What size grease trap does a commercial kitchen need in Australia?

Size depends on your fixtures. Add the fixture unit (FU) ratings for every fixture draining to the trap, then multiply by 100 litres. A mid-sized restaurant with 21 FU needs a 2,000 litre trap. Sydney Water sets a minimum of 1,000 litres and a maximum of 5,000 litres per trap.

How do you size a grease trap using fixture units?

Count every fixture that drains to the trap. Assign fixture unit ratings: kitchen sink = 3 FU, pot sink = 5 FU, rinse sink = 3 FU, wok burner = 1 FU, combi oven = 5 FU. Multiply total FU by 100 litres and match to the next trap size in the sizing table.

Can a dishwasher connect to a grease trap in Australia?

No. Dishwashers and glass washers must not connect to the grease trap. Hot water, detergents and surge flows break down the grease layer and stop the trap from working. They connect directly to the sewer through a separate floor waste.

What are the installation requirements for grease traps under AS/NZS 3500.2?

The outlet pipe invert must sit 150 mm lower than the inlet. The trap needs a rigid baffle at least 6 mm thick. A 100 mm vent must connect to the inlet line and terminate above the roofline. A hose tap with backflow prevention must be within 6 m. Above-ground traps need a platform and steps to AS 1657.

How often does a grease trap need to be pumped out?

Most councils require pump-outs every three months or when the trap reaches 25% capacity, whichever comes first. Your local water authority sets the actual frequency based on your business type and trap performance. A licensed waste transporter must do the pump-out.

Grease Trap Sizing and Installation

Section 1

What You Need to Know

Every commercial kitchen that produces greasy wastewater needs a grease trap. The trap sits between the kitchen drains and the sewer. It slows the water down so fats, oils and grease float to the top and get caught before they reach the public sewer. AS/NZS 3500.2 sets the plumbing rules. Your local water authority sets the trap size, pump-out schedule and approval process.

Get the size wrong, and the trap overflows or fails inspection. Get the location wrong, and pump-out trucks cannot reach it. A mid-sized restaurant kitchen typically needs a 2,000 litre trap. The minimum in Sydney Water’s area is 1,000 litres.

Section 2

The Rules

Every commercial kitchen fixture that produces greasy waste must drain through a grease trap (AS/NZS 3500.2)
The grease trap must hold wastewater for a minimum retention time of one hour (Water Corporation / Local Authority)
The outlet pipe invert must sit 150 mm lower than the inlet pipe invert (AS/NZS 3500.2)
The trap must contain a permanently fixed, rigid baffle at least 6 mm thick (AS/NZS 3500.2)
A high-level vent of 100 mm diameter must connect to the inlet line and terminate above the roofline (AS/NZS 3500.2)
Dishwashers and glass washers must not connect to the grease trap - hot water, detergents and surge flows break down the grease layer (Local Authority)
Above-ground traps need a platform and steps built to AS 1657 for safe maintenance access (Sydney Water / Local Authority)
A hose tap with backflow prevention must be within 6 m of the trap for cleaning (AS/NZS 3500.2)
In multi-storey buildings, a reflux valve is required between the trap outlet and the sanitary drainage system (AS/NZS 3500.2)
Sydney Water sets a minimum trap size of 1,000 litres for retail food businesses; maximum per trap is 5,000 litres (Sydney Water)

Section 3

What This Means in Practice

Take a mid-sized restaurant kitchen with the following fixtures draining to the grease trap:

Fixture	Count	FU Each	Total FU
Kitchen sink	2	3	6
Pot sink	1	5	5
Rinse sink	1	3	3
Wok burner	2	1	2
Combi oven	1	5	5
Total			21

Step 1 - Calculate volume using fixture units.
21 FU × 100 litres = 2,100 litres.

Step 2 - Match to the next available trap size.
The sizing table says 2,100 litres falls in the 1,701–2,600 L range. The required trap size is 2,000 litres.

Step 3 - Cross-check with peak flow (if known).
If the kitchen peaks at 0.5 L/s, that is 0.5 × 3,600 = 1,800 L/hour. The same sizing table gives a 2,000 L trap. Both methods agree.

Step 4 - Check local authority minimum.
Sydney Water requires at least 1,000 litres. The 2,000 litre trap exceeds this, so it passes.

The dishwasher in this kitchen does not connect to the grease trap. It connects directly to the sewer through a separate floor waste.

Section 4

Key Design Decisions

1

Fixture Unit Method vs. Peak Flow Method

Use the fixture unit method when the kitchen layout is known but flow data is not. Count every fixture, multiply by 100 litres, and match to the sizing table. Use the peak flow method when you have metered flow data or manufacturer flow rates. When both methods give different results, take the larger size.

Trade-off: The fixture unit method tends to oversize because it assumes all fixtures run at peak simultaneously. Oversizing costs more upfront and makes pump-outs more expensive, but it provides a buffer for future kitchen changes.

2

Single Trap vs. Multiple Traps

A single large trap keeps maintenance simple - one lid, one pump-out. But traps max out at 5,000 litres. For shared systems in shopping centres or food courts, you may need two or more traps in series, or a centralised system above 30,000 litres.

Trade-off: Multiple traps need more space, more access points, and more pump-out visits. A single larger trap is cheaper to maintain but needs a bigger pit and more structural support.

3

In-Ground vs. Above-Ground Installation

In-ground traps work well on flat sites with good sewer grades. They keep the trap out of sight and protect it from damage. Above-ground traps suit sites where the sewer invert is too high for gravity drainage, or where ground conditions make excavation expensive.

Trade-off: In-ground traps need excavation, waterproofing and a trafficable lid if vehicles cross over. Above-ground traps need a platform to AS 1657, take up floor space, and may need pumped drainage to the sewer.

4

Maintenance Access and Truck Clearance

The pump-out truck needs a clear path to within hose reach of the trap. Most trucks carry 30 m of suction hose. The trap lid must open fully, with vertical clearance at least equal to the trap depth. Plan for this access at design stage, not after the slab is poured.

Trade-off: Putting the trap close to the kitchen shortens drain runs and improves grease capture, but may conflict with loading docks, car parks or landscaping needed for truck access.

Section 5

Who Needs to Know What

Section 6

References

AS/NZS 3500.2:2025, Plumbing and drainage — Part 2: Sanitary plumbing and drainage
Plumbing Code of Australia (PCA), National Construction Code Volume Three
Sydney Water, Grease Traps - Installation Requirements
Sydney Water, Plumbing for Retail Food Businesses
Water Corporation (WA), Business Grease Arrestor Sizing
AS 1657:2018, Fixed platforms, walkways, stairways and ladders — Design, construction and installation
Local authority trade waste policies (vary by jurisdiction)

Grease Trap Sizing and Installation

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Fixture Unit Method vs. Peak Flow Method

Single Trap vs. Multiple Traps

In-Ground vs. Above-Ground Installation

Maintenance Access and Truck Clearance

Who Needs to Know What

Need this engineered for your project?

References

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