Drainage below ground

Also see:

5.3.4Foul and surface water disposal

Drainage systems shall be designed in accordance with relevant codes and standards to convey foul effluents and surface water satisfactorily to an appropriate outfall. Issues to be taken into account include:

  1. connections to sewers
  2. connections to surface water disposal systems
  3. rights of connection to disposal systems
  4. compatibility with other systems
  5. capacity of private sewers
  6. treatment plants for more than one home.

Connections to sewers

Connections to public sewers require the agreement of the responsible authority, which should be consulted as to the type and position of the connection.

Connections to private sewers require the agreement of the owners of the sewer. This should be obtained as part of the design process. Where the private sewer subsequently discharges into a public sewer, the local sewerage undertaker should be notified of the proposal.

Connections to surface water disposal systems

Surface water drainage is generally required to be separated from foul water drainage. Where permitted, surface water may be discharged into the main public surface water drains or directly into natural watercourses, ponds or soakaways, as appropriate.

Surface water should not discharge to:

  • septic tanks
  • cesspools
  • separate foul sewers.

For large or complicated homes, the volume of surface water to be disposed should be calculated in accordance with BS EN 12056-3.

Rights of connection to disposal systems

A legal right must exist when connecting drains to an outfall.

Compatibility with other systems

The drainage system should be compatible with the main sewerage system:
■ with separate systems for foul water and surface water
■ with separate systems where foul water is connected to the main sewer, while surface water disposal is by soakaways or other suitable means, or
■ as a combined system.

Where the sewerage undertaker permits surface water drains to be connected to a foul water system:
■ an interceptor should be installed on the surface water side of the foul sewer junction, or
■ trapped gullies should be used.

Where ground water drains are connected to surface water drains, there should be a silt trap on the ground water side of the junction.

Capacity of private sewers

Private drainage systems should be:

  • in accordance with BS EN 752
  • sufficient to cope with the intended capacity.

Where an existing private drainage system is to be extended, or where the capacity is to be increased, sufficient investigation, measurement and calculation should be undertaken to ensure that all parts of the private system are of adequate capacity.

Treatment plants for more than one home

Small sewage treatment works for more than one home should be designed in accordance with BS EN 12566.

Discharge from the waste water treatment plant should be:

  • sited at least 10m away from water courses and homes
  • designed by a suitably qualified engineer.