Key points
- Always size for peak flow, never for average flow.
- Roof drainage can triple your peak flow if the drains connect to the same network as floor drains.
- Most Quebec municipalities set the discharge limit at 15 mg/L of total hydrocarbons.
- Applications with detergents or high-pressure washing require a coalescing plate separator, not a simple gravity separator.
- Plan for a safety factor of 20 to 25% on the calculated peak flow before selecting the model.
Why proper sizing of an oil-water separator is critical
An oil-water separator is rated for a specific flow rate, expressed in GPM (gallons per minute) or L/min. This value assumes that contaminated water enters at a rate equal to or less than this flow, giving oil droplets enough residence time to rise to the surface before the effluent leaves the unit.
When the flow rate exceeds the nominal capacity, the residence time drops. Oil droplets no longer have time to rise and coalesce. The oil leaves with the effluent. Your discharge does not meet the standard. The separator appears to be working, no alarms are triggered, but your effluent is non-compliant. This is the classic trap of undersizing.
What are the three sizing variables for an oil-water separator?
1. Peak flow
This is the most important sizing figure. You need to calculate the worst-case simultaneous scenario, not the usual flow on a quiet Tuesday.
Peak flow comes from three main sources:
Actively used water — hoses, high-pressure washers, vehicle washing equipment. This is the dominant factor in workshops and car washes.
Roof drainage — in many facilities, roof drains connect to the same collector as floor drains. During heavy rain, roof drainage can be the main source of flow and overwhelm a separator sized only for wash water.
Groundwater infiltration — buried units on sites with high water tables can experience infiltration, especially after heavy rains.
Add up all simultaneous sources in the worst-case scenario.
2. Oil droplet size
Separation efficiency directly depends on droplet size. Large droplets rise quickly and are easy to capture. Small droplets, under 20 microns, resist any passive separator.
Two factors reduce droplet size:
- High-pressure or steam washing: pressure breaks oil into fine particles, much smaller than with low-pressure rinsing.
- Detergents and surfactants: they emulsify oil, making droplets chemically stable and resistant to coalescence.
If your facility uses detergents or high-pressure washing, specify a coalescing plate separator rather than a simple gravity tank. Plates treat droplets down to 20-60 microns. A simple gravity separator alone will not ensure compliant effluent under these conditions.
ERE's OlioSep™ range uses a coalescing plate design on all flow sizes. It treats both free oils and emulsified oils from washing operations.
3. Oil storage volume
Oil captured by the separator must be stored somewhere. When the collection chamber is full, oil bypasses the system and goes into the discharge.
Storage capacity must be sufficient to retain the oil generated by your facility between two scheduled cleanings.
Estimate your oil generation rate:
- A standard mechanical workshop generates 0.5 to 2 litres of oil per bay per day (drops, spills, washing residues).
- A washing bay with heavy contamination can produce 2 to 5 litres per day.
- A machine tool sump varies depending on the operation.
Recommended target: oil storage capacity corresponding to 90 days of maximum generation, to absorb seasonal variability and the emptying schedule.
How to size an oil-water separator: 4-step calculation
Step 1: Calculate peak flow
List all simultaneous water sources in the busiest scenario:
| Source | Flow |
|---|---|
| Hose A (3/4 in at 40 PSI) | 6 GPM |
| High-pressure washer B (cold, 3,500 PSI) | 3 GPM |
| Floor drain C (heavy rain, roof drainage) | 8 GPM |
| Peak Total | 17 GPM |
Step 2: Apply the safety factor
Separator efficiency decreases near nominal capacity. Add 20 to 25% to your peak flow:
17 GPM × 1.25 = 21.25 GPM minimum nominal capacity
Round up to the next available model in the OlioSep™ range. Here: 24 GPM.
Step 3: Check technology type based on oil nature
Does your facility use detergents, high-pressure washing, or both?
- No detergents, low pressure: a standard coalescing plate separator is suitable.
- Detergents used: confirm that the plate geometry is suitable for emulsified oils. Avoid simple gravity separators.
- Intense emulsification: consider a downstream polishing step (dissolved air flotation or media filter) if the discharge limit is strict.
Step 4: Check oil storage volume
Estimate daily oil generation × 90 days. Compare to the storage capacity of the selected model indicated in the technical data sheet.
Example: A 5-bay workshop generates approximately 8 litres of oil per day. Storage required for 90 days: 720 litres. Check that the OlioSep™ 24 GPM collection chamber exceeds this value, or plan for more frequent emptying (every 60 days).
Concrete sizing examples
Example A: 3-bay workshop, no vehicle washing
- 3 service bays, vehicles on lifts
- Occasional floor cleaning with mop and bucket
- Roof drainage not connected to the floor drain network
Peak flow: negligible mop cleaning (less than 0.5 GPM)
With safety factor: 0.5 × 1.25 = 0.625 GPM
Selection: OlioSep™ 2 GPM (oversized compared to the calculated minimum, for capacity reserve)
Estimated oil generation: 3 bays × 0.5 L/day = 1.5 L/day
Storage for 90 days: 135 litres required.
Example B: 4-bay workshop with 2 cold high-pressure washers
- 2 bays with cold high-pressure washers (3 GPM each)
- 2 mechanical bays only, no washing
- Roof drainage separate from the floor drain network
Peak flow: 2 × 3 GPM = 6 GPM
With safety factor: 6 × 1.25 = 7.5 GPM
Selection: OlioSep™ 8 GPM
Estimated oil generation: 4 bays × 1.5 L/day = 6 L/day
Storage for 90 days: 540 litres required.
Note: high-pressure washing generates finer droplets. Confirm that the selected coalescing plate model is suitable for this application.
Example C: Commercial car wash, 2 lanes, hot water + detergent, roof drainage connected
- 2 washing lanes simultaneously
- Hot water and detergents (emulsified oil)
- Roof drainage connected to the same collector
Flow of 2 lanes: 2 × 8 GPM = 16 GPM
Roof drainage (heavy rain peak): 12 GPM
Peak total: 28 GPM
With safety factor: 28 × 1.25 = 35 GPM
Selection: OlioSep™ 50 GPM (next available model above 35 GPM)
Important note: the use of detergents produces emulsified oils. Confirm that the coalescing plate geometry is suitable. Consider a downstream polishing unit if your municipality's discharge limit is below 15 mg/L.
OlioSep™ Quick Reference Chart
| Peak Flow (GPM) | With Safety Factor | OlioSep™ Model | Typical Application |
|---|---|---|---|
| ≤ 0.4 | ≤ 0.5 | OlioSep™ 0.5 GPM | Single floor drain |
| 0.5 to 1.6 | ≤ 2.0 | OlioSep™ 2 GPM | 1-2 bay workshop, no washing |
| 1.6 to 3.2 | ≤ 4.0 | OlioSep™ 4 GPM | 2-3 bay workshop, light washing |
| 3.2 to 6.4 | ≤ 8.0 | OlioSep™ 8 GPM | 3-4 bay workshop, active washing |
| 6.4 to 12.8 | ≤ 16.0 | OlioSep™ 16 GPM | 5-6 bay workshop |
| 12.8 to 19.2 | ≤ 24.0 | OlioSep™ 24 GPM | Large fleet, 6 bays and more |
| 19.2 to 24.0 | ≤ 30.0 | OlioSep™ 30 GPM | Bus depot, heavy industry |
| 24.0 to 40.0 | ≤ 50.0 | OlioSep™ 50 GPM | Commercial car wash, large industry |
What are the common mistakes in sizing an oil-water separator?
Sizing an oil-water separator is relatively simple when you have the right numbers. Problems almost always arise because you're working with the wrong input data. Here are the five most frequent errors in the field.
Using average flow instead of peak flow. A separator sized for your average flow will be undersized on a rainy Monday when three washing bays are operating simultaneously and roof drainage is active. Compliance is measured by the worst-case scenario, not the usual scenario.
Forgetting roof drainage. The connection of roof drains is easy to miss during a site visit. Check as-built plans, not just the floor drain schematic. In old buildings, surprising connections are common.
Treating all applications the same way. A dry mechanical workshop and a car wash with hot water and detergents have fundamentally different sizing requirements, even if the number of bays is identical. The type of washing activity changes everything.
Undersizing for emulsified oil. If you use detergents, a simple gravity separator will almost certainly not achieve effluent compliance. It is imperative to specify a coalescing plate separator. This is an expensive mistake to correct after installation.
Not planning for growth. If you're adding a bay in 18 months, size for that reality now. Adding an extra separator later is expensive and disruptive to operations. A larger model one size up from the start costs less than two separate installations.
Surface or buried unit: what's the difference for sizing?
When your peak flow calculation results in a large required capacity (30 GPM and above), the question often arises: should you opt for a surface unit or a buried tank?
Buried units offer greater oil and sediment storage capacity, and handle higher peak flows. They are suitable for commercial car washes, bus depots, and large fleet operations. The trade-off is the installation cost: excavation, concrete work, and manhole access easily add $15,000 to $60,000 CAD or more, depending on the configuration.
The OlioSep™ surface units at 30 and 50 GPM are a practical alternative when floor space permits and access for emptying is clear. Installation is simpler and significantly less expensive. For new high-flow installations, compare both options by integrating the total installation cost, not just the unit price.
Frequently asked questions about oil-water separator sizing
What happens if my oil-water separator is undersized?
The oil flows directly into your discharge. Your effluent will not meet the 15 mg/L limit required by most municipal sewer regulations. This is a compliance deviation that can go unnoticed until sampling during an inspection. The separator will appear to be working normally, but it will not separate effectively.
Can I connect multiple floor drains to the same oil-water separator?
Yes, this is even the standard configuration in most workshops. Add up the peak flow of all connected drains to get your total influent flow. Make sure all connections come upstream of the separator inlet, so that the unit receives the combined flow.
How do precipitation events affect oil-water separator sizing?
If your roof drains connect to the same collector as your floor drains, heavy rain can dramatically multiply the total flow. Systematically check if your floor and roof drain networks share a connection point upstream of the separator. This is particularly common in industrial buildings built before 1990 in Quebec.
My separator is rated for 8 GPM but I need more. Can I add a second one?
Yes. Two separators in series or parallel are a valid solution. Series mounting (one feeds the other) improves effluent quality. Parallel mounting (flow is distributed between two units) increases total capacity. For new installations, it is generally simpler and more economical to select a properly sized unit from the outset.
Is there a standard formula for sizing an oil-water separator?
Yes. API Publication 421 provides a design method for gravity separators. The key calculation is based on the rise rate of the smallest oil droplet to be captured, the effective cross-sectional area of the separator, and the flow rate. For coalescing plate separators, the plate geometry multiplies the effective surface area. The ERE technical team applies these calculations and can validate the sizing for your specific application.
Get the right sizing before ordering
A sizing error is much harder to correct after installation than before. If your peak flow calculation yields a result you're unsure about, the ERE technical team evaluates sizing questions free of charge.
Need help sizing an oil-water separator?
Describe your facility to us — expected flow, space constraints, municipal requirements. Our team will recommend the right OlioSep™ separator model and capacity.
→ Request a quote | 1-888-287-EREC | Browse OlioSep™ Separators | sales@ereinc.com
Read this article in English: Oil Water Separator Tank: How to Size It Right
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