Sounds like the biosands that everyone is so eager about- great for large scale, not so great for small scale (like a family well), and not at all portable.

Oh, and bad water was one of the worst killers of sailors in the Caribbean and on the big treasure routes during the age of sail, don't be so eager about water that is stored in the hope that age will "treat" it.

Very interesting topic.
I have a question. What if I'll use a reverse osmosis desalination device on a contaminated water? I guess it must make any water absolutely clean?

Yes, with a few caveats.

* They take a lot more pressure
* They're a lot more expensive
* Each type of membrane has its own chemical weaknesses -- for example, it may not be compatible with chlorine, or may require a certain pH range, or whatnot. They can be rather sensitive things.

But, when you use them right, they produce nearly 100% pure H2O.