This is a very general overview of how the filter works. For a fully detailed description, refer to CAWST website.
It is very important to understand that a properly constructed, installed and maintainer Bio Sand Filter is a living ecosystem. On no account should the user pour water into the filter that has been treated chemically. That of course includes water treated with chlorine. The bio layer will die rendering the filter significantly less efficient.
Dr. David Manz developed the household Bio Sand Filter in the 1990s at the University of Calgary, Canada. He also co-founded CAWST in 2001 to help support the professional services needed for the humanitarian distribution of the filter in developing countries. The Bio Sand Filter is a very effective, inexpensive and sustainable “technology” when constructed and installed correctly.
Water naturally contains many living things. Some of these living things are harmless and others can make people sick. Living things that cause disease are also known as pathogens. They are sometimes called other names, such as micro-organisms, microbes or bugs. Viruses, Protozoa and Helminths (worms) all come under the heading bacteria. Contaminated water may contain hundreds or thousands of pathogens per litre.
The physical characteristics of drinking water are usually things that we can measure with our senses: turbidity, colour, taste, smell and temperature. Turbid water looks cloudy, dirty or muddy. Turbidity is caused by sand, silt and clay that are floating in the water. Drinking turbid water will not make people sick by itself. However, viruses, parasites and some bacteria often attach themselves to the suspended particles in water. This means that turbid water usually has more pathogens. Drinking turbid water increases the chances of becoming sick.
The Bio Sand Filter can remove almost all the protozoa and worms, 98% of bacteria, and more than 70% of the viruses. [Source CAWST]
In the BSF, small microbes live in the top area of the sand. This is called the BIO LAYER. The bio layer is very important for making the water safe to drink. The bio layer takes about 30 days to grow. This 30 day period is required as a once off, only when the filter is installed for the first time. If the filter is used correctly is does not happen again. This ecosystem is the heart of the Bio Sand Filter.
The bio layer is the key component of the filter that removes pathogens. Without it, the filter removes about 30-70% of the pathogens through mechanical trapping and adsorption. The ideal bio layer will remove up to 99% of pathogens.
The bio layer is NOT visible – it is NOT a green slimy coating on top of the sand. The filtration sand may turn a darker colour, but this is due to the suspended solids that have become trapped.
When constructed, installed and used correctly the Bio Sand Filter will provide very good quality drinking water. It can be used in homes, schools and clinics. The filter is made from concrete and measures about 92cm high x 305mm diameter (inside) the wall thickness is about 50mm. The BSF used in our program in Cameroon is the round concrete JO-1 Design. It is filled with layers of specially prepared sand and gravel that go inside the concrete filter housing.
Health Promoters inform householders about the benefits of the filter as they visit members of the community.