Buchan trap
A Buchan trap is a device fitted in a domestic sewer pipe to prevent odours entering the pipe from the public sewer and permeating the house, a common problem before individual plumbing fittings were separately trapped.
The trap is made from fireclay and uses a water seal to prevent air passing from the sewer to 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 an air-inlet/access point on the 'house' side. It is located below the ground level, but can be accessed through the air-inlet and sometimes a rodding hole.
Blockage risk
The Buchan trap will collect solids, sludge and waste that is not in suspension. In recent times, non-paper based wipes have posed a particular problem, according to most water authorities, with the potential to create a partial or complete blockage. The sewage and rainwater then backs up the pipe and exits from the lowest connected appliance.This can be a significant problem in multi-level dwellings 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 recurring basis, such as annually.