Grease Trap Cleaning: How Often, Cost & Why It Matters

Updated July 2026 · 7 min read · Geelong Emergency Plumbing

A grease trap is the unglamorous but legally mandated component between a commercial kitchen and the sewer — it intercepts fats, oils and grease before they reach the drainage system, where they would congeal and cause exactly the kind of blockages that make fatbergs a news story. Neglected grease traps cause sewer blockages, council compliance violations, and the kind of drainage emergency that happens during the lunch service. Here's the maintenance picture.

How a Grease Trap Works

A grease trap is a tank installed between the kitchen drainage outlets and the sewer connection. Wastewater from sinks and dishwashers enters the trap, slows down (reducing turbulence), and the grease and fats — being lighter than water — float to the surface while the relatively clear water flows out the bottom into the sewer. Heavier food solids settle to the bottom. The trap retains both layers; only the middle layer of treated water exits to the sewer.

The trap's effectiveness depends on it having available capacity between the floating grease layer and the settling sludge layer. As both layers build up, the effective treatment zone shrinks, and eventually untreated grease begins exiting to the sewer — at which point the trap has failed its function and the sewer connection is being contaminated.

Residential Grease Traps

Residential properties don't typically have formal grease traps, but some older homes have a small interceptor trap on the kitchen drain line. These need cleaning when the grease layer becomes visible through the lid or the drain slows — typically every 1–3 years depending on cooking habits. A plumber cleans them during a drain service visit; some households maintain them with regular hot-water flushes and enzyme treatments.

For households with significant cooking grease production — households with large families, home-based food businesses, or properties on septic systems where grease causes particular problems in the absorption trench — a small under-sink grease trap is worth considering. A plumber can advise and install.

Commercial Grease Traps: The Compliance Obligation

Cafes, restaurants, commercial kitchens, food manufacturing facilities and any commercial operation discharging significant fats to the sewer are required by most Victorian councils to have a grease trap of appropriate size, and to maintain it at required intervals. City of Greater Geelong, like most metropolitan and regional councils, incorporates grease trap maintenance requirements into their trade waste agreements — the permit that governs commercial discharges to the sewer.

The cleaning frequency required under trade waste agreements typically ranges from monthly to quarterly depending on kitchen output, trap size and the council's assessment. Some councils require a service log to be kept on-site; inspectors check compliance. Fines for non-compliant grease trap maintenance (or absent traps) are real and not modest — and the cost of an emergency drain clear caused by grease trap overflow is the immediate consequence before any fine arrives.

Cleaning Frequency: The Practical Guide

The field rule used by most grease trap service operators: clean when the combined depth of floating grease and settled sludge equals 25% of the trap's total liquid depth. This is visible when you lift the lid and measure the layers. For a busy commercial kitchen, this commonly arrives monthly. For a smaller cafe with light output, quarterly may suffice. The compliance requirement is the floor; operational practice based on measured layer depth is the ceiling.

Never wait until the drain slows or backs up — at that point the trap has already been ineffective for some time, the sewer connection has been receiving grease, and a blockage in the drain line or sewer is likely.

What Grease Trap Cleaning Costs in Australia

Costs include pumping the contents, cleaning the trap walls, and disposing of the waste at an approved liquid waste facility. The disposal cost is a significant component — grease trap waste is classified liquid trade waste and can only be disposed of at licensed facilities, which is partly why service costs reflect more than just the labour.

Blockages Caused by Neglected Grease Traps

When a grease trap overflows or ceases to function, grease reaches the sewer line and cools into a waxy deposit on the pipe walls. In the drain line between the kitchen and the trap — or beyond the trap if overflow is persistent — grease blockages build up into solid masses that require hot-water jetting and sometimes mechanical cutting to remove. A hydro jet clear of a grease-blocked commercial drain line costs $400–$900 and takes the kitchen out of service for hours. The quarterly clean costs less and keeps the kitchen running.

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Related guides: Hydro jet drain cleaning · How to unblock a kitchen sink · Blocked drains Geelong