JP2009291744A - Membrane filtering method and filtration equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent membrane surface fouling with minimum energy without complicating the structure of a membrane module and increasing the equipment cost, and having a membrane filtration performance stable for a long time. And an apparatus.
SOLUTION: A filtration device 1 in which an external pressure type membrane module 10 is provided for one unit in the longitudinal direction, and water to be treated is passed along the membrane surface of the membrane module from the one end 10a side to the other end. This is a membrane filtration method in a filtration device that performs filtration by a cross flow method that flows to the side 10b and collects filtered water at the other end, and between the one end and the other end, The membrane surface flow velocity Uf of the water to be treated flowing along the membrane surface is larger than the limit membrane surface flow velocity Uf _req equal to the suction flow velocity Up into the pores 11a of the membrane 11, and the distribution of the membrane surface flow velocity in the longitudinal direction is the limit membrane. The membrane surface flow velocity is increased as it approaches the other end side from the one end side so as to be proportional to or approximate to the distribution of the surface flow velocity.
[Selection] Figure 1

Description

  The present invention relates to a membrane filtration method and a filtration device using a case-type filtration device, a tank-type filtration device, or an immersion-type filtration device, and is common to both pressure and suction methods. More specifically, the present invention is a filtration device for removing turbidity and sterilization of raw water such as river water, lake water, groundwater, seawater, domestic wastewater, industrial wastewater, and secondary treated water, or solid-liquid separation of activated sludge water. The present invention relates to a membrane filtration method and a membrane filtration device used in a membrane separation activated sludge device that performs a membrane.

  When using an external pressure type membrane module, a phenomenon called fouling (membrane clogging) occurs in which components such as suspended substances blocked by the membrane are deposited on the membrane surface and block the pores of the membrane. Since this phenomenon essentially reduces membrane filtration performance, it is very important to reduce and prevent fouling.

Fouling occurs when suspended substances or gel-like substances in the water to be treated enter into the pores on the membrane surface. Whether or not the suspended material enters the pores depends on the suction flow velocity Up (proportional to the operating flux) at the pore inlet and the longitudinal direction parallel to the membrane surface caused by the crossflow flow in the water to be treated. It depends on the magnitude relationship with the film surface flow velocity Uf. That is, if Up <Uf, the suspended substance is not sucked into the pores. On the other hand, if Up> Uf, the suspended substance is sucked into the pores and membrane clogging occurs. Here, the film surface velocity Uf is occurring by crossflow stream, a longitudinal flow velocity parallel to the membrane surface has a velocity distribution becomes smaller closer to the film plane, substantially by increasing the cross-flow mean velocity U _CF It tends to increase proportionally. Here, the crossflow average flow velocity _UCF is an average flow velocity calculated by dividing the total crossflow flow rate of the membrane module by the cross-sectional area of the module cross section perpendicular to the flow direction. Does not include the effects of eddy currents.

  Usually, in a filtration membrane, the cross-flow flow is effectively utilized in the longitudinal direction to prevent fouling on the membrane surface, so that the membrane module is long. Further, in order to increase the space utilization efficiency when installing the membrane module, the hollow fiber membrane is made thinner and the flat membrane is made smaller in the distance between the membranes. As a result, the flow path of the permeated water after passing through the membrane becomes long and narrow. For example, the conventional membrane filtration device 100 using the external pressure type hollow fiber membrane module 110 as shown in FIG. In this case, when the treated water side (right side in the figure) is pressurized or suction negative pressure is applied to the water collection part, the pressure inside the hollow fiber increases as the distance from the water collection side (left side in the figure) of the membrane module increases. The loss tends to increase and the permeate flow rate tends to decrease. The same applies to the case of a flat membrane.

  Moreover, in order to generate the cross-flow flow of water to be treated in the membrane module, there are a method using a pump and a method using an air lift action by aeration bubbles. In any of the methods, the energy used for the flow of the cross flow is as small as about 10% in which the energy input for transporting the fluid is used to prevent fouling. In other words, enormous waste of energy is incurred by applying an excessive crossflow flow velocity in a region other than the vicinity of the water collection side (the anti-water collection side or the middle portion in the longitudinal direction).

In Patent Document 1, by dividing the membrane module in the longitudinal direction, the non-uniformity of the permeation flow rate due to pressure loss after permeation of the membrane is reduced, and the substantial difference in flux between the counter-collecting side and the collecting side is reduced. ing. As a result, the difference between the water collecting side and anti collecting necessary water side cross flow mean velocity U _CF also reduced, which reduces the waste of energy by a cross-flow flow. Also in the conventional method as shown in FIG. 8A, if one unit of membrane module is divided into two or more units of membrane modules in the longitudinal direction and the total sum of membrane areas is made equal, the division is shortened. In the membrane module, the non-uniformity of the pressure loss after the membrane permeation is reduced, and the waste of energy is reduced. However, this method can save energy, but there is a problem that the number of membrane modules increases, the structure of the filtration device becomes complicated, and the equipment cost increases.

Patent Document 2 is an example of a spiral membrane element. Similar to Patent Document 1, by dividing the membrane module in the longitudinal direction and simultaneously making the number of inlets of the treated water equal to the number of divisions of the membrane module, the pressure loss when the treated water passes through the membrane element This reduces the longitudinal non-uniformity of the permeation flow velocity due to the water flow, and reduces the substantial difference in flux between the water collection side and the water collection side. The example of Patent Document 2 is not for the purpose of saving energy for cross-flow flow of water to be treated, but for the purpose of saving energy for supply pressure of water to be treated. There is a problem that the number of membrane modules increases, the structure of the filtration device becomes complicated, and the equipment cost increases.
Japanese Patent Laid-Open No. 08-024592 JP 2003-251154 A

  The object of the present invention is to solve the above-mentioned problems of the prior art, and to reduce fouling on the membrane surface (membrane clogging) with the minimum energy without complicating the structure of the filtration device and increasing the equipment cost. An object of the present invention is to provide a membrane filtration method and a filtration device that can be prevented and realize stable membrane filtration performance for a long time.

  The membrane filtration method according to the present invention is a filtration device in which one unit of an external pressure type membrane module is provided in the longitudinal direction, and water to be treated is disposed at one end of the membrane module along the membrane surface of the membrane module. A membrane filtration method in a filtration device that performs filtration by a cross-flow method that flows from the side to the other end side and collects filtered water at the other end side, between the one end and the other end , The membrane surface flow velocity of the water to be treated flowing along the membrane surface of the membrane module is larger than the limit membrane surface velocity equal to the suction flow velocity in the pores of the membrane constituting the membrane module, and the membrane surface velocity in the longitudinal direction is The film surface flow velocity is increased as the distribution approaches the other end side from the one end side so that the distribution is proportional or approximate to the distribution of the critical film surface flow velocity in the longitudinal direction.

  Further, in the filtration device according to the present invention, an external pressure type membrane module is provided for one unit in the longitudinal direction, and water to be treated is discharged from one end side of the membrane module along the membrane surface of the membrane module. A filtration device that performs filtration by a cross-flow method that flows to an end side and collects filtered water at the other end side, wherein the membrane surface flow velocity of the water to be treated that flows along the membrane surface of the membrane module is Means for increasing the speed as approaching the other end side from the part side, and the means for increasing the speed of the membrane between the one end part and the other end part is the pores of the membrane constituting the membrane module. It approaches the other end side from the one end side so that the distribution of the membrane surface flow velocity in the longitudinal direction is proportional to or approximates the distribution of the critical membrane surface flow velocity in the longitudinal direction. Increase the membrane surface velocity according to It is characterized in.

  The size of the critical membrane surface flow velocity is equal to the suction flow velocity in the membrane pores, so fouling (membrane clogging) can be reduced or prevented if the membrane surface flow velocity is larger than the critical membrane surface flow velocity. is there. In the membrane filtration method and the filtration device, the membrane surface flow rate of the water to be treated is increased as it approaches the other end side from the one end side of the membrane module. Since it is made larger, fouling (film clogging) on the film surface in the longitudinal direction can be prevented. In the membrane filtration method and the filtration apparatus, the membrane surface flow velocity distribution in the longitudinal direction is proportional to or approximates the distribution of the critical membrane surface flow velocity, so that fouling (membrane clogging) on the membrane surface is the minimum necessary. It can be prevented with limited energy, and stable membrane filtration performance can be realized for a long time. Further, in the filtration apparatus, the membrane module is provided for one unit in the longitudinal direction, so that the structure of the filtration apparatus to which the membrane filtration method is applied is not complicated and the equipment cost is not increased.

  In the membrane filtration method according to the present invention, the membrane surface flow velocity is increased by supplying the treated water from a plurality of regions in the longitudinal direction of the membrane module and increasing the cross flow average flow velocity of the treated water. Is preferred. Similarly, the filtration device according to the present invention has a case for housing the membrane module, and the case has a plurality of inlets of water to be treated between one end and the other end of the membrane module. The speed increasing means is preferably an inlet other than the inlet closest to the one end of the plurality of inlets.

  The membrane filtration method according to the present invention includes a water tank in which water to be treated is stored, and one end of the membrane module is located on the bottom wall side of the water tank, and the longitudinal direction of the membrane module is the bottom wall. The membrane module is arranged in the water tank so as to be in a direction substantially perpendicular to the water tank, and by generating an aeration air from the bottom wall side, the water to be treated flows from one end side to the other end side of the membrane module. In the case of a membrane filtration method in a filtration apparatus that performs filtration, the membrane surface flow rate is increased by generating an aeration air that is different from the above-mentioned aeration air between one end and the other end of the membrane module. Is preferable.

  Similarly, in the filtration device according to the present invention, the membrane module is disposed inside, and is provided for the water tank for storing the water to be treated and the membrane module, and is disposed in the water tank and stored in the water tank. A plurality of air diffusers for generating aeration air in the treated water, and the membrane module has one end located on the bottom wall side of the water tank and the other end on the water surface side of the treated water The plurality of air diffusers are disposed in the longitudinal direction of the membrane module with respect to the membrane module, and the means for increasing the speed is a plurality of air diffusers. A diffuser other than the diffuser closest to the bottom wall is preferred.

  According to the present invention, fouling to the membrane surface can be efficiently prevented with minimum energy in the entire longitudinal direction of the membrane module. For this reason, according to the present invention, the permeation flux of the membrane module can be stably maintained for a long time with the minimum energy, and the membrane filtration efficiency is greatly improved.

  The following embodiment is an example for explaining the present invention, and is not intended to limit the present invention only to this embodiment. The present invention can be implemented in various forms without departing from the gist thereof. Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is an explanatory diagram of a membrane filtration method according to an embodiment of the present invention. FIG. 1 shows the case of a case-type filtration device 1 configured by a case 20 in which a long membrane module 10 configured by bundling a plurality of membranes 11 and bonding and fixing both ends thereof is accommodated in a case 20. An example of the membrane 11 is a hollow fiber membrane. The hatched portions in FIG. 1 are end portions 10 a and 10 b that are bonded and fixed in the membrane module 10. In addition, although not shown in figure, the edge part 10b is comprised so that filtration treatment water can flow out. 1 (a), 1 (b), and 1 (c) schematically show various embodiments of the filtration device 1. Referred in the following description, FIG. 1 (a), FIG. 1 (b), the filtering device 1 shown in FIG. 1 (c), for convenience of explanation, the filtration device _{1 1,} a filtration device _{1 2,} also the filtration device _{1 3} In addition, the case 20 included in each filtering device may also be referred to as cases 20 ₁ , 20 ₂ , and 20 ₃ .

  The filtration device 1 uses an external pressure type membrane module 10, and membranes the water to be treated from one end (one end) 10 a side to the other end (other end) 10 b side of the membrane module 10. The water to be treated is treated by a so-called cross-flow method that flows along the membrane surface of the module 10 and collects the filtered water at the other end 10b side. And the to-be-processed water which flowed to the other end part 10b side is discharged | emitted from the to-be-processed water outlet 21 formed in the case 20, and the collected filtered water is discharged | emitted from the filtered-process water outlet 22 of the case 20. It has come to be. Thus, since the to-be-processed water is collected by the other end part 10b side of the membrane module 10, in this specification, the other end part 10b side of the membrane module 10 is also called the water collection side end part 10b, and one end part The 10a side is also referred to as a counter water collecting side end 10a.

  When using an external pressure type membrane module such as the membrane module 10, fouling (membrane clogging) in which components such as suspended substances blocked by the membrane are deposited on the membrane surface and block the pores of the membrane. The phenomenon called is generated. Since this phenomenon essentially reduces membrane filtration performance, it is very important to reduce and prevent fouling.

であれば、懸濁物質Ｍは細孔１１ａに吸い込まれない。逆に、

であれば、懸濁物質Ｍは細孔１１ａに吸い込まれて膜詰まりが生じる。ここで、膜面流速Ｕｆは、クロスフロー流によって生起され、膜モジュール１０を構成する膜１１の膜面に平行な長手方向の流速である。膜面流速Ｕｆは、いわゆる膜面近傍流速とすることができる。なお、懸濁物質Ｍの大きさが細孔１１ａより充分に大きければファウリング（＝細孔内への侵入）の恐れは無く、また、充分に小さければ細孔１１ａ内で捕捉されずに透過する。従って、ファウリングの防止対象となる懸濁物質Ｍの大きさは、細孔構造によって異なるが通常は細孔１１ａの直径以下から直径の２分の１程度以上である。例えば、細孔１１ａの平均孔径が０．１μｍの場合には、ファウリングの防止対象となる懸濁物質Ｍの大きさは０．０５〜０．１μｍ程度である。このとき、膜面近傍流速とは、膜面から懸濁物質Ｍの大きさと同程度の寸法、即ち、膜面から距離０．０５〜０．１μｍの範囲内での流速とすることができる。膜面流速Ｕｆは、クロスフロー平均流速Ｕ_ＣＦを大きくするとほぼ比例的に大きくなる傾向をもつ。つまり、次式が成り立つ。

ここで、クロスフロー平均流速Ｕ_ＣＦとは、膜モジュール１０のクロスフロー総流量を、流れ方向に垂直なモジュール断面の断面積で割って計算される平均流速のことであり、膜面近傍の局所的な渦流等の影響は含んでいない。 Here, the principle of occurrence of fouling and the principle of reducing / preventing fouling will be described with reference to FIGS. For simplification of description, in the description of FIGS. 2 and 3, components corresponding to the components of the membrane module 10 illustrated in FIG. In FIG. 3, fouling occurs when the suspended substance M or the gel-like substance in the water to be treated enters the pores 11 a on the surface of the membrane 11. Whether or not the suspended matter M enters the pores 11a is parallel to the suction flow velocity Up (proportional to the operating flux) at the inlet of the pores 11a and the membrane surface caused by the crossflow flow in the water to be treated. It depends on the magnitude relation with the film surface flow velocity Uf in the longitudinal direction. That means

If so, the suspended matter M is not sucked into the pores 11a. vice versa,

If so, the suspended matter M is sucked into the pores 11a, resulting in membrane clogging. Here, the membrane surface flow velocity Uf is a longitudinal flow velocity that is generated by the crossflow flow and is parallel to the membrane surface of the membrane 11 constituting the membrane module 10. The membrane surface flow velocity Uf can be a so-called membrane surface vicinity flow velocity. If the suspended substance M is sufficiently larger than the pores 11a, there is no fear of fouling (= intrusion into the pores). To do. Therefore, the size of the suspended matter M to be prevented from fouling varies depending on the pore structure, but is usually from the diameter of the pore 11a to about one-half of the diameter. For example, when the average pore diameter of the pores 11a is 0.1 μm, the size of the suspended matter M to be prevented from fouling is about 0.05 to 0.1 μm. At this time, the flow velocity in the vicinity of the membrane surface can be the same size as the suspended matter M from the membrane surface, that is, a flow velocity within a range of 0.05 to 0.1 μm from the membrane surface. Film surface velocity Uf has almost proportionally larger tendency by increasing the cross-flow mean velocity U _CF. That is, the following equation holds.

Here, the cross flow average flow velocity _UCF is an average flow velocity calculated by dividing the cross flow total flow rate of the membrane module 10 by the cross-sectional area of the module cross section perpendicular to the flow direction. Does not include the effects of typical eddy currents.

上記のような濾過膜１１においては、クロスフロー流を長手方向に有効に活用して膜面でのファウリングを防止するようにしているので、膜モジュール１０を長尺にしている。また、膜モジュール１０を設置する際の空間利用効率を上げるために、中空糸膜では糸を細く、また、平膜では膜間の距離を狭くしている。この結果、膜を透過した後の透過水の流路は長くかつ狭くなってしまい、例えば、図８（ａ）に示すような外圧型の中空糸膜モジュール１１０を利用した従来型の濾過装置１００の場合、被処理水側（図中、右側）を加圧するか、或いは集水部に吸引負圧を与えると、膜モジュール１１０の集水側（図中、左側）から遠ざかるほど、中空糸内部の圧力損失が大きくなって透過流速が減少する傾向がある。平膜の場合にも同様である。 Uf _req in the following equation (4) is the critical value of Uf in equations (1) and (2), and gives the limit value of the membrane surface flow velocity Uf in the longitudinal direction necessary for preventing fouling. If the membrane surface flow velocity Uf in the longitudinal direction exceeds the critical membrane surface flow velocity Uf _req over the entire membrane surface, that is, if the equation (5) is satisfied, the suction flow velocity Up at the inlet of the pore 11a will also exceed. Therefore, as a result, the suspended substance M can be prevented from entering the pores 11a, that is, fouling.

In the filtration membrane 11 as described above, the cross-flow flow is effectively utilized in the longitudinal direction to prevent fouling on the membrane surface, so that the membrane module 10 is elongated. Further, in order to increase the space utilization efficiency when installing the membrane module 10, the hollow fiber membrane is made thinner and the flat membrane is made smaller in the distance between the membranes. As a result, the flow path of the permeated water after passing through the membrane becomes long and narrow. For example, a conventional filtration device 100 using an external pressure type hollow fiber membrane module 110 as shown in FIG. In this case, when the treated water side (the right side in the figure) is pressurized or a suction negative pressure is applied to the water collecting part, the inside of the hollow fiber becomes farther away from the water collecting side (the left side in the figure) of the membrane module 110. The pressure loss increases and the permeation flow rate tends to decrease. The same applies to the case of a flat membrane.

Here, the results will be described that estimate the substantial flux F _R catchment side and the anti-water collecting side using membrane filtration model shown in FIG. 3 (a). As shown in FIG. 3 (a), the membrane 11 will be described here as a hollow fiber membrane. The closed side of the hollow fiber membrane 11 is the anti-water collecting side, and the open side corresponds to the water collecting side. FIG. 3A shows a state in which a filtration operation is performed. In the membrane 11 shown in FIG. 3A, the hollow fiber membrane is divided into N sections (N is an integer of 2 or more) in the longitudinal direction, and the i-th section (i is an integer from 1 to N). The pressure in the yarn at P (i) is expressed as P (i), the flow rate in the yarn as Q (i), and the flow velocity in the yarn as U (i). Further, the transmembrane pressure difference (= pressure loss of the pore 11a) in the i-th section is Pp (i), the total flow rate in the i-th section is Qp (i), and the suction flow velocity of the pore 11a is Up (i). The pressure outside the yarn is represented by Ph (i), the filtration pressure at the end of the water collecting side is represented by Po, the yarn outer diameter is represented by Df _out , the yarn inner diameter is represented by Df _in , and the yarn length of the section is represented by δL. Further, in FIG. 3A, for explaining the calculation model, the pores are schematically shown as linear pores, but the actual membrane pores have a maze structure like a sponge, It is not a simple linear hole as shown in FIG. Therefore, when the pore length was L _pore, pore length L _pore do not necessarily agree to the difference between the outer diameter and the inner diameter of the thread.

なお、ｉ＝１の場合は、集水側端部の位置での糸外部の圧力をＰｈ（０）として、
Ｐｏ−ｋ１・Ｕ（１）＋（Ｐｈ（１）−Ｐｈ（０））＝Ｐ（１）
を採用する。 The pressure balance in the yarn is expressed by the following equation (6).

In addition, in the case of i = 1, the pressure outside the yarn at the position of the water collection side end is Ph (0),
Po−k1 · U (1) + (Ph (1) −Ph (0)) = P (1)
Is adopted.

なお、ｉ＝Ｎの場合、
Ｑ（Ｎ）−Ｑｐ（Ｎ）＝０
とする。 The flow rate balance in the yarn is expressed by the following equation (7).

When i = N,
Q (N) -Qp (N) = 0
And

そして、糸内外の圧力と膜間差圧の関係は、次の（９）式で表される。

膜間差圧（細孔の圧力損失）は、次の（１０）式で表される。

また、細孔内の流速と流量の関係は、次の（１１）式で表される。

ｎ_ｐｏｒｅは、１ｍ当たりの細孔数である。この値は、直接的に測定することができないので、測定可能な表面開孔率ηと細孔径Ｄｐを使って、下式により推定する。

Furthermore, the relationship between the flow velocity in the yarn and the flow rate is expressed by the following equation (8).

The relationship between the pressure inside and outside the yarn and the transmembrane pressure is expressed by the following equation (9).

The transmembrane pressure difference (pressure loss of the pores) is expressed by the following equation (10).

The relationship between the flow velocity in the pores and the flow rate is expressed by the following equation (11).

n _pore is the number of pores per meter. Since this value cannot be measured directly, it is estimated by the following equation using the measurable surface opening ratio η and the pore diameter Dp.

The substantial flux F _R (i) in the i-th section is proportional to Up (i) and is represented by the following formula.

For example, in the calculation model shown in FIG.
Membrane length Lf of hollow fiber membrane: 2 m
Yarn outer diameter Df _out : 1.2φmm
Yarn inner diameter Df _in : 0.7φmm
Pore diameter Dp: 0.1 μm
Pore length L _pore : 0.3 mm
Surface open area ratio η: 35%
Number of pores per 1 meter n _pore : 1.7 × 10 ¹¹ / m
The number of section divisions N: 100. Under the condition that the water to be treated having a viscosity μ of 0.01 Pa · sec was operated at an average flux F _AV of 0.5 m ³ / m ² / d, the simultaneous equations of the equations (6) to (11) solving seeking up (i), by (13), substantially the flux _F try to estimate the _R, water collecting end is 0.88m ³ / ^m 2 / d at the maximum, the anti-water collecting side The edge was found to be a minimum of 0.33 m ³ / m ² / d. Under the above conditions, it shows a longitudinal detailed distribution of substantial flux F _R (the calculation results) in FIG. 3 (b).

  In order to prevent fouling over the entire film surface in the longitudinal direction, the film surface flow velocity Uf must be generated so that the above equation (5) is established over the entire region.

このことは、限界膜面流速Ｕｆ_ｒｅｑの長手方向の分布が、実質的なフラックスＦ_Ｒの分布（図３（ｂ））と同じ傾向になることを示している。実質的なフラックスＦ_Ｒが最も大きくなる集水側端部（の膜表面）では、細孔入口における吸い込み流速Ｕｐも最も大きくなるので、限界膜面流速Ｕｆ_ｒｅｑも長手方向で最大となる。また、Ｕｆ_ｒｅｑは、反集水側へ向かって集水側から遠ざかるに連れて、図３（ｂ）に示す実質的なフラックスＦ_Ｒと同じ傾向で小さくなっていく。実質的なフラックスＦ_Ｒの分布と限界膜面流速Ｕｆ_ｒｅｑの関係を図４（ａ），（ｂ）に示す。 By the way, the equation (13) shows the relationship between the substantial flux distribution F _R (i) and Up (i) in the i-th section, but when generalized, it is expressed as F _R = K · Up. be able to. Then, considering the critical condition (4), the following formula is established.

This longitudinal distribution limit film surface velocity Uf _req have shown that the same tendency as the distribution of the substantial flux F _R (Figure 3 (b)). In substantial flux F _R is most larger water collecting end (membrane surface), since the flow rate Up also maximized suction in the pore inlet, a maximum limit film surface velocity Uf _req also longitudinally. Moreover, Uf _req is, as the distance from the water collecting side to the counter-current water side becomes smaller at the same tendency as the substantial flux F _R as shown in FIG. 3 (b). Substantial flux _{F R} distribution and related limitations film surface velocity _{Uf req} FIG. 4 (a), the shown (b).

FIGS. 4 (a) shows a longitudinal distribution of substantial flux F _R, from the equation (14) in relation distribution limit film surface velocity Uf _req necessary anti-fouling also substantially since the same trend as the flux _{F R,} as shown in FIG. 4 _{(b), Uf req} is maximized location water collecting end (outlet vicinity), and _conversely, where _{the Uf req} is minimum Is the end of the water collection side (near the inlet). In order to prevent the membrane surface flow velocity Uf in the longitudinal direction from becoming excessive over the entire membrane surface, it is only necessary to give Uf having the same distribution as the curve of Uf _req in FIG. That is, by giving a Uf (longitudinal membrane surface flow velocity) larger than the Uf _req curve in FIG. 4 (b) in the entire membrane from the water collection side end to the counter water collection side end, the fau with minimum energy is obtained. Ring can be prevented. Alternatively, as shown in the relationship of the expression (3), since the cross flow average flow velocity U _CF is proportional to Uf, the distribution of the cross flow average flow velocity U _{CF in} the longitudinal direction is substantially (in the longitudinal direction of the membrane). be determined as proportional or to approximate specific flux distribution F _R, no waste in Uf of this time, it is possible to prevent the fouling with minimal energy.

FIG. 8A is a schematic diagram of an example of a conventional case-type filtration device 100 having an external pressure type hollow fiber membrane module as described above. The filtration device 100 is configured such that a hollow fiber membrane module 110 is accommodated in a case 120, treated water flows from a treated water inlet 123 on the anti-collection side, and the filtered treated water is filtered. The water to be treated flows out from the outflow port 121, and the treated water flows out from the outflow port 121. Ignoring the flow q passing through the hollow fiber membranes 111 constituting the hollow fiber membrane module 110 believes small, crossflow average flow velocity U _CF and the film surface velocity Uf in membrane module 110 is constant in the longitudinal direction. FIG. 9 shows the relationship between the membrane surface flow velocity Uf in the longitudinal direction and Uf _req necessary for preventing fouling in the conventional example of FIG. The membrane surface flow velocity Uf in the longitudinal direction becomes a constant value over the entire membrane surface, and this value needs to be the maximum value over the entire membrane surface, that is, Uf _OUT-req , which is Uf required at the water collection side end (near the outlet). Therefore, as shown by hatching in FIG. 9, Uf becomes significantly excessive over the entire film surface, and energy is wasted.

であるが、一般的に、ｑはＱに対して微少なので、ここでは、

と考える。ここで注目すべきは、図８（ａ）のような膜モジュール１１０を利用した従来の濾過装置１００では、膜面に沿って長手方向のクロスフロー平均流速Ｕ_ＣＦが、反集水側（又は流入口１２３）近傍から集水側（又は流出口１２１）近傍まで一定になっているか、若しくは、中空糸膜１１１を透過した流量ｑの分だけ遅くなっている点である。前述の通り、限界膜面流速Ｕｆ_ｒｅｑの分布は、膜面に沿って長手方向の分布が実質的なフラックスＦ_Ｒの分布（図４（ａ），図４（ｂ）参照）と同じなので、吸い込み流速Ｕｐが最も大きい集水側端部において最大値が必要とされ、集水側から遠ざかるに連れて、図４（ｂ）のように、実質的なフラックスＦ_Ｒと同じ傾向で小さくなる。従って、図８（ａ）のような従来方式のように、クロスフロー平均流速Ｕ_ＣＦを長手方向に一定にする場合には、膜面流速Ｕｆは、集水側端部のＵｆ_ｒｅｑを基準に決まってしまうので、集水側端部以外の（反集水側や長手方向の中間）部分ではＵｆが過剰になってしまうことになる。この様子が図９に示されており、集水側端部から反集水側端部まで、Ｕｆの過剰分が徐々に増加していることが判る。 This will be described more specifically. In FIG. 8A, the water to be treated flows in at a flow rate Q from the inlet 123 on the counter water collection side, and flows out from the outlet 121 on the water collection side at a flow rate Q ′. Since the outflow flow rate Q ′ decreases by the flow rate q that has passed through the hollow fiber membrane 111, strictly speaking,

However, in general, q is very small compared to Q, so here

I think. Here it should be noted, in the conventional filtration device 100 a membrane module 110 utilizes as in FIG. 8 (a), a cross-flow mean velocity U _CF longitudinally along the film surface, anti-water collecting side (or The point is that it is constant from the vicinity of the inflow port 123) to the vicinity of the water collection side (or the outflow port 121), or is delayed by the flow rate q that has passed through the hollow fiber membrane 111. As described above, the distribution of marginal film surface velocity Uf _req, the longitudinal distribution is the distribution of the substantial flux F _R along the film surface (FIG. 4 (a), the see FIG. 4 (b)) since it is the same as, suction flow rate up is maximum required in the largest water collecting end, as it gets farther away from the water collecting side, as shown in FIG. 4 (b), the smaller the same trend as the substantial flux F _R. Therefore, as in the conventional method, such as in FIG. 8 (a), in the case of a constant cross-flow mean velocity U _CF longitudinally, the film surface velocity Uf is based on the Uf _req water collecting end Since it is determined, Uf becomes excessive in the portion other than the water collection side end portion (on the side opposite to the water collection side or in the longitudinal direction). This state is shown in FIG. 9, and it can be seen that the excess amount of Uf gradually increases from the water collection side end to the counter water collection side end.

For example, in the example of FIG. 3B, the excess of Uf is estimated using numerical values. In FIG. 3 (b), about 2.7 times substantial flux _{F R} of the water collecting side end with respect to _{F R} of counter-current water-side end portion (0.88 ÷ 0.33) so also large, From the relationship between FIGS. 4A and 4B, the critical membrane surface flow velocity Uf _req at the water collection side end is also about 2.7 times larger than the Uf _req required at the counter water collection side end. That is, in the conventional method as shown in FIG. 8 (a), the necessary Uf _req is given constant at the end of the water collection side over the entire longitudinal direction. As a result, the Uf that is 1.7 times more excessive on the non-water collection side. Will be given. This ratio depends on the outer diameter, inner diameter, membrane length, aperture ratio, etc. of the hollow fiber, and does not always become a constant value.

In contrast, FIG. 1, the filtration device 1 ₁ according to an embodiment of the filtration device 1 shown in (a), the treatment water located at one end portion 10a of the case 20 ₁ has membrane module 10 In addition to the first inlet 23, a second inlet (speed increasing means) 24 is further provided between the first inlet 23 and the other end 10b side of the membrane module 10, and the one end 10a and the other end The film surface flow velocity Uf is made larger than the limit film surface flow velocity Uf _req with respect to the portion 10b, and the distribution of the film surface flow velocity Uf in the longitudinal direction is approximated to the distribution of the limit film surface flow velocity Uf _req .

That is, the first inlet 23, flowing a flow rate of the intermediate portion A second outlet 24 is located in the filtering device 1 ₁ in the longitudinal direction only generates Uf _A-req necessary anti-fouling. Furthermore, from the second inflow port 24, only the Uf _OUT-req necessary for the water collection side end portion 10b (or the vicinity of the outflow port 21) is generated by the sum of the treated water flowing in from the first inflow port 23. Inflow of additional flow. FIG. 5A shows the relationship between the longitudinal film surface flow velocity Uf generated as a result and Uf _req necessary for preventing fouling.

As shown in FIG. 5 (a), in the embodiment shown in FIG. 1 (a), a membrane surface is formed between the counter water collecting side end (one end) 10a and the water collecting side end (other end) 10b. The flow velocity Uf is greater than the critical membrane surface flow velocity Uf _req , and the distribution of the membrane surface flow velocity Uf is approximated to the distribution of the critical membrane surface flow velocity Uf _req . In other words, the membrane surface flow velocity Uf is increased stepwise as it approaches the water collection side end portion 10b according to the distribution of the critical membrane surface flow velocity Uf _req . 5 (a) and FIG. 9 are compared, the difference between the distribution of the membrane surface flow velocity Uf and the distribution of the critical membrane surface flow velocity Uf _req (the portion indicated by hatching) is smaller than that shown in FIG. Thus, it can be seen that an excessive amount of Uf can be saved while preventing fouling in the section from the intermediate portion A to the counter water collecting side end portion 10a. As described above, the film surface velocity Uf is proportional to the cross flow rate U _CF, adjusting the amount of treated water to flow from the second inlet 24 to be accelerated to the film surface velocity Uf as above This can be achieved. For example, since the limit film surface velocity Uf _req substantial flux F _R has the (14) equation relationship, it is sufficient to increase the cross flow rate U _CF in accordance with the substantial flux distribution in the longitudinal direction.

Next, an embodiment in the filtration device 1 ₂ is, the first flow of water to be treated which is located at one end 10a side of the case 20 ₂ has membrane module 10 of the filtration device 1 shown in FIG. 1 (b) In addition to the inlet 23, a second inlet 24 and a third inlet 25 are further provided between the first inlet 23 and the other end portion 10b side of the membrane module 10, according to the embodiment of FIG. Also, the distribution of the film surface flow velocity Uf in the longitudinal direction is approximated to the distribution of the limit film surface flow velocity Uf _req .

That is, the first inlet 23, flowing a flow rate of the intermediate portion B where the second inlet 24 is located in the filtering device 1 ₂ in the longitudinal direction by generating a Uf _B-req necessary anti-fouling. Further, from the second inlet 24, the sum of the for-treatment water which has flowed from the first inlet 23, the anti-fouling in intermediate portion C of the third inlet 25 is located in the filtering device 1 ₂ in the longitudinal direction An additional flow is added to generate the required Uf _C-req . Furthermore, from the 3rd inlet 25 (intermediate part C), the water collection side edge part 10b (or by the sum total of the to-be-processed water which flowed in from the 1st inlet 23 and the 2nd inlet 24 (intermediate part B) The flow rate is generated so as to generate the necessary Uf _OUT-req near the outlet 21). FIG. 5B shows the relationship between the longitudinal film surface flow velocity Uf generated as a result and Uf _req necessary for preventing fouling.

As shown in FIG. 5 (b), in the embodiment shown in FIG. 1 (b) as well, the membrane surface between the counter water collecting side end (one end) 10a and the water collecting side end (other end) 10b. The flow velocity Uf is greater than the critical membrane surface flow velocity Uf _req , and the distribution of the membrane surface flow velocity Uf is approximated to the distribution of the critical membrane surface flow velocity Uf _req . In other words, the membrane surface flow velocity Uf is increased stepwise as it approaches the water collection side end portion 10b according to the distribution of the critical membrane surface flow velocity Uf _req . When comparing FIG. 5B and FIG. 9, the difference between the distribution of the membrane surface flow velocity Uf and the distribution of the critical membrane surface flow velocity Uf _req (the portion indicated by hatching) is smaller than that shown in FIG. Thus, it can be seen that an excessive amount of Uf can be saved while preventing fouling in the section from the intermediate portion C to the counter water collecting side end portion 10a. As described above, the film surface velocity Uf is proportional to the cross flow rate U _CF, the the membrane surface velocity Uf To accelerated as described above to flow from the second inlet 24 and the third inlet 25 This can be achieved by adjusting the amount of treated water. For example, since the limit film surface velocity Uf _req substantial flux F _R has the (14) equation relationship, it is sufficient to increase the cross flow rate U _CF in accordance with the substantial flux distribution in the longitudinal direction.

Next, in one embodiment a is filtration device 1 ₃ of the filter 1, the first inlet 23 for introducing the water to be treated to one end 10a side of the membrane module 10 is provided as shown in FIG. 1 (c) and which, by using the filtration water outlet 22, the case 20 ₃ in which the processing water outlet 21 for discharging the treated water is formed for discharging the filtered treated water into the other end portion 10b side of the membrane module 10 ing. In the filtration device 1 _3, without additional inflow of water to be treated from an intermediate portion of the membrane module 10, through the inlet 23 (or counter water collecting end 10a) to the outlet 21 (or the water collecting end 10b) the cross-sectional area (inner diameter of the membrane module 10) by continuously changing at intervals, is accelerated cross-flow mean velocity U _CF, a result, the anti-water collecting end (one end) 10a and the water collecting side The membrane surface flow velocity Uf between the end portion (the other end portion) 10b is made larger than the limit membrane surface flow velocity Uf _req , and the distribution of the membrane surface flow velocity Uf in the longitudinal direction is made closer to the distribution of Uf _req necessary for preventing fouling. Is the method. As shown in FIG. 1 (c), changes in the cross-sectional area can be achieved by changing the inner diameter of the case 20 _3. Therefore, so that the case 20 ₃ is functioning as a means of accelerated the film surface velocity Uf.

Determine the cross-sectional area S _OUT (the cross-sectional area at the position of the II _out -II _out line in FIG. 1C) from the Uf _OUT-req required at the water collection side end 10b. In addition, the cross-sectional area S _IN from the Uf _IN-req required at the counter water collecting side end portion 10a to the counter water collecting side end portion 10a side (disconnection at the position of the line I _IN -I _IN in FIG. 1C) Area). In the embodiment of FIG. 1 (c), the intermediate part in the longitudinal direction shows an example in which these two places are linearly interpolated. Thereby, the film surface flow velocity Uf in the longitudinal direction changes in a quadratic function. FIG. 5C shows the relationship between the longitudinal film surface flow velocity Uf generated as a result and Uf _req necessary for preventing fouling.

As shown in FIG.5 (c), also in the Example shown in FIG.1 (c), it is a film | membrane surface between the counter water collecting side edge part (one end part) 10a and the water collecting side edge part (other end part) 10b. The flow velocity Uf is greater than the critical membrane surface flow velocity Uf _req , and the distribution of the membrane surface flow velocity Uf is approximated to the distribution of the critical membrane surface flow velocity Uf _req . In other words, the membrane surface flow velocity Uf is continuously increased according to the distribution of the critical membrane surface flow velocity Uf _req as it approaches the water collection side end portion 10b. When comparing FIG. 5C and FIG. 9, the difference between the distribution of the membrane surface flow velocity Uf and the distribution of the critical membrane surface flow velocity Uf _req (the portion indicated by hatching) is smaller than that shown in FIG. It can be seen that an excessive amount of Uf can be saved while preventing fouling over the entire film surface. In the present embodiment, the cross-sectional area from the water collecting side to the water collecting side is continuously changed, but it may be changed (discontinuously) on the stairs.

As described above, in the filtration device 1 exemplified as the filtration devices 1 ₁ , 1 ₂ , 1 ₃ , fouling to the membrane surface is efficiently prevented with a minimum energy in the entire longitudinal direction of the membrane module 10. Can do. From the viewpoint of efficiently reducing and preventing fouling, the membrane surface flow velocity Uf is preferably 1.5 times or more in the vicinity of the water collection side with respect to the vicinity of the counter water collection side. Further, in the filtration device 1, the membrane module 10 need only be provided for one unit in the longitudinal direction. Therefore, the structure of the filtration device 1 is simple and does not increase the equipment cost.

  The embodiment shown in FIG. 1 is a description of a case-type filtration device, but next, an embodiment of an immersion-type filtration device used in a membrane separation activated sludge device or the like will be described. In addition, the same code | symbol is attached | subjected and demonstrated to the element corresponding to the case of the Example shown in FIG.

  FIG. 6 is a schematic diagram illustrating an example of an immersion type filtration apparatus. The filtration device 2 has a water tank 30 for storing water to be treated. In the water tank 30, a plurality of membrane modules 10 and 10 are arranged such that one end portion 10a side of the membrane module 10 is located on the bottom wall 31 side of the water tank 30. Are arranged in parallel. FIG. 6 shows two membrane modules 10 and 10 as an example. A head 40 for allowing the membrane filtration water to flow out is attached to the other end portion 10b side of each membrane module 10 to constitute a membrane cartridge. In this configuration, one unit of the membrane module 10 is provided in the longitudinal direction of the membrane module 10, and a plurality of membrane modules 10 are provided in the direction orthogonal to the longitudinal direction. Further, as shown in FIG. 6, a baffle plate 50 is provided for the plurality of membrane modules 10, and a first air diffuser 61 is disposed on the one end portion 10 a side of the membrane module 10. The membrane filtration process by the cross flow method is possible by the swirl flow formed by the aeration air jetted from the first air diffuser 61. Further, an intermediate inlet 51 of the water to be treated is formed in the middle part of the baffle plate 50, and a second air diffuser 62 is disposed in the intermediate inlet 51.

  Here, with reference to FIG.9 (b), the conventional immersion type filtration apparatus 130 is demonstrated. FIG. 9 (b) is an example of a conventional immersion type filtration device 130 using the hollow fiber membrane module 110. The configuration of the filtering device 130 is different from the filtering device 2 in that an intermediate inlet is not formed in the baffle plate and no second air diffuser is provided.

The filtration device 130 performs filtration by suction from the water collection side (the head 40 side). The behavior of the filtrate after permeating through the membrane is the same as that of the case-type filtration device 100 of FIG. Exactly the same, the further away from the water collection side of the membrane module 110, the greater the pressure loss inside the hollow fiber, and the permeation flow rate tends to decrease. That is, the distribution of the substantial flux F _R is as shown in FIG. 3 (b) or FIG. 4 (a), the so also the distribution of the film surface velocity Uf required fouling in Figure 4 (b).

In addition, due to the swirl flow formed by the aeration air jetted from the first air diffuser 61 at the lower part of the membrane module, the water to be treated flows in from the lower part on the anti-collection side and flows upward along the membrane surface. A surface velocity is generated. Since the baffle plate 140 is provided on the side surface so that a swirl flow can be efficiently formed for several membrane module 110 groups, the inflow of water to be treated from the middle portion in the longitudinal direction of the membrane module 110 No. In other words, when the total amount of water to be treated supplied to the module group from the lower part of the module (opposite water collection side) is Q, the swirl flow is returned downward from the baffle plate 140 from the upper part of the module (collection side). that the total amount of the treated water is also Q becomes, in the conventional example of FIG. 8 (b), the cross-flow mean velocity U _CF is equal in membrane module lower (counter current water side) and the upper (collecting side). Accordingly, the relationship between the membrane surface flow velocity Uf in the longitudinal direction and Uf _req necessary for preventing fouling is the same as that of the conventional case type in FIG. 8A, and is as shown in FIG. As in the case-type conventional example, the membrane surface flow velocity Uf in the longitudinal direction is a constant value across the membrane surface from the bottom to the top of the membrane module, and this value is the maximum value across the membrane surface, that is, required at the upper water collection side end. Since Uf _OUT-req , which is a large Uf, is required, Uf becomes excessively large over the entire film surface, resulting in a large waste of energy.

On the other hand, in the embodiment of FIG. 6, in addition to the first air diffuser 61 at the lower part of the membrane module, the second air diffuser 62 at the intermediate part in the longitudinal direction and the swirl flow above the intermediate part are amplified. For this purpose, an intermediate inlet 51 serving as a treated water inlet is provided in the middle of the baffle plate 50. From the lower part of the membrane module, a total amount Q of water to be treated is generated so as to generate Uf _D-req necessary for preventing fouling in the intermediate part D where the intermediate inlet 51 is located in the longitudinal direction. Furthermore, from the intermediate inflow port 51, a flow rate sufficient to generate a necessary Uf _OUT-req at the water collection side end 10b (upper part of the membrane module) is caused to flow in by the sum of the treated water flowing in from the lower part. FIG. 7 shows the relationship between the longitudinal film surface flow velocity Uf generated as a result and Uf _req necessary for preventing fouling.

As shown in FIG. 7, even in the embodiment shown in FIG. 6, the membrane surface flow velocity Uf is between the counter water collecting side end (one end) 10a and the water collecting side end (other end) 10b. The distribution of the membrane flow velocity Uf is larger than the flow velocity Uf _req and approximated to the distribution of the critical membrane flow velocity Uf _req . In other words, the membrane surface flow velocity Uf is increased stepwise as it approaches the water collection side end portion 10b according to the distribution of the critical membrane surface flow velocity Uf _req . 7 and FIG. 9 are compared, the difference between the distribution of the membrane surface flow velocity Uf and the distribution of the critical membrane surface flow velocity Uf _req (the portion indicated by hatching) is smaller than that shown in FIG. It can be seen that an excessive amount of Uf can be saved while preventing fouling in the section from D to the counter water collecting side end 10a (lower membrane module).

As described above, the film surface velocity Uf is proportional to the cross flow rate U _CF, be accelerated the film surface velocity Uf as described above by adjusting the aeration stream generated by the second diffusion device 62 Can be realized. For example, since the limit film surface velocity Uf _req substantial flux F _R has the (14) equation relationship, an aeration flow generated by the second diffusion device 62 in accordance with the substantial flux distribution in the longitudinal direction adjusted may be increased cross-flow velocity U _CF. In addition, from the viewpoint of efficiently reducing and preventing fouling, the membrane surface flow velocity Uf is preferably 1.5 times or more in the vicinity of the water collection side with respect to the vicinity of the counter water collection side. Further, in the filtration device 2, the membrane module 10 need only be provided for one unit in the longitudinal direction, so the structure of the filtration device 2 is simple and does not increase the equipment cost.

  In the conventional example shown in FIG. 2 (b), in the actual apparatus, the rising speed of the bubbles gradually increases as it approaches the water surface. ) From the upper side (collection side), but due to the principle of mass balance, the upward total flow rate Q of the water to be treated does not increase. There is no difference in average cross flow velocity. In addition, the effect of preventing fouling due to the shearing effect when the bubbles caused by aeration rub against the membrane surface and the volume expansion effect accompanying the rising of the bubbles is also acting, but these effects are small and are excluded in the present invention. Think.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. For example, in FIGS. 1A and 1B, one or two inlets are further provided between the other end of the membrane module 10 and the first inlet. There may be three or more inflow ports. In the immersion type filtration device 2 shown in FIG. 6, it is also possible to dispose an air diffuser in addition to the second air diffuser 62.

It is explanatory drawing which shows the Example of the membrane filtration method by this invention. It is explanatory drawing of the fouling generation | occurrence | production principle. It is drawing which shows the trial calculation result which computed the membrane filtration model for calculating the substantial flux of a hollow fiber, and the longitudinal direction distribution of a substantial flux. It is drawing which shows the graph which shows a substantial flux and a limiting membrane surface flow velocity. It is drawing which shows the graph which shows the longitudinal direction distribution of the membrane surface flow velocity by each membrane filtration method shown in FIG. It is explanatory drawing which shows the other Example of the membrane filtration method by this invention. It is drawing which shows the graph which shows the longitudinal direction distribution of the membrane surface flow velocity by the membrane filtration method shown in FIG. It is explanatory drawing which shows the example of the conventional membrane filtration method. It is drawing which shows the graph which shows the longitudinal direction distribution of the membrane surface flow rate by the conventional membrane filtration method shown in FIG.

Explanation of symbols

1, 1 ₁ , 1 ₂ , 1 ₃ ... Filtration device, 11a... Fine pores of the membrane, 2... Filtration device, 10... Membrane module, 10a ... One end (anti-water collecting side end), 10b. (Water collecting side end), 11 ... membrane, 20, 20 ₁ , 20 ₂ ... case, 20 ₃ ... case (means for increasing speed), 23 ... inflow port, 24 ... inflow port (means for increasing speed), 25 ... Inlet (means for speeding up), 30 ... water tank, 31 ... bottom wall, 61 ... diffusing device, 62 ... diffusing device (means for speeding up).

Claims (6)

A filtration device in which an external pressure type membrane module is provided for one unit in the longitudinal direction, and water to be treated is passed from one end side to the other end side of the membrane module along the membrane surface of the membrane module. A membrane filtration method in the filtration device that performs filtration by a cross flow method to flow and collects filtered water at the other end side,
Between the one end and the other end, the membrane surface flow velocity of the water to be treated that flows along the membrane surface of the membrane module is equal to the suction flow velocity into the pores of the membrane constituting the membrane module As the membrane surface flow velocity is larger than the membrane surface flow velocity, and the membrane surface flow velocity distribution in the longitudinal direction is proportional or approximate to the distribution of the critical membrane surface flow velocity in the longitudinal direction, A membrane filtration method characterized by increasing the membrane surface flow velocity.   The membrane surface flow velocity is increased by supplying the treated water from a plurality of regions in the longitudinal direction of the membrane module and increasing the cross flow average flow velocity of the treated water. The membrane filtration method described in 1. A water tank in which water to be treated is stored; one end of the membrane module is positioned on the bottom wall side of the water tank, and the longitudinal direction of the membrane module is substantially perpendicular to the bottom wall The membrane module is disposed in the water tank, and the water to be treated is flowed from one end side to the other end side of the membrane module by generating an aeration air from the bottom wall side to perform filtration treatment. A membrane filtration method in the filtration device to perform,
The membrane surface flow velocity is increased by generating an aeration air different from the aeration air between the one end portion and the other end portion of the membrane module. Membrane filtration method. A cross flow system in which an external pressure type membrane module is provided for one unit in the longitudinal direction, and water to be treated flows from one end side to the other end side of the membrane module along the membrane surface of the membrane module A filtration device that performs filtration treatment and collects filtered water at the other end side,
Means for increasing the membrane surface flow velocity of the water to be treated flowing along the membrane surface of the membrane module from the one end side toward the other end side;
The speed increasing means is configured such that, between the one end portion and the other end portion, the membrane surface flow velocity is larger than a limiting membrane surface flow velocity equal to a suction flow velocity into the pores of the membrane constituting the membrane module, and Increasing the membrane surface flow velocity from the one end side toward the other end side so that the distribution of the membrane surface flow velocity in the longitudinal direction is proportional or approximate to the distribution of the critical membrane surface flow velocity in the longitudinal direction. A filtering device characterized by that. Having a case for housing the membrane module;
In the case, a plurality of inlets of the water to be treated are formed between the one end and the other end of the membrane module,
The filtration device according to claim 4, wherein the speed increasing means is an inlet other than the inlet closest to the one end portion among the plurality of inlets. The membrane module is disposed inside, and a water tank for storing the treated water;
A plurality of air diffusers that are provided for the membrane module and that generate aeration air to the treated water that is disposed in the water tank and stored in the water tank;
Further comprising
The membrane module is disposed in the water tank so that the one end portion is located on the bottom wall side of the water tank and the other end portion is located on the water surface side of the water to be treated.
The plurality of aeration devices are arranged in the longitudinal direction of the membrane module with respect to the membrane module,
The filtration device according to claim 4, wherein the speed increasing means is an air diffuser other than the air diffuser closest to the bottom wall among the plurality of air diffusers.

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