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Easy Guide to Oily Water

Oil Water Separator Selection – made simple

Never dealt with oily water treatment before? You’re not alone, most of our customers will only select an oily water separator once in their career. Maybe you’ve seen it go wrong and want to make sure you don’t waste money on the wrong system.

We replace dozens of broken, high maintenance, under-performing and non-compliant oily water separators every year. So we though we would share some information on oily water treatment that we’ve learnt over 30 years of working on oily water and more than 2000 systems installed.

1: Oily Water Basics

Oil contamination can be present in 4 different forms in water and each form requires a different treatment strategy.  So it is important to understand the 4 types of oily water pollution so that you can select suitable treatment options.

Obviously we cannot short list oil water separator types till we fully understand the oily water we need to treat.

Many oil water separator types will be ruled out because they simply cannot treat your particular oily water mix / type.

Below are the 4 ways that oil can present in water:-


Free Oil:

  • Quickly forms oil layers, slicks or sheens on the water surface
  • Large oil droplets greater than 150 micron
  • Easy to remove – lots of treatment options
  • Will block or overload many oily water separators if not removed first
  • European EN-858-1 ‘5 mg/l ‘ oil water test standard covers removal of free oil only

Emulsified Oil:

  • The most common type of oil contamination in many industries!
  • Oil droplets between 2 and 150 micron
  • Dispersed throughout the water
  • There are 3 types of emulsification:
    • Chemical – degreasers, fuel additives etc
    • Mechanical – pumps, valves etc
    • Biological – natural biological activity that breaks down oil layers
  • The smaller the oil droplet, the harder to separate
  • More powerful separation is required, especially for smaller oil droplets
  • Test standards such as the American ASTM standard measure removal of emulsified oil

Dissolved Oil:

  • Cannot be seen by the naked eye, in fact the water may appear to be ‘clean’
  • Often forgotten in oily water treatment design
  • Includes:
    1. Oil droplets less than 2 microns
    2. Detergents and cleaners which are often analysed as oil and grease
    3. Soluble hydrocarbons like benzene and toluene (BTEXs)
  • Biological degradation of ‘insoluble’ oil like diesel creates dissolved oils
  • Can be as high as 2000 mg/l oil and grease!
  • Cannot be removed with mechanical separators

Heavy Oil:

  • Oil with a density greater than water, oil that sinks in water (e.g. Bunker ‘C’ fuel oil)
  • Oil stuck to heavy solids
  • Won’t be removed by the many separators that require oil to be lighter than water

Not sure what mix of the 4 types of oil contamination you have? Let us help

Give us a call or send an email. Using our extensive data base of customers oil water pollution types we can put you in the picture in no time!

In addition you can take advantage of our extensive in-house oily water test facilities and laboratory. We are are able to analyse and characterise your oily water problem and give you a full report. In many cases we can do this for free!

2: Characterising Your Oily Water

There’s no ‘one size fits all’ oily water separator. Choosing an oil water separator without knowing what type of oily water it’s expected to treat is a recipe for disaster!

In the previous section we explained the different types of oil pollution you can experience in water, you can combine this knowledge with a few other key parameters listed below to accurately define the oily water that needs to be treated – this is typically called the influent.

Oil Droplet Size


Why’s it important?

  • It’s how oil separator performance is rated – the smaller the oil droplet a separator can remove, the more powerful it is
  • There is no one separator that can treat all oil droplet sizes

How do you measure it?

  • Measure it directly with a video particle analyser
  • Estimate it using data from similar applications
  • Estimate it from visual samples
  • Check out our blog on oil droplet size here for a detailed explanation

Oil Spill Load


Why’s it important?

  • You don’t want to overload the separator – feeding neat oil in is a problem for most separators
  • You want to size the oil collection tank correctly – you don’t want your separator to deal with a 200L spill but only have a 100L oil collection tank

How do you measure it?

  • Look at the infrastructure to decide what your biggest amount of gross oil spilt is that you expect the separator to deal with
  • Any spill above this amount would be classed as an incident, meaning the separator would be shut down and the spill dealt with manually
  • The separator would not be expected to deal with spills classed as an incident

Oil Density


Why’s this important?

  • Most separators use the density difference between the oil and water as their way of operating
  • The bigger this difference the better the separator will work

How do you measure it?

  • MSDS sheets when only a single oil is responsible for the contamination
  • Run a density test on the oil from collected from site – a weathered oil will have a different density to fresh oil
  • Combinations of oils will also result in different densities, i.e. petrol, diesel and engine oil all mixed together will have a different density to the individual oils

Dissolved Oils


Why’s this important?

  • Separators used to remove free and emulsified oils are generally not capable of removing dissolved oils without additional steps
  • If dissolved oils need to be treated the your vendor may need to introduce another treatment step to remove them
  • Not accounting for dissolved oils can be the difference between compliant and not compliant water
  • You water might look like drinking water, dissolved oils are not visible and could register as high 2000mg/L

How do you measure it?

  • Identify hydrocarbons with dissolvable components such as BTEX – i.e. petrol and light, aromatic hydrocarbons
  • Send a sample to a lab for a ‘blank’ analysis – i.e. filter an oily water sample to remove all oil particles >1μm, run the normal sample analysis after filtration to get a figure for the concentration of dissolved oils and other soluble contaminants
  • Detergents, degreasers and other surfactants are soluble and can show up as dissolved oils

There are a number of other parameters the can impact the treat-ability of oil water contamintation and the ability for the separator to perform as required and meet discharge compliance. Some of the other parameters that are relevant are temperature, water density, viscosity, etc.

These additional parameters are often interlinked such as temperature (a higher temperature will result in a lower water viscosity) and salt concentration which can lead to differing water densities.

Estimating Oil Droplet Size: Visually Compare Water Samples

You can also get a rough idea of your oil droplet size with a visual inspection of your sample. In general, if you can see the droplets with the naked eye, they are large, say 150 micron plus. If the water looks hazy, white or milky, the droplets are very small (less than 25 microns, often less than 10 micron). We have a data base of sample photos from thousands of applications with matching droplet sizes to give you a visual guide. Look for a sample that has similar characteristics to yours. If you need help, email us a photo of your sample and our oily water engineers will help you.

15 Micron
25 Micron
60 Micron
150 micron

3: Comparing Oil Water Separators

Now that you understand the 4 types of oil contamination; the particular mix of these type you have, and the importance of oil droplet size:

– you can begin to short list types of oil separator that might work for you

A good place to start is oil droplet sizes. You can narrow down your choice of oil separator technologies that suit your application based on what droplet size each separator technology is designed to treat.

Separator Technology Type300 micron150 micron60 micron25 micron15 micron
Ultraspin SeparatorsTick2Tick2Tick2Tick2Tick2
Media filtersCross2Tick2Tick2Cross2Cross2
Gas flotation with chemicalsCross2Tick2Tick2Tick2Tick2
Gas flotation without chemicalsCross2Tick2Tick2Cross2Cross2
Coalescing Plate Packs - VGSTick2Tick2Tick2Cross2Cross2
API gravity separatorTick2Tick2Cross2Cross2Cross2
European tanks / SPELTick2Tick2Cross2Cross2Cross2
Basic tanks, double or triple interceptorsTick2Tick2Cross2Cross2Cross2

Oil Water Separator Performance Curves

Now that you understand the importance of oil droplet size you will now be able to understand Oil Separator Performance Curves. Many factors affect the performance of an oily water separator system installation. For most customers a primary consideration is oily water separation ability because this defines final discharge water quality or oil recovery rates. Oil Separation efficiency can be expressed as follows:

Oil Separator Efficiency (%) = 100 x ( Cin – Cout/ Cin)

  • Cin = feed inlet concentration (mg/l)
  • Cout = discharge outlet concentration (mg/l)

More powerful separators have a number of advantages including obtaining better quality discharge water, meet or exceed Local and National Trade Waste discharge standards, meet or exceed future discharge standards, deal with accidental oil spills, tolerate higher strength non-“quick-break” detergents and cleaners.

Oil Water separation efficiency can be objectively accessed by examining the separator oil droplet size vs. separation efficiency curves. For higher quality separators these curves should be readily available from manufacturers. Manufacturers should also be able to confirm performance to these curves by independent testing. The purpose of oily water separators is to obtain clean water. However, claims of a separator being able to achieve less than 10 mg/l discharge are meaningless and deceptive without recourse to oil droplet size performance curves.


Caution: There are oily water vendors who can’t measure the oil droplet size as they don’t own the specialised equipment required. You should not trust vendors who can’t measure oil droplet size. They can only guess how their equipment will work in your application.

It’s like a pressure vessel manufacturer not being able to measure pressure, or an oven manufacturer not being able to measure temperature.  It’s ridiculous – but it happens!

Always ask your supplier for an objective oil droplet based performance curve like the one shown here. Make sure its one they have generated by measuring oil droplet size on their equipment, and not copied from another vendor like Ultraspin. Compare the curve to the oil droplet size you have or need. If a supplier can’t or won’t send you their own performance curve – be very wary!

4: Other Oily Water Fluid properties

Mean oil droplets size is the single most important factor for defining the dirty oil water, but there are several other factors that are also important.

Temperature Sym
Temperature Sym
Temperature Sym


The viscosity of the water falls as the water temperature rises. This allows oil droplets to move more easily through the water phase, thereby producing better oil separation with many oil water separators (but not all). Therefore, increased temperature improves separation efficiency for most oily water separators. The Ultraspin separator can be supplied to operate within the temperature range 0 to 120°C (32 to 248°F), at higher temperatures special materials may be required.

Oil Water Separators that work better at higher temperatures include:

  • Coalescing plate packs (5 to 80 oC)
  • Ultraspin Separators (5 to 120 oC)
  • Gravity tanks and vessels (5 to 90 oC)
  • Centrifuges (5 to 90 oC)

Oil water separators that may not separate as effectively at higher temperatures include:

  • Air flotation devices (DAF, CAL, IGF) (5 to 30 oC)
  • Media filters (5 to 45 oC)
  • Membranes (5 to 65 oC)
  • These types of separator may require water coolers to work effectively.

Density Difference

For many oil water separators the oil removal efficiency is dependent on the difference in density between the contaminant and the water. The separation efficiency increases as the difference in density increases. The density difference can increase if you have lighter oils or your water is salty, or both. Typical densities for Oil are 950 to 750 kg/m3 (0.034 to 0.027 lb/in3) and water : 1050 to 995 kg/m3.(0.038 to 0.036 lb/in3).

For these types of oily water separator performance improves as the density difference increases, and generally the density difference must generally be more than 25 kg/m3

  • Coalescing plate packs
  • Gravity tanks / SPEL / Triple and double interceptors
  • Ultraspin Separators

For these types of oil water separators are not greatly effected by small density differences (say less than 25 kg/m3):

  • Media filters
  • Membranes
  • Air flotation (DAF, IAF, CAF)

Inlet Concentration

Some types of oil water separator are NOT well suited to high oil feed concentrations and high feed oil concentrations overload or block the systems. For these types of system careful pre-treatment to remove oil will be required.

  • Coalescing plate packs (max 1% oil or 10,000 mg/l).
    • With high oil concentration for this type of separator the plates become coated in oil and sludge. This requires the plates to be removed and cleaning to restore oil removal efficiency.
  • Air flotation systems (DAF, IAF, CAF) (max 1% oil or 10,000 mg/l)
  • Media filters (typical max 100 mg/l oil )
  • Membranes (typical max 300 mg/l)

Other types of system can accommodate 100% oil concentration for a period of time:

  • Ultraspin systems
  • Centrifuges
  • Gravity tanks / SPEL / Triple and double interceptors
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