Can Surface Water Drain to a Foul Sewer?
When it comes to looking after your property’s drainage system, it is important to understand that not all water drains to the same place. All modern properties have separate drains for surface water and foul water, as these two types of water have their own particular treatment requirements, and cannot be processed the same way.
As such, it is your responsibility as the property owner to make sure that you understand the difference between a surface water drainage system and a foul water system, in order to make sure that you are not accidentally having a negative impact on the public sewer system.
Here, the drainage experts at Lanes Group will explain why surface water generally cannot run into foul sewers, as well as detailing the rare circumstances in which combined sewer systems are still used.
What is the difference between surface water drains and foul water drains?
Home drainage systems will primarily have to deal with two main types of water: surface water and foul water.
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 this extremely important 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.
What happens if surface water or rainwater drains into a foul water sewer?
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.
The inverse problem is much more serious: 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.
What are the rules for having a combined drainage system?
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 separated 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, who 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
- That 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
In order 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 serve to 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.
Find out more
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 surface water drainage guide.
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 Group about our domestic drainage services. You can also give us a call any time on 0800 526 488.
