What components are often found in a plumbing trap?

In plumbing, a trap is designed to prevent sewage gases from entering occupied spaces while allowing wastewater to flow through. The primary component of a plumbing trap is indeed a bend or U-shaped section of pipe that holds a small amount of water. This water acts as a seal, blocking odors and gases from the sewer system.

The U-shape ensures that, when wastewater flows into the trap, some of that water stays in the bend. This water seal is crucial for maintaining sanitation and air quality within a plumbing system.

The other components listed do not serve this purpose. Filters are typically used in different contexts to separate solids from liquids, pressure gauges measure pressure in a system but do not impede gas flow, and check valves allow flow in one direction but do not provide a seal against gases. Thus, the defining characteristic of a plumbing trap is that U-shaped bend which retains water to create an effective barrier against unpleasant sewage odors.