JP6939724B2 - Sewage treatment method and equipment - Google Patents

Description

The present invention broadly belongs to the field of sewage and wastewater, and particularly relates to a technique for removing nitrogen from wastewater.

A typical conventional treatment process for removing nitrogen is the circulating nitrification denitrification method. It consists of an oxygen-free tank equipped with a stirrer, an aerobic tank equipped with an air diffuser, and a final settling basin in the subsequent stage. In the aerobic tank, ammonia nitrogen in sewage is nitrified into nitrite nitrogen and nitrate nitrogen by the action of nitrifying bacteria. The nitrite nitrogen and nitrate nitrogen generated in the aerobic tank are sent as nitrification liquid to the oxygen-free tank in the previous stage using a circulation line, and denitrified by the denitrification reaction using organic carbon contained in the sewage. , Nitrogen is removed from wastewater. The activated sludge-containing liquid flowing out of the aerobic tank is solid-liquid separated in the final settling basin, and the sludge separated by sedimentation is returned to the reaction tank (anoxic tank or aerobic tank) through the return sludge line. The part is discharged to the outside of the system as excess sludge.

One of the circulating nitrification denitrification methods is disclosed in Patent Document 1. In this method, raw water is denitrified by denitrifying bacteria in an anaerobic tank, nitrified by nitrifying bacteria in an aerobic tank, and solid-liquid separated in a sedimentation tank, and the supernatant is discharged as treated water. In the activated sludge circulation modified method including the step, an anaerobic-aerobic tank equipped with a raw water agitation mechanism and an air blowing mechanism is provided between the anaerobic tank and the aerobic tank, and is calculated by calculation. Activated sludge characterized in that the anaerobic / aerobic ratio of the anaerobic-aerobic tank is controlled by appropriately driving the stirring mechanism or the air blowing mechanism from the ratio of the denitrification rate and the nitrification rate. This is a modified circulation operation control method. This anaerobic-aerobic tank is intended to efficiently treat wastewater by changing the balance between the anaerobic tank and the aerobic tank according to the raw water.

In addition, wastewater that removes nitrogen-based pollutants in wastewater by combining biological oxidation in an aerobic tank under aerobic conditions and biological denitrification in an oxygen-free tank under anoxic conditions. The wastewater is characterized in that at least one intermediate reaction tank is provided between the aerobic tank and the oxygen-free tank so that the wastewater can be operated under both aerobic conditions and oxygen-free conditions. (Patent Document 2) has also been developed. This method also makes it possible to use the intermediate reaction tank as either an aerobic tank or an oxygen-free tank depending on the quality of the raw water.

Further, it has a reaction tank consisting of an anaerobic tank, an oxygen-free tank and an aerobic tank, or a reaction tank consisting of an oxygen-free tank and an aerobic tank, and a settling reservoir in the subsequent stage, and the aerobic tank contains a microorganism-immobilized carrier. In a method for treating wastewater that is inherent and returns at least a part of sludge drawn from the sedimentation pond to the anaerobic tank or the oxygen-free tank, the microorganism is placed between the oxygen-free tank and the aerobic tank. By installing an intermediate reaction vessel without an immobilized carrier and equipped with an air diffuser and operating the air diffuser, the intermediate reaction vessel can be subjected to anaerobic conditions, aerobic conditions or conditions in which it coexists. A wastewater treatment method (Patent Document 3), which is characterized by operating underneath, has also been developed. This method also puts the intermediate reaction tank in an anaerobic state, an aerobic state, or a shared state thereof depending on the processing state of the oxygen-free tank.

Japanese Unexamined Patent Publication No. 7-16595 Japanese Unexamined Patent Publication No. 10-94796 Japanese Unexamined Patent Publication No. 2000-27993

As described above, the circulation type nitrification denitrification method returns the nitrification liquid from the aerobic tank to the oxygen-free tank, and all of the methods of Patent Documents 1 to 3 described above are provided with a nitrification liquid circulation line. This amount of circulating water has a large circulation ratio of about 2 to 2.5 times the amount of raw water, and the running cost of the circulation pump is considerable, and the vitrified solution containing a large amount of dissolved oxygen is sent to the oxygen-free tank. The decrease in anaerobicity in the oxygen-free tank has become a problem.

The present invention has been made to solve the drawback of "having a circulation line" of the conventional circulation denitrification method, and it is possible to remove nitrogen from wastewater efficiently and inexpensively with a smaller facility. The purpose is to provide a wastewater treatment method and equipment that can be performed.

In order to achieve a high nitrogen removal rate in a process that omits the nitrification liquid circulation line, a microaerobic tank set to a low DO is provided, and the nitrification reaction and the denitrification reaction are simultaneously performed in the microaerobic tank. This makes it possible to omit the installation of the circulation line. To do nitrogen removal at a high removal rate from sewage, denitrification in the anoxic tank of the NO _X contained in the return sludge is also important. In order to efficiently denitrify in the oxygen-free tank, it is important to increase the nitrification rate in the aerobic tank, and in order to achieve this with a small installation area, an aerobic tank to which a carrier is added is provided. In order to reduce the running cost of the process, it is important to reduce the aeration cost, and the aerobic tank DO for that purpose is operated so as to be 3.0 mg / L or less. Further, in order to promote the simultaneous nitrification and denitrification reaction in the microaerobic tank, the setting DO of the microaerobic tank is important, and it is preferable to operate the microaerobic tank DO at 1.0 mg / L or less. .. By doing so, a process that can remove nitrogen from sewage at a low running cost and in a small space is provided.

Therefore, the present invention includes at least an oxygen-free tank, a microaerobic tank, an aerobic tank, and a sedimentation pond installed in the subsequent stage, and the aerobic tank contains a microbial-immobilized carrier carrying nitrifying bacteria. In the sewage treatment method in which at least a part of the sludge drawn from the sedimentation pond is returned to the anoxic tank, the dissolved oxygen in the slightly aerobic tank is not returned from the aerobic tank to the anoxic tank. A sewage treatment method characterized in that the concentration is equal to or less than the dissolved oxygen concentration in the aerobic tank and the dissolved oxygen concentration in the aerobic tank is 1.0 mg / L or more and 3.0 mg / L or less.
It is provided with at least an oxygen-free tank, a microaerobic tank, an aerobic tank, and a settling pond installed in the subsequent stage, and the aerobic tank contains a microbial immobilization carrier carrying nitrifying bacteria, and the settling pond. In a sewage treatment device provided with a return line for returning at least a part of the sludge drawn from the oxygen tank to the oxygen-free tank, there is no circulation line for the nitrifying liquid from the aerobic tank to the oxygen-free tank, and the slightly aerobic tank has no circulation line. It is characterized by having a control mechanism for setting the dissolved oxygen concentration to be equal to or lower than the dissolved oxygen concentration in the aerobic tank and setting the dissolved oxygen concentration in the aerobic tank to 1.0 mg / L or more and 3.0 mg / L or less. It provides a sewage treatment device.

As described above, according to the present invention, the operating cost of the circulation pump is reduced by eliminating the circulation line of the nitrifying liquid and operating the aerobic tank DO at 3.0 mg / L or less in the wastewater circulation type nitrification denitrification method. It is possible to remove nitrogen from wastewater with a smaller facility than the conventional technology and at low cost.

It is a figure which shows the schematic structure of the sewage treatment apparatus which is one Embodiment of this invention. It is a graph which shows the relationship between the dissolved oxygen concentration of an aerobic tank and the cost of an aeration blower.

The anoxic tank applies the use of nitrate nitrogen and nitrite nitrogen oxygen by denitrifying bacteria in the absence of dissolved oxygen, and reduces these oxidized nitrogens in circulating water to nitrogen to remove nitrogen in the water. It is a tank to be used. A stirrer is installed in the tank to stir so that oxygen does not normally dissolve. The dissolved oxygen concentration (DO) of the inflow water to the anoxic tank is 0 to 0.1 mg / L in the present invention, and the DO of the mixed water of the vitrified liquid circulating water and the inflow water is usually 0. It will be about 3 to 1.5 mg / L. The aerobic tank is a tank in which nitrifying bacteria oxidize ammonia nitrogen to nitrate nitrogen or nitrite nitrogen in an aerobic state, and an air diffuser is usually provided in the tank. In order to efficiently carry out the nitrification reaction in this aerobic tank, a microbial immobilization carrier is contained in the aerobic tank. This microbial-immobilized carrier is a granular substance on which nitrifying bacteria and the like are immobilized. The microorganisms to be immobilized may be those used in the sewage treatment facility as they are or those used in the sewage treatment facility. For immobilization, a comprehensive method or a method by adhering microorganisms to the surface is inexpensive and preferable. For the carrier, synthetic polymers such as polyethylene glycol, polyacrylamide, and polyvinyl alcohol and polysaccharides such as agar, carrageenan, and starch are used in the case of the comprehensive method, and polypropylene is used in addition to the comprehensive method material in the case of adhesion. Polyethylene, polyvinyl formal, activated charcoal, inorganic materials and the like are used. The particle size is about 1 to 30 mm, usually about 3 to 10 mm.

The addition rate of the carrier is preferably 0.5% to 10%, more preferably 1% to 7%, and even more preferably 2% to 5% in terms of true volume with respect to the sewage in the aerobic tank.

The DO of the aerobic tank shall be 1.0 mg / L or more and 3.0 mg / L or less. If the aerobic tank DO is less than 1.0 mg / L, the nitrification performance by the carrier is significantly lowered, while if it exceeds 3.0 mg / L, the aeration cost is significantly increased, which is not preferable.

In FIG. 2, the horizontal axis is the aerobic tank DO, and the vertical axis is the ratio of the aeration blower power when the aeration tank DO is set to 3.0 mg / L and the aeration blower power is set to “1”. It is a graph. At this time, the DO of the microaerobic tank is 1.0 mg / L. As shown in FIG. 2, when the DO exceeds 3.0 mg / L, the aeration blower power increases remarkably. Therefore, the aerobic tank DO is preferably 3.0 mg / L or less.

The volume ratio between the oxygen-free tank and the aerobic tank varies depending on the inflow water quality and the amount of carrier input, but is about 4: 1 to 1: 3, and usually about 2: 1 to 1: 2.

The microaerobic tank is a tank that is installed between the anoxic tank and the aerobic tank and does not have a microbial fixation carrier, and has a DO concentration equal to or lower than that of the aerobic tank. An air diffuser is also provided in this microaerobic tank, but it is preferable that the air diffuser is provided in a offset manner so as to generate a swirling flow in the tank. A rectifying wall for swirling flow may be provided in the microaerobic tank.

The DO in the microaerobic tank is equal to or less than that of the aerobic tank, preferably 1.0 mg / L or less. This DO is a DO at a certain depth of the sensor unit of the DO meter.

In the DO meter for measuring this DO, it is preferable to install the sensor unit at 1/3 or more of the water depth of the microaerobic tank. This will be described next. Here, 1/3 or more of the water depth represents the shallow side of the water depth.

In the example of Patent Document 3, the MLSS is 2500 mg / L and the circulation ratio (this is considered to be the total of the circulation line and the return line, so it is regarded as the total circulation ratio), and the test sewage T- The N concentration ( _CTN ) is 30 mg / L. Therefore, assuming that the return ratio of the return line is 0.5 or 1, the circulation ratio of the circulation line is 1 or 0.5, and α (the ratio of nitrogen related to nitrification to the inflow nitrogen) is 0.61 (0 in the literature). A value of .7 to 0.8 is exemplified, but in reality, it is often lower than that, so it is set to a little less than 90% of the lower limit value), β (the amount of nitrogen denitrified in the microaerobic tank). The ratio to the amount of inflow nitrogen) is calculated as 0.11 and is as shown in Table 1. Assuming that the nitrification liquid circulation is stopped, the β increment is 0.2 when the return ratio of the return line is 0.5 and the circulation ratio of the circulation line is 1, and the return ratio is 1 and the circulation ratio is 0.5. The β increment in the case is 0.1. That is, when the nitrification liquid circulation is stopped, the denitrification rate required in the microaerobic tank increases, and the DO setting of the microaerobic tank becomes very important in order to promote the denitrification reaction in the microaerobic tank. You can see that.

The denitrification rate of activated sludge is 1.2 [mgN / gMLSS / h] (however, the BOD-SS load is 0.12 [kgBOD / kgMLSS / day]), and the nitrification rate of activated sludge is 2.3 [. Assuming [mgN / gMLSS / h], the ratio of the nitrification zone in which the DO meter is installed to the denitrification zone at the bottom is 0.34 in the nitrification zone: 0.66 in the denitrification zone. Therefore, when trying to secure the denitrification zone of the microaerobic tank firmly, it is necessary to install the DO meter at a depth of 1/3 or more of the water depth of the microaerobic tank. If the DO sensor unit is installed at a depth of 1/3 or less of the water depth and control is attempted, more DO will be brought into the deep part of the water tank, and a sufficient denitrification zone will be secured (formed) in the microaerobic tank. ) Therefore, the quality of treated water deteriorates due to high TN.

The nitrification zone in the above microaerobic tank is a site having a DO of 0.05 to 1 mg / L, and the denitrification zone is a site having a DO of less than 0.05 mg / L.

As described above, when nitrifying liquid circulation is performed, denitrification in the microaerobic tank is less important than when there is no nitrifying liquid circulation. In other words, when the nitrification liquid is circulated, the remaining NOx-N is sent to the oxygen-free tank by the nitrification liquid circulation even if the operation is performed with more emphasis on nitrification than denitrification in the microaerobic tank, where denitrification is performed. It can be carried out. Therefore, when there is nitrification liquid circulation, there is little concern that the amount of nitrification increases in the microaerobic tank, that is, the treated water quality deteriorates even if the nitrification zone becomes large.

Taking Table 1 as an example, the denitrification amount in the microaerobic tank may be 1/3 to 1/2 of the denitrification amount when there is a circulation line and when there is no circulation line, so the denitrification zone is 0. It may be 22 to 0.33, and the position of the DO meter useful for determining the size of the nitrification zone may be deeper than in the absence of the circulation line.

The volume ratio of the microaerobic tank to the aerobic tank is preferably about 3: 1 to 1: 3, preferably about 2: 1 to 1: 2.

A settling pond for precipitating sludge is provided on the downstream side of the aerobic tank, and the supernatant of the settling pond is discharged as treated water. On the other hand, a part of the subsided part is returned to the oxygen-free tank, and a part is drawn out as excess sludge. In the present invention, nitrate nitrogen and nitrite nitrogen produced in the aerobic tank are returned exclusively to the oxygen-free tank through this return line, where the denitrification reaction is carried out. The amount returned is preferably about 0.2 to 2, preferably about 0.5 to 1, as a volume ratio to the amount of raw water.

The returned sludge is returned to keep the MLSS concentration in the reaction tank at a predetermined value, and the returned sludge cannot be increased endlessly in order to increase the denitrification amount. The return rate is uniquely determined by the set reaction tank MLSS concentration and the concentrated sludge concentration in the final settling basin, and cannot be changed to adjust the denitrification amount.

A sewage treatment apparatus according to an embodiment of the present invention is shown in FIG. This device includes an oxygen-free tank 1, a microaerobic tank 11, an aerobic tank 2, and a final settling pond 3. The oxygen-free tank 1 is provided with a large propeller-type stirrer 4. The microaerobic tank 11 is divided into two chambers by a flat plate-shaped straightening vane 12, and the upper and lower portions of the straightening vane 12 form a passage between the two chambers. Then, an air diffuser 13 is provided in one of the chambers, and the water in the tank forms a swirling flow by the aeration from the air diffuser 13. An air diffuser 5 is also provided in the aerobic tank 2, and the microorganism-immobilized carrier 15 is charged in the aerobic tank 2. A screen 16 is attached to the outlet of the aerobic tank 2.

The sewage flows into the oxygen-free tank 1 from the raw water supply line 6, and then flows into the slightly aerobic tank 11 and the aerobic tank 2 in this order. The effluent from the aerobic tank 2 flows into the final settling pond 3, where sludge is settled, and the supernatant is discharged from the treated water discharge line 7 as treated water. On the other hand, a part of the settled sludge is returned to the oxygen-free tank 1 through the return sludge line 9, and a part of the sludge is discharged as surplus sludge 10.

The microaerobic tank 11 and the aerobic tank 2 are each provided with a DO meter. A flow rate control valve is provided in the air pipe connected to each of the air diffusers 13 and 5 of the microaerobic tank 11 and the aerobic tank 2, and the opening degree of the flow rate control valve is controlled by the DO meter of each tank. It is equipped with a pressure gauge that measures the discharge pressure of the aeration blower, and the operating frequency of the aeration blower is controlled by the pressure gauge.

The initial overflow running water of urban sewage having the water quality shown in Table 2 was treated with a water volume of ^{300 m 3 / day using the treatment device shown in FIG.}

[Example 1]
In the flow of the present invention, the slightly aerobic tank DO was operated at 1.0 mg / L and the aerobic tank DO was operated at 3.0 mg / L. The carrier filling rate was 4.0% (true volume).
The results obtained are shown in Table 3. The water tank capacity and Lancos in the table represent the relative values of the reaction tank capacity and the process running cost based on Example 1. As a result of operating the aerobic tank DO at 3.0 mg / L, the aeration blower power could be lowered, and in addition, since the circulation pump was not installed, the running cost of the process could be kept low.

[Example 2]
In the flow of the present invention, the slightly aerobic tank DO was operated at 1.0 mg / L and the aerobic tank DO was operated at 1.0 mg / L.
The results obtained are shown in Table 3.
As a result, the aeration blower cost could be further reduced as compared with Example 1. On the other hand, in order to secure the same treated water quality, it was necessary to set the carrier filling rate to 4.2%.

[Comparative Example 1]
A circulation line from an aerobic tank to an oxygen-free tank and a circulation pump were added to the configuration of the present invention. The circulation ratio was 1.5.
The results obtained are shown in Table 3.
Since the aerobic tank DO was 3.0 mg / L, the DO of the inflow water flowing into the anoxic tank was 1.5 mg / L, which affected the denitrification reaction. As a result, in order to obtain the same treated water quality as in Example 1, the capacity of the oxygen-free tank was 1.5 times larger. The treated water quality was similar to that of the examples. Increasing the circulation ratio should have increased the amount of denitrification, but the installed capacity has increased.

[Comparative Example 2]
In the flow of the present invention, the slightly aerobic tank DO was operated at 1.0 mg / L and the aerobic tank DO was operated at 3.5 mg / L.
The results obtained are shown in Table 3.
The carrier addition rate decreased slightly, but the power cost of the aeration blower increased.

[Comparative Example 3]
In the flow of the present invention, the slightly aerobic tank DO was operated at 1.0 mg / L and the aerobic tank DO was operated at 4.0 mg / L.
The results obtained are shown in Table 3.
The carrier addition rate decreased slightly, but the power cost of the aeration blower increased.

[Comparative Example 4]
The microaerobic tank DO was operated at 1.0 mg / L in a flow in which the aerobic tank and the carrier were removed from the flow of the present invention.
The results obtained are shown in Table 3.
Since the aerobic tank disappeared and only the slightly aerobic tank was set to the set DO, the power cost of the aeration blower was slightly reduced, but the nitrification capacity of the carrier was borne only by activated sludge. Therefore, the capacity of the microaerobic tank became very large in order to secure the same water quality as in the examples.

[Comparative Example 5]
In the flow of the present invention, the microaerobic tank DO was set to 2.0 mg / L and operated.
The results obtained are shown in Table 3.
Since the setting DO of the microaerobic tank became high, the power cost of the aeration blower increased slightly, the amount of denitrification in the microaerobic tank decreased, and the treated water quality deteriorated due to the high TN.

According to the present invention, in a sewage treatment facility in which a large amount of nitrifying liquid is returned from an aerobic tank to an oxygen-free tank, this nitrifying liquid circulation line can be eliminated, and the power of a circulation pump required for the circulation pump can be eliminated. Can be widely used in sewage treatment equipment having an oxygen-free tank and an aerobic tank.

1 Anoxic tank 2 Aerobic tank 3 Final settling basin 4 Stirrer 5 Air diffuser 6 Raw water supply line 7 Treated water discharge line 9 Return sludge line 10 Excess sludge 11 Slightly aerobic tank 12 Straightening plate 13 Air diffuser 15 Microorganism immobilization Carrier 16 screen

Claims (7)

It is equipped with at least an oxygen-free tank, a microaerobic tank, an aerobic tank, and a sedimentation pond installed in the subsequent stage.
The aerobic tank contains a microbial-immobilized carrier carrying nitrifying bacteria, and is absent from the aerobic tank in a sewage treatment method in which at least a part of sludge drawn from the sedimentation pond is returned to the anoxic tank. Without returning the nitrifying solution to the oxygen tank, the dissolved oxygen concentration in the microaerobic tank is equal to or less than the dissolved oxygen concentration in the aerobic tank, and the dissolved oxygen concentration in the aerobic tank is 1.0 mg / L or more 3 A sewage treatment method characterized by being 0.0 mg / L or less. The sewage treatment method according to claim 1, wherein the water flow in the tank generated by the air diffuser from the air diffuser installed in the microaerobic tank is a swirling flow. The sewage treatment method according to claim 1 or 2, wherein the dissolved oxygen concentration in the microaerobic tank is 1.0 mg / L or less. It is equipped with at least an oxygen-free tank, a microaerobic tank, an aerobic tank, and a sedimentation pond installed in the subsequent stage.
The aerobic tank contains a microbial-immobilized carrier carrying nitrifying bacteria, and is provided with a return line for returning at least a part of the sludge drawn from the sedimentation pond to the oxygen-free tank. In, there is no circulation line of the nitrifying liquid from the aerobic tank to the anoxic tank, and the dissolved oxygen concentration in the microaerobic tank is set to be equal to or lower than the dissolved oxygen concentration in the aerobic tank, and the dissolved oxygen concentration in the aerobic tank is set to be equal to or lower than that in the aerobic tank. A sewage treatment device including a control mechanism for 1.0 mg / L or more and 3.0 mg / L or less. The sewage treatment device according to claim 4, wherein the water flow in the tank generated by the air diffuser installed in the microaerobic tank is a swirling flow. The sewage treatment device according to claim 4 or 5, further comprising a control mechanism for reducing the dissolved oxygen concentration of the microaerobic tank to 1.0 mg / L or less. The sewage treatment apparatus according to any one of claims 4 to 6, wherein a dissolved oxygen concentration meter is installed in a microaerobic tank at a position of 1/3 or more of the water depth.

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