Dealing With FOG in Onsite Wastewater Systems

FOG in excessive amounts interferes with aerobic biological processes and leads to decreased treatment efficiency

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FOG (fats, oils and grease) is a constituent of sewage, typically originating from food stuffs (animal fats or vegetable oils) or consisting of compounds of alcohol or glycerol with fatty acids (soaps and lotions), usually measured in mg/L.

Sources of FOG
Fat found in onsite wastewater treatment systems is animal fat, oil from vegetable and cooking oils, and grease from petroleum-based soaps. FOG is generally treated in onsite wastewater treatment systems by separating them from the wastewater stream. At high temperatures, FOG is in a liquid state, but as the temperature cools, the fats component will solidify (see table below). FOG can be trapped in pretreatment components, such as septic tanks and grease traps, where it typically floats to the top of tanks. FOG is less dense and lighter than water.

It is important to try to contain FOG early in the system because it can accumulate inside pipes and lead to clogging of downstream components. FOG also contributes to BOD5 and TSS concentrations. FOG in excessive amounts interferes with aerobic biological processes and leads to decreased treatment efficiency. The expected levels of FOG concentration must be considered during wastewater treatment design. 

FOG in domestic wastewater will generally originate in the kitchen or bathroom. Kitchen FOG usually comes from disposing of animal- or vegetable-based food scraps and liquids down the sink. Households using garbage disposals will have 30 to 40 percent more FOG than households not using garbage disposals. Bath oils, suntan lotions, hair conditioners and moisturizing creams are bathroom sources of FOG that enter the wastewater stream. An increased use in cooking oils, lotions and hair conditioners will directly increase the FOG concentration in the wastewater.

Low FOG, although not considered a problem, could be the result of not using the kitchen or of higher than normal flows entering the system. Low FOG can also be attributed to the use of bar soap instead of liquid soaps.

Impact of FOG on systems
Fat -
Animal fat is relatively easy to hold in a tank because it’s quite sensitive to temperature. It becomes a solid at 80 degrees F, and wastewater temperature is usually cooler than that. Animal fat will break down in the soil, but it takes four times more energy to break down than the organic matter typically measured by BOD5. Fat is added to the system from cooking, cleanup and dishwashing, so commercial systems will typically have higher levels of fat than residential systems. If a system is supplied with a lot of animal fat, it will typically stay in the septic tank. If it is contained in the septic tank, it may not be observed in FOG measurements in downstream components.

Oils - Vegetable oil is not as sensitive to temperature as fat and can pass through the system. Oil can also be broken down through a biological process, but it takes 12 times more energy to break down oil than the organic matter typically measured by BOD5. There are many different types of oils used, but vegetable oil is the most common. Vegetable oil is often used in liquid form, but it can also be solid shortening. The liquid form is harder to hold in a tank.

The table above lists several different types of fats and oils that are commonly used and lists their physical properties. The ability of the oil to separate is influenced not only by temperature, but also by how the oil was generated and used. Free oil will rise to the wastewater surface and be easily separated when the mixture is allowed to become quiescent. Emulsified oil has been broken up into very small droplets and occurs either by mechanical or chemical action. An example of mechanical emulsification is when extremely hot water from a dishwasher is mixed with the oil. Given time and a decrease in temperature, this oil can be separated. Chemical emulsification occurs when detergents or cleaners produce a mix of oil and water. Degreasing compounds can generate dissolved oils, in which discrete oil particles are no longer present. Chemically emulsified oil will take a longer time to separate, increasing the risk of carrying it to downstream components unless long quiescent periods are available to allow separation.

Grease - Grease is petroleum-based and can be toxic to a system. Because grease is petroleum-based, it cannot be broken down, but it can be separated. Grease comes from lotions, hair products and soaps. Typically, there will be a higher percentage of grease in the FOG from residential systems when compared to most commercial systems. Grease can build up over time, coating components and inhibiting treatment of other constituents in the wastewater.

Dealing with FOG

  1. Evaluate the facility to determine the sources of FOG.
  2. Sample the effluent, whenever possible, within 18 hours of known peak usage from a pump tank (ideal) or septic tank effluent that is not in need of maintenance.
  3. Work with the owner to reduce the levels if possible.
  4. Install an external grease interceptor if possible with sufficient retention time (one to four days) in addition to septic tank capacity. This will need frequent maintenance to work properly.
  5. Design a system that can handle the measured or anticipated levels.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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