Grease trap vs grease interceptor is mostly a difference of size and of how the unit is rated. Both separate fats, oils and grease from kitchen wastewater before it reaches the sewer. A “grease trap” usually means a small unit indoors, rated by flow in gallons per minute. A “grease interceptor” usually means a large tank outdoors, rated by volume in gallons. The names are used loosely, so the label on your unit matters more than what anyone calls it.
The plumbing code does not rely on those two words. It describes a hydromechanical grease interceptor and a gravity grease interceptor, and knowing which one you have tells you how it should be sized, where it sits and how often it needs emptying.
What both of them do
Kitchen wastewater carries fats, oils and grease, known as FOG. When that water cools in the drain line, the grease hardens on the pipe wall and builds until the line blocks.
Both devices work on the same principle. Grease is lighter than water and floats, and food solids are heavier and sink. The unit slows the wastewater enough for that to happen. The floating layer and the settled solids stay behind, and the cleaner water in the middle flows on to the sewer.
Neither one destroys grease. It collects in the unit until someone removes it. A unit that is never emptied stops separating and passes grease straight through.
The names, and why they cause confusion
In everyday trade talk, “grease trap” means the small box under or near a sink, and “grease interceptor” means the big tank in the ground outside. That is how most restaurant operators, pumpers and many inspectors use the words.
The code uses different terms. California’s plumbing code, which is based on the Uniform Plumbing Code, defines two kinds of interceptor:
- A hydromechanical grease interceptor is identified by its flow rate and by its separation and retention efficiency, and is generally installed inside.
- A gravity grease interceptor is identified by volume, a 30 minute retention time, baffles, at least two compartments, a total volume of not less than 300 gallons, and gravity separation.
Those definitions are from the California Plumbing Code as published by IAPMO. The wording in the current edition may differ in detail, so check the edition your building department is enforcing.
In practice the everyday “grease trap” is a hydromechanical grease interceptor, and the everyday “grease interceptor” is a gravity grease interceptor. Some jurisdictions and some manufacturers use the words differently. If a notice or a permit uses one of these terms, ask the agency exactly what it means by it.
Grease trap vs grease interceptor compared
| Grease trap (hydromechanical) | Grease interceptor (gravity) | |
|---|---|---|
| Rated by | Flow, in gallons per minute | Volume, in gallons |
| Typical ratings | About 10 to 100 gpm | From 300 gallons, commonly 500 to several thousand |
| How it separates | Baffles and a flow control slow the water. Short retention | Large volume gives roughly 30 minutes of retention |
| Typical location | Indoors, under a sink or in the floor | Outdoors, buried, with manhole covers |
| What it serves | One fixture or a small group | The whole kitchen’s grease waste |
| Typical user | Cafe, small kitchen, single prep area | Full-service restaurant, hotel, hospital, cafeteria |
| How it is emptied | By hand or small pump, often by staff or a service | By a vacuum truck from a licensed hauler |
| How often | Frequently, sometimes weekly | Less often, on a schedule set by the sewer agency or by measurement |
| Installed cost | Lower | Higher, with excavation |
| Space needed | Small | A buried tank with truck access |
A third type, the automatic grease removal device, skims grease into a separate container on a timer. It is rated like a hydromechanical unit and still needs regular attention.
How they are sized
Sizing is where the two types differ most, and where under-sized units come from.
A hydromechanical unit is sized by the flow reaching it. The code’s method works from the drain pipe size or from the volume of the fixtures it serves, and gives a required flow rating. Units are tested and listed to standards such as PDI G-101 and ASME A112.14.3, and each has a rated flow and a rated grease capacity.
A flow control fitting is part of the installation. If it is missing, the unit will not perform to its rating.
A gravity unit is sized by the total drainage load of the kitchen. The code table gives a tank volume based on a 30 minute retention time. The local sewer agency often has its own minimum size and its own sizing sheet.
In both cases the sizing decision belongs to the designer, the building department and the sewer agency. It is not something to choose from a catalog by guessing.
Which one your kitchen needs
You rarely get to pick freely. The agency that controls your sewer connection decides what is acceptable.
In the Sacramento area, the Sacramento Area Sewer District’s ordinance says interceptors may be required for any facility used for the sale or preparation of food, and that they must be designed and installed according to the California Plumbing Code at the owner’s expense. Inside the City of Sacramento, the Department of Utilities runs the grease program and inspects food businesses. Its staff make unannounced visits and inspect grease traps and interceptors.
As a general pattern, a new full-service restaurant will be asked for a gravity interceptor outdoors. A small operation with limited grease, or an existing building with no room outside, may be allowed a hydromechanical unit. Dishwashers, garbage disposals and menu type all affect the decision.
If you are leasing a space for a restaurant, find out what is installed and whether it meets current requirements before you sign. Adding a buried interceptor to a building that was never designed for one is a major cost.
Maintenance: where most problems start
A correctly sized unit still fails if it is not emptied.
Hydromechanical units hold little grease, so they fill quickly. A busy kitchen may need to clean one every week or two. The lid comes off, the grease layer and the solids are removed, the baffles are cleaned, and the unit is checked and resealed.
Gravity interceptors are pumped by a licensed hauler with a vacuum truck. The whole contents should come out, not just the top layer. The City of Sacramento’s grease interceptor guidance says to have the interceptor pumped out completely at least once every three months.
Many agencies and haulers also use the 25 percent rule: service the unit when grease and solids together reach a quarter of its liquid depth, because separation drops off beyond that point. We did not find that rule in City of Sacramento or SacSewer documents, so treat it as a common industry practice and follow whatever your own agency requires. Our guide on how often a Sacramento restaurant should clean its grease trap covers the schedule in detail.
Keep the records. Pumping manifests and cleaning logs are what an inspector asks to see.
What goes wrong downstream
When a unit is too small, missing its flow control, or overdue for service, grease passes through and sets in the pipe after it. The first sign is usually slow floor drains, then a backup at the lowest drain in the kitchen during service.
Hot water and detergent make it worse. A dish machine discharging very hot, soapy water into a small unit keeps the grease liquid long enough to carry it through, and it then hardens further down the line.
When that happens, pumping the unit is only half the fix. The line between the kitchen and the unit, and the line after it, need cleaning too. The city’s guidance says the same: clean the sewer lines from the interceptor outlet to the main as well as the lines from the building to the interceptor. Our guide on a restaurant floor drain backing up explains how to tell a local clog from a grease problem downstream, and hydro jetting vs snaking covers which cleaning method removes grease from the pipe wall.
How to tell what you have
You can identify your unit without opening it.
- Look for a label or data plate. A hydromechanical unit is marked with a flow rating in gpm and a grease capacity in pounds. A gravity unit is described in gallons.
- Look at where it is. Under a sink or flush with the kitchen floor suggests hydromechanical. Manhole covers in the parking lot or service yard suggest a gravity tank.
- Check the plans or the permit. The plumbing permit for the kitchen lists the device and its size.
- Ask your hauler. The pumping manifest states the volume removed, which tells you roughly the size of the tank.
- Ask the sewer agency. They often have a record of the device registered at your address.
What not to do
Do not add enzymes, bacteria or emulsifiers to keep the unit “clean” unless your sewer agency allows them. Many agencies prohibit additives that push grease through the unit and into the public sewer.
Do not run very hot water or a garbage disposal through a small unit that was not sized for it.
Do not remove or bypass the flow control on a hydromechanical unit to make a slow sink drain faster.
Do not let staff open and clean a buried interceptor. It is a confined space with hazardous gases, and the work is for trained people with the right equipment.
Do not assume the unit the last tenant left is adequate for your menu.
Frequently asked questions
Is a grease trap the same as a grease interceptor? They do the same job. In common use a grease trap is a small indoor unit rated by flow, and a grease interceptor is a large outdoor tank rated by volume. The code calls them hydromechanical and gravity grease interceptors.
Grease interceptor vs grease trap: which is better? Neither is better in general. A gravity interceptor handles more flow and holds more grease, so it suits a full kitchen. A hydromechanical unit suits a small operation or a single fixture. Your sewer agency decides which is acceptable.
How do I know what size grease trap I need? Sizing follows the plumbing code and local agency rules, based on fixture drainage or pipe size. It should be calculated by a plumber or designer and approved by the building department and the sewer agency.
How often should a grease interceptor be pumped? Follow your sewer agency’s requirement. The City of Sacramento’s guidance says at least once every three months. Small indoor units need cleaning far more often, sometimes weekly.
Can a restaurant operate without a grease trap? In most places, a business that prepares or serves food and discharges to a public sewer is required to have grease control. Check with your sewer agency before opening or changing a menu.
Read the label, then the rules
What a unit is called matters less than what its label says and what your sewer agency requires. Find the rating, find the last service date, and find out who regulates your connection. Those three facts answer most grease questions.
We install, repair and replace grease traps and interceptors, and we clean the lines on both sides of them, for kitchens across the Sacramento area. See commercial kitchen plumbing services, or call (866) 981-1310 with the rating from the data plate and the date it was last serviced.