Grease Trap Sizing & Design Criteria

Calculating Grease Trap Sizes

1. Step One

To determine the appropriate grease trap size, calculate the total number of fixture units which will be connected to the trap. The following fixture unit counts shall be assigned to each different kind of fixture:

Indirectly Connected Plumbing Fixtures

Using a floor sink or hub drain.

Fixture Type	Trap and Trap Arm Size	Fixture Units
3-compartment sink	2", 3”, 4”	4, 6, 8
2-compartment sink	2”, 3”, 4”	4, 6, 8
1-compartment sink	2", 3”, 4”	4, 6, 8
Dishwasher	3”, 4”	6, 8
Garbage disposal/grinder	Prohibited	N/A
Wok stove	2", 3”, 4”	4, 6, 8
Hand sink	2", 3”, 4”	2, 3, 4
Mop sink	2", 3", 4"	2, 3, 4
Floor sink receiving cold, clear water (eg. condensate water, beverage station, etc.)	2", 3", 4"	2, 3, 4
Hub drain receiving cold, clear water (eg. condensate water, beverage station, etc.)	2", 3", 4"	0

Directly Connected Plumbing Fixtures

Fixture Type	Trap and Trap Arm Size	Fixture Units
Hand sink	2", 3"	0, 0
Mop sink	2", 3", 4"	2, 3, 4
Floor drains	2", 3", 4"	2, 3, 4
Floor sinks (no food service equipment directly above)	3", 4"	3, 4
Trench drains (receiving condensate from ice bins, coolers, beverage only, etc.)	2", 3", 4"	2, 3, 4
Trench drains (receiving grease-bearing wastes and solids)	2", 3", 4"	4, 6, 8

2. Step Two

Determine the minimum flow rating of the grease trap by multiplying the total fixture unit count times three gallons/minute:

Grease trap flow rating = Total fixture unit count x 3 gallons/minute
 

3. Step Three

Calculate the minimum liquid holding capacity of the trap by multiplying the grease trap flow rating (in gallons per minutes) times twelve minutes:

Grease trap liquid holding capacity (gallons) = grease trap flow rating x 12 minutes

Industrial Waste staff shall apply this grease trap sizing criteria in combination with best professional judgment as a part of the approval process. In many cases, a larger grease trap may be required than the one specified by the sizing criteria to adequately protect the City’s sanitary sewer and wastewater treatment plants. In rare circumstances, a smaller grease trap may be approved.

Grease traps/interceptors less than 100 gallons (minimum liquid holding capacity) will not be approved for installation.

Commercial Food Preparation facilities which utilize a dishwasher must install a minimum 500-gallon (liquid holding capacity) grease interceptor. This provides the 7-minutes retention time required in grease interceptor compartment 1, and 5-minutes retention time in grease interceptor compartment 2, equaling the 12-minutes total. This also helps retain high temperatures within the grease interceptor until cooling and separation of spent FOG (Fats, Oil and Grease) can occur, along with solids settling to prevent discharging these materials into the sanitary sewer. 

Grease Trap Design Criteria

Austin Water’s Office of Industrial Waste (IW) modified its grease interceptor policies in April 2021. Please see below for the updated requirements. 

Gravity Grease Interceptors (GGI) 
The approved design for GGIs shall be as follows: 

1. The grease interceptor (GI) must be constructed in accordance with the current plumbing codes adopted by the City of Austin, as amended, and installed in a manner acceptable to the Development Services Division (no exceptions); 
2. The GI shall have two compartments. Extremely large volume GIs may have three compartments; 
3. While operating at the GI's rated flow capacity, the first compartment must provide a retention time of no less than seven minutes, and the second compartment must provide a retention time of no less than five minutes (~60/40 split); 
4.  GI inverts and vents shall be external to the compartments.
5.  The GI flowline (upstream of inlet invert) must be at least 3 inches above the tank’s static water level; 
6.  The GI vent must be at least 3 inches above the static water level of the tank; 
7. The GI inlet must be near an elevation that is one half of the tank's static water level height, and the GI outlet must be at least 12 inches above the tank floor; 
8. Adequate flow diffusion features must be provided to evenly distribute flow throughout the GI. (Examples of such features would include a flow diverter plate in the primary compartment, "tee" piping between the two GI compartments and "tee" piping on the tank outlet); 
9. Be constructed of concrete or non-corrodible materials;
10. Each GI compartment shall be accessible for cleaning and inspection purposes (no exceptions). 
    
    Exceptions to the above criteria may be considered for approval in conjunction with the Building Plan Review process. In such cases, engineering drawings and supporting performance data must be submitted to and approved by the Director prior to GI installation. 
    Conceptual drawing of a City of Austin approved gravity grease interceptor design. (Adobe PDF)

Engineered GGIs 
For engineered GGIs, specific models shall meet the following requirements: 

1. Shall include a 60/40 split between compartments to allow for adequate retention time, similar to a GGI; 
2.  Shall be constructed of non-corrodible materials; 
3.  May have either internal or external inverts. 

The goal for engineered GGIs is to closely comply to the concrete GGI requirements but allow slight flexibility if site conditions are not ideal for a concrete GGI. 

Hydromechanical Grease Interceptors (HGI) 
Hydromechanical grease interceptors shall meet the following requirements: 

1. Must comply with American Society of Civil Engineers (ASME) A112.14.3 standard; 
2. Must be flow-rated at 75-gallons per minute (GPM) or higher; 
3. Must be constructed of non-corrodible materials; 
4. Must be installed per ASME A112.14.3 Type D standard (indirect connection without external flow control); 
5. Must have a minimum liquid capacity consistent with the IW approval letter specified minimum; and 
6.  HGIs greater than 500 gallons must have a 60/40 split between compartments. 

Generally, HGIs are ideal for low FOG-producing establishments such as small sandwich shops, coffee shops, and bakeries that use disposable flatware and do not utilize an automatic dishwasher, due to the HGI’s compact size. However, these requirements also allow larger HGI models if site conditions are not ideal for a Gravity Grease Interceptor. 

View contact information for approved grease trap manufacturers (Adobe PDF) that are familiar with these criteria. HOWEVER, DO NOT PURCHASE ANY GREASE TRAP WITHOUT FIRST VERIFYING WITH OUR OFFICE THAT THE SPECIFIC MODEL WILL BE APPROVED FOR INSTALLATION!

General Design Notes

Clear Water / Non-Grease-Bearing Waste Fixtures:

• Hand sinks are not required to be plumbed to the grease trap.
• Fixtures receiving non-grease-bearing wastes may be drained through a grease trap, but shall not be included for grease trap sizing unless routed to a floor sink (i.e., condensate from coolers, beverage only, etc.).
• Floor sinks with condensate/beverage are not counted as floor sinks if they have a 1" lip.

Grease-Bearing Waste Fixtures:

• For indirect waste systems where floor sinks and hub drains are used as receptors for dishwashers, 1-, 2- and 3-compartment sinks, etc., the fixture unit count shall be twice (2x) the floor sink or hub drain fixture unit count.
• Facilities using dishwashers or wok stoves are required to install 2-compartment grease traps or interceptors with a 12-minute total retention time.
• In most cases, grease trap sizing will include a 12-minute total retention time.  This is necessary to adequately protect the City’s sanitary sewer and wastewater treatment plants from grease or solids.
• In many cases, a larger grease trap may be required than the one specified by the sizing criteria. This is left to the discretion of Industrial Waste personnel.

Garbage Disposals/Grinders & Food Waste Collection Systems:

• As of January 2008, garbage disposals/grinders are prohibited in commercial kitchens and other industrial users of the sanitary sewer system. For additional information, refer to this 2011 Food Waste Disposal Memorandum.
• Food waste collection systems that require a potable make-up water supply shall:
  • Not exceed a flow rate of 1.28 gallons per minute for the make-up supply; and
  • Not include a pre-rinse spray valve.
• Food waste collection systems that do not require a potable make-up water supply, such as straining baskets, may include a pre-rinse spray valve provided it complies with the Uniform Plumbing Code (UPC) and local amendments to the UPC code.
• To receive approval to add a food waste collection system or food digestor, complete the Preliminary Request for Food Digestor Installation Form  and submit it to Austin Water Industrial Waste for plan review via mail or email:
    
  Mail: Industrial Waste – Austin Water
  6310 Wilhelmina Delco Dr., 3rd Floor
  Austin, TX 78752
  
  Email: IndustrialWaste@austintexas.gov

Grease Interceptor Installation Variance Request:

• The Grease Interceptor Installation Variance Request (GIIVR) form Sept 2024 is used to request to operate without a grease interceptor from the Industrial Waste program. This applies to only certain facilities including those serving pre-packaged foods in the original unopened container, providing drinks only or cutting fruit as some examples.
  
  If you feel this is applicable to your project, please fill out the GIIVR form and submit it along with the other required Plan Review documents. Industrial Waste staff will determine whether a grease interceptor is required upon initial submittal.
   
• After a grease interceptor waiver from Industrial Waste has been approved:
  • If the approved facility subsequently causes a Sanitary Sewer Overflow or issues within the wastewater collection system, they will be required to install a grease interceptor.
  • If the food type or food service changes, Industrial Waste staff will reassess whether a grease interceptor is needed based on the change in operations.

Disclaimer: Building Plans vary greatly from one project to another. Please keep in mind that the information above is applicable to most, but not all projects. Additionally, the information above is not intended to, and may not cover every situation, especially for unique or highly complicated projects.

For more information, visit the Industrial Waste Plan Review (Pretreatment Plan Review) webpage or contact the IW staff with questions by email at IndustrialWaste@austintexas.gov or by phone at 512-974-7293.