JP2017189725A - Water treatment method and water treatment system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water treatment technology capable of effectively removing fluoride ion and reducing the final amount of sludge even when a treatment for removing a fluoride ion in water to be treated containing a fluoride ion and a magnesium ion is conducted as one step treatment.SOLUTION: There is provided a water treatment method including an alkali addition process for adding at least one kind of alkali agent selected from a group consisting of a hydroxide of alkali metal and hydroxide of alkali earth metal to water to be treated containing a fluoride ion and a magnesium ion, a solid solution separation process for sold solution separating a dispersed material in which the fluoride ion is taken, generated by the alkali addition process, and an acid addition process for adding acid to a sludge derived from the solid liquid separated dispersion material, and the added amount as OH of the alkali agent to water to be treated in the alkali addition process (mg-OH/L) is 1.0 or more by a mass ratio to the fluorine concentration in water to be treated (mg-F/L), (OH amount/F amount).SELECTED DRAWING: Figure 1

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 oriented. In response to such problems, a method of treating exhaust gas using magnesium hydroxide instead of lime has been implemented. This method captures the sulfur content in the exhaust gas not as a solid substance such as gypsum, but as magnesium sulfate having a high solubility in water. It can be released.

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

  However, in this method, if magnesium ions and sulfate ions are present in the waste water, such as the waste water from the flue gas desulfurization apparatus using magnesium hydroxide, the fluoride ion removal rate by the calcium method There was a problem that decreased. This is thought to be because, in the case of such waste water, a large amount of magnesium ions and fluoride ions dissolve as a complex in the neutral pH range, and this causes no calcium fluoride to be generated.

  To solve this problem, when calcium ions are added to wastewater containing fluoride ions and magnesium ions to remove fluoride ions as precipitates, in Patent Document 2, the pH of the wastewater is set to 9.4 to 9.8. Patent Document 3 proposes adjusting the pH of the waste water to 8 to 10, respectively.

Japanese Patent Publication No.58-013230 Japanese Patent Application Laid-Open No. 08-057486 JP 2000-301165 A

  In the conventional methods as disclosed in Patent Documents 2 and 3, when calcium ions are added to the wastewater, the pH of the wastewater is set to 9.4 to 9.8 or 8 to 10 without precipitation of calcium fluoride. By adjusting to such a range, it is considered that magnesium ions dissolved as a complex in the wastewater precipitate and precipitate as magnesium hydroxide. The fluoride ions dissolved as a complex in the wastewater are taken into the magnesium hydroxide precipitate and precipitated, and the existing calcium ions and free fluoride ions react to react with the calcium fluoride. It is thought to precipitate as. Thus, it is thought that the removal rate of fluoride ions from wastewater can be improved.

  In the conventional method described above, magnesium ions are precipitated as magnesium hydroxide by raising the pH of the wastewater in addition to the precipitation of calcium fluoride caused by the addition of calcium ions to the wastewater. For this reason, the amount of sludge generated tends to increase. If the final amount of sludge becomes large, the sludge treatment cost may increase. In particular, when a large amount of calcium ions is added to the wastewater at once to improve the removal rate of fluoride ions in the wastewater, a larger amount of sludge is generated, and the wastewater contains a large amount of sulfate ions. In this case, a large amount of gypsum is generated as sludge. As a result, the sludge disposal cost increases, leading to an increase in the overall running cost related to water treatment. Considering the increase in processing costs accompanying the increase in the amount of sludge generated, at the actual site, the fluorine concentration in the wastewater is reduced to some extent by the first stage treatment, and then the fluorine concentration is reduced by the second stage treatment. It was usually considered to be performed in a two-stage process of lowering to the discharge standard level.

  However, as described above, when the treatment for removing fluoride ions in the water to be treated is performed in a two-stage treatment for the water to be treated containing fluoride ions and magnesium ions, it is compared with the case where the treatment is performed in a one-stage treatment. As the number of facilities increases, the facility costs and the installation space for the facilities increase. From the viewpoint of equipment costs and installation space of the equipment, the one-stage treatment is more advantageous than the two-stage treatment. However, as described above, the one-stage treatment is more suitable for the two-stage treatment as the amount of sludge generated increases. As a result of the increase in the overall running cost, it has been common technical knowledge to perform a two-stage process.

  Therefore, the present invention can effectively remove fluoride ions even in the case where the fluoride ions in the water to be treated containing fluoride ions and magnesium ions are removed by a single stage treatment, and the final sludge. Water treatment technology capable of reducing the amount of water is to be provided.

  The present invention provides an alkali addition in which at least one alkali agent selected from the group consisting of hydroxides of alkali metals and alkaline earth metals is added to water to be treated containing fluoride ions and magnesium ions. A solid-liquid separation step for solid-liquid separation of the suspended substance into which the fluoride ions are taken, and an acid added to the sludge derived from the suspended substance that has been solid-liquid separated. An addition amount of the alkaline agent as OH to the water to be treated in the alkali adding step (mg-OH / L) is a fluorine concentration in the water to be treated (mg-F / A water treatment method in which the mass ratio (OH amount / F amount) to L) is 1.0 or more is provided.

  According to the present invention, fluoride ions can be effectively removed and final sludge can be obtained even when the fluoride ions in the water to be treated containing fluoride ions and magnesium ions are removed in a single stage. It is possible to provide a water treatment technique capable of reducing the amount of water.

It is a schematic flowchart showing the water treatment method of one embodiment of the present invention. It is a schematic flowchart showing the water treatment method of another one Embodiment of this invention. It is a schematic flowchart showing the water treatment method of another one Embodiment of this invention. FIG. 5 is a schematic flowchart showing a water treatment method executed in Comparative Example 1.

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

  In one embodiment of the present invention, the water treatment method includes at least one selected from the group consisting of hydroxides of alkali metals and hydroxides of alkaline earth metals in the water to be treated containing fluoride ions and magnesium ions. A step of adding an alkaline agent (alkali addition step). By this alkali addition step, a suspended substance in which fluoride ions are taken into the treated water is generated. The water treatment method of the present embodiment includes a step of solid-liquid separation of a suspended substance into which fluoride ions are taken, which is generated by an alkali addition step (solid-liquid separation step), and a solid-liquid separated suspension. A step of adding an acid to the substance-derived sludge (acid addition step). By the solid-liquid separation process, fluoride ions taken into the suspended substance are removed from the water to be treated.

  Here, in order to effectively remove fluoride ions in the water to be treated, in the alkali addition step, a large amount of the specific alkaline agent is added to the water to be treated, A large amount of suspended matter in which fluoride ions are incorporated is generated. Specifically, the above-mentioned specific alkaline agent is added to the water to be treated, and the addition amount (mg-OH / L) of the alkaline agent to the water to be treated is the fluorine concentration (mg-F / L) in the water to be treated. Is added in an amount of 1.0 or more in terms of mass ratio (OH amount / F amount). This alkali addition step generates a large amount of sludge derived from the suspended solids separated in the solid-liquid separation step, so that more fluoride ions in the water to be treated are taken into this sludge. However, fluoride ions in the water to be treated can be effectively removed. At this time, a large amount of sludge is generated. However, in the water treatment method of this embodiment, by adding acid to the sludge separated into solid and liquid, a part of the sludge is dissolved, and the sludge that needs final disposal ( Hereinafter, in this specification, the amount of “final sludge” may be reduced. Therefore, the water treatment method of this embodiment can contribute to reducing the sludge disposal cost and reducing the overall running cost related to water treatment.

As described above, in the water treatment method of the present embodiment, the fluoride ion can be effectively removed and the final amount of sludge even when the fluoride ion removal treatment in the water to be treated is performed in a single stage treatment. Can be reduced. Therefore, the water treatment method of this embodiment can be expected to be put to practical use in the one-stage treatment of the removal of F ^{− from} the treated water for the treated water containing F ⁻ and Mg ²⁺ . Water to be treated F ^- if possible effectively removed in one stage processing, the number of equipment as compared with the case of a two-stage process requires less in narrow place equipment with possible to reduce the equipment cost installation It is also practically useful from the viewpoints of equipment costs and equipment installation space.

  The water treatment method of the present embodiment having practical advantages as described above can be executed by, for example, the water treatment system of one embodiment of the present invention. The water treatment system includes a reaction tank in which an alkali agent is added to water to be treated containing fluoride ions and magnesium ions, and a suspension in which fluoride ions generated by addition of the alkali agent to the water to be treated are incorporated. A solid-liquid separation tank for solid-liquid separation of turbid substances, and an acid addition tank for adding acid to sludge derived from suspended solids separated in the solid-liquid separation tank. In the water treatment system of this embodiment, at least one selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides is used as the alkali agent. And the addition amount (mg-OH / L) of the alkaline agent as OH to the water to be treated in the reaction tank is a mass ratio (OH amount / F amount) to the fluorine concentration (mg-F / L) in the water to be treated. 1.0 or more.

  Hereinafter, each process in the water treatment method of one embodiment of the present invention will be specifically described with reference to the drawings. In the drawings, portions common to the drawings are denoted by the same reference numerals, and description thereof may be omitted. FIG. 1 is a schematic flowchart showing a water treatment method according to an embodiment of the present invention.

As shown in FIG. 1, in the water treatment method of the present embodiment, a step of adding an alkali agent to raw water containing fluoride ions (F ⁻ ) and magnesium ions (Mg ²⁺ ) (raw alkali addition) Step) S11 is performed. As the alkali agent, at least one selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides is used. Through this alkali addition step S11, a suspended substance (preferably a precipitate) in which F ⁻ is taken in the water to be treated is positively generated.

From the viewpoint of generating suspended solids in which F ⁻ is taken into the water to be treated, in the alkali addition step S11, the pH of the water to be treated is adjusted within the range of 8.5 to 10.5 by adding an alkali agent. It is preferable to adjust, and it is more preferable to adjust in the range of 9.0-10.0. Through such an alkali addition step S11, Mg ²⁺ in the water to be treated is precipitated as magnesium hydroxide (Mg (OH) ₂ ), and suspended matter in which F ⁻ is taken into the precipitated Mg (OH) ₂ (preferably Can generate a precipitate more actively. Thus, fluoride ions can be removed from the water to be treated by removing suspended substances by solid-liquid separation described later.

  In the present specification, the pH of the water to be treated and sludge described later is a value at 25 ° C. or a converted value at 25 ° C. For example, when the temperature of the water to be treated is higher than 25 ° C., the pH 8.5 to 10.5 of the water to be treated in the alkali addition step is shifted to a value in a range lower than the actual measured 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 addition step is at 50 ° C. of the water to be treated. The pH value is adjusted to approximately 8.0 to 10.0. Moreover, when adjusting pH of to-be-processed water, sludge, etc., you may use well-known pH adjusters, such as an alkali and an acid which are generally used in the field of water treatment.

In the alkali addition step S11, sodium hydroxide (caustic soda) and potassium hydroxide (caustic potash) can be suitably used as the alkali metal hydroxide. Further, as the alkaline earth metal hydroxide, calcium hydroxide (slaked lime) and barium hydroxide can be suitably used. One kind of these alkali agents may be used alone, or two or more kinds may be used in combination. Furthermore, alkali agents other than alkali metal hydroxides and alkaline earth metal hydroxides may be used in combination. From the viewpoint of sufficiently precipitating magnesium ions in the water to be treated as magnesium hydroxide, sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca (OH) ₂ ) are more preferable as alkaline agents. Preferably, Ca (OH) ₂ is more preferable. When the water to be treated contains sulfate ions (SO ₄ ²⁻ ) in addition to F ⁻ and Mg ²⁺ , if Ca (OH) ₂ is used as an alkaline agent, in addition to the above-described precipitation of magnesium hydroxide, It is also possible to precipitate sulfate ions in the treated water as gypsum (calcium sulfate).

  In the water treatment method of the present embodiment, it is preferable to perform the fluoride ion removal treatment in the water to be treated in a one-stage treatment from the viewpoint of reducing the equipment cost and the installation space of the equipment. In that one-stage treatment, a specific amount of an alkaline agent is added to the water to be treated in order to remove fluoride ions to such an extent that the fluorine concentration in the water to be treated can satisfy the discharge standard. That is, in the water treatment method of this embodiment, the addition amount (mg-OH / L) of the alkaline agent as OH to the water to be treated is set to a mass ratio (mg-F / L) to the fluorine concentration (mg-F / L) in the water to be treated. (OH amount / F amount) is 1.0 or more. As a result, more suspended substances in which fluoride ions are taken into the water to be treated are generated, and the suspended substances are solid-liquid separated, whereby fluoride ions in the water to be treated can be further reduced.

  Since the water to be treated (raw water) may contain an inhibitor against the removal of fluorine, the amount of OH / F suitable for each target raw water during actual on-site operation It is desirable to investigate the quantity requirements in advance. Therefore, in the range where the mass ratio (OH amount / F amount) of the addition amount (OH addition amount) of the alkali agent as OH to the fluorine concentration in the treatment water is 1.0 or more, the water to be treated is obtained in advance. Therefore, it is preferable to determine the amount of alkali agent added as OH. That is, for the water to be treated, the mass ratio of the OH addition amount of the alkaline agent to the fluorine concentration in the water to be treated (OH amount / F amount) and sludge are separated in the solid-liquid separation step. Further, the relationship with the fluorine concentration (mg-F / L) in the supernatant water is obtained. And based on the relationship calculated | required previously, it is preferable to determine the addition amount as OH of the alkaline agent to the to-be-processed water in the alkali addition process with respect to the fluorine concentration in to-be-treated water. Specifically, based on the above relationship, the alkali agent as OH is used so that the fluorine concentration in the supernatant water separated from the sludge in the solid-liquid separation step is an OH amount / F amount that significantly decreases. The amount added can be determined. In addition, based on the above relationship, the OH of the alkaline agent is adjusted so that the fluorine concentration in the supernatant water separated from the sludge in the solid-liquid separation step is the amount of OH / F required to be a predetermined value or less. The amount of addition as can be determined. As the predetermined value of the fluorine concentration in the supernatant water, an arbitrary value can be selected, for example, 15 mg-F / L which is a regulation value when discharged into the sea area in the Water Pollution Control Law, and further for public use in the law It is possible to adopt 8 mg-F / L which is a regulation value when discharged into the water area. The preliminary test can be performed, for example, in a small-scale batch test in which water to be treated is placed in a beaker or the like and an alkali addition step and a solid-liquid separation step are performed. As will be described later, after the acid addition step, the final sludge after the addition of the acid is solid-liquid separated and the supernatant liquid contains fluorine, so the supernatant liquid is added to the alkali addition step. It is preferable to return to the water and treat it together with the water to be treated (raw water). In this case, the fluorine concentration in the treated water after returning the supernatant liquid usually tends to be higher (for example, about 15%) than the fluorine concentration in the initial treated water (raw water). In consideration of the increase in fluorine concentration in the water to be treated in that case, the OH of the alkali agent is set so as to be higher (for example, about 1.15 times) than the OH amount / F amount required based on the above relationship. It is more preferable to determine the addition amount as.

  The addition amount (mg-OH / L) of the alkali agent as OH to the water to be treated in the alkali addition step S11 is a mass ratio (OH amount / F amount) to the fluorine concentration (mg-F / L) in the water to be treated. If it is 1.0 or more, more suspended substances are produced as described above. Therefore, although the amount of sludge increases, the amount of sludge can be reduced by the acid addition step S32 described later. When the amount of alkali agent added to the water to be treated is 25.0 or more in the above mass ratio (OH amount / F amount), the removal ability of fluoride ions in the water to be treated is almost the same. It will not be. Therefore, from the viewpoint of suppressing the cost associated with the use amount of the alkaline agent, the mass ratio (OH amount / F amount) is preferably 25.0 or less, more preferably 20.0 or less, and still more preferably 16 0.0 or less. Further, if the sludge after the addition of the acid can be dehydrated as it is without performing the concentration treatment, it can contribute to further reduction of the overall running cost related to the water treatment. From this viewpoint, the mass ratio (OH amount / F amount) is preferably 3.0 or more, more preferably 4.0 or more, and still more preferably 5.0 or more. From the viewpoints of the cost associated with the use amount of the alkaline agent and the ease of dewatering of the sludge after acid addition, the above mass ratio (OH amount / F amount) is more preferably 3.0 or more and 25.0 or less. .

In the alkali addition step S11, it is preferable to add an alkali agent to the water to be treated so that a large amount of fluoride ions in the water to be treated is taken up by the sludge. For example, by adjusting the addition amount of the alkaline agent together with the residence time of SS (including SS that was originally contained in the raw water), the concentration C of the sludge that is solid-liquid separated in the solid-liquid separation step S22 described later _An alkaline agent can be added to the water to be treated so that _S1 is 10,000 mg / L or more.

  The alkali addition step S11 is preferably performed in the reaction vessel 11 as a vessel separate from the solid-liquid separation step S22 described later. Moreover, although not shown in figure, the reaction tank 11 is provided with the raw | natural water supply part for supplying to-be-processed water (raw water) to a reaction tank, and the alkaline agent supply part for adding an alkaline agent. More preferred. A raw | natural water supply part can be comprised with the supply pipe | tube and pump which send raw | natural water to the reaction tank 11 from the storage tank of raw | natural water, for example. The alkaline agent supply unit can be constituted by, for example, an alkaline agent supply pipe that sends the alkaline agent from the alkaline agent storage tank to the reaction tank 11 and a pump.

  As shown in FIG. 1, in the water treatment method of the present embodiment, a step of solid-liquid separation of the suspended matter (suspended matter in which fluoride ions in the water to be treated are taken up) generated in the alkali addition step S11 ( (Solid-liquid separation step) S22 is performed. This solid-liquid separation step S22 is preferably performed in a tank (solid-liquid separation tank) 22 that is separate from the tank (the aforementioned reaction tank) 11 for adding an 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. Among these, it is preferable to employ a coagulation / precipitation treatment from the point that solid-liquid separation can be performed as a suspended matter in which fluoride ions have been incorporated. In this case, using a precipitation tank such as a thickener, It is preferable to perform the solid-liquid separation step S22.

  Further, when the aggregation / precipitation treatment of the suspended substance is performed, a flocculant may be used to promote the aggregation / precipitation of the suspended substance. In this case, as shown in FIG. 1, it is preferable to perform step S21 of adding a flocculant to the water to be treated to which an alkaline agent has been added, between the alkali addition step S11 and the solid-liquid separation step S22. Moreover, it is preferable to perform process S21 which adds this flocculant in the tank (flocculant addition tank) 21 separate from the above-mentioned reaction tank 11 and the solid-liquid separation tank 22. FIG. As the flocculant, known inorganic flocculants such as polyaluminum chloride, aluminum sulfate, and iron salt flocculants, and polyacrylic acid ester flocculants, polymethacrylic acid ester flocculants, polyacrylamide flocculants, etc. These known polymer flocculants can be used.

  Through the solid-liquid separation step S22, sludge derived from the suspended substance is obtained. At this time, in the preferred water treatment method in the present embodiment, the sludge obtained by solid-liquid separation of the suspended substance is composed of a mineral phase mainly composed of magnesium hydroxide and has a fluorine content of 2 About 10% by mass sludge can be obtained.

It is preferable to adjust the concentration C _S1 of the sludge separated in the solid-liquid separation step S22 to 10000 mg / L or more. The concentration C _S1 of the sludge separated in the solid-liquid separation step S22 is the SS concentration contained in the treated water (raw water), the sludge retention time in the solid-liquid separation tank 22 (from the solid-liquid separation tank 22 to the sludge). It is possible to adjust the amount of the alkali agent and the addition amount of the alkali agent. It is more preferable to adjust the concentration C _S1 of the sludge separated in the solid-liquid separation step S22 to 20000 mg / L or more, and it is more preferable to adjust C _S1 to 30000 mg / L or more. On the other hand, if the concentration C _{S1 of the} sludge to be solid-liquid separated is too high, it becomes difficult to transfer the sludge. Therefore, the concentration C _S1 of the sludge can be adjusted to 100000 mg / L or less (more preferably 90000 mg / L or less). preferable. In addition, in this specification, the density | concentration of sludge (sludge) may be called SS density | concentration.

  The solid-liquid separation step S22 separates the suspended substance-derived sludge and the supernatant water. First, the sludge treatment method will be described.

As shown in FIG. 1, in the water treatment method of the present embodiment, a step (acid addition step) S32 of adding acid to sludge derived from suspended solids subjected to solid-liquid separation is performed. By this acid addition step S32, magnesium hydroxide which is a main component constituting the sludge is dissolved, 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 can remain in the sludge having a reduced fluorine content 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 reducing the final amount of sludge and from the viewpoint of leaving the fluorine content in the sludge, in the acid addition step S32, the pH of the sludge is adjusted to 3.0 to 8.5 by adding acid to the sludge. It is preferable to adjust within the range. From the viewpoint of sufficiently dissolving the magnesium hydroxide in the sludge and further reducing the amount of the final sludge, in the acid addition step S32, it is preferable to adjust the sludge to pH 8.5 or less, and to pH 8.0 or less It is more preferable to adjust, and it is further preferable to adjust to pH 7.0 or less. On the other hand, from the viewpoint of increasing the amount of fluorine remaining in the sludge so that the fluorine content in the sludge is not dissolved at a high concentration, in the acid addition step S32, it is preferable to adjust the sludge to pH 3.0 or more, and pH 4.0. It is more preferable to adjust to the above.

In the acid addition step S32, when the pH of the sludge is adjusted, if 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 the water. As a result of an examination experiment conducted by the present inventors, the solubility of Mg ²⁺ and sulfate ions (SO ₄ ^{2−) in} addition to F 2 ⁻ was confirmed as in wastewater discharged from a flue gas desulfurization apparatus suitable as water to be treated. ) In the high concentration of water to be treated, it was found that the fluorine content dissolves in the liquid in the range of about 200 to 600 mg / L in the pH range of sludge of 3.0 to 8.5. . From this, by the acid addition step S32, fluorine content dissolves in the liquid in the range of 200 to 600 mg / L together with magnesium hydroxide which is the main component of the sludge, thereby reducing the amount of sludge, It is considered that the fluorine content is precipitated as magnesium fluoride in the reduced sludge, and this allows the fluorine content to remain at a high concentration.

In addition to the main component magnesium hydroxide, the solid-liquid separated sludge contains magnesium fluoride (MgF ₂ ), magnesium hydroxide fluoride (MgFOH), etc. in which fluoride ions are incorporated into the precipitated magnesium hydroxide. Fluorine compounds can be included. Most of the fluorine content contained in the sludge is MgF ₂ or MgFOH. However, from the viewpoint of the solubility of magnesium fluoride described above, 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 S32, the solubility corresponding to this pH value. It is considered that the fluorine content is dissolved from MgFOH by this amount, and most of the other fluorine content is precipitated as magnesium fluoride. As a result, it is considered that magnesium fluoride remains at a high concentration in the final sludge obtained by solid-liquid separation of the sludge after addition of the acid in the acid addition step S32.

  The acid used in the acid addition step S32 is not particularly limited as long as a part of the sludge can be dissolved and the final amount of sludge can be reduced. Suitable acids include, for example, hydrochloric acid, sulfuric acid, and nitric acid. The amount of acid added to the sludge can be appropriately adjusted according to the SS concentration.

  As the type of acid, sulfuric acid is preferable to hydrochloric acid. When the pH of the sludge is adjusted using sulfuric acid, the final sludge concentration (amount) can be reduced and remains in the final sludge, compared to the case where the pH of the sludge is adjusted using hydrochloric acid. The amount of fluorine can be increased. In particular, by adding sulfuric acid to the sludge, the pH of the sludge is adjusted to 4.0 to 7.5, and more preferably, the pH is adjusted to about 4.0 to 6.5, so that a more remarkable effect can be stably obtained. be able to.

In the acid addition step S32, the solid-liquid separation is performed such that the sludge concentration C _S2 after the acid is added is lower than the sludge concentration C _S1 when the solid-liquid separation is performed in the solid-liquid separation step S22. Add acid to the sludge. In the acid addition step S32, the concentration C _S2 of the sludge after the acid is added is adjusted to be lower than the concentration C _S1 of the sludge when the solid-liquid separation is performed and in a range of 5000 mg / L to 50000 mg / L. It is preferable. Since the sludge after the addition of the acid can be dehydrated without being concentrated, the acid addition step S32 has a concentration C _S2 of the sludge after the addition of the acid of 10,000 mg / L or more (more preferably 15000 mg / L or more, more preferably 20000 mg / L or more). However, even in this range, the sludge concentration C _S2 after the acid is added is made lower than the sludge concentration C _S1 when the solid-liquid separation is performed. In particular, when the water to be treated contains sulfate ions and calcium hydroxide is used as the alkaline agent, the main components of the sludge after addition of the acid are gypsum and MgF ₂ , so that the sludge after addition of the acid The concentration process can be omitted.

  As shown in FIG. 1, the acid addition step S32 is preferably performed in an acid addition tank 32 to which the sludge separated in the solid-liquid separation step S22 is transferred. At this time, the sludge storage tank 31 in which the sludge separated in the solid-liquid separation step S22 is transferred and stored before the acid addition tank 32 so that the water treatment method of the present embodiment is easily performed in a continuous process. It is more preferable to provide. Therefore, the acid addition step S32 is more preferably a step of adding an acid in the acid addition tank 32 to the sludge transferred from the sludge storage tank 31 to the acid addition tank 32. Although not shown, the acid addition tank 32 is preferably provided with a sludge supply part for supplying sludge to the acid addition tank 32 and an acid supply part for adding acid. A sludge supply part can be comprised by the pipe | tube with which sludge passes, a pump, etc., for example. The acid supply unit can be configured by, for example, a supply pipe that sends acid from the acid storage tank to the acid addition tank 32, a pump, and the like.

  As shown in FIG. 1, the water treatment method of the present embodiment allows the sludge after the acid is added after the acid addition step S32 (after adding the acid to the sludge) so that it can be easily performed in a continuous process. It is preferable to further include a step (ripening step) S41 of stirring for a predetermined time. For stirring, a mechanical stirring device, a blower, or the like can be used, and it is preferable to use a mechanical stirring device provided with stirring blades. In the aging step S41, the sludge after the acid is added is stirred for a predetermined time in a state where the addition of the acid to the sludge is released. By this ripening step S41, the fluorine concentration in the supernatant separated from the final sludge after the step S41 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 is lowered. In the ripening step S41, the sludge is stirred for a predetermined time, but the stirring operation may also be performed in the alkali addition step S11, the solid-liquid separation step S22, the acid addition step S32, and the like.

  The water treatment method of the present embodiment is preferably performed by a continuous process in line with industrial use on the actual site from the viewpoint of good treatment efficiency. In the continuous process, since the acid addition step S32 is usually performed in the acid addition tank 32 to which the acid is continuously supplied, the aging step S41 is performed in a tank separate from the acid addition step S32 (aging). Tank) 41 is preferable. The sludge after the acid is added (reduced sludge) is transferred from the acid addition tank 32 to the aging tank 41 and the aging step S41 is performed, and then the supernatant liquid separated from the final sludge. It is possible to further reduce the fluorine concentration in the medium, and it is possible to shorten the total residence time of the sludge after the solid-liquid separation step S22. In the aging step S41, since the sludge is stirred, it is preferable to use the aging tank 41 having a stirring function. The above-described reaction tank 11, the flocculant addition tank 21, the solid-liquid separation tank 22, the sludge storage tank 31, the acid addition tank 32, and the like may be provided with a stirring function.

  The time (stirring time) of the aging step S41 is 30 to 180 minutes from the viewpoint of effectively reducing the fluorine concentration in the supernatant liquid separated from the final sludge thereafter and the equipment cost reduction. Is preferable, more preferably 60 to 150 minutes, and still more preferably 90 to 120 minutes. Further, in order to more effectively reduce the fluorine concentration in the supernatant, a plurality of aging tanks 41 for stirring the sludge after addition of the acid may be used, and the aging step S41 may be performed continuously for each aging tank 41. . In the ripening step S41, the fluorine concentration in the supernatant separated from the final sludge can be reduced, and as a result, the supernatant is returned to the alkali addition step S11 and processed. This leads to a reduction in the amount of alkali agent added, and can contribute to a reduction in running cost.

  As shown in FIG. 1, it is preferable that the water treatment method of this embodiment further includes a step (dehydration step) S51 of dewatering sludge to which an acid has been added after the acid addition step S32. When performing the above-mentioned ripening step S41, the dehydration step S51 can be performed after the ripening step S41. By the dehydration step S51, the final sludge can be recovered as a dehydrated cake. Moreover, it is preferable that the dehydrated filtrate generated in the dehydration step S51 is returned to the alkali addition step S11 and treated again with the water to be treated. The dehydrator 51 used for the dehydration treatment may be either a filtration type or a centrifugal type, but a filtration type is preferred. Suitable dehydrators 51 include a filter press dehydrator and a vacuum dehydrator, and it is more preferable to use a filter press 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 alkali agent in the alkali addition step S11, the final sludge dewaterability is improved. That is, in this case, since the moisture content of the final sludge is reduced, the amount of dehydrated cake is reduced, which can contribute to the compactness of the dehydrator 51 and the shortening of the operation time of the dehydrator 51. In the dehydration process, a pretreatment such as a tempering process or a concentration process may be performed so that water contained in the sludge (sludge) can be easily separated.

  In the water treatment method of the present embodiment, after the acid addition step, before the sludge to which the acid has been added is dehydrated, the sludge to which the acid has been added is concentrated and subjected to solid-liquid separation (concentration step). May be included. As described above, the water treatment method of the present embodiment can omit the concentration step, but the concentration of sludge after the acid is added is determined by a dehydrator (for example, a suitable filter press type dehydrator). When it is so low that it is difficult to dehydrate, it is preferable to perform a concentration step. Any of gravity concentration, mechanical concentration, and levitation concentration may be used as the method of concentration treatment.

  FIG. 2 is a schematic flowchart showing a water treatment method according to an embodiment of the present invention different from the embodiment shown in FIG. As shown in FIG. 2, in the water treatment method of one embodiment of the present invention, after the acid addition step S32 described above, a flocculant is added to the sludge after the acid is added (reduced sludge), A step (concentration step) S61 of concentrating the sludge and separating it into solid and liquid can be performed. Prior to the concentration step S61, the aging step S41 described above may be included. In the concentration step S61, it is preferable that the sludge after the acid is added is transferred to the coagulation tank 62, and the coagulant is added in the coagulation tank 62, and the sludge after the addition of the coagulant is concentrated from the coagulation tank 62. The sludge is preferably transferred to the tank 63 and concentrated in the concentration tank 63.

  By the concentration step S61, a final sludge and a supernatant separated from the sludge can be obtained. The final sludge is preferably recovered as a dewatered cake by the above-described dewatering step S51. Also in this case, as described above, it is preferable that the dehydrated filtrate generated in the dehydration step S51 is returned to the alkali addition step S11 and treated again with the water to be treated. Further, a part of the final sludge (sludge) obtained in the sludge concentration step S61 (concentration tank 63) may be returned to the aging step S41 (aging tank 41). By returning the sludge generated in the sludge concentration step S61 to the aging step S41, the fluorine concentration in the supernatant can be reduced by promoting the aging of magnesium fluoride contained in the sludge. is there.

  The supernatant in the concentration step S61 contains, together with the dissolved magnesium hydroxide, fluorine derived from magnesium hydroxide fluoride dissolved with a solubility corresponding to the pH value adjusted in the acid addition step S32. Therefore, it is preferable to return the supernatant to perform the above-described alkali addition step S11 and perform the treatment again with the water to be treated (see the broken line in FIG. 2). In this case, the fluorine concentration in the treated water after returning the supernatant liquid usually tends to be higher than the fluorine concentration in the initial treated water (raw water). It is preferable to increase the amount of the alkali agent added to the water to be treated in the above-described alkali addition step in accordance with the increase in the amount of water.

  So far, the treatment of the sludge derived from the suspended matter separated in the above-mentioned solid-liquid separation step S22 has been described. Next, the treatment of the supernatant water separated in the solid-liquid separation step S22 will be described.

  When the fluorine concentration in the supernatant water obtained in the solid-liquid separation step S22 of the suspended matter satisfies the discharge standard, the supernatant water can be discharged as treated water by adjusting the pH as necessary (FIG. 1 and FIG. 2). In addition, if the fluorine concentration in the diluted supernatant water satisfies the discharge standard when the supernatant water is diluted with another waste water, the pH of the diluted supernatant water is adjusted to the treated water as necessary. Can be released as As described above, in the alkali addition step S11 in the water treatment method of the present embodiment, when a specific amount of a specific alkaline agent is added to the water to be treated, the fluoride ion removal treatment in the water to be treated is performed in a single stage. In addition, fluoride ions can be effectively removed. Therefore, in the water treatment method of the present embodiment, it can be greatly expected that the fluorine concentration in the supernatant water obtained in the solid-liquid separation step S22 will be equal to or less than the discharge standard (for example, 8 mg / L).

  In the water treatment method of this embodiment, as described above, it is desirable to perform the fluoride ion removal treatment in the water to be treated, such as addition of an alkaline agent to the water to be treated, in a one-stage treatment. However, when the fluorine concentration in the supernatant water separated from the sludge after the solid-liquid separation step S22 is not sufficiently reduced, such as when the fluorine concentration in the water to be treated is high, The step of adding the alkali agent again may be performed, and the supernatant water may be returned to the alkali addition step S11. When the step of adding the alkaline agent to the supernatant water is performed again (in the case of two-stage treatment), the step can be performed in the same manner as the alkali addition step S11 described above, and the solid-liquid separation step after that step Etc. can be performed in the same manner as described above. FIG. 3 shows a schematic flow chart showing a water treatment method of still another embodiment of the present invention when treating in two stages.

  The water treatment method shown in FIG. 3 is performed on the supernatant water separated from the suspended solids in the solid-liquid separation step (first stage) S22 of the suspended matter produced in the above-described alkali addition step (first stage) S11. It further includes a step of adding the above-mentioned specific alkaline agent (second-stage alkali addition step) S12. In addition, this water treatment method includes a step of solid-liquid separation (second-stage solid-liquid separation step) S24 of the suspended matter in which the fluoride ions in the supernatant water are incorporated in the second-stage alkali addition step S12. In addition. The second-stage alkali addition step S <b> 12 is preferably performed in the second reaction tank 12, and the second-stage solid-liquid separation process S <b> 24 is preferably performed in the second solid-liquid separation tank 24. Also in this case, it is preferable to perform the step S23 of adding the flocculant in the flocculant addition tank 23 between the second-stage alkali addition step S12 and the second-stage solid-liquid separation step S24.

  When removing fluoride ions in the water to be treated in a two-stage treatment, the subsequent treatment on the sludge obtained in the solid-liquid separation steps S22 and S24 in each stage is performed together with the above-described acid addition as shown in FIG. Although it is preferable to perform step S32, the acid addition step may be performed separately.

  The water treatment method of the present embodiment described in detail above is suitable for waste water discharged from a flue gas desulfurization apparatus as water to be treated containing fluoride ions and magnesium ions, and uses magnesium hydroxide in exhaust gas. It is more suitable for waste water discharged from a flue gas desulfurization apparatus that removes sulfur. Moreover, the water treatment method of this embodiment is more suitable as a process with respect to the to-be-processed water containing a sulfate ion other than a fluoride ion and magnesium ion. Examples of such water to be treated include waste water discharged from a flue gas desulfurization apparatus using a flue gas desulfurization method implemented in a coal-fired power plant or a coke factory. Wastewater after flue gas desulfurization from coal-fired power plants and coke plants is discharged in large quantities, so the water treatment method of this embodiment reduces the amount of final sludge that requires dehydration, etc. Including the fluorine content in the final sludge at a high concentration that could not be achieved by the prior art has a great practical effect.

  In the water treatment method of the present embodiment, an alkali metal hydroxide and / or an alkaline earth metal hydroxide is added to the water to be treated, and the amount added as OH (mg-OH / L) is in the water to be treated. Since it is added in an amount of 1.0 or more in terms of mass ratio (OH amount / F amount) to fluorine concentration (mg-F / L), a large amount of suspended matter in which fluoride ions are incorporated in the water to be treated is generated. It is possible. By solid-liquid separation of this suspended substance, it becomes possible to remove more fluoride ions taken into the sludge derived from the suspended substance. And in the water treatment method of this embodiment, the amount of final sludge can be reduced by adding an acid to the sludge separated into solid and liquid. Therefore, the fluoride ion can be effectively removed by the water treatment method of the present embodiment even when the fluoride ion removal treatment water containing fluoride ion and magnesium ion is performed in one stage. It becomes possible to reduce the amount of final sludge.

  Therefore, according to the water treatment method of the present embodiment, it is possible to reduce the sludge disposal cost and contribute to reducing the overall running cost related to the water treatment. Moreover, with the water treatment method of this embodiment, it can be expected that the water to be treated containing fluoride ions and magnesium ions will be put to practical use in the one-stage treatment for removing fluoride ions in the water to be treated. If fluoride ions in the water to be treated can be effectively removed by one-stage treatment, the number of equipment can be reduced compared to the case of performing two-stage treatment, so that the equipment cost can be reduced and the equipment can be installed in a narrow place. It can be installed and is practically useful from the viewpoint of equipment cost and installation space.

As described above, the water treatment method of the present embodiment can have the following configuration.
[1] Alkali addition step of adding at least one alkaline agent selected from the group consisting of hydroxides of alkali metals and hydroxides of alkaline earth metals to water to be treated containing fluoride ions and magnesium ions And a solid-liquid separation step for solid-liquid separation of the suspended substance into which the fluoride ions are taken, which is generated by the alkali addition step, and an acid is added to the sludge derived from the suspended substance that has been solid-liquid separated An addition amount of the alkaline agent as OH to the treated water in the alkali adding step (mg-OH / L) is a fluorine concentration (mg-F / L) in the treated water. ) Is a water treatment method having a mass ratio (OH amount / F amount) of 1.0 or more.
[2] In the alkali addition step, the pH of the water to be treated is adjusted within a range of 8.5 to 10.5 by adding the alkali agent to the water to be treated. Water treatment method.
[3] The amount of the alkali agent added as OH to the water to be treated in the alkali addition step with respect to the fluorine concentration in the water to be treated is determined in advance as the mass ratio (OH amount / F amount) and The water treatment method according to [1] or [2], wherein the water treatment method is determined based on a relationship with a fluorine concentration in the supernatant water separated from the sludge in the solid-liquid separation step.
[4] The mass ratio (mg-OH / L) of the alkali agent as OH to the water to be treated in the alkali addition step with respect to the fluorine concentration (mg-F / L) in the water to be treated ( The water treatment method according to any one of [1] to [3], wherein the OH amount / F amount is 3.0 or more.
[5] The concentration C _S1 of the sludge that is solid-liquid separated in the solid-liquid separation step is adjusted to 10000 mg / L or more, and the concentration C _S2 of the sludge after the acid is added in the acid addition step The water treatment method according to any one of [1] to [4], wherein the water treatment method is adjusted to a range lower than C _S1 and in a range of 5000 mg / L to 50000 mg / L.
[6] In any of the above [1] to [5], in the acid addition step, the pH of the sludge is adjusted within a range of 3.0 to 8.5 by adding the acid to the sludge. The water treatment method as described.
[7] The water treatment method according to any one of [1] to [6], further including a step of stirring the sludge to which the acid has been added for a predetermined time after the acid addition step.
[8] The water treatment method according to any one of [1] to [7], further including a step of dehydrating the sludge to which the acid has been added after the acid addition step.
[9] The water treatment method according to any one of [1] to [8], wherein at least calcium hydroxide is used as the alkali agent.
[10] The water treatment method according to any one of [1] to [9], wherein the water to be treated is waste water discharged from a flue gas desulfurization apparatus.

The water treatment method according to any one of the above [1] to [10] can be executed by, for example, the following water treatment system.
[11] a reaction vessel in which at least one alkaline agent selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides is added to water to be treated containing fluoride ions and magnesium ions; A solid-liquid separation tank for solid-liquid separation of the suspended substance into which the fluoride ions are taken, which is generated by adding the alkaline agent to the water to be treated; and the suspension separated in the solid-liquid separation tank. An acid addition tank for adding acid to the sludge derived from the turbid substance, and the addition amount (mg-OH / L) of the alkaline agent to the treated water in the reaction tank as OH is the treated water. A water treatment system in which the mass ratio (OH amount / F amount) to fluorine concentration (mg-F / L) is 1.0 or more.

  In the water treatment system, each step (procedure) in the above-described 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 water to be treated. Can also be realized. Further, in the water treatment system, a program capable of executing each step (procedure) in the water treatment method described above is stored on various storage media or a network, and the control unit reads and executes the program. It is also possible to realize this water treatment method. In addition, it is also possible to make the apparatus containing the above-mentioned control part manage the addition amount of the alkali agent to the water to be treated, the addition amount of the acid to the solid-liquid separated sludge, and the like.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited to a following example.

(Treated water)
In the following test examples, wastewater discharged from a desulfurization facility that performs treatment of sulfur oxide (SOx) by a magnesium hydroxide method in a coal-fired power plant as treated water (raw water) to be treated (hereinafter referred to as “ Sometimes referred to as "desulfurization wastewater"). The pH of this desulfurization wastewater was 8.0. Table 1 shows the main components of desulfurization wastewater.

  Further, in this test example, the desulfurization wastewater was processed in a continuous process using a 1/1000 scale test apparatus. The test apparatus includes a fluorine removal test apparatus and a sludge treatment test apparatus, and is divided into them. In addition, since the regulation value of the fluorine concentration after treatment (treated water) varies depending on the country or region, in this test example, calcium hydroxide is added so that the total fluorine concentration after treatment is 15 mg / L or less. Evaluation was performed.

Example 1
In Example 1, the above-mentioned desulfurization wastewater was processed in a continuous process using a 1/1000 scale test apparatus capable of executing the process shown in FIG. The test apparatus used in Example 1 is a test apparatus for fluorine removal comprising a reaction tank (11), a flocculant addition tank (21), and a solid-liquid separation tank (22), a sludge storage tank (31), and an acid addition. A sludge treatment test apparatus including a tank (32), an aging tank (41), a coagulation tank (62), and a concentration tank (63) is provided (see FIG. 2). Hereinafter, the treatment performed in the fluorine removal test apparatus is referred to as “F removal process”, and the treatment performed in the sludge treatment test apparatus is referred to as “sludge volume reduction process”.

First, the desulfurization wastewater was subjected to the F removal process. Specifically, desulfurization waste water is supplied to the reaction tank (11), and in the reaction tank (11), 10 mass% calcium hydroxide aqueous solution is 4000 mg / L (about 1840 mg / OH as Ca (OH) ₂ ). L) (OH amount / F amount≈13.1). At this time, the pH of the desulfurization wastewater after the addition of calcium hydroxide was 10.0. The desulfurization waste water after the addition of calcium hydroxide is sent to the flocculant addition tank (21), and the flocculant (trade name “KE Flock KEA-776” manufactured by Nippon Steel & Sumikin Environment Co., Ltd. is also used for the following flocculants. ) Was added to the precipitation tank (22) as a solid-liquid separation tank. Then, the suspended matter (suspension material) generated by adding calcium hydroxide to the desulfurization wastewater is concentrated 15 times in the precipitation tank (22), and the suspension in which fluoride ions in the desulfurization wastewater are taken in. The material was separated into solid and liquid and recovered as sludge.

Next, after the sludge collected in the sedimentation tank (22) was once received in the sludge storage tank (31), it was subjected to a sludge volume reduction process to reduce the sludge volume. In the sludge volume reduction process, first, sludge is transferred from the sludge storage tank (31) to the acid addition tank (32), and in the acid addition tank (32), 75% by mass sulfuric acid aqueous solution with respect to sludge (91500 mg / L). Was added as H ₂ SO _{4 in} an amount of 57600 mg / L, and the pH of the sludge was adjusted to 7.0, then the sludge was transferred to the aging tank (41), and the sludge was stirred in the aging tank (41). . Thereafter, the sludge was transferred to a coagulation tank (62), an anionic polymer flocculant was added, and the sludge was transferred to a concentration tank (63) for solid-liquid separation, and the modified sludge was recovered. The supernatant liquid separated from the sludge in the concentration tank (63) was returned to the reaction tank (11) of the F removal process. Table 2 shows the residence time of each tank in the F removal process and the sludge volume reduction process. Table 3 shows the F concentration in the sedimentation tank (22) and the SS concentrations of the reaction tank (11), the sludge storage tank (31), and the aging tank (41).

(Comparative Example 1)
In Comparative Example 1, an F removal process in which calcium hydroxide is added in two stages using a test apparatus in which a reaction tank, a coagulation tank, and a precipitation tank are further added to the fluorine removal test apparatus used in Example 1 one by one. It was. Moreover, in the comparative example 1, the test apparatus which removed the acid addition tank from the sludge processing test apparatus used in Example 1 was used. A schematic flow chart showing the water treatment method executed in Comparative Example 1 is shown in FIG.

  In Comparative Example 1, the above-mentioned desulfurization waste water was processed in a continuous process using a 1/1000 scale test apparatus capable of executing the process shown in FIG. As shown in FIG. 4, the test apparatus used in Comparative Example 1 includes two reaction vessels (11, 12), a flocculant addition vessel (21, 23), and two solid-liquid separation vessels (22, 24). The apparatus includes a fluorine removal test apparatus and a sludge treatment test apparatus including a sludge storage tank (31), an aging tank (41), a coagulation tank (62), and a concentration tank (63). Hereinafter, the procedure performed in Comparative Example 1 will be specifically described with reference to the reference numerals in FIG.

The desulfurization waste water is supplied to the reaction vessel (11), and the desulfurization waste water in the reaction vessel (11) is 1200 mg / L (about 550 mg / L as OH) of 10 mass% calcium hydroxide aqueous solution as Ca (OH) _2. (OH amount / F amount≈3.9). At this time, the pH of the desulfurization waste water after the addition of calcium hydroxide was 9.5. The desulfurization waste water after adding calcium hydroxide was sent to a flocculant addition tank (21), and after adding the flocculant, it was sent to a precipitation tank (22) as a solid-liquid separation tank. Then, the suspended matter (suspension material) generated by adding calcium hydroxide to the desulfurization wastewater is concentrated 15 times in the precipitation tank (22), and the suspension in which fluoride ions in the desulfurization wastewater are taken in. The material was separated into solid and liquid and recovered as sludge. Moreover, the supernatant water (F density | concentration: 30 mg / L) isolate | separated from the sludge in the sedimentation tank (22) is sent to a reaction tank (12), and 10 mass% calcium hydroxide aqueous solution is added to a supernatant water in a reaction tank (12). Was added in an amount of 300 mg / L as Ca (OH) ₂ (about 140 mg / L as OH) (OH amount / F amount≈4.7). At this time, the pH of the supernatant water after addition of calcium hydroxide was 9.6. The supernatant water after adding calcium hydroxide was sent to the flocculant addition tank (23), and after adding the flocculant, it was sent to the precipitation tank (24). Then, the suspended matter (suspension material) generated by the addition of calcium hydroxide to the supernatant water is concentrated 15 times in the sedimentation tank (22), and the suspended matter in which fluoride ions are taken in is solid-liquid. Separated and recovered as sludge.

  Next, each sludge collected in the settling tanks (22, 24) was once received in the sludge storage tank (31) and put together. The sludge was transferred from the sludge storage tank (31) to the aging tank (41), and the sludge was stirred in the aging tank (41). Then, after the sludge was transferred to the coagulation tank (62) and the coagulant was added, the sludge was recovered by solid-liquid separation in the concentration tank (63). The supernatant liquid separated from the sludge in the concentration tank (63) was returned to the reaction tank (11) of the F removal process. Table 4 shows the residence time of each tank (see FIG. 4) in the test apparatus used in Comparative Example 1. Table 5 shows the F concentration in the settling tank (24) and the SS concentrations in the first and second stage reaction tanks (11, 12), the sludge storage tank (31), and the aging tank (41). Indicated.

  From the results of Example 1, with respect to desulfurization wastewater, the amount of calcium hydroxide added as OH is about 13.1 in terms of mass ratio (OH amount / F amount) to the fluorine concentration in the desulfurization wastewater. By adding, it was confirmed that the F concentration can be reduced to 15 mg / L or less even in one-stage treatment, and fluoride ions in the desulfurization wastewater can be effectively removed. Therefore, since the water treatment method of Example 1 can omit the equipment in the second stage F removal process as compared with Comparative Example 1, the equipment cost can be reduced and the installation space of the equipment can be reduced. it can.

  Moreover, in Example 1, since more suspended solids (SS) are generated than in Comparative Example 1, more fluoride ions are removed by incorporating fluoride ions in desulfurization wastewater into SS. Confirmed to get.

Furthermore, a dewatering test was performed on each of the modified sludge collected from the concentration tank (63) in Example 1 and the sludge collected from the concentration tank (63) in Comparative Example 1, and the dehydrated cake obtained. And the moisture content was measured. The dehydration test was performed under the conditions using a filter cloth having a pressure of 0.4 MPa and an air flow rate of 15 cm ³ / cm ² / sec. As a result, the amount of dehydrated cake was 5.08 g per liter of water to be treated in Example 1, and 6.72 g per liter of water to be treated in Comparative Example 1, and the water content of the dehydrated cake was 40% in Example 1. In Comparative Example 1, it was 65%. From the results including this dehydration test, in Example 1, the SS concentration increases, but by adding an acid to the solid-liquid separated SS (sludge), the SS concentration and the water content of the sludge can be reduced. It was confirmed that the final amount of sludge can be reduced as compared with Comparative Example 1. Therefore, the water treatment method of Example 1 can contribute to reducing the sludge disposal cost and reducing the overall running cost related to the water treatment.

  Note that a filter press type dehydrator is suitable as a dehydrator used for desulfurization treatment of desulfurized sludge, but an SS concentration of at least 20000 mg / L or more is recommended. In Example 1, since the SS concentration in the aging tank (41) was 20000 mg / L or more, the sludge after the aging process can be dehydrated as it is, and the coagulation tank (62) in the sludge volume reduction process and It was found that the concentration tank (63) can be omitted.

  Furthermore, the inventors calculated the equipment cost, running cost, and equipment installation area when the processes of Example 1 and Comparative Example 1 were applied to an actual machine. The equipment cost was 1.09 times, the running cost was 1.31 times, and the installation area of the equipment was 1.19 times. Therefore, it was confirmed that the process of Example 1 was superior in terms of processing cost and facility installation area.

(Examples 2 to 7)
In Examples 2 to 7, the amount of calcium hydroxide added to the desulfurization wastewater in Example 1 was changed to the amount shown in Table 6, and the amount of 75% by mass sulfuric acid aqueous solution added to sludge in the sludge volume reduction process was appropriately changed. The test was performed in the same manner as in Example 1 except for the change. The addition amount of the 75 mass% sulfuric acid aqueous solution with respect to sludge was adjusted suitably so that the pH of sludge might be set to 7.0. Table 6 shows the amount of calcium hydroxide (Ca (OH) ₂ ) added to desulfurized wastewater, the mass ratio of the amount of calcium hydroxide added as OH to the fluorine concentration in the desulfurized wastewater (OH amount / F amount), solid liquid The separated sludge concentration (in the sludge storage tank (31)) and the sludge concentration (in the aging tank (41)) after addition of the acid are shown together.

  As described above, the concentration of sludge that can be dewatered by a filter press-type dehydrator suitable for desulfurization treatment of desulfurized sludge is 20000 mg / L or more. Therefore, as a result of Examples 1-7, from the viewpoint of easy dewatering of sludge, the addition amount of the alkaline agent as OH to the water to be treated is a mass ratio (OH amount / F amount) to the fluorine concentration in the water to be treated. 4.0 or more is preferable, 5.0 or more is more preferable, and 6.0 or more is further preferable. Further, considering the ease of transporting sludge by a pump in an actual machine in a continuous process at an actual site, the sludge concentration is desirably 100000 mg or less. Therefore, as a result of Examples 1 to 7, from the viewpoint of ease of sludge transfer, the addition amount of the alkaline agent as OH to the water to be treated is a mass ratio (OH amount / F amount) to the fluorine concentration in the water to be treated. 1) or less is preferable, 15.0 or less is more preferable, 15.0 or less is more preferable, and 14.0 or less is further preferable.

(Examples 8 to 13)
In Examples 8 to 13, the calcium hydroxide for the desulfurization wastewater in Examples 2 to 7 was changed to sodium hydroxide, and the addition amount of the 75 mass% sulfuric acid aqueous solution to the sludge in the sludge volume reduction process was appropriately changed. Were tested in the same manner as in Examples 2-7. The addition amount of the 75 mass% sulfuric acid aqueous solution with respect to sludge was adjusted suitably so that the pH of sludge might be set to 7.0. About the addition amount of sodium hydroxide with respect to desulfurization wastewater, it was set as the quantity shown in Table 7 so that it may become the addition amount of the calcium hydroxide in Examples 2-7 when converted into OH. Table 7 shows the amount of sodium hydroxide (NaOH) added to the desulfurized wastewater, the mass ratio of the amount of sodium hydroxide added as OH to the fluorine concentration in the desulfurized wastewater (OH amount / F amount), and solid-liquid separation. The sludge concentration (in the sludge storage tank (31)) and the sludge concentration (in the aging tank (41)) after addition of the acid are also shown.

  As described above, the concentration of sludge that can be dewatered by a filter press-type dehydrator suitable for desulfurization treatment of desulfurized sludge is 20000 mg / L or more. Therefore, as a result of Examples 8 to 13, from the viewpoint of easy dewatering of sludge, when sodium hydroxide is used as the alkali agent, the amount added as OH to the water to be treated is based on the fluorine concentration in the water to be treated. It was found that even if the mass ratio (OH amount / F amount) was 16 or more, it was 20,000 mg / L or less. Further, from the viewpoint of the fluorine concentration of the supernatant water in the actual machine in a continuous process at an actual site, it is assumed that the OH amount / F amount is 16 or more. Therefore, as a result of Examples 8 to 13, when sodium hydroxide was used as the alkaline agent, the volume of sludge was reduced in order to obtain an SS concentration of 20,000 mg / L or more that can be dehydrated with a filter press type dehydrator. It is desirable to carry out a step of concentrating.

(Examples 14 to 29)
In Examples 14 to 29, after treatment (after the solid-liquid separation step) when an alkali agent was added to the water to be treated by changing the amount of OH / F in a small-scale batch test. A test was conducted to confirm the behavior of the fluorine concentration in the clear water (treated water). In this test, artificial wastewater having a pH of 7.5 prepared by assuming fluorine-containing wastewater from a flue gas desulfurization apparatus that removes sulfur in exhaust gas using magnesium hydroxide was used as water to be treated. Two types of artificial wastewater (artificial wastewater 1 and 2) having different fluorine concentrations were prepared. In Examples 14 to 21, artificial wastewater 1 was used, and in Examples 22 to 29, artificial wastewater 2 was used. The composition of artificial wastewater 1 and 2 is shown in Table 8.

  Each of the artificial wastewaters 1 and 2 was tested as follows. Artificial wastewater is put into a 200 mL beaker, and a 10 mass% calcium hydroxide aqueous solution is added so as to have an OH amount / F amount shown in Table 9 and stirred for 15 minutes. The suspended matter in which fluoride ions were taken in was separated into solid and liquid. And the supernatant water (process water) by which solid-liquid separation was carried out was extract | collected, and the fluorine concentration was measured. The measurement results are shown in Table 9. In addition, although detailed description is omitted about the sludge derived from the suspended solids separated into solid and liquid, it is subjected to the same sludge volume reduction process as that in Example 1 (however, the amount of sulfuric acid added to the sludge is the pH of the sludge). The final amount of sludge could be reduced.

  Regardless of the initial fluorine concentration of artificial wastewater, it was confirmed that when the OH amount / F amount was 6.0 or more, the fluorine concentration after treatment (supernatant water) could be significantly reduced. Moreover, in order to reduce the fluorine concentration in the supernatant water to 15 mg-F / L, which is a regulation value when discharged into the sea area in the Water Pollution Control Law, the OH amount / F amount is set to 10.0 or more. There was a need. However, since there is a possibility that an inhibitor against the removal of fluorine is contained in the desulfurization wastewater to be treated, it is possible to investigate in advance the optimum OH / F conditions for each wastewater during actual operation. desirable.

S11 Alkali addition process S22 Solid-liquid separation process S32 Acid addition process S41 Aging process 11 Reaction tank 22 Solid-liquid separation tank 32 Acid addition tank 41 Aging tank

Claims (11)

An alkali addition step of adding at least one alkali agent selected from the group consisting of hydroxides of alkali metals and alkaline earth metals to water to be treated containing fluoride ions and magnesium ions;
A solid-liquid separation step for solid-liquid separation of the suspended substance into which the fluoride ions are taken, which is generated by the alkali addition step;
An acid addition step of adding an acid to the sludge derived from the suspended solid that has been subjected to solid-liquid separation, and
The addition amount (mg-OH / L) of the alkali agent as OH to the water to be treated in the alkali addition step is a mass ratio (OH amount / OH) to the fluorine concentration (mg-F / L) in the water to be treated. The water treatment method is 1.0 or more in F amount).   The water treatment method according to claim 1, wherein in the alkali addition step, the pH of the water to be treated is adjusted to a range of 8.5 to 10.5 by adding the alkali agent to the water to be treated.   The amount of the alkali agent added to the water to be treated as OH in the alkali addition step with respect to the fluorine concentration in the water to be treated is determined in advance as the mass ratio (OH amount / F amount) and the solid-liquid separation. The water treatment method according to claim 1, wherein the water treatment method is determined based on a relationship with a fluorine concentration in the supernatant water separated from the sludge in the process.   The addition amount (mg-OH / L) of the alkali agent as OH to the water to be treated in the alkali addition step is a mass ratio (OH amount / OH) to the fluorine concentration (mg-F / L) in the water to be treated. The amount of F) is 3.0 or more. The water treatment method according to any one of claims 1 to 3. Adjusting the concentration C _S1 of the sludge to be solid-liquid separated in the solid-liquid separation step to 10000 mg / L or more;
In the acid addition step, the concentration C _S2 of the sludge after the acid is added is adjusted to be lower than the C _S1 and in a range of 5000 mg / L to 50000 mg / L. The water treatment method according to any one of the above.   The water treatment according to any one of claims 1 to 5, wherein in the acid addition step, the pH of the sludge is adjusted within a range of 3.0 to 8.5 by adding the acid to the sludge. Method.   The water treatment method according to claim 1, further comprising a step of stirring the sludge to which the acid has been added for a predetermined time after the acid addition step.   The water treatment method according to claim 1, further comprising a step of dehydrating the sludge to which the acid has been added after the acid addition step.   The water treatment method according to claim 1, wherein at least calcium hydroxide is used as the alkali agent.   The water treatment method according to any one of claims 1 to 9, wherein the treated water is waste water discharged from a flue gas desulfurization apparatus. A reaction vessel in which at least one alkali agent selected from the group consisting of hydroxides of alkali metals and alkaline earth metals is added to water to be treated containing fluoride ions and magnesium ions;
A solid-liquid separation tank for solid-liquid separation of the suspended substance into which the fluoride ions are incorporated, which is generated by adding the alkaline agent to the water to be treated;
An acid addition tank for adding an acid to the suspended matter-derived sludge separated in the solid-liquid separation tank,
The addition amount (mg-OH / L) of the alkaline agent as OH to the water to be treated in the reaction tank is a mass ratio (OH amount / F) to the fluorine concentration (mg-F / L) in the water to be treated. A water treatment system having an amount of 1.0 or more.

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