Filters

Bio Sand Filter

As of 2013 our favorite water filter has to be the BSF. It is easy to use, durable, effective and not very expensive. Below is information provided by CAWST (the Centre for Affordable Water and Sanitation Technology), an awesome Canadian organization founded by the inventor of the Biosand Filter.

What is a Biosand Filter?
A biosand filter (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 pathogen Biosand Filter Illustration

s 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.Pathogens – micro-organisms in water that make us sickSuspended Solids – dirt and other small pieces in the water (may also be called “turbidity”) Description of BSF Parts

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.

Removal_Mechanisms
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. CAWST has a summary of laboratory and field studies for the biosand filter:
Biosand Filter Studies – Literature Summary.

The biosand filter is best used as one step in a multi-barrier approach to safe drinking water. Different treatment methods remove different things from contaminated water. Although the water may look clear after filtration, there may still be some bacteria and viruses in the water. It is necessary to also disinfect the filtered water to ensure the safest drinking water possible. The most common low-cost methods used around the world to disinfect drinking water are:

Chlorine (liquid or tablet) Solar disinfection (SODIS) Boiling

Chlorine SODIS Boiling