Grease interceptors keep fats, oils, and grease out of the sewer, and getting the size and pump-out schedule right keeps your kitchen running and compliant. These answers cover sizing methods, gravity versus hydromechanical units, cleaning frequency, installation, and cost. Rules vary by city, so always confirm specifics with your local plumbing authority and FOG program.
Hydromechanical (point-of-source) interceptors are sized by flow rate in gallons per minute (GPM), not tank volume. A common method takes the drainage volume of the connected fixtures, divides it by a 1- or 2-minute drain time, and matches the result to a unit's rated GPM and pound grease capacity. Typical indoor units run 20-50 GPM, and most codes require a flow-control fitting to hold flow at the rating. Methods vary, so verify local code/AHJ. Our grease-trap sizing calculator gives a quick estimate.
Gravity interceptors are large in-ground or below-slab tanks sized by total volume, typically 500 to 2,000+ gallons. Sizing usually starts from the drainage load (number of seats, meals per day, or connected fixture units), applies the local code's formula, and rounds up to a standard tank. The tank must hold wastewater long enough for grease to separate and float, often around 30 minutes of retention. Because methods differ widely, sizes vary, so verify local code/AHJ and any FOG-ordinance minimum before ordering.
For a small interceptor, calculate the drainage volume of the fixtures it serves. For a three-compartment sink, multiply each bowl's length by width by depth, convert cubic inches to gallons by dividing by 231, take about 75% fill, and divide by the drain time to get GPM, then match that to a rated unit. Gravity tanks instead use meals-per-day or fixture-unit formulas. Exact factors differ by code, so treat any number as a starting point and confirm with your plumbing official.
These ratings describe liquid capacity, but the tanks are bulky. A 500-gallon tank is roughly 5-6 feet long; a 1,000-gallon tank often runs 6-8 feet long, 4-5 feet wide, and 5-6 feet deep. Larger units need excavation, traffic-rated lids if placed under a driveway, and clear truck access for pumping. Concrete, fiberglass, and plastic models differ, so confirm exact dimensions with the manufacturer's spec sheet before finalizing your site plan.
A 1,500-gallon gravity interceptor is a substantial in-ground tank, commonly around 8-10 feet long, 5-6 feet wide, and 5-6 feet deep, though dimensions vary by manufacturer and material (precast concrete, fiberglass, or HDPE). Tanks this size serve high-volume kitchens or multiple tenants and almost always sit outside. Plan for excavation, proper bedding, watertight risers to grade, and pump-truck access. Get the exact footprint from the maker's drawing before locking in your site plan.
Small under-sink hydromechanical units are often only 12-24 inches tall and fit in a cabinet or recessed in the floor. In-ground gravity interceptors are much deeper, typically 4-6 feet, with inlet and outlet pipes set at different heights so grease floats and solids settle between them. Burial depth also depends on the invert of your building's sewer line. Because the outlet must tie into existing drainage, have a plumber verify depths against your site conditions.
To spec or service a trap, note its rated flow (GPM) and grease capacity (pounds) from the nameplate, plus the physical length, width, and height. For pump-out planning, crews measure total depth and the grease-plus-solids layer to apply the 25% rule. If you are replacing a unit, measure the inlet and outlet pipe sizes and their center heights so the new trap lines up with existing plumbing. Keep the model number handy, since lids and parts are model-specific.
A grease interceptor tank is the large gravity separator, usually 500-2,000+ gallons of concrete, fiberglass, or plastic, installed underground or below the slab and typically outside the building. Wastewater from kitchen fixtures flows in, slows down, and grease rises to the top while food solids sink, so relatively clear water exits to the sewer. It does the same job as a small under-sink trap but with far more capacity and longer retention, which is why high-volume kitchens and FOG ordinances often require one.
A sample box (or sampling well) is a small access structure installed on the outlet side of a grease interceptor. It lets the sewer authority or an inspector draw a wastewater sample to confirm grease discharge stays under the local limit, often measured as FOG in milligrams per liter. Many municipal FOG programs require one on new or upgraded systems. Placement and design are set by local code, so confirm whether your jurisdiction mandates a sample port.
A grease interceptor room is a dedicated, often ground-floor or below-grade space built to house one or more indoor interceptors, common in high-rise or mall food courts where an outdoor in-ground tank is not feasible. It needs floor drainage, ventilation to manage odor, good lighting, and enough clearance for lids to open and for pumping hoses to reach. Requirements vary by building and plumbing code, so coordinate the room early with your plumber and AHJ.
Most jurisdictions tie pumping to the 25% rule: pump when combined floating grease and settled solids reach a quarter of the tank's liquid depth. In practice, small hydromechanical traps may need service weekly to monthly, while large gravity tanks are commonly pumped every 30 to 90 days. Fryer-heavy kitchens fill faster. Your local FOG program may set a hard maximum interval regardless of level, so keep pump-out records and check your permit.
Large interceptors should be fully pumped by a licensed grease hauler that removes all liquid, grease, and solids, scrapes the walls and baffles, and inspects for damage, not just skims the top layer. The waste is manifested and hauled to an approved facility. Small under-sink traps can sometimes be cleaned in-house by scooping grease and scrubbing baffles, but many operators still hire a service for thoroughness and recordkeeping. Never wash the contents down the drain.
A wet/dry shop vac can pull grease and water out of a small under-sink trap, but it is messy, the grease is hard on the machine, and you must dispose of the waste legally, never down a drain or storm sewer. It also will not satisfy jurisdictions that require a licensed hauler and a manifest. For anything beyond the smallest point-of-use unit, professional pumping is cleaner, safer, and gives you the documentation inspectors want.
A licensed hauler backs a vacuum truck to the trap, opens the lid, and vacuums out all grease, water, and solids. Good crews scrape the walls, baffles, and inlet and outlet tees, then often refill the tank with clean water to restore proper operation. They note the grease layer and tank condition and leave a manifest showing the volume removed and the disposal site. Keep that manifest, because inspectors and FOG programs frequently ask to see recent pump-out records.
Grease trap waste is the mix of fats, oils, grease, food solids, and water pumped from your trap, often called brown grease. It cannot go down the drain or in the dumpster. Licensed haulers take it to permitted facilities where it may be dewatered, sent to anaerobic digesters, rendered, or in some regions processed into biodiesel or fertilizer. This differs from clean used fryer oil (yellow grease), which is recycled separately. Disposal rules vary by state, so use a permitted hauler.
A well-maintained gravity interceptor tank can last 20-30+ years, since concrete and fiberglass are durable, though lids, risers, and baffles may need repair sooner. Small metal hydromechanical traps corrode faster, often 5-15 years, especially if grease sits and turns acidic. Regular pumping, keeping the tank refilled with water, and avoiding harsh drain chemicals all extend life. Cracks, leaks, or a failed baffle mean it is time to repair or replace the unit.
Small under-sink hydromechanical traps run roughly $200-$1,500 depending on GPM rating and material. Large in-ground gravity interceptors cost far more, commonly $2,000-$10,000+ for the tank alone, with total installed cost often $5,000-$20,000+ once excavation, plumbing, and traffic-rated lids are included. Concrete is usually cheaper than fiberglass but heavier to set. Prices vary widely by size, region, and site conditions, so get local bids. Size the unit first with our grease-trap sizing calculator.
A 1,000-gallon in-ground interceptor tank typically costs about $1,500-$5,000 for the tank itself, depending on material (precast concrete versus fiberglass) and region. Installed, expect roughly $4,000-$12,000+ once you add excavation, backfill, plumbing tie-ins, risers to grade, and traffic-rated lids if it sits under pavement. Rocky soil, a high water table, or long pipe runs push costs up. Because these swing widely, collect two or three local contractor bids before budgeting.
A 1,500-gallon gravity interceptor tank often runs about $2,500-$7,000 on its own, with fiberglass generally pricier than precast concrete. Fully installed, total cost commonly lands around $6,000-$15,000+ after excavation, setting the tank, plumbing, watertight risers, and any traffic-rated covers. Site access for both the delivery and future pump trucks matters. These figures are ballparks that vary by market and soil conditions, so always base budgets on local bids and your approved plans.
A small point-of-use grease trap (the kind that fits under a sink or next to a dish machine) typically costs about $200-$1,000 for the unit, plus installation. Higher-GPM models, stainless construction, and add-ons like flow-control fittings or automatic skimmers cost more. These indoor traps suit low-grease operations like coffee shops or small cafes, but many full kitchens need a larger interceptor. Check whether your local code sets a minimum size before buying the cheapest option.
Installing a small indoor trap can be a few hundred dollars if the plumbing is accessible. Installing a large in-ground interceptor is a construction project, commonly $3,000-$15,000+ for labor and site work alone, on top of the tank price. Cost drivers include excavation depth, soil and groundwater, concrete cutting, pipe rerouting, risers, traffic-rated lids, and permits. Get itemized local bids, and confirm the plumbing permit and any FOG-program approval are included in the quote.
A licensed plumber sizes the unit, pulls permits, and ties it into the kitchen drain line ahead of the sewer connection, keeping restrooms and other non-grease fixtures off the interceptor. Small traps mount under the sink or recess in the floor. Large tanks require excavation, a level compacted base, setting the tank, watertight risers to grade, and inlet and outlet piping at the correct elevations. An inspection and, in many areas, a FOG-program sign-off follow before use.
Yes. Large gravity interceptors are usually installed outside and underground, which keeps odor and bulk out of the kitchen and gives pump trucks easy access. They need a traffic-rated lid if placed under a driveway or parking area, watertight risers up to grade, and protection from freezing in cold climates. Small hydromechanical traps are typically indoors near the fixtures. Local code dictates what is allowed and where, so confirm placement with your plumber and AHJ.
Recessing a trap in a concrete floor means saw-cutting and removing the slab, excavating to the right depth, running the drain lines, setting the trap so its lid finishes flush with the floor, then backfilling and patching the concrete around it. The lid must seal against odor and be rated for foot or cart traffic. This is skilled plumbing and concrete work, so hire a licensed contractor and have the rough-in inspected before the slab is closed.
Yes, and it is often required when you take over a space, remodel, or change the menu to add fryers or a dish line. Retrofitting means finding room (under a sink, in the floor, or outside) and rerouting the kitchen drains through the trap without disturbing restroom waste lines. Space, slab work, and permits make it more involved than new construction. Expect plan review and inspection, so check with your building and sewer authorities before you start.
Match the type to your grease load and site. Low-grease shops (coffee, small cafe) may only need a small hydromechanical trap sized by GPM, while busy full kitchens with fryers usually need a large gravity interceptor sized by volume. Then confirm the local code minimum, available space (indoor versus underground), ease of pumping access, and material durability. Undersizing means constant backups and violations; oversizing wastes money. Size first, then pick a model, and verify the final choice with your AHJ.
After installation, fill the trap with clean water to the outlet level so it separates grease from day one, confirm the flow-control fitting (if required) is in place, and check that inlet and outlet tees and baffles are seated. Verify no restroom or storm drainage is connected. Label the unit, start a service log, and schedule the first pump-out interval. Many jurisdictions require an inspection and FOG-program registration before the kitchen opens, so line that up early.
An under-sink trap connects to the sink drain outlet ahead of the connection to the building drain, so the fixture drains into the interceptor inlet and the interceptor outlet ties into the sewer line. It should sit level, with a flow-control fitting if code requires one, and enough clearance to remove the lid for cleaning. Vent and trap arrangement must meet plumbing code. It is a straightforward job for a plumber but still typically needs a permit and inspection.
A commercial installation covers sizing to code, permitting, running or rerouting kitchen drain lines through the trap, setting the unit (under-sink, in-floor, or excavated in-ground), tying in inlet and outlet at correct elevations, adding risers and rated lids for interceptors, and testing for leaks. It ends with inspection and, in many cities, FOG-program approval. Because it combines plumbing, sometimes excavation and concrete, and code compliance, use a licensed contractor familiar with your local grease ordinance.
Yes. Shared interceptors are common in strip malls, food halls, and multi-tenant buildings, where several kitchens drain into one large in-ground tank. It can lower per-tenant cost, but responsibility for pumping, recordkeeping, and violations must be spelled out in the lease or a shared-maintenance agreement, since a neglected trap affects everyone. The tank must be sized for the combined peak load. Confirm the arrangement with your landlord, the sewer authority, and your local FOG program.