​Criteria for sizing an oil water separator

Cleanawater on 23 May 2019

Wash bays, refuelling areas, workshops, and construction sites have the potential for oil spills to contaminate wastewater. Failure to control the oil content of your wastewater can result in fines and site closures until the problems are resolved.

The combination of oil and water is a serious concern, and has many adverse consequences:

  • A small volume of oil spreads over a large distance on the surface of water.
  • Oil spilt into sewer and stormwater systems negatively impacts wastewater treatment plants.
  • All oil spills have a significant negative environmental impact.
  • Negative publicity from an environmental incident can result in the loss of reputation.

One of the fundamental steps to avoiding oil carry-over in wastewater is to use an oil water separator that is correctly sized for the application. Oil water separators are specified by water authorities in their permit conditions to ensure wastewater is treated to the required standard before entering the sewer.

Ensure wastewater compliance with the right oil water separator for your business. The team at Cleanawater will give you expert advice on sizing and installation.

Sizing an oil water separator

Traditional separation

The basic principle of a gravity oil water separator is based on the difference in density between oil and water. It's a fairly straightforward process:

  1. Oil is lighter than water and can be separated from a stream by allowing it to rise to the surface.
  2. Calculations determine how long it will take for oil droplets to rise to the surface based on the characteristics of the oil and other variables.
  3. Oil water separators are sized to ensure there is sufficient time for oil droplets to rise to the surface and be removed from the waste water stream.

sizing an oil water separator

As a rule of thumb, some experts recommend a ten minute retention time in a gravity oil water separator. To achieve this, the separator chamber should be sized at ten times the flow rate. For example, a 1,000-litre chamber is recommended for a flow rate of 100 litres per minute.

Modern separation

Modern oil water separators, like coalescing plate separators, speed up the process of gravity separators, and are best used as a permanent fixture at a facility. Here's how they work:

  1. Water and oil enters the systems and flows between plates of coalescing material.
  2. Oil droplets are trapped on the plates, allowing the water to flow freely around the plates.
  3. The clean water then exits the system via an outlet.

Coalescing plate separators have a much higher capacity and perform a more thorough separation than traditional oil water separators:

  • Coalescing plate separators have capacities ranging from 1,000 to 50,000 litres per hour.
  • These separators can remove oil up to 10ppm, meeting and often exceeding parameters.

Which factors impact sizing for an oil water separator?

This is a general guide to sizing an oil water separator, and many sizing factors depends heavily on the specific parameters of each application. Some factors affecting the size calculations for an oil water separator are detailed below.

1. Water flow rate

Oil water separators are designed to process a certain volume of water per hour. The flow rate has a direct influence over the amount of time a droplet of oil will remain in the chamber. This is therefore the amount of time available for it to separate out and rise to the surface.

If a system is located outdoors and captures rainwater, the flow rate will rise during a rain event. First flush diversion systems divert stormwater and untreated water to an oil water separator, then the sewer. Sizing the oil water separator must take into account peak load conditions to prevent oil carryover.

2. Oil content

oil water separator sizing criteria

The actual oil content, specifically gravity of the oil and droplet size, has a significant impact on how quickly the oil will separate from water. One of the design parameters for an oil water separator is the minimum size of oil droplet removed from the waste water stream.

  • Removing all oil droplets larger than 150 microns will yield an oil content of about 1,000 parts per million (ppm), which should be compared with permit conditions.
  • Standard discharge rates in Australia range from 10ppm-100ppm

3. Size of the collection pit

Many oil water separators draw intake from a supply pit which contains oily water. This is a typical configuration for wash bay applications. The larger the pit, the longer the settling time, which allows oil to begin rising to the surface. This reduces the load on the oil water separator, allowing a more rapid removal of oil.

4. Potential for site expansion

Many work sites make the mistake of sizing an oil water separator for current demands without taking into account future expansion.

It is more costly to remove an undersized system for replacement than to build a larger unit in the first place. Using an undersized oil water separator carries the additional risk that an oil carry-over may occur in peak load conditions causing an environmental incident.

5. Number and configuration of inlets

Oil water separators work best in non-turbulent conditions. The more inlets and the more turbulent the flow, the less efficient the oil water separator will be, requiring a larger separator to achieve the desired waste water quality. Site conditions can be improved to help lower the size requirement for an oil water separator, and therefore the cost.

Talk to Cleanwater for expert advice on oil water separators

Cleanawater has been involved in wastewater treatment in Australia for over 20 years. We offer oil technical services that meet the requirements of regulators. Our maintenance and inspection services also keep your oil water separator operating as designed, lowering the risk of an oil carryover.

Cleanawater has technical experts who are highly competent and experienced in designing oil water separators. Our consultants work with clients in most major water authority areas of Australia and can offer specific advice for compliance to permit conditions.

Contact our expert team today to arrange a consultation for your oil water separator needs.