JPH0477639B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for treating sewage containing organic matter and phosphorus, and more specifically, a method for extracting excess sludge in an activated sludge method with a biological phosphorus removal process; This relates to aeration treatment in dehydration treatment.

[Conventional technology]

Conventionally, one of the biological wastewater treatment methods using activated sludge is known to combine anaerobic and aerobic conditions to decompose BOD and remove phosphorus.

In this method, the primary treatment water sent together with the returned sludge from the initial settling tank (hereinafter referred to as initial settling) is passed through an anaerobic tank and then an aerobic tank for phosphorus removal alone, and a denitrification tank is also combined to remove phosphorus. Although there are methods that remove nitrogen in addition to removal, the common method for removing phosphorus is to remove phosphorus from the mixed solution by incorporating the phosphorus in the mixed solution into activated sludge and accumulating it. are doing.

The activated sludge that has accumulated a high concentration of phosphorus as described above is separated from the treated water by sedimentation in the final settling tank (hereinafter referred to as final settling), and a portion is returned to the initial settling side as return sludge, and the rest is recycled as surplus sludge. It is drawn out, concentrated, and sent to a solidification process through dehydration. The treated water, which is supernatant water, is sent to the drainage system. In addition, in the treatment system that extracts and dewaters excess sludge,
A liquid separated from the solids (hereinafter referred to as separated liquid) is produced, including the case where concentration treatment is performed during the process, and this separated liquid is generally returned to the initial settling side of the sewage treatment system.

By the way, it usually takes half a day to several days from the time excess sludge is drawn out to the time it is dehydrated.
If excess sludge is left under anaerobic conditions during this time,
There is a problem in that phosphorus is released from the activated sludge into the mixed liquid, causing phosphorus to return to the sewage treatment system via the separated liquid, resulting in the phosphorus removal process not functioning effectively.

Therefore, there is a method of adding a substance that insolubilizes phosphorus to excess sludge or separated liquid to create a phosphorus insoluble substance and disposing of it, or by keeping excess sludge in an aerobic state, the phosphorus released in the mixed liquid is recycled into activated sludge. In order to absorb the excess sludge, a method has been proposed in which the excess sludge is aerated for 2 to 18 hours (Japanese Unexamined Patent Publication No. 150487/1987).

However, the former method of phosphorus insolubilization requires a substance for this purpose, which increases costs, and the amount of dehydrated cake, etc. that must be disposed of increases due to the addition of the substance. Even with the 18-hour aeration method, it is not possible to store excess sludge for a long period of time, and if the aerated liquid is not dehydrated within a short period of time, phosphorus will be re-released.
Since aeration treatment cannot be carried out for more than 20 hours, the design of a dewatering treatment system for excess sludge is subject to time constraints due to these aspects.

[Problem that the invention seeks to solve]

In view of these problems with conventional methods, the present invention has been developed to reduce the concentration of phosphorus in the separated liquid to a low concentration in a dewatering treatment system for extracted excess sludge without being subject to restrictions on the storage time of excess sludge. The object of the present invention is to provide a sewage treatment method that allows dehydration treatment to be carried out as appropriate.

[Means for solving problems]

The gist of the sewage treatment method according to the present invention, which has been made to achieve this purpose, is as follows:
In the extraction and dewatering treatment of surplus sludge in the activated sludge method, which has a biological phosphorus removal process that combines anaerobic and aerobic conditions, the surplus sludge is
It is characterized by performing dehydration treatment at appropriate times while repeating intermittent aeration of aeration for ~6 hours and stopping aeration for 3~8 hours.

[Effect]

The reason why the present invention employs the above configuration is as follows.

In general, surplus sludge drawn from a wastewater treatment system that has the above-mentioned phosphorus removal process contains about 10 to 15 mg of phosphorus in the mixed liquid, for example, after being drawn out from the final sedimentation process, and it is stored under anaerobic conditions. If the activated sludge is stored at the bottom, a large amount of 350 to 400 mg of phosphorus will be released from the activated sludge into the mixed liquid.
If such a large amount of phosphorus separated liquid is returned to the wastewater treatment system, it will lead to the above-mentioned problem of phosphorus return.

However, even if the separated liquid contains phosphorus at a low concentration of, for example, 30 to 40 mg, in this case, problems may occur that would ultimately invalidate the effect of the phosphorus removal process as described above. I can think of no. This is because the amount of excess sludge is approximately 1% of the amount of raw water (sewage) flowing into the sewage treatment system, and even if the liquid separated from this excess sludge contains a low concentration of phosphorus, This is because the increase in the phosphorus concentration of raw water due to this will be about 10% or less.

On the other hand, according to the findings of the present invention, etc., in relation to the release and absorption of phosphorus from activated sludge that has accumulated phosphorus at a high concentration, it is possible to cause phosphorus absorption to some extent by aeration, and then to release phosphorus by stopping the aeration. It became clear that by repeating this process, it was possible to keep the phosphorus concentration in the mixture within a certain low concentration range.

Therefore, such intermittent aeration makes it possible to store excess sludge for a long time while keeping the phosphorus concentration in the mixed solution as low as about 30 to 40 mg/deg.

The amount of air for the intermittent aeration in the present invention is determined based on the concentration of excess sludge, the phosphorus concentration in the mixed liquid, etc., but is generally 0.1 to 2.0N.
/g・ds・hr preferably 0.5 to 1.0N/g・
ds·hr is appropriate, and the aeration time is preferably about 1 to 6 hours, preferably about 2 to 4 hours. If the aeration time is less than 1 hour, the phosphorus released into the mixed liquid may not be absorbed sufficiently into the activated sludge.
On the other hand, if the treatment is carried out for more than 6 hours, the phosphorus that has been absorbed into the activated sludge will be re-released and the phosphorus will no longer be able to be re-absorbed. Moreover, the time for stopping aeration is preferably about 3 to 8 hours, preferably about 4 to 6 hours. This is because if the aeration is stopped for less than 3 hours, it is essentially the same as continuing aeration, and as mentioned above, phosphorus is released in a form that cannot be reabsorbed.On the other hand, if the aeration is stopped for 8 hours or more, If the activated sludge is stopped, the excess sludge will be left in an anaerobic state for a long time, and more phosphorus will be released from the activated sludge, and the phosphorus concentration in the separated liquid will increase when the excess sludge is dehydrated in a timely manner. This is because it will not be effectively achieved.

Intermittent aeration may be performed on surplus sludge extracted from final settling, surplus sludge after temporary storage, or surplus sludge after concentration. Further, while the aeration is stopped, stirring may be performed to prevent the sludge from settling.

〔Example〕

FIGS. 1 to 3 each show an outline of the flow of implementation to which the sewage treatment method of the present invention is applied.

In each figure, 1 is the anaerobic tank, 2 is the aerobic tank, 3 is the final sedimentation, and the primary treated water 6 from the initial sedimentation (not shown)
The sludge flows into the anaerobic tank 1 together with the returned sludge 9, and is subjected to biological treatment under anaerobic conditions.
Biological treatment is carried out under aerobic conditions, which decomposes BOD in raw water (sewage) and accumulates phosphorus in activated sludge.

In the implementation flow shown in FIG. 3, a denitrification tank 12 is further provided between the anaerobic tank 1 and the aerobic tank 2, and a part of the secondary treated water from the aerobic tank is circulated through the circulating water line 13. The nitrogen in the mixture is removed together with it.

The secondary treated water flowing out from the aerobic tank 2 outlet is sent to the final settling 3 where it is sedimented and separated, the supernatant water is sent to the drainage system as treated water 7, and the other settled sludge is sent to the sludge drawing line 8. A part of it is used as the return sludge 9, and the remaining surplus sludge 1
0 flows into the sludge aeration tank 4 or the sludge storage tank 11. In addition, there is an excess sludge thickening tank (machine) 1 in front of these.
4 may be provided to send the concentrated excess sludge to the sludge aeration tank 4 or the sludge storage tank 11. Also, when using this concentration tank, final sedimentation 3
In addition to flowing the sludge from the aerobic tank 2, the secondary treated water from the aerobic tank 2 may be directly flowed into the thickening tank. 15 is the sludge aeration tank 4 from the excess sludge thickening tank 14
Alternatively, a thickened sludge drawing line for feeding sludge to the sludge storage tank 11 is shown.

In the sludge aeration tank 4, surplus sludge is stored while performing intermittent aeration, which is characteristic of the present invention, and the surplus sludge is appropriately fed from the sludge aeration tank 4 to the dewatering equipment 5 for dewatering treatment. In addition, the separated liquid generated during dehydration treatment is returned to the initial settling side. Example 1 Using sewage (BOD 160 mg/, total P 5.2 mg/) as raw water, the implementation flow shown in Figure 1 (without a concentration tank) was followed. processed and extracted from the final sediment.
MLSS: 9650mg/, Phosphorus concentration of mixture 35mg/
(The phosphorus concentration of the separated liquid obtained after centrifuging the surplus sludge was measured; the same applies hereinafter) was obtained.

This excess sludge is transferred to sludge aeration tank 4 at 0.3N
When aerated for 1 hour at /g・ds・hr, the phosphorus concentration in the mixture was 0.6 mg/hr, and after that, the aeration was
After stopping the time (the phosphorus concentration rose to 25 mg/), the operation of aerating for 1 hour (the phosphorus concentration rose to 1 mg/) was repeated.

The results of measuring changes in the phosphorus concentration in the mixed liquid in the sludge aeration tank 4 due to the above intermittent aeration operation are shown in FIG. 4A. As is clear from this figure, the surplus sludge stored in the sludge aeration tank 4 is constantly maintained at a low concentration of phosphorus over a long period of time due to intermittent aeration, and this is produced by dehydrating the surplus sludge as appropriate. Since the phosphorus concentration in the separated liquid is also kept low, there is virtually no negative effect of phosphorus regression.

On the other hand, Figure 4 (b) shows the change in phosphorus concentration in the mixed liquid when continuous aeration is continued for the excess sludge, and phosphorus release occurs when continuous aeration is performed for more than ten hours. This means that it cannot be stored for a long time.

Example 2 The same raw water as in Example 1 was treated according to the implementation flow shown in FIG. 3 (with surplus sludge thickening tank 14) to obtain surplus sludge. This excess sludge had an MLSS of 8500 mg/ in the sludge drawn from the final settling 3, an MLSS of 15060 mg/ in the thickened sludge drawn from the drawing line 15 of the excess sludge thickening tank 14, and a phosphorus concentration of 54 mg/ in the mixed liquid.

Next, the thickened sludge was stored in the sludge storage tank 11 for 24 hours (phosphorus concentration rose to 285 mg/), and then fed to the sludge aeration tank 4, with an aeration air volume of 0.5 N/g・ds・hr.
When intermittent aeration was repeated in the same manner as in Example 1, with the aeration time being 4 hours and the aeration stop time being 6 hours, the phosphorus concentration of the mixed solution was 3 mg/-35 mg/-2.5 mg/
......and always showed 40 mg/or less during the 50 hours of the test. The results are shown in FIG.

〔Effect of the invention〕

According to the sewage treatment method according to the present invention, the phosphorus concentration contained in the surplus sludge mixture can be maintained at a low concentration for a long time, and the phosphorus concentration in the separated liquid produced during dewatering treatment can also be reduced. This has the effect of being able to suppress the amount of phosphorus to a level that does not interfere with the phosphorus removal process in the treatment system.

Furthermore, according to the intermittent aeration of the present invention, surplus sludge is not exposed to excessive anaerobic or aerobic conditions and can be stored for a long time, allowing freedom in the design and operation conditions of dewatering equipment. This also has the effect of increasing the degree of freedom in designing the entire sewage treatment facility.

[Brief explanation of drawings]

Figures 1 to 3 are diagrams showing the implementation flow of sewage treatment equipment to which the method of the present invention is applied. FIG. 4A is a diagram illustrating the state of change in phosphorus concentration of the mixed liquid in the case of Example 1 of the present invention.
Figure B is a diagram showing how the phosphorus concentration changes when continuous aeration is performed. FIG. 5 is a second embodiment of the present invention.
It is a figure explaining the phosphorus concentration change state of the mixed liquid in the case of. 1: Anaerobic tank, 2: Aerobic tank, 3: Final settling, 4: Sludge aeration tank, 5: Dewatering equipment, 6: Primary treated water, 7: Treated water, 8: Sludge withdrawal line, 9: Return sludge, 1
0: surplus sludge, 11: sludge storage tank, 12: denitrification tank, 13: circulating water line, 14: surplus sludge thickening tank, 15: thickened sludge drawing line.

Claims (1)

[Claims] 1. In activated sludge having a biological phosphorus removal process that combines anaerobic and aerobic conditions, the excess sludge is extracted and dewatered,
A sludge treatment method characterized by carrying out dewatering treatment in a timely manner while repeating intermittent aeration of 1 to 6 hours of aeration and 3 to 8 hours of stopping aeration. 2. The sludge treatment method according to claim 1, wherein intermittent aeration is performed on temporarily stored surplus sludge. 3. The sludge treatment method according to claim 1 or 2, wherein intermittent aeration is performed on surplus sludge after concentration.