What is FOG (fats, oils, and grease) in plumbing?

FOG is any greasy material that leaves a kitchen with the dishwater. The fats come from animal sources like bacon grease, hamburger drippings, butter, and chicken skin. The oils come from frying oil, olive and vegetable oil, and oil-based dressings. The grease is the mix of the two that coats pans, plates, and cookware. Dairy products, gravies, sauces, mayonnaise, and batter all add to the load, and so do the bits of food rinsed off a plate or pushed through a disposal.

In a commercial kitchen, FOG enters the plumbing at almost every wet station. The main sources are the three-compartment sink where pots and pans are washed, the prep sinks, the dishwasher, the mop sink, and the floor drains that catch spills and washdown water. A busy restaurant can send a large volume of FOG toward the sewer every single day, which is why food service operations are the focus of local grease rules.

A home kitchen produces the same material, just less of it. Pan drippings, the oil from a deep fryer, bacon grease poured down the sink, and the greasy film rinsed off dishes all go into the drain. Hot tap water and dish soap make the grease look like it has washed away, but that is temporary. The grease simply moves down the line before it cools and settles.

It also helps to know what FOG is not. It is not the same as the soap scum or hair that clogs a bathroom drain, and it is not a chemical that dissolves on its own once it is in the pipe. FOG is a physical material that changes from liquid to solid based on temperature, which is exactly why it is so hard to flush out once it has set. Understanding that simple behavior is the key to keeping it out of a sewer line.

The trouble with FOG is a change of state. It enters the drain warm and liquid, then loses heat as it travels through the pipe. As it cools, it congeals and hardens, and the sticky layer clings to the inside walls of the sewer pipe. Each new pour adds another coat. The pipe's inside diameter shrinks like an artery filling with plaque, water flow slows, and eventually the line blocks. When a sewer main blocks, wastewater has nowhere to go and forces its way out the nearest low opening. That event is a sanitary sewer overflow, or SSO, and it can put raw sewage into homes, streets, yards, and waterways.

The scale of the problem is well documented. In its national review of overflow causes, the U.S. Environmental Protection Agency found that grease was the single most common cause of reported sewer blockages, at 47 percent, ahead of tree roots and ordinary debris. The same body of EPA research notes how often these overflows happen across the country. As the agency states, "There are approximately 23,000 to 75,000 SSO events per year in the United States." Blockages are the largest single cause of those events, and grease is the largest single cause of blockages.

This matters in a dry climate like Phoenix even more than the national average suggests. EPA's review found that nearly three-quarters of overflow events in the arid Southwest were caused by blockages, not by storms or pipe failures. A clogged grease-filled line is the kind of failure a kitchen can prevent, which makes FOG control a practical, controllable risk rather than an act of nature.

The damage is not limited to the public sewer. FOG builds up first in a building's own drain lines, where it traps food solids and creates the slow drains, foul odors, and recurring backups that close a kitchen. A single backup during service can shut a restaurant down, fail a health inspection, and cost far more than routine maintenance ever would.

The hardened grease also makes problems harder to fix once they start. A clean pipe can sometimes be cleared with a cable, but a line coated in solid grease often needs high-pressure water jetting to cut the buildup off the walls and restore the full diameter. The grease can also bond with other waste in the line, including soap residue and small food particles, forming a dense mass that resists a simple snake. The further FOG travels before it sets, the more pipe it can foul, which is why catching it early at the source is the only reliable approach.

The defense against FOG is a device that catches it before it reaches the sewer. A grease interceptor, also called a grease trap, sits between the kitchen drains and the building's sewer connection. Wastewater flows in and slows down inside the tank. Because grease is lighter than water, the FOG floats to the top, food solids sink to the bottom, and the cleaner water in the middle passes out to the sewer. The trapped grease and solids stay behind in the tank.

There are two common types. A hydromechanical grease interceptor is a smaller unit sized by flow rate in gallons per minute, often installed under or near a sink. A gravity grease interceptor is a large in-ground tank sized in gallons, used by higher-volume kitchens. The right type and size depend on the equipment connected to it and the volume of water that passes through. For the specifics of matching a unit to a kitchen, see our guide on what size grease interceptor a restaurant needs.

An interceptor only works while it has room to hold grease. Once it fills past a certain point, FOG passes straight through to the sewer as if no trap were installed. The widely used standard is the 25 percent rule: clean the unit when the combined fats and solids reach 25 percent of its capacity. The City of Portland's grease program puts firm minimums on top of that, requiring hydromechanical units to be cleaned at least every 30 days and gravity units at least every 90 days, with cleaning records kept for three years. Local intervals vary by city, so a kitchen should confirm its own rule. For how to set a schedule, see how often to pump a grease trap.

A common mistake is to treat the interceptor as a set-and-forget device. It is a holding tank, not a disposal, so everything it catches has to be pumped out and hauled away by a licensed service. Skipping a cleaning, or sizing the unit too small for the kitchen's flow, lets grease escape downstream and undoes the whole purpose of the equipment. Additives sold as grease "dissolvers" do not solve this either, because breaking grease into smaller pieces can simply move it past the trap and let it re-form in the public sewer.

For a food service business, FOG control comes down to two duties. The first is to keep FOG out of the drain in the first place. That means scraping plates and pans into the trash, wiping greasy cookware before washing, collecting used fryer oil in a rendering barrel for recycling, and using strainers on sink and floor drains to catch solids. Pouring or rinsing grease down a drain is what overloads the system. The second duty is to maintain the grease interceptor: pump it on schedule, keep it sized correctly for the kitchen, and hold onto cleaning records, because most local programs require them and inspectors ask to see them. State environmental agencies such as the Texas Commission on Environmental Quality run formal FOG programs for exactly this reason, since hardened grease in a public line is a regional pollution problem, not just one building's clog.

For a homeowner, there is no interceptor and no inspector, but the chemistry is identical. The simplest rule is to let grease cool, then throw it in the trash rather than rinse it down the sink. Wipe greasy pans with a paper towel before washing, scrape food scraps into the bin, and avoid relying on hot water to "melt" grease away, because it only moves the problem downstream. A garbage disposal grinds food but does nothing to stop grease, so the same cautions apply. For a fuller list of what belongs in the trash instead of the drain, see what not to put down the drain.

Whether the kitchen is a restaurant or a house, the goal is the same: stop fats, oils, and grease from entering the drain warm, because once they cool inside the pipe, they are the leading cause of the blockages and backups that EPA data ties to nearly half of all reported sewer clogs. Keeping FOG out of the line is far cheaper and cleaner than digging out a hardened blockage after the fact.