JP2004243248A - Nitrogen removing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitrogen removing device which can stabilize the removal ratio of T-N (total nitrogen) and maintain good filtration characteristics of water to be treated. <P>SOLUTION: In the nitrogen removing device 10 where the water to be treated, together with activated sludge, is circulated between a denitrification tank 12 and a nitrification tank 14 to biologically remove nitrogen components contained in the water to be treated by denitrifying bacteria and nitrifying bacteria contained in the activated sludge, a membrane separation means 22 having an aeration pipe 28 at the bottom is immersed in the nitrification tank 14, and the volume of the denitrification tank 12 is 1.5 to 10 times the total volume of the membrane separation means 22. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a nitrogen removing apparatus, and more particularly to a nitrogen removing apparatus configured to biologically remove nitrogen components contained in water to be treated by activated sludge.
[0002]
[Prior art]
As this type of nitrogen removing device, there is known a device in which water to be treated is circulated together with activated sludge between a denitrification tank and a nitrification tank. Activated sludge contains nitrifying bacteria and denitrifying bacteria, and in a nitrification tank, ammonia nitrogen in the water to be treated is nitrified by nitrifying bacteria into nitrate nitrogen under aerobic conditions. In the denitrification tank, nitrate nitrogen in the water to be treated is denitrified under the anaerobic condition by the action of denitrifying bacteria to form nitrogen gas, and this nitrogen gas is released to the atmosphere to remove nitrogen. As the concentration of activated sludge retained in the system is higher, the concentrations of nitrifying bacteria and denitrifying bacteria are relatively higher, and the treatment efficiency of nitrogen removal is improved.
[0003]
For this reason, in recent years, attempts have been made to immerse the membrane separation means in a nitrification tank and to separate the treated water from the activated sludge by this membrane separation means. That is, the water to be treated that has passed through the membrane is discharged out of the system as treated water, and the activated sludge separated by the membrane remains as it is in the circulation system between the denitrification tank and the nitrification tank. By employing such a method, the concentration of activated sludge in the circulation system can be reduced to about 10 g / L. This value of 10 g / L is about 5 times the concentration of activated sludge in the standard activated sludge method having a sedimentation basin, so that the treatment speed of the nitrification and denitrification reaction can be dramatically improved. Therefore, the capacity of the nitrification tank and the denitrification tank can be reduced accordingly, which contributes to the space saving of the required site area.
[0004]
In the nitrogen removing apparatus in which the membrane separation means is immersed in such a nitrification tank, the amount of air supplied to the nitrification tank increases because the concentration of the activated sludge is high. However, in this type of nitrogen removal apparatus, the properties of the inflowing water to be treated vary greatly. For this reason, under the operating conditions in which the oxygen consumption in the nitrification tank is smaller than originally planned, oxygen in the air that could not be completely consumed in the nitrification tank is brought into the denitrification tank, which should be an anaerobic condition without dissolved oxygen. In some cases, the denitrification reaction in the denitrification tank was adversely affected. Therefore, it is necessary to set the capacity of the denitrification tank on the assumption that oxygen is brought in from such a nitrification tank in advance. When setting the capacity of the denitrification tank, the capacity of the membrane separation means immersed in the nitrification tank seems to be related. Patent Literature 1 describes the results of an investigation on the relationship between the capacity of an aeration tank and the size of the membrane separation means for an aeration tank provided with a submerged membrane separation means.
[0005]
[Patent Document 1]
JP-A-8-267083
[Problems to be solved by the invention]
However, the contents described in Patent Literature 1 do not relate to the nitrogen removing device, and thus do not serve as a reference when setting the capacity of the denitrification tank. The present invention has been made under the above background, and an object of the present invention is to provide a nitrogen removing apparatus in which the capacity of a denitrification tank is appropriately set in accordance with the volume of a membrane separation means immersed in a nitrification tank. Is to provide.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the nitrogen removing apparatus according to the present invention circulates water to be treated together with activated sludge between a denitrification tank and a nitrification tank to remove nitrogen components contained in the water to be treated. In the nitrogen removal apparatus described above, the membrane separation means is immersed in the nitrification tank, and the capacity of the denitrification tank is set to 1.5 to 10 times the total volume of the membrane separation means.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an apparatus system diagram showing an embodiment of a nitrogen removing apparatus according to the present invention. The nitrogen removing device 10 mainly includes a denitrification tank 12 and a nitrification tank 14. Organic wastewater containing a nitrogen component is supplied to the denitrification tank 12 from a pipe 16. The nitrification liquid, which is the water to be treated from the nitrification tank 14, flows into the denitrification tank 12 through a pipe 18. The water to be treated that has passed through the denitrification tank 12 is sent from a pipe 20 to a nitrification tank 14. A membrane separation means 22 is immersed in the nitrification tank 14. The compressed air supplied from the blower 24 is diffused below the membrane separation means 22 from the air diffuser 28 through the conduit 26. A pipeline 32 is connected to the secondary side of the membrane element 30 of the membrane separation means 22, and a suction pump 34 is provided in the pipeline 32. A circulation pump 36 for circulating the nitrification liquid in the tank to the denitrification tank 12 is connected to a lower portion of the nitrification tank 14, and the nitrification liquid extracted by the circulation pump 36 is supplied to the denitrification tank 12 via the pipe 18. Circulated. The activated sludge is maintained at a high concentration (about 10 g / L) in the denitrification tank 12 and the nitrification tank 14, and circulates between the denitrification tank 12 and the nitrification tank 14 with the nitrification liquid. In addition, a pipe line 38 for extracting the activated sludge that has multiplied and becomes excessive is branched off on the discharge side of the circulation pump 36. The excess sludge extracted from the pipe 38 is usually reduced by a dehydration facility (not shown) and then discharged out of the system.
[0009]
In the above configuration, the water to be treated supplied to the denitrification tank 12 is mixed with the nitrification liquid circulated from the pipe 18. In the denitrification tank 12, nitrate nitrogen mainly caused by the nitrification liquid is denitrified by the action of denitrifying bacteria in the activated sludge to form nitrogen gas, and the nitrogen gas is released to the atmosphere to remove nitrogen. The denitrification reaction by the denitrifying bacteria actively proceeds under anaerobic conditions in which the dissolved oxygen in the liquid is substantially zero, and does not proceed when the dissolved oxygen is large. The water to be treated that has exited from the denitrification tank 12 is sent to the nitrification tank 14 through a pipe 20. In the nitrification tank 14, the ammonia nitrogen in the water to be treated is nitrified by the action of nitrifying bacteria in the activated sludge to become nitrate nitrogen. The nitrification reaction by the nitrifying bacteria actively proceeds under aerobic conditions in which the dissolved oxygen in the liquid is large. As means for supplying oxygen in the liquid, air from the air diffuser 28 is used. That is, the oxygen in the diffused air dissolves in the liquid, and the aerobic condition with a large amount of dissolved oxygen is maintained. The water to be treated which has been subjected to the nitrification treatment in the nitrification tank 14 undergoes membrane separation in the course of passing through the membrane separation means 22. The water to be treated that has passed through the membrane is sucked into the suction pump 34 from the secondary side of the membrane element 30 via the pipe line 32 and discharged out of the system as clear treated water. Further, the water to be treated, which has been subjected to the nitrification treatment in the nitrification tank 14, is withdrawn from the circulation pump 36 as a nitrification liquid, and circulated to the denitrification tank 12 via the pipe 18. The ratio between the amount of the nitrifying liquid circulated to the denitrification tank 12 and the flow rate of the water to be treated supplied to the denitrification tank 12 from the pipe 16 is called the nitrification liquid circulation ratio, and the nitrification liquid circulation ratio is usually about 2 to 3. Is set to
[0010]
On the other hand, solid matter mainly composed of activated sludge separated on the primary side of the membrane element 30 by the membrane separation by the membrane separation means 22 adheres and accumulates on the membrane surface of the membrane element 30 and causes blockage of the membrane surface. Therefore, the air diffused from the air diffuser 28 is also used for the purpose of separating and washing the solid matter adhered and deposited on the film surface. That is, the diffused air bubbles apply a shearing force to the membrane surface of the membrane element 30 in the process of floating by the buoyancy. The solids attached to and deposited on the film surface are peeled off by the shearing force of the bubbles. Further, the air diffused from the air diffuser 28 forms a circulating flow of the water to be treated in the nitrification tank 14 by the air lift function, and also plays a role of maintaining a good mixed state of the water to be treated and the activated sludge.
[0011]
As described above, the air diffused from the air diffuser 28 serves as a function as an oxygen supply source for maintaining dissolved oxygen in the water to be treated, a function to clean the membrane surface of the membrane element 30, and a circulation flow of the water to be treated. It has a function of forming, and in order to achieve these three functions simultaneously, there is a tendency that excessive diffusion is performed. For this reason, the case where the dissolved oxygen in the water to be treated in the nitrification tank 14 becomes unnecessarily high often occurs. The water to be treated having relatively high dissolved oxygen is withdrawn from the circulation pump 36 as a nitrification liquid as described above, and is circulated to the denitrification tank 12 via the pipe 18. As a result, dissolved oxygen in the nitrification liquid is brought into the denitrification tank 12, which may adversely affect the denitrification reaction in the denitrification tank 12. Therefore, it is necessary to set the capacity of the denitrification tank 12 on the assumption of dissolved oxygen brought in from the nitrification tank 14 via the nitrification liquid in advance.
[0012]
In the present embodiment, paying attention to the fact that the capacity of the denitrification tank 12 is closely related to the capacity of the membrane separation means 22 immersed in the nitrification tank 14, the capacity of the denitrification tank 12 is set to one of the total volume of the membrane separation means 22. It is set to 0.5 to 10 times. In addition, the total volume of the membrane separation unit 22 is a volume including a membrane element constituting the membrane separation unit 22 and an air diffusion unit below, and a case where a plurality of units of the membrane separation unit 22 are arranged in the nitrification tank 14. Is the sum of the volumes of each unit. That is, as shown in FIG. 2, when three units of the membrane separation means 22 having a width W, a length L, and a height H are provided in the nitrification tank 14, the volume of each unit is W × L. Since it is × H, the total volume V of the membrane separation means 22 is 3 × W × L × H. Therefore, the capacity of the denitrification tank 12 is set to 1.5 to 10 times the calculated total volume V.
[0013]
The reason for setting the value to 1.5 to 10 times will be described below. 3 and 4 show the results of one experiment by the present inventors. FIG. 3 defines the ratio of the capacity of the denitrification tank 12 to the total capacity of the membrane separation means 22 as a capacity ratio R, and uses this capacity ratio R as a parameter to obtain a total nitrogen removal rate (hereinafter referred to as a TN removal rate). It is the result of having investigated. According to the figure, when the capacitance ratio R is 1.5 times or more, the TN removal rate is stable at 80% or more. However, when the capacity ratio R is less than 1.5 times, the TN removal rate sharply decreases. From this result, it can be understood that the capacitance ratio R should be 1.5 times or more. When the capacity ratio R is less than 1.5 times, the TN removal rate sharply decreases as the dissolved oxygen in the nitrification solution is brought into the denitrification tank, thereby inhibiting the denitrification reaction, and the reaction time is reduced. Probably because of shortage. When the capacity ratio R is 1.5 times or more, the adverse effect of dissolved oxygen in the nitrification solution is covered by an increase in the residence time (reaction time), so that the TN removal rate is considered to be stable.
[0014]
FIG. 4 shows the results of examining the filtration characteristics of the water to be treated using the volume ratio R as a parameter. In the experiment, the water to be treated (concentration of activated sludge, about 10 g / L) in the nitrification tank 14 was collected, and 50 mL of the water to be treated was used as a No. And filtered through a filter paper of No. 5. The value shown on the vertical axis is the amount of filtrate 5 minutes after the start of filtration. According to the figure, when the volume ratio R is 10 times or less, the filtrate volume is about 20 mL, and is relatively stable. However, when the volume ratio R exceeds 10 times, the amount of filtrate starts to decrease rapidly. The rapid decrease in the amount of filtrate means that the membrane surface of the membrane element 30 is likely to be clogged at the time of membrane separation by the membrane separation means 22, which is a fatal phenomenon in maintaining stable operation. . The cause of this phenomenon is not fully understood. It is presumed that as a result of activated sludge staying for longer than necessary under anaerobic conditions in the denitrification tank 12, abnormally proliferated anaerobic bacteria actively secrete viscous substances and reduce filterability.
[0015]
In any case, in this embodiment, the capacity of the denitrification tank 12 is set to be 1.5 to 10 times the total volume of the membrane separation means 22, so that the TN removal rate of the nitrogen removal device 10 is stable. . In addition, since the filtration characteristics of the water to be treated are favorably maintained, the membrane separation by the membrane separation means 22 can be stably continued.
[0016]
【The invention's effect】
As described above, according to the nitrogen removal apparatus of the present invention, the capacity of the denitrification tank is set to 1.5 to 10 times the total volume of the membrane separation means, so that the TN removal rate is stable. In addition, since the filtration characteristics of the water to be treated are well maintained, the membrane separation by the membrane separation means can be stably continued, and the operation management of the apparatus is easy.
[Brief description of the drawings]
FIG. 1 is an apparatus system diagram showing an embodiment of a nitrogen removing apparatus according to the present invention.
FIGS. 2A and 2B show an example of an arrangement of membrane separation means immersed in a nitrification tank, wherein FIG. 2A is a plan view and FIG.
FIG. 3 is an experimental result diagram showing a relationship between a capacitance ratio R and a TN removal rate.
FIG. 4 is an experimental result diagram showing a relationship between a capacity ratio R and a filtration characteristic of water to be treated.
[Explanation of symbols]
10 nitrogen removal device, 12 denitrification tank, 14 nitrification tank, 22 membrane separation means, 24 blower, 28 diffuser tube, 30 membrane element, 34 ... Suction pump.

Claims (1)

In the nitrogen removal device that circulates the water to be treated together with the activated sludge between the denitrification tank and the nitrification tank and removes the nitrogen component contained in the water to be treated, while immersing the membrane separation means in the nitrification tank, A denitrification tank having a capacity of 1.5 to 10 times the total volume of the membrane separation means.

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