Development and evaluation of multistage filtration plants : an innovative, robust and efficient water treatment technology

The publication addresses the issue 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 suggested may be applicable in other parts of the world. The central hypothesis of the thesis is that the appropriate combinations of different types 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 water supply is safe. The hypothesis was initially tested in the Cauca River valley on a 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 on the 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. 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. 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 were found to be more appropriate and economical than so-called 'conventional' water treatment systems for small to medium-sized rural and urban communities.