A watermaker that is slowly making less water, or water that tastes or reads saltier than it used to, is one of the most common calls we get in the second half of a Mallorca season. Usually the fault is upstream of the membrane and cheap to put right. Sometimes it is the membrane itself. The readings tell you which, and crew can collect most of them.
How output is set
A reverse-osmosis watermaker pushes seawater against a membrane at high pressure. Fresh water passes through; the brine carries the salt away. How much fresh water you get depends on three things: the pressure at the membrane, the temperature and saltiness of the feed water, and the condition of the membrane.
That means some change is normal. Output rises in warm water and falls in cold water, so a watermaker will make more in August than in April. The Mediterranean is also saltier than the open Atlantic, which costs some output at the same pressure. What is not normal is a steady fall at the same time of year, or salinity creeping up.
Less water than usual
In rough order of likelihood:
- Blocked pre-filters or a dirty seawater strainer. By far the most common cause. Weed, sand and, in the Balearics, Posidonia fragments load the filters up, the feed pressure drops, and output follows.
- Air on the suction side. A loose filter housing, a cracked bowl or a tired O-ring lets air in. You will often see bubbles in the filter bowls, and the pressure may hunt.
- A weak boost or feed pump, or a high-pressure pump losing performance through wear or, on belt-driven units, belt slip.
- A pressure regulator or valve not holding its setting.
- A fouled membrane. Membranes left standing in seawater without being flushed grow biological fouling. Oil or diesel picked up when running in harbour does lasting damage. Watermaker manufacturers generally advise against running one inside a harbour or marina for that reason.
Saltier water than usual
Rising salinity in the product water usually points to one of these:
- Operating pressure too low, so the membrane is not rejecting salt as well as it should.
- Membrane damage. The usual causes are chlorine (flushing with dock water through an exhausted carbon filter), drying out during storage, or age.
- A leaking seal or O-ring inside the membrane housing, which lets feed water bypass the membrane.
- A dirty or faulty salinity probe. Worth ruling out before condemning a membrane.
Many systems divert product water overboard automatically when salinity is above the set limit. If the watermaker runs but nothing reaches the tank, that is often why.
What crew can check
- Clean the strainer and change the pre-filters. Check the filter bowls for air with the unit running.
- Write down the readings: operating pressure, product flow, product salinity and seawater temperature. Compare them with the commissioning figures or your log from earlier in the season. Without a baseline, we are guessing too.
- Check when the carbon filter on the freshwater flush was last changed, and when the membranes were last replaced or preserved.
Do not keep increasing the pressure to chase lost output. Running above the manufacturer's limit can damage membranes and pumps.
When to call us
Call us if output stays low with clean filters and no air, if salinity keeps rising, if the unit alarms on pressure, or if there is any leak on the high-pressure side. High-pressure leaks should be treated with respect; isolate the unit and leave the pipework alone.
What to send us
- the watermaker make and model, and a photo of the control panel;
- your readings: pressure, product flow, salinity and sea temperature, with the figures from when it was working well if you have them;
- when the pre-filters, carbon filter and membranes were last changed;
- where the yacht is.
We service Sea Recovery, Spectra, Schenker, Dessalator, HRO, EchoTec, Idromar and Aquatec watermakers on board in Mallorca and across the Balearics. See our watermaker service page, the Sea Recovery and Schenker pages, book online with photos, or call +34 971 251 374.