What is a grease trap? A grease trap is a plumbing device designed to capture fats, oils and grease (commonly referred to as FOG) and food solids before they enter the wastewater system. It is most commonly used in commercial kitchens where cooking, washing and food preparation generate greasy wastewater.
Grease traps are also known as grease interceptors, grease recovery devices or FOG traps, depending on the design and region. Regardless of the name, the purpose remains the same: to slow down wastewater so grease can separate and be collected instead of flowing into pipes and municipal sewer systems.
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Why Grease Traps Exist
The problem with fats, oils and grease in drains
When hot oils and fats are washed down sinks, they may appear harmless at first. As the wastewater cools, these substances thicken or solidify and begin to cling to pipe walls. Over time, this build-up restricts water flow and can lead to blocked drains, sewer overflows and costly plumbing failures.
Beyond individual buildings, grease build-up places strain on municipal sewer systems and wastewater treatment facilities. Excessive grease can damage infrastructure, disrupt treatment processes and increase the risk of environmental contamination if untreated wastewater is released.
Legal and operational requirements
Because grease is a major contributor to drain and sewer blockages, many municipalities require food service establishments to install and maintain grease traps. Health inspectors and local authorities may also require proof of regular cleaning and correct sizing. Even where enforcement is less strict, grease control remains essential for uninterrupted kitchen operations.
How a Grease Trap Works
Density separation explained
Grease traps rely on a simple physical principle. Fats, oils and grease are lighter than water, while food solids are heavier. When wastewater slows down, grease floats to the top, solids sink to the bottom, and relatively clean water remains in the middle.
What happens inside the trap
When wastewater enters the grease trap, its speed is reduced. This allows the mixture to cool and separate naturally. Grease rises to the surface and forms a layer, solids settle as sludge at the base, and the water between these layers exits the trap through an outlet pipe positioned to avoid disturbing the grease layer.
The role of baffles and flow control
Most grease traps include internal baffles that reduce turbulence and prevent grease from escaping with the outgoing water. These internal walls help direct the flow through the trap and maintain separation. In some systems, a flow control device is also used to prevent sudden surges that could force grease through the unit before separation occurs.
Where Grease Traps Are Installed
Inside the kitchen at point of use
Smaller grease traps are often installed directly under sinks or near dishwashers. These point-of-use units capture grease close to its source and are easier to access for frequent cleaning. Because of their limited capacity, they require more regular servicing.
Outside the building
Larger grease interceptors are commonly installed outdoors, either above ground or buried underground with access covers. These systems are designed for higher flow volumes and longer retention times. Outdoor placement also makes pump-out and servicing easier for larger commercial kitchens.
Types of Grease Traps and Interceptors
Passive hydromechanical grease traps
Passive hydromechanical grease traps rely on gravity and internal baffles to separate grease from water. They have no moving parts and must be cleaned manually. These units are commonly used in smaller kitchens due to their compact size and lower initial cost.
Gravity grease interceptors
Gravity grease interceptors are large tanks, often installed underground, that provide extended retention time. They are suitable for high-volume kitchens and facilities with multiple fixtures discharging wastewater. These systems typically hold more grease and solids, reducing the frequency of servicing compared to smaller units.
Automatic grease removal units
Automatic grease traps separate grease in the same way as passive systems but remove it mechanically on a scheduled basis. These units heat the grease and skim it into a collection container, reducing manual cleaning. While convenient, they still require monitoring and routine maintenance.
What Goes Into a Grease Trap (and What Shouldn’t)
Grease traps are designed to handle wastewater that contains small amounts of food residue and grease from normal washing activities. They are not intended to process large food scraps or solid waste. Plates and cookware should be scraped into bins before washing to prevent excessive solids from entering the system.
Pouring liquid grease directly down the drain should also be avoided. Even with a grease trap installed, excessive grease can overwhelm the system and reduce its effectiveness.
Cleaning and Maintenance
Why regular cleaning matters
A grease trap only functions properly while it has enough space to store separated grease and settled solids. As these layers build up, the available volume for wastewater decreases. Once the trap becomes too full, grease can bypass the system, flow into the sewer, or back up into the kitchen.
Neglected grease traps can cause odours, blockages, overflows and failed inspections. Routine cleaning is essential for reliable performance.
How often a grease trap should be cleaned
Cleaning frequency depends on trap size, kitchen activity and the volume of grease produced. Smaller units may need servicing every few weeks, while larger interceptors may operate on a longer schedule. A commonly used guideline is to clean the trap once the combined grease and solids reach around 25% of the unit’s capacity.
Regular inspections help determine the correct servicing interval and prevent unexpected problems.
What happens to collected grease
The waste removed from grease traps is often referred to as brown grease. This mixture of grease, water and decomposing food solids must be disposed of through approved waste management processes. Unlike clean used cooking oil, brown grease is generally not suitable for recycling without specialised treatment.
Common Grease Trap Problems
Unpleasant odours
Strong smells usually indicate decomposing food waste, excessive grease build-up or a damaged lid seal. Odours are often an early warning sign that cleaning or repairs are overdue.
Slow drains and recurring blockages
Slow drainage can occur when a grease trap is full, incorrectly sized or subjected to high flow surges. Persistent blockages may point to underlying grease management issues within the kitchen.
Overflows and backups
Overflows typically result from missed maintenance, poor sizing or improper use of sinks and drains. These incidents can disrupt operations, damage floors and equipment, and lead to compliance issues.
Choosing the Right Grease Trap
Selecting the right grease trap involves more than choosing a convenient size. Factors such as flow rate, number of fixtures, kitchen layout and operating hours all influence performance. Grease traps are available in a range of materials, including plastic, stainless steel, fibreglass and concrete, each suited to different environments.
Smaller cafés may operate successfully with point-of-use units, provided cleaning is frequent and consistent. High-volume kitchens, food courts, hotels and institutions generally require larger interceptors designed for heavy use.
What Is a Grease Trap? The Practical Takeaway
A grease trap is a simple but essential piece of infrastructure for any commercial kitchen. By slowing wastewater and allowing grease to separate, it protects plumbing systems, reduces the risk of blockages and supports compliance with local regulations. Its effectiveness depends on correct sizing, proper installation and consistent maintenance aligned with actual kitchen usage.
