If your auto shop drains to the municipal sewer — and virtually all do — you are legally required to treat that water before it leaves your property. In Canada, the National Plumbing Code mandates oil interceptors in all vehicle service facilities. Municipal sewer use bylaws add discharge limits that most untreated shop drainage exceeds by a wide margin.
This article covers the specific requirements that apply to auto shops, how to calculate the right separator size for your bays, and what happens when you get it wrong. For a broader overview of oil water separator types and technology, see the Oil Water Separators: Complete Industrial Guide.
What the Law Actually Requires
National Plumbing Code of Canada
The National Plumbing Code of Canada (NPC 2020), Clause 7.4.4.1 requires oil interceptors in the drainage system of any building used for:
- Motor vehicle repair or servicing
- Vehicle washing
- Parking garages (where oil drainage occurs)
- Fleet maintenance operations
This isn't optional — it's a building code requirement that applies at construction and applies retroactively when a municipality does a sewer use audit. Most municipalities enforce this through the building permit process, and inspectors check for an installed interceptor before issuing occupancy.
Municipal Sewer Use Bylaws
The NPC sets the equipment requirement. The sewer use bylaw sets the performance standard. Almost every Canadian municipality limits oil and grease in sewer discharge to 15 mg/L or less. Some urban municipalities are lower.
Untreated floor drain water from an auto shop routinely runs 50–500 mg/L oil and grease, depending on activity level. A properly functioning oil water separator brings this below the 15 mg/L threshold.
Federal Fisheries Act
Section 36 of the Fisheries Act prohibits the discharge of deleterious substances — including petroleum hydrocarbons — into any water frequented by fish, through any drainage system. This applies regardless of whether the discharge is intentional. A shop with an overflowing or failed separator that backs up into a storm drain can face Fisheries Act liability, not just a municipal fine.
Ontario-Specific Requirements
In Ontario, auto shops discharging to surface water (a ditch, stream, or storm drain that reaches a waterway) may require an Environmental Compliance Approval (ECA) under the Environmental Protection Act. Shops discharging only to the sanitary sewer typically don't need an ECA — they need to comply with their municipality's sewer use bylaw, which is enforced by the municipality.
The distinction matters: surface water discharge has stricter performance requirements (CCME guidelines at 0.1 mg/L PHC) than sewer discharge (typically 15 mg/L).
Quebec-Specific Requirements
Quebec auto shops are subject to municipal environmental bylaws and the Règlement sur les ouvrages municipaux d'assainissement des eaux usées (ROMAEU). Quebec municipalities vary in how actively they audit shop compliance, but the requirement is the same: no untreated petroleum-contaminated water to the sewer.
How to Size an Oil Water Separator for Your Shop
The single most common mistake is sizing based on the number of bays alone, without accounting for actual water flow.
Step 1: Identify Your Peak Flow Rate
Peak flow rate — measured in gallons per minute (GPM) — is the maximum rate at which water enters your drain system at any given moment. You need to size for the peak, not the average.
Two very different shop types:
Type A — Service and repair, no vehicle washing
Drip from raised vehicles, fluid spills, floor cleaning. These shops have low peak flows because water only enters the drain when someone is actively washing the floor with a mop or light hose.
Type B — Service plus active vehicle washing
Pressure washers, touchless wash bays, vehicle prep bays. These generate high flow rates continuously while washing is active.
The difference in sizing between Type A and Type B shops can be 4x or more.
Step 2: Calculate Flow Per Bay
Use these benchmarks as a starting point:
| Activity | Approximate Flow Rate |
|---|---|
| Standard hose (3/4" at 40 PSI) | 5–8 GPM |
| Pressure washer (cold water) | 2–4 GPM |
| Pressure washer (hot water / steam) | 2–4 GPM |
| Foam lance or soap gun | 1–2 GPM |
| Mop-and-bucket floor cleaning | 0.1–0.5 GPM (negligible) |
| Vehicle drip from parked unit | Negligible during normal service |
If three bays run hoses simultaneously, peak flow is 15–24 GPM. If one bay runs a pressure washer while two bays are doing dry service work, peak flow is 2–4 GPM.
Step 3: Account for Roof Drainage
If your floor drains connect to the same system as your roof drains — common in older buildings — you must include peak roof drainage in your flow calculation. In a heavy rain event, roof drainage alone can easily exceed your separator's rated capacity and cause bypass.
Check your building's plumbing diagram. If roof drains and floor drains share a common pipe before the separator, add the roof drainage volume.
Step 4: Apply a Buffer
Add 20–25% to your calculated peak flow before selecting a model. Separators operate at lower efficiency near their rated flow limit.
Worked Example
A 4-bay auto shop in Hamilton, Ontario. Two bays do active vehicle washing with cold water pressure washers (3 GPM each). Two bays do mechanical service only. All four bays can run simultaneously.
- Wash bays: 2 × 3 GPM = 6 GPM
- Service bays: negligible
- Peak flow: 6 GPM
- 25% buffer: 6 × 1.25 = 7.5 GPM
- Select: OlioSep™ 8 GPM
If the building also has roof drainage connected to the same floor drain system, recalculate with that added in.
OlioSep™ Sizing Guide for Auto Shops
ERE's OlioSep™ separators are available in sizes matched to common shop configurations:
| Shop Type | Peak Flow | OlioSep™ Model |
|---|---|---|
| 1–2 bay service (no washing) | 0.5–2 GPM | OlioSep™ 0.5 or 2 GPM |
| 1–2 bay service (light washing) | 2–4 GPM | OlioSep™ 2 or 4 GPM |
| 3–4 bay service (no washing) | 2–4 GPM | OlioSep™ 4 GPM |
| 3–4 bay service (active washing) | 6–10 GPM | OlioSep™ 8 GPM |
| 5–6 bay shop (active washing) | 10–18 GPM | OlioSep™ 16 GPM |
| Large fleet shop or bus depot (active washing) | 20–35 GPM | OlioSep™ 24 or 30 GPM |
Need help running the numbers for your specific shop? ERE's technical team does complimentary sizing recommendations — send your bay count, wash activity type, and province. See the contact block below.
Surface Mount vs. In-Floor: Which Fits Your Shop?
Surface mount OlioSep™ units install above the floor, near the drain they serve. Easier to access, inspect, and service. Most shops choose surface mount for new installations because a vacuum truck can reach the unit without special access provisions.
Flush mount OlioSep™ units sit below floor level with a surface-flush access lid. Better where a surface-mount unit would be in a traffic area or where aesthetics matter (customer-facing areas, showroom floors).
For older shops retrofitting a separator without major floor work, surface mount is usually the practical choice. Most surface mount units can be installed in a few hours with standard plumbing fittings.
What Happens Without a Separator
Municipal enforcement varies, but the consequences of non-compliance are real:
- Municipal sewer use violation: Warning, compliance order, escalating fines. Ontario municipalities can issue orders under the City of Toronto Act or equivalent requiring immediate compliance. Fines of $500–$10,000+ per day are not unusual for repeat violations.
- Building permit issues: A shop without a compliant separator may not pass an occupancy or plumbing inspection, blocking renovation permits.
- Insurance exposure: An oil spill that reaches a drain and causes downstream contamination may not be covered if the shop was operating without required equipment.
- Fisheries Act liability: If petroleum-contaminated water reaches a waterway, the exposure shifts from a bylaw fine to a federal offence.
A properly sized and maintained OlioSep™ unit costs far less than a single compliance order.
Not sure if your current separator is compliant? ERE's team can review your setup and tell you whether it meets current standards for your municipality. Contact us with your location and shop details.
Maintenance Requirements
An oil water separator in an auto shop requires regular maintenance to stay compliant. Most municipal permits or sewer use approvals require documented maintenance records.
At minimum:
- Monthly visual inspection — check oil accumulation level in the collection chamber
- Cleanout every 3–12 months — depending on activity level; busy shops with active washing need more frequent service
- Annual effluent check — sample your discharge and verify you're under your municipality's limit
A full cleanout means a licensed vacuum truck removing the accumulated oil and solids. This waste must be disposed of at an approved facility — not dumped on-site or into a dumpster.
For full maintenance procedures and scheduling guidance, see the Oil Water Separator Maintenance Guide.
Need help with oil water separators for auto shops?
ERE Inc. has been Canada's environmental equipment specialist for 30+ years.
→ Request a Quote | 1-888-287-EREC | Browse Oil Water Separators | sales@ereinc.com
Frequently Asked Questions
Is an oil water separator required for all auto shops in Canada?
Yes. The National Plumbing Code of Canada requires oil interceptors in any facility that services, repairs, or washes vehicles, and this requirement applies across all provinces. Additionally, every municipality with a sewer use bylaw limits oil and grease in discharge — typically to 15 mg/L — which you cannot achieve without a separator.
What size oil water separator does a 3-bay auto shop need?
It depends on whether you're washing vehicles. A 3-bay service shop with no active washing can often use a 4 GPM unit. A 3-bay shop with pressure washing or hose washing needs 8–16 GPM, depending on how many bays wash simultaneously. See the sizing table above.
Can I install an oil water separator myself?
The unit itself can be installed by a licensed plumber (or by the owner in jurisdictions that allow owner-operator work). The separator must be installed correctly — level, with proper inlet velocity, and with a sediment trap upstream if heavy solids are present. Incorrect installation destroys performance. ERE recommends using a licensed plumber familiar with industrial drain systems.
How do I know if my current interceptor meets requirements?
Check the manufacturer's rated flow (GPM) against your actual peak flow calculated from your bay activity. If the unit is undersized, or if it hasn't been cleaned in over a year, it's likely not meeting discharge standards. Contact ERE to assess your current setup.
Do car washes need the same separator as auto shops?
Car washes have similar requirements but different flow rates. A commercial car wash can generate 15–50 GPM and often uses detergents that emulsify oil, which requires a coalescing plate separator rather than a simple gravity unit. The OlioSep™ 30 and 50 GPM models cover most car wash applications.
Related articles
- Oil Water Separators: The Complete Industrial Guide
- Oil Water Separator Tank: How to Size It Right
- Oil Interceptors: How They Work and When You Need One
- Version française : Séparateur eau-huile pour garage et atelier mécanique
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