Oil Water Separator vs Grease Trap: Which Does Your Site Need?
Oil water separator vs grease trap comes down to one question: is the waste mineral oil or kitchen fat? An oil water separator treats mineral oil from vehicles and machinery, and a grease trap treats fats, oils and grease from food preparation, so the right one is decided by where the wastewater comes from. Workshop floor, wash bay, fuel forecourt: oil water separator. Commercial kitchen sinks, dishwasher, fryer area: grease trap. A site with both a workshop and a kitchen needs one of each, and no, one unit cannot do both jobs.
That covers most callers. The rest of this is for the people who have been told something different by a mate, a builder or a quote that looked suspiciously cheap.
The source of the waste decides it
The type of oil in the wastewater decides whether you need a separator or a trap, not the size of the site or what the drawings call the room. Mineral oil and animal or vegetable fat behave completely differently once they hit water, and each piece of equipment is built around one behaviour.
Mineral oil (engine oil, hydraulic fluid, diesel, gearbox oil) stays liquid at room temperature. It floats as droplets, some of them tiny. Kitchen fat goes in hot and liquid, then cools in the pipe and sets solid. Totally different problem.
Quick check by waste source
| Where the water comes from | What is in it | Equipment |
|---|---|---|
| Mechanical workshop floor washdown | Engine oil, coolant, grit | Oil water separator |
| Car, truck or plant wash bay | Oil, diesel, mud, detergent | Oil water separator |
| Service station forecourt | Fuel drips, oil | Oil water separator |
| Restaurant, cafe or pub kitchen | Fats, oils, grease, food scraps | Grease trap |
| Food court, bakery, butcher | Animal and vegetable fat | Grease trap |
| Depot with a workshop and a staff canteen | Both, from separate drains | One of each |
How an oil water separator works
An oil water separator pulls free mineral oil and suspended solids out of wastewater using coalescing plates, which catch the small oil droplets a plain tank would let straight through. Water enters an inlet chamber, heavy solids drop into a sludge hopper, and the flow then passes through a pack of closely spaced oleophilic polypropylene plates.
Oil sticks to those plates, small droplets merge into bigger ones, and the bigger ones float off to be skimmed. The Kwikflo KCPS range has three separate 50mm outlets: treated water to sewer, oil to a waste oil drum, sludge through a ball valve. It sits above ground on a galvanised frame, usually fed by a pump from a collection pit.
The full mechanics are in our guide to how an oil water separator works. Short version: it relies on oil staying liquid and floating. Feed it something that sets solid and it stops working.
How a grease trap works
A grease trap slows kitchen wastewater down long enough for it to cool, so fats rise and set into a crust on top while food solids sink to the bottom. Clearer water in the middle passes out to sewer under a baffle. No plates, no fine droplet capture, just volume and retention time.
That is why grease traps are sized on how many meals the kitchen turns out on a busy day, and why they are emptied by a licensed liquid waste contractor on a pump out schedule. The water authority sets the size through the trade waste consent. Our range runs from 1000L to 2000L in the grease arrestor collection.
Why you cannot swap one for the other
Kitchen fat will blind the plates in an oil water separator within weeks, and a grease trap will let most fine mineral oil droplets pass straight through to sewer. Either swap ends in a failed trade waste inspection, a blocked drain, or both.
Run kitchen waste into a coalescing plate separator and the fat cools on the plates, sets, and bridges the gaps. Water then short circuits around the blocked pack. The separator looks fine from outside and does nothing.
Run workshop wash water into a grease trap and the big slugs of oil float, sure. The small droplets, the ones that carry your oil and grease reading over the limit, leave with the water. Detergent makes it worse because it emulsifies the oil further. A grease trap has no mechanism to catch that.
We get asked this a lot by people trying to save on a second unit. The saving lasts until the first sample result comes back.
When a site genuinely needs both
A site needs both an oil water separator and a grease trap when it produces both mineral oil waste and food fat waste, and the two streams must run to their own unit on separate drainage. Mixing the drains first and treating after does not work for the reasons above.
Common examples:
- Transport depots with a truck wash and a staff kitchen or canteen.
- Service stations that also run a hot food counter.
- Council works depots, mine site camps and large industrial sites with a mess hall.
- Car dealerships with a workshop and a customer cafe.
Your hydraulic consultant or licensed plumber will lay out the drainage so each waste stream reaches the right unit. Do this at design stage. Splitting a combined drain after the slab is poured is expensive and annoying for everyone.
Cost comparison at a glance
An oil water separator generally costs more than a grease trap of similar volume because it carries a plate pack, a frame, outlets for three waste streams and often a pump and waste oil drum. Our KCPS separators run from $5,927 to $9,699, while our grease traps run from $2,999 to $5,399.
| Oil water separator (Kwikflo KCPS) | Grease trap (Kwikflo KGT) | |
|---|---|---|
| Treats | Mineral oil, fuel, silt | Food fats, oils, grease, food solids |
| Sizing input | Peak flow rate | Meals per busy day, set by the water authority |
| How it separates | Coalescing polypropylene plates | Retention time and baffles |
| Usual location | Above ground on a galvanised frame | Above or below ground |
| Sizes | 1000L, 1500L, 2000L, 3000L | 1000L, 1500L, 2000L |
| Price range | $5,927 to $9,699 | $2,999 to $5,399 |
| Routine maintenance | Hot water plate wash, drain sludge, empty oil drum | Pump out by a licensed liquid waste contractor |
Prices current as of October 2026, AUD including GST.
For separators, the four sizes are the KCPS-1000, KCPS-1500, KCPS-2000 and KCPS-3000. The KCPS-3000 ships without a junction box or control panel, on purpose, because the panel has to match the site's hazardous area rating.
What the water authority wants to see
Your local water authority wants the right pre-treatment for each waste stream written into your trade waste consent, and it will not accept a grease trap for workshop waste or a separator for kitchen waste. The authority sets the discharge limits and approval conditions. Not us, not your builder.
For a separator, expect questions about peak flow, bunding, and the Class 1 Zone 2 hazardous area that extends 500mm above the unit. For a grease trap, expect questions about meal numbers and pump out arrangements. Our page on oily wastewater discharge to sewer covers the separator side in more detail.
Frequently asked questions
What is the difference between an oil water separator and a grease trap?
An oil water separator removes mineral oil, fuel and silt from workshop, wash bay and forecourt wastewater, while a grease trap removes food fats, oils and grease from commercial kitchen wastewater. A separator uses coalescing plates to catch fine liquid oil droplets, and a grease trap uses retention time so fat cools, sets and floats.
Can a grease trap be used for a car wash in Australia?
No, a grease trap is not suitable pre-treatment for an Australian car wash, because it cannot capture the fine mineral oil droplets in wash water, especially once detergent is involved. A car or truck wash discharging to sewer needs an oil water separator, with the flow rating confirmed against the trade waste consent from your local water authority.
Can an oil water separator handle kitchen waste?
No, an oil water separator cannot handle kitchen waste, because food fat cools and sets solid on the coalescing plates and blocks them within weeks. Once the plates are blinded, water bypasses the plate pack and the separator stops treating anything. Kitchen wastewater needs a grease trap.
Do I need both a grease trap and an oil separator?
You need both a grease trap and an oil water separator if your site produces kitchen fat waste and mineral oil waste, such as a depot with a truck wash and a canteen. Each waste stream must run on its own drain to its own unit, laid out by your hydraulic consultant or licensed plumber.
Is an oil water separator more expensive than a grease trap?
Yes, an oil water separator usually costs more than a grease trap of similar volume, because it includes coalescing plate packs, a galvanised frame and separate outlets for water, oil and sludge. Waternex oil water separators range from $5,927 to $9,699 and grease traps from $2,999 to $5,399, prices current as of October 2026 including GST.
Not sure which one your site needs
Tell us what drains into the system, how many bays or meals you are dealing with, and which water authority issued the consent. We will tell you whether it is a separator, a trap or both, and give you a delivered price.
Working on a full site drainage package? We offer engineering design support, and contractors should ask about trade pricing.
This guide is general information for Australian commercial and industrial sites and is not a substitute for advice on your specific installation. Discharge limits, pre-treatment requirements and approval conditions are set by your local water authority through your trade waste consent. Hazardous area classification, electrical work and connection must be carried out by suitably qualified people. Confirm the required equipment and flow rating with your consultant or plumber before you order. Prices and specifications are current at the date of publication and change without notice.