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Page 125 Table 9.1 Alternative wastewater collection systems Type Description Motive force Waste character Grinder pump pessure sewers (GP) Small vaults at each connection store wastewater for periodic pumping into small pressure mains (min. dia. 30 mm) laid with uniform depth of burial Positive pressure provided by individual pumping units Ground fresh waste, which may become septic in extensive systems Septic tank effulent pumping pressure sewers (STEP) Interceptor tanks at each connection remove settleable solids and provide storage of settled waste for periodic pumping in to small pressure mains (min. dia. 30 mm) laid with uniform depth of burial Positive pressure provided by individual pumping units Septic settled waste Vaccum sewers Pneumatic valves at each connection release small volumes of wastewater and air into vacuum mains laid at shallow depth with 'sawtooth' pattern Negative pressure provided by central vacuum station Fresh oxygenated raw waste Small diameter gravity sewers (SDGS) Septic tank effluent free from grit, grease and large solids is collected in small mains (min. dia. 50mm) not designed for self-cleansing velocities Gravity Sepite settled waste where severe water conservation is practised or where long flat reaches are laid with few connections. Also, headworks (screens and grit removal) and primary sedimentation can be eliminated from the treatment plant as these unit processes are performed by the interceptor tanks at each connection. In addition, sludge volume and treatment are reduced because the interceptor tanks provide sludge digestion. Furthermore, as the sludges are segregated at each connection, they remain uncontaminated by commercial or industrial sludges, so reducing restrictions on sludge disposal. 9.2 Historical Development and Use Late in the 1950s, severe problems with surface seepage of wastes from failing septic tank systems in several rural subdivisions
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