Membranes are used to perform reverse osmosis, which consists in applying pressure on the salt water and passing it through a semi-permeable membrane that derives from the external side the water with a high concentration of salt and through an internal hole the water without salts suitable for consumption, having eliminated even viruses and bacteria in this process.

This membrane must be inserted into an airtight container that supports a maximum pressure of about 1000 PSI or 70 bar. With optimal working pressure the osmosis will be performed between 58 and 60 bar. Sea water in the Mediterranean has a salt concentration of about 37,000 ppm. Osmosis performed to obtain drinking water from sea water will convert this water to about 300 ppm when the membrane is new. If we work at a pressure below 55 bar we will get a higher concentration of salts in the water produced as well as if the membranes are worn by use or improper maintenance (see “winterizing”).

If the membranes are used with sea water without subsequent periodic rinsing with fresh water (one-week rinsing interval), these membranes will be unusable. It is important when buying a watermaker o desalinator that has an automatic fresh water flush when stopping the process of osmosis. There are systems that carry a weekly timer for automatic rinsing.

Something else to keep in mind are the containers for these membranes. It is strongly recommended that they be made of fiber, in the case of stainless steel containers these will end up oxidizing internally (remember that seawater is highly corrosive) and rust particles damage the membrane.

The membranes consist of an inner core of perforated polymer in which the microperforated polyamide is wound like a canvas. The quality of a membrane will be determined by the density of the polyamide and especially by its quality, which must be polyamide S.

The most commonly used membrane measurements in a boat watermaker are 2.5″x 21″ (6.35 cm x 53.34 cm) or 2.5″ x 40″ (6.35 cm x 101.6 cm). The nomenclature would be 2521 or 2540. There are also membranes for sea water of 4″ diameter with the same length or 8″.

It is highly recommended that when replacing a membrane the O-rings of NBR, which is the most seawater resistant material for this purpose, are also replaced.

At DESSAL we have high quality membranes for the most common brands of watermakers on the market.