Osmotic filters were a technology utilized in the life support system aboard 22nd century Starfleet vessels. The osmotic filters were required to be purged whenever a new gas tank was to be accessed and connected to ship's central ventilation.
In 2152, Doctor Phlox was required to perform this action on Enterprise NX-01, when rerouting the transfer lines to the ship's carbon dioxide tanks in order to evict a series of non-corporeal lifeforms called "Wisps" from the bodies of crewmembers of whom had been taken control. In order to do this, Phlox had to remove the support plate of an access tube, open two rotary valves and turn a lever to the three o'clock position, thereby releasing the gas. (ENT: "The Crossing")