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! A Pristine Pioneer Emergency filter or Aquamira Frontier Filter. You can buy one for about 15$. It can filter about 75 liters. STEP ONE: Ditch the cap. Using a knife, cut open the bottom of the (empty) bottle to make a funnel. STEP TWO: 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 THREE: 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 reduced.