Restaurant Grease Trap Compliance: UK Operator Guide
Running a London restaurant means juggling food safety, fire risk, licensing and drainage compliance all at once. Grease traps sit quietly under the sink or in the yard, but they are one of the most frequently flagged items during environmental health visits and Thames Water trade effluent audits. This guide explains what UK law actually requires from food service operators, how to size a grease trap correctly, how often it should be serviced, what records you must keep and what happens when an inspector finds a non-compliant unit on site.
Why UK restaurants are required to fit a grease trap
Fats, oils and grease (FOG) discharged into the public network cause hardened deposits that restrict flow, attract food waste and create the well-publicised “fatbergs” Thames Water removes each year. Three pieces of legislation underpin the requirement to capture FOG before it reaches the foul sewer.
The Water Industry Act 1991 (section 111) makes it an offence to discharge any matter likely to interfere with the free flow of a sewer or to prejudice sewage treatment. Building Regulations Part G3.3 and Part H require sanitary pipework and drainage to be designed so that hot, fatty waste from food preparation is intercepted before entering the foul drain. British Standard BS EN 1825 covers grease separator specification and sizing across Europe and is the document most consultants reference.
In short, a London restaurant kitchen that hot-washes pans and plates cannot legally discharge wash-water straight into the public sewer without some form of grease separation in the chain.
Thames Water trade effluent consent
Restaurants, takeaways, hotels and care homes within the Thames Water area discharging more than domestic-strength effluent typically require a trade effluent consent. The consent specifies maximum flow rate, permitted FOG concentration (often expressed as mg/l) and the type of pre-treatment required. Without consent, the sewerage undertaker can refuse to accept the discharge or impose enforcement.
You apply through the Thames Water Wholesale portal and submit a site drainage plan showing the proposed grease management equipment. Conditions are renewed periodically and an inspector may attend to verify what is actually installed matches the application.
Sizing a grease trap for restaurant volume
Undersizing is the single most common compliance failure. A 50-litre passive trap fitted under a single sink will not cope with a 60-cover bistro at lunchtime. BS EN 1825 sizes separators by nominal size (NS) calculated from peak flow, density factor (hot or cold water), temperature factor and a factor for detergents.
Rough sizing benchmarks
- Small cafe, up to 30 covers per service: passive trap 35-75 litres
- Pub kitchen or 50-cover restaurant: passive 100-200 litres or compact biological unit
- Hotel, 100+ covers, breakfast and dinner service: external below-ground separator NS 2 to NS 4
- Production kitchen, central cook-chill site: bespoke NS 7 to NS 15 external unit
An external below-ground separator should sit on its own branch to foul drain, not the surface water network. A site survey, normally including a CCTV drain survey of the existing connection, confirms whether the existing branch has capacity and gradient.
Maintenance frequency: typically monthly
The rule of thumb across Thames Water guidance, the British Water FOG code of practice and most environmental health teams is that a grease trap should be emptied when 25 percent of its capacity is reached. For a busy London restaurant that typically means monthly servicing, sometimes every two weeks for high-volume frying operations such as fish and chip shops.
Biological dosing systems (enzymes or microbial cultures) supplement but do not replace physical servicing. Operators that rely on dosing alone tend to fail inspection because the separator chamber still fills with solids.
Greater London Drainage offers scheduled drain cleaning for restaurant kitchens with documented removal volumes and digital service tickets.
Records the inspector will ask for
If Thames Water or the local environmental health officer attends site, they will normally request the following paperwork.
- Trade effluent consent reference and current conditions
- Most recent BS EN 1825 sizing calculation or manufacturer datasheet
- Service tickets covering the previous 12 months showing date, volume removed and waste carrier reference
- Waste transfer notes (Duty of Care) for the FOG removed
- Any CCTV reports relating to the kitchen branch or shared yard drain
Failure to produce records frequently triggers an enforcement letter even when the unit itself is in reasonable condition. Keep a folder behind the pass with paper copies and a cloud backup, especially if staff turnover is high.
Penalties for non-compliance
Discharging FOG in breach of section 111 of the Water Industry Act 1991 is a criminal offence carrying an unlimited fine on indictment. In practice Thames Water and London boroughs initially serve a notice requiring remediation within a defined period. Continued breach can lead to prosecution, recovery of clean-up costs (a single fatberg removal in 2024 cost a Soho operator over £40,000) and adverse publicity that hits trade hard.
Environmental health visits triggered by neighbour complaints (smells from a yard manhole or recurring blockages in a shared service yard) can also pull in food hygiene rating impacts.
Where a kitchen shares a yard with a forecourt or workshop, an additional petrol interceptor may be required upstream of the foul connection. A combined site drainage assessment avoids surprises later.
How to choose a maintenance contractor
Look beyond the price per visit. Cheap monthly servicing that fails inspection costs more than competent quarterly work that produces clean records. The minimum useful checklist:
- Licensed waste carrier (Environment Agency registration) for FOG removal
- Operatives trained to NRSWA or City and Guilds 6160 drainage qualifications
- Written method statement and risk assessment for confined space entry where applicable
- Digital service reporting with photos before and after
- Out-of-hours response in case the trap backs up mid-service
- Insurance covering trade effluent and environmental liability
- Familiarity with Thames Water trade effluent processes for London sites
- Ability to provide annual summary reports suitable for due diligence and lease renewal
Ask for two reference sites in the same operating model (independent restaurant, hotel chain, pub group) before signing. A contractor used to large central kitchens may not be the right fit for a 30 cover bistro in Camden, and vice versa.
Common operator mistakes
Reviewing the last twelve months of work across London hospitality sites, the same five issues come up again and again.
- Pot wash and dishwasher run into the same drain that bypasses the trap entirely, normally because the trap was retrofitted under one sink without rerouting the rest of the kitchen
- Operators relying solely on enzyme dosing and removing physical servicing, then failing a Thames Water audit when the chamber is half full
- Yard manhole sitting downstream of multiple units in a shared building, where ownership of cleaning is unclear and each tenant assumes the next is responsible
- Old units kept in service well past replacement age (15 years is the typical practical life of a passive trap)
- No annual deep clean of the actual chamber, leading to caked layers that reduce effective capacity
A walk through the kitchen with the contractor, sketching the actual layout from sink to manhole, catches most of these in 20 minutes.
When to call a professional
If you are buying or refitting a restaurant, fitting a new unit without a proper sizing calculation and survey is asking for an enforcement notice within 12 months. Book a CCTV drain survey on the existing connection, ask for a written sizing report and confirm the trade effluent consent is in your trading name before opening. Greater London Drainage covers Camden, Islington, Hackney and surrounding boroughs with engineers familiar with London restaurant compliance requirements.
Final thoughts
Grease trap compliance is one of those topics that feels boring until the kitchen floods at 8pm on a Friday or a notice arrives from Thames Water. Sizing the unit correctly, servicing it on schedule and keeping clean records makes the difference between a routine inspection and a written warning that affects the lease. Build the maintenance into the same calendar as your gas safety and fire risk assessment, keep the paperwork accessible and the cost of compliance stays predictable across the trading year.
