Traps, Interceptors, and Special Wastes
Traps
A trap holds a water seal of 2 to 4 inches. Each fixture is separately trapped except where the code permits a combined waste for a set of fixtures. Trap arms run level or at grade to the vent and connect within the length limit. Prohibited: S-traps, crown vents, bell traps, drum traps except where specifically allowed, double trapping, and any trap depending on moving parts.
Interceptors and separators
- Grease interceptors take kitchen waste before the sanitary line, sized by fixture capacity or the code formula, with scheduled maintenance and a flow-control fitting on the sink and dishwasher waste.
- Sand and oil interceptors serve garages, car washes, and shops.
- Laundry interceptors catch lint.
- Acid waste from labs is neutralized before discharge, and the neutralizing system is vented independently.
- Dental amalgam separators, plaster traps, and similar devices follow the manufacturer and local pretreatment rules.
Indirect and special waste
Indirect waste discharges through an air gap or air break into a receptor, never by direct connection. The air gap must be at least twice the diameter of the waste pipe. Steam, condensate, relief valve discharge, and food-service equipment discharge indirectly. Storm water, groundwater, and cooling water may not enter the sanitary system.
Worked example
A 3-inch condensate line drips into a floor sink with a 4-inch air gap. That gap is too small: twice the 3-inch pipe diameter requires 6 inches of unobstructed vertical separation between the pipe outlet and the receptor flood level rim.
Traps that cost marks
Treating an air break as an air gap, hard-piping a relief valve to a drain, and sizing a grease interceptor by sink count rather than capacity.