Implementation toolsWater PurificationHardwarePoint-of-Use Water TreatmentStraining and Filtration 
Straining and Filtration
- Compiled by:
- Raju Shrestha (Environment and Public Health Organization (ENPHO) )
Executive Summary
Straining is a very simple method of filtration. In this process, water is poured through a piece of cloth, which removes some of the suspended silt and solids and destroys some pathogens. After straining, water may not be perfectly safe for drinking but it can be a drinking water improvement step for people with no other treatment options. It is very important to use a clean cloth, as a dirty cloth may introduce additional pollutants into the water.
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A woman using a sari cloth to strain water. Source: AFRICDAY (2009)
Straining water through a piece of clean cloth is an extremely simple, low- resource method and widely used for household water treatment. Cloth filters have been used in many cultures for centuries. Typically in South Asia, a sari or saree (a strip of unstitched cloth ranging from four to nine meters in length that is draped over the body of women (it is also a traditional garment) is folded 7 to 8 times and used as a filter. In laboratory experiments using electron microscopy, it was found that an inexpensive sari cloth, folded four to eight times provides a filter of about 20 µm mesh size, was small enough to remove all zooplankton, most phytoplankton, all Vibrio Choleraeattached to the plankton and other particulates larger than 20 µm (COLWELL ET AL 2003, SAFE WATER INTERNATIONAL CLEARINGHOUSE N.Y.).The risk of cholera is therefore reduced of about 50% (HUQ et al., 1996). Water is poured through the folded sari cloth and collected in a pot underneath. The efficiency of straining depends on the weave of the cloth and the number of times it has been folded. Specific monofilament filter cloths are very efficient where guinea worm disease is prevalent. Such cloths remove organisms known as copepods, which act as intermediate hosts for the guinea-worm larvae. Dracunculiasis, more commonly known as guinea worm disease, is caused by drinking water hosting a parasite called Dracunculus medinensis (DPD 2008).
Very Effective For: | Somewhat Effective For: | Not Effective For: |
| Taste, smell, colour | Viruses Chemicals |
Effectiveness of straining. Adapted from CAWST (2009)
Comparison of electron micrographs of a single layer of New and Old sari. Source:COLWELL et al. (2002)
In a laboratory experiments, it was found that an old sari cloth made up of cotton is most effective in removing V. Cholerae. After several launderings, threads of an old sari become soft and loose, reducing the pore size compared to a new sari cloth (see pictures below) (COLWELL et al. 2003). Cloth filters do not remove chemical contaminants or dissolved compounds from water. After straining, additional treatment methods can further improve the safety of drinking water (see Household-level Drinking Water Treatment).
Applicability
Though the water collected from cloth filter is not perfectly safe, it is a drinking water improvement step for people with limited options. This procedure can be used as the first stage of treatment. Then water can be treated through available methods like sand filtering(see also biosand filter) or treated furtherwith disinfection methods such as SODIS, boiling, chlorination and others. Both cotton and nylon cloths are suitable for filtration. It is very important to use clean and dry cloth, as a dirty cloth may introduce additional pollutants or pathogens into the water.
Advantages
- Simple, low cost and easy technique
- Consumes little time
- Reduces turbidity from drinking water
- Known to reduce the risk of cholera if used correctly
- No particular equipment needed
Disadvantages
- Requires extra washing of sari after use
- Not completely effective for removal of bacteria
- Not effective for removal of viruses and chemical
References 
AFRICDEV (Editor) (2009): Africadev — Self Help Developments for Africa. Water. URL [Accessed: 05.08.2010].
DPD (Editor) (2008): Dracunculiasis — Guinea Worm Disease. Atlanta: Centers for Disease Control and Prevention, Division of Parasitic Disease (CDC DPD). URL [Accessed: 28.07.2010].
HUQ, A.; XU, B.; CHOWDHURY, M.A.R.; ISLAM, M.S.; MONTILLA, R.; COLWELL, R.R. (1996): A Simple Filtration Method to Remove Plankton-Associated Vibrio Cholerae in Raw Water Supplies in Developing Countries. In: Applied and Environmental Microbiology 62, 2508-2512. URL [Accessed: 05.08.2010]. PDF
COLWELL, R.R.; HUQ, A.; ISLAM, M.S.; AZIZ, K.M.A.; YUNUS, M.; KHAN, N.H.; MAHMUD, A.; SACK, R.B.; NAIR, G.B.; CHAKRABORTY, J.; SACK, D.A.; RUSSEK-COHEN, E. (2003): Reduction of cholera in Bangladeshi villages by simple filtration. In: Proceedings of the National Academy of Sciences of the United States of America (PNAS) 100, 1051-1055. URL [Accessed: 05.08.2010]. PDF
SAFE WATER INTERNATIONAL CLEARINGHOUSE (Editor) (n.y.): Sari Filtration. URL [Accessed: 10.07.2010].
HUQ, A.; YUNUS, M.; SOHEL, S.S.; BHUIYA, A.; EMCH, M.; LUBY, S.P.; RUSSEK-COHEN, E.; NAIR, G.B.; SACK, R.B.; COLWELL, R.R. (2010): Simple Sari Cloth Filtration of Water is Sustainable and Continues to Protect Villagers from Cholera in Matlab, Bangladesh. In: mBio 1. URL [Accessed: 05.08.2010]. PDF
CAWST (Editor) (2009): Straining. Fact Sheets - Academic. Calgary: Centre for Affordable Water and Sanitation Technology (CAWST). URL [Accessed: 03.08.2010]. PDF
HUQ, A.; YUNUS, M.; SOHEL, S.S.; BHUIYA, A.; EMCH, M.; LUBY, S.P.; RUSSEK-COHEN, E.; NAIR, G.B.; SACK, R.B.; COLWELL, R.R. (2010): Simple Sari Cloth Filtration of Water is Sustainable and Continues to Protect Villagers from Cholera in Matlab, Bangladesh. In: mBio 1. URL [Accessed: 05.08.2010]. PDF
Further Readings
CAWST (Editor) (2009): Straining. Fact Sheets - Simplified. Calgary: Centre for Affordable Water and Sanitation Technology (CAWST). URL [Accessed: 03.08.2010]. PDF
One-page factsheet containing information on effectiveness, appropriateness, acceptability and costs of straining.
CAWST (Editor) (2009): Straining. Fact Sheets - Academic. Calgary: Centre for Affordable Water and Sanitation Technology (CAWST). URL [Accessed: 03.08.2010]. PDF
A three-page factsheet containing introduction, operation procedure, treatment efficiency, operating criteria and other information related to straining.
GWEP (Editor) (2008): Guinea Worm Cloth Filter. Tamale: Guinea Worm Eradication Program (GWEP). PDF
Factsheet with technology description and relevant information ona manufactured commercial guinea worm cloth filter and its implementation in Ghana.
NWP (Editor) (2006): Smart Water Solutions. Examples of innovative, low-cost technologies for wells, pumps, storage, irrigation and water treatment. Amsterdam: Netherlands Water Partnership (NWP). URL [Accessed: 06.09.2011]. PDF
This booklet on water gives examples of innovations such as the use of sunlight to purify water, effective low-cost water filters, low-cost drip irrigation and locally produced hand pumps that are five times cheaper than imported pumps.
NWP (Editor) (2010): Smart Disinfection Solutions. Examples of small-scale disinfection products for safe drinking water. Amsterdam: KIT Publishers. URL [Accessed: 07.07.2010]. PDF
This booklet, part of the Smart Water Solutions series provides a wide range of methods and products for home water treatment in rural areas.
KAYAGA, S. (2005): Emergency Treatment of Drinking water at point-of-use. Geneva: World Health Organization (WHO). URL [Accessed: 06.08.2010]. PDF
This technical note is about simple treatment methods for drinking water at point-of use for people in an emergency situation. It also includes general steps in water treatment processes undertaken at household level with simple description on straining.
HUQ, A.; YUNUS, M.; SOHEL, S.S.; BHUIYA, A.; EMCH, M.; LUBY, S.P.; RUSSEK-COHEN, E.; NAIR, G.B.; SACK, R.B.; COLWELL, R.R. (2010): Simple Sari Cloth Filtration of Water is Sustainable and Continues to Protect Villagers from Cholera in Matlab, Bangladesh. In: mBio 1. URL [Accessed: 05.08.2010]. PDF
Follow-up study conducted after five years in Matlab, Bangladesh to determine the impact of simple sari cloth filtration on the incidence of cholera. Analysis of the data showed that 31% of the women used filters, of which 60% are sari filters for household water, reducing the incidence of cholera by 48%.
Case Studies
COLWELL, R.R.; HUQ, A.; ISLAM, M.S.; AZIZ, K.M.A.; YUNUS, M.; KHAN, N.H.; MAHMUD, A.; SACK, R.B.; NAIR, G.B.; CHAKRABORTY, J.; SACK, D.A.; RUSSEK-COHEN, E. (2003): Reduction of cholera in Bangladeshi villages by simple filtration. In: Proceedings of the National Academy of Sciences of the United States of America (PNAS) 100, 1051-1055. URL [Accessed: 05.08.2010]. PDF
This research was carried out in Matlab, Bangladesh in collaboration with the International Centre for Diarrhoeal Disease Research, Bangladesh. They found significant filtering of zooplankton (i.e. copepods) and colonial phytoplankton from household water by nylon and sari filtration, as well as reduction of cholera cases in the studied villages.
HUQ, A.; XU, B.; CHOWDHURY, M.A.R.; ISLAM, M.S.; MONTILLA, R.; COLWELL, R.R. (1996): A Simple Filtration Method to Remove Plankton-Associated Vibrio Cholerae in Raw Water Supplies in Developing Countries. In: Applied and Environmental Microbiology 62, 2508-2512. URL [Accessed: 05.08.2010]. PDF
Research conducted on removal of plankton and associated particulates by filter constructed from nylon net and sari materials. The result obtained indicates that 99% of Vibrio Cholerae, i.e., those cells attached to plankton was removed from the water sample during simple filtration, forepidemics strains from various geographical sources including Bangladesh, Brazil, India, and Mexico.
HUQ, A.; YUNUS, M.; SOHEL, S.S.; BHUIYA, A.; EMCH, M.; LUBY, S.P.; RUSSEK-COHEN, E.; NAIR, G.B.; SACK, R.B.; COLWELL, R.R. (2010): Simple Sari Cloth Filtration of Water is Sustainable and Continues to Protect Villagers from Cholera in Matlab, Bangladesh. In: mBio 1. URL [Accessed: 05.08.2010]. PDF
Follow-up study conducted after five years in Matlab, Bangladesh to determine the impact of simple sari cloth filtration on the incidence of cholera. Analysis of the data showed that 31% of the women used filters, of which 60% are sari filters for household water, reducing the incidence of cholera by 48%.
Training Material
CAWST (Editor) (2008): Household Water Treatment Manual. Calgary: Centre for Affordable Water and Sanitation Technology (CAWST). PDF
This manual on household water treatment system includes various useful topics on safe water and multi barrier approach (source protection, sedimentation, filtration, disinfection and safe storage).
Important Weblinks
http://www.who.int/ [Accessed: 21.07.2010]
This web link informs ondifferent program and research activities carried by the World Health Organisation on household water treatment options along with some information on ceramic filters.
http://news.bbc.co.uk/2/hi/health/2640307.stm [Accessed: 06.08.2010]
Link to the website of British Broadcasting Cooperation, which has covered an interesting news on cloth filter and its relation to cholera death with views of an expert.
http://www.africadev.org.uk/ [Accessed: 06.08.2010]
Website of Africadev - Self Help Developments for Africa, containing information on low cost water treatment methods.