December 2022 - Let’s Chew the Fat: A Brief Discussion on Sustainable FATS, OILS, and GREASE (FOG) Management

By Pedro DaSilva

Source: Calaveras County (CA) Water District

Fats, Oils, and Grease (FOG) are a common problem encountered in wastewater sanitary systems and typically come from a variety of foods, such as cooking oil, butter, fats in meat scraps, and processed food products such as mayonnaise and salad dressings.  FOG is commonly observed as the gelatinous substance floating at the top of grease traps and cleaned out from sanitary sewers. Although rancid and unsightly, FOG can be turned into biofuels to reduce energy costs or processed into biomaterials; however, if left unmanaged or disposed of improperly, it can cause serious damage to piping, sewer systems, and the environment. This article provides an overview of FOG and its properties and describes some best practices for FOG management.

When FOG enters the sewer system, it can congeal and harden to create deposits that can block piping and cause sewer overflows. In a 2004 report, the U.S. Environmental Protection Agency (EPA) estimated that 47% of sanitary and combined sewer overflows were caused by grease from industrial, domestic, and commercial sources. Controlling FOG in sewers can also reduce maintenance and cleanout costs for sewer utilities and wastewater treatment plants. Successful FOG management programs, such as the European Union’s RecOil and California’s CalFOG programs, have drastically reduced the amount of sewer blockages and supported reduction of greenhouse gas (GHG) emissions by utilizing grease waste to produce biofuels.

FOG Properties and Characterization

FOG is a blend of a variety of lipids, fatty acids, water, and other solids and impurities. FOG generated from food service establishments (FSEs) is generally comprised of lipids from vegetable oils and animal fats, whereas industrial sources can generate FOG derived from petroleum products. Using a combination of analytical methods, total FOG and total industrial FOG can be easily quantified. The difference between the two provides an approximation of the FOG from food sources.

FOG can additionally be characterized by measurement of its Dry Solids and Moisture Content, Free Fatty Acid (FFA) content, and Fatty Acid Profile. High quality FOG, also known as yellow or white grease, such as lightly used cooking oil, is primarily comprised of intact oils and has low solids, moisture, and fatty acid content. Yellow grease is the most desirable type of FOG as it requires little pretreatment to process into fuels or other products. Yellow grease prices in the US as of 2022 are about $0.60/pound. All lower quality FOG, such as grease cleaned out from grease traps or sewers, are typically referred to as brown or black grease. Its value is typically less than yellow grease and can range drastically based on the purification costs necessary to process.

Best Management Practices for FOG

Most FOG management follows the general waste management hierarchy: reduce, reuse, recycle. Ultimately, source reduction, or removing FOG from our cooking processes and shifting to more fat-free diets, would have the largest impact on these issues. Realistically, FOG will likely never be fully removed from diets as it is ingrained in many culinary practices, so it is more practical to emphasize proper disposal and recycling practices that divert FOG from sewers. Many water service authorities operate FOG management programs as part of a source control or pretreatment program, usually under the National Pretreatment Standard (40 CFR 403.5). The EPA’s National Pretreatment Program provides guidance for running these programs and includes the following objectives:

• Prevent the introduction of pollutants into a Privately Owned Treatment Works (POTW) that will interfere with its operation, including interference with its use or disposal of municipal sludge;

• Prevent the introduction of pollutants into a POTW that will pass through the treatment works or otherwise be incompatible with it; and

• Improve opportunities to recycle and reclaim municipal and industrial wastewaters and sludges.

Source: USEPA

Source control programs typically include education and outreach on FOG issues, such as training food service employees on proper FOG disposal, distribution of public information, and performing monitoring and maintenance of equipment and activities to ensure all procedures are being followed. Some recommendations for kitchen staff to reduce FOG introduction to sewers include wiping down pots and pans prior to washing, minimizing the use of detergents, and using cold water to perform dishwashing in the scullery. Source reduction programs have proven to be highly effective at reducing issues caused by FOG and increasing cost-savings from FOG recycling.

These programs also typically manage the operation and maintenance of grease traps or interceptors (GTIs) and other pretreatment devices. GTIs are the most common piece of equipment used to divert grease from sewers. Grease traps are typically devices under 55 gallons installed on FSE wastewater lines underneath sinks and grease interceptors are much larger pieces of equipment that are typically installed outside buildings but placed before the sewer main. GTIs function as a settling basin which allows the FOG to cool, coagulate, and gravity separate into a lipid layer that can be skimmed off. A sludge layer also forms as solid particles settle to the bottom and the middle water layer passes through to the sewer. GTIs can be as simple as a small tank or can have multiple chambers or baffles to aid separation. GTIs are a passive technology and require little to no operation, but the collected grease must be skimmed or pumped out periodically. GTIs are highly effective if maintained properly but require consistent and regular cleanouts. While cleanout frequency may be assessed on a case-by-case basis, many municipalities implement a 25% rule which states that GTIs need to be cleaned out once the combined grease and sludge volumes reach 25% of the device capacity. Depending on the production of the kitchen, this typically translates to monthly, quarterly, or semi-annual cleaning schedules.

Typical Grease Trap Configuration

 Benefits of Recycling FOG

It is much more desirable from a recycling standpoint to collect FOG as yellow grease before it enters a sewer or grease trap. Grease bins placed in kitchens to dispose of used kitchen oil are a great way of collecting yellow grease. Once grease enters the sewer or grease trap, it mixes with water and solids and begins microbial degradation turning into brown grease. Grease collecting companies typically collect all ranges of grease quality and some may also treat and refine their own grease.

Most yellow grease is processed into animal feed or detergents, but the amount of grease processed into biodiesel has been increasing considerably since the early 2000s driven by drastic increases in biofuel demand. Biodiesel or B100 is typically blended with petroleum diesel in 5% (B5) or 20% (B20) blends. Studies have shown that blends of up to B20 have similar performance properties as pure petroleum diesel and more machinery is being produced that can use biodiesel blends as fuel. Biodiesel is a much more sustainable source of fuel since it originates from biomass rather than fossil fuels and any replacement of petroleum diesel with biodiesel reduces GHG emissions that contribute to climate change. Currently, biodiesel is primarily produced from soybean oil, but there are ongoing debates about the feasibility and ethics of growing crops and utilizing a food source to produce energy. The use of yellow grease as a feedstock for biodiesel production has gained popularity due to its low cost and the fact that using a waste that would normally be disposed of bypasses the food-to-energy debate. New developments in the use of yellow grease as a feedstock for bioproducts, such as make-up, biodegradable polymers, and specialty chemicals, are expected to create competition and drive up the value of yellow grease.

Source: U.S. Energy Information Administration, State Energy Data System

The total amount of waste grease generated, as well as demand for biofuels, is expected to increase in the coming years. Currently, only yellow grease is economical to treat and refine into biofuel and bioproducts but increasing demand for biofuels could help facilitate processing of darker greases. Brown grease collected from grease traps or sewer cleanouts are usually treated for toxic contaminants, landfilled, or incinerated. New research is emerging into methods that can process darker greases into biodiesel more cost-effectively. New developments in anaerobic co-digestion also provide a promising sustainable disposal path for darker greases. This process involves the digestion of FOG and other food scraps in existing wastewater digesters and has shown promising results in improving methane production and sludge volume reduction.

Conclusion

Responsible and sustainable FOG management can provide both environmental and economic benefits. An FSE typically generates 35 - 250 pounds of grease per day (or roughly 0.1 pound of grease per meal).  At current prices, if that grease was collected and sold as used cooking oil, it could generate between $7,500 - $55,000 of revenue per year from grease sales. It would also be diverted from the wastewater collection system, saving utilities maintenance costs due to cleanouts and sewer overflows caused by FOG. Finally, if that grease is then processed into biofuel, it would also help decrease GHG emissions to the atmosphere. In the end, proper FOG management benefits us all.

PHE has been managing the implementation of wastewater source control pretreatment programs for federal clients for over 20 years. This includes services, such as ongoing grease trap inspections, development of strategies for reducing FOG build-up in the sewer collection system, coordination, and deployment of training programs to enhance awareness on best practices for FOG management, and FOG investigations. For more information on the services PHE offers with respect to wastewater source control pretreatment or FOG management, please contact Paul DiPaolo at paul.dipaolo@phe.com or 215-208-5434.