How do you pickle a marine watermaker?

To pickle a watermaker, you must first flush the system thoroughly with unchlorinated freshwater, then pump a biocide solution through the reverse osmosis membrane to stop biological growth. Leaving a watermaker unused for more than a week without pickling allows bacteria to foul the membrane, costing you upwards of £500 for a replacement.

The Freshwater Flush

Before any chemicals go near the system, you must purge all seawater. Never let chlorinated marina water touch your RO membrane. Chlorine instantly destroys the polyamide thin-film composite material. You have two options for the flush:

• Draw directly from your boat's freshwater tank, provided you have not recently shock-treated it with bleach.
• Use dock water, but pass it through an active carbon block filter (usually a 5-micron charcoal element) to strip out the chlorine.

Open the pressure relief valve on your high-pressure pump (or the Clark pump on your Spectra Ventura) so the system stops producing water. Run the feed pump to push freshwater through the system and out through the brine discharge for 15 to 20 minutes. This clears salt from the membrane, the high-pressure hoses and the raw water plumbing.

Mixing Pickling Solution

For a standard six-month lay-up in a mild climate like Greece, sodium metabisulfite (SMBS) is the standard pickling agent. Brands sell this under various names, such as Spectra SC-1. Generic food-grade SMBS works exactly the same and costs a fraction of the price. Avoid benzalkonium chloride solutions unless your manufacturer explicitly recommends them. They degrade certain pump seals.

Fill a clean 10-litre bucket with unchlorinated freshwater. Add three tablespoons (roughly 45 grams) of SMBS powder. Stir until completely dissolved. Do not mix the solution with seawater. The remaining salt reduces the effectiveness of the biocide.

Circulating the Biocide

With your system unpressurised, replace the freshwater inside the membrane housing with your pickling solution.

1. Remove your 20-micron and 5-micron seawater pre-filters from their housings. Throw the old pre-filters away. They cost £10 and are never worth saving. Leaving dirty filters in place during storage turns them into a breeding ground for black mould. Put the empty housings back on.
2. Disconnect your raw water intake hose after the seacock. Place it into the bucket of pickling solution. Alternatively, use your system's dedicated winterising three-way valve if fitted.
3. Ensure your product water line is isolated or has a check valve so pickling fluid cannot migrate back into your freshwater tank.
4. Start the feed pump. Watch the brine discharge overboard.
5. Once the bucket is nearly empty, switch off the feed pump immediately so you do not draw air into the system.
6. Close the raw water seacock and the brine discharge seacock to trap the solution inside the plumbing.