Differences among seawater desalination, softening, and desalting technologies

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This post was last edited by liuquan1100 on 2018-2-11 at 14:51. As water pollution becomes increasingly severe and water shortages become more apparent, technologies such as seawater conversion are gradually coming to the attention of people. So, what are the specific technologies involved in converting seawater? What are the differences between desalination, water softening, and desalting as these terms are used in the water industry? The salinity of water intended for drinking is much lower than that of seawater; since seawater has too high a salinity level, it cannot be consumed directly. So, what is seawater salinity? Seawater salinity refers to the ratio of all dissolved solids in seawater to the weight of the seawater, and it is usually expressed in grams per kilogram of seawater. Salinity is used to indicate the mass fraction of salt substances in seawater. The average salinity of the world’s oceans is 35‰, while the salinity of water used for drinking or industrial purposes is much lower than that of seawater. Furthermore, the carbonate hardness of natural seawater ranges from 70 to 90 dH, whereas the hardness of surface water used by us must not exceed 25 dH; therefore, seawater needs to be softened and desalinated before it can be utilized by us. The most obvious effect of seawater desalination is the reduction of the conductivity of the water; it targets substances other than water itself, including cations, anions, and various non-electrolytic substances. Seawater desalination involves removing salts from seawater to produce fresh water. It represents a technology that increases the available supply of freshwater, without being affected by time, space, or climate conditions. The resulting water has good quality, its price is becoming more reasonable, and it ensures a stable supply of water for drinking purposes for coastal residents as well as for use in industrial boilers. The main technical methods include distillation, electrodialysis, reverse osmosis, etc. ). Sea water softening has no direct relationship with conductivity; its main purpose is to reduce the hardness of water, such as calcium and magnesium ions. This can be achieved through ion exchange, using ions such as sodium or hydrogen to replace those ions, so its impact on conductivity is not significant. Below is a further analysis of the differences among seawater desalination, seawater softening, and desalting: Seawater desalination, simply put, is the process of purifying saltwater; its most obvious effect is a reduction in the conductivity of the water, with the aim of lowering its salinity. Seawater desalination refers to the process of removing salts from seawater to produce fresh water. It is an open-source, incremental technology for water resource utilization that can increase the total amount of fresh water, unaffected by time, space, or climate conditions; it ensures a stable supply of water for coastal residents’ drinking water needs as well as for industrial boilers. The process of obtaining fresh water from seawater is called desalination. The current methods for desalinating seawater include seawater freezing, electrodialysis, distillation, and reverse osmosis. Among these, reverse osmosis, which uses reverse osmosis membranes, has rapidly gained prominence in the market due to its advantages of simple equipment, easy maintenance, and modular design; it is gradually replacing distillation as the most widely used method. There are over 20 types of seawater desalination technologies worldwide, including reverse osmosis, low-pressure multi-effect distillation, multi-stage flash distillation, electrodialysis, vapor compression distillation, dew point evaporation, co-generation using hydropower, heat membrane co-generation, as well as desalination technologies that utilize nuclear, solar, wind, and tidal energy. In addition, there are various pre-treatment and post-treatment processes such as microfiltration, ultrafiltration, and nanofiltration. In terms of broad classification, they are mainly divided into two categories: distillation methods (thermal methods) and membrane methods. Among these, multi-effect distillation, multi-stage flash evaporation, and reverse osmosis membrane technology are the dominant technologies worldwide. Generally speaking, the low-pressure multi-effect process has advantages such as energy savings, lower requirements for seawater pretreatment, and higher quality of desalinated water; the reverse osmosis membrane method offers benefits like low investment costs and low energy consumption, but it requires extensive seawater pretreatment; the multi-stage flash distillation method features mature technology, reliable operation, and high production capacity, yet it has relatively high energy consumption. It is generally believed that low-threshold multiple-effect distillation and reverse osmosis membrane technology represent the future directions. Seawater softening, as the name implies, refers to the softening of hard water; it has no direct relation to conductivity and is primarily aimed at reducing the hardness of water. Seawater contains large amounts of calcium, magnesium, bicarbonate, and sulfate ions, which causes the formation of hard scale (CaSO4) and soft scale (CaCO3 and Mg(OH)2) on the walls of the heat exchange tubes or on the surface of membranes in seawater desalination processes. This leads to a reduction in heat transfer efficiency or membrane flux, thereby affecting the proper operation of the equipment. Therefore, softening seawater during the pretreatment stage to remove calcium and magnesium ions is of great significance for the long-term stable operation of equipment, as well as for saving energy and reducing economic costs. Desalination is the method or process of removing “chemical salts”. Desalination, in simple terms, is the method or process of removing “salt” – and by “salt” is meant the broader category of “chemical salts”, not just the common table salt; in other words, it involves removing the cations and anions from water. The production of fresh water from seawater, in other words, refers to seawater desalination.