Drainage - Introduction - Page 1 2 3

Separate and Combined Systems

A separate drainage system is one were the foul water and the surface water are always kept separate.

This is shown in the two previous diagrams.

When a separate system is used then the sewerage treatment plant will not get overloaded in periods of wet weather.


A combined system is no longer used and joins some or all of the surface water into the foul water drainage system.

This means that both surface water and foul water will discharge into the sewerage treatment plant. To avoid the treatment plant being overloaded, it may be possible to extract some foul water at various points in the drainage network. This can be achieved if the surface water is less dense than the foul water and tends to flow at the top in a drain. A separating device can be used to divert surface water into a storm water channel or drain.


It is generally agreed that the installation and running costs of sewerage treatment plant can be minimised if a separate system is adopted. For this reason the separate system is favoured by local authorities.

A typical combined system is shown below but not recommended.

Combined System is not recommended. The Separate system as shown on the previous page is now used.


Drainage - Introduction - Page 1 2 3