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Private drainage

Most properties in the UK are connected to a public main sewer which collects effluent from the foul drains and conveys it to a central sewage treatment works for processing. However, particularly in rural areas, some properties have private drainage installations because no public main sewer is available nearby. There are two main types of private drainage, which may be shared with adjacent properties, and may sometimes be sited on an adjoining site. These are septic tanks and cesspools (the latter are often referred to as cesspits). Much less common are self-contained treatment plants. Each of these installations must be located such that there is no risk of contamination of watercourses or wells nearby.


Septic tanks are of two basic types.

Traditional septic tanks

Traditionally the installation consists of two or more large underground settlement chambers built of either brickwork or concrete. Effluent is piped into the first chamber, where heavier solids sink to the bottom and lighter materials such as fat and grease float on the surface. Pipes within the chambers ensure that the surface layer (known as a crust) is not disturbed. This is important in order to ensure that the contents of the chamber are not in contact with the air. Anaerobic bacteria which cannot survive in contact with the air break down the materials within the chamber, causing a dense sludge to form at the bottom of the chamber. A system of pipes allows excess effluent to flow into the second chamber, where further settlement of finer solids and further treatment by bacteria occurs. Finally, the remaining effluent, which by this time is relatively clear in appearance, is drained to a series of underground perforated pipes which act as a soakaway. The effluent then seeps into the ground. Each settlement chamber requires a vent to the open air to allow sewer gases to dissipate.

Modern pre-formed septic tanks

In recent years it has been common practice to use a pre-formed unit which is installed below ground level and connected to inlet and outlet drains as for the traditional type of septic tank. The processes which occur within the unit are exactly the same, since the chambers are formed within the unit. This type of septic tank has the advantage at the time of installation that it is quicker and easier to install. A typical installation can be completed within a day.

Maintenance of septic tanks

Most septic tank installations work efficiently, and give little trouble. However, regular maintenance is required.

Whichever type of septic tank is in use, it is important to ensure that the system operates efficiently. If the system becomes over-loaded by an excess volume of effluent, this will pass through too quickly and will not be fully broken down. This may cause pollution of the surrounding sub-soil, and will probably result in solids being washed into the soakaway pipes, causing an obstruction to build up. It is therefore necessary to ensure that the system is of sufficient capacity to cope with the expected volume of effluent from the property. Storm water must not be discharged to the foul drains since the effluent will be diluted and greatly increased in volume such that the treatment process will be impaired.

The use of bleaches and other household cleaners which will be discharged with the effluent will tend to harm the bacteria which break down the effluent, and this will also result in solids passing through to the soakaway pipes. Special detergents are available which are suitable for use with septic tank installations.

Over a period of time, which will vary according to the volume of effluent treated, the sludge in the settlement chambers will build up to the point where the effective volume of the installation is significantly reduced. At this stage the chambers must be pumped out by a local contractor which specialises in such work. The sludge is taken away for disposal elsewhere.

It is prudent to inspect the installation at regular intervals in order to identify a need for attention at an early stage. If the level of effluent within the chambers is above the normal level this may be an indication that the soakaway pipes are not draining effluent away at an adequate rate. The pipes may be obstructed, or the local water table (the level of water within the sub-soil) may be raised such that effluent cannot be readily absorbed into the ground. The contractors engaged to empty the chambers can usually arrange for the soakaway pipes to be cleaned with high pressure water jets.

If the effluent is seen to be below the normal level this might indicate that there is a leak from the chamber. This is potentially very serious since significant pollution of the surrounding ground may occur. Leaks are most likely in the case of a very old installation.

If the pipes within the chambers are broken, the surface of the effluent will be disturbed, and this will hinder the normal treatment process. This will lead to the problems described above.

If any of the above problems are discovered, or any other advice is required, a qualified surveyor should be consulted. Any required repairs should be carried out by suitably qualified contractors.


In some cases there is insufficient space for soakaway pipes adjoining a septic tank, or the sub-soil is not sufficiently porous. In such cases a cesspool can be used.

A cesspool (commonly referred to as a cesspit) is simply a large underground chamber which collects effluent discharged by the foul drains. Older chambers may be built of brick or concrete, and more modern installations are usually pre-formed. When the chamber is full it must be emptied by a specialist contractor. The contents are taken away for disposal. In many cases the cesspool will need to be emptied at intervals of around 14 days, but this will vary with the volume of the chamber and the quantity of effluent discharged from the property.

If the cesspool becomes too full there is a risk of pollution of the surrounding area.


A further alternative is a self-contained treatment plant. Such installations are often shared by a small development of several properties. In simple terms a motorised mechanism assists in the treatment of the effluent, which is discharged to a soakaway drain at the end of the process. The KLARGESTER BIODISC is a typical example of this type of installation. This type of treatment plant requires regular maintenance, and the manufacturers will provide the relevant information and guidance.


It is important to ensure that only authorised persons are able to gain access to private drainage installations. Ideally the access covers should be secured such that adventurous children are excluded. Damaged covers must be replaced without delay to minimise the risk of persons falling into the chambers.


If a private drainage system is located on an adjoining site, or is shared with one or more other properties, you will need to establish the extent of your liability for repairs and maintenance. Your Legal Advisers will be able to assist in this regard.