Traps, interceptors, and special wastes
Domain 8 - Traps, interceptors, and special wastes
This domain covers trap fundamentals plus the interceptors that keep grease, sand, oil, lint, plaster, acid, and amalgam out of the drainage system. Items ask what must be trapped, what must be intercepted, and where the discharge may go.
Traps
- Every fixture needs a trap with a 2–4 in. seal, protected from siphonage by venting and from evaporation by a trap primer where the fixture is rarely used. Floor drains in mechanical rooms are the classic evaporation failure.
- Interceptors must be vented, accessible for cleaning, and fitted with removable covers. The interceptor does not replace the fixture trap.
- Prohibited: double trapping, S-traps, crown vents, and using the drainage line itself as a trap.
Interceptors and special wastes
- Grease interceptors are required for food-service and food-processing fixtures, including sinks, canning sinks, and mop sinks in kitchen areas, plus dishwashers where the code requires. Grease may not be diluted to escape the requirement.
- Sand and oil separators serve garage floor drains and car washes; lint traps serve laundries; plaster traps serve dental and art sinks; acid-neutralizing sumps serve laboratory sinks; dental offices also need amalgam separators under federal rules.
- Some wastes must be indirect: ice machines, steam tables, and refrigerator drains connect through an air gap or air break to a trapped receptor, never hard-piped into a trap arm.
- An indirect waste receptor located below grade needs backflow protection.
Worked example
An inspector finds an ice machine drain pushed into a floor drain branch below the trap, and a water heater T&P discharge ending 3 ft above the floor nearby. Reasoning: the ice machine requires an indirect connection with an air gap or air break above the flood-level rim of a trapped receptor, and it may not connect downstream of a trap. The T&P discharge may not be connected to the drainage system at all; it must terminate through an air gap within 6 in. of the floor or to the exterior. Both are rework items before a final inspection.
Where marks are lost
- Treating a grease trap as a substitute for the fixture trap.
- Directly connecting condensate or ice machine drains to a drainage line.
- Skipping trap primers and failing final inspection on a dry seal.