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designs have been based on design flows of 0.006-0.02 1/s per connection. Designs based on these flow criteria have been successful where watertightness of all tanks and piping is thoroughly tested during construction. Also, storage is available in the interceptor tank above the normal water level for household flows for short peak flow periods if they should occur (EPA, 1991). |
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Hydraulic computations for SDGS are the same as those used for conventional gravity sewers. Typically, Manning's pipe flow formula is used with a roughness coefficient (n) of 0.013. However, unlike conventional sewers, SDGS can alternate between open channel and pressure flow. Therefore, separate analyses must be made for each segment of the sewer in which the type of flow does not change. Where pressure flow occurs, the elevation of the energy grade line during daily peak flow conditions must be calculated to determine that no interceptor tank outlet invert lies below the grade line. If not, free-flowing conditions will not be maintained at those connections. Where this is determined to occur, the designer has several options: (a) the elevation of the downstream summit in the sewer can be lowered to lower the grade line; (b) the diameter of the main can be increased to reduce the frictional headloss and velocity head; or (c) a residential lift station or STEP unit can be installed to lift the wastewater into the collector. If short term surcharging above any interceptor tank outlet is expected, check valves on the individual service lateral may be sufficient to prevent backflow. |
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Experience has shown that the normal flows that occur in the systems are sufficient to keep the mains solids-free. The solids that enter the collectors from the interceptor tanks and the slimes which may grow within the sewer are easily carried when flow velocities of 0.15 m/s are achieved intermittently (Otis, 1986). Thus, SDGS need not be designed to maintain minimum flow velocities during peak flows, although many state agencies require that minimum velocities of 0.30.45 m/s be reached daily. |
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The pipe diameter is determined by hydraulic analysis. The minimum diameter is typically 100 mm, but 50 mm has been |
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