A Trap is device which puts a barrier in the sewer line in form of water seal and does not allow municipal sewer gases to escape into the house. The trap always remains full of water. If trap is not used in any of the sanitary fittings, the foul gases of municipal sewer will escape into the house and cause nuisance and pollute the atmosphere. Hence all sanitary fittings are always provided with a trap. The depth of barrier in the form of water is called water seal. The effectiveness of a trap depends upon the depth of water seal. The greater the depth of water seal, the more effective is the trap. The depth of water seal varies from 25mm to 100mm.
Types of traps: The traps can be classified according to shape or according to use. According to shape traps may be classified as P-type, Q-type and S-type. Whereas according to use they are classified as Floor or Nahani trap, Gully trap and Intercepting trap.
- Floor trap: It is made of cast iron or UPVC and provided at the entry point of waste water in the house. Hence these are used bathroom, kitchen, sink etc. A floor trap forms the starting point of waste water flow. A cover with grating is provided at its top so as to prevent the entry of solid matter into the underground drain. The cover can be removed for cleaning of the trap. This trap is also known as Nahani trap.
- Gully trap: This trap is usually made of stoneware and a cast iron grating is provided at its top. The gully is fitted inside a masonry chamber. A water seal of about 75mm is provided in this trap. Gully trap forms the beginning point of horizontal flow of sewage. It is normally provide near the external face of the wall and kept slightly higher or even in line with the ground. This trap takes sewage either to sewer or to inspection chamber or to manhole.
- Intercepting trap: This trap has comparatively deeper water seal i.e. 100mm. It is provided at the last inspection chamber or manhole of the house drainage system. Sewage coming from the house enters the public sewer after passing through intercepting trap. This trap is also known as interceptor or a disconnecting trap.