Rational design of domestic biosand filters

A biosand filtration (BSF) unit is an intermittently operated slow sand filter designed for household use. This paper reviews the practical application of BSF, identifies the important design considerations and proposes a systematic design procedure. The media properties, water requirements, filter cycle time and water temperature are identified as the most important design input parameters. The resultant specifications are the water dosage volume, water production rate and media bed dimensions. We propose two parameters for characterising the filtration rate, namely the initial and average clean bed filtration rate. Mathematical expressions for these two parameters and the filtration time are derived. Guideline values for the filtration rate and the ratio of the pore volume to the water dosage volume are established and used as design checks. It is noted that the filtration rate is determined solely by the properties of the water temperature and the media—customary constraints posed by the bed area and the bed depth had been eliminated. Therefore the heart of BSF design lies in the careful and appropriate selection of the filter media. The design procedure proposed is illustrated with an example for a typical rural household in Venda, South Africa.