Traps, Interceptors, and Special Wastes
Traps, Interceptors, and Special Wastes
This domain tests trap construction, trap seals, venting, interceptors, and special waste treatment. The exam expects you to know why traps lose seals, which traps are prohibited, and when grease, oil, sand, or acid waste must be intercepted.
Core rules
- A trap must maintain a water seal of at least 2 inches and not more than 4 inches. Trap arms must be vented within the allowed length. Prohibited traps include S-traps, drum traps, bell traps, and traps with moving parts, unless specifically approved.
- Each fixture must have its own trap except where a listed combination fixture is approved. Do not double-trap or trap in series.
- Grease interceptors are required for commercial cooking and dishwashing waste; oil/sand interceptors for garages and parking; acid neutralization for laboratory or dental waste. Size them from the adopted table and the fixture load.
- Interceptors need access for cleaning and must be maintained. Special waste must not be diluted to avoid treatment.
Worked example
A three-compartment sink discharges to a grease interceptor. The interceptor is undersized and has no accessible cover. The fix is not to add a second undersized unit in series; size the interceptor from the code table using the fixture volume and discharge rate, provide a gas-tight cover, and keep the outlet tee correct so grease is trapped, not carried downstream.
Traps
Do not install an S-trap or a trap with too shallow a seal. Do not run a trap arm too far to a vent. Do not route grease waste through a standard trap without an interceptor. Do not bury an interceptor without access.