Is water from a reverse-osmosis system safe to drink on a boat?
We have a seawater reverse-osmosis watermaker on our cruising yacht, feeding a 300-litre freshwater tank. Can we drink it straight from the galley tap, and do we need minerals, chlorine or another filter?
Answer
Yes, water from a properly maintained reverse-osmosis watermaker is safe to drink if it draws from suitable seawater and produces water within the manufacturer's specification. Check product-water salinity and the cleanliness of the tank and pipework first, because desalination does not guarantee that water stays clean on its way to your glass.
Check the watermaker output
A marine watermaker forces seawater through a membrane under pressure, separating fresh product water from concentrated brine. An intact membrane forms a strong barrier against microorganisms and removes most dissolved salts, but the complete installation is never permanently sterile.
Most automatic systems measure product-water conductivity, display salinity or total dissolved solids, and divert unacceptable water overboard. With a manual installation, the operator must check the output before directing it into the tank.
- Start with the outlet diverted to waste. Let the unit stabilise for the period specified in its manual, then check the reading before filling the tank.
- Know your normal reading. Many seawater units produce water containing a few hundred parts per million of dissolved solids. The acceptable limit and diversion setting vary by manufacturer and model.
- Investigate a rising reading. Check the measuring instrument, operating conditions, membrane seals and membrane condition before accepting more water.
A handheld conductivity or TDS meter is useful for checking the installed sensor. A low reading, though, measures dissolved ionic material; it does not prove that water is free of bacteria, fuel or other chemicals.
Protect the intake and membrane
Make water in clean, open seawater whenever possible. Avoid marinas, fuel docks, sewage-affected anchorages, muddy estuaries and water with an oily sheen. An ordinary marine watermaker is not a universal treatment plant for contaminated harbour water.
Inspect the seawater strainer and replace sediment prefilters at the intervals or pressure-drop limits specified by the maker. A common arrangement uses a 20-micron filter followed by a 5-micron filter, although some units use a different sequence. These filters protect the machinery and membrane; they do not disinfect drinking water.
Follow the prescribed freshwater-flush routine after use and the preservation procedure before an extended shutdown. The permitted idle period varies with the unit and water temperature. Do not assume an unused watermaker can sit for a whole season without preservation.
Keep chlorine-containing water and tank disinfectant away from the RO membrane unless the manufacturer explicitly permits them: chlorine damages many marine RO membranes. If the unit flushes with tank water, maintain the specified carbon dechlorination filter and replace it on schedule.