Combined Surface Water and Foul Drains

The problem with combining surface water and foul waste pump stations

When can you, or should you not combine surface water and foul waste into the same pump station?

The simple answer is just don’t do it. Pumping surface water and foul waste through the same pump station is something that should never be considered. It is tempting to think that combining the two will save on costs, but in reality, it would cost much more due to the problems it may cause.

Why you shouldn’t combine these two types of pump stations

To start with, it should only ever be discussed if the receiving drain is a combined sewer. Most, if not all, main sewers do not accept surface water being discharged into them. The main sewer and surface water drains are almost always separate, with the foul waste ending up at the local sewage treatment works, whilst the surface water ends up in streams and rivers etc. You obviously wouldn’t want sewage entering any watercourse, nor would the local sewage treatment works want a fluctuating flow rate every time it rains. So, for these basic reasons, there should be two separate drainage systems for surface water and foul waste.

There are some exceptions. Some very old sewers in parts of London, and other older large cities, still have combined sewers. But even in these rare applications where you are allowed to combine the two together, and a gravity fall cannot be achieved, you should not put them through the same pump station.

Foul waste 24 hour storage

Building Regulations state that 24-hour storage must be provided within the design for foul waste pumping stations. This is relatively easy to design and achieve. However, if surface water were to be discharged into the pump station, the designers would have to allow for 24-hour storage of rainwater too. As no one can predict how much rain is going to fall, the drainage engineers would have to allow for a worse case scenario, like a 1 in 100 year storm. This means the size of the chamber required to store both the foul waste and the surface water for 24 hours would be huge, making it very expensive. 

Combined waste pump stations are costly

It would be far cheaper to use separate pump stations, one for the foul waste with 24-hour storage, and one for the surface water where 24-hour storage is not required. Two pump stations including a medium-sized one for foul waste, and a small-sized pump station for surface water, would be more cost-effective to purchase, install and maintain than one large unit.

To summerise, in scenarios where combined foul waste and surface water is permitted, it’s worth considering opting for two separate pump stations in order to avoid the combined 24 hour storage requirement and high costs.

Whether you’re a business owner, or homeowner, and would like to discuss how one of our pump stations can benefit you, please reach out to the Solid Links team:

E: info@solidlinks.co.uk 

T: 01227 200801