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Septic Tanks, Sewage Treatment Plants (SPTP's) and Cesspools


Sedgemoor District Council can provide general advice on septic tanks, Sewage Package Treatment Plants (SPTP's or just STP's) and cesspools, but detailed technical advice should be obtained from a drainage contractor or the Environment Agency.

We sometimes receive complaints about smells from drainage systems or discharges from drainage systems on to land. The Environmental Health Pollution Team investigate and if the drainage system is not satisfactory the owner or owners may be required to resolve any problems. Whilst most cases are resolved informally, occasionally we resort to formal action.

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.

Percolation tests

Sometimes the surrounding ground is not suitable to allow effluent water to soak away, so it may not be suitable for a soakaway. The soil may not be permeable if there is a lot of clay or high water table. 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. Before constructing soakaways, you should seek advice from the Environment Agency, who will advise you on how to perform the percolation test and whether any permit may be required.

If the percolation tests show the ground is not suitable you may need to consider some other form of drainage.

Sewage Package Treatment Plant

These type of treatment plants work 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. Advice should be sought from the Environment Agency to see if a permit to discharge is required.


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. 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 tank overflows or leaks. Also, if it pollutes a watercourse, the Environment Agency may take legal action.

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.

How to report a problem

You can either call the number below or you can report online here:  Report a sewer or drain problem

Further information