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Water treatment

Unit processes or water

Unit processes or water treatment plant efficiencies are meaningless without reference to the quality of the raw and treated water. For this reason WHO (1993) provides not only water quality guideline values but also includes an example of performance efficiencies and treatment objectives for a multistage plant for small WS systems (table 2.15) based on some results of the present research work and other demonstration projects (Lloyd et al, 1991). This multistage plant consists of screening, plain sedimentation, CMF, SSF, and disinfection. Table 2.15 An example of performance objectives for removal of turbidity and faecal coliform bacteria in medium and small scale treatment plants (Lloyd et al, 1991; WHO, 1993) Turbidity Stage and process Removal 1 Average Maximum loading loading (%) (NTU) (NTU) Screening NA 2 NA NA Plain Sedimentation 50 60 600 Coarse (Gravel) filters 3 80 30 300 Slow Sand Filter >90 6 60 Disinfection NA

full-scale units. In this research, narrow filtering cells are used for preliminary or short-term work and bigger pilot or full-scale units are used for medium to long-term studies. Practically, all the reviewed work on CMF of natural waters is concentrated on suspended solids or turbidity and some on coliform removals. There is little consideration of other parameters such as iron, manganese or natural organic matter, which can be relevant to the behaviour of the SSF (table 2.6), the acceptability of the treated water by the community (sections 2.2 and 2.3; table 2.3), or the terminal disinfection reliability. This study includes work on suspended solids, turbidity and faecal coliform removal, together with considerations on other parameters relevant to the behaviour and application of the MSF technology. The appropriate development and transfer of the MSF treatment plants has been limited by the lack of side-by-side comparisons of the CMF stages of this technolgy for specific contaminant removal. "These on-site studies greatly reduce confounding influences induced by raw water quality variations and will make treatment technology comparisons more meaningful and equitable" (Collins, 1999). In fact, the limited amount of data with polluted natural surface waters together with the differences in raw water sources, design and O&M procedures makes it difficult to make comparative analyses of the different MSF options. This study includes work that allows comparative analysis of the DyGF (section 2.8.3) as the 1 st filtration stage under different operating conditions and the CMF alternatives (sections 2.8.4, 2.8.5, and 2.8.6) as the 2 nd filtration stage under similar design, operation and maintenance conditions and working in series with SSF. The results obtained will contribute to the understanding of the CMF technolgy and to identify the best combinations of CMF alternatives with SSF for different raw water quality ranges. The experience at bench and pilot scale is essential to obtain frequent and reliable data. However, it needs to be verified or complemented with observations on full-scale projects, which are built and maintained under representative conditions of the context in which the technology is being applied or transferred. This study includes long term monitoring of projects operated by local caretakers with low levels of schooling, under the supervision of community based organisations with some technical and managerial support. Finally, a few studies include some cost considerations on specific application of CMF alternatives but they do not allow comparative analisys. The present study includes cost considerations about construction, and O&M of the MSF technology to facilitate capital and running cost estimates, which should contribute to the selection of the more sustainable solutions under local conditions. 67