Final Report: An Integrated Approach to Understanding and Reducing Fat, Oil, and Grease (FOG) Deposit Formation for Sustainable Sewer Collection Systems

EPA Grant Number: R834264
Title: An Integrated Approach to Understanding and Reducing Fat, Oil, and Grease (FOG) Deposit Formation for Sustainable Sewer Collection Systems
Investigators: Ducoste, Joel
Institution: North Carolina State University
EPA Project Officer: Page, Angela
Project Period: August 1, 2009 through July 31, 2012 (Extended to July 31, 2014)
Project Amount: $569,568
RFA: Innovative and Integrative Approaches for Advancing Public Health Protection Through Water Infrastructure Sustainability (2008) RFA Text |  Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , Sustainability , Water

Objective:

The objectives of this study are as follows: 1) perform detailed bench scale tests that attempt to recreate FOG deposits and determine parameters that significantly influence their formation rate, 2) develop a numerical model that describes the FOG deposit formation kinetics, 3) perform bench scale tests to explore treatment options that improve FOG deposit chemical precursor removal with grease interceptors, 4) perform pilot scale experiments on a continuous flow sewer collection system that characterizes spatial variations in FOG deposit formation, and 5) develop a sewer collection system model to predict the high FOG deposit accumulation zones in sewer pipes.

Summary/Accomplishments (Outputs/Outcomes):

The results of this study have clearly shown that FOG deposits are calcium based fatty acid salts produced from a saponification reaction. Evidence of this reaction pathway for FOG deposits were provided by comparing samples of FOG deposits from the sewer collection systems at different locations to calcium based fatty acid salts produced with pure fats and FOG deposits created in the lab using grease interceptor effluent wastewater. Fourier Transform Infrared (FTIR) analyses showed that the bonds associated with the formation of calcium based saponified solids were present in field based FOG deposits, lab-based FOG deposits, and calcium based fatty acid salts created with pure oils and fats.
 
In this study, calcium, the dominant metal in FOG deposits, was found to be released at different rates from concrete under different pH conditions. Study results showed that calcium release from concrete structures could be a major pathway for the formation of FOG deposits in sewer systems and may explain why previous research did not find a correlation between water hardness and calcium content in FOG deposits. FOG was not only the source of free fatty acids when hydrolyzed, it also played a significant role as a transporter of free fatty acids in interface reactions that are responsible for FOG deposit formation in sewer lines. Lab tests showed that the formation of FOG deposits on the surface of concrete coupon blocks were not possible unless a residual amount of oil was also present at the air/water or water/concrete block interface.
 
The results of this study also showed that calcium content in the calcium based salts was a function of the solubility limit of the calcium source and was influenced by pH and temperature conditions. Calcium chloride, having the highest solubility among the three calcium sources studied, produced a soft, gel-like soap mass. Calcium hydroxide and calcium sulfate, on the other hand, displayed a granular texture. These semi-solid masses may harden and gain significant strength with time leading to maintenance challenges for their removal from the sewer pipe wall. Calcium based fatty acid salts displayed a certain level of stability indicating their limited flow properties when adhered to the sewer wall. The rheology of FOG deposits when compared to the calcium salts also validated results from previous studies and indicated that FOG deposit matrix could be layered combinations of debris and calcium based fatty acid salts with several calcium sources.
 
The effect of different fatty acids on surface reaction was also determined. The data indicated that stickier solids were formed on concrete surface and more severe concrete corrosion occurred when using unsaturated fatty acids than saturated fatty acids. However, prior research and research in the current study have shown that the predominant fatty acid in FOG deposits was saturated, primarily palmitic. Moreover, when lab based FOG deposits were produced using pure fats or oils, the results showed that the saponified solids displayed the same fatty acid profiles as the source fat or oil. The results of these lab based tests suggest that the large fraction of palmitic in FOG deposits is likely due to an alternative fate and transport of unsaturated free fatty acids (FFA) within the sewer collection system. The presence of FFAs in food service establishments (FSE) effluents, produced due to high temperature cooking and/or other fat hydrolysis processes, will lead to increased quantity and higher rates of FOG deposit formation. The results suggest that the high temperature operations in FSEs in cooking and/or sanitizing applications can have significant influence in the conversion of the unsaturated fats to saturated fats. The higher temperature conditions in grease interceptors may be a partial explanation for the increased level of palmitic acid in FOG deposits created from grease interceptor wastewater and the occurrence of this high palmitic fraction in field FOG deposit samples.
 
The pure detergent sources (i.e., non-anionic and anionic) increased the production of saponified solids, but did not display a significant change in the fatty acid profiles. However, the commercially available detergent used in household applications not only increased the production of saponified solids but also displayed significantly high fractions of palmitic in these solids. It’s possible that external chemical agents added to the surfactants that make-up the commercial detergents in households and/or FSEs dishwashing applications may need to be carefully investigated and/or modified to reduce FOG deposit formation potential in sewer systems. Lipase driven hydrolysis was also shown to enhance the production of FOG deposits. While selective removal of free fatty acids may be a strategy to reduce the FOG deposit formation, FSEs can play a significant role in the reduction of their formation by focusing on some key issues without compromising restaurant quality, public health, and sanitation: a) optimum usage of detergents; and b) use of a heat-exchange chamber prior to kitchen wastewater discharge into GI to significantly reduce the water temperature below 45 °C.
 
In the current study, the kinetics of lab-based saponified solids were determined to understand the kinetics of FOG deposit formation in sewers for two types of fat (Canola and Beef Tallow) and two types of calcium sources (calcium chloride and calcium sulfate) under three pH (7±0.5, 10±0.5, and 14) and two temperature conditions (22±0.5 and 45±0.5 °C). The results of this study displayed quick reactions of a fraction of fats with calcium ions to form calcium based saponified solids. Results further showed that increased palmitic fatty acid content in source fats, the magnitude of the pH, and temperature significantly affect the FOG deposit formation and saponification rates. The experimental data of the kinetics were compared with two empirical models: a) Cotte saponification model and b) Foubert crystallization model and a mass-action based mechanistic model that included alkali driven hydrolysis of triglycerides. Results showed that the mass action based mechanistic model was able to predict changes in the rate of formation of saponified solids under the different experimental conditions compared to both empirical models. The mass-action based saponification model also revealed that the hydrolysis of beef tallow was slower compared to liquid Canola fat resulting in smaller quantities of saponified solids.
 
Experimental tests were performed with metal coagulants and polymers to determine if these chemical additives could aid in the gravimetric separation of FOG in grease interceptors. Results showed that a significant amount of metal coagulant was needed to reach charge neutralization of the grease interceptor waste stream and was not practical from an enhanced separation performance strategy. The addition of cationic polymer was able to show some improved FOG separation performance. However, the polymer performance was dependent on the pH of the FSE waste stream. Tests with sudden change in pH from 9 to 4 were performed to simulate grease interceptor conditions when the influent had a high pH condition from the addition of detergents or surfactants for cleaning and the internal grease interceptor was acidic due to microbial activity. The results showed that the no polymer addition was significantly better than the addition of polymer under these test conditions. Overall, the results of utilizing polymer as a potential chemical additive to improve the FOG separation performance of grease interceptors met with mixed results and suggest that this strategy to enhance FOG separation would likely not work in full scale grease interceptors.
 
Pilot scale sewer collection system tests were performed to explore how different hydraulic structures (straight pipe and manholes), sewer pipe deformation (i.e., pipe sag), and roots intrusion impact the spatial deposition of FOG saponified solids. Results showed that FOG deposits preferentially accumulated on root surfaces and in pipe sag region. In addition, locations where the pipes had increased surface roughness, an additional amount of FOG deposits were visually observed. Results showed that while both pH conditions produced solid accumulation, the higher pH condition produced visually significant more solids. FTIR-ATR analysis on the solid samples confirmed that the solids were indeed calcium-based fatty acid salts. In addition, the results further showed that the accumulated calcium concentrations inside the solids were similar to the FOG deposits collected from full-scale sewer lines. However, the fatty acid profiles of the solids from the pilot system were similar to the fatty acid profile of the FOG released in the system and deviated from the profile found from FOG deposits from sewer systems, which were primarily saturated fatty acids. These results confirm the bench-scale work and suggest an alternative fate of unsaturated fatty acids that does not lead to their incorporation in FOG deposits in full-scale sewer systems.
 
A sewer collection system transport model that included the kinetics of FOG deposit formation was developed in this study. The model incorporated the mechanistic saponification reaction equations developed from the bench-scale tests. Two full scale sewer collection systems were simulated using Matlab Simulink software and CITYDRAIN 2.03 sewer collection system transport model. The model results showed that with parameter calibration, it could predict around 60 percent of the current high FOG deposit accumulation zones reported in these two collection systems. Deviations between the predicted and reported high FOG deposit accumulation zones was likely due to the model not incorporating specific hydraulic impacting features such as pipe sags, root intrusions, debris accumulation, and pipe roughness. In addition, the model’s prediction of high FOG deposit accumulation zones was sensitive to local flow and FOG loading variations. This data was not available for FSEs that were simulated and will likely impact the model’s prediction of these high accumulation zones. Further, no data was available on the amount of hydrolyzed FOG being released from FSE waste streams, which will impact the initial FFA concentration from these waste streams. The current model assumed an initial FFA concentration of zero. The model was able to demonstrate the impact of changing the location of FSEs on the predicted high FOG deposit accumulation zones and provided evidence that a sewer collection system transport model that incorporated FOG deposit formation kinetics could help pretreatment coordinators assess the impact of changing urban landscape on sewer collection system maintenance.

Overall, the results of this research study provided a comprehensive understanding of the mechanisms of FOG deposit formation in sewer systems. These insoluble hard solids hold tightly to the sewer pipe wall due to their rheological properties and contain a significant amount of calcium and primarily saturated long chain fatty acids. The process of saponification that these FOG deposits undergo is quite complex with many factors that can impact the rate of formation, their chemical constituents, and their rheological behavior. Ongoing research is needed to understand links between corrosion kinetics in sewer systems and FOG deposit formation, the mechanisms FOG hydrolysis and its rate beyond alkali hydrolysis in grease interceptors, FOG and FFA removal technologies that are non-intrusive in grease interceptors, and exploring more efficient ways to reduce temperature in grease interceptors.


Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 22 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Dominic CCS, Szakasits M, Dean LO, Ducoste JJ. Understanding the spatial formation and accumulation of fats, oils and grease deposits in the sewer collection system. Water Science & Technology 2013;68(8):1830-1836. R834264 (2012)
R834264 (2013)
R834264 (Final)
  • Abstract from PubMed
  • Full-text: ResearchGate-Full Text PDF
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  • Abstract: IWA Publishing
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  • Journal Article He X, Iasmin M, Dean LO, Lappi SE, Ducoste JJ, de los Reyes III FL. Evidence for fat, oil, and grease (FOG) deposit formation mechanisms in sewer lines. Environmental Science & Technology 2011;45(10):4385-4391. R834264 (2012)
    R834264 (2013)
    R834264 (Final)
  • Abstract from PubMed
  • Full-text: ES&T-Full Text PDF
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  • Abstract: ES&T-Abstract
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  • Other: ResearchGate-Full Text PDF
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  • Journal Article He X, de los Reyes III FL, Leming ML, Dean LO, Lappi SE, Ducoste JJ. Mechanisms of fat, oil and grease (FOG) deposit formation in sewer lines. Water Research 2013;47(13):4451-4459. R834264 (2012)
    R834264 (2013)
    R834264 (Final)
  • Abstract from PubMed
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  • Abstract: ScienceDirect-Abstract
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  • Other: ScienceDirect-Full Text PDF
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  • Journal Article Iasmin M, Dean LO, Lappi SE, Ducoste JJ. Factors that influence properties of FOG deposits and their formation in sewer collection systems. Water Research 2014;49:92-102. R834264 (2012)
    R834264 (2013)
    R834264 (Final)
  • Abstract from PubMed
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  • Abstract: ScienceDirect-Abstract
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  • Other: ScienceDirect-Full Text PDF
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  • Journal Article Iasmin M, Dean LO, Ducoste JJ. Quantifying fat, oil, and grease deposit formation kinetics. Water Research 2016; 88:786-795. R834264 (Final)
  • Abstract from PubMed
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  • Abstract: ScienceDirect-Abstract
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  • Other: ScienceDirect-Full Text-Full Text PDF
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