Comparable Water Purification Technology for Villages
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
Boiling is an unpractical drinking water purification technique for the rural poor because it requires too much energy to implement. In addition, it may be improperly implemented by "non-boiling cultures" (for example, non-tea-drinkers) because they risk not bring water to a full boil. Boiling has other risks too, for it increases the dangers of heavy metals in drinking water. As the water evaporates in the boiling process, higher concentrations of heavy metals accumulate in the drinking water. As a result, boiling is not a scalable drinking water solution for the rural poor. (back to top)
Wells seem like a great solution for the water problems of the rural poor. But they have major shortcomings in application. Often times dug and tube wells need purification because they may become contaminated during the rainy season. If the wells are poorly constructed or maintained, then they may be contaminated on a regular basis. For these reasons and more, a growing body of research questions the efficacy of wells:
Diwi Consult and Bureau d'Ingénierie pour le Développement Rural (BIDR)
(1994) Etudes d' Réhabilitation des Points d' Eau Existants.
DWD (2002a) Issue Paper 1: Overview of the Water Sector, Reform, SWAP and
Financial Issues. Directorate of Water Development, Ministry of Water, Lands
and Environment, The Republic of Uganda.
Hazelton, D. (2000) The development of community water supply systems using
deep and shallow well handpumps. WRC Report No, TT132/00, Water Research
Centre, South Africa.
Solar Water Disinfection ("SODIS") purifies drinking water inside 1 - 2 L plastic bottles placed on a corrugated steel sheet in direct sunlight. After the water reaches a temperature between 60 to 80°C (140 - 176°F) for at least 4 hours, it is ready to drink. Unfortunately, SODIS has major limitations.
SODIS is minimally effective at inactivating viruses and protozoa. For example, SODIS does not kill protozoa such as cryptosporidium or giardia. As a result, SODIS-treated water is not recommended for infants less than 18 months or for people with chronic gastrointestinal illness. In addition, the quality of the SODIS-treated water is very difficult to monitor or control. As a result, SODIS is not an effective drinking water solution for the rural poor. (back to top)
Reverse Osmosis (RO) purifiers bacteria, salts, sugars, proteins, particles, dyes, heavy metals, chlorine and other contaminants with a molecular weight greater than 150 – 250 daltons. RO systems require pressurized water that is not available in many parts of the developing world. In addition, RO membranes may foul unless the incoming water is carefully filtered before it enters the RO system. Furthermore, RO systems often require water softening equipment 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 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. For all of these reasons, RO is not a practical drinking water solution for the rural poor. (back to top)
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.
Ozone is the most effective disinfectant for all types of pathogens in drinking water. It leaves minimal or no residue in the water. However, ozonation systems are expensive to implement. (back to top)
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 light purifiers work by irradiating 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 UV 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. UV water purification systems do not leave residual disinfection compounds in the water. (back to top)
Slow sand technology works by filtering water through sand, albeit slowly. They are effective for removing bacteria, organics and protozoa from water; however, they require intense maintenance and a very fine sand not readily available in rural Africa or Asia. If a slow sand system is not carefully maintained by a well-trained crew, it is prone to contamination.
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