Monday, January 17, 2011

Intro to Biosand filters

I have often mentioned that water and sanitation is a big emphasis in our community development project.  Today I will introduce you to one of the appropriate technologies we are using here for clean water.  I am spending a lot of my time right now working on marketing and maintenance support for these, so I will probably be writing a lot about these.  This is the post you will want to read first, as it will give you the background on the device I call BSF.

The Biosand Filter (BSF) we are using was designed by a Canadian non-profit named CAWST (Center for Affordable Water and Sanitation Technology).  This organization has done an impressive design with research and development of this water filter, and they have diffused the technology in over 70 countries.

The best thing about the biosand filter is that it can be made with materials locally available in even the poorest countries.  There is a great amount of technical science in the specific design of the filter, but once the proper cement forms are made and proper gravel and sand is found, the filters are quite simple to produce, install and use.

The benefit of the BSF being reproducible with locally available materials is that then production and distribution can be sustained without assistance from NGO or other organizations.  If we want people’s access to clean water to remain after our project is gone, we have to avoid making them dependent on our assistance in maintenance and further distribution of filters.

Here is a descriptive diagram of the BSF that I have borrowed from the CAWST website  (  (Click on the picture to zoom in).

Water is poured in the top, passes through a slotted steel diffuser, into the sand.  The top 2-3 inches of sand becomes the bio-layer where predation of pathogens occurs.  Below the biolayer there is mechanical filtering as any remaining pathogens get stuck between the grains of sand.  Finally as the water seeps to the gravel layers at the bottom it flows into the exit pipe with hydraulic pressure that carries it upward.  The outlet pipe then allows the filtered water to drip into clean water containers.  A proper functioning BSF will produce 1 liter of clean water every 5-7 minutes.  If used throughout the day it can provide clean drinking water for a whole family.

That is it for today’s intro.  If you are interested in learning more about the BSF, I encourage you to visit the CAWST page on BSFs (  Next post I will tell you how we have gone about producing and distributing BSFs here in the villages.

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