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treatment facility (Kreissl et al., 1978) and more than 65 per cent of the total annual costs (Smith and Eilers, 1970). Less costly, but equally effective alternatives to conventional wastewater collection systems have been developed and have been shown to significantly reduce wastewater facility costs. Only recently have these alternatives been seriously considered. |
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Development of less-costly collection alternatives has centered around changing the motive force and/or the character of the wastes collected to reduce excavation and pipe costs. Table 9.1 lists the most common alternatives. Small diameter gravity sewers (SDGS) were the most recent to be introduced, but rapidly gaining in popularity. They have been called various names including variable grade sewers (Simmons et al., 1982), small bore sewers (Otis and Mara, 1985), and common effluent drains (South Australia Health Commission, 1986). They are all similar in design. Unlike conventional sewers, SDGS collect settled wastewater. Grit, grease and other troublesome solids which might cause obstructions are separated from the waste flow in septic or interceptor tanks upstream of each connection. The solids which accumulate in the tanks are removed periodically for treatment and disposal. |
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Although the term 'small diameter gravity sewer' has become commonly accepted, it is not an accurate description of the system. The mains need not be small in diameter (the size is determined by hydraulic considerations), nor are they 'sewers' in the sense that they carry wastewater solids. The most significant feature of SDGS is that primary pretreatment is provided in interceptor tanks upstream of each connection. This offers several advantages over conventional sewers. With the settleable solids and grease removed, it is not necessary to design the collector mains to maintain minimum self-cleansing velocities. Without the requirement for minimum velocities, the pipe gradients may be reduced and, as a result, excavation depths and the number of lift stations are also reduced. The need for manholes at all junctions, changes in grade and alignment, and at regular intervals is eliminated. The interceptor tank attenuates the wastewater flow rate from each connection which reduces the peak-to-average flow ratio below what is typically used for establishing design flows for conventional sewers. Without the requirement to carry solids, the depth of flow in the sewer is not critical. Therefore, SDGS can be employed without concern in areas |
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