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3.1 Conventional sewerage

Conventional sewerage is also termed deep sewerage. This term results from the fact that in actual practice the sewerage pipes are laid deep beneath the ground. There are a number of reasons for the relatively great depth of the pipes. A minimum velocity is needed to ensure that self-cleansing conditions occur at least once daily (usually 0.75 m/s). Combined with a minimum specified diameter (usually 150 mm internal diameter), the outcome is the requirement of steep gradients for the pipes. Added to this is the specification for a minimum depth of buried pipes to avoid interference with road traffic and other services (minimum of 0.9 to 1.2 m). Main sewerage trunks are therefore generally quite deep if gravity is relied upon as the driving force for flow. Figure 2.11 shows a typical layout for a deep sewerage system.

Pumping is generally required at various stages of the sewer pipe network, especially if the landscape is fairly flat. The larger the population served by the sewerage system, and the longer the planning horizon is to cope with future population increases, the larger the diameter of the final pipes becomes. The costs of the pipes, inspection manholes, pumps and pumping stations and their construction/installation are therefore high. The costs of operation and maintenance are correspondingly high.

The design procedure for conventional sewerage is well developed from its early beginnings in the provision of sewerage in the city of London and other European cities. It is now acknowledged that the design procedure for the conventional sewerage is based on very conservative assumptions.

Figure 2.11: Sewerage system for the city of Osaka (1) (lager image)

Figure 2.11: Sewerage system for the city of Osaka (2) (lager image)

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