And don't forget the simple, low-tech Slow Sand Filter, suitable for many homestead and survival situations: http://www.nesc.wvu.edu/ndwc/pdf/OT/TB/TB14_slowsand.pdf

If you're not looking for the most expensive, highest-tech, most-chemicals-required way to filter water, go with a method that is well over 100 years old, and is still used in some municipal facilities.

Some slow sand filters would look awfully good in many places in Japan right now...

Sue