Membranes are used to perform reverse osmosis, which consists of applying pressure to salt water and making it pass through a semipermeable membrane that diverts water with a high concentration of salt on the outside and, through an internal hole, salt-free water suitable for consumption, having eliminated even viruses and bacteria in this process.
Each membrane must be inserted into an airtight container that can withstand a maximum pressure of about 1000 PSI or 70 bar. With an optimal working pressure, osmosis will take place between 58 and 60 bar. It must be taken into account that seawater in the Mediterranean has a salt concentration of about 37,000 ppm. The osmosis carried out to obtain drinking water from seawater will convert this water to about 300 ppm when the membrane is new. If we work at a pressure lower than 55 bar, we will obtain a higher concentration of salts in the water produced, as well as if the membranes are worn out due to use or inadequate maintenance.
If the membranes are used with sea water without performing periodic rinses with fresh water (rinse interval of one week), these membranes will become unusable. When purchasing a water treatment or desalination plant for a boat, it is important to consider that it has an automatic rinsing system with fresh water when the osmosis process is stopped. There are systems that have a weekly timer for automatic rinsing.
Something else to consider is the containers for these membranes. It is highly recommended that they be made of fibre, as in the case of stainless steel containers, these will end up rusting internally (remember that sea water is highly corrosive) and the rust particles will end up damaging the membrane.
Membranes consist of an inner core of perforated polymer on which microperforated polyamide is wound like a canvas. The quality of a membrane will be determined by the density of the polyamide and above all by its quality, which must be polyamide S.
The most commonly used membrane sizes for a ship desalination plant 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 for the former will be 2521 or 2540. There are also 4” diameter seawater membranes of the same length or 8” ones.
It is highly recommended that when replacing a membrane, the NBR O-rings are also replaced, as this is the most seawater-resistant gasket material.
At DESSAL we have high-quality membranes for the most common brands of water treatment plants on the market.