Portable water purification

Portable water purification is used 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
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.

Filters
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.

Chemical disinfection
Iodine is added to water as a solution, crystallised, or in tablets. The iodine kills many &mdash; but not all &mdash; 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. , USAID is promoting a similar product (a sodium hypochlorite solution) which is sold in developing countries specifically for the purpose of drinking water disinfection.

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.

Ultraviolet purification
Ultraviolet (UV) light destroys DNA and thereby prevents microbes from reproducing, without reproduction the microbes become harmless. Germicidal UV light &mdash; between about 240 nm and 290 nm &mdash; 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, thus neutering the microbe.

This form of water purification has been used in metropolitan areas for many years but entered the portable water purification market in 1997 when Hydro-Photon released the SteriPEN. The product concept has proven to be a solid design function and reliable water purification system. Ultraviolet light has also been used by another portable water purification product named Aquastar.

The microbiological effectiveness, of SteriPEN, has been tested at three universities (U. Maine, U. Arizona, Oregon Health Sciences U.)and at an independent testing lab (A & L Laboratory Inc. In Auburn, Maine). 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.

SODIS
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 still
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 relies 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
Only in very high-use wilderness areas is it recommended that all waste be packed up and carted out to a properly designated disposal point. The vast majority of the time it is perfectly safe to bury human waste.