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Concrete Biosand Filter

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The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community water treatment for hundreds of years. The BSF is smaller and adapted for intermittent use, making it suitable for households.

Water treatment is carried out by the sand inside the filter. The filter container can be made of concrete, plastic or any other water-proof, rust-proof and non-toxic material.

 

The concrete BSF typically uses a box about 0.9 m tall by 0.3 m square, or about 0.3 m in diameter. The filter box is cast from a steel mold or made with pre-fabricated pipe. The container is filled with layers of sieved and washed sand and gravel (also referred to as filter media). There is a standing water height of 5 cm above the sand layer. Similar to in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the sand surface, which contributes to the water treatment.

 

A perforated diffuser plate or basin is used to protect the biolayer from disturbance when water is poured into the filter.

 

Pathogens and suspended material are removed from the water through a combination of biological and physical processes. These occur both in both the biolayer and within the sand bed. These processes include: mechanical trapping, adsorption/attraction, predation and natural death.

 

[[Image:CAWST_Biosand.png|thumb|right|150px| Cross-section through a concrete biosand filter]]

 

==History and social context==

 

==Suitable conditions ==

It is suitable to process batches of 12-18 litres. If four batches a day are filtered, this provides 24-72 litres of clean water.

{{Potential_Treatment_Capacity_table

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==Technical specification==

[[Image:CAWST_Biosand.png|thumb|right|150px| Cross-section through a concrete biosand filter]] ==What is a Concrete Biosand Filter?==The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community water treatment for hundreds of years. The BSF is smaller and adapted for intermittent use, making it suitable for households. Water treatment is carried out by the sand inside the filter. The filter container can be made of concrete, plastic or any other water-proof, rust-proof and non-toxic material.  The concrete BSF typically uses a box about 0.9 m tall by 0.3 m square, or about 0.3 m in diameter. The filter box is cast from a steel mold or made with pre-fabricated pipe. The container is filled with layers of sieved and washed sand and gravel (also referred to as filter media). There is a standing water height of 5 cm above the sand layer. Similar to in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the sand surface, which contributes to the water treatment.  A perforated diffuser plate or basin is used to protect the biolayer from disturbance when water is poured into the filter. ==How Does It Remove Contamination?=Operation=Pathogens and suspended material are removed from the water through a combination of biological and physical processes. These occur both in both the biolayer and within the sand bed. These processes include: mechanical trapping, adsorption/attraction, predation and natural death. ==Operation==

Contaminated water is poured into the top of the filter on an intermittent basis. The water slowly passes through the diffuser, and percolates down through the biolayer, sand and gravel. Treated water naturally flows from the outlet.

The treated water should be collected by the user in a safe storage container placed on a block or stand, so that the container opening is just under the outlet, minimizing the risk for recontamination.

==Treatment efficiency==

{{Treatment_Efficiency

|lab:bacteria=Up to 96.5% <ref name=buzunis>[[#buzunis95|Buzunis]] (1995)</ref><ref>[[#baumgartner06|Baumgartner]] (2006)</ref>

The taste, odour and colour of filtered water is generally improved. The treated water temperature is generally cooler than water stored in a plastic container.

==Operating criteria==*'''Flow The maximum recommended flow rate:''' <is 0.6 litres/minute. This is the maximum recommended flow rate; the actual flow rate will fluctuate over the filter cleaning cycle and between filters.*'''Batch volume:''' 12-18 litres*'''Daily water supply:''' 24-72 litres. This is based on 4 batches per day (i.e. morning, lunch, dinner, before bed)

A pause period is needed between uses to allow time for the microorganisms in the biolayer to consume pathogens in the water. The recommended pause period is 6 to 12 hours with a minimum of 1 hour and maximum of 48 hours.

==Robustness==There are no moving or mechanical parts to break. The piping is embedded in concrete, protecting it against breaks and leaks. Cracks can sometimes be repaired. Maintenance====

Concrete filters are heavy (70 – 75 kg for thin wall version and 135 kg for heavy wall version). Poor transportation of filters can lead to cracking and/or breakage. Filters should not be moved after installation. ==Estimated lifespan==The estimated lifespan is 30+ years; existing filters are still performing satisfactorily after 10+ years. Lids and diffusers may need replacement. ==Manufacturing requirements== =====Worldwide producers=====Free mold designs are available from CAWST =====Local production=====

Local production of filters is most common. Molds can be borrowed, rented, bought or constructed locally. Filters can be constructed at a central production facility, or in the community. Filter sand and gravel can be prepared (sieved and washed) on-site or nearby.

=====Materials and facilities=====required are

*Steel mold

*Sand, gravel, and cement

*Facilities: Workshop space for filter construction

=====Labour=====

A skilled welder required to fabricate molds. Anyone can be trained to construct and install the filter. Individual householders can assist in constructing their own filters.

=====Hazards=====

Working with cement and heavy molds is potentially hazardous and adequate safety precautions should be used. Concrete filters are heavy and difficult to move and transport.

==Maintenance requirements==Maintenance is required when the flow rate drops There are no moving or mechanical parts to a level that is inadequate for the household usebreak. Swirl and dump maintenance for the top layer of sand The piping is simpleembedded in concrete, takes a few minutes protecting it against breaks and leaks. Cracks can sometimes be done by household usersrepaired. The frequency  Concrete filters are heavy (70 – 75 kg for thin wall version and 135 kg for heavy wall version). Poor transportation of swirl filters can lead to cracking and dump depends on turbidity of inlet water/or breakage. Filters should not be moved after installation.  ====Estimated Lifespan====The outlet, lid estimated lifespan is 30+ years; existing filters are still performing satisfactorily after 10+ years. Lids and diffuser should be cleaned on a regular basisdiffusers may need replacement. ====Suppliers====Free mold designs are available from CAWST

==Cost==

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Note: Program, transportation and education costs are not included.

 

==Country experiences==

 

==Manuals==

 

==Movies==

 

==External links==

[http://www.cawst.org CAWST (Centre for Affordable Water and Sanitation Technology)]

==Footnotes==

==Acknowledgements==

This article is based on a factsheet from [http://www.cawst.org/ Centre for Affordable Water and Sanitation Technology (CAWST)], which is gratefully acknowledged.

 

==External Links==

[http://www.cawst.org CAWST (Centre for Affordable Water and Sanitation Technology)]

==References==

* <cite id=stauber06>Stauber, C., Elliot, M., Koksal, F., Ortiz, G., Liang, K., DiGiano, F., and M. Sobsey (2006). Characterization of the Biosand Filter for Microbial Reductions Under Controlled Laboratory and Field Use Conditions. Water Science and Technology, Vol 54 No 3 pp 1-7.</cite>

 

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