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Sanitary Drainage Systems - Volume Flow

Calculate expected volume flow load in sanitary drainage systems.

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The theoretical total load for a sanitary drainage system can be calculated by adding loads for every fixture in the system. Due to intermittent use of the fixtures - this method in general adds up to unrealistic high values for the main drainage lines.

  • realistic - or expected - total load is always lower than theoretical total load

Expected total load for sanitary drainage systems should instead be estimated by using empirical equations like

qet = k ( Σqn )1/2                                (1)


qet = expected total drainage load (gpm, l/s)

k = system coefficient describing the nature of the system

Σqn = total theoretical load - all fixtures summarized (gpm, l/s)

Note that the minimum expected total load can never be less than the load from the largest fixture.

The system coefficient compensates for the nature of the system. For systems serving large groups of people where the use is intermittent, like

  • hotels
  • hospitals
  • schools
  • theaters
  • wardrobes in factories
  • etc  

the coefficient - k - should be in the range 0.5 - 0.8. Closer to 0.8 for smaller systems with fewer fixtures and closer to 0.5 for larger systems with many fixtures.

For more ordinary systems where consumption patterns are more continuously, like

  • homes
  • offices
  • nursing homes

the coefficient - k - should be in the range 0.3 - 0.6. Closer to 0.3 for larger systems with many fixtures and closer to 0.6 for smaller systems with few fixtures.

Example - Sanitary Drainage System Hospital

If the theoretical total load from fixtures in a smaller hospital adds up to 50 l/s - the expected total load can be estimated to

 qet = 0.7 (50 l/s)1/2

    = 4.9 l/s

Assuming this is a smaller hospital the coefficient is set to 0.7.

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This page can be cited as

  • Engineering ToolBox, (2008). Sanitary Drainage Systems - Volume Flow. [online] Available at: [Accessed Day Mo. Year].

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