000 mg /L. By doing so, in the water to be treated , an alkali addition step of precipitating the magnesium ions as magnesium hydroxide, and generating a suspended substance in which the fluoride ions are incorporated into the precipitated magnesium hydroxide ,
A first solid-liquid separation step of performing solid-liquid separation on the suspended substance in which the fluoride ions are incorporated;
In the sludge, an acid is added to the sludge derived from the suspended solids that has been solid-liquid separated in the first solid-liquid separation step to adjust the pH of the sludge to a range of 3.0 to 8.5. An acid addition step of obtaining sludge after acid addition, in which the magnesium hydroxide of
After adding the acid, an aging step of stirring the sludge after the acid addition for a predetermined time,
After the aging step, the sludge after the acid addition is subjected to solid-liquid separation to obtain a final sludge and a second solid-liquid separation step of obtaining a final supernatant and a supernatant separated from the final sludge. seen including,
By the acid addition step and the aging step, the fluoride ions taken in the sludge after the acid addition is precipitated as magnesium fluoride, and the magnesium fluoride is left in the final sludge, water Processing method. A portion of the final sludge, water treatment method according to claim 3 to return to perform the aging step. The supernatant returned to perform the alkali addition step, the water treatment method according to any one of claims 2-4 for performing again the process with the water to be treated. The water treatment method according to any one of claims 1 to 5, further comprising a dehydration step of dehydrating the final sludge obtained in the second solid-liquid separation step. The acid addition step is performed in an acid addition tank,
And transferring the sludge after addition of the acid to the aging tank from the acid addition tank, water treatment method according to any one of claims 1 to 6 for the aging process in the aging tank. The water treatment method according to claim 7 , wherein a plurality of the aging tanks are used, and the aging step is continuously performed for each of the aging tanks. Before SL by adding an alkali agent on the supernatant water of the first solid-liquid the fluoride ions in the separation step is separated from the suspended solids from sludge taken, suspended in the second stage to the supernatant water A second stage alkali addition step to generate turbid substances,
A second solid-liquid separation step of performing solid-liquid separation of the second-stage suspended substance in which the fluoride ions in the supernatant water are incorporated by the second-step alkali addition step;
The water treatment method according to claim 1, further comprising: The water treatment method according to any one of claims 1 to 9, wherein the water to be treated is waste water discharged from a flue gas desulfurization device. By adding an alkali agent to the water to be treated containing fluoride ions and magnesium ions, wherein adjusting the pH of the water to be treated within the scope of 8.5 to 10.5, the fluoride ions the water to be treated a reaction vessel to produce a suspended matter is captured,
A first solid-liquid separation tank for performing solid-liquid separation on the suspended substance in which the fluoride ions are taken in the reaction tank;
An acid is added to the sludge derived from the suspended solids separated in the first solid-liquid separation tank, the pH of the sludge is adjusted to a range of 3.0 to 8.5, and the sludge after the addition of the acid is adjusted. An acid addition tank for obtaining
After adding the acid, an aging tank for stirring the sludge after the acid addition for a predetermined time,
After stirring the sludge after the acid addition for a predetermined time in the aging tank, the sludge after the acid addition is solid-liquid separated, and the final sludge and the final sludge are separated into a supernatant. and a second solid-liquid separation tank to obtain,
A water treatment system in which the supernatant is returned to the reaction tank and treated again with the water to be treated . By adding an alkali agent to the water to be treated containing fluoride ions and magnesium ions, wherein adjusting the pH of the water to be treated within the scope of 8.5 to 10.5, the fluoride ions the water to be treated a reaction vessel to produce a suspended matter is captured,
A first solid-liquid separation tank for performing solid-liquid separation on the suspended substance in which the fluoride ions are taken in the reaction tank;
An acid is added to the sludge derived from the suspended solids separated in the first solid-liquid separation tank, the pH of the sludge is adjusted to a range of 3.0 to 8.5, and the sludge after the addition of the acid is adjusted. An acid addition tank for obtaining
After adding the acid, an aging tank for stirring the sludge after the acid addition for a predetermined time,
After stirring the sludge after the acid addition for a predetermined time in the aging tank, the sludge after the acid addition is solid-liquid separated, and the final sludge and the final sludge are separated into a supernatant. and a second solid-liquid separation tank to obtain,
A water treatment system in which a part of the final sludge is returned to the aging tank . The pH of the water to be treated is adjusted within the range of 8.5 to 10.5 by adding an alkaline agent to the water to be treated which contains fluoride ions and magnesium ions and has a Mg concentration of 2000 to 20000 mg /L. By doing so, in the water to be treated, the magnesium ions are precipitated as magnesium hydroxide, and a reaction tank for generating a suspended substance in which the fluoride ions are incorporated into the precipitated magnesium hydroxide ,
A first solid-liquid separation tank for performing solid-liquid separation on the suspended substance in which the fluoride ions are taken in the reaction tank;
An acid is added to the sludge derived from the suspended solids separated in the first solid-liquid separation tank to adjust the pH of the sludge within a range of 3.0 to 8.5, An acid addition tank in which magnesium hydroxide is dissolved to obtain sludge after acid addition,
After adding the acid, an aging tank for stirring the sludge after the acid addition for a predetermined time,
After stirring the sludge after the acid addition for a predetermined time in the aging tank, the sludge after the acid addition is solid-liquid separated, and the final sludge and the final sludge are separated into a supernatant. and a second solid-liquid separation tank to obtain,
In the acid addition tank and the aging tank, the fluoride ions taken in the sludge after the acid addition is precipitated as magnesium fluoride, and the magnesium fluoride is left in the final sludge, water Processing system.

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