Filters bacteria, viruses, arsenic, etc. The sand and mold is relatively inexpensive, easy to produce and acquire, and a single system can provide enough water for a family of 8 over the span of many years. It simply requires stirring the top layer in order to clean.
To purchase commercially
Water quality test
While I'm at it I thought I would throw in the Katydyn product page
and a video on the Black Berkey water filters. Each Black Berkey will filter up to 3,000 gallons of water; and all you have you do for maintenance is wipe the outside down with a Scotch Brite pad. A pair of black Berkey's run for $99 and you can make your own drip containers out of sparkletts bottles or buckets. You can also purchase flouride and arsenic attachments for the filter (these are the white canisters you see below) I think the Berkey's even filter out radon!