Sizing a Small Desalination System for Islands, Fishing Villages and Remote Sites

Islands, fishing villages, offshore platforms and remote construction sites share one problem: no municipal mains. Shipping water in is prohibitively expensive and rainwater harvesting is weather-dependent. A small RO desalination system is usually the realistic answer — but these projects have traps, and a badly specified unit becomes expensive scrap metal in the sun.

1. Step one: calculate demand properly

Do not size by feel. Add up per-capita use and each water duty:

• Domestic use: residents 100–150 L/person/day; crew 80–120 L/person/day;
• Kitchens and public facilities: add 20–30% of domestic demand;
• Cooling, washdown and other utility water: from actual equipment requirements;
• Leakage and losses: add 10%;
• Then add 20–30% margin for peak season, maintenance downtime and low feed temperature.

Example: a village of 50 people at 120 L/person/day needs 6 m³/day, so with losses and margin a 8–10 m³/day unit is right. Err on the generous side: RO output falls about 30% at low seawater temperature.

2. Step two: the intake decides the pretreatment

• Beach well or infiltration gallery — best water quality because the sand layer filters it: low turbidity, low biology, simplest pretreatment and longest membrane life. Choose this if the site allows.
• Direct surface intake — high suspended solids, algae and oil; multimedia plus ultrafiltration is mandatory, and the intake must avoid aquaculture zones, sewage outfalls and vessel discharge areas.
• Intertidal intake — heavy sand and silt load; needs a settling pit and coarse screening, and the intake head must be designed against blockage.

3. Step three: equipment configuration

4. Step four: storage, power and discharge

Storage. Provide one to two days of demand in a food-grade PE or stainless tank. This lets the plant run during cheap-tariff hours or peak solar output instead of 24 hours a day.

Power. Small systems have high starting current, so generator capacity should be 2–2.5 times the rated high-pressure pump power — otherwise the breaker trips on every start. Where solar or wind is available, a PV + battery + RO off-grid arrangement can cut running cost by more than half: make water by day, use stored energy at night.

Brine discharge. Choose a well-flushed location away from coral, aquaculture and the intake's downstream recirculation zone. Submerge the outfall below 2 m and fit a diffuser — this is a hard requirement in most environmental approvals.

5. Five mistakes we see repeatedly

1. Specifying it like a mains-water plant. Seawater is extremely corrosive; a carbon steel frame rusts through in a year. Heavy coating or 316L is essential.
2. No spare cartridges. Resupply to an island is slow; a cartridge stock-out means a shutdown. Keep 6–12 months of consumables on site.
3. Unattended operation without remote monitoring. Faults are found late and small problems become overhauls.
4. Ignoring typhoon and salt spray. Equipment and control cabinets need sun, rain and salt protection, mounted above the historical high-tide level.
5. No operator training. Even good equipment fails within three months if nobody performs backwash and reads the pressure gauges. Train on delivery.

Shenzhen Duruofei Technology Co., Ltd. supplies skid-mounted and containerised desalination systems from 1 to 200 m³/day for islands, fishing villages and offshore platforms, configurable for off-grid solar operation, with installation, commissioning and operator training included. We ask for a water analysis report and a demand schedule up front — those two documents decide whether the design is right first time.