Abstract
1. This thesis addresses the major problem of providing 'safe' drinking water to small rural and urban populations, which are dependent upon polluted surface water sources. The problem is addressed in the context of the Andean highlands of Colombia and based on gravity flow systems, but the solutions proposed may be applicable in other parts of the world. 2. The central hypothesis of the thesis is that the appropriate combinations of different type of filtration stages provide a uniform quality of water of low risk to consumers. Where institutional and community development make chemical disinfection constantly reliable, the addition of low dose terminal disinfection should be enough to ensure that the drinking supply is safe. 3. The hypothesis was initially tested in the Cauca River valley at pilot scale. A range of designs of upflow, downflow and horizontal flow coarse gravel filters followed by slow sand filtration were compared in an intensive evaluation for their ability to remove key water quality parameters. A novel dynamic fine gravel filter was developed by the author and introduced in advance of the coarse gravel filters to cope with peaks of suspended solids. 4. Highly improved filtrates were achieved during the pilot scale trials and detailed comparative analyses demonstrated the strengths and weaknesses of all component stages. Bacteriological and turbidity removals were particularly impressive compared with earlier studies in Peru and elsewhere. The most efficient combinations of filtration stages consistently achieved five-log faecal coliform removal and this contributed to the full-scale application of the technology within the region. 5. Full scale systems in a range of locations in the Cauca Valley, and elsewhere in Colombia, have demonstrated convincingly that the combination of dynamic gravel filtration, single or multistage coarse gravel filtration and slow sand filtration provide a robust, reliable and efficient technology. It can be successfully operated and maintained at community level, and even without terminal disinfection, multistage filtration provides a consistently low risk drinking water. The inclusion of terminal disinfection provides a fail-safe barrier against occasional high peaks of pollution. 6. The work carried out during the course of this thesis has established multistage gravel filtration as an essential technology when dealing with polluted waters of highly variable quality, and more appropriate and economical than so-called 'conventional' water treatment for small to medium size rural and urban communities. 7. The research has introduced and established dynamic fine gravel filters as a vital and economical component in multistage filtration, which is capable of dealing with occasional high peaks of pollution within established routines of maintenance. 8. The studies on full-scale systems over the past 5 to 10 years have demonstrated that the technology is sustainable in the hands of local operators as a result of a participatory and joint- learning project approaches. 9. The limits of the technology with respect to extremes of raw water quality have been defined. A selection procedure to identify and combine filtration stages in a treatment plant is proposed. It is based on a set of treatment objectives, contamination levels in raw water sources, and removal efficiencies expected at each treatment stage.