How membranes are described
Marine RO membranes are cylindrical elements described by diameter and length in inches. Common marine sizes are 2.5 inch diameter elements at 21 or 40 inches long, and 4 inch diameter elements. A system may fit one, two or three of them in series or parallel.
More membrane area means more product water for the same pressure, which is why membrane count is the lever manufacturers pull to create a model range from one pump.
| Configuration | Typically found on | What it means |
|---|---|---|
| 1 × 2.5 × 20–21 in | Small energy-recovery systems | Modest output, very low power |
| 2 × 2.5 × 21 in | Mid energy-recovery systems | Doubles area without a bigger pump |
| 1 × 40 in | Single-membrane conventional systems | One long element, efficient use of a fixed pump |
| 2 × 21 in | Small-pack conventional systems | Less total area than a single 40 in — and it shows in the energy figure |
| 2 × 40 in | Large-pack conventional systems | Roughly doubles output on the same pump |
| 3 × 40 in | High-output framed systems | Charter and liveaboard territory |
Why membrane area decides efficiency
On a conventional system the high-pressure pump draws its rated wattage regardless of how many membranes are fitted. Add membrane area and you get more litres for exactly the same power. That is the whole explanation for the largest efficiency spread in our table.
| System | Power | Output L/hr | Draw W | Wh per litre | Price |
|---|---|---|---|---|---|
| Portable AC 140L Rainman | AC 240V | 100–154 | 1,250 | 9.8 | $5,329 |
| Portable AC 75L Rainman | AC 240V | 55–82 | 1,250 | 18.2 | $4,402 |
| Portable AC 70L Rainman | AC 240V | 50–77 | 1,250 | 19.7 | $5,296 |
Identical 1,250 W pump on all three. The only difference is the membrane pack, and the energy per litre roughly halves from bottom to top.
Standard or proprietary?
This is the question to ask before you buy, not when you need a replacement. If the membranes, prefilters and fittings are industry-standard sizes, you can source them from any water-treatment supplier anywhere in the world. If they are proprietary, you are tied to one supply chain, and a remote failure becomes a much bigger problem.
- Are the membrane elements a standard size, or specific to this manufacturer?
- Are the prefilter housings and cartridges standard?
- What does a replacement element cost, landed in Australia, and what is the lead time?
- If a proprietary controller or sensor fails, can the system still be run manually?
Life expectancy, honestly
Membranes operated and preserved correctly last a long time — often much longer than owners expect. There is no fixed calendar interval that applies to every boat, because life depends on water quality, how often the system runs, whether it is flushed and preserved properly, and whether it has ever been exposed to chlorine or run dry.
Anyone quoting you a firm number of years without asking those questions is guessing. Judge by performance — output at a known temperature and pressure, and product-water TDS — not by age.