JPH08323391A - Treatment of selenium-containing water - Google Patents

Abstract

PURPOSE: To provide a treating method of a selenium-containing water capable of stably and efficiency removing a selenium compd. to detoxicate the water with a simple device and operation. CONSTITUTION: The selenium-containing water 5 is introduced to an anaerobic treating tank 1 to bring it into contact with a biological sludge in an anaerobic state, and an oxidation-reduction potential(ORP) of contrasting silver/silver chloride electrodes is detected with an ORP meter 3, and a feed water introduce rate, an org. matter feed rate or returning sludge rate are controlled to reduce the selenium by controlling pumps 11, 12 or 13 so that the ORP value may be <=-50mV.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION 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 waste water containing selenium compounds such as Se ⁶⁺ and Se ⁴⁺ , there are methods such as coagulation and precipitation by iron salt and adsorption by ion exchange. The former requires a large amount of coagulant and has a problem that Se ⁶⁺ cannot be removed. On the other hand, the latter has a problem that the amount of adsorption is small, and that it is necessary to treat the recycled waste liquid.

As a biological reaction of a selenium compound, a lecture collection of Annual Meeting of the Japan Society for Water Environment, 1995, P176, Se ⁶⁺ and Se in the presence of lactose by a (sub) selenate-reducing bacterium.
It is reported that ⁴⁺ is reduced. However, in this method, it is necessary to separate and culture the (sub) selenate-reducing bacterium from the place contaminated with the selenium compound.

[0004]

An object of the present invention is to use selenium-containing water capable of stably and efficiently removing a selenium compound with a simple device and operation by using a microorganism which is easily available and used. It is to propose a processing method.

[0005]

The present invention is characterized in that selenium-containing water is anaerobically contacted with selenium-containing water under conditions where the oxidation-reduction potential is -50 mV or less with respect to a silver / silver chloride electrode as a control. It is a method of treating selenium-containing water.

In the present invention, "(selen) selenic acid" means "selenic acid and / or selenious acid". Also, “Se ⁶⁺ ”, “Se ⁴⁺ ”, “Se ⁰ ”, or “S
e ^2- "means each oxidation number + VI, + IV, zero or-
II means selenium. These may be simply described as Se. Further, in the present invention, “(nitro) nitric acid” means “nitric acid and / or nitrous acid”.

In the present invention, the selenium-containing material to be treated is contained.
Water is Se⁶⁺And / or Se ⁴⁺Of selenium compounds
It includes wastewater and other water. Se⁶⁺Or Se⁴⁺The selection
Examples of the phosphorus compound include (sub) selenic acid. Ingredient
As physical selenium-containing water, metal smelting industry wastewater, glass
Industrial wastewater, chemical industrial wastewater, and coal, oil or combustion
Examples include wastewater from the exhaust gas treatment process. these
In addition to selenium compounds, organic substances and nitrogen in selenium-containing water
Compounds, sulfates, etc. may be included.

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

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

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

The biological sludge containing such denitrifying bacteria and other bacteria capable of reducing (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. However, it can also be used by supporting it on a granular, fibrous, or other carrier having a high porosity. The carrier is not limited as long as it can support biological sludge, and examples thereof include sand, activated carbon, alumina gel, and foamed plastic. The biological sludge can be carried on the carrier 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 biological sludge in an anaerobic state,
(Selenite) in the selenium-containing water, that is, Se ⁶⁺ and / or Se ⁴⁺ is reduced to be harmless. At this time Se ⁶⁺
Is estimated to be reduced to Se ⁰ and / or Se ²⁻ via Se ⁴⁺ . In the present invention, the anaerobic state means a state in which oxygen is blocked, but it is permissible to mix in a small amount of oxygen to the extent that it does not inhibit the reduction of the selenium compound.

The above reaction requires an oxygen source and a nutrient source for respiration of biological sludge. As an oxygen source, since it is in an anaerobic state, not oxygen in the molecular form but oxygen that can be an oxidant contained in the form of (nitro) nitrite, carbohydrate, organic acid, sulfuric acid, etc. is used. As a nutrient source, organic substances contained in the reaction liquid or organic sludge are used as substrates. These oxygen sources and nutrient sources can be used as they are if 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 (s) selenic acid is reduced along with the decomposition thereof.

Explaining the case of biological sludge containing denitrifying bacteria, the presence of (nitrite) ions in the reaction system causes the denitrifying bacteria to appear in the biological sludge and keeps the activity high. Reduces selenate. When the (nitrite) nitrate ion is already present in the reaction system, it can be used as it is, but when it is not present, a (nitrite) salt or the like can be added. The (nitrite) nitrate may be added at a limit (about 1 to 10 mg / l as NOx) that maintains the activity of denitrifying bacteria.

When the raw water contains organic or ammoniacal nitrogen, in another nitrification step, the raw water is contacted with nitrifying bacteria to nitrify under aerobic conditions to convert organic or ammoniacal nitrogen into (nitrite) nitrate. It is converted to nitrogen, and the nitrification solution is anaerobically contacted with biological sludge containing denitrifying bacteria to denitrify and reduce (s) selenic acid. In this case, the selenium compound is (selenite) selenic acid in the nitrification step, but it is reduced in the denitrification step.

In the present invention, when selenium-containing water is contacted with biological sludge in an anaerobic state, the redox potential (ORP) with the silver / silver chloride electrode as a control is controlled to be -50 mV or less. This ORP indicates the anaerobic degree of the reaction system,
By setting the above ORP to an anaerobic degree of -50 mV or less, it becomes possible to reduce (selen) selenic acid. Even when ORP = 0, it is in an anaerobic state and the original function of biological sludge such as denitrification appears, but the reduction of (sub) selenate does not occur. Therefore, by making the anaerobic degree much stronger than that such as denitrification, it becomes possible to reduce (sub) selenic acid.

As a means for reducing the ORP to -50 mV or less, the amount of organic substances added, the concentration of biological sludge, and / or the residence time of selenium-containing water are controlled. The organic substance is added as a substrate, and usually methanol or the like is used.
It may be another organic matter source such as organic waste water. If the amount of such an organic substance added is increased, the ORP becomes low and the anaerobic level becomes high.

The biological sludge concentration is the biological sludge concentration of a system that performs an anaerobic reaction, that is, MLSS. When the MLSS is increased, the ORP is decreased and the anaerobic level is increased. In this case, the organic matter stored in the biological sludge is used as a substrate, and the sludge self-extinguishes. To control the residence time of raw water, 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 anaerobic reaction is carried out by introducing raw water and organic matter into the anaerobic reaction tank. An anaerobic reaction tank is used for the contact between the selenium-containing water and the biological sludge, and any method such as a floating method or a biofilm method can be adopted. The floating method is a method in which floc-like biological sludge containing denitrifying bacteria is stirred and brought into contact with each other in a floating state, and is performed in the same manner as the denitrifying step in the biological denitrifying 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 fixed bed type, fluidized bed type, upflow type, downflow type, etc. The same method as that used in the denitrification process can be adopted.

The residence time in the anaerobic reaction tank is the time required for the reduction of the (selen) selenate ion, but this can also be regarded as the time required for the decomposition of the organic substances present in the system, When performing denitrification in the system, the time required for denitrification can be 1.1 times or more. In the anaerobic reaction, a part of the reaction solution is extracted so as to have the above residence time, solid-liquid separation is performed, and the separated sludge is returned, and the sludge concentration is set to a predetermined concentration (500 to 50,000 mg / l, preferably 2000).
The reaction is carried out while maintaining ~ 20,000 mg / l).

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

By the above-mentioned biological reaction, (selenium) selenic acid in selenium-containing water is reduced to metal selenium to form a precipitate, which is separated by solid-liquid separation in a state of being attached to sludge.
It is discharged out of the system along 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 treatment can be obtained from the sludge used in the usual biological treatment method by acclimation, and its acquisition and use are easy, and pure separation and special culture conditions are not required. It is unnecessary. Such a biological sludge has a high selenium removing ability by being activated by supplying (nitrite) nitrate and the like.

[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 utilizing denitrifying bacteria. 1 is an anaerobic reaction tank, 2 is a solid-liquid separation tank, 3 is an ORP meter, and 4 is a controller.

As the treatment method, raw water as selenium-containing water is introduced into the anaerobic reaction tank 1 from the raw water channel 5, return sludge is introduced from the sludge return channel 6, and (nitro) nitrite ion (NOx) is introduced from the NOx introduction channel 7. Is introduced, and the organic matter is supplied from the organic matter introduction path 8 and mixed with the biological sludge containing the denitrifying bacteria in the tank, and the agitator 9
Gently stir with and contact in an anaerobic state.

At this time, the ORP in the reaction tank 1 is detected by the ORP meter 3 with the silver / silver chloride electrode as a reference, and the detection signal is input to the control device 4. In the controller 4, the ORP value is −
Pump 11, 12 or 1 so that it becomes 50 mV or less
Control 3 In this case, when the ORP is higher than −50 mV, whether to increase the liquid feed amount of the pump 12 to increase the organic substance addition amount or increase the liquid feed amount of the pump 13 to increase the return sludge amount to increase the MLSS. Alternatively, the amount of liquid fed by the pump 11 is reduced to reduce the amount of raw water introduced and the residence time is controlled to be longer. This control is performed by a preset program.

By the above treatment, in the anaerobic reaction tank 1, the denitrifying bacteria reduce NOx to N by using the organic matter as a substrate, thereby maintaining the denitrifying activity at a high state, and using the excess organic matter as the substrate in Se ⁶⁺ and Se. ^{Reduce 4+} to Se ⁰ etc. The generated N is released as a gas, and Se precipitates and adheres to the sludge.

A part of the reaction liquid is introduced into the solid-liquid separation tank 2 through the communication passage 14 for solid-liquid separation, and the separated liquid is discharged from the treatment water passage 15 as treated water. Part of the separated sludge is returned as return sludge to the anaerobic reaction tank 1 from the return sludge passage 6, and the rest 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 nitriding the electric power flue gas desulfurization wastewater containing a by aerobic treatment, denitrification and selenium reduction are performed by an anaerobic treatment.
Sludge was separated by solid-liquid separation. Residence time is 4 hours for aerobic treatment tank, 12 hours for anaerobic treatment tank, and 4 hours for solid-liquid separation tank.
Time. In the anaerobic treatment tank, anaerobic treatment was carried out with MLSS 4000 mg / l using the activated sludge of the sewage treatment equipment as seed sludge. As a result, ORP of aerobic treatment tank is +32
0 mV, ORP of anaerobic treatment tank is -40 to +20 mV, NH ₄ -N in treated water is 1 mg or less, NOx-N
Was able to denitrify at 1 mg / l or less, but Se is 1.1-
It was as high as 0.1 mg / l and was unstable.

Example 1 From the step of Reference Example 1, methanol was added to the anaerobic treatment tank at 20%.
When 0 mg / l was added, ORP gradually decreased, and
It became −50 mV after a week, and Se in the treated water was 0.04.
It became mg / l. When I continued driving, OR
P becomes −60 to −80 mV, and the treated water Se is always 0.
Stable below 1 mg / l. The denitrification effect was similar to that of 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 adjust the MLSS concentration to 8000.
mg / l. Initially ORP of anaerobic treatment tank is -40m
Although it was V, it immediately decreased to −50 to −60 mV, and the treated water Se was stable 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 below 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 state under the condition that the ORP is -50 mV or less, so that the selenium compound can be stably stabilized by a simple device and operation. It can be efficiently removed and rendered harmless.

[Brief description of drawings]

FIG. 1 is a system diagram showing a processing apparatus of an embodiment.

[Explanation of symbols]

 1 Anaerobic reaction tank 2 Solid-liquid separation tank 3 ORP meter 4 Control device 5 Raw water channel 6 Sludge return channel 7 NOx introduction channel 8 Organic substance introduction channel 9 Stirrer 11, 12, 13 Pump 14 Communication channel 15 Treated water channel 16 Sludge discharge channel

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[Procedure amendment]

[Submission date] June 2, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

FIG.

Claims (1)

[Claims] 1. A method for treating selenium-containing water, which comprises contacting selenium-containing water with biological sludge in an anaerobic state under conditions where the oxidation-reduction potential is −50 mV or less, using a silver / silver chloride electrode as a control.