How to Determine the Size of a Grease Trap for Your Restaurant

In the restaurant and food service industry, managing wastewater effectively is crucial to maintaining a clean and functioning plumbing system. Grease traps are required for facilities where food is prepared and served, as they help intercept fats, oils, and grease (FOG) before they enter the municipal sewage system. Local codes often mandate grease traps for commercial kitchens, including restaurants, cafes, and beverage shops, to prevent clogs and reduce sewer maintenance costs.

Calculation Methods

There are two primary methods for calculating grease trap sizing: the Gravity Method and the Hydromechanical (HGI) Method.

• Gravity Method: This traditional approach determines interceptor size based on the total flow rate of connected fixtures and a standard retention time (typically 30 minutes). It is commonly used for large underground Gravity Grease Interceptors (GGI) and follows plumbing code formulas like UPC and IPC.

• Hydromechanical (HGI) Method: This method, used for Hydromechanical Grease Interceptors (HGI), considers grease production, peak kitchen usage, and fixture type. It provides a more compact and efficient sizing approach, often following manufacturer-specific calculations such as Schier’s Grease Monkey Sizing Method.

Each method serves different applications, with GGIs being preferred for large commercial kitchens and HGIs for smaller, space-constrained establishments.

Steps to Calculate Grease Trap Size

(1) HGI Method:

1. Determine the Flow Rate of Fixtures:

   • Identify the plumbing fixtures that will drain into the grease trap. This typically includes sinks, floor drains, and other fixtures that may contribute grease to the wastewater.

2. Calculate the Total Flow Rate:

   • Let's use an example of a restaurant with a three-compartment sink.

   Example Calculation:

   • Sink Dimensions: 24 inches wide, 24 inches long, 14 inches deep

   • Calculate Volume: 24 × 24 × 14 = 8,064 cubic inches

3. Convert to Gallons: 8,064 ÷ 231 ≈ 34.9 gallons

4. Determine the Flow Rate:

   • Assume a 75% fill factor and a 2-minute drain time:

Flow Rate = (34.9 × 0.75) / 2 ≈ 13.1 GPM (Gallons Per Minute)

Select a Grease Interceptor with a Flow Rate greater than 13.4 GPM, or calculate the total required flow rate to choose the appropriate interceptor.

*SCHIER GB1: 20 GPM/70 LBS; GB3:50 GPM/272 LBS
SCHIER GB75 OUTDOOR: 75GPM/861 LBS

(2) Gravity Method:

For Gravity Interceptors, using GPM x30 equals volume size in gallons.:

1. Size the Grease Trap:

   • For passive grease traps, multiply the flow rate by a retention time factor (typically 30 minutes):

Grease Trap Size = 13.1 GPM × 30 mins ≈ 393 gallons Grease

1. Round Up to the Next Available Size:

   • In this case, a 500-gallon grease trap would be appropriate.

Additional Considerations

• Multiple Fixtures: If your restaurant has several fixtures, calculate the flow rate for each one separately and sum the results to get the total flow rate.

• Local Code Requirements: Different Authorities Having Jurisdiction (AHJ) have varying requirements regarding which plumbing fixtures must be connected to the grease trap. They are often willing to work with owners to find resolutions based on site constraints and the types of use.

• Typical Sizes for Smaller Beverage Shops: Smaller beverage shops usually have grease traps ranging from 25 to 75 GPM, depending on the number of fixtures and the types and locations of the traps.

• Consult Professionals: It's crucial to consult with an architect and plumbing engineer to ensure compliance with local regulations and accurate sizing.

Conclusion

Properly sizing your grease trap is vital for maintaining an efficient plumbing system in your restaurant. Always consult local regulations and a professional plumber for accurate sizing and compliance with local codes.

If you need assistance with grease trap sizing or any other plumbing needs, contact us for expert guidance and a free consultation. We’re here to help you navigate the complexities of plumbing requirements for your restaurant!

Reference: Grease interceptor Sizing Webnar by Plumbing & Drainage Institute