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The Slow Sand Filter (SSF) is a sand filter adapted for household use. Please note that although commonly referred to as the BioSand Filter (BSF), the BSF terminology is trademarked to one particular design, and this fact sheet encompasses all SSFs.
The version of the SSF most widely implemented consists of layers of sand and gravel in a concrete or plastic container approximately 0.9 meters tall, and 0.3 meters square. The water level is maintained to 5-6 cm above the sand layer by setting the height of the outlet pipe. This shallow water layer allows a bioactive layer (“schmutzdecke”) to grow on top of the sand, which contributes to the reduction of disease-causing organisms. A diffuser plate is used to prevent disruption of the biolayer when water is added. To use the SSF, users simply pour water into the SSF, and collect finished water out of the outlet pipe into a bucket. Over time, especially if source water is turbid, the flow rate can decrease. Users can maintain flow rate by cleaning the filter by agitating the top level of sand, or by pre-treating turbid water before filtration.