JP7633612B2 - How to make pure water that is safer than rainwater - Google Patents

Description

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

The specified rainwater is rainwater that falls and is collected as rainfall without coming into contact with ground components (objects on the ground surface such as soil, fields, roads, lakes, rivers, and swamps), and means rainfall that does not fall on land on the ground surface. The fine particle components dissolved in the rainwater include soil-derived components such as yellow sand (sand, dust, etc.), volcanic ash, sea salt components derived from seawater, sulfate particles derived from volcanic gas and exhaust gas, and bioaerosols containing organic components (bacteria, etc.), and the main components are considered to be fine particles called aerosols.

In this invention, pure water means water from which fine particle components 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 this invention. Rainwater that normally falls to the ground has an electrical conductivity of 20 μS/cm or more, and rainwater often contains aerosols and other fine particles as organic fine particles such as bacteria and pollen that do not contribute to electrical conductivity. For this reason, it cannot be said to be safe as rainwater.

Groundwater and other water are used as raw materials, and the purified water is used for various purposes such as agricultural water and industrial water depending on the degree of purification. There are many types of water as a resource, such as naturally occurring water and highly purified water (e.g., ultrapure water, etc.). Ultrapure water used in semiconductor manufacturing is considered to be the most highly purified water. However, water with a chemical structure close to _H2O is considered to be highly purified water. When the degree of purification from ultrapure water is reduced, it is classified as purified water, distilled water, rainwater, tap water, river water, lake water, groundwater, seawater, and domestic wastewater. The magnitude of the electrical conductivity of the target water roughly corresponds to these classifications.

Water that has been subjected to a high level of purification processing, i.e. purified water, can be produced by subjecting naturally occurring water (e.g. groundwater from springs) to purification processes such as distillation. However, this purification process requires external energy and equipment such as distillation. Using a purification method such as distillation cannot be said to be a cheap or energy-saving method.

Attempts have been made to produce highly purified water by using groundwater or tap water as the raw water and removing impurities and infectious microorganisms from it through sand filtration, chemical treatment, and membrane treatment. The purified water produced in this way has already been subjected to a type of processing at the raw water stage, namely, adsorption and elution by the strata, and sterilization processing during the production of tap water. For this reason, groundwater and tap water can no longer be considered safe water obtained naturally. Therefore, even when these waters are used as raw water for pharmaceuticals or as cleaning water, there are inherent uncertainties associated with processing. A method is needed to obtain cheap and safe water in a natural state without relying on high pressure, heat treatment, or chemical treatment.

Rainwater is generally considered to be a highly purified source of water. However, it may be contaminated with some bacteria and viruses that are classified as bioaerosols. In addition, it may be contaminated with fine particles generated in nature, such as yellow sand and decomposition products of pollen, and wear debris from man-made construction materials, posing a safety risk.

As mentioned above, when raw water is collected as natural rainwater, various pollutants are mixed into the raw water. Methods have been proposed to avoid contaminants being mixed in, or to obtain rainwater that is thought to reduce the possibility of contaminants being mixed in. 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 rainwater from the latter rainfall is classified 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.

Rainfall is contaminated by air pollutants. In other words, rainfall reflects the pollution of the local environment. For this reason, most research and development into preventing rainfall pollution is done with the aim of treating it as a global environmental pollution problem. For example, Non-Patent Document 2 shows a survey of rainwater conditions in Japan over a 10-year period from 1975 to 1985. It points out that qualitative changes in rainwater properties are causing acid rain, which has become a major social problem. However, these surveys also point out that the increase in air pollution raises questions about 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.

On the other hand, water with various levels of purification is required in the industrial world. For example, water at the level of a water source for industrial water, water at the level of agricultural water, etc. Industrial water, tap water, and ultrapure water used as raw water for beverages, water for manufacturing chemical products, water for manufacturing pharmaceuticals, and water for cleaning semiconductor integrated circuits, etc. In the present invention, rainwater is used as raw water. The level of purification of rainwater as raw water is currently unknown, but referring to the survey in Non-Patent Document 2, it is positioned as having a purification level ranging from industrial water to tap water or higher. This high level of purification of rainwater is ensured. In other words, the present invention proposes a method for ensuring this high level of purification and increasing safety.

In industry, a common technique for increasing the degree of purification of water is to physically remove particulate matter by utilizing the sieving effect of filtration, etc. 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 used to withstand the filtration operation are made of pressure-resistant metals, which increase in weight and size.

When using tap water as the raw water before treatment to increase the purification level 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 can new contamination occur when the water comes into contact with the adsorbent, but the adsorbent itself can also reduce the purification level due to dissolution, etc. Furthermore, when the chemical structures of the water contaminants are diverse, adsorption treatment is not a common method for purifying water.

The physicochemical phenomenon of phase separation is often used to increase the degree of purification of water. Distillation is commonly used as a 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 thermal energy, and because the water often comes into contact with the atmosphere and the distillation equipment, it is strongly affected by air and equipment contamination.

Because natural raw water is easy to obtain, 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 large amount of energy is required to increase the degree of purification.

There have been no examples to date of using pore diffusion membrane separation technology to treat raw water. This technology can be applied to treat 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 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 is unknown. Such impurities include microorganisms such as bacteria and viruses.

Membrane separation technology has been proposed in recent years that minimizes energy consumption and can easily and highly remove water-polluting particles, especially when the particles are large in diameter. Virus removal membranes are an example. Another example is a technology that has recently been 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 atmospheres 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 has a strain rate of 2/sec or more on the membrane surface, making it a viscous flow. The pore diffusion membrane separation technology is characterized in that the ratio of the flow rates of the secondary flow and the primary flow is small, at 0.1. (See Patent Documents 1 and 2.)

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 likely because this separation technology only emphasizes the effect of concentrating impurities in the water only in the center of the fluid flow. This is because it is generally expected to be used as a technology to remove impurities by removing only this portion outside the system. Here, the primary fluid in the pore diffusion flow corresponds to the components of the flow along the membrane surface of the fluid to be treated.

The pore diffusion membrane module is a concrete example of pore diffusion membrane separation technology in a module. There is a low transmembrane pressure difference and the primary flow exists along the membrane surface as a laminar viscous flow, while the secondary flow exists as a diffusion flow through the membrane. The membrane that is loaded is a porous membrane with clear pores.

Patent Publication No. 2017-000922 Patent Publication 2017-087097 Mitsuhiro Matsumoto, Hiroshi Ichikawa, Kunitoshi Ichimura, and Tatsumitsu Itano, Journal of the National Pollution Research Association, Vol. 8, No. 1, pp. 17-26 (1983) Motonori Tamaki, Environmental Technology, Vol. 14, No. 2, pp. 132-146 (1985) Osaka Chemical Marketing Center Co., Ltd., "Survey on Separation Membranes (Vol. 1)", 1980

Problem to be solved by the invention

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. Here, the pure water referred to in the present invention is the water defined above. On a global scale, it is constantly raining 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.

Even if it were possible to collect rainwater during rainfall using specified standards, it is unclear what specific technology would be used to apply subsequent technology to this rainwater to turn it into pure water. Conversely, unless the content of the subsequent technology is fixed, it would be impossible to even propose specific standards for the rainfall that should be collected. It is expected that membrane separation technology would be the most suitable subsequent technology, as it requires very little energy to purify water.

Even if membrane separation technology is fixed as the purification method after collecting rainwater, there are no set 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 will be collected based on the characteristics of the technology, and what specific membrane separation technology will be used to purify it, without clarifying what driving force is used to purify it, otherwise pure water will not 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.

Setting the transmembrane pressure difference to less than 1 atmosphere makes it possible to reduce energy consumption of the membrane separation device, and also reduces the weight of the materials used to make the device, lowering the pressure resistance requirements for the device. On the other hand, a decrease in the transmembrane pressure difference means that the expected treatment speed cannot be achieved unless the average pore size of the membranes loaded into the membrane separation device is increased. Increasing the average pore size of the membranes cannot be expected to increase the degree of purification after water treatment using the membrane.

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

The basis 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. Theoretical considerations show that once a raindrop's size is determined, it will have a constant falling speed. If the size of the raindrop is 0.1mm in diameter, the falling speed is 0.3m/sec. If the wind speed is 10m/sec, 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 on the ground surface of up to 17km.

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 Figure 1. As shown in Table 1, the electrical conductivity of rainwater decreases the further away from the coastline it is. For example, at Shimonoseki Port in Table 1, the electrical conductivity is 800μS/cm at sea, 90μS/cm at 1km from the coastline, 40μS/cm at 2km, 20μS/cm at 4km, 17μS/cm at 20km and 9μS/cm at 40km. However, if rainwater with a conductivity of 9μS/cm is stored for a long period of time, such as 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.

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 can be 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 (film, etc.) and then collected in a plastic gutter-like conduit, which is then stored 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 of the sea salt components.

The second feature of the present invention is that it removes only the initial rainfall from the rainfall in the area specified above. Here, initial rainfall means rainfall with a rainfall amount of 2 mm or less from the start of rainfall. By collecting only the rainwater excluding the rainfall amount of 2 mm or less from the start of rainfall, it is possible to prevent atmospheric pollutants from being mixed into the rainfall. It was discovered that most of the ionic components in the initial rainfall could hardly be removed by the composite membrane described below, which led to the need for the selection of the rainfall of the present invention.

Rainfall is collected on a flat object such as a roof and directed into a conduit such as a rain gutter, and the initial rainfall is directed into a tank of 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 given approximately 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 measured change in chloride ion concentration during the initial rainfall.

The rainfall volume is the volume between 1 cm2 of the ground surface area 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 in the saturated water content between 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 amount of 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 the 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 for a ground area of 1 cm2, 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. Meanwhile, 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.

To exclude only the initial rainfall and collect subsequent rainfall, two types of tanks, Tank A and Tank B shown in Figure 2, can be connected in series and in parallel to a conduit, and the initial rainfall and subsequent rainfall can 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.

It is preferable not to collect precipitation during the winter in Japan as rainwater to be used in this 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.

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 into 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 sharply 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 so-called porous membranes 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 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, and therefore 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.

The pore diffusion membrane separation module of the present invention has all the features shown in Patent Document 2, except for the porous membrane. The porous membrane 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 diffusion storage area consisting of the back surface of the flat membrane and three types of stripe walls; and (c) a primary fluid accumulation area where the flow paths are integrated.

Figure 3 shows a schematic diagram of a plastic cardboard-like module, 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-like plate with a rectangular cross-sectional hollow section. In other words, it has a cross-sectional structure similar to a harmonica, with two plate-like plastic plates joined in a stepped manner by many rectangular plates. Rainwater, the primary fluid, flows as a laminar flow through the hollow section of this harmonica. The loaded composite membrane and the entire module itself are made of plastic, which has the advantage of being lightweight and not corroded by rainwater.

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 numerous 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 portion (rib portion) of the structure, removing the plate-shaped portion of the plastic plate portion of the plate-shaped portion (liner portion), and replacing it with a composite membrane. Rainwater flows through the harmonica-shaped hollow portion, and this flow corresponds to the primary fluid, and is in a laminar flow state with a slight pressure difference between the membranes of 0.05 atmospheres or less.

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. The acidity of the accumulated rainwater is on the acid side, so there is almost no corrosion of the module due to this rainwater.

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 seen 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.

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 achieved without using artificial energy by changing the installation conditions of the modules (for example, by 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 differs 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 the driving force of a motor or the like to flow the primary fluid.

This invention presents a technology for producing pure water from rainwater. However, when this technology is realized, it has the 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 by collecting only specified rainwater and using a pore diffusion membrane separation module loaded with a specified membrane, it is possible to remove particulate 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 contain microorganisms with a diameter larger than that of a retrovirus. Viruses with a particle diameter of more than 160 nm include viruses (for example, coronaviruses) that are microorganisms that infect 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. To increase the rainwater collection rate, an average pore size of 80 nm or more is ideal.

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-like assembly, which is made into a filter paper-like flat membrane with a thickness of 170 μm to 400 μm, and a mixed solution of five components, consisting of an acetone solution of cellulose acetate and three poor solvents, methanol, cyclohexanol, and calcium chloride dihydrate, is applied to the surface of the flat membrane. 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 particles. The average pore size of the composite membrane is mainly determined by the ratio of cellulose acetate to acetone.

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 sea 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 air, so this air pollution 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.

By further separating the identified rainfall components using a pore diffusion membrane, infectious particulate components that may be mixed into the rainwater can be removed. This membrane separation can also 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 makes it possible to store the pure water for a long time. Pure water with improved safety can be provided by removing the 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.

Dependence of electrical conductivity of rainwater on rainfall time for short-term (10 hours) rainfall in Futajima, Wakamatsu Ward, Kitakyushu City (Figure a) and long-term (4 days) rainfall on rainfall time (Figure b). Measurement location: 25 meters above ground level (apartment terrace) in Futajima, Wakamatsu Ward, Kitakyushu City. 5 km north of the location is the coast (Hibiki-nada Sea), 0.7 km southwest is a river, 0.2 km south is Dokai Bay, and within 15 km east is land. The vertical arrows in the solid straight lines in Figure 1 (a) indicate rainfall with a north-northwest wind 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, and are predicted to be the levels of electrical conductivity of rainwater 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.

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 initial rainfall amount. For example, if the area of the rainwater collection surface (abbreviated as collection surface A) is S (unit: square meter) and the initial rainfall amount 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 be changed according to the amount collected. Conduit C: A conduit made of plastic pipe material that connects two types of tanks A and B and a flat material for collecting rainwater (collection surface A). 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.

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 part of the underside of the liner part of the plastic cardboard (the so-called lower part, shown by the 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 material 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 part 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 the sheet 2 that corresponds to this portion. Figure c: Cross-section of the module. Cross sections 1-1' and 2-2', 1-1' cross section shows the vertical cross section of the rib and is a cross section perpendicular to the flow direction of the primary fluid, while 2-2' cross section is a cross section determined by two directions: the direction of the rib surface that exists parallel to the longitudinal axis of the module and the thickness direction of the composite membrane. Along this cross section, the plane is also parallel to the flow of the primary fluid.

Model diagram of an apparatus for membrane treatment of collected rainwater using a plastic cardboard-shaped pore diffusion module: 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 long axis of the pore diffusion membrane separation module M is 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, the transmembrane pressure is given by (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 specific 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 part of the accumulated rainwater is returned to the receiver R1 by pump P or by manual power. The rainwater is separated by the composite membrane cm at the flow path s2 into a diffused liquid, which flows as d1 (indicated by the dashed arrow in the figure). The diffused liquid flows through a tube into d2 and is collected in the receiver R3 connected to the module.

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Rainwater after a certain period of rainfall (about 10 minutes) was collected in October and December 2022 at a site in Asakura Town, Fukuoka Prefecture (at an altitude of about 100 meters). These samples were designated as Sample 1 and Sample 2. The site is located about 45 km inland from the three coastlines of the Genkai Sea, Ariake Sea, and 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 arrows in Figure 1(a) indicate when the north wind was blowing at 2 meters per 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, the wind direction changes 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 in the rainwater at level 4 in Figure 1(b) will rise. If the distance of the rainwater collection point is further than 15 km from the coastline, the electrical conductivity in the rainwater is predicted to decrease even further.

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 rainfall was between 20mm and 50mm. The electrical conductivity of rainwater drops 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 due to the presence of organic foreign matter as shown below.

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 rainwater but slightly larger. Observation of the foreign matter under a polarizing microscope reveals that the foreign matter is composed of amorphous organic matter. Taking into account 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 subsequent pore diffusion membrane separation processing is essential to producing safe pure water.

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 to 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).

The treatment conditions for sample 1s are as follows. The composite cellulose membrane loaded in the membrane module is a porous membrane with an average pore size of 90 nm, a porosity of 80%, and a membrane 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 the 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 lower 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 in diameter. 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 is pure water as defined in this patent.

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.

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 after removing the initial rainfall (Figure 2) was constructed. The location is 40 km inland from the coastline. A polyethylene sheet (two layers of 0.05 mm thick) was assembled into a regular hexagonal umbrella shape to serve as the rain collection surface, 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. A polyvinyl chloride pipe (diameter 3 cm) was cut and glued to serve as conduit C, and a prototype was constructed as shown in Figure 2.

Rainfall on March 21, 2023 was collected using the device shown 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.

A pore diffusion membrane separation module shown in Figure 3 was produced 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 glued in 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 made of cellulose in the form of a flat membrane with an average pore size of 80 nm (measured by the filtration rate method) and a porosity of 72%. The composite membrane was produced 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.

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 treatment, the film was washed for 24 hours with running water and dried at 25°C at a constant length to produce a cellulose composite membrane.

The composite membrane produced by the above 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 atmospheres 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 thickness was 0.5 mm. In order to operate the transmembrane pressure difference at 0.05 atmospheres or less, the pressure resistance of the module is sufficient at 0.1 atmospheres, and the strength of the plastic board is sufficient to withstand this pressure.

Rainwater in tank B was treated with a membrane using a plastic cardboard-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 is 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 removing the membrane 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 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.

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 x (rib height)) as a strain rate, since the rib height 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 using membrane separation technology can be used. This is thought to have increased the transparency of the rainwater in receiver R3. The rainwater from tank B and the rainwater from 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 0.1 mm to 3 mm was generated from the rainwater in tank B. On the other hand, no floating matter was observed in the rainwater in R3. Based on these experimental results, we believe that aerosols present in rainwater are removed by the pore diffusion membrane treatment using the composite membrane.

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 receiver R2 into receiver R1. This makes it possible to perform membrane treatment of the rainwater by hole diffusion multiple times. The aerosol components dispersed within the rainwater can be concentrated by multiple membrane treatments.

Removal of fine particles dispersed in the air, such as salt components dissolved in rainwater and dispersible organic substances, is essential for quality control when using rainwater for drinking purposes or as water 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). Furthermore, when the fine particles are substances that have a strong effect on living organisms, such as infectious microorganisms (e.g., viruses, bacteria, etc.) or 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.

The meanings of the numbers, letters, etc. shown in FIGS. 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 over 20 m/s. 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.

Tank A: A tank that connects the rain that falls on the rain reservoir sheet to the pipe in series via pipe C. The tank's volume is set to a value equivalent to the amount of rainfall of 2 mm. The tank is made of plastic. Tank B: A tank that is connected in parallel via pipe C, and is a tank that 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.

e1; A rainwater inlet/outlet provided on sheet 1 of the hole diffusion module (a sheet through which the components of rainwater after membrane treatment that have been diffused or filtered through the membrane flow). e2; A 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; A 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 (constituting 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.

f: air sterilization filter installed at the air inlet. M: membrane separation module for pore diffusion. d1: flow of rainwater diffusion liquid permeated through 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 connecting 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 flow path for rainwater leading 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. When 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 (3)

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. The method for producing pure water according to claim 1, characterized in that as a method for removing rainfall of 2 mm or less from the initial rainfall in step (1), the method 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. 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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