How Does a Grease Trap Work? The Essential Guide for Commercial Kitchens

A grease trap works by separating fats, oils and grease (FOG) from wastewater before it enters the sewer system. Grease floats to the surface, solids sink to the bottom, and cleaner water flows safely away. This helps commercial kitchens prevent blockages, overflows, leaks and costly drainage issues while staying compliant with Section 111 legislation.

Grease traps play an essential role in commercial kitchen waste management by capturing FOG before it can enter drains and cause fatbergs in the sewer system. In this guide, we’ll explain how grease traps work, common grease trap problems and how proper maintenance helps keep your kitchen efficient and compliant.

Grease traps are a plumbing device designed to separate and capture fats, oils and grease (FOG) from wastewater before it enters the sewer system. As wastewater flows through the unit, FOG rises to the surface while heavier food particles sink to the bottom, allowing cleaner water to pass through safely.

FOG refers to the fats, oils and grease produced during cooking, food preparation and cleaning in commercial kitchens. Without proper grease management, these substances can build up inside pipework, causing blockages, overflows, leaks and expensive drainage problems.

They are essential for commercial kitchens because they help maintain efficient drainage systems, reduce the risk of fatbergs forming in sewers and support compliance with UK grease management legislation. For a more detailed breakdown, read our guide on what a grease trap is.

A grease trap may look like a simple tank on the outside, but it plays an essential role in commercial kitchen waste management. Designed to separate fats, oils and grease (FOG) from wastewater, grease traps help prevent blockages, overflows and costly drainage problems before they reach the sewer system.

Although grease traps are relatively simple systems, they use the natural properties of grease and water to separate waste effectively. Here’s what happens during each stage of a grease trap’s operation:

How a Grease Trap Works:

Wastewater, including the mixed-in FOG from cooking, cleaning and dishwashing activities, enters the grease trap. This initial mixture is typically warm, and the FOG particles are dispersed throughout the water.

As the warm mixture enters the grease trap, the temperature begins to drop which changes the state of the FOG. Cooler temperatures cause the FOGs, which were previously in a semi-liquid state, to start solidifying. This is vital for the separation process.

Inside the grease trap, a series of baffles (walls) slow the flow of wastewater and create a ‘calm’ zone. This gives the now semi-solid FOG time to rise, forming a floating layer at the top of the trap, while the heavier food particles and solids descend to form a sludge layer at the bottom.

The layering effect means the water in the middle, now largely cleared of FOG and solids, can move to the sewer. This ‘clear zone’ of water exits through an outlet valve positioned between the top layer of FOG and the bottom layer of sludge. It is this process that ensures the effluent leaving the trap is compliant with wastewater regulations.

Over time, the layer of FOG thickens, reducing the trap’s efficiency and capacity. To maintain functionality, this accumulated FOG must be manually removed at regular intervals. The frequency of this maintenance task is dependent on the volume of FOG processed by the kitchen and the capacity of the grease trap.

Proper disposal of FOG is also a critical step, often requiring specialised services to ensure environmental regulations are met.

The effectiveness of a grease trap is a direct result of its design and the precision with which each of these stages is managed. By exploiting the simplicity of natural separation based on density differences, engineers have created a system that is not only effective but also integral to the modern commercial kitchen.

The effectiveness of a grease trap is a direct result of its design and the precision with which each of these stages is managed. By exploiting the simplicity of natural separation based on density differences, engineers have created a system that is not only effective but also integral to the modern commercial kitchen. 

They help commercial kitchens stay compliant, protect plumbing systems and prevent fats, oils and grease (FOG) from entering the sewer network. Under Section 111 legislation, businesses producing FOG are responsible for managing wastewater correctly to reduce the risk of blockages, overflows and environmental damage.

Without a grease trap, grease can build up inside pipework and public sewers, increasing the likelihood of fatbergs, leaks, unpleasant odours and costly drainage issues. Proper grease management helps keep kitchens running efficiently while reducing the risk of disruption and expensive repairs.

Grease traps are commonly used across:

Any commercial kitchen producing FOG should have an effective grease management system in place.

They can develop issues if they aren’t properly maintained, leading to blockages, overflows, leaks, slow drainage and bad odours caused by FOG build-up.

These problems are often linked to poor maintenance, incorrect sizing or excessive grease entering the system. Left unresolved, they can disrupt kitchen operations and create compliance risks.

A grease trap keeps overflowing when it can no longer separate and store fats, oils and grease (FOG) effectively. This is usually caused by a build-up of waste or a system that isn’t suitable for the kitchen’s demand.

Common causes include a full trap that hasn’t been cleaned in time, a blocked outlet preventing proper flow, poor sizing for the volume of wastewater produced, or excessive FOG entering the system from cooking and washing activities.

Warning signs include slow draining sinks, foul odours, visible grease around the unit and wastewater backing up into sinks or floor drains.

The most effective solutions are regular emptying and cleaning, removing blockages, and ensuring the grease trap is correctly sized for the kitchen. In most cases, professional servicing is needed to fully resolve the issue and prevent it recurring.

A grease trap leaking is usually a sign that the unit is damaged, incorrectly installed or no longer sealing properly. Even small leaks can lead to odours, hygiene issues and potential compliance problems if left unresolved.

Common causes include cracked tanks, worn or damaged seals, corrosion over time, or poor installation that places stress on joints and pipe connections.

Signs of a leaking grease trap include pooling water around the unit, persistent smells, damp patches near pipework and visible cracks or deterioration.

Leaks should always be addressed quickly to prevent further damage and operational disruption. In most cases, professional repair is required to restore the system safely. 

Grease traps can produce strong odours when waste inside the system begins to decompose. As fats, oils and grease (FOG) break down, they release gases such as hydrogen sulphide, which causes the characteristic ‘rotten egg’ smell often associated with poorly maintained systems.

Poor ventilation can make this worse by allowing gases to build up and escape through sinks or around the unit instead of being safely dispersed.

A grease trap not draining properly is usually caused by system restrictions that prevent wastewater from flowing through as intended. This can quickly lead to backups, slow sinks and overflow issues in busy commercial kitchens.

Common causes include blocked pipework, an overloaded grease trap that hasn’t been emptied in time, or poor maintenance allowing FOG and solids to build up and restrict flow.

Regular servicing and cleaning are key to preventing drainage issues and keeping the system operating efficiently.

Fixing a leaking grease trap starts with identifying the source of the issue and addressing it quickly to prevent further damage, odours and compliance risks.

Check around the unit, pipework and surrounding floor area to pinpoint where wastewater is escaping, as this will help determine whether the issue is internal or external.

Inspect all seals and gaskets for wear, movement or deterioration, as damaged sealing is one of the most common causes of minor leaks.

Examine the grease trap body for cracks, corrosion or structural damage that may require repair or replacement.

Remove any built-up FOG or solid waste that could be causing pressure build-up and forcing wastewater out of weak points in the system.

If the leak continues or damage is identified, a professional engineer should inspect and repair the system to ensure it is safe, compliant and fully operational. For expert support, get in touch for our grease trap repair service.

In a busy commercial kitchen, the right grease trap isn’t just an accessory, it’s essential. Aqua Mundus offers a range of solutions designed to suit different kitchen sizes and operational demands, from compact under-sink units for smaller spaces to advanced automatic grease removal systems for high-volume environments. This ensures your kitchen remains efficient, compliant and environmentally responsible.

Correct installation and ongoing maintenance are key to reliable performance. Our expert engineers ensure every system is installed and correctly sized for your kitchen’s needs, helping to prevent common issues caused by poor fit or overloading. We also provide comprehensive servicing to keep systems running smoothly and reduce the risk of odours, blockages and breakdowns.

Partnering with Aqua Mundus means choosing a solution that supports compliance, sustainability and long-term operational efficiency.

The significance of a well-chosen, expertly installed and meticulously maintained grease trap cannot be overstated. It is the unseen hero of kitchen waste management, a role that Aqua Mundus is proud to support. By collaborating with us, you invest in more than just a product – you engage in a partnership that guarantees a greener, more efficient and regulation-compliant kitchen operation. 

For a comprehensive grease management solution that aligns with your kitchen’s performance and compliance needs, turn to Aqua Mundus. Get in touch with us today by emailing sales@aquamundus.co.uk, or use our easy online form and we’ll quickly get back to you.