Buchan trap
A Buchan Trap (alternative names: Bristol interceptor, interceptor trap and disconnecting trap) is a device made from fireclay located in a domestic sewer pipe to prevent vermin entering the pipe. The trap uses a water seal to prevent rats and mice climbing any further along the pipe. Waste flows from the house through a U-bend in the trap. This means that there is always water in the pipe preventing the passage of anything from the other direction. The device is a large clay U-bend with air-inlets and vents at the top. It is located below the ground level, but can be accessed through the air-inlet and a rodding hole. This hole allows drain rods to unblock anything located at the bottom of the U-bend.
Blockage risk
The Buchan trap will collect solids, sludge and waste that is not in suspension and, over time, can clog up. The overflow will back up the pipe and exit from the lowest connected appliance (sink, bath, dish washer, washing machine, etc.). This can be a significant problem in multi-level dwellings (e.g. tenements) where those at the lowest level will receive all the waste waters from those above. They should be checked, and cleaned if required, on a regular basis (say annually).
Traps of this type are widely believed by plumbers to stop the entry of rats but rats are good swimmers and can easily pass through the short flooded section of the trap. The purpose of these traps (as this article says) was to help prevent sewer air entering houses - foul air was thought by the Victorians to be the source of disease .
History
The Buchan trap was devised in the 19th century to stop the flow of sewer gases also known as miasmas. It was believed that the disease cholera was an air born infection, not water born. The Buchan trap is normally found in the bottom of manholes or drop-shafts. It normally denotes the end point of the domestic property's sewer before it joins the main public sewer.