Sanitary Sewers

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Sanitary Sewers

• Design Criteria

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Criteria

• minimum pipe size
• minimum maximum flow velocities
• minimum slope
• minimum cover depth
• location relative to other services
• manhole spacing

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Minimum Pipe Size

• sanitary sewers are normally smaller than
  stormwater sewers
• minimum sizes set for maintenance to prevent
  clogging
• service line from house (lateral) min. 100 mm
  (4)
• sanitary sewer min. 200 mm (8)

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Minimum Flow Velocity

• need to keep sanitary sewers flushed
• min. flow velocity 0.6 m/s
• there are trade-offs between depth of excavation
  and slope of sewers
• must keep sewage flowing to prevent anaerobic
  action and crown corrosion

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Maximum Flow Velocity

• due to high grit load maximum velocity is lower
  than storm sewers
• maximum flow velocity 3 m/s
• dont want to scour and erode inside of pipes
  prematurely

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Minimum Slope

• slope directly affects velocity
• must maintain sufficient slope to keep minimum
  velocity
• must limit slopes from causing flow velocities
  too high for pipes
• sometimes lift stations are needed to avoid
  deep excavations

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Minimum Cover Depth

• pipes need to be lower than house basements to
  accept sewage from all drains
• keep pipes below frost line
• where houses are below sanitary sewers, a pump is
  needed

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Other In-ground Services

• major concern involves leaks and contamination
• place below water supply
• place below storm sewers
• normally on centerline of roads

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Manhole Locations

• Similar criteria as storm sewers... manholes are
  need when
• sewer changes direction
• sewer changes pipe size
• sewer changes slope
• sewers converge
• maintenance cleaning requirements

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Now lets do some problems!