I should have probably introduced myself somewhere else first, but I saw this thread and I know a little from doing research about this.
Viruses don't usually go smaller than 20 nanometers (0.02 microns) and bacteria are usually bigger than 200 nanometers (0.2 microns). Most of the water filters I've seen out there will filter down to 0.2 microns, but not smaller. Many of the cheaper or smaller ones don't even do that. This means you're looking at filtering out particulate matter, bacteria, protozoa, etc., but
not viruses.
In order to kill viruses, you need to use purification tablets (cheap but taste nasty as they're usually based off chlorine. Iodine isn't near as effective as chlorine so avoid those). UV will also work, but you have to filter the water first, otherwise particulates in the water will literally stop the UV rays from penetrating far enough to purify all of the water. Also, UV purifiers seem to be very expensive, and keep in mind that they are electronic. Electronics can fail, so make sure you have tablets on you just in case. There are only two filter systems that I have found so far that can
mechanically filter down to the size of a virus. This is the Lifesaver bottle and the Sawyer 4l filter. The latter has received some reviews saying that it has failed quite quickly, and so I'm doubtful about the reliability of such a filter in a survival situation. The former seems to be mired by some negative reviews too, but they mostly originate out of misunderstanding.
In order to reduce chemicals and pharmaceuticals in your water, as well as chlorine (if you want to filter tap water, or filter after you've purified with tablets), you should use a filter that has an activated charcoal attachment. This is far cheaper, faster, and more practical than distilling water in a survival situation. The Lifesaver bottle has one of these, but their activated charcoal filter attachment is rated to only 250L, so you would need over a dozen if you plan to fully use the 4000L rating of the main filter.
Look for filters impregnated with silver. This will stop or minimise any bacterial growth. The Katadyn pocket has this I think, as does the Lifesaver bottle. I could be wrong on this though.
Also to consider: filter throuhput and method of use. For example, the lifesaver bottle is not practical for filtering large quantities of water. If all you want is drinking water, that will be fine. If you want to filter water for cooking, cleaning, brushing teeth, etc., then one of those Katadyn filters that attaches to water bladders is probably a more sensible variant.
Lastly, a word of warning with regards to micron ratings. What manufacturers usually list as their micron rating is the nominal (or average) rating. This is merely the average pore size. This means that there will be pores bigger, usually two to three times bigger. And since water likes to travel through the easiest place first, this is actually quite important. The minimal pore size they guarantee is called absolute rating. When you see a filter rated at 0.2 microns, be aware that the absolute rating is probably anywhere in between 0.5 and 0.9 microns, which is significantly bigger than the smallest bacteria. To complicate things further, the U.S. and International standards for testing water filters are different. The U.S. ones are (apparently) an order of magnitude
less stringent.
I've sent e-mails to several manufacturers trying to find out what their absolute rating is, including the Lifesaver bottle. I have only received one reply so far, that one concerning the Katadyns being between 0.5 and 0.9. This means they will still filter out most microbes, but not all.
If you combine both filtration and purification though, you should kill everything
Hope this helps!
