An oil separator (also called an oil-water separator or hydrocarbon separator) is a mandatory piece of infrastructure at professional car wash sites. It removes petroleum-derived substances — engine oil, fuel, grease, and hydraulic fluid — from wastewater before it enters the municipal sewer system or a closed-loop water recycling circuit.
How it works:
Oil separators exploit the density difference between water and hydrocarbons. Wastewater flows through a series of chambers where oil floats to the surface and is retained, while cleaner water exits from below. Coalescing plate separators add angled plates that accelerate oil droplet coalescence for higher efficiency.
Regulatory requirements:
In the EU, car wash oil separators must comply with EN 858 standards. Regular maintenance — including sludge removal and separator inspection — is required, typically quarterly. Documentation must be kept for environmental audits. See also REACH and EU Detergents Regulation for related compliance requirements.
Chemical compatibility:
Some surfactants can emulsify oil so effectively that it passes through the separator. Well-formulated products like Fortis Foam ECO are designed to clean vehicles while allowing oil separation to function correctly downstream.
Using biodegradable car wash chemicals reduces the overall pollutant load reaching the separator, extending maintenance intervals and improving discharge quality.
For a comprehensive guide on separator selection, legal requirements, and maintenance, see our article on oil separators for car washes.
Sizing guide
| Car wash type | Bays | Recommended NS (l/s) | Typical separator |
|---|---|---|---|
| Touchless 2-bay | 2 | NS 6–10 | Ø1000, 2000L capacity |
| Touchless 4-bay | 4 | NS 10–15 | Ø1200, 3500L capacity |
| Touchless 6-bay | 6 | NS 15–20 | Ø1500, 5000L capacity |
| Self-service 4-bay | 4 | NS 10–15 | Ø1200, 3500L capacity |
| Tunnel wash | 1 tunnel | NS 20–30 | Ø2000, 8000L+ capacity |
Always oversize rather than undersize. An undersized separator fails during peak hours, allowing oil breakthrough to the outlet — a compliance violation that can result in fines.
Frequently asked questions
What class oil separator does a car wash need?
Car washes require a Class I coalescing separator compliant with EN 858-1. Class I separators achieve outlet oil concentration below 5 mg/l, meeting discharge requirements for sewer systems. Class II (gravity only) separators — which only achieve < 100 mg/l — are insufficient for car wash wastewater and do not meet regulatory requirements in most jurisdictions.
How often should a car wash oil separator be emptied?
The sludge compartment should be emptied every 3–6 months, depending on traffic volume and season (winter generates more sediment from salt and sand). The coalescing insert should be replaced every 1–2 years. Outlet water should be tested by a laboratory every 6 months. All maintenance must be performed by licensed waste management companies, and records must be kept for environmental audits.
Can car wash chemicals damage the oil separator?
Yes — indirectly. Over-concentrated surfactants from over-dosed foam create stable oil-water emulsions that pass through the separator instead of separating. This is why correct dilution ratios and proper dosing pump calibration are critical. Products with high biodegradability also reduce the organic load on the separator. Our guide to chemical dosing mistakes covers this in detail. For maintenance procedure and costs see oil separator maintenance for car washes; for chemistry compatible with Class I coalescing separators see Fortis Foam PRO (touchless) and ECO (hand wash / detailing).
Do I need a separator if I have a closed-loop water system?
Yes. The oil separator is the first treatment stage in any closed-loop water recycling system. Without it, petroleum products accumulate in the recycling circuit, contaminating recycled water, damaging filtration equipment, and degrading wash quality. In fact, separators in closed-loop systems work harder because contaminants concentrate over recycling cycles — consider oversizing by one NS class.