Selenium removal method and selenium removal apparatus

Abstract

The present invention provides a selenium removal method and apparatus that can efficiently remove selenium contained in selenium-containing water and further reduce the selenium concentration in the treated water after treatment. [Solution] The selenium removal method includes a biological treatment step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment step of removing at least a portion of sulfite ions, which are interfering components in the biological treatment, before performing the biological treatment step.

Description

[Technical Field]

[0001] This disclosure relates to a selenium removal method and a selenium removal apparatus. [Background technology]

[0002] Conventionally, techniques for removing selenium from wastewater containing selenium are known. For example, Patent Document 1 discloses a method for treating selenium-containing wastewater using a stagnant reduction reactor comprising a contact reaction zone equipped with a contact reducing agent for reducing selenium within the reaction vessel, and a circulation path for returning the selenium-containing wastewater that has passed through the contact reaction zone back to the contact reaction zone for circulation. Patent Document 2 discloses a selenic acid-reducing bacterium belonging to the genus Pseudomonas that has the ability to reduce hexavalent Se. Patent Document 3 discloses a method in which a reducing iron compound is added to selenium-containing water, the selenium-containing water containing the reducing iron compound is introduced into a reaction vessel, the selenium in the liquid is reduced and precipitated by the reducing power of the iron compound under alkaline and non-oxidizing conditions, and the resulting precipitate is separated into solid and liquid to remove the selenium from the system. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6053593 [Patent Document 2] Patent No. 3025204 [Patent Document 3] Patent No. 7047459 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, the biological treatment method for removing selenium, as described in Patent Document 2, has the problem that it may not be able to sufficiently remove selenium depending on conditions such as the form of selenium contained and the types of coexisting components. Therefore, there is a need for a selenium removal method and apparatus that can efficiently remove selenium contained in selenium-containing water and further reduce the selenium concentration in the treated water after treatment. Furthermore, the chemical reduction method for removing selenium, as described in Patent Document 3, has the problem of increasing chemical costs and waste disposal costs because it requires the addition of many chemicals, and these added chemicals precipitate and become solid waste.

[0005] This disclosure was made to solve the above-mentioned problems and aims to provide a selenium removal method and a selenium removal apparatus that can efficiently remove selenium contained in selenium-containing water and further reduce the selenium concentration in the treated water after treatment. [Means for solving the problem]

[0006] To solve the above problems, the selenium removal method according to this disclosure includes a biological treatment step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium (hereinafter referred to as "metallic selenium") by biological treatment, and a pretreatment step of removing at least a portion of sulfite ions, which are interfering components in the biological treatment, before performing the biological treatment step.

[0007] The selenium removal method according to this disclosure includes a biological treatment step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment step of pre-treating the selenium-containing water by a Fenton reaction before performing the biological treatment step.

[0008] The selenium removal apparatus according to the present disclosure includes a biological treatment tank that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment tank that is disposed upstream of the biological treatment tank and removes at least part of sulfite ions that act as interfering components in the biological treatment performed in the biological treatment tank.

[0009] The selenium removal apparatus according to the present disclosure includes a biological treatment tank that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment tank that is disposed upstream of the biological treatment tank and pretreats the selenium-containing water by a Fenton reaction. [Effects of the Invention]

[0010] According to the selenium removal method and the selenium removal apparatus of the present disclosure, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced. [Brief Description of the Drawings]

[0011] [Figure 1] It is a diagram showing the configuration of the selenium removal apparatus according to the first embodiment of the present disclosure. [Figure 2] It is a diagram showing the configuration of the selenium removal apparatus according to the second embodiment of the present disclosure. [Figure 3] It is a diagram showing the configuration of a coagulation sedimentation treatment apparatus used in the selenium removal apparatus. [Modes for Carrying Out the Invention]

[0012] <First Embodiment> (Configuration of Selenium Removal Apparatus) Hereinafter, the selenium removal method and the selenium removal apparatus according to the embodiments of the present disclosure will be described with reference to Figure 1. Figure 1 is a diagram showing the configuration of the selenium removal apparatus according to the first embodiment of the present disclosure. As shown in Figure 1, the selenium removal device 100 comprises a biological treatment tank 2 and a pretreatment tank 3. The selenium removal device 100 is used to remove selenium contained in selenium-containing water, and is designed to reduce hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent metallic selenium by biological treatment. By reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or metallic selenium by biological treatment, the selenium contained in selenium-containing water can be removed from the system by coagulation and sedimentation treatment. The selenium removal device 100 may include a raw water tank 1 for storing the raw water, which is selenium-containing water 31. For example, the selenium-containing water 31 stored in the raw water tank 1 is supplied to the pretreatment tank 3 through the raw water supply line 11 by a liquid transfer pump 22.

[0013] The selenium-containing water 31 supplied to the pretreatment tank 3 undergoes pretreatment in the pretreatment tank 3 to remove at least some of the sulfite ions that interfere with the biological treatment carried out in the biological treatment tank 2. The pretreatment carried out in the pretreatment tank 3 may hereafter be referred to as the "pretreatment process". The pretreated selenium-containing water (hereinafter also referred to as "pretreated water 32") is supplied to the biological treatment tank 2 through the pretreated water supply line 12 by the liquid transfer pump 23.

[0014] The biological treatment tank 2 reduces the selenium contained in the selenium-containing water through the action of anaerobic microorganisms. That is, it reduces the hexavalent selenium contained in the selenium-containing water to tetravalent selenium or metallic selenium. The biological treatment carried out in the biological treatment tank 2 may hereafter be referred to as the "biological treatment process." The biological treatment tank 2 is filled with a carrier 21 to which denitrifying bacteria and selenium-reducing bacteria are attached and fixed. That is, the biological treatment tank 2 has a carrier 21 to which denitrifying bacteria and selenium-reducing bacteria are attached and fixed. The carrier 21 is not particularly limited, but for example, anthracite, sand, activated carbon, etc. can be used.

[0015] A hydrogen donor storage tank 4 serving as a hydrogen donor supply means is connected to the biological treatment tank 2 via a hydrogen donor supply line 14 with an injection pump 24 interposed therebetween. Here, the hydrogen donor refers to a substance that donates hydrogen to other substances in the biological treatment tank 2 to reduce them, and itself is dehydrogenated and oxidized. That is, the hydrogen donor is a compound supplied as a substrate for microorganisms such as denitrifying bacteria and selenium-reducing bacteria supported on the carrier 21 in the biological treatment tank 2. The hydrogen donor storage tank 4 stores an organic substance as a hydrogen donor or a compound containing an organic substance (hereinafter also simply referred to as "organic substance"). The organic substance is not particularly limited, and examples thereof include methanol, ethanol, acetic acid, formic acid and the like.

[0016] Under facultative anaerobic conditions, denitrifying bacteria perform respiration in the presence of an organic substance (such as methanol) as a hydrogen donor by using at least one bound oxygen of nitrate ion (NO3 - ) and nitrite ion (NO2 - ) (the bound oxygen being the O in NO3 - and the O in NO2 - ), so as to remove nitrogen from the wastewater. That is, respiration is performed by using at least one bound oxygen of nitrate ion (NO3 - ) and nitrite ion (NO2 - ) (the bound oxygen being the O in NO3 - and the O in NO2 - ), and at least one of nitrate ions and nitrite ions is reduced to form nitrogen gas. In addition, under facultative anaerobic conditions and in the presence of a hydrogen donor, selenium-reducing bacteria reduce selenium by performing respiration using at least one bound oxygen of selenate ion and selenite ion (the bound oxygen being the O in SeO4 2- and the O in SeO3 2- ).

[0017] A biological treatment water pipe 13 is provided at the outlet of the biological treatment tank 2. Selenium-containing water (hereinafter also referred to as "biological treatment water 33") that has been biologically treated by the biological treatment tank 2 is discharged through the biological treatment water pipe 13. The biologically treated biological treatment water 33 can be removed from the system by coagulation and sedimentation treatment of selenium in the biological treatment water 33, which can be performed as appropriate by conventionally known coagulation and sedimentation treatment. More specifically, tetravalent selenium can be coagulated and sedimented using coagulants such as inorganic coagulants and polymer coagulants, and can therefore be removed relatively easily by conventionally known coagulation and sedimentation treatment. Therefore, by biological treatment by the biological treatment tank 2, hexavalent selenium is treated by biological treatment into tetravalent selenium or metallic selenium that can be removed by coagulation and sedimentation treatment, thereby efficiently removing selenium contained in the selenium-containing water 31 and further reducing the selenium concentration in the treated water after treatment. There are no particular restrictions on the coagulation and sedimentation treatment performed on the biological treatment water 33, and it can be performed in accordance with conventionally known coagulation and sedimentation treatment methods. A coagulation and sedimentation treatment apparatus for performing the coagulation and sedimentation treatment will be described later.

[0018] In the selenium removal apparatus 100, before the selenium-containing water 31 is biologically treated in the biological treatment tank 2, sulfite ions (SO3), which are interfering components in the biological treatment, are treated in the pretreatment tank 3. 2- A pretreatment is performed to remove at least a portion of the sulfite ions. Sulfite ions cause metallic selenium (i.e., zero-valent selenium) to leach out. In the biological treatment tank 2, some of the selenium contained in the selenium-containing water 31 may accumulate as metallic selenium. Therefore, if the selenium-containing water 31 contains sulfite ions, the metallic selenium accumulated in the biological treatment tank 2 may form complex ions with sulfite ions and redissolve in the liquid phase, leaching out from the biological treatment tank and contaminating the biologically treated water 33. For this reason, the pretreatment step preferably includes a reaction that oxidizes the sulfite ions contained in the selenium-containing water and converts them to sulfate ions. By oxidizing the sulfite ions and converting them to sulfate ions, the leaching of metallic selenium by sulfite ions can be effectively suppressed.

[0019] The pretreatment performed in the pretreatment tank 3 is carried out, for example, by storing the selenium-containing water 31 supplied from the raw water tank 1 and adjusting the hydrogen ion concentration of the stored selenium-containing water 31. One example of pretreatment is adjusting the hydrogen ion concentration of the selenium-containing water 31 to a pH of 2-4. The method of adjusting the hydrogen ion concentration is not particularly limited, but one method is to add an acid such as hydrochloric acid or sulfuric acid to the selenium-containing water 31. For example, an acid such as hydrochloric acid or sulfuric acid can be supplied into the pretreatment tank 3 from an acid supply means (not shown) to adjust the hydrogen ion concentration of the selenium-containing water 31. It is more preferable to adjust the hydrogen ion concentration in the pretreatment to a pH of 2.5-3.5.

[0020] The pretreatment carried out in the pretreatment tank 3 may include aeration of selenium-containing water 31. That is, the pretreatment involves aeration of selenium-containing water 31 to remove sulfite ions (SO3 2- ) is oxidized to sulfate ions (SO4 2- The process may include a step to convert the selenium to a different state. The aeration of the selenium-containing water 31 is not particularly limited, but may be carried out, for example, until the sulfite ion concentration reaches several to several tens of mg / L. When aerating the selenium-containing water 31, it is preferable to adjust the hydrogen ion concentration of the selenium-containing water 31 to pH 7 to 11, and more preferably to pH 8.5 to 9.5. Although aeration of the selenium-containing water 31 is an optional process, aeration of the selenium-containing water 31 can shorten the time required for pretreatment, for example.

[0021] In addition to the sulfite ions described above, the interfering components in biological processing are carbonate ions (CO3). 2- ) or bicarbonate ions (HCO3) -) may also be included. If the selenium-containing water 31 contains carbonate ions or bicarbonate ions, the hydrogen ion concentration of the selenium-containing water 31 may change over time. For this reason, if the selenium-containing water 31 contains carbonate ions or bicarbonate ions, the hydrogen ion concentration of the selenium-containing water 31 may change to a concentration unsuitable for biological treatment. For this reason, it is preferable to remove the carbonate ions and bicarbonate ions contained in the selenium-containing water 31 in the pretreatment performed in the pretreatment tank 3. For example, a treatment to remove carbonate ions and bicarbonate ions is to adjust the hydrogen ion concentration of the selenium-containing water 31 and release the carbonate ions and bicarbonate ions in the selenium-containing water 31 into the gas phase. The adjustment of the hydrogen ion concentration is not particularly limited, but examples include adding an acid such as hydrochloric acid or sulfuric acid to the selenium-containing water 31. For example, an acid such as hydrochloric acid or sulfuric acid can be supplied into the pretreatment tank 3 from an acid supply means (not shown) to adjust the hydrogen ion concentration of the selenium-containing water 31 and release the carbonate ions and bicarbonate ions in the selenium-containing water 31 into the gas phase. As a pretreatment, for example, it is preferable to adjust the hydrogen ion concentration of the selenium-containing water 31 to pH 2 to 4, and more preferably to pH 2.5 to 3.5.

[0022] The pretreatment step may further include a step of pretreatment of selenium-containing water 31 by the Fenton reaction. The Fenton reaction is carried out under acidic conditions, involving hydrogen peroxide (H2O2) and divalent iron ions (Fe 2+ This reaction involves the reaction of ) and to produce a hydroxyl radical (·OH) with strong oxidizing power. In this reaction, hydrogen peroxide functions as an oxidizing agent. The iron catalyst containing divalent iron ions functions as a catalyst for hydrogen peroxide. Therefore, the Fenton reaction can also be described as a reaction that generates hydroxyl radicals using a redox reaction. In the pretreatment by the Fenton reaction, the aforementioned hydroxyl radical (·OH) can be generated to oxidize and decompose organic substances (for example, organic compounds containing selenium) contained in the selenium-containing water 31.

[0023] Selenium-containing water 31 contains selenium cyanide (SeCN- ) and other organic selenium compounds may be present. However, selenium cyanide and other organic selenium compounds cannot be treated by biological treatment using the biological treatment tank 2, and these compounds cannot be removed by coagulation and sedimentation treatment of the biologically treated water 33. On the other hand, by pre-treating the selenium-containing water 31 with the Fenton reaction, selenium cyanide and other organic selenium compounds contained in the selenium-containing water 31 can be oxidized and decomposed, converting them into forms that can be treated by biological treatment or coagulation and sedimentation treatment, such as tetravalent selenium or hexavalent selenium.

[0024] Furthermore, even when the sulfite ions in the selenium-containing water 31 are converted to sulfate ions by the pretreatment described above, some sulfite ions may remain in the selenium-containing water 31 (in other words, the pretreated water 32). By including a step of pretreatment of the selenium-containing water 31 by the Fenton reaction, it becomes possible to further oxidize the remaining sulfite ions with hydroxyl radicals, which have stronger oxidizing power, and thus further reduce the concentration of remaining sulfite ions in the selenium-containing water 31.

[0025] When the Fenton reaction is carried out in the pretreatment tank 3, for example, hydrogen peroxide can be supplied into the pretreatment tank 3 from a hydrogen peroxide supply means (not shown), and iron can be supplied into the pretreatment tank 3 from an iron supply means (not shown), thereby allowing the Fenton reaction to proceed within the pretreatment tank 3. As the iron can be supplied, divalent iron can be supplied, metallic iron can be supplied, etc. Furthermore, multiple types of raw materials can be mixed and used as the iron can be supplied. For example, although not particularly limited, examples of "divalent iron can be supplied" include iron(II) chloride, iron(II) nitrate, iron(II) sulfate, iron(II) hydroxide, and iron(II) oxide.

[0026] When the Fenton reaction is carried out in the pretreatment tank 3, the Fenton reaction may include a photo-Fenton reaction, which is performed under the irradiation conditions of light including ultraviolet light. The photo-Fenton reaction, also called the photo-Fenton reaction, consists of a cycle of oxidation of iron ions by hydrogen peroxide and photoreduction. By including the photo-Fenton reaction in the Fenton reaction carried out in the pretreatment tank 3, the amount of hydrogen peroxide and iron ions used for the Fenton reaction can be reduced, thereby lowering the running costs of the pretreatment process.

[0027] By pre-treating selenium-containing water 31 using the pre-treatment tank 3 as described above, and then biologically treating the pre-treated water 32 in the biological treatment tank 2, selenium contained in the selenium-containing water 31 can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0028] The selenium removal device 100 may further include a pH adjustment step between the pretreatment step and the biological treatment step, in which the hydrogen ion concentration of the pretreated water 32 obtained by pretreatment of selenium-containing water 31 is adjusted to pH 5 to 9. The biological treatment in the biological treatment step has a hydrogen ion concentration suitable for biological treatment, and by adjusting the hydrogen ion concentration of the pretreated water 32 to pH 5 to 9, selenium contained in the pretreated water 32 can be removed more efficiently. The pH adjustment before the biological treatment step is preferably adjusted to pH 5 to 9, and more preferably to pH 6.5 to 7.5. For example, the pH adjustment before the biological treatment step can be performed by adding alkaline industrial water or the like to the pretreated water 32 to dilute the pretreated water 32. Furthermore, although not particularly limited, the pH adjustment before the biological treatment step can also be performed by adding an alkali such as sodium hydroxide (caustic soda) or calcium hydroxide (slaked lime) to the pretreated water 32.

[0029] (Configuration of the selenium removal method) The selenium removal method according to the first embodiment can be carried out using the selenium removal apparatus 100 described above. The selenium removal method includes a biological treatment step and a pretreatment step. The biological treatment step is a step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or metallic selenium by biological treatment. The pretreatment step is a step of removing at least a portion of sulfite ions, which are interfering components in the biological treatment, before carrying out the biological treatment step.

[0030] (Configuration of the coagulation and sedimentation treatment apparatus) Next, the coagulation and sedimentation apparatus used in the selenium removal apparatus according to the first embodiment will be described with reference to Figure 3. Figure 3 is a diagram showing the configuration of the coagulation and sedimentation apparatus used in the selenium removal apparatus. As shown in Figure 3, the coagulation and sedimentation treatment apparatus 80 comprises a first coagulant addition unit 41, a second coagulant addition unit 42, and a coagulation and sedimentation tank 50.

[0031] The first coagulant addition section 41 and the second coagulant addition section 42 are devices for adding a coagulant to the water to be treated and for dissolving the added coagulant. More specifically, the first coagulant addition section 41 and the second coagulant addition section 42 are devices for adding a coagulant to biologically treated water 33 that has been biologically treated by the biological treatment tank 2 (see Figure 1) of the selenium removal device 100 (see Figure 1) and for dissolving the added coagulant. Biologically treated water 33 that has been biologically treated by the biological treatment tank 2 (see Figure 1) of the selenium removal device 100 (see Figure 1) is supplied to the first coagulant addition section 41. In the first coagulant addition section 41, an inorganic coagulant 61 and a pH adjuster 63 are added as coagulants. In the second coagulant addition section 42, a polymer coagulant 62 is added as a coagulant.

[0032] The first coagulant addition section 41 has a stirrer 43 for mixing and stirring the biologically treated water 33 with the inorganic coagulant 61 and the pH adjuster 63. The second coagulant addition section 42 has a stirrer 43 for mixing and stirring the biologically treated water 33 with the polymer coagulant 62.

[0033] The coagulation and sedimentation tank 50 is for separating solid matter contained in the treated water (i.e., biologically treated water 33) to which a coagulant has been added by coagulation and sedimentation. Here, the solid matter contained in the treated water includes not only solid components originally contained in the treated water, but also suspended matter formed by adding a coagulant to the treated water. The coagulation and sedimentation tank 50 separates the solid matter contained in the biologically treated water 33 to which a coagulant has been added in the first coagulant addition section 41 and the second coagulant addition section 42 by coagulation and sedimentation. The coagulation and sedimentation tank 50 has a stirrer 51 for stirring the biologically treated water 33, which is the treated water. Flocs (sludge) that have settled at the bottom of the coagulation and sedimentation tank 50 are discharged and removed from the tank as sludge 72. The treated water (biologically treated water 33 from which solid matter has been removed by coagulation and sedimentation) is discharged from the tank as treated water 71. Note that the coagulation and sedimentation treatment device used in the selenium removal device according to the first embodiment is not limited to the coagulation and sedimentation treatment device 80 shown in Figure 3, and conventionally known coagulation and sedimentation treatment devices can be applied.

[0034] (Effects and Benefits) According to the selenium removal device 100 and selenium removal method described above, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0035] <Second Embodiment> Next, a second embodiment of the selenium removal method and selenium removal apparatus according to the present disclosure will be described. In the second embodiment described below, components common to the first embodiment are denoted by the same reference numerals in the figures and their descriptions are omitted. The selenium removal apparatus 100A in the second embodiment differs from that of the first embodiment in the content of the pretreatment process in the pretreatment tank 3A.

[0036] Figure 2 shows the configuration of a selenium removal apparatus according to the second embodiment of this disclosure. The selenium removal apparatus 100A shown in Figure 2 comprises a biological treatment tank 2 and a pretreatment tank 3A, similar to the selenium removal apparatus 100 shown in Figure 1. However, in the pretreatment tank 3A, a pretreatment step of pretreating selenium-containing water by the Fenton reaction is performed as an essential pretreatment. For the pretreatment by the Fenton reaction, a treatment configured similarly to the pretreatment using the Fenton reaction described in the first embodiment can be used. For example, although not particularly limited, the pretreatment tank 3A may be equipped with a light irradiation source (not shown) for performing a photo-Fenton reaction as the Fenton reaction.

[0037] By including a step of pre-treating the selenium-containing water 31 by the Fenton reaction, selenium cyanide and other organic selenium compounds that cannot be treated by biological treatment in the biological treatment tank 2 can be oxidized and decomposed, converting them into forms that can be treated by biological treatment or coagulation and sedimentation, such as tetravalent selenium or hexavalent selenium. Furthermore, by including a step of pre-treating the selenium-containing water 31 by the Fenton reaction, the sulfite ion concentration in the selenium-containing water 31 can be effectively reduced by oxidizing sulfite ions with hydroxyl radicals, which have strong oxidizing power.

[0038] The pretreatment carried out in the pretreatment tank 3A may further include other pretreatments described in the first embodiment in addition to the pretreatment by the Fenton reaction. For example, before the pretreatment by the Fenton reaction, sulfite ions (SO3), which are interfering components in the biological treatment, may be included. 2- A pretreatment may be performed to remove at least a portion of the selenium-containing water 31. The pretreatment may also include aeration of the selenium-containing water 31. Furthermore, the pretreatment may include removing carbonate ions and bicarbonate ions contained in the selenium-containing water 31.

[0039] (Configuration of the selenium removal method) The selenium removal method in this embodiment can be carried out using the selenium removal apparatus 100A described above. The selenium removal method includes a biological treatment step and a pretreatment step. The biological treatment step is a step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or metallic selenium by biological treatment. The pretreatment step is a step of pretreating the selenium-containing water by the Fenton reaction before carrying out the biological treatment step.

[0040] (Effects and Benefits) As described above, the selenium removal device 100A and selenium removal method can efficiently remove selenium contained in selenium-containing water and further reduce the selenium concentration in the treated water after treatment.

[0041] As described above, several embodiments relating to this disclosure have been explained, but all of these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Examples]

[0042] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0043] (Example 1) Selenium was removed from selenium-containing water using a selenium removal device configured similarly to the selenium removal device 100 shown in Figure 1. Specifically, first, the following pretreatment steps were performed on the selenium-containing water before the biological treatment process. Next, the pre-treated water was subjected to biological treatment to reduce the hexavalent selenium contained in the selenium-containing water to tetravalent selenium or zero valent selenium through biological treatment. Subsequently, the biologically treated water was subjected to coagulation and sedimentation treatment using a coagulation and sedimentation apparatus configured similarly to the coagulation and sedimentation apparatus 80 shown in Figure 3 to remove the selenium contained in the selenium-containing water.

[0044] The pretreatment step in Example 1 was carried out by the following method. First, in the pretreatment step, sulfite ions (SO3) were used as interfering components in the biological treatment. 2- ), carbonate ions (CO3 2- ) and bicarbonate ions (HCO3) - The target of treatment was selenium cyanide (SeCN), which cannot be treated by biological methods. - ), and other organic selenium compounds were also treated.

[0045] First, selenium-containing water, to be treated, was prepared by dissolving soluble components of ash collected from an actual plant, simulating the treatment flow of an actual plant.

[0046] Next, regarding the selenium-containing water in the pretreatment step, sulfite ions (SO3) are considered as interfering components. 2-The concentration (mg / L) of hexavalent selenium and the total carbon dioxide concentration (mg / L as CO2), which is the sum of carbonate ions, bicarbonate ions, and carbon dioxide, were measured. In addition, the selenium concentration (mg / L as Se) in the selenium-containing water from the pretreatment process was also measured. The measurement results are shown in Table 1. In Table 1, "Total Carbon Dioxide Concentration" refers to the sum of the concentrations of carbonate ions, bicarbonate ions, and carbon dioxide (mg / L as CO2) as described above. In Table 1, "T-Se" in the "Selenium" column refers to the total selenium concentration (mg / L as Se) in the selenium-containing water. Furthermore, the breakdown of selenium contained in the selenium-containing water is shown in "Selenium Breakdown" in Table 1. In "Selenium Breakdown" in Table 1, the "Se(VI)" column refers to the concentration of hexavalent selenium (mg / L as Se), the "Se(IV)" column refers to the concentration of tetravalent selenium (mg / L as Se), and "SeSO3" refers to the tetravalent selenium concentration (mg / L as Se). 2- The column labeled "" contains selenosulfate ions (SeSO3 2- The concentration (mg / L as Se) is shown. Note that selenosulfate ion (SeSO3) 2- The concentration (mg / L as Se) of selenium cyanide (SeCN,org-Se, etc.) is shown in the column of Table 1, where it is calculated by subtracting the concentrations of hexavalent selenium (mg / L as Se) and tetravalent selenium (mg / L as Se) from the total selenium concentration (mg / L as Se). - This shows the concentration (mg / L as Se) per unit of selenium for ), other organic selenium compounds, and selenium whose structure has not been identified (hereinafter also referred to as "unknown selenium"). Sulfite ions (SO3 2- The concentrations of ) were measured by iodine titration. The total carbon dioxide concentration, which includes carbonate ions, bicarbonate ions, and carbon dioxide, is the value converted from inorganic carbon (IC) measured using a TOC meter (Shimadzu Corporation, TOC-LCPH). The concentrations of each type of selenium in the selenium-containing water were measured using inductively coupled plasma mass spectrometry (ICP-MS).

[0047] In Example 1, as a pretreatment step, the hydrogen ion concentration of the selenium-containing water to be treated was adjusted (hereinafter also referred to as "pH adjustment"). Specifically, hydrogen peroxide was first added to the selenium-containing water and stirred. At this time, the hydrogen ion concentration of the selenium-containing water was pH 3.0. During stirring in the selenium-containing water, the concentration of hydrogen peroxide was maintained at 180 mg / L. The concentration of hydrogen peroxide was checked periodically, and if the concentration of hydrogen peroxide decreased, hydrogen peroxide was added as needed to maintain the concentration at 180 mg / L. The stirring time after the initial addition of hydrogen peroxide was 5 hours.

[0048] Next, 10 mg / L of iron(II) sulfate was added to selenium-containing water, whose hydrogen ion concentration was adjusted to pH 3.0 and whose hydrogen peroxide concentration was maintained at 180 mg / L. Then, the selenium-containing water with added iron(II) sulfate was irradiated with UV light for 40 minutes to pretreat the selenium-containing water by the photofenton reaction.

[0049] The selenium-containing water was pretreated as described above. The selenium-containing water that underwent pretreatment was then treated to remove sulfite ions (SO3 2- The concentration was measured to be 1 mg / L or less. The measurement results are shown in Table 1. In Table 1, a "-" indicates that the concentration of the component corresponding to that column was not measured.

[0050] Next, the pre-treated selenium-containing water was diluted 2.8 times with pure water, and sodium hydroxide was added to the diluted selenium-containing water to adjust the hydrogen ion concentration to pH 7.0. After adjusting the hydrogen ion concentration of the selenium-containing water to pH 7.0, the selenium-containing water maintained a pH of 7.0 for 24 hours, and no fluctuation in hydrogen ion concentration was observed. Therefore, it was determined that carbonate ions and bicarbonate ions in the selenium-containing water were sufficiently removed by the above pre-treatment.

[0051] Next, selenium-containing water adjusted to pH 7.0 was subjected to biological treatment to reduce hexavalent selenium to tetravalent selenium or metallic selenium. Methanol and acetic acid were used as hydrogen donors in the biological treatment. Subsequently, the biologically treated selenium-containing water was subjected to coagulation and sedimentation treatment. The total selenium concentration (T-Se) of the treated water was measured to be 0.06 mg / L. The measurement results are shown in Table 1.

[0052] [Table 1]

[0053] (Comparative Example 1) In Comparative Example 1, selenium was removed from the selenium-containing water using the same method as in Example 1, except that the pretreatment step was omitted. In Comparative Example 1, the concentrations of each component in the selenium-containing water to be treated were measured, just as in Example 1. The measurement results are shown in Table 2. The values ​​in the "Before Treatment" column of Table 2 indicate the concentrations of each component in the selenium-containing water prepared as the water to be treated.

[0054] In Comparative Example 1, first, the hydrogen ion concentration of the selenium-containing water prepared as the water to be treated was adjusted to pH 7.0. Next, the selenium-containing water adjusted to pH 7.0 was subjected to biological treatment to reduce the hexavalent selenium in the selenium-containing water to tetravalent selenium or zero valent selenium. Subsequently, the biologically treated selenium-containing water was subjected to coagulation and sedimentation treatment.

[0055] The total selenium concentration (T-Se) in the treated water after coagulation and sedimentation treatment was measured to be 25 mg / L. As shown in Table 2, "Breakdown of Selenium," the concentration of hexavalent selenium (Se(VI)) was 1.5 mg / L, and the concentration of tetravalent selenium (Se(IV)) was 2.2 mg / L. Morphological analysis of the treated water after biological treatment revealed that the treated water contained selenosulfate ions (SeSO3) in addition to tetravalent selenium (Se(IV)) and hexavalent selenium (Se(VI)). 2-It was confirmed that selenium (Se) in a form believed to be ) was dissolved. Here, the morphological analysis described above was performed by separating the chemical species by morphology using ion chromatography, and then measuring the concentration of each chemical species by ICP-MS analysis.

[0056] [Table 2]

[0057] (result) As shown in Example 1, by performing a pretreatment step on selenium-containing water before the biological treatment step, it became possible to significantly reduce the total selenium concentration in the treated water after coagulation and sedimentation treatment. On the other hand, when the pretreatment step was not performed, although the concentrations of tetravalent selenium (Se(IV)) and hexavalent selenium (Se(VI)) decreased, selenium (Se) in forms other than tetravalent selenium (Se(IV)) and hexavalent selenium (Se(VI))) dissolved in the treated water, and the total selenium concentration in the treated water actually increased. In particular, selenosulfate ions (SeSO3) in the treated water... 2- The concentration of ) was 21 mg / L, indicating a significant increase.

[0058] <Note> The selenium removal method and selenium removal apparatus described in each embodiment can be understood, for example, as follows.

[0059] (1) A selenium removal method according to the first embodiment includes a biological treatment step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment step of removing at least a portion of sulfite ions which are interfering components in the biological treatment before performing the biological treatment step. The biological treatment process is a process that uses microorganisms such as denitrifying bacteria and selenium-reducing bacteria to perform biological treatment. The biological treatment process can be carried out using the biological treatment tank 2 of the selenium removal device 100 shown in Figure 1. The pretreatment process can be carried out using a pretreatment tank 3 located upstream of the biological treatment tank 2, as shown in the selenium removal device 100 in Figure 1. According to the selenium removal method of the first embodiment, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0060] (2) A selenium removal method according to a second embodiment includes a biological treatment step of reducing hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment step of pretreating the selenium-containing water by a Fenton reaction before performing the biological treatment step. The biological treatment process is a process that uses microorganisms such as denitrifying bacteria and selenium-reducing bacteria to perform biological treatment. The biological treatment process can be carried out using the biological treatment tank 2 of the selenium removal device 100A shown in Figure 2. The pretreatment process can be carried out using a pretreatment tank 3A located upstream of the biological treatment tank 2, as shown in the selenium removal device 100A in Figure 2. The Fenton reaction occurs when hydrogen peroxide (H2O2) reacts with divalent iron ions (Fe) under acidic conditions. 2+ This reaction involves the reaction of ) and to produce a hydroxyl radical (·OH) with strong oxidizing power. In this reaction, hydrogen peroxide functions as an oxidizing agent. An iron catalyst containing divalent iron ions functions as a catalyst for hydrogen peroxide. In the pretreatment step by the Fenton reaction, the aforementioned hydroxyl radical (·OH) can be generated to oxidize and decompose organic matter contained in selenium-containing water (for example, organic compounds containing selenium). According to the selenium removal method of the second embodiment, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0061] (3) The selenium removal method according to the third embodiment is the selenium removal method according to (1) or (2), wherein the pretreatment step includes a reaction that oxidizes sulfite ions contained in the selenium-containing water and converts them to sulfate ions. For example, in biological treatment using microorganisms such as denitrifying bacteria and selenium-reducing bacteria, some of the selenium contained in the selenium-containing water may accumulate in the biological treatment tank as metallic selenium. Therefore, if the selenium-containing water contains sulfite ions, the metallic selenium accumulated in the biological treatment tank may form complex ions with sulfite ions and redissolve in the liquid phase, leaching out from the biological treatment tank and contaminating the biologically treated water. By oxidizing sulfite ions and converting them to sulfate ions through the pretreatment step, the leaching of metallic selenium by sulfite ions can be effectively suppressed.

[0062] Furthermore, the pretreatment step may include aeration of selenium-containing water. That is, the pretreatment step involves aeration of selenium-containing water to remove sulfite ions (SO3 2- ) is oxidized to sulfate ions (SO4 2- This may include a process to convert to ).

[0063] (4) The selenium removal method according to the fourth embodiment is the selenium removal method according to any one of (1) to (3), wherein the pretreatment step includes a step of pretreating the selenium-containing water by a Fenton reaction. In the pretreatment step by the Fenton reaction, the above-mentioned hydroxyl radical (·OH) can be generated to oxidatively decompose organic matter (for example, organic compounds containing selenium) contained in the selenium-containing water.

[0064] (5) The selenium removal method according to the fifth aspect is the selenium removal method according to any one of (1) to (4), wherein the Fenton reaction includes a photo-Fenton reaction carried out under irradiation conditions of light including ultraviolet light. The photo-Fenton reaction consists of a cycle of oxidation of iron ions with hydrogen peroxide and photoreduction. When the pretreatment carried out in the pretreatment step includes a Fenton reaction, by making the Fenton reaction a photo-Fenton reaction, the amount of hydrogen peroxide and other substances used to carry out the Fenton reaction can be reduced, and the running cost of the pretreatment can be reduced.

[0065] (6) The selenium removal method according to the sixth embodiment is the selenium removal method according to any one of (1) to (5), wherein the pretreatment step includes a step of adjusting the hydrogen ion concentration of the selenium-containing water to pH 2 to 4. In the pretreatment step, it is preferable to adjust the hydrogen ion concentration of the selenium-containing water to around pH 3. Therefore, by adjusting the hydrogen ion concentration of the selenium-containing water to the above numerical range, sulfite ions (SO3) can be removed. 2- ) is oxidized to sulfate ions (SO4 2- This allows for suitable pretreatment to convert to ).

[0066] (7) The selenium removal method according to the seventh embodiment is the selenium removal method according to any one of (1) to (6), further comprising pH adjustment between the pretreatment step and the biological treatment step to adjust the hydrogen ion concentration of the selenium-containing water to pH 5 to 9. The biological treatment in the biological treatment step has a hydrogen ion concentration suitable for biological treatment, and by adjusting the hydrogen ion concentration of the selenium-containing water to the above numerical range, the selenium contained in the pretreated selenium-containing water can be removed more efficiently by the biological treatment step. pH adjustment before the biological treatment step can be performed by adding alkaline industrial water or the like to the selenium-containing water and diluting it. Alternatively, pH adjustment before the biological treatment step can also be performed by adding an alkali such as caustic soda or slaked lime to the selenium-containing water.

[0067] (8) The selenium removal method according to the eighth aspect is the selenium removal method according to any one of (1) to (7), further comprising a coagulation and sedimentation step after the biological treatment step, in which the selenium-containing water containing the reduced tetravalent selenium is subjected to coagulation and sedimentation treatment to remove the selenium contained in the selenium-containing water from the system. The coagulation and sedimentation step can be carried out in accordance with the coagulation and sedimentation step in conventionally known selenium removal methods. The coagulation and sedimentation step is not particularly limited, but can be carried out using, for example, a coagulation and sedimentation treatment apparatus 80 as shown in Figure 3. By further including the coagulation and sedimentation step, the selenium contained in the selenium-containing water can be efficiently removed and the selenium concentration in the treated water after treatment can be further reduced.

[0068] (9) The selenium removal apparatus according to the ninth embodiment, as shown in the selenium removal apparatus 100 in Figure 1, comprises a biological treatment tank 2 that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment tank 3 that is located upstream of the biological treatment tank 2 and removes at least a portion of sulfite ions that interfere with the biological treatment carried out in the biological treatment tank 2. The selenium removal apparatus according to the ninth embodiment can be used as a selenium removal apparatus for carrying out the selenium removal method described in any one of the above-described items (1) to (8). According to the selenium removal apparatus of the ninth embodiment, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0069] (10) The selenium removal apparatus according to the 10th embodiment, as shown in Figure 2, comprises a biological treatment tank 2 that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, and a pretreatment tank 3A located upstream of the biological treatment tank 2 that pretreatments the selenium-containing water by the Fenton reaction. The selenium removal apparatus according to the 10th embodiment can be used as a selenium removal apparatus for carrying out the selenium removal method described in any one of the above-described items (1) to (8). According to the selenium removal apparatus of the tenth embodiment, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0070] With the above configuration, selenium contained in selenium-containing water can be efficiently removed, and the selenium concentration in the treated water after treatment can be further reduced.

[0071] (11) The selenium removal apparatus according to the 11th paragraph is the selenium removal apparatus according to (9) or (10), further comprising a coagulation and sedimentation apparatus that removes selenium contained in the selenium-containing water containing the reduced tetravalent selenium from the system by coagulation and sedimentation treatment. The coagulation and sedimentation apparatus is not particularly limited, but for example, a coagulation and sedimentation apparatus 80 as shown in Figure 3 can be provided. By further comprising a coagulation and sedimentation apparatus 80 as shown in Figure 3, selenium contained in the selenium-containing water can be efficiently removed and the selenium concentration in the treated water after treatment can be further reduced. [Explanation of Symbols]

[0072] 1…Raw water tank 2…Biological treatment tank 3,3A…Pre-treatment tank 4…Hydrogen donor storage tank 11… Raw water supply line 12…Pretreatment water supply line 13…Biological treatment water piping 14…Hydrogen donor supply line 21…carrier 22…Liquid transfer pump 23…Liquid transfer pump 24…Injection pump 31…Raw water (selenium-containing water) 32…Pre-treated water 33…Biologically treated water 34…Hydrogen donor 41...First flocculant addition section 42...Second flocculant addition section 43, 44... Agitator 50…Coagulation and sedimentation tank 51…Agitator 61…Inorganic flocculants 62… High-molecular-weight flocculants 63... pH adjuster 71…Treated water 72...Sludge 80... Coagulation and sedimentation treatment equipment 100, 100A... Selenium removal device

Claims

1. A biological treatment process that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, A method for removing selenium, comprising a pretreatment step of removing at least a portion of sulfite ions that are interfering components in the biological treatment, before performing the biological treatment step.

2. A biological treatment process that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium by biological treatment, A method for removing selenium, comprising a pretreatment step of pretreating the selenium-containing water by a Fenton reaction before performing the biological treatment step.

3. The selenium removal method according to claim 1, wherein the pretreatment step includes a reaction that oxidizes sulfite ions contained in the selenium-containing water and converts them to sulfate ions.

4. The selenium removal method according to claim 1, wherein the pretreatment step includes a step of pretreating the selenium-containing water by a Fenton reaction.

5. The selenium removal method according to claim 2 or 4, wherein the Fenton reaction includes a photofenton reaction carried out under irradiation conditions of light including ultraviolet light.

6. The selenium removal method according to claim 1 or 2, wherein the pretreatment step includes a step of adjusting the hydrogen ion concentration of the selenium-containing water to pH 2 to 4.

7. The selenium removal method according to claim 1 or 2, further comprising a pH adjustment step between the pretreatment step and the biological treatment step, in which the hydrogen ion concentration of the selenium-containing water is adjusted to a pH of 5 to 9.

8. The selenium removal method according to claim 1 or 2, further comprising a coagulation and sedimentation step after the biological treatment step, in which the selenium-containing water containing the reduced tetravalent selenium is subjected to coagulation and sedimentation treatment to remove the selenium contained in the selenium-containing water from the system.

9. A biological treatment tank that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium through biological treatment, A selenium removal apparatus comprising a pretreatment tank positioned upstream of the biological treatment tank, which removes at least a portion of sulfite ions that interfere with the biological treatment carried out in the biological treatment tank.

10. A biological treatment tank that reduces hexavalent selenium contained in selenium-containing water to tetravalent selenium or zero-valent selenium through biological treatment, A selenium removal apparatus comprising a pretreatment tank positioned upstream of the biological treatment tank, which pretreatments the selenium-containing water by the Fenton reaction.

11. The selenium removal apparatus according to claim 9 or 10, further comprising a coagulation and sedimentation apparatus for removing selenium contained in the selenium-containing water from the system by performing coagulation and sedimentation treatment on the selenium-containing water containing the reduced tetravalent selenium.

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