Knowledge of how to make water safe for human consumption or use is vital. There are basically three methods: heat, chemicals and filtration.
Bacteria, viruses and most protozoan cysts (Giardia and E. Hystolytica) can easily be destroyed by heat. Time and temperature are inversely related. That means that the higher the temperature, the less time needed to disinfect the water.
I believe the best way to disinfect water is through boiling. Water is, generally, safe to drink after it reaches boiling point (100 degrees C). Just bringing the water to boiling temperature is killing pathogens. Just to make sure, boil the water for an additional minute. Giardia and E. Hystolytica have thermal death points at 60 degrees C, enteric viruses and bacteria die in seconds at 100 degrees C. The general rule is to boil water for a complete five minutes at sea level. There is a myth that you must boil water longer at higher altitudes. There is no need to significantly increase boiling time above 5 minutes, at altitude, because the thermal death points of most enteric pathogens is still within the 5 minute time window. Some negative sides of boiling water are the need for a heat source and fuel. Not everywhere is a good place for a fire and you may not always have cooking fuel. Heat is a one step process but doesn’t make the water look better if it is cloudy/murky.
Sterilization occurs after boiling for 10 minutes and kills spores, although spores are generally not know to cause GI problems. Pasteurization occurs when food or beverages are heated to 150 degrees Celsius for 30 minutes or 160 Celsius for 1-5 minutes.
Halogens (chlorine and iodine) are very good disinfectants. The thing to remember about using chemical to treat water is the concentration of the chemical and the time needed disinfect the water. Concentration (the amount of the chemical in the water) is measured in milligrams per liter (mg/L) or PPM (parts per million).
Chlorine is used by most cities to purify drinking water for people’s houses. Chlorine kills Giardia, E. hystolytica, viruses and bacteria. Generally, using chlorine, I advise a 60 minute contact time. The amount of time required for 99.9% kill of pathogens depends on water pH and temperature. Household bleach (5% hypochlorite) is chloride. Adding 4 drops (0.2 mL) of bleach to one liter of water produces adequate concentration of 10 PPM and should be left to site for one hour.
Iodine is another halogen that is widely used to treat water. The goal of concentration of iodine to treat water is 3-4 mg/Liter or 4 PPM, given adequate contact time. Iodine generally requires less time than chlorine. Iodine, at 3-4 mg/L, generally makes water safe after 30 minutes of contact time. Remember that cold water needs a longer contact time. The iodine tablets are very easy to carry and small.
A few negatives about using chemical water treatment is the bad taste of the water. I advise mixing some electrolyte powder after the water has been treated. Also, people with unstable thyroid disease or those with iodine allergy should not use iodine. Its use in pregnancy should be avoided, longer than 1 week.
Filtration is a very easy method to disinfect water. Most everybody knows somebody with a water filter. The thing to know about is pore size. This is the size, in diameter, of the particles than can pass through the filter.
Parasitic eggs and larvae are about 20 micro-meters
Giardia and E. Hystolytic are about 3-5 micro-meters
Bacteria are 0.4 micrometers
Filtration is ineffective against viruses because they are too small
Filters tend to clog, easily. This is especially true if you are filtering dirty/muddy water. Try to let the water settle for 30 minutes before filtering. This gives the larger particles time to drop to the bottom of the container via sedimentation. Filters can also be bulky to carry. There are some new filtration straws that work well, too.
Choosing which method to use is dependent on number of people in the group, time to devote to purifying water and equipment. Often times, a two-step process is best. Iodine with filtration is a very popular treatment that removes resistant cysts that iodine may not kill.
As for me, I prefer boiling, if I have the time and equipment. Actually, if possible, I try to filter and then boil. Filtration is my second favorite method, but takes up some extra room. Lastly, I carry iodine tablets for emergency. They are not my first choice, but I always have them nearby.