Continuous desalination of seawater

Abstract

When desalining seawater with RO membranes, membrane degradation due to fouling is a problem. Fouling by marine microorganisms is a major cause of this. [Solution] A MAX filter is used as a pretreatment to suppress fouling. The MAX filter can remove bacteria and microorganisms contained in the water. Subsequently, seawater desalination can be carried out continuously by RO membrane treatment.

Description

Technical Field

[0001] The present invention relates to the continuous treatment of seawater for the production of fresh water.

Background Art

[0002] As a technology for converting seawater into fresh water, the method of obtaining fresh water by thermal distillation uses reduced-pressure distillation to improve thermal efficiency. This method requires a large amount of thermal energy, making simple and mobile production of fresh water difficult. There is a membrane filtration method called the RO (Reverse Osmosis) method, which pressurizes seawater and concentrates it through a reverse osmosis membrane to obtain the permeated fresh water. Although this method consumes less energy, it has problems of clogging caused by fine particles (especially marine microorganisms) and membrane damage. In addition, there is a method of obtaining fresh water by atomization using ultrasonic waves, and the like.

Prior Art Documents

[0003]

Patent Documents

Patent Documents

Summary of the Invention

Problems to be Solved by the Invention

[0004] Many membranes with low energy consumption are used for producing fresh water from seawater. In particular, RO is an apparatus for producing fresh water from seawater by removing chloride ions in seawater through fine pores and chemically treated substances on the surface. Seawater, including freshwater, contains many fine dust particles and microorganisms. Furthermore, the diverse elements and molecular ions present in seawater damage the membrane's constituent materials, accelerating its deterioration. Suppressing this membrane performance degradation due to fouling is considered extremely important. Fouling can be caused by solid particles such as sand and dead microorganisms, inorganic precipitates such as calcium carbonate, organic matter (concentration of dissolved organic matter within the membrane), or biofouling (bacteria and the resulting biofilm formation). To solve this problem, a method using sand filtration, which is used for water purification, as a pretreatment was applied. However, the pretreatment section and the filtration membrane became clogged, requiring backwashing. Removal by sand filtration is slow, and relies heavily on the filtration effect of the filtration membrane created by microorganisms attached to the surface of the sand filter material. Therefore, the slow filtration rate is a problem, and increasing the filtration rate requires a vast area, which is also a problem. In RO membrane filtration, foulants in the water clog the pores and accumulate on the membrane surface, forming a cake layer that degrades membrane performance. Obtaining a certain amount of filtrate requires increasing the operating pressure and chemical cleaning, leading to increased costs. Furthermore, RO membrane filtration without pretreatment causes membrane clogging by microorganisms in the water, and heat treatment or disinfectant treatment also damages the membrane, failing to solve the problem. Therefore, we need to find a solution.

[0005] The goal was to invent a seawater desalination device that is compact, uses little energy, allows for variable production capacity with short maintenance times, and is capable of continuous operation. This invention <1> ~ <2> This method consists of the following steps. <1> As a pretreatment device, a MAX filter is used, which has a fast filtration rate and can remove particles larger than approximately 0.5 μm. (1) A process in which fine diatomaceous earth is used as a coating material for the MAX filter. (2) If the filtration pressure of the MAX filter increases, an automatic backwash is performed and the coating material is reattached to the MAX filter. (3) A process in which microorganisms can be removed using a MAX filter. The filtration device "MAX filter" used in this invention is a device manufactured by Monobe Engineering Co., Ltd. and is already in practical use. <2> The effect of the invention of a method for desalination by filtering the filtrate from a MAX filter with a PO membrane.

[0006] According to the present invention, since pretreatment is performed with a MAX filter, continuous filtration at a high speed is possible, and a method for removing particles and microorganisms larger than 0.5 μm can be provided. Continuous RO membrane filtration minimizes clogging of the filter membrane, thus extending the membrane life due to clogging. [Brief explanation of the drawing]

[0007] [Figure 1] Schematic diagram of the seawater desalination continuous production apparatus of the present invention [Figure 2] MAX filtration system and RO membrane desalination system [Modes for carrying out the invention]

[0008] The above invention will be described in detail below. Seawater contains microplastics (mostly 100 μm or larger), microorganisms such as Daphnia, and bacteria (mostly 1 μm or larger), which can be removed by the MAX filter. The MAX filter is spring-shaped, and the gap between the springs can be adjusted. It can be used for filtration with just the springs, but filter media can also be used. In either case, if the gap becomes clogged, it can be backwashed and the filter media removed, allowing it to be used again as a filter immediately.

[0009] Tables 1, 2, and 3 show the results of filtration tests using Artemia, Skeletonema, and Senedesmus, which are commonly found in water. Each test shows results with and without a filter medium. Note that Artemia ranges from 250 to 450 μm, Skeletonema ranges from 2 to 20 μm, and Skeletonema has a diameter of about 10 μm. Organisms smaller than aquatic microorganisms such as water fleas are bacteria, and the number of microorganisms and bacteria varies depending on many factors such as season, environment, and salt concentration. In general, even in bays with a relatively large number of bacteria, the number is 10 per 1 ml 3 to 10 4 , and most of them have a size of 0.5 μm or more. Table 3 shows the results of a filtration test using typical Escherichia coli. The test shows the cases with and without the use of a filter medium. In addition, although not bacteria, filtration using talc particles was also attempted, and the turbidity of the test results is also shown. [Examples]

[0010] A filtration test was conducted using a MAX filter with the gap of the MAX filter adjusted to 20 μm and 60 μm. Seawater with suspended Artemia was filtered with the MAX filter for each of the cases with and without the filter medium. The results are shown in Table 1. [Table 1] [Examples]

[0011] A filtration test was conducted using a MAX filter with the gap of the MAX filter adjusted to 60 μm. Seawater with suspended Skeletonema was filtered with the MAX filter using a filter medium. The results are shown in Table 2. [Table 2] [Examples]

[0012] A filtration test was conducted using a MAX filter with the gap of the MAX filter adjusted to 60 μm. Fresh water with suspended connected Scenedesmus was filtered with the MAX filter using a filter medium. Since Scenedesmus is a connected body of 3 to 4 individuals, one connected body was counted as one unit. The results are shown in Table 3. [Table 3] [Examples]

[0013] The gap of the MAX filter was adjusted to 20 μm, the diatomaceous earth filter medium was loaded, and 1x10 5 A filtration test was performed on seawater containing E. coli at a concentration of / ml using the MAX filter. The number of bacteria in the filtrate was determined by counting the number of colonies formed using LB plates. The results are shown in Table 4. [Table 4] [Example]

[0014] After the filter medium was installed in the MAX filter with a gap of 20 μm, seawater was filtered, and the turbidity and electrical conductivity of the filtrate were measured. The results at that time are shown in Table 5. [Table 5] [Example]

[0015] The MAX filter-treated seawater was subjected to treatment using a Toray TM810V RO membrane, and the pH, electrical conductivity, and desalination rate of the treated water were measured. The results are shown in Table 6. [Table 6]

Claims

1. A method for removing bacteria and microorganisms from water, including seawater, using a MAX filter.

2. A method of obtaining fresh water from seawater by filtering seawater treated with a MAX filter through an RO membrane.

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