Filtering is used for purifying water and refining feathers.

Filtering: Beginner

For more advanced filtering, you need to learn the knowledge Filtering: Beginner from Dora Fonti in Heidel City by completing her quest Filtering Feathers. The quest unlocks when you get access to the quest Learning Higher Processing Skills, which can be obtained in two ways: Have Gathering Apprentice 4 and get the quest directly from Ficy in Heidel City, or complete the questline started by the Black Spirit as you enter Heidel City.
The Black Spirit will ask you to go to Jemkas Wyrmbane, who will have the quest Because I Like You, which requires you to either have Gathering Beginner 7 or have completed the quest Go to the Northern Heidel Quarry.
Complete the rest of the questline to get access to Learning Higher Processing Skills and Dora Fonti's quest:

Filtering: Skilled


Product Ingredient #1 Ingredient #2 Skill Required
Fine Fancy Feather [1]
Fine Lightweight Plumex [2]
Fancy Feather [3]
Lightweight Plume [4]
Purified Water Processing [5]