1、 Mainstream process and equipment for seawater desalination
The seawater desalination process is mainly divided into two categories: thermal method (distillation) and membrane method (reverse osmosis). The equipment process is designed according to the quality of raw water and the demand for water production. The typical processes are as follows:
（1） Reverse osmosis (RO) seawater desalination process (membrane method)
Applicable scenarios: Small and medium-sized island water supply, municipal drinking water replenishment (accounting for over 60% of the global seawater desalination market).
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Seawater → Pre treatment system → High pressure pump → Reverse osmosis membrane module → Freshwater tank → Post treatment system → Finished water
Key equipment and process analysis:
pretreatment system
Coarse filtration: Remove large particle impurities such as seaweed and shellfish through a rotating filter screen (pore size 0.5-1mm) and a laminated filter to prevent membrane blockage.
Dosing device: Sodium hypochlorite (residual chlorine 0.5-1mg/L) is added for sterilization, and scale inhibitors (such as polyphosphate) are added to prevent CaCO3 and CaSO ₄ from scaling.
Fine filtration: Multiple media filters (quartz sand+activated carbon) or ultrafiltration (UF) are used to achieve inlet turbidity<0.2 NTU and SDI (pollution index)<3, meeting the requirements for RO membrane inlet.
Reverse osmosis host
High pressure pump: Provides a pressure of 6-8 MPa (seawater osmotic pressure is about 2.5 MPa) to push water molecules through the RO membrane, with a salt retention rate of>99.5%.
Membrane module: commonly used 8-inch brackish water membrane (such as Dow SW30HLLE), with a single membrane production capacity of about 25m ³/day, arranged in a first stage and two stages (the first stage membrane accounts for 2/3 of the total, and the second stage accounts for 1/3).
aftertreatment system
Degassing tower: Remove CO ₂ (pH about 5.5-6.0) from RO produced water, adjust the pH to 7.0-8.0 by adding NaOH, and prevent pipeline corrosion.
Mineralization device: By filtering limestone or adding minerals (such as Ca ² ⁺, Mg ² ⁺), the taste is enhanced (some projects are optional).
（2） Low temperature multi effect distillation (LT-MED) seawater desalination process (thermal method)
Applicable scenarios: Large municipal water supply and energy co production projects (such as power plants+seawater desalination).
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Seawater → Preheater → Evaporator (multi-stage series) → Condenser → Freshwater tank → Finished water
Key equipment and process analysis:
Preheating system
Plate heat exchanger: uses the condensation heat generated by distillation to preheat the raw seawater (from 25 ℃ to 60-70 ℃), improving thermal efficiency.
Evaporation condensation system
Evaporator: Usually 12-20 effects are set, with decreasing pressure per effect (from 0.1MPa to 0.01MPa). Heating steam (60-70 ℃) enters the first effect evaporator, and seawater evaporates to produce secondary steam, which in turn serves as the heat source for the next effect.
Condensation tube: The secondary steam condenses into fresh water outside the condensation tube, while the original seawater is heated and evaporated inside the tube, forming a “distillation condensation” cycle.
Concentrated water discharge: The salinity of concentrated water is about 1.5 times that of the original seawater, and some energy is recovered through energy recovery devices (such as turbines).
（3） Multi stage flash evaporation (MSF) seawater desalination process (thermal method)
Applicable scenarios: Large scale projects (such as the Middle East), with a single set of water production capacity of over 100000 tons per day.
Core principle: After preheating, seawater is introduced into the flash evaporation chamber (with a pressure lower than the saturation vapor pressure corresponding to the temperature), and instantly evaporates to produce steam, which condenses into fresh water. The process is similar to LT-MED, but the evaporation temperature is higher (80-120 ℃) and the energy consumption is slightly higher.
2、 Core technology comparison and energy consumption analysis
Process type: Reverse osmosis (RO), Low temperature Multi effect Distillation (LT-MED), Multi stage Flash Distillation (MSF)
Core equipment: RO membrane, high-pressure pump, pre-treatment filter evaporator, preheater, condenser flash chamber, heater
Energy consumption 3-5 kWh/ton of water (electricity consumption) 20-30 kWh/ton of water (heat consumption, equivalent to electricity consumption) 30-40 kWh/ton of water (heat consumption)
Water production cost 6-10 yuan/ton (coastal areas) 8-15 yuan/ton (relying on low-priced heat sources) 10-20 yuan/ton (high energy consumption)
Suitable for small-scale (100-10000 tons/day), large-scale (10000-100000 tons/day), and ultra large (over 100000 tons/day) applications
Water quality characteristics: conductivity<100 μ S/cm, low conductivity of trace elements<50 μ S/cm, taste closer to tap water conductivity<50 μ S/cm, similar to LT-MED
3、 Application fields of seawater desalination equipment
（1） Municipal and residential water use
Water supply for islands and coastal cities:
Case: China the Xisha Islands Seawater Desalination Plant (RO process, water output 500 tons/day), to solve the problem of drinking water for military and civilian stationed on the island; The Shuqaiq seawater desalination plant in Saudi Arabia (LT-MED process, with a production capacity of 880000 tons/day) supplies the capital Riyadh.
Emergency water supply: During disasters such as earthquakes and droughts, mobile seawater desalination equipment (such as containerized RO devices with a water production capacity of 50-200 tons/day) can be quickly deployed.
（2） Industrial water
Power industry: Boiler feedwater for power plants (requiring high-purity water with conductivity<0.1 μ S/cm) is often treated with “RO+EDI” deep treatment, such as the seawater desalination project of Beilun Power Plant in Ningbo, Zhejiang (with a production capacity of 20000 tons/day).
Petrochemical and Metallurgy: Circulating cooling water (required hardness<300mg/L), RO produced water is softened before use to reduce the risk of equipment scaling.
3） Special scenario applications
Offshore platforms and ships: Drilling platforms and cruise ships are equipped with small RO seawater desalination devices (with a production capacity of 10-50 tons/day) to meet the daily water needs of crew members (such as the “Haiyang Shiyou 981″ platform, which produces 300 tons of fresh water per day).
Military use: Navy vessels are equipped with reverse osmosis seawater desalination equipment (such as the Chinese Type 055 destroyer, which can produce hundreds of tons of fresh water per day) to ensure the water source for long-distance navigation.
（4） Ecological and Agricultural Fields
Ecological replenishment: Ecological restoration of coastal wetlands and saltwater lakes (such as using desalinated water to maintain water levels in the Palm Island artificial lake in Dubai, United Arab Emirates).
Agricultural irrigation: In arid coastal areas (such as the Negev Desert in Israel), desalinated water is used to irrigate high-value crops (such as strawberries and flowers), but the Na ⁺ concentration in the irrigation water should be controlled to be less than 200mg/L to prevent soil salinization.
4、 Typical case: Sorek seawater desalination plant in Israel
Scale: The world’s largest RO seawater desalination plant produces 624000 tons of water per day, supplying 4 million people in Tel Aviv and surrounding areas (15% of Israel’s residential water).
workmanship
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Seawater → Rotating filter screen (500 μ m) → Chlorination+coagulation sedimentation → Sand filtration+UF → RO (6.8MPa) → Degassing+pH adjustment → Municipal pipeline network
Technical highlights:
By using an energy recovery device (PX pressure exchanger), the energy consumption of the high-pressure pump is reduced to 3.2 kWh/ton of water at a cost of only 0.5 US dollars/ton.
The membrane module adopts anti pollution RO membrane, with a service life of more than 5 years and a cleaning cycle of more than 6 months.
5、 Development Trends and Challenges
technological innovation
Low energy consumption membrane technology: Develop ultra-thin composite membranes (desalination layer thickness<500nm) to reduce operating pressure to below 5MPa.
Hot film coupling process: such as the “LT-MED+RO” combined system, which uses the waste heat of the power plant to drive distillation, and the produced water is further treated by RO to improve the water recovery rate to over 80%.
Policy and Market:
The annual growth rate of the global seawater desalination market is about 7%, with the Middle East accounting for 45% of the market share. China and Southeast Asia are experiencing rapid growth in demand due to water shortages (such as the Shanghai water desalination project with a capacity of over 100000 tons per day planned for the 14th Five Year Plan in Hainan).
Challenge:
High cost: The cost of desalinated water on remote islands can reach 15-20 yuan/ton, requiring government subsidies;
Environmental impact: The discharge of concentrated water (salinity 3.5% → 7%) may exacerbate the increase in salinity near the coast, and it is necessary to install concentrated water dilution or discharge pipelines (such as at a distance of more than 1km from the coast and a depth of less than 20m).
Through technological iteration and large-scale application, seawater desalination is transitioning from “emergency water supply” to “conventional water sources” and will play a more important role in global water resource security in the future.
Post time: Jul-04-2025