It cools your wastewater so the fat floats and stays behind, which is why the vents are the working part, and why a 1,500 litre trap running too hot fails while looking perfectly healthy.
Skai Solutions read the water authority's own engineering guidance for this, not a supplier blog, and two findings in it contradict most of what gets written about grease traps. Below: the mechanism step by step, what must never go in, why your degreaser is working against you, the minimum sizes by venue, and what the device does badly.
A grease trap is a tank sitting between your kitchen drains and the sewer, and it works on nothing more complicated than cooling and density. Hot wastewater arrives carrying fats, oils and grease in suspension. Inside the tank the water slows down and cools. Cooled fat is lighter than water, so it floats to the top and stays there. Solids sink. What leaves through the outlet is the water in the middle.
The part that surprises most operators is that the cooling is not a side effect. It is the mechanism. In Sydney Water's own words, "vents allow the wastewater to cool so that the fats, oils and grease can separate from the wastewater", and "if the temperature remains too high, the trap won't work effectively". The vents are not an odour fitting. They are the working part of the device.
Which means a trap can be the right size, correctly installed, pumped out on schedule and still fail, purely because the water reaching it is too hot or the vents are blocked.
Everything from the dirty end of the kitchen goes through it. Food preparation areas, floor wastes, kitchen sinks, dishwashers and the garbage area all drain to the trap.
What must not go in is more interesting, because some of it is advice you will find given confidently online.
This is the single most useful line in the Sydney Water guidance and it is almost never repeated. Sydney Water advises using as little detergent as possible, because detergents break fats down so that they pass straight through the trap and into the sewer.
Read that again from the kitchen's point of view. A sink that is washed down with twice the degreaser produces wastewater in which the fat is emulsified. Emulsified fat does not float. It leaves through the outlet, cools in the sewer line instead of in your tank, and sets there. Your trap looks cleaner at the next inspection and your discharge is worse.
So the trap and the cleaning routine are one system, not two. A kitchen that has just moved to a stronger chemical and is now getting pressure from its water authority on discharge quality has probably found the cause.
There is no formula. Sydney Water's instruction is to size the trap "to meet your maximum hourly flow now and into the future", and then it publishes minimum sizes by business type instead. These are the Sydney figures; your own water authority sets its own.
| Business | Minimum trap |
|---|---|
| Take away, pizza shop, charcoal chicken | 1,000 L |
| Restaurant, 70 to 199 seats | 1,500 L |
| Restaurant, 800 to 1,000 seats | 5,000 L |
| Hospital or nursing home kitchen, 70 to 199 beds | 1,500 L |
| Supermarket with a chicken cooker | 1,500 L plus a grease removal device |
A single trap is between 1,000 and 5,000 litres. A complex can run multiple traps up to 30,000 litres across the site, which is six 5,000 litre tanks. Above that, centralised treatment such as dissolved air flotation becomes mandatory, with a discharge meter and a sampling point beside it.
The trap must drain to the sewer by gravity. Pumped systems are discouraged, because a pump failure overflows inside your building rather than outside it. Install it as close to the kitchen as you can: a long pipe run needs a heat trace or a steeper grade, or the grease sets in the pipe before it ever reaches the tank.
An explainer that only lists strengths is not much use, so here is the honest ledger.
The expensive outcome is not a fine. It is a tariff. Under the Sydney Water schedule for 2026 to 2027, a food business where pre-treatment is not maintained is charged A$20.061 a kilolitre, against A$2.826 a kilolitre for low strength food wastewater. That is 7.1 times the normal rate on every kilolitre you discharge, for as long as the trap is neglected. That arithmetic is ours, on two published Sydney Water rates.
Alongside it sit an agreement non compliance fee of A$152.31 and an additional inspection charge of A$264.94, plus the quarterly items that apply anyway: A$7.61 a quarter per grease trap and A$31.91 a quarter for the first trade waste process. The full picture of what servicing costs is on our grease trap cleaning cost page, and the wider trade waste charges page covers how the authority bills the rest.
If you are a cafe doing coffee and toasted sandwiches with no fryer and no char grill, your authority may not require a grease trap at all, and you should ask before you buy one. The obligation follows the grease load, not the signage. Food premises registration is the point at which this usually gets settled, which is covered on our food business registration cost page.
And if your trap is fine but your afternoons are not, the cheaper problem to fix is the phone. A Skai Solutions AI phone agent answers every call through prep and service, takes the booking into your system, and is live in five days. The trap will still need pumping out. At least the booking calls during the pump out will be answered.
How does a grease trap work?
By cooling and density, not filtering. Hot wastewater slows down inside the tank and cools, cooled fat floats to the top and stays there, solids sink, and the water in between leaves through the outlet. Sydney Water states that vents allow the wastewater to cool so the fats, oils and grease can separate, and that if the temperature stays too high the trap will not work effectively.
Do the vents on a grease trap actually matter?
They are the working part. Separation depends on the wastewater cooling inside the tank, and the vents are what allow it to cool. A blocked or undersized vent arrangement means hot water passes through still carrying its fat, which is a trap failing while looking fine.
Should I put enzymes or bacteria in my grease trap?
Not on a Sydney Water trade wastewater connection. Its list of substances not to add is solvents, enzymes, bacteria, odour control agents and pesticides. Products sold on the promise of digesting the grease in the tank fall inside that list.
Does detergent affect a grease trap?
Yes, and against you. Sydney Water advises using as little detergent as possible because detergents break fats down so they pass through the trap. Emulsified fat does not float, so it leaves the tank and sets in the sewer line instead.
What size grease trap does a restaurant need?
In Sydney, 1,000 litres minimum for a take away, pizza shop or charcoal chicken shop, 1,500 litres for a restaurant of 70 to 199 seats, and 5,000 litres for 800 to 1,000 seats. Single traps run 1,000 to 5,000 litres, a site can run multiple traps to 30,000 litres, and above that centralised treatment such as dissolved air flotation is required.
How often does a grease trap have to be pumped out?
Your water authority sets it, not you. In Sydney the frequency is fixed in the connection agreement from trap size, waste volume and wastewater quality, and the scheduling system accepts only 4, 8, 13 and 26 week intervals. Pump out must be done by an authorised transporter.
What happens if you do not maintain a grease trap?
In Sydney the cost arrives as a tariff rather than a fine. On the 2026 to 2027 schedule a food business where pre-treatment is not maintained is charged A$20.061 a kilolitre against A$2.826 for low strength food wastewater, which is 7.1 times the rate on every kilolitre discharged. An agreement non compliance fee of A$152.31 and an additional inspection charge of A$264.94 sit on top.
Can a grease trap be pumped to instead of draining by gravity?
It is allowed but discouraged. Sydney Water prefers gravity drainage because a pump failure causes the overflow to happen inside your property. If the pipe run to the trap is long it needs a heat trace or a steeper grade, since grease sets in a long cold run before reaching the tank.