Summary of water treatment knowledge

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Image 1: Explanation of Terms Image 1: 1. Raw water: Refers to natural water that has not undergone any treatment, or municipal tap water; it is also known as unprocessed water. 2. Clarified water: Water from which the suspended impurities in the raw water have been removed. 3. Deionized water: Water in which the cations and anions have been essentially removed or reduced to a certain level is referred to as deionized water. Methods for desalination include distillation, electrodialysis, reverse osmosis, ion exchange, etc. 4. Turbidity: It refers to the degree of turbidity in water, which is an optical effect resulting from the presence of certain suspended particles (including colloidal substances) in the water. The unit is expressed in NTU. Turbidity is one of the main characteristics used to determine visually whether water is polluted. The standard unit for turbidity is defined as 1 degree of turbidity being caused by 1 mg of SiO2. 5. Flocculant: A substance that causes colloidal particles to aggregate and bridge together, thereby leading to flocculation. 6. Total alkalinity: refers to the total amount of substances in water that can neutralize strong acids. 7. Acidity: It refers to the total amount of substances in water that can undergo neutralization with strong bases. 8. Hardness: Refers to certain metal ions in water that tend to form precipitates, usually referring to the levels of calcium and magnesium ions. 9. Conductivity: It is the conductivity of a solution between two parallel electrodes with an area of 1 square centimeter each and a distance of 1 centimeter apart, at a certain temperature. It can indirectly indicate the content of dissolved salts in water. 10. Resistivity: This is also an indicator of water’s ability to conduct electricity. The higher the resistivity of water, the worse its conductivity, and the fewer ions it contains. Its common unit is MΩ·cm. It has an inverse relationship with conductivity. For example: if the electrical conductivity of water is 0.2 μS/cm, then its resistivity is 1/0.2 = 5 (MΩ·CM). 11. TDS (Total Dissolved Solids): It refers to the inorganic substances that remain after suspended solids (SS) and colloids have been removed and all water has been evaporated. The units are ppm or mg/l, and it can be measured using a TDS meter. It also reflects the ion content in water. There is a rough correlation between it and conductivity: for the sodium chloride reference solution, a TDS value of 1 ppm corresponds to a conductivity of 2 μs/cm. 12. pH value: The relative amount of acids and bases in a solution. pH is a measure of the negative logarithm (log) of the hydrogen ion concentration in water. The pH value ranges from 0 to 14; a pH value of 7.0 indicates that the water is neutral ； If the pH value is less than 7.0, the water is acidic ； The pH value is greater than 7.0. Then the water is alkaline. 13. Alkalinity: Alkalinity refers to the amount of substances in water that can accept ions to undergo neutralization reactions with strong acids. The substances that contribute to alkalinity in water are primarily carbonate alkalinity produced by carbonates, bicarbonate alkalinity produced by bicarbonates, and hydroxide alkalinity resulting from the presence of hydroxides. 14. SDI: Pollution Index—used to measure the amount of suspended solids in the raw water used in reverse osmosis systems. 15. Ozone: An unstable, highly reactive form of oxygen that is produced by natural lightning or high-voltage charges passing through the air; it is an excellent oxidizing agent and disinfectant. 16. Residual chlorine: The effective chlorine remaining in water after it has been disinfected with chlorine and has been in contact with it for a certain period of time. 17. Total coliforms: Total coliforms refer to a group of aerobic and facultatively anaerobic, Gram-negative non-sporing bacilli that can ferment lactose and produce acid and gas within 24 hours when grown at 37°C. Total coliforms refer to the number of total coliforms per liter of water sample. 18. Recovery rate: refers to the ratio of the flow rate of the product water produced by the system to the inflow flow rate. 19. Desalination rate: A parameter that reflects the performance of the membrane; typically, the desalination rate of first-stage RO membrane systems is over 97%. It can be calculated simply as: (conductivity of raw water – conductivity of product water) / conductivity of raw water. 20. Salt content: The salt content of water, also known as mineralization, indicates the amount of salts present in the water. Since various salts in water generally exist in ionic form, the salt content can also be expressed as the sum of the amounts of various cations and anions in the water. 21. Precipitation: One of the technical methods for wastewater treatment. It can be divided into two mechanisms: physical precipitation and chemical precipitation. The precipitation referred to usually is physical precipitation, that is, a method of separation by gravity. 22. Reused water: There are various definitions; it is referred to as reclaimed water in wastewater treatment contexts, and as recycled water in industrial settings. The distinction is generally made based on water quality. It mainly refers to non-potable water that results from the treatment of urban or domestic wastewater to meet certain water quality standards, allowing it to be reused within a certain range. The quality of reclaimed water lies between that of potable water and wastewater; this is why it is called reclaimed water. The systems used to supply this reclaimed water are referred to as reclaimed water systems. 23. Organic pollution: refers to natural organic substances such as carbohydrates, proteins, amino acids, and fats, as well as certain other biodegradable synthetic organic substances. It mainly comes from domestic sewage and industrial wastewater. 24. Concentration polarization: During the operation of reverse osmosis, salts accumulate on the membrane surface, creating a concentration difference with those in the feed water. When the flow rate of the concentrated water is low, the water with high salt content cannot be removed in time, resulting in a very high concentration difference on the membrane surface. This hinders the diffusion of salts, and this phenomenon is known as concentration polarization. 25. Suspended solids (SS): Refer to solid substances suspended in water, including insoluble inorganic substances, organic substances, as well as sediment, clay, microorganisms, etc. The content of suspended solids in water is one of the indicators for measuring the degree of water pollution. It is the amount of solid obtained by drying the residue retained on the filter paper at a temperature of 103–105 degrees after filtering the water sample. Unit: mg/l. 26. Aeration: The process by which O2 in the air is transferred into the mixture for use by microorganisms. The purpose is to provide the dissolved oxygen required by microorganisms such as activated sludge, thereby ensuring the oxygen demand for their metabolic processes. 27. Biochemical Oxygen Demand (BOD): This refers to the amount of dissolved oxygen consumed by microorganisms during the decomposition and oxidation of organic matter in water, under specified time, temperature, and conditions. Typically, the testing period is 5 days at a temperature of 20°C. It is abbreviated as BOD5, with the unit being mg/L. 28. Chemical Oxygen Demand (COD): It refers to the amount of oxygen consumed in oxidizing the organic substances in wastewater using a strong oxidizing agent under certain conditions. The testing standard for wastewater generally uses potassium dichromate as an oxidizing agent, expressed in mg/L. 29. Water hammer: also known as shock wave in water. During the transportation of water (or other liquids), sudden changes in flow velocity occur due to reasons such as the abrupt opening or closing of valves, sudden shutdown of pumps, or rapid opening/closing of guide vanes; concurrently, there are significant fluctuations in pressure. 30. Adsorption: It refers to a method of using porous solids to adsorb one or several pollutants in wastewater, in order to recover or remove those pollutants and thereby purify the wastewater. 31. Enzymes: They are catalysts (biocatalysts) produced by living cells themselves. Its main component is protein, which is a substance that accelerates the rate of biochemical reactions. 32. Wastewater: Wastewater refers to the general term for water discharged during production and daily life activities. In their daily lives and industrial activities, humans use large amounts of water, which often becomes polluted to varying degrees; such polluted water is known as wastewater. 33. Wastewater treatment: It involves the use of various techniques and methods to separate and remove pollutants from wastewater, to recycle them, or to convert them into harmless substances, thereby purifying the water. 34. Wastewater reuse: The reuse of wastewater or effluent in production systems or for various domestic purposes after secondary and advanced treatment is known as wastewater reuse. When treated water meets specific reuse requirements and is reused, it can also be referred to as reclaimed water. 35. Scale: Solid deposits that form on the walls of tubes where water comes into contact with heated surfaces after prolonged operation, due to poor quality of the boiler water. 36. Slag: refers to the solid substances that are in suspension in the furnace water, as well as the sediment that accumulates in areas where the water flow is slow, such as at the bottom of the steam drum and lower header. Difference from scale: Water sludge is relatively loose, existing in a suspended or sedimented state, and part of it can be easily removed through the boiler’s blowdown process ； Scale, on the other hand, adheres firmly to the pipe walls and is difficult to remove. 37. Iron, manganese, aluminum: Even trace amounts of iron and manganese can cause problems such as staining, scaling, and off-flavors. In a reducing environment, iron exists in the form of water-soluble ferrous ions. Upon exposure to air, it gradually oxidizes into yellow-brown colloidal ferric ions, which eventually precipitate as brown-colored iron hydroxide. Manganese has properties similar to iron, and since the oxides of iron, manganese, and aluminum are also among the causes of fouling in RO membranes, it is necessary to analyze their contents. 38. Pure water: refers to water in which the strongly electrolytic substances that can be easily removed from water have been eliminated, and weak electrolytes such as silicic acid and carbon dioxide, which are difficult to remove, have also been reduced to a certain extent. The salt content of pure water is below 1.0 mg/L, and its conductivity is less than 3 μs/cm. 39. Ultra-pure water: Also known as high-purity water, it refers to water in which almost all conductive substances have been removed, and in which colloidal substances, gases, and organic compounds that do not dissociate in water are reduced to very low levels. The salt content of ultrapure water is below 0.1 mg/L, and its conductivity is less than 0.1 μS/cm. In addition to strict requirements regarding salt content or conductivity, pure water and ultrapure water also have strict limits on the levels of various metal ions in the water, as well as on the content of organic substances, particle size and quantity, and microbial counts. 40. Distilled water: This is water that is obtained by heating raw water to vaporize it, and then condensing the vapor back into liquid form. The conductivity of generally distilled water is around 10 μs/cm. By distilling primary distilled water once more to obtain secondary distilled water, and repeating this process multiple times, the conductivity can be reduced to very low levels of around 1.0 μs/cm. 41. Scale inhibitors: These are chemicals that are capable of dispersing insoluble inorganic salts in water, preventing or interfering with the precipitation and scaling of such salts on metal surfaces, thereby maintaining good heat transfer performance in metal equipment. 42. Ion exchange resin: An insoluble polymer compound with a network structure and functional groups (active groups that can exchange ions). They are usually spherical particulate matter. 43. Ions: These are atoms that, due to internal or external influences, lose or gain one or several electrons, thereby achieving a stable structure in which the number of electrons in the outermost shell is 8 or 2 (for helium atoms), or zero (for neutrons). This process is called ionization. 44. Water production rate (water flux): Refers to the capacity of the reverse osmosis system, that is, the amount of water that passes through the membrane per unit of time, and is usually expressed in tons per hour (t/h) or gallons per day (g/d). 45. EDI: Short for continuous electrodeionization, it is a new type of technology for producing ultra-pure water. It combines electrodialysis technology with ion exchange technology in a clever manner. Image 2: Explanation of the basic processes in pure water treatment. Image 1: Coarse filtration. Coarse filtration refers to mechanical filtration, which is used to remove suspended solids, colloids, turbidity, color, unpleasant odors, and other impurities from water. The main filtration methods include clarifiers, rapid filters, sand filters, sand filters, multi-media filters, activated carbon filters, disk filters, and high-efficiency fiber filters. 2. Fine filtration: Fine filtration uses filter membranes made of special materials, offering a high level of filtering precision. Commonly used are microfiltration membranes and filter element filtration. 3. Ultrafiltration: Ultrafiltration is a type of membrane filtration that removes large molecules, colloids, bacteria, and the like. It has high filtration precision; ultrafiltration membranes are commonly used. 4. Reverse Osmosis: Reverse osmosis, abbreviated as RO, operates on the principle that raw water passes through a reverse osmosis membrane under high pressure; the solvent in the water diffuses from areas of high concentration to those of low concentration, thereby achieving separation, purification, and concentration – as this process occurs in the opposite direction to natural osmosis. 5. Ion exchange: Various inorganic salts in water ionize to form cations and anions. As the water passes through a layer of hydrogen-type ion exchangers, the cations in the water are replaced by hydrogen ions; this is the principle behind salt removal in cation exchange resins ； As the water passes through the OH- type ion exchanger layer, the anions in the water are replaced by OH- ions; this is the principle behind desalination in anion exchange resins. A mixed-bed is an ion exchange device in which cation and anion exchange resins are mixed in a certain ratio and filled into the same exchange column. 6. EDI: EDI is a new desalination process that combines electrodialysis with ion exchange. It takes advantage of the strengths of both electrodialysis and mixed-bed ion exchange, using ion exchange for advanced treatment; no chemicals are required for regeneration, and the generation of H+ and OH- ions enables the regeneration of the resin. Image 3: Manufacturers of ultrafiltration membranes, reverse osmosis membranes, and EDI systems commonly used in engineering projects. Image 1: Ultrafiltration membranes – KOCH in the United States, Norelt in the Netherlands, Shanghai Huamo. 2: Reverse osmosis membranes – Hydranautics in the United States, DOW in the United States, KOCH in the United States, GE in the United States, Toray in Japan, S-Korea in South Korea. 3: EDI systems – GE (E-CELL) in the United States, IONPURE in the United States, Electropure in the United States, CANPURE in Canada (a European/American company that has been acquired by DOW), Zhejiang Dongda. Image 4: Common water treatment processes. Image 01: When the raw water is groundwater: sand filter + precision filter + reverse osmosis + mixed bed or EDI. 02: When the raw water is tap water: sand filter + activated carbon filter + precision filter + RO + mixed bed or EDI. 03: For surface water: ① multi-media filter + activated carbon filter + precision filter + RO + mixed bed or EDI; ② multi-media filter (or other types of filters) + ultrafiltration + precision filter + RO + mixed bed or EDI; ③ disc filter + ultrafiltration + precision filter + RO + mixed bed or EDI. Image 5: Common pipe materials used in water treatment projects. Image: Carbon steel pipes. Carbon steel pipes are used in the pipelines for supplying raw water. UPVC pipes: These are suitable for applications where the pipe diameter is less than DN150; they are easy to install. Stainless steel pipes: Used in applications with special requirements, often in small medical systems. Steel-lined rubber or plastic pipes: Used in large-scale projects; they are reliable but require complicated installation. Image 6. Various uses of pure water. Image: Pure water and ultrapure water are widely used in power plants, the electronics, pharmaceutical, and chemical industries. Harmful ions in the water are removed through filtration via various membranes or ion exchange processes. The desalinated water commonly used in power plants has the following key quality parameters: hardness approximately equal to zero, conductivity ≤ 0.2 μs.cm, and SiO2 ≤ 20 ppb. The chemical water used in chemical plants is diverse; generally, the quality requirements for such water are not higher than those for water used in power plants. However, certain ions may need to be controlled, which is why first-stage or second-stage reverse osmosis processes are commonly employed. The conductivity of the effluent water is 5~10 μs·cm or higher. For higher requirements, a mixed-bed or EDI can be added later. Pharmaceutical water has strict requirements regarding conductivity and bacteria, as well as regarding the materials used in the systems; stainless steel products are commonly chosen for this purpose. Usually, a sterilization and disinfection device is added after the pure water stage. The electronics industry has the highest requirements for water; most applications in this sector require water with a purity of 18 megohms. The requirements regarding resistivity constitute only a small fraction of those for water used in electronics; there are stringent requirements for many of the ions present in it. Therefore, there are special requirements for installation materials and piping. The selected process is also the most complex. Typically, after EDI, devices such as polishing mixed-bed systems, ultrafiltration units, sterilization systems, and nitrogen-sealed water tanks are required, which also results in high costs. Image 7. Commonly used processes based on water quality requirements
Image 1: When the required conductivity of the produced water is 10–20 μS/cm, RO pretreatment + single-stage reverse osmosis is employed (for chemical industry applications).
2: When the required conductivity of the produced water is 2–9 μS/cm, RO pretreatment + two-stage reverse osmosis can be used (for pharmaceutical and chemical industries); alternatively, RO pretreatment + water softening + single-stage reverse osmosis + EDI may also be utilized (for pharmaceutical and chemical industries).
3: When the required conductivity of the produced water is less than 0.2–2 μS/cm, RO pretreatment + single-stage reverse osmosis + mixed bed deionization is applied.
4: When the required resistivity of the produced water is 5–13 MΩ·cm, RO pretreatment + water softening + single-stage reverse osmosis + EDI is used; alternatively, RO pretreatment + two-stage reverse osmosis + EDI can also be employed (for pharmaceutical, chemical, electronic, and power generation industries).
5: When the required resistivity of the produced water is 13–17 MΩ·cm, RO pretreatment + water softening + single-stage reverse osmosis + EDI + mixed bed deionization is utilized; alternatively, RO pretreatment + two-stage reverse osmosis + EDI + mixed bed deionization may also be applied (for pharmaceutical, chemical, electronic, and power generation industries).
6: When the required resistivity of the produced water is 18 MΩ·cm, RO pretreatment + two-stage reverse osmosis + EDI + mixed bed deionization + sterilization + nitrogen blanketing is implemented. Image 8: Questions and Answers on Key Challenges in Pure Water Treatment Image 1: What are the main measures to reduce acid and alkali consumption? (1) Ensure the quality of incoming water ； (2) Ensure regeneration quality and extend the cycle of water production ； (3) Ensure the quality and purity of the regenerant solution; strictly control the regeneration operating procedures ； (4) Ensure the safe, reliable, and normal operation of the equipment. 2 What are the reasons for the stability of colloids in water? (1) Charging of colloid surfaces ； (2) There is a water level layer on the surface of the colloid ； (3) The surface of the colloid adsorbs certain substances that promote its stability. 3 What is the purpose of using a coagulant aid? Match 1) Improve the floc structure, making the particles larger, stronger, tougher, and heavier ； 2) Adjust the pH and alkalinity of the water to be treated to achieve optimal coagulation conditions, thereby enhancing the coagulation effect ； The coagulant aid itself does not have coagulating properties, but it can promote the coagulation process of impurities in water. 4. Basic concepts of coagulation? Since the colloidal particles present in water carry a negative charge, they repel each other. At the same time, they undergo continuous “Brownian motion” in water, remaining highly stable and unlikely to settle. When an appropriate amount of coagulant is added, these tiny colloidal particles become destabilized; this leads to adsorption and bridge formation, causing them to flocculate into clumps that quickly settle. This process is known as coagulation. 5 What are the main factors affecting the coagulation effect? 1) Water pH: When PAC is added, it hydrolyzes to produce Al(OH)3 colloids. When the pH is between 6.5 and 7.5, the solubility of these colloids is minimal, and the coagulation effect is also optimal ； 2) Alkalinity of water: When the alkalinity is insufficient, coagulants continuously produce H+ during hydrolysis, causing the pH value to drop and thus reducing the coagulation effect ； 3) Water temperature: At low temperatures, water has higher viscosity, which slows down the hydrolysis process; as a result, flocs form more slowly, and their structure is loose with small particles that do not settle easily ； 4) Composition of impurities in water: Their properties and concentrations have a great influence on the coagulation effect. 6. What is the relationship between the forms in which carbonic acid compounds exist in water and the pH value? 1) When the pH value ≤ 4.3, there is only CO2 (free) in water ； 2) When the pH value is between 8.3 and 8.4, over 98% of it is HCO3-; 3) When the pH value is greater than 8.4, there is no CO2 in the water. 7 What is the purpose of water treatment in boilers? 1) Prevent the accumulation of deposits and corrosion of water and steam in the boiler itself and its auxiliary systems during operation. Improve the heat transfer efficiency of boilers. 2) Ensure steam quality to prevent scaling and corrosion of turbine components; under conditions that ensure good water quality, reduce boiler blowdown losses and improve economic efficiency. 8 What is the working principle of a centrifugal pump? A centrifugal pump operates by using the rotation of an impeller to generate centrifugal force on water. Before starting the pump, it is necessary to fill both the pump casing and the suction pipe with water. Then the motor is started, causing the pump shaft to drive the impeller and the water to rotate at high speed. Under the effect of centrifugal force, the water is thrown toward the outer edge of the impeller and gathers inside the pump casing, flowing through the flow channels in the volute-shaped casing into the pressure pipeline of the pump. At the same time, a vacuum is created at the center of the water pump impeller as water is flung out, and the water in the suction tank is drawn into the impeller through the suction pipe under the effect of atmospheric pressure. As the impeller keeps rotating, water is continuously thrown out and then replenished. This results in the continuous water delivery by the centrifugal pump. 9 What is resin regeneration? After a period of softening or desalination operation, the resin loses its ability to exchange ions ； At this point, it can be reduced and regenerated using acids, bases, or salts to restore its exchange capacity; this process of restoring the resin’s capacity is known as resin regeneration. 10 What are the main factors affecting the working exchange capacity of resins? (1) Quality of water quality in the incoming water ； (2) Control indicators for exchange endpoints ； (3) Height of the resin layer ； (4) Water temperature and flow velocity ； (5) Effect of resinator regeneration, properties of the resin itself. 11 What are the chemical properties of resins? 1) Reversibility of ion exchange reactions, e.g.: RH + Na+ → RNa + H+. 2) Acidity and basicity: ROH → R+ + OH-; RH → R+ + H+. 3) Selectivity: Ion exchange resins have different adsorption capacities for various ions. 4) Degree of resin exchange capacity: Cation resins: Fe3+ > Al3+ > Ca2+ > Mg2+ > K+ ≈ NH4+ > Na+. Anion resins: SO42- > NO3- > Cl- > HCO3- > HSiO4. What are the types of contamination that can occur in mixed-bed resins? 1) Suspended pollution: It mostly appears in the form of cationic resins. Strengthen the pretreatment of raw water. 2) Organic contamination: Primarily occurs in strong-base cation exchange resins. Primary recovery method: Immerse the resin in a mixed solution of NaOH (1-4%) and NaCl (5-12%) for 24 hours. 3) Heavy metal ion iron contamination: It often forms in anionic resins, accelerates the rusting of pipes and equipment, reduces the Fe content in the incoming water, and necessitates additional iron removal measures. 13 What are the main reasons for the decline in RO membrane performance? 1) Chemical changes in the membrane itself: hydrolysis of the membrane, interference from free chlorine and active chlorine. 2) Physical changes in the membrane itself: compaction of the membrane, which leads to a decrease in water permeability and an increase in desalination efficiency ； Membrane contamination: Scaling, microorganisms, and solid particles cause contamination and blockage on the surface of the membrane or within it. 14 What is the working principle of a security filter? It involves mechanical filtration using 5um-pore PP filters, which traps or adsorbs the trace suspended particles and colloidal microorganisms remaining in the water on the surface of the filters as well as within their pores. As the water production time increases, the solid particles trapped in the filter element raise its resistance; when the pressure difference between the inlet and outlet reaches 0.1 MPa, the filter should be replaced ； The filter element of the filter is a replaceable cartridge filter. 15 How to prevent scaling on RO membranes? 1) Carry out proper pretreatment of the raw water to ensure that SOI is less than 4; meanwhile, disinfectants should be added to prevent the growth of microorganisms ； 2) During RO operation, it is necessary to maintain an appropriate operating pressure. Generally, as the operating pressure increases, the water production also increases; however, excessive pressure can cause membrane compaction. 3) During RO operation, the concentrated water should be kept in a flocculent state to reduce concentration polarization of the solution on the membrane surface, and to prevent the precipitation of insoluble salts on the membrane surface ； 4) When the RO unit is shut down, chemical flushing should be carried out in the short term, and CH2O protective solution should be used for protection in the long term. 5) When the RO product water volume decreases significantly or the salinity increases, as well as in case of surface scaling or contamination, chemical cleaning should be carried out. 16 What is the role of adding NaHCO3 during the desalination process in RO units? Eliminate or reduce the residual chlorine content in water to ensure the stability of RO elements; our company maintains a residual chlorine level of less than 0.1 mg/L. 17 What is the purpose of installing an electric slow-opening automatic valve in front of the 17 RO membrane modules? It prevents sudden start-up and shutdown of the high-pressure pump during RO operation, which could cause a rise in pressure and result in high-pressure shocks to the RO membrane elements, thereby damaging the membranes through water hammer effects. 18 What is a filtering cycle? How many steps are included? What is the role of each step? The filtration cycle is the actual operating time between two backwash cycles, and it includes three stages: filtration, backwashing, and rinsing. Backwashing is carried out to remove the contaminants that have accumulated during filtration, thereby restoring the filtering medium’s ability to trap impurities. Rinsing ensures the proper functioning of the filtration process A necessary step for ensuring water quality is passing the cleaning test; only after that can the system enter its normal operation mode for water production. 19 Principle of chlorine removal by activated carbon: The removal of residual chlorine by activated carbon is not due to physical adsorption, but rather a chemical reaction. As free residual chlorine passes through the activated carbon, it triggers a catalytic reaction on its surface; the free residual chlorine is quickly hydrolyzed to release oxygen atoms [O], which then react chemically with carbon atoms to form carbon dioxide. At the same time, HCLO present in the raw water is also rapidly converted into CO2 gas. Overall reaction: C + 2Cl2 + 2H2O → 4HCl + CO2↑ Based on this reaction, the amount of activated carbon in the container will gradually decrease depending on the level of residual chlorine in the raw water, and it should be replenished appropriately each year. 20 Principle of the reverse osmosis process: RO takes advantage of the property of semipermeable membranes, which allow water to pass through while preventing salts from doing so, in order to remove most of the salts from water. Pressure is applied on the raw water side of the RO system, causing a portion of the pure water in the raw water to pass through the membrane in a direction perpendicular to it; salts and colloidal substances in the water get concentrated on the surface of the membrane, while the remaining raw water carries away these concentrated substances in a direction parallel to the membrane. By having only a small amount of salt in the water, collecting this water achieves the purpose of desalination.