How to make pure water that is safer than rainwater

Abstract

To provide a method for producing pure water by collecting rainwater during rainfall, in which an electrical conductivity of the collected rainwater is 10 μS / cm or less and a concentration of fine particles having a diameter of 0.2 μm or more is at or below a specified value.SOLUTION: There is provided a method for producing pure water, in which rain falling from the sky is collected 15 km inland from a coastline, excluding an initial rainfall; and the rain is treated with a membrane in a pore diffusion module made of all plastic, which is filled with flat composite membranes made of cellulose with an average pore size of 20 nm or more and less than 200 nm, under specific conditions of a transmembrane pressure difference of 0.05 atmospheres or less.SELECTED DRAWING: Figure 3(a)

Description

[Technical field]

[0001] The present invention relates to a method for producing safe pure water in an energy-saving manner by collecting specified rainwater and removing particulate matter dissolved or dispersed in the rainwater.

[0002] Identified rainwater is rainwater that falls and is collected as rain without coming into contact with ground components (soil, fields, roads, lakes, rivers, swamps, and other objects on the ground surface). The term refers to rainfall that does not fall on land above. The fine particles dissolved in the rainwater include soil-derived components such as yellow sand (sand, dust, etc.), volcanic ash, sea salt components derived from seawater, volcanic gases and exhaust gases. These include sulfate particles derived from air gases, and bioaerosols containing organic components (such as bacteria), and the main components are thought to be fine particles called aerosols.

[0003] In the present invention, pure water means water from which fine particles with a diameter of 0.2 μm or more have been removed and whose electrical conductivity is 10 μS / cm or less. This definition of pure water is unique to the present invention. Rainwater that normally falls to the ground has an electrical conductivity of 20 μS / cm or more, and often contains aerosols and other fine particles as organic fine particles such as bacteria and pollen that do not contribute to electrical conductivity. Therefore, it cannot be said to be safe as rainwater. [Background technology]

[0004] Water such as groundwater is used as a raw material, and depending on the degree of purification, the purified water is used for various purposes such as agricultural water and industrial water. Water as a resource can be naturally occurring water or high-temperature water. There are many types of water, including water that has been purified to a high degree (e.g. ultrapure water). Ultrapure water, which is used in semiconductor manufacturing, is considered to be the most highly purified water. However, water that is closer to H2O in terms of its chemical structure is considered to be highly purified. When the degree of purification from ultrapure water is reduced, it can be classified as purified water, distilled water, rainwater, tap water, river water, lake water, groundwater, seawater, or domestic wastewater. These classifications roughly correspond to the level of electrical conductivity of the water in question.

[0005] Water that has been subjected to advanced purification treatment, i.e. purified water, can be produced by distilling naturally occurring water (e.g. groundwater). However, this purification requires external energy and equipment such as distillation. The use of distillation as a purification method cannot be said to be a cheap or energy-saving method.

[0006] Attempts have also been made to produce highly purified water by using groundwater or tap water as the raw water and removing impurities and infectious microorganisms from the raw water through sand filtration, chemical treatment, and membrane treatment. The purified water produced in this way has already been subjected to a certain type of processing at the raw water stage, namely, adsorption and elution by the soil layer and sterilization processing when producing tap water. Groundwater and tap water are no longer considered safe water obtained naturally. Therefore, even when these waters are used as raw water for pharmaceuticals or for cleaning, there are inherent uncertainties associated with processing. There is a need for a method to obtain clean water in a natural state without relying on high pressure, heat treatment, or chemical treatment.

[0007] Rainwater is generally considered to be a highly purified source of water. However, it may be contaminated with some bacteria and viruses classified as bioaerosols. In addition, rainwater may be contaminated with fine particles generated in nature, such as yellow sand and decomposition products of pollen, and wear particles from artificial constructions, posing a safety risk.

[0008] As mentioned above, when raw water is collected as natural rainwater, various pollutants are mixed into the raw water. Methods to avoid contaminants from being mixed in, or methods of obtaining rainwater that are thought to reduce the possibility of contaminants being mixed in, have been proposed. For example, according to Non-Patent Document 1, many of the human damage cases caused by acid rain occur at the beginning of rainfall (corresponding to early rainfall in this invention). For this reason, rainfall is classified into early rainfall and subsequent rainfall, and rainwater from early rainfall is broadly classified as early rainwater and the latter rainwater as late rainwater. This document points out that the ratio of early rainwater / subsequent rainwater can be an indicator of air pollution. This research example shows that when using rainwater, it is important to exclude early rainwater to avoid the effects of air pollution. However, this document has no intention of using rainwater as an industrial raw material, and does not propose a method for determining early rainwater. If rainwater is considered a resource, the less early rainwater to be removed, the more desirable it is, but it is not possible to specify the amount of early rainwater specifically.

[0009] Rainfall is contaminated by air pollutants. In other words, rainfall reflects the pollution of the local environment. Therefore, research and development on preventing rainfall pollution is mostly done from the perspective of the pollution problem of the global environment. For example, Non-Patent Document 2 shows a survey of rainwater in Japan over a period of about 10 years from 1975 to 1985. However, surveys of the actual situation have pointed out that the increase in air pollution is a problem in terms of the suitability of rainwater as a raw material for industrial use. From the standpoint of using rainwater as a water resource, research into the acid rain phenomenon has hardly produced any concrete measures.

[0010] On the other hand, the industrial world requires water with various levels of purification. For example, water with a level of purification for industrial water sources, water with a level of purification for agricultural use, etc. This includes industrial water, tap water, water used as a raw material for beverages, water for manufacturing chemical products, water for manufacturing pharmaceuticals, ultrapure water used for cleaning semiconductor integrated circuits, etc. Rainwater is used as water. The level of purification of rainwater as raw water is currently unknown, but according to a survey in Non-Patent Document 2, it is said to be at the level of tap water or lower than industrial water. This is a higher level of purification than the above. To ensure this high level of purification of rainwater. A method for improving the security is proposed in the present invention.

[0011] A typical technique used in industry to increase the degree of purification of water is to physically remove particulate matter by utilizing the sieving effect of filtration or other methods. With this method, as the diameter of the particles to be removed becomes smaller, the sieve mesh (i.e., the pores) of the material used for filtration become smaller. When the target for removal is at the molecular level, the pore size becomes 1 nm or less, and the pressure used during filtration becomes greater. For example, the average pore size of membranes used for reverse osmosis is about 0.3 nm, and the pressure applied to the membrane is about 50 atmospheres. In other words, not only does increasing the degree of purification by removing particles through filtration require a lot of energy, but the materials required to withstand the filtration operation are made of pressure-resistant metals, which increase in weight and size.

[0012] When using tap water as the raw water before treatment to increase the degree of purification of contaminated water, residual chlorine is already present in the raw water. Adsorption treatment using activated carbon is a common method for removing other chemical components dissolved in the water. Ion exchange resins are used to remove ionic metal components, among other chemical components. It should be noted that with adsorption treatment, not only will new contamination occur when the water comes into contact with the adsorbent, but the adsorbent itself may also reduce the degree of purification due to dissolution, etc. Furthermore, when the chemical structures of the water contaminants are diverse, adsorption treatment is not a common method of purifying water.

[0013] It is also common to use the physicochemical phase separation phenomenon to increase the degree of purification of water. Distillation is a commonly used method to increase the degree of purification of water. It is particularly often used in physicochemical experiments. This method requires phase separation, which consumes a lot of heat energy, and since the water often comes into contact with the air and the distillation equipment, it is strongly affected by air and equipment contamination.

[0014] Due to the ease of obtaining natural raw water, groundwater (including well water), tap water, and river water are used regardless of the level of contamination of the raw water. However, considering the energy costs required to increase the degree of purification, it is preferable for the raw water to have a small number of types and amounts of dissolved components. A common problem with conventional technologies for increasing the degree of purification of water is that little attention is paid to the degree of purification of the raw water. A typical example of this is seawater desalination technology. Seawater has too low a degree of purification as raw water. Non-Patent Document 3 gives examples of seawater desalination technology, but these technologies show that a great deal of energy is required to increase the degree of purification.

[0015] There have been no examples to date of using pore diffusion membrane separation technology in the treatment of raw water. This technology can be applied to the treatment of raw water that is highly purified but has safety issues. Rainwater is a natural water resource with a high degree of purification. It is believed that rainwater contains components that have been dissolved in the water or that have been mixed in from the atmosphere, or that are mixed in and dispersed in a state where they are not fully dissolved in the water. Some of the mixed components may exist as impurities whose existence state is unknown. Such impurities include microorganisms such as bacteria and viruses.

[0016] In recent years, membrane separation technologies have been proposed that minimize energy consumption and can easily and highly remove water-polluting particles with larger diameters. Virus removal membranes are an example. Another example is a recent technology called pore diffusion membrane separation technology. The latter technology is also used in the present invention. This technology is characterized in that the flow of the water to be treated on the membrane surface (called the primary flow) is laminar, the transmembrane pressure difference is 0.05 atm or less, and the movement of components other than water inside the membrane is a diffusion mechanism. The flow inside the membrane is defined as the secondary flow. The primary flow is characterized by a strain rate on the membrane surface of 2 / sec or more and a viscous flow. The pore diffusion membrane separation technology is characterized in that the ratio of the flow rate of the secondary flow to the primary flow is small at 0.1. (See Patent Documents 1 and 2.)

[0017] There have been no examples of pore diffusion membrane separation technology being used to treat raw water with the aim of removing small amounts of impurities dispersed or dissolved in the water and increasing the degree of purification. This is probably because this separation technology only emphasizes the effect of concentrating impurities in the water only in the center of the fluid flow. It is generally expected that this technology will be used to remove impurities by removing only this portion from the system. Here, the primary fluid in the pore diffusion flow corresponds to the component of the flow along the membrane surface of the fluid to be treated.

[0018] The pore diffusion membrane module is a concrete example of pore diffusion membrane separation technology. A low transmembrane pressure difference and a primary flow exist along the membrane surface as a laminar viscous flow, and a secondary flow exists as a diffusion flow through the membrane. The membrane to be loaded is a porous membrane with clear pores. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] Patent Publication 2017-000922 [Patent Document 2] Patent Publication 2017-087097 [Non-Patent Document 1] Matsumoto, Mitsuhiro; Ichikawa, Hiroshi; Ichimura, Kunitoshi; Itano, Tatsumitsu. Journal of the National Pollution Research Association, Vol. 8, No. 1, pp. 17-26 (1983). [Non-Patent Document 2] Motonori Tamaki, Environmental Technology, Vol. 14, No. 2, pp. 132-146 (1985) [Non-Patent Document 3] Osaka Chemical Marketing Center, Inc., "Survey on Separation Membranes (Vol. 1)", 1980 Summary of the Invention Problem to be solved by the invention

[0020] The object of the present invention is to propose a method for producing safe pure water using rainfall as naturally occurring water with almost no energy required. The pure water referred to in the present invention is the water defined above. On a global scale, rain falls all the time somewhere in the world. This rain is used as the source water for the pure water of the present invention, which is safe water with low electrical conductivity. Almost no energy is required to obtain rain. Even in Japan, it rains almost continuously. However, this rain contains airborne pollutants such as bacteria, pollen, and viruses, which are closely related to the vital activities of living organisms. The first problem to be solved is how to collect rainfall while avoiding air pollutants and infectious particles.

[0021] Even if rainwater can be collected according to a specified standard, it is unclear what specific technology would be used to convert this rainwater into pure water by applying subsequent technology. In fact, unless the content of the subsequent technology is fixed, it is not even possible to propose specific standards for rainwater to be collected. It is expected that membrane separation technology will be suitable as the subsequent technology, as it requires almost no energy to purify water.

[0022] Even if membrane separation technology is fixed as a purification method after rainwater collection, there are no established standards for the water properties suitable for this purification method. Ultimately, the only way to determine a method for producing pure water that requires almost no energy is to use rainwater as raw water and use energy-saving membrane separation technology as the purification method. When applying this technology, it is necessary to determine what kind of rainwater should be collected based on the characteristics of this technology, and what driving force is used to purify it using a specific membrane separation technology. Unless this is done, pure water cannot be produced by the energy-saving characteristics alone. In order to utilize the energy-saving characteristics of membrane separation technology, the transmembrane pressure difference must be set lower than the pressure used in conventional filtration technology.

[0023] Setting the transmembrane pressure difference to less than 1 atmosphere makes it possible to save energy in the membrane separation device, and also reduces the weight of the materials used in the device, lowering the pressure resistance requirements for the device. On the other hand, a reduction in the transmembrane pressure difference requires that the average pore size of the membranes loaded in the membrane separation device be increased in order to achieve the expected treatment speed. Increasing the average pore size of the membranes does not allow for an increase in the degree of purification after water treatment using the membranes. [Means for solving the problem]

[0024] The first feature of the present invention is that only rainfall that falls in a specified area is collected and stored as rainwater in a plastic receiver. The specified area here means an inland area 15 km from the coastline in Japan. Rainfall is collected in this inland area.

[0025] The reason for specifying the location 15km inland from the coastline is based on actual measurements of chloride ion concentration in rainwater during rainfall, actual data on the dependency of salt damage on buildings on distance from the coastline, and theoretical considerations regarding the falling speed of raindrops. There is a balance between the forces acting on a raindrop, which is the acceleration of gravity and the viscous force acting between the raindrop and the air. According to the results of theoretical considerations, raindrops have a certain falling speed when their size is fixed. If the size of a raindrop is 0.1mm in diameter, the falling speed is 0.3m / s. If the wind speed is 10m / s, the raindrop will be carried away at this wind speed, and if it falls from 500m into the sky, it will travel a horizontal distance of up to 17km on the ground surface.

[0026] Table 1 shows the average electrical conductivity of rainfall measured in Fukuoka, Yamaguchi and Oita prefectures. As an example in Fukuoka prefecture, the change in rainfall over time in Futajima, Wakamatsu-ku, Kitakyushu City is shown in detail in Figure 1. As shown in Table 1, the electrical conductivity of rainwater decreases as the distance from the coastline increases. For example, the electrical conductivity at sea shown in Shimonoseki Port in Table 1 is 800 μS / cm, while at Kitaminato, Wakamatsu-ku, it is 90 μS / cm at 1 km from the coastline, 40 μS / cm at 2 km, 20 μS / cm at 4 km, 17 μS / cm at 20 km, and 9 μS / cm at 40 km. However, if rainwater with a concentration of 9 μS / cm is stored for a long period of time of about one month, the generation of organic fine particles is observed. In other words, the safety of rainwater does not increase even if the degree of purification in electrical conductivity increases. This is thought to be due to the presence of aerosols in the rainwater.

[0027] By using rainwater as a water source, highly refined raw water can be obtained with almost no energy consumption. To collect only rainfall, for example, gutters on the roof of a house are used, and plastic gutters are preferably made of this material. If the goal is to actively collect only rainwater, the rain is collected using a plastic plate-like object (film, etc.), which is then collected in a plastic gutter-like conduit, which is then collected in a plastic tank. The droplets produced from the seawater are dried by the wind, etc., and sea salt components remain in the air. As the sea salt components float in the air, they corrode metal components. Some of the sea salt components remain in the raindrops, which causes corrosion of metal components due to rainfall. When collecting rainwater as a water source, plastic materials are used as materials to collect the rainfall in order to prevent corrosion of the metal components from the sea salt components.

[0028] The second feature of the present invention is that only the initial rainfall is removed from the rainfall in the specified area. Here, the initial rainfall means rainfall within 2 mm from the start of rainfall. By collecting only the rainwater excluding the rainfall within 2 mm from the start of rainfall, it is possible to prevent atmospheric pollutants from being mixed into the rainfall. It was found that most of the ionic components in the initial rainfall cannot be removed by the composite membrane described later, which led to the necessity of selecting the rainfall of the present invention.

[0029] Rainfall is collected on a flat object such as a roof and guided to a conduit such as a rain gutter, and the initial rainfall is guided to a tank of a fixed capacity connected in series to the conduit. Figure 2 shows two types of tanks: Tank A (a tank that holds the initial rainfall) connected in series to the conduit, and Tank B (a tank that holds rainfall following the initial rainfall) connected in parallel. The fixed capacity of Tank A is approximately given by the product (volume) of the area of ​​the rainfall surface and 2 mm of the initial rainfall. The initial rainfall volume of 2 mm that is to be removed here is determined from the theoretical value of the moisture content in the rainfall volume described below and the change in chloride ion concentration during the initial rainfall measured.

[0030] The rainfall volume is the volume between 1 cm2 of the ground surface and 500 meters above the ground (i.e. 0.05 cubic meters). The initial rainfall volume was calculated as the amount of excess water in this space. The amount of excess water is calculated based on the difference between the saturated water content at the ground surface temperature (20°C) and the sky temperature (0°C) (the difference between 17.3 g and 4.9 g). For example, if the dew point is reached at 200 meters above ground and the relative humidity at the ground surface is 70%, the excess moisture is 7.21 (=(17.3x0.7-4.9))g / m3, and this moisture exists from 200 to 500 meters above ground, and this excess moisture is what causes rain. This excess moisture exists in 0.03 m3 of air, which is the volume of air at a height of 500 to 200 meters with a ground area of ​​1 square centimeter, and this is what causes the initial rainfall. The amount of initial rainfall estimated in this way is 0.03x7.21≒0.22g / cm2, or 2.2mm. On the other hand, the actual amount of rainfall at which the measured chloride ion concentration of rainwater decreases to about 1 / 10 of the initial amount of rainfall was 2mm. Based on these calculated and measured values ​​of the amount of rainfall, 2mm was determined as the initial amount of rainfall to be removed.

[0031] To exclude only the initial rainfall and collect the subsequent rainfall, two tanks, Tank A and Tank B shown in Figure 2, should be connected in series and in parallel to the duct, and the initial rainfall and the subsequent rainfall should be collected separately in each tank. It is not clear what substances are contained in large quantities in the components of the initial rainfall, but it is assumed that it is sea salt.

[0032] It is preferable not to collect precipitation during the winter in Japan as the rainwater to be used in the present invention. It is preferable to exclude rainfall during the winter from November to February of the following year from the rainwater to be selected. This is because not only is there little rainfall during this period, but the electrical conductivity of rainwater during this period is generally high. For example, there is a lot of rainfall with an electrical conductivity of 50 μS / cm or more. In addition, rainwater during this period is likely to reflect abnormal air pollution such as yellow sand.

[0033] The third feature of the present invention is to collect the rainfall specified as described above, and to treat it with a pore diffusion membrane separation module having the following characteristics at a transmembrane pressure difference of 0.05 atm or less and a flow rate of the primary fluid on the membrane surface of 2 / sec or more. That is, the membrane loaded in the pore diffusion membrane module is a flat cellulose composite membrane with an average pore size of 20 nm or more and less than 200 nm, a membrane thickness of 600 μm or less and a porosity of 60% or more. The average pore size of the flat membrane needs to be less than 200 nm in order to have the function of removing bacteria, which are considered to be fine particles mixed in rainwater. In addition, the recovery rate of the recovered rainwater by pore diffusion increases rapidly when the average pore size is 20 nm or more. Here, pore diffusion means membrane separation in which the transport of components other than water through the pores inside the porous membrane is mainly carried out by a diffusion mechanism. This pore diffusion occurs only in a so-called porous membrane in which the average pore size of the loaded flat membrane is 10 nm or more and the presence of many pores is confirmed by an electron microscope. This is achieved by combining a porous membrane with a small transmembrane pressure difference and a primary fluid flow rate of greater than 2 / sec, resulting in laminar flow. The components that permeate the membrane do not contain particulate components, so the water components that pass through the membrane are almost completely free of particulate components. A purification process using pore diffusion membrane separation is essential to turn the accumulated rainwater into safe pure water.

[0034] The configuration of the pore diffusion membrane separation module in the present invention is a module having all the features shown in Patent Document 2, except for the porous membrane to be filled. The porous membrane to be filled is a flat cellulose composite membrane as described above. That is, (a) a primary flow path The module is composed of four regions: (a) a primary flow path area of ​​3 cm or more in length and 6 cm or more in length with flow separation; (b) a storage area of ​​the diffusion zone consisting of the back surface of the flat membrane and three types of stripe wall surfaces; and (c) a primary fluid accumulation area where the flow paths are integrated.

[0035] Figure 3 shows a schematic diagram of a plastic cardboard-shaped module, which is a typical example of a pore diffusion membrane separation module used in the present invention. The plastics used are polyethylene, polypropylene, and polycarbonate, which are easy to melt mold. These polymeric materials are molded into a cardboard shape to create a harmonica-shaped plate with a rectangular cross-sectional hollow. In other words, it has a cross-sectional structure similar to a harmonica, in which two plate-shaped plastic plates are joined in a stepped manner by many rectangular plates. Rainwater, which is the primary fluid, flows as a laminar flow through the hollow part of this harmonica. Since the loaded composite membrane and the entire module itself are made of plastic, it has the advantage of being lightweight and not corroded by rainwater.

[0036] The cross-sectional structure of the module in Figure 3 is distinctive. It has a cross-sectional structure resembling a plastic cardboard with a harmonica-shaped hollow structure, with many equally spaced rectangular columns (called ribs) sandwiched between two plastic plate-shaped sheets (called liners). For this reason, it is abbreviated as a plastic cardboard-shaped hole diffusion membrane separation module. A plastic cardboard-shaped module can be created by leaving the columnar part (rib part) of the structure, removing the plate-shaped part of the plastic plate part (liner part), and replacing it with a composite membrane. Rainwater flows through the harmonica-shaped hollow part, and this flow corresponds to the primary fluid, and is in a laminar flow state with a slightly pressurized state with a transmembrane pressure difference of 0.05 atmospheres or less.

[0037] When the above module is in operation, the primary fluid flows in a laminar state. This prevents clogging of the pores in the composite membrane due to particles in the fluid (rainwater in this case). A separation effect occurs within the primary fluid in the pore diffusion membrane separation module, and particle components gather in the center of the flow. The present invention is characterized in that pure water is produced by recovering only the particle-free water that has passed through the flat composite membrane. In addition, the entire module, including the composite membrane loaded into the pore diffusion membrane separation module and the circuit that creates the flow of the primary fluid, is made of plastic. Since the acidity of the accumulated rainwater is on the acid side, there is almost no corrosion of the module due to this rainwater.

[0038] The lower the transmembrane pressure difference, the lower the electrical conductivity of the aqueous solution recovered as pure water. However, the lower the pressure difference, the slower the rainwater recovery rate. For this reason, the transmembrane pressure difference must be increased to about 0.02 atmospheres. An increase in the transmembrane pressure difference leads to an increase in the rainwater recovery rate and a decrease in the degree of purification of the recovered water. The same relationship to this transmembrane pressure difference is also observed in the average pore size of the composite membrane loaded into the module. For example, if the average pore size of the membrane is not 20 nm or more, the recovery rate will decrease drastically, and if the average pore size exceeds 100 nm, the degree of purification of the recovered water will decrease.

[0039] The degree of purification of the recovered liquid increases when the transmembrane pressure difference is reduced to 0.02 atm and the flow rate of the primary fluid is increased to 10 / sec or more. However, the operating conditions of these modules can be realized without using artificial energy by changing the installation conditions of the modules (for example, increasing the angle of the flow direction of the primary fluid from the horizontal direction to change the flow rate of the primary fluid). The module is small and lightweight, and the condition settings can be easily changed without power. The pore diffusion membrane separation conditions can be changed by changing the transmembrane pressure difference and the primary flow rate. The module has the same shape and configuration as the module given in Patent Document 2. However, it is different in that only the loaded porous membrane is a flat membrane-shaped cellulose composite membrane given in the present invention. In addition, since the treatment target in the module is rainwater, the present invention is characterized in that it uses an energy-saving driving force realized only by the natural flow of water without using a driving force such as a motor to flow the primary fluid.

[0040] The present invention presents a technology for producing pure water from rainwater. However, when the technology is realized, it has an energy-saving feature of using natural energy and not using electrical driving force. The reason why electrical energy does not need to be used is that only the specified rainwater is collected and a pore diffusion membrane separation module loaded with a specified membrane is used to remove the fine particle components in the rainwater using only natural driving force (for example, a head difference of less than 1 meter as a head difference pressure across the membrane). The module can exhibit the characteristics of pore diffusion under specified operating conditions. The average pore size of the flat membrane loaded in the specified pore diffusion membrane separation module is preferably 160 nm or less. This is because even if infectious substances are present in the collected rainwater, they do not include microorganisms with a diameter larger than that of retroviruses. Viruses with a particle diameter of more than 160 nm include viruses (for example, coronaviruses) that are microorganisms that are transmitted through the air, such as tuberculosis bacteria. Setting the average pore size to a low value increases the safety of the collected rainwater and increases the possibility of using the rainwater as washing water for medical equipment. In order to increase the rate at which rainwater is collected, an average pore size of 80 nm or more is suitable.

[0041] The cellulose composite membrane loaded in the pore diffusion membrane separation module in the present invention is manufactured by the following manufacturing method. That is, natural cellulose short fibers with an aspect ratio of 20 or more are assembled into a flat membrane, and a filter paper-like flat membrane with a thickness of 170 μm to 400 μm is formed. The surface of the flat membrane is coated with a mixed solution of five components, including an acetone solution of cellulose acetate and three poor solvents, namely methanol, cyclohexanol, and calcium chloride dihydrate. This mixed solution causes microphase separation due to the evaporation of acetone, which is a good solvent. After the microphase separation, the membrane is washed with water, saponified with an alkaline solution, washed with water, and dried to produce a composite membrane of natural cellulose fibers and regenerated cellulose fine particles. The average pore size of the composite membrane is mainly determined by the ratio of cellulose acetate to acetone. Effect of the Invention

[0042] By using rainfall as a water source, raw water can be obtained in an energy-saving manner. By identifying the location where rainfall is collected, seawater components that are likely to be mixed into rainfall (sea salt components that remain after water evaporates from seawater spray, etc.) can be removed from the rainfall. In addition to sea salt components, the identified initial rainfall also contains soot, NOx components, and SOx components, which are environmental pollutants in the atmosphere, so these air pollutants can also be partially removed. By removing the initial rainfall, it is possible to reduce the chloride ion concentration in the rainwater to 1 / 10, making it suitable for producing highly purified pure water using the hole diffusion method.

[0043] By further separating the specified rainfall components using a pore diffusion membrane, infectious particulate components that may be mixed into the rainwater can be removed. Not only can the membrane separation remove undissolved particulate components in the rainwater, such as sea salt components. By removing these particulate components, organic fine particles present in the rainfall are almost completely removed. This allows the produced pure water to be stored for a long time. Pure water with improved safety can be provided by removing organic aerosol components present in the rain. Even if the electrical conductivity of rainwater is 10 or less, organic and amorphous precipitate components may occur if the rainwater is stored for a long period of time of one month or more, but this precipitate does not occur after the pore diffusion treatment. The pure water of the present invention can be used as washing water for medical materials or washing water in the bioindustry. [Brief description of the drawings]

[0044] [Figure 1]Dependence of the electrical conductivity of rainwater on rainfall time after a short rainfall period (10 hours) in Futajima, Wakamatsu Ward, Kitakyushu City (Figure a), and on the electrical conductivity of rainwater after a long rainfall period (4 days) in Futajima, Wakamatsu Ward, Kitakyushu City (Figure b). Measurement location: 25 meters above ground level (apartment terrace) in Futajima, Wakamatsu Ward, Kitakyushu City. 5 km north of this location is the coast (Hibiki-nada Sea), 0.7 km to the southwest is a river, 0.2 km to the south is Dokai Bay, and within 15 km to the east is land. The solid straight vertical arrow in Figure 1(a) indicates that the rainfall occurred with a north to northwest wind at a speed of 20 m / s. The horizontal solid lines 1, 2, 3 and 4 in Figures 1(a) and (b) indicate the levels of electrical conductivity, which are predicted to be the levels of electrical conductivity of rainwater that is thought to be precipitation from clouds above the land area of ​​Futajima (probably 500 m), above the land area of ​​Futajima and Dokai Bay (probably 500 m), above Dokai Bay (probably 500 m), and above Hibiki-nada Sea (probably 500 m), respectively.

[0045] [Diagram 2] A schematic diagram showing the principle of the system for removing initial rainfall. Tank A: A tank for selectively storing initial rainfall, with a capacity determined by the rainwater collection area and the specified initial rainfall. For example, if the area of ​​the rainwater collection surface (abbreviated as collection surface A) is S (unit: square meter) and the initial rainfall is 2 mm, the tank capacity is 2S liters. Tank B: A plastic tank for storing rainwater following the initial rainfall. The tank volume is designed according to the rainwater collection surface, but it is composed of a plastic bag whose capacity can change according to the amount collected. Conduit C: A conduit that connects the two types of tanks A and B to the flat material for collecting rainwater (collection surface A), a plastic pipe-like material. Collection surface A, tank A and conduit C are connected in series, and conduit C is connected in parallel to tank B. Initial rainfall is rainfall that resumes after a period of no rainfall for more than 4 hours, usually more than 10 hours, and refers to rainfall up to 2 mm from the start of rainfall. This precipitation often contains sea salt components from the atmosphere that have been transported by diffusion.

[0046] [Diagram 3]Schematic diagram showing the structure of a plastic cardboard (commonly known as Pladan)-shaped hole diffusion module. Figure a: External view of the hole diffusion module, consisting of three sheets (Sheet 1, Sheet 2, Sheet 3), inlet / outlet ports (e1, e2) for the primary fluid (rainwater), and outlet port (e3) for the diffusion liquid. Sheet 1 has an air inlet / outlet, and Sheet 2 is composed of a composite membrane and a packing section. Sheet 3 is a processed product of the plastic cardboard, consisting of inlet / outlet ports (e1, e2) for the primary fluid. Figure b: Plan view of the three types of sheets that make up the module, Sheet 1: Only a portion of the underside of the liner part of the plastic cardboard (the so-called lower part, shown by a dashed straight line in the figure) has been cut and removed. The composite membrane is loaded into the part of Sheet 2 that corresponds to this part. Sheet 2: A sheet-like object consisting of a packing section and a composite membrane section, with an embedding agent acting as an adhesive at the joint between the two. Sheet 3: Only a portion of the liner part of the plastic cardboard (the so-called upper liner part) has been removed. A composite membrane is loaded in the portion of sheet 2 that corresponds to this portion. Figure c: Cross-section of the module. Cross-sections 1-1' and 2-2'. In cross-section 1-1', the vertical cross-section of the rib appears, and it is a cross-section perpendicular to the flow direction of the primary fluid, while cross-section 2-2' is a cross-section determined by two directions: the direction of the rib surface that exists parallel to the longitudinal direction of the module, and the thickness direction of the composite membrane. Along this cross-section, it is also a plane parallel to the flow of the primary fluid.

[0047] [Figure 4]Model diagram of an apparatus that uses a plastic cardboard-shaped pore diffusion module to treat collected rainwater with membranes: Rainwater collected under specified collection conditions is filled in the receiver R1. The rainwater is treated by the pore diffusion membrane separation module M and then flows into the receiver R2. The pore diffusion membrane separation module M has its long axis inclined at a certain angle α with respect to a horizontal line (shown by a dashed line in the figure). By specifying α, the transmembrane pressure applied to the composite membrane in the module is specified. If the longitudinal length of the membrane is L, it is given by transmembrane pressure = (Δh1-Δh2) / 2 = L / 2· sin α. The flow rate of rainwater is determined by sin α and the height of the rib part of the module, as in the case of the transmembrane pressure. The flow direction of rainwater is indicated by solid arrows in the figure. The flows are indicated by s1, s2, s3, and s4. That is, the collected rainwater flows from the receiver R1 through the tube as flow s1, and then flows as flow s2 in the pore diffusion membrane separation module M. The rainwater that has been treated by the membrane in the module is accumulated in R2 via s3. A portion of the accumulated rainwater is returned to receiver R1 by pump P or by manual power. The rainwater is separated by a pore diffusion membrane through the composite membrane cm at flow path s2 to become a diffused liquid, which then becomes a flow (indicated by the dashed arrow in the figure) d1. The diffused liquid then becomes a flow d2 through a tube and is collected in receiver R3 connected to the module. The best way to invent

[0048] Rainwater after a certain period of rainfall (about 10 minutes) was collected in October and December 2022 at a location in Asakura Town, Fukuoka Prefecture (at an altitude of about 100 meters). These were designated as samples 1 and 2. The location is located about 45 km inland from the three coastlines of the Genkai Sea, the Ariake Sea, and the Suo-Nada Sea. The electrical conductivity of the collected rainwater was 9 μS / cm and 22 μS / cm for samples 1 and 2, respectively. As shown in the results of the electrical conductivity of rainfall exemplified in the Futajima area of ​​Wakamatsu Ward (a typical example is shown in Figure 1), the electrical conductivity of rainwater (proportional to the concentration of salts dissolved in the water) decreases rapidly with the duration of rainfall. The main component contributing to the electrical conductivity during the initial rainfall is sea salt. It is difficult to remove this component by pore diffusion membrane separation alone, so it is necessary to remove it efficiently at the rainwater collection stage. The solid arrow part in Figure 1(a) is when the north wind was blowing at 2 meters / second or more. At this time, the electrical conductivity of the rainwater rises sharply due to rainfall caused by clouds over Hibikinada, the northern coastline. The dashed arrows in Figure 1(b) indicate that when the rain stops, a change in wind direction occurs at the same time. It is predicted that level 4 rainfall shown in Figure 1(b) will not occur 15 km inland from the coastline. When rainfall occurs in the Futajima area in Figure 1, if the wind from the coastline is strong, the electrical conductivity of the rainwater at level 4 in Figure 1(b) will increase. It is predicted that the electrical conductivity of the rainwater will decrease even further if the distance of the rainwater collection point from the coastline is further than 15 km.

[0049] Table 1 shows the electrical conductivity of rainwater when the distance from the coastline to the rainwater collection points is changed. The rainwater collection points are distributed in Fukuoka, Yamaguchi, and Oita prefectures. The distance from the coastline to the collection points was measured on a 1:45,000 map. The rainwater collected was collected at a location where the precipitation amount was between 20mm and 50mm. The electrical conductivity of rainwater decreases rapidly the further away from the coastline, dropping to a level of 10μS / cm or less at distances of 15km or more. However, although the electrical conductivity of rainwater in Asakura, 40km from the coastline, is often below 10μS / cm, the rainwater (for example, the rainwater in sample 1) is unsafe as it contains organic foreign matter as shown below.

[0050] When the rainwater is left in a sealed plastic container at room temperature for a week, foreign matter grows to a size that can be seen with the naked eye. The foreign matter is light green to gray in color and has a specific gravity of 1.0, which is close to that of rainwater but slightly larger. Observation of the foreign matter with a polarizing microscope reveals that the foreign matter is composed of amorphous organic matter. Considering the specific gravity, amorphousness, and the cause of the rain, it is concluded that the foreign matter is an organic aerosol that has grown. Therefore, it is clear that the identified rainwater still contains amorphous organic foreign matter, and that the subsequent pore diffusion membrane separation process is essential to produce safe pure water.

[0051] Asakura is taken as an example of rainwater with low electrical conductivity among the rainwater collection points shown in Table 1. If the initial rainfall is excluded from the rainwater at this point, the electrical conductivity level is at the level of pure water, but it is clear that the particle removal performance does not meet the conditions of pure water. For example, if the rainwater is stored for several days, organic fine particles will be generated in the rainwater. Sample 1, which is rainwater collected at Asakura, was divided into two and stored at room temperature for two weeks. That is, the collected rainwater, which is one of the divided components, was transferred into a plastic bottle and stored at room temperature in a sealed state (this is abbreviated as sample 1b). The remaining rainwater was treated with a membrane using the hole diffusion module in Figure 3 (sample 1s). [Table 1]

[0052] The treatment conditions for sample 1s are as follows. The cellulose composite membrane loaded in the membrane module is a porous membrane with an average pore size of 90 nm, a porosity of 80%, and a thickness of 390 μm. Rainwater was separated using a pore diffusion membrane under operating conditions of a transmembrane pressure difference of 0.02 atm and a strain rate of rainwater as a primary fluid of 100 / sec. After treatment, the rainwater was poured into a plastic bottle, sealed, and stored at room temperature. The electrical conductivity of samples 1s and 1b was 8 to 9 μS / cm, respectively. Sample 1s was about 1 μS / cm smaller than 1B, but there was almost no difference between the two. However, a floating mass of white to gray cotton-like matter was generated at the bottom of sample 1b and grew to a size of 5 mm. On the other hand, the transparency of the rainwater in sample 1s was higher than that of sample 1b, and no cotton-like floating particles were observed. From this fact, it was concluded that sample 1s was pure water as defined in this patent.

[0053] In the present invention, pure water is defined as having an electrical conductivity of 10 μS / cm or less and containing no particles with a diameter of 0.2 μm or more. It is easy to experimentally measure the electrical conductivity of rainwater. However, it is impossible to prove that it is free of particles. This is because the amount of the target liquid that does not contain particles is not defined, so it is unclear whether it is on the scale of 1 ml or 1 kl. Considering that the condition "containing no particles" was included due to the requirement for safety to the human body, it was concluded that the necessary conditions of this section are satisfied in the present invention by processing using "pore diffusion membrane separation technology with a logarithmic removal coefficient Φ of 0.02 μm colloidal particles of 2 or more." Φ is given by the following formula. Φ = log N0 / Nf (1) Here, N0 is the concentration of colloidal particles before separation treatment with the pore diffusion membrane, and Nf is the concentration in the solution after treatment with the membrane. Example 1

[0054] The rainfall collection area was set at an altitude of about 60 meters in the Sugawa area of ​​Asakura City, Fukuoka Prefecture, and a tank system for storing rainwater with the initial rainfall removed, as shown in Figure 2, was constructed. The location is 40 km inland from the coastline. As the rain collection surface, polyethylene sheets (two layers of 0.05 mm thick) were assembled into a regular hexagonal umbrella shape, and a conduit was connected to the center, resulting in a rain collection area of ​​0.81 square meters. Tank A was a polyethylene bottle with a volume of 2 liters. Tank B was a polyethylene tank with a volume of 18 liters. As conduit C, a polyvinyl chloride pipe (diameter 3 cm) was cut and glued to create a prototype as shown in Figure 2.

[0055] Rainfall on March 21, 2023 was collected using the device in Figure 2. After confirming that the wind speed was within 10 m / s, the initial rainfall was collected. When about 2 liters of rainwater was stored in tank A as the initial rainfall, the rainwater automatically continued to be stored in tank B. The electrical conductivity of the rainfall in tank B was 24 μS / cm, while the electrical conductivity of the rainfall in tank B was 5 μS / cm. It can be seen that the components of the initial rainfall were stored in tank A and that the electrical conductivity of this component was high. It can be seen that removing the initial rainfall is essential to satisfy the necessary condition that the collected rainwater is pure water. Data on the initial rainfall in the Futajima area of ​​Wakamatsu Ward, Kitakyushu City (about 2 to 4 km inland from the coastline) shows that the contribution of the sea salt components that diffused into the atmosphere of the area of ​​interest from the sky above the sea is large for the components of the initial rainfall. Considering this view, the validity of the method of collecting rainwater to remove the initial rainfall is supported by collecting inland rainwater.

[0056] A pore diffusion membrane separation module shown in Figure 3 was fabricated using polycarbonate plastic cardboard (abbreviated as "pladan"). That is, a part of one side of the liner part of sheets 1 and 3 of the plastic cardboard shown in Figure 3 was cut off, and sheet-like material 2 was inserted and bonded to that part. The spacer part of sheet 2 (i.e., equivalent to the packing part) is composed of a packing sheet made of ethylene-propylene rubber with a thickness of 0.5 mm. The composite membrane part of sheet 2 is a flat membrane made of cellulose with an average pore size of 80 nm (measured by the filtration rate method) and a porosity of 72%. The composite membrane was fabricated by the following microphase separation method. Commercially available filter paper (made of cellulose, Whatman Grade 3, model number 1003917, thickness approximately 390 μm) was used as a flat membrane-shaped body of natural cellulose short fibers. The filter paper was divided into two equal pieces measuring 23 cm x 57 cm. One surface of each filter paper was treated with a steam iron, and this surface was used as the surface of the filter paper.

[0057] The casting solution was applied to the surface of this filter paper with an applicator to a casting thickness of 750 μm at 25°C. The casting solution used to induce microphase separation had a weight ratio of cellulose acetate / acetone / methanol / cyclohexanol / calcium chloride dihydrate of 0.07 / 0.57 / 0.15 / 0.15 / 0.06. After casting, the film was washed for 12 hours under running water over a period of 4 hours, and then saponified for 48 hours in a 1N potassium hydroxide aqueous solution. After the treatment, the film was washed for 24 hours with running water and dried at 25°C under constant length to produce a cellulose composite membrane.

[0058] The composite membrane produced by the above manufacturing method had a membrane thickness of 410 μm, an average pore size of 0.08 μm by the water filtration rate method, and a porosity of 72%. The logarithmic removal coefficient of this membrane at a transmembrane pressure difference of 0.015 atm for an aqueous solution of ferric hydroxide colloid particles (average particle size of 20 nm by dynamic light scattering method, manufactured by Nippon Tokushu Membrane Development Co., Ltd., colloid concentration of 1200 ppm) was 2.0. The composite membrane was molded into a sheet-like material shown in Sheet 2 (a commercially available solvent-based adhesive was used to bond the spacer part and the composite membrane). This was loaded into a polycarbonate plastic board-type pore diffusion module shown in Figure 3. The rib height was 3.5 mm, the rib spacing was 6.5 mm, and the liner part thickness was 0.5 mm. In order to operate the transmembrane pressure difference at 0.05 atm or less, the pressure resistance of the module is sufficient at 0.1 atm, and the strength of the plastic board is sufficient to withstand this pressure.

[0059] Rainwater in tank B was treated with a membrane using a plastic board-type hole diffusion module (effective membrane area 180 cm2) loaded with a composite membrane. The module was assembled into a membrane treatment device according to Figure 4. The long axis direction of the module (the length of the membrane is L, and this direction is the long axis) is inclined by α with respect to the horizontal direction. By setting this α, the transmembrane pressure difference is calculated as L / 2·sinα (expressed as a water column head difference). α was set to 1.3° and L = 40 cm. The calculated transmembrane pressure difference is 40 / 2·sin1.3°, and approximately 0.5 cmH2O (i.e., approximately 0.0005 atm) is loaded on the composite membrane. This transmembrane pressure difference causes only the water molecules in the rainwater to move through the composite membrane by a filtration mechanism. As this water moves, the diffusion of molecules other than water in the rainwater also occurs, and transport occurs through the membrane. Aerosols dissolved or dispersed in the rainwater are removed by membrane treatment. The flow velocity S2 at the membrane surface is approximately close to S3. The flow rate of S3 is determined by opening and closing the flow rate control cock installed in the circuit S3. The flow rate of S3 was set to 10 / sec. Rainwater flow rate The rainwater components collected by this membrane removal diffuse through the membrane and move to the back side of the membrane, becoming the diffused liquid flow d1. In the present invention, the rainwater components that become the diffused liquid flow d1 or d2 in the figure are collected. The diffused liquid flow passes through d2 and is collected in the receiver R3 in Figure 4. The electrical conductivity of the rainwater components after diffusion in R3 is about 4 to 5 μS / cm, which is reduced by about 1 μS / cm due to the membrane treatment. The transparency of the water was also increased by the membrane treatment to a degree that was obvious even when observed with the naked eye.

[0060] The flow of rainwater in tank B along the composite membrane was laminar, based on the flow velocity, the load conditions of the driving force of the flow, and the direct observation of the flow. The flow velocity is expressed as the strain rate. In other words, the flow velocity of S2 can be approximated as S2 / (4·(rib height)) as a strain rate because the height of the rib is 5 mm. The S2 value is determined by the control cock attached to S3. By setting the strain rate of S2 to 10 / sec, the particulate components in the rainwater flowing through S2 are concentrated in the center of the flow. Under these conditions, the flow separation mechanism of membrane separation technology can be used. It is believed that this is why the transparency of the rainwater in the receiver R3 has increased. The rainwater in tank B and the rainwater in R3 were stored in stoppered PET bottles at room temperature of 15°C for one week. As a result, greenish-gray cotton-like floating matter with diameters of 0.1mm to 3mm was generated from the rainwater in tank B. On the other hand, no floating matter was observed in the rainwater in R3. From these experimental results, it is considered that aerosols present in rainwater are removed by the pore diffusion membrane treatment using the composite membrane.

[0061] By installing a liquid flow rate control pump P in the rainwater flow circuit S4 of the membrane treatment device of the plastic board type hole diffusion module shown in Figure 4, it becomes possible to automatically pour the rainwater in the receiver R2 into the receiver R1. This makes it possible to perform membrane treatment by hole diffusion multiple times on the rainwater. The aerosol components dispersed inside the rainwater can be concentrated by multiple membrane treatments. [Industrial Applicability]

[0062] Removal of fine particles dispersed in the air, such as salt components dissolved in rainwater and organic substances dispersed therein, is essential for quality control when rainwater is used for drinking purposes or for cleaning industrial products. The pure water provided by the present invention is applicable not only to food and cleaning water, but also to chemical product manufacturing (e.g., urea water). In addition, when the fine particles are substances that have a strong effect on living organisms, such as infectious microorganisms (e.g., viruses, bacteria, etc.) and pollen component fine particles, their removal is also essential for health management. In industry, this technology can be used for purification in the manufacturing processes of pharmaceuticals, regenerative medicine products, food and cosmetics, etc. [Explanation of symbols]

[0063] The meanings of the numbers, letters, etc. shown in Figures 1 to 4 are summarized below. Dashed vertical arrows indicate an increase in the electrical conductivity of raindrops due to initial rainfall. Solid vertical arrows indicate an increase in electrical conductivity due to strong northerly or northwesterly winds of 20 m / s or more. Dashed vertical straight lines indicate an increase in electrical conductivity due to terminal rainfall. Horizontal solid lines 1, 2, 3, and 4 indicate a certain level of electrical conductivity. Solid line 1 indicates the level of electrical conductivity of rainwater during steady rainfall caused by rainclouds over Futajima, while solid lines 2, 3, and 4 are the predicted values ​​of the level of electrical conductivity of steady rainfall caused by rainclouds over the land part of Futajima and the western part of Dokai Bay, the level of electrical conductivity of steady rainfall caused by rainclouds mainly over the entrance to Dokai Bay and Kokura, and the level of electrical conductivity of steady rainfall caused by rainclouds over Hibiki-nada Sea, respectively.

[0064] Tank A: A tank that connects rainwater that falls on the rain reservoir sheet in series to the pipe C via pipe C. The tank's volume is set to a value equivalent to 2mm of rainfall. The tank is made of plastic. Tank B: A tank that is connected in parallel via pipe C and collects steady rainfall. The tank is made of plastic. Pole: A support that supports the rain reservoir sheet. Rigger lid: A mechanism to prevent rainwater from tank A from flowing back into tank B.

[0065] e1; rainwater inlet / outlet provided on sheet 1 of the hole diffusion module (the sheet through which the components of rainwater after membrane treatment that have diffused or been filtered through the membrane flow). e2; rainwater inlet / outlet provided on sheet 3 of the hole diffusion module before membrane treatment. cm; composite membrane. ad; adhesive that bonds the gap between the spacer (sp) and the composite membrane. sp; spacer, a membrane-like material made of ethylene-propylene copolymer with a thickness equal to the membrane thickness of the composite membrane. It prevents rainwater from flowing through the thickness of the composite membrane. ln; liner part of plastic cardboard (that makes up sheets 1 and 3). rb; rib part of plastic cardboard. se; sealing part. P; liquid pump for lifting rainwater from receiver R2 to receiver R1.

[0066] f: Air sterilization filter installed at the air inlet. M: Membrane separation module for pore diffusion. d1: Flow of rainwater diffusion liquid that has permeated the membrane in the module. d2: Components similar to the rainwater diffusion liquid create a flow in the module, and this flow flows out to the receiver R3. s1, s2, s3, and s4: Circuits that realize the flow of rainwater along the thin arrows, s1 is a circuit that connects the receiver R1 to the inlet of the membrane separation module, and s2 is a circuit that forms a flow from the inlet of the module to the outlet where the air filter f is located, and is the rainwater circuit in the module. s3 is a rainwater flow path that leads to the receiver 2, and the water column difference Δh2 is the driving force for the flow. s4 is a circuit for transporting the rainwater in the receiver R2 to R1. A liquid pump P is used for this transport, and the transport is automatic. If the transport from R2 to R1 is done manually, this circuit is not used. R1, R2, and R3: Plastic receivers for temporarily storing rainwater before membrane treatment. Δh1: hydraulic head difference between receivers R1 and R3, Δh2: hydraulic head difference between receiver R1 and the outlet of membrane separation module M.

Claims

1. A method for producing pure water from rainwater in an energy-saving manner through the following steps (1) to (4): step (1) collecting rainfall that falls from the sky and is collected in an inland area 15 km or more away from the coastline, excluding rainfall with a rainfall amount of 2 mm or less from the initial rainfall, step (2) passing the rainwater through an all-plastic pore diffusion module loaded with a flat membrane-like cellulose composite membrane having an average pore size of 20 nm or more but less than 200 nm and a membrane thickness of 200 μm or more but less than 600 μm, step (3) operating the module under conditions in which the transmembrane pressure difference is 0.05 atm or less and the flow of the primary fluid on the membrane surface is a laminar flow of 2 / sec or more when expressed in terms of strain rate, and step (4) recovering only the rainfall components that pass through the membrane to produce safe pure water.

2. The method for producing pure water according to claim 1, characterized in that the method for removing rainfall of 2 mm or less from the initial rainfall in step (1) comprises collecting the rainfall on a water-insoluble flat object such as a roof, guiding the collected rainfall to a conduit forming a waterway made of a water-insoluble material such as a gutter, guiding the initial rainfall to a tank of a certain capacity (abbreviated as Tank A) connected in series to the conduit, and allowing subsequent rainfall that exceeds the capacity of Tank A to overflow and be collected in a tank (abbreviated as Tank B) connected in parallel to the conduit, thereby removing only the initial rainfall.

3. A method for producing pure water according to claim 1 or 2, characterized in that rainfall collected in a period other than winter from November to February of the following year is used as the rainwater selected in step (1), and in step (2) the rainfall is collected using a polycarbonate plastic cardboard-type pore diffusion membrane module loaded with a cellulose flat-membrane composite membrane having an average pore size of 80 nm or more and less than 160 nm, and in step (4) components passing through the membrane are recovered under normal pressure.

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