Water Makers and Purification

Two ways to make fresh water

Reverse osmosis (RO) pumps filtered seawater at about 55 to 70 bar (800 to 1,000 psi) through spiral-wound membranes. Water passes through the membrane while salts stay in the brine. RO needs only electrical power and works regardless of engine load.

A freshwater generator boils seawater under vacuum at low temperature, using heat from the diesel engines' jacket cooling water. An ejector holds the vacuum, and a condenser recovers the vapour as distillate. It costs little to run while the engines carry steady load, but output falls when they do not.

Post-treatment

RO permeate and distillate are both soft and aggressive to pipes. Remineralisation raises hardness and pH. Chlorination keeps a disinfectant residual in tanks and lines. UV adds a disinfection barrier at the point of supply. Vacuum evaporation runs well below sterilising temperature, so distillate still needs disinfection.

Key specifications

Standards and what they require

Flag states and company standards set the potable water quality. Most base it on the WHO Guidelines for Drinking-water Quality and the WHO Guide to Ship Sanitation. ISO 15748 covers potable water supply on ships, and the Maritime Labour Convention requires good-quality drinking water for everyone on board. Class rules apply to pressure vessels, pumps and piping in the plant. Water makers are not usually class-essential, but the potable water system is surveyed. Expect a bacteriological and chemical test of the water after any new unit is installed.

Failure modes and inspection

• RO membranes. Fouling and scaling show as rising feed pressure or differential pressure for the same output. Oxidation by chlorine shows as rising salt passage. Polyamide membranes do not tolerate chlorine, so pre-treatment must remove it.
• High-pressure pump. Check hours, valve and seal wear, and vibration. Check the energy recovery device if fitted.
• Pressure vessels and fittings. Look for corrosion at end caps and couplings.
• Pre-treatment. Check multimedia filters and cartridge filters, and the silt density index of feed water.
• Freshwater generators. Look for scaling and leaks in the plate pack, gasket condition, ejector wear, brine pump and salinometer function.
• Dosing systems. Check pump function, and whether antiscalant and chlorine dosing records match operation.

A unit laid up wet without preservation chemicals can grow bacteria in membranes and pipework. Ask how it was stored.

Documentation

• Operating logs of pressures, flows and product salinity
• Membrane installation dates and cleaning records
• High-pressure pump service history
• Water quality test reports
• Material certificates for seawater-wetted parts
• Preservation record if stored

Integration and compatibility

An RO unit needs a seawater supply, a brine overboard line and power for the high-pressure pump. A freshwater generator needs jacket water flow and temperature from engines that run steadily, plus seawater for condensing and ejection. Both feed potable and technical tanks through post-treatment. Check the salinometer and dump valve that send off-specification water back to the sea instead of the tanks.

Refurbishment and selection

A used RO frame with new membranes is a common route, because membranes are a known replacement cost and the frame and pump last much longer. A freshwater generator suits units whose engines carry steady load. Buy new when you need a large step in capacity or a specific potable water approval. Tanks and distribution are under Water and sanitation, and effluent treatment under Sewage treatment plants.