Traps, Interceptors, Indirect, and Special Waste

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 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):

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):

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:

The rest of the trap rules are short and pointed:

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:

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 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 remaining interceptors are named by what they catch:

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:

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.

Indirect and Special Waste

Reference text for the indirect and special waste area. Every rule below is written from the printed

line of the 2024 International Plumbing Code (IPC 2024) that carries it, and each rule names the

section it comes from. Where the printed line does not state a value, no value is stated here.

1. Why indirect waste exists

There are drainage applications in buildings where a backup of liquid waste in a drainage system could

contaminate equipment and appliances. Chapter 8 covers the applications that require an indirect

discharge connection to the building's drainage system, and it carries the provisions for the types of

indirect connections and waste receptor configurations (IPC 2024, Chapter 8, About this chapter).

The chapter governs indirect waste piping and special wastes, and further controls food-handling

establishments, sterilizers, humidifiers, clear-water waste, swimming pools, methods of providing air

breaks or air gaps, and neutralizing devices for corrosive wastes (IPC 2024, 801.1). Devices,

appurtenances, appliances and apparatus intended to serve some special function — such as

sterilization, humidification, distillation, processing, cooling, or storage of ice or foods — and

that discharge to the drainage system, must be provided with protection against backflow, flooding,

fouling, contamination and stoppage of the drain (IPC 2024, 801.2).

2. Where an indirect waste connection is required

Food-handling equipment (in other than dwelling units), clear-water waste, humidifiers, dishwashing

machines and utensils, pots, pans and dishwashing sinks must discharge through an indirect waste pipe

as specified in Sections 802.1.1 through 802.1.7. Fixtures not required to be indirectly connected by

that section, and the exception to Section 301.6, must be directly connected to the plumbing system in

accordance with Chapter 7 (IPC 2024, 802.1). (Section 301.6 prohibits plumbing systems in an elevator

shaft or elevator equipment room, with an exception permitting floor drains, sumps and sump pumps at

the base of the shaft where they are indirectly connected to the plumbing system and comply with

Section 1003.4 — IPC 2024, 301.6 and Exception.)

The specific applications:

discharge through an indirect waste pipe by means of an air gap, and each well of a

multiple-compartment sink must discharge independently to a waste receptor (IPC 2024, 802.1.1).

food service and food establishments must be indirectly connected to the sanitary drainage system by

means of an air gap. Where a floor drain is located within an area subject to freezing, the waste

line serving the floor drain must not be trapped and must indirectly discharge into a waste receptor

located outside of the area subject to freezing. Exception: where protected against backflow by a

backwater valve, such floor drains must be indirectly connected to the sanitary drainage system by

means of an air break or an air gap (IPC 2024, 802.1.2).

discharge potable water to the building drainage system, the discharge must be through an indirect

waste pipe by means of an air gap (IPC 2024, 802.1.3).

deck drains discharge to the building drainage system, the discharge must be through an indirect

waste pipe by means of an air gap (IPC 2024, 802.1.4).

boilers discharge nonpotable water to the building drainage system, the discharge must be through an

indirect waste pipe by means of an air break or an air gap (IPC 2024, 802.1.5).

an air gap or air break into a waste receptor in accordance with Section 802.3 (IPC 2024, 802.1.6).

washing, rinsing or sanitizing of utensils, dishes, pots, pans or service ware used in the

preparation, serving or eating of food, must discharge indirectly through an air gap or an air break

to the drainage system (IPC 2024, 802.1.7).

3. Air gap against air break

The distinction the chapter draws is which method is required, not which is convenient:

The two connections are defined by measurement: the air gap between the indirect waste pipe and the

flood level rim of the waste receptor must be not less than twice the effective opening of the

indirect waste pipe (IPC 2024, 802.3.1), while an air break must be provided between the indirect

waste pipe and the trap seal of the waste receptor (IPC 2024, 802.3.2).

4. Materials and installation of indirect waste piping

Materials, joints, connections and methods used for the construction and installation of indirect

waste piping systems must comply with the applicable provisions of Chapter 7 (IPC 2024, 802.2).

Indirect waste piping must discharge through an air gap or air break into a waste receptor; waste

receptors must be trapped and vented and must connect to the building drainage system (IPC 2024,

802.3). Indirect waste piping that exceeds 30 inches (762 mm) in developed length measured

horizontally, or 54 inches (1372 mm) in total developed length, must be trapped. The exception: where

a waste receptor receives only clear-water waste and does not directly connect to a sanitary drainage

system, the receptor does not require a trap (IPC 2024, 802.3 and Exception).

5. Waste receptors

For other than hub drains that receive only clear-water waste and standpipes, a removable strainer or

basket must cover the outlet of waste receptors. Waste receptors must not be installed in concealed

spaces, and must not be installed in plenums, crawl spaces, attics, interstitial spaces above ceilings

and below floors. Ready access must be provided to waste receptors (IPC 2024, 802.4).

Sizing: a waste receptor must be sized for the maximum discharge of all indirect waste pipes served by

the receptor, and receptors must be installed to prevent splashing or flooding (IPC 2024, 802.4.1).

For the drainage load, the drainage fixture unit load of an indirect waste receptor receiving the

discharge of indirectly connected fixtures is the sum of the drainage fixture unit values of the

fixtures that discharge to the receptor, but not less than the drainage fixture unit value given for

the indirect waste receptor in Table 709.1 or 709.2 (IPC 2024, 709.4); and waste receptors such as

floor drains, floor sinks and hub drains that receive only clear-water waste from display cases,

refrigerated display cases, ice bins, coolers and freezers have a drainage fixture unit value of

one-half (IPC 2024, 709.4.1).

(25 mm) above a water-impervious floor (IPC 2024, 802.4.2).

but not greater than 42 inches (1067 mm) above the trap weir, and access must be provided to

standpipes and drains for rodding (IPC 2024, 802.4.3).

directly to a drainage system, a laundry tray waste line without a fixture trap may connect to a

standpipe for an automatic clothes washer drain. The standpipe must extend not less than 30 inches

(762 mm) above the weir of the standpipe trap and must extend above the flood level rim of the

laundry tray; the outlet of the laundry tray must not be greater than 30 inches (762 mm) horizontal

distance from the side of the standpipe (IPC 2024, 802.4.3.1).

6. Special wastes

Corrosive liquids, spent acids or other harmful chemicals that destroy or injure a drain, sewer, soil

or waste pipe, or create noxious or toxic fumes, or interfere with sewage treatment processes, must

not be discharged into the plumbing system without being thoroughly diluted, neutralized or treated by

passing through an approved dilution or neutralizing device. Such devices must be automatically

provided with a sufficient supply of diluting water or neutralizing medium so as to make the contents

noninjurious before discharge into the drainage system, and the nature of the corrosive or harmful

waste and the method of its treatment or dilution must be approved prior to installation (IPC 2024,

803.1).

A chemical drainage and vent system must be designed and installed in accordance with the code, and

chemical drainage and vent systems must be completely separated from the sanitary systems. Chemical

waste must not discharge to a sanitary drainage system until it has been treated in accordance with

Section 803.1 (IPC 2024, 803.2). The vent system for a chemical waste drainage system must be

independent of any sanitary drainage vent system, and its termination must be through the roof to the

outdoors or to an air admittance valve complying with ASSE 1049 (IPC 2024, 901.3).

Air admittance valves must not be installed in nonneutralized special waste systems as described in

Chapter 8, except where such valves comply with ASSE 1049, are constructed of materials approved in

accordance with Section 702.5, and are tested for chemical resistance in accordance with ASTM F1412

(IPC 2024, 918.8).

7. The indirect connection in the drainage load calculation

An indirect waste receptor is a drainage point of the building drain, so its load is computed as the

greater of the sum of the fixture units discharging to it and the value given for the receptor itself

in Table 709.1 or 709.2 (IPC 2024, 709.4). Where the discharge is known only as a flow rate, drainage

fixture unit values are computed on the basis that 1 gpm (0.06 L/s) of flow is equivalent to two

drainage fixture units (IPC 2024, 709.3).