It *was* intended that the Purification Works could be built in any system with an Oceanic planet OR a Terran planet OR a Jungle planet, but obviously the editor doesn't have an and/or system as I thought it did. If something is set as a requirement, then it MUST be present. I've already told Mike about the problem, so i'm hoping he will change it to allow the structure to be built in any system with any of the requirements, not just ALL of the requirements.
If he doesn't change it though, there are two possible courses of action; create three different Purification Works with a different planet type as a requirement, or simply have it as an Oceanic planet structure only.
Unfortunately, I haven't changed the requirements for this structure yet and i've already sent the update to Mike, so a fix WON'T be included in the Easter update. I'll make sure this is one of the things that I fix first after the Easter update is released though. I can't continue updating the game at the moment because my game files will be incompatible with those that are part of the Easter update, so I have to wait for that to be released before I can continue updating the game.
...You could always change it yourself though. Load up the editor, then click on the "Buildings" tab at the top of the screen (It has a pcture of the Yridian Intelligence building from BOTF on it). Then, in the panel on the right, scroll down until you find Purification Works; double-click on it. At the bottom of the screen you'll see a panel with all the different requirements. Simply uncheck the boxes next to Terran and Jungle planets, then close the editor and let it save the changes. Unfortunately, making such a change will make the auto-updater think your game fiels are out of date, so it will attempt to "update" your files and remove the change. You'll therefore have to stop it downloading updates. You'll have to let it download the Easter update if you want the bug fixes and content updates though - so you'll have to mod the game again once you've downloaded the Easter update.
If it helps, I created an animation to help show you how to use the editor. The animation is rather big though (And the images are a bit low quality because it is in .gif format) so you'll have to click on the thumbnail to see the full size version:
We'll announce the Easter update in the Supremacy Pre-release Download Thread once it is available so that you know when you can allow the game to update itself correctly.