A pre-filter is placed on top of the unit to remove most of the sediment. This is a simple pan with small nail holes in the bottom to allow the water, but not the sand, to pass. The sand from this filter can easily be removed and washed, protecting the larger filter. The slow sand filter actually ‘eats’ bacteria and viruses as they pass through. To do this, it grows algae on its surface (like you see on a sandy stream bed). The water must travel through a minimum of 75cm of sand to be effective. The outlet must also be above the level of the sand to make sure it is always under water.
Ordinary sand is used, though it needs very thorough washing. The filter is made of galvanised tin readily available in the bazaar. Other materials such as pottery could be used. Once a working design was settled on, several were placed in homes to see if there were social, cultural or other problems. Several improvements were suggested which are now included into the design.
This model is sold for US $11 (700,000 Afs) – the equivalent of five days’ wages for a manual labourer, so is quite affordable. The filter was taken to the metal workers’ bazaar where any ordinary tin smith had the ability and equipment to construct it.
The sand filter was promoted using video and on television. Filters were also placed in various public places around the city. Positive demand has brought income for the metal workers (SERVE are not involved at all in producing the filter) and hopefully effective water treatment will become accessible to everyone.
Brett Gresham is a civil engineer working for SERVE, an international NGO, on various appropriate technologies in northern Afghanistan. Their address is SERVE, PO Box 477, University Town, Peshawar, Pakistan.
Detailed instructions are available from Footsteps for anyone wishing to build this filter.