Why Not Just Drink the Water?
Excerpts of an article from rec.backcountry's Panel 9, by Alan Dove (firstname.lastname@example.org), Bill Tuthill (email@example.com), and edited by Eugene Miya (firstname.lastname@example.org)
In the Great Outdoors, there are potentially four dangers of drinking water straight from a source (assuming it's freshwater): chemical pollutants, protozoa and larger parasites, bacteria, and viruses. The oldest (and cheapest) method of purifying water is to boil it. Boiling for five minutes will kill any biological hazards you could expect to find. Most pathogens are actually long dead by the time the water boils, but the five minute boil will get them all (remember to add to this time at higher altitudes). Boiling will not neutralize chemical pollutants.
A carbon filter will remove chemical toxins. Chemicals in water could include inorganic contaminants (arsenic and other heavy metals) or organic toxins (fertilizers and pesticides, for example). In general, it is a bad idea to trust any purification system to remove these, as even small quantities could ruin your day in a hurry. The good news is that water sources in the backcountry are seldom contaminated with appreciable levels of toxic chemicals. Take a good look at the stream you're about to get water from. Are there fish in it? Is there algae on the rocks? Crawdads on the bottom? Insects skimming the surface? Plants growing along the banks? If yes, the water is probably non-toxic, chemically speaking. If you're hiking in the desert, though, and a trickle of water etching a groove in the rock is bubbling sulfur from its barren depths, you should probably avoid it.
There are a number of parasites, both multicellular and unicellular, which live in water. The most common ones in North America are Giardia lamblia and Cryptosporidium. Boiling will kill them. In third-world countries, the number of parasites in the water is staggering, hence the hackneyed advice, "Don't drink the water." Amoebae can cause dysentery ("Montezuma's Revenge"), whipworm causes diarrhea and possible complications if not treated, and roundworms (Ascaris lumbricoides) can be unpleasant, to name a few. In some areas, such as the Philippines and Africa, you should try to avoid any contact with river water, including swimming or washing in unpurified water, as Schistosoma sp. is prevalent in these areas. These tiny parasites bore directly into the skin, entering the bloodstream and eventually setting up shop in either the intestine or the bladder. If left untreated or incorrectly diagnosed (a common problem, as symptoms only become manifest weeks or months after contact), the complications can be severe. As with Giardia, though, all of these parasites are killed by boiling.
Bacteria are the second smallest pathogens in water. One frequently hears about water being tested for Escherichia coli. While strains of this bacterium can be pathogenic, the vast majority are not, and it is, in fact, one of the species required in the intestine for digestion to occur (without bacteria, we would all die). Since it is present in large quantities in sewage, it is a good indicator strain to show when water has been contaminated with sludge. There are plenty of other bacteria which are happy in the intestine, to the detriment of the host. All are killed by boiling.
The smallest parasites are viruses. In true wilderness areas, pathogenic viruses are seldom found in water, but the odds increase with population density and poor sanitation practices. Boiling is the most reliable way to do away with viruses, though, and is strongly suggested in third-world countries. The specific viruses you should worry about in water are Hepatitis A, Rotaviruses, Polioviruses, and Echoviruses. All of these will cause diarrhea, intestinal cramps, and discomfort about 48-72 hours after contact, and complications could range from liver damage (for hepatitis) to aseptic meningitis and encephalitis (for echoviruses), and paralysis or death (for polio).
Chemical purification involves the use of iodine or chlorine to kill the nasties in the water. This method is lightweight and relatively inexpensive, but will not neutralize chemical toxins. In addition, you must make sure that water at 25 deg. C (75deg. F) sits for 20-30 minutes with iodine in it for purification to take place. If the water is colder (as it usually is), you will need to let it sit longer - possibly overnight for cold stream water. Warm the water against your body or even on your stove if you want it to be purified faster. Once the appropriate time has elapsed, the "band-aid" taste of iodine can be neutralized with a small amount of ascorbic acid (vitamin C). Used properly, iodine will kill most protozoa and all bacteria and viruses in water. After prolonged use, some people develop thyroid problems, so be aware of this potential side effect.
The latest rage in water purification is the use of filters, and a large number of them are available (see the review later in this panel). There are a couple of things to bear in mind when shopping for filters. First, only a system which includes an iodine matrix will kill viruses (see below). Second, a filter with pores larger than 0.2 microns - note the location of the decimal point, as it is important - will let bacteria through. The advantages of a filter are quick processing time (don't have to wait for the pot to boil or the iodine to do its work) and clean- tasting water (no iodine or vitamin C flavor). Some systems also contain a carbon filter which will remove chemical toxins.
Boiling your water is cheap and easy, and kills all known pathogens.