Transport and Accumulation of Blockage-Related Solids in Urban Sewers: Wet Wipes and FOG (Fat, Oil and Grease)

Abstract

The accumulation of sewer solids significantly reduces the hydraulic capacity of urban sewer networks, ultimately leading to sanitary or combined sewer overflows. Recent studies and media coverage have identified wet wipes and FOG (fat, oil, and grease) deposits as two predominant solids contributing to sewer blockages; however, their transport and accumulation mechanisms remain poorly understood. This thesis aims to address this knowledge gap and propose mitigation strategies for blockages caused by wipes and FOG in sewers. First, the transport of wipes with different densities and sizes was systematically studied in a circular pipe (Chapter 2). Previous studies have primarily focused on quantifying wipe abundance in sewer networks, while their movement processes remain unexplored. The critical shear stress and flow velocity for the incipient motion of wipes were found to increase with increasing wipe density and with decreasing relative wipe size. Non-dimensional equations were developed to characterize the incipient motion using the critical Shields number and the particle Froude number. The mean wipe velocity and mean ambient flow velocity were accurately described by a power relationship. Wipe movement modes were classified based on the ambient cross-sectional average velocity. Next, wipe blockage due to a vertical obstruction was studied (Chapter 3). Blockages resulting from the interactions between instream solids and encountered obstacles have been extensively studied for other rigid materials; however, existing theories may not apply to flexible wipes in sewers. This study employed a vertical rod to simulate sewer obstructions. Stochastic interactions among wipes, turbulent flow, and the obstruction were studied in a circular pipe (diameter D = 25 cm), with systematic variations in flow Froude number (Fr), wipe length (L), flow depth (H), submerged rod length (hrod), and rod diameter (drod). The mean area ratio of wipes (ratio of projected area to original surface area) ranged from 0.14 to 0.30. The entrapment probability P (ratio of entrapped to released wipes) for a single wipe was strongly correlated with Fr, H/D, L/H, drod/H, and hrod/H. As more wipes were released, the influence of the obstruction on wipe accumulation processes became negligible. New equations were developed to characterize entrapment probability, blockage length, and backwater rise, which can be used to predict the development of wipe blockages and sewage levels. Afterwards, the influence of suspended solids on FOG deposit formation was studied (Chapter 4). The presence of suspended solids is a critical feature of wastewater, yet its effects have often been overlooked. This study examined the physicochemical properties of FOG deposits formed from oleic acid and palmitic acid in the presence and absence of suspended solids. Results show that palmitic acid solidified more rapidly and formed larger masses. Saponified FOG, metal silicates (calcium/magnesium silicates), and crushed concrete can trigger deposit formation by providing heterogeneous nuclei, facilitating adsorption, and leaching hydroxide/metal ions, respectively. Glass beads reduced deposit formation by 30% under a velocity gradient of 88.7 s-1 and 4-hour reaction time by forming a physical barrier. The asymmetric vibration of the carboxylate group was a suitable indicator for determining the percent saponification (PS). The saponification degree exhibited clear stratification, with PS reaching up to 69% in the inner layer of FOG deposits adjacent to the concrete surface and lower PS (7%–29%) in the outer layer. Last, the hydrodynamic effects on the FOG deposit formation were investigated (Chapter 5). Previous studies used constant flow conditions and did not correlate hydrodynamic parameters with FOG formation. This study analyzed the spatial distribution, growth behavior, and chemical composition of FOG deposits under varying flow conditions. Results show that the velocity magnitude (V), turbulent kinetic energy (TKE), and vorticity (|ω|) of the FFA-oil mixture were 15%, 22%, and 2% higher than those of water, respectively. Rather than accumulating in all low-velocity areas (V 20 s-1. Higher TKE (> 0.0037 m2/s2) near the concrete surface inhibits FOG accumulation. Increased TKE promoted outer layer growth and reduced inner layer thickness, indicating competitive formation processes. This thesis provides critical insights into sewer blockages caused by wipes and FOG. The findings can help municipalities and utility firms assess sewer blockage risks, optimize maintenance strategies, and implement targeted mitigation measures, ultimately enhancing sewer system efficiency and resilience