11:40 a.m. on September 21, 2009 (EDT)
mcgee55 said:
I previously believed it was also intended for whatever critters that make it past the filter
Well, viruses are critters.
On the "amount of bleach" statement - the effectiveness of any chemical treatment is dependent on a number of factors, making a thorough discussion "difficult to comprehend" in a single gulp. Temperature is a factor. pH is a factor. Concentration of the chemical is a factor. Time allowed is a factor. And more. Keep in mind that the most commonly used treatment in municipal water supplies in the US is chlorine in one of several forms. Swimming pool treatment is most often chlorine in one of several forms. (swimming pool treatment at the start of the season and periodically thereafter is called "shock treatment" because of the huge amount of chlorine added - I suspect you don't like drinking swimming pool water, even accidentally).
The link you give is basically correct, but a bit overstated. Note that the comment about the huge amount of bleach needed is in reference to heavily contaminated water, such as you would find in urban disasters, not what you more commonly encounter in wilderness situations. The source is Canadian, and in the Rockies and BC Ranges, the water at anything more than a couple miles from the trailhead has a pretty low concentration of nasty critters (just avoid beaver ponds).
Some people do not like even a slight chlorine taste (me for one). That's why I often just boil the water for drinking, at least on short, weekend length trips - costs a lot of extra fuel, but at least I don't get chlorine or iodine taste.
On the coffee filters - I have never tested coffee filters with water saturated with crypto or giardia, just used them to remove silt, sand, twigs, and such. However, a couple of friends who are MDs in the Wilderness medicine field tell me that at least some coffee filters are fine enough pore size to remove crypto, which are pretty large as are their oocysts. There are articles in the professional wilderness medicine journals to this effect. I would post the references, except we are still in the midst of moving into our new house and starting to unpack.
Your original question was about "Backup Water Treatment" and a lot of the discussion has been about emergencies where you somehow lost or broke your filter, the halogen pills got wet or lost, and maybe you didn't have fuel for your stove. To summarize the answers for such emergencies (emphasize - this is wilderness we are talking about, not areas where you have urban, industrial, and agricultural pollution, nor a disaster where municipal supplies are compromised or in 3rd world countries), you can get some distance toward potability (not sterile or pure in the technical sense) by using coffee filters, a T-shirt folded several times, "sun sterilization", and mostly judicious choice of source (e.g., avoid beaver ponds and scummy mudholes). In most high altitude locations where you might backpack, judicious choice will lower the probability of getting crypto or giardia. If you do get into such a situation, imbibing enough water to get you sick still allows about a week between drinking the water and the symptoms. So head out right away and talk to your doctor. Again, this was the emergency situation where you lost all other means of treating your water.
For the urban or 3rd world situation, I would say that carrying multiple backups and just putting up with the chlorine or iodine taste, combined with as judicious choice of water source is the way to go. For disaster preparation (hurricanes, floods, earthquakes), prepare your kit well ahead of time, including multiple alternatives stored in "grab kits" and several places (couple around the house, plus in each car in the family). Having lived in hurricane country (and been through one plus close to several tornados) and living now in earthquake country (been through one big one) plus urban wildfire country (only thing there is flee well ahead of the fire - don't even think about trying to save the house), just be very well prepared for the full range of eventualities.