I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely.”— Revelation 21:6, Bible (King James version)
As brainchild of James and Catherine, Project Purity was created prior to 2277. The project was originally supported by the Brotherhood of Steel when they first came to the Capital Wasteland; they would send knights and paladins to defend the project from super mutants and other threats in the D.C. ruins. However, the support eroded over time due to the projects lack of results and need for Brotherhood troops elsewhere, e.g. guarding the Citadel or purging the super mutants from the D.C. ruins. Later, more soldiers would be taken from the project in an attempt to make up for soldiers lost to the Brotherhood Outcasts splitting from the chapter. This support ended indefinitely after James left the project, thus ending almost all promising research and possibility of future success. After James' abandonment and no support from the Brotherhood, the other scientists abandoned the project; Doctor Madison Li and presumably what was left of the team left to the nearby Rivet City.
In 2277, James left Vault 101 in search of Doctor Li and with intent to restart Project Purity, but she refused and James moved on. Project Purity is what motivates him to eventually leave the vault in search of a G.E.C.K. in the hope of completing the project. Project Purity was briefly rekindled at the Jefferson Memorial, but the location was taken over by super mutants shortly thereafter. After enlisting the assistance of Doctor Li and her staff from Rivet City, the Lone Wanderer cleared the site before the project finally resumed progress. However, the Enclave seized the Jefferson Memorial shortly after many facets of the facility were repaired and brought up to code. Colonel Augustus Autumn attempted to persuade James to cede control to them. James refused and deliberately caused a spike of radiation, sacrificing himself so the Enclave couldn't take over the facility. it was all in futility as the Enclave took control of the facility immediately thereafter.
Though the Enclave managed to capture the Jefferson Memorial, a crucial component, the Garden of Eden Creation Kit, was still required to bring the project to fruition. Once the Lone Wanderer obtained the G.E.C.K., they are incapacitated by the Enclave and taken to Raven Rock. President John Henry Eden asks that the Lone Wanderer install the modified FEV when they reach the Project Purity control room, which will purge the Capital Wasteland of any beings "tainted" with radiation, regardless of severity. The Lone Wanderer either chose to insert the modified FEV, fulfill the original intent of Project Purity, or allow Sentinel Sarah Lyons to execute the latter. As a result, the Lone Wanderer is knocked unconscious by an explosion in the facility remains so for two weeks.
Same old killing, just a different ingredient.”— Talon Company mercenary
After the decision is made, the water of the Capital Wasteland is purified at last, and life begins anew in the Capital Wasteland. The Lone Wanderer can also, if they chooses, simply not do anything, allowing the purifier to self destruct, thus sacrificing the lives of everyone in the facility for no reason. The ending that follows is shorter than the normal ending.
In Broken Steel the purifier is activated, and the Brotherhood of Steel (along with the security team from Rivet City) begins distributing Aqua Pura (purified water) to the various regions of the Capital Wasteland. In addition, the small body of water directly in front of the Jefferson Memorial (the area before the bridge) known as the Tidal Basin is free of radiation. Any water beyond the bridge is still irradiated as Project Purity will take some time to purify all the water in the Potomac River. If the modified FEV was inserted into the purifier, then the Aqua Pura will be infected with it.
- The plan was to build a facility that could purify all the water in the Tidal Basin at once. No radiation, no muck, just clean water. - Doctor Li
- Revelation 21:6: I am Alpha and Omega, the beginning and end. I shall give unto him who is athirst the fountain of the waters of life freely. - The Bible quote that was effectively the inspiration, motto and password for the whole project.
 Related quests
- It is one of the locations shown in the Bradley-Hercules targeting data.
- If you look inside the water container behind the password entry pad once you start the purifier, you will see a statue of Thomas Jefferson. It can be clearly seen during Broken Steel.
- If you have started the purifier, but have not contaminated it with the FEV, a few plants around the Capital Wasteland will have shades of green.
 See also
- Occasionally, (after activating Project Purity) the tidal basin water will still be irradiated, even though it is supposedly "clean".
- The door to the rotunda sometimes may re-lock from the inside, long after the "Take It Back" Quest, locking the player in permanently.
- On some occasions a dead Mirelurk may appear to stand on water. If the bugged Mirelurk is shot with any weapon it will fall into the water. Requires the Broken Steel add-on.
- If you go to Project Purity before Take It Back!, you can jump onto the "generator" of the forcefield at the far-left, and slide into the grounds, and there will be some friendly Enclave NPCs, they can be spoken to, but you can only say goodbye. You can kill them. If you go inside, there will be hostile Enclave Soldiers, who can also be killed. Do not go into the rotunda, as the doors lock behind you.
- Sarah Lyons will continue to say "lets get this place locked down before we do anything else." After speaking into the intercom and will continue to do this even though all enemies are dead.