Grease interceptors are an important part of protecting both private plumbing systems and public wastewater infrastructure from fats, oils, and grease (FOG). But installing an interceptor is only part of the solution.
A grease interceptor that is not properly inspected, maintained, and cleaned can eventually become ineffective allowing the very material it was installed to capture to pass downstream.
That is why many municipal FOG programs rely on a simple concept known as the 25% Rule.
Understanding the rule is important for food-service establishments, facility operators, service providers, inspectors, and municipalities alike. More importantly, understanding why the rule exists can help everyone involved move beyond simply checking a compliance box and toward better FOG management.
What Is the 25% Rule?
In its most common form, the 25% Rule means that a grease interceptor should be serviced when the combined accumulation of floating FOG at the top and settled solids at the bottom reaches 25% of the interceptor’s operating or liquid depth.
Imagine the contents of a conventional gravity grease interceptor as three basic layers:
- Floating fats, oils, grease, and other material at the top;
- Wastewater occupying the middle; and
- Settled food particles and other solids at the bottom.
As the grease layer grows downward and the solids layer grows upward, the effective space available for wastewater separation becomes smaller.
Once those two accumulated layers reach the applicable 25% threshold, many FOG programs require the interceptor to be pumped and cleaned.
Municipalities including Salt Lake City, Dothan, Portland, Seattle and many others incorporate some form of the 25% concept into their FOG requirements or maintenance guidance.
However, there is an important distinction:
The 25% Rule should not be interpreted as one universal federal cleaning requirement.
Maintenance requirements are generally established by the local authority having jurisdiction (AHJ), applicable plumbing and sewer requirements, permit conditions, and, where applicable, manufacturer specifications. Some jurisdictions also establish maximum cleaning intervals—such as 30, 60 or 90 days—even when the interceptor has not yet reached the 25% threshold.
For a facility operator, the correct standard is therefore not simply, “We pump every 90 days.”
The better question is:
“What maintenance frequency keeps this particular interceptor operating properly while meeting the requirements of our local FOG program?”
Why 25% Matters
A conventional grease interceptor depends on separation.
Wastewater entering the interceptor must have sufficient conditions for lighter FOG to separate and rise while heavier solids settle. Proper sizing, installation, flow conditions and maintenance all contribute to that process.
The U.S. Environmental Protection Agency specifically identifies proper design, installation and maintenance as critical to effective FOG control and notes that maintenance frequency depends significantly on the amount of FOG generated and the Best Management Practices used by the establishment.
As accumulated grease and solids consume the interceptor’s usable space, the conditions necessary for effective separation can deteriorate.
That creates the possibility of FOG carryover—grease leaving the interceptor and entering downstream plumbing and eventually the public collection system.
In other words, an interceptor does not need to look “full” to need service.
That is one of the most important concepts for facility operators to understand.
The 25% Rule is a maintenance trigger, not a measurement of whether the tank has room for more waste.
The Calendar Should Not Be the Only Maintenance Tool
A scheduled maintenance program is important, but a calendar alone does not tell us how an interceptor is performing.
Consider two restaurants with similarly sized interceptors.
One may prepare several hundred high-fat meals every day, use multiple fryers, and generate significant food solids. Another may have a lighter menu, lower customer volume, excellent dry-cleaning practices, and relatively little FOG entering its wastewater.
Those facilities may require very different service frequencies.
EPA guidance recognizes this relationship, noting that required maintenance frequency depends heavily on the amount of FOG generated and the BMPs implemented by the establishment.
This is where inspection and measurement become valuable.
If an interceptor reaches 25% accumulation well before its scheduled service date, the facility may need more frequent service or may need to examine what is entering the system.
Conversely, some municipalities allow facilities with demonstrated performance and good maintenance records to seek modified service frequencies. That decision, however, belongs to the applicable authority—not simply to the operator or pumping company.
Maintenance Begins Before FOG Reaches the Interceptor
One of the most effective ways to manage a grease interceptor is to reduce the amount of FOG and solids entering it in the first place.
This is where good kitchen practices become part of infrastructure protection.
Common Best Management Practices include:
- Scraping food waste from plates, pans and cookware before washing;
- Dry-wiping grease from cookware and equipment;
- Using drain screens to capture food solids;
- Collecting and properly recycling waste cooking oil;
- Training employees on FOG procedures;
- Posting reminders near sinks and dishwashing areas;
- Cleaning spills with dry or absorbent methods before mopping; and
- Maintaining written inspection and service records.
EPA guidance has specifically identified dry cleanup, employee training and keeping FOG and food waste out of drains as practices that can reduce loading on grease removal equipment and potentially reduce maintenance costs.
Portland’s Environmental Services similarly recommends dry wiping, recycling waste cooking oil, keeping food waste out of drains and using drain screens as part of commercial FOG management.
The lesson is straightforward:
The best grease interceptor maintenance program begins in the kitchen—not at the interceptor.
What Proper Cleaning Should Accomplish
Proper service should be more than removing the grease floating at the top.
For conventional gravity interceptors, many municipal programs require complete evacuation during cleaning, including floating grease, wastewater and settled solids. Erie County, for example, specifies complete pumping and clearing of excessive solids from walls, baffles, and inlet and outlet areas as part of its maintenance guidance.
This matters because the material at the bottom of an interceptor counts just as much as the grease at the top when evaluating the 25% Rule.
Simply removing the visible grease cap while leaving accumulated solids behind does not restore the interceptor’s full operating condition.
A quality service should also provide an opportunity to identify problems such as damaged or missing components, excessive solids, unusual accumulation, blocked piping, structural deterioration, or other conditions that may affect performance.
Documentation Is Part of Maintenance
Good records serve both the facility and the municipality.
At a minimum, records may include the service date, service provider, condition of the interceptor, measurements or observations, volume removed, disposal information, repairs identified or completed, and the next anticipated service date.
Specific documentation requirements vary by jurisdiction. Salt Lake City, for example, requires commercial and industrial sewer users to retain grease-control-equipment maintenance, cleaning, and repair records for five years. Other jurisdictions establish different retention periods.
Documentation should therefore not be viewed merely as paperwork for an inspector.
Over time, good records tell a story.
They can reveal whether accumulation is increasing, whether cleaning frequency is appropriate, whether kitchen BMPs are working, and whether operational changes are affecting the FOG management system.
Measurement creates information. Information allows better decisions.
Different Interceptors May Require Different Approaches
The 25% Rule is particularly associated with conventional gravity grease interceptors, but FOG control equipment is not all designed or operated in the same way.
Hydromechanical grease interceptors, automatic grease removal devices, gravity interceptors and other technologies may have different capacities, operating characteristics, maintenance requirements, and manufacturer instructions.
Some municipal programs expressly distinguish between equipment types. For example, the Albuquerque Bernalillo County Water Utility Authority’s approved program materials establish different accumulation criteria for certain hydromechanical interceptors while retaining the 25% Rule for gravity interceptors and certain under-sink devices.
This is why USFOGA encourages municipalities and facility operators to avoid a one-size-fits-all approach.
Understand the equipment. Understand the manufacturer’s requirements. Understand the local ordinance. And understand the actual operating conditions of the facility.
Maintenance Is a Shared Responsibility
Effective FOG management does not belong to one participant.
Facility operators control much of what enters the wastewater stream and are typically responsible for maintaining their grease control equipment.
Service providers perform an essential role in inspecting, pumping, cleaning, documenting, transporting, and properly managing collected material.
Manufacturers design equipment to perform within defined operating parameters and provide specifications for its installation, operation and maintenance.
Municipalities and other authorities having jurisdiction establish local requirements, educate stakeholders, inspect facilities, monitor compliance, and protect the public wastewater system.
When these groups communicate and understand one another’s responsibilities, FOG management becomes much more effective.
That is an important part of USFOGA’s mission: bringing the people who regulate FOG, create it, manage it, and develop the systems and services used to control it into a common educational environment.
Compliance Should Be the Minimum—Not the Objective
At USFOGA, we believe effective FOG management works best when people understand why a requirement exists rather than simply being told what they must do.
The 25% Rule is a good example.
The objective is not to pump a tank because someone wrote “25%” into an ordinance.
The objective is to keep grease control equipment operating effectively, protect private plumbing, reduce FOG entering public infrastructure, prevent avoidable blockages and overflows, control operating costs, and create accountability throughout the FOG management process.
A well-maintained grease interceptor protects more than the facility where it is installed.
It protects the service provider called to maintain it. It supports the municipality responsible for the collection system. It protects ratepayers who ultimately bear the costs of public infrastructure. And it helps protect the environment and communities that infrastructure exists to serve.
That is the larger lesson behind the 25% Rule.
Measure before there is a problem. Maintain before there is a failure. Educate before enforcement becomes necessary.
When we approach FOG management that way, the 25% Rule becomes more than a maintenance requirement.
It becomes part of a responsible system for protecting our shared infrastructure.