Grease Interceptor Selection

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Grease Interceptor

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  • Grease Oil & Sediment Separation as per usage
  • Intercept greases and solids before they enter a wastewater disposal system

Grease Interceptor Selection

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intercept most greases and solids before they enter a wastewater disposal system.

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Grease Interceptor

Grease causes several problems when large amounts of it are discharged into a sanitary sewer system. Grease solidifies when it cools down, and this may block pipes and cause a sewage overflow. Exposure to sewage is a serious threat for public health, and repairing an overflow can be very expensive and difficult.

Grease interceptors are used to trap fat, oil and grease (FOG), preventing their discharge into the local sewer system. Since a sewage overflow  is very disruptive and health-threatening, local authorities mandate the use of grease interceptors in all applications where the wastewater contains FOG.

In terms of function, a grease interceptor and a grease trap are essentially the same device. However, the term “grease trap” is normally used for small units that operate at flow rates below 50 gpm and low pressure. On the other hand, the term “grease interceptor” is normally used for large units that handle a higher pressure and flow. The following table summarizes the main differences between grease traps and grease interceptors:

Grease Trap Grease Interceptor
Typical flow rate below 50 gpm Typical flow rate over 50 gpm
Low pressure High pressure
Installed inside kitchens, generally under sinks Installed outside, due to their large size
Require regular cleaning, typically once per month Typically cleaned every three months
More frequent cleaning may be necessary with heavy usage  


All types of FOG can block sewers, but issues are more likely when dealing with grease of animal origin. For this reason, grease interceptors are critical in any facility where animal products are processed.

How Does a Grease Interceptor Work?

The 2018 International Plumbing Code defines a grease interceptor as “a plumbing appurtenance that reduces non-petroleum fats, oils and greases in effluent by separation of mass and volume reduction.” Grease interceptors were invented in the 1880s, and they have experienced little change since then. Their basic operating principle is the following:

  • Grease floats on water because its density is 10 to 15 percent lower, and the two substances do not mix.
  • Fat, oil and grease (FOG) accumulate on top of the water inside a grease interceptor, and heavier particles fall to the bottom.
  • As food particles fall to the bottom of a grease interceptor, they form sludge.
  • Water is discharged from the middle of the interceptor, with a reduced content of FOG and food particles.

As a rule of thumb, grease interceptors are cleaned when they fill with grease to 25 percent of their total capacity. At this point, they can no longer separate water and grease effectively. However, some models are designed to accumulate a higher FOG percentage. Grease interceptors should be cleaned away from business hours, since the procedure releases unpleasant odors.

Grease that is suitable for recycling is called yellow grease. Used cooking oil is an example, since it has only been in contact with fresh food. Yellow grease has industrial applications, and it can also be processed into biofuel to generate heat and electricity. On the other hand, when grease has been in contact with rotten food particles or other types of waste, it is referred to as brown grease - this includes the FOG removed from a grease interceptor. Brown grease cannot be recycled, and it is generally sent to landfills or incinerated.

Cooking oil should be recycled whenever possible, since it can no longer be used when it reaches the grease interceptor. Recycling also reduces the accumulation rate of FOG inside the grease interceptor, allowing less frequent cleaning. Consider that a grease interceptor is designed for residual FOG; other forms of waste should be scraped off and discarded directly, instead of washing them down the drain.

Types of Grease Interceptors

Depending on their size and operating features, grease interceptors can be classified into several types:

  • Passive hydromechanical grease interceptors (HGI)
  • Gravity grease interceptors (GGI)
  • Automatic grease recovery systems
  • High capacity HGIs

Passive HGIs are essentially grease traps, designed for small-scale applications. They are normally located under sinks or embedded in the floor. HGIs trap grease passively, as their name implies, and they require frequent cleaning due to their small capacity.

HGIs are the most affordable type of grease interceptor, due to their simplicity and small size. However, their small capacity also means they need frequent cleaning. A kitchen can become non-compliant with local codes if its HGIs are not cleaned with enough frequency.

Gravity grease interceptors (GGI) are large units, with a typical liquid capacity of over 500 gallons. However, cleaning is normally needed when they are 25% filled with grease. GGIs are made of steel, fiberglass, plastic or concrete.

Since GGIs are large and buried underground, their replacement is expensive and heavy equipment is required. Typically, a GGI must be replaced after 15 years of use, or otherwise it can degrade and leak. This is a serious environmental and health hazard, which must not be allowed to occur.

Thanks to their large capacity, GGIs need less frequent cleaning than HGIs, typically every 90 days. Consider that cleaning a GGI requires special pumping equipment, due to the large volume involved.

Automatic grease recovery systems are also known as automatic grease removal devices. These interceptors operate just like a normal HGI, but they have an automatic mechanism to remove the accumulated grease, isolating it in a container. Thanks to this feature, an automatic grease recovery system has simpler maintenance needs than a conventional HGI. The automatic cleaning mechanism can be programmed to operate at regular intervals, based on the grease output of kitchen appliances and other sources.

In terms of space requirements and installation procedure, an automatic grease removal device is very similar to an HGI - it can be installed in a corner or under a sink. The upfront cost is higher due to the grease removal mechanism, but maintenance becomes much simpler.

Compared with a normal grease trap, an automatic unit is subject to additional performance standards. A normal HGI is only subject to a minimum grease removal efficiency, but an automatic unit must meet grease skimming requirements that are also standardized.

High capacity HGIs are normally used in applications with a high grease output, and where there is no space for a gravity grease interceptor. High capacity HGIs have gained popularity in recent years, and they are a common option for restaurants in commercial spaces that were previously used for other purposes. In these cases, there is rarely enough space for a gravity grease interceptor.

A high capacity HGI is normally made of plastic or fiberglass, and it is subject to standards just like a small grease trap. However, high capacity HGIs can accumulate a higher percentage of their volume as grease. While most grease interceptors require cleaning when they are 25% full, high capacity units can accumulate over 70% of their volume in grease. This means they require less frequent cleaning than normal units of the same size.

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Upgrading a Grease Interceptor

The first grease traps were made of concrete, since the material can be easily poured into various shapes and sizes. However, even when concrete is one of the main structural materials in modern construction, it is not suitable for grease interceptors:

  • When exposed to the acidic conditions of a grease trap, concrete degrades and fails more quickly. Grease traps made from other materials may last over 15 years, while a concrete trap will normally need a replacement in 5 to 10 years.
  • Concrete grease traps can also experience corrosion, and they are vulnerable to cracking when they are moved or serviced.
  • Due to its porosity, the surface of concrete is a breeding ground for harmful bacteria. These bacteria produce hydrogen sulfide, the substance responsible for the “rotten egg” smell. Hydrogen sulfide also becomes life-threatening as its concentration increases.

Modern grease interceptors are normally made of steel, fiberglass or plastic, since these materials eliminate the limitations of concrete. The material is an important factor when selecting a grease interceptor, since it affects cost, weight, installation difficulty and service life. Steel is more durable than concrete when used in grease interceptors, although it also degrades with acidic conditions. On the other hand, fiberglass resists corrosion, but it can crack with sudden movements. Plastic grease interceptors are typically the most durable, since they resists corrosion and cracking, while being lightweight and easier to move.

Sizing a grease interceptor correctly is very important for effective performance. Undersized and oversized units both have performance issues:

  • An undersized grease interceptor will reach its accumulation limit faster, and it will require frequent cleaning to prevent an overflow. As a consequence, the unit will have a higher maintenance cost.
  • On the other hand, an oversized grease interceptor represents a waste of capital. The unit also takes longer to reach its accumulation limit, which gives more time for grease and particles to disintegrate and reach the sewers.

Upgrading a grease interceptor can be a challenging project: the associated cost is high, and the smell released can cause plenty of disruption and discomfort. Selecting a suitable upgrade and careful planning are very important. By purchasing a grease interceptor with a long service life, you can forget about another replacement in a very long time. Some units are rated for over 20 years.

When selecting a grease interceptor, consider the total ownership cost instead of the upfront cost. Purchasing a cheap unit will be more expensive over time if it requires frequent cleaning and a replacement within a few years. Cleaning a grease interceptor is an unpleasant task, not to mention replacing the unit completely. Professional plumbing engineers and master plumbers will recommend a long-lasting unit that is easy to service, which is simpler and less expensive in the long run.

Using Grease Interceptors: General Recommendations

Ideally, a grease interceptor should be installed as close as possible to the fixture it serves. As the piping distance increases between FOG sources and a grease interceptor, maintenance issues become more likely. A grease interceptor should be installed in an accessible area that simplifies cleaning and maintenance. The cover should be free of obstructions, providing enough clearance for an easy removal.

Consider that a grease interceptor is designed for FOG - fat, oil and grease. This means that solid waste from sources like food grinders, dishwashers and toilets should not be directed to the grease interceptor.

Like with any other installation, a grease interceptor should be installed by a licensed plumbing contractor. An incorrect installation can lead to serious issues, and hiring contracts without licenses is against the law.

Adequate maintenance is important for all building systems, but it becomes critical in the case of grease interceptors. Coagulated grease is responsible for many sewage overflows, which are difficult to clean and very disruptive for the surrounding area. Poorly maintained grease traps also accumulate hydrogen sulfide, which produces an unpleasant smell in low amounts, and becomes deadly at higher concentrations. Grease interceptors should ideally have circular walls, since bacteria tend to accumulate in sharp corners.

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