blogTL;DR: Commercial kitchens in the UK must keep fats, oils and grease out of the public sewer, and drainage serving hot food premises should be fitted with a grease separator or another effective means of grease removal. Get the equipment sized correctly, keep the pipework accessible, service it on a schedule, and document everything. Book a commercial drainage assessment before a blockage forces the decision for you.
The drainage requirements for commercial kitchens catch more Cornwall food businesses out than any other part of a fit-out. Not because the rules are obscure, but because most operators inherit a kitchen rather than build one. The grease trap is already there (or it isn’t), the pipework runs somewhere under the floor, and nobody knows when it was last serviced. Then the wash-up sink backs up on a Saturday night in August, and a compliance question becomes a trading question.
South West Water cleared over 7,000 avoidable blockages from its network in the year to December 2025, removing roughly 450 tonnes of waste. Around three quarters of those blockages came down to two things: wet wipes, and fats, oils and grease. This guide covers what the regulations actually require, how grease management equipment is specified, how the pipework should be built and maintained, and what happens when it fails.
What Are the Drainage Requirements for Commercial Kitchens in the UK?
UK commercial kitchens must have drainage that removes wastewater hygienically, stays accessible for cleaning, and prevents fats, oils and grease from entering the public sewer. Building Regulations guidance recommends that drainage serving hot food premises is fitted with a grease separator or another effective means of grease removal. Discharging grease into the sewer is a criminal offence.
Three separate pieces of legislation apply at the same time, which is where the confusion usually starts.
- Building Regulations: Approved Document H sets the standard for foul drainage design, including pipe sizing, gradients, ventilation and access for clearing blockages. It also addresses grease separation in commercial hot food premises.
- Water Industry Act 1991: Section 111 makes it an offence to pass into a public sewer any matter likely to injure it or interfere with the free flow of its contents. Fats, oils and grease sit squarely inside that definition.
- Building Act 1984: Section 59 allows a local authority to serve notice on a building owner requiring satisfactory drainage provision. That can include ordering the installation of a grease trap.
FOG is the industry term for fat, oil and grease. It leaves the kitchen as a warm liquid, cools in the pipe run, and hardens onto the pipe wall. Food particles bind to it. The internal diameter shrinks. What started as a slow-draining sink becomes a solid blockage, and the same process repeats in the public sewer downstream.
The equipment question is where most operators want a straight answer, so let’s take that next.
Do You Legally Need a Grease Trap in a Commercial Kitchen?
There is no single UK law that says “every commercial kitchen must install a grease trap.” What the law says is that you must not allow fats, oils and grease to enter the public sewer, and Building Regulations guidance recommends a grease separator complying with BS EN 1825-1 (or another effective means of grease removal) on drainage serving commercial hot food premises. In practice, a hot food kitchen without grease management cannot meet that obligation.
A grease separator is a chamber fitted into the kitchen drainage line that slows the wastewater flow, allowing fats and oils to rise and food solids to settle, so that only the water layer passes on to the sewer. BS EN 1825-1 covers the design and performance of the unit itself. BS EN 1825-2 covers how it should be selected, sized and installed.
Water companies enforce this directly. South West Water and its compliance partner ECAS have worked with more than 2,200 food businesses across Devon and Cornwall on FOG management, checking kitchen practices and recommending grease-trapping equipment where it is missing. An inspection is not a formality. It is the step that comes before enforcement.
If your kitchen produces hot food and you have no grease management in place, treat that as an open compliance gap, not a grey area.
Choosing and Sizing Grease Management Equipment
Grease management equipment must be sized to the kitchen’s actual wastewater flow, not bought off a shelf by rough guess. BS EN 1825-2 sets out the calculation, which factors in flow rate, wastewater temperature, the density of the fats involved and the type of food being prepared. An undersized unit fails quietly, passing grease straight through.
Passive Grease Separators
These are static chambers, fitted under a sink or below ground outside the building. No power, no moving parts. They work well when they are correctly sized and emptied on schedule by a licensed waste carrier. They fail when a busy kitchen forgets about them for six months.
Grease Removal Units
A grease removal unit (GRU) is an automated version that skims separated fat into a collection container, which staff empty as part of the daily routine. Higher purchase cost, lower risk of the fat layer being left to build. These suit high-volume kitchens where wash-up runs continuously through service.
Biological Dosing Systems
Biological dosing systems release micro-organisms into the drain line to break down FOG before it hardens. They are a supporting measure. Used alone in a heavy fry kitchen, they will not carry the load. Used alongside a correctly sized separator, they help keep the downstream pipework clear.
Whichever route you take, sizing should follow a site survey covering menu type, cooking methods, covers per day and the existing pipework layout. The Water UK foodservice FOG management guide is the reference the water companies themselves work from.
How Commercial Kitchen Drainage Should Be Built
Kitchen drainage has to handle high-temperature wastewater, heavy solids loading and constant washdown, which means it is built differently to domestic drainage. Approved Document H sets the design principles: adequate pipe sizing, sufficient fall to keep solids moving, proper ventilation, and access points that let an engineer clear a blockage without excavating.
Floor Gullies and Channel Drainage
Wet kitchen areas need floor drainage that can be lifted and cleaned. Stainless steel gullies with removable baskets sit under fryers, kettles and combi ovens. Channel drainage runs along wash-up lines and pot wash areas. Both need to be flush with the floor finish and laid so the floor falls towards them, otherwise you get standing water and a hygiene problem on your next inspection.
Pipe Sizing, Falls and Access
Kitchen waste pipes are commonly 100mm, laid to a consistent fall so the flow keeps solids in suspension. Too shallow and material settles. Too steep and the water outruns the solids, which is just as bad. Rodding eyes and inspection chambers should be positioned at every junction and change of direction, and they need to remain reachable after the kitchen is fitted out. Building an access point in behind a fixed counter is a decision you pay for later.
Backflow and Odour Protection
Every gully and appliance connection needs a working trap seal to stop drain air entering the kitchen. Where a kitchen sits at or below the level of the sewer, a non-return valve or pumped system protects against sewage backing up into the building during heavy rainfall. Cornwall’s coastal towns see that scenario more often than most.
Good build quality only holds up if the system is maintained, which is where most kitchens lose ground.
How Often Should Commercial Kitchen Drains Be Cleaned and Surveyed?
Grease separators typically need emptying every four to six weeks, with high-volume fry kitchens requiring more frequent servicing. Drain lines serving a commercial kitchen should be jetted preventatively, usually every six to twelve months depending on output, and surveyed with a camera at least annually to check the condition of the pipework itself.
A planned maintenance schedule for a commercial kitchen looks like this:
- Daily: staff empty GRU collection containers, clear gully baskets, dry-wipe pans before wash-up and use sink strainers.
- Weekly: visual check of the grease separator for the fat layer depth, slow drainage or odour.
- Every four to six weeks: separator emptied and cleaned by a licensed waste carrier, with a waste transfer note retained.
- Every six to twelve months: High-Pressure Jetting of the kitchen drain run to remove hardened FOG that has bonded to the pipe wall.
- Annually: a CCTV Drain Survey to record pipe condition, identify cracks, displaced joints or root ingress, and produce a report you can hand to an inspector.
Reactive callouts cost more than planned work, and they arrive at the worst possible moment. A kitchen that closes for a day loses a day’s covers, and the blocked drain was building for months before it stopped the sink.
Where a survey finds a cracked or degraded pipe under a kitchen floor, Drain Pipe Relining repairs it from the inside. No excavation, no ripped-up flooring, no closing the kitchen for a week while a trench is dug through it.
Cornwall Kitchens Have Drainage Problems Other Regions Don’t
Commercial kitchens in Cornwall face drainage conditions that generic guidance ignores: seasonal demand spikes, ageing clay pipework under listed and granite-built premises, tight harbour-town access, high coastal water tables, and a significant number of rural food businesses that are not connected to a mains sewer at all.
The seasonal load is the one operators underestimate. A pub in Padstow or a restaurant on the Falmouth waterfront can run at four or five times its winter covers through July and August. FOG output scales with that. ECAS estimates that a typical pub restaurant generates around 1,750 litres of FOG a year, and in Cornwall a disproportionate share of that volume passes through the drains in a twelve-week window. Grease separator emptying intervals set for February will not hold in August.
Then there is the building stock. Many Cornish kitchens sit inside converted properties with original clay or pitch fibre drainage, laid at inconsistent falls and never designed for a commercial fit-out. Clay pipe joints open up. Roots find them. Add hardened grease and you have a blockage that a plunger and a bottle of drain cleaner will never touch.
Rural sites bring a different requirement entirely. A kitchen discharging to a septic tank or package treatment plant cannot send FOG into it under any circumstances: grease coats the bacterial layer that makes the system work, and the tank stops treating. If your premises runs off-mains, grease management is not a sewer protection measure, it is the only thing keeping your Septic Tank Maintenance costs from spiralling.
What Happens If Commercial Kitchen Drainage Fails Compliance?
Non-compliance carries criminal, financial and operational consequences. Under Section 111 of the Water Industry Act 1991, discharging matter that interferes with the free flow of a public sewer is an offence, and water companies can recover the cost of clearing blockages traced back to your premises. Local authorities can separately order drainage works, and food hygiene inspectors can act on drainage failures.
What that means in practice for a food business:
- Prosecution and fines under Section 111, plus recovery of the water company’s blockage clearance and pollution investigation costs.
- A notice under Section 59 of the Building Act 1984 requiring you to install or repair drainage, at your cost and on their timescale.
- Damage to your food hygiene rating, since drainage backing up into a food preparation area is a direct hygiene failure under the Food Safety Act 1990.
- Forced closure while the issue is resolved, which is the cost that actually hurts.
Inspectors ask for evidence. Service records, waste transfer notes, survey reports and staff training logs are what turn “we manage our grease” into a documented defence. Keep them.
Getting Your Kitchen Drainage Right
The drainage requirements for commercial kitchens come down to three commitments: install correctly sized grease management, build and maintain pipework that can be cleaned and inspected, and keep the paperwork that proves it.
- Grease separation is effectively mandatory for hot food premises under Approved Document H, and discharging FOG to the sewer is a criminal offence under Section 111.
- Sizing follows BS EN 1825-2 and a proper site survey, not guesswork.
- Planned jetting and annual CCTV surveys cost a fraction of an emergency closure.
Clear Stream Drainage Solutions provides commercial drainage assessments, planned maintenance contracts, High-Pressure Jetting and documented CCTV Drain Surveys for kitchens across Cornwall and Devon. No call-out fee, a fixed-price quote before any work starts, and an engineer on site within 1 to 2 hours anywhere in Cornwall. Every repair carries our 5-year guarantee.
Call our team on 01872 222555 to book a commercial kitchen drainage assessment.
Frequently Asked Questions
Is a grease trap a legal requirement in a UK commercial kitchen?
No single law names grease traps as compulsory, but the effect of the rules is close to it. Approved Document H recommends that drainage serving commercial hot food premises is fitted with a grease separator complying with BS EN 1825-1, or another effective means of grease removal. Section 111 of the Water Industry Act 1991 makes it a criminal offence to discharge fats, oils and grease into a public sewer. A hot food kitchen with no grease management has no realistic way of complying.
How often should a commercial kitchen grease trap be emptied?
Most UK kitchens empty their grease separator every four to six weeks. High-volume operations, particularly those with heavy fry output, need shorter intervals. The practical test is the fat layer: if it is approaching a quarter of the unit’s depth, the interval is too long. Emptying must be carried out by a licensed waste carrier and recorded on a waste transfer note.
What size grease trap does my kitchen need?
Sizing is calculated under BS EN 1825-2, which accounts for the peak wastewater flow rate, wastewater temperature, the density of the fats in use and the type of food being prepared. It cannot be worked out from covers alone. A site survey of your kitchen equipment, menu and drainage layout is the only reliable way to specify the correct unit.
Can I put a food waste disposal unit in a commercial kitchen?
Food waste disposal units add solids directly into your drainage and increase the loading on any grease separator downstream. Some water companies discourage them for exactly that reason, and they are not permitted at all on premises discharging to a septic tank or treatment plant. Check your position with your sewerage undertaker before installing one.
What happens if a blocked drain forces my kitchen to close?
You lose trading revenue for the duration, and the closure itself can trigger further scrutiny. Sewage backing up into a food preparation area is a hygiene failure under the Food Safety Act 1990, which can affect your food hygiene rating and, in serious cases, lead to a prohibition notice. Where the water company traces a sewer blockage back to your premises, it can also seek to recover its clearance costs.
Do commercial kitchens on septic tanks need grease management?
Yes, and arguably more urgently than kitchens on mains drainage. Fats and oils coat the bacteria that break down waste inside a septic tank or package treatment plant, which stops the system treating properly and leads to failure, odour and expensive emptying cycles. Any off-mains commercial kitchen needs grease separation before the tank.
How do I prove my drainage is compliant during an inspection?
Keep a drainage file. It should hold your grease separator specification and sizing calculation, servicing and waste transfer records, jetting reports, CCTV survey footage and reports, and staff FOG training records. Inspectors respond to documentation. A verbal assurance that the kitchen is managed well carries no weight without it.
