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Why Do I Need An Oil Water Separator

oil water separator

Why Do I Need An Oil Water Separator

Oil spills are not an infrequent guest on news coverages. These spills can be catastrophic to the environment and create long-lasting problems for the flora and fauna in that area. So it should go without saying that any business that uses an air compressor with oil should use an oil water separator to dispose of the oil properly. More specifically, you don’t have a choice but to use one when you discharge oily/ sediment-ridden wastewater into the sewer. 

The equipment that you need is an oil-water separator. Because oil is lighter than water, these machines remove oil, grease, and hydrocarbons and leave non-hazardous water to be safely disposed of through the discharge port. By successfully removing these contaminants, it meets the EPA requirements for dumping wastewater. However, that does not mean having one in your system automatically reaches the standards.

Oil water separators work through an application of Stokes’ Law. Now, what is this law? English mathematician George Stokes analyzed and described the relationship that dictates how the solid particles in a liquid will settle. This also applies to liquids of different weights in the same space, so oil and water. The equation he created can determine the velocity at which the particles rise or settle. 

As with all equations, there are multiple variables to be considered. Whether it is the specific gravity between the liquid and particles or the size of the particle, these variables can affect the exact calculation. When Stokes was doing his calculations, he made three key assumptions:

  • Particles are the same size
  • Particles are spheres
  • There is a laminar flow
    • This means the flow is gentle and smooth with no additional turbulence

Alongside Stokes’ Law, there are a few other factors that can have an impact on the efficiency, usage, and management of your equipment. These factors include but are not limited to the following:

  • Frequency and Intensity
  • Design Capacity
  • Emulsifying Agents
  • Maintenance Practices
  • Type of System
  • Quantity of Contaminants 

Now that we have a basic understanding of how these machines operate, let’s dive into the details of the oil removal process in an oil water separator.


How Exactly Do These Things Work?

When you are using an oil water separator, you can benefit from understanding how these machines operate and the processes that the wastewater needs to go through to be properly disposed of.

The process starts off as any compressed air system would and goes through the progression of creating compressed air. As this process occurs untreated condensate will start to collect in the system, hopefully not in the air lines but in the lines dedicated for condensate. 

The condensates are then passed through mufflers located in the integral expansion chamber. During this portion the condensates are collected by the mufflers through the process of depressurization. By removing pressure, it allows for Stokes’ Law to take effect on the condensate and allow the particles to separate more.

This depressurized condensate will then flow into “column A.” Once the condensate enters this column it is passed through an oleophilic media. This media works similarly to a coalescing filter in the way that the fibers absorb the oil but still let water pass through them. This filter floats at the top of the column to absorb the oil that floats to the top.

The more this filter becomes saturated, the heavier it becomes and it starts to sink down into the column. This behavior is advantageous because it means that the filter is constantly in contact with the surface of the water. This contact means the oil has nowhere else to go but into the filter as it rises to be above the water.

At the top of Column A is an indicator stick. This indicator shows the current status of the filter that is inside the column. This stick sinks alongside the filter media to give you an idea of where the filter is in the column. You will need to change the filter out just before the indicator stick is completely submerged.

After the condensate has passed through Column A, it is then moved into Column B (who would have thought). Rather than having a filter the water needs to pass through, this column is filled with activated carbon. Any oil that is left in the condensate will be absorbed into the activated carbon and the rest of the condensate will be able to pass through cleanly.

This clean water then exits through a disposable filter that is attached to the outlet port. This completes the process in the oil water separator and indicates that you can safely discharge the condensate. Although most of the oil has been removed, about 4mg/gal of oil is still present in the condensate. By achieving this level of oil content, the disposal of condensate can be done as usual with no risk to the environment.


How Does The Gunk Get In?

When compression occurs, all of the naturally occurring contaminants in the air like dust, water vapor and other particulates are mixed into the hot oil being used for lubrication. After compression, the heat will be removed from the air and when this is done contaminated condensate is formed. This condensate needs to be removed or else it will be detrimental to the equipment downstream.

Because of the nature of compressed air, this condensate is an unavoidable part of the process. This condensate can be collected through drain valves that will then drain into storage tanks to hold the condensate. You would then need to transport that contaminated water to a designated facility to have it properly taken care of. This method is both inconvenient and expensive, which makes it a less than ideal solution for your condensate.

The more common alternative for condensate treatment is properly filtering the condensate so that it is safe to dump it into the sewage system. The oil and particulates are removed when the condensate passes through the different levels of filtration and then it simply has to be dumped out into the sewer. By using this method, the oil water separator will pay for itself by reducing the costs of disposing of wastewater.


Types of Separation

Just as with compressors, oil water separators can use different methods of separation to remove contaminants from the condensate. These methods may vary in application, and depending on your system this can cause different quantities of oil to be present in the wastewater. 

There are four main types of oil separation that are used are:

  • Gravity Separation
  • Chemical Absorption
  • Non-Chemical Absorption
  • Adsorption

Right now those just may seem like fancy descriptors as to how the oil is being removed, so let’s dive into them so we can understand the different types of separation.


Gravity Separation

Gravity Separation is the simplest method of oil removal. By acting upon the natural weight difference between water and oil, oil is lighter so it will rise to the top and form a layer above the water. This allows for the oil to be skimmed off the top, but problems occur when the oil has become emulsified with the water.

Because some of the oil will make it through this process, the wastewater that is emitted may have a higher PPM of oil than is sufficient to meet the requirements necessary for properly disposing of compressed air condensate.


Chemical Absorption

Simply put, the media used for absorbing the oil is chemically attracted to oil and bonds with it while simultaneously repelling water. The oil becomes trapped in the filter media and the water continues along with the oil having been removed, even that which had become emulsified.

The wastewater that remains after this process meets the required PPM of oil and therefore can be dumped into the sewage system. 


Non-Chemical Absorption

Rather than having a chemical bonding process to remove the oil, non-chemical adsorption relies on the natural properties of the material. It must be hydrophobic (water-fearing) and oleophilic (oil loving). By attracting oil and repelling water, this style of absorption retains the oil within the material and the water passes through to be disposed of in the sewage. 



Adsorption and absorption are similar in name and in process. With absorption the condensate and oil is drawn into the center of the media, meanwhile adsorption keeps the oil on the surface of the material. The oil is attracted to the surface material and the many tiny pores on them to adsorb as much oil as possible while repelling the water.


Benefits to Oil Water Separators

Far and away the biggest benefit to having an oil water separator is the improved removal of oil. When given enough time and space, oily wastewater will separate into two different layers. Oil will float to the top due to being lighter than water and with a simple oil skimmer it can be removed easily and efficiently. 

Not only is it beneficial to remove the oil for the sake of environmental compliance, but by removing the oil before the water has been treated or filtered it increases the effectiveness and lifespan of the treatment and filtration equipment. By removing this oil effectively, it can become a potential revenue generator if your reclaimed oil can be sold.

When you are able to achieve the specific oil management challenges of your particular compressed air system you can achieve efficient oil separation and removal. By successfully doing this, you minimize the maintenance requirements for your oil water separator. This will keep your separator running smoothly and downstream treatment performing effectively. 


Maintenance of Oil Water Separator

Just like all of your compressed air equipment, oil water separators need to undergo consistent and proper maintenance to continue to operate at peak-performance while maintaining compliance with environmental standards. Oil water separators are mostly self-sufficient and have long intervals between maintenance as long as it is sized properly.

Trying to size these machines can be tricky as the climate can have a powerful impact on the efficiency and overall function of your oil water separator. You should match your model to the estimated humidity in your area, as long as it is within range it should not affect your operation.

Properly removing and disposing of oil is a problem both for the environment and economically for the individual. By minimizing the amount of particulates and oils that enter the OWS, through proper filtration in your compressed air system, lower quantities of sludge and oil will float to the top and need to be removed. This will give you longer periods between needing to move the oil or get maintenance on your equipment.


Before You Buy

Taking care of your oil water separator is the easiest way to ensure that it works. There are few final considerations for your equipment to just keep in mind. For starters, oil water separators should not be used to treat stormwater runoff and the water level should not exceed the specifications as it can cause the non filtered condensate to be pushed through.

It may not be the most glamorous or seemingly necessary piece of equipment, but oil water separators are incredibly important for the environment. Just as it is safe and responsible to wear a helmet while riding a bike, but not everyone follows that because to them it is merely a suggestion.

Most countries have requirements for water treatment and violating these can result in some hefty fines. These fines can impact the operator, business, or even both. These fines come from improperly disposing of oil or wastewater. The EPA guidelines for these types of fines are:

  • Negligent Violations
    • A fine of $2,500 to $25,000 per day of violation and/or imprisonment up to 1 year. While a second conviction is punishable by up to $50,000 per day of violation and/or imprisonment up to 2 years.
  • Knowing Violations
    • A fine of $5,000 to $50,000 per day of violation and/or imprisonment up to 3 years. For a second conviction, violators can pay up to a $100,000 fine per day of violation and/or imprisonment for up to 6 years.
  • Knowing Endangerment
    • Knowing violations of CWA requirements that put another person in danger of imminent death or serious bodily injury can be punished by up to a $250,000 fine and/or imprisonment of up to 15 years.
  • False Statements
    • Making false material statements, representations, or certifications may be punishable by up to a $10,000 fine and/or by imprisonment of up to 2 years. If convicted of making false statements, it is punishable by up to a $20,000 fine per day of violation and/or imprisonment of up to 4 years.

Now using an oil water separator is definitely the right thing to do, but that doesn’t mean you won’t go unpunished for not doing it. But everytime you dump improperly filtered wastewater filled with oil into a place it should not go you run the risk of being caught and having heavy fines and potential imprisonment. 

For how little work an oil water separator is to maintain and use properly, it just doesn’t make sense to not use one. The risks for not using one far outweigh the extra work needed to run one. Do your future self a favor and put in an oil water separator.

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