6.2 Grease, Oil & Special-Waste Interceptors and Separators

6.2 Grease, Oil & Special-Waste Interceptors and Separators

Quick Answer: IPC 2018 Section 1003.3.5.1 requires a grease interceptor to carry a grease retention capacity of 2 pounds for every 1 gpm of rated flow (Table 1003.3.5.1 — a 50 gpm unit retains 100 lb), a gravity grease interceptor is sized as peak drain flow (gpm) x 30 minutes of retention (Section 1003.3.7), and an oil separator at a repair garage needs 6 cubic feet for the first 100 square feet of drained area plus 1 cubic foot for each additional 100 square feet (Section 1003.4.2.2).

Interceptors and separators are the pretreatment devices that keep grease, oil, sand, glass, hair and other harmful substances out of the public sewer, the private sewage system and the treatment plant (Section 1003.1). Section 1003.2 adds two governing rules that shape almost every exam question in this area: the size, type and location of each device are set by the manufacturer's instructions plus this section, based on the anticipated conditions of use; and wastes that do not require treatment or separation shall not be discharged into any interceptor or separator. That second rule is why you never route a lavatory, a water closet or a clear-water waste line through a grease interceptor — the dilution flow just pushes accumulated grease out the outlet.

1. Where Grease Interceptors Are Required (IPC 1003.3.1)

A grease interceptor or an automatic grease removal device must receive the drainage from fixtures and equipment with grease-laden waste in food preparation areas — restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias and clubs. The code names the fixtures:

• Pot sinks and prerinse sinks
• Soup kettles or similar devices, and wok stations
• Floor drains or sinks into which kettles are drained
• Automatic hood wash units
• Dishwashers without prerinse sinks

Two limits complete the picture:

• Only grease-bearing waste. Interceptors and grease removal devices "shall receive waste only from fixtures and equipment that allow fats, oils or grease to be discharged."
• A food waste disposer shall not discharge to a grease interceptor (Section 1003.3.2). Ground food solids blind the baffles and destroy separation.
• Not required for individual dwelling units or private living quarters (Section 1003.3.4).

Where lack of space prevents installing or replacing an interceptor, Section 1003.3.1 permits one or more interceptors to be installed on or above the floor, upstream of an existing interceptor.

Additives (Section 1003.3.3): dispensing systems that inject emulsifiers, chemicals or enzymes into a grease interceptor are prohibited — they re-liquefy grease so it passes through and congeals in the sewer. Only microbial systems for aerobic bioremediation (or anaerobic pathogen inhibition) conforming to ASME A112.14.6 are permitted, and only where the interceptor manufacturer's instructions allow them.

Exam Trap: Candidates routinely flip the dishwasher and the disposer. The 2018 IPC requires a grease interceptor to serve a dishwasher without a prerinse sink (1003.3.1) and prohibits a food waste disposer from discharging to one (1003.3.2). If an option says "a food waste disposer may discharge to a grease interceptor sized for the additional load," it is wrong under the IPC regardless of the size claimed.

2. Hydromechanical Interceptors: Rating and Sizing (IPC 1003.3.5)

Hydromechanical grease interceptors, FOG (fats, oils and greases) disposal systems and automatic grease removal devices are sized, designed and tested to ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3, PDI G101 or PDI G102, and installed per the manufacturer's instructions. Every unit carries two ratings: a flow-through rating in gpm and a grease retention capacity in pounds, linked by Table 1003.3.5.1.

The relationship is exactly 2 pounds per gpm all the way down the table, and footnote a extends it: for flow-through ratings greater than 100 gpm, double the flow-through rating to get the grease retention capacity in pounds.

Rate-of-flow control (Section 1003.3.5.2): a grease interceptor must be equipped with a device that keeps the flow from exceeding the rated flow. The flow-control fitting must be vented, and the vent must terminate not less than 6 inches above the flood rim level (or be installed per the manufacturer's instructions). The vent admits the air that lets the orifice meter the flow instead of siphoning, and the 6-inch height keeps waste from overflowing out of the air intake.

Worked example — sizing a hydromechanical interceptor (PDI G101 method)

A restaurant has a three-compartment pot sink, each compartment 20 in. long x 18 in. wide x 12 in. deep, draining to an under-sink interceptor mounted in the kitchen.

1. Cubic content per compartment: 20 x 18 x 12 = 4,320 cubic inches
2. Gallons per compartment: 4,320 / 231 = 18.7 gallons (231 cubic inches = 1 gallon)
3. Total fixture capacity: 18.7 x 3 = 56.1 gallons
4. Actual drainage load (fixtures fill to about 75 percent; the wares displace the rest): 56.1 x 0.75 = 42.1 gallons
5. Flow rate, one-minute drainage period: 42.1 / 1 = 42.1 gpm
6. Select the next larger listed size: 42.1 gpm falls between the 35 and 50 gpm rows, so choose the 50 gpm unit, which Table 1003.3.5.1 credits with 100 lb of grease retention.

If site conditions permit a two-minute drainage period, 42.1 / 2 = 21.1 gpm, which rounds up to the 25 gpm / 50 lb unit. Use the one-minute period whenever the interceptor is close-coupled in the kitchen or serves an indirectly connected fixture; a two-minute period assumes only directly connected fixtures and a slower drain-down.

3. Gravity Grease Interceptors (IPC 1003.3.7)

Large in-ground tanks are sized on retention time, not on a flow rating:

Required capacity = peak drain flow into the interceptor (gpm) x 30 minutes

Gravity grease interceptors are designed and tested to IAPMO/ANSI Z1001 (add ASME A112.14.6 when a FOG disposal system is included). In practice they are built as two-chamber tanks: the first, larger chamber takes the shock load and lets grease rise and solids settle; the second polishes the effluent, and an outlet tee draws from below the floating grease layer. Discharge piping from any grease interceptor must be directly connected to the sanitary drainage system (Section 1003.3.8).

Worked example: a school kitchen has a calculated peak drain flow of 45 gpm. Required capacity = 45 x 30 = 1,350 gallons. Standard manufactured tanks run 500, 750, 1,000, 1,500, 2,000 gallons — so specify a 1,500-gallon interceptor. Never round down to the 1,000-gallon tank.

4. Oil Separators (IPC 1003.4)

Oil separators are required at repair garages where floor or trench drains are provided, car washing facilities, factories where oily and flammable liquid wastes are produced, and hydraulic elevator pits. Oil-bearing, grease-bearing or flammable wastes must pass through the separator before entering the building drainage system.

Exception: a separator is not required in a hydraulic elevator pit where an approved alarm system is installed — and that alarm may not shut down pumps used to keep the elevator available to firefighters.

Design requirements:

Section 1003.4.1 adds the physics: a mixture of light and heavy liquids of differing specific gravities must be separated in an approved receptacle — the separator works because oil (specific gravity about 0.85) floats on water (1.0) long enough to be skimmed. Section 1003.4.2.2 also carves out the exemptions: parking garages where no servicing, repair or washing is done and no gasoline is dispensed do not require a separator, and areas of commercial garages used only for automobile storage need not drain through one.

Worked example — oil separator capacity

A Maryland repair garage drains 2,400 square feet of shop floor to a trench drain.

• First 100 sq. ft. = 6 cubic feet
• Remaining area: 2,400 - 100 = 2,300 sq. ft.; 2,300 / 100 = 23 increments, at 1 cubic foot each = 23 cubic feet
• Required capacity = 6 + 23 = 29 cubic feet
• Converted: 29 x 7.48 gal/cu. ft. = about 217 gallons

5. The Other Special-Waste Interceptors

Venting (1003.9): interceptors and separators must be designed so they do not become air bound, and must be vented by one of the methods in Chapter 9. A device that traps air stalls flow and floods the fixture upstream.

Access and maintenance (1003.10): access must be provided to each interceptor and separator for service, and each must be maintained by periodic removal of accumulated grease, scum, oil, floating substances and settled solids. Cleaning frequency is not fixed by the IPC — it is set by the local sewer authority's FOG (fats, oils and grease) program. In Maryland that often means the county or municipal pretreatment coordinator, and in WSSC and Baltimore County territory the local plumbing code governs alongside the state code.

6. Hair, Plaster and Dental Amalgam Separators

The 2018 IPC contains no dedicated hair, plaster or dental amalgam interceptor section. Those devices are required through the general authority of Section 1003.1 (prevent the discharge of substances harmful or hazardous to the sewer) and Section 1003.2 (size, type and location approved based on anticipated conditions of use), plus the local sewer authority's rules. Do not go hunting for a code number that is not there.

Dental amalgam is the exception that carries a hard federal number. Under 40 CFR Part 441 (EPA's Dental Office Point Source Category, effective July 14, 2017), a dental office that places or removes amalgam and discharges to a publicly owned treatment works must operate an amalgam separator compliant with ANSI/ADA Specification 108 or ISO 11143, achieving at least 95 percent removal efficiency. Existing sources had to comply by July 14, 2020 and file a one-time compliance report by October 12, 2020; separators installed before June 14, 2017 may be grandfathered until June 14, 2027. Section 608.17.9 of the IPC covers the potable-water side of the same operatory: dental pumping equipment connections to the potable water system require backflow protection.