Understanding the spatial formation and accumulation of fats, oils and grease deposits in the sewer collection system

摘要

Wastewater flowing through the sewer pipes may contain fats, oils and grease (FOG). Researchers haveshown that the reaction between these FOG and calcium leads to the formation of insoluble calcium saltsof fatty acids, which start to accumulate inside these pipes. This buildup of solids leads to blockage of thesewer lines, which in turn produce sanitary sewer overflows. In addition, hydraulic conditions, which varyspatially within the sewer system due to different piping variations i.e., manholes, pipes, wet wells couldlead to spatial variations in the formation of FOG deposits. Further, other obstructions like root intrusionsin pipes or pipe deformations such as pipe sags may also enhance the detrimental buildup of FOGdeposits. The purpose of this study was to quantify the spatial variation in FOG deposit formation andaccumulation in a pilot-scale sewer collection system. The pilot system contained different pipeconfigurations (i.e., straight pipes, manholes), obstructions (i.e., root intrusions) and pipe deformation(i.e., pipe sag). This study was operated under two pH conditions - alkaline (pH=10) and neutral pH(pH=7.5). Fog and calcium were injected into the pilot system to achieve a sewer concentration of 200and 50 ppm, respectively. The alkaline condition was designed to simulate alkali hydrolysis of FOG thatwill produce free fatty acid leading to the formation of insoluble solids. Preliminary results showed thatwhile both the pH conditions produced solid accumulation, the higher pH condition producedsignificantly more solids. FTIR-ATR analysis on the samples confirmed that the solids formed wereindeed calcium-based fatty acid salts. The results suggest that if calcium hydroxide is released at theconcrete pipe surface in the presence of FOG, then there may be the potential for alkali hydrolysis ofFOG to occur and enhance the formation of insoluble calcium salts of fatty acid.