Swiss Water Process
also: Swiss Water, SWP
A solvent-free decaffeination method using water and activated carbon that preserves flavor well.
Swiss Water Process is a chemical-free way to make decaf. It uses only water, temperature, and activated carbon filters, no added solvents, which is why it is popular with roasters who want a clean label and with drinkers avoiding chemical residues.
The trick is a saturated liquid called Green Coffee Extract (GCE). A first batch of green-coffee is soaked to pull out both caffeine and flavor compounds; that water is then passed through carbon filters that catch the caffeine but let the flavor molecules through. The result is GCE that is full of coffee solubles but empty of caffeine. New green beans are soaked in this solution: because the GCE is already saturated with flavor, only caffeine migrates out of the beans into the water, leaving most of the taste behind.
It typically removes about 99.9% of the caffeine and is well regarded for retaining flavor. The trade-off is cost and complexity. For how it stacks up against solvent and CO2 methods, see ea-process and decaf-processes.