Filter Making

Biosand filter (BSF)

BSF is an adaptation of the traditional slow sand filter, which has been used for community drinking water treatment for 200 years. The biosand filter is smaller (about 1 m tall, 0.3 m wide on each side) and adapted so that it does not flow continuously, making it suitable for use in people’s homes. The filter container can be made of concrete or plastic. It is filled with layers of specially selected and prepared sand and gravel. The sand removes pathogens and suspended solids from contaminated drinking water. A biological community of bacteria and other micro-organisms grows in the top 2 cm of sand. This is called the biolayer. The micro-organisms in the biolayer eat many of the pathogens in the water, improving the water treatment.

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1. How Does the Biosand Filter Work ?

You can use any kind of water in the biosand filter – well water, borehole water, pond or river water, tap-stand water, or rainwater. This makes it very convenient for people because they can use whichever water source is closest to home, make it safe to drink. The water must not have been chlorinated though, or the chlorine will kill the biolayer. The water should also not contain any dangerous chemicals, because the biosand filter cannot remove most chemicals from water. Contaminated water is poured into the top of the biosand filter at least once per day (but not continuously). The water poured into the top of the filter slowly drips through the holes in the diffuser, and flows down through the sand and gravel. Treated water flows out of the outlet tube. No power is required - the filter works by gravity. It should take about 1 hour to get 12-18 litres of filtered drinking water. Pathogens and suspended solids are removed through biological and physical processes that take place in the sand. These processes include: mechanical trapping, predation, adsorption, and natural death.

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2. What can the biosand filter remove from water ?

The biosand filter has been studied in the field and in labs. It has been shown to remove the following from contaminated water: Up to 100% of helminths (worms) Up to 100% of protozoa Up to 98.5% of bacteria 70-99% of viruses The filter can also remove up to 95% of turbidity (dirt and cloudiness), and up to 95% or iron (which people often don’t like because it turns water, laundry and food red!). Like other filters, the biosand filter cannot remove dissolved contaminants or chemicals, such as salt, arsenic or fluoride. There is an adaptation of the biosand filter using rusty nails, called the Kanchan Filter, which can remove arsenic from water.