JP2013156227A - Method for removing radioactive material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for removing radioactive materials from active sludge containing the radioactive materials.SOLUTION: A method 1 for removing radioactive materials has an elution process 20 for solubilizing active sludge Sand eluting radioactive materials in the active sludge Sinto liquid, a solid body and liquid body separation process 30 for separating a liquid body Lfrom a solid body Safter the elution process 20, and an adsorption process 40 for adsorbing the radioactive materials in the liquid body Lseparated by the solid body and liquid body separation process 30 to adsorbent. Further, the method has a dehydration process 50 for dehydrating the solid body Sseparated by the solid body and liquid body separation process 30 and separating separate liquid Lfrom a dehydrated cake S, and a process for adsorbing the separate liquid Lseparated by the dehydration process 50 to the adsorbent.

Description

  The present invention relates to a method for removing radioactive substances from activated sludge containing radioactive substances.

The activated sludge in which radioactive substances such as radioactive cesium have been detected is considered to have been concentrated to a high concentration by repeating bioconcentration and mechanical concentration in the sewage treatment process.
In addition, since radioactive cesium is not detected in the supernatant liquid in the storage tank for storing activated sludge containing radioactive cesium, and in the separated liquid obtained by dehydrating the activated sludge, the activated sludge itself does not contain radioactive cesium in the cells of microorganisms. It is presumed that it is contained in a highly concentrated state.

As described above, the activated sludge can be maintained in a state where the radioactive substance is concentrated to a high concentration. However, at present, there are few known effective methods for removing the radioactive substance from the activated sludge containing the radioactive substance. For this reason, activated sludge containing radioactive substances is disposed of as follows.
For example, when sludge contains radioactive cesium as a radioactive substance, sludge with a radioactive cesium concentration of 8000 Bq / kg or less and its incinerated ash can be disposed of in landfill if waterproofing measures are taken on the premise that the former site will not be used for dwelling etc. . In addition, if the radioactive cesium concentration is low, it is possible to trade sludge as compost raw material or cement raw material, but the radioactive cesium concentration in the case of compost raw material is 200 Bq / kg or less, and the clearance level in the case of cement raw material Trading is difficult unless it is 100 Bq / kg or less.

  Thus, activated sludge containing radioactive substances cannot be disposed unless the concentration of radioactive substances is reduced to some extent. Therefore, there is a need for a method that can remove radioactive substances from activated sludge containing radioactive substances.

  As a method for treating radioactive substances, for example, there is a method in which radioactive wastewater containing radioactive substances is treated with activated sludge and solid-liquid separated into activated sludge (solids) containing radioactive substances and treated water from which radioactive substances have been removed. It has been proposed (see, for example, Patent Document 1).

JP 2007-64732 A

  However, the method described in Patent Document 1 has not yet removed the radioactive substance from the activated sludge containing the radioactive substance.

  This invention is made | formed in view of the said situation, and it aims at providing the method of removing a radioactive substance from the activated sludge containing a radioactive substance.

  As a result of intensive studies, the present inventors have found that if activated sludge is solubilized, radioactive substances such as radioactive cesium that have been incorporated into the cells of microorganisms can be eluted into the liquid, and radioactive substances can be removed from activated sludge. The present invention has been completed.

That is, the method for removing a radioactive substance of the present invention is a method for removing a radioactive substance from an activated sludge containing a radioactive substance, wherein the activated sludge is solubilized and the radioactive substance in the activated sludge is eluted in a liquid. And a solid-liquid separation step of separating the liquid and the solid after the elution step.
Moreover, it is preferable to have an adsorption | suction process which makes the adsorbent adsorb | suck the radioactive substance in the liquid isolate | separated by the solid-liquid separation process.
Furthermore, it is preferable to have a dehydration step of dehydrating the solid separated by the solid-liquid separation step and separating it into a separation solution and a dehydrated cake, and adsorbing the separation solution separated by the dehydration step to the adsorbent.
In addition, the method for removing a radioactive substance of the present invention is particularly suitable when the radioactive substance is cesium.
Furthermore, the method for solubilizing activated sludge is preferably one or more methods selected from the group consisting of alkali treatment, oxidation treatment, acid treatment, heating treatment, and ultrasonic treatment.
The adsorbent is preferably a graft product obtained by graft polymerization of a reactive monomer in the presence of a polymer substrate.

  ADVANTAGE OF THE INVENTION According to this invention, the method which can remove a radioactive substance from the activated sludge containing a radioactive substance can be provided.

It is a schematic block diagram which shows an example of the removal apparatus of the radioactive substance used for this invention. It is a schematic block diagram which shows the other example of the removal apparatus of the radioactive substance used for this invention. It is a schematic block diagram which shows the other example of the removal apparatus of the radioactive substance used for this invention. It is a schematic block diagram which shows the other example of the removal apparatus of the radioactive substance used for this invention.

The method for removing a radioactive substance according to the present invention includes an elution step of solubilizing activated sludge containing a radioactive substance and eluting the radioactive substance in the activated sludge into a liquid, and a solid-liquid separating the liquid and the solid after the elution step. A separation step.
In addition, the method for removing a radioactive substance according to the present invention includes an adsorption process in which a radioactive substance in a liquid separated by a solid-liquid separation process is adsorbed to an adsorbent, and a solid separated by a solid-liquid separation process is dehydrated to separate the separated liquid and It is preferable to have a dehydration step for separating the cake, and it is preferable that the separated liquid separated by the dehydration step is adsorbed on the adsorbent.
Hereinafter, an example of the method for removing a radioactive substance of the present invention will be specifically described with reference to the drawings. 2-4, the same code | symbol is attached | subjected to the same component as FIG. 1, and the description is abbreviate | omitted.

<Radioactive substance removal device>
FIG. 1 is a schematic configuration diagram showing an example of a radioactive substance removing apparatus used in the present invention. In this example, the radioactive substance removal apparatus 1 is, in order from the upstream side, the sludge storage means 10 for temporarily storing the activated sludge S ₀ containing the radioactive substance, and the activated sludge S ₀ solubilized to remove the radioactive substance in the activated sludge. The elution means 20 for elution in the liquid, the solid-liquid separation means 30 for separating the liquid L ₁ and the solid S _1, and the adsorption means 40 for adsorbing the radioactive substance in the separated liquid L ₁ to the adsorbent were separated. The solid S ₁ is dehydrated, and includes a dehydrating means 50 that separates the separated liquid L ₂ and the dehydrated cake S ₂ .

(Activated sludge)
Activated sludge S ₀ to which the present invention is the activated sludge used for normal sewage sludge and industrial waste water treatment, and includes a radioactive substance. In the present invention, the activated sludge before removal of radioactive substances may be referred to as “untreated activated sludge” or simply “activated sludge”.

  Although it does not restrict | limit especially as a radioactive substance contained in activated sludge, Radiocesium, radioactive strontium, radioactive zirconium, radioactive ruthenium, radioactive cerium etc. are mentioned. Among these, this invention is especially suitable when removing radioactive cesium from the activated sludge containing radioactive cesium.

(Sludge storage means)
Sludge storage means 10 is means for temporarily storing the activated sludge S _0.
The sludge storage means 10 includes a sludge storage tank 11. The sludge storage tank 11 is not particularly limited as long as it can store the activated sludge S _0.

(Elution means)
Elution means 20, activated sludge S ₀ solubilized, a means for eluting a radioactive material in the activated sludge in the liquid.
The elution means 20 in this example includes an alkali treatment means 21 and an oxidation treatment means 22 positioned downstream of the alkali treatment means 21.

Alkali treatment means 21, stirring with the first treatment tank 21a for storing while the activated sludge S ₀ sent from the sludge storage means 10 is warmed, the activated sludge S ₀ which is stored in the first treatment tank 21a comprising a stirrer 21b, and alkali addition means 21c for adding an alkali to the active sludge S ₀ which is stored in the first treatment tank 21a.
As the first treatment tank 21a, it is not particularly limited as long as it can store the activated sludge S _0, those hard material degraded by alkali or heating is preferable.
The stirrer 21b, are not particularly limited as long as it can stir the activated sludge S _0.
The alkali adding means 21c is not particularly limited as long as an alkali can be added.

Oxidation means 22 is stirred and second treatment tank 22a for storing while the activated sludge S ₀ sent from the alkaline treatment means 21 is warmed, the activated sludge S ₀ stored in the second treatment tank 22a comprising a stirrer 22b, the oxidizing agent addition means 22c for adding an oxidizing agent to the activated sludge S ₀ stored in the second treatment tank 22a.
The second treatment tank 22a, is not particularly limited as long as it can store the activated sludge S _0, those hard material degraded by oxidizing agents are preferred.
The stirrer 22b, are not particularly limited as long as it can stir the activated sludge S _0.
The oxidizing agent adding means 22c is not particularly limited as long as it can add an oxidizing agent.

(Solid-liquid separation means)
The solid-liquid separation unit 30 is a unit that separates the liquid L ₁ and the solid S ₁ .
Examples of the solid-liquid separation means 30 include a filter equipped with a filter, a centrifuge, and a membrane separation device. Also, the less the amount of activated sludge S ₀ to be processed, it may be used dehydrator.

(Adsorption means)
Suction means 40 is means for adsorbed by the adsorbent (not shown) the radioactive material in the liquid L in ₁ separated.
The adsorbent is not particularly limited as long as it can adsorb radioactive substances. For example, a graft product obtained by graft polymerization of a reactive monomer in the presence of a polymer base material is suitable. The graft product can be easily synthesized by, for example, a synthesis method described in Japanese Patent No. 4465447. Specifically, among the reactive monomers having a mono- or bifunctional phosphate group, two monomers having a monoester structure and a diester structure are simultaneously graft-polymerized on a polymer substrate to obtain a graft product. Examples of the polymer substrate include non-woven or fibrous polyethylene, polypropylene, and the like. On the other hand, examples of the reactive monomer include mono (2- (meth) acryloyloxyethyl) acid phosphate and di (2- (meth) acryloyloxyethyl) acid phosphate.
The adsorbent is not limited to the above-described graft product, and for example, a zeolite adsorbent or a Prussian blue adsorbent may be used.

(Dehydration means)
The dehydrating unit 50 is a unit that dehydrates the separated solid S ₁ and separates it into a separated liquid L ₂ and a dehydrated cake S ₂ .
As the dehydrating means 50, a known dehydrator for dewatering sludge can be used.
This example dehydration means 50 is adapted to be merged in the liquid L ₁ separated by the separating liquid L ₂ is a solid-liquid separation means 30. Then, the separation liquid L ₂ that has joined the liquid L ₁ is adsorbed by the adsorbent of the adsorbing means 40.

<Removal method>
Below, each process of the removal method of the radioactive substance of this invention is demonstrated in detail using the radioactive substance removal apparatus 1 shown in FIG.
First, once storing the activated sludge S ₀ to sludge storage tank 11 of the sludge storage means 10 (storing step). Activated sludge S ₀ By storing the sludge storage tank 11, since the supernatant (not shown) is separated, it is preferable to remove this supernatant. In addition, since the radioactive substance is taken in the activated sludge, it is not contained in the supernatant liquid. Therefore, the removed supernatant liquid can be discharged out of the system as it is.

(Elution process)
In the elution step, the activated sludge S ₀ once stored in the sludge storage tank 11 of the sludge storage means 10 and separated from the supernatant liquid is solubilized by the elution means 20, and the radioactive substance in the activated sludge is eluted in the liquid. It is a process.
Radioactive materials have been incorporated into the cells of the microorganism by solubilization activated sludge S ₀ is eluted into the liquid.

Examples of methods for solubilizing activated sludge include chemical treatments such as alkali treatment, oxidation treatment and acid treatment; treatment methods such as heating treatment; mechanical treatment such as ultrasonic treatment.
By solubilizing activated sludge by the above-described treatment method, the cell membrane of microorganisms in the activated sludge is easily destroyed, and as a result, radioactive substances are easily eluted in the liquid.

In the case where the activated sludge is solubilized by alkali treatment, examples of the alkali include sodium hydroxide.
The greater the amount of alkali added, that is, the higher the pH of the activated sludge after adding the alkali, the easier the cell membrane of microorganisms in the activated sludge is destroyed by the decomposition reaction with alkali. 20-60 mass parts is preferable with respect to 100 mass parts of solid content of activated sludge, and, as for the addition amount of an alkali, 30-50 mass parts is more preferable. If the amount of alkali added is 20 parts by mass or more, the cell membrane of microorganisms in the activated sludge can be sufficiently destroyed. In addition, even if the addition amount of the alkali exceeds 60 parts by mass, the cell membrane destruction effect reaches its peak.

When solubilizing activated sludge by oxidation treatment, an oxidizing agent may be added to the activated sludge. The oxidizing agent is not particularly limited as long as it has strong oxidizing power, and examples thereof include hydrogen peroxide, persulfate, and ozone.
As the amount of oxidant added increases, the cell membrane of microorganisms in the activated sludge is more easily destroyed by the oxidative decomposition reaction with the oxidant. The amount of oxidant added is the amount of saturated dissolved oxygen in the activated sludge after adding the oxidant to 1 L of liquid activated sludge when a chemical such as hydrogen peroxide or persulfate is used as the oxidant. The amount of 3000 to 7000 mg is preferred, and more preferably 4000 to 6000 mg. If the added amount of the oxidizing agent is 3000 mg or more, the cell membrane of microorganisms in the activated sludge can be sufficiently destroyed. In addition, even if the addition amount of the oxidizing agent exceeds 7000 mg, the cell membrane destruction effect reaches its peak.
On the other hand, when a gas such as ozone is used as the oxidizing agent, the amount of saturated dissolved oxygen in the activated sludge after addition of the oxidizing agent is preferably 4000 mg or more with respect to 1 L of liquid activated sludge. If the addition amount of the oxidizing agent is 4000 mg or more, the cell membrane of microorganisms in the activated sludge can be sufficiently destroyed.

When solubilizing activated sludge by acid treatment, examples of the acid include hydrochloric acid and sulfuric acid.
The greater the amount of acid added, that is, the lower the pH of the activated sludge after the acid is added, the more easily the microbial cell membrane in the activated sludge is destroyed by the acid decomposition reaction.

When solubilizing activated sludge by heating treatment, it is preferable to heat the activated sludge to 50 to 150 ° C. When heated to 50 ° C. or higher, the cell membrane of microorganisms in the activated sludge is easily destroyed. Moreover, although mentioned later in detail, when using a chemical process and a heating process together under atmospheric pressure especially, 50-90 degreeC is preferable, and when using a chemical process and a heating process under pressure together, it is 100-150. ° C is preferred.
The method for warming the activated sludge is not particularly limited.

  When solubilizing activated sludge by ultrasonic treatment, the activated sludge may be ultrasonically treated using, for example, an ultrasonic generator.

These alkali treatment, oxidation treatment, acid treatment, heating treatment, and ultrasonic treatment may be used alone or in combination of two or more. When using 2 or more types together, you may carry out simultaneously and may carry out sequentially. In particular, the chemical treatment such as alkali treatment, oxidation treatment, and acid treatment is preferably performed simultaneously with the heating treatment in order to facilitate the cell membrane decomposition reaction. However, when oxidation treatment is performed using a gas such as ozone as an oxidant, ozone added to the activated sludge volatilizes simultaneously with the heating treatment, and the oxidative decomposition reaction may not proceed sufficiently. Therefore, when the oxidation treatment and the heating treatment are performed simultaneously, it is preferable to perform the oxidation treatment using an agent such as hydrogen peroxide or persulfate as the oxidizing agent.
In addition, when ultrasonic treatment and treatment other than ultrasonic treatment (other treatment) are used in combination, ultrasonic treatment and other treatment may be performed at the same time, particularly alkali treatment, oxidation treatment, acid treatment, etc. When the chemical treatment and the ultrasonic treatment are used in combination, it is preferable to perform the ultrasonic treatment after the chemical treatment.

  In the elution means 20 shown in FIG. 1, after the first elution treatment is performed by using the alkali treatment means 21 together with the alkali treatment and the heating treatment, the oxidation treatment means 22 is used together with the oxidation treatment and the heating treatment. Then, the second elution process is performed.

Since the elution treatment time depends on the type of treatment method, treatment temperature, activated sludge condition, etc., it is usually judged visually whether the activated sludge has been solubilized and processed until the activated sludge is solubilized. I do.
The processing time tends to be shorter as the processing temperature is higher. Further, the treatment time tends to be shorter in the case where the activated sludge is solubilized by the oxidation treatment than in the case where the activated sludge is solubilized by the alkali treatment or the acid treatment. Moreover, when solubilizing activated sludge by ultrasonic treatment, 5 to 30 minutes is usually preferable, and 5 to 15 minutes is more preferable.

  Further, the pressure of the elution treatment is preferably from atmospheric pressure to 1.5 MPa, and may be determined according to the type of treatment method.

(Solid-liquid separation process)
The solid-liquid separation step is a step of separating the liquid L ₁ and the solid S ₁ by the solid-liquid separation means 30 after the elution step.
By separating the liquid L ₁ and the solid S ₁ after the elution step, the liquid L ₁ of radioactive material eluted, can be separated into a solid S ₁ containing the activated sludge after solubilization radioactive material has been removed , radioactive material is removed from the active sludge S _0.
Examples of a method for separating the liquid L ₁ and the solid S ₁ include a filtration method using a filter, a centrifugal separation method, and a membrane separation method. Also, the less the amount of activated sludge S ₀ to be processed, it may be used dehydrating methods.

(Adsorption process)
Adsorption step is a step of adsorbing the adsorbent radioactive materials in the liquid L in ₁ separated by solid-liquid separation step by the adsorption means 40.
The adsorption step, radioactive material because it is adsorbed by the adsorbent can be removed radioactive material from the liquid L _1.
The removal liquid L ₃ from which the radioactive substance has been removed by the adsorption process is discharged out of the system.

(Dehydration process)
The dehydration step is a step in which the solid S ₁ separated in the solid-liquid separation step is dehydrated by the dehydrating means 50 and separated into the separated liquid L ₂ and the dehydrated cake S ₂ .
Incidentally, in the solid-liquid separation step, by the performance of the solid-liquid separation means 30 is when the liquid L ₁ and the solid S ₁ is not completely separated, a part of the liquid L ₁ adheres to a solid S _1. Since the liquid L ₁ is included radioactive material removed from the active sludge, the solid S ₁ when a part of the liquid L ₁ is attached, radioactive substances eluted into the liquid L ₁ is the solid S ₁ It will be attached.
In such a case, if the dehydration step is performed, the separation liquid L ₂ and the dehydration cake S ₂ can be separated, so that even if a part of the liquid L ₁ adheres to the solid S ₁ in the solid-liquid separation step, the liquid L ₁ eluted radioactive material can be prevented from adhering to the dewatered cake S _2.

When performing dehydration step, it is preferable to a separate separation liquid L ₂ is adsorbed by the adsorbent. Thus, the radioactive substance contained in the separated liquid L ₂ is the adsorbent can be removed radioactive material from the separated liquid L _2.
In the dehydrating means 50 shown in FIG. 1, the separation liquid L ₂ is joined to the liquid L ₁ separated by the solid-liquid separation means 30 and then adsorbed on the adsorbent of the adsorption means 40.

<Effect>
As described above, according to the method for removing a radioactive substance of the present invention, the activated sludge containing the radioactive substance is solubilized and the radioactive substance in the activated sludge is eluted in the liquid, and then the liquid and the solid are separated. Since it isolate | separates, a radioactive substance can be easily removed from activated sludge.
Furthermore, if the liquid from which the radioactive substance is eluted is adsorbed by the adsorbent, the radioactive substance can be removed from the liquid.

Also, even if a part of the liquid adheres to the solid when the liquid and the solid are separated, if the solid is dehydrated and separated into a separated liquid and a dehydrated cake, the radioactive material eluted in the liquid adheres to the dehydrated cake. I can prevent it.
Furthermore, radioactive material can be removed from the separation liquid by adsorbing the obtained separation liquid to the adsorbent.

The activated sludge from which radioactive substances have been removed is recovered as a solid. Moreover, if this solid is dehydrated, it can be recovered as a dehydrated cake.
Since these solids and dehydrated cakes have a sufficiently reduced concentration of radioactive substances, they can be reused not only for landfills but also for compost materials and cement materials.

In addition, in order to raise the removal effect of a radioactive substance more, you may perform an elution process and a solid-liquid separation process again about the obtained solid and dehydrated cake (reprocessing). In the case of reprocessing, the solid or dehydrated cake may be returned to the sludge storage means or solubilization means.
The presence or absence of reprocessing may be determined by measuring the concentration of radioactive material in the solid or dehydrated cake.

<Other embodiments>
The method for removing a radioactive substance of the present invention is not limited to the method described above.
In the method described above, as shown in FIG. 1, activated sludge S ₀ is temporarily stored in the sludge storage tank 11 of the sludge storage means 10 and then supplied to the elution means 20. The activated sludge S ₀ is stored in the sludge storage tank. 11 need not be stored. However, once the activated sludge S ₀ is stored in the sludge storage tank 11, the supernatant liquid that does not contain the radioactive material can be separated from the activated sludge S ₀ before the solubilization step. The amount of processing in the process can be reduced.

  Moreover, in the radioactive substance removal apparatus 1 shown in FIG. 1, the elution means 20 is comprised by the alkali treatment means 21 which used the alkali treatment and the heating treatment together, and the oxidation treatment means 22 which used the oxidation treatment and the heating treatment together. However, for example, an ozone oxidation treatment means equipped with an ozone generator or the like may be used as the oxidation treatment means 22.

  In this example, after the first elution treatment (first time) is performed using both alkali treatment and heating treatment as the elution step, the second elution treatment (first treatment) is performed using both oxidation treatment and heating treatment. As described above, as the elution step, any one of alkali treatment, oxidation treatment, acid treatment, heating treatment, and ultrasonic treatment may be carried out once or twice or more as described above. Alternatively, two or more kinds may be used in combination, or may be performed once or twice or more.

When performing the elution treatment once, for example, as shown in FIG. 2, a radioactive substance removing apparatus 2 having an elution means 20 composed only of alkali treatment means 21 using both alkali treatment and heating treatment is used. As shown in FIG. 3, a radioactive substance removing device 3 having an elution means 20 composed only of an oxidation treatment means 22 using both an oxidation treatment and a heating treatment may be used.
Further, for example, as shown in FIG. 4, using the radioactive substance removing apparatus 4 provided with the elution means 20 provided with the oxidizing agent addition means 22c in the alkali treatment means 21, the alkali treatment, oxidation treatment and heating treatment are simultaneously performed. You may go.

2 to 4, the alkali treatment and / or oxidation treatment and the heating treatment are performed simultaneously, but only the alkali treatment and / or oxidation treatment may be performed without heating, You may perform only a heating process, without adding an oxidizing agent.
In addition, an ultrasonic generator may be provided in the alkali treatment means 21 and / or the oxidation treatment means 22 so that the alkali treatment and / or oxidation treatment, the heating treatment, and the ultrasonic treatment may be performed simultaneously. Only the ultrasonic treatment may be performed without adding the agent and without heating.
Furthermore, for example, an acid addition means may be provided instead of the alkali addition means 21c in FIG.

On the other hand, when the elution treatment is performed twice or more, the combination is not limited to the alkali treatment and the oxidation treatment as shown in FIG. 1, for example, alkali treatment and acid treatment, oxidation treatment and acid treatment, alkali treatment and oxidation treatment and acid treatment. Processing may be used. The order of the elution treatment is not particularly limited. For example, the alkali treatment means 21 and the oxidation treatment means 22 may be interchanged in FIG.
Furthermore, after chemical treatment such as alkali treatment, oxidation treatment, and acid treatment or heating treatment is performed at least once, mechanical treatment such as ultrasonic treatment may be performed.

Further, in the radioactive substance removing apparatus 1 shown in FIG. 1, after the separated liquid L ₂ separated by the dehydrating means 50 is merged with the liquid L ₁ separated by the solid-liquid separating means 30, the adsorbent of the adsorbing means 40 is used. Although it is adsorbed, the separated liquid L ₂ may be directly adsorbed to the adsorbent of the adsorbing means 40 without joining the liquid L ₁ , or an adsorbent is prepared separately from the adsorbing means 40, and this adsorbent the separated liquid L ₂ may be adsorbed.
Further, the separation liquid L ₂ may be added to the untreated activated sludge S ₀ , or may be supplied to any of the sludge storage means 10, the elution means 20, and the solid-liquid separation means 30.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these.
In the following examples and comparative examples, the dissolution rate of cesium into the liquid was determined. Since the elution rate of cesium is considered to be hardly affected by the type of isotope of cesium, the examples and comparative examples were performed using cesium 133 ( ¹³³ Cs), which is a stable isotope, for convenience.

[Adaptation of activated sludge containing cesium]
Batch activated sludge treatment was performed under the conditions shown in Table 1, and cesium was accumulated in the activated sludge as follows. In addition, as the raw water, adjusted soymilk (16 g / L) diluted with tap water was used. The concentration of cesium in this raw water was 50 ppb.
15 L of raw water was added to activated sludge with a capacity of 30 L, aerated for 15 hours, then discharged 2 L, the rest was allowed to stand for 1 hour to settle the activated sludge, and then 13 L of the supernatant was drained. This processing operation is defined as one cycle.
Next, 15 L of raw water was added to the residue after discharging the 13 L supernatant, and the same treatment operation was performed for a total of 21 cycles to acclimate the activated sludge. The activated sludge after the treatment operation of the 21st cycle was used as a sample for the cesium elution test.

  In Table 1, “BOD” is the biochemical oxygen demand, “MLSS” is the activated sludge suspended matter, “HRT” is the hydraulic residence time, and “SRT” is the sludge residence time.

[Example 1]
Using the cesium-containing activated sludge conditioned earlier, a cesium elution test was performed as follows.
First, the cesium-containing activated sludge was concentrated until MLSS became about 10000 mg / L. To 400 mL of this concentrate, 40% by weight of sodium hydroxide (NaOH) was added to 40% to DS (40 parts by weight with respect to 100 parts by weight of the solid content of the concentrate), processing temperature: 50 ° C., processing pressure: high. Atmospheric pressure, treatment time: Chemical treatment 1 (a combination of alkali treatment and heating treatment) was carried out under treatment conditions of 180 minutes to solubilize activated sludge, and cesium in the activated sludge was eluted (elution step). Then, the liquid and solid were isolate | separated using the centrifuge (solid-liquid separation process). The elution treatment conditions are shown in Table 2.
The MLSS and cesium concentrations in the solid liquid before the solid-liquid separation and the cesium concentration (elution cesium concentration) in the liquid after the solid-liquid separation were measured. In addition, MLSS was measured based on the "centrifugation method" described in the "sewage test method" (Japan Sewerage Association, 1997 edition). On the other hand, the cesium concentration was determined by measuring the cesium concentration in the decomposition solution by the ICP-MS method after decomposing the measurement sample with nitric acid according to JIS K 0133 according to the general rules of high-frequency plasma mass spectrometry.
Moreover, the elution rate of cesium was calculated | required from the following formula. These results are shown in Table 3.
Elution rate (%) = (elution cesium concentration in liquid / cesium concentration in solid liquid) × 100

[Example 2]
In the elution step, a cesium elution test was performed in the same manner as in Example 1 except that after chemical treatment 1, ultrasonic treatment with a frequency of 20 kHz and an output of 240 W was performed for 10 minutes as a mechanical treatment. The results are shown in Table 3.

[Example 3]
A cesium elution test was conducted in the same manner as in Example 1 except that the treatment temperature and treatment time of Chemical Treatment 1 were changed as shown in Table 2. The results are shown in Table 3.

[Example 4]
The treatment temperature and treatment time of the chemical treatment 1 were changed as shown in Table 2, and the chemical treatment 1 was followed by ultrasonic treatment with a frequency of 20 kHz and an output of 240 W for 10 minutes after the mechanical treatment 1 as in Example 1. The cesium dissolution test was conducted. The results are shown in Table 3.

[Example 5]
Instead of chemical treatment 1, in the elution step, the concentrate was charged with 5000 mg-O / L of hydrogen peroxide (H ₂ O ₂ ) having a concentration of 35% by mass (saturated dissolved oxygen in activated sludge after addition of hydrogen peroxide). Except that chemical treatment 2 (combination of oxidation treatment and heating treatment) was carried out under treatment conditions of treatment temperature: 150 ° C., treatment pressure: 1.5 MPa, treatment time: 5 minutes. A cesium elution test was conducted in the same manner as in Example 1. The results are shown in Table 3.

[Example 6]
In the elution step, a cesium elution test was conducted in the same manner as in Example 5 except that after the chemical treatment 2, ultrasonic treatment with a frequency of 20 kHz and an output of 240 W was performed for 10 minutes as a mechanical treatment. The results are shown in Table 3.

[Example 7]
In the chemical treatment 1, after adding sodium hydroxide to 400 mL of the concentrate, 5000 mg-O / L of hydrogen peroxide having a concentration of 35% by mass (the amount of saturated dissolved oxygen in the activated sludge after addition of hydrogen peroxide is 5000 mg) The cesium elution test was conducted in the same manner as in Example 1 except that the treatment temperature was 150 ° C., the treatment pressure was 1.5 MPa, and the treatment time was 5 minutes. The results are shown in Table 3.

[Example 8]
Instead of chemical treatment 1, in the elution step, ozone (O ₃ ) is added to the concentrate at a concentration of 5000 mg-O / L (amount of saturated dissolved oxygen in the activated sludge after addition of ozone is 5000 mg). A cesium elution test was conducted in the same manner as in Example 1 except that chemical treatment 3 (oxidation treatment) was performed under the treatment conditions of temperature: room temperature, treatment pressure: atmospheric pressure, treatment time: 720 minutes. The results are shown in Table 3.

[Example 9]
In the elution step, a cesium elution test was performed in the same manner as in Example 8 except that after chemical treatment 3, ultrasonic treatment with a frequency of 20 kHz and an output of 240 W was performed for 10 minutes as a mechanical treatment. The results are shown in Table 3.

[Example 10]
In the elution step, a cesium elution test was performed in the same manner as in Example 1 except that ultrasonic treatment with a frequency of 20 kHz and an output of 240 W was performed for 10 minutes instead of the chemical treatment 1. The results are shown in Table 3.

[Comparative Example 1]
The cesium-containing activated sludge was concentrated until MLSS reached about 10,000 mg / L, and then the liquid and the solid were separated using a centrifuge (solid-liquid separation step).
The MLSS and cesium concentrations in the solid liquid (concentrate) before solid-liquid separation and the cesium concentration (elution cesium concentration) in the liquid after solid-liquid separation were measured, and the elution rate of cesium was determined. These results are shown in Table 3.

[Comparative Example 2]
The cesium-containing activated sludge was concentrated until MLSS became about 10000 mg / L. Pure water was added to the concentrate and washed with centrifugation.
After removing the supernatant, pure water was added again, and the MLSS and cesium concentrations in the solid liquid were measured. Subsequently, the solid and the liquid were separated by centrifugal washing, and the cesium concentration (elution cesium concentration) in the liquid (supernatant liquid) was measured. Further, the dissolution rate of cesium was determined. These results are shown in Table 3.

As is clear from Table 3, in Examples 1 to 10, the cesium elution rate was as high as 90% or more.
That is, in Examples 1 to 10, it was found that 90% or more of the total cesium in the solid liquid before solid-liquid separation was eluted in the liquid.

On the other hand, in the case of Comparative Example 1 in which the elution step was not performed, the cesium elution rate was 50%.
Moreover, in the case of the comparative example 2 which performed washing | cleaning instead of the elution process, the elution rate of cesium was 12.5%.
That is, in the case of Comparative Examples 1 and 2, 50% or more of the total cesium in the solid liquid before the solid-liquid separation is not eluted in the liquid and is still taken into the activated sludge (solid). I understood.
In addition, since the elution rate of cesium is lower in Comparative Example 2 than in Comparative Example 1, cesium derived from raw water used for acclimatization of cesium-containing activated sludge is attached to the surface of cesium-containing activated sludge. I can guess. In the case of Comparative Example 2, cesium adhering to the surface of the cesium-containing activated sludge was removed in the first centrifugal cleaning, so that the cesium concentration in the solid liquid was lower than in Examples 1 to 10 and Comparative Example 1. Conceivable.

  From the above results, it was shown that, by solubilizing activated sludge, cesium in activated sludge is eluted in the liquid and radioactive substances can be removed from activated sludge by the present invention.

[Test example]
Hereinafter, test examples will be described.
In the following test examples, it was carried out in order to confirm whether the adsorbent can adsorb radioactive cesium eluted in the liquid.

100 g of incinerated ash obtained by incinerating activated sludge containing radioactive cesium 134 ( ¹³⁴ Cs) and radioactive cesium 137 ( ¹³⁷ Cs) was added to 1 L of 0.1 mol of hydrochloric acid, and the mixture was stirred for 1.5 hours. Solid-liquid separation into liquid. 500 mL of the separated liquid was filtered with 0.45 μm filter paper, and the filtrate was used as a test solution.
As a result of measuring the ¹³⁴ Cs concentration and the ¹³⁷ Cs concentration in this test solution using a germanium semiconductor detector, the ¹³⁴ Cs concentration was 15.9 Bq / L and the ¹³⁷ Cs concentration was 21.4 Bq / L. These results are shown in Table 4.

Five adsorbents of 1 cm × 1 cm were immersed in this test solution, and the adsorbing treatment was performed by stirring for 24 hours. Next, the adsorbent was removed, and the ¹³⁴ Cs concentration and ¹³⁷ Cs concentration of the test solution after the adsorption treatment were measured using a germanium semiconductor detector, and the removal rate of cesium was determined from the following formula. The results are shown in Table 4.
Removal rate (%) = {(cesium concentration in test solution before adsorption treatment−cesium concentration in test solution after adsorption treatment) / cesium concentration in test solution before adsorption treatment} × 100

In addition, the adsorption material manufactured as follows was used for adsorption processing.
First, the non-woven fabric was irradiated with radiation to generate reaction active sites. Irradiation was performed using an electron beam under conditions of a total dose of 200 kGy in a nitrogen atmosphere.
Subsequently, 10 (mono / di = 70% by mass / 30% by mass) of a mixed monomer of mono (2-methacryloyloxyethyl) acid phosphate and di (2-methacryloyloxyethyl) acid phosphate in the presence of the irradiated nonwoven fabric. Graft polymerization is carried out at 60 ° C. for 12 hours in a mixed solvent of methanol and pure water (methanol / pure water = 10 mass% / 90 mass%) at a concentration of mass% to obtain an adsorbent in which graft chains are introduced into the nonwoven fabric. It was.

As is clear from Table 4, it was found that radioactive cesium eluted in the liquid was adsorbed by the adsorbent.
Therefore, solubilize activated sludge containing radioactive material, elute the radioactive material in activated sludge into the liquid, separate the liquid and solid, and adsorb the radioactive material in the separated liquid to the adsorbent It was shown that radioactive material can be removed from the liquid.

1: Radioactive substance removal device, 10: sludge storage means, 11: sludge storage tank, 20: elution means, 21: alkali treatment means, 21a: first treatment tank, 21b: stirrer, 21c: alkali addition means, 22: oxidation treatment means, 22a: second treatment tank, 22b: stirrer, 22c: oxidant addition means, 30: solid-liquid separation means, 40: adsorption means, 50: dehydration means, S ₀ : activated sludge, S _1: solid, _{S 2:} dehydrated cake, _{L 1:} liquid, _{L 2:} the separated liquid, _{L 3:} removal liquid.

Claims (6)

A method for removing radioactive material from activated sludge containing radioactive material,
An elution step of solubilizing the activated sludge and elution of radioactive materials in the activated sludge into the liquid;
A method for removing a radioactive substance, comprising: a solid-liquid separation step of separating a liquid and a solid after the elution step.   The method for removing a radioactive substance according to claim 1, further comprising an adsorption step in which the radioactive substance in the liquid separated by the solid-liquid separation step is adsorbed by an adsorbent.   3. The method according to claim 1, further comprising a dehydration step of dehydrating the solid separated by the solid-liquid separation step and separating the solid into a separation solution and a dehydrated cake, wherein the separation solution separated by the dehydration step is adsorbed on an adsorbent. The removal method of the radioactive substance of description.   The method for removing a radioactive substance according to claim 1, wherein the radioactive substance is cesium.   The method for solubilizing activated sludge is at least one method selected from the group consisting of alkali treatment, oxidation treatment, acid treatment, heating treatment, and ultrasonic treatment. A method for removing a radioactive substance according to any one of the above.   The method for removing a radioactive substance according to claim 2 or 3, wherein the adsorbent is a graft product obtained by graft polymerization of a reactive monomer in the presence of a polymer substrate.

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