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Q&A on Water Treatment Knowledge and Techniques – Part 1: Terminology Explanations 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 suspended impurities have been removed. 3. Demineralized water: Water in which the cations and anions have been essentially removed or reduced to a certain level is referred to as demineralized 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 unit of turbidity corresponding to 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: 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 concentration of dissolved salts in water. 10. Resistivity: This is also an indicator of water’s conductivity. The higher the resistivity of water, the worse its conductivity, meaning there are fewer ions present in the water. 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 correspondence 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 produce alkalinity in water are mainly carbonate alkalinity generated by carbonates, bicarbonate alkalinity generated 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-spore-forming 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 product water produced by the system to the flow rate of the inlet water. 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 generally 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, and can 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 water-insoluble inorganic substances, organic substances, as well as sediment, clay, microorganisms, etc. The suspended solids content in water is one of the indicators used to measure the degree of water pollution. It is the amount of solid obtained by drying the residues on the filter paper after filtering water samples at a temperature of 103–105 degrees. 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): It refers to the amount of dissolved oxygen consumed by microorganisms in the process of breaking down and oxidizing organic substances in water under specified time, temperature, and conditions. The commonly used time is 5 days at a temperature of 20°C; this 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. Pure water: refers to water in which the strongly electrolytic substances that can be easily removed from water have been removed, 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, with a conductivity of less than 3 μs/cm. 30. 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. 31. 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, usually expressed in tons per hour (t/h) or gallons per day (g/d). 32. EDI: Short for continuous electrodeionization, it is a new type of technology for producing ultra-pure water. It cleverly combines electrodialysis technology with ion exchange technology. II. Explanation of the Basic Processes in Pure Water Treatment 1. Coarse filtration: Refers to mechanical filtration, which removes suspended solids, colloids, turbidity, color, unpleasant odors, etc., from water. The main filtration methods include clarifiers, rapid filters, sand filters, sand filters, multi-media filters, activated carbon filters, disc filters, and high-efficiency fiber filters. 2. Fine filtration: A filter membrane made of special materials, offering a high level of filtering precision. Commonly used are microfiltration membranes and filter element filtration. 3. Ultrafiltration: It is a type of membrane filtration that removes large molecules, colloids, bacteria, etc. It has high filtration precision; ultrafiltration membranes are commonly used. 4. Reverse Osmosis: Reverse osmosis is abbreviated as RO. Its principle involves forcing raw water to pass 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 hydrogen-type ion exchanger layer, 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: It is a new desalination process that combines electrodialysis with ion exchange. It leverages the advantages of both electrodialysis and mixed-bed ion exchange, using ion exchange for advanced treatment; no reagents are required for regeneration, and the generation of H+ and OH- through ionization is used to regenerate the resin. III. Common water treatment processes: For groundwater as the raw water: sand filter + precision filter + reverse osmosis + mixed-bed resin or EDI. For tap water as the raw water: sand filter + activated carbon filter + precision filter + RO + mixed-bed resin or EDI. For surface water: ① multi-media filter + activated carbon filter + precision filter + RO + mixed-bed resin or EDI. ②Multi-media filter (or other types of filters) + ultrafiltration + precision filter + RO + mixed-bed or EDI. ③Disk filter + ultrafiltration + precision filter + RO + mixed bed or EDI. IV. Various Uses of Pure Water 1. Pure water and ultra-pure water are widely used in power plants, the electronics industry, the pharmaceutical sector, and the chemical industry; harmful ions in water are removed through filtration using various membranes or through ion exchange processes. 2. 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. 3. The chemical water used in chemical plants comes in various forms; 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 primary or secondary reverse osmosis processes are commonly used. 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. 4. Water used in the pharmaceutical industry 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. 5. The electronics industry has the highest requirements for water, with most electronic applications requiring water with a purity of 18 megohms. The requirements regarding resistivity apply only to a small portion of water used for electronics; it has high demands on many of the ions present, which therefore imposes special requirements on the installation materials and pipes. Selecting the process is also the most complex. Typically, polishing mixed-bed systems, ultrafiltration, sterilization, nitrogen-filled water tanks and other devices need to be added after EDI, which also results in high costs. V. Q&A on Key Challenges in Pure Water Treatment 1. What are the main reasons that contribute to the deterioration of RO membrane performance? 1) Chemical changes in the membrane itself: hydrolysis of the membrane, interference from free chlorine, and oxidation by active chlorine. 2) Physical changes in the membrane itself: Compaction of the membrane 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. 2. 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 the RO elements. 3. 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 surface of the activated carbon, it triggers a catalytic reaction; the free residual chlorine is quickly hydrolyzed to release oxygen atoms [O], which then react 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. 4. 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 to 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 collecting the water that has passed through with only a small amount of salt remaining, desalination is achieved.