JPH06182377A - Method for treating sewage - Google Patents

Abstract

PURPOSE:To provide a method for treating sewage which yields stable dephosphorization and denitrification efficiency in a short hydraulic retention time. CONSTITUTION:In a sewage treatment method with an active sludge, sludge separated from liquid in the final sedimentation vessel 2 is treated anaerobically in an anaerobic vessel 3 and, after being treated aerobically in an aerobic vessel 4, is returned to an aeration vessel 1 as return sludge. Since the separated sludge is alone subjected to dephosphorization etc., its hydraulic retention time can be shortened, stabilizing the dephosphorization efficiency.

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 sewage having improved dephosphorization efficiency.

[0002]

2. Description of the Related Art As a method for treating sewage for the purpose of dephosphorization, the anaerobic / aerobic method shown in FIG. 5 has been conventionally known. In this method, sewage is put into the anaerobic tank 11 together with the returned sludge to discharge phosphorus from the bacterial cells, and then the aerobic tank 12
At the same time, the bacterial cells were over-ingested and the mixed solution was sent to the final settling tank 13 for solid-liquid separation, and the supernatant water was discharged as treated water, while part of the settling sludge was anaerobically used as return sludge. This is a method of returning to the tank 11.

However, in this conventional method, the anaerobic treatment is 1.5
It took 4.5 hours for the aerobic treatment, a long treatment time of about 6 hours (hydraulic retention time). In addition, since the anaerobic treatment time was short, when DO, NOx-N, etc. were mixed in the sewage, the ORP (oxidation-reduction potential) of the anaerobic tank 11 was not sufficiently lowered, and the dephosphorization efficiency was unstable. Furthermore, most of the organic nitrogen in the inflowing sewage is stored in the aerobic tank 12 as NH ₄ -N and NOx-N.
Since the solid-liquid separation is carried out after decomposing into soluble substances such as, the removal rate of nitrogen is as low as about 30%.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems, shortens the hydraulic retention time in the treatment tank, and can obtain stable high dephosphorization efficiency. Moreover, the present invention has been completed in order to provide a method for treating sewage which can also improve the denitrification efficiency.

[0005]

The present invention, which has been made to solve the above-mentioned problems, provides a method for treating sewage in which sewage is treated with activated sludge in an aeration tank and then solid-liquid separated in a final settling tank. The sludge that has been subjected to solid-liquid separation in anaerobic treatment and aerobic treatment is returned to the aeration tank as return sludge.

The present invention will be described in more detail below with reference to FIGS. FIG. 1 shows the flow of the present invention. 1 is an aeration tank, 2 is a final settling tank, 3 is a sludge anaerobic tank, 4
Is a sludge aerobic tank. Also in the present invention, sewage is treated with activated sludge in the aeration tank 1 together with the returned sludge, and thereafter, solid-liquid separation is performed in the final settling tank 2 as in the conventional case.

However, in the present invention, of the sludge drawn out from the final settling tank 2, the sludge to be returned is
The sludge anaerobic tank 3 anaerobically treats the sludge alone. Here, the ORP is -200 ~ -4 using the organic matter adsorbed by the sludge.
The anaerobic state of 00mV is maintained and phosphorus is sufficiently discharged. The actual residence time in this sludge anaerobic tank 3 is 1 to 3 hours (when the sludge return rate is 50%, the hydraulic retention time HRT is 0.
33 to 1.0 hours) is required. Although NOx-N is also denitrified here, since the substantial residence time is long, both phosphorus discharge and denitrification can be performed completely.

Next, the sludge is aerated in the sludge aerobic tank 4. Here, the sludge is activated by aeration, and at the same time, the organic nitrogen in the sludge is nitrified into NOx-N. Further, the phosphorus discharged in the sludge anaerobic tank 3 is absorbed here. Residence time here is 2-5 hours (sludge return rate is 50%
, A hydraulic retention time HRT of 0.67 to 1.7 hours is appropriate, and a longer time is preferable when nitrification is required,
Shorter is preferred when not needed. The aeration here can improve the adsorptivity of sludge in the next aeration tank 1.

The sludge discharged from the sludge aerobic tank 4 enters the aeration tank 1 together with sewage as activated sludge. Here, organic matter and organic nitrogen in sewage are adsorbed by sludge, and phosphorus is ingested excessively. The conditions for excessive intake of phosphorus are aeration tank 1
It is necessary to keep DO in 0.5 to 2.0 mg / L. DO is 0.5 mg /
If the amount is less than L, the efficiency of excessive intake of TP (Total-Phosphorus) in the aeration tank 1 decreases, and as shown in FIG.
P concentration increases. If the DO exceeds 2.0 mg / L, when the sludge is returned to the sludge anaerobic tank 3, it is ORed due to the effect of the carried-in DO.
P does not decrease sufficiently, and phosphorus is exhaled as shown in FIG.

The hydraulic retention time HRT here is 0.5
~ 2.0 hours is good, and if the residence time is insufficient, excessive intake of phosphorus and adsorption of organic matter (BOD) will be insufficient, and phosphorus and organic matter will remain in the treated water as shown in FIG. On the contrary, even if the residence time is longer than this, not only the treatment efficiency is not further improved, but also the decomposition of organic nitrogen into NH ₄ -N and NOx-N progresses, and the denitrification efficiency decreases. Further, when NOx-N is produced, the amount of NOx-N carried into the sludge anaerobic tank 3 increases, which adversely affects the discharge of phosphorus in the sludge anaerobic tank 3. The mixed solution discharged from the aeration tank 1 is subjected to solid-liquid separation in the final settling tank 2, the supernatant water is discharged as treated water, a part of sludge is withdrawn as excess sludge, and the rest is returned to the sludge anaerobic tank 3 again. to go into.

[0011]

As described above, in the present invention, sludge solid-liquid separated in the final settling tank 2 is anaerobically treated with sludge alone to discharge phosphorus, and then mixed with sewage to cause excessive intake of phosphorus. ing. Therefore, the hydraulic retention time can be shortened as compared with the conventional method in which dephosphorization is performed in the state of a mixed solution. Further, in the aeration tank 1, only the adsorption of organic matter and excessive intake of phosphorus are performed to secure the organic matter necessary for the discharge of phosphorus in the sludge anaerobic tank 3 after solid-liquid separation, and the ORP in the sludge anaerobic tank 3 is secured. It can be lowered sufficiently.
Further, by preventing the decomposition of the organic nitrogen in the aeration tank 1 and performing solid-liquid separation while adsorbing the organic nitrogen to the sludge, the organic nitrogen is extracted as surplus sludge to improve the denitrification rate. You can

[0012]

EXAMPLE Table 1 shows the result of comparison between the sewage treatment method of the present invention and the conventional anaerobic / aerobic method at a treatment scale of 1 m ³ / Hr. As is clear from Table 1, the processing time is reduced by 71% when the sedimentation tank is excluded, and by 21% when the sedimentation tank is included. Also,
The treated water quality is not only the same as that of the conventional method for BOD and P, but also the removal rate of N is improved.

[0013]

[Table 1]

[0014]

The effects of the present invention are listed below. The hydraulic retention time HRT excluding the final settling tank is 1.5-
In 4.7 hours, sewage organic matter and phosphorus can be removed efficiently. Therefore, the scale of the reaction tank can be reduced to 25-80% of the conventional one. Phosphorus is discharged only by sludge after solid-liquid separation in the final settling tank, so it is not easily affected by DO and NOx-N carried in sewage. Moreover, since the anaerobic time can be taken sufficiently, phosphorus can be sufficiently discharged. Therefore, excessive intake of phosphorus can be sufficiently performed, and stable phosphorus removal can be achieved. Organic nitrogen in sewage is removed by the steps of adsorption → solid-liquid separation → excess sludge extraction, and there is no decomposition process into NH ₄ -N and NOx-N, so the nitrogen removal rate is about 50% from the conventional 30%. Improves to 60%.

[Brief description of drawings]

FIG. 1 is a flow sheet of the present invention.

FIG. 2 is a graph showing the relationship between aeration tank DO and treated water TP.

FIG. 3 is a graph showing the relationship between an aeration tank DO and a sludge anaerobic tank ORP.

FIG. 4 is a graph showing the relationship between the hydraulic retention time of the aeration tank and the quality of treated water.

FIG. 5 is a flow sheet of a conventional anaerobic and aerobic method.

[Explanation of symbols]

 1 Aeration tank 2 Final settling tank 3 Sludge anaerobic tank 4 Sludge aerobic tank

Claims (3)

[Claims] 1. A sewage treatment method in which sewage is treated with activated sludge in an aeration tank and then solid-liquid separated in a final settling tank, and the sludge solid-liquid separated in the final settling tank is anaerobically and aerobically treated and then returned. A method for treating sewage, which comprises returning the sludge to the aeration tank. 2. The hydraulic retention time of the aeration tank is 0.5 to 2.0.
The method for treating sewage according to claim 1, characterized in that the time is set. 3. The method for treating sewage according to claim 1, wherein the DO in the aeration tank is controlled to 0.5 to 2.0 mg / L.