Seawater desalination technology is an important way to solve the shortage of freshwater resources. Associate Professor Zheng Xiaoying from Beijing Institute of Technology gave a detailed introduction to the most widely used commercial seawater desalination processes: low-temperature multi effect distillation, multi-stage flash distillation, and reverse osmosis. She analyzed their process principles, characteristics, development trends, and the current development status of China's seawater desalination industry.
Reverse osmosis is the reverse process of infiltration. Permeation is a natural phenomenon in which water molecules in a dilute solution rapidly diffuse towards the concentrated solution side through a semi permeable membrane, as shown in Figure 3. The liquid level on the freshwater side continues to decrease, resulting in a static water pressure difference. When this static water pressure difference causes the diffusion speed of water molecules on both sides to be equal, permeation reaches equilibrium, and the static water pressure difference at this time is called osmotic pressure (the osmotic pressure between the seawater and freshwater interface is about 2.4 MPa). The reverse osmosis process is exactly the opposite. In SWRO, applying pressure greater than seawater osmotic pressure on one side of seawater will cause water molecules in seawater to penetrate the membrane and retain salt. In theory, the greater the external pressure, the faster the reverse osmosis speed of water molecules in seawater.
The most crucial component for implementing reverse osmosis technology is the artificially synthesized reverse osmosis semi permeable membrane, which can almost only allow water molecules to pass through. The flat membrane of reverse osmosis membrane needs to be made into a certain structure in order to be used in water treatment engineering. At present, aromatic polyamide membrane elements are mainly used for seawater desalination. Sandwiched between two pairs of planar thin films, the thin film layer combines along the three edges of the thin film layer, and then combines with the entrance thin film layer to form a porous central tube. Finally, install perforated end caps at both ends and package them to form roller membrane elements. There are various specifications of commercial reverse osmosis membrane components, and the most commonly used membrane component in seawater desalination has a diameter of 200mm and a standard length of 1000mm. In the SWRO system, one or more roller membrane elements are connected and placed in a cylindrical pressure vessel. Multiple pressure vessels are connected and combined together to form a reverse osmosis device. Typically, a pressure vessel can contain 6-8 membrane components, while a medium-sized SWRO system consists of thousands of pressure vessels.
SWRO consists of a high-pressure pump, a pressure vessel, and an energy recovery device. After pre-treatment, seawater is pressurized by a high-pressure pump and transported to the pressure vessel of the reverse osmosis device. Seawater first passes through the first membrane element and flows through the inlet separation channel of the spiral winding of the membrane element. Under higher pressure, some water molecules continuously penetrate the membrane and enter the central tube of the roller membrane element through the production water distribution channel, producing product water. The remaining inlet water continues to flow along the direction of the water flow to the next membrane element. This process is executed in order. When the incoming water passes through the next membrane element, the concentration of the incoming water increases. When it passes through the last membrane element, the incoming water becomes concentrated water. The concentrated water flows out of the pressure vessel and is recovered by the energy recovery device before discharge.
Technical characteristics
Unlike thermal methods, SWRO does not have a phase transition process of evaporation and condensation. Its main energy consumption is the high-pressure pumping used to achieve the reverse osmosis process, which makes the water production cost of reverse osmosis lower than that of thermal method. In addition, compared with thermal methods, this equipment also has modular structural characteristics and higher process flexibility. The suspension and maintenance of local facilities will not affect the operation of the rest of the system. However, SWRO requires a complex and precise pre-treatment process, and different commercial membrane manufacturers have different requirements for the inlet SDI of their polyamide reverse osmosis membranes - strict limitations on indicators such as pH, temperature, and residual chlorine. When the pre-treatment does not meet the standards, fouling and scaling on the membrane surface will accelerate. During operation, the service life, energy consumption, and product water quality of membrane components will be affected, thereby increasing water production costs. The pre-treatment process of SWRO varies depending on the inflow method and seawater quality. For the open water inlet method, the current popular method is to add dissolved air flotation+ultrafiltration+5 before reverse osmosis μ M safety filter to ensure the water quality requirements of the reverse osmosis inlet. For a SWRO system with an average inflow TDS of 35000 mg/L, the inflow pressure is higher than 5.0 MPa, and the operational recovery rate (volume ratio of produced water to inflow) is between 40% and 60%. The energy recovery device is another key equipment, and the rapid development of SWRO is due to the continuous optimization of membrane materials and components, as well as the use of energy recovery devices with improved efficiency in reverse osmosis systems. At present, the efficient PX pressure recovery device can recover more than 95% of energy from concentrated water for pressurized raw seawater, which reduces the energy consumption of the reverse osmosis process by nearly half. The TDS of the first stage reverse osmosis effluent is about 300-500 mg/L, which meets the limit requirement of the World Health Organization for TDS indicators of drinking water (500 mg/L). SWRO has been widely used for drinking water supply in water scarce areas.
Current development trends
In the 1980s, SWRO began to compete with traditional thermal processes. Due to its advantages such as low equipment investment, short construction period, and low energy consumption, it has developed rapidly and has become the most important process in the global seawater desalination market. At present, primary SWRO is mainly used in the municipal industry, which is also the reason for the rapid growth of reverse osmosis technology production capacity. Future research on reverse osmosis technology will focus on developing more energy-efficient and durable new reverse osmosis membranes and membrane components, reducing operational energy consumption and maintenance costs, and lowering water production costs.
