What is an oil-water separator and how does it work? - Atlas Copco Lithuania
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What is an oil-water separator and how does it work?

Oil Water separators OSC

Oil-water separators play a crucial role in compressed air systems that use oil-injected compressors, such as the screw compressor. In the process of generating compressed air, these compressors also produce a condensate that contains trace amounts of oil. In order to protect the environment, the condensate must be treated – and the oil removed – before it can be disposed of.
 

That is where the oil-water separator comes in. Like the name says, it separates the compressor condensate’s water and oil to prevent the latter from getting into the wastewater.

What are oil-water separators?

An oil-water separator is a piece of equipment designed to separate oil and water by using a variety of filters. While different types of oil-water separators have different capabilities, they are used in a wide range of industries, including mining, construction, wastewater treatment, and food manufacturing. In this article, we will talk about how oil-water separators work and why they are essential for legal compliance and environmental protection. When it comes to oil-water separation in compressed air systems that use oil-injected compressors, the more the condensate is purified before it reaches the groundwater, the better it is for people and the planet. 

Why must oil and water be separated?

Removing oil from water is a very important job. Most people are probably familiar with the experiment that shows how one droplet of oil quickly spreads across a large water surface. In fact, one liter of motor oil can contaminate one million liters of groundwater. An oil slick that places itself on top of a body of water can prevent oxygen from reaching the plants and animals that live in it. The oil can also harm animals by impacting the insulating characteristics of fur-bearing animals and the water-repelling properties of a bird’s feathers.

There is a second important reason why oil-containing compressor condensate must be treated: In many places, it’s the law. More and more countries and other jurisdictions are enacting increasingly stringent environmental regulations that prohibit the dumping of oil-containing water. Violating these rules may result in costly penalties. Fortunately, as the experiment also demonstrates, oil and water don’t mix well and most of the oil will stay on the surface. In oily wastewater treatment, for example, both oil and suspended solids can be skimmed away. Unfortunately, however, that doesn’t mean that the oil residues found in compressor condensate can simply be skimmed off the top. Instead, oil-water separation in compressed air systems is an intricate process that is necessary to ensure legal compliance and minimize environmental pollution. Oil-water separators can eliminate approximately 99,5% of oil brought in by ambient air and the oil injected in the air by compression. Let’s take a closer look at how an oil-water separator works.

So how does an oil-water separator work?

Naturally, there are differences between oil-water separator brands and types. However, most involve multi-stage filtration and the principle of adsorption. Adsorption is a surface phenomenon where oil simply settles on the surface due to its lower density than water. Oil-water separators often use two or three stages of filtration which rely on different filtration media. Let’s go over each stage to better understand how a compressor’s condensate is treated.

Filtration stages in the oil-water separator

The oil-containing condensate from the compressor flows under pressure into the oil-water separator and through a first-stage filter, which is usually a pre-filter. A pressure relief vent usually helps reduce the pressure and avoid turbulence in the oil-water separator tank, allowing the gravitational separation of free oils. First-stage filters typically consist of polypropylene fibers, which adsorb the oil, but not the water. This means that oil droplets will stick to the surface of the polypropylene fibers. Due to their oil-attracting characteristics, these filters are aptly known as “oleophilic” filters. This type of filtration media usually floats on the surface of the water but gets heavier as it adsorbs oil droplets, sinking deeper near the end of its service life. 

 

After the first-stage filtration, the condensate flows through the main filters, which include second-stage and sometimes third-stage filters. These filters often rely on activated carbon (or organoclay for stronger emulsions) to purify and “polish” the condensate. In other words, depending on the type and size of the oil-water separator, the condensate will undergo one or two consecutive stages of filtration by activated carbon or organoclay.

 

At the end of this process, the last remaining oil residues in the condensate have been collected. At an ambient temperature of 20 ℃, while there is 1-2 g/m3 of oil in the condensate after the first-stage filtration, there is approximately 2-3 mg/m3 of oil remaining in the condensate after it leaves the oil-water separator.

 

The remaining water is sufficiently free from contaminants and can be safely discharged into the sewer system. The oil-water separator has done its job. And in the end, everybody benefits: The company that does the right thing and avoids fines and, most important of all, protects the environment.