October 2013, Vol. 25, No.10

Operator Essentials

What every operator should know about fats, oils, and grease

Rhonda Harris




A practical consideration  

What is FOG? 

FOG, which generally refers to “fats, oils, and grease,” is not a single compound with a single set of distinctive features. It is a family of chemicals with a common link of solubility in hexane, and various other features. 

FOG is 

  congealable — solidifies at room temperature; 

  floatable — specific gravity is less than 1 g/mL; 

  soluble — dissolves in water; and 

  emulsive — forms oil–water emulsions. 

FOG also can be 

  animal or vegetable in origin — usually biodegradable; 

  petroleum or mineral in origin — may carry volatile organics or
   other contaminants; 

  biodegradable — consumed by organisms as food; 

  nonbiodegradable — inert and not consumed as food; and 

  foaming — may be caused by surfactants and surface attractive

Where does FOG come from? 

FOG typically enters the municipal sewers from residential, commercial, and industrial establishments. These dischargers dump their waste FOG down the drain. This is most common for residences, either single-family or multifamily. 


Restaurants and other commercial and industrial establishments typically fall under a pretreatment program for their community and are not allowed to discharge their FOG wastes directly to the municipal sewer. 

Commercial and industrial establishments usually have grease traps or other collection systems, which are then pumped out and disposed of by a contractor. Many municipal or regional water resource recovery facilities accept this waste. This practice, at least, keeps FOG out of the sewers, where it can accumulate, plug pipes, and cause overflows. However, FOG still must be dealt with at the facility. 

When and where does FOG show up? 

FOG will show up in the collection system.  


FOG also will show up at the treatment facility. 


Some utilities accept FOG as a hauled waste at a specific location within the facility. 

There is a direct correlation in many cities between the amount of FOG in the collection system and the number of restaurants in a specific area, even though most restaurants have grease traps. FOG can build up in pipes, reducing the capacity of lines and even causing blockages and overflows, which must be handled on an emergency basis. This incurs extra expense for the utility, as well as the problem of dealing with regulatory agencies regarding the overflow. 


FOG can interfere with the influent pumps and wet wells or enter primary clarifiers as floating scum. 


This can work well if the FOG is used as a booster in the anaerobic digestion system for generating gas production. 


If this is the case, the facility may want to use a dual equalization tank for accepting FOG to allow time for separation, as well as allow for a continuous low rate feed to the digesters. 

Why is FOG a problem for operators?  

FOG can coat equipment and walls in the influent pump station, creating a nuisance that must be removed manually from the wet wells.  


If FOG is petroleum-based, it may contain some levels of benzene, toluene, ethylbenzene, xylene, and/or other volatile organic compounds. 

FOG may have entrapped materials that are toxic to the microorganisms in the biological treatment system. 


In this case, FOG can pose an explosion hazard to the sewer system and lift stations, so this type of FOG should be regulated and/or prohibited. 

How do we as operators deal with FOG? 

Most municipalities have regulations that help minimize FOG in the sewer, but some FOG always will make its way down to the treatment facility.  

It is helpful for the collection system operations staff to keep a map updated with the problem-prone areas for blockages and reduced capacity so that the appropriate authorities can go out to inspect and monitor these areas for “bad actors” — restaurants with overflowing grease traps, industries not disposing of their wastes properly, etc. 


This way, the operators hopefully can minimize the effects of FOG and reduce the number of sewer-plugging and overflow incidents. 


At the treatment facility, operators must minimize the effects by trapping FOG as early as possible and removing it from the process. 

What are some of the operational issues? 

When FOG reaches the treatment facility, if it does not congeal and is biodegradable, the only issues are excess biochemical oxygen demand loading and oxygen demand.  


Emulsified FOG and surfactants raise more questions and require additional investigation before being accepted. 

Operators always should determine FOG’s composition, how it is made, and whether it has any other special properties. 


If FOG is hauled in for introduction to an anaerobic digester, it can produce revenue, but only if the digester is designed to handle the FOG. This dosing must be monitored closely. 

Asking the right questions and being aware of the types of FOG coming into a system and treatment facility enables municipalities and/or utilities to begin to see FOG as an opportunity rather than a problem. The key to success in managing a FOG program is due diligence in this process. The bottom line: “Know your FOG!” 

Rhonda Harris is an “A” Operator, as well as director of consulting and training services, in the CH2M Hill (Englewood, Colo.) Operations Management Business Group in Dallas and Vancouver, British Columbia.