What is a Interceptor traps
An interceptor trap, often referred to simply as a sewer interceptor or a U-bend trap, is a plumbing device installed in a drainage or sewer system to prevent foul gases and odours from escaping from the public sewer into domestic or commercial properties. These traps function by maintaining a standing water seal in a curved section of pipe, usually shaped like a U, P, or S, which blocks the backflow of gases while allowing wastewater to pass through.
Interceptor traps are most commonly found at the interface between private drains and public sewers, particularly in older properties, and are designed to serve as a final barrier against unpleasant and potentially hazardous sewer gases such as hydrogen sulphide, methane, and ammonia.
How Interceptor Traps Work
The key feature of an interceptor trap is its water-seal bend — a curved pipe section that holds water at all times. As wastewater flows through the trap, a small volume remains in the bend, creating a liquid barrier. This barrier blocks air movement from the sewer back into the building, thus preventing odours and vermin from travelling upstream.
Some traps are designed as part of larger intercepting chambers, with access points for maintenance, while others are inline and embedded directly in the underground pipework. In domestic settings, they are usually found just upstream of the final discharge point into the main sewer.
Historical and Modern Use
Interceptor traps were commonly installed in Victorian and early 20th-century drainage systems, when sewer ventilation practices were less developed. At that time, there was a heightened concern about sewer gases carrying disease (a theory now outdated, though gases still pose nuisance and safety risks).
Today, modern drainage design standards typically rely on ventilated stack systems and sealed drainage layouts to manage air pressure and odour, reducing the need for intercepting traps. However, many older properties still have these features in place, and they remain relevant in specific design scenarios.
Key Functions
- Odour control: Prevents the backflow of foul-smelling gases from the public sewer into habitable spaces.
- Health and safety: Reduces exposure to potentially harmful or explosive gases.
- Vermin barrier: Prevents rodents and insects from entering buildings through the drainage network.
- Air pressure management: Helps stabilise air movement in unventilated or partially ventilated systems.
Design and Types
Interceptor traps come in several forms, depending on their location and purpose:
- U-bend traps: The most common, using a simple water seal to prevent gas movement.
- P-traps and S-traps: Variants of the U-bend found in above-ground plumbing fixtures.
- Bottle traps: Used in smaller domestic applications, such as beneath basins.
- Intercepting chamber traps: Found in older underground systems, often within a brick or concrete chamber fitted with an access point and rodding eye.
Some systems may include rodding access upstream or downstream of the trap, although many legacy installations lack this feature, making maintenance more challenging.
Maintenance Challenges
One of the primary issues with interceptor traps, especially those installed below ground, is that they can be difficult to inspect, clean, or unblock. Common problems include:
- Blockages in the U-bend due to solid waste, fat, or debris accumulation.
- Limited access, especially in older systems without nearby manholes or rodding points.
- Dry traps, where evaporation or siphoning causes the water seal to break, allowing odours through.
- Inadvertent removal during renovation or system upgrades, potentially creating open paths for gases.
Where maintenance is problematic and ventilation has been upgraded, some water authorities and engineers choose to remove or bypass interceptor traps during sewer rehabilitation projects.
Regulatory and Design Considerations
In modern UK drainage design, the use of interceptor traps is not typically required, and in many cases is discouraged unless there’s a specific need for odour or vermin control that cannot be managed through other means. Current guidance focuses on:
- Building Regulations Part H1 – Addresses the requirements for foul water drainage, including the use of traps at plumbing fixtures.
- BS EN 12056 and BS EN 752 – European standards for gravity drainage and wastewater systems.
When installing or retaining an interceptor trap, care must be taken to ensure adequate access for maintenance, correct alignment, and compatibility with surrounding pipework.
Conclusion
An interceptor trap is a legacy but still occasionally used device in drainage systems, designed to prevent foul sewer gases and vermin from entering buildings via underground pipes. By maintaining a water seal in a curved section of pipe, it provides a passive but effective form of protection. While modern systems favour other ventilation and odour-control methods, interceptor traps remain relevant in certain contexts, especially in older properties. However, due to maintenance and access challenges, their use is carefully considered within today’s drainage design standards.