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What is a Grease Trap?

What is a grease trap?

TZ-1826 Operation

Simply put, a grease trap is something kitchen wastewater flows through before entering the sewer waste system. This receptacle — technically defined as a grease interceptor — intercepts, captures, or "traps" grease. How?

Grease, the industry term for animal fats and vegetable oils, is 10 to 15 percent less dense than water. Grease also won't mix with water.

As a result, fats, oils, and grease float on top of water.

When kitchen wastewater flows through a grease interceptor, the grease and oils rise to the surface inside the trap and are trapped using a system of baffles. The captured grease and oils fill the trap from the top down, displacing “clean” water out of the bottom of the trap and into sewer line.

Peer into a grease trap and youll see a mat of grease. When this mat of grease gets deep enough, the trap must be cleaned out.

See how a grease interceptor like Trapzilla works!



Types of grease interceptors

There are four major types of grease interceptors found in most food service establishments:

  • Small passive Hydromechanical Grease Interceptors (HGI)

    Hydromechanical grease interceptors (most often referred to as grease traps) often sit inside the kitchen underneath the sink or in the floor.  They passively trap grease over time and need to be pumped frequently, as often as every week for high producing sites.  While they can be less expensive up front, they do cost more to clean due to the frequency.

    Most HGI's must be certified to a standard such as the ASME A112.14.3 or PDI-G 101 which require interceptors to retain a certain percentage of the grease which passes into the tank.

Little Dipper Grease Trap

  • Gravity Grease Interceptors (GGI)

    Gravity Grease Interceptors are often made out of concrete, but can be made out of steel, fiberglass, and plastic.  These interceptors are often greater than 500 gallons in liquid capacity, but hold a small percentage of their liquid capacity in grease and are not certified to meet any efficiency standards.  This is why cities require they be pumped out once the grease and solids amount to 25% of the contents.  Gravity Grease Interceptors are usually pumped every 90 days and can cost hundreds of dollars in maintenance each year.  GGI's made of concrete can fail as quickly as every 15 years depending upon the quality of the materials.

    Concrete Grease Interceptor

  • Automatic grease/oil recovery systems such as Big Dipper

    Automatic grease removal devices or recovery units offer an alternative to Hydromechanical Grease Interceptors in kitchens.  While their tanks passively intercept grease, they have an automatic or active mechanism for removing the grease from the tank and isolating it in a container.  These interceptors must meet the same efficiency standards as a passive HGI, but must also meet a standard such as the ASME A112.14.4 which proves they are capable of skimming the grease effectively.

    The upfront cost of these units is higher, but maintenance can be handled by the kitchen staff, eliminiating regular pumping charges and saving on operational costs.
    Big Dipper Automatic Grease Interceptor
  • Maximum Retention Hydromechanical Grease Interceptors such as Trapzilla 

    Max retention HGI's have become more popular in recent years as restaurants are opened in non-traditional sites without the space for a Gravity Grease Interceptor.  These interceptors take up less space and hold significantly more grease per their volume than other alternatives, often between 70-85% of their liquid capacity.  Most are made out of plastic while some are manufactured out of fiberglass.

    Like other hydromechanicals, these interceptors must meet efficiency standards and most manufacturers test beyond the minimum standard to the fail point to demonstrate the full capacity of the unit.  
    Trapzilla TZ-1826 Grease Interceptor

Traditional passive grease trap designs date back to 1885 when the first U.S. patent was issued. Today's large and small grease interceptors use the same basic operating design as the 1885 model. While they do capture some grease, they are often inefficient at retaining grease and removing the grease is a task left to the owner.

Small passive traps must be cleaned out by hand. Large pre-cast traps, on the other hand, must be cleaned out by a professionally operated vacuum or pump truck.

Why do I need a grease trap?

While sewer collection systems exist to take waste water to a treatment plant, there are some things is just was not designed to handle.  One of those is grease. 

Pipe Clogged with Grease

Grease, especially grease with animal fats, cools and solidifies at normal temperatures in pipes.  When this happens, blockages can form in the sewer pipes, eventually causing backups in the collection system called sanitary sewer overflows (SSO's).  SSO's are a significant health risk to the public, so it's in everyone's best interest to keep our pipes clear of grease.

For this reason, many cities require the use of grease traps, more technically referred to as grease interceptors at locations that prepare food items to ensure grease does not ultimately cause sanitary sewer overflows.

In some instances, installing a grease interceptor can save the food service establish as well.  Any location with long plumbing runs to the sewer collections system, such as a mall, hospital, or restaurant inside of a large building is in danger of creating blockages in the internal pipes which could lead to backups, fines, and perhaps even downtime as the internal plumbing is repaired.

Even If a restaurant has a traditionally designed passive trap which is not cleaned out on a timely basis, it will begin to allow too much grease into the sewer system, where it can cause blockages and sewage backups. This creates problems for wastewater system operators as well as the food service establishment.

Improperly maintained grease traps (or the failure to install a grease trap when required) often leads to fines, down-time, and can lead to bad publicity.

Need a grease trap, but not sure which one?  Check out Thermaco's new online sizing calculator and proposal request system.

Grease trap efficiency

While traditional grease traps and gravity interceptors have been the standard for more than 100 years, recent updates in this field are having a critical impact on the design and implementation of separator technology.  The size of concrete, steel, or fiberglass gravity grease interceptors makes them difficult and costly to locate, especially in urban environments.  Additionally, their retention efficiencies make them less effective as separators while newer technology allows for greater storage and less stagnant water which can easily turn into hydrogen sulfide.

As grease traps of the traditional design fill with grease, their efficiency at separating grease from wastewater decreases. When a trap is filled to capacity with fats and oil, separation no longer occurs and the trap no longer functions properly.

Traditional concrete traps must be pumped out after just 25 percent of their volume is filled, because after that they no longer work well enough to keep fats and oils out of the sewage systems.

Compact, highly efficient interceptors, such as Thermaco's Trapzilla systems, are engineered so they never lose efficiency. They can hold upwards of 90 percent of their volume in grease before they must be cleaned out.

TZ-1826 Separation Curve


Automatic Grease Interceptor Efficiency

Big Dipper systems work a bit differently than traditional traps. In them, grease is skimmed out automatically on a programmed schedule based on the amount of grease produced.

This automation means employees don't have to measure or check grease levels. The grease in these automatic systems accumulates in a separate chamber and is simply disposed of in a municipally approved waste container.

While more expensive to purchase, automatic grease interceptors give Food Service Establishments control over their grease waste management and save customers from paying service companies on a weekly or monthly basis to clean out the grease interceptor.

Now you know what a grease interceptor is, but are you ready to make a decision?  Click here now to get a FREE RESOURCE on factors to consider when choosing a grease interceptor.

Additional Resources

If you're getting ready to choose a grease interceptor for your next project, take a look at the resources below.

Replacing a grease trap? Here’s a few factors to consider
How to calculate the total cost of ownership of a grease trap
Grease interceptor design: a life-or- death decision
Why concrete interceptors ought to be left in the history books

Feel free to contact Thermaco directly for specific assistance on choosing the ideal grease interceptor for your commercial or institutional foodservice operation.