JP3358388B2 - Treatment method for selenium-containing water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for detoxifying selenium-containing water by biological treatment.

[0002]

2. ^{Description of the Related} Art As a treatment method for detoxifying wastewater containing selenium compounds such as Se ⁶⁺ and Se ⁴⁺ , there are methods such as coagulation precipitation with iron salts and adsorption by ion exchange. Among them, the former requires a large amount of a coagulant and has the problem that Se ⁶⁺ cannot be removed. On the other hand, the latter has problems in that the amount of adsorption is small, and that it is necessary to treat the regeneration waste liquid.

[0003] As biological reactions of selenium compounds, Proceedings of the Annual Meeting of the Japan Society on Water Environment, 1995, p.176, Se ⁶⁺ and Se by selenate-reducing bacteria in the presence of lactose are described.
It is reported that ⁴⁺ is reduced. However, in this method, it is necessary to separate and culture the selenite-reducing bacteria from the place contaminated with the selenium compound.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a selenium-containing water capable of stably and efficiently removing a selenium compound by using a microorganism which is easily available and used, and by a simple apparatus and operation. It is to propose a processing method.

[0005]

The present invention is characterized in that selenium-containing water is brought into contact with biological sludge in an anaerobic condition under a condition that an oxidation-reduction potential with respect to a silver / silver chloride electrode is -50 mV or less. This is a method for treating selenium-containing water.

In the present invention, "(selenous acid)" means "selenic acid and / or selenious acid." “Se ⁶⁺ ”, “Se ⁴⁺ ”, “Se ⁰ ” or “S
e ^2- ”is the respective oxidation number + VI, + IV, zero or −
II means selenium. These may be simply described as Se. In the present invention, “(nitrite) nitric acid” means “nitric acid and / or nitrous acid”.

[0007] In the present invention, selenium containing
Arizu is Se⁶⁺And / or Se ⁴⁺Selenium compound
Including drainage and other water. Se⁶⁺Or Se⁴⁺No
Examples of the compound include (sub) selenic acid. Ingredient
Physical selenium-containing water includes metal refining industrial wastewater and glass
Industrial, chemical, and coal, oil or combustion
Wastewater from the exhaust gas treatment process. these
Organic substances other than selenium compounds and nitrogen
Compounds, sulfates and the like.

The biological sludge used in the present invention is a biological sludge generated by maintaining selenium-containing water in an anaerobic state. The biological sludge used in the aerobic treatment of waste water such as the activated sludge treatment method (activated sludge) is used as the biological sludge. It can also be produced spontaneously by collecting and adding it to selenium-containing water and maintaining it in an anaerobic state. For such biological sludge (sub)
Bacteria that reduce selenate predominate, and such bacteria reduce (sub) selenic acid in selenium-containing water.

The biota generated in the biological sludge varies depending on the composition of the selenium-containing water, the conditions of the anaerobic treatment, and the like. For example, in a system in which (nitrite) nitrate ions are present in raw water or a reaction solution, denitrifying bacteria that perform nitrate respiration are dominant. In a system in which organic substances such as carbohydrates are present, acid fermenting bacteria appear, and in a system in which sulfates exist, sulfate reducing bacteria appear. Due to other substances present in the system, bacteria suitable for the decomposition appear,
The selenium compound is reduced along with their decomposition.
Of these, denitrifying bacteria are suitable.

The denitrifying bacterium which can be used in the present invention is a bacterium which decomposes organic substances by using oxygen of (nitrite) nitrate by nitrate respiration, and is found in facultative anaerobic bacteria such as Pseudomonas. Such a denitrifying bacterium is used in a denitrification step in a nitrification denitrification denitrification method utilizing a biological reaction of an ammoniacal nitrogen-containing wastewater. As the denitrifying bacterium that can be used in the present invention, in addition to the denitrifying bacterium in such a biological denitrification method, aerobic sludge (activated sludge) in a wastewater aerobic treatment method such as an activated sludge treatment method is collected,
It can also be generated spontaneously by maintaining it in an anaerobic state in the presence of organic matter and (nitrite) nitrate ions.

The biological sludge containing such a denitrifying bacterium and other bacteria that reduce (sub) selenic acid is usually a floc-like biological sludge. In the present invention, the floc-like biological sludge is used as it is in a suspended state. Alternatively, it can be used by being supported on a granular, fibrous, or other carrier having a large porosity. The carrier is not particularly limited as long as it can support biological sludge, and examples thereof include sand, activated carbon, alumina gel, and foamed plastic. In order to carry biological sludge on the carrier, the sludge can be carried by acclimation or treatment in the presence of the carrier.

The treatment method of the present invention comprises contacting selenium-containing water with the above-mentioned biological sludge in an anaerobic state.
The (sub) selenic acid, ie, Se ⁶⁺ and / or Se ⁴⁺ in the selenium-containing water is reduced and made harmless. At this time, Se ⁶⁺
Is presumed to be reduced to Se ⁰ and / or Se ²⁻ via Se ⁴⁺ . In the present invention, the term "anaerobic state" means a state in which oxygen is blocked, but a slight amount of oxygen that does not inhibit the reduction of the selenium compound is allowed.

The above reaction requires oxygen and nutrient sources for the respiration of biological sludge. As an oxygen source, not a molecular oxygen because it is in an anaerobic state, but oxygen that can be an oxidizing agent contained in the form of (nitrite), carbohydrate, organic acid, sulfuric acid or the like is used. As a nutrient source, an organic substance contained in a reaction solution or an organic substance contained in biological sludge is used as a substrate. These oxygen sources and nutrient sources can be used as long as they are contained in the selenium-containing water, but if they are not contained, they are added separately. As a result, the biological sludge is maintained at a high activity, and the (sub) selenic acid is reduced along with their decomposition.

The case of biological sludge containing denitrifying bacteria will be described. The presence of (nitrite) nitrate ions in the reaction system allows the denitrifying bacteria to appear in the biological sludge to maintain a high activity. Selenic acid is reduced. When (nitrite) nitrate ion is already present in the reaction system, it can be used as it is, but when it does not exist, (nitrite) nitrate can be added. (Nitrite) nitrate ions may be added to the extent that the activity of the denitrifying bacteria is maintained (about 1 to 10 mg / l as NOx).

When the raw water contains organic or ammonia nitrogen, in another nitrification step, the raw water is brought into contact with nitrifying bacteria to perform nitrification under aerobic conditions to convert the organic or ammonia nitrogen to (nitrite). Nitrogen is converted to nitrogen, and the nitrification liquid is brought into contact with biological sludge containing denitrifying bacteria under anaerobic conditions to carry out denitrification and reduce (sub) selenic acid. In this case, the selenium compound is (selenite) in the nitrification step, but is reduced in the denitrification step.

In the present invention, when the selenium-containing water and the biological sludge are brought into contact with each other in an anaerobic state, the oxidation-reduction potential (ORP) of the silver / silver chloride electrode is controlled to be -50 mV or less. This ORP indicates the anaerobic degree of the reaction system,
By setting the ORP to an anaerobic degree of -50 mV or less, it is possible to reduce (sub) selenic acid. Even in the case of ORP = 0, it is in an anaerobic state, and the original functions of biological sludge such as denitrification appear, but the reduction of (sub) selenic acid does not occur. Therefore, by making the anaerobic degree much stronger than the occurrence of denitrification or the like, it is possible to reduce (sub) selenic acid.

As means for reducing the ORP to -50 mV or less, the amount of organic matter added, the concentration of biological sludge, and / or the residence time of selenium-containing water are controlled. Organic substances are added as a substrate, and usually methanol or the like is used.
Other organic sources such as organic wastewater may be used. When the amount of such an organic substance is increased, the ORP decreases and the anaerobic level increases.

The biological sludge concentration is a biological sludge concentration of a system that performs an anaerobic reaction, that is, MLSS. When the MLSS is increased, the ORP decreases and the anaerobic level increases. In this case, the organic matter stored in the biological sludge is used as a substrate, and self-digestion of the sludge occurs. Controlling the residence time of raw water is to increase the residence time by reducing the amount of raw water introduced,
ORP decreases and anaerobicity increases. In this case, the quantitative relationship between the load and the biological sludge is the same as when the MLSS is increased, and the reaction forms are almost the same.

The above-described anaerobic reaction is performed by introducing raw water and organic substances into an anaerobic reaction tank. An anaerobic reaction tank is used for contacting the selenium-containing water with the biological sludge, and any method such as a floating method and a biofilm method can be adopted. The flotation method is a method in which a floc-like biological sludge containing denitrifying bacteria is brought into contact in a floating state with stirring, and is carried out in the same manner as the denitrification step in the biological denitrification method. The biofilm method is a method in which biological sludge is supported on a carrier to form a biofilm, and this is brought into contact with selenium-containing water, such as a fixed-bed type, a fluidized-bed type, or an upward-flow type or a downward-flow type. The same method as that employed in the denitrification step can be employed.

The residence time in the anaerobic reactor is the time required for the (sub) selenate ion to be reduced, which can be regarded as the time required for the decomposition of organic substances present in the system. When denitrification or the like is performed in the system, the time required for denitrification or the like can be 1.1 times or more. In the anaerobic reaction, a part of the reaction solution is withdrawn so as to have the above residence time, solid-liquid separated, the separated sludge is returned, and the sludge concentration is adjusted to a predetermined concentration (500 to 50,000 mg / l, preferably 2000 to 50,000 mg / l).
(〜20,000 mg / l) to carry out the reaction.

The ORP is controlled by installing an oxidation-reduction potentiometer in the anaerobic reaction tank as described above with a silver / silver chloride electrode as a control, and an organic substance supply system and sludge return so that the detection value indicates -50 mV or less. The system, the raw water introduction system, etc. are controlled to adjust the organic matter supply amount, biological sludge concentration, raw water introduction amount (residence time), and the like.

By the above-mentioned biological reaction, the (sub) selenic acid in the selenium-containing water is reduced to metallic selenium to form a precipitate, which is separated by solid-liquid separation in a state of being attached to the sludge.
It is discharged outside the system together with excess sludge. Then, the separated liquid from which selenium has been removed is discharged as treated water.

The biological sludge used in the above-mentioned treatment can be obtained by acclimation from sludge used in a usual biological treatment method, and it is easy to obtain and use it. Pure separation and special culture conditions are not required. Not required. Such biological sludge is activated by supplying (nitrite) nitrate ions and the like, thereby increasing the selenium removal ability.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram showing a processing apparatus of an embodiment using denitrifying bacteria. 1 is an anaerobic reaction tank, 2 is a solid-liquid separation tank, 3 is an ORP meter, and 4 is a control device.

The treatment method is as follows. Raw water as selenium-containing water is introduced into the anaerobic reaction tank 1 from the raw water passage 5, return sludge is introduced from the sludge return passage 6, and (nitrite) nitrate ion (NOx) is supplied from the NOx introduction passage 7. And the organic matter is supplied from the organic matter introduction passage 8 and mixed with the biological sludge containing denitrifying bacteria in the tank.
The mixture is gently agitated and brought into contact in an anaerobic state.

At this time, the ORP in the reaction tank 1 is detected by the ORP meter 3 using the silver / silver chloride electrode as a control, and the detection signal is input to the control device 4. In the control device 4, the ORP value is-
Pump 11, 12 or 1 so that it is 50 mV or less.
3 is controlled. In this case, when the ORP is higher than -50 mV, the amount of organic matter added is increased by increasing the amount of liquid supplied to the pump 12, or the amount of returned sludge is increased by increasing the amount of liquid returned by the pump 13 to increase the MLSS. Alternatively, control is performed such that the amount of feed of the raw water is reduced by reducing the amount of liquid supplied from the pump 11 to prolong the residence time. This control is performed by a preset program.

By the above treatment, in the anaerobic reaction tank 1, denitrifying bacteria reduce NOx to N using organic matter as a substrate, thereby maintaining a high denitrifying activity, and using excess organic matter as a substrate for Se ⁶⁺ and Se. ⁴⁺ is reduced to Se ⁰ or the like. The generated N is released as a gas, and Se precipitates and adheres to the sludge.

A part of the reaction solution is introduced into the solid-liquid separation tank 2 through the communication path 14 to be separated into solid and liquid, and the separated liquid is discharged from the treatment water path 15 as treatment water. Part of the separated sludge is returned to the anaerobic reaction tank 1 from the return sludge passage 6 as return sludge, and the remainder is discharged from the sludge discharge passage 16 as excess sludge.

Reference Example 1 NH ₄ —N 20 mg / l, hexavalent Se 0.9 mg / l
After the nitrification of the power flue gas desulfurization wastewater containing aerobic treatment, denitrification and selenium reduction by anaerobic treatment,
Sludge was separated by solid-liquid separation. The residence time was 4 hours for the aerobic treatment tank, 12 hours for the anaerobic treatment tank, and 4 hours for the solid-liquid separation tank.
Time. In the anaerobic treatment tank, the activated sludge from the sewage treatment apparatus was used as seed sludge and subjected to anaerobic treatment at MLSS 4000 mg / l. As a result, the ORP of the aerobic treatment tank is +32
0 mV, ORP of the anaerobic treatment tank is -40 to +20 mV, NH ₄ -N in the treated water is 1 mg or less, NOx-N
Denitrification was possible at 1 mg / l or less, but Se was 1.1 ~
It was as high as 0.1 mg / l and unstable.

Example 1 From the stage of Reference Example 1, methanol was added to the anaerobic treatment tank in an amount of 20%.
When 0 mg / l was added, the ORP gradually decreased to 1
It becomes -50 mV after a week, and Se in the treated water is 0.04 mV.
mg / l. After continuing driving, OR
P becomes -60 to -80 mV, and the treated water Se is always 0.1.
Stable at less than 1 mg / l. The denitrification effect was the same as in Reference Example 1.

Example 2 The addition of methanol in Example 1 was stopped, and sewage activated sludge was added to the anaerobic treatment tank to reduce the MLSS concentration to 8000.
mg / l. Initially the anaerobic treatment tank ORP is -40m
V, but immediately decreased to -50 to -60 mV, and the treated water Se was stabilized at less than 0.1 mg / l.

Example 3 In the state of Reference Example 1, the amount of the aerobic treatment liquid introduced from the aerobic treatment tank to the anaerobic treatment tank was reduced, and the residence time in the anaerobic treatment tank was set to 48 hours. As a result, the ORP of the anaerobic treatment tank becomes -50 to -90 mV, and the treated water S
e was always stable at less than 0.1 mg / l.

[0033]

According to the present invention, the selenium-containing water is brought into contact with the biological sludge in an anaerobic condition under the condition that the ORP is -50 mV or less. It can be efficiently removed and made harmless.

[Brief description of the drawings]

FIG. 1 is a system diagram illustrating a processing apparatus according to an embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Anaerobic reaction tank 2 Solid-liquid separation tank 3 ORP meter 4 Controller 5 Raw water path 6 Sludge return path 7 NOx introduction path 8 Organic matter introduction path 9 Stirrer 11, 12, 13 Pump 14 Communication path 15 Treatment water path 16 Sludge discharge path

Continuation of the front page (56) References JP-A-8-299986 (JP, A) JP-A-8-224585 (JP, A) JP-A-5-78105 (JP, A) JP-A-8-224586 (JP) , A) JP-A-8-309369 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C02F 3/28 WPI (DIALOG)

Claims (1)

(57) [Claims] 1. A method for treating selenium-containing water, wherein the selenium-containing water is brought into contact with biological sludge in an anaerobic condition under a condition that an oxidation-reduction potential with respect to a silver / silver chloride electrode is -50 mV or less.