marcward86 suggestion of a biosand filter is right on. Here is the guide for how to make one.The Biosand filter is fully described here: www.cawst.org complete with a downloadable manual.It is built with 2" of gravel at the bottom, then 2" of smaller pebbles, then 21" of sand on top. It is a version of a "slow Sand filter". The bio layer builds up over about 30 days and will then purify the water. Until that time you must boil, solar purify, or chemically purify the water before drinking it.The filter may be made in other containers than the concrete container that is described. Plastic barrels, trash cans, large diameter pvc pipe, and many other clean water tight containers can be used.Cities use "slow sand filters" on a larger scale to supply entire cities with safe, clean, pure, drinking water.