With BPA this, and mercury that, everyone is scared sh*tless when it comes to what is safe to drink.  Well, Tony Flynn with the World Vision charity and Potters for Peace must have been tired of listening to people go crazy over it, and has formulated a way for anyone to drink clean water anywhere, filtering it with materials we have on hand. via PlantGreen

Materials
1. crushed, dry clay
2. organic material(tea leaves, cofffee grounds, or rice hulls)
3. water
4. Cow manure

Instructions
1. Mix in enough water to make a stiff biscuit-like mixture
2. Form a cylindrical pot that has one closed end
3. Dry the pot in the sun
4. Surround the pot with straw and place it in a mound of cow manure
5. Light the straw and then top up the burning manure as required.
6. Filter will be completed in less than an hour.

According to ANU:

The organic materials are burned away during the firing process and create small passages in the filter that allow water, but not pathogens, to pass. This filter effectively removes 96.4-99.8% of E. Coli in water.