Ray Mears had a period where he looked a lot like BoB
There are lots of rubbish water purifier systems availible, quite a few so-so ones and a couple of really good ones, each with thier own advantages. Here are some thoughts on the best options I've used.
The MSR miniworks is highly regarded, although perhaps a little bulky, and has an amazing flow rate of 1 litre per minute. The miniwoks uses a filtration system down to 0.3 microns which is enough to remove all bacteria, protozoa, etc. Of course, the ceramic filter can do nothing about viruses. They must be killed pre or post use, by either high temperatures, ultraviolet light, or chemicals like iodine and chlorine. Where you believe the water to be virus free [and you're right] the filtration-only design comes into it's own avoiding the taste of iodine or chlorine.
Pre-Mac Trekker which, while is has an annoying flow rate of only 0.4 litres per minute, does have other things going for it. Designed 'to produce safe drinking water from contaminated sources by elimination of pathogenic organisms - bacteria, viruses and cysts' in tests by the London School of Hygiene and Tropical Medicine the Trekker achived total elimination of the bacteria Eschedria coli and a reduction of Poliovirus in excess of 99.9%. One clever feature is the use of a resin that has iodine in it, so very small amounts of Iodine are present in the filtered water - leave the water to stand for three minutes and everything's zapped. I like the Trekker a lot, while it pumps slower and has a slight taste of iodine, it packs smaller, and is the best choice for when you absolutely, positively, have to kill every virus in the cup.