Characterization of Grease Interceptors for Removing Fat, Oil and Grease (FOG) and Mechanisms of FOG Deposit Formation in Sewer Systems

Significant amounts of fat, oil and grease (FOG) in wastewater are discharged from Food Service Establishments (FSEs), multifamily housing, and single family homes. FOG must be separated from wastewater before it enters the sewage system, primarily due to its propensity to cause blockages in sanitary sewer collection lines. These FOG blockages, in the property owner’s sewer lateral or the town/city sewerage system lead to sanitary sewer overflows (SSOs) that cause untreated sewage to flow onto streets and travel to storm drains, creeks, and other surface waters. Of the estimated tens of thousands of sanitary sewer overflows (SSOs) that occur each year in the United States, approximately 48% are due to line blockages, of which 47% are related to FOG deposits that constrict the cross-sectional access of pipe. The presence of FOG in wastewater also results to significant problems in conventional biological treatment systems.;Reduction in the levels of FOG is thus highly desirable. Hence, grease interceptors (GIs) are installed between wastewater effluent points and the sewer system to allow FOG to be trapped. A potential approach for reducing the levels of FOG in the GI effluent is bioaugmentation. However, there have been very few long-term assessments of the physicochemical and microbial characteristics of full-scale grease interceptors (GIs). In the first phase of this study, Full-scale GIs were monitored over a year with and without bioaugmentation (treated and untreated cycles). Statistically significant differences between treated and untreated cycles were detected for several chemical and physical parameters. The treated cycles had lower BOD and COD at the grease interceptor outlet. While the combined treated cycle data did not show lower FOG concentrations in the GI outlet compared to the combined untreated cycle data, comparison of individual treated and untreated cycles show a positive effect due to the addition of product. Differences in the microbial community structure between treated and untreated cycles were detected by Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis and the clone and sequencing results. The shifts in the microbial composition were revealed. Taken together, the data shows that the addition of biological products results in changes in the GI chemistry and microbial ecology; these changes had no adverse effects, and in some cases positive effects, on COD, BOD, and FOG degradation in grease interceptors.;Despite the central role that FOG deposits play in SSOs, little is known about the mechanisms of FOG deposit formation in sanitary sewers. In the second phase of this study, FOG deposits were first formed under laboratory condition from the reaction between free fatty acids and calcium chloride. The FTIR data revealed that the FOG deposits (both laboratory-produced and natural FOG deposits) are metallic salts of fatty acids by comparisons with FOG deposits and pure calcium soaps. We demonstrated that calcium, the dominant metal in FOG deposits, was found to be released from concrete under different pH conditions. Oil played the role as a carrier of free fatty acids instead of the source of FFAs in surface reaction that is responsible for FOG deposit formation in sewer lines. The effect of different fatty acids on surface reaction was determined. The data indicated that stickier solid was formed on concrete surface and more severe concrete corrosion occurred when using unsaturated fatty acids than saturated fatty acids. Taken together, a comprehensive understanding of the mechanisms of FOG deposit formation in sewer systems was proposed.;By applying the knowledge of the proposed mechanisms, a pilot-scale pipe loop system was set-up to simulate gravity flow pipelines and pump-station wet wells, and to directly assess FOG deposit formation in sewer lines under controlled conditions.