Effect of source and environmental factors on properties and kinetics of FOG deposits in sewer collection systems

Fat, oil, and grease (FOG) deposits are calcium-based saponified solids that can clog sewer systems and cause sanitary sewer overflows (SSOs) that pose a significant threat to public health. USEPA has set a requirement to public water treatment facilities to spend 15 billion dollars on new pipes and wastewater infrastructure to control sewer overflows. Although investments on new pipes are necessary to cope with wastewater produced due to the increasing population size, the fundamental understanding of the formation of FOG deposits and their rate of formation is still unclear. This fundamental understanding is necessary to maintain a sustainable wastewater collection system. Therefore, the objective of this study was to identify and investigate potential source and environmental factors that may affect the formation mechanism of FOG deposits and their kinetics in sewer systems. Lab-scale studies were performed in which calcium-based saponified solids were prepared under alkali driven hydrolysis conditions with different source and environmental conditions. The conditions tested were: 1) types of fats used in restaurants, 2) types of calcium sources in sewer system, and environmental conditions in sewer system, i.e., 3) pH, and 4) temperature. Kinetic data was compared with two empirical models, i.e. a) Cotte saponification model, and b) Foubert crystallization model, and a mass-action based mechanistic model to capture the FOG deposit formation kinetics. The results of this study displayed differences in colors and appearances of saponified solids under different reaction conditions. Canola based saponified solids made with calcium sulfate appeared granular and rough in texture compared to those with calcium chloride. Under alkali driven hydrolysis, the fatty acid compositions of the final saponified solids were very similar to the source fats introduced at the beginning of saponification. The mass-action based mechanistic model was found to better predict the formation of saponified solids and could be an initial framework for incorporation in system wide sewer transport models.