JP3376903B2 - Intermittent aeration activated sludge treatment method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an intermittent aeration type activated sludge method for removing BOD and nitrogen by utilizing activated sludge by intermittent aeration.

[0002]

Biodenitrification is known as a method for treating organic wastewater containing BOD and nitrogen. In this method, the liquid to be treated is aerated in the aeration tank, and BOD is generated by the action of activated sludge.
Is removed, and further aerated in a nitrification tank to oxidize ammoniacal nitrogen or organic nitrogen to (nitro) nitrite nitrogen, and denitrification is carried out by maintaining an anaerobic state in the denitrification tank. As a modification of this method, there is also a method of introducing the liquid to be treated into the denitrification tank to remove BOD, and then nitrifying and circulating the nitrification liquid to the denitrification tank. In any of the above methods, it is necessary to provide a nitrification tank and a denitrification tank separately,
The device becomes larger.

As a method for improving this point, organic substance-containing water is continuously supplied to an intermittent aeration tank, and intermittent aeration is carried out in the presence of activated sludge to repeat an aerobic process and an anaerobic process. An intermittent aeration type activated sludge treatment method for removing BOD and nitrogen by using one aeration tank (hereinafter, may be simply referred to as an intermittent aeration method) has been proposed (for example, Japanese Patent Laid-Open Nos. 1-310798 and 4). -197497
issue). In this method, in the aerobic process, BOD is decomposed by the action of BOD-decomposing bacteria in the activated sludge by aeration, and nitrification is performed by the action of nitrifying bacteria to change ammoniacal nitrogen and organic nitrogen to (nitrite) It is oxidized to nitrogen, and in the anaerobic process, aeration is stopped and the action of denitrifying bacteria reduces (nitrite) nitrogen to nitrogen gas in the presence of a hydrogen donor for denitrification.

In the above anaerobic process, a hydrogen donor is required as a nutrient source for denitrifying bacteria, and the BOD component in the liquid to be treated flowing in is also used as a hydrogen donor in the denitrifying process, but this is insufficient. In this case, denitrification is performed by adding another hydrogen donor such as methanol. Since this hydrogen donor is added from the outside of the system, it becomes a factor that raises the processing cost, and therefore needs to be efficiently used for denitrification.

The hydrogen donor added here corresponds to the BOD corresponding to the (nitrite) ion produced in the aerobic process.
Although the amount is added, conventionally, the BOD amount necessary for the anaerobic process is intensively added within about 5 minutes immediately after the start of the anaerobic process. The reason is that in order to use the hydrogen donor efficiently, the longer the reaction time after the hydrogen donor is added, the better. Therefore, it is considered preferable to concentrate the addition in the early stage of the anaerobic process. It is due to the fact. For example, if the hydrogen donor is added in the latter stage of the anaerobic process, the added hydrogen donor is switched to the aerobic process before being used, and the hydrogen donor is wasted. It is thought that many hydrogen donors will be used for denitrification due to the increase in hydrogen.

[0006] In the conventional intermittent aeration method, the treatment is performed by the above method, but as the treatment is continued, the denitrification rate decreases and the treatment efficiency decreases. The denitrification rate per sludge is 0.08 kgN / kgVSS / d or more in the normal biological denitrification method using a nitrification tank and a denitrification tank as separate tanks, but when the intermittent aeration method is continued, it is 0.02 kgN / kgVSS / d.
It may decrease below. This denitrification rate is 1 of the normal method.
Since it is / 4 or less, the residence time in the denitrification step is 4 times or more. This is 4 of the conventional denitrification tank as an aeration tank.
This means that more than double the volume is required, which goes against the purpose of downsizing the device.

[0007]

An object of the present invention is to provide an intermittent aeration type activated sludge treatment method capable of increasing the denitrification rate in the denitrification step and efficiently removing BOD and nitrogen with a small apparatus. It is to propose.

[0008]

The present invention is the following intermittent aeration type activated sludge treatment method. (1) A liquid to be treated is supplied to an intermittent aeration tank, aeration is intermittently performed in the presence of activated sludge, and an aerobic process and an anaerobic process are repeated. In the aerobic process, BOD removal and nitrogen nitrification are performed.
In the anaerobic process, an amount of hydrogen donor necessary for denitrification is separately dispensed over a time period of 1/2 or more of the anaerobic process for denitrification, and the intermittent aeration activated sludge treatment method is characterized. (2) The liquid to be treated is supplied to the intermittent aeration tank, and intermittently aerated in the presence of activated sludge to repeat the aerobic process and the anaerobic process. In the aerobic process, BOD is removed and nitrogen nitrification is performed.
In the anaerobic process, a hydrogen donor is separately injected so that the average sludge load for 5 minutes is an addition rate of 0.6 kgBOD / kgVSS / d or less, and denitrification is performed, and the intermittent aeration type activated sludge treatment method is characterized.

In the present invention, "(nitro) nitric acid" means nitric acid and / or nitrous acid. Nitrate nitrogen is NO ₃ −
N, nitrite nitrogen is NO ₂ -N, (nitrite) nitrogen is N
O _{2 + 3} or NO _X -N, ammoniacal nitrogen NH ₃ -
N and total nitrogen may be represented by TN. Since organic nitrogen such as proteins and amino acids behaves similarly to ammonia nitrogen, ammonia nitrogen or NH ₃ —N may contain organic nitrogen. The "hydrogen donor" serves as a substrate for the denitrifying bacteria to respire nitric acid, and methanol is preferable, but other lower alcohols, lower fatty acids, and other organic substances may be used.

The liquid to be treated in the present invention is a nitrogen-containing liquid. Nitrogen includes ammoniacal or organic nitrogen. Examples of the liquid to be treated include sewage, domestic wastewater, and landfill leachate.
Landfill leachate is leachate when garbage or other organic matter is used for landfill. In particular, wastewater having a BOD concentration lower than the nitrogen concentration (BOD / N is 1.5 or less) is suitable for treatment.

The treatment method of the present invention is an intermittent aeration type activated sludge treatment method, and the basic constitution is the same as the conventional one, and by intermittently performing aeration in one or a plurality of intermittent aeration tanks. The aerobic process and the anaerobic process are repeated to remove BOD and denitrify. That is, by supplying the liquid to be treated to the intermittent aeration tank continuously or intermittently, by repeating aerobic process and anaerobic process by performing aeration intermittently in the presence of activated sludge, using a single aeration tank, Remove BOD and nitrogen.

In this method, in the aerobic step, BOD is decomposed by aeration by utilizing the action of BOD-decomposing bacteria in the activated sludge, and nitrification is performed by the action of nitrifying bacteria to remove ammoniacal nitrogen and organic nitrogen ) Oxidize to nitrate nitrogen,
In the anaerobic process, the aeration is stopped and the denitrification is performed by reducing (nitrite) nitrogen to nitrogen gas in the presence of a hydrogen donor by the action of denitrifying bacteria. In the anaerobic process, a hydrogen donor is required as a nutrient source for denitrifying bacteria, and the BOD component in the liquid to be treated that flows in is also used as a hydrogen donor in the denitrifying process. Denitrification is performed by adding the above hydrogen donor.

In this case, as described above, conventionally, the amount of hydrogen donor necessary for the anaerobic process is intensively added within about 5 minutes immediately after the start of the anaerobic process. Denitrification is performed by injecting an amount of the hydrogen donor for a time of ½ or more, preferably 80% or more of the anaerobic step. The injection is preferably performed at the same time as the start of the anaerobic process, but the dissolved oxygen remains immediately after the stop, so the injection may be started after several minutes, for example, 2 to 5 minutes. The injection is preferably carried out by averaging the required hydrogen donor amount described above over the injection time. The infusion may be continuous, intermittent or pulsed.

(1) The total amount of hydrogen donors injected in the anaerobic process depends on (nitrite) ions in the nitrification solution obtained in the immediately preceding aerobic process and (nitrite) ions in the liquid to be treated flowing into the anaerobic process. an amount substantially corresponding, raw nitrogen concentration _{(NH 4 -N, NO 2 -N} , NO 3 -N, organic nitrogen, or the like), raw BOD concentration, reaction vessel, is determined in consideration of MLSS concentration, etc. However, in general, the amount is 3 times the nitrogen load of the liquid to be treated, that is, 2 to 4 kgBOD / kgT-.
N. In the present invention, it is preferable to inject this total hydrogen donor amount over the entire injection time at an average injection rate. In this case, the injection rate is such that the average sludge load for 5 minutes is 0.6 kgBOD / kgVSS / d or less, preferably 0.4 kgBOD / kgVSS / d or less.

The above injection rate is defined by the following equation.

[Formula 1] Injection rate L _H [kgBOD / kgVSS / d]
= S _h / (X _r × 1/1000 × V _r × T _h × 1/144
0) where T _h = injection time of hydrogen donor during anaerobic process [minutes] S _h = injection amount of hydrogen donor during anaerobic process [kg as B
OD] X _r = concentration of MLVSS in reaction tank [mg / L] V _r = reaction tank volume [m ³ ] BOD of methanol is 1.0 [kg BOD / kg
Methanol].

As the intermittent aeration tank, a tank equipped with a stirrer and an aerator is used, and it is preferable to intermittently aerate with the aerator while stirring with the stirrer, but the stirrer may be stopped during the aerating, and the stirrer may also be used. The aerobic process may be distinguished from the anaerobic process depending on the difference in stirring strength. As the aerating device, any device such as an air diffusing type, a liquid diffusing type, and a gas-liquid mixing type can be used.

By intermittent aeration using the above intermittent aeration tank, B
Basic processing of OD removal and denitrification can be performed, but a small volume finishing nitrification tank and finishing denitrification tank are installed in it.
It is preferred to remove residual BOD and nitrogen.

Further, solid-liquid separation means can be provided after the finishing denitrification tank. A precipitation tank, a filtration device, a membrane separation device, or the like can be adopted as the solid-liquid separation means. A conventionally known membrane separator may be used, and it is preferable to use an MF membrane or a UF membrane.

The activated sludge concentration in the intermittent aeration tank is MLS.
S as 500 to 30,000, preferably 2,000
〜20,000, BOD tank load is 5 kg / m ³ / d or less, preferably 0.1-3 kg / m ³ / d, BOD sludge load is 0.3 kg / kg-VSS / d or less, preferably 0.15 kg / Kg-VSS / d or less, nitrogen load is 1k
gN / m ³ / d or less, preferably 0.05 to 0.6 kg
N / m ³ / d. Aeration time or aerobic process is 10
~ 720 minutes, preferably 20-60 minutes, aeration stop time or anaerobic step 10-720 minutes, preferably 2
It is preferably 0 to 60 minutes.

In the aerobic process, aeration is performed by an aeration device,
The BOD component in the liquid to be treated is decomposed, and the ammoniacal and organic nitrogen are nitrified to (nitrite) nitrogen, and the aeration amount and the aeration time are the amount and time required for this. In the anaerobic process, aeration is stopped and the hydrogen donor is injected to gently agitate the anaerobic state to reduce (nitro) nitrite nitrogen to nitrogen gas. After completion of the anaerobic process, aeration is restarted, and the process proceeds to the aerobic process, which is repeated.

During this time, the liquid to be treated continuously flows in and the liquid to be treated continuously flows out, so that the BOD component and the nitrogen content in the raw water and a part of the nitrification liquid flow out without being treated. Therefore, it is preferable to perform aeration in the finish nitrification tank to decompose the BOD component, nitrify the remaining nitrogen, and denitrify in the finish denitrification tank. After performing such treatment, solid-liquid separation is performed by solid-liquid separation means to obtain a treatment liquid, and part of the solid matter is returned to the intermittent aeration tank as return sludge. The excess sludge may be extracted from any position, but it is preferable to extract it from the intermittent aeration tank because the concentration is constant.

In the anaerobic process of the present invention, the hydrogen donor is not injected intensively in a short time, but is dispensed for a long time to denitrify the hydrogen donor in a low concentration state. Nitrogen (NO ₃ -N) is nitrite nitrogen (N
It exhibits a phenomenon of being directly reduced to nitrogen gas (N ₂ ) without passing through O ₂ —N).

In the conventional method in which a hydrogen donor is intensively charged in the early stage of the denitrification process and denitrification is performed in a high concentration state of the hydrogen donor, nitrate nitrogen is reduced to nitrite nitrogen. The bacteria become predominant, so that the reaction in which nitrate nitrogen is reduced to nitrite nitrogen takes precedence, and its rate is 0.10 times or more the denitrification rate that is reduced to nitrite nitrogen or nitrogen gas.
It was found to be about 2 kgN / kgVSS / d.
As a result, if the nitrate nitrogen becomes excessive for some reason, it is reduced to nitrite nitrogen in the anaerobic process and remains in the reaction tank, and is oxidized to nitrate nitrogen in the next aerobic process. Therefore, the denitrification process rapidly deteriorates, further nitrite nitrogen is left, and the quality of treated water deteriorates sharply.

For example, sludge concentration 10,000 mg / l a
s MLVSS, NH ₄ -N tank load 0.1 kg N / m ³ /
d, maximum denitrification rate 0.02 kgN / kgVSS / d =
0.2 kgN / m ³ / d, anaerobic process for 30 minutes, aerobic process for 30 minutes, methanol 1 added in one anaerobic process
As for the case of 2.6 mg / l, if it is processed successfully, NO ₃ -N concentration at the anaerobic process started 4.2 mg / l, and the anaerobic processes begin simultaneously with NO ₃ -
The reduction reaction from N to NO ₂ -N begins, and N takes about 3 minutes.
O ₃ -N is all reduced to NO ₂ -N, and after 30 minutes NO
All _2- N is also reduced and the denitrification reaction ends.

On the other hand, NO in the reaction tank₃-N is 20 mg / l
If it remains, NO at the start of the anaerobic process₃-N concentration
Becomes 20 mg / l, NO at the same time the anaerobic process starts₃−
N to NO₂The reduction reaction to -N begins, but after about 8 minutes,
11 mg / l NO₃-N is NO ₂-When it changes to N
The methanol is exhausted and the reaction stops. here
11 mg / l NO produced₂-N is nitrification reaction in aerobic process
Due to NO₃Since it is oxidized to -N, the denitrification reaction is eventually
Almost nothing happened and only methanol was consumed.
It During this time, the raw water continues to flow in and is loaded with nitrogen,
About 24 mg / l NO during the next anaerobic process₃-N concentration and
Becomes, NO with time₃-N accumulates. For this reason
As mentioned above, the denitrification rate per sludge is 0.02kgN
/ KgVSS / d or lower.

On the other hand, in the reaction in which the hydrogen donor is supplied little by little as in the present invention and the denitrification is carried out in a state where the hydrogen donor is in a low concentration, NO ₃ —N apparently turns out NO ₂ —N. It will be denitrified without going through, and NO ₃ -N will be generated due to temporary high load.
The residual concentration of NO ₃ —N can be lowered without the quality of the treated water being rapidly deteriorated even when the residual amount remains. As a result, the denitrification characteristics are improved, the denitrification rate per sludge becomes 0.1 kgN / kgVSS / d or more, and a reaction rate comparable to that of the conventional circulating nitrification denitrification method and the like can be obtained.

[0027]

According to the present invention, in the anaerobic process in the intermittent aeration type activated sludge treatment method, a hydrogen donor is dispensed over a long period of time to increase the denitrification rate in the denitrification process and to provide a compact apparatus. Can efficiently remove BOD and nitrogen.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram of a method for treating intermittent aeration activated sludge according to the embodiment. In FIG. 1, 1
Is an intermittent aeration tank, 2 is a finishing nitrification tank, 3 is a finishing denitrification tank,
4 is a solid-liquid separation tank, and 5 is a membrane separation device.

In the treatment method using the above apparatus, the liquid to be treated is introduced from the liquid to be treated 6 into the intermittent aeration tank 1 and the sludge passage 7 is returned.
The activated sludge is returned from the tank, and if necessary, acid such as sulfuric acid, hydrochloric acid or the like or alkali such as sodium hydroxide is injected from the chemical injection path 8 to adjust the pH to around 7 to 8, and air is sent from the air supply path 9. Aeration is performed from the aeration device 10 to perform aerobic treatment. In this aerobic process, BO is decomposed by BOD-decomposing bacteria in activated sludge.
The D component is decomposed and nitrifying bacteria oxidize ammoniacal nitrogen and organic nitrogen to (nitrite) nitrogen.

By stopping the aeration, the process moves to the anaerobic process, and while slowly stirring with the stirrer 11, a hydrogen donor such as methanol is injected from the chemical injection passage 8 and maintained in the anaerobic state for denitrification. Here, (nitro) nitrite nitrogen in the nitrification liquid is reduced to nitrogen gas. In this case, when the hydrogen donor is injected for more than half of the anaerobic process, the reduction from the nitrogen nitrate to the nitrogen gas is performed without passing through the nitrite nitrogen. However, the denitrification rate does not decrease due to residual nitrite nitrogen, and denitrification is performed at a high denitrification rate.

Upon completion of the anaerobic process, air supply is restarted from the air supply passage 9 to perform aeration, and the process proceeds to the aerobic process. This is repeated below. Since the liquid to be treated always flows in during the above operation, the denitrification treatment liquid in an amount corresponding thereto enters the finishing nitrification tank 2 through the system passage 12. Here, air is supplied from the air supply passage 13 and the aeration device 1
By aeration from No. 4, residual BOD is decomposed, ammoniacal or organic nitrogen is oxidized to (nitrite) nitrogen, and final nitrification is performed.

The nitrification solution in the finishing nitrification tank 2 enters the finishing denitrification tank 3 from the system passage 15, and while stirring gently with the stirrer 16, a hydrogen donor such as methanol is injected from the chemical injection passage 17 into an anaerobic state. By keeping it, (nitro) nitrite nitrogen is reduced to nitrogen gas, and finish denitrification is performed. The finishing nitrification tank 2 and the finishing denitrification tank 3 can be omitted when a high-level treatment is not required.

The denitrification treatment liquid enters the solid-liquid separation tank 4 through the system line 18, and if necessary, acid or alkali is injected from the chemical injection line 19 to adjust the pH to 7 to 8, and the immersion type membrane separation is performed. The device 5 performs solid-liquid separation. M as the separation membrane 20
An F membrane or a UF membrane is used, and the permeated liquid is taken out from the treatment liquid passage 21 as a treatment liquid. The partition wall 22 is provided, the aeration device 23 is arranged below the membrane separation device 5, and air is sent from the air supply passage 24 for aeration to form a gas-liquid circulation flow.
The separation membrane 20 is prevented from being clogged. A part of the concentrated liquid is returned to the intermittent aeration tank 1 as returning sludge from the returning sludge passage 7.
Excess sludge is appropriately discharged from the intermittent aeration tank 1 through the sludge passage 25.

[0034]

EXAMPLES Examples will be described below.

Example 1 and Comparative Example 1 Tests were conducted according to the flow shown in FIG. Intermittent aeration tank 800 l
iter, finishing nitrification tank 67 liter, finishing denitrification tank 133 l
Iter and solid-liquid separation tank 332 liter. Landfill leachate was used as test raw water, and water was passed at 800 liter / d. Raw water B
The OD concentration was 20 mg / l or less, and TN was adjusted to 100 to 250 mg / l by appropriately adding ammonium sulfate. NO
_x- N was 0 to 20 mg / l. The raw water was continuously calcium-removed by a sodium carbonate-added alkaline coagulation precipitation method and neutralized to pH 6.0 with sulfuric acid to pass water. The pH of the intermittent aeration tank and the solid-liquid separation tank was adjusted to around 7.2.

The intermittent aeration was operated as a 30-minute aerobic process and a 30-minute anaerobic process. However, DO is 2 mg / l even during the aeration process
Automatic control was performed so that aeration was temporarily stopped when the temperature exceeded the limit. Methanol was used as a hydrogen donor for denitrification and added in an anaerobic process. At this time, in the first half of the test, methanol was added in the first 5 minutes of the anaerobic process (Comparative Example 1), and in the latter half of the test, continuous addition was performed during the anaerobic process (Example 1).

FIG. 2 shows changes in the methanol sludge load per methanol addition time during the operation period. In Example 1, the BOD in the raw water was 1/20, which was extremely low as compared with the added methanol, so the BOD in the raw water as the hydrogen donor in the denitrification reaction was ignored. The transition of denitrification rate is shown in FIG. Here, the denitrification rate was sampled every 5 minutes at the same time as the start of the anaerobic process, and the NO ₂ -N concentration and NO ₃ -N concentration in the supernatant water of the aeration tank sludge were measured,
The denitrification rate is the rate of decrease of the NO ₂ -N concentration + NO ₃ -N concentration (referred to as NO _x -N concentration or NO _{2 + 3-} N concentration for convenience). Furthermore, an example of denitrification rate measurement is shown in FIGS. 4 and 5. FIG. 4 is a denitrification pattern of Comparative Example 1 when the methanol sludge load is high.
Is the state of. FIG. 5 is a denitrification pattern of Example 1 after the methanol sludge load was reduced and two months or more had passed, which is the state of 8/14 in FIGS. 2 and 3.

As is apparent from FIG. 3, when the methanol sludge load of Comparative Example 1 is as high as 0.8 or more and the methanol addition time is 20% or less of the anaerobic process, the denitrification rate per sludge is 0.02 to 0.02. The value is as low as 0.05 kgN / kgVSS / d. Also, during this period, as shown in FIG.
It can be seen that the reaction for reducing _3- N to NO ₂ -N has a denitrification pattern in which the reaction has priority. On the other hand, after the methanol sludge load of Example 1 was set to 0.4 or less (from June in the figure), the denitrification rate per sludge rapidly increased to 0.1 k.
Values above gN / kgVSS / d have been obtained. In the denitrification pattern at this time, as shown in FIG. 5, the reduction to NO ₂ -N has almost no priority, and the reduction to nitrogen gas is performed without passing from NO ₃ -N through NO ₂ -N. You can see that it has been denitrified.

From the above results, by using the present invention, the denitrification rate per sludge is 0.1 kgN / kgVSS /
It is clear that a high value of d or more and a good denitrification property in which reduction to NO ₂ —N is not given priority can be obtained.

[Brief description of drawings]

FIG. 1 is a system diagram showing an intermittent aeration type activated sludge treatment method according to an embodiment.

FIG. 2 is a graph showing changes in methanol sludge load per methanol addition time in Comparative Example 1 and Example 1.

FIG. 3 is a graph showing changes in denitrification rates of Comparative Example 1 and Example 1.

FIG. 4 is a graph showing denitrification characteristics of Comparative Example 1.

5 is a graph showing denitrification characteristics of Example 1. FIG.

[Explanation of symbols]

1 Intermittent aeration tank 2 Finishing nitrification tank 3 Finishing membrane nitrification tank 4 Solid-liquid separation tank 5 Membrane separation device 6 liquid path to be treated 7 Return sludge path 8, 17, 19 Drug injection route 9, 13, 24 Airway 10, 14, 23 Aeration device 11, 16 stirrer 21 Processing liquid path 22 partition 25 Drainage route

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Claims (2)

(57) [Claims] 1. A liquid to be treated is supplied to an intermittent aeration tank, and aeration is carried out intermittently in the presence of activated sludge to repeat an aerobic process and an anaerobic process. In the aerobic process, BOD removal and nitrogen nitrification are performed. In the anaerobic process, an amount of hydrogen donor required for denitrification is separately injected for ½ or more times of the anaerobic process for denitrification to perform the intermittent aeration activated sludge treatment method. 2. A liquid to be treated is supplied to an intermittent aeration tank, and aeration is carried out intermittently in the presence of activated sludge to repeat an aerobic process and an anaerobic process. In the aerobic process, BOD removal and nitrogen nitrification are carried out. In the anaerobic process, a hydrogen donor is separately injected so that the average sludge load for 5 minutes is an addition rate of 0.6 kgBOD / kgVSS / d or less for denitrification. Method.