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Application of reverse osmosis technology in drinking water purification in Russia Author: Zhang Shoukai Abstract: It discusses the necessity and feasibility of using reverse osmosis to purify drinking water. It introduces two characteristics of the reverse osmosis process used in Russia for purifying drinking water: 1. No pre-treatment is carried out before the reverse osmosis unit ； 2. The purified water quality meets health standards. Keywords: drinking water; reverse osmosis; Russia. With economic development and the improvement of living standards, there are increasing demands for drinking water, necessitating the design and adoption of new, advanced water purification technologies. To date, the approach taken to address this issue has been to add ozone and activated carbon adsorption units to conventional water purification systems, which greatly complicates the water purification process, **increases investment costs, with little improvement in water quality. One view is that groundwater is less affected by human activities and can be used as a substitute for surface water sources (even in large cities). However, most groundwater has high hardness and iron content, making it more complex and costly to purify using traditional water treatment methods (coagulation–sedimentation–filtration) compared to treating such water sources. With the advancement of technology, it has been discovered that for water supply stations with low flow rates, where the raw water comes from surface water, groundwater, or even seawater, membrane purification methods can solve this problem. To date, the membrane methods that have been applied and tested are electrodialysis and reverse osmosis. However, reverse osmosis has greater adaptability and future potential, as it can not only remove salts and other substances in ionic form but also suspended solids, organic matter, colloids, bacteria, and viruses. When the operating conditions of the purification process are disrupted, the water output of the device is reduced merely while maintaining the desired purification effect. In the past, reverse osmosis was considered to be unable to compete with other traditional purification methods—such as coagulation and sedimentation, aeration, and adsorption—in terms of economic efficiency. Most experts regarded it as a specialized and expensive method suitable only for industries such as pharmaceuticals, electronics, and food processing, owing to its high energy consumption (7.5–10 kW/m3 of treated water), high costs associated with membrane elements and pumping equipment, as well as the complexity and size of the pretreatment systems required. In recent years, membranes and membrane processes have continued to improve, and many types of high-pressure and low-pressure composite membranes have now been designed and are being manufactured for use in purifying raw water in various applications. When selecting solutions to purify surface water heavily contaminated with toxic substances and hard groundwater with high iron content, reverse osmosis is considered a good option for water treatment plants with small to medium capacity, as it is compact and simple, while also ensuring high quality of the treated water. Traditional water disinfection methods use chlorine oxides, but they have faced significant criticism and restrictions in recent years due to the formation of toxic trichloromethane, as well as their poor effectiveness against certain viruses. In addition to removing toxic organic and inorganic pollutants from water, reverse osmosis purification also ensures complete disinfection. Its capital and operating costs are becoming increasingly competitive with traditional methods, not to mention those specialized fields that require high-quality water. This is because reducing the operating pressure (0.7–1.6 Mpa) while increasing the water output per membrane unit leads to lower power consumption, as well as reduced costs for the pressure housing, pump equipment, hydraulic collection systems, and distribution systems. In recent years, the reverse osmosis method for purifying drinking water has also been widely used in Russia. In addition to importing reverse osmosis equipment from the United States and Germany, Russia also has its own factories that produce such equipment. The Small and Medium-Sized Reverse Osmosis Devices, developed through years of research, design, and manufacturing by the Russian Research Institute of Water Supply, Drainage, Hydraulic Structures, and Engineering Hydrogeology, can operate automatically; they require no large amounts of chemicals, need few maintenance personnel, and can be used in everyday living conditions. Russia has two important characteristics in applying reverse osmosis methods to purify drinking water. 1 No pretreatment is installed before the reverse osmosis unit. It is generally believed that substances present in water can cause membrane contamination, such as suspended solids and colloids, iron (iron oxide and ferrous iron), bacterial contamination, precipitation of poorly soluble substances (calcium sulfate and calcium carbonate), and organic pollutants (humic acid and fulvic acid). Removing these substances requires a more complex system design; due to the many components needed for pre-purification, it is not possible to create a simple and reliable automated system suitable for everyday use. Through research, the Russian Academy of Sciences has identified, both theoretically and through experimental studies, the patterns of scaling of various deposits on membranes, as well as the substances that have the most adverse effect on the properties of the membranes during operation. From the perspective of suppressing the formation and removal of precipitates, crystalline calcium carbonate poses the greatest threat to membranes. The most practical and effective way to remove these precipitates is to add 1–3 mg/l of scale inhibitor to the raw water, which can reduce the intensity of precipitation by 90%–95%. To remove the small amount of calcium carbonate deposits that form, the membrane can be cleaned periodically using organic acids and surfactants. The effectiveness of these cleaning solutions was evaluated, and it was found that under appropriate operating conditions, it is possible to completely remove the deposits and maintain the membrane’s performance at satisfactory levels. This led to the conclusion that an automated water purification system that requires no special maintenance or pretreatment can be developed. Intermediate tests were conducted on groundwater sources without pretreatment, during which periodic cleaning was used. Tests show that the performance parameters of the membrane after regeneration have, in fact, returned to their original levels due to the removal of a large amount of precipitates (calcium carbonate, suspended solids, and colloids). The Russian Academy of Sciences has conducted numerous studies and industrial tests on roll-type membrane reverse osmosis systems. It has been proven that the advanced purification of groundwater, surface water containing 10–15 mg/l of suspended solids and a colority of 30–40 degrees, as well as seawater and tap water, can be carried out without any pretreatment of the raw water; with 1–3 mg/l of scale inhibitor added, it is possible to operate continuously for 1000–3000 hours. Results of studies on surface water containing 5–10 mg/l of suspended solids show that the continuous cleaning cycle is 500–1000 hours; if operation continues without cleaning, the pressure loss increases by 0.4–0.5 MPa, thereby reducing the water production volume. Tests were once conducted using standard spiral-wound membrane elements with a diameter of 100 mm and a length of 1000 mm to treat river water containing 5–10 mg/l of suspended solids. The tests showed that the pressure loss increased as the flow rate through the device increased; therefore, for raw water with high concentrations of suspended solids and operating under harsh conditions, it is necessary to select the lowest possible water production rate. Based on the experience gained from experimental research, the Russian Academy of Sciences has designed reverse osmosis systems with a water production capacity of 1–10 m3/day that do not require pretreatment; it has also developed household units with a capacity of 100–200 L/h. 2 The quality of the purified water meets sanitary requirements. Although using reverse osmosis systems to improve well water and tap water has many advantages, the quality of the purified water is in serious conflict with the standards set by health authorities, namely the Guidelines on Health Aspects of Water Desalination WHO, ETS/80.4. Because desalinating water using reverse osmosis devices also removes the essential trace elements calcium, magnesium, and fluoride required by the human body, it can cause bone diseases and tooth decay. Water purified by reverse osmosis can remove a large amount of pollutants, while reducing the salt content in the water, that is, the concentration of ions in it, by 96% to 99%. As drinking water, there are certain requirements regarding the ions present in the purified water; it should contain a certain amount of salts, as well as calcium, magnesium, sodium, potassium, ammonium, chlorides, bicarbonates, sulfates, nitrates, and fluorides. For example, in the drinking water standards of Russia and the European Community, the allowable concentration of fluoride ions is 0.5–1.5 mg/L, while the World Health Organization recommends a calcium level of 1–2 mg-eq/L. Today, in the United States and Europe, **reverse osmosis systems are widely used to purify well water as well as remove fluoride, nitrate, and nitrite ions, which are very difficult to eliminate; the same is true in Russia. Taking the purification of groundwater with high fluoride levels near Moscow as an example, let’s see how Russia uses reverse osmosis systems to improve water quality while ensuring it meets drinking standards. While removing fluoride, it is necessary to ensure that the water hardness meets health standards; however, the challenge lies in the fact that reverse osmosis membranes are much less effective at retaining monovalent ions compared to divalent ions. This often happens: in an effort to reduce fluoride levels to the required amount, the hardness drops to a level that is not acceptable according to health standards ； When the total salt content and hardness have been reduced to the desired levels ； The content of individual ions, such as fluoride, also fails to meet the requirements. Therefore, it is necessary to select an appropriate membrane in order to achieve the desired level of water purification. The salt content of the purified water is also determined by the following parameters of the device: operating pressure, temperature, and water output rate (the ratio of the flow rate of the purified water to the total flow rate of water entering the device). The analysis results of Moscow’s groundwater with high fluoride content, using a computer program designed by an American company, are listed in Table 1. Table 1 Analysis results of groundwater with high fluoride content in Moscow *Raw water flow: 80 m3/h, effluent flow: 60 m3/h, efficiency: 75% **Raw water flow: 120 m3/h, effluent flow: 60 m3/h, efficiency: 50% The calculations above were carried out for various effluent volumes; it can be seen that the higher the effluent volume, the higher the concentrations of calcium and fluoride ions in the effluent, and the greater the pressure value. Based on the calculation results, the optimal pressure value is determined from several water output levels; the value selected must ensure that the concentrations of calcium and fluoride ions are as close as possible to the recommended values. The optimal operating parameters for the device can be obtained by plotting the data. The operating pressure at which the hardness and fluoride ion concentration are closest to the recommended values (the intersection point of the relationship curve) is the optimal pressure. Russian research has also found that using reverse osmosis systems to improve the quality of tap water often leads to excessive desalination, resulting in concentrations of calcium and fluoride ions—essential elements for life—in the water that are below safety standards. To improve the quality of tap water in Moscow, devices using nanofiltration membranes can be employed, in which case the hardness is reduced by 4/5 to 7/8 ； The total salt content is reduced by 1/3 to 1/2. In recent years, some cities in our country have seen a surge in demand for drinking pure water. Most of this pure water is produced using reverse osmosis systems; generally, the same equipment and processes used to produce industrial pure water are employed, and the pre-treatment systems are quite complex ； In the process of producing this type of water, the important health principle of retaining essential trace elements for the human body is also overlooked. The stones from other mountains can be used to polish jade; Russia’s experience in using reverse osmosis to produce drinking water is worth learning from.