Every commercial kitchen washes fats, oils and grease down its sinks: from pans, trays, plates and floor mops. In the drain, that grease cools, sets and sticks to the pipe walls. Over time it blocks drains and sewers. A grease trap is there to catch it first. This guide explains what a grease trap does, the common types and how to keep one working.
What a grease trap is
A grease trap, sometimes called a grease interceptor, is a tank fitted between the kitchen sinks and the drain. Waste water flows in and slows down. Fats, oils and grease are lighter than water, so they float to the top and are held back. Food solids sink to the bottom. The cleaner water in the middle flows on into the drain.
Traps come in different sizes. Small units often sit under a sink or dishwasher. Larger inline or below-ground units serve whole kitchens. Some newer units are automatic and skim off the grease into a separate container, but they still need emptying, cleaning and checking. No trap looks after itself.
What happens when it isn’t emptied
A grease trap only works while it has room. Once it fills with fat and solids, grease passes straight through into the drain. Common results include:
- slow-draining or blocked sinks
- bad smells in the kitchen and around drains
- drains backing up into the kitchen
- grease building up in the public sewer, adding to the large blockages known as “fatbergs”
Blocked drains and smells are a hygiene problem and something an EHO may pick up on. See EHOs and kitchen extraction.
Need it done? Duct Doctor cleans and tests kitchen extract and ventilation systems across London and the South East, with a certificate and photo report after every clean. Get a fixed quote.
How often to empty and clean it
There’s no single answer. It depends on the size of the trap and how much grease the kitchen sends down the sink. A busy kitchen with lots of frying and roasting fills a trap faster than a café making sandwiches. Signs that it’s overdue include smells, slow drainage and a thick layer of fat across the top of the trap. If the trap is hard to open or nobody on site knows where it is, that’s a sign too.
Many kitchens set a regular schedule and adjust it based on how full the trap is each time. Duct Doctor offers grease trap cleaning, which can be booked alongside an extract clean.
Good habits in the kitchen
- Scrape plates and pans into the bin before washing.
- Collect used cooking oil separately for proper disposal. Never pour it down the sink.
- Use sink strainers to catch food scraps.
- Don’t rely on very hot water or detergent to wash grease away. It sets again further down the drain.
- Keep a log of when the trap is emptied and cleaned.
Train new staff on these habits from day one. A trap that fills slowly is far easier to manage than one that fills in a week because nobody scrapes the plates.
Grease traps and extract grease
Grease traps and kitchen extract systems deal with different grease. The trap catches grease in waste water. The extract catches grease carried in cooking vapour. Both need looking after, and neglecting either causes problems. A well-run kitchen schedules both: regular grease trap emptying, regular filter cleaning and full TR19® Grease cleans of the extract system at the right interval. See how often to clean a kitchen extract.
Quick answers
Does every commercial kitchen need a grease trap?
Many do. Your water company or local council can tell you what applies to your premises.
Can I clean the grease trap myself?
Small under-sink units can be emptied by trained staff. Larger traps are usually best left to a contractor.
What’s a fatberg?
A large, hard mass of fat, oil, grease and other waste that builds up in a sewer and blocks it.
Related services
More advice
- What EHOs Look For in Kitchen Extraction
- Pub Kitchen Extract Cleaning: A Checklist
- Restaurant Kitchen Extract Cleaning: A Guide
- Takeaway and Dark Kitchen Extract Cleaning
- All advice
This guide is general information. It isn’t legal or insurance advice: check your own policy wording and lease, and speak to a qualified adviser about your fire risk assessment.