Desalination refers to the technology and process of obtaining fresh water from seawater. There are many types of seawater desalination technologies, including distillation, membrane methods (reverse osmosis, electrodialysis membrane evaporation, etc.), ion exchange methods, etc. However, the only methods suitable for large-scale seawater desalination are distillation and reverse osmosis.
Below is a detailed introduction to the process of using reverse osmosis for seawater desalination.
1. Seawater sterilization and algae removal
Due to the presence of a large number of microorganisms, bacteria, and algae in seawater. The proliferation of bacteria and algae, as well as the growth of microorganisms in seawater, not only brings many troubles to water intake facilities, but also directly affects the normal operation of seawater desalination equipment and process pipelines. Therefore, seawater desalination projects often use chemical agents such as liquid chlorine, sodium hypochlorite, and copper sulfate to kill bacteria and algae.
2. Coagulation filtration
In order to further improve the inlet water quality of reverse osmosis and reduce the turbidity of the inlet water, a multi-media filter is usually added after coagulation filtration to further remove small suspended solids and particulate matter in the water, ensuring further improvement in water quality.
3. Scale inhibitor and reducing agent
The composition of seawater is very complex, with high hardness and alkalinity. In order to ensure the better operation of the reverse osmosis system and maintain the system running without scaling, corresponding scale inhibitors need to be added according to the specific water quality. In addition, due to the addition of oxidizing agents for sterilization in the pre-treatment of reverse osmosis, reducing agents need to be added to reduce the residual chlorine in the influent of the reverse osmosis system to less than 0.1ppm (or ORP<200mV), meeting the requirements of the reverse osmosis system for the content of oxidizing substances in the influent.
4. Security filter
Due to the high salt content of seawater, the security filter requires the use of 316L material, with a filter element aperture of usually 5 μ m. The seawater before entering the high-pressure pump is filtered to block particles and impurities with a diameter greater than 5 μ m, ensuring the safe and long-term stable operation of the high-pressure pump, energy recovery device, and reverse osmosis membrane components.
5. High pressure pump and energy recovery device
High pressure pumps and energy recovery devices are important equipment for providing energy conversion and energy conservation for reverse osmosis seawater desalination equipment. Based on the required flow rate and pressure of the reverse osmosis seawater desalination equipment, the energy recovery device has a hydraulic turbine structure that can use the pressure of the concentrated seawater discharged from reverse osmosis to increase the inlet pressure by 30%, effectively utilizing the energy of the concentrated water and reducing energy consumption, thereby effectively reducing operating costs.
6. Reverse osmosis components and devices
The reverse osmosis membrane element is the core component of the reverse osmosis seawater desalination equipment. The seawater desalination membrane corresponding to the seawater desalination equipment system should be selected. Different membrane elements can be selected according to the different designs of the system. The reverse osmosis membrane element has high desalination rate, good pressure resistance, oxidation resistance, and pollution resistance. The material of the seawater desalination reverse osmosis equipment system should be made of 316L or higher stainless steel in the high-pressure section to prevent corrosion of high-pressure pipelines by the high salt content of seawater.
7. System control
The control system of reverse osmosis seawater desalination equipment usually uses a programmable logic controller (PLC) to form a decentralized sampling control and centralized monitoring operation control system. Set high and low pressure protection switches and automatic switching devices according to process parameters. When there are abnormalities in conductivity, flow rate, and pressure, automatic switching, interlocking alarm, and shutdown can be achieved to protect high-pressure pumps and reverse osmosis membrane components. Variable frequency control is used to start and stop the high-pressure pump, achieving soft operation of the high-pressure pump, saving energy consumption, and preventing damage to the high-pressure pump and membrane components caused by water hammer or back pressure. The program design can achieve low-pressure automatic flushing before and after the start-up and shutdown of the reverse osmosis device. Especially during shutdown, the metastable state of concentrated seawater will transform into sedimentation, polluting the membrane surface. The low-pressure desalinated water automatic flushing can replace the concentrated seawater, protect the membrane surface from pollution, and extend the service life of the membrane. The system's temperature, flow rate, water quality, water production and other related parameters can be displayed, stored, counted, tabular and printed. The dynamic process flow in the monitoring operation is clear and intuitive, and the system control simplifies manual operation, ensuring that the system can operate automatically, safely, and reliably.
