If you’re considering extended offshore living or sailing, one of the most crucial pieces of equipment you’ll need could be a RO watermaker. These devices can turn seawater into fresh, potable water, ensuring a steady supply even when you’re far from land. We’re surrounded by water but none of it is suitable for consumption.
What is a RO Watermaker?
A watermaker, also known as a desalinator, is a device that removes salt and other impurities from seawater to produce fresh water. It uses a process called reverse osmosis, where seawater is forced through a semi-permeable membrane at high pressure, leaving salt and other contaminants behind rejected as brine.
The Science Behind the Membrane
The heart of a watermaker is its semi-permeable membrane, typically made of thin-film composite polyamide. This membrane works on the principle of reverse osmosis (RO). In nature, osmosis occurs when water moves from an area of low solute concentration to an area of high solute concentration. Reverse osmosis, as the name suggests, reverses this process.
The RO membrane has pores so small (about 0.0001 micron) that they allow water molecules to pass through but block larger molecules like dissolved salts, bacteria, and other contaminants. By applying pressure (typically 55-70 bar for seawater) that exceeds the natural osmotic pressure, we force water molecules through the membrane, leaving behind a concentrated brine solution.
Salt rejection occurs because salt ions (Na+ and Cl-) are larger than water molecules (H2O) and carry an electrical charge. The membrane’s pores are not only smaller than these ions but also slightly negatively charged, further repelling the ions. This combination of size exclusion and charge repulsion results in typical salt rejection rates of 99% or higher in modern membranes.
It’s worth noting that while RO membranes are incredibly effective, they’re not perfect. A small amount of salt does pass through, which is why watermaker output is measured in parts per million (ppm) of total dissolved solids (TDS). For context, seawater typically has a TDS of about 35,000 ppm, while most watermakers produce water with less than 200 ppm TDS – well below the 1000 ppm standard for drinking water set by the World Health Organization.
Maintenance Tips
To keep your watermaker functioning optimally:
- Flush the system regularly with fresh water, especially if not in use, you dont want salt crystal to form inside the membrane
- Replace 5 microns pre-filters as recommended by the manufacturer, they are CHEAP compared to a membrane.
- Clean the membrane periodically to prevent scaling, ours recommends every end of season or 1000H running.
- Test your water quality regularly, you want to know early when something isn’t quite right
A watermaker can be a game-changer for offshore living, providing peace of mind and extending your range significantly, it’s like every other system on your boat, take good care of it and it should have a long life !
Would you be interested in knowing more about watermakers ? Ask, I might write some more !