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where A = i/t (i and t in minutes) |
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B = h/i (h and i in hours) |
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Crn = number of possible combinations of r units of a total of n |
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i = interval between successive discharges |
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t = duration of the discharge |
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h = duration of peak discharge |
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The equation was adapted to take account of the following considerations. Although the influence of toilet flushings on the maximum rate of discharge from interceptor tanks is small, they determine the actual maximum value, so the interval between successive uses of the toilet was taken as i. The maximum discharge value prevails only for an instant, but to be conservative the approximate time for the tanks to evacuate the volume of one flushing was adopted for t values. Values for h and i were based on observations and surveys. The curve of simultaneity for 2-hour peaks, i = 10 min and t = 1 min, is shown in Figure 10.1. Sensitivity analysis with different values for t and i, and with h varying between 1 and 2 hours produced very similar values for r/n. This means that the discharge loads and the capacity of the sewers computed by this method will normally not be exceeded. |
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As can be seen in Figure 10.1, the curve tends to be asymptotic to 23.5 per cent. In fact, with t = 30 seconds, it goes closer to 23 per cent and for values of t approaching 1 second the curve tends to drop to 4 per cent. It was considered too risky to use these low simultaneities, even though in reality the duration of maximum discharge from interceptor tanks lasts only a matter of seconds; so the more conservative values shown in Figure 10.1 were adopted for ASAS design. |
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Although sewers are usually laid above the watertable, infiltration flows are added to the design load with rates of 0.1 1/s per ha for vitrified clay pipe and 0.05 1/s per ha for plastic pipe. Storm drains are not permitted to be connected to tanks or sewers, but 0.5 1/s per ha is also added to design flows. These rates are half those specified in Cartagena for conventional sewers. It would be feasible to reduce the values for infiltration flows even more but, because the risk of unauthorized connections cannot be ruled out, conservative values are recommended. |
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