The 25% Rule for Grease Traps Explained: What Melbourne Businesses Must Know

What Is the 25% Rule for Grease Traps?

The 25% rule is one of the most important grease trap regulations that Melbourne food service businesses must understand. In simple terms, it states that your grease trap must be pumped out and cleaned before the combined layer of fats, oils, and grease (FOG) and settled solids reaches 25% of the trap’s total liquid depth.

This rule is enforced by Melbourne’s water authorities — Yarra Valley Water, South East Water, and City West Water — as part of your trade waste agreement conditions. It’s also a key component of the broader EPA Victoria grease trap regulations framework.

Ignoring the 25% rule doesn’t just risk fines — it compromises the effectiveness of your grease trap and can lead to FOG entering Melbourne’s sewer system, causing blockages, overflows, and environmental contamination.

How the 25% Rule Works in Practice

To understand the 25% rule, you need to understand how a grease trap functions. Inside the trap, wastewater separates into three layers:

Top layer (FOG) — Fats, oils, and grease float to the surface and accumulate over time.
Middle layer (effluent) — Relatively clear water sits between the FOG and solids layers. This is the treated water that exits to the sewer.
Bottom layer (settled solids) — Food particles and heavy debris sink to the bottom.

The 25% rule measures the combined thickness of the top FOG layer and the bottom solids layer as a percentage of the total liquid depth in the trap. When these two layers together reach 25% of the total depth, the trap has reached its effective treatment capacity and must be cleaned.

A Practical Example

Consider a standard 1,000-litre in-ground grease trap with a total liquid depth of 800mm:

25% of 800mm = 200mm
If the FOG layer measures 120mm and the settled solids measure 80mm, the combined total is 200mm — exactly 25%
At this point, the trap must be pumped out immediately

In practice, you should schedule cleaning before reaching the 25% threshold to maintain a safety margin. Most compliant Melbourne businesses schedule pump-outs when FOG levels reach approximately 15-20%.

Why Does the 25% Rule Exist?

The 25% threshold isn’t arbitrary — it’s based on the science of grease trap operation. Research has shown that when the combined FOG and solids layer exceeds 25% of the liquid depth:

Retention time decreases — There’s less volume of clear effluent in the trap, meaning wastewater passes through more quickly without adequate separation.
FOG carryover increases — Turbulence near the outlet can pull FOG into the effluent discharge, sending grease directly into the sewer.
Solids re-suspension occurs — The reduced clear zone means settled solids are more likely to be disturbed and carried out with the effluent.
Odour problems worsen — Anaerobic decomposition of trapped FOG produces hydrogen sulphide and other foul-smelling gases.

When a grease trap exceeds 25%, it’s essentially no longer functioning as designed. The wastewater leaving your trap may contain FOG concentrations that breach your trade waste agreement limits.

How FOG Levels Are Measured

During a grease trap pump-out service, qualified technicians measure FOG levels using a method called the “sludge judge” or dip test:

A clear tube or calibrated measuring stick is lowered vertically into the trap
The technician identifies the boundary between the FOG layer, clear effluent, and settled solids
Each layer’s depth is measured and recorded
The combined FOG and solids percentage is calculated against the total liquid depth

These measurements are recorded on your Waste Transport Certificate and maintenance log, forming part of your compliance documentation. Your water authority may request these records during an inspection.

How Often Should You Clean Your Grease Trap?

There’s no single answer to cleaning frequency — it depends entirely on how quickly your business generates FOG. Factors that affect FOG accumulation rate include:

Business type — A busy fish and chip shop generates far more FOG than a sandwich bar
Trading hours — Longer operating hours mean more wastewater and faster FOG build-up
Menu items — Deep-fried foods, cream-based sauces, and meat preparation produce more grease
Trap size — Larger traps can go longer between cleanings, all else being equal
Pre-treatment practices — Businesses that scrape plates and use sink strainers reduce FOG entering the trap

As a general guide for Melbourne businesses:

High-volume restaurants and takeaway shops — Every 4-6 weeks
Medium-volume cafes and bistros — Every 6-10 weeks
Low-volume sandwich bars and bakeries — Every 10-13 weeks

Use our compliance checker tool to get a personalised cleaning schedule recommendation based on your business details.

What Happens If You Exceed the 25% Limit?

If a water authority inspection finds your grease trap above the 25% threshold, consequences can include:

Formal warning — A first offence may result in a written warning with a compliance deadline.
Increased monitoring — Your water authority may require more frequent inspections at your expense.
Trade waste agreement conditions — Additional conditions may be imposed on your agreement, including mandatory increased cleaning frequency.
Financial penalties — Repeated breaches can trigger infringement notices and fines under EPA regulations.
Agreement cancellation — In serious cases, your trade waste agreement may be suspended or cancelled, effectively prohibiting you from operating.

Review our full regulations page for a complete overview of compliance obligations and consequences.

Best Practices for Staying Below 25%

Smart Melbourne businesses don’t just react to the 25% limit — they proactively manage FOG levels to stay well below it. Here are proven strategies:

1. Implement Kitchen Best Practices

Scrape all plates and cookware into bins before washing
Use sink strainers to catch food solids
Never pour cooking oil or grease down drains
Cool and collect used cooking oil in dedicated containers for recycling
Wipe greasy pans with paper towel before washing

2. Schedule Proactive Maintenance

Don’t wait until your grease trap approaches the 25% limit. Work with your service provider to establish a cleaning schedule based on your business’s actual FOG generation rate. A consistent schedule prevents emergencies and keeps you permanently compliant.

3. Monitor Between Services

Check your grease trap visually between scheduled cleanings. If you notice the FOG layer building up faster than usual — perhaps due to a busy holiday period or menu change — bring your next service forward.

4. Keep Accurate Records

Document every pump-out, including FOG levels measured and the total waste volume removed. These records help you identify trends and optimise your cleaning schedule over time.

Melbourne Grease Trap Cleaning provides scheduled grease trap pump-out services tailored to your business. We’ll monitor your FOG levels, optimise your cleaning schedule, and ensure you never breach the 25% rule. Contact us today to set up a compliant maintenance plan.