A grease trap catches fats, oils, and grease from a commercial kitchen before they reach any sewer. A septic tank treats all of a property's wastewater on site, then sends the liquid to a drainfield. A restaurant on septic needs both: grease left in the flow clogs the septic tank and drainfield. The trap is pumped far more often.
What does a grease trap do?
It keeps kitchen grease out of the pipes downstream. Hot, greasy water from sinks and dishwashers cools inside the trap. The grease floats to the top, and food solids settle. The cleaner water in the middle flows out. The plumbing code sets the purpose in Section 1003.1: interceptors "shall be provided to prevent the discharge of oil, grease, sand and other substances harmful or hazardous to the public sewer, the private sewage system or the sewage treatment plant or processes."
In Phoenix, every restaurant kitchen needs one. The city's FOG manual says "All Food Service Establishments that prepare food on site in the City of Phoenix are required to have a grease interceptor to prevent FOG from discharging to the city sewer." It sorts them into two sizes:
- Hydromechanical interceptors, which the manual says "are also commonly known as grease traps." These are small, usually indoors, and approved for kitchens "with up to four plumbing fixtures."
- Gravity grease interceptors, which "are generally much larger" and "are installed outdoors." Phoenix requires them for kitchens "with more than four plumbing fixtures" and for any kitchen with a garbage disposal or dishwasher.
A house doesn't need one. Section 1003.3.4 says a grease interceptor "shall not be required for individual dwelling units or any private living quarters." Our page on the difference between a grease trap and a grease interceptor covers the two commercial types in more detail.
What does a septic tank do?
It treats sewage on the property where there's no city sewer. The EPA describes the tank as "a buried, water-tight container usually made of concrete, fiberglass, or polyethylene." It says the tank's "job is to hold the wastewater long enough to allow solids to settle down to the bottom forming sludge, while the oil and grease floats to the top as scum."
That sounds like a grease trap, and it is, up to a point. The difference is what happens next. Bacteria in the tank digest the organic matter, and the liquid in the middle, called effluent, leaves through a baffled outlet into the drainfield. The EPA explains that the soil there "accepts, treats, and disperses wastewater as it percolates through the soil, ultimately discharging to groundwater." Arizona's design rule, R18-9-A314, requires a tank that is "Designed to produce a clarified effluent and provide adequate space for sludge and scum accumulations."
So a grease trap is a pretreatment device. It removes one problem, grease, and passes the rest along. A septic tank is treatment: it's the start of the process that returns the water to the ground. Our page on septic vs city sewer covers the bigger choice.
How do they compare side by side?
They differ in job, size, and schedule:
| Grease trap or interceptor | Septic tank | |
|---|---|---|
| Job | Catch fats, oils, and grease | Treat all sewage on site |
| What flows in | Kitchen drains only (sinks, dish machine, floor drains) | Every drain in the building |
| Where it sends water | To the sewer or the septic tank | To the drainfield |
| Who needs one | Commercial kitchens | Properties with no sewer connection |
| Bacteria's role | None intended; it separates, it doesn't treat | Digests organic matter |
| Typical Phoenix pumping | Every 30 days (small traps) or every 3 months (large interceptors) | Every 3 to 5 years (EPA) |
| Rules | City FOG program and plumbing code | Arizona on-site wastewater rules |
How often is each one pumped?
A grease trap fills in weeks. A septic tank fills in years.
For grease devices, Phoenix's manual sets the schedule. Hydromechanical interceptors "must be fully pumped out and cleaned every 30 days unless granted a variance by the Director," and "No hydromechanical interceptor may be allowed to go more than 90 days without maintenance." Gravity interceptors must be pumped "at a frequency of not less than once every 3 months unless granted a variance." There's also a fill limit: "If the FOG and solids occupy greater than 25% of the interceptor’s capacity, the FSE is required to perform a full cleaning of the interceptor." See how to tell when a grease trap is full.
For septic tanks, the EPA says "Household septic tanks are typically pumped every three to five years," and Maricopa County gives the same range. The EPA uses a similar percentage rule: a tank should be pumped "if more than 25 percent of the liquid depth is sludge and scum," or if the scum or sludge gets close to the outlet. Maricopa County also says to "clean the effluent filter inside the tank every six to twelve months." See how often to pump a septic tank.
Why does a restaurant on septic need both?
Because grease kills a septic system. A septic tank can hold some grease as scum. But a commercial kitchen sends far more than a household does. Grease that gets past the tank ends up in the drainfield. Maricopa County warns that poor maintenance "can cause solids or greases and oils in the tank to migrate into the drain field and clog the system." Once the soil is sealed with grease, effluent has nowhere to go. The EPA notes an overloaded drainfield "can flood, causing sewage to flow to the ground surface or create backups in toilets and sinks."
That's why the code's interceptor rule names "the private sewage system" alongside the public sewer. On a septic property, the kitchen drains go through the grease interceptor first, and only its outlet goes on to the septic tank. The interceptor has to be sized and maintained like any other, and it usually needs pumping far more often than the septic tank behind it.
### What about a home on septic?
A home doesn't need a grease trap, but it still needs to keep grease out. The EPA's list of things to never flush starts with "Cooking grease or oil," and Maricopa County's list of what not to put down the drain includes "Fat, Grease, Oil." Wipe greasy pans with a paper towel and put it in the trash, and scrape food scraps into the garbage rather than the disposal. The EPA points out that "everything that goes down your drain ends up in your septic system."
### Can one replace the other?
No, never. A grease trap does nothing about the rest of the sewage, and it has no drainfield, so it can't take the place of a septic system. A septic tank can't take the place of a grease interceptor either. It's built to hold sewage while bacteria work, not to catch a kitchen's grease load. Phoenix's FOG rule is written to protect the city sewer, and the plumbing code's interceptor rule covers private sewage systems too, so a restaurant needs a grease interceptor either way.
If a property switches from septic to city sewer, the grease interceptor stays and the old septic tank has to be properly taken out of service. Our page on converting from septic to sewer in Phoenix explains that process.
HQ Plumbing & Air installs grease interceptors and the kitchen drain lines that feed them, and repairs the building drains and sewer lines on both septic and city-sewer properties across the Phoenix area. We're licensed and insured, with free estimates and upfront pricing. See our commercial plumbing services.
The trap stops the grease. The septic tank treats everything else. A kitchen on septic needs the first to protect the second.