Grease Trap in a Commercial Kitchen: Trace What Actually Drains Into It

It is 9pm on a Friday. The last dockets are printing, a kitchenhand is hosing down the floor, and the pot sink is running hot with a tray of chicken drippings in it. It all goes down a drain. Somewhere under the floor or out the back sits a grease trap, and you assume it catches everything.

That assumption is where most grease trap problems start. A grease trap in a commercial kitchen only protects you from the fixtures plumbed into it. This guide shows you how to trace what feeds your trap, what bypasses it, and what that means for your service schedule.

What a Commercial Kitchen Grease Trap Does and Does Not Catch

A commercial grease trap slows wastewater down. Fats, oils and grease float to the top, solids settle to the bottom, and the cooler, calmer water in the middle flows on to the sewer. It is a simple physical separator. It does not break grease down, and it does not clean anything.

That matters because a trap only handles the water that reaches it. If a drain is plumbed around the trap, its grease goes straight to the sewer line. If too many fixtures push hot, fast water through a small trap, the grease has no time to separate.

Grease traps and grease interceptors

You will hear both terms. In everyday use, a kitchen grease trap usually means a smaller unit under a sink or in the floor. A larger in-ground unit outside the building is often called an interceptor. Your trade waste agreement and your plumber use the exact term for your site, so use theirs when you book a service.

Trace Your Pipes: A Fixture-by-Fixture Check

Most guides skip this step. Take 20 minutes with a clipboard, ideally with your plumber or someone who knows the building, and answer one question for each fixture: does this drain go through the trap?

The fixtures to check

• Pot wash and prep sinks. These are the heaviest grease sources in most kitchens. They should feed the trap.
• Floor drains. Many kitchens hose down the cooking line every night. Check whether those drains connect to the trap or run to a separate line.
• Dishwasher and glasswasher outlets. Hot water carries emulsified grease. Ask your plumber how your machine is plumbed and what your trade waste agreement says about it.
• Combi ovens, woks and cooking range drains. Some have their own waste lines. Confirm where each one ends up.
• Hand basins and toilets. These normally bypass the trap. If one runs through it, raise that with your plumber.

Mark each fixture on a rough floor plan as "trap", "not trap" or "don't know". Every "don't know" is a question for your plumber. The plumbing standard AS/NZS 3500 governs how this drainage is designed, so a licensed plumber can read your layout and tell you what is connected.

A worked example

Say you run a pub kitchen. Your pot sink and prep sinks feed the trap. Your nightly floor wash drains to a gully nobody has traced. You trace it and find it runs straight to the sewer, skipping the trap.

Your fix is not "clean the trap more often". The trap is doing its job on the fixtures it serves. The gap is the floor drain. Ask your plumber whether it needs rerouting, and ask your water authority whether your agreement allows the current setup. Monthly servicing will not fix a grease path that never reaches the trap.

Match the Trap to the Load, Not Just the Room

Once you know what feeds the trap, you can judge whether it suits your kitchen today. Kitchens change. A cafe adds a fryer. A pub adds a lunch service. A takeaway adds a second wok station. The trap in the ground stays the same.

Trap size and the volume of water entering it decide how well it separates grease. If your menu, hours or equipment have changed since the trap went in, tell your cleaner and your plumber. Your local water authority and plumbing requirements set the sizing rules, so check them for your site rather than relying on a rule of thumb.

For the cost side, the guide to grease trap cleaning for commercial kitchens covers cost, frequency and contractors in one place.

Know Your Agreement Before You Know Your Schedule

Your trade waste agreement with the water authority sets the pump-out frequency for your kitchen. Many operators do not know theirs. Most only think about the trap when it smells, backs up, or a council or Sydney Water inspection is due.

Find your agreement before you guess at a schedule. It tells you the required frequency and what the authority expects from your site. In Sydney, Sydney Water runs the trade wastewater process. Elsewhere, your local water authority holds the equivalent. For wider context on grease and wastewater rules, the NSW EPA publishes guidance, and the Water Services Association of Australia represents the urban water industry nationally.

If your trap is already showing warning signs, read 5 signs your grease trap needs cleaning. If you have let it go a long time, what happens if you never clean your grease trap explains the consequences.

Turn Your Trace Into Better Quotes

Your fixture map also helps you get fair prices. Cleaners quote on trap size, service frequency, access and disposal fees. Two kitchens with the same trap can get quite different prices because their access and frequency differ.

When you ask for quotes, give each cleaner the same details:

1. The trap type and where it sits. 2. What blocks access, such as a table, a stack of crates or a locked door. 3. The frequency in your trade waste agreement. 4. Any fixtures you suspect do not reach the trap.

Identical details let you compare like with like. For a Sydney kitchen, see grease trap cleaning for commercial kitchens in Sydney.

Grease Trap Quotes sends you three quotes from licensed cleaners by SMS, usually within about a minute of your request. The service covers Sydney today and is expanding to other Australian cities. Requests come from commercial kitchens: restaurants, cafes, pubs, clubs, takeaways and food courts.

Your Next Step

Do the trace this week. Draw your fixtures, mark the unknowns, find your trade waste agreement and note the frequency. Then get 3 grease trap quotes from licensed cleaners and give each one the same details. You will know what your trap covers, what it misses, and what a fair service looks like for your kitchen.

Licensed contractors who want to service these kitchens can apply to join GreaseTrapQuotes and see contractor territory pricing.

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*Written by Mario Lucas. Mario runs Grease Trap Quotes, which matches commercial kitchens in Sydney with licensed grease trap cleaners.*

Related reading: Grease Trap Installations: What to Check on the Day the Trap Goes In.

Related reading: Cooking Oil Disposal for Commercial Kitchens: 7 Steps in the Order to Do Them.

Related reading: Grease Trap Servicing: Why Your Schedule Goes Stale When Your Kitchen Changes.