Seawater contains about 35,000 mg/l of dissolved salts, far too much for drinking or industrial use. Desalination removes these salts to supply fresh water to islands, coastal factories, resorts and ships where freshwater sources are limited or unreliable.
Seawater reverse osmosis (SWRO)
Today most desalination plants use reverse osmosis. A high-pressure pump pushes seawater through special RO membranes at around 55–70 bar. The membranes let water pass and hold back more than 99% of the salt. Energy recovery devices reuse the pressure of the concentrate stream, which greatly reduces electricity consumption.
Pre-treatment is critical
Seawater contains algae, suspended solids and organic matter that quickly foul RO membranes. A reliable pre-treatment line usually includes intake screening, coagulation, multimedia filtration or ultrafiltration (UF), and 5 µm cartridge filters. UF in particular gives stable feed water quality (SDI below 3) even when sea conditions change.
Post-treatment and concentrate
RO permeate is very low in minerals and slightly aggressive, so it is remineralised and disinfected before use. The concentrate, about twice as salty as seawater, must be returned to the sea in a way that limits environmental impact.
Planning a desalination plant
- Seawater analysis, including temperature and seasonal changes
- Required capacity and water quality
- Available power supply and energy cost
- Corrosion-resistant materials for all wetted parts
- Operator skills and access to spare parts
TIPA supplies spiral wound RO and UF membranes and supports the design, installation and maintenance of desalination systems.
Small units or full systems?
The target for desalinated water is usually a TDS below 500 ppm, suitable for domestic or production use.
| Seawater filter unit | Desalination system | |
|---|---|---|
| Capacity | 10 – 500 l/h | 1 – 1,000 m³/h or more |
| Feed TDS | < 25,000 ppm | < 45,000 ppm |
| Fresh water recovery | < 30% | < 45% |
A typical open-channel RO desalination line
- Pre-treatment with multimedia sand filters and 10 µm cartridges; open-channel modules accept feed with SDI up to about 20
- RO modules with thin film composite polyamide/polysulfone membranes; only acid or alkali for pH adjustment
- 30 – 45% of the seawater becomes fresh water; the brine returns to the sea
- Membranes are cleaned about every 600 – 800 operating hours, with PLC-controlled cleaning sequences
Investment is significant: a 100 m³/day system typically costs around 10 – 20 billion VND, depending on water quality and equipment.
Other desalination methods
- Distillation (multi-stage, low-pressure or high-temperature evaporation): gives both fresh water and salt, but energy use is high
- Ion exchange for specific salts
- Solar desalination for very small, off-grid needs
Desalination plants in Vietnam
- Ly Son island, Quang Ngai: 200 m³/day design capacity, in operation since 2012
- Con Co island, Quang Tri: 300 m³/day, recovery 30 – 40%, Rochem TSRO modules, in operation since 2023
