[best for groups]
For treating large quantities of water, gravity filters are the way to go. Just fill the 4-liter bladder marked “dirty” with unfiltered water, hang it, and let the water drip through the hollow-fiber filter and into the clean bladder below. “Clear water moves through quickly (four liters in about five minutes), but turbid water can take up to three minutes longer to pass through,” says one tester.
Backflushing—simply hang the clean bladder above the dirty one for a few seconds to reverse the flow—kept the filter running strong through well over 200 liters in the silty Southwest. The filter protects against protozoa and bacteria (not viruses), and packs up to liter-bottle size. $120; 11.7 oz.; platy.com