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Percolation tests

Effluent from septic tanks can soak away into the ground, so long as the conditions are suitable. To work out if the ground is suitable, and how much land you will need, you must carry out a percolation test:

1. Excavate a hole 300mm square to a depth of 250mm below the proposed invert level (bottom of pipe) of the land drain fill

2. Fill the 300mm square section with water and allow to drain away overnight

3. Next day refill the test section with water to a depth of no more than 300mm and observe the time in seconds for the water to seep away completely

4. Divide this time by the depth of water in millimetres placed in the hole. This answer gives the average time required for the water to drop 1mm

5. This is the percolation value (V in seconds)

6. Carry out the test three times and take the average figure. Avoid extreme weather conditions like drought, frost or heavy rain

7. If the percolation value exceeds 100sec/mm the ground condition may be unsuitable for discharge from a septic tank system and if pursued may cause ponding of septic effluent due to inefficient soakage. You need to consider an alternative means of disposal

8. The floor area of land drains required may be calculated from this equation: A = P x V x 0.25  where P is the number of people served by the tank, V is the percolation value described above. A, of course, is the floor area.  ​


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