The water processing plant is found in Megaton; to reach it take a right after entering Megaton, it's directly upstairs from Craterside Supply. It is occupied by Walter in the morning, and Leo Stahl at night.
Walter is usually taking a smoke break outside or is inside tinkering with the various machines inside the plant. While talking to him he can initiate the unmarked quests Treatment and Walter's Scrap Metal. In the back room is Leo Stahl's desk, he can be spoken to about his drug problem or his desk broken into where he hides his chems.
The locked desk remains named as "locked desk" even after the lock is picked.
The water processing plant appears only in Fallout 3.
- pc Walter still describes problems with a possible water shortage and/or contamination even after Holy Water (Broken Steel add-on) is completed. [verified]
- pc When the pointer is aimed at an object or person in the plant then moved off that object, the cursor will flicker and show the drink option for a split second, and timed correctly you can drink from the object or non-player character and gain HP and a few rads but note the "drink" text is red and doing so is considered a crime. [verified]
- pc In case Walter is missing see his article for solutions. [verified]