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the particular drainage basin is estimated by assuming five residents per dwelling unit. If the saturation population method is not used, a maximum design period of 20 years is recommended. Although the service life of the sewers can be expected to be greater than 20 years, this shorter design period minimizes problems associated with uncertainty of forecasting population growth and water consumption and the high cost of maintaining large sewers with low flows. The shorter design period also avoids large front-end costs, thereby facilitating financing or enhancing the prospects of expanding sewerage service to more areas with a given amount of investment. |
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Design flows are estimated from available water meter readings, per caput estimates and assumed saturation populations. Initial flows are assumed to equal 80 per cent of the metered water supply in the area. Where no metering data are available, a recommended minimum flow in any reach of sewer is 1.5 1/s. Final flows are projected by assuming 120 persons per hectare in urban areas or five persons per dwelling. Per caput flows of 150200 1/day are used. The estimated average wastewater flows are multiplied by peaking factors to obtain an estimate for design. The initial flow is multiplied by 1.5 which represents a minimum day flow. The final flow is multiplied by 1.8 which is the product of the peaking factors for the maximum day (1.2) and the maximum hour (1.5) to obtain an estimate of the instantaneous peak flow. To these estimates, adjustments for potential clear water infiltration are added assuming 0.51 1/s per km of pipe. |
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The tractive tension method, as described above is used. A tractive tension of 1 N/m2 is assumed in the design procedure. Depths of flow are maintained between 20 and 75 per cent of the sewer diameter. |
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