# Percolation Tests Method to calculate area of drainage field for septic tanks and sewage treatment plant systems is as follows:

Step 1: Excavate a test hole 300mm square x 300mm deep below the proposed invert level of the drainage fields trench bottom,

Step 2: Fill the test hole with water and allow to drain away over night

Step 3: Refill to a depth of 300mm and note the time taken in seconds to drain away from 75% full to 25% full (i.e. 150mm drop in level from 225mm to 75mm)

Step 4: Repeat the procedure in two more test holes and calculate the average

Step 5: Calculate the Vp (average time in seconds for the water to drop 1mm) for example: If the average time for the 3 test holes was 2700 seconds,

• divide 2700 / 150mm (depth of water) = 18 Vp*  (* Vp should range between 12 and 100, if not then the system should be designed by a drainage specialist)
• Area of trench = number of persons to use property multiplied by Vp multiplied by 0.25 i.e. 5 persons X 18 x 0.25 = 13.5 meters squared
• To calculate the actual length of the area of the trench by the width of the trench i.e. 13 meters squared / 0.6m = 22m (Minimum permitted area is 30m long by 0.6m wide)