Sewer Backflow Protection and Non-Return Valve Design

Non-return valves are often suggested as a simple solution to sewer surcharge. In some properties, particularly where a single branch pipe serves a building, a push-fit non-return valve may be straightforward to install.

However, most drainage systems are more complex than they appear. Pipes branch, connect and sometimes serve multiple properties, meaning backflow does not always behave in the way people expect.

Installing a valve in the wrong location or incorrectly specifying the correct option may:

• Fail to protect the property from backflow
• Prevent the property draining during rainfall
• Cause internal or self-flooding
• Restrict access for maintenance

The drainage layout should be understood before specifying backflow protection.

Many Flood Surveys may 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 little point building a flood wall if water can flow straight under it into your defended area via the drainage system.

Designing backflow protection following a Flood Survey would fall under the category of Options Development and Design.

If designing a new drainage system, as part of a Detailed Drainage Design, backflow protection can be considered from the outset, depending on the risk, gradients, and site requirements.

FPS Environmental provides engineering review and design for backflow protection, as opposed to installation works.

This can include:

• Reviewing drainage drawings and asset records, such as CON29DW Reports
• Site inspection of drainage layouts
• Drainage Surveys
• Undertaking Flood Surveys
• Identifying surcharge risk and flood pathways
• Designing appropriate backflow protection measures

Where suitable, this may include specifying non-return valves, pumped systems, or in certain instances, a drainage redesign, such as in this case study: Oxfordshire: From Flood Investigation to Surface Water Drainage Strategy

Non-return valves can be appropriate where:

• The drainage system configuration is simple
• The valve location is accessible for maintenance
• The valve will not prevent the building draining during rainfall
• There is a clear risk of sewer surcharge

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

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

The correct solution depends on the drainage network serving the property.

In many cases a Drainage Survey or site visit as part of a Flood Investigation and Management Strategy is the first step in understanding the drainage system, before backflow protection can be designed. A site visit is usually the best starting point as this allows the FPS engineer or Flood Risk Consultant to understand the site before prepare a bespoke scope of works for the necessary Drainage Survey.

This may include:

• CCTV inspection
• Dye testing
• Review of public sewer asset information
• Confirmation of pipe routes, connections and manhole locations
• Identifying shared drainage or public sewer connections
• Assessing pipe locations and manhole depths to determine accessibility for installation and maintenance.

This information allows backflow protection measures to be designed correctly.