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(EPA, 1991). However, modification of conventional design standards have also been used successfully to reduce construction costs substantially. Modifications in design standards have been based on hydraulic theory, improved materials, satisfactory experience, and acceptable risk. 'Flat grade sewers' are one example which have been in use for approximately 80 years in Nebraska (Gidley, 1987). Because of flat terrain and highgroundwater table areas in Nebraska, conventional sewers were expensive. By reducing the minimum slopes or flow velocities, significant cost savings have been realized in construction (shallower sewers and manholes, less dewatering during construction, fewer lift stations) and in operation (lower pumping costs). Another example is 'condominial' sewerage developed in Brazil. Modifications in conventional design standards include reductions in minimum depth, minimum diameter, minimum slope, and changes in service connections. A third example is 'simplified sewerage', which is the subject of this chapter. |
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11.2
Origin and Development of Simplified Sewerage |
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Simplified sewers were developed in Brazil. They are the outcome of changes in several design parameters, including the standards for minimum diameters, minimum slopes, minimum depths, and the spacing and location of manholes. In addition, it makes use of design periods that are considerably shorter than those used in conventional sewerage. |
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The key impetus for its development was the realization that the application of the conventional design standards was making it difficult to expand coverage to middle- and lower-income communities. As in the USA, the prevailing design criteria were very similar to (and in some cases even more stringent than) those used by Waring in his design of the first separate sewer system in the USA constructed in 1880 (Otis, 1986). The 1880 sewer system, consisting primarily of 150 mm diameter pipes, was designed to carry peak flows at a minimum velocity of 0.6 m/s. Waring argued that, if that velocity was reached at least once a day, the system would remove the 'foul wastes' before they had a chance 'to do mischief' (Waring, 1889). However, to ensure complete removal of deposits, flush tanks were installed at |
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