Really, I think to make that simpler (but still a cost over time) would be charcoal (or using current game materials, coal). In a post-apocalyptic situation, a charcoal filter would be the most straightforward and most common way to purify water, so make water filters craftable, simple, but a consumable. Raise the output, but have the filters degrade over time and require replacing. Also, you should be able to purify murky water with them. That would be logical.