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tion is made that the sewer carries a flow of 6 × dry weather flow when running full). Analysis suggests that, with a Ks value of 6.0 in the ColebrookWhite equation, these gradients give maximum dry weather flow velocities ranging from about 0.45 m/s for 10 houses, through about 0.5 m/s for 40 houses and 0.55 m/s for 100 houses to 0.6 m/s for 200 houses. These velocities are lower than those which are normally specified but they assume a more realistic Ks value as locally made pipes may have a higher friction coefficient than normally assumed (the internal surfaces of Indian and Egyptian clay pipes are often decidedly rough, whereas Pakistani spun concrete pipes are smoother but those produced by small informal sector casting yards often have a rather uneven finish). |
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Studies in the UK (Lillywhite and Webster, 1979) suggest that gradient is not the main determinant of sewer blockages, which are affected more by the condition of the sewer and in particular of its joints. Poorly made joints are likely to cause blockages. However, the relevance of these studies, which were presumably conducted in relatively clean lengths of sewer, to the conditions found in South Asian sewers is debatable. Field observations over a number of years suggest that most conventional systems in Pakistan are obstructed to a greater or lesser extent, creating flow conditions which are certainly not self-cleansing and would be extremely difficult to represent theoretically. Solid waste and debris are common causes of obstruction, but another important factor is a build up of silt and sand in the inverts of sewers over time. Some of the sand may be washed into sewers from kitchens where it is used to clean utensils, but it is probable that the majority of the material is silt washed into the sewer with rainwater. Consideration of all these points leads to the conclusion that it is probably impossible to design sewers in low-income urban areas which are both maintenance-free and affordable. Some compromise between these two objectives will always be necessary. |
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This is perhaps a subject for further study but one response is to conclude that sewers should be designed so that the settlement of solids is concentrated at locations from which they can relatively easily be removed. Interceptor tank systems, designed to remove solids before flows enter the sewer, provide one means of achieving this objective. Another possible approach, which has been observed in both community and municipality built sewers |
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