JP4360204B2 - Organic wastewater treatment method - Google Patents

Description

  The present invention relates to a method for treating organic wastewater, and more particularly to a method for biologically treating organic wastewater containing phosphorus and nitrogen with activated sludge.

Conventionally, the A ₂ O method is well known as a biological treatment method for removing BOD components, nitrogen, and phosphorus in organic wastewater. In this method, wastewater is passed through an anaerobic tank, an oxygen-free tank, an aerobic tank, and a sedimentation tank in this order, the sludge separated in the sedimentation tank is returned to the anaerobic tank, and a part of the treatment liquid in the aerobic tank is anaerobic. Circulate in the tank.

  In the anaerobic tank, the BOD component in the wastewater is taken into the activated sludge returned from the settling tank under anaerobic conditions, and the activated sludge releases phosphorus. In the anoxic tank, denitrification treatment is performed by denitrifying bacteria in activated sludge under anoxic conditions. In the aerobic tank, phosphorus is taken into activated sludge under aerobic conditions, and nitrification is performed by nitrifying bacteria in the activated sludge. In the settling tank, the liquid to be treated from the aerobic tank is separated into treated water and activated sludge. Most of the activated sludge separated in the settling tank is returned to the anaerobic tank to perform the above-mentioned treatments, and the activated sludge accumulating the remaining phosphorus in a high concentration is extracted as excess sludge.

  On the other hand, in recent years, a membrane separation type activated sludge method in which solid-liquid separation is performed by a membrane unit immersed in a reaction tank has been put into practical use. In the membrane-separated activated sludge method, the treatment liquid containing the activated sludge in the reaction tank is subjected to membrane separation by a membrane unit, and the membrane permeated water is extracted as a clear treated water, and the activated sludge is left in the reaction tank. This method eliminates the need for a settling tank and allows the apparatus to be made compact. Moreover, since activated sludge can be kept in the reaction tank at a high concentration, there is an advantage that the treatment speed by activated sludge is fast.

Focusing on the advantages of this membrane separation type activated sludge method, a method of applying the membrane separation type activated sludge method to a conventional aerobic tank of the A ₂ O method has been proposed (for example, see Patent Document 1). According to this method, since the concentration of activated sludge to be circulated in the anaerobic tank, anaerobic tank, and aerobic tank can be maintained at about 10,000 mg / L, the treatment speed is dramatically improved, and the anaerobic tank and anoxic tank are improved. , The capacity of the aerobic tank can be reduced. Moreover, since a sedimentation tank becomes unnecessary, the apparatus can be made compact.
JP-A-2001-314890

However, the method of applying the membrane separation type activated sludge method to the aerobic tank of the A ₂ O method is because the activated sludge is kept at a high concentration in the aerobic tank as described above, and high load treatment is performed. The consumption of dissolved oxygen accompanying the reaction is intense. For this reason, if the amount of air diffused in order to maintain the inside of the aerobic tank in an aerobic state is insufficient, the dissolved oxygen is depleted and it is easy to shift to the anaerobic state. When the aerobic tank is in an anaerobic state, the phosphorus uptake reaction and nitrification reaction by the activated sludge described above do not proceed, and the treatment performance is significantly reduced.

  In the membrane unit immersed in the aerobic tank, the secondary side of the separation membrane is usually sucked by a suction pump, and membrane separation is performed using the suction negative pressure as energy. A constant flow type pump is used as the suction pump. For this reason, when the amount of inflow of wastewater decreases significantly, such as at night, or when the inflow of wastewater stops, the operation of the suction pump is temporarily stopped and the operation of the membrane unit is stopped. Further, even during steady operation, the suction pump may be intermittently operated (for example, 18 minutes of suction and 2 minutes of suction stop are repeated). Even in such intermittent operation, the operation of the membrane unit is a short time. Stops. In the time zone when the operation of the membrane unit is stopped, in order to save the aeration energy, there are two methods of stopping the aeration into the aerobic tank or continuing the aeration.

  According to the knowledge of the present inventors, when the aeration into the aerobic tank is stopped in the time zone when the operation of the membrane unit is stopped, the aerobic tank tends to become anaerobic as described above, and activated sludge accumulates. The released phosphorus is re-released in the aerobic tank. For this reason, there exists a problem that the phosphorus concentration in treated water becomes high when the operation of the membrane unit is resumed. On the contrary, if the air diffusion into the aerobic tank is continued in the time zone when the operation of the membrane unit is stopped, the air diffusion energy increases. Further, there is a problem that activated sludge flocs are refined by air aeration, and clogging of the separation membrane is promoted when the operation of the membrane unit is resumed.

  The object of the present invention is to improve the above-mentioned problems of the prior art, prevent re-release of phosphorus from activated sludge even when the operation of the membrane unit is stopped, and suppress the flocs of activated sludge from being refined. An object of the present invention is to provide a method for treating organic wastewater.

In order to achieve the above object, the organic wastewater treatment method according to the present invention passes organic wastewater containing phosphorus in the order of an anaerobic tank, an oxygen-free tank, and an aerobic tank, and is immersed in the aerobic tank. In the organic wastewater treatment method for separating treated water by the membrane unit, the aerobic tank performs intermittent aeration in a time zone when the operation of the membrane unit is stopped, and the intermittent aeration Is controlled based on the phosphorus concentration of the liquid to be treated in the aerobic tank .

In the above method, it is preferable that the time ratio of the intermittent aeration with respect to the time zone in which the operation of the membrane unit is stopped is 0.02 to 0.2 .

  According to the present invention, the intermittent aeration is performed during the time when the operation of the membrane unit is stopped, so that the aerobic tank is slightly aerobic during the time when the operation of the membrane unit is stopped. Can be maintained in a state. For this reason, it is possible to prevent re-release of phosphorus from the activated sludge and to suppress the flocs of the activated sludge from being refined.

  FIG. 1 is a system diagram for implementing the organic wastewater treatment method according to the present invention. The said apparatus 10 is equipped with the anaerobic tank 12, the anoxic tank 14, and the aerobic tank 16, and the organic waste water 18 is passed in this order. A first circulation pump 20 is disposed in the anaerobic tank 14, and the liquid to be treated in the anoxic tank 14 is circulated to the anaerobic tank 12 via the conduit 22. A second circulation pump 24 is disposed in the aerobic tank 16 and circulates the liquid to be treated in the aerobic tank 16 to the anoxic tank 14 via a pipe line 26. A membrane unit 28 is immersed in the aerobic tank 16. A coarse bubble diffusing means 30 is disposed below the membrane unit 28. The air pressurized by the blower 32 is supplied to the coarse bubble diffusing means 30 through the pipe 34, and the coarse bubbles from the coarse bubble diffusing means 30 are diffused toward the separation membrane constituting the membrane unit 28. . The coarse bubbles diffused from the coarse bubble diffusing means 30 come into contact with the separation membrane in the process of rising by buoyancy, and the membrane surface of the separation membrane is washed. Further, the swirling flow of the liquid to be treated is generated in the aerobic tank 16 by the floating energy of the coarse bubbles, and the mixing and stirring of the activated sludge and the water to be treated is promoted. Further, oxygen in the coarse bubbles comes into contact with the liquid to be processed and dissolves as dissolved oxygen in the liquid to be processed.

  However, since coarse bubbles have a small specific surface area, the oxygen dissolution efficiency for the liquid to be treated is low. For this reason, the fine bubble diffusing means 36 is disposed on the side of the membrane unit 28 at a position different from the coarse bubble diffusing means 30. The air pressurized by the blower 32 is supplied to the fine bubble diffusing means 36 through the pipe line 38, and the fine bubbles from the fine bubble diffusing means 36 are diffused. Since the fine bubbles have a large specific surface area, the oxygen dissolution efficiency for the liquid to be treated is high. Therefore, it is effective as a supply source of dissolved oxygen to the liquid to be processed.

  A conduit 40 is connected to the secondary side of the separation membrane of the membrane unit 28, and a suction pump 42 is connected to the conduit 40. By operating the suction pump 42, the secondary side of the separation membrane of the membrane unit 28 becomes negative pressure, and membrane separation is performed using the suction negative pressure as energy. The permeated water of the separation membrane is sucked by the suction pump 42 through the conduit 40 and discharged out of the system as clear treated water.

  A phosphorus concentration meter 44 capable of detecting the phosphorus concentration of the liquid to be treated is attached to the aerobic tank 16. The detection signal of the phosphorus concentration meter 44 is transmitted to the controller 46, and the controller 46 controls the drive of the blower 32 based on the received phosphorus concentration of the liquid to be processed. A pipe 48 is connected to the lower part of the aerobic tank 16, and a sludge extraction pump 50 is connected to the pipe 48.

  In the apparatus 10 having the above-described configuration, the organic waste water 18 containing phosphorus and nitrogen flows into the anaerobic tank 12 during normal operation. In addition, a liquid to be treated containing activated sludge in the anaerobic tank 14 at a high concentration is circulated into the anaerobic tank 12 through the conduit 22 by the first circulation pump 20. For this reason, in the anaerobic tank 12, the BOD component in the wastewater is taken into the activated sludge that circulates and flows under the anaerobic condition, and the activated sludge releases phosphorus. The liquid to be treated containing activated sludge overflowing the anaerobic tank 12 flows into the oxygen-free tank 14 at the next stage. In addition, the liquid to be treated containing activated sludge in the aerobic tank 16 at a high concentration is circulated into the anaerobic tank 14 through the pipeline 26 by the second circulation pump 24. For this reason, in the anaerobic tank 14, a denitrification process is performed by denitrifying bacteria in activated sludge under anaerobic conditions. The liquid to be treated containing activated sludge overflowing the anaerobic tank 14 flows into the aerobic tank 16 at the next stage. In the aerobic tank 16, phosphorus is taken into activated sludge under aerobic conditions, and nitrification is performed by nitrifying bacteria in the activated sludge. The liquid to be treated in the aerobic tank 16 is separated by the membrane unit 28. The permeated water separated by the membrane unit 28 is sucked by the suction pump 42 through the conduit 40 and discharged out of the system as clear treated water. Moreover, the activated sludge deposited in the lower part of the aerobic tank 16 is extracted by the sludge extraction pump 50 and discharged out of the system from the pipe 48 as surplus sludge.

  In order to maintain the aerobic tank 16 in an aerobic state, the air pressurized by the blower 32 is supplied to the coarse bubble diffusing unit 30 and the fine bubble diffusing unit 36 through the pipes 34 and 38. In the aerobic tank 16, the activated sludge is maintained at a high concentration of about 10,000 mg / L and a high load treatment is performed, so that the consumption of dissolved oxygen accompanying the reaction is intense. For this reason, if the amount of air diffused from each air diffuser is insufficient, a partial region in the aerobic tank 16 may become anaerobic. When at least a part of the aerobic tank 16 is in an anaerobic state, the phosphorus uptake reaction or nitrification reaction by the activated sludge described above does not proceed, and the treatment performance is significantly reduced. Therefore, in the present embodiment, the drive of the blower 32 is controlled based on the phosphorus concentration of the liquid to be processed detected by the phosphorus concentration meter 44. That is, when the phosphorus concentration of the liquid to be treated exceeds the reference value, the drive of the blower 32 is enhanced to control the amount of air to be diffused.

  Through the above treatment, the BOD component in the wastewater to be treated is mainly removed in the anaerobic tank 12 and the anaerobic tank 14, and the nitrogen component is denitrified in the anaerobic tank 14 and the nitrification treatment in the aerobic tank 16. Removed by. Further, the phosphorus component in the wastewater is mainly removed by the action of taking up phosphorus by the activated sludge in the aerobic tank 16, and the phosphorus is excessively accumulated in the excess sludge finally extracted by the sludge extraction pump 50. Is discharged out of the system.

  In the treatment method described above, the first circulation pump 20 and the second circulation are reduced when the inflow of the wastewater 18 is significantly reduced at night or when the inflow of the wastewater 18 is stopped, or during regular intermittent operation. The processing is substantially stopped by temporarily stopping the operation of the pump 24 and the suction pump 42. As a result, the operation of the membrane unit 28 is also stopped. In the processing method of the present embodiment, intermittent aeration is performed in the aerobic tank 16 during the time period when the operation of the membrane unit 28 is stopped. There are roughly two types of intermittent aeration methods.

  In the first method, intermittent aeration is performed at a rate of 1 minute every 10 minutes, for example, using a timer. In this case, the time ratio of intermittent aeration with respect to the time zone in which the operation of the membrane unit is stopped is 0.1. It is preferable that the aeration at this time be performed exclusively by the fine bubble aeration means 36. That is, in the time zone when the operation of the membrane unit 28 is stopped, the on-off valve 52 provided in the pipe line 34 is automatically closed, and only the on-off valve 54 provided in the pipe line 38 is opened. Then, only the fine bubbles from the fine bubble diffusing means 36 are diffused by the intermittent drive of the blower 32, and the diffusing from the coarse bubble diffusing means 30 is stopped. As described above, since the fine bubbles have high oxygen dissolution efficiency with respect to the liquid to be processed, they are effective as a supply source of dissolved oxygen to the liquid to be processed, and contribute to saving of aeration energy.

  In the second method of intermittent aeration, the drive of the blower 32 is controlled based on the phosphorus concentration of the liquid to be treated detected by the phosphorus concentration meter 44 as in the normal operation. In other words, when the phosphorus concentration of the liquid to be processed exceeds the reference value, the frequency of intermittent drive of the blower 32 or the air diffusion time per control is controlled to increase. Also in this method, it is preferable that the aeration is performed exclusively by the fine bubble aeration means 36.

  In the case of the second method, since the liquid to be treated in the aerobic tank 16 does not flow sufficiently during the intermittent aeration time, there is a high possibility that the phosphorus concentration varies. For this reason, there is a tendency that the phosphorus concentration detected by the phosphorus concentration meter 44 does not show an average value of the liquid to be treated, and the control may become unstable. Therefore, a more preferable method of intermittent aeration is performed by combining the first and second methods. That is, if the timer control as a base is performed by the first method and the phosphorus concentration of the liquid to be treated detected by the phosphorus concentration meter 44 exceeds the reference value, the aeration is enhanced as a temporary measure. Good. By adopting such a method, the aerobic tank 16 can be reliably maintained in a slightly aerobic state even during a time period when the operation of the membrane unit 28 is stopped. For this reason, it is possible to prevent re-release of phosphorus from the activated sludge. Further, the aeration energy can be suppressed to a minimum, and the flocs of activated sludge can be suppressed from being refined by air aeration.

  FIG. 2 is a graph showing an example of experimental results when the time ratio of intermittent aeration is changed. In FIG. 2, the horizontal axis indicates the time ratio of intermittent aeration with respect to the time zone in which the operation of the membrane unit 28 is stopped. The experiment was carried out by intermittently aeration with the fine bubble aeration means 36 at a frequency of once every 10 minutes during the time when the operation of the membrane unit 28 was stopped, and changing the aeration time per time. . That is, the intermittent aeration time ratio when the aeration time per time is 1 minute is 0.1, and the intermittent aeration time ratio when the aeration time per time is 2 minutes is 0.2. is there. The intermittent aeration time ratio when the aeration time per time is shortened to 12 seconds is 0.02. Using such an intermittent aeration time ratio as a parameter, an intermittent aeration operation for 2 hours is performed for each intermittent aeration time ratio, and immediately after the operation is finished, the phosphorus concentration of the liquid to be treated and the sludge filtration in the aerobic tank 16 I examined the sex. In the measurement, 50 mL of the liquid to be processed containing the activated sludge in the aerobic tank 16 at a high concentration was sampled for each experimental unit. It was filtered for 5 minutes with 5c filter paper. That is, the phosphorus concentration of the filtrate at this time was defined as the phosphorus concentration of the liquid to be treated, and the amount of the filtrate was used as an index for the filterability of sludge.

  In FIG. 2, the left vertical axis indicates the phosphorus concentration of the liquid to be treated, and the right vertical axis indicates sludge filterability. As is clear from FIG. 2, when the intermittent aeration time ratio is less than 0.02, the phosphorus concentration of the liquid to be treated increases rapidly. This confirms that at least a part of the aerobic tank 16 is in an anaerobic state and phosphorus is re-released from the activated sludge. When the intermittent aeration time ratio is increased from 0.02, the phosphorus concentration of the liquid to be treated decreases rapidly, and when the intermittent aeration time ratio becomes 0.2, equilibrium is almost reached. This confirms that if the intermittent aeration is properly selected, the inside of the aerobic tank 16 can be maintained in a stable microaerobic state, and re-release of phosphorus from the activated sludge can be prevented. On the other hand, the filterability of sludge gradually decreases when the intermittent aeration time ratio exceeds 0.2. This confirms that if the intermittent aeration time ratio is excessive, the flocs of activated sludge are refined by air aeration and the filterability is lowered. When the sludge filterability is lowered, a new problem of promoting the blockage of the separation membrane occurs when the operation of the membrane unit 28 is resumed. From the above experimental results, it has been found that it is preferable to select an intermittent air diffusion time ratio of 0.02 to 0.2. That is, if the intermittent air diffusion time ratio is selected within this range, release of phosphorus from the activated sludge can be prevented, and the flocs of the activated sludge can be suppressed from being refined by air aeration. In addition, aeration energy can be minimized.

It is an apparatus system diagram for enforcing the processing method of organic wastewater concerning the present invention. It is a graph which shows the experimental result when changing the time ratio of intermittent aeration.

Explanation of symbols

10 ......... Processing device, 12 ......... Anaerobic tank, 14 ......... Anoxic tank, 16 ......... Aerobic tank, 18 ......... Waste water, 20 ......... First circulation pump, 24 ......... Second circulation pump, 28 ......... Membrane unit, 30 ... Coarse bubble diffuser, 32 ......... Blower, 36 ......... Fine bubble diffuser, 42 ......... Suction pump, 44 ......... Phosphor densitometer, 46 ... Controller, 50 ... Sludge extraction pump.

Claims (2)

In the organic wastewater treatment method, organic wastewater containing phosphorus is passed through an anaerobic tank, an oxygen-free tank, and an aerobic tank in this order, and the treated water is separated by a membrane unit immersed in the aerobic tank. In the tank, intermittent aeration is performed during a time period when the operation of the membrane unit is stopped, and the degree of the intermittent aeration is controlled based on the phosphorus concentration of the liquid to be treated in the aerobic tank. A method for treating organic wastewater.   The method for treating organic wastewater according to claim 1, wherein a time ratio of the intermittent aeration to a time zone in which the operation of the membrane unit is stopped is set to 0.02 to 0.2.

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