Bison ran a decent post about water treatment/filtration yesterday and I thought I might follow his lead. I've covered this here on the blog a while ago as well as storage and preservation, but it's important enough with regard to emergency preparedness that it bears further discussion.
First off, in the interest of shameless self-promotion, I sell very nice water filters, so please buy from me. The most economical unit I have in my inventory is the KATADYN HIKER MICROFILTER for $59.95. It was chosen "Best of the bunch" by Backpacker magazine field testers not long ago. Watch a review here or a short official video from the manufacturer here. I don't currently sell the replacement filter cartridges, but they're available all over the internet - just Google "katadyn hiker filter replacement" and you'll be good to go.
I like the Katadyn filter because it's lightweight and portable, making it the perfect add-on to a survival kit.
For a more economical pseudo-DIY solution, I suggest the "Just Water" Ceramic Drip-Filter system. There is a great deal of info about this filter that can be found by reading this old thread from the LATOC Discussion Forums, but here is a summary:
* May not last as long as Berky filters, but cheaper with higher flow rate.
* Product used exclusively by the #2 relief organization in the world.
* Used by missionaries in 38 countries.
* Nominated for a World Health Humanitarian Award in 2008.
* Approved through testing at Johns Hopkins University and the National Sanitation Foundation, AEL Laboratories, Analytical Food Laboratories, British 5750 Quality Standard, and England’s Water Research Council standard.
As always, you need to use your own best judgment and decide your own course of action.