If you work in any industry that provides catering, like a commercial kitchen, office catering, restaurants, hotels, events hosting, healthcare or educational institutions, it’s vital that you know what a grease trap is and how they operate. A grease trap is a legal requirement for any commercial food business that cooks hot meals in any capacity, and failure to comply with strict legislation around grease traps can lead to blocked pipes, significant fines and even prosecution.

But don’t panic! With Aqua Mundus on your side, you’ll soon know all you need to know about these devices with our ultimate guide to answering the question, “What is a grease trap?”.

What Does a Grease Trap Do?

How Do Grease Traps Work?

What Does a Grease Trap Look Like?

What is a Grease Trap Used For?

Who Needs a Grease Trap?

What Types of Grease Traps Are There?

How Much Is a Grease Trap?

Where Does Grease Trap Waste Go?

How Often Should Grease Traps Be Cleaned?

Grease Trap vs Grease Interceptor

A grease trap is a plumbing device designed to intercept and separate fats, oils, grease and food solids from wastewater before it enters the drainage system. As wastewater flows through the trap, heavier solids settle at the bottom while lighter grease and oils float to the top, allowing cleaner water to exit safely. 

Without a grease trap, these substances can accumulate in pipes and cause serious issues such as blocked drains, sewer backups and noticeably foul odours. Left untreated, this waste can cause environmental contamination and harm waterways, which in turn increases the risk of costly repairs and regulatory penalties for businesses.

Grease traps have a longer history than you may think, but what is the purpose of a grease trap? To separate FOG from your wastewater before it reaches the sewer. How? It’s simpler than you might think and actually hinges on the natural separation of oil and water.

As your wastewater enters the grease trap it is slowed down by a series of baffles – essentially walls – within the trap. This allows time for the FOG to separate and float to the top of the tank, while the now cleaner water can pass through small gaps at the base of the tank, where it continues on to the public sewer system.

Those are the basics of grease trap operation, and we have a more comprehensive explanation of how grease traps work available.

The mixture of wastewater and FOG, whether from cleaning, cooking or dishwashing, enters the installed grease trap. 

This mixture is typically warm and begins to cool in the grease trap, which changes the form of the FOG into a semi-liquid state that is soon to solidify. 

Internal baffles (walls) slow the flow of the water and create a ‘calm zone’. The now semi-solid FOG rises and forms a floating layer in the trap, while heavier particles settle at the bottom and form sludge. 

Once separated, the area in the middle is now ‘clean’ water that is safe to move into the rest of the drainage system through an outlet valve. 

The FOG layer will start to thicken, thus reducing the trap’s efficiency and capacity. To prevent a breakdown, FOG needs to be manually removed at regular intervals. 

All FOG or sludge must be removed and disposed of in line with all appropriate regulations.

Grease traps vary in size and shape depending on their use, but they are generally simple, box-like or tank-style units. Smaller versions are often compact metal or plastic containers installed beneath sinks, while certain larger systems may be positioned underground outside a building to handle higher volumes of wastewater.

They are typically constructed from materials such as stainless steel, durable plastics or concrete. Each material offers different advantages, which could be varying strength levels, different resistances to corrosion and overall suitability for indoor or outdoor installation.

Choosing the right type depends on factors like the size of the kitchen, the amount of waste produced and where the unit will be installed, ensuring it performs efficiently. Seeking advice from grease trap experts can help you make the appropriate decision. 

So, what is a grease trap used for? Put simply, it’s a device that sits on your wastewater line that intercepts byproducts from the cooking process, namely fats, oils and grease – FOG for short. FOG can cause real problems in your pipes and the sewer system into which your pipes flow, not just for you but for everybody connected to the same sewer.

You may have heard of the modern phenomena of ‘fatbergs’ – giant conglomerations of FOG that cause major blockages in sewers, causing significant disruption and requiring expensive and time-consuming removal. A grease trap is an effective way of stopping FOG from getting into the sewer system.

There are multiple laws that define the legal requirements for having a grease trap installed, including the Water Industry Act 1991, the Environmental Protection Act 1990, the Food Safety Act 1990 and more. You can read more about that in our grease trap legal requirements blog. But for simplicity, we can sum it up here in one short sentence.

Any commercial kitchen that cooks hot food MUST have a grease trap installed.

It’s important to state that serving temperature isn’t relevant at all. You may serve your food cold, but if you had to cook the ingredients, then hot food has been created in the process. Whether your customers eat your food on-site or not is also irrelevant; the law doesn’t just apply to restaurants but to takeaways and other hot food caterers too.

If, in the course of your business activities, you create fats, oils and grease, you must legally intercept it before it makes its way into the sewer system. There are no exceptions to this, and it even applies to your home kitchen if you are using it to cook food for commercial purposes.

A grease trap is a device installed within a building’s plumbing system, typically positioned between kitchen appliances and the main drainage line. Its primary function is to intercept fats, oils and grease before they enter the sewer system, helping to prevent blockages and protect wider wastewater infrastructure from damage and other costly maintenance issues.

One of the unique challenges restaurants face is that they create a lot of fats, oils and grease through preparation, cooking and cleaning all year round. This means that a restaurant’s grease trap needs to be able to handle consistent usage and high capacity. Without a grease trap system, the FOG created by a restaurant will cause blockages, wastewater contamination, environmental damage, infrastructure damage and other issues like foul odours. 

Many food-related businesses are typically required to install grease traps due to the volume of fats, oils and grease they produce. This includes restaurants, cafés, takeaways, hotels, commercial kitchens and food processing facilities, where wastewater can quickly lead to blocked drains and sewer issues if left untreated. As outlined in the Water Industry Act of 1991, businesses that produce fats, oils and grease as part of their cooking processes are required to prevent those byproducts from entering the public sewer system. 

Knowing what a grease trap is and how it works isn’t necessary for all industries. Smaller premises like offices, businesses with minimal grease output or domestic kitchens may not require a grease trap.

If you are unsure whether you need a grease trap, consult your local authorities or check out our helpful guide. 

When you’re asking the question, “What is a grease trap?” it’s important to understand the different types of grease trap available. Manual grease traps, also known as passive grease traps, rely entirely on the natural separation of FOG from wastewater. They require regular manual cleaning and maintenance to remove built-up fats, oils and grease, making them ideal for lower-volume kitchens.

Automatic grease traps use mechanical components to separate and remove FOG from wastewater efficiently. These systems often feature self-cleaning capabilities, reducing manual intervention and maintenance frequency, making them a great choice for high-volume commercial kitchens needing continuous grease management.

Gravity grease traps are large-capacity systems designed to intercept significant volumes of FOG. They utilise gravity to separate fats and grease over time, making them suitable for industrial kitchens or facilities with extensive wastewater flow. They are often located undergroundand outside the premises and can be emptied by a vacuum tanker.

Grease trap costs will likely differ depending on the size of the unit, its design and the overall complexity of the installation itself. More compact models for smaller kitchens are typically less expensive, whereas larger systems built for commercial use or external installation come at a higher cost.

As well as the initial purchase, the costs of regular maintenance and cleaning should be factored into your budget to maintain efficient operation.

The waste collected inside a grease trap is known as brown grease, a mixture of fats, oils, grease and food solids. These waste by-products are typically pumped out during servicing, then transported by licensed waste contractors to approved processing facilities for safe treatment and disposal.

It cannot be discharged directly into drains, as this would lead to blockages, environmental damage and potential legal issues for businesses.

Cleaning frequency will all depend on the type of business and how much grease is produced, but for most commercial grease traps, it is standard practice to clean them a minimum of every 1–3 months to prevent blockages and maintain compliance. 

These timeframes should be considered as general guidelines, and actual cleaning intervals may vary depending on usage, trap size and local regulations. Our experts have compiled this helpful step-by-step guide on grease trap cleaning.

Grease traps and grease interceptors both remove FOG from wastewater, but they will differ in size, installation and practical usage. Grease traps are smaller units installed under sinks, suitable for low- to medium-volume kitchens, while grease interceptors are larger systems, often installed underground, designed for high-volume commercial kitchens and food processing facilities. 

Read more about the difference between grease traps and interceptors in our full guide, or see quick comparisons below:

This comparison helps businesses choose the right system based on kitchen size, grease output and available space.

Here at Aqua Mundus, our experts have been specialising in grease management for almost a quarter of a century. We have the knowledge and experience to ensure you always get the right grease trap for your business type and kitchen size.

To get an affordable and robust solution for your commercial catering operation, get in touch with us today right here online, on 01386 832205 or via emailus@aquamundus.co.uk.