JP6832685B2 - Treatment method of nitrogen-containing wastewater - Google Patents

Description

The present invention relates to a wastewater treatment method for purifying nitrogen-containing wastewater discharged from sewage or factories.

As a method for removing nitrogen contained in wastewater, nitrogen gas is obtained by oxidizing organic nitrogen / ammoniacal nitrogen in wastewater to nitrate nitrogen under aerobic conditions and then reducing nitrate nitrogen under anoxic conditions. The method of releasing into the atmosphere is known.

As a flow, raw water (wastewater discharged from sewage and factories) is introduced as treated water in the order of denitrification tank and nitrification tank, and a part of nitrification treated water flowing out of the nitrification tank is returned and circulated to the denitrification tank. Some (hereinafter referred to as "nitrification liquid circulation flow"). In some cases, raw water to be treated is introduced in the order of nitrification tank and denitrification tank, hydrogen donors such as organic substances are added to the denitrification tank for treatment, and excess hydrogen donor is treated in the reaeration tank (re-aeration tank). Hereinafter, it is described as "nitrification-denitrification-re-aeration flow"). Conventionally, activated sludge is mainly used, and in sewage treatment, it is common to operate the activated sludge concentration (MLSS) in each tank at 2,000 to 3,000 mg / L (Non-Patent Document 1). ).

When nitrogen in raw water exists in the form of nitrate nitrogen, a method is used in which nitrate nitrogen is reduced in a denitrification tank and released into the atmosphere as nitrogen gas under anoxic conditions. There is also a method of removing nitrogen by using activated sludge and a carrier in combination. For example, Patent Document 1 proposes a method of charging activated sludge and a carrier at a general concentration (MLSS: 2,500 mg / L) into a tank.

On the other hand, there is also a method of treating wastewater by microorganisms grown on the surface or inside of the carrier without putting activated sludge into the tank. According to this method, it is said that by retaining a high concentration of microorganisms, it is possible to have a high treatment capacity and provide a compact wastewater treatment (Non-Patent Document 2).

However, as described in Non-Patent Document 2, in the method of mixing the carrier with the activated sludge, the conventional carrier is merely an auxiliary agent for the activated sludge. Non-Patent Document 2 proposes a method using a polyvinyl alcohol gel carrier alone, and states that it has succeeded in sufficiently drawing out the capacity of the carrier.

In order to reduce nitrate nitrogen or nitrite nitrogen to nitrogen gas in the denitrification tank in the nitrogen removal step, the dissolved oxygen concentration (DO) in the tank is less than 0.5 mg / L, and the liquid is oxidized. It is necessary to operate with a reduction potential (ORP) of less than -0.1 mV. Further, it is said that the DO suitable for denitrification is less than 0.1 mg / L, and the ORP is about −300 mV to −50 mV.

Further, due to the dissolved oxygen contained in the raw water and the large amount of dissolved oxygen derived from aeration in the nitrification process, the denitrification tank cannot maintain the above-mentioned suitable DO and ORP conditions, and even if a carrier is added, the effect is exhibited. There was a problem that it could not be done and the denitrification process deteriorated. In the case of the nitrification liquid circulation flow, there is also a problem that the removal of the biochemical oxygen demand (BOD) becomes worse due to the deterioration of the denitrification treatment, and the treatment in the subsequent nitrification tank also deteriorates.

Japanese Unexamined Patent Publication No. 6-296991

Japan Sewerage Association, Sewerage Facility Planning / Design Guidelines and Explanations (Part 2) 2001 Edition, p.168 Kinki Chemical Association, Kinki Chemical Industry, June 2003, p.16

The present invention has been made to solve the above problems, and provides a wastewater treatment method having an excellent nitrogen removing ability.

The above object has a denitrification step in which denitrifying bacteria carried on a carrier and wastewater are brought into contact with each other in a denitrification tank under anoxic conditions, and the volume load (N) (mg / L · d) of nitrogen in the denitrification tank is provided. ) And MLSS (S) (mg / L) are solved by providing a method for treating nitrogen-containing wastewater, which comprises charging activated sludge into the denitrification tank so as to satisfy the following formula (1).

0.1 (d ^-1 ) ≤ S / N ≤ 2.0 (d ^-1 ) (1)

At this time, it is preferable that the carrier is a polyvinyl alcohol gel carrier.

Further, it is preferable to further have a nitrification step of introducing the treated water discharged from the denitrification tank into the nitrification tank and bringing it into contact with the nitrifying bacteria under aerobic conditions in the nitrification tank. At this time, it is also preferable to further have a circulation step of circulating a part of the treated water discharged from the nitrification tank to the denitrification tank (nitrification liquid circulation flow).

Further, it is preferable to further have a nitrification step of bringing the water into contact with the nitrifying bacteria under aerobic conditions in the nitrification tank, and to introduce the treated water discharged from the nitrification tank into the denitrification tank. At this time, it is also preferable to have a step of adding a hydrogen donor such as an organic substance to the denitrification tank for treatment, and then providing a re-aeration tank to treat the excess hydrogen donor (nitrification-denitrification-re-aeration). flow).

According to the present invention, it is possible to provide a method for treating nitrogen-containing wastewater having an excellent nitrogen removing ability.

It is a block diagram which shows the device of the nitrification liquid circulation flow. It is a block diagram which shows the device of nitrification-denitrification-re-aeration flow.

The present invention relates to a wastewater treatment method in which nitrogen-containing wastewater discharged from sewage or a factory is brought into contact with denitrifying bacteria in a denitrification tank under anoxic conditions. Here, the anoxic condition in the present specification means that the oxygen concentration (Dissolved oxygen: DO) dissolved in the liquid in the denitrification tank is less than 0.5 mg / L, and the oxidation-reduction potential of the liquid (Oxidation-reduction). potential: ORP) is less than -0.1 mV. It is said that the DO suitable for denitrification is less than 0.1 mg / L, and the ORP is about −300 mV to −50 mV. In addition, the raw water in this specification means wastewater generated from sewage and various factories. Since such wastewater may contain solids such as sand and dust, pretreatment such as removing these solids, adjusting the pH, and removing harmful substances such as heavy metals May be applied. Wastewater that has undergone such pretreatment before flowing into the treatment equipment is also called raw water. On the other hand, treated water refers to a liquid that has flowed into a treatment device and has been treated in a denitrification tank or the like.

In the method of treating wastewater with microorganisms carried on a carrier in a denitrification tank, it is more efficient to treat the wastewater only with the microorganisms carried on the carrier without returning the activated sludge to the denitrification tank. You should be able to do a good job. However, if the conditions necessary for denitrification are not met, the microorganisms that live on the carrier do not work and good treatment cannot be performed. On the other hand, if the carrier is mixed with activated sludge of a general concentration, microorganisms do not grow on the carrier, and the state becomes almost the same as the operation of only activated sludge, and efficient wastewater treatment cannot be performed.

As a result of diligent studies, the present inventors have found that in a wastewater treatment method having a denitrification step in which denitrifying bacteria carried on a carrier and wastewater are brought into contact with each other under anoxic conditions in a denitrification tank, nitrogen in the denitrification tank Activated sludge is charged into the denitrification tank so that the volume loading (N) (mg / L · d) and MLSS (Mixed Liquor Suspended Solids) (S) (mg / L) satisfy a specific relationship. It was found that the above problems can be solved by doing so.

In the treatment method of the present invention, the nitrogen volumetric load (N) (mg / L · d) and MLSS (S) (mg / L) in the denitrification tank are activated in the denitrification tank so as to satisfy the following formula (1). It is important to add sludge. Here, as the activated sludge to be charged into the denitrification tank, activated sludge generated in another flow can be used, but it is preferable to use activated sludge generated in the same flow. At this time, the sludge from the activated sludge tank or the settling tank can be returned.

0.1 (d ^-1 ) ≤ S / N ≤ 2.0 (d ^-1 ) (1)

In the above formula (1), when the S / N value is less than 0.1, the conditions necessary for denitrification are not met, the microorganisms living on the carrier do not work, and good treatment cannot be performed. The S / N value is preferably 0.15 or more. On the other hand, when the S / N value exceeds 2.0, microorganisms do not grow on the carrier, and the state becomes almost the same as the operation of only activated sludge, and efficient wastewater treatment cannot be performed. The S / N value is preferably 1.7 or less.

In the present invention, the raw water to be treated is not particularly limited in its component and concentration as long as it is wastewater that can be decomposed by microorganisms. Examples of nitrogen-containing wastewater include production wastewater from food factories, organic wastewater from chemical factories, and general sewage.

In the treatment method of the present invention, the carrier is preferably a polyvinyl alcohol gel carrier from the viewpoint that the tube of the treatment device is less likely to be clogged and the contact efficiency with microorganisms is excellent. Since the polyvinyl alcohol gel carrier has a large number of hydroxyl groups, it is highly hydrophilic and has a high affinity with living organisms, and is therefore suitable as a microbial carrier.

Further, unlike a foam such as a sponge, the polyvinyl alcohol gel carrier does not easily release water even if it is deformed by an external force, and can provide an environment suitable for the habitat of microorganisms.

The equivalent sphere diameter of the carrier is preferably 1 to 10 mm. If the equivalent diameter of the sphere is less than 1 mm, it may flow out of the tank. The equivalent diameter of the sphere is more preferably 2 mm or more. On the other hand, if the equivalent diameter of the sphere exceeds 10 mm, there may be a problem that bacteria cannot live inside and metabolites are difficult to be excreted outside the carrier because there is a distance from the surface of the carrier to the inside. Further, when the carrier is used in a fluidized state, if the equivalent diameter of the sphere of the carrier is too large, the fluidity is lowered, so that the contact efficiency with the wastewater is lowered and the efficiency of the wastewater treatment may be lowered. From this point of view, the equivalent diameter of the sphere is more preferably 6 mm or less.

The pore size of the pores communicating from the surface of the carrier to the inside can be freely controlled, but it is preferable that only bacteria can live inside the carrier. The pore size of the carrier is preferably 0.1 μm or more. If the pore size is less than 0.1 μm, problems such as the inability of bacteria to enter the inside may occur. The pore diameter is more preferably 0.5 μm or more. On the other hand, the pore diameter is preferably 100 μm or less. If the pore size exceeds 100 μm, large organisms other than bacteria may invade and the efficiency may decrease. The pore diameter is more preferably 50 μm or less.

The shape of the carrier is not limited, and can be any shape such as a cube, a rectangular parallelepiped, a columnar shape, a spherical shape, and a macaroni shape. Spherical is preferable in consideration of the fluidity of the carrier and the contact efficiency with waste water.

The specific gravity of the carrier is slightly larger than that of water, and it is preferable that the carrier has a specific gravity that allows it to swing in the reaction vessel so as not to be washed away from the reaction vessel. Therefore, the specific gravity of the carrier is preferably 1.001 or more, and more preferably 1.005 or more. On the other hand, the specific gravity is preferably 1.2 or less, and more preferably 1.1 or less.

The polyvinyl alcohol gel carrier in the present invention is more preferably an acetalized polyvinyl alcohol gel carrier from the viewpoint of being able to increase the retention amount of microorganisms and ensuring durability in repeated use.

The acetalized polyvinyl alcohol gel carrier can be obtained by an existing method, but it is an aqueous solution in which polyvinyl alcohol (PVA) and a water-soluble polymer polysaccharide are dissolved in that the structure is suitable for retaining microorganisms. Is formed into a spherical shape by dropping it into an aqueous solution containing polyvalent metal ions, and then acetalized with an aldehyde. Of these, a polyvinyl alcohol gel carrier that has been acetalized with a dialdehyde such as glutaraldehyde is preferable.

In the present invention, the amount of the carrier charged into the denitrification tank is not particularly limited, but is usually preferably 5 to 50% by volume, more preferably 5 to 40% by volume, based on the tank capacity of the denitrification tank. It is preferably 10 to 30% by volume, more preferably 10 to 30% by volume. Further, in the denitrification tank, the carrier can be flowed in order to improve the contact efficiency between the carrier and the wastewater. The method for flowing the carrier is not particularly limited, and examples thereof include a method of mechanical stirring and a method of aerating gas.

Further, in the present invention, the carrier can be charged into the nitrification tank from the viewpoint of further increasing the removal rate of nitrogen. The amount of the carrier added at this time can be the above-mentioned amount. In addition, the carrier can be flowed in order to improve the contact efficiency between the carrier and the waste water.

The bacteria used in the present invention can be any bacteria normally used for wastewater treatment. As a method of collecting bacteria, it is desirable to collect it as seed sludge from a reactor that is treating the target wastewater, but sewage sludge and industrial wastewater sludge are used as seed sludge, and the necessary bacteria grow. You can also wait for.

In the treatment method of the present invention, it is preferable to further have a nitrification step before or after the denitrification step from the viewpoint of further increasing the removal rate of nitrogen contained in wastewater.

First, an embodiment (method A) in which a nitrification step is further provided after the denitrification step will be described. In this case, it is preferable to introduce the treated water discharged from the denitrification tank into the nitrifying tank and bring it into contact with the nitrifying bacteria under aerobic conditions in the nitrifying tank.

FIG. 1 is a block diagram showing an example of the apparatus used in the method A. The apparatus shown in FIG. 1 includes a denitrification tank and a subsequent nitrification tank. In the treatment method using the apparatus of FIG. 1, the raw water is first introduced into the denitrification tank, the treated water discharged from the denitrification tank is introduced into the nitrification tank, and the nitrifying bacteria are formed in the nitrification tank under aerobic conditions. Make contact. At this time, from the viewpoint of denitrifying the liquid nitrified in the nitrification tank in the latter stage using the hydrogen donor in the raw water in the denitrification tank in the previous stage, a part of the treated water discharged from the nitrification tank is denitrified. It is preferable to further have a circulation step of circulating in (the nitrification liquid circulation of FIG. 1). Then, the treated water discharged from the nitrification tank is introduced into the activated sludge tank and then into the settling tank.

In the apparatus of FIG. 1, from the settling tank to the denitrification tank so that the volumetric load (N) (mg / L · d) and MLSS (S) (mg / L) of nitrogen in the denitrification tank satisfy the above formula (1). Activated sludge is added (sludge return (A) in FIG. 1). Further, the activated sludge may be put into the activated sludge tank from the settling tank (sludge return (B) in FIG. 1), and the increased sludge in the settling tank may be withdrawn as excess sludge.

Next, an embodiment (method B) in which a nitrification step is further provided before the denitrification step will be described. In this case, it is preferable to bring the wastewater into contact with the nitrifying bacteria in the nitrifying tank under aerobic conditions and introduce the treated water discharged from the nitrifying tank into the denitrification tank.

FIG. 2 is a block diagram showing an example of the apparatus used in the method B. The apparatus shown in FIG. 2 includes a nitrification tank and a denitrification tank in the subsequent stage. In the treatment method using the apparatus of FIG. 2, raw water is first introduced into a nitrifying tank, brought into contact with nitrifying bacteria under aerobic conditions in the nitrifying tank, and then introduced into a denitrification tank. It is preferable to put a hydrogen donor such as methanol into the denitrification tank. Then, the treated water discharged from the denitrification tank is introduced into the re-aeration tank and then into the settling tank.

In the apparatus of FIG. 2, from the settling tank to the denitrification tank so that the volumetric load (N) (mg / L · d) and MLSS (S) (mg / L) of nitrogen in the denitrification tank satisfy the above formula (1). Activated sludge is added (sludge return (C) in FIG. 2). In addition, activated sludge may be charged from the settling tank to the reaeration tank (sludge return (D) in FIG. 2), and the increased sludge in the settling tank may be withdrawn as excess sludge.

When a large amount of hydrogen donor is contained in the raw water, the method A is preferable because the hydrogen donor can be used for denitrification. Method B is preferable when the raw water does not contain a hydrogen donor, or when the amount of hydrogen donor is too small to be efficiently used for denitrification.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

(Carrier to be filled in the reaction tank)
The carriers charged into the denitrification tank and the nitrification tank are polyvinyl alcohol gel carriers acetalized with glutaraldehyde, having a sphere-equivalent diameter of 4.0 mm, a specific gravity of 1.025, and a water content of 93% by mass.

Example 1
A wastewater treatment test was carried out using the device of the nitrification liquid circulation flow (denitrification tank capacity: 250 L, nitrification tank capacity: 500 L, activated sludge tank: 150 L) shown in FIG. The denitrification tank and the nitrification tank were filled with the above-mentioned acetalized polyvinyl alcohol gel carrier at 10% of the tank capacity. Raw water (BOD: 450 mg / L, total nitrogen (TN): 150 mg / L) was continuously flowed into the denitrification tank at a flow rate of 1000 L / d. The circulation amount of the nitrifying solution was 3000 L / d.

Activated sludge is returned from the settling tank to the denitrification tank so that the nitrogen volume load (N) of the denitrification tank is 600 mg / L · d and the MLSS (S) is 500 mg / L (S / N = 0.83d- ^1). (Arrow A). In addition, the activated sludge was returned from the settling tank to the activated sludge tank so that the MLSS of the activated sludge tank was 2500 mg / L (arrow B). At this time, the increased sludge in the settling tank was extracted as excess sludge. No carrier was put into the activated sludge tank.

When operated under these conditions, the DO of the denitrification tank was less than 0.1 mg / L and the ORP was −120 mV, which were good conditions for denitrification. The amount of bacteria present per liter of the carrier in the denitrification tank was 5.8 × 10 ¹² , and sufficient bacteria inhabited the carrier. As a result, the BOD of the treated water was stable at 15 mg / L and the TN was 5 mg / L, and the excess sludge was very small at 1.3 kg (dry weight) in one month.

Comparative Example 1
In the apparatus shown in FIG. 1, the nitrogen volume load (N) of the denitrification tank is 600 mg / L · d and the MLSS (S) is 2000 mg / L (S / N = 3.3d- ^1). Wastewater treatment was carried out under the same conditions as in Example 1 except that the activated sludge was returned to the nitrogen tank.

When operated under these conditions, the DO of the denitrification tank was less than 0.1 mg / L and the ORP was −150 mV, which were good conditions for denitrification. But bacteria amount present in the carrier per liter of the denitrification tank is 3.1 × 10 ⁸ pieces, sufficient bacteria did not proliferate to the carrier. As a result, the BOD of the treated water was 20 mg / L and the TN was 105 mg / L. Although the BOD was treated, denitrification was not possible. The excess sludge weighed 6.1 kg (dry weight) in one month, which was comparable to that of normal activated sludge.

Comparative Example 2
In the apparatus shown in FIG. 1, wastewater treatment was carried out under the same conditions as in Example 1 except that sludge was not returned from the settling tank to the denitrification tank. The nitrogen volume load (N) of the denitrification tank was 600 mg / L · d. As a result, MLSS (S) was 36 mg / L (S / N = 0.06d- ¹ ).

When operated under these conditions, the amount of bacteria present per liter of the carrier in the denitrification tank was 1.1 × 10 ¹² , and sufficient bacteria were grown on the carrier. However, the DO of the denitrification tank was 1.5 mg / L and the ORP was +115 mg / L, which were not good conditions for denitrification. As a result, the BOD of the treated water was 18 mg / L and the TN was 123 mg / L. Although the BOD was treated, denitrification was not possible. The excess sludge was 1.5 kg (dry weight) in one month, which was very small.

Example 2
A wastewater treatment test was carried out using the device of the nitrification-denitrification-re-aeration flow (nitrification tank capacity: 500 L, denitrification tank capacity: 250 L, re-aeration tank: 150 L) shown in FIG. The denitrification tank and the nitrification tank were filled with the above-mentioned acetalized polyvinyl alcohol gel carrier at 10% of the tank capacity.

Raw water (BOD: 15 mg / L, total nitrogen (TN): 800 mg / L) was continuously flowed into the nitrification tank at a flow rate of 250 L / d. At this time, methanol was charged into the denitrification tank at 600 g / d as a hydrogen donor.

Activated sludge is returned from the settling tank to the denitrification tank so that the nitrogen volume load (N) of the denitrification tank is 800 mg / L · d and the MLSS (S) is 800 mg / L (S / N = 1.0d- ^1). Arrow C). Further, the carrier was not charged into the re-aeration tank, and the sludge was returned from the settling tank so that the MLSS was 2500 mg / L (arrow D in FIG. 2). The increased sludge in the settling tank was extracted as excess sludge.

When operated under these conditions, the DO of the denitrification tank was less than 0.1 mg / L and the ORP was -145 mV, which were good conditions for denitrification. In addition, the amount of bacteria present per liter of the carrier in the denitrification tank was 9.2 × 10 ¹² , and sufficient bacteria inhabited the carrier. As a result, the BOD of the treated water was stable at 19 mg / L, the TN was stable at 4 mg / L, and the excess sludge was very small at 0.9 kg (dry weight) in one month.

Comparative Example 3
In the apparatus shown in FIG. 2, the nitrogen volume load (N) of the denitrification tank is 800 mg / L · d and the MLSS (S) is 2400 mg / L (S / N = 3.0 d- ^1). Wastewater treatment was carried out under the same conditions as in Example 2 except that the activated sludge was returned to the nitrogen tank.

When operated under these conditions, the DO of the denitrification tank was less than 0.1 mg / L and the ORP was -155 mV, which were good conditions for denitrification. But bacteria amount present in the carrier per liter of the denitrification tank is 1.2 × 10 ⁸ pieces, sufficient bacteria did not proliferate to the carrier. As a result, the BOD of the treated water was 20 mg / L and the TN was 346 mg / L. Although the BOD was treated, denitrification was not possible. The excess sludge weighed 6.8 kg (dry weight) in one month, which was comparable to that of normal activated sludge.

Comparative Example 4
In the apparatus shown in FIG. 2, wastewater treatment was carried out under the same conditions as in Example 2 except that sludge was not returned from the settling tank to the denitrification tank. The nitrogen volume load (N) of the denitrification tank was set to 800 mg / L · d. As a result, MLSS (S) was 40 mg / L (S / N = 0.05d- ¹ ).

When operated under these conditions, the amount of bacteria present per liter of the carrier in the denitrification tank was 2.3 × 10 ¹² , and sufficient bacteria were growing on the carrier, but the DO of the denitrification tank was 1.2 mg / L and ORP were +85 mV, which were not good conditions for denitrification. As a result, the BOD of the treated water was 21 mg / L and the TN was 376 mg / L. Although the BOD was treated, denitrification was not possible. The excess sludge was 5.5 kg (dry weight) in one month.

As is clear from the above examples, according to the method for treating nitrogen-containing wastewater of the present invention, denitrification conditions can be adjusted while maintaining the microbial growth effect of the carrier, which is good for industrial wastewater treatment and sewage treatment. It can be processed and has high industrial value.

Claims (2)

A denitrification step of contacting with oxygen-free conditions and drainage and supported denitrification bacteria in the denitrification tank to the carrier,
A nitrification step of introducing the treated water discharged from the denitrification tank into a nitrification tank and bringing it into contact with nitrifying bacteria under aerobic conditions in the nitrification tank.
It has a circulation step of circulating a part of the treated water discharged from the nitrification tank to the denitrification tank.
The carrier is a polyvinyl alcohol gel carrier, and the volumetric load (N) (mg / L · d) and MLSS (S) (mg / L) of nitrogen in the denitrification tank satisfy the following formula (1). A method for treating nitrogen-containing wastewater, which comprises putting activated sludge into a denitrification tank.
0.15 (d ^-1 ) ≤ S / N ≤ 1.7 (d ^-1 ) (1) A nitrification process in which wastewater is brought into contact with nitrifying bacteria under aerobic conditions in a nitrification tank,
The process of adding a hydrogen donor to the denitrification tank and
The treated water discharged from the nitrification tank is introduced into the denitrification tank, and a drainage and denitrifying bacteria on a support and a denitrification step of contacting with oxygen-free conditions in the denitrification tank,
The carrier is a polyvinyl alcohol gel carrier, and the volumetric load (N) (mg / L · d) and MLSS (S) (mg / L) of nitrogen in the denitrification tank satisfy the following formula (1). A method for treating nitrogen-containing wastewater, which comprises putting activated sludge into a denitrification tank.
0.15 (d ^-1 ) ≤ S / N ≤ 1.7 (d ^-1 ) (1)

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