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Therefore, during peak hours all fixtures will be in use and the greatest possible discharge rate will occur when a toilet is flushed. Like all natural phenomena, the likelihood of several households producing their maximum discharge exactly at the same time is ruled by the law of probabilities. The simultaneity is determined by the intermittence in the use of the toilet. Because the statistical probability of exceeding the design load is low, sewers can be designed to flow full to obtain small diameters. |
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10.3.3
Criteria for Interceptor Tanks |
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As with the sewers, the design of the interceptor tanks depends on social considerations and cost and efficiency criteria. It is more convenient to adopt one size of tank for the unit house, rather than to have several sizes of tank depending on the number of occupants in each household. The size of the unit tank is determined according to the optimum period for desludging taking into consideration construction costs (this was 6 years for the Cartagena, Granada and San Zenon projects). As both construction and maintenance costs must be paid by the householders, the lowest cost must be calculated in terms of monthly rates. The monthly rate must cover construction costs and desludging costs. |
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10.3.4
Minimum Cost Parameters |
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The minimum cost of the system corresponds to the smallest possible interceptor tanks and the smallest possible sewer diameter. To determine that cost, iterative calculations are necessary because, while both the tank size and the sewer diameters are directly related to rates of flow and the solids characteristics, the dimensions of the tank themselves influence the rate of flow due to the tank's attenuating action on toilet flush-water flows and the characteristics of unretained solids. |
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Computations made in the Cartagena study, working with discrete particles of clay or lime (which are the commonly encountered materials in the areas under study) with a specific gravity of 2.65, enabled us to arrive at a set of basic parameters for the minimum cost of tanks and sewers. Further verification |
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