Marine Watermaker (RO Desalinator)

Overview

A marine watermaker produces drinking water from seawater by reverse osmosis: seawater is pressurised beyond its natural osmotic pressure and forced against a semipermeable membrane that passes water molecules while rejecting dissolved salt. Seawater at 35,000 ppm exerts about 27 bar of osmotic pressure, so practical systems run near 55 bar to drive useful flux through the membrane. A frame-mounted 100 L/h plant of the kind described here fits the machinery space of a cruising yacht or small commercial vessel and converts roughly a tonne of seawater into 100 L of product and 900 L of discharged brine every hour.

How it works

Raw seawater enters through a dedicated Intake Seacock and the Sea Strainer, which stops weed and shell. The Boost Pump then holds the line at 1–2 bar, both to push water through the pre-filters and to guarantee the high-pressure pump a flooded suction, a plunger pump that cavitates destroys its seals within hours. Feed passes the 20 µm Filter Cartridge and 5 µm Filter Cartridge in turn; the 5 µm final stage defines the largest particle allowed to reach the pump and membranes, and a Pressure Sensor across the housings warns when clogging cartridges starve the system.

The High-Pressure Pump is a triplex plunger pump: three ceramic Ceramic Plungers reciprocating in a 316L Pump Head (316L Manifold), driven through a Drive Belt by the 2.2 kW Pump Drive Motor. Positive displacement means the pump sets flow while the Back-Pressure Regulator on the brine outlet sets pressure, closing it raises system pressure toward the 55 bar working point, with the HP Relief Valve guarding the 69 bar vessel rating.

At pressure, feed sweeps axially through two spiral-wound Spiral-Wound RO Elements in series inside fibreglass Membrane Pressure Vessels. Water permeates the polyamide film into the central product tube; salt stays in the tangential flow and exits as brine. Only 10–15% of the feed is recovered as permeate. The low ratio is deliberate: high cross-flow velocity scours the membrane surface and limits concentration polarisation, the local salinity rise at the membrane wall that otherwise accelerates scaling and fouling.

Energy recovery

The brine leaves the last vessel still at nearly full pressure, carrying most of the pump's work. Small systems simply throttle it away; energy recovery models route it through the Brine Energy Recovery Unit, a piston-type pressure exchanger that transfers brine pressure directly into a portion of the incoming feed. Recovery cuts specific energy from roughly 6 kWh/m³ to near 3 kWh/m³, which on a yacht halves generator runtime per tank, the difference between a watermaker that runs off a small inverter bank and one that demands the genset.

Water quality and controls

Permeate from a healthy membrane stack measures 150–400 ppm total dissolved solids. The Salinity Probe reads conductivity continuously, and the Diversion Solenoid Valve dumps product overboard until it falls below the 500 ppm potable threshold, covering the first minutes of a run, when stagnant high-TDS water clears the system. Good water then passes the Product Flow Meter to the tank.

The Control System sequences all of this: boost pump first, high-pressure ramp after feed pressure confirms, diversion until salinity passes, and protective shutdown on low feed pressure, high system pressure, or persistent high salinity. After each run it opens the Freshwater Flush Valve and displaces seawater from pump and membranes with tank water passed through the Carbon Flush Filter, carbon matters because polyamide membranes are destroyed by even trace chlorine from municipally filled tanks. Programmed weekly flushes keep an idle system from biofouling without chemical pickling.

Maintenance

Consumables follow a predictable ladder: pre-filter cartridges every 20–50 hours depending on water clarity, pump HP Seal Kit and Check Valve Kit at 500-hour intervals, crankcase oil annually, and membranes after three to five years, signalled by rising permeate TDS or falling output that acid and alkaline cleaning cycles no longer recover. Output also tracks temperature, roughly 3% more water per degree Celsius, so the same plant making 100 L/h in the tropics yields nearer 70 L/h in 10 °C water at the same pressure. Long layups call for pickling with sodium metabisulphite to keep the membranes wet and sterile.