Restaurant Drainage: Grease Traps, Compliance & Maintenance Guide

A busy commercial kitchen puts more strain on a drainage system in a single day than most homes do in a month. The combination of fats, oils, grease (FOG), food waste, cleaning chemicals, and sheer volume of water creates conditions that domestic drainage simply isn't designed for.

Get it wrong and you're looking at potential fines, enforcement action, and — in the worst cases — forced closure. The good news? With the right setup and a proper maintenance schedule, restaurant drainage is entirely manageable.

How Grease Traps Work

A grease trap sits between your kitchen drains and the main sewer, intercepting fats, oils, and grease before they enter the public system. Wastewater flows in, FOG floats to the top and is retained, cleaner water passes underneath a baffle to the sewer, and food particles sink to the bottom. Over time, the trap fills up and needs emptying.

Types of Grease Traps

• Passive (manual) grease traps — rely on gravity. The most common type, from small under-sink units to large in-ground chambers. Need manual emptying on a schedule.
• Automatic grease removal units (GRUs) — use a heated element and mechanical skimmer. More expensive but more efficient, ideal for high-volume kitchens.

How Often Do Grease Traps Need Emptying?

Monthly emptying is typical for most commercial kitchens, but it varies based on food volume, cuisine type (fryers produce more FOG), trap size, and system type. The "quarter rule" is useful: empty when grease and solids reach 25% of the trap's depth.

Legal Obligations

Water Industry Act 1991

Discharging trade effluent into the public sewer without consent from your water company is an offence. Commercial kitchen wastewater is trade effluent, so you need a trade effluent consent to legally operate.

What This Means

• You must apply to your water company for consent
• The consent specifies limits on FOG content, temperature, and chemicals
• You must have adequate pre-treatment (a properly maintained grease trap)
• The water company can inspect your premises
• Non-compliance can result in fines and prosecution

If FOG from your kitchen causes a pollution incident, you could face prosecution by the Environment Agency with substantial fines.

Planned Preventive Maintenance (PPM)

A typical PPM schedule includes:

• Monthly grease trap emptying and cleaning
• Quarterly drain jetting of the full drainage run
• Annual CCTV drain survey to check pipe condition
• Documented records for every visit — your audit trail for compliance

What Happens When Grease Traps Aren't Maintained

• Internal blockages — backed-up drains flooding the kitchen, forcing closure during service
• Sewer blockages — the water company will trace it back to you
• Foul odours — not ideal in a food establishment
• Pest attraction — rats and insects drawn to accumulated grease
• Environmental Health intervention — improvement notices, prohibition, prosecution
• Insurance complications — insurers may not cover you without evidence of maintenance

How UK Drainage Services Helps

Our commercial drainage services include grease trap installation and maintenance, tailored PPM contracts, emergency drain unblocking, high-pressure jetting, and CCTV surveys. We work around your service hours and aim to resolve issues with minimal disruption.

To discuss a maintenance plan for your restaurant, call us on 0333 577 4242.