Also, once a filter becomes saturated with pollutants, it will allow additional pollutants in the water to pass through the filter. When selecting an activated carbon filter, a two-stage filter system with 0.1 to 0.3 micron filtering capacity is adequate. The first filter is a pre-filter that removes suspended particles, sand, rust and solids. The second filter removes the bacteria. There are many different brands of portable filters available.
Water is forced through a filter made of porous material.
The filter media filters out organisms that are bigger than their pore size, like parasites, Giardia, amoebas, cryptosporidia, organic material, etc. Many filters have pore sizes small enough to filter out bacteria. The pore size of portable hand operated filters is not small enough to filter out viruses. In general, viruses do not exist by themselves in water. They are usually found in clusters around organic material.