Water purification is a controversial issue: some mountain travellers will tell you they have never done anything to purify their water and in many years they've never had a problem. Others take a more cautious approach. Those who have suffered from the effects of giardia, for example, are likely to be numbered amongst the cautious! Personally, I'd rather be safe than sorry...
There are some basic precautions that you can take to minimise the risk of catching something nasty:
- Take water from a fast-flowing stream or river; only use a pool or lake when there is no alternative and then with great care.
- Walk upstream for a short way to check for dead animals or other contamination in the stream.
- If water is muddy, if possible let it settle before taking some; if it has bits in it, filter it roughly using a bandana or T-shirt before further treatment.
- Avoid using water in farming or lowland areas where agricultural chemicals are likely to be washed into surface water.
There are 4 main approaches to water purification: boiling, filtering, chemical treatment and UV light treatment.
Boiling water is usually regarded as a safe option, as long as any particulate matter has been removed and you boil it for long enough. General advice seems to be to bring the water to a rolling boil and keep it boiling for at least one minute. At higher altitudes, water boils at a lower temperature and a longer period such as 3 minutes is recommended (I guess at some point boiling ceases to be effective as the temperature reached is never high enough). Boiling affects the taste of water, it's a real pain in the neck, and uses lots of fuel. In my view it's best kept for emergencies!
Filtering has been very popular amongst outdoor folk. There are various types, including activated carbon and ceramic. This is a handy summary, from Katadyn, one of the leading manufacturers of water filters for travellers. Gravity filters are easy to use, but take a long time. Pump filters are more common, but even then the flow rate is not that marvellous. My own experience is that pump-filtering can become a real chore, especially when you are tired at the end of the day. It can be very tempting to not bother filtering, which could all too easily lead to an attack of the nasties. If you do decide to go down this route, then before purchasing a filter check its flow rate and work out how long it will take you to filter what you will need. On-demand filters, such as this one from Aquagear, can be useful during the day, as you fill up the bottle when convenient and only filter the water when you drink.
Chemical treatment is physically easier than boiling or filtration, but you need to be careful to use the right type of treatment at the right time. In the UK, we are used to our tap water being treated with chlorine: it's effective, but sometimes leaves a rather unpleasant taste, depending on how much chlorine our water company has had to add. Iodine is effective, but also suffers from a nasty aftertaste, and there are questions over its safety over long periods (3 to 6 months plus, depending on which resources you consult). Neutralising tablets or drops can be used to remove the bad taste, but only once treatment has completed. A variation of chlorine-based treatment is produced by McNett and used to be called Aquamira but is now known as Aquaventure. It is available in drop and tablet form, uses chlorine dioxide and leaves no aftertaste. Treatment takes about 20 minutes.
UV light treatment is technically interesting and a newcomer to the outdoors scene. Steripen is the brand that most are familiar with. Treatment is quick, takes just about no physical effort, and is reportedly very effective. On the negative side, you need to buy batteries (and make sure you don't run out) and there have been some reports of reliability issues.