What is FOG?
FOG refers to Fats, Oils, and Greases (FOG), even though both oils and grease are fats. Oils are liquid at room temperature, whereas greases are solid. But we can consider these all as fatty wastes. Food Service Establishments (FSEs) and food processing facilities, especially meat and dairy producers, are responsible for most FOG waste. However, oils and greases enter the sewer system from households as well. For instance, every time you rinse your dinner dishes or wash a greasy cooking pan in your sink, some fats wash down the drain and into the sewer system.
FOG Causes Problems in Sewer Systems
Unfortunately, FOG can cause big problems in the sewer system. For example, fats and oils coat the inside of sewers and reduce their capacity. Additionally, fatty wastes can combine with trash, such as “flushable” wipes and diapers, to form giant fatbergs that block sewage lines. Consequently, if there is a sudden increase in wastewater flow into the sewer, untreated sewage can be released into the environment. These sewage releases are called “overflows,” and they are dangerous to both human and ecological health. Specifically, the EPA estimates there are 23,000-75,0001 overflow events per year and nearly half of these are likely caused by FOG2.
Keeping FOG out of Sewers
FOG disposal is strictly regulated so that it cannot harm the sewer system or cause overflows. Ideally, producers capture these fatty wastes at the source before they can go down the drain. FSEs and other FOG producers capture two types of fatty wastes on-site. First, used cooking oil, also called “yellow grease,” is deposited into drums on-site. Second, grease traps are installed in sinks, basins, and dishwashers to remove any fats or oils washed into the drain. This “brown grease” is removed from the grease traps and also collected on-site for disposal.
Recycling and Disposal of Yellow and Brown Grease
Yellow grease is valuable because it can be sold to processors who use this for animal feed, biodiesel production, or the manufacture of soaps, cosmetics, and detergents. However, unlike yellow grease, brown grease doesn’t have much value. Instead, grease haulers collect the stored brown grease and transport it to either a landfill or a Wastewater Treatment Plant (WWTP) for disposal. Generally, landfilling of brown grease isn’t the best option because it is expensive and causes landfills to reach capacity faster. Subsequently, brown grease is usually brought to WWTPs for disposal3.
Options for Disposal of Brown Grease at WWTPs
Haulers deliver brown grease to a WWTP, where it is pumped through screens to remove any trash or debris. This waste is then added to an anaerobic digester, added to the wastewater treatment train, or dewatered and dried as solids. However, most WWTPs do not have anaerobic digesters and it is fairly energy-intensive to dewater and dry fatty wastes. WWTPs that dry and dewater brown grease ultimately handle a greater volume of solids.
Brown Grease in the Wastewater Treatment Train
Adding brown grease to the wastewater treatment train increases the overall costs and complexity of operation. For instance, fatty wastes do not dissolve in the wastewater but instead float on the surface. WWTPs skim FOG material off the surface of treatment tanks as “scum”. Additionally, most WWTPs use aerobic activated sludge tanks to treat organics in the wastewater. Activated sludge relies on microorganisms to break down organic material using oxygen which is provided from aeration. This means any extra organics, such as FOG, in the wastewater require more aeration. Extra organic material also increases the volume of activated sludge waste that is produced during treatment.
Treating FOG at WWTPs Increases Aeration Costs
The microorganisms in activated sludge require intense aeration of the wastewater to break down organics. In fact, aeration is the largest energy consumer in wastewater treatment at about 40-60% of total treatment costs. Biological Oxygen Demand (BOD) measures the amount of oxygen needed by microorganisms to break down organics in the wastewater. The higher the BOD of waste, the more aeration that is needed for treatment. Brown grease has an exceptionally high Biological Oxygen Demand (BOD) ranging from 7,000-10,000 BTUs/pound (4.5-6.5 kWh/kg)4.
Treating FOG at WWTPs May Require Bioaugmentation
Microorganisms in either activated sludge or an anaerobic digester have difficulty breaking down the organics in FOG. For this reason, some WWTPs need to use bioaugmentation to help the process. Bioaugmentation means adding special enzymes or microorganisms, including bacteria, yeast, and fungi, to help break down the FOG5-7. If enzymes or microorganisms are used, they need to be purchased and applied every time FOG is added to the treatment process as they do not persist in the treatment process.
Treating FOG at WWTPs Produces More Solids
Treating FOG waste at the WWTP increases the volume of solids produced and, therefore, the solids handling costs. In addition, many WWTPs may not have the capacity needed to dispose of FOG in the wastewater treatment process. For example, the 2021 Infrastructure Report Card by the American Society for Civil Engineers examined the more than 16,000 publicly owned WWTPs operating in the U.S. The report found, on average, treatment plants were functioning at 81% of design capacity, and 15% have already exceeded their design capacity8.
FOG can Cause Process Upsets in Anaerobic Digesters
WWTPs that have anaerobic digesters can use brown grease for co-digestion with municipal sludge to produce methane-rich biogas. However, there are only about 1,200 digesters operating at WWTPs in the U.S. which is less than 10% of all facilities9. In addition, brown grease has a high free fatty acid content, making it difficult to treat in an anaerobic digester. Co-digestion of brown grease can upset biogas production or even halt the entire process. For this reason, WWTPs can only provide 10-30% of the total waste fed to an anaerobic digester as brown grease10.
FOG Co-Digestion is a Waste of Energy
Adding FOG to a digester usually increases the production of methane. However, this improved methane production can vary widely from a 13-198% increase. Regardless, making more methane is only useful if the methane is used to recover energy. A little more than half of WWTPs with digesters recover energy from the biogas, mainly by burning it for heating purposes. Significantly, only about one-third of these facilities using biogas for energy generate electricity9.
Biodiesel from Brown Grease is More Sustainable
Perhaps a simple solution to treating brown grease at the WWTP is to make more valuable products than biogas. Although brown grease is not considered to have much value compared to yellow grease, it is still about 78% lipids11. Furthermore, the high free fatty acid content of brown grease isn’t a problem for REA’s patented technology which converts 75% of brown grease to biodiesel. Biodiesel has undoubtedly more energy than biogas. For instance, the biogas yield from FOG ranges from 20-25 million BTUs per ton (21-27 MJ per kg)10. In contrast, biodiesel has an energy density ranging from 32-35 million BTUs per ton (37-40 MJ per kg)11.
There is Enormous Energy Production Potential from Brown Grease
The U.S. Department of Energy estimates that nearly 1.7 million tons (over 7 billion kg) of FOG is available annually as brown grease12. Currently, virtually all of this waste is either incinerated or landfilled. However, brown grease waste could be converted to over 1 billion gallons of biodiesel (over 4 billion liters) annually which equals over 140 trillion BTUs (~150 billion MJs) of energy. For perspective, municipal WWTPs consume more than 30 Terawatt hours (over 100 billion MJs) of electricity every year13. While this is a staggering number, it represents less than the energy potential present in brown grease.
Do We Need to Shift our Thinking?
WWTPs can potentially change from net energy consumers to energy neutral, or to even net energy producers, by rethinking how waste FOG is treated. REA’s technology can divert and capture FOG in the sewage influent, combining it with hauled-in brown grease, to convert this waste material into B100 biodiesel in an automated and continuous-flow manner. Municipalities can utilize it to peak shave their energy consumption from the grid, or to eventually eliminate this consumption altogether. They can also utilize it to fuel their fleets and reduce respective carbon emissions by up to 74%. Biodiesel not consumed can be sold to local or regional fuel distributors for a considerable profit. Over the past 10 years, B100 biodiesel has been consistently priced higher than other alternative fuels.
1U.S. Environmental Protection Agency, Sanitary Sewer Overflows: National Pollutant Discharge Elimination System, November 2015
2U.S. Environmental Protection Agency, “Report to Congress on Impacts and Control of Combined Sewer Overflows and Sanitary Sewer Overflows,” EPA-833-R-04-001, August 2004
3U.S. Environmental Protection Agency, Waste Diversion, https://www.epa.gov/greeningepa/waste-diversion-epa
4Long, J.H., et al. (2012) “Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations,” Process Safety and Environmental Protection, 90: 231-245
5Monera Technologies Corporation, “How to Reduce Fat, Oil and Grease (FOG) In Drains, Plumbing and Sewer/Collection System Using Bioaugmentation,” https://www.moneratec.com/how-to-reduce-fog/
6NextTM Filtration Technologies, Inc., “Next F.O.G. STOP; Wastewater Benefits,” https://www.nextfiltration.com/wastewater-treatment/
7EnviroZyme®, “EnviroZyme® Eliminates Foam in Municipal Wastewater Trial,” https://www.envirozyme.com/resources/blog/envirozyme-eliminates-foam-in-municipal-wastewater-trial
8American Society for Civil Engineers, “Infrastructure Report Card,” 2021, https://infrastructurereportcard.org/
9U.S. Environmental Protection Agency, Anaerobic Digestion, https://www.epa.gov/anaerobic-digestion/types-anaerobic-digesters#WRRFdigesters
10Bhatt, A.H. and Tao, L. (2020) “Economic Perspectives of Biogas Production via Anaerobic Digestion,” Bioengineering, 7:74. https://www.mdpi.com/2306-5354/7/3/74/htm
11U.S. Department of Energy, Energy Efficiency & Renewable Energy, Alternative Fuels Data Center, https://afdc.energy.gov/fuels/biodiesel_basics.html
12U.S. Department of Energy, Bioenergy Technology Office, “Biofuels and Bioproducts from Wet and Gaseous Waste Streams: Challenges and Opportunities,” Washington, DC, USA, 2017. https://www.energy.gov/eere/bioenergy/downloads/biofuels-and-bioproducts-wet-and-gaseous-waste-streams-challenges-and
13U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, “Energy Data Management Manual for the Wastewater Treatment Sector,” DOE/EE-1700, December 2017
14U.S. Department of Energy, Energy Efficiency & Renewable Energy, Alternative Fuels Data Center, “Biodiesel Vehicle Emissions,” https://afdc.energy.gov/vehicles/diesels_emissions.html