1. Why multi-stage filtration?

In many developing countries, sanitary risks associated with surface water are increasing due to poor protection of water sources and inadequate wastewater and solid waste management. Surface water may show unpredictable and erratic changes in quality due to run-off during rainy periods. This is a critical problem in some Andean and Caribbean countries where 80% of water supply systems depend on surface water, which may require treatment to remove or reduce disease causing organisms.

Traditional approaches with chemical treatment or slow sand filtration (SSF) can be highly effective in removing pathogens. However, many SSF systems do not perform well because they become clogged by the amount of suspended material in the water, resulting in the need for frequent cleaning and the ultimate abandonment of many systems. Rapid sand filtration (RSF) requires complex levels of chemical treatment, making it an unsatisfactory solution for rural communities and small towns.

Multi-Stage Filtration (MSF) is an integrated solution for improving community water supply in rural and peri-urban areas. It uses a combination of coarse gravel filtration (CGF) and SSF, with two or three separate stages in the cleaning of the water.

This combination allows the treatment of water with levels of contamination well above that which can be treated by SSF alone. MSF retains the advantages of SSF as a robust and reliable treatment that can be maintained by operators with low levels of formal education. It is better suited than chemical water treatment to rural communities and small and medium-sized towns in developing countries and in more remote areas in industrialised countries. Other treatment processes such as sedimentation, sand traps and screens can precede MSF systems. Wherever possible, terminal disinfection should be included as a safety barrier after MSF.

MSF was developed in Latin America in the 1980s by the Cinara Institute of Valle University (CINARA) in Cali, Colombia, as part of an SSF research and development project, supported by IRC. However, despite its success, and efforts in the 1990s to develop and transfer this technology, it remains unknown in many countries. MSF is perhaps the only solution for many community water supply systems, which can be converted into MSF systems for better performance.

This Thematic Overview Paper (TOP) provides the basis for sector professionals to become aware of MSF technology and to learn more about it. It examines why MSF is a viable technological option and explains its advantages and disadvantages. One important lesson is that it is better, cheaper and easier to keep a water source clean than to implement complex systems to clean it up once it is polluted.

This TOP allows policy makers and lay people to understand the important principles of MSF, and gives sufficient detail to meet the information needs of specialists and water engineers. It looks at the technology, the processes taking place beneath the surface, cost issues, research, and the importance of achieving access to safe water. It also presents some compelling case studies of the technology in action.

Multi-stage filtration takes a proven technology of the past and makes it relevant to the present and future. It is fit for its purpose and ecological in its outlook. The TOP shows how and why.