Traps, Interceptors, and Special Waste
Traps, Interceptors, and Special Waste covers the devices that protect the drainage system from sewer gas, grease, oil, sand, and chemical waste. The exam tests trap seal integrity and whether a special waste needs pretreatment before it enters the sanitary drain.
Core rules
- A trap seal is normally 2 to 4 inches deep. Less than 2 inches allows sewer gas; more than 4 inches can retard flow.
- Prohibited traps include S-traps, drum traps, bell traps, crown vents, and double-trapped fixtures.
- Grease interceptors are required for commercial kitchen fixtures that discharge grease; oil/sand interceptors serve garages and similar areas.
- Acid-neutralizing traps or dilution systems are required for corrosive laboratory waste before it enters the drain.
- Trap primers are required where trap seals can evaporate, such as floor drains in unused areas.
Worked example
A floor drain has a 2-inch trap seal, but seasonal evaporation leaves only 1 inch of water. Is it compliant? No. The seal is below the 2-inch minimum, so sewer gas can pass. Reasoning: the trap’s performance depends on water depth, not on trap body size. Refilling once is temporary; the code solution is an approved trap primer or a design that maintains the seal.
Traps that cost marks
- Replacing an S-trap with another S-trap instead of a proper P-trap and vent.
- Sizing a grease interceptor by pipe size instead of fixture flow and code table.
- Connecting acid waste to cast iron without neutralization.
- Ignoring evaporation in floor drains.