Do you have to use the chemical or is it...
Do you have to use the chemical or is it just for extra precaution if you are taking water from a sketchy place.
Yes, you need to add the chemicals..otherwise you're going to get slightly filtered water that still has all your aquatic hitchhikers still in it..no bueno. Just add the solution.
The chemical solution is only to take care of virus's. If you are traveling in the US there is very little chance of getting sick from virus's in the water. If you travel outside the US I would say use it.
The pore size of the filter is 0.2 microns (or 0.2 micrometers). Anything smaller than 0.2 microns will pass through the filter, which would be any virus (hence the name). Giardia and Cryptosporidium are protozoa, large enough to be filtered out, and most all bacteria will be filtered as well.
The active ingredient in Viral Stop is sodium hypochlorite, a common chlorine-based disinfectant. Your tap water at home is likely disinfected with this chemical, but probably was not filtered with a 0.2 micron filter first.
filter size: 0.2 microns
viruses: 0.05 to 0.1 microns
bacteria: ~0.5 to 50 microns
Giardia: 10 microns
Cryptosporidium: 5 microns
So use your best judgement. If you suspect the water has lots of bacteria (perhaps from a muddy pool), you could add it to be on the safe side.
On Denali, we of course had no water to filter, but we melted water and added drops. No problems- except the person who didn't use drops got sick. I agree with others, if I think there are other people around my water source, I use the drops too. If it's pretty pristine, say back in the wilderness, I sometimes just filter. It's a judgement call, but other people are the most likely contaminate. If they are around, so are their germs.