How to size a drainage field for a Septic Tank?Cotterill Civils
When it comes to sizing a drainage field for the disposal of effluent from a septic tank or a small wastewater treatment system, it’s vital you get it right. Here’s our guide to what you need to do…
What is a Drainage Field?
Often referred to as septic drain fields or soakaways, these fields are an essential component of a septic tank system. They are designed to remove impurities and contaminants that come from the treated effluent of a septic tank. Whilst septic tanks do 45% of the work in treating effluent, drainage fields will do the remaining 55%.
They work by first allowing the treated effluent to infiltrate the ground at a controlled rate and then by further treating the effluent before it reaches groundwater level. A drainage field is designed to contain an arrangement of trenches that contain perforated pipework and gravel and they are often located below a soft landscaped area.
It is also important to note that a drainage field is not the same as a soakaway (used for surface water) even though it is often mistakenly referred to as this.
What regulations affect Drainage Fields?
Drainage fields must be a minimum of 10m from a watercourse, 50m from a water abstraction point and 15m from a building. They should also be sufficiently far away from any other drainage fields, mounds or soakaways. This is to ensure that the soakage capacity of the ground is not exceeded. A drainage field must also not be in an Environment Agency Groundwater Source Protection Zone. This is to protect abstracted drinking water.
It is illegal to discharge effluent from a septic tank to anywhere else other than a drainage field. Discharging to a watercourse is in breach of environmental law and the consequences of doing so could leave you liable to large fines.
How to size a Drainage Field?
Before you can install a drainage field, it is important to determine whether the ground is suitable or not. Ground that is not suitable includes:
- Ground that can become waterlogged
- Ground where water soaks away too quickly (such as sandy soils)
- Ground where water soaks away too slowly (such as clay soils)
To determine whether the ground is suitable, there are two tests you can undertake:
- Water table test – determines the location of the water table
- Percolation test – determines the rate of infiltration into the ground
The results of the percolation test will help to determine the size of the field. The better your percolation test results, the smaller the drainage field will need to be. The length of the drainage field is also determined by the number of bedrooms in the house and the occupants.
Using your percolation test results you can work out the floor area which your drainage field should cover using the below calculation;
At = p x Vp x 0.25
At is the floor area
p is the number of occupants in the home/building
Vp is the percolation value in secs/mm (i.e. the result from the percolation test)