Other Water Purification Technologies for Expeditionary Water Purification
There are many ways to purify water. Existing water purification techniques range from the simple and inexpensive to the very sophisticated and very expensive. As a result, existing technologies cover a wide range of effectiveness in treating waterborne pathogens, organic contaminants and inorganic contaminants.
Water Purification Technology Overview
BoilingBottled WaterSolar Water Disinfection (SODIS)Reverse Osmosis (RO)Chemicals: Iodine, Chlorine and Chloramines Ozone CeramicsUltraviolet Systems
Boiling
Boiling is a simple and common technique for water purification. It is extremely effective at inactivating pathogens from water; however, it is unpractical for daily use in many parts of the world. In natural disaster where fuel is not readily available, boiling requires too much energy. In addition, water may not be heated high enough to bring unsafe water to a full boil. Without a full boil, unsafe water remains unsafe. Furthermore, boiling increases the risks associated with heavy metals, since the concentration of heavy metals increases due to water loss during evaporation. (back to top)
Bottled Water
Water distribution via bottled water is extremely wasteful, energy-intensive and expensive. For many places in the world, it's simply unavailable. Even in places where it's available, it's far more efficient to purify existing ground water on site, than to truck it in from hundreds of miles away. Additionally, it's impossible to achieve the potential volume of onsite water purification with bottled water. Bottled water also has the added problem of what to do with the empty containers. In many parts of the world, empty water bottles are littered, potentially harming the environment. (back to top)
Solar Water Disinfection (SODIS)
A more recently developed technique is solar water disinfection, or SODIS. Unlike boiling, SODIS relies only on solar energy to disinfect the water. SODIS is a simple method to inactivate or kill pathogens using a combination of solar heat and sunlight. SODIS is used with 1 to 2 liter plastic bottles, preferably made of polyethylene terepthalate and preferably painted black on the non-sunlit back surface of the bottles. The bottles are completely filled with water and placed on a corrugated steel sheet in the sun. SODIS requires the water to attain a temperature of 60 to 80°C (140 - 176°F) for a minimum of 4 hours to remove the pathogens. Under cloudy conditions, the bottles must be placed in the sun for two consecutive days. SODIS is very inexpensive to implement, but is not as effective against viruses and protozoa. SODIS processed water is not recommended for infants less than 18 months or for people with chronic gastrointestinal illness. The quality of the purified water is very difficult to control. The technique does not work as well with even partial shade. SODIS does not kill protozoa such as cryptosporidium parvum oocysts. (back to top)
Reverse Osmosis
Reverse osmosis (RO) purifies bacteria, salts, sugars, proteins, particles, dyes, heavy metals, chlorine and related byproducts, and other contaminants with a molecular weight greater than 150 – 250 daltons. The RO requires pressurized water that is not available in many parts of the developing world. Reverse osmosis membranes may foul unless the incoming water is carefully filtered before the reverse osmosis system. The RO systems may also need water softening equipment upstream of the RO purifier where the water has high mineral content (hard water) to prevent membrane fouling.
There are two primary types of RO membrane: Thin Film Composite (TFC) and Cellulose Triacetate (CTA). TFC membranes filter out more contaminants than CTA membranes, but they are more susceptible to damage by chlorine. Since the RO membranes are subject to degradation by chlorine, iron, manganese, hydrogen sulfide, and to bacterial attack, a sediment filter and a granular activated carbon (GAC) pre-filter is often used ahead of the RO system. Additional treatment such as GAC is needed for volatile organic compounds such as benzene, MTBE, trichloroethylene, trihalomethanes, and radon.
The RO process is fairly slow and may require from 11.4 to 38L (3 - 10 gal.) of untreated water for each 3.8L (1 gal.) of purified water, making it problematic for use in areas where water is scarce. RO water treatment is not recommended for use without secondary treatment such as UV treatment for water that may contain biological contaminants such as viruses and bacteria. (back to top)
Chemicals: Iodine, Chlorine and Chloramines
Other more advanced water purification systems are readily available but have limitations as well. Both iodine and chlorine are effective at eradicating most bacteria, viruses, and protozoa. However, cryptosporidium parvum is one of several chlorine-resistant pathogens which is increasing in importance. Cryptosporidium parvum is an intestinal parasite that can be life threatening to infants, the elderly and people with compromised immune systems. Typically, it takes about seven days for symptoms of cryptosporidiosis to appear, long after the initial exposure occurred. The illness often can last up to two weeks. Removing protozoa like cryptosporidium parvum oocysts and giardia with chlorine purification is difficult because it requires a high product of chlorine concentration and application time. Since adding too much chlorine to drinking water can cause organ damage or death in humans, the concentration of chlorine that can be used to disinfect the water is limited. Therefore, the time required for chlorine disinfection of cryptosporidium is often prohibitive.
Chlorine has been shown to produce hazardous trihalomethanes when it is added to water with organic contaminants, as is typically found in natural sources such as rivers, lakes and streams. Trihalomethanes are also environmental pollutants, and many such as chloroform are considered carcinogenic. Additionally, chlorine is ineffective if the pH of the water is below 7.5. If the chlorine is from a bleach bottle more than six months old, it loses its potency.
Both iodine and chlorine can cause side effects in humans if used for an extended time. Iodine treated drinking water is not suitable for pregnant women or women over age 50 or people with thyroid problems.
Many modern water purification systems use chloramines instead of chlorine, adding increased sophistication to the treatment systems.
Chlorine dioxide is also used as a purification agent that kills most bacteria, viruses and protozoa. Due to the explosion hazard, it is typically manufactured at point of use, increasing purification system complexity and expense. Chlorine dioxide purification produces reaction by-products, the toxicity of which is unknown. (back to top)
Ozone
Ozone effectively disinfectants all types of pathogens in drinking water. It leaves little to no residue in the water. Unfortunately, ozonation systems are expensive and cumbersome. Furthermore, they may be too complex and fragile to operate in villages. As a result, they do not make good municipal water systems in remote villages. (back to top)
Ceramics
Other approaches rely on advanced ceramics or membranes instead of disinfectants to filter pathogens from the water. Ceramic filters are effective for filtering protozoa, but may clog easily due to particulates in the water. Typical ceramic filter elements have pores from 2 to 5 microns in size. Since bacteria such as cholera and salmonella are typically between 0.2 and 1.0 microns in size, bacteria pass through many of these filters. Viruses such as Hepatitis A and B, rotavirus, and the Norwalk virus are typically below 0.004 microns (1.57480315 × 10^(-7) in) in size, allowing them to pass easily through the ceramic filter element. These viruses and some bacteria may even penetrate reverse osmosis purifiers. (back to top)
Ultraviolet Systems
UVC purifiers work by irradiating the pathogens in the water, usually with low pressure mercury lamp(s) which emit a 253.7nm (9.98818898 × 10^(-6) in) peak wavelength. UVC has proven effectiveness in inactivating or killing a very wide range of viruses, bacteria, protozoa, helminthes, yeast, and mold. An advantage of UVC purification systems is that they are capable of treating the drinking water for all segments of the population, unlike other disinfection technologies such as iodine and chlorine. UVC systems do not leave residual disinfection compounds in the water.
Please see the World Health Organization's bibliography of point-of-use water disinfection.
Quick Link: Expeditionary Water Unit F.A.Q.