Fallout Shelter
19 Achievements
53,500 XP
Project Purity
Merge 3 Water Room
1,000 XP
46.92%
How to unlock the Project Purity achievement in Fallout Shelter - Definitive Guide
The best option is right from the start to put three water treatment rooms at 175 Credits each together in a row costing a total of 525 Credits to achieve.
This achievement definitely suffers from breaking. If it doesnt trigger once a 3 merged room is created, it will never trigger on that save file. Start a new vault in an empty slot and do as above.
If you would like a video guide for this though I will leave mine below
You can merge rooms by building the same type next to each other, to build a bigger room. The rooms have to be on the same level, which means if you have upgraded a small room and you build a room next to it so you will need to upgrade it too or it won't merge. Merged rooms are very useful, because you will be able to assign more Dwellers to work on them and they will produce more resources, you should aim to merge every rooms that you can.
This achievement can easily be obtained by building 3 unupgraded Water Treatment rooms next to each other so that they merge together to form 1 giant version. When you do so, this achievement shall be yours.