Squeezing out fresh water

Ocean Navigator|May/June 2020

Squeezing out fresh water
Modern watermakers range from hands-off automatic units to basic operator involved models
WAYNE CANNING

While watermakers have become increasingly easier to use with each generation of models, this installed unit has a set of laminated instructions handy.

The term “watermaker” is a bit of a misnomer, as these units do not really make water but rather turn undrinkable salt water into drinkable fresh water. The correct name is “reverse-osmosis desalination system” or “RO system,” but let’s face it: That simply does not roll off the tongue quite as easily as “watermaker.”

Understanding the correct name helps understand what these systems really do and how they work. Watermakers take salt water and push it through membranes that only allow the smaller water molecules to pass through, blocking and separating the larger salt molecules. The result is fresh water from salt water. Sounds pretty simple, but of course nothing on a boat is that easy.

Membranes are a semipermeable material that, in a sense, filters on a molecular level. In order to do this, salt water is pushed against the membranes at high pressure. The pressure is required to squeeze the water molecules through the membrane material. This is why watermakers use an energy-intensive, high-pressure pump. The resulting water that gets through the membrane is called the product, while what is left over is called brine or reject.

Almost all watermakers found on smaller boats will consist of a few basic components. To understand these and how they work together, let’s follow the flow of the water through a basic system. First, the water comes into the boat through a thru-hull fitting. This salt or “raw” water is pumped through at least two pre-filters of five and 20 microns by a small low-pressure or lift pump prior to going to the high pressure pump. These filters are required to remove larger particles that could damage the high-pressure pump or clog the membranes. Some systems will have additional filters along with an oil separator to further protect the system and membranes from contamination. Dirty water can result in frequent filter changes or damage to the membranes, so it is also best to operate a watermaker away from polluted harbors or bays if possible.

it’s a good idea to make sure the pre-filters are easily accessible.

Once the filtered raw water gets to the high-pressure pump, the real action begins. The high-pressure pump raises the water pressure to around 800 psi, which is the pressure required to push the water through the membranes. A pressure relief valve is also installed at this point to prevent over-pressurizing of the system. The hoses connecting the high-pressure side of the pump to the membranes will also need to be designed for this high pressure.

Membranes are key

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May/June 2020