TL;DR: Grease trap cleaning keeps fats, oils and grease out of your drains and out of the public sewer. Most commercial kitchens need a service every month, or sooner if the grease layer and settled solids reach a quarter of the trap’s working depth. UK law expects hot food premises to have working grease separation and documented waste disposal. Get on a planned schedule before a blockage closes your kitchen.

Grease trap cleaning is one of the smallest line items in a commercial kitchen budget, and one of the most expensive to skip. In 2024, South West Water cleared 6,400 sewer blockages across the South West, and 75% of those blockages were caused by a combination of fats, oils and grease and wet wipes (Water UK, citing South West Water 2024 data). Every one of those blockages started somewhere. A fair share started in a kitchen sink.

For a restaurant, pub, hotel, takeaway or café in Cornwall, the risk is not abstract. A blocked drain in the middle of August service means cancelled covers, an emergency callout, and a difficult conversation with your water company if the blockage reaches the public sewer. The good news is that this is one of the most preventable failures in a commercial kitchen.

This guide covers what a grease trap does, how often it needs cleaning, what UK law requires of food businesses, what a professional clean involves, what drives the cost, and how to keep FOG out of your drains between services.

What Is a Grease Trap and How Does It Work?

A grease trap is a chamber fitted between a commercial kitchen’s sinks and the drain. It slows wastewater down so fats, oils and grease float to the top and heavier food solids settle at the bottom. The cleaner water in the middle passes on to the sewer. Everything else stays in the chamber until it is removed.

FOG stands for fats, oils and grease: the residue from frying, roasting, dairy, sauces, dressings and dishwashing. It leaves the kitchen as a warm liquid. It arrives in your drain run as a solid.

UK Building Regulations use the term “grease separator” rather than grease trap, but food businesses use the terms interchangeably. Three formats dominate British kitchens:

Passive under-sink traps. Compact units plumbed directly beneath the pot-wash or prep sink. Low cost to install, high maintenance demand, because they fill quickly.
Below-ground grease separators. Larger chambers installed outside the building, sized to the kitchen’s flow rate under BS EN 1825. Common in hotels, pubs and high-volume restaurants.
Automatic grease recovery units (GRUs). Powered units that skim FOG continuously into a separate collection container, reducing (but not removing) the need for a full clean-out.

Whichever format you have, the principle is identical. The trap only works while there is space in it for grease to collect.

Why Grease Trap Cleaning Matters for Cornwall Food Businesses

A neglected grease trap stops separating. Once the grease layer and settled solids fill the chamber, FOG flows straight past and into the drain run, where it cools, hardens and binds with everything else in the pipe. The result is a blockage that builds quietly for weeks, then fails at the worst possible moment.

Cornwall makes that failure more expensive than it would be elsewhere. Trade is compressed into a short, intense season, so a kitchen closure in late July costs a multiple of what the same closure would cost in February. Many town-centre premises in Falmouth, Truro, Padstow and St Ives sit on older clay pipework with tight bends and narrow access, which gives hardened FOG plenty to cling to. And a good deal of the county’s foul network runs close to bathing waters, so a FOG-driven overflow is not only a drainage problem.

The commercial consequences stack up quickly:

Lost trade while the kitchen is closed and the drain is cleared
Emergency clearance costs, which are always higher than planned maintenance
Recharge from the water company where a blockage in the public sewer traces back to your premises
Environmental health attention if drainage failure creates a hygiene risk on site
Damage to pipework, which turns a cleaning job into a repair job

If your drains are already showing signs of FOG build-up, our blocked drain clearance in Cornwall team can clear the line and identify what caused it. Prevention is a planned schedule. Which raises the obvious question.

How Often Should You Clean a Grease Trap?

Most commercial kitchens need their grease trap emptied and cleaned every month. The working rule is the 25% test: the trap needs servicing before the combined grease layer and settled solids reach a quarter of its working depth. Kitchens with heavy frying, large pot-wash volumes or long service hours reach that point faster and need a shorter cycle.

Monthly is a baseline, not a universal answer. Frequency is driven by four things:

Menu. A fish and chip shop or a carvery generates far more FOG per cover than a sandwich bar.
Covers. Volume matters more than floor space. A small kitchen doing 200 covers a night fills a trap faster than a large one doing 60.
Trap capacity. An undersized passive unit under a pot-wash sink can fill in days, not weeks.
Season. Cornwall’s summer trade can double or triple output in July and August. Service intervals need to shorten with it.

Signs Your Grease Trap Is Overdue

Sinks and floor gullies draining slowly, especially after a busy service
Odour from the trap or from the drains around the kitchen
Visible grease at the trap outlet, meaning FOG is already escaping into the drain run
Drain flies in the wash-up area
Repeat blockages, which point to FOG that has already hardened downstream

By the time you notice any of these, FOG has been leaving the trap for a while. That is why fixed service intervals beat waiting for symptoms.

What Does the Law Say About Grease Traps in the UK?

UK law does not set a single “grease trap regulation” for food businesses. Instead, three separate legal duties apply together: Building Regulations expect grease separation in commercial hot food kitchens, the Water Industry Act 1991 makes it a criminal offence to let FOG interfere with the public sewer, and waste law governs how the material removed from your trap is disposed of.

Building Regulations: Approved Document H

Approved Document H (paragraph 2.21) states that drainage serving kitchens in commercial hot food premises should be fitted with a grease separator complying with BS EN 1825-1 and designed in accordance with BS EN 1825-2, or other effective means of grease removal (HM Government, Building Regulations 2010, 2015 edition). BS EN 1825 is the British and European standard covering the design, sizing, operation and maintenance of grease separators.

Water Industry Act 1991

Under section 111 of the Water Industry Act 1991, it is an offence to pass any matter into a public sewer that is likely to injure it or interfere with the free flow of its contents. FOG does exactly that. On conviction on indictment the penalty extends to imprisonment for up to two years, a fine, or both. Water companies can also recover the cost of clearing a blockage from the business responsible for it.

Waste Duty of Care

The material removed from your grease trap is business waste, and section 34 of the Environmental Protection Act 1990 places a duty of care on you as the producer. In practice, that means using a registered waste carrier and keeping the paperwork. GOV.UK confirms that a waste transfer note is required for each load of non-hazardous waste moved off your premises, and that you must keep a copy for two years and produce it for an enforcement officer from your local council or the Environment Agency on request. You can check a carrier’s registration on the Environment Agency public register before waste leaves your site.

Environmental health officers can also act under the Food Safety Act 1990 where drainage failure creates a hygiene risk. Documentation, in other words, is part of the job.

What Happens During a Professional Grease Trap Clean

A professional grease trap clean removes the entire contents of the chamber, cleans the internal components, restores the trap to working condition, and documents the waste that leaves site. Pumping out the top layer and leaving the rest behind is not a clean. It is a delay.

Here is what a proper service looks like:

Inspection. The engineer checks the grease and solids depth, the condition of the baffles and filters, and the state of the inlet and outlet.
Full pump-out. The complete contents are removed, not just the floating grease layer.
Scrape and wash-down. Walls, baffles and filter baskets are cleaned so the trap regains its full working volume.
Component check. Baffles, lids, seals and flow control devices are inspected and reseated.
Refill and recommission. The trap is refilled with clean water so it separates correctly from the next wash-up onwards.
Documentation. A waste transfer note is issued, and the service is logged for your records and for any inspection.

Where FOG has already escaped into the drain run, cleaning the trap alone will not fix the line. A CCTV Drain Survey shows exactly how far the build-up extends and whether the pipework underneath it is sound. High-pressure jetting then cuts hardened FOG off the pipe walls and restores the full bore of the drain. If the survey shows cracked or displaced pipework beneath the deposits, Drain Pipe Relining repairs the line from the inside, without excavating a kitchen floor or a customer car park.

What Affects the Cost of Grease Trap Cleaning?

Grease trap cleaning is priced on the size of the trap, how much material is in it, how accessible it is, and how often it is serviced. A below-ground separator at a hotel costs more per visit than a small under-sink unit at a café. Planned contracts are priced lower per visit than reactive one-off callouts.

The main cost drivers:

Trap type and capacity. Volume removed is the single biggest factor.
Service frequency. Regular servicing keeps volumes predictable. Long gaps mean heavier, harder deposits and longer visits.
Access. Traps behind fixed kitchen units, or below ground on a tight site with narrow vehicle access, take longer to reach and service.
Waste disposal. Grease trap waste must go to a licensed disposal point via a registered carrier, and that cost is built into the job.
Additional work. Jetting the drain run, or a CCTV survey where the line is already restricted, is separate from the trap service itself.

Compare that to the alternative. An emergency blockage during service means a callout, a closed kitchen, lost covers, and potentially a recharge from the water company. Clear Stream quotes a fixed price before any work begins, and there is no call-out fee. You know the number before an engineer lifts a lid.

Keeping FOG Out of Your Drains Between Cleans

Good kitchen practice reduces the load reaching your grease trap, extends the life of your drainage, and lowers your service costs. It does not replace the trap. Water companies do not accept careful habits as a substitute for working grease separation, because FOG still enters the system through dishwasher rinse water, floor cleaning and equipment washdown.

What works, day to day:

Scrape plates, pans and utensils into a food waste bin before anything reaches the wash-up sink
Dry-wipe greasy pans and surfaces with paper towel before washing
Fit and clean sink strainers on every kitchen plug hole and floor gully
Collect used cooking oil in sealed containers for collection by a registered waste carrier, never down a sink
Train every new starter on FOG disposal, and post reminders above the sinks
Keep a service log for the trap so the next inspection is a five-minute conversation

Two things to avoid. Never pour hot water or detergent through the trap to “shift” a build-up, because that simply pushes liquefied FOG into the drain run where it hardens again. And do not rely on enzyme or bacterial dosing as your grease management system, because dosing supports a trap, it does not replace one.

Rural sites need one extra consideration. Pubs, farm shops and cafés that are not on mains drainage send everything to a septic tank or treatment plant, and FOG is what kills those systems fastest. It coats the tank, blocks the outlet and drives premature failure. Regular Septic Tank Maintenance alongside grease trap servicing protects both.

Get Your Grease Trap on a Planned Schedule

The takeaways are simple. Clean the trap before the grease and solids reach a quarter of its working depth, which for most commercial kitchens means monthly. Keep the waste transfer notes. And treat the trap as one part of a drainage system that also includes the pipe run underneath your kitchen floor.

Clear Stream Drainage Solutions services grease traps, jets FOG-restricted drain runs and surveys commercial drainage across Cornwall and Devon, from Falmouth and Truro to Newquay, St Austell and Penzance. No call-out fee. A fixed price before work starts. An engineer on site within 1 to 2 hours anywhere in Cornwall when a blockage cannot wait.

Call our team on 01872 222555 to arrange a grease trap service or set up a planned maintenance schedule for your kitchen. Speak to Clear Stream today.

Frequently Asked Questions

How Often Should a Commercial Grease Trap Be Cleaned?

Most commercial kitchens need a grease trap service every month. The trigger is the 25% rule: empty the trap before the grease layer and settled solids together reach a quarter of its working depth. High-volume fryers, carveries and busy pot-wash areas reach that point sooner and need a shorter interval, particularly through the Cornish summer season.

Is a Grease Trap a Legal Requirement in the UK?

Building Regulations Approved Document H (paragraph 2.21) states that drainage serving kitchens in commercial hot food premises should be fitted with a grease separator complying with BS EN 1825-1, or other effective means of grease removal. Separately, section 111 of the Water Industry Act 1991 makes it an offence to discharge matter that interferes with the free flow of a public sewer, and FOG falls squarely within that.

Can I Clean a Grease Trap Myself?

Kitchen staff can skim and maintain a small under-sink trap between services, but the waste that comes out of it is business waste. Under section 34 of the Environmental Protection Act 1990 it must be transferred to a registered waste carrier, with a waste transfer note kept for two years. Most food businesses use a drainage contractor for the full clean because the pump-out, the disposal and the documentation come as one job.

What Happens if a Grease Trap Is Not Cleaned?

Once the chamber is full, the trap stops separating and FOG passes straight into the drain run, where it cools and hardens onto the pipe walls. That leads to slow drainage, odour, drain flies and eventually a full blockage. If the blockage reaches the public sewer, the water company can recover its clearance costs from the business responsible.

Does a Grease Trap Stop Drain Blockages Completely?

A correctly sized and regularly serviced grease trap removes the majority of FOG before it reaches your drain. It does not catch everything, and it does nothing about wipes, sanitary items or food solids flushed elsewhere on the premises. Grease trap servicing works best alongside periodic jetting of the drain run and a CCTV survey where flow is already restricted.

How Much Does Grease Trap Cleaning Cost in Cornwall?

Cost depends on the size of the trap, the volume of waste removed, how accessible the unit is, and whether the service is a planned visit or an emergency callout. Planned schedules cost less per visit than reactive work. Clear Stream provides a fixed-price quote before any work begins, with no call-out fee.

Grease Trap Cleaning: A Guide for Food Businesses