The routine, in order of how often you do it
| When | What | Why it matters |
|---|---|---|
| Every run | Check product-water quality before it goes to the tank | The one habit that stops a bad batch reaching your drinking water |
| After every run, if idle more than a day or two | Fresh-water flush | Displaces seawater so nothing is left to go biological inside the membrane |
| Regularly, more often in silty or harbour water | Inspect and change prefilters | A blocked prefilter starves the high-pressure pump and looks exactly like a failing membrane |
| Per the manufacturer's hours | Pump service kit | Seals and valves are consumables on a high-pressure pump |
| Before any lay-up of weeks or more | Preserve (pickle) with the specified chemical | Stops biological growth during storage; the wrong chemical can destroy a membrane |
Flushing: the habit that matters most
Seawater left stagnant inside a membrane at ambient temperature is an excellent place for biology to grow. A fresh-water flush displaces it, and on most systems takes a couple of minutes. If the boat is used regularly and the system runs most days, flushing is often unnecessary between runs; it is the sitting that causes the problem, not the running.
Flush water should be free of chlorine. Chlorine attacks thin-film composite membranes, which is why systems with automatic flush arrangements draw through a carbon filter. If you flush manually from a dockside hose, be aware that town water is normally chlorinated.
Prefilters: cheap, and the source of most confusion
Prefilters protect the high-pressure pump and the membrane from sediment. They are cheap, they are meant to be replaced, and how fast they load up depends entirely on the water. Clear ocean might give you many weeks; a silty river mouth or a plankton bloom can block one in a day.
The important point for diagnosis: a partially blocked prefilter reduces feed flow, which reduces output and can raise or lower pressure depending on where in the circuit you look. It presents almost exactly like a tired membrane. Always eliminate the filter first — it is the cheapest thing in the system and the most likely cause.
- Fit a pressure or vacuum gauge on the feed side if the system does not have one. It turns filter condition from a guess into a reading.
- Carry spares proportional to where you are going, not to how long you will be away.
- Note the date and the water conditions when you change one. Two or three entries tell you your own interval better than any general advice.
Pickling and lay-up
For anything beyond a short break, membranes need preserving with the chemical the manufacturer specifies, at the concentration they specify. This is not an area to improvise: the wrong chemical, or the right one at the wrong strength, can permanently damage a membrane, and some are incompatible with the materials in the rest of the circuit.
Two practical notes. First, a preserved system needs flushing thoroughly before you make drinking water again — know that procedure before you need it. Second, preservation solutions have their own shelf life inside the system; a boat pickled for a season may need re-preserving rather than being left indefinitely.
Membrane life, and the myth about it
Membranes last a long time when they are operated and preserved correctly. Replacing one because of its age alone is often unnecessary. Output falls for many reasons before the membrane is genuinely finished, and most of them are cheaper to fix.
Before concluding a membrane has failed, check the four things that change output: feed flow, operating pressure, seawater temperature and product-water quality. Cold water alone can cut output substantially with nothing wrong at all — a system rated in the tropics will underperform in Tasmania and that is physics, not a fault.
What to carry for remote cruising
The right kit depends on your specific system, so ask the manufacturer or your installer to specify it. Typically it covers sediment prefilters, pump service parts, O-rings, belts or impellers where fitted, spare hose and the preservation chemical. What matters more than the list is knowing which parts are standard sizes you can buy anywhere and which are proprietary — that is the difference between a delay and a ruined trip.