Oil/water separator

Method diagram

Method and installation description

The aim of a oil/water separator is to remove oil from wastewater. The theory is based on separation via differences in density. In general, the system consists of a concrete container, which is normally divided into three compartments. In some cases, the oil/water separator may also be made from plastic or metal. The wastewater flows into the first compartment of the oil/water separator. This is where the larger components (solid particles) sink and the lighter components float. This is often followed by a series of plastic plates. The oil particles come into contact with them and cling on to larger oil particles. Due to the lower density, they float upwards past the plates and form a floating layer on the water. The purified water (partly) flows out of the oil/water separator via a skimming wall.

In a coalescence filter, a carrier material with a large specific surface area (ca. 200 m²/m³) will be placed in the tank. Small undissolved particles stick to other larger particles on this surface, and then start to float. In doing so, a higher yield is realised for the removal of hydrocarbons.

Specific advantages and disadvantages

It is a straightforward installation with a simple construction and easy operation. Once installed, only the floating layers needs to be removed on a regular basis so that the contents (and thus retention time) are not excessively diminished. The system itself has no mechanical components and thus requires little maintenance.

An oil/water separator only works for oil-like substances that are water-insoluble, are difficult to mix with water and/or have a density that is similar to that of the water. If this is not the case, like in oil in water emulsions, the oil/water separator will not function.


The use of oil/water separators is standard practice in locations where wastewater or rain water could be polluted with mineral oil or other difficult to dissolve hydrocarbons. Installations may range from small systems (1 l/s) to very large ones (240 l/s).

Sectors in which oil/water separators are implemented, include oil refineries, food industry, slaughterhouses and car garages. Separators are also used as preliminary wastewater purifiers in restaurants, catering companies, etc.

Here are a few examples:

  • Rain water from car parks, fuel stations, scrap yards and storage sites for dangerous substances.   This often involves larger volumes;
  • Wastewater from car and truck washes;
  • Wastewater from barrel cleaning and tank cleaning;
  • Wastewater (e.g. floor cleaning) from car garages, car body workshops and other work places for machines and equipment;
  • Removal of oil from degreasing baths;
  • Wastewater released in oil refineries.

Boundary conditions

No specific boundary conditions.


The oil/water separator can primarily be implemented for the removal of oil-like substances.

If normal oil pollutants are present in wastewater, a yield of 50-90% can be realised. The yield is greatly determined by the flow profile in the oil/water separator and the physical/chemical properties of present oil-like components, e.g. density and solubility.

Support aids

In general, no support aids are used in an oil/water separator. If an oil/water separator is ineffective, because part of the oily components are in emulsion form, for example, a specific chemical pre-treatment (oil or emulsion breakdown) can sometimes prove helpful.

Environmental issues

The oily floating layer is removed from the wastewater.


The design (choice of material, sludge collection, shape, etc.) of the installation partly determines the cost price. The guide price for a simple grease separator in 2 and 10 l/s format, in HDPE and certified (KOMO or BENOR), amounts to ca. € 3.000-3.500 and € 5.500-8.500 respectively.


Analyses can demonstrate which components are present in a wastewater flow. This is important in order to make the right dimensioning choices for the oil/water separator.


The installation is simple.

Level of automation

The oil/water separator rarely needs automation.


  • EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
  • VITO-SCT, revision of technical notes WASS, 2009

 Version February 2010