What is the primary purpose of a trap in the waste drainage system?

The primary purpose of a trap in the waste drainage system is to block sewer gases. Traps are designed to hold a small amount of water, which acts as a barrier between the sewer system and the interior of a building. This water seal prevents harmful and foul-smelling sewer gases from entering living spaces, ensuring healthier indoor air quality.

While traps can facilitate proper drainage and help manage waste flow, their primary function is to create that vital barrier against gas intrusion. The design of traps—such as P-traps or S-traps—reinforces this purpose by retaining enough water to maintain a seal while still allowing wastewater to flow through.

Other aspects, like maintenance access or preventing siphoning, are important but secondary to the essential task of protecting occupants from unpleasant and potentially hazardous sewer gases.