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Unfortunately, there have been significant deterrents to their use from regulatory agencies, engineers, and the consumers. Engineers and regulatory agencies have been reluctant to promote SDGS because of the concern for long-term performance. Potential users have fought against their construction because of a perception that they are 'second rate'. Typical concerns are over backups, odours and potential for expansion. It has been proven, however, that these concerns can be overcome through proper planning and design. With time, it is expected that SDGS will receive full acceptance. |
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SDGS design practices have rapidly evolved in the USA as experience has been gained. The guidelines originally imported from Australia have gradually been modified to allow conduits as small as 50 mm in diameter, variable pipe gradients, depressed sections (below the hydraulic grade line), elimination of minimum flow velocities and reduction in peak-to-average flow factors. |
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9.3.1
Estimation of Design Flow. |
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The collector mains are sized to carry the maximum daily peak flow. Conventional sewer design typically assumes 380 litres per caput per day times a peaking factor of 4 to establish the necessary collector capacity. This estimate includes allowances for commercial flows and infiltration. Experience with SDGS has shown this design flow criterion is excessive because of the capacity of the interceptor tanks to attenuate the wastewater peaks. In residential dwellings, the rate of wastewater discharged from the building depends on the type of water fixtures and appliances used. Instantaneous peak flows may be 0.30.6 1/s. However, the interceptor tank can attenuate the peaks dramatically. Monitoring of individual interceptor tanks showed that outlet flows ranged between 0.03 and 0.06 1/s over periods of 3060 minutes (Otis, 1986). Long periods of zero flow also occur. The extent of attenuation depends on the design of the interceptor tanks and its outlet (Jones, 1975). Most recent SDGS |
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