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These carbon block filter cartridges remove any chlorine that might be present in the fresh water used to flush the watermaker when it is being shut down prior to a period of inactivity. Chloine can damage the watermaker's membranes, and is present to some extent in virtually all dockside water.
Fresh water flushing replaces saltwater within the watermaker - preventing possible corrosion of the internal parts, as well as reducing biological fouling of the membranes. This procedure is crucial to the longevity of the reverse osmosis membranes.
Note that fresh water flushing of the unit is not the same as the "membrance cleaning" procedure which uses chemicals to remove contaminants that build up in the plumbing and on the reverse osmosis membranes over time.