4.2 Grease Interceptors: Where Required, Sizing, Flow Control & Virginia's Disposer Rule

Why grease interceptors are required

Fats, oils and grease (FOG) cool and harden in sewers, causing blockages and sewer overflows. IPC 1003.1 requires interceptors and separators wherever oil, grease, sand or other substances would harm the public sewer, a private sewage system or the treatment process. 1003.2 adds that each interceptor's size, type and location follow the manufacturer's instructions and the code. Wastes that do not need treatment must not be discharged into an interceptor, so water closets, urinals and other sanitary fixtures never connect to a grease interceptor.

Where a grease interceptor is required (IPC 1003.3.1)

A grease interceptor or automatic grease removal device (AGRD) must receive drainage from fixtures and equipment with grease-laden waste in food preparation areas. Examples include restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias and clubs. The code lists:

• pot sinks and prerinse sinks;
• soup kettles and similar devices;
• wok stations;
• floor drains or sinks into which kettles are drained;
• automatic hood wash units;
• dishwashers without prerinse sinks.

Interceptors and AGRDs must receive waste only from fixtures and equipment that can discharge fats, oils or grease. Where space or other limits prevent installing or replacing an interceptor, one or more interceptors may be installed on or above the floor, upstream of an existing interceptor.

1003.3.4: a grease interceptor is not required for individual dwelling units or private living quarters.

Virginia's food waste disposer rule (VA 1003.3.2)

Virginia replaced the model 1003.3.2 with this text:

Where food waste disposers connect to grease interceptors, a solids interceptor shall separate the discharge before connecting to the grease interceptor. Solids interceptors and grease interceptors shall be sized and rated for the discharge of the food waste disposers. Emulsifiers, chemicals, enzymes, and bacteria shall not discharge into the food waste disposer.

Ground food solids would otherwise fill the interceptor quickly and reduce its effective capacity.

1003.3.3 additives: systems that dispense interceptor performance additives are not allowed, except systems that dispense microbes for aerobic bioremediation, or that inhibit pathogens by anaerobic methods. Even these are allowed only if the interceptor manufacturer permits them and the system meets ASME A112.14.6. Systems that discharge emulsifiers, chemicals or enzymes to grease interceptors are prohibited.

Hydromechanical grease interceptors (HGIs) and AGRDs (IPC 1003.3.5)

A hydromechanical grease interceptor separates grease continuously by air entrainment, buoyancy and interior baffling (IPC Chapter 2 definition). HGIs, FOG disposal systems and AGRDs are:

• sized to ASME A112.14.3, A112.14.4, A112.14.6, CSA B481.3 or PDI G101;
• designed and tested to ASME A112.14.3, A112.14.4, CSA B481.1, PDI G101 or PDI G102;
• installed per the manufacturer's instructions.

Table 1003.3.5.1 grease retention capacity:

Above 100 gpm, double the flow-through rating to find the retention capacity in pounds.

1003.3.5.2 flow control: interceptors must have devices that keep the flow at or below the rated flow. The flow-control device must be vented and terminate at least 6 inches above the flood rim level, or be installed per the manufacturer's instructions. A vented flow control draws in air, and the entrained air helps grease rise.

1003.3.6 AGRDs: an automatic grease removal device is located downstream of each fixture or group of fixtures per the manufacturer, is sized for the measured or calculated flow of all connected fixtures, and needs ready access for inspection and maintenance.

Sizing an HGI with the PDI G101 method

The IPC sends you to the sizing standards. PDI G101 uses the fixture's capacity and a drainage period:

1. Volume of each compartment in cubic inches = length × width × depth.
2. Total cubic inches ÷ 231 = gallons.
3. Multiply by 0.75, which allows for the sink not being filled to the brim and for displacement by dishes.
4. Divide by the drainage period (1 or 2 minutes) to get the required flow rating in gpm.
5. Pick the next standard rating at or above that flow, and confirm the retention capacity in Table 1003.3.5.1.

$$Q_{\text{gpm}} = \frac{\dfrac{L \times W \times D \times N}{231} \times 0.75}{t_{\text{min}}}$$

Worked example: a three-compartment pot sink, each compartment 24 × 24 × 14 inches.

• 24 × 24 × 14 = 8,064 cubic inches; × 3 = 24,192 cubic inches.
• 24,192 ÷ 231 = 104.7 gallons; × 0.75 = 78.5 gallons.
• 1-minute drainage: 78.5 gpm, so use a 100 gpm HGI with 200 pounds of grease capacity.
• 2-minute drainage: 39.3 gpm, so use a 50 gpm HGI with 100 pounds of capacity.

Gravity grease interceptors (GGIs) (IPC 1003.3.7)

A gravity grease interceptor is defined as having at least 500 gallons of capacity and separating grease by gravity during a retention time of at least 30 minutes.

• Capacity = peak drain flow into the interceptor (gpm) × 30 minutes.
• Designed and tested to IAPMO/ANSI Z1001. A GGI with a FOG disposal system meets both ASME A112.14.6 and Z1001.
• Installed per the manufacturer's instructions, or per ASME A112.14.6 and Z1001 if there are none.

Example: peak flow of 40 gpm × 30 minutes = 1,200 gallons of required capacity.

Connections and maintenance

• 1003.3.8: the discharge piping from a grease interceptor connects directly to the sanitary drainage system.
• 704.1: horizontal drainage piping upstream of a grease interceptor must slope at least 1/4 inch per foot, even when the pipe is 3 inches or larger.
• 802.1.7 and 802.1.6: pot, pan and dish sinks and commercial dishwashers discharge indirectly through an air gap or air break, so a floor sink often sits between the fixture and the interceptor.
• 1003.9: interceptors must not become air-bound and must be vented by a Chapter 9 method.
• 1003.10: each interceptor needs access for service, and must be maintained by periodically removing accumulated grease, scum, oil, floating substances and solids.