Traps, Interceptors, Backflow Prevention, and Cross-Connection Control
Traps and Interceptors
Traps protect the building from sewer gas; interceptors protect the sewer from grease, oil, sand, and other materials. Grease interceptors are required for commercial kitchens, and their sizing depends on fixture flow and retention time. Questions test where interceptors are required, how they are maintained, and why they must be accessible.
Backflow and Cross-Connections
A cross-connection is any actual or potential connection between potable water and a source of contamination. Protection methods include air gaps (the strongest), reduced-pressure zone assemblies, double-check assemblies, and vacuum breakers. The choice depends on the degree of hazard and local code. Traps in questions include using a device that is not approved for the hazard level, and failing to test backflow assemblies annually.
Maintenance and Testing
Backflow preventers must be tested by certified testers at installation and at required intervals. Interceptors must be cleaned and maintained. Records matter.
Rule of Thumb
Match the protection to the hazard: air gap for high hazard, RPZ for high hazard where an air gap is impractical, double check for low hazard. If a question describes a submerged inlet, the answer is an air gap or vacuum breaker.
Worked Example
A commercial dishwasher drains into a sink with a submerged inlet. The fix is an air gap or an approved air-gap device, not a simple check valve, because the hazard is high.