I like the way the newer versions require research to get landfill, but I don't like that they now require explosives to create. I think maybe setting the difficulty to easy would be a solution, but that's not a possibility for existing worlds. Is there any way to do this? I'm back to 0.0.7 now, if it's not possible I can just stay on that version.

Waterfill does not change its recipe difficulty based on the world setting. Rather it has its own recipe settings which you can change easily under "Mod Settings" (The one you are looking for is under the "Startup" tab called "Difficulty" below the "Noxys Waterfill" heading).

So yes this is absolutely possible.