This works - provided you boil the water for long enough. Some bugs are killed by bringing the water up to 70C, well below boiling, but some people claim you need to boil for at least 10 minutes. The author is not an expert on this, but suspects the 10 minutes bit is simply a precautionary overkill which has acquired legendary status. Research literature usually states that bringing the water to the boil will kill both E coli and G lamblia.

One point to watch with boiling is altitude. At 4000m the boiling point of water is depressed, so a little bit more time might be well advised. The major problems with this method are time, fuel and cooling the water down.

Microorganism Destruction Levels

(Ultraviolet energy at 253.7nm wavelength required for 99.9% destruction of various microorganism - in uwsec/cm²)

Bacillus anthracis (anthrax) 8,700 Shigella dysentariae (dysentery) 4,200 Corynebacterium diphtheriae, 6,500 Shigella flexneri (dysentery) 3,400 Dysentary bacilli (diarrhea) 4,200 Staphylococcus epidermidis 5,800 Escherichia coli (diarrhea) 7,000 Streptococcus faecaelis 10,000 Legionella pneumophilia 3,800 Vibro commo (cholera) 6,500 Mycobacterium tuberculosis (TB) 10,000 Pseudomonas aeruginosa 3,900 Salmonella (food poisoning) 10,000 Influenza (viruses) 6,600 Salmonella paratyphi (enteric fever) 6,100 Poliovirus (poliomyelitis) 7,000 Salmonella typhosa (typhoid fever) 7,000 Hepatitis 8,000