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surrounding the city of Adelaide in South Australia prompted the Department of Public Health to take corrective action. Three solutions were proposed: (a) restricted water use, (b) cartage, and (c) sewers (South Australia Department of Health, 1968). Restrictions on water use proved to be unsuccessful and the costs of cartage were prohibitive. However, conventional sewers also were too costly. As an alternative, 'common effluent schemes' were recommended based on limited experience in seaside towns. These schemes were small diameter 'drains' used to collect septic tank effluent from the existing septic tanks for subsequent treatment in oxidation ponds. They were designed with continuous negative gradients similar to conventional sewers. Cost-saving features were smaller conduit size, reduced pipe gradients, fewer manholes, lower pumping costs, and less operation and maintenance (South Australia Department of Health, 1968; Gutteridge, Haskins and Davey Ltd, 1977). |
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The first scheme was constructed in Pinnaroo, South Australia, in 1962. By 1986, over 80 schemes had been constructed. The largest scheme serves 4000 connections (South Australia Health Commission, 1986). Despite the lack of routine main flushing, obstructions have been infrequent. Where they have occurred, they were most often due to construction debris left in the conduits after installation. |
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Small diameter gravity sewers were not introduced in the USA until the mid-1970s. The first systems were in Mt Andrew, Alabama and Westboro, WI, which were small demonstration systems of 13 and 90 connections, respectively (Otis, 1986). The Mt Andrew system was constructed as a variable grade system with sections of sewer depressed below the static hydraulic grade line (Simmons et al., 1982). The Westboro system was designed with uniform gradients using the more conservative Australian guidelines (Otis, 1978). |
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Since the early 1980s the use of SDGS has rapidly increased in the USA primarily in existing rural communities. By 1990, over 200 systems were operating. More recently, they have been used increasingly for residential fringe developments, new subdivisions, and resort developments where the topography is favorable. Frequently, the systems built are hybrid gravity and pressure systems (Otis and Sirotiak, 1987). Experience with SDGS has been excellent and, as a result, the designs have become less conservative. |
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