What Size Grease Trap Do I Need? Melbourne Sizing Guide

How to Determine the Right Grease Trap Size for Your Melbourne Business

Installing a grease trap that is too small leads to frequent overflows, compliance failures and costly emergency pump-outs. Installing one that is too large wastes money on unnecessary capacity. Getting the size right is critical — and in Melbourne, it must comply with Australian Standard AS 1546.1, your local water authority’s trade waste requirements, and often specific conditions set by Melbourne Water or your council.

This guide explains the sizing methodology step by step, so you understand how professionals calculate the correct grease trap capacity for your kitchen. For a broader overview of all trap options, see our complete guide to grease trap types.

The AS 1546.1 Sizing Standard

AS 1546.1: On-site domestic wastewater treatment units — Septic tanks (which also covers grease traps) is the Australian Standard that governs grease trap sizing. While the standard provides the framework, Melbourne Water and individual councils may impose additional requirements or minimum sizes that override the standard’s calculated result.

Key Sizing Factors Under AS 1546.1

• Number and type of fixtures: Each sink, dishwasher, bain-marie and other fixture is assigned a fixture unit value
• Peak flow rate: The maximum simultaneous drainage flow from all connected fixtures
• Retention time: The minimum time wastewater must remain in the trap for effective separation
• Grease loading: The expected concentration of FOG based on your business type
• Safety factor: A multiplier applied to account for peak demand periods

Step-by-Step Sizing Process

Step 1: Identify All Connected Fixtures

List every fixture in your kitchen that will drain into the grease trap. Common fixtures include:

• Pre-rinse sinks
• Pot wash sinks
• Commercial dishwashers
• Bain-maries and steam tables
• Wok burners with water supplies
• Floor drains in food preparation areas

Note: Hand basins and toilet fixtures are not connected to grease traps.

Step 2: Calculate Fixture Units

Each fixture type is assigned a fixture unit (FU) value based on its drainage capacity. For example, a standard commercial sink may be rated at 2 to 6 FU depending on size, while a commercial dishwasher might be rated at 3 to 5 FU. Your plumber or trade waste consultant will reference the relevant tables in AS 1546.1 and AS/NZS 3500 to determine these values.

Step 3: Determine Peak Flow Rate

The peak flow rate is calculated by adding the fixture unit values of all fixtures that could discharge simultaneously during your busiest service period. This is expressed in litres per second (L/s) and represents the worst-case demand on your grease trap.

Step 4: Apply the Sizing Formula

The minimum trap capacity is calculated using:

Trap Volume = Peak Flow Rate x Retention Time x Safety Factor

For most Melbourne commercial kitchens, the minimum retention time is 15 to 20 minutes, and safety factors of 1.5 to 2.0 are commonly applied. This ensures the trap can handle peak loads without FOG bypassing the system.

Step 5: Check Against Melbourne Water Minimums

Melbourne Water publishes minimum grease trap sizes based on business type. Even if your AS 1546.1 calculation yields a smaller number, you must meet the water authority’s minimum. Typical minimums include:

• Small cafe (up to 50 seats): 1,000 litres minimum
• Medium restaurant (50-100 seats): 1,500 to 2,000 litres
• Large restaurant or hotel kitchen: 2,000 to 5,000+ litres
• Food court with multiple tenants: 3,000 to 10,000+ litres

Use our grease trap size calculator for a quick estimate based on your kitchen details.

Common Sizing Mistakes in Melbourne

Undersizing

The most frequent mistake is choosing a trap that is too small. This happens when businesses select a trap based on price rather than calculated capacity, or when fixture counts change after the initial sizing. An undersized trap fills with grease quickly, reduces separation efficiency and increases the risk of FOG entering the sewer — leading to potential fines from Melbourne Water.

Ignoring Future Growth

If you plan to add fixtures, extend trading hours or increase menu complexity, factor this into your sizing. It is far cheaper to install a slightly larger trap now than to replace an undersized one later.

Not Accounting for Dishwasher Heat

Commercial dishwashers discharge water at 60 to 80 degrees Celsius, which can melt solidified grease in the trap and reduce separation efficiency. Some sizing calculations apply an additional factor to account for this, or recommend installing a cooling chamber upstream of the trap.

The Role of Your Plumber and Trade Waste Officer

In Melbourne, grease trap sizing and installation must be carried out by a licensed plumber. Additionally, your local water authority’s trade waste officer will review and approve the proposed trap size as part of the trade waste consent process. The officer may require a larger trap than the AS 1546.1 calculation indicates, based on their experience with similar premises in your area.

What About In-Ground vs Above-Ground Sizing?

The sizing methodology is the same regardless of whether you choose an in-ground or above-ground trap. However, above-ground units are generally limited to smaller capacities (up to 200 litres), so if your calculation demands a larger trap, an in-ground installation is likely required.

Get Your Sizing Right the First Time

Incorrect grease trap sizing leads to compliance issues, higher maintenance costs and potential environmental penalties. Our team can conduct a full sizing assessment for your Melbourne kitchen, ensuring compliance with AS 1546.1, AS 4674 and all Melbourne Water requirements. Contact us for a free consultation and sizing quote.