Portable water purification
There is often a need to be able to treat water in remote or rural locations, or in emergency settings, to make it safe for drinking purposes. Although it is sometimes believed that water found in backcountry or wilderness areas is clean and potable, such water is often unsafe to drink.
Large rivers may be polluted with sewage effluent, surface runoff or industrial pollutants from sources far upstream. However even small streams, springs and wells may be contaminated by animal waste and pathogens. The presence of dead animals upstream is not uncommon.
In most parts of the world, water may contain bacterial or protist contamination originating from human and animal waste or pathogens which use other organisms as an intermediate host. Giardia lamblia and Cryptosporidium spp., both of which cause diarrhea (see giardiasis and cryptosporidiosis) are common pathogens. Less commonly in developed countries are organisms such as Vibrio cholerae which causes cholera and various strains of Salmonella which cause typhoid and para-typhoid diseases. Pathogenic viruses may also be found in water. The larvae of flukes are particularly dangerous in area frequented by sheep, deer or cattle. If such microscopic larvae are ingested, they can form potentially life threatening cysts in the brain or liver. This risk extends to plants grown in or near water including the commonly eaten watercress.
Portable drinking water systems or chemical additives are available for hiking, camping, and travel in remote areas.
Boiling water on a portable stove kills bacteria and viruses. At high elevations, though, the boiling point of water drops, so that several minutes of continuous boiling are required. At extremely high elevations, boiling ceases to be effective.
Portable pump filters are commercially available with ceramic filters that filter 5000 to 50,000 litres per cartridge, removing contaminants down to the 0.2 – 0.3 micrometre range. Some also utilize activated charcoal filtering. Most filters of this kind remove bacteria and protozoa, such as Cryptosporidium and Giardia lamblia, but not viruses, so disinfection by chemicals or ultraviolet light is required after filtration. Effective chemical additives include chlorine, chlorine dioxide, iodine, and sodium hypochlorite (bleach). There have been polymer and ceramic filters on the market that incorporated iodine post-treatment in their filter elements to kill viruses, but most have disappeared due to the unpleasant taste imparted to the water, as well as possible adverse health effects when iodine is ingested over protracted periods.
Small, hand-pumped reverse osmosis filters were originally developed for the military in the late 1980s for use as survival equipment, for example, to be included with inflatable rafts on aircraft. Civilian versions are available. Instead of using the static pressure of a water supply line to force the water through the filter, pressure is provided by a hand-operated pump, similar in function and appearance to a mechanic's grease gun. These devices can generate drinkable water from seawater.
While the filtration elements may do an excellent job of removing bacteria and fungi contaminants from drinking water when new, the elements themselves can become colonization sites. In recent years some filters have been enhanced by bonding silver metal nanoparticles to the ceramic element and/or to the activated charcoal to suppress growth of pathogens.
Iodine is added to water as a solution, crystallised, or in tablets. The iodine kills many — but not all — of the most common pathogens present in natural fresh water sources. Carrying iodine for water purification is an imperfect but lightweight solution for those in need of field purification of drinking water. Kits are available in camping stores that include an iodine pill and a second pill (vitamin C or ascorbic acid) that will remove the iodine taste from the water after it has been disinfected.
Chlorine-based bleach may be used for emergency disinfection. Add 2 drops of 5% bleach per litre or quart of clear water, then let stand covered for 30 to 60 minutes. After this it may be left open to reduce the chlorine smell and taste. Guidelines are available online for safe and effective use of bleach.
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Neither chlorine (e.g., bleach) nor iodine alone is considered effective against Cryptosporidium, and they are limited in effectiveness against Giardia. Chlorine is slightly better than iodine against Giardia.
Pur has developed a grayish powder from a combination of water purification chemicals. This "PUR Purifier of Water" is currently being used to clean drinking water in Kenya.  It has also been tested by the Centers for Disease Control and Prevention in Guatemala, Pakistan, and Liberia.  It was also sent to South Asia as part of the recovery from the Indian Ocean tsunami. 
The PUR Purifier of Water is a compound combining the primary active ingredients of calcium hyperchlorite and ferric sulfate. The first is a bleach that kills many harmful pathogens while the latter removes dirt, impurities, and further removes pathogens. It is known to remove the pathogens cholera, typhoid, and dysentery, toxic metals like mercury, lead, and arsenic, and pesticides such as polychlorinated biphenyl and DDT. 
Ultraviolet (UV) light destroys DNA and thereby prevents microbes from reproducing. Germicidal UV light — between about 240 nm and 290 nm — acts on thymine, one of the four base nucleotides in DNA. When a germicidal UV photon is absorbed by a thymine molecule that is adjacent to another thymine within the DNA strand, a covalent bond or dymer, between the molecules is created. This thymine dymer prevents enzymes from "reading" the DNA and copying it. Without the ability to replicate DNA, the microbe cannot reproduce and is rendered harmless.
The referenced company's microbiological effectiveness has been tested at three universities (U. Maine, U. Arizona, Oregon Health Sciences U.). When used as directed, the SteriPEN product has been shown to destroy over: 99.9999% of bacteria, 99.99% of viruses and 99.9% of protozoa (i.e. Giardia and Cryptosporidium). These levels of destruction meet or exceed the requirements of the US EPA’s Guide Standard and Protocol for Testing Microbiological Water Purifiers.
In SODIS (Solar Water Disinfection) microbes are destroyed by temperature and UVA radiation provided by the Sun. Water is placed in a transparent plastic bottle, which is oxygenated by shaking. It is placed for six hours in full sun, which raises the temperature and gives an extended dose of solar radiation, killing some microbes that may be present. The combination of the two provides a simple method of disinfection for tropical developing countries.
Solar distillation may use a pre-manufactured and easily portable still, commonly referred to as a solar still, but it has its roots in a makeshift still that can be constructed simply from readily available components. The solar still relys on sunlight to warm and evaporate the water to be purified. The water vapour condenses, usually on a plastic sheet suspended as an inverted cone, dripping into a collection dish placed beneath its center. Note that while the solar still shares exposure to UV and infra-red radiation with SODIS, it is essentially a completely different mechanism and the two should not be confused. In an extreme survival situation the solar still can be used to prepare safe drinking water from usually unsuitable sources, such as one's own urine.
Prevention of water contamination
To help minimise problems with drinking water in wilderness areas, it is strongly recommended that all waste be packed up and carted out to a properly designated disposal point. Burying human waste, even away from a watercourse, may be storing up serious problems for the next visitors to the area and also potentially to the local animal population.
- Water purification – large-scale methods
- Water filter
- Potability of backcountry water
- Appropriate technology#Water treatment – Water treatment section of Appropriate technology article.
- Waterborne diseases