JPS5814834B2 - Wastewater activated sludge treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating wastewater that produces a small amount of sludge.

The activated sludge method is widely practiced as a method for treating wastewater (in this application, a wide range of wastewater, such as urban sewage, various industrial wastewater, and livestock wastewater, is collectively referred to as wastewater).

However, in the conventional activated sludge method, since the amount of sludge produced is quite large, how to completely treat the sludge is a major problem.

Although attempts have been made to use the produced sludge as fertilizer, for example, there are various problems in terms of cost and technology, and it is difficult to say that the results are satisfactory.

Therefore, recently attempts have been made to avoid the problems of sludge treatment as much as possible by suppressing the amount of sludge produced.

For example, a method has been developed in which a wastewater treatment tank is divided into about three sealed treatment zones and a high oxygen concentration gas is mechanically stirred and mixed into the wastewater passing through these treatment zones.

However, since this method uses a closed treatment tank, there are serious problems such as a decrease in wastewater treatment efficiency due to a drop in pH caused by CO2 that accumulates in the treatment tank and the risk of explosion of volatile organic substances. exist.

Furthermore, since a mechanical stirring mechanism having a movable part is used in a high oxygen concentration gas or liquid in a closed tank, inspection and maintenance thereof is extremely difficult.

Therefore, the inventor of the present invention has conducted various studies in order to establish a wastewater treatment method that can suppress the amount of sludge produced without reducing treatment efficiency or risk of explosion, and as a result, has finally found the method described in the above claims. The present invention has been completed and its objectives have been achieved.

The present invention will now be described in more detail with reference to the accompanying drawings, which are flowcharts of one embodiment of the present invention.

In the method of the present invention, the aeration treatment of wastewater is carried out in three aeration treatment zones A, B and C.

In the first treatment area A, mixed water (hereinafter simply referred to as mixed water) consisting of raw waste water from line 2 and sludge returned from settling tank 32 through line 36 is treated.

The first treatment area A is of an open type, and here air from the line 4 and the pump 6 is blown into the mixed water with great force from the aeration pipe 8 to effect aeration.

Mixing and stirring of air and mixed water may be performed using an ejector, a premix nozzle, or the like.

The gas injection in treatment area A is 60% of the wastewater raw water BOD.
In order to remove preferably 70% or more, the dissolved oxygen concentration in the mixed water is maintained at 0.5 mg/l or more, the surface area is increased by making the sludge flocs finer, and the BOD components are adsorbed onto the surface of the flocs. and to increase the rate of oxidation of BOD components.

If the sludge flocs are sufficiently finely divided, the rate of respiration of aerobic microorganisms in the sludge will be more rate-limiting for mixed water treatment than the rate of oxygen transfer into the flocs.

Therefore, if the dissolved oxygen concentration is 0.5 m9/l or more, a sufficiently good wastewater treatment effect can be obtained.

Furthermore, by making the first treatment area A an open type, C02
Since the accumulation of volatile organic substances and the like is prevented, problems such as a decrease in the pH of the mixed water and the danger of explosion as described above can be completely avoided.

Note that the gas blown into the first treatment area A may be a gas containing oxygen at a higher concentration than air, although this has the disadvantage of increased cost.

Therefore, for example, if a large amount of plant waste gas containing high concentrations of oxygen is available on a regular basis, processing efficiency can be improved without increasing costs by using the waste gas alone or together with air. You can.

The residence time of the mixed water in the first treatment area A can vary greatly depending on the amount of raw wastewater, the quality of the raw wastewater, sludge concentration, the amount of gas blown, etc., but is usually in the range of 10 to 60 minutes. It's within.

In the open second treatment zone B, air or oxygen-containing gas is blown into the mixed water flowing from the first treatment zone A through the communication part 10 from the diffuser pipe 16 through the line 12 and the pump 14, Aeration is performed again.

The gas injection in treatment area B is 70% of the wastewater raw water BOD.
The purpose is to maintain a uniform mixed state of sludge and to flocculate the sludge in order to remove at least 80%, preferably at least 80%.

Therefore, it is desirable to blow gas slowly enough to maintain a uniform mixing state of the sludge in the mixed water, and to supply enough oxygen for the flocculation of the sludge and the accompanying oxidation of the BOD components. It is necessary to perform this so that the dissolved oxygen concentration in the mixed water is 2 m9/l or more.

The blown gas used in treatment zone B may be air, another oxygen-containing gas, or a mixture of both.

Examples of the oxygen-containing gas include plant waste gas and the like as in the first treatment zone A, and furthermore, exhaust gas from the third treatment zone C may be used.

In the second treatment area B, the generated CO2 is dissipated and removed outside the system by aeration under open conditions, so that the pH of the mixed water does not decrease.

Furthermore, the risk of explosion due to the accumulation of volatile organic substances etc. is completely prevented.

Also, always keep the mixed water neutral or slightly alkaline (pH 7~
8.5), and when treating wastewater containing substances for which the treatment rate with activated sludge is low (for example, coke oven wastewater containing thiocyanate ion SCN), the residence time is relatively short. In some cases, the first treatment area A alone may not be able to fully process the process, but in the present invention, C02 is dissipated by using the open second treatment area B, so the second treatment area Mixed water can be easily kept neutral or slightly alkaline, and therefore the above-mentioned substances existing in wastewater or adsorbed in sludge can be treated satisfactorily.

The residence time of the mixed water in the second treatment zone can also vary widely depending on other factors, but is typically in the range of 20 to 80 minutes.

In the closed type third treatment area C, oxygen-containing gas is blown into the mixed water flowing from the second treatment area B through the communication portion 18 through the aeration pipe 26, and further aeration is performed.

The purpose of gas injection in treatment zone C is to further promote flocculation of the sludge and to significantly reduce the volume of the sludge through self-oxidation of the sludge.

Therefore, in order to maintain a uniform mixed state of sludge in the mixed water and to maintain even the inside of the sludge aerobic due to the enlargement of flocs, a higher dissolved oxygen concentration than in the case of treatment area B is required. I need.

For this purpose, the dissolved oxygen concentration in the mixed water needs to be 5 mg/l or more, but on the other hand, if the dissolved oxygen concentration is too high, the sludge will become fine due to excessive self-oxidation, so it will take a long time for sedimentation and separation. The upper limit is set at 15 .mu./l because it requires a large amount of water and has the disadvantage that fine sludge flows out during treatment.

Within the range of 5 to 15 m9/l of dissolved oxygen, deterioration in the sedimentation property of the sludge due to gas generation, filamentous fungus generation, etc. that would occur when the inside of the sludge becomes anaerobic is completely prevented.

In order to promote flocculation of sludge in the third treatment zone C, it is necessary to maintain the dissolved oxygen concentration at 5 to 15 m9/1 and to stir gently to the extent that the mixed water is uniform. For this purpose, the oxygen concentration in the circulating gas should be adjusted to 35 to 55.
It is necessary to set it as vol%.

In other words, when stirring the liquid only by aeration of gas without using a mechanical stirring mechanism in which the movable part is in the liquid or in the gas with high oxygen concentration, the gas with an oxygen concentration of more than 55 vol. If you try to maintain the mixed state, the oxygen supply will be excessive and the dissolved oxygen concentration will be 15 mI? ，
It exceeds #.

On the other hand, if an attempt is made to maintain the dissolved oxygen concentration above 5 m9/l using circulating gas with an oxygen concentration of less than 35 vol%, it is necessary to perform vigorous circulation, which makes the sludge flocs finer and deteriorates their settling properties.

The oxygen concentration of the circulating gas in treatment zone C is set to 35 to 5.
In order to maintain the concentration at 5 vol % and to supplement the amount of gas discharged from line 28, high-concentration oxygen-containing gas is appropriately supplied from line 20.

The oxygen concentration and amount of the make-up gas are based on the amount of dissolved oxygen in treatment zones B and C, the oxygen consumption in treatment zone C, the composition and concentration of mixed water in treatment zones B and C, and the amount of oxygen from line 28. It is determined by the amount of emissions, etc., but the oxygen concentration is 40 vol%
It is preferable to set it as above.

In the treatment zone C, the blowing gas is circulated through the pump 22, the line 24, and the aeration pipe 26.

The residence time of the mixed water in the third treatment zone can also vary widely, but is typically in the range of 20 to 120 minutes.

Mixed water containing sludge that has undergone flocculation and self-oxidation is
Next, from the third treatment area C, the line 30 is passed to the sedimentation tank 32.
and sent to undergo sedimentation treatment.

The supernatant water is discharged from the system through line 34 and is further treated if necessary.

The settled sludge is transferred from the settling tank 32 to the line 36 and the pump 3.
Most of it is returned to the first treatment area A, and a small amount of surplus sludge (0.2 to 0.5 kgMLss/
kg BOD) is discharged from the system through line 40.

Various control factors (residence time, amount of blown gas, oxygen concentration, etc.) in the present invention are determined as follows.

First, from the water quality of the wastewater raw water to be treated (BOD value, composition of BOD components, etc.), the first Determine the residence time in the treatment area and the amount of air blowing.

Next, the amount of BOD removed in the second and third treatment areas, the required dissolved oxygen concentration (2 m9/l or more and 5 to 157%, respectively)
l/l), the time required for floc formation and self-oxidation, and the aeration conditions in both treatment zones, the residence time in both treatment zones, the oxygen concentration of the blown gas in both treatment zones, and the Determine the oxygen concentration and supply amount of the make-up gas to the third treatment zone.

Generally, in the case of highly reactive food factory wastewater, etc., the first
It is desirable to use very vigorous agitation in the first treatment zone to shorten the residence time, while increasing the residence time in the second and third treatment zones to ensure sufficient sludge floc growth and self-oxidation. On the other hand, in the case of chemical factory wastewater with low reactivity (e.g. steel factory wastewater), the residence time in the first treatment zone is lengthened, while the residence time in the second and third treatment zones is increased. It is desirable to have relatively short residence times.

In any case, the total processing time is significantly reduced compared to conventional methods.

According to the method of the present invention, the following effects are achieved.

(1) The amount of surplus sludge produced is extremely small.

(2) Unlike completely sealed treatment methods that use highly concentrated oxygen, no accumulation of CO2 and volatile organic substances occurs, so high treatment efficiency can be maintained at all times without the risk of explosion.

(3) It is essential to keep the mixed water neutral or weakly alkaline during activated sludge treatment, and wastewater containing substances that are treated at a low rate by activated sludge can also be sufficiently treated.

(4) Compared to a completely sealed treatment method that uses only high-concentration oxygen, a lower concentration of oxygen-containing gas can be used, and air can also be used, resulting in a significant reduction in cost. .

(5) Gas with a relatively low oxygen concentration (for example, 35 vol of oxygen
1% or less), the equipment cost becomes significantly large, and it becomes difficult to prevent the pH from dropping below the pH level due to CO2 accumulation and to maintain and manage the equipment.

In contrast, the present invention, which uses an open second treatment zone, has low equipment costs, does not cause pH drop, and is extremely easy to maintain and manage, which more than compensates for the disadvantages of disposable gas. Become something.

(6) When using a mechanical stirring mechanism that has a moving part in a high oxygen concentration gas or liquid in a closed tank such as the third treatment area C, maintenance inspection and maintenance of the mechanism is extremely difficult. However, in the present invention, such a difficulty does not exist because the dissolution of oxygen and stirring are performed simultaneously by blowing gas.

(7) Processing time can be greatly shortened by using the open type first and second treatment zones and the closed type third treatment zone, and by adopting the optimum amount of dissolved oxygen in each treatment zone. .

Examples of the present invention will be shown below to make the features of the present invention clearer.

Example 1 A coke oven was constructed using an open type first aeration tank of 1 m x 1.5 m x 2.5 m, an open type second aeration tank of the same size, and a closed type third aeration tank of 1 m x 3 m x 2.5 m. Wastewater was treated with activated sludge.

Wastewater that has undergone primary treatment in the pre-sedimentation tank (BODI 9 8p
pm, scN-concentration 20 ppm, pH 7. 3
) Return sludge to approximately 4 m3/H (sludge concentration 18000 mg
/l) about 2 m3/H and blow 150Nm3/H of air into the first aeration tank at a water temperature of 25°C to maintain the dissolved oxygen concentration (D.0.) at about 0.8mg/l. At the same time, the sludge was made finer.

The residence time in the first aeration tank is approximately 30 minutes, and the quality of the mixed water discharged from the first aeration tank to the second aeration tank is approximately 50 ppm BOD, lppm SCN, and pH 7. It was 2.

Air of 40 Nm 3 /H was blown into the second aeration tank from an aeration tube to perform aeration and stirring.

In the second tank, the D. 0. The flocs were formed while maintaining the concentration at about 2.1 .mu./l.

Residence time was 30 minutes.

The quality of the mixed water discharged from the second aeration tank to the third aeration tank was approximately 34 ppm BOD, no SCN was detected, and the pH was 7.2.

Gas with an oxygen concentration of 50 vol% was supplied to the third aeration tank, and circulating aeration was performed.

The oxygen concentration of the circulating gas is approximately 41 vol%, the circulating gas amount is approximately 1
D. in the mixed water by gentle stirring at 5Nm3/H. 0
．． Approximately 6. 2 mI? Floc formation and self-oxidation of the sludge were carried out with a residence time of 60 minutes while maintaining the temperature at a temperature of 1/7.

The quality of the mixed water at the outlet of the third aeration tank is BoD 24ppm.
, SCN was not detected, and the pH was 7.1.

The mixed water that came out of the third aeration tank was led to a sedimentation layer, where sludge and treated water were separated.

The SVI, which is a ring that indicates the sedimentation property of sludge, was 46 in the mixed water discharged from the third aeration tank, and the amount of surplus sludge generated was 0. 2
It was 8 kgMLss/kPBOD.

Example 2 1m x 1.5m x 3m open type first aeration tank, 1m x 1.
5m x 2m open type second aeration tank and 1m x 1.5m x 2
Urban sewage was treated with activated sludge using a closed type third aeration tank.

10 m3 of raw water (pH 7.2, water temperature 26°C) that has undergone primary treatment in a pre-sedimentation tank and has a BOD of approximately 136 ppm
Returned sludge (sludge concentration 14,000 mg/l was added to H at a return sludge rate of 40%, and 70N
Blow in air at m3/h, D. 0. about 0.9mg/l
The sludge was made finer while maintaining the sludge.

The residence time in the first aeration tank is 40 minutes, and the BOD of the mixed water discharged from the first aeration tank to the second aeration tank is approximately 32p.
pm and pH were 7.1.

A mixed gas of about 15 Nm3/H of exhaust gas from the third aeration tank and air is blown into the second aeration tank through an aeration pipe, and D. 0. 2. Flock formation was performed while maintaining the concentration at 1 m9/l.

BOD of mixed water discharged from the second aeration tank to the third aeration tank
was about 25 ppm, pH was 7.1, and residence time in the second aeration tank was about 40 minutes.

A gas of 1 Nm/H with an oxygen concentration of 5 vol% was supplied to the third aeration tank, and circulating aeration was performed.

The oxygen concentration of the circulating gas is approximately 36 vol%, and the D. 0.
Floc formation and autooxidation of the sludge were carried out while maintaining the concentration at approximately 5.3 mg/l.

The residence time in the third aeration tank was 40 minutes, and the mixed water (B01) that came out of the third aeration tank was 20 ppm, pH 6.9.
) was led to a settling tank, where sludge and treated water were separated.

SVI was 43 in the mixed water effluent from the third aeration tank.

In addition, the amount of surplus sludge generated is 0. 3 2 kyMLss/k
It was gBOD.

[Brief explanation of drawings]

The drawing is a flowchart outlining the method of the invention. A: Open type first processing area, B:
Open type second treatment area, C... Closed type third treatment area, 2... Wastewater raw water supply line, 4, 12
...Air supply line, 6, 14, 22...
... Pump, 8, 16, 26 ... Diffuser pipe, 20
... Oxygen supply line, 24 ... Oxygen-containing gas blowing line, 30 ... Aerated water delivery line, 32 ... Sedimentation tank, 34 ... ...
-Supernatant water discharge line, 36...Sludge removal line, 38...Pump, 40...Excess sludge discharge line.

Claims (1)

[Claims] 1. Three aeration treatment zones are used, and in the first treatment zone, which is an open type and accommodates mixed water consisting of wastewater and sludge returned from the settling tank, the amount of dissolved oxygen in the mixed water is reduced under agitation by air blowing. The sludge is made fine while maintaining the sludge at 0.5 mg/1 or more, and in an open second treatment zone following the first treatment zone, the first treatment zone is agitated by blowing air or oxygen-containing gas.
Sludge flocs are formed while maintaining the amount of dissolved oxygen in the inflow water from the second treatment zone at 2 mg/l or more, and in the closed third treatment zone following the second treatment zone, the oxygen concentration is 35 to 35.
Under agitation by blowing 55 vol% gas, while maintaining the amount of dissolved oxygen in the inflow water from the second treatment zone at 5 to 15 μ/l, the formation of flocs in the sludge is promoted and the flocs undergo self-oxidation. , characterized in that the outflow water from the third treatment area is led to a settling tank to settle the sludge, while the supernatant water is discharged outside the system, and a part of the settled sludge is returned to the first treatment area. Activated sludge treatment method for wastewater.