Grease interceptor sizing and maintenance field guide for plumbers

The grease interceptor, and the job it does

A grease interceptor is a tank or fitting that slows kitchen wastewater long enough for fats, oils, and grease to float to the top and food solids to drop to the bottom, so the water leaving for the sewer is clearer than the water that came in. That is the whole job. Slow the flow, let physics separate the grease, hold the grease, and let only the water move on.

It works because grease is lighter than water and barely mixes with it once the water cools and the flow calms down. Wastewater hits the inlet hot and turbulent. Inside the tank it spreads out, slows, and cools, and the grease rises while the heavy bits sink. A baffle or a tee on the outlet pulls clear water from the middle of the column, below the grease cap and above the solids.

What the interceptor is really protecting is the city's sewer main, not your floor drain. Grease that gets past it does not vanish. It congeals on the inside of the public pipe, builds up like cholesterol, narrows the main, and eventually causes the sanitary backup the city traces straight back to your kitchen. The interceptor is the device that keeps that bill from being yours.

Why is FOG regulated?

FOG is regulated because the grease that leaves a kitchen does not stay liquid. It cools in the sewer, hardens on the pipe walls, and combines with wipes and debris into the congealed mass crews now call a fatberg, which blocks the main and causes sanitary sewer overflows the city is on the hook for. So the city pushes the cost back to the source with a pretreatment program.

Most sewer authorities run a FOG program under the EPA pretreatment framework and the local sewer-use ordinance. A food service establishment has to pull a wastewater discharge permit, install and maintain an interceptor, and keep the grease in the discharge under a set limit. That limit is commonly written as 100 mg/L of FOG, often tied to EPA test method 1664, but the number and the testing vary by jurisdiction, so confirm your authority's ordinance.

Enforcement is real. The city can sample the discharge at your sample port, fine you for being over the limit, fine you for a missing or out-of-date pump-out record, and in a bad case pull the discharge permit, which can shut the kitchen. Treat the FOG program the way you treat the health inspector, because it has similar teeth.

Hydromechanical or gravity: which interceptor do you need?

Two families do the same job at different scales. A hydromechanical grease interceptor, the small unit most people still call a grease trap, sits indoors under the sink or on the floor near it, separates grease fast through baffles and air entrainment, and is rated by flow in gallons per minute. A gravity grease interceptor is a large in-ground or outdoor tank, rated by volume in gallons, that uses sheer water volume and long retention to let the grease separate slowly.

The hydromechanical unit is the small-kitchen, low-flow, tight-space answer. It is cheaper, fits indoors, and is serviced more often because it holds less. The gravity tank is the high-volume, full-restaurant answer, often 1,000 gallons and up, set in a parking lot or a yard where a pump truck can reach it. Many jurisdictions now require a gravity interceptor for any new full-service restaurant regardless of fixture count, so the choice is frequently made for you by the local ordinance.

The deciding factors are flow, space, and what the AHJ will accept. Low flow and no room outside points to a hydromechanical unit. A real cooking line and a place to bury a tank points to gravity. When the ordinance names a minimum tank size for your use, that overrides the fixture math.

How do you size a hydromechanical grease interceptor?

You size a hydromechanical interceptor by the drainage flow it has to handle, in gallons per minute, then confirm it has the grease capacity to match. The classic method works off the fixture it serves, usually the compartment sink: take the inside volume of the bowls in cubic inches, convert to gallons, take roughly 75 percent of that because nobody fills to the rim, and divide by the drain-down time, commonly one or two minutes, to get the peak GPM the trap must accept.

Match that flow to a rated unit. Hydromechanical interceptors come in standard ratings such as 20, 25, 35, 50, 75, and 100 GPM, and the trap is certified to remove most of the grease only up to its rated flow. Pick the rated size at or above your calculated peak. Then look at grease capacity: the PDI rating ties grease retention to flow at roughly two pounds of grease for every GPM of rating, so a 20 GPM unit is often listed around 40 pounds. That two-to-one figure is why a fast-flow kitchen still needs a real tank, not the smallest trap on the shelf.

The standards behind this are PDI G101 and ASME A112.14.3 for hydromechanical interceptors. Use the manufacturer's sizing tables and the GPM rating they certify, and confirm the method against the adopted plumbing code, because IPC and UPC handle the fixture math a little differently and the local FOG program can require a size above either.

How do you size a gravity grease interceptor?

You size a gravity interceptor by the kitchen's peak drainage flow and a required retention time, which lands you on a tank volume in gallons. The IPC approach is direct: take the peak drain flow into the interceptor in gallons per minute and multiply by a 30 minute retention time, so 35 GPM of connected fixtures works out to about 1,050 gallons, which rounds up to a standard 1,000 or 1,250 gallon tank.

The UPC and many health departments use a meals-based formula instead: meals served per peak hour multiplied by a waste flow rate, a retention factor, and a storage factor, which on a busy full-service kitchen routinely produces 1,000 to 2,000 gallons. Either path tends to land a real restaurant in the four-figure range, which is why 1,000 to 1,500 gallons is the common floor people quote. Gravity tanks come in standard steps from 300 gallons up, so you round up to the next listed size, never down.

Do not trust a single national formula here. The municipal FOG program very often publishes its own sizing worksheet, and it commonly demands an interceptor 50 to 100 percent larger than the bare code minimum, plus a stated minimum tank for any new restaurant. Size with the local worksheet first, the code formula second, and round up when they disagree.

Field example: sizing a tank for a 120-seat kitchen

Take a 120-seat full-service restaurant with a three-compartment sink, two prep sinks, a mop sink, and a couple of kitchen floor drains, with the high-temp dishwasher kept on its own line per the local rule. Add up the drainage flow the interceptor actually sees at peak and call it 35 GPM. That is the input the sizing turns on, and it is the number people guess at instead of working out.

Run the IPC method: 35 GPM times a 30 minute retention is 1,050 gallons of required volume. There is no standard 1,050 gallon tank, so you round up to a 1,250 gallon unit, never down to 1,000. Now check the local worksheet. If the FOG program wants the size bumped 50 percent for a new full-service kitchen, you are looking at closer to 1,500 to 2,000 gallons, and that bigger number is the one that gets permitted.

The lesson in the spread between 1,050 and 2,000 is the whole game. The conservative tank costs more up front and gets pumped less often, holds its retention through the dinner rush, and keeps you off the city's over-limit list. Undersize it to save a few hundred dollars and you buy a tank that loses retention every busy night and passes grease no matter how clean you keep the inlet.

What is the 25 percent rule?

The 25 percent rule is the maintenance trigger: pump and clean the interceptor when the floating grease cap plus the settled solids at the bottom reach 25 percent of the total liquid depth. Hit that point and you pump, whether or not the calendar says it is time. It is the single most useful number in the trade for knowing when service is actually due.

The reason is retention. The interceptor only separates grease because the water spends enough time in the tank to let the grease rise. Once a quarter of the working depth is grease and sludge, the clear water layer in the middle is too thin and moving too fast, the retention is gone, and grease starts riding the flow straight out the outlet to the sewer. The rule keeps a working volume of water in the tank at all times. The trigger is set conservatively below the point where tanks begin to lose retention, so there is a margin in hand before grease starts to carry over.

You measure it with a core sampler, a sludge judge or a dip stick that traps a column of the contents so you can read the grease and sludge layers against the water. Many ordinances pair the 25 percent rule with a hard backstop, commonly a 90 day maximum, and you service at whichever comes first. A high-volume kitchen can hit 25 percent in six to eight weeks, well before the quarterly date.

What drains to the interceptor, and what must not

Greasy fixtures go to the interceptor. That means the pot sink and three-compartment sink, the prep sinks, the wok and the dipper well, the pre-rinse station, and the kitchen floor drains and mop sink that catch the spills and the mop water. These are the sources of the FOG the device exists to catch, and routing them anywhere else is how grease reaches the sewer untreated.

What never goes to the interceptor is sanitary waste. Toilets, urinals, and any black-water fixture stay on the sanitary line and bypass the trap entirely. Run a toilet to a grease interceptor and you have built a device nobody can legally service and a code violation an inspector will catch on the riser diagram alone.

The dishwasher is the real argument. A high-temperature dishwasher discharges water well above 140°F, hot enough to melt the grease cap and push liquefied grease right through the tank and down the line, where it cools and hardens out of sight. Because of that, many codes, including the baseline UPC language, keep the dishwasher and the food waste disposer off the interceptor unless the AHJ specifically allows it, while some jurisdictions permit a low-temperature machine or a large gravity tank to take it. Check the adopted code and the local ordinance before you tie that line in, because guessing wrong here means a re-pipe.

Flow control and venting

A hydromechanical interceptor needs a flow control fitting ahead of it, and removing that fitting quietly defeats the trap. The fitting is an orifice with a vented air intake that limits the wastewater to the rated GPM no matter how fast the sink dumps, and it draws air in to help separate the grease. Without it, a full three-compartment sink empties faster than the rated flow, blows through the tank, and carries the grease out with the surge.

The air intake is the part people cap, plug, or leave off because it looks optional and it can gurgle. It is not optional. The vent is what lets the orifice meter the flow and what keeps the fitting from siphoning. If you have ever serviced a trap that was passing grease on a kitchen that swore it was maintained, look for a missing or capped flow control before you blame the unit.

Venting matters on the drainage side too. The interceptor sits in a drain-waste-vent system, and the trap arms, the interceptor vent, and the outlet all follow the same DWV rules as the rest of the plumbing. A poorly vented interceptor traps air, slows drainage, and disturbs the grease cap. Size and vent it like the drainage system it is part of, not as an island.

Installation: location, baffles, sample port

Install it where the pump truck can reach it and the kitchen can use it. A gravity tank goes in a yard or parking area with clear access for the vacuum hose and a manhole or pickable lid over each compartment. A hydromechanical unit goes under the counter or on the floor near the fixtures, with enough clearance to lift the lid and pull the baffles for cleaning. An interceptor you cannot open is an interceptor nobody maintains.

The internals do the separating. The inlet baffle or tee knocks down the incoming turbulence and sends flow deep into the tank, and the outlet tee draws clear water from mid-column, below the grease and above the sludge. A gravity tank runs two or more compartments in series so the second stage polishes what the first misses. Get the baffles or tees wrong and the tank short-circuits, sending inlet flow straight to the outlet with no retention.

Two installation details the FOG program checks. First, a sample port on the outlet, a small accessible structure where the city can draw a discharge sample to test against the limit, is required by many ordinances and is the inspector's first stop. Second, a tank under a parking lot or driveway needs an H-20 traffic load rating, designed to the AASHTO H-20 loading so it survives a loaded truck overhead. After setting a gravity tank, prove it watertight before backfill, the same hold-and-watch discipline as a hydrostatic pressure test, because a cracked or leaking interceptor leaks grease into the ground and fails the next inspection.

How often should you pump a grease trap?

You pump when the grease and solids reach 25 percent of the liquid depth, or when the ordinance backstop comes due, whichever is first. There is no single calendar answer because it tracks how much grease the kitchen makes. A small cafe on a hydromechanical trap might be weekly to monthly. A busy restaurant on a 1,000 gallon gravity tank is often monthly to quarterly. A high-volume fryer line can fill the 25 percent mark in well under two months.

Stop guessing and measure. Core-sample the tank on a schedule, log the grease and sludge depth, and let the trend tell you the real interval for that kitchen instead of copying a number off another job. The first few measurements after opening tell you whether the published quarterly interval is generous or already too slow for the volume.

Frequency is also a permit term. Many FOG programs write a maximum interval into the discharge permit, commonly 90 days for a gravity tank and tighter for a small trap, and being past that date is a violation on its own, even if the tank happens to be under 25 percent that week. Hold to the shorter of the measured trigger and the permitted interval.

The full pump-out, not the skim

A real service is a full pump-out: vacuum the entire contents out, scrape the grease off the walls, lid, and baffles, suck the settled solids off the bottom, and inspect the internals before refilling. The interceptor goes back into service essentially empty of grease and sludge. That is the only cleaning that restores the retention the 25 percent rule is protecting.

The scam in this trade is the dipper truck that skims the floating grease cap off the top, hands the kitchen a cheaper invoice, and leaves the settled solids and the grease-soaked baffles in place. The tank looks serviced for a week, then the retention is gone again because the solids layer was never touched. Skimming is not pumping, and a skim-only service against a 25 percent rule is a violation waiting to be sampled. If the invoice is suspiciously cheap and the truck is gone in ten minutes, you got skimmed.

Use a licensed grease hauler, not the kitchen staff and not a shop vac. Most jurisdictions require a permitted hauler with the right truck and a legal disposal site, and self-cleaning a tank can itself be a violation and a disposal liability. The hauler pumps the tank, hauls the FOG to a rendering or treatment site, and gives you the paperwork that proves it happened.

Where the grease goes: rendering and the manifest

The pumped grease does not go down another drain. The licensed hauler takes it to a permitted facility, often a rendering plant that recovers the FOG into feedstock for biodiesel, animal feed, or industrial products, or to a treatment site approved for grease waste. The point is a legal endpoint with a paper trail, not a back lot or a storm drain.

That paper trail is the manifest, the disposal receipt that names the generator, the hauler, the volume removed, the date, and the disposal site. It is the document that proves the grease left your tank and reached a legal destination, and it is the proof the FOG program asks for. Keep every manifest. The kitchen that pumps faithfully but cannot produce the receipts is, on paper, a kitchen that never pumped.

Do grease trap additives and enzymes work?

Enzyme and bacteria additives do not solve a grease problem, and most FOG programs prohibit them. The pitch is that you pour in a product that digests or emulsifies the grease so the tank needs less pumping. What actually happens is the additive breaks the grease into a fine, liquefied form that no longer floats, so it slips through the outlet with the water instead of being held in the tank.

That is the opposite of the job. The interceptor is supposed to hold the grease so it can be hauled away. An emulsifier just moves the grease downstream into the sewer main, where it cools, separates back out, and hardens on the public pipe, which is exactly the fatberg the program exists to prevent. Regulators have caught on. Many ordinances, including a number of major cities, ban enzyme and bacterial additives outright, and the regulatory line is that chemically or biologically liquefying grease is not removal.

Additives also cannot lift the settled solids off the bottom, so even where they slow the surface cap they do nothing for the sludge that drives the 25 percent reading. There is no product you pour in that replaces a full pump-out. If a sales rep promises one, they are selling you a violation.

Odor, H2S, and the neglected trap

A neglected interceptor announces itself by smell before it backs up. Grease and solids sitting too long go anaerobic and give off hydrogen sulfide, the rotten-egg gas, along with the sour, sewage smell that drifts up the floor drains and out of the trap lid into the kitchen and dining room. The odor is the early warning that retention is gone and the tank is overdue.

Hydrogen sulfide is more than a nuisance. In a tank or a confined space it is a real hazard, and over time it corrodes the concrete and metal of the interceptor and the surrounding pipe. An interceptor that reeks is usually one that blew past the 25 percent mark weeks ago.

The cure is the maintenance, not an air freshener or a chemical dose. Pump on the real interval, keep the solids out of the bottom, and the smell goes with the grease. If a tank still smells right after a service, suspect a skim-only pump-out that left the sludge behind.

Automatic grease removal units

An automatic grease removal unit, or AGRU, is a hydromechanical interceptor that skims its own grease on a timer into a separate collection container, so the floating cap is drawn off continuously instead of building until a pump truck comes. The water and solids still need handling, but the unit keeps the grease layer thin between full services.

The trade-off is up-front cost and moving parts against less frequent grease handling and a cleaner main interceptor. AGRUs suit a steady, predictable kitchen where the daily grease load is consistent and someone will empty and maintain the collection side. They are covered by their own standard, ASME A112.14.4, separate from the plain hydromechanical units under A112.14.3.

An AGRU does not erase the 25 percent rule or the solids. It manages the floating grease; the settled sludge still accumulates and still triggers a full pump-out, and the FOG program still wants the manifest. Treat it as a unit that stretches the interval, not one that ends the maintenance.

The records the city wants

The FOG program runs on records as much as on hardware. The discharge permit obligates you to prove the interceptor is maintained, and the proof is paper: the pump-out manifests from the licensed hauler, a maintenance log showing the service dates and the 25 percent readings, and the discharge sample results when the city tests the outlet. A clean tank with no records is, to the city, an unmaintained tank.

Most authorities want these kept for a stated period, commonly a few years, and many require the cleaning report filed with the sewer authority after each pump-out, sometimes by the hauler on the kitchen's behalf. When the inspector shows up, the first ask is for the file, not a flashlight.

This is where keeping the record at the moment of service pays off. A maintenance log built in the field, with the date, the hauler, the volume pumped, the 25 percent measurement, and the manifest attached, is the file that survives an audit. FieldOS exists to capture exactly that service record on site, so the manifest, the reading, and the photo land in one place the operator can hand the city, instead of a shoebox of receipts found after the violation notice.

What to document

Document the unit, the service, and the proof, every time. The interceptor that fails an audit is rarely the dirty one. It is the well-pumped one whose owner cannot produce the manifest. Capture enough that a stranger reading the file a year later can see the tank was sized right, serviced on time, and emptied to a legal site.

Record the unit type and size, the location and access, the flow control on a hydromechanical unit, and for each service the date, the hauler and their license, the volume pumped, the 25 percent reading that triggered it, and the manifest number. Tie the discharge sample results to the same file. That is the record the FOG program asks for and the one that answers the question of whether the tank was ever right.

Common mistakes

• Undersizing the tank to save money, so retention collapses every dinner rush and grease rides the flow to the sewer.
• Removing or capping the flow control fitting and its air intake, which floods a hydromechanical trap and passes grease.
• Pouring in enzyme or bacteria additives that emulsify the grease and move it downstream, which most FOG programs prohibit.
• Accepting a skim-only service that leaves the settled solids and baffle grease, instead of a full pump-out.
• Tying a high-temperature dishwasher into the interceptor where the code does not allow it, so hot water re-melts and passes the grease.
• Pumping faithfully but losing the manifests, so the file cannot prove the grease reached a legal disposal site.
• Letting the tank run past 25 percent or past the permitted interval before scheduling service.
• Setting a tank with no sample port, or under traffic without an H-20 rating, and failing the install inspection.

Field checklist

Standards and references

The installation framework is the adopted plumbing code, the IPC or the UPC depending on the jurisdiction, which sets where an interceptor is required, how it is sized, and what connects to it. The two codes differ on the dishwasher and on the sizing math, so the version your AHJ has adopted, with local amendments, controls the call. Do not assume the code you used on the last job is the code on this one.

The product standards are PDI G101 from the Plumbing and Drainage Institute and ASME A112.14.3 for hydromechanical grease interceptors, with ASME A112.14.4 covering grease interceptors that include an automatic grease removal device. These govern how a unit is rated and certified for flow and grease capacity. Cite them for the rating, not for the sizing of a given kitchen, which the code and the local worksheet drive.

The operating side lives in the municipal FOG program and the sewer-use ordinance, written under the EPA pretreatment framework, and administered by the sewer authority and often the health department. That program sets the discharge limit, commonly around 100 mg/L of FOG but verify locally, the permit, the maintenance interval, the manifest and recordkeeping rules, and the penalties. This is the most local and the most variable layer, and it is the one that fines you. Confirm every number here against the ordinance that actually applies.

Units, terms, and conversions

Grease control carries its own vocabulary, and the same device gets called different things across a permit, a spec, and a service ticket.

FOG is fats, oils, and grease, the regulated material. A grease trap and a hydromechanical grease interceptor, HGI, are the same small flow-rated unit, sized in gallons per minute. A gravity grease interceptor, GGI, is the large in-ground tank, sized in gallons. Flow control is the vented fitting that holds an HGI to its rated GPM. The discharge limit is read in milligrams per liter, mg/L. Retention time, the minutes the water spends in the tank, is what the gravity sizing turns on. The manifest is the disposal receipt that proves where the grease went.

FOG

Fats, oils, and grease, the regulated material the interceptor is built to capture

HGI / grease trap

Hydromechanical grease interceptor, the small flow-rated unit sized in gallons per minute

GGI

Gravity grease interceptor, the large in-ground tank sized in gallons of volume

Flow control

The vented fitting that limits flow to the rated GPM so the trap can separate grease

25 percent rule

The trigger to pump when grease plus solids reach 25 percent of the liquid depth

Retention time

The minutes wastewater stays in the tank, the basis of gravity interceptor sizing

PDI

Plumbing and Drainage Institute, source of the G101 interceptor rating standard

Manifest

The hauler's disposal receipt proving the grease reached a legal disposal site

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