JP6031376B2 - Selenium-containing wastewater treatment system and selenium-containing wastewater treatment method - Google Patents

Description

  The present invention relates to a selenium-containing wastewater treatment system and a selenium-containing wastewater treatment method for removing selenium from wastewater containing selenium, particularly wastewater having a high selenium concentration.

Selenium is an essential element for the human body, and there is no problem if it is in a trace amount, but it shows toxicity when it is increased to some extent compared to the required level. Therefore, selenium is an environmental standard designation item related to water pollution, soil pollution, and the like.
For example, since the coal ash treated in the ecocement production process generally contains chlorine, it must be washed with water before mixing as an ecocement raw material. The cleaning wastewater discharged at that time may contain selenium in addition to chlorine.

  In addition, selenium may also be contained in cleaning wastewater discharged when exhaust gas generated from a coal gasification combined cycle (IGCC) coal gasification process or the like is cleaned. In addition, selenium may also be contained in waste water generated by flue gas desulfurization. These effluents are, for example, purified until they satisfy a predetermined effluent standard, and then discharged into a river or the like.

In the case of wastewater containing selenium exceeding a predetermined wastewater standard, it is necessary to remove selenium during wastewater treatment. For example, selenium is removed until the selenium concentration is 0.1 mg / L or less. In wastewater containing selenium, for example, selenium exists mainly as tetravalent selenite ions (SeO ₃ ²⁻ ) or hexavalent selenate ions (Se0 ₄ ²⁻ ).

  As a method for removing selenium from selenium-containing wastewater, for example, a technique for removing selenium in wastewater by bringing iron into contact with wastewater containing selenium has been proposed (see, for example, Patent Documents 1 to 3).

  For example, fibrous iron as a contact reducing material is used, drainage is brought into contact with fibrous iron, air is blown out, hexavalent selenium is reduced to tetravalent selenium by oxidation of iron, and tetravalent selenium is produced. Has been proposed to precipitate with iron sludge (mainly iron hydroxide). Tetravalent selenium is more easily precipitated than hexavalent selenium, and is precipitated in a state where originally tetravalent selenium and selenium reduced from hexavalent to tetravalent adhere to iron sludge.

The waste water described above contains a Chemical Oxygen Demand component (hereinafter simply referred to as “COD component”) as an inhibitor that inhibits selenium removal. The COD component here is not an organic substance but an oxidizable component, for example, sulfite ion (SO ₃ ²⁻ ), thiosulfate ion (S ₂ O ₃ ²⁻ ) in water. , Dithionate ion (S ₂ O ₆ ²⁻ ) and the like. If these inhibitor concentrations in the wastewater are high, the selenium removal rate decreases.
Therefore, in the above-mentioned Patent Document 2, an inhibitor that inhibits selenium removal is removed before the selenium removal step.

  In addition, Patent Document 3 describes that when sulfur oxide or the like is contained in the waste water, the consumption of iron as a contact reducing material increases and the amount of iron sludge increases. In the selenium removal treatment, it is described that the supply of iron and the treatment of iron sludge become problems. Also in this case, it is possible to reduce iron consumption and iron sludge generation by removing sulfur oxides and the like before the selenium removal step.

JP 9-47790 A JP 2011-72940 A Japanese Patent No. 4812987

By the way, if the concentration of selenium contained in the wastewater increases, even if sulfur oxides are removed by pretreatment, the amount of iron consumed and the amount of iron sludge discharged will increase. For example, non-ferrous metal wastewater having a high selenium concentration is known. In this case, an increase in cost due to an increase in iron consumption and a cost for processing a large amount of iron sludge are required.
Non-ferrous metal wastewater containing selenium at a high concentration includes wastewater having a high concentration of sulfate ions (SO ₄ ²⁻ ) and dithionate ions as the above-mentioned COD components. Sulfate ions and dithionic acid are inhibitors that inhibit the removal of selenium, and it is necessary to remove COD components such as sulfate ions and dithionate ions before removing selenium.
For example, in Patent Document 2, coagulation precipitation treatment is performed as an inhibitor removal, that is, removal of sulfate ions and COD components, and then remains with an oxidizing agent (for example, ozone, hydrogen peroxide, potassium permanganate). As a COD component, for example, oxidation treatment of sulfite ion, thiosulfate ion, dithionate ion and the like is described.
Here, for example, when the content of dithionate ions in the waste water is high, there is a possibility that the removal of dithionate ions by oxidation treatment may be insufficient. That is, inhibitors such as sulfite ion, thiosulfate ion and dithionate ion are hardly decomposable and need to be oxidized in the presence of a strong oxidant as described above, but they are still sufficient. May not be removed (oxidized).
In this case, it is conceivable to use a catalyst or the like for the oxidative decomposition treatment, but this leads to an increase in cost.

  The present invention has been made in view of the above circumstances, and in the case of high selenium concentration and removal of selenium, when the concentration of an inhibitor of selenium removal to be removed in advance is high, the iron content used for selenium removal is effective. It aims at providing the processing system of the selenium containing waste water which can reduce cost by aiming at utilization, and the processing method of a selenium containing waste water.

In order to solve the above-mentioned problems, a selenium-containing wastewater treatment system according to the present invention comprises a COD component removal means for removing a COD component that inhibits removal of selenium from selenium-containing wastewater, and the selenium-containing wastewater after the removal of the COD component Reducing selenium of the selenium contained in the wastewater with iron, and selenium removing means for precipitating and removing the selenium contained in the selenium-containing wastewater together with the iron sludge oxidized during the reduction. Prepared,
In the COD component removing means, a heating oxidation process is performed in which the selenium-containing wastewater is heated with an acid to oxidize the COD component,
A part of the sludge generated in the selenium removing means is circulated between the selenium removing means and the COD component removing means by introducing the sludge generated in the selenium removing means into the selenium-containing wastewater. It is characterized by.

Further, the method for treating selenium-containing wastewater according to the present invention includes a COD component removal step for removing a COD component that inhibits removal of selenium from the selenium-containing wastewater, and the selenium contained in the selenium-containing wastewater after the removal of the COD component. A selenium removal step of reducing and reducing reducible selenium with iron, and precipitating and removing the selenium contained in the selenium-containing wastewater together with the iron sludge oxidized during the reduction,
In the COD component removal step, a heating oxidation step of oxidizing the COD component by heating the selenium-containing wastewater with an acid is performed,
Circulating the iron between the selenium removal step and the COD component removal step by putting a part of the sludge generated in the selenium removal step into the selenium-containing wastewater in the COD component removal step. It is characterized by.

According to such a configuration, the iron is used for the reduction of the reducible selenium, and the iron is used to precipitate the selenium by circulating the reduced selenium and the iron used to precipitate the originally reduced selenium. Thus, the amount of iron consumed can be reduced, and in the removal of selenium from the wastewater, the cost of the iron to be input to the selenium-containing wastewater can be reduced.
In addition, since the amount of generated sludge is reduced, it is possible to reduce the thermal energy required to dispose of the iron that has settled into sludge, thereby reducing the cost.

  The selenium dissolved in the waste water is mainly hexavalent and tetravalent selenium as described above, and the hexavalent selenium can be reduced to tetravalent using iron as reducible selenium. . During this reduction, iron is oxidized to divalent and trivalent iron ions, but tetravalent selenium is co-precipitated as sludge together with iron when iron is precipitated as iron hydroxide in approximately neutral to alkaline conditions. In addition, tetravalent selenium tends to precipitate, but hexavalent selenium hardly precipitates.

  Further, when iron sludge is disposed of, it may be burned in, for example, a kiln to reduce the volume. Although the heat energy used in this case is costly, by reducing the generated iron sludge, the heat energy required for the disposal of the iron sludge can be suppressed and the cost can be reduced.

In the selenium-containing wastewater treatment system (treatment method) according to the present invention, the COD component removal means (step) adds hydrogen peroxide and iron sulfate to the selenium-containing wastewater after the heating and oxidation step , thereby removing the COD component. It is preferable that a COD component removal (main) step for oxidation is performed, and the sludge is put into the selenium-containing wastewater in the heating oxidation step.

  According to such a configuration, the COD component removal step is divided into a heating oxidation step and a COD component removal (main) step, so that a part of the COD component is oxidized in the pre-treatment non-heating oxidation step. In addition, the COD component concentration sent to the COD component removal (main) step can be reduced, the load in the COD component removal (main) step can be reduced, and the processing efficiency can be improved.

At this time, as described above, the sludge that is introduced into the selenium-containing wastewater by the COD component removal means (COD component removal step) is introduced into the selenium-containing wastewater that is heated acidic. Thereby, for example, iron which is hydroxide in the sludge is dissolved in the wastewater heated under acidity, and becomes reusable.
For example, the iron ions dissolved in the heating oxidation process as described above can reduce the amount of iron sulfate charged in the COD component removal (main) process. Iron sulfate functions as a catalyst for generating hydroxy radicals from hydrogen peroxide.

In the selenium-containing wastewater treatment system (treatment method) of the present invention, before removing the COD component from the selenium-containing wastewater, sulfate ions that are contained in the selenium-containing wastewater and inhibit the removal of the selenium are removed. Equipped with a sulfuric acid removal means (a sulfuric acid removal step is performed),
In the sulfuric acid removal means (sulfuric acid removal step), calcium ions are introduced into the selenium-containing wastewater, and the sulfate ions are precipitated and removed as gypsum which is calcium sulfate.
The COD component removing means oxidizes the dithionate ion, which is the COD component, to make a sulfate ion, and the sulfate ion is added by the sulfuric acid removing means to form the gypsum by the calcium ion contained in the selenium-containing wastewater. It is preferable to remove it by precipitation.

  According to these configurations, sulfate ions that are inhibitors of selenium removal can be removed, and when dithionate ions as COD components that also serve as inhibitors are oxidized to sulfate ions, they are used to remove sulfate ions. Can be removed using the calcium ions. Thereby, the sulfate ion and dithionate ion which inhibit the removal of selenium can be removed, and the removal of selenium can be made efficient.

  According to the present invention, when removing selenium from selenium-containing wastewater, it is possible to reduce selenium with poor reducibility and reduce the consumption of iron used for precipitating selenium to reduce costs. it can.

It is process drawing for demonstrating the processing method of the selenium containing waste water using the processing system of the selenium containing waste water in embodiment of this invention.

Embodiments of the present invention will be described below.
The selenium-containing wastewater treatment system of this embodiment includes sulfuric acid removal means 1 that mainly removes sulfate ions from wastewater, and COD component removal means that mainly removes dithionate ions (S ₂ O ₆ ²⁻ ) from wastewater. 2 and selenium removing means 3 for removing selenium from the waste water.

The selenium-containing wastewater (for example, the non-ferrous metal wastewater described above) to be treated in this embodiment contains relatively high concentrations of tetravalent selenium and hexavalent selenium. As described above, these selenium is dissolved in the waste water as selenite ion (SeO ₃ ²⁻ ) or selenate ion (Se0 ₄ ²⁻ ).

This selenium-containing wastewater contains, for example, 20 mg / L or more in total of tetravalent selenium and hexavalent selenium, and may contain, for example, about 40 to 70 mg / L.
The selenium-containing wastewater contains a large amount of sulfate ions, for example, about 10000 to 70000 mg / L. The selenium-containing waste water contains about 80 to 250 mg / L of dithionate ions.

As described above, in this selenium-containing wastewater, the sulfate ion concentration is high, and this sulfate ion becomes an inhibitor that inhibits the removal of selenium, so it is necessary to remove the sulfate ion.
The sulfuric acid removal means 1 includes a removal tank 1a for removing calcium ion into sulfate ions and removing it as gypsum, and a precipitation tank 1b (solid-liquid separation device) for solid-liquid separation of gypsum.
In the sulfuric acid removal step in the sulfuric acid removal means 1, for example, calcium chloride (CaCl ₂ ) and sodium hydroxide (NaOH) are introduced into the selenium-containing wastewater in the removal tank 1a, and hardly soluble gypsum (CaSO ₄ .2H ₂ O). ) And the remaining sodium hydroxide is neutralized with hydrochloric acid, and gypsum precipitates and precipitates.

  The selenium-containing wastewater from which gypsum precipitates in the removal tank 1a is sent to solid-liquid separation means such as the precipitation tank 1b, and sulfate ions are removed as gypsum. The alkali to be added is not limited to sodium hydroxide, and may be calcium hydroxide (slaked lime) or other alkaline hydroxide.

  Moreover, the input amount of calcium chloride to the selenium-containing wastewater is, for example, 2000 to 70000 mg / L, and more preferably 5000 to 15000 g / L. Moreover, the input amount of sodium hydroxide (calcium hydroxide) to the selenium-containing wastewater is, for example, 150 to 4000 mg / L (250 to 8000 mg / L), and more preferably 3000 to 4000 g / L (5500 to 7500 g). / L).

  Moreover, the temperature of the selenium containing waste water in a sulfuric acid removal process should just be normal temperature (for example, 20-40 degreeC), and it is preferable that the pH of a selenium containing waste water is 10-12. Moreover, it is preferable that the sulfate ion concentration after a process is 10000 mg / L or less, for example.

  In the sulfuric acid removal step, as described above, calcium chloride and sodium hydroxide are charged into the selenium-containing waste water, and the solid-liquid separation of the gypsum generated thereby is performed. In addition, the input amount of calcium chloride exceeds the saturation amount, and the calcium chloride has a saturated concentration on the liquid side after the gypsum is solid-liquid separated.

  The COD component (dithionic acid) removing step in the COD component removing means 2 includes a heating acid decomposition step (heating oxidation step) as a pretreatment and a dithionic acid removal main step as a main treatment. The COD component removing means 2 includes, for example, a heating tank 2a for performing the heating acid decomposition step, an oxidation tank 2b for performing the dithionic acid removal main step, and a precipitation tank 2c for solid-liquid separation.

In the heating acid decomposition step, selenium-containing wastewater from which gypsum is separated into solid and liquid as described above flows into the heating tank 2a, and hydrochloric acid (HCl) and selenium-removing means 3 in the selenium-containing wastewater are solid-liquid as described later. Part of the separated sludge (mainly iron hydroxide sludge) is charged and heated. Thereby, the heating acid decomposition of the COD component containing dithionic acid is performed. In this case, mainly tetravalent sulfur is oxidized to hexavalent, for example, a part of dithionate ions are converted to sulfate ions. In addition, iron sludge containing iron hydroxide (Fe (OH) ₂ , Fe (OH) ₃ ) is dissolved by adding HCl.

The heating acid decomposition step in the COD component removing means 2 oxidizes a part of dithionate ions to sulfate ions, lowers the concentration of dithionate ions flowing into the dithionate removal main step, which is the main treatment, The dithionic acid removal process is a treatment for reducing the load in this step and dissolving iron sludge.
In addition, since sulfate ions and iron ions are generated in the selenium-containing wastewater, iron sulfate (FeSO ₄ ) is added to the selenium-containing wastewater.

The input amount of hydrochloric acid in the heating acid decomposition step is, for example, 25 to 400 mg / L, and more preferably 50 to 200 mg / L. Moreover, the input amount of the sludge in a heating acid decomposition process is 125-2000 g / L, for example, More preferably, it is 500-1000 g / L.
Moreover, it is preferable that the temperature of the selenium containing waste water in a heating acid decomposition process is 50-80 degreeC, and it is preferable that the pH of a selenium containing waste water is 1-3. Moreover, it is preferable that the dithionate ion concentration after a process is 50 mg / L or less, for example.

Dithionic acid removal In this step, hydrogen peroxide is added as an oxidizing agent in order to oxidize most of the dithionic acid ions into sulfate ions. In addition, by adding iron sulfate (FeSO ₄ ) as a catalyst for generating hydroxy radicals from hydrogen peroxide water, hydroxy radicals are generated and sulfuric acid of dithionate ions is compared with the case where only hydrogen peroxide is added. Oxidation to ions is promoted. Therefore, even if a high concentration of dithionate ions is present, the oxidation of dithionate ions can be prevented from being insufficient, and the dithionate ion concentration can be lowered.

In addition, as described above, in the heat acid decomposition step, by introducing iron sludge, iron is in a state of being charged, and the amount of iron sulfate charged can be reduced.
The heating acid decomposition step in this COD component removal means and the dithionic acid removal main step increase the sulfate ion concentration in the selenium-containing wastewater, but the calcium chloride charged in the sulfuric acid removal step is contained in the selenium-containing wastewater. It is contained at a saturated concentration, and the generated sulfate ions react with calcium ions to form gypsum. This gypsum precipitates in the precipitation tank 2c (solid-liquid separator) and is solid-liquid separated. Moreover, in the sedimentation tank 2c, the iron sludge which did not melt | dissolve among the input iron sludge is solid-liquid separated.

  At this time, most of the tetravalent selenium that easily precipitates compared to hexavalent selenium adheres to the iron sludge and precipitates together with the iron sludge. At this stage, the hexavalent selenium is not reduced and flows out to the selenium removal process side. In addition, although solid-liquid separation is also performed in the next selenium removal step, in the selenium removal step, the presence of SS (Suspended Solid) becomes an inhibiting factor, and therefore it is preferable to perform solid-liquid separation before the selenium removal step.

  Further, the iron sludge charged in the heating acid decomposition step of the COD component removing means 2 includes tetravalent selenium precipitated together with the iron sludge in the selenium removal step described later. The selenium-containing wastewater originally contains tetravalent selenium as described above. These tetravalent selenium is hardly oxidized in the heating acid decomposition treatment and dithionic acid removal main step of the COD component removing means 2, and most of them are precipitated and separated together with iron sludge. Further, tetravalent selenium that has not been separated by precipitation is precipitated together with iron sludge in the next selenium removal step. Here, if tetravalent selenium is oxidized to hexavalent, hexavalent selenium to be reduced using iron increases in the selenium removal step described later, and iron consumption increases. In order to cause an increase in iron sludge, it is not preferable that tetravalent selenium is oxidized to hexavalent.

Dithionic acid removal The input amount of hydrogen peroxide in this step is, for example, 50 to 1000 mg / L, and more preferably 200 to 400 mg / L. Moreover, the input amount of iron sulfate in the dithionic acid removal step is, for example, 100 to 1200 mg / L, and more preferably 500 to 900 mg / L.
Moreover, it is preferable that the temperature of the selenium containing waste water in this dithionic acid removal process is 50-80 degreeC, and it is preferable that the pH of a selenium containing waste water is 2-4. Moreover, it is preferable that the dithionate ion concentration after a process is 20 mg / L or less, for example.

The selenium removal means 3 for removing selenium includes a removal tank 3a for removing selenium, and a precipitation tank 3b (solid-liquid separation device) for solid-liquid separation of iron sludge containing tetravalent selenium generated during selenium removal.
In the selenium removal step, in a state where fibrous iron having a large surface area, oxygen in the air, and hexavalent selenium are present in water, the iron becomes divalent iron ions and dissolves in water. Oxidized to form trivalent iron ions and precipitate as Fe (OH) _{3 and the} like.

  In contrast, hexavalent selenium is reduced by iron that oxidizes as described above. That is, it is reduced from hexavalent selenium to tetravalent selenium that easily precipitates, and precipitates together with iron hydroxide. In addition, although iron hydroxide and tetravalent selenium are attached to the surface of the fibrous iron, deposits such as iron hydroxide on the iron surface are peeled off by blowing air to the fibrous iron. Iron sludge. This also exposes the surface of the iron that has not been oxidized, and the reduction of selenium proceeds.

  In the removal tank 3a, there is a portion where fibrous iron is held, and selenium-containing wastewater is circulated in this portion by a stirring device or the like. In the removal tank 3a, continuous treatment is basically performed, but the selenium-containing wastewater does not flow through the fibrous iron portion, but stays in circulation, and the retained selenium-containing wastewater is retained. It is discharged from the removal tank 3a.

The surface area per volume of fibrous iron in the selenium removal step is preferably 100 to 200 m ² / m ³ . Moreover, it is preferable that the temperature of the selenium containing waste water in a selenium removal process is 40-60 degreeC, and it is preferable that the pH of a selenium containing waste water is 4-6. Moreover, it is preferable that the residence time of the removal tank 3a of the selenium containing waste water in a selenium removal process is 10 to 50 hours, for example. Moreover, it is preferable that the average passing speed of the selenium containing waste water of the contact reaction zone in the removal tank 3a in which fibrous iron as a contact reducing material is disposed is 0.1 to 1.0 m / sec.

  As described above, a part of the sludge (iron sludge) generated in the selenium removal process is sent to the heated acid decomposition process of the COD component removal means 2 and is put into the selenium-containing waste water. Therefore, the iron in the sludge separated in the selenium removal process is circulated in the COD component removal process and the selenium removal process.

  In the selenium-containing wastewater treatment method in such a selenium-containing wastewater treatment system, the concentration of selenium contained in the selenium-containing wastewater is high, and the amount of iron required for the reduction from hexavalent selenium to tetravalent selenium. Even if the selenium removal process increases, the iron sludge generated in the selenium removal process is introduced into the selenium-containing wastewater by the COD component removal means 2 prior to the selenium removal process, that is, a part of the iron sludge with the selenium removal process Circulation with the previous COD component removing means 2 can reduce iron consumption and iron sludge generation.

  Here, the iron ions dissolved in the selenium-containing wastewater return to the selenium removal process from the COD component removal means 2, contributing to the precipitation of tetravalent selenium, and reducing the consumption of fibrous iron in the selenium removal process. Will do.

  Further, in response to the high concentration of dithionic acid in the selenium-containing wastewater, that is, the COD component concentration, in the oxidation of dithionic acid with hydrogen peroxide in the COD component removing means 2, hydrogen peroxide is used as a catalyst with divalent iron sulfate. When hydroxy radicals are generated from slag, the input amount of iron sulfate can be reduced by introducing iron sludge. This also makes it possible to reduce the amount of iron used.

  Further, the COD component removing means includes a heating acid decomposition step in which hydrochloric acid is introduced into the selenium-containing wastewater and heated before the main step of removing the COD component that oxidizes dithionic acid. In this heating acid decomposition step, By introducing iron sludge made of iron oxide, the amount of iron sludge dissolved in the selenium-containing waste water can be increased. That is, divalent and trivalent iron hydroxide can be dissolved by a hydrochloric acid solution.

  Further, in the heating acid decomposition step, a part of dithionate ion is oxidized to become sulfate ion, thereby reducing the load in this step of dithionate removal and promoting the oxidation of dithionate, The amount of usage can be reduced. Here, it is possible to reduce the operating cost by reducing the consumption of iron that occupies a large weight in the operating cost of the selenium-containing wastewater treatment system. In addition, by reducing iron consumption and reducing iron sludge generation, it is possible to reduce the thermal energy required to dispose of the generated iron sludge. Cost can be reduced.

  Further, since the COD component removing means 2 oxidizes sulfur of dithionate ions from pentavalent to hexavalent, tetravalent selenium in the selenium-containing wastewater and tetravalent selenium contained in the circulated iron sludge are oxidized. In the COD component removing unit 2, most of the tetravalent selenium remains tetravalent without being oxidized.

  Therefore, even if iron sludge containing tetravalent selenium is added to the COD component removing means 2, the tetravalent selenium is oxidized to hexavalent, so that it is necessary to reduce the hexavalent selenium to tetravalent. There will be no increase in iron consumption. Therefore, the iron consumption can be reliably reduced by circulating the iron sludge.

Examples of the present invention will be described below.
The experimental conditions of the comparative example and the example will be described below. The difference between the comparative example and the example is whether or not the above-described iron sludge is circulated, and other experimental conditions are set to be the same. In the comparative example and the example, the above-described sulfuric acid removal, dithionic acid removal, and selenium removal were performed, respectively.

Hereinafter, experimental conditions common to the comparative example and the example will be described.
The selenium-containing wastewater of this experimental example contains 37 mg / L of hexavalent selenium and 20 mg / L of tetravalent selenium. In addition, 61400 mg / L of sulfate ion and 92 mg / L of dithionate ion are included.
Moreover, the processing amount of the selenium containing waste water was 5 liters per hour.
As operating conditions in the sulfuric acid removal step, the temperature of the selenium-containing wastewater was 35 ° C., and the pH was 12.4. In addition, the input amount of calcium chloride to the selenium-containing wastewater was 69.3 g / L, and the input amount of calcium hydroxide as an alkali was 2.14 g / L.
By this sulfuric acid removal step, the concentration of sulfate ion in the selenium-containing wastewater was reduced from 61400 mg / L to 100 mg / L.

In the heating acid decomposition step of the COD component removing means 2, the selenium-containing wastewater from which sulfate ions have been removed is treated. As operating conditions in the heating acid decomposition step, the temperature of the selenium-containing wastewater was 60 ° C., and the pH was 3. The amount of hydrochloric acid charged into the selenium-containing waste water was set to 0.10 g / L per hour as a 35 wt% hydrochloric acid solution.
By this heating acid decomposition step, the dithionate ion concentration in the selenium-containing wastewater was reduced from 92 mg / L to 53 mg / L.

Dithionic acid removal in the COD component removal step In this step, the temperature of the selenium-containing wastewater was 30 ° C. and the pH was 3 as operating conditions. Moreover, the input amount of hydrogen peroxide to the selenium-containing wastewater was 64 mg / L, and the input amount of iron sulfate was 52 mg / L as iron in iron sulfate.
By this COD component removing means 2, the dithionate ion concentration in the selenium-containing waste water was reduced from 53 mg / L to 18 mg / L.

Dithionic acid removal in the selenium removal step In this step, the temperature of the selenium-containing wastewater was 30 ° C. and the pH was 3 as operating conditions. Moreover, the surface area per volume of the fibrous iron used as the contact reducing material was 101 m ² / m ³ . The residence time of the selenium-containing wastewater removal tank 3a in the selenium removal step was 5 hours. The average passing speed of the selenium-containing wastewater in the contact reaction zone of the contact reducing material was set to 0.3 m / sec.

  Under such experimental conditions, in the comparative example, the entire amount of sludge is discharged in the selenium removal step, and in the example, the heated acid decomposition step of the COD component removal means 2 so as to circulate half of the sludge discharged in the selenium removal step. To selenium-containing wastewater. That is, in the examples, the amount of sludge separated and discharged in the selenium removal step and the amount of sludge separated and sent to the heated acid decomposition step were set to 1: 1.

  Moreover, in the comparative example and the example, the selenium concentration in the waste water after the selenium removal step was set to 1 mg / L or less. Under such experimental conditions, the sludge discharge amount in the example was 80 mg / hr. Moreover, the discharge amount of sludge in the comparative example was 200 mg / hr.

  That is, the amount of sludge to be discharged can be reduced to half or less by circulating the sludge. In this case, since the discharge amount of sludge is reduced, the consumption amount of fibrous iron as the contact reducing material is also reduced. As a result, it is possible to reduce the cost required for iron input, and it is possible to reduce the cost by reducing the thermal energy required for sludge treatment. In the comparative example and the example described above, the amount of iron sulfate added in the COD component removal step is the same, but the amount of iron sulfate added in the example in which the iron sludge is added should be smaller than that in the comparative example. This can also reduce the amount of iron sludge generated.

  In addition, a continuous process is performed in the process in each tank. Moreover, you may perform solid-liquid separation by well-known solid-liquid separation methods other than the precipitation by a precipitation tank. Moreover, the inhibitory substance contained in a selenium containing wastewater is not restricted to a sulfate ion and a dithionate ion, For example, another sulfur oxide ion may be sufficient. Further, the COD component removing means 2, that is, the step of mainly removing pentavalent sulfur oxide ions may not necessarily be divided into the two steps of the heating acid decomposition step and the dithionic acid removal main step. In order to dissolve the iron sludge when circulating the iron sludge, it is preferable to have a heating acid decomposition step.

1 sulfuric acid removing means 2 COD (dithionic acid) removing means (COD component removing means)
3 Selenium removal means

Claims (6)

COD component removal means for removing a COD component that inhibits the removal of selenium from the selenium-containing wastewater; and reduction of the selenium contained in the selenium-containing wastewater after the removal of the COD component with iron, Selenium removing means for precipitating and removing the selenium contained in the selenium-containing wastewater together with the iron sludge oxidized during the reduction,
In the COD component removing means, a heating oxidation process is performed in which the selenium-containing wastewater is heated with an acid to oxidize the COD component,
A part of the sludge generated in the selenium removing means is circulated between the selenium removing means and the COD component removing means by introducing the sludge generated in the selenium removing means into the selenium-containing wastewater. A selenium-containing wastewater treatment system. In the COD component removing means, after the heating and oxidizing step, a COD component removing step of oxidizing the COD component by adding hydrogen peroxide and iron sulfate to the selenium-containing wastewater is performed, and the sludge is formed in the heating and oxidizing step. The selenium-containing wastewater treatment system according to claim 1, wherein the selenium-containing wastewater is introduced into the selenium-containing wastewater. Before removing the COD component from the selenium-containing wastewater, comprising sulfuric acid removal means for removing sulfate ions contained in the selenium-containing wastewater and inhibiting the removal of the selenium,
In the sulfuric acid removing means, calcium ions are added to the selenium-containing waste water, and the sulfate ions are precipitated and removed as gypsum which is calcium sulfate.
The COD component removing means oxidizes the dithionate ion, which is the COD component, to make a sulfate ion, and the sulfate ion is added by the sulfuric acid removing means to form the gypsum by the calcium ion contained in the selenium-containing wastewater. The selenium-containing wastewater treatment system according to claim 1 or 2, wherein the selenium-containing wastewater treatment system is removed by precipitation. A COD component removal step for removing a COD component that inhibits the removal of selenium from the selenium-containing wastewater; and reduction of the selenium contained in the selenium-containing wastewater after the removal of the COD component with iron, A selenium removal step of precipitating and removing the selenium contained in the selenium-containing wastewater together with the iron sludge oxidized during the reduction,
In the COD component removal step, a heating oxidation step of oxidizing the COD component by heating the selenium-containing wastewater with an acid is performed,
Circulating the iron between the selenium removal step and the COD component removal step by putting a part of the sludge generated in the selenium removal step into the selenium-containing wastewater in the COD component removal step. A method for treating selenium-containing wastewater. In the COD component removal step, after the heating oxidation step, a COD component removal main step is performed in which hydrogen peroxide and iron sulfate are added to the selenium-containing wastewater to oxidize the COD component, and the sludge is heated and oxidized. The method for treating selenium-containing wastewater according to claim 4, wherein the selenium-containing wastewater is introduced into the selenium-containing wastewater in a step. Before removing the COD component from the selenium-containing wastewater, a sulfuric acid removal step is performed to remove sulfate ions that are contained in the selenium-containing wastewater and inhibit the removal of the selenium,
In the sulfuric acid removal step, calcium ions are added to the selenium-containing waste water, and the sulfate ions are precipitated and removed as gypsum which is calcium sulfate.
In the COD component removal step, the dithionate ion, which is the COD component, is oxidized into sulfate ions, and the sulfate ions are added in the sulfuric acid removal step, and the calcium ions contained in the selenium-containing wastewater serve as the gypsum. The method for treating selenium-containing wastewater according to claim 4 or 5, wherein the selenium-containing wastewater is removed by precipitation.

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