Undersizing an oil water separator is the most expensive mistake you can make. A unit running above its rated capacity doesn't separate oil — it passes it straight through to your discharge. The result is a compliance failure, a fine, and a separator replacement. You pay twice.
Sizing an OWS correctly requires three inputs: peak flow rate, oil droplet characteristics, and required oil storage volume. This guide walks through each one with real calculations. For background on how oil water separators work and the types available, start with the Oil Water Separators: Complete Industrial Guide.
Why Tank Sizing Matters
An oil water separator is rated at a specific flow rate (GPM or L/min). That rating assumes the contaminated water enters at or below that rate, giving oil droplets enough residence time to rise to the collection surface before water exits the unit.
When flow exceeds the rated capacity:
- Residence time drops below what separation requires
- Oil droplets don't have time to rise and coalesce
- Oil exits with the effluent
- Your discharge fails the 15 mg/L limit most municipalities require
The separator looks fine. Nothing trips an alarm. But your effluent is non-compliant. That's the trap.
The Three Sizing Variables
1. Peak Flow Rate
This is the single most important number. Size for your worst-case peak, not your average or typical flow.
Peak flow comes from three sources:
Active water use — hoses, pressure washers, vehicle wash equipment actively discharging. This is your primary driver if you wash vehicles or equipment.
Roof drainage — in many facilities, roof drains connect to the same system as floor drains. During a heavy rainfall event, roof drainage can easily be the dominant flow source and overwhelm a separator sized only for wash water.
Groundwater infiltration — underground units in high-water-table sites may have groundwater entering the system, particularly after heavy rain.
Add up all simultaneous sources for your worst-case peak.
2. Oil Droplet Size
Separation efficiency depends on droplet size. Larger droplets rise faster and are easier to capture. Very small droplets — below 20 microns — are difficult for any passive separator to capture.
Two factors shrink droplet size:
- High-pressure or steam washing — breaks oil into smaller droplets than low-pressure rinsing
- Detergent or surfactant use — emulsifies oil, making droplets much smaller and chemically resistant to coalescing
If your application uses detergents or high-pressure washing, specify a coalescing plate separator rather than a simple gravity tank. Coalescing plates handle droplets down to 20–60 microns. A gravity-only separator may not achieve compliant effluent in these conditions.
ERE's OlioSep™ Surface Mount series uses a coalescing plate design across all flow rates. It handles both free-floating oil and emulsified oil from wash applications.
3. Oil Storage Volume
Oil that the separator collects must go somewhere. The collection chamber has a finite volume. When it's full, oil bypasses to the outlet.
The storage volume must be large enough to hold the oil your facility generates between scheduled cleanouts.
Estimate your oil generation rate:
- A typical automotive service bay generates 0.5–2 litres of oil per day (drip, fluid spills, oil residue from vehicle washing)
- An active vehicle wash bay with heavier contamination may generate 2–5 litres per day
- An industrial machining sump generates tramp oil at rates that vary by operation
Target: oil storage capacity ≥ 90 days of oil generation at maximum facility activity, to accommodate seasonal variation and cleanout scheduling.
The Sizing Calculation: Step by Step
Step 1: Calculate Peak Flow Rate
List every simultaneous water source at maximum activity:
| Source | Flow Rate |
|---|---|
| Hose A (3/4" at 40 PSI) | 6 GPM |
| Pressure washer B (cold, 3500 PSI) | 3 GPM |
| Floor drain C (heavy rain, roof drainage) | 8 GPM |
| Total peak | 17 GPM |
Step 2: Apply the Safety Factor
Separator efficiency drops near rated capacity. Add 20–25% to your peak flow:
17 GPM × 1.25 = 21.25 GPM minimum rated capacity
Round up to the next available model size. From the OlioSep™ series: 24 GPM.
Step 3: Check Oil Droplet Size Against Technology Type
Is your application using detergents, high-pressure washing, or both?
- No detergents, low pressure: Standard coalescing plate unit is appropriate
- Detergents used: Confirm the coalescing plate pack geometry handles emulsified oil. Avoid gravity-only units.
- Heavy emulsification: May need a downstream polishing step (DAF unit or media filter) to meet discharge limits
Step 4: Verify Oil Storage Volume
Estimate daily oil generation × 90 days. Compare to the selected model's oil storage capacity (listed in the spec sheet).
Example: A 5-bay shop generates an estimated 8 litres of oil per day. 90-day storage requirement: 720 litres. Verify the OlioSep™ 24 GPM oil storage chamber exceeds this, or plan for more frequent cleanouts (every 60 days).
Worked Examples
Example A: 3-Bay Auto Shop, No Vehicle Washing
- 3 service bays, vehicles raised on lifts
- Occasional floor cleaning with a mop and bucket
- No roof drainage to floor drain system
Peak flow: mop cleaning is negligible (< 0.5 GPM)
Safety factor: 0.5 × 1.25 = 0.625 GPM
Select: OlioSep™ 2 GPM (size up from minimum for reserve capacity)
Oil generation estimate: 3 bays × 0.5 L/day = 1.5 L/day
90-day storage: 135 litres required
Example B: 4-Bay Auto Shop, Active Vehicle Washing
- 2 bays with cold-water pressure washers (3 GPM each)
- 2 bays mechanical service only
- Roof drainage separate from floor drain system
Peak flow: 2 × 3 GPM = 6 GPM
Safety factor: 6 × 1.25 = 7.5 GPM
Select: OlioSep™ 8 GPM
Oil generation estimate: 4 bays × 1.5 L/day average = 6 L/day
90-day storage: 540 litres required
Example C: Commercial Car Wash
- 2 wash lanes operating simultaneously
- Hot water / detergent system
- Roof drainage enters drain system
Wash lane flow: 2 × 8 GPM = 16 GPM
Roof drainage (peak): 12 GPM
Total peak: 28 GPM
Safety factor: 28 × 1.25 = 35 GPM
Select: OlioSep™ 50 GPM (next available above 35 GPM)
Note: detergent use creates emulsified oil. Confirm the coalescing plate geometry is adequate. Consider a downstream DAF unit if the discharge limit is below 15 mg/L.
OlioSep™ Tank Sizing Quick Reference
| Peak Flow (GPM) | With Safety Factor | OlioSep™ Model | Typical Application |
|---|---|---|---|
| ≤ 0.4 | ≤ 0.5 | OlioSep™ 0.5 GPM | Single floor drain |
| 0.5–1.6 | ≤ 2.0 | OlioSep™ 2 GPM | 1–2 bay shop, no washing |
| 1.6–3.2 | ≤ 4.0 | OlioSep™ 4 GPM | 2–3 bay shop, light washing |
| 3.2–6.4 | ≤ 8.0 | OlioSep™ 8 GPM | 3–4 bay shop, active washing |
| 6.4–12.8 | ≤ 16.0 | OlioSep™ 16 GPM | 5–6 bay shop |
| 12.8–19.2 | ≤ 24.0 | OlioSep™ 24 GPM | Large fleet / 6+ bays |
| 19.2–24.0 | ≤ 30.0 | OlioSep™ 30 GPM | Bus depot / heavy industry |
| 24.0–40.0 | ≤ 50.0 | OlioSep™ 50 GPM | Commercial car wash / large industrial |
Common Sizing Mistakes
Using average flow instead of peak flow. A separator sized for your typical Tuesday is undersized for a rainy Monday when three wash bays run simultaneously and roof drainage is active.
Forgetting roof drainage. The roof drain connection is easy to overlook during a site visit. Check the as-built drawings, not just the floor drain layout.
Treating all applications the same. A dry-service auto shop and a pressure-wash car wash have fundamentally different sizing requirements, even at the same "size" facility.
Under-specifying for emulsified oil. If you use detergents, a gravity-only unit almost certainly won't achieve compliant effluent. Specify a coalescing plate design.
Not leaving room for growth. If you're adding a bay in 18 months, size for that now. Adding an additional separator later is expensive and disruptive.
Underground vs. Surface Mount for Tank Sizing
When your peak flow calculation results in a large required capacity (30+ GPM), the question often becomes whether to use an above-ground surface mount unit or an underground tank.
Underground units have larger oil storage capacity and handle higher peak flows and sediment loads. They're common for commercial car washes, bus depots, and large fleet operations. The tradeoff is installation cost — excavation, concrete work, and manhole access all add up. A 30–50 GPM underground OWS installation can run $15,000–$60,000+ installed.
Surface mount OlioSep™ units at 30 and 50 GPM are a practical alternative where the footprint allows and where cleanout access is available. Installation is simpler and significantly less expensive.
For a full comparison, see Above Ground vs. Underground Oil Water Separators.
Need help with oil water separator sizing?
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
What happens if my oil water separator is undersized?
Oil passes through to your discharge stream. Your effluent won't meet the 15 mg/L limit in most municipal sewer bylaws. This is a compliance failure that may not be visible until a sample is taken during an inspection. The separator will appear to be working — it just won't be separating effectively.
Can I connect multiple floor drains to one separator?
Yes, and this is common. Add the peak flow from all connected drains to get your total sizing input. Make sure all connections happen before the separator inlet so the unit sees the combined flow.
How does rainfall affect my oil water separator sizing?
If your roof drains connect to the same pipe as your floor drains (common in older facilities), heavy rain can multiply your total flow dramatically. Always check whether your floor and roof drain systems share a connection point upstream of the separator.
My separator is rated 8 GPM but I think I need more. Can I add a second unit?
Yes. Two separators in series or parallel are a valid solution. Series arrangement (one feeds the other) improves effluent quality. Parallel arrangement (flow split between two units) increases total flow capacity. For new installations, it's usually simpler to select the correctly sized single unit.
Is there a standard sizing formula I should use?
API Publication 421 provides a design methodology for gravity separators. The key calculation is based on the rise rate of the smallest oil droplet you need to capture, the cross-sectional area of the separator, and the flow rate. For coalescing plate separators, the plate geometry multiplies effective surface area. ERE's technical team applies these calculations and can validate sizing for your application.
Related articles
- Oil Water Separators: The Complete Industrial Guide
- Oil Water Separator for Auto Shops: Requirements & Sizing
- Oil Interceptors: How They Work and When You Need One
- Version française : Réservoir séparateur eau-huile — dimensionnement et choix
Une version française de cet article est à venir.