Seawater desalination refers to the technology and process of extracting freshwater from seawater. There are various desalination technologies, such as distillation, membrane processes (reverse osmosis, electrodialysis, membrane distillation, etc.), and ion exchange, but the methods suitable for large-scale seawater desalination are distillation and reverse osmosis.
Here, we will specifically introduce the reverse osmosis desalination process for seawater.
1. Seawater Sterilization and Algae Removal
Seawater contains a large number of microorganisms, bacteria, and algae. The proliferation of bacteria and algae and the growth of microorganisms in seawater not only create issues for the water intake facilities but also directly affect the normal operation of desalination equipment and process pipelines. Therefore, seawater desalination projects often use chemical agents such as liquid chlorine, sodium hypochlorite, and copper sulfate to sterilize and remove algae.
2. Coagulation and Filtration
To improve the quality of water entering the reverse osmosis system and reduce turbidity, a multi-media filter is typically added after coagulation and filtration. This process removes small suspended solids and particles from the water, further enhancing water quality before it enters the reverse osmosis system.
3. Anti-Scaling Agents and Reducing Agents
Seawater is complex in composition, with high hardness and alkalinity. To ensure optimal performance of the reverse osmosis system and prevent scaling, corresponding anti-scaling agents must be added based on the specific water quality. Additionally, since oxidizing agents are used during pre-treatment for sterilization, a reducing agent is added to the reverse osmosis feed water to neutralize the chlorine, ensuring that the residual chlorine content is less than 0.1 ppm (or ORP < 200mV), which meets the reverse osmosis system’s requirements for oxidizing substances in the feedwater.
4. Security Filter
Due to the high salinity of seawater, security filters are required to use 316L stainless steel materials. The filter's pore size is usually selected as 5μm to filter seawater before it enters the high-pressure pump, blocking particles larger than 5μm. This ensures the safe and stable operation of the high-pressure pump, energy recovery device, and reverse osmosis membrane elements.
5. High-Pressure Pump and Energy Recovery Device
High-pressure pumps and energy recovery devices are critical for providing energy conversion and energy savings in reverse osmosis seawater desalination equipment. The pump selection depends on the required flow rate and pressure. The energy recovery device typically uses a hydraulic turbine structure to utilize the pressure from the concentrated seawater discharge to increase the feedwater pressure by 30%, effectively recovering energy and reducing energy consumption, which in turn reduces operational costs.
6. Reverse Osmosis Membranes and Units
Reverse osmosis membrane elements are the core components of seawater desalination equipment. The membranes selected must correspond to the specific seawater desalination system. Depending on the system design, different types of membrane elements can be chosen. RO membranes are known for their high desalination rates, excellent pressure resistance, oxidation resistance, and anti-pollution properties. The materials used in the high-pressure parts of seawater desalination systems should be made of stainless steel 316L or higher to prevent corrosion due to the seawater's high salinity.
7. System Control
The control system of reverse osmosis seawater desalination equipment typically uses a programmable logic controller (PLC) to create a decentralized sampling control system, which monitors the operation of the system centrally. The system is equipped with high- and low-pressure protection switches, automatic switching devices, and alarm functions. When abnormalities occur in parameters such as conductivity, flow, or pressure, it can automatically switch, lock, or shut down the system to protect the high-pressure pump and RO membrane elements.
Variable frequency control is applied to start and stop the high-pressure pump, achieving soft operations, saving energy, and preventing damage caused by water hammer or back pressure. The program is designed to automatically flush the low-pressure water before and after startup and shutdown, particularly during shutdown, when the concentrated seawater is in a metastable state and may cause precipitation, polluting the membrane surface. Low-pressure flushing automatically displaces the concentrated seawater, protecting the membrane surface and extending its lifespan.
The system also allows for the display, storage, statistics, tabulation, and printing of related parameters such as temperature, flow rate, water quality, and water production. The dynamic process flow in the operational interface is clear and intuitive, simplifying manual operations and ensuring the system operates automatically, safely, and reliably.