Everything you need to know about oil water separators

Cleanawater on 20 August 2021

What is an oil water separator?

An oil water separator is a wastewater treatment solution to remove hydrocarbons from the water. Regulators often prescribe this technology for businesses that could have oil contamination of their wastewater, e.g. Auto shops and car washes. Oil water separators come in different types and sizes to suit various applications.

Difference between grease interceptor and oil water separator

A grease interceptor removes fatty solids from wastewater. They are typically found in commercial kitchens and restaurants, where they prevent fatty solids from clogging up the drainage system. Grease interceptors are much simpler than oil water separators in design. They slow down the flow of water and allow the solids to settle out for disposal.

Guidelines and regulations for oil and water

Guidelines and regulations for oil and water are Australian regulations that limit the amount of oil permissible in a wastewater stream. Limits can be set based on total oil content or individual hydrocarbon components. The regulations aim to prevent oil from entering the environment because of the damage it does to the natural ecosystem. Even a small volume of oil spreads out over a large area and, therefore, has the potential to impact large numbers of plants and animals.

Each business discharging wastewater into the sewer network must obtain a trade waste permit. Local water authorities issue these permits, which contain the specification limits for wastewater discharge, including oil content. Limits may vary from one local authority to another because of the different wastewater treatment plants in their network, the oil loading of the system, and the natural environment of the area.

Measuring oil in water

In cases of extreme contamination, oil is visible as a separate layer on the surface of the water. A lower oil content could appear as a rainbow sheen on the surface. Several laboratory tests exist for measuring the actual oil content of a sample. Gravimetric methods extract the oil using solvents, which are then evaporated. This method determines the total oil content but does not quantify the types of hydrocarbon in the sample. Other techniques, like gas chromatography, give a more detailed analysis of the components of the oil.

Choosing the right oil water separators

There are two main types of oil water separators, each with its own operating principle.


Hydrocyclones work by creating a rotational flow and using centrifugal force to separate oil from water. Heavier water molecules accelerate as they pass through the hydrocyclone and exit at the bottom, while lighter oil particles move to the centre and travel upwards, where they are separated and collected. Hydrocyclones need a minimum pressure drop across the equipment to function correctly. This method of oil removal is highly efficient, with levels of 10 to 15 microns achievable.

hydrocyclone oil water separator mining wash bay2


Coalescing oil water separators use a series of plates to accumulate oil droplets into larger and larger spherical particles. As the size of the oil particles grows, they tend to rise to the water's surface due to the difference in specific gravity. Coalescing plate oil water separators slow down wastewater flow to give time for the separation to happen. Clean water discharges from the base of the separator while oil collects in a chamber.

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Sizing an oil water separator

Sizing an oil water separator is an important consideration. Coalescing oil water separators work on the basis of retention time. It is generally accepted that a separator chamber should be approximately ten times the wastewater flow rate. This capacity allows a retention time of about ten minutes for the oil to separate from water. Hydrocyclone separators can be sized according to wastewater flow, expected incoming oil content and discharge specifications.

Oil water separators for different industries

While hydrocyclones offer the highest level of oil removal, this level of efficiency is not always necessary. Many applications across several industries can meet their requirements using a coalescing oil water separator.

Auto shops

Coalescing plate oil water separators are ideally suited to auto shops. It is crucial to identify the types of hydrocarbon that could be present in the wastewater of an auto shop. This information, together with wastewater volumes and trade waste permit conditions, can be used to select the best oil water separator solution.

Car washes

Car washes need oil water separators because of oil and grease that is washed off the vehicles. Using the correct detergents and degreasers is vital to ensure that they do not negatively impact the performance of an oil water separator.

Other industries

In general, hydrocyclones are most effective in applications with high flow rates or where the oil removal requirement is high. Coalescing plate oil water separators are effective in most other applications.

How do you build an oil water separator?

A typical oil water separator installation will include the following elements:

  • An underground collection pit to collect wastewater from the auto shop floor or car wash.
  • A float system to start and stop the supply pump to the oil water separator based on the level in the collection pit.
  • A skid-mounted oil water separator unit.
  • A waste oil collection drum.
  • A discharge to the sewer network.

The pump ensures a consistent flow of wastewater to the oil water separator and allows it to work in its design operating range. Sometimes, the discharge water from an oil water separator is collected in a pumping station so that it can be pumped to the effluent discharge point.

In some mining or industrial applications, it may be necessary to install an oil skimmer in the water collection pit. This pretreatment stage will remove large quantities of free oil that could overload an oil water separator.

How much does it cost to install an oil water separator?

common oil water separator problems and their solutions oil water separator components2

Oil water separator solutions vary in price from $2,500 to over $1 million. Factors influencing the final cost include the system's capacity, which technology you choose, the oil content of the wastewater feeding the unit and the specification on the discharge water. In general, hydrocyclone oil water separators are more expensive than coalescing plate separators, and they achieve a lower oil content on the discharge.

Cost savings of oil water separators

Even though the capital investment for an oil water separator may be significant, it is not as significant as failing to comply with regulations. Fines for excessive oil content in trade waste can be severe, and the cost of shutting down a business until it can demonstrate compliance is even more drastic.

Oil water separator problems and solutions

Oil water separators can have problems if there is a mismatch between the design and operating conditions. A high wastewater flow rate means that the retention time in the separator is reduced, and some oil may carry through with the water. This excursion could lead to fines or even business shutdown due to off-spec wastewater.

Another reason oil may not separate from water is the use of the wrong detergents and degreasers. These products help remove oil from vehicles and equipment, but they turn the oil water mixture into an emulsion. In this state, the oil will not coalesce in the separator and will carry through into the water discharge. Only quick break detergents and degreasers should be used so that the emulsion quickly breaks down and allows the oil to coalesce in the separator.

Failure to perform routine maintenance tasks can also lead to oil water separator problems. If the oil compartment in the separator overflow, it will cause oil to carry over with the discharge water.

Oil water separator servicing and maintenance

Local water authorities may require oil water separators to be regularly serviced and maintained as a condition of a trade waste licence. These activities can easily be overlooked, but neglecting an oil water separator could lead to problems with its operation and a release of high oil content wastewater. Some servicing and maintenance tasks to bear in mind are as follows:

1. Inspect silt traps and collection pits to prevent solids from carrying over into the separator.

2. Open the separator lid and inspect the water flow and build-up of oil on the surface.

3. Dispose of waste oil from the container regularly.

4. Remove and clean media packs on a quarterly basis.

5. Clean sludge from the collection pit.

How often should an oil-water separator be cleaned?

The oil collection container of an oil water separator should be checked weekly and cleaned out as often as required. However, if the separator has experienced a high oil content due to an oil spill, the system should be checked immediately.

Depending on its use, the oil water separator should be taken out of service and thoroughly cleaned every three to six months. This schedule will keep the unit running efficiently and prolong its life.

Oil water separator benefits

Cleanawater oil water separators enable our customers to comply with their regulatory requirements and protect the environment. Small amounts of oil can do large amounts of damage to the environment, and installing an oil water separator meets the duty of care requirement. This means that a business has taken all reasonable and practical steps to protect the environment.

Local water authorities may specify wastewater treatment as a requirement for trade waste discharge. Therefore, an oil water separator allows a business to comply with their trade waste conditions and avoid expensive fines or even a shutdown.

Talk to Cleanawater for expert oil water separator advice.

Contact Cleanawater for expert advice about your oil water separator needs. We offer hydrocyclone and coalescing plate options in both small and large solutions. Whatever your application, we are sure to have an oil water separator to meet your requirements. Cleanawater has more than 20 years of experience in the Australian market, and our leading experts can give you the advice you need.

Contact Cleanawater to find out more about our oil water separator solutions.