Traps, Interceptors, and Separators
Traps and trap seals
A trap must maintain a 2- to 4-inch seal. Trap seal loss occurs from evaporation, siphonage, backpressure, capillary action, and wind. Trap primers are required for floor drains and other traps that are not used regularly. A trap primer must be connected to a potable water source through an approved backflow preventer and must deliver enough water to maintain the seal.
Interceptors and separators
Grease interceptors receive kitchen waste and must be sized for the fixtures served, vented, and accessible for cleaning. They cannot receive sanitary sewage or storm water. Oil separators serve garages and similar areas; sand and grit interceptors serve car washes and similar discharges. An indirect waste receptor must have an air gap or air break and must be sized for the discharge. Clothes washer standpipes must be at least 18 inches and not more than 42 inches above the floor, with a trap and vent.
Worked example
A restaurant adds a three-compartment sink. The sink waste must discharge through an adequately sized grease interceptor before entering the building drain. If the interceptor is undersized, grease passes to the sewer; if the sink is connected downstream of the interceptor, the interceptor does nothing. The interceptor also needs a flow-control fitting and a vent.
Traps
Do not install a grease interceptor without a vent or access cover. Do not connect a dishwasher directly to a grease interceptor if the code requires an indirect connection. Do not forget trap primers in unused floor drains; a dry trap lets sewer gas enter.