Celia Way and Terry Thomas
12th International Rainwater Catchment Systems Conference, New Delhi, India, 2005
Slow sand filtration is a long-established technique for improving the biological and physical quality of water. In its standard form water is passed at about 200mm/hour down through a bed of sand about 1 metre deep. For use inside a household rainwater tank however we require a much cheaper and therefore shallower filter and we must accommodate the intermittent flow corresponding to water being drawn off for a few minutes several times a day. A tenth-scale model of such a filter was built in the laboratory and used to show that very shallow sand filters (as little as 5cm deep) are effective even when they are operated very intermittently – reductions in faecal coliform count of about 95% were obtained. Some field tests in Uganda then followed which confirmed the effectiveness of the technique. The paper describes the findings and also examines how slow-sand filtration could actually be incorporated in rainwater tank design. filters reported here.