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Developed for use in Bangladesh, the cloth filter is a simple and cost-effective appropriate technology method for reducing the contamination of drinking water. Water collected in this way has a greatly reduced pathogen count - though it will not necessarily be perfectly safe, it is an improvement for poor people with limited options.
The method used in Bangladesh is as follows: an old sari is folded to make four or eight layers. The folded cloth is placed over a wide-mouthed container used to collect surface water. It is usually sufficient to rinse the cloth and dry it in the sun for a couple of hours. In the monsoon seasons, it may be advisable to use a cheap disinfectant to decontaminate the material.
The preferred cloth is used cotton sari cloth. Other types of clean, used cloth can be used with some effect, though the effectiveness will vary significantly. Used cloth is more effective than new cloth, as the repeated washing reduces the space between the fibres.
The cloth is effective because most pathogens are attached to particles and plankton, particularly a type of zooplankton called copepods, within the water. By passing the water through an effective filter, most cholera bacteria and other pathogens are removed. It has been demonstrated to greatly reduce cholera infections in poor villages where disinfectants and fuel for boiling are difficult to get.
In sub-Saharan Africa where guinea worm dracunculiasis infections are endemic, infection is prevented by use of a nylon mesh with pore size of approximately 150 μm to filter out the copepods that host the parasite.
An old cotton sari, folded, creates a smaller effective mesh size (approximately 20-μm). This should be small enough to remove all zooplankton, most phytoplankton, and thus a large proportion of the cholera in the water (99%, according to laboratory studies). However, the nylon net with the larger mesh size was found to be "almost equally effective."
The cloth filter provides less than ideal purification on its own - usually filtering is an initial step, to be followed by further disinfection. However, where there are no other options, water professionals may consider that it is "of course, better than nothing" 
The cloth filter has been studied and reported on by Rita Colwell and Anwar Huq from the University of Maryland Biotechnology Institute, together with other researchers from the USA and Bangladesh. They report that:
- It is common practice in villages in Bangladesh to use cloth, frequently a flat, unfolded piece of an old sari, to filter home-prepared drinks.
The researchers studied the application of this technique to drinking water, with folded cloth. They studied the pore size of the cloth, the effect of folding the cloth on the effective pore size, the ability of the cloth to remove particles and plankton, as well as the effect on rates of cholera when used in a Bangladesh village.
- Moss J (January 27, 2003). "Cloth Filters Fight Cholera". Development Report (Voice of America).
- Colwell RR, Huq A, Islam MS, et al. (February 2003). "Reduction of cholera in Bangladeshi villages by simple filtration". Proc Natl Acad Sci USA. 100 (3): 1051–5. doi:10.1073/pnas.0237386100. PMC 298724. PMID 12529505.
- "Fighting the 'fiery serpent' in Sudan". ABC News (Australian Broadcasting Corporation). Reuters. June 6, 2008. "The Carter Center distributes cloth filters and plastic drinking pipes with gauze at one end to block the larvae."
- Hopkins DR, Ruiz-Tiben E, Diallo N, Withers PC, Maguire JH (October 2002). "Dracunculiasis eradication: and now, Sudan". Am. J. Trop. Med. Hyg. 67 (4): 415–22. PMID 12452497. "More than 278,000 cloth filters for household use in preventing the infection were distributed in Sudan in 1995 (compared with 93,000 the year before), and approximately 600,000 or more have been distributed each year since then. In 2001, however, in addition to distributing nearly 850,000 filters for household use, more than 7.8 million pipe filters were also distributed throughout endemic areas ...."
- Hogan J (13 January 2003). "Old clothes filter out cholera". New Scientist.
- Safe Saris - Bangladesh
- NSF Director Colwell Fights Spread of Cholera with Saris
- Sari filter stops cholera, with photo. ICDDR,B website.
- Photo showing woman using sari to filter drinking water - Source: Dr. Rita Colwell