A drainage field is a method of cleaning wastewater (for homes and businesses which aren’t connected to the mains sewer) before dispersing it into the ground.
A drainage field is a system of pipes placed in trenches, so that effluent can be discharged to the ground. The field must be constructed in a way which allows the effluent to be treated through aerobic digestion. For it to work correctly, it must be constructed in the top 700-800mm of the soil to ensure aerobic contact.
Some people get confused between a soakaway and a drainage field, and some use the terms interchangeably. But they are different things.
A drainage field is specifically for the dispersal of wastewater and a soakaway is for the dispersal of rainwater. A soakaway cannot legally be used for dispersal of wastewater (which includes water from toilets, sinks, washing machines, showers and dishwashers) as it doesn’t offer biological cleaning of the water, while a correctly constructed drainage field does.
Regulations around the design and build of a drainage field
The key legislation around the design and build of a drainage field is the Government’s Building Regulations 2010, which states that a drainage field must be:
- At least 10m from any watercourse or permeable drain
- At least 50m from the point of abstraction of any groundwater supply and not in any Zone 1 groundwater protection zone
- At least 15m from any building
- Sufficiently far from any other discharge to ground to ensure the overall capacity of the ground is not exceeded
- Down water/ slope of any groundwater source
In addition:
- No underground services or water pipes are allowed to be located within the dispersal area
- No access roads, driveways or paved areas should be located within the disposal area
- A percolation test must be carried out
- Drainage fields must ensure aerobic contact between liquid effluent and the subsoil. The minimum depth of the pipes should be 500mm below the surface
- Drainage fields should be constructed using perforated pipe laid in trenches of uniform gradient that is not steeper than 1:200
- The perforated pipes must be laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm
- Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt
- Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m wide being maintained between parallel trenches
- An inspection chamber should be installed between the septic tank and drainage field
A really key factor when designing a drainage field is that it must be big enough to treat and disperse the maximum amount of wastewater which the system will discharge to it. To work this out, you need to know the soil porosity – worked out from a percolation test – and the population of the household.
Some ground will be completely unsuitable to be a drainage field, particularly ground which becomes waterlogged, ground where water soaks away too quickly (such as sandy soils) and ground where water soaks away too slowly (such as clay soils).
Before conducting the percolation test, it is important to establish the position of the water table. To do this, dig a 1m2 trial hole to a depth of at least 1.5m below the invert level (the level of the inside of the bottom of the pipe) of the proposed drainage field pipework. The groundwater table shouldn’t rise to within 1m of the invert level. As groundwater levels vary at different times of the year, you must remember to also take this into account.
Percolation test
Before designing and installing your drainage field, it is essential to carry out a percolation test. This will establish the rate of infiltration into the ground on the land you intend to use for your drainage field.
It is impossible to design a drainage field which complies with the Building Regulations and the General Binding Rules for small sewage discharge to the ground without conducting a percolation test.
You may wish to do the percolation test yourself, but it might be better to get a professional to do it for you, as you can’t risk making any mistakes.
The test shouldn’t be carried out during heavy rain, heavy frost or drought, as these conditions will give an inaccurate result.
Step 1: Excavate a test hole 300mm square x 300mm deep below the proposed invert level of the drainage field trench bottom
Step 2: Fill the test hole with water and allow to drain away overnight
Step 3: Refill to a depth of 300mm and note the time taken in seconds to drain away from 75% full to 25% full (ie 150mm drop in level from 225mm to 75mm)
Step 4: Repeat the procedure in two more test holes and calculate the average of the three results as follows: Test 1 + Test 2 + Test 3 divided by 3 = average time taken
Step 5: Calculate the VP (average time in seconds for the water to drop 1mm) as follows:
For example: If average time above took 2700 seconds
(i) Divide 2700 seconds by 150mm depth of water
(ii) 2700 divided by 150 = 18 VP (VP must be between 12 and 100 to be successful)
Once you have the results of your percolation test, you can calculate the size of your drainage field, by multiplying it by the maximum occupancy of your household (rather than the number of people currently living there), then multiplying it by 0.25.
So for a three bedroom house with a maximum occupancy of five people, using the above example of VP 18:
18 x 5 x 0.25 = 22.5m2
To calculate the actual length of trench, divide the result above by the width of the trench required eg for a 600mm/ 0.6m trench:
22.5m2 divided by 0.6 = 37.5m
Designing the drainage field
Once you know the area needed, you will be able to use this to design the drainage field. You are required to design a loop system of pipes in the ground with the total area of the trenches equalling the area you have from your calculations eg 22.5m2. You must ensure there is a 2m gap between your trenches.
A drainage field is constructed with slotted 110mm drainage pipe (not flexible land drainage pipe). The pipes should be laid on clean shingle or broken stone, graded from 20mm to 50mm
There is no building control requirement for a vent on a drainage field. However, putting a vent near the end will allow airflow through the drainage system, which will aid the growth of aerobic bacteria and increase treatment quality, so it is recommended.
Generally speaking, it won’t be necessary to install a drainage field if you have a sewage treatment plant, only if you have a septic tank. This is because sewage treatment plants produce water which is almost clean, removing the need for secondary treatment.
With consent from the local authority, it is possible to install an infiltration system with a sewage treatment plant, which is much smaller and generally cheaper than a drainage field.
There will still be tests to perform based on the porosity of the ground etc to ensure there will be sufficient secondary treatment and adequate dispersal of the wastewater.
It is always important to check with a supplier and the local council that you are installing the right thing in line with the legislation. If you don’t install the correct drainage field, you risk problems with blockages, breaching the legislation and a costly installation before you are able to sell your house legally.
