Design, Operation, Applications, and Selection Considerations for Grease Interceptors
One of the principles we continue to emphasize at the United States Fats, Oils & Grease Alliance (USFOGA) is that better outcomes begin with better understanding.
FOG management brings together municipalities, wastewater professionals, inspectors, engineers, plumbers, manufacturers, service providers, restaurant operators, facility managers, and many others. Each approaches the issue from a different perspective and often with a different level of technical knowledge.
Effective communication requires us to recognize those differences.
Our responsibility at USFOGA is not simply to tell people what they should do. It is to help people understand why something matters, provide reliable information, and create a common language that allows regulators, businesses, and industry professionals to work together toward better outcomes.
Grease interceptors are a good example.
Most people involved in commercial food preparation understand the basic purpose of an interceptor: keep fats, oils, and grease out of the sanitary sewer system.
Understanding how different interceptors accomplish that—and how design, sizing, installation, operation, and maintenance affect performance—is another matter entirely.
Two of the most common categories are gravity grease interceptors (GGIs) and hydromechanical grease interceptors (HGIs).
Both are designed to separate FOG from wastewater before it reaches the sanitary sewer. They accomplish that objective differently, however, and understanding those differences is important for both the businesses operating them and the Authorities Having Jurisdiction (AHJs) responsible for protecting public infrastructure.
First, What Does a Grease Interceptor Actually Do?
Commercial kitchens generate wastewater containing varying amounts of fats, oils, grease, food particles, and other solids.
Without appropriate controls, some of that material can enter the sanitary sewer system.
As FOG travels through a collection system, it can cool, accumulate, combine with other materials, and contribute to restrictions and blockages. The result can include increased maintenance, sewer backups, sanitary sewer overflows, damage to infrastructure, and additional treatment costs.
The U.S. Environmental Protection Agency recognizes properly designed interceptor or collector devices as one method food service establishments can use to capture FOG before it enters the publicly owned treatment works. Proper design, sizing, installation, maintenance, and kitchen best management practices all contribute to successful FOG control.
At its most basic level, a grease interceptor creates an opportunity to separate FOG from wastewater before that wastewater leaves the facility’s grease-waste system and enters the public sewer system.
How that separation is accomplished is where gravity and hydromechanical systems begin to differ.
Gravity Grease Interceptors: Using Time and Volume
A gravity grease interceptor is generally a larger-capacity device that relies primarily on volume and retention time to allow separation to occur.
The basic principle is straightforward.
Grease-laden wastewater enters the interceptor and its movement slows. The interceptor provides space and time for materials of different densities to separate. FOG that is lighter than water can rise and accumulate toward the top, while heavier solids can settle toward the bottom. Wastewater then moves through the interceptor toward the outlet.
Baffles, compartments, inlet and outlet configurations, and other design characteristics help manage wastewater movement and separation.
Gravity interceptors are commonly installed underground and outside a building, although the specific installation depends on the facility, site conditions, applicable codes, product design, and local requirements.
Their larger physical volume can make them appropriate for operations generating substantial wastewater and FOG loads, but size alone should never be mistaken for performance.
A large interceptor that is improperly sized, poorly installed, inaccessible for service, overloaded with solids, or inadequately maintained can still create problems.
Hydromechanical Grease Interceptors: Managing Flow and Separation
A hydromechanical grease interceptor accomplishes the same fundamental objective using a different design approach.
HGIs use engineered flow characteristics, buoyancy, internal components, and other design features to facilitate the separation and retention of FOG from wastewater.
Unlike gravity interceptors, which historically have often been discussed primarily in terms of liquid volume and retention time, HGIs are commonly evaluated based on rated flow capacity, usually expressed in gallons per minute (gpm), along with grease storage or retention capacity and demonstrated performance.
That distinction is important.
A gravity interceptor and hydromechanical interceptor cannot necessarily be meaningfully compared simply by looking at their physical dimensions or total liquid volume. They represent different approaches to separation and may be evaluated under different design and performance criteria.
Standards such as ASME A112.14.3/CSA B481.1 establish testing and performance requirements for hydromechanical grease interceptors. Other recognized standards and certifications, including PDI G101, may apply depending on the equipment, installation, and locally adopted code.
For the AHJ, facility operator, engineer, or designer, those certifications matter because equipment selection should be based on recognized requirements and demonstrated performance—not simply a marketing description of a product.
A Hydromechanical Interceptor Isn’t Necessarily a Small Grease Trap
This deserves particular attention because terminology within the industry has evolved.
Someone entering FOG management may associate a hydromechanical grease interceptor with the small metal or plastic device traditionally installed beneath a sink.
Those products still exist, but the category has evolved considerably.
Modern hydromechanical systems are available in a broad range of capacities and configurations. Depending on the manufacturer and application, HGIs may serve individual fixtures, groups of fixtures, entire commercial kitchens, or significantly larger applications.
Some modern systems have liquid capacities measured in hundreds or even thousands of gallons.
There are even interceptor designs that have been performance tested and certified under standards applicable to both hydromechanical and gravity classifications.
For a newer AHJ or FOG professional, the important lesson is simple:
Don’t determine what an interceptor is—or how it should perform—based solely on its physical size or what similar equipment looked like twenty years ago.
Look at the design, rating, applicable standards, certification, installation requirements, intended application, and local code.
Gravity vs. Hydromechanical: Which Is Better?
This is probably the wrong question.
A better question is:
Which system is appropriate for this application?
There are applications where a properly designed gravity interceptor may provide an excellent solution. There are others where a properly selected hydromechanical system may be more appropriate.
Selection can depend on factors including:
- Type and volume of food preparation
- Expected FOG generation
- Peak wastewater flow
- Number and type of connected fixtures
- Available installation space
- Existing plumbing configuration
- Accessibility for inspection and service
- Expected solids loading
- Required service intervals
- Local plumbing and sewer requirements
- Manufacturer installation requirements
- Applicable testing and certification standards
- Requirements established by the AHJ
This is why USFOGA encourages collaboration among the facility, AHJ, engineer or designer when required, installer, equipment provider, and service professional.
Each sees a different part of the system.
Good decisions occur when those perspectives are communicated clearly and evaluated together.
Sizing Matters—but It Isn’t the Whole Answer
One of the easiest mistakes for someone new to FOG management is assuming that interceptor sizing is simply a matter of choosing a sufficiently large tank.
It isn’t.
Gravity and hydromechanical systems can use different sizing and performance methodologies. Flow, fixture loading, anticipated FOG generation, grease-storage capacity, solids, operational characteristics, and required service intervals can all become relevant considerations.
For a facility operator, this means an interceptor shouldn’t be selected simply because:
“That’s what we’ve always installed.”
For an AHJ, it also means that requiring a particular physical size without considering the application, adopted code, flow, loading, approved technology, and operating conditions may not always produce the desired result.
Sizing should reflect the system the interceptor is actually expected to serve.
Installation Is Only the Beginning
This may be the most important point in the entire discussion.
A grease interceptor isn’t a “set it and forget it” piece of infrastructure.
It is a FOG-management device that requires ongoing attention.
As grease and solids accumulate, the effective operating conditions within the interceptor change. Eventually those materials must be removed.
Required service frequency can vary considerably based on the amount of FOG and solids generated, interceptor design and capacity, facility operations, and the best management practices being followed.
That means two seemingly similar restaurants may require very different maintenance schedules.
Menu changes can matter.
Changes in customer volume can matter.
Kitchen practices can matter.
Employee training can matter.
The amount of food solids entering the system can matter.
Even a restaurant that operated successfully with a particular interceptor and service schedule for years can change enough operationally that its historical maintenance program is no longer appropriate.
This is why inspection and service records are valuable. They allow the facility and AHJ to move beyond assumptions and evaluate what is actually happening.
The Service Provider Is More Than a Pump Truck
One of the opportunities we see within the FOG industry is better communication between service providers, facilities, and municipalities.
The professional servicing an interceptor may observe conditions no one else regularly sees.
That can include excessive grease accumulation, unusual solids loading, damaged or missing components, inaccessible access points, evidence of unusual flow conditions, unexpected material entering the interceptor, or a service interval that simply doesn’t match actual operating conditions.
Those observations have value.
A quality service record shouldn’t simply say:
“Interceptor pumped.”
Where practical, documentation can help establish what condition existed when the interceptor was opened, what was removed, whether components appeared operational, and whether conditions suggest that something deserves further evaluation.
That information can help the food service establishment manage its responsibilities and help an AHJ better understand what is happening within its FOG program.
It also creates something we strongly value at USFOGA:
Shared accountability supported by useful information.
What Should an AHJ Look For?
An AHJ doesn’t need to become an interceptor manufacturer or plumbing engineer to manage an effective FOG program.
But an inspector or FOG coordinator should understand enough to ask good questions.
What type of interceptor is installed?
Is it gravity, hydromechanical, an automatic grease removal device, or another approved technology?
Was it appropriately sized for the application?
What fixtures and equipment discharge to it? What flow and FOG loads were anticipated? What methodology was used to size it?
Does the installation comply with locally adopted requirements?
National model codes and industry standards are valuable resources, but the locally adopted plumbing code, amendments, sewer-use ordinance, pretreatment requirements, and AHJ requirements ultimately need to be identified.
Is the interceptor accessible?
An interceptor that cannot reasonably be inspected or maintained creates an operational problem regardless of how well it performed when new.
Is it being maintained appropriately?
Don’t rely exclusively on a calendar. Service records and actual observed accumulation provide valuable information about whether the maintenance frequency matches the operation.
Have operations changed?
A change in menu, equipment, ownership, operating hours, production volume, seating, or facility use can materially change FOG generation.
What does the service history tell us?
Repeated downstream problems, excessive accumulation, high solids levels, unusually frequent pumping, or chronic maintenance issues may signal that a broader evaluation is warranted.
The AHJ does not necessarily need to have every answer.
It needs to know which questions should be asked and when additional expertise should be brought into the conversation.
Compliance Does Not Automatically Equal Performance
There is an important distinction between design compliance and operational performance.
An interceptor may have been properly specified, approved, and installed when a facility opened.
That doesn’t guarantee it will perform effectively forever.
Operations change.
Equipment changes.
Menus change.
Maintenance practices change.
People change.
Likewise, discovering FOG downstream of an interceptor does not automatically mean the interceptor technology itself has failed.
The appropriate response is to evaluate the system.
Was the interceptor properly sized?
Is the flow within its design parameters?
Is it being maintained?
Are excessive solids entering it?
Have kitchen operations changed?
Are the appropriate fixtures connected?
Has the interceptor been altered or damaged?
Are employees following appropriate FOG best management practices?
Good FOG management requires us to investigate before we assume.
That is as much a communication principle as it is an operational one.
Learning From the Industry Professionals Doing the Work
One of USFOGA’s strengths is the diversity of expertise represented within our membership and partner network.
Our industry includes municipalities, wastewater professionals, service providers, engineers, manufacturers, technology providers, commercial food operations, and others who encounter different parts of the FOG-management process every day.
That collective knowledge is valuable.
USFOGA partners Zurn, MIFAB, and Schier are examples of manufacturers with substantial experience in grease-interceptor technology and technical education.
Their published technical resources demonstrate the breadth of today’s interceptor market. Zurn offers both gravity and hydromechanical technologies and provides technical information addressing sizing, installation, certification, and maintenance. MIFAB offers hydromechanical systems ranging from smaller applications to large-capacity systems and publishes extensive specification and technical documentation. Schier provides another extensive technical library covering interceptor operation, sizing, hydromechanical separation, performance testing, installation, and maintenance.
These resources can be valuable educational tools for an AHJ, engineer, service provider, facility operator, or other professional trying to better understand interceptor technology.
However, manufacturer information should be one part of the educational process—not the entire decision-making process.
Product information should be considered alongside applicable plumbing codes, recognized standards, local ordinances and requirements, engineering considerations where appropriate, site conditions, actual facility operations, service requirements, and the judgment of the AHJ.
USFOGA’s role is not to declare one manufacturer or technology superior to another.
Our role is to bring credible voices together, improve access to education, and help everyone involved ask better questions.
Readers interested in learning more about the organizations supporting that work can visit the USFOGA Partner Network and explore the technical and educational resources available from our industry partners.
The Interceptor Is Only One Part of the FOG Management System
Perhaps the biggest misconception surrounding grease interceptors is believing that installing one solves the FOG problem.
It doesn’t.
An interceptor is one control within a much larger system.
Effective FOG management also involves kitchen best management practices, employee education, proper disposal of waste grease, solids management, routine inspection, appropriate service intervals, accurate documentation, clear municipal requirements, and consistent communication between everyone involved.
The best interceptor cannot compensate indefinitely for poor operating practices.
Likewise, strong operating practices cannot overcome an interceptor that is fundamentally inappropriate for the application.
The system has to work together.
Creating a Common Language Around FOG
At USFOGA, our goal isn’t to promote one technology over another.
Our role is to help municipalities, commercial facilities, service providers, manufacturers, engineers, and other industry professionals understand the technologies available, the standards governing them, and the responsibilities associated with using them effectively.
Gravity grease interceptors have an important place in FOG management.
Hydromechanical grease interceptors have an important place as well.
So do proper design, installation, maintenance, inspection, documentation, and education.
The right solution begins by understanding the application and asking the right questions.
As USFOGA continues expanding its education, certification, standards, and industry resources, we will continue approaching subjects like this with a commitment to clear communication, responsible decision-making, collaboration, and practical education.
Those values matter because FOG management crosses organizational boundaries. The decisions made by a facility can affect a service provider. The work performed by a service provider can provide important information to an AHJ. The requirements established by an AHJ affect facilities, engineers, installers, and manufacturers. Ultimately, all of those decisions can affect the public wastewater infrastructure we collectively depend upon.
Protecting that infrastructure isn’t the responsibility of any one person or organization.
It is a shared responsibility—and shared responsibility works best when everyone understands their role.
Technical References and Additional Resources
U.S. Environmental Protection Agency (EPA)
National Pretreatment Program – Fats, Oils and Grease. EPA resources provide guidance regarding FOG discharges from food service establishments, best management practices, and the role of grease-control devices.
International Code Council (ICC)
International Plumbing Code (IPC). Provides definitions and requirements addressing grease interceptors, installation, sizing, access, maintenance, and applicable standards. Always verify the code edition and amendments adopted by the applicable jurisdiction.
ASME A112.14.3/CSA B481.1
Hydromechanical Grease Interceptors. Establishes construction, testing, performance, and related requirements applicable to hydromechanical grease interceptors.
Plumbing and Drainage Institute (PDI)
PDI G101. A recognized testing and rating standard applicable to qualifying hydromechanical grease interceptors.
USFOGA Industry Partner Resources
USFOGA partners contribute technical expertise and educational resources covering numerous aspects of FOG management. Zurn, MIFAB, and Schier maintain technical libraries addressing grease-interceptor design, sizing, specifications, installation, testing, certification, operation, and maintenance. Readers can access these organizations and other industry resources through the USFOGA Partner Network.
USFOGA’s identification of partner resources is intended to further industry education and should not be interpreted as an endorsement of one manufacturer, product, or interceptor technology over another.
This article is intended for general educational purposes. Interceptor selection, sizing, design, installation, operation, and maintenance should comply with locally adopted plumbing codes, sewer-use requirements, manufacturer requirements, recognized standards, and requirements of the applicable Authority Having Jurisdiction. Engineering or other qualified professional review should be obtained where required.