What membranes are needed for seawater desalination?

Taken from the Internet--Seawater desalination refers to the process of treating seawater through desalination to convert it into usable freshwater. There are many different technical processes for desalinating seawater; the main methods that are currently mature and widely used are distillation and reverse osmosis. Regardless of the desalination technology, a heat source is required (obtained from electricity, nuclear energy, or solar energy). Distillation-based seawater desalination technologies, mainly including multi-stage flash distillation, low-temperature multi-effect distillation, and vapor compression distillation, involve distilling seawater to obtain fresh water. These technologies are currently used primarily in coastal power plants, where steam that has already done work in turbines is utilized as a heat source for desalinating seawater. The reverse osmosis seawater desalination technology involves pressurizing seawater and then using a reverse osmosis membrane to separate out the salts and obtain fresh water. It is widely adopted across various regions due to its advantages such as low investment, small land requirement, simple operation and maintenance, and the ability to operate on a large or small scale. Fresh water obtained through seawater desalination has relatively clean quality. Desalinated water by distillation is distilled water ； The desalinated water produced by reverse osmosis retains some minerals and microorganisms, and fully meets **drinking water standards. As desalination technology becomes increasingly mature, the scale of seawater desalination keeps expanding, and the costs associated with it are gradually decreasing. The UAE has built the world’s largest multi-stage flash seawater desalination plant, capable of producing 300,000 tons of desalinated water per day. It consists of 6 units connected in parallel (each unit having a desalination capacity of 57,700 tons), powered by generator sets with a total capacity of 732,000 kilowatts. The reverse osmosis seawater desalination plant built in Saudi Arabia produces 128,000 tons of desalinated water per day. Israel is building a reverse osmosis desalination plant with a production capacity of 330,000 tons per day. The reverse osmosis desalination plant built in the United States can produce 380,000 tons of desalinated water per day. To date, our country has built 20 reverse osmosis seawater desalination units and 2 distillation-based seawater desalination systems, achieving a desalination capacity of 30,000 tons per day. Among them: Tianjin Dagang Power Plant uses multi-stage flash seawater desalination units imported from the United States, with a daily desalinated water production of 3,000 tons ； The Huangdao Power Plant in Qingdao uses low-temperature multi-effect seawater desalination units to produce 60 tons of desalinated water per day ； Shandong Rongcheng Shidao Aquatic Products Group Company uses reverse osmosis seawater desalination units to produce 10,000 tons of desalinated water per day (the first unit with a capacity of 5,000 tons per day is already in operation) ； Shengsi County in Zhejiang Province uses reverse osmosis seawater desalination units, producing 2,000 tons of desalinated water per day (the first unit with a capacity of 1,000 tons per day is already in operation). The overall cost of seawater desalination (including equipment depreciation, etc.) has also decreased from 7 yuan in the 1990s to around 5 yuan today, thanks to the rising domestic production rate of desalination equipment and falling prices, as well as the continuous expansion of seawater desalination capacity. Experts predict that the overall cost per ton of water will continue to decline in the near future.

It seems that ultrafiltration membranes and reverse osmosis membranes should be used more often; the flash evaporation method is currently in the research phase

Reverse osmosis membranes are specifically used for seawater. There are devices in operation in the Middle East.

The membrane required for seawater desalination is a reverse osmosis membrane; more specifically, it is the membrane used for seawater desalination within the reverse osmosis system. The pressure of the water flowing into this membrane is generally between 4 MPa and 6 MPa. Companies capable of producing such membranes include Dow, Hydranautics, and Toray (whose membranes are made of aromatic polyamide). There is also a Japanese company, I think it’s Kubota, which uses cellulose acetate-based membranes for seawater desalination – they are the only ones currently using this type of membrane.