JP2005246283A - Operation method for membrane separation apparatus, and membrane separation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an operation method for a membrane separation apparatus and a membrane separation apparatus capable of suppressing increase in membrane pressure difference over a long period compared to the past, and consequently prologing the service life of the membrane. <P>SOLUTION: The membrane separation apparatus is provided with membrane modules dipped in a bioreactor for filtering active sludge mixed liquid in the bioreactor. In the operation method of the separation apparatus, filtering is started with a target penetration flow flux set to 50% or less of a target penetration flow flux, when alternately repeating filtering by the membrane module and washing of the membrane module, and the flow flux is stepwise or continuously increased to a target flow flux within a prescribed time from the start of the filtration, then, filtration is continued under maintaining the target flow flux until the next washing. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for operating a membrane separation apparatus and a membrane separation apparatus provided with a membrane module that is immersed in a biological reaction tank and filters an activated sludge mixed liquid in the tank.

  The membrane separation activated sludge method is a method for treating sewage such as municipal sewage and industrial wastewater. The membrane module is immersed in an aeration tank, the inside of the filtration membrane is sucked down with a suction pump, and the pressure is reduced. The liquid mixture (mixed liquid of sewage and activated sludge) is subjected to solid-liquid separation to obtain clear treated water (permeated water) from the secondary side of the membrane module.

  In this membrane separation activated sludge method, since the treated water is obtained by membrane filtration, clogging (fouling) of the membrane accompanying the operation of the membrane separation apparatus provided with the membrane module is unavoidable. In the filtration of the activated sludge mixed liquid, the membrane surface is constantly washed by the upward flow caused by aeration from the air diffuser disposed below the membrane module, but it is difficult to sufficiently suppress the progress of clogging. Therefore, the membrane surface is washed to recover the membrane performance when the membrane differential pressure rises or when filtration is performed for a certain period of time.

As a method for operating the membrane separator, there are a constant pressure method in which filtration is performed at a constant pressure and a constant flow method in which filtration is performed at a constant permeation flux. In the constant pressure method, the permeation flux F is decreased in accordance with the increase of ΔR in order to keep the membrane differential pressure ΔP constant under the increase in filtration resistance ΔR due to the adhesion of dirt on the membrane surface. Is performed (when the permeation flux F reaches the lower limit), the operation is temporarily stopped, the membrane is washed with a chemical solution to recover the permeation flux F, and then the operation is resumed. In the constant flow method, in order to keep the permeation flux F constant under an increase in filtration resistance ΔR due to adhesion of dirt on the membrane surface, the membrane differential pressure ΔP is increased as ΔR increases, When the filtration is performed (when ΔP reaches the upper limit value), the operation is temporarily stopped, the membrane is washed with a chemical solution to recover the permeation flux F, and then the operation is restarted. Recently, in order to improve the recovery rate of treated water, a constant flow method in which filtration is performed with a constant permeation flux is often employed in the operation of a membrane separator.
JP 11-276864 A (paragraph [0037])

  However, in the operation method of the membrane separation apparatus using the constant flow method described above, the filtration is performed with the target permeation flux set to a constant value from immediately after the chemical cleaning to immediately before the next chemical cleaning. Therefore, with the start of the filtration operation by sudden target permeation flux, sludge particles (fouling material particles) separated by a membrane such as suspended material particles adhere to the membrane surface rapidly. For this reason, immediately after the start of the filtration operation, the membrane differential pressure suddenly increases, and as a result, the increase in the membrane differential pressure due to the subsequent filtration operation also increases. The frequency of chemical cleaning was high.

  Therefore, the problem of the present invention is that it is possible to suppress an increase in the membrane differential pressure over a long period of time compared to the prior art, and thereby it is possible to obtain a sufficient amount of treated water stably over a long period of time, and the frequency of membrane cleaning. It is an object of the present invention to provide a method for operating a membrane separation apparatus and a membrane separation apparatus that can reduce the amount of chemicals used for cleaning as much as possible, and that can extend the life of the membrane.

  In order to solve the above problems, the present invention takes the following technical means.

  The invention of claim 1 is a method for operating a membrane separation apparatus comprising a membrane module that is immersed in an aeration tank and filters the activated sludge mixed liquid in the tank, and includes filtration by the membrane module and washing of the membrane module. When iteratively repeats, filtration is started with a permeation flux set to a value of 50% or less of the target permeation flux, and the permeation flux is gradually increased to the target permeation flux within a predetermined time from the start of filtration. The operation method of the submerged membrane separation apparatus is characterized in that it is continuously increased and then filtered while holding the target permeation flux until the next cleaning.

  The invention according to claim 2 is the operation method of the submerged membrane separation apparatus according to claim 1, wherein the permeation flux at the start of filtration is a value of 40% or less of the target permeation flux. Is.

  The invention of claim 3 is the operation method of the submerged membrane separator according to claim 1 or 2, wherein the permeation flux is increased to the target permeation flux in at least three stages within 24 hours from the start of filtration. It is what.

  Invention of Claim 4 is the membrane module immersed in the biological reaction tank, the suction pump interposed in the middle of the treated water outlet pipe connected to the secondary side of the said membrane module, and the said in the said treated water outlet pipe The treated water flow rate detector provided downstream of the suction pump and the set treated water amount are set to 50% or less of the target treated water amount at the start of filtration by the membrane module, and the target treated water amount within a predetermined time from the start of filtration. The target treated water amount is held until the next cleaning, and the treated water amount discharged from the suction pump measured by the treated water flow rate detector is the setting process. And a control device that controls the suction pump so that the amount of water becomes equal.

  According to a fifth aspect of the present invention, in the membrane separation apparatus of the fourth aspect, the set treated water amount is a value equal to or less than 50% of a target treated water amount at the start of filtration by the membrane module, and the target water within 24 hours from the start of filtration. It is a value that is increased in at least three stages until the amount of treated water, and then the target treated water amount is maintained until the next cleaning.

  The operation method of the membrane separation apparatus or the membrane separation apparatus of the present invention starts filtration with a permeation flux set to a value of 50% or less of the target permeation flux after washing, and the permeation flux from the start of filtration to the target The permeation flux is increased stepwise or continuously. As a result, the sludge particles adhering to the membrane surface gradually increase compared to the start of filtration by sudden target permeation flux, and at the start of filtration, sludge particles of various sizes are applied to the membrane. The speed of sludge particles when approaching, contacting, and sucking and adhering can be made substantially the same. Therefore, at the start of filtration, the membrane surface can be covered in a cake shape (aggregate shape) with as much sludge particles as possible, so that the small sludge particles that cause the clogging of the membrane to adhere to the membrane pores first. This can be greatly suppressed. Therefore, unlike the conventional case, it is possible to suppress an increase in the membrane differential pressure at the start of operation, and hence it is possible to suppress an increase in the membrane differential pressure due to the subsequent filtration operation with the target permeation flux. Compared to, the increase in membrane differential pressure can be suppressed over a long period of time, so that a sufficient amount of treated water can be obtained stably over a long period of time, and the frequency of cleaning the membrane can be reduced and the amount of chemical used for cleaning can be reduced. As a result, the lifetime of the film can be extended.

  The method or apparatus of the present invention starts the filtration with a permeation flux set to a value of 50% or less of the target permeation flux, thereby allowing the sludge particles of various sizes to approach the membrane → contact → suction. The speed of sludge particles when adhering can be made almost the same, so that small sludge particles concentrate on the membrane pores and block the membrane pores greatly, greatly increasing the membrane differential pressure. Can be suppressed. When filtration starts with a permeation flux set to a value that exceeds 50% of the target permeation flux, small sludge particles with low resistance received from the activated sludge mixed solution approach and adhere to the membrane pores, blocking the membrane pores. Therefore, it is difficult to stably obtain the effect of suppressing the increase in the film differential pressure. Note that the permeation flux at the start of filtration is preferably set to 40% or less of the target permeation flux with a width of 10% in consideration of the stability of the pump suction flow rate. In addition, the lower limit value of the permeation flux when starting filtration is the target permeation flux from the viewpoint of reducing the recovery rate of treated water and avoiding the complexity of the control device for controlling the permeation flux (treatment water amount). 10% or more is desirable.

  In the method or apparatus of the present invention, it is desirable to increase the permeation flux to the target permeation flux in at least three stages within 24 hours from the start of filtration. This is because if the permeation flux is increased to the target permeation flux in two stages at the filtration start time, the effect of suppressing the increase in the membrane differential pressure is not obtained stably and sufficiently. In order to avoid complication of the control device for controlling the permeation flux, the upper limit of the setting stage up to the target permeation flux is appropriately about five.

  Further, the time Hs when the permeation flux is increased stepwise from the start of filtration to the target permeation flux is preferably within 24 hours. This is because if the time exceeds 24 hours, the necessary processing amount per day may not be obtained. In general, when wastewater is treated, a margin is expected for about 24 hours in a flow rate adjusting tank or the like, and therefore it is set within 24 hours. In addition, from the viewpoint of obtaining the effect of suppressing the increase in the membrane pressure difference, the lower limit value of this time Hs is appropriately about 2 hours.

  FIG. 1 is a flow diagram of a sewage treatment facility equipped with a membrane separation apparatus for carrying out the method of the present invention.

  In FIG. 1, 31 is a flow rate adjusting tank, 32 is a pretreatment tank provided with a pretreatment screen 31a, and 33 is a denitrification tank. Reference numeral 34 denotes a nitrification tank as a biological reaction tank, and 2 denotes a plurality of hollow fiber membrane modules immersed in the activated sludge mixed solution in the nitrification tank 34. The hollow fiber membrane module 2 is an external pressure type module that has a large number of hollow fiber membranes extending in the vertical direction in the activated sludge mixed solution and is immersed in a standing posture. 9 is an air diffuser disposed below the hollow fiber membrane module 2, and 10 is a blower for supplying aeration air to the air diffuser 9. In the present embodiment, one membrane unit is configured by the plurality of hollow fiber membrane modules 2 arranged at intervals in the left-right direction in the drawing, and the plurality of membrane units are immersed in the nitrification tank 34. It has become. 3 is a water collection header provided for each membrane unit, and the secondary side (treated water outlet) of the plurality of hollow fiber membrane modules 2 constituting the membrane unit communicates with the water collection header 3. Yes. The water collection header 3 is connected to a treated water outlet pipe 5 in which an on-off valve 4, a suction pump 6 and a treated water flow rate detector 7 are interposed.

  Reference numeral 8 denotes a control device for the suction pump 6. The control device 8 is given a value of 50% or less of the target treated water amount at the start of filtration as the set treated water amount Qr, and a value that is increased stepwise to the target treated water amount within a predetermined time from the start of filtration. After that, a value as the target treated water amount is given until the next cleaning. In this embodiment, the control device 8 has an inverter circuit, and the suction pump 6 rotates so that the treated water amount Q discharged from the suction pump 6 measured by the treated water flow rate detector 7 becomes the set treated water amount Qr. By controlling the number, the treated water amount Q discharged from the suction pump 6 is maintained at the set treated water amount Qr. That is, the hollow fiber membrane module 2 starts filtration with the permeation flux set to a value of 50% or less of the target permeation flux by the control device 8, and the permeation flux is set within the predetermined time from the start of filtration. The permeation flux is increased stepwise or continuously, and then the target permeation flux is maintained until the next washing for filtration.

  The membrane separation device 1 is a plurality of hollow fiber membrane modules 2 immersed in a nitrification tank 34, and is disposed below these hollow fiber membrane modules 2, and is supplied with aeration air from a blower 10 outside the tank. An air device 9, a treated water outlet pipe 5 communicating with the secondary side of the hollow fiber membrane module 2 via the water collection header 3, a suction pump 6 interposed in the middle of the treated water outlet pipe 5, and a treatment The on-off valve 4 provided in the middle of the water outlet pipe 5 and upstream of the suction pump 6, and the treated water flow rate detector provided in the middle of the treated water outlet pipe 5 and downstream of the suction pump 6 7 and a control device 8 for controlling the number of rotations of the suction pump 6.

  Reference numeral 21 denotes a chemical liquid tank. A chemical liquid supply pipe 23 led from the chemical liquid tank 21 is connected to the water collecting header 3, and an open / close valve 22 is interposed in the chemical liquid supply pipe 23. Reference numeral 24 denotes a wash water tank, and a wash water supply pipe 26 led from the wash water tank 24 is connected to the downstream side of the on-off valve 22 in the middle of the chemical liquid supply pipe 23. An opening / closing valve 25 is interposed in the cleaning water supply pipe 26.

  In such a sewage treatment facility, sewage is sequentially introduced into the flow rate adjustment tank 31 and the pretreatment tank 32 to adjust the flow rate and remove impurities. Next, sewage from the pretreatment tank 32 is caused to flow into the denitrification tank 33 in which microorganisms (denitrifying bacteria) are present, and a nitrification solution containing nitrate nitrogen from the nitrification tank 34 in which microorganisms (nitrifying bacteria and the like) are present. By the circulation type nitrification denitrification method of circulating, nitrate nitrogen is reduced in the denitrification tank 33 and released into the atmosphere as nitrogen gas to remove nitrogen contained in the sewage.

  In addition to such nitrogen removal, during the filtration operation, the suction pump 6 sucks the inside of the hollow fiber membrane of the hollow fiber membrane module 2 to create a pressure difference between the inside and outside of the membrane, thereby enabling activated sludge in the nitrification tank 34. The treated water (permeated water) obtained by filtering the mixed solution through the hollow fiber membrane module 2 is taken out of the tank through the treated water outlet pipe 5 with the water collection header 3 and the on-off valve 4 opened.

  During this filtration operation, filtration is started with a permeation flux set to a value of 50% or less of the target permeation flux, and the permeation flux is gradually increased to the target permeation flux within 24 hours from the start of filtration. After that, the target permeation flux is maintained until the next washing, and the filtration is performed.

  Then, the hollow fiber membrane module 2 is washed as follows in order to recover the membrane performance at the stage of continuous filtration for a certain period of time. Washing was performed in a state where the hollow fiber membrane module 2 was immersed in the activated sludge mixed solution. At this time, aeration by the air diffuser 9 was stopped and cleaning was performed. Further, the on-off valve 4 of the treated water outlet pipe 5 is closed.

  First, the open / close valve 22 is opened, and in this embodiment, the chemical liquid from the chemical liquid tank 21 is pumped by a hydraulic head difference through the chemical liquid supply pipe 23 and the water collection header 3 to obtain the treated water in the direction opposite to the hollow fiber membrane. The module 2 is permeated from the inside to the outside of the hollow fiber membrane. The state in which the chemical solution is injected is held for a predetermined time. Next, the on-off valve 25 is opened (at this time, the on-off valve 22 is already closed), and the cleaning water from the cleaning water tank 24 is actually supplied through the cleaning water supply pipe 26, the chemical solution supply pipe 23 and the water collection header 3. In a form, it pumps by a head difference and permeate | transmits from the inside of the hollow fiber membrane of the hollow fiber membrane module 2 to the exterior in the direction opposite to when obtaining treated water. The purpose of cleaning with the cleaning water in addition to the chemical cleaning is to more reliably remove the membrane surface-attached sludge (fouling substance).

  In the above embodiment, the present invention is a hollow fiber membrane module as the membrane module. However, the present invention is not limited to this and may be a flat membrane module, and the type and shape of the membrane module can be selected as appropriate. is there. The biological reaction tank is a nitrification tank for the purpose of removing nitrogen in the above embodiment, but is not limited thereto, and may be a normal activated sludge tank for decomposing organic substances. Further, the cleaning of the membrane module is not limited to the cleaning method described in the above embodiment, but it is immersed in the biological reaction tank from the viewpoint of shortening the filtration operation stop time and the ease of the cleaning operation. It is preferable to clean the membrane module.

  An experiment was conducted by applying the method of the present invention to the sewage treatment facility shown in FIG. First, chemical cleaning was performed with the hollow fiber membrane module immersed in an activated sludge mixed solution (MLSS concentration (sludge concentration): about 10,000 mg / L) in the nitrification tank. As a chemical solution, an aqueous solution of sodium hypochlorite having a concentration of 5000 ppm was injected into each hollow fiber membrane module so as to permeate from the inside of the hollow fiber membrane of the hollow fiber membrane module to the outside, and held for 90 minutes. Thereafter, 90 liters of washing water was poured into each hollow fiber membrane module, and washing with washing water was performed.

After washing with the chemical solution, filtration with a hollow fiber membrane module was performed. Filtration is started by setting the permeation flux to 0.2 m ³ / m ² / day, and after maintaining this value for 1 hour, the permeation flux is 0.3 m ³ / m ² / day (holding for 1 hour) → permeation Flux 0.4 m ³ / m ² / day (1 hour hold) → Permeation flux 0.5 m ³ / m ² / day (1 hour hold) → Permeation flux 0.6 m ³ / m ² / day After increasing the permeation flux in five stages from the start of filtration to the target permeation flux 0.6 m ³ / m ² / day, the target permeation flux 0.6 m ³ / m ² / day is continuously maintained. Filtration was performed. The time from the start of filtration to the target permeation flux is 4 hours as described above. Further, in the comparative example, after performing chemical cleaning in the same manner as in the example, the permeation flux was set to the target permeation flux of 0.6 m ³ / m ² / day, and filtration was started. It was kept and continuously filtered. And the membrane differential pressure | voltage of the hollow fiber membrane module after 1 day progress and 30 days progress after the filtration start was measured. The results are shown in Table 1.

  As shown in Table 1, in the comparative example, since the filtration is started by sudden target permeation flux, small sludge particles suddenly adhere to the membrane pores at the start of filtration (at the start of operation). As a result, the membrane differential pressure increases, and due to the progress of clogging of the membrane at the start of filtration, the membrane differential pressure in the subsequent filtration operation also increases. For this reason, even if chemical cleaning is performed periodically, it becomes difficult to recover the performance of the membrane, and the membrane life of the membrane module is as short as several months (e.g., 8 months depending on the nature of the sewage to be treated).

  On the other hand, according to the embodiment, since an increase in the membrane differential pressure at the start of filtration can be suppressed, an increase in the membrane differential pressure due to the filtration operation at the target permeation flux thereafter can also be suppressed. It was possible to suppress an increase in the membrane differential pressure over a long period of time compared to the comparative example. As a result, even when chemical cleaning is performed as in the comparative example, the membrane life of the membrane module can be extended to several years (for example, 5 years).

It is a flowchart of the sewage treatment equipment provided with the membrane separation apparatus which enforces the method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Membrane separator 2 ... Hollow fiber membrane module 3 ... Water collection header 4,22,25 ... On-off valve 5 ... Process water outlet pipe 6 ... Suction pump 7 ... Process water flow rate detector 8 ... Control apparatus 9 ... Air diffuser DESCRIPTION OF SYMBOLS 21 ... Chemical solution tank 23 ... Chemical solution supply pipe 24 ... Wash water tank 26 ... Wash water supply pipe 33 ... Denitrification tank 34 ... Nitrification tank

Claims (5)

  A method for operating a membrane separation apparatus comprising a membrane module immersed in a biological reaction tank and filtering the activated sludge mixed liquid in the tank, when alternately performing filtration by the membrane module and washing of the membrane module, Filtration is started with a permeation flux set to a value of 50% or less of the target permeation flux, and the permeation flux is increased stepwise or continuously to the target permeation flux within a predetermined time from the start of filtration. Thereafter, filtration is performed while holding the target permeation flux until the next cleaning.   The method of operating a membrane separation apparatus according to claim 1, wherein the permeation flux at the start of filtration is a value of 40% or less of the target permeation flux.   3. The method for operating a membrane separation apparatus according to claim 1, wherein the permeation flux is increased to the target permeation flux in at least three stages within 24 hours from the start of filtration.   Provided on the downstream side of the suction pump in the treated water outlet pipe, the membrane module immersed in the biological reaction tank, the suction pump interposed in the middle of the treated water outlet pipe communicating with the secondary side of the membrane module The treated water flow rate detector and the set treated water amount are set to a value equal to or less than 50% of the target treated water amount at the start of filtration by the membrane module, and stepwise or continuously up to the target treated water amount within a predetermined time from the start of filtration. The increased value, and then the value of the target treated water amount until the next cleaning, and the treated water amount discharged from the suction pump measured by the treated water flow rate detector becomes the set treated water amount. And a control device for controlling the suction pump. The set treated water amount is a value that is 50% or less of the target treated water amount at the start of filtration by the membrane module, and is a value that is increased in at least three stages up to the target treated water amount within 24 hours from the start of filtration. The membrane separation apparatus according to claim 4, wherein the target treated water amount is set to a value until washing.

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