Septic tanks and cesspools

General advice on septic tanks and cesspools

North Devon Council can provide general advice on septic tanks and cesspools, but more specific advice should be obtained from a drainage contractor or the Environment Agency.

We may investigate complaints of nuisance from drainage systems, leaking tanks or defective drainage systems. As a result, the owner or owners may be required to resolve any problems. Failure to do so can result in legal action against the person.

Septic tanks

A septic tank is a type of biological sewage treatment system. Naturally occurring bacteria break down the solid matter, reducing its volume. The liquid flows out and is discharged through an underground drainage system called a soakaway. The accumulated solids need only be pumped out occasionally.

Like cesspools, they may be constructed in brick or blocks, although modern types are pre-formed in reinforced fibreglass.

The tank should ideally be away from housing. Our Building Control team can provide further advice.

To avoid problems, do not use excessive amounts of household detergents or bleaches. These upset the biological balance of the system. Also, do not overload the system by connecting rain or water drains to septic tanks or cesspools.


A cesspool or cesspit is a sealed underground storage tank that holds sewage until it is emptied out and disposed of. It may be brick or block construction or manufactured in steel or reinforced fibreglass and must have a minimum capacity of 18,000 litres.

It should be sited so that there is no risk of polluting water supplies, preferably as far as possible from any home.

It is an offence if the the tank overflows or leaks. Also, if it pollutes a watercourse, the Environment Agency may take legal action. This can lead to a fine of up to £20,000 and three months imprisonment.

If it leaks, a drainage engineer should be called out to remove the waste, clear up the sewage and resolve the leak.

To avoid problems, check the level in the tank regularly and do not let it overfill. Have it emptied at regular intervals - for example, these will become more frequent if you install a dishwasher.

Package treatment plant - Biodisk or Biobubble

These type of treatment plant works by allowing the natural bacteria to break down the sewage. They are similar to septic tanks but usually include some means of stirring the sewage or adding air to the effluent so that the bacteria can break it down more effectively. As a result, they often require a power supply. 

Percolation tests

Sometimes the surrounding ground is not suitable to allow effluent water to soak away. The soil may not be permeable if there is a lot of clay. Percolation tests need to be carried out to ensure that the water can be harmlessly disposed of from a septic tank or a similar installation. The tests should be carried out following BS6297: 1983 to determine the size of the soakaway. They must be sited six metres from a watercourse.

It may not always be possible to install soakaways, as soil conditions or other factors such as ground water levels may mean they would not be suitable. Instead, you will need to consider some other form of drainage. Before constructing soakaways, you must obtain a 'consent to discharge' from the Environment Agency, who will advise you on how to perform the percolation test.