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sea from 1998, is the planned provision of facilities for sludge incineration. |
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Bazalgette's original design data for London may be summarized as follows: |
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108 million gallons (23 700 m3) per day (24 hours) |
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286 million gallons (62 900 m3) per day (24 hours) |
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The sewers were constructed to carry one-half of that total quantity in 6 hours of the day. In other words, a peaking factor of 2 was allowed. Rainfall in excess of a quarter of an inch per day (approximately 6 mm/day) was diverted to the river. It was estimated at the time that the capacity of the sewers would not be exceeded on more than 12 days in a year. The system was designed as combined sewerage on cost grounds, although there were those who would have favoured separation of the foul and surface water flows. |
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At the turn of the century, a wide ranging presidential address to the Institution of Civil Engineers (Hawksley, 1901) included comment on the separate system of drainage, which was considered not only costly, but also 'in most cases undesirable, inasmuch as the washings from the surfaces of the streets must necessarily be of a foul nature so long as animal traction remains in use, whatever may be the case when, if ever, the employment of horses is entirely superseded by mechanical motors'. |
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With rapid and continuous growth of towns and cities in the UK, partially separate systems were developed which allowed the flow from back roofs and yards to be collected with the domestic sewage, and for storm water from other areas to be collected separately. Lloyd-Davies (1906) described improved methods of calculating such flows, and a Mr Fowler in discussion on that paper mentioned serious problems with flooding in Leeds. He advocated the use of leap-weirs to separate a proportion of the flood flow, and mentioned the benefits of traditional egg-shaped sewers, to give a better dry-weather-flow velocity. The type of overflow structure he described is illustrated, although no longer recommended, in a recent guide to the design of combined sewer overflow structures (Balmforth et al., 1994), and it is noted that precast concrete ovoid sections complying with current standards have recently become commercially available in the UK. |
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