JP6789779B2 - Wastewater treatment method, wastewater treatment system and coal gasification combined cycle equipment equipped with it - Google Patents

Description

The present invention relates to a wastewater treatment method, a wastewater treatment system, and an integrated coal gasification power generation facility including the wastewater treatment system.

Coal-fueled power generation includes conventional coal-fired power generation and integrated coal gasification power generation developed to improve coal power generation efficiency. In coal-fired power generation, coal is burned in an oxidizing atmosphere, and steam generated by the heat of combustion is used for power generation. In integrated coal gasification power generation, coal is steamed and burned under low oxygen conditions to cause a pyrolysis reaction to generate fuel gas, and the fuel gas is used for power generation.

As described above, the reaction conditions of coal differ between coal-fired power generation and integrated coal gasification power generation. It contains various harmful components contained in wastewater discharged from coal-fueled power generation facilities. The difference in the reaction conditions of coal affects the composition and form of harmful components contained in the wastewater discharged from each power generation facility. Wastewater with different compositions or forms of harmful components should be treated in a manner suitable for each. Patent Document 1 discloses a method for treating wastewater generated in a cleaning step of exhaust gas discharged from a coal gasification furnace.

Japanese Unexamined Patent Publication No. 2005-224771

The main water quality items to be treated for wastewater discharged from coal gasification combined cycle are COD component, selenium (Se), fluorine (F), heavy metals, SS (suspended solid), cyanide, thiocyanate (SCN), ammonia (NH). ₃ ), nitric acid (HNO ₃ ).

In Patent Document 1, an oxidizing agent is injected into wastewater, the pH is kept alkaline, and then an acid is added to decompose the COD component derived from thiosulfuric acid. After that, an oxidizing agent is added to the wastewater and irradiated with ultraviolet rays to decompose the COD component derived from formic acid.

When thiocyanate is contained in the wastewater, thiocyanate is changed to cyanate in the step of injecting an oxidizing agent as described above and changing the pH to alkali or acid. Cyan is decomposed by the subsequent addition of an oxidizing agent and irradiation with ultraviolet rays, but if the concentration of cyan is high, there is a concern that cyan that cannot be completely decomposed will remain and exceed the wastewater standard.

Further, the treatment method described in Patent Document 1 cannot remove nitrogen compounds such as nitric acid when the wastewater contains them. Further, Patent Document 1 does not describe the treatment of selenium.

The coal-fired power, because the coal is combusted in an oxidizing atmosphere, selenium contained in the waste water +4 selenium (selenite ^{_{ions, SeO 3 2-, Se (IV}} )) or +6 valent selenium (Selenium acid _ion, ^SeO 4 _2-, present as Se (VI)).

Meanwhile, in the coal gasification power generation, for gasifying coal in a reducing atmosphere, selenium is not present as +6, +4 lower ionic form than (seleno sulfate ions ^{_{(SeSO 3 2-, Se (II}} ) , etc. )) But it also exists.

Since the reaction between carbon (C) constituting coal and nitrogen (N) in the environment is promoted in a reducing atmosphere, cyanide is present in the wastewater from coal gasification combined cycle. Part of cyan is dissolved in the wastewater in the form of selenociic acid ions (SeCN ⁻ , Se (0)). Selenocinate ions are present in concentrations that require wastewater treatment.

The present inventors are studying a treatment method for removing selenium by coagulating and precipitating after adding an oxidizing agent to wastewater to change the selenium valence to +4 valence, and the present inventors have investigated total selenium (T-) in raw water. It was found that when the Se) concentration increases, a certain amount of +6 selenium is produced as a by-product when selenium is oxidized to +4 valence. Since + hexavalent selenium cannot be removed by coagulation precipitation, the total selenium concentration in the treated water increases as the by-product amount of + hexavalent selenium increases.

The present invention has been made in view of such circumstances, and is included in wastewater from a coal gasification power generation facility in wastewater treatment for removing selenium from wastewater by changing the selenium valence to +4 value. An object of the present invention is to provide a wastewater treatment method and a wastewater treatment system for efficiently and stably removing harmful components, particularly selenium compounds, nitrogen compounds and cyanide compounds.

In order to solve the above problems, the wastewater treatment method, the wastewater treatment system and the coal gasification power generation facility provided with the wastewater treatment method of the present invention adopt the following means.

In the present invention, acid is added to the wastewater of equipment for gasifying fuel containing selenium in a reducing atmosphere to make acidic wastewater, and then an oxidizing agent is added to the acidic wastewater to oxidize selenium and decompose cyanide. After the chemical oxidation step, the first coagulation sedimentation step of adding a coagulant to the wastewater to form agglomerates and precipitate, and the first coagulation sedimentation step, the wastewater is aerobic. After the first biological treatment step of contacting with activated sludge under the conditions and the first biological treatment step, the wastewater is brought into contact with activated sludge containing anaerobic microorganisms under anaerobic conditions in which organic substances are present to +6 in the wastewater. a second biological treatment step of Ru is reduced valence selenium tetravalent selenium, after said second biological treatment step, and a third biological treatment step of contacting the activated sludge under aerobic conditions the wastewater, the second organism Provided is a wastewater treatment method comprising a second coagulation-sedimentation step of adding a coagulant to the wastewater to form agglomerates and settling the wastewater after the treatment step.

In the chemical oxidation step, by adding an oxidizing agent to the wastewater, selenium is oxidized and the valence is changed to +4 or higher. More than 50% selenium is oxidized to +4 valence. Selenium may be oxidized up to 50% + 6 valence. The +4 valent selenium is precipitated and removed in the first coagulation precipitation step in the subsequent stage. The selenium oxidized to +6 valence is reduced in the second biological treatment step in the subsequent stage to become +4 valent selenium or 0 valent selenium, and is further precipitated and removed in the second coagulation precipitation step in the subsequent stage.

When the concentration of selenium in the wastewater is high, the amount of oxidant required to oxidize selenium also increases. As the amount of oxidant added increases, the amount of +6 selenium increases as shown in Table 1.

It is difficult to completely oxidize selenociic acid while suppressing the production of +6 valent selenium. +6 valent selenium is difficult to remove by coagulation precipitation. In the above invention, a sufficient amount of an oxidizing agent is added to almost completely oxidize the selenium in the waste water, and the +6 valent selenium produced at that time can be removed in the second biological treatment step and the second aggregation step. ..

In the chemical oxidation step, by adding an oxidizing agent after making acidic wastewater, selenocyanate ions can be separated into selenium (Se) and cyanide (CN), and the separated cyanide can be decomposed. The decomposed cyan becomes carbon dioxide and nitrogen and is removed from the wastewater. Cyan is also removed in the subsequent first biological treatment step. By removing cyan in two steps in this way, it is possible to reduce the cyan concentration to a concentration that can be discharged even in wastewater having a high cyan concentration.

In the first coagulation-precipitation step, SS, arsenic, fluorine and the like can also be removed in parallel.

In the first biological treatment step, the BOD component, COD component, cyanide, thiosian, hexane extractant, benzene and phenol can be decomposed and removed by aerobic microorganisms in activated sludge. In the first biological treatment step, ammonia is oxidized to nitrate nitrogen by aerobic microorganisms in activated sludge.

In the second biological treatment step, nitrate nitrogen can be reduced to nitrogen gas and decomposed and removed by the action of anaerobic microorganisms in activated sludge. In the second biological treatment step, thiosulfuric acid, which is one of the COD components, can be converted into sulfuric acid to be detoxified. Further, as described above, in the second biological treatment step, selenium can be reduced by the action of anaerobic microorganisms in the activated sludge. Selenium is mainly reduced from +6 valence to +4 valence.

In the third biological treatment step, COD components in wastewater can be decomposed and removed by oxidation by aerobic microorganisms in activated sludge. Here, what is mainly removed is the COD component derived from the organic substance in the second biological treatment step. Oxidation in the third biological treatment step has a weak oxidizing power, so + 6-valent selenium is not produced.

In the second coagulation-precipitation step, as described above, selenium reduced to +4 valence or 0 valence in the second biological treatment step can be precipitated and removed.

In the first biological treatment step to the second coagulation-precipitation step, not only selenium but also other components (BOD component, COD component, cyanide, thiocyanate, etc.) can be removed. Thiosulfuric acid and formic acid are trace components in wastewater. In Patent Document 1, ultraviolet irradiation or the like is carried out for the treatment of trace components, but in the present invention, the treatment can be performed in parallel with the removal of selenium. As a result, the equipment cost and the operating cost can be reduced as compared with the physicochemical treatment as in Patent Document 1.

In one aspect of the above invention, salt iron may be added to the wastewater in the chemical oxidation step. Thereby, the oxidation of selenium can be promoted.

In one aspect of the above invention, the second biological treatment step is after the nitric acid removing step of bringing the wastewater into contact with activated sludge to remove nitric acid under anaerobic conditions in which the first organic substance is present, and the nitric acid removing step. It is preferable to include a selenium reduction step of bringing the wastewater into contact with activated sludge to reduce selenium under anaerobic conditions in which a second organic substance is present.

When reducing +6 selenium in living organisms, nitric acid is an inhibitory factor. Therefore, when nitric acid is contained in the wastewater (for example, 20 mg / L or more), the second biological treatment step may be divided into a nitric acid removal step and a selenium reduction step. In the nitric acid removal step, a first organic substance suitable for reacting with nitrate nitrogen can be selected. In the selenium reduction step, a second organic substance suitable for selenium reduction can be selected. Anaerobic microorganisms in activated sludge consume oxygen in the order of oxygen, nitrate nitrogen, and selenium oxide. By carrying out the nitric acid removing step and the selenium reducing step in the above order, the decomposition efficiency of nitrate nitrogen and the reducing efficiency of selenium can be improved.

In one aspect of the above invention, an activated carbon treatment step of bringing the wastewater into contact with activated carbon may be further provided before the first biological treatment step.

By bringing the wastewater into contact with activated carbon, harmful substances that reduce biological activity are detoxified. COD components (hexane extractants), benzene, phenol, etc. can be removed. By removing the COD component before the biological treatment step, the efficiency of wastewater treatment in the subsequent stage can be improved.

Further, the present invention includes a first introduction route for guiding the wastewater of equipment for gasifying a fuel containing selenium in a reducing atmosphere, an oxidant addition section for adding an oxidant to the wastewater guided by the first introduction route, and an activated sludge. possess an acid addition section for adding an acid, with oxidizing the selenium in the drainage guided by said first introduction path, leading the chemical oxidation tank capable of degrading cyanide, the waste water via the chemical oxidation vessel A first coagulation sedimentation tank having a second introduction route and a coagulant addition portion for adding a coagulant to the wastewater guided by the second introduction route, and a first coagulation sedimentation tank that guides wastewater via the first coagulation sedimentation tank. It has 3 introduction paths and a first oxygen supply unit that supplies a fluid containing oxygen, activated sludge is housed therein, and the wastewater guided from the third introduction path is activated under aerobic conditions. It has a first biological treatment tank that is brought into contact with sludge and a fourth introduction route that guides wastewater via the first biological treatment tank, and the wastewater that is introduced from the fourth introduction route under anaerobic conditions inside . Activated sludge containing an anaerobic microorganism capable of reducing +6-valent selenium to +4-valent selenium is contained, and the wastewater derived from the fourth introduction route is brought into contact with the activated sludge under anaerobic conditions in which organic substances are present . It has a biological treatment tank, a fifth introduction path for guiding wastewater via the second biological treatment tank, and a second oxygen supply unit for supplying a fluid containing oxygen, and activated sludge is housed therein. A third biological treatment tank in which the wastewater derived from the fifth introduction route is brought into contact with the activated sludge under aerobic conditions, a sixth introduction route for guiding the wastewater via the second biological treatment tank, and the first. (6) Provided is a wastewater treatment system including a second coagulation sedimentation tank in which a coagulant is added to the wastewater guided by an introduction route to form agglomerates and settle.

In one aspect of the invention, the chemical oxidation tank may be provided with a salt of iron added portion of adding Shiotetsu to the drainage.

In one aspect of the above invention, the second biological treatment tank includes a nitric acid removing tank containing activated sludge under anaerobic conditions in which the first organic matter is present, and activated sludge under anaerobic conditions in which the second organic matter is present. It is preferable to have a selenium reduction tank containing sludge.

In one aspect of the above invention, an activated carbon treatment tank installed on the upstream side of the first biological treatment tank and containing activated carbon may be further provided.

The present invention also provides an integrated coal gasification power generation facility equipped with the wastewater treatment system of the above invention.

According to the present invention, after the chemical oxidation step, a step capable of removing +4 valent selenium (first coagulation-precipitation step) and a step capable of removing +6-valent selenium (second biological treatment step and second coagulation-precipitation step). By providing the above, the total concentration of selenium in the treated water is reduced to a desired value. Further, by carrying out the first biological treatment to the third biological treatment, the COD component, cyanide, thiocyanate, and ammonia can be removed. According to the present invention, harmful components contained in wastewater from coal gasification power generation facilities can be removed more efficiently and stably.

It is a schematic block diagram of the wastewater treatment system which concerns on 1st Embodiment. It is a treatment flow diagram of the wastewater treatment method which concerns on 1st Embodiment. It is a treatment flow diagram of the wastewater treatment method which concerns on 2nd Embodiment.

The present invention targets wastewater discharged from equipment that gasifies fuel containing selenium (Se) in a reducing atmosphere. The fuel containing selenium is, for example, coal. Coal contains a large amount of selenium depending on the place of origin. The facility for gasifying fuel containing selenium in a reducing atmosphere is, for example, a coal gasification power generation facility.

In a coal gasification facility, coal is steamed and burned in a reducing atmosphere to generate fuel gas. Exhaust from coal gasification combined cycle contains selenium and cyanide (CN). Selenium and cyan are dissolved in the wastewater when the gasified coal comes into contact with the wastewater.

In the waste water, selenium, selenite ion ^{_{(SeO 3 2-, Se (IV}} )) or +4 lower valence than divalent selenocyanate ^{(SeCN -, Se (0)} ), seleno sulfate ion (SeSO ₃ ^{2 -} , Se (-II)) exists.

In the effluent, cyanogen is cyanide (CN ⁻ ), selenocyanate ion (SeCN ⁻ , Se (0)), cyanogen chloride (CNCl ⁻ ), ferrocyanide compound ion ([Fe (CN) ₆ ] ^3- ), It exists in the form of ferrocyanide ion ([Fe (CN) ₆ ] ^4- ), thiocyan.

In the wastewater, there are other suspended substances (SS), arsenic (As), fluorine (F), mercury (Hg), chromium (Cr), BOD (Biochemical Oxygen Demand) component, COD component, and ammonia (NH). ₃ ) etc. are included. The COD component is a persistent substance in chemical oxidation treatment. Here, the persistent substance is thiosulfuric acid, methanol, acetic acid, formic acid, benzene, benzoic acid, phenol, chlorophenol, chloroaniline, aminobenzoic acid, hydantin and the like.

[First Embodiment]
FIG. 1 shows a schematic configuration diagram of a wastewater treatment system according to this embodiment. The wastewater treatment system 1 includes a chemical oxidation tank 2, a first coagulation sedimentation tank 3, a first biological treatment tank 4, a second biological treatment tank 5, a third biological treatment tank 6, a second coagulation sedimentation tank 7, and a control unit 8. I have.

The chemical oxidation tank 2 has a first introduction path 9 and an oxidant addition section 10. The first introduction path 9 is connected to the upstream side of the chemical oxidation tank 2, and the waste water W from the coal gasification power generation facility can be guided into the chemical oxidation tank 2. The oxidizing agent adding unit 10 is connected to the chemical oxidizing tank 2 so that the oxidizing agent can be added into the chemical oxidizing tank 2.

A salt iron addition unit 11, an acid addition unit 12, a pH measuring instrument (not shown), and a redox potential measuring instrument 13 are further connected to the chemical oxidation tank 2. The salt iron addition unit 11 can add salt iron to the waste water guided into the chemical oxidation tank 2. The acid addition unit 12 can add acid to the waste water guided into the chemical oxidation tank 2.

The first coagulation sedimentation tank 3 has a second introduction path 14 that guides wastewater that has passed through the chemical oxidation tank 2 into the first coagulation sedimentation tank 3, and a coagulant addition section 15. In FIG. 1, one end of the second introduction path 14 is connected to the chemical oxidation tank 2 and the other end is connected to the first coagulation sedimentation tank 3. The coagulant addition unit 15 is connected to the first coagulation sedimentation tank 3 so that the coagulant can be added into the first coagulation sedimentation tank 3. In FIG. 1, a plurality of coagulant addition portions 15a and 15b are provided so that different types of coagulants can be added to the wastewater in the first coagulation sedimentation tank 3 at arbitrary timings, but the coagulation is not limited to this. The agent addition unit 15 may be one.

A first pH adjuster supply unit 16 is further connected to the first coagulation sedimentation tank 3. The first pH adjuster supply unit 16 can supply alkali to the wastewater led to the first coagulation sedimentation tank 3 via the chemical oxidation tank 2.

The first biological treatment tank 4 has a third introduction path 17 and a first oxygen supply unit 18 that guide the wastewater that has passed through the first coagulation sedimentation tank 3 into the first biological treatment tank 4. In FIG. 1, one end of the third introduction path 17 is connected to the first coagulation sedimentation tank 3 and the other end is connected to the first biological treatment tank 4. The first oxygen supply unit 18 is connected to the first biological treatment tank 4 so as to supply a fluid containing oxygen and keep the inside of the first biological treatment tank 4 under aerobic conditions. Fluids containing oxygen include oxygen and air.

Activated sludge is housed in the first biological treatment tank 4. The first biological treatment tank 4 is preferably a fixed bed filled with a carrier to which activated sludge is attached. As the adherent carrier, sand, activated carbon, anthracite (anthracite), a plastic uneven plate, or the like can be used.

A pH adjusting unit 19 is further connected to the first biological treatment tank 4. The pH adjusting unit 19 can add an alkali or an acid to the first biological treatment tank 4 to adjust the pH in the first biological treatment tank 4 to 5 to 10, preferably 6 to 9.

The second biological treatment tank 5 includes a fourth introduction path 20, a nitric acid removal tank 21, and a selenium reduction tank 22 that guide the wastewater that has passed through the first biological treatment tank 4 to the second biological treatment tank 5.

In FIG. 1, one end of the fourth introduction path 20 is connected to the first biological treatment tank 4 and the other end is connected to the nitric acid removal tank 21. Activated sludge is contained in the nitric acid removing tank 21 under anaerobic conditions. The nitric acid removing tank 21 is preferably a fixed floor filled with a carrier to which activated sludge is attached. As the adherent carrier, sand, activated carbon, anthracite (anthracite), plastic particles, polyvinyl alcohol and polyethylene glycol can be used. The first organic matter adding portion 23 is connected to the nitric acid removing tank 21. The first organic substance addition unit 23 can add the first organic substance into the nitric acid removing tank 21.

The selenium reduction tank 22 is connected to the downstream side of the nitric acid removing tank 21 and includes a receiving route 24 for receiving wastewater via the nitric acid removing tank 21. Activated sludge is contained in the selenium reduction tank 22 under anaerobic conditions. The selenium reduction tank 22 is preferably a fixed bed filled with a carrier to which activated sludge is attached. A second organic substance addition section 25 is connected to the selenium reduction tank 22. The second organic substance addition unit 25 can add the second organic substance to the selenium reduction tank 22.

When the nitric acid removing tank 21 and the selenium reducing tank 22 have a fixed floor, a pipe for supplying backwash water for backwashing the fixed floor in the tank is provided at the bottom of each of the nitric acid removing tank 21 and the selenium reducing tank 22. (Not shown) should be installed. In that case, a pipe (not shown) for discharging the backwash drainage is provided on the ceiling of each of the nitric acid removing tank 21 and the selenium reducing tank 22.

The third biological treatment tank 6 has a fifth introduction path 26 that guides wastewater that has passed through the second biological treatment tank 5 into the third biological treatment tank 6, and a second oxygen supply unit 27. In FIG. 1, one end of the fifth introduction path 26 is connected to the selenium reduction tank 22 and the other end is connected to the third biological treatment tank 6. The second oxygen supply unit 27 is connected to the third biological treatment tank 6 so as to supply a fluid containing oxygen and maintain the inside of the third biological treatment tank 6 under aerobic conditions.

Activated sludge is housed in the third biological treatment tank 6. The third biological treatment tank 6 is preferably a fixed bed filled with a carrier to which activated sludge is attached. As the adherent carrier, sand, activated carbon, anthracite (anthracite), a plastic uneven plate, or the like can be used.

The second coagulation sedimentation tank 7 has a sixth introduction path 28 that guides the wastewater that has passed through the second biological treatment tank 5 to the second coagulation sedimentation tank 7, and a coagulant addition section 29. In FIG. 1, one end of the sixth introduction path 28 is connected to the third biological treatment tank 6 and the other end is connected to the second coagulation sedimentation tank 7. The coagulant addition section 29 is connected to the second coagulation sedimentation tank 7 so that the coagulant can be added into the second coagulation sedimentation tank 7. In FIG. 1, a plurality of coagulant addition portions 29a and 29b are provided so that different types of coagulants can be added to the wastewater in the second coagulation sedimentation tank 7 at arbitrary timings, but the coagulation is not limited to this. The agent addition unit 29 may be one.

A second pH adjuster supply unit 30 is further connected to the second coagulation sedimentation tank 7. The second pH adjuster supply unit 30 can supply alkali to the wastewater guided to the second coagulation sedimentation tank via the second biological treatment tank 5.

The arrangement order of the third biological treatment tank 6 and the second coagulation sedimentation tank 7 may be reversed.

The control unit 8 can control each configuration of the wastewater treatment system.
For example, the control unit 8 can receive the calculation result of the pH measuring instrument and adjust the amount of acid added from the acid addition unit 12 so that the wastewater contained in the chemical oxidation tank 2 has a predetermined pH.

For example, the control unit 8 receives the measurement result of the redox potential measuring device 13 and determines whether or not the redox potential of the waste water contained in the chemical oxidation tank 2 is within a predetermined range, and the result is determined. Based on this, the amount of the oxidizing agent added from the oxidizing agent adding unit 10 can be adjusted so that the redox potential of the wastewater contained in the chemical oxidation tank 2 is within a predetermined range. The control unit 8 can stop the addition of the oxidizing agent when the redox potential of the wastewater contained in the chemical oxidation tank 2 exceeds a predetermined range.

The control unit 8 is composed of, for example, a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), a computer-readable storage medium, and the like. As an example, a series of processes for realizing various functions are stored in a storage medium or the like in the form of a program, and the CPU reads this program into a RAM or the like to execute information processing / arithmetic processing. As a result, various functions are realized. The program is installed in a ROM or other storage medium in advance, is provided in a state of being stored in a computer-readable storage medium, or is distributed via a wired or wireless communication means. Etc. may be applied. Computer-readable storage media include magnetic disks, magneto-optical disks, CD-ROMs, DVD-ROMs, semiconductor memories, and the like.

Next, the wastewater treatment method according to this embodiment will be described. The wastewater treatment method according to the present embodiment is suitable for treating wastewater having a total selenium (T-Se) concentration of 100 mg / L or less, preferably 50 mg / L or less.

The wastewater treatment method according to the present embodiment includes a chemical oxidation step, a first coagulation sedimentation step, a first biological treatment step, a second biological treatment step, a third biological treatment step, a second coagulation sedimentation step and a discharge step. FIG. 2 shows a treatment flow chart of the wastewater treatment method according to the present embodiment.

(Chemical oxidation process)
First, acid is added to the wastewater to obtain acidic wastewater (S1). The acid is sulfuric acid, hydrochloric acid, nitric acid or the like. The pH of the acidic wastewater is adjusted to less than 1-7, preferably 3-6.

An oxidizing agent and salt iron are added to the acidic wastewater and stirred (S2). The order of addition is not particularly limited.

Oxidizing agent, hydrogen peroxide _{(H 2 O} 2), hypochlorous acid (ClO ^-), permanganate (MnO ₄ ^-), peroxomonosulfuric acid _(SO ^{5 2-),} peroxodisulfate _(S 2 _{O 8} It is selected from ^2-) or ozone _{(O 3).} The oxidizing agent is particularly preferably hydrogen peroxide. By adding an oxidizing agent to the acidic wastewater, the selenium of the selenociate ion is oxidized to +4 or more valent selenium (selenite ion and selenate ion), and the cyanion ion is separated from the selenociate ion. The separated cyan ions are finally decomposed into nitrogen (N ₂ ) and carbon dioxide (CO ₂ ) and removed from the wastewater.

The amount of the oxidizing agent added is appropriately set according to the concentration of cyanide or selenium in the wastewater.

When hydrogen peroxide is used as the oxidizing agent, the amount of hydrogen peroxide added is 20 mg / L or more, preferably 40 mg / L or more and 400 mg / L or less. If the amount of hydrogen peroxide added is too small, the oxidation of selenium does not proceed sufficiently and selenocyanate ions remain. On the other hand, even if hydrogen peroxide is added in excess, the redox potential of the acidic wastewater cannot be changed to match the amount of hydrogen peroxide added, and the cyanide decomposition effect is not so large.

When sodium hypochlorite is used as the oxidizing agent, the amount of sodium hypochlorite added is 200 mg / L or more, preferably 200 mg / L or more and 500 mg / L or less. If the amount of sodium hypochlorite added is too small, the oxidation of selenium does not proceed sufficiently and selenocyanate ions remain. On the other hand, if sodium hypochlorite is added excessively, the redox potential of acidic wastewater becomes too high. If the redox potential of acidic wastewater is set too high, the oxidation reaction of selenium is promoted and hexavalent selenium (selenate ion) increases. It is preferable not to increase the amount of selenate ion more than necessary.

The salt iron is ferric chloride, polyiron and the like. Salt iron promotes the oxidation of selenium, a selenocyanate ion, in acidic wastewater. The amount of salt iron added shall be such that the selenocyanate ion can be completely oxidized. For example, when ferric chloride is used as the salt iron, the amount of ferric chloride added is 10 mg / L or more and 1000 mg / L or less, preferably 50 mg / L or more and 400 mg / L or less as iron (Fe).

In the chemical oxidation step, the redox potential of the acidic wastewater may be controlled to a value within a predetermined range (S3). The redox potential of the acidic wastewater may be controlled so that the acidic wastewater becomes a solution having an oxidative tendency. Specifically, the redox potential of acidic wastewater is 200 mV or more and 1500 mV or less, preferably 200 mV or more and 1000 mV or less. If the redox potential of the acidic wastewater is too low, the oxidation of selenium does not proceed sufficiently and selenium cyanate ions remain, and the removal rate of selenium and cyanide decreases. If the redox potential of acidic wastewater is too high, hexavalent selenium (selenate ion) increases.

(First coagulation sedimentation step)
Alkali is added to the acidic wastewater that has undergone the chemical oxidation step and stirred to obtain neutral wastewater (S4). The alkali is sodium hydroxide (NaOH), slaked lime (Ca (OH) ₂ ), or the like.

A flocculant is added to the neutral wastewater to form agglomerates (S5). Then, it is allowed to stand for a predetermined time, the agglomerates are precipitated, and the supernatant is separated (S6).

The flocculant is an inorganic flocculant, or an inorganic flocculant and a polymer flocculant. When an inorganic flocculant and a polymer flocculant are used, the polymer flocculant is added as a coagulation aid after the inorganic flocculant is added.

The inorganic flocculant is polyaluminum chloride salt (PAC), aluminum sulfate, ferric chloride, or the like. The polymer flocculant is an anionic polymer flocculant, a nonionic polymer flocculant, or the like. The anionic polymer flocculant is, for example, Hishiflock H-305 (manufactured by Mitsubishi Hitachi Power Systems Environmental Solutions Co., Ltd.) or Hishiflock HA-510 (manufactured by Mitsubishi Hitachi Power Systems Environmental Solutions Co., Ltd.).

The formed aggregate contains tetravalent selenium (selenite ion). By removing the agglomerates, tetravalent selenium can be removed from the wastewater. It is difficult to remove +6 selenium with a flocculant. +6-valent selenium may remain in the wastewater after the first coagulation-precipitation step.

(S4) to (S6) may be carried out a plurality of times as needed. In FIG. 2, the first coagulation-precipitation step is repeated twice.

According to (S4) to (S6), other substances such as suspended substances (SS), arsenic (As) and fluorine (F) are also removed in the step of aggregating and separating tetravalent selenium.

The first coagulation-precipitation step can be carried out in parallel with the removal treatment of heavy metals such as mercury. In that case, a chelating agent is added to the neutral wastewater. In FIG. 2, a chelating agent is added to the neutral wastewater in the second first coagulation / precipitation step B. The chelating agent is, for example, Epoflock (registered trademark) L-1 (manufactured by Miyoshi Oil & Fat Co., Ltd.).

(1st biological treatment process)
The supernatant (supernatant drainage) separated in the first coagulation sedimentation step is brought into contact with activated sludge under aerobic conditions (S7). Here, a pH adjuster is added to the supernatant drainage to adjust the pH to 5-10, preferably 6-9. The pH regulator is sulfuric acid (H ₂ SO ₄ ) or sodium hydroxide (NaOH) and the like. Activated sludge is aerated by supplying a fluid containing oxygen with an air diffuser or the like to maintain aerobic conditions.

In the first biological treatment process, ammonia in the supernatant waste water by the action of nitrifying bacteria in the activated sludge is oxidized nitrate nitrogen (NO 3 ^_-) and nitrite nitrogen (NO 2 ^_-) is generated. As a result, ammonia can be removed from the wastewater. The BOD component, COD component, cyanide and thiocyanate in the supernatant wastewater are decomposed, oxidized and removed by aerobic microorganisms in the activated sludge. COD components include those derived from thiosulfuric acid and formic acid.

(Second biological treatment process)
In the second biological treatment step, the nitric acid removing step (S8) and the selenium reducing step (S9) are carried out in order.

In the nitric acid removal step, the supernatant wastewater (primary aerobic treated water) after the first biological treatment step is brought into contact with activated sludge under anaerobic conditions in which the first organic matter is present. The first organic substance is methanol (CH ₃ OH) or the like.

In the nitric acid removal step, the nitric acid nitrogen in the primary aerobic treated water reacts with the first organic matter and is decomposed into carbon dioxide, nitrogen and the like by the action of denitrifying bacteria in the activated sludge. As a result, nitrate nitrogen can be removed from the primary aerobic treated water.

In the nitric acid removal step, it is advisable to adopt a fixed bed type activated sludge. Unlike the floating method, the fixed bed method creates a concentration gradient of nitrate nitrogen with respect to the water flow direction in the fixed bed, and the denitrification action according to the nitrate nitrogen concentration in the wastewater is stable. Get up. For example, nitrate nitrogen load subjected to nitrate removal step is 5 kg / day from filler volume 1 m ³ per 0.5, if 2-3 (weight ratio) relative to the amount of methanol added nitrate nitrogen content in the waste water The nitrate nitrogen and nitrite nitrogen concentrations of the supernatant wastewater (primary anaerobic treated water) after the nitric acid removal step are 20 mg / l or less.

If thiosulfuric acid remains in the primary aerobic treated water, it is removed in the nitric acid removing step. Specifically, thiosulfuric acid is oxidized to sulfuric acid (SO ₄ ) by the action of denitrifying sulfur bacteria in activated sludge and detoxified.

In the selenium reduction step, the supernatant wastewater (primary anaerobic treated water) after the nitric acid removal step is brought into contact with activated sludge under anaerobic conditions in which a second organic substance is present. The second organic matter is ethanol, methanol, glycerol, lactic acid and its salt, acetic acid and its salt pyruvate and its salt, fumaric acid and its salt, malic acid and its salt, succinic acid and its salt, glutamate and its salt, One or more selected from citric acid and its salts, glucose, fructose and shoe cloth can be used.

In the selenium reduction step, by bringing primary anaerobic treated water having a small amount of nitrogen oxides into contact with activated sludge under anaerobic conditions, microorganisms consume oxygen of selenium oxide, and as a result, selenium is reduced. That is, selenium is reduced by the action of selenium-reducing bacteria. The selenium in the primary anaerobic treated water mainly exists in a +6 valent state. In the selenium reduction step, +6 selenium is reduced to tetravalent selenium or elemental selenium.

The amount of the second organic matter added is 0.3 to 50 (weight ratio of each organic matter) to the amount of selenium if the selenium concentration of the primary anaerobic treated water is about 5 mg / l or less (as hexavalent selenium). ), Preferably 2 to 40 (weight ratio of each organic substance) so that the selenium concentration of the primary anaerobic treated water (secondary anaerobic treated water) after the selenium reduction step is less than 0.1 mg / l. Can be done.

(Third biological treatment process)
The primary anaerobic treated water (secondary anaerobic treated water) after the selenium reduction step is brought into contact with activated sludge under aerobic conditions (S10). Activated sludge is aerated by supplying a fluid containing oxygen with an air diffuser or the like to maintain aerobic conditions.

In the third biological treatment step, organic substances (methanol, lactic acid, etc.) remaining in the secondary anaerobic treated water are decomposed into carbon dioxide and water by aerobic microorganisms in activated sludge. +4 valent selenium is hardly oxidized by aeration and remains as it is.

(Second coagulation sedimentation step)
Alkali is added to the secondary anaerobic treated water (secondary aerobic treated water) after the third biological treatment step and stirred to obtain neutral wastewater (S11). The alkali is sodium hydroxide (NaOH), potassium hydroxide (KOH) or slaked lime (Ca (OH) ₂ ) and the like.

A flocculant is added to the neutral wastewater to form agglomerates (S12). Then, it is allowed to stand for a predetermined time, the agglomerates are precipitated, and the supernatant is separated (S13).

As the coagulant, the same coagulant used in the first coagulation-precipitation step can be used. (S11) to (S13) may be carried out a plurality of times as needed.

In the second coagulation / precipitation step, the remaining +4 valent selenium can be coagulated / separated.

The second coagulation / precipitation step may be carried out before the third biological treatment step.

(Discharge process)
A pH adjuster is added to the supernatant (supernatant drainage) separated in the second coagulation-precipitation step to neutralize it. After that, it is released to the outside of the system.

[Second Embodiment]
The wastewater treatment system of the present embodiment has the same configuration as that of the first embodiment except that it includes an activated carbon treatment tank. The wastewater treatment method of the present embodiment is the same as that of the first embodiment except that it includes an activated carbon treatment step. FIG. 3 shows a treatment flow chart of the wastewater treatment method according to the present embodiment.

Activated carbon is housed inside the activated carbon treatment tank. The activated carbon treatment tank is installed on the upstream side of the first biological treatment tank. The activated carbon treatment tank of the present embodiment includes a seventh introduction route that guides the wastewater that has passed through the first coagulation sedimentation tank to the activated carbon treatment tank. One end of the seventh introduction route is connected to the upstream side of the activated carbon treatment tank, and the other end is connected to the downstream side of the first coagulation sedimentation tank.

In the present embodiment, the wastewater that has passed through the first coagulation sedimentation tank is subjected to the first biological treatment step after the activated carbon treatment step. In the activated carbon treatment step, the wastewater that has undergone the first coagulation sedimentation step is brought into contact with the activated carbon. Activated carbon also acts as a catalyst for detoxifying harmful substances that adsorb organic substances and reduce biological activity. As a result, hexane extract substances, benzene, phenol and the like can be efficiently removed.

1 Wastewater treatment system 2 Chemical oxidation tank 3 1st coagulation sedimentation tank 4 1st biological treatment tank 5 2nd biological treatment tank 6 3rd biological treatment tank 7 2nd coagulation sedimentation tank 8 Control unit 9 1st introduction route 10 Addition of oxidant Part 11 Salt iron addition part 12 Acid addition part 13 Oxidation-reduction potential measuring instrument 14 Second introduction path 15, 15a, 15b Aggregator addition part 16 First pH regulator supply part 17 Third introduction path 18 First oxygen supply part 19 pH Adjustment unit 20 4th introduction route 21 Nitrate removal tank 22 Selenium reduction tank 23 1st organic matter addition part 24 Acceptance route 25 2nd organic matter addition part 26 5th introduction route 27 2nd oxygen supply part 28 6th introduction route 29, 29a, 29b Coagulant addition unit 30 Second pH regulator supply unit

Claims (9)

After adding acid to the wastewater of equipment that gasifies fuel containing selenium in a reducing atmosphere to make acidic wastewater, an oxidizing agent is added to the acidic wastewater to oxidize selenium and a chemical oxidation step that decomposes cyanide. ,
After the chemical oxidation step, a first coagulation-precipitation step of adding a coagulant to the wastewater to form agglomerates and precipitate them.
After the first coagulation sedimentation step, a first biological treatment step of bringing the wastewater into contact with activated sludge under aerobic conditions,
Wherein after the first biological treatment process, +6 selenium tetravalent second biological treatment step of Ru is reduced to selenium in the said effluent is contacted with activated sludge containing anaerobic microorganisms under anaerobic conditions the presence of organic matter drainage When,
After the second biological treatment step, a third biological treatment step of bringing the wastewater into contact with activated sludge under aerobic conditions,
After the second biological treatment step, a second coagulation-precipitation step of adding a coagulant to the wastewater to form agglomerates and precipitate them.
Wastewater treatment method equipped with. The wastewater treatment method according to claim 1, wherein salt iron is added to the wastewater in the chemical oxidation step. The second biological treatment step is
A nitric acid removal step of contacting the wastewater with activated sludge to remove nitric acid under anaerobic conditions in which the first organic matter is present,
After the nitric acid removal step, a selenium reduction step of bringing the wastewater into contact with activated sludge to reduce selenium under anaerobic conditions in which a second organic substance is present,
The wastewater treatment method according to claim 1 or 2, which comprises. The wastewater treatment method according to any one of claims 1 to 3, further comprising an activated carbon treatment step of bringing the wastewater into contact with activated carbon before the first biological treatment step. A first introduction route that guides the wastewater of equipment that gasifies fuel containing selenium in a reducing atmosphere, an oxidizer addition part that adds an oxidant to the wastewater guided by the first introduction route, and an acid addition that adds an acid. possess a part, along with the oxidation of selenium in the drainage guided by said first introduction path, and chemical oxidation tank capable of degrading cyanide,
A first coagulation sedimentation tank having a second introduction path for guiding the wastewater via the chemical oxidation tank and a coagulant addition portion for adding a coagulant to the wastewater guided by the second introduction path.
It has a third introduction path that guides wastewater via the first coagulation sedimentation tank and a first oxygen supply section that supplies a fluid containing oxygen, and activated sludge is housed inside, and from the third introduction path. A first biological treatment tank in which the induced wastewater is brought into contact with the activated sludge under aerobic conditions ,
An anaerobic microorganism that has a fourth introduction route that guides wastewater via the first biological treatment tank and can reduce the +6 valent selenium of the wastewater derived from the fourth introduction route to +4 valent selenium under anaerobic conditions inside. A second biological treatment tank in which activated sludge containing the above is contained and the wastewater derived from the fourth introduction route is brought into contact with the activated sludge under anaerobic conditions in which organic substances are present .
It has a fifth introduction route that guides wastewater via the second biological treatment tank and a second oxygen supply unit that supplies a fluid containing oxygen, and activated sludge is housed inside, and from the fifth introduction route. A third biological treatment tank in which the guided wastewater is brought into contact with the activated sludge under aerobic conditions .
A sixth introduction route that guides wastewater via the second biological treatment tank, and a second coagulation sedimentation tank that adds a flocculant to the wastewater guided by the sixth introduction route to form agglomerates and precipitate them. ,
Wastewater treatment system equipped with. The wastewater treatment system according to claim 5, wherein the chemical oxidation tank includes a salt iron addition portion for adding salt iron to the waste water. The second biological treatment tank is
A nitric acid removal tank containing activated sludge under anaerobic conditions in which the first organic matter is present,
A selenium reduction tank containing activated sludge under anaerobic conditions in which a second organic matter is present,
5. The wastewater treatment system according to claim 5 or 6. The wastewater treatment system according to any one of claims 5 to 7, further comprising an activated carbon treatment tank installed on the upstream side of the first biological treatment tank and containing activated carbon inside. A coal gasification power generation facility including the wastewater treatment system according to any one of claims 5 to 8.

Images