Straight answers to the grease-trap and interceptor questions restaurant operators actually ask, from sizing and the 25% cleaning rule to venting, cost, and compliance. Estimate your unit with our grease trap sizing calculator, and always confirm the details with your local plumbing code and AHJ.
A grease trap, or grease interceptor, is a plumbing device that slows wastewater from your sinks and dishwasher so fats, oils and grease (FOG) cool and float to the top while heavier food solids settle to the bottom. Cleaner water passes out the middle to the sewer. Capturing FOG keeps it from congealing in your drain lines and the municipal system, where it causes blockages and backups.
They do the same job; the difference is size and location. "Grease trap" usually means a small hydromechanical unit installed indoors under or near a sink. "Grease interceptor" typically means a larger gravity tank, often 500 to 2,000-plus gallons, buried outside. Your flow rate and local code decide which you need. Some jurisdictions use the terms interchangeably, so confirm the definitions with your plumbing code and AHJ.
Almost certainly. Most food establishments that produce FOG, meaning anything with cooking, frying, or a dishwasher, must install grease control before discharging to the sewer. The size and type depend on your fixtures and menu. Requirements are set locally by your sewer authority or plumbing department, so verify the specifics for your area. See our local requirements lookup to start.
In most U.S. jurisdictions, yes. Commercial kitchens discharging FOG must have grease traps or interceptors under local plumbing codes and sewer-use ordinances. Rules vary widely on size, cleaning frequency, and recordkeeping, and a few areas exempt very low-FOG operations. There is no single national standard, so always confirm the exact requirement for your location with your plumbing code, sewer authority, or AHJ before you build.
Sizing is based on drainage flow, not guesswork. Small hydromechanical traps are rated in GPM and pounds of grease; large interceptors are sized in gallons from fixture drainage or seating. The common method calculates the flow from your three-compartment sink and other fixtures, then matches a trap that retains grease at that rate. Try our grease trap sizing calculator, then confirm with local code.
Find each bowl's volume: length by width by depth in inches, divided by 231, gives gallons. Add the bowls, take about 75% for actual fill, then divide by the drain time, usually one to two minutes, to get GPM. Match that flow to a trap's rated capacity. Our sizing calculator runs this for you. Local codes may specify a particular method, so confirm with your AHJ.
The widely used "25% rule" says you must clean or pump the trap once the combined floating grease and settled solids reach 25% of the unit's total liquid depth. For busy kitchens that often means small traps weekly to monthly and large interceptors every one to three months. Cleaning late lets FOG escape to the sewer. Your local ordinance may set a fixed maximum interval, so confirm with your AHJ.
Small under-sink traps can often be cleaned in-house: scoop out the FOG and solids, scrape the walls and baffles, and dispose of the waste properly, never down a drain. It is smelly, messy work, and some jurisdictions require licensed haulers with manifests, especially for larger units. Large in-ground interceptors almost always need a professional pump truck. Check your local rules before doing it yourself.
A small indoor trap can be cleaned in 15 to 45 minutes. A large in-ground interceptor pumped by a service truck typically takes 30 to 90 minutes depending on size and access. Full-service cleaning also includes scraping baffles, inspecting for damage, and hauling the waste, so schedule it during slow hours or after close, and keep the manifest receipts for your inspection records.
Once FOG and solids exceed capacity, the trap can no longer separate grease, so it flows straight into your drain lines and the sewer. That causes slow drains, foul odors, backups into the kitchen, and possible code violations or fines from the sewer authority. An overfull trap can also corrode and fail early. Sticking to the 25% cleaning rule is the way to avoid it.
Trapped FOG and food solids decompose without oxygen, releasing hydrogen sulfide, the rotten-egg smell, and other gases. Odor usually means the trap is overdue for cleaning, the vent or trap seal is compromised, or solids have built up. Regular cleaning on the 25% schedule, proper venting, and keeping the water seal intact control most odors. Persistent smells after cleaning suggest a venting or plumbing issue.
Decomposing grease can release hydrogen sulfide and methane. In normal, well-vented conditions the exposure is low, but in enclosed spaces or while cleaning large interceptors these gases can be hazardous and can even displace oxygen. Treat any confined-space entry seriously, ensure ventilation, and follow OSHA and manufacturer safety guidance. Leave large-interceptor entry to trained, properly equipped professionals.
Yes. Grease traps and interceptors need proper venting so wastewater flows freely and trap seals are not siphoned, which also controls odor and pressure. Venting requirements follow your local plumbing code, often based on the IPC or UPC. Improper venting causes slow drainage, gurgling, and smells. Confirm the exact venting and P-trap arrangement with your licensed plumber and AHJ.
Traps must sit between the grease-producing fixtures, such as the three-compartment sink, prep sinks, and dishwasher, and the sewer connection, and be accessible for cleaning. Small hydromechanical units go indoors near the sinks; large interceptors are usually buried outside near the kitchen. Some codes exclude the dishwasher upstream. Placement rules vary, so confirm the layout with your local plumbing code and AHJ.
A small indoor hydromechanical trap typically runs from a few hundred dollars up to roughly $1,500 for the unit. Large in-ground interceptors of 500 to 2,000-plus gallons commonly cost several thousand dollars installed, and excavation, plumbing, and permits add more. Ongoing cleaning is a separate recurring cost. Prices vary by size, material, and region, so get local quotes.
Cleaning a small trap in-house costs little beyond labor and disposal. Professional service for a small unit often runs roughly $100 to $300 per visit; large in-ground interceptors pumped by truck can run several hundred dollars or more depending on gallons and access. Frequency, driven by the 25% rule, is the biggest cost factor. Get local quotes and keep your manifests.
Grease-trap waste, called brown grease, is regulated and cannot go down any drain or into the trash in most areas. Licensed haulers pump it and take it to an approved treatment or rendering facility, giving you a manifest as proof. Keep those records for inspections. Yellow grease, meaning clean fryer oil, is separate and can often be recycled. Confirm disposal rules for your area with your AHJ.
A manifest is the documented record a licensed hauler gives you each time your trap or interceptor is pumped. It shows the date, the volume removed, the hauler's details, and the disposal destination. Many sewer authorities require you to keep manifests on-site, often two to three years, as proof of compliant cleaning, and inspectors ask for them. Confirm the required retention period with your local AHJ.
It depends on what you make. A cafe with espresso, milk, pastries, and a dishwasher still discharges some FOG and often needs at least a small trap. A full food menu increases the requirement. Some low-FOG operations may qualify for an exemption or a smaller unit. Because coffee-shop rules vary a lot, confirm the requirement for your area with your local sewer authority and AHJ.
Often, yes. Bakeries producing buttery, oily, or fried items and running dishwashers discharge FOG that most sewer authorities require to be intercepted. A low-grease bakery might qualify for a smaller unit or an exemption. As always, the requirement and size depend on your fixtures and local ordinance, so confirm with your plumbing code and AHJ before build-out.
You cannot simply remove required grease control; doing so typically violates the plumbing code and your sewer permit. If you are changing use, replacing an undersized unit, or reconfiguring plumbing, work through a licensed plumber and get approval from the AHJ. Abandoned in-ground interceptors usually must be properly pumped and decommissioned. Confirm the process locally before making changes.
Yes, when sized and maintained correctly. By slowing flow and letting grease float and solids settle, a properly sized trap captures the large majority of FOG before it reaches the sewer. They fail when undersized for the kitchen's flow, poorly vented, or cleaned too rarely, and grease then passes straight through. Correct sizing and the 25% cleaning rule are what make them effective.
An inspector or your service provider checks that the trap is the required size, properly installed and vented, not exceeding the 25% grease-and-solids level, and that you have current cleaning manifests. Sewer authorities may inspect periodically and issue violations for overdue cleaning or missing records. Keeping to your cleaning schedule and retaining paperwork is the easiest way to pass. Confirm inspection frequency locally.
A hydromechanical grease interceptor (HGI), also called a grease trap, uses flow control, baffles, and air entrainment to separate grease quickly in a compact unit. Rated in GPM and pounds of grease, HGIs are common indoors near the sinks and are sized to the fixtures' flow using standards such as PDI G101. Gravity interceptors are the larger buried alternative. Confirm which type your code requires.
A gravity grease interceptor (GGI) is a large tank, commonly 500 to 2,000-plus gallons, buried outside that relies on long residence time for grease to float and solids to settle. Because of their volume, many jurisdictions require GGIs for higher-flow kitchens. They are pumped by service trucks, and sizing follows local formulas based on drainage fixtures or seating. Confirm the required size with your AHJ.
Large gravity interceptors are sized in gallons using local formulas, often based on drainage fixture units, kitchen flow rate with a retention factor, or seats and meals per day. Hydromechanical units are sized by GPM from your fixtures. Undersizing is a common, costly mistake. Our grease trap sizing calculator gives a starting estimate; always confirm the required method and size with your AHJ.
Grease control is generally required wherever a commercial kitchen discharges FOG to the sanitary sewer, including restaurants, cafes, bakeries, and commissaries. Businesses on septic systems often face separate, sometimes stricter requirements. The exact triggers, sizes, and exemptions are set by local sewer-use ordinances and plumbing codes, which vary widely. Confirm what applies at your address with the local AHJ.
On septic, a grease interceptor is often even more important, because FOG quickly clogs the drain field and shortens tank life. Many health departments require a dedicated interceptor ahead of the septic tank for food establishments, and sizing and separation rules can be stricter than on municipal sewer. Confirm septic-specific requirements with your local health department and AHJ.
Yes. Fixtures draining into the grease trap still need proper P-traps to maintain water seals and block sewer gas, and the trap itself connects into a vented, trapped drainage system. The exact arrangement of traps, vents, and flow-control fittings is set by your plumbing code, whether IPC or UPC. Getting this wrong causes odors and slow drains, so have a licensed plumber confirm the layout.
Avoid enzymes, chemicals, or hot-water flushing meant to "dissolve" grease. They typically just liquefy FOG so it passes through and re-solidifies downstream in the sewer, which many ordinances prohibit. The only reliably compliant maintenance is physical removal on the 25% schedule. Some biological additives are marketed, but check they are allowed locally. When in doubt, confirm with your sewer authority.
There is no single answer; it depends on your fixtures and flow. Small full-service kitchens may use a hydromechanical unit rated around 20 to 50 GPM, while higher-volume operations often need in-ground interceptors of 1,000 to 2,000 gallons. Menu, dishwasher, and sink count all matter. Estimate yours with our sizing calculator, then confirm the required size with your local code and AHJ.
Hydromechanical traps are compact, often a foot or two across, to fit under or beside a sink. Gravity interceptors are large buried tanks; a 1,000-gallon unit is roughly the size of a small septic tank, several feet deep and wide. Exact dimensions vary by manufacturer and gallon rating. Your required capacity, not a fixed size, drives the footprint, so confirm it with local code.
Yes. Grease traps and interceptors are plumbing devices governed by the plumbing code, and installing or altering one generally requires a licensed plumber and a permit. Codes treat them as part of the drainage system with specific venting, trapping, and access rules. Because classification and permit requirements vary, confirm the process for your area with your local building or plumbing department and AHJ.
Generally no. Grease interceptors and floor sinks usually require proper individual or common venting rather than wet venting, and many codes specifically restrict it. The rules depend on whether you follow the IPC, the UPC, or a local amendment. Improper venting causes siphoned traps, odors, and slow flow. Have a licensed plumber confirm the allowed venting arrangement with your AHJ.