When Is a Grease Trap Required and How Do They Work?

Quick Answer: Grease traps (grease separators) are required by Building Regulations Part H and water company requirements for commercial kitchen drainage where fats, oils, and greases (FOG) would otherwise enter the drainage system. They must be sized to BS EN 1825 based on peak flow rate. Above-ground passive grease traps require emptying every 1–4 weeks; below-ground separators monthly. Failure to maintain them is an offence under water industry legislation.

Summary

Fats, oils, and greases (FOG) from commercial kitchens are among the most significant causes of sewer blockages in the UK. When hot cooking fat is washed down a sink, it is liquid. As it cools in the drainage pipework, it solidifies and accumulates on pipe walls, eventually causing total blockages — known as fatbergs in the sewerage press. Water companies spend hundreds of millions of pounds annually clearing FOG-related blockages.

Grease traps intercept the drainage flow from kitchen sinks, dishwashers, pot wash sinks, and floor drains before it reaches the sewer, retaining the FOG fraction and allowing cleaner water to pass through. Regular maintenance (emptying of retained FOG and food solids) is essential — a grease trap that is not maintained becomes a full grease trap that passes FOG through unimpeded.

In new commercial kitchen developments, the requirement for grease management is typically a planning condition, a requirement of Building Regulations Part H (drainage and waste disposal), and a condition of the water company's trade effluent consent. In existing businesses, water companies can issue notices requiring grease trap installation where FOG is causing blockages in the public sewer.

Key Facts

• Building Regulations Part H — Approved Document H requires that trade effluent (including FOG from commercial kitchens) is treated before discharge to the sewer; grease separators satisfy this requirement
• BS EN 1825 — the British/European Standard for gravity grease separators; covers design, sizing, installation, and maintenance
• BS EN 858 — covers light liquid separators (petrol interceptors); a related but separate standard; not for kitchen FOG
• Sizing formula (BS EN 1825) — NS (nominal size) = V_flow × F_r × F_d × F_t; where V_flow is peak flow rate, F_r is road factor, F_d is density factor, F_t is temperature factor; specialist calculation or manufacturer sizing tool required
• Peak flow rate — the primary sizing parameter; calculated from the number and type of appliances draining to the trap (sinks, dishwashers, pot wash)
• Above-ground passive grease traps — also called FOG traps; sit under the sink; typically hold 40–100 litres; must be emptied every 1–4 weeks; inexpensive but labour-intensive to maintain
• Below-ground grease separators — larger units (typically 300–3000 litres); installed in a chamber accessible via a manhole; should be emptied (desludged) every 1–4 weeks for grease layer; full desludge monthly or as required
• Automatic grease removal units (AGRU) — motorised skimming arms that continuously remove the grease layer; reduce emptying frequency to monthly or quarterly; more expensive capital cost but lower maintenance labour
• Trade effluent consent — discharge of trade effluent (kitchen wastewater) to sewer requires consent from the sewerage undertaker (Water Company); conditions typically require grease management
• Enforcement — water companies can issue trade effluent enforcement notices requiring businesses to install grease management; blocking a sewer through FOG is an offence under the Water Industry Act 1991
• Environmental Permit — where kitchen wastewater is discharged to surface water drainage or a soakaway (not typical but sometimes seen), an Environmental Permit from the Environment Agency may be required

Quick Reference Table

Detailed Guidance

How Passive Grease Traps Work

A passive grease trap operates on the principle that fat, oil, and grease are less dense than water and will float to the surface given sufficient retention time. The trap is a sealed or semi-sealed chamber with an inlet and outlet baffle. Wastewater from the kitchen enters through the inlet, slows down in the chamber (because the chamber is wider than the pipe), and the FOG fraction floats to the surface while food particles settle to the bottom. The clarified water layer in the middle passes through the outlet baffle and continues to the drain.

For this separation to work, the wastewater must retain sufficient velocity reduction in the trap and must not be mixed with large volumes of detergent (which emulsifies the fat and prevents separation). High-temperature water (above 65°C) dissolves fat rather than allowing it to solidify and float — this is why hot water and strong detergents, while useful for cleaning, also carry more FOG to the trap in dissolved form, which then solidifies downstream.

The trap must be emptied before the grease layer reaches the outlet baffle — once the grease layer is deep enough to pass through the outlet, the trap is effectively bypassed. Most manufacturers recommend emptying when the grease layer reaches 25% of the trap's working volume.

BS EN 1825 Sizing

BS EN 1825 provides a calculation method for sizing grease separators. The nominal size (NS) of the separator is calculated from the peak flow rate of wastewater entering it, modified by factors for:

• Temperature factor (F_t) — higher temperatures require a larger separator because hot water carries more dissolved fat
• Density factor (F_d) — reflects the type of food operation (fish and chips vs vegetarian cafe vs red meat restaurant)
• Road factor (F_r) — for road gullies draining to the separator; not relevant for kitchen installations

The peak flow rate is calculated from the number and type of drainage appliances: each type of appliance has a design flow rate (from BS EN 12056-2 for sanitary drainage design), and the peak flow rate is calculated from simultaneous use factors.

In practice, most grease trap suppliers provide online sizing tools or manual sizing guides based on the number of covers (seats) in the restaurant, or the number of meals served per day. These provide a reasonable starting point, but BS EN 1825 calculation is required for compliance.

Installation Requirements

Above-ground units: Under-sink grease traps must be installed with:

• Easy access for cleaning — positioned where the lid can be fully removed
• Venting to prevent anaerobic gas build-up and smell (typically through the drain vent stack)
• The unit level (to ensure the separation chamber works correctly)
• Access for inspection by the water company if required

Below-ground separators (BS EN 1825): Must be installed:

• In a watertight chamber with a manhole cover accessible for emptying
• With the inlet and outlet at the correct relative levels (the separator body is typically installed with the inlet 50mm higher than the outlet)
• With vent pipe to atmosphere (via the vent stack or dedicated vent)
• With inlet and outlet connections that maintain the correct depth and orientation
• Below the frost line to prevent freezing (in unheated external areas)

The manhole cover must be rated for the loading (foot traffic vs vehicle traffic). Below-ground separators in car parks or loading areas require heavy-duty covers rated to the relevant load class.

Maintenance Requirements

Grease trap maintenance is a legal obligation, not a best practice. Under the Water Industry Act 1991, blocking a sewer through failure to maintain grease management equipment is an offence. The consequences range from a substantial clean-up cost (charged to the business) to criminal prosecution in serious or repeated cases.

Maintenance intervals should be based on the fill rate of the trap (measured during initial commissioning) rather than a fixed time interval, but as a guide:

• Under-sink passive traps: empty at least every 2 weeks; weekly in busy kitchens
• In-ground separators: desludge when grease layer is 25% of effective capacity; typically monthly
• Units with AGRU: FOG removed automatically by skimmer; manual desludge monthly or quarterly

Waste from grease traps is classified as a waste (typically non-hazardous trade waste for domestic kitchen operations) and must be removed by a licensed waste carrier and disposed of at an approved facility. Records of emptying must be kept for inspection by the water company.

Trade Effluent Consent

Discharging trade effluent (wastewater from a trade or industry that is not domestic sewage) to the public sewer requires consent from the sewerage undertaker (typically the water company for the area) under Section 119 of the Water Industry Act 1991. Kitchen wastewater is trade effluent.

The consent application must describe the volume, composition, and nature of the discharge. The water company will set conditions including:

• Maximum flow rate and daily volume
• Quality standards (BOD, suspended solids, pH, FOG content)
• Grease management requirements (trap type, maintenance schedule)
• Record-keeping obligations

Operating without consent, or in breach of consent conditions, is a criminal offence. For new commercial kitchen developments, the trade effluent consent should be applied for as part of the planning and building control process.

Environmental Permits for Non-Sewer Discharge

Where a commercial kitchen cannot connect to the public sewer (rural locations, listed buildings where connection is impractical), kitchen wastewater may need to be discharged via a septic tank or sewage treatment plant. If kitchen FOG is included in this discharge, the effluent quality requirements are more stringent than for domestic sewage. An Environmental Permit from the Environment Agency (in England) may be required.

In practice, most commercial kitchen operators connected to any form of public or private drainage system will require some form of grease management. Consult the relevant water company or the Environment Agency at the earliest planning stage.

Frequently Asked Questions

My takeaway is getting notices from the water company about FOG. What are my legal obligations?

You are required to take reasonable steps to prevent fat, oil, and grease entering the public sewer. The water company can issue a notice under Section 59 of the Water Industry Act 1991 requiring you to install and maintain grease management equipment. Failure to comply with a Section 59 notice is a criminal offence. Engage a grease management supplier immediately, notify the water company of your intended actions, and implement the agreed measures promptly.

Can I use biological dosing instead of a grease trap?

Biological dosing systems introduce bacterial cultures that break down FOG within the drainage pipework. They can reduce FOG levels but are not generally accepted as an alternative to a physical grease separator by water companies. They may be used as a supplementary measure alongside a grease trap. Check with your water company before specifying dosing as the sole FOG control — most will not accept it as a substitute for a physical separator.

How do I know if my kitchen needs a grease trap before I open?

Check with:

1. Your local planning authority — if you required planning permission, check the conditions
2. Building Control — Approved Document H; the BCO may require evidence of grease management
3. Your water company — apply for trade effluent consent; the consent will specify grease management requirements
4. Environmental Health — the food premises inspection may identify requirements

For any business cooking greasy food (fried chicken, kebabs, burgers, fish and chips), a grease management system is almost certain to be required.

Does a domestic kitchen extension require a grease trap?

No. Grease trap requirements apply to commercial food premises. Domestic kitchens are not subject to the same requirements, even if the kitchen is larger than typical. However, in a residential flat building with multiple kitchens, some water companies impose FOG management requirements in the conditions for the connection to sewer — check with the water company.

Regulations & Standards

• Building Regulations Part H (Approved Document H) — drainage and waste disposal; trade effluent treatment before sewer discharge

• Water Industry Act 1991 — trade effluent consent; offences for blocking sewers; enforcement powers

• BS EN 1825:2004+A1:2011 — grease separators; principles of design, performance, testing, marking, and quality control

• Environmental Permitting (England and Wales) Regulations 2016 — Environmental Permits for discharges not to public sewer

• Water UK — FOG Best Practice Guidelines — industry guidance on FOG management for food businesses

• Environment Agency — Trade Effluent — permitting requirements for non-sewer discharge

• Thames Water — Grease Management — practical guidance including sizing and approved products

• CIBSE — Drainage System Design — BS EN 1825 and Part H design guidance

• underground drainage — general drainage design and Part H compliance

• pump stations — when gravity drainage to a grease trap is not possible

• part h drainage — full overview of Building Regulations Part H