Grease Trap Sizing & Design Criteria
All fixtures with the potential to carry grease-bearing wastewater or solids must be plumbed to the grease trap or interceptor.
Note: Do not purchase a grease trap or interceptor before obtaining building plan approval and an Industrial Waste approval letter specifying the required size and design. Purchasing before approval creates risk of rejection and replacement costs. Grease traps and interceptors with a minimum liquid holding capacity below 100 gallons will not be approved for installation.
- Calculating Grease Trap Sizes
Step 1 — Determine fixture units
Calculate the total number of fixture units connected to the trap using the counts below.
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 (e.g., condensate water, beverage station) 2", 3", 4" 2, 3, 4 Hub drain receiving cold, clear water (e.g., condensate water, beverage station) 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) 2", 3", 4" 2, 3, 4 Trench drains (receiving grease-bearing wastes and solids) 2", 3", 4" 4, 6, 8 Step 2 — Calculate flow rating
Multiply the total fixture unit count by three gallons per minute:
Grease trap flow rating = total fixture units × 3 gal/min
Step 3 — Calculate minimum liquid holding capacity
Multiply the grease trap flow rating by 12 minutes:
Grease trap liquid holding capacity (gallons) = flow rating × 12 minutes
Industrial Waste staff will apply this sizing criteria alongside professional judgment during the approval process. A larger trap than specified may be required in many cases; a smaller trap may be approved in rare circumstances.
Note: Commercial food preparation facilities that use a dishwasher must install a minimum 500-gallon (liquid holding capacity) grease interceptor. This provides the required seven-minute retention time in compartment 1 and five-minute retention time in compartment 2, for 12 minutes total. It also helps retain high temperatures until cooling and separation of spent fats, oils and grease (FOG) and solids can occur before discharge to the sewer.
- Grease Trap Design Criteria
Austin Water's Industrial Waste program updated its grease interceptor policies in April 2021.
Gravity Grease Interceptors
The approved design for gravity grease interceptors (GGIs) is as follows:
- 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 Austin Development Services — no exceptions
- Must have two compartments; extremely large volume GGIs may have three compartments
- The first compartment must provide a retention time of no less than seven minutes and the second no less than five minutes (approximately 60/40 split) while operating at rated flow capacity
- Inverts and vents must be external to the compartments
- The flowline (upstream of inlet invert) must be at least 3 inches above the tank's static water level
- The vent must be at least 3 inches above the static water level of the tank
- The inlet must be near an elevation equal to half the tank's static water level height; the outlet must be at least 12 inches above the tank floor
- Adequate flow diffusion features must be provided to evenly distribute flow throughout the GGI — examples include a flow diverter plate in the primary compartment, "tee" piping between the two compartments and "tee" piping on the tank outlet
- Must be constructed of concrete or non-corrodible materials
- Each compartment must be accessible for cleaning and inspection — no exceptions
Exceptions may be considered through the Building Plan Review process. Engineering drawings and supporting performance data must be submitted to and approved by the Director before installation.
Conceptual drawing of a City of Austin-approved gravity grease interceptor design (PDF)
Engineered Gravity Grease Interceptors
Engineered GGIs must meet the following requirements:
- Must include a 60/40 split between compartments for adequate retention time
- Must be constructed of non-corrodible materials
- May have either internal or external inverts
The goal for engineered GGIs is to closely follow the concrete GGI requirements while allowing slight flexibility when site conditions are not ideal for a concrete GGI.
Hydromechanical Grease Interceptors
Hydromechanical grease interceptors (HGIs) must meet the following requirements:
- Must comply with ASME A112.14.3 standard
- Must be flow-rated at 75 gallons per minute (GPM) or higher
- Must be constructed of non-corrodible materials
- Must be installed per ASME A112.14.3 Type D standard (indirect connection without external flow control)
- Must have a minimum liquid capacity consistent with the Industrial Waste approval letter minimum
- HGIs greater than 500 gallons must have a 60/40 split between compartments
HGIs are generally suited for low-FOG establishments such as small sandwich shops, coffee shops and bakeries that use disposable flatware and do not use an automatic dishwasher, due to their compact size. Larger HGI models are also permitted when site conditions are not ideal for a gravity grease interceptor.
Approved grease trap manufacturers list (PDF) — these manufacturers are familiar with Austin Water's criteria.
Note: Do not purchase any grease trap without first confirming with Industrial Waste staff that the specific model will be approved for installation.
- General Design Notes
Clear water and non-grease-bearing waste fixtures:
- Hand sinks are not required to be plumbed to the grease trap
- Fixtures receiving non-grease-bearing wastes may drain through a grease trap but must not be included in grease trap sizing unless routed to a floor sink (e.g., condensate from coolers, beverage-only fixtures)
- Floor sinks with condensate or beverage that have a 1-inch lip are not counted as floor sinks for sizing purposes
Grease-bearing waste fixtures:
- For indirect waste systems using floor sinks and hub drains as receptors for dishwashers and compartment sinks, the fixture unit count is twice (2x) the floor sink or hub drain fixture unit count
- Facilities using dishwashers or wok stoves must install two-compartment grease traps or interceptors with a 12-minute total retention time
- In most cases, grease trap sizing requires a 12-minute total retention time to protect the sanitary sewer and wastewater treatment plants from grease and solids
- In many cases, a larger trap than specified by the sizing criteria may be required at the discretion of Industrial Waste staff
Garbage disposals, grinders and food waste collection systems:
- As of January 2008, garbage disposals and grinders are prohibited in commercial kitchens and all other industrial users of the sanitary sewer system. See the 2011 Food Waste Disposal Memorandum for more information.
- Food waste collection systems that require a potable makeup water supply must not exceed a flow rate of 1.28 gallons per minute and must not include a pre-rinse spray valve
- Food waste collection systems that do not require potable makeup water, such as straining baskets, may include a pre-rinse spray valve provided it complies with the Uniform Plumbing Code (UPC) and local amendments
- To add a food waste collection system or food digestor, complete the Preliminary Request for Food Digestor Installation Form (PDF) and submit to:
Austin Water Industrial Waste
6310 Wilhelmina Delco Drive, 3rd Floor
Austin, TX 78752
Email: IndustrialWaste@austintexas.gov
Grease interceptor installation variance:
- The Grease Interceptor Installation Variance Request form (PDF) is used to request permission to operate without a grease interceptor. This applies only to certain facilities — such as those serving pre-packaged foods in original unopened containers, providing drinks only or cutting fruit. Submit the completed form along with other required Plan Review documents. Industrial Waste staff will determine whether a grease interceptor is required upon initial submittal.
- If a variance is approved and the facility subsequently contributes to a sanitary sewer overflow or other collection system issue, a grease interceptor will be required
- If food type or service operations change, Industrial Waste staff will reassess whether a grease interceptor is needed
- Use of Enzymes, Bacteria and Other Agents
Under Austin City Code § 15-10-22(23), the use of enzymes, chemicals or other agents to cause oil, grease and solids to pass through a pretreatment facility is prohibited.
The following guidelines clarify how this prohibition applies:
- Direct addition to a grease trap is prohibited. The grease trap sizing and design criteria are based on gravimetric separation. Adding enzymes or chemical emulsifying agents interferes with this separation. Adding bacterial or other microbial agents to convert the trap into a biological reactor is also prohibited.
- Addition to floor drains or kitchen fixtures for line maintenance is allowable only if the agents do not interfere with grease trap performance. This use is prohibited if any of the following signs of interference are present:
- The agent extends the time before oil, grease and solids accumulate to effective trap operating capacity
- The agent causes emulsification in the grease trap, shifting the grease from a stratified layer with a dense crust to a uniform pancake batter-like consistency
- Free-floating oil or grease appears downstream of the grease trap
- A grab sample result for fats, oils and grease exceeds the instantaneous maximum concentration limit of 200 mg/L
Violations are subject to penalties of up to $2,000 per violation per day. If misuse of these agents contributes to a sanitary sewer overflow, the violator may also be required to pay cost recovery for cleanup and repair.
This policy does not apply to conventional dishwashing detergents, cleaning agents or food processing chemicals used for dishwashing, kitchen cleaning or food preparation.
Note: Building plans vary from project to project. The information on this page applies to most projects but may not cover every situation, particularly for unique or highly complex projects.
For questions, visit the Industrial Waste Plan Review page or contact Industrial Waste staff at IndustrialWaste@austintexas.gov or 512-974-7293.