Traps, Interceptors, and Special Waste
Traps, Interceptors, and Special Waste
This domain covers the water seal, the devices that keep grease and solids out of the drainage system, and the waste that must never be discharged directly into it.
Traps
Every fixture must be trapped, except fixtures with integral traps and the specific exceptions the code lists. The water seal must be at least 2 in. and not more than 4 in. deep, and the trap must be self-cleaning: level, with a smooth weir, set so the drop from fixture outlet to weir does not encourage siphonage. Prohibited: S-traps, drum and bell traps, crown vents, double trapping, and two fixtures on one trap unless the code permits a combination fixture. Seldom-used floor drains need trap primers so the seal does not evaporate.
Interceptors
- Grease: required for commercial kitchens and any discharge carrying fats, oils or grease; sized from the code table with a flow-control fitting to protect retention time, plus venting and access.
- Oil/sand: garages, car washes, repair shops.
- Lint, acid neutralizers, plaster and similar: where the code requires them for the specific waste stream.
Special waste and indirect connections
Steam condensate, boiler blowdown, cooling water and equipment using potable water must reach the drain through an air gap or air break into a properly sized receptor - never by direct connection. Prohibited discharges include gasoline, flammable liquids and anything that would damage the piping.
Worked example. A restaurant adds a three-compartment sink and a dishwasher. The sink discharges through a flow-control fitting into a grease interceptor; the dishwasher's high-temperature sanitary discharge enters a receptor through an air gap or air break, not a direct connection. A small under-sink "grease trap" does not satisfy the interceptor requirement.
Traps. Direct-connecting indirect waste; omitting trap primers; burying an interceptor without access; sizing a grease interceptor by guesswork instead of the table.