Irrigation brackish water desalination device and irrigation water treatment method

Abstract

The invention belongs to the field of water treatment, and particularly relates to an irrigation brackish water desalination device and an irrigation water treatment method. The brackish water desalination device for irrigation comprises a desalination system and a backwashing system, the desalination system is composed of a sand filtration unit, a rough filtration unit, a microfiltration unit, an ultrafiltration unit and a nanofiltration unit which are sequentially arranged, and the treatment units are connected through pipelines; and the backwashing system consists of a pipeline and a plurality of valves which are independently arranged. According to the irrigation water treatment method, brackish water serves as water of a water source, qualified irrigation water is obtained through treatment of the brackish water desalination device for irrigation, and the irrigation water treatment method has the advantages of being low in operation pressure, large in assembly, large in water flux, capable of saving energy, environmentally friendly and the like.

Description

Technical Field

[0001] The present invention belongs to the field of water treatment, and specifically relates to a brackish water desalination device for irrigation, and an irrigation water treatment method using the brackish water desalination device for irrigation. In addition, the present invention also relates to a hollow fiber nanofiltration membrane used in a nanofiltration unit in the brackish water desalination device for irrigation. Background Art

[0002] Brackish water mainly refers to lake water, river water and groundwater with a salt content of 1000-5000 mg / L. The main reasons for the formation of brackish water are: 1) excessive irrigation water for a long time, resulting in increased salt content in the water; 2) brackish water formed by surface water flowing through saline-alkali land; 3) due to crustal movement, the water level is lower than the sea level, and seawater backflows. Brackish water contains many heavy metals and harmful impurities, tastes bitter, and is difficult to drink directly. Long-term drinking leads to gastrointestinal disorders and low immunity. In addition, excessive salts and impurities in brackish water are very harmful to the human body, and the root cause of many endemic diseases lies in drinking water. It is very common to cause digestive system diseases and is a high-incidence area for gastric cancer, esophageal cancer, cirrhosis, liver cancer and stone disease. This is caused by excessive mineralization in drinking water, excessive nitrites, sulfates and organic matter humus. In addition, water shortage has become a factor restricting the sustainable development of agriculture. For this reason, some water-scarce areas have to use brackish water directly for farmland irrigation, but this will deteriorate groundwater resources, leading to secondary salinization of the soil and affecting the growth of crops. Therefore, it is urgent to desalinate brackish water.

[0003] The desalination of brackish water is actually the desalination of salt water, so that it can reach the standard of irrigation or domestic water after treatment. At present, the main methods for desalination of brackish water are distillation, electrodialysis, reverse osmosis and nanofiltration.

[0004] Distillation is the earliest method used to desalinate brackish water. Its principle is to heat brackish water to make it boil and evaporate, and then condense the steam into fresh water. The main advantages of distillation are simple structure, easy operation, and good quality of fresh water. However, its disadvantages are high power consumption, high operating costs, easy corrosion and scaling, and difficulty in large-scale production.

[0005] Electrodialysis is a separation technology that uses ion exchange membranes to separate anions and cations in salt water under the action of an electric field, thereby reducing the salt concentration in the fresh water chamber to obtain fresh water. Electrodialysis brackish water desalination technology has the main advantages of simple process, high desalination rate, low water production cost, convenient operation, and no pollution to the environment. However, it has the disadvantages of strict requirements on water quality, need for pretreatment of raw water, small scope of application, inability to remove organic matter and bacteria in water, and high energy consumption of equipment operation, which limits its application in brackish water desalination projects.

[0006] The reverse osmosis (RO) method is to add a higher pressure than the natural osmosis on one side of the concentrated solution, reverse the natural osmosis direction, and press the ions in the solution to the other side of the semipermeable membrane. This is opposite to the normal osmosis process in nature, so it is called "reverse osmosis". Its principle is that under the action of external force, with the help of the interception effect of the semipermeable membrane, the solute and solvent in the solution are forced to separate, thereby achieving the purpose of concentration, purification or separation. The reverse osmosis brackish water desalination method can remove more than 90% of soluble salts and more than 99% of colloidal microorganisms and organic matter from water. Compared with other water treatment methods, the reverse osmosis method has the advantages of no phase change, normal temperature operation, simple equipment, low investment, high efficiency, small footprint, convenient operation, low energy consumption, wide adaptability, high degree of automation and good water quality. However, the reverse osmosis method has high pressure, which makes it expensive, and there is a problem of using concentrated water, and there is a phenomenon of "over-filtration" in the reverse osmosis process, so that some valuable components are also intercepted.

[0007] Nanofiltration (NF), also known as low-pressure reverse osmosis, is a pressure-driven membrane separation technology between reverse osmosis and ultrafiltration. The operating range of nanofiltration membranes is between ultrafiltration and reverse osmosis. It can intercept nanoscale (0.001μm) substances and has an interception capacity of 20-98% for dissolved salts. In particular, nanofiltration has a higher removal efficiency for high-valent ions in water than low-valent ions. A certain amount of low-valent ions (such as Na+, K+, etc.) that are beneficial to the human body can be allowed to pass through, while hardness ions that are harmful to the human body, including chlorination disinfection byproducts, low-molecular organic matter, etc., can be effectively removed. Therefore, nanofiltration can be used to produce direct drinking water, and a certain amount of low-valent ions are still retained in the effluent water and the treatment cost is low. However, the current disadvantages of nanofiltration are large one-time investment, high operating and maintenance costs, short life of domestic membranes, expensive imported membrane components, and high requirements for pre-treatment of raw water. Summary of the invention

[0008] Aiming at the technical problems of high investment cost, short life and high cost of membrane materials in existing brackish water treatment technology, the present invention develops a brackish water desalination device with low operating pressure, large permeability, moderate desalination rate and regenerable filter membrane.

[0009] In order to achieve the above object, the present invention adopts the following technical solution.

[0010] A brackish water desalination device for irrigation comprises a desalination system and a backwashing system; the desalination system is composed of a sand filter unit, a coarse filter unit, a microfiltration unit, an ultrafiltration unit and a nanofiltration unit which are arranged in sequence, and each treatment unit is connected by a pipeline; wherein a booster pump is arranged between the ultrafiltration unit and the nanofiltration unit, and valves are separately arranged in a raw water inlet pipeline, a pipeline for raw water flowing through each unit, and a concentrated liquid outlet pipeline of the microfiltration unit, the ultrafiltration unit and the nanofiltration unit, and the concentrated liquid is combined and discharged from the concentrated liquid outlet device; the backwashing system is composed of a pipeline and a plurality of separately arranged valves, and the backwashing system removes the deposited dirt in each filter membrane by backwashing the coarse filter membrane, the microfiltration membrane and the ultrafiltration membrane to restore the membrane flux and separation performance of each filter membrane in the coarse filter unit, the microfiltration unit and the ultrafiltration unit.

[0011] In the brackish water desalination device for irrigation, the nanofiltration unit is mainly composed of a nanofiltration membrane assembly, and the nanofiltration membrane assembly contains a filter element, which is formed by stacking a number of bundles of hollow fiber nanofiltration membranes in parallel along the water flow direction.

[0012] Furthermore, the nanofiltration membrane assembly includes: a tubular membrane shell with openings at both ends, a plurality of purified water discharge ports are arranged on the outer side of the membrane shell, and the inner side of the membrane shell is a tubular chamber; a filter element, which is composed of a plurality of bundles of hollow fiber nanofiltration membranes stacked and distributed in parallel along the axial direction of the tubular chamber on the inner side of the membrane shell, and tightly fills the entire chamber; and two end covers that close the openings at both ends of the membrane shell, one of the end covers is provided with a raw water inlet, and the other end cover is provided with a concentrated liquid outlet.

[0013] Furthermore, the materials of the membrane shell and the end cap are any one of aromatic polyethersulfone composite materials, aromatic polypropylene composite materials, aromatic polyester composite materials, aromatic polyamide composite materials, aromatic polyurethane composite materials and aromatic polyvinylidene fluoride composite materials.

[0014] In the nanofiltration membrane assembly described above, the hollow fiber nanofiltration membrane filled in the inner tubular chamber of the filter element is mainly composed of cellulose triacetate, with a pore inner diameter of 0.1-1mm and a wall thickness of 0.05-0.3mm; the hollow fiber nanofiltration membrane has a membrane flux of 50-60L / m at an operating pressure of 0.3-0.4MPa. 2 ·h, membrane strength 9-11MPa.

[0015] The brackish water desalination device for irrigation of the present invention is provided with a stainless steel shell and a base, and a universal roller for easy movement is provided under the base.

[0016] The present invention also relates to a method for treating irrigation water, wherein the irrigation water comes from brackish water, and then the brackish water desalination device for irrigation is used to treat the water to obtain irrigation water that meets the water quality requirements of the Agricultural Irrigation Standard (GB 5084-2021). Specifically, the brackish water is pumped in by a centrifugal pump, and then sequentially treated by a sand filter unit, a coarse filter unit, a microfiltration unit and an ultrafiltration unit, and then sent to a nanofiltration unit by a booster pump, and the purified water obtained after treatment by the nanofiltration unit is collected through a purified water discharge port; the concentrated liquids discharged by the microfiltration unit, the ultrafiltration unit and the nanofiltration unit are combined and discharged through a concentrated liquid outlet; during the working process, the valves in the water inlet pipelines of the sand filter unit, the coarse filter unit, the microfiltration unit, the ultrafiltration unit and the nanofiltration unit are all in a fully open state; the valves in the concentrated liquid outlet pipelines of the microfiltration unit and the ultrafiltration unit are in a slightly open or closed state; the valve in the concentrated liquid outlet pipeline diverted in the water inlet pipeline entering the nanofiltration unit is in a 1 / 3 open state.

[0017] The brackish water desalination treatment device for irrigation of the present invention is provided with a backwashing system, which can remove dirt deposited on the surface of each unit filter membrane during the use of the device, thereby regenerating the coarse filtration membrane, microfiltration membrane and ultrafiltration membrane, and restoring the membrane flux and separation performance of each filter membrane in the coarse filtration unit, microfiltration unit and ultrafiltration unit.

[0018] Furthermore, the backwashing operation method is as follows: turn off the pump and the booster pump, connect the flushing water inlet to the tap water system with a hose, use the tap water pressure, and then control the sand filter membrane, coarse filter membrane, microfiltration membrane and ultrafiltration membrane to be backwashed separately through a number of separately set valves; during the backwashing process, first turn all switches to the closed state, then open the sand filter unit control valve, and the flow direction of the flushing water in the pipeline is opposite to the direction of the desalinated water to backwash the sand filter membrane of the sand filter unit, and after the backwashing is completed, close the sand filter unit control valve; then open the coarse filter unit control valve to backwash the coarse filter membrane of the coarse filter unit, and after the backwashing is completed, close the coarse filter unit control valve; then open the microfiltration unit control valve to backwash the microfiltration membrane of the microfiltration unit, and after the backwashing is completed, close the microfiltration unit control valve; finally, open the ultrafiltration unit control valve to backwash the ultrafiltration membrane of the ultrafiltration unit, and all backwashing water is discharged through the flushing water outlet.

[0019] The hollow fiber nanofiltration membrane described above in the present invention is prepared by the following method:

[0020] 60-80 parts by volume of sulfolane solvent are heated to 35-50° C., and then 50-70 parts by mass of cellulose triacetate, 0.6-2 parts by mass of composite salt and 2-3 parts by mass of polyethylene glycol 400 are added in sequence, stirred to form a paste mixture and then continued to stir for 25-40 minutes, and then the paste mixture is heated to 60-70° C., granulated after swelling and curing treatment for 50-70 minutes, cooled and relaxed for 8-15 hours, and the obtained membrane material is spun at a constant temperature of 50° C., and then the spinning product is heated at 140° C. to 200° C. to evaporate the sulfolane solvent, then stay in cold air for 20-40 seconds, and then enter a 10-15° C. coagulation bath to obtain a nanofiltration membrane, and finally the nanofiltration membrane is washed with water, and the washed nanofiltration membrane is then subjected to glycerol curing and physical and chemical treatment to obtain a hollow fiber nanofiltration membrane product.

[0021] Furthermore, in the above-mentioned preparation method, the composite salt is a mixed salt composed of sodium chloride, magnesium chloride and zinc chloride, and the mass ratio of sodium chloride, magnesium chloride and zinc chloride in the mixed salt is 0.1-0.6:0.5-1.2:0.02-0.05.

[0022] Compared with the existing domestic nanofiltration membrane technology, the brackish water desalination device for irrigation, the irrigation water treatment method, the hollow fiber nanofiltration membrane and the nanofiltration membrane assembly of the present invention have the following significant technical features:

[0023] (1) Among the existing brackish water desalination methods, the distillation method consumes a lot of energy, has high operating costs, and is prone to corrosion and scaling; the reverse osmosis method has high requirements for raw water pretreatment, and the filter material needs to be replaced regularly according to the raw water quality and effluent requirements, which increases operating costs; the electrodialysis method causes scaling in the cathode chamber, blocks the flow channel in the concentration chamber, and forms a scaling layer on the membrane surface, affecting the membrane stack treatment performance and service life. The brackish water desalination treatment device for irrigation of the present invention can overcome the defects of the above-mentioned existing brackish water treatment methods, and each filter unit can be disassembled. The overall device has the advantages of small footprint, convenient installation and use, easy maintenance and replacement, and low investment and operating costs.

[0024] (2) The brackish water desalination device for irrigation of the present invention is used to desalinate brackish water, and the total salt content of the desalinated water obtained is less than 930 mg / L and the water recovery rate is not less than 80%. The water quality meets the requirements of the agricultural irrigation standard (GB 5084-2021), which fundamentally solves the problem that brackish water cannot be used for farmland irrigation and soil secondary salinization. The preferred application example shows that the various indicators of vegetables irrigated with desalinated water obtained by the method of the present invention are better than those of vegetables directly irrigated with brackish water. According to the irrigation conditions of the vegetable test base in the application example, the yield of the planted crops has been significantly increased. Therefore, the irrigation water treatment method of the present invention has obvious technical competitive advantages, significant social and economic benefits, and has a high value for promotion and application.

[0025] (3) In the method of the present invention, the nanofiltration membrane is prepared by using cheap and readily available main raw materials such as triacetyl cellulose, sulfolane and polyethylene glycol, and a phase inversion method is used to prepare an ultra-large nanofiltration membrane component with ultra-low starting pressure, large membrane flux, appropriate desalination rate (maintaining a sodium ion removal rate of about 80% and a calcium and magnesium ion removal rate of more than 96%) and high membrane strength. Its main performance indicators are: large mesopore diameter, inner diameter D0 - 0.5-1mm; wall thickness δ - 0.05-0.3mm; membrane flux reaches 51L / m at an operating pressure of 0.37MPa. 2 .h. The membrane strength reaches 10Mpa, which is significantly innovative compared with nanofiltration membranes at home and abroad, and provides a new type of nanofiltration membrane material for membrane separation technology.

[0026] (4) In the method of the present invention, the nanofiltration membrane assembly is prepared by an integrated casting process. The size of the cast nanofiltration membrane assembly is 160mm×1000mm, which makes it possible to cast super-large assemblies. This solves the problem that the aspect ratio of the nanofiltration membrane assembly in the currently commonly used nanofiltration membrane device is relatively low. In order to improve the processing capacity of the device, multiple stages are required to be connected in series, which makes the overall device occupy a large area and the operation process complicated. In addition, the nanofiltration membrane assembly provided by the method of the present invention has a flux of 50-60L / m under a low pressure of 0.3MPa to 0.4MPa. 2 h, which is in line with the technical development trend of nanofiltration technology with low pressure and high membrane flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a cross-sectional view of the mesoporous fiber nanofiltration membrane assembly of the present invention;

[0028] Figure 2 It is a three-dimensional schematic diagram of the mesoporous fiber nanofiltration membrane assembly of the present invention;

[0029] Figure 3 This is a schematic diagram of the process of the device for desalinating brackish water for irrigation of the present invention;

[0030] Figure 1 , Figure 2 In the figure, 1-raw water inlet; 2-purified water outlet; 3-membrane shell; 4-filter element; 5-end cover; 6-concentrate outlet; Figure 3 Among them, 1-18 are valves. DETAILED DESCRIPTION

[0031] In the following examples 1 to 5, CTA represents cellulose triacetate; PEG represents polyethylene glycol. The following preferred specific embodiments illustrate the specific implementation of the present invention. In the following examples, all volume parts mentioned are in milliliters (ml); all mass parts mentioned are in grams (g).

[0032] Example 1

[0033] Heat 60 parts by volume of sulfolane solvent to 40°C, then add 55 parts by mass of cellulose triacetate, 0.6 parts by mass of composite salt and 3 parts by mass of polyethylene glycol 400 in sequence, mix into a paste, stir for 30 minutes, then heat to 70°C, granulate after swelling and solidification for 60 minutes, cool and relax for 12 hours, heat the membrane material to 50°C for spinning, evaporate the solvent in the 140°C to 200°C zone, then stay in the cold air layer for 35 seconds, enter the 15°C condensation bath for cooling, and obtain a nanofiltration membrane. Wash the membrane with water and perform glycerol solidification physical and chemical treatment to obtain an opaque, milky, uniform tubular or filamentous hollow fiber nanofiltration membrane product.

[0034] In this embodiment, the determination methods of the technical indicators not mentioned are the general determination methods in the art. The determination methods of the membrane flux, working pressure and membrane strength among the technical indicators of the nanofiltration membrane are as follows:

[0035] (1) Membrane flux measurement method: A nanofiltration membrane of a specified area is bundled and encapsulated in a membrane module. The water flux is measured under the maximum working pressure. The measurement time is not less than 1 hour, and the flux per square meter per hour is calculated.

[0036] (2) Working pressure measurement method: A fiber membrane of a certain area is bundled in a closed membrane assembly, and ultrafiltration water is passed through. Pressure is gradually applied to push the membrane, and the water permeability is measured at the same time. At the beginning, the flux increases with the pressure. When the pressure reaches a certain value, the flux decreases. The pressure at this time is the maximum pressure P max , the corresponding maximum working pressure is 0.65P max .

[0037] (3) Method for measuring membrane strength (pressure value that causes membrane damage per unit area of ​​fiber membrane): A fiber membrane of a certain area is bundled in a closed membrane assembly, and ultrafiltration water is passed through. Pressure is gradually applied to push the membrane, and the water permeability is measured at the same time. At the beginning, the flux increases with increasing pressure. When the pressure reaches a certain value, the flux decreases. If the pressure is continued to be increased, the flux suddenly increases. The pressure value at this time is the membrane strength value.

[0038] The results of measuring the technical indicators of the hollow fiber nanofiltration membrane prepared in this example are shown in Table 1.

[0039] Example 2

[0040] 70 parts by volume of sulfolane solvent are heated to 35°C, then 50 parts by mass of cellulose triacetate, 2 parts by mass of composite salt and 2.5 parts by mass of polyethylene glycol 400 are added in sequence, after being mixed into a paste, stirred for 30 minutes, then heated to 60°C, granulated after swelling and solidification for 60 minutes, cooled and relaxed for 12 hours, the membrane material is heated to 50°C for spinning, evaporated and removed the solvent in a 140°C to 200°C heating zone, then stayed in a cold air layer for 30s, entered a 15°C condensation bath and cooled to obtain a nanofiltration membrane. After the membrane is washed with water, glycerol solidification is carried out to obtain an opaque, milky, uniformly tubular or filamentous hollow fiber nanofiltration membrane finished product. The method for determining the technical indicators of the hollow fiber nanofiltration membrane obtained in this embodiment is shown in Example 1, and the measurement results are shown in Table 1.

[0041] Example 3

[0042] 80 parts by volume of sulfolane solvent are heated to 50 ℃, then 70 parts by mass of cellulose triacetate, 1 part by mass of composite salt and 3 parts by mass of polyethylene glycol 400 are added successively, after mixing into a paste, stirring for 30 minutes, then warming up to 60 ℃, swelling and solidification after 60 minutes of granulation, cooling and relaxing after 12 hours, the membrane material is heated to 50 ℃ of spinning, evaporated in 140 ℃ ~ 200 ℃ heating zone to remove solvent, then sent into a cold air layer to stay for 20s, then enter 10 ℃ of condensation baths to cool, and obtain a nanofiltration membrane. After washing the membrane, glycerol solidification physical and chemical treatment is carried out to obtain an opaque, milky, uniform tubular or filamentous hollow fiber nanofiltration membrane finished product. The determination method of the technical indicators of the hollow fiber nanofiltration membrane obtained in this embodiment refers to Example 1, and the determination results are shown in Table 1.

[0043] Example 4

[0044] 65 parts by volume of sulfolane solvent are heated to 45 ℃, then 65 parts by mass of cellulose triacetate, 0.8 parts by mass of composite salt and 2 parts by mass of polyethylene glycol 400 are added successively, after being mixed into a paste, stirred for 30 minutes, then warmed to 60 ℃, granulated after 60 minutes of swelling and solidification, cooled and relaxed for 8 hours, the membrane material is heated to 50 ℃ of spinning, evaporated and removed the solvent in a 140 ℃ to 200 ℃ heating zone, then stayed in a cold air layer for 38s, and then entered a 12 ℃ condensation bath for cooling to obtain a nanofiltration membrane. After the membrane is washed with water, glycerol solidification is carried out to obtain an opaque, milky, uniformly tubular or filamentous hollow fiber nanofiltration membrane finished product. The determination method of the technical indicators of the hollow fiber nanofiltration membrane obtained in this embodiment is shown in Example 1, and the determination results are shown in Table 1.

[0045] Example 5

[0046] 75 parts by volume of sulfolane solvent are heated to 35 ℃, then 50 parts by mass of cellulose triacetate, 1.5 parts by mass of composite salt and 3 parts by mass of polyethylene glycol 400 are added successively, after mixing into a paste, stirring for 30 minutes, then warming up to 60 ℃, granulation after swelling and solidification for 50 minutes, cooling and relaxing after 8 hours, heating the membrane material to 50 ℃ spinning, evaporating and removing the solvent in a 140 ℃ ~ 200 ℃ heating zone, then staying in a cold air layer for 40s, and then entering a 10 ℃ condensation bath to cool, to obtain a nanofiltration membrane. After washing the membrane, glycerol solidification physical and chemical treatment is performed to obtain an opaque, milky, uniformly tubular or filamentous hollow fiber nanofiltration membrane finished product. The determination method of the technical indicators of the hollow fiber nanofiltration membrane obtained in this embodiment is shown in Example 1, and the determination results are shown in Table 1.

[0047] Table 1 Technical indicators of hollow fiber nanofiltration membrane

[0048] project unit Example 1 Example 2 Example 3 Example 4 Example 5 Membrane flux <![CDATA[L / m 2 ·h]]> 51.3 50.1 53.5 60.0 55.7 Maximum working pressure MPa 0.37 0.40 0.31 0.39 0.36 Membrane strength MPa 10.62 10.98 9.13 11.0 10.13

[0049] Example 6

[0050] A series of mesoporous fiber nanofiltration membrane components were prepared using the mesoporous fiber nanofiltration membranes prepared in Examples 1 to 5 as filter element materials. Figure 1 , Figure 2 As shown. Figure 1 is a cross-sectional view of the nanofiltration membrane assembly of the present invention, Figure 2 It is a three-dimensional schematic diagram of the nanofiltration membrane assembly of the present invention.

[0051] like Figure 1 , Figure 2 As shown, the nanofiltration membrane assembly of the present invention comprises a membrane shell 3, a filter element 4 and two end caps 5 that seal the openings at both ends of the membrane shell 3. The membrane shell 3 is a tubular structure with openings at both ends, and a plurality of purified water discharge ports 2 are arranged on the outside thereof, and a tubular chamber is arranged on the inside thereof, and the filter element 4 tightly fills the entire chamber space. Different from the prior art, the present invention does not provide a central tube for raw water or treated liquid to flow in and out, but rather distributes a plurality of bundles of hollow fiber nanofiltration membranes prepared by the present invention in parallel and densely stacked along the axial direction of the tubular chamber on the inner side of the membrane shell 3. In the nanofiltration membrane assembly of the present invention, the openings at both ends of the membrane shell 3 are sealed by two end caps 5. Of the two end caps 5, one of the end caps 5 is provided with a raw water inlet 1, and the other end cap 5 is provided with a concentrated liquid outlet 6.

[0052] The material of the membrane shell 3 and the two end caps 5 that close the openings at both ends of the membrane shell 3 in the nanofiltration membrane assembly of the present invention is preferably a polymer material, such as an aromatic polyethersulfone composite material, an aromatic polypropylene composite material, an aromatic polyester composite material, an aromatic polyamide composite material, an aromatic polyurethane composite material, and an aromatic polyvinylidene fluoride composite material.

[0053] A brackish water desalination device for irrigation was constructed using the above-mentioned nanofiltration membrane assembly, and its structural schematic diagram is shown in the figure. Figure 3 As shown. Figure 3 The device of the present invention comprises a sand filtration unit, a coarse filtration unit, a microfiltration unit, an ultrafiltration unit and a nanofiltration unit which are arranged in sequence; wherein a booster pump is provided between the ultrafiltration unit and the nanofiltration unit, and the nanofiltration unit adopts the nanofiltration membrane assembly prepared in Example 6.

[0054] The process of treating brackish water using the above-mentioned device is as follows: brackish water is pumped in through a centrifugal pump, sequentially passes through sand filtration, coarse filtration, microfiltration and ultrafiltration, and then is sent to nanofiltration through a booster pump. After nanofiltration treatment, the desalinated water is discharged through a rotor flowmeter, and the concentrated liquids discharged from the microfiltration unit, ultrafiltration unit and nanofiltration unit are combined and discharged through the concentrated liquid outlet.

[0055] like Figure 3 As shown, a booster pump is provided between the ultrafiltration unit and the nanofiltration unit, and separate valves are provided in the raw water inlet pipeline, the pipeline for raw water to flow through each unit, and the concentrated liquid outlet pipelines of the microfiltration unit, ultrafiltration unit, and nanofiltration unit. The concentrated liquid is combined and discharged from the concentrated liquid outlet. During operation, Figure 3 As shown in the figure, valves 1 to 5 are all fully open, valves 6 to 7 can be slightly open or not open (because the recovery rates of microfiltration and ultrafiltration are very high), and valve 8 is opened 1 / 3 to control the pressure of the booster pump and play a pressure dividing role. During the test, the booster pump pressure is controlled at about 0.37Mpa.

[0056] In the above device, the positions of the raw water inlet and the concentrated water outlet do not necessarily have to adopt the above mode. A more preferred embodiment is that the raw water inlet pipe and the concentrated water outlet can be placed in the raw water bucket, so that the concentrated water can be repeatedly processed, which increases the utilization rate of water and simplifies the operation steps.

[0057] The device of the present invention is designed with a backwashing system to backwash the coarse filtration membrane, microfiltration membrane and ultrafiltration membrane to restore the membrane flux and separation performance. For the nanofiltration membrane, since the recovery rate is appropriate, the concentration polarization is small, and the membrane pollution is small after pretreatment, microfiltration and ultrafiltration, and ordinary backwashing does not have a good effect, backwashing is not set. When the effluent water quality decreases significantly, the membrane assembly can be replaced. Figure 3As shown, the backwashing operation method is: turn off the pump and the booster pump, connect the flushing water inlet to the tap water system with a hose, and use the tap water pressure to backwash the sand filter, coarse filter, microfiltration and ultrafiltration separately; during the backwashing process, first turn all switches to the closed state, then open valve 14 and valve 9, and the flushing water passes through valve 14 in the opposite direction of the desalinated water to backwash the sand filter membrane of the sand filter unit, and close valve 14 and valve 9 after the backwashing is completed; then open valve 15 and valve 10 to backwash the coarse filter membrane of the coarse filter unit, and close valve 15 and valve 10 after the backwashing is completed; then open valves 16, 17 and valve 11 to backwash the microfiltration membrane of the microfiltration unit, and close valve 17 and valve 11 after the backwashing is completed; finally, open valves 16, 18 and valve 12 to backwash the ultrafiltration membrane of the ultrafiltration unit, and all backwashing water is discharged through the flushing water outlet.

[0058] The brackish water desalination treatment device for irrigation of the present invention has the advantages of small footprint, convenient operation (just press a button to start the pump to start the treatment), easy mobility (rollers are provided underneath), convenient water volume metering (a rotor flowmeter is connected when the nanofiltration water comes out), and convenient maintenance and replacement of membrane components.

[0059] Example 7

[0060] This embodiment uses the brackish water desalination treatment device for irrigation provided in Example 6 to perform brackish water desalination treatment, wherein the brackish water uses brackish water from deep wells in the Sanba Irrigation Area of ​​Fenhe River in Shanxi Province, and the nanofiltration membrane assembly used in the nanofiltration unit uses the mesoporous fiber nanofiltration membrane prepared in Example 1 as the filter element material. The device of the present invention is used to perform water treatment tests at an operating pressure of 0.37 MPa and a water temperature of 16°C. The water quality data before and after treatment are shown in Table 2 (Note: the data in Table 2 are the average values ​​of four experiments).

[0061] Table 2 Data before and after water treatment

[0062] Test items unit Raw water Desalinated water Desalination rate (%) <![CDATA[Na + ]]> mg / L 109.752 20.962 80.9 <![CDATA[K + ]]> mg / L 43.065 12.575 70.8 <![CDATA[Ca 2+ ]]> mg / L 35.323 1.212 96.6 <![CDATA[Mg 2+ ]]> mg / L 223.406 7.0958 96.8 <![CDATA[CO3 2- ]]> mg / L 0.22 Not detected 100.0 <![CDATA[HCO3 - ]]> mg / L 0.37 Not detected 100.0 <![CDATA[Cl - ]]> mg / L 832.81 82.85 90.1 <![CDATA[SO4 2- ]]> mg / L 2243.32 57.83 97.4 Total salt content mg / L 3435.875 172.492 95

[0063] From the data in Table 2, it can be seen that the Cl — The content of salt is high, up to 3435.9 mg / L, which will increase the osmotic pressure of the soil solution, hindering the absorption of water by plants, and increase soil salinization. The monovalent ions such as K in the desalinated water obtained by the device of the present invention are + and Na + The desalination rate is less than 85%, and divalent ions such as Ca 2+ Mg 2+ and SO4 2—The desalination rate is as high as 95%, which is in line with the general characteristics of nanofiltration membranes, that is, the desalination rate of divalent ions is significantly greater than the removal rate of monovalent ions. At the same time, after being treated with the device of the present invention, the desalination of Cl in water — The content of desalinated water was reduced from 832.81 mg / L to 82.85 mg / L, which is much lower than the 250 mg / L in the irrigation standard. The total salt content was also reduced from 3435.875 mg / L to 172.5 mg / L after treatment, which is much lower than the 1000-2000 mg / L requirement in the irrigation standard. Therefore, the desalinated water obtained after treatment can be used in the field of agricultural irrigation.

[0064] Example 8

[0065] In order to further verify the actual effect of nanofiltration membrane in desalinating brackish water, the Vegetable Research Institute of Shanxi Academy of Agricultural Sciences was commissioned to conduct a vegetable irrigation test on the desalinated water provided in Example 7.

[0066] The specific implementation methods and effects of the present invention are described in detail below using Chinese cabbage and radish as examples.

[0067] (I) Chinese cabbage irrigation experiment

[0068] The Chinese cabbage variety is Jincai No. 3, and the experimental plan is as follows:

[0069] ①The area of ​​test area A and B is 1 square meter;

[0070] ② The soil at a depth of 1 meter in the A and B test areas was isolated from the surrounding soil with plastic film;

[0071] ③The area of ​​test area A and B is 14 square meters. Ten cabbages were selected from each test area, and all the plants were weighed.

[0072] The whole growth period of the experimental Chinese cabbage in this embodiment is 85 days. During this period, due to the abundant rain, only six irrigations were performed. From the plant ecology observation during the growth period, the plants generally grew well in the seedling stage. After entering the continuous growth period, the differences gradually became more obvious from the ecological comparisons such as plant height and leaf size. After the heading period, the differences were even greater. The cabbages irrigated with untreated bitter and salty water and tap water had relatively small plants and more yellow leaves, but no plant death occurred. The test data are shown in Table 3.

[0073] Table 3 Cabbage experimental data of the Academy of Agricultural Sciences

[0074]

[0075] The experimental data in Table 3 show that: 1) the various indicators of the cabbage irrigated with treated bitter and salty water are generally better than those of the cabbage irrigated directly with bitter and salty water, especially from the more intuitive and obvious indicator of equivalent per mu yield, the treated water area produced 125.5 kg of clean vegetables, equivalent to 6275 kg per mu; the untreated water area produced 111.9 kg of clean vegetables, equivalent to 5595 kg per mu. Through simple calculation, it can be seen that the desalinated water obtained by the bitter and salty water desalination treatment device of the present invention is used for cabbage irrigation, which can increase the yield by 680 kg per mu, and the yield increase rate is about 13%.

[0076] (II) Radish irrigation experiment

[0077] The radish variety is Chunhong No. 1, and the experimental plan is as follows:

[0078] ① Dig a 2m×5m×1m pit in the experimental field, lay a plastic film to isolate it from the surrounding soil, then backfill the soil, tamp it down and harrow it flat, and soak it with untreated water to sow seeds;

[0079] ② In the same plot of land, 2m×5m×1m, flat ridges are used and the seeds are sown after being soaked with treated water.

[0080] ③ The brackish water and treated water provided in Example 7 of the present invention were used for irrigation during the entire growing period, and other cultivation managements as in ① and ② were completely consistent, and the process was repeated twice.

[0081] The whole growth period of the experimental radish in this embodiment is 60 days. From the observation of plant ecology during the growth period, the growth of the plants in the seedling stage is generally good, and no plant death occurs during the growth period, but as the date goes by, the growth indicators such as leaf color and leaves gradually produce obvious differences. After harvest, the shape, skin color, skin surface, average root length, diameter, single root weight and other indicators of the fleshy roots are compared, indicating that the commercial quality and quality flavor of the radish irrigated with treated water are significantly better than those irrigated with bitter and salty water directly. The test data are shown in Table 4.

[0082] Table 4 Comparison of the effects of different water qualities on Chunhong No. 1 radish

[0083]

Claims

1. A brackish water desalination device for irrigation, characterized in that: Including desalination system and backwash system; The desalination system is composed of a sand filtration unit, a coarse filtration unit, a microfiltration unit, an ultrafiltration unit and a nanofiltration unit arranged in sequence, and each treatment unit is connected by a pipeline; wherein a booster pump is arranged between the ultrafiltration unit and the nanofiltration unit, and valves are separately arranged in the raw water inlet pipeline, the pipeline for raw water to flow through each unit, and the concentrated liquid outlet pipelines of the microfiltration unit, the ultrafiltration unit and the nanofiltration unit, and the concentrated liquid is combined and discharged from the concentrated liquid outlet device; The backwashing system is composed of a pipeline and a plurality of separately arranged valves. The backwashing system controls the coarse filtration membrane, the microfiltration membrane and the ultrafiltration membrane through the control of the plurality of separately arranged valves to backwash the coarse filtration membrane, the microfiltration membrane and the ultrafiltration membrane, removes the dirt deposited in each filter membrane and restores the membrane flux and separation performance of each filter membrane in the coarse filtration unit, the microfiltration unit and the ultrafiltration unit.

2. The brackish water desalination device for irrigation according to claim 1, characterized in that: The nanofiltration unit is mainly composed of a nanofiltration membrane assembly, and the nanofiltration membrane assembly contains a filter element (4). The filter element (4) is formed by stacking a plurality of bundles of hollow fiber nanofiltration membranes in parallel along the direction of water flow.

3. The brackish water desalination device for irrigation according to claim 2, characterized in that: The nanofiltration membrane assembly comprises: A tubular membrane shell (3) with openings at both ends, wherein a plurality of purified water discharge ports (2) are arranged on the outer side of the membrane shell (3), and the inner side of the membrane shell (3) is a tubular chamber; A filter element (4), wherein the filter element (4) is composed of a plurality of bundles of hollow fiber nanofiltration membranes as described in claim 2 which are stacked and distributed in parallel along the axial direction of the tubular chamber on the inner side of the membrane shell (3) and tightly fill the entire chamber; and two end covers (5) closing the openings at both ends of the membrane shell (3), wherein one of the end covers (5) is provided with a raw water inlet (1), and the other end cover (5) is provided with a concentrated liquid outlet (6).

4. The brackish water desalination device for irrigation as claimed in claim 3, characterized in that: The material of the membrane shell (3) and the end cover (5) is any one of an aromatic polyethersulfone composite material, an aromatic polypropylene composite material, an aromatic polyester composite material, an aromatic polyamide composite material, an aromatic polyurethane composite material and an aromatic polyvinylidene fluoride composite material.

5. The brackish water desalination device for irrigation according to any one of claims 2 to 4, characterized in that: The hollow fiber nanofiltration membrane is mainly composed of cellulose triacetate, with a pore inner diameter of 0.1-1 mm and a wall thickness of 0.05-0.3 mm. The hollow fiber nanofiltration membrane has a membrane flux of 50-60 L / m at an operating pressure of 0.3-0.4 MPa. 2 ·h, membrane strength 9-11MPa.

6. The brackish water desalination device for irrigation according to any one of claims 1 to 4, characterized in that: The brackish water desalination treatment device is provided with a stainless steel shell and a base, and a universal roller for easy movement is provided under the base.

7. A method for treating irrigation water, wherein the irrigation water is brackish water, and the treatment device is the brackish water desalination device for irrigation according to claim 1, characterized in that: The brackish water is pumped in by a centrifugal pump, and then processed by a sand filter unit, a coarse filter unit, a microfiltration unit and an ultrafiltration unit in sequence, and then sent to a nanofiltration unit by a booster pump. The purified water obtained after the treatment by the nanofiltration unit is collected through a purified water discharge port (2); the concentrated liquids discharged by the microfiltration unit, the ultrafiltration unit and the nanofiltration unit are combined and discharged through a concentrated liquid outlet (6); during the operation, the valves in the water inlet pipelines of the sand filter unit, the coarse filter unit, the microfiltration unit, the ultrafiltration unit and the nanofiltration unit are all in a fully open state; the valves in the concentrated liquid outlet pipelines of the microfiltration unit and the ultrafiltration unit are in a slightly open or closed state; and the valve in the concentrated liquid outlet pipeline diverted from the water inlet pipeline entering the nanofiltration unit is in a 1 / 3 open state.

8. The irrigation water treatment method according to claim 7, characterized in that: The operation method of backwashing is: turn off the pump and the booster pump, connect the flushing water inlet to the tap water system with a hose, use the tap water pressure, and then use a number of separately set valves to control the sand filter membrane, coarse filter membrane, microfiltration membrane and ultrafiltration membrane to backwash separately; during the backwashing process, first turn all switches to the closed state, then open the sand filter unit control valve, and the flow direction of the flushing water in the pipeline is opposite to the direction of the desalinated water to backwash the sand filter membrane of the sand filter unit. After the backwashing is completed, close the sand filter unit control valve; then open the coarse filter unit control valve to backwash the coarse filter membrane of the coarse filter unit, and close the coarse filter unit control valve after the backwashing is completed; then open the microfiltration unit control valve to backwash the microfiltration membrane of the microfiltration unit, and close the microfiltration unit control valve after the backwashing is completed; finally, open the ultrafiltration unit control valve to backwash the ultrafiltration membrane of the ultrafiltration unit, and all backwashing water is discharged through the flushing water outlet.

9. A hollow fiber nanofiltration membrane used in a nanofiltration unit in a brackish water desalination device for irrigation according to any one of claims 1 to 6, characterized in that: Prepared by the following method: 60-80 parts by volume of sulfolane solvent are heated to 35-50° C., and then 50-70 parts by mass of cellulose triacetate, 0.6-2 parts by mass of composite salt and 2-3 parts by mass of polyethylene glycol 400 are added in sequence, stirred to form a paste mixture and then continued to stir for 25-40 minutes, and then the paste mixture is heated to 60-70° C., granulated after swelling and curing treatment for 50-70 minutes, cooled and relaxed for 8-15 hours, and the obtained membrane material is spun at a constant temperature of 50° C., and then the spinning product is heated at 140° C. to 200° C. to evaporate the sulfolane solvent, then stay in cold air for 20-40 seconds, and then enter a 10-15° C. coagulation bath to obtain a nanofiltration membrane, and finally the nanofiltration membrane is washed with water, and the washed nanofiltration membrane is then subjected to glycerol curing and physical and chemical treatment to obtain a hollow fiber nanofiltration membrane product.

10. The hollow fiber nanofiltration membrane used in the nanofiltration unit of the brackish water desalination device for irrigation according to claim 9, characterized in that: The composite salt is a mixed salt composed of sodium chloride, magnesium chloride and zinc chloride, and the mass fraction ratio of sodium chloride, magnesium chloride and zinc chloride in the mixed salt is 0.1-0.6:0.5-1.2:0.02-0.05.

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