Multi-stage filtration for developing world surface water treatment

Abstract

Gravel pre-filters and slow sand filters are robust and reliable unit processes with the potential for broad small community application in the developing world as they are able to provide physical and microbiological treatment. A multi-stage system comprising gravel pre-filters, slow sand filters and terminal disinfection conforms with the multiple barrier concept in relation to pathogen removal. Employing upflow gravel pre-filters as a form of pre-treatment also considerably reduces the risk of the slow sand filter blocking as a result of the surface straining of particles present in the raw water. The performance of a multi-stage treatment system, comprising shallow upflow gravel pre-filters and fabric-enhanced slow sand filters, was investigated both in service and during cleaning operations in a UK-based research project. The pre-filters provided microbial and physical treatment capacity and attenuated peaks in the raw water to the benefit of the slow sand filter. The slow sand filter achieved high levels of pathogen removal in addition to providing further physical treatment. The novel raised floor configuration adopted for the upflow pre-filters enabled high cleaning efficiencies to be achieved by rapid draindown. In parallel with the research work, a multistage system was constructed at Nyabwishongwezi, Rwanda, to provide a supply for new communities settling in the northern section of the Akagera Game Park following the 1994 genocide and war. The Nyabwishongwezi Treatment Plant and Supply Network Project was promoted by the United Nations High Commissioner for Refugees, Oxfam and Minitrape, Rwanda. It was constructed broadly in keeping with the advice given to the parties promoting the project by the Centre for Environmental Health Engineering (CEHE), University of Surrey, during research visits to Rwanda in the late 1990s. In October 2003, CEHE research staff visited Nyabwishongwezi to support the Rwandan organisation (the Regie) operating the multi-stage plant and distribution system. The multi-stage system was found to be achieving substantial levels of microbial and physical improvement of the raw water obtained from the River Umuvumba.