|IASMIN, MAHBUBA - North Carolina State University
|DUCOSTE, JOEL - North Carolina State University
Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2015
Publication Date: 11/10/2015
Citation: Iasmin, M., Dean, L.L., Ducoste, J.J. 2015. Quantifying fat, oil, and grease deposit formation kinetics. Water Research. 88:786-795.
Interpretive Summary: Sanitary sewage blockages are often the result of fats and oils used for cooking being disposed of improperly using sinks in household and restaurant kitchens. Once in the sewers, these materials combine with calcium from the concrete conduits to form insoluble residues due to saponification. This report describes the kinetics of the saponification process using three models. The best predictor of the formation of the blockages where different amounts of calcium were present and where the acidity of the water was different was one where the breakdown of the fat was controlled by the more alkaline conditions. This showed that solid fats were reduced at a slower rate than liquid oils and less able to be saponified and form blockages. From this modeling, prediction of blockage formation could be followed using software programs by municipal sewer systems.
Technical Abstract: Fat, oil, and grease (FOG) deposits formed in sanitary sewers are calcium-based saponified solids that are responsible for a significant number of nationwide sanitary sewer overflows (SSOs) across United States. 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 levels (7 ± 0.5, 10 ± 0.5, and at 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. This mechanistic saponification model, with its ability to track the saponified solids chemical precursors, may provide an initial framework to predict the spatial formation of FOG deposits in municipal sewers using system wide sewer collection modeling software.