How to produce Freshwater at sea pt.2

Reversed osmosis unit in freshwater production

Reversed osmosis units are the second most common method to produce fresh water on board the vessel. The first article about freshwater production You can find here. What is osmosis and why it has to be reversed? Osmosis occurs spontaneously as a result of the passage of a solvent, through a semi-permeable membrane from lees concentration substance to higher concentration substance until equalizing both sides. In simple words, if we have fresh water on one side of the filter and seawater on the other side, the membrane will slowly release salt to freshwater. You are aware of how osmosis work, what happened if we reverse this process?

The physical explanation of reversed osmosis

Osmosis occurs spontaneously as a result of the passage of a solvent through a semi-permeable membrane from a solution with a lower concentration of a solute to a solution with a higher concentration. Leads to an equalize concentration of both solutions. In simple words, osmosis tries to equal concentration from both sides of the filter.

Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low-solute concentration by applying a pressure in excess of the osmotic pressure. The largest and arguably most important application of reverse osmosis is the separation of pure water from seawater and other brackish waters: The seawater, or brackish water, is pressurized against one surface of the membrane, causing the transport of salt-depleted water across the membrane and creating pure water on the low-pressure side.

Complete system with Reversed Osmosis Unit

From a physical point of view, reversed osmosis isn’t a more complicated process than evaporation and condensation. RO units are actually less complicated devices. You can buy an RO unit for Your home use purposes as well. Of course for marine use, this is a little bit more complicated equipment because of size.

The system basically looks like in the picture above. Contaminated water is coming to the system, through the filters. Sediment filter which is removing big contamination like dust and rust. Next is a carbon filter, which reduces volatile organic compounds and chlorine. Next filtration is done by the semi-permeable membrane in the RO unit. The pressure of incoming water is used as pressure which pushes water from hi contaminated side to the low contaminated side. Clean water travels through the next carbon filter to storage tanks and consumption points. The system is really easy. The biggest advantage of this system is redundancy because a couple of RO units can be installed onboard easily.


I expect that evaporators will become less attractive in the next years, because of RO units. Especially that research about (1.)Low-fouling membranes, NanoH2O membranes, biomimetic membranes are going to improve this good solution even better.

For more information about RO units and their development, I can highly recommend the paper of Lyndsey Wiles and Elke Peirsegaele.

1).Reverse Osmosis: A history and explanation of the technology and how it becamy so important for desalination.


  • Mateusz Białas

    Researcher of Autonomous and Remotely Controlled ships. Since 2014 in Maritime Industry. Experience gained at Ro-Ro, Ferry, Container, and Heavy Lifts, design of Navy Ships.

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,Narasimharao v
,Narasimharao v
3 years ago

Very beautifully demonstrated.we have also U.V.unit installed in some ships after Carbon Filter.

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