How a Marine RO Watermaker Makes Fresh Water: Five Subsystems and the Full Process

An ocean-going vessel can be at sea for weeks. Carrying all the fresh water from port is impractical: crew drinking, washing and galley use, plus main engine and auxiliary cooling make-up, add up to several tonnes — sometimes tens of tonnes — per day. A marine RO watermaker — called a freshwater generator or FW generator on merchant ships — is the vessel's own water plant, turning seawater over the side into potable water on demand.

1. The principle: reversing natural osmosis

Under natural conditions water crosses a semi-permeable membrane from the dilute side towards the concentrated side. Reverse osmosis inverts that: a high-pressure pump applies 6.0–7.0 MPa to the seawater side, well above the osmotic pressure of seawater (about 2.5–3.5 MPa), forcing water molecules against the concentration gradient. Salt ions, colloids, bacteria and heavy metals stay behind and leave with the concentrate.

Open-ocean seawater is around 35,000 ppm TDS. A marine RO element typically rejects more than 99.5%, holding product TDS below 700 ppm, which satisfies the WHO/GB 5749 drinking water guideline. Two-pass machines take product below 50 ppm for boiler make-up or more demanding duties.

2. Five subsystems, none optional

Pretreatment — this is what decides membrane life

• Sea chest strainer — stops seaweed, shells and plastic debris;
• Booster feed pump — delivers 0.2–0.3 MPa positive suction pressure so the high-pressure pump does not cavitate;
• Multimedia filter — quartz sand (sometimes with activated carbon) removing solids above roughly 30 microns, backwashable;
• 20 µm and 5 µm cartridge filters — the last barrier before the membranes.

Practical rule: if the pressure gauge downstream of the cartridge filter reads close to zero, the cartridges are blocked. Change them — running on, you will damage the high-pressure pump through insufficient feed.

High-pressure desalination — the core

Comprising the high-pressure plunger pump, FRP or stainless pressure vessels, seawater RO elements, concentrate regulating valve and high-pressure cut-out switch. Pump discharge is normally set at 6.5–6.8 MPa. The regulating valve sets the recovery ratio, usually 20–35% on marine units; push recovery much higher and concentration polarisation accelerates scaling.

Electrical and control system

A PLC with inline conductivity meter, flow meters, pressure switches and solenoid valves automates three actions: product only enters the tank when salinity is within limit (otherwise it diverts overboard); low suction or overpressure stops the machine; tank high level stops the machine.

Clean-in-place (CIP) system

A cleaning tank and pump with a three-way valve let cleaning solution circulate through the elements. Calcium carbonate scale comes off with citric acid; organic and biological fouling needs alkaline cleaner plus biocide; rinse to neutral with permeate afterwards.

Chemical dosing

Antiscalant to control calcium carbonate and sulphate scaling, plus a small biocide dose to limit biofilm. Dose according to feed quality and recovery — slightly under is safer than over, because excess antiscalant itself fouls membranes.

3. The complete process flow

Sea chest → strainer → booster pump → multimedia filter → 20/5 µm cartridges → dosing → high-pressure pump → RO membranes → permeate → UV steriliser → potable tank. Concentrate → regulating valve / energy recovery device → overboard.

Note that concentrate discharge backpressure should generally stay below 0.48 MPa. If the discharge line runs below the unit, fit a vacuum breaker so the system cannot siphon itself into vacuum.

4. Why marine units need energy recovery

The high-pressure pump raises seawater to 6.5 MPa and roughly 60–70% of that pressure energy leaves with the concentrate. A turbine or pressure-exchanger type energy recovery device captures it and pre-pressurises the feed, saving 30–40% of the electrical load. On a 10 m³/d marine unit that is 3–5 kWh per day; on a 100 m³/d unit the annual saving is a serious number.

5. What to specify

• Daily capacity — from crew numbers plus engine make-up, with 20–30% margin;
• Minimum feed temperature — check membrane area at 5–10 °C for northern routes;
• Power supply — 380V/50Hz or 440V/60Hz three-phase is common; state it on the order;
• Materials — 316L stainless wetted parts, FRP vessels, heavy-duty anti-corrosion frame;
• Class — ocean-going vessels usually need CCS, BV or ABS approval, and the flag state may add requirements.

Shenzhen Duruofei Technology Co., Ltd. builds marine RO watermakers from 1 to 100 m³/day with energy recovery and PLC control as standard, with skid dimensions adapted to the vessel. Space in a ship is expensive, so these units are normally arranged as compact vertical skids that can be moved in through engine-room access.