Seawater Desalination Systems · Safeguarding healthy drinking water for ocean-going vessels and island residents 海水淡化装置 · 为远洋船舶、海岛人员健康饮水保驾护航

Brackish water in China is mainly found in the northwest, north and eastern coastal regions. Three methods are commonly used: reverse osmosis (RO), which presses brackish water through a semi-permeable membrane under high pressure — the mainstream choice thanks to low energy use, small footprint and high automation; electrodialysis, suitable for lower-salinity water; and distillation (MED/MVC), suitable where waste heat is available. For brackish water of 1–10 g/L salinity, RO is the first choice, achieving a salt rejection rate above 97% with output that is safe to drink.

Seawater desalination has two main technical routes: membrane (reverse osmosis) and thermal (distillation). RO pushes seawater through a membrane at high pressure, removing over 97% of the salt; it is the most widely used and most energy-efficient method, at only 3–5 kWh per ton of water. Thermal processes — multi-stage flash (MSF), multi-effect distillation (MED) and mechanical vapor compression (MVC) — suit large power or chemical plants with usable waste heat. Electrodialysis and freezing methods exist but are rarely used. Choose by scale, energy conditions, and capital/operating cost; for small-medium, marine and island projects, RO is the preferred option.

Cleaning falls into physical and chemical methods. In daily operation the system automatically performs low-pressure flushing with permeate at set intervals to wash deposits off the membrane surface. When salt rejection drops or the pressure difference rises by 5–10%, chemical cleaning (CIP) is required: first circulate an acidic solution (e.g. citric acid, pH ≈ 2) to dissolve carbonates and iron oxides, then an alkaline solution (pH 10–11) to remove organics and colloids; soak for 2–4 hours and rinse back to neutral to restore performance. Keep the solution below 35°C, carry out cleaning only with the unit stopped and valves switched, and follow the manual or have trained personnel do it.

Large island desalination units (50+ tons/day) suit inhabited islands with stable mains or diesel power: village centralized water supply, resort and guesthouse clusters, aquaculture bases, border posts and island stations. Container-type or modular designs are easy to transport and install and resist humidity and salt fog; paired with solar or battery systems they can also run off-grid. Size the unit by the island's resident population — allow 100–150 L per person per day, so an island of 500 residents needs roughly a 50–80 ton/day unit to cover all freshwater needs.

A seawater desalination plant is a complete system that removes salt and impurities from seawater to produce freshwater. It consists of three parts: pretreatment (multi-media and cartridge filters that remove silt, colloids and suspended solids to protect the RO membranes), the main desalination section (high-pressure pump + RO membrane system, the core stage with salt rejection above 99%), and post-treatment (pH adjustment, remineralization and disinfection) so the output meets the national drinking water standard GB 5749-2022. The whole system runs automatically and is widely used on vessels, islands, drilling platforms and in water-scarce coastal areas.

Marine desalination units free vessels from shore-based water supply — producing fresh water on board greatly cuts resupply cost and port time. They are compact and lightweight, with horizontal designs that fit narrow engine-room spaces; fully automatic one-button operation needs no dedicated operator; they withstand rolling (±22.5°), humidity and salt fog with marine-grade anti-corrosion components; and the output directly covers crew drinking water, boiler feed and equipment cooling. With an energy recovery device, power consumption is only 3–4 kWh per ton — economical to run.

RO seawater desalination offers: ① high salt rejection — over 99% single-stage, output TDS below 500 mg/L, meeting drinking water standards; ② low energy consumption — it runs at ambient temperature without phase change, and with an energy recovery device only 3–5 kWh per ton, far below distillation; ③ high modularity — small footprint, quick installation, easy capacity expansion; ④ high automation — PLC monitoring throughout with unattended operation; ⑤ long membrane life (3–5 years), low maintenance cost and reliable operation. RO is therefore the mainstream desalination technology with over 80% market share worldwide.

Pretreatment is the foundation of stable desalination operation — its goal is to remove suspended solids, colloids, algae, bacteria and large particles from the raw water to prevent membrane fouling and clogging. A typical train: seawater intake → coagulation/settling (removes coarse silt) → multi-media filter (retains suspended solids) → cartridge filter (5 μm, protects the high-pressure pump and membranes). For high-turbidity or high-organic seawater, flotation or ultrafiltration units are added; antiscalant is dosed to prevent scaling and a reducing agent neutralizes residual chlorine. The target is a silt density index (SDI) below 5, ideally below 3. Good pretreatment extends membrane life beyond 5 years and significantly cuts operating costs.