The watermaker makes less water, or salty water

A watermaker (also called a reverse-osmosis desalinator) pushes seawater at high pressure through a membrane that only lets fresh water through. When it produces less than before, or the water it makes is no longer fresh enough, the cause is usually simple: a clogged filter, a fouled intake, colder water. But a badly stored membrane or a tired pump can also be to blame. This guide goes from the most common cause to the rarest. Your unit manufacturer’s manual always has the last word, especially for pressures, cleaning products and storage times.

1. What you notice

• The output drops little by little from one week to the next, at the same setting.
• The output drops all at once, sometimes with a change in the pump’s sound or bubbles in the hoses.
• The product water is too salty: the salinity meter (the small device that measures dissolved salt, often in ppm) goes above the threshold given by the manufacturer, or the water tastes brackish.
• The high pressure rises more than usual for the same output or, on the contrary, no longer reaches its normal value.
• More output than before and saltier water: a particular sign, which points to an internal leak (see below).

2. Safety first

The high-pressure circuit works at several tens of bars. Never open a fitting, a membrane housing or a pressure gauge until the unit is switched off and the pressure has dropped back to zero. Also cut the power supply before touching the pump or its motor. And above all: water that is too salty must not go to the tank. Leave it on the reject line until it is back below the threshold; a few litres of salty water are enough to make a whole tank undrinkable, which would then have to be emptied and rinsed.

3. The probable causes, from most to least common

• Clogged pre-filters — the cartridges that stop sand, silt and plankton before the pump clog all the faster when the water is dirty. The pump is starved of water, the output drops, and it can cavitate (draw air and vacuum), which wears it. This is by far the most common cause.
• Fouled strainer or intake — weed, shells, a plastic bag over the through-hull, an intake seacock not fully open, or an air leak on an intake fitting (bubbles in the clear hose give it away).
• Colder or saltier water — this is not a fault. At the same pressure, a membrane produces noticeably less in cold water than in tropical water; rated outputs are given for a reference temperature. Saltier seas also reduce production slightly.
• Fouled membrane — a biofilm forms when the unit stands idle without flushing or storage treatment; lime or mineral deposits can also scale it. The membrane then needs more pressure for less water. A chemical clean or sterilisation, with the manufacturer’s products and procedure, often gives it back part of its output.
• Wrong pressure setting — too low, the membrane produces little and lets more salt through; too high, it gets damaged. The regulating valve may have moved, or the gauge may be wrong.
• Leak — a visible leak on the high-pressure side makes the pressure drop; an internal leak, at the O-ring of a membrane housing for example, lets seawater join the product water: normal or higher apparent output, but salty water.
• Tired high-pressure pump or booster pump — worn seals, valves or pistons, a slipping belt, a failed booster pump (the small pump that feeds the big one): the pressure is no longer reached.
• Damaged or worn-out membrane — by chlorine (dock or tank water passed through without a charcoal filter), by frost during winter lay-up, by drying out, by hydrocarbons, or simply by age. It then lets too much salt through, whatever the setting.
• Faulty salinity meter — a fouled probe can show salty water that is not. A handheld salt tester settles the question.

4. What you can check yourself, safely

• Note the output, pressure, salinity and water temperature at start-up, and compare them with your previous readings or the values in the manual. Without a point of comparison, you cannot tell whether the unit has really dropped.
• With the unit off, look at the pre-filters: a brown or grey cartridge must be replaced. Always keep spare cartridges on board.
• With the unit off and the seacock closed, clean the raw-water strainer and check that the intake seacock opens fully.
• Look for bubbles in the intake hoses and hand-tighten the accessible clips on the intake side.
• Go round the fittings to spot a leak or white salt marks, and check the belt tension if the pump has one.
• When restarting, let the water go to the reject line for the first few minutes, then check its salinity before sending it to the tank.
• If the manual provides for it, do a fresh-water flush following its procedure, always through the charcoal filter: chlorine in mains water destroys the membrane.
• Only turn the pressure valve within the range given by the manufacturer, never beyond the maximum pressure.

5. When to stop and call a professional

Stop there if the water stays salty after changing the filters and cleaning the strainer, if the output does not come back after a membrane clean done according to the manual, if a leak appears on the high-pressure side, if the pump makes a new noise or if the pressure becomes unstable. Opening a membrane housing, replacing pump seals or deciding that a membrane is finished takes tools and experience. Write down your readings before the job: a technician who can see how output and salinity have changed saves a lot of time.

6. Stopping it from coming back

A membrane is damaged mostly while standing idle. Run the watermaker regularly, or flush it with fresh water at the interval given by the manufacturer, and put it into storage with the intended product as soon as the stop exceeds the time it sets — during winter lay-up in particular, when it must also be protected from frost (see winterising and recommissioning). Do not run it in harbour water, in a silty estuary or near an oil slick: wait for clear water, offshore or at anchor. Change the pre-filters before they turn black. For a long passage, carry filters, seals and storage product (see preparing for an Atlantic crossing).

In the Hullteam log, record every filter change, every cleaning or storage treatment, the date, the running hours and your output and salinity readings: that is what will show a buyer the condition of the unit, and what will make the boat’s value at resale (see which proofs count).