Project Purity is an abandoned water filtration facility in Fallout 3, which is activated during the game's main quest. In the expansion, Broken Steel, players can observe how the availability of clean water affects the inhabitants of the Wasteland.

Project Purity was an initiative whose purpose was to build a water purification facility to provide clean water to the Wasteland. The project was originally supported by the Brotherhood of Steel, who later abandoned it due to lack of manpower.

During the main quest line of Fallout 3, the player discovers that his missing father left Vault 101 with the intention of continuing Project Purity. After enlisting the help of Doctor Li from Rivet City, the project was reinitiated at an abandoned water treatment plant near the Jefferson Memorial. At the end of the game, the player must decide whether to enter the Project Purity control room and activate the system.

In Broken Steel, the third downloadable expansion pack for the game, begins two weeks after the events of the main storyline. Project Purity has been successfully activated, no matter choice the player made in the main story. The Brotherhood of Steel has stepped up and is coordinating distribution of the purified water to the Wasteland's inhabitants. Project Purity has transformed into a central hub of activity and a number of related water quests are now available.Fallout Wiki: Project Purity 1up.com: Bethesda Reveals New Details...for Fallout 3: Broken Steel (Project Purity info) (April 21, 2009)

The Project Purity journals are holotapes found throughout the main quest line that document James' various efforts to launch Project Purity. In total, there are 13 journals, seven of which are "personal" entries. There is also a related note found on James' desk in the Jefferson Memorial sub-basement. It is a recording made by the player's mother, Catherine, as she discusses her work on the project.Fallout Wiki: Project Purity Journals