• Description

What is a cesspool?

A cesspool is a large holding tank into which all the sewage from a building drains. It holds both solid waste and liquids and therefore needs to be emptied on a regular basis by a tanker lorry.

What is a septic tank?

Like a cesspool, a septic tank is basically a holding tank into which the sewerage from a dwelling drains. It differs in that the liquid part of the waste is separated from the solids and is discharged from the tank to a system of underground land drains. Because the tank holds only the solid waste it can be smaller than a cesspool, and while it will still need to be emptied, this will usually be needed less frequently.

septic tank discharge

How to I obtain a cesspool/septic tank?

It is possible to construct a tank on site but you would need to seek expert technical advice before attempting to do so. The easiest solution is to purchase the tank form a local builders merchants who will be able to supply you with a factory made tank. It is important to check that the product has an Agreement (BBA) Certificate as the guidance provided by this certificate will provide assurance that the product will operate successfully.

How do I know which type of tank I need?

Ask yourself the following:
(a) Is the site steeply sloping?
(b) Does the site have a high water table?
(c) Will the site containing the septic tank be too small to accommodate at least 9 square metres of outfall drain?
If the answer to either (a), (b) or (c) is ‘Yes’, then a septic tank will be unsuitable and a cesspool would appear to be the only option

What size tank will I need?

This will depend on the number of occupants of the building – the minimum size allowed is 2.7 cubic metres (2700 litres) for a septic tank and 18 cubic metres (18,000 litres) for a cesspool.

Where is the best place to site the tank?

It is recommended that the tank should be a minimum 15 metres from any building but must be within 30 metres of a road/driveway capable of supporting the emptying tanker. You must also ensure that the outfall drains from the tank are not laid within 10 metres of any known ditch, stream or watercourse and not laid within 50m of a well, borehole or spring. It should be located as far from the property and the site boundaries as possible and downslope from the property.

If I choose a septic tank, how do I know if the ground is suitable for the outfall drains?

A test known as a percolation test must be carried out on the area of land chosen and the test results forwarded to your Building Control Officer. To carry out a percolation test follow these steps:

1. A trial hole should be dug to determine the position of the standing ground water table. The trial hole should be a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework. The ground water table should not rise to within 1m of the invert of the proposed effluent distribution pipes. If the test is carried out in summer, the likely winter groundwater levels should be considered. A percolation test should then be carried out to assess the further suitability of the proposed area.

2. Excavate a hole 300mm square to a depth of 300mm below the proposed invert level of the effluent distribution pipe. Where deep drains are necessary the hole should conform to this shape at the bottom, but may be enlarged above the 300mm level to enable safe excavation to be carried out. The holes should be roughly in the area where you propose to lay the outfall drainage.

3. Fill the 300mm square section to a depth of at least 300mm with water and allow to seep away overnight.

4. Next day, refill the test section with water to a depth of at least 300mm and observe the time in seconds, for the water to seep away from 75% full to 25% full (i.e. a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.

5. The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken. The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.

6. Drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and hole tests have been favourable.

This minimum value ensures that untreated effluent cannot percolate too rapidly into groundwater. Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field. However, provided that an alternative form of secondary treatment is provided to treat the effluent from the septic tanks, it may still be possible to discharge the treated effluent to a soakaway.

How do I calculate the required length of outfall drains?

To convert your test result into the floor area of subsurface drainage trench (m²), use the following formula:-

Number of persons served by the tank X the percolation value (Vp) X 0.25

eg If the tank serves five persons and the trench is to be 0.6 metres wide then the length of the drain needed would be -

Building Control will need to see these results, together with a diagram showing your proposed drainage layout before approving any septic tank installation.

How are outfall drains constructed?

The outfall land drains should generally be laid in a closed herringbone design, using  rigid perforated pipe laid in trenches of a uniform gradient which should not be steeper than 1/200.  Pipes should be laid on a 300mm layer of clean shingle or broken stone between 20mm and 50mm.  The trenches should be filled to a level 50mm above the pipe with an impervious sheeting laid over the aggregate to separate the soil.  Drainage trenches should be from 300mm to 900mm wide and areas of undisturbed ground 2.0m wide should be maintained between parallel trenches. 

field drain

Please check with your local building control department before using information contained in this page to ensure that they comply with your local building regulations.