JP2004195424A - Removing method of nitrogen oxides such as nitric acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the expansion of granule sludge at a low cost, to prevent bonding of granules with each other and to improve the removing efficiency of nitrate nitrogen (nitrogen oxides) relating to a removing method for removing the nitrogen oxides such as nitric acid in a liquid to be treated in biological denitrification equipment filled with denitrifying bacteria granules or the like. <P>SOLUTION: In the removing method of removing the nitrogen oxides such as the nitric acid by passing the water to be treated containing the nitrogen oxides such as the nitric acid through to the biological denitrification equipment and denitrifying it, the passing speed of the water to be treated to the biological denitrification equipment is turned to 2-8 m/h. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a removal method for removing nitrogen oxides such as nitric acid in a liquid to be treated by a biological denitrification apparatus filled with denitrifying bacteria granules and the like. More specifically, the present invention relates to a method for denitrifying wastewater containing cations that react with carbon dioxide such as calcium to form carbonate.
[0002]
[Prior art]
In general, groundwater, river water, lake water, etc. may contain nitric acid or nitrous acid.For example, when used as raw water for water supply, there are standards that should contain nitric acid or nitrous acid. It is necessary to remove nitric acid and the like from raw water.
[0003]
As a method for removing such nitric acid, various methods have conventionally been adopted, and for example, inventions such as Patent Literature 1 and Patent Literature 2 have been made.
[0004]
[Patent Document 1]
JP-A-9-103799 [Patent Document 2]
JP 2000-70986 A
In these inventions, raw water containing nitric acid is separated into concentrated water and demineralized water by an electrodialysis device, and the separated concentrated water is supplied to a biological denitrification device filled with denitrifying bacteria granules to denitrate. Nitric acid treatment is used to remove nitric acid.
[0006]
[Problems to be solved by the invention]
By the way, in wastewater containing a large amount of calcium (Ca), it reacts with CO ₂ generated by a biological denitrification reaction to generate calcium carbonate (CaCO ₃ ) crystals.
In the biological denitrification reaction, OH ^- ions are also generated, and the pH in the biological denitrification device rises, so that the production of CaCO ₃ is promoted.
[0007]
Granulated sludge contains a mixture of denitrifying bacteria and CaCO ₃ crystals.When the amount of CaCO ₃ increases, the specific gravity of the granular sludge increases, and the granule sludge becomes difficult to expand and floats at the conventional water flow rate. It becomes difficult. Thereby, the contact between the granules and the water to be treated becomes poor, and the treatment efficiency is reduced. Further, when the granules are combined with each other to form a lump, a poor contact portion is further generated, and the efficiency is significantly reduced.
[0008]
In order to prevent the generation of scale, there is a method of lowering the pH by injecting an acid, as described in Patent Document 2, but such an acid injection causes an increase in running cost. Also.
[0009]
SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and at the same time, can improve the expansion of granule sludge at a low cost, can prevent the bonds between granules, and can remove nitrate nitrogen (nitrogen oxide). The task is to increase efficiency.
[0010]
[Means for Solving the Problems]
The present invention has been made to solve such problems, and a means for solving the problems is to pass treated water containing nitrogen oxides such as nitric acid to a biological denitrification apparatus. In the removal method for removing nitrogen oxides such as nitric acid by performing a denitrification treatment, the flow rate of the water to be treated to the biological denitrification apparatus is set to 2 to 8 m / h.
[0011]
If the flow rate of the water to be treated is less than 2 m / h, the denitrifying bacteria granules cannot be suitably expanded, and hence the contact between the water to be treated and the denitrifying bacteria is not performed well. This is because the object removal efficiency cannot be increased.
In particular, in the case of the water to be treated containing a calcium (Ca) content of 50 mg / l or more, it is preferable that the water content be 2.5 m / h or more.
[0012]
On the other hand, if the flow rate of the water to be treated exceeds 8 m / h, the denitrifying bacteria granules may inadvertently flow out of the biological denitrification apparatus, and a circulation pump for obtaining the flow rate is required. A great deal of power is required for that. Further, when a circulation pump is provided, a problem may occur due to scaling of CaCO ₃ or the like in the pump.
[0013]
In particular, if the flow rate of the water to be treated is set to 6 m / h or less, it becomes unnecessary to install a circulation pump in the system.
[0014]
From the above viewpoint, the more preferable range of the flow rate of the water to be treated is 2.5 to 6 m / h, and the most preferable range is 3 to 5 m / h.
[0015]
Another means for solving the above-mentioned problem is to remove nitrogen oxides such as nitric acid by passing treated water containing nitrogen oxides such as nitric acid through a biological denitrification apparatus and performing denitrification treatment. In the removal method, the water to be treated is passed through a biological denitrification device so that the expansion rate of the denitrification bacteria granules in the biological denitrification device is 2% or more, and more preferably, the denitrification bacteria granules The water to be treated is passed through a biological denitrification apparatus such that the expansion rate of the water is 3% or more.
[0016]
If the coefficient of expansion is less than 2%, the contact between the water to be treated and the denitrifying bacteria is not performed well, so that the efficiency of removing nitrogen oxides cannot be increased.
[0017]
Here, the expansion coefficient means that the height of the granule-filled layer at the time of stopping water flow is H _0, and the height of the granule-filled layer whose interface with the liquid layer has risen after water flow is H _1. , (H ₁ −H ₀ ) / H ₀ × 100.
[0018]
In the present invention, the upper limit of the expansion rate is not specified, but it is essential that the denitrifying bacteria granules do not inadvertently flow out of the biological denitrification apparatus. For example, the initial granule packed bed height H ₀ If is half the height of the biological denitrification device, theoretically expansion rates up to 100% would be acceptable. Therefore, even if the upper limit of the expansion rate is not determined, the invention described in claim 3 does not become unclear.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, in the embodiment of the present invention, when explaining a nitric acid removing method of one embodiment, a configuration of an apparatus used in the method will be described with reference to the drawings.
[0020]
In FIG. 1, reference numeral 1 denotes a raw water tank in which raw water containing nitric acid such as groundwater is stored.
[0021]
Reference numeral 2 denotes an electrodialysis device as a membrane separation device for supplying raw water from the raw water tank 1 and concentrating nitrate ions and the like in the raw water. The electrodialysis device includes a cation exchange membrane, an anion exchange membrane, and the like. This is for separating a certain nitric acid-containing water into treated water obtained by desalination treatment and concentrated water having a high concentration of nitric acid. The electrodialysis apparatus 2 is of a polarity conversion type that can convert electrodes on both sides.
[0022]
Reference numeral 17 denotes a concentrated water storage tank for storing the concentrated water separated by the electrodialysis device 2.
[0023]
Reference numeral 3 denotes an upward sludge blanket (USB) type biological denitrification apparatus for denitrifying the concentrated water stored in the concentrated water storage tank 17, and the bottom of the apparatus is provided with an inlet through which the concentrated water is introduced. A tube 4 is provided.
[0024]
Reference numeral 5 denotes a granule deposition layer containing denitrifying bacteria, which is laminated on the bottom of the biological denitrification device 3. Reference numeral 6 denotes a gas collision portion for separating nitrogen gas generated from the denitrifying bacteria contained in the granule deposition layer 5, and is formed in an umbrella shape.
[0025]
Reference numeral 7 denotes a gas collection unit for collecting gas generated in the biological denitrification device 3, which is formed in a substantially truncated conical shape with an open bottom surface, and is provided slightly above the gas collision unit 6. . Then, the nitrogen gas collected by the gas collecting unit 7 is discharged from the drawing pipe 8.
[0026]
Reference numeral 9 denotes a treated water outflow pipe for allowing treated water in the biological denitrification device 3 to flow out of the biological denitrification device 3, and is provided near the upper liquid level of the biological denitrification device 3. Reference numeral 10 denotes an oxidation-reduction potential measuring device for measuring the oxidation-reduction potential in the biological denitrification device 3, which is configured to be able to detect the oxidation-reduction potential by a sensor.
[0027]
Reference numeral 11 denotes a methanol storage tank for storing methanol to be supplied to the biological denitrification apparatus 3 as a hydrogen donor. The methanol storage tank is smaller than the theoretical injection amount required for the denitrification reaction (in this embodiment, the theoretical injection amount). (50-80% of the hydrogen donor) to the biological denitrification device 3, and methanol while controlling according to the oxidation-reduction potential measured by the oxidation-reduction potential measurement device 10. It is connected to the flow path 14 of the concentrated water between the electrodialyzer 2 and the biological denitrification apparatus 3 via two flow paths including a second flow path 13 for injection. The first channel 12 and the second channel 13 are provided with pumps 15 and 16, respectively.
[0028]
Then, a nitric acid removing method for removing nitric acid using the nitric acid removing apparatus having such a configuration will be described below.
[0029]
First, raw water such as groundwater is supplied from the raw water tank 1 to the electrodialysis apparatus 2. The raw water contains nitric acid and calcium. Depending on the quality of the raw water, pretreatment such as coagulation sedimentation and sand filtration is performed before the electrodialysis device 2.
[0030]
The raw water supplied to the electrodialysis device 2 is separated into a desalted treated water and a concentrated concentrated water, and the treated water is used as it is, and the concentrated water is supplied to the concentrated water storage tank through the flow path 14. Supplied to 17.
[0031]
The concentrated water supplied to the concentrated water storage tank 17 is then passed through the flow path 14 to the biological denitrification device 3. In the present embodiment, water is passed at a flow rate of 3 m / h.
[0032]
The concentrated water that has been passed through the biological denitrification device 3 flows in the biological denitrification device 3 in an upward flow from the bottom to the top. At this time, since the granular deposition layer containing the denitrifying bacteria is laminated on the bottom of the biological denitrification device 3, the raw water comes into contact with the denitrifying bacteria granules in an upward flow.
[0033]
While contacting the denitrifying bacteria granules, the nitrogen oxides in the raw water are converted into nitrogen gas by the denitrifying bacteria, and the concentrated water flows upwards of the biological denitrification device 3 while being purified as treated water.
[0034]
In this case, in the present embodiment, since the concentrated water is passed through the biological denitrification device 3 at a flow rate of 3 m / h, the expansion of the denitrifying bacteria granule sludge in the biological denitrification device 3 is promoted, The good contact between the concentrated water and the denitrifying bacteria improves the nitrate nitrogen removal efficiency. In addition, it is possible to prevent the granules from becoming large in size or agglomerated by physical action such as collision between granules.
[0035]
As a result of passing the concentrated water at such a flow rate, the expansion rate was 3% in the present embodiment. Therefore, the contact between the concentrated water and the denitrifying bacteria was performed well, and the efficiency of removing nitrate nitrogen was improved. improves.
[0036]
The nitrogen gas converted in the biological denitrification device 3 is moved to the upper part of the biological denitrification device 3 by the upward flow of the raw water and the floating gas. The treated water is discharged from the treated water discharge pipe 9 as purified water. The discharged purified water is subjected to a process such as pH adjustment as necessary and discharged to a river or the like.
[0037]
In the biological denitrification device 3, the reaction for converting nitric acid to nitrogen is not sufficiently performed if the amount of the hydrogen donor causing the reaction is insufficient. Therefore, methanol as a hydrogen donor is injected into the biological denitrification device 3 from the methanol storage tank 11.
[0038]
Methanol from the methanol storage tank 11 is continuously injected into the concentrated water flow path 14 via the first flow path 12.
[0039]
On the other hand, methanol injected into the concentrated water flow path 14 via the second flow path 13 is supplied from the methanol storage tank 11 while being controlled by the oxidation-reduction potential measuring device 10.
[0040]
In this case, the amount of methanol continuously injected through the first flow path 12 is set to 50 to 80% of the theoretical amount of methanol required to cause a denitrification reaction in the biological denitrification device 3. Further, the control value of the reduction potential measuring device 10 when methanol is injected while being controlled through the second flow path 13 is set to -100 mv to -200 mv.
[0041]
In the above embodiment, the flow rate of the concentrated water to the biological denitrification apparatus 3 is set to 3 m / h. However, the water flow rate is not limited to this, and in the present invention, the flow rate is 2 m / h. Water will be passed at a flow rate in the range of ~ 8m / h.
[0042]
The more preferable range of the water flow rate is 2.5 to 6 m / h as described above, the more preferable range is 3 to 5 m / h, and the most preferable range is 3 to 3.5 m / h.
[0043]
Further, in the above embodiment, since the polarity conversion type electrodialysis device capable of changing the polarity of the electrode is used as the electrodialysis device 2, the pH of the concentrated water to be separated becomes almost neutral, and the pH of the next pH adjustment is adjusted. However, the use of a polarity conversion type electrodialysis apparatus is not an essential condition of the present invention.
[0044]
Further, in this embodiment, the electrodialysis device 2 is used as a membrane separation device for separating raw water into treated water and concentrated water, but the type of the membrane separation device is not limited to this, and for example, a reverse osmosis membrane It is also possible to use a device or the like.
[0045]
Further, the use of such a membrane separation device is not an essential condition for the present invention. For example, raw water can be directly passed through the biological denitrification device 3 as water to be treated. Incidentally, it is preferable to use a membrane separation device for the treatment of tap water as in the above embodiment, but it is not necessary to use a membrane separation device when applying the present invention to factory wastewater.
[0046]
Further, in the above embodiment, a USB type biological denitrification device provided with a gas collision unit 6, a gas collection unit 7, and the like is used as the biological denitrification device 3, but the type of the biological denitrification device is not limited to this. It is not done. In short, any USB type biological denitrification apparatus using granules may be used.
[0047]
Furthermore, in the above embodiment, methanol was used as the hydrogen donor to be injected into the biological denitrification device 3, but the type of hydrogen donor is not limited to this, and the type of bacteria in the biological denitrification device 3 In other words, any material may be used as long as it causes a denitrification reaction of nitrogen oxides such as nitric acid as a hydrogen donor. For example, it may be hydrogen itself or acetic acid.
[0048]
Further, in the above embodiment, as a means for controlling the injection amount of the hydrogen donor, the control is performed by measuring the oxidation-reduction potential in the biological denitrification apparatus 3, but the control may be performed by other means. It is.
[0049]
Further, in the above embodiment, the first flow path 12 for continuously injecting methanol as a hydrogen donor, and the second flow path 13 for injecting methanol while being controlled by control means such as the oxidation-reduction potential measuring device 10 are provided. Since the methanol is injected from the two channels, the sensor of the oxidation-reduction potential measuring device 10 as the control means does not sense and even if the methanol is not injected from the second channel 13, the first Since methanol is continuously injected from the passage 12, a favorable effect of being able to cope with an unexpected situation was obtained, but providing a flow passage for continuously injecting a hydrogen donor is essential to the present invention. Not a condition.
[0050]
Further, the type of nitrogen oxide to be treated according to the present invention is not limited to nitric acid of the above-described embodiment, but may be nitrous acid.
[0051]
Further, the type of treated water to be treated is not limited to the raw tap water of the embodiment, and the present invention can be applied to treated water containing nitrogen oxides such as factory wastewater.
Further, in the above embodiment, the case where the present invention is applied to the water to be treated containing Ca has been described. However, in addition to Ca, a cation that reacts with CO ₂ to form carbonic crystals in the tank is contained. It is also possible to apply the present invention to the water to be treated.
[0052]
【Example】
Hereinafter, examples of the present invention will be described.
[0053]
(Example 1)
The concentrated wastewater having a pH of 7.5, 60.3 mg / l of NO ₃ -N, 141 mg / l of Ca, 38.8 mg / l of Mg, 0.2 mg / l or less of phosphorus, and 0.5 mg / l of DOC is used in the above embodiment. The concentrated waste water was denitrified by passing water through such a biological denitrification apparatus.
[0054]
The water flow rate was 3 m / h as in the above embodiment, and the expansion rate was 3%.
[0055]
As a result of such a denitrification treatment, in this example, NO ₃ —N could be reduced to 1.0 mg / l or less.
[0056]
(Example 2)
In this embodiment, the tested change in removal rate and granules expansion rate of NO ₃ -N accompanying the change in water flow rate using a denitrifying bacteria granules containing CaCO ₃ to about 85%. The same concentrated wastewater as in Example 1 was used as the water to be treated.
[0057]
The water flow rates were 0.7m / h, 2.0m / h, 3.0m / h, 4.7m / h, 6.0m / h and 17.5m / h.
[0058]
The result is shown in FIG.
[0059]
As is clear from FIG. 2, the NO ₃ -N removal rate was about 40% at 1 m / h or less, but at 2 m / h, it exceeded 50% and was close to 60%, and further 3 m / h. NO ₃ -N removal rate becomes more than the above 90%.
[0060]
【The invention's effect】
As described above, the present invention provides a method for removing nitrogen oxides such as nitric acid by subjecting water to be treated containing nitrogen oxides such as nitric acid to a biological denitrification apparatus and denitrifying the water. The flow rate of the water to be treated to the biological denitrification apparatus is 2 to 8 m / h, or the expansion rate of the granular sludge in the biological denitrification apparatus is 2% or more. Since the treated water is passed through the biological denitrification device, the expansion of the denitrifying bacteria granules in the biological denitrification device is promoted, and the contact between the raw water and the denitrifying bacteria is performed well, so that the nitrate nitrogen is removed. There is an effect that the removal efficiency is improved.
[0061]
In addition, since the water flow rate does not exceed 8 m / h, there is no danger of the denitrifying bacteria granules inadvertently flowing out of the biological denitrification device.
[0062]
Further, when the flow rate of the water to be treated to the biological denitrification apparatus is set to 2.5 to 6 m / h, the above effect is more improved, and particularly when the flow rate is set to 3 to 5 m / h, a better effect is obtained. Will be done.
[0063]
Furthermore, when the flow rate of the water to be treated to the biological denitrification device is set to 6 m / h or less, it is not necessary to install a circulation pump in the system, so no extra power is required. There are advantages.
[Brief description of the drawings]
FIG. 1 is a schematic side view showing an apparatus used for a nitric acid removal method as one embodiment.
FIG. 2 is a graph showing the correlation between the flow rate of water through a biological denitrification apparatus, the removal rate of NO ₃ —N, and the expansion rate of granules.
[Explanation of symbols]
3. Biological denitrification equipment

Claims (3)

In a removing method for removing nitrogen oxides such as nitric acid by passing treated water containing nitrogen oxides such as nitric acid through the biological denitrification device 3 and performing denitrification treatment, A method for removing nitrogen oxides such as nitric acid, wherein the flow rate of treated water is 2 to 8 m / h. In a removing method for removing nitrogen oxides such as nitric acid by passing treated water containing nitrogen oxides such as nitric acid through the biological denitrification device 3 and performing denitrification treatment, A method for removing nitrogen oxides such as nitric acid, wherein the flow rate of treated water is 2.5 to 6 m / h. In a removing method for removing nitrogen oxides such as nitric acid by passing treated water containing nitrogen oxides such as nitric acid through the biological denitrification apparatus 3 and performing denitrification treatment, A method for removing nitrogen oxides such as nitric acid, characterized in that the water to be treated is passed through a biological denitrification device so that the expansion rate of the nitrifying granules is 2% or more.

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