Oil Interceptors: How They Work and When You Need One

The National Plumbing Code of Canada requires an oil interceptor in the drainage system of any auto shop, garage, or vehicle washing facility. If you have floor drains that can collect petroleum-contaminated water and your building connects to a municipal sewer, an oil interceptor is not optional.

This article explains what oil interceptors are, how they work, when they're legally required, and how they relate to the broader category of oil water separators. For a full technical guide to oil water separators across all sizes and applications, see the Oil Water Separators: Complete Industrial Guide.

What Is an Oil Interceptor?

An oil interceptor is a plumbing device that removes petroleum hydrocarbons from wastewater before it enters the municipal sewer or a septic system. It sits in the drain line between your floor drains and the sewer connection.

The term "oil interceptor" comes from the plumbing code world. The National Plumbing Code of Canada (NPC) uses this term specifically for the passive separating devices required in commercial automotive facilities. In engineering, environmental consulting, and equipment supply, the same device is often called an oil water separator (OWS) — the two terms describe the same type of equipment, used interchangeably in practice.

Grease interceptors are a different category, designed for animal fats and vegetable oils from food service facilities, not petroleum products. The two types are not interchangeable. A grease interceptor will not adequately remove motor oil or fuel from automotive drainage. An oil interceptor will not remove food-service fats and grease effectively either.

How an Oil Interceptor Works

The physics are straightforward. Petroleum-based oil has a specific gravity below 1.0 — it floats on water. An oil interceptor creates a chamber where water velocity slows enough for oil to rise to the surface while water continues to the outlet.

Gravity Separation (Basic Interceptors)

The simplest oil interceptors are two-chamber gravity separators. Contaminated water enters the first chamber, velocity drops, oil floats to the top and is retained. Partially treated water flows under a baffle into a second chamber for further settling, then exits to the sewer.

Gravity interceptors are inexpensive and have no moving parts. Their limitation: small oil droplets (below 60–150 microns) don't rise fast enough at typical flow rates. If your application uses pressure washing or detergents, a basic gravity interceptor may not achieve the 15 mg/L effluent quality that most municipalities require.

Coalescing Plate Interceptors

ERE's OlioSep™ Surface Mount separators use a coalescing plate design. A corrugated plate pack inside the unit dramatically increases the effective surface area for separation. Oil droplets travel only a short distance before contacting a plate surface, coalescing into larger droplets, and rising to the collection chamber.

Coalescing plate units handle smaller oil droplets than gravity-only interceptors, which makes them appropriate for vehicle wash applications and facilities using detergents. They achieve effluent quality of less than 10–15 mg/L total petroleum hydrocarbons under normal operating conditions — within the discharge limits of most Canadian municipalities.

When Is an Oil Interceptor Required?

National Plumbing Code of Canada (NPC 2020)

Clause 7.4.4.1 of the NPC 2020 requires oil interceptors in buildings used for:

• Motor vehicle servicing or repair
• Automobile service stations
• Vehicle washing
• Parking garages where drainage may be contaminated with oil

This requirement applies across all provinces and territories. It is enforced at the building permit stage — no permit for a new shop or a renovation that involves drain work will issue without an interceptor specified on the drawings.

Municipal Sewer Use Bylaws

The NPC tells you what equipment to install. Your municipal sewer use bylaw tells you how clean the effluent must be. Most Canadian municipalities set a maximum oil and grease concentration of 15 mg/L in the sewer. Some urban municipalities are stricter.

Untreated drainage from an auto shop or vehicle wash typically runs 50–500 mg/L — well above any municipal limit. The interceptor closes that gap.

The Fisheries Act

At the federal level, Section 36 of the Fisheries Act prohibits discharge of deleterious substances into waters frequented by fish, through any drainage system. This captures shops near storm drains, ditches, or any drainage path that ultimately reaches a natural waterway. The Fisheries Act doesn't care whether the discharge was intentional — if your failed or overflowing interceptor sends contaminated water toward a waterway, you face potential federal liability.

What Facilities Need an Oil Interceptor

The NPC is clear on the facility types. In practice, the following all require an oil interceptor:

Auto shops and vehicle service centres: Any facility that services, repairs, maintains, or inspects vehicles where floor drains are present. This includes oil change shops, transmission shops, brake shops, alignment centres, and general repair shops.

Car washes: All commercial car washes. Tunnel washes, self-serve bays, and touch-free washes all have high-flow, oil-contaminated drainage that requires treatment.

Gas stations: The forecourt drainage at a fuel station collects fuel spills and oil drips. These require interceptors on the drain system.

Parking garages: Covered and underground parking structures where oil drips from vehicles accumulate on the floor and enter the drainage system.

Fleet maintenance depots: Bus garages, truck maintenance facilities, municipal works yards — all require interceptors. High-capacity units (30–50 GPM range) are common in this category.

Dealership service departments: Same requirements as independent repair shops.

Oil Interceptor vs. Oil Water Separator: What's the Difference?

The terms describe the same thing, used in different contexts:

When a plumber says "the code requires an oil interceptor," and an engineer says "you need an oil water separator," they mean the same device category. The ERE OlioSep™ series serves both contexts — code-compliant oil interceptors for commercial automotive facilities and industrial-scale OWS for larger applications.

The only practical distinction: "oil interceptor" in the NPC context usually implies a smaller, passive unit for a standard automotive facility. "Oil water separator" in engineering practice includes large API separators, centrifugal units, and systems for refinery or marine applications that go well beyond plumbing code requirements.

For a full breakdown of OWS types by technology and scale, see Oil Water Separators: The Complete Industrial Guide.

How to Size an Oil Interceptor

Sizing follows the same logic as any oil water separator. The key variable is peak flow rate — the maximum GPM your drain system will carry at any given moment.

Quick sizing rules for automotive facilities:

For a detailed sizing methodology with step-by-step calculations, see Oil Water Separator Tank: How to Size It Right.

Maintenance Requirements for Oil Interceptors

An oil interceptor that isn't maintained stops intercepting. The oil storage chamber fills, and once full, oil bypasses the collection zone and exits with the effluent. Many municipalities require documented maintenance records as part of their sewer use compliance program.

Minimum maintenance for a commercial automotive facility:

• Monthly: Visual inspection of oil level in the collection chamber. If the chamber is more than 75% full, arrange immediate cleanout.
• Every 3–12 months: Full cleanout by a licensed vacuum truck service. Busy shops with active vehicle washing need quarterly cleanout; light-use shops may manage with annual service.
• Annually: Inspect the coalescing plate pack for fouling (coalescing plates that are coated with solids lose efficiency). Inspect inlet and outlet fittings.

Cleanout waste — the accumulated oil and sludge — must be disposed of at a licensed facility. It cannot be discharged to a septic system, storm drain, or dumpster.

Most municipalities that actively enforce sewer use bylaws will ask for cleanout records during an inspection. Facilities without records are assumed to be non-compliant.

What Happens If You Don't Have One

Municipal enforcement of oil interceptor requirements is inconsistent but real. Consequences for non-compliance include:

• Compliance order: Written order to install a compliant interceptor within a specified timeline (typically 60–90 days). Non-compliance with the order escalates to fines.
• Fines: Sewer use bylaw fines in Ontario range from $500 to $10,000+ per day for continuing violations.
• Building permit delays: A shop renovation that triggers a plumbing inspection will stall until an interceptor is in place.
• Fisheries Act liability: In the worst case — a failed interceptor discharging to a storm drain that reaches a waterway — federal charges under the Fisheries Act are possible.

The equipment cost of a properly sized OlioSep™ interceptor for a 3–4 bay shop runs $2,500–$6,000 installed. A single compliance order and remediation event typically costs far more.

Frequently Asked Questions

Is an oil interceptor the same as a grease trap?

No. An oil interceptor is designed for petroleum hydrocarbons (motor oil, diesel, fuel). A grease trap or grease interceptor is designed for animal fats and vegetable oils from food service. They are different products for different applications and are not interchangeable. The NPC requires oil interceptors for automotive facilities and grease interceptors for food service establishments — the two requirements are separate.

Can I install an oil interceptor myself?

The installation itself must comply with the NPC and your local plumbing code. In most provinces, plumbing work on commercial buildings requires a licensed plumber. The interceptor must be installed level, with proper inlet velocity and a sediment trap upstream if heavy solids are present.

How do I know what size oil interceptor I need?

Calculate your peak flow rate (GPM) from all drain-connected water sources at maximum simultaneous use. Add a 20–25% buffer and select the next model size up. See the full sizing guide at Oil Water Separator Tank: How to Size It Right for a step-by-step walkthrough.

Does my existing interceptor meet current NPC requirements?

If it was installed before NPC 2020 and hasn't been updated, it may not. Key questions: Is the flow rating adequate for your current operations? Has the coalescing plate pack (if present) been inspected recently? Is the unit being maintained with documented records? ERE's team can assess your existing setup — contact us with your unit model and facility details.

What does an oil interceptor cost in Canada?

A surface mount OlioSep™ unit for a typical 3–4 bay shop runs $2,500–$6,000 for the unit plus $500–$1,500 installation. Larger units for car washes or fleet depots run higher. For a full cost breakdown, see the Oil Water Separator Cost Guide.

Related articles

• Oil Water Separators: The Complete Industrial Guide
• Oil Water Separator for Auto Shops: Requirements & Sizing
• Oil Water Separator Tank: How to Size It Right
• Version française : Intercepteur d'huile — fonctionnement et réglementation

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