Quantifying fat, oil, and grease deposits formation kinetics in sewer collection systems

Fat, oil and grease deposits in sewer systems are known as calcium based fatty acid salts that are responsible for a significant number of sanitary sewer overflows (SSOs) throughout the United States. Since 2007, clean water act (CWA) requires publicly owned treatment works (POTWs) to invest more than 15 billion dollars in new pipes and infrastructure to eliminate the overflows. However, little research has been performed to identify the potential key factors and quantify their effects that could lead to FOG deposit formation even in new pipes, such as types of fats used in restaurants, types of calcium sources, pH, temperature, and mixing conditions inside sewer systems. The objective of this study was to determine the kinetics of FOG deposits and exploring effects of the factors previously described. Laboratory scale saponification experiments were performed and kinetic data was collected under nearstoichiometric conditions. The results displayed a significant influence of pH and temperature on kinetics of calcium-based fatty acid salts since they influence the solubility of calcium sources in water. The kinetic data also suggested a fraction of fast soap formation either due to presence of some initial free fatty acid (FFA) in source fats and/or high intensity mixing condition of the constituents. The kinetic data was compared with two empirical models: a) saponification and b) crystallization. The results showed that both empirical models performed equally well. However, both models required modifications to the empirical constants based on changes to experimental conditions. Therefore, a mechanistic model was prepared that is expected to better elucidate the chemistry of the FOG deposit formation process. Future research includes investigation of more potential factors, such as use of detergent types in restaurants, presence of initial FFA and effect of microbial activities during the formation process.