JPH10277588A - Biological treatment of selenium-containing waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for the biological treatment of selenium- containing waste water capable of removing hexavalent selenium (Se<6+> ) and ammonia nitrogen from waste water containing them without exerting adverse effect on nitrification reaction in a biological nitrification and denitrification apparatus. SOLUTION: When waste water containing hexavalent selenium (Se<6+> ) and ammonia nitrogen is biologically treated, selenium is removed through a Se treatment apparatus 40 wherein waste water is passed through a biological nitrification and denitrification apparatus 50 to performs nitrification and denitrification under a condition not substantially advancing the reduction of hexavalent selenium (Se<6+> ) and hexavalent selenium (Se<6+> ) is reduced to tetravalent selenium (Se<4+> ) and metal selenium (Se<0> ) by the action of activated sludge and a reducing agent to be removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for treating wastewater containing hexavalent selenium and ammonia nitrogen, such as industrial wastewater discharged from a coal-fired thermal power plant or the like.

[0002]

2. Description of the Related Art In a thermal power plant, a large amount of water is used for desulfurization of gas generated by burning raw materials,
Discharge. In this effluent, substances such as trace metals in exhaust gas, EP (Electric Precipitator), and ammonia used in flue gas desulfurization / denitration equipment are dissolved. When discharging wastewater, the concentration of substances contained in the wastewater is regulated by the Water Pollution Control Law and the regulations of each local government, and it is necessary to perform wastewater treatment until the concentration falls below the regulated value. FIG. 5 shows a system flow of wastewater treatment in a thermal power plant. According to this system, the coagulation filtration tower 103, the ion exchange resin tower 104, the activated carbon adsorption tower 105, the biological nitrification denitrification apparatus 1
09 (nitrification tank 110, denitrification tank 111, re-aeration tank 112,
(Including blower 113 and sedimentation tank 114), coagulation and sedimentation equipment 115, etc. to separate and remove nitrogen (ammonia), metal salts, organic matter, etc. contained in the wastewater, and regulate each component. After the treatment is performed so as to be equal to or less than the value, the water is discharged through the treated water pit 116. In FIG. 5, 101 is a raw water adjustment tank, 102 is a raw water pump, 1
Reference numerals 06 to 108 are liquid feed pumps.

On the other hand, due to the recent tightening of wastewater treatment regulations, metal salts such as selenium (Se) have become newly regulated substances, and their treatment has been required. The selenium concentration in wastewater generated from coal-fired thermal power plants is about 2 pp
m (hexavalent selenium occupies most in normal combustion method)
Since the selenium drainage regulation value is 0.1 ppm or less, selenium treatment is indispensable, and measures must be taken. Selenium is contained in exhaust gas (including ash) and discharged when burning coal, and finally selenate (S) is contained in wastewater.
eO ₄ ²⁻ and SeO ₃ ²⁻ ) are dissolved. Among these selenates, tetravalent selenium (mainly in the form of SeO ₃ ^2- ) forms a hardly soluble metal salt with iron, calcium, etc., and is removed by physical means such as coagulation and precipitation with iron salts. For example, it can be removed in the coagulating sedimentation device 115. However, hexavalent selenium (mainly Se
O ₄ ^2- ) does not form sparingly soluble salts with metals, and the ion-exchange resin tower 104, the activated carbon adsorption tower 10
At present, there is no effective means for physicochemically removing hexavalent selenium in wastewater since adsorption at 5 does not occur.

The present inventors have been conducting various studies on such a method for treating selenium-containing wastewater. In the denitrification tank of the biological nitrification denitrification apparatus, reduction of some hexavalent selenium may be partially performed depending on the treatment conditions. Found that it is happening. Furthermore, it was found that the addition of an organic acid (or a salt thereof) such as acetic acid, which is considered to be easy for microorganisms to assimilate, to the denitrification tank promotes the reduction of hexavalent selenium. An outline of the biological nitrification and denitrification process will be described by taking as an example a case where a one-sludge type apparatus shown in FIG. 6 is used and sodium acetate having a strong reducing action is used as a reducing agent. The biological nitrification and denitrification apparatus 109 includes a nitrification tank 110, a denitrification tank 111, a re-aeration tank 112, and a sedimentation tank 114, and activated sludge is circulated in each tank in this order. The selenium-containing wastewater is sent to a nitrification tank 110 according to a conventional biological nitrification and denitrification process. Here, phosphoric acid and an alkaline agent are added from the phosphoric acid tank 121 and the alkaline tank 123 by the phosphoric acid pump 122 and the alkaline pump 124, and the pH is adjusted to 6.6 to 7.0 by the pH controller 127.
The ammonia in the wastewater is oxidized to nitrate ions by the action of nitrifying bacteria and sent to the denitrification tank 111. In the denitrification tank 111, sodium acetate as a hydrogen donor (reducing agent) is supplied from a reducing agent tank 125 via a reducing agent pump 126, and nitrate ions are reduced to nitrogen gas by the action of denitrifying bacteria. At the same time as the denitrification reaction, hexavalent selenium in the wastewater is reduced to tetravalent selenium or metal selenium (zero-valent selenium).

[0005] The wastewater from the denitrification tank 111 is sent to a re-aeration tank 112 to decompose excessively added sodium acetate, and the sodium acetate in the wastewater is aerobically decomposed to remove other organic matter in the wastewater. Removed with. Re-aeration tank 112
Is discharged into a settling tank 114 for separating activated sludge and treated water. Here, the activated sludge is settled by gravity sedimentation and separated from the treated water. One part of the activated sludge is removed as excess sludge, the remaining part is returned to the nitrification tank 110 by the return pump 120, and the treated water is sent to the subsequent process. Can be In FIG. 6, reference numeral 113 denotes a nitrification tank 110 and a re-aeration tank 11.
It is a blower that sends air to 2. Among the selenium contained in the treated water, selenium reduced to tetravalent or zero-valent can be appropriately removed in a later step by a method such as coagulation sedimentation. However, when unreduced hexavalent selenium remains, it is difficult to remove. is there.

The present inventors have focused on the selenium reduction effect in the denitrification tank of the above biological nitrification denitrification apparatus in the process of treating selenium-containing wastewater. When is recycled between the nitrification tank and the denitrification tank, if the tetravalent selenium or metal selenium generated by the reduction of hexavalent selenium in the denitrification tank is mixed into the nitrification tank along with the return of the sludge, It has been found that the nitrification reaction is inhibited and the nitrification performance may be reduced. The nitrification reaction is a reaction in which ammonia nitrogen is oxidized to nitric acid by a microorganism (nitrifying bacterium). In the flows of FIGS. 5 and 6, the following equation (1) performed in the nitrification tank 110 of the biological nitrification denitrification apparatus 109 is used. ) And (2).

## STR1 ## ^{_{NH 4 + +3/2 · O 2 →}} 2H + + NO 2 - + H 2 O (1) NO 2 - +1/2 · O 2 → NO 3 - (2)

[0007]

As described above, it is difficult to treat selenium, a new substance to be regulated, to a regulated value or less by a conventional biological nitrification and denitrification apparatus. In addition, selenium in the wastewater is a regulated substance and the days are short, so there has been little study on the method of treating selenium in the wastewater, and selenium and ammonia, like wastewater from coal-fired thermal power plants, have not been studied. No method has been found that can efficiently treat both selenium and ammonia nitrogen from wastewater containing nitrogen. The present invention has been made in view of the above circumstances of the prior art, and has been intended to remove hexavalent selenium (Se ⁶⁺ ) and ammonium nitrogen from wastewater containing nitrogen without adversely affecting a nitrification reaction in a biological nitrification denitrification apparatus. An object of the present invention is to provide a biological treatment method for selenium-containing wastewater that can be produced.

[0008]

The present invention provides hexavalent selenium (S)
In the biological treatment of wastewater containing e ⁶⁺ ) and ammonium nitrogen, the reduction of hexavalent selenium (Se ⁶⁺ ) does not substantially proceed by passing the wastewater through a biological nitrification and denitrification apparatus. Under the conditions, nitrification and denitrification were performed, and then activated sludge and a reducing agent were used to ^convert hexavalent selenium (Se ⁶⁺ ) to tetravalent selenium (S
e ⁴⁺ ) or metallic selenium (Se ⁰ )
A biological treatment method for selenium-containing wastewater, wherein selenium is removed through an e-treatment device.

A preferred embodiment of the Se treatment apparatus provided downstream of the biological nitrification and denitrification apparatus is a packed tower for reducing and removing selenium by supplying wastewater and a reducing agent to a packed tower filled with an activated sludge carrier. There is a Se treatment apparatus of the type or a floating treatment tank type in which wastewater and a reducing agent are supplied to a treatment tank in which activated sludge floats to reduce selenium and precipitate and remove selenium in a sedimentation tank.

[0010] If necessary, the selenium removal efficiency can be improved by adding a coagulant when removing selenium in the Se treatment apparatus.

As the reducing agent used in the Se removing device, for example, cysteine hydrochloride, sodium sulfide,
One or a mixture of two or more selected from the group consisting of sodium hydrosulfite, monosaccharides, polysaccharides, organic acids and salts thereof, and alcohols.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a system flow of a biological treatment method for wastewater containing selenium containing ammonia nitrogen by combining a biological nitrification denitrification apparatus and a Se treatment apparatus according to the present invention. The biological nitrification denitrification apparatus 50 in FIG. 1 is the same as the conventional one, and includes a nitrification tank 43, a denitrification tank 44, a re-aeration tank 45, a blower 47, and a sedimentation tank 46 as main components. In the method of the present invention, the wastewater containing ammonia nitrogen and hexavalent selenium is first passed through a biological nitrification and denitrification apparatus, and the conditions under which the reduction of hexavalent selenium does not substantially proceed, that is,
In a denitrification tank, methanol having a low reducing power is used in a proper amount as a hydrogen donor (reducing agent) (usually C / N
(At a ratio of about 3.0), the treatment is performed with the residence time within an appropriate time, and nitrification and denitrification are performed. Here, the condition under which the reduction of hexavalent selenium does not substantially proceed means that tetravalent selenium or zero-valent selenium generated by the reduction of hexavalent selenium accumulates in the sludge in the biological nitrification and denitrification apparatus and enters the nitrification tank. It means a condition that is returned and does not inhibit the nitrification reaction.

The wastewater containing hexavalent selenium, which has been treated in the biological nitrification and denitrification apparatus 50 and from which ammonia nitrogen has been removed by nitrification and denitrification, is supplied to the Se treatment apparatus 40 by the liquid feed pump 42, where activated sludge and By the action of the reducing agent 41, hexavalent selenium is reduced to tetravalent selenium or zero-valent selenium and removed.

The Se treatment apparatus installed downstream of the biological nitrification and denitrification apparatus uses conventional activated sludge for selenium removal. Generally, wastewater is discharged to a packed tower filled with activated sludge adhering carriers. And a reducing tower to supply selenium to reduce and remove selenium, or a floating tank to supply effluent and a reducing agent to a treatment tank where activated sludge floats to reduce selenium and remove selenium in a sedimentation tank A processing tank type processing apparatus can be used. In each case, a carbon source such as sodium acetate, acetic acid, formic acid, and methanol is added in an amount required for selenium reduction as a hydrogen donor (reducing agent) required for reducing hexavalent selenium. The reduction reaction from hexavalent selenium to tetravalent or zero-valent selenium is presumed to be as shown in the following formula (3).

Embedded image SeO ₄ ^2- → SeO ₃ ^2- → (... → Se ⁰ ) (3) In this case, it is considered that the selenate ion functions as the final electron acceptor for the respiration of microorganisms (Selenium). Breathing).

Representative examples of the reducing agent include the above-mentioned sodium acetate, acetic acid, formic acid, methanol and the like, and cysteine hydrochloride, sodium sulfide, sodium hydrosulfite, monosaccharides, polysaccharides, organic acids, and the like. One or two selected from the group consisting of salts and alcohols
Mixtures of more than one species can be used. When sodium sulfide or the like is used as a reducing agent, methanol or the like is used in combination as a carbon source necessary for growing sludge.

If tetravalent selenium ions are not fixed to the activated sludge carrier or the activated sludge and remain in the treatment solution, a metal salt such as iron or calcium, for example, a coagulant such as ferric chloride is added. Selenium may be precipitated and removed.

As described above, in the biological nitrification denitrification apparatus, nitrification denitrification is carried out under the condition that reduction of hexavalent selenium does not substantially proceed, so that inhibition of the nitrification reaction by the reduced selenium can be prevented while reducing ammonia nitrogen. By removing selenium and removing it in the form of a precipitate in the Se treatment device provided on the downstream side, wastewater containing ammonia nitrogen and hexavalent selenium can be efficiently treated.

[0018]

EXAMPLES The method of the present invention will be described more specifically with reference to the following examples. (Example 1) [Example in which a packed tower-type Se treatment apparatus is provided downstream of a biological nitrification denitrification apparatus] Se-treatment of a packed tower type before a biological nitrification denitrification apparatus shown in FIG. A mini-model device of a biological treatment system for selenium-containing wastewater equipped with a device was used. 1 sludge type biological nitrification denitrification equipment 5
0 denotes a nitrification tank 51, a denitrification tank 52, a re-aeration tank 53, and a sedimentation tank 5.
The sludge is returned from the sedimentation tank 54 to the nitrification tank 51 by the return pump 55 at a constant flow rate. The capacities of the nitrification tank 51, the denitrification tank 52, the re-aeration tank 53, and the precipitation tank 54 were set to 3, 1.5, 1.2, and 8 liters, respectively. A wastewater containing ammonia nitrogen and hexavalent selenium is supplied from a raw water tank 74 by a liquid sending pump 78 and the nitrification tank 5
1 and the re-aeration tank 53 are ventilated with an air pump 56 by an air balloon 56, and a nitrification tank 51 is provided with a pH controller 57, and an alkali agent is automatically administered from an alkali tank 58 by an alkali pump 59 to adjust the pH to 7.5. Kept.

The denitrification tank 52 was slowly stirred by a stirrer 60, and methanol was continuously added from a reducing agent tank 61 by a reducing agent pump 62 as a carbon source (reducing agent). The addition amount of methanol was set to C / N = 3.0, and the residence time of the wastewater in the biological nitrification and denitrification apparatus was about 10 hours (the residence time in the denitrification tank was about 3 hours). In the sedimentation tank 54, the activated sludge is settled by gravity sedimentation and separated from the treated water. The activated sludge is returned to the nitrification tank 51 from the return pump 55, and the treated water is sent to the subsequent process. The sludge concentration was 3000 for the nitrification tank 51, the denitrification tank 52, and the re-aeration tank 53.
５5000 ppm.

As the Se treatment apparatus 40, a cylindrical Se treatment tank 71 (capacity: 1.5 liters) is filled with the activated sludge-adhering carrier, and the wastewater which has been subjected to nitrification and denitrification by the biological nitrification and denitrification apparatus 50 is sent. The liquid is sent by the liquid pump 75 and the Se treatment tank 71
Water was passed into the tank from the bottom side of the tank. Sodium acetate as a carbon source (reducing agent) is supplied from a reducing agent tank 72 to a reducing agent pump 73.
To the Se treatment tank 71 from the bottom side. The amount of addition is (1/3) of the amount of methanol added to the denitrification tank 52.
It was doubled. As a result, hexavalent selenium was reduced to tetravalent selenium or zero-valent selenium by the action of microorganisms. Ferric chloride (FeCl ₃ ) as coagulant at the same time
Is continuously added from the bottom of the Se treatment tank 71 from the iron salt tank 76 by the iron salt pump 77 to precipitate tetravalent selenium.
It was fixed in the processing tank 71. The added amount of FeCl ₃ was set to be Se: Fe = 1: 3. Tables 1 and 2 show the raw water flow conditions and raw water composition. The NH ₄ —N is 75 ppm and the selenium concentration is 2 ppm as Se ⁶⁺ .

[0021]

[Table 1]

[0022]

[Table 2]

The test was performed by measuring the concentration of selenium at the entrance and exit of the Se treatment tank 71, and measuring the residual ammonia concentration in the nitrification tank 51. Table 3 shows the results of the water passage test for 23 days. From Table 3, it is 23 from the start of water supply
During the day, the Se processing capacity in the Se processing tank 71 is good,
(Exit Se ⁶⁺ + Exit Se ⁴⁺ ) <0.1 ppm, which satisfies the Se emission regulation value. Further, the processing capacity of ammonia in the nitrification tank 51 was also good.

[0024]

[Table 3]

(Example 2) [Example in which a floating Se treatment apparatus is provided downstream of a biological nitrification denitrification apparatus] A floating treatment tank type is installed downstream of a biological nitrification denitrification apparatus shown in FIG. A mini-model device of a biological treatment system for selenium-containing wastewater provided with a Se treatment device was used. Biological nitrification denitrification device 50
Is the same as that described in the first embodiment. Liquid pump 8
In 8, wastewater containing ammonia nitrogen and hexavalent selenium was supplied to the biological nitrification and denitrification apparatus 50, and treated in the same manner as in Example 1.

The Se processing apparatus 40 is a floating processing tank type Se processing apparatus.
The wastewater composed of a treatment tank 81 and a sedimentation tank 82 and subjected to nitrification and denitrification by the biological nitrification denitrification device 50 was fed by a liquid sending pump 83 and supplied to the Se treatment tank 81. The sludge accumulated in the sedimentation tank 82 is returned to the Se treatment tank 81 via the return pump 89. The capacity was 1.5 liters for the Se treatment tank 81 and 1 liter for the sedimentation tank 82. Sodium acetate was supplied as a hydrogen donor (reducing agent) from a reducing agent tank 84 to the Se treatment tank 81 by a reducing agent pump 85, and hexavalent selenium was reduced to tetravalent selenium or zero-valent selenium by the action of microorganisms. At the same time, ferric chloride (FeCl ₃ ) was added from an iron salt tank 86 by an iron salt pump 87 to precipitate tetravalent selenium and zero-valent selenium, and to store them in the Se treatment device 40. At this time, the addition amounts of sodium acetate and FeCl ₃ were the same as in Example 1. The raw water flow conditions and raw water composition were the same as in Example 1.

In the test, the selenium concentration was measured at the inlet and outlet of the Se treatment tank 81 in the same manner as in the first embodiment.
Then, the measurement was performed by measuring the residual ammonia concentration. Table 4 shows the results of the water passage test for 23 days. From Table 4, Se in the Se treatment tank 81 for 23 days from the start of water flow
The processing capacity is good, (Exit Se ⁶⁺ + Exit Se ⁴⁺ ) <0.
1 ppm, which satisfies the Se emission regulation value. Further, the processing capacity of ammonia in the nitrification tank 51 was also good.

[0028]

[Table 4]

(Comparative Example) As a comparative example, a selenium-containing wastewater flow test was conducted according to the flow of FIG. 4 in which no Se treatment device was provided downstream of the biological nitrification denitrification device 50. In this example, selenium reduction is performed simultaneously with denitrification in the denitrification tank 52 of the biological nitrification denitrification apparatus 50. The biological nitrification and denitrification apparatus 50 was the same as in Examples 1 and 2, and a wastewater containing ammonia nitrogen and selenium was supplied from a raw water tank 91 by a liquid feed pump 92. The reducing agent to be added to the denitrification tank 52 was acetic acid, and the amount of addition was C / N = 4.0. Other raw water flow conditions and raw water compositions were the same as in Examples 1 and 2.

The test was performed by measuring the residual ammonia concentration in the nitrification tank 51 and the selenium concentration at the inlet and outlet of the denitrification tank 52. Table 5 shows the results of the 23-day water flow test.
Shown in From Table 5, the selenium treatment in the denitrification tank 52 became good 9 days after the start of water supply, and Se ⁶⁺ <0.1 pp.
m, but at the same time, NH ₄ —N remained in the nitrification tank 51, indicating that the nitrification performance was reduced. Compared to Examples 1 and 2, the selenium concentration in the sludge is 100 times or more. In addition, tetravalent selenium is also present in the nitrification tank 51, and is contained in sludge or solution.
It is considered that the nitrification reaction was inhibited by selenium (0) or selenium (0).

[0031]

[Table 5]

[0032]

According to the present invention, a Se treatment apparatus is provided on the downstream side of a biological nitrification denitrification apparatus, and wastewater containing ammonia nitrogen and hexavalent selenium is first purified in a biological nitrification denitrification apparatus. Nitrogen denitrification is performed under the condition that reduction of hexavalent selenium does not progress in advance, and then selenium-containing wastewater is removed by reducing the hexavalent selenium to tetravalent selenium or metal selenium by the action of activated sludge and a reducing agent in a Se treatment apparatus. According to this method, selenium-containing wastewater in which ammonia nitrogen coexists can be treated without adversely affecting the nitrification reaction in the biological nitrification and denitrification apparatus. This method is particularly effective when the treatment in the biological nitrification and denitrification apparatus is a one-sludge type in which the same sludge is circulated and used in a nitrification tank and a denitrification tank.

[Brief description of the drawings]

FIG. 1 is a diagram showing a flow of a biological selenium reduction treatment method of the present invention.

FIG. 2 is an explanatory view showing an outline of a biological selenium reduction treatment test apparatus used in Example 1.

FIG. 3 is an explanatory view showing an outline of a biological selenium reduction treatment test apparatus used in Example 2.

FIG. 4 is an explanatory view showing an outline of a biological selenium reduction treatment test apparatus used in a comparative example.

FIG. 5 is a flowchart showing an example of a conventional thermal power plant wastewater treatment facility.

FIG. 6 is a flowchart showing an outline of a conventional biological nitrification and denitrification apparatus.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Naoyuki Uejima 1-1-1 Wadazakicho, Hyogo-ku, Kobe-shi, Hyogo Inside Mitsubishi Heavy Industries, Ltd. Kobe Shipyard

Claims (5)

[Claims] In the biological treatment of wastewater containing hexavalent selenium (Se ⁶⁺ ) and ammonia nitrogen, the wastewater is passed through a biological nitrification denitrification apparatus to substantially remove hexavalent selenium (Se ⁶⁺ ). Nitrification and denitrification are performed under the condition that the reduction of Se ⁶⁺ ) does not proceed, and then hexavalent selenium (Se ⁶⁺ ) is converted to tetravalent selenium (Se ⁴⁺ ) or metal selenium (S 4
A biological treatment method for selenium-containing wastewater, wherein selenium is removed by passing through a Se treatment device that reduces and removes selenium to e ⁰ ). 2. The Se treatment apparatus according to claim 1, wherein the Se treatment apparatus is a packed tower type Se treatment apparatus for supplying wastewater and a reducing agent to a packed tower filled with activated sludge adhering carriers to reduce and remove selenium. 2. The biological treatment method for selenium-containing wastewater according to 1. 3. The Se treatment apparatus supplies effluent and a reducing agent to a treatment tank in which activated sludge floats to reduce selenium,
2. The biological treatment method for selenium-containing wastewater according to claim 1, wherein the Se treatment apparatus is a floating treatment tank type Se treatment apparatus for removing selenium in a sedimentation tank. 4. The method for biologically treating selenium-containing wastewater according to claim 2, wherein a coagulant is added when removing selenium in the Se treatment apparatus. 5. The method according to claim 1, wherein the reducing agent is cysteine hydrochloride, sodium sulfide, sodium hydrosulfite, a monosaccharide,
The selenium-containing wastewater according to any one of claims 1 to 4, wherein the selenium-containing wastewater is one or a mixture of two or more selected from the group consisting of polysaccharides, organic acids, salts thereof, and alcohols. Biological treatment method.