Practical experiences at 5 slow sand filtration plants in South Africa

Slow sand filtration (SSF) is an effective physical and biological technology that employs fine sand beds and low filtration rates to treat polluted and contaminated raw water. Its effectiveness lies in the capabilities of the Schmutzdecke (a thin dirty layer formed at the top of the sand bed) to trap suspended solids as water flows through, and to support organisms that kill pathogens. SSF is simple in design and construction and it usually uses locally available labour and materials. The operation and maintenance are simpler than that for high-rate filtration plants. However, in practice SSF do not benefit for the potential advantages. The aim of this paper is to discuss the practical application of experiences of SSF in South Africa. It is based on findings from visits to some SSF plants. These are compared to the theoretical expectations to draw out some learning points.