Grease Traps 101
The City of Abilene has experienced a large number of sanitary sewer overflows (SSOs) for the past few years. An SSO results from a clog or blockage in the sewer line. When the line is clogged, the sewage must go somewhere and so it overflows to the next available outlet-a sink, manhole, dishwasher, etc. Most of the SSOs have been due to Fats, Oils, and Grease (FOG) in the sewer. In the photo at right, grease from a restaurant has overflowed outside. It can not only kill the grass and harm the environment, but kill a restaurant's budget.
- What Is a Grease Trap
- Why They Are Necessary
- Good Kitchen Practices
- Pros & Cons of Small Grease Traps
A grease trap is a device that separates grease and solids from wastewater before it enters the sanitary sewer. Grease traps are installed as part of the plumbing at food service facilities such as restaurants and cafeterias.
There are two types of grease traps:
- Small grease traps (50 gallons or less) are much smaller and are often located indoors under the three-compartment sink.
- Large grease traps (more than 50 gallons) are usually located outside and typically have manhole type lids.
They both operate essentially the same way. Dirty, greasy water from kitchen sinks, dishwashers, etc. goes into the grease trap along with any food scraps that get washed down the drain. The mixture separates into three layers as it cools. Food scraps and solids sink to the bottom, grease floats to the top, and water ends up in the middle. As the grease layer and solids layer build-up, the water goes through a series of baffles and eventually to the outlet of the trap.
Some facilities also have a holding tank to recycle used frying oil, but this is not a grease trap since it’s not connected to the sewer system.
Without a grease trap, grease and food scraps go directly into the private sewer line, which then connects to the public sanitary sewer system. Over time, grease coats the inside of these pipes and forms a clog. When a sewer line becomes clogged, it's an expensive cleanout for a restaurant and the City of Abilene!
In order for a grease trap to function properly, it must be cleaned regularly. When the grease layer inside the trap gets too thick, grease will start to leave the trap with the water since there's no more room for it to remain in the trap. When the grease leaves the trap, it coats the pipes and eventually forms a clog, which will lead to a backup or overflow of sewage.
Likewise, if the solids are not cleaned out regularly, they will also contribute to potential problems. Food scraps and other solids will eventually begin to decompose and give off a gas called hydrogen sulfide or H2S. The H2S forms sulfuric acid when it mixes with the water. Eventually, the acid will eat a hole in the pipes or even the grease trap itself. Keeping the trap clean can prevent overflow and costly repairs.
Cool It. Can It. Trash It! applies to businesses as well as residents. One way to keep pump out frequencies to a minimum is to reduce the amount of grease and food scraps going into the trap in the first place.
- Make sure your employees are not pouring oil or grease down the drain (including floor drains).
- Scrape food scraps into the trash.
- Wipe greasy pans and dishes with a paper towel before putting them in the sink or dishwasher.
- Use a sink strainer in the drain to catch smaller bits of food.
By following good kitchen practices and maintaining your grease trap, you can avoid costly repairs and clean up from a backup or overflow.
Small grease traps are often favored because they have little upfront cost and take up less room than the large grease traps. In some situations, this may be the only option. However, many facilities are simply looking at the initial cost and don’t realize that a smaller grease trap is going to require more time and maintenance in order for it to function efficiently. Small grease traps are usually less effective than large traps for one or more of the following reasons:
- The flow control device is not present. This device restricts the flow of water leaving the trap so that the grease has time to cool and separate. However, by restricting the flow, the drain time from the sink is also slowed down, which is inconvenient in a busy restaurant. Without a flow control device, the grease is simply flushed through the trap along with the water.
- It is not cleaned often enough. A smaller grease trap fills up faster than a larger one and will, therefore, need to be cleaned more frequently. Odor also becomes a problem since most small grease traps are located indoors.
- It is too small to remove the grease generated by the facility. While a small grease trap may work well for a small, quiet coffee shop, it may be completely overloaded by a busy greasy food place. The greasy food place would have to spend quite a bit of time and effort to keep a small grease trap working efficiently.
Because of these issues, many cities are phasing out small grease traps.