Traps, Interceptors, and Special Wastes
Traps
A trap seal is 2 to 4 inches deep; deeper seals need additional protection against siphonage. Each fixture is separately trapped except for listed combinations, and certain traps are prohibited: S-traps, bell traps, drum traps with mechanical seals, crown vents, and two fixtures sharing one trap. Traps must be level, accessible, sized to the fixture outlet, and self-cleaning. Floor drains in rarely used areas lose their seal to evaporation and need trap primers.
Interceptors
- Grease interceptors serve food-service sinks and are sized from the connected fixture-unit load or the code's sizing formula; they need a vent, an accessible cover, and a flow-control fitting where required.
- Sand and oil separators serve garages and repair bays; lint traps serve laundries; acid-neutralizing interceptors serve laboratory waste.
- Solids interceptors and backwater valves address debris and sewer surcharge.
- Interceptors are not substitutes for traps; each fixture still needs its own trap unless the code says otherwise.
Special wastes
Corrosive, toxic, flammable, or high-temperature wastes are treated or diluted before entering the drainage system. Boiler blowdown, condensate, and pool backwash go through indirect waste. Dental offices use amalgam separators.
Worked example
A restaurant has a three-compartment sink, a prep sink, and a mop sink. All three discharge to a single grease interceptor sized on the combined fixture-unit load. The interceptor requires a vent, an accessible cover for cleaning, and the correct discharge outlet arrangement. Undersizing it is the classic failure, because a smaller unit passes grease downstream and creates a maintenance problem the inspector will cite.
Traps
- Treating a floor drain as a grease interceptor.
- Burying the interceptor cover in a finished floor without access.
- Forgetting the vent on an interceptor.
- Allowing two fixtures to share one trap.