When it comes to maintaining your property's drainage system, it’s essential to recognise that not all water drains to the exact location.
Modern properties have separate systems for surface water and foul water because they require different treatments and cannot be combined for processing.
As a property owner, it’s your responsibility to understand the difference between these two systems. This knowledge ensures that you don’t inadvertently disrupt the public sewer system.
In this guide, the drainage experts at Lanes Drainage Services UK will explain why surface water should not enter foul sewers and discuss the rare cases where combined sewer systems are still in use.
Home drainage systems will primarily have to deal with two main types of water: surface water and foul water.
The definitions for these are:
The term “surface water” describes any water found above ground. The phrase can relate to water from rivers, ponds, lakes and streams, but from a drainage perspective, it mostly refers to rainwater that is carried away from gutters, patios, driveways or roads and needs to be discharged somewhere.
Because this water is generally free from pollutants, the purpose of a surface water sewer is to transport it to a stream, a river or a soakaway.
Meanwhile, foul water drainage deals with wastewater that is carried through the pipes connected to your bathroom, kitchen, utility room or washing machines.
Unlike surface water, foul water is dirty or contaminated and needs to be purified in a sewage treatment plant before it can be allowed to pass through.
Because of the significant difference in the function of foul drains and surface water drains, these two systems of drainage need to be kept separate, and your local water company will have strict rules in place to ensure this.
There are no serious consequences if surface water enters a foul water sewer, as the water will all pass through a water treatment facility before it is released back into the environment. However, this is not necessary, and can waste time and resources for sewage plants by forcing them to treat water that could otherwise have flowed straight back into a natural watercourse.
Discover where your surface water drains to in our article ‘How do I know where my surface water drains to?’.
Allowing foul water to escape into a surface water drainage system is a serious problem that contributes directly to the pollution of our waterways and natural ecosystem.
This is because surface water drains lead directly to streams and rivers, and when foul water is allowed to enter, this mixture will be released straight into the environment without treatment.
The same principle applies both for large-scale public sewage treatment plants and private septic tanks; in the latter case, it may have the additional effect of clogging your septic tank system and polluting your own drainage field.
As such, the strict rules imposed by local authorities on keeping foul water and surface water drainage systems separate need to be respected, as they will ultimately help to prevent you from accidentally causing pollution or damaging the public sewers and your own property.
Discover more information on how septic tanks work in our article ‘Homeowners' guide to cesspits and septic tanks’.
The only exceptions to the rules on separating surface water drainage and foul water drainage systems are for properties with septic tanks or those that are located in areas with no access to separate drainage pipes.
In certain parts of the country, the only available public sewer nearby may still be a Victorian-era combined sewer, meaning they will have no choice but to set up a combined connection.
Establishing a combined drainage system requires special permission from the local authority, which will require you to provide evidence that establishing separate foul water drainage and rainwater drains is impossible on your premises. You must be able to demonstrate the following:
That there are no separate foul and surface water sewers in your vicinity
Your property is unsuitable for setting up a soakaway system as an alternative method of surface water drainage
That the foul water and rainwater drainage flow is kept separate up to the point where it leaves the boundary of your property, to ensure that your connection can be upgraded in future
To do this, the water company will ask you to provide the following, to conform with building regulations:
An extract from the relevant ground investigation report results
The results of a percolation test
An email or letter from Building Control confirming the situation
The exacting nature of the rules surrounding surface water drainage and foul water drainage should underline just how important they are to the health of the environment and the public sewer network, as well as the importance of keeping stormwater drains and sewage treatment plants properly maintained.
For more information on your responsibilities as a septic tank owner, read our article ‘Septic tank maintenance – What you need to know’.
To learn more about who is responsible for surface water drainage and keeping your drainage system in the best working condition, take a look at our ‘Guide to surface water drainage and flooding’.
If you are looking for advice and practical support with any aspect of installing, maintaining or repairing domestic drains, get in touch with the experts at Lanes Drainage Services UK about our domestic drainage services.
You can also contact us by phone at any time on 0800 526 488.