All appliances which discharge waste or soil must have a water trap either as an integral part of the appliance, as in the case of a water closet, or as a fitting directly below the appliance drain. In certain cases a group of appliances may have a single trap. A trap is a water seal which prevents gases from the drain entering the building through the open drain connection of an appliance. The required depth of water with the trap is dependent upon the type of drainage system used and there are a number of different designs of trap. Traps can be made of different shapes. Most common shapes of traps are P, Q and S.
Gully Trap – Gully traps disconnect the sullage drain of the house from the main drain. They are employed to receive sullage or waste water from wash basins, sinks and baths and pass it on to the house drains. Gully traps should be fixed near the surface level of the floor. There should be a grating on top of the trap to block solid matter. Pipes should be connected to the gully trap below the grating. In combined system of sewage the rain water is also passed into the house drains through the gully traps.
Intercepting Trap – It is also known as ‘Sewer trap’. It disconnects the house drain from the street sewer. It can be fixed in the last manhole of the house or in a separate small chamber between the lowest end of the house drain and the street sewer. It has a comparatively deeper water seal than ordinary trap and on opening at the top of the trap known as ‘cleaning eye’. This opening is closed with a plug. The intercepting traps should be properly flushed out otherwise they will not be of much use.
Grease Trap – Grease traps are used where large quantities of oily wastes are let out into the drain like hotels, restaurants and some industries. These oily substances deposit solids in the house drains and municipal sewers. To avoid this, grease traps are used. Grease trap can be a small cast iron or masonry chamber with a bent pipe as an outlet. The oily substances float to the surface to form a scum on top. The bent pipe allows the sewage to flow out while retaining the scum. This scum can be removed out from the top.
Silt Trap – Washing of kitchen utensils, sometimes, involves ash and pulverized brick pieces. These particles need steep slopes and higher velocities in the drain for movement. These should be trapped and removed in slit traps. These silt traps are required where extensive utensil washing is carried out like hotels and restaurants. Silt and grease traps can be combined.