Characterization of Washoff Behavior of In-Sewer Deposits in Combined Sewer Systems

In-sewer deposits in combined sewer systems (CSSs) are closely related with the behavior of first foul flush and combined sewer overflows. The artificial flushing experiment separating the washoff of in-sewer deposits from the inflow of surface pollutants was carried out to simulate first foul flush in a CSS. The washoff behaviors of each pollutant including chemical pollutants, bacterial indicators, and enteric viruses were intensively investigated. By using several morphological analyses, some of which were suggested through this study, the characteristics of first foul flush were examined. As a result, the washoff behaviors of each pollutant were characterized according to their (i) event load ratios (ELRs), (ii) time-series concentration and load curves, (iii) concentration vs. flow rate curves, and (iv) dimensionless runoff concentrations (DRCs). The first foul flush patterns of each parameter were categorized into 3 typical groups: the strong-, partial-, and no first foul flush group. The order of these groups signifies their different physicochemical properties of in-sewer deposits in CSSs, their strength of first foul flush phenomena, and the washoff priority as well.