Oil Water Separator Guide: How They Work and Which Type to Use
How an oil water separator removes fuel and oil from runoff, how the main types compare, and what to check before choosing one.
What an oil water separator does
An oil water separator takes fuel, oil and other light liquids out of water before it leaves a site. It sits in the drainage line from places where hydrocarbons end up on the ground or in the wash water: refuelling areas, workshops, depots, substations and similar sites. Without one, a small leak or a major spill can run straight into the stormwater drain or the sewer.
There are three types you will see specified in Australia: the triple interceptor, the coalescing plate separator and the Class 1 coalescing separator. They all rely on oil being lighter than water, but they differ in how much oil they leave in the water, how much space they take and how they cope with a spill. This guide explains each one. Mullaly makes the Aquator Class 1 oil water separator, so we say where it fits and where another type may suit better.
How does an oil water separator work?
Every oil water separator starts with gravity. When oily water slows down in a tank, oil droplets rise to the surface because oil is less dense than water, and grit and silt sink to the bottom. The oil is held in the tank and the cleaner water leaves through an outlet set below the oil layer.
How well this works depends on the size of the droplets. Large droplets rise quickly and are easy to catch. Small droplets rise so slowly that many would be carried out with the water before they reach the surface. That is why better separators add a coalescing stage: a surface or media that the small droplets touch and stick to, where they merge into bigger drops that float up.
Oil in water comes in three forms:
- Free oil. Separate droplets that float given time. This is what separators are built to remove.
- Emulsified oil. Droplets so fine and so stable, often because of detergents, that they stay mixed in the water. Gravity separators, with or without coalescing, are not designed to break these.
- Dissolved oil. Hydrocarbons dissolved in the water. A separator does not remove them.
Triple interceptors, coalescing plate separators and Class 1 separators
Triple interceptor
A triple interceptor is a tank, usually below ground, divided into three chambers in series. Water passes from one chamber to the next, and at each stage heavy solids settle and floating oil is held back. There are no moving parts and nothing to clog, and they are common under workshop floors and wash bays where mud and grit come with the oil. Because a triple interceptor relies on gravity alone, fine droplets pass through, so it does not reach the outlet quality of a coalescing separator, and how well it works depends on how full of oil and sludge it is allowed to get.
Coalescing plate separator
A coalescing plate separator (CPS) adds a pack of closely spaced, angled plates. Water flows between the plates and oil droplets only have to rise a short distance before they touch a plate, gather and run up into the oil layer. That makes a CPS more compact than a plain gravity tank for the same flow, and better at catching small droplets. The plates need regular cleaning, because silt and sludge build up between them and reduce performance.
Class 1 coalescing separator
A Class 1 separator is one made and tested to meet the Class 1 outlet limit in BS EN 858-1 (see below). The Aquator oil water separator is one: it has two chambers in one fibreglass tank. Hydrocarbons and silt are captured and stored in the first chamber, and a coalescing filter in the second chamber removes the remaining oil, bringing the light liquid content at the outlet below 5mg/L (5ppm).
Oil water separator types compared
| Feature | Triple interceptor | Coalescing plate separator | Class 1 coalescing separator (such as the Aquator) |
|---|---|---|---|
| How it separates | Gravity, across three chambers in series | Gravity, with inclined plates that gather small droplets | Gravity in the first chamber, then coalescing media in the second |
| Fine droplets | Mostly pass through | Many caught on the plates | Caught by the coalescing filter |
| Outlet quality | No set outlet class. Depends on size, flow and how often it is emptied | Depends on the plate pack and flow. Check the maker's tested figure | Aquator: less than 5mg/L light liquid at the outlet (BS EN 858-1 Class 1) |
| Solids | Good at settling grit and mud | Silt between the plates reduces performance | Aquator: silt captured and stored in the first chamber |
| Spill containment | Holds what fits in the chambers | Depends on the design | Aquator: emergency stop valve closes automatically on a large spill or when the oil store is full |
| Typical sites | Workshops, wash bays, small depots | Industrial process water, larger sites with limited space | Service stations, refuelling areas, substations, mining and heavy vehicle sites |
| Power | None | None for the separator itself | Aquator: none. It is gravity operated |
General descriptions. Performance of any separator depends on its design, sizing and maintenance, so ask for the tested figures for the unit you are buying.
Class 1 and Class 2 under BS EN 858-1
BS EN 858-1 is the European standard for separators for light liquids such as petrol and diesel. It sets two classes by the oil left in the water at the outlet under test conditions:
- Class 1: less than 5mg/L of light liquid.
- Class 2: less than 100mg/L of light liquid.
Class 1 is the tighter limit, and it generally needs a coalescing stage to reach. Which class your site needs is up to the authority that receives the water: the local water utility for discharge to sewer under a trade waste agreement, or state environment rules and the council for discharge to stormwater. Check before you choose a separator.
Aquators are made to meet or exceed the requirements of BS EN 858-1:2002. The Aquator has been through third-party, independent testing to verify that it complies, and is independently tested to make sure it continues to meet it.
Choosing an oil water separator for your site
Before asking for a quote, work out the following, or ask your hydraulic engineer:
- Where the water goes. Stormwater or sewer, and what that authority requires.
- Flow rate. Set by the catchment area draining to the separator and the design rainfall, or by the process or wash water flow.
- Spill volume. The largest spill the separator should hold, for example from a fuel delivery or a damaged tank.
- What is in the water. Fuel and oil, silt and grit, and whether there are detergents or other chemicals.
- Location. Under a trafficked area or not, groundwater level, and access for a pump-out truck.
- Power. Whether power is available, or the site needs a separator that works without it.
For runoff from fuel areas, the Aquator oil water separator handles flow rates up to 50L per second, with spill capture from 1,500L to 9,000L as standard. Higher flow rates and spill capacities are possible, and custom-made tanks can be supplied on request.
Car wash and wash bay oil water separators
Wash bays and car washes are the hardest case for any separator. Detergents and degreasers are designed to break oil into fine droplets and keep it mixed in the water, which is the opposite of what a separator needs. Hot water and high-pressure washing do the same. Emulsified oil passes through gravity and coalescing separators alike.
Wash water usually goes to sewer, so the local water utility's trade waste requirements decide what treatment the wash bay needs. Those requirements often cover the detergents allowed, solids settling and the type of pre-treatment. Check them first, then tell us what will be washed, with what, and at what flow. We will advise whether an Aquator is the right unit for it.
The Aquator and related Mullaly products
The Aquator is Mullaly's dual chamber Class 1 commercial oil water separator, made in fibreglass with integral ribs from virgin resin and glass fibre reinforcement. It is used at service stations, refuelling areas, power stations, substations and switchyards, mining and heavy vehicle maintenance sites, wind farms, waste transfer depots and asphalt plants. All collected liquid is treated, with no bypass.
If your site's fuel tanks are coming from Tank Solutions, the group Mullaly belongs to, you will also find the Aquator on the Tank Solutions website, so the separator can be ordered with the tanks.
Treated runoff often goes on to storage: see our stormwater retention tanks, industrial water tanks and underground water tanks. For wastewater held on site, see wastewater storage tanks, and where water has to be lifted to the drain, stormwater pump stations.
Maintaining a separator
A separator only works while it has room for oil and silt. As the oil layer thickens and sludge builds up, the water passes through faster and less oil is caught, and in a spill there is less space to hold it. Every type needs:
- Regular inspection of the oil and sludge levels, and after heavy rain or a spill.
- Pumping out by a licensed liquid waste contractor before the oil and sludge storage is full.
- Cleaning of plates, filters or coalescing media to the maker's instructions.
- Records of inspections and pump-outs, which trade waste agreements and site licences often ask for.
The Aquator operating, installation and maintenance manual (PDF) sets out the inspection and service steps for an Aquator, and the Aquator technical data sheet (PDF) has the product details. More documents are in the resource centre.
Oil water separator questions
How does an oil water separator work?
Oily water slows down in a tank, oil rises because it is lighter than water, and silt sinks. The oil is held back and cleaner water leaves through an outlet below the oil layer. Better separators add a coalescing stage that joins small droplets into bigger ones so they float out too.
What is a triple interceptor?
A below-ground tank with three chambers in series. Solids settle and floating oil is held back in each chamber as the water passes through. Triple interceptors are simple and common under workshops and wash bays, but they rely on gravity alone, so fine oil droplets pass through.
What is a coalescing plate separator?
A separator with a pack of closely spaced, angled plates. Small oil droplets touch the plates, merge and rise to the oil layer, so the separator catches finer droplets than a plain gravity tank and takes less space. The plates need regular cleaning.
What is the difference between a Class 1 and a Class 2 separator?
Under BS EN 858-1, a Class 1 separator leaves less than 5mg/L of light liquid at the outlet under test, and a Class 2 separator less than 100mg/L. The authority receiving the water decides which class a site needs.
Can an oil water separator treat car wash or wash bay water?
Detergents emulsify oil into fine, stable droplets that gravity and coalescing separators are not designed to remove. Check your water utility's trade waste requirements for wash water, then tell us what is washed and how, and we will advise whether an Aquator suits.
How often does a separator need emptying?
It depends on the site and how much oil and silt reach it. Inspect it regularly and after heavy rain or a spill, and have it pumped out before the oil and sludge storage is full. Follow the maker's manual and any conditions in your trade waste agreement or licence.
Ask about an oil water separator
Tell us the site, where the water goes, the catchment or flow rate and the spill volume, and our team will recommend the right Aquator. We reply to every enquiry within 2 business days.
Or call 02 4964 8270 or email sales@mullaly.com.au