An oil water separator that is undersized, improperly installed, or left unmaintained doesn't just fail — it creates a compliance liability that can result in fines, permit revocation, and remediation costs that dwarf the cost of the equipment itself.
This guide covers how oil water separators work, the types available for industrial and commercial applications, how to size one correctly for your facility, what Canadian regulations require, and what a proper maintenance program looks like.
How an Oil Water Separator Works
An oil water separator (OWS) exploits the physical difference in density between oil and water to cause separation. Petroleum hydrocarbons and most non-emulsified oils have a specific gravity below 1.0 and float. Water has a specific gravity of 1.0 and sinks. A well-designed OWS lets this natural density difference do the work before treated water is discharged.
Industrial OWS systems use three separation principles:
Gravity Separation
The oldest and most common method. Contaminated water enters a chamber where velocity is reduced, allowing time for oil droplets to rise to the surface and free water to settle to the bottom. Separated oil accumulates in a collection zone and is periodically removed. The American Petroleum Institute (API) separator design is the industrial standard for gravity-based units.
Gravity separation works well for free-floating oil but is limited with emulsified oil or very small droplets (below 60 microns). Flow rate must be kept low enough — typically below the rise rate of the smallest oil droplets you need to capture.
Coalescing Plate (Parallel Plate) Separation
Coalescing plate packs outperform basic gravity tanks for most commercial applications. Corrugated polypropylene or stainless steel plates installed at an angle increase the effective surface area inside the separator. Small oil droplets rise a shorter distance to contact a plate surface, coalesce into larger droplets, and float to the surface. Water with sediment slides down the inclined surfaces.
Coalescing plate separators handle higher flow rates in a smaller footprint than gravity-only units. They separate droplets down to approximately 20–60 microns depending on plate geometry and flow rate. ERE's OlioSep™ Surface Mount series uses a coalescing plate design scaled to commercial and light industrial flow rates.
Centrifugal (Hydrocyclone) Separation
Centrifugal force — not gravity alone — drives separation. Contaminated water enters the unit tangentially, creating a vortex. The denser water phase is forced outward and downward; the lighter oil phase migrates to the center and exits through an upper outlet.
Hydrocyclone separators can process high flow rates in a compact unit with no moving parts, but they are less effective on emulsified oils and require consistent inlet pressure. They are most common in oilfield and offshore marine applications.
Types of Oil Water Separators
Surface Mount (Above-Ground) Units
Installed above the floor, typically connected to floor drains or sumps. Easier to access for maintenance and inspection. Best suited for indoor applications — auto shops, wash bays, compressor rooms, light industrial facilities.
ERE's OlioSep™ Surface Mount series covers 0.5 GPM to 50 GPM, sized for individual floor drains up to multi-bay service facilities. The 0.5 GPM unit handles a single floor drain; the 16 GPM and 30 GPM units are typical for 3–6 bay automotive shops with active vehicle washing.
Flush Mount (In-Floor) Units
Installed flush with or below the floor surface, with a removable lid for access. Preferred where floor-level aesthetics matter or where vehicle traffic passes over the drain area. The OlioSep™ Flush Mount Plastic separator integrates into floor drain systems with minimal above-grade footprint.
Underground (In-Ground) Tanks
Buried below grade, typically precast concrete or high-density polyethylene (HDPE). Common in outdoor vehicle wash facilities, parking structures, and sites where a large volume buffer is needed. Underground units handle high peak flow rates and large sediment loads. They require more extensive installation (excavation, backfill) and more rigorous inspection access planning.
Portable Units
Trailer-mounted or skid-mounted separators for temporary applications: construction dewatering, emergency spill response, tank cleaning operations, pipeline maintenance. The OlioSep-Mini™ coolant and oil/water separation system covers small-volume applications where a permanent installation is impractical.
Grease Interceptors
A specialized category for food service and food processing applications where animal fats, vegetable oils, and grease — not petroleum — are the target contaminant. The OlioSep™ Grease Interceptors line is designed for restaurants, commercial kitchens, meat processing, and institutional cafeterias. Grease interceptors are typically required under the National Plumbing Code of Canada for any food service establishment connected to municipal sewer.
DNAPL Separators
Dense Non-Aqueous Phase Liquid (DNAPL) separators handle chlorinated solvents, PCE, TCE, and other contaminants with specific gravity above 1.0 — they sink, not float. ERE's DNAPL™ Oil Water Separator reverses the geometry of a standard OWS to collect the denser contaminant phase from the bottom of the separator vessel, not the top.
How to Size an Oil Water Separator
Undersizing is the most common and costly OWS mistake. A separator operating above its rated flow rate has insufficient residence time for separation — oil passes through and ends up in your discharge.
The Three Variables That Drive Sizing
1. Peak flow rate (GPM or L/min)
This is not your average flow — it's your worst-case peak. Account for roof drainage entering floor drains during heavy rain, simultaneous vehicle washing, and fire suppression water if the drain system is connected.
A practical rule for automotive bays: assume 1–2 GPM per active wash bay, plus a buffer for roof drainage. A 3-bay shop that washes vehicles simultaneously typically needs a 4–8 GPM separator.
2. Oil droplet size
Emulsified oil (from pressure washing with detergent, or from turbulent conditions) generates much smaller droplets than free-floating oil from a leaking vehicle. Coalescing plate units handle smaller droplets than gravity separators. If you use high-pressure steam or detergent-based washing, specify a coalescing plate unit.
3. Contaminant concentration
High-concentration influent (e.g., a sump that collects drip from multiple engines) requires either more frequent cleanout or a larger collection chamber. Size the oil storage volume to hold at least 90 days of accumulated oil between scheduled cleanouts, based on your facility's contamination load.
Quick Reference: OlioSep™ Sizing Chart
| Application | Typical Flow Rate | OlioSep™ Model |
|---|---|---|
| Single floor drain | 0.5 GPM | OlioSep™ 0.5 GPM |
| Small repair shop (1–2 bays) | 2–4 GPM | OlioSep™ 2 or 4 GPM |
| Mid-size auto shop (3–4 bays) | 8 GPM | OlioSep™ 8 GPM |
| Large shop / fleet service (5–6 bays) | 16–24 GPM | OlioSep™ 16 or 24 GPM |
| Bus depot / heavy vehicle wash | 30 GPM | OlioSep™ 30 GPM |
| Large industrial / commercial car wash | 50 GPM | OlioSep™ 50 GPM |
| CNC coolant / small workshop | N/A | OlioSep-Mini™ |
These are guidelines. Consult with ERE's technical team for site-specific sizing — flow rates vary based on drain design, facility layout, and wash protocols.
Canadian Regulations: What Is Required
Regulatory requirements for oil water separators in Canada operate at three levels: federal, provincial, and municipal. Most facilities are subject to all three.
Federal Level
The Canadian Environmental Protection Act (CEPA) and the Fisheries Act set the framework. Section 36 of the Fisheries Act prohibits the deposit of deleterious substances (including petroleum hydrocarbons) into waters frequented by fish, directly or through any drainage system. Violations carry significant penalties — up to $12 million for a first offence by a corporation under CEPA amendments.
The Canadian Council of Ministers of the Environment (CCME) has established guidelines for petroleum hydrocarbons in soil and water. For surface water discharge, the CCME water quality guideline for petroleum hydrocarbons (PHC) is 0.1 mg/L (CCME, 2008). Many provincial standards reference CCME guidelines as the benchmark.
Provincial Level (Ontario Example)
In Ontario, the Environmental Compliance Approval (ECA) process under the Environmental Protection Act governs industrial stormwater and process water discharge. Automotive businesses, fleet operators, and vehicle washing facilities typically require an ECA if discharging to surface water.
The Ontario Water Resources Act regulates discharge to groundwater and municipal sewers. Municipal sewer use bylaws typically set maximum allowable concentrations for oil and grease (typically 15 mg/L in Ontario municipalities), pH, temperature, and other parameters. Discharge above these limits without treatment constitutes a sewer use violation.
In Quebec, the Règlement sur les ouvrages municipaux d'assainissement des eaux usées (ROMAEU) and municipal environmental compliance requirements govern discharge to the municipal wastewater system.
Municipal Level
Most municipalities require:
- Oil interceptors in the floor drain plumbing for any auto shop, vehicle service facility, or car wash under the National Plumbing Code of Canada (NPC 2020, Clause 7.4.4.1)
- Grease interceptors for all food service establishments connecting to municipal sewer
- Periodic inspection reports (typically annually) confirming the interceptor is being maintained and is operating within specification
Note: Requirements vary by municipality. Before specifying a separator, confirm local requirements with your building department or environmental consultant. ERE's team regularly assists clients in identifying the applicable standard for their jurisdiction — contact us with your location and application.
Oil Water Separator Installation
Surface Mount Installation
- Locate the separator in a sheltered area, accessible for maintenance — typically near the floor drain or sump it serves
- Connect inlet plumbing from the drain/sump (ensure no significant solids are entering without a strainer or settling basin upstream)
- Connect outlet plumbing to the discharge point (municipal sewer or further treatment)
- Ensure the unit is level — OWS performance depends on gravity; even slight tilting of a coalescing plate unit can reduce efficiency
- Install a bypass valve for periods of very high flow (above rated capacity) if flooding risk exists
Underground Installation
- Excavate to the unit dimensions plus working clearance on all sides
- Install on a level compacted gravel or concrete pad
- Provide manhole access for inspection and cleanout
- Ensure adequate venting to prevent pressure buildup
- Confirm backfill material and compaction meet manufacturer specifications
Common Installation Errors That Kill Performance
- No sediment trap upstream: Heavy solids entering the separator coat the coalescing plates, reducing efficiency over weeks
- Piping too large or too small: Inlet velocity must be controlled — too fast prevents separation; too slow causes settling inside the inlet pipe
- Installed in a frost-exposed location without insulation: Water inside an above-grade unit in Canadian winters will freeze and burst
- No access for cleanout equipment: If a vacuum truck can't reach the unit, it won't get cleaned. If it doesn't get cleaned, it fails.
Oil Water Separator Maintenance
An oil water separator requires regular maintenance to maintain performance. A separator that isn't cleaned has a finite capacity — once the oil storage chamber is full, oil bypasses to the outlet. Many regulatory inspections include a check of cleanout records.
Maintenance Schedule
| Task | Frequency |
|---|---|
| Visual inspection of oil level in collection chamber | Monthly |
| Full cleanout (vacuum truck) | Every 3–12 months depending on facility load |
| Inspection of coalescing plate pack for fouling | Annually |
| Effluent sampling to verify discharge quality | Annually or as required by permit |
| Inspection of seals, gaskets, and inlet/outlet connections | Annually |
Signs Your Separator Needs Immediate Attention
- Visible oil sheen in discharge
- Oil storage chamber full to within 25% of capacity
- Unusual odours from the separator (hydrogen sulfide suggests stagnant, septic conditions)
- Physical damage to coalescing plates or housing
- Inlet or outlet blockage
For detailed maintenance procedures, see our Oil Water Separator Maintenance Guide which covers cleaning intervals, cleanout contractors, and discharge sampling protocols.
Industry Applications
Automotive Service and Repair
The most common application in Canada. Every provincial building code and municipal plumbing standard requires oil interceptors in the drainage system of vehicle service facilities. A standard 3-bay shop needs at minimum a 4–8 GPM separator. Facilities that wash vehicles require sizing up due to wash water volume.
For detailed requirements and sizing examples, see Oil Water Separator for Auto Shops: Requirements & Sizing.
Car Wash Facilities
Commercial car washes generate high flow rates and typically use detergents that emulsify oil, making separation more challenging. Recirculation car washes require separators on the recirculating water system in addition to any discharge stream. Municipal requirements for car washes typically include a separator plus often a pH adjustment system.
Industrial Manufacturing
Machining operations, metalworking, and coolant use generate tramp oil in coolant sumps. The OlioSep-Mini™ is specifically designed for this application — removing tramp oil from water-based metalworking fluids extends coolant life, cuts disposal costs, and improves cutting performance.
Fleet Maintenance and Bus Depots
Large vehicle depots with wash bays and multiple service pits need high-capacity separators (30–50 GPM range) and typically require underground units to handle the volume. For a full comparison of installation types, see Above Ground vs. Underground Oil Water Separators. Transit agencies and municipal fleet operators frequently require regulatory approval documentation.
Commercial and Institutional Kitchens
Grease interceptors (a specialised OWS variant) are mandatory under the National Plumbing Code for all food service operations. The OlioSep™ Grease Interceptors are sized based on the fixture units served and the meal-per-day capacity of the kitchen.
Environmental Site Remediation
DNAPL contamination on former industrial sites — dry cleaners, solvent users, gas stations with chlorinated solvent releases — requires a separator designed for dense-phase liquids. The DNAPL™ Oil Water Separator from ERE is engineered for this application, separating contaminants that standard OWS units would allow to pass.
Cost: What to Expect in Canada
Oil water separator costs in Canada range widely based on size, type, and installation complexity:
| System Type | Unit Cost (CAD) | Installation | Total Installed |
|---|---|---|---|
| Small surface mount (0.5–2 GPM) | $800–$2,500 | $300–$800 | $1,100–$3,300 |
| Mid-size surface mount (4–8 GPM) | $2,500–$6,000 | $500–$1,500 | $3,000–$7,500 |
| Large surface mount (16–30 GPM) | $6,000–$15,000 | $1,000–$3,000 | $7,000–$18,000 |
| Underground tank system | $8,000–$40,000 | $5,000–$20,000 | $13,000–$60,000 |
| Portable / rental unit | $200–$800/month rental | Minimal | Per project |
These ranges assume standard installation. Sites requiring excavation in rock, complex plumbing modifications, or regulatory permitting will have higher total costs.
For a detailed breakdown with Canadian-specific pricing factors, see our Oil Water Separator Cost Guide.
Frequently Asked Questions
Do I legally need an oil water separator in Canada?
If your facility discharges water that contacts petroleum products — automotive shop floor drains, vehicle wash areas, industrial equipment wash pads — yes, in most cases. The National Plumbing Code of Canada requires oil interceptors in automotive service facilities, and most municipalities enforce additional sewer use requirements. The Fisheries Act applies at the federal level to any discharge that could reach waterways. Contact your local building department and check your municipal sewer use bylaw to confirm your specific requirements.
What is the difference between an oil water separator and an oil interceptor?
The terms are often used interchangeably in the field. Technically, "oil interceptor" typically refers to smaller, passive units used in commercial plumbing applications (shops, gas stations), while "oil water separator" is a broader term covering all sizes and technologies including large industrial units. Grease interceptors are a separate category for fats, oils, and grease (FOG) from food service. See our Oil Interceptors guide for a full comparison.
How often does an oil water separator need to be cleaned?
It depends on contamination load. A lightly used shop might need cleanout once or twice a year; a busy fleet wash facility may need quarterly service. The rule: inspect monthly, and schedule a vacuum truck cleanout before the oil storage volume is 75% full. Never let the separator go more than 12 months without professional service.
What GPM rating do I need for a 3-bay auto shop?
A 3-bay shop running simultaneous vehicle washing typically needs 8–16 GPM, depending on hose flow rates and whether roof drainage enters the same drain system. A shop that only drips from vehicles (no active washing) can often use a 4 GPM unit. See the Oil Water Separator Tank Sizing Guide for a full sizing methodology with worked examples.
Can I use a standard oil water separator for grease from a restaurant kitchen?
No. Standard OWS units are designed for petroleum hydrocarbons, which are lighter than water. Animal fats and vegetable oils require grease interceptors sized to the kitchen's fixture units and meal capacity. Using a standard OWS on a grease stream will result in poor performance and likely regulatory non-compliance.
What is a coalescing plate oil water separator?
A coalescing plate (or parallel plate) separator uses a pack of angled plates inside the separation chamber. Oil droplets rise a short distance to contact a plate surface, coalesce into larger droplets, and float to the surface more efficiently. This design achieves better separation than a simple gravity tank in a smaller footprint and handles smaller oil droplets — important when detergent-based washing is involved.
What discharge standard does an oil water separator need to meet?
Most Canadian municipalities set oil and grease limits of 15 mg/L in the sewer use bylaw. Direct discharge to surface water triggers Fisheries Act considerations and CCME guidelines (0.1 mg/L for petroleum hydrocarbons in most water quality contexts). A properly maintained coalescing plate separator routinely achieves 10–15 mg/L effluent quality under normal operating conditions.
Where can I get an oil water separator in Canada?
ERE Inc. supplies the OlioSep™ line of oil water separators across Canada, with technical support for sizing, installation, and regulatory compliance. Request a quote or sizing consultation through our team.
Next Steps
The right oil water separator comes down to three things: peak flow rate, contamination type, and the discharge requirements for your jurisdiction. Get those three right and the rest is straightforward.
ERE Inc. has supplied oil water separators across Canada for over 30 years. Our technical team provides sizing recommendations, regulatory guidance, and equipment selection at no charge.
Ready to spec an oil water separator for your facility?
ERE Inc. has been Canada's environmental equipment specialist for 30+ years. Send us your flow rate (GPM), application (auto shop / car wash / industrial), number of bays or drains, and province/city — we'll respond with a sizing recommendation within 1 business day.
→ Request a Quote | 1-888-287-EREC | Browse OlioSep™ Oil Water Separators | sales@ereinc.com
Lire en français : Séparateurs eau-huile : guide complet pour l'industrie
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