There is a lot of information about water purification systems, and a lot of the info is purposely misleading because companies want you to buy their products. Which water purification method to use depends on your situation and the threats present.
Boiling is a good way to treat water in most common disaster situations, such as grid outages when the water treatment center stops working. Most sources (including FEMA and the Red Cross) say that you need to boil water for 1 minute to kill bacteria, viruses, parasites, etc.
Probably the most common chemical treatment for water is chlorine bleach. Make sure it is unscented bleach with no added cleaners. It should say 8.25% sodium hypochlorite on the label.
Water filters are considered a type of “mechanical” water treatment because they trap contaminants that go through them. There is a lot of variability in water filters, so you must be careful when choosing one. The most important thing to look at is the micron size rating.
Unlike the materials used in mechanical water filters, which “catch” impurities, activated carbon absorbs impurities. This unique property makes activated carbon the only water treatment method that will remove organic chemicals such as benzene and pesticides.
Ultraviolet light causes the DNA in bacteria, viruses, and parasites/protozoa to scramble. They will still be in the water, but they will be harmless.
When it comes to treating water, reverse osmosis is one of the most effective single methods. The only impurities the system won’t remove are organic chemicals (such as fluoride, chlorine, and lead).