JP3944981B2 - Method for treating selenium and nitrogen-containing water - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for detoxifying selenium and nitrogen-containing water by biological treatment.
[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, a report on the Annual Meeting of the Water Environment Society, 1995, P176 reports that Se ⁶⁺ and Se ⁴⁺ are reduced in the presence of lactose by (sub) selenate-reducing bacteria. 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]
For this reason, when the selenium-containing water contains organic or ammoniacal nitrogen, the organic matter is decomposed by the nitrification process using a nitrifying bacterium and aerated, and the organic and ammoniacal nitrogen are oxidized to (sub) nitric nitrogen. Then, the selenium compound can be reduced together with denitrification by anaerobic treatment using denitrifying bacteria, and if necessary, the excess organic matter can be decomposed by re-aeration treatment.
[0004]
However, in the nitrification process and the re-aeration process, the selenium compound reduced in the denitrification process is oxidized again and eluted, so that the treatment becomes incomplete, and the oxidized selenium compound is used as a substrate to reduce again. A large amount of organic matter is required. For example, when the treatment is performed by combining the nitrification step and the denitrification step, when the sludge from the denitrification step is returned to the previous nitrification step and the nitrification treatment is performed, the selenium compound insolubilized by the reduction is oxidized and eluted. In order to reduce the eluted selenium compound again in the denitrification step, the amount of organic substance added as a substrate increases.
[0005]
In this way, when combining the nitrification process and the denitrification process, in order to prevent the elution of the selenium compound and increase the selenium removal rate, and to reduce the amount of organic matter to be added, a solid-liquid is separately provided after each process. A method of providing a separation step and returning the separated sludge to the immediately preceding step has been proposed (Japanese Patent Laid-Open No. 9-1180). In this method, since the sludge in the denitrification process is returned only to the denitrification process, elution of insolubilized selenium is prevented, the selenium removal rate is increased, and the amount of organic substances used is also reduced.
[0006]
However, in such a processing method, since a solid-liquid separation step is provided for each step and sludge is returned to the immediately preceding step, the apparatus and operation become complicated and the processing cost increases.
[0007]
On the other hand, as a method for denitrification of nitrogen-containing water, generally the nitrification process, denitrification process, re-aeration process, and solid-liquid separation process are combined in this order, and a part of the separated sludge in the final solid-liquid separation process is nitrified. The device is designed and processed to be returned to the process. The reason for providing the solid-liquid separation step in the final stage is to concentrate and improve the efficiency of the apparatus and operation for solid-liquid separation. The reason why the separated sludge is returned to the nitrification process is to secure the necessary amount of sludge because the sludge growth rate in the nitrification process is low, and the total amount including the amount returned to the denitrification process and the re-aeration process. Returned to the nitrification process.
[0008]
When selenium is removed using an existing processing apparatus employed in such a conventional denitrification method, the insolubilized selenium is eluted as described above, and the processing efficiency is lowered. Providing a solid-liquid separation tank in each process in order to prevent this is a significant change in the apparatus and is difficult to cope with.
[0009]
[Problems to be solved by the invention]
The object of the present invention is to prevent elution of selenium compounds and increase the removal rate of selenium and nitrogen and reduce the amount of organic substances to be used even when combining a nitrification process and a denitrification process. The present invention is to propose a method for treating selenium and nitrogen-containing water that can remove selenium and nitrogen compounds stably and efficiently at low cost, and can use existing equipment as it is. .
[0010]
[Means for Solving the Problems]
The present invention is the following method for treating selenium and nitrogen-containing water.
(1) a nitrification step in which selenium and nitrogen-containing water are contacted with a nitrifying bacterium fixed to a carrier and aerated for nitrification;
A denitrification step in which the nitrification solution is brought into contact with denitrifying bacteria under anaerobic conditions , organic matter is added to perform denitrification, and selenium is reduced;
The denitrified solution, without going through the nitrification step, by performing the aeration required degree of the decomposition of organic matter, and re-aeration step of decomposing the excessively added organics,
A solid-liquid separation step for solid-liquid separation of the re-aeration treatment liquid ;
The portion of the separated sludge, how to process the selenium and nitrogen-containing water and a returning step for returning to the denitrification step without going through the nitrification process.
(2) The method according to (1) above, wherein the addition amount of the organic substance in the denitrification step is 3 to 5 times as much as BOD with respect to the nitrogen amount of (nitrous) nitric acid.
(3) The method according to (1) or (2) above, wherein the amount of aeration in the re-aeration step is such that DO is 1 to 3 mg / l and the residence time is 0.5 to 4 hours.
[0011]
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, respectively. These may be simply described as Se.
In the present invention, “(nitrous) nitrile” means “nitric acid and / or nitrous acid”.
[0012]
The selenium and nitrogen-containing water to be treated in the present invention are Se ⁶⁺ and / or Se ⁴⁺ selenium compounds, and waste water and other water containing ammoniacal and / or organic nitrogen compounds. ^{Examples of} Se ⁶⁺ or Se ⁴⁺ selenium compounds include (sub-) selenoic acid. Specific selenium and nitrogen-containing water include metal refining industrial wastewater, glass industrial wastewater, chemical industrial wastewater, and wastewater from coal, petroleum or combustion exhaust gas treatment processes. These selenium and nitrogen-containing water may contain other organic substances and inorganic substances in addition to selenium and nitrogen compounds. When water containing only a selenium compound or only a nitrogen compound is produced separately, these can be mixed and treated.
[0013]
The present invention is a method of biologically treating such selenium and nitrogen-containing water, and in particular, the treatment is performed utilizing nitric acid respiration. Thus nitrogen compounds and nitrification in the nitrification process, resulting (sub) also selenium during denitrification by nitrate action of denitrifying bacteria by adding organic matter in the denitrification step the insolubilized by reducing, denitrified liquid By performing aeration to the extent necessary for the decomposition of organic matter in the re-aeration step without passing through the nitrification step, the excessively added organic matter is decomposed and the re-aeration treatment solution is solidified without going through the re-aeration step. It separates in the liquid separation process, and a part of the separated sludge is returned to the denitrification process without going through the nitrification process . Nitrifying bacteria used in the nitrification process are nitrifying bacteria used in ordinary nitrification and denitrification treatments, including nitrobacter and the like, which oxidize organic or ammonia nitrogen to (nitrite) nitrate nitrogen.
[0014]
In the nitrification process, raw water containing selenium and nitrogen is contacted with nitrifying bacteria fixed on a carrier and aerated to decompose organic matter under aerobic conditions, and nitrification is performed to convert organic or ammonia nitrogen ( Sub-) Convert to nitrate nitrogen. As the nitrification tank used in the nitrification process, an aeration tank equipped with a conventionally used aeration device can be used.
[0015]
As a carrier for fixing nitrifying bacteria, a granular carrier having a specific gravity of 0.5 to 1.5 such as sponge or gel is preferable, but a fixed bed type carrier may be used, and in some cases, polyvinyl alcohol or polyethylene glycol is used. Inclusive immobilized nitrifying bacteria may be used. The existing nitrification tank can be used in the nitrification tank of the present invention simply by introducing a granular carrier such as sponge or gel. The amount of carrier used is about 10 to 40% of the tank volume.
[0016]
In order to fix the nitrifying bacteria to the carrier, raw water is introduced into the nitrification tank, mixed aeration with the carrier is performed, and further aeration is performed after the organic matter is decomposed. Contained sludge grows on the carrier. At the stage of starting up the apparatus, activated sludge may be introduced from another apparatus and inoculated, or sludge may be returned and propagated.
When nitrifying bacteria are fixed to the carrier in this way, nitrifying bacteria necessary for nitrification are retained, and thereafter nitrification can be performed without returning sludge.
[0017]
In the nitrification process, the raw water is introduced into the nitrification tank and brought into contact with the nitrifying bacteria fixed on the carrier and aerated, so that organic substances in the raw water are aerobically decomposed, and organic or ammoniacal in the raw water Nitrogen is oxidized to (nitrite) nitrate nitrogen. In the nitrification process, organic matter in raw water is decomposed, and the minimum amount of aeration necessary to oxidize organic or ammonia nitrogen to (sub) nitrite nitrogen is performed. Generally, the amount of aeration is dissolved oxygen. The residence time is about 2 to 48 hours so that (DO) is 1 mg / l or more.
[0018]
In the denitrification step, denitrification is performed by bringing the nitrification solution of the nitrification step into contact with denitrifying bacteria under anaerobic conditions, and (selenium) selenate is reduced. In this case, the selenium compound is (selenic) selenate in the nitrification step, but is reduced by anaerobic treatment in the denitrification step.
The denitrifying bacteria used in the denitrifying process are bacteria that decompose organic substances by utilizing oxygen of (nitrite) nitrate ions by nitrate respiration, and include facultative anaerobic bacteria such as Pseudomonas. Such denitrifying bacteria can be grown spontaneously by mixing the nitrification liquid containing sludge flowing in from the nitrification process with organic matter and keeping it anaerobic, but by introducing activated sludge from other processing equipment It can also be connected.
[0019]
The denitrification process is performed in the same way as the denitrification process in the normal nitrification denitrification method. The nitrification liquid is introduced into the denitrification tank, mixed with sludge containing denitrification bacteria, and maintained in an anaerobic state. The (nitrogen) nitric nitrogen in the inside is reduced to nitrogen gas, and at the same time, (nitrogen) selenic acid is reduced to an insoluble selenium compound. At this time, the separated sludge is returned from the solid-liquid separation step and mixed. In the denitrification step, denitrifying bacteria may be fixed to the carrier as in the nitrification step, but the treatment is usually performed without fixing.
[0020]
In the denitrification step, (nitrite) selenate, that is, Se ⁶⁺ and / or Se ⁴⁺ is reduced and insolubilized by contacting the nitrification solution with denitrifying bacteria in an anaerobic state. 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 denitrification step 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. In the denitrification step, the presence of (nitrite) nitrate ions in the reaction system makes it possible to maintain a high activity of denitrifying bacteria against selenium, thereby reducing (nitrous) selenate efficiently.
[0021]
In the denitrification process, an oxygen source and a nutrient source are required for respiration of denitrifying bacteria. Since the oxygen source is in an anaerobic state, oxygen contained in the form of (nitrous) nitric acid is used instead of molecular oxygen. As a nutrient source, organic substances such as methanol are added and used as a substrate together with organic substances contained in the returned sludge. As a result, the denitrifying bacteria are maintained at a high activity, and (sub) selenoic acid is reduced along with their degradation.
[0022]
It is preferable that the organic substance added as a nutrient source is an organic substance amount necessary for the reduction of (nitrous) nitric acid and (nitrous) selenate or a small excess thereof. As a result, even if the re-aeration process subsequent to the denitrification process is omitted or a re-aeration process is provided, the amount of selenium compound oxidized by re-aeration can be reduced. The addition amount of the organic substance is generally 3 to 5 times as much as BOD with respect to the nitrogen amount of (nitrous) nitrite.
[0023]
The residence time in the denitrification tank is the time required for the reduction of (nitrous) nitric acid and (sulphite) selenate ions, but this can also be taken as the time required for the decomposition of organic substances present in the system. The time required for performing only denitrification can be 1.1 times or more. The sludge concentration in the denitrification tank is preferably maintained at 500 to 50000 mg / l, preferably 2000 to 20000 mg / l.
[0024]
The treatment liquid in the denitrification step is re- aerated in the re - aeration step without going through the nitrification step, and decomposes the excessively added organic matter. In the re-aeration process, the treatment liquid of the denitrification process is introduced into the re-aeration tank together with the sludge and aeration is performed, so that facultative anaerobic bacteria contained in the sludge decompose organic matter under aerobic conditions. The aeration here can prevent the oxidation of selenium by making it necessary to decompose organic matter. The amount of aeration is 1 to 3 mg / l DO, and the residence time is about 0.5 to 4 hours.
[0025]
A solid-liquid separation process is operation which isolate | separates a re-aeration process liquid into a process liquid and sludge. The treatment solution is discharged as it is or after-treatment as necessary. A part of the separated sludge is returned to the denitrification process as a return sludge without passing through the nitrification process, and the remaining part is sent to a selenium recovery device or the like as sludge containing selenium. Examples of the solid-liquid separation means include a precipitation separation device, a filtration separation device, and a membrane separation device.
[0026]
The return step is a step of returning the amount necessary for the denitrification step of the separated sludge to the denitrification step, thereby maintaining the denitrification tank at a predetermined sludge concentration and maintaining the reducing power of nitrogen and selenium.
[0027]
In the present invention, by using nitrifying bacteria immobilized on a carrier in the nitrification step, the required amount of nitrifying bacteria can be retained in the nitrification tank, thereby making it unnecessary to return the sludge to the nitrification tank, and the solid-liquid in the nitrification process Separation can be eliminated. Therefore, the separated sludge in the solid-liquid separation process can be returned to the denitrification process without going through the nitrification process, preventing selenium oxidation in the nitrification process and minimizing the amount of organic substances added in the denitrification process. be able to.
[0028]
Even when a re-aeration process is provided, oxidation of the selenium compound in the re-aeration process can be prevented by minimizing the amount of aeration in the re-aeration process necessary for the oxidation of organic substances. Therefore, even if the separated sludge after the re-aeration process is returned to the denitrification process, the amount of organic matter added in the denitrification process can be reduced, and the installation of a solid-liquid separation tank dedicated to the denitrification process can be omitted. it can.
[0029]
For this reason, selenium and nitrogen-containing water can be treated simply by introducing a carrier into a nitrification tank using an existing nitrification denitrification apparatus provided for conventional nitrification denitrification. Costs can be reduced, and the operation can be performed with almost the same operation as in the case of conventional nitrification and denitrification alone.
[0030]
【The invention's effect】
As described above, in the present invention, nitrification is performed using the nitrifying bacteria immobilized on the carrier in the nitrification step, the nitrification solution is brought into contact with the denitrifying bacteria under anaerobic conditions in the denitrification step, and organic matter is added to perform denitrification. At the same time, selenium is reduced, and the denitrification solution is aerated in the re-aeration process without passing through the nitrification process, thereby decomposing excess organic substances and re-aeration treatment. The liquid is separated into solid and liquid in the solid-liquid separation process, and part of the separated sludge from the solid-liquid separation process is returned to the denitrification process without going through the nitrification process. Combine the nitrification process and the denitrification process. Even in this case, it is possible to prevent the elution of selenium compounds by a simple device and operation to increase the removal rate of selenium and nitrogen, and to reduce the amount of organic substances to be used. And it is possible to remove at low cost, it is possible to accept the existing device.
[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 the processing method of the embodiment. In FIG. 1, 1 is a nitrification tank, 2 is a denitrification tank, 3 is a re-aeration tank, and 4 is a solid-liquid separation tank.
[0032]
In the method for treating selenium and nitrogen-containing water, raw water is introduced into the nitrification tank 1 from the raw water channel 5, brought into contact with biological sludge containing nitrifying bacteria fixed to the carrier, and air is passed from the air channel 6 through the air diffuser 7. When introduced and aerated, it decomposes organic matter and nitrifies organic and ammoniacal nitrogen to (nitrite) nitrate nitrogen. The nitrification tank 1 is provided with a screen 8 to prevent the carrier from flowing out.
[0033]
The nitrification solution in the nitrification tank 1 is taken out through the screen 8, introduced into the denitrification tank 2 from the system 9, mixed with the return sludge returned from the return sludge path 10 and sludge in the tank and brought into contact with the denitrification bacteria, A denitrification process is performed by supplying an organic substance such as methanol from the supply path 11 and gently stirring with a stirrer 12 to maintain an anaerobic state. Thereby, (nitrogen) nitric nitrogen is reduced to nitrogen gas, and (nitrogen) selenic acid is also reduced to an insoluble selenium compound.
[0034]
The denitrification liquid in the denitrification tank 2 is introduced into the re-aeration tank 3 from the system path 13 together with the sludge, and is aerated by the air introduced through the air diffuser 15 from the air path 14 to decompose the remaining organic matter. Here, oxidation of the selenium compound is prevented by limiting the aeration intensity to a level necessary for the oxidation of the organic matter.
[0035]
The processing liquid in the re-aeration tank 3 is introduced into the solid-liquid separation tank 4 from the system path 16 to perform solid-liquid separation, and the separated liquid is discharged out of the system from the processing water path 17 as processing water. Part of the separated sludge is returned from the return sludge passage 10 to the denitrification tank 2, and the remainder is discharged out of the system from the waste sludge passage 18 as excess sludge.
[0036]
In the above treatment, the nitrifying bacteria necessary for nitrification can be retained in the nitrifying tank 1 by introducing the carrier into the nitrifying tank 1 and fixing the nitrifying bacteria, and a solid-liquid separation device dedicated to the nitrifying tank is provided. Nitrification can be performed without returning sludge. The sludge adhering excessively to the carrier is peeled off and sent to the denitrification tank 2 together with the nitrification liquid, and finally discharged as surplus sludge together with other sludge.
[0037]
In the denitrification tank 2, (s) selenic acid is also reduced by adding more organic material than is necessary for the reduction of (nitrite) nitrate. The re-aeration tank can be omitted by limiting to the minimum amount required. Even when an existing re-aeration tank is used, it is better to add a small amount of organic matter.
[0038]
The amount of aeration in the re-aeration tank 3 is limited to the limit necessary for the oxidation of the remaining organic matter, thereby preventing elution due to oxidation of the insolubilized selenium compound to increase the selenium removal rate and returning it to the denitrification tank 2. The amount of organic matter added to reduce selenium contained in the sludge can be reduced.
[0039]
The amount of returned sludge to be returned in the return step is preferably the amount of sludge required for the reduction of (nitrous) nitric acid and (selenic) selenate in the denitrification step. In this case, the returned sludge is removed without passing through the nitrification tank 1. By returning to the nitriding tank 2, the selenium compound can be used for denitrification without being oxidized. By placing the denitrifying bacteria in an anaerobic-aerobic cycle in this way, the reducing activity of (nitrous) nitric acid and (nitrous) selenate can be maintained high. The sludge taken out from the waste sludge path 18 can be subjected to selenium recovery and other processes in the sludge treatment process.
[0040]
As the above-mentioned treatment apparatus, an existing apparatus can be used as it is as a nitrification denitrification treatment apparatus, and the denitrification and selenium removal treatment of selenium and nitrogen-containing water can be performed simply by introducing a carrier into the nitrification tank of the existing apparatus. Therefore, it can be used as it is.
[0041]
【Example】
Examples of the present invention and comparative examples will be described below.
Comparative Example 1
Ni 2 tank 4 liter, denitrification tank 2 liter, re-aeration tank 0.7 liter, precipitation tank 2 liter, Na ₂ SO ₄ : 5,000 mg / l, MgSO ₄ · 7H ₂ O: 1,000 mg / l, KH _The synthetic waste water containing ₂ PO ₄ : 50 mg / l and NH ₄ Cl: 250 mg / l was passed at a rate of 5 ml / min. In addition, sodium selenate was added to the synthetic wastewater as selenium to a concentration of 1 mg / l.
The water temperature of the nitrification tank was maintained at 20 ° C. Further, methanol was continuously passed through the denitrification tank at a rate of 2 g / day. The DO in the nitrification tank was maintained at 2-6 mg / l. Return sludge was returned from the sedimentation tank to the nitrification tank at a rate of 5 ml / min.
Excess sludge was extracted from the nitrification tank so that the MLSS was 4,000 mg / l. The amount of excess sludge withdrawn was 70 ml per day.
As a result, selenium contained in the raw water at 1 mg / l sometimes decreased to about 0.1 mg / l, but generally varied in the range of 0.2 to 0.5 mg / l.
[0042]
Example 1
In the above apparatus, the returned sludge was applied to the denitrification tank, and 400 ml of urethane sponge chopped into 3.5 mm squares was added to the nitrification tank. One week after the addition of the sponge, nitrification began to proceed almost 100%.
It extracted from the re-aeration tank so that MLSS of a denitrification tank might be set to 4,000 mg / l. The withdrawal amount was 70 ml per day.
The removal of selenium was good, and it was 0.1 to 0.15 mg / l for treated water compared to 1 mg / l raw selenium.
[0043]
From the above results, it can be seen that the selenium removal rate can be increased by introducing the carrier into the nitrification tank to retain the nitrifying bacteria and returning the returned sludge to the denitrification tank.
[Brief description of the drawings]
FIG. 1 is a system diagram of a processing method according to an embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Nitrification tank 2 Denitrification tank 3 Re-aeration tank 4 Solid-liquid separation tank 5 Raw water path 6, 14 Air path 7, 15 Air diffuser 8 Screen 9, 13, 16 System path 10 Return sludge path 11 Substrate supply path 12 Stirrer 17 Process Waterway 18 Waste sludge path

Claims (3)

A nitrification step in which selenium and nitrogen-containing water are contacted with a nitrifying bacterium fixed to a carrier and aerated for nitrification;
A denitrification step in which the nitrification solution is brought into contact with denitrifying bacteria under anaerobic conditions , organic matter is added to perform denitrification, and selenium is reduced;
The denitrified solution, without going through the nitrification step, by performing the aeration required degree of the decomposition of organic matter, and re-aeration step of decomposing the excessively added organics,
A solid-liquid separation process for solid-liquid separation of the re-aeration treatment liquid ;
A method for treating selenium and nitrogen-containing water, including a returning step of returning a part of the separated sludge to the denitrification step without going through the nitrification step . The method according to claim 1, wherein the amount of the organic substance added in the denitrification step is 3 to 5 times as much as BOD with respect to the nitrogen amount of (nitrous) nitric acid. The method according to claim 1 or 2, wherein the amount of aeration in the re-aeration step is such that DO is 1 to 3 mg / l and the residence time is 0.5 to 4 hours.

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