Portable water purification
Untreated water may contain potentially pathogenic agents, including protozoa, bacteria, viruses, and some larvae of higher-order parasites such as liver flukes and roundworms.
Water contaminated by aluminium or zinc cannot be treated in this way using a strong alkali as higher pHs re-dissolve the metal salts.
Most portable treatment processes focus on mitigating human pathogens for safety and removing particulates matter, tastes and odours.
Significant pathogens commonly present in the developed world include Giardia, Cryptosporidium, Shigella, hepatitis A virus, Escherichia coli, and enterovirus.
In backcountry areas of the United States and Canada they are sometimes present in sufficient quantity that water treatment is justified for backpackers,[3] although this has created some controversy.
In general, more human activity up stream (i.e. the larger the stream/river) the greater the potential for contamination from sewage effluent, surface runoff, or industrial pollutants.
The WHO states bringing water to rolling boil then naturally cooling is sufficient to inactivate pathogenic bacteria, viruses and protozoa.
Most filters of this kind remove most bacteria and protozoa, such as Cryptosporidium and Giardia lamblia, but not viruses except for the very largest of 0.3 μm and larger diameters, so disinfection by chemicals or ultraviolet light is still required after filtration.
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.
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.
Water passing through activated carbon is commonly used in concert with hand pumped filters to address organic contamination, taste, or objectionable odors.
Chemical disinfection with halogens, chiefly chlorine and iodine, results from oxidation of essential cellular structures and enzymes.
Of note, Cryptosporidium oocysts, likely Cyclospora species, Ascaris eggs are extremely resistant to halogens and field inactivation may not be practical with bleach and iodine.
[14] Similarly to potassium iodide (KI), sufficient consumption of tetraglycine hydroperiodide tablets may protect the thyroid against uptake of radioactive iodine.
[21] Halazone tablets were thus commonly used during World War II by U.S. soldiers for portable water purification, even being included in accessory packs for C-rations until 1945.
The EPA recommends two drops of 8.25% sodium hypochlorite solution (regular, unscented chlorine bleach) mixed per one quart/liter of water and leave to stand covered for 30 to 60 minutes.
[22][6] The Centers for Disease Control & Prevention (CDC) and Population Services International (PSI) promote a similar product (a 0.5% - 1.5% sodium hypochlorite solution) as part of their Safe Water System (SWS) strategy.
The product is sold in developing countries under local brand names specifically for the purpose of disinfecting drinking water.
Sodium dichloroisocyanurate tablets are available in a range of concentrations to treat differing volumes of water[25] to give the World Health Organization's recommended 5ppm[26] available chlorine.
The primary disadvantage of silver ion/chlorine dioxide-based techniques is the long purification times (generally 30 minutes to 4 hours, depending on the formulation used).
Another concern is the possible deposition and accumulation of silver compounds in various body tissues leading to a rare condition called argyria that results in a permanent, disfiguring, bluish-gray pigmentation of the skin, eyes, and mucous membranes.
One recent study has found that the wild Salmonella which would reproduce quickly during subsequent dark storage of solar-disinfected water could be controlled by the addition of just 10 parts per million of hydrogen peroxide.
Germicidal UV-C light in the short wavelength range of 100–280 nm acts on thymine, one of the four base nucleotides in DNA.
Prolonged exposure to ionizing radiation can cause single and double-stranded breaks in DNA, oxidation of membrane lipids, and denaturation of proteins, all of which are toxic to cells.
In the event that such UV-treated water containing neutered microbes is exposed to visible light (specifically, wavelengths of light over 330-500 nm) for any significant period of time, a process known as photo reactivation can take place, where the possibility for repairing the damage in the bacteria's reproduction DNA arises, potentially rendering them once more capable of reproducing and causing disease.
[33] UV-treated water must therefore not be exposed to visible light for any significant period of time after UV treatment, before consumption, to avoid ingesting reactivated and dangerous microbes.
UV-C LED systems address disadvantages of mercury-based technology, namely: power-cycling penalties, high power needs, fragility, warm-up time, and mercury content.
In solar water disinfection (often shortened as "sodis"), microbes are destroyed by temperature and UVA radiation provided by the sun.
In theory, a solar (condensation) still removes all pathogens, salts, metals, and most chemicals but in field practice the lack of clean components, easy contact with dirt, improvised construction, and disturbances result in cleaner, yet contaminated water.
Such filters can do little, if anything, to mitigate germs and other harmful constituents and can give a false sense of security that the water so produced is potable.
