Traps, Interceptors, and Special Wastes
Traps
The trap seal must be at least 2 inches and no more than 4 inches deep. Traps must be set level so the seal is not spilled, must be self-cleaning, and must be accessible for cleaning. Each fixture gets its own trap - no double trapping, no double trapping of a set of fixtures, no trap on a trap. A trap may not be installed to serve fixtures on different floors.
The trap arm carries the fixture waste to the vent. Its length is limited by pipe size, and the arm must slope uniformly at no more than ¼ inch per foot, because the vent must be taken off before the arm falls far enough to act as a drain. Common maximum trap-arm lengths: 1¼ inch = 5 ft, 1½ inch = 6 ft, 2 inch = 8 ft, 3 inch = 12 ft, 4 inch = 16 ft.
Interceptors
Grease interceptors are required where kitchen or food-processing waste discharges to the drainage system; they are sized by flow rate and grease capacity, must be accessible for cleaning, and must not receive sanitary waste. Sand and oil interceptors serve garages and repair shops. All interceptors need venting and a clean, watertight cover.
Special wastes
Chemical and acid waste may require neutralization or dilution before entering the building drain. Boiler drains, steam condensate, and swimming pool filter backwash have separate rules. Where a fixture is below the level of the next upstream manhole, a backwater valve protects against sewage backup. Cooling water and unpolluted industrial waste may need separate systems.
Rule of thumb
If the question asks for a trap seal depth, the answer is a number between 2 and 4 inches - most commonly 2 inches for a lavatory or sink.