Water treatment device, method for cleaning biofilm in water treatment device, and method for evaluating biofilm thickness in water treatment device

Abstract

To provide a water treatment apparatus capable of exhibiting high treatment performance in accordance with load changes of a liquid to be treated by properly maintaining a film thickness of a biofilm formed in a gas permeable film.SOLUTION: A water treatment apparatus 10 comprises: a treatment tank 11 to be supplied with a liquid S to be treated; a hollow fiber membrane 12 having gas permeability and soaked in the liquid S to be treated in the treatment tank 11; and a biofilm 30 formed on an outside surface of the hollow fiber membrane 12 and utilizing oxygen-containing air supplied into the hollow fiber membrane 12. The water treatment apparatus treats the liquid S to be treated by using the biofilm 30. The water treatment apparatus 10 comprises: an air diffusion pipe 13 positioned under the hollow fiber membrane 12 and cleaning the biofilm 30 by discharging a cleaning gas; and an oxygen concentration meter 14 for measuring oxygen concentration of discharged air after passing through the hollow fiber membrane 12. In the air diffusion pipe 13, cleaning strength of the biofilm 30 is controlled based on oxygen concentration measured by the oxygen concentration meter 14.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a water treatment device including a gas-permeable membrane immersed in a liquid to be treated in a treatment tank and having gas permeability, and a biofilm formed on the outer surface of the gas-permeable membrane and utilizing an oxygen-containing gas supplied into the gas-permeable membrane, as well as a method for cleaning the biofilm and a method for evaluating the membrane thickness in the water treatment device. [Background technology]

[0002] Conventionally, an example of this type of water treatment device is a hollow fiber membrane bioreactor as shown in Patent Document 1. The hollow fiber membrane bioreactor in Patent Document 1 includes a casing immersed in the liquid to be treated in a treatment tank and open at the top and bottom, a plurality of gas-permeable hollow fiber membranes arranged inside the casing, a gas supply means for supplying gas to the inside of the hollow fiber membranes, and an aeration means installed below the casing for supplying gas to the outside of the hollow fiber membranes. The hollow fiber membrane bioreactor in Patent Document 1 has a biofilm formed on the outer surface of the hollow fiber membranes that utilizes the gas supplied to the inside of the hollow fiber membranes. Furthermore, the hollow fiber membrane bioreactor in Patent Document 1 is configured so that an upward flow is generated inside the casing by the gas supplied to the outside of the hollow fiber membranes.

[0003] In the hollow fiber membrane bioreactor described in Patent Document 1, the contact of air bubbles with the hollow fiber membrane surface allows for efficient cleaning of the enlarged biofilm, and stable, high treatment performance can be obtained over a long period of time. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2006-101805 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the hollow fiber membrane bioreactor described in Patent Document 1 lacks a method for real-time monitoring of the amount of microorganisms adhering to the hollow fiber membrane surface (biofilm thickness) or its alternative indicators. Therefore, the amount of microorganisms adhering to the hollow fiber membrane surface (biofilm thickness) cannot be controlled in response to fluctuations in the inflow load of the treated liquid. Therefore, the hollow fiber membrane bioreactor described in Patent Document 1 suffers from a significant decrease in treatment capacity and the failure to obtain the desired treated water quality if the amount of microorganisms adhering to the hollow fiber membrane surface (biofilm thickness) cannot be appropriately maintained. Furthermore, insufficient control of the amount of microorganisms (biofilm thickness) reduces the oxygen transfer efficiency in the hollow fiber membrane, preventing the energy-saving benefits of the MABR (membrane aeration bioreactor). This increases the membrane area required for the hollow fiber membrane, resulting in a larger device size. Furthermore, the increased membrane area required for the hollow fiber membrane also increases the amount of gas supplied to the hollow fiber membrane.

[0006] An object of the present invention is to provide a water treatment device that can maintain an appropriate thickness of a biofilm formed on a gas-permeable membrane and exhibit high treatment performance that follows load fluctuations of the liquid to be treated. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, the water treatment device of the present invention comprises a treatment tank to which the liquid to be treated is supplied, a gas-permeable membrane immersed in the liquid to be treated in the treatment tank and having gas permeability, and a biofilm formed on the outer surface of the gas-permeable membrane and utilizing an oxygen-containing gas supplied into the gas-permeable membrane, and is a water treatment device that treats the liquid to be treated using the biofilm, and is also equipped with a cleaning unit located below the gas-permeable membrane that cleans the biofilm by ejecting cleaning gas, and a measuring unit that measures the oxygen concentration in the primary side gas after passing through the gas-permeable membrane, and the cleaning strength of the cleaning unit against the biofilm is controlled based on the oxygen concentration measured by the measuring unit.

[0008] This allows the increase or decrease in oxygen consumption by the organisms in the biofilm to be determined from the oxygen concentration measured by the measuring unit, and the cleaning strength against the biofilm can be controlled after determining whether the thickness of the biofilm is appropriate, so that the biofilm formed on the outer surface of the gas-permeable membrane can be maintained at an appropriate thickness.

[0009] In the water treatment device of the present invention, the cleaning section increases the cleaning strength of the biofilm when the oxygen concentration after cleaning the biofilm decreases compared to the oxygen concentration before cleaning the biofilm, and decreases the cleaning strength of the biofilm when the oxygen concentration after cleaning the biofilm increases compared to the oxygen concentration before cleaning the biofilm.

[0010] According to this, the cleaning strength of the biofilm is controlled in accordance with the increase or decrease in oxygen concentration before and after cleaning of the biofilm, so that the biofilm can be maintained at an appropriate thickness with precision.

[0011] In the water treatment device of the present invention, the cleaning strength of the cleaning section against the biofilm is controlled by changing at least one of the following requirements: the frequency of ejection of the cleaning gas against the biofilm, the amount of the cleaning gas ejected per unit time against the biofilm, and the ejection time of the cleaning gas against the biofilm.

[0012] This allows the method of controlling the cleaning intensity for the biofilm to be changed depending on the state of the biofilm.

[0013] The water treatment device of the present invention includes a film thickness evaluation device that evaluates the film thickness of the biofilm relative to the outer surface of the gas-permeable membrane based on the oxygen concentration measured by the measurement unit.

[0014] This makes it possible to determine whether the thickness of the biofilm is appropriate by understanding the increase or decrease in the amount of oxygen consumed by the organisms in the biofilm from the oxygen concentration measured by the measuring unit.

[0015] The method for cleaning a biofilm in a water treatment device of the present invention comprises a treatment tank to which a liquid to be treated is supplied, a gas-permeable membrane immersed in the liquid to be treated in the treatment tank, and a biofilm formed on the outer surface of the gas-permeable membrane and utilizing an oxygen-containing gas supplied into the gas-permeable membrane, and the method treats the liquid to be treated using the biofilm, in which the biofilm is cleaned by ejecting cleaning gas from below the gas-permeable membrane, the oxygen concentration in the primary side gas after passing through the gas-permeable membrane is measured, and the cleaning strength for the biofilm is controlled based on the measured oxygen concentration.

[0016] This allows the increase or decrease in oxygen consumption by the organisms in the biofilm to be understood from the oxygen concentration in the primary gas after passing through the gas-permeable membrane, and the cleaning strength against the biofilm can be controlled after determining whether the thickness of the biofilm is appropriate, so that the biofilm formed on the outer surface of the gas-permeable membrane can be maintained at an appropriate thickness.

[0017] In the method for cleaning a biofilm in a water treatment device of the present invention, the oxygen concentration in the primary gas after passing through the gas-permeable membrane is measured before and after cleaning the biofilm, and if the measured oxygen concentration after cleaning the biofilm is lower than the oxygen concentration before cleaning the biofilm, the cleaning intensity for the biofilm is increased, and if the measured oxygen concentration after cleaning the biofilm is higher than the oxygen concentration before cleaning the biofilm, the cleaning intensity for the biofilm is decreased.

[0018] According to this, the cleaning strength of the biofilm is controlled in accordance with the increase or decrease in oxygen concentration before and after cleaning of the biofilm, so that the biofilm can be maintained at an appropriate thickness with precision.

[0019] In the method for cleaning a biofilm in a water treatment device of the present invention, the cleaning strength of the biofilm is controlled by changing at least one of the following requirements: the frequency of ejection of the cleaning gas onto the biofilm, the amount of the cleaning gas ejected onto the biofilm per unit time, and the ejection time of the cleaning gas onto the biofilm.

[0020] This allows the method of controlling the cleaning intensity for the biofilm to be changed depending on the state of the biofilm.

[0021] The method for evaluating the thickness of a biofilm in a water treatment device of the present invention comprises a treatment tank to which a liquid to be treated is supplied, a gas-permeable membrane immersed in the liquid to be treated in the treatment tank, and a biofilm formed on the outer surface of the gas-permeable membrane and utilizing an oxygen-containing gas supplied into the gas-permeable membrane, and the method treats the liquid to be treated using the biofilm, and evaluates the thickness of the biofilm relative to the outer surface of the gas-permeable membrane based on the oxygen concentration in the primary gas after passing through the gas-permeable membrane.

[0022] This allows the increase or decrease in oxygen consumption by the organisms in the biofilm to be determined from the oxygen concentration in the primary gas after passing through the gas-permeable membrane, and then the appropriateness of the biofilm thickness can be evaluated, thereby allowing the biofilm formed on the outer surface of the gas-permeable membrane to be maintained at an appropriate thickness.

[0023] In the method for evaluating the thickness of a biofilm in a water treatment device of the present invention, the biofilm is cleaned by ejecting cleaning gas from below the gas-permeable membrane, and the oxygen concentration in the primary gas after passing through the gas-permeable membrane is measured before and after cleaning the biofilm.If the measured oxygen concentration after cleaning the biofilm is lower than the oxygen concentration before cleaning the biofilm, it is determined that the thickness of the biofilm is thicker than that suitable for treating the liquid to be treated.If the measured oxygen concentration after cleaning the biofilm is higher than the oxygen concentration before cleaning the biofilm, it is determined that the thickness of the biofilm is thinner than that suitable for treating the liquid to be treated.

[0024] This allows the appropriateness of the biofilm thickness to be evaluated based on the increase or decrease in oxygen concentration before and after cleaning of the biofilm, making it possible to accurately maintain the biofilm at an appropriate thickness. [Effects of the Invention]

[0025] According to the water treatment device, the method for cleaning a biofilm in a water treatment device, and the method for evaluating the thickness of a biofilm in a water treatment device of the present invention, the cleaning strength of the biofilm is controlled based on the oxygen concentration in the primary gas after passing through the gas-permeable membrane. Therefore, the biofilm formed on the outer surface of the gas-permeable membrane can be maintained at an appropriate thickness by understanding the increase or decrease in oxygen consumption by the organisms in the biofilm. This makes it possible to provide a water treatment device that can demonstrate high treatment performance that follows load fluctuations in the treated liquid. Furthermore, because the oxygen transfer efficiency in the hollow fiber membrane is improved, the membrane area required for the hollow fiber membrane can be minimized, thereby minimizing the size of the water treatment device and reducing the amount of gas supplied to the hollow fiber membrane. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic configuration diagram of a water treatment device according to an embodiment of the present invention. [Figure 2] These are enlarged cross-sectional views of the hollow fiber membrane and biofilm in the water treatment device, where (A) shows a case where the biofilm is properly formed on the hollow fiber membrane, (B) shows a case where the biofilm has grown larger than the hollow fiber membrane, (C) shows a case where the biofilm tends to grow larger than the hollow fiber membrane, and (D) shows a case where the biofilm has become thinner than the hollow fiber membrane. [Figure 3] FIG. 1 is a diagram showing the change in oxygen concentration (off-gas oxygen concentration) measured by an oxygen concentration meter relative to the treatment time (elapsed time) of the liquid to be treated in a water treatment device, in which the oxygen concentration decreases after cleaning (scouring) of the biofilm. [Figure 4] FIG. 1 is a diagram showing the change in oxygen concentration (off-gas oxygen concentration) measured by an oxygen concentration meter versus the treatment time (elapsed time) of the liquid to be treated in a water treatment device, in which the oxygen concentration increases after cleaning (scouring) of the biofilm. [Figure 5] FIG. 1 is a graph showing changes in oxygen concentration (off-gas oxygen concentration) measured by an oxygen concentration meter with respect to treatment time (elapsed time) of a liquid to be treated in a water treatment device. DETAILED DESCRIPTION OF THE INVENTION

[0027] The water treatment device of the present invention will be described below.

[0028] As shown in FIG. 1, a water treatment device 10 according to one embodiment of the present invention is mainly composed of a treatment tank 11 to which a liquid to be treated S is supplied, a hollow fiber membrane 12 (an example of a "gas-permeable membrane") immersed in the liquid to be treated S in the treatment tank 11, an aeration pipe 13 (an example of a "cleaning section") located below the hollow fiber membrane 12 and discharging cleaning air (an example of a "cleaning gas"), and an oxygen concentration meter 14 (an example of a "measuring section") that measures the oxygen concentration in the exhaust air (an example of a "primary-side gas") after passing through the hollow fiber membrane 12.

[0029] In the treatment tank 11, the liquid to be treated S (raw water) is supplied from the bottom of the treatment tank 11, and the treated liquid to be treated S (treated water) flows out from the top of the treatment tank 11. The liquid to be treated S (raw water) is supplied to the treatment tank 11 from the initial settling outflow channel 15 via the raw water tank 16 and first piping 17. The supplied liquid to be treated S is circulated by a circulation pump 18. The liquid to be treated S (treated water) treated in the treatment tank 11 flows out into the treated water tank 20 via second piping 19.

[0030] A plurality of hollow fiber membranes 12 are arranged in parallel so as to extend in the vertical direction of the treatment tank 11. The hollow fiber membranes 12 are gas permeable and can selectively allow oxygen in the air to pass through. The hollow fiber membranes 12 are mainly composed of non-porous membranes, but may also be non-porous and porous composite membranes. As shown in Figures 1 and 2, air (an example of an "oxygen-containing gas") is supplied to the inside of the hollow fiber membranes 12 from a first blower 21 provided outside the treatment tank 11 via a third pipe 22. The air supplied by the first blower 21 passes through the membrane surface of the hollow fiber membranes 12 without forming bubbles and dissolves in the liquid S to be treated in the treatment tank 11. The exhaust air after passing through the hollow fiber membranes 12 is discharged via a fourth pipe 23.

[0031] 2, a biofilm 30 is formed on the outer surface of the hollow fiber membrane 12. In the biofilm 30, organisms in the biofilm 30 utilize oxygen contained in the air from the hollow fiber membrane 12 to biologically remove substances to be treated (e.g., organic matter, nitrogen compounds, etc.) in the liquid to be treated S.

[0032] As shown in Figure 2(A), the biofilm 30 is formed from the outer surface of the hollow fiber membrane 12 to a predetermined thickness M. As the thickness M of the biofilm 30 increases, the amount of organisms present in the biofilm 30 increases in proportion to the increase in thickness M.

[0033] For example, as shown in FIG. 2(B), in the case of a biofilm 30A having a thickness M1 that is thicker than the thickness M of the biofilm 30, the biomass of the biofilm 30A is greater than that of the biofilm 30 by the amount of the increased thickness. Therefore, the treatment performance of the biofilm 30A for the liquid to be treated S is improved compared to the biofilm 30. However, the outer surface of the biofilm 30A is separated from the outer surface of the hollow fiber membrane 12 by the amount of the increased thickness. Therefore, oxygen is less likely to be supplied from the hollow fiber membrane 12 to the outer surface of the biofilm 30A, and the contact efficiency with oxygen is reduced. Therefore, the oxygen transfer efficiency of the biofilm 30A is lower than that of the biofilm 30, and the treatment efficiency of the liquid to be treated S relative to the amount of air supplied is poor.

[0034] On the other hand, as shown in FIG. 2(C), in the case of biofilm 30B having a thickness M2 that is thicker than the thickness M of biofilm 30 but thinner than the thickness M1 of biofilm 30A, the distance between the outer surface of biofilm 30B and the outer surface of hollow fiber membrane 12 is closer than that of biofilm 30A. Therefore, oxygen is more easily supplied from the hollow fiber membrane 12 to the outer surface of biofilm 30A than to biofilm 30A, improving the efficiency of contact with oxygen. However, the biomass of biofilm 30B is greater than that of biofilm 30 due to the increase in membrane thickness. Therefore, the amount of oxygen consumed by organisms in biofilm 30B is greater than that of biofilm 30. Therefore, if the amount of air (oxygen) to be supplied to hollow fiber membrane 12 (biofilm 30B) is the same as that of biofilm 30, the amount of oxygen supplied to biofilm 30B will be insufficient.

[0035] Furthermore, as shown in FIG. 2(D), in the case of a biofilm 30C having a thickness M3 that is thinner than the thickness M of the biofilm 30, the biomass of the biofilm 30C is less than that of the biofilm 30 due to the reduction in the thickness. Therefore, the amount of oxygen consumed by organisms in the biofilm 30C is less than that of the biofilm 30. Therefore, if the amount of air (oxygen) to be supplied to the hollow fiber membrane 12 (biofilm 30C) is the same as that of the biofilm 30, sufficient oxygen is supplied to the biofilm 30C. However, since the biofilm 30C has fewer organisms to treat the liquid S than the biofilm 30, the treatment performance of the liquid S is lower than that of the biofilm 30. Furthermore, since the biofilm C is supplied with more oxygen than the amount of oxygen consumed by the organisms in the biofilm 30C, the oxygen transfer efficiency is lower than that of the biofilm 30.

[0036] In this way, the thickness M of the biofilm 30 is set in consideration of conditions such as the contact efficiency between the biofilm 30 (organisms in the biofilm 30) and oxygen, and the transfer efficiency of oxygen in the biofilm 30.

[0037] As shown in FIG. 1 , the air diffuser 13 intermittently discharges cleaning air from below the hollow fiber membrane 12. The cleaning air discharged from the air diffuser 13 is supplied to the air diffuser 13 from a second blower 24 provided outside the treatment tank 11 via a fifth pipe 25. The air diffuser 13 supplies the cleaning air as bubbles into the liquid S to be treated. The air diffuser 13 cleans the biofilm 30 by creating turbulence or shear force on the surface of the biofilm 30 with the supplied air bubbles and the upward flow generated by the supply of bubbles. Note that the gas discharged from the air diffuser 13 is not limited to cleaning air, and any gas capable of cleaning the biofilm 30 may be used, such as nitrogen gas, biogas, or recycled exhaust air (off-gas) flowing through the fourth pipe 23.

[0038] The oxygen concentration meter 14 is provided in the fourth pipe 23 and measures the oxygen concentration in the exhaust air flowing through the fourth pipe 23. In the water treatment device 10, based on the oxygen concentration measured by the oxygen concentration meter 14, an increase or decrease in the amount of oxygen consumed by the organisms in the biofilm 30 is determined, and an increase or decrease in the thickness of the biofilm 30 formed on the outer surface of the hollow fiber membrane 12 is evaluated.

[0039] As described above, in the biofilm 30, the organisms in the biofilm 30 use the oxygen supplied from the hollow fiber membrane 12 to biologically remove the substance to be treated in the liquid to be treated S. Therefore, when the biofilm 30 tends to enlarge (the biomass of the biofilm 30 tends to increase) (as in FIG. 2(C)), the amount of oxygen consumed by the organisms in the biofilm 30 increases, and the concentration of oxygen contained in the exhaust air after passing through the hollow fiber membrane 12 decreases.

[0040] On the other hand, if the biofilm 30 becomes too thick (as in Figure 2(B)) or if the biofilm 30 becomes too thin (the biomass of the biofilm 30 decreases too much) (as in Figure 2(D)), the amount of oxygen consumed by the organisms in the biofilm 30 decreases, and the concentration of oxygen contained in the exhaust air after passing through the hollow fiber membrane 12 increases.

[0041] For this reason, in the water treatment device 10, the oxygen concentration contained in the exhaust air after passing through the hollow fiber membranes 12 is measured by the oxygen concentration meter 14 to determine an increase or decrease in the amount of oxygen consumed by the organisms in the biofilm 30, and thereby to determine an increase or decrease in the thickness of the biofilm 30 formed on the outer surface of the hollow fiber membranes 12. That is, if the oxygen concentration measured by the oxygen concentration meter 14 is lower than a predetermined oxygen concentration (the oxygen concentration when the thickness M of the biofilm 30 is the appropriate thickness), it can be determined that the thickness of the biofilm 30 is thicker than the appropriate thickness M and that the biofilm 30 is tending to thicken. On the other hand, if the oxygen concentration measured by the oxygen concentration meter 14 is higher than the predetermined oxygen concentration, it can be determined that the thickness of the biofilm 30 is thinner than the appropriate thickness M and that the biofilm 30 is peeling off too much and is tending to become thin, or it can be determined that the thickness of the biofilm 30 is sufficiently thicker than the appropriate thickness M and that the biofilm 30 is excessively thickened.

[0042] Furthermore, in the water treatment device 10, the suitability of the thickness of the biofilm 30 is evaluated based on the increase or decrease in the thickness of the biofilm 30 determined from the measurement results of the oxygen concentration meter 14, and then the cleaning intensity of the biofilm 30 by the aeration pipe 13 is controlled. Here, the cleaning intensity of the biofilm 30 is controlled by changing at least one of the following: the frequency of the cleaning air discharged from the aeration pipe 13 to the biofilm 30; the discharge amount per unit time of the cleaning air discharged from the aeration pipe 13 to the biofilm 30; and the discharge time of the cleaning air discharged from the aeration pipe 13 to the biofilm 30.

[0043] As shown in Figure 3, if the oxygen concentration (off-gas oxygen concentration) measured by the oxygen concentration meter 14 decreases after the biofilm 30 is scoured (scouring) by the aeration pipe 13 (the arrow in Figure 3), it is believed that the biomass of the biofilm 30 formed on the hollow fiber membrane 12 has increased, resulting in an increased oxygen consumption by the organisms. This suggests that the thickness M of the biofilm 30 is excessive. Therefore, in the water treatment device 10, the swollen biofilm 30 is peeled off by increasing the cleaning intensity of the biofilm 30 by the aeration pipe 13, and the thickness of the biofilm 30 is adjusted to an appropriate thickness M.

[0044] On the other hand, as shown in Figure 4, if the oxygen concentration (off-gas oxygen concentration) measured by the oxygen concentration meter 14 increases after the biofilm 30 is scoured (scouring) by the aeration pipe 13 (the arrow in Figure 4), it is believed that the biomass of the biofilm 30 formed on the hollow fiber membrane 12 has decreased, resulting in a decrease in the amount of oxygen consumed by the organisms. This suggests that the film thickness M of the biofilm 30 is too small. Therefore, in the water treatment device 10, the cleaning intensity of the biofilm 30 by the aeration pipe 13 is reduced to promote growth of the biofilm 30 until the thinned film thickness of the biofilm 30 reaches an appropriate film thickness M.

[0045] Next, the control of the cleaning strength of the biofilm 30 by the aeration pipe 13 will be described. FIG. 5 shows the change in the oxygen concentration (off-gas oxygen concentration) measured by the oxygen concentration meter 14 relative to the treatment time (elapsed time) of the liquid S to be treated in the water treatment device 10. That is, FIG. 5 shows the change in the oxygen concentration (off-gas oxygen concentration) measured by the oxygen concentration meter 14 relative to the treatment time (elapsed time) of the liquid S to be treated in the water treatment device 10 (the change in the amount of oxygen consumed by the organisms in the biofilm 30, and the change in the biomass of the biofilm 30). In the water treatment device 10, the biofilm 30 is cleaned by the aeration pipe 13 at predetermined intervals (e.g., once every six hours), thereby controlling the thickness of the biofilm 30 to an appropriate thickness M. Note that in FIG. 5, cleaning (scouring) of the biofilm 30 by the aeration pipe 13 is performed at the times indicated by the arrows in the figure.

[0046] As shown in Figure 5(a), if the off-gas oxygen concentration increases after scouring, i.e., if the biomass of the biofilm 30 decreases after scouring, it is determined that the biofilm 30 has been detached more than the appropriate film thickness M by scouring. As a result, as shown in Figure 5(b), by reducing the cleaning intensity of scouring, the biofilm 30 is encouraged to grow until the thinned film thickness of the biofilm 30 reaches the appropriate film thickness M. For example, the cleaning intensity of scouring is reduced by changing the frequency of scouring (the frequency of blowing cleaning air onto the biofilm 30) from once every six hours to once every 12 hours.

[0047] By reducing the frequency of scouring, the biofilm 30 grows without peeling off, and as shown in Figure 5(c), the thickness M of the biofilm 30 increases, and the off-gas oxygen concentration decreases.

[0048] As shown in (d) of Figure 5, if the off-gas oxygen concentration decreases further after scouring, the biofilm 30 is not sufficiently peeled off by scouring, so the film thickness of the biofilm 30 is thicker than the appropriate film thickness M and the biomass of the biofilm 30 is greater than the appropriate amount.

[0049] Furthermore, as shown in Figure 5(e), when the off-gas oxygen concentration, which had decreased after scouring, increases, as shown in Figure 2(B), the biofilm 30 becomes too thick, the contact efficiency between the organisms in the biofilm 30 and oxygen becomes poor, and the organisms in the biofilm 30 do not consume oxygen, resulting in an increase in the off-gas oxygen concentration. Therefore, the thickness of the biofilm 30 is controlled to an appropriate thickness M by increasing the amount of cleansing air discharged per unit time from the aeration pipe 13 or by lengthening the discharge time of cleansing air from the aeration pipe 13.

[0050] Furthermore, as shown in Fig. 5(f), if the off-gas oxygen concentration increases after scouring, i.e., if the biomass of the biofilm 30 decreases after scouring, the biofilm 30 will be in a state where it has been detached too much by scouring, compared to the appropriate film thickness M. Here, when a high load of the liquid to be treated S flows into the treatment tank 11, an oxygen-consuming reaction progresses in the biofilm 30, reducing the off-gas oxygen concentration and causing the biofilm 30 to grow, as shown in Fig. 5(g).

[0051] Furthermore, as shown in Figure 5(h), if the off-gas oxygen concentration increases after scouring without decreasing as in Figure 5(d), the biofilm 30 peels off without thickening, resulting in a film thinner than the appropriate film thickness M. Note that, as shown in Figure 5(i), the off-gas oxygen concentration tends to increase, which indicates that the biofilm 30 is thinner than the appropriate film thickness M without thickening.

[0052] Thus, in a water treatment device 10 as shown in Figure 5(a) in which an oxygen concentration meter 14 measures an off-gas oxygen concentration equal to or greater than a predetermined amount, the oxygen consumption efficiency in the biofilm 30 can be improved by reducing the frequency of scouring (scouring cleaning strength) and decreasing the off-gas oxygen concentration, as shown in Figure 5(b).

[0053] As described above, according to this embodiment, the cleaning strength of the biofilm 30 is controlled based on the oxygen concentration in the air after passing through the hollow fiber membranes 12. This allows the biofilm 30 formed on the outer surface of the hollow fiber membranes 12 to be maintained at an appropriate thickness M by understanding the increase or decrease in the amount of oxygen consumed by the organisms in the biofilm 30. This makes it possible to provide a water treatment device 10 that can demonstrate high treatment performance that follows load fluctuations in the treated liquid S. Furthermore, because the oxygen transfer efficiency (oxygen consumption efficiency) in the hollow fiber membranes 12 is improved, the membrane area required for the hollow fiber membranes 12 can be minimized, making it possible to reduce the size of the water treatment device 10 and the amount of air supplied to the hollow fiber membranes 12. [Explanation of symbols]

[0054] 10 Water Treatment Equipment 11 Treatment tank 12 Hollow fiber membrane (gas permeable membrane) 13 Aeration pipe (cleaning section) 14 Oxygen concentration meter (measurement unit) 30 Biofilm S Liquid to be treated

Claims

1. a treatment tank to which the liquid to be treated is supplied; a hollow fiber membrane immersed in the liquid to be treated in the treatment tank; a pipe for supplying an oxygen-containing gas into the hollow fiber membrane from one end side of the hollow fiber membrane; a biofilm formed on the outer surface of the hollow fiber membrane and utilizing an oxygen-containing gas supplied into the hollow fiber membrane; Equipped with A water treatment device that treats the liquid to be treated by the biofilm, a cleaning unit located below the hollow fiber membrane and configured to clean the biofilm by discharging a cleaning gas; a measuring unit that measures the oxygen concentration of the gas inside the other end side of the hollow fiber membrane, the gas being supplied from one end side of the hollow fiber membrane and having passed through the hollow fiber membrane in a direction along the membrane surface; Equipped with The cleaning unit controls the cleaning strength against the biofilm based on the oxygen concentration measured by the measuring unit. A water treatment device comprising:

2. The cleaning unit includes: If the oxygen concentration after cleaning the biofilm is reduced compared to the oxygen concentration before cleaning the biofilm, the cleaning intensity for the biofilm is increased; If the oxygen concentration after cleaning the biofilm increases compared to the oxygen concentration before cleaning the biofilm, the cleaning intensity for the biofilm is reduced. The water treatment device according to claim 1 .

3. The cleaning unit controls the cleaning strength of the biofilm by changing at least one of the following requirements: a frequency of ejection of the cleaning gas to the biofilm, an amount of the cleaning gas ejected per unit time to the biofilm, and a time period for ejecting the cleaning gas to the biofilm. The water treatment device according to claim 1 or 2, characterized in that

4. a membrane thickness evaluation device that evaluates the membrane thickness of the biofilm relative to the outer surface of the hollow fiber membrane based on the oxygen concentration measured by the measurement unit; The water treatment device according to any one of claims 1 to 3, characterized in that

5. A method for cleaning a biofilm in a water treatment device comprising: a treatment tank to which a liquid to be treated is supplied; a hollow fiber membrane immersed in the liquid to be treated in the treatment tank; a pipe for supplying an oxygen-containing gas into the hollow fiber membrane from one end side of the hollow fiber membrane; and a biofilm formed on the outer surface of the hollow fiber membrane and utilizing the oxygen-containing gas supplied into the hollow fiber membrane, wherein the water treatment device treats the liquid to be treated with the biofilm, The biofilm is cleaned by discharging a cleaning gas from below the hollow fiber membrane; measuring the oxygen concentration in the gas after it is supplied from one end side of the hollow fiber membrane and passes through the hollow fiber membrane in a direction along the membrane surface, the gas being in the membrane on the other end side of the hollow fiber membrane; and controlling the cleaning intensity for the biofilm based on the measured oxygen concentration. A method for cleaning biofilms in a water treatment device.

6. A gas is supplied from one end side of the hollow fiber membrane and passes through the hollow fiber membrane in a direction along the membrane surface, and the oxygen concentration in the gas inside the other end side membrane of the hollow fiber membrane is measured before and after cleaning of the biofilm, If the measured oxygen concentration after cleaning the biofilm is decreased compared to the oxygen concentration before cleaning the biofilm, the cleaning strength for the biofilm is increased, If the measured oxygen concentration after cleaning the biofilm increases compared to the oxygen concentration before cleaning the biofilm, the cleaning intensity for the biofilm is reduced.

6. The method for cleaning a biofilm in a water treatment device according to claim 5,

7. Controlling the cleaning intensity of the biofilm by changing at least one of the following: the frequency of the cleaning gas being discharged to the biofilm; the amount of the cleaning gas being discharged to the biofilm per unit time; and the duration of the cleaning gas being discharged to the biofilm.

7. The method for cleaning a biofilm in a water treatment device according to claim 5 or 6,

8. A method for evaluating the thickness of a biofilm in a water treatment device, comprising: a treatment tank to which a liquid to be treated is supplied; a hollow fiber membrane immersed in the liquid to be treated in the treatment tank; a pipe for supplying an oxygen-containing gas into the hollow fiber membrane from one end side of the hollow fiber membrane; and a biofilm formed on the outer surface of the hollow fiber membrane and utilizing the oxygen-containing gas supplied into the hollow fiber membrane, wherein the water treatment device treats the liquid to be treated with the biofilm, measuring the oxygen concentration in the gas after it is supplied from one end side of the hollow fiber membrane and passes through the hollow fiber membrane in a direction along the membrane surface, the gas being in the membrane on the other end side of the hollow fiber membrane; If the measured oxygen concentration is lower than the oxygen concentration when the biofilm thickness is appropriate, it is determined that the biofilm thickness is thicker than the appropriate thickness. If the measured oxygen concentration is higher than the oxygen concentration when the biofilm thickness is appropriate, it is determined that the biofilm thickness is thinner than appropriate or that it has grown too thick. A method for evaluating the thickness of a biofilm in a water treatment device.

9. A method for evaluating the thickness of a biofilm in a water treatment device comprising: a treatment tank to which a liquid to be treated is supplied; a hollow fiber membrane immersed in the liquid to be treated in the treatment tank; a pipe for supplying an oxygen-containing gas into the hollow fiber membrane from one end side of the hollow fiber membrane; and a biofilm formed on the outer surface of the hollow fiber membrane and utilizing the oxygen-containing gas supplied into the hollow fiber membrane, wherein the water treatment device treats the liquid to be treated with the biofilm, The biofilm is cleaned by discharging a cleaning gas from below the hollow fiber membrane; The gas is supplied from one end side of the hollow fiber membrane and passes through the hollow fiber membrane in a direction along the membrane surface, and the oxygen concentration in the gas inside the other end side of the hollow fiber membrane is measured before and after cleaning of the biofilm; If the measured oxygen concentration after cleaning the biofilm is lower than the oxygen concentration before cleaning the biofilm, it is determined that the thickness of the biofilm is greater than a thickness suitable for treating the liquid to be treated, If the measured oxygen concentration after cleaning of the biofilm increases compared to the oxygen concentration before cleaning of the biofilm, it is determined that the thickness of the biofilm is thinner than a thickness suitable for treating the liquid to be treated. A method for evaluating the thickness of a biofilm in a water treatment device.

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