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How To Size A Soakaway For A Septic Tank Or Sewage Treatment Plant

If you are installing a septic tank and some times a sewage treatment plant you will have to also construct a soakaway for the waste water to discharge to.

To determine the size of area required for a drainage soakaway you need to carry out a percolation test;
Percolation test method
Excavate a hole 300mm square to a depth 300mm below the proposed invert level of the soakaway pipes

Fill the 300mm square hole to at least the 300mm mark and allow to soakaway over night.
On the next day, fill to the 300mm mark again and observe the time it takes to soakaway in seconds, from 70% full to 25% full.
Divide the time taken by 150.  The answer gives you the average time in seconds (V).
Carry out this test at least 3 times in at least 2 different places.
Drain fields or soakaways should only be installed when the results are between 12 -100.

How to Perform the Percolation Test Calculations:

V = The amount of time in seconds for the water in the test hole to drop by 1mm.
P = The maximum number of persons that the tank or plant is designed to serve
Area (A) = V X P X 0.20  for sewage treatment units
Area (A) = V X P X 0.25 for septic tanks
Design And Construction Of The Soakaway
The minimum size a soakaway should be constructed to is 30m.
Soakaways should be always constructed with perforated pipe, and laid in trenches with the same gradient not steeper than 1:200
Pipes should be laid on a 300mm layer of shingle or medium of up to 50mm.
The trenches must be filled 50mm above perforated pipe and covered with a membrane and then filled in with soil.
The trenches for the soakaway should be between 300mm and 900mm wide with a distance of 2m between trenches.