Technical guidance: Sewage disposal

Most people do not need to worry about where their sewage goes. However, houses outside the water authority main sewer areas have to have a properly designed drainage system. Three basic methods of sewage disposal are available: septic tanks, cesspools, and package sewage treatment plants.

Septic tanks

A septic tank system consists of the tank itself and a drainage field. Raw sewage is fed to the tank, and settled sewage is discharged to the drainage field. Sludge accumulates at the bottom of the tank and has to be removed periodically.

The drainage field typically consists of either a soakaway or a system of sub-surface irrigation pipes, which allow the effluent from the tank to percolate into the surrounding soil.

Due to the poor soil porosity in the north of the District this system is not usually suitable. A “Percolation Test” must be undertaken in all cases to determine the length of distribution drain required.

Despite the simplicity of operation and maintenance, it is generally recognised that proper care is rarely given to septic tanks. This is believed to be due to the costs of desludging and lack of knowledge of maintenance requirements.

Cesspools

A cesspool is a watertight tank, installed underground, for the storage of sewage. No treatment is involved. Cesspool capacity is specified in terms of 45 days holding time, with a minimum capacity of 18m3. The cesspool should be sited at least 15m from habitable dwellings, and preferably downslope and downwind.

Cesspool liquors are strong and usually in a septic condition. It is essential that cesspools are emptied frequently. The disadvantages of cesspools are the high emptying costs and the risk of damage that would allow sewage to escape from the tank.

Package Sewage Treatment Plants

Modern package plants are compact and those using the activated sludge process produce less sludge than other methods. Primary settlement can also be eliminated and secondary sludge can be stored aerobically for long periods with negligible odour. These advantages over septic tanks and cesspools come at extra cost: initial outlay and maintenance costs are higher and power is required to operate the plants.

As the discharge is of a higher quality it is usually piped direct to a watercourse. This will generally require the consent of the Environment Agency before any work is undertaken. This method of disposal is often the only option available in areas with clay subsoil as soil porosity is not a concern.

Aftercare

Responsibility for satisfactory performance rests with the owner of the installation, and several control mechanisms exist to prevent and address problems of pollution, nuisance and health hazard. These include the planning process, building control, environmental health legislation and the regulatory function of the Environment Agency to protect water quality.

Building Regulations

The installation or replacement of a sewage disposal system is subject to the requirements of the Building Regulations. An application must be submitted prior to undertaking any work.

Please note that discharge from a wastewater treatment plant system is likely to require consent from the Environment Agency.