CAPILLARY DISTILLATION METHOD AND DEVICE

Abstract

The present invention introduces a capillary distillation method comprising the following steps: heating wastewater, subsequently directing the heated wastewater to one end of a water-diversion fiber material. Through capillary action, the heated wastewater permeates the water-diversion fiber material, diffusing downward along microchannel pathways within the material. Upon reaching the surface of the water-diversion fiber material, the wastewater evaporates. Water vapor is then directed to a condensation module, where it undergoes condensation and collection, resulting in the production of fresh water.

Claims

1. A capillary distillation method, comprising following steps of: heating wastewater, subsequently contacting the heated wastewater to one end of a water-diversion fiber material such that, by means of capillary action, the heated wastewater enters an inside of the water-diversion fiber material from the one end of the water-diversion fiber material and gradually diffuse downward along micropore channels in the water-diversion fiber material and toward a surface of the water-diversion fiber material and evaporate after diffusing to the surface of the water-diversion fiber material; contacting water vapor to a condensation module such that the water vapor is condensed and collected, thereby obtaining distilled water.

2. The capillary distillation method according to claim 1, wherein the water-diversion fiber material is selected from one or more of cotton, hemp fiber and polyvinyl alcohol.

3. The capillary distillation method according to claim 1, wherein the water-diversion fiber material is in one or more forms of a sheet, a bundle, a mesh or a sponge.

4. The capillary distillation method according to claim 1, wherein the water-diversion fiber material has a water contact angle of <90.

5. The capillary distillation method according to claim 1, wherein the water-diversion fiber material has water-diversion micropore channels, and the micropore channels have an average pore diameter of 0.1 m-100 m.

6. The capillary distillation method according to claim 1, wherein the wastewater is heated by a heating module to a temperature of 40 C.-90 C.

7. A capillary distillation device using the method according to claim 1, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; and an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

8. The capillary distillation device according to claim 7, wherein the capillary distillation device further comprises a wastewater storage tank (1) for storing wastewater to be treated, and the return pipe (12) is connected with the wastewater storage tank (1).

9. The capillary distillation device according to claim 7, wherein the condensation module (9) is a plate-type condenser.

10. The capillary distillation device according to claim 7, wherein the diffusion chamber (4) is communicated with the wastewater storage tank (1) through a thermal-insulation pipe (11).

11. The capillary distillation method according to claim 1, wherein the water-diversion fiber material (6) has water-diversion micropore channels, and the water-diversion micropore channels have an average pore diameter of 1 m-50 m.

12. A capillary distillation device using the method according to claim 2, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; and an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

13. A capillary distillation device using the method according to claim 3, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; and an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

14. A capillary distillation device using the method according to claim 4, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; and an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

15. A capillary distillation device using the method according to claim 5, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

16. A capillary distillation device using the method according to claim 6, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), and the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

17. A capillary distillation device using the method according to claim 11, comprising: a heating assembly (3), comprising a heating device for heating wastewater feed; and an evaporation assembly, comprising a diffusion chamber (4) and an evaporation and condensation chamber (5), wherein the diffusion chamber (4) is communicated with the evaporation and condensation chamber (5) through a water-diversion fiber material (6), and the evaporation and condensation chamber (5) is communicated with a fresh water collection chamber (7), wherein the water-diversion fiber material (6) and a condensation module (9) for condensing water vapor are arranged in the evaporation and condensation chamber (5), an end portion of one end of the water-diversion fiber material (6) is connected with the diffusion chamber (4) and extends into the diffusion chamber (4), and an end portion of another end of the water-diversion fiber material (6) is connected with a return pipe (12), and a gap is provided between the water-diversion fiber material (6) and the condensation module (9).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic structural diagram of a capillary distillation device.

[0035] FIG. 2 is a schematic structural diagram of an evaporation assembly and an evaporation and condensation chamber.

[0036] FIG. 3 a schematic diagram of the principle of capillary distillation.

[0037] FIG. 4 is a data diagram of a water flux and a salt rejection rate in the treatment process of embodiment 3.

[0038] FIG. 5 is a data diagram of a water flux and a salt rejection rate in the treatment process of embodiment 4.

[0039] FIG. 6 is a data diagram of a water flux and a salt rejection rate in the treatment process of embodiment 5.

[0040] FIG. 7 is a schematic structural diagram of an air-gap membrane distillation device in Comparative Examples 1 to 3.

[0041] FIG. 8 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 1.

[0042] FIG. 9 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 2.

[0043] FIG. 10 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 3.

[0044] FIG. 11 shows contact angles of cotton, hemp fiber and polyvinyl alcohol materials.

[0045] In the figures, 1wastewater storage tank; 2feed pump; 3heating assembly; 4diffusion chamber; 5evaporation and condensation chamber; 6water-diversion fiber material; 7fresh water collection chamber; 8fresh water pump; 9condensation module; 10fresh water storage tank; 11thermal-insulation pipe; 12return pipe.

DESCRIPTION OF THE EMBODIMENTS

[0046] The following embodiments are for exemplary illustration only and are not to be construed as a limitation of the present invention. In order to better illustrate the embodiments, some components in the accompanying drawings are omitted, enlarged or reduced, and do not represent the dimensions of the actual product. It can be understood for those skilled in the art that some well-known structures in the accompanying drawings and descriptions thereof may be omitted. The positional relationships described in the accompanying drawings are for exemplary illustration only and are not to be construed as a limitation of the present invention.

[0047] When describing the embodiments of the present invention, the orientation or positional relationship indicated by the terms length, width, thickness, height, longitudinal, lateral, upper, lower, front, back, left, right, vertical, horizontal, top, bottom, inside, outside and the like is based on the orientation or positional relationship shown in the relevant accompanying drawings, and is used only for the convenience of describing the present invention and simplifying the description, rather than to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, so the above terms cannot be construed as a limitation of the present invention.

Embodiment 1 Capillary Distillation Method and Device

[0048] As shown in FIG. 1 to FIG. 2, a device using a capillary distillation method includes: a wastewater storage tank 1 for storing wastewater to be treated, a heating assembly 3 and an evaporation assembly. The heating assembly 2 includes a heating device for heating a wastewater feed. The evaporation assembly includes a diffusion chamber 4 and an evaporation and condensation chamber 5. The diffusion chamber 4 is communicated with the evaporation and condensation chamber 5 through a water-diversion fiber material 6. The evaporation and condensation chamber 5 is communicated with a fresh water collection chamber 7. The water-diversion fiber material 6 and a condensation module 9 for condensing water vapor are arranged in the evaporation and condensation chamber 5. An end portion of one end of the water-diversion fiber material 6 is connected with the diffusion chamber 4 and extends into the diffusion chamber 4, and an end portion of another end is connected with the diffusion chamber 4 is connected with a return pipe 12. A gap is provided between the water-diversion fiber material 6 and the condensation module 9.

[0049] In this embodiment, the return pipe 12 is connected with the wastewater storage tank 1. The unevaporated wastewater flowing from the water-diversion fiber material 6 returns to the wastewater storage tank 1 through the return pipe 12.

[0050] In this embodiment, the condensation module 9 is a plate-type condenser. There may be 6 plate-type condensers.

[0051] In this embodiment, a partition is arranged between the diffusion chamber 4 and the evaporation and condensation chamber 5. The partition is arranged at a bottom of the diffusion chamber 4 and a top of the evaporation and condensation chamber 5. The partition closes and isolates the diffusion chamber 4 and the evaporation and condensation chamber 5, so that the wastewater in the diffusion chamber 4 can enter the evaporation and condensation chamber 5 only through the water-diversion fiber material 6.

[0052] In this embodiment, the diffusion chamber 4 is communicated with the wastewater storage tank 1 through a thermal-insulation pipe 11.

[0053] In this embodiment, a feed pump 2 is further arranged between the wastewater storage tank and the heating assembly. A fresh water pump 8 is further arranged between a fresh water storage tank 10 and the fresh water collection chamber 7.

[0054] Further, this embodiment further provides a capillary distillation method, including the following steps: heating wastewater, subsequently contacting the heated wastewater to one end of a water-diversion fiber material such that, by means of capillary action, the heated wastewater enters an inside of the water-diversion fiber material from the end of water-diversion fiber material and gradually diffuse downward along micropore channels in the water-diversion fiber material and toward a surface of the water-diversion fiber material and evaporate after diffusing to the surface of the water-diversion fiber material; the water vapor being condensed and collected, thereby obtaining distilled water.

[0055] Further, FIG. 3 shows a schematic diagram of the principle of capillary distillation. As shown in FIG. 3, the principle of the capillary distillation method is as follows. Wastewater in the wastewater storage tank is heated by the heating device (to 60 C.) and then enters the diffusion chamber through the thermal-insulation pipe, and the heated wastewater entering the diffusion chamber contacts the end portion of one end of the water-diversion fiber material and enters the inside of the water-diversion fiber material from this end through capillarity. The heated wastewater gradually diffuses downward along the micropore channels in the water-diversion fiber material and toward the surface of the water-diversion fiber material and then evaporates in the evaporation and condensation chamber after diffusing to the surface of the water-diversion fiber material. Water vapor after evaporation of the heated wastewater contacts the condensation module (20 C.) such that the water vapor is condensed. Subsequently, the condensate water slides down along the condensation module into the fresh water collection chamber, and then enters the fresh water storage tank for storage.

[0056] In this embodiment, the water-diversion fiber material is a polyvinyl alcohol sheet material (provided by Hunan Kerun Membrane Industry Co., Ltd.).

[0057] In this embodiment, the water-diversion fiber material is hydrophilic, and specifically has a contact angle of 40.250.53 (as shown in FIG. 11) and micron-sized micropore channels (specifically 1 m).

[0058] Limited by the necessity of using microporous hydrophobic materials, the existing membrane distillation technology will inevitably have the problems of membrane wetting, membrane contamination and membrane fouling, making it still at the stage of research and development in the laboratory application and not suitable for commercial application in the market. The microporous hydrophobic membrane, serving as a mass transfer medium for water vapor, provides mass transfer channels for water vapor, and various small molecule substances and impurities contained in the wastewater will contaminate the membrane pores, causing wetting or blockage, thereby greatly affecting the efficiency of membrane distillation. Through long-term research, the inventors use the water-diversion fiber material as the mass transfer medium to replace the traditional hydrophobic fiber material membrane for distillation treatment of wastewater. According to the present invention, the water-diversion fiber material is used as a medium for transporting, carrying and evaporating feed wastewater; the wastewater diffuses inside the water-diversion fiber material after entering the diffusion chamber through the capillarity and then evaporates on the surface of the material; water vapor after evaporation of the heated wastewater forms fresh water under the action of the condensation module, and the fresh water is recovered into the fresh water storage tank and stored therein. By replacing the hydrophobic membrane material with the water-diversion fiber material as the medium for mass transfer and evaporation of water vapor, the present invention avoids the problems of wetting, fouling and scaling of the hydrophobic membrane material during the membrane distillation process, and moreover, by using cotton, hemp fiber and polyvinyl alcohol as the main material, the preparation method is mature and has low cost. The distillation based on the water-diversion fiber material in the present invention can utilize low-grade heat sources, has the advantages of compact equipment, no secondary pollution and low investment and operating costs, and is more conducive to realizing large-scale industrial application.

Embodiment 2 Capillary Distillation Method and Device

[0059] This Embodiment is different from Embodiment 1 in that: the water-diversion fiber material used in this embodiment is a cotton fiber sheet (provided by Hunan Kerun Membrane Industry Co., Ltd.) having a contact angle of 45.280.98 and an average pore diameter of 50 m.

Embodiment 3

[0060] The capillary distillation method and device in Embodiment 1 are used to treat wastewater from a factory. The factory wastewater has a salinity of 30000 mg/L, a total organic carbon (mainly engine oil) of 1000 mg/L, a pH of 7 and a temperature of 25 C.

[0061] The specific treatment process is as follows.

[0062] (1) The factory wastewater is introduced into an oil removal tank such that oil on the surface is removed. The oil-removed wastewater feed enters the wastewater storage tank, and is heated by the heating device to 60 C., and then, the heated wastewater feed is slowly lifted into the diffusion chamber by the feed pump.

[0063] (2) After entering the diffusion chamber, the heated wastewater feed contacts the end portion of one end of polyvinyl alcohol fiber sheet in the diffusion chamber. Through capillarity, the heated wastewater enters the polyvinyl alcohol fiber sheet and uniformly diffuses from top to bottom. The wastewater transported to the polyvinyl alcohol fiber sheet evaporates on the surface of the polyvinyl alcohol fiber sheet. Water vapor diffuses to the air gap between the fiber sheet and the condensation module. The water vapor after evaporation of the heated wastewater diffusing into the air gap contacts the condensation module (20 C.) and condenses into fresh water. The fresh water slides down from the condensation module under the action of gravity into the bottom of the evaporation and condensation chamber. Then, the fresh water is drawn by the fresh water pump to the fresh water storage tank for recycling.

[0064] (3) The unevaporated fresh water in the fiber sheet enters the return pipe, and then enters the wastewater storage tank through the return pipe for repeated evaporation. Step (1) and step (2) are repeated until the salinity of the wastewater in the wastewater storage tank reaches 200000 mg/L.

[0065] FIG. 4 is a data diagram of a water flux and a salt rejection rate in the treatment process of Embodiment 3. According to the results in FIG. 4, the initial flux of the capillary distillation is 2.565 L/(m.sup.2.Math.h); and after the capillary distillation continuously runs at a stable and high flux for 24 h, the salt rejection rate is up to 99.99%, and the finally produced water has a total organic carbon (TOC) of 0.28 mg/L and a measured salinity of 4.62 mg/L, which meets the criteria for distilled water.

Embodiment 4

[0066] The capillary distillation method and device in Embodiment 1 are used to treat wastewater from a factory. The factory wastewater has a salinity of 35000 mg/L, a surfactant content (mainly sodium lauryl sulfate) of 30 mg/L, a pH of 7 and a temperature of 25 C.

[0067] The specific treatment process is as follows.

[0068] (1) The factory wastewater is introduced into a cartridge filter such that suspended solid impurities in the wastewater are removed. Then, the wastewater feed enters the wastewater storage tank, and is heated by the heating device to 60 C., and then, the heated wastewater feed is slowly lifted into the diffusion chamber by the feed pump.

[0069] (2) After entering the diffusion chamber, the heated wastewater feed contacts the end portion of one end of polyvinyl alcohol fiber sheet in the diffusion chamber. Through capillarity, the heated wastewater enters the polyvinyl alcohol fiber sheet and uniformly diffuses from top to bottom. The wastewater transported to the polyvinyl alcohol fiber sheet evaporates on the surface of the polyvinyl alcohol fiber sheet. Water vapor after evaporation of the heated wastewater diffuses to the air gap between the fiber sheet and the condensation module. The water vapor diffusing into the air gap contacts the condensation module (20 C.) and condenses into fresh water. The fresh water slides down from the condensation module under the action of gravity into the bottom of the evaporation and condensation chamber. Then, the fresh water is drawn by the fresh water pump to the fresh water storage tank for recycling.

[0070] (3) The unevaporated fresh water in the fiber sheet enters the return pipe, and then enters the wastewater storage tank through the return pipe for repeated evaporation. Step (1) and step (2) are repeated until the salinity of the wastewater in the wastewater storage tank reaches 150000 mg/L.

[0071] FIG. 5 is a data diagram of a water flux and a salt rejection rate in the treatment process of Embodiment 4. According to the results in FIG. 5, the initial flux of the capillary distillation is 2.632 L/(m.sup.2.Math.h); and after the capillary distillation continuously runs at a stable flux for 24 h, the salt rejection rate is up to 99.99%, and the finally produced water has a measured salinity of 4.16 mg/L, which meets the criteria for distilled water.

Embodiment 5

[0072] The capillary distillation method and device in Embodiment 1 are used to treat wastewater from a factory. The factory wastewater has a salinity of 35000 mg/L, a Ca.sup.2+ and SO.sub.4.sup.2 content of 30 mmol/L, a pH of 7 and a temperature of 25 C.

[0073] The specific treatment process is as follows.

[0074] (1) The factory wastewater is introduced into a cartridge filter such that suspended solid impurities in the wastewater are removed. The oil-removed wastewater feed enters the wastewater storage tank, and is heated by the heating device to 60 C., and then, the heated wastewater feed is slowly lifted into the diffusion chamber by the feed pump.

[0075] (2) After entering the diffusion chamber, the heated wastewater feed contacts the end portion of one end of polyvinyl alcohol fiber sheet in the diffusion chamber. Through capillarity, the heated wastewater enters the polyvinyl alcohol fiber sheet and uniformly diffuses from top to bottom. The wastewater transported to the polyvinyl alcohol fiber sheet evaporates on the surface of the polyvinyl alcohol fiber sheet. Water vapor after evaporation of the heated wastewater diffuses to the air gap between the fiber sheet and the condensation module. The water vapor diffusing into the air gap contacts the condensation module (20 C.) and condenses into fresh water. The fresh water slides down from the condensation module under the action of gravity into the bottom of the evaporation and condensation chamber. Then, the fresh water is drawn by the fresh water pump to the fresh water storage tank for recycling.

[0076] (3) The unevaporated fresh water in the polyvinyl alcohol fiber sheet enters the return pipe, and then enters the wastewater storage tank through the return pipe for repeated evaporation. Step (1) and step (2) are repeated until the salinity of the wastewater in the wastewater storage tank reaches 150000 mg/L.

[0077] FIG. 6 is a data diagram of a water flux and a salt rejection rate in the treatment process of Embodiment 5. According to the results in FIG. 6, the initial flux of the capillary distillation is 2.582 L/(m.sup.2.Math.h); and after the capillary distillation continuously runs at a stable flux for 24 h, the salt rejection rate is up to 99.99%, and the finally produced water has a measured salinity of 3.76 mg/L.

Comparative Example 1

[0078] An air-gap membrane distillation device, as shown in FIG. 7, is used to treat the same production wastewater as in Embodiment 3. The factory wastewater has a salinity of 30000 mg/L, a total organic carbon (TOC) of 1000 mg/L, a pH of 7 and a temperature of 25 C. In this process, a polyvinylidene fluoride hydrophobic membrane having a pore diameter of 0.45 m is used.

[0079] The specific treatment process is as follows.

[0080] (1) The factory wastewater is introduced into an oil removal tank such that oil on the surface is removed. The oil-removed wastewater feed enters a feed storage tank, and is heated by a heating device to 60 C., and then, the heated wastewater feed is slowly lifted into a feed chamber by a feed pump.

[0081] (2) In the feed chamber, the wastewater feed directly contacts the polyvinylidene fluoride hydrophobic membrane, and under the driving force of the vapor partial pressure difference between both sides of the membrane, the water vapor passes through membrane pores and enters a condensation chamber. Under the action of a condensation module (20 C.), clean condensate fresh water is formed, enters into the fresh water storage tank and is collected.

[0082] (3) In the feed chamber, the feed after evaporation and concentration returns to the feed storage tank. If the flux decreases significantly or the salt rejection rate increases significantly, the operation is stopped.

[0083] FIG. 8 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 1. According to the results in FIG. 8, the flux of the membrane distillation continuously decreases from 2.633 L/(m.sup.2.Math.h) to 0.325 L/(m.sup.2.Math.h) during the operation process, with a flux loss rate of up to 87%. This indicates severe contamination on the surface of the membrane, so that the efficiency of the treatment process drops sharply and tends to 0 L/(m.sup.2.Math.h), which eventually leads to the failure of the membrane distillation.

Comparative Example 2

[0084] The air-gap membrane distillation device in Comparative Example 1 is used to treat the same factory wastewater as in Embodiment 4. The factory wastewater has a salinity of 35000 mg/L, a surfactant content (mainly sodium lauryl sulfate) of 30 mg/L, a pH of 7 and a temperature of 25 C. In this process, a polyvinylidene fluoride hydrophobic membrane having a pore diameter of 0.45 m is used.

[0085] The specific treatment process is as follows.

[0086] (1) The factory wastewater is introduced into a cartridge filter such that suspended solid impurities in the wastewater are removed. Then, the wastewater feed enters the wastewater storage tank, and is heated by the heating device to 60 C., and then, the heated wastewater feed is slowly lifted into the feed chamber by the feed pump.

[0087] (2) In the feed chamber, the wastewater feed directly contacts the polyvinylidene fluoride hydrophobic membrane, and under the driving force of the vapor partial pressure difference between both sides of the membrane, the water vapor passes through membrane pores and enters the condensation chamber. Under the action of the condensation module (20 C.), clean condensate fresh water is formed and, enters into the fresh water storage tank and is collected.

[0088] (3) In the feed chamber, the feed after evaporation and concentration returns to the feed storage tank. If the flux decreases significantly or the salt rejection rate increases significantly, the operation is stopped.

[0089] FIG. 9 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 2. According to the results in FIG. 9, the initial flux of the membrane distillation is 2.533 L/(m.sup.2.Math.h); and only after 80 min of operation, the flux continuously increases to 3.712 L/(m.sup.2.Math.h), and the salt rejection rate decreases from 99.99% to 60.42%, which indicates severe wetting of the hydrophobic membrane. The feed directly permeates to the fresh water side through the hydrophobic membrane (water at the non-feed side evaporates), causing the contamination of the fresh water. The results show that the membrane distillation fails because the hydrophobic membrane is wetted.

Comparative Example 3

[0090] The air-gap membrane distillation device in Comparative Example 1 is used to treat the same factory wastewater as in Embodiment 5. The wastewater has a salinity of 35000 mg/L, a Ca.sup.2+ and SO.sub.4.sup.2 content of 30 mmol/L, a pH of 7 and a temperature of 25 C. The polyvinylidene fluoride hydrophobic membrane has a pore diameter of 0.45 m.

[0091] The specific treatment process is as follows.

[0092] (1) The factory wastewater is introduced into a cartridge filter such that suspended solid impurities in the wastewater are removed. Then, the wastewater feed enters the wastewater storage tank, and is heated by the heating device to 60 C., and then, the heated wastewater feed is slowly lifted into the feed chamber by the feed pump.

[0093] (2) In the feed chamber, the wastewater feed directly contacts the polyvinylidene fluoride hydrophobic membrane, and under the driving force of the vapor partial pressure difference between both sides of the membrane, the water vapor passes through membrane pores and enters the condensation chamber. Under the action of the condensation module (20 C.), clean condensate fresh water is formed, enters into the fresh water storage tank and is collected.

[0094] (3) In the feed chamber, the feed after evaporation and concentration returns to the feed storage tank. If the flux decreases significantly or the salt rejection rate increases significantly, the operation is stopped.

[0095] FIG. 10 is a data diagram of a water flux and a salt rejection rate in the treatment process of Comparative Example 3. According to the results in FIG. 10, the initial flux of the membrane distillation is 2.544 L/(m.sup.2.Math.h); and due to the strong concentration polarization effect on the surface of the membrane, the fouling on the surface of the membrane gradually accumulates, so that the flux drops to 1.202 L/(m.sup.2.Math.h).

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the protection scope of the present invention. A person of ordinary skill in the art can make other variations or changes in different forms on the basis of the above description and idea, and it is unnecessary and impossible to exhaust all the embodiments here. Any modification, equivalent substitution, or improvement made within the spirit and principle of the present invention shall fall into the protection scope of the claims of the present invention.