The seawater desalination equipment is a specialized device that processes high salinity seawater (with a salt content of about 35000mg/L) into water that meets the standards for domestic and industrial use (with a salt content of ≤ 500mg/L and drinking water ≤ 1000mg/L). The core technology revolves around “desalination”. According to mainstream technologies (mainly reverse osmosis and distillation), their structure and function are as follows:
1、 The core structure of mainstream seawater desalination equipment
（1） Reverse osmosis seawater desalination equipment (widely used, accounting for over 60% of global desalination capacity)
The reverse osmosis method utilizes the selective permeability of semi permeable membranes (reverse osmosis membranes) to separate water molecules from salt ions under high pressure, making it suitable for small and medium-sized scale (daily water production of hundreds to tens of thousands of tons).
1. Pre treatment system (protecting the core reverse osmosis membrane)
Seawater intake device
Construction: Shore water intake pipe (material: 316L stainless steel or HDPE resistant to seawater corrosion), grille filter (intercepting large particle impurities such as marine organisms and sediment, with a pore size of 5-10mm).
Special design: Some equipment is equipped with “anti biological attachment devices” (such as ultraviolet or copper alloy filters) to prevent algae and shellfish from breeding and blocking in the pipeline.
Coagulation sedimentation/clarification unit
Components: coagulant dosing device (such as polyaluminum chloride to remove colloidal impurities), inclined tube sedimentation tank (inclined PVC inclined tube at 60 ° to accelerate sediment settling).
Function: Reduce seawater turbidity (from 10-30NTU to below 1NTU), and prevent particles from scratching the reverse osmosis membrane.
Multi-media filter
Filling layer: upper layer of quartz sand (filtering suspended particles), lower layer of anthracite (adsorbing organic matter), partially equipped with activated carbon layer (removing residual chlorine and odor).
Automatic backwash function: When the resistance of the filter layer is too high, the filter material is flushed with reverse water flow to restore the filtration capacity.
Security filter (precision filtration)
Construction: Polypropylene (PP) melt blown filter cartridge (pore size 5 μ m), installed on the last barrier in front of the reverse osmosis membrane.
Function: Retain tiny particles remaining after pretreatment, absolutely protecting the reverse osmosis membrane from scratches.
Seawater pretreatment auxiliary system
Scale inhibitor dosing device: Special seawater scale inhibitors (such as those containing phosphonates) are added to prevent calcium and magnesium ions from scaling on the membrane surface after seawater concentration (high concentration of calcium and magnesium in seawater can easily generate calcium carbonate and calcium sulfate scales).
Reducing agent dosing device: If seawater is disinfected with chlorine (such as ship seawater systems), sodium bisulfite is added to remove residual chlorine (chlorine will oxidize the reverse osmosis membrane).
2. Core reverse osmosis system
high-pressure pump
Features: Centrifugal pump resistant to seawater corrosion (impeller material: duplex steel or titanium alloy), providing high pressure of 5.5-6.5MPa (seawater osmotic pressure is about 2.5MPa, requiring ultra-high pressure to push water molecules through the membrane).
Control: Equipped with a frequency converter, the flow rate is automatically adjusted according to the inlet pressure to stabilize the operating pressure of the membrane component.
Reverse osmosis membrane module
Membrane material: Aromatic polyamide composite membrane (resistant to seawater corrosion, desalination rate ≥ 99.5%), made into roll type membrane elements (diameter 8 inches or 4 inches, single membrane element water production 50-100m ³/d).
Arrangement method: Multiple membrane elements are connected in series to form a membrane shell (material: fiberglass or 316L stainless steel), and the membrane shells are connected in parallel to form a membrane array, which is combined according to the water production demand (such as dozens of membrane elements required for equipment with a daily output of 1000 tons).
Energy recovery device (key energy-saving component)
Construction: A common “pressure exchanger” (PX valve) or “turbine energy recovery machine” is connected to the reverse osmosis concentrated water outlet (the concentrated water still has a high pressure of about 5MPa).
Function: Transfer the high-pressure energy of concentrated water to the inlet water, reducing the energy consumption of the high-pressure pump (which can save 30-40% energy and is a standard configuration for large reverse osmosis equipment).
3. Post treatment system (improving water quality applicability)
Degassing tower
Function: Remove carbon dioxide from reverse osmosis wastewater (with low pH and high solubility of CO ₂ after seawater desalination), and avoid pipeline corrosion.
Principle: By aeration or vacuum degassing, CO ₂ is decomposed into gas and discharged, increasing the pH of the produced water from 5-6 to 7-8.
Mineralization/conditioning device
Construction: Add food grade minerals such as calcium chloride and magnesium sulfate, or pass through a limestone filter bed (allowing water to come into contact with calcium carbonate).
Function: Supplement the missing minerals in reverse osmosis water (such as calcium and magnesium content that meet WHO standards when drinking), improve taste and health.
Ultraviolet
Component: High power UV lamp (wavelength 254nm) illuminates the produced water to kill microorganisms (such as bacteria and viruses) that may penetrate the membrane, ensuring the safety of drinking water.
（2） Distillation based seawater desalination equipment (suitable for large-scale applications such as the Middle East)
Distillation method evaporates seawater by heating, and fresh water is obtained by condensing the steam. It is divided into types such as “multi-stage flash evaporation” and “multi effect evaporation”. Taking multi-stage flash evaporation (MSF) as an example:
1. Heating system
Steam heater: uses power plant steam turbine extraction (or gas boiler steam) to heat seawater to 100-120 ℃ (below boiling point to avoid scaling).
2. Flash evaporation chamber (core unit)
Construction: 10-30 flash chambers connected in series, with pressure gradually decreasing (from 0.5 bar in the first chamber to 0.05 bar in the last chamber).
Principle: High temperature seawater enters the low-pressure flash evaporation chamber and evaporates instantly (“flash evaporation”). The steam condenses into fresh water on the chamber wall, and the concentrated seawater that has not evaporated enters the next stage of lower pressure flash evaporation chamber to repeat the process.
3. Condensation and heat exchange system
Each flash evaporation chamber is equipped with a condenser tube: using the cold seawater entering the system as a cooling medium, it absorbs steam heat to condense and preheat the cold seawater (energy-saving design).
4. Salt discharge system
The concentrated high salinity seawater (with a salt content of over 70000mg/L) is usually discharged back into the ocean (slow discharge is required to avoid excessive local seawater salinity).
2、 The main function of seawater desalination equipment
1. Solve the problem of freshwater resource shortage
Provide domestic water for coastal areas, islands and ocean going ships (such as the seawater desalination stations in Zhoushan Islands and the Xisha Islands in China, with daily water output of hundreds of tons to meet the needs of residents).
Provide production water for industries such as power plants (boiler makeup water requires low salt water), petrochemical enterprises (cooling water), steel plants (cleaning water), etc., especially suitable for coastal industrial areas lacking fresh water.
2. Ensure water safety in special scenarios
In the military field, when stationed on islands and sailing on ships, small seawater desalination equipment is used to achieve self-sufficiency in fresh water and eliminate logistical dependence.
Emergency relief: When disasters such as tsunamis and droughts cause interruptions in freshwater supply, mobile seawater desalination equipment (such as vehicle mounted reverse osmosis devices) can be quickly deployed.
3. Improve the efficiency of water resource utilization
Seawater is an inexhaustible resource (97% of the Earth’s water is seawater), and desalination technology breaks through the limitations of traditional reliance on surface water and groundwater, alleviating the pressure of water resource depletion.
Some equipment can be linked with power plants and chemical plants (such as “electricity water cogeneration”), using industrial waste heat to heat seawater and reduce desalination energy consumption (such as large-scale power plant desalination integration projects in the Middle East).
3、 Comparison of different technologies
Technical advantages, disadvantages, and applicable scenarios
The energy consumption of reverse osmosis method is relatively low (about 3-5 kWh/m ³), the equipment is compact and the membrane is prone to contamination (requiring strict pretreatment), and the cost of membrane replacement is high, medium, and small-scale (islands, ships)
Multi stage flash evaporation (MSF) has strong pollution resistance, stable water production, high energy consumption (about 10-15 kWh/m ³), and large-scale equipment (in Middle Eastern countries and coastal power plants)
summarize
The seawater desalination equipment achieves the conversion of seawater to freshwater through the process of “pretreatment core desalination post-treatment”, which is a key technology to solve the global water resource crisis. Reverse osmosis has become mainstream due to its high efficiency and energy saving, while distillation is still irreplaceable in large-scale industrial scenarios. Its core function is to break through geographical limitations, convert “unusable” seawater into “usable” freshwater, and support the water demand of coastal areas and special industries. It is an important component of the water resources security system in the 21st century.
Post time: Jul-19-2025