JP3837757B2 - Method for treating selenium-containing water - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method in which selenium-containing water is brought into contact with biological sludge in an anaerobic state to reduce selenium, and an insolubilized selenium compound is adsorbed on the biological sludge and removed.
[0002]
[Prior art]
As a treatment method for detoxifying wastewater containing a selenium compound such as Se ⁶⁺ or Se ⁴⁺ , there is a biological treatment method in which selenium-containing water is brought into contact with biological sludge in an anaerobic state to reduce the selenium compound. For example, as a biological reaction of selenium compounds, it is reported that Se ⁶⁺ and Se ⁴⁺ are reduced in the presence of lactose by (sub-) selenate-reducing bacteria in the Annual Meeting of the Water Environment Society of Japan, 1995, P176. Has been. This method isolates (sulphite) selenate-reducing bacteria from places contaminated with selenium compounds and uses them to reduce selenium compounds. In addition, denitrifying bacteria, sulfate-reducing bacteria, acid-producing bacteria, etc. Can be used to reduce selenium compounds under anaerobic conditions.
[0003]
In such anaerobic treatment, the selenium compound is insolubilized by reduction and adsorbed on the biological sludge. Therefore, the selenium compound is removed by separating and discharging the sludge by solid-liquid separation. Reduction of selenium compounds requires the presence of organic substances as a substrate, and excess added organic substances remain after the anaerobic treatment process, so the treatment liquid after anaerobic treatment is aerobically treated to decompose excess organic substances There is also.
[0004]
[Problems to be solved by the invention]
By the way, in the above treatment, the selenium compound reduced and insolubilized in the anaerobic treatment step is adsorbed and removed by the biological sludge, but since selenium is toxic, the activity of the biological sludge decreases when the selenium content in the biological sludge increases. However, there is a problem that the selenium removal rate and the organic matter removal rate are lowered.
[0005]
The object of the present invention is to prevent the biological sludge activity from being lowered by the selenium compound insolubilized in the anaerobic treatment, and to maintain the biological sludge activity at a high level, thereby stably and efficiently removing the selenium compound. It is to propose a method for treating possible selenium-containing water.
[0006]
[Means for Solving the Problems]
The present invention is the following method for treating selenium-containing water.
[1] In a method in which selenium-containing water is brought into contact with biological sludge in an anaerobic state to reduce selenium, and the insoluble selenium compound is adsorbed on the biological sludge and removed.
An anaerobic treatment process in which selenium-containing water and organic substances are supplied and contacted with biological sludge in an anaerobic state; and
A control process for controlling the amount of organic matter supplied to the system so that the selenium content in the biological sludge represented by SeX in the following formula (1) does not exceed 20 wt % ( vs. MLSS) ;
A method for treating selenium-containing water, comprising:
SeX = ΔSe / ΔX (1)
ΔX = ΔBOD · Y (2)
However, SeX: Selenium content in sludge (-)
ΔSe: removed selenium (mg / l)
ΔX: Sludge production (mg / l)
ΔBOD: removed BOD (mg / l)
Y: Sludge conversion rate
[2] The method according to [1], further including an aerobic treatment step in which biological sludge is transferred in the anaerobic treatment step to perform an aerobic treatment.
[0007]
In the present invention, “(selenic acid)” means “selenic acid and / or selenious acid”. “Se ⁶⁺ ”, “Se ⁴⁺ ”, “Se ⁰ ”, or “Se ²⁻ ” means selenium having an oxidation number of + VI, + IV, zero, or −II. These may be simply described as Se.
In the present invention, “(nitrous) nitrile” means “nitric acid and / or nitrous acid”.
[0008]
The selenium-containing water to be treated in the present invention is waste water or other water containing Se ⁶⁺ and / or Se ⁴⁺ selenium compounds. ^{Examples of} Se ⁶⁺ or Se ⁴⁺ selenium compounds include (sub-) selenoic acid. Specific selenium-containing water includes metal smelting industrial wastewater, glass industrial wastewater, chemical industrial wastewater, wastewater from coal or petroleum combustion exhaust gas treatment processes, and the like. These selenium-containing waters may contain organic substances, nitrogen compounds, sulfates and the like in addition to selenium compounds.
[0009]
In the present invention, such selenium-containing water is subjected to anaerobic treatment by contacting it with anaerobic biological sludge in an anaerobic treatment step to reduce the selenium compound.
The biological sludge used in the anaerobic treatment process is a biological sludge produced by maintaining selenium-containing water in an anaerobic state, and it collects biological sludge (activated sludge) in an aerobic treatment method for wastewater such as the activated sludge treatment method. It can also be generated spontaneously by adding it to selenium-containing water and maintaining it in an anaerobic state. In such biological sludge, bacteria that reduce (sub) selenic acid are dominant, and such bacteria reduce (sub) selenic acid in selenium-containing water.
[0010]
The biota produced in the biological sludge in the anaerobic treatment step varies depending on the composition of 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 the raw water or the reaction solution, denitrifying bacteria that perform nitrate respiration predominate. In systems where organic substances such as carbohydrates are present, acid-fermenting bacteria, hydrogen-producing bacteria, etc. appear, and in systems where sulfates are present, sulfate-reducing bacteria appear. Bacteria suitable for the decomposition appear by other substances present in the system, and the selenium compound is reduced along with the decomposition. Of these, denitrifying bacteria are particularly suitable.
[0011]
Denitrifying bacteria that can be used in the anaerobic treatment process are bacteria that decompose oxygen by utilizing the oxygen of (nitrite) nitrate ions by nitrate respiration, and are found in facultative anaerobic bacteria such as Pseudomonas. Such a denitrifying bacterium is used in a denitrification step in a denitrification method by nitrification denitrification using a biological reaction of ammoniacal nitrogen-containing wastewater.
As the above-mentioned denitrifying bacteria, the denitrifying bacteria in such a biological denitrification method can be used as they are, and aerobic sludge (activated sludge) in aerobic treatment of waste water such as activated sludge treatment method is collected and used as organic matter and It can also be generated spontaneously by maintaining an anaerobic state in the presence of (sub) nitrate ions.
[0012]
Biological sludge containing such denitrifying bacteria and other (sulphite) selenate-reducing bacteria is usually floc-like biological sludge. In the present invention, floc-like biological sludge can be used as it is in a suspended state. However, it can also be used in the form of particles, fibers, or other carriers 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. In order to carry biological sludge on a carrier, it can be carried by acclimatization or treatment in the presence of the carrier. Moreover, you may fix the microbe which reduces a selenic acid in gels, such as polyvinyl alcohol and polyethyleneglycol.
[0013]
In the anaerobic treatment step, the selenium-containing water is brought into contact with the biological sludge as described above in an anaerobic state, whereby (sub-) selenic acid, that is, Se ⁶⁺ and / or Se ⁴⁺ in the selenium-containing water is reduced and insolubilized. At this time, Se ⁶⁺ is estimated to be reduced to Se ⁰ and / or Se ²⁻ via Se ⁴⁺ . The anaerobic state in the anaerobic treatment step means a state where oxygen is blocked, but a slight amount of oxygen that does not inhibit the reduction of the selenium compound is allowed.
[0014]
The above-described anaerobic treatment step requires an oxygen source and a nutrient source for respiration of biological sludge. As an oxygen source, oxygen that can be an oxidizing agent contained in the form of (nitrous acid), carbohydrates, organic acids, sulfuric acid, etc. is used instead of molecular oxygen because it is in an anaerobic state. As a nutrient source, organic substances contained in the reaction solution or organic substances contained in biological sludge are used as a substrate. 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, methanol or the like is added separately. As a result, the biological sludge is maintained at high activity, and (selenic) selenate is reduced along with the decomposition thereof.
[0015]
In the case of biological sludge containing denitrifying bacteria, the presence of (nitrogen) nitrate ions in the reaction system causes denitrifying bacteria to appear in the biological sludge and keeps the activity high. Reduce. (Nitrite) nitrate ions can be used as they are when they are already present in the reaction system, but (nitrite) nitrates can be added when they are not present. (Nitrous) nitrate ions may be added within the limit for maintaining the activity of denitrifying bacteria (about 1 to 10 mg / l as NOx).
[0016]
When the raw water contains organic or ammonia nitrogen, the raw water is contacted with nitrifying bacteria in advance in the nitrification process and nitrified under aerobic condition to convert the organic or ammonia nitrogen into (sub) nitric nitrogen. The nitrification solution is brought into contact with biological sludge containing denitrifying bacteria under anaerobic conditions to perform denitrification and to reduce (sub) selenic acid. In this case, in the nitrification process, the selenium compound is (selenic) selenate, but is reduced in the denitrification process as the anaerobic treatment process.
[0017]
The anaerobic treatment step is performed by introducing raw water and organic matter into the anaerobic treatment tank.
An anaerobic treatment 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 in a floating state, and is performed in the same manner as the denitrification step in the biological denitrification method.
The biofilm method is a method in which a biosludge is supported on a carrier to form a biofilm, and this is brought into contact with water containing selenium, such as a fixed bed type, a fluidized bed type, an upflow type, a downflow type, etc. A method similar to that employed in the denitrification process can be employed.
[0018]
The residence time in the anaerobic treatment tank is the time required for the reduction of the (selenium) ion, but this can also be taken as the time required for the decomposition of the organic substances present in the system. When performing nitrogenation etc., it can be 1.1 times or more of the time required for denitrification.
In the anaerobic treatment, the reaction is carried out while maintaining a predetermined sludge concentration (500 to 50000 mg / l, preferably 2000 to 20000 mg / l) so as to achieve the above residence time.
[0019]
When the selenium compound is reduced in the anaerobic treatment step, an insoluble selenium compound is generated and adsorbed on the biological sludge. The selenium compound adsorbed on the biological sludge is removed from the system by separating and discharging the biological sludge by solid-liquid separation.
[0020]
The anaerobic treatment liquid taken out from the above anaerobic treatment step contains excessively added organic matter and other oxidizable substances. Therefore, in order to oxidize and decompose these, after the anaerobic treatment step, An aerobic treatment process can be provided. Examples of such an aerobic treatment step include an activated sludge treatment step for decomposing excess organic substances used as a substrate in the anaerobic treatment step.
[0021]
The biological sludge used in such an aerobic treatment process can use normal activated sludge, and can be generated spontaneously by aeration of the liquid to be treated, but in sewage treatment equipment and other activated sludge treatment equipment The generated activated sludge can also be used as biological sludge. In the aerobic treatment step, the organic matter and other oxidizable substances are oxidized by bringing the anaerobic treatment liquid into contact with such biological sludge in an aerobic state, that is, aeration. The treatment conditions are the same as those for normal activated sludge treatment.
[0022]
When denitrifying bacteria are used as the biological sludge in the anaerobic treatment process, the denitrifying bacteria are facultative anaerobic bacteria of the same type as the biota contained in normal activated sludge, so without separating the biological sludge in the anaerobic treatment process. The aerobic treatment can be carried out by transferring it to the aerobic treatment process as it is. In this case, the amount of aeration is limited so that the selenium compound insolubilized in the anaerobic treatment step is not re-eluted by the aerobic treatment, and the oxidation-reduction potential (ORP) in the aerobic treatment step is maintained at 160 mV or less. Is preferred. Here, ORP is a value relative to a silver / silver chloride electrode.
[0023]
In the above treatment, the selenium compound adsorbed on the biological sludge does not affect the activity of the biological sludge when the selenium content is 20% by weight (vs. MLSS) or less as selenium, but it is toxic if it exceeds 20% by weight. Begins to appear and biological activity decreases. The same applies to both the anaerobic treatment process and the aerobic treatment process. In the present invention, by controlling the amount of organic matter supplied to the system so that the selenium content does not exceed 20% by weight, the amount of sludge produced is changed and the selenium content in the biological sludge is maintained at 20% by weight or less. This reduces the toxicity of selenium and increases the removal rate of selenium and organic matter.
[0024]
The selenium content in the biological sludge is represented by SeX in the following formula (1) .
SeX = ΔSe / ΔX (1)
ΔX = ΔBOD · Y (2)
However, SeX: Selenium content in sludge (-)
ΔSe: removed selenium (mg / l)
ΔX: Sludge production (mg / l)
ΔBOD: removed BOD (mg / l)
Y: Sludge conversion rate [0025]
In the present invention, the organic substance (BOD) is supplied so as to give ΔX that SeX does not exceed 0.2 (20 wt%). From the viewpoint of removing selenium, the smaller the amount of organic matter such as methanol supplied to the removed selenium, the better the efficiency.However, the smaller the amount of organic matter supplied, the less sludge is produced and the higher the selenium content. The organic substance is supplied so as not to exceed 20% by weight. In this case, an oxygen source (for example, NOx) for decomposing excess organic matter is also added in the same manner.
[0026]
In the denitrification reaction, BOD about 3 times as much as the amount of denitrification N is required, and 10 to 30% by weight is Δx with respect to this BOD. In the organic acid fermentation and hydrogen production reaction, the sludge conversion rate varies greatly depending on the type of organic matter, and Δx is in the range of 5 to 50% by weight of the removed BOD. In the aerobic treatment, 20 to 50% by weight of the removed BOD is Δx.
[0027]
The sludge generation amount also changes depending on the BOD sludge load, and the sludge generation amount increases if the BOD sludge load increases, and decreases if the BOD sludge load is low. This is expressed by the following equation.
Δx = a · ΔBOD−Kd · X (3)
However, Kd: autolysis rate constant (day ⁻¹ )
X: Sludge amount (g)
[0028]
From the above relationship, the organic substance addition amount is controlled so that the selenium content in the sludge does not exceed 20% by weight. Such an organic substance addition amount is controlled so that the selenium content in the sludge does not exceed 20% by weight in both the anaerobic treatment process and the aerobic treatment process. When the anaerobic treatment process and the aerobic treatment process are operated in independent biota, only the anaerobic treatment process can be controlled, but when the biological sludge is transferred between the processes, each process contains selenium. It is necessary to control so that the rate does not exceed 20% by weight.
[0029]
Control for maintaining the selenium content in the sludge as described above is performed not only in the case of treatment with floating biological sludge, but also in the case of treatment with a fixed bed type, biofilm filtration type, or the like. In the case of a fixed bed type or a biofilm filtration type, an air diffuser is provided in the middle of the packed bed in a single tank, and the lower part can be an anaerobic treatment process and the upper part can be an aerobic treatment process. In these cases, the addition of oxygen sources such as (nitrite) ions should be slightly less, and the organic matter should be added in excess of the amount of organic matter necessary for denitrification, etc., and the excess amount of organic matter added should be controlled by the above formula. Is preferred.
[0030]
In this way, by controlling the supply amount of organic matter so that the selenium content in the biologically treated sludge does not exceed 20% by weight, it is possible to prevent the inhibition by selenium and maintain the activity of the biological sludge high, Thereby, the removal rate of selenium and organic substances can be increased, and high-quality treated water can be obtained.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a system diagram showing a processing apparatus according to an embodiment of the present invention. In FIG. 1, 1 is an anaerobic treatment tank, 2 is an aerobic treatment tank, 3 is a solid-liquid separation tank, and 4 is a control device.
[0032]
In the treatment method of FIG. 1, raw water (selenium-containing water) is introduced into the anaerobic treatment tank 1 together with the return sludge from the return sludge path 6 and mixed with the biological sludge in the tank gently stirred by the agitator 7. Then, anaerobic treatment is performed to reduce the selenium compound. At this time, an organic substance such as methanol is introduced from the organic substance path 8 as a substrate, and an oxygen source such as (nitrite) nitrate is introduced from the oxygen source path 9. These do not need to be introduced when present in raw water.
[0033]
When using denitrifying bacteria as biological sludge, (nitrogen) nitrate is introduced as an oxygen source, when using sulfate-reducing bacteria, sulfate is introduced, and when using acid-producing bacteria, these are not necessary. Underwater organic substances are used as they are. In the anaerobic treatment tank 1, using these oxygen sources, oxygen in the selenium compound is also utilized when respiration without molecular oxygen such as nitric acid respiration is performed, and the selenium compound is reduced and insolubilized.
[0034]
Selenium insolubilized in the anaerobic treatment tank 1 is adsorbed by biological sludge to form sludge. The liquid in the tank containing such sludge is introduced into the aerobic treatment tank 2 from the transfer path 11, is aerated by the air introduced from the air path 13 through the air diffuser 14 by the blower 12, undergoes aerobic treatment, and remains. Organic substances and other oxidizable substances that oxidize and are decomposed.
[0035]
The liquid mixture in the aerobic treatment tank 2 is extracted from the transfer path 15 to the solid-liquid separation tank 3, where it is separated into solid and liquid, and the separated liquid is discharged from the treatment water path 16 as treated water. A part of the separated sludge is returned from the return sludge passage 6 to the anaerobic treatment tank 1, and the remaining portion is discharged from the waste mud passage 17 as surplus sludge.
[0036]
In the above processing, the selenium content rate signal 21 of the sludge obtained by analysis is input to the control device 4, the organic matter supply amount is calculated based on the signal, and the organic matter supply pump 22 is controlled to control the organic matter supply amount. To do.
[0037]
As a result, the selenium content in the biological sludge in the anaerobic treatment tank 1 and the aerobic treatment tank 2 is maintained so as not to exceed 20% by weight, and the activity of the biological sludge does not decrease. In the aerobic treatment tank 2, selenium is not eluted due to aeration so that the oxidation-reduction potential is 160 mV or less, and organic substances and other oxidizable substances are oxidized and decomposed. The selenium compound thus reduced and insolubilized in the anaerobic treatment tank 1 maintains an insoluble state in the aerobic treatment tank 2 and is separated as a solid in the solid-liquid separation tank 3, and a part thereof is discharged as excess sludge. The
[0038]
In FIG. 1, the anaerobic treatment liquid in the anaerobic treatment tank 1 is directly transferred to the aerobic treatment tank 2, but another solid-liquid separation tank is provided after the anaerobic treatment tank 1, and the separated sludge is returned to the anaerobic treatment tank 1. By doing, you may process the anaerobic processing tank 1 and the aerobic processing tank 2 with the independent biological sludge. Although the anaerobic treatment tank 1 and the aerobic treatment tank 2 are floating biological sludge, a treatment using a fixed biological sludge such as a biofilm method may be performed.
[0039]
【Example】
Examples of the present invention will be described below. In each example,% is% by weight.
Example 1
One synthetic wastewater containing Na ₂ SeO ₄ 40 mg / l, NaNO ₃ 600 mg / l, KH ₂ PO ₄ 50 mg / l, K ₂ HPO ₄ 50 mg / l, MgSO ₄ .7H ₂ O 50 mg / l, methanol 500 mg / l .2 was introduced into a liter anaerobic treatment tank and subjected to anaerobic treatment at pH 7.4 and residence time of 24 hours. The mixed liquid was subjected to solid-liquid separation in a 2 liter solid-liquid separation tank, a part of the separated sludge was returned to the anaerobic treatment tank, and the remainder was discharged as excess sludge.
As a result, the treated water Se was stable at 0.25 mg / l or less and the Se removal rate was 95% or more for 60 days. MLSS in the anaerobic treatment tank 30 days after the start of operation was 3800 mg / l, MLVSS / MLSS was 77%, and the selenium content of sludge was 12% (vs. MLSS).
[0040]
Example 2
The treatment was performed under the same conditions as in Example 1 except that Na ₂ SeO ₄ was changed to 60 mg / l. As a result, the treated water Se was 0.25 mg / l or less, the Se removal rate was 95% or more, and stable treatment was possible for 30 days. The MLSS in the anaerobic treatment tank 30 days after the start of operation was 3900 mg / l, MLVSS / MLSS was 68%, and the selenium content of sludge was 19%.
[0041]
Example 3
The treatment was performed under the same conditions as in Example 1 except that Na ₂ SeO ₄ was changed to 80 mg / l. Se removal became good about 10 days after the start of operation, and the treated water Se was 0.1 mg / l or less at the best, but Se removal deteriorated after about 30 days, and it can be completely removed at 6 weeks. lost. MLSS in the anaerobic treatment tank 30 days after the start of operation was 3900 mg / l, MLVSS / MLSS was 63%, and the Se content of sludge was 21%. After the removal, methanol in the synthetic wastewater was increased to 750 mg / l and NaNO ₃ was increased to 900 mg / l. As a result, the Se content of the sludge decreased from 22% to 19%, and the treated water Se increased from 12 mg / l to 1 mg. / L.
[0042]
From the above results, it can be seen that by removing the Se content in the sludge so as not to exceed 20 mg / l, the Se removal rate can be increased and stable treatment can be performed.
[0043]
【The invention's effect】
According to the present invention, by controlling the amount of organic matter supplied so that the selenium content in the biological sludge does not exceed 20% by weight, the biological sludge can be kept at a high activity by preventing the biological sludge from decreasing its activity. There is an effect that the removal rate of selenium and organic substances is increased, whereby selenium and organic substances can be stably removed.
[Brief description of the drawings]
FIG. 1 is a system diagram showing a processing apparatus according to an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Anaerobic processing tank 2 Aerobic processing tank 3 Solid-liquid separation tank 4 Control apparatus 5 Raw water path 6 Return sludge path 11, 15 Transfer path 12 Blower 13 Air path 14 Air diffuser 16 Treatment water path 17 Waste mud path 21 Selenium content rate signal 22 Organic substance supply pump

Claims (2)

In a method of reducing selenium by bringing selenium-containing water into contact with biological sludge in an anaerobic state and adsorbing the insoluble selenium compound to the biological sludge,
An anaerobic treatment process in which selenium-containing water and organic substances are supplied and contacted with biological sludge in an anaerobic state; and
A control process for controlling the amount of organic matter supplied to the system so that the selenium content in the biological sludge represented by SeX in the following formula (1) does not exceed 20 wt % ( vs. MLSS) ;
A method for treating selenium-containing water, comprising:
SeX = ΔSe / ΔX (1)
ΔX = ΔBOD · Y (2)
However, SeX: Selenium content in sludge (-)
ΔSe: removed selenium (mg / l)
ΔX: Sludge production (mg / l)
ΔBOD: removed BOD (mg / l)
Y: Sludge conversion rate The method of Claim 1 including the aerobic treatment process which transfers the biological sludge in an anaerobic treatment process and performs aerobic treatment.

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