Backcountry streams, rivers, and springs look fresh and inviting, and just might be the best-tasting and most satisfying water you'll ever drink. Until a few days later when you spend all your time in the bathroom, and possibly the hospital. Not really worth it.
How do you get sick from such 'pure' sources? Microorganisms known as Giardia and Cryptosporidium live in most untreated water around the world, and when they reach your gastrointestinal tract, they make themselves to home. Viruses can also live in water, but are rare in North America.
To avoid the bugs, it's best to drink only from a trusted source like municipal or bottled water. The further you get from the road, however, the harder that will be. Killing or removing contaminants is important for health and as a result, many options are available. Boiling is always the guaranteed, no-questions purifier, but a stove, fuel, and pot are bulky and time consuming for a day hike.
For many years every backpacker carried a little glass bottle of iodine tablets, and every liter of water from a natural source would get one tablet, be shaken occasionally, and 30 minutes later the water is declared 'potable.' Now, the tablets are chlorine dioxide and individually packaged, but the process is about the same. This chemical treatment will address giardia and crypto cysts as well as viruses, and is very effective on water with few physical contaminants. Iodine and other forms of chlorine are still available, but chlorine dioxide is considered the safest and most effective.
When exposed to high intensity ultraviolet light, the enzymes of a microorganism cannot access its DNA, ending reproduction and eliminating the risk from exposure. Many municipal treatment plants use some form of UV exposure because of its efficiency and lack of added taste. In personal use, exposure of 90 seconds will purify a liter of water. Because the light cannot penetrate skin, there is no risk to the user. Just like chemical treatment, UV will not address physical contaminants.
When you're not sure exactly where your water has been, it's a good idea to filter it, removing chemicals and other toxic substances not addressed by chemical treatment. Common practice is to draw water from the top of moving water or allow it to settle in a collapsible container and pump directly into drinking bottles or camp supply. The industry standard is a filter media pore size between 0.3 and 0.02 microns. For reference, the finest hair is about 40 microns thick. With such a small filter size, many biological contaminants are also caught, and some manufacturers claim that their filters eliminate the need for chemical treatment.
The time available to filter your water, group size, and space and weight are all considerations when selecting your filter. They will be in three categories; Personal, Pump, and Gravity
Personal filters are built into a bottle or installed in the drinking tube of a hydration system, and are designed for immediate use with small quantities for short terms. They will still have the media quality and treatment path, but generally a smaller surface area and thus may clog sooner.
Pump filters will be sized for small (2-3) or medium (3-5) size groups and are best used in a scenario where some quantity of water will be treated for future use, and require some preparation and arrangement of the source and destination conatiners, but the pump will be fairly quick to use and should be back-washed and maintained after each use.
Gravity systems use the same filter cartridge as a pump, or a slightly larger version to increase surface area, and consist of a large (4-to-8 liter) reservoir placed at some height, then the water is filtered as it feed through the outlet hose. This type is excellent for large groups with facility to suspend a heavy load (a liter weighs 2.2 pounds) and a constant need for water in a base camp situation.
With experience, you will find a treatment system that you are comfortable with, but before your first adventure, it can be a bit tricky to figure out which is right for you.
- Filtering can be the most effective and most filter will last through several hundred (or thousand) liters, but requires time and space to do the work, meaning you will want to do a substantial quantity at each session.
- Chemicals are very convenient and can be used on-the-go, but the treatment supply will be limited, require time, and will always add some flavor to the water.
- UV adds no flavor and is much quicker, but relies on batteries/recharging and equipment that can be lost or broken.
As you consider your itinerary, terrain, and group size, the decision might be easier, since with a group the filtering can be done all at once, but when traveling alone you'd rather fill your own bottle and zap it before drinking. If the water will be 'mostly reliable,' throwing a strip of chlorine dioxide tabs in your first aid kit will get the job done in a pinch and you'll feel better.