Method for operating reverse osmosis membrane device, and reverse osmosis membrane device

Abstract

Provide is a reverse osmosis membrane device that is capable of treating raw water containing membrane-fouling substance in large quantities, such as MBR-treated water, in a stable manner while preventing decreases in permeate flow rate, and a method for operating thereof. A method for operating a reverse osmosis membrane device that treats raw water containing high-molecular-weight organic matter, wherein the raw water contains high-molecular-weight organic matter having a molecular weight of 10,000 or more at a concentration of 0.01 ppm or more, wherein the reverse osmosis membrane device is equipped with a spiral-type reverse osmosis membrane element that has a membrane surface area satisfying the below formula (1), and wherein the reverse osmosis membrane device is operated at a permeate flux of 0.6 m/d or less:
membrane surface area(m.sup.2)?n.sup.2?(11/16)(1)
wherein n represents a diameter (inches) of the reverse osmosis membrane element.

Claims

1. A method for operating a reverse osmosis membrane device that treats raw water containing high-molecular-weight organic matter, wherein the raw water contains high-molecular-weight organic matter having a molecular weight of 10,000 g/mol or more at a concentration of 0.01 ppm or more, wherein the reverse osmosis membrane device is equipped with a spiral-type reverse osmosis membrane element that has a membrane surface area?44 m.sup.2 (square meters), and a raw water spacer having a thickness of 0.2-0.6 mm, wherein the reverse osmosis membrane device is operated at a permeate flux of 0.45 m/d or less, and wherein the reverse osmosis membrane device is operated with a flow rate of concentrate being 0.1 m/s or more in linear velocity.

2. The method for operating a reverse osmosis membrane device according to claim 1, wherein the raw water is MBR-treated water.

3. The method for operating a reverse osmosis membrane device according to claim 1, wherein the reverse osmosis membrane device is further equipped with a permeate spacer having a thickness of 0.1-0.25 mm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1a, 1b, and 1c are flow diagrams illustrating embodiments of methods according to the present invention for treating biologically treated water.

(2) FIG. 2a is a graphical representation of relationships between permeate flux and concentration factor in a reverse osmosis membrane separation process in which the raw water is aqueous solution of NaCl or water containing high-molecular-weight organic matter having an average molecular weight of 10,000. FIG. 2b is a graphical representation of relationships between average linear velocity and concentration factor in the same process.

(3) FIG. 3 is a schematic cross-sectional view of a flat-sheet membrane cell used in Examples.

DESCRIPTION OF EMBODIMENTS

(4) The following describes embodiments of the present invention in detail.

(5) <Raw Water>

(6) In the present invention, the raw water subjected to reverse osmosis membrane separation with the reverse osmosis membrane device contains 0.01 ppm or a higher concentration of high-molecular-weight organic matter having a molecular weight of 10,000 or more. High-molecular-weight organic matter having a molecular weight of 10,000 or more, in particular, biological metabolites such as polysaccharides and proteins, is likely to foul the membrane and therefore often causes decrease in permeate flux. The raw water to be treated by the present invention contains the high-molecular-weight organic matter at 0.01 ppm or more, for example 0.05 to 0.5 ppm. Such raw water tends to lower greatly permeate flux of a reverse osmosis membrane.

(7) The raw water containing high-molecular-weight organic matter may be water recovered from various kinds of discharged water, and biologically treated water such as MBR-treated water.

(8) The method for measuring the concentration of the high-molecular-weight organic matter having a molecular weight of 10,000 or more in the water is not limited. Instruments that measure TOC or similar through molecular weight fractionation using chromatographic techniques such as LC-OCD (liquid chromatography-organic carbon detection) and HPLC (high-performance liquid chromatography) can be used for the measurement. The concentration can also be measured by separating substances having a molecular weight of 10,000 or more and substances having a molecular weight of less than 10,000 using a UF membrane with a cutoff molecular weight of 10,000, and by subsequently conducting TOC analysis on the concentration of the substances having a molecular weight of 10,000 or more.

(9) <Spiral-Type Reverse Osmosis Membrane Element>

(10) The spiral-type reverse osmosis membrane element installed in the reverse osmosis membrane device according to the present invention has a spiral-type membrane unit having a flat envelope-like membrane wound spirally, a raw water spacer arranged at a primary side (one surface) of the membrane, and a permeated water spacer arranged in the membrane (a secondary side).

(11) The diameter of the spiral-type reverse osmosis membrane element is not limited. Elements with diameters of 4 inches, 8 inches, and 16 inches are commercially available. Usually used is an 8-inch spiral-type reverse osmosis membrane element.

(12) The length of the element is usually, but not limited to, approximately 1 m.

(13) The spiral-type reverse osmosis membrane element used in the present invention has a membrane surface area that satisfies the formula (1) below:
Membrane surface area(m.sup.2)?n.sup.2?(11/16)(1)

(14) where n represents the diameter (inches) of the reverse osmosis membrane element.

(15) When the membrane surface area of the spiral-type reverse osmosis membrane element does not satisfy the formula (1) above, and the membrane surface area(m.sup.2) is smaller than n.sup.2?(11/16), the permeate flux for the same permeate flow rate cannot be reduced, and the object of the present invention is unattainable.

(16) The membrane surface area(m.sup.2) of the spiral-type reverse osmosis membrane element is preferably n.sup.2?11/16 or more, more preferably n.sup.2?13/16 or more. The membrane surface area is preferably n.sup.2?16/16 or less for the purpose that thickness of the reverse osmosis membrane, the raw water spacer, and the permeate spacer have a required amount.

(17) The material of which the reverse osmosis membrane is made is not limited. An aromatic polyamide membrane synthesized on a substrate using phenylene diamine and an acid chloride is preferred because a membrane with a high rejection is preferred. The commonly used range of thicknesses of the reverse osmosis membrane is 0.1 to 0.15 mm.

(18) The shape of the raw water spacer and the permeate spacer is not limited. Commonly used is a mesh spacer that is composed of regularly spaced wires made of plastics such as polyethylene, polypropylene. The wires have the same or different diameters, and are stacked to cross each other at an angle of 45? to 90?.

(19) Examples of raw water spacers now in common use include those having a thickness of 0.69 mm (26 mil), 0.71 mm (28 mil), or 0.86 mm (34 mil). In the present invention, it is preferred to use a raw water spacer having a thickness of 0.6 mm or less, for example, approximately 0.2 to 0.6 mm, so that the formula (1) above is satisfied. Too large a thickness of the raw water spacer leads to a small membrane surface area per element, which makes the formula (1) above no longer satisfied. Too small a thickness of the raw water spacer makes the occlusion of channels with suspended matter.

(20) It is preferable that the mesh-like raw water spacer has the porosity of 60% to 95% and the mesh size of 1.0 to 4.0 mm. The mesh-like raw water spacer having such a porosity and a mesh size contributes to suppression of concentration polarization by providing a raw water-mixing effect while maintaining sufficient water flowability. The porosity is the percentage of the spatial volume minus the volume of the wires divided by the spatial volume. The mesh size is the spacing between the wires.

(21) The thickness of the permeate spacer is not limited. The thickness is preferably 0.1 to 0.25 mm. Too large a thickness of the permeate spacer leads to a small membrane surface area per element, as likely as the raw water spacer. Too small a thickness leads to a large differential pressure and therefore a small permeate flow rate.

(22) <Permeate Flux>

(23) In the present invention, the reverse osmosis membrane device with the spiral-type reverse osmosis membrane element having the membrane surface area as specified above is operated at a permeate flux of 0.6 m/d or less.

(24) The pure water permeate flux of a reverse osmosis membrane device at a standard operation pressure is generally 0.7 to 0.85 m/d, and it is customary to set the permeate flux at approximately 0.5 to 0.7 m/d when applying raw water containing inorganic salts or organic matter.

(25) The inventors have found experimentally that high-molecular-weight organic matter having a molecular weight of 10,000 or more fouls the reverse osmosis membrane and that the concentration of the high-molecular-weight organic matter concentration on the membrane surface exceeding 1 ppm leads to significant decreases in permeate flux. The inventors have found that when the raw water contains 0.01 ppm or more high-molecular-weight organic matter having a molecular weight of 10,000 or more, a concentration factor for the membrane surface concentration exceeding 100 leads to significant decreases in permeate flux. Ensuring the concentration factor does not exceed 100 requires that the permeate flux is 0.6 m/d or less. Thus in the present invention, the reverse osmosis membrane device is operated at a permeate flux of 0.6 m/d or less, preferably 0.45 m/d or less. Excessive reduction of the permeate flux is, however, uneconomical because it increases the number of membranes needed. It is thus preferable that the permeate flux is 0.2 m/d or more.

(26) The flow rate of the concentrate in an 8-inch spiral-type reverse osmosis membrane element is usually 2.0 to 8.0 m.sup.3/h, and the linear velocity in this condition is 0.05 to 0.15 m/s. In the present invention, the hydraulic linear velocity of the concentrate preferably maintained at 0.1 m/s or more in order not to increase the membrane surface concentration, although the range depends on the permeate flux. Reducing the linear velocity to less than 0.1 m/s is not preferred because it leads to a concentration factor on the membrane surface exceeding 100 and therefore significant decreases in permeate flux. A linear velocity exceeding 0.2 m/s is not preferred because it leads to too large a pressure loss and also increases the possibility of membrane breakage. A more preferred linear velocity is thus 0.1 to 0.2 m/s.

(27) When an 8-inch spiral-type reverse osmosis membrane element having a membrane surface area smaller than n.sup.2?(11/16)=44 m.sup.2 is operated under the conditions satisfying the above, the permeate flow rate and recovery rate of water are decreased. Satisfying the above permeate flux and linear velocity conditions therefore requires the use of the spiral-type reverse osmosis membrane element having the membrane surface area of 44 m.sup.2 or more, namely n.sup.2?(11/16) or more.

(28) <Treatment of Biologically Treated Water>

(29) The reverse osmosis membrane device according to the present invention is particularly suitable for use in reverse osmosis membrane separation of biologically treated water.

(30) FIGS. 1a, 1b, and 1c are flow diagrams illustrating embodiments of methods according to the present invention for treating biologically treated water, in which a reverse osmosis membrane device according to the present invention is used.

(31) In FIG. 1a, the biologically treated water is treated through aerobic and/or anaerobic biological treatment means 1, flocculation and aggregation treatment means 2, solid-liquid separation means 3 such as dissolved-air pressure floatation, filtration means 4, a safety filter 5, and a reverse osmosis membrane device 6.

(32) In FIG. 1b, water treated by the biological treatment means 1 is directly fed to the filtration means 4 such as a membrane filtration device, and the resulting water is introduced into the reverse osmosis membrane device 6 and subjected to reverse osmosis membrane separation. In FIG. 1c, water treated by an MBR (immersion membrane separation device) 7 is directly introduced into the reverse osmosis membrane device 6 and treated. Methods according to the present invention are not limited to these methods.

EXAMPLES

(33) The present invention is described in more detail with reference to Reference Examples, Examples, and Comparative Examples.

Reference Example 1

(34) Tables 1 and 2 below show the relationship between a thickness of a raw water spacer and a membrane surface area, and the relationship between permeate flux and linear velocity at a constant flow rate in a spiral-type reverse osmosis membrane element having a diameter of 8 inches (n.sup.2?(11/16)=44).

(35) As is clear from Tables 1 and 2, reducing the thickness of the raw water spacer increases the membrane surface area per element, thereby reducing decreases in flux caused by fouling. Too small a thickness of the raw water spacer, for example, 0.2 mm or less, leads to high hydraulic resistance, which heightens the risk of occlusion with suspended matter or similar.

(36) TABLE-US-00001 TABLE 1 <Fixed conditions of permeate flow rate: 1.1 m.sup.3/h, and concentrate flow rate: 5.1 m.sup.3/h> Thickness of Membrane raw water spacer Permeate flux Linear velocity surface area (m.sup.2) (mm) (m/d) (m/s) 33.6 1.00 0.79 0.086 37.1 0.86 0.71 0.091 41.8 0.71 0.63 0.097 44 0.65 0.60 0.101 50.7 0.50 0.52 0.114 63.6 0.30 0.42 0.151 72.8 0.20 0.36 0.198

(37) TABLE-US-00002 TABLE 2 <Fixed conditions of permeate flow rate: 1.1 m.sup.3/h, and concentrate flow rate: 3.6 m.sup.3/h> Thickness of Membrane raw water spacer Permeate flux Linear velocity surface area (m.sup.2) (mm) (m/d) (m/s) 33.6 1.00 0.79 0.060 37.1 0.86 0.71 0.063 41.8 0.71 0.63 0.067 44.0 0.65 0.60 0.070 50.7 0.50 0.52 0.079 63.6 0.30 0.42 0.105 72.8 0.20 0.36 0.137

Reference Example 2

(38) The relationship between permeate flux or average linear velocity and concentration factor (membrane surface concentration/average bulk concentration) was analyzed in a reverse osmosis membrane separation process in which the raw water was an aqueous solution of NaCl or water containing high-molecular-weight organic matter having an average molecular weight of 10,000. The results are illustrated in FIGS. 2a and 2b.

(39) As can be seen from FIGS. 2a and 2b, the concentration of high-molecular-weight organic matter on the membrane surface significantly rises with increasing permeate flux and decreasing average linear velocity in general, as compared with low-molecular-weight substances such as NaCl, although with some differences between polymer species.

Example 1

(40) The inventors assumed the use of an 8-inch spiral-type reverse osmosis membrane element having a membrane surface area of 44.0 m.sup.2. A flat-sheet membrane piece was cut out from a reverse osmosis membrane (a thickness of 0.13 mm) ES20 of Nitto Denko Corporation to 50 mm in width?800 mm in length, and the piece was installed in a test flat-sheet membrane cell illustrated in FIG. 3 together with a 0.65 mm-thick mesh spacer of polypropylene as a raw water spacer (porosity 92%, mesh size 2.6 mm) and a 10 mm-thick ceramic (sintered porous ceramic) spacer as a permeate spacer.

(41) The flat-sheet membrane cell illustrated in FIG. 3 has a structure in which a membrane unit is held in a space formed by channel-forming members of acrylic resin 21, 22, and 23 and pressure reinforcement members of SUS 24 and 25. The membrane unit is provide with a raw water spacer 11, a permeate spacer 12, and a reverse osmosis membrane 10 arranged therebetween.

(42) The raw water flows from a raw water inlet 13 into the primary side of the reverse osmosis membrane 10 and flows along the raw water spacer 11. The water permeated through the reverse osmosis membrane 10 flows along the permeate spacer 12, and is taken out from a permeate outlet 15. The concentrated water is taken out from a concentrate outlet 14.

(43) Biologically treated water was used as raw water after being treated by a flocculation and aggregation process and a filtration process. The raw water was fed such that a permeate flux became 0.6 m/d, and the concentrated water flowed at a linear velocity of 0.11 m/s. The permeate flow rate was measured after passing water for 500 hours.

(44) The initial permeate flow rate was 1.10 m.sup.3/h and the initial concentrate flow rate was 5.66 m.sup.3/h, both expressed as the flow rate of an 8-inch element.

(45) The concentration of high-molecular-weight organic matter having a molecular weight of 10,000 or more in the raw water was 0.05 ppm. The results are shown in Table 3.

Comparative Example 1

(46) The same example as Example 1 was conducted except that a 0.86 mm-thick raw water spacer of polypropylene was used. The inventors assumed the use of an 8-inch reverse osmosis membrane element having a membrane surface area of 37.1 m.sup.2. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 0.93 m.sup.3/h and the initial concentrate flow rate was 6.32 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 2

(47) The same example as Example 1 was conducted except that the permeate flux was made 0.70 m/d. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.28 m.sup.3/h and the initial concentrate flow rate was 5.66 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 3

(48) The same example as Comparative Example 1 was conducted except that the permeate flux was made 0.70 m/d. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.08 m.sup.3/h and the initial concentrate flow rate was 6.32 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 2

(49) The same example as Example 1 was conducted except that the flow rate of the concentrated water was made 0.09 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.10 m.sup.3/h and the initial concentrate flow rate was 4.63 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 4

(50) The same example as Comparative Example 1 was conducted except that the flow rate of the concentrated water was made 0.09 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 0.93 m.sup.3/h and the initial concentrate flow rate was 5.17 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 5

(51) The same example as Comparative Example 2 was conducted except that the flow rate of the concentrated water was made 0.09 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.28 m.sup.3/h and the initial concentrate flow rate was 4.63 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 6

(52) The same example as Comparative Example 1 was conducted except that the permeate flux was made 0.70 m/d and that the flow rate of the concentrated water was made 0.09 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.08 m.sup.3/h and the initial concentrate flow rate was 5.17 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 3

(53) The same example as Example 2 was conducted except that a 0.50 mm-thick raw water spacer of polypropylene was used. The inventors assumed the use of an 8-inch spiral-type reverse osmosis membrane element having a membrane surface area of 50.7 m.sup.2. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.27 m.sup.3/h and the initial concentrate flow rate was 4.11 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 4

(54) The same example as Example 2 was conducted except that a 0.40 mm-thick raw water spacer of polypropylene was used. The inventors assumed the use of an 8-inch reverse osmosis membrane element having a membrane surface area of 56.4 m.sup.2. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.41 m.sup.3/h and the initial concentrate flow rate was 3.65 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 5

(55) The same example as Example 3 was conducted except that the permeate flux was made 0.50 m/d and that the flow rate of the concentrated water was made 0.11 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.06 m.sup.3/h and the initial concentrate flow rate was 5.02 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 6

(56) The same example as Example 4 was conducted except that the permeate flux was made 0.45 m/d and that the flow rate of the concentrated was made 0.11 m/s in linear velocity. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.06 m.sup.3/h and the initial concentrate flow rate was 4.47 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Example 7

(57) The same example as Example 1 was conducted except that a 0.30 mm-thick raw water spacer of polypropylene was used. The inventors assumed the use of an 8-inch spiral-type reverse osmosis membrane element having a membrane surface area of 63.6 m.sup.2 and that the permeate flux was made 0.40 m/d. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.06 m.sup.3/h and the initial concentrate flow rate was 3.78 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

Comparative Example 7

(58) The same example as Comparative Example 3 was conducted except that the raw water had the concentration of high-molecular-weight organic matter having a molecular weight of 10,000 or more of 0.005 ppm. The permeate flow rate was measured after passing water for 500 hours. The initial permeate flow rate was 1.08 m.sup.3/h and the initial concentrate flow rate was 6.32 m.sup.3/h, both expressed as the flow rate of an 8-inch element. The results are shown in Table 3.

(59) TABLE-US-00003 TABLE 3 Initial Initial Permeate Retention rate Membrane Thickness of Concentrate Concentration of permeate concentrate flow rate at of initial surface raw water Permeate linear velocity high-molecular-weight flow rate flow rate 500 hours permeate area (m.sup.2) spacer (mm) flux (m/d) (m/s) organic matter (ppm) (m.sup.3/h) (m.sup.3/h) (m.sup.3/h) flow rate (%) Example 1 44.0 0.65 0.60 0.11 0.05 1.10 5.66 1.04 94.5 Comparative 37.1 0.86 0.60 0.11 0.05 0.93 6.32 0.88 94.6 Example 1 Comparative 44.0 0.65 0.70 0.11 0.05 1.28 5.66 0.97 75.8 Example 2 Comparative 37.1 0.86 0.70 0.11 0.05 1.08 6.32 0.82 75.9 Example 3 Example 2 44.0 0.65 0.60 0.09 0.05 1.10 4.63 1.01 91.8 Comparative 37.1 0.86 0.60 0.09 0.05 0.93 5.17 0.85 91.4 Example 4 Comparative 44.0 0.65 0.70 0.09 0.05 1.28 4.63 0.90 70.3 Example 5 Comparative 37.1 0.86 0.70 0.09 0.05 1.08 5.17 0.75 69.4 Example 6 Example 3 50.7 0.50 0.60 0.09 0.05 1.27 4.11 1.16 91.3 Example 4 56.4 0.40 0.60 0.09 0.05 1.41 3.65 1.29 91.5 Example 5 50.7 0.50 0.50 0.11 0.05 1.06 5.02 1.03 97.2 Example 6 56.4 0.40 0.45 0.11 0.05 1.06 4.47 1.06 100.0 Example 7 63.6 0.30 0.40 0.11 0.05 1.06 3.78 1.06 100.0 Comparative 37.1 0.86 0.70 0.11 0.005 1.08 6.32 1.05 97.2 Example 7

(60) Table 3 clearly indicates the following.

(61) In Examples 1 to 7, a high permeate flow rate is obtained in a stable manner even after passing water for 500 hours. In particular, no decrease in permeate flow rate had occurred even after 500 hours had elapsed in Examples 6 and 7.

(62) In Comparative Examples 1 and 4, a little decrease in permeate flow rate occurred, but the permeate flow rate after 500 hours is low since the initial permeate flow rate is low. In Comparative Examples 2 and 5, in which the initial permeate flow rate is high, the large decrease in permeate flow rate results in the permeate flow rate after 500 hours being low. In Comparative Examples 3 and 6, the large decrease in permeate flow rate leads to the permeate flow rate after 500 hours being low likewise. With raw water in which the concentration of high-molecular-weight organic matter having a molecular weight of 10,000 or more was low as in Comparative Example 7, the decrease in permeate flow rate was slow.

INDUSTRIAL APPLICABILITY

(63) The present invention, which is applicable to various reverse osmosis membrane devices used in applications such as desalination of seawater, production of ultrapure water, processing of industrial water, and recovering of discharged water, is particularly suitable for reverse osmosis membrane devices that process biologically treated water, in particular, MBR-treated water.

(64) Although the present invention is herein detailed on the basis of a particular embodiment, it will be understood by those skilled in the art that various changes are possible without departing from the spirit and scope of the present invention.

(65) The present invention is based on Japanese Patent Application No. 2013-031032 filed on Feb. 20, 2013, which is hereby incorporated by reference herein in its entirety.

REFERENCE SIGNS LIST

(66) 1 Biological treatment means 2 Flocculation and aggregation treatment means 3 Solid-liquid separation means 4 Filtration means 5 Safety filter 6 Reverse osmosis membrane device 7 MBR (immersion membrane separation device) 10 Reverse osmosis membrane 11 Raw water spacer 12 Permeate spacer