Traps, Interceptors, and Special Wastes
Trap basics
A trap seal must be between 2 in and 4 in deep. Traps must be self-cleaning, accessible, and level. Prohibited types include the S-trap, drum trap, bell trap, and any trap that depends on internal partitions for its seal. Only one trap per fixture, and no double-trapping. A trap subject to evaporation (floor drain, infrequently used fixture) needs a trap seal primer.
Interceptors and special wastes
Grease interceptors serve food service; oil/sand interceptors serve garages; neutralizing interceptors serve acid or chemical waste; lint traps serve laundries; plaster traps serve construction. Interceptors require accessible covers, correct sizing by flow rate and retention time, and scheduled maintenance. Special wastes that could damage the system or the treatment works must not discharge to the sanitary drain without treatment - use indirect waste with an air gap or air break, or a separate system.
Indirect waste
An indirect waste receptor receives waste through an air gap or air break, never a direct connection. Air gap is the stronger protection; air break is a physical separation without a full gap.
Common traps
- Trap seal measured as pipe size rather than water depth.
- Two fixtures sharing one trap.
- A grease interceptor installed without a flow-control fitting.
- Discharging a chemical waste directly into the sanitary system.
- Assuming an interceptor replaces the need for a trap.
Rule of thumb
If a question mentions repeated stoppages, odors, or siphoning at one fixture, suspect a missing vent, an over-long trap arm, or an evaporated seal.
Worked example: A floor drain in a rarely used mechanical room has a dry trap and sewer odor. The remedy is a trap primer or a deep-seal trap - not a larger trap.