When I’m off hunting and fishing at the camp, I usually bring clean water. But I like to have backup option. The thing is that « real » and safe filters are way too expensive to use merely as backups. That’s why I like to use chlorine dioxide (by Pristine), that give a better taste than Iodine. Also, the steripen (the Classic model) offers quick, easy and very safe way to treat water. But I was a little disapointed with its prefilter, that is 40 microns wide.

Here’s a quick way to transform an emergency straw type filter into a gravity flow drip system that can filter nasties down to 3 microns. Exit the tubing, valve, collapsible water container, glue, CamelBak, and ... technology.
Keep it simple and light!

Step 1: What You Need

one 2 liters soda bottle (5¢!);
A Pristine Pioneer Emergency filter or Aquamira Frontier Filter. May not be the safest filters out there, but remember; I use this setup as a prefilter and a backup, before treating the water with UV or chemicals. You can buy one for about 15$. It can filter about 75 liters (enough for a fishing season!)
That’s it.

Step 2: The Funnel

Ditch the cap. Using a knife, cut open the bottom of the (empty) bottle to make a funnel.

Step 3: The Filter

Ditch the straw. Insert the filter tightly in the bottleneck, making sure the output end of the filter is on the outside of the bottle. With the bottle I used, the seal was quite tight. I guess one could use a length of tape to make it really watertight.

Step 4: The Drink

Fill the funnel with water, and put it over your bottle. It drips slowly, but take a deep breath...

Now you’re ready to drink, or to treat your water more thorougly with chemicals or UV. If the water is very dirty, you can cover the input end of the filter with a coffee filter and a rubber band, in which case the flow rate will probably be reduce.

If you don't want the filter to stand deep in the bottle (like on the picture above), you could make a stand out of a smaller soda bottle. Cut the bottom and the neck to make a tube, and put it between the Nalgen and the funnel.

Also, one could attach some wire on the funnel, so you can suspend the filter above your container.