| Removes | Bacteria, protozoa, sediment |
| Usually misses | Viruses |
| How | Physical straining through a membrane |
| Types | Squeeze, gravity, pump, straw |
A water filter removes bacteria, protozoa (like Giardia), and sediment from backcountry water by physically straining it through a fine membrane or element. It makes natural water safe to drink in most of North America, but most filters don’t remove viruses, so where viral contamination is a risk a purifier or chemical treatment is needed.
Filter formats
Squeeze, gravity, pump, and straw filters all strain out Giardia and bacteria.
Filter vs purifier
For viruses you need purification — see water filter vs purifier.
Frequently asked questions
How does a water filter work?
A filter forces water through a membrane or element with pores small enough to trap bacteria, protozoa, and sediment while letting water pass. Formats include squeeze, gravity, pump, and straw filters. The trapped pathogens stay in the filter, so it must be cleaned and eventually replaced.
Do water filters remove viruses?
Most don't — viruses are smaller than the pores of typical filters. In most of North America's wilderness, viruses aren't the main concern, so filters suffice. But where human/animal contamination makes viruses a risk (often internationally), you need a purifier or chemical/UV treatment.
What's the difference between a filter and a purifier?
A filter physically removes bacteria, protozoa, and sediment but usually not viruses; a purifier also eliminates viruses (via finer media, chemicals, or UV). If viruses are a concern for your water source, choose a purifier or add chemical treatment.
Sources
- Backcountry water treatment — CDC
- Water treatment — American Hiking Society