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to be a compromise to avoid excessive depth and pumping, and not necessarily to achieve a self-cleansing regime. Where topography permits, steeper gradients are preferred in conventional practice. |
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8.3.5
Improved Separate Systems and Source Control |
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Developments in sewerage systems in Europe are likely to reflect the requirements of the Urban Waste Water Treatment Directive. Discharges from combined sewer overflows, together with the polluted nature of surface water runoff in urban areas, often produce an adverse environmental impact on water courses. Construction Industry Research and Information Association (1992) describes scope for control of urban runoff, by methods such as infiltration techniques. Proposals in the Netherlands include an 'improved' separate system, whereby the more polluted portion of flow from the stormwater system is accepted into the foul system for eventual full treatment. These developments often appear more costly to the individual householder or developer, unless considered in an overall environmental sense. |
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8.4
Unconventional Sewerage. |
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The review of conventional sewerage above shows how practice has evolved, and is likely to continue to develop, both at the level of overall concepts, and concerning detailed design criteria. Similarly, there may be the need for practice to be modified in different situations. Cotton and Franceys (1991) discuss the provision of infrastructure for low-income sites in developing countries, and report successful sewerage design using a minimum velocity criterion of 0.5 m/s. Pickford (1995) summarizes unconventional methods of sewerage whereby a saving in capital cost has been achieved, by one or more of the following: |
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using smaller diameter pipes, |
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laying pipes at flatter gradients; |
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