Very quick summary - hope it is useful....
Water contamination can be simply divided into 3 categories:
Particulates
Biological (Bacteria, viruses, parasites, etc)
Chemicals and heavy metals
The standard methods of water treatment are as follow:
Straining
Boiling
Distillation
High efficiency filtration
Chemical treatment
Reverse osmosis
Particulates can be removed by collecting water careful without disturbing the sediment - or if the water is turbid, by simply straining through a cotton T shirt, sock or specialist device like a millbank bag.
Biological contaminants , from a practical point of view could be considered in terms of their size as particle size ultimately affects the efficacy of the shop bought filtration systems.
1 to 2 microns and bigger can be filtered out by devices like those made by lifestraw, Sawyer, katadyne & MSR. These types of filter are affective against most bacteria and parasites such as ecoli, salmonella, cryptosporidium etc
Virus are commonly sub 1 to 2 microns and, in general, filtration systems struggle to remove them.
Chemical treatment such as Chlorine dioxide tablets are affective against most viruses (as well as bacteria and protozoa) and so where there is high risk of viral infection or you want to sterilise large quantities of water (e.g. 30 litres) quickly, they are very useful. Dont taste great, but effective.
Boiling is affective against viruses, bacteria & protozoa but not chemicals or particulates
Chemicals and heavy metals are not filtered out using ceramic and fine pore filtration systems, however those using activated charcoal do have some ability to remove a wide range of chemicals even some in solution.
In practice however if you are not drinking from the same source for very long you are unlikely to be adversely affected by chemical pollutants (unless their concentration is exceptionally high).
It is good practice to combine a couple of techniques together in areas where water quality is suspect e.g. filter out the sediment using a T shirt and then boil/treat with chlorine dioxide tablets.
My choice for an exped? Bearing in mind I dont know where you are going or how many you are treating water for)
1. Cotton T shirt
2. Sawyer filter or katadyne / MSR pump filter
3. Boil
I recommend you watch MCQbushcrafts latest video - it should answer many questions regarding kit