Reverse Osmosis

medium 133 models total

A reverse osmosis plant forces seawater through semi-permeable membranes at 55-70 bar to reject dissolved salts, trading the low-grade waste heat an evaporator needs for a heavier electrical load and a stricter feed-water pre-treatment regime.

Read more — Reverse Osmosis explained ▾

What sets reverse osmosis apart from thermal evaporators

A freshwater generator on most merchant ships uses jacket-cooling water heat to flash-evaporate seawater under vacuum. Reverse osmosis (RO) does the opposite: it stays cold and uses a high-pressure pump to push raw seawater across a semi-permeable membrane, forcing water molecules through while rejecting salt, bacteria and most dissolved solids. Where an evaporator is limited by available waste heat and shuts down when the main engine is stopped or running light, an RO plant only needs electrical power, so it keeps producing water alongside the engine, in port, or on a diesel-electric vessel with no spare heat at all. The trade-off is a much higher specific energy consumption per cubic metre unless an energy recovery device is fitted, and a feed water that must be pre-filtered to protect membranes that an evaporator does not need.

Reverse osmosis flow schematic — Flow diagram of a reverse osmosis plant from seawater feed through the pre-filter and high-pressure pump into the membrane module, splitting into permeate to the product tank and concentrate through the energy recovery device that boosts the feed pressure before overboard discharge.

Main components

High-pressure feed pump

A multistage centrifugal or positive-displacement pump raises seawater to 55-70 bar for typical single-pass seawater RO. It is the largest single power consumer in the plant and the component most exposed to wear from any grit that pre-filtration misses.

Membrane vessels

Spiral-wound thin-film composite membrane elements sit inside pressure vessels, usually six or seven elements per vessel. Rejection rate is typically 98-99.5% of dissolved salts on a well-maintained set.

Pre-treatment

Coarse strainers, cartridge filters down to 5 microns, and often a dosing system for anti-scalant protect the membranes from suspended solids, algae blooms and scale-forming minerals.

Energy recovery device

A pressure exchanger or turbocharger recovers hydraulic energy from the concentrate (brine) stream leaving the membranes and feeds it back into the high-pressure side, cutting specific power consumption roughly in half on modern units.

Post-treatment

Product water passes through a remineralisation or pH-correction stage and UV or chlorination before it is fit to enter the potable water tanks, since RO permeate on its own is aggressive and near-mineral-free.

Selection and sizing

Daily production capacity in cubic metres per day, matched to crew size and any cargo or process water demand.
Feed seawater temperature and salinity range for the trading area, since colder or more saline water needs higher pump pressure for the same output.
Available electrical power and whether an energy recovery device is fitted, which changes specific consumption from roughly 4-6 kWh/m3 down to 2-3 kWh/m3.
Membrane material and housing pressure rating, since brackish-water elements will not survive full seawater pressure.
Deck space and weight, particularly on smaller vessels retrofitting a plant into an existing engine room.

Regulations and class

Potable water quality is generally checked against WHO drinking water guidelines rather than a single maritime convention, and flag or port health authorities may sample it. Classification societies require periodic survey of the pressure vessel and piping under their machinery survey regime, and the high-pressure pump and its relief valve are treated as a pressure system needing routine inspection. Ballast Water Management Convention issues do not apply to freshwater generators, but any RO plant drawing from a ballast tank rather than a sea chest needs separate scrutiny.

Typical faults

Fault	Cause	Consequence
Falling permeate flow	Membrane fouling by biofilm or scale	Reduced daily production, higher differential pressure across vessels
Rising salt passage	Membrane degradation or a torn O-ring	Product water fails salinity limit, must be dumped to bilge
High-pressure pump trip	Blocked cartridge filters starving the pump	Loss of production until filters are changed
Rapid membrane fouling	Anti-scalant dosing pump failure	Premature membrane replacement, often within months

What to look for in a supplier

Availability of spare membrane elements and cartridge filters in the ports the vessel actually calls at, not just at the yard.
Documented specific energy consumption figures at stated feed conditions, not a single best-case number.
Compatibility of the energy recovery device with the pump manufacturer, since retrofitting one later is expensive.
Remote monitoring or trending of conductivity and differential pressure, which gives early warning of fouling before it becomes a rejection failure.

Track membrane differential pressure and permeate conductivity from day one; a slow upward trend over weeks is normal fouling and cleanable with a CIP flush, but a sudden step change usually means a torn membrane or a failed O-ring seal that needs the element pulled.

Inspector Mode Show All 133 total

7 manufacturers · 133 models

Danfoss

58 ✓ 58 verified

APP 0.6 ✓ verified

Application

Seawater Reverse Osmosis (SWRO) / Brackish Water Reverse Osmosis (BWRO)

Common Failures & Inspection Points

Area: Feed Water Silt Density Index (SDI) Exceeds 3.0
Check: Measure SDI before RO membrane inlet every 4 weeks using standard SDI test kit (0.45 µm filter, 15 minutes). SDI must remain below 3.0. If exceeding 3.0, pretreatment (multimedia filter, carbon filter, cartridge filter) requires backwashing and cartridge replacement
Area: Membrane Fouling Detection - Differential Pressure Rise or Permeate Flow Decline
Check: Monthly baseline pressure differential monitoring across membrane stages. A sudden spike in differential pressure (> 10% increase from baseline) indicates fouling. Visual inspection for yellow slime (biofouling) or white crystals (scaling). Inspect feed spacer geometry and membrane autopsy if fouling suspected
Area: Pump Pressure and Performance Degradation
Check: Verify APP/PAH pump discharge pressure at standard flow rates weekly. Compare with baseline. Monitor power consumption (kW input). Check for cavitation noise or abnormal vibration. Verify pump inlet pressure 0.5-5 bar (7-72 psi). Inspect for seal leaks and corrosion on stainless steel surfaces
Area: Energy Recovery Device (iSave/MPE) Efficiency Loss or Biofilm Fouling
Check: Monitor ERD inlet/outlet pressures and flows monthly. Compare energy recovery efficiency to baseline (typically 92%). Inspect for scaling/fouling by measuring pressure drop across ERD. Visual inspection of high-pressure spacer for biofilm buildup. For MPE 70, verify motor rotation speed (650-875 rpm) and electrical current
Area: Cartridge Filter Clogging and Breakthrough
Check: Monitor prefilter differential pressure weekly (must stay below 1.5 bar; replace when reaching limit). Inspect cartridge visual condition (melt blown polypropylene for 3 µm stage, 10 µ absolute for main stage). Replace cartridges immediately if pressure differential exceeds maximum or after 12 months operation. Verify Beta 5000 rating for main filters (10 µ absolute minimum)

Typ-universelle Inspektionspunkte fuer Freshwater Generators (verifiziert, 2026-06).