Preserving your reverse osmosis membranes for long-term storage is quite easy, and we strongly recommend this procedure if you will be unable to flush your watermaker with fresh water for more than three weeks. This procedure will protect your membranes for up to six months. If you wish to keep your system preserved for longer than six months, you will have to thoroughly flush, then re-preserve your system every six months. Extended storage can cause some rubber and plastic parts to decay, so don’t over do it. Membrane preservative (or storage chemical) is available in two forms:
- Cartridge Form
- Powder Form (bulk powder)
The type of preservative you use depends on your system design. I will discuss how to use both types of pickling chemical.
Cartridge Form Preservative
First, lets discuss how to use the cartridge form of storage chemical. This type requires your desalinator system to have a recirculating loop to cycle the brine discharge back into the water inlet before the low pressure boost pump. Some watermakers (like those made by Village Marine) include a valve that creates a recirculation loop. Village Marine calls this valve the “cleaning valve.” Here is a sample plumbing diagram for Village Marine Little Wonder Modular watermaker:
Note the cleaning valve, and how it returns the discharge to the boost pump inlet. Before preserving your membranes, first thoroughly flush your watermaker with fresh water. Then remove your prefilter from the housing and discard it. If you have two prefilters, remove the 20 micron filter. Install the preserving cartridge in place of the prefilter. Note that the cartridges we sell fit both standard and “Big Blue” 10″ housings. Switch the cleaning valve to the cleaning (recirculate) position. If you do not have a cleaning valve, disconnect the brine discharge hose and connect it to the boost pump inlet.
Close the seacock. Make sure the product water will not go into your tanks. Also, make sure the system pressure is reduced to ZERO before running with chemicals in the system. Use your fresh water flush valve to purge any trapped air from installing the preserving cartridge. Run your desalinator for 30 minutes to thoroughly disperse the preserving chemical. Shut it off, and leave it for up to six months!
Powder Form Preservative
Now let’s talk about how to use the powder form of the preserving chemical. Use this method for larger desalinators and systems that do not have a recirculating loop. After thoroughly flushing your watermaker with fresh water, disconnect the brine discharge hose and run it to a clean bucket filled with fresh, dechlorinated water. You can use RO product water or tap water that has been run through a carbon filter.
Disconnect your boost pump inlet hose (make sure you close the seacock) and run it to the same bucket. Make sure you don’t allow the boost pump to lose its prime. Add the preserving chemical to the bucket so that you end up with a 2% solution overall. The preservative needs to be 2% by weight of the total water in the system, not just the bucket! Here is a quick way to estimate the total water in your system:
- Start with the water in your pressure vessels:
- 2.5 x 21″ Membranes – About 3 Lbs each
- 2.5 x 40″ or 4.0 x 21″ Membranes – About 6 Lbs each
- 4.0 x 40″ Membranes – About 15 Lbs each
- Then add the water in your prefilters:
- 2.5 x 10″ Standard Housings – About 2 Lbs each
- 2.5 x 20″ Standard Housings – About 4 Lbs each
- 4.5 x 10″ Big Blue Housings – About 5 Lbs each
- 4.5 x 20″ Big Blue Housings – About 10 Lbs each
- Then add the water in the plumbing system and your bucket:
- Plumbing – About 1-2 Lbs
- Bucket – 8 Lbs per gallon
For example, a system with two 2.5 x 40″ membranes and two 2.5 x 10″ prefilter housings will have about 17 pounds of water, not including the bucket. If you add two gallons for the bucket, the total water in your system will be about 33 pounds. Calculate the amount of preservative powder to use:
Total Water (Lbs) x 0.02 = Preservative (Lbs) In our example, 33 Lbs x 0.02 = 0.66 Lbs, or 2/3 of a 1 Lb Jar Carefully mix the preservative chemical in the bucket of fresh water. Wear safety goggles and a mask, and work in a well ventilated area. This stuff is pretty harsh. Set the system pressure to ZERO, and make sure the product water will not go into your tanks. Run your watermaker boost pump and feed pump to recirculate the preservative for 30 minutes.
Freeze Protection
To preserve your watermaker for cold climates, it is best to use Food-Grade Propylene Glycol (Non-Toxic Antifreeze will work) to protect your system. For best freeze protection, a 60% propylene glycol to 40% water ratio should be used. Calculate how much PG to use by the method above. Please note that PG alone will not prevent organic growth. Non-toxic antifreeze contains some growth inhibitors, and will provide protection of your desalinator for up to six months.
Recommissioning After Storage
After long-term storage, you will need to flush the chemicals out of your watermaker thoroughly before making any product water. Switch your cleaning valve to “Run,” or reconnect your brine discharge and boost pump inlet hoses. Replace your prefilter(s) with new, and replace your fresh water carbon filter. This is a good time to consider changing your high-pressure pump oil, also.
Open the seacock, and run your desalinator at ZERO pressure, making sure the product water (if any) gets dumped overboard. Let the system run at ZERO pressure for 30 minutes to flush the storage chemicals out of the nooks and crannies. Slowly increase the operating pressure to 800 PSI (or to the maximum rated product flow if you are floating in brackish water). Make sure the product water is dumped overboard, and let the system run for another 30 minutes.
Compare your product water quality and flow rate with your benchmark values from when the membranes were new. (You did write that down, didn’t you?) Write the new values in your watermaker log. After adjusting for temperature and salinity differences in the raw water, if your product flow rate has reduced by more than 10% you should run a cleaning cycle – first with organic detergent, then with scale remover. The procedure is almost exactly the same as for preserving your membranes.
Happy cruising,
Tim