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How Grease Traps/Interceptors Work

Learn how and why grease traps or interceptors work to keep your plumbing functioning properly


In case you haven't heard, it's a really bad idea to pour fats, oils, or grease (FOG) down the drain. The problems that can result are usually gross and expensive - and not just for you. FOG can not only clog sewer lines (a great way to make friends in the neighborhood), it can create problems at water treatment facilities, as well. Just as with your arteries, care must be taken to stay healthy and avoid blockages: you can’t just pour the stuff down your gullet, or the drain!

Zurn grease trap

At home, it's easy enough to dispose of FOG properly: just pour it into an empty jar or can and put it in the trash (you should dry-wipe pots and pans, too). But what about restaurants, cafeterias, and similar larger-scale establishments? The FOG that these kitchens need to dispose of is far greater in volume and frequency, requiring a specialized device to ensure pipes and the things they connect to remain functional and safe. It's called a grease interceptor (or "grease trap"), and it's one of the more important features of a commercial kitchen.

Interceptors come in smaller versions designed for indoor connection to individual sinks and other fixtures (usually with a total flow less than 50-100 gpm), and larger ones that are installed outdoors and underground, to service entire establishments. Both the "trap" and "interceptor" labels are used interchangeably for these different types, but you'll sometimes see the smaller specifically referred to as a trap and the larger as an interceptor.

In the most basic terms, a grease trap works by slowing down the flow of warm/hot greasy water and allowing it to cool. As the water cools, the grease and oil in the water separate out and float to the top of the trap. The cooler water - minus the grease - continues to flow down the pipe to the sewer. There are two primary methods that grease traps/interceptors use to separate out the FOG, each of which is suited to certain applications.

Called "hydromechanical grease interceptors", the smaller, indoor types utilize internal baffles to lengthen the path of flow, providing more time and space for separation. Vented flow control devices regulate the amount of wastewater entering the device, and mix air in with it to efficiently separate FOG from solids and water. The FOG that solidifies at the top of the tank, as well as the relatively heavy food solids that fall to the bottom, need to be cleaned out often, usually by employees of the establishment.

The larger "passive" or "gravity" interceptors rely more on time and gravity to separate FOG, which is about 80% lighter than water. Larger tank sizes, internal baffling, and standpipes increase the "retention time" on these passive units, allowing gravity to work its magic. These interceptors are usually cleaned out on a routine basis by professionals.

When possible, interceptors should be PDI (Plumbing & Drainage Institute) certified to ensure the best quality and performance. Traditionally only applicable to hydromechanical units, some new gravity interceptors are made to this standard.


Sizing a Grease Trap or Interceptor

The most important consideration when it comes to an interceptor is the size, which is tied to the rating for maximum allowable flow in gallons per minute. Hydromechanical interceptors can usually handle up to a 100 gpm flow; anything beyond that is best handled by a gravity interceptor. But just how do you determine that flow and the proper size? There are a few ways: sizing according to the waste pipe's diameter, sizing according to the total volume of draining fixtures, or sizing according to Drain Fixture Unit values (DFU), which are defined for specific fixtures by the Uniform Plumbing Code.

Some manufacturers have charts available listing the GPM flow based on pipe diameter (at the standard 1/4" slope) for their grease traps, making this method relatively simple provided you know which type of interceptor you need and the size of your waste pipes. Sizing according to DFU values is similarly straightforward: just add up the DFU values for the fixtures draining into the interceptor.

Determining the size based on the capacity and flow of the actual fixtures being used is of course more involved, and requires some basic math:

  1. You'll need to measure the fixture's length, width, and height to determine the fixture's capacity in cubic inches (LxWxH).
  2. Next, convert that number to gallons by dividing it by 231 (1 gallon = 231 cubic inches).
  3. Because the fixture will likely never be filled to the brim before draining, a good average load is 75% of the fixture's capacity, so you'll need to multiply the total gallons by 0.75.
  4. Finally, you'll need to determine the flow rate by filling the fixture up three-quarters and timing how long it takes to drain completely. Dividing the result from Step 3 by the time it takes to drain will give you the gallons per minute.

Adding these values up from all the fixtures to be connected will give you a ballpark interceptor size. You'll want to consult with the manufacturer or a professional - as well as the authority having jurisdiction - to be sure you have the correct interceptor for your needs. Too small, and the unit will allow FOG to pass through freely unless it's cleaned constantly. Too large, and you run the risk of creating sulfuric acid within the tank, which can damage things downstream. You'll also be paying more for maintenance and cleaning than may be necessary. Beyond size, there are also establishment-specific issues: for example, vegetable oils require a longer retention time to separate than animal fats.

Additionally, be aware that according to the Uniform Plumbing Code, dishwashers are not to be connected to an interceptor unless specifically required by the authority having jurisdiction, due to the detergents and other chemicals used in them which can emulsify FOG, rendering the interceptor useless. Garbage disposals are permitted to discharge directly into drainage systems. Be sure to check with your local code authority before sizing and purchasing.

Whatever the type of interceptor you install, you'll want it as close as possible to the fixtures being served to minimize the risk of problems in the pipe leading to it, and to maintain efficient separation. Keep in mind the maintenance people, or the employees that will be cleaning it out: make sure there's enough room to easily open covers and perform maintenance.

FOG is an important component of some of our favorite foods, but can quickly turn dangerous. Be it your building’s plumbing or your own, protecting against coagulation is vital. There’s nothing as easy and efficient as a grease interceptor for our arteries, but you can at least do right by your community by managing your FOG output responsibly. The proper interceptor, installed correctly, will not only keep you on the right side of the law, but also help you avoid the disruptive and potentially catastrophic plumbing issues caused by FOG in the sewer line. Size appropriately, choose wisely, and rest easy!


Need a new grease trap or interceptor? We've got a great selection.



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Frequently Asked Questions

Q. "Why does my grease trap back up and overflow? If I buy a new one from you will it backup and clog less?"
A. Grease traps back up either because they need to be cleaned or because of an outlet blockage. Buying a large unit would mean you'll need to clean it less often as there is more storage capacity of grease. But please understand that all grease traps will clog if you don't clean the grease regularly.

Q. "Does a grease trap prevent blockages and stoppages?"
A. Grease traps do help prevent stoppages after the trap but blockages can (and do) occur as a result of insufficient (not frequent enough) grease trap line maintenance. A grease trap or drain line which is not periodically maintained (you must take the grease out of the trap) will eventually clog.

Q. "My local governmental code wants to know how long the water stays in the grease trap?"
A. The way some (many operate differently) health/safety/engineering departments figure the grease trap size needed is that they wish for you to keep the water in the trap for a certain period (length) of time. What that means is: If you have a 3 gallon per minute drain flow, and a 30 gallon grease trap, some agencies will consider the 3 gallons as having a 10 minute "hold" time. You calculate this by dividing the gpm by the gallon volume of the trap.

Not all governmental bodies operate the same way and so, please, contact them before you order a grease trap. Make certain that the size that you are ordering is the size that they will accept. Note that all grease traps have a maximum flow rate. Also note that for sanitation reasons, (all brands) grease traps are sealed and not open to the elements. Some brands of grease traps are difficult to open (not ours) but, in all cases, all grease traps are sealed. Again, generally (not always), the size of the grease trap versus the gallons per minute of your drainage is what determines the size that you are required to have. Please be certain of the size that is required by your local code or governmental regulatory agency prior to ordering. Shipping is very expensive for this product, and we do not accept returns on grease traps.

When in doubt, do not order these until you are certain of what you need. The codes are designed to try to take the most grease out of the water. If you under size your grease trap, some of the grease could pass through and also you might need to pump out (take the grease out of) the trap quite frequently.

Q. "How long should the holding time of the wastewater in the grease trap be?"
A. All of the grease traps that we offer come with a flow control device, so in most situations you shouldn't need to be concerned with that issue. The flow control limits the volume/speed, thereby generating the holding time that is required of the wastewater sitting inside of the greasetraps.

Q. "Yesterday I had our grease trap pumped clean and today the trap overflowed. Why did this happen"
A. Hopefully you had it pumped before it was stopped up. Pumping the grease trap alone does not prevent stoppages down the line. While pumping grease, grease can get into your main line if the pumping is done wrong. Also, if you had it pumped because you had a stoppage and so assumed it was due to the grease trap, it might be that you had a partial stoppage already in your main line.



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