Traps, Interceptors and Separators

A trap is the part of a drainage system that keeps sewer gas on the sewer side of the fixture, and an interceptor or separator is the part that keeps grease, oil, sand and other harmful matter out of the public sewer. The 2024 International Plumbing Code (IPC 2024) governs the material and the installation of all three, and it governs them in one chapter: the chapter on traps, interceptors and separators (IPC 2024, 1001.1). This text is written from the printed lines of that chapter, together with the printed lines that size a trap, that require a waste receptor to be trapped, and that vent the traps and the interceptors. Every rule names the address it comes from. Where the printed line gives no value, none is given here.

1. The trap under every fixture

Each plumbing fixture is trapped separately by a liquid-seal trap, unless the code permits otherwise (IPC 2024, 1002.1). Two distances govern how the fixture outlet may be connected to the trap, and both are drawing dimensions:

The drop from the fixture outlet down to the trap weir is at most 24 inches.

The run from the centreline of the fixture outlet to the centreline of the trap inlet is at most 30 inches (IPC 2024, 1002.1).

The same section states that the height of a clothes washer standpipe above a trap follows Section 802.4.3, and that a fixture is not double trapped (IPC 2024, 1002.1). Four exceptions are printed with it (IPC 2024, 1002.1):

8.Fixtures with integral traps are not covered by the rule.

9.A combination fixture may stand on one trap where one compartment is no more than 6 inches deeper than the other and the waste outlets are no more than 30 inches apart.

10.Floor drains in multilevel parking structures discharging to a building storm sewer need not be trapped individually; where such drains must discharge to a combined building sewer, they need not be trapped individually either, provided they connect to a main trap as Section 1103.1 requires.

11.A hydromechanical grease interceptor may serve a food utensil, dishes, pots and pans sink in accordance with the manufacturer's instructions, with an emergency floor drain downstream of the interceptor connection on a branch serving only that drain and the interceptor; where the interceptor serves a combination sink of not more than three compartments, the drop from the fixture outlet to the interceptor inlet is at most 30 inches and the developed length of waste pipe from the most upstream fixture outlet to the interceptor inlet is at most 60 inches, and such a sink connects directly to the interceptor.

2. How a trap must be built

A fixture trap scours itself. It has no interior partitions, except where the trap is integral with the fixture or is made of an approved material that resists corrosion and degradation. Where slip joints are used they are made with an approved elastomeric gasket, and they are placed only at the trap inlet, the trap outlet and inside the trap seal (IPC 2024, 1002.2).

Six types of trap are prohibited outright (IPC 2024, 1002.3):

15.A trap that depends on moving parts to hold its seal.

16.A bell trap.

17.A crown-vented trap.

18.A trap that is not integral with the fixture and that depends on interior partitions for its seal, except a trap made of an approved material resistant to corrosion and degradation.

19.An "S" trap.

20.A drum trap.

Drum traps used as solids interceptors, and drum traps serving chemical waste systems, are not prohibited (IPC 2024, 1002.3).

The seal itself is a measured quantity: every fixture trap holds a liquid seal of at least 2 inches and at most 4 inches, or deeper where a special design serves an accessible fixture (IPC 2024, 1002.4). Where a seal can evaporate, it is protected by one of the five methods the chapter names (IPC 2024, 1002.4.1), and each of the five has its own printed requirement:

A potable water-supplied trap seal primer valve supplies water to the trap, conforms to ASSE 1018, and its discharge pipe connects to the trap above the trap seal on the inlet side (IPC 2024, 1002.4.1.1).

A reclaimed or graywater-supplied trap seal primer valve does the same, conforms to ASSE 1018, is supplied with reclaimed or graywater of a quality matching the valve manufacturer's requirements, and connects above the trap seal on the inlet side (IPC 2024, 1002.4.1.2).

A wastewater-supplied trap primer device supplies water to the trap, conforms to ASSE 1044, and connects above the trap seal on the inlet side (IPC 2024, 1002.4.1.3).

A barrier-type trap seal protection device keeps a floor drain trap seal from evaporating, conforms to ASSE 1072, and is installed as the manufacturer directs (IPC 2024, 1002.4.1.4).

A fixture drain from a lavatory or hand sink serves as trap seal protection for an emergency floor drain, a trench drain or a floor sink in the same room, and a drinking fountain fixture drain serves that purpose for such a fixture in the same room or in an adjacent room; the drain is not routed on or above the floor surface and connects below the flood level rim and above the inlet to the trap of the receiving fixture (IPC 2024, 1002.4.1.5).

The rest of the trap rules are short and pointed:

A fixture trap is big enough to drain its fixture quickly and is never smaller than the size the fixture-unit table prints for that fixture, and it is never larger than the drainage pipe it discharges into (IPC 2024, 1002.5).

A building trap, also called a house trap, is prohibited (IPC 2024, 1002.6).

Traps are set level with respect to the trap seal, and where they need it they are protected against freezing (IPC 2024, 1002.7).

A recess made to connect an underground trap, such as the recess serving a bathtub cast in a slab, has sides and a bottom of corrosion-resistant, insect-proof and vermin-proof construction (IPC 2024, 1002.8).

A brittle vitrified clay or similar acid-resisting trap installed underground is embedded in concrete that extends 6 inches beyond the bottom and the sides of the trap (IPC 2024, 1002.9).

In mental health centres, pipes and traps are not left exposed (IPC 2024, 1002.10).

3. Trap sizes read against the fixture-unit table

Trap sizes are not chosen freely: the same table that gives the drainage fixture unit value of a fixture carries the smallest trap that fixture may have (IPC 2024, 709.1). Values read from it:

Fixture	Drainage fixture units	Minimum trap size (inches)
Automatic clothes washer, commercial	3	2
Automatic clothes washer, residential	2	2
Bathtub	2	1 1/2
Bidet	1	1 1/4
Combination sink and tray	2	1 1/2
Dental lavatory	1	1 1/4
Dishwashing machine, domestic	2	1 1/2
Drinking fountain	1/2	1 1/4
Emergency floor drain	0	2
Floor drain	2	2
Kitchen sink, domestic	2	1 1/2
Laundry tray, one or two compartments	2	1 1/2
Lavatory	1	1 1/4
Service sink	2	1 1/2
Sink	2	1 1/2
Wash sink, each set of faucets	2	1 1/2

All of those columns are read from the printed table (IPC 2024, Table 709.1). For a water closet or urinal the table prints no fixed trap size but requires the trap to be consistent with the fixture outlet size, and it prints that for traps larger than 3 inches the other table is used (IPC 2024, Table 709.1). A fixture the table does not list takes its load from its outlet size, and its smallest trap is that outlet size but never under 1 1/4 inches (IPC 2024, 709.2). Where an indirect waste receptor receives the discharge of indirectly connected fixtures, the load of the receptor is the sum of the fixture unit values discharging to it, and never less than the value the tables give for the receptor itself (IPC 2024, 709.4); and a receptor such as a floor drain, floor sink or hub drain that receives only clear-water waste from display cases, refrigerated display cases, ice bins, coolers or freezers is worth one-half fixture unit (IPC 2024, 709.4.1).

4. Interceptors and separators: when they are required

An interceptor or a separator is required in order to stop oil, grease, sand and other substances that are harmful or hazardous to a public sewer, a private sewage system, a sewage treatment plant or its processes from being discharged into it (IPC 2024, 1003.1). The size, the type and the location of each interceptor and each separator are designed and installed to the manufacturer's instructions and to Section 1003, based on the anticipated conditions of use; wastes that need no treatment or separation are not discharged into an interceptor or separator at all (IPC 2024, 1003.2).

5. Grease interceptors and automatic grease removal devices

A grease interceptor or an automatic grease removal device is required to receive the drainage from fixtures and equipment that produce grease-laden waste in food preparation areas — restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias and clubs among them — and the fixtures and equipment named in the printed line include pot sinks, prerinse sinks, soup kettles or similar devices, wok stations, floor drains or sinks into which kettles are drained, automatic hood wash units, and dishwashers without prerinse sinks (IPC 2024, 1003.3.1). Such interceptors and devices receive waste only from the fixtures and equipment that allow fats, oils or grease to be discharged; where space or another constraint prevents installing or replacing a grease interceptor, one or more interceptors may be installed on or above the floor and upstream of an existing interceptor (IPC 2024, 1003.3.1).

Around that requirement sit five short rules:

A food waste disposer does not discharge to a grease interceptor (IPC 2024, 1003.3.2).

A dispensing system that adds interceptor performance additives does not go in, except where it dispenses microbes to enhance aerobic bioremediation of grease and other organic material, or to inhibit the growth of pathogenic organisms by anaerobic methods; such a microbial system is installed only where the interceptor manufacturer allows it and where it conforms to ASME A112.14.6, and systems that discharge emulsifiers, chemicals or enzymes are prohibited (IPC 2024, 1003.3.3).

Neither an interceptor nor an automatic grease removal device is required for an individual dwelling unit or any private living quarters (IPC 2024, 1003.3.4).

Hydromechanical grease interceptors, fats, oils and greases disposal systems, and automatic grease removal devices are sized and designed and tested to the listed standards — ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.1, CSA B481.3, PDI G101 or PDI G102, as the printed line distributes them — and are installed to the manufacturer's instructions, or in their absence to ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101 (IPC 2024, 1003.3.5).

An automatic grease removal device is located downstream of the fixture or fixtures it serves, following the manufacturer's instructions, is sized to pretreat the measured or calculated flows of all connected fixtures or equipment, and is given ready access for inspection and maintenance (IPC 2024, 1003.3.6).

A gravity grease interceptor, with or without a fats, oils and greases disposal system, is sized by multiplication rather than by table: the peak drain flow into the interceptor in gallons per minute is multiplied by a retention time of 30 minutes (IPC 2024, 1003.3.7). Those interceptors are designed and tested to IAPMO/ANSI Z1001, and where a disposal system is part of the assembly, to ASME A112.14.6 as well, and they are installed to the manufacturer's instructions or, in their absence, to those two standards (IPC 2024, 1003.3.7). The discharge piping from a grease interceptor connects directly to the sanitary drainage system (IPC 2024, 1003.3.8).

A hydromechanical grease interceptor must hold a grease retention capacity that matches its flow-through rating. The printed table gives, in total flow-through rating against grease retention capacity in pounds: 4 gpm — 8 pounds; 6 — 12; 7 — 14; 9 — 18; 10 — 20; 12 — 24; 14 — 28; 15 — 30; 18 — 36; 20 — 40; 25 — 50; 35 — 70; 50 — 100; 75 — 150; 100 — 200 (IPC 2024, Table 1003.3.5.1). For a flow-through rating above 100 gpm, the retention capacity in pounds is twice the flow-through rating (IPC 2024, Table 1003.3.5.1). Every grease interceptor is equipped with a device that controls the rate of water flow so that the flow does not exceed the rated flow; that flow-control device is vented and terminates at least 6 inches above the flood rim level, or is installed as the manufacturer directs (IPC 2024, 1003.3.5.2).

6. Oil separators, sand interceptors and the rest

An oil separator is installed where floor or trench drains are provided in a repair garage, at car washing facilities, in factories that produce oily and flammable liquid wastes, and at hydraulic elevator pits. Oily, greasy or flammable wastes are discharged into it before they reach the building drainage system or another point of disposal (IPC 2024, 1003.4). A hydraulic elevator pit is excepted where an approved alarm system is installed, and such an alarm must not stop the pumps used to maintain emergency operation of the elevator by firefighters (IPC 2024, 1003.4). Where light and heavy liquids of different specific gravities are mixed, treated or untreated, they are separated in an approved receptacle (IPC 2024, 1003.4.1).

Oil separators are either listed and labeled, or designed as the section sets out (IPC 2024, 1003.4.2):

The separator has a depth of at least 2 feet below the invert of the discharge drain, and its outlet opening has a water seal of at least 18 inches (IPC 2024, 1003.4.2.1).

Where automobiles are serviced, greased, repaired or washed, or where gasoline is dispensed, the separator holds at least 6 cubic feet for the first 100 square feet of area drained, plus 1 cubic foot for each additional 100 square feet of drained area. A parking garage in which servicing, repairing or washing does not happen and in which gasoline is not dispensed needs no separator, and areas of commercial garages used only to store automobiles need not be drained through one (IPC 2024, 1003.4.2.2).

The remaining interceptors are named by what they catch:

A sand or similar interceptor for heavy solids is designed and located so that it can be reached for cleaning, and it holds a water seal of at least 6 inches (IPC 2024, 1003.5).

A clothes washer discharges through an interceptor fitted with a wire basket or similar removable device that keeps solids of 1/2 inch or larger, string, rags and buttons out of the drainage system. Clothes washers in individual dwelling units, and a single clothes washer designed for a dwelling unit but installed elsewhere, are not required to discharge through an interceptor (IPC 2024, 1003.6).

A bottling plant discharges its process wastes into an interceptor that separates broken glass and other solids before the waste reaches the drainage system (IPC 2024, 1003.7).

Slaughtering room and dressing room drains are equipped with approved separators that keep feathers, entrails and other clogging material out of the drainage system (IPC 2024, 1003.8).

Interceptors and separators are designed so that they cannot become air bound, and they are vented by one of the methods of the chapter on vents (IPC 2024, 1003.9).

Access is provided to every interceptor and separator for service and maintenance, and each is maintained by the periodic removal of the accumulated grease, scum, oil or other floating substances and of the solids deposited in it (IPC 2024, 1003.10).

Materials close the chapter: the materials and the methods used to build and install traps, interceptors and separators comply with Chapter 10 and with the applicable provisions of the chapters on fixtures and on sanitary drainage, and the fittings used have no ledges, shoulders or reductions that could slow or block the flow (IPC 2024, 1004.1).

7. Traps in the indirect waste system

Where a fixture discharges indirectly, the trap belongs to the receptor, not to the fixture. Indirect waste piping discharges through an air gap or an air break into a waste receptor; receptors are trapped and vented and connect to the building drainage system (IPC 2024, 802.3). The one printed exception is a receptor that receives only clear-water waste and does not connect directly to a sanitary drainage system — that receptor needs no trap (IPC 2024, 802.3).

The two connections are separated by how they are built. An air gap is a distance: the space between the indirect waste pipe and the flood level rim of the receptor is at least twice the effective opening of the indirect waste pipe (IPC 2024, 802.3.1). An air break is a relationship: it is provided between the indirect waste pipe and the trap seal of the receptor (IPC 2024, 802.3.2).

Three receptor types carry their own trap or standpipe geometry:

A hub drain is formed as a hub or as a pipe rising at least 1 inch above a water-impervious floor (IPC 2024, 802.4.2).

A standpipe is trapped individually, rises at least 18 inches but no more than 42 inches above the trap weir, and is accessible, together with the drains, for rodding (IPC 2024, 802.4.3).

As an alternative to connecting a laundry tray directly to a drainage system, a laundry tray waste line without a fixture trap may connect to the standpipe of an automatic clothes washer; the standpipe rises at least 30 inches above the weir of the standpipe trap and above the flood level rim of the laundry tray, and the laundry tray outlet is no more than 30 inches horizontally from the side of the standpipe (IPC 2024, 802.4.3.1).

Waste receptors themselves are not concealed: apart from a hub drain receiving only clear-water waste and a standpipe, the outlet is covered by a removable strainer or basket, and a receptor is not installed in a concealed space, a plenum, a crawl space, an attic, or an interstitial space above a ceiling or below a floor, and ready access is provided to it (IPC 2024, 802.4). A receptor is sized for the greatest discharge of all the indirect waste pipes it receives and is installed so that it neither splashes nor floods (IPC 2024, 802.4.1).

One prohibited locational rule refers a trap question back to this chapter: plumbing systems may not be placed in an elevator shaft or an elevator equipment room, but floor drains, sumps and sump pumps are permitted at the base of the shaft provided they are indirectly connected to the plumbing system and comply with Section 1003.4, the oil separator requirement (IPC 2024, 301.6).

8. Vents for traps and for interceptors

A trap that is vented badly is a trap that loses its seal, so the venting rules and the trap rules are read together. The plumbing system carries a system of vent piping that admits or emits air so that no fixture trap seal is subjected to a pressure differential greater than 1 inch of water column (IPC 2024, 901.2), and traps and trapped fixtures are vented by one of the methods the venting chapter sets out (IPC 2024, 901.2.1).

The distance a trap may sit from its vent is printed as a table: 1 1/4-inch trap at 1/4 inch per foot — 5 feet; 1 1/2-inch trap at 1/4 inch per foot — 6 feet; 2-inch trap at 1/4 inch per foot — 8 feet; 3-inch trap at 1/8 inch per foot — 12 feet; 4-inch trap at 1/8 inch per foot — 16 feet (IPC 2024, Table 909.1). A self-siphoning fixture such as a water closet is not limited by that table (IPC 2024, 909.1). Two rules keep the connection sound: the total fall of a fixture drain from its own slope may not exceed the diameter of that drain, and the vent connection to a fixture drain, other than a water closet, is not below the trap weir (IPC 2024, 909.2). A vent is also not installed within two pipe diameters of the trap weir (IPC 2024, 909.3). Where a dry vent runs vertically, it rises at least 6 inches above the flood level rim of the highest trap or trapped fixture it serves, and the exception to that rule is a vent for an interceptor located outdoors (IPC 2024, 905.4).

Interceptors and separators have a venting rule of their own: they are designed so that they do not become air bound, and they are vented by one of the methods of the venting chapter (IPC 2024, 1003.9). Two prohibitions sit close to that subject. The vent system serving a chemical waste drainage system is independent of any sanitary drainage vent system, and it terminates through the roof to the outdoors or to an air admittance valve conforming to ASSE 1049 (IPC 2024, 901.3). And an air admittance valve is not installed in a non-neutralized special waste system, except where it complies with ASSE 1049, is constructed of a material approved under Section 702.5, and is tested for chemical resistance to ASTM F1412 (IPC 2024, 918.8). The chemical system itself is kept apart from the sanitary one: chemical drainage and vent systems are separate from sanitary systems, and chemical waste does not enter a sanitary drainage system until it has been treated as Section 803.1 requires (IPC 2024, 803.2).

9. The grade that leads to an interceptor

One slope rule exists because of interceptors. A horizontal drain is laid in uniform alignment at a uniform slope, at least the value the slope table gives for its size (IPC 2024, 704.1). Where the drain is upstream of a grease interceptor, the slope is not less than 1/4 inch per foot, which is a 2 percent slope, and the note to the slope table applies that value to piping draining to a grease interceptor (IPC 2024, 704.1, IPC 2024, Table 704.1).

10. What is checked on a trap-and-interceptor installation

An installation of this kind is checked in a fixed order, because each check depends on the one before it. First, that every fixture is trapped separately, within the drop and run limits of Section 1002.1, and not double trapped (IPC 2024, 1002.1). Second, that no prohibited trap has been used and that the trap scours itself (IPC 2024, 1002.3, IPC 2024, 1002.2). Third, that the seal is between 2 inches and 4 inches deep and that any seal exposed to evaporation is protected by one of the five printed methods (IPC 2024, 1002.4, IPC 2024, 1002.4.1). Fourth, that the trap size is the size the fixture-unit table prints and is not larger than the drain it discharges into (IPC 2024, 1002.5, IPC 2024, Table 709.1). Fifth, that the interceptor or separator that the use demands is present, sized and installed — the grease interceptor with its retention capacity against its flow rating and its flow-control device, the oil separator with its depth, water seal and drained-area capacity, the sand interceptor with its 6-inch seal, and the clothes washer, bottling and slaughterhouse interceptors against their own rules (IPC 2024, 1003.3.5.1, IPC 2024, 1003.3.5.2, IPC 2024, 1003.4.2.1, IPC 2024, 1003.4.2.2, IPC 2024, 1003.5, IPC 2024, 1003.6, IPC 2024, 1003.7, IPC 2024, 1003.8). Sixth, that the interceptor is vented so that it cannot become air bound, and that the traps it serves sit within their vent distance (IPC 2024, 1003.9, IPC 2024, Table 909.1). And seventh, that it can be reached and emptied, because an interceptor nobody can service is an interceptor that will not work (IPC 2024, 1003.10, IPC 2024, 918.5). Every one of those checks was read from a printed line of the 2024 edition, and none of them was inferred from practice.

Traps, Interceptors and Separators