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perhaps where water is scarce, the relatively small cost reductions do not put vacuum sewerage in the low-cost category. |
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Typical design criteria used for vacuum sewerage are as follows: |
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0.2 per cent in flow direction, with regular lifts |
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Similar to conventional UK sewerage |
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The sewage may be conveyed at velocities
of about 5 m/s to the vacuum collection
vessel. |
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In developed countries, the original benefits to public health from a comprehensive sewerage system may often be taken for granted. Public concern has perhaps shifted towards wider environmental considerations. Operations have become automated, and work forces reduced. Performance criteria for design and operation reflect a balance between convenience, control and cost that is judged to be acceptable and affordable to the customers. As these design criteria for foul sewerage have evolved to suit this context, it is argued that they should not necessarily be mindlessly transferred to other situations. |
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Conventional hydraulic criteria for foul sewerage have been described and discussed, with a historical perspective from the UK. It has been demonstrated that the conventionally adopted self-cleansing velocity does not reflect a single absolute theoretical value, but that conditions for sediment movement depend upon the type of sediment and sewer. |
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The hydraulic design of settled or solids-free sewerage has been contrasted with conventional design. The resulting use where appropriate of smaller, shallower pipes that may follow the natural ground contours, should offer scope for providing the main benefits of effective sanitation at a considerably lower cost than conventional sewerage. |
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