Designing Backflow Protection

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Flood Zones

Backflow protection is now more vital than ever, with sewer surcharge an ever increasing risk, particularly in built up urban areas where drainage systems are overwhelmed by short bursts of heavy rainfall, as witnessed in the July 2021 London Floods.

What is Sewer Surcharge?


“Sewer backflow” refers to the movement of sewage or wastewater against intended flow. Water can back up into homes or other properties that are connected to the sewer system when the public sewer system is backed up or overloaded.

Sewer surcharge is a particular risk to below ground properties, such as basement flats. This is because the invert level of their drainage connection is usually close to the main sewer, and when the system surcharges they are often the lowest point for water/sewage to backflow out of and alleviate pressure in the system.  When a sewer system is overloaded, it may overflow into a building.

Sewer backflow can be a severe issue, not just from the immediate flooding issue, but also because it can spread pathogens and harmful bacteria, potentially putting occupants’ health at danger. Additionally, it may harm property. Consequently, it is crucial to take action to help prevent sewage backflow.

Sewer surcharge can also occur in flood mitigation schemes where other Property Flood Resilience (PFR) measures have been installed. If a flood barrier is installed, this can create a head of pressure one side of the barrier, effectively encouraging sewer surcharge through the artificially created pressure differential. It is vital to understand the drainage layout at the site, prior to investigating management options, such that they can be designed and located correctly. A Drainage Survey may be require prior to the installation of backflow measures.

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How to Prevent Sewer Backflow?

The installation of a non-return valve in the drainage network system is one efficient method of preventing wastewater backflow. Non-return valves will allow water to flow in one direction, but not back the other way. These are usually in the form of flap valves intended to stop wastewater from flowing into a site or property. When a Flood Survey is undertaken, the drainage network must be considered as it can have defining factors for the overall scheme. By working with FPS Environmental Ltd you can rest assured you are working with the industry leaders within flood risk management and backflow control.

Drainage System

Choosing the Correct Non-Return Valve


Unfortunately, there is generally a poor understanding across the industry of drainage network backflow protection, along with where to place a non-return valve, and which style to recommend.

At FPS Environmental Ltd we pride ourselves on our knowledge and understanding of backflow protection and will work with you to ensure the correct system is recommended. This will also consider long term maintenance aspects, following the CIRIA Code of Practice.

Many flood risk surveys will simply state ‘Non-Return Valve’ or ‘NRV’ without specifying the style, variety or quantity. This is usually because further information is required from a drainage plan. Although this is an additional step in the process, it is vital that it is conducted because drainage backflow is one of the most important aspects in managing flood risk. There is no point building a flood wall if water can flow straight under it into your defended area via the drainage system.

Sewer surcharge for both foul and surface water drainage is often overlooked in both Property Flood Protection (PFR) and perimeter protection schemes.

When a flood survey is conducted, the construction and drainage must be considered alongside the risk and client requirements.

At Flood Protection Solutions Ltd we have in-house Civil Engineers who can advise on backflow protection for both residential and commercial projects.

Options are dependent on the drainage configuration, however may include:

  • Full Port Non-Return Valve in inspection chamber
  • Inline Non-Return Valve in pipe
  • Push Fit Non-Return Valve such as Buffalo
  • Wastewater Non-Return Valves
  • Panseal (last resort)

Additional Complexities

Unfortunately there are often additional complexities such as manholes being owned by water authorities, ‘self-flooding’ whereby water cannot escape, or due to the size/depth of chambers for access.


Your Helpful Partner

Fortunately, FPS Environmental Ltd are experts in the field of backflow protection and over the last 10 years have designed and specified 100s of backflow protection systems, not just protecting your property from the disgusting risk of other people’s sewage entering your property, but also future proofing your property and ensuring it is more saleable, thus providing a great Return on Investment.

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Whilst our main capacity is in assessing drainage systems and designing/specifying the correct non-return valves, we can also assist in overseeing installation works to ensure they are performed correctly.

Click on the image below to watch our very own Simon Crowther explaining all about Drainage Backflow and Sewer Surcharge

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Client Testimonials

Here are some of our latest testimonials from satisfied customers. If you'd like to discuss your requirements further, please don't hestitate to get in touch.

Hannah Rogers

FPS Environmental were great to work with. They responded immediately to my initial enquiry and followed up quickly with a quote. A very friendly, efficient and professional service. Thanks!

James Langton

Karen was a pleasure to deal with and entirely professional. Thanks!

Suzy Foster

FPS completed a flood risk assessment for me. Really helpful company. Very flexible with the many amendments needed! The final report is excellent.

Alex Duncan

Good to work with, Karen always helpful, and provided a detailed and professional report in a timely fashion.

Peter Blackadder

Excellent phone based advice on a prospective property purchase.


Mr Ashcroft


“Simon is a very experienced flood protection specialist. He was very professional and knowledgeable in his services.”

Mr Lattimer


“The survey and report were very thorough and really gave me what I needed”


Mr. Blakely


“The report is professional and informative, as indeed were the two surveyors who attended our property”

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