5.3 Grease Interceptors, Oil Separators & Other Interceptors (IPC 1003)

Why this section matters

Interceptors protect the public sewer and treatment plant from grease, oil, sand and solids. Exam questions ask which fixtures must drain to an interceptor, which must not, and how to size one. All of this is in Section 1003 of the 2021 IPC.

General rules (1003.1–1003.2)

• Interceptors and separators are required to keep oil, grease, sand and other harmful substances out of the public sewer, private sewage system or treatment plant.
• Each one is sized, typed and located per the manufacturer's instructions and this section.
• Wastes that do not need treatment may not be discharged into an interceptor. That includes toilets, urinals and other sanitary waste.

Grease interceptors

Where required (1003.3.1)

A grease interceptor or automatic grease removal device (AGRD) must receive the drainage from grease-laden fixtures and equipment in food preparation areas, such as:

• restaurants;
• hotel, hospital and school kitchens;
• bars, factory cafeterias and clubs.

Listed fixtures include:

• pot sinks and prerinse sinks;
• soup kettles and wok stations;
• floor drains or sinks that receive kettle drainage;
• automatic hood wash units;
• dishwashers without prerinse sinks.

An interceptor may receive only fixtures and equipment that discharge fats, oils or grease. Where space is tight, interceptors may be installed on or above the floor upstream of an existing one.

• 1003.3.4: Interceptors are not required for individual dwelling units or private living quarters.
• 1003.3.2: A food waste disposer may not discharge to a grease interceptor. Ground food fills the unit and degrades separation.
• 1003.3.3: Additives are prohibited except approved microbial systems conforming to ASME A112.14.6 and allowed by the interceptor maker. Emulsifiers, chemicals and enzymes are banned because they push grease downstream.
• 704.1: Drain piping upstream of a grease interceptor must slope at least 1/4 inch per foot.

Hydromechanical grease interceptors (1003.3.5)

These compact units are designed and tested to ASME A112.14.3, ASME A112.14.4, CSA B481.1, PDI G101 or PDI G102. They rely on flow control and baffling.

• 1003.3.5.2, flow control: The interceptor needs a device that limits flow to its rating. The flow control must be vented, terminating at least 6 inches above the flood rim, or installed per the manufacturer.
• Table 1003.3.5.1, grease retention capacity: 2 pounds per gpm of flow-through rating (4 gpm = 8 lb, 20 gpm = 40 lb, 50 gpm = 100 lb, 100 gpm = 200 lb). Above 100 gpm, double the gpm rating to get pounds.

Sizing example (PDI G101 method). A three-compartment pot sink has bowls of 24 × 24 × 14 inches.

1. Volume = $3 \times (24 \times 24 \times 14) = 24{,}192$ cubic inches, or $24{,}192 \div 231 = 104.7$ gallons.
2. Assume 75% fill: $104.7 \times 0.75 = 78.5$ gallons.
3. For a 2-minute drain period: $78.5 \div 2 = 39.3$ gpm. Select the next standard size, 50 gpm.
4. Per Table 1003.3.5.1, a 50-gpm unit must retain 100 pounds of grease.

Gravity grease interceptors (1003.3.7)

Large in-ground tanks are designed and tested to IAPMO/ANSI Z1001. Required capacity:

$$\text{Volume (gal)} = \text{peak drain flow (gpm)} \times 30\ \text{minutes}$$

A 40-gpm peak flow needs at least 1,200 gallons. Gravity units with fats, oils and grease (FOG) disposal systems also meet ASME A112.14.6.

Other installation rules

• 1003.3.6: An AGRD is installed downstream of the fixtures it serves, sized for their measured or calculated flow, with ready access.
• 1003.3.8: The interceptor discharge connects directly to the sanitary drainage system.
• 1003.9: Interceptors and separators must not become air-bound and must be vented by a Chapter 9 method.
• 1003.10: Each needs access for service. Accumulated grease, scum, oil and solids must be removed periodically.

Many sewer authorities enforce a rule that a grease interceptor must be pumped when grease and solids reach a set fraction of its depth, often 25%. That is a local pretreatment rule, not an IPC section.

Oil separators (1003.4)

Where required: in repair garages where floor or trench drains are provided, car washes, factories producing oily or flammable liquid waste, and hydraulic elevator pits. Oil-bearing and flammable wastes must pass through the separator first.

• In elevator pits, an approved alarm system may replace the separator. The alarm may not shut down pumps that firefighters need for emergency elevator operation.
• Exempt: parking garages with no servicing, repairing, washing or fuel dispensing, and storage-only areas of commercial garages.
• 1003.4.2.1, design: depth at least 2 feet below the invert of the discharge drain, and an outlet water seal of at least 18 inches. Listed and labeled separators may be used instead.
• 1003.4.2.2, capacity where vehicles are serviced, repaired or washed, or fuel is dispensed: at least 6 cubic feet for the first 100 square feet drained, plus 1 cubic foot for each additional 100 square feet.

Example. 1,000 square feet of service bay floor needs $6 + 9 \times 1 = 15$ cubic feet, which is about $15 \times 7.48 = 112$ gallons.

Other interceptors

Exam traps

• Disposers never discharge to grease interceptors.
• Dishwashers without prerinse sinks are on the list of equipment that must go to a grease interceptor. Check the manufacturer's temperature limits.
• 2 pounds per gpm for hydromechanical units. 30 minutes of retention for gravity units.
• Oil separator: 6 cubic feet for the first 100 square feet plus 1 cubic foot per additional 100 square feet, with an 18-inch outlet seal.