Grease Interceptors: Complete Sizing and Selection Guide for Canada

A grease interceptor for a commercial kitchen is sized by flow rate (GPM), not by tank volume alone. Canada's National Plumbing Code and most municipal sewer-use bylaws base the minimum interceptor capacity on the fixture-unit flow from the kitchen's sinks, dishwashers, and floor drains — get that number wrong and the interceptor either overflows during a lunch rush or gets rejected at inspection. ERE stocks OlioSep™ gravity grease interceptors in three flow-rate classes for restaurants, hotels, schools, and food-processing facilities across Canada.

What Does a Grease Interceptor Do?

A grease interceptor is a watertight tank, installed underground or at grade between a commercial kitchen's drain lines and the municipal sewer connection, that separates fats, oils, and grease (FOG) from wastewater before it leaves the property. It works entirely on gravity and density difference — no pump, no power draw, no moving parts. Wastewater slows down as it enters the tank, heavier solids settle into a built-in sludge trap, and grease (lighter than water) rises and accumulates on the surface, while the clarified water underneath discharges to the sewer.

Any commercial kitchen producing FOG-laden wastewater is a candidate: restaurants, hotel and institutional kitchens, school and hospital cafeterias, bars, laundries, and food-processing plants. Left uncontrolled, FOG cools and solidifies in sewer lines downstream of the property, which is a well-documented contributor to municipal sanitary sewer blockages and overflows.

Grease Interceptor vs. Grease Trap vs. Oil Interceptor

"Grease trap" and "grease interceptor" describe the same function at different scales — a grease trap is typically a small, under-sink hydromechanical unit rated under about 50 GPM for light-duty kitchens, while a grease interceptor is the larger, in-ground gravity tank sized for full commercial-kitchen flow. Both are unrelated to an oil interceptor, which separates petroleum products (fuel, motor oil, hydraulic fluid) from automotive, industrial, or vehicle-wash wastewater rather than food-grade fats and oils. If your facility handles both a commercial kitchen and a vehicle bay or shop area, you need both systems — they are not interchangeable, and a bylaw inspector will check for the correct one on each drain line.

How Do You Size a Grease Interceptor?

Sizing starts with the total drainage fixture units (DFU) connected to the interceptor — every sink, dishwasher, floor drain, and pre-rinse station feeding it — converted to a peak flow rate in GPM using the fixture-unit tables in the National Plumbing Code of Canada, then cross-checked against your local municipality's sewer-use bylaw, which may set a higher minimum retention time or grease-storage capacity than the code baseline.

1. List every fixture that drains into the interceptor (3-compartment sink, pre-rinse sink, dishwasher, floor drains) and its DFU rating from the plumbing code fixture table.
2. Sum the DFU total and convert to peak flow rate (GPM) using the code's fixture-unit-to-flow conversion, applying the 24-minute drainage period used for kitchen loads (not the shorter period used for fixture-only sizing).
3. Compare against your municipal bylaw — many Canadian municipalities require a larger interceptor than the plumbing-code minimum, or set a fixed minimum size (commonly 10-20 GPM) regardless of a small kitchen's calculated flow.
4. Select the interceptor class that meets or exceeds the higher of the two numbers, then confirm inlet/outlet pipe diameter matches your existing drain line before ordering.

These figures are the OlioSep™ Grease Interceptor line's published capacities — a totally watertight, corrosion-resistant tank with a built-in sludge trap, polyethylene protective covers, and the option to build a bypass directly into the unit for facilities that need to route flow around it during maintenance. Larger commercial kitchens with higher fixture counts should have their DFU total calculated against the full code table rather than estimated from the table above.

Where Does Canadian Code Require a Grease Interceptor?

Grease interceptors are a code requirement, not an optional upgrade, anywhere FOG-laden wastewater discharges to a municipal sanitary sewer. The National Plumbing Code of Canada sets the baseline requirement for grease interceptors on commercial kitchen waste lines, and municipalities layer their own sewer-use bylaw on top — typically specifying minimum interceptor size, required pump-out frequency, and a maintenance-log requirement the facility must produce on inspection. FOG discharged past an undersized or poorly maintained interceptor is one of the most common contributors to sanitary sewer blockages and overflows that municipal wastewater programs track under frameworks like the Canadian Council of Ministers of the Environment's municipal wastewater guidance — which is why bylaw enforcement in this area tends to be strict and inspection-driven rather than complaint-driven.

Facilities required to install a grease interceptor typically include:

• Restaurants and commercial kitchens
• Hotels and institutional food service (hospitals, schools, seniors' residences)
• Bars and catering facilities
• Food-processing plants
• Any site producing a high volume of fats, oils, or grease in its wastewater

How Do You Maintain a Grease Interceptor?

A grease interceptor needs no structural upkeep — the tank material is built for high mechanical resistance and corrosion resistance — but it does need a regular inspect-empty-clean-refill cycle to keep the grease layer from building past its storage capacity and carrying over into the sewer line. Most municipal bylaws tie the required pump-out interval to a percentage-full threshold (commonly 25%) rather than a fixed calendar schedule, so higher-volume kitchens need more frequent service than the interval alone suggests. The maintenance principles are the same ones covered in ERE's broader oil water separator maintenance guide — inspection frequency, sludge/grease removal, and refill with clean water before returning the unit to service.

ERE supplies the full oil/water separator range, including grease interceptors, oil interceptors, and above/below-grade separator configurations, for facilities across Canada.

Frequently Asked Questions

What size grease interceptor do I need?

Size is based on the peak flow rate (GPM) from every fixture draining into it — sinks, dishwashers, pre-rinse stations, floor drains — calculated from the drainage fixture units in the National Plumbing Code of Canada and cross-checked against your municipal bylaw's minimum. Most small-to-mid commercial kitchens fall in the 10-20 GPM range; larger operations need a full fixture-unit calculation rather than an estimate.

Is a grease interceptor required by code in Canada?

Yes. The National Plumbing Code of Canada requires a grease interceptor on commercial kitchen waste lines discharging FOG-laden wastewater, and municipalities add their own sewer-use bylaw requirements on top — typically minimum size, pump-out frequency, and a maintenance log the facility must produce on request.

How often should a grease interceptor be cleaned?

Most municipal bylaws tie pump-out frequency to a fill-level threshold, commonly 25% full, rather than a fixed calendar interval — so a high-volume kitchen may need service every few weeks while a lower-volume one can go longer. Check your local bylaw for the specific threshold and required logging.

What's the difference between a grease interceptor and an oil interceptor?

A grease interceptor separates fats, oils, and grease (FOG) from commercial-kitchen wastewater. An oil interceptor separates petroleum products — fuel, motor oil, hydraulic fluid — from automotive, industrial, or vehicle-wash wastewater. They serve different waste streams and are not interchangeable; a facility with both a kitchen and a shop or wash bay needs one of each.

Related articles

• Oil Water Separators: The Complete Industrial Guide
• Oil Interceptors: How They Work and When You Need One
• Oil Water Separator Maintenance Guide
• Above Ground vs. Underground Oil Water Separators

Lire en français : Intercepteurs à graisse : guide de dimensionnement pour le Canada