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Straining and Filtration

Author/Compiled by
Raju Shrestha (Environment and Public Health Organization (ENPHO) )
Dorothee Spuhler (seecon international gmbh)
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.

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
In Out

Freshwater

Drinking Water

 

A woman using a sari cloth to strain water. Source: AFRICDAY (2009)

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:

Helminths

Protozoa

 

Turbidity

Bacteria

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)

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.

Library references

Dracunculiasis — Guinea Worm Disease

DPD (2008): (= Parasitic Disease Information Factsheets ). Atlanta: Centers for Disease Control and Prevention, Division of Parasitic Disease (CDC DPD) URL [Accessed: 28.07.2010]

Reduction of cholera in Bangladeshi villages by simple filtration

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) In: Proceedings of the National Academy of Sciences of the United States of America (PNAS). 100 ,1051-1055 . URL [Accessed: 05.08.2010]

Straining. Fact Sheets - Academic

A three-page factsheet containing introduction, operation procedure, treatment efficiency, operating criteria and other information related to straining.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets - Academic ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]
Further Readings

Straining. Fact Sheets - Simplified

One-page factsheet containing information on effectiveness, appropriateness, acceptability and costs of straining.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets - Simplified ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

Straining. Fact Sheets - Academic

A three-page factsheet containing introduction, operation procedure, treatment efficiency, operating criteria and other information related to straining.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets - Academic ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

Guinea Worm Cloth Filter

Factsheet with technology description and relevant information ona manufactured commercial guinea worm cloth filter and its implementation in Ghana.

GWEP (2008): (= Household Water Treatment and Safe Storage (HWTS) Product and Implementation Fact Sheet ). Tamale: Guinea Worm Eradication Program (GWEP)

Smart Water Solutions

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 (2006): Examples of innovative, low-cost technologies for wells, pumps, storage, irrigation and water treatment. (= Smart water solutions ). Amsterdam: Netherlands Water Partnership (NWP) URL [Accessed: 06.09.2011]

Smart Disinfection Solutions

This booklet, part of the Smart Water Solutions series provides a wide range of methods and products for home water treatment in rural areas.

NWP (2010): Examples of small-scale disinfection products for safe drinking water. (= Smart water solutions ). Amsterdam: KIT Publishers URL [Accessed: 07.07.2010]

Emergency Treatment of Drinking-Water at the Point of Use. Technical Notes on Drinking-Water, Sanitation and Hygiene in Emergencies

Normally, drinking water supplies need to be treated during and after an emergency to make them safe and acceptable to the user. This technical note describes some of the most common and simple treatment options suitable for use during an emergency.

KAYAGA, S. ; REED, B. ; (2013): (= Technical Notes on Drinking-Water, Sanitation and Hygiene in Emergencies , 5 ). Geneva: World Health Organization (WHO) URL [Accessed: 26.08.2013]

Conservation et Traitement de l Eau a Domicile

This practical guide provides a review of different processing techniques and adequate water conservation at home and is structured around 10 key questions that should be posed before choosing a suitable solution.

DESILLE, D. ; (2013): Paris: Programme Solidarite Eau (PSeau) URL [Accessed: 06.06.2013]
Case Studies

Reduction of cholera in Bangladeshi villages by simple filtration

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) In: Proceedings of the National Academy of Sciences of the United States of America (PNAS). 100 ,1051-1055 . URL [Accessed: 05.08.2010]
Training Material

Household Water Treatment Manual

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).

CAWST (2008): Calgary: Centre for Affordable Water and Sanitation Technology (CAWST)

Alternative Versions to

No Structure Described.