JP2000254688A - Biological denitrification treatment of waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To treat for a long period of time without the degradation in the nitric acid reduction effect of denitrification bacteria by adding an inorganic acid to waste water before the introduction thereof into a denitrification tank, thereby suppressing the deposition of calcium carbonate at the time of the biological denitrification treatment of the nitrogen-containing waste water which contains calcium ions. SOLUTION: In the case of the denitrification treatment of the waste water, a carrier carried with the denitrification bacteria is packed in the denitrification tank 1 and is maintained at a fluidization state by the waste water introduced from the lower part of the tank. At this time, a microbial cell peeling device 3, which is composed of agitating vanes having guides on the circumference and peels part of the denitrification bacteria adhered to the carrier by applying shearing force to the carrier floating to the surface, is actuated. In addition, part of a circulating liquid is withdrawn and the inorganic acid is added in addition to methanol necessary for the denitrification reaction at need in a circulation tank 5. The liquid withdrawn from the circulation tank 5 is again sent by a circulating pump 6 to the denitrification tank 1. The inorganic acid is added into the waste water in the manner described above, by which the deposition of the calcium carbonate is suppressed and the stable denitrification reaction is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for the biological denitrification of wastewater, and more particularly, to a method in which carbon dioxide gas generated during a biological denitrification reaction is combined with calcium ions in wastewater to form insoluble carbon dioxide. It relates to a method for producing calcium and preventing it from adhering to the denitrifying bacteria and reducing its action.

[0002]

2. Description of the Related Art It is well known to convert organic nitrogen or ammonia in wastewater containing no calcium ions into nitrate and biologically denitrify it. The principle of this method is that in a denitrification tank where denitrifying bacteria are cultured,
With the help of the added hydrogen donor such as methanol, nitrate is reduced, converted to nitrogen and released into the atmosphere. However, in this reaction, a hydroxyl group is generated, so that the treated wastewater becomes alkaline. Therefore, in order to maintain the action of the denitrifying bacteria, it is necessary to add an appropriate acid to neutralize the generated hydroxyl groups.

[0003] On the other hand, when nitrogen-containing wastewater containing high-concentration calcium ions is denitrified and removed by the above-mentioned method, the carbon dioxide gas generated during the reaction becomes carbonate ions and combines with the calcium ions to form the hardly soluble calcium carbonate. Is generated, and the generated alkalis are all neutralized by hydrogen ions generated during the generation of carbonate ions, so that the addition of the acid as described above is not required. However, in this case, the calcium carbonate precipitates on the cells in the carrier or sludge carrying the denitrifying bacteria and inhibits its function. There are problems such as deposition and adhesion that cause blockage of the flow path and increase in water treatment cost due to an increase in the amount of generated sludge. Among the above-mentioned problems, regarding the inhibition of the action of denitrifying bacteria by calcium carbonate, the amount of precipitated calcium carbonate is, of course, a certain amount relative to the amount of denitrifying bacteria, as well as other problems, in the case of precipitation in large amounts. Even if it exceeds, it cannot be ignored. Therefore, in the biological denitrification treatment of nitrogen-containing wastewater containing calcium ions, calcium carbonate generated in the treatment process is a factor that significantly affects the denitrification efficiency. Therefore, suppressing the production of the substance is one of the important issues.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems that occur when treating nitrogen-containing wastewater containing calcium by a biological denitrification method, and reduces the nitrate reduction action of denitrifying bacteria. It is another object of the present invention to provide a method that enables long-term processing without using any method.

[0005]

According to a first aspect of the present invention, there is provided a method for biologically denitrifying nitrogen-containing wastewater containing calcium ions, wherein an inorganic acid is added to the wastewater before the wastewater is introduced into a denitrification tank. This is a biological denitrification treatment method for wastewater, wherein precipitation of calcium carbonate is suppressed by the method.
The present invention according to claim 2 is the method according to claim 1, wherein the inorganic acid is hydrochloric acid. The present invention according to claim 3 is the method according to claim 1, wherein the inorganic acid is added in a range of 0.1 to 0.6 chemical equivalent to nitrate contained in the wastewater. Claim 4
The present invention according to claim 1 is the method according to claim 1, wherein the biological denitrification treatment is performed by a fluidized bed type denitrification apparatus.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific examples of the nitrogen-containing wastewater to which the present invention is applied include a nitrification reaction solution, a nitrate waste solution, and the like. The present invention provides a method for biologically denitrifying a nitrogen-containing wastewater containing calcium ions, by adding an inorganic acid to the wastewater under predetermined conditions to suppress the precipitation of calcium carbonate and stably It is intended to provide a method for enabling a denitrification reaction.

[0007] A denitrifying bacterium plays an important role in the biological denitrification process. Therefore, in this method, how to hold the denitrifying bacteria in the denitrification tank constitutes a feature of the process. One of the best-known methods is the denitrifying bacteria activated sludge method in which sludge containing denitrifying bacteria is returned. In this method, the concentration of denitrifying bacteria in the treatment tank is kept constant by returning the settled sludge (denitrifying bacteria) separated in the sedimentation tank to the treatment tank in which the denitrification reaction is performed by the denitrifying bacteria existing in a floating state. How to maintain. In this method, the specific gravity of the sludge is increased by the formation of calcium carbonate, which facilitates the separation of the sludge in the sedimentation tank and helps to increase the sludge concentration in the denitrification tank.
However, if the amount of precipitated calcium is not suppressed, the function of the denitrifying bacteria is inhibited.

[0008] As a second method, there is a method in which denitrifying bacteria are carried on a carrier and accumulated in a denitrification tank at high density.
In this method, the carrier may form a fixed bed or may form a fluidized bed. In a fixed bed, calcium carbonate precipitated on a carrier is hardly separated from the carrier by a treatment such as an increase in the amount of circulating fluid or a degree of recycling of carbon dioxide gas, which is different from denitrifying bacteria. For this reason, the carrier is covered with calcium carbonate, and eventually causes the passage to be blocked. This tendency is remarkable when the particle size of the carrier packed in the fixed bed is small. In such a case, it is necessary to make the precipitation of calcium carbonate almost zero. On the other hand, when the carrier forms a fluidized bed, by providing a mechanical peeling means in the denitrification tank, calcium carbonate precipitated on the carrier is constantly peeled off from the carrier like the bacterial cells,
Since the sludge is discharged to the outside, the amount of calcium carbonate can be kept almost constant in the denitrification tank, similarly to the denitrification bacteria. Therefore, in this case, the amount of precipitated calcium carbonate may be adjusted to such an extent that the precipitated calcium carbonate does not inhibit the function of the cells.

In the present invention, the influence of the amount of generated calcium carbonate on the denitrifying bacteria is evaluated using the weight ratio of the amount of calcium carbonate and the amount of organic matter (Ca / microbial cell value) of the substance adhering to the sludge or the carrier as an index. And The amount of calcium carbonate was defined as the weight obtained by subtracting the ignition loss at 600 ° C. from the dry weight at 105 ° C. of the adhered substance on the carrier, and the amount of organic matter was expressed as the ignition loss at 600 ° C. The present inventors have found that this Ca
/ It has been found that when the cell value exceeds 1, the function of the denitrifying bacteria starts to decrease. Therefore, in order to stably maintain the biological denitrification treatment method for wastewater according to the present invention, it is necessary to always keep the Ca / cell value at 1 or less. In wastewater having a composition in which this value exceeds 1, the precipitated calcium carbonate adversely affects the function of the denitrifying bacteria, and a sufficient denitrification reaction cannot be expected. FIG. 1 is a schematic diagram showing the results obtained from a denitrification test of a simulated wastewater containing NO ₃ ⁻ and Ca ²⁺ by the present inventors. The hatched area in this figure indicates that the drainage within this composition range does not sufficiently function as a denitrifying bacterium. That is, in the shaded region, the Ca / cell value exceeds 1, indicating that many of the denitrifying bacteria are included in the precipitated calcium carbonate, thereby inhibiting the action of the denitrifying bacteria.
In this figure, if the concentration of nitrate is very low or very high even in the region where the calcium concentration is very high, the denitrifying bacterium enters the region where the performance of the denitrifying bacterium works normally (that is, the normal performance). It is considered that the reason is that when the nitrate concentration is low, the amount of generated carbon dioxide gas is also small, and the amount of generated calcium carbonate falls below the solubility. Also, it is considered that when the nitrate concentration is high, the amount of generated carbon dioxide gas is large, so that all calcium ions are dissolved in the form of calcium bicarbonate.

[0010] In the case of performing a denitrification treatment of wastewater having a composition having a Ca / bacterium value exceeding 1, that is, a composition in a hatched region in Fig. 1, measures for stably performing this treatment were studied. The first conceivable method is to dilute the wastewater. Normal performance can be achieved by diluting Ca / cell concentration to 1 or less calcium ion and nitrate ion concentration. However, it requires a large amount of water and is suitable for industrial implementation. Not. The next conceivable method is to increase the pressure of carbon dioxide gas by making the denitrification tank a closed type and pressurizing the inside of the tank. In this method, since the concentration of bicarbonate ions increases and the concentration of carbonate ions decreases, precipitation of calcium carbonate can be prevented. However, this method is not a preferable method because the denitrification tank needs to have pressure resistance and the carbon dioxide gas pressure needs to be adjusted.

Therefore, the present inventors have studied to develop a simple method capable of stably treating wastewater having the composition of the shaded region in FIG. As a result, they found that by adding a small amount of an inorganic acid to the wastewater in advance or during the denitrification treatment, it was possible to operate with normal performance. That is,
By adding the inorganic acid to the wastewater before introducing it into the denitrification tank, the amount of generated calcium carbonate can be reduced, and the operation can be performed in a state where the performance of the denitrifying bacteria normally works. This is because some of the hydroxyl groups generated in the denitrification reaction are neutralized by inorganic acids, and the amount of acid required for neutralization of the remaining hydroxyl groups is reduced. It is thought that it stays as an ion. Moreover, the pH of the treated wastewater remains neutral regardless of whether an acid is added or not, so that the addition of the acid has no adverse effect on the denitrifying bacteria. Examples of the inorganic acid used in the present invention include hydrochloric acid and sulfuric acid, and hydrochloric acid is particularly preferred. When sulfuric acid is added, sulfuric acid-reducing bacteria grow in the denitrification tank, and this may consume excess methanol to generate hydrogen sulfide, which is a malodorous substance.

The amount of the inorganic acid to be added is in the range of 0.1 to 0.6, preferably 0.2 to 0.4 chemical equivalent, based on the sulfate group in the waste water. If the amount of the inorganic acid is less than 0.1 chemical equivalent, the formation of calcium carbonate cannot be effectively suppressed. On the other hand, as for the upper limit of the addition amount, since the formation of calcium carbonate can be suppressed by adding 0.6 chemical equivalent, even if an inorganic acid in an amount exceeding 0.6 chemical equivalent is added, a considerable effect is not obtained. When 0.1 chemical equivalent of an inorganic acid is added, a small amount of calcium carbonate is produced, but the Ca / cell value is kept at 1 or less. The location and timing of adding the inorganic acid to the wastewater may be any time before entering the denitrification tank, may be added to the tank storing the wastewater, or may be added to the wastewater immediately before the denitrification tank. When the denitrification tank is of a recycle type, it may be added directly to the denitrification tank or may be added to an appropriate place on the recycling line.

The apparatus used to carry out the method of the present invention may be any of a denitrification type activated sludge method (sludge circulation type), a type filled with denitrification bacteria supported on a carrier, and the like. Is preferably a fluidized bed type rather than a fixed bed type. FIG. 2 shows one embodiment of a fluidized bed type denitrification apparatus. The apparatus comprises a denitrification tank (1), a fluidized bed of a carrier carrying a denitrifying bacteria (2), a cell stripper (3), a screen for preventing carrier overflow (4), a circulation tank (5), and a circulation pump (6). , A drainage tank (7), a drainage supply pump (8), and a treated water outlet (9) as main components. Denitrification tank (1)
Is a carrier carrying denitrifying bacteria (for example, about 1.5 mm in diameter).
Degree of spherical activated carbon), and becomes fluidized by drainage introduced from the lower part of the tank. The microbial cell detaching device (3) is constituted by a stirring blade with a guide around the periphery, applies a shearing force to the carrier that has come up, and exfoliates a part of the denitrifying bacteria adhering thereto. By adjusting the rotation speed of the stirring blade, the level of the fluidized bed of the denitrifying bacteria-adhering carrier can be kept constant.

As the carrier, in addition to the above-mentioned activated carbon, those generally used in water treatment, for example, silica, glass, alumina and the like, and plastic particles such as polypropylene and polystyrene can be used. In addition, the carrier overflow prevention screen (4) is a screen with a fixed opening, and has an eye not larger than the diameter of the carrier, so that the treated water is discharged together with the bacteria detached from the carrier, but the carrier overflows. Is prevented. In the circulation tank (5), a part of the circulating liquid is withdrawn, and if necessary, an inorganic acid is appropriately added in addition to methanol necessary for the denitrification reaction. The liquid (treated water) extracted from the circulation tank is sent again to the denitrification tank by the circulation pump (6).

[0015]

Next, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples. Example 1 The wastewater was denitrified using the fluidized bed type denitrification apparatus shown in FIG. The denitrification tank (1) having an effective volume of 30 L was filled with immobilized denitrifying bacteria particles having an average particle size of about 2.5 mm so that the height of the stationary tank was 60%. The immobilized denitrifying bacteria particles are obtained by forming a simulated wastewater containing methanol and nitrate ions using a spherical activated carbon having an average particle size of 1.5 mm as a carrier to form a denitrifying bacteria biofilm on the surface of the carrier. Simulated drainage (composition: NaNO ₃ 5.3 g / L, CaCl ₂ 4.2 g / L, KH ₂ PO ₄
(0.04 g / L, CH ₄ OH 2.4 g / L) was introduced at a flow rate of 10 L / hour, and a fluidized bed was formed by setting the circulation rate to 900 L / hour. From the start of operation, a 10% aqueous hydrochloric acid solution was added to the circulation line at a rate of 0.08 L / hour. At this time, the added amount of hydrochloric acid is equivalent to 0.36 chemical equivalent to the nitrate group in the waste water. A denitrification treatment was performed under these conditions.

As a result, the amount of precipitated calcium carbonate was about 15% of the amount of calcium in the treated water, and a denitrification rate of 99% or more could be stably maintained for 1500 hours or more. Further, the pH of the treated water was in the range of 6.4 ± 0.2 throughout the entire operation, and it was shown that there was no influence on the denitrifying bacteria due to the pH fluctuation. Further, after the operation was completed, a part of the carrier carrying the denitrifying bacteria was taken out, only the deposits were separated from the carrier, and the deposits were dried and analyzed for calcium and organic carbon contents. The Ca / cell value defined in
It was 0.83.

Comparative Example 1 After the operation of Example 1 was completed, the operation was continued under the same conditions as in Example 1 except that the aqueous hydrochloric acid solution was not added to the circulation tank. When hydrochloric acid is not added, p
H was slightly higher than in Example 1 and was maintained in the range of 6.8 ± 0.2. However, it was found that the calcium ion concentration of the treated water was lowered, and the precipitated amount of calcium carbonate was clearly increased significantly. Table 1 shows the properties of the treated water discharged from the denitrification tank.

[0018]

[Table 1] Table 1 Denitrification rate (%) pH Ca ²⁺ concentration (% by weight) During acid addition operation 99.8 6.4 0.13 No acid addition Day 1 99.9 6.4 0.08 Day 2 100 6.4 0.05 Day 4 99.8 6.5 0.03 6th day 98.0 6.5 0.03 8th day 87.0 6.5 0.02 10th day 65.4 6.5 0.02 12th day 30.9 6.6 0.02 14th day G 33.4 6.5 0.02

As a result, it was found that the denitrification rate began to decrease from the sixth day after the addition of the acid was stopped, and that the performance of the denitrification tank was remarkably reduced. After the 12th day, the denitrification rate was 30
% Range. This was because the precipitated calcium carbonate was deposited on the carrier, the bacterial cell surface was covered, and the operation of adjusting the amount of methanol was not performed in response to the decrease in the amount of bacterial cells. It is considered to have received. Then, when the amount of added methanol was decreased in accordance with the decrease in the number of cells, the denitrification rate was restored to the order of 60%. However, when the denitrification treatment was performed without adding an inorganic acid, it was inevitable that the denitrification bacteria were reduced due to the precipitation of calcium carbonate, and the denitrification treatment performance was reduced.
It was confirmed by an electron microscope that the surface of the carrier was covered with calcium carbonate and the amount of denitrifying bacteria was reduced.

Example 2 A denitrification treatment was performed using the apparatus shown in FIG. The apparatus includes a denitrification tank (10), a cylindrical cylindrical nonwoven fabric carrier (15 mm in diameter, 15 mm in length, 2 mm in thickness), a filling tank (11), a wire mesh for preventing overflow of the carrier (12), and a drainage tank ( 13), drainage supply pump (1
4) It comprises a treated water outlet (15), a blower (16) for supplying headspace gas to the bottom of the tank and mixing the inside of the tank by aeration, and a gas disperser (17). Denitrifying bacteria are immobilized on the carrier of the filling tank (11) at a density of about 10,000 mg / L based on the effective volume of the denitrifying tank. This denitrification tank, the simulated wastewater _{(composition: NaNO 3 16.7g / L, CaCl} 2 1
1.5 g / L, KH ₂ PO ₄ 0.12 g / L, CH ₄ OH 6.9 g / L, HCl
(3.6 g / L) at a flow rate of 10 L / hour from the lower part of the tank. 0.36% by weight of hydrochloric acid was previously added to this simulated wastewater.
(Equivalent to 0.50 chemical equivalents relative to the nitrate in the wastewater). As a result of continuous operation, almost no precipitation of calcium carbonate was observed during the operation, and a stable denitrification rate of 95% or more
Operation could be continued for more than 000 hours. In addition,
The pH of the treatment liquid during the entire operation period was in the range of 6.3 ± 0.2, and there was no influence on the denitrifying bacteria due to the pH fluctuation. On the other hand, when denitrification treatment was similarly performed on the simulated wastewater to which hydrochloric acid was not added, 90% or more of the calcium ions contained in the wastewater were precipitated as calcium carbonate, deposited on the carrier, and denitrified. Decreased significantly. From the above, it was shown that by adding hydrochloric acid to wastewater, long-term normal operation was possible while maintaining a high denitrification rate.

Example 3 Using the same denitrifying apparatus as in Example 1, the wastewater was denitrified by gradually changing the amount of hydrochloric acid to be added. First, a denitrifying bacterium was attached to spherical activated carbon having a diameter of about 1.5 mm, and this was grown in a wet state until the diameter became about 2.5 mm. The immobilized denitrifying bacteria particles were filled in a denitrification tank at a stationary bed height of 60%, and the simulated wastewater used in Example 1 was supplied thereto at a flow rate of 10 L / hour. The liquid circulation rate of the denitrification tank was set at 900 L / hour. The amount of the 10% hydrochloric acid aqueous solution to be added to the circulation tank was set to 0.061 L / hour, which corresponds to 0.3 chemical equivalent to the concentration of nitrate in the wastewater at the beginning of the operation. Under these conditions, the denitrification rate of the treated water was 99.8%, the pH was 6.4, and the calcium carbonate precipitation rate was about 15% as in Example 1.

After operating for 600 hours under the above conditions, when the amount of the aqueous hydrochloric acid solution was reduced to 0.04 L / hour (corresponding to 0.2 chemical equivalent to nitrate), the precipitation rate of calcium carbonate was 25%. The denitrification rate remained almost unchanged at 99.6%. Therefore, after operating under these conditions for 1000 hours, the amount of the aqueous hydrochloric acid added was reduced to 0.02 L.
/ Hour (corresponding to 0.1 chemical equivalent to nitrate). As a result, the calcium carbonate precipitation rate increased to about 37%. However, the denitrification rate decreased by 1000 after the conditions were changed.
Even after a lapse of time, the rate was 98%, indicating that a sufficient denitrification reaction had been performed. Therefore, after the operation for 1000 hours, the operation was performed again with the addition amount of the 10% hydrochloric acid aqueous solution reduced to 0.01 L / hour (corresponding to 0.05 chemical equivalent to nitrate). At this time, the precipitation rate of calcium carbonate reached 50%, the denitrification rate also began to decrease rapidly, and after 800 hours of operation, the denitrification rate dropped to 83%. FIG. 4 shows the measurement results of the denitrification rate and the calcium carbonate precipitation rate when operated under the above-described conditions. As a result, it was found that the denitrification activity could be maintained at a normal value by adding an inorganic acid in an amount of 0.1 chemical equivalent or more to the nitrate groups in the wastewater.

[0023]

According to the present invention, in a method for biologically denitrifying nitrogen-containing wastewater containing calcium ions, the precipitation of calcium carbonate is suppressed by adding an inorganic acid to the wastewater. Normalize the performance of denitrifying bacteria,
Stable operation can be continued for a long time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the results of a denitrification test of simulated wastewater containing nitrate and calcium ions.

FIG. 2 shows an example of a denitrification reactor used in the present invention.

FIG. 3 shows another example of the denitrification reaction apparatus used in the present invention.

FIG. 4 is a graph showing changes in the denitrification rate and calcium precipitation rate of treated water during the operation period in the method of Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Denitrification tank 2 Fluidized bed of carrier carrying denitrifying bacteria 3 Cell peeling device 4 Screen for preventing carrier overflow 5 Circulation tank 6 Circulation pump 7 Drain tank 8 Drainage supply pump 9 Treatment water outlet 10 Denitrification tank 11 Filling of carrier carrying denitrifying bacteria Layer 12 Wire mesh 13 Drain tank 14 Drain supply pump 15 Treated water outlet 16 Blower 17 Gas dispersion pipe

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Shimokawa 2-1-1, Tsurumichuo, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture Inside Chiyoda Kako Construction Co., Ltd. (72) Tokihiko Koyama Tsurumi-chuo, Tsurumi-ku, Yokohama-shi, Kanagawa Chome 12-1 Chiyoda Chemical Works, Ltd. (72) Inventor Sumita Emi 2--12-1, Tsurumi-chuo, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture F-term (reference) 4D003 AA12 AB02 EA01 EA25 4D040 BB02 BB42

Claims (4)

[Claims] 1. A method of biologically denitrifying a nitrogen-containing wastewater containing calcium ions, wherein the precipitation of calcium carbonate is suppressed by adding an inorganic acid to the wastewater before introducing the wastewater into a denitrification tank. Biological denitrification treatment method. 2. The method according to claim 1, wherein the inorganic acid is hydrochloric acid. 3. The method according to claim 1, wherein the inorganic acid is added in a range of 0.1 to 0.6 chemical equivalent to nitrate contained in the waste water. 4. The method according to claim 1, wherein the biological denitrification treatment is performed in a fluidized bed type denitrification apparatus.