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An example quoted was the sewered septic tanks or common effluent drains in South Australia. These arise where during the initial development, houses are served by septic tanks and soakaways. At a later stage, these septic tanks are connected to a small bore sewerage system, with economy over a conventional installation. Design data quoted from South Australia were as follows: |
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3 times the average dry weather flow |
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100 mm diameter at 1 in 150
150 mm diameter at 1 in 250
200 mm diameter at 1 in 300 |
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Pipes were required not to flow more than half full at any time. Annual flushing was recommended. It is understood that these systems operate very satisfactorily. The saving in construction cost relates to the use of smaller and shallower pipes than for conventional sewerage. However, the velocity and gradient requirements quoted are still close to conventional sewerage criteria, and do not take full advantage of the solids-free nature of the sewage. The required half full limit, appears similar to the UK building drainage requirement (BS8301: BSI, 1985) that the peak flow in a foul drain should not exceed a proportional depth of 0.75 to ensure adequate air flow and avoid the risk of induced trap siphonage. However, this conflicts with the possible use of the inflective gradient approach. |
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In some flat areas of the UK, where provision of conventional gravity sewers is difficult and costly, vacuum systems have been employed. Stanley and Mills (1984) show cost savings of over 20 per cent, while others (Ashlin et al., 1991) quote more modest savings. Satisfactory operation relies on effective elimination of surface water flows. |
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Although these results appear to show a promising application of unconventional sewerage particularly in flat regions, and also |
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