Reverse osmosis filtering provides an extremely fine level of filtering. Typical backpacking filters remove contaminants 0.2 microns and larger. These filters work well for most water sources and easily filter bacteria and other small organisms. The next level of filters remove contaminants 0.02 microns and larger. These filters remove the ultra-small organisms such as viruses. Reverse Osmosis filters remove contaminants down to 0.0001 microns.

Most reverse osmosis filters have multiple stages that perform different levels of filtering. The “normal” filters remove the larger contaminants before the water reaches the semi-permeable membrane. This membrane uses holes that are so small that they allow only the water molecules to pass through. A reverse osmosis filter can remove dissolved solids and chemicals such as fluoride, led, and even salt. Desalination plants normally use reverse osmosis to purify salt water.