I would collect the rain water in some sort of tank. Install a ultraviolet light and then use the a series of cartridge filters (available at hardware stores) for filtering.
You can get a ultraviolet light at larger pet supply stores - they use it to kill algae in ponds; don't know if it would work with diesel, and I doubt it would be "approved" for human use...but I betcha it would kill all kinds of bugs floating past it.

Check with laws/regulations regarding collecting rainwater. I know that around my parts, it is not allowed.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D