Is two step water purification needed? This could be in the event of a collapse of society, or on a hiking / camping trip.
There is a common misconception about filtration and purification. Filtration removes bacteria, protozoa and cyst, but does nothing to viruses. Purification affects bacteria, protozoa, cyst and viruses. Purification can be by any means – UV light, filters that remove a virus, chemical treatment.. etc.
Most water filters on the market have a pore size of 0.1 microns. The CDC recommends a maximum pore size of 0.01 microns for viruses. If a purifier says it has a pore size of .1 or .2 microns, it is only a filter. A virus will slip through a .1 or .2 micron filter.