JP2006272218A - Two-stage membrane filtration system and operation method of two-stage membrane filtration system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for improving the permeated water recovery rate of the entire water treatment system from the aspects of lowering environmental loads, lowering costs and saving energy, designing a two-stage membrane filtration plant suitable for the quality of water to be treated and stably performing operation, maintenance and management. <P>SOLUTION: For the two-stage membrane filtration system satisfying a prescribed relational expression and its operation method, the operation method comprises: a first stage filtration process of executing the membrane filtration of the water to be treated of average turbidity T<SB>1</SB>by a membrane filtration flux F<SB>1</SB>and the permeated water recovery rate R<SB>1</SB>in a first stage membrane filtration device composed of an ultrafiltration membrane module or a microfiltration membrane module; a physical cleaning process having a process of reversely washing the first stage membrane filtration device by feeding water from the permeated water side of the first stage membrane filtration device to the side of the water to be treated; and a second stage filtration process of executing the membrane filtration of physical washing drainage of average turbidity T<SB>2</SB>discharged from the first stage membrane filtration device at the time of performing the physical washing process by the membrane filtration flux F<SB>2</SB>and the permeated water recovery rate R<SB>2</SB>in a second stage membrane filtration device composed of an ultrafiltration membrane or a microfiltration membrane. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention provides a highly efficient and low-cost limit for improving the permeate recovery rate in water treatment fields such as water purification for drinking water production, industrial water production, sewage / drainage treatment, and pretreatment for seawater desalination. The present invention relates to a two-stage membrane filtration system including an outer filtration membrane module or a microfiltration membrane module, and an operation method of the two-stage membrane filtration system.

  In recent years, membrane filtration has been used in various water treatment fields such as water purification for drinking water production, industrial water production, food industry, sewage / drainage treatment, and seawater desalination. This is because the membrane filtration method has a high-precision separation function, automatic operation of the equipment is possible, operation maintenance management is easy, installation space is small, and other processes such as existing processes This is because the water treatment system can be combined and has a high-performance water treatment system.

  Especially in the field of drinking water production, that is, water purification treatment, since the 1990s, chlorine-resistant pathogenic microorganisms such as Cryptosporidium and Giardia derived from livestock manure cannot be treated at the water purification plant and have been mixed into the treated water since the 1990s. Since it has become apparent, membrane filtration has been introduced as an alternative to conventional rapid filtration methods based on coagulation sedimentation and sand filtration.

  On the other hand, microfiltration membranes and ultrafiltration membranes have innumerable small pores called pores. Basically, substances larger than these pores are removed by the membrane, and clear permeated water is obtained and removed. The contaminated material accumulates on the membrane, causing a phenomenon of reduced filtration performance called “fouling”. In the case of microfiltration membranes and ultrafiltration membranes, filtration operation is repeated while repeating physical cleaning called backwashing with clarified water (hereinafter referred to as backwashing) or air scrubbing cleaning using air every few tens of minutes to several hours. To continue stably. The soiling substance peeled off from the membrane by this physical cleaning is discharged out of the membrane module system as physical cleaning waste water. Although the filtration operation can be continued by the physical washing as described above, there is a problem that the permeate recovery rate of the entire water treatment system is lowered because a part of the permeate is used for the physical washing.

  In recent years, the amount of water treatment plants using membranes has been increased to tens of thousands of tons per day and hundreds of thousands of tons per day to reduce costs. Physical washing wastewater with a scale of several thousand tons will come out, and from the viewpoint of low environmental load, low cost, and energy saving, the decline in permeate recovery rate has become one of the important issues that cannot be ignored.

  Patent Document 1 discloses a wastewater treatment apparatus including a membrane filtration device capable of concentrating washing wastewater from a membrane discharged when membrane water is filtered through a river until the solids concentration of sludge is 5 to 10%. Yes. Here, it is stated that the membrane washing wastewater is concentrated by a membrane device for concentration treatment and then introduced into a dehydrator, thereby enabling stable dewatering treatment throughout the year and improving the permeate recovery rate. However, Patent Document 1 does not describe any relationship between the recovery rate of the first-stage and second-stage membrane filtration devices, the membrane filtration flux, and the quality of the water to be treated, and the optimum two-stage according to the quality of the water to be treated. No technology is disclosed for designing and maintaining a membrane filtration plant.

Also in Patent Document 2, the water to be treated is pretreated and then subjected to membrane filtration, and the physical washing wastewater is filtered by a second-stage membrane filtration device so that the permeate recovery rate of the entire system is 99.9% or more. Although the technology is disclosed here, if the physical washing wastewater of the first stage membrane filtration device is concentrated in the second stage membrane filtration device, stable concentration of sludge is possible without adding a flocculant as a pretreatment. Is merely a description of a two-stage membrane filtration system conventionally performed in actual plants. There is no proposal for a special new technology, and there is no description about the relationship between the recovery rate of the first and second stage membrane filtration devices, the membrane filtration flux, and the quality of the water to be treated, as in Patent Document 1. .
JP-A-11-57434 Japanese Patent Laid-Open No. 2004-267887

  The present invention is designed to improve the permeate recovery rate of the entire water treatment system from the viewpoint of low environmental load, low cost, and energy saving, and to design a two-stage membrane filtration system suitable for the quality of water to be treated. The purpose is to provide technology for stable operation and maintenance.

  The present invention for solving the above problems is achieved by the following configurations (1) to (12).

(1) 1) First-stage filtration means comprising an ultrafiltration membrane module or microfiltration membrane module having unique parameters a, b, c 2) From the permeate side of the first-stage filtration means to the treated water side 3) Physical washing means for backwashing the first-stage filtration means, and 3) Filtering the physical washing drain discharged at the time of washing by the physical washing means with specific parameters d, e, and f Second-stage filtration means comprising an outer filtration membrane module or a microfiltration membrane module, 4) T ₁ , the average turbidity of water to be treated for membrane filtration by the first-stage filtration means, and membrane filtration by the second-stage filtration means when the average turbidity of treatment water was T _2, the membrane filtration flux F ₂ and transmission in membrane filtration flux F ₁ and permeate recovery R _1, and the second stage filtration means in the first stage filtration unit water recovery rate _{R 2} , The following equation _F ¹ = _aT 1 ^-b
F ₂ = dT ₂ ^−e
R ₁ = 1-cF ₁
R ₂ = 1−fF ₂
A two-stage membrane filtration system having means for controlling membrane filtration conditions so as to substantially satisfy the parameters a, b, c, d, e, unique to the ultrafiltration membrane module or the microfiltration membrane module. f is 1.5 ≦ a ≦ 10
0.3 ≦ b ≦ 1
0.005 ≦ c ≦ 0.04
1.5 ≦ d ≦ 6
0.1 ≦ e ≦ 1
0.005 ≦ f ≦ 0.02
Is a two-stage membrane filtration system.

  (2) The two-stage membrane filtration system according to (1), having means for controlling the permeate recovery rate R of the entire two-stage membrane filtration system to 99.9% or more.

  (3) The two-stage membrane filtration system according to (1) or (2), wherein the membrane module of the second-stage membrane filtration apparatus is an immersion type membrane module.

  (4) The two-stage membrane filtration system according to any one of (1) to (3), wherein the membrane module of the first-stage membrane filtration apparatus is a pressure-type membrane module.

  (5) The two-stage membrane filtration system according to (4), wherein the membrane module of the first-stage membrane filtration apparatus is a total filtration system.

(6) 1) The unique parameters a, b, c are
1.5 ≦ a ≦ 10
0.3 ≦ b ≦ 1
0.005 ≦ c ≦ 0.04
The membrane to be treated having an average turbidity T ₁ is subjected to membrane filtration with a membrane filtration flux F ₁ and a permeate recovery rate of R _1. 1st-stage filtration process, 2) Physical washing process including the process of feeding water from the permeate side of the 1st-stage membrane filtration apparatus to the treated water side and backwashing the 1st-stage membrane filtration apparatus, and 3) its inherent parameter d , E, f
1.5 ≦ d ≦ 6
0.1 ≦ e ≦ 1
0.005 ≦ f ≦ 0.02
In the second-stage membrane filtration apparatus comprising an ultrafiltration membrane module or a microfiltration membrane module, the physical washing wastewater having an average turbidity T ₂ discharged from the physical washing step is permeated with a membrane filtration flux F _2. An operation method of a two-stage membrane filtration system having a second-stage filtration step of membrane filtration with R ₂ , wherein the average turbidity T ₁ , T ₂ , the membrane filtration fluxes F ₁ , F ₂ , and The permeate recovery rates R ₁ and R ₂ are substantially
F ₁ = aT ₁ ^-b
F ₂ = dT ₂ ^−e
R ₁ = 1-cF ₁
R ₂ = 1−fF ₂
The operation method of the two-stage membrane filtration system which is operated so that the permeated water is recovered from the first-stage filtration step and the second-stage filtration step.

  (7) The operation method of the two-stage membrane filtration system according to (6), wherein the permeate recovery rate R of the entire two-stage membrane filtration system is 99.9% or more.

  (8) The operation method of the two-stage membrane filtration system according to (6) or (7), wherein the membrane module of the second-stage membrane filtration apparatus is an immersion type membrane module.

  (9) The operation method of the two-stage membrane filtration system according to any one of (6) to (8), wherein the membrane module of the first-stage membrane filtration device is a pressure type membrane module.

  (10) The operation method of the two-stage membrane filtration system according to (9), wherein the membrane module of the first-stage membrane filtration apparatus is a total filtration system.

(11) Parameters a, b, c, d, e, and f unique to the ultrafiltration membrane module or the microfiltration membrane module are
5 ≦ a ≦ 8
0.4 ≦ b ≦ 0.8
0.016 ≦ c ≦ 0.03
2.5 ≦ d <5
0.2 ≦ e <0.4
0.01 ≦ f <0.016
The two-stage membrane filtration system according to any one of (1) to (5), wherein

(12) Parameters a, b, c, d, e, and f unique to the ultrafiltration membrane module or the microfiltration membrane module are
5 ≦ a ≦ 8
0.4 ≦ b ≦ 0.8
0.016 ≦ c ≦ 0.03
2.5 ≦ d <5
0.2 ≦ e <0.4
0.01 ≦ f <0.016
The method for operating a two-stage membrane filtration system according to any one of (6) to (10), wherein

  ADVANTAGE OF THE INVENTION According to this invention, the water treatment system which consists of an ultrafiltration membrane or a microfiltration membrane which raised the permeated water collection | recovery rate which was about 95% conventionally to 99.9% is provided. Designing a two-stage membrane filtration system suitable for the quality of the water to be treated and providing a technology for stable operation and maintenance of the system, providing excellent water treatment from the viewpoint of low environmental load, low cost, and energy saving A system is obtained. Especially in large membrane filtration plants with large throughput, the risk is high when stable operation is not possible, but according to the present invention, the risk is suitably distributed between the first-stage membrane filtration device and the second-stage membrane filtration device. It is also possible to make it.

  The two-stage membrane filtration system of the present invention will be described in detail using the example of FIG. The present invention is not limited to the two-stage membrane filtration system shown in FIG.

  The treated water led to the treated water tank 1 is supplied to the first-stage membrane filtration device 3 by the pump 2 of the first-stage membrane filtration device, and the permeated water is stored in the permeated water tank 6. A portion of the permeate stored in the permeate tank 6 is fed from the permeate side of the first-stage membrane filtration device 3 to the treated water side using the backwash pump 7 to backwash the first-stage membrane filter device. I do. In the case where the membrane module is a hollow fiber membrane module, air scrubbing cleaning in which air is introduced from the lower part of the membrane module to shake the hollow fiber membrane is generally performed. These physical washings such as back washing and air scrubbing washing are performed every several tens of minutes to several hours, and usually physical washing wastewater is generated at this degree of physical washing. The generated physical washing wastewater is guided to the second stage membrane filtration device 4, filtered using the pump 5 of the second stage membrane filtration device, and sent to the permeate tank 6.

In the two-stage membrane filtration system shown in FIG. 1, the present inventors have determined the average turbidity T _{1 of the} water to be treated introduced into the water tank 1 to be treated and the average turbidity of physical washing wastewater from the first-stage membrane filtration device 3 ( Membrane feed water average turbidity to the second stage membrane filtration device) T ₂ , membrane filtration flux F _{1 of} the first stage membrane filtration device, membrane filtration flux F _{2 of the second} stage membrane filtration device, first stage membrane filtration device As a result of intensive studies on the relationship between the permeated water recovery rate R ₁ and the permeated water recovery rate R ₂ of the second-stage membrane filtration device, the first-stage membrane filtration device has a low permeated water recovery rate with a high membrane filtration flux. By making the membrane filtration device of the eye a high permeate recovery rate with a low membrane filtration flux, the total membrane area of the membrane area of the first-stage membrane filtration device and the membrane area of the second-stage membrane filtration device is reduced, and The membrane filtration device and the second-stage membrane filtration device can continue to operate stably. As a result, there is a risk for stable supply of permeated water at low cost. Two-stage membrane filtration system less found that obtained.

Here, the membrane filtration flux is a value obtained by dividing the permeate flow rate (m ³ / day) of the membrane module by the membrane area (m ² ), and the unit is m / day. The permeate recovery rate is calculated by subtracting the amount of permeated water obtained in one filtration cycle from the amount of permeated water obtained by filtration in one cycle of the membrane filtration device minus the amount of physical washing wastewater generated in physical washing. The value is obtained by multiplying the measured value by 100, and the unit is%. The permeate recovery rate R of the entire two-stage membrane filtration system is a value obtained by multiplying the value obtained by dividing the average flow rate of the permeate flowing out of the permeate tank 6 by the inflow rate of the treated water flowing into the treated water tank 1 by 100. It is. Conventionally, the permeate recovery rate of the membrane filtration device was about 95%, but a permeate recovery rate of 99.9% or more can be obtained by using the two-stage membrane filtration system of the present invention described in detail below.

  If the first-stage membrane filtration device of the main system of the overall flow rate of the two-stage membrane filtration system is a low membrane filtration flux, the membrane area of the first-stage membrane filtration device increases, the membrane module costs increase, and the installation area of the entire system In addition, the number of pipes, flanges, and motor-operated valves that connect the membrane module increases, resulting in an increase in equipment costs. Furthermore, if the first stage has a low membrane filtration flux, it will be necessary to set the first stage permeate recovery rate higher, the membrane feed water average turbidity of the second stage membrane filtration device will be higher, and stable operation of the second stage will be possible. It becomes difficult. Moreover, even if the membrane permeation flux of the second-stage membrane filtration device of the recovery system, which is a slight processing amount of the entire processing flow rate of the two-stage membrane filtration system, is increased, the membrane area of the entire two-stage membrane filtration system is slight. Only the membrane area can be reduced, and not only the contribution to the total cost reduction is small, but also the stable operation is difficult because the membrane feed water average turbidity of the second-stage membrane filtration device is high.

As described above, the first-stage membrane filtration device has a high permeate filtration flux and a low permeate recovery rate, and the second-stage membrane filtration device has a low membrane filtration flux and a high permeate recovery rate. In order to achieve a filtration system, a membrane module capable of stable operation with a high membrane filtration flux is installed in the membrane module of the first stage membrane filtration device, and a high average turbidity membrane feed water is supplied to the second membrane filtration device. It is necessary to adopt a membrane module capable of high permeate recovery rate operation. Various studies were made on the relationship between the membrane filtration flux and the permeate recovery rate of the membrane filtration devices in the first and second stages. As a result, a membrane module that satisfies the following relational expression was adopted and the following relational expression was satisfied: A low-cost two-stage membrane filtration system suitable for the quality of water to be treated can be obtained by a two-stage membrane filtration system comprising filtration means capable of controlling the membrane filtration flux and the permeate recovery rate. The present inventors have found out.
F ₁ = aT ₁ ^-b
F ₂ = dT ₂ ^−e
R ₁ = 1-cF ₁
R ₂ = 1−fF ₂
1.5 ≦ a ≦ 10
0.3 ≦ b ≦ 1
0.005 ≦ c ≦ 0.04
1.5 ≦ d ≦ 6
0.1 ≦ e ≦ 1
0.005 ≦ f ≦ 0.02
Here, a, b, c, d, e, and f are parameters specific to the membrane module.

  a and d are parameters that are considered to express the potential of dirt resistance to turbidity of the membrane module itself. If it is large, it can be operated with a high membrane filtration flux, and if it is small, it is difficult to operate with a high membrane filtration flux. Represents that. In view of this, the constant a inherent to the first-stage membrane module requiring high membrane filtration flux is 1.5 or more and 10 or less, preferably 3 or more and 9 or less, more preferably 5 or more and 8 or less. The constant d inherent to the membrane module is 1.5 or more and 6 or less, preferably 2 or more and 5.5 or less, and more preferably 2.5 or more and less than 5.

b and e are parameters that are considered to express the amount of change in the maximum membrane filtration flux for stable operation of the membrane module with respect to changes in turbid mass. If the amount is large and small, the amount of change is not large. The constant b inherent to the first-stage membrane module that requires stable operation at a high membrane filtration flux is 0.3 or more and 1 or less, preferably 0.35 or more and 0.9 or less, more preferably 0.4 or more and 0.8. The constant e unique to the second-stage membrane module is 0.1 or more and 1 or less, preferably 0.15 or more and 0.7 or less, more preferably 0.2 or more and less than 0.4. C and f are This parameter is considered to represent the amount of physical washing required for stable operation. If it is small, the amount of physical washing required is small, and it is possible to operate with a high permeate recovery rate even in a high membrane filtration flux. This means that the permeate recovery rate that can be stably operated is greatly reduced as the membrane filtration flux increases. The constant f inherent to the second-stage membrane module requiring high permeate recovery rate operation is 0.005 or more and 0.02 or less, preferably 0.008 or more and 0.018 or less, more preferably 0.01 or more and 0.016. The constant c inherent to the first-stage membrane module is 0.005 or more and 0.04 or less, preferably 0.01 or more and 0.035 or less, and more preferably 0.016 or more and 0.03 or less.

  In particular, a is 5 or more and 8 or less, d is 2.5 or more and less than 5, b is 0.4 or more and 0.8 or less, e is 0.2 or more and less than 0.4, and c is 0.016 or more and 0.00. 03 or less, and by setting f to 0.01 or more and less than 0.016, the first stage membrane filtration device has a low permeate recovery rate with a high membrane filtration flux, and the second stage membrane filtration device has a low membrane filtration flow. It is possible to realize a high permeate recovery rate with a bundle, which is preferable.

Next, how to obtain the constants a, b, c, d, e, and f will be described. First, regarding the relationship between the average turbidity T of the water to be treated and the membrane filtration flux F, stable operation is possible when the average turbidity T of the water to be treated is changed using the membrane module employed in the two-stage membrane filtration system. The upper limit membrane filtration flux F is obtained, and the constants a and b are determined by fitting the experimental result to a function of F = aT− ^b . The constants are determined using at least 3 points, preferably 4 points, and more preferably 5 points or more of data of different average turbidity of water to be treated. It is preferable to use the method of least squares for function fitting.

  Next, the relationship between the membrane filtration flux F and the permeate recovery rate R is as follows. For this, the average turbidity T of the water to be treated is changed using the membrane module employed in the two-stage membrane filtration system, and the previously obtained coverage is obtained. Membrane filtration is performed with the upper limit membrane filtration flux F capable of stable operation with respect to the average turbidity T of the treated water, the upper limit recovery rate R capable of stable operation at this time is obtained, and the experimental results are fitted to a function of R = 1-cF. To determine the constant c. The above procedure is performed in both the first-stage membrane filtration apparatus and the second-stage membrane filtration apparatus, and the relational expressions necessary for designing the second-stage membrane filtration system are obtained.

  In addition, as an ultrafiltration membrane module or microfiltration membrane module having such a, b and c, and an ultrafiltration membrane module or microfiltration membrane module having such d, e and f, known It may be obtained by a method.

Next, using the ultrafiltration membrane module or the microfiltration membrane module, in the two-stage membrane filtration system of the present invention, the membrane filtration fluxes F ₁ and F ₂ and the permeation of the _first- stage membrane filtration apparatus and the second-stage membrane filtration apparatus. An operation method of a two-stage membrane filtration system that performs membrane filtration by controlling the water recovery rates R ₁ and R ₂ to operating conditions so as to maintain a specific relationship will be described.

From the water to be treated an average turbidity _{T 1} using the relationship of _F ¹ = _aT 1 ^-b seek membrane filtration flux _{F 1} of the first stage membrane filtration device. Next, the permeated water recovery rate R ₁ of the _first- stage membrane filtration apparatus is obtained using the relationship of F ₁ to R ₁ = 1−cF ₁ . When R ₁ is determined, the membrane feed water average turbidity T _{2 of the} second-stage membrane filtration device is determined from the relationship of T ₂ = T ₁ / (1-R ₁ ). If T ₂ is Kimare _F 2 = _dT ² from ^-e relationship can decide membrane filtration flux _{F 2} of the second stage membrane filtration apparatus, from the _{F 2} using the relationship _{R 2} = 1-fF 2 permeate recovery of second stage membrane filtration units R ₂ is determined. The gist of the present invention is an operation method of a two-stage membrane filtration system that performs membrane filtration while controlling F ₁ , F ₂ , R ₁ , and R ₂ determined by the above procedure. In addition, the control of each operation condition of the first stage filtration process, the second stage filtration process, and the physical cleaning process is performed in advance by a PLC (programmable controller) built in each device (each means) constituting the second stage membrane filtration system. It can be recorded and automatically executed.

  When the membrane filtration flux and the permeate recovery rate are determined by the above method, only one optimum membrane filtration flux and permeate recovery rate can be obtained according to the average turbidity of the water to be treated. Become. However, in practice, there is a certain range of membrane filtration flux and permeate recovery rate that can be stably operated. Therefore, in the present invention, the object of the present invention can be achieved by operating so as to substantially satisfy the above relational expression. Here, substantially satisfying means that the value of the membrane filtration flux and the permeate recovery rate is in the center, the value of the membrane filtration flux is within a range of ± 10%, and the value of the permeate recovery rate is ± 2%. The range value is controlled as the membrane filtration flux and permeate recovery rate of the first-stage membrane filtration device and the second-stage membrane filtration device of the present invention.

In addition, while observing the transition of the rate of increase in the differential pressure of the first-stage membrane filtration device and the second-stage membrane filtration device, membrane filtration in the range of ± 10% for membrane filtration flux and ± 2% for permeate recovery rate Change the flux and permeate recovery rate, for example, operating conditions so that the rate of increase in the filtration differential pressure of the first-stage membrane filtration device and the second-stage membrane filtration device becomes small even when temporarily treated with high turbidity treated water. It is also preferable to change and control the above, and it is included in the scope of the present invention.
Examples of the shape of the separation membrane used in the membrane filtration device of the present invention include a hollow fiber membrane, a tubular membrane, and a flat membrane, and any shape can be used in the present invention. A hollow fiber membrane that can increase the effective membrane area per unit unit volume can be preferably used in the field, that is, water treatment applications such as water purification processes, water production, and wastewater treatment. In the hollow fiber membrane module, a hollow fiber membrane is housed in a container, and a pressurized membrane module that obtains permeated water by supplying pressurized water to be treated with a pump or the like, and the membrane supply water into a tank or the like. There are roughly two types of submerged membrane modules in which the membrane module is directly immersed and suction filtered, and any membrane module can be employed in the present invention. However, the point of the present invention is that the first stage membrane filtration device has a high permeate recovery rate with a high membrane filtration flux, and the second stage membrane filtration device has a high permeate recovery rate with a low membrane filtration flux. For this purpose, a pressurized membrane module that is advantageous for high membrane filtration flux in the first stage membrane filtration device, and a high permeate recovery rate operation suitable for high average turbidity treated water in the second stage membrane filtration device It is preferable to employ an immersion membrane module.

The separation membrane used in the present invention is a separation membrane classified as a so-called microfiltration membrane or ultrafiltration membrane having a pore diameter of 1 nm to 10 μm. Generally, reverse osmosis membranes and nanofiltration membranes are not included in the present invention because they are not physically washed, and the system design philosophy is different. Here, the pore diameter of the separation membrane is measured by the method described below. That is, the membrane filtration rate J _v Permeability L _p and water separation membrane, obtained by calculation using the following relation.

J _v = L _p · ΔP
L _p = (H / L) · (R _p ² / 8η)
Here, ΔP is the transmembrane pressure difference, H is the average membrane porosity, L is the film thickness, R _p is the average pore diameter, and η is the viscosity of water.

  Examples of the material for the microfiltration membrane or ultrafiltration membrane include inorganic materials such as polyacrylonitrile, polysulfone, polyether sulfone, polyphenylene sulfone, polyphenylene sulfide sulfone, polyvinylidene fluoride, cellulose acetate, polyethylene, polypropylene, ceramic, etc. Although not particularly limited in view of the gist of the present invention, polyvinylidene fluoride having high chemical durability, polyacrylonitrile and cellulose acetate which are hydrophilic materials and are difficult to get dirty are preferable.

  The treated water in the present invention is a membrane supply water supplied to the membrane module of the first-stage membrane filtration device. For example, in the water purification treatment field, natural water such as river water, lake water, and ground water is mainly treated water. Become. The treated water may be sewage secondary treated water or seawater.

  The membrane filtration device is a device for producing permeated water by providing at least a pressurizing means on the stock solution side or a suction means on the permeate side. A pump may be used as the pressurizing means, or a pressure due to a water level difference may be used. Moreover, what is necessary is just to utilize a pump and a siphon as a suction means. A system in which this membrane filtration device is combined in two stages and the physical washing wastewater of the first stage membrane filtration device is filtered by the second stage membrane filtration device is the two-stage membrane filtration system of the present invention.

  The operation of the membrane separator includes constant flow filtration and constant pressure filtration, but the present invention provides a technique for suitably determining the membrane filtration flux and the permeate recovery rate, and is limited to the constant flow filtration operation. The

  In addition, the total amount of filtration operation that filters the total amount of water to be treated and the cross-flow filtration operation that returns a part of the water to be treated supplied to the separation membrane module to the water to be treated. There is. Speaking from the gist of the present invention, any filtration operation method may be used, but the total amount filtration operation is simpler and easier to manage, and all the membrane feed water other than the physical washing wastewater can be obtained as permeate. This is advantageous because it leads to a reduction in energy costs.

  The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.

  The performance evaluation of the two-stage membrane filtration system was performed by the drivability evaluation using Lake Biwa water described below. The change in filtration pressure difference between 2 weeks and 3 months was measured, and the daily average filtration pressure increase rate (kPa / day) was calculated and compared using the following formula.

Filtration differential pressure increase rate (kPa / day) = (P ₁ −P ₀ ) / T
Here, the filtration differential pressure immediately after the physical cleaning at the start of the operation evaluation is P ₀ (kPa), the filtration differential pressure at the end of the operation evaluation is P ₁ (kPa), and the operation period is T (day).

  If the filtration differential pressure increase rate is about 0.35 kPa / day or less, the filtration differential pressure increase is about 60 kPa after 6 months of operation. Considering that the initial filtration differential pressure is about 20 to 30 kPa and the standard for chemical cleaning is about 100 to 150 kPa, the filtration differential pressure increase rate is 0.35 kPa / day. Thus, it can be said that the hollow fiber membrane has high stain resistance and can be stably operated.

<Example 1>
As a first-stage membrane filtration apparatus, one module of a hollow fiber microfiltration membrane module “HFS-2020” made by polyvinylidene fluoride with a membrane area of 72 m ² is used as a first-stage membrane filtration apparatus, and as a second-stage membrane filtration apparatus, polyfluoride produced by Toray Industries, Inc. A two-stage membrane filtration system using one module of an immersion type hollow fiber membrane module having a membrane area of 22 m ² produced using a hollow fiber membrane of vinylidene fluoride hollow fiber microfiltration membrane module “HFS-2020” was produced.

The relationship between the treated water average turbidity T _{1 of the} hollow fiber microfiltration membrane module “HFS-2020” manufactured by Toray Industries, Inc. and the upper limit membrane filtration flux F ₁ capable of stable operation is F ₁ = 5.8 T ₁ ^{− 0.55,} the relationship _{F 1} and stable operation can limit the permeate recovery ratio _{R 1} was _{R 1} = 1-0.021F 1. Moreover, the membrane feed water average turbidity T _{2 of} a submerged hollow fiber membrane module having a membrane area of 20 m ² produced using a hollow fiber membrane of a hollow fiber microfiltration membrane module “HFS-2020” made of polyvinylidene fluoride manufactured by Toray Industries, Inc. the stable operation can relationship upper membrane filtration flux _{F 2} and _F 2 ₌ ^{2.5T 2 -0.35,} the relationship _{F 2} and stable operation can limit the permeate recovery _{R 2} is _R 2 = 1-0.014F ₂ . Lake Biwa water with an average turbidity of 3 degrees was filtered using this two-stage membrane filtration system.

From the above formula, the membrane filtration flux of the first-stage membrane filtration device is 3.2 m / day, the permeate recovery rate is 93.3%, the membrane filtration flux of the second-stage membrane filtration device is 0.7 m / day, and the permeation rate The water recovery rate was 99.0%. At this time, the average turbidity of the physical washing wastewater of the first stage membrane filtration device, that is, the average turbidity of the membrane supply water of the second stage membrane filtration device is 45 degrees, the treated water flowing into the treated water tank of the second stage membrane filtration system The flow rate was 230 m ³ / day, the average flow rate of the permeate flowing out from the permeate tank was 229.85 m ³ / day, and the permeate recovery rate of the entire system was 99.93%.

  As a result of the operability evaluation, the filtration differential pressure increase rate of the first-stage membrane filtration device is 0.20 kPa / day, and the second-stage membrane filtration device has no increase in filtration differential pressure for 50 days. Could be done.

<Comparative Example 1>
In a similar two-stage membrane filtration system as in Example 1, a two-stage membrane filtration system in the same operating conditions as in Example 1 except that the membrane filtration flux F ₁ of the first stage membrane filtration units and 3.8 m / day A drivability evaluation was conducted. In this comparative example, F ₁ is not included in ± 10% of F ₁ obtained from F ₁ = 5.8T ₁ ^−0.55, and does not satisfy the requirements of claims 1 and 6. As a result, the rate of increase in the filtration differential pressure of the first-stage membrane filtration device is 0.65 kPa / day, and the increase in the filtration differential pressure is faster than in Example 1, and the frequency of the chemical cleaning of the first-stage membrane filtration device is thought to increase. It was.

<Comparative example 2>
In the same two-stage membrane filtration system as in Example 1, the operating conditions are the same as in Example 1 except that the membrane area of the second-stage membrane filtration device is 15 m ² and the membrane filtration flux F ₂ is 1.0 m / day. The operability evaluation of the two-stage membrane filtration system was carried out. In this comparative example, F ₂ is not included in ± 10% of F ₂ obtained from F ₂ = 2.5T ₂ ^−0.35, and does not satisfy the requirements of claims 1 and 6. Here, the submerged hollow fiber membrane module of the second stage membrane filtration apparatus has the same hollow fiber membrane effective length and the same hollow fiber membrane filling rate as the submerged hollow fiber membrane module of Example 1, and only the diameter of the module. It was made smaller to reduce the number of hollow fiber membranes. For this reason, the constants d, e, and f were the same as those in Example 1. As a result of the drivability evaluation, the rate of increase in the filtration differential pressure of the second stage membrane filtration apparatus was 1.0 kPa / day, and the increase in the filtration differential pressure was faster than that in Example 1, and the chemical cleaning frequency of the second stage membrane filtration apparatus was high. Was thought to increase.

<Comparative Example 3>
In the same two-stage membrane filtration system as in Example 1, until the membrane filtration fluxes F ₁ and F ₂ of the first-stage membrane filtration device and / or the second-stage membrane filtration device do not satisfy the requirements of claims 1 and 6 If it is lowered, it is necessary to increase the number of membrane modules, which is not economical and disadvantageous compared to Example 1 in terms of membrane module cost, installation area, equipment cost, and the like.

<Comparative example 4>
In a similar two-stage membrane filtration system as in Example 1 enhanced the permeate recovery ratio R ₁ of the first stage membrane filtration units and 97.0%. _{R 1} in this comparative _example, F ^{₁ = 5.8T} _{1 ^-0.55,} and _R 1 = 1-0.021F not included in of ± 2% _{R 1} derived from _1, claims 1 and 6 Requirements Will not be satisfied. Also, if the recovery rate of the second stage membrane filtration device is increased to 97.5% even if the recovery rate of the first stage membrane filtration device is increased, the permeate recovery rate of the entire system can be maintained at 99.93%, The permeate recovery rate of the membrane filtration device was lowered to 97.5%. Other than that, the operability evaluation of the two-stage membrane filtration system was performed under the same conditions as in Example 1. As a result, the rate of increase in the filtration differential pressure of the first-stage membrane filtration apparatus was 0.68 kPa / day, and the increase in the filtration differential pressure was faster than that in Example 1. In addition, since the permeate recovery rate of the first stage membrane filtration apparatus was increased, the average turbidity of physical washing wastewater of the first stage membrane filtration apparatus, that is, the average turbidity of the membrane supply water of the second stage membrane filtration apparatus was 102 degrees. The rate of increase in the filtration differential pressure of the membrane filtration device was as high as 1.3 kPa / day, and stable operation was not possible.

<Comparative Example 5>
In a similar two-stage membrane filtration system as in Example 1, it was lowered permeate recovery ratio R ₁ of the first stage membrane filtration units to 90.0%. _{R 1} in this comparative _example, F ₁ = 5.8T _{1 ^-0.55,} and _R 1 = 1-0.021F not included in of ± 2% _{R 1} derived from _1, claims 1 and 6 Requirements Will not be satisfied. In addition, if the recovery rate of the second-stage membrane filtration device is lowered to 99.93%, the permeate recovery rate of the entire system cannot be increased to 99.93%, so the second-stage membrane filtration device cannot be increased to 99.93%. The permeate recovery rate of the membrane filtration device was increased to 99.93%. Other than that, the operability evaluation of the two-stage membrane filtration system was performed under the same conditions as in Example 1. As a result, the rate of increase in the filtration differential pressure of the first-stage membrane filtration device was 0.16 kPa / day, and the increase in the filtration differential pressure was slow and stable compared to Example 1. However, since the permeate recovery rate of the first-stage membrane filtration device was lowered, the average turbidity of physical washing wastewater of the first-stage membrane filtration device, that is, the average turbidity of membrane supply water of the second-stage membrane filtration device was reduced to 31 degrees. However, since only a few physical washings could be performed, the filtration differential pressure increase rate of the second-stage membrane filtration device was as high as 0.9 kPa / day, and the increase in the filtration differential pressure was faster than in Example 1, The chemical cleaning frequency of the second stage membrane filtration device was considered to increase.

  The two-stage membrane filtration system of the present invention can be used for water purification treatment for drinking water production, industrial water production, sewage / drainage treatment, pretreatment for seawater desalination, and the like.

An example of the two-stage membrane filtration system which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Water tank to be treated 2 Pump of first stage membrane filtration device 3 First stage membrane filtration device 4 Second stage membrane filtration device 5 Pump of second stage membrane filtration device 6 Permeate tank 7 Backwash pump 8 Valve

Claims (12)

1) First-stage filtration means comprising an ultrafiltration membrane module or a microfiltration membrane module having unique parameters a, b, c
2) Physical washing means for backwashing the first-stage filtration means by feeding water from the permeate side to the treated water side of the first-stage filtration means,
3) A second-stage filtration means comprising an ultrafiltration membrane module or a microfiltration membrane module having specific parameters d, e, and f, which membrane-filters the physical washing wastewater discharged during washing by the physical washing means,
4) When the average turbidity of the water to be treated filtered by the first stage filtration means is T ₁ and the average turbidity of the water to be treated filtered by the second stage filtration means is T ₂ , the first stage filtration is performed. The membrane filtration flux F ₁ and permeate recovery rate R _{1 in} the means, and the membrane filtration flux F ₂ and permeate recovery rate R ₂ in the second-stage filtration means are expressed by the following formula: F ₁ = aT ₁ ^-b
F ₂ = dT ₂ ^−e
R ₁ = 1-cF ₁
R ₂ = 1−fF ₂
A two-stage membrane filtration system having means for controlling membrane filtration conditions so as to substantially satisfy the parameters a, b, c, d, e, unique to the ultrafiltration membrane module or the microfiltration membrane module. f is 1.5 ≦ a ≦ 10
0.3 ≦ b ≦ 1
0.005 ≦ c ≦ 0.04
1.5 ≦ d ≦ 6
0.1 ≦ e ≦ 1
0.005 ≦ f ≦ 0.02
Is a two-stage membrane filtration system. The two-stage membrane filtration system according to claim 1, comprising means for controlling the permeate recovery rate R of the entire two-stage membrane filtration system to 99.9% or more. The two-stage membrane filtration system according to claim 1 or 2, wherein the membrane module of the second-stage membrane filtration apparatus is an immersion type membrane module. The two-stage membrane filtration system according to any one of claims 1 to 3, wherein the membrane module of the first-stage membrane filtration device is a pressure type membrane module. The two-stage membrane filtration system according to claim 4, wherein the membrane module of the first-stage membrane filtration apparatus is a total filtration system. 1) The unique parameters a, b, c are
1.5 ≦ a ≦ 10
0.3 ≦ b ≦ 1
0.005 ≦ c ≦ 0.04
The membrane to be treated having an average turbidity T ₁ is subjected to membrane filtration with a membrane filtration flux F ₁ and a permeate recovery rate of R _1. First stage filtration process,
2) Physical washing step including a step of feeding water from the permeate side of the first-stage membrane filtration device to the treated water side and backwashing the first-stage membrane filtration device,
3) The unique parameters d, e, f are
1.5 ≦ d ≦ 6
0.1 ≦ e ≦ 1
0.005 ≦ f ≦ 0.02
In the second-stage membrane filtration apparatus comprising an ultrafiltration membrane module or a microfiltration membrane module, the physical washing wastewater having an average turbidity T ₂ discharged from the physical washing step is permeated with a membrane filtration flux F _2. An operation method of a two-stage membrane filtration system having a second-stage filtration step of membrane filtration with R ₂ , wherein the average turbidity T ₁ , T ₂ , the membrane filtration fluxes F ₁ , F ₂ , and The permeate recovery rates R ₁ and R ₂ are substantially
F ₁ = aT ₁ ^-b
F ₂ = dT ₂ ^−e
R ₁ = 1-cF ₁
R ₂ = 1−fF ₂
The operation method of the two-stage membrane filtration system which is operated so that the permeated water is recovered from the first-stage filtration step and the second-stage filtration step. The operation method of the two-stage membrane filtration system according to claim 6, wherein the permeate recovery rate R of the entire two-stage membrane filtration system is 99.9% or more. The operation method of the two-stage membrane filtration system according to claim 6 or 7, wherein the membrane module of the second-stage membrane filtration apparatus is an immersion type membrane module. The operation method of the two-stage membrane filtration system according to any one of claims 6 to 8, wherein the membrane module of the first-stage membrane filtration apparatus is a pressure type membrane module. The operation method of the two-stage membrane filtration system according to claim 9, wherein the membrane module of the first-stage membrane filtration apparatus is a total filtration system. Parameters a, b, c, d, e, and f inherent to the ultrafiltration membrane module or microfiltration membrane module are
5 ≦ a ≦ 8
0.4 ≦ b ≦ 0.8
0.016 ≦ c ≦ 0.03
2.5 ≦ d <5
0.2 ≦ e <0.4
0.01 ≦ f <0.016
The two-stage membrane filtration system according to claim 1, wherein: Parameters a, b, c, d, e, and f inherent to the ultrafiltration membrane module or microfiltration membrane module are
5 ≦ a ≦ 8
0.4 ≦ b ≦ 0.8
0.016 ≦ c ≦ 0.03
2.5 ≦ d <5
0.2 ≦ e <0.4
0.01 ≦ f <0.016
The operation method of the two-stage membrane filtration system according to claim 6, wherein the two-stage membrane filtration system is an operation method.

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