CLEARMASTERS
The short version
Do commercial kitchens need a grease trap?
In practice, yes. It’s an offence to put anything into a public sewer that stops it flowing, and that includes fat, oil and grease. Building Regulations guidance says commercial hot-food kitchens should have a grease separator. A trap only works if it’s the right size and emptied regularly, and you should keep a waste transfer note for every collection.
Interactive
How a grease trap works
Tap a step to see what happens inside.
- Greasy water flows inHot water from sinks and dishwashers carries melted fat and food.
- It slows and coolsThe tank is wide, so the flow slows down and the water cools.
- Fat floatsFOG is lighter than water, so it rises and forms a layer on top.
- Solids sinkFood scraps settle to the bottom as sludge.
- Cleaner water leavesWater is drawn from the middle, under the fat. When the fat and sludge layers grow too deep, FOG escapes, which is why emptying matters.
The rules
What the law says about FOG
Three pieces of law and guidance matter most for a commercial kitchen in England.
Don’t block the sewer
It’s an offence to put anything into a public sewer that is likely to interfere with its free flow. Water companies can trace blockages back, recover their costs and prosecute.
Fit a grease separator
Guidance says drainage serving commercial hot-food kitchens should have a grease separator to BS EN 1825, or another effective way of removing grease. This matters for new premises and refits.
Duty of care for waste
Grease trap waste and used oil are business waste. Use a registered waste carrier and keep waste transfer notes for at least two years.
Your local water company may also have its own FOG advice and inspection programme. Not sure who owns the drains you discharge into? See who is responsible for drains. This guide is general information, not legal advice.
Options
Types of grease trap
Under-sink grease trap
A small passive box fitted under or next to a sink. Common in cafés and small kitchens.
- Size
- Small
- Cleaning
- Often daily to weekly
- Best for
- Low volumes
Grease removal unit (GRU)
An automatic unit that heats and skims off the fat into a collection pot, so the main tank needs emptying less often.
- Size
- Small to medium
- Cleaning
- Empty the pot, daily
- Best for
- Busy kitchens
Below-ground grease separator
A large tank buried outside, built to BS EN 1825. Needed for bigger kitchens and often for new hot-food premises.
- Size
- Large
- Cleaning
- Tanker, at least monthly
- Best for
- Restaurants, hotels, sites
Enzyme or bacterial dosing
Liquids dosed into the drain to break FOG down. Can help keep pipes clear, but it is not a substitute for a trap and regular emptying.
- Size
- n/a
- Cleaning
- Refill as needed
- Best for
- Extra help only
Emptying
How often should a grease trap be emptied?
Below-ground separators: BS EN 1825 calls for them to be emptied and cleaned at least monthly, and more often for busy kitchens.
Under-sink traps: often daily to weekly, depending on size and how much you cook.
A useful rule of thumb: once the fat and solids take up about a quarter of the trap, it’s time. Beyond that, fat starts escaping into the drain.
We empty traps and separators with our own tankers, on a regular schedule, and leave a waste transfer note every time.
Interactive
FOG compliance check
Ten quick questions. The most important ones count for more.
There’s a grease trap, GRU or separator on the kitchen drains
It’s sized for how busy the kitchen is now
It’s emptied on a schedule (at least monthly for a separator)
You keep waste transfer notes for every collection
Used cooking oil is collected separately by a licensed collector
Staff scrape plates and pans into the bin before washing
Sinks and floor drains have strainers
Staff know what can and can’t go down the drain
You keep a cleaning and maintenance log
Drains are jetted when needed, before they block
Day to day
FOG good practice in the kitchen
Do
- Scrape plates, pans and trays into the food bin
- Wipe greasy pans with paper before washing
- Collect used oil in a sealed container for a licensed collector
- Fit strainers to sinks and floor gullies
- Keep a log of checks and emptying
Don’t
- Pour oil, fat or gravy down sinks or gullies
- Use hot water or detergent to “wash” fat away: it sets further down
- Put food waste down a macerator to the drain
- Take the lid off a trap and leave it open
- Rely on dosing alone instead of emptying
For runs that build up fat, we offer planned drain jetting. If it’s already blocked, see our guide to blocked drains.
Questions answered
Grease trap and FOG FAQs
Is a grease trap a legal requirement in the UK?
There isn’t one law that says every kitchen must have a grease trap, but it is an offence under the Water Industry Act 1991 to put anything into a public sewer that interferes with its flow, which includes fat, oil and grease. Building Regulations guidance also says drainage from commercial hot-food kitchens should have a grease separator. In practice, most commercial kitchens need one.
How often should a grease trap be emptied?
It depends on the size of the trap and how busy the kitchen is. BS EN 1825 calls for below-ground grease separators to be emptied at least monthly. Small under-sink traps often need cleaning daily or weekly. If the fat layer is more than about a quarter of the trap’s depth, it’s overdue.
What happens if a restaurant puts fat down the drain?
Fat builds up in drains and sewers, causing blockages, smells and sewage flooding. Water companies investigate blockages and can trace them back to the businesses responsible. They can recover clean-up costs and prosecute under the Water Industry Act.
What is the difference between a grease trap and a grease separator?
The terms are often used interchangeably. “Grease trap” usually means a smaller unit under or near a sink. “Grease separator” usually means a larger tank, often below ground, built to BS EN 1825, for bigger kitchens.
Can I empty a grease trap myself?
Staff can clean small under-sink traps, following the maker’s instructions, but the waste must still be disposed of properly with a waste transfer note. Below-ground separators need a tanker and a registered waste carrier.
Do enzyme dosing systems replace a grease trap?
No. Dosing products can help keep pipes clear, but some water companies warn they can just break fat into smaller pieces that set again further down the sewer. They don’t replace a properly sized trap that’s emptied regularly.
What paperwork do I need for grease trap waste?
You should get a waste transfer note for every collection and keep it for at least two years, as part of your duty of care for business waste. Only use a registered waste carrier.
Can you set up regular grease trap emptying?
Yes. We can agree a regular emptying schedule based on the size of your trap and how busy you are, with a waste transfer note after every visit. Call us on 03333 233566 to arrange it.
Got a different question? Call 03333 233566 or send us a message.