What is the purpose of the trap in plumbing systems?

The purpose of the trap in plumbing systems is to create a water seal against sewer gases. This is vital for maintaining indoor air quality and preventing harmful gases, such as methane and hydrogen sulfide, from migrating into living spaces. Traps are designed to hold a certain amount of water, which acts as a barrier that blocks these gases while still allowing wastewater to flow through the system.

While the other options may seem relevant to plumbing, they do not accurately describe the primary function of a trap. For instance, while traps can prevent the escape of water to some extent, their main role is not about retaining water but rather maintaining that seal against gas. Similarly, facilitating constant water flow is more related to the overall plumbing design and pipe structure rather than the specific function of the trap. Lastly, filtering out solid waste is not a function of the trap; that role is typically served by other components in the plumbing system, such as strainers or grinders in specific systems.