There has been quite a bit of discussion on this topic. Most recent sources advise that bringing water to the boil (making allowance for lower boiling point at altitude) is sufficient to kill any potentially harmful bacteria. This is from the UNHCR providing advice to aidworkers for providing potable water in emergency situations, which in fact advices against prolonged boiling:theorsmeister said:It is not infact true that purifying simply requires bring to the boil.
You should infact boil the water for as long as possible, but after ten minutes there are no microorganisms left to kill. So atleast ten and add an extra minute for every 1,000 feet above sea level, as water boils at a lower temperature higher up.
If you cannot afford the time and effort, boil for as long as possible, bringing it to the boil will still kill most microorganisms, but it will not be completely safe.
Use this as a guide, I believe to be true, but do not hold me responsible for anything that may go wrong.
source: http://www.aidworkers.net/UNHCR/water-09.html92. Boiling is the surest method of water sterilisation. At low altitudes, water that is simply brought to the boil can be assumed to be free of pathogenic bacteria. Boiling should, however, be continued for one minute for every 1,000 metres of altitude above sea level, as the boiling temperature reduces with altitude. Prolonged vigorous boiling is often recommended but is not necessary to destroy the faecal-orally transmitted pathogens; it wastes fuel and increases the concentration of nitrates in the water. Water with high nitrate content is dangerous for very young babies. Domestic fuel supplies may, in the longer term, be the determining factor: boiling requires about 1 kg of wood per litre of water