Oil separator for car washes: requirements, classes and sizing

01Does a car wash need an oil separator?

Yes. Wash water from a car wash facility contains oil, fuel residues, heavy metals and particles that cannot be discharged untreated to the public sewer or receiving water. Local discharge regulations set requirements for the discharge of oily wastewater, and the local authority issues a discharge permit with specific limit values. For car washes this means in practice a class I coalescing separator, because the wash water often contains emulsified (finely dispersed) oil that a simpler gravity separator does not capture well enough.

02What is the difference between class I and class II?

EN 858-1 divides oil separators into two classes according to how clean the water is leaving the separator:

Class I uses a coalescing filter that makes small oil droplets merge into larger droplets that float up and are held back. This is the class required when the water contains emulsified oil, such as in a car wash.

03Why must the sand trap come before the separator?

A sand trap removes sand, grit and sludge by sedimentation before the water reaches the oil separator. It protects the coalescing filter from clogging and keeps the separation efficiency high. The sand trap is sized under EN 858-2, and is always placed before the separator in the flow direction. In many facilities the sand trap and oil separator are supplied as a tailored unit.

04How is an oil separator sized?

Sizing follows EN 858-2 and is based on:

• Maximum water flow through the facility, given in litres per second
• Any stormwater flow from roofs and impermeable surfaces (rainfall x area x runoff factor)
• Whether the water contains emulsified oil, which requires class I
• The density of the oil to be separated out

An undersized separator cannot keep up with separating the oil at high load, while a heavily oversized solution is needlessly expensive. Norrloop sizes the separator and sand trap for the specific facility and supplies it ready-tailored.

05Why choose a PE oil separator over concrete or steel?

Norrloop supplies oil separators in rotationally moulded polyethylene (PE). PE is corrosion-free and needs no sacrificial anodes, unlike steel tanks. The material is light, watertight and withstands the aggressive environment of oily wastewater over time.

06What does the regulation require?

The most important frameworks for a car wash facility:

• Local discharge regulations - requirements for the discharge of oily wastewater
• The local authority's discharge permit - local limit values and sampling requirements
• EN 858-1 and -2 - classification, performance and sizing of separators and sand traps

Many local authorities require documented sampling and regular inspection of the separator. It pays to clarify the discharge permit requirements with the local authority early in the project.

07How often must the oil separator be emptied?

Emptying is governed by the level of sludge and oil in the separator, not by a fixed date. A level monitor or alarm warns when the sludge or oil layer approaches the limit for how much the separator can store. Regular inspection and emptying as needed ensures the separator keeps meeting the treatment requirement, and that the sludge is not carried further into the facility.

08Frequently asked questions