JPH06304589A - Treatment of return water - Google Patents

Abstract

PURPOSE:To provide the method for treatment of return water to be generated from a secondary treatment stage of a sewer treatment plant capable of removing the phosphorus in the return water and decreasing the burden of a water treatment system. CONSTITUTION:The excess sludge after the secondary treatment of the sewer is separated to the return water and thickened sludge by a gravity thickening vessel 4. A part of the thickened sludge is added to the return water at such a ratio at which the concn. of the MLSS thereof attains >=100mg/L. The return water is then aerated in a phosphorus removing vessel 6. As a result, the thickened sludge changes from an anaerobic to aerobic state and, therefore, the phosphorus in the return water can excessively be taken, by which the phosphorus in the return water is removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating return water generated from a secondary treatment process in a sewage treatment plant, and more particularly to a method for treating return water for removing phosphorus from the return water. Is.

[0002]

2. Description of the Related Art As shown in FIG. 4, sewage is first passed through a first settling tank 1, an aeration tank 2, and a final settling tank 3, and the supernatant water of the final settling tank 3 is discharged as secondary treated water. Known from. And the sludge from the final sedimentation pond 3
Part of it is returned to the aeration tank 2 as return sludge, but most of it is subjected to solid-liquid separation by gravity concentration in the gravity concentration tank 4 as excess sludge, and the concentrated sludge is dehydrated by the dehydrator 5, while the return water is It was returned to the aeration tank 2 as it was.

However, since the excess sludge becomes anaerobic when it is gravity-concentrated in the gravity thickening tank 4, there occurs a phenomenon in which phosphorus taken up in the excess sludge is released into the return water, which causes the return flow. There is a problem that the load of the water treatment system is increased when the phosphorus concentration in the water becomes high and the phosphorus is returned to the aeration tank 2.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned conventional problems and provides a method for treating return water by removing phosphorus in the return water to reduce the load on the water treatment system. It was completed in order to do.

[0005]

The present invention, which has been made to solve the above-mentioned problems, has a method of solid-liquid separating excess sludge after secondary treatment of sewage into return water and concentrated sludge in a gravity concentration tank. , A part of the concentrated sludge is added to the returned water so that the MLSS concentration is 1000 mg / L or more, and aeration is performed in the phosphorus removal tank, and the concentrated sludge is excessively ingested with phosphorus in the returned water. It is characterized by removing phosphorus.

The present invention will be described in more detail below with reference to the flow sheet of FIG. As shown in FIG. 1, the excess sludge after the secondary treatment of the sewage by the first settling tank 1, the aeration tank 2, and the final settling tank 3 is gravity-concentrated in the gravity thickening tank 4 in the same manner as the conventional one, and is condensed with the return water. It is separated into sludge. The sludge concentrated here may include a part of the initial sludge generated from the first settling basin 1.

In the present invention, a part of the concentrated sludge is added to this return water. The amount of addition is 1000 MLSS concentration of the return water.
It should be more than mg / L. Also, it is appropriate to add about 1/4 to 1/5 of the total volume of concentrated sludge. Then, the returned water to which the concentrated sludge is added is aerated in the next phosphorus removing tank 6 for about 1 to 3 hours. At this time, DO is preferably about 2 ppm or more. As described above, the sludge becomes anaerobic and starved in the gravity concentration tank 4, but when it is aerated in the phosphorus removal tank 6 and changes to an aerobic state, it excessively ingests phosphorus in the return water, and Phosphorus is removed.

The concentrated sludge discharged from the gravity thickening tank 4 is a dehydrator 5.
The dehydrated cake is separated into a dehydrated cake and a dehydrated filtrate, but this dehydrated filtrate can be sent to the phosphorus removing tank 6 together with the return water as shown by a broken line.

Returned water from the phosphorus removal tank 6 is a solid-liquid separation tank 7
Is separated into treated water and excess sludge in the phosphorus removal tank, and the treated water is returned to the aeration tank 2. Since this treated water is one from which phosphorus has been removed, the load on the water treatment system can be reduced more than before. Since phosphorus is released when the solid-liquid separation tank 7 becomes anaerobic, it is preferable to use centrifugal concentration or the like, which does not make the sludge anaerobic.

[0010]

EXAMPLES Examples of the present invention will be shown below. The excess sludge of 5000 mg / L discharged from the final settling tank 3 in Fig.
Gravity concentration was carried out for an hour to perform solid-liquid separation, and the sludge was concentrated to 20000 mg / L. One-fifth of this concentrated sludge was added to the return water and the dehydrated filtrate from the gravity concentration tank 4 and aerated in the phosphorus removal tank 6 to obtain treated water. Aeration time is 2 hours and MLS at this time
S is 1200 mg / L. As a result, phosphorus in return water is 15pp
Although it was m, the phosphorus in the treated water decreased to 1 ppm, and the BOD in the return water was 100 ppm, but the BOD in the treated water was
Was reduced to 10 ppm.

FIG. 2 is a graph showing the relationship between the aeration time of the phosphorus removal tank 6 and the phosphorus concentration in the treated water in this case. When the aeration time is 1 hour or less, sufficient phosphorus intake is not performed, and conversely 3 This indicates that the phosphorus removal rate remains almost unchanged even after aeration for a longer time.

Further, FIG. 3 shows the MLS in the phosphorus removing tank 6.
It is a graph which shows the relationship between S and the phosphorus concentration in treated water,
The higher the MLSS is 1000 mg / L or more, the larger the removal amount is, but 10
It turns out that 00 to 1500 mg / L is sufficient. If the MLSS is made higher than necessary, the load of the solid-liquid separation step in the latter stage becomes high, which is not preferable.

[0013]

As described above, according to the present invention, a part of the concentrated sludge is added to the return water from the secondary treatment step of the sewage and aerated in the phosphorus removal tank, and the concentrated sludge is returned to the return water. Since it was made to excessively ingest phosphorus, there is an advantage that phosphorus in the return water can be removed to reduce the load on the water treatment system.

[Brief description of drawings]

FIG. 1 is a flow sheet showing the processing steps of the present invention.

FIG. 2 is a graph showing the relationship between the aeration time of a phosphorus removal tank and the phosphorus concentration in treated water in an example.

FIG. 3 is a graph showing the relationship between the MLSS of the phosphorus removal tank and the phosphorus concentration in the treated water in the examples.

FIG. 4 is a flow sheet showing a conventional sewage treatment process.

[Explanation of code] 1 First settling tank 2 Aeration tank 3 Final settling tank 4 Gravity concentration tank 5 Dehydrator 6 Phosphorus removal tank 7 Solid-liquid separation tank

Claims (1)

[Claims] 1. The excess sludge after the secondary treatment of sewage is subjected to solid-liquid separation into return water and concentrated sludge in a gravity concentration tank, and then the concentration of MLSS in the return water is 1000 mg / L or more. A method for treating return water, which comprises removing part of the return water by aeration in a phosphorus removal tank and causing the concentrated sludge to excessively ingest phosphorus in the return water.