Watermaker (Desalinator)

What sets an RO watermaker apart

A reverse-osmosis desalinator forces pre-filtered seawater through a spiral-wound semi-permeable membrane at high pressure, typically 55 to 70 bar for seawater feed. The membrane passes water molecules but rejects dissolved salts and most organics, so permeate on the low-pressure side comes out as fresh water while a concentrated brine stream is discharged overboard. This differs fundamentally from the older evaporator (flash or plate) plants still found on steam and larger motor ships, which boil off fresh water using waste heat from jacket cooling water and need no high-pressure pump, but are heavier, slower to start and less suited to small vessels or intermittent operation.

Main components

Pre-filtration

Sediment and cartridge filters ahead of the high-pressure pump remove particulates that would otherwise scour or foul the membrane surface.

High-pressure pump

A piston or multistage centrifugal pump that raises feed pressure to membrane operating range; this is the highest-wear, highest-energy component in the system.

Membrane vessels

Pressure vessels holding one or more spiral-wound membrane elements in series, sized by the daily production rate required.

Energy recovery device

On larger units, a pressure exchanger or turbine recovers energy from the brine discharge stream and feeds it back into the high-pressure side, cutting specific energy consumption significantly compared with a plant with no recovery.

Product water monitor

A conductivity or salinity sensor with an automatic diversion valve that dumps out-of-specification permeate overboard instead of into the potable tank.

Selection and sizing

Sizing runs off daily production requirement in litres or cubic metres per day against crew and passenger complement, feed water temperature and salinity at the vessel's typical trading area, and available electrical power. Warmer, less saline water yields higher flux for the same membrane area, so a unit sized for the Baltic will underperform in the Red Sea unless margin is built in. Redundancy matters more than on most auxiliary machinery: loss of the only watermaker on a long passage becomes a provisioning emergency, so many owners specify two smaller units over one large one.

Regulations and class

Product water quality is generally checked against WHO or equivalent potable water guideline values rather than a single maritime-specific standard, and class societies typically require the discharge piping and pressure vessel design to meet standard piping class rules rather than imposing desalinator-specific class notations. Flag state and port health authorities may require periodic bacteriological testing of the potable water system downstream of the watermaker, particularly for passenger vessels.

Typical faults

• Membrane fouling from inadequate pre-filtration — production rate drops and required feed pressure climbs, eventually forcing early membrane replacement.
• Biological growth during idle periods — a plant left full of seawater without flushing or pickling develops biofouling and odour, degrading product quality on restart.
• High-pressure pump seal wear — leads to falling pressure and, on piston pumps, oil contamination of the feed side if seals fail completely.
• Salinity probe drift — an uncalibrated sensor can pass out-of-specification water into the potable tank instead of diverting it overboard.

What to look for in a supplier

• Membrane and pressure vessel materials rated for continuous seawater duty, not brackish-water equivalents sold at a lower cost.
• Availability of spare membrane elements and high-pressure pump seals through a marine parts network, not only direct from the manufacturer.
• A documented flushing and pickling procedure for lay-up periods.
• Energy recovery efficiency figures quoted at the vessel's actual feed water temperature range, not only at optimum test conditions.

Flush the membrane with fresh water before any lay-up longer than a few days — a plant left sitting full of seawater is the single most common cause of premature membrane replacement on yachts and small vessels.