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 have shown that the reaction between these FOG and calcium leads to the formation of insoluble calcium salts of fatty acids, which start to accumulate inside these pipes. This buildup of solids leads to blockage of the sewer lines, which in turn produce sanitary sewer overflows. In addition, hydraulic conditions, which vary spatially within the sewer system due to different piping variations i.e., manholes, pipes, wet wells could lead to spatial variations in the formation of FOG deposits. Further, other obstructions like root intrusions in pipes or pipe deformations such as pipe sags may also enhance the detrimental buildup of FOG deposits. The purpose of this study was to quantify the spatial variation in FOG deposit formation and accumulation in a pilot-scale sewer collection system. The pilot system contained different pipe configurations (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 200 and 50 ppm, respectively. The alkaline condition was designed to simulate alkali hydrolysis of FOG that will produce free fatty acid leading to the formation of insoluble solids. Preliminary results showed that while both the pH conditions produced solid accumulation, the higher pH condition produced significantly more solids. FTIR-ATR analysis on the samples confirmed that the solids formed were indeed calcium-based fatty acid salts. The results suggest that if calcium hydroxide is released at the concrete pipe surface in the presence of FOG, then there may be the potential for alkali hydrolysis of FOG to occur and enhance the formation of insoluble calcium salts of fatty acid.