Elucidation of the Physical Separation and Accumulation Mechanisms of Three Distinct Layers in Grease Traps

Grease traps are essential for intercepting fats, oils, and grease (FOG) to prevent sewer blockages. However, the internal mechanisms governing their separation efficiency and layer accumulation remain poorly defined. This work investigates the dynamics of three distinct internal phases inside grease traps, i.e., top-floated scum, middle-clarified liquid, and bottom-settled sludge, through a 32-day site investigation (SI) and 24 laboratory column tests (CTs). The SI revealed that the settled sludge layer was 10 times thicker at the inlet chamber and 72 times thicker at the outlet chamber than the floated scum, indicating that sludge accumulation is the primary driver of desludging frequency and a key indirect influence on grease trap performance. CT results further demonstrated that separation kinetics were governed by solids content, with low-solid wastewater achieving settling velocities of up to 3.04 m h^-1, significantly higher than those of high-solid wastewater (0.87 m h^-1). These differences highlight the solid content in the grease trap governing the separation rates. Based on these findings, this work proposes optimized design criteria, including a surface overflow rate (SOR) of 20.9 m³ d^-1 m^-2, a minimum surface area (A_s) of 1.25 m², and a 1-h hydraulic retention time (HRT). Ultimately, these results provide a systematic assessment framework to refine grease-trap design standards based on the physical behavior of all three internal waste layers.