All You Need to Know About Grease Traps

Why Grease Traps Matter More Than You Think

If you’ve ever heard of “fatbergs”, you’ll know they’re no joke. These vast, solidified masses of fats, oils and grease (FOG) lurk in sewer networks, choking pipes and flooding properties. They don’t just appear – they’re caused by waste unwisely flushed down drains.

Cranfield University estimates that 75% of the 200,000 sewer blockages in the UK every year are caused by FOG waste. Water UK put the number even higher – 300,000 blockages a year, resulting in 3,000 flooded homes and costing the UK around £200 million annually.

The good news is that grease traps offer a simple and reliable way to stop FOG at source.

What Is a Grease Trap?

A grease trap is a containment device that captures and holds fats, oils and grease before they can enter your drainage system. Positioned between the kitchen’s waste pipes and either a mains sewer or off-mains treatment system, it works by slowing down wastewater flow. This allows FOG – which is lighter than water – to rise to the top, while heavier solids drop to the bottom. Only relatively clean water passes out to the drain.

Grease traps come in two general forms:

• Small internal traps: ideal for low-volume applications such as pubs, cafés or domestic homes.
• Larger underground grease separators: suitable for busy commercial kitchens such as those in hotels or restaurants.

Although you might hear terms like grease separator, grease interceptor or fat trap, these all refer to the same principle: keeping FOG out of your pipes.

How Do Grease Traps Work?

Inside the unit, baffles or barriers slow down the wastewater and encourage separation. These devices usually form three chambers:

• Inlet chamber – slows the wastewater and begins separation.
• Settlement chamber – allows FOG to rise and solids to settle.
• Outlet chamber – releases water with significantly reduced FOG content.

Smaller models might include flow control valves or vented devices to regulate input. Larger underground versions rely purely on gravity and are often called passive separators.

It's worth remembering that grease traps are not food disposal units. They can handle small particles, but not large amounts of leftover food, sauces or gravies. Those should always go in the bin.

Grease Trap Maintenance

Traps must be emptied regularly. For under-sink models, this could mean daily emptying or periodic cleaning, depending on usage. Some include containers that store extracted grease to reduce maintenance frequency.

Larger underground systems, on the other hand, require professional attention—particularly because built-up FOG can generate hazardous gases over time. Depending on your setup, this cleaning might only be needed every few weeks or months.

In high-use environments, biological grease treatment fluids can be added to break down FOG between clean-outs.

Grease Management Legal Requirements in the UK

In commercial food preparation settings, grease management is a legal obligation under multiple UK regulations. These include:

• The Water Industry Act 1991

Drainage systems must not discharge harmful substances.

• Environmental Protection Act 1990

Improper FOG disposal can be classed as a statutory nuisance.

• Food Safety and Hygiene (England) Regulations 2013

These impose cleanliness requirements on food premises, including disposal systems.

Failure to comply can result in heavy fines, prosecution or closure by environmental health officers.

Where Grease Traps Should Be Installed

Grease traps are mandatory in most commercial kitchens. This includes:

• Restaurants and takeaways
• Cafés and bakeries
• Hotels and pubs
• Schools, colleges, hospitals and other institutions
• Industrial food preparation facilities

Even in domestic kitchens, where FOG volumes are lower, a grease trap can still prevent blocked drains, especially if you're off-mains and relying on a septic tank or sewage treatment plant. FOG can coat and block soakaway fields, leading to total system failure.

Space is a consideration. If you’re short on room beneath your sink, an underground separator, installed between the waste pipe and treatment system, may be a better long-term solution.

Choosing the Right Size of Grease Trap

The right grease trap depends on your situation. In small kitchens with light FOG output, a compact under-sink trap may be sufficient. For larger premises with multiple sinks, dishwashers, ovens and surface washing, a high-capacity grease separator is usually essential.

However, not every appliance should be connected. For example, dishwashers often use detergents that can emulsify FOG, reducing a trap’s effectiveness. Always consult your supplier to ensure your grease management system is properly configured.

Need expert advice on the right grease trap?

Contact Drainfast today for friendly guidance and high-quality products, delivered nationally from our well-stocked depots.

Useful Links

• Cranfield University - https://blogs.cranfield.ac.uk/environmental-technology/fatbergs-in-our-sewers/
• Water UK - https://www.water.org.uk/fatsblockpipes
• Water Industry Act 1991 - https://www.legislation.gov.uk/ukpga/1991/56/contents
• Environmental Protection Act 1990 - https://www.legislation.gov.uk/ukpga/1990/43/contents
• The Food Safety and Hygiene (England) Regulations 2013 - https://www.legislation.gov.uk/uksi/2013/2996/contents/made