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5.2.1 Small Diameter Gravity Sewer
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5.2.1 Small Diameter Gravity Sewer

Description:
Description:
Information

Small diameter gravity sewer (SDGS) is a piped network conveying wastewater by gravity. It is generally built to accommodate maximum growth that may occur.

1

Figure 5.1 Schematic of a gravity sewer system

(Source: WERF, 2010)

 

Key design criteria of SDGS include:

 

  • If the slope is greater than 0.5% and less than 0.75%, small grit traps should be installed between the house and the main sewer lateral to prevent solids from entering and clogging the sewer lines.
  • If the slope of the main lateral sewer systems is less than 0.5%, either a residential or community interceptor tank shall be installed on the branch line upstream of the low gradient main sewer line to prevent clogging.
  • Recommended pipe size is 100mm for less than 25 household connections, 150mm for less than 250 household connections, and 200mm for less than 1500 household connections.
  • A minimum diameter of 100 mm is recommended for laterals and a minimum of 150 mm for main sewer lines.
  • The depth of the piping should be the minimum necessary to prevent damage from anticipated earth and truck loadings and freezing.
  • Since hydraulic flushing is sufficient to cleans the lines of accumulated organic solids, cleanouts are recommended in lieu of manholes, except at major junctions, because the latter are more costly and are a source of infiltration, inflow and grit.
  • Cleanouts should be located in all lines between the house and sewer mainline connections, and at 75 m intervals along the main sewer lines.
  • As the topography is rarely conductive to purely gravity flow, the lift stations are often needed.
  • Air release risers are required at or slightly downstream of summits in the sewer profile.

 

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