Desalination: How It Works And Which System Is Right For You

December 15, 2017

About 2.5 percent of the water on Earth is fresh. Of that, only about 1 percent is easily accessible. When you’re on a boat in the middle of the ocean, no fresh water is accessible at all. Fortunately, you’re surrounded by water and we have the technology to make that water usable. The technology isn’t new. Aristotle described a desalination technique in the 4th century BC in Meteorologica. There’s even a mention of desalination (although vague) in the Bible (Exodus 15:22-26). Large-scale desalination was developed in the 1930s and has been a critical source of fresh water in many parts of the world since then.

In this article, we’ll discuss the basic principles of desalination with regards to the two main types: reverse osmosis and thermal. We’ll talk about which type of desalination system is used for boats and some things to consider when choosing your system.

Thermal Desalination

Thermal desalination was the first type to be used on a large scale in the 1930s. There are 3 main types of thermal desalination— vapor compression (VC), multi-effect distillation (MED), and multi-stage flash distillation (MSF). The different types of thermal desalination vary in their efficiency, but they all follow the same basic principles. Essentially, thermal desalination systems mimic the natural water cycle. The water is evaporated, accumulated, and condensed. The condensed water is then free of salt and impurities. Because thermal desalination requires heat, it can often be linked to power plants and refineries, which produce heat as a by-product of existing processes. On the other hand, this heat requirement makes thermal desalination less than ideal for marine use.

Reverse Osmosis

Instead of evaporation, reverse osmosis uses a filtering system to remove salt and impurities. Salt water is forced through a semi-permeable membrane. Salt and impurities are trapped in the membrane and the water that comes through is fresh. The process requires high amounts of pressure (600-1200 psi for saltwater), but requires less energy than thermal desalination.

Watermaker On Board

Most marine desalinator units that are currently available for smaller cruising vessels use reverse osmosis. There are small evaporation units out there. When choosing the right system for your boat, there are some important questions to ask. The first is whether you should be looking at 120/220-volt AC systems or 12- or 24-volt DC systems. If your boat will have its own generator, you should be shopping for the former. Smaller boats that aren’t using a generator should probably opt for a 12- or 24-volt DC system. With good weather, a wind generator or solar panel should be able to keep up.

The next thing to consider is the size (output) of your system. The U.S. Coast Guard recommends having 30 gallons of fresh water available for each person on board per day. For example, if you have 4 on board your vessel, you’ll need about 120 gallons a day. You’ll then need to determine how many hours a day you’d like to run your water maker. Most people run their water makers between 2 and 5 hours per day. So, for the example above, if you’re willing to run your watermaker for 2 hours a day, you’ll need a water maker that’s capable of producing 60 gallons of fresh water per hour.


Desalination has been around for thousands of years in one form or another. With advances in technology, it has become practical and trusted enough to be relied upon as the only source for fresh water. Reverse osmosis has made reliable desalination possible for owners of small and large ocean-going vessels. It’s important to choose the correct system for your boat, but it’s a relatively simple process once you know the right questions to ask. For more information regarding your fresh water making needs, visit FCI Watermakers or view our full marine and commercial product line – we are the true innovator of marine reverse osmosis systems.

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