Septic Tanks

When Can a Septic Tank be Used?

Septic tanks are mainly used in locations where tests (see Percolation section at bottom of this page) have demonstrated the subsoil is suitable for the discharge and disposal of treated waste water, and where approval has been granted by the appropriate local authority, or exceptionally, where written approval has been granted by the Scottish Environment Protection Agency (SEPA) to discharge to a stream.

If a septic tank does not function properly it can cause an odour nuisance, flooding and pollution, which can result in additional expense for you.

If you follow the simple tips outlined on this page you should experience trouble-free and cost-effective waste disposal.

How Does a Septic Tank Work?

The purpose of a septic tank is to treat waste water from your property which is generally not connected to the public waste water system. It is usually either a large rectangular box made of brick, stone or concrete, or a spherical / cylindrical GRP tank buried underground not far from the property it serves.

A septic tank works like a simple waste water treatment works and the treated waste water drains from the septic tank’s outlet pipe to a soakaway .

Waste material (sludge) is allowed to settle in the tank and is digested by natural bacteria breeding in the tank. Over time this sludge builds up on the bottom of the tank. This sludge has to be removed regularly to ensure that the tank continues to work properly and to prevent the soakaway becoming choked.

How Often Does Your Tank Need to be De-sludged?

De-sludging should normally take place every twelve months. However, experience has shown that depending on the tank size and usage, this period may be extended, but it is not normally beyond two years. If you know where the septic tank outlet drains to, check that the discharge is a light grey colour. If the liquid includes dark solid material or recognisable sewage solids, this shows that the septic tank needs de-sludging.

Why ECS?

At ECS, we are able to offer two types of Septic Tanks, the traditional Spherical unit, often referred to as an “onion” and the horizontal cylindrical type, often referred to as “low profile”.

The spherical units are available in 3 sizes, 2800 litre, 3800 litre and 4500 litre capacity.

The cylindrical “low profile” units are available from 3,000 litres capacity upto 240,000 litres capacity.


Please ask our sales team to advise you on the size required, which will be calculated based on the current British Water recommendations.

PPG4 11 Appendix A: Percolation Test

Avoid carrying out this test in extreme weather conditions such as drought, frost and heavy rain.

a) Excavate at least two (three in Northern Ireland) holes 300mm square to a depth 300mm below the proposed invert level (bottom of pipe) of the infiltration pipe and space them evenly along the proposed line of the subsurface irrigation system.

b) Fill each hole with water to a depth of at least 300mm and allow to seep away overnight.

c) Next day, refill each hole 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).

d) Divide this time by 150. This answer gives the average time in seconds (Vp) required for the water to drop 1mm.

e) 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. This is the percolation value Vp (in seconds).

f) The average figure for the percolation value (Vp) is obtained by summing all the values and dividing by the number of values used.

g) Drainage field disposals should only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable.

h) The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into ground water.

i) Where Vp is above the limit of 100 effective treatment is unlikely to take place in a drainage field as there will be inefficient soakage in this location which may lead to sewage ponding on the surface.

j) For domestic premises, the floor area of the drainage field (A in square metres) required may be calculated from:

A = p x Vp x 0.25 for septic tanks A = p x Vp x 0.20 for package sewage treatment plants

Where; p is the number of people served by the tank (this should be the maximum number of people that could live in the house).

Vp is the percolation value described above.

If in doubt, consult your professional advisor or local authority building control officer for advice.