How a Grease Trap Actually Works
A grease trap does not filter anything. It is a settling tank that uses density and time. Wastewater enters, slows, and separates: solids sink, fats, oils and grease, commonly written FOG, float to the top, and the relatively clear water in the middle leaves through an outlet that draws from below the floating layer.
The whole thing depends on retention time, meaning how long a given gallon of water stays in the tank before it exits. Slow water separates. Fast water does not. This is why flow rate matters as much as tank volume. A trap is rated in gallons per minute, and it is sized against the drainage fixture units connected to it: a 3 compartment sink, a prep sink, a wok range, a dishwasher, a mop sink. Undersize the unit relative to actual flow and the water blows straight through with grease still in suspension, no matter how clean the tank is.
Temperature matters too. A dishwasher discharging 160 degree water into a small interceptor keeps grease liquid and emulsified all the way through the tank and out the outlet. It then cools and hardens in the building drain.
Under Sink Interceptor Versus In Ground Exterior Trap
They are not the same appliance and they fail differently.
An under sink interceptor, sometimes called a hydromechanical grease interceptor, is typically a 20 to 50 pound rated steel or polyethylene box sitting under or beside the sink, often with an integral flow control fitting and vented air intake ahead of it. That fitting exists to throttle flow to the trap's rated gpm. Removing it, which people do when the sink drains slowly, destroys the trap's function immediately. Interceptors are cleaned by hand, frequently, sometimes weekly, and they are the ones that fail from neglect between busy shifts.
An in ground exterior trap or gravity grease interceptor is a 1000 to 3000 gallon concrete or fiberglass tank in the parking lot, with access covers, an inlet tee, an outlet tee and a sample tee on the discharge side for authority testing. Retention time is long and service is done by a licensed hauler with a vacuum truck. These fail from corrosion, not neglect.
Many downtown restaurant spaces in converted mill buildings never had room for an exterior tank, so they run undersized interior units on kitchens far busier than the original design contemplated. That mismatch, not sloppy staff, is usually what a chronic grease problem comes down to.
The 25 Percent Rule
The industry standard, and the one most local authorities write into their FOG programs, is that a grease trap is serviced before the combined thickness of the floating grease layer and the settled solids layer reaches 25 percent of the tank's total liquid depth.
The reason is retention time. At 25 percent you have lost a quarter of your effective volume, water moves through the remaining space faster, and separation efficiency falls off sharply. Past that point the outlet draws from a zone still carrying suspended grease, and you are discharging into the sewer lateral untreated.
How to actually measure it:
- Open the access cover and use a sludge judge or a dowel with a coring sleeve to take a full depth sample.
- Measure the floating FOG layer at the top and the settled solids at the bottom.
- Add them together and divide by the total liquid depth.
- Log the number and the date every time.
That log is the point. Two or three measurements tell you your building's actual accumulation rate, and you set a schedule from data instead of guessing. A kitchen that hits 25 percent in 34 days gets a 30 day interval. FOG programs generally require pumping manifests to be kept on site and produced on inspection.
What Fails First
In order of how often it shows up:
The outlet baffle or outlet tee. This is the component that keeps the floating grease layer from reaching the outlet pipe. In an interceptor it is a removable baffle plate; in a tank it is a sanitary tee turned down into the liquid. It cracks, corrodes at the fasteners, or gets knocked loose during a careless pumping. Once it is gone, grease discharges straight into the lateral and the trap is decorative. The kitchen sees no symptom at all until the building drain blocks.
The sample tee. The tee on the discharge side used to draw a compliance sample is often the most corroded fitting on the system. If it fails, an inspector cannot pull a sample and the site fails on paperwork.
The lid and hardware. Anaerobic bacteria in the settled solids produce hydrogen sulfide gas. In the humid headspace it converts to sulfuric acid on the underside of surfaces, eating concrete, corroding steel lids and seizing bolts. A lid that will not open, or crumbling concrete at the rim, is a hydrogen sulfide problem and a safety issue: a rotted lid in a parking lot is a fall hazard into a confined space.
The inlet tee and internal partitions. Less common but harder to fix, since a failed partition in a two chamber tank short circuits separation entirely.
Diagnosing It Before the Kitchen Shuts Down
Odor is almost always the first symptom. A sulfur or sour smell around the floor drains, the mop sink, or the parking lot cover means the trap has gone anaerobic, the solids layer is thick, and separation has already degraded. Odor is a maintenance alarm, not a nuisance to be masked.
Other early signals worth training staff to report:
- A 3 compartment sink that drains slower at the end of service than at the start
- Gurgling at the floor drain when the dishwasher discharges
- Standing water around the exterior access cover
- Visible grease in the sample tee or at the outlet side of the trap
The failure you do not want skips these entirely: a blown outlet baffle sending grease into the lateral for months, ending in a sewer backup through the kitchen floor drains during dinner service. That is a closure, a Category 3 water remediation in a food space, and a conversation with the health department at once. It is also a discharge violation, and the Greater Lawrence Sanitary District treats what leaves the building.
Building a Schedule That Holds
A workable program for most kitchens:
- Daily: dry wipe pans and scrape plates, screens in every sink basket.
- Weekly: hand clean the under sink interceptor, inspect the baffle, confirm the flow control fitting is present.
- Monthly: measure FOG and solids depth in the exterior trap, log it.
- At the interval your log tells you: full pumpout with the tank emptied completely, not just the top layer skimmed. Skimming leaves the solids that generate the hydrogen sulfide.
- Annually: inspect baffles, tees, lid and hardware with the tank empty, and jet the kitchen branch lines and building drain.
Do not use enzyme or bacteria additives as a substitute for pumping. They emulsify grease so it passes through the trap, which moves the problem downstream into the lateral where it costs far more to fix.
Frequently Asked Questions
How often does a grease trap need to be pumped? Pump when combined FOG and solids reach 25 percent of liquid depth, which for most exterior 1000 to 1500 gallon tanks means every 30 to 90 days and for small under sink interceptors means weekly to biweekly. Measure for two or three cycles and set the interval from your own accumulation rate rather than a generic recommendation.
Why does the kitchen smell even right after the trap was pumped? Because the tank was skimmed rather than fully emptied, or because the odor is coming from the drain lines rather than the trap. Grease coated branch lines and a building drain with a hardened FOG layer produce hydrogen sulfide of their own. A full pumpout plus jetting the lines usually clears it.
Can a restaurant clean its own grease trap? Staff can and should hand clean a small under sink interceptor. An in ground exterior trap needs a licensed hauler with a vacuum truck, both because of the volume and because opening the tank is confined space work with hydrogen sulfide present. Repairs to baffles, tees or the connected drainage piping are plumbing work and need a licensed plumber.
Restaurants that want the trap, the branch lines and the building drain handled as one system should start with our [Commercial Plumbing](https://arosoplumbinglawrence.site/services/commercial-plumbing) page. Aroso Plumbing services kitchens along Essex Street and Broadway and can set a measured schedule for your building rather than a guessed one.
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